The Mechanism of Carbon Black Reinforcement of SBR and NR Vulcanizates

Gum and black-filled vulcanizates of styrene-butadiene rubber (SBR) and natural rubber (NR) were prepared. Strengths in uniaxial tension were determined for specimens with and without edge-cuts. Normal tensile strengths (no cut) of the gum NR, filled NR and filled SBR were similar; whereas, with pre-cuts, even as small as 0.1 mm, strengths of these were markedly different. The SBRs exhibit a steady decrease in strength with increased cut size, while the NR vulcanizates show a drop in strength at a critical cut size ccr. With small cuts, when the gum NR substantially crystallizes in bulk, its strength greatly exceeds that of the filled SBR. The opposite is true with large cuts, due to the large drop in strength of gum NR at ccr. Remarkably, above ccr, the gum NR is only about twice as strong as the gum SBR. Rupture in the gum SBR, gum NR and filled SBR all take place by simple lateral cut growth. At all cut sizes, the filled NR is the strongest material by at least a factor of two, and prior to catastrophic fracture it exhibits auxiliary cracking. It is proposed that crystallization and filler act synergistically, resulting in sufficient anisotropy to cause longitudinal cracking. This provides a super-blunting mechanism, which augments resistance to catastrophic cut growth.