Modelling anomalous moisture uptake, swelling and thermal characteristics of a rubber toughened epoxy adhesive

Abstract

Experimental studies have been undertaken to characterise the diffusion, swelling and thermal characteristics of a rubber toughened epoxy adhesive. Gravimetric experiments were carried out for three different thicknesses of bulk adhesive. The bulk adhesives were exposed to 81.5%, 95.8% RH and water immersion, all at 50°C. Dual stage moisture uptake profiles were observed for thin specimens at all levels of exposure. It was found that a dual stage uptake model fits the anomalous uptake response excellently. The swelling of the adhesive was characterised using the same exposure environment. Swelling was slow at low levels of moisture uptake and increased gradually before becoming linear with increasing moisture uptake. The initial slow swelling of the adhesive was attributed to the diffusion of water into the existing free volume. A moisture-dependent swelling coefficient was determined using finite element modelling, where the dual stage uptake model and moisture-dependent mechanical properties of the adhesive were taken into account. The thermal expansion of the bulk adhesive was determined using a bi-material curved beam specimen. The curvature of the beam reduced when it was exposed to a wet environment. This behaviour was successfully modelled using coupled moisture diffusion and stress analysis incorporating the derived moisture-dependent swelling behaviour.