Accelerated aging versus realistic aging in aerospace composite materials. II. Chemistry of thermal aging in a structural composite

Samples of an aerospace structural epoxy composite (8552/IM7) were subject to long-term (approximate to 1 year) thermal aging at temperatures of 70 degrees, 120 degrees, 170 degrees, and 200 degrees C (in air). The changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques, including gravimetric analysis, Fourier transform infrared (FTIR), and dynamic mechanical analysis (DMA) to compare the effects of different severities of degradation treatment. The results highlighted the large differences in chemical effects between the surface and the interior of the composite with very minor changes in the latter even at quite high aging temperatures and long aging times. The oxidative changes at the surface, however, varied from highly selective molecular changes for particular chemical groups at the lower aging temperatures (70 degrees and 120 degrees C), to quite general and extensive oxidative degradation at the higher aging temperatures (170 degrees and 200 degrees C). The results indicated that the mechanical changes in an aged composite of this type will vary greatly with the material thickness and surface protection as well as the aging temperature the composite is exposed to. (c) 2006 Wiley Periodicals, Inc.