ARTICLE pubs.acs.org/IECR
Effects of Operational Parameters on Particle Size Distributions in Methyl Methacrylate Suspension Polymerization Odinei H. Gonc) alves,*,† Andre L. Nogueira,‡ Pedro H. H. Araujo,‡ and Ricardo A. F. Machado‡ † ‡
Post-graduation Program of Food Technology (PPGTA), Federal University of Technology - Parana, Campo Mour~ao/PR - Brazil Department of Chemical Engineering, Federal University of Santa Catarina, Florianopolis/SC - Brazil ABSTRACT: The correct manipulation of particle size distribution in suspension polymerization is important because particle size determines the product applicability and processability. Methyl methacrylate suspension polymerization was studied in order to determine the effect of operational parameters in manipulating the particle size distribution (PSD) while keeping suspension stability. The effects of stirring rate, stabilizer concentration, and stabilizer addition time were evaluated on particle size distribution, Sauter mean diameter, and suspension stability. Stabilizer concentration affected particle sizes but at the expense of system stability as low amounts of stabilizer led to uncontrolled coagulation of the particles. Changing the stirring rate was efficient in manipulating particle sizes although its increase led to the broadening of the PSDs while reducing the Sauter mean diameter. The moment stabilizer was fed has shown to be a very efficient way to manipulate the PSD as delaying the stabilizer addition increased the Sauter mean diameter and narrowed the distributions. However, there was a limit in increasing the addition time as adding the stabilizer at higher conversions compromised the suspension stability.
’ INTRODUCTION Important commercial resins are manufactured by batch suspension polymerization, including poly(vinyl chloride), styrenic resins like EPS (expandable polystyrene), HIPS (high impact polystyrene) and SAN (styrene acrylonitrile resin), poly(methyl methacrylate), poly(vinyl acetate) and their copolymers.1 In a suspension polymerization, relatively water-insoluble monomers are dispersed in water (the continuous phase) by a combination of strong stirring and the use of small amounts of stabilizers. Polymerization is started with the addition of an oil-soluble initiator, and the monomer droplets are converted from a highly mobile liquid state through a sticky stage to hard solid polymer particles. This point is known as the Particle Identification Point (PIP) in which particle breakage and coalescence cease due to the very large viscosity of the monomer/polymer particles.1,2 The final particle size distribution (PSD) results from the dynamics of breakage and coalescence of monomer droplets/polymer particles that change during the reaction due to the increase in the viscosity of the dispersed phase with monomer conversion to polymer. At the beginning of the reaction, low viscosity monomer droplets break due to the interaction with the turbulent flux, while coalescence could be minimized with the use of stabilizers. As the viscosity of the particles increases breakage rate decreases as the viscous forces inside the particles become higher than the turbulent forces generated by the impeller. In the sticky stage, when the action of the stabilizers is not enough, the coalescence of particles increases. In the last stage, particles become rigid and the PSD remains the same until the end of the reaction.3,4 The product PSD affects important quality attributes, for instance, handling, storage, bead impregnability, and morphology after expansion, processability, insulation capability, and mechanical resistance.3 Despite its importance, with a few exceptions,5,6 experimental studies on particle size distributions focused on nonreactive systems710 or on extremely low r 2011 American Chemical Society
dispersed to continuous phase ratio.7,8,1012 Several detailed reviews on suspension polymerization have been published,1,1317 and there is an agreement that the full understand of the relationship between particles properties and operational parameters are still a work in progress. Particle size distribution is influenced by a number of factors including geometry (stirrer type, baffles, reactor dimensions), interfacial phenomena, polymerization kinetics, rheological behavior of the polymer/ monomer particles, and others. It is of key importance to find ways to efficiently manipulate the PSD in order to produce monodisperse particles in each polymerization batch. Moreover, one should be able to obtain particles with distinct average diameters from batch to batch using the same reactor and with minimal process adaptation.2 Correctly manipulating PSD is also important in industrial suspension polymerization processes due to safety issues.18 Methyl methacrylate radical polymerization is highly exothermal and presents a pronounced autoacceleration period known as gel effect.19 Keeping suspension stability is necessary during the gel effect period since the heat of reaction must be rapidly transferred from the monomer/polymer droplets to the continuous media (water) and then to the reactor walls.18,20 The manipulation of the particle size distribution must be accomplished while avoiding particle coagulation and the risk of thermal runaway.21,22 Small amounts of stabilizers are added to the suspension to minimize the coalescence of the particles. A number of stabilizers are listed in the literature23 from inorganic compounds like sodium tripolyphosphate to water-soluble polymers like poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP). It is common practice at industry to feed the stabilizer, as PVP, to the Received: January 31, 2011 Accepted: June 20, 2011 Revised: May 25, 2011 Published: June 20, 2011 9116
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Industrial & Engineering Chemistry Research reactor at the beginning of the reaction when the viscosity of the monomer droplets is very low leading to the stabilization of small droplets that are going to originate small polymer particles. Adding the stabilizer at later stages of polymerization could be an interesting way to manipulate the PSD because the remarkable changes in the viscosity of the monomer/polymer droplets throughout the reaction affects the mechanisms of coalescence and breakage of the monomer droplets/polymer particles. The objective of this work is to evaluate the effect of some operational parameters (stirring rate, stabilizer concentration, and stabilizer addition time) on the particle size distribution in methyl methacrylate batch suspension polymerization. These parameters were chosen because they can be easily implemented in industrial conditions with minimal process adaptation and operational cost. Histograms of the particle size distributions, Sauter mean diameter, and dimensionless standard deviation were used to compare the effect of these operational parameters on polymer particle sizes.
’ MATERIAL AND METHODS Materials. Methyl methacrylate (MMA, Rohm and Haas) with a minimum purity of 99.6% and inhibitor (topanol-A) concentration of 12 ppm was used as monomer. Poly(vinyl pyrrolidone) (PVP, 360,000 g/mol, SulPolimeros) was used as stabilizer agent. Benzoyl peroxide (BPO, Sigma-Aldrich, 98% purity) was used as initiator. Distilled water was used as continuous medium in the suspension polymerization. P-Benzoquinone (Sigma-Aldrich, 99% purity), chloroform (98% purity, Vetec), sodium chloride (98% purity, Vetec), and ascorbic acid (99% purity, Vetec) were used as received. Polymerization Procedure. The reactions were carried out in a 1-L jacketed glass reactor with a total organic load of 22%vol/vol. Nitrogen was injected in the reactor during all experiments to keep the reaction medium free of oxygen. The reactor was equipped with a reflux condenser to avoid water/monomer loss. A mechanical stirrer was equipped with a three-blade propeller impeller. Initially, the reactor was fed with distilled water (505 g), ascorbic acid (0.789 g, to minimize inhibition of the polymerization by oxygen), and sodium chloride (25.25 g, to decrease the water solubility of MMA). After ten minutes the monomer (141.45 g) was fed and the reactor temperature raised. When the reaction temperature reached 70 °C, the stirring rate was adjusted from 600 rpm to the desired value (400, 600, or 900 rpm), and the initiator (0.386 g) was added guaranteeing better control of the polymerization time and the stabilizer addition times. The moment when BPO was added was arbitrarily defined as the beginning of the reaction. It is worth mentioning that the small amount of stabilizer added was not enough to fully prevent the coalescence being the monomer droplets continuously subjected to constant breakage and coalescence at the first stages of the reaction promoting the distribution of the initiator through the monomer droplets. The moment that stabilizer (0.20, 0.50, or 1.00 g/Lwater) was fed varied according to the experiment (0, 30, 60, or 85 min after the beginning of the reaction). The system was kept under isothermal conditions (70 °C) with a constant stirring rate for approximately four hours. Table 1 presents the experimental conditions used in the suspension polymerizations. Characterization. Monomer conversion was determined gravimetrically using 0.01%wt p-benzoquinone as inhibitor. Samples of approximately 10.0 mL were withdrawn from the system
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Table 1. Experimental Conditions for MMA Polymerizations run experimental conditions
1
2
3
4
5
6
7
8
PVP (g/Lwater)
0.20 0.50 1.00 1.00 1.00 1.00 1.00 1.00
stirring rate (rpm)
600
600
600
600 600
600
400 900
stabilizer addition
60
60
60
0
85
0
30
0
time (min)
at regular intervals of time for analyses. Sampling was carefully accomplished using a sampling device with a large enough aperture (70 mm diameter) to guarantee a representative sample of the reaction medium. Samples were dried at 60 °C until constant weight. Weight average molecular weight of the polymer at the end of the polymerization was measured by size exclusion chromatography (SEC). SEC was performed using a Waters apparatus equipped with three Styragel columns in series (HR 2, HR 4, HR 6; effective molecular weight ranges of 5 1022 104, 5 1036 105, and 2 1051 107 g/mol, respectively), at 35 °C with tetrahydrofuran (THF) as eluent (flow rate was 1.0 mL min1). A Waters 2410 refractive index detector was used, and molar masses were determined from a calibration curve based on PS standards (average molecular weights varying from 580 to 11,200,000 g/mol). The viscosity of the organic phase (polymer/monomer mixture) was calculated using the correlation proposed by Singh and Gupta (2008).24 Polymer particles were dried and separated through sieving using standard sieves with nominal aperture diameters of 44, 106, 300, 500, 590, 710, 1000, 1180, 1680, and 2360 μm. Results are presented as histograms of retained mass percentages in each sieve. Sauter mean diameter (d32) and dimensionless standard deviation (σ) were calculated using eqs 1 and 2, respectively25 n
d32 ¼
∑ Δji i¼1 Δji ̅ i i¼1 D n
∑
¼
1 Δji ̅ i i¼1 D n
∑
ð1Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n
σ ¼
∑ ½ðD̅ i d32 Þ2 3 Δji i¼1 d32
ð2Þ
where Δji is the mass fraction retained in sieve i, and Di is the average diameter considering sieves i and (i+1).
’ RESULTS AND DISCUSSION Influence of Stabilizer Addition Time. The moment that the stabilizer was fed to the reactor was evaluated in order to provide control over the particle size distribution. After the initiator addition (arbitrary defined as the beginning of the reaction) the system was allowed to react, and the stabilizer was added at 0, 30, 60, or 85 min after the initiator addition. Conversion profile is showed in Figure 1, while Table 2 presents the viscosity of the organic phase (monomer/polymer mixture) and the approximated degrees of conversion at each stabilizer addition times. The viscosity of the organic phase was calculated assuming a weight average molecular weight of 920,000 g/mol (obtained by 9117
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Industrial & Engineering Chemistry Research SEC). It is worth noting that the time of PVP addition did not affect the evolution of monomer conversion and polymer
Figure 1. Monomer conversion profile (70 °C; 0.11%mol BPO).
Table 2. Monomer Conversion and Viscosity of the Organic Phase at a Given Stabilizer Addition Time
a
stabilizer addition
conversion
viscosity of the organic
time (minute)
(%)
phase (Pa.s)a
0 30
0 3
0.6 9.0
60
8
136.8
85
14
3056.2
Calculated values.24
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molecular weights as the particles were too large (>10 μm) to present any effect of radical compartimentalization. Thus, the effect of the moment that PVP was fed to the reactor on the polymerization kinetics is negligible.1,26,27 Figure 2 presents the final particle size distributions, and Figure 3 presents the Sauter mean diameter and dimensionless standard deviation. The delay in the stabilizer addition led to a remarkable increase in the Sauter mean diameter. It was possible to obtain particles presenting Sauter mean diameters from 600 μm (PVP at 0 min) to 1800 μm (PVP at 85 min) just changing the time in which the stabilizer was added to the reactor making this parameter an interesting variable to manipulate the particle sizes. The change in the viscosity of the dispersed phase is remarkable when the formed polymer is highly soluble in the monomer.4,24,28 One can expect a great variation in the particle sizes when the stabilizer addition is delayed since the change in the viscosity of the dispersed phase is very pronounced during the reaction. In Table 2 one can observe that after 30 min the viscosity of the dispersed phase was 15 times higher than the viscosity at the beginning of the reaction. The increase in the viscosity of the dispersed phase was even greater after 85 min (5000 times higher). The delay in the stabilizer addition means that at the initial polymerization stages the monomer/polymer droplets were not stabilized against coalescence while being continuously broken by the turbulence generated by the stirring of the reaction medium.29 The increase in the monomer conversion led to an
Figure 2. Final PSDs for different stabilizer addition times (0.11%mol BPO; 70 °C; 1.0 g/Lwater PVP; 600 rpm). 9118
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Figure 3. Influence of stabilizer addition time on (9) d32-Sauter mean diameter and (0) σ-dimensionless standard deviation (0.11%mol BPO; 70 °C; 1.0 g/Lwater PVP; 600 rpm).
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increase in the dispersed phase viscosity and also to an increase in the maximum stable particle size as the higher viscosity of the dispersed phase opposes the deformation of the monomer/ polymer particles due to turbulence that could lead to particle breakage.6 The result was that the average diameter of the monomer/polymer particles increased during the reaction. The later the stabilizer addition the greater were the droplet sizes and thus the final polymer particles. On the other hand, the addition of the stabilizer at the beginning of the reaction led to the stabilization of small droplets which led to small polymer particles. Another effect was the decrease in the values of the dimensionless standard deviation as the PSD shifted to higher particle sizes. It means that the relative weight of the standard deviation to the average size decreased with increasing average sizes which is very important when producing suspension polymers in industrial scale.2 As discussed by Alvarez et al. (1994)3 and pointed out by Jahanzad et al. (2004),30 the increase in the monomer/polymer droplets viscosity reduces the breakage rate more significantly
Figure 4. (a), (b), (c) Final PSDs for different stirring and (d) influence of stirring rate on (9) d32 - Sauter mean diameter and (0) σ-dimensionless standard deviation rates (0.11%mol BPO; 70 °C; 1.0 g/Lwater PVP at 0 min). 9119
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Figure 5. (a), (b), (c) Final PSDs for different stabilizer concentration and (d) influence of stirring rate on (9) d32 - Sauter mean diameter and (0) σ-dimensionless standard deviation rates (0.11%mol BPO; 70 °C; 600 rpm; PVP added at 60 min).
than drop coalescence. This can explain the narrowing of the PSD when the stabilizer was added in the later stages of polymerization. Results also suggested that a long delay (for this system, 150 min) can lead to suspension instability as the increase in viscosity becomes very fast at conversions higher than 20% and the particles become sticky without any stabilizer increasing the efficiency of particle coalescence and the overall coalescence rate. On the other hand, the increase in the viscosity increases the resistance of the particle to break resulting in uncontrolled particle coagulation. Influence of Stirring Rate. Figure 4 (a), (b), and (c) presents the particle size distributions for the methyl methacrylate suspension polymerization using different stirring rates. The stabilizer (PVP at 1.00 g/Lwater) was added just after the initiator addition. Figure 4 (d) shows the Sauter diameter and dimensionless standard deviation. The Sauter mean diameter decreased with increasing stirring rates when the stabilizer concentration was kept constant as expected.9 However, Chatzi and Kiparissides (1984)9 reported the narrowing of the PSD for increasing stirring rates. The discrepancy could be due to the fact that they studied a nonreactive system with no significant changes in the dispersed phase viscosity as observed in suspension polymerization systems.1,31,32
Bimodal distributions were also found in all experiments. This behavior was also reported by Konno and co-workers5 in suspension polymerizations using dispersed to continuous phase ratio from 0.1 to 0.5. Bimodal distributions can be explained considering that at any given time PSDs are determined by the interplay of breakage and coalescence of the dispersed monomer droplets/polymer particles. During the reaction, the kinetics of coalescence and breakage changes with the viscosity of the dispersed phase as well as the minimum and maximum stable particle size. This is a dynamic process that does not reach the equilibrium for the PSD at any conversion (below PIP) as the conversion is always increasing with time. Another point is that at higher conversions, erosion becomes the main breakage mechanism leading to the formation of smaller particles.5,11,33 Bimodal distributions can be also explained by the nonhomogeneity of the turbulent flow inside the reactor.10,13,14,34 It is worth noting that increasing the stirring rate from 400 to 900 rpm Sauter mean diameters decreased from 1000 to 400 μm. It means that changing the stirring rate could be an efficient way to manipulate the particle size distribution (PSD). However, in Figure 4 one can observe that the increase in the stirring rate also led to a remarkable increase in the dimensionless standard deviation. Broad particle size distributions are not desired in industrial scale production of suspension-made polymers since it can lead to material loss, reduced processability, and increasing operational costs.2 9120
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Industrial & Engineering Chemistry Research Influence of Stabilizer Concentration. Figure 5 (a), (b), and (c) shows the particle size distributions when the stabilizer concentrations used were 1.00, 0.50, and 0.20 g/Lwater. Stirring rates were kept constant at 600 rpm, and the stabilizer was added 60 min after the initiator addition. Sauter mean diameter and dimensionless standard deviation are presented in Figure 5 (d). It can be observed in the histograms that the decrease in the stabilizer concentration led to an increase in the particles size. However, a great amount of the particles were retained in the upper sieve (2360 μm) when 0.20 and 0.50 g/Lwater were used which could lead to lower values of Sauter mean diameter than the actual values. The same stands for the dimensionless standard deviation. When 0.20 and 0.50 g/Lwater PVP were used some degree of suspension destabilization was observed (particle coagulation and formation of non spherical particles). Under the evaluated experimental conditions only the stabilizer concentration of 1.00 g/Lwater was able to maintain suspension stability. Suspension instability must be avoided in industrial polymerization processes because it may lead to uncontrolled coalescence of the polymer particles decreasing the heat exchange capacity thus compromising operation safety and the final particle morphology.
’ CONCLUSION Changing the stabilizer concentration was not efficient in manipulating the particle size distributions since it showed a minor effect on the Sauter mean diameter under the evaluated experimental conditions. Moreover, concentrations below 1.00 g/Lwater led to suspension instability. Stirring rate was an efficient parameter in the manipulation of the particle sizes since a broad range of Sauter mean diameters were obtained when the stirring rate was changed from 400 to 900 rpm. However, high stirring rates led to an increase of the dimensionless standard deviation which is undesirable in most industrial suspension polymerization processes. The moment stabilizer was fed has shown to be a very efficient way to manipulate the PSD as delaying the stabilizer addition increased the Sauter mean diameter and narrowed the distributions which is a very desirable characteristic. However, there was a limit in increasing the addition time as adding the stabilizer at higher conversions compromised the suspension stability. ’ AUTHOR INFORMATION Corresponding Author
*Fax: +55 44 35234156. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank CNPq - Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and CAPES - Coordenac) ~ao de Aperfeic) oamento de Pessoal de Nivel Superior for the financial support. ’ REFERENCES (1) Yuan, H. G.; Kalfas, G.; Ray, W. R. Suspension Polymerization. Macromol. Sci. - Rev. Macromol. Chem. Phys. 1991, C31, 215. (2) Bishop, R. B. Pratical Polymerization for Polystyrene; Cahners Publishing Co.: Massuchusetts, 1971. (3) Alvarez, J.; Alvarez, J.; Hernandez, M. A Population Balance Approach for the Description of Particle Size Distribution in Suspension Polymerization Reactors. Chem. Eng. Sci. 1994, 49 (1), 99.
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