The mechanism for the exceptionally high tear strength of carbon black/Hevea natural rubber vulcanizates

Abstract

Natural rubber vucanizates containing 0-50 phr of a fine carbon black (N115,d=27nm) were prepared and tensile strengths of normal (no pre-cut) and edge pre-cut specimens were determined. Normal tensile strengths of all vulcanizates were similar. At the relatively slow strain rate experienced wholesale by normal uncut specimens, all vulcanizates, prior to crack initiation, strain-crystallized sufficiently to be strong. However, pre-cut specimens experience increased strain rate at a cut tip. Magnification of the strain rate increases as cut depth c increases. Fracture in the gum NR and vulcanizates with up to 14 phr of black occurred by simple forward crack growth from a cut tip, and all exhibited a critical cut size, above which strength dropped abruptly. Furthermore, for these lightly filled samples, strength and critical cut size decreased with increased black content. This indicates less strain-crystalliation before rupture of pre-cut specimens, when levels of black are low. This effect is attributerd to rapid straining at a cut tip and hindering of the chain mobility necessary for crystallization. When black content was increased to 15 phr, with 1 mm<c<2 mm, about 50% of specimens retained simple lateral fracture and were weak, but the other 50% developed deviated cracks (knotty tearing) and were significantly stronger than corresponding pre-cut gum specimens, especially at large c. High strength with sufficient black levels are attributed to increased strain-crystallization and super-blunting (multiple cracks) at a cut tip. These inhibit forward crack growth. For carbon black to enhance strain-crystallization relative to the gum, it appears there must be enough of it to form a bound rubber/black network. If the black concentration is less than this percolation threshold, strain-crystallization is hindered at a cut tip.