Spatially resolved measurements of the diffusion of water in a model adhesive/silicon lap joint using FTIR-transmission-microscopy

The diffusion of D2O in adhesive joints was measured by spatially resolved FTIR-transmission spectroscopy. This method was developed to model the local uptake of water in technical metal adhesive joints. Silicon wafers were used as IR transparent model substrates and a two component epoxy amine system as an adhesive. The D2O uptake was measured as a function of the distance to the edge of the joint, the exposure to the D2O atmosphere and the temperature. The temperatures during exposure were varied below and above the glass transition temperature of the adhesive. It could be shown that the water diffusion strictly follows Fickʹs second law. The measured diffusion constants were constantly similar but slightly lower than those obtained in the literature by techniques that study the swelling of adhesives as free standing films or as supported films. This shows that the special structure of an adhesive layer between two solid substrates can increase the mobility of water molecules. By varying the exposure temperature, the temperature dependency of the diffusion coefficient is given by a straight line in the Arrhenius plot, thus an activation energy for the diffusion of about 72 kJ/mol could be calculated. Remarkably, the temperature dependency traversed smoothly the glass transition temperature of the adhesive without any discontinuities. The developed measurement technique is of high interest for the study of the influence of additives and surface modifications on the diffusion kinetics of water in real adhesive joints.