Single fiber properties - a key to the characteristic defibration patterns from wood to paper fibers

Abstract

This study approaches the phenomena of thermomechanical defibration of wood by examining single fiber properties. A hypothesis was formed based on literature: The defibration patterns due to impacts during fiber separation and high-consistency refining are related to the morphological properties of fibers. This is because: The effects caused by the action of defibration vary in morphologically different fibers, and

The defibration action in the plate gap is influenced by the properties of the fiber material in the plate gap and changes as the properties of the fiber material in the plate gap change. In other words: the character of the fiber material affects the defibration result through two routes: firstly the defibration patterns of fibers are related to their properties, and secondly the character of the fiber bed in the plate gap influences the defibration action. The defibration patterns of fiber shortening, fiber wall thickness reduction and changing of fiber wall internal structure are discussed. The experimental part focuses on the defibration effects which are the measurable deformations in the fibers as a result of defibration actions. Defibration patterns are a set of defibration effects that develop the stiff wood fibers into papermaking fibers and fines particles. These concepts can be applied to all mechanical pulping processes, but this thesis focuses on their application to the TMP process and spruce as raw material. Fiber shortening was the result of cutting of fibers during the fiber separation stage. Fast-grown and thinner-walled fibers were more resistant than thick walled slow growth fibers towards the harsh conditions prevailing during fiber separation. Faster warming up of the fiber wall as a result of compression and relaxation of the material as well as encountering fewer shear forces than thick-walled incompressible fiber material were suggested as explanations for the different response of these fibers. The gradual peeling off of layers from the fiber surface resulted in reduction of fiber wall thickness. Different types of fibers produced different types of fines. The fibrillar fines were formed mainly from thick-walled fibers, and 50-100% of the fibrillated fibers originated from latewood. The flake-like fines originated both from outer layers of latewood and earlywood fibers during fiber separation stage but also from pieces of the thinwalled earlywood fibers during the later stages of the whole defibration process. Only half of the fines fraction was formed as a result of peeling off of fiber wall. The rest consisted of ray cells, pieces of fibers and fiber wall formed as a result of fiber cutting or splitting. The differences in the fiber wall thickness did not explain the flexibility differences between initially refined and highly refined samples. From this was concluded that the fiber wall flexibility increased and fiber wall structure loosened during the defibration. Local swelling of the fiber wall was revealed using optical sectioning by confocal laser scanning microscopy. As a result of this inhomogeneity both fiber wall swelling and fiber conformability varied along the fiber length. Removal of the outer fiber layers increased fiber flexibility by decreasing fiber elastic properties and lowering the moment of inertia.