Analysis of thermally treated wood samples using dynamic FT-IR-spectroscopy

Thermal treatment of wood has been performed from thevery beginning – e.g., Stamm (1956), Kollman andSchneider (1963), Rusche (1973) – with the aim toenhance the durability of wood for outdoor constructionsand indoor humid areas. Extensive studies by VTT (Viita-niemi et al. 1995) in this area have led to the developmentof Thermowood which is obtained by heat treatment attemperatures between 1608C and 2308C in an oxygen-free atmosphere for several hours. As an effect of thethermal treatment, Thermowood obtains a characteris-tic darker colour and develops a higher resistance tohumidity as well as reduced swelling and shrinkagebehaviour. The new technical properties follow fromchanges of chemical nature in the wood-structure, suchas a reduction in hemicellulose content, which reducesthe water absorption capability.The following improvements have been obtained forthermally treated wood: higher shape stability (swellingand shrinking could be reduced to 50% of the untreatedmaterial); reduction of equilibrium moisture content (themoisture content of Thermowood is up to 50% lowerthan for normal wood); reduction of internal stresses; andhigher durability against fungi.Also, negative changes by the thermal treatment areobserved: the density is reduced up to 15% which caus-es reduced fire resistance; the strength properties are upto 20–60% reduced; breaking by splintering is more like-ly; the colour does not resist UV radiation so that theThermowood must be treated by UV blockers similar tonon-treated wood; Thermowood becomes grey byweathering similar to untreated wood.The main goal of this research was to further increasethe understanding regarding the molecular changesoccurring due to thermal treatment in relation to the vis-coelastic behaviour of the wood. FT-IR spectroscopy iswell established for characterising the chemical natureof substances. With the combination with dynamicmechanical analysis, it has been shown that informationregarding the molecular structures taking the load inpolymer materials may be analysed (Noda 1990). Dynam-ic FT-IR is also applicable to studies of cellulose andwood fibre materials (Salme´n et al. 2005). For this reason,dynamic FT-IR spectroscopy was used for characterisingthe response of wood components to thermal treatment.The samples investigated were taken from commerciallyavailable beech Thermowood .