Effect of mechanical treatment on softwood kraft fiber properties

The aim of this work was to gain a better understanding of the changes occurring in softwood strength properties and fiber wall structure after mechanical treatment was applied to the chip/fiber matrix during kraft cooking. Mechanical treatment of fibers during industrial and laboratory production of pulp is common. In the pulp mill the pulp is mixed, pumped and shear forces are applied to the fiber at different temperatures and under different chemical conditions. The main objective of this research was to investigate the reasons for fiber strength loss and to examine the changes that really affect fiber strength. Fiber damage, changes in the fiber wall structure, reduced single softwood kraft fiber strength and fiber deformations (curl, kinks and dislocations) all affected the fiber network properties. Mechanical treatment at the end of kraft cooking conditions resulted in fiber damage such that single fiber strength was reduced. This increased with increasing treatment temperature. It was concluded that both mechanical treatment at the end of cooking and homogenisation at room temperature of kraft pulp fibers increased the number of fiber deformations. The increase in fiber deformations did not reduce single fiber strength but did affect the strength properties of the fiber network. The fiber damage induced by mechanical treatment also decreased the z-directional strength of the fiber network, which was concluded to be due to separation of the structural elements on the fiber surface layer. The separation of structural elements could enhance the irregularities on the fiber surface layer and so reduce the bonding area. The bulking of the fiber surface layer, e.g. more separated layers, might reduce the ability to resist z-directional stresses. The effect of mechanical treatment on spruce fibers was more severe than on the corresponding pine fibers. The spruce fibers developed more deformations and damage as a result of mechanical treatment. The reasons for the lower bonding and strength properties of the damaged fibers could be due to differences in the fiber wall structures of spruce and pine. Overall the results suggested that fiber damage induced by mechanical treatment during cooking changed the fiber wall pore structure in such a way that the number of links (between fibrill aggregates) in the fibril (aggregate) skeleton of the fiber wall decreased. The reduced contact in the fibril aggregate skeleton of the fiber wall (because of fewer restrictions) affected the cell wall structure so that it could no longer support stresses in the fiber network. The zero-span tensile strength of fibers was not dependent on the degree of fiber deformation, but according to the hypothesis presented above, on the 3-dimensional arrangement of the structural elements in the fiber wall. This 3-dimensional arrangement of the structural elements in the fiber wall defines the axial load bearing ability of the softwood kraft fiber.reviewe