Thermodynamic analysis and injection molding of hierarchical superhydrophobic polypropylene surfaces

In this study, thermodynamic analysis of the hierarchical structure of the 3D cylinder-textured surface model was performed. The wetting states at different penetration depths, the effects of three components on the wetting properties, and all equilibrium contact angle of the hierarchical structure were investigated. It was found that the interaction between micropillars and nanopillars can affect the transition energy barrier and the transition pitch in the wetting-state transition process. This showed that all components would play a key role in enhancing the surface hydrophobicity. Polypropylene (PP) surfaces with mono micropillars and hierarchical structures were both fabricated by injection molding. Mold inserts for hierarchical structures were obtained by the combination of a punching plate and an anodized aluminum alloy plate. The static contact angle (CA) and the roll-off angle of injection-molded PP surfaces were measured and analyzed from the perspective of thermodynamic analysis. With the hierarchical structures, a static CA of about 10 degrees as well as a roll-off angle of about 5 degrees was approached. Compared with a mono micropillar-structured PP surface, the hierarchical-structured PP surface has a larger static CA and a smaller roll-off angle. The work demonstrates an inexpensive and reproducible technique to fabricate function-designed controlled hierarchical structures on PP material.