The effect of high voltage electrostatic field (HVEF) treatment on bonding interphase characteristics among different wood sections of Masson pine (Pinus massoniana Lamb.)

Abstract High voltage electrostatic field (HVEF) treatment has been investigated as an optimization method for enhancing the bonding performance of wood via increasing its polarization degree and improvement of the penetration of phenol formaldehyde (PF) adhesive. As the wood surfaces from cross cut (C), radial cut (R) and tangential cut (T) behave differently, five cut combinations formed the samples to be tested, namely C-C, R-R, R-T, T-T (always parallel to grain) and T-T⊥, where the grains were perpendicular to each other. The gluing and HVEF treatments were performed simultaneously. The sample surfaces were characterized by electron spin resonance (ESR) spectroscopy, dynamic contact angle (CAdyn) measurements, X-ray densitometry, fluorescence microscopy, Fourier-transform infrared (FTIR) spectroscopy and measurements of compression shear bonding strength (CSBS). An increased surface energy led to decreased CAdynS in the following order: cross section<tangential section<radial section. Obviously, the triggered free electrons of the HVEF treatments changed the wood surfaces. The penetration depth of PF into wood cell decreased significantly and the maximal density increased after the HVEF treatment. The lower CAdyns also contributed to the better reaction of the wood surface with the PF resin. The CSBS of the five sample combinations was enhanced owing to a better performance of adhesive aggregation, which was increased by 18% (C-C), 24% (T-T), 26% (T-T⊥), 31% (R-T) and 42% (R-R), respectively. Pore size and pore size distribution contributed a lot to the bonding properties of HVEF-treated wood sections.