Gaskets for valves are often used to prevent leakage between two objects from pressure, corrosion, and the natural thermal expansion and contraction of piping. Since machined surfaces cannot be perfect, gaskets are used to fill irregularities. Gaskets are usually made of sheet materials such as paper gaskets, rubber, silicone rubber, metal, cork, felt, neoprene, nitrile rubber, fiberglass, or plastic polymers such as PTFE. Application-specific gaskets may contain asbestos.
