Contribution of tung oil to the resistance of heat-induced wood shrinkage during thermal modification

Abstract

Thermal modification is commonly employed in the wood industry as an effective commercial strategy. However, heat-induced shrinkage accompanied by treatment processes may lead to a decreased wood recovery and devaluation of the products. In this study, tung oil was used to relieve heat-induced shrinkage during the thermal modification process, and the anti-shrinkage mechanism was investigated. The results indicated that the primary cause of heat-induced shrinkage in wood was the thermal degradation of cell wall components. There was a significant correlation (Spearman correlation coefficient = 0.974, p < 0.01) between the weight percent change (WPC) and heat-induced shrinkage (HS) in the air-thermally modified samples (TMA wood). Tung oil thermally modified samples (TMO wood) presented a lower shrinkage compared to TMA wood. The thermal modification process also influenced the relative content of wood components, leading to a shift of the cell wall mesopores in TMA wood towards larger sizes as the temperature increased. On the contrary, tung oil could increase the thermal degradation temperature, improve thermal stability and decelerate thermal degradation of wood, thus preventing the enlargement of cell wall mesopores. The impregnation of tung oil into the internal structures of wood, such as vessels, fibers, and ray cells, provided additional support to the cell lumen. In addition, confocal laser scanning microscopy (CLSM) presented that tung oil was uniformly distributed in the wood cell walls and occupied portions of mesopores, reducing the total pore volume of nitrogen absorption. Therefore, tung oil is a green and renewable modifier that effectively resisted heat-induced shrinkage during thermal modification. The anti-shrinkage approach exhibits considerable potential for improving wood recovery, conserving forestry resources, and promoting the development of a sustainable wood industry.