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Fig. 14 Schematic of producing lignin-based thermosetting polyester coatings. This fi gure has been adapted/reproduced from ref. 19 with permission from American Chemical Society, copyright 2018.
polyacrylonitrile. 162 The authors concluded that silylation improved the compatibility for surface coatings and lms.
hydroxyl groups on the lignin. Development in this area has potential, as polyesters tend to exhibit better biodegradability than polyole ns. 3.5.5. Lignin-based acrylate coatings. Lignin-based acry- lates rely on the gra ing of acrylate moieties, as these are not inherent to lignin. For example, methacrylation of Kra lignin was done to produce UV-curable coatings. 145 The authors concluded that incorporating lignin into the formulation improved thermal stability, cure percentage, and adhesive performance. An elaborate study on aging of lignin-containing polymer materials was conducted by Goliszek et al. 30 The authors gra ed Kra lignin with methacrylic anhydride and further polymerized the product with styrene or methyl meth- acrylate. Low amounts of lignin (1 – 5%) showed incorporation into the network, whereas higher concentrations showed a plasticizing and more heterogeneous e ff ect. High lignin loadings also enhanced the detrimental e ff ects of aging, which may seem counterintuitive, as other reports frequently state a UV-protective ability of lignin. Still, increase absorption of UV light can also amplify the detrimental e ff ects thereof. A combination of epoxy and acrylate was used to develop dual- cured coatings with organosolv lignin. 160 The lignin was rst reacted with epoxy resin and subsequently with acrylate to form a prepolymer. In a second step, the prepolymer was mixed with initiators and diluent to be coated onto tinplate substrates. All in all, lignin-based acrylate coatings appear to have reached su ffi cient technological maturity, yet the advantage of adding lignin is sometimes unclear. 3.5.6. Other approaches. Szabo et al. gra ed Kra lignin with p -toluenesulfonyl chloride, whose product was then graf- ted onto carbon bers. 161 The results suggested an improved shear tolerance of the modi ed carbon bers in epoxy or cellulose-based composites. Silylation was furthermore per- formed of Kra lignin, which was further co-polymerized with
4. Lignin in technical applications – a critical commentary The development high-value products from lignin has been a topic of great interest for some years, and is still gaining popularity. 163 Added-value applications are being pursued, ranging from asphalt emulsi ers or rubber reinforcing agents, to the production of aromatic compounds via thermochemical conversion. 164 The question arises, however, if including lignin in a coating can really lead to a better overall product? Comparison with state-of-the-art formulations is frequently omitted, benchmarking lignin-based solutions only to a refer- ence case with low performance. “ Attributing value to waste ” is one of the primary motivations behind lignin-oriented research. For example, bioethanol production from lignocellulose biomass o en gives rise to a lignin-rich byproduct. The overall economics of such biore neries could be improved if the lignin- rich residue could be marketed at a value. Still, to establish a new product on the market, this product also needs to compete with existing solutions in terms of performance and price. This point is o en overlooked in literature, in particular concerning lignin-based surfaces and coatings. Harnessing lignin's inherent properties is key, as this can create synergies and yield an advantage over other biopolymers. It comes to no surprise that the dominant use of technical lignin is in water-soluble surfactants, as polydispersity is a key feature here. 3 As has been pointed out, the performance of surfactant-blends o en outperforms single surfactants in real- world applications, since the mixture can preserve its function over a wider range of environmental conditions. A second example of key properties would be lignin's polyphenolic
RSCAdv. , 2023, 13 , 12529 – 12553 | 12545
© 2023 The Author(s). Published by the Royal Society of Chemistry
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