Analyses and control of interphase structures and adhesion properties of epoxy resin/epoxy resin for development of CFRP adhesion systems

• Adhesion boundary regions were labelled with deuterated and fluorinated epoxy adhesives. • Boundary regions were identified through confocal Raman scattering. • Boundary regions were distributed into penetration and interphase regions. • Formation of a penetration phase was crucial to the adhesion properties. • Substrate curing temperature and adhesive molecule size impacted penetration thickness and adhesive strength. In the adhesion of carbon fiber reinforced plastics (CFRP), the boundary regions were composed of only epoxy resins of CFRP matrix and adhesives, and their similar epoxy structures pose significant difficulty in studying the adhesion mechanism. Herein, we focused on structure and properties of laminates of epoxy resin substrates and adhesives with different curing conditions of substrates. We prepared laminates using deuterated epoxy adhesives or fluorinated epoxy adhesives, and their boundary regions were identified through confocal Raman scattering measurements. The regions were distributed into “penetration” and “interphase”. In particular, the penetration phase is a key of the adhesion properties. The penetration behaviors strongly depended on the crosslinking densities of the adherends, suggesting that the penetration regions would be directly impacted by the curing conditions of the substrates and molecular size of epoxy adhesive precursors. Our findings provide insights into novel designs of the reliable adhesion-based manufacturing systems of CFRP.