Evaluation of thermal ageing of a carbon fibre reinforced bismalemide

Polyimides and bismaleimides are studied widely, particularly in polymer-matrix composites, for their thermal and mechanical behaviour. These studies have concentrated on the mechanism of thermo-oxidative degradation and the extent of microcrack formation, material loss and the accompanying deterioration in mechanical properties with thermal ageing. Specimen size, fibre orientation, the ratio of cut surface to total-surface area of specimens are also considered in composite materials. This paper examines the thermal characteristics of T300 plain-weave carbon fibre fabric reinforced Cycom 5250-4 bismaleimide resin laminates. The thermal ageing process involves placement of cut specimens in air-circulating ovens set at 210, 230 and 250 °C for various periods of time up to 2000 h. The results indicate weight-loss fluxes of approximately 3.7 × 10−5 mg/(mm2 h) for 210 and 230 °C, and 6.3 × 10−5 mg/(mm2 h) for 250 °C up to 500 h ageing time. It is calculated that the degradation process requires activation energy in the range of 151–270 kJ/mol, depending on the type of gaseous environment used and the level of degradation reached. Over the conditions of thermal ageing considered, the flexural and the interlaminar shear strengths deteriorate by a factor of 2 as a result of matrix loss and microcrack formation. The ageing process also produces postcuring of the matrix influencing mechanical properties at high temperatures.