Scanning electron microscopy studies on mechanism of tear fracture of styrene-butadiene rubber

Abstract

Scanning electron microscopy (SEM) studies on tear fracture of styrene-buatadiene rubber (SBR) vulcanizates have been made with respect to the following parameters: (a) the nature of cross-links (flexible sulphur (-S_x-) type and rigid carbon-carbon (-C-C-) type); (b) the effect of addition of reinforcing filler, ISAF carbon black (N 220) in both cross-linking systems; and (c) the effect of cross-link density in the case of sulphur-cured vulcanizates. It was observed that peroxide-cured SBR (unfilled) vulcanizate undergoes brittle fracture whereas the optimum cross-linked sulphur-cured unfilled vulcanizate undergoes fracture in the shear planes and with branching of the tear path. In the case of sulphur-cured SBR (unfilled) vulcanizate with a higher cross-link density the fracture pattern is similar to that of the vulcanizate which has an optimum cross-link density. When the cross-link density is increased or the nature of cross-link is changed to the rigid C-C type, stress dissipation becomes difficult and hence the tear resistance decreases. Peroxide-cured SBR (filled) vulcanizate shows a typical stick-slip mode of fracture as well as steady tear and microfolds over the surface. Sulphur-cured SBR (filled) vulcanizates show short and curved tear lines. Fillers help to arrest crack growth and to increase stress dissipation, thereby increasing tear resistance.