Application of Modern NMR Spectroscopic Techniques to Structural Studies of Wood and Pulp Components: Dissertation

Various NMR spectroscopic techniques in both solid and liquid state were applied to investigate the polymeric components of wood and pulp. The aim of the work was to obtain new structural information on the changes occurring in the morphology of cellulose and in the lignin structure during various chemical pulping related processes in order to help optimising the conditions for efficient delignification and bleaching reactions without compromising pulp yield or strength properties. Mainly the effects of kraft pulping and oxygen delignification were investigated, but some samples of sulphite, PS-AQ and soda-AQ pulping were also studied. One of the main objectives of this research was to investigate if some parts of the spruce fibre are affected unequally by kraft pulping. Therefore, the residual lignin structure and the crystallinity of cellulose in ray cells and on the fibre surface were compared to the corresponding structures inside the fibre by isolating fines fractions before and after refining of kraft pulp. Mainly softwood components were considered throughout this thesis. According to the 13C CPMAS measurements the crystallinity of cellulose increases during pulping and the metastable cellulose Iα is converted to the more stable cellulose Iβ polymorph. Various pulping methods or hemicellulose contents were not, however, observed to affect cellulose crystallinity. A slightly lower degree of cellulose crystallinity was found in birch pulps compared to the corresponding pine pulps, and the birch pulps were also assumed to contain larger amounts of well-ordered xylan. After kraft pulping, cellulose crystallinity was found lower in ray cells and on the fibre surface compared to the long fibre fractions. Refining was observed slightly to facilitate the cellulose crystallization as well as the cellulose fibril aggregation during drying due to the better swellability of the fibres. A slight increase in cellulose crystallinity during oxygen delignification was also observed, whereas during the QPZP-bleaching sequence the degree of cellulose crystallinity decreased slightly.The results of 13C CPMAS as well as 2D HSQC and 3D HSQC-TOCSY easurements show that most of the original structures identified in MWL are still present in technical lignins, although their relative proportions vary after kraft pulping and oxygen delignification. The cleavage of aryl ether linkages during kraft pulping and the preserving effect of oxygen delignification on aryl ether linkages were observed in both solid state and in solution NMR studies. However, some reactive structures, e.g. β-O-4 and dibenzodioxocin structures were still found in residual lignin after kraft pulping and some of the reactive structures were shown to survive even in the dissolved spent liquor lignin. Similarly, residual lignin is still to a certain extent phenolic after oxygen delignification. The reactivity of those functionalities may thus be hindered by their involvement with less reactive condensed structures or LCcomplexes. According to the dipolar dephasing measurements, the condensed aromatic lignin structures were found to enrich into fibres during pulping and oxygen delignification, whereas the less condensed lignin structures were removed already in the early stage of pulping. However, condensed diphenyl methane structures supposed to be formed in kraft pulping conditions, could not be found in the residual lignins. Evidence of residual lignincarbohydrate complexes was obtained indirectly by ordinary 13C CPMAS measurements as well as by proton spin-lattice relaxation measurements. According to these results, interactions between cellulose and lignin are possible. In ray cells and on the fibre surface the suggested interactions are more prominent than in the fibres. Otherwise, no significant differences in residual lignin structure between fines and long fibres were observed. Residual lignin of fines was found to be less phenolic and only very slightly more condensed than the residual lignin of long fibres.