Nitrocellulose Particle Stability: Coagulation Thermodynamics

Pergamon
0043-1354(94)E0107-H
War. Res. Vol. 29, No. 1, pp. 49-59, 1995
Copyright © 1994ElsevierScienceLtd
Printed in Great Britain.All rights reserved
0043-1354/95$7.00+ 0.00
NITROCELLULOSE PARTICLE STABILITY: COAGULATION
THERMODYNAMICS
D. GRASSO, 1. J. C. CARRINGTON,2~ " P. CHHEDA l and B. KIM 3@
tDepartments of Civil Engineering and Chemical Engineering and The Environmental Research
Institute, The University of Connecticut, Storrs, CT 06269-3037, U.S.A., 2U.S. Army Engineer District,
Philadelphia, 100 Penn Square East, Philadelphia, PA 19107, U.S.A. and 3U.S. Army Construction
Engineering Research Laboratory, P.O, Box 4005, Champaign, IL 61820, U.S.A.
(First received December 1993; accepted in revised form April 1994)
Abstract--The production of nitrocellulose (NC) creates large quantities of waste NC fines in wash water
streams. Current processing techniques attempt to remove these fines by cross-flow microfiltration, air
flotation, settling, centrifugation and lime precipitation. To date, no fundamental study investigating the
interfacial thermodynamics of these fines has been reported. This work explored the stability of NC fines
in water employing the DLVO model and the more recently developed extended DLVO (ExDLVO) model
as described by van Oss and colleagues [van Oss C. J., Chaudhury M. K. and Good R. J. (1988) Chem.
Rev. 88, 927-941]. Both models predicted that NC fines will become unstable and coagulate, with the
addition of a 1: 1 indifferent electrolyte at ionic strengths greater than approx. 5 mM. The ExDLVO
theory, which accounts for Lewis acid-base (AB) interactions, predicted greater attractive potential energy
availability. Experimental studies corroborated modeling predictions. Results indicate that NC fines may
be easily destabilized allowing potential reuse.
Key words--acid-base interactions, coagulation, contact angle, DLVO, electrophoresis, electrostatic
interactions, ExDLVO, flocculation, Lifshitz-van der Waals interactions, munitions, nitrocellulose,
particle size, particle stability, propellants
INTRODUCTION
primary wastewater effluents. The first waste stream
is contaminated with nitric and sulfuric acids (pH
between 1 and 2). The second stream is at neutral
pH and derives from rinse operations used to wash
excess acid from newly nitrated cellulose (Hefflnger
and Worrell, 1992; Wendt and Kaplan, 1976). Wastewater production from the latter stream ranges from
16 to 100 gallons per pound of NC manufactured
(Patterson et al., 1976). Both waste streams contain
NC fibers, most of which are of colloidal size
(Huibregtse et al., 1978). In this study, NC particles
with diameters greater than 1/~m are referred to as
fibers while those colloidal sized, diameters between
l/am and 1 nm, as fines. The fines, thought to be
extremely difficult to separate from suspension, must
be removed to meet site specific regulatory discharge
limits.
At the Radford (Virginia) Army Ammunition
Plant (RAAP), current removal methods for suspended NC fines are undergoing study and modification. The United States Army Construction
Engineering Research Laboratories (USACERL),
Champaign, II. has been conducting research on NC
fines separation, treatment, and disposal methods.
Treatment processes under consideration for implementation include anaerobic digestion, acid and
alkaline hydrolysis, biodegradation by white rot
fungus, the use of chemical additives to coagulate
fines, direct microfiltration, and rotary vacuum
Nitrocellulose (NC), first prepared by Braconnot in
1833 and improved by Schonbein in 1846, is a single
base, low-smoke powder used as a propellant in conventional military weapons. NC is a nitrate of cellobiose polymers, containing two successive units of
glucose linked by a/~-glycosidic bond, with a general
chemical formula (Quinchon and Tranchant, 1989):
m
m
cu~ o ~
H
o ~c,.
B
ri
I
C~o.
o R_
I
I
/ r ~ C ~
OR
H"~ C ~ O ~
I
H
I
OR
o/
I
Clt 2 OR
where, R = H or NO2. The degree of substitution
(number of hydroxyl groups substituted by nitrate
groups per monomer unit) and degree of polymerization control the quality of NC and indirectly determine the character of waste products (Mudrack,
1966).
The production process requires copious amounts
of water. NC manufacturing typically results in two
*Author to whom all correspondence should be addressed.
tAt the time of this study, J. C. Carrington was a Master's
student at Connecticut.
49
50
D. GRASSOet al.
filtration followed by microfiltration. Of these
methods, only microfiltration and coagulation by an
indifferent electrolyte may allow effective recycling of
the removed N C . Physicochemical investigations to
date have left the quantification of NC surface properties largely unaddressed. This study attempts to
explore and quantify NC surface thermodynamics in
order to evaluate effective solid/liquid separation
techniques allowing potential reuse.
In order to effectively remove colloidal particles
from a suspension, they must be aggregated into
large, readily separable flocs amenable to solid/liquid
separation. Particle-particle potential energies of
interaction typically provide insight to efficacious
destabilization techniques. In this study, both traditional Derjaguin, Landau, Verwey and Overbeek
(DLVO) (Derjaguin and Landau, 1941; Verwey and
Overbeek, 1948) and extended DLVO (ExDLVO)
(van Oss et al., 1988) models were employed to
identify and quantify potential energy characteristics.
These models were used to characterize the stability
of NC fines under varying ionic strength conditions.
The ExDLVO model is recent "cutting edge" work in
particle stability which expands the DLVO model by
accounting for Lewis acid-base interactions (AB)
between particles. An experimental protocol was
designed and implemented to validate particle stability predictions with an indifferent electrolyte.
THEORY
Proper summation of interfacial forces acting on
colloidal particles in aqueous media wilt indicate if
particles are thermodynamically driven to coagulate
or remain in suspension. Interactive forces have
traditionally been described as the result of attractive
electrodynamic or van der Waals forces and repulsive
electrostatic forces (Shaw, 1980). The DLVO theory
has been the classic model applied to describe colloid
stability (Hiemenz, 1986; Shaw, 1980). Many researchers have discovered that this theory does not
universally apply to colloidal suspensions, especially
in aqueous media (Grasso and Chheda, 1994; van Oss
et al., 1990; Xu and Yoon, 1989). Recent work by van
Oss and his colleagues has improved the theory of
colloid stability by including the effects of AB reactions thereby extending traditional DLVO theory
applicability.
Efforts to quantify interfacial forces in colloidal
suspensions dates back to 1873, when van der Waals
was researching certain properties of non-ideal gases
and liquids. He proposed the concept of attractive
interaction between neutral atoms. These forces were
later described by three related but different etiologies: (1) dipole-dipole interactions (orientation) as
reported by Keesom (1915); (2) dipole-induced
dipole (induction) interactions, reported by Debye
(1920); and (3) induced dipole--dipole (dispersion)
interactions, described by London (1930). Litshitz's
continuum theory showed that the interactions of
condensed media were similar to van der Waals
interactions of gas atoms (Lifshitz, 1956). This work
devised a method for the ab initio incorporation of all
van der Waals (LW) interactions (Norris et al., 1992;
van Oss et al., 1988).
Initial theories on hydrophobic colloid interactions
assumed that stability was the result of both long
range van der Waals attraction forces and electrostatic (EL) repulsions. Derjaguin and Landau (1941),
and Verwey and Overbeek (1948) did extensive
research on these initial concepts, which, when
combined, resulted in what has been accepted as the
general theory on colloid stability. The model estimates van der Waals energy and electrostatic potential energy deriving from charged surfaces as a
functions of distance between particles. These two
energies are summed resulting in the total interaction
energy as a function of interparticle separation distance. Varying the electrolyte concentration allows
estimations of the critical coagulation concentration
(CCC), the concentration of coagulation onset
(Shaw, 1980).
In addition to van der Waals attraction and electrostatic repulsion, many researchers have postulated
that other physical forces may effect colloidal suspensions (van Oss et al., 1987a; Xu and Yoon, 1989). Of
these forces the most pronounced is called "hydrophobic interaction" when attractive and "hydration
pressure" when repulsive. It has been reported that
these forces, which are polar but not electrostatic or
electrodynamic, may have energies up to two orders
of magnitude greater than those routinely calculated
using the traditional DLVO theory (van Oss et al.,
1988; van Oss, 1993). These polar forces derive from
AB interactions between polar particles in polar
media. Failure to recognize and account for these AB
interactions in the past may have caused erroneous
explanations of polar media interfacial interactions
(van Oss et al., 1988; van Oss, 1993). The inclusion
of these forces is essential in allowing aqueous interactions to be calculated on a more reasonably accurate scale (van Oss et aL, 1990).
It is clear that each interaction, LW, EL and AB,
is important and must be properly determined to
accurately predict particle stability. The total energy
balance of a particle is the additive result of each
interaction energy.
Therefore, the DLVO total energy equation may be
expressed as:
AG = AG Lw + AG EL
(1)
and the ExDLVO total energy equation as:
AG = AG tw + AG EL+ AG aa
(2)
Lifshitz-van der Waals (L W ) interactions
Lifshitz-van der Waals free energies of interaction
between two materials immersed in a liquid can be
directly obtained from the surface tensions, 7, of the
compounds as a function of geometry. AG Lw, the
Nitrocellulose particle stability
apolar free energy of interaction for two crossed
cylinders at 90 ° is given by:
AR
AG Lw = - - 6d
(3)
where A, the Hamaker constant is, (Hiemenz, 1986)
A = 24ny see
w (d0)2
(4)
resulting in
AGLW=
4rc7~w(d°)2R
d
(5)
potential at the particle surface, V. The reciprocal of
the Debye length, x, may be expressed as:
/ lO00e2N 2
x = ;
~cizi
~/ eoEkT i
7~ w = [7x/7;~s
Lw - yxf~-~[w]2
(6)
or rewritten as
kw
(7)
where y Lw is the LW surface tension of the liquid,
mJ/m 2, and yLWis the LW surface tension of the solid,
mJ/m 2.
If YL, the surface tension of the liquid is unavailable
in the literature, a variety of experimental methods
may be used to determine its value (Hiemenz, 1986;
Shaw, 1980). The non-polar surface energy component, yLW, is determined using the Y o u n g - G o o d Girifalco-Fowkes equation:
7L w -- "~L(I + COS 0 ) 2
(8)
4
where 7L is the surface tension of an apolar liquid,
mJ/m 2, and 0 is the contact angle between the solid
and the apolar liquid.
Electrostatic (EL) interactions
The electrostatic or double-layer potential between
particles of the same charge is always repulsive. The
electrostatic interaction energy or free energy (AG EL)
as a function of the distance, d, between the surface
of two cylinders crossed at 90 ° is predicted by:
AG EL= •E 0R(~'o) 2 In[l + exp(-- xd)]
(9)
where E is the dielectric constant of the medium, 78.55
for water at 25°C; E0 is the dielectric permitivity of the
medium in a vacuum, 8.854 x 10-~2 C/V- m; R is the
radius of the cylindrical particle, nm; and t/'0 is the
(10)
where e is electron charge, C; k is Boltzmann's
constant, J/K; T is absolute temperature, K; N is
Avagadro's number; l/M; c; is the concentration of
counterions, M; and zi is the valence of counterions
(van Oss et al., 1990).
For water at 25°C, x becomes
x=3.2725x/~
where do, nm, is the minimum equilibrium distance
which derives from the Born repulsion, calculated as
0.158 nm; d is the separation distance, nm; and R is
the radius of the cylinder, nm (van Oss, 1994). (Note:
all equations given are to replicate the conservative
assumption of interaction between NC fines, which
are best described as two crossed cylindrical particles
at 90 ° described later in Materials and Methods.) The
LW component of surface tension between the solid
(NC) and the liquid (water), YSL,
LW can be determined
with the Good-Girifalco-Fowkes combining rule
which applies to all exclusively LW interactions
(van Oss et al., 1988):
ysLw = ysLw + 7LLw -- 2 ~
51
(nm -I)
(11)
where I = 5z; c;z~ the ionic strength of water.
A c i d - b a s e ( A B ) interactions
For AB forces, the surface energy parameters
include all Lewis electron-acceptor/electron-donor
interactions. Hydrogen bonding interactions are included in AB interactions because hydrogen-donor
and hydrogen-acceptor interactions may be considered a subset of Lewis AB interactions (Jensen,
1980; Gutmann, 1978). The equation describing the
AB free energy for two crossed cylinders at 9if as a
function of distance, d is:
AGAB = 27tRZAGAdoa e x p [ ~ ]
(12)
where R is the radius of the cylindrical particle, nm;
2 is the decay length of the liquid molecules, approx.
1 nm for water (van Oss et al., 1990), and AGdA,~(J/m2)
is the change in free energy due to hydrogen-bonding
interactions at the minimum equilibrium distance do.
Contact angle measurements can be used to
determine AG,~0s (van Oss et al., 1990). The total
non-electrostatic surface energy may be divided into
two additive components, i.e. apolar and polar, as
(van Oss et al., 1990):
7 = 7 LW -~- ?AB
(13)
For a given polar substance, the etectron-acceptor
(7 +) and the electron-donor ( 7 ) characteristics are
typically different. Consequently, these parameters
are individually quantified and subsequently used to
determine the polar component (7 AB) of the surface
energy of compound i are expressed as (van Oss et al.,
1990):
7~ B
=
2~/7,+ 7 .
(14)
The polar components of the free energy of interaction between a solid, S, and a liquid, L, may be
expressed as:
AG~,B = k G s ~ = - 2 ( ~ + x / 7 ~ 7 ~ )
(15)
which accounts for electron-acceptor interaction of
the solid (S) with the electron-donor of liquid (L) and
vice-versa. AGsA~is the change in free energy due to
acid-base interactions between a solid and a liquid at
minimum equilibrium distance (van Oss, 1994).
D. GRASSO et al.
52
Using the D u p r 6 (1869) e q u a t i o n which is applicable to b o t h polar a n d apolar c o m p o u n d s :
AB
AB ~ B _ _ ~ B
AGdo
= AG AB
sL = 7 SL --
(16)
c o m b i n i n g e q u a t i o n (16) with equations (6, 13, 14 a n d
15) results in:
= (
+
+
(17)
E m p l o y i n g the Y o u n g - D u p r 6 e q u a t i o n (Nir, 1976):
(1 + cos 0)? L = - , -sLA~°ta'
,v
(18)
A~total
= AGEw + AGs~
•.s SL
(19)
where,
the following relationship is obtained:
(1 + cos 0)~& = - A G Lw - AGs~
(20)
which, w h e n c o m b i n e d with equations (7, 15) a n d the
D u p r 6 equation, becomes
(1 + cos 0)~ L = 2[X/)'LW~ Lw
+X/7~-Tf+X/7+TS]
Millipore filter. The filter samples were air dried overnight.
All samples were sputter coated (Polaron Model E5100
Series II Sputter Coater) for I min with gold. The samples
were randomly analyzed using a Coates and Welter HPS50b
Field Emission SEM and micrographs taken with a 4200
line photometer. Based on visual inspections of the micrographs, the shape of the NC fines was determined to be
cylindrical (Carrington, 1994). The use of SEM micrographs
to determine the diameter of the NC fines resulted in
samples that were singly truncated, i.e. diameters below
37.5 nm could not be observed. A method for simplifying
the estimators for normal distribution when a sample is
singly truncated was applied (Cohen, 1959). The NC fines
diameter yielded a truncated log normal distribution with a
mean of 4.895 and a standard deviation of 0.474 (Svarovsky,
1990). This mean value represents a radius (R) value of
67 __ 1.606 nm which was subsequently used in model predictions. Normality was tested by regression analysis, a
procedure which was essentially an equivalent to the
Shapiro-Wilk test (Filliben, 1975). Before transformation,
the truncated distribution of NC fines, which was near
normal (regression analysis yielded r 2 = 0.98), had a mean
radius of 122.7 + 88.0 nm. Knowing the standard deviation
and the mean diameter, the degree of polydispersity can be
predicted. Polydispersity may be defined as the ratio of the
area-mean and length-mean diameters:
Pd =
(21)
In order to solve e q u a t i o n (21) for the three u n k n o w n
variables, ), sEw , ~,~ a n d ~ s , a set o f three simultaneous
equations is structured from experiments with polar
a n d apolar liquids a n d subsequently solved (Norris
et aL, 1992).
MATERIALS AND METHODS
Materials
RAAP provided the NC (nitrogen c o n t e n t ~ 13.1%)
samples used in this study. Samples provided were final
product NC, and 5 gallon of wastewater samples with
suspended NC fines taken from different locations in the
manufacturing process. The wastwater samples examined
were: poacher decant, pulp, 1.5h settle after 2 h boil;
blender decant after 1.5 h settle; Jordon beater, P-5 pulp,
after 2 h settle; and centrifuge effluent. The initial ionic
strength of the wastewater was 2.4 mM.
Diiodomethane and glycerol were analytical grade obtained from Sigma Chemical Company, St Louis, Mo.
Water was distilled and deionized using a Millipore Milli-Q
system.
Particle size analysis
Traditional particle analysis techniques for colloidal dispersions typically provide Stokes' radius, which can differ
from the actual radius of non spherical particles. Therefore,
scanning electron microscopy (SEM) was used to determine
the shape and mean radius of the NC fines present in
wastewater samples. Two analysis techniques were applied
to ensure proper size distribution and random sampling
(Laxen and Chandler, 1982, 1983; Nomizu and Mizuike,
1986). The first technique involved placing 20 ml drops of
water containing suspended NC fines on aluminum foil
mounted on SEM holders. For each suspension, three sets
of drops were taken; one set with no dilution, one at 1:5
dilution with deionized water, and the other at 1:10 dilution. The drops were air dried overnight (approx. 16 h).
The second technique involved passing 5 ml of a 1 : 1 0 0
dilution for each suspension of NC fines through a 0.2 mm
~
(22)
where Pd is the degree of polydispersity; a is the standard
deviation and d,, is the arithmetic mean diameter (Hunter,
1987). For the truncated log normal distribution values, this
resulted in a Pd = 1.0, indicating that the log of the NC
diameters can be considered monodisperse. Using the nontransformed distribution a Pa = 1.23 was obtained, indicating significant polydispersity. A system can be considered
monodisperse if the coefficient of variation is less than 10%
(Hunter, 1987). For the truncated log normal distribution
values this results in a coefficient of variation of 1.2%.
However, when using values for the distribution without
transformation, the coefficient of variation is 72%. Thus,
diameters of NC fines would be considered polydisperse
when the distribution is not transformed, but monodisperse
for a log transformed distribution.
Electrophoretic mobility
Colloidal dispersions used for electrophoresis experiments
were prepared by siphoning off and centrifuging the supernatant of the final product NC suspended in Milli-Q water.
Centrifugation was carried out at 4000 rpm for 30 min using
an ultracentrifuge (Ivan Sorvall lnc., Model SS-3, Norwalk,
Conn.). After centrifuging, the supernatant was siphoned off
and the solids collected. Two suspensions of approx.
300 mg/l each were prepared using NaCl solution to yield
final ionic strengths of l0 -2 and 10-4 M.
To create a colloidal dispersion of final product grade
NC, the solid and Milli-Q water were blended for approx.
30sec at 1500 rpm using a commerical blender (Waring
Products Division, New Hartford, Conn.). This mixture
was allowed to settle for 48 h and the resulting supernatant
was a stable suspension of colloidal NC fines. To obtain
wastewater dispersion samples, individual 5 gallons sample
containers from RAAP were gently agitated and samples
were siphoned from the center.
The electrophoretic mobility of the NC particles was
measured by laser diffraction techniques (Malvern Instruments Inc., Zetasizer IIc, Southborough, Mass.). Aliquots of
final product NC suspensions were directly injected into the
Zetasizer after the addition of an electrolyte, NaC1. The
values reported here are the averages of 36 measurements
obtained at varying voltages (ll0-160 V) with respective
varying field strengths. All measurements were performed at
Nitrocellulose particle stability
25°C. Zeta potential values, ~', corresponding to EM
measurements were calculated using Henry's equation,
3r/u
- - 2EEofl( x R )
(23)
where r/is viscosity of the medium, N . s/m2; u is electrophoretic mobility of the NC fines, /~m.cm/s.V; and f~ is
described by the following equations for low values of x R
(<20) (Hiemenz, 1986);
(~-R)2 5(KR)3 (KR)4
.fi(~R)= 1 +
+ -16
48
96
[(KR) 4 (KR)6"I . n . rKSe-'dt
~-Jexp(x/<)J~ ~-(24)
and for high values of x R (>20) (Hunter, 1981):
9
75
fI(KR)=1.5-2(~+2(~c2R2)
330
~3Ra
(25)
The surface potential, ~0, was then computed
~0 = ~(1 + :)exp(~:b)
53
Critical coagulant concentration (CCC)
To verify that the surface characteristics of the wastewater
suspended NC fines were not drastically different than those
of the final product, both sample types were studied. The
following approach was utilized to determine the CCC
values. Final product and wastewater sample suspensions of
NC fines, were dispersed into a series of 100 ml graduated
cylinders. Varying amounts of NaC1 were added and the
cylinders were sealed with parafilm. The contents of the
cylinders mixed by inversion 7 times. After quiescent sedimentation for varying lengths of time, a portion of the
supernatant was carefully removed and analyzed for optical
density. Optical density was measured spectrophotometrically (Varian, Cary 1, Sugarland, Tex.) using a 1 cm cell at
500 nm. Optical density was subsequently plotted vs NaC1
concentration. Inflection points of the traces corresponded
to the CCC.
Inflection points were determined by fitting a function
using Jandel Scientific Tablecurve Version 2.14 (AISN
Software, Corte Modera, CA) and setting the second derivative equal to zero.
(26)
where b is the distance between the surface of the charged
particle and the slipping plane, usually taken to be about
0.5 nm (van Oss et al., 1990). The value of r was calculated
using equation (11). Stokes' radius of the particles, ct, was
log normally distributed with a range of 515-2600 nm and
a mean radius of 1140 nm as determined by laser diffraction
techniques. (Note the difference between actual mean radius
67 nm and Stokes' mean radius 1140 nm.)
Contact angle measurements
A smooth film of NC was needed to conduct contact
angle measurements for the determination of surface parameters. A sample of wet final product NC (approx. 20 g)
was filtered to remove water and then dried for 3 h in a
desiccator. Varying amounts of NC were dissolved in 100 ml
of acetone, yielding solutions of 10, 5 and 2% NC by weight.
A cleaned glass slide was vertically dipped into these
solutions and removed in a steady perpendicular manner. As
the acetone evaporated, a thin smooth film of NC formed
on the slide.
The objective of this study was to examine the stability of
NC fines in an aqueous matrix, consequently hydrated films
were required. The NC films were allowed to dry for approx.
3 h and then soaked in Milli-Q water overnight, approx.
16 h. After soaking, the films were allowed to dry. The dry
film on the glass slide was placed on a Goniometer (Rame
Hart Inc., Model 100-00-115, Mountain Lake, N.J.) at 20°C
and 65% humidity. Using 10,ul stainless steel syringes
(Hamilton), a drop of liquid was placed on the surface of
the NC film and the advancing contact angles (left and right
sides) were immediately (approx 7 s after the drop contacts
the film) measured. Contact angle measurements were taken
at 15min intervals of drying. When the contact angle
remained constant at two consecutive intervals, this was the
time used to obtain a hydrated NC film, representative of
NC fines in a water dispersion. Experiments revealed contact angle measurements were only effective with the 2% NC
solution. The 5 and 10% NC solutions created a film which
was too porous, absorbing deposited liquid drops.
Contact angles were measured using drops of diiodomethane to determine the value of 7sLw of the NC film
using equation (6). (Contact angles using ct-bromonaphthalene drops were attempted but no observable angle could
be measured.) Contact angles measured with the polar
liquids, water and glycerol were used to determine the
contributions of the two parameters, 7 ÷ and ~ -, of the polar
surface tension 7sAB of NC. Contact angle measurements
were also determined for water at ionic strengths of 100, 500
and 1000mM NaCI. The values reported here are the
averages for a minimum of 30 measurements per liquid.
RESULTS AND DISCUSSION
Electrophoresis measurements yielded ( values o f
- 3 5 . 7 9 and - 2 7 . 5 4 m V at ionic strength o f 1 0 - 4 M
and 10 -2 M, respectively; resulting in corresponding
surface potential values o f
-36.38mV
and
- 3 2 . 4 3 mV. This resulted in an average surface
potential o f - 3 4 . 4 0 mV. The surface parameters for
the three liquids used for contact angle measurements
are listed in Table 1. Contact angles o f these liquids
on s m o o t h thin films o f hydrated N C are given in
Table 2. Table 3 summarizes values for ysLw and, ),~
and 7 s parameters o f 7 AR for NC. Van Oss et al.
(1987a) reported values o f 38, 0, 18 m J / m 2 for 7sLw, ),~
and Ys, respectively for non-military grade ( < 12%
nitrogen content) hydrated N C acquired from
alternative manufacturing techniques. The slightly
higher values o f these parameters reported in the
Table 1. Componentsof surface tensionfor apolar and polar liquids
used to measure contact angles (compliedfrom van Oss et al., 1990)
~L
7Lw
;'~
;'x
Liquid
mJ/m2
mJ/m2
mJ/m2
mJ/m:
Diiodomethane
Glycerol
Water
50.8
64.0
72.8
50.8
34.0
21.8
. . . . .
3.92
57.4
25.5
25.5
Table 2. Advancing contact angles of various
liquids on NC films
Liquid
Contact angle, 0
Diiodomethane
3I. 18
Glycerol
37.60
Water
45.68
Water w/0.1 M NaCI
46.52
Water w/0.5M NaCI
46.89
Water w/I M NaC1
48.01
Table 3. Surface tensioncomponentsof
NC as determined by contact angle
measurements
Component
Value, mJ/m2
7Lw
43.73
7~
1.415
7s
24.43
D. GRASSOet al.
54
10
-10
-20
-30
[-
..40
-50
U
r~
-6O
-70
-80
-90
-100
-110
-120
-130
0
i
I
I
I
I
1
2
3
4
5
=
I
,
I
6
m
7
I
I
8
9
10
Surface to Surface Distance, nm
Fig. 1. LW component of change in free energy of interactions for NC fines (R = 67 nm).
present study can be attributed to variability in
maufacturing techniques, varying nitrogen contents,
and different sample preparation techniques. These
values indicated a pseudo-monopolar surface behavior (van Oss et aL, 1987c) of N C substrate employed
in the present work.
Particle stability model predictions
The change in free energy for each type of interactions, i.e. EL, LW and AB, was calculated for N C
suspensions, using varying concentrations of NaCI,
as a function of distance between the surfaces of two
cylindrical particles perpendicular to each other as a
conservative estimate. The minimum contact distance, d o , between the two cylindrical particles was
taken as 0.158nm (van Oss et al., 1988). Figure 1
shows the free energy curve for LW interactions
which is identical for all of ionic strengths. Figure 2
illustrates the free energy curves for EL interactions.
The AB interactions are displayed in Fig. 3.
In the classical D L V O theory the free energy of
particle interaction is a function of only LW and EL
10
A
A
9
8
7
mM
ofNaCIadE
6
4
3
2
1
0
-l
I
0
1
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
10
Surface to Surface Distance, am
Fig. 2. EL component of change in free energy of interactions for NC fines (R = 67 nm) in various
concentrations of NaCI solution.
Nitrocellulose particle stability
55
-50
-100
==
-150
/
-200
/
D = 1000
-250
J
-300
0
[
1
'
2
'
3
'
4
I
I
I
I
I
5
6
7
8
9
10
Surface to Surface Distance, nm
Fig. 3. AB component of change in free energy of interactions for NC fines (R = 67 nm) in various
concentrations of NaC1 solution.
lation than predicted by the traditional theory. Due
to this total energy shift, the ExDLVO theory
predicts that formed flocs may be stronger than
predicted by DLVO theory. This energy shift is a
result of AB interactions being dominate but relatively short ranged (Van Oss, 1993). In addition to
reducing EL forces, the strength of the AB forces is
impacted by NaCl, the more electrolyte added the
more hydrophobic the NC fines become. As expected,
the electron accepting/donating capability of the NC
surface is modulated with the addition of NaC1.
Values of AG~0a [equation (12)], and ?~ increased
forces (Figs 1 and 2). The total DLVO energy is
represented by Fig. 4. A total energy value less than
1.5 kT typically represents an unstable suspension
(van Oss et al., 1987b). EL forces are predominant at
long ranges but are overcome at short ranges by LW
forces. As expected, the addition of an electrolyte
reduces the EL forces. DLVO theory predicts that
NC should be unstable, i.e. flocculation should occur
between 1 and 5 mM of 1:1 indifferent electrolyte
added. The ExDLVO theory, as depicted in Fig. 5,
which for NC includes attractive polar AB forces
(Fig. 3), predicts more energy available for coagu7
6
5
4
f-
3
2
mM of NaCI added
A = 0.001
B = 0.01
A
C=0.1
D=I
C
E=5
F=IO
G = 100
D
H = 1000
i
01
-1
-2
-3
-4
-5
-6
-7
1
2
3
4
5
6
7
8
9
10
Surface to Surface Distance, nm
Fig. 4. Change in the total free energy of interactions obtained using classical DLVO theory for NC fines
(R = 67 nm) in various concentrations of NaCI solution.
D. GRASSOet al.
56
mM of NaCI added
A = 0.001
B = 0.01
C=0.1
D=I
E=5
F=10
G ffi 100
H = 1000
4
3
E-
2
1
AA ~
/
~,,,,~'[~
0
-1
F
~-
-2
-3
.4
-5
-6
-7
,
I
1
,
I
|
3
2
4
5
6
7
8
9
10
Surface to Surface Distance, um
Fig. 5. Change in the total free energy of interactions obtained using extended DLVO theory for NC fines
(R = 67 nm) in various concentrations of NaCI solution.
with the addition of NaC1 from - 1 . 6 6 to
-6.01 mJ/m 2 and 1.415 to 1.601 mJ/m 2 for electrolyte concentrations ranging from 0 to 1 M, respectively. Values of 7s decreased from 24.43 to
21.65 mJ/m 2 for a similar electrolyte concentration
domain. The decrease in electron donor parameter
and the increase in electron acceptor parameter of
NC with increasing NaC1 concentration may be
potentially attributable to association of Na with the
surface. The ExDLVO also predicts incipient flocculation at l m M < l < 5 m M .
The reason the two
theories yield similar predictions for CCC is that for
NC, in a zero electrolyte solution, 7 - = 24.43 mJ/m 2
which is near the transitional value of 28.3 mJ/m 2
indicating that NC is on the precipice of hydrophylicity (van Oss, 1993). Higher concentrations of
added NaCI result in greater total negative interfacial
energy causing instability as supported by both Figs 4
and 5 for 5, 10, 100 and 1000mM concentrations.
Critical coagulation concentration (CCC)
Figure 6 shows the O.D. vs NaCI concentration for
final product NC. After 1 h, the O.D. of the 10, 50
and 100 mM had increased. This phenomenon results
0.I0
|
;
,
g
,,~
0.00
.001
.01
•
after I Hr
•
after 6 Hr
--
after 90 Hr
.1
~~
~ ,
1
10
-
tO0
CONCENTRATION of NaCI, mM
Fig. 6. Optical density vs NaCI concentration for a colloidal dispersion of final product NC fines.
Nitrocellulose particle stability
57
0-51
J
0.4
g
03
--,I,
;,;
z
0.2
~
~
after 6 Hr
after 24 Hr
after143Hr
~
~
k
~
,,J
~J
0.0
-
.001
.01
.1
10
1
100
CONCENTRATION of NaCl, mM
Fig. 7. Optical density vs NaCI concentration for blender decant wastewater with suspended NC fines
(R = 67 nm).
from the aggregation o f N C fines in these suspensions
w i t h o u t a d e q u a t e settling time. After 3 h, the O.D. o f
the 10, 50 a n d 100 m M suspensions decreased, indicating c o a g u l a t i o n a n d settling o f N C fines. After
90 h the results remained similar. The 90 h trace was
fitted with a n arc tangent function. Setting the second
derivative o f this function equal to zero yields the
inflection p o i n t or the C C C which was 6.8 raM.
Figure 7 shows the O.D. vs NaCI c o n c e n t r a t i o n
a d d e d to blender decant wastewater from R A A P .
A l t h o u g h all wastewater samples were examined, the
blender wastewater results are reported as typical
...':'...
.,,..',
X 10 .7 .... :.. ...-'"'"
,....
5~ .....'
o.
.,........,-,"!'".
E
~a~
"'..:.
•
•.,'
..t...
"...
i'"'"'...,.
"....
..........
'o
~
¢r
. .... i ....
~'~~ ~ "'''''""
.,.."'" .-'"i'''"
1-
""
....i,::;;//
6
x 10 .9
10
-6
Log Ionic Str. (M)
Particle Separation Distance (m)
Fig. 8. Impact of NC radius on ExDLVO theory predictions of stability. The surface represents an energy
of 1.5 kT for which a critical NC particle radius exists. At a NaCI concentration, separation distance, and
radius above this surface a stable suspension exists, while at values below this surface the suspension will
be unstable due to dominance for attractive LW and AB forces. (The response surface was limited to
300 nm for better resolution.)
58
D. GRASSOet al.
findings. The ionic strength of the blender wastewater
was 2.4 mM. This sample exhibited nearly the same
results as the final production NC sample with the
major difference being the time required for flocs to
settle. Fitting a curve to the data and setting the
second derivative of the equation equal to zero
resulted in a inflection point of 4.77 mM. The inflection point value added to the initial ionic strength
yields a true CCC value of 7.17 mM. The history of
the various traces clearly shows that the flocculation
process is kinetically limited.
Observed CCC values between final product NC
and wastewater containing NC fines are similar,
suggesting no substantial change in surface characteristics at various process intervals. The actual CCC
values are slightly higher than those predicted by
the DLVO and ExDLVO theories but are within
excepted tolerance. This difference may be attributed
to simplifying assumptions made in the model
development.
The observed CCC values were dominated by
perikinetic flocculation. To enhance particle contact,
orthokinetic flocculation is necessary. When perikinetic and orthokinetic rates of flocculation are equal
and flocculation is in transition from Brownianmotion dominated kinetics to fluid-motion kinetics,
particle size and velocity gradient can be related. For
most cases of fluid motion caused by stirring or
baffled flow, the perikinetic rate dominates for particles less than 1/~m, and orthokinetic rate dominates
for particles larger than 1 #m (Ives, 1990). For NC
fines, the fine length as supported by SEM micrographs revealed lengths in excess of 1 #m. Consequently, the minor addition of an indifferent
electrolyte and mechanical energy input may enhance
solid/liquid separation allowing ultimate reuse of
the NC colloids. Flocculation studies are presently
underway in our laboratories.
Role o f effective particle radius
As discussed earlier, the particle radius, R, of the
NC fines is directly related to each component of
surface energy, EL, LW and AB [see Eqs (5, 9 and
12)]. Because NC fines are polydisperse, this relationship was studied to determine the impact of the radius
on the stability behavior of NC particles. Setting
AG = 1.5 kT and solving for R yields:
below this surface the suspension will be unstable
due to dominance of attractive LW and AB
forces. Thus, at low concentrations of NaC1 and
separation distances greater than approx. 4 nm, NC
fines with radii less than 10 nm will be unstable while
fines with larger radii will remain stable. (The response surface was limited to 300 nm for better
resolution.) Analyzing the boundaries of the colloid
domain, i.e. diameter between 1 and 1000 nm, fines
with R = 0.5 nm will settle at any ionic strength while
fines with R =500rim will begin to settle at
1 <I<5mM.
CONCLUSIONS
This study explored and quantified NC surface
thermodynamics which to date were inadequately
addressed. A proper understanding of the
thermodynamics enables exploration of effective
solid/liquid separation techniques to achieve
potential recycling of NC fines. Both DLVO and
ExDLVO models were employed to identify
and quantify potential energy characteristics.
These models were used to characterize the
stability of NC fines under varying ionic strength
conditions. ExDLVO theory more completely
describes interfacial forces acting on NC fines.
In this study, the difference in CCC predictions
between DLVO and ExDLVO models is not
as pronounced as in other studies (Grasso and
Chheda, 1994; van Oss et al., 1990). However,
the magnitude and spatial variability of potential
energy profiles between the two models were
disparate. This study demonstrated that NC fines
can be removed from a wastewater stream via
destabilization by an in-different electrolyte. An
experimental protocol validated particle stability
predictions with NaC1, NC fines dispersed in
production wastewater became unstable when the
ionic strength of the wastewater was greater than
or equal ca 7 mM (compared to model predictions
of 1-5mM). The results of this study provide
the groundwork for future research to explore
techniques to enhance particle contact allowing
efficacious solid/liquid separation and potential
recycle.
1.5 kT
R =
(ee0~Z01n[l+exp(--xd)]) -
A plot of radius as a function of NaC1 concentration and particle separation distance, d, is shown in
Fig. 8. The response surface in this plot represents an
energy of 1.5 kT for which a critical NC particle
radius exists. At a NaCI concentration, separation
distance, and radius combination above this
surface, a stable suspension exists. At values
do - d'] _
LW 2
2r~2AGdA0aexp 2 , ] ( ~ )
(27)
Acknowledgements--This research was supported in part by
United States Army Construction Engineering Research
Laboratory, Champaign, I11., and the Pollution Prevention
Research and Development Center at the University of
Connecticut. We would like to thank Mr James Heffinger,
Radford (Virginia) Army Ammunition Plant for providing
samples and discussing production process issues; Dr Uwe
Koehn, Department of Statistics, for his assistance in
Nitrocellulose particle stability
preting particle size distributions and Dr Marie Cantino, Ms
Lamia Khairallah and Mr James Romanow, Department of
Chemistry for their assistance in electron microscopy analysis, all of the University of Connecticut.
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