Hydrolytic Degradation and Its Effect on Mechanical Properties of HFPE-II-52 Polyimide: Preliminary Results
Polyimides and fiber reinforced polyimide matrix composites are used in demanding applications requiring mechanical performance at high temperatures (300 + °C). When exposed to moisture and elevated temperature for extended periods of time polyimides may
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Hydrolytic Degradation and Its Effect on Mechanical Properties of HFPE-II-52 Polyimide: Preliminary Results Yi Xu and Alan T. Zehnder Abstract Polyimides and fiber reinforced polyimide matrix composites are used in demanding applications requiring mechanical performance at high temperatures (300 + C). When exposed to moisture and elevated temperature for extended periods of time polyimides may undergo hydrolytic degradation in which bonds are broken at a rate dependent on the temperature and moisture absorbed into the material. These broken bonds will be reflected in reductions in mechanical properties such as moduli, glass transition temperature and flow strength. In this project the polyimide HFPE-II-52 is aged under fully moisture saturated conditions at temperatures up to 250 C. Following the temperature and moisture exposure, cube shaped samples are tested in compression at room and elevated temperatures to measure the reductions in stiffness and yield stress. This research is aimed at providing a means for monitoring and predicting hydrolytic degradation and its effect on mechanical performance. Keywords Polyimide • Hydrolysis • Damage • High temperature • Mechanical behavior
9.1
Introduction
Polyimide matrix composites (PiMCs) extend the role of traditional polymer matrix composites (PMCs) to high temperatures due to their high glass transition temperature (Tg around 350 C), toughness and stiffness. However, polyimides or PiMCs, when exposed to hot, high humidity environments, can suffer significant degradation and can fail by mechanisms such as blistering and delamination [1, 2]. One of the underlying mechanisms of moisture degradation is hydrolysis, or breaking of bonds due to reaction with water. Hydrolysis can be reflected in reductions of mechanical properties such as elastic moduli and flow strength. In early work on this topic the tensile strength of Kapton polyimide film immersed in water at temperatures from 20 to 100 C was shown to decrease as a function of immersion time [3]. In subsequent studies the degradation temperature was extended beyond 100 C by using a heated, sealed pressure chamber partially filled with water. When heated, the environment equilibrates to at the steam saturation pressure. Studies of the timetemperature threshold of hydrolysis have been performed for the polyimides AFR700B, PETI-5, K3B and others [4, 5]. In this study, we apply the pressure chamber method to investigate the hydrolytic degradation of the polyimide HFPE-II52. We expose the polyimide samples to fully saturated conditions at temperatures up to 250 C, and for time spans from several hours to a week. The exposed samples are then tested in compression at both room and high temperature (285 C) to probe the resulting changes in moduli and strength. This is a work in progress, but preliminary results are given here and discussed.
9.2 9.2.1
Experimental Method Sample Preparation
The samples used for hydrolytic degradation tests were manufactured by compression molding of HFPE-II-52 polyimide powd
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