Effect of zinc oxide distribution on vulcanization efficiency and mechanical properties of rubber nanocomposites

INTRODUCTION The production of rubber composites in tire industries involves a compounding phase and a curing process. To get highly performant materials, a homogeneous distribution of chemicals, reinforcing fillers and curing agents in rubber is required. Among them, ZnO is the most efficient worldwide employed curing activator, even though its poor dispersion in rubber matrix generally implies a high demand of ZnO. However, as ZnO undergoes leaching during tire life-cycle, the reduction of ZnO amount is becoming an urgent issue. In this context, the present work aims at improving the dispersion of ZnO in rubber nanocomposites, simultaneously reducing the employed ZnO amount. The impact of a different distribution was studied by comparing the vulcanization efficiencies and the mechanical properties of cured rubber nanocomposites prepared by using either a novel double function filler, ZnO/SiO2, constituted by ZnO nanoparticles (NPs) anchored to silica or the conventional micro-crystalline ZnO (m-ZnO). MATERIAL AND METHODS ZnO/SiO2 was prepared through a sol-gel procedure; ZnO NPs (3-5 nm) were anchored on silica particles, as demonstrated by the morphological and structural characterization. ZnO/SiO2 and m-ZnO were used to prepare two polyisoprene (IR) nanocomposites with a brabender. The compounds were calendered at 50°C and vulcanized in a compression molding press. A dynamic-mechanical analysis was performed at room temperature in tensile loading conditions: a 0.1% of amplitude dynamic strain was superimposed to a static deformation, at a frequency of 10 Hz. Uniaxial tensile tests were also performed at constant nominal strain rate and at room temperature, up to failure. RESULTS AND DISCUSSION ZnO/SiO2 showed higher curing efficiency in sulfur cross-linking of IR composites compared to m-ZnO and the cured materials presented dynamic-mechanical properties suitable for tire production at lower Zn amount. Besides, different mechanical behaviors were registered for the two rubber nanocomposites using a different distribution of ZnO. The higher curing efficiency of ZnO/SiO2 was explained by a more homogeneous distribution of zinc in rubber and higher accessibility of the Zn2+ ions during the process. The immobilization of ZnO NPs was responsible of a different reaction mechanism, which accounts for the higher cross-linking density and different mechanical properties of the cured material.