Iclal Avinc Akpinar, Simay Bayramoglu, Salih Akpinar
In materials science, the increasing use of lightweight and multi-material structures has made improving the interfacial bonding characteristics of polymer-based adhesive systems increasingly important. Accordingly, chemical surface activation and interfacial engineering strategies have attracted considerable attention for enhancing polymer–fiber compatibility and adhesion performance. However, the combined effects of fiber content, surface treatment, and orientation on adhesion behavior remain insufficiently understood. In the present study, natural fibers obtained from the rachis part of the palm tree were chemically modified and incorporated into an epoxy adhesive matrix to investigate the effect of surface functionalization on polymer–fiber interfacial adhesion. In the first stage, the effects of fiber ratios (5–20 wt%) and chemical surface treatments (methanol cleaning and methanol +2–6% HNO3) on adhesion behavior were evaluated. Tensile tests showed that specimens treated with methanol cleaning followed by 4% HNO3 oxidation and containing 10 wt% fiber exhibited an approximately 48% increase in failure load compared to neat joints. In the second stage, the influence of fiber orientation (0–90°) was examined using the optimized parameters. The results indicate that interfacial load-transfer capability increased as the fiber orientation approached perpendicular alignment, reaching maximum performance at 90°. Based on SEM observations, nitric acid treatment was found to increase the surface roughness of the fibers and strengthen the polymer–fiber interfacial bond. FTIR, XPS and contact angle measurements suggested the development of oxygen-containing surface functionalities and improved wettability, consistent with enhanced interfacial adhesion. These findings demonstrate that appropriate chemical surface treatment, fiber content, and orientation can effectively enhance the interfacial adhesion and bonding efficiency of epoxy-based adhesive systems, providing practical guidance for the design of high-performance bonded structures.