Cellulose
are generated during refining in the stock preparation process and have a strong fibrillar character, as they mainly consist of fibrils torn from the fibre wall structure during pulp refining (Retulainen et al. 1993). Besides this highly fibrillar material, fibre fragments resembling primary fines are also produced during refining due to fibre cutting and shortening. A certain proportion of fibrillar material, however, is also present in primary fines. Hence the distinction between primary and secondary fines is to some extent more a definition of origin of the pulp fines than of morphological properties, although it will always reflect the morphology to large part as well. In order to improve the strength properties of paper it is necessary to increase the relative bonded area and/ or specific bonding strength (Dasgupta 1994; Mota- median et al. 2019). This increase can be achieved by sheet densification, which is positively influenced by addition of fine cellulosic materials such as micro or nanofibrillated celluloses or pulp fines, therefore leading to an increase in tensile properties (Ba¨ckstro¨m et al. 2008; Taipale et al. 2010). In contrast to such micro or nanofibrillated celluloses (Sandquist 2013), which need substantial amounts of energy in produc- tion, fines may be directly accessible from process streams like filtrates, or could be separated from the pulp using suitable aggregates (Hinck and Wallendahl 1999). Due to their different morphological character, primary and secondary fines have a different effect on sheet properties. As the mainly fibrillar material of the secondary fines shows higher surface area and stronger swelling, it has a stronger effect on bonded area and therefore a stronger influence on strength properties of paper compared to the more blocky primary fines (Ferreira et al. 2000; Fischer et al. 2017; Mayr et al. 2017a, b; Motamedian et al. 2019; Odabas et al. 2016). Still, also fine cellulosic materials defined as secondary fines can differ in their fibrillar material content, respectively their fibrillar area (Mayr et al. 2017a). These differences are on the one hand caused by the pulp type being refined and on the other hand by different refining conditions in terms of specific edge load and varying refining intensities (Mayr et al. 2017b). While for secondary fines their effect on strength properties has extensively been reported by several researchers, this is not the case for primary fines, where only a few studies are available focusing on the effects of their removal or addition on paper
technological properties (Ba¨ckstro¨m et al. 2008; Chauhan et al. 2012; Ferreira et al. 1999, 2000; Mayr et al. 2017b; Motamedian et al. 2019). Due to the differences in experimental designs, these studies did not always show consistent results. Chauhan et al. (2012), for example, show that the addition of up to 10 and 20% w/w of primary fines, separated from bleached hardwood kraft pulp by means of a Bauer McNett classifier, prior to refining leads to reduced strength properties of papers produced from these pulp blends after PFI refining compared to the reference without the addition of primary fines. Ferreira et al. (1999), on the other hand, show that removal of primary fines from unbleached hardwood kraft pulp, leads to a decrease in tensile and burst index, but an increase in tear index. In this case, primary fines were removed by a Bauer McNett classifier equipped with a 200 Mesh screen. Results by Ba¨ckstro¨m et al. (2008) in an experimental design, where primary fines were added to the pulp, again show that the addition of either primary or secondary fines to refined fibres allows an improvement of tensile index, burst index and TEA. Fines were separated with a Celleco laboratory filter equipped with a 100 l m mesh size cloth. The fine fraction was further separated by a 20 l m cloth and the passing fraction discarded to exclude colloidal fines material from the investigation. The fines content was determined acording to TAPPI Standard T261. Our work focuses on the effect of different addition rates of primary fines of a softwood kraft pulp to the same pulp and on the effect of this addition rate on pulp and paper properties. The rates of addition are chosen in a range that corresponds to amounts of fines that may also be produced by refining in industrial stock preparation. The blends of pulp and primary fines are evaluated in the unrefined as well as in the refined refined state. Thereby the evolvement of e.g. tensile properties due to addition of fines and/or refining is accessible and allows the assessment of possible savings in refining energy through the addition of in our case primary fines.
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