Sustainable superhydrophobic and self-cleaning wood via wax within Epoxy/PDMS nano-composite coatings: Durability related to surface morphology

The stability of wood materials in harsh environments requires a durable superhydrophobic (SH) coating. In this paper, a sustainable SH coating, suitable for interior and exterior applications of wood, was produced by reinforcing with bio based carnauba-wax two matrices, i.e. Epoxy resin or polydimethylsiloxane (PDMS). Fluorine-free dodecyl-terminated silica nanoparticles (DDT-SiO2), prepared through a chemical procedure, have been used to increase the hydrophobicity of all coating formulations. A novel approach for the wood substrate preparation was proposed and tested. The results showed that long-term water-repellency of the SH coatings can be enhanced by solvent/ultrasonic pre-treatments, controlling the chemical and morphological characteristics of the wood substrate before coating. The formulation containing wax exhibited a contact angle of 172° and a sliding angle <3° when applied on a clean/micro-structured wood surface subjected to solvent pre-treatments. The PDMS-based coating displayed excellent durability to water impact test, 120 cycles outdoor weathering/aging and severe environments (i.e. chemicals, ultrasonic and sea-water) than Epoxy one. While wax decreased the transparency of Epoxy-based coatings, the synergistic effect on the durability of the coating was evident. The superior characteristics of non-wetting, mechanically durable and resistance to finger-touch test of wax-PDMS-DDT-SiO2 coating were attributed to the hierarchical structure caused by carnauba-wax self-assembly and interwoven morphology. In fact, the wax helps keep the “PDMS-NPs” lotus-like structures tightly connected, producing a robust surface. The self-cleaning test, carried out using commonly used liquids and colored water, confirmed the admirable performance of the PDMS-based coating, guaranteeing also aesthetic property. The surface treatment obtained can be considered “eco-friendly” as it is composed of vegetable carnauba-wax containing nanoparticles modified with non-fluorinated compounds.