Variations in mechanical properties of an epoxy adhesive on exposure to warm moisture

The detrimental effects of a humid environment on the mechanical properties of adhesives have been investigated for many years. However, from early studies to recent contributions most of the interest has been focused on the reduction of strength related to plasticity associated with moisture uptake, interfacial weakening, etc. Much less attention has been paid to variations of elastic constants, which influence both the stiffness of the joint and the distribution of stresses. The goal of this study was to measure the effects of a humid and warm environment on tensile strength, Young's modulus and Poisson's ratio of a two-component epoxy adhesive, Henkel Hysol 3425. The measurements have been carried out on bulk specimens of dogbone shape, instrumented with two-grid (axial/transverse) strain gauge rosettes and tested in tension. The conditions of exposure, generated in a climatic cabinet, were 100% relative humidity and 50?°C. To relate the exposure time to the moisture uptake, the weight of the specimens was monitored. It has been noticed that most of the water uptake occurs in the first week of exposure; however, at progressively slower rate, the phenomenon is noticeable almost until the fourth week and then saturation is achieved. Over the same period, the mechanical properties decay as moisture uptake continues; at the end, the loss in strength is about 75% whilst for the elastic moduli the loss is approximately 20%. No clear evidence is found about the Poisson's ratio, which exhibits a non-monotonic behaviour: stable in the early weeks, then increasing and decreasing of a few per cent. In accord with previous works, the behaviour of the mechanical properties seems to be governed by the amount of moisture uptake.