batch and continuous emulsion copolymerization of ethyl acrylate

8/13/2019 Batch and Continuous Emulsion Copolymerization of Ethyl Acrylate

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Batch and ContinuousEmulsion Copolymerizationof Ethyl Acrylate andMethacrylic AcidGlenn L. Shoaf a & Gary W. Poehlein ba Chemical Engineering Georgia Institute of

Technology , Atlanta, Georgi ab Res earch and Graduate Studies , CentennialResearch Building 400 Tenth Street, N. W.,Atlanta, Georgia, 30332Published online: 04 Oct 2006.

To cite this article: Glenn L. Shoaf & Gary W. Poehlein (1989) Batch and

Continuous Emulsion Copolymerization of Ethyl Acrylate and MethacrylicAcid, Polymer-Plastics Technology and Engineering, 28:3, 289-317, DOI:10.1080/03602558908048601

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POLYM.-PLAST. TECHNOL. ENG., 28 3), 289-317 1989)

BATCH AND CONTINUOUS EMULSIONCOPOLYMERIZATION OF ETHYL ACRYLATE

AND METHACRYLIC ACID~ _ _ _ _ _ _ ~

GLENN L. SHOAF

Chemical EngineeringGe org ia Institute of TechnologyAtlanta, Georgia

GARY W. POEHLEIN*

Research and Graduate StudiesCentennial Research Building400 Tenth Street, N .W .Atlanta, Georgia 30332

Abstract

Emulsion copolymerization of a moderately water-soluble monomer(ethyl acrylate) with a completely water-soluble monomer (methacrylicacid) was initially examined in a batch reaction system. The reactionrates, copolymer composition, and physical properties of the latex prod-uct were characterized. Batch reactions were run with various monomer

ratios. The latex stability was strongly dependent on both the tempera-ture and the overall fractional conversion. Several continuous processesinvolving a tubular reactor and/or a continuous stirred-tank reactor weredesigned and utilized so as to produce a latex product with propertiessimilar to the batch product.

*To whom correspondence should be addressed.

289

Copyright 989 by Marcel Dekker, Inc

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290

1 INTRODUCTION

SHOAF AND POEHLEIN

The reaction mechanisms of emulsion polymerization involving monomerswith significant water solub ilityare much m ore complex than the conventionalscheme depicted by Harkins[11. Priest 121 and Fitch and Tsai 131 revealed thata significant degree of particle nucleation may o ccur in the aqueous phase by aprocess called homogeneous nucleation. When a highly water-soluble mono-mer such a s a carboxy lic acidis present, a significant amount of polymeriza-tion may also occur in this phase. T he reaction rates then become dependentonthe partition of the monomers between the two phases. Copolymer systemsincrease the complexity even further.

Some theoretical work has been reportedon modeling the homogeneousnucleation of particles and the reaction rate for systems involving monomerswith moderate water solubility(


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EMULSION COPOLYMERIZATION OF EA AND MAA 29 1

Removal of inhibitor from monomers is not required for batch emulsionpolymerization systems because after a brief period early in the reaction inwhich the inhibitor is consumed, the reaction will proceed in a normal manner.Inhibitor is continuously added, however, to continuous reactors and thepolymerization can be significantly reduced. However, MAA and EA were soreactive that high conversions and fast reaction rates were obtained withoutremoval of the inhibitor even in the continuous runs.

Potassium persulfate, sodium bisulfite, ferrous sulfate, and hydroquinonewere used as received from Fisher Scientific Company. An anionic liquidsurfactant, ABEX-JKB, was obtained from Alcolac.

The nitrogen used was extra dry. Deionized water was used as receivedfrom the deionization unit.

6. Experimental Apparatus and Operatin g Procedures

1. Batch Reactor Setup

The batch reactor system was comprised of a 1-L glass reactor with a con-denser, nitrogen purge line, sampling port, mercury thermometer, and teflon

stirrer. A temperature-controlled water bath was employed to heat the reactor.

2. Batch Polymerization Procedure

The recipe for a typical MAA/EA copolymerization is given in Table 1. TheMAA/EA monomer ratio was varied among different runs. Deionized waterand emulsifier were added to the reactor and mixed with a paddle agitator.Heating was begun by slightly immersing the reactor into a 72C water bath.

TABLE 1Typical MAA/EA Copolymerization Recipe

Component Weight fraction

Water 0 821

Monomers 0.157

Surfactant 0.022

Ferrous sulfate 8 .6 4 E-7

(MAA/EA ratio range = 0.053-3.0)

Potassium persulfate 0. 00 03 2

Sodium bisulfite 0.00017

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292 SHOAF A N D POEHLEIN

MAA and EA monomers were added to the reactor. Ferrous sulfate solutionwas injected with a syringe. The reactor was continuously purged with a slowbubbling flow of nitrogen. Agitation was maintained for a minimum of 30 minprior to initiation to ensure preemulsification of the monomers. After thereactor temperature leveled off at 49 C, potassium persulfate and sodiumbisulfite initiator solutions were quickly injected into the reactor from separate20-mL syringes to start the reaction.

Samples were drawn from the reactor over the conversion period withsyringes connected to a tube which extended into the reactor. These sampleswere injected into preweighed plastic vials each containing 5.0 mL of 2.0%hydroquinone solution. The vials were capped, shaken, and placed in an icewater bath. Gravimetric analysis was later uscd to determine the conversion ofeach sample. The reactor was held at the peak exotherm temperature (usually70-74C) for 1 h.

3. Continuous Reactor Setup

A schematic diagram of the general experimental setup for the continuoussystem is shown in Fig. 1. Preemulsified monomer and initiator solutions werepumped from separate tanks by a Milton Roy volumetric pump to a glasspremixer placed just before the tubular reactor. This premixer contained asmall magnetic stimng bar which was spun by a compressed-air-driven stirrer.Both premixer and stirrer were submerged in a heated water bath (maintainedwithin 1C of the set temperature), as was the tubular reactor. The premixerwas used to minimize phase separation as well as to mix the monomer andinitiator solutions before they entered the tubular reactor. Immediately follow-ing the premixer was a tee where nitrogen was injected to form alternatingplugs of nitrogen and latex, each normally about an inch long. The tubularreactor was operated in this plug-flow mode to minimize back-mixing and wasreferred to as a plug-flow reactor (PFR). Tube inner diameters ranged from l/s

to 3/16 in.Following the PFR was a glass continuous stirred-tank reactor (CSTR)

which was stirred, purged with nitrogen, and heated by internal stainless steelcoils. A temperature controller maintained the temperature of the CSTR towithin 0.5 C of the set temperature. The CSTR possessed a working capacityof about 490 mL. Effluent from the first CSTR overflowed into a second 1500-mL glass CSTR which was used as a nitrogen-purged, stirred, storage vesselthat was heated by an external water bath.

Several modifications to the system were explored in various experimentalruns. The particular flow systems investigated are listed in Table 2. Variousoperating temperatures were also used.

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294 SHOAF AND POEHLEIN

TABLE 2Various Reactor Systems Investigated

ID Res. time StorageNo. PFR (in.) (min) N, plugs CSTR vessel

Yes NoNo

Yes Yes. No

3. Yes 10 Yes No No

4. Yes 10 Yes N o Yes

5 . Yes 21 N o No Yes

6. Yes '/4 21 N o N o Yes

7. Yes /4 4.4 N O Yes Yes

8 Yes /x + 6 10 + 9 No No Yes9. Yes 4. 6. and 14 No Yes Yes

4. Continuous Polymerization Procedure

Water was initially added to the monomer and initiator vessels and nitrogenpurged for 1 h. Surfactant, monomer, andFeS04 solution were added to themonomer vessel and strongly agitated for30 min to preemulsify the mono-mers. Initiators were added to the initiator vessel. If the CSTR and storagevessels were utilized, they were filled with about500 m L of deionized waterand purged with nitrogen as well as heatedto the desired temperature. Aboutfour to six CSTR residence times were needed to thoroughly flush the initialwater cha rge from the CSTR. An additional four or five residence times we re

needed to flush out the storage vessels.Th e volumetric pum p flow rates w ere setso that the total reaction mixtureexiting the glass premixer flowed at either15.0 mL/min or 30.0 mL/m in. Theproportion of each component in this mixture was identicalto that used in thebatch system as listed in Table 1 with an MAA/EA ratio of 2 . 2 . After thepumps were started, sam ples of effluent latex were taken from thePFR and/orCS TR an d analyzed for conversion ev ery0.5 or 1 mean residence time. Fifteenmilliliters of the sample were added to a plastic vial containing2 .0 mL of a1 O hydroquinone solution and then placed inan ice bath. Most runs were

designed to continue for 10 to 12 residence times in order to ensure steadystate. After all flow stop ped , the product in the storage vessel was postreactedat 70C for an additional 30 to 45 min.

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EMULSION COPOLYMERIZATION OF EA AND MAA 295

C. Analyt ical Measurement Techniques

1. Conversion

Gravimetric analysis was routinely used to determine overall conversion.Conversion of individual monomers was determined with a VARIAN 3300 gaschromatograph. Standards of MAA and EA (0.02-0.10 M ontaining aninternal standard, amyl alcohol, were injected. Good peak separations wereattained. Calibration curves of monomer areal internal standard area versusknown monomer concentration were plotted and fitted by a fourth-order poly-

nomial. The reactor samples were injected (after dilution and addition ofinternal standard) and the monomer concentrations were determined for eachmonomer based on the calibration curves. The injection temperature was170 C, and the column temperature was 220C. Helium was the carrier gas,and the 10-ft column was packed with Chromsorb 254 by Alltech.

2. Viscosity

The viscosity was measured at room temperature by a Brookfield model LVT

viscometer with a No. 1 spindle at a speed of 60 rpm.

3. pH

A glass electrode, after proper preparation and calibration, was simply im-mersed into the sample. The pH was read directly.

4. Part ic le Size

An average particle size was obtained by diluting a latex sample and analyzingit in an N-4 Coulter Counter (nanosizer). Samples were tested in triplicate. Anaverage particle size was reported as well as a standard deviation which servedas a measure of the width of the particle size distribution.

5. Centri fuge Stability

This test was designed as an accelerated settlement test. Ten milliliters of thelatex sample were placed in a centrifuge tube and centrifuged at about 2000rpm for 15 min. The solution was poured off, and after slight packing with astimng rod, the amount of sediment was measured with a ruler.

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6. Thermal Stabili ty

This test was designed to accelerate the aging of the latex. Two hundredmilliliters of a latex sample were added to a tall beaker with a magnetic stirringbar. The beaker was placed on a hot plate, stirred, and heated to 75C for 1 h.The sample was cooled to 25C and the viscosity was remeasured as describedearlier. The percent change from the original viscosity was a measure of thethermal stability of the latex.

7. Mechanical Stabi lit y

This test was designed to simulate the mechanical action of a centrifugal pumpworking against a closed valve. A 100.0-g sample was placed in a Waringblender and agitated for 5 min at the high setting. The sample was thenstrained through a tea strainer. The amount of coagulum collected was ameasure of the latex mechanical stability.

111 RESULTS AND DISCUSSION

A. Batch Reaction Studies

The reaction of methacrylic acid (MAA) and ethyl acrylate (EA) is very fastrelative to many emulsion copolymer reactions which require several hours toreach high conversion. Conversion versus time curves for the recipe given inTable 1 (MAA/EA ratio equal to 2.2) are shown in Fig. 2. Almost completeconversion was achieved in about 10 to 12 min for three duplicate runs. Thesigmoidal shape of these curves reveals a slow initial reaction rate over the first3 to 4 min, then a sharp increase in rate during the next 3 to 4 min, and finally a

decrease over the last few minutes as the reaction reaches complete conver-sion. All batch reactions, excluding the isothermal reactions, were essentiallyadiabatic. They were initiated at 49 C, and the temperature increased to 70-74C in the manner shown in Fig. 3 .

It is well documented that the rate of reaction for carboxylic acids decreasesdramatically when significant amounts of the acid are present in the dissociatedform [7-9]. All reactions reported in this study were run at a low pH so that anyeffect of the dissociated acid was negligible.

Values for the reaction rates based on the conversion and temperature plots

were calculated by two different methods. One involved a heat balance aroundthe lab reactor with estimates for heat transfer coefficients and heat capacities.The weighted average value of the heats of reaction of the monomers and

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TIME (MINUTES)

FIG. 2. MAA/EA batch reaction conversion data for three identical runs.

7 5

% 7 0 -l

W 3

i22 55-

8 65

60

5 0 - MAAm = 2.2/1.0

45 I I I0 2 4 6 0

TIME (MINUTES)

FIG. 3. MAAIEA batch reaction exotherm .

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reactor volume were known. The slope of the temperature-time curve dTldt)was determined graphically at different points and the rate of reaction, R,, wasthen calculated directly from a heat balance equation. The second methodinvolved writing a simple monomer balance equation, extracting the slopesdMT/d r )rom a plot of total unreacted monomer versus time, and inserting the

value for reactor volume. R p was then calculated directly. Figure 4 shows thebatch reaction rates averaged over several runs and plotted against time.Calculations of these rates were dependent on extracting slopes from plotteddata. This type of differential approach can often lead to significant errors. Acheck on the accuracy of these two methods was obtained by numericallyintegrating the area under the rate versus time curves, obtaining the totalconcentration of polymer after 12 min, then averaging the values obtainedfrom both methods. Assuming 100% conversion, the result should have corre-sponded to the total monomer feed at the beginning of the reaction. The percentdifference for four different runs was relatively small, ranging from 2 to 7%.In addition, fairly good agreement was obtained between the two methods.

B. Copolymer Composition Equation

The following copolymer composition equation often provides a good estimatefor the instantaneous composition of a copolymer over the complete range ofconversion.

f is the instantaneous fraction of monomer i in the reaction mixture, F i s the

instantaneous fraction of monomer i in the copolymer, and r l and r are thereactivity ratios for monomers 1 and 2 , respectively. This equation is oftenapplied to cationic, anionic, and free-radical reactions as well as bulk, solu-tion, and emulsion processes. However, according to Odian [ 101, the reactivityratios, r l and r 2 ,may depend on which specific initiation system is used as wellas on pH, solvent type and solvent concentration, and gel effect. Zilbermanand co-workers [ 11 also reported that r values may change with conversion forhigh-conversion polymers. Despite these limitations, the copolymer equationcan be very useful for obtaining a qualitative description of the composition of

many copolymers.The overall fractional conversion can be calculated as a function off, and F ,

from Eq. 2 ) .

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EMULSION COPOLYMERIZATION OF EA AND MAA

50

40

zX

J

CJ3c

z

kV

W

0a

2(LL

UIl-

Ka

1c

0

0 =HEAT BALANCE METHOD

A =MONOMER BALANCE METHOD

299

0 2 4 6 8 10

TIM E(M NUTES)

FIG. 4. Batch reaction rate transients.

where CON is the overall fractional conversionof the reaction mixture, r is avariable of integration, and f,, is the initial fractionof monomer 1 in thereaction m ixture. Individual m onom er concentrationswere obtained by ana-lyzing sam ples taken ov er the conversion period ina typical batch run with gaschromatography. Experimental value s for f, were then calculated withEq 3).

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where [ M i ] s the concentration of monomer i (mol/L reaction mixture). Thecopolymer compositions based on both theoretical and experimental relation-ships off, and overall fractional conversion were then calculated and plotted asa function of overall fractional conversion in Fig. 5. Reactivity ratios wereassumed to remain constant over the entire range of conversion. Reportedvalues of 5.68 and 0.12 were used for r (MAA) and r 2 (EA), respectively. TheMAA reacted rapidly over a large percentage of the overall conversion period.The copolymer equation underestimated the initial extent to which MAAreacted as compared to EA. Polymerization in the aqueous phase most likelycontributed to the discrepancy between the theoretical and experimental re-sults. Over about the last 10% conversion, the EA then reacted almost exclu-sively with itself since most of the MAA had been depleted. As a result, thecopolymer consisted of a high degree of consecutive MAA units with some EAunits intermixed in the chain, plus either EA units capping the end of the chainand/or EA homopolymer chains formed separately from the other copolymerchains.

Schuller [ 121 reveals that for a more quantitative and accurate analysis, onemust account for the distribution of the monomer between the water and oilphases as well as the monomer/water ratio. The locus of polymerization in

conventional emulsion polymerization is the swollen polymer particles. How-ever, both monomers in the MAA/EA system have significant water solubility.EA is approximately 2.5% soluble in water by weight and MAA is completelywater soluble. Therefore, a significant amount of polymerization most likelyoccurred in the aqueous phase. However, the different solubilities of themonomers probably leads to different concentrations of each monomer in theaqueous phase as opposed to the interior of the particles such that the composi-tion of the water-phase polymer differs from that formed inside the latexpolymer particles. Schuller [12] further states that a growing oligomer radical

may enter a latex particle and be exposed to different concentrations of mono-mers such that the composition of the copolymer may differ significantly alonga single chain. The result is a block-like structure with hydrophilic and hydro-phobic parts corresponding to the segments polymerized in the aqueous phaseand in the latex particle, respectively. This most likely occurs to some extent inthe MAA/EA system. It is even more likely to occur if MAA is replaced byacrylic acid (AA) since AA is more hydrophilic than MAA, which would causeit to have a greater concentration difference between the aqueous and particlephases. Titration studies with carboxylated emulsion systems have indeed

shown that AA is more concentrated in the aqueous phase or at the particlesurface than is MAA [13, 141.

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= F l - T H E O R E T I C A L M A A IN P O LY M E R

n = F l - A C T U A L M A A I N P O LY M E RA - F 2 - A C T U A L E A I N P OLYM ER

P = F 2 - T H E O R E T I C A L E A I N P O LY M E R

MAAIEA=2.211.1 = Wl 0

0 0.2 0.4 0.6 0.8

F R A C T I O N A L C O N V E R S I O N

FIG. 5. Theoretical and experimental copolym er composition.

C. Characterization o f Particles

Measurements of the MAAIEA latex revealed that the final particle size wasrelatively small at about 120 nm diameter. The particle size of samples takenover the conversion period was also measured. The rather surprising results are

shown in Fig. 6. As the fractional conversion increased from 0.0 to 0.6, theparticle size unexpectedly decreased from large initial values. As the fractionalconversion increased from about 0.6 to 1.0, particle size then increased as

would be expected with normal particle growth. The particle size measured at0.9 to 1.0 fractional conversion was the expected 120 nm.

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302

150

125

100 -

75

50

25

00

SHOAF AND POEHLEIN

MAA/EA = z.z/i.o

I I I0.2 0.4

0 6 0.8

FIG. 6. Particle size measurements from a typical batch reaction.

One explanation for measurement of large particles at low conversionsmight be that the nanosizer was actually measuring monomer droplets. How-ever, the samples were well diluted with water before measurem ent, and m ostof the mo nom ers should have been removed since both have significant w ater

solubilities. Th e most probable explanationof the results is that after removalfrom the reactor, particles from the low-conversion samples coagulated.

Previous work has dem onstrated that PM AA serves as a stabilizerof poly-mer particles [ 151, Loncar and co-workers[141 show ed that early incorporationof MAA into their MAA/EHA latex particles assisted in stabilization. Priest(21 mad e studies with non-micelle-forming stabilizers including w ater-solublepolym ers such as polymethacrylic acid. He stated that water-solub le colloidsmay act by enve loping the particles in a hull of w ater-soluble material whichminimizes contact of the interior of the droplets on collision. Likew ise,Muroi [16] proposed that copolymerized acid tends to concentrate at thesurface layerfor carboxylated EA latices, and when the amount of copolym er-ized acid increases, the surface layer thickness increases, providing a greater

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concentration of negatively charged carboxyl groups on the particle surface,and hence a greater degree of stabilization.

MAA may play a significant role in the stabilization of the MAA/EA latexparticles. Therefore, the steady decrease in particle size with initial conversionsupports the supposition that as more MAA reacted and combined with theparticles through either physical adsorption or chemical bonding, the particlesbecome more stable and less coagulation occurred after sample removal.

D. Effect of Temperature on Particle Size

Several isothermal batch runs were performed with the MAA/EA system inorder to examine the effect of temperature on the final particle size of thecopolymer. Four runs were made at temperatures of 50 C, 55 C, 60 C, and70C. The temperature of the reacting mixture was held within 2 of the settemperature despite the exotherm experienced with each run. All other condi-tions remained the same as in previous runs. The final products of each runwere analyzed for particle size. The results are shown in Fig. 7. Particle size

40 50 60 70 80

TEMPERATURE (DEGREESC)FIG. 7 Effect of temperature on particle size for isothermal copolymerization.

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increased dramatically with increasing temperature. A 20C increase in tem-perature produced almost a twofold increase in particle diameter (which corre-sponds to an eightfold increase in particle volume due to the cubic dependenceof volume on diameter).

This result was somewhat surprising since previous workers reported that,in general, particle size tends to decrease with increased temperature [2, 171.The main reason for the expected decrease in particle size with temperature isthat more free radicals are formed at higher temperatures that increase theinitiation rate and thus generate more particles. The increase in the number ofparticles results in a net decrease in size.

Two explanations may account for the antithetical behavior observed withthe MAA/EA system. First, as the initiation rate increased with increasingtemperature, more particles were indeed formed. However, a larger number ofparticles requires more stabilizer to prevent coagulation. Priest [2] notes that ifthe stabilizer is not efficient or not present in sufficient amount, coagulationmay occur. Stabilizer for the MAA/EA system refers to both surfactant andcopolymerized acid. A larger number of particles results in a smaller amount ofboth species on the particle surface, thus leading to a less stable system.Another factor related to an increase in particle number is that more inter-

particle collisions could occur, thus increasing the chances for interparticlecombination.A second explanation involves the fact that the reactivity ratios of MAA and

EA may have changed with temperature. A substantial increase in the reac-tivity ratio of EA relative to MAA could cause the rate of particle growth toincrease significantly relative to the reaction and subsequent combination ofaqueous-phase carboxyl groups onto the particle surface. If the carboxylgroups are indeed important for particle stabilization, then less of these speciesalong the particle surface during the critical growth period could lead to

increased coagulation and thus larger particles. More experimental data areneeded in order to clarify this situation.

Latex from batch runs containing particles with diameters near 120 nmpossessed a characteristic light-blue tint resulting from the scattering of lightby the small particles. As the particle size increased, the latex appeared morechalky and the characteristic blue tint was much less apparent. The appearanceof the final latex, therefore, provided some indication of the final particle size.

E. Effect of Mixin g Intensi ty on Partic le Size

Mixing intensity may affect the size of the polymer particles formed in someemulsion reaction systems. Mixing can be maintained at a constant level in a

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batch reactor. The mixing intensity in a continuous reactor, however, is not aseasily controlled, especially if a tubular reactor is used. Mixing intensity inthese reactors usually differs from that of a batch system. Therefore, it wasnecessary to examine the influence of mixing on the final particle size for theMAA/EA system before designing possible continuous processes. Four batchreactions were run with a wide range of stirrer speeds (0 to 700 rpm) with allother conditions remaining the same. The monomer was effectively preemulsi-fied before adding the initiators in each run. The results listed in Table 3 showthat 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 120 nmvalue that was observed with the normal batch product. A slight increase inparticle size was observed at the higher mixer speeds where high shear forceswould tend to overcome the stabilizing effect of the surfactant.

F Reaction Rates wi th Different Monomer Ratios

The batch reaction procedure was repeated for a series of different mono-mer ratios. The total monomer weight fraction of 0.157 was held constant.Conversion-time curves for these runs are shown in Fig. 8. The pure EA

reaction occurred very quickly, reaching high conversion in 4 to 5 min. WhenMAA was added, the initial reaction became sluggish. As the ratio of MAA toEA was increased, the sluggishness in the initial rate became more pro-nounced.

Several factors may have contributed to the decreasing initial reaction ratewith increasing MAA/EA ratio. MAA is completely water soluble so thatinitial reaction most likely occurred in the aqueous phase forming MAA-richcopolymer. As the concentration of MAA was increased, more radicals reactedwith MAA monomer in the aqueous phase before penetrating the monomer

droplets or micelles, thus delaying the initiation of the more reactive EA

TABLE 3Effect of Mixing Intensity on Particle Size

Rate Particle sizeof agitation RPM (nm)

No agitation 0 119 30

Low 40 120 30Medium 400 124 2 30

High 700 132 2 40

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306

1

0.8

z

v

Uw

z

2

> 0.6

8

F 0.4

a

z

a:LL

a

0.2

0

SHOAF AND POEHLEIN

0 2.5 5 7.5 10

TIME (MINUTES)

FIG. 8 Conversion transientsfor varying MAA/EA ratios.

12.5

monom er. H ow eve r, as the M AA was depleted, more radica., penetrateb theloci of high E A concentration and the reaction rate then accelerated as particlesand EA-rich copolymer were formed.

An increased concentration of MAA in the feed may also have led to athicker acid layer surrounding the particles earlier in the reaction. Radicalsmust penetrate this layerin order to reach the interiorof the particles w here theEA concentration is the highest. A thicker layer would tend to slow thediffusion of the radicals into the particles which could result in slower initialreaction rates.

An additional factor which must be considered is that MAA contained agreater level of inhibitor than the EA. The increased feed concentration of

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EMULSION COPOLYMERIZATIONOF EA AND MAA 307

MAA may have required a longer induction period for the inhibitor to beconsumed. Unfortunately, this point could not be adequately examined sinceMAA homopolymerizations could not be achieved at a 0.157 monomer weightfraction without complete gelation of the reaction contents.

G. Physical Property Data

Average physical properties for the final latex product formed in the batchprocess from the recipe listed in Table 1 are summarized in Table 4.

H. Continuous Reaction Studies

A series of continuous reactions were carried out using various reactor designsand reactor conditions. The objective was to prepare a product with the sameproperties as the batch MAAJEA copolymer product, but in a continuoussystem. The various system designs included a continuous stirred-tank reactor(CSTR); a CSTR followed by a simulated storage vessel, which was heated,stirred, and purged with nitrogen; a plug-flow tubular reactor (PFX) only; aPFR plus a simulated storage vessel; PFRs with variations in lengths and inner

diameters; and a PFR followed by a CSTR. The storage vessel was used toreact residual monomers. Changes in reactor conditions primarily involvedchanges in temperature and flow rates.

Several continuous runs were made using a CSTR followed by a heated,

TABLE 4Physical Property Data of the MAA/EA

Emulsion Copolymer

ROPefiY Average value

Viscosity 7 CP

PH 2.7

% solids 15.7

Centrifuge stability Trace of solids

Thermal stability 1-2% viscosity change

Mechanical stability No coagulation

Particle diameter 120 nm

AoDearance White emulsion with bluish tint

aL.ess than 25% viscosity change was desired.

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stirred, and nitrogen-purged storage vessel. The flow rate was15.0 mL/minsuch that the average residence time of the CSTR was 32.7 min. Both vesselswere heated to 70C. Th e CSTR effluent possessed a 97% average conv ersion,with a final product conversion of100%. Th e product was c halky w ith particlesizes ranging from 220 to 250 nm. The conversion from the CSTR wasunsteady as shown in Fig. 9. The initial rise in conversion over the first5residence times sim ply represents the time needed to flush the initial charge ofwater from the reactor.

Conversion oscillation is common in many conventional emulsion polymer-ization systems in whicha CSTR is utilized. Particle growth often requires all

of the incoming surfactant for stabilization, preventing the formation of mi-celles and the generationof new particles. Eventually, the particles wash ou t ofthe reactor and the conversion drops . Incoming surfactant creates new m icellesin which new particles are generated; the conversion begins to rise once more ,and the cycle repeats. Th is phenomenon p robably contributed significantly tothe conversion oscillation experienced with the M A A /E A sy stem . Since con-version oscillation produces an inconsistent product, all future runs with theCS TR were modified to include a tubular prereactor to generate particle s eed sfor the CSTR feed.

The conditions in a CSTR are radically different from that of a batchreactor. A steady-state CS TR op erates at one levelof conversion which doesnot change with time. The final product, therefore, often possesses differentproperties than the batch product. However, the product from a PFR is ordi-narily very similar to that of the batch product. Th e reaction occ ursas he fluidmoves so that a conversion gradient is established throughout the length of thetube. The conversion for an individual plug of fluid should change with timemuch like that of the batch reaction.

Initial continuous runs with a PFR utilized the formationof plugs by

injecting nitrogen a ta tee just a fter the premixer. The se plugs w ere about1 to 2in. long and were designed to minimize back-mixing. Each plugof reactingmixture thus constituted a small batch reactor traveling through the teflontubing. The mean reactor residence time could be vaned by adjusting thenitrogen flow. All PFR runs were isothermal. Initial runs with nitrogen plugswere made at 70C. This temperature was chosen to ensure high conversionwith a 10-min residence time. The average conversion attained was about85%. The product was white and chalky with particle diameters 2 to3 timeslarger than desired. Additionally, the high temperature change of the nitrogenfrom am bient to 70C caused an expansionof the gas as it traveled through thetube creating nonuniform pressure gradients. The fluid velocity tended tofluctuate in a cyclic fashion and was very difficult to control.

The nitrogen plugs were removed to eliminate the cyclic flow behavior, and

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EMULSION COPOLYMERIZATION OF E AND MAA 309

1

0.95

0.90

TEMPERATURE = 71.0 DEG CMA- = 2.2/1.0MEANRES. TIME = 32.7 MINUTESCONVERSION IS AT THE CSTR EXIT

0.85

0.80

0 i 2 3 4 5 6TIME (HOURS)

FIG. 9. CSTR conversion profile.

the emulsion was run straight through the tube. Calculation of the Deannumber [18] showed that secondary ffows such as stagnant eddies and spirall-ing flows should be negligible at the flow rates of interest if no nitrogen plugswere utilized. Without plugs, the original PFR had a residence time of 20.9min. Runs at 70C resulted in nearly 100% conversion of the product exitingthe tube.

The physical properties of the final product obtained in these initial continu-ous runs were very similar to those of the batch products as listed in Table 4except for the appearance and the particle diameters. The initial latexes pro-duced in the PFX were chalky, with particle diameters 2 to 3 times larger thandesired.

One major difference in the batch and continuous runs was that the batch

runs were adiabatic, while the continuous runs were isothermal. The tempera-ture changed throughout the conversion period from 49C to 72C in the batchruns. While in the continuous runs, the temperature was maintained at aconstant level. As indicated in the batch experiments, low temperatures must

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31 0 SHOAF AND POEHLEIN

be used with isothermal run s to attain small particles in the M A A /E A system .Therefore, the temperature of the continuous runs were modified in order todecrease the siz e of the particles in the final product. Subse que nt runs at55Cand a flow rate of 15.0 m L/m in produceda final product with a distinct bluishtint and an average particle size of140-150 nm, Conversion was essentiallycomplete.

Some runs at this low flow rate resulted in phase separation and eventualplugging of the tube. The refo re, a %-in. inne r diameter tube was connected tothe end of the %-in. inner diameter tube. The flow rate was then doubled toincrease the mixing of the fluid as it traveled through the tube. The residencetime in this PFR setup was 19.0 min. T he conversionof the emulsion enteringthe larger tube was great enough that no noticeable coagulationor phaseseparation occurred. T he completely converted pfoduct possessed a bluish tintand had a particle s ize of 119 nm when run at 55C. W hen run at a highertemperature, 62.5 C, the final particle size was larger,207 nm, as was ex-pected.

The PFR was also operated at various lengths in order to determine therelationship between residence time and conversion for con tinuo us, isothermaloperation. The results are summarized in Fig. 10.

A series of continuous runs were made witha PFR-CSTR combination atabout 55C. The storage vessel was also included in these runs to ensure thathigh conversion was achieved. The PFR residence time in the initial run was4.25 min and the effluent conversion was0.27. Th is conversion wastoo low toproduce stab le particlesfor the CSTR feed. As a result, the final product wascha lky, w ith large particles,262 nm. An extended PFR residence time(6 min)yielded a product w ith sma ller particles,2 11 nm . W hen the PFR residence timewas increased to 14 min, the conversion of the exiting reaction mixture wasbetween 60% and 62%. (The PFR temperature was 55.5 C.) The average

diameter of the particles exiting the PFR w as100 nm. Th e reaction mixture inthis case was run directly into the storage vessel. Complete conversion wasachieved in the storage vessel, and the final particle size was134 nm. Thissmall particle size indicated that the particles exiting the PFR were sufficientlystabilized so that little coagulation occurred despite the fact that the reactantmixture was fed into the final storage vessel (which was maintained at arelatively high temperature, 70C) with only60% to 62% conversion.

Both isothermal continuous and isothermal batch reactions produced rela-tively small particles only if the temperature was keptlow (


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EMULSION COPOLYMERIZATION OF EA AND MAA 31 1

FIG. 10. Conversion-residence time relationship for the plug-flow reactor.

be about 60% even if the PFR effluent was fed directly to a postreact vesselmaintained at 70C. The minimum in particle size shown in Fig. 6 for the batch

reaction also occurred at about a 60% conversion, These results suggest that acritical conversion of about 60% may be required for latex particles toremain stable. Comparison of Fig. 2 and Fig. 3 reveals that a 60% conversionfor the adiabatic batch reaction occurs before the temperature of the reactionmixture exceeds 60C. Therefore, the conversion in the adiabatic batch reac-tion reaches the point at which the particles become stabilized before thereaction mixture reaches temperatures where destabilizing effects becomeimportant. The possible effects of high temperatures on the particle stabilitywere discussed earlier.

1 Scaleup of Continuous Reactor Designs

The true merit of any process design is determined by how practically it can beachieved on an ndustrial scale. Any scaleup of a chemical process from the lab

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to the plant involves many unknowns which may be identified only by actualexperimentation with the plant-scale setup. However, theoretical scaleup cal-

culations are still essential in the initial design of the large-scale process. Thetheoretical design for a batch process is rather straightforward since fewfundamental changes from the lab-scale process are usually required. Continu-ous systems, however, are more complex, making direct scaleup more diffi-cult. Therefore, the feasibility of producing the MAAlEA latex continuouslyon a large scale based on the findings of the laboratory studies was addressed.

The results from the laboratory studies suggested that two reactor designswould be feasible for producing the MAA/EA copolymer with the desiredproperties in a continuous process. These designs included a PFR with astorage vessel and a PFR-CSTR combination. First, an arbitrary productionrate (350,000 lb/week) for the plant scale was assumed. Equations werederived relating this production rate to inner tube diameter, tube length, flowrate, residence time, and Reynolds number. Plots of the corresponding Rey-nolds numbers, tube lengths, and Dean numbers are shown in Figs. 11-13.

4000

3000

zm 2000

*wP= 1000

2

n

0

BASIS : 350 000 EE/WEEK

0 2 4 6 8 10

INNER TUBE DIAMETER (INCHES)FIG. 11. Reynolds number-diameter relationship for commercial-size tubular re-

actors.

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I500R E S TIMEp 14 MIN. 60 CONV.

A = RES. TIME= 20 MIN. = 100 CONV.

BASIS : 350.000 LBS/WEK

10

0 2 4 6 8

INNER TUBE DIAMETER (INCHES)FIG. 12. Length-diameter relationship for commercial-size tubular reactors.

400

300

p:w

g 200

I3z4

100

=COIL DIAMETER= 3 FEETA = COIL DIAMETER= 6 FEET

= COIL DIAMETER 9 FEETV = COIL DIMARER= 12 FEET

I I I0 2 4 6 8 10

INNER TUBE DIAMETER (INCHES)

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(The Dean number is a measure of secondary flows in coiled tubes. A Deannumber less than 10 signifies that secondary flows are probably negligible.)Tube lengths are computed based on both 14-min and 20-min residence timescorresponding to the times needed to obtain 60% and 100% conversion from aPFR operated at 55C. Dean numbers are calculated for four coil diametersranging from 3 to 12 ft.

The Reynolds number for the continuous laboratory PFR runs was 28.Larger Reynolds numbers would probably be desired for a plant-scale systemto minimize the chances of phase separation. However, reasonable inner tubediameters and tube lengths must be employed. The Dean number is greaterthan 10 for all coil diameters investigated, with inner tube diameters as high as10 in . The large Dean numbers imply that secondary flows are significant in theplant-scale PFRs. Dean numbers calculated for the lab-scale reactor systemwere all much less than 10. However, since mixing had little effect on the finalproduct properties, increased secondary flows should not adversely affect thefinal product of the plant-scale reactor as long as stagnant regions on the insidewall of the coiled tubing do not develop.

A PFR-CSTR system requires a PFR tube with a minimum residence time of14 min in order to give high enough conversion to stabilize the particles.

Corresponding tube sizes and lengths are presented in Figure 12. The size ofthe CSTR reaction vessel in the PFR-CSTR system depends only on the desiredresidence time. The laboratory CSTR possessed a residence time of 16.3 and32.7 min for flow rates of 30.0 mL/min and 15.0 mL/min, respectively. Forthe arbitrarily selected flow rate of 350,000 lb/week, the required CSTR size is84.0 gallons, 112.0 gal, 140.0 gal, and 168.0 gal for residence times of 15min, 20 min, 25 min, and 30 min, respectively.

J. Plant-Scale Process Design for Continuous EmulsionCopolymerization of M A A I E A

A practical plant-scale process design was generated by utilizing the results ofthe experimental lab-scale runs and the results obtained from the scaleupcalculatic 7s (Fig. 14). The design is based on the assumed production rate of350,000 lb/week for 24 h/day operation, 5 days/week. The identification andthe size of each of the vessels are given in Table 5.

Two options are presented for the plant-scale design. Option 1 utilizes onlythe PFR plus the storage vessel. Option 2 utilizes a PFR-CSTR combination

followed by the storage vessel. Option I is most feasible with this particularcomonomer system, for it is more cost-effective to purchase a 200-ft longertube to achieve high-conversion product exiting the PFR than to continually

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E.-

EM

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TABLE 5Definition of Symbols and Vessel Sizes for the Plant-Scale

Continuous Emulsion Copolymerization System

Symbol usedin Fig. 14 Definition Size

T-1 Monomer premix vessel

T-2 Monomer holding vessel

T-3 Initiator holding vessel

T-4 Initiator premix vessel

T-5 CSTR

T-6 Postreact vessel

PFR Plug-flow reactor

FM Flow meter

HE Heat exchanger

P Pressure indicator

PlT enclosure

T Temperature indicator

(2.0 in. inner diameter)

Heated enclosureof PFR

5000 gal

5000 gal

125 gal

125 gal

250 gal

400 gal

Option 1 Option 2

670 f t 470 ft_ _ _ _ _ _

All sizes are based on a flow rateof 350,000 b/week with operation24 hrtday, 5 daystweek.

operate an additional 250-gal vessel. Ho wever, many other conventional emul-sion polymer systems require a much longer reaction time such that high-conversion cannot be achieved without utilizing an extrem ely longPFR ube.The tube in these cases would be used primarily to create seed particleswithonly a 10% o 20% overall conversion.Most conventional emu lsion systemsprod uce stab le particles at this low conv ersionso that the PFR effluent could befed into a CSTR or series of CSTRs wh ere the majority of the reaction w ouldoccur. A postreaction vessel could again be used to ensure com plete conv er-sion, if desired. Option 2 would, therefore, be more feasible with mostconv ention al em ulsion polym er system s which require relatively long reactiontimes.

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IV. CONCLUSIONS

The batch copolymerization of MAA and EA is very fast relative to moreconventional emulsion systems. The latex particle size increases dramaticallywith temperature for isothermal batch copolymerizations. MAA contributessignificantly to the stabilization of the latex particles. A PFR or a PFR/CSTRcombination operated at 55C (or lower) may be utilized to make an MAA/EAcopolymer with the same properties as the batch product made under adiabaticconditions. An overall conversion of 60% or greater is required to produce aPFR effluent with small, stable particles. The most feasible continuous reactordesign for this product consists of a long tubular reactor producing an effluentwith a 90% to 100 conversion, followed by a postreact vessel.

REFERENCES

111

121

[31

141~ 5 1161

W . D. Harkins, 1. Polym. Sci . , 5, 217 (1950); J Am. Chem. SOC. ,69, 1428(1947).W . J. Priest, J . Phys . Chem. , 56, 1077 (1952).

R. M. Fitch and C. H. Tsai, in Polymer Colloids (R. M. Fitch, ed.), PlenumPress, New York, 1971.M. Litt and V. Stannett, J. Polym. Sci. A - 1 , 8, 3607 (1970).R. L. Zollars, J. Appl. Polym. Sci . , 24 , 1353 (1979).R. K. Greene, R. A. Gonzalez, and G. W . Poehlein, ACS Symp. Ser. , No. 24,341 (1976).A. Katchalsky andG. Blauer, Faraday SOC. Trans. , 47 1360 (1951).S. H. Pinner, J . Polym. Sci . , 9, 282 (1952).K. Plochocka,J. Macromol. Sci .-Rev. Macromol. Che m.,C20(1), 67 (1981).G. Odian, Principles of Polymerization, 2nd ed., Wiley, New York, 1981.

Y. N . Zilberman, R. A . Navolokina, and0 . P. Kuvatzina, Polym. Sci. USSR,A22(9), 2006 (1980).H. Schuller, Copolymerization in Emulsion, inPolymer Reaction Engineering(K.-H . Reichert andW. Geiseler, eds.), Huthig and Wepf, Germany,1986.S. Muroi, K. Hosoi, and T . Ishikawa, J . Appl . Polym. Sci . , 11, 1963 (1967).F. V . Loncar, M. S . El-Aasser, and J. W. Vanderhoff, Emulsion Polymeriza-tion, Preprints of American Chemicals Society Mee ting, New York, April1986.P. Molyneux, Wa ter Soluble Synthetic Polym ers: Prop erties and Behavior, Vol.1, CRC Press, 1983.S. Muroi, J. Appl . Polym. Sci . , 10, 713 (1966).Emulsion Polymerization of Acrylic Monomers, Publication CM-104, Ncf,Rohm and Haas Co.W. R. Dean, Phil . Mag. S . , 7(4), 20 (July 1927).

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batch and continuous emulsion copolymerization of ethyl acrylate

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