ENVIRONMENTAL AGING OF HIGH-PERFORMANCE POLYMERIC COMPOSITES - EFFECTS ON DURABILITY

The effect of sub-T-g environmental aging on the durability of two high-performance polymeric composites has been investigated. The material systems under study were a thermoplastic-toughened cyanate ester resin (Fiberite 954-2) and a semicrystalline thermoplastic resin (Fiberite ITX), and their respective carbon-fiber composites, IM8/954-2 and IM8/ITX. Specimens were aged for periods of up to 9 months in environmental chambers at 150 degrees C and in one of three different gas environments: nitrogen, a reduced air pressure of 13 . 8 kPa (2 psi air) or atmospheric ambient air (14 . 7 psi air). The glass transition temperatures, T-g, of the two resin systems were monitored as a function of aging time and environment. The changes in T-g showed effects of both physical aging and chemical degradation; the latter appeared to be sensitive to the oxygen concentration in the aging environment. Flexure rests were performed on 8-ply unidirectional (90 degrees) IM8/954-2 and IM8/ITX composites, aged up to 6 months in the three gas environments at 150 degrees C. The samples showed a 30-40% loss in the bending strength after aging. These strength reductions were sensitive to the oxygen concentrations in the aging environment. Stress/strain tests were also conducted on the same composites to measure the ultimate properties of the materials before and after aging in the three different environments at 150 degrees C. The results showed a decrease of 40-60% in the ultimate strain to failure with aging. The modulus of both composite systems on the other hand increased by up to 20% after aging for 6 months, possibly as a consequence of the physical aging phenomena. In both systems greatest reduction in 'useful' mechanical properties occurred in the ambient air environment, while the least reduction occurred in nitrogen. Weight loss in the plain resin and composite samples was monitored as a function of aging time and environment Typically, all of the samples showed 1-2% weight loss after 9 months of aging at 150 degrees C, and the composite samples lost much more weight (on a polymer basis) than unreinforced resin specimens over the same aging period. The weight loss data as well as all the above-mentioned observations were indicative of an oxidation process in the composites.