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.
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