Epoxidised natural rubber/silica hybrid nanocomposites by sol-gel technique: Effect of reactants on the structure and the properties

This paper reports the effects of various reaction parameters such as solvent, mole ratio of water to tetraethoxysilane and temperature on the structure and the properties of epoxidised natural rubber/silica organic-inorganic hybrid nanocomposites, prepared by sol-gel technique. The sol-gel reaction was conducted at a constant concentration of tetraethoxysilane (45 wt% with respect to the rubber), used as the precursor for silica under a constant pH of 1.5. Infrared spectroscopic studies and the ash content data indicated the maximum silica generation in tetrahydrofuran compared to chloroform and carbon tetrachloride, which are less polar and had low affinity towards water than the former. The silica particles prepared from tetrahydrofuran were scattered within the rubber matrix with an average dimension of 100 nm, as evident from the transmission electron microscopic study. Dispersion of nanosilica within the composites was obtained when the tetraethoxysilane to water mole ratio was maintained up to 1:2, beyond which the resultant composites showed phase separation due to the agglomeration of the silica particles. High gelling temperature of the hybrids also resulted in phase separated morphology, probably due to the accelerated condensation reaction in the composites. All the phase separated composites showed higher infrared optical density and transmission loss compared to the nanocomposites. Poor mechanical reinforcement was observed from the dynamic mechanical analysis of the uncured composites having larger silica particles. On the contrary, better mechanical properties were achieved with the nanocomposites containing 90–100 nm silica. The nanocomposite prepared with 1:2 tetraethoxysilane to water mole ratio in tetrahydrofuran under room temperature showed the highest tensile strength and 100% tensile modulus, probably due to better polymer-filler interaction, in the uncrosslinked state and after crosslinking.