Tearing of black-filled (N660) natural rubber vulcanizates

Tear resistance and crack patterns of carbon black N660-filled natural rubber vulcanizates were studied at 25-80 °C. Results are compared with previous ones for similar vulcanizates filled with a finer carbon black N115.;At room temperature, vulcanizates with up to 15 phr of N660 show an abrupt drop in strength σbc at a critical cut size, ccr. This results in an exclusion zone, where specimens never fail. When cut size c is smaller than ccr, bulk crystallization take place prior to catastrophic failure. When c > ccr, crystallization only occurs at a crack tip and specimens are quite weak.;Surprisingly, a small amount of N660 weakens the NR instead of strengthening it. When the black concentration is low (0–15 phr), σbc and ccr decrease as black content increases.;With 18 and 22 phr of N660, an exclusion zone and ccr are absent; strength decreases continuously with increasing cut size. Bound rubber tests show that formation of a coherent gel is the reason for the disappearance of these behaviors.;At 23 phr of N660, knotty cracks develop at room temperature. With more black a larger fraction of specimens fail with knotty cracks. Thus, with enough black, strain-induced crystallization is promoted and reinforcement begins.;At elevated temperatures, σbc is lower than that at room temperature. At 40 °C, with up to 12 phr of N660, a ccr is apparent and all specimens fail by growth of a single lateral crack. However, ccr is smaller at 40 °C than at room temperature.;With increased black or increased temperature, a portion of specimens show crack deflection (serrated single crack or knotty cracks) before failure. A serrated single crack is a transition pattern between a single lateral crack and knotty cracking. As black concentration increases, the onset temperature for knotty tearing decreases: 80 °C for NC0, 70 °C for NC3 and NC6, and 50 °C for NC12.;Less energy is released at elevated temperature. Therefore, σ bc of specimens with knotty cracks tears are not significant higher than those with only a single crack, especially at high temperatures.;The effect of carbon black type on σbc is as significant as the effect of temperature. Results of NR vulcanizates filled with carbon black N115 are similar to those filled with N660, although N115-filled vulcanizates exhibit knotty cracks at lower carbon black concentration and temperature.