Abstract
Technical lignins are widely available as side streams from pulping and biorefining processes. The aromatic structure of such lignins could be exploited in coating formulations to provide antioxidant or UV-blocking functionalities to packaging films. In this study, six technical lignins sourced from different plant species by given isolation/modification methods were compared for their composition, molar mass, and functional groups. The lignins were then used to prepare thin spin-coated films from aqueous ammonia media. All the lignins formed ultrathin (<12 nm), smooth (roughness < 2 nm), and continuous films that fully covered the solid support. Most of the films contained nanometer-sized particles, while those from water-insoluble lignins also presented larger particulate features, which likely originated from macromolecular association during solvent evaporation. These latter films had water contact angles (WCAs) between 40 and 60°, corresponding to a surface energy of 42–48 mJ/m2 (determined by Zisman plots). For comparison, the water wettability measured on lignin pellets obtained by mechanical compression tracked closely with the WCA obtained from the respective thin films. Considering the widely diverse chemical, molecular, and structural properties of the tested lignins, comprehensively documented here by using a battery of techniques, the solubility in water was found to be the most important and generic parameter to characterize the thin films. This points to the possibility of developing lignin coatings with predictable wetting behavior.
Highlights
Lignin is the second most abundant biopolymer on Earth and, together with cellulose and hemicelluloses, one of the three main components of the plant cell wall
The term “technical lignin” is commonly used to refer to lignin extracted from biomass by pulping and other extraction methods
Softwood kraft lignin isolated through the LignoBoost process was obtained from UPM Biochemicals (Finland)
Summary
Lignin is the second most abundant biopolymer on Earth and, together with cellulose and hemicelluloses, one of the three main components of the plant cell wall. Lignin is synthesized from three monolignol precursors (sinapyl, coniferyl, and pcoumaryl alcohols) that produce syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units.[1] These phenylpropane units form a highly cross-linked and amorphous macromolecule whose composition largely depends on the botanical origin of the plant. Lignin in softwoods contains more than 95% G units with minor amounts of H units, hardwoods contain mainly S and G units (often close to 50−50% ratio), and grasses contain all three (S, G, and H) units in varying ratios.[2] The β−O−4 bond is generally accepted to be the main linkage between the lignin units, but several other bonds (e.g. 4−O−5, β−5, β−1, β−β, and 5−5) exist. The term “technical lignin” is commonly used to refer to lignin extracted from biomass by pulping and other extraction methods
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