FT-IR and UV Raman spectroscopic studies on thermal modification of Scots pine wood and its extractable compounds

Abstract

The aim of this study was to examine, in some detail, the thermally induced changes in Scots pine (Pinus sylvestris) wood with Fourier transform infrared (FT-IR) and UV resonance Raman (UVRR) spectroscopies. These techniques were also utilised to identify lipophilic and hydrophilic extracts from heat-treated and native wood samples. Furthermore, the molecular structures of the extractable compounds characteristic for Scots pine wood were comprehensively studied with UVRR spectroscopy.

Spectroscopic data revealed that the structure of thermally treated wood was extensively modified at temperatures above 200 °C. These modifications included the depolymerisation and condensation of lignin, degradation of hemicelluloses as well as the removal and/or decomposition of the wood resin components. UVRR and FT-IR spectral data indicated that thermally modified wood treated at temperatures above 200 °C had increased durability against fungal attack and damage. The structure of lignin on the surface of thermally treated wood was less vulnerable to weathering than that of untreated wood. A diminished content of hemicelluloses contributed, at least partially, to the improved fungal and weather durability of heat-treated wood, as did the lower equilibrium moisture content and swelling. It is likely that the modified structure of lignin in the heat treated wood partially retarded UV light-induced degradation reactions of lignin during natural weathering.

All unsaturated model compounds of wood resin were resonance enhanced by UV excitation. Spectra recorded from solid heartwood, knotwood, and branchwood samples included medium to strong Raman signals due to the unsaturated structures of wood resin. Moreover, characteristic bands for particular extractable compounds were observed in the UVRR spectra of the lipophilic and hydrophilic extracts and solid wood samples. Of these, pinosylvin had a useful band in the fingerprint region at 997 cm−1 and was identified as being present sometimes in significant quantities.