Cell wall changes under compression combined with steam treatment in relation to wood hygroscopicity

Abstract

Abstract Compression combined with steam (CS) treatment is postulated to be an environmentally friendly and efficient modification method to improve the dimensional stability, durability, and mechanical strength of wood. The influences of CS treatment with different radial compression ratios (RCRs) (25% and 50%) and different steam temperatures (140, 160 and 180°C) on chemical components, porosity, and hygroscopicity of earlywood and latewood in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) were investigated respectively on a cellular level by imaging Fourier Transform Infrared (FT-IR) microscopy, Confocal Raman Microscopy (CRM), nitrogen adsorption and dynamic vapor sorption (DVS). The results indicated that the degradation of carbonyl groups of the glucuronic acid component of xylan in earlywood and latewood was mainly responsible for the low hygroscopicity of CS-treated wood. Also, a significant decrease in the amount of C=O and C=C linked to the lignin aromatic skeleton involved in either crosslinking reactions or the degradation reactions could be another contributor to the reduction in wood hygroscopicity. CS-treated wood with a steam temperature of 180°C possessed a lower hygroscopicity that correlated well with the depolymerization of crystalline and amorphous cellulose. A more deformed structure of CS-treated wood led to the formation of greater amounts of mesopores in the cell walls, which could lead to increased degradation of the chemical components of wood cell walls. Furthermore, a higher equilibrium moisture content (EMC) level was found for CS-treated wood with a 50% compression ratio compared to a 25% compression ratio.