Different approaches to tailoring chemical pulp fibres

Abstract

The objective of this thesis work was to examine different approaches to tailor chemical fibres of different raw materials. The focus in searching for new approaches was on pressure screen fractionation, selective treatment of each fraction, mechanical pre-treatment before refining, refiner loadability and its link to fibre properties and filling design, and on-line quality control of fibre properties. The evaluation is based on the impacts on fibre properties, filtration, refining and the resulting paper properties. Tailoring of fibres using pressure screen fractionation was found to produce a long and coarse (reject) fraction offering high dewatering efficiency, homogeneous and energy-efficient refining, and better strength properties, such as tear index, bulk and fracture toughness, in pure and mixed sheets with other fibres. Although the accept fraction contains short and thin fibres and has a high fines content, it proved possible to use the accept fraction to increase the bonding and scattering of once dried softwood and to reduce the refining energy input needed to reach a certain tensile strength level. A new mechanical pre-treatment was examined and found to promote lumen collapse and de-swelling of fibres, and hence to improve the strength-dewatering combination of softwood kraft pulp. The treatment involved application of linear loads, heat, and shear forces over multiple passes. In refining, the pre-treated fibres produced better dewatering and a consolidated structure with less cutting, fines creation and external fibrillation compared to never dried fibres. The pre-treated fibres offer better potential for developing a higher tensile index, stiffness and Scott bond than once dried fibres at a certain degree of refining. Refiner loadability and gap movement are strongly related to fibre properties and filling design. Fibre properties together with pulp consistency contribute to the size and strength of flocs building up inside the refiner, where big and strong flocs are loaded earlier and maintain a wide gap with less floc size changes. Here, pulp consistency was found to have a smaller effect than fibre properties. Filling design, reflected in the cutting edge speed, was found to contribute strongly to the gap movement and refiner loadability. An increase in edge cutting speed caused the refiner gap width to decrease linearly, thus enhancing different refining effects such as fibre cutting, fibrillation and fibre swelling. A factor network linking on-line measured fibre properties, calculated factors and predicted paper properties was found to be an effective tool for monitoring changes in pulp quality, such as different raw materials with different average fibre lengths. The model was built off-line and tested against on-line mill operation and found to be effective in predicting paper properties of both never and once dried pulp. The refining model was tested with a laboratory refiner and used to explain strength properties such as tensile index, to monitor changes in paper properties due to refining and to determine the optimum refining conditions and different refining effects such as bonding, straightening and fibre cutting.