Non-classical water diffusion in an industrial adhesive

Experimental studies were performed on an industrial rubber toughened epoxy adhesive to characterise its diffusion behaviour in water at various temperatures. Gravimetric experiments and density measurements were carried out at 25, 40, 60 and 70 °C on bulk specimens. Non-classical mass uptake and swelling were observed. Differential scanning calorimetry and surface fracture analyses were undertaken to understand this specific behaviour. Experiments showed that adhesive glass transition was crossed during aging, and that cavities developed during sorption. Part of the water diffused in the matrix causing the decrease of the adhesive Tg while the other part entered into cavities. To model the water uptake and swelling of the adhesive, it had been chosen to separate the water mass fraction into two parts. A thermo-diffuso-mechanical modelling based on a thermodynamic framework was proposed to solve the diffusion problem. The model was implemented in a user subroutine for ABAQUSTM and a 2D finite element modelling was designed to obtain the simulated diffusion behaviour of the adhesive. Mass uptake and swelling were simulated for different aging conditions and confronted to experimental results. Although a slight deviation was observed between experimental and modelled swellings at 60 and 70 °C, numerical results were satisfactory at mass uptake at 25, 40, 60 and 70 °C so confirming previous hypotheses.