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.