Studies of the Vulcanization of High Elastic Polymers. I. The Vulcanization of Natural Rubber with Thiuram Disulfides. Part 1

Abstract

Abstract The present paper, which is concerned with the clarification of the mechanism of thiuram vulcanization, although the studies are still in progress, nevertheless leads us to the following fundamental facts. 1. In the vulcanization of natural rubber by thiuram disulfides in the presence of excess zinc oxide, the zinc dialkyldithiocarbamate at any stage is the principal, and probably even the sole, reaction product that can be extracted with organic solvents. 2. There are no experimental proofs that make probable the presence of thiuram monosulfide in the extracts. 3. The maximum attainable conversion in vulcanization of any particular tetralkylthiuram disulfide to the corresponding zinc dialkyldithiocarbamate amounts to about 66 per cent, and this is independent of the temperature. 4. The maximum conversion of the thiuram disulfide is practically independent of its concentration in the rubber, provided that the differences in the concentrations involved are not excessive. 5. The extracts of thiuram vulcanizates contain free thiuram disulfide in addition to the zinc dithiocarbamate. The concentration of the thiuram disulfide decreases rapidly with the time of vulcanization and approaches zero asymptotically. This is exemplified by the results with tetrapropylthiuram disulfide. 6. It is shown to be probable that the vulcanization continues to proceed slowly during extraction of the vulcanizate with boiling solvents, so that the analytical results of the hot extracts are not such as to throw light on the kinetics of thiuram vulcanization. 7. The decrease of concentration of the free thiuram disulfide in the vulcanizates evidently takes place more rapidly than the formation of the zinc dithiocarbamate. It is, therefore, probable that the vulcanization of rubber by thiuram disulfides can be separated into two partial processes. There is a rapid reaction of the rubber with the thiuram disulfide, which is presumably “vulcanization-inert”, and there is an additional, slower reaction which involves a splitting off of dithiocarbamic acid from a reaction product between rubber and thiuram disulfide and which leads to the vulcanization. 8. All exhaustively extracted vulcanizates contain sulfur. Apparently the sulfur content in vulcanizates of short reaction times is greater than in those of longer periods of vulcanization. 9. The formation of the zinc dithiocarbamate as sole reaction product in the thiuram vulcanization can hardly be explained otherwise than as a dehydrogenation of the rubber, concerning the mechanism of which we still know, to be sure, scarcely anything. It is reasonable to conceive of a reaction of the disulfides with the α-methyl groups, in which chain radicals may be formed which give rise to cross-linking over C—C bonds. The investigations are being continued and a further communication will appear in the near future.