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onto cellulose nanocrystals. 110 The product was cast into thin lms, which showed nanostructured morphologies with increased water resistance and the ability to form self- supported hydrogel- lms. In another publication, Hambard- zumyan et al. simply mixed the cellulose nanocrystals with lignin in solution, a erwhich lms were cast onto quartz slides and dried by evaporation. 111 The authors found that optically transparent lms with UV-blocking ability could be produced. It was concluded that increasing the CNF concentration allowed for better dispersion of the lignin macromolecules, dislocating the p – p aromatic aggregates and hence yielding a higher extinction coe ffi cient. An elaborate work on lignin-starch composite lms was conducted by Baumberger. 39 The lms were produced via oneof two methods: (1) powder blending of thermoplastic starch and lignin, followed by heat pressing and rapid cooling, and (2) dissolution in water or dimethyl sulfoxide followed by solvent- casting and solvent evaporation. The author concluded that the lignin acted either as ller or as extender of the starch matrix, where the compatibility was favored by medium relative humidity, high amylopectin/amylose ratios, and low molecular weight lignin. Lignosulfonates formed good blends and imparted a higher extensibility onto the starch lms, likely due tobene cial interactions between sulfonic and hydroxyl groups. Non-sulfonated lignin, on the other hand, improved water- resistance to a greater extent. Three recent studies have found that incorporating lignin into a molded pulp materials can reduce the wettability of the material, as witnessed by an increase in contact angle or a decrease in water-uptake. 8,36,88 The advantage of such imple- mentation is that high temperature and pressure will promote densi cation, as the lignin can ow into cavities. High densities of up to 1200 kg m − 3 were reported, where the uptake of water is hindered not only by limiting mass-transport, but also by con ning the swelling of cellulose bers. 88 Various researchers have included lignin in the formulation of paper-sizing agents. In one implementation, Javed et al. blended Kra lignin with starch, glycerol, and ammonium zirconium carbonate to produce self-supporting lms and paperboard coatings. 112 The mechanical lm stability was better
all-biobased materials and coatings. For example, eucalyptus Kra lignin and cellulose acetate were combined in solution and cast onto beech-wood, which produced a protective coating similar to bark. 37 However, the authors did not determine the mechanical properties of the product, which would be impor- tant to address, as the potential brittleness could impart prac- tical use. On the other hand, the biodegradation of lignin is indeed more challenging than that of cellulose and hemi- cellulose, 105 which may hence contribute to an improved resis- tance against certain fungi and bacteria. In addition, the lignin- based veneer may add functionalities such as water-repellence, UV-protection, and improved abrasion resistance, 106 but still a comparison with established treatment agents is lacking. Cellulose nano brils (CNF) and (cationic) colloidal lignin particles was cast into lms by Farooq et al. , yielding improved mechanical strength as compared to the CNF alone. 107 A sche- matic of the proposed interactions is given in Fig. 10. The authors concluded that the lignin particles acted as lubricating and stress transferring agents, which additionally improved the barrier properties. The discussed e ff ects could also be induced by the lignin acting as a binder, hence lling gaps and providing an overall tighter network. 36,88 Riviere et al. combined lignin- nanoparticles and cationic lignin with CNF, however, the oxygen barrier and mechanical strength were lower than the CNF without added lignin. 108 This e ff ect was likely due to a disruption of the binding between CNF networks. The poly- phenolic backbone of lignin generally provides less opportuni- ties for hydrogen bonding than compared to the cellulose macromolecule. The authors work on solvent extraction of lignin from hydrolysis residues is noteworthy, however, and the work showed promising potential for antioxidant use. LCC were combined with hydroxyethyl cellulose, producing free-standing composite lms. 109 In this study, the addition of LCC enhanced the oxygen barrier properties and could also improve the mechanical stability and rigidity. A better e ff ect of LCC was noted than combining lignosulfonates with hydrox- yethyl cellulose alone. Synergies could hence arise from carbo- hydrates that are covalently bond onto the lignin. An interesting approach was taken by Hambardzumyan et al. , who Fenton's reagent to partially gra organosolv lignin
Fig. 10 Schematic illustration of proposed interaction between CNF and di ff erent lignin morphologies. 107
RSCAdv. , 2023, 13 , 12529 – 12553 | 12539
© 2023 The Author(s). Published by the Royal Society of Chemistry
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