Effect of fumed silica surface area on silicone rubber reinforcement

The effects of fumed silica on the processability and vulcanizate properties of silicone rubber have been studied using fumed silicas with surface areas ranging from 90 to 417 m(2)/g at a constant loading of 38 phr. The results show that all rheological properties of the compounds and mechanical properties of vulcanizates are strongly affected by the silica surface area. With increasing surface area, the viscosity of the compounds and the modulus of the vulcanizates increase markedly, the degree of dispersion of the filler, the tear strength and elongation at break are deteriorated, and the compression set is somewhat improved. While the bound rubber and tensile strength rise to a maximum at a surface area between 170 and 250 m(2)/g, the extrudate appearance is poorer in this range. It was also found that lower surface area fumed silica favors discoloration.