Chemical interaction and mechanism of activation of binary systems of vulcanization accelerators

Abstract

The efficiency and mechanism of the combined action of vulcanization accelerators are considered from the standpoint of their chemical structure. An investigation of the vulcanization of various types of butadiene‐styrene and natural rubbers showed that double accelerator systems may be divided into three groups; namely: systems with mutual activation of the accelerators, systems with activation of one of the accelerators, and systems with additive action of the accelerators. The vulcanization kinetics of the first two (nonadditive) groups of accelerators differ from those for the separate application of the accelerators by a retardation in the initial period and a sharp increase in rate of the main period of the vulcanization process. During interaction of accelerators of which one contains the benzothiazolyl group and the other is a hydrogen donor the formation of 2‐mercaptobenzothiazole is observed. Formation of this compound may also be noted on interaction of the accelerators in the rubber medium, the yield in this case being considerably higher than on separate interaction of the accelerators with the rubber. In the presence of sulfur (disulfide‐sulfenamide or disulfide‐organic nitrogenous base systems) a considerable rise in the yield of 2‐mercaptobenzothiazole is observed, accompanied by the formation of hydrogen sulfide and its subsequent interaction with the disulfide. The activity of double accelerator systems is the higher, the more intensive the formation of 2‐mercaptothiazole. The mutual activation of the accelerators is explained by the formation of intermediate complexes, decomposing to free radicals that initiate the interaction between sulfur and rubber and polymerization processes of the latter. A scheme of such reactions is presented.