Hexamethylene diamine-modified epoxidized natural rubber and its effect on cure characteristics and properties of natural rubber blends

Compatibility is a serious problem in silica-added natural rubber (NR) because of the difference in the polarities of silica and NR. Thus, a coupling agent, such as silane, or a compatibilizer must be incorporated. In this work, amine-functionalized NR was synthesized by an in situ epoxidation process using performic acid and later modified with hexamethylene diamine (HMDA) by a ring-opening reaction. The effects of amine-modified epoxidized NR (ENR-HMDA) content on the cure characteristics, physical and mechanical properties of various NR blends (NR/ENR-HMDA) without filler were studied. ENR-HMDA enhanced the vulcanization process. The maximum torque of compounds increased with the corporation of ENR-HMDA and scorch time and cure time decreased. The cure rate index of NR/ENR-HMDA increased by 98%. All vulcanized rubber blends showed the strain-induced crystallization at high extension. Mechanical properties and crosslink density of the blends were higher than those of NR and NR incorporated with unmodified ENR. Tensile strength was 61% and 43% higher than that for NR and NR/ENR, respectively. The NR/ENR-HMDA blends showed a single T(g)that indicated the miscibility of both components. The developed ENR-HMDA compound can be used to improve the compatibility of NR and fillers which would enable the fabrication of a wide range of rubber products.