Neutron Reflectometry Study on the Interfacial Layer of Epoxy Resin To Improve Adhesion Strength

The performance of structural adhesives is strongly influenced by the interfacial structure between the adhesive and the adherend. In this study, we developed a nanometrically stable interfacial layer derived from an epoxy resin system containing an excess of hardener. This layer, formed on solid substrates, exhibited excellent structural stability, even after extensive solvent washing. Neutron reflectometry was employed to investigate its structure under both dry and humid conditions, revealing a robust and well-defined interface that suppressed moisture accumulation at the adherend interface. FT-IR spectroscopy confirmed an excess of unreacted amino groups in the stable interfacial layer, enabling further curing reactions with freshly applied epoxy resin. Recoating experiments demonstrated that the interfacial layer remained intact after thermal curing, suggesting strong interfacial compatibility. Notably, the presence of the stable interfacial layer led to a significant enhancement of the adhesive performance. Lap shear tests showed that the adhesion strength of the epoxy resin nearly doubled when the stable interfacial layer was introduced at the interface. This approach does not rely on external coupling agents or primers but instead utilizes a homologous material system to tailor interfacial properties through molecular-level design. The strategy presented here offers a practical and scalable method for improving the adhesion in epoxy systems. By directly engineering the adhesive interface using components intrinsic to the epoxy formulation, this method enables a reliable and enhanced bonding performance without altering the bulk material.