Synergistic enhancement of hygroscopicity and micromechanical properties of wood cell walls through joint tung oil and thermal modification

High hygroscopicity is an important factor that influences the durability and service life of wood structures. Joint modification of tung oil impregnation and thermal modification (OTM) has been acknowledged as an effective technique to reduce wood hygroscopicity and preserve mechanical properties. However, the contribution of this approach to the effect of wood cell wall properties at the microscopic level remains unclear. In this study, the surface morphology, micromechanical properties, and dynamic moisture absorption of OTM-modified wood cell walls were characterized. The results of morphological observation clearly demonstrated and quantified the spatial distribution of tung oil within wood pores, including porous capillary structures and wood cell walls. It is also observed that an interpenetrating polymer network was formed between tung oil and the cell wall components. The cell wall roughness of all thermally modified samples increased as the temperature rose, while the oven-dried density and equilibrium moisture content decreased. Notably, the immersion of tung oil in cell wall structures significantly alters the surface morphology of cell wall, transitioning from ellipsoidal protrusions in the control group to spiky protrusions with rounded tips in the OTM samples. Moreover, 210 ℃ was identified as a critical turning point for Young's modulus and hygroscopicity in cell wall of thermally modified wood. Below 210 ℃, Young's modulus showed an upward trend as the temperature increased, while a precipitous drop occurred in air thermally modified (TM) samples at 210 ℃. However, due to the impregnation of tung oil, the effect of thermal modification on Young's modulus of wood was attenuated, especially at high temperatures. The OTM wood (7.87 ± 0.79 GPa) at 210 ℃ showed a larger Young's modulus compared to TM wood (3.03 ± 0.42 GPa), corresponding to an increase of 159.7%. Additionally, TM samples at 210 ℃ also showed increased water absorption, water absorption rate, and reduced hygroscopic hysteresis in comparison to the control group. Conversely, the moisture content of OTM samples continued to reduce at 210 °C. In conclusion, the formation of a polymer network between wood components and tung oil could further reduce the hygroscopicity of thermally modified wood while maintaining the micromechanical properties of the cell walls.