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Purpose The surface characteristics of thermally and chemically modified wood, such as surface roughness, surface free energy (SFE) and wettability, are important properties that influence further manufacturing processes such as gluing and coating. The aim of this paper was to determine the influence of the surface roughness of thermally and chemically modified teak wood on their SFE, wettability and bonding quality for water-based acrylic and solvent-based alkyd varnishes. In addition, durability against subterranean termites in the field of these modified teak woods was also investigated to give a valuable information for their further application. Design/methodology/approach The woods tested in this study were fast-growing teak woods that were prepared in untreated and treated with furfuryl alcohol (FA), glycerol maleic anhydride (GMA) and thermal. SFE values were calculated using the Rabel method. The wettability values were measured based on the contact angle between varnish liquids and wood surfaces using the sessile drop method, and the Shi and Gardner model model was used to evaluate the wettability of the varnishes on the wood surface. The bonding quality of the varnishes was measured using a cross-cut test based on ASTM 3359-17 standard. In addition, durability against subterranean termites in the field of these modified teak woods was also investigated according to ASTM D 1758-06. Findings The results showed that furfurylated and GMA-thermal 220°C improved the durability of teak wood against termites. The furfurylated teak wood had the roughest surface with an arithmetic average roughness (Ra) value of 15.65 µm before aging and 27.11 µm after aging. The GMA-thermal 220°C treated teak wood was the smoothest surface with Ra value of 6.44 µm before aging and 13.75 µm after aging. Untreated teak wood had the highest SFE value of 46.90 and 57.37 mJ/m2 before and after aging, respectively. The K values of untreated and treated teak wood increased owing to the aging treatment. The K values for the water-based acrylic varnish were lower than that of the solvent-based alkyd varnish. The untreated teak wood with the highest SFE produced the highest bonding quality (grades 4–5) for both acrylic and alkyd varnishes. The solvent-based alkyd varnish was more wettable and generated better bonding quality than the water-based acrylic varnish. Originality/value The originality of this research work is that it provides evaluation values of the durability and SFE. The SFE value can be used to quantitatively determine the wettability of paint liquids on the surface of wood and its varnish bonding quality.

An innovative eco-friendly technology, applied to improve the quality of the short rotation teak, was wood modification by heat treatment. The study was to investigate the effects of heat treatment at 180°C, 200°C, and 220°C on the characteristics of the short rotation teak. The results show that the short rotation teak degraded hemicellulose and increased αcellulose and lignin after the heat treatment. Anti-swelling efficiency value after heat treatment was improved ranging between 12.9% and 46.7%, indicating an improvement in dimensional stability. The MOE and MOR values decreased after heat treatment. The heat treated sample at 180°C after graveyard test was the lowest in weight loss, which presented good durability against termite (rating 9) and decay (rating 10). The surface roughness and SFE values decreased as the heating temperature increased. The heat treated at 220°C provided the lowest K-values (0.04 for acrylic and 5.04 for alkyd paint) which indicate the poorest wettability.

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Short rotation teak (SRT) has a diameter of around 30 cm at the age of 15 years; however, the wood still has low quality, especially in dimensional stability and biological durability. The objective of this study was to improve dimensional stability and biological durability of SRT through modification treatment using non-biocide chemicals, citric acid (CA) and benzophenone tetracarboxylic acid (BPTCA). SRT sapwood was impregnated by 20% and 40% of CA or BPTCA followed by heating in the oven at 150 °C for 2 h. Dimensional stability, strength, surface characteristics and durability against termite attacks of the modified woods were evaluated. Results show that dimensional stability as anti-swelling efficiency (ASE) increased by an average of 46% (CA) and 50% (BPTCA) after treatments. Durability against termites showed that the modified woods were categorized to be class 1 (very durable). Fourier Transform Infrared (FTIR) and X-ray Diffraction observations indicated a possible chemical reaction between the wood components with CA and BPTCA. The water-based acrylic and oil-based alkyd contact angle values of the modified woods were lower than 90°, indicating their high wetting ability against the varnish liquids. The treatment can be applied to improve the quality of SRT for high-quality wood product utilization.