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INDUSTRIAL AND ENGINEERING CHEMISTRY
(5) Berlin, H., Ibid., 1,988,475(Jan. 22, 1935). (6) Bormans, E.,Ibid., 1,885,563(Nov. 1, 1932). (7) Brother, G. H.,and McKinney, L. L., IND.ENO.CHEM.,30, 1236-40 (1938). (8) Ibid., 31, 84-7 (1939). (9) Brother, G. H., and McKinney, L. L., Modern Plastics, 16,‘41-3 (1938). (10) Burk, R. E.,Thompson, H. E., Weith, A. J., and Williams, I., “Polymerization”, A. C. S. Monograph 75,p. 215,New York, Reinhold Pub. Corp., 1937. (11) Chase, H.,Brit. Plastics, 7,516 (April, 1936). (12) Collet, R.,French Patent 734,335 (Oct. 19, 1932). (13) Ellis, Carleton, “Chemistry of Synthetic Resins”, p. 415. New York, Reinhold Pub. Corp., 1935. (14) Frood, H.. British Patent 176,405(1920). (15) Fuhrmann, L. J., U. S. Patent 2,006.736(July 2, 1935). (16) Goldsmith, B. B., British Patent 14,098 (June 18, 1907). (17) Zbid., 412 (Jan. 7, 1910). (18) Goldsmith, B. B., U. S. Patent 840,931(Jan. 8,1907). (19) Ibid., 924,057(June 8,1909). (20) Ibid., 964,964(July 19, 1910). (21) Zbid., 965,137(July 19. 1910). (22) Ibid., 1,027,122(May 21, 1912). (23) Ibid., 1,076,417(Oct. 21, 1913). (24) Grodzinski, P.,K u m t s t o f e , 26, 141-4 (1936). (25) Hansen, D.W.,U. S. Patent 2,047,961(July 21, 1936). (26) Herald Akt. Ges., German Patent 530,134 (Dec. 29, 1927). (27) Hotta, K., and Nakajimi, K., Japanese Patent 42,075 (March 24, 1922).
VOL. 32, NO. 7
(28) I. G. Farbenindustrie Akt.-Ges., British Patent 282,635 (Dec. 23, 1926). (29) I. G. Farbenindustrie Akt.-Ges., French Patent 700,411 (Aug . 11, 1930). (30) Jaroslaw’s Erste Glimmerwaren-Fabrik, British Patent 272,947 (June 17, 1926). (31) Jaroslaw’s Erste Glimmerwaren-Fabrik, French Patent 635,745 (June 10,1927). (32) Kuhl, H., German Patent 280,648 (Aug. 21, 1913). (33) Laurin, L., and Bidot, E., French Patent 751.798 (SeDt. 9.. , 1933). (34) Laurin, L.,end Bidot, E., U.S. Patent 1,969,932(Aug. 14,1934). (35) Lougee, E.F., Modern Plastics, 13,13-15 (April, 1936). (36) Pabst, F., French Patent 652,615(April 12,1928). (37) Peakes, G. L.,Modern Plastics, 14, 39-41 (1937). (38) Peakes, G. L.,Plastic Products, 10,53-7 (Feb., 1934). (39) Zbid., 10,93-8 (March, 1934). (40) Ibid., 10, 133-6 (April, 1934). (41) Plauson, H., U. S. Patent 1,395,729(Nov. 1, 1921). (42) Rossi, L.M., and Peakes, G. L., Zbid., 2,066,016(Deo. 29,1936). (43) Satow, S.. Ibid., 1,245,980(Nov. 8 , 1917). (44) Sturken, O.,Ibid., 2,053,850(Sept. 8, 1936). (45) Taylor, R. L.,Chern. & Met. Eng., 43, 172 (1936). (46) Tripet, R.,French Patent 808,497(Feb. 8, 1937). (47) Weygang, C.,British Patent 111, 171 (Nov. 22, 1916). (48) Wiechmann, F. G.. U. S. Patent 1,061,346(May 13. 1913). (49) Ibid., 1,080,188(Dec. 2, 1913). (50) Zbid., 1,135,340(April 13,1915). (51) Ibid., 1,218,146(March 6 , 1917).
Vulcanization of Rubber Compounds Effect of Hydrogen E. W. BOOTH AND D. J. BEAVER Monsanto Chemical Company, Nitro, W. Va.
M
Sulfide on the Rate of Vulcanization
ANY articles in the literature give theories of the vul-
canization of rubber; some have claimed that the formation of hydrogen sulfide during the cure is essential, others have denied the formation of any hydrogen sulfide during soft rubber vulcanization. I n 1923 Bedford and Gray (8) presented a summary of previous theories and stated that hydrogen sulfide is liberated during vulcanization, which r e acts with zinc oxide present to form zinc sulfide. They further state that the accelerating action of zinc dithiocarbamates are retarded by hydrogen sulfide. I n 1936 Fisher and Schubert (4) stated that their data “appear to show that in the formation of hard rubber, when the amount of sulfur mixed with the rubber is approximately the theoretical (32.02 per cent), i t adds to the rubber hydrocarbon until saturation is complete; very little, if any, substitution takes place”. If, then, saturation with sulfur must be complete before substitution can take place, little or no hydrogen sulfide can be formed during soft rubber vulcanization. In 1939 Fisher (3) proposed a new theory of vulcanization, the basis of which is that hydrogen sulfide adds to the double bond of rubber to form a mercaptan. Should this theory represent the actual mechanism of the vulcanization process, then the addition of hydrogen sulfide to a compounded stock should increase the rate of vulcanization and possibly produce an improved vulcanizate. Therefore, the tests which form the basis for this paper were carried out to determine the effect of hydrogen sulfide on the rate of vulcanization of accelerated rubber compounds.
The rubber compounds used in these tests were milled, vulcanized, and tested according to the specified procedure given by the A. S. T. M. ( I ) . The uncured compounds were divided into two portions, and from each portion sheets were prepared (4.5 X 16.5 X 0.25 cm.). Half of the sheets were placed on shelves of 1-cm. wire screen in a closed drum, and hydrogen sulfide was passed through. I n most cases the time of treatment was 15 hours at room temperature (25” C.), which was sufficient to saturate the rubber compounds. Tests were also carried out in which the time of treatment was varied from 2 to 72 hours and the temperature from 25” to 100” C. To obtain reproducible results, the treated sheets must be vulcanized immediately after treatment because of the rapid loss of hydrogen sulfide on standing. Table I shows the results obtained with rubber compounds containing practically every commercial type accelerator, many of which are used in combination with diphenylguanidine. All are retarded in rate of vulcanization by the hydrogen sulfide treatment. However, the physical properties of a majority of the compounds are not permanently affected; that is, a longer time of vulcanization of the treated compounds has produced maximum modulus and tensile figurer:, comparable with the untreated compounds. Aldehyde-amines, dithiocarbamates, thiuram sulfides, and litharge are permanently affected; -that is, the maximum modulus and tensile
JULY, 1940
IKDUSTRIAL AND ENGINEERING CHEMISTRY
1007
figures obtainable are markedly poorer than those of the untreated compounds. (Formula: rubber 100.0, zinc oxide 10.0, sulfur 3.0, stearic acid 0.5, accelerator as shown: HzS-treated 15 hours The ratio of mercaptobena t 25' C . ) zothiazole to diphenylguani--Untreated-HIS-TreatedVulcanivulcanidine in the last three AcceleratorMax. Max. zation % on Max. tensile Max. zation compounds in Table I is that modulus tensile time modulus time rubber Name required to produce the di-Ky./eq. c m . 7 Min. cKg./eq.c m . 7 Min. . . phenylguanidine salt of mer227 191 180 0.75 95 60 Mercaptobenzothiazole 106 78 219 240 235 1.00 118 90 Benzothiazyl disulfide captobenzothiazole. Accord228 92 217 240 1.00 98 90 Zinc mercaptobenzothiazole 214 95 218 240 1.00 118 90 Dibenzothiazyldimethylthiolurea ing to Bedford and Gray (2) 221 88 218 240 109 1.00 90 Benzothiazyl thiobeneoate hydrogen sulfide reacts with 337 102 0.50 90 85.5 257 180 Cyclohexylaminothiobenzothiazole 237 0.75 90 98.5 86 207 180 Diphenylguanidine benzothiazyl disulfide (and Reaction product of butyraldehyde 0.50 282 309 90 109 251 240 and butylideneaniline presumably with other mer256 305 45 0.25 60 192 120 Tetramethyl thiuram disulfide captobenzothiazole derivatives piperidine salt of cyclopenta30 264 335 100 148 120 0.25 met yf ene dithiocarbamio acid) such as benzothiazyl thioReaction product of carbon disulfide 30 23 1 321 0.25 100 197 120 and methylene dipiperidine benzoate) to give free mer221 10.0 299 6 0 75 156 180 Litharge (6.5% sulfur) c a p t o b e n z o t h i a z o l e which Benenthiazvl disulfide 137 350 45 121 283 180 A35 Diphenyl ;anidme would then form a salt with ~Iercapto~cnzothiazole 141 336 45 119 306 180 Diphenylguanidine diphenylguanidine. The reZinc mercaptobenzothiazole 117 333 45 104 262 180 sults obtained with the treated Diphenylguanidine Cyclohexylaminothiobenzothiazole 135 153 342 45 portions of these compounds 321 90 ::E Di henylguanidine D i g enzothiaeyldimethylthiolurea show no evidence of any such 111 333 45 105 302 180 Diphenylguanidine reaction. I n addition to the 0.44 Mercaptobenzothiazole 151 144 352 6 0 315 180 Diphenylguanidine tests shown in the table, Bensothiazyl disulfide 154 60 119 352 350 180 0.56 Diphenylguanidine one sheet of these treated Benzothiazyl thiobenzoate 126 306 180 compounds containing benE] 131 338 6 0 Diphenylguanidine zothiazyl disulfide, diphenylguanidine, or benzothiazyl thiobenzoate was placed in a vacuum oven for 2 hours a t 25" C.; curing produced the same modulus and tensile figures as the untreated portions. Rubber dissolves one per cent of hydrogen This is further evidence that these accelerators did not react with the hydrogen sulfide. sulfide at room temperature. This amount Table I1 shows the effect of varying the time and temperaof gas is sufficient to retard the rate of vulture of hydrogen sulfide treatment on the physical properties canization of all types of accelerators. of the stock, together with variations in the weight of gas However, the physical properties of the absorbed. The cure test results show that the retardation of hydrogen-sulfide-treated compounds conthe rate of vulcanization is least after Phour treatment a t 25" C., is slightly more after 4 hours a t 50" to 60' C., and is taining aldehyde-amines, dithiocarbamthe maximum after 15 hours a t 25" C. Likewise, the perates, thiuram sulfides, and litharge centage increase in weight of gas absorbed shows that more were poorer, no matter how long the time hydrogen sulfide was absorbed after 4 hours at 50" to 60" C. of vulcanization was increased; but simithan after 4 hours at 25" C.; but 15 hours a t 25" C. showed larly treated compounds containing the the maximum hydrogen sulfide absorption, which was apmercaptobenzothiazole type and diphenylproximately one per cent. I n other words, the greater the absorption of hydrogen sulfide, the greater the retardation. guanidine accelerators produced physical Tests with rubber showed a one per cent increase in weight properties equal t o the untreated comdue to 15-hour hydrogen sulfide treatment a t 25" C. Vulpounds on prolonging the time of vulcanicanization in an atmosphere of hydrogen sulfide (1.75 kg. zation. There was little or no evidence of per sq. cm.) indicated that all accelerators were retarded the formation of zinc sulfide or of the permanently. The results in Table I11 prove that no appreciable amount reaction of sulfur with hydrogen sulfide. of zinc sulfide is formed as a result of the hydrogen sulfide treatment, since the increase in weight is approximately the TABLEI. EFFECTOF HYDROGEN SCLFIDE O N THE RATE OF VULCANIZATION COMPOUNDS CONTAINING ACCELERATORS
::A ::A !;:A
OF
RUBBER
1 1 1 1 1
~~
TABLE11. EFFECT OF TIMEAND TEMPERATURE OF HYDROGEN SULFIDETREATMENT (Formula: rubber 100.0, zinc oxide 10.0,sulfur 3.0. stearic acid 0.5, accelerator as shown; temperature of vulcanization, 134' C.) HnS-Treated -Untreated--4 Hr. at 25' C . -15 Hr. a t 25' C.-4 Hr. a t 5O-6O0,C.--Accelerator LMax. Max. Vulcani- Max. Max. VulcaniInMax. Max. VulcaniInMax. Max. Vulcanl- Inyo on modten- zation mod- tenzation crease mod- ten- zation orease modteneation crease Name rubber ulus sile time ulus sile time in wt." ulus sile time in wt.4 ulus sile time in wt.0 - K g . / ~ q . cm.M i n . -Kg./aq. c m . 7 M i n . % -Kg./sq. c m . 7 Min. % -Kg./sq. cm.- M i n . % ' .Mercaptobenzothiasole 0.75 95 226 60 93 212 120 0.315 78 191 180 0.97 88 212 150 0.72 Beneothiazyl thiobensoate 1.00 109 220 90 106 218 120 0.318 88 218 240 1.01 97 219 180 0.78 Dibenzothiasyl dimethylthiol urea 1.00 119 214 90 107 218 120 0.318 95 218 240 0.98 98 216 180 0.76
.
5
Based on rubber content after HzS treatment.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1008
VOL. 32, NO. 7
This retardation is not specific to any one accelerator. The v u l c a n i z e d s h e e t s of t h e (Temperature of vulcanization, 134' C.) UntreatedHnS-Treated treated compounds in each ---Accelerator--VulcaniVulcanicase showed porosity and Materials yo on RIax. A1a.x. zation Max. Max. aation Increase Compound compared rubber modulus tensile time modulus tensile time in wt. within a few hours after vul- K g . / s q . em.-. Man. - K o . / s q . cm.Min. % canization showed a sulfur Zinc oxide 4.0 A" 295 120 66 101 120 1.65 bloom. 1.51 179 Sulfur The data in Table V show B" Zinc oxide 11,5) 2.0 198 290 120 2s so 120 1.78 Sulfur that the rate of combination of sulfur with rubber is reCb Zinc oxide 239 285 120 53 123 120 1.70 Sulfur tarded by the presence of Db.c Zinc oxide 10.0 95 227 60 7s 191 180 1.00 E b Zinc sulfide 10.0 21 100 120 Not tested hydrogen sulfide in the compound. They also show that Formula: rubber 100, carbon black 50, stearic acid 3, pine t a r 2, benzothiazyl thiobeneoate 1.05, antioxidant 1.0, diphenylguanidine phthalate 0.70. zinc oxide and sulfur a s shown: HZS-treated 72 hours at 25' C. aldehydeamine accelerators b Formula: rubber 100.0, sulfur 3.0, stearic acid 0.5, mercaptobenzothiazole 0.75, zinc oxide and zinc sulfide as shown. and litharge react chemically C HIS-treated 15 hours a t 25' C. with hydrogen sulfide, since in compounds containing these two materials the a b s o r p TABLE IV. EFFECTOF HYDROGEN SULFIDEON THE RATEOF VULCANIZATION OF CARBON BLACKCOMPOUNDS tion of the gas is greater and (Formula: rubber 100.0, carbon black 50.0, zinc oxide 5.0, sulfur 3.0, stearic acid 3.0, pine t a r 2.0, accelerator as part of it is retained perH2S-treated 15 hours a t 25' C . ; temperature of vulcanization, 134' C.) shown; manently. Compounds conUntreatedHIS-Treated Accelerator VulcaniVulcanitaining carbon black also yo on Max. Max. zation Max. Max. aation Increase show a permanent absorption Name rubber modulus tensile time modulus tensile time in wt. - K g . / s q . cm.Min. c-Kg./sq. c m . 7 Min. % of part of the hydrogen sulMercaptobenzothiazole 1.0 222 318 90 56 1OG 240 1.65 fide. All compounds which Diphenylguanidine +,2.4-diindicate a permanent absorpnitrophenylbenzothlazole 1.25 190 314 90 67 11.5 240 1.78 tion of hydrogen sulfide (as shown by degassing in a vacuum) also indicate a retardation in the rate of cure and a permanent effect on the physical same for a stock containing 10 per cent zinc oxide as i t is for properties of the rubber compounds. pure rubber. Comparison of the B and C compounds shows that there is no reaction between hydrogen sulfide and sulfur; doubling the sulfur content has not appreciably affected Conclusions the retarding action due to hydrogen sulfide because the 1. Rubber dissolves approximately one per cent hydrogen maximum modulus and tensile figures are obtained in 120 sulfide when saturated a t room temperature. minutes in each case. Comparing the physical properties of 2. All types of commercial accelerators are retarded in the D compound after hydrogen sulfide treatment with the rate of vulcanization as a result of the hydrogen sulfide treatphysical properties of the untreated E compound shows that ment, and the retardation is directly proportional to the hythe former still contains sufficient zinc oxide to activate the drogen sulfide content. accelerator. This result indicates that there is little reaction 3. The physical properties of mercaptobenzothiazole types between hydrogen sulfide and zinc oxide. of accelerators and diphenylguanidine are not permanently The results in Table IV together with the three tread comaffected by hydrogen sulfide, but dithiocarbamates, thiuram pounds (A, B, C) in Table I11 show that hydrogen sulfide sulfides, aldehyde-amines, and litharge are permanently treatment of these compounds has permanently retarded the affected. vulcanization owing to the larger amount of absorbed gas, 4. Rubber compounds containing mercaptobenzothiazole which is approximately 70 per cent more than for rubber. types of accelerators or diphenylguanidine, which have been treated with hydrogen sulfide and then degassed in a vacuum oven, show normal rate of vulcanization. Compounds conTABLEV. EFFECTOF HYDROGEN SULFIDETREATMENT ON taining aldehyde-amines, litharge, or carbon black show perCHEMICAL AND PHYSICAL PROPERTIES OF RUBBERCOMPOUNDS manent retarding even after degassing. (HnS-treated 15 hours at 25' C.) 5. Increased zinc oxide or sulfur have no appreciable J K L M N Formula effect on the retardation. 100 100 100 100 100 Rubber ... 10 10 10 5 Zinz oxide 6. Little or no zinc sulfide is formed due to the hydrogen ... . . . . . . . . . . 50 Carbon black sulfide treatment. ... 3 3 6.5 3 Sulfur Pine t a r ... . . . . . . . . . . 2 7 . Hydrogen sulfide treatment of rubber compounds reStearic acid . . . 0.5 0.5 ... 3 . . . 0.75 . . . . . . 0.75 Mercaptobenzothiazole tards the rate of combination of sulfur with rubber. Litharge ... . . . . . . . 10 .... 8. I n no case has the hydrogen sulfide treatment improved Butyraldehyde-butylideneaniline ... .... 0.5 . . . . . . . the physical properties of the vulcanizate. % free S at max. modulus OF ZINC OXIDE, ZINCSULFIDE, AND SULFUR TABLE111. EFFECTOF DIFFERENTAMOUNTS 7 -
7 -
': E]
0
7 -
T
0.560
.. .. ..
0.60 1.10
1.4 0.4
1.9 0.9
1.72 0.76
... ...
0.425
increase in wt. (based on rubber) ' i t e r treatment After degassing 2 hr. at 5 mm.
0.96 0.05
1.023 0.05
Hours a t room temp. t o reach constant wt.
8
8
33
8
50
Max. modulus, kg./sq. cm. Untreated Treated and degassed
., ,
152 147
269 176
225 133
202
Untreated Treated
0.59
0.395
Literature Cited
, , ,
82
(1) Am. SOC. Testing M a t e r i a l s , Rept. of Comm. D-11, Nov., 1939. (2) Bedford and Gray, IND. EKG.CHEM.,15, 720 (1923). (3) F i s h e r , Ibid., 31, 1386 (1939). (4) Fisher and Schubert, Ibid., 28,211 (1936). PRESENTED before the Division of Rubber Chemistry a t t h e 99th Meeting of the American Chemical Society, Cincinnati, Ohio.
