Tear strength of filled rubbers

The strength of black-filled rubbers has been investigated under a variety of test conditions using various compounding formulations, of both strain-crystallizing and non-strain-crystallizing elastomers. The enhancement in the tear strength is substantial when knotty tearing occurs. In knotty tearing, the crack tip grows perpendicularly to the general direction of propagation. This effectively increases the tip diameter and thus the tear strength. A strong correlation between tearing energy and knot diameter (measured in the unstrained state) has been found in the present investigation. Factors which affect the development of knotty tearing were investigated. It was found that knotty tearing is affected by the degree of strain-crystallization, molecular mobility, nature and concentration of crosslinks, the type and concentration of carbon black, temperature and tear rate. The onset of knotty tearing appears to be related to the development of strength anisotropy at the tip of the tear. The effect of this anisotropy on the energy to propagate tearing in the direction of pre-straining was investigated using split tear test-pieces. The tearing energy for crack propagation in the direction of molecular orientation gives a quantitative measure of strength anisotropy developed in the vulcanizate as a consequence of pre-straining. It was found that, in a stretched vulcanizate, the tearing energy to propagate tearing in this direction was a factor of about 20 lower than the tearing energy of the unstretched vulcanizate. In a certain type of black-filled vulcanizate, the anisotropy introduced during pre-stressing to large extension still persisted even after the pre-stretch was removed. The present investigation shows that the anisotropy introduced by pre-stressing is associated with the set. The tearing energy of a pre-stressed vulcanizate was found to correlate strongly with the set