The most common seal material used for ball valves is pure PTFE (high chemical resistance), or PTFE with glass fibre filler (increased abrasion resistance), graphite (high temperature resistance) and even stainless steel (abrasion and high temperature resistance). There are also seals made of PEEK (high strength and temperature resistance), and PA or POM used in valves for hydraulic oils. Specialized ball valves come with pure graphite or all-metal seals.
Concentric butterfly valves and diaphragm valves are sealed with NBR, EPDM, FKM and VMQ elastomer seals or plastic seals such as PTFE bonded to an elastomer.
Eccentric butterfly valves, on the other hand, often have metal (lamellar) seals, or metal and PTFE+graphite laminated seals.
In industrial poppet valves and gate valves, a metal-to-metal seal is often implemented, where the closing element (poppet, gate), seals on a metal seat. Such a combination, however, may not ensure 100% valve tightness.