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polyurethanes yielded a limited degree of biodegradability. 148 A di ff erent approach was made by Rahman et al. , 149 who synthe- sized waterborne polyurethane adhesives with aminated lignin. The tensile strength and Young's modulus improved with increasing ratios of aminated lignin, which could be due to an increased cross-linking density. Still, the overall percentages of lignin in the coatings were comparably low, as the authors added only between 0 – 6.5 mol% lignin. It is curious to note that the authors proclaimed better storage stability of aminated lignin dispersions, yet only the weathering resistance of the nal coating was measured. Some of the challenges with lignin in polyurethane materials include reactivity and a high cross-linking density. Due to the latter, polyurethane formulations are frequently limited to low percentages of lignin, typically 20 – 30 wt% at max, as higher ratios can yield brittle and low-strength materials. 150 One approach is to increase the degree of substitution is depoly- merization of lignin, but other chemical modi cations or frac- tionations may be equally applicable. In this context, the work byKlein et al. should be mentioned, who reported polyurethane coatings with lignin ratios of up to 80%. 151 A comparably low curing temperature of 35 °C was used, which could also entail incomplete reaction. Curiously, there is no data on the mechanical strength of the lms. In addition, the authors measurements of hydroxyl groups via ISO 14900 and 31 p-NMR are widely divergent. In two other publications by the same author, the antioxidant properties and antimicrobial e ff ect of such lms were studied. 13,152 In a di ff erent study, methyl- tetrahydrofuran was used to extract the low-molecular weight portion from Kra lignin. 153 The authors used between 70 to 90 wt% lignin in the nal formulation at NCO/OH molar ratios of 0.16 – 0.04. While providing a good adhesive strength, the lms elastic modulus is within the same range of the fraction- ated lignin, whereas no information on the material strength was provided. It would thus appear that the elevated cross- linking density may be circumvented, i.e. , simply by reacting only a sub-fraction of the available hydroxyl groups of lignin. Still, it has yet to be demonstrated that such coatings are also competitive in mechanical strength and abrasion resistance. 3.5.2. Lignin-based phenolic resin coatings. Lignin may also be used as a phenol-substituent in phenol-formaldehyde resins. 20 This approach was utilized by Park et al. to produce cardboard composites by spray coating. 154 The authors reported that lignin puri cation by solvent extraction yielded better results than by acid precipitation. Substituting with 20 – 40wt% lignin surprisingly accelerated the curing kinetics, compared to the lignin-free case. The coated cardboard showed lower water absorption; however, the contact angle was also lower, which could be due to a change in surface chemistry and morphology. It would be interesting to study even higher degrees of substi- tution and to delineate with the mechanical strength. Still, it appears that coatings with lignin-phenol-formaldehyde have so far been aimed at providing a water-barrier. For example, the work by Rotondo et al. coated superphosphate fertilizers with hydroxymethylated lignin resins, 119 which signi cantly slowed the phosphate release.
3.5.3. Lignin-based epoxy resin coatings. Similar to lignin- containing polyurethanes, epoxy resins also target a reaction with the hydroxyl groups. In analogy to that, chemical condi- tioning such as depolymerization can potentially improve the nal material. For example, Ferdosian et al. tested di ff erent ratios of depolymerized Kra or organosolv lignin in conven- tional epoxy resin formulations. 155 The authors showed that large amounts of lignin retarded the curing process particularly in the late stage of curing. At the right dosage (25%), the lignin- based epoxy exhibited better mechanical properties than the neat formulation, while improving adhesion on stainless steel. Both e ff ects appear plausible considering lignins macromolec- ular and polydisperse composition. In this context, a recent patent by Akzo Nobel should also be mentioned, which describes the use of lignin and potential epoxy crosslinker for functional coatings. 156 Adi ff erent approach was chosen by Hao etal. , who carboxylated Kra lignin rst, followed by its reaction with PEG-epoxy. 157 The coatings possessed a lignin content of 47%. In addition, the self-healing ability was demonstrated by transesteri cation reaction in presence of zinc acetylacetonate catalyst. Crosslinking of nanoparticles is an interesting approach, as the coagulation to nanoparticles may favor a di ff erent ratio of functional groups at the surface than in the bulk lignin. In addition, this approach can produce composite materials, which exhibit di ff erent characteristics than a homogeneous polymer. For instance, Henn et al. combined lignin- nanoparticles with an epoxy resin, i.e. , glycerol diglycidyl ether, to treat wood surfaces. 106 The coatings showed nano- structured morphology, which still preserved the breathability of the wood, hence drawing advantage from lignin's nano- particle formation. Zou et al. coprecipitated so wood Kra lignin together with bisphenol-a-diglycidyl ether to produce hybrid nanoparticles. 158 The particles were either cured in dispersion for further cationization or directly tested in their function as wood adhesives. The use of lignin-based nano- particles in curable epoxy resins is hence promising, as it can generate new functionalities, but the maturity of this tech- nology still needs to be advanced. 3.5.4. Lignin-based polyester coatings. While the use of lignin in polyester coatings is technologically feasible, few publications were found to this topic. One reason for this could be the slow reaction kinetics of direct esteri cation. Coupled with lignin's structure and chemistry, polyester-based coatings would be less straight forward than polyurethanes or epoxy resins, which involve highly reactive coupling agents. As dis- cussed previously, chemical modi cation of lignin may improve this circumstance, for example by depolymerization or intro- duction of new reactive sites. As such, oxidative depolymeriza- tion and subsequent membrane fractionation has been suggested to produce a raw material, which can be utilized in subsequent polyester coatings. 159 A second example would be solvent-fractionated lignin, which has been carboxylated by esteri cation with succinic anhydride. 19 As illustrated in Fig. 14, the modi ed lignin reportedly underwent self-polymerization, where the gra ed carboxyl groups reacted with residual
12544 | RSCAdv. , 2023, 13 , 12529 – 12553
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