Cellulose (2021) 28:5807–5826
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Fig. 2 Scheme of experiments. In the fractionation, the values refer to wire meshes used (ASTM series). The R30, R100, R200, and R400 fractions were collected from the wires and vessels
after the fractionation. An additional sedimentation phase was added after the fractionation, and this yielded the ‘pass’ fraction. The bold text indicates the samples taken for further analysis
Scanning electron microscopy
solution at least 18 h before the measurements were made. Some of the coarsest fractions could not be measured according to the recommended procedure due to the poor water retention of the fiber cake and resulting water overflow. A filter paper (Macherey- Nagel MN 616) was therefore used, as instructed in the manual. Use of the filter paper was however found to affect the absolute zeta potential value.
The PCC-CTMP samples were characterized by scanning electron microscopy (SEM) using a Hitachi SU3500 equipped with a wolfram filament and backscatter electron (BSE) detector to capture micro- graphs of the samples. Composition (COMP) mode was used in order to obtain good contrast between PCC and the cellulosic fiber; the PCC particles or PCC rich areas therefore show lighter than areas free from PCC. The imaging was done on non-coated samples using the variable pressure (VP-SEM) mode at 60 Pa, 15 kV acceleration voltage and a working distance of 10 mm. These images are labeled with the text VP- SEM. The coated samples (sputter with an Au/Pd target) were imaged using secondary electron imaging (labeled SEM in the images), an acceleration voltage 15 kV and a working distance of 10 or 15 mm.
Surface charge density (cationic-demand titration)
The charge density of the pulps and fractions was measured using a cationic-demand titration procedure (PCD 02, BTG Mu¨tek GmbH and Mettler DL25, GWB for titrant dosing). Titration was done as back titration as recommended by the manufacturer, although the amount of sample was modified. The amount of fiber slurry containing approxi- mately 0.2 g of fiber was filtered on a polycarbonate membrane (GE Water & Process technologies, 3.0 l m) and rinsed into a decanter using deionized water to obtain a 5.0 g sample. The amount of poly(diallyldimethylammonium chloride) (PDAD- MAC, 1.0 mN, M = 107 000 g/mol, BTG) solution added was 20 g. Fibers were removed from the residual PDADMAC by centrifugation (6400 g). Titration was carried out using polyethene sodium sulfonate (PES-Na, 1.0 mN and M = 19 100 g/mol, BTG). After the titration, the fiber weight was determined by drying the fiber overnight at 105 C. The results are given as averages of three or four measurements.
Handsheet testing
The handsheets were conditioned (23 C and 50% RH) and tested according to the SCAN standards SCAN P6:75 (grammage), SCAN P 7:96 (density and thickness), and SCAN P 38:80 (tensile index, tensile stiffness, and elastic modulus). The tensile and tensile stiffness indices were additionally calculated using a fiber-weight-based index, i.e. the ash weight was subtracted from the grammage used in the calculation. When the fiber-weight-based index is discussed, this is indicated in the text. Brightness and opacity were measured using Elrepho (L&W) according to standards ISO 2470-1
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