Hygrothermal Properties of Highly Toughened Epoxy Adhesives

Abstract

The absorption and desorption of water in two different rubber-toughened epoxy adhesives was measured gravimetrically over a relatively wide range of temperature and relative humidity (RH). The data were fitted to a new diffusion model in which Fick's law was assumed to act in two sequential stages, each with its own diffusion coefficient and saturated water concentration. This “sequential dual Fickian” (SDF) model and a Langmuir-type diffusion model were both able to model the absorption behaviour. The dependence of the five SDF model parameters on temperature and RH was investigated in detail. The two diffusion coefficients were found to be largely independent of RH, while the fractional mass uptake values for each stage increased with RH. The absorption temperature only had a significant effect on the diffusion coefficient of the first stage and the fractional mass uptake of the second stage. Water desorption from the two epoxies was modeled accurately using Fick's law. A significant difference was observed between the amounts of retained water in the two adhesives after drying. The results can be used to predict the water concentration distribution in adhesive joints exposed to environments of changing temperature and RH.