The tensile and tear properties of edge-cut specimens of natural rubber vulcanizates containing varying amounts of a coarse (particle size: 250 nm, surface area: 13 m2/cc) carbon black (N990) were determined at different temperatures.
The properties of these vulcanizates at 25°C were compared with those determined by other researchers that contained N115 (particle size: 27 nm, surface area: 200 m2/cc) and N660 black (particle size: 55 nm, surface area: 63 m2/cc). At low carbon black loadings (6-12 phr), N990 black gave a higher tear strength than that with the finer N115 and N660, but, the converse was true at higher carbon black loadings (25-75 phr)!
Multiple cracks at the tip of an edge-cut were first observed at 55 phr of N990 while 18 phr and 40 phr have been reported previously for N115 and N660, respectively. Furthermore, unlike in the case of the N115- and N660- vulcanizates, multiple crack formation in edge-cut N990 samples did not give much reinforcement; even a vulcanizate with100 phr loading of N990 was only marginally stronger than the gum vulcanizate at all the cut depths.
For a given carbon black loading, crack complexity generally increased with increase in temperature, while tear resistance decreased.