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Fig. 8 Coatings comprising lignin particles produced by evaporation-induced self-assembly (a) and vertical cross-section of the obtained layers (b). 85
3.2.1. Lignin-ester derivatives. Esteri cation of lignin with fatty acids has been investigated by several authors. This approach bears potential, as it combined two bio-derived (macro-)molecules. The lignin contributes a backbone for gra ing and may improve dispersibility and adhesion of the fatty acids on lipophobic surfaces. The fatty acids can in turn render the lignin more hydrophobic, improving the water barrier, e.g. , on paper substrates. To improve the reaction yield, reactive intermediates are frequently used. Several publications have studied the use of lignin esteri ed with fatty acid-chlorides as hydrophobization agents for paper and pulp products. 78,93 The coating a ff ected both the surface chemistry and morphology, as illustrated in Fig. 9. The result is usually a decrease in water-vapor transmission rate (WVTR), oxygen transmission rate (OTR), and an increase in aqueous contact angle. Oxypropylation with propylene carbonate has been used as an alternative esteri cation approach, which yielded a similar hydrophobization and barrier e ff ect on recycled paper. 94 A downside of oxypropylation is the use of toxic reac- tants, i.e. , propylene oxide, and the requirement for high pres- sure during the reaction. While fatty acid chlorides do not need high pressures, these chemicals are highly corrosive and require the absence of water. All mentioned aspects can stand in the way of commercial implementation. Hua et al. reacted so wood Kra lignin with ethylene carbonate to convert phenolic hydroxyl units to aliphatic ones, 95 as these are considered more reactive. The samples were further esteri ed with oleic acid and spin- or spray-coated onto glass, wood, and Kra pulp sheets. The authors showed that hydro- phobic surfaces with contact angles ranging from 95 – 147°were possible. The pulp boards furthermore showed a more uniform surface a er the coating. Esteri cation with lauroyl chloride was also used by Gordobil et al. , who studied their application as wood veneer by press-molding and dip-coating. 96 While the feasibility to treat wood and wood-based products was demonstrated on a technological level, the comparison to established treatment agents is frequently lacking. For example, linseed oil is an established wood-treatment agent, which undergoes self-polymerization in the presence of air. Paper- sizing agents can be based on compounds that are similar in
3.1.2. Applied research. An example for applied research would be paper and pulp products, which can be rendered less hydrophilic by surface-sizing. Application of the lignin can be done via an aqueous dispersion or alternatively by impregna- tion a er dissolution in a solvent. 35,88 A similar approach was used to treat beech wood with lignin nanoparticle via dip- coating, which improved the weathering resistance of the wood. 89 Such dip-coating may preserve breathability of the substrate due to the porous structure. In this context, the patent application WO2015054736A1 should be mentioned, which discloses a waterproof coating on a range of substrates including paper. 90 In this invention, the lignin is coated onto the substrate a er at least partial dissolution, followed by heat or acid treatment. However, as discussed above, the lignin by itself is not a hydrophobic material. While lignin-nanoparticles may alter the surface morphology of pulp products, an improvement in long-term water-resistance may be mostly determined by a ff ecting mass-transfer kinetics. Deposition of lignosulfonates on nylon has been demon- strated, which improved the ultraviolet protection ability of the fabric. 91 This deposition took place from aqueous solution and under heating, reportedly yielding a chemical bonding of lignin's OH groups to the NH groups of nylon 6. Such bonding would indeed be necessary, as the lignosulfonate would other- wise be easily washed away. Zheng et al. coated micro brillated cellulose with Kra lignin and sulfonate Kra lignin, which promoted re retard- ancy of the material. 42 At last, iron-phosphated steel was rendered more resistant to corrosion a er spray coating with lignin, which was rst dissolved in DMSO and other commer- cial lignin-solvents. 92 While proven in the lab, these two appli- cations must be considered with care, as unmodi ed lignin is a brittle material, which can limit the long-term durability of such products. 3.2. The use of chemically modi ed lignin Chemical modi cation of lignin is frequently done to improve or enable the processability in blends with materials. In addi- tion, chemical modi cation may add or alter functionalities as required in speci c applications.
RSCAdv. , 2023, 13 , 12529 – 12553 | 12537
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