Shear adhesive strength between epoxy resin and copper surfaces: a density functional theory study

Adhesive strength varies greatly with direction; various adhesion tests have been conducted. In this study, the shear and tensile adhesive strength of epoxy resin for copper (Cu) and copper oxide (Cu₂O) surfaces were estimated based on quantum chemical calculations. Here, density functional theory (DFT) calculations with dispersion correction were used. In the tensile process, the entire epoxy resin is peeled off vertically, whereas in the shear process, a force parallel to the adhesive surface is applied. Then, a bending moment acts on the adhesive layer, and a total force (stress) inclined at an angle θ with respect to the adherend surface is applied to the adhesive interface. We computed adhesive stress-displacement curves for each θ exhaustively and discussed the changes. When θ equals 90°, it corresponds to a tensile process. As θ decreases from 90°, the shear adhesive stress on both surfaces decreases slowly. When θ is less than 30°, the constraint to the surface causes periodic changes in the adhesive stress curves. The constraint to the Cu₂O surface is especially strong, and this change is large. This periodicity is similar to the stick-slip phenomenon in tribology. To further understand the shear adhesive forces, force decomposition analysis was performed, revealing that the periodicity of the adhesive stress originates from the DFT contribution rather than the dispersion one. The procedure proposed in this study for estimating shear adhesive strength is expected to be useful in the evaluation and prediction of adhesive and adherend properties.