Water absorption by an epoxy resin and its effect on the mechanical properties and infra-red spectra

A filled epoxy resin used as a structural adhesive and based on the diglycidyl ether of bisphenol A cured with dicyandiamide has been subjected, in its bulk form, to ageing at 40, 55 and 70°C and ca. 100% relative humidity. Gravimetric, viscoelastometric and FTi.r. studies have been effected after various times of exposure. Water absorption in the polymer is essentially Fickian, although closer inspection reveals the finer behaviour to be sigmoidal. The activation energy for diffusion is of the order of 80 kJ mol⁻¹, but there appears to be no clear relationship between equilibrium absorption values and temperature. Viscoelastometry has shown that T_g diminishes from ca. 115°C before ageing to ca. 90°C at saturation, 1% of water uptake corresponding to ca. 8°C reduction in T_g. Reductions in Young's modulus were observed both in the glassy and rubbery states after ageing and the latter is associated with molecular chain scission. FTi.r. analysis has shown several modifications occurring due to water absorption, the main one being an increase in intensity of the band at 1740 cm⁻¹. It is concluded that water absorption leads both to plasticization effects and chemical modification of the epoxy resin.