Cure and Mechanical Behavior of Rubber Compounds Containing Ground Vulcanizates. Part I—Cure Behavior

SBR compounds containing ground vulcanizates of known composition and cure state were prepared and the cure behavior studied. Decreases in scorch times and maximum rheometer torques were observed when ground vulcanizates were added to the SBR compounds. Two primary phenomena are proposed to explain these findings: (1) migration of sulfur from the matrix rubber to the ground vulcanizate (causing torque reduction) and (2) migration of accelerator fragments from the ground vulcanizate to the matrix (causing decreased scorch time). The first proposal is based on direct measurement of sulfur concentrations in both ground particles and the matrix. The second is based on the detection, by high performance liquid chromatography, of mercaptobenzothiazole in the extract from ground vulcanizates. The second also is inferred from the fact that ground vulcanizate particles cured with peroxide do not alter scorch time. Moreover, a compound containing sulfur and ground (accelerated-sulfur vulcanized) rubber, but no added accelerator, nonetheless exhibits acceleration of cure.