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|Jazyk zpracování závěrečné práce:||Angličtina|
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|Název práce:||Thermal Modification of Wood: Process and Properties|
|Abstrakt:||Thermal modification of wood has been developed to increase the biological durability and dimensional stability of wood. The aim of this thesis was to extend the basic knowledge about these methods and provide a better insight into several details. Several investigations have been carried out in the small-scale laboratory chamber using different wood species and temperature levels. The European beech (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst.), white poplar (Populus alba L.) and English oak (Quercus robur L.) have been selected and thermal modification at 180 °C, 200 °C and 230 °C has been applied. The research was divided into two sections, the analysis of process and its influence on wood properties. In general, thermal modification of wood has to be optimized separately for each wood species. The process itself is characterized by usage of elevated temperatures (180--230 °C) and their effect on the wood properties. From this point of view, the employed temperatures play a dominant role in the changes of properties. The temperature distribution in the wood during the process has been intensively studied. The temperature profiles of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L. Karst.) samples (80×80×200 mm3) proved presence of the exothermic reactions occurring at high temperatures ( 200°C). The temperature overshoots depended on the species used. Due to the exothermic reaction, the sample temperatures increased to 240 °C (beech) and 215 °C (spruce), while temperature in the chamber was maintained at 200 °C for 3 hours. The numerical simulation of the thermal modification process describing heat transfer in wood has been verified by experimental data. The numerical model has been enhanced to pyrolysis model, mainly based on literature results, describing external heat source based on the degradation of the main chemical compounds of wood. The predicted and experimental data are in good agreement and the numerical error is lower than 5%. The results of the studied material properties of thermally modified wood were evaluated in comparison to reference samples. The moisture behaviour of the thermally modified wood is improved, which is basically a known fact from the literature. The equilibrium moisture content (EMC) decreased when temperature increased. The result show an improvement of about 50--70% (depending on species used and the temperature applied). The mechanical properties are influenced by the degradation of the main chemical compounds and changes in the anatomical structure. The mass or density loss is not often higher than 10%. The major disadvantage of thermally modified wood is its mechanical behaviour. The decrease in strength and toughness is a permanent issue for the future studies and improvements. Bending strength is often one of the most negatively affected properties. The results show the MOR value about 20--30% lower, which potentially decreases the usability of thermally modified wood as a material for structural constructions or certain applications. It is believed that for several reasons the thermally modified wood could be used as a replacement for expensive or rare imported timbers. The improvement of the sorption behaviour, artificial aging, decreasing of long storage are the major reasons. Studies dealing with the acoustic behaviour appear very seldom. The acoustic characteristics of thermally modified wood species have been studied in this research. For example the acoustic constant and the modulus of elasticity are often recommended as useful characteristics for wood acoustic evaluation. Thermally modified wood shows an improvement of the acoustic constant of up to 25%. A good acoustic quality is manifested by spruce (not much influenced by modification) and partly oak modified at 180 °C. The best performance has been found for poplar modified at 200 °C. Based on the results, thermally modified wood has a good potential to be used as a material for some kinds of musical instruments (guitars, violins, etc.). The wood modification is still a growing area and the demand for thermally modified wood in the European countries increased about 80% during the last decade. It is a good motivation for research and development (R&D) to keep improving and upgrading this modification technique.|
|Klíčová slova:||degradation, FEM model, simulation, acoustic properties, heat distribution, heat treatment, chemical changes, innovation, moisture behaviour, pyrolysis, strength, thermal modification, temperature profile|
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