Cellulose
Funding
Open access funding provided by Graz University of
sheets) at a similar or slightly increased level of dewatering resistance.
Technology.
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Conclusion and outlook
For unbleached softwood kraft pulp the addition of primary fines originating from the same pulp leads to an increase in tensile strength in the unrefined state as well as at low refining intensities. The maximum achievable level of tensile strength on the other hand is not increased by the addition of primary fines, but stays at the level of the reference. However, in terms of refining energy the addition of primary fines shows the potential to reach a certain level of tensile strength at lower refining energy consumption at similar or only slightly increased dewatering resistance. At the same time the apparent density is slightly lower at a given tensile strength in the unrefined state and at low refining intensities. At higher refining intensities the addition of primary fines does not contribute to the level of tensile strength, as the fibre network is already highly densified due to fibre flexibilization in PFI refining. Here, additionally added primary fines mainly tend to block pores and thereby increase dewatering resistance and reduce air permeability. In this laboratory study, primary fines showed a positive contribution to fibre fibre bonding at lower refining intensities. A direct comparison to the effect of secondary fines generated during refining is not possible based on this study, as only a very low amount of secondary fines was produced during the PFI refining. This will have to be observed, when it comes to the applicability of our results under industrial conditions, which is planned as a next step in our evaluations. An industrial refiner will produce a higher amount of additional secondary fines and might also show some fibre shortening during refining. It will therefore be interesting to see, to which extent the results of these trials in an industrial setting differ from the findings reported in this lab study. We are grateful for the support by our industry partners in the frame of the FLIPPR 2 project, Mondi, Sappi, Zellstoff Po¨ls AG, a member of Heinzel pulp, and Papierholz Austria. The K-Project FLIPPR 2 is funded as part of COMET-Competence Centers for Excellent Technologies promoted by BMVIT, BMWFW, Styria and Carin-thia. The COMET program is managed by FFG. Acknowledgments
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