THE INDIA RUBBER WORLD 



but decomposes on standing. The metallic salts of dithiocarbamic 

 acids are much more stable according to Krulla.= 



The aniline salt, by loss of hydrogen sulphide, produces 

 thiocarbanilide. 



The stable dithocarbamates above mentioned lose hydrogen 

 sulphide in a similar manner when heated to the temperatures 

 used in the vulcanization of rubber and produce thiourea deriva- 

 tives. It is, therefore, quite possible that they may function as 

 curing agents in the same manner as thiocarbanilide. 



Andre Dubosc' has stated that thiourea derivatives can "furnish 

 in a colloidal state all the sulphur necessary for vulcanization." 

 In checking up this statement it appeared at first that Dubosc 

 was correct, but the cures obtained were soon shown to be due 

 to free sulphur present as an impurity in the accelerator used. 

 Pure thiocarbanilide and pure dithiocarbamates do not vulcanize 

 rubber in the absence of free sulphur either in a pure gum or 

 high zinc oxide stock. We, therefore, are unable to agree with 

 this investigator that the sulphur of such compounds is avail- 

 able for vulcanization. 



There are some dithiocarbamates which liberates the free base 

 by a heat decomposition, such, for example, as the liberation of 

 dimethylamine by heating its carbon disulphide reaction product 

 Since these free bases are evidently good curing agents, it is pos- 

 sible that they may function as accelerators after being liberated 

 by a heat decomposition of the dithiocarbamates. It is. however, 

 not the object of this paper to discuss the mechanism of the ac- 

 tion of thiourea derivatives as accelerators. What we wish to 

 show is that there are many accelerators not ordinarily, classed 

 with thiocarbanilide but which undoubtedly produce thiourea 

 derivalives by reason w^ith sulphur during the vulcanization pro- 

 cess, so that they may be classed as being similar to thiocar- 

 banilide in their ultimate action. 



In June, 1913, J. Bastide was granted a patent* wherein he 

 claimed methylene and ethylene compounds of aliphatic and aro- 

 matic amines as vulcanization accelerators. As specific examples 

 he mentions methylene-diphenyl-diamine and phenyl-imino- 

 mcthane. the latter being otherwise known as anhydro-formalde- 

 hyde-aniline or methylene-aniline. These two accelerators easily 

 react with sulphur to form thiourea derivatives. 



Methylene-aniline easily polymerizes to the di-, tri-, and prob- 

 ably higher polymers. We have found it convenient to consider 

 it as the dipolymer and to take 210 as the molecular weight. It 

 has been found that 210 grams of methylene-aniline will react 

 with four atomic weights of sulphur, whereby one molecular 

 weight of carbon bisulphide and one molecular weight of hydro- 

 gen sulphide are lost and that about 95 per cent of the product 

 is thiocarbanilide. 



This reaction starts at about 130 degrees C. and proceeds best 

 at ISO degrees C. The amount of carbon bisulphide liberated may 

 be determined by condensing as much as possible and weighing. 

 Any uncondensed carbon bisulphide vapors may be caught in 

 aniline wash bottles which have previously been saturated with 

 hydrogen sulphide. The amount of hydrogen sulphide may be 

 determined by absorption in caustic. A small amount of aniline 

 and other products are formed by side reactions. 



It is quite probable from the above data that methylene-aniline, 

 when compounded as such, will generate carbon bisulphide during 

 the cure. In the presence of basic amido compounds this carbon 

 bisulphide should at once generate dithiocarbamates, similar to 

 those which have been show-n by Ostromislensky to have such 

 high curing power. The curing power of methylene-aniline may, 

 therefore, be due in part to the formation of dithiocarbamates 

 formed from carbon bisulphide liberated slowly in the cure and 

 a subsequent reaction with amido compounds which may be 

 present, bul its chief curing power is evidently due to the direct 

 formation of thiocarbanilide. It must not be assumed, however, 



■"Cheynische BericUten." volume 46. page 2669 

 •"The India Rubber World," February 1. 1919. 

 •French Patent No. 470.883. 



that methylene-aniline should therefore have as strong a curing 

 power as thiocarbanilide. That this is not the case is undoubtedly 

 due to the lag of the sulphur reaction during the cure. 



Methylene-diphenyl-diamine produces several reaction products 

 when heated with sulphur, the reaction proceeding easily at 140 

 degrees— 150 degrees C. 



(a) A certain amount of the thiocarbanilide is formed, but the 

 yield is comparatively low. 



(b) Methylene-diphenyl-diamine by heat alone loses aniline, 

 probably by a semidine reaction with itself. 



This reaction may be continued until the condensation has pro- 

 ceeded so far that one mole of the original compound has lost 

 one mole of aniline. By reaction with sulphur before compound- 

 ing and removal of such free aniline as may be formed, there is 

 produced an accelerator of much greater curing power than the 

 original material and which shows curing properties very similar 

 to those of thiocarbanilide. One of the constituents of this re- 

 action is apparently a sulphur reaction product. Another very 

 closely resembles thiocarbanilide and is probably one of the re- 

 action products which is formed from methylene-diphenyl-diamine 

 when this compound is used as an accelerator for the vulcaniza- 

 tion of rubber. 



(c) Methylene-diphenyl-diamine in the presence of aniline, 

 either added as such or formed by reaction (6), undergoes a semi- 

 dine transformation with the aniline at temperatures even lower 

 than milling temperatures. Paramido-benzyl-aniline is a liquid 

 which is crystallizable with difficulty and forms so easily from 

 the other reaction product that there is no difference in the curing 

 power of the two compounds. Paramido-benzyl-aniline reacts 

 easily with sulphur to produce para-amido-thiobcnzanilide. 



The latter is another compound very similar to thiocarbanilide, 

 to which may be attributed a portion of the curing power of the 

 original accelerator. 



The main reaction of this type of methylene accelerators is evi- 

 dently to substitute thiocarbonyl groups for methylene groups. 

 This produces compounds very similar to thiocarbanilide and 

 which may be considered as being derived from thiocarbanilide 

 by similar condensation and semidine reactions, although we 

 have been unable to prepare them directly from thiocarbanilide. 



In the interaction of hexamethylenc-tetramine with sulphur dur- 

 ing the cure, we have another possibility of the formation of car- 

 bon bisulphide reaction products with amines. Hexamethylene-tet- 

 ramine reacts very readily with sulphur at curing temperatures, 

 producing a multitude of products including hydrogen sulphide, 

 ammonia and carbon bisulphide in large amounts. Dubosc^ has de- 

 scribed the sulphur reaction products of hexamethylenc-tetramine, 

 but for some unaccountable reason has absolutely overlooked two 

 of the main reaction products, ammonia and carbon bisulphide. 

 The accelerating action of hexamethylenc-tetramine may therefore 

 be explained by the interaction of this ammonia and carbon bisul- 

 phid to form a dithiocarbamate. This allows us to classify hex- 

 amethylenc-tetramine as a thiourea accelerator. 



With a large majority of accelerators there is no possibility of 

 the formation of thiourea derivatives by a reaction with sulphur. 

 .\s far as is known, all accelerators containing methylene groups, 

 similar to those described, react easily with sulphur at curing 

 temperatures to produce thiourea derivatives. This does not 

 include, however, the methylene groups of such compounds as 

 piperdine, or penta-methylenc-diamine which on heating loses 

 ammonia and forms piperidine. 



SUMMARY. 



1.— Organic accelerators containing methylene groups, similar 

 to those described, readily react with sulphur to produce thiourea 

 derivatives. 



2.— These sulphur reactions take place at curing temperatures 

 and may throw some light on the mechanism of the reactions of 

 these accelerators during vulcanization. 



■'Locus 



