How do oxygen and ozone cause natural rubber to crack over time?

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truck tires being removed from their molds — rubber Truck tires being removed from their molds.

Oxygen and ozone degrade natural rubber through distinct chemical processes that compromise its structural integrity.

Oxidation
Oxygen triggers free-radical oxidation reactions within the rubber. These reactions can lead to two detrimental outcomes: molecular rupture, which causes the material to soften, or excessive interlinking, which results in hardening as the rubber ages.1

Ozone Cracking
Ozone reacts readily with the carbon-carbon (C=C) double bonds present in unsaturated elastomers. When rubber is under slight tension—specifically when stretched by more than 10 percent—this reaction causes the molecules on the surface to break. This process creates small, deep fissures known as ozone cracks.1

To mitigate these effects, manufacturers incorporate protective additives:
* Antioxidants: Chemicals like amines and hindered phenols are used to disrupt oxidation reactions.1
* Antiozonants: Certain diamines protect the rubber by reacting with ozone before it can attack the C=C bonds.1
* Waxes: Paraffin wax is often added to form a protective skin on the surface of the rubber.1