Characterisation of thermally modified hard- and softwoods by 13C CPMAS NMR

The changes induced by thermal modification in the chemical structure of spruce [Picea abies (L.) Karst.], birch (Betula pendula), aspen (Populus tremula) and oak (Quercus robur) were studied by 13C CPMAS NMR spectroscopy. Spruce, birch and aspen were thermally modified at ∼195 °C and oak at ∼160 °C, under steam, according to an industrial-scale heat treatment process. In both hard- and softwood samples, 13C CPMAS NMR measurements revealed a degradation of less ordered carbohydrates (i.e. hemicelluloses and amorphous cellulose) in the thermally modified wood, which resulted in an increase in the cellulose crystallinity. Furthermore, thermal modification induced changes in the lignin structure by a cleavage of the β-O-4 linkages. In the softwood lignin, a decrease also occurred in the methoxyl group content leading to a more condensed lignin structure.