Studies on the ultrastructure of natural fibres and its effects on the fibre utilization

The ultrastructure of plant fibers has a significant effect on the properties of plants as well as on the behavior of the plant material undergoing various processing conditions, which logically affects the the final applications of plant-based materials. In this thesis, the ultrastructure of natural fibres was analysed using several complementary techniques to gain new insights into the ultrastructure of both wood and non-wood fibres. The hierarchical structure of the natural fibre cell wall was found to play a vital role in cell wall reactivity and the material properties of fibres. Fibre defects, disruptions in the cell wall hierarchy, were found to have significantly increased susceptibility to chemical reactions when compared to intact fibre sections. An increase in the amount of defects was also found to slightly decrease the fibre strength properties. Another hierarchical feature, the microfibril angle was found to govern several properties of fibres and wood material. High strain to failure could be seen in both paper handsheets and samples of juniper wood, a species with a high microfibril angle. The effect of microfibril angle overrides the effects of chemistry and fibre dimensions when explaining the fibre properties. The distribution of the cell wall components were, in fact, found to be similar in all of the wood species analysed. However, differences could be found in the distribution of lignin functional groups between spruce and pine. Differences in coniferyl aldehyde group distributions were seen also in size exclusion chromatography measurements of isolated lignin samples. Such deviation between pine and spruce was used to explain the differences in their behaviour during thermomechanical pulping. Size exclusion chromatography also revealed differences in the ultrastructures of similarly processed eucalyptus and birch pulps. Eucalyptus pulp was found to have considerable amounts of lignin associated with the cellulose fraction of the pulp, whilst in birch most of the lignin was associated with hemicelluloses. Such differences can be of use when optimizing bleaching processes to decrease the consumption of chemicals.