Summary Thermal modification of wood produces a wood material with many interesting properties, such as enhanced dimensional stability, lower equilibrium moisture content and increased biological durability. Changes in the chemical structure of pine (Pinus sylvestris) caused by thermal treatment were investigated by studying various components of wood using 13C CPMAS NMR spectroscopy. Electron spin resonance (ESR) spectroscopy on the same set of samples was used to study the formation and stability of free radicals formed during the treatment. The most remarkable changes revealed by solid state NMR were the increase in relative crystallinity of cellulose and destruction and deacetylation of hemicelluloses. Changes in the lignin fraction were mostly registered as diminishment in the methoxyl content, although the intensity of the aromatic region increased relative to the carbohydrate fraction during the treatment. Increase in the intensities of the ESR signals from thermally treated wood samples proves the formation of stable free radicals. In addition, radical formation is believed to take part in condensation reactions leading to crosslinks within the lignin and possibly between lignin and other wood components. Both of the methods used indicate that the changes are most remarkable when the treatment temperature is over 200°C.

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Scots pine samples, heat-treated (225°C under steam) and reference (kiln-dried), were exposed to natural weathering for 7 years in Espoo, Finland. The weathered and unweathered samples were examined with FTIR, UV resonance Raman, and 13 C CPMAS NMR spectroscopies. The spectroscopic results revealed that the lignin contents of the weathered heat-treated and especially of the weathered reference softwood samples diminished significantly. The surface of the weathered heat-treated sample was still rich in aromatic and conjugated carbonyl structures, whereas the surface of the reference sample was enriched with cellulose. These results indicated that weathering products of lignin were leached out with water from the reference sample, whereas in the heat-treated wood they were largely unleachable. The structure of the heat-treated wood was modified and degradation products did not leach out as easily as in the case of the reference sample. The weathering also resulted in a decreased content of amorphous polysaccharides of the reference sample, whereas the changes in the polysaccharide contents between weathered and unweathered heat-treated samples were not as dramatic because the amorphous carbohydrates were already degraded in the heat treatment. The results indicated that heat-treated wood is more resistant to natural weathering than untreated wood.

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Heat treatment reduces hygroscopicity and accompanying dimensional changes in wood. Prior to coating, pine and spruce boards were heat treated at 225°C for six hours under steam, in order to achieve dimensional stability and durability of wood substrate. The panels were coated surface finishes which are commonly used on exterior cladding, joinery and fences in Finland. Performance of the coated heat‐treated and untreated panels was monitored during five years’ outdoor exposure. Without coating the heat‐treated wood is not weather resistant. The original dark brown colour of the uncoated heat‐treated wood panels was not stable when exposed to weather, turning grey. Cracking of the heat‐treated wood without coating was at the same level as that of the untreated wood despite the lower moisture content of the heat‐treated wood. The unpigmented or low build stains and oils did not prevent cracking of the heat‐treated wood. Weather resistance of the heat‐treated wood was improved by the water‐ or solvent‐borne paints. Wood heat treated by means of this process is comparable to untreated wood as a substrate for coatings and no alterations in coating recommendations are needed when considering coating of heat‐treated wood.

Wood is thermally modified by heating and steaming in order to change its properties, e.g., to improve the biological resistance and to increase the hardness of wood. The structure of thermally modified Scots pine (Pinus sylvestris) was studied using wide-angle, small-angle and ultra-small-angle X-ray scattering methods. Modification temperatures varied from 100 to 240°C. No marked changes in the microfibril angle distribution were observed. The mass fraction of crystalline cellulose in wood (the crystallinity of wood) and the size of cellulose crystallites increased above 150°C. After modification at 230°C for 4 h the thickness of the cellulose crystallites increased from 3.1 to 3.4 nm. Thermal modification had no effect on the orientation of the voids, but an increase in the porosity of the cell wall was observed. The distance between cellulose crystallites was approximately 4.7 nm in hydrated wood and a decrease in order between microfibrils was observed at 160–200°C.

Summary Thermal modification is a technique to produce wood with increased dimensional stability and lower equilibrium moisture content. 2H NMR relaxation measurements and pulsed field gradient (PFG-NMR) methods are non-invasive spectroscopic techniques that can be used to measure the response of liquid confined in porous materials and yield information on the size and distribution of the pores. These methods were used to study the structure and changes in structure of thermally modified Scots pine wood. The 2H longitudinal relaxation measurements of wood samples at different moisture contents showed different relaxation times and relaxation time distribution in the thermally treated samples. The effect of the thermal treatments on the cell size in wood samples was studied by PFG-NMR measurements with different dwell times. The PFG-NMR measurements showed no clear change in the cell dimensions of the thermally modified samples compared with control samples taken from the same log.

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