Development of a Low Toxicity PMR-type Polyimide for Advanced Composite Applications

The thermosetting polyimide, PMR-15, is the leading contender for use in polymer matrix composites intended for high temperature applications. Although the properties of PMR-15 are generally good, its widespread use in aircraft and aeroengines has been hampered by a number of drawbacks. Among these are microcracking due to thermal cycling, irreproducibility of processing and the toxicity of the methylene diamine comonomer. The aim of the work reported here was to develop a PMR-type polyimide with properties equivalent to conventional PMR-15 but with none of the drawbacks. A key target of the work was to reduce the hazards associated with the use of methylene dianiline (MDA). Consequently, much of the work involved screening formulations in which MDA was replaced by a diamine of reduced toxicity. One formulation, coded B1, has been identified of which initial results look extremely promising. This formulation uses a partially fluorinated, four-ring diamine of low toxicity. This material was synthesized in-house and subsequently scaled-up in a semi-commercial process. Prepreg was manufactured on commercial equipment. Cure cycles were developed using mechanical spectrometry (RDS). Laminates were made by autoclave molding, and mechanical properties before and after high temperature ageing were determined. Preliminary results show that the B1 formulation compares favorably with conventional PMR-15.