Degradation mechanism of fiber-reinforced plastics and its implications to prediction of long-term behavior

Short-term measurements on fiber reinforced plastics (FRPs) appeared to indicate that their degradation in aqueous media has simple (linear or sublinear) kinetics with Arrhenius-type temperature dependence, permitting easy design of accelerated tests and development of long-term predictive models of FRP degradation. However, upon more prolonged exposure, delayed increases in the rate of degradation were observed. Such non-linear effects greatly complicate predictions of long-term behavior. These effects were postulated to result from interface degradation involving alkali leaching from the fibers, enhanced hydrolytic extraction of monomeric acrylic acid from the matrix, fiber/matrix debonding, microcracking, and ensuing enlargement of the effective area of the composite exposed to further attack, leading to enhanced corrosion. Measurements of fiber dissolution as a function of time demonstrated the existence of accelerated degradation phenomena. The pH at the fiber/matrix interface was shown to rise using an indicator. Hydrolytic depolymerization of the matrix surrounding the fibers was demonstrated using Raman spectroscopy and leachate analysis. Direct evidence of interface cracking and matrix degradation during exposure to aqueous media was gathered using environmental scanning electron microscopy (E-SEM). The effects of porosity as well as of the presence of microcracks were demonstrated by measuring the extent of silica dissolution from the fibers embedded in the FRP before and after the porosity had been altered using various thermal treatments. The results of these measurements were integrated to give a coherent interpretation of the observed delayed increases in degradation rates of FRPs exposed to aqueous environments. (C) 1999 Kluwer Academic Publishers.