The heat treatment of wood is an environmentally friendly method for wood protection. The progressive thermal degradation of the wood components at process-temperatures above 180°C leads to the reduction of hydroxyl groups. For this reason hygroscopicity, dimensional stability and durability are improved. In addition, heat treatment results in varying weight loss, depending on the treatment parameters, such as maximum treatment temperature, heating rate, holding time at the maximum temperature and the medium in the atmosphere, such as water vapour, oil or nitrogen. The increasing mass loss leads to progressive decrease in density. Here, the early and late wood growth zone of a tree ring can be altered to varying degrees. This ultimately affects the properties of the thermally modified wood. Therefore the aim of the present investigation was to detect the change of density of oak (Quercus robur L.), due to the influence of a thermal modification. The reduction of the density was recorded by High-Frequency Densitometry and compared to measurements based on the Gravimetric Method. To achieve a comparison of the initial density before and the density in result of thermal treatment, the examination was carried out on the same sample. The evaluation of the methods allows to conclude, that the High-Frequency Densitometry Method shows less deviations in their results than the Gravimetric Method. In addition this method provides reasonably good results in respect to the intended localisation of the reduction of density within an annual- ring. As to be expected, in contrast to the Gravimetric Method the High-Frequency Densitometry Method proved to be applicable to localise the change of the tree-ring density. It can be summarized that the measurement of the local density change is possible and the density of earlywood was significantly more reduced compared to that of the latewood. In parallel, changes occur in the microstructure in the form of cell wall delamination.
Keywords: High-Frequency Densitometry, wood density, Thermal Treatment
Authors
Clauder L.
Department of Wood Science and Technology, Eberswalde University for Sustainable Development – University of Applied Sciences, Eberswalde, Germany
Shchupakivskyy R.
Ukrainian National Forestry University, Department of Woodworking and Technology, Lviv, Ukraine
Pfriem A.
Department of Wood Science and Technology, Eberswalde University for Sustainable Development – University of Applied Sciences, Eberswalde, Germany
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