Investigation of Radical and Polar Mechanisms in Vulcanization Reactions

Abstract

Abstract The results obtained with free radical traps in a variety of curing systems for SBR rubber give evidence for both polar and free radical mechanisms. There is apparently no one mechanism even for sulfur vulcanization and the relative free radical or polar character of the crosslinking reaction varies over a broad range including situations in which both types occur in the same formulation. This may be due to competing reactions which function independently, or it may be the result of a combination of polar and free radical reactions in a single complex mechanism. For example, certain accelerators may initiate the reaction by dissociation to free radicals which attack the S8 molecule and form polysulfides which then decompose by a polar mechanism to initiate the propagation stage of the reaction. Based on the information obtained in this study and other supporting evidences, it is possible to group the familiar curing systems according to the apparent polar or free radical character of the reaction as observed in SBR. It is interesting to note that the accelerated sulfur curing systems which most closely approximate a polar mechanism either involve an amine (DPG) or a compound which is either present as a zinc salt (dithiocarbamate) or can easily form zinc salts at the curing temperature (thiuram disulfide and mercaptobenzothiazole systems). All curing systems studied which include free sulfur appear to involve a polar mechanism.