On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible. A common and low cost antiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are also widely used. Ozone cracks were commonly seen in automobile tire sidewalls, but are now seen rarely thanks to the use of these additives.

The problem can be prevented by adding antiozonants to the rubber before vulcanization. Silicone rubber, Hypalon and polyurethanes are also ozone-resistant. Attack is less likely because double bonds form a very small proportion of the chains, and with the latter, the chlorination reduces the electron density in the double bonds, therefore lowering their propensity to react with ozone. Ozone-resistant elastomers include EPDM, fluoroelastomers like Viton and polychloroprene rubbers like Neoprene. The higher the concentration, the greater the number of cracks formed. Exposed surfaces are attacked first, the density of cracks varying with ozone gas concentration. Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible. Nitrile rubber is a copolymer of butadiene and acrylonitrile units, but the proportion of acrylonitrile is usually lower than butadiene, so attack occurs. Every repeat unit in the first three materials has a double bond, so every unit can be degraded by ozone.

Tiny traces of ozone in the air will attack double bonds in rubber chains, with natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber being most sensitive to degradation. Only a trace amount of the gas is needed to initiate cracking, and so these items can also succumb to the problem.

However, it does occur in many other safety-critical items such as fuel lines and rubber seals, such as gaskets and O-rings, where ozone attack is considered unlikely. The problem was formerly very common, especially in tires, but is now rarely seen in those products owing to preventive measures. Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking.

Ozone 5 crack