Nanomaterials 2023 , 13 , 1931
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according to TAPPI T204 [67] whereas the ash content was determined by calcination at 525 ◦ C, according to TAPPI T211 [68]. Soluble and insoluble lignin, pectin, cellulose and hemicellulose of raw materials were obtained following the NREL/TP-510-42618 stan- dard [69]. A weight of 300 mg of the material was hydrolyzed with 3 mL of 72 wt.% H 2 SO 4 for 1 h in a water bath at 30 ◦ C. Then, 84 g of deionized water was added and introduced in an autoclave at 121 ◦ C for 1 h. The hydrolyzed samples were vacuum-filtered. Insoluble lignin remained in the filter whereas the soluble lignin fraction was obtained by measuring the absorbance of the filtrate in the UV-Visible spectrophotometer at 240 nm. Hemicellulose, cellulose and pectin content were analyzed in the filtrate after neutralization with CaCO 3 and passed through a 0.2 μ m filter by using a modular HPLC device Jasco series 2000 (Jasco, Tokyo, Japan) [70]. 2.2.4. Characterization of CMNFs The aspect ratio was obtained by the simplified gel point ( GP ) methodology based on the sedimentation of the fibers by using increments of the derivative at the origin of the curveC o vs. Hs / Ho , as Equation (1) shows [71]. To determine the GP value, a 250 mL CMNF suspension was prepared using deionized water and 200 μ L of crystal violet 0.1 wt.% to favor the sediment visualization [72]. The aspect ratio was calculated with Equation (2) according to Varanasi et al. (2013) [73], assuming a density of fibers around 1500 kg/m 3 and the crowding number theory described by Martinez et al. (2001) [74]:
∅ o ( i ) − ∅ o ( 0 ) ( H s / H o ( i )) − ( H s / H o ( 0 ))
∅ o ( i ) ( Hs / Ho ( i ))
d ∅ o d ( H s / H o ) ≈
GP = lim
(1)
=
H s / H o → 0
Aspect ratio = 6.0 ·
1000 GP kg
(2)
m 3
To characterize the morphology of CMNFs, optical microscopy (OM) and transmission electron microscopy (TEM) were used. Micro- and nanofibrils were visualized under 5 × magnification using a Zeiss Axio Lab.A1 optical microscope and a color microscope camera Zeiss AxioCam eRc 5s (Carl Zeiss Microscopy GmbH, Göttingen, Germany). TEM analyses were carried out at the Centro Nacional de Microscop í aElectr ó nica (Madrid, Spain) with a JEM 1400 microscope from JEOL (Tokyo, Japan). Samples were prepared adding 15 μ L of 10% Poly-L-Lysine solution on a copper grid covered with a Formvar/carbon continuous layer. Then, 12 μ L of 0.005 wt.% of CMNF suspensions were deposited and left to dry before TEM analysis [75]. To process the images, the program of public domain Image J was used to measure the diameter range of the different suspensions and evaluate the heterogeneity of the CMNFs. Transmittance readings of 0.1 wt.% diluted suspensions were measured in the wavelength of 600 nm on a UV–Vis Shimadzu spectrophotometer UV-160A using distilled water as reference. Finally, the carboxyl content of the suspensions was determined by conductometric titration according to Xu et al. (2022) [76]. 2.2.5. Hand Sheet Preparation and Testing The CTMP pulp was soaked in hot water for around 1 h to soften the fibers (and favor fiber swelling). Then, they were hot disintegrated at 2% in solids using a PTI pulp disintegrator (Vorchdorf, Austria) at 30,000 revolutions according to ISO 5263-3 [77]. Hand sheets of 100 g/m 2 were prepared using a Rapid Köthen sheet former (Paper Testing Instruments, Pettenbach, Austria) according to ISO 5269-2 [78]. The sheets were dried in the drying plates in the Rapid Köthen until they reached a dry content of 65–70%, and thereafter put in a sealed plastic bag to maintain their moisture level. They were kept at 4 ◦ C until hot-pressing. Figure 2 shows a scheme of the process of hand sheet production.
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