Nanomaterials 2022 , 12 , 790
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In this paper, different CMF/CNF hydrogels with various fibrillation degrees have been characterized and dispersed at different stirring speed levels. The behavior of the obtained samples was analysed based on the Ø g data. TEM images and AR theories have been used to validate the effectiveness of the Ø g parameter to characterize the dispersions. Based on this method, the optimal stirring-speed conditions of CNFs have been applied for an industrial application. Therefore, the effect of the dispersion degree of CNF hydrogels, before the mixture of nanocellulose with the pulp on the reinforcement of recycled old corrugated cardboard (OCC) sheets has been evaluated by measuring mechanical (bursting index, short-span compressive test index, tensile index, tear index) and physical properties (bulk and porosity) of the handsheets. In addition, the effect of disintegrating CNF and OCC together in a pulper, without a previous dispersion of CNF, was also studied.
2. Materials and Methods 2.1. Materials
CMFs/CNFs form a three-dimensional network structure within the hydrogel that is modified when the suspensions are prepared at different dispersion degrees. Four types of CMFs or CNFs have been used in this study from three different sources. The analysis of samples with different fibrillation degrees is crucial to validate the hypothesis of the study. The first sample was a microcellulose obtained from grinded cotton linters (C-CMF) with high purity cellulose (Sigma-Aldrich, St Louis, MO, USA). C-CMF were prepared without any pretreatment and mechanically fibrillated in a laboratory homogenizer PANDA PLUS 2000 manufactured by GEA Niro Soavy (Parma, Italy) using six progressive passes of homogenization from 300 to 900 bars. Commercial recycled newspaper with an ash content of 14% was used to prepare two types of products with different fibrillation, CMF (R-CMF) and CNF (R-CNF). Both were disintegrated at 30,000 revolutions and 3.0 wt.% consistency in a pulp disintegrator (PTI, Vorchdorf, Austria). To obtain R-CMF, the cellulose pulp was refined at 5000 rev- olutions in a PFI mill (Hamjern Maskin AS, Hamar, Norway) and six steps of homoge- nization at 600 bars were applied. R-CNF was obtained by a chemical pre-treatment with 2,2,6,6-tetramethylpiperidin-1-oxyl-radical (TEMPO) according to Saito et al. [42] using 1 mmol NaBr and 10 mmol NaClO/g of pulp. Once the pulp was oxidized, a filtration cleaning process was performed using distilled water and four steps of homogenization at 600 bars have been applied. The third raw material was obtained from Eucalyptus globulus ECF bleached kraft pulp, kindly supplied by Torraspapel, S.A. (Zaragoza, Spain), with 74% of cellulose and 18% of hemicellulose as its main components. The CNF (E-CNF) were obtained by TEMPO- mediated oxidation with 2.5 mmol of NaClO/g of dry pulp. Then, six steps of homoge- nization at 600 bars were applied. All hydrogels were stored at 4 ◦ C at ~1 wt.% to avoid their aggregation, after adding some drops of glutaraldehyde as bactericide (5 drops/L) to avoid bacterial growth, until they were used. The raw materials and the pretreatments used to obtain the CMFs/CNFs were selected to obtain a large variation of the properties. Dry CMFs/CNFs composition was character- ized before the HPH treatment. Cellulose, hemicellulose, lignin (soluble and insoluble), extractives, and ashes were measured. Firstly, extractives of the samples were determined via Soxhlet extraction and ash content was determined via calcination according to TAPPI T204 and T211, respectively. Cellulose, hemicellulose and lignin content were obtained following NREL/TP-510-42618. 300 mg of sample was hydrolyzed for 1 h with 3 mL of 72wt.%H 2 SO 4 . Then, 84 g of water was added in the sample and then introduced in the autoclave for one hour at 121 ◦ C. The hydrolyzed samples were vacuum filtered, the insoluble lignin remained in the filter and the soluble part was obtained in the UV-visible spectrophotometer by measuring the absorbance of the filtrate. Hemicellulose and cellulose content were analyzed using high performance liquid chromatography (HPLC) from the filtrate after neutralization with CaCO 3 and filtered. This process was not carried out in
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