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Heat-resistant composite materials based on polypyromelliteimide

Термостойкие композиционные материалы на основе полипиромеллитимида
Polypyromellitimides (PPMI) belong to the class of cross-linked thermoset polyimides with a structure extremely rich in imide cycles; therefore, they exhibit the best temperature characteristics: PPMI molding parts are suited to continuous operation up to 260 °C, and can withstand short-term heating to 482 °C [1]. Comparing PPMI with similar materials, we can see that only a small number of polymers, including polyetheretherketone (PEEK) and polytetrafluoroethylene (PTFE), have a similar performance. However, PPMI far exceeds PEEK and PTFE in such important operational parameter as the heat distortion temperature (HDT), and has much lower coefficient of thermal expansion. This allows the use of PPMI in the areas, where heavy demands are made on precision manufacturing and dimensional stability of molding parts, for example, bushings, bearings, sockets and other tribological parts. Other important features of PPMI are flammability and radiation resistance, which allows PPMI to be used in the structures, where fire resistance (instrument panels and trim cabin aircraft) and radiation resistance (nuclear reactors and other equipment operating under elevated levels of radiation) are required. However, despite the above advantages, PPMI molding parts are used rather rarely due to elaborate manufacturing process (in contrast to PPMI films), and, as a consequence, expensive resulting material: about 1000 dollars per kg (for comparison, PPMI film costs about 65 dollars per kg). The difficulties of molding a fully imidized PPMI due to the following factors: inert powder [2]; a weak ability to soften and flow, i.e. maintaining a very high melt viscosity (108 Pa•s at 400 °C); the structurization processes [3] that hamper effective powder sintering. Therefore, to produce the PPMI parts, commonly, polyamide acid is derived from the solution in the form of thin films, powders, coatings on glass fiber, etc.; then, partial imidization of intermediate products is performed chemically or thermally, and then, they are processed into parts by hot pressing, sintering or other techniques. Partially imidized PPMI powder has a higher activity and ability to soften/flow of fully imidized one; however, there is another problem: the imidization water, which is released in the compacted polymer mass during subsequent high-temperature imidization, activates thermal degradation processes and has a detrimental effect on the product properties. Consequently, the development of new effective methods of production of fully imidized PPMI parts is an important problem. Commercial fully imidizated PPMI molding parts are produced by a few number of companies. Among them, DuPont is the most famous. It produces the PPMI parts for construction and tribological application (Vespel trademark of SP series). The molding parts produced in Russia are based on PI-PR-20 polyimide. The latter is modified linear aromatic polyimide with improved melt flow, which provides excellent sintering and high quality molding parts; however, as compared to the Vespel parts, they exhibit much lower HDT values. The waste of PPMI films production is of particular interest to us as a source of low-cost raw material for fabricating PPMI molding powders with a high degree of imidization and, then, molding parts similar to those of a Vespel SP series. The main objectives of our scientific research is to obtain PPMI powder with a high degree of imidization and it’s modification to improve the efficiency of molding process and improve the quality of the final product.
[1] J.A. Brydson, Plastic materials handbook, seventh ed., Butterworth, Heinemann, 1999, pp. 516–521. [2] D.M. Bigg, A study of the effect of pressure, time, and temperature on high-pressure powder molding, Polymer Engineering & Science. 17 (1977) 691-699. [3] N.A. Adrova, M.I. Bessonov, L.A. Laius, Polyimides - a new class of heat-resistant polymers, Israel Program for Scientific Translations in Jerusalem, 1969.

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