April, 1934
I NDUSTR IAL X SD
EN GIN EE R I N G CH E M ISTR Y
culiar properties of unmilled rubber may be attributed to a “crude rubber structure.” This structure appears to be essentially different from the vulcanized structure. The data indicate that stiffeners help to retain the crude rubber structure or possibly to build up a similar one. No evidence n-as found to show that they act as vulcanizing agents. Reclaims obviously differ from both crude rubber and from unreclaimed vulcanized rubber. The softeners and pigments present undoubtedly have considerable effect on the results obtained in some of these tests. The experiments seem to indicate that, during reclaiming, the vulcanized structure is partly broken down and the flow characteristics are increased by softeners added or formed during the process. When uncured gas black stocks are compared with vulcanized rubber in a similar manner, it is found that the black stocks resemble unvulcanized rubber in their behavior on the mill, and in the hot water and the hysteresis set tests. These are all “flow characteristics.” The solubility and modulus are similar to those of well-vulcanized rubber while the tensile strength is similar to that of slightly vulcanized rubber. Here is a combination of properties which is different from that in either vulcanized or crude rubber. It seems,
437
therefore, that there exists here a third type of structure, the “pigment structure.” Investigators who have studied the reenforcing action of gas black generally attribute the effect to interfacial phenomena between the individual pigment particles and the rubber matrix (9). There are, therefore, a t least three distinct types of structure: (1) a crude rubber structure, possibly having its origin in the latex particles; (2) a pigment structure established by interfacial phenomena between the hydrocarbon and the pigment particles; and (3) a vulcanized structure built up within the hydrocarbon by chemical reaction. Each of these three structures causes distinctive combinations of solubility, flow characteristics, and tensile properties. LITERATURE CITED (1) Garvey, B. S., and White, W. D., IND.ENQ.CHEM.,25, 1042
(1933). (2) Kindscher, E. (in K. Memmler’s Handbuch der Kautschukwissenschaft), p. 379, S. Hirzel, Leipsig, 1930; Stevens, W. H., J. SOC.Chem. Ind., 48,60T (1929). (3) Shepard, N. A. (in Alexander’s “Colloid Chemistry”). Vol. IV, p. 309, et seq., Chemiral Catalog, 1932. RECEIYED Septeniber 28, 1933.
...*.
IV. Vulcanizing Agents Other than Sulfur The vulcanizing action of sulfur chloride, mdinitrobenzene, selenium, tetramethylthiuram disulfide, and benzoyl peroxide has been inz~estigated by the use of a set of tests developed ,for measuring the degree of vulcanization with sulfur-cured compounds. These tests show that all of these malerials are true vulcanizing agents. Comparison of the various vulcan izates with different types of unculcanized rubber
P
REVIOUS papers in this series have covered the development of a set of tests for measuring vulcanization (IO) and the use of these tests for studying the function of sulfur during vulcanization (Q), and for comparing vulcanized rubber with unmilled rubber and uncured gas black stocks (8). This report covers the use of the same set of tests for a comparison of vulcanization by sulfur with that by other materials generally accepted as vulcanizing agents. The recipes for the compounds used are as follows: SULFURCHLORIDE Compound 1 First latex crepe was Compound 2. First latex cre e was calendered in the lahocalendered an$ cured ratory to 0.038 em (vapor process) in the (0.015 inch) and cured factory. the sheet was in the factory by the 0.023 ek. (0009 inch) vapor process followed thick. by treatment with ammonia. COMPOEXD 4. SELENIUM [Cures: 15, 30, 60, 120, 240, aud 480 min. a t 149’ C. (300’ F.)1 First latex crepe 100 Vandex (selenium) 28 @-Naphthylamine 4 P a r a 5 n wax 4 - Litharge 25 123 Zinc oxide 50 COMPOUND 5, TETRAMETHYLTHIURAM DISULFIDE
-
COMPOUND 6, BENZOYL PEIIOXIDE
104
109
The dinitrobenzene compound is based on a formula used by Fisher and Gray (7) ; that for selenium is based on a recom-
shows that the outstanding characteristic of the vulcanized structure is its resistance io flow under u wide variety of conditions. The vulcanized structure appears to be a composite one which varies with the rate of a reaction catalyzed by the culcanizing agenf, fhe rate of combination of the vulcanizing agent, the nature of the addition product, and possibly with differences in the nature of the catalyzed reaction. inendation by Boggs (5). The mixing, curing, and testing procedure was the same as that described in detail in the earlier paper (IO). I n Table I are given the testing data for each compound on the uncured stock and on the cured sheet haring the highest tensile strength in the range selected.
DISCESSION OF RESULTS Comparison of these data with the corresponding data on sulfur compounds, reported in the earlier paper, shows that all of these compounds have undergone the changes characteristic of vulcanization with sulfur. Therefore, according to the criteria used here, the materials tested are true vulcanizing agents. A comparison of the properties of the different types of vulcanized rubber with each other and with tough crude rubber and uncured gas black stocks shows that all of the vulcanizates have a marked resistance to flow in all of the tests. On the other hand, both the gas black stocks and crude rubber show a comparatively small resistance to flow in one or more of the following tests: hot water, milling, retentivity, thermoplasticity, hysteresis set. With regard to tensile and modulus, both of these types give values as high or higher than do some types of vulcanizate. Uncured gas black stocks, like vulcanized rubber, show low solubility in benzene. In ice water, some vulcanized, high accelerator-low sulfur compounds freeze as badly as crude rubber. It is thus apparent that the outstanding characteristic of the vulcanized structure is its resistance to flow under a wide variety of conditions, especially a t high temperatures. Vulcanization, then, is a reaction which primarily snp-
438
INDUSTRIAL ANI) E N G I N E E R I N G CHEMISTRY p r e s s e s t h e flow characteristics of uncured rubber. This change always seems t o be accompanied b y a marked decrease in solubility and by an increase, to a greater or less extent, in the tensile strength and modulus.
Vol. 26, No. 4
genation of the rubber (1'7). Tetramethylthiuram disulfide probably decomposes and liberates a small amount of very active free sulfur (16). I n all of these cases the amount of addition product is small, either because of low tendency of formation or because of the small amount of reagent available. With sulfur alone there is reason to believe that amounts of combined sulfur up to one per cent are not sufficient to cause more than a very slight vulcanization (9). This is probably also the case with equivalent amounts of addition product of these other reagents. I t seems probable] therefore] that in all of these cases a catalyzed reaction of the hydrocarbon plays an important part in the vulcanization. I n the case of selenium (4),nitro compounds (?'), and benzoyl peroxide ( 7 ) , as with sulfur, there is evidence that this catalyzed reaction is not THEORETICAL one involving a change in unaaturation. DISCUSSION Thus there appears to be in all of these cases the probaI n t h e s e c o m - bility of a t least two reactions: (1) a combination of vulpounds the several canizing agent with rubber and (2) a reaction of the hydroproperties change a t carbon catalyzed by the vulcanizing agent. The product of d if f e r e n t relative the combination would, of course, be different vith different rates. R h e r e reagents. Possibly, also, there are differences in the nature similar differences of the catalyzed reaction, though in most cases it seems to were observed in the be one which does not involve a change in the unsaturation case of accelerated of the hydrocarbon. The catalytic activity of the wlcansulfur compounds, izing agents undoubtedly depends on the initial composition they were attributed and also on the stability, and thus varies considerably among to differences in the the different reagents. The vulcanized structure seems to be a composite one built vulcanized structure depending on the up by the catalyzed reaction and modified more or less by relative rates of two the addition reaction. structure - f o r m i n g reactions, sulfur adLITERATURE CITED dition, and a reac(1) Bernstein, G., Kolloid-Z., 11, 185 (1912). tion Catalyzed by ENQ.CHEM.,22, 740 (1930). (2) Blake, J. T., IND. sulfur (9). The ex(3) Blake, J. T., Ibid., 22, 737, 744 (1930). tension of this ex(4) Boggs, C. R., Ibid., 10, 117 (1918). p l a n a t i o n t o the (5) Boggs, C. R., U. S. Patent 1,249,272 (1917). (6) Bunschotten, E., Chem. Weekblad, 15, 527 (1918) ; Indaa Rubber nonsulfur vulcanizW o r l d , 60,559 (1919). i n g a g e n t s is i n (7) Fisher, H. L., and Gray, A. E., IND.E m . CHEX,20, 294 (1928). agreement with the ( 8 ) Garvey, B. S., Jr., Ibid., 26, 434 (1934). chemical eyidence (9) Garvey, B. S., Jr., and Thomson, G., Ihid., 25, 1292 (1933). (10) Garvey, B. S., Jr., and White, W. D., I b i d . , 25, 1042 (1933). available. Henrichsen, F. W., and Kindscher, E., Kolloid-Z., 6, 202 (1910). With sulfur chlo- (11) (12) Henriques, R., Chem-Ztg., 17, 634 (1893); 18, 701, 1155 ride there is an ad(1894). d i t i o n r e a c t i o n (13) Kirchhof, F., Kolloid-Z., 14, 35 (1914). which probably re- (14) hleyer, K. H., and Mark, H., Ber., 61, 1939 (1928). I., J. Russ. Phgs. Chem., 47, 1453, 1462, 1467, s u l t s i n b r i d g e (15) Ostromislensky, 1885, 1898, 1904 (1915) ; Handbuch der Kautschukwissenformation between Echaft (K. Memmler), p. 373, S. Hirsel, Leipsig, 1930. double bonds of the (16) Romani, E., Giorn. chim. i n d . applicata, 3, 197 (1921); Rubber Aoe . ) . 10. 13 (1921). " iY. . A.Yvan, hydrocarbon ( I , 11Dekker, P.,'and Prawirodipoero, R . S., Kaut1 4 , l Q ) . With nitro (17) Rossem, schuk, 7, 202, 220 (1931). compounds the re- (18) Stevens, H. P., J. SOC.Chem. I n d . , 36, 107 (1916). agent is destroyed (19) Weber, C. O., I b i d . , 13, 11 (1894). d u r i n g c u r e , a n d RECEIVED November 23, 1933. there appears to be a small amount of some sort of addition BEKZESEEXPORTS ASD PRODUCTION HIGHER.Exports of product formed (8, benzene from the United States in 1933 totaled 8,439,456 gallons, 6,15,18). Selenium valued at $1,594,075, compared with 3,241,317 gallons, valued at a p p a r e n t l y com- $611,656, in 1932, according t o the Department of Commerce. bines to a limited Benzene, an important industrial chemical product obtained extent with rubber in the distillation of coal tar, is used extensively as a motor fuel constituent. Other consuming outlets are explosives, dyes, (5,4),Benzoyl per- plastics, and a number of other finished coal-tar products. o x i d e f o r m s some Some shifts have occurred in the markets for American benzene. addition or substitu- Shipments to the Central and South American area increased t i o n p r o d u c t and 20 per cent compared with 1932, Argentina taking 71,000 gallons of a total of 130,000 gallons shipped t o that area. Consignments probably also causes to the Far East practically quadrupled in 1933 compared with some bridge forma- the preceding year, the largest Far Eastern purchaser being tion by d e h y d r o - Japan.
