A plastic, film or foam, that maintains stability in high temperature and is itself flame-retardant. Polyimide films are used in CPU heatsink interfaces, and foams are used in insulation and firewall applications. Polyimide is more expensive than PET (films) or fiberglass (insulation), so it is limited to applications where temperature or weight is a critical factor (e.g. on ships - polyimide foam is a U.S. Navy favorite.)