Kurzbeschreibung
(Englisch)
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Heat treatment of wood has been known as an effective method for modifying the properties of wood. In most treatments the temperature is high between 160 and 250°C. Although durability and hygroscopicity of wood improve considerably, but undesired side effects like loss of strength, rigidity,increase the brittleness and the odor limit strongly its utilization as load bearing elements and at indoor applications. For this purpose, in this project the effects of mild temperature heat treatment, between 110 and 150°C, will be studied on the wood chemical degradation, property and the parameters of the heating process are optimised for an improved wood properties with reduction and even elimination of the undesired effects. With respect to the modified property, this heated wood is similar to the aged wood.
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Abstract
(Englisch)
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Heat treatment of wood has been known as an effective method for modifying the properties of wood. In most treatments, the temperature varies from 160 to 250°C. Under these temperatures the properties of wood such as hygroscopicity, dimensional stability and resistance to microorganisms improve considerably. But, heat treatment at high temperature also produces some side effects, the mechanical properties and more particularly the tensile resistance of wood along the radial direction degrades severely and wood becomes more brittle. Consequently, high temperature heat treated wood loses its aptitude to be considered as load-bearing elements. Besides, chemical degradation of a range of wood constituents at high temperature produces an odour, which limits its usage in indoor applications. In this project, the chemical and mechanical characteristics of spruce (Picea abies (L.) Karst.), treated under mild temperature has been studied. The aim was to understand the combined effect of temperature-moisture and time on the chemical changes of wood polymeric constituents and physical and mechanical behaviour of wood. It is tried to optimize the processing parameters to reduce and even eliminate the undesired effects of heat treated wood such that to be usable as load bearing elements and capable being used in indoor application. Spruce wood was treated under the conditions of: temperatures of 110, 130 and 150°C; relative humidities (RH) of 0, 10 and 25% and treatment times of 5, 11 and 26 days. The samples were tested in radial tensile failure, masse loss, longitudinal Young's modulus and the colour was analysed. Among these characteristics radial failure stress showed highly sensible to the heat treatment parameters. Its value reduced with increasing of: temperature, RH and treatment time. A series of radial failure stress tested samples was selected and two methods were implemented for chemical characterization: 1- using NIR spectroscopy and PCA to understand the modifications in the structure of spruce wood samples during the thermal and hydro-thermal treatment. These modifications were more pronounced with the increase of the treatment period, increase of the temperature and also with higher amount of relative humidity involved in the treatment. The treatment was affected especially the hemicelluloses and amorphous carbohydrates. Water vapours together with the released low molecular compounds from the wood structure (due to the temperature) act as catalysts and accelerate the degradation process. Although the temperature was quite low, we still observed release of radicals, hydrolysis reactions and also rearrangements in the wooden structure. 2- In the second method a series of solvent extractions followed by acid hydrolysis was performed in order to determine the amounts of carbohydrates, lignin and extracts. The result showed that: In general, the lesser values in the radial failure stress in thermo-hydro treated spruce are correlated to a higher amount of extracts. The extracts represent the cleaved polymers from the wood structure. At more severe conditions, i.e. higher temperature, longer treatment times and higher relative humidity, the failure stress is lower and a higher amount of total extracts is also found. Therefore, it can be said that the loss in failure stress is related probably to the de-polymerization of the wood components. Physically speaking, it can be said that the cut of the chains at the molecular level may weaken the resistance of heat treated wood to tensile force. At temperatures of 110°C, and 0% RH, radial failure stress reduces only 7% after 26 days treatment. Besides, its color shows an interesting correlation with the natural aged spruce wood.
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