Thin coating of silica/polystyrene core-shell nano/microparticles with hierarchical morphology onto polymeric films for fabrication of superhydrophobic surfaces

A novel coating process applied directly onto polymeric films offers a long-term alternative to short-term surface treatment of low surface energy polymers. In this paper, we report (i) an improved synthesis of a coating of raspberry-like particles directly onto polymeric surfaces and (ii) a novel method to obtain films with out-of-the ordinary superhydrophobic and self-cleaning properties. Inspired by the ‘lotus effect’ observed in nature, superhydrophobic surfaces were fabricated by combining dual-scale hierarchical coatings with low surface energy fluoroalkanes. Raspberry-like core-shell microparticles (MPs) composed of polystyrene (PS) core coated with silica (SiO2) shell (SiO2/PS) were synthesized by in situ dispersion polymerization of styrene directly on the surface of corona-treated polypropylene films. The PS MPs covered surface was then coated with SiO2 nano/microparticles (N/MPs) forming a hierarchical morphology on the surface. Biomimicry of the lotus leaf morphology is achieved by the hierarchical structure formed by dual-scale N/MPs that cover both PS particles and bare polypropylene surface, allowing air to be trapped in cavities formed. Adjustment of the diameters of the PS and SiO2 N/MPs allows the surface roughness of the coating to be controlled and optimized. 1H,1H,2H,2H-perfluorododecyltrichlorosilane was applied to lower the surface free energy, allowing the polymeric films to exhibit excellent superhydrophobic properties such as water contact angle above 170° and rolling angle of 0. Here, the application of the hierarchical coating was innovatively improved by direct synthesis of each layer onto the polymeric film. The treated surface portrays self-cleaning—soil placed over the surface is efficiently washed away by water, leaving clean trails. The durability of the microstructure and wetting properties of the coating were investigated. The coated films were observed to withstand a tape peeling test for up to 20 repetitions and sandpaper abrasion (1.96 kPa) for a distance of 2 m while retaining superhydrophobicity.