Nanomaterials 2023 , 13 , 2536
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Table5. Morphological characteristics of the pulps pretreated with refining, enzymatic hydrolysis, and TEMPO-mediated oxidation prior to the HPH.
Pretreatment Untreated pulp
Length w ( μ m)
Diameter ( μ m)
Coarseness (mg/m)
Fines (%) 37.9 ± 2.1 47.2 ± 7.5 38.3 ± 1.1 53.6 ± 4.0 51.2 ± 4.6 61.0 ± 3.9
881 ± 18 715 ± 95 541 ± 22 351 ± 34 258 ± 11 249 ± 11
24.6 ± 0.1 22.3 ± 0.6 23.9 ± 1.3 23.7 ± 1.7 20.0 ± 1.0 18.1 ± 0.7
0.155 ± 0.0003 0.156 ± 0.011 0.144 ± 0.001 0.098 ± 0.020 0.046 ± 0.017 0.037 ± 0.012
Mec
Enz_80
Enz_240
TEMPO_5 TEMPO_15
Length w : length weighted in length.
Figure 2. Optical images of the initial recycled pulp without pretreatment ( a ) and the pulps after different pretreatments: mechanical pretreatment (refining) ( b ); enzymatic hydrolysis with 80 mg/kg ( c ); enzymatic hydrolysis with 240 mg/kg ( d ); TEMPO-mediated oxidation with 5 NaClO/g ( e ); and TEMPO-mediated oxidation with 15 mmol NaClO/g ( f ). Figure 2 shows the optical images of the original and pretreated pulps. The original fibers (Figure 2a) exhibited a smooth surface, without notable external fibrillation. In contrast, the Mec pulp image (Figure 2b) shows an extremely high degree of fibrillation, which favors fine-and-fiber interaction, as well as the formation of smaller networks. During mechanical refining, fiber hornification is reversed since fibers recover part of their swelling ability, which increases their coarseness [46]. Fibrillation was also observed in the Enz_80- and Enz_240-pretreated pulps, albeit to a lesser extent (Figure 2c,d). This fact was also observed by other authors [27], so enzymes have been proposed as an alternative to refining to reduce the hornification effect. It is noticeable that Enz_240 induced greater fibrillation compared to Enz_80, resulting in a reduction in fiber length (Table 5). In contrast, TEMPO-mediated oxidation led to the lowest external fibrillation, as this reaction specifically oxidizes the cellulose chains, conferring electrostatic repulsive forces on interfibrils that reduce the energy required for nanofibrillation (Figure 2d,e), but without changing the fiber morphology during the chemical treatment. However, at high doses of NaClO, some external microfibrillation on the fibers can be observed due to the high electrostatic repulsive forces among chains due to the generated carboxyl groups in combination with hydrodynamic forces during pulp washing. This produces some CNFs
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