Evaluation of the colour parameters on hygroscopicity and dimensional stability of thermally modified Populus tomentosa Carr.

The water vapour sorption and viscoelastic characteristic of thermally modified poplar wood (Populus tomentosa Carr.) based upon colour changes were studied to provide insights into the changes in hygroscopicity and dimensional stability properties by using dynamic vapour sorption (DVS) and dynamical mechanical analysis (DMA). The thermal treatments at 180, 200, and 220 degrees C for 2, 4, 6, 8, and/or 10 h under air atmosphere were applied to cause the component degradation and crosslinking reactions within the cell wall. As the thermal treatment temperature and time increased, the lightness (L*) value of wood showed a gradual decrease. After the removal of hydroxyl groups, the hydrophobicity and dimensional stability had been significantly improved, especially at low L* value. A gauss function (R2 up to 0.99) relationship was observed between water adsorption property and L* value. Based on the analysis of the multiple sorption parameters against L* value, thermal treatment degraded the hemicellulose and reduced the moisture sorption of wood cell wall and the diffusion of water through the cell wall matrix. As the L* value declined, the trend that slow moving of side chains, increasing glass transition point temperature of hemicellulose, and more additional covalent bonds resulted in increased E '' and decreased E ' values was becoming evident. From these perspectives, it could be deduced that reduced water diffusion and high polymer stiffness confined expansion of the cell wall to endow thermally modified wood with improved hydrophilicity and dimensional stability.