Morphological and Wettability Properties of Thin Coating Films Produced from Technical Lignins

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.