The adhesion strength between epoxy resin adhesive and polymer adherends was found to be almost constant from 2.3 mN/m to 15.3 mN/m of surface free energy region. It increases linearly in the region above 15.3 mN/m. These results were obtained by using polymer adherends and low surface free energy adherends such as PTFE particle dispersion coated adherend. However, the adhesion mechanism that explains this experimental result remains unclear. The reason for this is that adherends with different surface free energies have different elastic moduli and Poisson’s ratios, making it difficult to explain the relationship between adhesion strength and surface free energies in a unified manner.

In this study, the authors regard the epoxy resin adhesive and polymer adherend as mono material with the average physical properties between epoxy resin and polymers to apply Griffith’s theory to this mono material. In the lower surface free energy region, the adhesion strength is reduced because of poor wettability of epoxy resin to the polymer. The authors revised the Griffith formula taking into the account the wettability factor. This theoretical formula agrees with the experimental value in the region where the surface free energy is more than 15.3 mN/m. The adhesion strength when the surface free energy is 2.3 mN/m can be explained from the fact that the adherend surface is a PTFE particle dispersed material.