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Fig. 7 Overview of di ff erent application modes for producing functional surfaces and coatings with lignin.
angles ranging from 40 – 60°. Despite widely diverse composi- tions, the solubility in water was found to be the parameter governing the properties of the thin lms. Similar results were obtained by Notley and Norgren, who found that lignin coatings prepared from diiomethane or formamide yielded even lower contact angles at about 20 – 30°. 34 The approach was further re ned by Souza et al. , who treated the spin-coated lignin lms via UV radiation or SF6 plasma treatment in addition. 84 While the UV treatment reduced the contact angle from about 90° to 40°, the plasma treatment produced superhydrophobic surfaces with contact angles exceeding 160°. The latter was also shown to induce major surface restructuring with a strong incorporation of CF x and CH x groups, which would account for the large increase in contact angle. Coatings with lignin-based nano- particles can also be made by evaporation-induced self- assembly, whose properties and morphology are strongly gov- erned by the drying conditions and evaporation rate. 85 An example of the obtained morphologies is shown in Fig. 8. Based on these reports, research is generally concurring on the fact that lignin by itself is not a hydrophobic substance. Harsh treatments, chemical modi cations, or ne-tuning of surface morphology are necessary to invoke hydrophobicity. Spin-coated lms of milled-wood lignin have furthermore been investigated for enzyme adsorption. 86 Similarly, the adsorption of proteins on colloidal lignin has been studied by Leskinen et al. , who produced protein coronas on the lignin particles via self-assembly. 87 The authors further showed that this deposition was governed by the amino acid composition of the protein, as well as environmental parameters such as the pH and ionic strength. The use of lignin for protein-adsorption is an interesting implementation, as it can provide di ff erent surface chemistries than its lignocellulosic counterparts. Still, the compatibility with in vivo environments is questionable, as biodegradation is not given here.
or inert materials, (3) the blending of lignin in thermoplastic materials, and (4) the use of lignin as a precursor for synthe- sizing thermoset polymers. An overview of the di ff erent approaches for formulation and application is given in Fig. 7. These will be discussed in more detail further on. While surface layer or coating are usually applied onto another material, there are also implementations that include lignin as part of the overall base-matrix. Examples for the latter include lignin-derived biocarbon particles for CO 2 capture or wastewater treatment, polyurethane foams, and lignin as an internal sizing agent in pulp products. 36,43,79,80 The predominant way of using lignin in functional surfaces is by blending with other substances. Such formulations o en include agents, which are established for a particular application, e.g. , starch for paper sizing or clay for controlled-release urea fertilizers. 81,82 Formulations in polymer synthesis usually draw on speci c functional groups that are found in lignin, for example, the hydroxyl groups as polyol replacement in polyurethane or the aromatic moieties as phenol replacement in phenol- formaldehyde resins. 18,33 3.1. Surfaces and coatings with neat lignin Applying technical lignin by itself is a simple approach, as no co-agents are required. While some degree of adhesion to the substrate is o en given, pressure and heat may be applied in addition. Publications pertaining to this topic can be grouped into two categories, i.e. , fundamental research studying the formation and properties of lignin-based lms and coatings, as well as applied research, which is usually focused on a speci c end-use. 3.1.1. Fundamental research. A fundamental study was performed by Borrega et al. , who prepared thin spin-coated lms from six di ff erent lignin samples in aqueous ammo- nium media. 83 The lms exhibited hydrophilicity with contact
12536 | RSCAdv. , 2023, 13 , 12529 – 12553
© 2023 The Author(s). Published by the Royal Society of Chemistry
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