The Vulcanization of Elastomers. 17. The Role of the Oxide in Vulcanization with Thiuram Compounds. II

Abstract

Abstract The reaction of tetraethylthiuram disulfide with zinc oxide, in which significant amounts of zinc dithiocarbamate are formed, was investigated quantitatively. The decrease in thiuram disulfide and increase in dithiocarbamate were followed by conductometric titration with HCl (dithiocarbamate) and with CuSO4 in the presence of hydroquinone (thiuram disulfide) as well as by a spectrophotometric method. Both methods gave as the limiting value for the dithiocarbamate formation, 90 mole %, calculated on the initial thiuram disulfide. This value is independent of the reaction temperature. Judging from this evidence on dithiocarbamate formation, it seems clear that some condition holds here other than that which holds in the case of a thiuram vulcanization. In the latter case, the limiting value for dithiocarbamate formed, also independent of temperature, is 66 mole % based on the initial thiuram disulfide. In incompletely reacted mixtures of ZnO and thiuram disulfide, the spectrophotometric method indicates higher dithiocarbamate content than does the conductometric method of analysis. This shows that spectrophotometry shows at least one (unknown) compound which conductometry does not detect. The spectrophotometric analyses of thiuram vulcanizates are in agreement with the results of conductometric titrations of the vulcanizate extracts; both methods indicate 66 mole % as being the limiting value for dithiocarbamate formation; likewise, the same kinetic relations are found by the two methods. The use of the spectrophotometric method was carefully checked with control tests.