Nanomaterials 2022 , 12 , 790
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C-CMF due to the sample was not pretreated and directly was homogenized. CNFs/CMFs from recycled paper present a higher amount of ashes from the fillers added in the previous paper manufacture. On the other hand, the amount of cellulose is slightly reduced in the chemical pretreatment of R-CNF at the same time the insoluble lignin is also reduced. This is produced by the removal of lignin and amorphous cellulose during TEMPO-mediated oxidation. In the same way, the amount of cellulose also is slightly reduced in E-CNF respect to the initial cellulose value. However, this effect occurs to a lesser extent due to the lower amount of NaClO used in E-CNF pretreatment. CMFs/CNFs were characterized according to Balea et al. 2019 [10] and listed in Table 1. As for the chemical parameters, the number of carboxyl groups differs greatly between the CNFs and the CMFs, the latter with almost zero content since these samples have not been oxidized in the pretreatment. The superficial cationic demand obtained show also the same trend as carboxyl groups. The TEMPO-mediated oxidation influences on the fibrillation of the samples, with a higher transmittance of these samples that reach the range of nanofibrils. On the other hand, C-CMF from cellulose powder shows a low polymerization degree in a similar way than CNFs, however the diameter size is much larger than the other samples, with a low aspect ratio value. This effect is due to the ground of the sample that breaks the cellulose chains and decreases the aspect ratio without reaching the nanofibrillation of the sample. Diameter average of the other samples is in the nanoscale, although the R-CMF show a higher number of fibers in the microscale as nanofibrillation yield indicates.
Table1. Characterization of CMFs/CNFs.
C-CMF
R-CMF
R-CNF
E-CNF
Dry composition
Cellulose (%)
>99.9
56 ± 1 13 ± 1
50 ± 1 18 ± 1
72 ± 1 18 ± 1
Hemicellulose (%) Soluble lignin (%) Insoluble lignin (%)
- - - -
4.3 ± 0.5 10.0 ± 0.5 6.0 ± 0.5
12.5 ± 0.5 5.3 ± 0.5
-
Extractives (%)
1.8 ± 0.1
2.0 ± 0.2
0.3 ± 0.1
Ashes (%)
<0.1 *
12.5 ± 0.3 14.0 ± 0.5 3.0 ± 0.5 Chemical parameters
Carboxyl Groups (mmol/g)
0.06 0.06
0.07 0.04
0.81 0.62
0.59 0.80
Superficial cationic demand (meq/g)
Morphological parameters
Transmittance 400 nm (%) Transmittance 800 nm (%) Polymerization Degree (monomeric units) Nanofibrillation Yield (%)
2.1 9.2
1.8 8.7
15.4 35.7
83.5 94.8
229
703
201
440
<5
39
78
89
Diameter (average)
~5 μ m 44nm 19nm 28nm
* In accordance with the instructions of the manufacturer.
2.2. Methods 2.2.1. Determination of Gel Point in Suspensions
The dispersion of nanofibrils was evaluated before its application, according to the methodology described by Martinez et al., based on the settling of samples to calculate Ø g [35]. They analyzed the relation between the sediment concentration and the compress- ibility effects, redefining the Ø g and developing Equation (1), which later was simplified in Equation (2). To prepare sedimentation experiments, suspensions with different CNF hydrogels were prepared using deionized water and 200 μ L of crystal violet 0.1 wt.% to
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