-ELASTOMERS-Latex(11) Damon, K. G., Anal. Chem.. 21, 1066 (1949). (12) Gils, G. E. van, Arch. Rubbemilt. A‘ederZand. Indie, 25, 383 (1941). (13) Ibid.,27,139 (1950). (14) Gils, G. E. van, Trans. Inst. Rubber Ind., 23, 74 (1947). (15) Gordon, M., and Dunlop Rubber Co., Brit. Patent 634,879 (March 29,1950). (16) Hayes, K. W., Trans. I n s f .Rubbe- Ind., 26,223 (1950). (17) Homans, L. N. S., and Gils, G. E. van, Proc. Rubber Tech. Conf. London, p. 292 (1948). (18) I. G. Central Rubber Organization at Leverkusen, Office of Technical Services, U. S. Dept. Commerce, CIOS R e p t . XXXIII-49 (1945). (19) International Latex Processes, Ltd., Brit. Patent 639,886 (July 5,1950). (20) Jordan, H. F., PTOC.Rubber Tech. Conf.London, p. 111 (1938). (21) Lepetit, F.. Trans. I n s t . Rubber I n d . , 23, 104 (1947). (23) Linscott, C. E., and U. S. Rubber Co., U. S. Patent 2,534,359 (Dee. 19,1950). (23) McColm, E. M., Rubber A g e , 62, 655 (1948). (24) McKeand, D. J., IND.ENG.CHEM.,43,415 (1951). ( 2 5 ) MrKeand, D. J., Moss, c., and Dunlop Rubber Co., Brit. Patent Appl. 24,403 (Oct. 6, 1950). (26) McKeand, D. J., Newnham, J. L. hl., and Dunlop Rubber Co., Ibid.,23,640 (Sept. 27, 1950). (27) Madge, E. W.,Collier, H. M.. and Peel, J. D., Trans. Inst. Rubberlnd., 26,305 (1950). (28) Martin, G., Ret*.g6n. caoutchouc, 18, 90 (1941); translated in Rubber Chem. TechnoZ., 19, 494 (1946), 20, 608 (1947). (29) Miserentino, C. O., presented before the Division of Rubber Chemistry, 120th Meeting, AM.CHEM.SOC.,New York, N. Y. (30) Monsanto Chemical Co., “Toxicology and Dermatology of Pentachlorphenol and Sodium Pentachlorphenate,” 1948.
(31) Murphy, E. A., Proc. Rubber Tech. Con/., London, p. 1st (1938). (32) Murphy, E. 8., Trans. I n s t . Rubber Ind., 18, 173 (1942). (33) Murphy, E. A., Madge, E. W., and Pounder, D. W., Proc. .Rt.bber Tech. Conf.,London, p. 303 (1948). (34) Kederlandsch-Indisch Instituut voor Rubberonderzoek, Brit. Patent 633,904 (Dee. 30, 1949). (35) Panich, R. M., Kal’yanova, K. A,, and Voyutsky, 9. S.,K o l l o i d Zhur.. 12,50 (1950). (36) Paton, F. J., private communication. (37) Philpott, M. W., private communication. (38) Pinazzi, C., Rev. gBn. caoutchouc, 27, 725 (1950). (39) Prokofiev, A. A., U q e k h i SovremennoE Biol., 27, 421 (1949). (40) Rhines, C . E., Linscott, C. E., and U. S. Rubber Co., U. S. Patent 2,534,370 (Dec. 19, 1950). (41) Rubber Stichting, Brit. Patent 633,403 (Dee. 19, 1949). (42) Ibid., 634,241 (March 15, 1950). (43) Rumbold, J. S.,and U. S.Rubber Co.. C. S. Patent 2,534,374 (Dee. 19, 1950). (44) Ibid., 2,534,375 (Dee. 19, 1950). (45) Stacey, M . , Chemistry & Indiistry, 45, 727 (1950). (46) Stamberger, P., India Rubber W-odd, 103, 35 (1940). (47) Stamberger, P., Rubber A g e (A!. Y.)? 66, 291 (1949). (48) Stamberger, P., and International Latex Corp., U. S. Patent 2,321,111 (June 8, 1943). (49) Sutton, S.D . . I n d i a R u b b e r J . , 112,626 (1947). (50) Talalay. L., Rubber Aqe (’\‘. Y,), 6 8 , 713 (1951) (abstract). (51) Van Buskirk, E. C., Butsch, P. V., and C . S.Rubber Co., U. 9. Patent 2,484,434 (Oct. 11. 1949). (52) W a l l Street J . , Oct. 11, 1949. (53) Wingfoot Corp., Brit. Patent 652.839 (May 2, 1951). (,54) W r e n W. G., Trans. Inst. Rubber I n d . , 18,91 (1942). RECEIVED for review September 17, 1961.
.ACCEPTED January 29, 1952.
SYNTHETIC RUBBER LATEX DEVELOPMENTS L. H. HOWLAND, V. C. NEKLUTIN,R. W. BROWN, AND H. G. WERNER United States Rubber Co.,Naugatuck Chemical Division, Naugatuck, Conn.
This work was carried out with two main objects in view-to apply the well-known advantages of low temperature synthetic rubber polymerization to products designed to be used in latex form, and to develop new types of latex to fill specific needs of industry. Several new low temperature latices have been developed, some of which are polymerized to 50% solids and heat-concentrated to 60% solids. Two variations of a substantially iron-free formula have been carried to the production stage, and several modifications designed, to satisfy special demands have been made on a pilot plant scale. Latex stability problems in products containing high Mooney viscosity polymers have been overcome. Subfreezing polymerizations have been studied on a laboratory scale. Other new products developed include latices containing no permanent electrolyte and emulsified with volatile base soaps, and latices prepared with cationic emuIsifiers for coating and impregnating applications. The production of low temperature synthetic rubber latices is a big advance toward freeing the consuming industries from their dependence on natural rubber latex with its wide variations in cost and supply. For the consumer, this should mean a greater supply of latex-derived products, such as foam sponge, a t m o r e stable prices. Also, the newer types of latices have given improvements in some consumer products and their unique properties are expected to open unexplored fields of latex applications. 762
T
HERE has long been a need for relatively inexpensive and
readily available synthetic rubber latex t o supplement natural latex. During the last war this need became acute and the synthetic rubber industry started work toward the development of such materials. The first of these latices were Types I, 11, and I11 which have been described previously ( 6 ) , and are familiar to most people in the latex applications field. Types I and I1 have a solids content of 26 t o 28% while Type I11 is made to 37 to 39% solids. Tvio further variations of these latices in commercial production are X-381 (9),a latex similar to Type I11 but containing polymer of low-er-25 to 40 instead of 70 t o 100Mooney viscosity (hIL-4), and X-523 (la),which is similar to Type I1 except that it is stabilized $7-ithammonia at the end of its manufacturing operation. Since some of the trade required higher solids latices and since i t was costly t o ship the extra Tater in low solids products, research continued t o seek higher solids formulas. Some other latices developed under 50% solids were Type I V (27), X-409 (ZO),and X-446 (21). Type IV i s similar to Type I11 except it is prepared a t 39.1 to 42% solids. GR-S X-446 latex, being similar and superior to X-409, quickly replaced i t in most applications. This latex is made at 47 t o 49% solids by using a low water recipe-95 parts water based on 100 parts of monomers charged. It is otherwise quite similar to Type ITi, the main exception being a slightly higher ratio of butadiene t o styrene in the charge recipe, and the addition of electrolyte to control viscosity. T h e starting polymerization temperature is 104” F. and i t is
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Latexraised to 140" F. at 40% solids. The use of this latex has recently reached rather large volume. Since some latex applications, such as foam sponge, require solids above 50%, products in this range were also developed ( 3 ) . Some of th'e production latices (16,16) of this type which will be briefly disoussed are X-276 (Type V) (18), X-359 (Type VI) (7), X-370 (Type VII) (8), and X-270 (Type VIII) (16). The X numbers given are the original designations of experimental latices, which later became standardized into the types given in paren theses. Type V latex is polymerized to 59% minimum solids with low emulsifier content t o give large particle size, approximately 3000 A. diameter, and relatively low viscosity, 250 cp. maximum at &yosolids. It was developed primarily for production of foamed sponge, but has also been used in other applications requiring high solids latex. The original formula for making a latex similar to Type VI polymerized very rapidly and required a condenser on the reactor t o aid cooling ( 3 ) . The formula later was redesigned for a slower reaction in order to obtain larger particle size and thus lower viscosity, and at the same time to allow elimination of the condensers or heat exchangers. Type VI latex is now made at a 55-45 butadiene-styrene ratio t o 60 to 63% solids using 60parts of charge water to 100 of monomers, with Dresinate 731 and Daxad 11 for emulsification, Additional Dresinate 731 is added in a booster mixture a t 45 t o 48% solids in order to carry the polymerization t o aa near 100% conversion as possible and t o increase stability of the latex. The polymerization is started at 140" F. and finished at 150"F. Since the conversion never actually proceeds much over 95%, the unreacted monomers are removed by steam distillation. No shortstop or antioxidant is added to the latex. Type VI1 latex formula is somewhat similar t o t h a t for Type VI, One of the major differences is the use of an 80-20 butadiene-styrene ratio in order to give with good flexibility at subfreezing temperatures, and another is the use of fatty acid soap instead of Dresinate soap in the booster and stabilizing' solution, which is added t o the polymerization system at 40 to 45% solids. Final solids are 60 to 6370. Type VI11 latex is quite similar to Type VI, e x c e p t t h a t it is made to 50 t o 55% solids. Several cold latices have been d e s c r i b e d previously (22,dS). Of these a fairly high solids cold latex X547 (19, 22, $3) and a low solids cold LLEOTROLYlf latex X-570 (90) have been commerFigure 1. Effect of Varying cially produced for Electrolyte on Latex Stability some time (see Tahle I). The Mooney viscosity values given in Table I and elsewhere were determined with a large rotor at 212' F. after 4 minutes in t h e machine (ML-4). The screens value is a measure of macroscopic coagulum present, expressed as the percentage of the latex sample retained on an 80-mesh screen. The free styrene determination was made by an ultraviolet absorption test. The bound styrene content was obtained by measuring the refractive index of the polymer after extraction with solvent to remove nonhydrocarbon material. Average particle diameter was determined by a 90"light scattering method based on relations derived by Debye ( 4 ) . Viscosity of the latex was measured by a standard method ( 1 4 ) . I n addition t o the research conducted t o obtain the above
.April 1952
Table I.
Iron-Activated L o w Temperature Latex Formulae A,
Charge formula Water! total Butadiene Styrene Neo-Fat K-242Potassium hydroxide Potassium RR soap Daxad 12-Kb Triton R-1000 Potassium sulfate Potassium chloride SulfaleeB-8d
MTM
DIBHf CHPP Ferrous sulfate Sodium sulfhydrate Potassium pyrophosphate Stabilizing so@, potassium oleate Shortstop, DMDTCC, as 5 % solution Conditions and remesentative
x-547 65
70
30 1.25 0.02 1.5 0.4 0.11 0.22
0.10 0.052 0.25 1.75 0.2
B, X-570 180 71
29 4.5 0.1 0.4
0.135 0.10 0.10
C, X-619 65 70 30 1.25 0.02 1.5 0.4 0.03 0.22
0.15
0.10 0.052 0.26
0.2
1.75 0.2
50 41 50 60 60 6.5 47 1%-160 100-120 70-100 60-63 24-25 47.5-49.5 9.7 9.2 9.7 Nil gzeens % 0.1 0.03 0.02 Free stirene, % 0,005 23.8 23.3 Bound styrene, % 24.8 30 Viscosity cp 15 Aipprox. 800 2000 Partiole &e,'A. 2000 a Potassium soap of stabilized fraction of distilled tall oil. b Potassium salt of condensed alkyl naphthalenesulfonic acid. Sodium salt of condensed alkyl naphthalenesulfonic acid. d tert-Dodecyl mercaptan. e Mixture of CIS, C I P ,and CIZtertiary mercaptans. I Diisopro ylbenzenehydroperoxide. 0 Cumene Kydroperoxide. Added as 20% solution at end of reaction.
72
e
Dimethylammoniurndimethyldithiocarbamate.
latices, u,ork has been conducted to produce a number of new specialty latices as well as new and improved cold latices. The present paper discusses developments by the writers in the fields of conventional low temperature latex recipes, volatile base latices at both high and low temperatures, and cationic latex recipes at both high and low temperatures. CONVENTIONAL LOW TEMPERATURE LATEX RECIPES
Cold latex production started in June 1949, in three reactors a t t h e government synthetic rubber plant at Naugatuck, Conn. These facilities permitted latex consumers to obtain large enough quantities of latex for commercial evaluation. Although some consumers found immediate application for available cold latices, t h e industry as a whole took considerable time t o study the merits of these products, and in many cases special modifications of existing latices had to be made to meet their requirements. However, by the time natural latex became high priced, cold latex development and evaluation had reached the stage where it could replace considerable natural rubber latex in blends, and, in some large applications, replace natural latex 1 0 0 ~ too produce high quality finished products. I n view of this, a large demand has developed for cold latices of the GR-S type, and greatly expanded facilities are being provided far their manufacture for such uses as foam sponge, tire cord treatment, certain dipping processes, and many other applications. The great demand for these latices shows t h a t products of considerable merit have been devised. As is well known, the manufacture of synthetic rubber latices offers something never possible until recently-namely, the tailoring of latices t o meet specific uses-and it is expected that this should result in the development of latices superior to the natural product in many new applications. The following discussion reviews the results of recent research t o make products suitable for various segments of the industry. Formulas and Properties of Latex Polymerized at 41' to 50" F. X-570 is a cold version of Type I1 latex in that i t is a standard
INDUSTRIAL AND ENGINEERING CHEMISTRY
763
-ELASTOMERS-LatexTable 11.
Polyamine-Activated Low Temperature Latex Formulas
Charge formula Water, total Butadiene Styrene Potassium oleate Potassium hydroxide Daxad 12-Ka Daxad l l b Potassium chloride Sodium sulfate Sulfole B-8C DIBHde DETA Stabilizing soap/, potassiuin oleate Shortstop, KDTCB, as 5 % solution
A
B, X-633
76 70 30 1.5 0.04 1,5
76 50 50 1.5 0.04 1.5
0.55
0.55
0.20 0.3 0.16 1.5 0.2
0.15 0.3 0.5 1.5 0.2
C, X-635 80
70 30 1.5 0.04 1.5 0.55 0.20 0.3 0.15 1.5 0.2
Conditions a n d representaiive properties 50 Polymerization temp., F. 50 50 80 Conversion, % 80 80 50 39 44 Av. reaction time, hr. 70-100 70-100 h o o n e y vis. of polyiiier 70-100 62.3 60.1 49.0 Solids, % 10.0 9.8 10.3 pH 0,022 Screens % 0.076 0.022 0.05 Free s t i r e n e % 0.02 0.07 6 6.2 Surface tension, dynes/cm. 60 50.6 0.970 0.975 0.987 Specific gravity 2 6 .0 28.0 4 3 . 5 Bound styrene, 70 124 13.6 49 6 Viscosity cp. 1700 1730 2020 Particle s'ize, A. a Potassium salt of condensed alkyl naphthalenesulfonic acid. b Sodium salt of condensed alkyl naphthalenesulfonio acid. C tert-Dodecyl mercaptan. d Diisopropylbenzenehydroperoxide. e Diethylenetriamine. f -4dded as 20% solution a t end of reaction. Q Potassium diinethyldithiocarhamate.
polymer supplied in latex form a t the low solids level normally encountered in dry polymer recipes. Unlike Type I1 latex, however, it is made t o a high Mooney viscosity (110 f 10 instead of 50) and is shortstopped with a nondiscoloring stopper, dimethylammoniumdimethyldithiocarbamate. A variety of this type of latex made to the same high Mooney viscosity on an X-565, ironfree, polyamine-activated recipe (10)has been developed a t the pilot plant as 5-4411 and has been undergoing tests for a long time. Continued development of high solids types of cold latex has led to a number of new products. Reports by several customers that latices with high Mooney viscosity polymers give iniprovements in quality gave impetus to development of unmodified or slightly modified recipes. B s a result, a high Riooney viscosity (150 to 170) X-547-type latex which is heat concentrated from about 48.5% to 60 to 63% solids is now being produced as X-619 ( 2 1 ) . Production of these latices containing high RIooney viscosity polymers has brought about unexpected problems in latex stability during manufacture. In an earlier paper (13)it was pointed out t h a t ferrous sulfide-activated X-547-type latices are rather difficult to produce, and that a simpler formula for manufacture was being developed, based on polyamine activation. The formula used a t that time is reproduced as recipe A in Table 11. More recently several modifications of this recipe have been developed which involve such items as butadiene-styrene ratios from 100-0 to 50-50, varying sodium-potassium ion ratios, Mooney viscosities
Table 111.
Effect of Polymerization Temperatures on Latex Film Tensiles
Polymerization temp., ' F. Monomer ratio, butadienestyrene % conversion Mooney viscosity Stress-strain propertiesa
X-446 104-140
J-3639-4 50
53-47 100 70-100
50-50 80 89
T b E O 1900 685 Average of two cures. b T tensile strength, pounds per square inoli. 0 E:elongation, %.
a
764
T 3015
E
720
from 70 to 170, and latices containing polymer modeiatelTcross-linked with divinylbenxene. In addition, modifications have been made for subfreezing polymerizations down to 0' F. One of these, recipe B, Table 11, is now produced as X-633. It is based on a 50-50 butadiene-styrene ratio and polymerized to 49.5y0 solids This latex has given vulcanized film tensile5 on the order of 1000 pounds per square inch higher than can be obtained with a similar hot latex. Table I11 s h o w a comparison of latex film tensile data from this latex and a similar hot latex. The original polyamine-activated recipe, 5-3478 (23),using mi all-potassium system-Le., potassium salts of the soap, electrolyte, and secondary stabilizing agent-was costly, had too small a particle size, and would not coagulate easily during certain manufacturing operations. To overcome these difficulties, the potassium salts were partially replaced by their sodium counterparts. Although for this system the reaction time to 80% conversion was increased from 40 to 50 hours when the potassium sulfate and Dayad nere replaced with sodium salts, the resulting latex was improved from the customers' viewpoint. I n testing various ratios of sodium to potassium ions, the mixture used in 5-3802 and later in 5-4248 was found to be optimum for many applicationb This consisted of a potassium soap and sodium salts of the electrolyte and secondary stabilizer. This recipe was more economical and gave latex with larger particle size which possessed excellent coagulation characteristics. The effect of various mixtures of sodium and Dotassium ions on average - .aarticlr size is listed i n Table IV.
Table IV.
Effect of Mixtures of Sodium and Potassium Ions on Latex Particle Size
Soap Potassium Potassium Potassium Sodium
Daxad Potassium Sodium Sodium Sodium
Electrolyte Potassium Potassium Sodium Sodium
Average Particle Size, A . 1700 1815 2020 2060
One of the best recipes to date, 5-4248 (authorized for production as X-635, Recipe C, Table 11),based on a 70-30 butadienestyrene monomer ratio, is polymerized a t 50" F. to 80% conversion, 70 to 100 Mooney viscosity, and an unvented solids content of 48.6%. Upon removal of the unreacted butadiene and styrene with the aid of a Wallier-Wallac~heat exchanger in order to minimize dilution of the latex, the solids are 53 to 5491,. This latex is then concentrated to 60 to 6291, solids. Several foam manufacturers have reported excellent 100% synthetic rubber latex sponge from this product. A similar latex, but of still higher Mooney viscosity (130 to 160 ML-4) has been made as 5-4473. I n addition, low styrene latices such as 5-4174 (90-10 butadiene-styrene) and 5-4437 (87-13 butadiene-styrene) have been produced and heat concentrated to 60% solids for applications where good low temperature properties are required. Although these recipes use soape of oleic acid a i t h low linoleic acid content (Emersol 233LL, Emery Industries) as the primary emulsifiers, systems using other emulsifiers are currently in the development stage. Two promising materials are the potassium soap of Rubber Reserve fatty acid and the potassium soap of a mixture of dimer and oleic acids (Emery 259-113-R fatty acid). The potash Rubber Reserve soap is not as active as potassium oleate in supporting polymerization and requires a larger amount in the initial charge in order to obtain the same polymerization rate; however, i t does result in very stable latex with less foaming than latices emulsified with potassium oleate during stripping and heat concentration. The potassium soap from the mixture of dimer and oleic acids is also outstanding in t h a t the particle size produced is considerably higher than with either the oleic or Rubber Reserve soaps. Values of average particle diameter as
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
high as 2400 A. have been found for latices prepared with this emulsifier. This results in a less viscous latex a t 60 to 6301, solids. The light-scattering method used for particle size determination was developed for particles smaller than those contained in many of these high solids latices, and results may be seriously in error for particles above 2000 A. in diameter. Electron microscope photographs have given values of average particle diameter considerably larger than shown by light-scattering measurements. For example, a sample of X-619, having an average diameter of 2200 A. as meamred by light scattering, had an actual weight average particle size of 5900 A. from electron microscope photographs. Only the initial soap used in the charge has thus far been mentioned. During or a t the end of the polymerization, an equivalent amount of soap is added as a stabilizer. The stabilizing soap can be the same as the charge soap, or i t can be changed so as t o conform to the customer's requirements. That soap added during polymerization is added in one or more increments, I n the development of l a t i c e s containing polym e r s of h i g h Mooney viscosity, i t was found that latex stability during polymerization and processing greatly decreased as the Mooney viscosity s--. .. .was increased by Figure 2. Effect of Varying reducing the Electrolyte on Particle Size amount of modifier in the s y s tem. Formulas which gave satisfactory stability a t norma] modifier levels developed massive amounts of coagulum when the modifier was reduced or elimina#,ed. The most satisfactory method to date of stabilizing unmodified latex has been found to be an increase in electrolyte concentration in the aqueous phase, Figure 1 shows the amounts of coagulum formed per 100 grams of monomers in a series of bottle polymerizations with varying amounts of electrolyte and different water contents in a formula similar to recipe C in Table 11, except for elimination of the modifier. It is evident that a t each water level there is a minimum amount of electrolyte which prevents prefloc formation. This same recipe with 70 parts of water and a normal amount of modifier gives a stable latex with 0.5 part of electrolyte, which is the amount required for a satisfactorily low viscosity latex. The greater stability a t higher 'electrolyte levels is presumably due to increased average particle size with a consequent reduction of surface area t o be covered by the emulsifier and secondary stabilizer present in the system, Figure 2 shows the nearly linear increase in particle size with electrolyte as determined by lightscattering measurements. Further improvement in stability of this material, as measured by latex filterability, is gained by increase in the emulsifier level, as shown in Table V. The filterability test, which measures microscopic coagulum, is essentially similar to the strainability test of Brass and Slovin (9), in that the amount of latex passing through a 0.5-inch disk of felt before plugging occurs in the quantity determined. The test used here differs in that filtration is carried out not under vacuum but under a constant pressure head of 12 inches of latex. In this range the increased emulsifier has no appreciable effect on latex particle size, indicating that in this high electrolyte recipe the amount of soap available for forming polymer particles is, to a large extent, limited by the electrolyte concentration rather than by the total amount of soap present. Higher soap levels (3 t o 4 parts) have given smaller particle size. At first i t was thought that the decrease in stability was due to smaller particle size in the unmodified recipes, but investigation
____
CISOIRNYTD
April 1952
Table V. E%kt of Increased Emulsifier on Latex Fiftmbility and Particle Size Potassium oleate 1.25 Filterability, at 50% solids 0.8 electrolyte 5 1.2 electrolyte 26 Particle size A. 0.8 electrdlyte .1740 1.2 electrolyte 1960
1.50
1.75
16 118
17 315
1700 2030
1640 1960
2.0
82 220 + 1880, av. 1740 1920, av. 1970
showed this not to be the case, since both modified and unmodified recipes gave the same average particle size, within experimental error a t corresponding electrolyte concentrations. Tread Vulcanizate Properties of Polymer from High Solids Latex Polymerized a t 50' F. I n order t o determine whether the polymer prepared in high solids (low water) latex recipes is equivalent t o standard low temperature dry polymers prepared in high water systems, a sample of X-547 latex was flocculated and finished in the usual manner, and then was compared in a tread-type vulcanizate with GR-S 10, X-526 (a standard 41 F. dry polymer), and 5-3168 ( a pilot plant preparation using the same activator system and emulsifier as X-547, but polymerized with 180 parts of water instead of 55 parts of water as used in X-547 manufacture). The polymers were compounded in a tread-type recipe with 40 parts of E P C black and cured a t 292' F. Physical test data, summarized in Table VI, show that X-547 is substantially equivalent to standard low temperature polymers and exhibits the expected superiority over GR-S 10 polymerized a t high temperatures.
Table VI* Tread Vulcanizate Properties of Polymer from Latex Polymerized at 50 F. O
Time, Minutes
X-547 5-3168 X-526 99 108 57 Mt4 93 106 68 TENBILE DATA,UNAQED 300% modulus 25 370 550 400 50 710 980 870 100 1070 1530 1460 Tensile, Ib./sq. inoh 25 2380 3060 2820 50 3760 4080 4100 100 3880 3730 3780 Elongation at break, % 25 950 800 940 50 750 690 710 100 610 510 540 TENBILE DATA,AGED 96 HOURS AT 2 1 2 O F. 390 3oo% modulus 25 330 470 550 50 450 580 100 470 620 550 25 2880 2390 2240 lbJsq. inch 50 2220 2280 1850 100 2190 2130 2190
R~~ M L - ~
Elongation at break, 70
25 50 100
Rebound, %, at room temp, At 212O F. *lex inch crack per growth kilocycle (O.Ool-
410 280 230 47 54
av. three cures Una ed Age3 Abrasion resistance rating Variable slip (1)
u. s.
0.2 1.4
122b 120
290 240 230 47 57
0.3 2.3 121b 124
310 210 230 46 56
0.3 1.8 126b 112
GR-S loa 53 62 430 750 1240 1650 2890 3350 790 700 570 450 610 500 1870 1970 2080 260 210 270 38 48
0 7 4 8 lOOb
100
Control polymeriaed at 1220 F. b Average of two determinations.
a
,
Formulas for Polymerization a t Subfreezing Temperatures. I n order to obtain the further advantages from still lower temperature polymerization which have previously been shown possible in the latex field ( 2 2 ) , research has been carried out on the development of high solids recipes for subfreezing polymerization.
INDUSTRIAL AND ENGINEERING CHEMISTRY
765
-ELASTOMERS-Latex-
Pigure 3.
Moyno Pump
So far, satisfactory formulas have been worked out for bottle scale polymerizations a t both 14" and 0 " F. Evaluation i l l larger reactom is currently under way. BIethanol is used as the internal antifreeze and reaction rates are maintained a t the lower temperatures by increased emulsifier levels and catalyst-activator charges. At 0 " F. the more active polyamine, tetraethylenepentamine, is used for activation instead of the diethylenetriaminca used in other recipes. Iron and sequestering agents are employrd when needed to adjust polymerization rates. Table VI1 gives generalized formulas for both 14' and 0" F. polymerizations, in recipes A and B, respectively.
half, while the concentrating t8imes to 48.57, solids for X-547 were cut by tn-o thirds of the usual time. I n fact, with the heat exchanger it was possible to vent, strip, and concentrate to 62% solids in less time than the same operation required t o reach 48.5% solids without the heat exrh anger. Since this installation in the plant, a type of pump other than the Dc Lava1 1.hl.O. type has been found satisfactory in pumping the hot latex out of a high vacuum at low static head without destabilizing the latex by mechanical means. This is the Moyno pump, manufactured by Robbins and Meyers. The Sanitary-type model is preferred because it can be dismantled, cleaned, and p u t back toget,her in a very short period of time. The pump is self-priming and has a positive displacement. It works on the progressing cavity principle, in vvhich a helically shaped rotor is revolved on an off-center axis through a stator which contains a double internal helical opening. A cut-away drawing of a standard model pump is shonm in Figure 3. VOLATILE BASE LATEX RECIPES
For some finished products, polymer which is free from soup and other water solubles is required, particularly where water absorption must be a t a minimum. Methods have been described for t,he isolation of such polj~niersfrom standard latex (IS). For some applications where it is desired to apply polymer directly from emulsion, latices are required which on evaporation will deposit polymer free from water-soluble contaminants. In order t o Table VII. Formulas for Polymerization a t Subfreezing Temperatures prepare such latex, it is necessary t h a t the soap used in polymerA B ization decompose during drying and curing of the deposited Charge formula 68 13 Water film, This is accomplished by the use of ammonium or volatile Methanol 12 25 Butadiene 70 70 amine soaps (25). I n addition, if electrolyte is used to control Styrene 30 30 latex viscosity during polymerization, it too should volatilize on Potassium oleate 3.5 3 5 Daxad 12-Ka 1.0 1 0 evaporation. Ammonium carbonate and acetate have been Potassium sulfate 0.4 Potassium phosphate 0 3 found to be satisfactory in this respect. Formulas for both high DIBH: 0.5 0 5 and low polymerization temperatures will be discussed. DETA 0.3 TEPAd 0.3 High Temperature Recipes. .4lthough a number of morc or Conditions less satisfactory volatile base recipes employing conventional Polymerization temp., F. 14 0 Conversion, 70 60 60 persulfate initiation at temperatures above 100' F. have been Av. reaction time, hr. 40 48 knolvn for some time, they have suffered from several disadvanPotassium salt of condensed alkyl naphthalenesulfonic acid. b Diisopropylbeneenehydroperoxide. tages-primarily, slowness of reaction rate and instabi1it.y of the C Diethylenetriamine. d Tetraethylenepentamino. latex to stripping. I n addition, the persulfate initiator or the sulfate resulting from its decomposition always leads to the presence of some electrolyte in the latex and deposited polymer. -4program was initiated, therefore, to develop new catalysts and Latex Finishing. The method of heat concentrating late^ emulsifiers for volatile base latices. Table IX shows two recipes with the aid of a Model H.E.R.E. Walker-Wallace heat exchanger which have given good results a t polymerization tcmperatures has been discussed in a previous article (23). Since that time the above 100" F. equipment has been installed in the plant and has proved to be Recipe A employed a free radical-type catalyst system and highly successful, not only in concentrating latices, but also in gave excellent rates of polynierization and latex properties coniwarming the latex in order to vent the unreacted butadiene and to pared to earlier persulfate recipes. It stmillsuffered from soap prepare for steam stripping of unreacted styrene. A summary of hydrolysis and loss of amine during stripping, which necessitated plant stripping and concentrating times is given in Table VIII. addition of diethylamine during the operation to prevent coaguThe time required to vent the butadiene has been reduced by lation. Although this recipe was promising, the catalyst, is expensive and is believed to leave a toxic residue Table VIII. Effect of Walker-Wallace Heat Exchanger on on decomposition. Further investigation led to) Concentration Times recipe B which is considerably more versatile and Butadiene Styrene ConcentratConcentratTotal Finishing Stripping Stripping ing to 48.5% ing to 62% Cycle, Time, economical than recipe A. Furthermore, recipe Time, Hr. Time, Hr. Solids, Hr. Solids, Hr. Hr. uses lower amounts of water in the init.ial charge, x-547 1.6 8 4.0 13.6 permitting higher final solids. These higher solids, X-619 1.6 7 11.1 19.7 x-547" 4.0 8 10.4 22.4 as well as the excellent electrical properties of t h e final film, permit the use of the latex directly witha X-547 without Walker-Wallace heat exchanger. out. the necessity of creaming. It has been carried r -
766
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44,. No. 4
-ELASTOMERS-LateTable IX. Hiah Temperature Volatile Base Latex Formulas A
Charge formula Water
Bu-ta-diene Styrene Diethylamine laurate Morpholine laurate Ammonium carbonate MTMa a,a'-Bisazoisobutyronitrile DIBHb DETAG Ethylenediaminetetraacetic acid Stabilizing soapd Diethylamine laurate hlorpholine oleate Shortstop, D M D T C e C:onditions and properties Polymerization temp., F. Conversion, 70 Reactor solids. 'Z Av. reaction time, hr. Mooney vis. of polymer Solids (final), %
.nes/cm.
120 50 50 4.0 0.14 0.2
2.0
150 88 41.6 40 60 51 10.4 0.02 0.10 32.8 0.98 46 44
$050 5
6 0
d e
B
90 50 50
3.0 0.45 0.23 0.10 0.10 0.008 1.0 0.2 115 91 47.6 40 83 57.6 8.7 0.001 0.08 44.4 0.99 46 29.5 2065
Mixture of CIS Cir, and CIPtertiary mercaptans. Diisopropylbe Aaenehydroperoxide. Diethylenetriamine. Added a t end of reaction. Dimethylammoniumdimethyldithiocarbamate.
through the 80-gallon reactor stage with no difficulties in reaction rate or latex stability, and appears very promising both as t o manufacture and finished product characteristics. This recipe is a high temperature adaptation of previously developed low temperature volatile base recipes t o be discussed. It employs a conventional hydroperoxide-polyethylene polyamine activator system of the type described by Whitby et al. ($4) for low temperature polymerization. Reaction a t the higher temperatures is controlled at almost any desired rate by varying the concentration of iron-chelating agent, ethylenediaminetetraacetic acid. It has been shown t h a t very small amounts of iron are necessary to catalyze the polymerization-initiating peroxide-polyamine reaction, indicating t h a t the chelating agent controls the speed of polymerization by regulating the concentration of iron available for catalysis (6). The amount of iron necessary is so minute that i t is always present in any but the most carefully purified reagents. Use of morpholine as the volatile base in this recipe gives the desired balance between stability to steam stripping and soap decomposition on final drying of the latex film. Unlike recipes employing diethylarhine or other more volatjle amines, no extra base needs to be added during stripping to keep the latex from flocculating. Although the most satisfactory of the above volatile base latices have been carried through only the pilot plant stage, no obstacles are ~ e e nt o their manufacture in the present synthetic rubber plants. It is believed t h a t the nature of these products will lead t o suggestions for their applications and t h a t latex consumers will find how to make excellent finished articles from them. Low Temperature Recipes. Because of the generally improved properties of low temperature polymers, it was of interest to develop volatile base latex recipes for low temperature polymerization. The polyamine-activated formula was found to be well suited to this type of recipe because of its siniplicity and resistance t o inhibition by amines or ammonia, even in excessive amounts. Table X shows three recipes which have been developed through the research stage for low temperature volatile base polymerization. Recipe A gives low solids latex which can be stripped of unreacted monomers if the process is carefully controlled, although prolonged stripping leads t o complete flocculation because of soap April 1952
hydrolysis, unless ammonia is continually added t o the system. Recipe B, with morpholine as the soap-forming base, is quite stable to stripping. Recipe C was developed for the production of high solids latex. Like A, it is not stable t o extended stripping or concentration but can be stabilized by addition of a moderately volatile base such as morpholine or diethylaminoethanol either after polymerization or as a replacement for part of the ammonia in the charge. An excess of base, preferably ammonia, has been found necessary for reasonable rates of polymerization in all of these low temperature recipes. Figure 4 shows the variation in per cent solids of recipe C after 48 hours of polymerization as a function of excess ammonia (parts per hundred of monomers). The function of the excess ammonia does not appear to be purely to increase the p H PWTS A M W U of the. system, for Figure 4. Effect of Excess Amwith some fatty monia on 41" F. Volatile Base acids, particularly Recipe lauric, much smaller amounts of excess base than shown in Figure 4 are needed. Possibly the excess ammonia serves to suppress salt formation between the soap-forming acid and polyamine activator. These low temperature volatile base recipes have progressed only through the research stage and are not yet ready for commercial production.
1
i
CATIONIC LATEX RECIPES
Latex prepared with cationic emulsifiers is of particular interest in coating and impregnating processes. Many organic fibers, e.g., wool and cellulose, are normally electronegative and, therefore, strongly attract the positively charged latex particles, giving faster and more complete exhaustion of latex on the fibers than do conventional latices emulsified with anionic agents. High Temperature Recipes. A high temperature cationic latex, 5-2298, has been produced in the pilot plant in a 500-gallon reactor on several occasions, and improved recipes for both high and low temperature polymerization have recently been carried
Table X. Low Temperature Volatile Base Formulas A
Charge formula Water 180 Butadiene 71 Styrene 29 Ammonium laurate 5.0 Morpholine soap of mixed f a t t y acids Ammonium soap of mixed fatty acids Ammonia 0.5 Ammonium acetate 0.7 Ammonium carbonate MTM" 0.15 DIBH; 0.15 DETA 0.15 TEPAd Ethylenediaminetetracetic acid Shortstop, D M D T C e 0.2 Conditions Polymerization temp., F, 41 Av. reaction time, hr. 12 Conversion, % 60 Mixture of Cl8 Cl4, and Clz tertiary mercaptans. b DiisopropylbeAzenehydroperoxide. 0 Diethylenetriamine. d Tetraethylenepentamine. e
C
B 180 71 29
70 50 50
5.0 0.5
2.5 0.5
0.5 0.15 0.15 0.15
1.2 0.15 0.20
0.2 41 15 60
0.15 0.002 0.2 41 45 60
Dimethylammoniumdimethyldithiocarbamate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
757
-ELASTORXERS-LatexTable XI.
.
High Temperature Cationic Latex Formulas
Charge formula Water total Butadiene Styrene Dodecylamine hydrochloride Cis amine hydrochloride Herjadeoyl trimethylammonium chloriae Aluminum chloride.6H20 Potassium chloride teit-Hexadecyl mercaptan tert-Butylhydroperoxide DIBHQ Ferrous sulfate Stabilizing soa b , dodecylamine hydrochloricfz Shortstop TMDHAC DTBHQd Conditions a n d representative properties Polymerization temp., O F. Conversion, % Av. reaction time, hr. hlooney vis. of polymer Solids, %
A 120 50 50 5.0
B
C
120 50
120
50
50
50
3.0
0 io
0.10
1.0
122 85 22
50 41
2.7 Eeens Si1 Free styrene, % 0.03 Surface tension, dynesjcm. 50.7 Specific gravity 0.99 Bound styrene 49.5 Viscosity c p 22.5 Particle i i z e , ' ~ . 950 a Diisopropylbenzenehydroperoxide. b Added as 10% solution at end of reaction. C Polymerieed trimethyldihydroquinoline. d Di-tert-butylhydroquinone.
122
8.5 36 35
42 1.7
768
do
11.4
,b
B
180 70 30 3.0
0.3 0.1
120 50 50
3.0 0.5 0.3
0.15
0.05 0.05 0.2
0.15
0.10 0.10 0.2
41
41
60
80
26
60 40
12 60
24
40
through the research stage. Table XI gives general information on three hot recipes currently under investigation. Recipe A with dodecylamine hydrochloride as the emulsifier is satisfactory as far as reaction rate and latex properties are concerned, but suffers from the high cost of dodecylamine. Recipe B, 5-4430, employs a much less expensive emulsifier, tallow amine (Armeen T, Armour and Co.), consisting largely of octadecyl- and octadecenylamines. The rate of polymerization of this formula is somewhat slower than recipe A, but is still fast enough for practical use. This recipe has been carried through the 5-gallon reactor stage. Other catalyst systems have resulted in faster rates of polymerization. Recipe C with a quaternary ammonium salt as emulsifier is expected to give advantages in stability over the amine-emulsified latices. The emulsifier is compatible with many anions which precipitate the high molecular weight amines and is also effective in both acid and alkaline media. It has not yet been carried beyond the research stage. Low Temperature Recipes. I n order t o apply 90 the advantages of low temperature polymerizaBO tion t o cationic systems a k number of cold recipes have been carried through the research stage. The key to rapid low temperature polymerization 40 in cationic recipes was 15 io I -5 found t o be careful conDH trol of the P H of the Figure 5. Effect of pH on aqueous phase. Figure 5 Conversion shows the variation of conversion with p H in a low solids dodecylamine-emulsified recipe a t 41 O F. (Recipe A, Table XII). Since the amines are rather strongly basic it is extremely difficult to control p H in the critical region-pH 5.0 t o 6.0-if strong acids are used for neutralization. Best results have been obtained by using acetic acid or a mixture of hydrochloric and acetic for neutralization. This recipe may be adjusted to give extremely rapid rates of polymerization. Conversions of L
A Charge formula Water Butadiene Styrene Dodecylamine acetate C1s amine acetate Aluminum chloride Potassium chloride JIThIa tert-Hexadecyl mercaptan DIBHb Ferrous chloride Shortstop, DTBHQC Conditions Polymerization temp.. O F Conversion, 70 Av. reaction time, hr. hlooney vis. of polymer Solids. ?'G
85 24
44.6
Y
XII. Low Temperature Cationic Latex Formulas
122
0.03
1::
Table
above 90% have been obtained in 18 hours a t temperatures as low as 0" F. in the presence of methanol antifreeze in bott,le polymerizations. Rosin Amine D (Hercules Powder Co.) and tallow amine have also been used successfully in this recipe a t 41 O F. These low solids recipes have been carried through the 5gallon reactor stage in the pilot plant. Recipe B (Table X I I ) was developed to give medium solids cold latex corresponding to the high temperature 5-4430 discussed previously. Like recipe A, it can be adapted to higher 01' lower polymerization temperatures by adjusting the catalystactivator level. This recipe has not yet progressed beyond Dhe research stage. The cationic latices described are the results of research and pilot plant scale development and have not yet reached plant scale manufacture. Although it is believed t h a t the best of these products can be manufactured commercially, certain changes in the plants will have t o be made before their production can be realized on a practical basis. One of the main requirement>s would be the isolation of reactors and a finishing line devoted entirely to acid-side latices in order to avoid difficulties t h a t would arise from cross-contamhation of acid-side and alkaline-side latices. Undoubtedly, t,his can be done as soon as the present rush for production has subsided. ACKNOWLEDGMENT
The authors wish to express their appreciation to Miss 1Iaiy Foy and t'o J. A. Reynolds and the personnel of the Research Polymerization Laboratory for their valuable assistance in thie project. Thanks are also due to T. D. Ramsey of the Lot.01 Development Department for supplying film tensile data. The cooperation of Robbins and Meyers, Inc., and mi. G. Cleavcs of Hayes Pump and Machinery Co. is also g r a t e f ~ ~ l lacknos-ly edged. The authors wish to thank t,he Office of Rubber Reserve for permission to publish this paper. LITERATURE CITED
(1) Adams, J. W., unpublished work, ( 2 ) Brass, P. D.! and Hlovin, D. G., Anal. Chem., 20, 172 (19481. (3) Chittenden, F. D., lIcCieary, C. D., and Smith, H. S.,Isn. EKG.CHICK,40, 337 (1948). (4) Debye, P., J . Applied P h y s . , 15, 338 (1944). (5) Hobson, R. W., C l o w n e y , 5. Y . ,Kelly, A. L., a n d D'Ianni, J. D., Goodyear Tire and Rubber Co., private communication. ( 6 ) Howland, L. H., Peaker. C. R., and Holmberg, A . W., I n d i a Rubber World, 109, 579 (1944). (7) I n d i a Rubber World, 115, 801 (1947). (8) Ibid., 116, 392 (1947). (9) Ibl'd., p. 513. (10) Ibid., 117, 226 (1948). (11) Ibid., 118, 245 (1948). (12) Ibid., 120, 476 (1949). (13) Madigan, J. C . , Howland, L. H.. Burns, E. R., and %awn, C. V., I N D . ENG.CHEX, 40, 2384 (1948).
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 44, No. 4
-ELASTOMERS-LatexMooney, M. S.,and Ewart, R. H., Phgsics, 5 , 350 (1934). Reconstruction Finance Corp., Office of Rubber Reserve, “Classified List of Currently Produced GR-S Polymers,” 1951. (16) Ibid., “General Sales and Distiibution Circular for Government Synthetic Rubber,” February 1950. (17) Ibid., “GR-S Latex Type I11 and GR-S Latex Type I V , General Purpose Synthetic Latices,” March 1948. (18) Ibid., “GR-S Latex Type V,” June 1947.
Smith, H. S.,Werner, H. G., Madigan, J. C., and Howland, L. H., IND.ENG.CHEM.,41, 1584 (1949). (23) Smith, H. S., Werner, H. G., Westerhoff, C. B., and Howland, L. H., Ibid., 43, 212 (1951). (24) Whitby, G. S., Wellman, N., Flouty, V. W., and Pteven, H. S.,
(19) Rubber Age, 65, 681 (1949). (20) Ibid., 67, 337 (1950). (21) Ibid., 68, 726 (1951).
RECEIVED for review July 17, 1951. ACCEPTED October 29, 1951. Presented before the XIIth International Congress of Pure and Applied Chemistry, New Ybrk, N. Y .
(14) (15)
(22)
Ibid., 42, 4 4 5 (1950). (25)
White, L. M. (to United States Rubber Co.), U. S. Patent 2,393,133 (Jan. 15, 1946).
Film from Mixtures of Natural PHYSICAL PROPERTIES R. M. PIERSON, R. J. COLEMAN, T. H. ROGERS, JR., D. W. PEABODY, AND J. D. D’IA”1 The Goodyear Tire and Rubber Co., Akron, Ohio
T
HE principal tonnage Because the use of cold rubber latices may be extended comparative lack of uniu s e s of G R - S - t y p e into the field of latex applications as 100% synthetic, formity in the colloidal and latices, aside from tire cord rather than as an extender for natural rubber latex, a other nonpolymer properties adhesives, are in products in study was undertaken to evaluate the effect of several of the latices will, i t is realwhich natural rubber latex is polymerization variables on the properties of blends conized, fail t o bring out the taining 0 to 100% synthetic rubber. I t was found that maximum potential properthe principd or an important component, T h e a m o u n t blends containing 70% or more natural rubber latex had ties of many of the polymers. and type of synthetic latex little effect of the stress-strain properties of the mixture: Such compromises were necused in blends of commercial that cold rubber latices gave higher stress-strain values essary in a preliminary proof natural rubber blends than did hot rubber latices: gram of this nature in order importance have been inand that low conversion synthetic polymers produced to keep the variables within fluenced by economic factors higher stress-strain properties than the high conversion a practical number. It was and by the processing and physical properties of the polymers in the blends. The physical properties of a felt, moreover, t h a t t h e product, but more often the natural rubber stock are superior to those of any of the evaluation should roughlyapsynthetics have been looked synthetic rubber latices so far tested. proximate the conditions of upon chiefly as “extenders” use, and that methods t h a t €or the natural latex. were completely unattainable However, with the introduction of the so-called cold rubber in commercial practice should not be employed. This elimilatices, there has been much evidence that the use of synthetic nated from consideration certain techniques which had been rubber latices may now be extended into these applications as observed to give improvements in sheet properties, such as cast100% synthetic, and the products will have useful physical ing the sheet in special atmospheres, drying in vacuum desiccaproperties somewhat comparable to those made using naturaltors, curing at low temperatures, and the like. synthetic rubber blends. An important property which has a considerable influence on No comprehensive evaluation of the effect of the several polythe technical usefulness of a latex, but which is not easily conmerization variables on the vulcanized film properties of natural trolled or susceptible of precise measurement, is its ability to form rubber-GR-S blends has been reported, although studies on some well-knit, strong sheets during drying or when treated with gelling aspects of this problem have been described (3, 11). A preagents-Le., prior t o cure. This property is commonly known liminary study of the variables of butadiene-styrene ratio, polyas wet gel strength, and reflects the effects of a large number of merization temperature] percentage conversion, and Mooney variables of the polymer, latex colloidal properties, pretreatment, viscosity, on the properties of blends varying from 0 to 100% synviscosity, drying environment, and so on. Natural rubber latex thetic content, has been undertaken. I n this initial phase of the is outstanding in exhibiting high wet gel strength under a n exstudy attention has been confined to those variables affecting tremely wide variety of conditions of use, whereas GR-S synonly the polymer properties, and the so-called latex propertiesthetics have been very erratic in this respect. Several techamount and type of surface active agents, catalysts, particle surniques have been developed which help to overcome this defiface unsaturation, pH, etc.-have been varied, of necessity, over ciency of the GR-S synthetic latices in the manufacture of fairly wide limits in order to make possible the preparation of frothed sponge ( 2 , 8, I S ) . The attainment of high vulcanizate latices of the desired range of polymer characteristics. Further, strength in cast latex sheets is usually tied in with the attainment the study was confined t o a more or less uniform evaluation proof high wet gel strength under the conditions of use. Thus, high cedure, rather than an attempt to develop casting, drying, and vulcanizate strengths on cast latex films of cold GR-S rubbers curing conditions which are optimum for each latex, The have been obtained in some cases (3,IO), but they have often evaluation technique used adheres closely to procedures which been found difficult to reproduce from one laboratory t o the next. have been widely practiced in the latex industry for years (4). Also, widely variable results have been obtained by the same inThe restriction of the scope of the evaluation proce‘dure and the vestigators on supposedly similar latices, as has been the experiApril 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
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