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

A commercial epoxy adhesive based on DGEBA cured with dicyandiamide (D DA) and containing fillers has been aged in water vapour (ca. 100% RH) at various elevated temperatures. Viscoelastic characterisation was e ffected after ageing for various times up to saturation and after subs equent drying. Young's modulus in the glassy state is reduced after wa ter sorption but returns virtually to its original value after drying, suggesting physical weakening due to plasticisation whereas, in the r ubbery state, the observed diminution is only slightly restored on dry ing. An analysis based on the theory of rubber elasticity leads to the conclusion that chemical degradation is occurring by hydrolysis and c hain scission. Gravimetric measurements effected on the polymer expose d to vapour for a fixed time and then dried to (virtual) equilibrium s how an increase in weight followed by a decrease and finally a weight loss as exposure time is increased. A theory is proposed to explain th e phenomena by initial chemical combination of water followed by leach ing 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 a dhesive to water leads to both reversible (physical) and irreversible (chemical) degradation of the material.