Thermal treatment is one of the most promising methods for upgrading wood durability properties. In the hydrothermal modification process, not only the chemical properties and structure of wood are changed, but also physical properties such as colour, mass, volume and mechanical strength. The heating of wood in the water vapour medium influences mainly the three basic components of wood – cellulose, hemicelluloses and lignin. As a result of the thermal treatment, the content of hydrophilic groups in wood decreases dramatically. Hence, the thermally modified wood binds much less water than untreated wood. The hygroscopicity of thermally modified wood is affected mostly by the hemicelluloses degradation, some break-up of the amorphous part of cellulose and changes in the lignocarbohydrate complex.
In the present study, the chemical changes in soft deciduous wood – aspen, grey alder and birch as a result of hydrothermal treatment were investigated. For modification, an experimental laboratory modification device was used. Wood boards under pressure in water vapour medium were subjected to the treatment at 140, 160 and 180ºC. Holding time at the maximum temperature was 1 h. With increasing treatment temperature, density decreases, mass losses grow, which can be explained by the evaporation of more volatile components, wood dehydration and hemicelluloses decomposition to low-molecular compounds, with the formation of CO2, formic acid, acetic acid, furfural, etc. With increasing temperature, the amount of acids, esters, sugars and tannins in the condensates grows. The Kürchner-Hoffer method was applied for cellulose determination, the Klason method for lignin but the hemicelluloses content was determined indirectly. For assessing the changes in the functional groups of wood, FTIR spectroscopy was used. With increasing treatment temperature, as a result of the degradation of less ordered carbohydrate (hemicelluloses), the degree of crystallisation of cellulose increases; the amount of the acetone extractable compounds in wood also grows. Applying the water vapour sorption method, the specific surface accessible for water vapours was determined. It has been found that the obtained wood hydrophobicity is partially reversible and decreases in elevated moisture conditions.
Keywords: aspen, birch, grey alder, hydrothermal modification
Authors
Grinins J.
Latvian State Institute of Wood Chemistry, Riga, Latvia
Biziks V.
Latvian State Institute of Wood Chemistry, Riga, Latvia
Andersons B.
Latvian State Institute of Wood Chemistry, Riga, Latvia
Andersone I.
Latvian State Institute of Wood Chemistry, Riga, Latvia
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