Effect of mastication, antioxidants and crosslink density on the mechanical properties of gum and black-filled natural rubber vulcanizates

The tensile behavior of black-filled natural rubber (NR) vulcanizates, as well as a corresponding gum compositions were studied. The effect of mastication, antioxidants and crosslink density on tensile strength and cut growth were determined. Dumbbells, with and without edge cuts, were strained continuously to rupture. Crack growth was determined at various strains. Catastrophic rupture paths and micro-cracking patterns were examined using scanning electron and optical microscopy. Cracking patterns were related to strength. To facilitate discussion, tensile strengths refer to breaking stress for uncut specimens, while tear strengths are for specimens with precuts.;Black-filled vulcanizates containing antioxidants exhibit higher strengths, compared to those without antioxidants, both before and after oxidative aging. Rubber breakdown, shown by the lowering of Mooney viscosity and/or molecular weight does not markedly lower tensile and tear strength of the vulcanizates with antioxidants, before and after aging. Mastication decreases the tear strength of black-filled vulcanizate without antioxidants before and after aging at 70°C for 24 h. In addition, mastication markedly reduces the critical cut size of gum vulcanizate with no antioxidants after aging at 100°C for 12 h. All black-filled samples, except those with no antioxidants aged at 100°C for 24 h, show longitudinal cracking during deformation, while all gum vulcanizates exhibit simple lateral crack growth. Slow crack growth has a large influence on strength.;In other experiments, crosslink densities were varied in gum and black-filled vulcanizates. Tensile and tear strength of black-filled vulcanizates are not as sensitive as gum vulcanizates to changes in crosslink density. Vulcanizates containing low and moderate crosslink density show a change in the slope of plots of strength versus cut length at a critical cut size whereas highly crosslinked vulcanizates, exhibit a continuous drop in strength. The discontinuity at the critical cut size of gum vulcanizates is attributed to the lack of substantial bulk crystallization. In addition, highly crosslinked gum vulcanizates have a nearly constant tearing energy for various cut sizes. Unlike gum vulcanizates, all black-filled vulcanizates. develop longitudinal cracks near the cut tip. The extent of longitudinal cracks is lower as the crosslink density and/or cut size is increased, corresponding to decreased strength. Intrinsic flaw sizes of gum samples pass through a minimum with increasing crosslink density.