Polymers 2024 , 16 , 110
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Various strategies have been developed to overcome these obstacles to enhance the affinity of the paper mass to CaCO 3 . One of the strategies is the addition of crosslink- ers, which can improve the interaction between the cellulose fibers and CaCO 3 particles. Crosslinkers are chemical compounds that can form bonds between the cellulose fibers, creating a more stable network that can better resist the disruptive effects of fillers [7,8]. The addition of fillers such as SiO 2 [9], TiO 2 [10], h-BN [11], and h-BN-OH [12] has been shown to be effective in improving the affinity of the paper mass to CaCO 3 . These compounds can form bonds with both the cellulose fibers and CaCO 3 particles, creating a stronger network that improves the strength, stiffness, and printability of the paper. Therefore, using crosslinkers in papermaking is an important strategy for improving the quality and performance of paper products. Silicon dioxide (SiO 2 ) can form covalent bonds with both the cellulose fibers and CaCO 3 particles. The SiO 2 particles can also serve as a connector between the cellulose fibers and CaCO 3 particles, helping to strengthen the inter-fiber bonding and improve the stiffness and strength of the paper. In addition to its role as a crosslinker, SiO 2 can also improve the drainage and retention of the paper mass. The SiO 2 particles are hydrophilic, which means that they can help absorb water and improve the flow of the paper mass through the paper machine. This can lead to faster drying times and increased productivity. SiO 2 can be added to the paper mass in various forms, including colloidal silica, precipitated silica, and silica fume [9,13]. Titanium dioxide (TiO 2 ) as a crosslinker can also improve the optical properties of paper products. TiO 2 is a white pigment that can reflect and scatter light, leading to a brighter and more opaque paper. This can be particularly beneficial in applications where high brightness and opacity are expected. In addition, TiO 2 can also improve the drainage and retention of the paper mass—similar to SiO 2 . This can lead to faster drying times and increased productivity. TiO 2 can be added to the paper mass in various forms, including rutile, anatase, and nano-sized TiO 2 particles. Rutile and anatase are two crystalline forms of TiO 2 , with rutile being the most commonly used form in papermaking due to its higher refractive index and opacity. Nano-sized TiO 2 particles have a smaller particle size and can provide additional benefits such as improved printability and ink adhesion [10,14]. Hexagonal boron nitride (h-BN) is a layered material that consists of hexagonally arranged boron and nitrogen atoms. It has a high surface area and unique surface chemistry that make it attractive for various applications. When added to the paper mass, h-BN can form covalent bonds with both the cellulose fibers and CaCO 3 particles, improving the interaction between them and creating a more stable network. The h-BN particles can also serve as a connector between the fibers and fillers, enhancing the stiffness and strength of the paper. h-BN-OH is a derivative of h-BN that has been functionalized with hydroxyl groups. The hydroxyl groups increase the surface energy and wettability of the h-BN particles, allowing them to interact with the cellulose fibers and CaCO 3 particles more effectively. The hydroxyl groups can also provide additional chemical functionality, allowing for further modifications and improvements in the paper properties [12,15]. In this work, the impact of different inorganic fillers on calcium carbonate content in the paper was investigated. For this reason, mesoporous SiO 2 nanoparticles, TiO 2 nanopar- ticles, h-BN nanoflakes, and hydroxylated h-BN nanoflakes (h-BN-OH) have been explored as additives. They have been introduced to the paper pulp in the form of a polyethylene glycol (PEG) mixture to induce bonding between the inorganic structures and paper pulp components. Various characterization methods were employed to determine the chemical structure and morphology of prepared samples, including TEM, SEM, XRD, Raman spec- troscopy, and TGA. The ash content (residual solid particles after the combustion process) was evaluated according to ISO 1762:2001. It allowed us to select mesoporous silica as the most efficient filler, enhancing the retention of the fillers by 12.1% in respect to unmodified paper sheets.
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