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RSC Advances
a summary of the current developments in research, where focus was placed on the formulation and nal applications. Overall, coatings with neat lignin or blends of lignin with other active ingredients appear the most practical. Reduced wetting is hereby achieved, as the lignin can alter the surface morphology, hinder mass-transport, and con ne swelling of enclosed bers. The lignin itself is not considered a hydro- phobic material, because the contact angle is usually below 90°. On the other hand, hydrophobicity can be induced by plasma surface treatment, blending with other agents, or chemical modi cation. For the latter, gra ing or esteri cation of lignin with alkyl-containing moieties is a frequently taken approach. Chemical modi cation may also be used to improve the compatibility with ole nic thermoplastics. Addition of lignin can ne-tune the characteristics thermoplastics and improve adhesion to other materials. On the downside, embrittlement frequently limits this technology to low percentages of lignin. Thermoset coatings with lignin can be based on chemistries such as polyurethanes, phenolic resins, epoxy resins, polyesters, and polyacrylates. Various synthesis routes have been proposed in literature, which can bene t to some degree of the inherent properties of lignin. Both the formulation and processing depend on the nal application of the coating or surface functionalization. The use of lignin with cellulose-based substrates is frequently sug- gested, as this can yield all-biobased materials. Lignin can improve the resistance to wetting of paper and pulp products. In addition, it can add UV protection and oxygen-scavenging capabilities in packaging applications. Lignin-based surfaces have also been proposed for adsorbents for wastewater treat- ment, wood veneers, and corrosion inhibitors for steel. The biomedical eld has also explored lignin-based biomaterials, which draw on its potential antimicrobial properties. A great number of publications also reports on agricultural uses, where a lignin-based coating may account for slower release of fertil- izer. At last, general-purpose polymer coatings can be tailored via the inclusion of lignin, and the resistance to fouling of membranes can be improved. All mentioned applications were discussed critically in this review, placing emphasis on the bene t that adding lignin may provide. While introduction of functionalities may be possible, publications frequently do not compare to a well-performing reference case, hence limiting the assessment of the true potential. In addition, the ratio of lignin in thermoset coatings is usually quite low. Higher levels may be achieved a er chemical modi cation, but such synthesis can also have negative implications on the economic and environ- mental cost of the nal product. In conclusion, the advancement of functional surfaces and coatings with lignin has yielded promising results. However, there also must be a bene t of using lignin compared to other biopolymers or existing petrochemical solutions. Only by har- nessing lignin's inherent properties, can solutions be developed that are competitive and value-creating. These properties include lignin's polyphenolic structure, a higher C/O ratio than, e.g. , polysaccharide biopolymers, its ability to self-associate into nano-aggregates, and its thermoplasticity. These properties are
structure, which is not found in common polysaccharides. Lignin has hence been investigated as a UV blocking additive in e.g. , sunscreen products or packaging. 165 However, compati- bility of the resulting product with human- or food-contact is addressed insu ffi ciently by many authors. A similar situation was given in case of lignin as antioxidant additive in cosmetics, 166 where the dark color and smell may limit the nal use. Compared to cellulose or hemicellulose, lignin has a higher carbon-to-oxygen ratio. Due to this and its polyaromatic struc- ture, it would indeed be a better raw material for producing carbonaceous materials. Research on activated carbon, graphitic carbon, and carbon bers has indeed being conduct- ed. 167 A key step toward lignin-based carbon ber production was identi ed as removal of b -O-aryl ether bonds. 60 In addition, the charring ability of lignin has been proposed as a bene t in re retardants. 168 Still, lignin-based re retardants o en use chemical modi cations, such as phosphorylation. If chemical modi cation is necessary, the question arises if such chemis- tries really need to be based on lignin, since other bio- macromolecules may possess a higher reactivity and number of reactive sites. Lignin can be readily precipitated from solution into nano- particles and nanospheres. Various applications have been suggested based on this, such as functional colloids and composite materials with uses in ame retardancy, food pack- aging, agriculture, energy storage, and the biomedical eld. 169 A more speci c example would be nanoparticulated lignin in poly(vinyl alcohol) lms with increased UV absorption. 170 While this technology appears straight-forward, its nal use has yet to be proven. At last, technical lignin is usually thermoplastic, exhibiting glass-transition temperatures in the range of 110 – 190 °C. 24 The use of lignin as polymeric ller or in thermoplastic blends is hence promising. In some cases, chemical modi cation may be necessary to improve compatibility, e.g. , with polyole ns; 26 however, the use as simple ller material would not necessitate modi cation. Additional strength could also be derived from added cellulose bers, which could potentially bene t from added lignin as compatibilizer. In summary, one needs to build on the inherent properties of lignin, such as polydispersity, poly-aromaticity, a higher C/O ratio than for polysaccharides, and thermoplasticity. Only by utilizing characteristics that set lignin apart from other biopolymers, can solutions be developed that are innovative and market competitive. Chemical modi cation is a useful tool for tailoring; however, each processing step will add an economical and environmental cost to the nal product. In other words, the simplest approach is o en the best – some- thing that is frequently disregarded when developing complex synthesis protocols for lignin. 5. Summary and conclusion Functional surfaces and coatings can be formulated in a variety of ways, which includes the use of neat, chemically modi ed, blended, and cross-linked lignin. This review provides
12546 | RSCAdv. , 2023, 13 , 12529 – 12553
© 2023 The Author(s). Published by the Royal Society of Chemistry
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