Mechanism of the effect of nano-silica on crack growth and wear resistance of natural rubber-based composites

In this work, by systematical engineering of polymer–filler interface, the mechanism of the effect of silica on the tear and wear resistances of natural rubber (NR) is comprehensively investigated under various loading range. In particular, the roles played by the untreated silica are compared with a type of sacrificial physical interactions and also with chemical interactions at the interface. The novel composite with sacrificial interactions was developed by grafting maleic anhydride onto the NR backbone and grafting amine–amide functionalities onto the silica surface. Full covalent bonding at the interface was also provided by treatment of silica using the bis[3-triethoxysilylpropyl]tetrasulfide silane (TESPT). Results indicated that the crack growth resistance of silica-filled NR composites in the unfilled and low-filled ranges is mostly governed by the nature of rubber network, and thus, all types of the filler-polymer interactions modify crack growth through altering the cross-link density. The viscoelastic loss was, however, the effective mechanism in the highly filled range for all types of filler-polymer interactions. The adverse interaction between NR and pristine silica substantially reduces the crack growth resistance, the strength and the wear resistance of the respective system at high loadings, and the results were even worse than the unfilled case. Compared to the system with covalent interactions, the presence of sacrificial physical bonds at interface dramatically improves the polymer–filler interactions and the silica dispersion. This composite presumably benefits from a type of tightly bonded rubber that is responsible for a well balance of crack growth resistance, mechanical strength, wear resistance and more importantly stable dynamic behavior. In the high-loading range, the tear resistance of the system containing the sacrificial bonds revealed a significant growth of about 100% compared to the raw rubber system filled with the pristine silica, suggesting a promising design scenario for preparation of durable rubber composites.