Reinforcing ability of fractionated softwood kraft pulp fibres

Although fractionation has been widely studied, research on the reinforcement ability of different fibre fractions has not been published earlier. In this study bleached softwood kraft pulp was fractionated using hydrocyclones and pressure screens. The fractionated pulps were blended with TMP and GW and the results showed that the type of mechanical pulp (TMP/GW) and the proportion of chemical pulp had the major effect on the blend sheet properties. The long fibre fraction and the thick-walled fibre fraction gave in the mixture with mechanical pulp higher freeness values than the unfractionated pulp. In addition, when the thick-walled fibre fraction was blended with mechanical pulp, the tear index increase was 12-15% for TMP, and 9-20% for GW, depending on the kraft pulp proportion and the tensile index of the kraft pulp. Corresponding increase in tear index of the blend sheets when using the long fibre fraction was up to approximately 10% in TMP and GW blend sheets. This study also showed that it was possible to increase tear index with up to 16% and fracture toughness index with up to 23% of the GW blend sheet by separate refining of the kraft pulp fractions