Bonding behavior of urea-formaldehyde adhesive with wood tangential section under a high-voltage electric field

Wood composites often exhibit weak interaction between urea-formaldehyde (UF) and wood, leading to edge warping and cracks at the bonding interphase. To address this issue, the high-voltage electric field (HVEF) has been effectively employed to enhance the bonding strength of wood composites. This study aims to further investigate the bonding process and the mechanism of bonding enhancement under HVEF treatment by examining the physicochemical characteristics, curing reaction of UF adhesive, and dynamic wetting behavior on the wood tangential section. The chemical reaction and bonding strength at the bonding interphase were also examined. The results revealed that the HVEF treatment increased the polarization extent of the adhesive molecules, resulting in variations in the physical properties of the adhesive. Particularly, a higher enthalpy during the adhesive curing reaction was observed, with an 85% increment observed under the N-P(- ) condition. The FTIR spectra showed an increased peak intensity of UF main chemical bonds and a higher extent of chemical interaction between wood and UF functional bonds under HVEF treatment. These findings can be attributed to higher degrees of molecule activation and cross-linking between wood and UF. Furthermore, the HVEF treatment led to a decreased absorption ratio of UF and an increased wet/dry wood bonding strength of the two-layer composite. The absorption ratio of UF exhibited the highest decrement of 36%, while the wet/dry wood bonding strength showed the highest increment of 85%/100% under the N-P(- ) condition. This study emphasizes the significant role of higher extents of UF polarization, activation, and cross-linking with wood in enhancing the bonding strength of HVEF-treated wood composites.