Cellulose
30 minutes. After the addition of methylene blue to the sample for enhanced contrast of the fines in imaging, microscopy sample holders were prepared with three droplets of the stained fines suspension and dried on a heating plate. Three sample holders per blend were prepared and 200 pictures were taken for each sample. Figure 3 shows an exemplary image depicting fibrils and flakes. Automated image analysis was applied based on MATLAB to obtain values for average equivalent circle diameter of either fibrils and flakes as well as the fibril area (in %) (Mayr et al. 2017a). In addition to the microscopy based characterisa- tion described above we used an Alicona Infinite Focus Measurement device allowing imaging using so called shape from focus. Thereby imaging is possible at 50 fold magnification (corresponds to 0 : 625 l m = p ixe l ) without the usual problems with depth of focus arising when you look at three dimensional particles at high resolution. Imaging was done based on the same samples described above. These imaging method (Fig. 4) gives a much more detailed qualitative impression of the fines and their morphological characteristics.
Fig. 4 Microscope image of separated primary fines taken with an Alicona infinite focus measurement with a magnification of 50
first four hand sheets were discarded to reach an equilibrium of fines in the recirculated white water and to achieve a 100% retention of the primary fines (Giner et al. 2015).
Hand shee t s f orming
Hand sheets of 80 g = m 2 using the unrefined and refined reference pulp and the unrefined and refined pulp / primary fines blends were prepared according to ISO 5269-3 on a Rapid Ko¨then hand sheet former using white water recirculation to avoid losses of primary fines due to retention issues (Giner et al. 2015).The
Hand shee t s p ro p er t ies
Hand sheet testing was performed after conditioning the samples for at least 24 hours in the climate room at 23 C and 50% relative humidity. Apparent sheet density was determined according to ISO 534, tensile properties according to ISO 1924-2 and Gurley air permeability according to TAPPI 460 om-16.
Results and discussion
Pulp properties of reference pulp and pulp/primary fines blends
Figure 5 shows the cumulative length weighted fibre length distribution of the separated primary fines with the L&W fibre Tester þ , showing that 94% of the material is smaller than 200 l m with the remaining 6% of the separated material being below a maximum length of 400 l m.
Fig. 3 Exemplary microscope image showing flakes and fibrils, a flakes, b fibrils
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