High-voltage electric field-induced decreased absorption efficiency of phenol formaldehyde adhesive in different sections of Chinese fir wood

High-voltage electric field (HVEF) treatment is one of the most effective methods to improve the bonding strength of wood-based composites. In order to further investigate the adhesion mechanism during HVEF treatment, the adhesive reaction and dynamic wetting behavior of wood in different sections were investigated in detail based on the wetting model. Chinese fir samples with radial, tangential and cross sections and phenol formaldehyde (PF) adhesive were selected as test materials. The study of conductivity, zeta potential, FTIR and dissolved oxygen variations showed the production of polarized molecules, dipoles and oxygen-containing chemical bonds in the PF adhesive leading to extensive curing reactions. The average adhesive absorptivity, its decrease rate (K_a) and initial slope (IS_a0) of the wetting model decreased regardless of the changes in the contact angles and its related parameters (K_θ, IS_θ0). The order of decrease in adhesive absorptivity of different wood sections was as follows: tangential section (16.23%) < cross section (18.60%) < radial section (21.20%). For the radial-combined composites, the highest increment of bonding strength was obtained, which was consistent with the previous study and the relationship between absorptivity and its bonding strength was significantly correlated (R² = 82%). However, higher dispersion degree of absorptivity was observed for longitudinal sections, which is attributed to the type and size of pores among wood sections.