POLYMER REACTION ENGINEERING Vol. 11, No. 3, pp , 2003

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1 POLYMER REACTION ENGINEERING Vol. 11, No. 3, pp , 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; ak04@ lehigh.edu. 359 DOI: /PRE Copyright D 2003 by Marcel Dekker, Inc (Print); (Online)

2 360 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

3 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 %, 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 DI water 26.0 BMA BMA (450 cm 3 ) SLS (4 mm) a NMA 24.0 KPS (3 mm) a NaHCO (9.2 mm) a a Concentration in mmol per dm 3 water. b Final amount of polymer is ca wt% of latex.

4 362 Krishnan et al. Table 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) (+1) 150 ( 1) ( 1) 400 (+1) (+1) 400 (+1) *The numbers in the parentheses are the encoded levels. **At a shear-rate of 461 s 1.

5 Seed and Feed Stage Agitations of BMA and NMA 363 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 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

6 364 Krishnan et al. 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

7 Seed and Feed Stage Agitations of BMA and NMA 365 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 ( )/( ) = 0.6 and an agitator speed of 300 rpm corresponds to the encoded level of ( )/( ) = 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 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

8 366 Krishnan et al. 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

9 Seed and Feed Stage Agitations of BMA and NMA 367 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,

10 368 Krishnan et al. 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 to 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

11 Seed and Feed Stage Agitations of BMA and NMA 369 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 ) 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

12 370 Krishnan et al. 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 13:1x s þ 1:35x f error ¼ 0:35 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

13 Seed and Feed Stage Agitations of BMA and NMA 371 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

14 372 Krishnan et al. 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 0:605x s þ 0:045x f error ¼ 0:195 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.

15 Seed and Feed Stage Agitations of BMA and NMA 373 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 0:003x f error ¼ 0:018 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

16 374 Krishnan et al. 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 wt%). The diluted latexes were ultracentrifuged at 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

17 Seed and Feed Stage Agitations of BMA and NMA 375 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.

18 376 Krishnan et al. 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. REFERENCES Ali, S. I., Zollars, R. L. (1987). Changes in coagulum configuration resulting from shear-induced coagulation. J. Colloid Interface Sci. 117(2):

19 Seed and Feed Stage Agitations of BMA and NMA 377 Ali, S. I., Zollars, R. L. (1988). Generation of self-preserving particle size distributions during shear coagulation. J. Colloid Interface Sci. 126(1): Arai, K., Arai, M., Iwasaki, S., Saito, S. (1981). Agitation effect on the rate of soapless emulsion polymerization of methyl methacrylate in water. J. Polym. Sci., Polym. Chem. Ed. 19(5): Chern, C. S., Hsu, H., Lin, F. Y. (1996). Stability of acrylic latexes in a semibatch reactor. J. Appl. Polym. Sci. 60(9): Dave, M. N. (1998). Effect of agitation on scale-up of reactors for emulsion polymerization. M.S. thesis. Bethlehem, PA: Lehigh University. Husband, J. C., Adams, J. M. (1992). Shear-induced aggregation of carboxylated polymer latexes. Colloid Polym. Sci. 270(12): Koh, P. T. L., Andrews, J. R. G., Uhlherr, P. H. T. (1984). Flocculation in stirred tanks. Chem. Eng. Sci. 39(6): Krishnan, S. (2002). Effects of agitation in the emulsion polymerization of n- butyl methacrylate and its copolymerization with N-methylol acrylamide. Ph.D. dissertation. Bethlehem, PA: Lehigh University. Krishnan, S., Klein, A., El-Aasser, M. S., Sudol, E. D. (2003). Agitation effects in emulsion copolymerization of n-butyl methacrylate and N-methylol acrylamide. Polym. React. Eng. 11(3): Kusters, K. A., Wijers, J. G., Thoenes, D. (1997). Aggregation kinetics of small particles in agitated vessels. Chem. Eng. Sci. 52(1): Lowry, V., El-Aasser, M. S., Vanderhoff, J. W., Klein, A. (1984). Mechanical coagulation in emulsion polymerizations. J. Appl. Polym. Sci. 29 (12 Pt. 1): Lowry, V., El-Aasser, M. S., Vanderhoff, J. W., Klein, A., Silebi, C. A. (1986). Kinetics of agitation-induced coagulation of high-solid latexes. J. Colloid Interface Sci. 112(2): Matejicek, A., Pivonkova, A., Kaska, J., Ditl, P., Formanek, L. (1988). Influence of agitation on the creation of coagulum during the emulsion polymerization of the system styrene-butyl acrylate-acrylic acid. J. Appl. Polym. Sci. 35(3): Neuman, R. C. (1997). Experimental Strategies for Polymer Scientists and Plastics Engineers. Munich: Hanser Publishers. Ni, H., Du, Y., Ma, G., Nagai, M., Omi, S. (2001). Mechanism of soap-free emulsion polymerization of styrene and 4-vinylpyridine: characteristics of reaction in the monomer phase, aqueous phase, and their interface. Macromolecules 34(19): Ottewill, R. H. (1997). Stabilization of polymer colloid dispersions. In: Lovell, P. A., El-Aasser, M. S., eds. Emulsion Polymerization and Emulsion Polymers. Chichester, U.K.: John Wiley and Sons, pp Utracki, L. A. (1973). Mechanical stability of synthetic polymer latexes. J. Colloid Interface Sci. 42(1):

20 378 Krishnan et al. Van t Riet, K., Smith, J. M. (1975). Trailing vortex system produced by Rushton turbine agitators. Chem. Eng. Sci. 30(9): Van t Riet, K., Bruijn, W., Smith, J. M. (1976). Real and pseudo-turbulence in the discharge stream from a Rushton turbine. Chem. Eng. Sci. 31(6): Varela de la Rosa, L. (1991). Emulsion polymerization in an automated reaction calorimeter. M.S. thesis. Bethlehem, PA: Lehigh University. von Smoluchowski, M. (1917). Mathematical theory of the kinetics of the coagulation of colloidal solutions. J. Chem. Soc. 112(II): Zollars, R. L., Ali, S. I. (1986). Shear coagulation in the presence of repulsive interparticle forces. J. Colloid Interface Sci. 114(1): Zollars, R. L., Ali, S. I. (1987). Particle concentration effects on shear coagulation in the presence of repulsive interparticle forces. Colloids Surf. 24(2 3): Received August 14, 2002 Accepted February 4, 2003

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