Effect of nano zinc oxide on the cure characteristics and mechanical properties of the silica-filled natural rubber/butadiene rubber compounds

With the increasing interest in environmental and health issues, legal restrictions, such as European Union (EU) End of Life Vehicle Directives, were strengthened. This led us to incorporate nano zinc oxide (nano-ZnO), with particle sizes of 30-40 nm and specific surface areas of 25.0-50.0 m²/g, instead of conventional ZnO into natural rubber (NR)/butadiene rubber (BR) compounds to decrease the content of zinc in the formulation. In the unfilled system, only a 20 wt % nano-ZnO content, compared to conventional zinc oxide content, showed the cure characteristics and mechanical properties of the same level. This was because the increase in the specific surface area of the nano-ZnO led to an increase in the degree of crosslinking. The effect of nano-ZnO on the cure characteristics and mechanical properties was more pronounced in the silica-filled system than in the unfilled system. This was mainly because of the dispersing agent used in the silica-filled system, which also improved the dispersion of nano-ZnO. The silica-filled NR/BR compounds containing 0.3-3.0 phr of nano-ZnO showed improved curing characteristics and mechanical properties, such as optimum cure time, 100 and 300% modulus, tensile strength, and tear strength compared to the compound with 5 phr of conventional ZnO. The optimum amounts of nano-ZnO and stearic acid were only 1.0 and 0.1 phr, respectively.