Self-shrinking Ailanthus altissima substrate obtained by ultrasonic-assisted treatment: Density and pore structure characteristics

The pore structure characteristics of wood are important factors affecting permeability. The main objective of this study was to evaluate the density and pore structure characteristics of self-shrinking Ailanthus altissima substrate obtained by ultrasonic-assisted treatment. In the experiment, 320 W and 25 kHz ultrasound combined with alkali solution (1%, 4% and 8% NaOH) was applied. The pore structure, chemical groups, macroscopic and microscopic morphology of samples were investigated by mercury intrusion porosimetry, Fourier transform infrared spectroscopy, stereomicroscopy, and scanning electron microscopy. The results indicated the cell-wall components (particularly hemicelluloses) were degraded or transformed. Thus, the density (oven-dried, skeletal and bulk) and pore structure of the wood were altered. In the condition of 1% and 4% NaOH condition, a uniform self-shrinking wood block with relatively-high oven-dried density and bulk density, but lower porosity than the water-immersion samples, was obtained by ultrasonic-assisted treatment. In comparison with the waterimmersion samples, the distribution of pores of samples in alkali condition shifted towards larger diameters, particularly ultrasonic-assisted one. The relative content of macropores (> 0.5 mu m) increased, and the relative content of ultrasonic-1% NaOH-treated samples reached 86.10%.