Advanced solid state NMR spectroscopic techniques in the study of thermally modified wood

Wood is thermally modified to obtain desired properties such as dimensional stability, low equilibrium moisture content and increased resistance to biodegradation and weather. Our detailed understanding of the changes in the chemical structure of wood induced by thermal treatment is not entirely complete. Furthermore, the effects of thermal modification on the chemistry of wood biodegradation and weathering are not well known despite the widespread use of thermally modified wood as exterior material. Various solid state C CPMAS NMR measurements were applied to investigate the changes in chemical structure induced by a Finnish industrial-scale heat treatment process. Several wood species, including pine, spruce, birch, aspen and oak were studied. Wood samples were thermally modified under normal pressure using water vapour as a shielding gas, in the temperature range of 160-240°C. Some of the thermally modified and unmodified wood samples were further exposed to soft or brown rot fungi and weathering. Besides conventional C CPMAS NMR measurements, advanced solid state NMR techniques, i.e., spin-locking and dipolar dephasing, were applied to investigate the changes in the main components of wood: cellulose, hemicelluloses and lignin. The effect on wood of this type of thermal modification has not previously been studied by advanced solid state NMR techniques. According to conventional C CPMAS NMR spectra, degradation and deacetylation of hemicelluloses occurred during thermal modification of both softwoods and hardwoods. However, overlapping of the signals of different wood components makes the investigation of the cellulose content difficult. The subspectra of cellulose and the lignin-hemicellulose matrix were separated by the spin-locking technique and linear combination. The results showed the cellulose crystallinity index (CrI) to increase in thermal modification for every wood sample, as a consequence of the preferred degradation of the less ordered carbohydrates. Both before and after thermal modification, the CrI values were higher for softwoods than hardwoods: values for softwood samples ranged from 51% before to 65% after, and values for hardwood samples from 41% before to 54% afterwards. Sample pairs were taken from the same tree trunk in order to eliminate unnecessary errors due to heterogeneity of the wood structure. Differences in the CrI values of samples taken from the same trunk were found to be minor. The accuracy of the spin-locking technique was evaluated by comparing cellulose crystallinity values obtained by the spin-locking technique and the wood crystallinity values obtained by X-ray scattering measurements for the same set of pine and spruce wood samples. The values were in good agreement. Dipolar dephasing technique was applied to study the lignin quaternary carbons and the degree of condensation. Besides the cleavage of the β-O-4 linkages and demethoxylation in both hardwood and softwood lignin, condensation was observed in softwood lignin after the thermal modification. C CPMAS NMR spectral data were interpreted by principal component analysis (PCA) to study thermally modified and unmodified pine wood samples that had been exposed to either soft or brown rot fungi. With PCA wood samples could be classified according to their weight losses and heat treatment temperatures. Weight losses were largest for the unmodified wood samples (varying from 40% to 60%) and least for the samples heat treated at over 220°C (~2-3%). Exposure of unmodified pine to fungi resulted in a drastic decay of the cell wall polysaccharides. Soft rot fungus attacked cellulose more extensively, while brown rot fungus degraded mainly hemicelluloses. The changes in the lignin structure were minor. Increased biological resistance of thermally modified pine was observed. Conventional C CPMAS with dipolar dephasing was applied to examine thermally modified and unmodified pine wood samples that had been exposed to natural weathering for seven years. The NMR spectral data revealed a significant decrease in lignin content of the weathered thermally modified and especially the weathered unmodified samples. According to the results, the surface of the thermally modified sample was still rich in aromatic and conjugated carbonyl structures, whereas the surface of the unmodified sample was enriched in cellulose. The structure of the thermally modified wood sample was modified and the degradation products were not leached out with water as easily as from the unmodified sample. Increased weather resistance was observed for thermally modified pine.