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No single physical property of rubbers is responsible for the successful performance of an oil seal or ‘O’ ring. The ultimate tensile strength, breaking elongation, modulus, shore hardness, creep and stress relaxation in tension and compression loads are all important physical properties that characterize a seal or ‘O’ ring. Compression strength and set together with stress relaxation or decay are important for effective sealing. The difference in these properties in a swollen seal is highly critical. An optimum swelling value in a fluid medium is a desirable feature. De-swelling decreases the seal pressure against the wall of the housing where the seal is fixed, leading to leakage. Over swelling minimizes the physical properties of the rubber. Seals made of polysulfide rubbers have extreme fuel resistance but undesirably high compression set. The effect of temperature on the seal is an important factor. Swelling under stress can increase at higher temperatures and a suitable compounding technique should be adopted to reduce this effect.