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of the paper, as aging and yellowing may be promoted by the presence of acids and chromophores. Layer-by-layer self-assembly was used by Peng et al. to produce superhydrophobic paper coated with alkylated ligno- sulfonate and poly(allylamine hydrochloride). 40 Alternatively, Lit et al. deposited such layers on cellulose bers by combining lignosulfonates with the divalent copper cation instead of a polycation. 41 A similar e ff ect on surface morphology was noted, while contact angles within the hydrophobic regime could be achieved. The utilization of lignosulfonate-polycation assemblies for cellulose hydrophobization is somewhat coun- terintuitive, since polyelectrolyte complexes tend to be hydro- philic and can swell in water. The long-term stability of such coatings in water is hence questionable, still for short contact times the modulation of surface roughness and chemistry can be bene cial. Solvent casting was employed by Wu et al. , using ionic liquids to dissolve cellulose, starch, and lignin. 117 The biopoly- mers were coagulated by addition of the non-solvent water, further being processed into exible amorphous lms. The process appears similar to the production of cellulose regener- ates. Utilizing other biopolymers than cellulose, i.e. , lignin, hemicellulose, and starch, is an interesting approach for ne- tuning the desired lm properties. Zhao et al. used evaporation induced self-assembly of lignin nanoparticles and CNF, which were subsequently oxidized at 250 °C and then carbonized at 600 – 900 °C. 79 These nano- and micro-sized particles could be used for CO 2 adsorption, where synergistic e ff ects between the CNF and lignin nanoparticles were noted. An illustration of the particles is shown in Fig. 11. Agrochemical formulations with lignin-based coatings predominantly involve fertilizer formulations, i.e. , for controlled release of nutrients. The lignin can be part of a coating, which then acts as a mass-transfer barrier that delays the dissolution of nutrients. 82,118,119 The focus is usually on urea as nitrogen fertilizer or calcium phosphate as superphosphate fertilizers. 82,118 – 120 An advantage of using lignin, apart from being biodegradable and water-insoluble, is the potential function as soil amendment. 121
when using ammonium zirconium carbonate as a cross-linking agent, in addition to reducing the water-transmission rate. Both the lignin and the ammonium zirconium carbonate also reduced leaching of starch when in contact with water. In a second publication, the author further developed the formu- lation's use in pilot trials. 81 Johansson et al. coated paperboard, aluminium foil, and glass with mixtures of latex, starch, clay, glycerol, laccase enzyme, and technical lignin. 113 The authors found that the oxygen scavenging activity was greatest for lignosulfonates, as compared to organosolv, alkali or hydrolysis lignin. This e ff ect was explained by a greater ability of the lac- case to introduce cross-linking on the lignosulfonate macro- molecules. In another publication, Johansson et al. also combined lignosulfonates with styrene-butadiene latex, starch, clay, glycerol, and laccase enzyme. 114 The results showed that both active enzyme and high relative humidity were necessary for good oxygen scavenging activity. Laccase-catalyzed oxidation of lignosulfonates furthermore resulted in increased sti ff ness and water-resistance of the starch-based lms. Winestrand et al. prepared paperboard coatings using a mixture of latex, clay, lignosulfonates, starch, and laccase enzyme. 115 The lms showed improved contact angle with active enzyme and oxygen- scavenging activity for food-packaging applications. While the results for paper-sizing with addition of lignin show promising potential, food packaging applications may impose additional requirements. For example, stability of the coatings may not be given in environments that contain both moisture and lipids. In addition, to the best of our knowledge, no study addressed the migration of sizing-agents into food. Still, the utilization of lignin as oxygen scavenger is promising, as this utilizes one of lignins inherent properties, which are found in few other biopolymers. As an alternative to technical lignin, Dong et al. applied alkaline peroxide mechanical pulping e ffl uent in paper-sizing, which comprised 20.1 wt% lignin and 16.5 wt% extractives based on dry matter weight. 116 Blended with starch, the e ffl uent improved the tensile index and reduce the Cobb value of paper, while providing contact angles of 120° and higher. Such implementation can, however, also aggravate certain properties
Fig. 11 Lignin-based porous particles obtained by oxidation and carbonization for carbon capture. SEM images of lignin particles carbonized at low (a) and high (b) pre-oxidation rate, yielding two distinct morphologies. This fi gure has been adapted/reproduced from ref. 79 with permission from Elsevier B.V., copyright 2017.
12540 | RSCAdv. , 2023, 13 , 12529 – 12553
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