Spectrochemical characterization by FT-Raman spectroscopy of wood heat-treated at low temperatures: Japanese larch and beech

Test samples of Japanese larch (Larix leptolepis) heartwood and Japanese beech (Fagus crenata) sapwood were heated for 22h at constant temperatures (50 degrees-180 degrees C) under three water content conditions. Raman spectra of the samples were recorded before and after the heat treatments, and spectral changes in the range from 1000cm(-1) to 1800cm(-1) were evaluated using the difference spectrum method. For both wood species, the Raman band intensity at 1655-1660cm(-1) due mainly to the C=C and C=O groups in lignin clearly decreased with increasing heat-treatment temperature (HTT). The spectral change was thought to reflect the progress of condensation reactions of lignin molecules during the heat treatment. Moreover, the decrease in band intensity was considerably facilitated by the presence of water in the cell wall, suggesting that the condensation is closely related to the softening of lignin. From the spectral changes in the wavenumber region of 1200-1500cm(-1), it was considered that wood constituents are partially decomposed at the higher HTT.