The effect of hydration–dehydration cycling on the properties of a rubber modified, mineral filled epoxy resin is reported. This resin was based on diglycidyl ether of bisphenol A and cured with dicyandiamide. The material was aged in deionized water and a 5% (w/w) NaCl solution at 65°C. Gravimetric measurements, transmission electron microscopy, and diffusive reflectance IR Fourier transform spectroscopy were used to monitor the water sorption and physical and chemical changes occurring in the material. Two-stage absorption was observed with deionized water, and the quasiequilibrium-state water content was independent of the hygrothermal history. The equilibrium weight after dehydration depended on the hygrothermal history due to the presence of irreversibly absorbed water and leaching of material. The first stage of the absorption was found to be Fickian and was interpreted as being related to the process of saturation of the epoxy matrix. The driving force for the second stage was the osmotic pressure, and it was not observed when the material was aged in NaCl solution. The water caused irreversible damage to the resin through microcavity formation, and part of the water was molecularly dispersed in the epoxy matrix and part resided in microcavities. Absorption–desorption cycling resulted in an increased rate of absorption during subsequent rehydration. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3468–3476, 2001
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