Emulsion Polymerization by Continuous Feed of Reactants LABORATORY SCALE PRODUCTION OF HIGH SOLIDS LATEX PAUL FRAM, GRAHAM T. STEWART, AND ANDREW J. SZLACHTUN Army Prosthetics Research Laboratory, Walter Reed Army Medical Center, Washington 12, D . C.
A
SEMIBATCH emulsion polymerization technique has been developed for the production on a laboratory scale of high solids latex of butyl acrylate-acrylonitrile copolymer. I n contrast to the “batch” method, in which the ingredients are introduced initially into the reactor, the semibatch method employs equipment designed t o add the reactants automatically and continuously t o the aqueous phase in the reactor. D a t a are reported for the polymerization and physical properties of copolymers of butyl acrylate and acrylonitrile which were prepared by both batch and semibatch methods. Techniques employed t o produce synthetic polymers by emulsion polymerization are reported in many publications (3, I S , 17, 20, 22, 26, 28, SO) and patents ( 4 , 9 , 14, 19, 2 1 ) . With the widespread use of emulsion polymerization techniques to obtain products of great industrial potential, laboratory methods should be provided which lend themselves to research and development programs. When monomers t h a t boil below room temperature are used, it is necessary to use closed pressure-type equipment. Both Fryling ( I S ) and Kolthoff ( 1 7 ) have reported in detail laboratory emulsion polymerization studies, employing closed bottles as reactors, by the method generally referred to as “bottle” polymerization. Starkweather ($8)reported the use of thick-walled borosilicate glass tubes as reactors. When monomers are used that have boiling points appreciably above the temperature of polymerization, as in the presept case, the reaction may readily be run in stirred vessels ( 1 1 , So), preferably so equipped as to maintain an inert atmosphere over the liquid phase. Whereas most laboratory emulsion polymerization studies are carried out by batch Polymerization, in which all the ingredients are initially introduced into the reactor, it is often desirable and in some cases necessary to add the reactants continuously during the reaction. Most of the processes described in the literature ( 1 , 4, 8, 9, 16, 24, 25, 2 9 ) for adding reactants continuously t o the reactor utilize large scale plant equipment designed for continuous-type operation. Monomers that polymerize very exothermically-Le., the lower acrylate esters-are conveniently emulsion-polymerized by either continuous or intermittent addition of the monomer to the reactor to yield a high solids emulsion. This latter process, termed “semibatch” emulsion polymerization, should possess some advantages of both the batch process and the true continuous process. Several authors, such as Wall and coworkers (SI), Denbigh (5,6),and de Nie (4),have pointed out t h a t one of the advantages of the true continuous process is the possibility of obtaining a copolymer product with a more homogeneous composition. I n contrast, the copolymer product formed during a batch polymerization is compositionally heterogeneous, primarily because the copolymer composition formed a t any instant is different from that of the monomer mixture from which it is being formed. I n many uses and commercial applications of emulsion copolymerization products i t is highly undesirable t o use materials t h a t are nonhomogeneous in composition. T h k applies particularly t o copolymers in which the properties change to a large extent with small variations of the ratio of monomer units contained in the copolymerization product.
According to de N e ( 4 ) ,copolymers of vinyl chloride-vinylidene chloride containing from 5 t o 15% b y weight of vinylidene chloride change solubility in acetone rapidly in this range of copolymer composition. At the lower per cent of vinylidene chloride content the product is substantially insoluble in acetone, while a t the upper per cent it is completely soluble. For use in lacquers it is apparent t h a t the solubility of the copolymer is of essential importance. The copolymer composition is also critical in the use of a comonomer in a synthetic fiber, such as the polyacrylonitrile type, to promote dyeability. Nonuniformity in dyeability may be attributed t o a variation in the content of the dye-attracting comonomer. I n the third example, t h a t of the copolymers studied in this paper, butyl acrylate-acrylonitrile copolymers show a rapid transition in some properties with increase in acrylonitrile content. For instance, abrasion resistance and low-temperature flexibility ( 1 2 ) of these copolymers change significantly in the range from 25 t o 35% acrylonitrile. I n an effort t o prepare reproducible high solids latices of these copolymers on a laboratory scale, equipment was designed to feed the reactants automatically into the vessel and to carry out the reaction under carefully controlled and recorded conditions. No attempt is made in this paper to treat the characteristics of the semibatch emulsion polymerization technique on a theoretical basis. For a very lucid treatment of the theory of emulsion polymerization, reference is made t o Flory’s book ( I O ) . It can be deduced from the quantitative theory of emulsion polymerization developed by Smith and Ewart ( 2 7 ) t h a t there are certain advantageous features inherent in the semibatch technique. The conditions required for optimum rate and degree of polymerization are: (I) a large number of particles per unit volume (and therefore small particle size) for a high rate; and (2) a low initiation rate for a given particle size for a high degree of polymerization. These two conditions in a normal batch process stand in opposition t o one another, since a low initiation rate favors a smaller number of particles during t h e particle-generating phase of t h e polymerization (85, 27). A possible advantage of t h e semibatch technique, then, appears t o be t h e opportunity t o employ a high rate of initiation and high emulsifier concentration during t h e initial phases of the polymerization t o form a high number of polymer particles. During the subsequent phases of t h e polymerization, by controlling the feed rates of the initiator ingredients, a lower initiation rate may be obtained. Utilization of t h e semibatch technique theoretically should allow the production of polymers of a high degree of polymerization a t a high rate of conversion. As a corollary, a semibatch latex should have a smaller particle size than a latex produced by t h e batch process having the same polymer content and degree of polymerization. SEMIBATCH POLYMERIZATION E Q U I P M E N T
The components of the equipment used t o maintain and record accurate temperature control were used interchangeably for batch
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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RECYCLE TO BATH COOUNG WATER
COOLING WATE
Figure 1.
Schematic diagram of equipment for semibatch emulsion polymerization of high boiling polymers
and semibatch reactions, and the data this paper. Figure is a schematic diagram of the semibatch emulsion polymerization nitrile copolymers. ~h~
*
for these are reported in the equipment used for of butyl acrylate-acrylomust be modified for
the emulsion polymerization of monomers whose boiling points are above the reaction temperature. Three reactant feed tanks are shown, each connected to a proportioning pump, from which reactive ingredients, either in solution form or as liquid monomers, were metered into the reactor. The pumps were set t o meter the liquids into the reactor at predetermined rates. The pumps employed a reciprocating action having an adjustable displacement capacity of from 50 t o 1650 ml. Per hour. T h e reproducibility of the pump . . rates, a t the pressure head used, was about flO%%e., a maximum variation of f 2 0 t o 25 minutes in a 4-hour pumping cycle. T h e polymerization vessel, a 5-liter, 4-necked glass flask, was placed in a stainless steel cooling jacket 10 inches in inside diameter and 10 inches high. The flask was fitted with a variablespeed stirrer, a condenser, a dip tube for nitrogen flushing, and two thermocouples-one for recording the reaction temperature and another for controlling the flow of cooling water in the jacket. An adapter was designed for connecting the lines from the pumps t o the flask in such a way t h a t the catalyst and activator solutions were not mixed until after they had entered the aqueous emulsion. An electronically operated solenoid valve, activated by the temperature controller, allowed cooling water t o enter the bottom of t h e jacket and flow out through an overflow port. T h e required mixture of monomers was added to the monomer feed tank and the corresponding pump was set a t a predetermined rate, generally t o introduce the monomers into the reaction flask in a 3- t o 6-hour cycle. The monomer feed tank, a 3-liter, bottom-outlet flask, was equipped with a reflux condenser and a dip tube for nitrogen flushing. Catalyst and activator were dissolved in separate portions of deionized water and placed in their respective feed tanks. The pumps were adjusted to add the desired amounts of each ingredient in about the same time as was em-
ployed t o add all the monomer. The feed tanks for the aqueous solutions of catalyst and activator were 500-ml. graduated separatory funnels, fitted with inlet tubes for pressure equalization. TYPICAL PROCEDURE FOR SEMIBATCH LATEX PREPARATION
The butyl acrylate monomer (Rohm & Haas) was freed of inhibitor by alkaline u.ashings, as described b y Leonard and coworkers ( I 8 ) . The acrylonitrile (American Cyanamid co.) was used as received without further treatment. All chemicals were, unless specified otherwise, of A.C.S. grade purity. The water was deionized by means of a Barnstead Bantam demineralizer (Barnstead Still and Sterilizer Co.), and oxygen-free nitrogen (Southern Oxygen cO.) was to purge the system of a v . Santomerse D (Monsanto Chemical Co.), t h e emulsifier generally used, was reported to be the sodium salt of a n alkylaryl sulfonate. The following procedure was successfully used by the semibatch method to prepare about 1 gallon of latex of 70-30 butyl acrylate-acrylonitrile copolymer (Run D, No. 85-36), for which the charge recipe is shown in Table I.
Table I.
Typical Recipes Used in Batch and Semibatch Polymerizations A B C DQ E Fa
Run h-o. 83-22-2 83-34 Method B B Butyl acrylate 70 73 Acrylonitrile 30 27 82 Water 82 2 Santomerse D 1.2 Potassium persulfate 0.01 0.01 Sodium thiosulfate 0.01 0.01 Potassium chloride 0.25 0.25 Feed period, hr. ... ... Reaction pH 2.0 1.8 One fourth of required m onomer.
85-25 SB 70 30 82 2
85-36 SB 70 30 82 2
83-27-2
B
65 35 82 1.2
85-41 SB 65 35 84 2
0.0294 0.0275 0.01 0.0265 0.0292 0.01 0.25 0.25 0.25 3.7 3.25 , .. 2.2 2.8 2.0 catalyst, and activator added
0.0209 0,0227 0.25 4.7 2.0 initially.
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Vol. 47,No. 5
TO REACTION- EQUIPMENT
Figure 2. Schematic diagram of equipment employed to maintain slight nitrogen pressure in reactor
r - l LAYER
T ATMO
DEWAR FLASK
DIP LJNE SUBMERGE 2 ' IN MERCURY
One fourth of the total amount of the monomers was added to the reactor, which contained t h e aqueous solution of the Santomerse D and potassium chloride. T h e p H was adjusted t o a value ranging from 2 to 3 by means of dilute sulfuric acid. T h e rate of t h e monomer pump was adjusted to 587 ml. per hour. An excess amount of catalyst solution was prepared by dissolving a calculated amount of potassium persulfate in 350 ml. of water. A small quantity of concentrated sulfuric acid (1.2 grams) was added to the catalyst solution. T h e activator solution was also prepared in excess by dissolving a calculated amount of sodium thiosulfate in 350 ml. of water. T h e two solutions were metered in during the run a t about t h e same rate (64 ml. per hour) until the desired amount of each ingredient was added, and then the pumps were stopped. Dip lines inserted in the catalyst and activator solutions were used for flushing with oxygen-free nitrogen for 45 minutes. During this time, the emulsion in t h e reactor and the remainder of the monomer were freed of oxygen by evacuating and filling with nitrogen several times. Then the feed tanks were placed under a slight head of nitrogen pressure, and the bottom outlet stopcocks were opened, t o allow t h e lines leading to the reactor t o fill. T h e stopcocks were then closed. After the temperature of t h e reaction flask had been adjusted t o about 2 " below the desired reaction temperature, one fourth of the total amount of each of the initiator ingredients was added in the form of two separate 25-ml. aqueous solutions. This was the starting point of the polymerization cycle. T h e stopcocks of the feed tanks were opened and t h e pumps started approximately 10 minutes after t h e first indication of reaction, as determined by a temperature rise seen on the recorder or by the appearance of a bluish opalescence in t h e emulsion. h'itrogen flushing of the emulsion in t h e reactor was allowed to continue for 45 minutes after the start of t h e cycle. Then, by turning off the nitrogen flushing line and employing the equipment shown in Figure 2, the reactor was placed under a nitrogen pressure of 2 inches of mercury for the duration of the run. EXPERIMENTAL RESULTS
From considerable experience a t this laboratory and a t the Government Laboratories a t the University of Akron with batch-type emulsion polymerization of various acrylate elastomers, i t was known t h a t high solids latices could be prepared by polymerization a t a pH between 2 and 3 and a t temperatures ranging from 0' t o 50" C. by employing a redox initiation system (11). Table I shows the charge formulas employed t o produce some of the latices referred to in the following comparison of various batch and semibatch emulsion polymeri5ations. These formulas contained the monomers of butyl acrylate and acrylonitrile in an approximate weight ratio of 70 t o 30. The reaction conditions (Tables I and 11) were selected with the view of
XYGEN-FREE NITROGE
yielding materials of high molecular weight in the form of stabe latices containing 55% theoretical solids. Generally, conversion curves obtained for batch runs were fairly linear (see Figure 3 ) up to about 50% conversion, where the rate of reaction usually slowed down considerably. A long polymerization cycle of the order of 24 to 7 2 hours was required to produce the desired solids content. The initial slopes of these curves for batch runs gave a rate of reaction ranging from 10 to 15% conversion per hour. Reproducibility of rate of reaction and total conversion was somewhat difficult. I n a n attempt to prepare a high solids latex in a shorter period of time under controlled conditions, the method of semibatch emulsion polymerization wm investigated. B y means of this method, the catalyst and activator solutions were either partially or entirely proportioned into the reactor during a 3to 6-hour cycle. Not only did this method of continuous addition allow the safe use of larger quantities of catalyst, b u t i t was probably responsible for a lower percentage of loss of active persulfate catalyst due to reduction during the polymerization
Table 11. Polymerization Data for Typical Batch and Semibatch Runs Run A B C D a E F " NO. 83-22-2 83-34 85-25 85-36 83-27-2 85-43 Metliod B B SB SB B SB Temp. control methodc NAC AC AC BC NAC A C 20 20 20 20 Initial reaction temp., O C. 20 25 27 21 21 Maximum temp., O C. 3ld 2 0 . 5 30 NF h-P NP KjP NF TP Purging method' Induction time, hr. 0.7 0 75 1.0 0.3 1 5 0.3 22 22 10f 22 22 51 Total time agitated, hr. Total solids after 22 hr., 46.4 52.0 51.2 53.0 54.0 48.2 70 Total solids after stripping, 52.2 54.5 54.4 51.9 54.8 51.2 700 Adjusted p H 8.5 9.5 8.9 9.8 7.4 9.7 Latex viscosityh 100 250 78 107 290 155 (30 r,p.m.), c.p.8 2 1 2 1 2 2 Spindle no. a One fourth of required monomer, catalyst, and activator was initially added. B. Batch method. SB. Semibatch method by continuous feed of reactants. c AC. Automatic control equipment. NAC. Nonautomatic control of temperature. d Cooling water a t 18O C . was insufficient t o control temperature rise. e N F . Nitrogen flushing continued for first 6 hours. N P , Nitrogen pressure (2 inches of mercury) after initial evacuation, nitrogen filling, and brief flushing. f Frozen stirrer due to excess coagulation. 0 Latex stripped 1 hour a t 5-mm. pressure a t 25O C . after alkaline adjustment of p H . h Brookfield viscosity determined on alkaline stripped latex.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cycle, since the thiosulfate reducing agent was also slowly added during the cycle. I n the charge formula for R u n D, No. 85-36 (Table I), 0.0275 p a r t of persulfate was used. When the persulfate was added t o the reactor in the manner specified, it took 1.5 hours t o raise the amount of persulfate in the reactor to 0.01 part, the total amount added initially in the case of batch runs. Theoretical per cent total solids were calculated using the data for the pumping rates of the monomer mixture, the activator solution and the catalyst solution, and the solids originally present in the reactor, assuming t h a t all the monomer in the reactor a t any particular time was instantaneously converted to solid polymer. I n Figure 3, the conversion curves for run D, No. 85-36, and a typical batch run, R u n A, No. 83-22-2, may be compared. I n spite of the lesser catalyst content during the initial stages of the run, the semibatch method gave a significantly higher initial rate. This may be attributed to the higher ratio of emulsifier to monomer. Kevertheless, this method of adding the catalyst and activator had no obvious disadvantage. T o determine if the higher total parts of catalyst and activator and the method of reactant addition influenced the molecular weight of the resulting copolymer, intrinsic viscosities were obtained from measurements of dilute solution viscosity of the copolymers dissolved in methyl ethyl ketone. Table I11 shows t h a t irrespective of the method of reactant addition the intrinsic viscosity values remained nearly constant (ranging from 5.0 to 5.6) for the copolymers prepared according to the information in Tables I and 11.
60
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SEMI -!BATCH THEORETICAL
-
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0
cn
-
t-
z W
0
E20t I/ , o v 7 0 : 3 0 BUTYL ACRYLATE
:
I
ACRY LONlTR ILE
Figure 3. Conversion curves for typical 70-30 butyl acrylate-acrylmitrile copolymer
The polymerization data obtained from each run listed in Table I are described in Table 11. Batch runa A and E were carried out in equipment which did not include the automatic temperature control equipment shown in Figure 1. Manual control in these runs normally gave a reaction temperature range of 3 ~ 5 'C. or more. I n runs B, D, and F, use of the automatic temperature control equipment with a proper supply of cooling water gave a temperature variation of i1O C. or less. I n run C, No. 85-25, the temperature of the t a p water used for cooling was 18" C., which during the initial stages of the cycle gave too small a temperature differential t o maintain proper control at 20" C., and the reaction temperature rose t o a maximum of 31' C. before being brought under control. Examination of t h e data in Table I1 shows t h a t the batch runs gave significantly more variations than the semibatch runs-for instance, considerable coagulum formed in batch runs B and E
Table 111.
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Properties of High Acrylonitrile Content Butyl Acrylate-Acrylonitrile Copolymers A B C D E F 83-22-2 83-34 85-25 85-36 83-27-2 85-41 B B SB SB B SB
Run No. Methoda Nominal BA: AN monomer ratio A X in copolymer, % Ult. tensile strength, lb./sq. innh
300% mcdulus, 1bJ
sq. inch Tear resistance, lb./ sq. inch Abrasion resistance, rubs Intrinsic viscosity
70:30
73:27
70:30
70:30
65:35
66:36
25.0
26
29.4
29.0
31.5
35
G i n
id50 60
1,590 440
1,640 430
1,590 380
1,660 300
110
230
300
400
740
1,600
42
55
93
85
141
249
1600 5.4
8400 30,000 21,000 30,000 29,000 5.6 5.5 5.2 5.6 5.0
19 17 14 9.6 2.3 0.18
90 89 75 60 15 0.44
Tpp., C.
Stiffness in flexure, lb./aq. inch X 10-8 a
)_I -10
0 (+IO
44 36 32 31 28 3
100 ,
.
65 64 40 7.3
75 60 57 40 39 14
.. .
110 i40 87 38
B. Batch method. SB. Semibatch method by continuous feed of reactants.
Two films were rubbed against each other until one developed a hole or they resisted 30,000rubs.
which interfered with the operation of, the paddle-type stirrer. In addition, the solids content determined after 22 hours' reaction time was generally lower for batch runs, varying from 46.4 to 51.2%. The total solids content for semibatch runs in each case was a t least 52% after 22 hours, equivalent t o about 95% conversion. I n Figures 4 and 5, the first-, third-, and fifth-hour temperature control data were plotted for a typical batch and semibatch run, respectively. During the third hour, the rate of heat liberation as a result of the more rapid conversion of the semibatch run was reflected by the greater number of times the cooling system was turned on. During the third hour this occurred six times in the case of the semibatch run, compared to twice for the batch run. I n spite of the higher monomer concentration present during the fifth hour of the batch reaction, the amount of heat liberated was not significantly different from that of the semibatch run during the same period of the reaction cycle. From the manner in which the reaction temperature was controlled it appeared t h a t the equipment for automatically maintaining a uniform reaction temperature was adequate when cooling water of a sufficiently low temperature was employed. Naturally, there are many variations possible in recipe and procedure in both batch and semibatch methods. I n one of t h e applications intended for these latices (18)-namely, production of hollow, flexible replicas by casting in artificial stone moldstwo of the requisites were high solids content and low latex viscosity. It was found t h a t the viscosity of the polymerized latices was lowered by adjustment of the p H to a slightly alkaline value. The viscosity of the latices was determined with a Model LVF Brookfield viscometer at spindle speeds of 6, 12, 30, and 60 r.p.m. I n Table I1 are shown only the values of viscosity in centipoises for the various latices measured at a spindle speed of 30 r.p.m. It appeared from the viscosity data t h a t both batch and semibatch methods could be employed t o give useful, low-viscosity (less than 2000 centipoises) latices of butyl acrylateacrylonitrile copolymers. I n Figure 6 are plotted two conversion curves for the semibatch preparation of 100:2 ethyl methacrylate-glycidyl methacrylate copolymer latices. These runs, for which the polymerization data are not included in this paper, were charged to yield a 48% theoretical solids content. I n one run the emulsifier content was 4 parts and in the other it was 2 parts. It is of interest to note the significant increase in the rate of polymerization at the higher emulsifier concentration.
Vol. 47, No. 5
INDUSTRIAL AND EN,GINEERING CHEMISTRY
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I st
HOUR
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20 OC. 15
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20 30 40 50 (MINUTES) REACTOR TEMPERATURE -------_BATH TEMPERATURE IO
--
1
60
FigureJ4. Automatic temperature control during first, third, and fifth hours of batch run R u n B, 83-34
T h e glycidyl methacrylate comonomer was employed in these preparations t o effect cross linking of the polyethyl methacrylate. Erickson (7) has described the use of such a polyfunctional comonomer to insolubilize polystyrene. The two dissimilar polymerforming groups contained in glycidyl methacrylate are an epoxide group and a n olefinic group; they form polymers by entirely different mechanisms. T h e monomer was subjected first t o free radical type copolymerization with ethyl methacrylate through the olefinic group, and then the resulting copolymer wae further made t o react under different conditions through the unaffected epoxide group, so t h a t a n insoluble polymer was formed. T o effect a uniform distribution of the polyfunctional comonomer along the copolymer chain, especially for two monomers with significantly different reactivities, the semibatch method probably holds more promise than the batch method, while the true continuous method of polymerization undoubtedly will yield the optimum distribution of the comonomer. The latter method did not lend itself from a practical standpoint on a laboratory scale to the preparation of high solids latices.
undoubtedly because the higher vapor pressure of acrylonitrile accounted for nearly all of the difference in copolymer composition. I n the case of the other runs (B, C, D, and F) the copolymer composition was kept closer t o the charge ratio of the monomers by using a purging technique which consisted of consecutively evacuating the filling with nitrogen. The equipment used t o accomplish this is shown schematically in Figure 2. T o obtain a short induction period in these runs a minimum of about 45 minutes of nitrogen flushing appeared to be sufficient t o remove the inhibiting effect of dissolved oxygen. Although sufficient data on physical properties were not available to demonstrate the superiority of the copolymers produced by batch or semibatch techniques, it was demonstrated from this study and information included in previous publications (11, 18) t h a t increase of acrylonitrile content increased the mechanical strength and low-temperature stiffness properties and decreased solubility and tackiness characteristics. Copolymers of equivalent acrylonitrile content and molecular weight, prepared by either method, appeared to have similar physical properties according to the tests which were performed. (The abrasion test employed t o determine the resistance of the various copolymers to breakdown when one film was rubbed over another film of the same material shox-ed t h a t a rapid rate of increase of abrasion resistance occurred with an increase in acrylonitrile content in the range from 25 t o 30y0.) I n addition, the 300% modulus and the temperature a t which the copolymers became brittle increased rapidly in this range. It was essential, therefore, for optimum abrasion resistance and low temperature behavior t h a t the technique used to prepare these copolymers of high acrylonitrile content yield reproducible results.
25 20.
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P H Y S I C A L P R O P E R T I E S OF COPOLYMERS
The technique of casting films and the test methods employed
to obtain the physical properties of these film-forming copolymers have been described by Blevins and coworkers (2). I n Table I11 are listed some of the mechanical properties measured a t room temperature for films cast from latex in artificial stone molds. Also listed are the stiffness moduli measured b y means of a Tinius Olsen stiffness tester a t various low temperatures. Shown in Table 111, the analyzed acrylonitrile content was in some cases appreciably different from the amount in the monomer mixture, in contradiction with what one would expect for such high conversion polymerizations. This discrepancy was traced t o t h e manner in which the system was flushed with nitrogenfor example, the copolymers obtained from runs A and E contained significantly less acrylonitrile than the charge ratio of the monomer. During these runs the nitrogen flushing was allowed to continue for several hours. B y means of dry ice traps (see Figure 2) and analysis of the entrapped material i t was determined t h a t the amount of acrylonitrile lost in the stream of nitrogen was considerably more than t h a t of butyl acrylate,
I
I
IO
- - ---
I
1
20
30
I
40
I
50
( MINUTES 1 REACTOR TEMPERATURE BATH TEMPERATURE
I
60
Figure 5. Automatic temperature control during first, third, and fifth hours of batch run R~~ D, 85-36 SUMMARY
One reason for investigating t h e semibatch method of polymerization was t h e potentially easier temperature control in comparison with the batch method. This was especially important in the case of extremely exothermic polymerizations. For instance, the batch preparation of high solids latices of butyl acrylateacrylonitrile copolymers frequently resulted in considerable coagulation, especially at the reactor wall, attributed t o excessively cold water in the cooling jacket and poor heat transfer conditions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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THEORETIC A L
4 P#RTS EMULSIFIER 2 PARTS EMULSIFIER
1005
influence on the homogeneity of the copolymer composition and resultant physical properties. T h e semibatch method gave definitely improved uniformity of conversion curves and higher rates of reaction. Conditions of polymerization were investigated for preparing b y both methods copolymers of similar molecular weight as indicated by intrinsic viscosity measurements. ACKNOWLEDGMENT
SEMI-BATCH 100- 2 ETHYL METHACRYLATE : GLYCIDYL METHACRYLATE
Figure 6.
Effect of emulsifier content on polymerization
Build-up of coagulum on the reactor wall caused a further decrease in the rate of heat transfer, and, thus, poor temperature control. The general feasibility of using the semibatch technique on a laboratory scale has been demonstrated. Compared to the batch method, the semibatch method offered considerable versatility and appeared to have special application for the preparation of high solids latices. B y closely controlling the polymerization procedure within narrow limits, useful high solids latices were prepared which contained butyl acrylate-acrylonitrile copolymers with high acrylonitrile content. Temperature control in this study was important in determining the factors influencing the preparation of high solids latices. Equipment was developed for achieving excellent temperature control in both batch and semibatch emulsion polymerizations. The charge formulas, polymerization data, and copolymer physical properties were determined for copolymer latices having monomers in a weight ratio of about 70 parts of butyl acrylate to 30 parts of acrylonitrile. I n considering the relative merits of the batch and semibatch methods, the subject of heterogeneity of copolymer composition was emphasized. During the course of a batch process the ratio of the monomers in the unreacted mixture has been reported t o vary because of monomer reactivity differences. Wall and coworkers (SI)have analyzed fractionated samples of styrene-methyl methacrylate copolymers prepared by batch- and continuous-type processes, and reported t h a t the batch process of polymerization gave a copolymer possessing much greater compositional heterogeneity. Although the semibatch method would not be expected to yield as homogeneous a copolymer composition as the true continuous method, i t should be capable of producing by the discriminating adjustment of monomer feed rates a more homogeneous copolymer composition than is possible in a batch process. The usefulness of many properties of copolymers is dependent to a critical extent on small variations in the percentage of monomer constituents contained in the copolymer. The copolymer system of butyl acrylate and acrylonitrile showed rapid changes in abrasion resistance, solubility, flexibility, and tackiness with increased acrylonitrile content. T h e reproducibility of copolymer acrylonitrile content was aided in both batch and semibatch techniques of emulsion polymerization b y reducing the time of nitrogen flushing and employing a n improved purging technique. Test results obtained were inconclusive in showing one method of polymerization t o be preferable to the other in
The work discussed was performed as part of a research project of The Office of the Surgeon General, Department of the Army, in connection with the government amputee research program in cooperation with the Advisory Committee on Artificial Limbs, National Research Council. The authors wish t o express gratitude to Fred Leonard of this laboratory for his advice and cooperation, and to W. K. Taft, M. H. Reich, and R. W. Laundrie of the Government Laboratories a t the University of Akron for their efforts in scaling up the reactions in plant size equipment. The assistance of M. G. DeFries, Louis Schneider, and A. L. Beat is gratefully acknowledged, REFERENCES
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