A CP/MAS 13C-NMR study of supermolecular changes in the cellulose and hemicellulose structure during kraft pulping

During kraft pulping of spruce wood, the structure and interactions of the polysaccharides present in tracheid cell walls are affected at the supermolecular level. Two consecutive changes in the supermolecular structure were observed in the CP/MAS C-13-NMR spectra of the pulps as cooking proceeds: an increase in cellulose crystallinity and a change in order and/or structure of hemicellulose. The incidence of these structural transformations seems to be closely associated with changes occurring in the kinetics of both the hemicellulose and lignin extraction. The cellulose crystallinity was suddenly increased when the cooking temperature reaches 170 degreesC and this event coincided with the transition from initial to bulk phase. A gradual change in the spectral behaviour of the hemicelluloses was observed during the kraft pulping. This gradual change appears to occur as a consequence of changes in intermolecular aggregation of the hemicelluloses.