Abstract
Thermal modification is an environmentally friendly method to improve dimensional stability, durability, and aesthetic properties of wood. Changes in lignin as one of the main wood components markedly influence wood product properties and recycling possibilities of thermowood at the end of its life cycle. Teak and iroko wood samples were thermally treated at the temperatures of 160 °C, 180 °C and 210 °C following the Thermowood process. Dioxane lignin was isolated from treated and untreated wood and analysed by nitrobenzene oxidation (NBO), size exclusion chromatography (SEC) and Fourier transform infrared spectroscopy (FTIR). The yields of both acid-insoluble and dioxane lignins increased with an increasing treatment temperature. Dioxane lignins are GS-types containing more guaiacyl units compared to syringyl ones with S/G ratios of 0.91 and 0.84, respectively. In the process of thermal modification, several degradation and condensation reactions were observed. The cleavage of methoxyl groups and side chains, oxidation reactions, cleavage of the β-O-4 ether linkage and cross-linking radicals arising at higher temperatures were all confirmed. However, during the thermal treatment, teak lignin changed in a different way than iroko lignin, e.g., the molecular weight of iroko lignin decreased at all applied temperatures while it increased at 180 °C and 210 °C in teak lignin, and the change in S/G ratio and the cleavage of alkyl-aryl bonds are different in both wood species.
Highlights
Wood is a natural, renewable, mostly inexpensive and accessible material
The apparent insoluble lignin content increased during thermal modification, when compared to the reference sample and the sample treated at the temperature of 210 ◦ C by 14% and 27% for teak and iroko, respectively
During the thermal treatment of teak and iroko wood, the apparent insoluble lignin content increased, when compared to the reference sample and the sample treated at the temperature of 210
Summary
Modification is applied to overcome weak points of the wood material that are mainly related to moisture sensitiveness, hardness, low dimensional stability, low resistance to wear, UV irradiation, to bio-deterioration against fungi, termites, and marine borers [1]. Wood thermal treatment is an economical and environment friendly pretreatment. It can be performed in range of 160 to 260 ◦ C in different environments, e.g., air, vacuum, nitrogen and oil. Its main objective is to improve dimensional stability and durability, but sometimes thermal treatment is used to change the aesthetic properties of wood. The wood colour becomes darker depending on the treatment temperature, time, and environment [2,3,4]
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