Thermal Modifications in Softwood Studied by FT‐IR and UV Resonance Raman Spectroscopies

Scots pine planks were heat‐treated under steam in a temperature range of 100–240°C at every 20°C. Changes in chemical structure of the wood samples were examined with UV resonance Raman (UVRR) and Fourier transform infrared (FT‐IR) spectroscopies. Prior to the FT‐IR and UVRR analysis, the heartwood part of wood samples were ground and extracted with acetone. Both the extracts and the extracted samples were analyzed. In addition, Klason lignin contents of the samples were determined. The FT‐IR and UVRR spectroscopy were powerful techniques to monitor chemical changes in the heat‐treated wood samples. Results confirmed most of what determined previously by other research groups using different techniques. The UVRR and FT‐IR spectra of acetone extracts showed that lignin became partly acetone soluble after a heat treatment of 180°C or greater. Increased levels of free phenolic hydroxyl groups were detected in lignin, probably due to cleavages of β‐O‐aryl ether interunit linkages. The amount of extractable lignin increased with increasing temperature, while the resin in the extracts decreased. UVRR spectra of the wood samples extracted with acetone indicated that the structure of unextractable lignin remained unchanged when heated up to 200°C. Formation of new carbonyl structures was observed in solid wood samples and their extracts. These structures were possibly formed from the degradation products of lignin and hemicelluloses. Lignin content of the samples was increased as a result of degradation of wood hemicelluloses, which started below 200°C.