Improved Breaking Length Development of Unbleached Softwood Kraft Pulp in PFI Refining by Addition of Primary Fines

Abstract

Nowadays the so called fines fraction is experiencing increasing interest for the papermaking society, as an essential component of any papermaking pulp. It shows distinctive properties affecting both production process and product properties to a large extent. Several research groups have experimented with either primary and/or secondary fines to assess their corresponding properties in the recent years. When it comes to the influence of primary fines on paper and process, these studies do not always show consistent results, attributed to different raw materials considered, to retention issues during sheet forming or maybe to formation issues arising at higher dosages. This work focuses on the clarification, how primary fines of unbleached softwood kraft pulp (Kappa number ~27) influence the product and process parameters, especially if the total fines amount is risen compared to the original stock. Primary fines are separated from the pulp using a laboratory pressure screen to be added again in controlled amounts afterwards. Thereby three pulp blends, showing a primary fines content of around 5%, 9% and 12% where prepared. These pulp blends were refined in a PFI mill at 1000, 4500 and 6000 revolutions and compared with the unbeaten reference. The refining treatment mainly resulted in fiber flexibilisation and internal fibrillation while barely any secondary fines were produced. Because retention of the fines material might be an issue, a Rapid-Köthen sheet former with white water recirculation was used. The results of paper testing show that the tensile index develops at lower specific refining energy when adding primary fines prior to refining due to increased densification of the sheets. The results also show increased dewatering resistance (Schopper-Riegler) at a given tensile index, while densification and air permeability (Gurley) are comparable. Considering the linear relationship between tensile index and sheet density – independent of the fines content – it can be concluded that fibre flexibilisation and primary fines both enhance fibre-fibre bonding and that both strategies result in the same increase in mechanical strength with the downside of slightly reduced dewatering in case of the introduction of primary fines.