Ozone Cracking and Protection of Rubber

Abstract

Abstract Studies of the growth of single ozone cracks show that a critical severity of deformation is required for growth to occur. Above this deformation the rate of single crack growth is essentially independent of strain and for a number of rubbers at normal temperatures is proportional to the ozone concentration. The mechanics of cracking in stretched surfaces is more complex, the rate of growth being dependent on strain and on various other factors such as the gas flow conditions and the test piece size. It is shown that these effects are consistent with diffusion control of the rate of attack; this occurs because the rapidity with which ozone is destroyed by stretched rubber leads to a reduced concentration adjacent to a surface. Certain chemical antiozonants can completely prevent cracking, even at very high deformations. Current investigations suggest that this is due to the formation of a protective layer on the surface of the rubber by direct reaction of the antiozonant with ozone; the diffusion of unreacted antiozonant is an important factor influencing layer development. It appears that the amount of antiozonant required for protection may be determined by the rate at which a coherent layer can be formed in relation to the rate at which ozone is attacking the surface.