Bonding performance of Chinese fir heartwood and sapwood with different coatings under a high voltage electric field

Inadequate adhesion performance has been observed in the heartwood with high extract content, leading to poor wood durability. To address this issue, we employed a high voltage electric field (HVEF) treatment that combines activation and polarization functions to enhance the bonding strength between wood and the coating layer. The heartwood and sapwood samples were prepared from Chinese fir lumber. Two types of coatings were applied urea formaldehyde (UF) and polydimethylsiloxane (PDMS). The results revealed a higher absorptivity of UF and PDMS on the sapwood compared to the heartwood, attributed to the larger diameter of tracheids and lower extract content, resulting in lower contact angles on the sapwood. Following the HVEF treatment, a reduction in absorptivity was observed for UF on the heartwood, while a decrease in absorptivity was observed for PDMS on the sapwood. This disparity can be attributed to the differential activation and polarization effects of the HVEF treatment on the two types of coatings. The changes in absorptivity were corroborated by the mass gain rate of UF- and PDMS-coated wood samples, with a strong positive correlation observed between the mass gain rate and absorptivity, yielding a correlation coefficient ≥ 79%. The HVEF treatment significantly enhanced the bonding strength of UF-coated heartwood under N-P(-) condition and PDMS-coated sapwood under N-P(+), resulting in the highest increments of 71% and 75%, respectively. These results were attributed to the fact that more activated functional groups recombination of the two coatings, originating from the collision of activated charges with their molecule structures under the HVEF treatment, providing more chances for the chemical reaction with the wood sample at the coating interphase by detecting from the notable variations of chemical bonds and exothermic reaction procedure in the spectra and thermography, especially for the UF-coated heartwood sample under N-P(-) condition.