Applicability of fractionation of softwood and hardwood kraft pulp and utilisation of the fractions: Dissertation

Fractionation of final or semi-finished fibres gives more advanced possibilities to design pulps with unique fibre characteristics. The objectives of this thesis were to clarify the applicability of fractionation of softwood and hardwood kraft pulp and utilisation of the fractions. Fractionation of softwood, birch and eucalyptus pulps gave fractions with substantially different physical and chemical properties. The contents of lignin, extractives and some metals were higher in the birch and softwood accept fraction because of the primary fines. Removal of primary fines from the oxygen-delignified birch kraft pulp gave a higher final brightness at slightly lower active chlorine consumption in DEDeD bleaching and improved brightness stability. When the birch fines were bleached using a ZeQP sequence, the extractives content of the fines fraction was reduced by 40%. The hydrocyclone- and pressure screen-fractionated softwood pulps were blended with thermomechanical (TMP) and groundwood (GW) pulp. The softwood long fibre fraction and the thick-walled fibre fraction gave in the mixture with mechanical pulp higher freeness values than the unfractionated pulp. The thickwalled fibre fraction gave clearly higher tear index values than the feed pulp in the mixture both with GW and TMP. Also, the long fibre fraction gave somewhat higher tear index values, especially when mixed with GW. Softwood thin- and thick-walled fibre fractions were refined separately. Fractions that were separately refined and after refining re-combined were then blended with GW. Separately refined fibre fractions in all cases gave higher freeness and higher fibre length of the chemical pulp-GW mixture than the unfractionated pulp-GW mixture. It was possible to increase the tear index by up to 16% and the fracture toughness index by up to 23% of the GW blend sheets by separate refining of the softwood kraft pulp fractions. Through hydrocyclone fractionation of birch pulp, a coarse fraction was obtained with a high tensile stiffness. The fine fraction had a high bonding ability. The coarse fraction could be used in the top layer of board or in fine paper. The fine fraction obtained by hydrocyclone and screen fractionation could be used for bonding in the middle layer of board making it possible to use coarser mechanical pulp. The vessel-picking tendency of eucalyptus pulp was significantly reduced by removing vessel elements from the pulp using hydrocyclone, and also by refining the vessel-rich fraction.