Many elastomers are rich in unsaturated
double bonds, which can react with ozone present in the air in process known as
ozonolysis. This reaction breaks the polymer chains, degrading the mechanical properties of the material. The most obvious effect of this is cracking of the elastomer (ozone cracking), which is exacerbated by mechanical stress. The rate of degradation is effected both by the chemical structure of the elastomer and the amount of ozone in the environment. Elastomers which are rich in double bonds, such as
natural rubber,
polybutadiene,
styrene-butadiene rubber and
nitrile rubber are the most sensitive to degradation, whereas
butyl rubber,
polychloroprene,
EPDM and
Viton are more resistant.
Ground-level ozone is naturally present, but it is also a product of
smog and thus degradation is faster in areas of high air pollution. All of these factors make vehicle
tires particularly vulnerable, as they contain a high level of unsaturated groups, operate in areas prone to air pollution and are subjected to significant mechanical stresses. == Protection of elastomers ==