Influence of surface roughness of ten tropical woods species on their surface free energy, varnishes wettability and bonding quality

Abstract

PurposeThe surface characteristics of wood such as surface roughness, surface free energy (SFE) and wettability are important properties influencing further manufacturing processes such as gluing and coating. The purpose of this study is to determine the influence of surface roughness of ten tropical woods on their SFE, wettability and bonding quality for water-based acrylic and solvent-based alkyd varnishes.Design/methodology/approachThe woods tested in this study were fast-growing teak, afrika, sungkai, mindi, merbau, durian, lamtoro, pulai, acacia and kempas. Wood surfaces were prepared in unsanded and sanded using an abrasive paper of 120 grits. SFE values were calculated based on the Rabel method. Wettability values were measured based on the contact angle between varnish liquids and wood surfaces using the sessile drop method, and the S/G model was used to evaluate the wettability of the varnishes on the woods surface. The bonding quality of the varnishes was measured using a cross-cut test based on the ASTM 3359-02 standard.FindingsThe results show that unsanded kempas wood had the roughest surface with a Ra value of 16.24 µm, whereas sanded lamtoro wood has the smoothest surface with a Ra value of 6.86 µm. The unsanded afrika wood had the highest SFE value of 53.61 mJ/m2, whereas sanded fast-growing teak had the lowest SFE value of 36.17 mJ/m2. Sanded merbau woods had the lowest K value of 0.022 for the water-based acrylic varnish, whereas unsanded afrika wood had the highest K value of 9.253 for the alkyd varnish. Afrika wood with the highest K values (highest wettability) for both acrylic and alkyd varnishes produced the highest bonding quality (grade 4-5). Compared to the water-based acrylic varnish, the solvent-based alkyd varnish was more wettable and generated better bonding quality.Research limitations/implicationsImproving the quality of fast-growing wood from plantation by painting could be considered to increase their use for higher value wood products.Practical implicationsCompared to water-based acrylic varnish, solvent-based alkyd varnish was more wettable and generated better bonding quality.Originality/valueThe originality of this research is to evaluate the values of surface free energy. SFE could be used to quantitatively determined the wettability of paints liquid in the surface of wood