Styrene Drop Size and Size Distribution in an Aqueous Solution of

When the molecular weight of the PVA increases, the drop size decreases and the ... concentration, for a constant drop size, was found to be equal to ...
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Ind. Eng. Chem. Res. 2000, 39, 2085-2090

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GENERAL RESEARCH Styrene Drop Size and Size Distribution in an Aqueous Solution of Poly(vinyl alcohol) Bin Yang, Koji Takahashi,* and Makoto Takeishi Department of Materials Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan

The dispersion of styrene monomer (StM) in an aqueous solution of a poly(vinyl alcohol) (PVA) stabilizer has been studied experimentally. Parameters affecting the initial StM dispersion, such as stirring time, type and concentration of the suspending agent, and the volume fraction of styrene, have been studied in detail and the results compared with existing theories. The transitional time for a dynamic equilibrium to be established in the dispersions was found to be 150 min. When the molecular weight of the PVA increases, the drop size decreases and the StM drops become more stable toward coalescence. With one of the PVAs (PVA-2), the critical PVA concentration, for a constant drop size, was found to be equal to 0.013%, 0.023%, and 0.043% respectively, when the volume fraction of StM correspondingly equaled 0.05, 0.1, and 0.2. 1. Introduction Suspension polymerization is an important industrial process. In this process, monomer(s) immiscible with or slightly soluble in water is first dispersed by stirring in an aqueous phase containing a stabilizer to form tiny drops. Then the polymerization reaction is initiated in the monomer drops by the use of a monomer-soluble initiator. It has long been recognized that the drop size and size distribution in the early stages of suspension polymerization play an important role in determining the final polymer particle size distribution. The drop size and the size distribution at any instant result from a dynamic equilibrium between the drop breakage and coalescence, which, in turn, depend on the turbulent intensity and the type and concentration of the suspending agent. The most commonly used suspending agents for styrene suspension polymerization are watersoluble polymers such as hydroxy ethyl cellulose or poly(vinyl alcohol) (PVA) and water-insoluble inorganic powders such as tricalcium phosphate. The adsorption of such molecules at the monomer-water interface may not only increase the breakage of drops by reducing the interfacial tension but also slow down the coalescence of drops by forming a thin colloidal film at the styrenewater interface to give the drops better elastic properties.1 The effect of the elastic properties is enhanced by increasing the concentration of the suspending agent until a certain surface coverage of the drops is reached. At that point, a “critical surface coverage” is established and a further increase in the suspending agent concentration will have very little effect on the drop stability.2 For such a “stabilized dispersion”, if agitation of a liquid-liquid dispersion is maintained for a sufficiently long time, a local dynamic equilibrium is established between drop breakup and coalescence.3 At equilibrium, * To whom correspondence should be addressed. Tel.: +810238-26-3156. Fax: +81-0238-26-3414. E-mail: takahashi@ ckokushi.yz.yamagata-u.ac.jp.

breakage and coalescence occur at the same rate and the average size and size distribution of the formed drops depend on the extent of agitation as well as on the physical properties of the liquid mixture. Although the behavior of a stabilized drop dispersion in the early stages of suspension polymerization plays a major role in determining the final polymerized particle size and size distribution, there is very little published work on the early stages of the reaction where unreacted drops are present. The purpose of this research is to characterize the formation of styrene monomer dispersions in the presence of polymeric drop stabilizers and to examine the effect of changing the chemical nature of the stabilizer. Effect of PVA. PVA is made by partially hydrolyzing poly(vinyl acetate) and is used as a stabilizer in different industrial fields, for instance, in textiles, adhesives, and coatings, especially in the suspension polymerization of styrene. It has been established that the stabilizing properties are dependent on the concentration, the degree of hydrolysis, and the molecular weight. The stabilizing properties are also dependent on the stage of the styrene polymerization when PVA is added. In regards to the hydrolysis degree, it has been agreed upon that PVA with a hydrolysis degree of 87-89% is the best suspension agent for the suspension polymerization of styrene by many researches.4,5 In regards to the molecular weight of PVA, Mendizabal et al.4 reported that low molecular weight PVA produced more stable suspensions than did high molecular weight PVA when the stability was determined by testing how long a water-styrene suspension lasted after the stirring was stopped. On the contrary, when the polymerization process was carried out with high molecular weight PVA, it gave single, well-defined particles, while low molecular weight PVA gave clusters. To study this phenomenon, Olayo et al.6 researched the interfacial tension of the water-(PVA)/styrene system and showed that the change of temperature, physical properties of

10.1021/ie990709i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

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Table 1. Physical Properties of Styrene Monomer and Deionized Water (25 °C) density (kg/m3)

viscosity (NS/m2)

996.95 901.13 992.24a 987.43a 977.8a

9.03 × 10-4 6.71 × 10-4 9.81 × 10-4 b 1.067 × 10-3 b 1.273 × 10-3 b

deionized water styrene monomer styrene-water (φ ) 0.05) styrene-water (φ ) 0.1) styrene-water (φ ) 0.2)

a Mean density was calculated by Heuven et al.’s (1971) expression: FM ) φFd + (1 - φ)Fc. b Mean viscosity was calculated by Vermeulen (1955)’s expression: µM ) µc[1.5µdφ/(µd + µc) + 1]/ (1 - φ).

Table 2. Physical Properties of Polyvinyl Alcohol (PVA) PVA type

av mol wta

hydrolysis degree (%)

PVA-1 PVA-2 PVA-3

85 000-146 000 31 000-50 000 13 000-23 000

87-89 87-89 87-89

a Values of molecular weight were supplied by Aldrich Chemical Company, Inc.

the dispersed phase, and stirring would influence the interfacial tension dramatically, thus, affect the stability of PVA to styrene suspension. There has been little research about the effect of the molecular weight of PVA on the styrene suspension in the early stage of styrene suspension polymerization until now. What kind of molecular weight of PVA would make the styrene suspension most stable? This will be investigated in the present work.

Figure 1. Experimental setup.

2. Experiment 2.1. Vessel and Impeller. Experiments were carried out in a cylindrical vessel with a flat bottom of which an inside diameter T equaled 0.144 m. The vessel was fitted with four equally spaced stainless steel baffles, each 1/10th the tank diameter. The impeller is a vertical six-blade disk-style impeller (D ) 0.072 m, W ) 0.0144 m). The agitated vessel was put into a thermal water tank, and the temperature was kept at 25 °C. 2.2. Continuous Phase and Dispersed Phase. The continuous phase consisted of deionized water and a suspending agent, PVA with different molecular weights. The styrene monomer was the dispersed, and the volume fraction of which equaled 0.05, 0.1, and 0.2, respectively. Some physical properties of styrene monomer and water used in this work are shown in Table 1, and the physical properties of PVA are summarized in Table 2. It should be noted that the system we studied is a nonreactive system because there is no initiator in the vessel. 2.3. Drop Size Measurement. Drop size and size distribution in this work are obtained by a sampling method. At constant time intervals from the beginning of the experiment, a 5 mL sample liquid was drawn off with a glass pipet from a position A as shown in Figure 1. The sample droplets were transferred immediately to a laboratory dish containing a 3.0 wt % aqueous solution of PVA (the average polymerization degree ) 15 000) to prevent the droplets from coalescing. The entire sampling procedure takes about 1 s. After the sample was collected, photographs of these droplets (about 300-400 drops) were taken by a microscope camera (Olympus-PM-10-M(C-35)), and from these the transient droplet diameter distribution and mean diameter were measured. Here, the mean diameter is the Sauter mean diameter.

Figure 2. Drop size at various positions in a stirred vessel (φ ) 0.05, CPVA-2 ) 0.05%).

At last, a study was performed to investigate the efficiency of the experimental procedure. There are two issues to resolve. Does the surfactant stop the coalescence and breakup of the dispersed phase drops? Can the drop size of the sample drawn from the position, A, represent the average value in the whole vessel? A dispersion of styrene/water-PVA was stirred at 650 rpm for 3 h where the dispersed phase fraction was φ ) 0.2. A sample of the dispersion was withdraw and placed into the 3.0 wt % PVA solution, and the sample was observed under the microscope for 20 min. No drop movement, coalescence, or breakup was observed. Therefore, it was concluded that the surfactant was found to be quite sufficient at stabilizing the drop size against coalescence and breakup. To investigate the second issue, another study was undertaken by collecting and measuring samples from three different positions in the vessel (A, B, and C, as shown in Figure 1). Their average drop diameters at different agitated speeds are shown in Figure 2. The figure shows that only a slight change in the drop size is observed despite the wide range of the impeller speed (note that the same results are obtained in the cases of

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Figure 3. Variation of drop size with stirring time (CPVA-2 ) 0.05%).

φ ) 0.1 and 0.2). This observation implies that the stirred vessel used in our research is almost homogeneous spatially. In general, even a fully developed turbulent flow field in an agitated vessel is far from homogeneous. For example, significant differences can exist between the dispersed phase interactions in the impeller and circulation regions, and it has been postulated in the literature that the impeller region is dominated by drop breakage and the recirculation region by drop coalescence. However, Chatzi et al.7 suggested that homogeneous can be looked at for vessels with short circulation times and low coalescence rates, that is, systems with a low dispersed phase volume fraction, in the presence of protective colloids. However, the validity of the spatial homogeneity assumption regarding the drop size distribution of a dispersion depends on the experimental conditions. We demonstrated the homogeneity at our experimental conditions; thus, we can safely say that the average drop size at any point (for example, point A) in a vessel can be taken as the average value in the whole vessel. 2.4. Experimental Procedure for Different Types of PVA. To investigate the effect of PVA of different molecular weight on the stability of the styrene drops toward coalescence, an experiment for each PVA was carried out by the following procedure: PVA was dissolved in the distilled water to make up a solution with a concentration of 0.05% and the solution was put into the agitated vessel described in section 2.1. Then styrene with a volume fraction of 0.1 was added to the vessel. When the agitated vessel was warmed to 25 °C, the agitation speed was adjusted to the desired value and the experiment began. The agitation was maintained for 180 min to reach a steady state. After that, the sample was drawn and photographed and the experiment was over. The agitation speeds investigated were 250, 450, and 650 rpm, respectively. After the experiment of an agitation speed of 250 or 450 rpm, the agitated vessel and the stirrer were washed clean and were ready to be used to do the experiment of the next speed from the step of making up the solution. However, after the experiment of 650 rpm, the stirrer speed was suddenly reduced to 250 rpm and the agitation was maintained for another 180 min. Then the drops were sampled and photographed, and the experiment of 650 rpm was finished. 3. Results and Discussion 3.1. Styrene Drop Size and Size Distribution with Time Elapse. The stirrer was run at 350 rpm,

Figure 4. Variation of styrene drop size distribution with stirring time (φ ) 0.1, CPVA-2 ) 0.05%).

Figure 5. Variation of dimensionless drop size with stirring time (CPVA-2 ) 0.05%).

and the volume fractions of the dispersed phase equaled 0.05, 0.1, and 0.2, respectively. The dispersion samples were drawn and photographed after stirring times of 5, 30, 60, 90,120, 150, and 180 min, respectively. Figures 3 and 4 show the variation of average drop size and drop size distribution with agitation time. From Figures 3 and 4 it can be seen that the drops are large initially, and their size decreases significantly with time during the first 150 min to stabilize thereafter regardless of the volume fraction of the dispersed phase. After 150 min of stirring, the drop size distribution remains more or less the same. The initial decrease in drop size shows that the breakage rate exceeds the coalescence rate during the initial period of liquid-liquid dispersions. This process continues (for 150 min) until a dynamic equilibrium is reached. The unsteady-state time of 150 min agrees with the observation of Tanaka et al.8 but is less than the result of Apostolidou et al.,9 who obtained unsteady times lasting up to 4 h. Considering that the unsteady-state time is drastically affected by the type of PVA, the type of PVA used in the present work is different from what was used in Apostolidou’s research, which may lead to the different unsteady-state time. The unsteady-state time of 150 min is much longer than the values cited in the literature for other liquidliquid systems without a suspending agent (usually a few minutes). Pan et al.5 showed that the interfacial tension decreases relatively slowly with time in the presence of PVA as a suspending agent. This slow decrease toward equilibrium may explain the relatively long time taken by the liquid-liquid systems to stabilize; the PVA molecules adsorbed at the styrene-water interface take a finite time to rearrange themselves and protect the styrene drops against coalescence.

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Figure 6. Variation of drop size distribution with agitation speed for PVA-1 (Mw ) 85 000-146 000, φ ) 0.1, CPVA ) 0.05%, stirring time t ) 180 min).

Figure 7. Variation of drop size distribution with agitation speed for PVA-2 (Mw ) 31 000-50 000, φ ) 0.1, CPVA ) 0.05%, stirring time t ) 180 min).

Hong et al.10 proposed the following relation:

d32 d/32

) 1 + Re-βt

(1)

/ is a steady-state drop size (constant for a where d32 given system). If this equation is valid, plots ln[(d32 / / )/d32 ] of vs time should be linear. Figure 5 shows d32 / / that the plots of ln[(d32 - d32 )/d32 ] against t of different dispersed phase fractions are linear, confirming the validity of eq 1 and suggesting that the decrease of drop size indeed follows an exponential decay rule very closely. The coefficients R and β for this equation were found to be equal to 0.3315 and 0.016 66 for φ ) 0.05, 0.4725 and 0.020 43 for φ ) 0.1, and 0.4313 and 0.023 81 for φ ) 0.2, respectively. 3.2. Influence of the Type of Suspending Agent on the Drop Size and Size Distribution of Styrene. The types of PVA used in this experiment are described in Table 2. The purpose of the experiment is to investigate the effect of PVA of different molecular weight on the stability of the styrene drops toward coalescence by analyzing the variation of the drop size distribution with the agitation speed. The experimental procedure is described in section 2.4. The variation of the drop size distribution with the agitation speed for PVA-1, PVA-2, and PVA-3 is shown in Figures 6-8, respectively. Figure 6 shows that, with PVA-1, at the agitation speeds investigated, the drop sizes are small and the drop size distributions are narrow; that is, the agitation speed does not have a

Figure 8. Variation of drop size distribution with agitation speed for PVA-3 (Mw ) 13 000-23 000, φ ) 0.1, CPVA ) 0.05%, stirring time t ) 180 min).

Figure 9. Variation of drop size distribution at different PVA concentrations (PVA-2, Mw ) 31 000-50 000, φ ) 0.1, N ) 450 rpm, stirring time t ) 180 min).

great influence on the drop size and size distribution. If a narrow size distribution is produced at 650 rpm and the agitation speed is then reduced to 250 rpm, Figure 6 shows that the drop size and size distribution are almost unaffected by this decrease of agitation speed. These indicate that PVA-1 has a strong capability of accelerating breakage of drops and stabilizing drops from coalescence. Figure 7 shows that, with PVA-2, an increase in the agitation speed provides a small drop size and a narrow size distribution. When the stirring speed is reduced from 650 to 250 rpm, the drop size and size distribution are unaffected. When PVA-3 was used as the suspending agent, Figure 8 shows that although an increase in agitation speed leads to an decrease in drop size, the drop sizes are large and the size distributions are broad compared with those in Figures 6 and 7. In this case, the stability toward coalescence is also very low as shown by the broadening of the size distribution when the agitation speed is reduced from 650 to 250 rpm. Thus, from these observations, we can conclude that an increase in the molecular weight of the suspending agent will strengthen the stability of the drops toward coalescence and the ability of the drop breakage as well as lead to a small drop size and narrow drop size distribution. However, in industrial fields, the PVA with a molecular weight of about 40 000 (PVA- 2) is always used as a stabilizer for styrene suspension polymerization to meet the demand for both the particle size and the size distribution of the products.5 3.3. Effect of the Suspending Agent Concentration on the Volume and Size Distribution of Styrene Drops. To clarify the effect of the suspending agent concentration on the drop size and stability,

Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 2089 Table 3. Critical Surface Coverage for Different Agitation Speeds at Different Column Fractions of Styrene N (rpm)

φ

d32

Sc (10-6 g/cm2)

250 350 450 550 650 250 350 450 550 650 250 350 450 550 650

0.05 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2

134 90 69 50 44 160 108 83 66 51 187 128 94 72 65

6.01 3.71 2.63 2.02 1.61 6.04 3.73 2.62 2.00 1.62 5.99 3.67 2.61 2.01 1.61

and the size distribution is wide. As the PVA concentration is increased, the quantity of PVA adsorbed on the surface of drops increases and the coalescence efficiency decreases; thus, the drop size is small, and the size distribution is uniform. From Figure 10 we also can see that there is a critical PVA concentration above which the drop sizes remain steady, and this critical concentration does not appear to be greatly affected by the agitation speed, which may be because the quantity of PVA absorbed at the styrene-water interface is limited but appears to increase when the volume fraction of the dispersed phase is increased. They are about 0.013%, 0.023%, and 0.043% respectively with volume fractions of the dispersed phase equal 0.05, 0.1, and 0.2 correspondingly. The existence of critical concentration may be because there is a critical surface coverage of PVA above which the coalescence is completely eliminated. Leng et al.2 calculated the critical surface coverage using the following expression by assuming that all of the stabilizer has been adsorbed on the surface of the drops.

Sc )

Figure 10. Variation of drop size with PVA (PVA-2, Mw ) 31 000-50 000) concentration (stirring time t ) 180 min).

experiments were performed using 0.005-0.05 and 0.3 wt % concentration of PVA (PVA-2). The volume fractions of the dispersed phase were 0.05, 0.1, and 0.2, and the agitation speeds were 250, 350, 450, 550, and 650 rpm, respectively. Figure 9 shows the variation of the drop size distribution with PVA concentration. Figure 10 shows the variation of the Sauter mean diameter, with PVA concentration at different agitation speeds and different volume fractions of styrene. It can be seen that an increase in the PVA concentration narrows the size distribution (see Figure 9) and, at low PVA concentration, the drop diameter decreases when the PVA concentration increases (see Figure 10). According to the mechanism of the suspending agent function, it might be expected that, at low PVA concentration, there is not enough PVA to form films covering the styrene-water interface of drops completely or enough concentration to prevent their coalescence, so the drop size is large

(1 - φ)d32Ccrit 6φ

(2)

where Ccrit is the critical concentration, that is, the minimum PVA concentration necessary to obtain a constant drop size. Knowing the value of the critical concentration, we can calculate the critical surface coverage (Sc) at different dispersed phase fractions and drop diameters. The calculated results are also shown in Table 3. This indicates that when the agitation speeds (turbulent pressure forces) are constant, the critical surface coverages to keep coalescing are the same. It also shows that the critical surface coverages at high agitation speeds are higher than those at low agitation speeds to keep drops from coalescing. This result agrees with Shinnar’s breakup-coalescence mechanism.11 4. Conclusions For the dispersion of styrene in a continuous aqueous phase containing PVA, agitation must be maintained for a relatively long time (150 min) before a dynamic equilibrium between breakage and coalescence is established. The type of suspending agent has a great influence on the drop size and drop stability. When the molecular weight of PVA is increased, the drop size decreases and the StM drops become more stable toward coalescence.

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An increase in the PVA concentration decreases the mean drop size and narrows the drop size distribution. The critical PVA (PVA-2) concentration to be used in StM dispersion were found to be equal to 0.013%, 0.023%, and 0.043% respectively when the volume fractions of styrene monomer were 0.05, 0.1, and 0.2 correspondingly (for agitation speeds between 250 and 650 rpm). Further increases in PVA concentration, above the critical value, do not have a great effect on the drop size and volume distribution. Nomenclature

(2) Leng, D. E.; Quarderer, G. J. Drop dispersion in suspension polymerization. Chem. Eng. Commun. 1982, 14, 177-201. (3) Shinnar, R.; Church, J. M. Statistical theories of turbulence in predicting particle size in agitated dispersions. Ind. Eng. Chem. 1960, 52, 253-256. (4) Mendizabal, E.; Castellanos-Ortega, J. R.; Puig, J. E. A method for selecting a poly(vinyl alcohol) as stabilizer in suspension polymerization. Colloids Surf. 1992, 63, 209-217. (5) Pan, Z. R.; Weng, Z. X.; Huang, Z. M. Suspension polymerization; Beijing, 1997. (6) Olayo, R.; Garcia, E.; Garcia-corichi, B.; Sanchez-vazquez, L.; Alvarez, J. Poly(vinyl alcohol) as a stabilizer in the suspension polymerization of styrene: the effect of the molecular weight. J. Appl. Polym. Sci. 1998, 67, 71-77.

C ) off-bottom clearance of vessel Ccrit ) critical PVA concentration CPVA ) concentration of PVA d ) drop diameter d32 ) Sauter mean diameter d/32 ) steady-state Sauter mean diameter D ) impeller diameter N ) impeller speed Sc ) critical surface coverage T ) vessel diameter W ) blade width

(7) Chatzi, E. G.; Kiparissides, C. Steady-state drop-size distributions in high holdup fraction dispersion systems. AIChE J. 1995, 41 (7), 1640-1652.

Greek Letters

(9) Apostolidou, C.; Stamatoudis, M. Transient behavior of drop sizes in stabilized agitated dispersions. Chem. Ing. Tech. 1991, 63, 66-68.

R, β ) constants µc ) viscosity of the continuous phase µd ) viscosity of the dispersed phase µm ) average dispersion viscosity of the liquid-liquid mixture Fc ) density of the continuous phase Fd ) density of the dispersed phase Fm ) average density of the liquid-liquid mixture φ ) volume fraction of the dispersed phase

Literature Cited (1) Rosen, M. J. Emulsification by surfactants. In Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989.

(8) Tanaka, M.; O’shima, E. Dispersing behavior of droplets in suspension polymerization of styrene. Can. J. Chem. Eng. 1988, 66, 29-35.

(10) Hong, P. O.; Lee, J. M. Unsteady-state liquid-liquid dispersions in agitated vessels. Ind. Eng. Chem., Process Des. Dev. 1983, 22, 130-135. (11) Shinnar, R. On the behavior of liquid dispersions in mixing vessels. J. Fluid Mech. 1961, 10, 259-275.

Received for review September 27, 1999 Revised manuscript received February 29, 2000 Accepted February 29, 2000 IE990709I