Mari Nuopponen

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

Abstract 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.

Ultra Violet Resonance Raman Spectroscopy in Lignin Analysis: Determination of Characteristic Vibrations of p-Hydroxyphenyl, Guaiacyl, and Syringyl Lignin Structures:

Raman spectroscopy of wood and lignin samples is preferably carried out in the near-infrared region because lignin produces an intense laser-induced fluorescence background at visible excitation wavelengths. However, excitation of aromatic and conjugated lignin structures with deep ultra violet (UV) light gives resonance-enhanced Raman signals while the overlapping fluorescence is eliminated. In this study, ultra violet resonance Raman (UVRR) spectroscopy was used to define characteristic vibration bands of model compounds of p-hydroxyphenyl, guaiacyl, and syringyl lignin structures at three excitation wavelengths (229, 244, and 257 nm). The intensities of each band, relative to the intensity of the aromatic vibration band at 1600 cm−1, were defined and the most suitable excitation wavelength was suggested for each structure. p-Hydroxyphenyl structures showed intensive characteristic bands at 1217–1214 and 1179–1167 cm−1 with excitation at 244 nm, whereas the bands of guaiacyl structures were more intensive with 257 nm excitation. Most intensive characteristic bands of guaiacyl structures were found at 1289–1279, 1187–1185, 1158–1155, and 791–704 cm−1. Syringyl structures had almost identical spectra with 244 and 257 nm excitations with characteristic bands at 1514–1506, 1333–1330, and 981–962 cm−1. The characteristic bands of the three structural units were also found from the compression wood, softwood, and hardwood samples, indicating that UVRR spectroscopy can be applied for the determination of chemical structures of lignin.

Carbon-thirteen cross-polarization magic angle spinning nuclear magnetic resonance and Fourier transform infrared studies of thermally modified wood exposed to brown and soft rot fungi.

Thermally modified wood has many technically interesting properties, such as increased dimensional stability, low equilibrium moisture content, and enhanced biological and weather resistance. This paper describes solid-state nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopic studies on the decay of heat-treated and untreated pine (Pinus sylvestris) by brown (Poria placenta) and soft rot fungi. Both techniques combined with multivariate data analysis proved to be powerful tools for the study of wood degradation by fungi. When untreated pine was exposed to brown or soft rot fungi, a drastic decay of the cell wall polysaccharides was observed. Brown rot fungus degraded mainly hemicelluloses while soft rot fungus attacked cellulose more extensively. The aromatic region of 13C cross-polarization magic angle spinning (CPMAS) NMR spectra revealed that the structure of lignin was also altered. New carboxylic structures were formed as a consequence of the decay. The increased biological resistance of pine wood heat-treated at >220 °C was observed in the 13C CPMAS NMR and IR spectra.