Surface Chemistry and Adhesive Bonding of Tannin-rich Woods to a Poly(vinyl acetate) / Linseed Oil Coating: The Role of Thermal Modification

Abstract

Effects of thermal modification were investigated relative to the surface and adhesive bonding of coatings to tannin-rich oak (Quercus petraea) and chestnut (Castanea sativa) woods. Samples were heat-treated for 2 hours at different temperatures (160 °C, 180 °C, and 200 °C) and then coated sequencially with linseed oil followed by poly(vinyl acetate) (PVAc) varnish. Increasing heat treatment temperature led to a significant darkening of the wood and a decrease in gloss and surface roughness. While thermal modification alone caused a reduction in adhesion strength, the subsequent application of linseed oil was effective in mitigating this adverse effect. The oil created a more favorable bonding interface by reducing the negative influence of tannins and hydrophobicity on the adhesive. The combined thermal modification and linseed oil treatment achieved favorable adhesive bonding performance of tannin-rich wood species with the varnish. This study suggests that low heat treatment temperatures combined with a linseed oil coating can be recommended for applications where good adhesion strength is desired.