Effect of sulfur to accelerator ratio on crosslink structure, reversion, and strength in natural rubber

Abstract

Anew relationship among the sulfur to accelerator (S/A) ratio, the degree of reversion or the net loss of crosslinks at the prolonged cure time, and the tensile strength and crosslink structure of gum natural rubber (NR) vulcanizates is described here. To study this, N,N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS), N-cyclohexyl-2-benzothiazole sulfenamide (CBS), 2,20-dithiobisbenzothiazole (MBTS), and tetrabenzylthiuram disulfide (TBzTD) were used as accelerators. The results showed that for all four tested accelerators, the degree of reversion and tensile strength of the vulcanizates did not simply increase with increasing S/A ratios within the range of 0.26-6.67 by weight. This was because the proportion of polysulfidic linkages playing an important role on these properties was not simply proportional to the S/A ratios but turned out to pass through a maximum and then decline with further increasing S/A ratios for the DCBS, CBS, and MBTS cure systems. Nevertheless, when considering the effect of crosslink structure on the thermal and mechanical properties, it was observed that for all four tested accelerators, the increase in the extent of polysulfidic linkages gave the vulcanizate with the lower reversion resistance but the higher tensile strength. Therefore, the generalization that it is the high concentration of polysulfidic linkages in the network that causes a decrease in the reversion resistance but an increase in the tensile strength is seemingly still applicable.