When PTFE washers are used in systems using water-reactive acidic or basic gases, their moisture can cause the formation of corrosive acids or alkaline liquids in the system. For most specialty gases, CTFE is recommended over PTFE because it is less permeable to oxygen and moisture, and evidences less cold flow. This leads to better leak integrity and allays contamination concerns. In some applications, PTFE mixed with a solid filler material—such as calcium fluoride or brass—is used to reduce the cold flow and permeation rates of the PTFE. This is especially important when PTFE washers are being used in acid gas service where the elimination of moisture contamination is critical. Lead or annealed copper washers are most often used in fluorine and other reactive fluoride service.

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