Sulfur and accelerators play very important roles in curing natural rubber, influencing the properties of rubber vulcanizates. Such properties are also associated with the microstructure of vulcanized natural rubber. In this study, the relationships between tensile properties, strain-induced crystallization behavior, and structural inhomogeneity were investigated with special attention to the ratio of sulfur to accelerator (S/Acc). Increasing the S/Acc ratio increased crosslink density, simply from having more of the crosslinking agent. The highest tensile strength was obtained at unit ratio (=1), which was associated with SIC behavior based on wide angle x-ray diffraction measurements. Reductions in both the tensile strength and crystallinity were noticed when the crosslink density was relatively high. Structural inhomogeneity of network structures induced by crosslinking was investigated by means of small angle x-ray scattering, which showed the increased size and improved homogeneity of distribution with increased S/Acc ratio.

Keywords: natural rubber, tensile properties, microstructure, strain-induced crystallization