Surface Modification for Enhanced Lap Shear Strength of the Epoxy-Bonded Joints Consisting of Metallic Adherents and Similar/Dissimilar Materials

Epoxy adhesives play an important role in manufacturing various components and products in industrial and daily applications. By modifying the surface of adherents with poly­(glycidyl methacrylate) (PGMA), the lap shear strengths of the epoxy adhesive-bonded joints were significantly improved. However, the influence of the thickness of the PGMA layer on the reinforcement of lap shear strength remains unclear. Hence, in this study, the PGMA-grafted metallic adherents, e.g., pure aluminum (Al) or stainless steel (SUS), are derived from phosphonic acid/metal surface interaction and amine–epoxy addition reactions, achieving PGMA-modified samples with a similar grafting density and different thickness of polymeric layer. A single-lap shear test on the joints using similar materials (SUS/SUS and Al/Al) demonstrated that the ultimate lap shear strength of the joints using an adherent with the PGMA-based polymeric layer is almost twice stronger than that of the bare adherents when the epoxy resin was utilized as an adhesive. Moreover, the thickness of the PGMA layer exhibits a negative influence on this reinforcement. Meanwhile, the carbon-fiber-reinforced polyetheretherketone (CFR-PEEK) adherents with polymer brushes were surface-initiated photopolymerization of GMA. And the lap shear strength of the joints using dissimilar materials (Al/CFR-PEEK) was significantly enhanced by the surface modification of both metal and CFR-PEEK surface with PGMA.