  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.
