Design of Sol–Gel Hybrid Bio-sourced Lignin/Silica Hydrophobic Nanocomposites through a Dip-Coated Evaporation-Induced Self-Assembly Method

Lignin–silica hybrid nanocomposite films were prepared on aluminum alloys 2024 through a sustainable dip-coating process combined to an evaporation-induced self-assembly methodology using Kraft lignin and tetraethyl orthosilicate as the organic and the inorganic sources, respectively. Dip coating parameters like withdrawal speed and relative humidity were optimized to create the best coating deposition. Two different synthesis pathways, direct (DR) and non-direct (NDR), were evaluated with respect to an ecofriendly approach. The NDR synthesis involved a pre-coating functionalization of the substrate by a 3-(Triethoxysilyl) propylsuccinic alkoxysilane linker, which provides a significant improvement and enhancement of the film stability due to the creation of anchorage sites for the hybrid composite. The surface morphology of the material, its chemical composition, and wettability were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and water contact angle measurements. The results show a hydrophobic behavior of the hybrid film coating improved by the self-assembly of lignin nanoparticles on the surface, with the contact angle increasing from 28° for the pure Al alloy substrate to 95° for the lignin-based hybrid coating.