Effects of ultrasonic pretreatment on physicochemical properties and drying behavior of Cunninghamia lanceolata and Eucalyptus grandis × urophylla

Ultrasonic treatment offers promising potential for improving wood drying efficiency and permeability, yet its effects on the coordinated structural and drying responses of different species remain underexplored. This study assessed the impact of 1-5 h ultrasonic treatments (320 W, 40 kHz) on the physicochemical properties, pore structure, permeability, and drying behavior of Cunninghamia lanceolata and Eucalyptus grandis × urophylla. Results showed that ultrasonic pretreatment did not alter cellulose crystallinity type but reduced crystallinity and free hydroxyl content, while increasing mass loss, microfibril angle, and hygroscopic dimensional stability. With increasing treatment time, cell double wall thickness initially decreased and then stabilized, and Shore hardness declined across all three sections. SEM and MIP analyses revealed that ultrasound gradually disrupted pit membranes, increased the proportion of macro- and mesopores, total pore volume, and porosity, thereby enhancing gas permeability. Treatments of 1-4 h showed the most significant improvements, while excessive treatment (5 h) led to structural collapse or pore closure. Permeability was enhanced by up to about 63 % and 60 % in radial and tangential directions of C. lanceolata, and by about 42 % and 33 % in E. grandis, respectively. Drying time was shortened, and drying rate and diffusion coefficient were increased, while volumetric shrinkage was reduced. Drying rates were raised by about 10 % and 13 % after 2 h (C. lanceolata) and 4 h (E. grandis) treatments, respectively. However, bound water was found to be less responsive to ultrasonic stimulation than free water. These findings provide a theoretical and practical basis for optimizing ultrasonic parameters to improve the drying performance and permeability modification of fast-growing plantation wood.