Experimental and numerical study on water uptake of epoxy resins under high temperature and pressure conditions

This work explores the potential of recycling carbon-fiber–reinforced epoxy com- posites, focusing on the fundamental transport phenomena governing the interaction between water and the polymeric matrix. An experimental campaign was conducted to investigate the water uptake behavior of a commercial epoxy resin (HexFlow® RTM6) under hydrothermal conditions at 100 °C, 150 °C, and 200 °C. Gravimetric measurements were analyzed using analytical solutions of Fick’s second law, both in one-dimensional and cylindrical geometries, to estimate the apparent diffusion coefficients. The results confirmed the thermally activated nature of diffusion: the diffusion coefficient increases signif- icantly with increasing temperature, especially in the jump from 150 to 200, where the increase corresponds to approximately one order of magnitude. At 200 °C, it was also observed that the onset of chemical degradation suggested the occurrence of early-stage solvolysis. To complement the experimental results, a molecular dynamics (MD) approach is proposed to model water diffusion at the atomistic scale in a crosslinked epoxy net- work. The MD framework aimed to reproduce the same temperature and pressure conditions as the experimental tests and obtain a diffusion coefficient value from the post-processing of these simulations that was comparable to the experimental values. This has been deeply useful to elucidate the microscopic mechanisms of water transport—such as hydrogen bonding and free-volume diffusion—that drive macroscopic moisture uptake. The integration of molecular modeling and experimental analysis provides a multi- scale perspective for the optimization of solvolysis-based recycling of epoxy resins.