E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
use in direct contact with water because of the hydrophilic nature of cellulose. Bio-based solutions to tackle the lack of wet strength are hence needed to limit the use of synthetic polymers. Replacing synthetic wet strength additives with natural ones participates to the increased biodegradability of the products. In addition, the use of cellulose pulp can keep recycling rates high, which is a circumstance not found for regular composites of bio-based fibers and a thermoplastics matrix. To improve the wet strength of cellulosic materials and more specifically paper, different wet strength agents (WSA) have been used throughout the years. In wet conditions, the main mechanism of breakage is fiber pull-out, while the integrity of the fibers is mostly maintained ( Schäfer et al., 2021 ), as the fibers become more flexible in the water. The role of the WSA is to prevent water from disrupting the hydrogen bonds, dipole, and Van der Waals interactions between the fibers. There are different mechanisms that can lead to the increased wet strength, including the protection of existing fiber-fiber bonds or the creation of new covalent bonds ( Schäfer et al., 2021 ; Singh et al., 2024 ). Typical wet strength agents are synthetic polymers, such as polyamide amine-epichlorohydrin (PAE), polyethyleneimine (PEI), polyvinylamine (PVAm), and melamine formaldehyde (MF) which are not environmentally friendly ( Francolini et al., 2023 ; Singh et al., 2024 ). Some natural polymers like chitosan and starch can also be used as WSA, but these are less effective than synthetic polymers and often more expensive. Lignin is a natural polymer that can act as a binder in the fiber network. It is a polyphenolic branched polymer with varying structures depending on its source and extraction method ( Balakshin et al., 2021 ). Lignin can be used as a filler to increase the density of paper, and it has been shown to improve both the dry and wet strength when added as a binder in cellulose based products ( Jiang et al., 2020 ). The lignin naturally present in mechanical pulps has also been used in combination with hot pressing to increase the strength of the paper ( Zhao et al., 2020 ). Pressing at temperatures above the glass transition temperature can soften the lignin and allow densification of the network before stiffening again at low temperatures. At high temperatures and under pressure, changes in the chemical structure of lignin can also occur. Depolymerization and recondensation reactions may happen simultaneously ( Li et al., 2015 ; Zhao et al., 2020 ). Condensation between lignin units strengthens the fiber network. Zhao et al. (2020) analyzed the changes in the chemical structure of natural lignin during hot pressing. They concluded that a strengthening effect happens with the increase of moisture content during pressing due to the increase in condensation reactions. Lignin has been used to strengthen cellulosic materials by cross-linking and thermal curing ( Zhang et al., 2022b ; Huang et al., 2023 ). Ionic cross-linking is based on the addition of multivalent metal cations or cationic polymers to bond the lignin to the oxidized cellulose, while covalent cross-linking often requires additional chemicals to react with either lignin or cellulose. Thermal curing, however, can also create covalent bonds using only thermal energy. Control of the temperature is important during thermal curing, as it is necessary for the self-bonding of lignin, while excessive high temperatures can degrade the cellulose and hemicelluloses. Both residual (natural) lignin ( Zhao et al., 2020 ; Mattsson et al., 2021 ; Wang et al., 2021a , 2021b ) and the addition of technical lignin as additives ( Jiang et al., 2020 ; Sanchez-Salvador et al., 2024 ) have been considered as wet strength agents. The advantage of using technical lignin is that chemical fibers are softer and allow better compaction during paper formation than mechanical pulps. Moreover, the amount of added lignin and its distribution in the fiber network can be better controlled. Mattsson et al. (2021) showed that the optimal lignin content in paper for wet strength was 7 %–12 % ( w / w ). Jiang et al. (2020) impregnated cellulose paper with lignin dissolved in acetone. They obtained paper with high wet strength, improved thermostability, and UV-blocking capacity. However, the use of organic solvent requires additional washing steps and is not environmentally friendly. Lignin powder can be dispersed in water and added to pulp before the wet cake formation; however, it does not naturally interact with cellulose in this implementation. Therefore, to improve its retention and distribution within the fibers, flocculants need to be added. When adding lignin particles as wet strength agents in bulk paper sheets at the filtration stage, significant loss can be observed during the paper formation ( Sanchez-Salvador et al., 2024 ). This again highlights the importance of adding a flocculant. Flocculants are molecules that can bridge fillers or fines, for example, to increase their retention and improve filtration efficiency in the paper industry ( Blanco et al., 2009 ). In this case, a positively charged polymer acts as a bridge between negatively charged fibers and lignin. Polyacrylamides are common flocculants used in the paper industry, but natural flocculants, such as cationic starch and chitosan, are also available. Maximova et al. (2001 ; 2004 ) studied the adsorption mechanism of lignin on cellulose fibers, using cationic starch and polydiallyldimethylammonium chloride (PDADMAC) as cationic polymers to complex the lignin. Depending on the order of addition, the interfacial properties of the lignin were different. Adding the cationic polymer first to the cellulose fibers allowed the deposition of lignin on the surface, while first mixing the cationic starch with lignin led to the formation of polyelectrolyte complexes with lignin in the core ( Maximova et al., 2004 ). Starch can also interact with lignin as it can act as co-binder and plasticizer ( Diaz-Baca and Fatehi, 2024 ). Cationic starch was thus selected for this study. Paper sizing with starch and lignin has been previously performed, and synergies were observed when measuring the water absorption and contact angle on the surface of the paper, which was coated with both starch and lignin ( Kopacic et al., 2018 ). In combination with the wet strength agent, the strength of fiber-based products can be increased by tuning the process parameters during formation. The moisture content is a key parameter during the pressing of fiber-based materials. It was shown that increasing moisture content during hot pressing led to an increase of tensile strength ( Zhao et al., 2020 ; Sanchez-Salvador et al., 2024 ). The moisture influences the plasticity of bleached fibers, which can improve the packing of the fibers and the density of the substrate. Moisture at high temperatures also influences the softening of the lignin and lignin condensation reactions ( Bouajila et al., 2006 ). Zhao et al. (2020) studied the strength of chemi-thermomechanical pulp (CTMP) thermoformed substrates at different humidity levels and obtained a linear correlation between dry strength and moisture content as well as density and moisture content during pressing. However, no information about water interactions and wet strength was shown. Therefore, two different moisture contents during pressing were used in this study.
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