The influence of mechanical, physical and chemical pre-treatment processes of wood surface on the relationships of wood with a waterborne opaque coating

Abstract

The selection of the wood surface pre-treatment process prior to finishing for an optimal wood-coating relationship can be challenging, but has a great significance for the properties of the coated products. The present study compares the surface properties of beech (Fagus sylvatica L.) wood after mechanical (MT), physical (PT) and chemical (CT) treatment and shows the relationships of the pre-treated wood with original (O) and diluted (D, 10% water addition) waterborne opaque coatings. Wood was found to have the most oxidized surfaces (O/C ratio of 0.7) and a pronounced polar character (water contact angle 24°) after PT with atmospheric plasma discharge, followed by surfaces exposed to MT (O/C ratio 0.6, 52°) and surfaces exposed to CT (O/C ratio 0.4, 94°), representing surface treatment with an organic solvent. Changes in the cell structure of the wood and reduced surface roughness (Sa decreased by −2.0%) were observed only after CT. The best wettability with coatings was observed on wood exposed to PT and CT, reaching the values of advancing contact angles of 94° and 89° for O coating and 71° and 75° with D coating, respectively. This shows that it is not the surface condition of the wood that has the greatest influence on the wettability with the coatings used, but the properties of the coatings play a major role. After applying the coatings to the pre-treated wood surfaces, the thicker (between 60 μm and 41 μm) coating films were found on the wood coated with O coating than on the wood coated with D coating (between 47 μm and 37 μm). Despite some differences in the determined coating penetration depths, no general statement could be made about the influence of the type of surface pre-treatment. The highest adhesion strengths of the O coating and the D coating were found on the wood surfaces after CT (6.4 MPa and 5.4 MPa), MT (4.6 MPa and 4.5 MPa) and the smallest on the surfaces after PT (2.2 MPa and 2.5 MPa). The reasons for the contradictions between the wettability with coatings and the adhesion strength could be due to the formulation of the coatings studied, which seems to adhere better to wood substrates pre-treated with an organic solvent.