Identification of Textile Fibers (Woodhead Publishing Series in Textiles) (

Identification of textile fibers
The Textile Institute and Woodhead Publishing
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Woodhead Publishing in Textiles: Number 84
Identification of
textile fibers
Edited by
Max M. Houck
Cambridge
New Delhi
Published by Woodhead Publishing Limited in association with The Textile Institute
Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington
Cambridge CB21 6AH, England
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Contents
Contributor contact details
Woodhead Publishing in Textiles
xi
xv
Part I: Textile fiber structure and characteristics
1
1
Introduction to textile fiber identification
M M Houck, West Virginia University, USA
References
3
Ways of identifying textile fibers and materials
M M Houck, West Virginia University, USA
Introduction
Identification and comparison of fibers
Classification of fibers
Pyrolysis gas chromatography
Analysis of fiber colors and dyes
Future trends
References
6
1.1
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Natural animal textile fibres: structure,
characteristics and identification
S R Tridico, Australian Federal Police, Australia
Introduction
Animal fibre growth, structure, composition and properties
Types of natural animal fibres
Natural animal fibre characteristics
Identification of natural animal fibres
Future trends
Sources of further information and advice
Acknowledgements
References
5
6
8
9
19
22
22
23
27
27
28
35
38
44
61
62
65
67
v
vi
Contents
4
Synthetic textile fibers: structure, characteristics and
identification
K Kajiwara, Otsuma Women’s University, Japan and
Y Ohta, Toyobo Co. Ltd, Japan
Introduction
Fundamental characteristics of fibrous materials
Common synthetic fibers
Crystal structure of synthetic fibers
Identification of synthetic fibers
References
4.1
4.2
4.3
4.4
4.5
4.6
5
5.1
5.2
5.3
5.4
5.5
5.6
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
High performance fibers: structure, characteristics
and identification
Y Ohta, Toyobo Co. Ltd, Japan and K Kajiwara,
Otsuma Women’s University, Japan
Introduction
The primary structure and physical properties of HPFs
Identification of high strength and high modulus fiber
Alternative methods for analyzing higher-order structure
Sources of further information and advice
References
The use of classification systems and production
methods in identifying manufactured textile fibers
K L Hatch, The University of Arizona, USA
Introduction
Polymer origins and fiber classification
PLA/polylactide fiber
Fiber subclasses
Multicomponent fibers
Future trends
Sources of further information and advice
References
68
68
72
84
86
87
87
88
88
88
95
107
109
109
111
111
112
115
119
123
126
127
128
Part II: Methods of fiber identification
131
7
133
7.1
7.2
7.3
7.4
Optical microscopy for textile fibre identification
M Wilding, The University of Manchester, UK
Introduction
Practical and quality control considerations
Initial identification based on physical appearance
Identification based on properties
133
134
137
139
Contents
7.5
7.6
7.7
7.8
Examples of more advanced microscopic techniques
Future trends
Sources of further information and advice
References
8
The use of spectroscopy for textile fiber
identification
M M Houck, West Virginia University, USA
Introduction: spectroscopy of fibers
Categorizing methods by nature of excitation
Categorizing methods by measurement process
Common methods of spectroscopy
References
8.1
8.2
8.3
8.4
8.5
9
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
10
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
11
Microspectrophotometry for textile fiber color
measurement
S Walbridge-Jones, Bureau of Alcohol, Tobacco,
Firearms and Explosives, USA
Introduction
An understanding of spectroscopy
Microspectrophotometer design
Types of microspectroscopy
Perception of color: human vs. machine
Metamerism
Applications of microspectroscopy in fiber analysis
Limitations, strengths, and future trends
References
Alternative and specialised textile fibre
identification tests
P H Greaves, Microtex, UK
Introduction
Alternative methods of fibre identification
Scanning electron microscopy
Further techniques
Benefits of scanning electron microscopy compared to
a light microscope
Quantitative aspects
Sources of further information and advice
References
Analysis of dyes using chromatography
S W Lewis, Curtin University of Technology, Australia
vii
147
150
152
156
158
158
159
159
160
163
165
165
166
167
169
172
175
175
178
180
181
181
181
187
195
197
199
200
201
203
viii
Contents
11.1
11.2
11.3
11.4
11.5
11.6
11.7
Introduction
Dyes
Forensic analysis of dyes
Conclusions
Sources of further information and advice
Acknowledgments
References
12
DNA analysis in the identification of animal fibers
in textiles
P F Hamlyn, BTTG Ltd, UK
Introduction
Extraction of DNA from animal fibers
Development of methods for using DNA analysis to
identify animal fibers
Effect of fiber processing on DNA analysis and the use
of DNA amplification technology
Future trends
Sources of further information and advice
References
12.1
12.2
12.3
12.4
12.5
12.6
12.7
Part III: Applications
13
13.1
13.2
13.3
13.4
13.5
13.6
14
14.1
14.2
14.3
14.4
14.5
14.6
14.7
Identifying plant fibres in textiles: the case
of cotton
S Gordon, CSIRO Materials Science and Engineering,
Australia
Introduction
Cotton fibre structure and composition
Cotton fibre properties
Future trends
Sources of further information and advice
References
The forensic identification of textile fibers
M M Houck, West Virginia University, USA
A forensic mindset
Microscopy of fibers
Manufactured fiber production and spinning
Polarized light microscopy
Fluorescence microscopy
Conclusions
References and further reading
203
204
206
219
220
220
220
224
224
226
227
229
233
235
236
237
239
239
241
245
256
256
256
259
259
261
262
266
271
272
273
Contents
15
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
15.9
15.10
15.11
15.12
15.13
15.14
15.15
15.16
15.17
15.18
16
16.1
16.2
16.3
16.4
16.5
16.6
16.7
16.8
Identifying and analyzing textile damage in
the textile industry
W D Schindler, University of Applied Sciences
Hof, Germany
Introduction: importance of and reasons for textile damage
analysis in the textile industry
Main types, manifestations and causes of textile damage
Methods of identifying and analyzing textile damage
Damage analysis according to the type of fiber
Damage analysis of cellulosics, especially cotton
Damage analysis of wool
Damage analysis of silk
General types of damage to synthetics
Analysis of damage to polyester fibers
Analysis of damage to nylon fibers
Analysis of damage to acrylic fibers
Analysis of damage to elastane (spandex) fibers
Analysis of damage to polyolefin fibers, especially
polypropylene
Special types of textile damage and their analysis
Sources of further information and advice
Conclusions
Acknowledgment
References
ix
275
275
278
280
292
292
295
303
304
306
309
311
312
317
320
325
326
327
327
The role of fibre identification in textile conservation
P Garside, University of Southampton, UK
Introduction
Analytical techniques
Conservation strategies
Case studies
Future trends
Sources of further information and advice
Acknowledgements
References
335
Index
366
335
337
349
351
356
357
357
358
This book is dedicated to my father, Max W. Houck (1917–2008).
‘Dad, I’m sorry you didn’t get a chance to read this one.’
Contributor contact details
(* = main contact)
Chapters 1, 2, 8 and 14
Chapter 4
M. M. Houck
Forensic Science Initiative
Forensic Business Research and
Development
West Virginia University
Morgantown
West Virginia
USA
E-mail: [email protected]
Professor K. Kajiwara*
Faculty of Home Economics
Otsuma Women’s University
12 Sanban-cho
Chiyoda-ku
Tokyo 102-8357
Japan
E-mail: [email protected]
Chapter 3
Silvana R. Tridico
Biological Criminalistics
Australian Federal Police
PO Box 401
Canberra
ACT
Australia 2601
E-mail: [email protected]
Dr Y. Ohta
Toyobo Co. Ltd
Toyobo Research Center
1-1 Katata 2-Chome
Otsu
Shiga 520-0292
Japan
E-mail: [email protected]
xi
xii
Contributor contact details
Chapter 5
Chapter 9
Dr Y. Ohta*
Toyobo Co. Ltd
Toyobo Research Center
1-1 Katata 2-Chome
Otsu
Shiga 520-0292
Japan
E-mail: [email protected]
S. Walbridge-Jones
Forensic Chemist-Trace Evidence
Bureau of Alcohol, Tobacco,
Firearms and Explosives
Forensic Science Laboratory
355 N. Wiget Lane
Walnut Creek, CA 94598
USA
E-mail: [email protected]
Professor K. Kajiwara
Faculty of Home Economics
Otsuma Women’s University
12 Sanban-cho
Chiyoda-ku
Tokyo 102-8357
Japan
E-mail: [email protected]
Chapter 6
Professor Kathryn L. Hatch
Department of Agricultural and
Biosystems Engineering
The University of Arizona
Shantz Building
Room 403
1177 E. 4th Street
Tucson, AZ 85721-0038
USA
E-mail: [email protected]
Chapter 7
Dr M. Wilding
School of Materials
The University of Manchester
Sackville Street Building
PO Box 88
Manchester M60 1QD
UK
E-mail: mike.wilding@manchester.
ac.uk
Chapter 10
Dr P. H. Greaves
Microtex
7 Newall Carr Road
Otley LS21 2AU
UK
E-mail: [email protected]
Chapter 11
S. W. Lewis
Curtin University of Technology
GPO Box U1987 Perth
Western Australia 6845
Australia
E-mail: [email protected]
Chapter 12
Dr P. Hamlyn
BTTG Ltd
Unit 14
Wheel Forge Way
Trafford Park
Manchester M17 1EH
UK
E-mail: [email protected]
Contributor contact details
Chapter 13
Chapter 16
S. Gordon
Cotton Research Unit
CSIRO Materials Science and
Engineering
Henry Street
Belmont
Victoria
Australia 3216
E-mail: [email protected]
P. Garside
Textile Conservation Centre
University of Southampton
Winchester Campus
Park Avenue
Winchester SO23 8DL
UK
E-mail: [email protected]
Chapter 15
Professor em. Wolfgang D.
Schindler
Fichtelgebirgsstraße 17
D-95126 Schwarzenbach
Germany
E-mail: [email protected]
xiii
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1
Introduction to textile fiber identification
M M HOUCK,
West Virginia University, USA
Abstract: The identification of fibers is critical to a number of industries,
including textiles, forensic science, fashion, and design. The actual
identification, however, varies with industry and method. Changes in
textile technology create a constant need to improve identification
methodology. The old methods – despite the increased pace of new
technology – are often the best. Microscopy still dominates the field for
analytical methods and provides a range of analysis barely possible with
any other method. Combined with spectroscopy, microscopy is the
quintessential fiber identification tool.
Key words: fiber identification, microscopy, methodology, spectroscopy.
Manufacturers use set methods to ensure a quality product fit for purpose.
This implies a market-based taxonomy with a company-product orientation, a supply chain of raw and processed materials, and explicit rules on
categories. For example, the American Association of Textile Chemists and
Colorists (AATCC) lists the following specified methods:
• colorfastness to commercial laundering and to domestic washing
• flammability of clothing textiles
• smoothness of seams in fabrics after repeated home laundering
• electrostatic propensity of carpets
• wrinkle recovery of fabrics: appearance method
• dimensional changes in textiles other than wool.
The titles to these methods indicate what is important to define in their
products. AATCC lists microscopy as a method to identify fibers but notes
that it should be used with caution since manufactured fibers are frequently
produced in a number of modifications which alter their appearance.1 The
Association also lists ‘reaction to flame’ (Table III) as a test method with
the following diagnostics: melts near flame, shrinks from flame, burns in
flame, etc.
By contrast, ASTM International lists the following in their methods:
•
•
•
flame resistant materials used in camping tentage
pile retention of corduroy fabrics
elastic properties of textile fibers
3
4
•
•
Identification of textile fibers
performance specifications for underwear fabrics, woven, men’s and boys’
commercial moisture regains for textile fibers.
While the view is similar to that of AATCC, ASTM International lists
infrared spectroscopy as the preferred method for fiber identification:
‘additional physical properties of the fibers such as density, melting point,
regain, refractive indices, and birefringence . . . are useful for confirming
the identification’ (Volume 7.01, D276). Both organizations treat the
samples as bulk for purposes of the possible identifying tests; after all, if
you make them, sample size should not be a problem.
By comparison, the forensic sciences treat microscopy as the primary
method for fiber identification:
Microscopic examination provides the quickest, most accurate, and least
destructive means of determining the microscopic characteristics and polymer
type of textile fibers. Additionally, a point-by-point, side-by-side microscopic
comparison provides the most discriminating method of determining if two or
more fibers are consistent with originating from the same source.2
Forensic fiber examiners use microscopy first and then other methods,
such as infrared spectroscopy, as confirmatory techniques. Why not start
with infrared spectroscopy, as do the manufacturers? A company that spins
fibers knows what products it makes; therefore, the universe of possible
answers is sufficiently limited. For a forensic scientist, however, fibers found
at a crime scene could conceivably be from nearly any source and you can’t
make assumptions. A rayon fiber and a cotton fiber will both show up as
‘cellulose’ on an infrared spectrum; a microscope easily distinguishes
between them. Two nylon 6,6 fibers may be chemically identical but have
different diameters, cross-sections, or birefringences.
The orientation of the analysis is different (comparison), although the
goal is partially the same (identification). Forensic fiber analysis routinely
has minimal samples with which to work and this structures what tests can
be used. Forensic fiber analysis is also interested in traits of which the
manufacturers may not be aware. For example, delustrants, typically titanium dioxide, are added to deluster or dull otherwise bright fibers. The
manufacturers add the delustrants during the fiber spinning process at a
certain rate to achieve the desired end goals with little regard for the distribution of the granules within the fiber. For a forensic scientist, however,
that distribution can be a significant comparator between two otherwise
similar samples: one fiber with large, aggregated granules is dissimilar to
one with small, evenly-distributed granules, all other factors being the same.
The difference could be batch-to-batch variation, plant-to-plant variation,
or some other node along the supply chain. Suffice it to say, the fiber manufacturer did not intentionally distribute the delustrant in such a way as to
aid the forensic scientist. Forensic scientists, therefore, add additional infor-
Introduction to textile fiber identification
5
Production analytical methods
Market taxonomy
company-product orientation
supply web
explicit rules on categories
Forensic analytical methods
After-market taxonomy
end use
as used
implicit rules on categories
Sharing methodologies
Different approaches due to different
goals – quality at lowest price for
manufacturing, reconstruction and
product tracking for forensics
Manufacturing
Forensics
1.1 Analytical methods used by manufacturers and forensic scientists.
mation to the market-based taxonomy with their own forensic taxonomy of
products. As the physicist P.W. Bridgman succinctly said, ‘The concept is
synonymous with the corresponding set of operations’,3 meaning, the
methods you use frame the orientation of your analysis from the start.
The analytical schemes used by both manufacturers and forensic scientists have value, although they may have different goals (Fig. 1.1). Some
methods are used exclusively by one group, others are shared, while some
shared methods have greater or lesser utility for the analyst. This book
strives to provide a broader perspective about the methods available for
fiber identification to create a fuller toolbox for the fiber analyst, regardless
of their scientific orientation.
1.1
References
1. American Association of Textile Chemists and Colorists, AATCC Technical
Manual, Research Triangle Park, NC: AATCC, 2008.
2. Scientific Working Group for Materials Analysis, Forensic Fiber Examination
Guidelines, 1999, Forensic Science Communications 1(1), online at www.fbi.gov.
3. Bridgman PW. The Logic of Modern Physics. New York, NY: McMillan Publishers,
1928; page 23.
2
Ways of identifying textile fibers
and materials
M M HOUCK, West Virginia University, USA
Abstract: The identification of fibers is critical to a number of
industries, including textiles, forensic science, fashion, and design.
The actual identification, however, varies with industry and method.
Changes in textile technology create a constant need to improve
identification methodology. The old methods – despite the increased
pace of new technology – are often the best. Microscopy still
dominates the field for analytical methods and provides a range
of analysis barely possible with any other method. Combined
with spectroscopy, microscopy is the quintessential fiber identification
tool.
Key words: fiber identification, microscopy, methodology, spectroscopy.
2.1
Introduction
The identification of fibers is an important component to the textile industry, forensic science, fashion designers, and the automotive industry, among
others. The process, however common across disparate industries, is conducted very differently in each. A quick look at the methods published by
various disciplines demonstrates what characteristics and properties are
important (Table 2.1). The American Association of Textile Chemists and
Colorists (AATCC) lists microscopy as a method to identify fibers but notes
that it must be used with caution on manufactured fibers since they are
produced in a variety of modifications which alter the appearance.1 AATCC
also lists ‘reaction to flame’ as a test method with the following categories
for test reactions: ‘melts near flame’, ‘shrinks from flame’, and ‘burns in
flame’. ASTM International, in their volumes on textiles, lists infrared
spectroscopy as the preferred method for fiber identification and adds,
‘additional physical properties of the fibers such as density, melting point,
regain, refractive indices, and birefringence . . . are useful for confirming
the identification’.1 This stands in stark contrast to the methods listed for
forensic casework which maximize analysis on minimum sample sizes; most
methods center on microscopy which the other groups find ‘useful’. Forensic
scientists typically get very little sample with which to work, often one or
6
Ways of identifying textile fibers and materials
7
Table 2.1 Various published methods for textile testing and fiber identification
Methods from the American Association of Textile Chemists and Colorists:
• Colorfastness to commercial laundering and to domestic washing
• Flammability of clothing textiles
• Smoothness of seams in fabrics after repeated home laundering
• Electrostatic propensity of carpets
• Wrinkle recovery of fabrics: appearance method
• Dimensional changes in textiles other than wool
Methods from ASTM International:
• Flame resistant materials used in camping tentage
• Pile retention of corduroy fabrics
• Elastic properties of textile fibers
• Performance specification for underwear fabrics, woven, men’s and boys’
• Commercial moisture regains for textile fibers
Methods from Scientific Working Group on Materials Analysis (forensic):
• Microscopy of textile fibers, including polarized light and fluorescence
microscopy
• Thin-layer chromatography of non-reactive dyes in textile fibers
• Pyrolysis-gas chromatography of textile fibers
• Infrared analysis of textile fibers
two individual fibers, while fiber producers have by comparison nearly
unlimited samples.
Analytical differences at the application level mask a deeper truth about
the textiles being tested. Materials used in commercial manufacturing have
to be fit for purpose or else they will not be economically viable as end-use
products.2,3 This extends even to natural products, such as wool or silk, which
can be thought of as ‘designed’ through domestic breeding programs or
feedback from customers which leads to intentional selection of raw materials. If the raw goods are selected for their product-specific desirable characteristics, then how much more so the methods employed to analyze them?
To the users of AATCC’s methods, smoothness of seams after laundering
is an important trait; customers do not like sewn textiles that pucker and
ruin the lines of a garment and will not buy them. In determining if a homicide suspect left fibers from his sweater on the victim’s body, however,
smooth seams are not a priority. It is more important to determine the
individual fiber’s morphological, optical, and chemical properties as minutely
as possible to be able to make as close an association as possible with the
textile in question. The two scientists have different goals, so they use different methods and thereby reveal separate analytical realities of the
materials.
8
Identification of textile fibers
Testing regime 1
Testing regime 2
2.1 Various analytical methods (rectangles) make up analytical
regimes (shades) which in turn create product taxonomies. These
taxonomies differ depending upon which traits are tested for and with
which methods. Often taxonomies do not intersect, like testing for
consumer goods versus testing for forensic casework; some methods
do overlap between disciplines (dark gray). The methods employed
for characterizing a fiber as ‘low moisture regain’ will differ from
those that identify it as ‘negative sign of elongation’.
A taxonomy of goods is constructed from such analytical realities (Fig.
2.1). One testing regime will place a particular product in one set or sets
while another might reclassify it elsewhere, although not necessarily radically. This is explained in Fig. 2.1.
2.2
Identification and comparison of fibers
The process of fiber analysis can be thought of in two phases – identification
and comparison. Although the methods used in these processes may be
similar, the goals of each are quite different. Identification is a process of
classification4 or placing the fiber into a group or set with shared characteristics. This involves observing the physical and chemical properties of the
fiber that help put it into sets with successively smaller memberships. These
properties can be observed by a combination of microscopy and chemical
analysis. Identification tests are performed prior to comparisons and every
effort should be made to conserve fibers for later comparison if the quantity
is limited.5,6
Ways of identifying textile fibers and materials
9
Comparison of fibers involves observing any correlation between fibers
from a questioned source (from a crime scene, a defective sample, or counterfeit seizure, for example) and fibers from a known source (such as an
item of clothing from the suspect, a quality assurance sample, or a known
manufacturing source). Although the goal of this analysis is to determine if
the two fibers could have come from the same source, each comparison test
is performed with an eye towards looking for significant differences between
the known and questioned fibers. Recognition of counterfeit fibers or textiles, for example, involves a set of known parameters against which the
suspect fibers are tested. If the suspect fibers do not compare favorably with
the known parameters, then they are more likely to be fakes. It is only when
all of the testing is complete and no significant differences are found that
is it possible to conclude that the known and suspected fibers exhibit the
same microscopic, optical, and chemical properties and therefore could
have come from the same source – that is, they are genuine.
2.3
Classification of fibers
A textile fiber is a unit of matter, either natural or manufactured, that forms
the basic element of fabrics and other textile structures. Specifically, a textile
fiber is characterized as having a length at least 100 times its diameter and
a form that allows it to be spun into a yarn or made into a fabric by various
methods. Fibers differ from each other in chemical structure, cross-sectional
shape, surface contour, color, as well as length and width.7–9 The diameter
of textile fibers is small, generally 0.0004 to 0.002 inch (in.), or 11–51
micrometers (μm). Their length varies from about 7/8 in. or 2.2 centimeters
(cm) to many miles. Based on length, fibers are classified as either filament
or staple fiber. Filaments are a type of fiber having indefinite or extreme
length, such as synthetic fibers which can be made to any length; silk is the
only naturally occurring filament. Staple fibers are natural fibers or cut
lengths of filament, typically being 1.5 to 8 in (3.75 to 20 cm) in length.
The size of natural fibers is usually given as a diameter measurement in
micrometers. The size of silk and manufactured fibers is usually given in
denier (in the US) or tex (in other countries). Denier and tex are linear
measurements based on weight by unit length. The denier is the weight in
grams of 9000 meters of the material fibrous. Denier is a direct numbering
system in which the lower numbers represent the finer sizes and the higher
numbers the larger sizes. Glass fibers are the only manufactured fibers that
are not measured by denier. A 1-denier nylon is not equal in size to a 1denier rayon, however, because the fibers differ in density. Tex is equal to the
weight in grams of 1000 meters (one kilometer) of the fibrous material.7
Fibers themselves are classified into two major classes: natural and manufactured. A natural fiber is any fiber that exists as such in the natural state,
10
Identification of textile fibers
such as cotton, wool, or silk. Manufactured fibers are made by processing
natural or synthetic organic polymers into a fiber-forming substance; they
can be classified as cellulosic or synthetic. Cellulosic fibers are either made
from regenerated or derivative cellulosic (fibrous) polymers, such as wood
or cotton. Synthetic fibers are formed from substances that, at any point in
the manufacturing process, are not a fiber; examples are nylon, polyester
and saran. No nylon or polyester fibers exist in nature and they are made
of chemicals put through reactions to produce the fiber-forming substance.10
The generic names for manufactured and synthetic fibers were established
as part of the Textile Fiber Products Identification Act enacted by the US
Congress in 1954.
2.3.1 The Textile Fiber Products Identification Act
Pursuant to the provisions of section 7(c) of the Textile Fiber Products
Identification Act [16 CFR Part 303], the US Federal Trade Commission
thereby established the generic names for manufactured fibers, together
with their respective definitions, as set forth in this section, and the generic
names for manufactured fibers, together with their respective definitions,
set forth in International Organization for Standardization (ISO) Standard
2076: 1999(E), ‘Textiles – Man-made fibres – Generic names’.
(a)
(b)
(c)
(d)
Acrylic. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 85% by weight
of acrylonitrile units.
Modacrylic. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of less than
85% but at least 35% by weight of acrylonitrile units except fibers
qualifying under paragraph (j)(2) of this section and fibers qualifying
under paragraph (q) of this section.
Polyester. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 85% by weight
of an ester of a substituted aromatic carboxylic acid, including but not
restricted to substituted terephthalate units, and para substituted
hydroxy-benzoate units. Where the fiber is formed by the interaction
of two or more chemically distinct polymers (of which none exceeds
85% by weight), and contains ester groups as the dominant functional
unit (at least 85% by weight of the total polymer content of the fiber),
and which, if stretched at least 100%, durably and rapidly reverts substantially to its unstretched length when the tension is removed, the
term elasterell-p may be used as a generic description of the fiber.
Rayon. A manufactured fiber composed of regenerated cellulose, as
well as manufactured fibers composed of regenerated cellulose in
Ways of identifying textile fibers and materials
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
11
which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups. Where the fiber is composed of cellulose
precipitated from an organic solution in which no substitution of the
hydroxyl groups takes place and no chemical intermediates are
formed, the term lyocell may be used as a generic description of the
fiber.
Acetate. A manufactured fiber in which the fiber-forming substance
is cellulose acetate. Where not less than 92% of the hydroxyl groups
are acetylated, the term triacetate may be used as a generic description of the fiber.
Saran. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 80% by weight
of vinylidene chloride units (—CH2—CCl2—).
Azlon. A manufactured fiber in which the fiber-forming substance is
composed of any regenerated naturally occurring proteins.
Nytril. A manufactured fiber containing at least 85% of a long chain
polymer of vinylidene dinitrile (—CH2—C(CN)2—) where the vinylidene dinitrile content is no less than every other unit in the polymer
chain.
Nylon. A manufactured fiber in which the fiber-forming substance is
a long-chain synthetic polyamide in which less than 85% of the amide
linkages are attached directly to two aromatic rings.
Rubber. A manufactured fiber in which the fiber-forming substance
comprises natural or synthetic rubber, including the following
categories:
(1) A manufactured fiber in which the fiber-forming substance is a
hydrocarbon such as natural rubber, polyisoprene, polybutadiene, copolymers of dienes and hydrocarbons, or amorphous
(noncrystalline) polyolefins.
(2) A manufactured fiber in which the fiber-forming substance is a
copolymer of acrylonitrile and a diene (such as butadiene) composed of not more than 50% but at least 10% by weight of
acrylonitrile units. The term lastrile may be used as a generic
description for fibers falling within this category.
(3) A manufactured fiber in which the fiber-forming substance is a
polychloroprene or a copolymer of chloroprene in which at least
35% by weight of the fiber-forming substance is composed of
chloroprene units.
Spandex. A manufactured fiber in which the fiber-forming substance
is a long chain synthetic polymer comprised of at least 85% of a segmented polyurethane.
Vinal. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 50% by weight
12
Identification of textile fibers
of vinyl alcohol units (—CH2—CHOH—), and in which the total of
the vinyl alcohol units and any one or more of the various acetal units
is at least 85% by weight of the fiber.
(m) Olefin. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 85% by weight
of ethylene, propylene, or other olefin units, except amorphous (noncrystalline) polyolefins qualifying under paragraph (j)(1) of this
section. Where the fiber-forming substance is a cross-linked synthetic
polymer, with low but significant crystallinity, composed of at least
95% by weight of ethylene and at least one other olefin unit, and the
fiber is substantially elastic and heat resistant, the term lastol may be
used as a generic description of the fiber.
(n) Vinyon. A manufactured fiber in which the fiber-forming substance is
any long chain synthetic polymer composed of at least 85% by weight
of vinyl chloride units (—CH2—CHCl—).
(o) Metallic. A manufactured fiber composed of metal, plastic-coated
metal, metal-coated plastic, or a core completely covered by metal.
(p) Glass. A manufactured fiber in which the fiber-forming substance is
glass.
(q) Anidex. A manufactured fiber in which the fiber-forming substance is
any long-chain synthetic polymer composed of at least 50% by weight
of one or more esters of a monohydric alcohol and acrylic acid,
CH2=CH—COOH.
(r) Novoloid. A manufactured fiber containing at least 85% by weight of
a cross-linked novolac.
(s) Aramid. A manufactured fiber in which the fiber-forming substance
is a long-chain synthetic polyamide in which at least 85% of the amide
linkages are attached directly to two aromatic rings.
(t)
Sulfar. A manufactured fiber in which the fiber-forming substance is
a long-chain synthetic polysulfide in which at least 85% of the sulfide
(—S—) linkages are attached directly to two aromatic rings.
(u) PBI. A manufactured fiber in which the fiber-forming substance is a
long chain aromatic polymer having reoccurring imidazole groups as
an integral part of the polymer chain.
(v) Elastoester. A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer composed of at least 50% by
weight of aliphatic polyether and at least 35% by weight of polyester,
as defined in paragraph (c) of this section.
(w) Melamine. A manufactured fiber in which the fiber-forming substance
is a synthetic polymer composed of at least 50% by weight of a crosslinked melamine polymer.
(x) Fluoropolymer. A manufactured fiber containing at least 95% of a longchain polymer synthesized from aliphatic fluorocarbon monomers.
Ways of identifying textile fibers and materials
(y)
13
PLA. A manufactured fiber in which the fiber-forming substance is
composed of at least 85% by weight of lactic acid ester units derived
from naturally occurring sugars.
2.3.2 Natural textile fibers
A number of important vegetable fibers, such as cotton, jute and sisal, and
animal fibers, such as wool, camel and silk, appear as evidence. Vegetable
fibers are characterized primarily by microscopy11–13 as are animal fibers,
which, except for silk, are hairs.13–15
Vegetable fibers
The three major sources for fibers derived from plants are the seed, the stem
and the leaf, depending upon which source works best for a particular plant.
Plant fibers are found in two principal forms: the technical fiber, used in
cordage, sacks, mats, etc., or individual cells, as in fabrics or paper. The examination of technical fibers should include a search for internal structures, such
as spiral elements or crystals, and the preparation of a cross-section; additionally, a chemical test for lignin may be done. Technical fibers should be
macerated, the fabrics teased apart, and paper re-pulped for the examination of individual cells. The relative thickness of the cell walls and the size,
shape and thickness of the lumen, cell length, and the presence, type, and
distribution of dislocations should be noted. The direction of twist of the
cellulose in the cell wall can also be determined by the dry twist test.6 Other
characteristic cells should be noted and compared to reference specimens.
The most common plant fibers encountered are cotton, flax, jute, hemp,
and sisal.7,8 Bast fibers are those derived from the stems of the plant, leaf
fibers from the leaves and seed fibers from in and around the reproductive
pod. The basic plant materials are extensively processed before being incorporated into the final product.7,8,11,16 For additional information about plant
fiber identification, the interested reader should consult references in
Gaudette’s chapter6 and Mauersberger.16
Bast fibers
Flax (linen) is derived from the stem of Linum usitatissimum. It has a
clockwise twist and may range in diameter from 40 to 80 μm. The ultimates
are polygonal in cross-section, with thick walls and small lumina.
Microscopically, the fibers have dark dislocations that are roughly perpendicular to the long axis of the fiber. Flax may be ‘cottonized’, a process
similar to mercerization, and may present a cottony appearance. Flax is
most often found in clothing and household textiles.
14
Identification of textile fibers
Jute (Corchorus capsularis) appears bundled microscopically and may
have a yellowish cast. The ultimates are polygonal but angular with mediumsized lumina. It can be distinguished easily from flax by its counterclockwise twist. The dislocations appear as angular X’s or Y’s and may be
numerous. Jute is used in cordage, rugs, and hardware cloth among other
products.
Ramie (Boehmeria nivea) has very long and very wide ultimates, with the
width ranging from 25 up to 75 μm. The walls are thick and appear flattened
in cross-section. Ramie has frequent, short dislocations and longer transverse striations. In cross-section, radial cracks may be present. Ramie may
be used in ropes, sacking as well as some clothing items.
With more bundled ultimates, a wider lumen and fewer nodes, hemp
(Cannabis sativa) is easy to distinguish from flax. Hemp also has a counterclockwise twist. Cross-sectioning hemp helps in distinguishing it from jute
in that hemp’s lumina are rounder and more flattened than jute’s. Hemp
may also have a brownish cast to it. The products hemp is most often found
in are ropes, bags, and, occasionally, clothing.
Leaf fibers
Sisal (Agave sisilana) is relatively easy to identify due to its irregular lumen
size, acicular crystals, spiral elements and annular vessels. Sisal has a counterclockwise twist. In cross-section, sisal looks somewhat like cut celery. Sisal
is used in carpets, ropes, twine and floor mats.
Although potentially difficult to distinguish from sisal on a slide mount,
abaca (Musa textilis) has many characteristics that help to identify it. Its
ultimates have a uniform diameter and a waxy appearance; often it is darker
than sisal; also they are polygonal in cross-section and vary in size. Abaca
may present spiral elements but often will have stegmata which are visible
as small crown-like structures. Abaca, like sisal, has a counter-clockwise
twist. Ropes, cordage and floor mats are typical sources of abaca.
Seed fibers
Cotton is by far the single most common textile fiber and accounts
for approximately half of all textile fibers processed each year. Mature
cotton has a flat twisted ribbon-like appearance which is easy to identify.
Cotton fibers are made up of several spiralling layers around a central
lumen. About half of the world’s cotton is grown in the United States. The
appearance of cotton can be modified by chemical finishing to produce a
desired result. Mercerization, for example, is the process where cotton fibers
are soaked in sodium hydroxide and this causes the fibers to untwist
and swell.
Ways of identifying textile fibers and materials
15
Kapok fiber is used primarily for life preservers and upholstery padding.
The fibers are hollow, producing very buoyant products, but are brittle,
which precludes spinning or weaving. Various synthetics, such as polyester
and polyurethanes, are the main competition for kapok.
Coir (Coco nucifera) comes from the husk of the coconut and, accordingly, is a very dense, stiff fiber easily identified microscopically. On a slide
mount, coir appears very dark brown or opaque with very large, coarse
ultimates. Coir also has a distinctive cross-sectional shape. Coir is usually
found in floor or door mats.
Animal fibers
Textile fibers derived from animal sources are typically the hairs of mammals,
such as sheep’s wool. Animals produce three main types of hairs: vibrissae
(whiskers), guard, and fur. Guard hairs are the relatively long, thick hairs
which cover the main portion of an animal’s body. Guard hairs are the most
useful in identifying and comparing animal hairs. Fur hairs, by contrast, are
small, thin hairs that provide bulk and warmth; microscopically, they are
not very distinctive and may appear similar between otherwise dissimilar
animals. The microscopy of hairs has been covered elsewhere14,17–19 and the
interested reader is directed to those references.
The importance of silk, produced by the Bombyx silkworm, should not
be underestimated, however. Recently, silk, especially spider silk, has been
of intense research interest for its tensile strength and other phenomenal
physical properties.20–22
2.3.3 Manufactured textile fibers: physical and
optical analysis
Microscopic analysis
The microscope is the primary tool for fiber analysis5,6,23–28 and the applications range from simple stereomicroscopy through higher power optical
and polarizing microscopy to scanning electron microscopy. Instrumentation
routinely has integrated microscopes to analyze small samples. Despite the
power of modern computerized instrumentation, microscopy should always
come first.26 Low- and high-power optical microscopy is the most commonly
employed of all of the microscopic techniques. It is also the most discriminating method because most textile fibers can be excluded from a known
sample by size, shape, color, or some other easily observable microscopic
characteristic. Stereomicroscopes, polarized light microscopes, comparison
microscopes, and fluorescent light microscopes are all used in the identification of fibers as well as in comparison of known and questioned fibers.
16
Identification of textile fibers
1.7
polyester 0.153
1.65
nparallel
1.6
polyethylene 0.042
1.55
polypropylene
0.03
nylon 0.048
rayon 0.03
modacrylic 0.015
acrylic 0.003
1.5
acetate 0.003
1.45
1.45
1.47
1.49
1.53
1.51
nperpendicular
1.55
1.57
1.59
2.2 Average refractive indices and birefringence (next to fiber type) for
manufactured fibers. Birefringence is defined as nparallel minus
nperpendicular.
A polarized light microscope is a central tool for the identification and
analysis of manufactured fibers. Many characteristics of manufactured fibers
can be viewed in non-polarized light, however, and these provide a fast,
direct and accurate method for the discrimination of similar fibers. Given
proper training and experience, a fiber examiner can identify a fiber’s
generic class simply by its microscopic characteristics and optical properties.29,30 Refractive index and birefringence are the two most distinguishing
features for the identification of a fiber’s generic class5,30–34 (Fig. 2.2). A
comparison light microscope is required to confirm whether the known and
the questioned fibers truly present the same microscopic characteristics.5
The cross-section is the shape of an individual fiber when cut at a right
angle to its long axis. The shapes of manufactured fibers vary with the
desired end result, such as the fiber’s soil hiding ability or a silky or coarse
feel to the final fabric.7 Some fiber types tend to stay within certain crosssectional families, for example acrylics tend to appear as bean-shaped fibers
and rayon tends to be irregular. The particular cross-section also may be
indicative of a fiber’s intended end-use: many carpet fibers have a lobed
shape to help hide dirt and create a specific visual texture to the carpet. A
physical cross-section should be prepared and numerous approaches have
been published for cross-sectioning. The method outlined by Palenik
and Fitzsimmons35,36 is simple, inexpensive and conservative of sample.
Additional methods, including fiber microtomes, fibers suspended in epoxyfilled pipette tips, and Teflon-coated slides, have been published.5,37,38
Ways of identifying textile fibers and materials
17
r
R
2.3 Modification ratio is defined as R/r.
The modification ratio of a fiber is a geometrical measurement used in
the characterization of trilobal fiber cross-sections. The modification ratio
is the difference in size between the outside diameter of the fiber and the
diameter of the core (Fig. 2.3). Many manufacturers use modification ratios
in the descriptions of their fibers for patent purposes.6,39,40
The way a fiber’s diameter is measured is dependent upon its crosssectional shape; there is more than one way to measure the diameter of a
non-round fiber. Manufactured fibers can be made in diameters from about
6 μm (so-called microfibers40) up to monofilament fibers used in fishing line,
which can be as large as 1 mm.7–9 By comparison, natural fibers vary in
diameter from cultivated silk (10–13 μm) to US sheep’s wool (up to 40 μm
or more) and human head hairs range from 50–100 μm. A manufactured
fiber greater than 40 μm is probably a carpet-type fiber.41
Some manufactured fibers retain air-pockets or voids after production.
For example, wet-spun fibers, such as acrylics, may have voids that range in
size from submicron up to several microns. Voids are created when pores
in the solidifying fiber are filled with a mixture of solvent and non-solvent
fluids.9 The size, shape, distribution, and concentration of voids are related
to the composition and production methods of the fiber and are an important comparative feature. Inclusions are materials or discontinuities that are
placed or occur in fibers. These may be accidental inclusions, such as the
draw marks sometimes seen in melt spun fibers or intentional inclusions,
such as large clumps of delustrant or anti-static materials.7,10
Delustrants are finely ground particles of materials, such as titanium
dioxide, that are put into the fiber as it is made. These particles diffract
light passing through the fibers, reducing their luster. Fibers are classified
in the textile industry as bright, semi-bright, or dull;10,16 forensic scientists
classify fibers as slightly, moderately, or heavily delustered.6 The size,
shape, distribution, and concentration of delustrant granules should be
noted.
18
Identification of textile fibers
A fiber’s construction is an important indication of its production and end
use. Examples are bicomponent fibers (two or more polymer types spun in
a sheath/core or bilateral relation), biconstituent fibers (two different polymers spun together from a homogeneous solution), or microfibers. These
specialty fibers are distinctive and particular attention should be paid to
their construction and composition if they are recovered as evidence.42
A polarizing light microscope will be necessary for determining a fiber’s
optical properties. These properties include refractive indices, retardation,
birefringence, sign of elongation, and dichroism. There are a number of
excellent works that describe the use of polarized light microscopy for the
analysis of fibers.26,43 In his chapter on fiber analysis in Maehly and Williams’
Forensic Science Progress, Grieve44 included a thorough discussion of microscopic methods. Gaudette also presented microscopic techniques is his
chapter on fiber analysis in Saferstein’s Forensic Science Handbook.6
Schemes of analysis for textile fibers by polarized light are used in training
and have been published.28
Another important microscopic technique is thermal microscopy, which
can be accomplished conveniently using a commercial hot stage. In this
apparatus, a computer-controlled hot stage is mounted on the stage of the
microscope. A very small piece of the fiber is then placed on a special slide
and inserted into the hot stage. The melting point range can be observed
and recorded. This technique can be used to distinguish between certain
subclasses of synthetic fibers that differ only in polymer structure,45 such as
some nylons and polyesters.46 Table 2.2 contains the softening and melting
points for some common fibers.
Thermal microscopy can also be used to accurately determine the
refractive indices of fragments of fiber. The fiber is mounted in a liquid with
a refractive index slightly above that of the fiber and then put in the
Table 2.2 Softening and melting points for some synthetic
fibers5–7,9,10,11,16
Softening
Acetate
Triacetate
Nylon
Polyester
Olefin
Acrylic
Modacrylic
Saran
Melting
°F
°C
°F
°C
364
482
340
445–490
260–320
473–490
300
285
184
250
171
229–254
127–160
245–254
149
141
500
550
415–509
450–500
275–338
–
370
334
260
288
213–265
250–260
135–170
–
188
168
Ways of identifying textile fibers and materials
19
hot stage. As the temperature of the hot stage is increased, the refractive
indices of the liquid and the fiber decrease, but the decrease in the refractive
index of the fiber is negligible compared to that of the liquid. When the
Becke line disappears, the computer records the temperature. Some programs are capable of storing the standard refractive index and coefficient
of temperature, and will thus keep a running track of the refractive index
of the liquid as the temperature is raised. Dispersion curves can also be
generated this way, if suitable interference filters or a monochromator are
available.6,26,30,32,43
At times, it may be necessary to examine the ends or the surface of a
fiber or textile at higher power than is available with light microscopy. The
scanning electron microscope (SEM) can be used to visualize the surface
of fibers and textiles for cross-sectional shape; it is also very useful for
characterizing damage.47–50 In assessing the type of damage evident on a
textile and evaluating the possible instrument(s) that could have caused it,
empirical testing is crucial. Many papers outline various methods of causing
and evaluating fabric and fiber damage. The book by Hearle, Lomas and
Cooke48 offers a wealth of visual and descriptive data and it should be in
the library of any laboratory that performs fabric examinations.
Chemical analysis
Before the development of modern instrumentation, solubility tests were
used widely to characterize fibers. While destructive and not as specific as
instrumental tests (such as infrared spectroscopy), solubility tests are easy
and quick to perform. A small portion of the questioned fiber is placed under
a coverslip and viewed microscopically. A drop of the appropriate solvent is
applied with a pipette and the changes are recorded. Schemes for solubility
tests have been published and the specific test employed depends upon the
resources, materials and protocols of the particular laboratory.6,45,46 There
are several instrumental tests for characterization and comparison of
synthetic fibers. These include pyrolysis-gas chromatography, infrared
spectroscopy, Raman spectroscopy and, less often, mass spectrometry,
to determine or confirm the generic polymer class and/or sub-class.
Microspectrophotometry in the ultraviolet and visible wavelength ranges is
used to characterize the color of dyed or pigmented fibers. And, finally,
fluorescence microspectrophotometry and high performance liquid chromatography are employed in the analysis of the dyes used to color fibers or
textiles.
2.4
Pyrolysis gas chromatography
Most synthetic fibers are polymers that contain one or two monomers. If
such a fiber is heated to high enough temperatures in an inert environment,
20
Identification of textile fibers
the polymers will decompose. If pyrolysis is carried out under controlled
conditions in the inlet of a gas chromatograph (GC), the resultant array of
peaks will be highly reproducible, although some variation will exist from
day to day and among different instruments. This array of peaks is highly
specific and sensitive to small differences in the polymer backbone.5 In
order to identify a polymer class or subclass, it is necessary to analyze
known fibers under the same conditions (consecutively with the questioned
fibers, if possible) and compare these results with the results of the questioned fibers. Many forensic laboratories have pyrolysis GC instruments,
making it readily available to trained personnel. A noted problem with
pyrolysis GC as a forensic analysis is that it is destructive. As with any
chromatography technique, shifts in the retention times due to injection
techniques or instrument maintenance limits the use of reference
chromatograms. The addition of mass spectrometry to identify individual
pyrolytes somewhat compensates for this problem.
2.4.1 Infrared spectroscopy
Substances absorb infrared (IR) energy characteristic of the atoms and
bonds that make up their molecules. Because each substance is made up of
different molecules, the wavelengths of infrared radiation are different from
all other substances. The absorption of infrared radiation corresponds to
the vibrating and rotating of all or parts of the molecules. Infrared spectra
can be quite complex and, because of their specificity, allow for the easy
identification of a fiber’s generic polymer class and subclass.29,51,52
In modern instruments, energy from an infrared source is directed through
the fiber by an interferometer. A computer keeps track of the wavelength
and amount of light transmitted through the fiber at each wavelength, the
result being a plot of wavenumber (cm−1) against intensity. Infrared spectra
are highly reproducible from day to day and from instrument to instrument,
making the development of spectral libraries feasible. Numerous polymer
libraries are available commercially.
Most of the recent work in IR has been devoted to distinguishing subclasses of generic fiber classes.29,42,51–53 The application of Fourier transforms
to forensic fiber analysis has been well documented and these instruments
are now commonplace in laboratories. Studies have highlighted the usefulness of FT-IR in casework, particularly as a confirmation method for microscopy work and as a technique for finer discrimination of subgeneric classes
of fibers, such as acrylics and nylons.54–56 FT-IR currently serves as an important tool in the analytical scheme of fibers for forensic purposes.5
Fiber analysis by FTIR is usually carried out on single fibers. This requires
that a microscope be interfaced with the IR instrument. In most instruments, the microscope is mounted in an auxiliary module that is connected
Ways of identifying textile fibers and materials
21
to the FTIR via a light pipe or a set of mirrors. The microscope may have
its own detector or may use the one in the main instrument. Another consideration is sample preparation: the fiber may have to be flattened to
reduce the effects of spectral distortion due to the diffraction caused by the
fiber’s cross-section. There are a number of techniques available for flattening fibers. A metal roller is an effective and easy way to flatten fibers. The
fiber is flattened on the smooth portion of a clean glass microscope slide
with the roller; the fiber is then carefully transferred to the specimen holder
and then to the microscope stage.
Sometimes the available fiber evidence is too short or too thin to obtain
an FTIR spectrum by conventional means. In such cases, attenuated total
reflectance (ATR), also called internal reflection, spectroscopy, may be
used. In this technique, the fiber is placed in tight contact with an IR transparent, high refractive index crystal. When IR radiation is passed into the
sample and crystal at angles greater than the critical angle, total reflection
takes place, and an excellent IR spectrum can be obtained.
2.4.2 Raman spectroscopy
Raman spectroscopy is an IR technique different from, but complementary
to, traditional IR spectroscopy. It involves the measurement of bond vibrations by a light scattering method. Many infrared modes that are weak or
not permitted in IR are very strong in Raman. For example, IR spectroscopy requires that a vibration causes a change in dipole moment in the
molecule. Nonpolar bonds have vibrations that do not result in this change
and are thus IR inactive but strong in Raman. Raman spectroscopy has
been in existence since 1928 but has recently come into its own as a spectroscopic method due to technological advances in monochromators, lasers,
filters and CCD devices.57–59
An early paper on the application of Raman spectroscopy characterized
natural and synthetic fibres, organic and inorganic in composition. Both
methods were used in tandem with a diamond cell used to obtain FTIR
spectra.59 Dye or pigment information is also available with some Raman
methods.60,61 The largest collaborative forensic study undertaken to date on
Raman was conducted by the European Fibres Group (EFG), a working
group of fiber experts from across Europe.62 Three dyed fiber samples, two red
acrylic and one red wool, were analyzed by six different makes and models of
Raman instrumentation from 458 nm to 1064 nm. Blue (488 nm) and green
(532 nm) lasers produced the best overall results for spectral quality.63 The
authors recommend a Raman instrument that can be tuned over the ranges
listed previously to account for luminescence effects. This study also shows
that Raman spectroscopy can identify the main dye type present in a color
fiber but minor dye(s) may be obscured and difficult to identify.
22
Identification of textile fibers
2.5
Analysis of fiber colors and dyes
The vast majority of fibers used in commercial applications are colored.
Fiber color is one of the most important properties in the comparison of
fibers and is thus a critical test in the analytical scheme. Synthetic dyes and
pigments belong to 29 different chemical categories with more than a dozen
different application methods. Even simple dyes might require between
eight and ten processes to convert the raw materials into a finished dye.
Given that the total annual production of any particular dye might not
amount to more than ten tons and that small process batches are becoming
the rule in the dyeing industry,64 color becomes a powerful discriminating
property. The selection of dyes is based upon many factors that, while not
based on the final desired color, nevertheless affect the textile’s appearance.65 Color is particularly significant when the gamut of colors is considered: literally, millions of shades are possible in textiles. When these colors
are spread out across the range of garments and carpeting produced in any
one year, and ‘multiplied’ by the number of garments and carpets produced
in previous years, the importance of color cannot be underestimated.
Besides polymer class, color may be the single most discriminating trait the
fiber examiner observes.
There are a variety of methods for characterizing either the color of
and/or the dye(s) in the fibers. They fall into three major categories: visual,
chemical, and instrumental. The visual method is the simple observation
and comparison of the fiber colors by use of the unaided eye. Visual comparison is easy, fast, accurate and non-destructive. It is a crucial first step
in any fiber comparison, as many otherwise similar fibers can be excluded
from consideration by color.66 Chemical methods, which include thin-layer
chromatography and high performance liquid chromatography, address the
make-up of the dyes used to color the fiber. This latter statement is an
important distinction: analyzing the color of a fiber is not the same as
analyzing the dyes used to color that fiber. Instrumental analyses include
microspectrophotometry in the UV and/or visible ranges and, more rarely,
spectrophotometric measurement of fluorescence.
2.6
Future trends
Keeping up with fiber technology is all but impossible. Changes in technology create potential new products and applications and, of course,
consumer demands for the next big thing drive the entire system:
Every market into which the consumer’s fashion sense has insinuated itself is,
by that very token, subject to this common, compelling need for unceasing
change in the styling of its goods . . . No single style of design, no matter how
brilliantly it is conceived, can claim any independent fashion significance at all,
nor can it possess more than a fugitive lease on life.67
Ways of identifying textile fibers and materials
23
The fugitive life of textile fibers creates a swirl of technology, fashion,
analysis, and interpretation. New applications in medical textiles, micro- and
nano-fibers, geotextiles, and specialty fibers, such as Spectra®, broaden and
deepen the analytical world of the fiber expert.
Yet, as far as analysis goes, the old methods are the best. Microscopy still
dominates the field for analytical methods and provides a range of analysis
barely possible with any other method. Combined with an analytical bench,
such as infrared or Raman, it is the quintessential fiber identification tool.
Microscopy should be the first method of choice for any fiber scientist.
2.7
References
1. D-276-87 Standard Test Methods for Identification of Fibers in Textiles, I. ASTM,
Editor. 2003, ASTM International: Philadelphia, PA.
2. Molotch, H., Where stuff comes from: How toasters, toilets, cars, computers, and
many other things come to be as they are. 2003, New York: Routledge.
3. Norman, D., The psychology of everyday things. 1988, New York: Basic
Books.
4. Thornton, J., Ensembles of class characteristics in physical evidence examination. Journal of Forensic Sciences, 1986. 31(2): p. 501–503.
5. SWGMAT, Forensic Fiber Analysis. Forensic Science Communications, 1999.
1(1).
6. Gaudette, B.D., Forensic Fiber Analysis, in Forensic Science Handbook, Volume
3, R. Saferstein, Editor. 1993, Prentice-Hall, Inc.: Englewood Cliffs, NJ.
7. Hatch, K., Textile Science. 1993, St. Paul, MN: West Publishing.
8. Yeager, J. and L. Teter-Justice, Textiles for Residential and Commerical Interiors.
2001, New York: Fairchild Publications.
9. Ziabicki, A., Fundamentals of Fibre Formation. 1976, New York City, N.Y.:
John Wiley and Sons, Inc.
10. Kroschwitz, J., Polymers: Fibers and Textiles, A Compendium. 1990, New York:
John Wiley & Sons.
11. The Textile Institute, Identification of Textile Materials. 1975, Manchester, UK:
The Textile Institute.
12. Catling, D. and J. Grayson, Identification of Vegetable Fibres. 1982, London, U.K.:
Chapman & Hall, Ltd.
13. Cook, J., Handbook of Textile Fibers I: Natural Fibers. 1969, Metuchen, NJ:
Textile Book Service.
14. Brunner, H., The Identification of Mammalian Hair. 1974, Melbourne: Inkata
Press.
15. Palenik, S., Light microscopy of medullary microstructure in hair identification.
Microscope 1983. 31: p. 129–137.
16. Mauerberger, H.R., Matthew’s Textile Fibers. 6th ed. 1954, New York City,
New York: John Wiley and Sons, Inc.
17. Hicks, J.W., Microscopy of Hairs, Federal Bureau of Investigation. 1977,
Washington, D.C.: Federal Bureau of Investigation.
18. Robertson, J., ed. Forensic Examination of Hair. Forensic Science Series, ed.
J. Robertson. 1999, Taylor and Francis: Philadelphia, PA.
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Identification of textile fibers
19. Bisbing, R., Forensic Hair Comparisons, in Forensic Science Handbook,
R. Saferstein, Editor. 2002, Prentice-Hall: Englewood Cliffs, NJ.
20. Garrido, M., et al., Active control of spider silk strength: comparison of drag line
spun on vertical and horizontal surfaces. Polymer, 2001. 43(4): p. 1537–1540.
21. Hinman, M., J. Jones, and R. Lewis, Synthetic spider silk: A modular fiber.
TIBTECH, 2000. 18(September): p. 374–379.
22. Arcidiacono, S., et al., Purification and characterization of recombinant spider
silk expressed in Escherichia coli. Applied Microbiology and Biochemistry, 1998.
49(1): p. 31–38.
23. Hinsch, J., The Technology of the Polarized Light Microscope. Fiber Producer,
1983. 11(3): p. 10–20.
24. Locard, E., The analysis of dust traces. The American Journal of Police Science,
1930. I: p. 176–249.
25. Locard, E., Manual of Police Techniques. 3rd ed. 1939, Paris: Payot.
26. McCrone, W.C., L.B. McCrone, and J.G. Delly, Polarized Light Microscopy. 1978,
Ann Arbor, MI: Ann Arbor Science.
27. Siegel, J.A. and M.M. Houck, Forensic Textile Fiber Analysis, in Forensic Sciences,
C. Wecht, Editor. 2001: Philadelphia, PA.
28. Stoeffler, S.F., A flowchart system for the identification of common synthetic
fibers by polarized light microscopy. Journal of Forensic Sciences, 1996. 41:
p. 297–299.
29. Tungol, M., A. Montaser, and E. Bartick, Analysis of single polymer fibers by
fourier transform infrared microscopy: The results of case studies. Applied
Spectroscopy, 1990. 44: p. 1655–1658.
30. Coyle, T., R. Robson, and P. Bauer, Identification of lyocell using dispersion
staining. Science and Justice, 2002. 2: p. 75–79.
31. Gorski, A. and W. McCrone, Birefringence of fibers. Microscope 1998. 46:
p. 3–16.
32. Heyn, A.N.J., Observations of the birefringence and refractive index of synthetic
fibers with special reference to their identification. Textile Research Journal,
1952. 22: p. 513–522.
33. Johri, M.C. and D.P. Jatar, Identification of some synthetic fibers by their
birefringence. Journal of Forensic Sciences, 1979. 24: p. 692–697.
34. Thetford, A. and S.C. Simmens, Birefringence phenomena in cylindrical fibres.
Journal of Microscopy, 1969. 89: p. 143–150.
35. Palenik, S.J. and C. Fitzsimons, Fiber Cross Sections: Part 1. Microscope, 1990.
38: p. 187–195.
36. Palenik, S.J. and C. Fitzsimons, Fiber Cross Sections: Part 2. Microscope, 1990.
38: p. 313–320.
37. Grieve, M. and Paterson, Preparation of fiber cross sections. Laboratory Practice,
1967. 13: p. 167–171.
38. Grieve, M.C., Identification of polyester fibres in forensic science. Journal of
Forensic Sciences, 1977. 22: p. 390–402.
39. Deadman, H.A., The importance of trace evidence, in Trace Evidence Analysis:
More Cases from Mute Witnesses, M.M. Houck, Editor. 2003, Academic Press:
San Diego, CA.
40. Clayson, N. and K. Wiggins, Microfibers – a forensic perspective. Journal of
Forensic Sciences, 1997. 42: p. 4–7.
Ways of identifying textile fibers and materials
25
41. Deadman, H.A., Fiber evidence and the Wayne Williams trial: Part I. FBI Law
Enforcement Bulletin, 1984. 53(3): p. 12–20.
42. Grieve, M., J. Dunlop, and T. Kotowski, Bicomponent acrylic fibres – their
characterization in the forensic science laboratory. Journal of the Forensic
Science Society, 1988. 28: p. 25–34.
43. Carroll, G.R., Forensic Fibre Microscopy, in Forensic Examinations of
Fibres, J. Robertson, Editor. 1992, Ellis Horwood: New York City, NY. p. 99–
126.
44. Grieve, M., ed. Fibers and their examination in forensic science. Forensic Science
Progress, ed. A.M.a.R.L. Williams. 1990, Springer-Verlag: New York.
45. Hartshorne, A., F. Wild, and N. Babb, The discrimination of cellulose di- and
triacetate fibers by solvent test and melting point determination. Journal of the
Forensic Science Society, 1991. 31(4): p. 457–461.
46. Grieve, M., The Use of Melting Point and Refractive Index Determinations to
Compare Colorless Polyester Fibers. Forensic Science International, 1983. 22:
p. 31–48.
47. Choudry, M., The use of scanning electron microscopy for identification of cuts
and tears – an observation based on criminal cases. Scanning Microscopy, 1987.
1: p. 119.
48. Hearle, J., B. Lomas, and W. Cooke, Atlas of Fibre Fracture and Damage to
Textiles. 1998, Boca Raton, FL: CRC Press.
49. Monahan, D. and H. Harding, Damage to clothing – cuts and tears. Journal of
Forensic Sciences, 1990. 35: p. 901–912.
50. Stowell, L., The use of scanning electron microscopy to identify cuts and tears
of a nylon fabric. Journal of Forensic Sciences, 1990. 35: p. 947–950.
51. Tungol, M., E. Bartick, and A. Montaser, The Development of a Spectral
Data Base for the Identification of Fibers by Infrared Microscopy. Applied
Spectroscopy 1990. 44: p. 543–549.
52. Tungol, M., E. Bartick, and A. Montaser, Forensic analysis of acrylic copolymer fibers by infrared microscopy. Applied Spectroscopy, 1993. 47: p.
1665–1658.
53. Bartick, E. and M. Tungol, Infrared microscopy and its forensic applications, in
Forensic Science Handbook, R. Saferstein, Editor. 1993, Prentice-Hall: Englewood
Cliffs, NJ.
54. Grieve, M.C., Another Look at the Classification of Acrylic Fibres, Using FTIR
Microscopy. Science and Justice, 1995. 35: p. 179–190.
55. Grieve, M.C., Forensic Examination of Fibres. Forensic Science Progress, 1990.
4: p. 41–125.
56. Grieve, M. and L. Cabiness, The recognition and classification of modified acrylic
fibers. Forensic Science International, 1985. 29: p. 129–146.
57. Greive, M., R. Griffin, and R. Malone, Characteristic dye absorption peaks
found in the FTIR spectra of coloured acrylic fibres. Science and Justice 1998.
27–37: p. 27.
58. Jochem, G., Fiber-plastic fusions and related trace material in traffic accident
investigation, in Trace Evidence Analysis: More Cases in Mute Witnesses, M.M.
Houck, Editor. 2004, Academic Press: San Diego, CA.
59. Lang, P., et al., The Identification of Fibers by Infrared and Raman
Microspectrophotometry. Microchemical Journal 1986. 34: p. 319–331.
26
Identification of textile fibers
60. Munro, C., W. Smith, and P. White, Qualitative and semi-quantitative trace analysis of acidic monoazo dyes by surface enhanced resonance Raman scattering.
Analyst 1995. 120: p. 993–1003.
61. Thomas, J., et al., A further look at Raman spectroscopy for the forensic
examination of fibres. Forensic Science International, 2003. 136: p. 125–136.
62. Wiggins, K., The European Fibres Group (EFG) 1993–2002: Understanding and
Improving the Evidential Value of Fibres. Analytical and Bioanalytical Chemistry,
2003. 376: p. 1172–1177.
63. Massonnet, G., et al., Evaluation of Raman Spectroscopy for the Analysis of
Coloured Fibres: A Collaborative Study. Journal of Forensic Sciences, 2005. 50:
p. 1028–1038.
64. Apsell, P., What Are Dyes? What Is Dyeing?, in AATCC Dyeing Primer. 1981,
American Association of Textile Chemists and Colorists: Research Triangle
Park, North Carolina.
65. Park, J. and J. Shore, Dye and fibre discoveries of the twentieth century, Parts I
and II. Journal of Society of Dyers and Colourists, 1999. 115: p. 157–167.
66. Houck, M.M., Intercomparison of unrelated fiber evidence. Forensic Science
International, 2003. 135: p. 146–149.
67. Robinson, D., The Meaning of Fashion, in Inside the Fashion Business, J. Jarnow,
B. Judelle, and M. Guerreiro, Editors. 1965, John Wiley & Sons: New York,
p. 52.
3
Natural animal textile fibres: structure,
characteristics and identification
S R TRIDICO, Australian Federal Police, Australia
Abstract: The use of natural animal fibres in textile materials began
before recorded history. Animal fibres of the most significant economic
value in the textile market today are those made from wool, mohair,
Angora rabbit, cashmere, camel, alpaca and cultivated silk. Natural fibres
sourced from the pelage of animals exhibit a variety of morphological
features which may be used to identify the particular family the hair
originated from, which contrasts to the processes involved in the
identification of silk. This chapter details the growth, structure and
properties of animal fibres which affords each animal fibre type different
and unique properties enabling industry to manufacture a plethora of
textiles destined for a variety of end-uses.
Key words: natural animal textile fibres, growth, structure, composition
and properties of animal hairs and silk fibres, morphology of animal
hairs and silk fibres, identification of animal hairs and silk fibres, silk
production.
3.1
Introduction
The use of natural fibres of animal origin for textile materials began before
recorded history. Textile fibres can be classified into two main categories,
natural and man-made (see Fig. 3.1). Wool is generally accepted by the
textile industry as a term referring to animal fibres originating from sheep;
accordingly, this convention is used in the remainder of the chapter.
Fibres from wool, mohair, angora, cashmere, camel and alpaca have the
most significant economic value in the textile market today. The other
significant animal fibre is cultivated silk originating from the silkworm
Bombyx mori (B.mori). Accordingly, the following sections will predominantly focus on these seven fibre types.
Section 3.2 details the growth, structure and properties of animal fibres.
All mammalian hairs grow from follicles embedded in the skin, in contrast
to the silks which are extruded from silk moth larvae. The chemical composition of all animal hairs is the same; they are made from the protein
keratin, whereas silk fibres consist of the protein fibroin. This difference
affords each animal fibre type different and unique properties which result
in a plethora of textiles destined for a variety of end-uses.
27
28
Identification of textile fibers
Fibre
Natural
Animal
(protein fibres)
Plant
Mineral
Man-made
Regenerated
Synthetic
Wool
sheep
Hair
Goat family (Bovidae)
(mohair, cashmere)
Camel family (Camelidae)
(camel, alpaca, vicuna)
Other fur-bearing animals, in particular the
rabbit family (Leporidae)
(angora)
Silk
3.1 Classification of textile fibres.
Sections 3.3 and 3.4 detail the different types of hairs found on the pelage
of animals, the types of silk fibres produced and the range of morphological
characteristics exhibited by these fibres.
Section 3.5 focuses on the manner in which animal hairs may be identified
as originating from a particular animal family or species and the contrast
to the processes involved in the identification of silk.
Sections 3.6 and 3.7 recommend sources of further information and
future trends. The most significant future trend is the production of OptimTM
fibres created from stretched wool which produces a thinner more luxuriant
fibre akin to silk. The impact this fibre will have regarding identification is
that the OptimTM fibres microscopically look like silk fibres and as a result
may be erroneously identified.
3.2
Animal fibre growth, structure, composition
and properties
3.2.1 Hair growth
All mammalian hair fibres are of similar structure, chemistry and physical
behaviour differing only in fine detail between the species; as Wildman
stated,1 ‘it will help the reader to an understanding of the unique structure
of animal fibres, their reactions to reagents, and the principles employed in
Natural animal textile fibres
29
their identification if he follows. . . . how they are developed in the
skin. . .’
If a piece of skin was to be sectioned at right angles to the skin surface
to produce a thin vertical section, a microscopic examination would reveal
that the skin consists of two main portions; an ‘underskin’ or dermis and
overlying this a thinner ‘outerskin’ or epidermis. These two major components of the skin will retain their separate identities throughout the growth
of the animal. When a hair fibre is going to develop, a series of changes
begins in the skin which results in the formation of a little plug or follicle.
Hair follicles develop in utero as a downgrowth or invagination of the epidermis into the dermis and it is from the bottom of this structure that a new
hair fibre starts its growth. The hair follicle is a dynamic organ in which
division, differentiation and migration of cells occur in the various tissues
of which it is composed. The mature hair fibre contains at least two cell
types, the surface layer or cuticle, consisting of flattened overlapping cells,
whose free margins point towards the tip of the hair fibre, and the main
central cortex, or inner ‘body’, made up of spindle-shaped cortical cells. The
hair may also possess a third and central structure consisting of an open
meshwork of condensed cells called a medulla (or air space). The main cellular features and processes of a mature, growing hair follicle are illustrated
in Fig. 3.2. As the cells of the immature forming hair fibre are pushed
upward from the base of the follicle, their central nuclear bodies become
reduced in size; whilst this is happening there is deposited in the cell a
material which is an intermediate product in the formation of the protein
keratin of the mature hair fibre. This keratinisation, or hardening, process
of follicular structures proceeds faster and lower down, particularly in the
cells forming the outermost layer of the inner root sheath, than it does
in the more central layers which form the main ‘body’ of the hair fibre.
The presence of this comparatively rigid inner root structure around
the soft young hair fibre cells and the direction of growth are important
factors in determining the shape of the hair fibre when it later emerges from
the skin.
Attached to the hair follicle is the arrector pili muscle which upon contraction causes the hair to ‘stand on end’, and to some follicles one or more
sebaceous glands. The wax or sebum produced from the sebaceous gland
facilitates the movement of the growing hair fibre as it pushes its way
through the cells in the follicle and ultimately, through the horny dermis.
Thus, the mammalian hair fibre is the product of a delicately adjusted
living organism and results from a series of intricate growth structures
genetically designed to produce a variety of hairs of finite lengths. The type
of mammal and its hereditary or genetic constitution can have an effect
on the appearance of these three main parts of the hair fibre which will
ultimately dictate the end-use of that fibre.
30
Identification of textile fibers
Epidermis
Dermis
Mature hair
Arrector pili muscle
Sebaceous gland
Medulla
Zone of hardening of hair
fibre
Disulphide bonding,
resorption and dehydration
Inner root sheath
Cuticle
Cortex
Cortical cells
Outer root sheath
Dermal sheath
Basement
membrane
Follicle bulb
Dermal papilla
Keratin gene expression
Cell proliferation and
differentiation
3.2 Schematic diagram of a hair follicle showing the various features
and major areas of cell proliferation and keratinisation (courtesy of
Thomson publishers2).
3.2.2 Silk production
Silk is an animal fibre but instead of being grown from a follicle embedded
beneath the skin in the form of hair, it is produced by insects during the
construction of their webs, cocoons or climbing ropes. Two types of silk
fibres are utilised in the textile industry: cultivated silk (produced through
the process of sericulture), from the mulberry silkworm B. mori, the mainstay of the silk industry comprising 95% of the world’s silk production; and
wild or tussah (tasar) silk mainly produced by various species of the silkworm of the Antheraea genus which live in the wild, feeding mainly on oak
leaves. As a result of their varied diets B. mori produces silk filaments which
are usually white in colour and highly lustrous in contrast to the tussah
silkworms which produce silk filaments ranging green or tan in color and
lack the high lustre of the cultivated silk.
Natural animal textile fibres
31
Irrespective of the type of genus of silkworm used to produce cultivated
or wild silk, the mode in which the animal produces the fibre is the same.
The silkworm larva possesses a pair of modified salivary glands (sericiteries) which produce a clear, viscous, proteinaceous fluid that is extruded
through openings or spinnerets on its mouthparts. As this fluid is exposed
to the air it hardens; this hardened silk filament is then used by the larva
to wrap the fibre around itself in the form of a cocoon.
3.2.3 Animal fibre structure and composition
The composition of all the animal fibres are the same in that they are made
up amino acid chains joined together through condensation to form the
polymeric molecule protein. However, the protein constituting wool and
hair fibres (keratin) varies enormously to the protein comprising the silk
(fibroin). The major difference being that keratin fibre proteins are highly
cross-linked by disulphide bonds, whereas the secreted silk fibroin fibres
tend to have no cross-links and a more limited array of less complex amino
acids. Irrespective of this significance difference in their composition and
structure all fibres of animal origin are all fibres of character; each one
exhibiting unique properties which ensures it a position of special significance as a textile fibre.
All animal fibres consist exclusively of proteins and, with the exception
of silk, constitute the fur or hair of animals. Proteins are nitrogen-containing
substances which are essentially chain-like molecules formed by the union
of α-amino acids joined together by peptide linkages which retain one terminal amino (NH+) group and one terminal carboxylic acid (CO−) group
resulting in the elimination of water (condensation). Although amino acids
may have other formulae, those in proteins invariably have a general
formula as illustrated in Fig. 3.3.
Each amino acid consists of a single carbon atom to which is attached a
carboxyl function (–COOH), an amino function (–NH2), a hydrogen atom
and a side-chain (R) which defines each particular amino acid and its chemical character. Over 20 amino acids with different side groups (R) are known;
the difference between proteins arises from the differences between these
side groups attached to the main chain illustrated in Fig. 3.4.
H
H2NCCOOH
R
3.3 Amino acid structure.
32
Identification of textile fibers
O
CHCNH
R
n
3.4 Protein structure.
Two major classes of natural protein fibres exist and include keratin,
found in hair and fur, and fibroin secreted (insect) fibres. In general, the
keratin fibres are proteins highly cross-linked by disulfide bonds from the
cystine, (–CH2SSCH2–), residues in the protein chain which comprises some
10–15% of wool fibres. Although the exotic fibres alpaca, cashmere, Mohair,
angora and camel are chemically similar in their composition to wool, their
cystine content in their protein chains differ, which can be up to 24% in
some fibres. The keratin fibres tend to have helical, intermittent helical sections within the protein sequence and are extremely complex in structure
with the inclusion of a cortical cell matrix surrounded by a cuticle sheath
laid on the surface as overlapping scales. The cell matrix, or cortex, of some
hair fibres may contain a central cavity or medulla. The keratin fibres tend
to be round in cross-section with an irregular crimp along the longitudinal
fibre axis.
Raw silk, whether cultivated or wild, contains about 75% fibre and 25%
of a globular protein called sericin. The sericin is usually left on the silk filaments to protect them from mechanical damage during processing. The silk
yarn or fabric is degummed to remove the sericin, resulting in a silk fibre
which is essentially pure fibroin.
The protein fibroin, as illustrated in Table 3.1, has a markedly different
amino acid composition from that of keratin. Fibroin, unlike the keratin
fibres, has a more limited array of less complex amino acids; glycine (H–)
and alanine (CH3–) constitute in total some 60% of the amino acids comprising this protein. Literature citations regarding glycine, alanine, tyrosine
and serine as the major amino acids comprising fibroin accord with the
notable of exception of the amino acid cystine. Needles3 and Cook4 report
that fibroin lacks the amino acid cystine whereas Kushal and Murugesh5
report that fibroin contains negligible amounts of this amino acid.
Thus, with the virtual absence of cross-links and with limited bulky side
chains present in the amino acids, fibroin molecules align themselves parallel to each other and hydrogen bond to form a highly crystalline and oriented ‘pleated-sheet’ or ‘beta’ structure. Because of its high cost, silk finds
a limited use in textiles; a minor amount of wild or tussah silk is produced
for specialty items. The silk fibres comprising the ‘wild’ silks differ from
those of cultivated silk in colour and texture; however, the wild silk and
Natural animal textile fibres
33
Table 3.1 The content of the α-amino acid side-groups (R) in wool and
silk protein fibres (grams of amino acid per 100 g protein). Data from
‘Textile Fibres, Dyes, Finishes and Process – A Concise Guide’3 with
the exception of * value which is taken from ‘Studies on Indian Silk’5
α-Amino acid
Wool keratin
Silk fibroin (cultivated)
Inert
Glycine
Alanine
Valine
Leucine
Isoleucine
Phenylalanine
5–7
3–5
5–6
7–9
3–5
3–5
36–43
29–36
2–4
0–1
0–1
1–2
Acidic
Aspartic acid
Glutamic acid
6–8
12–17
1–3
1–2
Basic
Lysine
Arginine
Histidine
0–2
8–11
2–4
0–1
0–2
0–1
Hydroxyl
Serine
Threonine
Tyrosine
7–10
6–7
4–7
13–17
1–2
10–13
Miscellaneous
Proline
Cystine
Methionine
Tryptophan
5–9
10–15
0–1
1–3
0–1
0.00 / 0.13*
0.0
0–1
cultivated silk fibres are sufficiently similar in chemical composition and
structure as to be considered as homogeneous fibre types.
3.2.4 Physical and chemical properties of protein fibres
Hair fibres are related to wool in their chemical structure; they all comprise
keratin. But they all differ from wool, and from each other, in their physical
(and morphological) characteristics; they are of different length and fineness and have different shapes and internal structures. With silk fibres, on
the other hand, despite several species of silkworms being used in their
production, the construction of their cocoons are sufficiently alike for the
silk to be regarded as a fairly homogeneous material.
The configuration and orientation of the individual molecular chains
within each protein fibre, in conjunction with its the overall shape, will
affect the fibre properties. In protein fibres, like other natural fibres, the
34
Identification of textile fibers
orientation of the molecules within the fibre is determined by the biological
source during the growth or production, and the maturity process of
the fibre.
According to Needles3 there are several essential ‘primary’ properties
that any polymeric material must possess in order to produce a fibre adequate enough for its intended final product. These properties are fibre
length to width ratio, fibre uniformity, fibre strength and flexibility, fibre
extension and elasiticity and fibre cohesiveness. However, the polymeric
material should also exhibit additional characteristics in order to increase
their desirability and value in its intended end uses. Such properties include
moisture absorption characteristics, fibre resilience, density, lustre and
chemical resistance. Man-made fibres are specifically manufactured in order
to meet these essential critera; however, nature ensures that the protein
fibres it produces are ‘ready made’ to fulfil these requirements. In relation
to fibre lengths, hairs are grown to genetically determined finite lengths and
as such, they are regarded as staple fibres and treated as such in the production of wool products; silk fibres, however, are extruded as extremely long
continuous filaments and therefore regarded as a filamentous fibre in the
textile industry.
Protein fibres are generally fibres of moderate strength, resilience and
elasticity and at moderate humidities do not build up significant static
charge.
Wool, like other hair fibres, contains a substantial amount of the amino
acid cystine. Cystine residues play a very important role in the stabilisation
of the fibre structure due to the cross-linking action of their disulphide
bonds, which holds the polymer chains together not only in wool, but also
in other animal fibres. The disulphide bonds are responsible for relatively
good wet strength of wool. Wool is resistant to attack by acids but is readily
attacked by weak bases even at low dilutions and is irreversibly damaged
and coloured by dilute oxidising bleaches such as hypochlorite. Reducing
agents will cause reductive severance of the disulphide bonds within the
wool, evetually causing it to dissolve. However, this property is exploited in
the textle industry as under controlled conditions, reducing agents can be
used to partially reduce the wool and flat set or set permanent pleats in
the wool.
Wool, unless chemically treated, is susceptible to attack by several species
of moths which are able to to dissolve and digest wool fibres. However, it
is reasonably resistant to attack by other biological agents such as mildew.
Wool fibres have excellent resiliency and recovery rate from deformation
except under high humidity, it is insoluble in all solvents with the exception
of those capable of breaking the disulphide cross-links. Wool is a good heat
insulator due to its low heat conductivity and bulkiness, which permits the
air to become trapped in the fibres comprising the textile constructions.
Natural animal textile fibres
35
Wool and other hair fibres are unique amongst natural fibres for their
possession of overlapping cuticular scales present on the outermost surface
of the fibre, their position akin to overlapping tiles on a roof. This characteristic scaly outer surface is vital in the process of felting; a process which
is unique to wool and other animal fibres. Felting is the consolidation of
these fibrous materials by the application of heat, moisture and mechanical
action, causing the scales on the wool and hair fibres to interlock and mat
together. The fabric shrinks and undergoes characteristic changes in its
structure. The fabric becomes thicker and the fibres are matted into closely
packed masses. The outline and character of the yarn pattern in the fibre
becomes indistinct and the fabric loses much of its elasticity; the surface of
the fabric is covered by fibres, and its appearance is altered.
The outer scales on the wool or hair fibre are aligned such that their edges
point towards the tip end of the fibre. Felting appears to most amenable
with wool or hair fibres which bear prominent cuticular scales, probably due
to the fact that the felting treatment tends to bend the scale edges and fibres
into loops, which during the mechanical action with repeated fabric compression, causes the loops and scales to ‘travel’ and become interlocked and
entangled; unlike bonded fabrics, felts do not require adhesive for their
production. Felted fabrics are used in the hat industry, apparel and drapery,
in industry for insulation, packing and polishing materials and felt padding
is used in apparel and furniture. Cashmere fibres are almost identical to
wool in relation to their chemical composition; however, owing to fineness
and better wetting properties, cashmere fibres are more susceptible to
chemical damage, especially with respect to alkalis.
Silk, due to the virtual absence of cystine, is not as resistant to acids as
wool, but is more resistant to alkalis. Silk is very resistant to organic solvents
but soluble in hydrogen bond breaking solvents such as cupammonium
hydroxide. Unlike other natural fibres silk is more resistant to biological
attack. Strong oxidising agents such as hypochlorite will cause silk to rapidly
discolour and dissolve, whereas reducing agents have negligible effect
except under extreme conditions. Silk fibres are relatively stiff and show
good to excellent resiliency and recovery from deformation depending on
the temperature and humidity conditions. These fibres exhibit favourable
heat-insulating properties but owing to their moderate electrical resistivity,
tend to build up static charge.
3.3
Types of natural animal fibres
3.3.1 Mammalian hairs
The pelage, or coat, of animals comprises various hair types which are
illustrated in Fig. 3.5. Close examination of the pelage will reveal that some
36
Identification of textile fibers
(a)
(b)
(c)
3.5 Examples of hair types which may be found on the pelage of
animals (a) over hair, (b) guard hair and (c) under hairs (courtesy of
Dr Hans Brunner).
sparsely distributed hairs are distinctly longer than the hairs comprising the
bulk of the coat; these longer hairs are called overhairs.
The larger or coarser hairs forming the bulk of the pelage are termed
guard hairs. These hairs generally exhibit a variety of sizes in one pelage,
ranging from the coarse and long to those that cannot be distinguished from
the underhairs. The guard hairs may be of uniform diameter along the hair
shaft, tapering to a tip. However, some guard hairs are specialised into a
type described as shield hairs. In shield hairs the distal (tip region) of the
hair is noticeably wider and flattened, forming a shield.
The underhairs are shorter and much finer than the overhairs and guard
hairs, these hairs are usually found close to the body and serve to insulate the
animal. In general, underhairs are wavy and retain a uniform diameter along
the length of the hair with the exception of the tip which tapers to a point.
The classification of hair types, as outlined above, predominantly relies
upon the appreciation of the profile or general outline of the hair, e.g.
straight or wavy. Some examples of the types of profiles which may be seen
on the pelage of animals are illustrated in Fig. 3.6.
Natural animal textile fibres
(a)
(b)
37
(c) (d)
3.6 Examples of hair profiles (a and b) guard hairs (c) under hair,
(d) magnified view of a constriction in an under hair (courtesy of
Dr Hans Brunner).
Carpets
Bedding
Upholstery
Blankets
Woollen fabrics
Worsted fabrics
17–24
25–27
28–30
31–33
34–40
41+
Fibre diameter in microns
3.7 Wool fibres and their uses in the textile industry based on their
diameters (modified from FAO Agricultural Bulletin 1227).
The coarseness or fineness of the animal hairs determines their end use
in the textile industry and as such animal hair fibres need to be graded
according to their fibre diameter. For example, the major end uses of wool
are apparel products, bedding and carpets. Figure 3.7 illustrates how the fibre
diameters determine the end product, with the coarser fibres being used
38
Identification of textile fibers
for carpets and the finer fibres being used for apparel fabrics which need to
be softer against the skin. In general wool fibres coarser than 21 microns in
diameter cannot normally be processed into yarns destined to produce
lighter, softer fabrics that are both functional as well as aesthetically
pleasing.
3.3.2 Cultivated and wild silk fibres
Silkworm is a common name for the silk-producing larvae several species
of moths; however, the mulberry silkworm B. mori is the most common
moth used in the commercial production of silk. B. mori feeds exclusively
on the leaves of the mulberry tree and has flourished only where conditions
are suitable for large numbers of leaf-bearing mulberry trees. However, this
moth has been cultivated over many centuries and is no longer found in the
wild, and today is totally dependent on humans for its existence and as such
the silk it produces is known as cultivated silk. Unlike silk produced from
silkworms living in the wild, cultivated silk is harvested from a cocoon as a
continuous silk filament approximately 1000 m in length. This is achieved
by killing the pupa prior to its emergence as a moth during which the
pupa secretes an alkali which dissolves the cocoon threads thereby ruining
the silk. Silk is a continuous filament around each cocoon and is freed
by softening the cocoon in water, locating the free end and harvesting the
silk thread.
Wild or tussah silk, on the other hand, is produced by silkworms which
live in an environment free to feed on a variety of leaves and complete their
life cycles, with the pupa contained in the cocoon being allowed to live and
emerge as a moth. As a consequence of this ‘wild’ existence the integrity of
the single silk filament produced by some species of larvae to build its
cocoon is broken, resulting in silk which consists of numerous strands. This
silk is generally coarser than the cultivated silk because the wild silk consists
of numerous strands, rather than a single seamless one, and is also variously
coloured due to the uncontrolled diet of the insects.
3.4
Natural animal fibre characteristics
3.4.1 Animal hairs
Morphological characteristics exhibited by animal hairs are usually examined using the optical microscope, which is excellent for examining the
interior of the hair shaft; however, the exterior of the hair, i.e. the cuticle,
is best examined using a scanning electron microscope (SEM) as the resolution of the image is far higher than that attainable with the optical microscope, resulting in a much more detailed image. The use of these two
Natural animal textile fibres
39
Scale
Cuticle
Medulla
Cortex
3.8 Diagrammatic representation of the major structural components
which may be exhibited by hairs (courtesy of Dr Hans Brunner).
microscopes in the examination of animal hair is detailed in the identification section of the chapter and unless otherwise stated, any references to
microscopy will be in relation to the use of the optical microscope.
As seen in preceding sections, animal hair fibres have a unique structure
consisting of the outermost scale cuticle, an inner cortex and, in some hair
fibres a central medulla, as diagrammatically illustrated in Fig. 3.8.
All mammalian hairs bear morphological characteristics typical to the
family of the particular species. These morphological characteristics may be
seen on the outside of the hair shaft as cuticular scale patterns, inside the
cortex as medullary patterns or at the root end which, in some animals,
bears a characteristic shape; however, for the majority of animal fibres used
in the textile industry the root will be absent, with exception of the coarse
kemp fibres, which may be found in some fabrics; these fibres are medullated and exhibit a brush-like root.
The works of Wildman1 and Brunner and Coman6 are considered as
seminal works and standard references in relation to the idenification of
animal hairs based on their classification of morphological features and
characteristics on the basis of their microscopical appearances. Wildman,
and Brunner and Coman classified the hair characteristics on the basis of
their cuticular scale patterns and medullae; Brunner and Coman further
classified hair characteristics on the basis of their cross-sectional shapes. As
illustrated in Fig. 3.8 the cuticle or outer layer of the hair shaft comprises
a single layer of overlapping cells, arranged like tiles on a roof, with the free
edges pointing towards the tip. Unlike human hairs, animal hairs exhibit a
variety of cuticular scale arrangements to form distinct patterns. In relation
to the cuticular scale patterns exhibited by animal hairs, Brunner and
40
Identification of textile fibers
Form of scale margins
(a) Smooth
(b) Crenate
(c) Rippled (d) Scalloped (e) Dentate
Distance between scale margins
(f) Distant
(g) Near
(h) Close
Scale patterns
(i) Simple
coronal
(o) Regular
wave
(j) Diamond
petal
(p) Irregular
wave
(k) Narrow
diamond petal
(l) Broad
petal
(m) Regular
mosaic
(n) Flattened
irregular
mosaic
(q) Single
chevron
(r) Double
chevron
(s) Streaked
(t) Transitional
3.9 Cuticular scale patterns as classified by Brunner and Coman
(courtesy of Dr Hans Brunner).
Coman divided the classification into three criteria, each based on the following main features: terms which describe the form of the scale margin,
terms which describe the distance between the external scale margins and
terms which are descriptive of the general scale patterns as illustrated in
Fig. 3.9.
As depicted in Figs 3.10 and 3.11 Wildman similarly classified the cuticular scale patterns with the exception that the dentate scale margin, coronal
Natural animal textile fibres
Mosaic (regular)
Interrupted regular wave
Single chevron
(a form of regular wave)
Mosaic (irregular)
Mosaic (irregular wave)
Double chevron
41
Simple regular wave
Wave (medium depth)
Streaked wave
(a variety of interrupted wave)
3.10 Cuticular scale patterns as classified by Wildman (courtesy of
BTTG Ltd).
and transitional scale patterns are not represented. Owing to the differences
in the terminologies of these two main works it is recommended to use one
reference or the other when identifying animal textile fibres in order that
consistency of terms and descriptors is maintained and to provide the
source used in the identification process.
42
Identification of textile fibers
Irregular petal
(a form of interrupted irregular wave)
Lanceolate
(a form of fine pectinate
and also of regular wave)
Coarse pectinate
(a form of regular wave)
Diamond petal
Narrow diamond petal
3.11 Cuticular scale patterns as classified by Wildman (courtesy of
BTTG Ltd).
Brunner and Coman defined four major structural groups of animal hair
medullae: unbroken, broken, ladder and miscellaneous. They further subdivided each of these groups into a total of 12 distinct types as depicted in
Fig. 3.12. The top half of each type details the structure seen in a hair in
which the medulla is filled with air; the lower half illustrates animal hairs
which have been treated in order to facilitate the observation of the medulla.
The medulla consists of shrunken cells, the spaces between these shrunken
cells are usually filled with air. Under the microscope these appear as
obvious black, opaque structures which can obscure the structure or pattern
of the medulla. If these hairs are treated in such a way as to allow mounting
medium to enter the cortex and infiltrate these air spaces, viewing the
medulla shape and form is facilitated.
Wildman, on the other hand, classified the medulla types into the following four broad categories: (a) unbroken (wide) lattice, (b) and (c) simple
Natural animal textile fibres
(a)
Narrow
medulla
lattice
(g)
(b)
(c)
Wide
medulla
lattice
Narrow
aeriform
lattice
(h)
Fragmental
Uniserial
ladder
(i)
Multiserial
ladder
(d)
Wide
aeriform
lattice
43
(e)
(f)
Simple
Interrupted
(j)
(k)
(l)
Globular
Stellate
Intruding
3.12 Medulla types as classified by Brunner and Coman (courtesy of
Dr Hans Brunner).
unbroken, (d) interrupted and (e) fragmental as illustrated in Fig. 3.13. The
following medulla types by Brunner and Coman are illustrated in Fig. 3.12:
narrow medulla lattice, uniserial ladder, multiserial ladder and globular, stellate and intruding being absent. The latter three comprise the ‘miscellaneous’ major structural group being found in animals such as seals, wombats
and platypus. The different classifications of medulla types and scale patterns may reflect the different aims of each of the works. Wildman’s work
details morphological characteristics of animal fibres of importance in the
textile industry; whereas Brunner and Coman deal with the morphological
characteristics of a variety of mammalian hairs for use by animal ecologists
and in the examination of animal hairs found as contaminants in food.
Brunner and Coman noted that in addition to the differences in medullae
types and cuticular scale patterns exhibited by animal hairs, significant
44
Identification of textile fibers
(a)
(b)
(c)
(d)
(e)
3.13 Medulla types as classified by Wildman (courtesy of BTTG Ltd).
differences are also apparent in the cross-sectional shapes of animal hairs.
In Fig. 3.14, Brunner and Coman depict the most common cross-sectional
shapes enountered in animal hairs, the dark, central features representing
the medulla.
3.4.2 Silk fibres
Silk, although produced by an animal and a protein-based fibre, is not generally regarded as a true animal fibre since it comprises fibroin and not
keratin, nor does it grow from a follicle embedded beneath the skin but is
extruded from modified salivary glands from a larva. As such it does not
bear any of the morphological characteristics exhibited by the true keratin
animal fibres. Under the microscope silk has the appearance of a glass-like
filament of uniform diameter which may bear striations along its length. In
the raw state the colours of the fibres may reveal if the fibre has been produced as cultivated silk or as the product of wild silk; cultivated silk, once
degummed, has a high natural lustre and sheen white in colour. Wild silks
vary in colours such as, but limited to white, cream, green, brown, and
amber. The variety of colours is attributable to the variety of leaves consumed by the various wild silk moth species.
3.5
Identification of natural animal fibres
3.5.1 Keratin-based animal fibres
All animal hairs bear morphological characteristics and features which not
only allow differentiation between them but also their identification to a
species or family level. Animal hairs used in the textile industry, despite
Natural animal textile fibres
Circular
medium size
medulla
45
Circular
large medulla
Oval
large medulla
Oval
medium size
medulla
Oval
medulla absent
Eye-shaped
Oblong
large medulla
Oblong
medium size
medulla
Cigar-shaped
Concavo-convex
divided
medulla
Concavo-convex
bilobed medulla
Concavo-convex
large medulla
Reniform
Dumb-bell
shaped
3.14 Cross-sectional shapes as classified by Brunner and Coman
(courtesy of Dr Hans Brunner).
being processed and dyed, may retain sufficient characteristics to determine
the possible source.
The identification of animal hair fibres begins with the determination that
the hair is animal not human in origin. This is generally easy to achieve and
Table 3.2 details the morphological characteristics human and animal hairs
show, which enable their differentiation. Clearly, in animal hairs used
to manufacture textile products, the banding characteristic may not be
commonly seen.
Once a hair has been identified as animal in origin, a further more
detailed macroscopic examination is performed to determine a number of
46
Identification of textile fibers
Table 3.2 Characteristics which may be used to differentiate between human
and animal hairs
Feature
Human hair
Animal hair
Colour
Relatively consistent along
hair shaft
Medulla
Less than 1/3 of the width of
the hair shaft
Amorphous, mostly not
continuous when present
Pigment
distribution
Cuticular scales
Even, slightly more towards
the cuticle
Imbricate, similar along the
shaft from root (proximal
end) to tip (distal end)
Usually bulbous (club
shaped) and indistinct
Often showing abrupt,
profound colour changes
known as banding
Usually greater than 1/3 of
the width of the hair shaft
Continuous, often varying in
appearance along the
shaft, defined structure
Central or denser towards
the medulla
A variety of patterns often
showing variation in
structure from root to tip
Variety of shapes and forms,
usually distinct
Root
features such as the profile of the hair, e.g. the length and appearance of
the hair, i.e. straight or wavy, and determine whether the hair is likely to be
a guard hair or an underhair. A preliminary examination of the animal fibre
with a stereomicroscope (up to 100× magnification) may reveal the width
and gross morphology of the medulla characteristic to a particular hair type
and possible animal or origin, e.g. a hair with a brush-like root and an
obvious wide unbroken medulla would strongly indicate the presence of a
kemp fibre. In general the largest of the guard hairs (primary guard hairs)
are of paramount importance in the identification of animal hairs, for it is
these hairs which generally exhibit the most diagnostically useful characteristics. The underhairs are generally of little diagnostic value in determining the identification of animal fibres.
The identification of animal hair fibres predominantly relies upon the
morphological features present inside the cortex, such as the medulla and
on the outside of the hair from the cuticular scale patterns, as the size and
shape of these scales and their pattern of arrangement around the hair are
useful criteria for identification purposes.
In relation to observing the cuticular scale patterns present on animal
fibres for identification purposes, a number of methods can be employed,
each with their own merits and limitations.
The scale patterns may be visualised by mounting the hairs in a semipermanent mounting medium with a refractive index (RI) lower than that
of keratin (RI 1.55) which facilitates the viewing of the external scale patterns but slightly masks the internal features. Figure 3.15 illustrates the
efficacy of three semi-permanent mounting media each with varying RIs.
Natural animal textile fibres
(a)
(b)
47
(c)
3.15 Photomicrographs of the same woollen fibre mounted in cedar
oil RI 1.513 (a), liquid paraffin RI 1.470 (b) and glycerine and water RI
1.403 (c) (courtesy of BTTG Ltd).
The hair will need to be dried and/or cleaned of the semi-permanent
medium following the examination.
Scale casts may also be made by coating a cover slip with a medium such
as clear nail polish, laying the hair in the polish, once the polish is dry, the
hair is removed, the cover slip inverted and placed on a microscope slide;
cuticle scale cast, embedded in the nail polish, can be viewed under a compound microscope. This method does not require the hair to be dried or
cleaned and the cast is a permanent record.
Examination of the external features of animal hair fibres may be achieved
with the use of a scanning electron microscope (SEM). The SEM uses secondary electrons to ‘view’ the surface characteristics of a fibre, the surface
of which is usually coated with a thin layer of gold to assist the speedy
display of the scanned surface. The major advantage of the SEM, over the
optical microscope, is its very high resolution (down to 2 nanometres) and
the relatively large depth of field. This enables the complete surface of a
fibre to be seen in high detail thus enabling a better discrimination of the
48
Identification of textile fibers
surface characteristics. However, unlike the optical microscope, no internal
features are visible and due to the thin gold coating, the hair cannot be
examined further.
If the textile of origin is unknown, determining the width or diameter
of the animal fibre may assist in determining its textile provenance, e.g.
a coarse wool fibre greater than 40 microns may indicate that it originated
from carpet; or if a garment is a blend of wool and cashmere, determining the diameters of the fibres may assist in their differentiation and
identification.
Although Brunner and Coman regard the cross-sectional shape of a hair
as ‘undoubtedly the most single important criterion used in . . . hair identification’ it is a destructive technique and as such should be used with
caution. Wildman does not illustrate cross-sectional shapes of animal hairs;
he does, however, discuss their value and use for animal hair identification
and for studies on the micro-structure of the hairs. Seta8 makes the following comments in relation to the use of cross-sectioning of hairs regarding
their identification:
The variable shape of the shaft gives a clue to the identification of
species. . . . Many hair examiners have adopted the cross-sectional shape for
characterising hairs. . . . For this examination some investigators used the longitudinal mount without preparing a cross section. . . . This may be justifiable
from the following points:
1. The consumption of the . . . hair would be minimal
2. The cross-sectional shape is variable from hair to hair and from point to
point on the same hair.
3. The cross-sectional shape does not have as much validity as has been
thought.
4. The production of suitable cross-sections depends completely on the
experience and ability of the examiner.
The preceding sections detail a plethora of medullae types and the cuticular scale patterns which may be seen on the animal fibre. The medulla types
and scale patterns not only vary from hairs of different species but these
characteristics may vary along the length of the same hair fibre and between
hairs comprising the pelage of the one animal. The widest point of the hair
fibre provides the most diagnostic medulla type. Tables 3.3 and 3.4 illustrate
the most significant, general features and characteristics present in fine and
coarse animal fibres for identification purposes.
Wool, like all other fibres of animal origin, consists of a cuticle of scales,
a cortex and in some instances a medulla. Very fine fibres, for example those
produced by merino sheep, have no detectable medullae but consist of
cuticle and cortex as depicted in Fig. 3.16. The images of representative
types of wool, illustrated in Figs 3.17 and 3.18 show that there is a variation
in thickness not only between the fibres, i.e. inter-species variation but also
None
Circular to
elliptical
15–24 μm
Medulla
Cross-section
Simple broken
Circular to oval
Irregular waved
mosaic
Mohair
Uniserial ladder
Oval to rectangular
Single or double
chevron
Angora rabbit
Waved mosaic
near to distant
scale margins
Not observed
Circular to oval
Camel
Coronal, distant
scale margins
smooth
Non medullated
Circular to oval
Cashmere
Varies
Almost circular
Irregular waved
mosaic
Alpaca
Fibre
diameter
Cross-section
Medulla
Cuticular
scale
pattern
30–36 μm
Up to 40+ μm
Fragmental or
unbroken to
wide unbroken
lattice
Circular to oval
Irregular mosaic
and simple
waved pattern
Mohair (Angora
goat)
Regular mosaic and
irregular mosaic;
smooth, near
margins
Wide lattice
Simple unbroken
narrow or
fragmented
narrow
Round to elliptical
Wool
Dumb-bell
ovoid
TBA
Wide unbroken
multi-serial
ladder
Double
chevron
Angora rabbit
Up to 120 μm
Circular to oval
Irregular waved
mosaic; near
and smooth
margins
Simple unbroken
(fine lattice)
Camel
mfd 80–86 μm
Simple broken or
unbroken
medulla
(medium
diameter)
Circular to oval
Irregular waved
mosaic; near
margins
Cashmere goat
40–60 μm
Varies
Irregular waved
mosaic;
smooth; near
margins
Varies
Alpaca
Table 3.4 Morphological features present on coarse wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres
Fibre diameter
Simple,
coronal
Cuticular scale
pattern
Wool
Table 3.3 Morphological features present on the finer wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres
50
Identification of textile fibers
3.16 Photographs showing the scale structure of fine woollen fibres
(courtesy of BTTG Ltd).
3.17 Photographs showing the structure of coarse woollen fibres
(courtesy of BTTG Ltd).
3.18 Photographs showing the structure of coarse woollen fibres
(courtesy of BTTG Ltd).
Natural animal textile fibres
(a)
51
(b)
3.19 Diagrammatic representations illustrating the extremes of
uniformity and irregularity of woollen fibre diameters. (a) Crosssectional shapes of fine, high quality woollen fibres; (b) crosssectional shapes of the coarser, lower quality woollen fibres (courtesy
of BTTG Ltd).
along the length of each fibre. The inter-species variation is mainly due to
genetic or inherited characteristics, i.e. the intra-species variation is generally due to nutritional feasts and famines during the period of fibre growth.
The lower quality and coarser wool fibres tend to become medullated with
the wide lattice type occuring in the very coarse fibres, including the kemps;
this medullation is commonly seen in carpet wool fibres. The simple unbroken medium to narrow type of medulla is seen in many fibres from longwools and cross bred wools. The fragmental type of medulla often occurs
in wool fibres, but it is often significantly smaller in relation to the rest of
the fibre.
Fibre diameters of wool fibres can vary from uniform to irregular fibre
diameters, as illustrated in Fig. 3.19. The higher qualities of wool fibres
exhibit a smaller mean fibre diameter but also less variation in the fibre
thickness; the lower quality and coarser fibres have an increased variation
in fibre diameter.
The coarser wool fibres as well as being medullate, exhibit a regular
mosaic-type scale pattern illustrated in Fig. 3.20, which may alternate with
short lengths of irregular waved pattern; in contrast to the fine wool fibres
which exhibit the same scale pattern type irrespective of the breed of sheep
they originated from (see Figs 3.21 and 3.22).
Alpaca is from the fleece of the alpaca Lama pacos which belongs to
the llama family, so that alpaca fibres and llama fibres have many morphological features and characteristics in common which may be seen in
the preceding figures. Alpaca fibres produced, range in diameter from
24–26 μm. These fine alpaca fibres bear scales which are smooth-margined
52
Identification of textile fibers
3.20 Cuticular scale patterns which may be exhibited by coarser
woollen fibres, all of which are of the regular mosaic pattern with the
exception of (d) which shows the regular mosaic pattern merging into
an irregular waved mosaic form (courtesy of BTTG Ltd).
Natural animal textile fibres
53
3.21 Scale pattern exhibited by fine woollen fibres all of which have
the same type of scale pattern (irregular-waved mosaic), with scale
margins which are smooth and distant (courtesy of BTTG Ltd).
18 μm
3.22 SEM image of a scale pattern of a merino fibre (courtesy of
CSIRO Textile and Fibre Technology).
54
Identification of textile fibers
25 μm
3.23 SEM image of the scale pattern of an alpaca fibre (courtesy of
CSIRO Textile and Fibre Technology).
as illustrated in Fig. 3.23. The coarser fibres (fibres 50–60 μm or over) have
scales which form an interrupted irregular wave pattern as illustrated in Fig.
3.24. Regarding alpaca fibre cross-sections, these animals produce a spectacular array of shapes and forms which can be seen in Figs 3.25 and 3.26
and are characteristic of not only the alpaca but also the llama.
The pelage of the angora rabbit, like that of other animals, has two major
fibre types, the outer guard hair and the shorter fur or underhair. In the
textile industry, angora rabbit fibres may be used alone or blended with
wool or nylon. The angora rabbit fibre bears a medulla which is characteristic of the lagomorph family to which it belongs (which includes ‘domestic’
rabbits and hares). The coarser angora rabbit hairs have a wide, unbroken,
mulitserial ladder medulla; the finer hairs, in general, bear a uniserial ladder
medulla. The cuticle shows a single or double chevron scale pattern as seen
in Fig. 3.27. The dumb-bell cross-sectional shape is typical for rabbit fibres
as seen in Fig. 3.28.
Mohair comes from the angora goat Capra hircus aegragus; these fibres
are very regular in thickness along their lengths and have smooth outlines,
which cause the scale margins to be difficult to detect in profile. The outlines
of mohair are, in this respect, sharply distinct from those of wool fibres with
Natural animal textile fibres
55
3.24 Scale pattern found on a coarse alpaca fibre (courtesy of BTTG
Ltd).
which they may be mixed. The cuticular scale pattern of coarse mohair is
illustrated in Fig. 3.29.
Cashmere originates from the cashmere goat, Capra hircus laniger; the
outstanding characteristic of the very fine cashmere fibres is that the majority of scale margins are distant, as illustrated in Figs 3.30 and 3.31. This
characteristic of distant smooth margined scales, together with the even
fibre outline and thickness makes them easily recoginisable by the experienced examiner. In contrast, very coarse cashmere fibres in the basal half
of the hair exhibit irregular waved mosaic with near and crenate rippled
margins as seen in Fig. 3.32.
Like cashmere only the soft underhair (or underwool) or down hair of
the camel Camelus bactrianas is used in the production of yarn. For the
56
Identification of textile fibers
Fibres of fine to medium thickness
Outline of fibre section almost circular, outline of medulla section
almost circular and relatively narrow.
Type A
Medium to coarse fibres (first type) (45–50m diameter)
Outline of fibre section approaching circularity, outline of
medulla section almost circular and relatively narrow.
Type B
Medium to coarse fibres (second type)
Outline of fibre section ovoid, medulla elongated in section and
in a direction along the major axis of the section.
Type C
Rather coarse fibres
Outline of fibre section ovoid to angular.
Medulla section characteristically dumb-bell shaped in outline.
This type is frequently seen in the coarser grades of brown, white
and black alpaca.
Type D
Coarse to very coarse fibres
A coarse fibre whose fibre section outline is approximately triangular,
but with two of the sides almost equal to each other in length, i.e.
almost the shape of an isosceles triangle. Found in samples of white
alpaca. Medulla section appoximately T-shaped.
Type E
3.25 Excerpt from ‘The Microscopy of Animal Textile Fibres’1 showing
the various cross-sectional shapes exhibited by alpaca fibres (courtesy
of BTTG Ltd).
Natural animal textile fibres
57
3.26 Photomicrographs showing the cross-sectional shapes exhibited
by alpaca fibres (courtesy of BTTG Ltd).
camel, the colour of the hairs collected or harvested range from reddish to
light brown with variants from brown to grey (white hairs may occur but
these are extremely rare). The camel fibres possess certain features which
help in their identification. With the use of low power microscopy camel
hairs, unlike wool fibres, are seen to be very regular in outline or profile
and to exhibit a uniform diameter along their lengths; the cuticular scale
edges project so very slightly from the hair shaft that its profile appears
58
Identification of textile fibers
3.27 Scale pattern characteristic of rabbit fibres (courtesy of Dr Hans
Brunner).
3.28 Photomicrograph depicting cross-sectional shapes exhibited by
rabbit fibres (courtesy of BTTG Ltd).
Natural animal textile fibres
59
3.29 Photomicrographs depicting scale patterns present on mohair
fibres (courtesy of BTTG Ltd).
almost a straight line (see Figs 3.33 and 3.34). This particular feature is
extremely useful in distinguishing camel fibres from wool fibres with which
they may be mixed or blended.
According to Wildman1 the medulla of the coarse camel hair fibre is
of the unbroken type and is quite narrow in relation to the diameter of
the fibre. This tendency to have a relatively narrow medulla is characteristic
of the most coarse camel fibres and is a useful characteristic for
identification.
3.5.2 Non-keratin silk fibres
In contrast to the animal hair fibres, the identification of silk is generally
easy to achieve. Degummed silk filaments are smooth-surfaced and semi
transparent. Kushal and Murugesh5 found that the mulberry and nonmulberry silks exhibit an entirely different cross- and longitudinal sectional
shapes and varieties which are illustrated in Figs 3.35, 3.36 and 3.37; the
mulberry silks show a more or less triangular cross-section and a smooth
surface, which markedly differ from the the non-mulberry (wild silk)
varieties.
Until recent times, cultivated silk was easily distinguished from all other
fibres by its narrow diameter, but the advent of microfibres has changed
this. The identification of silk must now be approached with care as nylon
60
Identification of textile fibers
3.30 Photomicrograph showing the smooth distant scale margins
present on cashmere (goat) fibres (courtesy of BTTG Ltd).
Natural animal textile fibres
61
10 μm
3.31 SEM image of the scale pattern of a cashmere fibre (courtesy of
CSIRO Textile and Fibre Technology).
microfibres and silk can be confused because of the similarities in their
diameters and their infrared spectra. Silk, however, is normally less regular
in appearance along its length than a microfibre. An easy way to view this
irregularity is between crossed polars using the interference colours in the
same way that one views a topographic map. The most definitive difference,
if difficulties are encountered, is to place a short segment or cross-section
from the fibre in question in the hot stage. Nylon will melt while silk will
not. Colour is the principal point of comparison once it has been established
with certainty that the fibre is silk and what type of silk it is. Fluorescence
microscopy may provide additional features based on any fluorescence of
the dyes.
3.6
Future trends
The most significant breakthrough in wool technology since the development of shrink-resistant technology in the 1960s is touted to be the OptimTM
fibres created by the Textile and Fibre Technology Division of the
Commonwealth Industrial Research Organisation (CSIRO) in Australia.
Woollen fibres are stretched and then set, these ‘parent’ woollen fibres
are then processed which causes them to rearrange themselves, resulting in
fibres which possess a structure akin to that found in silk fibres. A wool fibre
with a diameter of 19 microns will be reduced to a fibre with a diameter of
62
Identification of textile fibers
3.32 Photomicrograph showing the scale pattern on a coarse
cashmere fibre (courtesy of BTTG Ltd).
15–16 microns. OptimTM fine is smoother than wools as in the stretching
process become elongated along the wool fibre which gives the fibre a
smoother appearance and feel; the cross-sectional also changes from round
to oval found in untreated wools to triangular which resemble those of silk
fibres as illustrated in Figs 3.38 and 3.39.
3.7
Sources of further information and advice
Australia and New Zealand represent the major contributors of wool used in
the textile industry. The following internet sites may be accessed for further
Natural animal textile fibres
63
3.33 Photomicrographs showing the cuticular scale arrangement on a
camel fibre (courtesy of BTTG Ltd).
10 μm
3.34 SEM image of a camel fibre (courtesy of CSIRO Textile and Fibre
Technology).
64
Identification of textile fibers
3.35 SEM images showing the various longitudinal scale patterns
exhibited by mulberry silk fibre and wild silk fibres (courtesy of Wiley
Publishers).
3.36 SEM image showing cross-sectional shapes of mulberry silk
fibres (courtesy of Wiley Publishers).
Natural animal textile fibres
65
3.37 SEM image showing the cross-sectional appearance of wild silk
(tussah/tasar) fibres (courtesy of Wiley Publishers).
information: CSIRO at http://www.csiro.au and the Wool Research Organisation of New Zealand at http://www.woolresearch.com.
There are several publications which deal with the identification of
animal fibres. The publication produced by Wildman1 is a very comprehensive and significant reference guide solely in relation to animal textile fibres
featuring numerous photographs depicting the various morphological characertistics exhibited by many animal hairs used in the textile industry in
the 1950s and describes various methodologies employed for their examination; despite being published over 50 years ago the principles of the examination of animal textile fibres and the basis of their identification apply
today.
In 1978 H.M. Appleyard9 produced a ‘Guide to the Identification of
Animal Fibres’ which was a concise version of ‘The Microscopy of Animal
Textile Fibres’ by A.B. Wildman;8 the purpose of the publication being to
act more as a laboratory manual to assist practitioners who may not require
all the details given in ‘The Microscopy of Animal Textile Fibres’ in relation
to laboratory techniques. The book describes the morphological features
exhibited by 49 different animal species, which includes those described by
Wildman.
3.8
Acknowledgements
I am indebted to the following people for their invaluable assistance:
Ms Tahnee Dewhurst (VPFSC document section) for her patience in the
production of images, Ms Tracey Archer (VPFSC Librarian) for her tenacity
and skills in obtaining reprints, my dear friend Dr Hans Brunner for his
66
Identification of textile fibers
Merino wool
36 μm
(a)
Optim wool
36 μm
(b)
3.38 SEM images of the cross-sectional shapes of unprocessed wool
fibres (a) and the processed OptimTM wool fibres (b) (courtesy of
CSIRO Textile and Fibre Technology).
Natural animal textile fibres
Silk
67
18 μm
3.39 SEM image of the cross-sectional shapes of mulberry silk fibres
(courtesy of CSIRO Textile and Fibre Technology).
‘carte-blanche’ approach to my republication requests. I would also like to
thank Ms Heather Forward and Margaret Pate (CSIRO-Textile and Fibre
Technology Division, Melbourne), Lyndon Arnold (RMIT Melbourne) and
Dr Matthew Fleet (SARDI, South Australia) who were generous in devoting their time and efforts in assisting a person whom they had never met.
3.9
References
1. Wildman A B (1954), The Microscopy of Animal Textile Fibres, Leeds, Wool
Industries Research Association (WIRA).
2. Harding H and Rogers G (1999), ‘Physiology and Growth of Human Hair’, in
Robertson J (Ed), Forensic Examination of Hair, London, Taylor and Francis,
6.
3. Needles H L (1986), Textile Fibres, Dyes, Finishes, and Processes A Concise Guide,
New Jersey, Noyes.
4. Cook J G (1984), Handbook of Textile Fibres Natural Fibres, Durham, Merrow.
5. Kushal S and Murugesh B K (2004),‘Studies on Indian Silk. I. Macrocharacterization
and Analysis of Amino Acid Composition’, Journal of Applied Polymer Science,
92, 1080–1097.
6. Brunner H and Coman K (1974), The Identification of Mammalian Hairs,
Melbourne, Inkata.
7. Petrie O J (1995), ‘Harvesting of textile animal fibres’. Food and Agriculture
Organisation (FAO) of the United Nations, Bulletin 122.
8. Seta S, Sato H and Miyuke B (1988), ‘Forensic Hair Investigation’, Forensic
Science in Progress, 2.
9. Appleyard H M (1978), Guide to the identification of Animal Fibres, Leeds, Wool
Industries Research Association (WIRA).
4
Synthetic textile fibers: structure,
characteristics and identification
K KAJIWARA,
Otsuma Women’s University, Japan and
Y OHTA, Toyobo Co. Ltd, Japan
Abstract: Various types of conventional synthetic fibers are reviewed in
terms of their structure, mechanical and physical properties. The
identification method for each fiber is also briefly described for practical
convenience.
Key words: synthetic fibers, chemical fibers, polyester fiber, polyamide
fiber, polyacryl fiber.
4.1
Introduction
By 1930, most scientists were convinced that polymers were, in fact, covalently linked macromolecules. Although a polymer has a high molecular
weight, its primary structure is not particular complicated and is composed
of multiple simple units (monomers). Synthetic polymers are widely used
in fibers and plastics, which can be seem in clothes, molded parts and tyres,
and nowadays we cannot imagine our life without them. However, originally the polymer industry utilized mainly natural polymeric materials
including wood, leather, resins, fibers and rubber. Polymeric materials
support our food, clothing and housing needs, and are indispensable even
in advanced technology as demonstrated by optical fiber, carbon fiber,
polymer battery, artificial kidney, and ion-exchange membrane. The polymers used for non-structural materials are often referred to as functional
polymers. The synthetic polymer industry was already in existence in the
early 1930s when nylon was invented. The production of nylon 66 began in
1938 (Carothers, Du Pont) and nylon 6 in 1942 (Schlack, IG, Germany). The
chemical formulae are written as:
H − [ HN ( CH 2 )m ⋅ NHCO (CH 2 )n CO]x − OH (nylon m, n + 2 ) or,
H − [ HN ( CH 2 )n CO]x − OH (nylon n + 1)
Nylon is melt-spun to make a filament which is said to be thinner than a
spider’s web but stronger than steel. Polyvinylalcohol (PVA) was first synthesized in 1924 in Germany, but its industrial production had to wait until
68
Synthetic textile fibers
69
a PVA that was stable in boiling water was developed in 1950 by Sakurada
using thermal treatment and partial formylation. Whinfiled and Dickson
developed polyester fiber in 1940. Although Carothers was the first to
attempt to synthesize polyester using various combinations of polyol and
aliphatic polybasic acid, he failed to achieve a high melting point and
switched his strategy to the combination of polyol and polyamine. Whinfield
and Dickson (Calico Printers) employed aromatic polybasic acid (terephtharic acid = 1,4-diformylbenzene) to achieve a high melting temperature. The commercial production of polyester fiber started in 1948 in the
USA (Du Pont) and in 1952 in the UK (ICI). Nowadays, polyester is the
most widely produced synthetic fiber. Acrylic (PAN) fiber is made from
the copolymer of the main component acrylonitrile (CH2=CHCN over
50%) with the addition of acrylic acid ester (CH2=CH ⋅ COOR), vinyl
acetate (CH2=CHOCOCH3), acrylamide (CH2=CH-CONH2), and methacrylic acid ester (CH2=C(CH3)COOR) to improve dyeability, solubility
and transparency. PAN fiber is suitable for knitting and blanket material,
and is a precursor for carbon fiber, which is produced in two steps: PAN
fiber is heat-treated with air at 250°C, and then carbonized into graphite at
a higher temperature without oxygen. These are the three major synthetic
fibers, in contrast to the three major natural fibers: cellulose, wool and silk.
The fiber-forming polymers possess common molecular characteristics such
as a large intermolecular interaction through polar groups, a symmetric
structure for high crystallinity, an appropriate range of melting temperatures and glass transition temperatures, a high molecular weight and a good
drawability.
Table 4.1 compares the performance of the most important fibers. Here
the tensile strength denotes the weight (g) to break 1 d fiber. Denier (d)
corresponds to the weight (g) of a fiber that is 9000 m in length. A silk filament is about 1 denier. The SI unit (Le Système International d’Unités, the
basic units of which are summarized in Table 4.1) for tensile strength and
modulus is given by GPa (1 GPa = 1010 dyne/cm3 = 1.0197 × 104 kg/cm2),
which is related to g/d as:
g d=
11.3
× GPa
density ( g cm 3 )
The tensile strength required for apparel applications is at most a few g/d.
For example, a conventional nylon is 5 g/d, while steel wire is 3.5 g/d.
However, the strength per cross-section is given as 50 kg/mm2 for nylon and
248 kg/mm2 of steel because of the density difference.
The elongation is defined as
Elongation (%) =
Δ
× 100
3.8∼5.3
2.6∼4.4
3.7∼4.4
2.2∼4.4
4.2∼5.7
4.4∼5.7
3.5∼5.7
Polyester (PET)
Polyester (PBT)
Polyester (PTT)
Polyacrylonitrile
Nylon 6
Nylon 66
Vinylon (PVA)
2.8∼4.6
4.0∼5.3
3.7∼5.2
1.8∼4.0
2.6∼4.4
3.7∼4.4
3.8∼5.3
1.14
1.26∼1.30
12∼26
1.14
1.14∼1.17
1.31
1.34
1.38
Specific
gravity
25∼38
28∼45
25∼50
20∼40
20∼40
20∼32
dry
dry
wet
Elongation
(%)
Tensile strength
(cN/dtex)
Table 4.1 Fiber performance
53∼79
26∼46
18∼40
34∼75
18∼35
23
79∼141
Young’s
modulus
(cN/dtex)
5.0
4.5
4.5
2.0
0.4
0.4
0.4
Moisture
regain
(%)
Hot pyridine,
phenol, cresol,
conc. formic
acid
Phenol, mcresol, conc.
formic acid
N,N′-dimethylformamide
Dimethylsulfoxide
Phenols
m-cresol (hot),
o-chlorophenol (hot), nitro
benzene (hot),
dimethyleforamide
(hot)
Specific solvent
Vat dye, metal
complex dye,
sulphur dye,
direct dye,
pigment
Cationic dye,
disperse dye
Acid dye, metal
complex dye,
disperse dye,
reactive dye
Acid dye, metal
complex dye,
disperse dye,
reactive dye
Disperse dye,
cationic dye
for basicdyeable
polyester
Disperse dye,
pigment
Dyeing agent
DuPont
(Nylon®)
<1939>
Nihon Gosei
Sen-I <1946>
I.G. (PerlonL®) <1942>
Teijin <1979>
Asahi
Chemical
(Solotex®)
<1999>
DuPont
(Orlon®)
<1950>
ICI (Terylene®)
<1955>
Company
(trade name)
<Year>
4.0∼6.6
4.4∼7.9
0.5∼1.1
1.5∼2.0
1.1∼1.2
2.6∼3.5
0.9∼1.5
2.6∼4.3
Polypropylene
Polyethylene
Polyurethane
Viscose rayon
Triacetate
Silk
Wool
Cotton
2.9∼5.6
0.7∼1.4
1.9∼2.5
0.6∼0.8
0.7∼1.1
0.5∼1.1
4.4∼7.9
4.0∼6.6
3∼7
25∼35
15∼25
25∼35
18∼24
450∼800
8∼35
30∼60
1.54
1.32
1.33
1.30
1.50∼1.52
1.0∼1.3
0.94∼0.96
0.91
60∼82
10∼22
44∼88
26∼40
57∼75
35∼106
8.5
15
11.0
3.5
11.0
1.0
0
0
Copper
ammonium
Methylene
chloride,
glacial acetic
acid
Copper
ammonium
Tetra chloroethane, carbon
tetrachloride,
cyclohexane,
monochloro
benzene,
xylene,
tetralin,
toluene
Pyridine (hot),
phenol,
phenols
Copper ammonium, copper
ethylene
diamine
Reactive dye,
direct dye, vat
dye, naphthol
dye, sulphur
dye
Acid dye, metal
complex dye,
reactive dye
Reactive dye,
direct dye, vat
dye, naphthol
dye
Reactive dye,
direct dye, vat
dye, naphtol
dye, sulphur
dye, pigment
Disperse dye,
acid dye
Pigment
Pigment,
disperse dye
I.G. (PerlonU®) <1941>
Montecatini
<1959>
Courtaulds
(Courlene®)
<1952>
72
Identification of textile fibers
Δ艎 corresponds to the elongated length. Most fibers have an elongation of
over 10%.
Young’s modulus (E) is an intrinsic parameter of the materials, defined
as the ratio of the stress per unit cross-section (S) to the strain (ε).
E=S ε
A high Young’s modulus indicates a hard material.
4.2
Fundamental characteristics of
fibrous materials
Fiber is a general term for materials which are characterized by a long and
thin shape. Fibrous materials may be organic, inorganic or metal, but should
possess a small cross-sectional diameter in comparison with the length.
Because of this characteristic, fibrous materials possess flexibility as well as
high strength. Fibrous materials are a fundamental part of our lives, not
only as the materials for clothes to protect us from the environment, but
also as part of our bodies, which are made of fibrous materials that sustain
flexibility and strength. Commercially available fibrous materials can be
classified into two categories: natural fibers and chemical fibers. Natural
fibers include plant fibers (such as cotton, hemp and pineapple fiber), animal
fibers (such as wool, mohair and silk), and mineral fibers (such as asbestos).
Chemical fibers include regenerated fibers (such as rayon), semi-synthetic
fibers (such as acetate) and synthetic fibers (such as the organic fibers of
nylon, polyester and acrylonitrile, and the inorganic ones of glass fiber,
metal fiber and carbon fiber).
The commercial success of nylon ignited the development of synthetic
fibers. In the early days of the polymer industry, most R&D effort was targeted at finding new synthetic fibers. Tables 4.2 and 4.3 summarize the
molecular structure and thermal properties of polymers for fibrous materials. New polymers were created, and now polymers are applied in various
fields such as fibers, plastics and elastomers.
The application of fibrous materials is not limited to apparel. Non-apparel
(industrial) use of fibers can be found in ropes, fishing nets and composites.
Industrial synthetic fibers include (i) the fibers that were first developed for
apparel but were unsuitable and so were converted for industrial use (e.g.,
PVA, PP), (ii) apparel fibers applied to industrial applications (PET, nylon,
PAN), and (iii) the fibers specially developed for industrial use (hightenacity/high-modulus fibers). The performance of fibers depends heavily
on spinning, drawing and further processing. For example, polyester for
apparel should possess easy dyeability and a good hand, while polyester for
tyre cords requires a high tenacity/modulus, toughness for repeated deformation and thermal stability. The fiber production technology has been
Synthetic textile fibers
73
Table 4.2 Molecular structure of polymers for fibrous materials
Polymer
[Polyolefin/vinyl
polymers]
Polyethylene
(PE)
Polytetrafluoroethylene
(PTEE)
Polypropylene
(PP)
Molecular structure
CH2 -CH2
n
CF2 -CF 2
n
CH2 -CH
n
CH3
Polyvinyl alcohol
(PVA)
CH2 -CH
n
OH
Polyvinyl
chloride (PVC)
CH2 -CH
n
Cl
Polyacrylonitrile
(PAN)
CH2 -CH
n
CN
Poly-4-methyl-1pentene
(P4M1P)
CH2 -CH
CH2
n
CH
CH3 CH3
[Nylon/aliphatic
polyamide]
Nylon 6
=
(CH2)5 -C-NH
O
Nylon 66
Nylon 46
NH
O
C
O
O
NH- C
C
O
=
=
NH- C
n
=
NH
O
=
NH
=
NH-(CH2)6-NH-C-(CH2)4-C
=
O
O
n
PPTA
C
=
NH- C
n
=
[Aramid/
aromatic
polyamide]
Poly-mphenyleneiso-phthalamide
(PMIA)
Poly-p-phenylene
terephthalamide
(Kevlar; PPTA)
Aramid
copolymer
(Technora)
n
O
O
n
n
n=0.5
74
Identification of textile fibers
Table 4.2 Continued
Polymer
=
=
O
O
=
O-(CH2)3 -O- C
C
=
O
O
O-(CH2)4 -O- C
C
C
n
n
=
=
O
n
O
O
=
Poly-ε-caprolactone
O- (CH2)2 -O- C
O
-C-O-(CH2)2-O
=
[Polyester]
Polyethylene
terephthalate
(PET)
Polytrimethylene
terephthalate
(PPT)
Polybutylene
terephthalate
(PBT)
Polyethylene
naphthalate
(PEN)
Molecular structure
C
n
=
O -(CH2)5-C
n
O
Polylactic acid
CH3
O
=
O
C
C
n
H
[Polyarylate
(aromatic
polyester)]
Vectran(X:Y=7:3)
CH3
O
=
Poly([R]-3hydroxybutyrate);
P(3HB)
O
C
CH2
C
n
H
O
O
C
=
C
=
O
O
X
O
C
O
=
O
[Other heterocyclic polymer]
Poly-p-phenylene benz-bisoxazole
(PBO)
Poly-p-phenylene benz-bisthiazole
(PBT)
Poly(benz
imidazole)(PBI)
Poly(phenylene
sulfide)(PPS)
O
O
N
N
X
Y
n
N
S
S
N
n
NH
H
N
N
N
S
n
n
C
C
=
O
Y
=
Econol
O
O
Z
Synthetic textile fibers
75
Table 4.2 Continued
Polymer
Molecular structure
Polysulfone
(PSF)
CH3
SO2
O
C
O
n
CH3
Poly(ether
sulfone)(PES)
SO2
O
n
Poly(ether ether
ketone)(PEEK)
O
O
C
=
n
O
[Polymide]
Polymide
(P84; PI)
CH3
CO
N
CO
CO
Ar
Ar :
n
CH2
O
C
=
=
O
C
C
C
O
N
O
N
[Cellulose]
Cellulose
n
=
=
O
[Amorphous
polymer]
Polymethyl
methacrylate
(PMMA)
CO
N
or
Poly(promellitic
imide)
(Capton; PPI)
CO
H
OH
OH H
H
H
O
CH2OH
H
H
O
CH2OH
O
H
OH H
H
O
H
n
OH
CH3
CH2
C
C= O
O
CH3
Polycarbonate
(PC)
O
n
CH3
C
O
C
=
CH3
O
n
developed to a high degree of sophistication, controlling the fiber and fabric
characteristics to match the specified design requirements.
A single filament is composed of fibrils (an assembly of polymers), and
a basic unit of fibrils is known as a microfibril, which is dimensionally similar
for all fibers as shown for three natural fibers (Table 4.4). Synthetic fibers
also possess a similar structural element. Figure 4.1 shows the high-order
structural model of a synthetic fiber proposed by Peterlin.1 Here the
76
Identification of textile fibers
Table 4.3 Thermal properties of polymers for fibrous materials
Polymer
Tg (°C)
Tm (°C)
T 0m (°C)*
Heat of fusion
(cal/g)
PE
PTFE
PP (isotactic)
PVA
PVC
PAN
P4MP1
POM
Nylon 6
Nylon 66
Nylon 46
PMIA
PPTA
−36
−73
−3
85
−19
105
29
−82
40
76
82 (R.H. 0%)
270∼280
260∼270
125∼135
330
165∼173
Not known
200∼240
Not known
230∼240
179
215∼220
250∼260
290∼319
370
No melting point
(560)**
No melting point
(>500)**
255∼265
272
60
270 (transition to
liquid crystal)
No melting point
(630∼650)**
No melting point
(630∼650)**
280∼290 (>440)**
334∼345
373
No melting point
(459)**
141.1
332
187.5
265
272.8
–
–
–
260
301
70.0
19.6
49.4
38.6
42.1
–
33.7
45∼80
54.9
61.0
280
337
–
33.5
24.5
–
4.8
Technora
PET
PEN
PCL
Vectran
69∼81
115
−60
150
PBO
(300∼350)
PBT
(300∼350)
PPS
PEEK
PES
PBI
85∼90
143
162
–
* Equilibrium melting point temperature, which corresponds to the melting
temperature of an ideal crystal free from the surface energy.
** The value in brackets denotes the temperature at thermal degradation.
microfibrils of ca. 10–20 nm in diameter combine in parallel to form fibrils
of 0.1 μm in diameter, and then the fibrils combine into a filament.
Hess and Kiessing2 (Fig. 4.2) proposed the fringed micelle model to
explain schematically the structure of microfibrils of synthetic fibers. Here
the microfibril is composed of a crystalline region (where polymer chains
are oriented in the fiber axis direction) and an amorphous region (where
polymer chains are less oriented), and a single polymer chain penetrates
through the two regions. However, polymer chains are found to fold to
Synthetic textile fibers
Table 4.4 Wide-angle X-ray diffraction pattern from various fibers
Fiber
Crystallographic data
Cotton
(Cellulose I)
Monoclinic
a = 0.835 nm, b =
1.034 nm, c = 0.79 nm
a = 90°, b = 84°, g = 90°
rc = 1.592 g/cm3 (Z)
1 (101), 2 (101̄), 3 (002),
4 (021)
Silk
Viscose rayon
(Cellulose II)
Polyethylene
Polypropylene
Polyacrylonitrile
(PAN)
Monoclinic
a = 0.965 nm, b =
1.040 nm, c = 0.695 nm
a = 90°, b = 90°, g = 62.40°
rc = 1.45 g/cm3
1 (100), 2 (002), 3 (201),
4 (112)
Monoclinic
a = 0.814 nm, b =
1.034 nm, c = 0.914 nm
a = 90°, b = 62°, g = 90°
rc = 1.583 g/cm3 (Z)
1 (101), 2 (101̄), 3 (002),
4 (021)
X-ray diffraction pattern
4
1 2
4
2
1
3
4
1 2 3
Orthorhombic
a = 0.740 nm, b =
0.493 nm, c = 0.253 nm
a = 90°, b = 90°, g = 90°
rc = 1.00 g/cm3 (Z)
1 (110), 2 (200), 3 (210),
4 (020), 5 (310), 6 (011),
7 (111), 8 (201), 9 (211)
Monoclinic
a = 0.665 nm, b =
2.096 nm, c = 0.650 nm
a = 90°, b = 99°20′, g = 90°
rc = 0.936 g/cm3, ra =
0.85 g/cm3 (H)
1 (110), 2 (040), 3 (130),
−−
4 (131) (041), 5 (022)
(112)
Orthorhombic
a = 2.10 nm, b = 1.19 nm,
c = 0.504 nm
a = 90°, b = 90°, g = 90°
rc = 1.592 g/cm3 (Z)
1 (400) (200), 2 (620),
3 (040)
3
6 7
8 9
1 23 4 5
5
4
1 23
1
23
77
78
Identification of textile fibers
Table 4.4 Continued
Fiber
Crystallographic data
Polyvinylalcohol
(PVA)
Monoclinic
a = 0.718 nm, b =
0.252 nm, c = 0.551 nm
a = 90°, b = 91°42′, g = 90°
rc = 1.35 g/cm3, ra =
1.29 g/cm3 (Z)
−
1 (100), 2 (001), 3 (101),
4 (101), 5 (200), 6 (301),
7 (202), 8 (110), 9 (111)
−
(111)
Nylon 6 (α type)
Nylon 66
Poly-L-lactic
acid (PLA)
Carbon fiber
X-ray diffraction pattern
8
45
1 23
3
12
Orthorhombic
a = 1.06 nm, b = 0.61 nm,
c = 2.88 nm
a = 90°, b = 90°, g = 90°
rc = 1.592 g/cm3 (H)
1 (011), 2 (110) (200),
3 (211), 4 (120), 5 (103),
6 (014), 7 (113), 8 (016),
9 (116), 10 (216)
Hexagonal
a = 0.2462 nm,
b = 0.2462 nm,
c = 0.6707 nm
a = 90°, b = 90°, g = 120°
1 (002), 2 (100)
67
1 23
Monoclinic
a = 0.956 nm, b =
1.724 nm, c = 0.801 nm
a = 90°, b = 67°30′, g = 90°
rc = 123 g/cm3 (Z)
1 (200), 2 (002), 3 (202)
Triclinic
a = 0.49 nm, b = 0.54 nm,
c = 1.72 nm
a = 48°30′, b = 77°,
g = 63°30′
rc = 1.24 g/cm3,
ra = 1.09 g/cm3 (Z)
1 (100), 2 (010) (10),
3 (002)
9
8
5
6
7
1
2
1
9 10
2
3
4
Synthetic textile fibers
79
Table 4.4 Continued
Fiber
Crystallographic data
Poly(ethylene
terephthalate)
(PET)
Triclinic
a = 4.56 nm, b = 5.94 nm,
c = 10.75 nm
a = 98°30′, b = 118°,
g = 112°
rc = 1.455 g/cm3,
ra = 1.335 g/cm3 (Z)
−
1 (010), 2 (110), 3 (100),
−
−
4 (011), 5 (111), 6 (011),
−
−
−
7 (111), 8 (112), 9 (103),
−
10 (013), 11 (003)
Poly(trimethylene
terephthalate)
(PTT, 3GT)
Triclinic
a = 0.459 nm, b =
0.621 nm, c = 1.831 nm
a = 98°, b = 90°, g = 111.7°
rc = 1.428 g/cm3, ra =
1.314 g/cm3 (H)
−
1 (002), 2 (010), 3 (012),
4 (012), 5 (101), 6 (102)
−
−
−
(112), 7 (113), 8 (113),
−
−
9 (104), 10 (114)
X-ray diffraction pattern
11
9 10
8
6
5
4
7
3
1
2
3
1
4
6
5
2
8
7
9
10
Poly(butylene
terephthalate)
(PBT, 4GT)
Poly(ethylene
naphthalate)
(PEN)
Triclinic
a = 0.483 nm, b =
0.59 nm, c = 1.159 nm
a = 99.7°, b = 15.2°,
g = 110.8°
rc = 1.396 g/cm3,
ra = 1.281 g/cm3
−
1 (010), 2 (110), 3 (100),
−
−
−
4 (120), 5 (011), 6 (101),
−
7 (011), 8 (111), 9 (101),
10 (001)
Triclinic
a = 0.651 nm, b =
0.597 nm, c = 1.32 nm
a = 81.33°, b = 144°,
g = 100°
rc = 1.407 g/cm3,
ra = 1.328 g/cm3
−
1 (010), 2 (100), 3 (110)
10
11
56
1
1 2 3
789
23 4
80
Identification of textile fibers
Fibril
Fibril (~0.1 mm)
Fibril end (defect)
Microfibril
(~10 nm)
4.1 High-order structural model of a synthetic fiber.
L
4.2 Fringed micelle model.
L
Synthetic textile fibers
81
(c)
(d)
(a)
(b)
(a) Amorphous
(b) Folded crystal
(c) Extended chain crystal
(d) Fringed micelle
4.3 Various phases of polymer chains.
ca. 10 nm in length in a single crystal of polyethylene,3 and the folded crystal
structure is now considered to be the basic structure of a crystalline polymer
rather than the fringed micelle. The condensed phase of polymer is composed of folded chains as illustrated in Fig. 4.3.4
Drawing is an essential process for fiber formation. Folded microfibrils
contain tie molecules linking the folded crystal (indicated by arrow A in
Fig. 4.4); the tie molecules designated as B in Fig. 4.4 connect the folded
microfibrils. When the fiber is drawn, the folded parts deform and align in
the drawing direction (schematically shown in Fig. 4.5). The polymer chains
tying the crystal regions form the amorphous phase with the chain ends.
The mesophase (the intermediate phase between the crystalline and amorphous phases) may exist, but no definite proof can be given.
As yet, no synthetic fiber possesses as highly organized and complex a
structures as that of natural fibers. The fibrils of natural fibers have a more
sophisticated structure, as illustrated in Figs 4.6, 4.7 and 4.8, which show
images of the microfibril structures of cotton, wool and silk respectively.
Cotton fiber is made of cellulose and has a hollow centre (lumen). A
single filament is twisted 80 to 120 times. The thin primary cell membrane
(ca. 0.1 μm in thickness) covers the secondary cell membrane (ca. 4 μm in
thickness), which has a hollow centre (lumen) for transferring water and
nutritious substances (Fig. 4.6). A cotton fiber is naturally twisted, and
therefore is elastic and has a good texture. The air trapped in the lumen
functions as an insulator. Hemp is also a cellulose fiber, but its surface
82
Identification of textile fibers
Microfibril
B
B
A
A
A
4.4 Folded microfibrils.
A
L
A
A
A
L1
L
Before drawing
Natural drawing
4.5 Schematic representation of drawing.
structure is different from that of cotton. The surface of hemp fiber is composed of fibrils arranged in parallel, so that the fiber is hard and straight
(not twisted).
Wool is an animal fiber (protein) produced in cells. The main protein
component is keratin, and the fiber has a sophisticated high-order structure
as shown in Fig. 4.7. The hydrophilic main fiber component (cortex) is
Synthetic textile fibers
83
Secondary cell membrane
(thickness 4 μm)
Primary cell membrane (0.1 μm)
Lumen
Outer skin
Winding
(thickness ca. 1 μm)
Cellulose net
4.6 Structure of cotton fiber.
Epi-cuticle
Exo-cuticle
Low-s protein
Nucleic residue
Endo-cuticle
Cuticle
High-s protein
Left-handed
coil-coil bundle
Right-handed α-helix
Amorphous
matrix
Cell membrane
complex
Microfibril
(0.3 μm in diameter)
Microfibril
(80 Å in diameter)
Cortex
Cuticle
Vapor
Vapor
Water droplet
Ortho
Para
Cortex
Water droplet
Vapor
4.7 Structure of wool fiber.
84
Identification of textile fibers
(100 Å)
(1 μm)
(ca. l d)
Average 2.8 d
4.8 Structure of silk fiber.
covered with a hydrophobic cuticle (scales). Since the cortex has a bilateral
structure, where the ortho-cortex and para-cortex are stacked together,
wool is crimped.
Raw silk has a characteristic structure composed of two types of protein:
fibroin and sericin (Fig. 4.8). Sericin glues two fibroin filaments together,
but dissolves in hot water. Silk is customarily used as an example of a fibroin
filament, and its characteristic lustre and scroop are the result of a particular
triangular cross-section with a lateral slit.
4.3
Common synthetic fibers
4.3.1 Nylon
Nylon is a common name for aliphatic polyamide, and is classified according
to its number of constituent aliphatic carbons. However, nylon is conventionally referred to as polyamide fiber. Nylon 66 and Nylon 6 are the most
common nylons, which are produced by polycondensation of diamine
(hexamethylene diamine) and dicarbooxylic acid (adipic acid), and by polycondensation of ω-aminocarboxylic acid or ring-opening polymerization of
lactam (ε-caprolactam), respectively. Nylon 66 and Nylon 6 possess similar
characteristics except for their melting points (see Table 4.3), and are used
for both apparel and industrial (non-apparel) products because of their
lightness, appropriate elasticity and elastic recovery, good heat set property
and dyeability.
Other commercially available nylons include Nylon 610 (poly(hexamethylene sebacamide), mp 210°C), Nylon 11 (poly(11-aminoundecanoic
Synthetic textile fibers
85
acid), mp 186°C) and Nylon 12 (poly(12-aminododecanoic acid lactam), mp
177°C), but these nylons are used mainly for industrial applications. Nylon
4 (poly-2-pyrolidone) has similar moisture-absorbing characteristics to
cotton, but is not yet used commercially because of its severe water-free
polymerization condition.
4.3.2 Polyester
Polyethylene terephthalate (PET), which is the most successful synthetic
fiber, is composed of ester links of aliphatic (ethylene diol) and aromatic
(terephthalic acid) groups. The aromatic group is rigid and is considered as
a long virtual chemical bond, so that the extended chain assumes a large
zigzag form, resulting in a lower intrinsic Young’s modulus (ca. 110 GPa)
of the crystalline region in the molecular axis direction in comparison with
the more extended polyethylene chain (ca. 280 GPa).5 When the aliphatic
carbon number increases, the 3D structure of molecular chains then changes
and the Young’s modulus decreases. Polytrimethylene terephthalate (PTT
or 3GT) (see Table 4.2) and polytetramethylene terephthalate (PBT or
4GT) possess 3 methylene and 4 methylene groups, respectively. PTT and
PBT exhibit a slightly lower Young’s modulus (23 cN/dtex) than Nylon 66.
Polytrimethylene terephthalate (PTT) fiber is produced by melt-spinning
of the polycondensate of terephthalic acid and 1,3-propane diol. PTT has
excellent stretching characteristics and its elastic recovery (88% at 20%
elongation) is one of the best among synthetic fibers.
Aliphatic polyester fiber is in general biodegradable, although its melting
point is low and its application is limited. Polylactic acid (PLA) is produced
by melt-spinning the polycondensate of lactic acid, obtained by fermentation of corn starch. Another example of aliphatic polyester is poly(3hydroxybutyrate) (P(3HB)) produced by marine bacteria. The melting
point of these aliphatic polyesters is low, between 170 and 175°C. The tensile
strength of polylactic acid fiber could be as high as 4–5 cN/dtex, the Young’s
modulus is about half that of PET and the glass transition temperature is
57°C, while the tensile strength of P(3HB) can reach 10 cN/dtex but its glass
transition temperature is lower (4°C).
4.3.3 Polyacrylonitrile
Polyacrylonitrile is atactic, so that large cyanic groups stick out randomly
from the main chain and prevent it from having a close packing density.
Thus the crystallinity of polyacrylonitrile fiber is low. Since the dyeability
of pure polyacrylonitrile fiber is poor, methyl acrylic acid or methyl methacrylic acid is copolymerized into acrylonitrile in commercial fibers to
improve it.
86
Identification of textile fibers
4.3.4 Polyolefin
Polyethylene and polypropylene are two major polyolefin fibers that are
available on the market. Polyethylene has a simple chemical structure consisting only of methyl groups. The most stable conformation is an all-trans
planar zigzag structure, and its intrinsic Young’s modulus in the molecular
axis direction is calculated as 280 GPa. However, this simple chemical structure results in thermal internal rotation around the C—C bond, even at
lower temperatures, which disturbs the all-trans planar zigzag conformation, so that the polyethylene fiber is soft and has a low Young’s modulus.
If the all-trans planar zigzag conformation could be maintained, the thermal
conductance in the chain axis direction would be as good as that of a
diamond.6
Polypropylene has a methyl side group on each ethylene unit. Methyl
side groups may attach on the same side (isotactic) or the opposite side
(syndiotactic) of the main backbone chain when it is stretched on the
plane parallel to ethylene backbone units. The methyl side groups should
appear on every ethylene repeat unit (head-tail structure), but in some
cases the methyl groups are found on two adjacent units (head-head structure) or no methyl groups are found on two adjacent units (tail-tail structure). As a consequence, the characteristics of polypropylene fiber depend
on its stereo regularity (isotactic or syndiotactic) and the head-head/tail-tail
to the head-tail ratio. At present, commercial polypropylene fiber mainly
consists of isotactic polypropylene. The polypropylene chain assumes a
helical conformation composed of trans-gauche alternate unit arrangements due to the steric hindrance of bulky methyl side groups. Since the
methyl groups attach directly to the main chain, the steric repulsion between
methyl groups is strong enough to prevent the trans-gauche transformation.
The helical structure of the polypropylene’s main chain behaves like a
spring and suppresses the intrinsic Young’s modulus, which is much lower
than that of polyethylene and nylon. The Young’s modulus of polyethylene
and nylon is relatively high because of the planar zigzag main chain
structure.
4.4
Crystal structure of synthetic fibers
The crystal structures of synthetic fibers are well documented. Table 4.4
summarizes some crystal structures and corresponding schematic wideangle X-ray diffraction patterns. The crystal system (monoclinic, triclinic,
orthorhombic or hexagonal), the lattice constants (three axial lengths a, b
and c, and three axial angles a, b and g), the density of the crystal region
(rc) and the amorphous region (ra), the chain conformation (Z: a planar
zigzag conformation, H: a helical conformation), and the Miller indices are
Synthetic textile fibers
87
Table 4.5 Infrared spectroscopic absorbance bands from synthetic fibers
Fiber
Characteristic absorbance bands (cm−1)
PET
Nylon 6
Nylon 66
Polyacrylonitrile
Polyethylene
Polypropylene
Polyvinyl alcohol
Cotton
Wool
Silk (fibroin)
Rayon
Triacetate
1730–1410 1340 1250 1120 1100 1020–870 730
3300 3050 2950 2850 1630 1530 1450 1250 680 570
3300–2950 2850 1630 1530 1470 1270 930 680 570
2950 2250 1730 1450 1360 1220 1160 1060 540
2900 2850 1470 1460 1370 740 720
2970–2940 2850 1450 1370 1160 990 970 840
3400–2950 1430–1400 1090–1050 1020 850 790
3450–3250 2900 1630 1430 1370 1100–970 550
3400–3250 2900 1720–1600 1500 1220
3300 2950 1710–1630 1530–1500 1440 1220 610 540
3450–3250 2900 1650 1430–1370 1060–970 890
3500 2950 1750 1430 1370 1230 1040 900
shown, together with the schematic diffraction pattern for each synthetic
(and natural) fiber. Synthetic fibers can be identified by comparing the Xray diffraction patterns.
4.5
Identification of synthetic fibers
Synthetic fibers can be identified by a combination of various tests including
microscopic observation, specific gravity measurement, infrared spectroscopic analysis, the burning test, the coloration test and the dissolving test.
Table 4.5 summarizes the characteristic absorbance bands for synthetic
fibers to facilitate their identification. Other characteristics of the respective
fibers can be seen in Tables 4.1 to 4.4.
4.6
References
1. A. Peterlin, J. Polymer Sci., C9, 61 (1965); J. Polymer Sci., A-2, 7, 1151 (1967).
2. K. Hess, H. Kiessing, Naturwissenschaft., 31, 171 (1943); Z. Physik. Chem., A193,
196 (1944).
3. A. Keller, Phil. Mag., 1171 (1957).
4. B. Wunderlich, Ber. Bunsenges., 74, 772 (1970).
5. K. Tashiro, Prog. Polym. Sci., 18, 377 (1993).
6. A. Yamanaka, T. Kashima, Sen’I Gakkaishi, 56, 128 (2000).
5
High performance fibers: structure,
characteristics and identification
Y OHTA, Toyobo Co. Ltd, Japan
and K KA JIWARA, Otsuma Women’s University, Japan
Abstract: Structural and mechanical properties of various high strength
and high modulus fibers (HPFs) can be characterized mainly by their
high tensile strength, which is at least twice that of conventional
fibers. The detailed methods to identify each HPF are also
described.
Key words: high strength and high modulus fiber, aramid fibers, ultra
high molecular weight polyethylene fiber, spinning, super fibers.
5.1
Introduction
A high strength and high modulus fiber is identified by a tensile strength
of over 2 GPa, and is often referred to as a high performance fiber (HPF)
or super fiber. Inorganic fibers such as carbon fiber are included in the
extended definition of HPF, but this chapter will focus on HPFs based on
organic polymers. Organic HPFs are produced not by conventional melt
spinning, but by gel spinning and liquid crystal spinning (considered as a
special type of solution spinning), originally developed for ultra-high molecular weight polyethylene (UHMW-PE) fiber and aramid fiber, respectively.
These fibers have been developed for various industrial uses including civil
engineering, the aerospace industry, the automobile industry and sports
goods. This chapter focuses mainly on organic HPFs, and reviews their
structures and physical properties.
5.2
The primary structure and physical
properties of HPFs [1,2]
5.2.1 Classification of HPFs
Table 5.1 and Fig. 5.1 summarize the mechanical and physical properties of
organic HPFs together with their primary structure. These HPFs are commercially available or will soon appear on the market. HPFs are conventionally classified according to their chain rigidity, because the rigidity of
the chain determines the spinning method and thermal properties such as
88
Flexible chain
Polyketone
PVA
UHMW-PE
Polybenzazole
K2
Zylon AS
Zylon HM
M5
DyneemaSK60
DyneemaSK71
Spectra900
Spectra1000
1.39
1.47
Polyarylate
Technora
Vectran
HPF
Rigid chain
1.14
1.38
1.43
1.45
1.44
1.45
1.47
1.44
1.45
–
–
Kevlar 29
Kevlar 49
Kevlar 119
Kevlar 129
Kevlar 149
Twaron
Twaron HM
PA6
PET
p-Aramid
Conventional
1.54
1.56
1.70
0.97
0.97
0.97
0.97
(g/cm3)
Density
Fiber
Polymer
Type
37
37
23
26–32
>35
26
30
16–20
14–18
25
22
8
8
20
20
21
23
16
21
21
(cN/dtex)
5.8
5.8
3.9
2.6–3.2
>3.5
2.6
3.0
3.4
3.2
0.9
1.1
2.9
2.9
3.1
3.4
2.3
3.0
3.0
(GPa)
Strength
1150
1720
1940
880–1230
>1230
1235
1765
192–360
520
494
35
106
485
838
379
688
970
500
720
(cN/dtex)
180
270
330
88–123
>123
120
171
71
91
4
14
70
135
55
99
143
72
105
(GPa)
Modulus
3.0–5.0
3.0–5.0
3.5
2.7
3.5
2.5
4.6
10–20
10–30
3.6
2.8
4.4
3.3
1.5
3.5
2.7
(%)
Elongation
Table 5.1 Mechanical and physical properties of HPFs in comparison with those of conventional fibers
650
↑
530
145
∼155
146
500
327–331
223
265
560
↑
↑
↑
↑
>500
>500
(°C)
Melting/
decomposition
temp.
68
↑
>50
18
25
28
29
29
29
(–)
LOI
90
Identification of textile fibers
N
N
O
O
N
H
n
Zylon®
O
O
C
C
N
H
n
Kevlar®, Twaron®
O
C
N
H
O
C
N
H
N
H
O
N C
H
O
C
n
O
Technora®
H
H
O
C
O
n
Vectran®
C
O
C
O m
H
H
C
OH
C
n
Polyvinyl alcohol
H
m
H
C
H
n
Dyneema®, Spectra®
5.1 Chemical structures of HPFs.
heat resistance and flame resistance. UHMW-PE and aramid polymers
represent HPFs made from a flexible polymer and a rigid polymer, respectively, as shown in Table 5.1. Generally, HPFs made from rigid polymers
have superior heat resistance and flame resistance compared with HPFs
made from flexible polymers. The details of the manufacturing process for
each fiber will not be discussed, but it depends strongly on the chain rigidity.
Gel spinning [3] is applied mainly to flexible chain polymers, while HPFs
from rigid chain polymers are produced by liquid crystal spinning [4]. More
details of the respective spinning processes can be found in the reviews
[1, 2].
5.2.2 The primary structure and physical properties of
HPFs made from rigid chain polymers
Aramid fiber
The pioneer of HPF was aramid fiber, developed in the 1970s by DuPont.
Aramid is a general term applied to the polycondensates composed of
aromatic dicarbooxylic acid and aromatic diamine. Numerous combinations
of carboxylic acid and diamine can be used to produce aramids, including
the AB-type polycondensates made from monomers containing both carboxylic acid and diamine groups in a molecule. The commercially available
aramids such as Kevlar® and Twaron® are poly(p-phenylene terephthalamide) (PPTA), which is a polycondensate of terephthalic acid and p-
High performance fibers
91
Tensile modulus (GPa)
400
CF(HM)
300
Zylon-HM
CF(Reg.)
Zylon-AS
Steel
200
Spectra1000
CF(HT)
Kevlar 149
100 Dyneema SK60
PET
Vectran
Kevlar 29
0
0.0
2.0
4.0
6.0
Tensile strength (GPa)
8.0
5.2 Mechanical properties of HPFs.
80
Limiting oxygen index (LOI)
Zylon-HM
60
PPS
40
PBI
PI
p-Aramid
m-Aramid
20
PET
0
0
200
400
600
800
Melting or decomposition temp. (°C)
5.3 Heat resistance and anti-flammability properties of HPFs.
phenylene diamine as shown in Fig. 5.1. The aramid fiber is at least twice
as strong in terms of tensile strength as conventional fibers (Fig. 5.2), and
is thus referred to as a super fiber. Aramid fiber also exhibits excellent creep
resistance, and has better mechanical properties in terms of fatigue resistance for bending and impact resistance/absorption as it has a higher elongation at break (3–4%) than carbon fiber. These characteristic features of
Kevlar® have been used in bullet-proof vests. Since Kevlar® is composed
of extremely rigid molecular chains, the melting point of the crystalline part
is higher than its decomposition temperature. Thus, besides its outstanding
mechanical properties, Kevlar® (p-Aramid) exhibits high heat resistance
(the decomposition temperature is 560°C) as shown in Fig. 5.3. It is highly
92
Identification of textile fibers
flame-resistant and is therefore applied to flame-retardant garments and
insulators where high heat resistance is required. It is chemically stable and
resistant to organic solvents and alkaline solutions, although it is less resistant to acid, such as sulfuric acid, which is used as a spinning solvent. Kevlar®
is less resistant to UV light and other outdoor exposure than conventional
polyester, and care should be taken to shield it sufficiently from the light
by applying a coating or coloring to increase its useful life. Kevlar® is
hygroscopic (4–6%), and may be hydrolyzed causing mechanical deterioration; care should therefore be taken to store and use Kevlar® in relatively
dry conditions.
In recent years, the production process has been much modified to
improve fatigue resistance and tenacity, and a variety of aramid fibers have
been produced with additional functions including dyeability and crimp.
Copolymer-type aramid fiber
Since polymer-composed aramid fiber is highly cohesive and rigid, it cannot
be melt-spun and is generally dissolved into strong acid, such as sulfuric
acid, for solution spinning (liquid crystal spinning). Strong acid restricts the
process technologically, and many attempts have been made to solubilize
aramid in an organic solvent by copolymerization with flexible or less linear
monomers. The Teijin Co. has succeeded in developing and commercializing
an aramid fiber called Technora®. that is soluble in an organic solvent.
Technora® is a random copolymer composed of terephthaloyl dichloride
(TPC), p-phenylenediamine (PPDA) and 3,4′-diaminodiphenyl ether (3,4′DAPE) in the ratios 100 (TPC) : 50 (PPDA) : 50 (3,4’-DAPE) as shown in
Fig. 5.1. Unlike PPTA fiber, Technora® this copolymer dissolves into an
organic solvent and can be drawn after being spun [5,6]. The less linear
molecular structure of Technora® results in a lower tensile modulus than
that of PPTA fiber, but its tensile strength is higher due to its improved
molecular orientation and crystallinity from drawing. Since Technora® is
composed of less rigid molecular chains, its durability, including fatigue
resistance, is superior to that of PPTA fiber. The heat resistance and other
characteristics of Technora® are similar to those of other aramid fibers.
Wholly aromatic polyester fiber
HPF can also be produced from wholly aromatic polyester made up of ester
linkages. An ester linkage is relatively flexible for internal rotation, so that
wholly aromatic polyester can be molecular-designed to have a melting
point lower than the decomposition temperature by introducing suitable
copolymeric units. Wholly aromatic polyester forms a thermotropic liquid
crystal phase when it melts, and can be melt-spun without any solvent. In
High performance fibers
93
order to improve the tensile strength, the molecular weight should be
increased by solid phase polymerization at a higher temperature after being
spun. Vectran® is one of the wholly aromatic polyester fibers that are
commercially available (see Table 5.1). Vectran® is a copolymer made up
of 70% ABA (p-hydroxy-benzoic acid) and 30% ANA (6-hydroxy-2naphthoic acid) [7]. Its fatigue resistance is high, and it is applied to ropes
and nets used in contact with water. The high strength monofilament of
Vectran® has been developed by giving it a sheath-core structure and used
as printing screen application.
Heterocyclic polymer fiber
Heterocyclic polymers were intensively investigated in the 1990s for high
heat resistance, mostly by the US Air Force Research Institutes. These
polymers have a more rigid chain structure than aramid fibers. Poly(pphenylene-benzo-bisoxazole) (PBO) has been developed from that research
into a high performance fiber, which is now commercially available as
Zylon®. The PBO fiber has an extremely rigid and linear chain structure,
and its molecular cross-section is small. As a result, as-spun PBO fiber
(Zylon®-AS) is approximately twice as strong as aramid fiber in terms of
tensile strength and modulus. By heat-treating as-spun PBO fiber (high
performance PBO fiber; Zylon®-HM), its modulus increases further as
shown in Fig. 5.2. Zylon®-HM is a high performance organic fiber, which
has a higher tensile strength and modulus than conventional carbon fiber
in GPa units (for the same cross-sectional basis). The decomposition temperature of PBO is approximately 100°C higher than that of PPTA fiber,
and PBO exhibits an excellent flame-resistance, with its LOI (limiting
oxygen index) being 68%, the highest value among commercially available
organic polymer materials. With these characteristics, PBO fiber is referred
to as a second-generation super fiber [8, 9]. The chemical characteristics of
PBO fiber are similar to those of PTTA fiber. PBO fiber exhibits a resistance to organic solvents and alkaline solutions at room temperature, but is
less resistant to strong acid. Under some conditions, PBO fiber deteriorates
more quickly from UV and other light than aramid fiber. Although the
moisture absorption (official moisture regain 2–3%) of PBO fiber is
lower than that of PTTA fiber, adequate care should be taken for its storage
and use, since its strength will drop considerably when stored in a hightemperature and high-humidity environment for an extended period.
Another heterocyclic polymer, poly{2,6-diimidazo [4,5-b:4′,5′-e]
pyridinylene-1,4 (2,5-dihydroxy) phenylene} (PIPD:‘M5’) (see Table 5.1) is
being developed as an ‘M5’ fiber for commercial use, where a benzene
ring is replaced by a pyridine ring [10]. ‘M5’ fiber is supposed to possess
similar mechanical and thermal characteristics to a PBO fiber. As a result
94
Identification of textile fibers
of intermolecular hydrogen bonds from hydroxyl groups introduced onto
the benzene rings, ‘M5’ fiber has a high compression strength (approximately 1 GPa), an improvement which make it about twice as strong as
aramid fiber.
5.2.3 The primary structure and physical properties of
HPFs made from flexible polymer
Ultra-high molecular weight polyethylene fiber
HPFs made from rigid polymers were developed between the 1970s and
1980s. In the late 1980s, the flexible polymer ultra-high molecular weight
polyethylene (UHMW-PE) was used to produce HPFs by an innovative
spinning process known as gel spinning, and now HPFs made from UHMWPE are commercially available as Dyneema® and Spectra®. The details of
gel spinning are discussed in the review [3]. UHMW-PE fiber is extremely
light (its specific gravity is less than 1), while its tensile strength and modulus
are high enough to be a HPF. Since polyethylene is flexible, UHMW-PE
fiber has high impact strength as well as high abrasion and fatigue resistance. Polyethylene is also chemically stable over a wide pH range and has
excellent weather resistance. However, polyethylene swells in organic
solvent at high temperatures. The melting point of high strength polyethylene is increased to 147°C from its equilibrium melting point of 141°C due
to extended chain crystallization by a high drawing ratio, but for normal
use, the temperature should be kept below 100°C. Polyethylene is completely hydrophobic, and increases its strength at lower temperatures. These
characteristics are utilized for marine ropes, tag ropes, fishing lines and
other products used in watery environments or at extremely low temperatures, e.g. bobbins for superconducting coils (the size change caused by
temperature could be reduced by utilizing a negative thermal expansion
coefficient characteristic) [11].
Other flexible polymers
The commercial success of UHMW-PE fiber stimulated the application of
gel spinning to ultra-high molecular weight polyvinyl alcohol (PVA), polyacrylnitril (PAN) and polyethylene terephthalate (PET). By controlling the
coagulating process with a non-aqueous solvent as a coagulant, 2 GPa fiber
is produced from PVA. Gel spinning has been intensively applied to produce
HPF from the copolymer of polyolefin and carbon monoxide (polyolefin
keton) in recent years. This fiber was first developed by Akzo Nobel Co.
[12], but its commercialization was abandoned. It is only recently that
Asahi Kasei Co. has restarted the project to develop super fiber from
polyolefin ketone.
High performance fibers
95
5.2.4 Other high strength and high modulus fiber
Professor Kikutani’s group has been developing HPF from commodity
polymers including PET as part of a national project. By controlling the
chain entanglements in a melt state, they have succeeded in improving the
tenacity of PET up to 1.7 GPa [13]. Among natural polymers, cellulose
resembles PET with respect to molecular structure and usage amount in
the world as both cellulose and PET are composed of semi-rigid chain
molecules and are commonly used for various applications in large quantities. The theoretical strength and modulus calculated from the molecular
structure predict that cellulose could satisfy the requirements of HPF fiber.
In fact, 2 GPa cellulose fiber has been obtained by liquid crystal spinning
from its polyphosphoric acid solution, but no commercial product is available as yet [14]. Spider silk extracted from goats’ milk proved to have a
high tenacity and high modulus; genetic engineering was used to splice a
spider gene into a goat’s milk gland [15]. The spider silk that was obtained
exhibited a high elongation at break, unlike other HPFs.
5.3
Identification of high strength and
high modulus fiber
5.3.1 Mechanical and thermal characteristics
HPF is markedly different from other fibers in its tensile strength and
modulus. As shown in Table 5.1, the tensile strength of HPF should be over
2 GPa. Care should be taken when measuring the tensile strength, because
there will probably be slippage at the jaw of the tension tester due to the
high tension at the measurement point, and the low friction coefficient of
the fiber surface resulting from the extended chain structure. Since the distribution of strength among single filaments affects the strength of the total
assembly, this should be taken into account in evaluating the tensile strength.
The procedure for the evaluation of the tensile strength of super fiber is
reviewed in detail by Hagege et al. [16].
The heat resistance of HPFs is evaluated in terms of melting temperature
and thermal decomposition temperature by a conventional DSC or TGA,
or by the microscopic observation of fiber at the heating condition with the
hot plate stage equipment. HPF is generally classified into two types: HPF
composed of rigid chain polymer and HPF composed of flexible chain
polymer. UHMW-PE fiber represents a HPF composed of flexible chain
polymer, and Fig. 5.4 shows its DSC endothermic peak pattern at melting
point [17]. UHMW-PE fiber has a melting point around 145–150°C.
However, its apparent melting point shifts, dependent mainly on the
fiber’s restraint conditions during the DSC measurement. In some cases,
96
Identification of textile fibers
.
Q
Chopped (1 mm)
Fixed (in epoxy)
T (°C)
120
100
140
160
5.4 DSC chart for UHMW-PE fiber [3].
100
In air
Residual weight (%)
80
60
Zylon-AS
Zylon-HM
40
Aramid
Aramid-HM
20
Copoly-Aramid
0
0
100 200 300 400 500 600 700
Temperature (°C)
5.5 TGA charts for HPFs.
conventional polyethylene fiber (with tensile strength less than 1 GPa)
shows a melting point slightly lower than that of HPF, so that molecular
weight measurement and primary structure analysis would have to be performed in order to identify polyethylene HPF. Rigid polymer exhibits no
DSC endothermic peak from crystal melting, but its thermal decomposition
temperature can be detected from the weight loss by TGA. Figure 5.5 shows
the weight loss curve observed by TGA for various HPFs in air. Most heatresistant HPFs have a thermal decomposition temperature above 500°C as
shown in Fig. 5.5. The HPF could be identified from the absolute value of
High performance fibers
97
the thermal decomposition temperature listed in Table 5.1. However, exact
identification should be performed by analysis of the primary structure.
5.3.2 Analysis of chemical (primary) structure
The most authentic way of identifying a HPF is the determination of the
primary structure by various chemical analyses. The chemical analyses
include infrared (IR) spectroscopy, Raman spectroscopy and highresolution nuclear magnetic resonance (NMR) for examining the chemical
species and linkage modes, and intrinsic viscosity measurement and GPC
for determining the molecular weight. The identification of a HPF could be
assisted by measuring the specific gravity, thermal decomposition temperature and solubility in various solvents.
The chemical analyses and corresponding sample preparations of HPFs
are performed in the same way as for conventional synthetic fibers. However,
many HPFs do not dissolve in ordinary solvents, and thus the applicability
of conventional chemical analyses will be restricted. For example, UHMWPE dissolves only into organic solvents of relatively low polarity including
decalin, tetralin and liquid paraffin at higher temperatures. PPTA and PBO
polymers dissolve only in strong acid such as sulfuric acid and methanesulfonic acid, respectively. Therefore, fewer reports are available for the
NMR and GPC measurements on those polymers.
IR spectroscopy could be applied generally to the analysis of HPF primary
structures. The ATR (attenuated total reflection) method affords a simple
measurement for HPFs. The characteristic IR absorption is summarized in
Table 5.2 for each HPF. For example, the blending ratio of meta-aramid and
Table 5.2 The typical IR absorption values for HPFs
Polymer
Fiber
Producer
IR absorption (cm−1)
p-Aramid
Kevlar®
Twaron®
DuPont
Teijin Twaron
m-Aramid
Polyarylate
Nomex®
Cornex®
Vectran®
DuPont
Teijin
Kuraray
Polybenzazole
Zylon®
TOYOBO
M5
Magellan
Dyneema®
Spectra®
TOYOBO/DSM
Honeywell
3500–3200, 1660, 1545, 1515,
1405, 1320, 1265, 1115,
1020, 830
3500–3200, 1660, 1610, 1540,
1490, 1420, 1310, 1240, 780
3050–3100, 1930, 1740, 1600,
1260, 1160, 1050, 1010, 890,
760
3010–3120, 2830–2990, 1630,
1410, 1365, 1115, 1055,
1010, 850, 700
3000–3600, 1570, 1500, 1360,
1280, 1200, 960, 720–870
2920, 2850, 1470, 720
UHMW-PE
98
Identification of textile fibers
para-aramid can be determined from the intensity ratio at absorption peaks
[18]. However, the mode of monomer sequence or the molecular weight
cannot be determined easily from IR spectroscopy. The determination of
the monomer sequence by NMR is reported for Technola [19].
GPC is suitable for molecular weight determination, but cannot be
applied in practice for aramid or PBO because of their corrosive solvents,
sulfuric acid or methanesulfonic acid. The molecular weight and branching degree can be evaluated for UHMW-PE fiber by applying high temperature GPC. UHMW-PE has a high molecular weight, over 500 000 dl/g,
while the molecular weight of conventional polyethylene is less than
100 000 dl/g.
The intrinsic viscosity is the simplest criterion for the molecular weight.
It can be measured in the same manner for HPFs as for conventional polymers, although the type of solvent that can be used is limited. Care should
be taken in the solvent preparation because, for example, the viscosity of
PBO solution will vary greatly with the water content in the solvent and/or
the addition of salt [20].
5.3.3 Analysis of higher-order structure
Wide-angle X-ray diffraction (WAXD) and small-angle
X-ray scattering (SAXS)
HPFs are basically composed of crystalline polymer, and many reports have
been published on the details of the respective crystalline structures, which
enable the type of HPF to be identified. Wide-angle X-ray diffraction
(WAXD) is one of the main tools for the identification of the crystalline
structures. Figure 5.6 shows the WAXD pattern and the corresponding
crystal structure for UHMW-PE fiber (a, b) [21–23], PPTA fiber (c, d)
[24, 25], PIPD fiber (e, f) [26] and PBO fiber (g, h) [27, 28], respectively. The
crystal lattice constants of the respective fibers are summarized in Table 5.3.
Fewer reports are found for the crystal structure analysis of copolymeric
HPF including Vectran® and Technola® because of their structural ambiguity [29–31]. The WAXD pattern yields strong spot reflections basically in
the equatorial plane for any HPF as shown in Fig. 5.6, suggesting a high
degree of crystallinity and a higher crystal orientation in the fiber axis. This
high degree of crystallinity and higher crystal orientation characterize the
high-order structure of HPFs.
UHMW-PE fiber exhibits extremely strong spot-like scattering intensity
in the equatorial direction, reflecting its high degree of crystallinity (over
90%) and higher crystal orientation. The crystal structure is characterized
by an orthorhombic crystal as generally observed in conventional isothermal polyethylene crystal prepared at normal temperature. In fewer cases,
High performance fibers
99
(a)
¼
HIV
HIII HIII′
HII
¼
¼
¼
¼
HIV′
HI HI′
HII′
¼
HIV′
b
¼
¼
¼
¼
HIII′
HII′
HII
HI
HIV
HI′
a
HIII
(b)
5.6 (a) WAXD profile for UHMW-PE fiber [21]; (b) crystalline structure
for UHMW-PE fiber [23]; (c) WAXD profile for PPTA fiber [24];
(d) crystalline structure for PPTA fiber [24]; (e) WAXD profile for PIPD
fiber [26]; (f) crystalline structure for PIPD fiber [24]; (g) WAXD
profiles for PBO fiber: as-spun fiber (left side) and heat treated fiber
(right side) [9]; (h) crystalline structure of PBO fiber [27].
100
Identification of textile fibers
(c)
b
a
C
M
D
b
Projection parallel to the c-axis
Projection parallel to the a-axis
(d)
(e)
5.6 Continued
High performance fibers
c
b
a
(f)
AS: As-spun PBO fiber
HM: Heat treated PBO fiber
(g)
5.6 Continued
101
102
Identification of textile fibers
(h)
5.6 Continued
Table 5.3 Crystalline structure of HPF fibers
HPF
PPTA
Kevlar®
UHMW-PE*
Spectra®900
PBO
As-spun
PIPD
Heat treated
Reference
Type
A
(nm)
B
(nm)
C
(nm)
a
(degrees)
b
(degrees)
g
(degrees)
Chain number in
single unit cell
Crystal
(g/cm3)
density
24
Monoclinic
0.780
0.519
1.29
90
90
90
2
22
Orthorhombic
0.7422
0.4944
0.2550
90
90
90
2
27
Monoclinic
1.120
0.354
1.205
90
90
101.3
2
26
Monoclinic
1.260
0.348
1.201
90.0
108.6
90.0
2
1.5
0.974
1.64
1.77
* setting angle = 44.7°
UHMW-PE fiber indicates the existence of a small amount of monoclinic
crystal, which might result from drawing in the production process. The
crystal structure of PTTA fiber is reported as monoclinic as shown in Fig.
5.6 (d). Figures 5.6 (f) and (h) show the proposed crystal structures for PIPD
fiber and PBO fiber, respectively. Here PPTA fiber and PIPD fiber form
intermolecular hydrogen bonds, whereas PBO fiber has no such intermolecular hydrogen bond.
Traditionally, the crystal structure was determined by trial and error by
adjusting the calculated diffraction pattern from a model crystal structure
to the observed X-ray diffraction pattern. In recent years, computational
chemistry has developed to such an extent that the details of a crystal
structure can be discussed in terms of the positional probability of each
atom composing the crystal by potential energy calculations [32].
High performance fibers
103
Since a rigid chain polymer has less freedom for constituent molecular
chains than polyethylene, a small distortion occurring in the crystal can only
be released by axial shift. In consequence, the ordering deteriorates gradually in the molecular chain direction. The crystal structures summarized
above are those of an ideal structure within a short distance order, but
include in reality the molecular chain distortion and/or the non-unique
structural states in the molecular chain direction. When the chain distortion
progresses further, the chain disorder develops into an amorphous state. In
this context, the amorphous state of rigid polymers differs from the amorphous state composed of random coils in flexible polymers with respect to
the structure and molecular motion. Although the amorphous structure is
different, there is a difference in electron density between the amorphous
and crystal phase, so that the space distribution of the amorphous region
can be analyzed by SAXS. A common feature in the SAXS profile from
HPF is that no clear long period structure (normally observed for commodity fibers) is observed in the fiber axis direction. A ‘pleats structure’ is
proposed for PPTA fiber from the SAXS profile and observation by a
transmission electronic microscope, where the c axis is inclined 0° and 10°
alternately in the period of 150 nm to 250 nm with respect to the fiber axis
as shown in Fig. 5.7 [33, 34]. UHMW-PE fiber exhibits a strong streak characteristic to the fully extended chain structure in the equatorial direction
[21], but no such clear long period pattern is observed in the meridian direction as with conventional polyethylene fiber (Fig. 5.8). This SAXS profile is
thought to be the result of the structural characteristics of UHMW-PE fiber
which assumes an extended chain structure, resulting in large crystalline
5.7 Pleat structure proposed for PPTA fiber [33].
104
Identification of textile fibers
5.8 SAXS profile for UHMW-PE fiber [21].
sizes in the fiber axis direction, and is also made of an almost 100% crystalline region, resulting in the disappearance of the electron density contrast
between the crystalline and amorphous region.
A four-point interference long period pattern is observed for PBO fiber
(especially PBO HM fiber) as shown in Fig. 5.9 [9]. Computational chemistry
indicates that the crystalline region of PBO fiber is mainly constituted of
crystal domains as shown in Fig. 5.10, characterized by the inclined structure
of liquid crystal domain units formed during the spinning process [35].
The average image for a higher-order structure of fiber is composed of the
ordinary crystalline structure with superimposed inhomogeneity in the radial
direction of a monofilament. The crystal structures of PPTA fiber, UHMWPE fiber and PBO fiber are analyzed in accordance with this image. Here the
analysis is performed hierarchically over a wide range of dimensions from
nano to sub-micron and micron size. Thus the identification of fibers could be
done from the analysis of an overall structure as fiber possesses a hierarchical
structure as demonstrated by the representative examples below.
Optical microscopy and electron microscopy
Figure 5.11 shows the optical microscopic photographs under (a) a bright
field condition and (b) a crossed-Nicols condition (with crossed polarizers
AS fiber
HM fiber
5.9 SAXS profiles (contour plot) for PBO fiber: as-spun fiber (top),
heat treated fiber (bottom) [9].
5.10 Microstructure model proposed for heat-treated PBO fiber [35].
106
Identification of textile fibers
20 μm
Dyneema® SK60
Spectra® 1000
Kevlar® 29
Technola®
Vectran®
(a)
Zylon® AS
A
20 μm
Dyneema® SK60
Spectra® 1000
Kevlar® 29
Technora®
Vectran®
(b)
Zylon® AS
P
5.11 (a) Micrograph for HSFs under bright field condition;
(b) micrograph for HSFs under crossed-Nicols condition.
placed at 45° with respect to the fiber axis, respectively) of the monofilaments of UHMW-PE fiber (Dyneema® SK60 and Spectra® 1000), PPTA
fiber (Kevlar® 29), PBO fiber (Zylon® AS), Vectran® and Technora®. All
the fibers exhibit strong brightness under a crossed-Nicols condition, and
confirm the high orientation of molecular chains (crystal) in the fiber axis
direction. The high orientation of molecular chains is one of the common
features of HPF. HPF commonly assumes a so-called skin-core structure;
that is, a different structure at the surface or in the middle part of fiber.
High performance fibers
107
5.12 SEM observation of the surface of UHMW-PE fiber [21].
When we observe the photograph of Kevlar carefully, horizontal lines can
be observed, corresponding to a ‘pleat structure’. However, information
obtained from optical microscopy is, in general, not enough to distinguish
individual HPFs. The resolution of the optical microscope is limited to the
sub-micron order of size, and so the electron microscope is useful for analysis at the nano level. The surface of an HPF is as smooth as conventional
synthetic fibers, but streaks were observed on the surface of UHMW-PE
fiber (Dyneema® SK60) by a scanning electron microscope (SEM) as seen
in Fig. 5.12. Analysis of the microcrystal structure inside fiber can effectively
be done by observation through a transmission electron microscope. The
micro-structural analyses of internal monofilaments are reported for PPTA
fiber using detailed observations by optical and electron microscopes [36,
37]. Figure 5.13 shows another example of PBO fiber, where the crystal
lattice was directly observed by an electron microscope [9].
5.4
Alternative methods for analyzing
higher-order structure
High-resolution solid-state NMR has been applied to HPF in order to
understand the correlation between the higher-order structure and molecular motion. Since solid-state NMR affords only limited information, a few
applications are found where, for example, the higher-order structure of
UHMW-PE fiber is discussed in terms of the crystal phase, the amorphous
phase and the intermediate phase from the difference in the relaxation
times. The molecular chain motion in the crystal phase is discussed in detail
108
Identification of textile fibers
Microfibril
Microvoid
Surface
The a-axis of crystal is radially
oriented in a fiber.
Cross-section
Void-free region
< 0.2 μm
Longitudinal section
Microfibril
Microvoid
PBO molecules are highly
oriented in the microfibril.
(orientation factor > 0.95)
5.13 Microstructure model proposed for PBO fiber [9].
using the results of solid-state NMR. However, solid-state NMR should be
regarded as a complementary tool for the identification of a higher-order
structure [38, 39].
Young et al. apply Raman spectroscopy extensively. A specific Raman
shift was observed from the fiber under tension, and the amount of this shift
enabled the microscopic stress exerted on a specific bond to be estimated.
Although it would be difficult to fully identify the chemical structure of a
fiber, the higher-order structure can be analyzed with respect to the mechanical characteristics [40–42].
A strong beam source from synchrotron radiation has been available for
a few years. Here an incident beam is focused at the micron level, and is
being used for the analysis of the microscopic structure, at the monofilament surface and inside it, by WAXD and/or SAXS [43].
High performance fibers
5.5
109
Sources of further information and advice
Many reviews and monographs are available on the production, structural
characteristics and applications of high performance fiber (HPF). The monographs edited by Bunsell et al. provide a good introduction to reinforcing
fibers for composite materials [44]. Descriptions of the production of various
HPFs have been compiled by several authors including Ward [1, 2]. The
pioneer of HPF, aramid fiber, is reviewed by Yang [4], who has been engaged
in the development of PPTA fiber from the beginning [1, 2]. Although the
function of HPF is specific, common methods are applied for the characterization and identification of HPF, and no preference for any particular
method is given in this review. The characterization of molecular oriented
materials is reviewed by Ward [45]. The application and development trends
for HPF can be found in the monograph by Hongu [46].
5.6
References
[1] ‘Advanced Fiber Spinning Technology’ (Ed. T. Nakajima), Woodhead, 1994.
[2] ‘Developments in Oriented Polymers-2’ (Ed. I. M. Ward), Elsevier, 1987.
[3] H. Yasuda, K. Ban and Y. Ohta, ‘Advanced Fiber Spinning Technology’ (Ed.
T. Nakajima), Woodhead, 1994, p.172; J. Lemstra, R. Kirschbaum, T. Ohta and
H. Yasuda, ibid, p.39.
[4] H. H. Yang and S. R. Allen, ‘Kevlar Aramid Fiber’, John Wiley & Sons, 1993.
[5] S. Ozawa, Polym. J., 19, 119 (1987).
[6] H. Matsuda, T. Asakura and Y. Nakagawa, Macromolecules, 36, 6160 (2003).
[7] J. Nakagawa, ‘Advanced Fiber Spinning Technology’ (Ed. T. Nakajima),
Woodhead, 1994, p.160.
[8] K. Yabuki, High Strength High Modulus Fibers in Progress in Textiles: Science
& Technology, Vol. 2 Fibers (Ed. V. K. Kothari), p.615, IAFL Publications, New
Delhi (2000); K. Yabuki, Look Japan, Aug.1995, p.24.
[9] T. Kitagawa, H. Murase and K. Yabuki, J. Polym. Sci., Polym. Phys. Ed., 36, 39
(1998).
[10] D. J. Sikkema, Polymer, 39(24), 5981 (1998); M. Lammers et al., Polymer, 39(24),
5999 (1998).
[11] T. Kashima, A. Yamanaka, E. S. Yoneda, S. Nishijima and T. Okada, Adv. Cryog.
Eng., 41, 441 (1996); A. Yamanaka, T. Kashima, K. Hosoyama, IEEE. Trans.
Appl. Superconductivity, 11, 4061 (2001).
[12] B. J. Lommerts et al., J. Polym. Sci., Polym. Phys. Ed., 31, 1319 (1993); C. C.
Copel et al., ANTEC, Vol.2, 1800 (1997).
[13] T. Kikutani, PPS-22 Proceedings 22nd Annual Meeting of the Polymer
Processing Society, p.144 (2006).
[14] S. J. Eichhorn, R. J. Young, R. J. Davies and C. Riekel, Polymer, 44, 5901 (2003).
[15] Nexia Biotechnologies Inc., http://www.nexiabiotech.com
[16] R. Hagege and A. R. Bunsell, ‘Fiber Reinforcements for Composite Materials’
(Ed. A. R. Bunsell), Elsevier, 1988, p.479.
[17] P. J. Lemstra, R. Kirschbaum, T. Ohta and H. Yasuda, ‘Developments in Oriented
Polymers-2’ (Ed. I. M. Ward), Elsevier, 1987, p.39.
110
Identification of textile fibers
[18] M. Nanbu, Jpn. Res. Assn. Text. End-Uses, 37, 357 (1996).
[19] H. Matsuda, T. Asakura and Y. Nakagawa, Macromolecules, 36, 6160 (2003).
[20] D. B. Roitman, R. A. Wessling and J. McAlister, Macromolecules, 26, 5174
(1993).
[21] A. W. Saraf, P. Desai and A. S. Abhiraman, J. Appl. Polym. Sci., Appl. Polym.
Symp. 47, 67 (1991).
[22] Y. Fu, W. Chen, M. Pyda, D. Londono, B. Annis, A. Boller, A. Habenschuss,
J. Cheng and B. Wunderlich, J. Macromol. Sci.–Phys., B35(1), 37 (1996).
[23] H. Tadokoro, ‘Structure of Crystalline Polymer’, John Wiley & Sons, New York
(1979).
[24] M. G. Northolt and J. J. van Aartsen, J. Polym. Sci., Polym. Letters Ed., 11, 333
(1973); M. G. Northolt, Euro. Polym. J., 10, 799 (1974).
[25] S. J. Krause, D. L. Vezie and W. W. Adams, Polym. Commun., 30, 10 (1989).
[26] E. A. Klop and M. Lammers, Polymer, 39(24), 5987 (1998).
[27] A. V. Fratini and W. W. Adams, Am. Cryst. Assoc. Abs., 13, 72 (1985); A. V.
Fratini, P. G. Lenhert, T. J. Resch and W. W. Adams, ‘The Materials Science and
Engineering of Rigid-Rod Polymers’ (Ed. W. W. Adams et al.), Materials
Research Society, 1990; Material Research Society Symposia Proceedings 134,
465 (1984).
[28] D. C. Martin and E. L. Thomas, Macromolecules, 24, 2450 (1991).
[29] J. Blackwell, R. A. Cageao and A. Biswas, Macromolecules, 20, 667 (1987).
[30] K. Tashiro, Y. Nakata, T. Ii, M. Kobayashi, Y. Chatani and H. Tadokoro, Sen’i
Gakkaishi, 43(12), 627 (1987).
[31] T-M. Wu and J. Blackwell, Macromolecules, 29, 5621 (1996).
[32] K. Tashiro, J. Yoshino, T. Kitagawa, H. Murase and K. Yabuki, Macromolecules,
31(16), 5430 (1998).
[33] M. G. Dobb, D. J. Johnson and B. P. Saville, J. Polym. Sci., Polym. Phys. Ed., 15,
2201 (1977).
[34] H. H. Yang and S. R. Allen, ‘Kevlar Aramid Fiber’, John Wiley & Sons, 1993.
[35] S. Ran, C. Burger, D. Fang, D. Cookson, K. Yabuki, Y. Teramoto, P. M. Cunniff,
P. J. Viccaro, B. S. Hsiao, B. Chu, NSLS Activity Report 2001, Science Highlight,
2-147.
[36] R. Hagege, M. Jarrin and M. Sotton, J. Microscopy, 115, 65 (1979).
[37] M. Panar, P. Avakian, C. Blume, K. H. Gardner, T. D. Gierke and H. H. Yang,
J. Polym. Sci., Polym. Phys. Ed., 21, 1955 (1983).
[38] K. Kuwabara and F. Horii, Macromolecules, 32, 5600 (1999).
[39] S. Bourbigot, X. Flambard and B. Revel, Euro. Polym. J., 38, 1645 (2002).
[40] W. F. Wong and R. J. Young, J. Mater, Sci., 29, 510 (1994); ibid, 520 (1994).
[41] K. V. Prasad and D. T. Grubb, J. Polym. Sci., Polym. Phys. Ed., 27, 381 (1989).
[42] Y. Takahashi, J. Polym. Sci., Polym. Phys. Ed., 39, 1791 (2001).
[43] C. Riekel, A. Cedola, F. Heidelbach, and K. Wagner, Macromolecules, 30(4),
1033 (1997).
[44] ‘Fiber Reinforcements for Composite Material’ (Ed. A. R. Bunsell), Elsevier,
1988.
[45] ‘Structure and Properties of Oriented Polymers’ 2nd edn, (Ed. I. M. Ward),
Chapman & Hall, London, 1997.
[46] ‘New Fibers’ (Ed. T. Hongu and G. O. Phillips), Ellis Horwood, New York,
1990.
6
The use of classification systems and
production methods in identifying
manufactured textile fibers
K L HATCH, The University of Arizona, USA
Abstract: This chapter explains the connection between polymer origins
and fiber classification and the connection/distinction between fiber
classes and fiber subclasses. Then, this chapter focuses on the recently
approved generic fiber class called PLA or polylactide fiber, the recently
approved subclasses of fibers including lyocell, elasterell-p (also known
as elastomultiester), lastol, and then various types of multicomponent
fibers. The final topic of the chapter is future trends in fiber manufacture.
Readers are referred to a series of books which detail the synthesis of
polymers used to create fibers and the production of fibers from those
polymers.
Key words: PLA, polylactide, lyocell, elasterell-p, elastomultiester, lastol,
multicomponent fiber.
6.1
Introduction
Every day in laboratories around the world, polymer and textile fiber
scientists are at work seeking ways to modify existing polymers and textile
fiber manufacturing processes to enhance fiber performance, reduce manufacturing costs, and create more environmentally friendly processes. These
scientists are also developing new polymers to form into entirely new textile
fibers which require new methods of manufacture. While much of what is
discovered and/or underlies commercial production is probably not reported
in the public domain, an incredible amount of outstanding information
on polymer synthesis and manufactured fiber production is in the public
domain. A continuous procession of books and monographs have been
published in recent years which include those by Lewin and Preston (1985),
Mukhopadhyay (1993), Hongu and Phillips (1990 and 1997), Hatch (1993
and 2006), Klein (1994), Nakajima (1994), Masson (1995), Lewin and Pearce
(1998), Hearle (2001), Woodings (2001), McIntrye (2004), Blackburn (2005),
and Hongu et al. (2005). Countless articles, a few of which are cited in this
chapter, have appeared in scientific journals. Myriads of internet sites
provide information about textile polymers and textile fiber production
including the most recent developments.
111
112
Identification of textile fibers
Because there is so much information available on the manufacture of
manufactured textile fibers, no attempt is made in this chapter to summarize
that information. Rather, the approach is to discuss several fundamental
concepts, describe the manufacture of recently approved generic class and
subclasses of fiber, and types of multicomponent fibers. Then, future trends
and further sources of information and advice follow.
6.2
Polymer origins and fiber classification
As is well known, the fundamental unit of all textile fibers, whether natural
or manufactured, is a polymer. Polymers in natural fibers are biosynthesized. Polymers in manufactured fibers may be (a) extracted from living
plants or animals that biosynthesized the polymeric material or (b) manufactured (synthesized) from monomers obtained from petroleum or from
living plants and animals. Based on this fundamental difference in the origin
of polymer, manufactured fibers are listed in two tables. Table 6.1 lists fibers
manufactured from naturally occurring polymers which may be cellulosic,
protein, or other polymer chemistries. Table 6.2 lists fibers manufactured
from lab-synthesized polymers which may be synthesized using monomers
from petroleum or using monomers extracted from living plants and
animals.
Where confusion seems to occur is the tendency to call some manufactured fibers ‘natural fibers’; specifically, these are fibers made with naturallyoccurring polymers (fibers listed in Table 6.1) as well as fibers whose
monomers were extracted from plants and animals (fibers listed in the righthand column of Table 6.2). The argument for calling these fibers ‘natural’
is because the polymers or monomers for polymer creation come from a
natural source. However, none of the fibers listed in Tables 6.1 and 6.2 are
natural fibers because they do not exist as such in the natural state, as is
the case for cotton, flax, hemp, wool, mohair, and asbestos fibers. Rather all
fibers listed in Tables 6.1 and 6.2 are derived by a process of manufacture
from a substance(s) which, at any point in the manufacturing process, was
not a fiber.
A second point of some confusion centers on the terms generic class and
generic subclass. Part of the confusion probably arises because the United
States Federal Trade Commission (FTC) which oversees the Textile Fiber
Products Identification Act (TFPIA, (http://www.ftc.gov/os/statutes/textile/
rr-textl.htm) places the name and definition for a generic subclass within
the appropriate generic class definition and never uses the term ‘subclass’
in the definition. In contrast, in Europe the International Bureau for the
Standardization of Man-Made Fibres (BISFA, www.bisfa.org) only lists
and defines generic classes of fibers. For example in the TFPIA, polyester
fibers are:
The use of classification systems and production methods
113
Table 6.1 Fibers manufactured from naturally occurring polymers
Cellulose polymer
fibers (polymers
obtained from cotton
fiber, trees, bamboo
stalks, other plants)
Protein polymer fibers
(polymers extracted
from milk (casein),
peanuts, soybeans,
etc.)
Fibers from other polymers
(obtained from sources
given below)
Rayon/Viscose, Modal,
Cupro
Lyocell (subclass)
Acetate
Triacetate (subclass)
Azlon
Alginate (polymer from
seaweed)
Rubber (polymer from
Brazilian rubber tree and
guayule-desert shrub and
potentially from genetically engineered
sunflowers)
Chitosan (polysaccharide
polymer from shellfish),
not a generic class
Table 6.2 Fibers manufactured from lab-synthesized polymers
Monomers for polymer synthesis from
petroleum
Monomers for polymer synthesis
from living plants and animals
Polyester
Elasterell-p (sub)/
Elastomultiester
Nylon/Polyamide
Olefin
Lastol (subclass)
Acrylic
Modacrylic (subclass)
Spandex/Elastane
Rubber
Lastrile (subclass)
Anidex
Aramid
Elastoester
Carbohydrate
sources
Protein sources
PLA/polylactide
Synthetic silk, not
yet a generic
class
Fluorocarbon/
Fluorofiber
Sulfar
PBI/polyimide
Novoloid
Nytril
Vinal
Vinyon
Saran
manufactured fiber(s) in which the fiber-forming substance is any long-chain
polymer of at least 85% by weight of an ester of a substituted aromatic carboxylic acid, including but not restricted to substituted terephthalate units
[formula omitted here] and parasubstituted hydroxybenzoate units [formula
omitted here]. Where the fiber is formed by the interaction of two or more
chemically distinct polymers (of which none exceeds 85% by weight), and
114
Identification of textile fibers
contains ester groups as the dominant functional unit (at least 85% by weight
of the total polymer content of the fiber), and which, if stretched at least 100%,
durably and rapidly reverts substantially to its un-stretched length when the
tension is removed, the term elasterell-p may be used as a generic description
of the fiber.
Elasterell-p is the subclass even though not distinctly called that in the
definition. In contrast, BISFA lists and defines polyester and separately
lists and defines elasterell-p (actually elastomultiester). The definition for
elastomultiester is:
fiber formed by the interaction of two or more distinct macromolecules in two
or more distinct phases (of which none exceed 85% by mass) which contains
ester groups as dominant functional unit (at least 85%) and which after suitable treatment when stretched to one and a half times its original length and
released recovers rapidly and substantially to its original length.
www.bisfa.org/booklets/Terminology_2006.pdf
To further emphasize the important distinction between generic class and
subclass as used in the TFPIA and in this chapter, note these statements
taken from the TFPIA: A generic class is ‘a grouping of fibers having similar
chemical compositions or specific chemical characteristics’. A generic fiber
subclass is created when ‘a manufactured fiber (1) has the same general
chemical composition as an established generic fiber category; (2) has distinctive properties of importance to the general public as a result of a new
method of manufacture or substantially differentiated physical characteristics, such as fiber structure; and (3) the distinctive feature(s) make the fiber
suitable for uses for which other fibers under the established generic name
would not be suited, or would be significantly less well suited. (http://www.
ftc.gov/os/2002/11/16cfrpart303amend.htm)
The bottom line is that the FTC and BISFA have approved the ‘same’
fiber names to appear on textile product labels. Most of those names appear
in Tables 6.1 and 6.2 except for the inorganic fibers: metallic, glass, carbon,
and metal. Subclass names are identified by including ‘subclass’ after the
name. Where the TFPIA name and BISFA name differ, the TFPIA name
is given first and the BISFA name follows a slash.
For clarity, the term fiber variant (fiber type, specialty fiber) needs to be
mentioned, as it has close ties to generic subclass. Variant refers to fibers
resulting from a change in the polymer or to the basic spinning process,
which are designed to improve one or more fiber properties. Modifications
include a) altering fiber fine structure by altering polymer length and or
orientation and cystallinity within the fiber, b) changing the cross-sectional
shape of the fiber, c) incorporating a chemical between polymer chains, d)
incorporating a small proportion of a monomer in the polymer chains, and
e) grafting and incorporating bulky reactive groups along polymer chains.
The use of classification systems and production methods
115
Table 6.3 Production of viscose rayon fiber variants
Description of change
Variant name
Inclusion of new chemical prior to or in spinning solution
a. Titanium dioxide
Delustered rayon
UV protective rayon
b. Optical whitening compound
Optically whitened rayon
c. Dye molecules
Producer-dyed rayon
Acid-dyeable rayon
d. Protein or polymers containing –NH2
groups
e. Flame retardants
Flame-resistant rayon
f. Water-holding polymers (such as sodium Super-absorbent rayon
polyacrylate or sodium carboxy methyl
cellulose)
g. Nano-scale-engineered silver-bearing
Anti-odor/anti-microbial
nano-particles
Change in spin bath conditions
a. Increase of zinc sulfate concentration in
spin bath
b. Increase of spin bath temperature
c.
Reduction of acid concentration and an
increase of temperature in the spin bath
Multiple changes
a. Use of a higher grade of cellulose,
elimination of ageing and ripening,
lowering of chemical concentrations in
spin bath to preserve polymer chain
length
1
High-tenacity rayon
Crimped high-performance
(crimped HWM rayon1)
Self-crimped rayon
High wet-modulus (high
performance) rayon
In BISFA, HWM rayon is called Modal.
Standard practices for identifying a fiber variant/type in the consumer
marketplace are to (a) place a trademarked name before the generic name
of the fiber; for example, Qiana® nylon and (b) use a descriptive adjective
before the generic class name; for example, delustered rayon fiber and
static-free nylon fiber. Usually, fiber manufacturers have a numbering
system to identify fiber variants they are selling to yarn and fabric manufacturers. Tables 6.3–6.6 provide the names of common fiber variants and a
description of modifications made for rayon, acrylic, and melt-spun fibers.
Note that Table 6.4 outlines fiber variants within the lyocell subclass.
6.3
PLA/polylactide fiber
PLA/polylactide fibers are ‘manufactured fiber(s) in which the fiber-forming
substance is composed of any long-chain synthetic polymer at least 85%
by weight of lactic acid ester units derived from naturally occurring sugars’
116
Identification of textile fibers
Table 6.4 Production of solvent-spun rayon fiber variants
Description of change
Variant name
Multiple
Men’s wear fiber
Additive to spinning solution
Non-fibrillating fiber
High-fibrillating fiber
Delustered/UV-light scattering fiber (with TiO2)
Skin-healthy fiber (with seaweed powder)
During fabric finishing
Peach-skin surface
Smooth (clean) surface retention
Resin-treated
Plasma surface treated
Table 6.5 Production of melt-spun fiber variants
Description of change
Variant name
Polymer modification
Introduction of side-groups
Dyeable olefin
Spin solution alteration
Polymerize to specified
length/degree
Add substitutents
Additive to melt
Titanium dioxide
Anti-static compounds
Optical whitener
Titanium dioxide
Nanoscale-engineered silverbearing nanoparticles
Change in spinneret shape
Trilobal, flat/ribbon, square, multichanneled, etc.
Post draw parameters (speed, etc.)
High and low tenacity
Hydrophilicity increase
Delustered
UV-reflecting (protective)
Anti-static
Optically whitened
UV-resistant
Anti-odor/anti-microbial
Greater luster, improved resilience,
enhanced moisture transport, etc.
(http://www.fitfibers.com/cross_
sections.htm)
Staple and filament lengths (texturing)
Reduction of fiber size (denier)
(FTC-TFPIA). This generic classification of fibers is the most recently
approved generic class in the United States and Europe.
The PLA generic classification is the first classification for a manufactured synthetic fiber whose origin is a renewable natural resource – corn,
and more precisely corn kernels. The sequence of events involved in the
manufacture of PLA polymers is: harvesting corn, removing the kernels
from the cob, fermenting the kernels to extract dextrose, converting dextrose to lactic acid, forming dimers of lactic acid called lactide, and then
The use of classification systems and production methods
117
Table 6.6 Production of acrylic fiber variants
Description of change
During polymerization
Those with 85% or more acrylonitrile
composition.
Streams of two different acrylonitrile
polymer solutions merge to form one
fiber. See last section of this chapter
for further detail.
Fibers were originally this until the
introduction of basic dyes which
became the standard placing aciddyeable fibers as the specialty
(variant) fiber.
Use of only acrylonitrile polymer, the
formation of higher molecular weight
polymers than found in ‘textile-grade’
applications, and higher draw ratio
during spinning of the fiber.
Copolymers of acrylonitrile with small
amounts of monomers such as
carboxylic acids or vinyl bromide and
alteration in spinning.
Additions to the polymer solution
Inclusion of an optical whitener in the
spinning solution.
Addition of colored pigment to the
polymer solution prior to fiber
spinning.
Addition of titanium dioxide as a delustrant to the polymer solution prior to
fiber spinning.
Contact of acrylic fiber with a concentrated solution of cationic dyes solution
during many stages of wet-spinning
except to dried fiber.
Other methods
Incorporation of stable voids in the fiber
structure to hold water. Manufacturers
use different processing procedures to
obtain this end result.
Production of a fiber with a corrugated
surface, a structure that allows loose
fiber to move out of the yarn before it
entangles itself with other fibers, thus
not producing a pill (an entangled
mass of fiber) on a fabric’s surface.
Variant name
Flame-retardant
Bicomponent
Acid-dyed fibers – the result of the
‘original’ production of acrylic
fibers, now not often produced
Reinforcing (asbestos
replacement)
Carbon fiber precursor
Optically whitened
Pigmented
Delustered
Producer dyed
Moisture absorbent
Pill-resistant
118
Identification of textile fibers
polymerizing the lactides to form the polylactic acid (PLA) polymer.
Formation of PLA textile fibers then involves the extrusion of a melt of
PLA polymers through a spinneret. PLA polymers can also be used to coat
paper, make films and injection molded plastics, among other useful products. Cargill-Dow operates the world’s first industrial-scale PLA production
manufacturing facility (plant) in Blair, Nebraska, a facility having the capability of producing 300 million tons of fiber annually.
The following claims have been made about PLA fibers:
1. The properties of PLA fiber are unique so new fabric performance
results.
2. Woven and knit apparel fabrics; bedding products including fiber-fill
for pillows, comforters, and pillow-top mattresses; carpeting including
recyclable carpeting, disposable products such as diapers and wipes, and
simulated suede and leather can be made.
3. Eco-efficient and environmentally friendly textile products result
because:
a. The fundamental (starting) chemical in PLA fibers is obtained from
an annually renewable natural resource (corn) rather than from a
diminishing supply of oil which takes millions of years to replenish/
renew.
b. All chemicals used in processing PLA are recognized as environmentally safe.
c. The process of producing PLA polymer uses 30–50% less fossil fuel
than usually required to produce oil-based manufactured synthetic
polymers (such as polyester and nylon and olefin).
d. Making PLA polymer generates about half the greenhouse gas emissions as making petroleum-based polymers.
e. PLA fibers are biodegradable (other manufactured synthetic fibers
are not) so PLA fibers can be composted.
f. The products (chemicals) from composting can be used to grow more
corn, beets, rice etc. for possible future conversion to PLA.
g. PLA fiber can be recycled (ground up, melted) and formed into new
fiber or film.
h. PLA fiber is highly sustainable because it decomposes to CO2 and
H2O by natural processes. ‘PLA fiber offers superior sustainability
and lower environmental impact than any other non-cellulosic synthetic fiber, and is possibility superior to some natural fibers.’
4. Making PLA fiber creates a new market for corn (the most common
starting material) and therefore more income for farmers who produce
the corn.
5. PLA fabrics can be offered at the same price per yard as polyester
fabrics because the specific gravity of PLA is only 1.25 which is less than
The use of classification systems and production methods
119
polyester. Because fibers are sold by the pound and converted into
fabrics sold by length or area, it is possible to sell PLA fabrics at the same
price as polyester fabrics even when PLA fiber is priced 12% higher.
PLA fiber is being used in the following products:
a)
Woven and knit fabrics for apparel including socks, shirts, and sports
clothing (activewear).
b) Home furnishing products particularly fiberfill for pillows, bed comforters, mattress pads, fiber beds (similar to feather beds), and pillowtop mattresses.
c) Synthetic suede and leathers produced with splitable segmented fiber
production (see variants section of this chapter).
d) Recyclable carpeting because both the face fiber and backing can be
made of PLA.
e) Disposable products such as diapers and wipes.
While all PLA fibers are composed of polylactic acid polymers, fibers for
various uses will be composed of PLA polymers differing in spatial configuration. This results because lactic acid, the monomer, exists as L-lactic acid
and D-lactic acid. The most common form is L-lactic acid.
Following polymerization, when the predominant lactide in the polymer
chains is L, the internal structure of the fiber is highly crystalline. When
polymer chains are composed of 15% or more of D-lactide units, then
the internal fiber structure may be described as amorphous. It is also
possible to control the sequencing of D-lactide and L-lactide units along
a polymer chain.
Currently, most PLA fiber is available as a smooth rod (round crosssection) fiber but is also available in a cross section that enhances its wicking
ability and is present in several multicomponent fibers. For further details
about PLA fiber refer to Lunt (2000), Lunt and Bone (2001), Yang and
Huda (2003), Gupta et al. (2006), and Hatch (2006).
6.4
Fiber subclasses
Prior to 1995, the fiber subclasses in the TFPIA were triacetate (subclass of
acetate) and lastrile (a subclass of rubber). Since 1995, three subclasses have
been approved: lyocell as a subclass of rayon on 15 April 1996, elasterell-p
as a subclass of polyester on 7 November 2002, and lastol as a subclass of
olefin on 17 February 2003. The Federal Trade Commission is currently
considering a petition to create triexta as a subclass of polyester. A rich
source of information about the chemistry and properties of new fibers is
contained in the petition documents submitted by fiber manufacturers to
the FTC. These documents can be found on the FTC’s homepage (www.ftc.
gov) along with all documents generated during the approval process.
120
Identification of textile fibers
6.4.1 Lyocell fiber
Rayon fibers are ‘manufactured fiber(s) composed of [100%] regenerated
cellulose, as well as manufactured fiber of regenerated cellulose in which
substituents have replaced not more than 15% of the hydrogens of the
hydroxyl groups. Where the fiber is composed of cellulose precipitated from
an organic solution in which no substitution of the hydroxyl groups takes
place and no chemical intermediates are formed, the term lyocell may be
used as a generic description of the fiber.’ (FTC-TFPIA) Fibers which are
supposed to be marketed as lyocell are often marketed (incorrectly) as
Tencel®, the trademark of the producer. Like viscose rayon fibers, lyocell
fibers are 100% cellulose but the average polymer length is higher in lyocell
fibers than in viscose rayon fibers.
Lyocell fibers are manufactured using the solvent spinning manufacturing
process, a process with beginnings in the 1970s. The source of cellulose for
lyocell fibers is wood pulp from trees grown especially for this purpose on
managed tree farms. This quality wood pulp is mixed with amine oxide
(N-methylmorpholine-N-oxide) in water. The mixture is then passed to a
continuous dissolving unit where a clear, viscous solution is formed. The
solution is then extruded through a spinneret into a dilute aqueous solution
of amine oxide, which precipitates the cellulose as fiber. After washing and
drying, the fiber is ready for finishing processes that are tailored to develop
performance required in various end-uses. The diluted amine oxide from
washing is purified and after removal of excess water is reused. The materials used in the process are environmentally clean, and recycling of the
solvent is an integral part of the process. Waste products are minimal and
non-hazardous. Virtually all of the solvent is reclaimed and reused making
the process environmentally friendly.
The ‘fundamental’ process is used to make fiber for women’s garments.
This process has been altered in a number of ways to produce a menswear
fiber, and also altered to produce a number of other variants shown in Table
6.4. Lyocell fiber may also be modified to change fiber surface appearance
and properties during fabric finishing. Selected references about the production and properties of lyocell fiber include Bullio (1992), Cole (1992),
Kumar (1994), Watkins (1995), Nechwatal et al. (1996), Hongu and Phillips
(1997), Keesee (1998), Kumar and Harden (1999), Karypidis et al. (2001),
Taylor et al. (2001), and Hatch (2006).
6.4.2 Lastol and elasterell-p/elastomultiester
Lastol and elasterell-p, are considered together because they are both
elastomeric fibers (a classification based on common distinctive property);
those fibers that extend noticeably when a tensile force is applied and
The use of classification systems and production methods
121
recover quickly and almost completely to their original length when that
tensile force is released. Fibers of the spandex, rubber (natural and synthetic), and anidex (no longer produced) classes are also elastomeric
fibers.
Lastol fiber was developed by Dow Fiber Solutions of the Dow Chemical
Companies to compete with spandex fiber primarily in the manufacture
of easy-care comfort-stretch fabrics/garments but also to compete where
power stretch is desired. The statement added to the end of the olefin fiber
class definition in the FTC/TFPIA is as follows: ‘When the fiber-forming
substance is a cross-linked synthetic polymer, with low but significant crystallinity, composed of at least 95 percent by weight of ethylene and at least
one other olefin unit, and the fiber is substantially elastic and heat resistant,
the term lastol may be used as a generic description of the fiber.’ Other
names associated with lastol fiber include: (a) CEF, an acronym for Crosslinked Elastic Fiber; (b) XLATM Freedom Fiber, a Dow Fiber Solutions
tradename; and (c) DCC-0001, a temporary name assigned by the FTC
while the Dow petition was under review.
The structure of lastol fiber differs from the structure of conventional
olefin fibers because:
•
•
•
the polymers in lastol are short branched, not linear as in other olefin
fibers,
the polymers are crosslinked, and
there is a low but significant crystallinity of 12–16% in lastol which is
in contrast to conventional olefin fiber with a crystallinity of >50 and to
rubber fiber which has no crystalline structure.
Further, the polymer in lastol is a co-polymer, not a homopolymer as in
conventional olefin fibers. Most interesting is that each lastol polymer is
essentially composed of ethylene monomers but a co-monomer is inserted
into the ethylene chain on occasion. Scientists know that substantially every
lastol polymer in a fiber has the same ethylene to co-monomer ratio and
that each polymer within the lastol fiber has about the same molecular
weight. This magic of polymer formation is achieved by homogeneous
or single site catalyst systems known as a metallocene or a constrained
geometry catalyst system. This is even more complex than the Ziegler-Natta
method used to make other olefin polymers. Excellent information about
the properties of these fibers is available online: http://www.ftc.gov/os/
statutes/textile/info/petition_dowsubclass.pdf and www.dowxla.com.
Elasterell-p fiber’s definition is ‘where the fiber is formed by the interaction of two or more chemically distinct polymers (of which none exceeds
85% by weight), and contains ester groups as the dominant functional unit
(at least 85% by weight of the total polymer content of the fiber), and which,
if stretched at least 100%, durably and rapidly reverts substantially to its
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Identification of textile fibers
un-stretched length when the tension is removed, the term elasterell-p may
be used as a generic description of the fiber.’
Each elasterell-p fiber is a side-by-side bi-component fiber with a distinctive ‘snowman’ cross sectional shape. The polymers on one side of each fiber
are ‘regular’ commercial polyester polymers and the polymers forming the
other side differ by one methylene unit. (Note: it is not known whether
this means one more or one less unit). When heated, the fiber becomes
helical due to differential shrinkage of the two polyester polymers forming
the fiber.
The primary market for elasterell-p fiber is comfort-stretch apparel,
apparel that fits close to the skin and therefore needs to elongate (stretch)
to allow initial movement and to recover as movement is reversed to retain
garment shape (dimension/fit). Elasterell-p fabrics compete with fabrics
containing spandex fibers and with polyester stretch-textured fabrics. A
current manufacturer of elasterell-p is Invista. Elasterell-p was known as
T-400 during its development and initial sale and assigned the name DP0002
elastic fiber during FTC petition review. For further information, an
excellent online source is www.ftc.gov/os/statutes/textile/fedreg/020215comments.pdf
6.4.3 Potential ‘triexta’
In 2006, the FTC was petitioned to allow fiber content labels of carpeting
to indicate the percentage of fiber in the product made with PPT polyester
to be given as triexta rather than as PPT polyester or triexta polyester. The
request was to add the following definition to the end of the polyester fiber
definition: ‘and where specifically the glycol used to form the ester consists
of at least ninety mole percent 1,3-propanediol, the term triexta may be used
as a generic description of the fiber.’
In this particular situation, the focus of the argument for triexta to be a
new subclass is on showing that carpeting made from PPT has distinctively
better performance than carpeting made with other polyester fibers. And
that being allowed to label the product in a manner that consumers would
not relate the performance of the carpet to the inferior performance of
polyester carpeting was essential.
To follow the approval process for triexta fiber, begin with reading the
petition: www.ftc.gov/os/statutes/textile/info/PTT001_Petition.pdf.
Some history of the development of PPT polymers and PPT fibers provides insight into the reason that PPT polyester was not previously used for
carpeting. Although a process was developed and patented in the late 1940s
to synthesize PPT polymers, textile fibers were not commercially produced
for a long time even though the fibers had good physical and chemical
properties and there was potential for their use in textile products was that
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123
PPT polymers and fibers were just too expensive to produce. Two starting
chemicals (monomers) are needed: PDO (1,3-propanediol) and PTA
(purified terephthalic acid) with PDO being the expensive monomer.
Shell Chemical started to commercially produce PDO in the 1960s but
stopped in the early 1970s because of cost and quality issues. In the early
1990s, Shell developed an innovative way to produce PDO economically.
Later, it would be discovered how to produce PDO biochemically with
chemicals from corn.
The next step was to work on methods to make PPT fiber and study PPT
fiber properties. After Shell acquired a PET manufacturing facility from
Goodyear in 1992, it had a facility to not only produce PET but PPT as
well. In other facilities Shell owned, Shell began to manufacture PPT fiber.
Shell then partnered with the largest carpet manufacturer in the world
and with one of the world’s biggest producers of textured yarns. In May
1995, Shell Chemical introduced PPT fiber into the marketplace with the
tradename ‘CorterraTM’.
6.5
Multicomponent fibers
A multicomponent fiber (conjugate fiber) is one that contains more than
one type of polymer (ASTM D4466-02). Most multicomponent fibers are
bicomponent, ‘a fiber consisting of two polymers which are chemically different or physically different or both’ (ASTM, 2006). A tricomponent fiber
is one ‘consisting of three polymers which are chemically different, physically different, or any combination of such differences’ (ASTM, 2006). The
manufacture of a multicomponent fiber is usually accomplished by extruding the two or three types of polymers simultaneously through the same
spinneret orifice. When a fiber is composed of polymers are from different
polymer classes such as polyester and polyamide or polyester and polypropylene, it may be called bicomponent-bigeneric (preferably) or biconstitutent (a deprecated term).
Multicomponent fibers are further described by the physical placement
(arrangement) of the polymers within the fiber. Fibers composed ‘of two or
more polymers at least two of which have a continuous longitudinal external surface’ are called lateral (preferably) or side-by-side. A fiber ‘consisting
of a continuous envelope which encases a continuous, internal region’ is
called sheath-core (preferably). A fiber ‘in which one or more polymeric
fibrous materials is dispersed in another’ is called a matrix fiber (preferably)
or islands-in-the-sea (a deprecated term). Three excellent sources of detailed
information about the formation and uses of these fibers are Jeffries (1971),
Placek (1971), and Hersh (1985), and this URL: www.eng.utk.edu/mse/
pages/Textiles/bicomponent%20fibers.htm.
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Identification of textile fibers
Four major reasons for the manufacture of multicomponent fibers are
to (a) produce fibers having three-dimensional crimp, (b) make binder
fibers, (c) make fabrics composed of micro-fibers, and (d) make suede-like
fabrics. These processes are now described briefly.
6.5.1 Fibers having three-dimensional crimp
These manufactured fibers have either a lateral arrangement of two components with different water sensitivities or have a sheath-core arrangement of two components with different melting points.
Bicomponent acrylic fibers are the primary example of 3-D fibers in the
marketplace today having a lateral arrangement of two components with
different water sensitivities. These fibers are modeled after wool fiber, a
natural fiber which has three-dimensional crimp because the protein polymers on one side of its cortex region (ortho side) differ from the protein
polymers on the para side of the cortex region. The polymers in the cortex
region have different sensitivities to water. When the fiber is wet, the fiber
straightens. As the fiber dries, the fiber crimps placing the ortho cortex on
the inside of the crimp provided the fiber is free from constraint.
While not all 3-dimensionally crimped acrylic fibers are manufactured
using the same components consider the manufacture of the 3-D crimped
acrylic fibers successfully sold under duPont’s trademarks SayelleTM and
WintukTM. These fibers were manufactured by dry spinning of polyacrylonitrile (PAN) polymer and a sulfonate copolymer through a spinneret.
Although the spinneret holes were circular, the forces acting on each
polymer in gellation produced mushroom-shaped filaments comprising a
‘cap’ of PAN and a ‘stem’ of the copolymer. Crimp was developed during
dyeing and drying operations.
The sulfonate copolymer by virtue of its many ionic sulfonate groups is
hydrophilic and shrinks as the water is removed, whereas the PAN component of the fiber is stable. This difference in length creates the spiral crimp
with the copolymer portion being on the inside of the spiral. The mechanical
forces creating the crimp are of low magnitude which means that the yarn
or garment made with the fiber must be relatively free of restraint for full
crimp development in the fibers. When a garment containing this type of
bicomponent acrylic is laundered – the hydrophilic portion elongates,
straightening out the crimp, then on tumble drying, the crimp redevelops.
An example of a 3-dimensionally crimped fiber having a sheath-core
arrangement of components with different melting points is a 100% PLA
fiber. Because the PLA polymer can be polymerized with control over the
content and arrangement of the three stereoisomers, polymers with varying
melt temperatures can be made. In fact, polymers can be made with
melt temperatures ranging from about 120oC/248oF and 175oC/347oF. When
The use of classification systems and production methods
125
polymers with different melt temperatures are extruded in a sheath-core
configuration, with the core being the lower melt temperature component,
3D crimped fibers result. Self-crimping fibers are straight as they are produced but after drawing and heat-setting and heating the fiber above
its heat-set temperature causes the fiber to shrink. Because the two
polymers shrink at different rates, the fiber curls into a helical shape, providing bulk.
6.5.2 Binder fibers in nonwovens
Binder fibers are the most efficient way to impart strength to a nonwoven
fabric. Binder fibers are bicomponent fibers with a sheath of one polymer
and a core of another polymer. The binder fibers’ sheath melts at a relatively
low temperature, and the core melts at a higher temperature. Nonwoven
fabrics made with binder fibers can be bonded together simply by heating
the fabric to melt the sheath but not the core of the binder fiber. Upon
cooling, the molten sheath freezes, gluing the other fibers together and
producing a strong fabric. Binder fibers may be composed entirely of PLA
polymers, of polyethylene and PET polyester polymers, or of coPET and
PET polymers.
6.5.3 Fabrics composed of microfibers
Microfibers are those that have a denier (the weight in grams of 9000 meters
of fiber) less than one which makes them as fine as cultivated silk fiber.
While making microfibers to subsequently spin into yarns and make into
fabric is not impossible, it is a challenge due to the fragility of the fibers.
Therefore, the usual method for making microfiber fabrics is to extrude fine
polyester (or nylon) filaments into a thin stream of polystyrene. Each filament fiber – part styrene and part filaments in a matrix arrangement – is of
standard denier. These filament fibers, gathered into a filament yarn, are
woven or knitted to form a fabric. The next step is to pass the fabric through
a bath of solvent to remove the polystyrene, thus liberating the nylon or
polyester fine filaments. A major drawback is that organic solvents have
to be used to remove the polystyrene, a process that creates environmental
and flammability concerns. Replacement of the poly-styrene with watersoluble polymers has been tried but it is expensive and filaments become
tacky when moisture is absorbed from aqueous fiber finishes or from
the air.
The development of PLA polymers provided another method of making
microfiber fabrics that are 100% nylon or 100% polypropylene (PP) but
not 100% polyester microfiber fabrics. Here, nylon or polypropylene microfilaments are extruded into a stream of PLA polymer making a fiber having
126
Identification of textile fibers
the matrix arrangement. The PLA polymer can be easily removed with
three minutes exposure in a bath of hot, 3% caustic soda, conditions similar
to those used in commercial bleaching and scouring operations. Fabric
composed of 0.6 DPF fibers can be simply and cost effectively made starting
with a 3-denier per-filament fiber. The reason that 100% PET polyester
microfiber fabrics cannot be made in this manner is because PET will
hydrolyze under the conditions of hydrolyzing the PLA.
6.5.4 Making simulated suede and leather fabrics
Traditionally, simulated leather and suede fabrics, a type of nonwoven
fabric, are made from bicomponent, (3-denier filament) fibers containing
alternating segments of PET-polyester and nylon. The filaments are carded
into a loose web. Hydro-entanglement, a process of using water jets to cause
the fibers to entangle themselves into a coherent structure and to cause
each fiber to split into 0.2 denier filaments (micro-fibers), follows. After
fabric finishing, the nonwoven fabric’s surface has the feel of natural
suede. The white fabric is then dyed twice, once with dyes that will color
the nylon micro-fibers and then with dyes that will color the PET-polyester
micro-fibers.
A newer way to produce simulated leather and suede fabrics involves
making a bicomponent fiber with alternating segments of PLA and PET
polymers. These two polymers easily split apart during hydro-entanglement
and the fabric can be dyed in one dye bath because both are dyeable with
disperse dyes. This means greater efficiency and improved uniformly of the
dyeing.
6.6
Future trends
Future trends for textile fibers are numerous. Two excellent lengthy descriptions of future trends can be found in New Fibers 2nd edition (Hongu and
Phillips, 1997) in a chapter titled ‘Fibers for the next millennium’ and in a
2005 book titled New Millennium Fibers (Hongu et al. 2005). The authors
classify textile fiber developments as ‘new frontier fibers’, ‘superfibers’, and
‘high function fibers’. They also include chapters that summarize fiber developments for specific applications: a) expected advancements to enhance
human heath and comfort, b) specific advancement in fibers for medical
healthcare, and describe possible advancement using nanotechnology to
create a variety of nano-fibers. Interestingly, the authors also write about
fiber for health and nutrition, a topic that diverges from textile fiber.
Another trend to be noted is that of greater utilization of chemicals from
agricultural products for the synthesis of polymers. Certainly, the extraction
of lactic acid from corn and other agricultural products to produce PLA
The use of classification systems and production methods
127
polymers and the production of PLA fiber is a major example of the beginning of this trend. Another example is the extraction of latex from plants,
such as sunflowers and guayule, to product rubber fiber and other rubber
products (Wood, 2002).
Further, greater utilization of polymers that exist in naturally-occurring
abundant materials is anticipated. A recent example is the extraction of
cellulose from bamboo and the use of this cellulose polymer to produce
‘rayon from bamboo’. Likewise, crab and shrimp shells, are an abundant
source of chitin, a polysaccharide which can be made into fibers, fabrics,
sutures, and nonwoven fabrics. Research continues to improve production
and find additional uses.
Not to be forgotten is the interest in and research activity related to
genetically engineering polymers for fibers. Currently, there is considerable
interest in genetically engineering dragline spider silk because here is a fiber
that has high strength (is stronger per unit weight than steel), is light-inweight, has high extensibility, has high water absorption, and is biodegradability. The properties of spider silk make it suitable for everything from
surgical sutures to body armor (Mahish and Laddha, 2005). However, harvesting spider silk from spider webs is not practical. Genetic engineering is
being used to make spider silk as chemical synthesis is not viable at the
present time. Mahish and Laddha (2005) describe several ways that scientists have approached the synthesis of dragline spider silk including the
most recent and promising way which is the insertion of silk genes into goats
and cows to produce silk protein in their milk. A method of spinning spider
silk from solution on a large scale has not been developed. Some problems
yet to be solved include: the spider spinning dope is about 50% protein,
making the dope too viscous, and the silk is not soluble in water.
6.7
Sources of further information and advice
Sources of information about the development of new polymers for fibers
and methods for spinning fibers are abundant. These include online and
print scientific journals and scientists at universities with polymer and fiber
development departments, at fiber manufacturers, and at major textile fiber
related associations. Specific examples follow.
6.7.1 Major scientific journals
Journal of Applied Polymer Science (http://www3.interscience.wiley.com/
cgi-bin/jhome/30035),
Journal of Engineered Fibers & Fabrics (www.jeffjournal.org),
Chemical Fibers International http://www.chemical-fibers.com/),
Textile Research Journal (http://trj.sagepub.com/),
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Identification of textile fibers
AATCC Review (formerly Textile Chemist and Colorist and American
Dyestuff Reporter and Textile Chemist and Colorist) (http://www.aatcc.
org/magazine/aatccReview.cfm).
6.7.2 Organizations that focus on polymer and
textile fiber production
The American Manufactured Fiber Association (http://www.fibersource.
com/afma/afma.htm)
Numerous Global Manufactured Fiber Associations listed at the following
URL: http://www.fibersource.com/f-tutor/assoc.htm
6.7.3 United States universities with polymer and
fiber development programs
Polymer and Fiber Engineering program at Auburn University (http://www.
eng.auburn.edu/txen/)
School of Materials Science and Engineering at Clemson University (http://
mse.clemson.edu/index.htm)
School of Polymer, Textile and Fiber Engineering at Georgia Tech University
(http://www.tfe.gatech.edu/)
Textile Engineering, Chemistry and Science program at North Carolina
State University http://www.tx.ncsu.edu/departments/tecs/index.html
6.7.4 Book publishers
Marcel Dekker (www.dekker.com)
Woodhead Publishing (www.woodheadpublishing.com)
6.8
References
ASTM, ‘Standard Terminology for Multicomponent Textile Fibers,’ ASTM Standard
D4466-02, Annual Book of ASTM Standards, Vol 7.02, ASTM International,
Conshohocken PA, 2005.
Blackburn R S (2005), Biodegradable and sustainable fibres, Cambridge, Woodhead
Publishing Ltd.
Bullio P G (1992), ‘The fiber of tomorrow: special characteristics place Tencel, a new
cellulosic fiber, in a class of its own’, Fiber World, Sept 16–18.
Cole D J (1992), ‘A new cellulosic fibre – Tencel’, in Mukhopadhyay S K, Advances
in Fibre Science, Cambridge, The Textile Institute, 25–44.
Federal Trade Commission, Textile Fiber Product Identification Act, Code of Federal
Regulations, Title 16, Section 303.7, 2006. http://www.ftc.gov/os/statutes/textile/rrtextl.htm
The use of classification systems and production methods
129
Gupta B, Revagade N, Anjum N, Atthoff B, Hilborn J (2006) Preparation of
poly(lactic acid) fiber by dry-jet-wet spinning. II. Effect of process parameters
on fiber properties; J Applied Polymer Science; 93, 3774–80.
Hearle J W S, High Performance Fibres, Woodhead Publishing Limited, Cambridge
England, 2001.
Hatch K L, Textile Science, West Publishing Co. Minneapolis MN, 1993.
Hatch K L, Textile Science, Tailored Text Custom Publishing, Apex NC, 2006.
Hersh S P (1985), ‘Polyblend fibers’, in Lewin M and Preston J, Handbook of Fiber
Science and Technology Volume III Part A, New York and Basel, Marcel Dekker,
Inc, 2–50.
Hongu T, Takigami M, Phillips G O, New Millennium Fibers, Woodhead Publishing
Limited, Cambridge England, 2005.
Hongu T and Phillips G O, New Fibers, 1st ed., Woodhead Publishing Limited,
1990.
Hongu T and Phillips G O, New Fibers, 2nd ed., Woodhead Publishing Limited, 1997.
Chapter 8 Cellulosic Fibers, 191–208.
Jeffries R (1971), Bicomponent Fibres, London, Merrow Publishing Co. Ltd.
Karypidis M, Wilding M A, Carr C M (2001), ‘The effect of crosslinking agents and
reactive dyes on the fibrillation of lyocell’, AATCC Review, 1(7), 40–4.
Keesee S H (1998), ‘Troubleshooting in wet processing: Acetate, rayon/lyocell and
spandex blends’, Textile Chemist Colorist, 30(6), 10–11.
Klein W (1994), Man-made Fibres and Their Processing, Cambridge, Woodhead
Publishing Limited.
Kumar A (1994), Lepola M, Purtell C. ‘Enzymatic finishing of man-made cellulosic
fabrics’, Textile Chemist Colorist, 26(10), 25–7.
Kumar A, Harden A (1999). ‘Cellulase enzymes in wet processing of lyocell and its
blends’, Textile Chemist Colorist/Am Dyestuff Reporter, 1(1), 37–45.
Lewin M and Pearce E M (1998) Fiber Chemistry (2nd edition), New York, Basel,
and Hong Kong, Marcel Dekker Inc.
Lewin M and Preston J (1985) Handbook of Fiber Science and Technology Volume
III Part A, New York and Basel, Marcel Dekker, Inc.
Lunt J ‘(2000), Polylactic acid polymers for fibers and nonwovens’, Inter Fibers J,
June, 48–52.
Lunt J, Bone J (2001), Properties and dyeability of fibers and fabrics produced from
polylactide (PLA) polymers’. AATCC Review, 1(9), 20–3.
Mahish S, Ladda S K (2005), ‘Spider silk: the miracle material’, AATCC Review,
5(1), 14–16.
Masson J C (1995), Acrylic Fiber Technology and Applications, New York, Basel,
and Hong Kong, Marcel Dekker.
McIntyre J E (2004), Synthetic Fibres: Nylon, polyester, acrylic, polyolefin, Woodhead
Publishing Limited, Cambridge England.
Mukhopadhyay S K (1993), ‘High performance fibers’, Textile Progress, 25 3/4,
1–85.
Nakajima T (1994), Advanced Fiber Spinning Technology, Cambridge, Woodhead
Publishing Limited.
Nechwatal A, Nicolai M, Mieck K-P (1996), ‘Crosslinking reactions of spun-wet
NMMO fibers and their influence on fibrillability’, Textile Chemist Colorist, 28(5),
24–7.
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Placek C (1971), Multicomponent fibers, Park Ridge NJ, Noyes Data Corporation.
Taylor J N, Bradbury M J, Morehouse S (2001), ‘Dyeing Tencel and Tencel A100
with poly-functional reactive dyes’, AATCC Review 1(9), 21–4.
Watkins P (1995), Tencel: The mystery explained’, Apparel International, 36(1),
21–4.
Wood M (2002), Sunflower Rubber?, Agricultural Research, 50(6), 22.
Woodings C R (2001), Regenerated Cellulose Fibres, Cambridge, Woodhead
Publishing Inc.
Yang Y, Huda S (2003), ‘Comparison of disperse dye exhaustion, color yield, and
colorfastness between polylactide and poly(ethylene terephthalate)’, J Applied
Polymer Science, 90(12), 3285–90.
7
Optical microscopy for textile
fibre identification
M WILDING, The University of Manchester, UK
Abstract: This chapter examines some of the most important methods in
optical microscopy for the identification of fibres, briefly discussing the
underlying optical phenomena on which they are based and indicating
areas where each might be advantageously applied. In addition, it
considers some of the practical issues involved in preparing fibre
specimens for microscopy, and straightforward procedures that can
greatly simplify the process of making an unambiguous identification.
A list of resources and further reading is provided.
Key words: Optical microscopy of fibres, refractive index and
birefringence in fibre identification, fluorescence microscopy in fibre
identification, confocal microscopy in fibre identification, specimen
preparation for fibre microscopy.
7.1
Introduction
It may be said with some justification that the simplest and most obvious
way to identify a given fibre is to look at it. In this regard it is useful to
recall the Textile Institute’s definition of a fibre: ‘. . . a unit of matter characterised by length, fineness and a high ratio of length to thickness’. This
property of fineness (many fibres being so delicate as to be barely visible
to the naked eye) makes meaningful examination impossible in most cases
without the aid of magnification, and therefore some form of microscope.
However, there is a wide diversity of instruments available, based on a
variety of different principles; and each type will have its own particular
strengths and weaknesses so that it may not always be easy to decide on
the best technique to use in any specific case. This chapter examines some
of the most important methods, briefly discussing the underlying optical
phenomena on which they are based and indicating areas where each might
be advantageously applied. Of potentially equal value, it also considers
straightforward procedures that can greatly simplify the business of making
an unambiguous identification.
There are several excellent reference sources: I have found the book by
Greaves and Saville (1995) to be particularly valuable, and would certainly
recommend it as a suitable starting point for readers intending to pursue
any of the experimental techniques described here.
133
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Identification of textile fibers
7.2
Practical and quality control considerations
Before examining specific microscopic methods that can be used in relation
to fibre identification, it is appropriate to consider a number of general
points associated with the nature of fibrous materials themselves, sampling
methods and specimen preparation. In addition, it should be realised that
any microscopic method being contemplated is likely to be covered by at
least one internationally recognised standard. It is therefore important to
ensure that, wherever possible, the relevant prescribed procedure is followed. Sources of information concerning the major standards organisations can be found in Section 7.7.
Whilst the operation of a given type of microscope will demand specific
procedures, there are nevertheless some general principles that should be
adhered to, as well as points to consider, examples of which will now be
discussed briefly.
7.2.1 Destructive versus non-destructive examination
A question which should be asked at an early stage in the investigation is:
does it matter whether the test is destructive? In forensic studies, for
example, it is not uncommon for the amount of material available to
be minute; and this may be required in its original form at some future date
– either as evidence or to enable further testing should this become necessary. The answer to this question will therefore have a bearing on what
procedures and techniques may be suitable.
If the subject must be preserved, then it is most likely that it will be the
sole (or nearly-so) individual available for inspection. Thus, those methods
which, for example, involve immersing the specimen in a refractive-index
liquid will probably be ruled out because once the fibre has been contaminated in this way, it will be virtually impossible to recover it in its original
pristine state. Similarly, methods which call for physical sectioning, melting,
etc., will obviously have to be avoided. On the other hand, sampling is
clearly not a relevant issue in such cases, which does at least simplify matters
somewhat.
7.2.2 Sampling
If a relatively large quantity (say 100 individuals or more) is available, then
a wider range of techniques becomes possible. However, it should be appreciated that fibres, particularly those of natural origin, can be considerably
more variable in character than many other common materials. This variability is not limited to obvious features such as dimensions, but extends
to physical (e.g., mechanical, electrical and thermal), and in some cases
Optical microscopy for textile fibre identification
135
chemical, properties also. For this reason it will normally be necessary to
examine a representative sample of fibres, rather than a single specimen.
How many individuals should be included, and the sampling procedure
adopted to ensure the sample really is representative then become important additional considerations.
In this regard, an important factor is the supposed (or known) source of
the fibre(s) to be identified. In many cases this will be a textile of some
description. Textile fibres can exist either in ‘raw’ form (e.g., bale cotton,
fluff, lint and dust) or as part of a manufactured structure such as a roving,
sliver, yarn, knitted or woven fabric, or finished article. Moreover, a textile
fibre’s origin could be an article of clothing or some other domestic item
(carpet, curtain, upholstery, etc.), but equally it might be an industrial
product such as a nonwoven filter or vehicle tyre cord. Each such structure
will have its own distinctive characteristics, irrespective of the fibre type(s)
it contains, and these will tend to dictate the sampling procedure that should
be followed.
A further complication is that very many textile products contain not one,
but a mixture (blend) of different fibre types. This intensifies the need to
employ the proper sampling method to ensure that these differing components will be correctly represented.
It is not the intention to describe detailed sampling methods here. An
account of the main procedures in use for preparing samples for microscopy
from loose fibre, yarns, fabric and made-up articles can be found in Chapter
2 of Greaves and Saville (1995). Additionally, most standard texts that deal
with the physical testing of textile materials also cover issues relating to
sampling; for example, the books by Booth (1983) and Saville (1999).
7.2.3 The conditions of temperature and humidity
The variability of textile fibres has already been referred to. Of equal importance is the fact that most of them are, to a greater or lesser degree, sensitive
to temperature and humidity. With only very few exceptions, fibres (whether
strictly ‘textile’ or not) are composed of carbon-based polymers. Some of
these (e.g. polyesters, polyamides and polyolefins) are thermoplastic, which
means they soften as the temperature rises, but other properties can
also be affected. Moreover, the temperature need not be particularly high
for discernible effects to occur: in some cases even raising the temperature
from 20 °C to 30 °C will produce measurable changes in physical properties.
Many fibres (wool, cotton and silk being notable examples) are in addition
hydrophilic. Thus, variations in the surrounding humidity can have a
similarly dramatic effect. Since it is the physical properties that are likely
to be used in the identification process, it is vital to maintain the temperature and humidity of the testing laboratory as close as possible to the
136
Identification of textile fibers
appropriate internationally agreed standard (e.g., 20 ± 2 °C; 65 ± 2% relative
humidity).
Testing the specimen in the prescribed laboratory atmosphere is not
sufficient in itself, however. Fibres tend to absorb (and desorb) moisture
relatively slowly. Thus, if a subject is transferred from one environment to
another, it may take a significant time to reach equilibrium. This necessitates
a procedure called ‘conditioning’, in which, typically, the specimen is left in
the controlled atmosphere for a period of up to 24 hours prior to testing or
examining.
7.2.4 Specimen preparation
A vital component of practical microscopy is, of course, preparation of the
specimen itself. Space does not allow a detailed account of all the various
sample-mounting procedures that may be employed, but it is important to
highlight some basic principles. There are essentially two main aspects from
which a fibre specimen can be viewed: (a) longitudinally (i.e. side-on); or
(b) transversely (i.e. in cross-section).
Except when making very superficial examinations (using, for instance, a
stereo zoom microscope), the specimen will generally be mounted between
a glass slide and cover slip. A few drops of an appropriate mounting liquid
should be included in order to ensure good optical contact, thus improving
image clarity. Common examples are xylene, liquid paraffin (‘nujol’), cedarwood oil, ‘Permount’ (a mixture of piccolyte: a B-pinene polymer; and toluenene polymer) and phytohistol. Moreover, the refractive index of the
liquid must be known if quantitative information is required. It is also very
important to note that many refractive-index liquids are not only toxic in
the conventional sense, but also carcinogenic.
Whatever mounting method is used, caution must be taken not to damage
the specimen in the process, as even the tiny forces associated with normal
handling can cause permanent physical changes. Tweezers and mounting
needles will probably be used to assist in preparation, but care still needs
to be exercised.
Longitudinal specimens
The specimen (either a single length of fibre or, for example, a small tuft)
is usually placed on the microscope slide using tweezers. In the case of a
tuft, this should be carefully teased out with a mounting needle so that as
far as possible the entire specimen is a single fibre in thickness. If a liquid
is to be used it can be added at this stage using a dropper. It may be necessary to tease the specimen again with the needle to ensure thorough wetting.
If the fibres should absorb some of the liquid, then extra drops can be added
Optical microscopy for textile fibre identification
137
as appropriate, but be sure not to use more than is needed to fill the space
between cover slip and slide. The cover slip is then gently applied, using the
tip of the needle to lower it down in the manner of closing a lid. The liquid
should spread evenly under the cover slip, but if any air bubbles appear,
they can sometimes be removed by pressing down very gently onto the top
surface with the needle, and easing it slightly from side to side.
Transverse (cross-sectional) specimens
Preparing transverse sections does require some skill, but is not particularly
difficult to do. If sufficient fibre is available it can be achieved using the
‘plate method’, in which a bundle of fibres is pulled, using strong thread,
through a small hole in a metal plate. The bundle must be thick enough so
that it is a snug fit in the hole. The protruding fibres on both surfaces are
then sliced off flush with the plate, using a razor blade or scalpel. Alternatively
a microtome, such as the Hardy microtome, can be used.
Palenik and Fitzsimmons (1990) describe a relatively simple method for
preparing cross-sections. In their procedure the fibre to be examined is
sandwiched between two thin films of polyethylene, the whole being
mounted on a glass slide with a cover slip, and placed on a microscope hotplate. The composite specimen is heated just sufficiently to melt the polyethylene, and light pressure is exerted on the cover slip to ensure effective
embedding of the fibre in the molten polymer. The sandwich is then allowed
to cool, after which the cover slip is removed. Aided by a stereo zoom
microscope, thin slices are taken using a razor blade. These must be thin
enough to lie on a microscope slide with the original fibre axis vertical.
Polyethylene is chosen because it does not adhere to glass. It should be
pointed out, though, that its melting temperature is around 145 °C, which
to some extent limits the range of fibres for which the technique is
applicable. An alternative embedding medium such as epoxy resin may
be considered where the specimen is particularly temperature-sensitive,
although this requires a different procedure. For further guidance on preparing specimens, see, for example, Greaves and Saville (1995) (Chapters 2
and 5).
7.3
Initial identification based on
physical appearance
7.3.1 Stereo zoom and simple light microscopy
Considering the definition of a fibre given in the introduction, one might
be forgiven for thinking that all fibres are of generally similar appearance
– i.e., long thin cylinders. Most synthetic (i.e. artificial) fibres do admittedly
138
Identification of textile fibers
tend to have this geometry, but it is certainly not the case for the vast majority of natural fibres or, for that matter, many of the regenerated fibres. These
classes of fibre encompass considerable diversity of form, and usually
display clearly distinguishable features.
For this reason it is often best to begin with a straightforward examination using a simple instrument such as a stereo zoom or a conventional light
microscope, which will enable these characteristics to be identified very
quickly. Ideally, the microscope should be equipped with a vernier-type
graticule to facilitate measurement of apparent thickness, etc. A small
sample of the material should be prepared so that short lengths (say a few
mm) of individual fibres will be observable. This can be placed with very
little fuss on to a microscope slide, using tweezers; a few drops of a suitable
mounting liquid, such as liquid paraffin, can be added before the cover-slip
is lowered. This should give good optical definition, although it may not be
necessary.
The appearance of the fibres is then noted, and possibly a tentative identification made. For instance, wool and other ‘hair’ fibres nearly always
display cuticular scales on their surface. These overlap in the manner of roof
tiles. It might not be possible on first examination to state with confidence
that one has a particular breed of wool – or indeed wool from a sheep at
all – but it would generally be obvious that the subject belongs to the
broader category of hair fibres. At the very least, if it has overlapping scales
then its animal origin is essentially confirmed.
This example brings to mind the Holmesian Maxim (Conan Doyle, 1890):
‘When you have eliminated the impossible, whatever remains, however
improbable, must be the truth.’ Adopting this principle, it is often easier to
proceed on the basis of what a fibre is not, rather than what it is. Thus, if it
does not have overlapping scales then we can deduce that it probably is not
wool. (One does need to exercise caution, however, since it is possible for
wool to be chemically treated so as to remove the scales; additionally, continued wear may have rendered the fibre surface smooth.)
Many other natural, and man-made, fibres can be identified (or eliminated) in this way, by seeking out their most distinctive features – convolutions in the case of cotton, striations in viscose rayon, and so on. Moreover,
in some instances this quick method of initial identification can also be
applied to the visually plainer artificial fibres; polyesters, polyamides
(nylons) and so on. Whilst these are most commonly manufactured to be
cylindrical, this is not by any means always the case. Melt-spun synthetic
fibres, in particular, are often produced with non-circular cross-sections:
e.g., elliptical, triangular, trilobal and ‘propeller’; again, these features are
relatively easy to observe. However, it may be necessary to prepare crosssection samples (as distinct from the longitudinal ones assumed above). The
Optical microscopy for textile fibre identification
139
transparency/opacity of the fibres may be another factor to be noted, as
well as any colouration.
Even in the case of cylindrical fibres, all is not lost as regards making a
useful initial examination: there are many different detailed specifications
that may be encountered, and which can therefore readily be used as identifiers. These include the diameter, and whether or not the fibre is hollow; if
it is, then a further factor will be the relative thickness of the fibre wall. The
reason why such features are so useful is that, whilst textile fibres as a whole
have a reputation for being variable, this judgement is based mainly on the
natural fibres. For most synthetics the extrusion and other manufacturing
conditions are generally well-controlled so that one would not expect large
variations in dimensions within the same batch. Therefore if, for example,
the diameter of a synthetic fibre under observation is significantly different
from that of a reference sample, the Holmesian Maxim would suggest
that it is not from the same batch, irrespective of whether it is the same
fibre type.
The first stage in the identification process, then, would normally be to
attempt to place the subject into its correct group within the overall fibre
classification scheme: natural-animal; natural-vegetable; man-made (regenerated); synthetic. Greaves and Saville (1995; p. 9) present a flow-chart
describing a useful procedure whereby this might be accomplished.
7.4
Identification based on properties
Although the ‘quick look-see’ approach is very useful in making the initial
examination, it will inevitably be necessary to seek more detailed clues as
to a fibre’s exact identity. An obvious case in point is where a specimen has
been narrowed down to being synthetic, based on its physical appearance
under the microscope, but where the exact polymer type remains unknown.
We may not be able to tell, for example, if it is polyester, polyamide, polyolefin or acrylic, all of which can appear the same in a simple microscopic
observation. The key here is to utilise certain physical fibre properties that
are related to the specific chemical composition and which can be observed
using microscopic techniques. Among the most important of these are:
refractive index, usually considered together with its related quantity,
birefringence; melting (or otherwise) behaviour; and solubility.
7.4.1 Refractive index and birefringence
Before discussing how fibres might be identified on the basis of their refractive index, it is important to appreciate that they are seldom optically isotropic (or indeed isotropic in any respect). This arises ultimately because
140
Identification of textile fibers
the chemical repeat, or monomer, groups that make up the constituent
polymers of most fibres are themselves anisotropic. The anisotropic nature
of the fibre as a whole results from a greater or lesser degree of orientation
within its structure. This may occur naturally, as in the helical fibrils of
cotton and the aligned cortical cells of wool; alternatively, it may be a result
of deliberate stretching or other processes applied during manufacture in
order to improve performance properties. (This is routinely the case for
melt-spun synthetics such as nylons and polyesters.) It can even be induced
inadvertently during use.
Simple polarised light microscopy
As a result of the structural orientation referred to above, the vast majority
of fibres possess a property known as ‘birefringence’, implying, in practical
terms, that their longitudinal and transverse refractive indices differ.
A birefringent specimen will rotate the plane of polarisation of planepolarised light passing through it.
Useful information can often be gained quite quickly from such subjects
by using a relatively simple polarising microscope. This is in essence just
a standard microscope to which has been added one polarising filter (the
‘polariser’) between the light source and specimen, and a second (the
‘analyser’) after the specimen. Both of these may be capable of being
rotated, but the analyser certainly must be. The polariser ensures that the
light incident on the specimen is plane-polarised. In use, the analyser is set
to transmit only light which is polarised at 90 ° to that of the incident beam.
This arrangement is generally referred to as ‘crossed polars’. Thus, in the
absence of a specimen no light would reach the eyepiece and the entire
field would appear dark. If a birefringent fibre is placed on the stage,
however, it will rotate the plane of polarisation of the incoming light by an
amount related both to its birefringence and to its orientation with respect
to the electric field vector. Light leaving the specimen which is polarised at
any angle other than 90 ° to the analyser will be partially transmitted, giving
rise to an image. Figure 7.1 illustrates this schematically. For simplicity the
lenses and other standard optics of the microscope have been omitted.
Thus, if the sample stage is rotated through 360 °, the intensity of the
image will be observed to change. In particular, there will be four symmetrically spaced positions of maximum brightness and four for which the image
is essentially completely dark.
‘Polarisation colours’ will also usually be seen, varying in hue and intensity not only with the degree of rotation, but also across the fibre. Moreover,
the specific colours and patterns of variation will differ from one fibre type
to another. This can provide a very useful means for making a tentative
identification.
Optical microscopy for textile fibre identification
Light transmitted
No light transmitted
141
Some light transmitted
Analyser
Uncrossed polars
Crossed polars
Polariser
Unpolarised light
Without specimen
With birefringent fibre
7.1 Schematic illustration of the effect of rotation of polarisation plane
by a birefringent fibre.
Polarised light microscopy is most commonly performed in the transmission mode, as implied in the above, but for opaque specimens it may be
possible to gain similar information by using the reflection mode, where the
subject is illuminated from above.
Optical anisotropy in more detail
The approach just described is rather limited owing to its being purely
qualitative. For a more conclusive identification it will generally be necessary to make measurements, either of the birefringence or the individual
refractive indices of the fibre, for which more sophisticated instrumentation
is needed. It is also important to examine optical anisotropy in rather
more detail.
Possibly the most significant advance in understanding of the optical
anisotropy of fibres was that provided by Ward (1962), who devised a model,
referred to as the ‘aggregate theory’, to account for the observed optical
(and, indeed, mechanical) behaviour of partially crystalline fibrous polymers. In essence, the solid material was imagined to comprise a large collection of identical anisotropic, orientable units. The model was successfully
applied to several different polymeric fibre types (Pinnock and Ward, 1964,
1966, Hadley et al., 1964, 1969). The full analysis is rather complicated,
but it is appropriate to highlight several key aspects here.
The molecular property that controls light-refraction is called ‘polarisability’. This is a second-rank tensor quantity, which therefore has (formally) nine components. A typical polymer monomer unit will have only
six independent components, however, which can be further reduced to
three principal values by choosing a particular set of orthogonal spatial axes
142
Identification of textile fibers
referred to as ‘principal axes’. In nearly all fibre-forming polymers, one of
these axes will be closely parallel to the main chain axis, and the polarisability in this direction is usually significantly greater than in the two transverse directions, since it corresponds to the direction of covalent bonding.
The two transverse components will generally also differ from one another,
but to a much smaller degree. Polarisability being an extensive property
(indeed, having the dimension of volume), it is essentially additive. This
means that in larger assemblies, its magnitude will be in proportion to the
number of ‘molecules’ contained. Based on knowledge of the unit cell size
and shape, details of the chemical bonding (including secondary interactions such as H-bonding) and packing within and between chains, it is possible to determine the mean polarisability components for structural units
occurring within the fibre. Thus, an elementary ‘crystallite’ or fibril, say, will
typically have three different principal polarisability components. To a reasonable approximation, these can be related to three corresponding orthogonal refractive indices via the Lorenz-Lorenz equation, shown here in an
inverted form:
ni =
1 + 2α i V
1 + αi V
In the above, ni (i = 1, 2 or 3) is any one of the three refractive indices and
αi is the corresponding principal component of polarisability. The subscript,
i, refers also to the electric-field direction (polarisation direction) of the
incident light. V is the molar volume occupied by the chosen ‘unit-cell’.
Figure 7.2 represents an elementary crystal, in which the principal axes
have been set up in such a manner that the long- (i.e., molecular chain, in
most cases) axis is labelled ‘3’, with the two transverse directions being ‘1’
and ‘2’, respectively.
If a fibre could be assumed to consist entirely of a random collection of
identical anisotropic units of the kind described – thereby being isotropic
overall – it would have effectively only one refractive index, which may be
called niso, numerically equal to the average of the three individual values:
niso =
1
(n1 + n2 + n3 )
3
[7.1]
The problem is how to deal with the departure from isotropy encountered in real fibres. Most commonly, and certainly for the majority of manmade and synthetic fibres, it can be assumed that the distribution of
orientation is such that the chain axes of the elementary units are preferentially inclined towards the fibre axis direction, but with no preferred orientation transversely. The symmetry associated with this type of orientation
is referred to as ‘cylindrical’ or ‘uniaxial’. Owing to this, most fibres display
Optical microscopy for textile fibre identification
143
3
Crystal or fibril
Refraction controlled by n3
Refraction
controlled by n3
Refraction controlled by n1
2
Refraction
controlled by n2
1
Electric vector of plane-polarised light
7.2 A crystalline fibril with three principal polarisability components
has three corresponding refractive indices.
only two distinct refractive indices at the macroscopic level, conventionally
denoted by: n// for light polarised parallel to the fibre axis; and n⊥ for light
polarised perpendicularly to the fibre axis. (Technically, the fibre still has
three principal refractive indices, but two of them are identical, and equal
to n⊥.)
The ‘birefringence’ (Δn) of a fibre is defined as the difference between
the measured values of its two independent refractive indices:
Δn = n/ / − n⊥
[7.2]
The birefringence of a given fibre may be positive, approximately zero
or – exceptionally – negative. In any event, the magnitude of Δn is a valuable indicator of the degree of orientation within a fibre’s structure.
Although there are microscopic methods whereby the birefringence can
be estimated directly, the most reliable techniques tend to be based on
measurements of the two individual refractive indices. The measurement of
n// and n⊥ normally entails the use of a polarising microscope, by means of
which a fibre sample can be illuminated, in turn, with light which is polarised
parallel to, and at right angles to its longitudinal axis. One method frequently used is to immerse each of a set of nominally-identical samples in
a different liquid of known refractive index. The larger the difference
between the refractive index of the liquid and that of the sample, the higher
will be the definition and contrast of the image formed. In principle, a
perfect match would render the sample invisible. In practice, this is very
unlikely to be achieved; instead, the refractive index of the liquid giving the
closest match would generally be taken as that of the fibre. If a suitable set
of liquids is available it should be possible to interpolate for better precision. Clues as to whether a particular sample has a higher or lower refractive index can be gained by using techniques such as the Becke test: the
144
Identification of textile fibers
‘Becke’ line is a bright fringe which appears around the edges of a slightlyout-of-focus image. As the objective lens is moved away from the subject
the line appears to move towards the medium with the higher refractive
index.
From the above discussion, it is apparent that if one is primarily interested in identifying the polymeric type of a fibre based on its refracting
power, then orientation will complicate matters: there are no unique values
either of birefringence or of the individual refractive indices that can be
assigned to a given type. However, suppose for the present that the simple
fibre-structure model discussed above is applicable. It may be appreciated
that whereas the individual refractive indices and the birefringence will in
general all change with differing degrees of orientation, the average refractive index should remain reasonably constant – and equal to niso – provided
that orientation is the only variable feature of the fibre. Owing to the uniaxial symmetry generally present, the practical definition of niso is slightly
different from that given earlier; viz:
niso =
1
( n/ / + 2n⊥ )
3
[7.3]
The range of isotropic refractive index values presented by textile and
other fibres as a whole is not great, but significant nonetheless. It is at any
rate sufficiently wide to allow a degree of differentiation between the major
fibre types. Figure 7.3 presents typical values of isotropic index for a number
of fibres.
It is important to note that the values shown are only typical, because
even the isotropic refractive index is in fact a non-unique property: the
simple model previously discussed ignored several important aspects of
fibre structure which need to be addressed. For example, there will usually
be an amorphous, or at least poorly-ordered, phase present in addition to
the so-called ‘crystalline’ phase. This disordered material will also refract,
and the observed value of niso will therefore depend on the relative abundance of the crystalline and non-crystalline material. Because the molecular
packing density is greater in the crystalline than in the amorphous regions,
it follows that raising the crystallinity will result in increased values of n//
and n⊥, and hence of niso.
A further complication arises because the amorphous material may itself
be oriented independently of the ordered phase, and thus contribute towards
the birefringence. Moreover, in many fibres the crystalline phase, in particular, will be inhomogeneous: it is very likely to comprise a distribution of
objects of varying states of crystalline perfection, size and shape.
Notwithstanding the above complications, however, with caution a tentative identification (or elimination) can often be made. For example, if the
refractive index is greater than 1.5 the fibre in question is probably not
Optical microscopy for textile fibre identification
145
1.650
1.630
Isotropic refractive index
1.610
1.590
1.570
1.550
1.530
1.510
1.490
1.470
Fl
ax
W
oo
Po
l
ly
es Sil
k
te
r(
PE
T)
se
lu
el
C
C
el
lu
lo
lo
se
tri
ac
et
a
Po dia te
ly ce
pr ta
op te
yl
en
Ac e
St Mo ryl
an d ic
da ac
rd ryl
vi ic
C sco
hl
or se
of
N ibre
yl
on
66
N
yl
on
C 6
ot
to
n
1.450
7.3 Typical values of (isotropic) refractive index for fibres; calculated
from published data (Greaves and Saville, 1995).
acetate; and if it is less than 1.57 then polyester is an unlikely candidate. On
the other hand, it is clear (Fig. 7.3) that there are groups of fibres for which
the values of refractive index lie very close to one another; but often, one
or more of these may be recognised on the basis of its visual appearance.
For instance, Nylon 66, Nylon 6 and cotton have similar refractive indices,
but cotton has convolutions whereas nylon does not. Similarly, it is unlikely
that acrylics and viscose would be confused owing to the longitudinal striations on viscose fibres. Admittedly, distinguishing between fibres such as
Nylon 66 and Nylon 6 can be less straightforward: it will generally necessitate the measurement of some other property, such as the melting point.
There are several ways in which refractive index and birefringence can
be estimated. For example, in appropriate cases the individual values of n//
and n⊥ can be determined via the ‘double-immersion’ technique using an
interference microscope. With care, this method can be very accurate.
However, it is not straightforward, and since it requires a reasonable supply
of the material to be investigated, it is not really suitable where a single
fibre specimen is all that is available. More often – and certainly more conveniently – fibre identification on the basis of birefringence is carried out
using a ‘quartz wedge compensator’, the commonest of which is undoubtedly the Berek. This is, in essence, a device for measuring the optical retar-
146
Identification of textile fibers
dation produced by a birefringent specimen, based on a tiltable calcite plate.
The technique entails a polarising microscope capable of incorporating the
compensator. The fibre is mounted in the usual way, using a refractive-index
liquid, and viewed under crossed polars. The illumination must be of known
wavelength, and so either a monochromatic light source or a suitable filter
must be employed. The stage is first rotated until the fibre image appears
dark. It is then further rotated by 45 ° (clockwise, say). The compensator is
inserted and the tilt angle adjusted to bring the zero-order (dark) interference fringe to the centre of the image. The stage is then rotated by 90 ° in
the opposite sense and the measurement repeated. This procedure has the
effect of eliminating any ‘off-set’ error. The retardation is determined from
the mean of the two angle values, using calibration tables provided with the
compensator. In order to convert the retardation into birefringence, the
optical path-length through the specimen must be known. Hence the fibre
thickness must be measured. A more detailed description of the compensator method can be found at, for example: http://www.olympusmicro.com/
primer/techniques/polarized/berekcompensator.html.
Hamza and co-workers (1992) also give a detailed account of the
determination of the refractive indices and birefringence of fibres using a
microinterferometric technique.
7.4.2 Melting behaviour
Observation of the melting behaviour is another approach that can be taken
in the fibre-identification exercise. This may be done using either a standard
or polarising light microscope fitted with a hot-stage incorporating the
means to control and measure the specimen’s temperature to within a few
degrees Celcius. Greaves and Saville (1995; p. 16) give more detail regarding the techniques used.
Not all fibre types melt, and this in itself can be a valuable means of
eliminating candidates. Those that do generally belong to the category of
thermoplastic fibres – most commonly encountered as the melt-spun synthetics – which includes polyolefins (polyalkene), polyamides, polyesters,
acetates and vinyl fibres, along with certain of their copolymers and other
variants. Figure 7.4 shows some typical values.
7.4.3 Solubility
The extent to which a fibre dissolves in various solvents offers a further
means of identification, and a suitable procedure for this is described by
Greaves and Saville (1995; pp 12–16). As these authors emphasise, however,
safety (of both operator and equipment) is of paramount importance, and
must be the first consideration when this technique is being contemplated.
Optical microscopy for textile fibre identification
147
350
Melting point/ °C
300
Lower
Upper
250
200
150
Po
ly
Po eth
ly yle
Po pro ne
p
ly
(la ylen
ct
e
ic
ac
N id)
yl
on
C
1
hl
or 1
of
C
i
br
el
e
lu
lo Ny
se
lo
n
di
ac 6
et
a
Po Ny te
lo
l
y
C
el est n 6
6
er
lu
lo
se (PE
T
tri
ac )
et
at
e
100
7.4 Typical values of melting temperature for fibres; data for
poly(lactic acid) from Malmgren and co-workers (2006); all other
data from Greaves and Saville (1995).
There, are of course, many characteristics of fibres additional to those
discussed, that can be useful in effecting their identification, but space does
not permit further discussion. Reference, to the resources listed, especially
Greaves and Saville (1995) is again recommended.
7.5
Examples of more advanced
microscopic techniques
The most straightforward methods of identification tend to be those using
the conventional types of light microscope already referred to, such as
stereo zoom, polarising, and interference instruments. However, the information these provide can be limited, and it is appropriate to consider, albeit
briefly, several more sophisticated, recent developments.
7.5.1 Fluorescence microscopy
Certain organic materials display the phenomenon of ‘fluorescence’. This is
a process in which light of one characteristic wavelength (and hence colour)
is emitted following excitation of the molecule by light of another, shorter,
wavelength. It is quite commonly observed in biological systems, where it
can give rise to a very reliable means of identification. The same can hold
148
Identification of textile fibers
true for textile fibres. It may be that the constituent polymer itself contains
one or more fluorescent species or that certain additives within the fibre
(including dyes and pigments) fluoresce. A fluorescence microscope makes
us of this emitted light, from which an image of the fibre, or part thereof, is
formed.
The wavelength of the light stimulating fluorescence varies from one
molecular species to another, as does the fluorescence itself. Thus, a given
species will fluoresce with a characteristic colour, provided light of the
appropriate wavelength is used to stimulate it. In the most basic system, the
illumination source is a bright1 white light; a set of filters and a dichroic
beam-splitter is then used to ensure that only the excitation wavelength
reaches the specimen, and that only the expected fluorescence colour
reaches the observer (or detector). Figure 7.5 is a simple representation of
Eyepiece
Fluorescence
Barrier
filter
Dichroic
beam-splitter
Excitation
filter
Objective
lens
Sample
7.5 The principal components of a simple fluorescence microscope.
(Based on a diagram appearing at http://www.seas.upenn.edu/
~confocal/epi-fluor.html.)
1
The fluorescence is invariably very weak compared to the light stimulating it, which means a
particularly bright source is generally needed.
Optical microscopy for textile fibre identification
149
a fluorescence microscope. Some of the more advanced types of instrument
employ a tuneable laser as the illumination source.
A single fibre specimen, then, may yield a series of images which differ
in colour and in form, corresponding to the various fluorescent species it
contains, and the wavelengths selected for its illumination. In some experimental studies, specimens are prepared that have been selectively stained
with a fluorescent dye in order to highlight specific features within the
structure. For example, Thomson et al. (2007) used the technique in a study
of the fibre–fibre interface regions in spruce cellulose. Fluorescence microscopy is also rapidly emerging as a crucial tool in the medical and biological
sciences. Palomero et al. (2006), for instance, have used it to investigate
free-radical generation in muscle fibres.
7.5.2 Confocal microscopy
Ideally, an optical microscope image should be formed exclusively from
light originating from the plane within the specimen upon which the objective is focussed. Inevitably, however, stray light arising from out-of-focus
regions will also reach the eye (or, more generally, detector). This produces
blurring of the image and, in consequence, degradation of the information
obtained. It can be particularly severe in fluorescence microscopy, where
the whole specimen may be contributing to the fluorescent effect. Confocal
microscopy was developed in order to eliminate this problem. The basic
principle upon which it is based dates back to an invention by Minsky
(1988). In essence, a pinhole is used to block out the unwanted light. Figure
7.6 illustrates very simplistically how this is achieved.
Suppose the microscope objective is focussed on the point within the
specimen marked F in the diagram. A pinhole is located where the real
image is expected, in front of the ocular (point F¢). Provided the pinhole is
not too small, all the light originating from F will thus be transmitted, and
so an image will be observed. Other points within the specimen also potentially produce images, though. Consider one of these, marked P. This
Screen
with pinhole
Sample
P
P´
F
F´
7.6 The essential principle of confocal microscopy; light from the
point of focus enters the detector via the pinhole. Light from
elsewhere in the specimen is largely rejected.
150
Identification of textile fibers
happens to lie beyond F, and so will produce a real image lying somewhere
behind F¢ (shown as point P¢). Normally, most of the light from P¢ would
also enter the eyepiece, thus blurring the intended image, but the screen
effectively prevents it from being transmitted. The same would be true, to
a greater or lesser extent, for all other points within the specimen.
This is all well and good, but there is a problem: not only will light from
points lying behind or in front of F be rejected, but also that from any point
within the same plane as F. That is, the screen blocks light originating from
everywhere except F, which means the observable image comprises a single
point (actually an Airy disk) rather than a 2-dimensional picture. In order
to produce a pictorial image of the entire plane containing F, it is necessary
to arrange for the incident light to scan across the specimen in two perpendicular in-plane directions. The image has then to be built up gradually from
the information obtained for each point in the scan. It is not generally
observed directly; instead, a CCD (charge-coupled device) detector is used,
and a bitmapped pixel image displayed on a monitor.
Currently, the most important practical confocal instrument is the ‘laserscanning fluorescence microscope’, a very informative description of which
may be found at http://www.physics.emory.edu/~weeks/confocal/. As with
fluorescence microscopy more generally, the exploitation of the confocal
technique is currently increasing very rapidly.
A significant feature of this method is the ability, for suitable subjects, to
build up a 3-dimensional picture by superimposing images generated from
successive planes within the specimen (a process referred to as ‘virtual
sectioning’): Albrechtova and coworkers (2007), for example, used this
approach to study the effects of acid rain on fibres of Norway spruce; and
Kubinova et al. (2004) present a useful review of this field of image
analysis.
7.6
Future trends
Nowadays, a wide range of sophisticated analytical methods other than
optical (light) microscopy is available for use in fibre identification; atomic
force microscopy, electron microscopy, vibrational spectroscopy and so on.
Nevertheless, there is no doubt that imaging methods using light in one
form or another will remain crucial. This final section describes briefly some
of the more recent developments taking place.
7.6.1 Multiphoton fluorescence microscopy
This is a relatively new technique for imaging based on the simultaneous
absorption by a fluorophore of two (in almost all practical instruments)
photons and subsequent emission of a single, shorter-wavelength, photon.
Optical microscopy for textile fibre identification
151
The energy of the emitted photon is approximately equal to the sum of the
energies of the exciting photons, which means that comparatively low
energy excitation can be employed (typically within the infrared region of
the spectrum). This is a considerable advantage, particularly where live
biological specimens are being examined, because it greatly reduces the risk
of cell damage, or photodegradation. In practical instruments the exciting
photons are provided by two separate lasers of either equal or unequal
wavelengths. The beams from these lasers, which are sometimes tunable, are
combined at the point of focus within the sample. Because the probability
of simultaneous absorption of two photons is so small, it tends to occur only
where the beams meet, and where the intensity is highest, which is at this
focal spot. This effectively eliminates unwanted fluorescence in much the
same way that the pinhole does in a confocal microscope, so that these
instruments also lend themselves to virtual sectioning.
Although multiphoton microscopy was developed specifically for the
bio-sciences, it is likely that it will become more widely exploited, and
should prove valuable in relation to fibre identification. Many forensic
subjects, for example, are vulnerable to radiation damage when viewed
under high-intensity visible light.
An exhaustive review of two-photon fluorescence microscopy for
biological studies is given by Diaspro and co-workers (2005)
7.6.2 Overcoming the classical resolution limit
The classical (Abbe) diffraction limit for a light microscope is determined
by the wavelength of the illumination and the angle subtended by the objective lens. In the best practical case this results in a resolution, for white light,
of approximately 200 nm. It is possible to reduce this to some extent by, for
example, using confocal methods, and by employing various modifications
to the microscope optics. Even then, though, 100 nm is about the best that
can be expected. However, recent developments have taken place, making
use of the nonlinear properties of fluorescent dyes, which promise resolutions down to as little as 10 nm. Successful use of such techniques, which
include so-called stimulated emission depletion (STED), is reported in
publications by Willig et al. (2006a, 2006b).
7.6.3 Optical coherence tomography
Originally reported in 1991 (Huang et al., 1991), optical coherence tomography (OCT) is a non-invasive technique for through-specimen imaging
based on interferometry. Because of its ability to produce high-resolution
three-dimensional images at penetration depths of the order of millimetres,
it has become increasingly exploited in the biomedical field, especially
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Identification of textile fibers
ophthalmic optics (Ko et al., 2001). Significant recent advances in OCT
instrumentation have taken place through the introduction of broad-band
light sources, including high-output LEDs and femtosecond pulsed lasers,
and it is likely that OCT will prove to be valuable in relation to fibre
studies.
7.6.4 Other developments
Other areas for development include methods of compensating for
variations in fibre thickness and refractive index, as well as the dispersion
effects often associated with thicker specimens. See, for instance:
http://www.hhmi.org/janelia/pdf/july_workshop.pdf.
7.7
Sources of further information and advice
7.7.1 Books
Booth, J. E. (1983) Principles of Textile Testing, London, NewnesButterworth. ISBN: 0408014873.
Diaspro, A. (Ed.) (2001) Confocal and Two-Photon Microscopy: Foundations,
Applications and Advances, New York, Wiley-Liss. ISBN-10: 0471409200.
ISBN-13: 978-0471409205.
Greaves, P. H. & Saville, B. P. (1995) Microscopy of Textile Fibres, Oxford,
U.K., Bios Scientific Publishers. ISBN: 1 872748 24 4.
Heath, J. P. (2005) Dictionary of Microscopy, Wiley. ISBN-10: 0470011998.
ISBN-13: 978-0470011997.
Matsumoto, B. (Ed.) (2002) Cell Biological Applications of Confocal
Microscopy (2nd Edn.), Academic Press. ISBN-10: 0125804458. ISBN-13:
978-0125804455.
Murphy, D. B. (2001) Fundamentals of Light Microscopy and Electronic
Imaging, Wiley-Liss. ISBN-10: 047125391X. ISBN-13: 978-0471253914.
Paddock, S. W. (Ed.) (1999) Confocal Microscopy Methods and Protocols,
Humana Press. ISBN-10: 0896035263. ISBN-13: 978-0896035263.
Pawley, J. B. (Ed.) (1995) Handbook of Biological Confocal Microscopy
(2nd edn.), Springer. ISBN: 0306448262.
Rochow, T. G. and Tucker, P. A. (1994) Introduction to Microscopy by
Means of Light, Electrons, X Rays, Or Acoustics, Springer. ISBN:
0306446847.
Rost, F. W. D. (1991) Quantitative Fluorescence Microscopy, Cambridge
Universtiy Press. ISBN-13: 9780521394222. ISBN-10: 0521394228.
Saville, B. P. (1999) Physical Testing of Textiles, Cambridge, Woodhead, with
The Textile Institute. ISBN: 1855733676. Online version also available at:
http://www.knovel.com/knovel2/Toc.jsp?BookID=925&VerticalID=0.
Optical microscopy for textile fibre identification
153
7.7.2 Electronic resources
http://micro.magnet.fsu.edu/primer/photomicrography; accessed 21 August,
2007
http://www.fz-juelich.de/inb/inb-1/Two-Photon_Microscopy/; accessed 22
August, 2007
http://www.futureimage.com/; accessed 24 August, 2007
http://www.hhmi.org/janelia/pdf/july_workshop.pdf; accessed 24 August,
2007
http://www.lightlabimaging.com/oct.html; accessed 24 August, 2007.
http://www.loci.wisc.edu/multiphoton/fastmp.html; accessed 24 August,
2007
http://www.loci.wisc.edu/multiphoton/mp.html; accessed 24 August, 2007
http://www.microscopyu.com/articles/polarized/polarizedintro.html;
accessed 23 August, 2007
http://www.physics.emory.edu/~weeks/confocal/; accessed 17 August, 2007
http://www.rsc.org/chemistryworld/Issues/2007/March/TheMillionDollarMi
croscope.asp; accessed 22 August, 2007
http://www.seas.upenn.edu/~confocal/epi-fluor.html; accessed 17 August,
2007
http://www.stjapan.de; accessed 21 August, 2007
7.7.3 Suppliers of microscopes and accessories
Brunel Microscopes Ltd
Unit 2, Vincients Road
Bumpers Farm Industrial Estate
Chippenham
Wiltshire SN14 6NQ
UK
Helpline: 0044 (0)1249 462655
Fax: 0044 (0)1249 445156
http://www.brunelmicroscopes.co.uk/; accessed 24 August, 2007
Carl Zeiss (UK) Ltd.
Head Office
15–20 Woodfield Road
Welwyn Garden City
Hertfordshire AL7 1JQ
UK
Phone: +44 1707 871200
Fax: +44 1707 330237
http://www.zeiss.co.uk/; accessed 25 August, 2007.
154
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GX Optical
http://www.gxoptical.com; accessed 21 August, 2007
Division of GT Vision Ltd (UK, Europe, Africa, Asia, Australiasia):
Hazel Stub Depot
Camps Road
Haverhill
Suffolk CB9 9AF
UK
Tel: (from UK) 01440 714737 (outside UK) +44 1440 714737
Fax: (from UK) 01440 709421 (outside UK) +44 1440 709421
Division of GT Vision LLC (USA, Canada, S and Central
America):
10205 Easterday Court
Hagerstown,
MD 21742
USA
Tel: +1 240 235 4118
Fax: +1 240 235 4120
Leica Microsystems
International Headquarters:
Leica Microsystems GmbH
Ernst-Leitz-Strasse 17–37
35578 Wetzlar
Phone +49 6441 29-0
Fax +49 6441 29-2590
http://www.leica-microsystems.com/
Microscope Systems Scotland
11 Strathblane Road
Glasgow G62 8DL
UK
Phone: +44 (0) 141 563 9696
Fax: +44 (0) 141 563 9696
Email: [email protected]
http://www.microscopesales.co.uk; accessed 25 August, 2007.
Olympus
http://www.microscopy.olympus.eu; accessed 25 August, 2007.
Prior Scientific
Toll Free: 800-877-2234
http://www.prior.com/; accessed 25 August, 2007.
Optical microscopy for textile fibre identification
155
Selectscience.net
http://www.selectscience.net/; accessed 25 August, 2007.
7.7.4 Supporting organisations
The American Microscopical Society, Inc.
http://www.amicros.org/; accessed 25 August, 2007.
ASTM International
(formerly The American Society for Testing and Materials (ASTM))
100 Barr Harbor Drive,
PO Box C700,
West Conshohocken, PA 19428-2959
USA
http://www.astm.org/cgi-bin/SoftCart.exe/index.shtml?E~mystore; accessed
24 August, 2007.
The Australian Microscopy and Microanalysis Society
http://www.microscopy.org.au/; accessed 25 August, 2007.
British Standards Online
http://www.bsonline.bsi-global.com/server/index.jsp; accessed 24 August,
2007
The European Microscopy Society (EMS)
http://www.eurmicsoc.org/; accessed 25 August, 2007.
The International Organization for Standardization (ISO)
1, ch. de la Voie-Creuse, Case postale 56
CH-1211 Geneva 20
Switzerland
Telephone +41 22 749 01 11; Fax +41 22 733 34 30
http://www.iso.org; accessed 25 August, 2007.
Olympus Microscopy Resource Center
http://www.olympusmicro.com/primer/techniques/polarized/
berekcompensator.html; accessed 17 August, 2007
The Royal Microscopical Society
37/38 St Clements
Oxford OX4 1AJ
UK
Tel +44 (0)1865 248 768 or +44 (0)1865 254760
156
Identification of textile fibers
Fax +44 (0)1865 791 237
http://www.rms.org.uk; accessed 25 August, 2007.
The Textile Institute
1st Floor, St James’s Buildings,
Oxford Street,
Manchester M1 6FQ
UK
Tel: +44(0)161 237 1188
Fax: +44(0)161 236 1991
http://www.texi.org; accessed 25 August, 2007.
Directories of international societies and of suppliers may also be available
at: http://www.mwrn.com/default.aspx; accessed 25 August, 2007.
7.8
References
Albrechtova, J., Janacek, J., Lhotakova, Z., Radochova, B. and Kubinova, L. (2007)
Novel efficient methods for measuring mesophyll anatomical characteristics from
fresh thick sections using stereology and confocal microscopy: application on
acid rain-treated Norway spruce needles. Journal of Experimental Botany, 58,
1451–1461.
Booth, J. (1983) Principles of textile testing, London, Newnes-Butterworth.
Conan Doyle, A. (1890) The Sign of Four.
Diaspro, A., Chirico, G. and Collini, M. (2005) Two-photon fluorescence excitation
and related techniques in biological microscopy. Quarterly Reviews of Biophysics,
38, 97–166.
Greaves, P. H. and Saville, B. P. (1995) Microscopy of Textile Fibres, Oxford, U.K.,
Bios Scientific Publishers.
Hadley, D. W., Pinnock, P. R. and Ward, I. M. (1964) Mechanical Anisotropy in
Oriented Polymers. Polymer, 5, 384–385.
Hadley, D. W., Pinnock, P. R. and Ward, I. M. (1969) Anisotropy in Oriented Fibres
from Synthetic Polymers. Journal of Materials Science, 4, 152–&.
Hamza,A.A., Sokkar,T. Z. N. and Ramadan,W.A. (1992) On the Microinterferometric
Determination of Refractive Indices and Birefringnce of Fibres. Pure and Applied
Optics, 1, 321–336.
Huang, D., Swanson, E. A., Lin, C. P., Schuman, J. S., Stinson, W. G., Chang, W., Hee,
M. R., Flotte, T., Gregory, K., Puliafito, C. A. and Fujimoto, J. G. (1991) Optical
Coherence Tomography. Science, 254, 1178–1181.
Ko, T. H., Ghanta, R. K., Hartl, I., Drexler, W. and Fujimoto, J. G. (2001) Ultra-high
resolution optical coherence tomography for quantitative topographic mapping
of retinal and intraretinal architectural morphology. Investigative Ophthalmology
& Visual Science, 42, S793–S793.
Kubinova, L., Janacek, J., Karen, P., Radochova, B., Difato, F. and Krekule, I. (2004)
Confocal stereology and image analysis: Methods for estimating geometrical
Optical microscopy for textile fibre identification
157
characteristics of cells and tissues from three-dimensional conflocal images.
Physiological Research, 53, S47–S55.
Malmgren, T., Mays, J. and Pyda, M. (2006) Characterization of poly (lactic acid)
by size exclusion chromatography, differential refractometry, light scattering
andthermal analysis. Journal of Thermal Analysis and Calorimetry, 83, 35–40.
Minsky, M. (1988) Memoir on Inventing the Confocal Scanning Microscope.
Scanning, 10, 128–138.
Palanik, S. and Fitzsimmons, C. (1990) Fiber cross-sections: Part II. A simple method
for sectioning single fibers. The Microscope, 38, 313–320.
Palomero, J., Pye, D., Kabayo, T. and Jackson, M. J. (2006) In situ detection and
measurement of intracellular ROS and nitric oxide generation in isolated mature
skeletal muscle fibres by real-time fluorescence microscopy. Free Radical Research,
40, S77–S77.
Pinnock, P. R. and Ward, I. M. (1964) Mechanical and Optical Anisotropy in
Polyethylene Terephthalate Fibres. British Journal of Applied Physics, 15,
1559–&.
Pinnock, P. R. and Ward, I. M. (1966) Mechanical and Optical Anisotropy in
Polypropylene Fibres. British Journal of Applied Physics, 17, 575–&.
Saville, B. (1999) Physical Testing of Textiles, Cambridge, Woodhead with The Textile
Institute.
Thomson, C. I., Lowe, R. M. and Ragauskas, A. J. (2007) Imaging cellulose fibre
interfaces with fluorescence microscopy and resonance energy transfer.
Carbohydrate Polymers, 69, 799–804.
Ward, I. M. (1962) Optical and Mechanical Anisotropy in Crystalline Polymers.
Proceedings of the Physical Society of London, 80, 1176–&.
Willig, K. I., Keller, J., Bossi, M. and Hell, S. W. (2006a) STED microscopy resolves
nanoparticle assemblies. New Journal of Physics, 8.
Willig, K. I., Kellner, R. R., Medda, R., Hein, B., Jakobs, S. and Hell, S. W. (2006b)
Nanoscale resolution in GFP-based microscopy. Nature Methods, 3, 721–723.
8
The use of spectroscopy for textile
fiber identification
M M HOUCK, West Virginia University, USA
Abstract: The use of spectroscopy for fiber analysis is widespread and
ranges from simple identification of polymer type(s) to structural
information. Colorants used on textiles, be they dyes or pigments, are
also the subject of spectroscopic analysis. Of necessity, the spectroscopy
of fibers and related materials is a broad and complicated topic. This
chapter will be limited, therefore, to basic concepts, applications, and
future improvements in some of the spectroscopic analysis of textile
fibers.
Key words: fibers, spectroscopy, identification.
8.1
Introduction: spectroscopy of fibers
The use of spectroscopy for fiber analysis is widespread and ranges from
simple identification of polymer type(s) to structural information. Colorants
used on textiles, be they dyes or pigments, are also the subject of spectroscopic analysis. Of necessity, the spectroscopy of fibers and related materials
is a broad and complicated topic. This chapter will be limited, therefore,
to basic concepts, applications, and future improvements in some of the
spectroscopic analysis of textile fibers.
A spectroscope is an optical instrument for producing spectral lines and
measuring their wavelengths and intensities, originally producing colored
bands relevant to the elements of the sample captured on photographic
film. This spectrum shows narrow (also called sharp) lines distributed in
energy across the visible spectrum. For example, sodium emits two close
lines in the yellow region of the spectrum, while cadmium emits a strong
red and a strong green line. Spectroscopy is the study of any measurement
produced by a spectroscope; a plot of the response as a function of
wavelength or frequency (more common) is referred to as a spectrum.
Spectrometers and spectrometry are often conflated with spectroscopes and
spectroscopy. Thus:1
Spectroscopy: The study of physical systems by the electromagnetic radiation with which they interact or that they produce.
Spectrometry: The measurement of such radiations as a means of obtaining
information about the systems and their components. In certain types of
158
The use of spectroscopy for textile fiber identification
159
optical spectroscopy, the radiation originates from an external source and
is modified by the system, whereas in other types, the radiation originates
within the system itself.
Spectroscope: A device which enables visual observation and evaluation of
optical spectra (usually confined to the visible spectral region).
Spectrometer: A general term for describing a combination of spectral
apparatus with one or more detectors to measure the intensity of one or
more spectral bands.
Typically, practitioners recognize the practical, if nominal, differences
between methods and instruments.
8.2
Categorizing methods by nature of excitation
The method of spectroscopy applied in identifying or analyzing fibers
depends on the physical quantity to be measured, usually an intensity
either of energy absorbed or produced. Optical spectroscopy involves interactions of matter with electromagnetic radiation or light. Ultraviolet-visible
spectroscopy is an example Electron spectroscopy involves interactions
with electron beams, such as those produced by an electron microscope,
either scanning or transmission. Mass spectroscopy (more typically called
mass spectrometry) involves the interaction of charged species with
magnetic or electric fields. The spectrum produced has the mass m as the
variable, but the measurement is essentially one of the kinetic energy of
the particle.
8.3
Categorizing methods by measurement process
Spectroscopic methods are differentiated as either atomic or molecular
based on which structure they measure; additionally, they can be further
classified on their interaction with the sample. Absorption spectroscopy
measures in the range of the electromagnetic spectrum where a substance
absorbs; this includes atomic absorption and other molecular techniques,
such as the infrared region (infrared spectroscopy) and the radio wave
region (nuclear magnetic resonance or NMR). Emission spectroscopy, by
contrast, exploits the range in which a substance radiates or emits. Given
that, it is a prerequisite that the substance first absorbs energy and this can
be from a number of sources but each determines the title of the subsequent
emission (fluorescence spectroscopy, for example). Scattering spectroscopy
measures the light scattered by the sample at specific wavelengths, incident
angles, or polarization angles. Raman spectroscopy is one application of
scattering spectroscopy.
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Identification of textile fibers
8.4
Common methods of spectroscopy
Fluorescence spectroscopy excites a sample with high energy photons and
the sample then emits lower energy photons. The species being examined
will have a low energy state and an excited state of higher energy. Within
each of these electronic states are various vibrational states. Photons of high
frequency have higher energy than those of lower frequency light. When
the photons are absorbed by the molecules in the sample, the molecule
gains or emits the energy of the photon and the photon carries some of the
energy of the molecule away. As molecules fall into any of various vibrational levels in the ground state, the emitted photons will have different
energies that are indicative of the species in the material. Analyzing the
different frequencies and relative intensities of light emitted in the fluorescence spectrum determines the structure of the sample.
Atomic absorption spectroscopy (AA) vaporizes the sample in a flame or
graphite furnace. The temperature of the flame is low enough that the
flame itself does not excite sample atoms from their ground state. Instead,
lamps exposed to the flame at various wavelengths for each type of
analyte of interest excite the sample. The amount of light absorbed after
it passes the flame is proportional to the analyte’s concentration in the
sample. Among other applications, AA has been used to analyze metal
ions in fibers.2–4
Atomic emission spectroscopy (AES) also uses flame excitation but at a
higher temperature than atomic absorption spectroscopy. The analyte
atoms are directly excited by the flame and, thus, no elemental lamps are
needed to shine into the flame. This high-temperature atomization provides sufficient energy to promote the atoms into high energy levels. The
emission lines in the spectra are narrow because the transitions occur
between distinct energy levels. AES excites all the atoms in a sample
simultaneously; therefore, they can be detected simultaneously and this
is a major advantage of AES compared to atomic-absorption; high resolution is required for this type of analysis as the spectra of multi-elemental
samples can be very crowded and complicated. Plasma emission spectroscopy is a more modern version of this method. AES has been used in
textile conservation,5 analysis of antimicrobial agents in textiles,6 and the
treatment of textile effluents,7 among other topics.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is a
method of emission spectroscopy that excites atoms and ions with a
plasma, causing it to emit electromagnetic radiation at wavelengths characteristic of a particular element. Intensity of the emission is proportional
to the element’s concentration in the sample. ICP-AES may be referred
to as Inductively Coupled Plasma Optical Emission Spectrometry (ICP-
The use of spectroscopy for textile fiber identification
161
OES). ICP-AES has been applied to the analysis of metals in textiles for
health and safety,8 forensic science,9 and the kinetics of dye chemistry,10
among other analyses.
Laser induced breakdown spectroscopy (LIBS), also called laser-induced
plasma spectrometry (LIPS), is a type of atomic emission spectroscopy
which uses a high energy laser pulse to excite the source. The main advantage of LIBS is that it can analyze a sample regardless whether it is solid,
liquid, or gas. At sufficiently high temperatures, all elements will emit
light; therefore, LIBS can detect any element, depending on the power
of the laser and the sensitivity of the detector. Given this flexibility, LIBS
has been applied to, among many other areas, art and colorants,11,12 characterization of textile materials,13 and more specifically, nanofibers and
electrospinning.14
8.4.1 Visible spectroscopy
A thorough discussion of visible spectroscopy is given elsewhere in this
book and so will not be repeated here. Ultraviolet and visible range spectroscopy is useful in the analysis of colorants in textiles and is routinely
used in the forensic analysis and comparison of fiber-related evidence.
8.4.2 Infrared spectroscopy
Infrared spectroscopy offers much to the textile chemist: minor sample
preparation, fast analysis, small sample size, and detailed information about
the different types of bonds present in the sample. The infrared portion of
the electromagnetic spectrum is divided into three regions; the near-, the
mid-, and the far-infrared, based on their position relative to the visible
spectrum. The three regions are not distinctly divided (by exact molecular
or electromagnetic properties) and have different spectroscopic utilities:
•
•
•
the far-infrared (400–10 cm−1) has low energy and is used for rotational
spectroscopy,
the mid-infrared (4000–400 cm−1) is used to analyze the fundamental
vibration and rotational-vibrational structures, and
the near-IR (14 000–4000 cm−1) is used for overtone or harmonic
vibrations.
Infrared spectroscopy exploits the fact that molecules rotate or vibrate
at specific frequencies depending on their discrete energy levels. The shape
of the molecular energy surfaces, the masses of the composite atoms, and
the associated vibrational couplings determine the frequencies at which the
molecule will move. IR active vibrational modes in a molecule are associated with changes in the permanent dipole. The resonant frequencies are
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Identification of textile fibers
related to the strength of the bond and the mass of the atoms at either end
of it. This relationship allows for the bond type to be discerned through its
IR active components.
The complexity of the molecule and its bonds determine the types of
‘movement’ it can make when excited in an IR region. Simple diatomic
molecules have only one bond, for example, which may stretch; more
complex molecules more bonds, some of which may be conjugated. The
complexity leads to infrared absorptions at specific, characteristic frequencies related to chemical groups or classes.
The IR spectrum of a sample is collected by passing a beam of infrared
light through the sample over a range of energies within the IR region;
either a monochromatic beam (changing wavelength over time) or a Fourier
transform15 instrument (FTIR; measuring all wavelengths at once) is used.
The transmitted signal indicates how much energy was absorbed at each
wavelength. The spectrum can be displayed as either transmittance or
absorbance data. Because of its multiple advantages, virtually all modern
infrared spectrometers are FTIR instruments. Spectra may also be obtained
by a variety of alternative IR techniques. Other techniques include micro
internal reflection spectroscopy (MIR), which differs from attenuated total
reflectance (ATR) in that the infrared radiation is dependent upon the
amount of sample in contact with surface of the prism.16
IR spectroscopy works well in conjunction with a microscope allowing
for the analysis of single textile fibers. The fibers must be flattened in preparation for analysis. Because the flattening is destructive of morphology, the
minimum length of fiber necessary for the analysis should be used (as a
typical IR microscope is optimized for a 100 μm spot size, analytical performance will not necessarily be improved with the use of fibers greater
than 100 μm in length). The flattened fiber may be mounted across an aperture, on an IR window, or between IR windows. Common IR window
materials used for this purpose include but are not limited to KBr, CsI,
BaF2, ZnSe, and diamond. Consensus guides for fiber sample preparation
and analysis by FTIR are available.17
8.4.3 Raman spectroscopy
Much like infrared spectroscopy, Raman spectroscopy analyzes vibrational
and rotational modes of molecules. It does so through the so-called Raman
effect, which occurs when laser light impinges upon a molecule and interacts
with the electron cloud of the bonds of that molecule. The molecule is
excited from the ground state to a virtual energy state and then relaxes into
a vibrational excited state. This relaxation generates Stokes Raman scattering; molecules in an already elevated vibrational energy state generate
anti-Stokes Raman scattering. A change in molecular polarizability is
The use of spectroscopy for textile fiber identification
163
required for the molecule to demonstrate a Raman effect. The amount of
polarizability change determines the intensity of the effect.
Raman spectroscopy lends itself to microscopic analysis for the following
reasons:
• specimens do not need to be fixed or sectioned,
• a very small sample volume (<1 μm in diameter) can be analyzed,
• raman spectroscopy identifies the species present, and
• water does not interfere significantly.
Thus, Raman spectroscopy is suitable for the microscopic examination of
a wide variety of materials, such as minerals, polymers, ceramics, and proteins. Raman microscopy generally uses near-infrared lasers, which reduce
damage to the sample because of their relatively low power.
8.5
References
1. IUPAC Compendium of Chemical Terminology (2nd Edition)(the ‘Gold Book’)
compiled by Alan D. McNaught and Andrew Wilkinson, Blackwell Science,
Oxford, 1997, all on page 387.
2. Masri, M. and F. Reuter (1974). ‘Interaction of wool with metal cations.’ Textile
Research Journal 44(4): 298–300.
3. Izumi, S., Y. Shimizu, et al. (2002). ‘Absorption Behavior of Metal Ions on Chitin/
Cellulose Composite Fibers with Chemical Modification by EDTA.’ Textile
Research Journal 72(6): 515–519.
4. Bozzi, A., T. Yuranova, et al. (2004). ‘Self-cleaning of wool-polyamide and
polyester textiles by TiO2-rutile modification under daylight irradiation at
ambient temperature.’ Journal of Photochemistry and Photobiology A: Chemistry
172(1): 27–34.
5. Hardin, I. and F. Duffield (1992). ‘Microanalysis of Persian textiles.’ Iranian
Studies 25(1–2): 43–59.
6. Nakashima, H. and T. Ooshima (2007). ‘Analysis of Inorganic Antimicrobial
Agents in Antimicrobial Products: Evaluation of a Screening Method by X-ray
Fluorescence Spectrometry and the Measurement of Metals by Inductively
Coupled Plasma Atomic Emission Spectroscopy.’ Journal of Health Science
53(4): 423–429.
7. Fersi, C., L. Gzara, et al. (2005). ‘Treatment of textile effluents by membrane
technologies.’ Desalination 185(1–3): 399–409.
8. Rezic, I. and I. Steffan (2006). ‘ICP-OES determination of metals present in
textile materials.’ Microchemical Journal 85(1): 46–51.
9. Bisbing, R. (2006). ‘A trace evidence primer for microscopists.’ Microscopy and
Microanalysis 12: 12–14.
10. Dordevic, D., J. Cerkovnik, et al. (2006). ‘The comparison of the kinetics of
hydrolysis of some reactive dyes before and after purification.’ Fibres and
Textiles in Eastern Europe 14(2): 85–88.
11. Oujja, M., A. Vila, et al. (2005). ‘Identification of inks and structural characterization of contemporary artistic prints by laser-induced breakdown spectroscopy.’
Spectrochimica Acta Part B: Atomic Spectroscopy 60(7–8): 1140–1148.
164
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12. Clark, R. (2002). ‘Pigment identification by spectroscopic means: an arts/science
interface.’ Comptes Rendus Chimie 5(1): 7–20.
13. Andrews, D. (2007). Optical Techniques for the Analysis and Characterization
of Chemicals and Materials. The Optics Encyclopedia. T. Brown, K. Creath, H.
Kogelnik et al. New York, John Wiley and Sons.
14. Ayutsede, J., M. Gandhi, et al. (2006). ‘Carbon Nanotube Reinforced Bombyx
mori Silk Nanofibers by the Electrospinning Process.’ Biomacromolecules 7:
208–214.
15. A Fourier transform transforms one function into another, which is called the
frequency domain representation of the original function, often a time-domain.
For more information, the interested reader is directed to R. N. Bracewell, The
Fourier Transform and Its Applications, 3rd ed., Boston, McGraw Hill, 2000.
16. Bartick, E. G., Tungol, M. W. and Reffner, J. A. A new approach to forensic
analysis with infrared microscopy: Internal reflection spectroscopy, Analytica
Chimica Acta (1994) 288:35–42.
17. Scientific Working Group for Materials Analysis (1999). ‘Forensic Fiber Analysis.’
Forensic Science Communications 1(1); available online at www.fbi.gov.
9
Microspectrophotometry for textile fiber
color measurement
S WALBRIDGE-JONES,
Bureau of Alcohol, Tobacco, Firearms and Explosives, USA
Abstract: In a scientific setting when an optical property such as color
needs to be measured with some standard of accuracy, the science of
spectroscopy must be applied. This chapter discusses how and why
spectroscopy is the method of choice for color measurement. When
sample sizes are reduced, Microspectroscopy is utilized and is the main
method discussed. Microspectroscopy is a non-destructive technique
used to measure the chemical properties of microscopic samples. The
non-destructive analysis by this technique makes it the primary
application for textile fiber color measurement. The strengths and
limitations of microspectrophotometry and future trends in textile
analysis are discussed.
Key words: microspectrophotometry (MSP), dyed textile fibers,
spectroscopy of color, metameric fibers, color matching, forensic analysis
of dyed textile fibers.
9.1
Introduction
Color is an integral part of our daily lives. The color choices we make reflect
our inner moods and feelings. We are subjective when choosing the color
of our automobiles to the colors we paint our homes. Human perception of
color will always be subjective; it is not measurable. The human brain ‘perceives’ color; but it cannot measure color. The ocular sensitivity of humans
is limited to only the visible region of the electromagnetic spectrum: red to
violet with maximum ideal ocular sensitivity in the green region. Therefore,
in a scientific setting when an optical property such as color needs to be
measured with some standard of accuracy, the science of spectroscopy must
be applied. This chapter will discuss how and why spectroscopy is the
method of choice for color measurement. When sample sizes are reduced,
Microspectroscopy is utilized and will be the main method discussed.
Microspectroscopy is a non-destructive technique used to measure the
chemical properties of microscopic samples. The non-destructive analysis
by this technique makes it the primary application for textile fiber color
measurement. The strengths and limitations of microspectrophotometry
and future trends in textile analysis will also be discussed.
165
166
Identification of textile fibers
White light
Red
Green
Blue
9.1 Dispersing prism.
9.2
An understanding of spectroscopy
The electromagnetic spectrum ranges, in increasing wavelength, from x-rays
to radio waves. The word spectrum as defined by Merriam-Webster is a
‘continuum of color formed when a beam of white light is dispersed (as by
passage through a prism) so that its component wavelengths are arranged
in order.’ The branch of science called spectroscopy studies how the wavelength of light changes after it interacts with a sample. The prism in Fig. 9.1
is the ‘sample’ for this demonstration. The resultant color spectrum provides
information about the sample.
Hence the information gained here is that the ‘sample’ (a prism) has the
ability to disperse white light into its respective wavelengths (each wavelength has its own color). For a given chemical compound, every light and
matter interaction is unique. Spectroscopy is a branch of science that allows
us to examine this light/matter interaction. This interaction is measured by
a spectrophotometer, an instrument which measures the intensity (or wavelength) of light after interaction with a sample. The result of this interaction,
displayed as a spectrum, provides information about the sample on a molecular level. A spectrophotometer has the ability to analyze a sample in the
ultraviolet (UV), visible (Vis), and near infrared region (NIR) spectral
regions. This capability makes a spectrophotometer a principal tool for
studying the structures of molecules and their environment. In contrast,
human perception of the ‘light-matter interaction’ is a perception and is
only accomplished in the visible region of the spectrum. The differences
between human color perception and instrumental color measurement will
be discussed later.
The theory of spectroscopy is based on quantum mechanics. Energy in
matter and light is quantized. Atoms and molecules have discrete energy
levels known as quantum levels. Translated, atoms and molecules may exist
only in certain allowed energy states. Spectroscopy as previously stated is the
study of light and matter interactions or the study of the transitions between
Microspectrophotometry for textile fiber color measurement
167
the allowed energy states of atoms and molecules. When energy is transferred
between quantum levels radiation is either emitted or absorbed.
Transitions can involve atoms or molecules and the choice of the energy
of light (light source) controls which transitions take place. Thus, it is possible
in the application of spectroscopy to manipulate a sample by exciting it with
a known source of energy and measuring the expected energy transition. A
majority of molecular transitions occur in the UV-Vis region. The UV-Vis
region is a part of the electromagnetic spectrum where interactions between
light and matter can be observed. Dyes which produce the colors in our
favorite shirts, pants, carpets and automobiles are substances that are easily
excitable and absorb light in the visible region. Some dyes and pigments also
absorb light in the UV and NIR range. The chemistry behind color requires
that for absorption of radiation (light) in the visible region to occur some
groups of conjugated double bonds must be present in the dye molecule.
Conjugated bonds increase the likelihood of excitation, a requirement for
spectroscopy. Molecules containing systems of conjugated bonds which
absorb specific wavelengths of light are called chromophores. Auxochromes
are any side groups that when attached to a chromophore change the wavelength and intensity of absorption. Dye compounds are made from the
combination of a chromophore with an auxochrome attached to a benzene
ring or rings. The chromophore provides the color of the dye compound and
the auxochrome affects the strength of the dye. UV-visible-NIR spectra of
dyes can provide information about the chromophore itself, the chemistry
that the chromophore molecules may have undergone, and even the effects
that temperature and substrates may have on the chromophore. Because the
majority of the textile environment is dyed or pigmented, the usefulness of
spectroscopy prevails when the molecular structure of these dyes and pigments are studied.
Although hand-held spectrophotometers exist, an accurate and precise
analysis of a dye and/or pigment can best be evaluated on single fibers with
the help of a microspectrophotometer. Microspectroscopy is the study of
light and matter interactions in microscopic samples and a microspectrophotometer combines the microscope’s ability to magnify small samples with
the capabilities of a spectrophotometer. Microspectrophotometry is commonly used in forensic science laboratories as a quick and non-destructive
means for examining colored microscopic items such as fibers, paint and ink.
Microspectrophotometry is also a suitable method for dye examination and
quality control of dye production in the textile science field.
9.3
Microspectrophotometer design
A microspectrophotometer is a combination of a microscope with a
spectrophotometer. A basic understanding of the microscope’s optics is
168
Identification of textile fibers
Dispersing
element
*
Light source
Focusing
element
Focusing
element
Sample
CCD array
*Entrance slit
9.2 How a spectrophotometer works (diagram provided by Paul
Martin, CRAIC Technologies and reprinted with permission).
necessary to utilize a microspectrophotometer. A microscope is a complex
system that uses lenses and mirrors to form a magnified image of a sample.
The light ray paths, lenses and diaphragms used in the microscope’s system
are all interrelated and require optimal ‘tuning’ for effective illumination.
A good reference for understanding proper illumination and set-up for a
microscope is necessary, as the techniques are not intuitive.1
The function of the microscope in a microspectrophotometer system is
to transmit light energy while magnifying a sample. The spectrophotometer
of a microspectrophotometer system is the device which measures the
change in light intensity versus wavelength after the light has interacted
with the sample. A spectrophotometer can be used to measure the light that
is transmitted through a sample, the light that is reflected off a sample, or
the light that was emitted from a sample. The basic design of a spectrophotometer is shown in Fig. 9.2. A light source, typically a xenon lamp, is focused
on a sample. The sample will absorb some wavelengths of light while transmitting others. The transmitted wavelengths are separated by a monochromator into spectral bands which in turn create a spectrum. The wavelengths
of light separated by the monochromator are actually photons of light. Thus,
a device is needed in order to detect these photons and convert them into
an electrical signal. Now with a general idea of how each individual component works independently, the combined system, the microspectrophotometer can be discussed.
Figure 9.32 depicts a microspectrophotometer. A lamp in the microscope
unit emits white light which is focused, via a condenser lens, on the sample
which is sitting on the sample stage. Transmitted light rays which are not
absorbed, reflected, scattered, or diffracted by the sample/system are collected by the microscope objective. The objective lens focuses the light onto
the entrance aperture of the spectrophotometer. Most of the light is reflected
by a mirrored aperture into a digital imaging system which allows you to
1
For example, McCrone, W., Delly, J. and McCrone, L. (1978) Polarized Light Microscopy, Ann
Arbor Science Publishers, Ann Arbor, MI is a standard reference in this area.
2
Courtesy of Paul Martin, CRAIC Technologies.
Microspectrophotometry for textile fiber color measurement
Holographic
grating
High resolution
digital imaging
169
TE cooled CCD
detector
Mirrored aperture
UV-visible-NIR
microscope
objective
Sample
Condenser
UV-visible-NIR
light source
9.3 Basic design of a transmission microspectrophotometer (diagram
provided by Paul Martin, CRAIC Technologies and reprinted with
permission).
visualize the point in which spectra is collected. The light, which is not
reflected, passes through the spectrophotometer aperture and is separated
by an optical grating into wavelength components. A charged coupled
device (CCD) is the detector that measures the intensity of these wavelengths and converts them into an electrical signal. A computer serves in
data processing and recording.
9.4
Types of microspectroscopy
As mentioned previously a spectrophotometer can be used to measure the
light that is transmitted through a sample, the light that is reflected off a
sample, or the light that is emitted from a sample. A microspectrophotometer is capable of measuring the transmittance, reflectance and/or fluores-
170
Identification of textile fibers
cence in the ultraviolet-visible (UV-vis) range of microscopic samples such
as individual fibers. Transmission microspectroscopy is the method of choice
for analyzing individual colored fibers. The basics of this method were
explained above and can be referenced in Fig. 9.3. A dark scan, a reference
scan, and a sample scan must all be collected. A dark scan collects the
background ‘noise’ of the system. A reference scan collects the mounting
or support materials of the sample plus the system. A sample scan is of the
sample plus mounting or support materials. The data is analyzed by calculating the ratio of the sample scan minus the dark scan to the reference scan
minus the dark scan. The resultant data, in the form of an absorbance spectrum, represents the sample only with all system artifacts ratioed out.
Transmittance microspectroscopy measures how much light was transmitted through the sample or, more accurately, how much light was absorbed
by the sample.
Figure 9.4 represents reflectance microspectroscopy. Reflectance microspectroscopy can be utilized for opaque samples such as paint chips. The
Holographic
grating
High resolution
digital imaging
UV-visible-NIR
light source
TE cooled CCD
detector
Mirrored aperture
Beamsplitter
UV-visible-NIR
microscope
objective
Sample
9.4 Basic design of a reflectance microspectrophotometer (diagram
provided by Paul Martin, CRAIC Technologies and reprinted with
permission).
Microspectrophotometry for textile fiber color measurement
Holographic
grating
171
TE cooled CCD
detector
Mirrored aperture
High resolution
digital imaging
Barrier filter
UV-visible-NIR
light source
Dichroic filter
Excitation filter
UV-visible-NIR
microscope
objective
Sample
9.5 Basic design of a fluorescence microspectrophotometer (diagram
provided by Paul Martin, CRAIC Technologies and reprinted with
permission).
lighting technique is different for an opaque sample than a transparent
sample. In this process the light is reflected off the sample rather than
transmitted through it; however, the sampling procedure is similar. A reference material with similar reflectance properties to the sample material is
required to ensure that the system’s artifacts are correctly ratioed out of
the final result. The result is a reflectance spectrum representing the ‘color’
reflected from the sample’s surface which is the same ‘color’ we see when
macroscopically examining the sample in our environment.
Fluorescence microspectroscopy provides information about lightabsorbing specimens. Fluorescent samples will absorb light energy at one
wavelength and then emit energy usually at a longer wavelength. No reference scan is taken during this technique and thus there is no correction to
a standard. A dark scan is subtracted from the sample scan with the resulting spectrum being measured in counts. Figure 9.5 represents the basic setup for microfluorometry. Optical brighteners are highly fluorescent and
172
Identification of textile fibers
often added to textile fibers. During a textile fiber examination, fluorescent
samples are often visually evaluated first by incident fluorescence. When
fluorescent fibers are observed under the microscope, an examiner will most
likely continue the fiber comparison using fluorescence microspectroscopy.
Reflectance microspectroscopy is seldom used for the examination of
colored textile fibers. Transmission microspectroscopy is the most common
method of choice when analyzing and comparing colored textile fibers.
9.5
Perception of color: human vs. machine
The textile industry and consumers alike are concerned with product
appearance. When an individual buys a black business suit, it is anticipated
that the product, both pants and jacket, will be the same color black. A
rack of suits may have shades which appear slightly different, however,
somewhere in the production process, this ‘batch’ of suits passed a color
test within a certain tolerance and is now being sold as a group of ‘black’
suits from the vendor. In the forensic laboratory, if two of these suits were
to be compared by microspectrophotometry the analyst might find that
the fibers which make up the two visually ‘identical’ suits are not dyed
with an analytically indistinguishable set of dyes. Two people may observe
these suits and describe them all as being ‘black’. Unlike a spectrophotometer, humans lack the ability to specifically measure the outcome of the
interaction of light and an object (matter). What humans can do though is
experience the interaction between light and an object.
The color detectors in the eye are the cones. Individual cones are sensitive
in three defined frequencies of light: red (‘L’, maximum 564 nm), green (‘M’,
maximum 534), and blue (‘S’, maximum 420 nm). The brain via the optic
nerve processes the eye’s detection of the three primary colors and the
result is the perception of color (what color we see). The rods in the eyes
are sensitive in dim light and thus help in the ability to detect grays. Figure
9.6 illustrates the human eye color response and also includes the absorption spectra for the rod cells ‘R’. If the cones and rods were at 100% functionality in all humans, all people would perceive color in an identical
manner. Human color vision, however, is a direct expression of the wavelengths of light absorbed by each eye [1]. The eye-brain system translates
the absorbed wavelengths into a color. For example, when looking at a red
apple, the maximum wavelength perceived is red, as the skin of the apple
absorbs the majority of the rest of the spectrum except red. Not every
person may describe the red apple as being the same red. One reason is the
influence of environment: The brightness and color of the foreground in
which an object is being observed will influence the observer’s interpretation of the brightness and color of the object being observed. For a visual
textile color comparison, it is optimal that the comparison be carried out
Microspectrophotometry for textile fiber color measurement
498
Normalised absorbance
420
173
534 564
100
50
S
R
M
L
0
400
Violet
Cyan
Blue
500
Green
600
Yellow
700
Red
Wavelength (nm)
9.6 Human color response.
9.7 Color matching light box.
under a defined illumination and that the physical conditions of the environment are identical and reproducible. These requirements have pushed
the textile industry to use color matching boxes that are lined in neutral
gray and use a defined illuminant. Figure 9.7 is a photograph of a color
matching booth. A second reason why color recognition is subjective is that
the eye-brain response and recognition of color can be influenced by such
factors as fatigue, mood, and appetite. Thus, the perception of color not only
varies between observers but can also vary for the same observer.
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Identification of textile fibers
Communicating about color is a subjective experience. As such, it often
results in errors in textile quality control when two people communicate
about one or more colors. Various systems for defining and measuring color
have been developed. One is the use of color atlases, such as the Munsell
Book of Color. Albert Munsell, an artist, recognized that color is a three
dimensional concept having distinct properties of hue, value, and chroma
[1]. Hue is the color name used to describe a specific wavelength of light
[1]. There are six primary hues: red, orange, yellow, green, blue, and violet.
The light and dark properties of a color are defined as their value [1]. A
color’s hue can be changed in value by adding white or black. Chroma is
the intensity of a color [1]: bright colors have a high intensity or what could
be described as deepness in color. White, black, and gray are achromatic
colors because they are colors with no brightness (that is, no intensity).
Munsell developed a three-dimensional system for communicating about
color by using printed chips arranged by hue, chroma, and value. These color
chips in the form of either a color tree or color book can be used to reduce
variance in color communication. The chance of asking two individuals to
describe the color ‘red apple’ with exact concurrence is unlikely; using
physical samples in a color ordering system with specific designations
(2YR/3) is helpful but this still can fall short in accuracy of color communication – the sample may fade over time and the perception of the color
chip can vary, depending on the observer and the lighting conditions.
A Munsell Book of Color is only one example of how color can be
defined and communicated. Mathematical methods of describing color have
been developed by the CIE: Commission Internationale de L’Eclairage
(International Commission on Illumination). One of these methods, CIE
L*a*b* (CIELAB), is the most complete color model used conventionally
to describe all the colors visible to the human eye. A full discussion of this
model, and color systems in general, are beyond the scope of this book.3
The three basic coordinates represent the lightness of the color (L*, where
an L* of zero yields black and an L* of 100 indicates white), its position
between red/magenta and green (a*, where negative values indicate green
and positive values indicate magenta), and its position between yellow and
blue (b*, negative values indicate blue and positive values indicate yellow).
The L*a*b* color model is meant to be a device-independent model as a
reference; because of this, it is important to remember that the visual representations of the entire range of colors in this model cannot be accurate.
Since the L*a*b* model is a three-dimensional model, it can only be
represented properly in a three-dimensional space.
3
For a fuller discussion, see, for example, Kuehni, R. (2001) ‘Color space and its divisions,’
Color Research & Application 26(3):209–222.
Microspectrophotometry for textile fiber color measurement
175
The environment and any color vision deficiencies of the observer affect
visual color comparison regardless of whether the sample is whole textile
items or single fibers. Microspectrophotometry is an approach to defining
and measuring color which is more accurate in fiber color comparison.
Microspectrophotometry has many advantages over visual comparison of
colored fibers. Microspectrophotometry has the ability to control the illumination and environment of the samples. The instrument can detect small
intensity and wavelength variations. In addition, the instrument has the
ability to detect differences between two colored samples outside the visible
range (approx. 400–700 nm).
9.6
Metamerism
Color matching is essential in the textile industry but even with effective
communication along the design and production process relating to a product’s color, a visual match between two colored samples does not imply that
the two samples were dyed with identical dyes. Textile materials are dyed
in batches that vary in size and dye content. Common practice in dye houses
is to ‘top up’ a dye bath when the volume has reduced [2]. This often means
adding a different set of dyes to still achieve the same original requested
color. Textile objects that appear the same colors under the same conditions
of light but have different spectral curves and can appear not to match
under a different lighting condition are called metameric pairs [3].
Metamerism is often resolved on larger textile items by using different light
sources when color comparing. However, metamerism in smaller items such
as textile fibers is difficult to resolve because a microscope, which is needed
for observation of these small fibers, has only a single illumination source.
Metameric textile fibers will therefore appear similar macroscopically and
microscopically but can be distinguished by Microspectroscopy. In a fiber
analysis scheme, the ability to recognize metamerism in fibers is imperative.
Therefore, Microspectroscopy is an essential technique to be used in fiber
comparisons, especially when comparing metameric fibers.
9.7
Applications of microspectroscopy in
fiber analysis
9.7.1 Sample preparation
Considerations for sample preparation are dependent upon the type of
microspectroscopy used; regardless of the method, a well-prepared sample
will reduce the uncertainty in spectral data. It is recommended that a
stereomicroscope be used when choosing fibers for sample preparation.
When selecting known fibers for comparison, a representative sampling
176
Identification of textile fibers
based on available sample size is necessary. The sample population chosen
should include all apparent color and shade variations. Factors such as:
laundering, extent of wear, bleaching, and thermal, biological and/or
mechanical degradation can cause artifacts in the spectral data and should
be considered. The absorption spectrum of colored fibers is dependent on
the dyestuff as well as the fiber. The absorption spectrum of natural fibers
will exhibit more intra-sample variation compared to man made fibers.
Based on the factors mentioned the number of fibers selected for examination is not easily standardized and may vary.
For transmission analysis, a representative sample of fibers is placed on
a clean glass microscope slide. The fibers should be teased apart, straightened and flattened on the slide reducing the amount of fiber twist. A drop
or two of mounting media is added to the slide and the fibers are covered
with a cover slip. The same number of questioned and known fibers should
be prepared separately, if possible, but as above. Sample preparation for
fluorescence microspectroscopy requires that the sample be mounted in a
non fluorescing media such as glycerin. Considerations should be made of
any fluorescent coating on the glass slides used with quartz slides being a
good substitute for eliminating this problem. Reflectance microspectroscopy does not require the use of mounting media.
9.7.2 Data collection
The initial steps for setting up a microspectrophotometer are identical for
transmission, reflectance and fluorescence work. The first step is to turn on
the instrument and allow it to thermally stabilize. Typically this takes at least
30 minutes but a user should follow their instrument manufacturers’ recommendations. Proper illumination and set up of the microscope should be
performed before use. The condenser aperture setting and objective lens
selection are based on the microscope and type of analysis respectively. For
transmission microspectrophotometry with single fibers, a 20× objective is
common.
Prior to data collection, an instrument calibration is performed. Instrument
calibration should be performed before daily use. Each individual laboratory may have additional quality control requirements that will also need
to be followed. Instrument calibration for a microspectrophotometer
involves checking wavelength and photometric accuracy. Wavelength accuracy is checked by using holmium oxide and didymium filter glasses. These
glasses can be certified by the manufacturer with National Institute of
Standards and Technology (NIST) traceable standards. Using NIST traceable standards is recommended for forensic analysis where instrument calibration data may be scrutinized in a court of law.
Photometric accuracy is checked using a set of neutral density filters.
Usually sets contain three calibrated filters at 10% (1.1A), 20% (0.78A),
Microspectrophotometry for textile fiber color measurement
177
and 30% (0.56A). These filters are used to check spectral linearity and are
also certified with NIST traceable standards. Additional calibration may be
required in a laboratory setting depending on the application of the microspectroscopy analysis. For example, if tristimulus values X, Y, Z and chromaticity coordinates x, y, z are critical, then using a set of transparent filters
for the blue, green and red part of the spectrum would calibrate for colorimetry work [2].
The procedure for any of the above calibrations is as follows. After the
instrument has optimized and a reference slide is in focus, a dark scan is
taken. The light source can be blocked by placing a black piece of plastic
over the field diaphragm. Unblock the light source and collect a reference
scan. Run the wavelength standards followed by the photometric standards.
Compare data to the NIST standards and/or previous instrument calibration data to determine if the instrument is functioning properly.
Once the microspectrophotometer has stabilized and the wavelength and
photometric calibrations have been verified; collecting transmission microspectroscopy data from colored fibers is a quick, non-destructive method.
Mounted fibers are placed on the stage, brought into best focus and Koehler
illumination is set. A dark scan is taken following the directions above. A
reference location, just off the sample in an area containing only slide,
mounting media and coverslip, is chosen for the reference scan. Following
the reference scan, focus on an area of interest on the sample and collect
spectra. The resultant spectrum will be an ‘absolute’ spectrum of the fiber’s
chemical components (dyes).
9.7.3 Evaluating microspectroscopy data
The collection of absorbance spectra is straightforward but the same can
not always be said for the data evaluation. When comparing questioned and
known fiber samples the evaluator must ask themselves, what determines
a significant difference? Or where is the quality assurance threshold for this
colored textile? Where should the tolerance level be set for color differences? Several approaches in evaluating spectral data can be taken in an
effort to answer these types of questions. A general suggested approach for
a fiber comparison is to begin by collecting a number of spectra of the
known fibers and evaluating variance within the known. If little or no variance is observed then the analyst should collect a number of spectra of the
questioned fibers. If variance is observed in the question fibers then the next
step would be to analyze more known fibers to evaluate the range of variation observed in the question fibers. Fiber spectra can be compared from
the absorbance and/or transmittance plots. When dealing with single colored
fibers, comparison of absorbance plots is visually easier and common in a
forensic fiber comparison. In a quality assurance textile situation, individual
questioned fiber spectra may be compared to a larger set of target (known)
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Identification of textile fibers
spectra. Another approach would be to calculate the standard deviation of
the sample sets. This type of mathematical function is likely to be part of
the software package that accompanies the microspectrophotometer.
In any spectral comparison, the overall shape of the spectral curve is a
good place to start. A difference in the overall shape between spectra of
what are presumed to be the absorbance patterns of ‘identically’ dyed fibers
most often can be considered a significant difference. This is especially true
when the absorbance patterns are completely different.
An evaluator of such spectra need not be an expert in the spectroscopy
field to understand a basic pattern matching difference. The slopes of the
absorbance curves are another comparison point as well as any shoulders
or points of inflection present/absent in the spectra. Differences in the slope
of an absorbance curve, for example, may or may not be considered a difference or may be calculated as falling within the set tolerance for that fiber
and that dye. Artifacts in the data caused by such factors as thermal, chemical, biological and mechanical degradation of the fibers, will weigh into a
color comparison and evaluation. Relative intensities of absorbance spectra
can vary but this variance is not always an indication of a color difference.
The uneven dye uptake typical of cotton fibers is an excellent example of
this. A good knowledge of textile fibers and dyeing will assist the evaluator
in the interpretation of microspectroscopy data.
Determination of the ‘threshold,’ ‘tolerance,’ and/or ‘significant difference’ between two colored textiles analyzed by microspectrophotometry is
fluid and based on the application. In forensic science laboratories, textile
fibers are often evaluated using a variety of instruments such as polarized
light microscopy, Fourier transform infrared spectrometry, fluorescence
microscopy and microspectrophotometry, to determine any differences
between known and questioned fibers.
9.8
Limitations, strengths, and future trends
For a meaningful comparison/analysis of fiber color there is a need for a
careful examination of the sampling population. Factors to consider when
choosing a fiber sampling scheme were discussed in the sample preparation
section of this text. One problem mentioned was the influence the chemical
make-up of colored fibers can have on the absorption spectrum. Spectra
can vary between colored fibers from the same source. This influence seldom
exhibits itself in man-made fibers such as polyesters and polyamides. These
types of fibers are more chemically homogeneous, resulting in limited intrasample variation reflected in the spectra. However natural fibers, such as
cotton and wool, are not chemically homogeneous but instead are composed of many different chemical components, many of which are unevenly
distributed throughout the fiber. The result of adding a dye to a fiber with
Microspectrophotometry for textile fiber color measurement
179
a chemically unbalanced make-up is color variation. This type of intrasample variation in natural fibers is quite abundant, making forensic fiber
comparisons difficult. When color comparing natural fibers, a complete and
careful sampling of questioned and known can help in reducing the inhomogeneity of the resultant spectra.
How color is applied and absorbed along the length of the fiber is an
important comparison characteristic in both natural and man-made fibers
but more so in man-made fibers. Often several dyes are used to give a fabric
a desired color. Color can be applied to the surface of a fabric; a common
procedure for printed fabrics. Yarns can be dyed and then spun to make a
fabric. Individual fibers can be colored prior to being spun into yarns.
Variations such as these can result in dye defects and dye uptake variability.
Dye uptake variability was discussed above in relation to natural fibers but
may also be found in man-made textiles that are dyed after being constructed especially in tight woven fabrics.
The cross-sectional shape of the fibers can also influence a color assessment especially during a quality control assessment of a particular fabric
dye using a microspectrophotometer. For example, consider two sample sets
of fibers, identically dyed and analyzed by microspectrophotometry. The
first fiber sample set are round blue (nylon) fibers while the second fiber
sample set turn out to be tri-lobal blue (nylon) fibers. The absorbance curves
for each fiber set analyzed could differ depending on the QC definition of
‘difference.’ In this example any possible variations in the absorbance
curves between the two fiber sets would most likely be caused by the difference in thickness of the trilobal fiber versus the round fiber. However,
the result from this situation could cause the analyst, unknowing of the fact
identical dyes were used, confusion in thinking that a different blue dye was
used during the second dye batch for the set of nylon fibers or that a
problem exists with that particular dye. Even processing techniques such as
draw ratio can affect the resulting dyed color.4
A fiber’s thickness can limit the use of microspectrophotometry. When
analyzing smaller diameter fibers, such as micro fibers, by microspectrophotometry, the light focused on these fibers has less surface area to interact
with. This causes much of the light to scatter resulting in weak spectral
curves. Thus, the absorbance data for colored micro fibers will not provide
the same information about the chemical dye(s) typical with larger diameter colored fibers.
Despite some limitations in the application of microspectroscopy to fiber
color, this technique has many benefits when compared to visual color
4
For example, see Maillo, J. et al. (2008) ‘The relationship between dyeing behavior and fine
structure in polyamide 6 fibers – the influence of drawing ratio,’ American Association of
Textile Chemists and Colorists Review, March: 39–43.
180
Identification of textile fibers
assessment. The instrumental measurements are absolute, objective, and
repeatable. Color is measured at each wavelength along an extended range
versus human vision which ‘perceives’ color all at once and in the visible
region only. Measurement methods can be standardized with a microspectrophotometer. In addition, the results from this technique are quantitative
which allows for statistical treatment of the data. The resulting data is also
linear data which is simple to visually assess. The spectrophotometers currently being used for color measurement range from bench top to handheld models. However, these instruments are limited to larger sample
sizes. Loose fibers are difficult to measure with repeatability and there is
always the threat of the fibers falling in to the sampling port and interfering
with measurement [4]. Using a microspectrophotometer would relieve this
difficulty.
As mentioned early in this chapter, color and color design is an emotional
choice. Modern consumers are more aware of how products look and equate
good visual design with product quality. In turn higher aesthetic standards
are set driving competition among industry to produce the ‘best quality
product’. Companies have to consider the potential devastation a color
difference can make by the time it is repeated in a product. An example of
this often presents itself in the carpeting industry. The number one consumer complaint in the carpeting industry is color inconsistency [5].
Consumer complaints such as this can cause high monetary loss for the
carpeting manufacturer. Utilizing microspectrophotometry to assist in a
problem such as dye absorbance in carpet fibers could only be beneficial.
Research in this arena is beginning to emerge in the textile industry. In
the forensic world, microspectrophotometry is a critical aspect to a color
analysis and can add enormous significance to a fiber comparison.
9.9
References
1. http://daphne.palomar.edu/design/color.html.
2. Adolf F and Dunlop J (1999), ‘Microspectrophotometry/Colour Measurement’,
in Robertson J and Grieve M, Forensic Examination of Fibres, London, Taylor
& Francis, 251–289.
3. Eyring M (2002), ‘Visible Microscopical Spectrophotometry In The Forensic
Sciences,’ in Saferstein R, Forensic Science Handbook Volume 1, New Jersey,
Prentice Hall, 322–387.
4. http://www.datacolor.com.
5. Connelly R L (1997), Colorant formation for the textile industry. In: Color
Technology in the Textile Industry. American Association of Textile Chemists and
Colorists, Research Triangle Park, North Carolina, pp. 91–96.
10
Alternative and specialised textile fibre
identification tests
P H GREAVES, Microtex, UK
Abstract: The less frequently used methods of fibre identification are
summarised, from elementary burning, staining and solubility tests, to
density, melting point and refractive index determinations. More
advanced instrumental techniques are explained: differential scanning
calorimetry (DSC) and thermal analysis, thermogravimetric analysis
(TGA), pyrolysis and briefly transmission electron microscopy (TEM).
Scanning electron microscopy (SEM) is considered in some detail.
Principles, specimen preparation, factors affecting the image and
information obtained are considered. Advanced scanning electron
microscopy techniques are noted, and the optimal instrument choice
between the scanning electron and light microscopes compared.
The principles of quantitative analyses of fibre mixtures are
introduced, for blends of both chemically equivalent and chemically
different fibres.
Key words: fibre identification, alternative identification techniques,
scanning electron microscopy (SEM) of textiles, specimen preparation,
quantitative analysis of fibre mixtures.
10.1
Introduction
The two most frequently used methods of fibre identification are arguably
those of light microscopy and infrared spectroscopy – or a combination of
the two. There are, however, other techniques available, and these range
from simple physical and chemical tests, to complex instrumental methods.
The purpose of the chapter is to provide an outline of the principles of
some of the more important of these alternative methods of identification,
and then to describe in more detail the theory and application of one of
most vital means of obtaining detailed images of fibres – that of scanning
electron microscopy.
10.2
Alternative methods of fibre identification
10.2.1 Burning tests
Originally considered to be the first step towards classifying a fibre type,
the increased use of fibre blends and fire retardant fibres has led to the
181
182
Identification of textile fibers
burning test being of only minor value today. A further disadvantage is
that the test is destructive and requires a considerable amount of material
to be available. The burning test relies on the fact that the chemical composition of a fibre will largely determine its behaviour when exposed to a
flame. The nature of contraction, smell, combustion and ash of a fibre will
normally provide an indication of its generic type – classic examples being
animal fibres, cellulosic fibres, acetates and other thermoplastic synthetics.
Burning tests are still used in industry, where, for example, the composition
of an unlabelled bale of raw material may be in question. One of the most
distinctive odours produced by burning a sample of fibre is that of nylon,
where the smell is traditionally described as being like that of ‘burning
celery’.
It should be noted that burning tests remain subjective, and are essentially non-reproducible. There is the added hazard that toxic fumes may be
generated in such tests (e.g., with acrylic fibres).
10.2.2 Staining tests
The different chemical composition and reactivity of different fibre types
causes them to have particular affinities for specific dye or stain types. This
has led to the development of mixtures of dyes which are specifically prepared as fibre identification stains, and which produce known colours for
individual fibre types.
A further refinement is the compilation of stain mixtures for distinguishing variants of the same fibre type, e.g. acrylics, of which Meldrum’s Stain
(Meldrum, 1961) is an example.
The chief limitation of staining tests is that they can only be used on
undyed material, and chemical damage may also affect the nature of staining – though in certain cases this can be used as a diagnostic feature when
attempting to classify types of fibre damage (Park and Shore, 1982).
Nevertheless staining tests can be very useful – particularly if combined
with light microscopical examination.
10.2.3 Chemical solubility
An indication of the chemical composition of a fibre can be readily gained
by establishing in which solvent it dissolves. This can be achieved by immersing fibres in a prescribed series of solvents, usually of ascending chemical
reactivity, until the first is found in which it dissolves. Confirmatory tests
may then be carried out using solvents of known behaviour for the initially
identified fibre type. A fresh specimen must be used for each solubility
determination, to avoid any risk of cumulative damage effects.
Alternative and specialised textile fibre identification tests
183
Table 10.1 Examples of some commonly applied solvents in solubility tests
Fibre type
Soluble in
Nylon
Acrylic
80% Formic acid
Boiling dimethyl
formamide
Viscose/Modal/Lyocell Cuprammonium hydroxide
Polyester
Dichloroacetic acid
Polyolefins
Boiling xylene
Insoluble in
Xylene
80% Formic acid
Xylene, 80% formic acid
Xylene, 80% formic acid,
dimethyl formamide
Cuprammonium hydroxide,
80% formic acid
Solubility tests may be carried out in small tubes on a macro scale, or
more usefully while the specimen is under observation on the stage of a
microscope (ideally kept for just such tests). There are two advantages of
the microscopical method:
•
as the fibres are under observation during the test, mixtures of fibres
may be identified and
• fractions of millilitres of solvent are used, thus minimising safety risks.
Tables providing examples of solvents for particular fibre types, and supporting confirmatory solvent tests are available (The Textile Institute, 1985;
Greaves and Saville, 1995).
Examples of some commonly applied solvents in solubility tests are
shown in Table 10.1.
10.2.4 Melting point determination
The thermoplastic nature of many chemical (‘synthetic’) fibres causes them
to soften and shrink on the application of heat. The temperature at which
these changes occur can provide a useful diagnostic feature of the fibre type.
Even where true melting does not take place, the test can serve to eliminate
certain fibre types.
Instruments specifically designed for determining the melting point of
fibres or plastics are available, or the test may be carried out under the light
microscope using a specially adapted hot stage. As with the solubility tests
this latter method permits positive recognition of different component
fibres in a blend. Most fibre science books will have tables of melting point
values. Some examples are shown in Table 10.2.
Note that the measured value of a fibre’s melting point may vary according to the apparatus and conditions used, and the dimensions (e.g. denier)
of the fibre. Calibration tests should be conducted.
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Identification of textile fibers
Table 10.2 Melting points of some manufactured fibres
Fibre type
Melting point (°C)
Acrylic
Does not melt but decomposes with
discolouration
Does not melt
250–255
290–300
185–210
Does not melt but decomposes with
discolouration
210–216
252–260
250–260
133
160–165
Aramid
Cellulose diacetate
Cellulose triacetate
Chlorofibre
Modacrylic
Nylon 6
Nylon 6.6
Polyester
Polyethylene
Polypropylene
10.2.5 Refractive index measurements
The molecular structure of textile fibres makes them optically anisotropic,
i.e. they have two refractive indices – one for light polarised along the fibre’s
axis and one for light polarised across it. The fibres are thus said to be
birefringent.
For colourless (undyed) fibres, the refractive index for each direction may
be found by simply mounting the fibre in different liquids of known refractive index until the two are found which causes the fibre to become invisible
for light polarised along the fibre axis, and for light polarised across it (normally referred to as ‘parallel’ and ‘perpendicular’). The difference between
these values is the birefringence of the fibre, and constitutes a distinct identification feature.
While the most accurate method of determining the refractive index of
a fibre is to use monochromatic light and an interference microscope, it is
quite possible to achieve satisfactory results with white light and a rotatable
polarising filter attached to an ordinary light microscope.
The accuracy with which polarised light microscopy may be used to
identify fibres has been documented by Carroll (1992), while detailed
descriptions of identification by FTIR microscopy may be found in Tungol
et al. (1990).
10.2.6 Density measurements
The polymers of which fibres are made have different densities, and this
can be used as a diagnostic test. Values may range from below 1.0 g/cm3 for
polyolefins to over 2.5 g/cm3 for glass fibres.
Alternative and specialised textile fibre identification tests
185
In its simplest form, a density gradient column is made which consists of
a vessel of liquid having a near linear density increase from the top to the
bottom of the column. Ethanol/water mixtures, or varying strengths of salt
solutions are examples of possible liquids.
It is vital that any medium does not cause sorption or swelling to the fibre
under test as this will result in a change of density.
Fibres of known density are placed in the column and allowed to reach
their stable positions in the liquid, i.e. where the density of the fibre matches
that of the liquid. The test fibre is introduced into the system and the
point at which the sample stops sinking is taken as the point of equivalent
density.
Variants of the density gradient technique are titrimetric methods,
where the volume of liquid adjusting the density is metered through a
burette and the ‘end point’ of no movement of the sample is recorded,
centrifugally accelerated determinations and kinetic (rate of fall)
measurements.
With all density based determinations, accurate control of temperature
is critical.
10.2.7 Differential scanning calorimetry and
differential thermal analysis
Chemical and physical changes induced into a fibre polymer by the application of heat result in changes in enthalpy and entropy. Differential scanning
calorimetry (DSC) monitors the difference in the relative heat flow into the
sample compared with that into a blank (inert) system as the basic heat
source experienced by both is linearly raised.
Changes in the flow of heat will occur at those temperatures at which the
sample undergoes either a physical or chemical change. DSC is primarily a
quantitative tool in the form of a calorimeter. This permits the size of the
enthalpy and entropy changes to be positively identified. The temperatures
at which they arise are less reliably pinpointed.
In contrast to DSC, differential thermal analysis (DTA) monitors the
temperature differences between the sample and the inert system as the
enthalpy and entropy changes that accompany physical and chemical transformations of matter take place as both are linearly heated. This provides
accurate values for the temperature of such induced changes; however, only
limited quantitative information is available.
Examples of heat-induced changes in fibrous polymers are second order
or glass transition (Tg), desorption of moisture, crystallisation, fusion, chemical reactions and irreversible decomposition processes. A recently obtained
DSC curve for a particular type of polyester is shown in Fig. 10.1.
186
Identification of textile fibers
Sample: Weak green
Size: 6.3250 mg
DSC
Method: Ramp
Comment: N2 Purge 200 cm3 / minute
–0.4
168.77 °C
Flie: C:\TA\Data\DSC\Data.052
Operator: PHIL G
Run Date: 5-Aug-02 12:28
Heat flow (W/g)
–0.6
–0.8
–1.0
–1.2
252.86 °C
–1.4
0
Exo Up
50
100
150
200
250
300
350
Temperature (°C) Universal V2.6D TA Instruments
10.1 DSC curve for printed polyester fibre from upholstery fabric.
10.2.8 Thermogravimetric analysis
Thermogravimetry involves the monitoring of the sample mass on a
thermal balance, as the temperature is linearly raised. The sample mass–
temperature curves enable the occurrence of all thermally induced mass
changes to be quantitatively recorded. These changes may be due to drying,
chemical reaction, chemical degradation or volatilisation. The degree of
mass change and the temperature of occurrence can be positively identified
and are characteristic of the heated sample. This method should allow
all types of fibre – whether or not thermoplastic – to be definitively
characterised.
10.2.9 Pyrolysis
The flash pyrolysis of a fibre in an inert atmosphere, either in a micro-oven
or by electrically heating a thin coil of wire on which the sample is mounted,
produces a breakdown of volatile components that may be analysed by gas
chromatography. If the conditions of pyrolysis are accurately controlled
then the volatile products of the reaction may be reproducibly obtained
and a characteristic pyrolysis chromatogram or ‘pyrogram’ may be used as
a fingerprint to aid in the identification of the fibre sample.
Key areas of importance in pyrolysis gas chromatography are the rate or
speed of the pyrolysis, the carrier gas which serves as the inert atmosphere
Alternative and specialised textile fibre identification tests
187
to transfer the volatile pyrolysis products on to the chromatography column
and the medium of the column allowing separation of the components.
The theory and practice of gas chromatography has been described in
detail (Harris and Habgood, 1966).
10.2.10 Transmission electron microscopy (TEM)
Although not generally used for generic identification of fibre type, the
vastly superior resolution of the TEM compared with that of the light
microscope can be employed to provide information about the fine interior
structure of a material examined in thin section.
The instrument may be considered loosely analogous to the projection
light microscope, in that the illuminating and image forming radiation is
passed though the material via condensing lenses. The emergent beam,
having interacted with the specimen is then enlarged by magnifying lenses
and projected onto a screen or means of image recording. By using electrons instead light, the magnification and resolution of the instrument can
be increased many thousands of times over that of the light microscope.
Transmission electron microscopy is able to resolve distances comparable
with inter-atomic spacings – though to do so the specimen must necessarily
be very thin – less than 100 nm.
Apart from the great expense of the instrument itself and the very specialised and precisely controlled specimen preparation techniques which
are required to use the TEM (Chescoe and Goodhew, 1984), a disadvantage
when compared with the light microscope is the need to operate under high
vacuum. It is also normally only possible to study a small part of a thin layer
of the material in question.
Fibre investigations using the TEM have traditionally concentrated
on resolving the structure of crystalline regions in the material by
studying their electron diffraction patterns (Johnson, 1965; Dobb, 1970)
and it is not as such used for straightforward fibre identification (Sikorski,
1975).
An exception to this is in distinguishing between different asbestos fibre
types (Park and Shore, 1982), where the extreme fineness of the fibrils,
and their specific diffraction patterns are suitable for analysis in the TEM.
10.3
Scanning electron microscopy
If the TEM can be considered roughly analogous to the projection light
microscope, the closest approximation to the scanning electron microscope
(SEM) is usually said to be the reflected light stereo microscope. The SEM
is the instrument whose images of surfaces are so frequently seen as dramatic ‘3-D’ features of stunning quality in the semi-technical press, ‘serious’
188
Identification of textile fibers
television news and scientific advertising features. In actuality, the closest
optical analogue to the SEM is probably the confocal laser scanning reflected
light microscope – though limited numbers of textile scientists are wholly
familiar with the technology of this instrument.
10.3.1 Principle of operation of the SEM
In order to understand how the SEM forms images of such magnification
and clarity, it is first necessary to appreciate some fundamental concepts.
The limiting factor of resolving power – wavelength
The maximum useful magnification attainable with a light microscope is
governed firstly by the numerical aperture (NA) of the objective lens. The
second and most restrictive factor controlling resolution is that of the wavelength of the imaging radiation, i.e. light. For a shorter wavelength, the
distance resolvable between two points becomes smaller. For light this
wavelength is generally between the range of 400–700 nm.
It is generally accepted that these constraints limit the maximum useful
magnification of the light microscope to around ×1000–×1200. This translates to an approximate distance of around 0.3 microns (1 micron = 10−6
metre). Further magnification may be produced, but contains no additional
detail and is said to be ‘empty’ magnification.
Electrons are small, negatively charged particles and whose wavelengths
are orders of magnitude shorter than those of light.
The performance of the light microscope may be increased vastly by
using electrons as the imaging radiation. The wavelength of a free electron
is given by De Broglie’s equation:
λ=
h
mv
where h = Planck’s constant
m = mass
v = velocity
For a microscope operating at (say) 10 000 volts (10 kV), this will produce an
electron wavelength in the order of 12 pm – several thousand times shorter
than that of light. The NA of the system can thus be relatively small.
Interactions of electrons with matter
An electron incident on a specimen surface may undergo a number of
reactions.
Alternative and specialised textile fibre identification tests
189
If the material is very thin, the electron may pass through unaffected.
This happens to a large degree in the TEM and may necessitate methods
of contrast enhancement.
The electron may displace another electron from an outer orbital of an
atom, which is emitted by the specimen. These are called secondary electrons and have a low energy compared with the incident electron beam.
Their emission is highly dependent on surface topography.
If the electron is elastically scattered by the atomic nucleus, a small
number may be deflected back by a high angle. These are called backscattered primaries.
The majority of elastically scattered electrons however will proceed to
spread out and strike further atoms within the specimen without losing
much energy in the process. Some of these electrons will find their way back
to the surface of the material where they may be collected. The emission
of these backscattered electrons is highly dependent on atomic number.
When the secondary electron is displaced from the atom, the resulting
energy change and replacement transfer from a higher energy orbital produces a characteristic X-ray photon. These are of specific wavelength and
energy to the atom from which they have been generated.
The incident electron beam may produce light, and this is used to form
the cathodoluminescence image.
A sometimes troublesome effect of the electron beam is the generation
of heat, which can lead to the melting or decomposition of thermoplastic
or delicate specimens. The effect can be controlled to a large extent by
coating (see page 193) or by specially cooled stages or specimen holders.
Other interactions may also occur, such as transmission of electrons, atom
displacement, diffraction and current generation. For the analysis of fibres,
however, it is the secondary electrons which are most useful for topographical image formation. The backscattered electrons and characteristic X-rays
are also important. Typical electron–specimen interactions are shown in
Fig. 10.2.
The mechanics of SEM operation
The key to the functioning of the SEM is synchronisation.
In essence, a fine electron probe, of typically 10–20 kV, is scanned across
the surface of the specimen in a raster fashion:
I___________________
___________________I
I___________________
___________________I
I___________________
190
Identification of textile fibers
Electron
beam
Secondary
electrons
Backscattered
primaries
X-Rays
} 10 nm Escape depth
for secondaries
} 1 μm
10.2 Schematic diagram of some of the possible interactions of
electrons with a specimen (e.g., a fibre).
Electrons may be generated from a heated tungsten filament, a lanthanum
hexaboride tip, or a field emission gun. The signal generated by this is used
to modulate the intensity of a simultaneously scanning beam in either a
cathode ray tube or on an image recording medium. Thus when the probe
encounters a feature which produces a strong signal (for example, a ‘peak’
generating an increase in secondary electron emission), the intensity of the
corresponding CRT spot is increased.
Magnification in the SEM is achieved by making the scanned area of the
specimen small in relation to the scanned area of the CRT or image recording medium.
The scanning of the two synchronised beams may thus be likened to a
pantograph of they type used to produce enlarged drawings. A 1 mm2 area
of the specimen scanned and synchronised with the modulated beam scanning a 10 cm × 10 cm CRT screen will produce a magnification of ×10 000
for example.
Unless there are special circumstances, the whole system must be operated under high vacuum, to avoid deflection of the beam, contamination
effects and loss of both incident and generated signal strength.
10.3.2 Factors affecting the image
The most commonly used signal of the SEM in fibre work is the secondary
electron, or emissive mode. The generation of secondary electrons is highly
dependent on surface topography, and a spike or thin edge will emit con-
Alternative and specialised textile fibre identification tests
191
Electron
probe
Detector
Specimen
10.3 Effect of topography on secondary electron image.
siderably more electrons than a flat area – which will in turn emit more
than a trough or hollow (Fig. 10.3). These lower energy secondaries, primarily from the thin surface layer of the specimen are collected by a detector to which is applied a slight bias to attract the electrons.
The low penetrating power of secondary electrons means that their area
of origin is comparable to that of the diameter of the electron probe itself,
and this provides the highest resolution images of the SEM.
Backscattered electrons are those elastically scattered by the atomic
nucleus, and thus are highly dependent on the atomic number of the
element(s) from which the specimen is composed. These high energy electrons are not easily deflected by charging effects (see Section 10.4), and
their high energy means they can originate from deeper in the specimen
where the interaction volume is larger.
For uncoated specimens it is also possible to use the backscattered electrons to obtain an image based on the atomic number of the specimen –
where the contrast is due to different elements with different atomic weights.
Purpose-made backscattered detectors collect from a much greater solid
angle than the normal collector and thus a better signal to noise ratio is
obtained. Because the number of backscattered electrons increases with the
accelerating voltage of the probe, it is often advantageous to use a higher
kilovoltage than when imaging with secondary electrons.
Characteristic X-rays are produced as a result of energy transfers between
electron orbitals during the generation of secondary electrons. These X-rays
are characteristic of the element from which they have originated, and their
energies and/or wavelengths can be used to characterise and quantify the
elements present.
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Identification of textile fibers
10.3.3 Specimen preparation
Cleanliness
Unless a fibre is being examined specifically to determine the presence or
nature of a surface residue, it is important that the specimen placed in the
SEM chamber should be as clean as possible. There are several reasons for
this, but the most important is to avoid contamination of the column with
any extraneous volatile organic matter that may be degraded by the electron beam, and to ensure that the surface studied is that of interest and not
some additional residue which has been acquired inadvertently (e.g., from
handling). Samples whose compositions are required to be analysed by
backscattered electron or X-ray emissions may be polished, so that surface
roughness does not interfere with the elemental information generated.
Fixation (chemical, cryofixation)
The electron beam, and some preparatory processes may generate heat in
insulating specimens. Fibres of low melting point (e.g., polyolefins) or some
natural (biological) fibres are particularly prone to damage by this means.
Such materials may be stabilised by chemical treatment, where the more
vulnerable groups are modified or replaced by molecules of similar size but
greater thermal stability. It is vital, however, that any such procedures do
not change the inherent nature of the material or fibre to be studied.
An alternative to chemical fixation is thermal fixation, i.e. to stabilise the
specimen by rapid freezing, so that its structure remains unchanged. Care
must be taken to ensure that cooling conditions do not allow the formation
of ice crystals in the material, and that the specimen is protected against
damage by the beam in the SEM itself. This may be achieved by using a
nitrogen cooled stage, and specimens viewed in such ways may often be left
uncoated.
Mounting
Whereas in the light microscope, the choice of mounting medium and cover
glass thickness are the two major concerns, specimen mounting in the SEM
offers many more options. In general, the fibre or textile sample is fixed to
a small metal support called a stub. The sample must be securely attached
to this, so that it remains fixed during manipulations in the chamber.
There are several media which may be used to hold the specimen to its
stub, and these include:
•
Double-sided tape (metal based). This is perhaps the most easily used
method, though care must be taken to ensure that no air bubbles are
Alternative and specialised textile fibre identification tests
193
trapped in mounting. The tape may give an unwanted background X-ray
signal, or may crack and shrink during examination.
• Carbon adhesive. This has the advantage of being a poor electron
emitter, so will not hide the specimen, but can migrate and be
absorbed.
• Silver paint (‘Dag’). Possibly the most widely used medium, silver paint
has excellent conductive properties, dries rapidly and provides a good,
bright background to the SEM image. The paint is relatively expensive,
however, particularly if frequent thinning of the suspension is needed.
Coating
The electron beam which is used to obtain the SEM image is composed of
negatively charged particles. Unless the specimen to be examined is a conductor (e.g., a metal), it is necessary to render the surface layer of the fibre
specimen conductive. This is applied in the form of a coating.
There are two reasons why insulating materials must be coated prior to
SEM examination:
(1) To avoid build-up of negative charge which may be destructive (beam
damage) and which may sporadically discharge and deflect the incident beam. The negative charge can also prevent the collection of
generated secondary electrons. These unwanted phenomena are collectively known as ‘charging effects’.
(2) To provide a higher secondary electron-emitting surface than the base
specimen material, so that high energy beams and long scan times
are not needed to obtain a clear SE signal of the specimen’s surface
topography.
The most usual coating materials are metals (e.g., gold, or goldpalladium) or carbon. The latter has the advantage of permitting X-ray
analysis. These coatings are applied by sputter, evaporative or thermal
deposition means and their control is critical. The balance must be struck
between potential damage effects (e.g. thermal – particularly in sputter
coaters), rate and thickness of deposition and uniformity of coating
(Chapman, 1986).
A specimen coated for routing morphological studies may have a coating
thickness of 15–30 nm thickness (Postek, 1980), though for finer detail
the coating would need to be far thinner. An optimum coating should be
continuous from the specimen to the stub, and of uniform thickness.
As an alternative to metal or carbon deposition, it is possible to use proprietary sprays which are applied to the specimen to provide a conductive
coating. Similar in operation to ‘anti-static’ products, these can be useful
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Identification of textile fibers
where the specimen is so delicate that it may not withstand the thermal,
mechanical or vacuum condition of the coating unit. Such sprays will not
be as effective as metal or carbon, but if used with a low KeV can provide
a useful substitute in certain circumstances. Clearly, if the layer of interest
lies just beneath the specimen surface, and X-ray analysis is required,
coating with metal or any extraneous matter is futile.
Many further examples of the principles of correct specimen preparation
for the SEM may be derived from texts aimed primarily at the TEM, as
their methods of operation and crucial parameters are broadly similar
(Goodhew, 1973).
10.3.4 Examination
Once the specimen has been prepared in a satisfactory manner, and any
special conditions accounted for (e.g., melting point, cleanliness, conductivity, information required, etc.), then it can be placed in the chamber of the
instrument, vacuumed down and stabilised for examination.
Among factors to consider when attempting to obtain the optimum
image are:
Accelerating voltage: Interactions of signal strength, penetration of probe,
noise, beam damage, spot size.
Working distance: Objective lens to specimen. Depth of field vs. resolving
power.
Spot size: Resolution vs. signal to noise ratio. Charging.
Tilt (w.r.t collector): Signal strength vs. geometry.
Signal collection: SE topography vs. BE atomic number. Charging.
Vacuum: Dehydration, charging, preservation of hydrated areas.
Final aperture: Resolution, signal strength, depth of field.
Image composition: Ratio of SE to BSE signal. Filtering. Topography vs.
atomic number. Contrast.
Scan time: Signal strength and resolution vs. charging and beam damage.
Corrections for electron lens aberrations: Chromatic, spherical, astigmatism.
This list is intended merely to cite examples of parameters which must be
adjusted in order to obtain the best quality image for the particular specimen being examined in the SEM. There are many more factors and possible
interactions which may be involved, but which are outside the scope of this
chapter. It should be apparent, however, that the operation of the SEM is
somewhat more complex than for the operation of the equivalent light
microscope in Brightfield mode.
Alternative and specialised textile fibre identification tests
195
Conversely, many of the above SEM considerations are now automated,
while the mathematical wave theories, depictions and consequent practical
application techniques of (e.g.) multiaxial indicatrix figures, Nomarski differential interference contrast image manipulation and metrology in the
microscopy of reflected and transmitted polarised light and multiple beam
interferometry remain somewhat of a specialised art.
10.4
Further techniques
10.4.1 High resolution SEM
While the normal range of resolution for an SEM is within the range of
around 5–10 nm, the development of field emission guns and high excitation
probe-forming ‘immersion’ objective lenses has allowed the development
of systems capable of achieving spatial resolutions of less than 1 nm.
The small source size and high brightness of the field emission gun is
able to produce a probe spot of sub-nanometre size, which with the low
energy spread gives less chromatic aberration than a Lab6 source. This
probe contains sufficient current for secondary electron imaging, while
the immersion type objective lens is also of low aberration and ensures the
smallest probe diameter.
Additional benefits of this type of lens are the screening out of external
magnetic fields and the elimination of relative vibrational motion between
the sample and the optic axis.
For high resolution it is, of course, necessary that the pixel size of the
image is smaller than the area of emitted secondary electrons which are
generated within a few nanometres of the point where the probe enters the
specimen (Joy, 1989).
Modern tungsten filament gun SEMS will have resolutions nearer
3.0 nm at 30 kV if operating under very high vacuum (Hitachi Corporation,
2006).
10.4.2 Environmental SEM (ESEM)
Although traditionally the SEM has operated only under high vacuum,
which has meant dehydration of water-containing specimens, and the possible introduction of artefacts, recent years have seen the introduction of
variable pressure or environmental SEMs. Essentially, the electron column
is operated under vacuum as normal, until just above the specimen. Here
in a small chamber with aperture to permit access of the beam, it is possible
to have a gaseous or water vapour medium.
The signals collected include interactions of the beam with the gas – in
particular ionising collisions of emitted secondary electrons to both amplify
196
Identification of textile fibers
the signal and provide a source of dynamic charge control (Stokes, 2006).
In general it is possible to image specimens in conditions much closer to
their natural environments – and even to see globules of water or oil on
some surfaces. The images from these instruments working with pressure
will normally never be as of ultimately high resolution as those obtained
from a system under high vacuum, but the environmental SEM can be an
extremely useful instrument for specimens whose features may be changed
by the conditions imposed by vacuum.
Typical gas pressures for the ESEM are 102–104. This is lower than atmospheric pressure, but sufficiently high to permit the observation of live insect
specimens (Ford and Stokes, 2006). A review of the principles of ESEM has
been provided (Thiel, 2004).
10.4.3 X-ray microanalysis
As previously explained, one of the results of the interaction of an electron
beam with a specimen is the generation of X-rays. A proportion of these
X-ray have wavelengths and energies that are characteristic of the element
from which they have been emitted. It is thus possible to use the SEM to
carry out elemental analysis of specimens.
There are two different types of X-ray microanalysis systems – energy
dispersive and wavelength dispersive. The essential features of each may be
broadly listed as below:
Energy dispersive (EDX)
•
•
•
•
•
•
rapid (qualitative analysis)
display of entire spectrum in same format
high collection efficiency
low sensitivity to alignment or topographical effects
low cost and relatively compact
simultaneous analysis of full X-ray spectrum above a given Z value.
Wavelength dispersive
• high resolution (element separation)
• analysis can be quantitative
• wide range of elements analysed (Be to U)
• high sensitivity
• the energy dispersive systems are perhaps the most commonly used in
the SEM.
An EDX spectrum of nylon in an upholstery fabric is shown in
Fig. 10.4.
The count rate of EDX (or EDS) analysis is an important factor. If the
number of X-rays reaching the detector is low (e.g., in thousands per unit
Alternative and specialised textile fibre identification tests
197
cps
80 C
60
40
O
20
F
0
0
P
2
Ti
4
6
8
Energy (keV)
10.4 EDX spectrum of nylon fibres in treated upholstery fabric.
time) a long period of analysis is required. This may result in damage to the
specimen and an increased level of background counts (noise).
Conversely, if the count rate is high (perhaps hundreds of times the frequency of a low count rate), than as a signal is processed by the analogue
to digital converter, a discriminator rejects all other pulses and these are
not included in the spectrum sent to the multi channel analyser. At count
rates of around 2000 per second the converter processes pulses at an
acceptable rate and few are rejected.
At higher rates more pulses will be rejected and this leads to an unacceptably large proportion of ‘dead time’ – when the processor is closed to
additional signals. A high amount of dead time should be avoided as it leads
to a bias in the analysis. 5% or below is generally considered a satisfactory
amount of dead time.
10.5
Benefits of scanning electron microscopy
compared to a light microscope
The main purpose of the SEM is to provide high resolution and high depth
of field images of the surfaces of specimens. It can thus operate at much
higher useful magnifications than the light microscope, but will not normally
show interior detail.
One well-known area where the characteristics of the SEM are put to
good use is in the identification of animal fibres. Here, the great depth of
field and higher magnification can permit the measurement of scale edge
thickness. This feature is used in the distinction between cashmere and
wool, and in the quantitative analysis of blends of these and other animal
fibres (Kusch and Arns, 1986; Wortmann and Arns, 1986; Weidemann
et al., 1987). It must be remembered, however, that as interior features are
198
Identification of textile fibers
not shown, the instrument must be supplemented by light microscopy
where, for example, the nature of the ladder medullae of fur fibres are to
be studied.
This proviso is acknowledged in the new ISO Standard 17751: Quantitative
Analysis of Cashmere, Wool and Fibre Mixtures (International Organisation
for Standardisation, 2005).
Comparative micrographs using the LM to illustrate the interior features
and the SEM to illustrate surfaces of the same samples of fibre have been
produced, and form valuable reference works (Sich, 1989; Greaves and
Saville, 1995). When the fibre examination is combined with elemental
analysis by X-ray microanalysis, the SEM becomes a very powerful tool
indeed. It can, for example, show the nature and distribution of an anti-soil
fluorocarbon finish on a single fibre or the elemental distribution of contaminants on a sample of fabric.
Further areas where the SEM can provide information not easily accessible with light microscopy are in the examination of surface coatings, the
distribution of shrink resist polymers (Greaves, 1991) and detailed studies
of fibre damage (McCarthy and Greaves, 1988).
One fact that must be noted, however, is that the SEM will not show
colour (though it is quite possible to add ‘pseudo colour’ to images
electronically).
The ideal system would be to use light and electron microscopies as complementary techniques – each revealing information not readily accessible
to the other, but which together provide the most detailed picture (Langley
and Kennedy, 1981). An example of the usefulness of the SEM’s high resolving power and depth of field is shown in Fig. 10.5. This is the surface of a wool
Mag = 10.00 K X
2 μm*
EHT = 3.00 kV
WD = 5 mm
Signal A = Inlens
Photo No. = 213
Date: 10 May 2006
Time: 16:45:13
10.5 Secondary electron image of treated wool fibre surface.
Alternative and specialised textile fibre identification tests
199
fibre, with scale distal edge prominent. The micrograph was taken in order
to establish the presence of ‘nanosphere’ anti-staining treatment on the
fabric of a school uniform. The fine surface asperities confirm the treatment
has been applied. These were not visible with light microscopy.
Finally, one field in which the SEM has recently proved invaluable, is in
the study of the so-called microfibres. Examinations with the instrument
have shown clear images of the constituent parts of both the precursors for
the different types, and the morphology of the end product (Hongu and
Phillips, 1997).
10.6
Quantitative aspects
Although a full description of the quantitative analysis of fibre blends is
outside the scope of this work, it will nevertheless be helpful for the reader
to have a basic appreciation of the principles involved, since today the
number of products made from mixtures of different fibres is likely to
exceed those made from a single fibre type only.
The determination of the percentage by weight of different components
in a material may be required for authenticating identification, labelling,
costing or processing of the material. Broadly, unless the mixture is of
clearly separable individual components (e.g., the warp and weft yarns of a
fabric) methods of analysis are divided into those where the fibres mixed
are chemically the same, and those where they are chemically different.
10.6.1 Mixtures of chemically equivalent fibres
Where fibres have the same chemical compositions, but are of different
generic types, it is necessary to use physical means to distinguish them. The
most commonly applied method of carrying this out is to use a microscopical identification, measurement and counting procedure – either with the
light microscope or the SEM. Measurement is necessary because straight
numerical counting would not take into account different diameters of
fibres and thus not produce an accurate weight percentage.
The first application of this technique was developed for the analysis of
animal fibre blends (Wildman, 1953) and is still in use today, though in
sometimes modified form. The technique permits the determination of proportions in blends such as the commercially valuable mixtures of cashmere
and wool, mohair and wool, wool and angora, etc. The procedure can also
be adapted for use with blends of vegetable fibres, and mixtures of natural
and regenerated cellulosics.
Other techniques that may be applied to blends of fibres chemically the
same are X-ray diffraction (Chidamberswaran et al., 1987) and analysis by
DNA (see Chapter 12).
200
Identification of textile fibers
10.6.2 Mixtures of chemically different fibres
The traditional means of separating individual components from a mixture
of chemically different fibres is to use selective dissolution. In essence, the
more soluble component in the blend is removed using an agent which is
known not to cause any damage or weight loss to the component(s) which
are to remain. As an example, to determine the percentage of cotton in a
cotton/polyester blend (as used in shirtings and bed linens, etc.), after pretreatment to remove any non-fibrous matter and accurate determination of
the original weight of the material, the cotton would be dissolved from the
blend using 75% sulphuric acid. After complete dissolution the polyester
residue would be rinsed, dried and weighed, and the proportions of the
two components determined – either on a dry weight basis, or, more usually,
with the agreed allowances for moisture regain.
Strict procedures for the determination of fibre composition by chemical
dissolution are laid out in International and National Standard Documents
(ISO, 2006; British Standards Institute, 2002; AATCC, 2003). Other
methods of analysing chemically different fibres are gaining increased
acceptance after continuing development, particularly that of near infrared
spectroscopy, infrared spectroscopy and nuclear magnetic resonance spectroscopy. There are no formal standard test procedures for these techniques
as yet, however.
10.7
Sources of further information and advice
The Textile Institute
International Headquarters
First Floor
St James’s Buildings
Oxford Street
Manchester M1 6FQ UK
Tel. 44 (0) 161 237 1188
Fax 44 (0) 161 236 1191
Email [email protected]
(Publications, Conferences, Committees, Meetings, Consultancy Register,
Special Interest Groups)
The Royal Microscopical Society
37/38 St Clements
Oxford OX4 1AJ UK
Tel. 44 (0) 1865 254760
Email [email protected]
Alternative and specialised textile fibre identification tests
201
(Publications, Courses, Meetings, Exhibitions, Journals, Liaison with Other
Institutes)
Society of Dyers and Colourists
Perkin House
Grattan Road
Bradford BD1 2JB UK
Tel. 44 (0) 1274 390955
Fax. 44 (0) 1274 392888
Email [email protected]
(Publications, Meetings, Lectures, Reference Materials)
10.8
References
American Association of Textile Chemists and Colorists. AATCC Method 20A.
AATCC, NC USA (2003).
British Standards Institution. BS4407. Quantitative Analysis of Fibre Mixtures B.S.I.
London (1988 (2002)).
Carroll GR. Forensic Fibre Microscopy in Forensic Examinations of Fibres,
J Robertson (Ed.) Ellis Horwood, New York City, NY (1992).
Chapman SK. Working with a Scanning Electron Microscope. Lodgemark Press,
Kent (1986).
Chescoe D and Goodhew PJ. The Operation of the Transmission Electron
Microscope. Royal Microscopical Society Handbook. Oxford University Press
(1984).
Chidamberswaran PK et al. Textile Research Journal (1987) 57, 167.
Dobb MG. Journal of the Textile Institute 61, 232 (1970).
Ford BJ and Stokes DJ. Bug’s Eye View. In Focus 2006. 3. Royal Microscopical
Society, Oxford.
Goodhew PJ. Specimen preparations in materials science in Practical Methods
in Electron Microscopy. AM Glauert (ed). North Holland Publishing Co.,
New York and Oxford (1973).
Greaves PH and Saville BP. Microscopy of Textile Fibres. Royal Microscopical
Society Handbook. Bios Scientific Publishers, Oxford (1995).
Greaves PH. Proceedings International Conference on Fibre and Textile Science,
Ottawa, Canada. The Fibre Society and The Textile Institute (1991).
Harris WE and Habgood HW. Programmed Temperature Gas Chromatography.
John Wiley & Sons Inc. (1966).
Hitachi High Technologies Corporation. Microscopy and Analysis 115 (2006).
Hongu T and Phillips GO. New Fibres (Second edition). Woodhead Publishing Co.
Abington, Cambridge (1997).
International Organisation for Standardisation. ISO 17751 Final Draft (2005).
International Organisation for Standardisation. ISO 1833 Quantitative Analysis of
Binary and Ternary Fibre Blends (2006).
Johnson DJ. PhD Thesis, University of Leeds (1965).
202
Identification of textile fibers
Joy DC. High Resolution Scanning Electron Microscopy. Institute of Physics
Conference Series No. 98, Chapter 10. IOP Publishing Group, Bristol and New
York (1989).
Kusch P and Arns W. Schriftenriche des Deutsches Wollforschungsinstitut 87
Deutches Wollforschungsinstitut, Aachen, Germany (1986).
Langley KD and Kennedy TA. Textile Research Journal (1981) 51.
McCarthy BJ and Greaves PH. Wool Science Review (1988) 65.
Meldrum, K. Journal of the Society of Dyers and Colourists (1961) 77, 22.
Park J and Shore J. Review of Progress in Colouration (1982) 12, 43.
Postek MT et al. Scanning Electron Microscopy – A Student’s Handbook. Postek MT
Jr and Land Research Industries Inc. (1980).
Sich J. Proceedings 2nd International Conference on Speciality Fibres. DWI Aachen,
Germany (1989).
Sikorski J. Structural Studies of Mammalian Keratin (in Structures of Fibrous
Biopolymers. Colton Papers 26. Editors EDT Atkins and A Keller) (1975).
Stokes DJ. Progress in the Study of Biological Specimens using ESEM. In Focus
2006. 2. Royal Microscopical Society, Oxford.
The Textile Institute. Guide to the Identification of Textile Materials 7th edition.
Manchester, UK (1985).
Thiel BL. Image Analysis in Materials Science by Low Vacuum SEM. International
Materials Reviews (2004) 49 (2) 109–122.
Tungol MW, Monaster A and Bartick EG. Analysis of Single Polymer Fibres by
Fourier Transform Infrared Microscopy. The Results of Case Studies. Applied
Spectroscopy 44, 543 (1990).
Weidemann E et al. SAWTRI Bulletin (1987) 21.
Wildman, AB. The Microscopy of Animal Textile Fibres. Wool Industries Research
Association, Leeds (1953).
Wortmann F and Arns W. Textile Research Journal (1986) 56.
11
Analysis of dyes using chromatography
S W LEWIS,
Curtin University of Technology, Australia
Abstract: The colour of fibre, present naturally or imparted through use
of a dye or pigment, can be extremely discriminating for the forensic
scientist. One approach for colour analysis available to the forensic
scientist is chromatography and, while partially destructive of the
evidence, it can provide a much greater degree of discrimination than
physical or optical methods alone. This chapter will give an overview of
chromatographic methods applied to the forensic analysis of dyes in
fibres focusing on thin-layer chromatography (TLC). Other approaches
to instrumental chromatographic analysis of dyes, including high
performance liquid chromatography (HPLC) and capillary
electrophoresis (CE) will also be discussed.
Key words: forensic fibre dye analysis, thin-layer chromatography,
extraction, high performance liquid chromatography, capillary
electrophoresis.
11.1
Introduction
A key element of a fibre’s functionality is its colour, which can be extremely
discriminating for the forensic scientist. This colour may be present
naturally or can be imparted through use of a dye or pigment. As described
in Chapter 9, microspectrophotometry can provide a good deal of
differentiation; however, in a small number of cases, different dyes may
produce the same or similar colour (Wiggins et al., 2005). One approach
available to the forensic scientist is chromatography and, while partially
destructive of the evidence, it can provide a much greater degree of discrimination than physical or optical methods alone. Chromatographic
methods require the extraction of the colourant from the fibre followed by
analysis. This chapter will give an overview of chromatographic methods
applied to the forensic analysis of dyes in fibres, focussing on thin-layer
chromatography (TLC) which is the most widely used chromatographic
technique applied in this area. Other approaches to instrumental chromatographic analysis of dyes that have been suggested, including high performance liquid chromatography (HPLC) and capillary electrophoresis (CE)
will also be discussed.
203
204
Identification of textile fibers
11.2
Dyes
Dyes have been used to alter the appearance of cloth and fabric by
changing their colour since ancient times (Garfield, 2000). Dyes can be
defined as any substance which has an affinity for a substrate and, on
binding to that substrate, alters the way in which the material absorbs
light, and hence changes the colour that is subsequently observed. Dying
of cloth with a wide variety of vegetable and animal dyes has been carried
out for thousands of years. Famous historical dyes include the rare and
expensive Royal purple, extracted from certain species of mollusc and used
by the Roman emperor and his household; cochineal, the crimson dye
obtained from the dried and pulverised bodies of cactus insects; and indigo
sourced from the leaves of certain plants (Garfield, 2000). The birth of
modern dyestuffs occurred with the discovery of mauveine by William
Perkin in 1856 and today synthetic dyes predominate (Gregory, 2000,
Garfield, 2000).
Colour is due to the preferential absorption of certain wavelengths of
light, the observed colour being those wavelengths of light that are reflected.
The particular bands of light absorbed, and hence the colour observed, by
a particular dyed fabric is due to the chemical structure of the dye. Light is
absorbed by a molecule when it has sufficient energy to promote an electron from a ground state orbital, where electrons within a molecule normally reside, to an unoccupied higher energy orbital. Dyes are organic
molecules with extensive areas of high electron density due to the presence
of double bonds. When the double bonds alternate along the structure the
molecule is said to be conjugated. This leads to overlapping π orbitals, which
results in decreased energy gaps between ground state and higher energy,
unoccupied orbitals. The areas of a molecule that absorb visible light are
termed chromophores, the wavelengths of light they absorb will depend on
their structure and the presence of other groups, termed auxochromes,
which alter the electron density within the conjugated group.
For a chemical to act as a dye it must have some affinity for the substrate
to which it is being applied. Dyes can be bound either physically, through
a variety of different attractive forces such ionic, van der Waals, and hydrogen bonding, or chemically, through covalent bonds (Gregory, 2000).
Classification and identification of individual dyes is complicated by the
bewildering array of chemical structures, methods of application, and commercial names (Gregory, 2000). One of the best known sources of information on dyes is the Colour Index, which was first published by the Society
of Dyers and Colourists in 1925 and is now available in an on-line version
(www.colour-index.org, 2007). This utilizes a dual system of numbers and
names to classify dyes and pigments, based primarily on the method of
application, which are listed in Table 11.1.
Analysis of dyes using chromatography
205
Table 11.1 Dye classes and associated fibres
Class
Fibre type
Method of application
Acid
Wool, silk, polyamide, protein,
polyacrylonitrile,
polypropylene
Cotton, viscose
Acidic dyebaths
Azoic
Basic
Direct
Disperse
Polyacrylonitrile, modified
acrylic, polyester, polyamide
Cotton, viscose
Metallized
Polyester, polyacrylonitrile,
polyamide, polypropylene,
acetate/triacetate
Wool, polypropylene
Reactive
Cotton, wool, polyamide
Sulfur
Cotton
Vat
Cotton
Fibre impregnated with coupling
component and treated with
stabilized diazonium salt
Acidic dyebaths
Neutral or slightly alkaline
dyebath containing additional
electrolyte
Aqueous dispersion of dye, often
applied at high temperatures
or in the presence of a carrier
Formation of a metal complex,
dye, metal (mordant) may be
applied before, after or at
same time as dye
Dye reacts chemically with fibre
forming covalent bonds, may
be under acid or alkaline
conditions
Dyes reduced in alkaline
medium to produced leuco
form of dye which penetrates
fibre and then is oxidized back
to insoluble form of dye
Complex process similar to
sulfur dyes
Sources: Gregory (2000), Wiggins (1999)
The following examples illustrate classification of the dyes illustrated in
Fig. 11.1. Dye A is CI Vat Green 1, CI Number 59825, indicating that its
method of application is via a vat dying process. Dye B is CI Direct Red
45, CI Number 14780, which is applied directly to the fibres.
One disadvantage of this classification process is that it does not provide
information concerning the chemical structure of the dye, which is essential
in developing analytical chemistry procedures for their extraction, separation and identification. In terms of chemical properties the unique identifier
for the each dye is the Chemical Abstracts Service (CAS) Registry number,
for example for Dye A in Fig. 11.1 this is 128-58-5. This number can be
used for searching in on-line databases like Scifinder Scholar.
206
Identification of textile fibers
OCH3
OCH3
SO3Na
H3C
OH
S
N
O
N
N
SO3Na
Dye A
O
Dye B
11.1 Chemical structures of selected dyes.
Due to the variation in chemical structure of fibres, some dyes are more
suited for use with certain fibres. For example acid dyes are particularly well
suited to the dyeing of wool and nylon due to the presence of the amino
groups that are charged under acid conditions thus attracting the dye anion
(Wiggins, 1999). The Metropolitan Laboratory of the Forensic Science
Service in the UK carried out a study on fibre/dye combinations encountered during a 12 month period from 1993 to 1994 and discovered that while
certain combinations were more prevalent, it was clear that many dyes are
available for each of the fibre types commonly encountered in forensic
examinations (Wiggins, 1999). It should also be noted that most textiles are
dyed with multiple dyes, and due to the complexity of the dying process,
this occurs via a batch process. This leads to variability in colour between
batches (Connelly, 1997, Wiggins et al., 1987). This may be useful to the
forensic scientist, but also leads to complications in developing protocols
for the analysis of dyes in unknown samples.
11.3
Forensic analysis of dyes
As stated above, fibres are most likely to be dyed with a combination of
several different dyes. This combination has the potential to be characteristic of a particular source of fibres. Forensic analysis of dyes is thus generally carried out to compare recovered samples with controls, rather than to
absolutely identify a particular dye. The two issues which face the forensic
analyst when considering the analysis of fibre dyes are:
(i)
the sheer complexity of the problem with the wide variety of dyes and
fibres and potential combinations thereof, and
(ii) the destructive nature of chromatographic analysis due to the extraction of the dye from the fibre.
It is for these reasons that extraction and analysis of dye from fibres is only
carried out once the other physical and optical methods of analysis have
been exhausted.
Analysis of dyes using chromatography
207
11.3.1 Extraction
The first step in the analytical process is extraction of the dye/dyes from
the fibre. A number of different extraction schemes utilising a variety of
solvents have been proposed (Beattie et al., 1979, 1981a, 1981b, Cheng
et al., 1991, Hartshorne and Laing, 1984, Home and Dudley, 1981, Laing
et al., 1990, 1991, Macrae and Smalldon, 1979, Resua, 1980, West, 1981,
Wiggins, 1999). These schemes vary in detail, but have essentially the same
goal of using a sequence of extractions, which not only provide an extract
of the dye but can also be used to classify the dye. A comprehensive
approach is that developed by the Forensic Science Service in the United
Kingdom which involves different solvent sequences depending on the
identity of the fibre, which had been established by microscopy and/or
infrared spectrophotometry (Wiggins, 1999). Subsequent separation and
identification of the dyes was by thin layer chromatography (TLC). Details
of the various extraction schemes are summarized in Figs 11.2 to 11.7
(Wiggins, 1999).
The extraction process itself is very simple. A single fibre is placed in a
glass tube (2.5 cm × 1.5 mm i.d.), sealed at one end. Solvent (around 10 μL,
it should be sufficient to completely immerse the fibre) is added and the
tube is heat sealed prior to incubation in an oven for the requisite time and
temperature, as described in the extraction procedure (Wiggins, 1999). If
the fibre is very pale, a larger sample than a single fibre is needed, due to
sensitivity issues. Control samples are utilized in order to establish dye class
and the best extraction procedure prior to analysis of recovered fibres
(Wiggins, 1999). Once classification of the dye has occurred via the schemes
described in Figs 11.2 to 11.7, extraction for TLC analysis can be carried
out as outlined in Tables 11.2 and 11.3.
Care must be taken when using this approach to characterise dye classes
in unknown samples, with dyes being classified as ‘being equivalent to, or
Stage 1: Pyridine/water (4:3)
100 °C 10 min
Good
extraction
ACID DYE
Good
extraction
METALLIZED DYE
Poor/no
extraction
Stage 2:
2% aqueous oxalic acid 100 °C
20 min
Poor/no
extraction
REACTIVE DYE
11.2 Extraction and classification of dyes from wool fibres (Wiggins, 1999).
208
Identification of textile fibers
Stage 1:
Glacial acetic acid
100 °C 20 min
Poor/no
extraction
Good
extraction
AZOIC DYE
Stage 2:
Pyridine/water (4:3) 100 °C 20
min
Poor/no
extraction
Good
extraction
Stage 3:
Dithionite/polyvinylpyrrolidone*
100 °C 20 min
Extract applied to TLC plate
Fibre colour changed
No coloured spot/
spot not original
fibre colour
REACTIVE DYE
DIRECT DYE
Fibre colour unchanged
No coloured spot/
spot not original
fibre colour
INGRAIN DYE
Fibre colour changed
coloured spot original
fibre colour
Stage 4:
New fibre, 10–14% sodium
hypochlorite
100 °C 10 min
Fibre colour changed
SULFUR DYE
Fibre colour unchanged
VAT DYE
*sodium dithionite (80 mg) polyvinylpyrrolidone (30 mg) sodium hydroxide
(10%, 450 μL) water (9 mL); use immediately and discard excess
11.3 Extraction and classification of dyes from cotton and viscose
fibres (Wiggins, 1999).
Analysis of dyes using chromatography
209
Stage 1:
Formic acid/water (1:1)
100 °C 20 min
Good
extraction
Stage 2:
TLC procedure – methyl acetate eluent
No movement
Movement
Stage 3:
TLC procedure – methanol eluent
Sharp line at solvent front
ACID DYE
DISPERSE DYE
Little or no movement or smeared
BASIC DYE
11.4 Extraction and classification of dyes from polyacrylonitrile fibres
(Wiggins, 1999).
Stage 1:
Chlorobenzene
150 °C 15 min
Poor/no
extraction
Good
extraction
DISPERSE DYE
Stage 2:
Pyridine/water (4:3) 100 °C 20 min
Good
extraction
Stage 3:
TLC procedure – methanol eluent
Sharp line at solvent front
ACID DYE
Poor/no
extraction
REACTIVE OR
DIAZO DYE
Little or no movement or smeared
BASIC DYE
11.5 Extraction and classification of dyes from polyamide fibres
(Wiggins, 1999).
210
Identification of textile fibers
Stage 1:
Chlorobenzene
130 °C 10 min
Poor/no
extraction
Good
extraction
DISPERSE DYE
Stage 2:
Dimethyl formamide/formic acid (1:1)
100 °C 20 min
Good
extraction
BASIC DYE
11.6 Extraction and classification of dyes from polyester fibres
(Wiggins, 1999).
Stage 1:
Methyl acetate/water/acetic
acid (5:5:1) 100 °C 20 min
Poor/no
extraction
Good
extraction
DISPERSE DYE
Stage 2:
Pyridine/water (4:3) 100 °C 20 min
Some extraction
Stage 3:
2% Aqueous oxalic acid 100 °C 20 min
then pyridine/water (4:3) 100 °C 20 min
Improved
extraction
METALLIZED DYE
No extraction
PIGMENT
No improvement
ACID DYE
11.7 Extraction and classification of dyes from polypropylene fibres
(Wiggins, 1999).
Analysis of dyes using chromatography
211
Table 11.2 Extraction and disruption solutions for fibre dye analysis
Solution
Composition and preparation
Pyridine/water
Formic acid/water
Aqueous oxalic acid
4 : 3 v/v, prepare 100 mL and use until exhausted
1 : 1 v/v, prepare 100 mL and use until exhausted
0.2 g in 100 mL water, use immediately and discard
excess
0.5 M acetic acid, prepare 100 mL and use until
exhausted
Cellulase (Penicillium funiculosum) 1.6 mg/mL in
sodium acetate buffer (0.1 M in water adjusted to
pH 5 ± 0.2 with glacial acetic acid), prepare 50 mL
and discard at end of each week
0.75 M in water, prepare 100 mL and use until
exhausted or end of one month
3.0 M in water, prepare 100 mL and use until
exhausted or end of one month
Acetic acid
Cellulase
Sodium hydroxide for
wool
Sodium hydroxide for
cotton
Source: Wiggins (1999)
acting as, a particular dye class’ (Wiggins, 1999). This covers the possibility
if new dyes are introduced. This scheme as originally proposed did not
account for reactive, vat, ingrain and sulfur dyes which were not considered
extractable.
Some additional approaches to extraction of reactive dyes from wool and
cotton fibres have been suggested. The extracts obtained are not true dye
extracts, as the dyes are covalently bound to the fibres (Wiggins, 1999). The
fibre structure needs to be disrupted to release a coloured solution. The
procedures are rather more involved than the simple extraction procedure
outlined above. For wool, fibres are placed in a glass tube with 3 μL sodium
hydroxide solution (see Table 11.2 for details), the tube is sealed and then
incubated at 45°C for 24 hours with continuous agitation. This is followed
by the addition of 2 μL citric acid, mixing and centrifugation at 7000 rpm
for 5 minutes. Cotton fibres are placed in a more concentrated sodium
hydroxide solution (5 μL, see Table 11.2 for details) at 0°C for 4 hours. The
solution is then discarded and the fibres re-suspended in 5 μL acetic acid
for 20 seconds. The resulting solution is again discarded and 3 μL cellulase
solution added. The tube is re-sealed and incubated at 45°C for 24 hours
with continuous agitation. This is followed by the addition of 3 μL methanol,
mixing and centrifugation at 7000 rpm for 5 minutes (Wiggins, 1999).
Once classification of the dye has occurred via the schemes described in
Figs 11. 2 to 11.7, extraction for TLC analysis can be carried out as outlined
in Table 11.3.
212
Identification of textile fibers
Table 11.3 Choice of extraction solutions for fibre dye analysis
Dye class
Fibre type
Extraction solution
Acid
Wool, silk, polyamide, protein,
polyacrylonitrile, polypropylene
Cotton, viscose
Polyacrylonitrile, modified acrylic
Polyester, polyamide
Cotton, viscose
Polyester, polyacrylonitrile,
polyamide, polypropylene,
acetate, triacetate
Wool, polypropylene
Pyridine/water
Azoic
Basic
Direct
Disperse
Metallized
Pyridine/water
Formic acid/water
Pyridine/water
Pyridine/water
Pyridine/water
Aqueous oxalic acid then
pyridine/water
Source: Wiggins (1999)
11.3.2 Separation
After extraction, separation of the coloured components is required, and
this has generally been achieved through the use of chromatographic techniques. Chromatography is defined by IUPAC as a ‘physical method of
separation in which the components to be separated are distributed between
two phases, one of which is stationary (stationary phase) while the other
(the mobile phase) moves in a definite direction’ (MacNaught and Wilkinson,
1997). The chromatographic techniques that have been used for forensic
fibre dye analysis are thin layer chromatography (TLC)(Beattie et al., 1981b,
Home and Dudley, 1981, Laing et al., 1990, Shaw, 1980, Wiggins, 1999,
Wiggins et al., 2005) and high-performance liquid chromatography (HPLC)
(Griffin and Speers, 1995, 1999, Griffin et al., 1994, Huang et al., 2004, 2005,
Laing et al., 1988, Oxspring et al., 1994, Petrick et al., 2006, Speers et al., 1994,
Wheals et al., 1985, Yinon and Saar, 1991). More recently a non-chromatographic separation technique, capillary electrophoresis (CE) has also been
proposed (Oxspring et al., 1994, Robertson, 1999, Siren and Sulkava, 1995,
Tetler et al., 1994, Xu et al., 2001).
Thin layer chromatography (TLC)
In TLC the stationary phase is present as a thin layer upon an inert rigid
support. Izmailov and Schraiber first suggested this approach in the 1930s,
but TLC in the form it is recognized today was not established until the
1950s (Robards et al., 1994). Today there is a wide range of pre-made plates
with a variety of different stationary phases available to the scientist. A
typical TLC separation involves application of a solution of the sample to
Analysis of dyes using chromatography
213
be analyzed as a discrete spot or smear upon the chromatographic plate.
Once the solvent in which the sample was applied has evaporated, the plate
is developed by allowing a mobile phase to move by capillary action through
the stationary phase, carrying the components contained within the
sample with it. These components are retained to different extents by the
stationary phase thus leading to separation. The elution characteristics of
compounds can be reported as Rf values, which are a measure of the relative
distance travelled by each compound from the origin with respect to the
solvent front.
TLC is essentially a variant of liquid chromatography or HPLC, with the
mobile phases and stationary phases utilised being common to both. There
are some key differences between the two approaches. In TLC, unlike
HPLC, there is no need for the sample solvent to be compatible with the
mobile phase as it is evaporated off when applying the sample to the plate.
Similarly there is no requirement for the mobile phase to be compatible
with detection as is the case with HPLC. TLC is an ‘open bed’ method, with
retention being observable at all stages, unlike in HPLC, where if a component is strongly retained, the only indication will be the lack of a peak.
In TLC standards and samples can be run simultaneously on a single plate
under identical conditions. A key advantage of TLC is its relative simplicity
when compared to other chromatographic techniques. The steps in a TLC
analysis have been defined as (Robards et al., 1994):
(i)
selection of a suitable chromatographic stationary phase
(ii) application of the sample
(iii) selection of a mobile phase
(iv) development
(v) visualization and detection.
This can also be followed by quantification, however this is not generally
utilized in forensic fibre dye analysis (Wiggins, 1999).
There are, however, some disadvantages to TLC. Owing to reliance on
capillary action for development, flow rate of mobile phase is difficult to
control with any precision. Detection limits are also an issue, with HPLC
being far better in this regard. In addition there is a certain amount of
manual dexterity required to load (or ‘spot’) the samples onto the plate and
this can introduce some variability.
A range of stationary phases is available for TLC, which can be classified
on the basis of their composition and primary mechanism of retention. The
most popular phases in common use are silica gel and alumina, the main
retention mechanism for both being due to an adsorption process involving
affinity of polar and polarisable compounds to the stationary phase (Robards
et al., 1994). Non-polar components are not retained and move further
along the TLC plate. Aromatic compounds are readily polarizable; hence
214
Identification of textile fibers
this mode of TLC is well suited to the separation of fibre dyes that have
extensive aromaticity and many other polarisable functional groups.
The stationary phase is laid down as a thin layer (100–2000 μm) thick
upon an inert rigid support, such as glass or aluminium (Robards et al.,
1994). While in earlier times the scientist had to undertake the laborious
and time consuming process of preparing these plates from scratch, there
are now many commercial sources of pre-made plates. A specific type of
plate, which has been previously been recommended for forensic analysis
of dye fibres, is aluminium-backed silica gel 60F 254, with dimensions of 5
× 7.5 cm available from Merck (Wiggins, 1999). It has been recommended
that prior to use these silica plates are either stored in desiccator or heated.
This step is to ensure that any water that may be present is driven off, as
this will alter the activity of the silica stationary phase (Wiggins, 1999). The
term activity describes the particular properties of the adsorbent, which
give rise to retention of a given substance, it is determined by both the
chemical structure of the sorbent and any adsorbed species. Adsorbed water
has a great effect on both retention, which is decreased with increasing
humidity, and also may alter the selectivity of the separation. The efficacy
of heating has been questioned as re-adsorption of water occurs extremely
rapidly, with a plate becoming significantly re-equilibrated with ambient
humidity within five minutes of removal from a desiccator or other humidity controlled environment (Robards et al., 1994). What is clear is that the
plates need to be stored and prepared in a controlled and reproducible
manner, preferably in a humidity-controlled environment.
Appropriate application of the sample is highly important in obtaining
good separations. In principle it is very simple, using a glass capillary or
micropipette, the dye extract is applied as spot of approximately 2 mm in
diameter, typically about 1 cm from the bottom edge of the plate (Wiggins,
1999). To ensure good results there are a number of practical issues that
need to be faced. Care needs to be taken not to damage the adsorbent layer
or to produce an excessive spot size. The solvent used to load the sample
should be able to wet the stationary phase and it should also be volatile so
that it can be driven off prior to development of the plate. This process can
be accelerated through the application of heat, either applied using a hair
dryer or by having the plate rest upon a hot surface. To increase the amount
of dye material on the plate, thus providing a more detectable result,
repeated application of sample on the initial spot could be carried out. It is
recommended that both questioned samples, known samples and standard
dye mixtures are run at the same time, with the questioned sample being
bracketed by known samples or standard dyes (Wiggins, 1999). The applied
spots should not be too close to each other, or to the edge of the plate.
There are a number of different approaches to developing TLC plates,
with varying levels of complexity. However the simplest, and most widely
Analysis of dyes using chromatography
215
used approach is the ascending linear method (Robards et al., 1994). Once
the plate is completely dry, it placed in a pool of eluent contained within a
covered chamber. This can be a simple as a beaker with a petri dish cover,
or alternatively a commercially available development chamber. Prior to
use the eluent needs to stand for a few minutes in the covered chamber to
allow the chamber to become saturated with eluent vapour (Robards et al.,
1994). The depth of eluent should be such that when the plate is placed in
the chamber, the eluent level should be below the applied spots, a distance
of at least 0.5 cm has been recommended (Wiggins, 1999). The eluent will
then move up the plate by capillary action, this process takes only a few
minutes and separation of the components within the applied dye spots will
be observed. Development should be carried out until the eluent has travelled around 2 cm beyond the origin where the spots were applied (Wiggins,
1999). Any further than this may lead to the separated spots become diffuse,
which makes visualization and interpretation problematic.
The eluents used for TLC of fibre dyes will depend on the class of dye
being analysed, Table 11.4 summarizes the composition of eluent mixtures
and their application to particular dye classes. It has been recommended
that a minimum of two eluents should be applied to the analysis of a particular dye extract (Wiggins, 1999). Issues which need to be considered when
deciding whether a particular eluent system is suitable for a dye extract
include; separation and sharpness of bands, how far the bands have moved
from the origin, how close to the separated bands are to the solvent front
and the strength (concentration) of the dye extract from the recovered
(questioned) fibres (Wiggins, 1999).
Interpretation of the developed TLC plates is carried out by comparing
the band position and colours of the questioned fibre extracts with control
fibre extracts and standard dye mixtures (Table 11.5). This can be done
under normal laboratory lighting or the use of ultraviolet light. Rf values
can be calculated; however, they are of limited value when comparing
between plates due to subtle differences in development that lead to different retention.
A recovered fibre may be too pale in colour or too short to enable the
dye extract to be concentrated enough to obtain a successful TLC. There is
no simple way to predict whether a fibre is going to be long enough; analysis
of an equivalent control fibre is therefore highly recommended (Wiggins,
1999). In these circumstances the more sensitive approach of HPLC may
yield more successful results.
High performance liquid chromatography (HPLC)
Since its introduction in the late 1960s HPLC has become perhaps the most
widely used analytical separation technique in use today. In HPLC, the
216
Identification of textile fibers
Table 11.4 Eluents for TLC analysis of extracted fibre dyes
Eluent no.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Solvents (proportions v/v)*
Fibre type
Dye class
n-butanol, acetone, water,
ammonia (5 : 5 : 1 : 2)
Wool or silk
Cotton or viscose
Cotton
Cotton or viscose
Cotton
Cotton or viscose
Polyester
Polyester
Acid or
metallized
Direct
Basic
Reactive
Acid or
metallized
Direct
Reactive
Direct
Reactive
Azoic
Disperse
Disperse
Polyester
Disperse
Polyester
Disperse
Polyamide
Acid
Polyamide
Acid
Polyacrylonitrile
Basic
Polyacrylonitrile
Basic
Wool
Reactive
Cotton
Reactive
Cotton
Reactive
Pyridine, amyl alcohol, 10%
ammonia (4 : 3 : 3)
n-Butanol, ethanol, ammonia,
pyridine, water (8 : 3 : 4 : 4 : 3)
Methanol, amyl alcohol, water
(5 : 5 : 2)
Toluene, pyridine (4 : 1)
Chloroform, ethyl acetate,
ethanol (7 : 2 : 1)**
n-Hexane, ethyl acetate, acetone
(5 : 4 : 1)
Toluene, methanol, acetone
(20 : 2 : 1)
n-Butanol, acetic acid, water
(2 : 1 : 5)***
n-Butanol, ethanol, ammonia,
pyridine (4 : 1 : 3 : 2)
Chloroform, butanone, acetic
acid, formic acid** (8 : 6 : 1 : 1)
n-Butanol, acetic acid, water
(4 : 1 : 5)***
Propan-1-ol, methanol, water,
ammonia (6 : 3 : 1 : 4)
n-Butanol, ethanol, ammonia,
pyridine, water (8 : 3 : 4 : 4 : 6)
n-Butanol, ethanol, ammonia,
pyridine, water (6 : 3 : 2 : 6 : 6)
Cotton or viscose
Polyacryonitrile
Wool
Wool or silk
* Ethanol used is 99%, ammonia is 0.880 SG unless otherwise stated, eluents
discarded weekly except for ** which should be discarded daily.
*** These eluent combinations form an upper and lower phase; use upper phase
as eluent.
Source: Wiggins (1999)
stationary phase, typically particles in the range 3–5 μm, are contained
within a column with dimensions of 3–25 cm in length and 3–5 mm internal
diameter. The mobile phase is pumped through the stationary phase at high
pressure (up to 6000 psi) with flow rates of 0.1–10 mL/min. Samples are
introduced into the system before the separation column using a variety of
injection devices which can be manual or automated. A number of different
Analysis of dyes using chromatography
217
Table 11.5 Standard dye mixtures for TLC analysis of fibre dyes
Standard dye mixture*
Eluent no.
A
Solway green G (CI acid green 25)
Solway blue RNS (CI acid blue 47)
Naphthalene fast orange (CI acid
orange 10)
1, 2, 3, 4, 9, 10, 12, 13, 14, 15
B
Superacet fast orange (CI disperse
orange 3)
Superacet fast violet B (CI disperse
violet 8)
Superacet scarlet 2G (CI disperse
orange 1)
5, 7, 8
C
Superacet fast orange (CI disperse
orange 3)
Superacet fast violet B (CI disperse
violet 8)
6
D
Solway green G (CI acid green 25)
Superacet fast orange (CI disperse
orange 3)
Superacet fast violet B (CI disperse
violet 8)
11
* Approximately 5 mg of each dye made up to a final volume of 25 mL with
pyridine/water 4 : 3. Use until exhausted.
Source: Wiggins (1999)
Sample
injector
Mobile phase
reservoir
Pump
Column
Detector
Data
recorder
11.8 Schematic diagram of the components of an HPLC system.
approaches can be used to detect the eluting analytes; the most common is
ultraviolet-visible absorbance spectrophotometry; however, mass spectrometry has also been used in the analysis of fibre dyes. The resulting separation
is recorded as a graph, formerly on a chart recorder but now electronically
using a computer, which is referred to as a chromatogram. A schematic of
a typical HPLC system is presented in Fig. 11.8.
There are a number of different forms of HPLC depending on the type
of retention mechanism involved. By far the most common is partition
218
Identification of textile fibers
chromatography, which can be further divided into reversed phase chromatography and normal phase chromatography. The stationary phase for partition chromatography is chemically bonded to silica particles. For reversed
phase chromatography the bonded phase is relatively non-polar with polar
compounds eluting first from the column, while for normal phase chromatography, which utilizes relatively polar stationary phases, non-polar compounds are the first to elute.
HPLC is an extremely powerful separation technique that is widely used
in all areas of analytical science. A wide range of stationary phases is available to the analyst in combination with an almost limitless range of mobile
phase compositions. It is this flexibility that also increases the complexity
of the development process for a new analytical separation. HPLC also has
high initial set-up costs and significant running costs when compared with
TLC. As samples are introduced in solution into an HPLC system, another
issue is the potential incompatibility between the extraction solvent and the
separation conditions (Griffin and Speers, 1999).
HPLC, however, has the advantages of far greater efficiency separations,
being able to resolve the components of very complex mixtures, and greater
sensitivity. This latter is a particular advantage when analyzing extremely
small samples of material. The powerful combination of HPLC coupled
with mass spectrometry has been applied to forensic fibre dye analysis
(Huang et al., 2004, 2005, Petrick et al., 2006, Yinon and Saar, 1991). This
allows the determination of molecular structure information on eluted
bands and thus provides an extra dimension of information.
Capillary electrophoresis (CE)
The term capillary electrophoresis (CE) is used to describe a group of
techniques where separation of the components of a chemical mixture
occurs in a narrow bore capillary under the influence of an electric field. In
its simplest form, separations in CE are based upon the differential migration of charged species through a fused silica capillary filled with the background electrolyte. Detection is typically by ultraviolet-visible absorbance
spectrophotometry, resulting in graph, which looks similar to a chromatographic separation and is often referred to as an electropherogram. A
schematic of typical CE instrument is presented in Fig. 11.9. Dedicated
capillary electrophoresis instruments, which automate many of the processes in carrying out an analysis, are available. For neutral species separations are made possible by addition of surfactants to the background
electrolyte above the critical micelle concentration. This leads to the formation of micelles, which act as a ‘pseudo-stationary’ phase, separation being
achieved by interaction between the analytes and the micelles. This hybrid
Analysis of dyes using chromatography
Capillary filled with electrolyte
219
Detector
Data recorder
and instrument
control
Buffer reservoirs
Electrodes
High voltage
supply
11.9 Schematic diagram of a CE system.
of chromatography and CE is termed micellar electrokinetic chromatography (MEKC).
Highly efficient, rapid separations are possible and CE in its various
modes is a well-established analytical technique with over 30 000 references
to date in a diverse range of application areas. Forensic application of CE
was first demonstrated by Weinberger and Lurie and since then has been
the subject of a number of reviews and book chapters. What makes CE so
attractive for forensic analysis is its exceptional separation power coupled
with rapid analysis, minimal sample preparation, low reagent usage and
waste production. That being said, there has been little use of this technique
for the forensic analysis of fibre dyes, with only a few applications reported
in the literature (Oxspring et al., 1994, Robertson, 1999, Siren and Sulkava,
1995, Tetler et al., 1994, Xu et al., 2001). In a similar fashion to HPLC there
is the issue of compatibility between the extraction solvent and the separation conditions. Robertson in an early report of the application of CE to
the separation of dyes also noted issues with reproducibility and dyes
becoming trapped in the capillary (Robertson, 1999). Nevertheless CE has
tremendous potential due to its unrivalled separating power and this area
is worthy of further research.
11.4
Conclusions
Despite the rise of more modern instrumental methods such as HPLC and
CE, TLC still has a key role to play in the forensic analysis of dyes. Its simplicity and significant body of knowledge concerning the separation of dyes
by this technique ensure that it remains a key tool. HPLC and CE will also
have a place, particularly where sensitivity is an issue. However, more
220
Identification of textile fibers
research is required, particularly with the latter technique. One area, which
appears not to have had much attention in the published literature over the
last decade, is extraction of dyes from fibres. Many of the solvents used in
the extraction schemes described earlier in this chapter are hazardous and
it would be advantageous to replace them with safer alternatives. Approaches
to automating the process would also be an aid in terms of increasing
throughput, reducing exposure to hazardous solvents and releasing personnel for other less onerous duties. There are indications that work is commencing in this area; however, there have not been any publications on this
subject to date.
11.5
Sources of further information and advice
A good overview of the basics of chromatography including TLC and
HPLC can be found in Skoog, D.A., Holler, F.J. and Crouch, S.R (2007)
Principles of Instrumental Analysis, 6th Edition, Belmont, Thomson Brooks/
Cole. The same volume also provides a review of CE. A more in-depth
study of can be found in Robards, K., Haddad, P. R. and Jackson, P. E.
(1994) Principles and Practice of Modern Chromatographic Methods,
London, Academic Press.
Information on dyes and their application can be found in Gregory, P.
(2000) Dyes and Dye Intermediates, Kirk-Othmer Encylopedia of Chemical
Technology, John Wiley & Sons. The Colour Index can be found on-line
at www.colour-index.org.
A key resource for forensic dye analysis by TLC, with particular reference to the extraction schemes developed by the Forensic Science Service
in the United Kingdom is Wiggins, K. G. (1999) Thin Layer Chromatographic
Analysis of Fibre Dyes, in Robertson, J. and Grieve, M. (Eds) Forensic
Examination of Fibres, 2nd edn, London, Taylor and Francis.
11.6
Acknowledgments
The author would like to than Dr Susan Bennett (New South Wales Forensic
Services Group) for guidance in the early stages of preparation of this
chapter and Emma Patton (Curtin University of Technology) for useful
comments on the draft manuscript.
11.7
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technique. Journal of Chromatography, A, 717, 149–55.
Skoog, D.A., Holler, F.J. & Crouch, S.R (2007) Principles of Instrumental Analysis,
6th Edition, Belmont, Thomson Brooks/Cole.
Speers, S. J., Little, B. H. & Roy, M. (1994) Separation of acid, basic and dispersed
dyes by a single-gradient elution reversed-phase high-performance liquid
chromatography system. Journal of Chromatography, A, 674, 263–70.
Tetler, L. W., Cooper, P. A. & Carr, C. M. (1994) The application of capillary
electrophoresis/mass spectrometry using negative-ion electrospray ionization to
areas of importance in the textile industry. Rapid Communications in Mass
Spectrometry, 8, 179–82.
West, J. C. (1981) Extraction and analysis of disperse dyes on polyester textiles.
Journal of Chromatography, 208, 47–54.
Wheals, B. B., White, P. C. & Paterson, M. D. (1985) High-performance liquid
chromatographic method utilizing single or multi-wavelength detection for the
comparison of disperse dyes extracted from polyester fibers. Journal of
Chromatography, 350, 205–15.
Wiggins, K. G. (1999) Thin Layer Chromatographic Analysis of Fibre Dyes. In
Robertson, J. & Grieve, M. (Eds) Forensic Examination of Fibres, 2nd edn,
London, Taylor and Francis.
Wiggins, K. G., Cook, R. & Turner, Y. J. (1987) Dye Batch Variation in Textile Fibres.
Journal of Forensic Sciences, 33, 998–1007.
Wiggins, K. G., Holness, J.-A. & March, B. M. (2005) The importance of thin
layer chromatography and UV microspectrophotometry in the analysis of reactive
dyes released from wool and cotton fibers. Journal of Forensic Sciences, 50,
364–8.
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Analysis of dyes using chromatography
223
Xu, X., Leijenhorst, H., Van Den Hoven, P., De Koeijer, J. A. & Logtenberg, H.
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Journal of Chromatography, 586, 73–84.
12
DNA analysis in the identification
of animal fibers in textiles
P F HAMLYN, BTTG Ltd, UK
Abstract: Animal hair fibers of commercial importance include wool and
speciality fibers such as alpaca, mohair and cashmere. Commercially the
most important speciality fiber is cashmere. However, cashmere apparel
has frequently been found to be adulterated with cheaper fibers. A
major problem facing the speciality fiber industry has been the lack of a
fast and reliable analytical technique to differentiate between expensive
fibers and lower-cost contaminants. This chapter summarizes recent
developments in molecular biology allowing the use of DNA analysis
for the identification of unknown animal fiber blends and leading to
the provision of a commercial service for speciality fiber analysis.
Key words: animal hair fibers, cashmere, deoxyribonucleic acid (DNA),
DNA analysis, identification of fibers.
12.1
Introduction
Animal hair fibers of commercial importance include wool from various
breeds and cross breeds of sheep, and speciality fibers. Speciality animal
hair fibers (hereafter referred to as speciality fibers) also known as luxury
fibers due to their softness, lustre and scarcity value may be defined as all
types of animal hair fibers used in textiles other than sheep’s wool. The
principal speciality fibers are alpaca, angora from Angora rabbits, camel
hair obtained primarily from the undercoat of the Bactrian or two-humped
camel, mohair from the fleece of Angora goats (Capra hircus aegagrus) and
cashmere. New types of speciality fibers may occasionally be introduced
into the marketplace. For example, cashgora, a fiber derived from crosses
between cashmere-bearing goats and Angora (mohair-bearing) goats, is
now recognized as a speciality fiber in its own right.
Commercially the most important speciality fiber is cashmere, which
commands some of the highest prices in the world of textiles. Cashmere is
defined as the fine (dehaired) undercoat fibers produced by cashmere goats
(Capra hircus laniger). As with most commodities, fiber supply and demand
varies with time and socio-economic factors. Increased competition and
demand for cashmere over recent years has resulted in increased product
contamination. There is a mandatory requirement in many countries to
define the fiber content of clothing and related domestic textile materials
224
DNA analysis in the identification of animal fibers in textiles
225
and breach is a criminal offence. Cashmere, in particular, has frequently
been found to be adulterated with much cheaper fibers such as fine wool,
angora, yak hair and occasionally mohair. Angora and mohair are speciality
fibers in their own right but their lower prices relative to cashmere make
them suitable candidates for substitution. Positive identification of animal
fibers is important not only in textiles where regulations govern the labeling
of fiber content, but also in forensic science and archaeology.
A major problem facing the speciality fiber industry has been the lack of
a fast and reliable analytical technique, which will differentiate between
high-quality, often expensive fibers and lower-cost contaminants. To date,
the most important means of identifying animal fibers has been microscopic
analysis. Chemically there is little difference between fibers from different
animal species. Therefore, physical parameters such as fiber diameter, cuticle
scale height and surface characteristics (e.g., scale frequency and scale patterns) have to be assessed. However, there is considerable overlap in parameters such as fiber diameter between different types of fiber due to the range
of variation within species and the scale patterns of animal fibers can suffer
mechanical damage and chemical erosion during processing. Detailed measurements have to be made on many individual fibers for each sample
received for analysis. Even skilled personnel working in this area, however,
may have difficulty in positively identifying the components of certain
fiber blends.
There is one chemical that can be used to differentiate between animal
species and even individuals of the same species. Deoxyribonucleic acid
(DNA) contains the unique genetic instructions or code for the development and functioning of a living organism offering the potential for the
identification of any species through an analysis of its DNA. Some of the
most important advances in DNA analysis were made in the 1980s. In 1984
traces of DNA were successfully isolated from a 150-year-old museum
specimen of the Quagga, an extinct subspecies of the plains zebra. This
‘ancient’ DNA could not only be extracted but was also sequenced (Higuchi
et al., 1984). The invention of DNA or genetic fingerprinting was announced
by Sir Alec Jeffreys at the University of Leicester in 1985. Three years later
by combining DNA fingerprinting with a new technique called the polymerase chain reaction (PCR) the sensitivity of DNA fingerprinting could
be increased to allow the identification of an individual from the minute
amounts of DNA that could be extracted from a single hair (Higuchi et al.,
1988). Using the PCR technique DNA could be analyzed from the root
region of shed, as well as freshly plucked single hairs. A breakthrough in
speciality fiber analysis was made when it was demonstrated that DNA was
not only present in hair roots but could also be recovered from animal hair
shafts (Kalbe et al., 1988). This was an important development since some
fibers such as wool are shorn rather than combed.
226
Identification of textile fibers
Following on from these advances in molecular biology BTTG set out to
apply DNA analysis to the identification of unknown fiber blends although
initial progress was slow because the techniques were difficult and expensive (Hamlyn et al., 1990). However, the advent of automated procedures
for performing PCR and the development of DNA extraction kits led
to the provision of a commercial service for speciality fiber analysis (see
Section 12.6).
12.2
Extraction of DNA from animal fibers
Animal hair fibers are constructed from cellular material. In the living cells
of animals the DNA is contained inside spherical bodies known as nuclei
and smaller cellular bodies called mitochondria. The DNA inside nuclei
contained within immature hair follicles can be made visible using a technique known as in situ DNA hybridization. As the follicle matures, the
nuclei become elongated but the signal does not extend into the more
mature parts of the fiber where keratinization has taken place (Fig. 12.1).
During keratinization the cells die, but traces of DNA from both nuclei and
mitochondria remains trapped inside the fiber. The recalcitrant and hydrophobic (waterproof) nature of fibers protects the DNA from chemical and
biological degradation.
Various methods have been developed for the recovery of DNA from
animal hair fibers. DNA has been extracted from cuticle cells removed from
the surface of fibers by washing in an aqueous solution of sodium dodecyl
sulphate (Fig. 12.2), from cryogenically milled fibers and from whole fiber
snippets treated with a suitable proteolytic enzyme. A disadvantage of
12.1 Direct visualization of nuclei inside a single hair follicle using the
technique of in situ hybridization and a total genomic DNA probe
(from Broadbent, 1997).
DNA analysis in the identification of animal fibers in textiles
227
kb
23
2.3
2.0
0.56
λ Hind III
CASHMERE
MOHAIR
WOOL
YAK
ANGORA
CAMEL
12.2 DNA extracted from the cuticle of speciality fibers and visualized
by UV fluorescence after resolution on an agarose gel (from Nelson
et al., 1992).
using cuticle cells is that they are more susceptible to damage by processing
treatments. Improved DNA extraction procedures and methods of DNA
analysis resulted in the requirement for only 20 milligrams of a fiber sample
for routine analysis at BTTG whereas originally 20 grams was needed. A
standard protocol (reproduced from Hamlyn et al., 2001) is given below:
Fibers are chopped into small fragments using clean and sterile scissors and
approximately 25 mg weighed into a microtube. Extraction buffer (600 μl) and
papain (20 μl) is added to each tube and the reagents mixed and centrifuged
at 8000 rpm (MSE minifuge) for a few seconds. The mixture is then incubated
at 50°C for up to 48 hours. DNA is extracted from the enzyme-treated fibers
using a DNA extraction kit for hard tissues (Nucleon Biosciences). It is important to avoid using the ethanol precipitation step at the end of the manufacturer’s protocol since this causes natural pigments present in the fibers to bind
to the extracted DNA making subsequent DNA analysis extremely difficult.
Instead the DNA is purified and concentrated using Microcon 100 microconcentrators (Millipore). The extracted and purified DNA should be stored
at −20°C until required.
12.3
Development of methods for using DNA analysis
to identify animal fibers
12.3.1 Selection of target DNA sequences
A prerequisite to the DNA profiling of animal fibers is the identification of
short DNA sequences that are unique to each species. It is an advantage if
228
Identification of textile fibers
these sequences are present as multiple copies, since this increases the sensitivity of the test. Once species-specific DNA sequences have been located
(e.g., from databases containing published sequence information), complementary DNA sequences known as oligonucleotides can be constructed,
which, under carefully controlled conditions, specifically hybridize to the
target DNA molecule giving a positive signal, confirming the presence of a
particular fiber type. Oligonucleotides based on the repetitive Satellite II
DNA region are available that can be used to distinguish between DNA
samples from closely related species such as sheep and goats. Satellite II
DNA is present in high concentrations in the nuclei of many higher animals.
Evolutionary pressure has produced a series of mutations in this DNA,
however, resulting in natural variations between species.
Mitochondrial genes such as cytochrome b are also present in high copy
numbers (up to 1000 mitochondria are present for each nucleus in an
animal cell) and have a faster evolution rate than nuclear genes. These have
proved useful in the design of species-specific primers for the analysis of
processed fibers (Bayliffe, 1999).
12.3.2 Conventional DNA hybridization analysis
Conventional DNA hybridization analysis can be carried out using a simple
dot-blot technique. Initial attempts at analysing the DNA recovered from
animal hair fibers involved the use of total genomic DNA probes (Kalbe
et al., 1988). However, a major limitation of this approach in relation to fiber
profiling was the inability to differentiate between DNA recovered from
fibers of related genera such as sheep, goats and yak, although more distant
species such as rabbit were readily distinguished. Both sheep’s wool and
yak hair have been used to adulterate cashmere, so it is important to be
able to distinguish between all three fiber types. Therefore, species-specific
oligonucleotides were specifically designed for use as probes to distinguish
between DNA extracted from raw wool, yak hair and cashmere fibers
(Hamlyn et al., 1992; Nelson et al., 1992).
The dot-blot technique is illustrated in Fig. 12.3. DNA isolated from fiber
samples together with control samples of genomic DNA is denatured by
boiling to convert the native double-stranded DNA molecules into single
strands and then spotted onto a nylon membrane (Bio-Rad). A singlestranded preparation of the oligonucleotide to be used as the probe is then
incubated with the membrane to allow hybridization to take place between
the probe and any complementary strands of DNA present in the sample.
After free unhybridized probe has been removed by washing the membrane several times, the hybrid double-stranded DNA molecules can be
visualized by several different methods according to how the probe has
been labeled (i.e., by using radioactive, colorimetric or luminescent labels).
DNA analysis in the identification of animal fibers in textiles
229
Sample
Isolation of
double stranded DNA
Immobilization of single
stranded DNA onto membrane
(dot blotting)
Hybridization (binding)
with labeled probe
Washing to remove
unbound probe
Visualization of
labeled DNA
12.3 Analysis of DNA by the dot-blotting technique.
For example, a sheep-specific oligonucleotide probe can be used to detect
the presence of wool but does not give a positive signal with DNA extracted
from other types of fiber (Fig. 12.4). Thus the presence or absence of wool
in a bale of raw cashmere can be quickly determined.
12.4
Effect of fiber processing on DNA analysis and
the use of DNA amplification technology
Although standard hybridization methods can be used for the identification
of samples of raw fibers, with processed materials (scoured, bleached or
dyed) and finished garments the quality and quantity of DNA present
within the fiber is much reduced and it is not possible to use conventional
methods of DNA analysis. Only small quantities of DNA can be extracted
from processed fibers, typically less than 1 ng DNA per gram of sample.
Therefore, in vitro DNA amplification technology known as the polymerase
chain reaction (PCR) which allows for the analysis of minute quantities of
DNA was developed at BTTG for animal fiber analysis (Hamlyn et al.,
1996). DNA is first extracted from the fiber sample and purified then the
purified DNA is amplified prior to analysis.
The PCR technique was conceived by Kerry Mullis along with other
researchers in 1983 while working at the Cetus Corporation in Emeryville,
CA. In essence, PCR reproduces what happens when the DNA inside a cell
is replicated during cell division and is able to duplicate DNA fragments
230
Identification of textile fibers
Cashmere Mohair Wool
Yak Angora Camel
Sample 1
Sample 2
Genomic DNA
Goat Sheep Bovine Rabbit Llama
12.4 Hybridization of a sheep-specific oligonucleotide probe to DNA
extracted from various animal fibers. A positive signal was only
obtained with wool DNA (from Hamlyn et al., 1992).
12.5 Thermal cycling machine used for carrying out the polymerase
chain reaction.
many times producing sufficient amplified DNA for subsequent analysis. By
using an automated temperature cycling block (Fig. 12.5) this ‘enzymatic
copying system’ can be used to produce millions of identical copies of a
specified DNA sequence after only 1–2 hours (Fig. 12.6). These copies
provide sufficient DNA for further analysis (e.g., sequencing), or can simply
DNA analysis in the identification of animal fibers in textiles
231
Denaturation of DNA strands at 95°C
Binding of primers at 55°C
After each cycle of
heating and cooling
the number of copies
of the specified DNA
sequence is doubled.
After 1–2 hours
millions of copies
have been produced.
Replication of DNA starting
from the primers at 72°C
12.6 Amplification of DNA using the polymerase chain reaction. The
primers are synthetically prepared oligonucleotides having sequences
complementary to the DNA on either side of the segment of DNA to
be amplified.
confirm the presence of the target sequence in the sample. Species-specific
primers were designed to give different sized amplification products with
wool, yak hair and cashmere, and these can be visualized by ultraviolet fluorescence after resolution on an agarose gel containing ethidium bromide
(Fig. 12.7). Thus the presence or absence of an adulterant can be quickly
determined and this result can later be confirmed by DNA sequencing.
The resulting DNA sequence can be compared to published sequences
232
Identification of textile fibers
No DNA
Wool
Yak
Cashmere
300
200
100bp
12.7 DNA amplification products obtained from wool, yak hair and
cashmere using different primer sets (from Hamlyn et al., 1996).
providing courts with evidence of an exact match that leaves no room
for doubt. Because of the prodigious sensitivity of PCR it is essential that
negative controls are included with the samples and go through the
same extraction process. In practice, each test is run as a multiplex containing a second universal set of primers (12S ribosomal DNA) which coamplify a non-specific 403 bp product from all mammalian DNA samples
thereby confirming that the extracted DNA is of sufficient quality for analysis and that no inhibitory substances such as pigments are present (Fig.
12.8). Using this approach, DNA has been successfully amplified from
samples of dyed material that did not yield detectable levels of DNA using
conventional extraction techniques and it has been demonstrated that the
PCR technique is capable of amplifying and therefore allowing rapid detection of minor components in mixed DNA samples (Hamlyn et al., 1996,
2001). Although this type of analysis is not quantitative, it is a simple and
rapid test providing useful criteria for the confirmation of fraudulent substitution allowing us to state with absolute certainty whether contamination
by other fibers is present at any stage of processing from raw material to
finished garment.
There are alternative strategies for carrying out DNA analysis using the
PCR technique. At BTTG the use of species-specific PCR primers allowed
the development of a rapid and relatively inexpensive test for commercial
application. Using PCR primers designed from conserved regions and a
PCR-RFLP technique Subramanian et al. (2005) reported that they could
differentiate between cashmere and wool fibers. RFLP or restriction fragment length polymorphism involves cutting the amplified DNA into fragments using suitable enzymes (endonucleases), which only cut the DNA
molecule at specific sites (restriction sites). The DNA fragments produced
by enzymatic cleavage are then separated according to length by agarose
DNA analysis in the identification of animal fibers in textiles
1
2
3
4
5
6
7
8
9
10
233
11 12
-- Internal Control (403 bp)
-- Camelid DNA (330 bp)
12.8 Analysis of DNA from various fiber samples using Camelid
primers1.
Lanes 1 & 12:
Lane 2:
Lane 3:
Lane 4:
Lane 5:
Lane 6:
Lane 7:
Lane 8:
Lane 9:
Lane 10:
Lane 11:
DNA ladder in multiples of 100 bp2
alpaca sample A
alpaca sample B
no DNA
blank extraction
cashmere
wool
yak hair
rabbit angora
natural camel hair
blonde camel hair
1
The Camelid primers give an amplification product of 330 base pairs.
The DNA ladder consists of differently sized DNA fragments having
lengths in multiples of 100 base pairs that serve as visual molecular
weight standards for use in qualitative agarose gel analysis. The 600
base pair band has increased intensity to serve as a reference point.
2
gel electrophoresis resulting in a unique RFLP profile. Appleby et al. (1997)
used degenerate PCR primers for the selective amplification of a polymorphic region of the ribulose-1, 5-bisphosphate carboxylase (RuBisCo)
small subunit genes to devise a new DNA-based test for identifying natural
products (e.g., fibers, food products and timber) derived from plants.
With highly processed samples, where fibers have undergone harsh chemical treatments, it is conceivable that it may not be possible to isolate any
intact DNA suitable for amplification and therefore subsequent DNA analysis will not be possible. In our experience this has only been found in a
couple of cases which both involved fibers that had been recycled and
heavily dyed (Hamlyn, unpublished).
12.5
Future trends
The main limitations of DNA analysis are the inability to carry out quantitative analysis of unknown fiber blends and the lack of species-specific
234
Identification of textile fibers
primers for some types of fiber. For example, it is currently not possible to
distinguish between the different types of goat fiber – cashmere, cashgora
and mohair using DNA analysis. In a detailed study on the phylogenetic
history of the domestic goat, six keratin loci, both coding and non-coding,
were examined in a broad spectrum of contemporaneous goat breeds
exhibiting different fiber types. However, no polymorphisms uncovered
at the keratin loci appeared to exhibit breed or fiber type specificity
(Brown, 2003).
In the case of the South American Camelids, although alpaca is the major
fiber type available commercially, there is much interest in developing
higher value fibers. The South American Camelids consist of two wild
species (vicuña and guanaco) and two domestic forms (alpaca and llama).
Crosses between all four species produce fertile offspring, although it is
unlikely that the wild species interbreed naturally. The wild vicuña was
hunted and killed until recent times for its soft, fine coat but is now protected by the Peruvian government. Vicuña fiber, whose color ranges from
chestnut to fawn, is the finest and most luxurious of the llama group but is
only produced in relatively small quantities. In addition, breeding trials are
being carried out to investigate genetic crosses of alpaca and vicuña, the
so-called Paco-vicuña. Unfortunately, the genetics of the domestic camelids
is very complex. Work by Jane Wheeler (1995) on mummified llamas and
alpacas from pre-Spanish conquest times indicate that extensive crossbreeding between llamas and alpacas occurred following the Spanish conquest leading to the loss of the original fine fiber breeds. The result is that
today fiber from these animals is very heterogeneous with regard to color,
fleece types and fiber diameter. It is therefore likely to be a difficult task to
design suitable PCR primers that are specific enough to allow differentiation between DNA isolated from Camelid fibers currently in use or under
development. Although DNA sequencing could potentially be employed
by making a comparison to published Camelid sequences this may be
problematical with fiber blends.
If DNA analysis could be carried out on individual intact fiber shafts, this
would enable the quantitative determination of the composition of unknown
fiber blends. However, investigations at BTTG have indicated that in situ
DNA analysis of single fiber shafts is not practical because the DNA is
encapsulated inside the waterproof environment of the keratinized cells
and attempts to get at this DNA leads to the dissolution of the fibers
(Broadbent, 1997). Therefore, extraction-based techniques have to be
employed. A key strategy will be to improve the efficiency of recovery of
DNA from whole fibers. Proteolytic enzymes such as proteinase K (Sigma)
can partially degrade animal fibers but the cells of the cortex remain intact
even after several days incubation with the enzyme (Fig. 12.9). A second
problem relates to the large number of fibers that will have to be analyzed
DNA analysis in the identification of animal fibers in textiles
235
12.9 Degradation of wool after extended incubation with proteinase K.
The spiked shaped fragments are released cells of the cortex (from
Broadbent, 1997).
to give an accurate result making any test prohibitively costly using manual
techniques. However, automated systems have been developed for both
DNA extraction and amplification, and chip-based systems will ultimately
lead to low cost procedures that can handle large numbers of samples.
The latter are starting to have an impact in medical diagnostics and
related areas.
12.6
Sources of further information and advice
BTTG can analyze fiber, yarn or fabric samples for goat DNA to confirm
the presence or absence of goat fibers (the DNA test cannot distinguish
between cashmere, cashgora or mohair), sheep DNA (for sheep’s wool) and
yak DNA (for yak hair). The DNA test is qualitative only. We therefore
cannot report a percentage figure for the level of each fiber type in blends.
The results are reported as the presence or absence of goat DNA, sheep
DNA or yak DNA. The presence of the DNA can be used to infer the presence within the product of fiber from the target animal. Samples can also
be analyzed for rabbit DNA (for angora) and alpaca (for Camelid fibers).
Commercially, the present position at BTTG is that fiber blend analysis is
carried out using the projection microscope for quantification (i.e., the different types of fibers in the blend are identified, their diameters measured
and used to calculate the percentage by weight of the components of the
blend) and DNA analysis is employed to objectively confirm the nature of
those fiber types. This joint approach provides an objective speciality fiber
236
Identification of textile fibers
analysis service that is currently available to our customers. Please contact
the author of this article for further details.
12.7
References
Appleby J M, Nelson G, McPherson, M J and Hamlyn P F (1997), ‘PCR amplification
of the RuBisCo small subunit genes and their novel application to plant tissue
identification’, Heredity, 79(6), 557–563.
Bayliffe A I (1999), Speciation of livestock in textile and food industries: application
of DNA diagnostics, PhD thesis, University of Leeds, UK.
Broadbent E L (1997), The identification and quantification of the constituents in
processed animal fibre blends, PhD thesis, University of Leeds UK.
Brown R J P (2003), Genetic variation in the domestic goat Capra Hircus, PhD thesis,
University of Leeds, UK.
Hamlyn P F, Nelson G and McCarthy B J (1990) ‘Applied molecular genetics – new
tools for animal fibre identification’, Speciality Fibres: Scientific, Technological and
Economical Aspects (A Körner, F J Wortmann, G Wortmann and H Höcker,
eds), Schriftenreihe des Deutschen Wollforschungsinstitutes, a.d. Technischen
Hochschule Aachen, e.v.,106, pp. 249–258.
Hamlyn P F, Nelson G, and McCarthy B J (1992) ‘Wool Fibre Identification using
Novel Species-specific DNA Probes’, Journal of the Textile Institute 83(1),
97–103.
Hamlyn P F, Ramsbottom S, McCarthy B J and Nelson G (1996), ‘Analysis of
speciality fibres using DNA amplification techniques’, Metrology and Identification
of Speciality Animal Fibres (J P Laker and F J Wortman, eds), European Fine
Fibre Network, Occasional Publication No. 4, Macaulay Land Use Research
Institute, Aberdeen, Scotland, pp. 59–68.
Hamlyn P F, Nelson G, Asghar N and McCarthy B J (2001) ‘Identification of
speciality animal fibres using DNA profiling’, 3rd European Symposium and
SUPREME European Seminar Progress in South American camelids research,
Göttingen, Wageningen Pers, pp. 117–121.
Higuchi R, Bowman B, Freiberger M, Ryder O A and Wilson A C (1984), ‘DNA
sequences from the quagga, an extinct member of the horse family’, Nature 312,
282–284.
Higuchi R, von Beroldingen C H, Sensabaugh G F and Erlich H A (1988) ‘DNA
typing from single hairs’, Nature, 332, 543–546.
Kalbe J, Kuropka R, Meyer-Stork S, Sauter S L, Loss P, Henco K, Riesner D, Höcker
H and Berndt H (1988), ‘Isolation and characterisation of high-molecular mass
DNA from hair shafts’, Biol. Chem. Hoppe-Seyler, 369, 413–416.
Nelson G, Hamlyn P F and Holden L (1992) ‘A Species-specific DNA Probe for
Goat Fibre Identification’, Textile Research Journal, 62(10), 590–595.
Subramanian S, Karthik T and Vijayaraaghavan N N (2005). ‘Single nucleotide
polymorphism for animal fibre identification’ J. Biotechnol., 116(2), 153–158.
Wheeler J C (1995), ‘Evolution and present situation of the South American
Camelidae’, Biol. J. Linnean Society, 54, 271–295.
13
Identifying plant fibres in textiles:
the case of cotton
S GORDON,
CSIRO Materials Science and Engineering, Australia
Abstract: This chapter discusses techniques and test methods for
identification of cotton fibre properties and the existence of cotton fibres
in textile products. Identification of cotton fibres by their physical,
chemical and genetic attributes serves various commercial purposes
along the cotton supply chain. In the first part of this chapter the
physical attributes of raw cotton, upon which the fibre is initially valued,
are described. These have a large effect on cotton’s processing ability
and are of particular importance to the spinner and fabric manufacturer.
Whilst appreciation of these physical attributes is often overlooked by
the final consumer, high value is often attached to perceptions of the
fibre’s origin or genetic make-up. In this case, identification of cotton
fibre and origin of fibre in retail products is important, but often quite
difficult to test. Current and new techniques for identifying cotton fibre
in fabric and its origin are discussed in the second part of this chapter.
Key words: cotton fibre properties, identification of cotton fibres, cotton
fibre origin, genetically modified cotton, quantitative analysis of cotton
in textiles.
13.1
Introduction
For spinners who buy their raw cotton from growers, or more usually cotton
merchants, identification of the physical properties in the cotton they buy is
essential for optimising processing efficiency and end-quality in the spinning
process. Fibre quality specification for the spinner represents one aspect of
fibre identification; where quality, and usually the origin of cotton is specified, classed and checked on delivery as part of the sales contract between
the merchant and spinner. The importance of this specification is based on
the high cost of cotton in terms of mill operating costs and the adverse
consequences of having cotton outside specification in the manufacturing
process. Accordingly, in this part of the supply chain where there is high risk,
there is good information available to the spinner about the cotton they buy.
However, this information on the quality and origin largely disappears by
the time the finished cotton product appears in a retail outlet.
While many surveys show consumers prefer to wear clothing made of
cotton, and that consumers think fibre content is an important factor when
choosing garments (Cotton Incorporated, 2006), this awareness does not
239
240
Identification of textile fibers
extend to a detailed appreciation of the origin and physical properties of
the fibre in the product they choose. Consumer preference is by-and-large
driven by price and the consumer’s own perception of quality, which depend
more on emotive attributes such as design, colour and style. Technical
quality often comes a distant second in the quality consideration stakes.
However, it can be said high quality fibre is generally converted into high
quality fabric and garments, and that poor quality fibre is generally, although
not always, converted into lower quality fabric and garments.
The connection between origin, quality and price comes to the fore in
situations when the price of the product is determined by the ‘type’ of
cotton used. For example, products labelled as being made from ‘Egyptian’
cotton suggest to the purchaser they should expect their purchase to be
made from high quality extra long staple (ELS) Egyptian grown Gossypium
barbadense cotton, which is not always the case. In this type of situation the
market requires the fibre in the garment to be identified, or the percentage
of a particular fibre in the garment to be measured, in order to bring assurance to the licensing of the product Trademark particularly between the
retail, wholesale and manufacturing segments. The tests for origin and presence are more forensic-like in their scope and represent the other end of
fibre identification in the cotton supply chain.
In this chapter the importance of identifying cotton fibres in terms of
their quality, for the spinner, and their traceability from origin or quality in
finished goods for the consumer, are discussed. A brief introduction of the
cotton supply chain and cotton market is given, before the structure and
physical properties linked to the identification of cotton, for the two most
widely cultivated species, i.e. Gossypium hirsutum known as Upland cotton,
which constitutes around 94% of world production, and G. barbadense,
referred to widely as Egyptian or Pima type cotton and which makes up
3–4% of world production, are discussed.
Many of the techniques used to identify and measure cotton fibres are
applied widely to other fibres and these have been discussed in earlier
chapters. In this chapter, test methods specific to the measurement of cotton
fibre quality are discussed along with the most often used methods to identify cotton fibres from yarn or textile samples of unknown origin, including
analysis of remnant DNA extracted from cotton fibres, which has recently
been reported. The chapter concludes with a brief discussion on future
trends in the identification of cotton fibre.
13.1.1 Cotton supply chain
The significance of cotton in the world textile market is evident by its majority share among fibres for traditional textile goods such as apparel and
home furnishings. In this market cotton is used in more than 60% of the
Identifying plant fibres in textiles: the case of cotton
241
Grower (cotton/module) → Ginner (bale) → Merchant (bale lots) → Spinner
(yarn) → Knitter/Weaver (fabric) → Dyeing/Finishing (dyed finished fabric)
→ Design/Cutting/Sewing (product) → Wholesale (product distribution)→
Retail (product sale)
13.1 The cotton supply chain and the points at which fibre specification or identification testing usually occurs.
staple yarn (Oerlikon, 2007), the intermediate that is knitted or woven into
fabric for traditional textiles. The dominance of cotton in this market is in
part due to the economics of production, distribution and manufacture but
also the structural and physical properties of the fibre. Figure 13.1 illustrates
the cotton supply chain for traditional textile products. The importance of
fibre to fabric engineering from the perspective of the manufacturer is discussed in comprehensive detail in papers by El Mogahzy (1992a, 1992b).
For this discussion it is sufficient to say the specification of fibre quality
during manufacture is paramount for achieving good processing performance and high quality levels of end-product at the lowest cost possible.
Whilst the bulk of world cotton is sold into traditional textile markets,
there is a range of non-textile end-uses where cotton fibres are used. These
include dissolving fibres for pulp from which cellulose intermediates and
synthetic fibres and casings can be made, the production of felts for cushions, pads, automotive upholstery and furniture upholstery, the production
of absorbent medical grade cotton for cotton balls and swabs and fibre pulp
for specialised papers including fine writing paper, filter paper, currency,
sanitary products and battery separators. In many of these products cotton
fibres are used as a pure source of cellulose, e.g. as pulp for specialised
papers, and as such is no longer recognisable in its fibre form, although its
cellulose structure might still be recognisable. In other products such as
wadding in upholstery and mattresses, felts and pads the fibre used is often
sourced as a by-product of other cotton fibre conversion processes, e.g. short
fibres or cotton noil from the combing process before spinning, or sourced
from recycled (shredded) cotton fabric waste. In these situations, whilst the
fibre is recognisable as cotton, the original physical quality of the fibre is
no longer obvious.
13.2
Cotton fibre structure and composition
Cotton is a unicellular fibre that grows from the epidermis cells on the
surface of cotton seeds. From a structural perspective the cotton fibre is one
242
Identification of textile fibers
of the largest and simplest plant cells; a singularly discrete, elongated cell
that has no junctions or inter-cellular boundaries that compromise the
physical and chemical processing of other vegetable fibres. Figure 13.2
shows G. hirsutum fibres elongating from the seed epidermis at 2 to 3 days
post anthesis (dpa). Fibre growth is continuous and extends for 50 to 60 dpa
(a)
(b)
13.2 (a) Gossypium hirsutum seed (ovule) at 2 to 3 days post anthesis
and (b) new fibres elongating from seed epidermis (CSIRO Plant
Industry).
Identifying plant fibres in textiles: the case of cotton
243
in two nominally separate growth phases. Initially from flowering, the cell
elongates for 20 to 30 dpa. The realised fibre length during this period is
dependent upon the plant’s genetics and the environment, in particular on
plant water stress. There is some question as to whether fibre fineness
or more correctly the perimeter or biological fineness (Ramey, ca 1982),
changes during this phase. Investigators in the past have assumed it remains
constant according to the genetics of the variety or species. More recent
thinking proposes the perimeter might also expand during this phase.
The elongation phase is followed by a cell wall thickening phase where
the secondary cell wall of the fibre is laid down in a series of concentric
growth rings or lamellae that reflect diurnal temperature fluctuations during
development (Kerr, 1937). At maturation the cotton fibre remains hollow
as a result of the remnant protoplasm, which is referred to as the lumen;
the dimensions of which become complementary to measurements of the
fibre’s cell wall dimensions, which in turn have an important influence on
fibre processing ability.
The fully developed cotton fibre consists of a waxy cuticle enveloping the
fibre, a cell wall that is differentiated into primary (outer) and secondary
(inner) layers, and the lumen. Some regard the cuticle and primary wall
layers as a single layer; however, others (Maxwell et al., 2003) have revealed
the primary wall is distinct from the waxy cuticle. The wax layer surrounding
the outside of the cell wall is essential for the efficient processing of cotton
fibre into spun yarn. It provides a lubricating layer that reduces fibre-tometal friction and therefore fibre breakage during mechanical processing.
The downside is this layer also acts as an impermeable barrier to the entry
of water and dye molecules into the fibre. The chemical composition of the
wax is complex and contains a number of lipid classes including wax alkanes,
fatty acids, fatty alcohols, plant steroids and mono, di and triglycerides
(Fargher and Probert, 1924, Fargher and Higginbotham, 1924, Hornoff and
Richter 1964). There appears to be no significant inter-species differences
in lipid content, although Amin and Truter (1972) found the waxes extracted
from G. barbadense fibre contained a lower proportion of hydrocarbon. For
successful dyeing, this barrier and particularly the wax alkanes must be
removed by scouring and/or bleaching.
The fibre cell wall itself represents one of the purest sources of cellulose.
The cellulose content of raw cotton ranges between 86 and 96% by dry
weight depending on its maturity and the method of determination
(Mauersberger, 1954). There is little inter-species or inter-varietal difference
in cotton cellulose content. The other main but smaller components of
cotton excluding water, are pectins (0.7–1.2%), proteins (1.1–1.9%), waxes
(0.4 – 1.0%) and minerals measured as ash content (0.7–1.6%). After scouring and bleaching, which removes most of these, dried cotton converts to a
purity of nearly 99% cellulose. In comparison, bast fibres such as flax, jute,
244
Identification of textile fibers
ramie, kenaf and cannabis, from the stems of these plants are about
three-quarters cellulose. Wood fibre including coniferous, deciduous and
eucalyptus, contain 40–55% cellulose, with other plant parts and species
containing much less cellulose. The cellulose ‘monomer’ in cotton fibres,
i.e. a β-1,4-D(+)-glucopyranose molecule linked by 1,4-glucodic bonds, is
polymerised to form the highest molecular weight compound of all plant
fibres with a degree of polymerisation between 3500 and 10 000 units (Liang
and Marchessault, 1959, Weiss, 1972).
The spatial arrangement of cellulose fibrils and larger aggregates determines the morphology and properties of a fibre and in maturing cotton, the
arrangement of these fibrils differs, depending on the state of fibre development. The fibrils in the primary wall are not well aligned and have a crystalline index of 30% in contrast with 70% for mature fibres measured at
42 dpa (Bolyston and Hebert, 1995). In mature fibres the secondary wall
comprises about 94% of the fibre material (Goynes et al., 1995) and thus
dominates the mechanical and physical chemical properties of the fibre. An
impression of the overall structure of the cotton fibre described by Jefferies
et al. (1969) is shown in Fig. 13.3. In the secondary wall cellulose fibrils are
laid down in helical layers on the inside of the fibre at the protoplasm
boundary, with helix angles changing from around 35° in the outer layers
to 20° in the inner S3 layers.
Lumen
S3 20°
S2 20–30°
Reversal
S1 20–35°
Primary wall
Pectin
Fats
Waxes
13.3 Representation of cotton fibre structure by Jefferies et al. (1969).
Identifying plant fibres in textiles: the case of cotton
245
13.4 Cross-sections of cotton fibres embedded in methyl-butyl
methacrylate resin showing cross-section shape, cell wall and lumen
(CSIRO Textile and Fibre Technology).
When a cotton boll opens, the fibres are long, cylindrical and turgid cells
containing living protoplasm. With opening there is a loss of water and the
fibres dehydrate and become flattened and twisted. The twists that arise are
referred to as convolutions and are dependent principally upon wall thickness, which in turn is dependent upon variety, fibre maturity and drying
conditions. The cross-section of the cotton fibre after dehydration is generally kidney-shaped, although the shapes range from near circular in mature
fibres to a flattened shape in immature fibres. The kidney shape is an inherent phenomenon due to zones of different density in the secondary wall
layers (Kassenbeck, 1970). Figure 13.4 shows cross-sections of cotton fibres
embedded in methyl-butyl methacrylate resin featuring the variations in
shape around the typical kidney-shaped cross-section.
13.3
Cotton fibre properties
The structure of cotton cellulose in the fibre cell wall is comparatively
uniform across species and varieties and this is significant in considering the
range of physical and chemical processing properties in cotton fibres. The
structural uniformity in mature fibres across varieties and to a certain extent
species means the physical and chemical differences between cotton of
246
Identification of textile fibers
different origins are quite small in comparison with differences within other
natural fibres. The consequences of this natural uniformity with regards to
processing include the ability to continuously rather than batch process along
the cotton supply chain, the relative ease in blending cotton of different
origins and a requirement for accurate and precise fibre testing methods.
The main physical properties measured and specified in cotton are length
(measured today as the upper half mean length (UHML) (Woo, 1968)),
bundle strength and fineness (measured as the Micronaire value), and these
alone can explain up to 93% of the variation in yarn strength (Bell and
Gilham, 1989), an important measure of yarn quality. Other fibre properties
important in processing include short fibre content, i.e. percentage of fibres
less than 12.5 mm, maturity (the degree of cell wall thickening), elongation
and trash content.
These physical properties are expressions of the particular cotton variety
and the conditions under which it was grown. The cotton grower’s selection
of variety and the decisions with respect to nutrients, pests, water, harvesting
and ginning all affect the final quality of fibre produced. In the future, these
decisions are likely to be more complex particularly as interactions between
these inputs, and new variables such as genetically modified varieties that
produce enhanced fibre quality come to light.
Table 13.1 lists the differences in physical dimensions and properties
between cotton species, and between flax, ramie and polyester. Whilst the
broad structure of cellulose in mature cotton fibres is similar, differences in
terms of cellulose fibril spiral angle have been measured between different
species of cotton (Meredith, 1951). These differences are reflected to some
degree in different stress and strain and birefringent (optical) properties of
the fibre.
Colour differences between species are explained by the existence of pigments, pectins, waxes and proteins in the cuticle and primary layer of the cell
wall, and are removed by the scouring and bleaching processes applied to all
cotton. Differences in dye uptake are largely dependent on the maturity of
the cotton sample whatever the species, although fibre fineness also plays a
role in dyed appearance. Immature fibre, that is fibre with little or no fibre
wall thickening, is associated with non-uniform dyeing of fabrics (Pal and
Esteve, 1959, Smith, 1991) and increased relative wax content (Gordon et al.,
2002). Mature fibre with more cellulose and therefore binding sites for dye
molecules is able to hold more dye and will appear darker dyed against
immature fibre. Fibre maturity also affects the lustre of the fibre.
13.3.1 Measuring differences in processing ability
For the three short-staple spinning systems used by the cotton textile industry today, i.e. ring, rotor and air-jet spinning, the fibre properties listed in
Identifying plant fibres in textiles: the case of cotton
247
Table 13.1 Typical fibre properties for cotton of different species, variety
and origin
Species/Variety
Gossypium
hirsutum
US CA1
US MS1
Australia2
China3
UHML*
(mm)
29
28
29
28
Diameter
(μm)
Linear
density
(mtex)
Bundle
strength
(cN/tex)
Birefringency
n1 − n2**
13–14.7
14.2–16.8
13–14.7
14–16
160–190
180–220
175–215
180–210
>33
>28
>30
>28
0.046
0.044
G. barbadense
Giza var. ELS4
Giza var. LS4
Indian ELS
US Pima1
36
32
>32
35
11.5–13
12–14
11.5–13
12–14
125–140
155–170
140–165
155–170
>43
>35
–
>40
0.049
G. arboreum
Bengalense var.
<20
>20
>300
18.5
0.043
Linum
usitatissimum
Flax (linen)5
>27***
8–25***
22–36
0.060
Boehmeria nivea
Ramie
Polyester
>60***
var
28–35***
var
up to 75
0.160
50–250
* UHML = upper half mean length, which is nominally equivalent to staple
length
** n1 = axial refractive index and n2 = transverse refractive index
*** dependent on degree of retting, decortication and hackling
1. http://www.ams.usda.gov/cotton/mncs/
2. http://www.austcottonshippers.com.au/
3. http://www.egyptgizacotton.com/
4. Gordon et al., 2004
5. Sampaio et al., 2005
Table 13.2 Major fibre parameters in short-staple spinning systems in
order of importance to productivity and quality
Importance rank
Ring
Rotor (open-end)
Air-jet (inc. MVS)
1
2
3
4
Length
Strength
Fineness
Strength
Fineness
Length
Trash
Length
Trash
Fineness
Strength
Table 13.2 are considered especially important. In combination, the listed
parameters describe fibres that are sufficiently flexible to accommodate the
continuous rearrangement of fibers during drafting in spinning; have a high
length to diameter ratio to permit flexibility, effective consolidation and
248
Identification of textile fibers
inter-fibre coherence; and have surface properties that allow smooth
drafting.
Until 50 years ago a sample of cotton taken from the ginned bale was
assessed for its processing ability against physical cotton standards by a
human cotton classer. Comparison with physical standards, whether those
of the United States Department of Agriculture (USDA) Agricultural
Marketing Service (AMS), to which there are over 20 signatory cotton
associations from around the world, or other national country standards, is
still the predominant method for measuring the value of cotton fibre. There
is currently active organisation from within the international cotton industry to promote objective test measurement of important fibre parameters.
It is estimated that between 30 and 40% of the world cotton crop today is
classed using objective instrument test methods (Qaud, 2008).
Subjective classing by hand and eye with the help of physical reference
standards has been the predominant method of grading cotton fibre quality
since cotton trading began. Establishment of formal cotton classification
standards occurred in the United States after the 1907 International Cotton
Congress, at a time when the commercial trade of raw cotton and fabrics
made from cotton yarns reached significant volumes. Even at that time it
was recognised that cotton destined for the manufacture of textiles for
household and clothing products, demanded the development of measurements for predicting yarn and fabric attributes.
The first cotton classification standards for fibre colour and length grades
were established in 1909 by the USDA. The standards for colour were at
first based on physical samples that exhibited a range of colour. Cotton fibre
length was judged by a human cotton classer using a manual technique that
involved pulling fibres away from small bundles into a spread of fibres from
which the fibre length, or staple length as it is still known, could be determined. The staple length and the UHML are regarded as being close but
not identical measurements of the length of the longest fibres in cotton.
A Universal Cotton Standards Agreement was established in 1923
between the USDA and 23 other cotton associations from 21 countries. The
USDA Universal Cotton Standards now cover strength, length, uniformity
index, Micronaire, colour grade and procedures used to achieve agreement.
Other nations have developed or are in the process of developing their
own official cotton standards and descriptions, e.g. Chinese cotton grade
is classed according China’s Cotton Colour Characterisation Chart that
describes grades with similar relativity to the USDA Colour Grades. China
is also currently developing quality standards for strength, fineness,
maturity and uniformity applicable to high volume instrument (HVI) testing
(Butterworth and Xinping, 2004).
A range of objective physical test methods now exist to class cotton and
determine its value for the grower and spinner. The instruments used to
Identifying plant fibres in textiles: the case of cotton
249
13.5 High volume instrument lines used to class cotton objectively
(Australian Classing Services).
conduct these tests were developed in the early twentieth century and initially involved the use of microscopes, weighing scales and comb sorters
to literally measure fibre dimensions. Demand for quicker objective test
methods by growers seeking more transparent classing results, and spinners
demanding more information to optimise quality and productivity in
their mills led to the development of the HVI lines used today (Fig. 13.5).
The instruments in these lines measure length, length uniformity (index),
strength, extension, the Micronaire value, colour and trash content. Whilst
test instruments in these high volume lines are nominally calibrated with
direct, first principal reference methods, the fibre properties are measured
indirectly via strain gauges, light meters, air-flow meters, pressure gauges,
digital scanners and capacitors. The indirect methods introduce some
questions about the cause of effects observed; however, the accuracy and
precision particularly for length and strength parameters are generally
well accepted. Questions about the accuracy of objective measurement of
fineness by the Micronaire instrument (Lord, 1956, Lord and Heap, 1988)
and short fibre content (Heap, 2004 and Robert et al., 2005) are well
documented.
The colour of cotton lint has always played a major part in assessing and
marketing fibre value, although for nearly all base grade export cotton, e.g.
USDA 31-3 or Xinjiang 129, traded each year colour is largely irrelevant
as cottons of USDA Classing Grade 31 and higher show very little difference between each other in terms of processing ability.
250
Identification of textile fibers
Table 13.3 HVI properties for export cottons into South East Asian spinning
mill laydowns sampled during 2004
Origin
UHML
(mm)
UNI (%)
SFC
(%)
STR
(g/tex)
EXT
(%)
COL
(Rd)
YEL
(+b)
MIC
(μg/in)
USA SJV
USA TX
USA CA
Australia
China
W. Africa
29.0
28.6
28.1
28.7
28.5
28.0
82.3
81.9
81.8
82.1
82.7
82.0
9.3
10.3
10.5
10.1
8.1
9.0
31.3
29.7
28.0
29.3
28.1
27.9
7.2
7.4
7.35
7.6
7.5
7.4
78.2
77.0
78.0
78.3
80.1
76.1
9.2
7.8
9.0
8.6
9.0
9.1
4.22
4.18
4.25
4.30
4.05
3.90
Today, a HVI line, which incorporates instruments for length and length
uniformity, fineness, bundle strength and extension and colour, measures
825 samples in a 7-hour and 20-minute shift (Ghorashi, 2006). Since 1980
there has also been the development of low volume test instruments to
measure other important fibre properties such as stickiness, maturity, and
the distribution of fibre properties such as length and trash. Table 13.3 lists
the HVI properties measured on export-grade cottons destined for the
same laydowns in South East Asian spinning mills during 2004 (Gordon
et al., 2004). Notable is the consistent average quality between growths
of different origin.
13.3.2 Identification of cotton fibre origin in textiles
After fabric manufacture, cotton as an apparel or home furnishing product
strongly retains its identity, although this identity is not usually linked to
the specific country of origin nor, as described earlier, to its specific quality.
The exception to this is where a particular growth brands and controls the
supply chain through to the finished garment using a licensing system, e.g.
Supima®, the promotional organisation of the American Pima cotton
growers. Without the onus of this type of system cotton is usually blended
with a range of growths on the basis of price, quality and availability.
Classification and authentication of cotton geographic origin is therefore
important to brand owners and to governments that regulate international
cotton trade. The development of methods and in particular the potential
of genetic-based methods to identify the cotton fibre content of finished
textiles would represent a significant opportunity for license holders to
control their brand and for governments to improve their ability to enforce
compliance with trade agreements between nations.
Even comparisons based on physical fibre properties do not provide a
clear-cut distinction of long, fine G. barbadense fibre from ‘lesser’ quality
Identifying plant fibres in textiles: the case of cotton
251
13.6 Comb-sorter staple array of ginned cotton (CSIRO Textile and
Fibre Technology).
G. barbadense or Upland cotton. Fibre from yarn pulled from a nominated
cotton product and untwisted to extract enough fibres (>60 mg) can be used
to build a comb sorter array (American Society for Testing and Materials
(ASTM) Standard Test Method D1440, 2007), which provides some distinction of cotton type on the basis of length characteristics. However, this
approach is unable to clarify whether or not a fibre is from a particular
origin, only that it is likely to be of a particular quality and hence species.
Figure 13.6 shows a comb sorter array for an ELS type cotton prior to
combing. The mean of the longest fibres is the most telling in this test as
this length measurement corresponds with the length values observed in
Table 13.1. Using this approach, high quality ELS cotton, with very long
staple length, can be distinguished with reasonable confidence from lesser
quality Pima and longer staple Upland cottons. Calculation of the short
fibre content in the array also allows comments to be drawn about whether
or not the fibre has been carded or combed.
The diameters of fibres drawn from the same specimen can also be measured and the values compared with the expected ranges listed in Table 13.1
for various varieties and species. Figure 13.7 shows a compilation of comb
sorter (effective length) and diameter test results measured on fibre specimens from the pile yarns of bathing towels labelled as being ‘Egyptian’ in
origin. The graph shows the cut-off points for Upland and Egyptian-type
(G. barbadense) cottons on the basis of the expected length and diameter
ranges. The effective length is on average 4 to 5 mm less than the comb
sorter equivalent of the UHML.
Another identification issue relates to the current and future production
of genetically modified (GM) cotton, e.g. Roundup Ready® and ‘Bt’ cotton.
To date nine countries have commercialised GM cotton, and around 36%
of the world cotton area in 2006/07 was planted to GM varieties (Townsend,
2007). The realisation of new cotton varieties via GM with unique fibre
Identification of textile fibers
Effective length (mm)
252
35
33
31
29
27
25
23
21
19
17
15
10
11
12
13 14 15 16 17
Diameter (microns)
G. barbadense
18
19
20
G. hirsutum
13.7 The relationship between comb sorter (length) and diameter test
results for fibre specimens drawn from towel pile yarn.
quality characteristics, e.g. fibre with improved extensibility or moisture
absorption abilities, whilst not likely in the next ten years, would mark a
dramatic and irrevocable change along the cotton supply chain. There are
currently no trade barriers for fibre from GM cotton, largely because of the
difficulty to date of measuring the difference between non-GM and GM
fibre, but also because cotton fibre from GM plants has shown no adverse
quality or health effects to the manufacturer or consumer. Issues also exist
with the identification and certification of organic cotton, which according
to US organic and other international standards do not allow the use of
biotech (read GM) cotton (Wakelyn and Chaudhry, 2007).
Identification of remnant DNA in fibre cell protoplasm using the
Polymerase Chain Reaction (PCR) is thought of as the optimal solution in
the authentication of cotton’s origin (species). With the development of GM
cotton, consolidation in the number of seed cotton production companies
and the increase in market value of growths with a labelled origin, the
stakes in fibre and seed cotton authentication have grown. To this end a
number of biotech companies have sought to be able to provide a test for
genetic authenticity.
Extraction of identifiable DNA from seed and other plant matter, e.g.
leaves, has been achievable since techniques for DNA extraction and purification first appeared. However, until recently no one company or research
group had reported being able to successfully extract and amplify the
DNA from mature cotton fibre to enable identification. In fact extraction
efficiency of DNA material from fibre specimens using the widely used
cetyltrimethylammonium bromide (CTAB) extraction method (Rogers and
Identifying plant fibres in textiles: the case of cotton
253
Bendlich, 1994) with subsequent DNA precipitation, relied on cotton fibres
being raw, i.e. not washed (read scoured) or wet treated in any way, and
there being seed material contained within the specimen.
However, according to a recent press release (Applied DNA Sciences
Inc., January 2008), Applied DNA Sciences Inc., a company that specialises
in “DNA-based security solutions”, has developed a new extraction protocol that improved DNA isolation ‘one thousand-fold’ over previous
methods. The extract of cotton (fibre) cell nuclear and chloroplast genomes
could be assayed to identify the difference between Pima (G. barbadense)
and Upland cotton. The company had been sponsored by Supima® growers
to develop an authentication method to identify and confirm the Supima®
cotton content of branded apparel and home furnishing products.
Measurement of the method’s sensitivity in detecting DNA from processed
fibre, and its ability to distinguish between varieties of the same species
were not detailed in the press release.
13.3.3 Identification tests for cotton fibre in textiles
For situations where the presence of cotton in a fabric or garment needs
to be identified, there are a range of standard qualitative test methods
that use techniques and measures that will have been described in earlier
chapters. These standards or their equivalents in other countries are
used to identify fibres under various product identification, labelling and
trade regulations throughout the world. For example, the Federal Trade
Commission (FTC) in the USA identifies generic fibre names described in
legislature but relies upon the American Association of Textile Chemists
and Colourists (AATCC) Test Method for identifying those fibres. The
standards are also used by yarn and fabric buyers to check and accept
purchases.
The most widely used standards in this regard are the AATCC Test
Method 20, which describes a range of physical, chemical and microscopical
tests for identifying commercially used textile fibres and the ASTM Standard
Test Method D276, which contains similar techniques as well as descriptions
for the use of infrared spectroscopic techniques. It is noted that where the
infrared spectrum indicates native (raw) cellulose, then it is desirable to
resort to microscopical analysis to elucidate the type of fibre.
Under a light microscope cotton fibres are recognised by the presence
of the lumen and convolutions, i.e. twists along the length of the fibre.
Gossypium barbadense varieties contain fewer convolutions than G. hirsutum varieties although the difference is small and variable (Meredith, 1951).
Other features of convolutions, e.g. the convolution angle, show no difference between species, although a statistical albeit speculative relationship
254
Identification of textile fibers
13.8 Cotton fibres under polarised light microscopy showing reversals
marked with arrows at reversal boundary.
exists between the convolution angle and the angle of cellulose fibril orientation (Duckett and Cheng, 1972). Another unique feature of cotton fibres
are the reversals in the direction of the spiral (fibril) structure or helix along
the length of the fibre. At the point of reversal, the fibrils for a short interval
lie parallel with the fibre axis. They are clearly highlighted as changes in
transmitted interference colours under polarised light microscopy when
using a first order red compensator plate, e.g. as per the ASTM Standard
Test Method D1442. Figure 13.8 is a photo showing the reversals in mature
and immature cotton under a polarised light microscope. Cross-sectional
analysis also reveals the unique ‘kidney’ cross-sectional shape of the cotton
fibre (see Fig. 13.4).
Microscopical analysis is also useful in elucidating various treatments
that have been applied to cotton fibre. For the most part the resolution of
the light microscope (up to ×400 magnification) is suitable for identifying
most treatments and conditions associated with cotton fibres. For example,
microscopical examination easily reveals whether fibre has been mercerised
in concentrated sodium hydroxide (18–25% NaOH). Mercerised cotton
appears smoother along its length because the swelling effect of the process
makes the convolutions less easy to observe, although the reversals are still
apparent under polarised light microscopy. The cross-sections of mercerised
cotton fibres are more circular, which results in a more uniform reflecting
Identifying plant fibres in textiles: the case of cotton
255
surface and increased fibre lustre. The lumen is typically smaller in a
mercerised fibre.
Examination of cavitomic cotton fibres, i.e. fibre damaged by microorganisms such as cellulolytic bacteria and fungi, under a light microscope
can reveal fungal hyphae and fractures in the surface of the fibre. Chiefly,
cavitomic cotton is greyer and creates dye uptake problems in processing
because of the damage to cotton cellulose. Swelling cavitomic fibres in
concentrated NaOH causes differential swelling at fracture points along the
length of the fibre for identification purposes. Likewise the presence of
shrink-proofing and permanent crease resins can be seen under a light
microscope. For greater detail scanning electron microscopy can be used to
observe treatments and changes to the cotton fibre form.
13.3.4 Quantitative analysis of cotton in textiles
Quantitative tests for determining the cotton fibre blend composition of
mixtures of fibres are described in AATCC Test Method 20A and ASTM
Standard Test Method D629. Both also describe procedures for the estimation of the amount of moisture and non-fibrous materials in textiles. The
use of these tests follows the textile labelling laws in each country. For
example, according to the USA FTC each fibre type in a fabric must be
labelled by percentage in the order of preponderance by weight, as long as
it is greater than 5% of the total fabric weight. Percentages of fibres at less
than 5% of the fabric (garment) can be labelled at the discretion of the
manufacturer or brand owner. Fibres included as decorative or technical
aspects, e.g. elastic ribbon or tie cords, of the garment do not need to be
specified.
The particular test used depends on the fibre blend, the intimacy of the
fibre blend and the type of non-fibrous material to be measured. For cotton,
the situation is often that the fibre blend, usually with polyester but also
with rayon, spandex or wool in an intimate blend, needs to be tested to
check fibre proportions. Substitution of cotton by other fibres and viceversa is common particularly for polyester blends where the price of one
regularly varies along a world price parity line against the other. Because
cotton, unlike nearly all other fibres, does not undergo any chemical processing until fabric finishing, interest in the content of non-fibrous material
in cotton is unusual.
The analytical procedure for the quantitative analysis of cotton blend
textiles typically involves dissolving the nominated or suspected fibre in the
intimate blend using a solvent specific for that fibre. Cotton cellulose is
difficult to dissolve and there are a limited range of solvents. As a result
hydrolysis in a strong mineral acid, e.g. 70% sulphuric (H2SO4) acid is more
common. After dissolving or disintegrating the nominated fibre, the residual
256
Identification of textile fibers
fibre component is washed and dry-weighted to determine the percentage
of fibre in the blend. Blends of cotton with rayon fibres, which are extruded
from amorphous cellulose that is more reactive and easily hydrolysed, are
subject to a less concentrated mineral acid solution (59.5%) that ‘dissolves’
the rayon and leaves the cotton fibres.
13.4
Future trends
Specification of fibre quality in industrial countries, e.g. the USA, will occur
in the gin rather than at classing laboratories. According to Ghorashi (2006),
the future fibre testing system will be implemented in the gin. The testing
will be fully automated and installed inline with the process flow of the gin.
The system of the future will have remote monitoring, calibration and will
measure all pertinent fibre qualities a multiplicity of times.
Cotton identification in the future on the basis of recovered genetic
information extracted from the raw and processed fibre is contingent upon
satisfactory DNA-extraction procedures and assays sensitive enough to
reveal differences in the extracted genetic material. It remains to be seen
whether the genetic variation between varieties of the same species is large
enough to be a measurable point of differentiation. Determining the point
of origin or place of production will remain unlikely without the application
of a DNA-based, electro or nano-based labelling procedure.
13.5
Sources of further information and advice
Identification of Textile Fibres, The Textile Institute, Manchester, England,
7th Edition (1972).
Steadman, R. G., Cotton Testing, Textile Progress, 27(1), 66 pp (1997).
Gordon, S. G. and Hsieh, Y. L. (eds), Cotton Science and Technology,
Woodhead Publishing in Textiles, Cambridge, 547 pp (2007).
13.6
References
AATCC Test Method 20-1995, Fiber analysis: Qualitative, 38–61 (1997).
Amin, S. A. and Truter, E. V., Cotton lipids: A preliminary survey, Journal of the
Science of Food and Agriculture, 23, 39–44 (1972).
Applied DNA Sciences Inc., Applied DNA Sciences Receives Milestone Payments
from Supima Company to Exhibit at Supima Trade Show January 22–24, 2008,
press release published on January 10th 2008.
ASTM D276-00a Standard test methods for identification of fibers in textiles.
ASTM D629-99 Standard test methods for quantitative analysis of textiles.
ASTM D1440-07 Standard test method for length and length distribution of cotton
fibers (Array Method).
Identifying plant fibres in textiles: the case of cotton
257
ASTM D1442-06 Standard test method for maturity of cotton fibers (Sodium
Hydroxide Swelling and Polarized Light Procedures).
Bell, T. M. and Gilham, E. M., The World of Cotton, ContiCotton EMR, Washington
DC, 395–403 (1989).
Bolyston, E. K. and Hebert, J. J., The primary wall of cotton fibers, Textile Research
Journal, 65, 429–431 (1995).
Butterworth, J. and Xinping, W., Cotton and Products: Update on China’s Cotton
Classification Reform, USDA Foreign Agricultural Service, Gain Report No.
CH4039 (December 2004).
Duckett, K. E. and Cheng, C. C., The detection of cotton fiber convolutions by the
reflection of light, Textile Research Journal, 42, 263–270 (1972).
El Mogahzy, Y. E., Optimising cotton blend cost with respect to quality using HVI
fiber properties and linear programming, Part I: Fundamentals and advanced
techniquies of linear programming, Textile Research Journal, 62, 1–8 (1992a).
El Mogahzy, Y. E., Optimising cotton blend cost with respect to quality using HVI
fiber properties and linear programming, Part II: Combined effects of fiber
properties and variability constraints, Textile Research Journal, 62, 108–114
(1992b).
Fargher, R. G. and Probert, M. E., Alcohols Present in the Wax of American Cotton,
Journal of the Textile Institute, 15, 337–346T (1924).
Fargher, R. G. and Higginbotham, L., Constituents of Wax from Egyptian
Sakellarides Cotton, Journal of the Textile Institute, 419, 419–433T (1924).
Ghorashi, H., The Universal Transition from Manual to Instrument Cotton Classing,
Report to ITMF HVI Working Group, Bremen (2006).
Gordon, S. G., Evans, D., Church J., Petersen, P., Thom, S. L. and Woodhead, A., A
Survey of Cotton Wax Contents in Australian Cotton, Report to the Australian
CRDC, 34 pp (November 2002).
Gordon, S. G., Van Der Sluijs, M. H. J. and Prins, M. W., Quality Issues for Australian
Cotton from a Mill Perspective, Report to the Australian Cotton Industry,
Australian Cotton CRC (pub.), 54 pp (July 2004).
Goynes, W. R., Ingber, B. F. and Triplett, B. A., Cotton fiber secondary wall
development – time versus thickeness, Textile Research Journal, 65, 400–408
(1995).
Heap, A. S., Relative Short Fibre Content, Presentation to ITMF Length Working
Group, Bremen (2004).
Hornoff, G. V. and Richter, H., Chemical composition of cotton fibres originating
from various areas, Fasterforsch. Textiletech., 15, 165–177 (1964).
Jefferies, R., Jones, D. M., Roberts, J. G., Selby, K., Simmens, S. C. and Warwicker,
J. O., Current ideas on the structure of cotton, Cellulose Chemistry and Technology,
3, 255–274 (1969).
Kassenbeck, P., Bilateral structure of cotton fibers as revealed by enzymatic
degredation, Textile Research Journal, 40, 330–334 (1970).
Kerr, T., Cotton Hair Growth Rings: Structure, Protoplasma, 27, 229–241 (1937).
Liang, C. Y. and Marchessault, R. H., Infrared spectra of crystalline polysaccharides.
I. Hydrogen bonds in native celluloses, Journal of Polymer Science, 37, 385–395
(1959).
Lord, E., Air through plugs of textile fibres, Part II. The Micronaire Test for Cotton,
Journal of the Textile Institute, 47, T17–T47 (1956).
258
Identification of textile fibers
Lord, E. and Heap, S. A., The Origin and Assessment of Cotton Fibre Maturity,
International Institute for Cotton (pub.), 40 pp (1988).
Mauersberger, H. R. (ed), Matthew’s Textile Fibers, 6th Edition, John Wiley, New
York (1954).
Maxwell, J. M., Gordon, S. G. and Huson, M. G., Internal structure of mature and
immature cotton fibers revealed by scanning probe microscopy’, Textile Research
Journal, 73, 1005–1012 (2003).
Meredith, R., Cotton fiber tensile strength and x-ray orientation, Journal of the
Textile Institute, 42, T291–T299 (1951).
Oerlikon, The Fiber Year 2006/07, A World Survey on Textile and Nonwovens
Industry, Oerlikon (pub.), Issue 7 (May 2007).
Pal, P. N. and Esteve, R. M., A study of the relationship between dye absorption and
cototn fiber properties at equilibrium, Textile Research Journal, 29, 811–815
(1959).
Qaud, M., (Chair of ITMF HVI Working Group), personal communication (April
2008).
Ramey, H. H., The Meaning and Assessment of Cotton Fibre Fineness, International
Institute for Cotton (pub.), 40 pp (ca 1982).
Robert, K. Q., Dunn, M. C., Cui, X. L. and Price, J. B., Method for determining
broken fibre content in ring yarn, Proceedings of the Beltwide Cotton Conferences,
New Orleans (2005).
Rogers, S. O. and Bendlich, A. L., in Molecular Biology Manual, Gelvin, S. B. and
Schilperoort, A. R. (eds), Kluwer, Dodrecht, 2, 1–8 (1994).
Sampaio, S., Bishop, D. and Jinsong, S. S., ‘Physical and chemical properties of flax
fibres from stand-retted crops desiccated at different stages of maturity’, Industrial
Crops and Products, 21, Issue 3, 275–284 (May 2005).
Smith, B., A review of the relationship of cotton maturity and dyeability, Textile
Research Journal, 61, 137–145 (1991).
Cotton Incorporated, Textile Consumer, Global Consumer Apparel Shopping
Trends, 39 (Fall 2006) found at http://www.cottoninc.com/TextileConsumer/
TextileConsumerVolume39/.
Townsend, T., in Cotton: Science and Technology, Gordon, S. G. and Hsieh, Y. L. (eds),
Woodhead Publishing in Textiles, Cambridge, 425–456 (2007).
Wakelyn, P. J. and Chaudhry, M. R., in Cotton: Science and Technology, Gordon,
S. G. and Hsieh, Y. L. (eds), Woodhead Publishing in Textiles, Cambridge, 130–174
(2007).
Weiss, A. H., Conversion of solid waste to liquid fuel Textile Research Journal, 42(9),
526–533 (1972).
Woo, J. L., An appraisal of the length measures used for cotton fibres, Journal of the
Textile Institute, 59, 557–572 (1968).
14
The forensic identification of textile fibers
M M HOUCK, West Virginia University, USA
Abstract: The professional perspective of the analyst shapes which
characteristics are important for identification of the materials of
interest. In the case of fibers and forensic science, the mindset is focused
on limited sample size, murky origins, and potentially uncertain
provenance. Microscopy, therefore, becomes the primary method of
choice for forensic fiber identification.
Key words: forensic, microscopy, investigations.
14.1
A forensic mindset
The forensic mindset began long ago and is intimately tied to the medical
mindset of diagnosis. Three names routinely repeat in the historical literature on this topic: Giovanni Morelli, Sigmund Freud, and Arthur Conan
Doyle (Ginzburg, 1979; Vakkari, 2001). All of their notions of what might
loosely be called ‘detection’ hinge upon the identification and recognition
of seemingly common or insignificant bits of information that lead to the
questions at issue. Interestingly, all three were doctors or had studied medicine. Morelli created a ruckus in the art world by publishing a critique of
Italian paintings in Munich (under a pseudonym, Ivan Lermolieff) that
chastised the status quo for misidentifying the artist of several paintings.
Morelli based his method on details normally overlooked by other critics,
such as how hands or ears were shaped and rendered (Vakkari, 2001); he
felt that in these details, the artists were not being mindful of their style
and let their individuality shine through. Morelli made hundreds of reattributions of authorship and was judged as correct in over half of them.
Freud had read Morelli’s work (as Lermolieff) had commented:
It seems to me that his method of inquiry is closely related to the technique
of psychoanalysis. It, too, is accustomed to divine secrets and concealed things
from unconsidered or unnoticed details, from the rubbish-heap, as it were, of
our observations (as cited in Wind, 1963).
Freud later acknowledged that Morelli had an influence on his method
of psychoanalysis. Doyle’s connection to this triad is conceptual in that he
was also a medical doctor and learned the art and science of diagnosis in
medical school. One of Doyle’s instructors, in fact, Joseph Bell, who was
particularly adept at using minor details to accurately diagnose disease
259
260
Identification of textile fibers
conditions, was an influence for the character of Sherlock Holmes (Ginzburg,
1979). Things, then, are heavily coded as to their origins (who is the author
of this painting, what is the source of this mental aberration, who committed
this crime?) and it is up to the investigator to discern the subtle indicators
which reveal the hidden information. The forensic mindset, therefore, comes
from the appreciation that minor details (Holmes’ trifles) are the signs
pointing to the de-coding of the material’s origins.
The mindset of a method is inherent in the approach it takes and the
observations it makes. As Henry Marcuse said, the theory is consistent in
the corresponding set of operations: If length is measured, then length is
important for some reason integral to the material to be analyzed. As well,
whether the measurement is in millimeters or inches, kilometers or miles is
important. The approach taken with fiber identification reflects what is
important to those doing the testing. The American Association of Textile
Chemists and Colorists methods manual (AATCC, 2007) lists among its
standard analyses the following:
• colorfastness to commercial laundering and to domestic washing
• flammability of clothing textiles
• smoothness of seams in fabrics after repeated home laundering
• electrostatic propensity of carpets
• wrinkle recovery of fabrics: appearance method
• dimensional changes in textiles other than wool.
The AATCC technical manual lists microscopy as useful for identification
of fibers but ‘[i]t must be used with caution on man-made fibers since they
are frequently produced in a number of modifications which alter the . . .
appearance.’ (page 20). The Technical Manual also lists ‘reaction to flame’
(Table III) as a test method with the categories for results as: melts near
flame, shrinks from flame, and burns in flame, among others.
Another example of professional orientation to analysis comes from
ASTM, International, Volume 7 lists the following as methods for fiber
identification:
•
•
•
•
•
flame resistant materials used in camping tentage
pile retention of corduroy fabrics
elastic properties of textile fibers
performance specifications for underwear fabrics, woven, men’s and
boys’
commercial moisture regains for textile fibers.
ASTM lists infrared spectroscopy as the ‘preferred method’ for fiber
identification, noting that: ‘additional physical properties of the fibers such
as density, melting point, regain, refractive indices, and birefringence . . . are
useful for confirming the identification’ (ASTM D276). Most of the tests
The forensic identification of textile fibers
261
relate to physical properties but treat fibers as a bulk material with certain
performance characteristics.
Compare these approaches with that taken by forensic scientists. Rather
than starting from the premise of knowing what the fiber is and determining
its properties, a forensic scientist has no real idea of what the fiber might
be and cares not a bit about its pile retention or ability to regain water. The
fiber presented to a forensic microscopist could be anything – almost
literally – and the scientist must approach it with an open mind. Therefore,
microscopy becomes the method of choice, not just useful or cautionary, for
the forensic scientist (Heyn 1952; Longetti and Roche, 1958; McCrone 1982;
Rouen and Reeve, 1970; Stoeffler, 1996). Typically, forensic scientists do not
receive much material with which to work; one or two fibers may be all
there is for evidence. A conservative perspective and preservation of sample
is a central concern. Other methods apply and supplement or enhance the
analysis, such as infrared spectroscopy or fluorescence microscopy, but
the central approach for the forensic identification comes from discerning
the subtle microscopic characteristics of fibers.
14.2
Microscopy of fibers
Manufactured fibers differ physically in their shape, size, internal properties
and appearance. Some of the microscopic characteristics of certain fibers
may indicate a polymer class or a particular end use. The term manufactured fibers refers to various families of fibers produced from fiber-forming
substances, which may be synthesized polymers, modified or transformed
natural polymers or glass. Synthetic fibers, by contrast, are those manufactured fibers which are synthesized from chemical compounds (for example,
nylon or polyester). Therefore, all synthetic fibers are manufactured, but
not all manufactured fibers are synthetic.
Because fibers begin as unorganized masses of monomers and end up as
organized linear materials, the polymers in the fibers become oriented
mostly parallel to the longitudinal axis of the fiber. Orientation, then, refers
to the degree of parallelism of the polymers in a fiber. When a majority of
the polymers are aligned with the fiber axis, the fiber is described as highly
oriented.
Crystallinity is the degree to which a fiber consists of crystalline regions,
where the polymers are in a tightly packed spatial arrangement, rather than
amorphous regions, where the polymers are randomly arranged and loosely
structured. All fibers have both types of regions; the degree to which a fiber
is crystalline or amorphous, however, affects its physical properties and
end-uses. Natural fibers are internally structured that precludes useful
examination under polarized light.
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Identification of textile fibers
Although they are often correlated, it is important to remember that
crystallinity and orientation are two separate entities; for example, PET
fibers slowly drawn at low temperatures result in highly oriented but amorphous fibers. The degree of fiber orientation depends on the draw ratio,
drawing conditions (wet or dry) and the composition of the spinning dope:
crystallinity is a temperature-dependent phenomenon.
14.3
Manufactured fiber production and spinning
The bonding together of monomers to form polymers is called polymerization. The reactions that build polymers for synthetic manufactured fibers
occur by either a condensation (step-growth) or an addition (chain-growth)
mechanism. In a condensation reaction, each bond that occurs involves the
release of water or some other simple substance, such as ethylene glycol in
polyester production. Condensation reactions yield a product in which the
repeating unit has fewer atoms than the monomer or monomers. In an
addition reaction, the resulting polymer has sub-units which have molecular
formulae identical to those of the monomer. Combination occurs by rearrangement of the combined monomer units. The molecular weight of the
polymer is the sum of the molecular weight of all of the monomers in the
chain.
Synthetic fibers are formed by extruding the fiber-forming substance,
called spinning dope, through a hole or holes in a shower head-like device
called a spinneret; this process is called spinning. The spinning dope is
created by the rendering of solid monomeric material into a liquid or semiliquid form by a solvent or heat. The four major methods of fiber spinning
are dry, wet, melt and gel.
Dry spinning extrudes the spinning dope into a heated chamber to remove
the solvent leaving the solid filament behind. Acetate, acrylic, modacrylic,
and triacetate are examples of dry-spun fibers. Wet spinning extrudes the
spinning dope into a liquid coagulating medium where the polymer is
regenerated. Acrylic, modacrylic, and rayon are examples of wet-spun fibers.
Melt spinning differs in that the spinning dope is melted and extruded into
air or other gas, or into a liquid, where it cools and solidifies. Nylon, polyester, olefin are examples of melt-spun fibers. Gel spinning is a process in
which the primary mechanism of solidfication is the gelling of the polymer
solution by cooling to form a gel filament consisting of precipitated polymer
and solvent. The solvent is then washed off in a bath. Gel-spun fibers have
a high tensile strength and modulus.
After the fibers are spun, they may go through a number of steps before
they are ready for construction into yarns or shipment as fibers. Fibers are
typically drawn to increase their length, strength, and form; this has an effect
on their optical properties. Also, they may be treated with chemicals to yield
The forensic identification of textile fibers
263
a desired property, such as increased apparel comfort or stain-resistance.
Fibers may be crimped to alter their physical form. Microscopic properties,
such as cross-section and diameter, are important characteristics for the
initial comparison of fibers. Very often, the physical traits of a fiber may
suggest whether it is an acrylic or rayon fiber, from a garment or carpet or
intended for household or industrial use.
The examination of the optical properties of manufactured fibers can
yield a tremendous amount of information about their chemistry, production, end-use and environment. Careful measurements and analysis of these
properties is a crucial step in the identification and later comparison of
textile fibers. Optical properties, such as refractive index, birefringence, and
color, are those traits that relate to a fiber’s structure or treatment revealed
through observation. Some of these characteristics aid in the identification
of the generic polymer class of manufactured fibers. Others, such as color,
are critical discriminators of fibers that have been dyed or chemically finished. A visual and analytical assessment of fiber color must be part of every
fiber comparison. The fluorescence of fibers and their dyes is another useful
point of comparison.
Thermal properties relate to the softening and melting temperatures for
manufactured fibers and the changes the fiber exhibits when heated. Not
all manufactured fibers are thermoplastic, or capable of melting, and so it
is important to observe the fibers as they undergo increasing temperature.
This is done by fitting a special thermal stage to a microscope, which gradually heats up the fiber while the microscopists observes the changes. The
range of temperatures within which a fiber is altered should be recorded.
Table 14.1 gives ranges of melting temperatures for the more common
manufactured fibers.
Based upon a fiber’s polymer composition, it will react differently to
various instrumental methods, such as Fourier transform-infrared spectroscopy (FT-IR) or pyrolysis-gas chromatography (P-GC), and chemicals, such
as acids or bases. These reactions yield information about the fiber’s molecular structure and composition.
A polarized light microscope is the primary tool for the identification and
analysis of manufactured fibers. Many characteristics of manufactured fibers
can be viewed in non-polarized light, however, and these provide a fast,
direct and accurate method for the discrimination of similar fibers. A comparison light microscope is required to confirm whether the known and the
questioned fibers truly present the same microscopic characteristics.
The cross-section is the shape of an individual fiber when cut at right
angles to its axis. Shapes for manufactured fibers varies with the desired
end result, such as the fiber’s soil hiding ability or a silky or coarse feel to
the final fabric. Some fiber types tend to stay within certain cross-sectional
families; for example, bean-shaped fibers tend to be acrylics and rayon tends
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Identification of textile fibers
Table 14.1 Melting temperatures for some
fiber types1
Fiber type
Temperature (°C)
Acetate
Acrylic
Aramid
Modacrylic
Nylon
6
6,12
6,6
Olefin
polyethylene
polypropylene
Polyester (PET)
Rayon
Saran
Spandex
Triacetate
Vinal
224–280
Does not melt
Does not melt
204–2252
213
217–227
254–267
122–135
152–173
256–268
Does not melt
167–184
231
260
200–260
1 From Carroll, 1992.
2 Some members of this class do not melt.
14.1 Cross-sections of various fibres.
to be irregular (Fig. 14.1). The particular cross-section may also be indicative of a fiber’s intended end-use: many carpet fibers have a lobed shape to
help hide dirt and create a specific visual texture to the carpet. A fiber’s
cross-sectional shape can be gleaned from a visual inspection by focusing
through the fiber: as the focal plane moves through the fiber, the viewer can
see changes that relate to its physical shape. If the cross-sectional shape is
difficult to discern from an optical cross-section, or it appears that the cross-
The forensic identification of textile fibers
265
section may help identify or distinguish the fiber, then a physical crosssection should be prepared. Numerous approaches have been published for
cross-sectioning but the method outlined by Palenik and Fitzsimmons
(1990) is simple, inexpensive, and conservative of sample.
The modification ratio of a fiber is a geometrical measurement used in
the characterization of non-round fiber cross-sections. The modification
ratio is the difference in size between the outside diameter of the fiber and
the diameter of the core. Many manufacturers use modification ratios in the
descriptions of their fibers for patent purposes. This characteristic may assist
the examiner in providing information to fiber manufacturers during
product queries.
The way a fiber’s diameter is measured is dependent upon its crosssectional shape; there is more than one way to measure the diameter of a
non-round fiber. Manufactured fibers can be made in diameters from about
6 μm (so-called microfibers) up to a size limited only by the width of the
spinneret holes. By comparison, natural fibers vary in diameter from cultivated silk (10–13 μm) to US sheep’s wool (up to 40 μm or more) and human
head hairs range from 50–100 μm. A manufactured fiber greater than 40 μm
is probably a carpet fiber.
Some manufactured fibers retain air-pockets or voids after production.
For example, wet-spun fibers, such as acetates, may have voids that range
in size from submicron up to several microns. Voids are created when pores
in the solidifying fiber are filled with a mixure of solvent and non-solvent
fluids (Frushour and Knorr, 1998). It can be possible to distinguish between
wet-spun and dry-spun acrylics by the size and number of voids (Masson,
1995). The size, shape, distribution, and concentration of voids is related to
the composition and production methods of the fiber and is an important
comparative feature.
Inclusions are materials or discontinuities that are placed or occur in
fibers. These may be accidental inclusions, such as the draw marks sometimes seen in melt spun fibers, or intentional inclusions, such as large clumps
of delustrant or anti-static materials. Delustrants are finely ground particles
of materials, such as titanium dioxide, that are introduced into the spinning
dope. These particles help to diffract light passing through the fibers and
reduce their luster. Fibers can be classified as bright, semi-bright and dull,
although other categories may be denoted, such as slightly, moderately and
heavily delustered. The size, shape, distribution, and concentration of delustrants should be noted.
A fiber’s construction is an important indication of its production and
end-use. Examples are bicomponent fibers (two or more polymer types
spun in a sheah/core or bilateral relation), biconstituent fibers (two different polymers spun together from a homogeneous dope), or microfibers
(fibers with a denier of less than 1.0). Fibers may be constructed for specific
266
Identification of textile fibers
traits. These specialty fibers are distinct and particular attention should be
paid to their construction and composition.
14.4
Polarized light microscopy
Polarized light microscopy is an easy and quick non-destructive way to
determine the generic polymer class of manufactured and synthetic textile
fibers. Beyond the immediate characteristics used to discriminate between
polymer types, the examination of fibers in polarized light provides valuable
information about the production and finishing of the fiber after spinning.
14.4.1 Light and fibers
Light is composed of electromagnetic waves and a change in velocity is
associated with the polarization (positive or negative charge) that occurs
under the influence of an electric field. The outer electrons of molecules,
which are taking part in covalent bonds, are thus affected. It is therefore
possible to assign a polarization (or bias) to each chemical bond. The polarization will also vary with the direction of the electric field.
The optical properties of fibers depends on the formation of oriented
molecular aggregates in the fiber when it is spun; these aggregates are called
micelles. The configuration of the polymer(s) involved, as well as the individual properties of the micelles, also plays a part. The orientation and
crystallinity of the micelles determines how light will be affected as it passes
through the fiber. By viewing a fiber with polarized light, the internal structure of the fiber, and, thus, its polymer make-up, may be deduced.
14.4.2 Refractive index
Fibers vary in shape but are almost always thicker in the center than near
the edges. Thus they act as crude lenses, either concentrating or dispersing
the light that passes through them. This alteration of light can be calculated
as the ratio of the speed of light in a vacuum to the speed of light in the
fiber (or any other medium). This ratio is called the refractive index. If a
fiber has a higher refractive index than the medium in which it is mounted,
it acts as a converging lens, concentrating light within the fiber. If the fiber
has a lower refractive index than the mounting medium, it acts as a ‘diverging’ lens, and the light rays diverge from the fiber.
In most fibers, the light rays only slightly converge or diverge and thus
appear as a thin bright line, called the Becke line, after the French mineralogist Ferdinand Becke who first described it in 1893, at the interface between
the fiber and the mounting medium. While observing the fiber, the working
distance on the microscope is increased (the stage is moved down). If the
fiber has the higher refractive index, the Becke line moves toward the fiber
The forensic identification of textile fibers
267
as the working distance is increased. If the mounting medium has the higher
index, the Becke line moves toward the medium (away from the fiber) as
the working distance is increased. If fibers are mounted in Permount, which
has a refractive index of 1.52, then a fiber can be described as being greater
than, equal to or less than 1.52.
Manufactured and synthetic textile fibers have two optical axes, one
parallel to the long axis of the fiber and one perpendicular to the long axis
(this could be considered the ‘short’ axis). Internally, manufactured and
synthetic fibers have crystalline and amorphous regions and these are more
or less oriented to the long axis of the fiber. This orientation creates a difference between the speed of light passing through the long axis of the fiber
and the light passing through the ‘short’ axis. Light passing along one axis
is impeded more than light passing through the other axis because of the
greater electrical polarization of the molecules in that direction. Thus,
manufactured and synthetic fibers are said to be anisotropic, meaning that
light is affected differently in the two directions. Because of this difference,
two distinct refractive indices are created. These refractive indices are
called n|| (pronounced, ‘n parallel’), for the refractive index when the long
axis is parallel to the orientation of the polarizing filter, and n⊥ (pronounced,
‘n perpendicular’), for the refractive index when the long axis is perpendicular to the polarizing filter’s orientation (Table 14.2).
Table 14.2 Refractive indices of manufactured and synthetic
textile fibers
Fiber type
n||
n⊥
n|| − n⊥
Acetate
Dicel
1.478
1.476
1.473
1.473
0.005
0.003
Triacetate
Tricel
Arnel
1.469
1.469
1.469
1.468
0
0.001
Acrylic
Acrilan 36
Orlon
Acrilan
1.511
1.51
1.52
1.514
1.512
1.525
−0.003
−0.002
−0.005
Modacrylic
Dynel
Teklan
SEF
Verel
1.535
1.52
>1.52
1.535
1.533
1.516
>1.52
1.539
0.002
0.004
−[low]
−0.004
Vinyon
Fibravyl
Rhovyl
Vinyon HH
1.54
1.541
1.528
1.53
1.536
1.524
0.01
0.005
0.004
268
Identification of textile fibers
Table 14.2 Continued
Fiber type
n||
n⊥
n|| − n⊥
Rayon
Viscose (regular)
Viscose (regular)
Viscose (high tenacity)
Vincel (high wet modulus rayon)
Fortisan
Fortisan 36
Cuprammonium
Tencel
1.542
1.545
1.544
1.551
1.547
1.551
1.553
1.57
1.52
1.525
1.505
1.513
1.523
1.52
1.519
1.52
0.022
0.02
0.039
0.038
0.024
0.031
0.034
0.05
Olefin
Courlene (PP)
Polypropylene
SWP (PE)
Courlene X3 (PE)
Polyethylene
1.53
1.52
1.544
1.574
1.556
1.496
1.492
1.514
1.522
1.512
0.034
0.028
0.03
0.052
0.044
Nylon
Enkalon (6)
ICI nylon (6,6)
Qiana
Rilsan (11)
Nylon 6
Nylon 6,6
Nylon 11
1.575
1.578
1.546
1.553
1.568
1.582
1.55
1.526
1.522
1.511
1.507
1.515
1.519
1.51
0.049
0.056
0.035
0.046
0.053
0.063
0.04
Silk (degummed)
1.57
1.52
0.05
Aramid
Nomex
Kevlar
1.8
2.35
1.664
1.641
0.136
0.709
Polyester
Vycron
Terylene
Fortrel/Dacron
Dacron
Kodel
Kodel II
1.713
1.706
1.72
1.7
1.632
1.642
1.53
1.546
1.535
1.535
1.534
1.54
0.183
0.16
0.185
0.165
0.098
0.102
Spandex
Lycra/Vyrene
1.561
1.56
0.001
Others
Vicara (Azlon)
Teflon
Calcium alginate
Saran
Novoloid
Kynol (drawn)
Kynol (undrawn)
Polyacrylostyrene
Darvan (Nytril)
Polycarbonate
1.538
1.38
1.524
1.61
1.5−1.7
1.658
1.649+
1.56
1.464
1.626
1.536
1.34
1.52
1.61
1.5−1.7
1.636
1.649
1.572
1.464+
1.566
0.002
0.04
0.004
0
0
0.022
<0.001
−0.012
0
0.06
Source: AATCC, 1996; ASTM, 1996; McCrone et al., 1979; Perry, 1985;
Rouen and Reeve, 1970.
The forensic identification of textile fibers
269
The refractive indices of a fiber can be measured directly by placing the
fiber in a series of liquids of specific refractive indices until the refractive
indices of the fiber and liquid are the same. At this point, the fiber ‘disappears’
because it and the liquid are now isotropic, meaning that light is traveling at
the same speed through both the fiber and the liquid. This method is not
always practical in the forensic examination of fibers; a relative refractive
index assessment can be made instead. The refractive index of the mounting
medium must be known and of a value very near 1.5 (for example, Permount
= 1.525), as this provides easy discrimination between polymer types.
If n|| is greater than n⊥, that is, n|| − n⊥ would equal a positive number, the
fiber is said to have a positive sign of elongation. This means that light is
traveling faster along the long axis of the fiber than it is along the short axis.
If n|| is less than n⊥, the fiber is said to have a negative sign of elongation.
Here, light is traveling faster along the short axis than it is along the long
axis, and n|| − n⊥ would equal a negative number. To determine the sign of
elongation, it is necessary to add another polarizing filter.
14.4.3 Crossing polarized filters
By inserting an additional polarizing filter, which has an orientation perpendicular to the first, between the eyepiece and the objective, any natural
light is effectively prevented from traveling to our eye. Since the polarized
light undergoes no change in its original direction of vibration, it is stopped
at the second filter. This second filter is called the analyzer for reasons which
will become clear later. A fiber placed parallel to either orientation would
appear black because none of the light vibrations could meet the criteria
of both filters. These positions are called extinction points. If we place a
fiber at 45° to the polarizer and the analyzer, some light is allowed to pass
through, and we see a bright fiber on a black background. This is because
the fiber is anisotropic and has two RIs, therefore, the light is traveling
through the fiber at two different speeds causing the light to be resolved
into two mutually perpendicular planes of vibration. These speeds are
determined by the distance the light has to travel (for example, the fiber’s
diameter) and the orderliness of the fiber’s molecular construction (orientation and crystallinity). Since the light waves are in different planes, they
cannot interfere with each other. As the light passes through the fiber, it is
bent by the RIs of the fiber and becomes out-of-phase. This means that the
light waves are now free to interfere with each other.
When these out-of-phase waves of light strike the analyzer, it diffracts
them into various colors depending on the wavelengths being added and
subtracted through interference; therefore, they are called interference
colors. The colors produced are indicative of the fiber’s polymer type and
organization.
270
Identification of textile fibers
If a fiber shows interference colors, it has a positive sign of elongation.
For fibers that show a grey color, we must move the scale up or down to
determine which direction the colors go. By placing a 550 nm wave plate
between the fiber and our eye, we move the fiber’s value up by 550 nm in
either the yellow/orange range (−) or the blue range (+), depending on
where it started. The fiber must be in the additive position for this procedure. The plate retards the light by an additional 550 nm, so slow + slow =
higher order color (add) or slow + fast = lower order color (subtract).
When the two rays are recombined in the analyzer, this retardation
causes, to a greater or less degree, destructive interference for certain wavelengths of light. The remaining wavelengths combine to give the interference colors we see when we cross the polars. Interference colors vary in
hue with the retardation according to a characteristic sequence of colors
known as Newton’s series. The Newton’s series is not a spectrum; it is
divided into orders, with the end of each order marked by a red-violet color
representing one full wavelength of retardation. Each order equates to
550 nm, so first order colors fall at 550 nm, second order colors at 1100, and
so on. The colors become less intense as the orders increase. A printed chart
of the Newton series of colors is called the Michel-Levy interference color
chart. The chart shows the colors produced by the interference of light
waves, relating thickness with wavelength and retardation. Retardation
increases linearly with both thickness and birefringence.
The difference in the speed of light along the two rays is called the retardation or path difference, because of the difference in lengths of the paths
the rays traverse. The amount of retardation depends on the difference in
velocities (determined by the RIs of the fiber) and the thickness of the
material (how much fiber the ray had to travel through). This relation is
expressed by:
R = T ( n − n⊥ ) ,
where R is the retardation, T the thickness of the fiber in nanometers, and
(n|| − n⊥) the birefringence.
Interference colors allow an examiner to characterize fibers based upon
their optical properties, which relate to their chemistry and production. One
the more distinctive traits of a fiber is its birefringence. Because the velocity
of the two polarized beams differs, they suffer unequal refraction during
passage through the fiber, which is said to doubly refracting or birefringent.
The birefringence of a fiber can be determined with the polarizing microscope by examining the fiber between crossed polars. The characteristic
birefringence of a given substance is the numerical difference between the
maximum and minimum refractive indices, that is (n|| − n⊥) for uniaxial
substances. Birefringence will be greatest when the polymers in the fiber
The forensic identification of textile fibers
271
are lined up parallel to the longitudinal axis of the fiber and will be zero if
they are randomly organized.
To determine the birefringence, we must find the path difference between
n|| and n⊥. To do this, the slow ray must be compensated for by artificially
increasing the thickness of the fiber. With this compensation, either a wedge
or a tilting disk, the thickness is increased until the fast ray is as slow as the
slow ray. At this point, the middle of the fiber turns black, indicating that
the fiber is isotropic at that point. The influence of the fiber upon the polarized light is therefore reversed in a controlled, measurable manner. It is
important to measure the thickness of the fiber at the same point where the
retardation is measured; that is, along the optical path your eye takes
through the fiber.
14.5
Fluorescence microscopy
Many dyes used to color textiles have fluorescent components to them.
These components, called fluorophores, and their response to certain wavelengths of light can be useful in comparing textile fibers. Not all textile dyes
fluoresce but fluorescence comparisons should be performed regardless: If
the questioned and known fibers both fail to fluoresce, that is another point
of meaningful comparison.
Fluorescence occurs when a substance is excited by specific wavelengths
of light. A light of relatively short wavelength illuminates a substance and
the substance absorbs and/or converts (into heat, for example) a certain,
small part of the light. Most of the light which is not absorbed by the substance is re-emitted, which is called fluorescence. The fluorescent light has
lost some of its energy and its wavelength will be longer than that of the
source light.
Certain dye combinations may produce fluorescence of a particular
intensity and color, both of which should be noted during the examination.
Fibers dyed with similar dyes should exhibit the same fluorescence characteristics, unless the fiber and/or dye(s) have been degraded by UV exposure,
bleaching or some other similar means. It is important to consider these
factors when collecting known samples.
In a fluorescence microscope, the specimen is illuminated with light of a
short wavelength, for example ultraviolet or blue, produced by placing a
filter, called an excitation filter, between the light source and the sample. A
series of wavelength filter are useful to cover the range of fluorescence seen
in textile dyes. The resulting fluorescent signal, however, is weak compared
to the strength of the illuminating light. Therefore, the excitation light must
be filtered out by a filter placed between the specimen and the eye which
is a barrier to all light except that emitted by the specimen. This barrier
272
Identification of textile fibers
filter ought to be opaque to the excitation wavelength and transparent to
the longer wavelengths to transmit the fluorescence. The fluorescent fiber
is therefore bright against a dark, preferably black, background.
14.6
Conclusions
Identifying fibers in a forensic setting requires a knowledge of fiber manufacture, materials, and methods but rarely an application of standard textile
industry methods. The goals, the mindset, and the materials are all different.
In quality assurance work or in quality control problem solving, a forensic
mindset may work better than that of the production laboratory because
the questions are similar – what is this unknown, where did it come from,
why is it here? Methods may be unique to one industry, to another, or they
may overlap (Fig. 14.2) but that all depends on the goals and materials
under study. Although forensic scientists routinely turn to the fiber manufacturing industry for information, it may be that the fiber manufacturers
may benefit from forensic techniques and methods.
Manufacturing
analytical methods
Forensic analytical
methods
Aftermarket taxonomy
Market taxonomy
• company-product
orientation
• end use
• supply web
• implicit rules on
categories.
• explicit rules on
categories.
• as used
Some sharing of methodologies, protocols
Different approaches due to different goals – quality
at lowest price for manufacturing, reconstruction and
product tracking for forensics
Analogous to experimental vs. historical sciences
14.2 Comparison of manufacturing and forensic methods of identifying fibres.
The forensic identification of textile fibers
14.7
273
References and further reading
AATCC (1996) Technical Manual, Research Triangle Park, NC: American
Association of Textile Chemists and Colorists.
AATCC (2007) Technical Manual, Research Triangle Park, NC: American
Association of Textile Chemists and Colorists.
ASTM, D 276-87 (1996) Standard Test Methods for Identification of Fibers in
Textiles, Philadelphia, PA: ASTM.
Carroll, G (1992) Forensic fibre microscopy. In: Forensic Examinations of Fibres.
(Ed: Robertson, James) Ellis Horwood, New York, 99–126.
Delly, JG (1973) Microscopy’s color key. Ind. Res. 15 (11, October), 44–50.
Frushour, B and Knorr, R (1998) Acrylic Fibers, in M Lewin and EM Pearce (eds)
Handbook of Fiber Chemistry, 2nd edn. Marcel Dekker, Inc.: New York.
Ginzburg, C. (1979) ‘Clues: Roots of a Scientific Paradigm’, Theory and Society 7
(3), 273–288.
Heyn, A (1952) Observations of the birefringence and refractive index of synthetic
fibers with special reference to their identification. Text. Res. J. 22, 513–522.
Hinsch, J (1983) The technology of the polarized light microscope. Fiber Producer
11 (3, June), 10–20 (6 pages).
Johri, M and Jatar, D (1979) Identification of some synthetic fibers by their
birefringence. J. Forensic Sci. 24, 692–697.
Longhetti, A and Roche, G (1958) Microscopic identification of man-made fibers
from the criminalistics point of view. J. Forensic Sci. 3, 303–329.
Masson, J (ed) (1995) Acrylic Fiber Technology and Applications, Marcel Dekker,
Inc.: New York.
McCrone, W (1982) Microanalytical tools and techniques for the characterization,
comparison and identification of particulate (trace) evidence. Microscope 30,
103–117.
McCrone, W, McCrone, L and Delly, J (Eds) (1978) Polarized Light Microscopy.
Ann Arbor Science, Ann Arbor, Michigan. 251 pages.
McCrone, W, Delly, JG and Palenik, SJ (1979) The Particle Atlas, Chicago, IL:
McCrone Associates.
Palenik, S and Fitzsimons, C (1990) Fiber cross sections: Part I and Part II,
Microscope 38, pp. 187–195 and 313–320.
Perry, DR (ed.) (1985) Identification of Textile Materials, Seventh Edition, Manchester, UK: The Textile Institute.
Raheel, M (1996) Modern Textile Characterization Methods. Marcel Dekker, Inc.:
New York.
Robinson, P and Bradbury, S (1992) Qualitative Polarized-Light Microscopy. Oxford
University Press, Royal Microscopical Society, Oxford, UK.
Rost, F (1992) Florescence microscopy, Volume I, New York City, NY: Cambridge
University Press.
Rouen, R and Reeve, V (1970) A comparison and evaluation of techniques for
identification of synthetic fibers. J. Forensic Sci. 15, 410–432.
Smith, B and Block, I (1982) Textiles in Perspective. Prentice-Hall, Inc.: Englewood
Cliffs, NJ.
Stoeffler, S (1996) A flowchart system for the identification of common synthetic
fibers by polarized light microscopy. J. Forensic Sci. 41, 297–299.
274
Identification of textile fibers
Thetford, A and Simmens, S (1969) Birefringence phenomena in cylindrical fibres.
J. Microscopy 89, 143–150.
Vakkari, J (2001) ‘Giovanni Morelli’s “Scientific” Method of attribution and its
reinterpretations from the 1960s until the 1990s,’ Konsthistorisk Tidskrift 70(1–2),
46–54.
Wind, E (1963) Art and Anarchy, Faber and Faber: London.
Ziabicki, A (1976) Fundamentals of Fibre Formation. John Wiley and Sons: New
York.
15
Identifying and analyzing textile damage
in the textile industry
W D SCHINDLER,
University of Applied Sciences Hof, Germany
Abstract: Textile damage identification is in many respects a special
application of fiber analysis. This chapter begins with an overview on the
complex subject of textile damage in the textile industry. It then
discusses the favored methods of textile damage identification and
analysis, first in general and then according to the type of fiber.
Key words: identification and analysis of damage to textiles; mechanical,
chemical, biological and heat (thermal) fiber damage identification; main
types, manifestations and causes of textile damage; methods of
identifying textile damage, especially textile microscopy and infrared
spectroscopy (IR, IRS); unwanted deposits on textiles, especially stains.
15.1
Introduction: importance of and reasons for
textile damage analysis in the textile industry
The identification of fibers is an essential part of textile damage analysis.
Without the knowledge of the type of fiber most demanding damage examinations, identifications and interpretations would not be possible. Only a
few relatively simple tests, such as strength and abrasion loss or some
uncomplicated faults of the production of yarns and fabrics may be discussed without the knowledge of the involved type of fibers and their
corresponding specific physical and chemical behavior.
Textile damage analysis is, like forensic investigations, a very interesting
application of fiber identification. Beyond that, damage analysis is in some
aspect a continuation, deepening and further development of the fiber
identification. The first and main step of fiber analysis is the identification
of the standard type of fiber. The second step could be the further investigation of the fiber subtype, e.g. fiber modification or, in special cases such as
acrylic or elastane fibers, even finding out the producer (by the type of
comonomers, additives and spinning process). The third step or level of fiber
analysis could be the analysis of changes of well-known fibers by production
and usage, the damage analysis.
This chapter intends to give an overview on the broad subject of
textile damage and the usual methods of textile damage identification and
analysis. The concept of this chapter does not attempt an exact description
275
276
Identification of textile fibers
of experimental details, but the references will be a help for those who are
interested in more information.
The analysis of damage to textiles is a fascinating special area of testing
of textiles. It has significant practical relevance, considerable charm but also
many difficulties. Determining the exact cause of damage can often be a
real challenge. Those who carry out damage analysis need wide-ranging
knowledge and some experience but also intuition and the ability to reason
and weigh up evidence like a detective. Further requirements for successful
analysis of damage to textiles and the knowledge and skills which successful
analysts should have are:
•
understanding of textile technology, including textile physics and textile
chemistry
• extensive knowledge of textile fibers, their conversion to yarns and
fabrics, dyeing and finishing, making-up processes and typical usage
• familiarity with the most important methods used in damage analysis,
such as microscopy, chromatography, IR spectroscopy and thermal
analysis
• ability to get peripheral information, starting with the conditions used
in producing, dyeing and finishing the damaged textile, including the
processes and machines used, further processing stages, storage, transport and, where appropriate, usage.
Analysis of damage to textiles is not usually an exact science, although it
does use scientific methods. In many cases several different tests are necessary. Their results can sometimes be contradictory. These then have to be
evaluated and weighed up against each other very critically, whereby comparison samples, experience with similar cases and information about the
circumstances of the damage can be useful. In many ways this process is
similar to a court trial when only circumstantial evidence is available but
fortunately in the laboratory the damage can often be imitated and the
evidence thus verified.
15.1.1 Practical importance of textile damage assessment
and reasons for the analysis of causes of damage
Textiles can become damaged during their production as well as during
distribution and usage. Experience shows that consumers are less likely to
make complaints than are those involved in the chain of textile production.
For example, textile manufacturers often dispute with textile dyers and finishers about who might be responsible for faults, whereby this can usually
be clarified quickly by means of a surface imprint as described in Section
15.3.8. As well as the question of who is responsible and therefore who has
to bear the costs, it is naturally also of interest to know how the damage
Identifying and analyzing textile damage in the textile industry
277
can be repaired and also how it can be avoided in future. In the case of
large lots or continuous production it is often important to find out very
quickly what the cause is in order to stop producing the fault as soon as
possible. In this respect damage analysis plays an essential role in quality
assurance. Lack of quality and subsequent complaints should not be underestimated as an image problem that they present for the supplier. Experts
for damage analysis are thus often employed by well-known companies.
In spite of modern process control, optimization and quality control
faults cannot be avoided completely. They occur at all stages of textile production including storage and transportation. The experts for damage analysis at fiber, dyestuff and auxiliary agent producers, as well as those at
testing and research institutes, still have their hands full in dealing with
many cases of damage where the costs caused by the fault can be quite high.
Some disputes about damage are settled by courts but more usually the
customer and producer agree on a settlement. Depending on the importance of the business relationship, fair dealing and price discounts play an
important role here. Occasionally unjustified complaints are also made in
the hope of obtaining just these benefits. In order to remain in charge of
investigations of complaints most of the West European producers of fibers,
dyestuffs and auxiliary agents have their own testing laboratories with
damage analysis experts. The alternative, namely to let testing institutes
carry out this work, is usually rejected because, amongst other reasons, the
companies would have to reveal too much detailed knowledge and also
because it often takes too long. Some producers of textile auxiliary agents
choose another, risky path. They take care of complaints simply by giving
price reductions without carrying out their own laboratory investigations.
Most textile companies do not have the personnel or equipment required
for the clarification of complicated faults. On the other hand they do have
the best insight into peripheral information required for damage analysis.
However, when they hand over the fault to an institute or, more usually, to
their suppliers of fibers, dyestuffs or auxiliaries for investigation they often
only pass on part of this information. Analysis of the fault is then often
regarded as technical customer service in order to promote and cement
customer relationships. The intensity of these external analyses of damage
then sometimes reflects that of the business relationship.
Although there are no figures available on the economic importance of
textile damage analysis, it can be roughly estimated from the expenses
incurred by producers of fibers, dyestuffs and auxiliary agents in dealing
with complaints and analyzing damage. These expenses for personnel and
equipment are quite high, even though some decades ago they were even
higher. The author knows of at least two dozen such laboratories in Germanspeaking countries, in industry and in testing and research institutes. If it is
assumed that on average two or three employees with a corresponding
278
Identification of textile fibers
annual budget of about 300 000 to 400 000 euros per laboratory are involved,
this would amount to a total of up to 10 million euros just for Germany,
Switzerland and Austria.
15.2
Main types, manifestations and causes of
textile damage
Faults are defects which lower the value and usefulness of goods. Faults can
be defined as any undesirable kind of deviation from prescribed requirements. Damage is the disadvantage arising from faults. Thus damage analysis is a wider-ranging term than analysis of faults. It is damage which leads
to complaints and, as a rule, to demands to repair the damage or compensate for it. Damage analysis is an important part of quality control and
contributes to quality assurance.
15.2.1 Main types of damage
Mahall divided his book Quality Assessment of Textiles – Damage Detection
by Microscopy1 entirely on the basis of types of damage, namely in chapters
on:
•
•
•
•
•
•
•
•
chemical damage
mechanical damage
thermal and thermo-mechanical damage to synthetics
streaks and bars in textile fabrics due to yarn differences and technological reasons
causes of the formation of tight threads and their effects
defects caused by deposits and encrustations on the fiber material
other defects in the quality of textiles
microbiological damage to fibers.
There should be added:
•
•
biological damage, especially to natural fibers
and last but not least light damage.
Some explanations and examples are described in Section 15.4, as well as
some possibilities of identifying these types of damage. Special types of
damage, such as deposits (especially stains), streaks, barriness and biological
damage are discussed in Section 15.5.
15.2.2 Manifestations of damage
The manifestation of damage to textiles can vary widely. It ranges from
obvious, for example, visible or easily recognizable defects to hidden ones,
Identifying and analyzing textile damage in the textile industry
279
for example defects that are hard to detect or those that can only be
detected later. It is the latter type which is generally the reason for giving
guarantees.
Perception and description of the fault are the first steps in damage
analysis. Damage can be perceived visually, macro- and/or microscopically,
often only with a specific type of illumination, such as reflected, oblique or
transmitted light, or with a specific light source, for example UV or polarized light. Some faults are detected by other senses, usually in the form of
a handle assessment, or they can be registered by measurements. Examples
of the latter are color measurement, tensile and abrasion strength, extensibility, shrinkage and fastness properties. These technical properties can be
supplemented by thermophysiological comfort properties and care requirements, where significant deviations from the agreed or specified values can
be claimed as faults.
Damage is manifested most commonly as stains followed by streaks and
barriness. Other types of damage manifestation, such as differences in hue
or depth of shade, unevenness of dyeing, lack of strength, deposits, abrasion
and holes, occur with less frequency.2
It is important here to describe the fault as exactly as possible. All
the typical characteristics and peculiarities, their frequency and possible
regularity have to be noted and also whether they can be localized to individual fiber or thread systems. This makes it easier or even at all possible
to determine the cause of the damage later.
15.2.3 Damage causes
There are mechanical, thermal, chemical and biological causes of damage
to textiles. They may be attributed to different causal agents, in particular
to textile manufacturers, dyers and finishers, garment producers, distributors or consumers. Although there are cross-relations between all the
above-named causes and causal agents, it is usual, for example, to attribute
mechanical damage to the textile manufacturer and chemical damage to
the dyer and finisher.
In contrast to the types of damage manifestation, there is no especially
typical cause of damage. In the statistical investigation mentioned above,
based on 550 cases of damage, there were 81 different causes of damage
where more than 10 cases were registered. Their proportion ranged from
two per cent to a maximum of six per cent.2 Other sources also report a
similarly wide range of damage causes with a relatively low percentage in
each case.3 This enormous range of causes is typical for damage to textiles
and makes its analysis considerably difficult.
This statement is also applicable to the causes of the most frequent manifestation of damage, namely stains. Statistical analysis of 258 cases of damage
280
Identification of textile fibers
with stains resulted in 29 types of cause each represented in more than
seven cases.2 The most common types of cause gave frequencies in the range
from seven to eight per cent. These are, for example, stains caused by
mechanical and chemical influences, dyestuffs, grease or oil, silicone and
dead or immature cotton.
15.3
Methods of identifying and analyzing
textile damage
The methods of textile damage analysis are mostly similar or even identical
to those of fiber identification: microscopy, spectroscopy and chromatography are very important for both applications. Each of these powerful
methods is described in a separate chapter of this book. Therefore here only
methods or aspects of methods that are of specific interest for textile damage
analysis will be mentioned.
15.3.1 Chemical and physical assessment of
textile damage
Of the many possibilities for investigating damaged textiles, two main
groups can be ascertained: chemical and physical testing. Corresponding to
this, two types of textile laboratories are often to be found, namely chemical
and physical testing laboratories. Each offers different advantages for
damage investigation. Physical testing does not require any chemicals and
often gives results which are easily interpretable and may allow a direct
quality assessment such as ‘just acceptable’, ‘second choice’ or ‘reject’. On
the other hand, with chemical testing, the cause of the damage can be ascertained, which is what is actually meant by analysis of damage. But this
advantage often has to be paid for in the form of higher costs for personnel
and equipment.
15.3.2 Procedure for textile damage analysis
Unfortunately there are no hard and fast rules on how to proceed with
textile damage analysis. The variety of cases and causes is too great for this.
Nevertheless, some companies and institutes use their own preprinted forms
with long lists of tests for this purpose. This has the advantage that none of
the rare tests is overlooked but also the disadvantage of inflexibility and
unnecessary work with certain types of damage. Such preprinted forms may
be of help to less experienced testers but experienced testers tend to have
their own specific procedures, depending on the case, and are often guided
by their intuition. As a general rule preliminary tests are made, followed by
more painstaking specific tests.
Identifying and analyzing textile damage in the textile industry
281
The usual steps taken in an investigation are illustrated here using the
example of stain analysis:
1. Manifestation of the damage with a description of the type, distribution
and possible regularity of the stains.
2. Microscopy with increasing magnification, beginning with a magnifying glass or stereomicroscope and progressing to 300- to 1000-fold
magnification. Use of different types of illumination and contrast,
such as reflected, oblique and transmitted light, UV, polarization or
fluorescence.
3. Preliminary tests such as solubility and staining tests.
4. Isolation of the substance causing the stains (for example, by extraction
in a Soxhlet apparatus) and concentration of the extraction residue. It
is recommended here to cut out the stained areas from the specimen
and to extract an equivalent quantity of unstained material for comparison. If the stain is in the form of insoluble deposits, an attempt can
be made to dissolve the fiber material and thus isolate the stain
substance.
5. Comparison and identification, usually by means of thin layer chromatography and/or IR spectroscopy. Comparison is made with the blank
sample (extract from unstained areas) and with authentic substances
which could have caused the stain. If the stain cannot be extracted, IR
spectra from stained and unstained areas can be compared and the
spectra subtracted in order to identify the stain substance. The limits of
detection of typical stain substances in IR spectra and a comparison of
IR methods have been given in the literature.4,5
6. Reproduction, if possible, of the damage in order to verify the findings,
for example with authentic stain substances on the same textile material,
using conditions as close as possible to those used with the damaged
sample.
7. Further verification, for example, if possible, by means of consultation
with the persons concerned in the stage of production suspected of
causing the damage. It is important here to consider alternatives and to
test the plausibility of the findings critically.
8. Summary of the findings, discussion of the results. If the cause has not
been clearly identified, the results should be formulated carefully and
alternatives mentioned. Documentation including photographs and possibly samples.
9. If possible, hints should be given on how to avoid such stain formation
in future and also on how to remove the stains (suitable solvents and
procedures).
The last steps in this stain analysis demonstrate that comparative samples
and information from peripheral areas of the case can be particularly useful
282
Identification of textile fibers
aids in damage analysis. An archive with similar cases of damage can also
be very useful here. If the archive is very extensive, retrieval of information
on all damage cases according to different criteria should be possible, such
as type of textile and fiber, damage manifestation, cause of damage, method
and procedure leading to clarification and, if available, the source and client
for the analysis.
In many cases, but unfortunately not in all, about halfway through
these steps, or at least after the preliminary tests, the cause of the damage
may be suspected and this may lead to a working hypothesis. This hypothesis then has to be either unequivocally verified or rejected on the basis
of further tests. It is often hard to give up a hypothesis and look for a
better one. Self-criticism and experience can be helpful here as well as
literature searches and discussions with interested colleagues or other
experts.
15.3.3 Preliminary examination
The subject of preliminary examination is as varied as a later part of this
section, namely miscellaneous methods. There may also be no clear-cut
separation between the two. Preliminary examination is composed of simple
tests, carried out in a short time and with little effort, which give the first
clues for damage analysis.
It is usual to begin with an exact visual examination, if possible in comparison to an undamaged sample. Notice should be made of any peculiarities in appearance. Sometimes abraded and raised areas, holes, thin places
and pressure marks can be easily recognized without optical aids. With the
use of a magnifying glass they can be seen more clearly and in more detail.
The same is true for many visible deposits of foreign matter such as silicates,
calcium and magnesium salts (for example, oxalate, phosphate), polyester
oligomers and mildew spots.
As a next step, easily determinable differences between the damaged
sample and undamaged comparison samples can be sought, for example
handle assessments, wetting behavior (TEGEWA drop test6) or pH value.
The latter can be determined with moist pH paper or more accurately by
adding a drop of liquid indicator or with a flat-bottomed pH electrode. The
microscopic detection of acid residues by the formation of methyl orange
crystals is described in Section 15.4.2. Simple tests of mechanical strength
(stretching between the thumbs), crocking fastness tests (dry and wet) and,
where appropriate, a test of wet fastness, for example in cold and then
heated dimethylformamide, also belong in this group.
After marking the damaged area, woven fabrics can be separated into
warp and weft threads and knitted fabrics unravelled in order to investigate
the isolated threads more thoroughly. Do the threads from the damaged
Identifying and analyzing textile damage in the textile industry
283
area show differences in diameter, twist level or yarn composition? Are
stray fibers discernible?
Fiber identification
Determination of the fiber type, including checking of the stated fiber type,
is one of the most important preliminary tests. If no IR spectrometer is
available, standard fibers can be most readily identified using the characteristic reactions according to Stratmann.7 For modified fibers or highperformance and specialty fibers the more complicated classification on the
basis of solubility groups and their subdivisions7,8 or other methods of
analysis9 can be used.
Further preliminary tests
Sometimes it can be appropriate to carry out preliminary tests for readily
detectable elements or compounds, for example the Beilstein test for organically bound chlorine (green coloration of the flame when heating a copper
wire with a small fiber sample in the non-luminous region of a gas flame).
In this way fibers containing vinyl chloride can be easily recognized.
A further example is the detection of iron in the combustion residue of
a textile sample or directly on the fiber material. Hints on the presence of
silicone deposits are given by the adhesive strip test (lowering of the adhesion due to silicone) and the foam test (marked formation of foam when
the textile sample is shaken with chlorinated hydrocarbons). Detection by
means of IR spectroscopy is mentioned in Section 15.3.6.
15.3.4 Microscopy
Microscopy is certainly the most important method for damage analysis of
textiles. Without microscopy and its supporting techniques the elucidation
of most cases of damage would not be possible. The extensive literature on
this topic also demonstrates the great importance of textile microscopy for
textile damage analysis.1,7–26 A number of well-known textile microscopists
who also worked intensely on elucidation of faults deserve mention here:
A. Herzog (Vienna, Dresden), H. Reumuth (Jena, Mannheim, Karlsruhe),
P.-A. Koch (Dresden, Zürich, Krefeld), M. Stratmann (Krefeld), N.
Bigler (Basle), K. Mahall and I. Goebel (Düsseldorf) and G. Schmidt
(Ludwigshafen).
Many microscopic methods are used in the textile industry to investigate
raw materials, for product development and analysis of competitors’ samples,
to check production and control effects and quality. Textile microscopy is
indispensable in dealing with complaints and analyzing damage as well as
284
Identification of textile fibers
in avoiding faults and repudiating unjustified claims. A short survey on
applications of stereo, compound and fluorescence microscopy, including
cross-section and grinding techniques for textile damage analysis is described
in Chapter 8 of Fan’s book Chemical Testing of Textiles.27
15.3.5 Chromatography, preferably thin
layer chromatography
With textile damage analysis the possibility of identifying the separated
substances by comparing them with authentic samples is often as important
as the separation itself. This identification is successful when the separation
behavior in one, or preferably more, separation systems is the same and
when additional findings such as the same staining or reaction behavior
show that the substances are identical. A prerequisite for such chromatographic identification is that the identity of the substance is already suspected so that the relevant substances can be chromatographed at the
same time for comparison. An even more fundamental prerequisite is that
the substances to be analyzed are soluble in the mobile phase. Naturally
it is also important that a suitable separation system is known or can be
developed.
Of the many chromatographic methods used in analysis the one preferably used in textile damage analysis is thin layer chromatography (TLC).
The reason for this is that TLC delivers results quickly, simply and cheaply,
with usually sufficient accuracy for elucidation of damage cases. Dyestuffs,
optical brighteners, soluble textile auxiliaries and fiber finishes are especially suitable for TLC. Many pigments are also sufficiently soluble. Crosslinked finishes, fibers, coatings and other polymers are unsuitable. For the
analysis of these insoluble or polymeric substances the more costly pyrolysis
gas chromatography (P-GC) and variations of high performance liquid
chromatography (HPLC) or, often simpler in the case of almost homogeneous polymers, infrared spectroscopy can be used.
TLC is particularly useful for analyzing cases of damage with soluble
stains or with oily or greasy soiling. Further examples of interest are TLC
of carriers for polyester28,29 and polyester oligomers.30,31 Many low molecular weight textile auxiliary agents can be analyzed with TLC and this can
be useful in elucidating certain cases of damage. Known examples are stains
caused by carriers or softeners. Surfactants can be separated using TLC and
identified by comparison with authentic surfactant.32,33 When mixtures of
surfactants with different ionic forms are present it is recommended to
separate them first with an ion exchanger. Ethoxylated products can be
separated by TLC into the individual species up to an ethoxylation degree
of 20 to 25. This results in an impressive row of spots like a pearl necklace
Identifying and analyzing textile damage in the textile industry
285
and the size of the spots gives a rough idea of the distribution of the degrees
of ethoxylation.34–36
15.3.6 Infrared spectroscopy
Infrared spectroscopy (IRS) is often a useful supplement to TLC especially
in the analysis of insoluble or macromolecular substances. However, with
mixtures of substances the superimposed IR spectra are frequently so
complex that they can hardly be interpreted. A previous separation, including that with TLC, is very useful for IR investigations.
IRS is a particularly powerful method for damage analysis. With this
method fibers, coatings and other deposits, textile auxiliaries and substances
causing stains can be identified. Chemical damage to fibers can also be
detected by means of specific structural changes. In damage analysis the
composition of mostly liquid extraction residues is of particular interest. As
well as qualitative IRS, quantitative applications are also available, where
the determination of the concentration of dissolved substances, the blend
ratio in fiber mixtures or estimation of the comonomer content in copolymers is possible.
On account of its high energy flow and favorable signal to noise ratio
modern Fourier Transform IRS (FT-IRS) enables the use of very low energy
analysis methods such as IR microscopy37 and many interesting reflection
methods including directed and diffuse reflection as well as attenuated total
reflection (ATR). Namely single reflection diamond ATR is of particular
interest. In the literature5,38,39 the advantages and disadvantages of the different IR sampling and measuring techniques suitable for textile applications, especially for damage analysis, can be found.
The following examples serve to give an idea of the many possibilities
for the use of IRS in investigating damage to textiles. The identification of
fibers, including unwanted foreign fibers, of coatings, stains and other fiber
deposits are the applications which spring most easily to mind. The IR
spectra of polyester oligomers are almost identical with those of polyester
fibers. The oligomers can be investigated directly with diffuse reflection or
the diamond ATR method.
Silicone stains still occur quite often and with the aid of IRS they can be
readily and fairly sensitively detected by means of the Si-CH3 rocking band
at 800 cm−1, several intensive Si-O-Si bands in the range from 1020 to
1120 cm−1 and Si-CH3 bending vibrations at 1260 cm−1. Table 15.1 lists the
suitability of these bands for the detection of silicones depending on their
superimposition with fiber bands.4 Other types of stains such as paraffins,
sizes, softeners and carriers are somewhat more difficult to identify with
IRS.4 On account of the intensive F-C main bands at 1200 and 1150 cm−1
fluorocarbon finishes can be detected by IRS at application levels of
286
Identification of textile fibers
Table 15.1 Suitability of IR bands for identifying silicone stains,
depending on the type of fibre4
Wave number (cm−1)
Cotton
Viscose
Wool
Nylon 6.6
Polyester
770–800
1020–1120
1260
2965
++
–
+
0 (+)
++
–
+
0
++
+
0 (+)
–
++
+
–
–
0 (+)
–
–
0 (+)
Explanation of symbols:
++ indicates very good suitability (single, non-overlapping bands)
+ good suitability.
0 means that because of superimposition silicone can only be detected
by the markedly higher intensity of the bands.
– means that no increase in intensity of the superimposed bands is
recognizable.
fluorocarbon polymer from about 0.3 to 1.2%, depending on the overlapping with fiber bands.5 When dealing with complaints or analyzing competitors’ samples concerning bonded nonwovens, the composition of the binder
is of interest as well as the type of fiber. The binder can also be determined
with IRS.40
15.3.7 Thermal analysis
Thermal analysis (TA) is the comprehensive name for a group of analytical
methods in which physical or chemical properties of a sample are measured
as a function of temperature and time. TA has been indispensable in polymer
technology and research for a long time. In the area of textiles it has
increased in importance due to the fast-growing market segment of technical textiles, for example in quality control and in analysis of products, competitors’ samples and damage. In the other textile market segments, namely
apparel and household textiles, TA is especially important in damage
analysis.
A database research using the keywords textile and TA (including the
methods DSC, TGA and TMA, which are explained below) showed a
marked difference in their relevance for textiles. By far the most important
is differential scanning calorimetry (DSC), followed by thermogravimetric
analysis (TGA) and finally thermomechanical analysis (TMA).
Determination of melting point
The determination of melting point as used for fiber identification can itself
be included under TA, because here the phase change is determined as a
Identifying and analyzing textile damage in the textile industry
287
function of temperature. It can be carried out very easily using a Kofler hot
bench.41 The melting point or melting range can be determined in a few
minutes with an accuracy of about ±2°. This is usually sufficient for fiber
identification. With intimate fiber blends a melting point or hot stage microscope with polarized light is necessary. The melting behavior of all the types
of fibers can be observed consecutively. Even if a fiber does not melt when
heated up to 300 °C, for example, this can still be a useful piece of evidence
for its identification.
Differential scanning calorimetry
With DSC not only the temperature range for melting (Tm) or for decomposition (Td) and, during cooling, that of crystallization (Tc) can be determined but also the corresponding enthalpies (heat of fusion Hm, heat of
decomposition Hd and heat of crystallization Hc). Furthermore the characteristic temperature for the amorphous areas, the glass temperature (Tg),
and the so-called effective temperature or middle endotherm peak temperature (MEPT) can be determined. By comparison of the measured heat
of fusion with the theoretical value the purity or content can be determined.
This is particularly relevant for fiber recycling.
For damage analysis of textiles made from polyester the MEPT is especially interesting because it gives insight into the thermal prehistory of the
fibers.42–44 The MEPT is the maximum temperature of a small endothermic
peak between the small endothermic stage of glass transition and the large
endothermic melting peak. The position of this MEPT peak is variable and
depends, besides other facts, on the temperature of thermal pretreatments
(Tp). Its size depends on the intensity, and thus mainly on the duration of
the thermal treatment and also on the tension, for example during setting.
The measured MEPT usually lies several degrees above the temperature
of a preceding thermal treatment (MEPT > Tp). It gives useful information
for damage analysis.
For example, it is possible to determine from this temperature whether
polyester goods were dyed at the boil (with carrier), under high temperature (HT) conditions or using the thermosol process. Conclusions about
setting temperature are also possible, in particular differences in setting
conditions can be determined exactly. Unfortunately, this thermal memory
also has notable restrictions. It can be superimposed by mechanical influences; for example, differences in tension also affect the MEPT. The way in
which the heat was applied (steam, hot air, conducted heat) also causes
significant differences. Finally, a strong thermal influence should not have
taken place subsequently as this extinguishes the memory for the weaker
influence. The model of Jeziorny45 for explaining the phenomenon of MEPT
helps to make these factors understandable. According to this model, small
288
Identification of textile fibers
increasingly ordered areas, the so-called micro-crystals, are formed under
increased heat input from initially unordered structures on the surface of
the crystalline areas of the fiber. These then melt during the DSC measurement. If the sample is cooled after the first DSC measurement and then
subjected to a second run, a MEPT peak is no longer found. Comparison
of the first and second run thus makes it easier to find and interpret this
thermal event. If the DSC instrument is sufficiently sensitive, a MEPT peak
can also be found with nylon fibers. Its allocation to thermal pretreatments
is, however, more difficult than with polyester because, among other reasons,
the rate of crystallization is higher with nylon.
DSC is also useful for characterizing bicomponent fibers, film-forming
finishes and coatings. For example, with polysiloxane a very low glass temperature (about −120 °C) is characteristic, followed by a crystallization peak
(at about −100 °C) and a large melting peak (−40 °C). As a matter of interest
the enthalpies obtained by integration of these peak areas enable the crystalline ratio to be calculated: (Hm − Hc):Hm. This calculation is also possible
with polyester, especially with granules, on account of their higher amorphous content.
Thermogravimetric analysis
With TGA the change in weight of the samples on heating is determined
(usually possible up to 1000 °C). In a nitrogen atmosphere the decomposition of the sample can thus be studied, in air the ability to be oxidized is
additionally determined. In this way fibers modified to be flame-resistant
can be distinguished from standard fibers. In fiber composites, for example
fiber-reinforced rubber, it is thus possible to determine the proportions of
the components with relatively little effort: moisture and softeners in the
first stage of weight loss up to about 220 °C, then the fiber and rubber components up to 500 °C and finally after changing from a nitrogen atmosphere
to air the carbon used as a filling burns and above 700 °C the non-burnable
inorganic filling remains. The first derivative of the weight loss curve, the
derivative TG (DTG), enables a more exact determination. By coupling
TGA with a mass spectrometer or a FT-IR spectrometer the decomposition
products can be analyzed. Because of the higher costs such methods are
only used in exceptional cases for textile damage analysis.
Thermomechanical analysis
TMA investigates the changes in the dimensions of a sample as a function
of the temperature, for example shrinkage or extension of fibers.46 It is
easier to work here with filaments than with staple fibers. Fiber composites
and other materials are also analyzed with dynamic loading. This dynamic-
Identifying and analyzing textile damage in the textile industry
289
mechanical analysis (DMA) enables, for example, the glass temperature of
elastomers to be determined exactly. But in textile damage analysis TMA
is seldom used.
15.3.8 Further methods
There are a large number and variety of methods which can be used for
damage analysis of textiles and these methods can naturally also be combined. It is economic restraint which most affects the imagination of the
damage analyst. In other words, any method can be considered for damage
analysis if it is or could be useful, does not cost too much and does not take
too long.
In addition to the important methods of damage analysis described above
three further methods will be briefly described here.
Techniques for surface imprints
Imprint techniques have been a proven and important method in damage
analysis of textiles for a long period of time. It is often advantageous not
to investigate the original object under the microscope but rather the negative imprint of its surface:
•
In an imprint it is often possible to see if a fault in a colored textile was
caused during textile manufacture or during dyeing and finishing.
Spinning faults, such as use of different fiber counts or differences in
yarn twist, and faults in fabric production can be seen in the imprint as
well as in the original (and in the same location). On the other hand,
faults arising from dyeing or printing are eliminated in the imprint.
• The imprint is transparent and the color of the sample does not interfere. Thus with dark-dyed wool fibers the cuticle scales can only be easily
recognized in the imprint. The same applies to abraded places and other
types of mechanical damage to the surface of dark dyeings.
• Since the surface imprint is very thin (about 0.02 mm) the depth of focus
is usually much better than in direct microscopy of the uneven, threedimensional textile surface and possible fiber lustre and transparency
do not interfere. In direct microscopy with reflected light the image is
usually not sharp because the fiber interior and the underside of the
fiber also reflect light.
• Since the transparent imprints are examined in transmitted light, it is
not necessary to have a microscope with reflected light. In addition, the
original sample remains unchanged.
There are two different widely used imprint methods in damage analysis,
namely imprints on gelatine-coated plates47 and on thermoplastic films,
290
Identification of textile fibers
Table 15.2 Comparison of the advantages of the most important surface
imprint methods
Imprint with gelatine-coated plates
Imprint with thermoplastic films
No thermal influence on the sample
No special equipment necessary
No swelling of hydrophilic fibers
With a commercial instrument49
larger areas (approximately 20 ×
30 cm) can also be tested
Detection of grease, oil or wax
deposits possible due to diffusion
into and dulling of the film
No false indication of structural
differences, arising from diffusion of
grease, oil or wax deposits (possible
effect on films)
usually polypropylene or polystyrene.1,48 In Table 15.2 the most important
advantages of these complementary imprint methods27 are compared.
Extraction methods
A typical textile laboratory is characterized by several Soxhlet extractors
standing in the fume cupboard. That is to say, the extraction of textile
samples is a routine or standard procedure. During extraction, substances
soluble in organic solvents or water are removed from the textile, then, as
a rule, concentrated by distillation, and the extraction residue is analyzed
qualitatively and/or quantitatively. Examples of extracted substances are
stains, fiber spin finishes, lubricants, residual grease in wool, residues of
surfactants and other chemicals such as acids, bases or thickeners, soluble
finishes, dyes and optical brighteners, pesticides and other biocides, carriers,
heavy metals, salts and formaldehyde. Stepwise extraction using solvents of
increasing polarity (for example, first hexane, then methylene chloride, then
absolute alcohol and finally water) can give a first indication of the nature
of the extracted substances.
Different extraction methods and apparatus can be used. They are especially useful for damage analysis. Some of the procedures are standardized.50 Mini versions of the Soxhlet extractor, are preferred for very small
samples such as stains. As alternatives to the Soxhlet extractor, automated
apparatus have been developed. The Morapex rapid extractor51,52 enables
the test sample to be extracted non-destructively in a very short time with
either water or organic solvents.
Average degree of polymerization of fibers
Many types of damage, including chemical, thermal, photolytical, biological
and some types of mechanical damage, are based on degradation of
Identifying and analyzing textile damage in the textile industry
291
the polymer chains in the fiber. Thus determination of the average degree
of polymerization (DP) gives a direct scale for assessing the extend of
such damage but not for its cause. The time and cost of determining DP
is, however, so great that, whenever possible, simpler but less accurate
methods are preferred. Examples of these are loss of tensile strength
and abrasion resistance or the pinhead reaction with cotton. An advantage
of DP determination is that it allows quantitative estimation of the
damage.53,54
Of the many methods known for determining DP, measurement of viscosity according to Staudinger is the one preferred in damage analysis because
it can be carried out in any textile laboratory. The viscosity is measured
indirectly via the times taken for the polymer solution and the solvent
to run through an Ubbelohde or Ostwald viscosimeter. A prerequisite is
that a suitable solvent is available for the fiber and that the corresponding
constants for the calculation are available. The solvent must not damage
the fibers and it should be easy to handle. Sometimes, however, healthdamaging m-cresol has to be used (polyester, nylon). Schefer55 has listed
solvents and viscosity constants for 16 undamaged fiber types.
In order to give an idea of the other methods which are used for damage
analysis1,56 the following examples are listed:
•
•
•
•
detection of metals, such as Fe, Cu, Ca, Mg, and nonmetals, such as N, P,
S, Cl, F, which can help to elucidate the damage
swelling and solubility tests, especially, but not only, with natural
fibers
determination of concentration, for example by means of titration,
gravimetry or colorimetry
staining tests which mark, for example, setting differences, oil and grease
deposits or fungi. They generally have the disadvantage that the samples
have to be undyed or only lightly dyed. It is time-consuming if the original dyeing first has to be removed in order for staining with test dyes to
be carried out and there is also the danger that additional damage may
occur during stripping of the dyeing.
Choice of the most suitable method is made more difficult if there is very
little sample material available. An ideal method should be highly sensitive,
reproducible and give clear-cut results. A common combination of methods,
especially when analyzing stains, begins with microscopy, followed by concentration after extraction. The extract from an undamaged area serves for
comparison. For identification the preferred methods are TLC (if authentic
samples are available for comparison) and IR spectroscopy. Reference
spectra are also very useful here but it is possible to identify the substances
causing damage by direct interpretation of the spectra.
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Identification of textile fibers
15.4
Damage analysis according to the type of fiber
The extensive subject of damage analysis of textiles can be divided into
typical cases of damage depending on the stage of processing or the
technology of yarn and fabric production such as:
•
•
•
•
•
filaments, threads and yarns57,58
woven fabrics59
knitted fabrics60–63
nonwovens
textile composites, coated fabrics.
An additional group of damages occurs during cleaning operations, such
as washing64 and dry cleaning. Although this division may be useful for
many typical types of faults, a division according to the type of fiber appears
even more suitable. In this way the types of damage typical for a particular
fiber can be meaningfully grouped. For example, cellulosic, protein and
synthetic fibers each have their own characteristic strengths and weaknesses, which are enlightening when analyzing damage to them. In the following section the most important methods of damage analysis will be
mentioned for the respective types of fiber.
15.5
Damage analysis of cellulosics, especially cotton
As opposed to wool, cellulosic fibers are relatively stable to alkali but sensitive to acid. In addition, cellulosic fibers can be damaged by strong oxidizing
agents, excessively high temperatures and microorganisms. The extent of
this damage (the damage factor) can, among other means, be assessed by
viscosimetric determination of the degree of polymerization. A much easier
method is the following reaction.
15.5.1 ‘Pinhead’ reaction with cotton65,66
This rapid microscopic test indicates chemical damage to cotton and enables
the degree of damage to be roughly estimated. The cotton fiber to be tested
is cut with very sharp scissors or a razor blade to snippets of about 1 mm
length and embedded with 15% sodium hydroxide on a glass slide. After
covering the sample with a cover slip and leaving for 2 to 3 minutes, the
formation of ‘pinheads’ at the cut ends is evaluated. In Table 15.3 the
appearance of the ‘pinheads’ is described corresponding to different stages
of damage and approximate ranges of the degree of polymerization.17,65
If the pinhead reaction demonstrates chemical damage the next point of
interest is to determine the exact cause. The tests described below for
damage due to chemicals or fungi are applicable to all cellulosic fibers.
Identifying and analyzing textile damage in the textile industry
293
Table 15.3 ‘Pinhead’ reaction and damage to cotton17,65
Type
Formation of pinheads
DP range*
1
Undamaged
>1500
2
Clearly damaged
3
Heavily damaged
4
Very heavily damaged
5
Extremely damaged
Well-rounded pinheads on about ¾
of all fiber snippets
Mostly flat protuberances with
some semi-rounded pinheads
Cut ends mostly flat with some flat
protuberances
All cut ends smooth with varying
width of lumen, fibers partially
convoluted
Surface notches, longitudinal and
transverse splits, fibrillation and
marked deformation
1600–1000
1100–700
800–400
<400
* DP = Average degree of polymerization
15.5.2 Detection of acid damage with Fehling’s solution
Since cellulosic fibers are sensitive to acids, they can be easily damaged by
the acid catalysts used in easy-care, silicone, fluorocarbon and flameretardant finishes as well as by drops of concentrated acid or faulty dyeing
of cellulose/wool blends. Carbonization of wool is based on this sensitivity
of cellulose to acid. Acid damage is also known to occur in sulphur dyeings
(cleavage of sulphuric acid during storage) and by contact between cellulose textiles with easy-care finishes and chlorinated bathing water or
detergents containing chlorine bleach (chloroamines are formed, which
decompose to hydrochloric acid and oxygen).
The 1,4-glucosidic bonds in the cellulose chains are cleaved hydrolytically
by acids. Cellulose damaged in this way, so-called hydrocellulose, is characterized by the shorter length of the molecules and thus a higher concentration of end groups. One end of the chain has a ring-forming hemiacetal
structure in equilibrium with the open-chain aldehyde form. This chemical
test for damage with Fehling’s solution is based on the higher concentration
of aldehyde groups after acid cleavage. The formation of a red color due to
precipitated copper (I) oxide indicates acid damage. In principle during this
redox reaction the aldehyde groups of the cellulose chains are oxidized to
carboxylic acid groups and the copper is reduced from copper (II) to copper
(I). It is important here to compare with acid-damaged reference samples
and undamaged goods, since the latter also show a slight red coloration. The
aldehyde groups may already have been partially oxidized by atmospheric
oxygen. Only a marked red coloration is a sure sign of acid damage. In cases
of doubt this test should be repeated. The following reverse conclusion may
294
Identification of textile fibers
also be useful here: if the pinhead reaction indicates chemical damage and
the Oxycarmine test described below is negative there is a high probability
that acid damage has occurred.
The detection of aldehyde groups with ammoniacal silver solution, as
recommended in the literature,56,67 is dangerous because the reagent can
explode after standing for longer periods.
15.5.3 Detection of oxidative damage with Oxycarmine
Cellulosic fibers are often oxidatively damaged when the most commonly
used bleaching agent, namely hydrogen peroxide, decomposes catalytically.
This unwanted decomposition to aggressive radicals is catalyzed especially
by heavy metals. Traces of iron, for example (abraded metal, rust from water
or steam pipes), are sufficient to cause severe catalytic damage during peroxide bleaching. The damage is manifested as a marked loss of strength or
even formation of holes. Naturally, it is difficult to detect traces of iron
around the edge of these holes.
Oxidatively damaged cellulose, known as oxycellulose, is characterized
by polymer chain degradation and an increased concentration of carboxylic
acid groups. These are the basis for the chemical detection of oxidative
damage. They bind basic dyes via salt links, whereas these dyes do not stain
undamaged cellulose. A well-known example is the staining test with
Methylene Blue.68 Staining with the test dye Oxycarmine69 is said to be four
times as sensitive as with Methylene Blue66 and is specific for oxycellulose.
The test dye has to be freshly prepared from two storeable components and
can only be used for one day. The dyeing conditions are simple and explained
in the product information. It is also important here to have comparative
stainings on undamaged and oxidatively damaged goods in order to
interpret the blue coloration correctly.
15.5.4 Damage by microorganisms
Cellulosic fibres are more often damaged by fungi than by bacteria. The
microscopic detection of fungi is based on the staining with Lactophenol/
Water Blue (see Section 15.5.3, detection of biological damage to wool).
Storage of moist fabric overnight in a warm environment can be sufficient
to cause fungal attack, leading to variously colored mildew patches, which
are difficult to remove.
15.5.5 Faulty quality in cotton, in particular neps from
immature and dead cotton
Differences in fiber quality in one lot of cotton can lead to unlevel fabric
appearance. These differences cause higher costs in dyeing and finishing and
Identifying and analyzing textile damage in the textile industry
295
represent a difficult challenge for the dyer. They are also often the reason
for customer complaints. Immature and dead cotton as well as the so-called
hard or red cotton (with high concentrations of calcium or iron compounds)
are not themselves typical forms of damage but are a frequent form of
faulty quality with similar negative effects.
There are many publications on the subject of neps and their detection,
for example Furter and Frey70 and on the determination of the degree of
maturity of cotton.71 Here, only the principle of the so-called red/green test
will be mentioned, since it is easy to carry out in any textile laboratory1,56,72
and it gives a quick idea of the proportion of immature cotton. It does not
correlate well with other tests of maturity.73–78 On the other hand, cotton
samples which show the same degree of maturity after the usual maturity
test with the air-flow method often show subsequent differences in depth
of dyeing,74 thus underlining the importance of this dyeing test.
The red/green test was introduced by Goldthwait, Smith and Barnett and
is thus also known as the GSB test.72 It was developed for raw cotton.
However, Denter et al. recommend previous alkaline scouring.74 If the red/
green test is used on yarns and fabrics neps become more clearly visible as
small green spots. Using a mixture of a red direct dye with small molecules
and a green direct dye about twice as large the mature, thick-walled cotton
fibers are dyed mostly red and the immature, thin-walled fibers, but also
very fine mature fibers, are dyed mostly green. Commercially available
replacement dyes for those originally used are listed in the literature.27 It is
of interest for damage analysis that the red/green test can also be used for
the detection of differences in cotton, caused by alkali treatment or
mercerization.79,80
15.6
Damage analysis of wool
15.6.1 Wool chemistry and types of damage
The chemistry of protein fibers, especially that of wool, is more complex
than that of most other types of fiber. The amide groups which link the
repeat units are also to be found in nylon fibers. Amides can be cleaved
hydrolytically by strong acids and bases. However, the inter- and intramolecular bonds in wool are particularly numerous. There are hydrogen and
disulfide bonds, salt links and hydrophobic attraction. All of these bonds
are important for understanding chemical damage to wool and the possibilities for its detection. Wool is particularly sensitive to alkali, but is also
damaged by strong acids, reducing agents and oxidizing agents.
Indications of wool damage
First indications of possible damage to wool are a harsh handle or yellowing,
both, where possible, in comparison to a wool sample before the potential
296
Identification of textile fibers
damage. Heavy felting and change of shade in dyed fabrics are also signs
of possible damage. Microscopic indications are usually more unequivocal,
since the characteristic scale structure of wool fibers not only enables their
identification and characterization but also analysis of damage to them.
Changes in the scale structure can be seen particularly well in surface
imprints (see Section 15.3.8). It is important here that several samples (at
least 10, in critical cases up to 100) taken from different parts of the fabric
are examined in order to gain a representative impression. Even in highquality wool fabrics some individual damaged fibers can be found. Easily
recognizable signs of damage under the microscope are:
•
•
•
•
•
•
•
•
abraded scale edges or other types of scale damage (mechanical damage,
kinks, rupture)
projecting, bulky scales (for example, after damage by acids, alkalis,
chlorine or light)
longitudinal striations in the interior of the fiber (damage due to alkali
or bleaching)
pore-like openings, especially visible in embedding agents with a similar
refractive index to wool (nD 1.55, for example Canada balsam), caused,
for example, by too heavy chlorination81
fibrillar delamination of the wool hairs with release of the spindle cells
(heavy chemical or enzymatic damage)
frayed fiber ends (brush ends, damage by tearing, possibly indicating
shoddy wool)
root ends and skin remnants (possibly indicating slipe wool)
crescent-shaped bites (insect attack).
15.6.2 Staining tests
Many dyestuffs have been described in the literature as staining tests for
wool damage. Examples are:
•
•
•
•
•
•
•
Methylene Blue/C.I. Basic Blue 9356 (0.1% aqueous solution, 3 minutes
at room temperature),
Neocarmin W1 and Neocarmin MS69 (5 minutes at room temperature),
Cotton Blue/Water Blue B or Telon Blue AGLF(DyStar)/
Lactophenol,1,56
Indigocarmin (cold saturated solution and 40 ml/l sulphuric acid),82
Sirius Light Grey GG/C.I. Direct Black 77 and Sirius Light Red 4BL/C.I.
Direct Red 79 (0.5% solutions, 3 minutes at the boil),
Rhodamine B/C.I. Basic Violet 1068,81 (1% aqueous solution, 1 minute
at the boil),
Congo Red/C.I. Direct Red 28.83
Identifying and analyzing textile damage in the textile industry
297
It is recommended to separate the sample into the individual fibers before
the staining test and to treat these for 10 minutes in a solution of about 0.5%
wetting agent. After staining the sample should be thoroughly rinsed.
A common factor with these staining tests is that damaged wool usually
takes up more dye. Most types of damage destabilize the wool structure so
that deeper dyeings take place than with undamaged wool. Crosslinking
wool protecting agents can reduce the amount of dye uptake. These staining
tests should only be regarded as a rough indication of wool damage, compared with the methods described below. Furthermore, they are restricted
to undyed or only slightly dyed wool.
15.6.3 The Pauly reaction as an important indication
of damage
The diazo reaction according to Pauly1,56 is especially useful in damage
analysis of wool. It shows up mechanical, chemical and biological damage
and demonstrates destruction or peeling off of the cuticle (scale layer). The
Pauly reaction is based on the fact that the cuticle layer contains considerably fewer aromatic amino acids than the cortex layer lying beneath it.
These aromatic amino acids, especially tyrosine, react with the Pauly reagent
(diazotized sulphanilic acid) to form a red monoazo dyestuff. The intensity
of the coloration thus obtained, ranging from yellow-brown via orangebrown to red-brown, corresponds as a rule to the degree of damage. A more
detailed description is given in recent literature.27
By counting fibers under the microscope the Pauly reaction can be evaluated semi-quantitatively.56,81 However, Mahall1 has reported that after very
heavy alkaline damage the color intensity of the Pauly reaction can decrease.
For this reason also this test dyeing should always be combined with a
microscopic examination. The Pauly reaction can also be carried out with
dyed wool. However, the red coloration is difficult to see on deeply dyed
wool, for example at the edges of the fibers.1 The Pauly reaction also works
with other animal hairs. In addition it shows up hair roots and skin particles,
which can sometimes be present in wool yarns.
15.6.4 The Krais, Markert and Viertel reaction for detecting
acid or alkali damage
Whereas the Pauly reaction is a useful indicator for every type of damage to
the cuticle layers of wool the swelling reaction with ammoniacal potassium
hydroxide according to Krais, Markert and Viertel (KMV reaction) enables
the cause of the damage to be determined, namely whether damage due to
acid or alkali is present.1,56,84,85 A further advantage of this swelling reaction
is that it can also be carried out on dyed fibers. For example, fibers dyed by
298
Identification of textile fibers
Table 15.4 Time until commencement of swelling of wool with KMV
reagent56,81,84
Time until commencement of
KMV reaction
Conclusions about type and extent of damage
1 to 2 minutes
2 to 6 minutes
6 to 10 minutes
10 to 12 minutes
Very heavy acid damage
Acid damage
Treated with acid but scarcely damaged
Normal undamaged wool, depending on
fineness and origin
Possibly treated with alkali but without
noticeable damage
Treated with alkali, in most cases also
alkaline damage
15 to 30 minutes
More than 30 minutes
the Pauly reaction can be used here (after drying) so that this analysis of
damage can be concentrated on the more heavily damaged fibers.
Procedure for the KMV swelling reaction: the wool fibers to be tested
are laid on a microscope slide, a few drops of the KMV reagent are added
and the fibers observed under the microscope at intermediate magnification. The time is noted from the addition of the reagent to the appearance
of the first small, hemispherical blisters on the fiber surface. The evaluation
of the findings is given in Table 15.4. The irregular small protuberances
slowly become larger until they finally break up. With coarse wool fibers
the time taken for the appearance of the first blisters is longer than that for
fine fibers. Again this illustrates that a comparison with undamaged wool
and wool with defined damage is a useful aid to interpretation. In particular,
the comparison of samples before and after a potentially damaging process
enables a more exact explanation of the damage. When acid and alkali
damage have taken place sequentially the results of the KMV reaction are
not unequivocal. Long swelling times can occur without alkaline damage if
the wool has been treated with aldehydes or other crosslinking agents (as
fiber protecting agents).
This qualitative analysis of acid and alkaline damage is complemented
by quantitative tests as described in a sub-section below. Short swelling
times in the KMV reaction are a sensitive and discriminating indication of
acid damage. The longer swelling times after alkaline damage provide less
discerning evidence. A further indication of alkaline damage is the particularly heavy staining with Lactophenol/Water Blue, but this is not specific,
since other chemical, biological and mechanical forms of damage also
cause deeper staining. Rolling up of the fiber ends to crosier forms or even
more pronounced ringlet forms is, on the other hand, alone typical of
alkali-damaged wool.1
Identifying and analyzing textile damage in the textile industry
299
15.6.5 Detection of alkali and acid residues, including
formation of methyl orange crystals
A simple indication of possible damage can be obtained by measuring the
pH value of the wool (liquid indicator, flat-ended pH electrode) or that of
the aqueous extract, according to IWTO-2 or DIN 54275 and 54276, for
example. The determination of the alkali content (IWTO-21) and acid
content (IWTO-3, DIN 54280) are standardized methods.86 In cases where
only small amounts of sample are available for damage analysis, microscopic investigations with methyl orange can be useful as an alternative to
these methods of direct pH measurement. A few fibers on a microscope
slide are covered with a cover slip, 0.1% aqueous methyl orange solution
is then added from the side. In the presence of acid thin orange-colored
needles are rapidly formed at the fiber surface. They consist of the free acid
of methyl orange, which at pH values of 4 or below is hardly soluble in
water.17,21 This elegant method of detecting acid residues is also applicable
to all other types of fiber.
15.6.6 Detection of damage due to heat and light
After longer periods of exposure to heat wool textiles can discolor above
temperatures of about 120 °C. The allowable temperature for rapid pressing
is about 160–170 °C.87 At higher temperatures and longer periods of time
wool yellows. In strong light wool is at first bleached (photo bleaching) and
then yellows (photo yellowing). This effect is intensified by additional
thermal stress, for example with wool textiles in automobiles. UV absorbers
lessen this damage. Alkali residues increase the extent of yellowing by heat.
Further indications of this type of damage are markedly lowered peracetic
acid/ammonia and urea/bisulphite solubility as well as lower cystine content.
Alkali solubility and Methylene Blue uptake are increased slightly by dry
heat treatments and markedly by damage due to light and moist heat.88
Peracetic acid/ammonia solubility enables a differentiation between damage
due to dry heat (markedly reduced solubility) and due to light (increased
solubility). Additionally this is said to be a good indicator for crosslinked
wool (reduced solubility). The tryptophan content can also be used as
an indicator for photodegradation.89,90 These quantitative methods are
mentioned in a following sub-section.
15.6.7 Detection of biological damage to wool
Bacterial damage to animal fibers is described more often than fungal
damage.1,81,91 Dyed fabric damaged by bacteria, for instance, often has light
spots or streaks which cannot be repaired. Typical observations on the
300
Identification of textile fibers
damaged fibers are intensified striations and splitting up into cortex cells.
The fact that only the orthocortex has been attacked and the paracortex
remains is characteristic.1 In order to differentiate from acid damage, in
which the cortex cells can also be exposed, the KMV reaction, which is very
sensitive to acid damage, is recommended. The extent of damage by bacteria can be assessed with the Pauly reaction. Bacteria can be stained with
Lactophenol/Water Blue (as can fungi) and also with carbolfuchsin.91
However, the bacteria are often washed off during dyeing or scouring so
that only the changes in the fiber, as mentioned above, remain as evidence.
Mahall has described in detail the procedure and the problems of detecting
bacterial damage.1 The same is true for fungal damage.
Wool can also be damaged by insects. For example, the larvae of the
clothes moth (Tineola biselliella), the furniture carpet beetle (Anthrenus
flavipes) and the black carpet beetle (Attagenus) feed on keratin. Treatment
with moth- and beetleproofing agents can largely prevent such damage. The
damage caused by insects to non-treated wool can be easily recognized
under the microscope.63 If fibers with crescent-shaped bites are found at the
edge of holes, the holes can be said to originate from the furniture carpet
beetle, which sometimes also leaves behind typical hairs. According to
Peter, moth larvae usually bite off the end of the fiber and leave behind in
the vicinity of the fiber end typical traces of a transparent, voluminous
substance, which exudes from the mouth of the larva when feeding in the
vicinity of the fiber end.17 Moth bites are sometimes also found on cellulosic
and synthetic fibers, the fiber substance then being excreted without being
digested. Excrement is further evidence of moth damage.
15.6.8 Quantitative analysis of wool damage
The quantitative methods mentioned here enable the degree of wool
damage to be estimated and sometimes also the type of damage. The principle of solubility tests is to determine the weight loss after an exactly
defined treatment and to compare this with the weight loss of undamaged
wool. The weight loss is always calculated on the basis of the dry weight of
the fibers and given in percent. As a rule, the greater the difference to the
weight loss of undamaged wool the more extensive is the damage. By
analogy, this also applies to the methods where the content of certain amino
acids (cystine, cysteine, cysteic acid, lanthionine) or the amount of dye
uptake (for example, Methylene Blue) is determined. Here also the difference to the reference value for undamaged wool is used to evaluate the
damage. Table 15.5 lists the most important quantitative methods for wool
damage analysis.
More complicated methods are only used for wool damage analysis in
exceptional cases. They often involve the chromatographic separation of
Principle
Treatment with 0.1 N NaOH
at 65 °C, 60 minutes
Treatment with a urea/
bisulfite solution at
65°C, 60 minutes
Oxidation of cystine with
peracetic acid and treatment with 0.2 N ammonia
Method
Alkali solubility
IWTO-4 (1)
ASTM D-1283 (2)
BS 3568 (3)
SNV 195 587 (4)
DIN 54281 (5)
Urea/bisulphite
solubility
IWTO-11
DIN 54279
Peracetic acid/
ammonia solubility
SNV 195 586
Decrease after treatment with
alkali, crosslinking agents or
dry heat
Increase after treatment with
acids or oxidizing agents;
marked decrease after
treatment with alkali or
crosslinking agents
Increase after treatment with
acids or oxidizing agents,
also after steaming above
100 °C
Main conclusions
Table 15.5 Important quantitative methods for wool damage analysis88
90 to 86%
40 to 50%
12 to 17%
Value for undamaged
wool
60 to 40%
about 5% or
80 to 90%,
respectively
50 to 80%
Value with very
heavy damage
Total hydrolysis, electrophoretic separation and
colorimetry or FTIR
preferably with ATR
Staining with Methylene Blue
and colorimetry of the
residual liquor
Cysteic acid
IWTO-23
DIN 54286
and
IR spectroscopy
Methylene
Blue-uptake
SNV 195 588
International Wool Textile Organisation
American Society for Testing and Materials
British Standards Institution
Schweizerische Normen-Vereinigung
Deutsches Institut für Normung
Eidgenössische Materialprüfanstalt
Hydrolysis of wool with
sulfuric acid, reduction of
cystine with sulfite,
colorimetric determination
of the blue color formed
with phosphotungstic acid
Cystine and cysteine
content
IWTO-15
EMPA-Methode (6)
(1)
(2)
(3)
(4)
(5)
(6)
Principle
Method
Table 15.5 Continued
Global parameter, increase
with almost all types of
chemical damage to wool
Increase after heavy oxidation,
for example with peroxide
or chlorine
Decrease in cystine content
after treatment with alkali,
peroxide, reducing agents
(not with acids), increase in
cysteine content after
reduction
Main conclusions
3 to 5 mmol / 100 g
0.2 to 0.5%
Cystine 11 to 12.5%
Cysteine 0.3 to 0.4%
Value for undamaged
wool
25 to 35 mmol /
100 g
3 to 5%
Cystine 6 to 8%
Cysteine 1 to 2%
Value with very
heavy damage
Identifying and analyzing textile damage in the textile industry
303
the total hydrolysate of wool samples, which is available in automated form.
Schefer92 names some exceptions to the rule that the results of quantitative
wool damage analysis correlate well with mechanical textile tests, for
example after crosslinking reactions.
15.6.9 Cumulative wool damage
Damage to wool is often due to a combination of several causes, whereby
according to Doehner and Reumuth81 the effects can be developed, added,
multiplied or even increase exponentially. When more than one chemical
acts on wool, as is usually the case in dyeing and finishing, damage can occur
which would have been harmless if each of these chemicals had acted alone.
This effect is known as cumulative damage. For example, intensive peroxide
bleaching and dyeing in strong acid conditions can each be carried out alone
in such a way that the performance of the wool is not lessened and other
undesirable effects are also avoided. If, however, these treatments are
carried out under the same conditions in sequence the wool suffers considerable damage. Sanger93 explained this by the catalytic effect of the cysteic
acid formed during oxidation on the acid hydrolysis of the neighboring
amide group. The proton from the cysteic acid is transferred via a hydrogenbond-stabilized six-membered ring structure to the amide nitrogen, which
makes this amide group easier to hydrolyze.
15.7
Damage analysis of silk
Reports on faults with silk fabrics are more common than would be expected
from their share of the world fiber market (about 0.1%). One reason for
this is that silk is particularly delicate. Mechanical treatment, especially in
the wet state (dyeing and finishing as well as household laundering), causes
irreparably abraded areas, the so-called blanched places. Defibrillation and
splitting off of the silk filaments causes incident light to be scattered diffusely. The silk luster is no longer present, the fabric appears dull and
lighter. This is particularly noticeable with dark dyeings. The abraded fibers
can be identified in direct microscopy and even better in surface imprints.
The same is true for silk lousiness (exfoliation), which is not actually a case
of damage but does represent a quality problem. Silk lousiness is the term
for pills formed from fine fibrils which become separated from the filaments
of the silk. They are laid bare during degumming and during further processing they are twisted together into pills.94 Again they represent an aesthetic problem especially with dyed fabrics.
Mahall and Goebel have reported that the Pauly reagent as used in wool
damage analysis can also detect mechanical and chemical damage to silk,
although the silk fiber does not have a cuticle layer but only a thin fibroin
304
Identification of textile fibers
sheath.1,94 With silk the Pauly reaction thus has to be carried out for a short
time and with cooling (1 to 2 minutes, ice-cooled). To help verify the results
it is recommended to carry out comparisons on undamaged silk and on silk
with defined damage. In addition, before this analysis is made the Bombyx
mori silk has to be carefully checked to see if it is completely degummed,
since residues of sericin are stained red similarly to damaged fibroin.
Undamaged Bombyx mori silk is stained yellow. Degummed tussah silk is
stained yellow-brown by the Pauly reagent and bleached tussah silk light
orange, thereby indicating a slight degree of bleaching damage.
Similar staining tests have been described for the detection of sericin and
for controlling the degumming process.27,95 In all these staining tests the
fibroin is colorless after rinsing and the sericin is stained red. Checking of
the stained samples under the microscope is in all cases useful for verifying
the findings.
Since weighted silk is more easily damaged mechanically and the detection of weighting is an important aspect of quality control, this subject will
also be mentioned briefly. Weighting with minerals, usually with tinphosphate-silicate, results in a light-colored skeleton residue of ash after
burning. At higher levels of weighting the structure of the yarn or fabric is
retained in the ash. Agster56 has described conventional chemical detection
reactions for weighting. Silk can also be weighted by grafting-polymerization with methacrylamide,96 which can be detected by conventional chemical analysis or by IR spectroscopy.97
15.8
General types of damage to synthetics
As opposed to the natural fibers only a few simple test methods are known
for damage analysis of synthetic fibers. On the other hand, the standard
synthetic fibers are also generally much less heavily damaged by acids,
alkalis and microorganisms than are cellulosic and protein fibers. But there
are also typical faults and types of damage with synthetic fibers, as is briefly
described in this section.
15.8.1 Thermal damage
Thermal damage is one of the most frequent causes of complaints about
synthetic fibers, especially if they have a relatively low melting point. It
causes, amongst other things, hardening of handle, yellowing, loss of strength,
uneven fabric appearance (light reflection) and dyeing behavior (spots,
streaks and other types of uneveness such as warp splashes). Thermal
damage can occur at many stages of processing. Examples are texturizing,
setting, singeing, pressing and sewing. During texturizing the originally circular fiber cross-sections are usually flattened to polygons. When setting is
Identifying and analyzing textile damage in the textile industry
305
at too high a temperature or for too long, the yarns are flattened at the
interlacing points. During singeing of staple fiber blends with cellulosics or
wool protruding synthetic fibers can melt to form small balls, which cause
a hard handle and which dye more deeply in exhaust processes (small dark
spots, deeper dyeing being caused by the high amorphous content and the
decrease in relative surface area) and after continuous dyeing are lighter
than the undamaged fibers (the diffusion time is too short for the melt
balls).1 Pressing at too high a temperature causes flattening and bonding of
thermally sensitive synthetic fibers. Thermal damage also occurs through
friction, impact, striking, cutting or punching out during textile production
and garment manufacture. Mahall has shown many typical examples of
this.1 The most important methods of investigation for thermal damage are
microscopy in longitudinal view (also with surface imprints) and on crosssections, dyeing tests and thermal analysis. Buchanan and Hardegree
described the influence of heat and tension (for example, during drawing,
texturizing and occasionally dyeing) on faults in yarns made of polyester,
nylon 6.6 and polypropylene.98
15.8.2 Damage by light
According to their chemical constitution and stabilization, synthetic fibers
show differing degrees of sensitivity to light, for example aliphatic and
especially aromatic polyamides are more sensitive and polyacrylonitrile
fibers less so. Apart from some technical fibers, synthetic fibers are usually
delustred with titanium dioxide. This catalyses photolytic degradation (photolysis), which can be recognized by an apparent coarsening of the grains
of the delustrant. The delustrant pigments appear larger because they are
surrounded by a sphere of degraded fiber substance with a different refractive index.21,22 Damage by light can usually be detected by fiber-specific
reactions and also by non-specific effects such as yellowing, loss of strength
and decrease in the average degree of polymerization.
15.8.3 Mechanical damage
Mechanical damage to synthetic fibers can be just as easily detected under
the microscope as with natural fibers. In the case of thermoplastic fibers
mechanical damage is often accompanied by thermal damage. This causes
deformations which can be clearly seen in the longitudinal view and in fiber
cross-sections. The change in reflection of light in the damaged area interferes with the uniform appearance of the fabric. Hearle studied the structure of ruptured fibers (polyester, nylon, acrylic) under the light microscope
and with electron microscopy and also damage caused by abrasion and
torsional stress, the samples mainly coming from damage in use.99
306
Identification of textile fibers
Differences in drawing ratio, fineness or texturizing of synthetic fibers are
often the reason for streaks and barriness in fabrics made from them. They
can be identified by marking the threads, stripping off of the dye and redyeing, in which case they reappear at the marked places. These faults can
usually be recognized under the microscope (surface imprints), differences
in drawing ratio can be seen particularly well in polarized light.100
15.8.4 Chemical damage
The chemical weak spots and the corresponding types of damage vary
greatly with synthetic fibers depending on their structure. A general difference in chemical stability exists between fibers formed by polymerization
and polycondensation. At extreme pH values polycondensate fibres are
hydrolytically degraded, for example by cleavage of ester or amide bonds
between the constitutive elements. The carbon backbone chain of the polymerisate fibers is stable to hydrolysis but sensitive side groups such as the
nitrile groups of acrylic fibers can be hydrolyzed.
15.8.5 Microfibers
Microfibers are especially sensitive to light damage, owing to their large
specific surface. Light damage is intensified by the catalytic effect of some
dyes and their light degradation products. Also a much higher dyestuff
concentration is needed for microfibers, compared to the same shade on
normal fibers. Fastness problems can also arise from residual size that is
usually not completely washed off from the microfibers.
15.9
Analysis of damage to polyester fibers
Polyester fibers (polyethylene terephthalate, PET) are chemically relatively
stable, damage due to acid or alkali is therefore rare. At extreme pH values,
however, hydrolytic degradation occurs, for example as used in the alkaline
titre reduction of polyester textiles. Thermal damage is more common,
caused by setting, pressing, pleating or singeing fiber blends containing PET
or thermally bonding nonwovens at too high temperatures. Another form
of thermal damage is the thermal deformation due to heat of friction, which
can occur during sewing and cutting or punching out. Mahall has described
many examples of this.1 The damage due to excessive heat and mechanical
effects, for example during primary spinning,101 secondary spinning102 or
tension during setting, primarily causes structural differences in the PET
fibers which lead to barry dyeings and shade differences across the fabric.
As a test dyeing for detecting structural differences in PET C.I. Disperse
Blue 79 (for example, Dianix Navy Blue NNG, Foron Navy S-2GL or
Identifying and analyzing textile damage in the textile industry
307
Table 15.6 Validity of test methods for changes in structure of PET due to heat
and tension106
Test method
Effect of heat
Effect of
tension
Equipment
costs
Effort
Dye uptake during HT
dyeing*
Residual shrinkage**
(hot air)
Tensile strength
Elongation at break
Work at 5–10% elongation
Elongation at 200–300 cN
Residual extension
Critical solution time*
(phenol)
Density (graduated
column107)
Differential scanning
calorimetry (DSC)
Thermomechanical
analysis** (TMA)
X-ray (long period)
–
–
large
large
+
++
low
low
–
–
–
–
–
–
–
+
++
++
++
–
large
large
large
low
low
low
low
low
low
low
low
large
++
–
medium
medium
++
–
large
low
+
++
large
low
+
–
large
large
Effects of heat and tension: * simultaneously, ** separately.
Validity: − none, + good, ++ very good.
Ostacet Navy Blue 2 GLS) has been recommended, for example for 30
minutes at 130 °C.103 Another test dyeing uses a mixture of a blue dye which
shows up structural differences very markedly and a yellow dye which does
not have this property.104
Differences in draw ratio in PET fibers can be analyzed by microscopic
determination of their specific birefringence, whereby higher tension results
in a higher birefringence whereas the influence of temperature varies.105
Section 15.3.7 described how the thermal prehistory of PET can be analyzed with the aid of the so-called effective temperature or middle endotherm peak temperature (MEPT) determined by differential scanning
calorimetry. The term ‘effective temperature’ was introduced by Berndt and
Heidemann.106 In this publication a review of many other testing methods
for analyzing structural differences in PET is given, including an evaluation
of how well they respond to the influence of heat and tension. Table 15.6
shows that the methods of thermal analysis are fairly well suited for this
purpose.
Photolytically damaged polyester fibers can be recognized under the
microscope by means of the apparent coarsening of delustrant grains,108 as
308
Identification of textile fibers
described in Section 15.4.4, and by the fluorescence in UV light of the degradation products oxy- and 2,5-dihydroxy terephthalic acid.109,110 This fluorescence microscopy enables a differentiation between photo- and thermal
damage, but only if optical brighteners do not superimpose on the weak
fluorescence of the terephthalic acid derivatives.
15.9.1 Polyester oligomers as a cause of damage
PET oligomers are low-molecular-weight side-products of the polycondensation reaction with two to about ten repeat units. These linear and cyclic
oligoesters are possibly also formed during the melt spinning process. The
total content of oligomers is 1 to 3%; 70 to 90% of this amount occurs in
the form of the cyclic trimer cyclo-tris-ethylene glycol terephthalic acid
ester, c(G-T)3. A large percentage of the cyclic trimer diffuses onto the fiber
surface under high temperature (HT) dyeing conditions and from there into
the dyebath. c(G-T)3 is practically insoluble in water up to 100 °C and at
100 to 140 °C only to the extent of 1 to 5 mg/l. Thus during dyeing and in
particular during cooling down of the dyebath it can precipitate and deposit
on the goods being dyed or on the interior surfaces of the dyeing vessel.
The problems which thus arise are particularly great when dyeing loose
fibers or yarns. These problems include dust formation, poor running properties, difficult spinnability and increased wear on thread guides and needles.
With piece dyeings the oligomer deposits can cause light spots, especially
due to a filtration effect on the HT beam dyeing machine (possible perforation imprints on the fabric roll),111 but less so with knitted goods in jet
dyeing machines. In addition, the fabric then lacks brilliance, a cloudy greyness gives an uneven fabric appearance. Oligomers which adhere to the
fiber surface particularly interfere (surface oligomers, as opposed to core
oligomers). They are often mixed with spinning oils and other auxiliaries or
with dyes and can be difficult to remove. PET oligomers still repeatedly
cause complaints.
The first indication of damage due to PET oligomers is often the appearance of the fault: light grey deposits, which are partially easy to remove
mechanically and form dust. In addition, when an isolated sample is shaken
in isopropanol a marble-like suspension with mother-of-pearl lustre is
formed.112 Detection methods for PET oligomers, especially for c(G-T)3:
•
•
•
IR spectra resemble that of the PET fiber;
TCL detection, especially by comparing with authentic samples;
melting range 305 to 327 °C, according to their purity (marked difference to PET fiber dust, which melts at 250 to 255 °C), possible confirmation by means of mixed melting points: a mixture of approximately
equal parts of sample and authentic cyclotrimer should not show any
marked depression of the melting point;
Identifying and analyzing textile damage in the textile industry
•
309
microscopic detection by recrystallization on the microscope slide (dissolve in dichloromethane, c(G-T)3 crystallizes out after concentration
by evaporation of the solvent112 or formation of crystals after melting
the sample on the microscope slide.1 The crystals thus formed are usually
hexagonal, sometimes also in the form of needles or star-shaped and
they appear golden yellow to multicolored in polarized light.112
15.10 Analysis of damage to nylon fibers
In terms of quantity the standard nylon fibers were overtaken by polyester
fibers around 1975. The reason that the latter fibers are now by far the most
important group of synthetic fibers has also to do with the fact that nylon
6 and 6,6 fibers are more sensitive to light and less stable to hydrolysis than
polyester fibers. Nylon fibers also have a greater tendency to yellowing in
heat, nylon 6 in particular being less stable to heat than PET. On the other
hand, the abrasion resistance and bending strength of nylon fibers is very
high. Hearle has described the microscopic analysis of fatigue appearance
in nylon fibers.99 During extreme conditions of use and also during dyeing
and finishing oxidative damage can occur, often intensified by heat and/or
light. UV radiation damages aliphatic polyamides and especially the aramids.
With nylon 6 and 6,6, acid damage and, less common, alkaline damage are
usually the result of inappropriate dyeing and finishing treatments. At
extreme pH values their amide bonds are cleaved hydrolytically. Comparison
of the sensitivity to damage of the standard nylon fibers shows that nylon
6 is more sensitive to hydrolysis and heat whereas nylon 6,6 is more easily
damaged by oxidation.
15.10.1 Detection of acid damage
According to Bubser and Modlich113 acid damage to nylon fibers can be
detected in a simple way by a staining test with Rhodamine B (C.I. Basic
Violet 10, available from Aldrich, Fluka, Sigma). Depending on the degree
of acid damage, staining of the sample ranges from colorless with weak
damage via pale pink to dark red with heavy damage (Table 15.7). Nylon
fibers damaged by heat, light or oxidation are not stained in this test.
15.10.2 Detection of thermal damage
When treatments such as setting, pressing, pleating or singeing of nylon
fibers are carried out at too high temperatures, and also during thermal
bonding of nonwovens or during cutting and sewing, fiber deformation,
fiber bonding and formation of melt balls at the fiber ends can occur, all of
310
Identification of textile fibers
Table 15.7 Extent of damage and staining of acid-damaged nylon
fibers with Rhodamine B113
Extent of acid damage
Staining
Loss of strength
No damage
Weak damage
Medium damage
Strong damage
Very strong damage
Colorless
Pale pink
Pink
Red
Deep red
0%
About
About
About
About
5%
15%
30%
70%
which can be easily recognized under the microscope. The melt balls usually
dye more deeply114 and give rise to a hard handle. Setting differences in
carpet yarns giving streakly dyeings can be rapidly detected by spectroscopy
in the near-infrared region.160
15.10.3 Detection of damage by light
Despite photostabilization during fiber production and, in some cases, additionally during finishing, damage by light (especially UV light) is fairly
common with nylon fibers. The complex photolysis reactions are accelerated by delustrants as well as by contaminants within and external to the
fiber and, among many other things, also by nitrous gases.115–117 These reactions cause yellowing and loss of strength and can be recognized under the
microscope by the apparent coarsening of the delustrant grains.
15.10.4 Detection of damage by oxidation
With a ninhydrin staining reaction a darker color than normal is obtained
after oxidative damage.118 Using modified zinc chloride solutions (Frotté
reagent I and II) nylon fibers can be distinguished and identified by the
so-called Frotté reaction according to Koch19 (crenellated, finely structured
transverse folds and cracks on the fiber surface, which gradually change to
coarser structures and then dissolve). With solutions of zinc chloride, potassium iodide and iodine (zinc chloride-iodine reagent), modified according
to Bubser and Modlich, the Frotté reaction only occurs after oxidative
damage, but not after damage by acid, heat or light.113
This is described in more detail in Table 15.8 and is shown in Table 15.9
in a review together with other methods for detecting damage to nylon
fibers. The conclusions which can be drawn from these microscopic methods
are unfortunately often not exact enough and can only serve as a rough
guide,66 probably also because in practice different types of damage are
superimposed, for example light and heat or light and oxidation (photooxidation) or even all three causes of damage together.
Identifying and analyzing textile damage in the textile industry
311
Table 15.8 Damage to nylon fibers and Frotté reaction with modified zinc
chloride-iodine reagent according to Bubser and Modlich113
Type of damage
Loss of strength
Swelling behavior after 3
minutes at room temperature
None (gray fabric)
Weak acid damage
Medium acid damage
Strong acid damage
0%
About 5%
About 15%
About 35%
Very strong acid damage
Heat set
Overset
About 70%
About 15%
About 30%
Thermal damage
Damage by light
About 60%
About 40%
Oxidative damage
About 80%
Marked Frotté reaction
Marked Frotté reaction
Moderate Frotté reaction
Marked flattening of the
crenellations
No Frotté reaction
Marked Frotté reaction
Marked flattening of the
crenellations
No Frotté reaction
No Frotté reaction, dulling of
the unswollen part
Marked Frotté reaction
Table 15.9 Review of detection methods for damage to nylon fibers113,114,118
Damage due to
Acid
Heat
Light
Oxidation
Rhodamine B staining
Swelling in modified zinc
chloride-iodine reagent
Ninhydrin color reaction
Embedding in olive oil
Staining with acid dye
+
–
–
–
–
–
–
+
darker
lighter
normal
lighter
dull
darker than normal
darker than normal
15.11 Analysis of damage to acrylic fibers
Acrylic fibers (polyacrylonitrile, PAN) for the home furnishings and garment
sector are one of the fiber types with the greatest range of variation in their
commercial types, similarly to the elastane fibers. Two possibilities for variation in the manufacture of acrylic fibers are characteristic for this: the type
and amount of comonomers added to make the structure more flexible and
to enable dyeing with basic dyes and secondly the possibility for wet or dry
spinning with different solvents. Additional special features are dope dyeing
and gel dyeing. The resulting variety of types is a cause of many cases of
damage due to mistaken types and dyeing faults.
PAN fibers have good resistance to acid up to medium concentrations
and somewhat poorer resistance to alkalis. Their biological resistance is
good. PAN fibers are very resistant to nonpolar organic solvents, oxidizing
agents and weathering. Their abrasion resistance is low because of a ten-
312
Identification of textile fibers
dency to fibrillation.119 Resistance to heat is also relatively low, above 150 °C
they show yellowing, and, depending on the type of fiber, from 200–250 °C
an exothermic cyclization takes place (discoloration from yellow to brown
and then black, due to naphthyridine structures). This short review shows
that with PAN fibers one can expect damage from heat, abrasion, polar
solvents and effects of strong alkalis to be predominant. In addition, damage
is also caused by too great or uneven shrinkage, the reason for which can
be found in fiber manufacture, textile production or dyeing and finishing.
15.11.1 Thermal damage
The influence of cold or hot stretching in combination with heat setting has
been investigated with many physical and chemical methods, which are also
suitable for corresponding damage analysis (amongst others porosity,
density, strength, extensibility, differential thermal analysis, molecular
weight, electron microscopy).120 By measuring the critical solubility time in
common solvents (dimethylformamide, dimethylsulphoxide, dimethylacetamide) Gacén and Arias121 developed a simple test which shows differences
in fiber structure and shrinkage behavior sensitively and accurately (the less
the fibers have been shrunk the shorter the solubility time). This method
was also used for development and endurance testing of technical textiles,
including those made of PAN fibres.122
15.12 Analysis of damage to elastane (spandex) fibers
Elastic fibers are usually elastane fibers, which according to ISO 2076 are
made up of at least 85% by weight of segmented polyurethanes. In the USA
they have the generic name spandex. Although the share of elastane fibers
on the world fiber market is still less than 1% they have achieved increasingly greater importance in the last decades. In the meantime about half of
all clothing fabrics contain elastane fibers and around 85% of elastane fibers
are used for clothing. This increasing distribution is the first reason for the
frequency of complaints relating to textiles containing elastane. The second
reason is that they can be overstrained during production, dyeing, finishing
and in use. This already suggests the third reason, namely the relatively high
sensitivity of elastane fibers to certain types of damage. They are particularly susceptible to thermomechanical damage (by heat and tension) and
are also attacked and degraded by hydrolysis (acids and alkalis) and photolysis (especially by UV light). When the conditions are not too aggressive
they are stable to oxidizing and reducing agents, except for the heavy
damage caused by chlorine.
The urethane link, which binds the segments of the elastane repeat units,
can be described as half ester and half amide of a carboxylic acid. The sta-
Identifying and analyzing textile damage in the textile industry
313
bility of polyurethanes is thus similar to that of polyamides and polyesters.
The latter are more stable because of their high crystallinity and compactness, which is reflected in their density (elastane 1.2 to 1.3 and PET 1.38 g/
cm³). As with all elastic substances elastane fibres are crosslinked at wide
intervals. They consist of so-called soft and hard segments. The latter are
the fixed points in the network, they determine elasticity (recovering force),
setting and heat behavior. The hard segments are highly crystalline and
contain urethane and, in some cases, additional urea groups, both groups
form strong intermolecular hydrogen bonds (crosslinking via secondary
valency). The soft segments are the flexible structures with little order,
located between the fixed points, and they are responsible for the extensibility. With molecular weights in the range from 1000 to 3000 they are
relatively large123 and consist mainly of aliphatic polyethers (such as polytetrahydrofuran) or aliphatic polyesters. The former are more stable to hydrolysis whereas the latter are more resistant to oxidation (for example, chlorine
or photooxidation). The hard segments contain mainly aromatic structures
which, as with the aramids, increase cohesion between the chains because
of their mutual attraction. On the other hand, because they absorb UV light
strongly, they contribute greatly to damage by light. This review is intended
to show the main structural weaknesses in elastane fibers. They are the fiber
class with the greatest variation of types. This can lead to mistaken identity
and thus cause faults if the specific properties of each type are not given
sufficient attention during processing.
15.12.1 Mechanical damage
Elastic multifilament yarns are less damaged during sewing than monofils
because usually only a few of the primary filaments are damaged by the
needle and the remainder is sufficient to hold the yarn together. Although
the extensibilty at break of elastane fibers is 400 to 800% mechanical
damage can occur with overstretching. This does not necessarily mean that
a tear occurs, large residual extension (set) and lower recovery forces can
be reasons for complaint caused by overstretching. Drastic damage includes
thread breaks due to too high mechanical stress during knitting or weaving.
A tear in the elastane core of a covered elastic yarn is referred to as a core
break. It occurs particularly often with knitted fabrics; with woven fabrics
loss of elasticity is the greater problem in practice.124 This type of damage
is easy to recognize visually. Mechanical overstraining and fatigue (ageing)
can be analyzed by mechanical testing methods, such as stress/strain behavior and hysteresis curves.123 From their experience with many complaints
concerning elastane Gähr and Lehr recommend that conclusions about the
cause of damage should not be made from the hysteresis behavior of an
elastic thread isolated from the fabric.124 They justify this with the extension
314
Identification of textile fibers
limit of 300% in the test and with the fact that stresses which occur before
the actual damage takes place can have a large effect on the hysteresis
behavior.
15.12.2 Thermal damage
Depending on the type of elastane fiber noticeable thermal damage can
occur when the fibers are heated above about 170 °C. Mainly yellowing
accompanied by loss of strength and elasticity occurs. Under the microscope
deformed elastomer fibers can then frequently be seen and others which
have taken on the contours of the covering component or adhere to it.124
The temperature range for softening of elastane fibers lies between 170 and
230 °C, depending on the type, and for melting and commencement of degradation between 230 and 290 °C.123 Water lowers the bonding in the hard
segments (secondary valence bonding by means of hydrogen bonds); hydrothermal treatments therefore cause more damage than dry heat at the same
temperature. Additional tension also increases the thermal damage considerably (thermomechanical damage). Especially if undamaged material is
available for comparison, damage by heat can be investigated by means of
the stress/strain behavior mentioned above or with thermomechanical
methods of analysis:
•
An elastane yarn set under excessive extension has a stress/strain curve
lying to the ‘left’ of the curve for undamaged yarn, it has lower tenacity
and much lower extensibility.124
• By determination of the hot breaking time, this is the time taken for an
elastomer thread stretched by 100% to break at 193.5 °C.
• By determination of the heat distortion temperature (HDT); this is
the temperature at which an elastomer thread under a pretension of
0.2 mN/tex and heated at a rate of 20 K/minute reaches an extension of
0.25%.123
The HDT differs greatly depending on the type of elastane fiber and often
lies between 170 and 190 °C, with melt-spun fibers it can be lower. In order
to avoid damage the setting temperature should not noticeably exceed the
HDT of the respective elastane fiber type.124,125
15.12.3 Damage by light (photolysis)
In spite of the incorporation of stabilizers, elastane fibers are frequently
damaged by light, especially when it has a high UV component (for example,
when wearing sport or bathing textiles outdoors). Photolysis causes discoloration as well as loss of strength and elasticity or even fiber breakage. The
UV stabilizers can be partially washed out during dyeing and finishing.123
Identifying and analyzing textile damage in the textile industry
315
Photolysis can be accelerated by oils and skin creams as well as sebaceous
oils. The latter effect was demonstrated by Küster and Herlinger with model
substances (squalene and linoleic acid methyl ester) on the basis of the
decrease in relative viscosity of the damaged elastane fibers dissolved in
dimethylacetamide. Perspiration, on the other hand, is said not to accelerate
photolysis.126
15.12.4 Chemical damage
Acids: elastane fibers are stable to dilute mineral and organic acids at room
temperature. At higher concentrations and higher temperature damage
occurs including dissolution. The structural relationship to nylon fibers
becomes apparent here, as it also does in respect to dyeing behavior.
Alkalis: at room temperature, elastane fibers are astonishingly resistant
to alkalis. According to Hueber127 it is possible to causticize and mercerize
cotton/elastane blends at low temperature. At the boil damage occurs with
more than 2 g/l of soda.
Reducing and oxidizing agents: under the usual conditions of dyeing and
finishing elastane fibers are relatively resistant. The limits for peroxide
bleaching in blends with cotton are said by Naroska128 to lie at pH 11 and
100 °C. The resistance to chlorinated water in swimming pools is said to be
good,123 on the other hand cases of damage have been reported here.
Chlorine bleach causes heavy damage as does ozone.
Exhaust gases, especially nitrous gases (NOx) cause damage by yellowing
and loss of strength. Yellowing can also occur when nitrous gases from
stenters directly heated with gas react with spinning oils, fiber lubricants
and knitting oils used with elastane fibers, if these have not been completely
removed by scouring.129
Oils and fats such as mineral oil, paraffin, wax, unsaturated fatty acids
(spinning oils125), cosmetic oils and sun protection agents are absorbed by
elastane fibers and can lead to loss of strength and elasticity due to loosening of the fiber structure. Some of these products also accelerate photolysis.126 If dry-cleaning is carried out carefully with the usual solvents such as
perchloroethylene or benzene, no damage occurs to elastane, except when
stabilizers to light are extracted.123 Highly polar organic solvents such as
dimethylformamide, dimethylacetamide, cyclohexanone, butyrolactone and
phenols damage elastane fibers due to swelling or even dissolution.
15.12.5 Further types of damage, sources of faults and
their detection
Many of the types of damage already mentioned are worsened in combination. Known examples are photooxidation and thermomechanical damage.
316
Identification of textile fibers
Table 15.10 Test methods for damage to elastane fibers123–125,130
Method
Comments
Stress/strain behavior
Also as a simple manual
method for determining
elongation at break
Hysteresis or tensile-elastic
behavior
For all types of damage, especially for analyzing
mechanical and thermal damage. Flat form of
the curve shows loss of elasticity, decrease in
tensile strength and elongation at break.
Loss of elasticity and residual extension (set)
with almost all types of damage. During textile
production modified to such an extent that this
method is not recommended for analyzing
damage in dyeing and finishing or in use.124,125
Testing of the temperature dependence of the
extension and shrinkage behavior at low
(TMA) or oscillating (DMA) yarn tension: glass
temperature, elasticity and other parameters
for exact differentiation of elastomeric fibers.
The HDT is the temperature at which a pretensioned elastane thread at a defined rate of
heating reaches a certain extension (usually
around 200°C). Suitable for determining heat
setting conditions and for testing thermal
damage.
The time taken for an elastane thread stretched
by 100% to break at 193.5°C (usually >20
seconds), especially suitable for analysis of
thermomechanical damage.
Quantitative analysis of chain degradation and
degree of damage possible. Used for analysis
of photochemical damage,126 also suitable for
analyzing other degradation reactions, for
example with acids, alkalis, chlorine, exhaust
gases or heat.
Thermomechanical analysis
(TMA) and dynamic
mechanical analysis
(DMA)
Heat distortion temperature
(HDT)
Hot breaking time
Relative viscosity and
average degree of
polymerization
Resistance to ageing also results from a mixture of many types of influence,
for example mechanical, thermal and chemical influences such as atmospheric oxidation and the detergents and cleaning agents commonly used
in laundering. As already described for wool, elastane fibers also appear to
suffer from cumulative damage. For example, wet processing treatments
during dyeing and finishing which normally have tolerable effects lead to
noticeable fiber damage after intensive presetting.125 Dyeing of polyester/
elastane blends, often also with a wool component, is relatively problematical. Elastane fibers are damaged above 115 °C (as is also wool) causing
softening of the elastane and loss of elasticity.124 The alternative, namely
to dye with carriers at lower temperatures, is also difficult because many
carriers can cause swelling of elastane fibers and reduce thereby their elasticity. During heat setting of piece goods the elastane fiber component is
irreversibly damaged by too high temperatures and tension together with
Identifying and analyzing textile damage in the textile industry
317
excessive duration (stretching without recovery).124 Recommendations for
finishing treatments for elastic textiles have been published, for example
by Naroska128 and Hueber.127
A frequent cause of faults are silicone stains on textiles containing elastane. During primary spinning elastane fibers require 2–6% of spinning
oils,123 this being 6 to 8 times more than on other yarns. These oils contain
a large percentage of silicone oil, which in a normal pre-scour (without
special detergents) is only partially removed. This is particularly a problem
with cotton/elastane blends because cotton also retains a large amount of
silicone. The silicone residues assist the thermomigration of dyes, which can
lead to poor crocking fastness and the dyes can deposit as stains on the
fabric. Silicone stains are often first noticed after coloration and are difficult
to remove. Their detection is described in Section 15.3.6.
Table 15.10 gives a review of the methods commonly used for analysis
of damage to elastane fibers. They usually require a relatively large effort
or high costs. There is a disproportional relation between the many possibilities for damage to elastane fibers and the small number of simple detection methods known and suitable for this purpose.
15.13 Analysis of damage to polyolefin fibers,
especially polypropylene
Of the two polyolefin fibers polypropylene (PP) and polyethylene (PE)
polypropylene has by far the greater importance. PP is the second most
important man-made fiber type after polyester and continues to grow at a
remarkable rate.
PP fibers are relatively cheap. According to Schmenk et al.131 they are
very resistant to acids, alkalis and organic solvents at room temperature.
Damage can be caused by oxidizing substances, such as chlorine bleach and
concentrated nitric acid at higher temperatures, as well as hydrocarbons
and chlorinated hydrocarbons above 100 °C (swelling and dissolution).
Their abrasion resistance is high. Tensile strength and extensibility can be
varied within a wide range during fiber production. A great disadvantage
of polyolefin fibers, which is particularly noticeable with PE fibers, is the
large amount of deformation under stress, so-called creep. This is also the
reason for their low degree of elastic recovery after compression, an important property with carpets. Further weaknesses are their low resistance to
heat and light, especially UV light, which can lead to loss of strength. Heat
stabilizers can increase the temperature for long-period thermal resistance
from the usual 80 °C up to as high as 125 °C. Stabilizers to light and UV
enable PP textiles to be used outdoors. Unmodified PP fibers cannot be
dyed with the usual dyeing methods. Dope dyeing with pigments gives high
fastness but is only economical for large quantities. For analysis of damage
318
Identification of textile fibers
it is important to know that PP fibers exist in many further modifications,
such as:
•
•
•
those based on Ziegler-Natta catalysts (ZN-PP) or on metallocene catalysts (mPP), the latter having a more uniform chain length and being
more highly isotactic with a melting point about 15 °C lower;
as microfibres and hollow fibers;
antimicrobial, flame-resistant or antistatic modifications.
Recently elastic polyolefin fibers have also been introduced (generic name:
lastol), they are crosslinked and stable at temperatures up to 220 °C and
above.132 High-tenacity polyethylene fibers, with ultra high molecular weight
(UHMW-PE, Dyneema) have been available for some time; they are produced by a gel-spinning process at high dilution.
This review serves to illustrate that PP fibers are fairly commonly damaged
by heat and light, by mechanical overstraining and long periods of strain
and by oxidation, including photooxidation. PE fibers are even more sensitive to heat than PP fibers, but are less damaged in principle by light and
oxidation. This last point is difficult to generalize because their behavior is
strongly dependent on the type and amount of added stabilizers to light
and oxidation.
15.13.1 Mechanical damage to polyolefin fibers
Polyolefin fibers have relatively good abrasion resistance. Their tensile
strength and extensibility can be varied over a wide range by means of the
chain length and draw ratio. An extreme example is high-tenacity UHMWpolyethylene Dyneema, which is used, amongst other things, for bulletproof vests. Mechanical damage to polyolefine fibers can occur at many
stages during processing and also in use. Sewing damage to PP knitted
goods has been investigated by Wang et al.133 Mechanical damage during
needling of PP nonwovens has been described by Qian and Chu, who investigated the dynamic creep behavior and ageing of PP geotextiles as a function of the type of bonding of the web.134 Residual extension and deformation
after longer periods of strain are also typical types of mechanical damage
to polyolefin fibers. Mechanical damage can be detected by the usual physical testing methods and under the microscope, preferably in the form of
surface film imprints.
15.13.2 Thermal and thermomechanical damage to
polyolefin fibers
The low temperatures for the softening ranges of PP (150 to 155 °C) and
especially for PE (105 to 120 °C) are the reason for many cases of damage
Identifying and analyzing textile damage in the textile industry
319
caused by excessive heat, often combined with mechanical stress, as can
occur in heat setting, sewing or pressing. According to Chidambaram et al.135
the loss of strength in PP fibers during thermal bonding of nonwovens is
due more to the temperature than to the mechanical strain. With the aid of
the methods for thermal analysis (DSC, TGA, TMA) polyolefin fibers and
their modifications can be easily identified. The melting range enables, for
example, differentiation between LD-PE and HD-PE as well as ZN-PP and
mPP. By comparing the measured value for the latent heat of melting with
the theoretical value the purity of raw and recycled material can be determined. The degree of crystallinity can also be evaluated in this way. The
stability to oxidation of PE fibers can be determined with isothermal DSC
at 200 °C by measuring the time (oxidation induction time OIT) until commencement of oxidation (onset of the exothermal reaction). Buchanan and
Hardegree investigated the influence of spinning conditions on the shrinkage behavior of PP fibers by means of TMA.98 With PE a thermal memory
has also been noted, an effect which is particularly interesting for the analysis of damage. Thermal treatments cause a so-called melting gap, usually
just before the DSC melting curve reaches its maximum. This effect has
been explained by the fact that during thermal treatments the amorphous
areas of the fiber form crystallites with a sufficiently high melting point.
After complete melting of the fiber this thermal prehistory is erased, so that
a comparison of the curves for the first and second run of the DSC can
improve the validity of the interpretation.
Thermal degradation of PP fibers can be analyzed quantitatively by viscosimetric determination of the chain length in decalin at 135 °C. More
common is the determination of the melt flow index MFI, which can be
carried out in an automated form.131 The fact that marked damage is possible by thermolysis is demonstrated by the decrease of the chain length of
PP granulate or PP chips to about half their initial value during melt spinning of PP fibres.136 The molecular weight of PP fibers is given as 150 000–
600 000, but usually 200 000–300 000;131 this corresponds to an average
degree of polymerization of 3600–14 300, or usually 4700–8300.
15.13.3 Damage by light and oxidation to polyolefin
fibers, including photooxidation
The relatively high sensitivity of PP fibers to light and oxidation arises,
amongst other reasons, from the fact that radical intermediate products are
energetically favored. The methyl groups on the tertiary carbon atom are
weak electron donors. They thus stabilize free electrons on the tertiary C
atoms. Cleavage by radicals of the C-H bonds of the tertiary C atoms is thus
favored. The necessary activation energy can be supplied by heat (thermolysis), light (photolysis) or by reaction with free radicals. In the presence
320
Identification of textile fibers
of oxygen peroxide radicals are formed at the tertiary C atoms in the chain.
These react with other tertiary C-H groups, forming hydroperoxides and
new PP radicals. The hydroperoxides decompose with chain cleavage,
whereby carbonyl and alkene structures are formed.137 Heat stabilizers are
radical catchers, for example sterically hindered phenols or phenol-free
compounds. Phosphites, for example, which reduce the peroxides, are used
as antioxidants. Hindered amine stabilizers, HALS, can be used as UV
stabilizers.131
The above-mentioned types of damage are manifested by chain degradation and yellowing, accompanied by brittleness and loss of strength. They
can be detected and analyzed by the appropriate methods described in the
preceding sections. However, loss of fiber strength often does not correlate
with viscosity or chain length. Martin explains this by the supposition that
with damage by light the amorphous areas are preferentially degraded.138
Pezelj et al.139 have investigated damage to PP fibers by ozone and light,
whereby low concentrations of ozone sufficed to cause brittleness, loss of
strength and increase in hydroperoxide content.
Unfortunately there are not enough simple methods for the detection of
damage to polyolefin fibers. On the other hand, there are also not so many
cases of complaint, based on damage to PP and PE fibers, which are difficult
to analyze, although these fibers are increasingly present in all three
segments of the textile market.
15.14 Special types of textile damage and
their analysis
The selection of special types of damage causes described here is restricted
to the investigation of deposits on fibers, especially stains, the detection of
the causes of streaks and barriness, and to biological damage.
15.14.1 Analysis of unwanted deposits on textiles,
especially stains
Since these deposits are usually not distributed evenly on the fibers and
textile fabrics they often consist of more or less large stains, spots or streaks.
They are one of the most frequent causes of damage. Deposits of lime or
PET oligomers show up as greyness on white fabric or light-colored structures on dyed fabric. The identification of PET oligomers is described in
Section 15.4. Lime is soluble in acid and can be washed off with, for example,
acetic acid or sequestering agents. Calcium ions can be detected by precipitating them as oxalate.56 Mahall has described a simple microscopic detection method for lime.1
Identifying and analyzing textile damage in the textile industry
321
Detection of oil, grease, paraffin and wax deposits
These hydrophobic substances can often be marked and detected by staining with oil dyes.1 Nowadays are available Sudan Red 7B and C.I. Solvent
Red 27 as Oil Red O, both from Aldrich and Fluka. Oily deposits can generally be distinctly seen on account of this coloration.
Even more sensitive than these staining tests for detecting grease and
oil contamination is an imprint on thermoplastic films (see Section 15.3).
During production of the imprint hydrophobic deposits diffuse into the
film and can usually be easily recognized by the local cloudiness they thus
cause. The natural waxes of cotton do not interfere here because they are
evenly distributed. Spots caused by pigments or disperse dyes are also
transferred onto the film imprints and are then easier to investigate
microscopically.1
In addition, grease, oil, waxes and paraffins can be detected by IR spectroscopy, either by a direct comparison of spectra from the stain and from
unstained areas, with the possibility of subsequent subtraction of spectra,
or by spectroscopy of the extraction residue (after extraction and concentration). In the latter case it is also recommended to compare extracts from
a stained area and from a similarly sized area without stains. Long alkyl
chains are characteristic for these compounds, which can thus be identified,
for example, by the intensive C-H bands at about nearly 3000 cm−1. Apart
from these stretching bands intensive deformation bands at about 1500 cm−1
and a weaker band at 720 cm−1 are also found, the last one being characteristic for a chain structure with more than three methylene groups. The
extracted fats and oils can also be analyzed more exactly by thin-layer
chromatography. An indication for oil deposits is given by fluorescence in
UV light,140 on the fabric (if it has not been optically brightened) as well as
in the extraction residue and on the TLC plate.
Detection of unwanted film-like deposits
This kind of deposit interferes by causing, for example, a harsh handle, dye
reservation or other optical effects, and also chalky streaks when the fabric
is scratched. Typical causes are size residues, printing paste thickeners which
have not been washed off or unevenly distributed finishing agents. They can
usually be identified with the aid of film imprints, since they often show up
in the form of flat cakes or crumbly deposits.1 Film-like deposits usually
cause a somewhat blurry appearance of the surface imprint, for example
blurred scale structures in wool. Size residues can be detected by color
reactions on the fabric or in an extract (combined with precipitation reactions).141 An advantage of the imprint method here is that the textile fabric
can be investigated without being separated into individual fibers, which
322
Identification of textile fibers
would destroy film-like deposits. However, deposits can sometimes be
better analyzed by means of staining tests and the preparation of crosssections of yarn or fabric. Mahall1 investigated fiber adhesion and size
residue (including distribution of size and over-sizing) in this way.
Detection of other deposits in the form of stains
The reasons for the occurrence of stains and their chemical compositions
are numerous. This complicates their analysis. A typical method of approach
is explained in Section 15.3.2. Löffel28 has described the comparative selective extraction of stains with subsequent identification, preferentially with
TLC and IRS. In Section 15.3.6 information is given on the identification
of silicone stains and fluorocarbon deposits using IRS. The detection of silicones with fluorescence microscopy is described in the literature.142 Schindler
et al.4 have published a comprehensive review of the relevant literature and
of types and causes of stains formed during production, dyeing and finishing
of textiles (see Table 15.11). In this review the fiber-dependent limits of
detection by IRS of stains caused by mineral oil and paraffin, sizes based
on polyacrylate, fabric softeners and polyester carriers are described. Stains
which arise during textile usage are often easier to analyze because the
circumstances of their occurrence is mostly known or is fairly easy to determine.144 Illing-Günther and Hanus have described a stain analysis with
microspectrophotometry.25
As well as the most common form of stain, namely that caused by deposits of foreign substances, there are two further kinds. One occurs due to
localized effects of chemicals which modify the fibers in such a way that
they reflect light or take up dye differently, for example splashes of caustic
soda on cellulose. In addition, residues of the chemical which caused the
stain can sometimes be detected directly on the fabric or in an extract. With
the third type of stain as a result of mechanical influences the local reflection of light is modified in such a way that a manifestation of damage in
the form of a stain occurs. This can best be detected under the microscope,
for example by starting with a stereomicroscope and different types of
illumination.
Identification of the substances which caused the stain is usually the
prerequisite to determining who is responsible, who carries the blame and
how best to repair the damage. Optimal removal of the staining substance
requires knowledge of the type of fiber involved in order to avoid further
damage to the textile during stain removal. As a rule small individual
stains are removed by stain removing agents. With larger or more frequent
stains dry cleaning or scouring, depending on the type of stain, is carried
out with possible addition of surfactants, sequestering agents, enzymes, acids
or bases.
Identifying and analyzing textile damage in the textile industry
323
Table 15.11 Types of stains and the processing stage where they occur4
Occurrence
Type of stain and cause
Production of yarns
and fabrics
Oil and paraffin stains (usually wet paraffinizing), often
together with abraded metal, which darkens the stain
Small spots due to fly and clumps of foreign fibers
Residues of size (usually widely distributed on the warp,
blurry warp streaks)
Residues of sizing auxiliaries such as paraffins, oils,
waxes, fats, softeners and smoothing agents
Preserving agents (often inhibit enzymes)
Silicone stains from antifoaming agents
Acid and alkali stains
Antifoam stains based on mineral oils or silicones,
sometimes also containing silicic acid
Stains caused by PET carriers and oligomers
Precipitation of auxiliaries with opposite charges
Lime and phosphate deposits
Spots due to undissolved or precipitated dye
Stain-like lighter dyeing due to air bubbles in wound
packages
Stains due to drips of water or chemicals (change in dye
affinity)
Silicone stains (often darker, from softening, stretch or
hydrophobic finishes and antifoaming agents)
Softeners, hydrophobic agents, flame retardants and
other finishing agents which have precipitated due to
faulty treatment conditions, usually colorless and
uncommon
In addition to soiling, stains due to yellowing, caused by
antioxidants in plastic films and cartons together with
nitrous oxides in the air (combustion engines) and
cationic substances143
Pretreatment
Dyeing
Finishing
Storage and
transport
15.14.2 Detection of the causes of streaks and barriness
in woven and knitted fabrics
Streaks and bars are second only to stains as one of the most common
manifestations of damage. They occur in numerous forms,145 for example:
•
•
•
•
parallel or oblique to the warp or weft direction
with a repeat pattern or irregularly
in bands or bars
running along short or long sections of thread or across differing numbers
of wales or courses.
The cause of the fault can usually be clarified here with the aid of a
microscope and film imprint. The causes are as numerous as the forms the
324
Identification of textile fibers
faults take. This is illustrated by the 24 relevant examples in Mahall’s book1
and the 10 examples in Goebel’s publication on the formation of streaks.145
As a rule streaks and bars are caused by faults in textile production.
Examples for this are:
•
•
•
•
mistaken material, usually use of the wrong yarn
differences in yarn count, yarn bulk, yarn twist, thread tension, plying,
pile opening, hairiness, inhomogeneous blends
faults during texturizing or mercerizing
with pile fabrics more deeply incorporated tuft rows or differences in
needling.
Faults arising from dyeing and finishing are also known:
•
•
•
wet abrasion and other types of mechanical damage in jet dyeing
machines
plaiting-down faults in cotton pretreatment: squashed fibers, notches,
cracks and splits in the fibers which occur when the goods, swollen with
alkali, are packed down too densely
greasy deposits and resinated mineral oil, which have a carrier effect on
polyester, leading to deeper dyeing.
15.14.3 Detection of biological damage
As well as damage to wool by the larvae of clothes moths and carpet beetles,
microbiological damage to fibers is of interest here. This damage is usually
caused by fungi and, less commonly, by bacteria. Bacterial damage to wool
is known to occur, it causes fiber degradation and an unpleasant odor.
Bacteria often live in symbiosis with fungi on fibers. Both types of microorganism can feed on natural fibers and many types of textile auxiliary
based on natural substances, for example sizes, spinning oils, fabric softeners, starching agents and stiffening agents, printing paste thickeners and
other types of digestible agents. Synthetic fibers are not completely resistant
to microorganisms, for example elastane fibers and polyurethane coatings
can be damaged by them. Humidity, warmth and time favor microbial
damage. It leads to loss of strength and occasionally to mildew stains,
unpleasant handle, odors and loss of color. Microbial damage frequently
occurs after lengthy transport of goods packed when damp or containing
size, or when damp fabric is stored overnight or over a warm weekend in
a textile dyeing and finishing mill. Antimicrobial treatments can prevent
such damage but it still occurs repeatedly in practice.
Musty-smelling mildew stains are often a first indication of fungal attack.
They occur particularly frequently on cellulosic textiles and their color
varies depending on the type of fungus from black to olive green, reddish
Identifying and analyzing textile damage in the textile industry
325
brown to orange and yellowish brown. According to Nopitsch146,147 actual
detection of the fungus is best made under the microscope by staining with
Lactophenol Blue reagent. The Cotton Blue dye which gave this reaction
its name is no longer available. Mahall1 recommends the still available
substitute Telon Blue AGLF (DyStar) and in addition he mentions a 0.5%
solution of Methylene Blue as a possible alternative. In this book1 there are
many well-illustrated practical examples of fungal damage and also three
examples of bacterial attack on wool.
Bacterial damage to wool is also favored by warmth, humidity and time.
A neutral to weakly basic environment supports bacterial growth, low pH
values inhibit it. Level souring-off of the fabric is the simplest method of
protection against bacteria when wool has to be stored moist for longer
periods of time. With wool not only the fungi and bacteria cultures are
stained with the Lactophenol Blue reagent but also the damaged areas of
the wool are clearly and specifically more deeply stained. This is helpful in
damage analysis when the actual microorganisms have been washed out
during scouring. In bacterial damage, longitudinal striations first appear on
the wool fiber and the spindle-shaped cells of the orthocortex are then laid
bare. This results in a characteristic appearance for bacterial damage.1 Only
after further, more extensive damage are the spindle cells of the paracortex
laid bare, since these contain a greater concentration of stabilizing disulphide links. Since wool is also fibrillated by acid damage, it is recommended
to differentiate from bacterial damage by carrying out the KMV reaction
with ammoniacal potassium hydroxide (see Section 15.4). Macroscopically,
stains caused by bacterial attack appear lighter because the fibrillation leads
to a greater scattering of incident light.
15.15 Sources of further information and advice
A very useful book, in which many typical cases of damage are described
and illustrated, has been published by Mahall.1 In this book he has summarized his years of experience in damage analysis and his many publications in such a way that readers receive valuable stimulation for their own
work. Further books which may be of assistance in damage analysis include
those from Hearle et al.,10 Agster,56 Stratmann,7 Greaves and Saville,11 the
Textile Institute12 and Fan.27 Chapter 8 of Fan’s book gives much more
detailed information on the subject of this chapter here, including experimental details.
Most of the articles on damage analysis published in journals are not
recent.13–15,88,148–154 This is also true of company brochures on this topic.16,17,155
In the selection mentioned here, it is the last article which is cited if it is
part of a series of articles. Citations for previous articles in the series can
be obtained there.
326
Identification of textile fibers
TESS, an expert system for textile damage analysis
Another particularity of textile damage analysis is the expert system TESS.
This abbreviation stands for ‘Textiles Experten-System für Schadensfälle’
(Textile Expert System for Cases of Damage). It will be briefly described
here because this gives an idea of the complexity and problems of damage
analysis on textiles. TESS was developed from about 1993 by the
Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA) in St
Gallen, Switzerland, in close cooperation with a dozen project partners and
has been in industrial use since 1998.156–159 TESS is a Windows-based diagnostic system for all stages of textile production including dyeing and finishing. The knowledge gained from numerous experts was continuously
structured and implemented in a knowledge base. This consists of a network
of about 2000 nodes. The initial nodes of the network are five simplified
manifestations of damage (stains, streaks, holes, surface differences and
differences in handle). These are further subdivided according to size, direction, frequency, color and position of the fault. For an exact determination
of the cause of the damage further investigations are requested, for example
observation of the fault in reflected and transmitted light and possibly UV
light, surface film imprint, determination of whether the fault runs parallel
to the threads, determination of count and fastness or extraction. In the
form of a dialogue TESS suggests stepwise further tests, delineates the area
of possible causes and, if successful, names the cause of the fault and ways
to repair it and avoid it in future. Further advantages of TESS are that it
supports and relieves experts during damage analysis, and that it is especially useful in training new staff. It is always available, it considers very
many possibilities and notes the steps taken (transparent logic). It serves
to preserve the specialized knowledge of experts who retire and sometimes
enables shorter diagnosis times and earlier recognition of the cause of
faults. Disadvantages of TESS are that until now it has mainly been successful with faults arising from textile production and it appears to be
limited in its suitability for the numerous types of damage connected with
textile dyeing and finishing. In spite of the large amount of work invested
in its development much more experience has to be included. Since this
continuing effort appeared to be too time-consuming and expensive, further
work on this difficult project ceased in 2002. In the long run TESS was not
able to cope with the enormous variety of cases of damage in textile dyeing
and finishing, the complexity of the manifestations of damage and, in particular, their causes. On the other hand, this emphasizes the importance and
illustrates the performance of experienced damage analysts.
15.16 Conclusions
The previous section again shows the great variety and complexity of
damage analysis on textiles in general and textile-chemical damage analysis
Identifying and analyzing textile damage in the textile industry
327
in particular. These challenges correspond to the demands made on damage
analysts in terms of broadly based, thorough knowledge, great experience
and the right combination of logical and intuitive approaches depending on
the problem at hand. In addition, these experts require many kinds of
information, not only concerning the particular case of damage but also on
approaches and solutions to similar cases. Private collections of cases of
damage, study of the literature and exchange of ideas with colleagues are
always helpful. Useful ideas (sometimes generated just by aside comments),
that help in interpreting one’s own work can arise while reading the many
published cases of damage in practice. Well-known experts have described
in these publications their often very individual approaches based on their
particular experience in analyzing textile damage. Occasionally they are
honest enough to confess that in spite of much effort a case could not be
solved under the given circumstances.
15.17 Acknowledgment
The author wishes to thank his colleague, Professor Elizabeth Finnimore,
for her careful translation.
15.18 References
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Identification of textile fibers
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35 Bey K, ‘Die dünnschichtchromatographische Analyse auf dem Gebiet der
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42 Wiesener
E,
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Untersuchungen
an
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45 Jeziorny A, ‘Mechanism of the appearance of the mobile peak on the thermograms of polyethyleneterephthalate fibres’, Acta Polymerica, 1986, 37, 237–240.
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50 for example DIN 54278, Prüfung von Textilien – Auflagerungen und Begleitstoffe
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66 Evidence gathered at the Textile Chemical Laboratory of the University of
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67 Sommer H, Monatsschrift für Textilindustrie, 1927, p. 158.
68 Available for example from Merck, Darmstadt, Germany.
69 Producer FESAGO, Chemische Fabrik Dr. Gossler GmbH, D-69207
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79 Schmidt G, personal communication and Textilien unter dem Mikroskop,
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81 Doehner H and Reumuth H, Wollkunde, Berlin, Paul-Parey-Verlag, 1964.
82 Herzog A, ‘Nachweis von Wollschädigungen mit Indigokarmin’, Melliand
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83 Still available, for example Merck, Darmstadt, Germany, Art.Nr. 105233.
84 Krais P, Markert H and Viertel O, Forschungshefte der Deutschen
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85 Zahn H, ‘Schädigung beim Karbonisieren und Nachweismethoden (I)’, Textil
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86 IWTO is the International Wool Textile Organisation, IWTO Regulations available from The Woolmark Company, West Yorkshire, England.
87 Zahn H, Wulfhorst B and Külter H, Faserstoff-Tabellen nach P-A Koch: Wolle
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88 Schefer W, ‘Schädigungen von Wolle durch Veredlungsoperationen’,
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89 Schäfer K, ‘Determination of the amino acid tryptophan in protein fibres’,
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90 Schäfer K, Föhles J and Höcker H, ‘Lichtschädigung bei Wolle und anderen
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91 Rockstroh E et al., Prüfen von Textilien. Band II: Mikrountersuchungen, 2. Aufl.,
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92 Schefer W, ‘Schädigung von Wolle durch Veredlungsoperationen, insbesondere
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93 Sanger F, Biochem. Journal, 1949, 45, 563 (quoted from Chwala A, Anger V,
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94 Mahall K and Goebel I, ‘Fortschritte auf dem Gebiet des Entbastens der Seide’,
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95 Knott J, Freddi G and Belly M, ‘Analytische Untersuchungen zum Entbasten
von Seide’, Melliand Textilberichte, 1983, 64, 481–483.
96 Gavet L, Ambroise G and Giorgio A, ‘Organic weighting of silk by grafting’,
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332
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97 Schindler W and Wiesel U, ‘Nachweis der Methacrylamid-Erschwerung von
Seide’, Melliand Textilberichte, 1999, 80, 546–548.
98 Buchanan D R and Hardegree G L, ‘Thermal stress analysis of textile yarns’,
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99 Hearle J W S, ‘Fibre fracture and textile durability – part 2’, Textiles Magazine,
1999, 28, 15–21.
100 Bigler N, ‘Ein Beitrag zur Erkennung von Fehlern in Flachzwirngarnen’,
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101 Nanal S Y, ‘Problems occuring in manufacture of polyester staple fibre’, Journal
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102 Topf W, ‘Fehler bei der Verarbeitung von Chemiefasern’, Textil Praxis
International, 1983, 38, 641–644.
103 Zimmermann R, DyStar, personal communication, 2003.
104 Schaich B, ‘Bestimmung der Fixierunterschiede von Polyestergewebe, vorzugsweise durch Polarisationsmikroskopie’, Undergraduate Thesis, University of
Applied Sciences at Münchberg, 1993.
105 Nettelnstroht K, ‘Schadensuntersuchung im Textilbetrieb mit dem
Polarisationsmikrokop’, Melliand Textilberichte, 1983, 64, 918–921.
106 Berndt H-J and Heidemann G, ‘Fehlerursachen und Erkennungsmethoden von
Farbstreifigkeit in Polyester-Material’, Deutscher Färber-Kalender, 1972, 76,
408–479.
107 Wiley R E, ‘Setting up a density gradient laboratory’, Plastics Technology,
March 1962, and Österreichische Chemiker-Zeitung, 1965, 66, 65–72.
108 Weber R,‘Lichtmikroskopische und elektronenmikroskopische Untersuchungen
an lichtgeschädigten Polyesterfasern’, Textilveredlung, 1970, 5, 703–708.
109 Valk G, Kehren M-L and Daamen I, ‘Über die Photooxidation von Polyäthyle
nglykolterephthalat-Fasern’, Angewandte Makromolekulare Chemie, 1970, 13,
97–107.
110 Day M and Wiles D M, ‘Photochemical Degradation of Poly(ethylene terephthalate). III. Determination of Decomposition Products and Reaction
Mechanism’, Journal of Applied Polymer Science, 1972, 16, 203–215.
111 Nettelnstroth K, ‘Mikroskopische Fehlererkennung an Textilien’, Textil Praxis
International, 1983, 38, 572–574, 668–671.
112 Anonymous (probably Höhn W), Oligomere – Phänomen, Analyse und Minimierung von PES-Olgomeren, Firmenschrift Dr. Th. Böhme, Geretsried, 1997.
113 Bubser W and Modlich H, ‘Erkennung und Unterscheidung von Schäden an
Polyamidfasern (Perlon und Nylon)’, Textil Praxis International, 1959, 14, 1041–
1043 and 1152–1158.
114 Goebel I, Cognis/Henkel, personal communication, 1994.
115 Kratzsch H and Hendrix H, ‘Zur Unterscheidung von Licht- und Säureschäden
bei Polyamidfasern’, Melliand Textilberichte, 1964, 45, 1129–1133.
116 Reinert G, ‘Zur Photostabilität der Polyamidfaser’, Melliand Textilberichte,
1988, 69, 58–64.
117 Bever M, Breiner U, Conzelmann G and von Bernstorff B-S, ‘Protection of
polyamide against light’, Chemical Fibers International, 2000, 50, 176–178.
118 Küppers H., ‘Textiluntersuchung – nicht aufwendig’, Textilveredlung, 1974, 9,
67–71.
119 Gries T, Rixe C, Steffens M and Cremer C, ‘Faserstofftabellen nach P-A Koch:
Polyacrylfasern’, Melliand Textilberichte, 2002, 83, 795–816.
Identifying and analyzing textile damage in the textile industry
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120 Sarhadjieva V, Nankova Z and Dimov K, ‘Influence de l’étirage et du traitement
thermique sur les propriétés des fibres acryliques’, Ann. Sci. Text. Belges, 1975,
164–183.
121 Gacén J and Arias J M, ‘Die kritische Auflösezeit für Acrylfasergarne – The
critical solution time of acrylic fibre yarns’, Melliand Textilberichte, 1980, 61,
533–536.
122 Rossbach V and Karunaratna N, ‘Beständigkeitsprüfung technischer Textilien
mit der Methode der kritischen Lösezeit’, Melliand Textilberichte, 1985, 66,
223–226.
123 Fabricius M, Gries T and Wulfhorst B, ‘Faserstofftabellen nach Koch P-A:
Elastanfasern’, 2nd edn, Melliand Textilberichte, 1995, 76, 980–990.
124 Gähr F and Lehr T, ‘Bedeutung thermischer und hydrothermischer Prozesse
als Ursache von Elastanschädigungen’, Melliand Textilberichte, 2001, 82,
722–725.
125 Gähr F and Lehr T, ‘Ursachen von Elastanschädigungen bei der Veredlung von
Maschenwaren’, Maschen-Insustrie, 2002, 52, 38–41.
126 Küster B and Herlinger H, ‘Untersuchungen zum photochemischen Abbau von
Elastomerfasern’, Textil Praxis International, 1981, 36, 15–21.
127 Hueber H, ‘Praktische Aspekte bei der Veredlung von elastischen Textilien’,
Melliand Textilberichte, 1998, 79, 243–246.
128 Naroska D, ‘Chancen und Risiken beim Veredeln elastischer Textilien’, Melliand
Textilberichte, 1999, 80, 611–615.
129 Freiberg H, ‘Indirekte Gasheizung für Spannrahmen und Hotflues’, Melliand
Textilbderichte, 2002, 83, 330–334.
130 Falkai B v, Synthesefasern, Weinheim, Verlag Chemie, 1981.
131 Schmenk B, Miez-Meyer R, Steffens M, Wulfhorst B and Gleixner G, ‘Fiber
tables according to Koch P-A: Polypropylene fiber table’, 2nd issue, Chemical
Fibers International, 2000, 50, 233–253.
132 Anonymous, ‘Dow XLA-Faser jetzt Lastol’, Melliand Textilberichte, 2003, 84,
242 and the quotations given therein: Chemical Fibers International, 2002, 52,
373 and Melliand International, 2003, 9, 4.
133 Wang Y, Feng X and Sivakumar M, ‘Improving sewability of PP knitted fabric’,
The Indian Textile Journal, 1998, 109, 86–88.
134 Qian C, Chu C, ‘Fatigue properties of the two composite geotextiles’, Journal
of Dong Hua University, English edition, 2002, 19, 53–55.
135 Chidambaram A, Davis H and Batra S K, ‘Strength loss in thermally bonded
polypropylene fibers’, Joint INDA-TAPPI Conference, INTC 2000, International
Nonwovens Technical Conference, Book of Papers, Dallas USA, 2000,
19.0–19.23.
136 Wulfhorst B and Meier K, ‘Faserstofftabellen nach Koch, P-A: Polypropylenfasern’, 1st edn, Chemiefasern/Textilindustrie, 1989, 39/91, 1083–1090.
137 Ripke C, ‘Polypropylen-Fasern und ihre Pigmentierung’, Chemiefasern/
Textilindustrie, 1980, 30/82, 30–35 and 110–114.
138 Martin E, ‘Lichtbeständigkeit der Faserstoffe’, Textilveredlung, 1983, 18, 222–
225.
139 Pezelj E, Cunko R and Andrassy M, ‘The influence of repeated maintenance
treatments on chemical and thermal properties of polypropylene fibers’, 78th
World Conference of the Textile Institute, 5th Textile Symposium of SEVE and
SEPVE, volume III, 395–400, Thessaloniki, Greece, 1997.
334
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140 Hesse R and Pfeifer H, ‘Fluoreszenzmikroskopie zur Erkennung von
Fehlerursachen an Textilien’, Textilveredlung, 1974, 9, 82–87.
141 Dugal S and Schollmeyer E, ‘Systematisierung der Analytik von Schlichtemitteln
auf Geweben aus Cellulosefasern und deren Mischungen mit Synthesefasern’,
Textil Praxis International, 1984, 39, 252–254.
142 Schindler W and Drescher P, ‘Fluoreszenzmarkierung von applizierten Siliconen
zur Kontrolle ihrer Verteilung’, Melliand Textilberichte, 1999, 80, 67–68.
143 Hemmpel W-H, ‘Zum Problem der Lagervergilbung unverpackter, freihängender Textilien’, Textil Praxis International, 1983, 38, 261–264 and 354–355.
144 Diener H, Fleckentfernung – aber richtig, 10. Aufl., Leipzig, Fachbuchverlag,
1980.
145 Goebel I, ‘Streifenbildung – Identifizierung häufig auftretender Fehler in
Geweben und Wirkwaren’, Textilveredlung, 1993, 28, 389–394.
146 Nopitsch M, ‘Beitrag zum Nachweis von Schimmel auf Baumwolle und von
Wollschädigungen im allgemeinen’, Melliand Textilberichte, 1933, 14, 139–142.
147 Nopitsch M, Textile Untersuchungen, Stuttgart, Kohlhammer, 1951.
148 Hemmpel W-H, ‘Gewährleistungshaftung – Produkthaftung – Umwelthaftung
– drei wichtige Gründe für die Qualitätssicherung’, Textil Praxis International,
1991, 46, 130–132, 134–137.
149 Hemmpel W-H, ‘Reklamation des Endverbrauchers an Färber und Appreteur
– verdeckte Mängel im Textilgut’, Textil-Betrieb, 1983, 101, 22–26.
150 Lancendorfer T, ‘Möglichkeiten der Mikroskopie bei der Qualitätsbeurteilung
und Fehlererkennung an Fasern und Flächengebilden’, Melliand Textilberichte,
1990, 71, 493–497.
151 Kunze W, ‘Reklamationsfälle aus der Veredlungsindustrie und ihre Ursachen’,
Textil Praxis International, 1974, 29, 315–324.
152 Pehl F, ‘Fehler in Textilien und die Ermittlung ihrer Ursachen – Schadensfälle
aus der Textilindustrie’, Taschenbuch für die Textilindustrie, 1984, 415–429.
153 Various authors: Vom Textillabor zur Textilpraxis, Schweizerische Vereinigung
von Färbereifachleuten, SVF, Basel.
154 Anonymous, SVF-Lehrgang für den Textilveredler, S 1 – S 107, Basel,
Schweizerische Vereinigung von Färbereifachleuten, 1967–1969.
155 Anonymous, Schadenfälle in der Textilindustrie, Ciba AG, Basel.
156 Gerbig M, ‘EMPA News – TESS – Textiles Experten-System für Schadenfälle’,
Textilveredlung, 1993, 28, 121.
157 Gerbig M and Hufenus R, ‘TESS – Textiles Experten-System für Schadenfälle’,
Textilveredlung, 1993, 28, 365–369.
158 Hufenus R and Gehring S, ‘TESS – Wie man aus Schaden klug wird’,
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50–55.
159 Gehring S and Hufenus R, ‘Expertensystem für Schadenfälle’, Melliand
Textilberichte, 1998, 79, 77–79.
160 Gosh S and Rodgers J E; ‚Schnelle Identifizierung von hitzefixierten
Teppichgarnen durch Reflexionsanalyse im nahen Infrarotgebiet’, Melliand
Textilberichte, 1988, 69, 361–363.
16
The role of fibre identification in
textile conservation
P GARSIDE, University of Southampton, UK
Abstract: Accurate identification of fibres is vital to textile conservators.
Such knowledge, in conjunction with an understanding of the properties
and usage of textile materials, will inform conservation, display and
storage strategies. It may further help to annotate biographical detail
concerning the origins of the textile and related history. Microscopy has
traditionally afforded the principal means of identifying historic fibres,
alongside simple chemical and physical tests. However, these latter
techniques are destructive and can require relatively large samples,
considerations which are particularly problematic when dealing with
fragile and valuable cultural artefacts. Consequently, they are gradually
being superseded by instrumental analytical methods which are noninvasive or require just microsamples, such as spectroscopy and
chromatography, and advanced microscopic techniques often combined
with sophisticated computational analysis. Potentially these approaches
can reveal even more about the constituents and may offer clues as to
their state of preservation. While the intricate construction and multimedia nature of artefacts often make these investigations particularly
challenging, and aesthetic and ethical considerations add another
complicating dimension, nonetheless successful material characterisation
is essential to the well being and continued enjoyment of our textile
heritage.
Key words: fibre identification, textile conservation, fibre microscopy,
fibre spectroscopy.
16.1
Introduction
The ability to accurately identify fibres is vital to textile conservators, and will
influence the way in which an artefact is both understood and treated.
Knowing the composition of an object will inform conservation strategies,
enabling aspects of particular concern to be highlighted, and to determine not
only those treatments that are suitable for the item but also – and perhaps
more importantly – allowing those that are particularly inappropriate to be
avoided. This knowledge will also inform handling, display and storage decisions. Furthermore, the identity of the fibres and other components in an
artefact will help to confirm its provenance, and will bring to light areas of
alteration or earlier conservation work; similarly, the information can be used
to distinguish original pieces from more modern reproductions.
335
336
Identification of textile fibers
Traditionally microscopy, alongside chemical and physical tests (such as
stains specific to certain fibre types, or burn and solubility tests), have been
used in textile conservation, as these require little equipment or training
and are generally inexpensive, although they do have many limitations. The
value of microscopy means that it is unlikely to be displaced, but the other
techniques are now being superseded by analytical methods such as spectroscopy, chromatography and X-ray analysis, which provide more detailed
information with less intervention. An important factor that will influence
the choice and scope of investigative techniques is the availability of
resources. Although modern analytical instruments are available to the
larger museums and institutions, many conservators will not have access
to equipment more sophisticated than a simple microscope, which will
necessarily limit the range and detail of information that can be determined.
The analyses are further constrained by the considerations of cost, time
and training.
In order to use any of these analytical methods effectively, and to be able
to accurately interpret the results, it is necessary to have more than an
understanding of the techniques themselves – an appreciation of the history
of textiles, of the changing uses of materials over time, and of their chemistry and composition is also important. This knowledge will necessarily influence the choice of conservation treatments – of the best methods of cleaning,
washing, reshaping, consolidating and supporting as object – and will help
to ensure that none of these processes cause further damage.
In this regard it is also important to understand the difference between
conservation and restoration – although the terms are open to interpretation, in general conservation refers to the process of limiting the effects of
past damage and ensuring that future deterioration is prevented as far as
possible, with the minimum of intervention, whereas restoration involves
returning an object to its original (and often functional) state. Expressed in
these terms, the two processes can be considered as the extremes of a continuum, and both are influenced by the requirements to display objects,
to stabilise them for long-term storage, and the availability of suitable
techniques, materials and equipment.
In textile conservation, fibre identification may be complicated by ethical
considerations that are not necessarily encountered in other fields: is it possible to take samples from an object without damaging it, either physically
or aesthetically, or jeopardising its long term stability? Is the information
obtained from the analysis likely to be of sufficient value to justify the interference? Is it acceptable to take samples at all, even if doing so will not cause
damage? These concerns have led to an increasing interest in the potential
for non-invasive, non-sampling methods of fibre characterisation.
This chapter will describe those techniques which are of particular use to
textile conservators, and demonstrate, with reference to specific case studies,
The role of fibre identification in textile conservation
337
the ways in which this knowledge can then be used to inform conservation
strategies.
16.2
Analytical techniques
16.2.1 Sampling
As noted above, the ethics of sampling are a significant concern in textile
conservation. Ideally, no samples would be taken at all, and all analyses
would be carried out in situ, using non-invasive techniques. Obviously this
is impractical in many cases, of course; in these situations, a compromise
must be sought, balancing the benefits that can be derived from analytical
sampling and the extent to which this will inform conservation strategies,
with the necessity of intervening to the minimal possible extent.
Where samples must be taken, they should preferably be removed as
fibres or fragments of yarn which are already detached from the bulk object
due to pre-existing damage. If this is not practical then it may be possible
to sample from seams, hems or internal areas which will not affect the aesthetic properties of the item. Sampling from obvious, undamaged regions
should always be avoided.
16.2.2 Microscopy
Microscopy is usually the initial method of characterising the fibres and
other components that comprise an artefact.1–4 An initial, relatively low
magnification survey of an object can readily be carried out with stereomicroscopy, which can highlight areas of particular concern and interest, which
will also give an indication of the general composition and weave structure,
and does not require samples to be taken (Fig. 16.1). A more detailed examination of particular components can then be performed with techniques
such as transmission and reflectance microscopy, if it is ethically acceptable
for samples to be taken. Light microscopy allows the identification of fibres
based on their distinctive morphologies (Fig. 16.2)1–13 and is of particular
value in identifying natural fibres as many of these materials have well
defined, characteristic structures, such as the surface scales of wool, the convolutions of cotton and the nodes of flax and hemp. A detailed knowledge
of the structures of these fibres allows more subtle differentiations to be
made based on associated structures (so called ‘guide elements’) which may
be visible – the species, and sometimes the breed of the animal from which
wool or hair has been taken can be determined; superficially similar bast
fibres can be distinguished; wild and domestic silks can be separated.
A comparison of these general features can readily be made, noting
characteristics such as the fibre shape and structure, physical dimensions
338
Identification of textile fibers
16.1 The use of a stereomicroscope on an articulated arm to carry out
an initial examination of an object.
Plant
Silk
Wool
10 μm
Synthetic
16.2 Typical plant (jute), wool, silk and synthetic fibres, as observed
via transmitted light microscopy.
The role of fibre identification in textile conservation
339
and indicators of overall condition, including the presence of dyes, dirt or
degradation. Longitudinal samples are easily prepared by laying fibres flat
on a slide. Transverse specimens typically require the sample to be set in
resin, or other similar suitable medium, before sectioning to reveal the
desired aspect. The interpretation of these examinations relies either on the
experience of the user, or access to a library of suitable reference materials
or micrographs.
Various techniques can be used to gain more information. By ensuring
that the mounting medium is of the same refractive index as the exterior
of the fibre, thus rendering it more-or-less transparent, it may also be possible to examine internal structures in greater detail.1 Ashing can be used
to look for characteristic mineral residues, crystals and other inorganic
particles, often found in natural fibres, and is achieved by removing the
organic components through pyrolysis in a furnace.4 Maceration may also
be used in the preparation of plant fibres, employing a solution of acetic
acid and hydrogen peroxide to break down intercellular matrix that holds
fibre bundles together to yield the individual cells.4,9 Surface structures may
be revealed more clearly by the use of fibre casts, forming an impression of
the fibre surface using wax or a suitable thermoplastic polymer;1,11 this is
particularly useful in the study of animal fibres (wool and hair) in which
dyeing has obscured the pattern of surface scales, but is also applicable to
any fibre with distinct surface features.
Light microscopy may be refined by using polarised light;1,4,9,14–17 at its
simplest this can be used as a contrast technique, highlighting the various
structures of a fibre (Fig. 16.3), but it can also reveal considerably more
about the crystallinity and composition of the sample. Fibres tend to be
10 μm
Transmitted light
10 μm
Polarised light
16.3 Flax fibre viewed via conventional and polarised transmitted light
microscopy.
340
Identification of textile fibers
optically anisotropic, due to the strong alignment of the component polymers with the fibre axis, and so possess two principal refractive indices, one
parallel to the fibre axis (n||) and the other perpendicular to it (n⊥), which
are characteristic of the fibre type. These indices may be determined in a
variety of ways, perhaps most simply by employing a range of mounting
media of known refractive index, illuminating the sample either parallel or
perpendicular to the axis with polarised light, and determining the medium
in which the fibre becomes invisible; this can be facilitated by the Becke
line test, which indicates whether a given medium has a refractive index
higher or lower than that of the sample.1
Another refinement, fluorescence microscopy, is of particular use in the
identification of chemical modifications to the fibre, including dyes, brightening agents and the like, and of the effects of ageing;1,3,9,14,18 fibre blends
are particularly easy to identify by this technique. The addition of fluorescent dyes (fluorochromes) to the sample can also aid the investigation of
fibres, particularly when specific moieties or structures within the fibre are
of interest.
Problems with characterisation by microscopy can arise from several
sources, particularly variations in the production, treatment and processing
of the fibres, including factors such as spinning, weaving, bleaching and
dyeing, alongside the degradation and other damage that accumulates over
time. All of these factors can reduce or alter the potential information which
can be derived from the fibre, often to the point where the sample can only
be identified in general terms, such as ‘bast’ (thus including flax, hemp,
ramie, and others). Synthetic fibres are often problematic to distinguish via
conventional microscopy, as many morphological features such as crosssectional shape, diameter, etc., are indicative but not uniquely characteristic;
however, polarised light microscopy generally offers a reliable method of
identifying these materials, due to their particular optical properties.19–24
16.2.3 Electron microscopy and X-ray microanalysis
Scanning electron microscopy (SEM) allows the fibre surface to be studied
in great detail (Fig. 16.4), and is often combined with energy dispersive Xray spectroscopy (EDS or EDX), which can give an indication of elemental
composition and distribution.1,3,6,7,16,25–29 Electron microscopy is broadly
analogous to light microscopy, with an electron beam replacing the light
source and electromagnets taking the role of the optical elements; information is derived from the electrons or radiation emitted from the sample as
it interacts with the beam, and captured by appropriate detectors. In terms
of micrography, the two most useful techniques are secondary electron and
backscattered electron imaging. The former relies on low energy secondary
electron, generated through the influence of the primary electron beam on
The role of fibre identification in textile conservation
341
1 μm
16.4 Electron micrograph of a flax fibre, exhibiting defibrillation and
surface damage.
the sample, and emitted from the surface layer; it provides a high resolution
(∼5 nm) method of observing the surface topology of the specimen. The
latter technique utilises high energy primary electrons which are scattered
through atomic interactions. As these electrons can escape from greater
depths within the sample, the resultant images are of lower resolution
(∼100 nm) and contain less information about surface morphology;
however, as more massive elements are more efficient scatterers, regions of
higher average atomic mass appear brighter in the resultant images, and the
technique can also indicate the boundaries of crystalline domains.
The incident electrons may also result in the production of X-rays, which
are of energies characteristic of the element from which they are generated
– EDS exploits these emissions to provide an elemental analysis of the
specimen.1,16,25,30,31 However, the detection of low mass elements (such as
those that comprise the majority of organic fibres: C, N, O) is poor; therefore
this technique is principally of use in assessing those components in which
heavier elements are present (for example: in dyes, mordants and pigments;
in the salts of tin, iron, lead and other metals used for silk weighting; in
metal threads; in the metals, glasses and ceramics of buttons and other fittings; in soiling, dirt, rust stains and the like). The data produced by this
technique is analogous to that derived from X-ray fluorescence (XRF)
experiments, and can be used to characterise the same kinds of materials.
In terms of conservation, SEM is particularly useful for looking for
subtle signs of damage in textiles, such as the initial stages of defibrillation
342
Identification of textile fibers
Undamaged gilt surface
5 μm
‘Blistered’ corrosion surface
200 μm
5 μm
16.5 A metal thread, examined by light and electron microscopy.
of plant fibres, or the microfractures and plasticiser migration observed
in synthetic materials; the morphology of fracture surfaces can also prove
informative.27,28 In combination with EDS, the technique can provide invaluable information in the investigation of metal threads (an important component of many high quality historical textiles), as it can be used to
investigate not only those morphological features that indicate construction, use and deterioration, but also to investigate the composition of
the metal components and the nature and extent of any corrosion products
(Fig. 16.5).3,32–39
The technique does possess a number of disadvantages, however.1,16 The
first of these is that SEM will only provide surface information about a
sample – internal structures, such as the hollow lumen of plant fibres, cannot
be seen. A greater problem arises from the conditions associated with the
equipment: the experiment is carried out under vacuum, which can result
in dehydration, dimensional changes and damage in the sample; damage
can also be caused by charging effects and heating by the electron beam.
This latter problem can be avoided by coating the specimen with a suitable
conductive material, usually carbon or gold, but this means that the sample
cannot be recovered in its original state, which may have ethical considerations in its own right and will also limit the range of subsequent experiments that can be carried out on the specimen, an important concern when
it is necessary to take the smallest possible sample and derived the maximum
information from it.
The role of fibre identification in textile conservation
343
Some of these problems can be avoided by using low vacuum or environmental scanning electron microscopy (ESEM);40,41 the lower vacuum
does not lead to the same degree of desiccation, and cations, generated by
collisions between the electron beam and gas in the chamber, serve to dissipate charge build-up. The main drawback of the technique is that resolution is generally lower than in a conventional SEM system, though recent
developments are improving the performance of these machines.
16.2.4 Chemical and physical tests
These tests have a long tradition in textile conservation as they require little
in the way of equipment and are generally easy to carry out.3,8,11,13,42 They
rely on the characteristic chemical and physical properties of fibres in
order to identify them, or to highlight damage or the effects of processing.
However, they have a number of significant disadvantages, as a result of
which they are gradually being replaced by other, more modern techniques:
in general, these tests are destructive or will at least significantly modify
the sample, a problem which is exacerbated by the requirement for relatively large specimens. Furthermore many of the tests are specific to certain
fibre types, so different protocols are required for materials of synthetic,
plant and animal origins; as a result, the less is known about the sample
initially, the greater the number and range of tests required, which can
become extremely time consuming and, of course, significantly increases the
amount of material required. All of these tests are complicated by the presence of fibre blends, and are unlikely to be optimised to deal with uncommon fibre types; treatments (including dyes, fire retardants and brightening
agents) or other additional components will also influence the results of
these experiments, as will degradation.
A particularly wide range of chemical tests exist for the identification of
plant fibres.4,9,43 The phloroglucinol test selectively stains lignin, and can thus
be used to distinguish fibres on the basis of their characteristic lignin content.
Zinc chloroiodide (the Herzberg reagent) has the effects both of highlighting structures within fibre cells and differentially staining cellulose, lignin
and lignified cellulose. Cuprammonium hydroxide (Schweitzer’s reagent)
causes the swelling and dissolution of cellulose; as the content and distribution of cellulose varies between species, the pattern and rate of this process
is characteristic. Damage to these fibres can also be assessed with staining
tests, highlighting features that may otherwise not be apparent:11,13 Fehling’s
solution precipitates red copper(I) oxide in regions of acid damage. The
Clibbens and Geake test causes damaged regions to darken. The Turnbull
Blue test stains regions of acidic or oxidative damage dark blue. The Congo
Red test dyes the interior of the fibre more strongly than the exterior, thus
revealing areas where the cell wall has swollen or split.
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Identification of textile fibers
Some commercial analytical stains are also available, such as the
Shirlastain range, which rely on dye mixtures designed to stain different
fibre types specific colours;11,13 generally each stain is designed for a certain
category of fibre – for example, distinguishing between individual cellulosic
fibres, or determining the general origin of a sample (synthetic, animal or
plant). Problems can arise from the qualitative interpretation of the resultant colour, and this colour will of course also be influenced by dyes and
surface treatment as well as discolouration due to age or damage.
Solubility tests can be used to systemically categorise a fibre sample
based on its chemical nature, using readily available reagents,3,8,11 but require
large specimen sizes and will not distinguish chemically similar materials
such as plant fibres (primarily composed of cellulose) or wools and hairs
(composed of keratin proteins).
The physical properties of a fibre can also be investigated: burn tests rely
on the differing ways in which fibres undergo pyrolysis when subjected to
a flame.3,5,11,13 Various factors are taken into consideration, including whether
or not the specimen melts, whether it continues burning when removed
from the flame, the resultant smell, the final appearance of the material, etc.
Buoyancy or specific gravity tests assess the characteristic density of a material.10,13 The twist test, particular to plant fibres, exploits the helical winding
of the cellulose in the cell wall, which causes the fibres to characteristically
twist clockwise or anticlockwise when allowed to dry after wetting;5,8–10,13
the information provided by the test is limited, but can differentiate between
fibres that are morphologically difficult to distinguish, such as flax and
hemp.
16.2.5 Spectroscopy
A more specific approach to fibre identification can be taken using spectroscopic methods, particularly vibrational spectroscopy. These techniques rely
on the stimulation and observation of bond vibrations at characteristic frequencies, and thus identifying specific chemical moieties in the sample; it
can be used not only to distinguish fibres, but also to assess their state of
degradation and to confirm processing methods, dye treatments, silk weighting and the like.44–54 Identification is generally achieved either by correlating
the bands arising from specific constituents with the known composition of
materials, or, more commonly, by comparing the spectrum of an unknown
material with a suitable spectral library, to identify the fibre by its characteristic ‘signature’ (Fig. 16.6). Spectroscopic identification is of particular
use in the case of those specimens that have undergone surface degradation
or wear to the point where examination by microscopy provides little or
no useful information (a situation often encountered with archaeological
samples).44,51,55–59
The role of fibre identification in textile conservation
345
Cotton
Intensity
Silk
Polyester
4000
3500
3000
2500
2000
1500
1000
Wavenumber/cm–1
16.6 Characteristic FT-IR spectra of three common textile fibres.
The most widely available spectroscopic technique, Fourier-transform
infrared (FT-IR) spectroscopy, can be used both to characterise fibres and
to investigate their condition. Materials with broadly different chemistries,
such as synthetic fibres, wools, silks and plant fibres, can be readily distinguished; the subtle chemical differences between the more closely related
fibres can then be exploited to differentiate these materials – for example,
it has been shown that it is possible to identify plant fibres on the basis of
their lignin content,43 exploiting the same properties that are used in the
phloroglucinol test; similarly, the sub-classes of, for example, nylon (Nylon
6, Nylon 66, Nylon 12 and so on) can be distinguished. Band assignments
may be established for particular fibre polymers, such as cellulose60–62 and
silk,63–65 and degradation can be assessed, either through the loss of identifiable chemical components and the accumulation of distinctive degradation
products (such as carbonyl containing species produced by oxidative processes), or by changes in microstructure deduced from spectroscopically
derived crystallinity indices.29,48,65–67 The technique is also of use in the identification of dyestuffs and pigments, as well as other treatments such as
optical brightening agents or flame retardants.44–46
Traditionally, FT-IR spectroscopy required a relatively large sample that
would be ground to a fine powder before being made up as either a KBr
disc or a mull in an appropriate mineral oil; these processes require large
sample sizes and, obviously, are destructive, so are not particularly suited
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Identification of textile fibers
to the requirements of textile conservation. Attenuated total reflectance
(ATR) spectroscopy provides a more appropriate sampling method, requiring only a small fibre sample (often a millimetre or so of a single yarn);68–70
spectra are derived from the radiation that interacts with the specimen at
the interface with a crystal window (often diamond or germanium), and the
nature of this interaction means that the data is obtained from the outer
layer of the material (the sampling depth is of the order of the wavelength
of the radiation, so usually a few microns). Another suitable method is
provided by microspectroscopy,1,16,52,71–73 in which a spectrometer is attached
to a set of specially designed microscope optics, which allows spectra to be
recorded from samples as small as a few tens of microns in diameter. This
offers many advantages, particularly where specimens are limited in size, or
are found with other materials (such as contaminants, other fibres in the
blend, surface films, etc.) as spectra can be recorded from discrete components of the sample. Problems may be encountered due to the generally
low power of these systems resulting in low signal to noise ratios and, particularly in the case of single fibres, the roughly cylindrical nature of the
specimen can cause lensing effects which will also diminish the quality of
the data.
As an alternative to the mid-infrared region exploited by conventional FT-IR spectroscopy, near-infrared spectroscopy may be employed
instead,74,75 and has been shown to be useable with proteinaceous,76–82 cellulosic54,83 and synthetic84–89 fibres, as well as identifying blends90–92 and processing treatments.93 This technique has a variety of advantages, perhaps the
foremost of which, for the purposes of textile conservation, is its ability to
be readily used with fibre optic probes (opaque to mid-infrared radiation);
this enables spectra to be readily recorded in situ, with the minimum of
disturbance to the object in question (Fig. 16.7). In addition spectral accumulation is generally rapid, and under the right conditions spectra can be
recorded through glass or an air-gap (a benefit when dealing with framed
objects). The main drawback with the method lies in the nature of the
spectra themselves – the bands observed in the data are combinations and
overtones of those seen in mid-infrared spectra, and are generally associated with vibrations involving hydrogen; this complicates spectra to the
point that band assignments are often impossible and at best ambiguous.
Dark or reflective surfaces can also prove problematic. Therefore the technique is best used either by comparison of spectra to a library of suitable
references, or in conjunction with chemometric analyses that allow subtleties in the data to be drawn out through statistical approaches.74,94–97
Another vibrational technique, Raman spectroscopy, has also proven to
be of value in conservation, as it is typically non-destructive, requires little
sample preparation, is of use for both organic and inorganic materials and
has a good spatial resolution.98–101 It has been employed to differentiate
The role of fibre identification in textile conservation
347
16.7 The use of an optical probe with a near-infrared spectrometer to
carry out a non-invasive analysis.
untreated plant fibres (ramie, jute, flax, cotton, kapok, sisal and coir) on the
basis of peak ratios derived from the associated C-H stretches and glycosidic C-O-C stretches,55 and has shown that the degradation of these materials may be monitored.102 Similar studies have also investigated silk fibres
from various sources and at various stages of processing,64,103–105 including
spider silk,106 and wool, subjected to treatments such as bleaching and to
physical changes induced by stretching.56,107 However, the Raman technique
is not routine for many conservation science laboratories and, in any case,
luminescence can prove problematic, particularly with historic materials.
The relatively high power of the lasers used in Raman spectrometers also
potentially leads to the risk of localised burning to the sample.
It is also possible to gain an understanding of the microstructure and
crystallinity of fibres through spectroscopic techniques; the simplest
approach involves defining crystallinity indices based on peak ratios, a
technique that has successfully been used with materials such as cellulosic
fibres62,67,108,109 and silk.110,111 A more sophisticated method of investigating
these properties may be achieved by introducing an infrared polariser into
the system; bonds are only observed when they are aligned with the electric
vector of the polarised radiation, and the intensity of bands in the resultant
spectra reflect the long range ordering, if any, of the various components of
the fibre. This has been successfully used to examine the helical winding of
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Identification of textile fibers
cellulose in the cell walls of plant fibres,112,113 distinguishing otherwise similar
fibres on the basis of the unique angle and sense of wind, and to assess the
links between crystallite orientation and physical integrity in silk fibres.114
16.2.6 Dye analysis
Dye analysis can provide a valuable means of assessing the provenance of a
fibre. Dyes may be broadly categorised as either natural or synthetic, the
latter being first developed in the mid 19th century. Natural dyes were often
highly specific to particular locations, or available to a wider market only
after trade routes were established, which may give a useful indication of the
history or a textile. Synthetic dyes were frequently only used for a handful
of years before being superseded by dyestuffs that were either technically
more advanced (for example, easier to apply, possessed of better light or
wash fastness, or less damaging to the fabric) or more suited to the demands
of changing fashions; as these substances are often recorded in detailed
patent applications, it may be possible to pin-point their dates of likely use
with some accuracy. By understanding the nature of a dye it is possible not
only to gain a better appreciation of the origins of a textile, but also how the
material has been processed in the past and how it may respond to conservation treatments. For example, some dyes will change colour or solubility as
pH varies, in which case the acidity of any wash solution must be carefully
controlled to ensure that the dyestuff will not alter or bleed; similarly the
metal mordants may exchange with other soluble ions in a wash solution,
again potentially leading to the loss or alteration of a dye.
Dye analysis has traditionally been carried out by chromatographic techniques, a specialised procedure that requires experience, dedicated equipment and a substantial reference library, but which can provide very accurate
results.115–117 Attempts have also been made to use FT-IR, Raman and UVvisible spectroscopic techniques, with varying degrees of success; Raman
spectroscopy in particular has proven useful to this end.44,118–120
In some situations, a precise identification of a dye is not necessary. For
example, if a support fabric is to be dyed to match an object, to ensure that
it is as unobtrusive as possible, the colour of the textile is important rather
than the precise formulation of the dye; in these cases, a simpler and more
easily determined colour-space measurement may be appropriate, using a
colorimeter.
16.2.7 Additional components
‘Textile’ artefacts, such as clothing, upholstery, banners and embroideries,
are also likely to contain many other non-textile components, including:
buttons, zips and other fastening; decorations, such as beads and sequins;
The role of fibre identification in textile conservation
349
paints, lacquers and varnishes; and support materials in the form of foam,
stuffing or frames. It is often as important to identify these materials as it
is the fibres of the fabric, both to achieve a greater understanding of the
history of an object, and to inform conservation treatments. Many of the
techniques already discussed are also appropriate to these additional components – FT-IR spectroscopy, for example, is ideally suited to the examination of organic materials, and EDS or XRF can provide valuable information
about the composition of inorganic aspects.
16.2.8 Analysis and bulk properties
Another important consideration is to be able to gain an understanding of
the physical and chemical condition of an artefact, as this will not only
influence the choice of conservation methods to be used, but will also help
to indicate how the object may behave over time, thus suggesting what
handling, display and storage strategies might be the most appropriate. At
its simplest, this may be done by looking for visible signs of deterioration
via microscopy, or for chemical changes associated with degradative processes, such as oxidation or hydrolysis, through spectroscopic methods. A
more systematic approach, and one on which a range of recent research has
concentrated, involves drawing correlations between spectroscopic or
chromatographic signatures, and measurable physical properties, such as
tenacity or elasticity. This not only provides a means of more fully understanding the links between the chemical and microstructural characteristics
of a fibre and the bulk behaviour of a fabric into which it is incorporated,
but also potentially offers a non-invasive or microsampling method of
determining the large-scale condition of an object destined for conservation; additionally it may lead to in situ analyses to determine which objects
in a collection are most at risk and in particular need of urgent intervention,
or if a particular display strategy is, for example, liable to place an object
under undue strain.
To this end, surrogate materials are often employed; these are either
naturally or artificially aged specimens that mimic the properties of the
object itself and can be used in a sacrificial manner to determine how the
item is likely to respond to various conservation strategies or display conditions. In order to choose or prepare these materials, it is necessary to have
a good understanding not only of the composition of the object, but also its
state of deterioration and of the chemistry of the components so that the
mechanisms that have lead to its present state might be appreciated.
16.3
Conservation strategies
The principal aim of conservation is to ameliorate the effects of preexisting damage and prevent future deterioration, with the minimum of
350
Identification of textile fibers
intervention, often with the requirement the object is in a suitable state for
display or further investigation; conservation does not generally involve
replacing missing areas of fabric, renewing faded or fugitive dyes, restoring
lost function or otherwise modifying the item.3,121–123 Above all else, these
treatments are intended to cause no further damage to the object and, as
far as possible, to be reversible, thus ensuring that they will not interfere
with any future interventions; historically not all conservation strategies
have abided by these principles. The techniques involved in conservation
include the following.
Cleaning, to remove dirt, soiling, stains, dye bleeding or other material
such as mould spores. It may be desirable to remove other substances as
well, such as adhesives which have spread, leading to disparate parts of an
object being bound together and not only resulting in the loss of original
form but also introducing damaging stresses. Cleaning may simply involve
the removal of surface dirt, through low-powered suction, or for more
robust fabrics, gentle mechanical action with a suitable brush or cloth, or it
may use an aqueous or organic solvent wash; in this latter case it is necessary to have an understanding of the nature of the various components, in
order to choose a wash solution of appropriate solvency and pH that will
remove the unwanted dirt, but that will not react with or cause the dissolution of any aspect of the object, and similarly will not cause bleeding or
colour changes in dyes. As with all aspects of conservation, it is preferable
to err on the side of caution and leave soiling rather than risk damaging
the underlying object by removing it. It should also be borne in mind that
in some cases soiling is intrinsic to the history of the artefact, and it is not
appropriate to remove it.
Reshaping is important as many objects are encountered in a heavily
creased or crumpled state, which not only means that their original form
is lost, but may also place it under undue stress. Humidification is often
involved in this process, as water acts as a plasticiser for most natural and
some synthetic fibres, restoring flexibility to these materials and allowing
them to be reshaped with minimal risk of causing damage. However,
some materials do not respond well to humidification (heavily weighted,
‘shattered’ silk, for example), so correct identification of materials is
important.
Support involves affixing a fragile or fragmentary fabric to a suitable inert
backing or overlying material (often a silk, nylon or polyester net, dyed to
ensure that it is as unobtrusive as possible), using either a suitable, reversible adhesive or careful stitching. This supports the item, thus minimising
further physical damage. Cosmetic in-fills may also be used, to make areas
of damage less obvious or to aid the intelligibility of an incomplete design
or section of text; in this case, the intent is to not to make the object appear
undamaged (and the infill should be recognisable as such on close examina-
The role of fibre identification in textile conservation
351
tion), but to give a general impression of the missing area, to better to
understand the item as a whole.
Pest control is an important consideration in those artefacts that may
harbour mould spores or insect eggs, capable of remaining dormant on a
fabric for extended periods; to some extent cleaning will remove these
contaminants, but other measures might also be necessary. The most common
procedures are freezing and anoxia, and again a knowledge of the composition of the object is important – on freezing some materials may be taken
below their glass transition temperature and thus be at risk of physical
damage on subsequent handling, whilst anoxia can induce reactions, particularly colour changes in certain classes of dye. Pesticides are to be avoided
as they may interact unfavourably with the artefact and are likely to leave
residues.
Mounting is usually the final stage of conservation, and provides the
object with appropriate structure to minimise stresses whilst, usually, allowing it to be presented in a manner suitable for display. The structures should
be purpose built for the artefact in question, and a well designed support
may minimise or avoid the need for interventive conservation. In all cases
it is important to ensure that the materials used are compatible with those
of the object itself and will not interact in a deleterious manner.
16.4
Case studies
16.4.1 George Mallory
In 1924 George Mallory, along with Andrew Irvine, led an expedition to
climb Everest – if they achieved their goal, they would have made the first
successful ascent of the mountain. Mallory and Irvine were last seen on the
afternoon of 8 June, at a height of almost 28 000 feet, disappearing into the
mist. Mallory’s body was found in 1999, and it became evident that he had
died after a severe fall; a DNA sample was recovered, to confirm his identity,
along with samples of clothing and equipment, before the body was buried
on the mountain (Fig. 16.8).
An analysis of the clothing was carried out at the Textile Conservation
Centre, on behalf of the Mountain Heritage Trust, in order to determine
the style and structure of the garments, as well as the nature of the component fibres and the presence, or otherwise, of weatherproofing treatments
or the like. The composition of the jacket, trousers and three different shirts
were of particular interest, and were examined by a combination of light
and electron microscopy and FT-IR spectroscopy to determine the characteristic morphologies and chemistries of the specimens; solvent extractions
on small sections of yarn were also carried to look for evidence of weatherresistant treatments. The jacket and trousers were made of cotton, which
352
Identification of textile fibers
FRONT
TCC 2689.4
16.8 Front panel from Mallory’s jacket.
appeared to be treated with some form of oil-based weatherproofing; each
of the shirts was woven from a different material – wool, domesticated silk
and tussah (wild) silk. Microscopy also allowed the dimensions, ply and spin
of the yarns to be established, along with the weave structures. These results
informed decisions that were made about the most appropriate way to store
the items, and highlighted those aspects that may be in particular need of
future intervention.
Subsequently this information, along with the details of construction of
the garments, also allowed replicas to be made and tested by the University
of Leeds Performance Clothing Research Centre. This revealed that for the
kind of climb Mallory intended – a rapid ascent and descent – the performance of the clothing was comparable to that of modern equipment. This
not only provided valuable information about the history of extreme weather
clothing, but also helped refute the suggestion that Mallory may have died
on Everest because his choice of clothing was not suitable for the task.
The role of fibre identification in textile conservation
353
17 m
head
foot
24 m
16.9 The fore topsail of the HMS Victory.
16.4.2 The Victory sail
The only surviving sail from the HMS Victory, Nelson’s flagship at the
Battle of Trafalgar in 1804, is held by the Royal Navy at the Portsmouth
Historic Dockyard (Fig. 16.9). The Navy wished to display the sail to mark
the bicentenary of the battle, and commissioned the Textile Conservation
Centre both to work on the item and also to suggest the most appropriate
method of display.124–126 This work was supported by the Society for Nautical
Research and the Ministry of Defence.
The sail itself was a substantial object (24 × 16 m in size, and weighing
nearly half a tonne), and had been extensively damaged during the battle,
with numerous holes from cannon and musket fire and a long tear caused
by a falling mast. Analysis of the materials, by light and electron microscopy
and infrared spectroscopy, showed that the sail itself was constructed from
linen, whilst the rope that bound its edge was of hemp; in addition to the
obvious damage it was also apparent that the fabric had suffered from more
subtle chemical deterioration over time along with extensive mould growth
in certain areas.
354
Identification of textile fibers
16.10 Working on the Victory sail.
The conservation itself involved surface cleaning using low-powered
vacuum suction and gentle mechanical action to remove debris and mould
spores (Fig. 16.10). The presence of these spores represented a potential
health hazard to the conservators, so breathing masks were worn. The size
of the object also presented problems, as it was necessary to walk and sit
on the sail, so the weight of the conservators and equipment was distributed
on sheets of rigid PlastozoteTM; to further protect the sail, it was raised on
a well-ventilated platform, to ensure free circulation of air. The sail’s size
also meant that it could not be fully laid out in the space available, so was
partially rolled on a specially constructed inflatable boom; to move the sail
this boom was used in conjunction with a heavy canvas sling. Throughout
the cleaning programme, spot tests were carried out to ensure the effectiveness of the process.
In parallel to this, work was carried out to determine the most appropriate method of displaying the sail; a suggested means of doing this was to
hang the sail from a yard, as it would have originally been employed on the
ship, and it was necessary to determine if this would subject the item to
damaging stresses. Permission was given to remove yarn samples from
regions of pre-existing damage, along with one small piece of loose sailcloth.
By characterising the nature and condition of these specimens, it was possible to prepare artificially aged surrogate materials using a modern linen
sailcloth. The original specimens, along with the surrogates were then subjected to mechanical testing to determine their physical properties; this not
only gave an indication of the condition of the sail itself, but also confirmed
that the surrogates were in a sufficiently similar state to the original materials that they could be used to assess the bulk properties of the object, tests
that could not accurately be carried out on single yarns, nor on the sail itself
The role of fibre identification in textile conservation
355
for fear of causing permanent damage. The samples were also examined by
Raman spectroscopy, which suggested a potential link between certain
spectral changes and the observed deterioration of the material.102 These
analyses revealed that the tenacities of the individual yarns were roughly
uniform across the sail, and that although as a whole it still had enough
residual strength to support its own weight, to allow it to do so would risk
permanent deformation. Therefore the recommendation was made that the
sail was displayed either flat or at a shallow angle, on a suitable solid mount,
to minimise undue and prolonged stresses.
16.4.3 ‘Parachute silk’ slip
A variety of artefacts in the Hampshire County Museums and Archives
Service collection were investigated by staff from the Textile Conservation
Centre.127 The analyses were carried out as part of a pilot study to assess
the value of near infrared spectroscopy as an in situ analytical technique.
Amongst the items studied was a slip dating from the 1940s and believed
to be constructed from parachute silk. During the Second World War and
the years that followed, rationing and the diversion of goods to the war
effort limited the availability of many commonplace and luxury items,
including fabrics. As a result materials were often adapted from other
sources to fill this gap; in particular, silk from discarded or surplus parachutes was often employed to make wedding gowns and under-garments.
The slip in question had been donated to the collection as such. However,
the spectra recorded from the garment, when compared with a comprehensive library of known materials, actually revealed the fabric to be nylon
(and probably Nylon 6) rather than silk.
This investigation highlights the benefits of a rapid analytical technique
that is non-invasive and can be carried out without removing the object
from the collection. The information will allow the garment to be stored
and displayed in a manner best suited to the long-term stability of its component materials, and its history and origins are more fully understood.
16.4.4 Freddie Mercury
A pair of red faux leather trousers were received by the TCC to be made
safe for storage and occasional display;128 these trousers had originally
belonged to Freddie Mercury, the lead singer of Queen, and had been
purchased at auction by the client (Fig. 16.11). NIR spectroscopy showed
that the fabric consisted of a thin layer of polyester polyurethane, bonded
to a brushed cotton base fabric. The trousers were in generally good condition, but the polyurethane layer had started to become cracked and was
peeling from the underlayer; in addition it was sticky, and had begun to
356
Identification of textile fibers
5 mm
16.11 Freddie Mercury’s faux leather trousers.
adhere to itself in areas where the fabric was folded. In addition the metal
fittings – a zip and rivets – were heavily corroded. The deterioration of the
polyurethane is likely to be due to a combination of oxidation and plasticiser migration, whilst the damage to the metal components probably arises
from the action of both volatile acidic degradation products of the polyurethane and sweat. Although some work has been done on consolidating
materials of this sort,129 it was felt that with the current state of knowledge
the best interim measure was to internally support the trousers with custom
made forms to prevent further creasing and delamination, and to lacquer
the rivets to protect them from further corrosion.
16.5
Future trends
It is likely that modern analytical techniques, such as spectroscopy, chromatography and advanced microscopic analysis, will become more widely used
in future, gradually replacing traditional methods, particularly the destructive and often unreliable analyses like stain and burn tests. The trend will be
driven by the greater availability of these analytical approaches, the reduction in cost and complexity, and a wider appreciation of the benefits of these
methods in the conservation community. It is likely that there will be a
The role of fibre identification in textile conservation
357
greater emphasis on microsampling or non-invasive techniques, especially
those which allow in situ examinations to be carried out, thus obviating
the need to remove objects from collections or disturb displays, reducing the
risk of causing damage through handling. Some techniques can potentially
further limit the need for intervention – NIR spectroscopy, for example, can
be used through glass, theoretically allowing items such as framed banners
or textile art to be assessed without interfering with the setting. A further
probable development will be the drive to derive the maximum possible
information from an analysis, not only identifying the component materials,
but also using the investigation to determine the chemical and physical condition of the sample and the manner in which this reflects the properties of
the object as a whole. This will be achieved either through direct correlation
of analytical signatures with measurable physical properties, or through
methods such as chemometrics, which employ multivariate analysis techniques to derive statistical correlations between sets of data; these approaches
are potentially very powerful, and are able to exploit subtleties in data too
small to be readily observed through conventional assessments.
However, there will always be the call for simple, inexpensive methods
that can readily be employed by a conservator working alone and without
the support of a fully equipped laboratory.
16.6
Sources of further information and advice
Chemical Principles of Textile Conservation (A. Tímár-Balászy and D.
Eastop; Butterworth-Heinemann, Oxford, UK, 1998) offers a good general
grounding in the role of chemistry and the use of analytical techniques in
textile conservation, and provides many useful case studies. As an introduction to the study of plant fibres, particularly by microscopic techniques,
Identification of Vegetable Fibres (D.M. Catling and J.E. Grayson; Archetype
Publications, London, UK, 1998) is invaluable, and the Handbook of Fiber
Chemistry (M. Lewin and E.M. Pearce, eds; Marcel Dekker, Inc., New York,
USA, 1998) provides detailed information about the majority of commonly
encountered fibres and textile materials. Conservation Science: Heritage
Materials (E. May and M. Jones, eds; RSC Publications, Cambridge, UK,
2006) is another valuable resource, highlighting the value of scientific and
technical understanding in the wider field of conservation, dealing with a
broad range of different materials. The National Parks Service (USA) is currently constructing an online Microscope Slide Fiber Reference Database.
16.7
Acknowledgements
The author would like to thank his colleagues at the TCC, who carried out
much of the research presented in the case studies and whose support and
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Identification of textile fibers
encouragement has been invaluable, and in particular Nell Hoare (Director
of the TCC) for permission to publish; Peter Goodwin (keeper and curator
of HMS Victory) and the commanding officer of HMS Victory for permission to publish the research on the sail, and the Society for Nautical Research
for supporting the work through the ‘Save the Victory’ fund; Mary Rose
and the Mountain Heritage Trust for permission to publish the results of
the investigations on the Mallory clothing; Hampshire County Museums
and Archives Service for permission to publish the work on the ‘parachute
silk’ slip; J. Mitchell for permission to publish the work on Freddie Mercury’s
trousers; and the AHRC, which supported much of the author’s research
through his position as a Research Fellow at the AHRC Research
Centre for Textile Conservation and Textile Studies; images were reproduced by permission of the Textile Conservation Centre, University of
Southampton.
16.8
References
1 P.H. Greaves & B.P. Saville; ‘Microscopy of Textile Fibres’; Bios; Oxford, UK;
1995.
2 M. Sawbridge & J.E. Ford; ‘Textile Fibres under the Microscope’; Shirley
Institute; UK; 13–20; 1987.
3 A. Tímár-Balászy & D. Eastop; ‘Chemical Principles of Textile Conservation’;
Butterworth-Heinemann; Oxford, UK; 1998.
4 D.M. Catling & J.E. Grayson; ‘Identification of Vegetable Fibres’; Archetype
Publications; London, UK; 1998.
5 M. Goodway; ‘Fibre Identification in Practice’; Journal of the American Institute
for Conservation; 26; 27–44; 1987.
6 J.W.S. Hearle & R.H. Peters (eds); ‘Fibre Structure’; The Textile Institute;
London, UK; 1963.
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68
69
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127 E. Richardson, G. Martin & P. Wyeth; ‘Collecting a Near Infrared Spectral
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128 D. Lovett; ‘Another One Bites the Dust? A Case Study of Freddie Mercury’s
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129 T. Bechthold; ‘Wet look in 1960s furniture design: degradation of polyurethanecoated textile carrier substrates’; in: C. Rogerson & P. Garside (eds); ‘The Future
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Index
abaca 14
absorption spectroscopy 159
acetate 11
acid damage
cotton 293–4
elastane (spandex) 315
nylon 309, 310, 311
wool 297–9
acid dyes 205, 206
acrylic (polyacrylonitrile or PAN) 10, 69,
70, 77, 85
damage analysis 311–12
extraction and classification of dye
from 209
fibre variants 117
addition reactions 262
additional components 348–9
adhesive strip test 283
adulterants 224–5, 231–3
after-market taxonomy 4–5, 272
aggregate theory 141–6
agricultural product chemicals 126–7
aliphatic polyester 85
alkali damage
elastane (spandex) 315
wool 297–9
alpaca 49, 224, 234
identification 51–4, 55, 56, 57
alternative identification techniques 181–7
American Association of Textile Chemists
and Colorists (AATCC) 3, 4, 6, 7,
253, 255, 260
amino acids 31, 32, 33
angora rabbit 49, 54, 58, 224, 225
anidex 12, 121
animal fibres 15, 27–67
characteristics 38–44
DNA analysis see DNA analysis
future trends 61–2, 66, 67
identification 44–61, 62, 63, 64, 65
physical and chemical properties 33–5
SEM 38–9, 47–8, 197–8
366
structure and composition 31–3
types of 35–8
see also animal hairs; silk
animal hairs 15, 27, 28
characteristics 38–44, 45
growth 28–30
identification 44–59, 60, 61, 62, 63
types of 35–8
anoxia 351
appearance 137–9
Applied DNA Sciences Inc. 253
aramid 12, 90–2
arrector pili muscle 29, 30
ascending linear method 214–15
ashing 339
ASTM, International 3–4, 6, 7, 253, 255,
260–1
atomic absorption spectroscopy (AA) 160
atomic emission spectroscopy (AES) 160
attenuated total reflectance (ATR)
spectroscopy 21, 162, 285, 346
automated DNA analysis 235
automated testing of cotton 256
auxochromes 167, 204
average degree of polymerisation 290–1
azlon 11
backscattered electrons 189, 190, 191,
340–1
bacterial damage 324–5
bamboo 127
bars/barriness 278, 323–4
bast fibres 13–14, 243–4
Becke line test 143–4, 266–7, 340
Beilstein test 283
Bell, Joseph 259–60
Berek quartz wedge compensator 145–6
bicomponent fibres 18, 123, 124, 265
biconstituent fibres 18, 265
binder fibres 125
biological damage 278
wool 299–300, 324–5
Index
birefringence 16, 139–46, 184, 247, 267–8,
269, 270–1
bulk properties 349
bundle strength 246, 247
burning tests 3, 6, 181–2, 260, 344
calibration 176–7
camel hair 49, 55–9, 63, 224
camelids 28, 234
capillary electrophoresis (CE) 218–20
carbon adhesive 193
carbon coating 193
carbon fibre 78
carpeting 180
cashgora 224
cashmere 35, 49, 55, 60, 61, 62, 224–5
cathodoluminescence 189
cavitomic cotton 255
cell wall 243, 244
cellulose 95
content in cotton 243–4
cellulosic fibres 10
damage analysis 292–5
see also cotton
chain rigidity 88–90
characteristic X-rays 189, 190, 191
Chemical Abstracts Service (CAS) Registry
number 205
chemical analysis 19–22
conservation 343–4
damage 280
HPFs 97–8
see also under individual techniques
chemical damage 278, 306
elastane (spandex) 315
see also acid damage; alkali damage;
oxidative damage
chemical fibres 72
see also synthetic fibres
chemical fixation 192
China Cotton Colour Characterisation
Chart 248
chitin 127
chlorine 283
chroma 174
chromatogram 217
chromatography 22, 203–23
CE 218–20
damage analysis 284–5
extraction of dyes 207–12, 220
HPLC 22, 213, 215–18, 219–20
TLC 22, 212–15, 216, 217, 219, 284–5
chromophores 167, 204
CIELAB colour model 174
classification of fibres 9–19, 112–23
generic classes 10–13, 112–14, 115–19
manufactured fibres 15–19
natural fibres 13–15
polymer origins and 112–15, 116, 117
367
subclasses 112–14, 119–23
Textile Fiber Products Identification Act
10–13, 112–14
variants 114–15, 116, 117
classification standards for cotton 248
cleaning (in conservation) 350, 354
cleanliness 192
clothing
Freddie Mercury’s trousers 355–6
George Mallory’s 351–2
‘parachute silk’ slip 355
coating, conductive 193–4, 342
cochineal 204
cocoon 38
coir 15
colour
analysis of 22
dyes see chromatography; dyes
human vs machine perception 172–5
microspectrophotometry see
microspectrophotometry
colour atlases 174
Colour Index 204–5
colour matching light boxes 173
comb sorter array 251, 252
commercial hot stage 18–19
comparison 4, 8–9
compensator 145–6
complaints 277
condensation reactions 262
conductive coating 193–4, 342
confocal microscopy 149–50
conjugate (multicomponent) fibres 123–6
conjugated double bonds 167, 204
conservation 335–65
analytical techniques 337–49
additional components 348–9
bulk properties 349
chemical and physical tests 343–4
dye analysis 348
electron microscopy and X-ray
microanalysis 340–3
optical microscopy 337–40
sampling 337
spectroscopy 344–8
case studies 351–6
Freddie Mercury’s trousers 355–6
George Mallory’s clothing 351–2
HMS Victory sail 353–5
‘parachute silk’ slip 355
future trends 356–7
strategies 349–51
construction, fibre 18, 123–4, 265–6
conventional DNA hybridisation analysis
228–9, 230
convolution angle 253–4
convolutions 245, 253
copolymer-type aramid fibre 92
Corterra 123
368
Index
cortex 29, 30, 39
cosmetic in-fills 350–1
cotton 14, 71, 77, 81, 83, 239–58
damage analysis 255, 292–5
extraction and classification of dye from
208, 211
fibre properties 245–56
differences in processing ability 246–50
identification of fibre origin 250–3
identification tests for cotton fibre in
textiles 253–5
quantitative analysis 255–6
future trends 256
non-textile applications 241
structure and composition 241–5
supply chain 240–1
crimp, three-dimensional 124–5
cross-sectional shapes
animal hairs 43–4, 45, 48, 49
alpaca 54, 56, 57
cotton 245
manufactured fibres 16, 263–5
microspectrophotometry 179
cross-sectional specimens 137
crossed polars 140, 141, 269–71
cryofixation 192
crystal structure
HPFs 98–104, 105
synthetic fibres 77–9, 86–7
crystallinity 261–2
indices 347
cultivated silk 30, 38
cumulative damage 303, 316
cuticle 29, 30, 39, 243
extraction of DNA from cuticle cells
226–7
cuticular scale patterns 39–42, 46–7, 49,
138
wool 51, 52
cylindrical symmetry 142–3
cystine 34
damage analysis 19, 275–334
according to type of fibre 292
biological damage 278, 299–300, 324–5
causes of damage 279–80
importance and reasons for 275–8
main types of damage 278
manifestations of damage 278–9
methods 280–91
average degree of polymerisation
290–1
chemical and physical assessment 280
chromatography 284–5
extraction methods 290
IR spectroscopy 285–6
microscopy 283–4
preliminary examination 282–3
procedure 280–2
surface imprint techniques 289–90
thermal analysis 286–9
natural fibres 292–304
cotton 255, 292–5
silk 303–4
wool 295–303, 324–5
streaks and barriness 278, 323–4
synthetics 304–20
acrylic 311–12
elastane (spandex) 312–16, 317
general types of damage 304–6
nylon 309–11
polyester 306–9
polyolefin 316–20
TESS expert system 326
unwanted deposits and stains 278,
279–80, 281–2, 320–3
dark scan 170, 171, 177
De Broglie’s equation 188
dead time 197
decomposition temperature 89, 95–7
decorations 348–9
degree of polymerisation, average 290–1
delustrants 4, 17, 265
denier 9, 69
density, linear 9, 69, 184–5, 246, 247
density gradient column 185
deposits, unwanted 278, 320–3
destructive examination 134
detection 259–60
diameter, fibre 17, 139, 265
animal hairs 48, 49
cotton 247
microspectrophotometry 179–80
differential scanning calorimetry (DSC)
185–6, 287–8
differential thermal analysis (DTA) 185
dispersing prism 166
disulphide bonds 34
DNA amplification technology 229–33
DNA analysis 224–36
conventional DNA hybridisation analysis
228–9, 230
cotton 252–3, 256
effect of fibre processing 229–33
extraction of DNA from animal hairs
226–7
future trends 233–5
selection of target DNA sequences 227–8
dot-blot technique 228–9, 230
double-immersion technique 145
double-sided tape 192–3
Doyle, Arthur Conan 259–60
drawing 81, 82
dry spinning 262
dyes 22, 70–1, 167, 204
analysis and conservation 348
chemical structures 205–6
chromatography see chromatography
Index
classification 204–5
extraction 207–12, 220
forensic analysis 206
uptake variability 178–9, 246
Dyneema 89, 90, 94
see also ultra-high molecular weight
polyethylene (UHMW-PE)
dynamic-mechanical analysis (DMA)
288–9
effective temperature (MEPT) 287–8, 307
Egyptian cotton 240, 251, 252
elastane see spandex
elasterell-p (elastomultiester) 114, 119,
120–2
elastoester 12
electromagnetic spectrum 166
electron microscopy
conservation 340–3
HPFs 107, 108
SEM see scanning electron microscopy
TEM 187
electron spectroscopy 159
electrons 188
interaction with matter 188–9, 190
electropherogram 218
elongation 69–72
sign of 269, 270
eluents for TLC 215, 216
emission spectroscopy 159
energy-dispersive X-ray microanalysis
(EDX) 196–7, 340, 341–2
energy states 166–7
energy transitions 167
environmental SEM (ESEM) 195–6, 343
European Fibres Group (EFG) 21
extinction points 269
extra long staple (ELS) cotton 240, 251
extraction
DNA from animal fibres 226–7
dyes 207–12, 220
methods and damage analysis 290
eye, and colour perception 172–3
fasteners 348–9
Fehling’s solution 293–4
felting 35
fibre casts 339
fibre variants 114–15, 116, 117
fibrils 75–6, 80, 244
fibroin 27, 31, 32, 33, 84
field emission guns 195
filament 9
film-like deposits 321–2
filter glasses 176
flax 13
flexible chain-based HPFs 88–90
primary structure and physical properties
94
369
fluorescence microscopy 147–9, 171–2,
271–2, 340
fluorescence spectroscopy 160–1
fluoropolymer 12
foam test 283
folded crystal structure 81
follicle 29, 30
forensic analysis 4–5, 6, 7, 259–74
dyes 206
fluorescence microscopy 271–2
forensic mindset 259–61
manufactured fibre production and
spinning 262–6
microscopy 261–72
polarised light microscopy 266–71
Fourier transform infrared (FTIR)
spectroscopy 20–1, 162, 285, 345–6
freezing 351
Freud, Sigmund 259
fringed micelle model 76–81
Frotté reaction 310, 311
fungal damage 324–5
fur hairs 15
gel spinning 262
gelatine-coated plate imprints 289–90
generic fibre classes 10–13, 112–14
PLA/polylactide 115–19
generic subclasses 112–14, 119–23
genetic engineering 127
genetically modified (GM) cotton 251–2
glass fibre 12
goat fibres 28, 234
see also cashmere; mohair
GPC 98
grease deposits 315, 321
guard hairs 15, 36, 46
hairs
animal see animal hairs
human 45, 46
heat distortion temperature (HDT) 314
heat stabilisers 320
hemp 14, 81–2
heterocyclic polymer fibre 93–4
high performance fibres (HPFs) 88–110,
318
classification 88–90
identification 95–108
analysis of higher-order
structure 98–108
analysis of primary structure 97–8
mechanical and thermal
characteristics 95–7
primary structure and physical properties
90–5
high performance liquid chromatography
(HPLC) 22, 213, 215–18, 219–20
high resolution SEM 195
370
Index
high volume instrument (HVI) testing
248–50
Holmesian Maxim 138, 139
hot stage 18–19
hue 174
human hair 45, 46
humidification 350
humidity 135–6
hybridisation
conventional DNA hybridisation analysis
228–9, 230
in situ DNA hybridisation 226
hydrocellulose 293
hydro-entanglement 126
hydrolysis 255–6
identification, and comparison 4, 8–9
immersion type objective lens 195
imprint techniques 289–90
in situ DNA hybridisation 226
inclusions 17, 265
indigo 204
inductively coupled plasma atomic emission
spectroscopy (ICP-AES) 160–1
infrared polariser 347–8
infrared (IR) spectroscopy 4, 6, 20–1,
161–2, 181, 260
absorbance bands from synthetic fibres
87
damage analysis 285–6
FTIR 20–1, 162, 285, 345–6
HPFs 97–8
NIR 346, 347, 357
insect damage 300, 351
instrument calibration 176–7
interference colours 269–70
internal reflection spectroscopy (ATR) 21,
162, 285, 346
International Bureau for the
Standardization of Man-Made Fibres
(BISFA) 112, 114
iron 283
Irvine, Andrew 351
isotropic refractive index 142, 144–5
Jeziorny model for MEPT 287–8
jute 14
kapok 15
keratin 27, 31, 32, 33
see also animal hairs
keratinisation 226
Kevlar 89, 90–2
knitted fabrics 323–4
Krais, Markert and Viertel (KMV) reaction
297–8
lab-synthesised polymers 112, 113
lactic acid 119
laser induced breakdown spectroscopy
(LIBS) 161
laser-scanning fluorescence microscope 150
lastol 12, 120–2, 318
lastrile 119
lateral (side-by-side) fibres 123
leaf fibres 14
leather, simulated 126, 355–6
licensing system 250
light damage 278, 305
elastane (spandex) 314–15
nylon 310, 311
polyolefin fibres 319–20
wool 299
linear density 9, 69, 184–5, 246, 247
llamas 234
longitudinal specimens 136–7
Lorenz-Lorenz equation 142
lumen 243, 244
lyocell 115, 116, 119, 120
M5 (PIPD) 89, 93–4, 98–102
maceration 339
Mallory, George 351–2
manufactured fibres 9–10, 15–21, 27, 28,
111–30, 261–2
chemical analysis 19–22
fibre subclasses 112–14, 119–23
future trends 126–7
generic classes 10–13, 112–14
polylactide fibre 115–19
instrumental tests 19–21
microscopic analysis 15–19
multicomponent fibres 123–6
polymer origins and fibre
classification 112–15, 116, 117
production and spinning 262–6
refractive index 267–8
see also synthetic fibres
manufacturers’ analytical methods 3–5, 272
market taxonomy 4–5, 272
mass spectroscopy/spectrometry 159, 217,
218
matrix fibres (islands-in-the-sea fibres) 123
mauveine 204
mechanical damage 278, 305–6
elastane (spandex) fibres 313–14
polyester fibres 306, 307
polyolefin fibres 318
mechanical properties
HPFs 89, 95–7
synthetic fibres 69–72
medulla 29, 30, 39
medullae types 42–3, 44, 48, 49
melamine 12
Meldrum’s Stain 182
melt spinning 262
fibre variants 116
melting points 18, 183–4, 286–7
Index
HPFs 89, 95–7
manufactured fibres 263, 264
microscopy 146, 147
mercerised cotton 254–5
Mercury, Freddie 355–6
metal coating 193
metal roller 21
metallic fibres 12
metamerism 175
methyl orange crystals 299
micellar electrokinetic chromatography
(MEKC) 218–19
micelles 266
Michel-Levy interference colour chart
270
micro internal reflection (MIR)
spectroscopy 162
microbiological damage 278, 294
wool 299–300, 324–5
microfibres 18, 199, 265, 306
microfibre fabrics 125–6
microfibrils 75–81
microscopy 3, 4, 6, 23, 260
animal fibres 38–9, 46–8, 61, 225
conservation 336, 337–43
damage analysis 283–4
forensic analysis 261–72
HPFs 104–7
manufactured fibres 15–19
mixtures of chemically equivalent fibres
199
see also under individual techniques
microspectrophotometry 19, 22, 165–80
human vs machine colour
perception 172–5
limitations and strengths 178–80
metamerism 175
microspectrophotometer design 167–9
microspectroscopy 165, 167, 346
applications in fibre analysis 175–8
data collection 176–7
data evaluation 177–8
sample preparation 175–6
types of 169–72
middle endotherm peak temperature
(MEPT) 287–8, 307
mitochondria 226
mitochondrial genes 228
mixtures of fibres 199–200
chemically different fibres 200
chemically equivalent fibres 199
cotton blends 255–6
modacrylic 10
modification ratio 17, 265
mohair 49, 54–5, 59, 224, 225
moisture regain 70–1
Morapex rapid extractor 290
Morelli, Giovanni 259
mounting (in conservation) 351
371
mounting media 46–7, 136, 339
SEM 192–3
multicomponent (conjugate) fibres 123–6
multiphoton fluorescence microscopy 150–1
natural fibres 9–10, 13–15, 72, 81–4, 112
damage analysis 292–304
microspectrophotometry 178–9
see also animal fibres; plant fibres; and
under individual names
naturally occurring polymers 112, 113, 127
near-infrared (NIR) spectroscopy 346, 347,
357
neps 294–5
neutral density filters 176–7
Newton’s series 270
nitrous gases 315
non-destructive examination 134
nonwoven fabrics 125
normal phase chromatography 218
novoloid 12
nylon 11, 68, 70, 72, 78, 84–5
damage analysis 309–11
distinguishing silk from nylon microfibres
59–61
extraction and classification of dye from
209
nytril 11
oil deposits 315, 321
olefin see polyethylene; polyolefin;
polypropylene
oligonucleotides 228
optical anisotropy 139–46, 184
aggregate theory 141–6
optical coherence tomography (OCT)
151–2
optical microscopy 133–57, 181
advanced techniques 147–50
animal fibres 38–9, 46–7, 61, 225
conservation 337–40
cotton 253–5
future trends 150–2
HPFs 104–7
identification based on properties 139–47
melting behaviour 146, 147
refractive index and birefringence
139–46
solubility 146
manufactured fibres 15–19
overcoming the classical resolution limit
151
polarised light microscopy see polarised
light microscopy
practical and quality control
considerations 134–7
destructive vs non-destructive
examination 134
sampling 134–5
372
Index
specimen preparation 136–7
temperature and humidity conditions
135–6
SEM compared to 197–9
stereo zoom and simple light microscopy
137–9
optical spectroscopy 159
Optim fibres 28, 61–2, 66
organic HPFs see high performance fibres
orientation 261, 262
origin (of cotton) 240, 250–3
overhairs 36
oxidative damage
cotton 294
elastane (spandex) 315
nylon 310, 311
polyolefin fibres 319–20
oxycarmine test 294
‘parachute silk’ slip 355
paraffin deposits 315, 321
partition chromatography 217–18
path difference (retardation) 270, 271
Pauly reaction 297
PBI 12
PDO 123
pelage 35–6
pest control 351
pH value 282
photolysis 314–15
photometric accuracy 176–7
photooxidation 319–20
physical properties 16–18, 263–6
animal fibres 33–5
cotton 246, 247
HPFs 89, 95–7
synthetic fibres 69–72
physical testing
conservation 344
damage 280
‘pinhead’ reaction 292–3
PIPD (M5) 89, 93–4, 98–102
plant fibres 13–15
see also under individual names
pleat structure 103, 107
polarisability 141–2
polarised light microscopy 18, 139–46, 184,
339–40
crossed polars 140, 141, 269–71
forensic analysis 266–71
poly-3-hydroxybutyrate (P(3HB)) 85
polyacrylonitrile (PAN) see acrylic
polyamide see nylon
polybutylene terephthalate (PBT) 79, 85
polyester 10, 69, 70, 85, 112–14
damage analysis 306–9
extraction and classification of dye from
210
see also polyethylene terephthalate
polyester oligomers (PET oligomers)
308–9
polyethylene 71, 77, 86
UHMW-PE 89, 90, 94, 95, 96, 98–102,
103–4, 107
polyethylene terephthalate (PET) 79, 85
damage analysis 306–9
HPF from 95
polylactide (PLA) 13, 78, 85, 115–19
microfibre fabrics 125–6
polymer chains 76–81
polymerase chain reaction (PCR) 225,
229–33, 252
polymers
molecular structure of polymers for
fibres 72, 73–5
polymer origins and fibre classification
112–15, 116, 117
thermal properties 72, 76
see also high performance fibres;
manufactured fibers; synthetic fibres
polyolefin 12, 86
damage analysis 316–20
see also polyethylene; polypropylene
polyolefin keton 89, 94
polyphenylene-benzo-bisoxazole (PBO) 89,
90, 93, 98–102, 104, 105, 107, 108
polypropylene 71, 77, 86
damage analysis 316–20
extraction and classification of dye from
210
polytrimethylene terephthalate (PTT) 79,
85
polyurethane 71, 355–6
polyvinylalcohol (PVA) 68–9, 70, 78
PPT 122–3
PPTA 98–102, 103, 104
preliminary examination 282–3
pressing 305
price 240
primary wall 243, 244
principal axes 141–2
processing
DNA analysis and processed fibres
229–33
processing ability of cotton 246–50
product taxonomies 7–8
protein fibres see animal fibres
proteinase K 234, 235
proteins 31–2
pupa 38
pyrolysis gas chromatography (GC) 19–20,
186–7
quagga 225
quality control 277
quality faults 294–5
quantitative analysis
cotton in textiles 255–6
Index
mixtures of fibres 199–200
wool damage 300–3
quantum levels 166–7
quartz wedge compensator 145–6
rabbit, angora 49, 54, 58, 224, 225
Raman spectroscopy 21, 108, 162–3, 346–7
ramie 14
rayon 10–11, 116
‘reaction to flame’ 3, 6, 181–2, 260, 344
reactive dyes 205
extraction 211
red/green test (GSB test) 295
reference scan 170, 177
reflectance microspectroscopy 170–1, 172
refractive index 16, 18–19, 139–46, 184, 340
forensic analysis 266–9
reshaping 350
resolution
optical microscopy 151
SEM 188, 195
restoration 336
restriction fragment length polymorphism
(RFLP) 232–3
retardation (path difference) 270, 271
reversals (cotton) 244, 254
reversed phase chromatography 218
rigid chain-based HPFs 88–90
primary structure and physical properties
90–4
royal purple dye 204
rubber 11, 121
sail, HMS Victory 353–5
sample scan 170, 171, 177
sampling
conservation and 337
microspectroscopy 175–6
optical microscopy 134–5
saran 11
Satellite II DNA 228
Sayelle 124
scale casts 47
scanning electron microscopy (SEM) 19,
187–99
animal fibres 38–9, 47–8, 197–8
benefits compared to optical microscopy
197–9
conservation 340–2
ESEM 195–6, 343
examination 194–5
factors affecting the image 190–1
further techniques 195–7
mechanics of operation 189–90
principle of operation 188–90
specimen preparation 192–4
scattering spectroscopy 159
Scientific Working Group on Materials
Analysis 7
373
sebaceous gland 29, 30
secondary electrons 189, 190–1, 340–1
secondary wall 243, 244
seed fibres 14–15
selective dissolution 200
sericin 32, 84
setting 304–5
sheath-core fibres 123
Shell Chemical 123
shield hairs 36
sign of elongation 269, 270
silica stationary phase plates 214
silicone stains/deposits 283, 285, 286, 316
silk 15, 27, 28, 67, 71, 77
characteristics 44
cultivated and wild silk 30, 38
damage analysis 303–4
identification 59–61, 64, 65
physical and chemical properties 33, 35
production 30–1
structure and composition 32–3, 81, 84
silk lousiness 303
silkworms 38
silver paint 193
simulated leather/suede 126, 355–6
singeing 305
sisal 14
slip, ‘parachute silk’ 355
small-angle X-ray scattering (SAXS) 103–4,
105
softening points 18
solid-state nuclear magnetic resonance
(NMR) 107–8
solubility tests 19, 146, 182–3, 344
solvent spinning 120
solvent-spun rayon fibre variants 116
solvents 70–1, 183
Soxhlet extractor 290
spandex 11, 121
damage analysis 312–16, 317
speciality fibres 224–5
see also DNA analysis
species-specific DNA sequences 227–8
specific gravity 70–1
specimen preparation
optical microscopy 136–7
SEM 192–4
Spectra 89, 90, 94
see also ultra-high molecular weight
polyethylene (UHMW-PE)
spectral comparisons 177–8
spectrometer 159
spectrometry 158–9
spectrophotometer 166, 168
spectroscope 159
spectroscopy 158–64, 166–7
categorising methods 159
by measurement process 159
by nature of excitation 159
374
Index
conservation 344–8
infrared see infrared (IR) spectroscopy
Raman 21, 108, 162–3, 346–7
visible 161
see also microspectrophotometry;
microspectroscopy
spider silk 15, 95, 127
spinning 90, 262–6
cotton 246–8
staining tests 182, 291
conservation 343–4
wool damage 296–7
stains 278, 279–80
analysis of 281–2, 320–3
standard dye mixtures 215, 217
staple fibres 9
stationary phase 213–14, 215–16
stereo zoom microscopy 136–8
stereomicroscopy 46, 337, 338
stimulated emission depletion (STED) 151
streaks 278, 323–4
subclasses, fibre 112–14, 119–23
suede, simulated 126
sulfar 12
Supima cotton 250, 253
supply chain, cotton 240–1
support (in conservation) 350
surface imprint techniques 289–90
surface treatments
cotton 254–5
SEM and examination of 198–9
surrogate materials 349, 354
synthetic fibres 10, 68–87, 261, 262
crystal structure 77–9, 86–7
damage analysis 304–20
fundamental characteristics 72–84
identification 87
performance 69–72
refractive index 267–8
see also manufactured fibres; and under
individual names
taxonomies, product 7–8
technical fibres 13
Technora 89, 90, 92
temperature
decomposition temperature 89, 95–7
heat distortion temperature 314
melting points see melting points
MEPT 287–8, 307
optical microscopy conditions 135–6
tensile strength 69, 70–1
HPFs 89, 91, 95
TESS expert system 326
testing institutes 277
tex 9
textile damage see damage analysis
Textile Fiber Products Identification Act
(TFPIA) 10–13, 112–14
texturising 304
thermal analysis 286–9
thermal cycling machine 230
thermal damage 278, 304–5
acrylic 312
elastane (spandex) 314
nylon 309–10, 311
polyester 306, 307
polyolefin 318–19
wool 299
thermal microscopy 18–19
thermogravimetric analysis (TGA) 186, 288
thermomechanical analysis (TMA) 288–9
thermomechanical damage 278, 318–19
thermoplastic film imprints 289–90
thin layer chromatography (TLC) 22,
212–15, 216, 217, 219
damage analysis 284–5
three-dimensional crimp 124–5
tight threads 278
titrimetric methods 185
transmission electron microscopy (TEM)
187
transmission microspectroscopy 169, 170,
172, 177
transverse (cross-sectional) specimens 137
triacetate 71, 119
tricomponent fibres 123
triexta 119, 122–3
trousers, Freddie Mercury’s 355–6
tussah (wild) silk 30, 38
Twaron 89, 90
twist test 344
ultra-high molecular weight polyethylene
(UHMW-PE) 89, 90, 94, 95, 96,
98–102, 103–4, 107
underhairs 36
uniaxial symmetry 142–3
Universal Cotton Standards Agreement 248
Upland cotton 251, 252
UV stabilisers 320
upper half mean length (UHML) 246, 247
value (colour) 174
variants, fibre 114–15, 116, 117
Vectran 89, 90, 93
Victory sail 353–5
vicuña 234
vinal 11–12
vinyon 12
virtual sectioning 150
viscose 208
viscose rayon 71, 77
fibre variants 115
viscosity 98, 291
visible-range spectroscopy 161
see also microspectrophotometry;
microspectroscopy
Index
visual colour/dye comparison 22
visual examination 282
voids (air pockets) 17, 265
wavelength (of electron) 188
wavelength accuracy 176
wavelength dispersive X-ray microanalysis
196
wax deposits 315, 321
weighted silk 304
wet spinning 262
wholly aromatic polyester fibre 89, 90, 92–3
wide-angle X-ray diffraction (WAXD)
98–102
wild (tussah) silk 30, 38
Wintuk 124
wool 34–5, 71, 81, 82–4
applications 37–8
damage analysis 295–303
375
biological damage 299–300, 324–5
cumulative damage 303
indications of damage 295–6
quantitative analysis 300–3
extraction and classification of dye from
207, 211
identification 48–51, 52, 53
woven fabrics 323–4
X-ray diffraction 77–9, 86
X-ray microanalysis 196–7, 340,
341–2
X-rays, characteristic 189, 190, 191
Young’s modulus 70–1, 72
Zylon 89, 90, 93
see also polyphenylene-benzo-bisoxazole
(PBO)