A study of resin removal and fixation of radiata pine wood by microwave-superheated steam treatment

Radiata pine is a valuable resource for construction and furniture manufacturing. However, its heartwood, which has a high resin content (RC), is limited in its applications. This is because the exudation of resin can cause issues such as unsightly appearance, degumming, or paint bubbling, posing a significant challenge to its use. Hence, the heartwood of radiata pine was taken as the research object, and the resin in wood removed and fixed by microwave-superheated steam (MS) treatment was studied. The moisture content (MC), RC, relative content of turpentine, and the anti-resin-exudation performance were determined; the changes in the morphology of resin in the resin canals, the Young's modulus of the latewood cell wall, and the chemical composition were analyzed. The findings show that MS can remove some of the pine resin in the wood, but not all. Microscopic observations confirmed that some resin remained in the resin canals of the MS treatment samples. In addition, the turpentine content decreased dramatically after MS treatment. For instance, the average turpentine content of latewood in samples after treatment at 120 °C was reduced to 9.7%, only 0.3% after treatment at 140 °C, and no turpentine was detected after treatment at 160 °C. This may explain why the wood after MS treatment showed good anti-resin-exudation, as the volatilization of turpentine would lead to the increase in softening point and thermal stability of resin. In addition, the Young's modulus of latewood cell wall increased with superheated steam (SS) treatment temperature. The resin was oxidized, dehydrogenated, and isomerized after MS treatment. These findings confirmed that MS treatment improves the anti-resin-exudation performance of radiata pine by increasing the softening point of residual resin and fixing it inside the wood. Through this strategy, the treated radiata pine heartwood shows greater potential for application.