Gaskets are mechanical seals between two objects, usually used to prevent pressure, corrosion, and natural thermal expansion, contraction, and leakage of pipelines between two objects. Due to the impossibility of perfect machined surfaces, the use of gaskets can fill in the irregularity. Gaskets are usually made of sheet-like materials, such as paper, rubber, silicone rubber, metal, cork, felt, chloroprene rubber, nitrile rubber, glass fiber, or plastic polymers (such as polytetrafluoroethylene). Gaskets for specific applications may contain asbestos.