Softwood kraft pulp fines: application and impact on specific refining energy and strength properties

Daniel Mandlez,Wolfgang Bauer,Rene Eckhart,Lukas Zangl-Jagiello

doi:10.1007/s10570-020-03467-1

Abstract

Along with the emergence of micro and nanofibrillated celluloses and their application in papermaking, the influence of the so called fines fraction of pulps on both process and product properties has received increasing research interest in recent years. Several researchers have experimented with primary and/or secondary pulp fines to assess their effects on paper properties with not always consistent results. Our work focuses on the targeted application of the primary fines fraction of an unbleached softwood kraft pulp. The primary fines are separated from the pulp to be subsequently added to achieve blends of 5%, 9% and 12% primary fines content. These blends were then refined in a PFI mill to evaluate the effect of the primary fines on refining as well as on paper properties of hand sheets prepared from these pulps. It is shown that the addition of primary fines enhances tensile strength in the unrefined and slightly refined state, while the maximum tensile strength of the highly refined reference pulp is not increased. A slightly increased dewatering resistance (Schopper Riegler) at comparable air permeability (Gurley) for a given tensile strength was also observed. The linear relationship between tensile index and apparent sheet density seems to be affected in the unrefined and slightly refined state where the breaking length of the fines enriched samples is higher for a given apparent density.

Highlights

In the evaluation of fibre morphological properties of any given pulp it is standard to differentiate between the coarse and the fines fraction
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
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

Summary

Introduction

In the evaluation of fibre morphological properties of any given pulp it is standard to differentiate between the coarse and the fines fraction. The fines fraction is commonly either defined as material passing a 76 lm screen (200 mesh) in a Britt Dynamic Drainage Jar Device (according to SCAN-CM 66:05), or particles smaller than 200 lm in case of determination by automated optical analysis (according to ISO 16065-2) (Mayr et al 2017b). Depending on their origin, pulp fines have different morphological character and are usually divided in primary and secondary fines (Odabas et al 2016). The distinction between primary and secondary fines is to some extent more a definition of origin of the pulp fines than of morphological properties, it will always reflect the morphology to large part as well

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Conclusion