Properties of natural rubber/recycled ethylene-propylene-diene rubber blends prepared using various vulcanizing systems

The role of various vulcanizing systems on the curing characteristics, mechanical properties, morphology and dynamic mechanical analysis of natural rubber and recycled ethylene-propylene-diene rubber blends was investigated. Accelerated sulfur-vulcanizing systems (semi-EV and EV), peroxide, and mixed sulfur/peroxide-vulcanizing systems (semi-EV/peroxide and EV/peroxide) were observed and compared. The blends were processed on a two-roll mill, and a fixed amount of carbon black was also incorporated. Amongst the blends, accelerated sulfur-vulcanizing systems exhibited higher torques, state of cure, tensile strength and elongation-at-break, in comparison with the peroxide-vulcanizing system, whereas the tensile modulus, hardness and cross-link density showed lower trend. In the mixed sulfur/peroxide-vulcanizing systems, it showed intermediate behavior to the individual sulfur- or peroxide-vulcanizing systems. This was associated to the interference of peroxide during the cross-linking formation. SEM micrographs of semi-EV-vulcanizing system exhibited more roughness and cracking path indicating that higher energy was required towards the fractured surface. The high cross-link density observed from the swelling study could be verified from the storage modulus (E') where peroxide vulcanized blends provided a predominant degree of cross-linking followed by semi-EV, semi-EV/peroxide, EV and EV/peroxide-vulcanizing systems, respectively. The glass transition temperature (T-g) depicted at maximum peak of mechanical loss factor (tan delta(max)), indicating that semi-EV-vulcanizing system showed highest T-g value. The T-g values can be ordered as semi-EV > peroxide > semi-EV/peroxide > EV > EV/peroxide-vulcanizing systems. The T-g of rubber vulcanizates can be increased due to the restriction of molecular movement such as cross-link density.