Teflon Gaskets

Teflon -based gasket materials are able to withstand significantly more 
aggressive chemical environments (including highly oxidizing environments) than standard non-asbestos gasket materials. However their temperature, pressure and mechanical performance is inferior to standard non-asbestos gasket materials. Metal inserted and expanded PTFE are able to operate at higher pressures. 
Teflon is principally used as a gasket material because of it's capability to 
remain unaffected by the majority of chemicals used in industry. It is also 
classified, in most cases, as a food grade material, making it acceptable to 
both medical and food production industries. There are three basic types of 
Teflon gasket materials. Virgin Teflon,filled Teflon and expanded Teflon. 
Virgin Teflon is by far the least expensive of the three varieties,and is used 
in most general applications. Filled Teflon® is a gasket material with a virgin. Teflon base to which a substance is added during manufacture to improve the sealing characteristics of Teflon . This "filler" usually is glass or graphite particles although ceramics has sometimes been used. The resulting gasket material is strong, yet highly compressible with very good, though not universal, chemical resistance. 
Expanded Teflon is virgin Teflon to which a stress resistance has been added by means of molecular rearrangement during manufacture. This process ensures that the molecular chains from which the Teflon is constructed, do not arrange themselves into set patterns. This means that expanded Teflon possesses no structural weakness (grain) and expands and contracts equally in all directions. Because no additives are required, expanded Teflon® also possesses the unique chemical resistance of virgin Teflon.