Semi-Batch Emulsion Copolymerization of Vinyl Acetate and Butyl ...

2614
Macromol. Chem. Phys. 2001, 202, 2614–2622
Full Paper: Detailed kinetic studies on semi-batch emulsion
copolymerization of vinyl acetate/butyl acrylate
(VAc/BuA) (80:20) at 808C were carried out using alkyl
polyglucoside nonionic surfactants and ammonium persulfate
as initiator. The polymerization recipe was varied
with respect to the amount of initiator initially charged
and continuously fed to the reactor, the surfactant concentration,
the electrolyte concentration, and the monomer
addition rate. Various types of alkyl polyglucosides with
different hydrophilic and hydrophobic chain lengths were
tested. Latex particle stabilization was shown to depend
significantly on the surfactant structure and its adsorption
characteristics. In general, the latex particle size distribution
evolved from a bimodal to a unimodal distribution
while latex destabilization was observed at high surfactant
or/and high initiator concentrations. Based on experimental
observations it was found that the most important factor
for imparting particle stability was the hydrophilic/
hydrophobic chain length ratio of the surfactant which
actually controlled the balance between surface coverage
and stabilizing capability.
The effect of surfactant structure on the average particle size
(surfactant concentration 1 wt.-%; initiator addition policy
0.25–0.25 wt.-%).
Semi-Batch Emulsion Copolymerization of Vinyl
Acetate and Butyl Acrylate Using Oligomeric Nonionic
Surfactants
Nikos Lazaridis, 1 Aleck H. Alexopoulos, 2 Costas Kiparissides* 1, 2
1
Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 472, 540 06 Thessaloniki, Greece
2
Chemical Process Engineering Research Institute, Aristotle University of Thessaloniki, P.O. Box 472,
540 06 Thessaloniki, Greece
Introduction
Emulsion polymers of vinyl acetate are widely used in
many applications (i.e., exterior and interior architectural
coatings, adhesives, and paints) due to their good durability
and availability at low cost. In these applications a
continuous film is formed upon drying of the latex. The
mechanical properties of the polymer film depend on the
molecular properties (e.g., molecular weight distribution,
copolymer composition, etc.) and morphological properties
(e.g., particle size distribution) of the latex.
Copolymerization of vinyl acetate with other vinyl
monomers can lead to the production of latex particles
having a wide range of molecular and particle-morphological
properties depending on the molecular structure of
the comonomer and the copolymer composition. One of
the most important industrial latexes, widely utilized in
the architectural coatings market, is the vinyl acetate
(VAc)/butyl acrylate (BuA) emulsion copolymer with a
BuA composition of 15–25%. The emulsion copolymerization
of the VAc/BuA system is characterized by large
differences in reactivity ratios (r VAc = 0.05 and r BuA =
5.5), water solubilities (25 g/L for VAc and 1–1.5 g/L for
BuA), propagation rate constants (k p (VAc) = 4000 L/
mol/s and k p (BuA) = 200 L/mol/s), and glass transition
temperatures (T g (VAc) = 328C and T g (BuA) = –548C). [1]
Due to these significantly different monomer properties,
latexes having a wide range of molecular and particlemorphological
properties can be obtained by varying the
comonomer composition, thus influencing the glass transition
temperature and the minimum film-forming temperature
of the latex paints.
The type of copolymerization process (semi-batch or
batch) is also an important factor affecting the final latex
[2, 3]
properties. In batch copolymerization of BuA with
Macromol. Chem. Phys. 2001, 202, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1208–2614$17.50+.50/0

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2615
VAc, BuA is depleted very rapidly due to its high reactivity
ratio, while the polymerization rate of VAc prominently
increases after the consumption of BuA. [1] Generally,
this leads to heterogeneous particle structures with a
soft core, rich in the more hydrophobic butyl acrylate,
and a harder hydrophilic shell, rich in the softer vinyl
acetate. On the other hand, a semi-continuous emulsion
copolymerization process can produce latex particles
[1, 2, 4]
which are nearly homogeneous. Thus, copolymer
composition control in terms of optimal monomer
addition policy has widely been studied. [2, 5, 6] It has been
shown that homogeneous copolymers having a desired
composition can be obtained by feeding the more reactive
comonomer into the reactor according to an optimal
[3, 7]
control law. Semi-batch polymerization can prolong
the nucleation period and result in a decrease of particle
growth rate under monomer starved conditions, leading
to the production of small sized, high solid-content
latexes. These latex products exhibit increased emulsion
viscosity which is frequently desirable in paint
formulations. Moreover, based on the structural differences
of the latex particles, the semi-continuous polymerization
of VAc/BuA has been found to lead to the production
of latexes having improved film forming properties.
[1]
Another important feature in the emulsion copolymerization
of VAc/BuA is the significant difference in the
water solubilities of the two monomers which strongly
affects the respective monomer partition coefficients in
the polymer and water phases. The high water solubility
of VAc indicates that water phase polymerization will be
dominant at low conversions. Thus, particle nucleation
will predominately occur by the homogeneous nucleation
mechanism. [2] At higher monomer conversions the particle
phase volume increases and gradually becomes the
main locus of polymerization. [1, 8] Kinetic studies on the
semi-continuous emulsion polymerization of VAc/BuA
have shown that the number of radicals per particle is
large and, thus, the polymerization does not follow the
classical Smith-Ewart kinetics. [9] The number of radicals
per particle was determined to be between 1 and 7 for
BuA compositions between 20 and 100%. [1] On the other
hand, in batch copolymerizations the number of radicals
per particle was initially large but decreased below 0.5 at
higher monomer conversions after the consumption of
BuA. [1]
In semi-batch emulsion polymerizations the initiator
can initially be charged or/and continuously fed during
polymerization. Increased initiator concentrations affect
not only the polymerization kinetics but also the stability
of the particles by increasing the ionic strength of the
continuous aqueous phase. Vandezande and Rudin [4]
observed a nearly linear relationship between the ionic
strength of the aqueous phase and the latex particle surface
area. Moreover, they reported that the final monomer
conversion increased when a continuous initiator addition
policy was employed.
In general, particle stabilization is achieved by the use
of anionic surfactants, which are extensively employed in
many emulsion polymerization systems. However, the
limited stabilizing effectiveness of anionic surfactants at
high solids (e.g., A40%) and electrolyte concentrations
has led to the development of alternative stabilization
techniques, including the combination of anionic and
nonionic surfactants. [10] VAc/BuA latex particles produced
in batch reactors can have a core-shell morphology.
Such core-shell latex particles are difficult to stabilize
due to poor surfactant adsorption characteristics of
the hydrophilic VAc surface. [11] This problem appears
even with the production of homogeneous particles in
semi-batch reactors and has led to the development of
alternative stabilization methods. These include the use
of functional comonomers containing weak or strong acid
groups (e.g., acrylic acid), the use of steric stabilizers [12]
(e.g., PVOH, HEC, PEG) and the use of polymerizable
surfactants. [13, 14] Unfortunately, the use of alternative stabilization
methods has often led to the appearance of
other problems (e.g., deterioration of film formation
properties). As a result, there is a persisting need for the
development of new surfactants for stabilization of high
solids-content latexes.
A limited number of publications, dealing with the use
of nonionic surfactants for latex stabilization, have
appeared in the open literature. [12] In general, nonionic
surfactants provide stabilization that is lower than that
obtained by anionic surfactant systems, often leading to
the formation of multi-modal and broad particle size distributions.
[10] Vandezande and Rudin [4] described a seeded
semi-batch emulsion polymerization process for the production
of high solid VAc/BuA latexes, using the anionic
surfactant sodium dodecyl sulfate, (SDS). The production
of high solid-content latexes (e.g., up to 55–60%) could
only be achieved by employing a nonionic surfactant in
addition to SDS. However, Vandezande and Rudin
observed a reduction in the polymerization rate in the
presence of a nonionic surfactant, which was attributed to
the inhibition of oligomer radical entry rate by the viscous
surfactant layer. In a similar study, Bataille et al. [9]
employed polyoxyethylene-b-polypropylene nonionic
surfactants with limited success.
In the present work, a new generation of nonionic surfactants
(e.g., alkyl polyglucosides) was employed for
the production of high quality environmentally benign
water-borne VAc/BuA latexes. All emulsion polymerization
experiments were carried out in the presence of alkyl
polyglucosides. The main goal of this work was to investigate
the effect of the surfactant molecular structure and
concentration as well as the initiator addition policy on
the polymerization kinetics, latex stability and particle
size distribution.

2616 N. Lazaridis, A. H. Alexopoulos, C. Kiparissides
Experimental Part
Materials
Vinyl acetate (Aldrich, +99%, 3–5 ppm MEHQ) and butyl
acrylate (Aldrich, +99%, 10–55 ppm MEHQ), were used as
received without any further purification. Ammonium persulfate
(Merck, 98%) was used as initiator, sodium bicarbonate
(Merck) as buffer and hydroquinone (Merck, +99%) as
inhibitor to quench the polymerization. The nonionic surfactants
(e.g., oligomeric alkyl polyglucosides) were supplied
by Akzo-Nobel. They had a very low solubility in the
organic phase while their solubility in the aqueous phase
exhibited a small temperature dependence. The adsorption
and particle stabilization properties of the surfactants
depended on the number of hydrophilic glucoside segments,
N S , and hydrophobic alkyl segments, N C (see Table 1). The
general chemical formula of a straight-chain alkyl polyglucoside
can be written as: H1[C 6 O 5 H 10 ] Ns 1O1[CH 2 ] Nc 1H.
Notice that as N S increases or N C decreases, the hydrophile to
lipophile balance (HLB), which is proportional to the (N S /
N C ) ratio, increases (Table 1).
Table 1.
Surfactant
Alkyl polyglucoside surfactant properties.
CMC
g=L
N S
Hydrophilic
segments
N C
Hydrophobic
segments
N C /N S
Ratio
HLB
S1 0.8 1.9 9.1 4.8 13.6
S2 0.08 1.9 12/14 6.3 12.1
S3 1.8 4.0 9.1 2.3 14.4
S4 1.6 8 5.0 13.3
S5 3.9 8 2.1
Polymerization Process
Emulsion copolymerization experiments were carried out in
a 500 ml jacketed glass reactor (Normschliff Gerätebau
Wertheim) equipped with a condenser and a nitrogen purge
line. Two streams consisting of the monomer and initiator
solutions were fed into the reactor using two piston pumps.
The reaction mixture was thermostated to within l0.058C
with the aid of a constant temperature bath (Julabo F32 HC)
provided with an external temperature sensor. The reactor
was equipped with a six-blade impeller. The agitation rate
was set equal to 150 rpm to provide adequate mixing and
minimize coagulum formation.
The emulsion copolymerization of VAc/BuA (80:20) was
carried out at 808C following a standard recipe. Initially, the
surfactant/buffer aqueous solution (2 wt.-% with respect to
the total mass of monomers) was introduced to the reactor
and thermostated at 808C while being purged with nitrogen
for half an hour. Subsequently, a specified amount of the
initiator dissolved in water was added to the surfactant solution.
During the following four hours, the initiator and monomer
solutions were pumped at constant rates into the reactor.
According to our standard recipe, the amount of initiator
initially charged as well as the total amount of initiator continuously
fed into the reactor were 0.25 wt.-% on total monomers
while the amount of surfactant added was 2 wt.-% on
total monomers. The monomer and initiator solutions were
also purged with nitrogen prior to the polymerization process
for about half an hour.
Instantaneous Monomer Conversion and Latex Particle Size
Measurements
Latex samples of about 1 ml were withdrawn from the reactor
in half-hour intervals and mixed immediately with a 0.02
wt.-% aqueous hydroquinone solution at a 9:1 w/w ratio and
then placed in an ice bath. The total solid-content was determined
gravimetrically using an IR-moisture analyzer (Sartorius
MA40). Thermogravimetric analysis (TGA) measurements
were also employed to verify the accuracy of the of
the IR results. The instantaneous conversion of the two
monomers at time t, defined by the ratio of the polymer mass
over the total mass of the monomers fed to the reactor up to
time t, was determined from the measurement of the total
mass of solids. The rate of polymerization was calculated
from the differentiation of the conversion versus time data.
Finally, the overall monomer conversion was defined as the
ratio of the polymer mass over the total mass of the monomers
in the standard recipe.
The latex samples were first diluted in deionized water to
remove the monomer from the swollen particles and, then,
the particle size distribution of the unswollen particles was
measured by photon correlation spectroscopy (Malvern
Autosizer Lo-C). The instrument covered a size range from
30 to 3000 nm and was equipped with a 4 MW internal laser
at 670 nm and a 64 channel digital correlator. It should be
noted that the error in the experimentally measured values of
instantaneous monomer conversion and average particle diameter
was less than l10%.
Results and Discussion
Effect of the Molecular Structure of the Nonionic
Surfactant
The molecular structure of the nonionic surfactants is
characterized by the number of repeating segments of the
hydrophilic and hydrophobic moieties (Table 1). These
surfactant characteristics influence the physical properties
such as surface coverage, surfactant volume fraction and
thickness of the adsorbed surfactant layer, which are crucial
for the latex particle stabilization.
The VAc/BuA emulsion copolymerization investigated
in this work presented certain similarities to the results of
Piirma [15] who studied the emulsion polymerization of
styrene and MMA using a series of short-chain blockcopolymers
(e.g., PS-b-PEO and PMMA-b-PEO) as stabilizers.
For the styrene emulsion polymerization, a fourfold
increase in the length of the adsorbing hydrophobic
chain segment caused severe particle destabilization,
resulting in a three-fold increase in the particle diameter.
For the MMA emulsion polymerization, it was found that
an optimum ratio of the adsorbing hydrophobic to stabilizing
hydrophilic segments existed for maximum stabilization
of particles. Particle destabilization occurred in the

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2617
Figure 1. The effect of surfactant structure on (a) the instantaneous
monomer conversion and (b) the average particle size
(surfactant concentration 1 wt.-%; initiator addition policy
0.25–0.25 wt.-%).
presence of polymeric stabilizers with a high PEO content
(e.g., a high ratio of hydrophilic/hydrophobic moieties)
due to the decrease of surface coverage. On the
other hand, particle destabilization was also observed at a
low PEO content of polymeric stabilizers and was attributed
to the insufficient amount of stabilizing ethylene
oxide segments per adsorbed surface area. [15] Furthermore,
an increase in the surfactant molecular weight
resulted in a shift in the optimum value of the hydrophilic/hydrophobic
chain length ratio and gave rise to a
slight increase of the average particle size and polymerization
rate.
Figure 1a and 1b illustrate the effect of the molecular
structure of the surfactant on the instantaneous monomer
conversion and average particle size of latexes produced
using the standard recipe. As can be seen in Figure 1a,
the polymerization initially proceeds under non-starved
conditions. After an approximate polymerization time of
90–120 min, depending on the surfactant type, the polymerization
continues at an almost maximum rate (e.g.,
the instantaneous monomer conversion is higher than
80%). It should be noted that surfactants S2 and S3
resulted in higher polymerization rates than that of surfactant
S1 (Figure 1a). The lower polymerization rate
observed for surfactant S1 was attributed to the reduced
oligomer radical entry rate through the “viscous layer” of
the adsorbed nonionic surfactants. [4] A similar behavior
has been observed for mixed surfactant systems in which
the monomer conversion was found to be lower than that
obtained with simple anionic surfactants even though the
former provided better stabilization and an increased particle
number. [4]
The effect of the surfactant molecular structure on the
latex particle size was found to be significant (Figure
1b). The final average particle size varied by more than
100 nm depending on the surfactant structure (e.g., the
values of N C and N S ). It is noteworthy to notice that surfactant
S1, despite having a shorter adsorbing chain segment,
N C , than surfactant S2 and a shorter stabilizing
chain, N S , than surfactant S3, provided the most effective
particle stabilization. Assuming that surfactants S1 and
S2 have similar surface coverages, the observed increase
in particle stabilization in the presence of surfactant S1
can be attributed to the larger number of stabilizing
chains per unit adsorbed area. On the other hand, because
of the substantially different values of the hydrophobic/
hydrophilic segment ratios for surfactants S1 and S3
(e.g., 4.5 and 2.3, respectively) the surface coverage of
surfactant S3 will be lower than that of surfactant S1,
resulting in a reduced particle stabilization ability.
The above arguments were confirmed by comparison
of the experimental data to the predictions of a comprehensive
mathematical emulsion copolymerization model
that included the effect of steric stabilization. [16] The
values of the saturated surface coverages were obtained
on the basis of best fit of theoretical stabilization model
predictions to experimental data. It was found that the
surfactant volume fraction of the adsorbed layer, f, varied
with the hydrophobic/hydrophilic segment ratio, N C /N S ,
according to the following expression:
u ¼ N C
N S
0:107 0:0111 N
C
0:135
N S
ð1Þ
Notice that for a value of (N C /N S ) ratio of 4.8, Equation
(1) exhibits a maximum value of 0.123 with respect to the
volume fraction u. Furthermore, it was found that the diffusional
radical entry rate to the particles increased with a
decrease of the volume fraction of the adsorbed layer, u. [16]
Based on the above observations, surfactant S1 will provide
the best stabilization, for it has the largest value of u
and exhibits the lowest polymerization rate due to the largest
“viscous layer” resistance to the radical entry rate.
Effect of Surfactant Concentration
Figure 2a and 2b illustrate the effect of concentration of
surfactant S1 on the instantaneous conversion and the

2618 N. Lazaridis, A. H. Alexopoulos, C. Kiparissides
Figure 4. The effect of surfactant concentration on the average
particle size: (surfactant S3; initiator addition policy: 0.25–0.25
wt.-%).
Figure 2. The effect of surfactant concentration on (a) the
instantaneous monomer conversion and (b) the average particle
size (surfactant S1; initiator addition policy: 0.25–0.25 wt.-%).
Figure 3. The effect of surfactant concentration on the average
particle size (surfactant S2; initiator addition policy: 0.25–0.25
wt.-%).
average particle size. It can be seen that as the concentration
of surfactant S1 increases from 0.03 to 1.8 wt.-% the
polymerization rate increases (Figure 2a) while the particle
stabilization is improved (Figure 2b). However, at
very high surfactant concentrations (e.g., 3.7 wt.-% on
total monomers) particle destabilization and lower polymerization
rates were experienced. Particle destabilization
at high surfactant concentrations was also observed
for surfactant S2 but to a smaller extent (Figure 3). It is
important to point out that no particle destabilization was
observed in the presence of surfactant S3 even at high
concentrations (Figure 4). The particle destabilization
observed at high concentrations of surfactant S1 can be
attributed to a depletion mechanism caused by displaced
surfactant molecules or micelles that may trigger the
nucleation of a second generation of particles, leading to
small-large particle agglomeration.
The effect of surfactant concentration on the evolution
of the particle size distribution (PSD) is illustrated in Figure
5. It can be seen that the PSD generally evolves from
a bimodal distribution to a unimodal one. The bimodal
distribution at low conversions is a manifestation of the
combined action of the micellar and homogeneous
nucleation mechanisms. When the surfactant concentration
is above the CMC (see Figure 5b and 5c) the PSD
initially exhibits a bimodal form. The first peak of the
distribution at 50–60 nm can be attributed to micellar
nucleation of particles while the second peak at 65–
75 nm can be related with the homogeneous particle
nucleation mechanism. Because of the increased stability
of latex particles formed by micellar nucleation, the first
peak of the distribution is narrower and includes smallersize
particles than the homogeneous nucleation peak. The
two peaks gradually merge together with time due to particle
coalescence. It should be pointed out that the most
stable and narrowest PSD was obtained at an intermediate
surfactant concentration (Figure 5b). On the other hand,
at large surfactant concentrations (Figure 5c) particle
destabilization occurred while the bimodal character of
the distribution was retained for longer polymerization
times, leading eventually to broader final PSD’s (Figure
5c).
Effect of Initiator Addition Policy
The effect of initiator addition policy on the instantaneous
monomer conversion and the average particle size
was also investigated. In the experiments carried out, the

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2619
Figure 6. The effect of initiator addition policy on (a) the
instantaneous monomer conversion and (b) the average particle
size (surfactant S1 concentration 2 wt.-%).
Figure 5. The effect of surfactant concentration on the particle
size distribution. Surfactant S1 concentration: (a) 0.06, (b) 0.9,
and (c) 1.8 wt.-% to monomers (initiator addition policy: 0.25–
0.25 wt.-%).
initial amount of initiator or the amount of initiator continuously
fed to the reactor was varied. Figure 6a and 6b
depict the time evolution of the instantaneous monomer
conversion and average particle size for various initiator
addition policies. Notice that the rate of polymerization
and the final monomer conversion increase as the total
amount of initiator added to the reactor (e.g., initially
charged and continuously fed) increases up to the total
recipe value of 0.5 wt.-%. An increase in the initiator
amount initially charged or/and in the initiator amount
continuously fed to the reactor above the respective
recipe values (e.g., of 0.25 wt.-% initially charged and
0.25 wt.-% continuously fed) led to severe destabilization
and reduction of the final monomer conversion.
It was observed that when the initiator amount initially
charged was increased above its recipe value of 0.25
wt.-%, destabilization occurred early in the polymerization
(Figure 6b). On the other hand, when the initiator
amount continuously fed was increased above its recipe
value, destabilization occurred later in the reaction. In all
other cases, the effect of charged and fed initiator
amounts could not be distinguished as evidenced by the
experimental results obtained with 0.12 wt.-% initially
charged and 0.13 wt.-% continuously fed initiator
amounts (see Figure 6a and 6b). The extent of particle
destabilization caused by an increase in the initiator concentration
was less significant in the presence of surfactant
S2 (Figure 7) and insignificant for surfactant S3 (Figure
8).
The time evolution of the PSD at constant surfactant
concentration and different initiator addition policies is
depicted in Figure 9. The bimodal to unimodal evolution

2620 N. Lazaridis, A. H. Alexopoulos, C. Kiparissides
Figure 7. The effect of the initial initiator concentration on the
average particle size (surfactant S2: 1 wt.-%).
Figure 8. The effect of the initial initiator concentration on the
average particle size (surfactant S3: 1 wt.-%).
of PSD which was observed with the nominal initiator
addition policy (Figure 9a), was also observed for the
case of an increased initiator addition policy (e.g., 0.45
wt.-% initially charged and 0.25 wt.-% continuously fed)
(Figure 9c) but led to a narrower PSD located at larger
particle sizes. However, when the amount of initiator
continuously fed increased from 0.25 wt.-% to 0.53
wt.-%, the PSD remained bimodal and evolved to a
multi-modal PSD located at larger particle sizes (Figure
9b).
Other investigators [4] have reported a similar reduction
in the average particle size as a result of a decrease in the
initiator concentration in the presence of anionic or/and
cationic surfactants. This behavior was attributed to the
decrease of the ionic strength of the reaction medium
with the concomitant improvement of the stability of the
primary particles. It should be noted that even with nonionic
surfactants the ionic strength could influence the
steric stabilization of the particles by changing either the
thickness of the adsorbed surfactant layer or the surfactant
surface coverage.
In order to elucidate whether or not the ionic strength
of the reaction medium was the main mechanism for the
Figure 9. The effect of initiator addition policy on the particle
size distribution (surfactant S1, 1 wt.-%; initiator addition policy
(a) 0.25–0.25 wt.-%, (b) 0.25–0.53 wt.-%, (c) 0.45–0.25
wt.-%).
observed destabilization of latex particles at high initiator
concentrations, three experiments were carried out by
varying the initiator and electrolyte concentrations but
keeping the total ionic strength of the reaction medium
constant. The ionic strength of the medium, I, was calculated
based on the concentrations of the ionic species of
initiator (1:2) (C I : Na 2 S 2 O 8 ), and those of electrolyte
(1:1) (C E : NaHCO 3 ). Accordingly, the total ionic strength
of the reaction medium will be equal to: I = 3 N C I + C E . It

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2621
Figure 10. The effect of initial initiator and electrolyte concentrations
at constant ionic strength on the average particle size
(surfactant S2: 1 wt.-%; 0.25 wt.-% initiator continuously fed).
was found that even at constant ionic strength, differences
in the average particle size were observed (Figure 10)
implying a possible interaction mechanism between
initiator and surfactant, which influences the latex particle
stabilization or/and the radical entry rate. The most
stable case was observed when using a low initiator concentration
and a high electrolyte concentration (e.g., C I =
0.013 mol/L and C E = 0.023 mol/L electrolyte). Notice
that increased initiator concentrations eventually lead to
larger particle sizes.
Effect of Monomer Addition Rate
The existence of “pseudo-steady states” in semi-batch
emulsion copolymerization has been investigated and
reported in the literature for various polymerization systems.
[5, 6, 17–19] According to these reports, there is a critical
monomer addition rate above which the monomer concentration
in the latex particles becomes saturated. Thus,
above this critical value, the polymerization rate remains
constant irrespectively of the monomer addition rate provided
that the monomer diffusion rate through the aqueous
phase is larger than the polymerization rate. On the
other hand, for monomer addition rates below the critical
addition rate, the polymerization rate will be equal to the
monomer addition rate. The existence of the critical
monomer addition rate has been experimentally tested
and confirmed with monomers having different water
solubilities such as styrene and vinyl acetate.
In the present study, the effect of monomer addition
rate was examined using surfactant S3. It was observed
that the polymerization rate was initially increased (e.g.,
in the first 30–60 min) and then reached a constant value
which was equal to the respective monomer addition rate
(Figure 11a). The small overshoot observed in the polymerization
rate at about 90 min was attributed to the
polymerization of the excess monomer accumulated in
the aqueous phase during the non-starved period (Figure
Figure 11. The effect of monomer addition rate on (a) the
polymerization rate and (b) the average particle size. Dashed
lines indicate the monomer addition rates (surfactant S3: 2
wt.-%; initiator addition policy 0.25–0.25 wt.-%).
11a). Furthermore, it was observed that an increase in the
monomer addition rate resulted in an increase in the average
particle size (Figure 11b). Notice that larger monomer
addition rates result in higher monomer concentrations
in the latex particles and, thus, in larger particle
growth rates. The above results for the polymerization
rate (Figure 11a) and the average particle size (Figure
11b) are in agreement with the results of Dimitratos et
al. [5, 6] obtained for the semi-batch emulsion copolymerization
of VAc/BuA (80:20) at 608C.
Conclusions
For all the investigated alkyl polyglucoside surfactants, it
was found that the latex particle stability and the rate of
polymerization increased with an increase in the surfactant
concentration up to the standard recipe value. However,
excessive surfactant concentrations resulted in particle
destabilization. As a result, the final average particle
size exhibited a “U-shape” behavior with respect to the
surfactant concentration. A possible cause for the
observed particle destabilization at high surfactant con-

2622 N. Lazaridis, A. H. Alexopoulos, C. Kiparissides
centrations may be attributed to bridging or/and micellar
depletion flocculation. Particle destabilization was also
observed when either the fed continuously or/and initially
charged initiator amounts were increased above the
respective recipe values. The latex particle size distributions
usually evolved from bimodal to unimodal shapes.
Furthermore, it was shown that latex particle stabilization
significantly depended on the surfactant structure.
The key factor for imparting stability was the hydrophilic/hydrophobic
chain length ratio of the APG surfactants
represented by the segment number ratio N S /N C . For small
values of the N S /N C ratio, the density of the stabilizing
chains in the adsorbed surfactant layer was insufficient to
provide stabilization while large values of N S /N C led to a
decreased surfactant surface coverage. Thus, optimum
particle stabilization could be achieved at a specific
hydrophilic/hydrophobic chain length ratio. The results of
the present investigation provide useful guidelines for
choosing the best surfactant from an homologous series
of APG’s for optimum latex particle stabilization.
Acknowledgement: The authors gratefully acknowledge the
DGXII of EU for supporting this work under the BRITE/EURAM
Project BE 95-1214.
Received: August 2, 2000
Revised: December 8, 2000
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