Diffusion of water into an epoxy adhesive: comparison between bulk behaviour and adhesive joints

摘要

The diffusion of water into a bulk epoxy adhesive at 70℃ under ～ 100% relative humidity has been studied by classic gravimetric analysis. As diffusion progresses, the elastic modulus decreases. This same effect has also been observed with torsional adhesive joints but the phenomenon seems to occur more rapidly. Since gravimetric analysis is not feasible with practical bonded joints, a 'composite' model has been developed in which water ingress may be estimated by changes in overall elastic behaviour of the polymer. Elastic moduli are due to a combination of rigidities resulting from the relative fractions of 'wet' and 'dry' polymer under load. The composite model gives values of the coefficient of diffusion, D, for the bulk adhesive in satisfactory agreement with those obtained by gravimetric analysis. However, the value of D for torsional joints is considerably greater. It is proposed that a phenomenon of 'capillary diffusion' exacerbates water ingress. Surface tension effects near the metal(oxides)-polymer interfacial region increase the effective driving force for water penetration.