Effects of extractives removal on the performance of clear varnish coatings on boards

In this work, the effects of extractives removal on the performance of used varnishes were studied. The experimental samples were prepared using defect-free alder (Subcordate alnus) and ironwood (Zelkova carpinifolia) with moisture content of 12% and were coated with polyester and two-part polyurethane (urethane alkyd) varnishes. Removal of extractive materials followed Tappi Test Methods using hot water and ethanol. Variable parameters were based on the solvent type (extraction method), wood species, and varnish type. Other parameters such as moisture content, sanding process, and dimensions were kept constant. Pull-off adhesion and dynamic absorption tests were used to assess adhesion and performance of the two clear varnish coatings. Sanding process was employed prior to coating, which helped prepare surfaces virtually free of damage. In general, the extractive-free samples had better adhesion strength and wettability compared with the untreated (control) ones. Based on the results of this study, it can be pointed out that all the variable parameters, including the type of wood, varnish, and solvent, had significant effect on the adhesion of varnishes, applied on the wood surface. The difference in wettability between extracted and unextracted samples is due to blocking of the free hydroxyl groups by extractive materials. In addition, it was found that the removal of extractives had the effect of increasing the surface wettability of both species, an important consideration for the varnish coating. The highest adhesion was obtained from polyurethane varnish, applied on ironwood specimens. It seems that the diffuse-porous anatomical structure of ironwood along with its high density of 0.79 g/cm³ could be responsible for its higher adhesion strength than alder species.