Effect of rapid interfacial moisture penetration on moisture distribution within adhesive layers

• Modeled 2D moisture diffusion in adhesive layers with distinct coefficients via FDM. • Determined interfacial diffusion coefficients and analyzed thickness distributions. • Investigated thickness-wise moisture distribution through numerical calculations. In this study, we numerically calculated the moisture distribution through the thickness of the adhesive layer in an adhesive joint and considered rapid interfacial moisture diffusion. Additionally, the effects of the diffusion coefficient and layer thickness were investigated. An accurate understanding of moisture diffusion in adhesive joints is essential for enhancing their long-term performance and reliability, particularly in applications demanding high environmental durability. However, the moisture behavior near interfaces, which causes faster moisture penetration into the adhesive layer than into the bulk is still not clearly understood. In this study, a model was developed using the finite difference method, considering both bulk and interfacial diffusion properties. The diffusion coefficient near the interface was back-calculated using the experimentally measured moisture distribution in the adhesive layer, which revealed that the moisture distribution in the thickness direction was almost uniform for thin adhesive layers, and parabolic distribution became more pronounced as the adhesive layer became thicker.