Bound water in wood of the main tree species of Siberia: thermal analysis and sorption

This paper deals with the hygroscopic properties of the wood of the Siberian tree species. We analyzed Siberian larch Larix sibirica Ledeb., Scotch pine Pinus sylvestris L., Siberian spruce Picea obovata Ledeb., Siberian fir Abies sibirica Ledeb., drooping birch Betula pendula Roth., and wood samples of aspen Populus tremula L. using sorption models including the Brunauer–Emmet–Teller (BET), Frenkel–Halsey–Hill (FHH), Zimm–Lundberg (ZL), and micropore volume filling (MVF) theories along with accurate methods, namely thermogravimetry (TG), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC). The isotherms of moisture sorption by wood were used to quantify the most important hygroscopic characteristics. We used BET to calculate monolayer capacity (um), specific inner surface area (Ssp) and ZL equation to find water vapor content and relative pressure when water clusters start to form in the wood samples, FHH model and MVF theory to obtain fractal dimension of «sorption surface» and characteristic sorption energy (Ес), respectively. Application of Ozawa–Flynn–Wall method to TG/DTG data allowed us to identify a dependence of water desorption activation energy on the stage of nonisothermal wood drying and to determine maximum moisture loss rate. Based on the measurement values obtained from DSC experiments, we found the energy spent for the removal of bound water when heating samples at a constant rate. The values of um, Ssp and Ес indicated that moisture sorption differed considerably (8.8 % for um and Ssp and 31.4 % for Ес) among the woody species of interest. A kinetical analysis revealed that the wood of birch, aspen, and larch had higher values of moisture desorption activation energy as compared to pine, spruce and fir. These differences agreed well with the values for moisture evaporation heat determined by DSC method.