PHYSICAL AND CHEMICAL EFFECTS IN AN EPOXY-RESIN EXPOSED TO WATER-VAPOR

A commercial epoxy adhesive based on DGEBA cured with dicyandiamide (DDA) and containing fillers has been aged in water vapour (ca. 100% RH) at various elevated temperatures. Viscoelastic characterisation was effected after ageing for various times up to saturation and after subsequent drying. Young's modulus in the glassy state is reduced after water sorption but returns virtually to its original value after drying, suggesting physical weakening due to plasticisation whereas, in the rubbery state, the observed diminution is only slightly restored on drying. An analysis based on the theory of rubber elasticity leads to the conclusion that chemical degradation is occurring by hydrolysis and chain scission. Gravimetric measurements effected on the polymer exposed to vapour for a fixed time and then dried to (virtual) equilibrium show an increase in weight followed by a decrease and finally a weight loss as exposure time is increased. A theory is proposed to explain the phenomena by initial chemical combination of water followed by leaching of severed chain segments. Calculations of average inter-crosslink molecular weight using both viscoelastic and gravimetric data are in good agreement. It is concluded that long-term exposure of the epoxy adhesive to water leads to both reversible (physical) and irreversible (chemical) degradation of the material.