A Brief Overview of Theories of PVC Plasticization and

A Brief Overview of Theories of PVC Plasticization and
Methods Used to Evaluate PVC-Plasticizer Interaction*
Paul H. Daniels
ExxonMobil Chemical Company, 4500 Bayway Dr. Baytown, TX 77520-2101
This paper reviews the most widely used models for
explaining how plasticizers render PVC flexible. These
models include the gel, lubricity, and free volume theories; kinetic theories; and mathematical models which
predict on the basis of plasticizer structure how much
a plasticizer will lower the polymer glass transition in a
flexible PVC compound. Since plasticization results
from interactions between plasticizer and polymer,
methods which have been used to study either the
strength or the permanence (or both) of those interactions are also briefly discussed. Tools which have often
been used to study plasticizer-PVC interactions include
infrared and nuclear magnetic resonance spectroscopy, compression and humid-aging tests, dynamic
mechanical analysis, torque rheometer tests, plasticizer-resin clear point temperature measurements,
plastisol gelation/fusion by hot stage measurements,
and others. J. VINYL ADDIT. TECHNOL., 15:219–223, 2009.
ª 2009 Society of Plastics Engineers
POLY(VINYL CHLORIDE)
Poly(vinyl chloride): A Versatile Thermoplastic
PVC products range from rigid to very flexible and
from nondurable items like packaging to products with
service lives measured in decades or longer. PVC is used
in construction products such as pipe and fittings, window frames, siding, flexible roofing membranes, and
wire and cable coatings. It is used in home and office
floorings and as upholstery, wall coverings, and laminates of various types. But PVC is also widely used in
more short-lived products, including toys, fishing lures,
wearing apparel, advertising banners and signs, and
many others. PVC owes its wide utility to its ability,
unique among high-volume thermoplastic addition polymers, to accommodate many different additives at many
*Paper presented orally at SPE Vinyltec 2008, Rosemont, IL, October
13-15, 2008.
Correspondence to: P. Daniels; e-mail: [email protected]
DOI 10.1002/vnl.20211
Published online in Wiley InterScience (www.interscience.wiley.
com).
Ó 2009 Society of Plastics Engineers
JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—
—2009
different loading levels. These additives can modify and
enhance the performance of the PVC compound. The
structural characteristic of the PVC polymer which
allows it to be so accommodating is its polarized carbon-chlorine bond. This polarity allows the polymer to
interact with polarized additives via electrostatic interactions including van der Waals forces and dipole-dipole
interactions. These forces, though weak compared to primary chemical bonds, are nevertheless strong enough to
keep additive and polymer intimately associated for the
(sometimes long) life of the product.
Vinyl chloride monomer (VCM) was first reported by
Justus von Liebig and his student Henri Victor Regnault
in 1835. In 1872 German chemist Eugen Baumann synthesized PVC from VCM. The American chemist Waldo
Semon serendipitously discovered how to plasticize PVC
in 1926. All of these discoveries together with the development in 1930 of heat-stabilizing additives for PVC,
which made processing of PVC compounds possible, led
to the commercial production of PVC. PVC was produced
by emulsion polymerization in 1930 and suspension polymerization in 1934. Worldwide manufacturing capacity
for PVC grew to 2 million tons per year in 1960 and to
more than 30 million tons per year by 2003 [1].
PVC Structure and Microstructure
Commercial PVC is produced by three types of processes: suspension polymerization, emulsion polymerization and, to a lesser extent, mass polymerization.
Suspension polymerization is the most important process used to produce PVC for rigid end uses and for flexible products which are processed as melt compounds.
Vinyl chloride monomer is introduced under pressure to a
closed, water-containing reaction vessel to which has been
added primary dispersants (suspending agents, e.g., polyvinyl alcohols) and a VCM-soluble free-radical initiator.
The contents of the vessel are agitated by mechanical stirrers as the mixture is heated. The free-radical polymerization of the VCM is largely the idealized head-to-tail type
of addition. However, there are some defects in the PVC
molecular structure, including chlorine atoms located on
allylic carbon atoms and chlorine atoms located on terti-
ary carbon atoms. (Both of these types of structures negatively impact the heat stability of the polymer.) The finished resin particles are porous, irregularly shaped
(‘‘popcorn shaped’’), and have an average diameter of
about 140 microns. Resin particles consist of clusters of
primary particles roughly one micron in diameter surrounded by a thin (0.5 to 5 micron) pericellular membrane. The polymer may have a number-average molecular weight ranging from 30,000 (low-medium-MW PVC)
to 60,000 (high-MW PVC) up to 150,000 atomic mass
units or higher (ultra-high-MW PVC). (Note that the molecular weight of the polymer is commonly indicated by
its K value, its intrinsic viscosity in some solvent.) Within
the primary resin particles, the polymer exists largely in
amorphous form. Short, random, mostly syndiotactic
sequences in the repeating units in the polymer allow it to
form some crystal structures which are quite important to
the performance of the polymer. Roughly 8% of the PVC
consists of crystallites, and the average spacing between
these crystallites (as determined by x-ray diffraction) is
0.01 microns [2, 3, 4].
Emulsion PVC is used primarily in ‘‘pastes’’ or plastisols which are dispersions of PVC resin in plasticizer.
The process used to make these resins is similar to that
used to make PVC suspension resins. Vinyl chloride
monomer is again introduced to a closed, water-containing pressurized vessel. Likewise, a VCM-soluble free-radical precursor is used to initiate the polymerization reaction. However, in the emulsion resin process, soaps (e.g.,
sodium lauryl sulfate) are used instead of dispersants.
When the reaction vessel and its contents are stirred, an
emulsion forms, and VCM is sequestered within the
micelles formed by the soap molecules. When the monomer polymerizes, the PVC resin particles assume the
dimensions of the interior of these micelles, roughly one
micron in diameter and spherical. Differences between
emulsion and plastisol resins may be only in the structure,
or gross morphology, of the resin particles. The microstructure (polymer MW, crystallinity, number of chain
defects, etc.) of resins made by either of these processes
can be virtually the same. The structural differences in
the resin particles, however, lead to markedly different
processability characteristics of the PVC compounds made
with each of them.
Mixing PVC suspension resins with a quantity of plasticizer equal to as much as one hundred percent (or more)
of the mass of the resin under controlled conditions produces a dry-blended PVC compound in which plasticizer
has penetrated the amorphous part of the PVC resin and
solvated it. Dry-blended PVC compounds are free-flowing
powders. Mixing emulsion resins, also under carefully
controlled conditions, with similar quantities of plasticizer
produces a dispersion of resin particles in plasticizer, a
plastisol[2]. PVC plastisols are liquids, viscosities of
which vary with the types and levels of plasticizers used
and with the types and levels of other viscosity-modifying
additives.
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Mass-polymerized PVC resins are similar in particle
size, porosity, and microstructure to the suspension resins
and are used in many of the same end uses. The difference between the suspension resin process and the mass
resin process is that the latter does not use water as a
reaction medium. Since the PVC polymer is insoluble in
liquid VCM, VCM is used instead. The reactor used in
the mass polymerization process has two stages. In the
first stage, PVC resin seed particles are formed by a freeradical polymerization process under agitation conditions
similar to those used in the suspension reactors. When the
polymerization reaches about 12% conversion, the reaction mixture is transferred to a second vessel which contains a low-speed mixer more suitable for mixing solids.
Unlike the suspension resin particles, mass-polymerized
PVC resin particles do not contain any residual dispersant,
nor do they have a pericellular membrane. In theory,
these differences make mass resins both cleaner and easier to dry-blend with plasticizers. There are, however, a
number of practical difficulties with the mass process
which have limited its use. The process produces a great
number of fine particles which are difficult to separate
from the rest. Also, it is more difficult to strip residual
VCM from mass resins than from suspension resins, and
since PVC is a good thermal insulator, it is difficult to get
heat into the second stage of the reaction vessel [2].
PLASTICIZERS
Plasticizers are used to make PVC (and other materials) flexible. Internally plasticized PVC consists of VCM
which has been copolymerized with another monomer
(e.g., vinyl acetate or ethyl acrylate). The resulting copolymer is softer and has a lower modulus and a lower tensile
strength than the corresponding PVC homopolymer. Externally plasticized PVC consists of PVC homopolymer in
which the amorphous part of the polymer has been solvated by additives, typically esters.
The Council of the International Union of Pure and
Applied Chemistry defined a plasticizer as a substance or
material incorporated in a material (usually a plastic or
elastomer) to increase its flexibility, workability, or distensibility. (By this definition) A plasticizer may reduce
the melt viscosity, lower the temperature of a secondorder transition, or lower the elastic modulus of the product.
Mechanisms of Plasticization
Semon discovered external plasticizers for PVC when
he was seeking to increase the adhesion of the polymer to
metal by dehydrohalogenating it in high-boiling solvents.
Some of these solvents turned out to be plasticizers. How
external plasticizers effect their changes in PVC has been
a matter of conjecture and a subject of research since the
1940s. Early researchers had the advantage of being able
to refer to studies of plasticized non-PVC polymers and
DOI 10.1002/vnl
vulcanized rubber dating back to the mid-1800s. By 1950,
two theories for the mechanism of plasticizer function in
PVC had been developed, the lubricity theory and the gel
theory.
The lubricity theory was developed by Kilpatrick [5],
Clark [6], and Houwink[7], among others [8]. It holds that
plasticizer in PVC acts as a molecular lubricant allowing
the polymer chains to move freely over one another when
a force is applied to the plasticized polymer. It assumes
that the unplasticized polymer chains do not move freely
because of surface irregularities. In this model, one segment of the plasticizer is strongly attracted to the polymer, while other segments are not. The former acts as a
solvent for the polymer, while the latter acts as a
lubricant.
The gel theory of plasticization was developed by Aiken
and others [8, 9]. It holds that the polymer molecules are
loosely tied together at varying intervals. Added plasticizer
increases the random motion of the polymer chains in the
nonassociated regions of the polymer. Aiken thought that
the gel structure could be formed either by permanent
intermolecular ties or by ties which form and disappear in
dynamic fashion as plasticizer solvates then desolvates
these areas. This theory explained why externally plasticized PVC compounds soften more with increasing temperature than do internally plasticized compounds. The
external plasticizers are free to solvate and desolvate different sites on the polymer molecule to which they are
attracted and to which other polymer molecules might be
attracted. Internally plasticized systems lack this freedom.
Likewise, the gel theory explains how nonsolvents for the
PVC molecule (e.g., some secondary plasticizers) can
soften it. They increase the space between polymer molecules, thereby reducing polymer-polymer interactions at
sites where polymer chains could associate.
The free volume theory of plasticization sought to
explain the reduction in polymer glass transition temperature upon the addition of plasticizer. It had been observed
that the specific volume of polymers decreased linearly
with decreasing temperatures until the Tg was reached.
After that, the specific volume decreased more slowly. It
was assumed that the increased specific volume above the
glass transition temperature was attributable to ‘‘free volume’’, space between molecules. Today, free volume is
typically defined as the difference in specific volume at
some temperature of interest and some reference temperature, usually absolute zero. (It is difficult to determine
what the volume of a plastic would be at absolute zero,
so that figure is an approximation.) Free volume in the
polymer could come from several sources, motion of
polymer end groups, motion of polymer side groups, and
internal polymer motions. Below their glass transition
temperature, polymers show limited motions of these
types. That is why unplasticized PVC is hard and rigid.
When plasticizer is added to the polymer, motions in the
plasticizer molecule, like motions in the polymer, create
free volume [8, 10].
DOI 10.1002/vnl
Kinetic or mechanistic theories of plasticization see the
association between polymer and plasticizer and between
plasticizer and plasticizer as transient and ever-changing.
Associations form, disappear, then reform. Some plasticizers form stronger associations with the PVC polymer than
others. At low plasticizer loadings in the PVC, plasticizer-polymer associations predominate. At high plasticizer levels, plasticizer-plasticizer associations predominate [8].
Mathematical models for plasticization, such as those
developed by Mauritz and Storey [11], attempt to predict
the Tg of a plasticized PVC from the glass transition temperatures of the polymer and the plasticizer. The plasticizer efficiency at reducing this glass transition temperature is based on structural features of the plasticizer such
as the length and branchiness of the side chains.
PVC-Plasticizer Interaction.
All of the models of PVC plasticization imply some
chemical interaction between the plasticizer and the polymer.
Plasticizers must be attracted to the polymers that they
modify nearly as strongly as they are to other plasticizer
molecules. Without this sort of plasticizer-polymer interaction, plasticizers in plasticized PVC would tend to self-associate, form increasingly larger micropools within the
PVC, and eventually exude. Fortunately, every repeating
unit in the PVC polymer chain contains a polarized carbon-chlorine bond. This structural feature makes it possible
for the polar parts of a plasticizer molecule (e.g., the aromatic ring and the ester linkages in a phthalate ester) to
interact with the polymer via van der Waals forces and
dipole-dipole interactions. In plasticized PVC these interactions permit the plasticizer in a finished flexible product to
solvate the amorphous part of the polymer but not the
tightly self-associated crystalline part of the polymer. These
crystalline crosslinks between polymer molecules serve a
purpose in flexible PVC similar to that of the crosslinks in
elastomers or the crosslinks in thermoplastic olefins. They
increase the elastic modulus of the polymer, thus giving
the strained form some memory for its original shape.
Plasticizer-PVC interaction in flexible PVC begins with
the mixing step, which proceeds in predictable stages [8].
Upon mixing a dry compound (plasticizer with suspension- or mass-polymerized PVC), the plasticizer first wets
the resin particles and then, with externally added heat
(either heat generated by the friction of intense mixing or
heat from some sort of heating jacket on the mixer), diffuses into and solvates the amorphous part of the PVC
polymer. The heat provided is usually sufficient to raise
the temperature of the PVC resin to its Tg (approximately
808C). at which point it dry-blends much more rapidly
but insufficiently to raise its temperature much beyond
1108C. At that point the PVC resin particles can begin to
sinter. Incompletely dry-blended PVC (in which some of
the plasticizer is poorly dispersed and some plasticizer is
self-associated, not solvating the amorphous part of the
JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—
—2009 221
polymer) typically produces finished products in which
there are regions of high concentrations of plasticizer.
This situation eventually leads to exudation.
In mixing a plastisol, the plasticizer again wets the
resin particles, but in this case, temperatures are maintained well below the Tg of the resin, and the plasticizer
does not diffuse into the resin. Mixing can be either highintensity for low-viscosity plastisols or low-intensity for
high-viscosity plastisols. The resulting mixture is a dispersion of solid resin particles in plasticizer. These plastisols
can be quite stable. Plastisol inks for textile printing, for
instance, are often maintained in inventory for several
months. Poorly dispersed plastisols contain agglomerated
resin particles and have viscosities higher than expected.
This condition can make further processing of incompletely mixed plastisols difficult.
Like the finished products produced from incompletely
dry-blended PVC solids, finished products made from
inadequately mixed plastisols can also show regions of
high plasticizer concentrations which will eventually lead
to exudation.
When plastisols are processed, the next step in plasticization is similar to the second step in dry-blending suspension resins; with added heat (up to, say 1108C), plasticizer diffuses into and solvates the amorphous part of the
PVC polymer. This solvation swells the resin particles to
the point at which all of the liquid plasticizer has been
absorbed. At this point the plastisol is said to be gelled.
The next stages of PVC plasticization are the same for
plastisols and dry compounds. With added heat, plasticizers (partially) dissolve the crystalline parts of the polymer.
Depending on the solvating strength of the plasticizer, temperatures of 150-1808C are typically required. At this point
the plasticized PVC is in a melt stage where it can be
formed to a desired shape. Boundaries between what were
originally separate resin particles have been largely obliterated. Note that the PVC is typically not completely melted.
Plasticized PVC suspension resin, for example, flows
through the final zone of an extruder in primary particles
about one micron in diameter. These particles contain bundles of about 10 million PVC polymer molecules. With
cooling, the structure of the polymer is reestablished. Crystalline crosslinks reform, rapidly at first and continuing for
up to 48 hours, so that crystalline crosslinks are present in
the finished product to approximately the same degree as
they were in the PVC resin [2, 8].
PREDICTING AND MEASURING
PVC-PLASTICIZER INTERACTIONS
Plasticizers may be either primary or secondary types.
Primary plasticizers are the additives largely responsible
for making PVC flexible and are materials which can
function without ancillary additives. The main functions
of primary plasticizers have already been described. Secondary plasticizers by themselves are not completely
compatible with the polymer but are made compatible by
222 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—
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the presence of the primary plasticizer. Secondary plasticizers are used to reduce compound costs or to improve
compound properties such as low-temperature flexibility,
flame retardancy, or processability. Examples include aliphatic, cycloaliphatic, and partially aromatic hydrocarbon
oils and chlorinated paraffins.
Several methods have been used to predict the strength
of the interactions with PVC of either primary or secondary plasticizers. Among these are several interaction parameters. The Hildebrand solubility parameter is described
as the square root of the cohesive energy density, the
energy which holds molecules of a substance together [12].
It can be estimated by dividing the heat of vaporization of
a substance by its molar volume or, more commonly, by
using additive constants for the functional groups in organic molecules. Solubility parameters have proven useful
for comparing compatibilities of plasticizers within a homologous series (e.g., phthalates) but are less useful in
comparing PVC-plasticizer interactions of plasticizers from
different chemical families [8, 12]. Polarity parameters for
plasticizers are determined by ratioing the number of apolar carbon atoms in the plasticizer to the number of polar
groups present and multiplying this number by the molar
mass of the plasticizer [8, 13]. Flory-Huggins interaction
parameters have been used for plasticizers with PVC. This
mathematical approach uses an interaction factor, chi (v),
along with numbers of moles and molar volumes of plasticizer and PVC to predict the energetics of plasticizer-polymer mixing (free energy of mixing) [8, 14].
Experimental methods have also been used to assess
the strength of the plasticizer-PVC interactions. FTIR
methods typically look at the spectral shift of the plasticizer carbonyl group (assuming that the plasticizer is an
ester) and the PVC resin carbon-chlorine bond. The carbonyl and carbon-chlorine bond group absorption frequencies can each be shifted to frequencies several wavenumbers lower (lower energy) when the ester group interacts
with the PVC [8, 15–17]. The intermolecular interaction
between the polar parts (carbonyl group) of the plasticizer
and the PVC (carbon-chlorine bond) slightly weakens the
intramolecular bonding forces in the interacting functional
groups. Solid state C-13 NMR cross-polarization magicangle spinning (CPMAS) has also been used to study
PVC-plasticizer interactions. Chemical shifts and line resolutions as well as spin-lattice and spin-spin relaxation
times for the plasticizer carbonyl carbon and alkyl carbons are measured as it interacts with the polymer [8, 18–
20]. The strongest polymer-PVC interactions appear to be
between the electronegative region of the plasticizer (carbonyl linkage) and the electropositive atoms of the PVC.
Interactions appear to be electrostatic in nature, and polymer flexibility results from free volume created within the
polymer as the result of thermal motion of plasticizer
alkyl side chains [21].
Nonspectroscopic experimental methods for studying
plasticizer-PVC interactions include torque rheometer
tests [22] (ASTM D 2396, D 2538), a test for plasticizer
DOI 10.1002/vnl
compatibility under humid conditions (ASTM D 2383),
dynamic mechanical analysis [23], and plasticizer-PVC
resin clear point and hot bench gelation tests. Each of
these tests indicates how strongly plasticizer and PVC
interact by determining the temperature at which the resin
dry blends or gels or fuses, or the conditions under which
the plasticizer becomes incompatible with the polymer,
Over the past eight decades, perhaps 20,000 compounds have been screened as PVC plasticizers. There are
presently around 300 plasticizers which are manufactured
and perhaps 100 which are of commercial importance. Of
these commercially important primary plasticizers, nearly
all are esters. Moreover, just three of these esters, di-2ethylhexylphthalate (DOP), diisononylphthalate (DINP)
and diisodecylphthalate (DIDP) account for 75% of the
PVC plasticizers used in the world today [8, 24]. Plasticizers are almost invariably esters because of their specific requirements for interacting with the polymer. Plasticizers in flexible PVC must be closely associated with the
amorphous part of the polymer at room temperature, and
plasticizers must be fairly permanent. The plasticizer must
not self-associate in preference to solvating the polymer.
The plasticizer must act as a solvent for the crystalline
part of the PVC at flexible PVC processing temperatures
but not at lower temperatures. Also, the plasticizer must
not react with the PVC. (Because of its labile chlorine,
PVC is very susceptible to thermal decomposition.) Molecules used as plasticizers should ideally be odorless, colorless, liquid (for ease of mixing), relatively nonvolatile,
non-water-soluble, and, of course, nontoxic. Taken together, all of these requirements point to high-MW esters
containing certain functional groups.
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