Natural rubber vulcanizates (NR) have high tear resistance, excellent flexibility and high resilience. All tires contain black-filled natural rubber compounds in the location (belt region) of maximum stress concentration.
Edge-cut tensile specimens of gum natural rubber vulcanizates show an abrupt drop in strength at a critical cut size and an abrupt decrease of normal (no cut) tensile strength at a critical temperature. These effects are due to changes in bulk strain crystallization at small cut sizes or low temperatures. The addition of carbon black affects tearing; this depends on the type and concentration of black. Natural rubber vulcanizates filled with
high concentrations carbon black have very high tear resistance. This is due to a high degree of crystallization around a cut tip.
A series of natural rubber vulcanizates containing a very fine carbon black (N115, surface area: 123 m2/g) were torn at 25, 60 and 100°C. Amazingly, the addition of low concentrations of carbon black decreases the tear resistance at 25 and 60°C. Thus, there is a critical level of black needed for “reinforcement”. This amount of carbon black is also the concentration for incipient bound rubber formation; a continuous filler network seems to be necessary for the onset of reinforcement. Tearing in the transition zone depends on
temperature. At 100°C, black-filled natural rubber vulcanizates behave like amorphous vulcanizates; they show a steady increase in strength as carbon black content increases.