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Fig. 9 SEM images of (a) uncoated paperboard and (b) paperboard coated with lignin-fatty acid ester. This fi gure has been adapted/reproduced from ref. 78 with permission from Elsevier B.V., copyright 2013.
used in wood-varnish formulations with a higher technological maturity. 3.2.3. Other approaches. A variety of other modi cations has been proposed to develop coatings from lignin. For example, Dastpak et al. reacted lignin with triethyl phosphate to spray-coat iron-phosphated steel for corrosion protection. 44 Coating of aminosilica gel with oxidated Kra lignin was per- formed by electrostatic deposition, which improved the adsorption capacity for dyes from wastewater. 102 Wang et al. phenolated lignosulfonate, followed by Mannich reaction with ethylene diamine and formaldehyde to produce slow-release nitrogen fertilizers. 103 The nal product exhibited elevated contact angles, however, an increased surface roughness likely also contributed to this e ff ect, as the phenolated and aminated lignin exhibited nanoparticle structures. A di ff erent approach was taken by Behin and Sadeghi, who acetylated lignin with acetic acid to coat urea particles in a rotary drum coater. 104 The use of lignin in slow-release fertilizers can be useful, as lignin can have a soil-conditioning e ff ect. However, biodegradability also must be considered, which can be negatively a ff ected by chemical modi cation. Self-healing elastomers were synthesized by Cui et al. , who gra ed lignin with poly(ethylene glycol) (PEG) terminated with epoxy groups. 31 The authors concluded that a new material was developed with potential application for adhesives, but the ultimate stress was comparably low at 10 – 12 MPa. The material was named as a self-healing elastomer; however, the appear- ance and rheological properties suggest a thixotropic gel instead. 3.3. Blends of lignin with other substances In the context of this review, the largest number of publications was found for lignin-blends with other substances. The advan- tage of this approach lies in the ease of implementation, exi- bility for later adjustments, and potential synergies with other co-agents. The lignin and other additives may be mixed right before or during surface modi cation, hence not requiring lengthy preparations such as the synthesis of chemically modi ed lignin or a pre-polymer. To facilitate better overview, this section was subdivided into several sub-section, which were distinguished by the application area or formulation-approach. 3.3.1. Cellulose bers and other wood-based products. The use of lignin in combination with cellulose bers, brils, or derivatives has received considerable attention, as this can yield
function to fatty acids, such as resin acids or alkenyl succinic anhydride. Considering these examples, the questions arises whether modifying lignin bears an advantage over using established coatings or sizing agents. In the light of this discussion, the acid-catalyzed transesteri cation of lignin with linseed oil should be mentioned. 77 According to the authors, a suberin-like lignin-derivative was produced, which introduced hydrophobicity on mechanical pulp sheets, while being more compatible with the bers than linseed oil alone. The proposed process is simple in setup and reactants, which facilitates ease of implementation. In addition, the lignin is prescribed a key function, i.e. , acting as a compatibilizer between the bers and the triglycerides. At last, controlled-release fertilizers with lignin-fatty acid gra polymers have been proposed. Wei et al. crosslinked sodium lignosulfonate with epichlorohydrin, followed by esteri cation with lauroyl chloride. 97 Sadehi et al. reacted lignosulfonate with oxalic acid, proprionic acid, adipic acid, oleic acid, and stearic acid. 98 The modi ed lignin was further used to spray-coat urea granules. Both implementations showed enhanced hydrophobicity and the ability to coat urea for slower release of nitrogen. Still, it would be important to compare such approaches with established coating or blends of lignin and natural waxes or triglycerides, which do not require an elaborated synthesis. 3.2.2. Enzymatic modi cation. Enzymatic modi cation of lignin has the advantage of comparably mild reactions condi- tions, which can have a positive impact on process economics. On the downside, enzymes are comparably expensive and imposes higher technological demands. In addition, the variety of lignin-compatible enzymes is somewhat limited. Enzymatic treatment can induce a number of changes to lignin, such as oxidation, depolymerization, polymerization, and gra ingwith other components. 99 For example, Mayr et al. coupled ligno- sulfonates with 4-[4-(tri uoromethyl)phenoxy]phenol using laccase enzymes. 100 A er successful coupling, the lignosulfo- nate lms exhibited reduced swelling and an increase in aqueous contact angle. Fernandez-Costas et al. performed laccase-mediated gra ing of Kra lignin on wood as a preser- vative treatment. 101 While the reaction itself was deemed a success, the desired antifungal e ff ect was only obtained a er inclusion of additional treatment agents, such as copper. It is hence questionable if enzymatically coupled lignin poses as a competitive wood-treatment agent, as the lignin could also be
12538 | RSCAdv. , 2023, 13 , 12529 – 12553
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