Physical, mechanical, and viscoelastic properties of natural rubber vulcanizates cured with new binary accelerator system

Natural rubber was vulcanized with a new binary accelerator system based on 1-phenyl-2,4-dithiobiuret (DTB) and dicyclohexyl benzothiazyl sulfenamide. A significant reduction in cure time was observed with the addition of DTB. The cure kinetics was investigated and the activation energy was determined. The mechanical properties were improved by adding DTB, and the maximum values are shown at an optimum concentration. Estimates of the concentration of crosslinks and of the relative proportions of different types of crosslinks were made by the chemical characterization of the vulcanizates. The crosslink densities obtained from swelling measurements and stress-strain measurements were compared with those obtained from modulus measurements. All of them follow a similar trend and support the observed mechanical properties. A dynamic mechanical analysis of the mixes was carried out and the activation energy was determined from Arrhenius plots. The glass-transition temperature was found to increase with the increase in crosslink density and frequency of measurements. The stress-strain curves were found to not deviate from the strain crystallizing nature of natural rubber. The mechanical properties, network characterization, and processing characteristics were used to optimize the DTB concentration. (C) 2003 Wiley Periodicals Inc.