Miniemulsion Copolymerization in Batch and Continuous Reactors

1792

Ind. Eng. Chem. Res. 1999, 38, 1792-1800

Miniemulsion Copolymerization in Batch and Continuous Reactors Charles J. Samer and F. Joseph Schork* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Conventional emulsion polymerization (also called macroemulsion polymerization) has been widely used in a variety of industries whereas nucleation and polymerization directly in submicron monomer droplets (i.e., miniemulsion polymerization) has been primarily an academic curiosity. However, much interest has recently been generated on the basis of the prospects of incorporating very hydrophobic ingredients into a water-based system via miniemulsion polymerization. In this work, a very hydrophobic acrylic monomer, 2-ethylhexyl acrylate, is copolymerized with methyl methacrylate using a miniemulsion recipe. The goal of this paper is to examine emulsion copolymerization kinetics for monomers of very different water solubilities. It is believed that the effects of water solubility differences between comonomers are minimized using a miniemulsion recipe. In this case, the copolymer composition is more uniform in batch miniemulsion copolymerization experiments, relative to batch macroemulsion copolymerizations. The same behavior is, however, not observed in a continuous-stirred tank reactor. At even moderate steady-state conversions, there is a substantial driving force for monomer transport that favors incorporation of the more water-soluble comonomer. 1. Introduction Copolymerization refers to the process by which two monomers (M1 and M2) are simultaneously polymerized. Odian1 describes four different ways in which each monomer can be incorporated into the copolymer chain. Copolymers can combine the more desirable properties of two monomers more efficiently than a simple blend of two homopolymers. Therefore, many commercial emulsion polymers are based on copolymerization, as well as terpolymerization (where three monomers are reacted simultaneously). In order to more fully benefit from copolymerization, scientists and engineers have long been interested in understanding the mechanisms and kinetics of such reactions. One of the most challenging aspects of copolymerization is controlling the relative incorporation of each monomer in the resulting copolymer chain. Staudinger1 observed that the relative copolymerization rates of monomers were sometimes very different from their relative rates of homopolymerization. Mayo and Lewis1 developed the following equation to describe copolymerization kinetics:

d[M1] d[M2]

)

[M1](r1[M1] + [M2]) [M2]([M1] + r2[M2])

(1)

where [Mi] is the molar concentration at the site of propagation. The reactivity ratios, r1 and r2, are defined by the homopropagation rate constants (kpii) divided by the crosspropagation rate constants (kpij) such that

r1 )

kp11 kp12

and r2 )

kp22 kp21

(2)

where r1 and r2 are determined experimentally, typically from bulk or solution polymerization experiments. Dif* Author to whom correspondence should be addressed. Phone: 404-894-3274. Fax: 404-894-2866. E-mail: [email protected].

ferent types of copolymerization behavior are observed, depending on the values of r1 and r2. Equation 1 is rewritten in a more useful form, where the comonomer compositions (f1 and f2) are related to the instantaneous copolymer compositions (F1 and F2):

r1f1 +1 F1 f2 ) F2 r2f2 +1 f1

(3)

where

f1 [M1] F1 d[M1] and ) ) F2 d[M2] f2 [M2] Equation 3 is known as the copolymerization equation. It is often necessary to calculate the average copolymer composition as a function of the total monomer fractional conversion, x. In this case, the comonomer mole fractions, f1 and f2, will change as the monomer is converted to a polymer:

(1 - x)1+r1r2-r1-r2 )

() () ( f1 f10

r2-r1r2

f2 f20

r1-r1r2

×

)

(1 - r2)f20 - (1 - r1)f10 (1 - r2)f2 - (1 - r1)f1

1-r1r2

(4)

where fi0 is the initial mole fraction of Mi1. The average copolymer composition, F h 1, is then calculated at a specific value of x and f1 using the following equation:

F h1 )

f10 - f1(1 - x) x

(5)

The instantaneous copolymer composition, F1, given earlier by eq 3, becomes increasingly different from F h1 as the conversion increases. Equation 4 applies to a

10.1021/ie980706q CCC: $18.00 © 1999 American Chemical Society Published on Web 04/16/1999

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1793

batch reactor. A slightly easier equation will be derived later for the copolymer composition as a function of conversion in a continuous reactor. A miniemulsion is similar to a conventional emulsion (also called a macroemulsion) where a small amount ( F1Homogeneous for all f1. Guillot5 observed this behavior experimentally for the common comonomer pairs of styrene-acrylonitrile and butyl acrylate-vinyl acetate. Both acrylonitrile and vinyl acetate are relatively water-soluble (8.5 and 2.5 wt %, respectively) whereas styrene and butyl acrylate are relatively water-insoluble (0.1 and 0.14 wt %, respectively). However, in spite of the fact that styrene and butyl acrylate are relatively water-insoluble, monomer transport across the aqueous phase is normally fast enough to maintain equilibrium swelling in the growing polymer particle and therefore allow use of the monomer partition coefficient. There are a variety of empirical and theoretical treatments for determining the partition coefficients, yet Schuller found that the copolymer composition is rather insensitive when K1 ) ∞ and K2 ) 1. This is a reasonable assumption for styreneacrylonitrile and butyl acrylate-vinyl acetate. A list of water solubilities for some common emulsion polymerization monomers is given in Figure 2. The relative water solubilities indicated at the top of the figure are arbitrarily chosen for this work and are based on several factors that will become evident in the following sections. In the previous discussion the focus was on emulsion copolymerization of monomers of different water solubilities, such that one monomer came from the relatively water-soluble category and the other monomer from the relatively water-insoluble category. This work also focuses on emulsion copolymerization of monomers of different water solubilities; however, the comonomers are taken, one each, from the relatively water-insoluble and extremely water-insoluble categories. 2.1. Extremely Water-Insoluble Comonomers. Macroemulsion Copolymerization. Reimers and Schork3 examined emulsion copolymerization of MMA

1794 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999

Figure 2. Water solubilities for various emulsion polymerization monomers.

Figure 3. Copolymer composition for vinyl stearate in a batch macroemulsion copolymerization reaction. Fraction of vinyl stearate (F h 2) in a copolymer for batch macroemulsion copolymerization as function of total conversion (ref 3). Data indicated by the × sign. Model I (‚‚‚): K1 ) 1; K2 ) ∞. Model II (s): K1 ) K2 ) ∞. Model III (- - -): K1 ) ∞; K2 ) 1. The homogeneous copolymerization reactivity ratios are r1 ) 2.35 and r2 ) 0.02.

in a batch reactor with extremely water-insoluble comonomers, such as vinyl stearate (VS), vinyl hexanoate (VH), vinyl decanoate (VD), vinyl 2-ethylhexanoate (VEH), and p-methylstyrene (pMS). These experiments are of interest here to gain some insight into the kinetics of emulsion copolymerization of extremely water-insoluble comonomers and apply this information to similar reactions in a continuous-stirred tank reactor (CSTR). Although Schuller had developed eq 6 for monomers of different water solubilities, this particular situation is quite different from what Schuller had considered. For the comonomer pair of MMA-VS, K1 , K2 so that Schuller’s eq 6 predicts

F h 2Emulsion > F h 2Homogeneous The data of Reimers and Schork3 for the batch macroemulsion polymerization of MMA-VS is given in Figure 3 along with three different variations on the copolymerization equation:

model I f K1 , K2 (‚‚‚) model II f K1 ) K2 ) ∞ (s) model III f K1 . K2 (- - -)

The dotted line (model I) is based on Schuller’s partition coefficient model (eq 6) and shows that these predictions are in fact much worse than the predictions based on homogeneous polymerization reactivity ratios given by the solid line (model II). The dashed line (model III) shows the best fit with the data and is discussed below. In order to explain the poor agreement with models I and II in Figure 3, one must consider the relationship between water solubility and monomer transport. It has long been accepted that monomer transport is not a rate-limiting step in the conventional emulsion polymerization of relatively water-insoluble monomers, such as MMA, styrene, and butyl acrylate. However, the water solubility of VS is as much as 3 orders of magnitude smaller than these typical emulsion polymerization monomers. In this case, VS cannot readily cross the aqueous phase to saturate the growing polymer particles. (The major transport resistance is actually from the monomer droplets into the aqueous phase.) Therefore, Schuller’s partition coefficient model cannot be used here, since this assumes that the particles are saturated with both monomers, M1 and M2. The homogeneous polymerization model is not useful either since the monomer concentration in the particles is not identical to the bulk monomer concentration. Therefore, a pseudo-partition coefficient, κ, is proposed for extremely water-insoluble comonomers to assist in better understanding the data. κ2 is not a partition coefficient and does not have any physical significance. κ2 as used in this work is simply an adjustable parameter that replaces K2 in eq 6. Model III is based on this pseudo-partition coefficient model, where K1 ) ∞ and κ2 ) 1. Since this monomer cannot readily cross the aqueous phase, it is conceivable that a significant portion of the extremely water-insoluble monomer polymerizes directly in the large monomer droplets, separately from the MMA-rich latex particles. In the most extreme case, this could lead to submicron poly-MMA particles and micronsized poly-VS particles, although no experimental evidence is provided for such a bimodal distribution nor to the fact that a mixture of homopolymers would be obtained in that case. Miniemulsion Copolymerization. Extremely waterinsoluble comonomers are only selectively used in emulsion polymerization because of concerns about monomer transport limitations. Typically, copolymer composition can be manually adjusted by slowly feeding in the more reactive monomer throughout the reaction; however, this may not be helpful when trying to overcome monomer-transport limitations. Therefore, Reimers and Schork3 performed identical copolymerization experiments in miniemulsions, where monomer transport is less significant, to determine what effect this would have on the evolution of the copolymer composition. In their experiments, the extremely water-insoluble comonomer also acted as the hydrophobic swelling agent, and after sonication effected predominant droplet nucleation. As a result of nucleation and polymerization directly in submicron monomer droplets, these systems show very different behavior than macroemulsions, with respect to the copolymer composition. This difference is illustrated in Figure 4 for the miniemulsion copolymerization of MMA and VS. In this case, the miniemulsion recipe shows a significant increase in the fraction of the extremely water-insoluble comonomer incorporated at the very beginning of the reaction.


Figure 4. Copolymer composition for vinyl stearate in a batch mini- and macroemulsion copolymerization reaction. Fraction of vinyl stearate (F h 2) in a copolymer in a batch macroemulsion (×) and miniemulsion (b) copolymerization as a function of total conversion (ref 3). The homogeneous copolymerization reactivity ratios are r1 ) 2.35, and r2 ) 0.02 (s). The corresponding emulsion copolymerization reactivity ratios, derived from eq 6 where K1 ) ∞, κ2 ) 1, and ψ ) 0.41, are r′1 ) 13.01 and r′2 ) 0.013 (- - -).

The VS copolymer composition for the miniemulsion reaction given in Figure 4 is above what is predicted from the homogeneous polymerization reactivity ratios. This was attributed to the formation of micelles. Micelles act as sinks for the more water-soluble monomer. However, the total surface area of the droplets is still much greater than that of the micelles, and therefore the primary site of polymerization is richer in the more water-insoluble comonomer. In spite of this result, uniformity in copolymer composition throughout the miniemulsion polymerization reaction opens the door for a variety of new and unique emulsion polymers. 3. Experimental Section Most fundamental studies of polymerization reactions are done using batch reactors. Batch reactors are cheaper to build and to operate and are more efficient in terms of the time required to run a single reaction. However, polymerization studies in a CSTR can be useful for the following reasons: (1) Data analysis is easier and more accurate in a CSTR. (2) CSTRs are more discriminatory for the elucidation of certain process mechanisms. (3) Some commercial products are produced in continuous reactors. A flow diagram of the reactor system is shown in Figure 5. Two separate feed streams, emulsion and initiator, were fed at constant rates by an FMI Model QG-20 valveless piston pump and a MasterFlex variable flow-metering pump, respectively. These pumps maintained an aqueous volume fraction of approximately 0.7 in both reactors. The emulsion and initiator feed tanks are 5 and 3 L round-bottom flasks, respectively. In the miniemulsion configuration (i.e., hydrophobe included in recipe), the emulsion stream is continuously sonicated via a Fischer Model 30 sonic dismembrator. The flow cell has a working volume of approximately 20 mL and is jacketed to remove any heat generated by the action of the sonicator. The CSTR is a two-piece baffled resin kettle agitated by a three-bladed propeller operating at about 300 rpm. An overflow weir maintains the volume of this perfectlymixed CSTR at about 240 mL. Salt tracer experiments

were used to validate the ideal mixing model assumed for this CSTR, but are not reported here. A YSI glass conductivity cell was immersed into the CSTR and conductivity was recorded on a YSI Model 35 conductivity meter. Upon exiting the CSTR, the emulsion is fed to a 25 mL quench vessel operating at approximately 20 °C. A slip stream of this latex is pumped to the densitometer via another MasterFlex variable flow pump. Density is measured using an Anton-Paar DMA densitometer. Conversion is calculated using this density measurement and verified with random samples for gravimetric analysis. Conductivity was measured with a standard conductivty probe. Monomer droplet and particle diameters were measured off-line with a Malvern Autosizer IIc, using a helium-neon laser at 629 nm gathering light scattered at a 90° angle. The procedure for measuring droplet/ particle sizes for these systems is given in Fontenot and Schork.6 Particle concentrations were calculated in the following manner using the measured conversion (x) and particle diameter (dpi):

(mass Lmonomer ) aq

6(x) Np )

πd h p3Fp

(7)

where Fp is the polymer density and d h p is the volume average (or root mean cube (RMC)) diameter of the particles. Particle diameters are measured using a lightscattering technique described earlier. The volume average diameter is calculated from the particle size number distribution as

d hp )

(

∑dp 3Np ∑Np i

)

i

i

1/3

(8)

Monomer droplet concentrations are calculated in a similar manner, although the variability of such measurements is almost (50% in some cases. Samples for monomer droplet size were taken at the exit of the sonicator flow cell before and after each miniemulsion experiment to obtain an approximate value for the monomer droplet diameter in the feed stream. Monomer droplet sizes were not measured for macroemulsion polymerization reactions. Copolymer compositions were determined by NMR.3 Both feed tanks were charged with deionized water and sparged for 30 min with pure nitrogen to remove dissolved oxygen before any chemicals were added. During the reaction, a very slow nitrogen purge stream was used to eliminate oxygen in the head space of the feed tanks. In addition, the CSTR was purged with nitrogen for at least 30 min prior to the reaction and during the reaction. The sparger was then removed from the emulsion feed tank, the SLS was then added to the water, and the mixture was then gently agitated to allow the surfactant to completely dissolve. If required, hexadecane was added to the necessary monomer charge and sparged with pure nitrogen for 5 min before being added to the surfactant-water solution in the emulsion feed tank. The emulsion was agitated vigorously using an anchor-type agitator blade for at least 1 h. The agitator speed was adjusted to create a significant vortex in the emulsion feed tank without causing excessive foaming. Potasium persulfate (KPS) is then


Figure 5. Schematic of a CSTR for continuous miniemulsion polymerization reactions. Macroemulsion polymerization reactions are performed in the same apparatus where the sonicator is taken off-line.

added to the initiator tank and allowed to completely dissolve via a magnetic stirrer. Temperature control for the CSTR was provided by an Omega Series 6100 temperature controller, a Type-J thermocouple, and two immersion resistance heaters. The thermocouple measures the emulsion temperature inside the reactor, which is compared to the temperature setpoint programmed into the temperature controller and, if necessary, the temperature controller increases the voltage signal to the two resistance heaters, which heat the water bath around the CSTR. Effective temperature control was possible without an active cooling cycle because of the continuous feed of the cooler emulsion and initiator streams. Throughout this work the temperature in the reactor was 45 ( 2 °C. The reactor overflow was initially sealed and the reactor was purged with nitrogen before and during the reaction. The cooling water flow to the condenser and quench vessel could be turned on at the start of the reaction. Both the initiator and emulsion feed lines were primed and their flows verified for the desired experiment. The water bath around the CSTR was preheated. The reaction begins when the initiator and emulsion feed pumps were turned on. The reaction began from an empty reactor. After 1 unit of residence time, the reactor overflow was unsealed so that the product begins to fill the quench vessel. Methyl methacrylate was supplied, with the inhibitor, by Rohm and Haas. The inhibitor, methyethyl hydroquinone (45 ppm MEHQ), was removed by vacuum distillation and stored at 5 °C prior to use. 2-Ethylhexyl acrylate, inhibited with 10 ppm MEHQ, was purchased from Aldrich Chemical. The inhibitor was removed by washing with a 10 wt % NaOH solution saturated with NaCl and stored at 5 °C prior to use. Potassium persulfate (KPS, 99%), sodium lauryl sulfate (SLS, 98%) and hexadecane (HD, 99+%) were purchased from Aldrich Chemical Co. Polymethyl methacrylate was purchased from PolySciences (Mn ) 300 000) and was used, in some experiments, in place of HD to stablize

the miniemulsions. Ultra-high-purity nitrogen (99+%) was purchased from Air Products and Chemicals. The apparatus used in the batch experiments was similar to the CSTR; however, the procedure was very different. The two-piece resin kettle was used for these batch experiments where the overflow nozzle was sealed with a rubber septa. Deionized water was sparged with pure nitrogen for 15 min before the emulsifier was added. For the miniemulsion experiment, monomer and hydrophobe were mixed together for 10 min before being added to the soap solution. The emulsion mixture was sonicated for 6 min at 60% of maximum sonicator power. A magnetic stirrer was used for bulk mixing during sonication. The miniemulsion (or macroemulsion) was then added to the reactor and heated to 45 °C. The reaction begins by injecting the initiator solution into the reactor. Conductivity was measured during these batch experiments to probe the mechanism of nulceation; however, the density was not measured during the reaction. Several samples were taken during the reaction for gravimetric determination of conversion. 4. Batch Emulsion Copolymerization of 2-Ethylhexyl Acrylate The goal of this work was to analyze the copolymerization kinetics of MMA (M1) and 2-ethylhexyl acrylate (M2) in a CSTR for miniemulsions and macroemulsions. 2-Ethylhexyl acrylate (EHA) was chosen for this work because of its extremely low water solubility and because it is a commercially important monomer. Also, several important kinetic parameters are readily available in the literature. Batch miniemulsion and macroemulsion copolymerization experiments were performed first to verify the conclusions of Reimers.3 Some literature references are given for the emulsion polymerization of EHA for co- and terpolymerization reactions with very water-soluble monomers in batch reactors.7,8 This previous work focuses on the mechanical and physical properties of these EHA co- and

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1797 Table 1. Emulsion Recipe for Batch Miniemulsion and Macroemulsion Copolymerization Reactions variable

value

temperature agitator water MMA EHA HD

45 °C 300 rpm 250 mL 90 mL 8 mL 2 mL

[S]a ) [I]a

0.02

(Lmolaq)

terpolymers and does not include any information on the evolution of the copolymer composition. Masa et al.9 investigated the terpolymerization of EHA/styrene/ methacrylic acid in a semi-continuous reactor for miniand macroemulsions. They found that the rate of polymerization is faster in macroemulsions than in miniemulsions; however, the miniemulsions had better ionic stability and lower viscosities as a result of an increase in carboxyl groups at the surface of the miniemulsion latex particles. They concluded that the surface of the miniemulsion latex was richer in acidfunctional groups since the miniemulsion particle is more hydrophobic as a result of the addition of hexadecane (HD). In light of more recent information, this result may also be due to the fact that EHA is the slowest monomer to cross the aqueous phase and therefore polymerizes last in macroemulsions, thus burying the acid-functional polymer. For the miniemulsion latex, the acid-functional monomer is concentrated at the surface of the particle since monomer transport is not an issue. This result would be similar regardless of whether HD was included. Miniemulsions are characterized by the presence of a cosurfactant, more accurately referred to as a hydrophobe. The hydrophobe is soluble in the organic phase, but insoluble in the aqueous phase, and stabilizes the submicron monomer droplets against diffusional degradation by lowering the Gibbs energy of the dispersed phase. In the work of Reimers,3 the extremely waterinsoluble comonomers also acted as the hydrophobe. However, in this present work, EHA is not sufficient to produce stable submicron monomer droplets. With EHA alone, the resulting miniemulsion degraded within 15 min, thus preventing effective miniemulsion polymerization. Therefore, a small amount of HD was added to the organic phase to insure predominant droplet nucleation. The batch recipe is given in Table 1. The copolymer compositions for the batch emulsion copolymerization of MMA and EHA are presented in Figure 6 as a function of the total monomer conversion for a macroemulsion recipe (one experiment, ×) and a miniemulsion recipe (two experiments, b and O). As expected, the copolymer is richer in the more waterinsoluble comonomer in miniemulsions than in macroemulsions. Furthermore, the miniemulsion copolymerization data more closely follow the copolymerization equation, given by the solid line in Figure 6, where the reactivity ratios are reported by Patnaik10 from bulk copolymerization experiments. The macroemulsion copolymerization data show good agreement with Schuller’s equation, given by the dashed line, where K1 ) ∞ and κ2 ) 4. It is important to recall that κ2 is not truly a partitition coefficient, since EHA does not readily saturate the growing polymer particles in the macroemulsion experiments as a result of limited monomer transport.

Figure 6. Copolymer composition for 2-ethylhexyl acrylate in batch miniemulsion and macroemulsion copolymerization reactions. Fraction of 2-ethylhexyl acrylate (F h 2) in a copolymer for batch macroemulsion polymerization (×) and miniemulsion polymerization (b and O) as a function of total conversion where f20 ) 0.041. The homogeneous copolymerization reactivity ratios are r1 ) 1.85 and r2 ) 0.42 (s). The corresponding emulsion copolymerization reactivity ratios, derived from eq 6 where K1 ) ∞, κ2 ) 4, and ψ ) 0.41, are r′1 ) 3.03 and r′2 ) 0.26 (- - -).

Although the difference in copolymer composition is minimal, there is a distinct advantage to using miniemulsions to improve the uniformity of the copolymer with respect to the composition of the extremely waterinsoluble comonomer incorporated. In this case, EHA is slightly more water-soluble than originally believed, which may be attibuted to the presence of MMA solubilized in the aqueous phase. This result may also contribute to the fact that EHA does not make for a very good hydrophobe. 5. Continuous Emulsion Copolymerization of 2-Ethylhexyl Acrylate The copolymer experiments presented in the previous section suggest that batch microemulsion copolymerization leads to an increase in the amount of the more water-insoluble monomer (EHA) incorporated into the copolymer, relative to batch macroemulsion copolymerization. In order to further example the prospects of incorporating extremely water-insoluble ingredients into a water-based system, miniemulsion and macroemulsion copolymerization experiments were examined in a single CSTR. The mole fraction of EHA in the comonomer mixture was varied between 0.05 and 0.033, where all other conditions remain constant. Miniemulsions included a small amount of hexadecane (HD), as well as continuous sonication, to ensure predominant droplet nucleation. Specific recipes and operating conditions are given in Table 2. All experimental measurements included in this section were obtained as steady state. 5.1. Steady-State Conversion and Particle Concentration. Steady-state total monomer fractional conversion is plotted in Figure 7 for macroemulsions (×) and miniemulsions (b) as a function of the comonomer composition in the feed stream. There is little evidence of an increase in steady-state converion with feed composition. One might be expected since the propagation rate constant for EHA is approximately 1 order of magnitude greater than that of MMA.11 It is unclear that the polymerization rate for miniemulsion copolymerization is significantly greater than that for macro-


Figure 7. Total number fractional conversion for macroemulsion (×) and miniemulsion polymerization (b) in a CSTR as a function of the inital EHA mole fraction. The broken lines refer to the homopolymerization of MMA under similar conditions for HDstabilized miniemulsions (- - -) and PMMA-stabilized miniemulsions (‚‚‚), and macroemulsions (-‚-).

Figure 8. Particle concentration for macroemulsion (×) and miniemulsion polymerization (b) in a CSTR as a function of the initial EHA mole fraction. The broken lines refer to the homopolymerization of MMA for HD-stabilized miniemulsions (- - -) and PMMA-stabilized miniemulsions (‚‚‚) and macroemulsions (-‚-).

Table 2. Basic Recipe for Continuous HD-Stabilized Miniemulsion Copolymerization Reactions variable

value

temperature sonicator duty EHA mole fraction

45 °C 80% 0.05, 0.112, 0.177, 0.33

hydrophobe

0.025

agitator

300 rpm

emulsion

0.3

[S]a [I]a

(Lmolaq) (Lmolaq)

θ (min)

g of HD g of MMA

L emulsion L aq

0.02 0.02 20 min

emulsion copolymerization, regardless of the initial comonomer concentration. Steady-state particle concentrations are plotted in Figure 8 for macroemulsions (×) and miniemulsions (b). As expected, the particle concentration increases significantly from micellar nucleation to droplet nucleation. This increase is attributed to the fact that particle nucleation and particle growth are decoupled via predominant droplet nucleation. On the basis of the differences in conversion and particle concentration between miniemulsion and macroemulsion copolymerization observed in Figures 7 and 8, it is believed that miniemulsion copolymerization is characterized by predominant droplet nucleation. This is consistent with the continuous homopolymerization results reviewed below. The broken lines in Figures 7 and 8 correspond to the homopolymerization of MMA under similar conditions for HD-stabilized miniemulsions (- - -), polymer-stabilized miniemulsions (‚‚‚), and macroemulsions (-‚-). This information provides further evidence that these continuous copolymerization reactions behave as expected in miniemulsions and macroemulsions. The miniemulsion copolymerization data appears to fall between HD- and poloymer-stabilized miniemulsions, which suggests that the initial monomer droplet concentration is between that of HD-stabilized and PMMA-

Figure 9. Average copolymer composition for macroemulsion (×) and miniemulsion (b) copolymerization in a steady-state CSTR as a function of total monomer fractional conversion for f20 ) 0.053. The homogeneous copolymerization reactivity ratios are r1 ) 1.85 and r2 ) 0.42 (s). The corresponding emulsion copolymerization reactivity ratios, derived from eq 6 where K1 ) ∞, κ2 ) 2, and ψ ) 0.41, are r′1 ) 4.05 and r′2 ) 0.19 (- - -).

stabilized miniemulsions. However, the evidence still suggests predominant droplet nucleation. With this is mind, the next section will examine the differences in copolymer composition between miniemulsions and macroemulsions, much like what was done earlier for batch copolymerization experiments. 5.2. Copolymer Composition. On the basis of the batch copolymerization experiments given earlier, and also on the fact that droplet nucleation dominates continuous miniemulsion copolymerization, one would expect the average compositions to follow the relation F2Mini > F2Macro. However, this same behavior is not observed in a CSTR. The steady-state copolymer compositions for macroemulsions (×) and miniemulsions (b) are plotted versus total monomer conversion for f20 equal to 0.053 and 0.177 in Figures 9 and 10, respectively. Although the steady-state conversion increases significantly from macroemulsions to miniemulsions, the copolymer composition is nearly identical for both reactions. In this case it does not appear that droplet nucleation leads to an increase in the amount of the


Figure 10. Average copolymer composition for macroemulsion (×) and miniemulsion (b) copolymerization in a steady-state CSTR as a function of total monomer fraction conversion for f20 ) 0.177. The homogeneous copolymerization reactivity ratios are r1 ) 1.85 and r2 ) 0.42 (s). The corresponding emulsion copolymerization reactivity ratios, derived from eq 6 where K1 ) ∞, κ2 ) 2, and ψ ) 0.41, are r′1 ) 4.05 and r′2 ) 0.19 (- - -).

extremely water-insoluble comonomer incorporated into the copolymer, as observed in batch reactors. These experimental copolymer compositions are compared with the predicted copolymer compositions calculated from eq 3 where the reactivity ratios are obtained from homogeneous copolymerization experiments (s) and where the pseudo-partition coefficient, κ2, is adjusted to fit the data using eq 6 (- - -). In this analysis, the experimental values of total monomer conversion and copooymer composition are used together, in a steady-state material balance, to calculate the compositions of the unreacted monomers. These comonomer compositions are then used in thhe copolymerization equation with the appropriate reactivity ratios to determine the theoretical copolymer compositions for these figures. Surprisingly, both the macroemulsion and miniemulsion data show good agreement with Schuller’s modified reactivity ratio model (eq 6) where K1 ) ∞ and κ2 ) 2 (- - -). This includes both mini- and macroemulsion copolymer compositions at different initial comonomer compositions. Although it was found earlier that κ2 ) 4 in batch macroemulsion copolymerization experiments, the fact that κ2 ) 2 in these experiments does not necessarily suggest that EHA is more water-insoluble in a CSTR relative to a batch reactor. Recall that κ is not a true partition coefficient and therefore is not independent of the external environment, such as the type of reactor, or total conversion. Since the water solubility cannot change, it is suggested that monomer transport, or at least the relative differences between monomer transport for MMA and EHA, changes between batch and continuous miniemulsion copolymerization reactions. In order to evaluate this theory, a simple steady-state particle number model is developed here to simulate miniemulsion copolymerization in a CSTR. These continuous model results are then considered in light of the earlier batch copolymerization results. 6. Conclusions Miniemulsion copolymerization in a CSTR of monomers of very different water solubilities is characterized by predominant droplet nucleation. This conclusion is

based on the fact that the steady-state particle concentration and polymerization rate for a miniemulsion copolymerization recipe are significantly greater than those for a macroemulsion copolymerization recipe. These results are consistent with homopolymerization experiments in a CSTR for HD-stabilized miniemulsions and PMMA-stabilized miniemulsions and macroemusions. The difference in copolymer composition between miniemulsion and macroemulsion copolymerization in a batch reactor is not observed in a CSTR. In this case, the copolymer composition for the extremely waterinsoluble comonomer in a miniemulsion recipe decreases from a batch reactor to a CSTR. This difference can be attributed to the fact that monomer transport is enhanced in the steady-state CSTR where fresh monomer droplets are in contact with “monomer-starved” particles. The comonomers cannot cross the aqueous phase at similar rates because of their water-solubility differences; therefore, this favors incorporation of the more water-soluble comonomer. In a batch reactor all of the droplets are nucleated at roughly the same time; therefore, little monomer is available to quench monomerstarved particles. However, this does not preclude miniemulsion copolymerization in a CSTR for extremely water-insoluble comonomers. In spite of the fact that the copolymer composition in the continuous miniemulsion is less than that predicted using the homogeneous copolymerization reactivity ratios, the miniemulsion copolymer might be more uniform than the macroemulsion copolymer where the possibility of significant droplet nucleation could lead to two separate homopolymers or, at the very best, copolymers of various composition. Therefore, it is very important to use CSTR data to scale up a continuous miniemulsion copolymerization product to take into account the different particle growth kinetics for batch and continuous reactors. Acknowledgment The financial support of the National Science Foundation under Grants CTS-9417306 and CTS-9224813 is gratefully acknowledged. Literature Cited (1) Odian, G. Principles of Polymerization; Wiley: New York, 1991. (2) Ugelstad, J.; El-Aasser, M. S.; Vanderhoff, J. W. Emulsion Polymerization: Initiation of Polymerization in Monomer Droplets. J. Polym. Sci.: Polym. Lett. Ed. 1973, 11, 503. (3) Reimers, J. L.; Schork, F. J. Miniemulsion Copolymerization Using Water-Insoluble Comonomers as Cosurfactants. Polym. React. Eng. 1996, 2 (2-3), 135. (4) Schuller, H. Copolymerization in Emulsion. In Polymer Reaction Engineering; Reichert, K., Geisler, W., Eds.; Huthig and Wepf Verlag: Heidelberg, 1986. (5) Guillot, J. Computer Simulation of Emulsion Processes for Monomers of Different Water Solubility. In Polymer Reaction Engineering; Reichert, K., Geisler, W., Eds.; Huthig and Wepf Verlag: Heidelberg, 1986. (6) Fontenot, K. F.; Schork, F. J. Batch Polymerization of Methyl Methacrylate in Mini/Macroemulsions. J. Appl. Polym. Sci. 1993, 49, 633. (7) Loncar, F. V.; El-Aasser, M. S.;Vanderhoff, J. W. Emulsion Copolymerization of 2-Ethylhexyl Acrylate with Acrylic Acid and Methacrylic Acid. Polym. Mater. Sci. Eng. 1986, 54, 453. (8) Pourahmady, N.; Bak, P. I. Structure and Properties of Terpolymers from Emulsion Polymerization of Vinyl Chloride, 2-Ethylhexyl Acrylate and 2-Hydroxyethyl Acrylate. Polym. Mater. Sci. Eng. 1993, 63, 427.

1800 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 (9) Masa, J. A.; Lspez De∼Arbina, L.; Asua, J. M. A Comparison Between Miniemulsion and Conventional Emulsion Terpolymerization of Styrene, 2-Ethylhexyl Acrylate and Methacrylic Acid. J. Appl. Polym. Sci. 1993, 48, 205. (10) Patnaik, M.; Choudhary, V.;Varma, K. I. Structural and Thermal Characterization of Methyl Methacrylate/Alkyl Acrylate Copolymers. Eur. Polym. J. 1992, 28 (11), 1433. (11) Beuermann, S.; Paquet, D. A.; McMinn, J. H.; Hutchinson, R. A. Determination of Free-Radical Proagation Rate Coefficients

of Butyl, 2-Ethylhexyl, and Dodecyl Acrylates by Pulsed-Laser Polymerization. Macromolecules 1996, 29, 4206.

Received for review November 9, 1998 Revised manuscript received February 10, 1999 Accepted February 18, 1999 IE980706Q

Miniemulsion Copolymerization in Batch and Continuous Reactors

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