Thermostable heterocyclic polymers

Towards the end of the 50s, the project for an American civil supersonic aircraft flying at mach 3 was at the origin of intense research activity in the field of thermostable polymers. It was in aerospace applications that the weight savings offered by low-density polymers were of greatest interest. The National Aeronautic and Space Administration's (NASA) Apollo project ensured a degree of continuity in research by bringing together the work of industrial groups and universities. But since the early 1980s, the electronics industry as a whole has become the driving force behind R&D work on these high-tech polymers. In the form of films, protective coatings, adhesives and matrices for the manufacture of printed circuits, heterocyclic polymers can be found in all electronics applications, from the manufacture of semiconductors to functional systems (radio, television, radar, etc.).

The thermal stability of macromolecular materials is much lower than that of metals or mineral compounds such as graphite, quartz or ceramics. Thermoplastic polymers, for example, lose their mechanical properties when heated above their melting temperature for crystalline polymers, or their glass transition temperature (t _g ) for amorphous polymers. Like all organic compounds, polymers behave dynamically in relation to heat. This means that thermostability is measured not as a function of temperature alone, but as a function of the time-temperature pair.

By convention, polymers are said to be thermostable if they can be used safely for :

• 30,000 hours at 200 °C ;

• 1,000 hours at 300 °C ;

• 10 hours at 400 °C ;

• or a few minutes at 500°C.

The thermal resistance of organic materials also depends on the atmosphere in which they are to be used. If the atmosphere is inert (vacuum, nitrogen, carbon dioxide, etc.), thermal degradation is purely a pyrolysis phenomenon, and macromolecular chains are broken when the thermal energy supplied is sufficient to rupture the covalent bonds that form the backbone of the polymer. In air or oxygen, on the other hand, oxidation reactions become predominant, and occur at a much lower temperature than that measured during pyrolytic degradation.