Nanomaterials 2022 , 12 , 790
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dye the fibres [32]. Samples were agitated at several velocity gradients (G) up to 3000 s − 1 using an overhead stirrer Heidolph RZR 2051 (Heidolph Instruments GmbH & Co. KG; Schwabach, Germany). Then, 250 mL of each suspension were settled into graduated cylinders until the sediment reached a steady value, which differs depending on the size of the fibrils, and to obtain the complete deposition of the suspension. Figure 2 shows the graduated cylinders of R-CMF stirred at different agitations.
Figure2. Graduated cylinders at different stirring speeds.
The C o was the same for all the samples to favor the comparison between the dis- persion conditions. The C o was chosen to obtain a sediment height of approximately 4–12% of the total height, due to the difficulty to measure the height accurately at lower sedimentation values. On the other hand, higher sedimentation heights would cause the increment of concentrations that appears in Equation (2), for which the derivative has been substituted, moves away from the limit Hs/Ho to close to zero [39]. 2.2.2. Transmission Electron Microscopy To analyze the morphology of the fibers, a small sample of each CMF/CNF suspen- sions was collected after being stirred. They were analysed in the Centro Nacional de Microscop í a (Madrid, Spain) by TEM with a JEM 1400 microscope (JEOL, Tokyo, Japan). To prepare TEM samples, CMF/CNF suspensions were diluted until 0.01 wt.%, and a drop was settled on a copper grid covered with a carbon coating and dried. Image J software was used to process the images. 2.2.3. Preparation and Characterization of Reinforced Cardboard Sheets E-CNF was used to validate the effect of CNF hydrogel dispersion on paper strength. Ø g was used as a tool to determine the best conditions of papermaking application to enhance mechanical properties. Recycled OCC was used in the preparation of sheets due to the importance to increase the strength of cardboards for packaging [43]. OCC and CNF were disintegrated separately and then blended. OCC pulp was prepared through disinte- gration of 70 g of dry OCC in 2000 mL of tap water (3.5 wt.%) by using a pulp disintegrator according to ISO 5263-1 standard [44]. The OCC was left to soak 24 h before disintegration to favor fiber swelling. Separately, the E-CNF solutions (1 kg/m 3 ) were prepared with CNF and tap water in the high-speed overhead stirrer for 10 min at several G values from 3 to 3000 s − 1 . To prepare the handsheets, OCC-CNF pulp suspension (1.0 wt.%) were prepared from both products stirred separately and mixture with a ratio 95.5 wt.% OCC/4.5 wt.%
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