Valve gasket is usually used to prevent two objects from pressure, corrosion, and pipeline natural thermal expansion and contraction leakage. Since machined surfaces cannot be perfect, the irregularities can be filled with gaskets. Gaskets are usually made of sheet materials, such as pad paper, rubber, silicone rubber, metal, cork, felt, neoprene, nitrile rubber, glass fiber or plastic polymer (such as polytetrafluoroethylene). Gaskets for specific applications may contain asbestos.