Relative Importance of the Effects of Seed and Feed Stage ...

POLYMER REACTION ENGINEERING 

Vol. 11, No. 3, pp. 359–378, 2003 

Relative Importance of the Effects of Seed and Feed 

Stage Agitations on Latex Properties in Semibatch 

Emulsion Copolymerization of n-Butyl 

Methacrylate and N-Methylol Acrylamide 

S. Krishnan, # A. Klein, * M. S. El-Aasser, and E. D. Sudol 

Emulsion Polymers Institute and Department of Chemical Engineering, 

Lehigh University, Bethlehem, Pennsylvania, USA 

ABSTRACT 

The effects of agitation in a ca. 24% solids semibatch emulsion 

copolymerization of n-butyl methacrylate and N-methylol acrylamide in 

a2dm 3 reactor are reported. A Rushton turbine with 8 cm tip-to-tip 

diameter was used as the agitator. The agitation speeds during the seed 

and feed stages of the semibatch process were varied at two levels. The 

final latexes obtained from the four experiments were characterized for 

the size of the polymer particles, viscosity, amount of water-soluble 

polymer, and the amount of coagulum at the end of the reaction. A 

# Current address: S. Krishnan, Materials Science and Engineering Department, 

Cornell University, Ithaca, New York, USA. 

*Correspondence: A. Klein, Emulsion Polymers Institute and Department of 

Chemical Engineering, Lehigh University, 111 Research Drive, EPI, Iacocca Hall, 

D325, Bethlehem, PA 18015, USA; E-mail: ak04@ lehigh.edu. 

359 

DOI: 10.1081/PRE-120024419 

Copyright D 2003 by Marcel Dekker, Inc. 

1054-3414 (Print); 1532-2408 (Online) 

www.dekker.com

360 Krishnan et al. 

higher agitation speed nucleated a greater number of polymer particles 

during the in situ seed formation step (seed stage). In the absence of any 

secondary nucleation during the monomer-feeding stage, the final latexes 

had a higher number of particles when the agitation speed during the 

seed stage was higher. The amount of coagulum increased with an 

increase in the agitation power-input. The amount of water-soluble 

polymer was influenced mainly by the agitation during the seed stage of 

the process, through the effect of the latter on the number of polymer 

particles. However, the pooling of the BMA monomer during the feed 

stage, because of poor mixing and shear in the reactor, resulted in an 

increased water-soluble polymer formation. Latexes prepared using a 

higher agitation speed during the seed stage had a higher viscosity. 

Key Words: Agitation; Seeded emulsion polymerization; Semi-batch 

emulsion polymerization. 

INTRODUCTION 

The objective of this work is to investigate the effects of seed stage and 

feed stage agitation in a semibatch emulsion copolymerization of n-butyl 

methacrylate and N-methylol acrylamide. As seen in the previous paper 

(Krishnan et al., 2003), agitation affects the amount of water-soluble 

polymer in the final latex. Experiments using psuedoplastic latexes with 

relatively high solids content showed that the incorporation of the NMA 

monomer in the polymer particles increased with an increase in the 

agitation power. This could be attributed to the combined effect of two 

factors: first, the effect of agitation (bulk mixing and shear rate) during the 

feed stage, and second, the effect of agitation on particle nucleation during 

the seed stage. When the latex viscosity was high, mixing showed an effect 

on the water-soluble polymer formation even using a 2 dm 3 scale reactor. 

In this paper, we describe the effects of agitation on the properties of a 

24% solids latex with a lower viscosity and a Newtonian viscosity behavior. 

The relative effects of seed stage and feed stage agitations were determined 

using a simple 22 factorial design of experiments (Neuman, 1997). 

MATERIALS 

n-Butyl methacrylate inhibited by 10 ppm monomethyl ether of 

hydroquinone (MEHQ), N-methylol acrylamide (48 wt% solution in water) 

inhibited with 30 ppm of MEHQ, and potassium persulfate (ACS reagent) 

were obtained from Sigma-Aldrich. Sodium lauryl sulfate (Ultrapure

Seed and Feed Stage Agitations of BMA and NMA 361 

Bioreagent) was obtained from J. T. Baker, and sodium bicarbonate, from 

Mallinckrodt Baker. Nitrogen gas (Zero Grade 0.5, minimum purity 

99.998%, oxygen < 0.5 ppm) was obtained from Airgas. Deionized water 

was used for preparing the emulsions. The BMA monomer was passed 

through an inhibitor-removal column (Sigma-Aldrich) before use, and all 

the other chemicals were used as received. 

PROCEDURES 

Latex Synthesis 

The details of the reactor and the experimental setup were given in the 

previous paper (Krishnan et al., 2003). A monomer-starved semibatch 

process with in situ seed formation was developed for a ‘‘low-solids’’ 

copolymer latex of BMA and NMA. The recipe is given in Table 1. 

The initial reactor charge consisted of BMA, and a solution of SDS 

and NaHCO 3 in ca. 930 g of DI water. The emulsion was heated to 70°C (in 

ca. 15 min) under the seed stage agitation speed. Nitrogen was bubbled 

through the emulsion during the heating and emulsification steps, which 

lasted for ca. 30 min. 20 g of the initiator solution (the recipe amount of KPS 

in DI water) was injected near the agitator using a stainless steel needle. 

Nitrogen was bubbled into the emulsion throughout the seed stage, and the 

nitrogen tube was raised above the liquid level in the reactor during the feed 

stage. The seed stage and feed stage durations were 30 min and 90 min, 

respectively. A 30 min post-feeding time was allowed, after which, ca. 1 mL 

of 1% aqueous hydroquinone solution was injected into the reactor. The 8 cm 

diameter agitator was used in all the experiments. The agitation during the 

Table 1. 

Recipe for emulsion copolymerization of BMA and NMA. 

Seed stage (70°C, 30 min) 

Feed stage (70°C, 90 min) b 

Ingredient Amount (g) Ingredient Amount (g) 

DI water 950.00 DI water 26.0 

BMA 50.00 BMA 241.4 (450 cm 3 ) 

SLS 1.104 (4 mM) a NMA 24.0 

KPS 

0.769 (3 mM) a 

NaHCO 3 

0.736 (9.2 mM) a 

a Concentration in mmol per dm 3 water. 

b Final amount of polymer is ca. 24.4 wt% of latex.


Table 2. 22 Factorial design of experiments to study the effects of seed stage and feed stage rotational speeds. 

Expt. 

Seed stage 

rpm* 

Feed stage 

rpm* 

Coagulum 

(pphm) 

Dv 

(nm) 

Water-soluble 

species in final 

latex (g) 

Viscosity** 

(cP) 

1 150 ( 1) 150 ( 1) 0.0248 194.2 17.48 6.88 

2 400 (+1) 150 ( 1) 0.0337 168.7 17.96 7.24 

3 150 ( 1) 400 (+1) 0.1095 197.6 16.66 6.91 

4 400 (+1) 400 (+1) 0.1662 170.7 16.36 7.20 

*The numbers in the parentheses are the encoded levels. 

**At a shear-rate of 461 s 1 .


seed stages and feed stages (‘‘factor’’) was varied by varying the speed of the 

agitator at two levels. When the values of x factors are varied at n levels, a 

total of n x different experiments can be performed. Thus, the latexes were 

prepared using all the 2 2 possible permutations of agitation speeds. No 

monomer pooling was observed at any stage of the reaction, when the 8 cm 

Rushton turbine was used. After their synthesis, the latexes were characterized 

for the ‘‘responses’’, viz., the amount of coagulum, the particle 

diameter, the amount of water-soluble polymer, and the viscosity. The agitation 

conditions and the results are shown in Table 2. 

Determination of Coagulum Amount 

For the determination of the amount of coagulum (aggregated polymer 

particles), the polymer on the reactor inserts were carefully scraped, 

collected, washed using a sieve with 53 mm pore size, dried in an oven at 

75°C, and weighed. The latex was also filtered through the sieve, but the 

amount of coagulum in the latex was negligible. 

Characterization of Serum-Polymer 

Ultracentrifugation (Beckman, L8-55M ultracentrifuge) was used to 

separate the aqueous phase of the latex, commonly called the serum, 

from the polymer particles. It was found that ultracentrifugation of the 

latex diluted with an equal mass of water resulted in a good separation 

of the serum and the particles. Centrifugation was carried out at 

37000 rpm and 4°C, for 6 h. Let x be the mass of a sample of the 

original latex, w, the mass of water used for dilution, and y, the mass 

fraction of solids in the aqueous phase of the diluted latex. The mass 

fraction of solids in the serum of the original latex, f ws , was calculated 

using Eq. 1: 

f ws ¼ 

 

y 

1 y 

 

f w þ w 

x 

ð1Þ 

where f w is the mass fraction of water in the original latex. If f a is 

the mass-fraction of non-polymeric solids in the serum (SDS, NaHCO 3 , 

KPS, and unreacted NMA), the mass of water-soluble polymer in the 

serum, M wsp (based on the total mass, M tot , of the recipe) is given by: 

M wsp ¼ M tot ðf ws f a Þ ð2Þ


Figure 1. 

1 H-NMR spectrum of water-soluble species in the latex. 

1 H-NMR spectra of the solids in the serum showed that there are 

negligible amounts of SDS and unreacted NMA in the serum of the final 

latex. Figure 1 shows the spectrum for the water-soluble species in the 

serum of the latex obtained from Run 4 (cf. Table 2). The serum was 

separated from the latex by ultracentrifugation. H 2 O was evaporated (at 

room temperature) from the serum by bubbling nitrogen into a glass vial 

containing the serum. The solids remaining in the vial were then 

redissolved in D 2 O. On comparison of this spectrum with that reported in 

the previous paper (Krishnan et al., 2003), it is observed that the peaks 

corresponding to SDS are less pronounced in the former. This is expected 

because of the lower concentration of SDS in the latexes prepared using 

the recipe in Table 1, than that in the latexes of the previous paper 

(Krishnan et al., 2003). 

For the comparison of the effect of agitation on water-soluble polymer 

formation, we use the values of the total amount of water-soluble species, 

M ws obtained using Eq. 3. All other species in the serum (SDS, KPS, 

NaHCO 3 , any unreacted NMA) are expected to be in the same amounts in 

the serum for the different latexes, since the dilution of the latexes (for 

ultracentrifugation) was the same. The dependence of the amount of SDS in 

the serum, on the total surface area of the particles in the latex, is not 

expected to alter the results significantly. 

M ws ¼ M tot f ws ð3Þ


Determination of Particle Size Distribution and 

Viscosity of Latex 

The particle diameters were measured using capillary hydrodynamic 

fractionation (CHDF 1100, Matec Applied Sciences). The latex viscosities 

(at room temperature) were measured using a Bohlin rheometer, and the 

cup and cylinder arrangement. 

Data Analysis 

The objective is to develop equations that relate the responses and the 

factors. For first order effects, the equation is of the form: 

y ¼ h þ m s x s þ m f x f 

ð4Þ 

where y is the response, h is the numerical average of the four responses, 

m s is the first order effect of independent variable (factor) S, m f is the first 

order effect of independent variable F, x s and x f are encoded levels of 

variables S and F, respectively. S is the rotational speed of the agitator in 

the seed stage, and F is the speed during the feed stage. To simplify the 

analysis, the independent variable levels are encoded as follows: 

High level = +1 

Low level = 1 

Thus, an agitator speed of 200 rpm corresponds to the encoded level of 

(200–275)/(275–150) = 0.6 and an agitator speed of 300 rpm corresponds 

to the encoded level of (300–275)/(400–275) = 0.2, where 275 rpm is the 

mid level. The encoded levels of agitation speeds are enclosed in the 

brackets in Table 2. 

If y 1 , y 2 , y 3 , and y 4 are the outputs (responses) of the variable y from 

the four runs, the first order effects can be calculated using Eqs. 5–7. 

S LO ¼ y 1 þ y 3 

2 

S HI ¼ y 2 þ y 4 

2 

m s ¼ S HI S LO 

2 

ð5Þ


F LO ¼ y 1 þ y 2 

2 

F HI ¼ y 3 þ y 4 

2 

m f ¼ F HI F LO 

2 

h ¼ y 1 þ y 2 þ y 3 þ y 4 

4 

The experimental error is calculated as follows: 

E LO ¼ y 2 þ y 3 

2 

E HI ¼ y 1 þ y 4 

2 

m e ¼ E HI E LO 

2 

error ¼ absðm e Þ 

ð6Þ 

ð7Þ 

ð8Þ 

The calculation of the first order effect involves all four experimental 

data. The averaging reduces the influence of an error in any datum. The 

coefficients m s and m f can be directly compared to determine the relative 

importance of each independent variable. For a measured effect to be 

significant, it has to be greater than the experimental error. The error also 

includes the deviation of the experimental effect from the simple model 

assumed (Eq. 4). 

RESULTS 

Effect of Agitation on Coagulum Formation 

Shear-induced coagulation in latexes has been studied by several 

researchers (Ali and Zollars, 1987, 1988; Husband and Adams, 1992; 

Kusters et al., 1997; Lowry et al., 1984, 1986; Matejicek et al., 1988; 

Utracki, 1973; Zollars and Ali, 1986). Smoluchowski’s expression for 

orthokinetic (shear-induced) flocculation is normally used to describe the 

rate of shear-induced aggregation. The collision frequency in the absence of 

interparticle forces, for uniform particles approaching each other along 

rectilinear paths, was predicted by Smoluchowski to be (16/3)N 2 p a3 g where 

N p is the number of particles per unit volume, a is the particle radius, and g 

is the shear rate (von Smoluchowski, 1917). If each collision results in


aggregation, this also gives the rate of shear-induced aggregation. However, 

the surfactant molecules on the surface of the latex particles provide 

stability to the particles, by electrostatic or steric repulsion, and only those 

collisions with force sufficient to overcome the repulsive force barrier will 

be effective. Husband and Adams (1992) have studied the orthokinetic 

flocculation of carboxylated latex particles and have found that a certain 

minimum shear rate was required to initiate aggregation. The minimum 

shear rate could be predicted by equating the electrostatic repulsive force 

between a pair of particles and the hydrodynamic shear force opposing it. 

Thus, in the presence of repulsive interparticle forces, the rate of decrease 

in the number concentration of particles is given by: 

dN p 

dt 

¼ 

16 3 N2 p a3 g 

W 

where W is a stability factor. When the colliding particles coalesce and 

form spherical aggregates, Eq. 10 can be expressed in a pseudo-first order 

form (Koh et al., 1984): 

dN p 

dt 

¼ 

4f gN p 

pW 

ð9Þ 

ð10Þ 

where f=4pa 3 N p /3 is the particle volume fraction, which remains constant 

during a batch flocculation process. Integration of Eq. 10 with respect to 

time gives 

 

ln 

N p 

N p;0 

 

¼ 

4f g 

pW t 

ð11Þ 

where N p,0 is the number of particles at t =0. If c is the fraction of particles 

coagulated at time t, then 

or, 

lnð1 cÞ ¼ 4f g 

pW t 

ð12Þ 

c 4f g 

pW t 

ð13Þ 

for c less than ca. 10%. Thus, the amount of coagulum is expected to be 

proportional to f, the volume fraction of the particles in the suspension, g, 

 

the shear rate, the time t for which the latex is sheared, and inversely 

proportional to the stability factor, W. 

Based on the assumptions of homogeneous isotropic turbulent flow in 

a stirred vessel, a frequent assumption is that the average shear rate, g 

 

avg ,


is proportional to the square root of the power input per unit volume (Koh 

et al., 1984; Kusters et al., 1997; Lowry et al., 1984, 1986): 

 

g avg ¼ 

 

2P 0:5 

ð14Þ 

15mV R 

where P is the power-input, V R is the volume of the liquid in the reactor, 

and m, the viscosity of the fluid. For a 40, 45, and 50% solids batch 

emulsion copolymerizations of vinyl chloride and ethyl acrylate in a 2 

gallon reactor, Lowry et al. (1984, 1986) found that the amounts of 

coagulum at the end of the polymerization could be correlated to the 

agitation power-inputs using Eqs. 12 and 14. Similarly, for semibatch 

emulsion polymerization of BMA and its copolymerization with NMA, 

Dave (1998) found that the mass of coagulum was proportional to P 0.5 ,as 

expected from Eqs. 13 and 14. 

Some experimental results that do not conform to Eqs. 9 or 13 must 

also be pointed out. Eq. 13 indicates that amount of coagulum is directly 

proportional to the time to which the suspension is subjected to the shear 

rate g. However, using viscosity measurements, Utracki (1973) has found 

that the high rate macrocoagulation process was preceded by an induction 

period called the coagulation time, t c , during which the coagulation energy 

necessary to overcome the threshold energy of coagulation, accumulated in 

the system. Microcoagulation (formation of doublets and triplets) occurred 

during the induction period. Instead of the exponential decrease of the 

particle number with time as suggested by Eq. 11, Utracki proposed that the 

number of particles decreased linearly with time during the induction 

period. His theoretical equation relating t c to the shear rate, g, and volume 

fraction of dispersed phase, f, was confirmed by the experimental data for 

poly(vinyl chloride) and poly(vinyl acetate) latexes. Also, Zollars and Ali 

(1987) found that when the repulsive forces (arising from adsorbed 

surfactant, or chemically bound surface groups) between the latex particles 

were high, the shear coagulation rate decreased with an increase in the 

particle volume fraction, f, in contrast to Eq. 10. This unusual result was 

attributed to the decrease in the collisional efficiency with increasing 

particle volume fraction. However, when the latex particles possessed only 

weak repulsive forces, the coagulation rate increased linearly with f, as 

expected. Their work involved very low particle volume fractions, ranging 

from 4.2910 5 to 6.8610 4 . Matejicek et al. (1988) have reported that 

during a 52% solids semibatch emulsion terpolymerization of styrene, n- 

butyl acrylate, and acrylic acid, in a 25 dm 3 stirred reactor, the amount of 

coagulum decreased with an increase in P/V R below a specific power-input 

(P/V R ) of 80 W/m 3 . They attributed this coagulum formation to poor


homogenization of the emulsion in the reactor at the low power-input 

values. However, above 80 W/m 3 , the coagulum increased with an increase 

in P/V R . However, instead of the expected order of dependence of 0.5, they 

found that the coagulum was proportional to (P/V R ) 1.25 . Chern et al. (1996) 

have studied the stability of methyl methacrylate (MMA) and n-butyl 

acrylate (BA) copolymer latexes, and have found that the amount of 

coagulum increased with the agitation speed, but the effects of other 

variables like electrolyte concentration, amount of surfactant in the 

monomer feed, relative amounts of MMA and BA, and solids content, 

were more significant. 

Eq. 15 relates the amount of coagulum to the agitation speed during the 

seed stage (x s ) and the feed stage (x f ) of our experiments: 

% coagulum ¼ 0:0836 þ 0:0164x s þ 0:0543x f 

error ¼ 0:0120 

ð15Þ 

The positive values of m s and m f confirm the expected result that the 

amount of coagulum increases with the agitation speed. It is seen from the 

magnitudes of the coefficients of x s and x f that the feed stage agitation has a 

stronger effect on the coagulum than the seed stage agitation. This is 

expected for the following reasons: 1) the volume fraction of solids is lower 

during the seed stage; 2) the surface-coverage of the particles is higher 

during the seed stage, providing greater electrostatic repulsion between the 

particles; 3) the surface coverage decreases during the feed stage as the 

particles grow in surface area; 4) the larger particles of the feed stage will 

be more sensitive to shear-induced aggregation than the smaller particles of 

the seed stage (Ottewill, 1997); and 5) the feed stage duration is longer (90 

min compared to the 30 min of seed stage). 

In all our experiments, it was observed that most of the coagulum was 

on the agitator blades, and the reactor inserts. The amount of coagulum in 

the latex was negligible. This suggests two possibilities. First, all the 

coagulum is formed near the agitator. Second, the coagulum is formed in 

the bulk of the reactor, and the aggregates are captured by and adhere to the 

agitator blades when they pass through the agitator region in the reactor. 

The shear rate in a stirred reactor is not uniform throughout the reactor. 

It is higher in the region near the agitator. Van’t Riet and Smith (1975) 

showed that in a turbine stirred vessel, a trailing vortex pair existed behind 

each stirrer blade and that the trailing vortices are associated with 

centrifugal accelerations and high shear rates. They have found that the 

trailing vortex system leaving the blade tips deviates greatly from isotropic 

conditions (Van’t Riet et al., 1976). As an example, a 5 cm diameter 

Rushton turbine stirring a Newtonian liquid with a viscosity of 1 cP and a


density of 1 g/cm 3 at 600 rpm (10 rotations/s) in a 15 cm diameter reactor, 

with a liquid height of 15 cm, can be considered. For a power number of 5, 

the power-input per unit volume can be calculated to be 5895 erg s 1 

cm 3 . Using Eq. 14, this corresponds to an average shear rate of 280 s 1 . 

The Reynolds number (Re) of the flow, given by ND 2 r/m, is 25,000. For Re 

between 15,000 and 90,000, Van’t Riet and Smith (1975) showed that the 

shear rate in the region immediately behind the blade can reach values as 

high as 90 N. In the above example, this corresponds to a shear rate of 900 

s 1 . Thus, the trailing vortices behind the agitator blades can be the main 

loci for coagulum formation. The shear rate at a given point in the trailing 

vortex is a function of Re only. Hence, the scale-up criterion of constant Re 

seems more logical than constant (P/V R ). It is also important how 

frequently the fluid passes through the agitator region. The frequency of 

fluid circulation in the reactor will be proportional to ND 3 . If the circulation 

rate is much higher than the coagulation rate, the maximum shear rate, 

which is in the agitator region, will control the coagulation process. During 

emulsion polymerization of BMA in a stirred reactor, we found that the 

order of dependence of the amount of coagulum on the agitation speed was 

recipe dependent, and significantly different from 1.5 (Krishnan, 2002). 

However, because of the combined effect of the above-discussed factors, 

an order different from the value of 1.5 based on Eq. 14 (P/N 3 D 5 ) is 

not unexpected. 

Effect of Agitation on Particle Diameter 

Eq. 16 shows a strong dependence of the particle diameter in the final 

latex on the seed stage rpm. 

D v ðnmÞ ¼182:8 

error ¼ 0:35 

13:1x s þ 1:35x f 

ð16Þ 

The negative coefficient indicates that volume-average particle diameter in 

the final latex decreases as the seed stage rpm increases. Thus, more 

particles are nucleated under higher agitation speed, which is consistent 

with the results reported in the previous paper (Krishnan et al., 2003). The 

comparatively smaller coefficient for the feed stage rpm indicates that 

secondary particle nucleation or shear-induced aggregation were negligible 

during the feed stage. Figure 2 shows the particle size distributions in the 

seed and final latexes corresponding to experiments 1 to 4 in Table 2. The 

distributions were obtained by capillary hydrodynamic fractionation. The 

particle diameters of the final latexes are given in Table 2. It is seen that


Figure 2. 

Table 2. 

Particle size distribution in seed and feed latexes for the experiments in 

the particle diameter in the seed was smaller when the agitation speed was 

higher. The figure shows a fairly good reproducibility of the particle size 

distributions. The final latexes show peaks at higher or lower particle 

diameters, corresponding to the particle sizes in the seed latexes. The 

diameter in the final latex is not affected by the feed stage agitation. 

The decrease in the particle diameter with an increase in the agitation 

is a recurring feature for particle nucleation in systems with surfactant 

concentrations near the critical micelle concentration. Arai et al. (1981) 

have observed an increase in the number of particles with the increase in 

the agitation speed during surfactant free emulsion polymerizations of 

methyl methacrylate using KPS initiator at 65°C. Varela de la Rosa (1991) 

has reported the heat evolution rates during emulsion polymerizations of 

styrene using KPS initiator at 70°C at an SDS concentration of 10 mmol 

dm 3 for agitator speeds of 300 rpm and 500 rpm. The particle diameter 

was smaller and the reaction rate higher, when the agitator speed was 500 

rpm. In the absence of any oxygen impurity in the reactor, this effect can 

be explained by the interfacial nucleation mechanism recently proposed by 

Ni et al. (2001). According to this mechanism, under the influence of 

shear, minidroplets are formed at the interface of the monomer droplets 

and water. These minidroplets result in the nucleation of the polymer 

particles. The greater the agitation, the higher the number of these minidroplets, 

and the resulting polymer particles. Trace amounts (ppm levels) of 

oxygen impurity in the reactor headspace and the agitation dependent mass


transfer rate of oxygen from the headspace into the emulsion can also result 

in a higher number of polymer particles at a higher agitation speed 

(Krishnan, 2002). 

Effect of Agitation on NMA Incorporation 

Eq. 17 correlates the effect of agitation speed on the amount of watersoluble 

species in the latex serum. 

water-soluble species ðgÞ ¼ 17:115 

error ¼ 0:195 

0:605x s þ 0:045x f 

ð17Þ 

The negative coefficient for the seed stage agitation speed implies that the 

amount of water-soluble species decreases with an increase in the seed 

stage agitation speed. The effect of feed stage agitation is negligible, and is 

below the limits of experimental error. This may seem surprising, because 

the copolymerization reaction occurs only during the feed stage, and a 

greater influence of the feed stage agitation on the amount of water-soluble 

polymer would be expected. This can be explained on the basis of the 

higher number of particles nucleated at higher agitation speed during the 

seed stage. The copolymerization of NMA in the polymer particles has to 

compete with the aqueous phase polymerization of NMA. Because of the 

hydrophilicity of the NMA monomer, the polymer formed in the aqueous 

phase will remain water-soluble as long as it contains a significant number 

of NMA units. If the number of polymer particles is higher, the extent of 

copolymerization of NMA at the particle loci (within the particles or at the 

surface of the particles) will be higher. This would result in a lower amount 

of water-soluble polymer. The 1 H-NMR spectra of the solids in the serum 

show that the proportion of the BMA units to the NMA units in the serum 

polymer is very small, that is, the serum polymer is mainly homopolymer 

of NMA. 

The latexes prepared using the recipe in Table 1 had lower viscosities 

than the high-solids latexes reported previously (Krishnan et al., 2003). The 

viscosities, especially at the lower shear rates, were at least 2 orders of 

magnitude lower. The plots of viscosity vs. shear rate are relatively flat in 

contrast to the strong shear-thinning behavior of the high-solids latexes. 

Also, the 8 cm diameter agitator was used in the low-solids emulsion 

copolymerization experiments. This agitator could maintain good top-tobottom 

uniformity in the reactor, even at 150 rpm. These factors resulted in 

the non-dependence of the amount of water-soluble polymer on the feed 

stage agitation.


Figure 3. Effect of agitation speeds on the viscosities of the final latexes. The pairs 

of numbers in the figure are the seed stage and feed stage agitation speeds, 

respectively. The solid curves show the viscosity values as the shear rate was 

increased, and the dashed curves correspond to the decreasing shear rates. 

Effect of Agitation on Latex Viscosity 

Figure 3 compares the viscosities of the latexes prepared under 

different agitation conditions. Eq. 18 shows the viscosity of the final latex 

(at a shear rate of 461 s 1 ) as a function of the seed stage and feed stage 

agitator speeds. 

Latex viscosity ðcPÞ ¼ 7:058 þ 0:163x s 

error ¼ 0:018 

0:003x f 

ð18Þ 

On comparing the coefficients of x s and x f , it is clear that the agitator speed 

during the seed stage has a greater influence on the viscosity of the final 

latex, compared to the rotational speed during the feed stage. The increase 

in viscosity with an increase in the seed stage agitation is attributed to the 

higher number of particles in the final latexes, as discussed in the previous 

paper (Krishnan et al., 2003). 

Molecular Weight of the Serum-Polymer 

The molecular weight distribution of the serum-phase polymer was 

determined using gel permeation chromatography (GPC, Waters 515 HPLC


Figure 4. The GPC detector response as a function of elution time, for the polymer 

in the aqueous phase of the latexes. The area under the curve is proportional to the 

total amount of water-soluble polymer in the injected sample. 

Pump, Tosoh Biosep TSK-GEL columns GMPWXL and PWXL, and 

Waters 410 Differential Refractometer detector). Figure 4 shows the 

response of a refractive index detector as a function of the elution time. A 

0.01 M NaNO 3 solution was used as the eluant. The final latexes were 

diluted with equal masses of water (to a solids content of ca. 12.2 wt%). 

The diluted latexes were ultracentrifuged at 37000 rpm and 4°C for 5 h. 

The solids content in the serum was ca. 0.7 wt%. The serum was directly 

injected into the GPC column. The area under the response curve is 

proportional to the total amount of water-soluble polymer present in the 

serum. The data in Figure 4 are consistent with the gravimetric results 

shown in Table 2. The amount of water-soluble polymer is higher when the 

seed stage agitation speed is lower. 

Monomer Emulsification 

In the experiments reported in Table 2, there was no pooling of the 

BMA monomer at any point of the reaction. However, when a Rushton 

turbine with 4 cm diameter was used at 400 rpm (both seed and feed


Figure 5. 

conditions. 

The amount of water-soluble species in the latexes for different agitation 

stages), the BMA monomer started to form a pool at ca. 60 min of the feed 

stage, and by ca. 80 min, a ca. 0.5 cm monomer layer accumulated at the 

top of the reactor. The solids content in the final latex was ca. 23 wt%. 

Figure 5 compares the amount of water-soluble species in the serum formed 

when the 4 cm agitator was used, with those in the latexes prepared using 

the 8 cm agitator. Clearly, the amount of water-soluble solids is higher. 

Thus, a poor emulsification of the BMA monomer results in a slower rate 

of transfer of the BMA monomer to the polymer particles. The proportion 

of the added NMA monomer that homopolymerizes, is greater compared to 

the amount copolymerizing with BMA. Thus, a greater amount of watersoluble 

polymer is formed. 

The frequency at which the fluid recirculates through the agitator 

region is proportional to ND 3 . Thus, the fluid passed through the agitator 

region less frequently with the 4 cm agitator than with the 8 cm agitator. 

Emulsification of the added BMA occurs in the high-shear agitator region. 

The size of the BMA droplets will be lower with a higher shear rate in the 

agitator region, and more frequent recirculation through this region. Thus, 

the BMA droplets produced by the 4 cm agitator will be larger compared 

to those produced by the 8 cm agitator. The surfactant coverage of the 

polymer particles and the monomer droplets decreases during the feed stage 

because of growing surface area (and no added surfactant). The monomer 

droplets rise to the surface of the reactor and coalesce to form a pool.


CONCLUSIONS 

Agitation has a strong effect on the properties of the latex prepared by 

emulsion polymerization. More particles are nucleated under a higher 

agitation speed. This observation is consistent with the mechanism of 

interfacial particle nucleation (Varela de la Rosa, 1991), wherein the shear 

stress generated by the agitation results in the formation of minidroplets 

near the surface of the monomer droplets, and polymer particles are formed 

by the nucleation of these minidroplets. Although the reactions were carried 

out under a constant flow of nitrogen through the reactor, trace amounts of 

oxygen impurity in the reactor headspace can also result in a higher particle 

concentration at a higher agitation speed (Krishnan, 2002). 

The main factor influencing the amount of water-soluble solids in 

the latex serum is the number of particles produced at the end of the seeding 

stage. The greater the number of particles, the lower was the amount of watersoluble 

polymer. More NMA monomer copolymerizes with BMA in the 

polymer particles when the number of particles is higher. The agitation 

during the feed stage did not have a significant effect unless the 

emulsification of the added BMA monomer was poor and pooling occurred 

(as with the 4 cm Rushton turbine). The low viscosity of the latex, and the 

good mixing with the 8 cm agitator even at 150 rpm, were the reasons for no 

effect of the feed stage agitation on the water-soluble polymer formation. 

As expected, the amount of coagulum increased with the agitation 

intensity. Almost all coagulum formed during the reaction was on the 

agitator blades, baffle, etc. If the trailing vortices behind the agitator blades 

are the sites of coagulum formation, then the amount of coagulum is 

expected to scale with the Reynolds number. 

The viscosity of the final latex was more sensitive to the number of 

polymer particles than the amount of water-soluble polymer. The latex 

viscosity was higher when the latexes were prepared using a higher agitation 

speed during the seed stage and thereby contained more polymer particles. 

ACKNOWLEDGMENTS 

The help of Mr. William Anderson with NMR data acquisition and 

interpretation, and the financial support from the Emulsion Polymers 

Liaison Program is greatly appreciated. 

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Received August 14, 2002 

Accepted February 4, 2003

Relative Importance of the Effects of Seed and Feed Stage ...

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