Silica-reinforced natural rubber compounds compatibilized through the use of epoxide functional groups and TESPT combination

Epoxidized natural rubber (ENR) and bis-(3-triethoxysilylpropyl) tetrasulfide (TESPT) were used to improve the properties of silica-filled NR. The use of ENR containing 51 mol% epoxide groups (ENR-51) as a compatibilizer without TESPT was optimized at 7.5 phr, based on the results of Payne effect and tensile strength. By using 7.5 phr of ENR-51 with varying amounts of TESPT in a range of 2 to 5 wt% relative to the silica, the properties of compounds were compared to those of the ones with optimum TESPT content (i.e. 8.6 wt% relative to the silica) and without. The addition of TESPT to the ENR-51 compatibilized silica-filled NR compound had no effect on Mooney viscosity but lowered the Payne effect to the same level as that of the silica/TESPT compound, and significantly decreased both scorch and optimum cure times. The silica-filled NR with ENR and the small amount of TESPT combination showed a further increase in tensile strength to match that of the optimized silica/TESPT system, while maintained the elongation at break. This work demonstrates that the use of ENR as compatibilizer clearly enhances the properties of silica-filled NR compounds, and that such properties can be further improved by adding TESPT at a half or less amount of TESPT normally needed for silica-filled compounds.