Coupled diffusion/large-deformation behavior of epoxy matrix resin in corrosive environments

Since thermoset resins that contain ester groups (including vinylester, polyester, and anhydride-cured epoxy) are susceptible to hydrolytic degradation, amine curing agents with epoxy are a widely used alternative because they are much more stable against water or alkaline solutions and also allow for curing at lower temperature compared to anhydride. However, amine groups react with inorganic acids to form a salt which can have detrimental effects on the epoxy resin depending on the composition and curing conditions. Experimental data pertaining to diffusion kinetics of sulfuric acid in aminecured epoxy resin is lacking in the literature, but the available studies have shown that it forms a step-like concentration profile, rather than a Fickian-Type gradient, and its mass-change tends to be proportional to the square-root of time, similar to Fickian diffusion. Although usually no cracking or mass-loss is observed, strength and stiffness decrease over time with almost no warning, making it a very dangerous durability problem. The current study examines the unique high-swelling phenomenon of bisphenol-F epoxy (DGEBF) cured by polyamine in hot/wet and low-pH environments. Most studies of epoxy or vinylester matrix resins report swelling between 0.5-3%, within small-strain approximation for coupled diffusion-deformation models. However, considerable swelling of DGEBF/polyamine is observed, reaching a total volume increase over 30% for the most aggressive condition. This creates considerable internal stresses that in turn severely affect the mechanical properties of the resin in the shortterm time scale (i.e., pre-saturation). The step-like concentration profile of sulfuric acid creates a moving swelling front where rubbery and glassy phases are separated by a small transition region; compressive stress is distributed through the swollen layer while the glassy core experiences tensile stress as it constrains some swelling, and the transition layer has some associated shear stress. This affects long-Term strength and overall material performance.