Pro Ligno (Dec 2018)
Changes in Hygroscopic Properties of Poplar and Black Pine Induced by Thermal Treatment
Abstract
The aim of the present research is to comprehend the response of poplar (Populus sp.) and pine (Pinus nigra L.) wood to thermal modification at the temperature of 180°C and 200°C, for 3 different treatment durations of 3, 5 and 7 hours, in the presence of air, by examining their physical and hygroscopic properties and comparing these species properties one another. Specifically, in this research, the mass loss of the samples due to the treatment, the density, the equilibrium moisture content (EMC) and the hygroscopic properties (radial, tangential and longitudinal swelling and absorption percentages) were examined. As the treatment intensity increased, the mass loss of pine and poplar boards due to treatment also increased, which corresponds to the loss of moisture, and the chemical constituents of wood due to their degradation (evaporation of volatile extracts, loss of products through thermal degradation etc.). The thermal treatment has led to a reduction in the recovery rate of the pine and poplar plates of their original weight due to reduced moisture recovery, which is more pronounced as the treatment temperature and duration increase. The EMC of all heat-treated pine and poplar samples was reduced compared to the corresponding control value, even in the case of the less intensive treatment (180°C-3h) and as the intensity increased, EMC decreased further (20.73%-38.43% for pine and 18.76%-49.20% for poplar). Thermal treatments reduced the density of pine (0.76-20.24%) and poplar (1.04-24.42%), and as the treatment intensity increased, larger reduction was recorded. The hygroscopic properties of heat-treated pine were improved in every case due to thermal treatments. The most intensive treatment tended to slightly increase the adsorption and radial-tangential swelling rates, without approaching the control levels. Referring to poplar, all treatments, even the less intensive ones, improved the hygroscopic nature of wood, reducing the adsorption and tangential-radial swelling rates compared to the control sample. Therefore, confirming the results of literature, the thermal treatment was found to improve the hygroscopic behavior of pine and poplar and to further reduce tangential swelling levels in relation to radial direction, contributing to the limitation of wood anisotropy.
