Vulcanization. Part VII. Kinetics of Sulfur Vulcanization of Natural Rubber in Presence of Delayed-Action Accelerators

The scheme and treatment of the kinetics of scorch-delay vulcanization proposed in a previous paper were applied to natural rubber sulfur vulcanization accelerated by 2,2′-thiobisbenzothiazole (MBTS), 2-(N-cyclohexyl)benzothizaolesulfenamide (CBS), 2-(N-morpholinothio)benzothiazole (MOR), or 2-(N,N-diisopropyl)benzothiazolesulfenamide (DPBS). According to the scheme (see PDF for diagram) where the subscript x = a positive integer. The specific rates k 1 and k 2 and the ratio k 4 /k 3 ′ were related to starting concentrations of accelerator, sulfur, and stearic acid. In general, an increase in stearic acid concentration or a decrease in accelerator concentration induces an increase in k 1 or k 4 /k 3 ′ but a decrease in k 2 . An exception to this is that, when the highly hindered sulfenamide DPBS is used as the accelerator, k 1 does not change with changes in starting concentrations. It is interesting that k 2 does not respond appreciably to changes in sulfur concentration when sulfenamides are used, unless the sulfur concentrations are extremely low. In contrast to this, k 2 increases linearly with the square root of the sulfur concentration when MBTS is used. The changes in rates which occur with changes in the starting concentrations were attributed to the formation of chelates between zinc ion (brought into solution by stearic acid) and accelerator, intermediate reaction products, or crosslink precursors. The differences noted between the rates obtained with the three sulfenamides were attributed to differences in steric hindrance and stability.