Nanomaterials 2023 , 13 , 1931
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2. Due to a more hydrophobic surface character of the paper after hot-pressing technol- ogy [101]. 3. New water-resistant bonds such as covalent bonds are formed during hot-pressing [102].
Figure12. Influence of fibrillated cellulose on wet strength after hot-pressing technology.
Furthermore, CMNFs can retain more water using hydrogen bonding and within the 3D network than pulp fibers due to their large specific area and the high number of carboxyl groups they contain. Consequently, its addition can improve the hydrogen bonds between the cellulose fibers during the pressing process, thus increasing the web strength. 3.2.4. Tear Resistance One of the drawbacks of hot-pressing technology is that the tear index decreases because the paper obtained has a higher density and it is more rigid. It was decided to test if the addition of fibrillation material could avoid at least partially this loss in tear index. As can be seen in Figure 13, tear index increases with the addition of CMNFs maintaining the fibrillation degree in PS2 and further increases with the fibrillation degree maintaining the CMNF dose at 1.5% (Figure 13). Further improvements were achieved than previous studies by Guan et al. (2019) and Balea et al. (2019) demonstrated, which reported that NC content did cause a slight increase in the tear index [59,103]; however, this effect was lost after hot-pressing as the material was still very rigid, limiting the application of hot-press papers to those applications in which tear index values are not critical.
Figure13. Influence of CMNFs on tear resistance. 4. Conclusions
The synergy between the use of fibrillated cellulose as a bulk additive and hot-pressing has been assessed. This synergy effect is very significant, further improving the properties of hot-pressed papers. Hot-pressing and fibrillated cellulose highly decrease air permeability (by 80% and 68%, respectively) for refining pretreated samples, due to the increased fiber flexibility, which increases up to 90% due to the combined effects. In the case of dry strength, improvements up to 118% can be obtained while individual improvements were 31% by
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