Aqueous epoxy-based superhydrophobic coatings: Fabrication and stability in water

Superhydrophobic (SH) surfaces have numerous application forms; however, perhaps the most commonly used form is as coatings. Most of coatings apply organic compound as solvents thus are not environment-friendly. In this study, we successfully fabricated SH coatings using waterborne epoxy resin emulsion. Such coatings possess an excellent adhesive strength with substrates and a high surface hardness; however, their SH properties were less durable when immersed in water, e.g. a Cassie-Wenzel transition occurred. Although this phenomenon is reversible following surface drying, it seriously limits the application of SH surface in areas such as anti-icing, self-cleaning, drag reduction, etc. XPS and SEM characterizations confirmed that this instability or transition was caused by an increase in the number of exposed hydrophilic groups embedded within the coating surface, but not by changes associated with the surface microstructure. By enhancing the macromolecular weight of the epoxy resins, for example, by reducing their epoxy value and reducing the number of cross-linking agents, the number of exposed hydrophilic groups such as hydroxyl (-OH) and amidocyanogen (-NH) functionalities decreased, which increased the SH stability in submerged conditions. This study will be helpful in improving the practical application of SH coatings.