Effect of curing system on reinforcing efficiency of silane coupling agent

Abstract

Silane coupling agent has long been used to enhance degree of reinforcement of silica. The mechanical properties of silica-filled vulcanizates are greatly improved in the presence of silane coupling agent, mainly due to an increase in rubber–filler interaction. Recently, various types of silane coupling agent have been developed and commercialized. In the present study, two types of silane coupling agent, i.e. bis-(3-triethoxysilylpropyl) tetrasulfane (Si-69) and 3-thiocyanatopropyl triethoxy silane (Si-264) were selected for comparison of their reinforcing action in various curing systems: conventional vulcanization (CV), semi-efficient vulcanization (semi-EV) and efficient vulcanization (EV). The results reveal that the addition of silane coupling agents not only improves compound processability, but also enhances the mechanical properties of the rubber vulcanizates. The effect of silane type on processability is found to be independent of the curing system. Compared with Si-69, Si-264 can react with the silanol groups on the silica surface more readily, due to its less bulky structure and lower viscosity. In other words, Si-264 can reduce the filler–filler interaction to a greater extent than Si-69, leading to better compound processability. The effect of silane type on the vulcanizate properties, on the other hand, depends greatly on the curing system. For CV and semi-EV systems, Si-264 imparts greater degree of reinforcement than Si-69. This might be attributed to the combined effects of better rubber–filler interaction, better filler dispersion and higher state of cure when Si-69 is replaced with Si-264. However, for the EV system, contrary results are obtained, i.e. Si-69 provides better reinforcement than Si-264. Explanation is given to the effect of sulfur contribution of Si-69 which is dominant only in the EV system.