Several different plastics developed in recent years that maintain mechanical and chemical integrity above 204°C for extended periods are frequently referred to as superpolymers. They are polyimide, polysulfone, polyphenylene sul-fide, polyarylsulfone and aromatic polyester.
In addition to their high-temperature resistance, all these materials have in common high strength and modulus, and excellent resistance to solvents, oils, and corrosive environments. They are also among the highest priced plastics, and a major disadvantage is processing difficulty. Molding temperatures and pressures are extremely high compared with conventional plastics. Some of them, including polyimides and aromatic polyester, are not molded conventionally. Because they do not melt, the molding process is more of a sintering operation. Because of their high price, superpolymers are largely used in specialized applications in the aerospace and nuclear energy field.
Indicative of their high-temperature resistance, the superpolymers have a glass transition temperature well over 260°C as compared to less than 177°C for most conventional plastics. In the case of polyimides, the glass temperature is greater than 427°C, and the material decomposes rather than softens when heated excessively.
Polysulfone has the highest service temperature of any melt-processible thermoplastic. Its flexural modulus stays above 2040 MPa at up to 160°C. At such temperatures it does not discolor or degrade.
Aromatic polyester does not melt, but at 427°C can be made to flow in a nonviscous manner similar to metals. Thus, filled and unfilled forms and parts can be made by hot sintering, high-velocity forging, and plasma spraying. Notable properties are high thermal stability, good strength at 316°C, high thermal conductivity, good wear resistance, and extra-high compressive strength.
