Seals with an elastomeric element may also be susceptible to degradation by environmental factors such as ozone and oxygen. The combination of material stress and ambient ozone can cause cracks to develop in some materials. Figure 32 shows cracking of a shaft seal’s elastomeric lip.

Nitriles, for example, are at high risk for this type of degradation. This is because the chemical backbone of nitrile contains a double bond, and double bonds are the primary attack sites for ozone. In actuality, environmental cracking is most often seen in nitrile seals prior to installation. To help avoid this, seals should be stored away from sunlight, UV light, and ozone-generating devices such as welders and electric motors. In addition to compromising a sealing lip’s ability to maintain proper contact with the shaft, cracks also act as minute leak paths through which fluid can escape.

There are several different ASTM tests designed to gauge a material’s resistance to ozone degradation. ASTM D 1149 calls for 20% elongation of a material sample that is then exposed to a prescribed ozone/air mixture. This exposure takes place within an enclosed ozone chamber heated to 104° F. Following a specified length of exposure, the sample is examined for the presence of surface cracks. ASTM D 1171 utilizes largely the same procedure, with the difference that the material specimens are triangular and are wrapped around a mandrel (rather than elongated).

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