REVERSION BEHAVIOR, MECHANICAL AND THERMAL AGING PROPERTIES OF GUM NATURAL RUBBER VULCANIZATES WITH DIFFERENT SULFUR TO ACCELERATOR RATIOS AND ACCELERATOR TYPES

Effects of sulfur to accelerator (S/A) ratio on reversion behavior, mechanical and thermal aging properties of gum natural rubber vulcanizates were studied. Various types of accelerator including N, N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS), N-dicyclohexyl-2-benzothiazole sulfenamide (CBS), 2.2'-Dithiobisbenzothiazole (MBTS) and Tetrabenzylthiuram disulfide (TBzTD) were used. For each accelerator, a series of five compounds was formulated to have about the same crosslink density using different S/A ratios. It was found that for all accelerators but TBzTD, the percentage of reversion after reaching maximum torque 20 min initially increased with increasing S/A ratio and then decreased with further increasing S/A ratio. So there was a certain S/A ratio for each accelerator which gave the highest reversion. For compound cured with TBzTD, reversion initially increased with increasing S/A ratio but eventually it leveled off with further increasing S/A ratio. Plots of tensile strength as a function of S/A ratios for all accelerators except DCBS looked like a convex curve. This means the highest tensile strength was obtained at intermittent S/A ratio. For vulcanizate cured with DCBS, tensile strength tended to decrease with increasing S/A ratio. The concentration of polysulfidic, disulfidic and monosulfidic linkage of each vulcanizate was investigated. The results showed that for all accelerators the extent of polysulfidic linkage was not directly proportional to increasing S/A ratio but it passed through maximum with increasing S/A ratio. Interestingly, it was observed that for all accelerators, the S/A ratio giving the highest extent of polysulfidic linkage was similar to that causing compound the highest reversion. Although vulcanizates cured with the highest S/A ratio did not give the highest reversion, relative tensile strength of these vulcanizates after thermal aging was the lowest for all accelerators.