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Polyimide (sometimes abbreviated PI) is a polymer of imide monomers. The structure of imide is as shown.
Thermosetting polyimides are commercially available as uncured resins, stock shapes, thin sheets, laminates and machines parts. Thermoplastic polyimides are very often called pseudothermoplastic. There are two general types of polyimides. One type, so-called linear polyimides, are made by combining imides into long chains. Aromatic heterocyclic polyimides are the other usual kind, where R′ and R″ are two carbon atoms of an aromatic ring. Examples of polyimide films include Apical, Kapton and Kaptrex. Polyimide parts and shapes include Meldin, Vespel and Plavis. Polyimides have been in mass production since 1955.