Because plastic parts are being used more and more as alternatives to metal components, you should be aware that sealing against a plastic surface may present some special problems. The hardness or chemical makeup of an O-ring can cause fine surface cracks to form in adjacent plastic parts. This “crazing” of the plastic is often the result of ester plasticizers (chemical substances added to increase softness, provide low temperature flexibility, and improve processing) in the O-ring compound and may ultimately cause the plastic part to fail. Ethylene propylene, chloroprene (Neoprene®), nitrile, and silicone have all been used successfully in conjunction with plastics such as Noryl® (modified polyphenylene oxide, or PPO), Cycolac T® (ABS thermoplastic), and Lexan® (thermoplastic polycarbonate).

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