Nanomaterials 2022 , 12 , 790
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CNF. Then, cationic starch (0.5 g CS/100 g dry pulp) was added as retention agent and stirred for 30 min at low speed previous the sheet formation. Finally, pulp suspensions were prepared with basis weight of 80 g/m 2 according to standard ISO 5269-2 by using a Rapid Köthen sheet former [45] (PTI, Vorchdorf, Austria). Handsheets were conditioned at 25 ◦ C and 50% humidity for at least 24 h before physical and mechanical characterization. In addition, to compare the effect of OCC and CNF disintegrated by separate or together, OCC and CNF were stirred at the same time in the pulper at 3000 s − 1 for 10 and 60 min (30,000 and 180,000 revolutions) with the same rate CNF/OCC as previously. To characterize the handsheets, mechanical properties were calculated using the av- erage grammage of handsheets. Then, bursting strength index (kPa · m 2 /g), SCT index (N · m/g), tensile strength index (kN · m/kg) and tear index (mN · m 2 /g) were evaluated. Tensile strength was measured in an MTS Criterion Mode 43 from MTS Systems Corpo- ration (Eden Prairie, MN, USA), following ISO 1924-3 standard [46]. Bursting strength was determined using a Messmer Büchel digital hydraulic board burst tester (Veenendaal, Netherlands) according to ISO 2759 standard [47]. A short span compression tester (Mess- mer Büchel, Veenendaal, Netherlands) was used to measure the short-span compressive test (SCT) according to TAPPI T826 standard [48]. Finally, tearing resistance was measured with a Tearing Tester from Lorentzen & Wettre (Stockholm, Sweden) according to ISO 1974 standard [49]. Physical properties measured were porosity, basis weight, and thickness. Porosity ( μ m/Pa · s) was evaluated in a Bendtsen Porosity Tester number 8699 from Anders- son & Sørensen (Copenhague, Denmark) according to ISO 5636-3 [50]. Basis weight and thickness were determined according to ISO 536 and ISO 534, respectively [51,52]. Finally, bulk (cm 3 /g) indicates the thickness in relation to the basis weight.
3. Results and Discussion 3.1. Evaluation of the Dispersion Degree of CMF/CNF Suspensions
Ø g results were obtained by sedimentation of the CMFs/CNFs until the formation of a stable deposit. In the case of R-CNF and E-CNF with smaller sized fibers, the clear zone at the top of the cylinder is not formed at the low concentrations required for Ø g . This fact is due to the small size of fibers and a slow sedimentation, not allowing the distinction of the clear and coalescence layer at short times [32]. Therefore, the sedimentation curve obtained is quite different to the conventional one and only the interphase that indicates the formation of deposits is observed, in the base of the cylinders. At the beginning, the deposits increase over the time due to the compaction of part of material from the coalescence layer until all material has a compaction grade enough to be visible. In this point, we observe a clear separation of the compacted material that continues to compress until the sample has completely sedimented [32]. The height of this layer decreases until the height does not vary with time, and then, Ø g is calculated. On the other hand, C-CMF and R-CMF show a conventional sedimentation. Figure 3 shows the Ø g of the different CMF/CNF hydrogels at different stirring speeds. To facilitate the study of dispersion, Equation (3) was used to simplify the experimental labour using a unique C o , the same for all stirring speeds but different between hydrogels, having as long as possible a H s /H o from 4% to 12%. C o used were 1.5 kg/m 3 forC-CMF, 0.15kg/m 3 for R-CMF, 0.25 kg/m 3 for R-CNF and 1 kg/m 3 for E-CNF. The use of the same C o to calculate the Ø g does not allow its estimation with precision at the higher speed of C-CMF, R-CMF, and R-CNF, since the sediment height was very scarce. Therefore, these values were not shown in Figure 3 although they would be used in the estimation of AR in Figure 4. Increasing the velocity gradient, we can observe a decrease in the Ø g decrease until a minimum value, then at a certain velocity gradient, Ø g increase again. In those samples that presented a very low deposit at high speed (2500 s − 1 for R-CMF and R-CNF and500 s − 1 for C-CMF) the Ø g should be higher than the last value calculated in Figure 3 for each hydrogel.
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