Nanomaterials 2023 , 13 , 1931
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becomes more flexible and easier to compress, reaching a high sheet density, and this effect is accentuated with the presence of CMNFs. NC also acts by blocking the voids in the 3D network, reducing the ease of air-volume passage within, since compacted NC provides longer tracks for air molecules to pass through [95]. Air-permeability reduction is also related to a decrease in porosity with NC addition [96].
Figure6. Influence of CMNF dosages on air permeability.
It is remarkable that it is possible to achieve a significant air permeability reduction at very-low-refining NC dosages (1.5%), decreasing by 40% and 32% in the NHP and HP papers, respectively. This suggests that a low NC dosage is enough to achieve the desired properties. It was also observed that the best results were obtained using a refining process during the pretreatment to produce CMNFs instead of a tempo-mediated oxidation process (TMO); this can be explained by the fact that with the refining process more flexible fibers with a higher fine content were obtained, leading to an easier network structure to compress. At this low NC dosage (1.5%), a series of tests were performed to calculate the influence of the fibrillation degree on the reduction in air permeability using the fibrillated cellulose obtained using the refining pretreatment. Figure 7 shows that air permeability slightly decreases when the fibrillation degree increases but the improvement does not justify the use of highly fibrillated NC such as TMO. This is important because low-cost fibrillated NC could be used in hot-pressed papers. On the other hand, by increasing refining, it is possible to obtain denser products; this is due to the improvement in fiber flexibility and the higher content of fines.
Figure7. Influence of CMNFs on air permeability at dosage 1.5%.
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