Effect of vulcanization system on properties of thermoplastic vulcanizates based on epoxidized natural rubber/polypropylene blends

Epoxidized natural rubber with mole percentage epoxide of 30 was synthesized and used to prepare thermoplastic vulcanizates based on 75/25 ENR/PP blends with Ph-PP compatibilizer. Influences of various curing systems (i.e., sulfur, peroxide and a mixture of sulfur and peroxide-cured systems) were investigated. We found that the mixing torque, shear stress, shear viscosity, tensile strength and elongation at break of the TPVs using the mixed-cure system exhibited higher values than those of the sulfur and peroxide-cured systems, respectively. This may be attributed to a formation of S-S, C-S combination with C-C linkages in the ENR phase. In the sulfur-cured system only S-S linkages are formed, whilst in the peroxide curing system more stable C-C linkages are formed. However, during shearing at high temperature of the peroxide and mixed-cure systems, the peroxide caused degradation of the polypropylene molecules. Higher level of DCP was used in the peroxide-cured system and caused greater influence on properties. In the mixed-cure system, lower influence of PP degradation and influence of formation of more stable C-C linkages overcomes the drawback. Therefore, we observed the highest values of those properties using the mixed-cure system. The curing systems did not affect the hardness properties and solvent resistance of the TPVs. We also found that the dispersed vulcanized rubber domains of TPV with the peroxide-cured system were smaller rubber particles than those of the mixed and sulfur-cure systems.