Cellulose
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Fibril Area [%]
AV ECD Fibril [ m]
AV ECD Flake [ m]
60% of the projected area is fibrillar content. In comparison the fibril area of bleached softwood secondary fines is in a range of up to 75% when measured with the given method (Mayr et al. 2017b). The average equivalent circle diameter (ECD) of fibrils and flakes are of compareable size. As there is a considerable amount of fibrillar material also present in primary fines (see also Fig. 4), the positive effect of primary fines on strength properties should not be underestimated. The cumulative length weighted fibre length dis- tribution of the reference pulp and the pulp/primary fines blends in the unrefined and the refined state obtained with the L&W fibre Tester þ are depicted in Fig. 8. While the influence of primary fines addition is highly significant, a comparably low influence of refining on the fibre length distribution is visible. This shows that only a low amount of secondary fines was generated and that hardly any shortening of long fibres occurred during PFI refining, indicating that the PFI mill treatment leads mainly to fibre flexibilisation due Fig. 7 Characterization data of separated primary fines showing average equivalent circle diameter of fibrils and flakes as well as the fibril area (errorbars indicate 95 % confidence interval)
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Fig. 5 Cumulative length weighted fibre length distribution of separated primary fines
This cumulative fibre length distribution clearly shows that the separated material consists almost exclusively of fines and contains hardly fibres at all. Still, the evaluation of fines in such a flow cell is prone to misinterpretation of the material due to two reasons. On the one hand the fines already are—at least in one dimension—close to the minimum resolution of such a measurement device. On the other hand, especially fibrilar material is highly swollen and tends to show too low contrast to be accessible for such a device. Therefore the method based on microscopy and subsequent image analysis (Mayr et al. 2017b) is necessary, to assess the morphological properties of the fines material. This method (Fig. 6) shows, that besides the expected flake like material also a significant amount of fibrillar material is present in the separated primary fines. This is also confirmed by the the high value for fibril area (Fig. 7), showing that
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Fig. 8 Cumulative length weighted fibre length distribution of blends after refining in comparison to the unrefined references
Fig. 6 Microscope image of the separated primary fines
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