Moisture Degradation Effects on the Mechanical Properties of HFPE-II-52 Polyimide: Experiments and Modeling

Polyimide resins are widely used as a matrix for high temperature fiber reinforced composites. However, when operated or stored in a hot wet environment, polyimides may undergo significant moisture degradation, greatly reducing the mechanical performance of the polyimide or polyimide matrix composite. The mechanisms of moisture degradation explored in this paper are twofold. First, hydrolytic degradation may occur, in which chemical bonds break at a rate that depends on temperature and moisture level. Hydrolytic degradation will be reflected in permanent decreases to the stiffness and strength of the material. Second, water vapor can act as a plasticizer at high temperature and cause instantaneous but reversible reduction of mechanical properties. A set of experiments designed to study the hydrolytic degradation and plasticization of the polyimide HFPE-II-52 are described here. The experiments consist of exposing the polyimide to high temperature, moisture saturated conditions over a range of times and temperatures. Following moisture exposure, compression tests are performed to measure the reductions of stiffness and yield stress. Temperature and moisture dependent kinetic models are developed and fit to the data. These experiments and models aim to provide a means for predicting the onset and rate of moisture degradation and its effect on the mechanical performance of polyimides and polyimide matrix composite laminates.