Nanomaterials 2023 , 13 , 2536
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board, on the properties of the resulting CNFs and the associated energy consumption remains unexplored. Recently, Ang et al. (2021) produced several CNF nanopapers from DIP using different combinations of mechanical refining (10,000, 30,000, and 50,000 revolutions in PFI) and HPH at 1000 bar (1 pass and 3 passes) [24]. They found that the nanofibrillation efficiency for recycled fibers was lower than that for virgin bleached eucalyptus kraft pulp due to hornification during papermaking. Therefore, results from virgin pulps cannot be transferred to recycled pulp, since the pulp composition is different, as paperboard contains more lignin and ashes, and the fibers are hornified due to the drying process. Therefore, recycled pulps will behave differently during CNF production. In terms of energy consumption, Josset et al. (2014) conducted an energy-related study on the direct fibrillation of recycled newspaper and wheat straw through a grinding process [16]. They compared the CNFs obtained from these raw materials with CNFs derived from a bleached wood pulp (ECF) by evaluating the mechanical properties and specific surface areas of the fibrillated materials. The study found that the energy inputs required to achieve optimal mechanical properties in the prepared films from recycled newspaper were higher compared to the ECF, ranging from approximately 4 to 5 kWh/kg. Another study by Ozola et al. (2019) estimated that the production of CNFs from recycled pulp, without deinking, had higher specific energy consumption (0.865 kWh/kg waste) compared to the production of other products such as egg packaging or cardboard [29]. Overall, these studies demonstrate that the production of CNFs from different types of recycled papers is feasible in terms of improving the quality of the final product. However, it is important to consider the energy consumption associated with the specific production processes for effective resource management and sustainability. This research aims to generate new knowledge on the effects of different treatment combinations on the CNF properties obtained from recovered paperboard. Five different pretreatments are considered, including a mechanical pretreatment through PFI refining (20,000 revolutions), two enzymatic hydrolysis approaches (80 mg/kg and 240 mg/kg), and TEMPO-mediated oxidation under two different conditions (5 mol NaClO/kg and 15 mol NaClO/kg). The study explores five different conditions for the HPH mechanical process for each pretreatment method. Furthermore, the energy requirements associated with the different treatment processes are assessed. This information will facilitate the selection of a fibrillation process based on the required CNF properties.
2. Materials and Methods 2.1. Materials
OCC was selected as the recycled cellulose source as it is one of the most significant types of recovered papers for recycling, and it is listed in the European List of Standard Grades of Recovered Paper and Board (EN 643) as Group 1 in ordinary qualities (1.05). Liner and fluting, used in proportions of 35 and 65% for OCC pulp preparation, were kindly provided by SAICA (Zaragoza, Spain). This fluting/liner ratio is the most common for cardboard boxes. The chemical composition of the recycled pulp is presented in Table 1. A commercial monocomponent enzyme cocktail, Novozym 476, was kindly supplied by Novozymes A/S (Bagsværd, Denmark). This enzyme cocktail contains 2% ( w / v ) of endo- β -1,4-glucanases, with an enzyme activity of 341 U/mL. The other reagents utilized in this research were purchased from Sigma Aldrich (Madrid, Spain). 2.2. Cellulose Nanofiber Production OCC pulp was prepared from a mixture of 35% liner and 65% fluting OCC paper by disintegration at 3% at 3000 rpm for 20 min using a standardized 2 L lab pulper. The resulting pulp was then adjusted to the desired pretreatment consistency by diluting it with distillated water or filtering it through a qualitative paper filter using a Buchner funnel. The production of CNFs involves a two-step process, beginning with a pretreatment stage
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