Polymer Latexes - American Chemical Society

Chapter 9

Production of Vinyl Acetate—Butyl Acrylate Copolymer Latexes of Narrow Particle Size Distribution Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch009

Part 2, Effect of Reaction Variables in Seeded Reactions Gerald A. Vandezande and Alfred Rudin Institute for Polymer Research, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Seeded reactions using a monomer starved feed method were used to produce a range of narrow particle size distribution (PSD) 85/15 weight percent vinyl acetate / butyl acrylate copolymer latexes at 40 and 55 percent solids. Using ammonium persulfate as the initiator and sodium dodecylbenzene sulfonate (DS-10) as surfactant, a 40 percent solids latex was produced. Seeded reactions produced narrow PSD latexes with diameters between 194 to 1370 nm. Seeded reactions were also used to produce a range of narrow PSD particle sizes at 55 percent solids with diameters from 250 to 800 nm. These latexes were produced by maintaining the seed latex at 55 percent solids throughout the reaction. Seeded reactions are difficult to carry out due to the need to use nonionic as well as anionic surfactants in the polymerization. Experiments show that a significant factor in delaying the production of second generation particles is the concentration of the seed latex. Further studies indicate that the ionic strength in the aqueous phase of the seed latex can also be an important factor in delaying the formation of second generation particles. Latexes of vinyl acetate copolymers are used in a variety of applications such as adhesives and architectural coatings. The particle size and size distribution of the latexes may affect the end use properties of the product. Narrow P S D latexes of vinyl acetate / butyl acrylate copolymers are being studied in this connection in our laboratory. Various parameters that affect the particle size and size distributions of 85/15 weight percent vinyl acetate / butyl acrylate latexes were discussed in Chapter 8, "Production of Vinyl Acetate-Butyl Acrylate Copolymer Latexes of Narrow Particle Size Distribution: Part 1, Effect of Reaction Variables". Within the constraints of the systems studied, the influences of these parameters on the particle size and size distribution were as follows. Increasing the ionic strength of the aqueous phase via additions of Na2CU3 buffer and ammonium persulfate initiator, increases the particle size and widens the particle size

0097-6156/92/D492-0134$06.00/0 © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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9. VANDEZANDE & RUDIN

Vinyl Acetate-Butyl Acrylate Copolymer Latexes 2 135

distribution of a latex. Buffers are needed to offset the acidity generated by persulfate initiators (1-8). The reactivity ratios of vinyl acetate and butyl acrylate are approximately 0.04 and 5.50, respectively (9), and therefore in batch polymerizations heterogeneous copolymers are produced, which is an undesirable effect. As a result of this, monomer starved feed methods are used to produce homogeneous latexes. Batch additions of afractionof the total pre-emulsion at the beginning of the reaction affects the latex by decreasing the size and increasing the size distribution of the particles. Larger particles may be produced via monomer starved feed seeded reactions. Seeded reactions require introduction of a preformed latex into the reactor. Monomer is slowly added concurrently with initiator so that the existing particles are increased in size by continual polymerization of monomer in the particle. This results in a narrow PSD latex and an increase in particle size. In the production of copolymers of vinyl acetate and butyl acrylate a variety of stabilizing agents are employed, the most common ones being sulfonated dodecyl phenols as ionic surfactants, and ethoxylated alkyl phenols as nonionic or steric stabilizers (8). The type of initiator affects the stability of the latex particles (10). Anionic initiators such as ammonium persulfate produce anionically ended oligomeric radicals. When these oligomers are incorporated into the polymer particle they add stability to the particle. Ionic surfactants play a large part in controlling the latex particle size while the nonionic surfactants allow for the production of vinyl acetate latexes with concentrations above 40 percent solids by providing a steric barrier to prevent particles from coalescing. A research program in our laboratory comprises a study of the effects of latex particle size and size distribution on the properties of water-based coatings. This article reports practical procedures for production of narrow PSD vinyl acrylic latexes with a range of particle sizes and concentrations similar to those of commercial polydisperse materials that are used in such applications. Experimental Latex Preparation and Analysis. The polymerizations were carried out using the apparatus described in the companion paper, "Effects of Reaction Variables in the Production of Narrow Particle Size Distribution Vinyl Acetate/Butyl Acrylate Copolymer Latexes (I)". The pH measurements of the latexes, the monomer concentration determination during the reaction and the NMR characterization of the copolymers were also discussed. Latex Particle Size and Size Distribution Measurements. The particle size and distribution of the latexes were obtained using an ICI-Joyce Loebl Disk Centrifuge as described in the companion paper. Particle sizes below 80 nm may not be reliably measured on the disc centrifuge and particles below 40 nm may not even be detected. Smaller particles can be detected however, by the visual observation of the spinning disc soon after the injection of the latex onto the disc. This is especially useful in detecting the onset of a second generation of particles in monodisperse latex systems. The method used in this work to detect second generation particles was as follows. The highest rotational speed of the disc was selected and the spin fluid, water with a methanol gradient, was injected. The latex, which had previously been diluted to approximately 1 percent using an 80/20 water/methanol mixture, was injected into the spinning disc, onto the spin fluid. After 5 minutes, when the larger particles had migrated to the outer edge of the

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

136

POLYMER LATEXES

disc leaving a clear solution, a light blue ring could be seen near the top of the fluid indicating the presence of a second of generation particles. This ring slowly migrated to the outer edge of the disc, but upon migration, the ring was diluted and thus became very faint. The analytical technique was unable to detect these particles. Using this method, the existence of a second generation of particles could be confirmed by the visual observation of the spinning disc, but the size and size distribution could not be recorded.

r


Surface Tension Measurements. Measurements of the surface tension of selected latexes at 22°C were made using a Du Nouy ring tensiometer. Narrow PSD Latex and Subsequent Seeded Reactions at 40% Solids Narrow PSD Seed Latex. In order to produce a small-particle-size narrow PSD seed latex, the following reaction was performed (Table I; Bqperiment 117). Surfactant was added to the reactor with deionized water. Tne addition of surfactant slightly increased the particle polydispersity by increasing the length of the nucleation period, but dramatically decreased the particle size. The ammonium persulfate was added to the reactor concurrently with the preemulsion, monomer dispersed in water using DS10, and Na2CC>3 was not added at all. This procedure ensured minimum coagulation during the nucleation period which therefore resulted in the formation of very small particles. Only one sixth of the pre-emulsion was added. The product particle size was not measured, since the particle size was below 80 nm. The disc centrifuge may roduce unreliable results below this particle size. The particle size of this seed itex was calculated from the measured particle size of subsequent seeded reactions. Without the addition of Na^COj the latex was not buffered, so hydrolysis would take place if the reaction had oeen taken to completion. In order to circumvent this problem the reaction was only continued until one-sixth of the pre-emulsion was added, which resulted in a 13% solid latex. At this point the reaction was allowed to post react for 1/2 hr. and the pH was measured. The pH had not dropped significantly (pH 6 at the start to pH 5.5 at the end) and thus not much hydrolysis could have taken place. A portion of this product was then used as a seed for a subsequent reaction (Taole I; Experiment 118) to increase the particle size and solids concentration. Since the particle size and PSD was determined by the seed produced in Experiment 117 NaHCOU could be added to the pre-emulsion without affecting the size or PSD of the latex in experiment 118 and thus hydrolysis could be kept to a minimum.

E

Seeded Reactions. Seeded reactions were performed as listed on Table I. After the initial seeded reaction (Experiment 118), increases in particle size were obtained by adding 200g of the previous latex to the reactor (e.g.. Experiment 119). Further reactions, Experiment 120-123 were done as Experiment 119 by using 200 g of the latex of the previous reaction as a seed to produce larger particles. The seeded reactions contained Na2C03 in the pre-emulsion to maintain the pH above 5. A low polydispersity was retained in the products of these seeded reactions (Table III). The initiator was fed concurrently with the monomer to eliminate the possibility of coagulation of the latex by large additions of ionic initiator. It is very difficult to predict the effects of initiator concentration (11,12), and consequently, also the rate of polymerization, in the reactor, especially when the initiator concentration is increased with time. A simple way to ensure a low monomer content in the reactor throughout the reaction time is to vary the




Table I. Final latex and subsequent seeded reactions EXPERIMENT #

117

118

119


REACTOR CHARGE 168.08

200.00

37.0*

200.0**

180.0 32.0 0.225 71.0

176.4 31.1 0.400 74.0 0.46

180.0 31.9 0.500 74.0 0.46

20.0 0.50

12.5 0.41

12.5 0.41

Deionized Water (g) 225.00 Sodium dodecyl benzene sulfonate (DSlO-Alcolac Chemical) (g) 0.225 Seed Latex (g) MONOMER FEED Vinyl Acetate (e) Butyl Acrylate (g) DS10(g) Deionized Water (g) N a H C 0 (g) 3

INITIATOR FEED Deionized Water (e) Ammonium persulfate (g) * **

From Experiment 117 From Experiment 118

emulsion feed rate and keep the initiator feed rate constant so as to keep the monomer concentration in the reactor below 0.5 weight percent. The feed policy for the pre-emulsion which was finally obtained was a step increase of monomer flow rate from 0.6 to 3.6 ml/min over 3 hrs. The initiator was added concurrently with the pre-emulsion at a constant rate over 3 hrs. This feed policy gave monomer concentrations of no more than 0.2 weight percent during the reaction and essentially a monomer starved feed reaction was obtained. Confirmation that the reaction was monomer starved was obtained from NMR analyses to determine the vinyl acetate centered triad fractions. The reaction was essentially monomer starved for the seeded reactions as can be seenfromthe NMR results in Table II. The theoretical monomer starved feed and batch values for vinyl acetate centered triadfractions,calculated by Pichot (10) for the mole ratio of monomers used in this study, are also given. Deviations may be expected from the feed comonomer composition, since at the beginning of the reaction the polymer formed is richer in butyl acrylate as a result of the differences in reactivity ratios. The monomer is fed into the reactor at 85/15 weight percent, but the actual vinyl acetate concentration in the reactor is very much greater than this, as confirmed by gas chromatography. During the time the monomer composition comes to equilibrium, the monomer composition is expected to deviate from the theoretical composition. Experiment 120 in particular resulted in significant deviations. This may be due to the fact the number of NMR scans to obtain the spectra was only 2/3 of the others. This gave a poorer resolution and thus an inaccurate portrayal of the copolymer composition.


138

POLYMER LATEXES

Table II. Microstructure of vinyl acetate / butyl acrylate copolymer latexes produced by monomer starve feed seeded reactions where A = vinyl acetate and B = butyl acrylate in vinyl acetate centered triad fractionsfromNMR analysis


Triad theoretical monomer starve fed theoretical batch 118 119 120 121 122 123

AAA

BAA

BAB

0.78 0.78 0.79 0.78 0.86 0.82 0.82 0.81

0.20 0.13 0.18 0.21 0.12 0.16 0.16 0.17

0.01 0.08 0.03 0.01 0.00 0.02 0.02 0.02

Table III. Particle size and size distributions of seeded reactions with 0.5g DS10 in the pre-emulsion

Latex

117 118 119 120 121 122 123

Size (nm.)

Poly dispersity

54 194 298 452 676 1011 1328

1.034 1.029 1.028 1.02 bimodal bimodal

Standard Coefficient Surface Deviation of Variation Tension _ (dyne/cm^) (nm) (%)

20.5 29.2 42.5 51.8

10.6 9.8 9.4 7.7

42.5 38.5 36.0 35.2 35.2 35.0

Successive seeded reactions produced larger narrow PSD particles. The size and size distributions are listed in Table III. In the reaction to produce the 1011 nm latex in Experiment 122, a second generation of particles was obtained. This was probably a result of excessfreesurfactant and oligomers in the aqueous phase (13). In a subsequent set of seeded reactions the surfactant charge to the preemulsion was reduced to 0.07g in the pre-emulsion (experimental procedure as Experiments 118 to 122). This was done in order to delay the onset of the second generation of particles by reducing the amount of surfactant fed to the reactor and thus reduce the coverage of the particles by surfactant (14). The results are reported in Table IV. The onset of second generation particle formation was delayed although only enough to produce larger narrow PSD particles to 1050 nm. The initiator plays a significant role in the production of surface active agents and thus decreasing the surfactant can only play a limited




Table IV. Particle size and size distributions of seeded reactions with 0.07g DS10 in the pre-emulsion


Latex

117 124 125 126 127 128 129

Size (nm.)

54 206 305 454 690 1050 1489

Poly dispersity

1.035 1.033 1.029 1.025 1.010 bimodal

Standard Deviation (nm)

Coefficient Surface of Variation Tension (%) (dyne/cm ) 2

10.5 10.0 9.6 8.9 5.9

21.7 30.6 43.5 61.4 61.6 —

49.0 47.0 45.0 43.0 42.5 45.0

role in the delay of second generation particle formation. It is interesting to note that the surface tension of the latexes significantly increased when a second generation of particles was formed in the recipe with low surfactant concentration, but did not change noticeably when a second generation of particles were formed with higher surfactant concentrations. Finally, seeded reactions were performed without diluting the latex with water after the initial seeded reaction. Thus, by keeping the pre-emulsion at 40 percent by weight monomer, the latex was kept at 40 percent solids throughout the reaction. This resulted in the formation of monodisperse particles from 200 to 1368 nm, presumably due to a higher N a H C 0 and initiator ion concentration as compared to the diluted latex. This concept is more fully explored in the next sections. 3

Results and Discussion of 55 Percent Solids Latex Production of 55 Percent Solids Narrow PSD Latex. Since industrial processes produce vinyl acetate / butyl acrylate latexes at 55 percent solids, attempts were made to produce monodisperse latexes at 55 percent solids. Initial attempts to increase the solids of the latex by increasing the sodium dodecylbenzene sulfonate (DS10) and monomer concentration failed. Latexes produced with only DS10 as surfactant did not produce stable latexes at concentrations greater than 44 percent solids. The use of Rexol 25/407 (Hart Chemical; 70 weight percent solids in water; a nonyl phenol ethoxylated surfactant with an average of 40 moles of ethylene oxide), a surfactant that acts as a steric stabilizer, allowed for the production of latexes at 55 percent solids (25). Initially a latex was made by the following method (Table V; Experiment 130). Deionized water and DS10 were charged to the reactor before the addition of any other ingredients. A pre-emulsion of monomer, water, and surfactant (DS10 and Rexol 25/407) was added slowly to the reactor over a period of 4.0 hours. The pre-emulsion feed rate was slowly increased stepwise from 0.6 ml/min to 4.8 ml/min over 4 hours so as to keep the monomer concentration in the reactor very low at the beginning of the reaction. The initiator was added concurrently with the pre-emulsion at a constant rate over 4 hours. The latex produced had a polydispersity of 1.09.


140

POLYMER LATEXES Table V. Seeded reactions of 55 percent latexes

EXPERIMENT #

130

131

125.0 0.24

125.0 0.24 100.0

316.0 56.0 0.58 150.8 15.0 0.714

Deionized Water (g) 23.6 Ammonium persulfate (g) g) 0.714

132

133

134

115.0 0.24 200.0

115.0 0.0 100.0

115.0 0.0 100.0

316.0 56.0 0.58 150.8 15.0 0.714

316.0 56.0 0.58 150.8 15.0 0.714

316.0 316.0 56.0 56.0 0.24 0.38 150.8 150.8 4.4 8.8 0.714 0.714

23.6 0.714

23.6 0.714

23.6 0.714

bimodal

bimodal

bimodal bimodal


REACTOR CHARGE Deionized Water (g) DS10(g) Seed Latex (g) (Exp. 130) MONOMER FEED Vinyl Acetate (c) Butyl Acrylate &> Deionized Water (g) Rexol 25/407 (g) NaHC0 (g) 3

INITIATOR FEED 23.6 0.714

SIZE AND DISTRIBUTION D (nm.) 183 Polydispersity 1.09 Standard Deviation (nm) 30.7 Coef. of Variation (%) 16.8 n

Seeded Reactions. Seeded reactions at 55 percent solids are difficult to carry out since a large amount of nonionic surfactant is needed to stabilize the latex. The nonionic surfactant provides a steric barrier to the migration of oligomers to the surface of the particle compared to the anionic surfactants. This is due to the high viscosity at the surface imparted by the nonionic surfactants (16). The initial latex produced at 55 percent gave a unimodal distribution (Table V; Experiment 130). A subsequent reaction, with this latex as the seed latex, produced a bimodal distribution (Experiment 131). A series of experiments was done in order to determine a method to delay the formation of a second generation of articles, but in each case bimodality was discovered very early in the reaction, irst, the seed latex charge was increased (Experiment 132). This was done so that the surface area of the seed latex was increased and thus adsorption of oligomers would increase. It was theorized that this would in turn delay the formation of a second generation of particles. A bimodal distribution of particles was obtained. Secondly, the DS10 charge to the reactor was eliminated and the DS10 and Rexol 25/407 concentrations in the pre-emulsion were reduced (Experiment 133). This was done in order to decrease the stability of

P




Table VL Seeded reactions with seed at 55 percent and monomer feed at 55 percent

EXPERIMENT*

135

136

137

116.0 37.0*

100**

100***

360.5 63.5 0.82 173.0 8.50 0.85

360.5 63.5 0.82 321.0 8.5 0.85

360.5 63.5 0.82 321.0 8.5 0.85

25.0 0.81

25.0 0.81

25.0 0.81

253 1.023 22.3 8.8

535 1.025 47.0 8.8

800 1.019 61.3 7.7


REACTOR CHARGE Deionized Water (g) Seed Latex* (g) MONOMER FEED Vinyl Acetate (c) Butyl Acrylate (g)

DSto(g) Deionized Water (g) Rexol 25/407 (g) NaHC0 (g) 3

INITIATOR FEED Deionized Water (g) Ammonium persulfate (g) SIZE AND DISTRIBUTION Polydispersity Standard Deviation (nm) Coef. of Variation (%) * From Experiment 117; Table I ** From Experiment 135 *** From Experiment 136 the precipitating oligomers, forcing them to coalesce with existing particles. This also produced a bimodal distribution. Thirdly, the DS10 and Rexol 25/407 concentrations were further reduced (Experiment 134). The resulting latex was bimodal and a small amount of coagulum was obtained. Further decreases in surfactant concentration produced a very unstable latex which resulted in massive coagulation. This is expected when the surfactant used is reduced significantly (14). In order to create a more narrow PSD latex and to increase the final particle size, another series of reactions, using less Rexol 25/407 and a latex from Experiment 117 Table I as a seed, were performed. The latex produced in this manner had a polydispersity of 1.023 (Table VI; Experiment 135). It is interesting to note that for the reactions containing 15 grams Rexol 25/407 a second generation of particles was not detected at the end of the first seeded reaction but they were detected almost immediately after the start of the



142

POLYMER LATEXES

second seeded reaction. The production of second generation particles in the second seeded reactions may have been due to dilution of the latex at the start of the reaction. To test the hypothesis that dilution of a latex may be the cause of a second generation of particles, a latex was made without diluting the seed latex, which was at 55% solids, and using a pre-emulsion at 55 percent weight monomer (Table VI Experiment 13? and 137). As a result the latex in the reactor remained at 55 percent throughout the reaction and a narrow PSD product was obtained with a diameter of 535 nm. Another seeded reaction was performed with the 535 nm latex as a seed. Samples were drawn from the reactor at 1/2 hr. intervals and the particle size was analyzed on the disk centrifuge in order to detect the onset of second generation particle formation. Second generation particles were detected when the seed latex reached a diameter of 800 nm. This method of seeding allowed a seeded reaction to produce a monodisperse latex with particle sizes from 250 nm up to 800 nm at 55 percent solids, without changing the surfactant concentration. The surface area for the seed latexes in experiments 134 (Table V) and 137 (Table VI) were calculated, to determine if the total surface area is responsible for the delay in the formation of second generation of particles when seeded reactions are performed with seed latexes at 55 percent solids (Experiment 137) as opposed to diluted seed latexes (Experiment 134). It is expected that an increase in the surface area would decrease the rate of addition of surfactant per unit surface area and thus facilitate the adsorption of oligomers on the surface of the existing latex particles delaying the production of second generation particles. The seed latex in experiment 134 had a surface area three times that of the seed latex in experiment 137 at the start of the reaction. Since the surfactant concentration in the latex of experiment 134 is only double that in experiment 137, the surface of the latex in experiment 134 is more sparsely covered. Even with half the amount of surfactant in the pre-emulsion feed a second generation of particles was formed immediately in experiment 134, while in experiment 137 no bimodality occurred until a particle size of 800 nm was reached. It appears therefore that the available surface area is not the overriding factor in the delay of production of a second generation of particles in this series of experiments. It is also probable that the interparticle distance may be a factor in producing a second generation of particles. Poehlein (17) gives the following equation for interparticle distance: dj = [.707((100x2* ) / ( 3 x V ) ) l / 3 - l ] d p

(1)

where dj is the interparticle distance, dp is the particle diameter and V is the volume percent of particles in the latex or: dj = [(( 100 x 2l/2 *

P

p

) / ( 6 x W)) 1/3 -1 ] dp

(2)

where W is the weight percent polymer in the latex and p is the polymer density. Equation 2 was used to calculate the interparticle distance of the two seed latexes for the experiments 137 and 134. The seed latex particle sizes were 535 and 183 nm, respectively. The 535 nm latex was not diluted and had a interparticle distance of 87.5 nm while the 183 nm latex was diluted with 115 grams of deionized water and after dilution had an interparticle distance of 91.8 nm. It is expected that similar interparticle distances would facilitate similar rates of capture of oligomers. D F





Clearly it is not just the interparticle distance that is the main influence in the delay of formation of second generation of particles, since the smaller diluted latex has a similar interparticle distance when compared to the larger undiluted latex, yet a second generation of particles were formed in the diluted latex. It was expected that similar distances between particles would maintain similar rates of collapse of oligomers onto existing particles, and thus delay second generation particle formation. The trend does not appears to be the case here. The only plausible explanation for the delay in formation of a second generation of particles is that the ionic strength in the aqueous phase is a factor in the delay in second generation particle formation. Hansen (18) expresses similar ideas when he states that the difftision rate of oligomers onto polymer particles is reduced by an electrostatic repulsion factor and new particles are formed when repulsive factors are high. The electrostatic repulsive factor is reduced under higher ionic strengths and therefore the formation of second generation particles is delayed. As has been noted earlier in the companion aper "Effects of Reaction Variables in the Production of Narrow Particle Size Hstribution Vinyl Acetate/Butyl Acrylate Copolymer Latexes (I) , increasing the ionic strength in the aqueous phase decreases the stability of the primary particles. Dilution of the latex decreases the ionic strength and thus allows for a more stable environment for the ionic ended oligomers and surfactant to form primary particles. This decreases the rate of collapse of oligomers onto existing particles and a second generation of particles can form.

E

11

Conclusions Seeded reactions using a monomer starved feed method may be used to produce a range of narrow PSD vinyl acetate / butyl acrylate copolymer latexes at 40 and 55 percent solids. These reactions are affected by surfactant concentrations. In reactions with ammonium persulfate initiator and sodium dodecylbenzene sulfonate surfactant to produce a 40 percent solids latex, the anionic emulsifier has only a limiting role m the delay of second generation of particle in seeded reactions. The charged polymeric end groups produced by the ionic initiator appears to have a major role. Seeded reactions can produce narrow PSD latexes at 40 percent solids from 194 to at least 1368 nm. Seeded reactions to produce a range of monodisperse particle sizes at 55 percent solids are difficult to carry out because of the need to use nonionic surfactants as well as anionic surfactants in the polymerization. Experiments show that a significant factor in delaying second generation particles is the concentration of the seed latex. Further studies indicate that the ionic strength in the aqueous phase of the seed latex can be an important factor in delaying second generation particles. Narrow PSD latexes were produced with diameters of 250 to 800 nm without changing the surfactant concentration by maintaining the latex concentration at 55 percent solids throughout the reaction. Acknowledgments This research was supported by the Natural Sciences and Engineering Research Council of Canada. Literature Cited 1 2

Lichti, G.; Gilbert, R.G.; Napper, D.H. J. Polym Sci., Polym. Chem. Ed. 1977, 15, 1957. Dunn, A.S.; Chong, L.C. Br. Polym. J. 1970, 2, 49.


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Okubo, M.; Mori, T. Colloid and Polymer Science 1988, 266, 333. Kolthoff, I.M.; Miller, I.K. J. Am. Chem Soc. 1951, 73, 3055 El-Aasser, M.S.; Makgawinata, T.; Misra, S. In Emulsion Polymerization of Vinyl Acetate, El-Aasser, M.S., Ed; Vanderhoff, J.W., Ed. Applied Science Publishers, New Jersey, 1981, p. 240 6 Litt, M.H.; Chang, K.H.S. In Emulsion Polymerization of Vinyl Acetate, El-Aasser, M.S., Ed.; Vanderhoff, J.W. Applied Science Publishers, New Jersey, 1981, p. 151 7 Goodall, A.R.; Hearn, J.; Wilkinson, M.S. Br. Polym. J. 1978, 10, 141. 8 Nguyen, B.D. Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, 1986. 9 Pichot, C.; Llauro, M.; Pham, Q. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 2619. 10 Fitch, R.M.; Tsai, C. J. Polym. Sci., Polym. Lett. Ed. 1970, 8, 703. 11 Chatterjee, S.P.; Banerjee, M. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 1517. 12 Okubo, M.; Mori, T. ColloidPolym.Sci.1988, 266, 333. 13 Kiparissides, C.; MacGregeor, J.F.; Hamielec, A.E. J. Appl. Polym. Sci. 1979, 23, 401. 14 Sudol, E.D.; El-Aasser, M.S.; Vanderhoff, J.W. J. Polym. Sci., Polym. Chem. Ed. 1986, 24, 3499. 15 Woods, M.E.; Dodge, J.S.; Krieger, I.M.; Pierce, P.E. J. Paint Tech. 1968, 40, 541. 16 Blackley, D.C.; Emulsion Polymerization, Applied Science Publishers, London, 1975. 17 Poehlein, G. Advances in Emulsion Polymerization and Latex Technology, 5th Annual Short Course, 1974. 18 Hansen, F.K.; Ugelstad, J. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 3033. Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch009

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