Influence of high voltage electrostatic field (HVEF) on bonding characteristics of Masson (Pinus massoniana Lamb.) veneer composites

For the first time, it was experimented with high voltage electrostatic field (HVEF) for the optimization of the bonding process of urea formaldehyde (UF) resin to wood. Varying HVEF treatment times (0-8min) and intensities (5-20kV) were applied. Wood surface characteristics, polymerization extent of UF, and bonding interphase of UF to wood were examined by electron spin resonance (ESR) spectroscopy, dynamic contact angle goniometers, Fourier transform infrared spectroscopy (FTIR), X-ray densitometer, fluorescence microscope and bonding strength measurement. The results indicated that HVEF treatments applied to wood veneer induced a decrease in contact angles and an increase in the free radicals of wood, and surface energy varied with treatment times and intensities. These varieties were because triggered free electrons, broken chemical bonds and new ions appeared on the wood surface with HVEF treatment. An aggregated performance was obviously obtained in micrographs and vertical density profiles (VPDs). It was explained that resin penetration into wood structure was significantly shallow, and resin concentration and density at bonding interphase were more enhanced than the control. This is due to the increment of surface energy and lower contact angles providing more chances for possible reactions of wood surface with the UF resin. Resin penetration depth and density varied with different treatment times and intensities. Increased cross-linking extent of UF was obtained under varied conditions of HVEF treatment. The bonding strength was measured as a function of treatment times and intensities respectively; it increased by 85% and the wood failure ratio significantly increased by 70% under HVEF treatment (20kV, 8min) due to the aggregation performance and more cross-linked UF resin. It was manifested that both treatment time and intensity play a significant role in HVEF treatment, and HVEF treatment has a significant influence on the characteristics of the wood surface and the bonding properties of wood composites.