Enhancing the properties of <i>Eucalyptus urophylla</i> wood through low-temperature thermal modification and the synergistic effect of thermal modification and impregnation techniques

Cultivated eucalyptus wood is widely used in industry. However, its application in high-value-added products is restricted by its inherent dimensional instability. This study comprehensively assessed the effects of thermal modification (TM) at 150 °C, paraffin (P) or tung oil (T) impregnation, and their combinations (TM + P, TM + T). The results showed that all single treatments (TM, P, T) improved dimensional stability and reduced moisture absorption, although they were accompanied by noticeable colour changes (ΔE* = 5–23.62). Paraffin impregnation provided greater hygroscopic stability than tung oil impregnation and TM alone. The combined TM + P treatment exhibited the most excellent synergistic effect, reducing hygroscopic swelling by 26%–37% and increasing the modulus of elasticity (MOE) by 19%–22% relative to the T and TM + T groups. The optimal treatment (150/4P) achieved the most remarkable improvements: a 64% reduction in dimensional instability, 61% and 58% decreases in equilibrium and saturated moisture content, respectively, a 23% increase in MOE, and an 18% improvement in the modulus of rupture (MOR). FTIR spectra confirmed the incorporation of paraffin or tung oil in the modified wood. Low-temperature TM alone caused minor chemical changes, whereas impregnation induced significant chemical changes.