Water Uptake in Polymer Composites with Voids

Abstract

Long-term durability assessment of polymer matrix composite materials exposed to humid air or liquid water requires a firm understanding of the mechanisms governing water uptake and the chemical interactions involved in degradation of the matrix and the fiber/matrix interface. The added mass of water will cause swelling of the matrix, which tends to degrade bonding between fiber and matrix. Voids provide space where water may accumulate, and if they are located at the fiber/matrix interface, the accumulated water may cause hydrolysis. Voids, furthermore, are geometrical irregularities that cause stress concentration, and reduction of strength. Fickian diffusion is considered valid to analyze the moisture uptake in polymers and void-free composites. For composites containing voids, however, deviations from Fickian diffusion are observed. Voids in the form of capillaries along the fibers define flow channels and promote ‘wicking’, which is a rapid water uptake mechanism. Experimental studies on water uptake in polymer matrix composites show a strong dependence of voids. A composite containing even a small void volume fraction will absorb moisture in excess of that contained in the matrix resin. The moisture uptake in a composite immersed in seawater has been modeled using a combination of diffusion of moisture in the matrix resin, combined with the capillary flow in the voids. The predictions qualitatively agree with experiments, but saturation times are off. More detailed characterization of the void structure and improved flow modeling are required to accurately predict the dynamics of water uptake in a composite.