Nanomaterials 2023 , 13 , 1931
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modifiers [34], aerogels and foams [35], food industry [36], energy [37], environmental [38], papermaking [39] and many others [40,41]. One of the most promising NC applications is in papermaking, especially in bulk and surface applications [39]. NC has been applied in papermaking due to its strong properties, mainly its high strength and mechanical properties; dry and wet strength; Z-strength and fracture toughness [42,43]; decreased porosity and permeability; enhanced barrier proper- ties towards air, grease, water vapor and liquids; high oxygen-barrier performance [44]; its low density, enabling grammage reduction while increasing tensile strength [45]; increased filler retention and reduced surface roughness and water penetration [46]; decreased linting problems in printing papers [47]; and increased coloring efficiency of colored papers [48], water treatments [49], etc. Furthermore, as NC has a relatively high number of carboxyl groups, their reactivity make it possible to modify the hydrophilicity and charge surface to improve their potential applications in papermaking [39,50,51]. In addition, among all potential applications in papermaking, the use of NC as a reinforcement additive has been the most studied. NC-reinforced paper has the potential not only to improve conventional packaging paper, but also to progress to the substitution of non-biodegradable plastics obtained from fossil resources, as in the case of food packaging. All published market studies about NC show that the market is expected to grow from USD 300 million currently to USD 800 million by 2026, which means an annual growth rate of more than 30% in the coming years, and this will certainly continue to grow in the future driven by the sustainability trend [52]. Therefore, the industrial and academic community are making a tremendous effort to unlock the potential for engineered sustainable materials based on NC, but several challenges remain related to the production, characterization and application of these prod- ucts [53,54]. Factors such as process optimization and cost-effective production methods need to be further addressed. In this aspect, the in situ production and application of NC is considered. From this point of view, NC produced from fibers containing lignin, called lignocellulosic micro/nanofibers [55–58], and from recycled fibers, is attracting more attention since paper improvements were observed in z-strength, tensile index, tear index, burst index, E-modulus, strain at break, tensile stiffness, and air resistance [59,60]. The aim of this manuscript is to investigate the synergic effect of using fibrillated cellulose, containing micro and nanofibers (CMNFs), as a bulk additive, and hot-pressing to improve the properties of the paper. The cellulose source to produce CMNFs was the waste from a liner paper machine using 100% recovered paper. To our best knowledge, the use of NC to further enhance paper properties in conjunction to hot-pressing is a novel approach that has not been reported in the literature yet.
2. Materials and Methods 2.1. Materials
Waste liner from a paper machine using 100% recycled paper was used as cellulose raw material to produce CMNFs. The reagents used for the TEMPO-mediated oxidation pre- treatment were 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) reagent (98 wt.%) supplied by Sigma-Aldrich (St. Louis, MO, USA), as well as NaBr (>98.5 wt.%), NaOH (>98 wt.%) and NaClO (12% w/v , previously titrated to obtain the actual concentration) which were supplied by Panreac AppliChem (Barcelona, Spain). Other reagents used for the charac- terization of CMNFs were NaCl (>99.5 wt.%), NaOH (>98 wt.%), H 2 SO 4 at 98 wt.% and crystal violet (>90.0 wt.% anhydrous basis) supplied by Merck (Madrid, Spain), and 0.1 wt.% Poly-L-Lysine solution with a molecular weight > 70,000 by Electron Microscopy Sciences (Hatfield, PA, USA). For HYP production, a standard bleached flash dried spruce CTMP (CSF 420 mL) from SCA Östrand mill (Timrå, Sweden) was used to prepare the paper hand sheets.
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