Hygroscopic properties of hardwoods

Abstract

Hygroscopic properties of nine hardwoods from Hungary were studied by moisture sorption isotherms, methods of thermogravimetry (TG) and differential scanning calorimetry (DSC). The studied wood species included oak, beech, hornbeam, black locust, lime, ash, maple, alder, and aspen. The parameters of the wood — water sorption system were calculated using the equations of Brunauer — Emmett — Teller (BET), Guggenheim — Anderson — de Boer (GAB), Frenkel — Halsey — Hill (FHH), Zimm — Lundberg (ZL), and the theory of the volume filling of micropores (TVFM). Within the frameworks of these models, the monolayer capacity, the apparent specific surface area, the parameters of clustering of water in wood, the characteristic energy of adsorption, and other parameters were calculated. Based on the data of TG and the fourth derivative of the mass loss rate contour during drying, the «fractional composition» of hygroscopic moisture was established. According to the Ozawa — Flynn — Wall isoconversion method, the dependence of the activation energy of moisture thermal desorption on the degree of drying was calculated. The enthalpy of vaporization of hygroscopic moisture ΔНvap was measured using DSC. The complex of measured and calculated parameters of the wood — water system made it possible to differentiate the studied hardwoods regarding the hygroscopic properties of wood. According to the value of the apparent specific surface area (BET, GAB) the wood species found to be in the following order: lime < black locust < beech < oak < hornbeam < ash < alder < maple < aspen. Hardwoods were ranked as follows by the value of the characteristic energy of adsorption Ec (TVFM): lime < beech < hornbeam < ash < black locust < oak < maple < alder < aspen. The intrinsic stresses in wood change in the same row as Ec. An increase in Ec indicates an increase in intrinsic mechanical stresses. Analysis of the enthalpy of vaporization of hygroscopic moisture during non-isothermal drying of wood revealed three sub-ranges of moisture content (0 — ~0,04; 0,04 — ~0,08 and > 0,08 g H2O/g dry weight). This, on the one hand, «unites» the wood species, on the other, differentiates them by the bonding energy of water in wood.