Experimental investigation of moisture penetration in adhesive layers under varying bonding conditions using near-infrared spectroscopy

The mechanical properties of adhesive joints are affected by the moisture content, making it necessary to understand the moisture diffusion behavior in the adhesive layer. The diffusion coefficient of the bulk material is generally used in the numerical calculation of the penetration distance; however, the joint strength often decreases beyond the expected value. Therefore, it is believed that moisture penetrates much faster in the adhesive layer than in the bulk, although there have been no direct observations of this behavior. In this study, the effects of the adhesive thickness and surface wettability of the substrate on moisture penetration into the layer were investigated. Near-infrared spectroscopy was applied to non-destructively reveal the in-plane moisture distribution in the adhesive layer. The moisture distribution in the adhesive layer sandwiched between quartz glasses was monitored for six weeks by immersing it in room-temperature water. Two adhesive layers with different thicknesses (0.3 and 0.1 mm) were prepared, and it was revealed that the thinner the adhesive layer, the faster the moisture diffusion. A numerical analysis based on Fick's law showed that the diffusion coefficient of a 0.1 mm-thick adhesive layer was approximately 1.6 times greater than that of the bulk. In addition, the different surface wettability resulted in a change in the diffusion behavior at the corners of the adhesive layer, although there was little difference in the case of unidirectional penetration.