The effect of different hygrothermal aging conditions on the glass transition temperature of a six-ply quartz-fiber-reinforced bismaleimide composite is investigated via dynamic mechanical analysis. Bismaleimide is a polymer matrix material suitable for high-temperature structural and electrical applications such as the radar-protecting structure on supersonic aircraft. This particular material is usually subjected to water absorption due to humid air, precipitation, condensation, and accumulation of water in the interior of its constituent structure. In the fully dry, un-aged condition, the glass transition temperature of the laminate is approximately 380°C. Hygrothermal aging conditions are simulated by full-immersion of laminate specimens in distilled water ranging from 30 to 200 days at 25, 40, 60, 75, and 90°C. Specimens immersed at 40°C for 200 days showed the largest depression in glass transition temperature, to approximately 365°C. A subtle, secondary transition marked by a depression and recovery in storage modulus is consistently present in specimens exposed to 75 and 90°C conditions, independent of total immersion time, near 260°C. However, the specimens aged at 75 and 90°C did not exhibit a significant decrease in glass transition temperature as expected. Results indicate variations in glass transition temperature as a result of water absorption in BMI/quartz laminates are dependent on hygrothermal aging history, rather than solely a function of moisture content.
