Exposure to ozone (O3) and other atmospheric contaminants can cause tiny cracks to form on the O-ring’s surface. Running perpendicular to the direction of stress, these cracks are visible evidence of the fact that ozone weakens the O-ring compound by attacking unsaturated (double) bonds and breaking apart the polymer chains. This breakage is known as chain scission.

Cracking (also known as crazing) may be prevented (or at least limited) by using materials with fully-saturated bonds that are less susceptible to chemical attack. Weather- and ozone-resistant elastomers such as silicone, fluorocarbon (Viton®), EPDM, polyurethane, polyacrylate, fluorosilicone, ethylene acrylic, and epichlorohydrin are all good choices. In some instances, an elastomer whose ozone resistance is inherently poor can be supplemented with antiozonant additives.

Keep in mind that stretching an O-ring more than 5% increases its exposure to chemical attack. Storage around ozone-generating equipment (such as electric motors), especially in a stretched (installed) condition, will lead to rapid deterioration of the elastomeric compound, often in as little as a few days. Figure 138 is an example of O-ring failure due to weather and ozone cracking.