Nanomaterials 2023 , 13 , 1931
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The high differences between the hot-pressed and non-hot-pressed samples could be explained because of the collapse of the fiber network causing a higher density. In fact, the density of the hot-pressed papers reaches values around 800 kg/m 3 , almost double the average density of the non-hot-pressed papers 450 kg/m 3 . However, this is not the only factor as no differences were obtained in the densities of the non-hot-pressed sheet independently of the NC dosage or of the fibrillation degree. The reduction of bulk with NC addition can be also associated with intrinsic NC properties, the better fiber network accommodation entails more compacted papers [97]. The synergy between the use of fibrillated cellulose as a bulk additive and hot-pressing is very significant. Hot-pressing and fibrillated cellulose highly decrease the air perme- ability (80% and up to 68%, respectively) for refining the pretreated samples, due to the increased fiber flexibility, which increase up to 90% using the combined effect. Figure 8 shows SEM images of different samples; as it can be observed, the surface of the samples containing CMNFs is covered by the fibrillated cellulose reducing and closing the voids formed in the 3D fiber network by the larger fibers and, as consequence, these papers have a lower air permeability. 3.2.2. Dry Strength Figure 9 shows the increase in tensile index (tensile strength divided by the grammage of the sheet) when increasing the dosage of fibrillated cellulose produced with a PS2 homogenization sequence. The fiber network strength increased by increasing the number of bonds. The tensile index increased from 28.3 ± 1.2 kN · m/kg to 37.2 ± 1.0 kN · m/kg (31.4%) in non-hot-pressed papers and from 54.6 ± 2.7kN · m/kg to 61.9 ± 1.8kN · m/kg (13.4%) in hot-pressed papers when adding refining CMNFs. Due to their high aspect ratio and large relative surface area, CMNFs can enhance the bonding between fibers and even form a more crosslinked network [98]. The synergy between the use of fibrillated cellulose as a bulk additive and hot-pressing is very significant. In the case of dry strength, improvements up to 118% are obtained while individual improvements are up to 31% by CMNFs and 92% by hot-pressing (see Figure 9). It should be noted that the addition of fibrillated cellulose also increases the tensile index after hot-pressing but to a lower level. However, this increment is small in comparison to the effect of the hot-press itself, which almost doubles the tensile index. This effect is associated in part to the presence of lignin in the HYP [18]. Therefore, the fiber bonds are not the only responsible aspect of this increase after hot-pressing, supporting the theory that paper is strengthened by new covalent bonds, possibly in a crosslinking structure within the lignin and/or between the lignin and carbohydrates. This can also be demonstrated when applying 1.5% of refining CMNFs at different fibrillation degrees as it is shown in Figure 10. As expected, compression strength (SCT) followed the same pattern as tensile index as shows in Figure 11. 3.2.3. Wet Strength It is well known that independently of the type of pulp used to produce the paper, the inter-fiber bonding is unstable in water in untreated papers. As fibrillated cellulose has a high capacity to absorb water, normally the wet strength of paper containing NC decreases except in the cases in which the fibrillated material is treated to become hydrophobic.
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