Accelerated ageing versus realistic ageing in aerospace composite materials. III. The chemistry of thermal ageing in bismaleimide based composites

Samples of two Aerospace grade bismaleimide composites (Cytec Fiberite 5250-4/IM7 and 5250-4/ RTM) were subject to long-term (approximate to 1-year) thermal ageing in air at temperatures of 70, 120, 170, 200, and 250 degrees C. The changes to the chemical and physicochernical structure of the composite were then analyzed by a range of different techniques, including gravimetric analysis, Fourier Transform infrared spectroscopy, and dynamic mechanical analysis, to compare the effects of the different ageing temperatures and to see if much simpler and more fundamental testing techniques could provide more informative forecasting information than the standard mechanical testing methods. The results emphasize the strong variation in chemistry that takes place between the surface and the interior of the composite rnaterials at all temperatures tested. The results also confirmed the significant variations in chemical and physicochemical ageing mechanisms (e.g. glass transition temperature, that occur between the more realistic ageing temperatures encountered in service (similar to 120 degrees C) and the accelerated ageing conditions often used for ageing studies (> 170 degrees C). This article highlights the lack of agreement in scientific literature on the basic chemistry of bismaleimide cure. (c) 2007 Wiley Periodicals, Inc.