Characterisation of moisture diffusion and strength degradation in an epoxy-based structural adhesive considering a post-curing process

Adhesive bonding techniques have been widely used in many industrial fields, such as aerospace, vehicle and civil engineering, etc., while the bonded structures are designed to perform satisfactorily under complex service conditions. This paper presented an experimental insight into the moisture diffusion process in Araldite 2015 adhesive considering the effect of post-curing process under elevated temperature. Polytetrafluoroethylene mould was adopted for the curing process of dumbbell adhesive specimens and both distilled and salt water were introduced to simulate a severe immersion environment. Quasi-static testing was conducted on both unaged and aged bulk adhesive after certain period of immersion to reveal the mechanical properties variation. Measurements on specimen weight and thickness were performed with different time intervals during the immersion process. It is shown that the strengths of bulk adhesive were significantly degraded due to the presence of water in both immersion conditions, where higher moisture content led to greater strength degradation regardless of the curing and ageing conditions applied. In addition, distilled water environment led to notably higher saturation content and hygroscopic swelling compared to salt water. However, the rates of water diffusion and hygroscopic swelling processes in salt water were detected to be far above the ones in distilled water. It is worth noting that slight increase was observed in moisture saturation content and swelling expansion for post-cured adhesive, while the coefficients of hygroscopic swelling showed no difference for all sample groups studied. Scanning Electron Microscopy observation further revealed the transition in microscopic topography from unaged to saturated samples.