Polymers 2024 , 16 , 110
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2. Experimental Part 2.1. Chemicals
Eucalyptus-derived cellulose fibers, wood-derived cellulose fibers, chemical thermo- mechanical pulp cellulose (CTMP), cationic starch, calcium carbonate, and potassium amyl xanthate (PAX) were derived by Arctic Paper Kostrzyn SA. Polyethylene glycol 2000 (PEG, M = 1800–2200 g/mol) was purchased from Carl Roth, Karlsruhe, Germany. Titanium dioxide was kindly supplied by Grupa Azoty Police (Police, Poland). Tetraethyl orthosil- icate (TEOS) and boron nitride (BN, ~1 μ m, 98%) were purchased from Sigma Aldrich (Poznan´, Poland). Ammonia solution (NH 4 · OH, 25%), sulfuric acid (H 2 SO 4 , 95%), nitric acid (HNO 3 , 65%), and potassium permanganate (KMnO 4 ) were delivered from Chempur (Piekary S´la˛skie, Poland). 2.2. Synthesis of Silicon Dioxide (SiO 2 ) SiO 2 nanoparticles were prepared according to a modified Stöber method [16]; 5.8 mL of TEOS and 150 mL of ethanol were initially mixed in a round bottom flask and stirred for 10 min at room temperature (RT). Next, 7.8 mL of ammonia solution was added, and the mixture was further stirred for 12 h. After that, the product was separated by centrifugation, washed a few times with ethanol, and dried. 2.3. Synthesis of Hexagonal Boron Nitride (h-BN) A total of 0.2 g of bulk boron nitride was placed in a flask, and then, 200 mL of ethanol was added. Afterward, the obtained mixture was sonicated using an ultrasonic homogenizer for 12 h. The final product was washed with distilled water and dried at 80 ◦ C. 2.4. Synthesis of Oxidized Hexagonal Boron Nitride (h-BN-OH) To prepare h-BN-OH, a modified Hummers method was applied [12]. Briefly, 0.2 g of h-BN powder was placed in a round-bottomed flask, and then, 13.5 mL of H 2 SO 4 and 4.4 mL of HNO 3 were added. The mixture was stirred to obtain a homogenous dispersion. After that, 1.2 g of KMnO 4 was partially introduced, and finally, the mixture was heated to 90 ◦ C and kept at this temperature for 12 h. Next, the mixture was cooled to RT, filtrated, and washed a few times with distilled water until the pH value approached 7. Finally, the product was dried at 80 ◦ C overnight. 2.5. Preparation of PEG Solution In the experiments, the different polyethylene glycol (PEG) variants (PEG 2000, PEG 4000, PEG 10000, and PEG 20000) were tested to choose the optimal one. During the preparation of paper sheets using the Rapid-Köthen machine (Lodz, Poland) different performance characteristics of the paper with the tested PEG variants were evaluated. The final selection of PEG 2000 was based on its superior performance during the paper sheet preparation process. PEG 2000 exhibited the best solubility in the chosen solvent and its integration with paper pulp. An appropriate PEG 2000 amount was added to 1 L of distilled water, and with the use of a magnetic stirrer, it was completely dissolved. The amount of PEG 2000 was calculated in 1 ton of dry pulp to obtain a concentration of 1 kg of PEG/ton of dry cellulose (typical commercial procedure). 2.6. Preparation of Paper Sheets As a reference sample, a sheet of paper without the addition of inorganic fillers was prepared. The reference paper sample contained only standard commercial components used for the production of paper sheets. Paper with a grammage of 80 g/m 2 was created by combining three types of cellulose fibers: short-fiber cellulose pulp (eucalyptus derived), long-fiber cellulose pulp (birch-derived), and chemical thermomechanical pulp cellulose (CTMP) with a mass ratio of 70/20/10, respectively. The cellulosic mass was mixed in a plastic container using a mechanical stirrer for 15 min. Subsequently, PAX and a cationic
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