-ELASTOMERS-Synthetic Fielding, J. H., IND. ENG.CHEM.,41, 1560 (1949). Flory, P.J., Ibid., 38,417 (1946). Flory, P.J., J. Am. Chem. SOC.,69, 2893 (1947). Flory, P. J., Rabjohn, N., and Shaffer, M., J. Polymer Sci., 4, 226 (1949). Forman, D. B., Radcliff, R. R., and Mayo, L. R., IND.ENG. CHEM.,42,686 (1950). Frank, R. L., et al., Zbid., 40,420,879 (1948). Gee, G., Trans. Insl. Rubber Ind., 18, 266 (1943). Gehman, S. D., Woodford, D. E., and Wilkinson, C. S., Jr., IND.ENG.CHEM.,39,1108 (1947). Hampton, R. R., Anal. Chem., 21, 923 (1949). Hanson, E. E., and Halverson, G., J. Am. Chem. Soc., 70, 779 (1948). Hart, E, J., and Meyer, A. W., Zbid., 71, 1980 (1949). Howland, L. H., Messer, W. E., Neklutin, V. C., and Chambers, V. S., Rubber A g e , 64,459 (1949). Johnson, B. L., IND. ENG.CHEM.,40, 351 (1948). Johnson, B. L., and Wolfangel, R. D., Ibid., 41, 1581 (1949). Johnson, P.H., and Bebb, R. L., Ibid., 41, 1677 (1949), Juve. A.E..Ibid.. 39. 1494 (1947). Jive: A. E.: et al.,’Zbid., 39,i490 i1947). Kemp, A. R., and Straitiff, W. G., Ibid., 26,707 (1944). (34)Kolthoff, I. M.,Lee, T. S., and Mairs, M. A., J . Polymer Sci., 2, 199 (1947). (35)Lansing, W. D., and Kraemer, E. O., J. Am. Chem. SOC.,57, 1369 (1935). (36) Laundrie, R. W.,Feldon, N., and Rodde, A. L., India Rubber WorZd, 122,683 (1950). (37) Marvel, C. S.,Inskeep, G. E., Deanin, R., Juve, A. E., Sohroeder, C. H., and Goff, M. M., IND.ENG. CHEM.,39, 1486 (1947), (38) Meyer, A.W., Zbid., 41,1570 (1949). (39) Mochel, W. E., and Nichols, J. B., Zbid., 43, 154 (1951). (40) Mochel, W.E., Nichols, J. B.. and Mighton, C. J., J. Am. Chem. SOC.,70,2185(1948). (41) Owen, H.P.,Rubber Age (N.Y.),66, 544 (1950). (42)Rabjohn, N.,Bryan, C. E., Inskeep, G. E., Johnson, H. W., and Lawson, J. K., J . Am. Chem. SOC.,69,314 (1947).
Rubber-
(43) Rinne, W. W., and Rose, J. E., IND.ENQ. CHEM.,40, 1437 (1948). (44) Saffer, A., and Johnson, B. L., Zbid., 40, 538 (1948). (45) Salomon, G., and Amerongen, G. J. van, J . Polymer Sci.. 2, 365 (1947). (46) Salomon, G., and Koningsberger, C. J., Ibid., 2, 522 (1947). (47) Salomon, G., van der Schee, A. C., Ketelaar, J. A. A,, and van Eyk, B. J., Discussions Faraday Soc., No. 9, 281 (1950). (48) Schoene, D. L., Green, A. J., Burns, E. R., and Vila, G. R., IND. ENG.CHEM.,38,1246 (1946). (49) Schulre, W. A., and Crouch, W. W., J . Am. Chem. SOC.,70,3891 (1948). (50) Schulze, W.A,, Crouch, W. W., and Lynch, C. S., IND.ENQ. CHEM.,41,414 (1949). (51)‘Starkweather, H. W.,et al., Ibid., 39, 210 (1947). (52) Storey, E. B., and Williams, H. P., Rubber Age ( N . Y.), 68,671 (1951). (53)Sutherland, G. B. B. M., and Jones, A. V., Discussions Faradau Soc.. No. 9.281 (1950). (54) Swart,’G. H.; Pfau, E. S., and Weinstock, K. V., I n d i a Rubber World, 124,309 (1951). (55) Sweitzer, C. W., Goodrich, W. C., and Burgess, K. A.,Rubber Age (N.Y.), 65,651 (1949). (56) Syrkin. Y. K., and Dyatkina, M. E., “Structure of Molecules.” p. 282,New York, Interscience Publishers, 1950. (57) Vanderbilt Rubber Handbook, R. T. Vanderbilt Co., New York, N. Y., p. 51,1948. (58) Welch, L. M.,Nelson, J. F., and Wilson, H. L., IND.ENQ. CHEM.,41,2835 (1949). (59)White, L. M., Zbid., 41,1554 (1949). (60)Yakubchik, A. I., Vasiliev, A. A., and Zhabina, V. M., R u b b n . Chem. and Technol., 18,780 (1945). (61) Yanko, J. A , , J . Polymer Sci., 3, 576 (1948). (62)Zapp, R. L., and Baldwin, F. P., I N D . ENQ. CHEM.,38, 948 (1946). (63) Zapp, R. L.,and Gessler, A. M., Zbid., 36, 656 (1944). RECXIVE~D for review September 17, 1951. ACCBPTED January 25, 1962. Contribution 187,Research Laboratory, Goodyear Tire a n d Rubber Co.
POLYMERIZATION RATES IN EMULSION SYSTEMS MAURICE MORTON, P. P. SALATIELLO, AND HAROLD LANDFIELD University of Akron, Akron, Ohio
T
HE mechanism of emulsion polymerization, as
proposed by Harkins and his collaborators (1, 2 ) explains the unique characteristics of this system. I n the case of relatively insoluble monomers, the initiation of polymer nuclei supposedly occurs almost entirely in the monomer which is solubilized in the interior of the soap micelles, rather than in the individual monomer droplets or in the aqueous solution. One of the obvious reasons for this is the relatively high number of micelles with diameters of approximately 50 A., as compared with the monomer droplets with diameters of approximately 10,000 A. Since the free radicals generally arise from a n initiator in the aqueous phase, the micelles would capture far more of these radicals than either of the other two phases. However, the formation of new polymer nuclei does not continue indefinitely, because of the gradual disappearance of the soap micelles which are depleted by the adsorption of soap molecules on t h e surface of the polymer nuclei.
April 1952
Hence, at a relatively early stage of the polymerization new particles cease to b e formed, more or less, and the p o l y m e r i z a t i o n proceeds largely by a process of growth of t h e p a r t i c l e s a l r e a d y formed, which are also very small, about 200 A., compared to the monomer droplets. It is this process of polymerization in a large and constant number of isolated loci-about 10’6 per ml.which lends to the emulsion system its special and unique characteristics. These particles imbibe monomer, which diffuses from the free droplets, and thus become polymer-monomer particles in which most of the polymerization occurs. On this basis it has been possible for Smith and Ewart (8) t o develop a n interesting kinetic treatment of emulsion polymerization. The most striking conclusion of their treatment is t h a t in the ideal case the number of particles in the latex represents twice the number of growing radicals during the steady state. Hence, from a knowledge of the polymer-
T h e emulsion polymerization of .butadiene and styrene was studied with several different initiator systems. Styrene behaved according to the predictions of the theory of emulsion polymerization, in that the rate of growth of the latex particles was independent of variables other than temperature. Hence, at a given temperature, the rate can be related to the particle size. For butadiene, however, the various initiator systems showed varying degrees of efficiency, the only one which behaved according to theory being the peroxamine type. Hence the rate versus particle size relation is different for each system. The only reliable method for evaluating the relative efficiency of an initiator system is to determine the rate of polymerization per particle of latex formed.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
739
-ELASTOMERS-Synthetic
Rubber-
monomer ratio in the particles, it should be possible to obtain absolute values of the propagation rate constant. This has been done for the persulfate-initiated styrene polymerization by Smith ( 7 ) , who found that this system appears to behave in a more or less ideal fashion.
I
,
2
I 3
I
I
I
I
4
5
6
7
TlME
IN
gave much poorer rates with the diazothioether recipe than did ordinary distilled water. It is probable that the TDN initiator depends on traces of oxygen. Butadiene was obtained from the Government Laboratories, Akron, Ohio, and was produced by the Koppers Co. from alcohol. It was found that the petroleum butadiene, such as obtained from the Phillips Petroleum Co., showed a very marked induction period with the dichromate-arsenite system, hence the alcohol butadiene was used throughout. It was stated to have a 99.5% butadiene content. It was freed from inhibitor by distillation, as previously described (4). Soap was of the potassium, Office of Rubber Reserve, variety as prepared by Swift and Co., with a 92% solids content. Potassium dichromate was Mallinckrodt Analytical Reagent, Arsenious oxide was Baker C.P. reagent. It was dissolved in potassium hydroxide and neutralized to a pH of 7 with hydrochloric acid. TDN (p-tolyldiazothio-2-naphthylether) was prepared a t the Government Laboratories, Akron, Ohio, and recrystallized once, It was charged as a benzene solution using 0.5 ml. solution per bottle.
J
STYRENERECIPES (Temperature, 40' C.) Recipe 1 Recipe 2 180 180 2 2 100 100 0.3 0.3 0.1 0.1
HOURS
Figure 1. Polymerization Rates for Butadiene C h r o m a t e - a r s e n i t e catalyst
I n some recent papers ( 4 4 ) a similar study has been extended to butadiene, isoprene, and styrene, using some variations in initiator types. This study has revealed that although styrene shows a polymerization rate per particle of latex which is independent of the type and amount of initiator, this is not the case for butadiene and isoprene. Butadiene polymerization has been studied in four different initiator systems and only one of these was found t o behave in the ideal fashion, from which a propagation rate constant could be calculated. The results of such a calculation for the three monomers is described in the papers (4-6). The present paper deals with a comparison of the rate per particle obtained in the different emulsion systems for butadiene and styrene, and points out the extent of deviation from the ideal type of system. Such information, although of little value from theoretical considerations, is of practical importance, because it affords a quantitative measure of the efficiency of each of the initiator systems studied. I n this investigation, the following systems were studied for t h e emulsion polymerization of butadiene: persulfate-mercaptan; hydroperoxide-polyamine ( 9 ) ; dichromate-arsenite (3); and diaaothioether. I n addition, the behavior of butadiene is compared with that of styrene in the systems persulfate; persulfate-ferrous iron; persulfate-sulfite; and hydroperoxide-polyamine. EXPERIMENTAL
Polymerization. Polymerizations were carried out in 4-ounce bottles and the rates were determined from t h e change in total solids in samples obtained by means of the hypodermic needle method. The techniques and materials are described elsewhere (4). However, the following new recipes and materials were involved.
Recipe 3 Water 180 Potassium ORR soap 2 Styrene 100 Potassium persulfate 0.3 Cumene hydroperoxidea Triethylenetetrsminea Sodium sulfitea 0.1 FeSOc7HzOb 0.05 a Bdded a t about 30% conversjon, a f t e r temperature changed to 5' C. b Added a t about 30% conversion.
The water used was ordinary distilled water, freed from oxygen by boiling and stored under Linde high purity nitrogen, Styrene was obtained from the Dow Chemical Co. and contained tert-butyl catechol as inhibitor. It was freed from inhibitor by washing with sodium hydroxide solution, drying, and distilling under reduced nitrogen pressure. Sodium sulfite was Merck Reagent Grade. Ferrous sulfate was Mallinckrodt Analytical Reagent. Cumene hydroperoxide was supplied by the Hercules Powder Co. and was stated to contain 67.2% hydroperoxide. I t was charged as a benzene solution, 0.5 ml. per bottle. Triethylenetetramine was supplied as a technical grade by Carbide and Carbon Chemicals Corp. It was distilled at 18-mm. pressure, and the main fraction, boiling point 153" t o 154.5" C., was used. Particle Size Determination. The technique used to determine the average particle diameter of a latex was the Maron soap titration method previously described ( 4 ) . RESULTS AND DISCUSSION
Butadiene Polymerization. The rate curves obtained in butadiene polymerizations with the dichromate-arsenite and the T D K type of initiators are s h o m in Figures 1 and 2. As in the previous work booster injections of the initiators were used at about 30% conversion, where it was assumed t h a t no new particles could be formed. I n the case of the TDW system, this injection
CAT. CHARGE
CAT,
INJECTION
0 2 AT
BQTADIEXE RECIPEB
Water Butadiene Potassium ORR soap Po tassium dichromate Arrienious oxide X I TLIU
(Parts by weight) DichromateArsenite 180 100 6
60' C
30%
NONE
Diazothioether 180 100 5
I
ITater used for the diazothioether recipe was ordinary distilled water. However, for the dichromate-arsenite recipe, it was found that water redistilled over alkaline permanganate, and in a nitrogen atmosphere showed a marked decrease in the induction period of the polymerization. On the other hand, this redistilled water
740
I
2
3 TIME
Figure 2.
4 IN
5
6
HOURS
Polymerization Rates for Butadiene
p-Tolyldiazothio-2-naphthyle t h e r catalyst
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic Table I.
Particle Size and Number in Polybutadiene Latex
Type of catalyst %,conversion of latex Initial catalyst charge Injected catalyst charge of latex, Initiation temp. Propagation temp. of latex, C. O C. Polymerization rate, % per hour Particle diameter, A. (OS) Particle diameter, A. (Dn calc.) No. of particlea/ml. water ( X 10 -16) Polymerization rate, (gram/second/ml.,HaO X 106) Rate/particle, (gram/second X 1021) a
Chromate-Arsenite 62.4 48.8 60.5 0.3/0.1 0.3/0.2 0.3,'O.l 40.0 40.0 31.8 540
61.0 0.2 0.2 50.0 50.0 9.6 470 640
TDNa 71.5 0.2 0.2 60.0 60.0 18.7 480 660
61.0 0.2 0.2 60.0 50.0 8.8 470 640
6.3
6.0
5.7
30.0 30.0
400.
440.0 0.0 24.5 640 470
8.3
6.1
6.4
4.9
3.8
1.9
1.5
2.9
1.4
5.9
6.2
3.2
2.4
4.8
2.4
12.4 650 480
p-Tolyldiazothio-2-naphthyl ether.
Rubber-
still is only about one half as fast. Another feature to observe is the dependency of the rate per particle on the number of particles present a t a given soap charge. It can be seen here that the mutual system shows a marked dependency of the rate per particle on the number of particles, as has been previously discussed (4). No conclusions can be drawn about the behavior of the T D N system in this connection, since no variation in the number of particles could be obtained. However, the chromate-arsenite system exhibits a similar behavior to that of the peroxalnine system, in that the rate per particle does not change with the number of particles.
Table 11. Rate of Emulsion Polymerization of Butadiene Peroxamine Potassium DichromateArsenic Oxide
STYRENE POLYMERIZATION
Smith (8) has shown by means of seeded polymerization of styrene that the rate per particle is independent of o,05 0.2 0.2 40 50 60 10 10 persulfate concentration within fairly 10 10 50 50 50 8.5 13.3 Fide limits. I n a previous paper ( 6 ) 5.1 9.6 8.8 5.1 8.8 quite good agreement between the ab8 . 3 6 . 1 5.8 6.3 5.7 solute propagation rate constant ob2.6 2.3 5.9 6.2 1.3 2.4 2.4 tained for styrene with two different 20.5 18.2 initiator svstems (Dersulfate and Dera Diisopropylbenzenemonohydroperoxide-triethylenetetramine. oxamine) was found. This independence of the propagation rate of styrene on the type of initiator used is also strikingly demonstrated in the data in Table 111. These rehelped to maintain a linear rate, which would otherwise show a sults show the effect of various combinations of the persulfate tendency to die out. On the other hand, the dichromate-arsenite and peroxamine systems on the rate per particle a t 40" and 5" C. system, which is capable of maintaining a linear rate, shows a n In the one case where the latex was initiated by persulfate a t 40"C. anomalous dying out when an injection is made. This is the and then changed to a peroxamine system a t 5' C., the change first instance found where an injection of initiator has this effect, was made a t about 30% conversion. I t can be seen that the and may possibly be due to a sensitivity of this redox system to imbalance in'proportions of the two ingredients, This is also illustrated by the fact that an increase in arsenite to dichromate ratio results in a decreased rate, as shown by the rate curves in Figure 1. From the linear portions of each of the rate curves a rate could be calculated, and this rate was related to the number of particles present in each latex as determined from particle diameters. These data are shown for butadiene in Table I. As in the previwas calcuous work, the number-average particle diameter (Dn) lated from the soap-average diameter (volume/surface average, D,)by using the factor 0.735 which was found generally applicable for particle diameters in the vicinity of 1000 A. The initiation temperature of a latex refers to the temperature at the start/,/ , Le.,up to about 30% conversion-while the propagation tempera2 3 4 5 T I M E IN H O U R S ture of the latex refers to the temperature during the remainder of the polymerization-Le., up to about 60% conversion. Figure 3. Effect of Adding ReducI n the chromate-arsenite system the number of particles is detant a t 3 0 k Conversion in Persulfatependent on the initiation temperature as well as on the initiator Styrene Polymerization charge. Within the limits shown, the rate is proportional to the number of particles. I n the case of the T D N system the surprising result was obtained that the number of particles initiated a t values for the rate per particle a t 5' C. agree quite well regardless 60" C. was the same as that obtained a t 50" C. Hence no data of the system used. Furthermore, from the rate per particle are shown on the relation between number of particles and rate values a t 40" and 5" C. it is possible to calculate an activation per particle. energy value of about 8 kg.-cal. per mole for the propagation rate, which agrees with the previous values ( 6 ) obtained over a narHowever, in Table 11, an over-all comparison is made between rower range of temperature. the results obtained with the above two systems and those previously found for the mutual and the peroxamine systems ( 4 , 5 ) . As a furthkr check on the question of the dependence of rate per The four systems are arranged in order of increasing efficiency as particle on type of initiator, the rate studies illustrated in Figure indicated by the values of the rate per particle of latex. It is 3 were carried out. These involved the injection of reducing obvious that both the mutual and T D N recipes show a markedly agents, such as ferrous salt or sodium sulfite, into a persulfatelower rate per particle than the peroxamine system, by a factor of catalyzed styrene latex a t about 30% conversion, where no new 10 to 20 , The chromate-arsenite system can be said to approach particles would ostensibly be formed. Harkins ( 8 ) has stated, the type of rate obtained with the peroxamine recipe, although it on the basis of limited data, that the rate was approximately
Mutual Initiator system Initiator charge of monomer, part/100 rubber 60 Initiation temperature, O C. Propagation temperature, O C. 50 4.9 Propagation rate, % per hour No. of particles/ml. Hz0 ( X 10-16) 13.7 Rate/partiole, gram/second 0.54 ( X 10") Rate/particle a t 50' C., calcd.
April 1952
(4)
TDN
D TETA' IBP(1 to 1 Ratio)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
74 1
Rubber-
-ELASTOMERS-Synthetic
CONCLUSION
Table 111. Rate of Emulsion Polymerization of Styrene (Two parts of soap) Initiator system Initiation temperature of latex,
c.
Propagation system Propagation temperature of latex C. convkrsion of latex. % Polymerization rate, Yoper hour Psrticle diameter, -4. (D,) Particle diameter, A . (Dn ralc.) No. of partioles/ml. €110 ( X Polymerization rate, grains/second/ml. HzO X 105 Rate/particle, grams/second X 1090
*
CHPTETA'
40 40 KzSs08 CHP-TETAa
CHP-TETAa
Same 5 74 17.0 1000
830
5 67 9.2 1150 850
0.89
1.18
1 89
1.42
2.6
1.2
1.4
40 72 13.3 1500 1100
0 54
10-15)
KaSrOsCHPTETAb
Potassium Persulfate
5 50
7.0
1130
3.5
1.1
6.5
1.2
5
5
740
Curncne hydroperoxide-triethyienetetrarnine. persulfate-cumene hydroperoxide-triethylenetetramine.
b .Potassium
doubled by the addition of a ferrous salt to such a system. It can be seen from Figure 3 that the addition of free fm-ous ion results in a definite though small increase in rate of polymerization, the rate changing from 24 to 30% per hour. Similarly, for the sulfite addition, the rate increases from 21.5 to 26.5% per hour. I t is entirely likely that the addition of an active reducing agent into a styrene system a t 40" C. may lead to the formation of some new particles from styrene in the aqueous phase or in the free monomer phase, which would account for the increased rate. Although it would be of great interest to confirm such a formation of new particles, the soap titration method used for particle size determination is not considered sensitive enough to detect such relatively small changes in number of particles. Smith (8) also found that the rate of polymerization of a seeded latex showed a small increase when excessive amounts of persulfate were added.
The emulsion polymerization of styrene, with the various initiator systems which have been tried, appears t o follow the theoretical prediction that the rate of polymerization depends only on the number of particles and on the temperature. It is therefore possible to correlate particle size and rate of polymerization of polystyrene latex. In the case of butadiene, however, the different initiator systems tried exhibit varying degrees of efficiency in the propagation process, the peroxamine system being the only one which appears to conform to theory. Hence, for butadiene, the particle size and rate of polymerization can only be correlated separately for each initiator type. It is obvious that the only reliable method of evaluating the efficiency of any initiator system is to determine the rate per particle of latex. LITERATURE CITED
(1) H a r k i n s , R. D., J . Am. Chem. Soc., 69, 1428 (1947). (2) H a r k i n s , W.D., J . Polvmer Sci., 5 , 217 (1950). (3) Kolthoff, I. &I., a n d Meehan, E. J., private communication to Office of R u b b e r Reserve. (4) h f o r t o n , A I . , Salatiello, P. P., a n d Landfield, H., J . Polymer Sci., 7, 111 (1952). (5) Ibid., p. 215. (6) I b i d . , in press. (7) S m i t h , W. V., J . Am. Chem. Soc., 70,3695 (1948). (8) Smith, W. V., a n d Ewart, R. H.. J . Chem. Phus., 16, 592 (1948). (9) Whitby, G. S., Wellman, N., Flouta, V. TT., a n d Stephens, H. L., I X D . ENG. CHEM., 42,445 (1950).
RECEIVED for review November 5 , 1951.
ACCEPTED December 11, 1951. This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corp., in connection with the government synthetic rubber program.
Sonie Effects of Polymerization Temperature andMolecular Weight J. R. BEATTY, Research Center, B. F. Goodrich Co., Brecksville, Ohio B. M. G.ZWICKER, B. F. Goodrich Chemical Co., Cleveland, Ohio The advantages of 5" C. cold rubber over hot GR-S in high-black vulcanizates are well established. This paper describes some changes in micro and macro structure observed in butadiene-styrene copolymers made a t five polymerization temperatures (--18', -IO", 5", 20°, and 50" C.) and their relation to changes in vulcanizate properties as a function of polymerization temperature and molecular weight. The major differences in polymer structure of cold rubber as compared with GR-S appear to be: a narrower molecular weight distribution with less soupy polymer, a slight increase in crystallization tendency, and an increase in homogeneity of the polymer composition. Improvement in stress-strain and resilience with reduction of polymerization temperature from 50,. t o 5" C. was proportionately much greater than that resulting from further reduction to -18" C. Reduction in the amount of low molecular weight polymer softener in cold rubber as compared with GR-S increased its sensitivity to hot aging and changed the cure rate. Vulcanizates of fractionated polymers showed a marked decrease in hysteresis as molecular weight increased. There was a simultaneous increase in dynamic rate or modulus, but 742
the product, as measured by temperature rise, decreased with increasing molecular weight in nearly all cases. Dynamic flexibility and crystallization tendency a t -25" C . of OR-S vulcanizates are advemely influenced by the presence of low molecular weight material.
S
EVERAL authors (13-16) have reported that as poly-
merization temperature is reduced, butadiene-styrene copolymers display improved vulcanizate properties in treadtype recipes. The quality advantages of the cold rubber vulcaniaates are offset somewhat by inferior factory processability as compared to hot GR-S polymers. A fundamental study was undertaken to characterize the changes in polymer structure which might explain the observed differences. POLYMER PREPARATIONS
Polymerization. The polymers evaluated in this program were prepared in pilot plant equipment with recipes designed to make polymerization temperature the primary variable over the range of -18' to +50" C. Several charges mere prepared in a 175-gallon reactor a t each temperature and the latices were
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
