ure of a material’s elasticity after prolonged action of compression (Figure 1), either under ambient conditions, or while being exposed to elevated temperatures. The results are used to relate the elastomer’s compression set result to its propensity to leak in service. The higher the compression set, the higher the risk of a leak. An elastomer’s compression set performance is dictated by two factors: the elastomer (polymer type, crosslink type and quantity, filler type and process aids) and test procedure. Under compression the elastomer is subject to both thermal and chemical attack. The rate of thermal attack is dependent on the temperature of the test oven. Chemical attack can occur in a variety of ways, with crosslinks forming or breaking or through degradation of the polymer chain. As temperatures are increased, or test times extended, the degree of thermal or chemical attack increases, thus reducing the ability of the rubber to recover to its original shape (see Figure 2 on page 13). Therefore, the compression set test can be used as a measure of the cure of the elastomer. By testing at elevated temperatures, the degree of compression becomes a measure of the strength of crosslinking within the elastomer.