Nanomaterials 2023 , 13 , 2536
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fibrillation. The most significant impact of HPH on morphology was observed in TEMPO_5 and TEMPO_15 CNFs. In both cases, the number of visible elements (Figure 3) decreased with increasing HPH intensity when observed through optical microscopy. This reduction was attributed to the intense internal nanofibrillation resulting from the combination of shearing forces and electrostatic repulsive forces. Figure 7 shows how the pretreatment determines the potential of HPH to enhance the properties of CNFs. In all cases, CNFs produced solely through mechanical pretreatment and HPH exhibited the lowest values of yield, transmittance, and CD, but the highest values of aspect ratio, measured by GP. Mechanical pretreatment caused external fibrillation and increased the accessibility of cellulose for subsequent treatments. However, it had limited impact on the yield and transmittance of CNFs. The energy consumption for each HPH sequence depends on the applied pretreatment. Enzymatic hydrolysis and TEMPO- mediated oxidation are more efficient in reducing energy consumption for homogenization as shown at Figure 7. The most efficient was TEMPO_15, as expected from the high anionic groups introduced on the fibers that generate electrostatic repulsive forces among cellulose chains helping nanofibrillation. Nanofibrillation yield and transmittance increased notably with energy consumption and with NaClO used in TEMPO-mediated oxidation. However, differences caused for the enzyme dose were not significant except for aspect ratio. This could indicate that the effect of the enzyme is limited by other variables, for example, due to poor accessibility of the enzyme to the cellulose chains due to the lower swelling ability and the presence of lignin. Figure 6 shows that Enz_80 and Enz_240 CNFs have different behavior with water. The latter tends to release water when it is deposited in a Petri dish. The interaction with water is key for GP determination because it affects the settling process. This differs from the results obtained for virgin pulps by Sanchez-Salvador et al. (2022) who observed a notable increase in transmittance, yield, and cationic demand by increasing the dose of enzyme threefold [27]. This evidences the effect of raw material composition in CNF production by enzymatic hydrolysis. The CD of CNFs is not solely attributed to the presence of anionic groups on the nanofibril surface, but it is also influenced by the surface area available for PDADMAC adsorption, which depends on the external specific surface of the CNFs. This is evident from the increase in CD with the intensity of HPH treatment and with the level of fibrillation. However, the main factor affecting CD is the pretreatment itself. It is not possible to achieve a high CD for TEMPO CNFs with the studied combinations of pretreatment followed by HPH, due to the substantial number of anionic charges generated by TEMPO-mediated oxidation. TEMPO CNFs also exhibited higher yield and transmittance compared to others. However, similar yields and transmittance to TEMPO_5 CNFs can be obtained using enzymatic hydrolysis followed by a more intense HPH treatment but with around 50% higher energy consumption. Even mechanical refining of the recycled pulp followed by the most intense HPH studied can produce CNFs of the same order of magnitude in terms of yield and transmittance, although with a lower value than the less fibrillated TEMPO_5 CNF. These findings differ from previous studies on Aspen pulp [28], where TEMPO-mediated oxidation followed by highly intense HPH enabled a yield of 100% and a transmittance around 95%, which were significantly higher than the values obtained with recovered paper. In this case, pulp composition is different due to the higher percentage of ashes and the presence of hornified fibers, which affects their capacity for being fibrillated and their interaction with water and chemicals and some dissolved and colloidal material. The effect of HPH on aspect ratio is clearly influenced by the pretreatment. In the case of a mechanical pretreatment, the aspect ratio of CNFs increased with the energy con- sumption of HPH. A small effect on aspect ratio was observed for enzymatic pretreatments and a negligible effect was obtained in the case of TEMPO_5. However, the aspect ratio decreased with HPH intensity for TEMPO_15 CNF. These CNFs formed a hydrogel whose consistency increased with HPH intensity, which indicates that there was a 3D network of CNFs interacting with water. If the real aspect ratio of all the CNFs in TEMPO_15 was as low as that shown in Table 6 and Figure 7, the formation of a network would be unlikely.
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