Comparison of the Strength of Normal and Edge-Cut Tensile Specimens of Styrene-Butadiene Rubber and Natural Rubber with Similar Crosslink Density - Part I

Edge-cuts of various depths, c, were introduced into tensile specimens of similarly crosslinked (r_c » 6 (10^-5) moles of crosslinks/mL) gum (unfilled) natural rubber (NR) and gum styrene-butadiene rubber (SBR). When specimens contained no intentional cut (c = 0, i.e., normal tensile strength), the NR is about ten times as strong as the SBR. This difference is due to the stereo-regularity (cis 1,4-polyisoprene) of NR, enabling it to strain-crystallize when deformed. On the other hand, SBR has an irregular, amorphous microstructure that renders it incapable of crystallization. However, for testpieces that contain an edge-cut, the strength, σ_bc, of the NR relative to that of the SBR depends strongly on cut depth. When c » 0.2 mm, the strength of both vulcanizates is reduced about 50%, and hence the ratio of strengths remains about an order of magnitude. But, with a further increase in c, the SBR exhibits a steady decrease in σ_bc, while the strength of the NR drops discontinuously by nearly a factor of four when c » 1.7 mm. Now, the NR is only about two and one-half times as strong as the SBR. Extrapolation of σ_bc to larger c results in a predicted cut depth c » 4.3 mm at which the strength of the NR and SBR would be similar. At sufficiently large cut depth, it appears that strain rate at the cut tip is high enough and breaking strain low enough that rupture occurs before significant strain-crystallization commences.