Sulfur Vulcanization with Two Plateaus—Formation of an Inhibitor during Vulcanization?

Abstract Efficient sulfur vulcanizing systems based on thiazolesulfenamide accelerators derived from secondary amines show unusual and complex vulcanization kinetics in NR. This feature does not appear to have been reported previously. Depending on the nature and purity of the accelerator, the ratio of accelerator to sulfur, the cure temperature, the type of rubber being used, and the presence of other additives, two distinct periods of crosslinking may be evident, separated by a more or less well-defined plateau. This plateau is longer and more distinct if the accelerator is pure, its ratio to sulfur is high, the cure temperature is low, and there are no other additives present. Sulfenamide accelerators derived from both primary and secondary amines may give rise to this behavior in rubbers other than NR. The observed behavior has been shown to be due to the formation, during the first cure period, of an extractable crosslinking inhibitor. The structure of this inhibitor is still in doubt, but the present balance of evidence favors it being an aminodithiobenzothiazole [Structure (1)] or a dithiobisdialkylamine [Structure (2)] or a polythio variant of those structures.