Abstract:In the pulp and paper industry different types of pulp or fiber fines are generated during the pulping (primary fines, mechanical fines), and/or the refining process (secondary fines). Besides fibers, these cellulosic microparticles are a further component of the paper network. Fines, which are defined as the fraction of pulp that is able to pass through a mesh screen or a perforated plate having a hole diameter of 76 µm, are known to influence the properties of the final paper product. To better understand the effect and properties of this material, fines have to be separated from the pulp and investigated as an independent material. In the present study, fines are isolated from the pulp fraction by means of a laboratory pressure screen. To allow for further processing, the solids content of the produced fines suspension was increased using dissolved air flotation. Morphological properties of different types of fines and other cellulosic microparticles, such as microfibrillated celluloses (MFC) are determined and compared to each other. Furthermore, handsheets are prepared from these materials and properties, such as apparent density, contact angle, modulus of elasticity, and strain are measured giving similar results for the analyzed types of fines in comparison to the tested MFC grades. The analysis of the properties of fiber fines contributes on the one hand to a better understanding of how these materials influences the final paper products, and on the other hand, helps in identifying other potential applications of this material.

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The rheological behaviour of homogenised fibres originally having different lengths was evaluated. For this purpose, mixtures of pulp fibres and fines were fibrillated mechanically without pretreatment and characterised with regard to morphology and viscosity. It was found that, for all samples, a similar number of homogenisation passes was needed to reach a viscosity plateau. However, the value of the final viscosity differed significantly: homogenised suspensions derived from fines achieved only about 60 % of the viscosity of suspensions derived from pulp. Already after a few homogenisation cycles, no differences between the samples could be measured using optical devices, indicating that fibrillation on the nanometre scale was responsible for the distinct rheological behaviours. Atomic force microscopy measurements indicated significantly reduced fibril lengths for the suspensions derived from fines, which explains their reduced viscosity.

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The swelling behavior of pulp fibers has a significant influence on process and product properties. The water uptake of fibers is controlled by a different mechanism. While fiber charge is a driving factor for swelling, the swelling is hindered by the solid structure of the fiber wall. In the case of the fines fraction of pulps, this structure is broken to some extent and the fines are able to swell two to three times more compared to fibers. Thus fines are an important factor regarding the swelling behavior and water retention of pulps, although, at least for chemical pulp, their mass fraction is only between 4 and 15%. For this reason, it is of interest to investigate not just the swelling behavior of pulps, but also of the fiber and fines fractions separately. Swelling is often characterized using the water retention value (WRV) based on a centrifugation technique. WRV measurement is a standardized method for the measurement of the amount of water retained in a given pulp sample. For fine cellulosic materials the standardized procedure cannot be performed. Thus, various modifications of the standard method have been applied by different groups for the evaluation of these materials. Due to these modifications the values obtained cannot be related to the standardized method. In this work a novel approach to determining the WRV of the fines fraction in a given pulp based on the standard procedure will be presented. This allows a quantitative investigation of the contribution of the fibers and fines fraction to the WRV of any given pulp sample.