The effect of hydroxy-methylated-resorcinol treated veneer on bonding interface and bonding strength of carbon fiber/poplar veneer composite

Limited comprehension of differences between tight and loose poplar veneer surfaces affected the bonding performance of carbon fiber (CF)/poplar (P) veneer composites. This study assessed the impact of veneer contact angle variation and hydroxy-methylated resorcinol (HMR) coupling agent treatment on the bonding interface and strength of CF/P veneer composites. Four kinds of epoxy resin-based 3-ply CF/P composite were created: P surface/CF/P surface (SCS), P back/CF/P back (BCB), HMR-treated P surface/CF/HMR-treated P surface (HSCS), and HMR-treated P back/CF/HMR-treated P back (HBCB). Tight surface contact angle was bigger than loose surface angles, and the wettability of loose surface is better than that of tight surface. HMR treatment enhanced epoxy wettability on both sides of veneer, minimizing differences between both sides. After-HMR treatment of veneer, the bonding strength of HSCS group increased notably with 77% higher dry strength and 80% higher wet strength compared to SCS and the peeling rate of HSCS and HBCB was less than 5%. Meanwhile, HMR treatment on veneer effectively balanced epoxy adhesive distribution in bonding interface of CF/P veneer composites and obviously minimized the difference of bondline thickness of the CF/P composite caused by difference between the surface and back of veneer enhancing the bonding strength of the composite.