Heat-treated softwood exposed to weathering

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 ¹³C 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.