Nanomaterials 2023 , 13 , 1931
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2.2. Methods 2.2.1. Cellulose Pretreatments
To produce the CMNFs, first, liner was left to soak for 24 h to favor fiber swelling before disintegration into a pulp disintegrator (PTI, Vorchdorf, Austria) at 30,000 revolutions. Two pretreatments were used to obtain the CMNFs. On the one hand, refining was used as the mechanical pretreatment using a PFI mill manufactured by Hamjem Maskin AS (Hamar, Norway). The pulp consistency was adjusted to 10 wt.% and the pulp was subjected to 20,000 revolutions [61,62]. On the other hand, a TEMPO-mediated oxidation (5T-Oxidation) chemical pretreatment was performed in a 5 L glass reactor at 1 wt.% cellulose consistency with 0.1 mmol TEMPO/g pulp and 1 mmol NaBr/g pulp [63,64]. Then, 5 mmol NaClO/g pulp was added to start the reaction which was conducted at room temperature and with stirring conditions at 200 rpm. During the process, pH was controlled to 10, adding a 2 M NaOH solution dropwise until pH remained constant [65,66]. 2.2.2. Cellulose Treatment To produce CMNFs, pretreated pulps were mechanically fibrillated using a high- pressure laboratory homogenizer PANDA 2000 PLUS (GEA Niro Soavy, Parma, Italy) using pulp suspensions with a consistency around 1 wt.%. Figure 1 shows a scheme of the production of the CMNFs from the raw material. To achieve different nanofibrillation yields, four pressure sequences (PS) were carried out in the high-pressure homogenization (HPH). The PSs are described below from less intensive to more intensive: • PS0: No homogenization of the fibers, only the pretreated samples. • PS1: 3 passes of HPH at 300 bars. • PS2: 3 passes of HPH at 300 bars and 3 additional passes at 600 bars. • PS3: 3 passes of HPH at 300 bars, 3 passes at 600 bars and 3 passes at 900 bars.
Figure1. Scheme of CMNF production. 2.2.3. Chemical Characterization of the Raw Material Used to Produce CMNFs First, the broke liner was dried to obtain the total solid content of the sample. Ex- tractives, insoluble and soluble lignin, pectin, cellulose, hemicellulose, and ashes were measured by triplicate. Extractives of the sample were determined by Soxhlet extraction
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