Influence of vulcanization system on curing, mechanical, dynamic and morphological properties of maleated natural rubber and its thermoplastic vulcanizate with thermoplastic copolyester elastomer

Abstract

Maleated natural rubber (MNR) was prepared and compounded with four alternative vulcanization systems: sulfur, peroxide, phenolic, and mixed sulfur-peroxide vulcanization systems. It was found that the peroxide and mixed sulfur-per-oxide cured systems show a plateau curing curve, while the sulfur-cured system exhibits reversion, and the phenolic cured system gives marching cure behavior. Also, thermoplastic vulcanizates (TPVs) based on dynamically cured MNR and thermoplastic copolyester elastomer (TPC-ET) blends (i.e., MNR/TPC-ET TPVs) by using these vulcanization systems were also prepared and characterized. The height and area underneath the dynamic vulcanization peaks in the mixing torque-time curves directly relate to the degree of vulcanization and strength along with the nature of crosslink structures, and these had the rank order peroxide > mixed sulfur-peroxide > phenolic > sulfur by vulcanization system. This is in good agreement with the maximum torque, tensile strength, hardness, moduli, and toughness of MNR compounds. The locations of the peaks also correspond to the scorch time (ts1), and the sulfur and phenolic cured systems with scorch times of 2.31 and 2.44 min showed peaks at 3.3 and 3.4 min, respectively, offering improved process safety. The sulfur-cured TPV had sulfidic linkages and the smallest-sized vulcanized rubber domains, giving it the highest tensile strength, elongation at break, storage modulus, and rubber elasticity (i.e., the lowest tension set and tan δ). However, the phenolic cured TPV had the highest stiffness, Young’s modulus, and large vulcanized rubber domains, which relate to low content of Chroman ring structures in the crosslinked MNR vulcanizates, with a comparatively low interfacial area and hence weak interfacial interactions between MNR and TPC-ET phases. Furthermore, the mixed sulfur-peroxide and peroxide curing systems gave intermediate tension set, tan δ, and elongation at break.