Nanomaterials 2022 , 12 , 790
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hydrogel. At this low agitation speed, R-CNF is not well dispersed with clusters in the suspension. The nanofibrils are not effectively separated and the surface area low with the highest diameter geometric mean. Increasing velocity gradient up to 70 and 125 s − 1 , the lowest Ø g are obtained and the length of fibers barely changes but the number of nanofibers with high diameter (>30 nm) decreases, indicating that clusters disappeared and the network is slightly spongier. Since the length of the fibers is maintained and the mean diameter decreases, the highest AR is obtained. Higher gradients as 500 s − 1 produce an increase in Ø g associated with the shortening of fibers and, to a lesser extent, a reduction in diameter. Extreme agitation (around 2500 s − 1 ) almost halves the length of the fibers and sedimentation deposits are very compact, with the reduction of the percentile 95th below 30nm. R-CMF shows the same trend as R-CNF but at different stirring speeds. Low agitation (50 s − 1 ) shows the higher length as the same time the higher diameter due to clusters and fibers not separated. Then, with the intermediate agitation (500 s − 1 ), the minimum Ø g , fiber length is almost maintained but shows a drastic decrease in the diameter distribution due to the separation of the biggest fibers producing a spongy network. The effect of refining produces the separation of the microfibrils but not their breakage, resisting a higher speed than R-CNF pretreated by TEMPO mediated oxidation in which there are less bundles of fibers around the primary structures that allow the creation of networks. However, as in R-CNF, higher velocity gradients than the minimum Ø g (around 900 s − 1 ) show the break of the open network with the shortening of the length of nanofibrils while the diameter is maintained. This effect is also observed with an extreme agitation that halves the fiber length. In short, the agitation (stirring speed) at which the minimum Ø g is produced for each CMF/CNF hydrogel would be the best agitation conditions to avoid aggregation of CMF/CNF on the one hand, and the shortening of the fibrils on the other. 3.3. Effect of CNF Dispersion on the Mechanical and Physical Properties of Paper The effect of CNF dispersion degree before being added to the OCC pulp disintegrated has been studied on the reinforcement of cardboard. In addition, the effect of the dispersion of CNF and the disintegration of OCC pulp at the same time has also been studied. E-CNF dispersed at different levels, the same used to determine the minimum Ø g (Figure 3), has been studied before the addition in the OCC pulp that was previously disintegrated at 30,000 revolutions (around 3000 s − 1 ) in a pulp disintegrator. Figure 7 shows physical (porosity and bulk) and mechanical properties (tensile, bursting, SCT and tear indexes) of the handsheets prepared when CNF was stirred separately. In addition, Ø g andARwere included in Figure 7a,c, respectively, to favor the comparison of the samples. On the other hand, Figure 8 shows the results obtained when the dispersion of CNF and disintegration of OCC was carried out at the same time. OCC blank has the highest Bendtsen porosity of 10.1 μ m/Pa · s. Substituting 4.5 wt.% OCC by CNFs previously stirred and added after OCC pulping, porosity is reduced in more than 60% in all cases. This value depends on the agitation speed of CNFs, with a lower reduction at 3 s − 1 , due to CNFs not being well dispersed yet. However, at 100 s − 1 it is produced the higher reduction of the porosity up to 1.4 μ m/Pa · s that represents a reduction of more than 86%. This fact suggests a higher occupation of pores when CNFs are applied and well dispersed in the handsheets [54,55]. This porosity is almost maintained with a slightly increase until 500 s − 1 . Then, porosity increases again at 3000 s − 1 until 2.5 μ m/Pa · s. This fact is produced at the same time Ø g increases again, due to the shortening of the nanofibers and the break of the network leading to a decrease in the occupation of pores. In addition, a decrease in the CNF size could produce a higher loss of CNFs during the production of the handsheets in the sheet former, moving a higher CNF propertion to the process water. As for the other physical property tested, the bulk, all samples studied have the same result as Figure 7b shows, indicating the replace of a low content of OCC by CNFs do not have influence on the thickness neither the basis weight of the handsheets.
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