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M. N. A. MOHAMMAD TAIB ET AL.
et al. 2021). Traditional paper faces challenges of achieving maximum mechanical properties while main- taining good barrier properties. Moreover, when paper is used for an extended period, it tends to shrink, which can render it unusable and lead to a deterioration of its physical and mechanical properties. There is a need to use a filler or additives to overcome these issues. The first use of filler comes from mineral additives that have been added to papermaking to improve its properties for different paper applications (Ma et al. 2016). The mineral additives also reduce the cost of paper manufacturing and are considered cheaper (Fortună et al. 2020). Functional fillers report properties such as brightness, opacity, sizing control, and an increase in wet and dry strength, among others (Ehman, Evangelina Vallejos, and Cristina Area 2025). Even with a substantial improvement, these types of fillers severely wear paper machine parts and printing cylinders and increase the cost of maintenance (Samyn et al. 2018). The abrasion from this filler influences the hardness, fineness, and structures of particles. Even when used at a low loading content, these fillers are still very abrasive (El-Sherbiny, El-Sheikh, and Barhoum 2015; Lourenço et al. 2015). Therefore, it is important for nanofillers to have desirable properties to avoid these negative effects. Nano additives have become a popular trend and play an important role in manufacturing the highest quality of paper and cost efficiency in papermaking (Fortună et al. 2020). Nanoscale materials with high surface area and function- ality are widely investigated for various technological applications, it also can be incorporated into pulp and paper matrices to improve their structural, barrier, or functional performance. They are broadly categorized into bio-based, inorganic, and carbon-based nanomaterials. The nanofillers have been added in pulp and paper processes as nano pigments for coating, nano polymer additives, nano sizing agents, nano retention systems, fiber nano-coating, and nano-based smart paper (Taib 2023). The addition of nano additives varies from 5% to 37% based on the qualities and grades of papers (Song et al. 2018). Some of these nano additives, even though they improve some properties such as opacity and brightness, compromise mechanical properties such as paper strength due to this filler weakening of inter-fiber bonding (Lourenço et al. 2013). This is because a higher content of filler could lead to poor interaction and paper bulk loss (Lourenço et al. 2013). Proper nanofillers need to be considered before being used in paper based on final applications. The area of nanotechnology has gained popularity and attention among researchers with the use of nanofillers with dimensions of 1–100 nm in pulp and paper processing (Chauhan and Chakrabarti 2012). This nanosized dimension presents a better aspect ratio, ease of modification, desirable mechan- ical properties, and a more active surface available to be bonded with pulp and paper processing (Shen et al. 2010). It exhibits good functionality when integrated in pulp and paper processing. They are broadly categorized into bio-based, inorganic, and carbon-based nanomaterials. Bio-based nanofillers, such as nanocellulose (cellulose nanocrystals [CNC], cellulose nanofibrils [CNF], bacterial nanocellulose [BNC]) and lignin nanoparticles, offer biodegradability and compatibility with cellulosic fibers. Inorganic nanofillers, including nano-silica, montmorillonite (MMT) clay, and metal oxide nanoparti- cles such as TiO 2 and ZnO, provide barrier, antimicrobial, and UV-blocking properties. Carbon-based nanofillers like graphene oxide (GO), carbon nanotubes (CNTs), and carbon black offer conductivity and mechanical reinforcement (Bajpai 2018; Naijian et al. 2019). It can also add value to the final properties of pulp and paper and thus be used in different fields of application. These nanofillers can be added through various methods to pulp and paper processing. The methods, such as co-mixing with flocculation of fixation chemicals or direct in-situ precipitation with various precursors, can be used to add nanofillers (Fortună et al. 2020; Shen et al. 2010). Some of nanofiller extraction involves high technologies and machines, as well as expensive chemicals that make the cost of production high, thus affecting the total cost of manufacturing pulp and paper products. The uniform distribution of nanofillers in pulp and paper processing is another aspect that needs to be controlled to get good final properties, mostly when using the highly hydrophilic nanofillers such as nanocellulose and nanoclay (Shen et al. 2010). Some modifications can be performed to alter the polarity of nanofillers, such as using chemical treatments (esterification, silylation, oxidation, and others) (Börjesson and Westman 2015; Chung and Washburn 2016). Surface surfactants such as cationic surfactant (CTAB) or anionic surfactant (sodium oleate) can also be added to improve the properties and get uniform distribution in pulp and paper processing (T. Wang et al. 2021). The bad interactions between nanofillers and papermaking at the wet-end processing could affect the strength and sizing agents and also pose challenges in dye processing (Dhiman et al. 2025). Moreover, the pores in pulp and paper
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