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gain, which biopolymers have over fossil-based polymers and llers. At last, slow-release fertilizers can be prepared from ther- moplastics and lignin. Li et al. blended poly(lactic acid) (PLA) withKra lignin samples, some of which had been chemically modi ed by esteri cation or Mannich reaction. 144 Urea particles were then coated by solvent casting or dip-coating, where the alkylated lignin yielded improved barrier properties and better compatibility with PLA. Microscope images of the coated urea particles are shown in Fig. 13. While lignin-PLA blends can potentially be more biodegradable than lignin-based resins, the biodegradation in soil may still be insu ffi cient. Our recom- mendation is hence to favor blends of unmodi ed lignin with biopolymers, such as starch, cellulose, or carrageenan, as this will not contribute to microplastics pollution. 3.5. Lignin as a precursor to thermosets The four most common applications of lignin in thermosets are polyurethane, epoxide resins, phenolic resins, and polyesters. 11 Unsurprisingly, formulations of lignin-based thermoset coat- ings are o en derived from such chemistries. The lignin can also be rendered compatible with other formulations, e.g. , with polyacrylates by gra ing with methacrylic acid. 145 Suchgra ing reactions are indeed instrumental to overcome some of the traditional challenges of lignin; 146 however, they can also be accompanied by unwanted side-e ff ects, such as poor biodegradability. 3.5.1. Lignin-based polyurethane coatings. Lignin utiliza- tion in polyurethanes is done as polyol replacement, where lignin's hydroxyl groups are reacted with isocyanate groups acting as cross-linker. The lignin may even be soluble in the polyol, which aids straight-forward substitution. Lignin deriv- atization to improve the compatibility and performance includes hydroxyalkylation ( e.g. , with propylene oxide, propylene carbonate, or epichlorohydrin), esteri cation with unsaturated fatty acids, methylolation, and demethylation. 28 Chen etal. blended alkali lignin and PEG, which were further polymerized with hexamethylene diisocyanate in presence of silica as leveling agent. 147 Experiments were limited to 60 wt% lignin, as higher ratios yielded an embrittlement. The mixtures were processed into lms, which showed some potential for biodegradation. These results indeed corroborated by other authors, which also state that lignin incorporation in
The UV absorbance was improved by both nanoparticle formation and pretreatment with the CatLignin process. The latter was explained by partial demethylation and boosting of chromophoric moieties. While lignin may conveniently replace fossil-based and non-biodegradable UV actives, other factors also need to be tested for such a product to become feasible, e.g. , non-hazardousness, safety for human contact, and skin tolerance. Other applications that can pro t from this property include UV-protective clothing, 91 packaging materials, 83,107 agrochem- ical formulations, 122 and personal protective equipment. 134 It should be mentioned, however, that enhanced UV absorbance is not always bene cial, as it can also lead to faster degradation of the lignin-containing materials. 12 3.4. Lignin as part of thermoplastic materials Lignin is a thermoplastic material with glass-transition temperatures in the range of 110 – 190 °C. 24 As such, it is straight forward to use technical lignin as a ller material in, e.g. , thermoplastics or bitumen admixtures. 38 Potential advan- tages of lignin in thermoplastic polymer coatings have been discussed by Parit and Jiang, 21 i.e. , by adding UV-blocking and antioxidant activity as required in packaging applications. In general, the addition of lignin in thermoplastics can increase sti ff ness, but at the expense of extensibility. 38 Chemical modi- cation (alkylation) may be required to improve both tensile sti ff ness and strength of ole nic polymers. 26 On the other hand, lignin's amphiphilic make-up can impart advantages, e.g. , by improving the adhesion of polypropylene coatings. 143 Another example would be the use of lignin in biocomposites from polypropylene and coir bers. 45 While no signi cant e ff ect on tensile strength of the composites was found, adding lignin reportedly delayed the thermal decomposition. Coatings with polymers are frequently used to protect the mechanical integrity of the underlying substrate. For added lignin to be advantageous, the mechanical characteristics of the polymer blend must hence be improved. While publications in this area frequently focus on the added functionalities, some also reported improvements in the mechanical strength of the coatings. 12,26 On the downside, the addition of lignin is o en limited to low ratios and chemical modi cation may be required. 12 These factors can limit the overall sustainability
Fig. 13 SEM images of PLA-lignin coated urea pellets showing (a) the coating layer over urea core and the coated pellet (b) before and (c) after urea release to surrounding water. This fi gure has been adapted/reproduced from ref. 144 with permission from Elsevier B.V., copyright 2017.
RSCAdv. , 2023, 13 , 12529 – 12553 | 12543
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