emulsion polymerization and copolymerization in continuous reactor systems

Polymer International 30 (1993) 243-251

Emulsion Polymerization and Copolymerization in Continuous

Reactor Systems*

Gary W. Poehlein School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0370, USA

(Received 3 February 1992; accepted 30 April 1992)

Abstract: Continuous reactors are important for commercial production of latex products and they can be useful tools for fundamental polymerization kinetics studies. Results are presented for four projects: (i) elimination of conversion oscillations in continuous stirred-tank reactors (CSTR) by use of a tubular pre- reactor; (ii) use of a seed-fed CSTR to measure free radical transport from monomer-swollen latex particles; (iii) development of a continuous process as an alternative to a commercial batch process; and (iv) determination of copolymerization kinetic parameters with a steady-state CSTR system.

Key words: emulsion polymerization, continuous reactors, kinetics, latex manufacture.

INTRODUCTION

Latex products are manufactured in various types of batch, semi-batch and continuous processes. Semi-batch (sometimes called semi-continuous) reactors are the most common because they are flexible and they permit control of polymerization rates and important latex charac- teristics such as particle sizes and morphology as well as molecular weights and compositions. Batch processes cannot be used for many products simply because of heat removal limitations, especially with large reactors. Control of particle morphology and other latex charac- teristics is also very limited with batch reactors.

Continuous reactor processes have been used for the manufacture of selected latex products since the 1940s. A variety of reactor systems have been described in the literature,' including stirred tanks, tubular circulation loops, single-pass tubes, a pulsed packed column; and combinations of tubes and stirred tanks. The primary reasons for studying emulsion polymerization in continuous reactors are for development of commercial

* Presented at International Symposium on Polymeric Micro- spheres, Fukui, 23-26 October 1991.

processes and for the elucidation of fundamental reaction mechanisms and kinetic models.

Continuous commercial processes can offer economic advantages, uniform quality products and high production rates. Continuous research reactors comprised of stirred tanks operating at steady state provide for simple measurement of polymerization rates and copolymer composition under fixed conditions-algebraic rather than differential equations. In addition, the broad distribution of residence times makes the continuous stirred tank an ideal reactor for studying competitive growth of different size latex particles.

The purpose of this paper is to demonstrate the utility of continuous reactor studies for examination of fundamental reaction phenomena and for process development. Four specific research studies will be briefly described: two related to solving commercial process problems and two associated with kinetic modeling.

STABLE OPERATION OF CSTR SYSTEMS

One of the major problems associated with the use of continuous stirred-tank reactor (CSTR) systems for the manufacture of polymer latexes is the prevention or

243 Polymer International 0959-8103/92/$05.00 0 I992 SCI. Printed in Great Britain

244 Gary M Poehlein

control of limit-cycle oscillations which seem to occur naturally in single CSTRs or in the first CSTR in a series of such reactors. Greene et ~ 1 . ~ reported significant monomer conversion oscillations in a single CSTR used for emulsion polymerization of methyl methacrylate.

Figure 1 illustrates that these oscillations can be of large magnitude and that the time scale for a complete cycle can be quite long. Please keep in mind that the feed streams (rates and compositions) to this reactor were held constant during the run shown in Fig. 1. The time scale shown is real time divided by the mean residence time in the reactor (0 = reactor volume/volumetric effluent flow rate). One cycle in the conversion oscillation requires about eight reactor turnovers. The experiment was stopped after about 14 mean residence times because the supply of feed materials was depleted.

Ley & Gerrens4 reported oscillations in both monomer conversion and surface tension in the effluent of a CSTR for a high-conversion styrene emulsion polymerization experiment. These data are shown in Fig. 2. The small but clearly evident monomer conversion oscillations follow the same trends as the rate at which droplets of the effluent stream exited from a capillary tube-a parameter related to latex surface tension. Nearly five complete cycles are shown and there appears to be no damping of the magnitude of the oscillations. The time for one complete cycle or oscillation was about 370min or 9.2 mean residence times (reactor turnovers).

Berens' studied the emulsion polymerization of vinyl chloride in a single CSTR. He observed transient behavior in the particle size distribution in the effluent latex, as shown in Fig. 3. The different particle size distribution (PSD) curves shown seem to indicate different periods of particle nucleation.

The unsteady cyclic behavior shown is generally agreed to be caused by coupled particle nucleation and growth phenomena. The nucleation of new particles when free surfactant is present is very rapid. Fast nucleation

50k

0 I I I I I I I I I ' I I I

DIMENSIONLESS TIME, we) Fig. 1. Conversion transients for MMA emulsion polymerization in an unseeded CSTR (Greene et d3). t=Time; @= mean residence time. 6 = 20 min; T = 40°C; monomer: H,O = 0.43; [ sur fac tan t ] = [ in i t ia tor ] = 0.01 mol/ l i t e r ;

[NaHCO,] = 0.01 mol/liter; [NaCl] = 0.02 molhiter.

0 2 4 6 8 1 0 1 2 1 4

_"/"'.. I 1 1 1 1 ~ 1 ........................... ... . . . . . . . . . . . . . . . . . . . . . . $ 1 . ?I.. . . ... . . . \ . . . . . . . . .. ..

3~ z 5 11 0k-h & I 4 : I & ; I & O ! ' 1 u- 0 5 I5 25 s '5 50

UPPER SCALE - - TIYE(min) LOWER SCALE - - DIMENSIONLESS TIME

Fig. 2. Conversion and surface tension transients for styrene emulsion polymerization in an unseeded CSTR (Ley &

Gerrens41.

I m 1ooo 1m 2 s o D 3 a x ) I ' l l ' l ' t l l l l l l l l ' l l I t l ' l ' I I t , I I

PARTICLE MAMETE3

Fig. 3. Particle size transients for VCI emulsion polymerization in an unseeded CSTR (Berens5).

followed by particle growth generates additional surface area which adsorbs surfactant and this serves as a mechanism for removing free surfactant from the continuous phase. When the particle surface area exceeds the surfactant supply, the nucleation of new stable particles stops. Hence, the nucleation period is probably very short compared to the length of one cycle.

The CSTR system, however, includes a steady feed stream which contains surfactant and an effluent stream which contains unsaturated particle surface area. Eventu- ally the added surfactant and the surface area (i.e. the particles) removed return the system to one containing free surfactant and new particles are again nucleated rapidly. The short-term nucleation period followed by particle growth and removal from the reactor is repeated again and again to sustain the observed limit cycles in conversion, particle size and number, and in surface tension of the latex in the reactor. The nucleation phenomena that trigger the oscillations are rapid but the total time of one complete cycle is generally 6-10 mean residence times.

A number of solutions have been proposed for this CSTR oscillation problem, including on-line control systems which measure turbidity or surface tension, on- line control coupled with by-passing the first CSTR with some of the recipe ingredients, and removal of nucleation

POLYMER INTERNATIONAL VOL. 30, NO. 2,1993

Emulsion polymerization and copolymerization in continuous reactor systems 245

I I I I I d I

2 4 4

Fig. 4.-Experimental continuous reactor system. 1, Monomer emulsion; 2, initiator solution; 3, duplex volumetric pump; 4, water bath; 5, tubular pre-reactor; 6, CSTR; 7, syringe pump; 8,

nitrogen supply.

from the CSTR by incorporating a seed latex in the feed stream or by generating a seed in a tubular reactor upstream of the CSTR. The use of seed or a pre-tubular reactor are the most certain methods for ensuring easily controlled steady-state operation of a CSTR. Properly designed and operated systems avoid nucleation in the CSTR with particle concentrations being controlled by seed addition to the feed stream or with the tubular pre- reactor. These techniques have been shown to be effective with a variety of polymerization recipe^.^-^

Gonzalez6 and Berens have all demonstrated that feeding a seed latex to prevent significant nucleation in the CSTR completely eliminates the oscillation problem. Stone used a spiral flow vessel as a pre-reactor. Berens simply incorporated a professional seed latex into the feed stream. Gonzalez6 and Lee7 utilized a plug-flow tubular pre-reactor process, as illustrated in Fig. 4.

The tubular pre-reactor was comprised of &in Teflon tube about 120ft long. Plug flow was achieved by injection of N, gas into a tee at the entrance to the reactor tube. The mean residence time in the tube could be varied independently from the CSTR mean residence time by changing the N, flow rate. Figure 5 shows stable operation with a recipe similar to that shown in Fig. 1 when the pre-reactor was not used. Stable operation with the tube-CSTR system has been demonstrated for a number of different polymers and copolymers.

A seed-fed CSTR offers at least one other advantage. Steady-state operation with a CSTR without seed (if steady state can be achieved) will produce fewer particles than a batch or tubular reactor when the same recipe is used. The particle number in an unseeded CSTR will be at least a factor of two lower and an order of magnitude decrease in particle concentration is possible, with a corresponding increase in particle size. Hence, the seed- fed CSTR system eliminates the cycling problem and

5 2 3 4 5 6 7 0 OO I

DIMENSIONLESS TIME ( m i Fig. 5. Stable conversion profile for MMA emulsion polymerization in a tube-CSTR system (Greene et ~ 1 . ~ ) . CSTRB= 30 min; [S] = [I] = 0.01 mol/liter; [NaHCO,] = 0.01 mol/liter; [NaCI] = 0.02 mol/liter; M/ W ratio = 0.43; tube effluent conver-

sion = 11.2% (CSTR feed).

produces more particles with smaller average size than a standard unseeded CSTR.

TRANSPORT OF FREE RADICALS OUT OF PARTICLES

Experimental processes such as shown in Fig. 4 can also be useful for fundamental kinetic studies. The so-called desorption of free radicals from monomer-swollen latex particles, for example, is an important phenomenon in emulsion polymerization that is reflected in the kinetic models utilized for fundamental studies and reaction simulations. The steady-state seed-fed CSTR system is a good tool for studying this phenomenon. Dubner et al." have developed a kinetic model for a seed-fed CSTR which can be used to predict monomer conversion and particle size distribution (PSD) in the effluent latex. All model parameters except the radical desorption coefficient can be obtained from the literature. Hence, experimental measurements of reactor conversion and/or seed and effluent particle size distributions can be used to study radical desorption.

The transport of growing radical oligomers out of particles is not likely because of entanglements and the hydrophobic nature of the species. Hence, desorption is thought to follow a chain transfer reaction in which a small, mobile free radical is generated. Lee' and Poehlein et a/." studied the influence of the added chain transfer agents, carbon tetrachloride and a series of normal mercaptans, on radical desorption. An example of the measured PSDs of the tube effluent (seed) and the CSTR effluent (product) are shown in Fig. 6. In this figure M/ Wis the monomer/water weight ratio, TIM is the weight ratio of chain transfer agent carbon tetrachloride to monomer, 0, is the CSTR mean residence time, the ( d ) values are average diameters, and XI and X , are the monomer


246 Gary K Poehlein

zl- 7

nl EW-STCZ9 M / l r = 28/72 T/Y - 0 2 2 [S]. = 0 0 2 Y z [I]. - 002 Y T. = 60 'C e. = 3 6 1 MIN

<d.> = 38 0 nm <d,> - 53 0 nm

X1 9 . 2 5 X2 = 25.1 $

I I

0 1.0 2.0 3.0 4.0 5.0 E DIMENSIONLESS DIAMETER

EW-STCZ9 M / l r = 28/72 T/Y - 0 2 2 [S]. = 0.02 Y [I]. - 002 Y T. = 60 'C e. = 36.1 MIN

<d.> = 38 0 nm <d,> - 53 0 nm

X1 9 . 2 5 X2 = 25.1 $

- 1.0 2.0 3.0 4.0 5.0 E DIMENSIONLESS DIAMETER

Fig. 6. Particle size distributions in CSTR feed and effluent streams/styrene emulsion polymerization (Lee7). (Feed is

generated in tubular pre-reactor.)

conversions in the seed effluent and the CSTR, respectively.

The steady-state CSTR model is used to fit the effluent PSD (and/or reactor monomer conversion). The fit is obtained by adjusting the desorption rate coefficient. Figure 7 shows the results for a typical experiment. The goodness-of-fit was judged by the area between the simulation curve and experimental histogram. Dimensionless diameter is the actual measured diameter divided by the average diameter of the particles in the seed latex. The parameters y, a:, p and r, are dimensionless

EXP-STCZ9 DEVIATION = 16.776 Z 7 = 2.8 x lo-'

a,/=769X

Y. =o.o = 10.3X lo-*

EXPERIMENTAL

S I Y U L A T I O N x 1.0 2.0 3.0 4.0 5.0 6.0 DIMENSIONLESS DIAMETER

Fig. 7. Comparison of model simulated and experimental PSDs/styrene emulsion polymerization (Lee').

LO

A Ftl V

LO

I I I I 10

4 6 8 10 12 NUMBER OF CARBON ATOMS IN MERCAPTAN

Fig. 8. Influence of mercaptan chain length on the radical desorption coefficient and average number of free radicals per

particle (Lee7). [RSH],/[M]o = 0.005, T = 60°C.

groups from the ii theory developed by Dubner et al." y, a: and r, are equivalent to the parameters rn, a' and Y in the Ugelstad & Hansen" treatment of the 6 theory for batch reactions with monodisperse particles. y includes the free radical desorption coefficient, or: includes initiation rate and particle size, and Y, accounts for water- phase termination. p is a new dimensionless group for the CSTR which includes the reactor mean residence time. Water-phase termination does not significantly influence the particle size distribution, which does depend on y, a: and p.

Figure 8 shows the influence of primary mercaptans of different length on the radical desorption coefficient and the average number of free radicals per particle. The molar ratio of mercaptans to monomer was maintained constant so the different ki and ii values were somewhat unexpected. Separate molecular weight measurements demonstrated that the rate of mercaptan transport to the reaction site in the particles was a function of chain length (or water solubility). The chain transfer reactions with the higher molecular weight mercaptans in the particles were limited by diffusion from the monomer-mercaptan droplets through the aqueous phase to the reaction sites in the monomer-swollen particles. These results clearly demonstrate the utility of a CSTR for emulsion polymerization kinetic studies.

CONTINUOUS EMULSION COPOLYMERIZATION OF ETHYL ACRYLATE (EA) AND METHACRYLIC ACID (MAA)

The MS thesis project of Glenn Shoaf was to design a continuous process for an existing low-solids latex


Emulsion polymerization and copolymerization in continuous reactor systems

B 2oo- 3 175-

w d 150-

Ei

125-

TABLE 1. Typical MAA/EA copolymerization recipe

Component

Water Monomers

S u rfacta nt Ferrous sulfate Potassium persulfate Sodium bisulfite

(MAA/EA ratio range = 0.053-3.0)

Weight fraction

0.821 0.1 57

0.022 8.64 E-7 0.000 32 0.000 17

copolymer product which was manufactured for captive use in a batch reactor. The recipe is shown in Table l .13*14

Most of the experiments were carried out at an MAA/EA weight ratio of 2.211.0. The batch production reactions were initiated at about 50°C and allowed to run nearly adiabatically with the large amount of water serving as a heat sink. The reaction was completed in about 10 min to produce a latex with average particle diameters in the range 120-130 nm. Typical batch reaction conversion time curves are shown in Fig. 9 and the corresponding temperature profiles are given in Fig. 10.

When the same recipe was fed to an unseeded CSTR, the high-conversion effluent had a particle concentration which was an order of magnitude lower and an average particle diameter more than twice as great as the batch product. This result is an example of the phenomenon mentioned earlier where an unseeded CSTR produces considerably fewer particles with the same recipe. Particle nucleation in a batch reaction produces enough surface area for adsorption of all the added surfactant early in the reaction and the surface of the final latex is not completely covered with surfactant. The effluent from an unseeded steady-state CSTR is saturated with surfactant and hence the particle number is smaller and the average particle size is larger. In this case, the CSTR product did not perform well in the application so different reactor configurations were examined.

TIME (MINUTES) Fig. 9. Conversion profiles for MAA/EA in three identical

batch reactions (Shoaft3).

0 2 4 6 0 TIME (MINUTES)

247

!

Fig. 10. Temperature profiles for commercial batch reactions shown in Fig. 9 (Shoaf13).

Application tests suggested that latex particle size was important for satisfactory performance. Hence, before further continuous reactor runs were carried out, the influence of agitation intensity and temperature was examined in batch reactions. Isothermal batch reactions were conducted at 50,55,60 and 70°C. Temperatures were controlled to within 2°C of these target values. The particle size results are shown in Fig. 11. Temperature has a significant influence on nucleation and, therefore, any continuous isothermal process will need to operate near 50°C in order to produce the desired small particles.

This result was somewhat surprising since previous workers reported that, in general, particle size tends to decrease with increased tempera t~re . '~ . '~ The main reason for the expected decrease in particle size with temperature is that more free radicals are formed at higher temperatures and thus more particles are generated. The increase in the number of particles results in a net decrease in size.

250 7

225 s P

I I I

40 50 60 70

TEMPERATURE (DEGREES C) D

Fig. 11. Effect of temperature on average particle size for isothermal batch MAA/EA copolymerization (Shoaf13).


248 Gary W Poehlein

Two explanations may account for the antithetical behavior observed with the MAA/EA system. First, as the initiation rate increased with increasing temperature, more particles were indeed formed. However, a larger number of particles requires more stabilizer to prevent coagulation. Priest” notes that if the stabilizer is not efficient or not present in sufficient amount, coagulation may occur. Stabilizer for the MAA/EA system refers to both surfactant and copolymerized acid. A larger number of particles results in a smaller amount of both species on the particle surface, thus leading to a less stable system. Another factor related to an increase in particle number is that more interparticle collisions could occur, thus increasing the chances for interparticle combination.

A second explanation involves the fact that the reactivity ratios af MAA and EA may have changed with temperature. A substantial increase in the reactivity ratio of EA relative to MAA could cause the rate of particle growth to increase significantly relative to the reaction and subsequent combination of aqueous-phase carboxyl groups onto the particle surface. If the carboxyl groups are indeed important for particle stabilization, then less of these species along the particle surface during the critical growth period could lead to increased coagulation and thus larger particles. More experimental data are needed in order to clarify this situation.

Mixing intensity may also affect the size of the polymer particles formed in emulsion reaction systems. Mixing can be maintained at a relatively constant level in a batch reactor. The mixing intensity in a continuous reactor, however, is not as easily controlled, especially if a tubular reactor is used. Mixing intensity in these reactors usually differs from that of a batch system. Therefore, it was necessary to examine the influence of mixing on the final particle size for the MAA/EA system before designing possible continuous processes. Four batch reactions were run with a wide range of stirred speeds (0-700 rpm), with

TABLE 2. Effect of mixing intensity on particle size

Particle size (nm)

Level of agitation rPm

-

No agitation 0 119+30 Low 40 120+30 Medium 400 124k30 High 700 132k40

all other conditions remaining the same. The monomer was effectively pre-emulsified before adding the initiators in each run. The results listed in Table 2 show that mixing had very little effect on particle size. Both the low- and the no-mixing cases produced particles with an average diameter very near the 120nm value that was observed with the normal batch product. A slight increase in particle size was observed at the higher mixer speeds, where high shear forces would tend to overcome the stabilizing effect of the surfactant. After these batch reactions were carried out, a series of continuous runs were made with a tube-CSTR combination at about 55°C. Both $- and &in tubes were used with and without N, plugs. Some runs at low flow rates resulted in phase separation and eventual tube plugging.

When this low flow rate was avoided, satisfactory products were produced in isothermal continuous reactors operated in the 50-60°C temperature range. The reaction could be carried out to near 100% conversion in a single-pass tubular reactor with a residence time of about 20 min. A tube-CSTR system also proved to be satisfactory. The important process conditions were nucleation at low temperatures (50-60°C) and the use of a tubular reactor (or pre-reactor) to achieve high particle concentrations and small diameters. A flow diagram of the proposed commercial continuous process is shown in Fig. 12.

T-4

Option 2

r-, ~2

Fig. 12. Flow diagram for proposed commercial process. FM = Flow meter; HE = heat exchanger; SM = static mixer (Shoaf l’).

POLYMER INTERNATIONAL VOL. 30, NO. 2.1993

Emulsion polymerization and copolymerization in continuous reactor systems 249

FUN D A M ENTAL COPOLY M ERlZATlON STUDIES

Copolymerization studies in a seed-fed CSTR system were carried out for the monomer pairs styrene-methyl acrylate: styrenePacrylonitrile,8 styrene-MAA,"*" styrene-acrylic acid (AA)"," and styrene-sodium styrene sulfonate." The water solubility of styrene is quite low (-0.005 g mol/liter), whereas the solubilities of the comonomers listed above vary from about 0.65 g moljiter for methyl acrylate to complete water miscibility for the acid and sulfonate monomers. Hence, one must know how these more water-soluble monomers are partitioned among the phases in order to develop satisfactory reactor models. Both methacrylic acid (MAA) and acrylic acid (AA) are completely soluble in water but they partition very differently, as shown in Figs 13 and 14.

The reactor models include consideration for partitioning of the monomers among the different phases in the reaction media, the transport of free radicals between the aqueous phase and the monomer-swollen particles, and the reactions occurring in the aqueous and particle phases. The partition data were used to establish correlations that were used with mass balances in the CSTR copolymerization simulations. When reactions are carried out with functional monomers, one can have changes in rate due to four parameters: (1) monomer concentrations in the particles, (2) effective propagation rate constant, (3) average number of free radicals per particle, and (4) number of particles. Research publica- tions do not often sort out which of these parameters are responsible for the observed experimental rate changes.

A seed-fed CSTR system can be useful in determining which parameters are responsible for experimental

0.5

w 4 0.4

E

t? 0.3

0.2 B 2 0.1

0.0 0.55 0.10 0.15 0.20 025 0.30

MONOMEFtflAmR RATIO Fig. 13. Partition of acid monomers between styrene and water (Lange'*). (Weight fractions, %s and ratios are used.) 0 = 10% MAA; a=5% MAA; .=2.5% MAA; V=lO% AA;

-+ = 5 % AA; x =2.5% AA.


0.040 I 1

M O N O M ~ F ~ P A ~ RATIO Fig. 14. Partition of acrylic acid between styrene and water (Lange"). (Weight fractions, %s and ratios are used.) 0 = 10%

AA; A = 5% AA; = 2.5% AA.

observations. If particle nucleation is absent, for example, one can compare polymerization rates among systems with different comonomer compositions but with the same particle number concentration. Figure 15 illustrates conversion-time profiles for styrene and styrene-acid copolymerizations in a seed-fed CSTR.'* The steady-state conversions predicted by the reactor model are also shown. Lange18 has demonstrated that the reductions in rate caused by the addition of acid monomer result from

I a'

0.0 3 2 3 4 5 6 7 8

DIMENSIONLES TIME Fig. 15. Comparison of steady-state simulations and experimental conversion profiles for styrene and styrene-acid monomer emulsion polymerizations in a seed-fed CSTR (LangeI8). Simulation exp. - - - - -, 0 = 0% acid monomer; _-- , A = 5% acrylic acid; = 5% methacrylic

acid.

250

1

OD -

0.6 - 0.4 -

M-

0 1

52 XAA

0 a e f i e 0

D

D

0 A A A

1 1 I I 1

Gary K Poehlein

-103 -I0]

0.8 4 D

D

'i i i i~ i ir i i i DIME3JsIONLESSTlME

Fig. 16. Conversion profiles of individual monomers for styrene and M A A in a seed-fed CSTR (Lange"). 0 =Overall conversion; a = M A A conversion; = styrene conversion.

acid monomer chain transfer reactions in the particles which enhance radical desorption and lower the average number of free radicals per particle.

The adjustable parameters in the CSTR model were determined from seeded batch reaction experiments. The resulting radical desorption coefficients, t?; (in cm2/s) were 2.2 x (styrene), 8.0 x (styrene-AA) and 3.0 x

(styrene-MAA). The 6 value is higher for MAA than for AA because more MAA is partitioned in the monomer-swollen particles. This increases the rate of radical transfer to MAA monomer and enhances the desorption rate, thus lowering f i and the rate of polymerization.

SX M

Fig. 18. Free radical desorption coefficients for styrene-methyl acrylate emulsion copolymerization reactions (Mead').

The partitioning also influences the reactions taking place in the continuous and particle phases and the conversions of the different monomers. Figures 16 and 17 illustrate such data for the styrene-MAA and styrene- AA systems. The MAA conversion is higher than that of AA because it is more strongly partitioned in the particles where most of the reaction takes place.

The average desorption coefficients, 6, given above are a function of conditions within the reaction system, including the concentrations of the two monomers in the particles. This dependency on individual monomer concentrations is illustrated in Fig. 18 for the styrene- methyl acrylate system studied by Mead.' The solid dots in Fig. 18 represent desorption coefficients determined from experimental measurements of conversion and particle size distributions in a seed-fed CSTR. The network shown is simply an attempt to show a surface through the data points. Nomura et all9 relate the average desorption rate coefficient to those of the individual monomers, as shown in the following equation:

where L = kpAArBIMBlp

kpBBYAIMAIp

k p A A and kpBB are the propagation rate constants, rA and rB

are the copolymerization reactivity ratios, and [MB], and [MA], are the monomer concentrations in the polymer particles. Lange'' used this relationship with measured 6 values to evaluate the individual desorption coefficients.

Fig. 17. Conversion profiles of individual monomers for styrene His results were kdsty = 2.2 x 10- ", K M A A = 9.4 x lo-" and AA in a seed-fed CSTR (Lange'*). 0 = Overall conversion; and &AA = 9-1 x 10- lo cm2/s. These individual co-

efficients are in the expected order Sty < MAA < AA. = AA conversion; = styrene conversion.


Emulsion polymerization and copolymerization in continuous reactor systems 25 1

SUMMARY

The four examples described in this paper demonstrate the utility of continuous reactor systems for process development and for fundamental kinetic studies. Seed- fed steady-state CSTR systems are especially attractive for research and for use in commercial practice because of their inherent stability and ease of control.

ACKNOWLEDGEMENTS

Financial support from the National Science Foundation under Grants CBT-8717926 and CTS-9023240, from the Dow Chemical Co. and from the Georgia Institute of Technology is gratefully acknowledged. The US Govern- ment has certain rights to this material.

REFER EN CES

1 Poehlein, G. W., in Encyclopedia of Polymer Science and Engineering, Vol. 6, 2nd edn. J. Wiley & Sons, New York, 1986, p. 1.

2 Hoedemakers, G. F. M., PhD dissertation, Technische Universiteit, Eindhoven, The Netherlands, 1990.

3 Greene, R. K., Gonzalez, R. A. & Poehlein, G. W., in Emulsion Polymerizution, ed. I. Piirma & J. Gardon. ACS Symp. Series No. 24, Washington, DC, 1976, Chapter 22.

4 5 6

7

8

9

10

11

12

13

14

Ley, G. & Gerrens, M., Mukromol. Chem., 175 (1974) 563. Berens, A. R., J. Appl. Polym. Sci., 18 (1974) 2379. Gonzalez-P., R. A,, MS thesis, Dept of Chem. Engng, Lehigh University, Bethlehem, PA, 1974. Lee, H. C., PhD dissertation, School of Chem. Engng, Georgia Institute of Technology, Atlanta, GA, 1985. Mead, R. N., PhD dissertation, School of Chem. Engng, Georgia Institute of Technology, Atlanta, GA, 1987. Stone, J. M., US Patent 3 600 349, assigned to Copolymer Rubber and Chemicaf Corp. (Aug. 1971). Stone, J. M., US Patent 3 730928, assigned to Copolymer Rubber and Chemical Corp. (May 1973). Dubner, W., Lee, H. C. & Poehlein, G. W., Brir. Polym. J., 14 (1982) 159. Poehlein, G. W., Lee, H. C. & Chern, C. S., in Polymerization Reuciion Engineering, ed. K.-H. Reichert & W. Geisler. Huthing and Wepf Publ., Heidelberg, 1986, p. 59. Shoaf, G. L., MS thesis, School ofChem. Engng, Georgia Institute of Technology, Atlanta, GA, 1986. Shoaf, G. L. & Poehlein, G. W., Po1ym.-Plasi. Technology Engng, 28 ~. - - (1989) 289.

15 Priest, W. J., J. Phys. Chem., 56 (1952) 1077. 16 Emulsion Polymerization ofAcrylic Monomers. Publication CM-104,

17 Shoaf, G. L., PhD dissertation, School of Chem. Engng, Georgia

18 Lange, D. M., PhD dissertation, School of Chem. Engng, Georgia

19 Nomura, M., Kubo, M. & Fujita, K., J. Appl. Polym. Sci., 28 (1983)

20 Ugelstad, J. & Hansen, F. K., Rubber Chem. Technol., Rubber Rev., 49

Ncf, Rohm and Haas Co., Philadelphia, PA.

Institute of Technology, Atlanta, GA, 1989.

Institute of Technology, Atlanta, GA, 1991.

2767.

(1 970) 530.


emulsion polymerization and copolymerization in continuous reactor systems

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