Reinforcement Studies—Effect of Silica Structure on Properties and Crosslink Density

Abstract

Abstract This work was undertaken to study the effect of silica filler structure on the effective crosslink density and on mechanical properties. Vulcanizates of silicone, SBR, and EPM were included in the work. A compression modulus method was used for crosslink density determination. Samples were swollen prior to measurement in two environments : (a) toluene, (b) toluene in an NH8 atmosphere (designed to cleave polymer-filler linkages if they were present). The results showed that, in peroxide-cured silicone and EPM elastomers, the crosslink density increased with filler structure, but after cleavage of polymer-filler bonds by the NH3-modified swelling environment, there was very little change of crosslink density with filler structure increase. The effect of increasing filler structure on crosslink density in the SBR elastomers was very slight, showing a small increase in progressing from discrete particles to low structure and thereafter no change. The quanitative results of this study are important in that they provide excellent support for Medalia's theory of the role of occluded volume in reinforcement. The results show that polymer-filler bonds are required, in combination with occluded volume, in order to achieve a substantial increase in both 200% modulus and crosslink density.