Stable Bionic Superhydrophobic Coating Surface Fabricated by a Conventional Curing Process

By Z. Luo, Z. Z. Zhang, L. Hu, W. M. Liu, Z. G. Guo, H. J. Zhang, W. J. Wang. 2008

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In nature, various plants, including the lotus leaf, exhibit the unusual phenomenon of superhydrophobicity. The surfaces of these leaves usually have binary structures on the micrometer and nanometer scales, resulting in low water sliding angles (WSAs) and high water contact angles (WCAs) up to 162° ± 5°. This is because air can be trapped between the droplets and the wax crystals at the plant surface, which minimizes the real contact area. Water repellency is important in daily life as well as in many industrial and biological processes, such as the reduction of frictional drag on ship hulls, antiicing and self-cleaning. Artificial superhydrophobic surfaces with WCAs larger than 150° and very low WSAs have been prepared by various processing methods through controlling the surface topography of expensive hydrophobic materials, using techniques such as etching and machining. However, these methods for creating superhydrophobic surfaces typically use expensive materials or severe conditions, limiting the application of superhydrophobic surfaces. Bionic polymer surfaces with superhydrophobicity have been prepared by a one-step casting process under ambient atmosphere. Erbil et al. employed a very simple method involving solvent evaporation to fabricate a superhydrophobic surface, which provides a new, promising method to fabricate artificial superhydrophobic surfaces on polymer surfaces. However, it is well known that the various superhydrophobic surfaces were only fabricated on glass substrates at room (or low) temperature, and the cohesion between the polymer coatings and glass substrates was weak, allowing the films to be easily scraped off, and no longterm stability over a wide pH range was achieved in comparison with the high-temperature process. These superhydrophobic surfaces cannot withstand low-to-high temperature changes. Conventional fluorine-polymer hydrophobic coatings have been used as antisticking and antifouling surfaces and to reduce drag and flow noise for long time because of their low surface free energy. However, the ordinary fluorine-polymer hydrophobic surfaces have WCAs of merely 110°–125°, i.e., they are not superhydrophobic. In this Communication, we demonstrate that bionic poly(tetrafluoroethylene)/poly(phenylene sulfide) (PTFE/PPS) superhydrophobic coatings with long-term stability, high cohesional strength, and resistance to temperature change can be prepared by a simple, inexpensive, and conventional curing process. A superhydrophobic surface with a porous network, micrometer–nanometer-scale binary structure (MNBS) roughness, and the lowest surface energy hydrophobic groups (–CF3) was fabricated using commercially available PTFE and PPS on stainless steel and engineering materials. The fabrication of a superhydrophobic coating by our conventional curing process is reported in this Communication for the first time to our knowledge. It is expected that this technique will make it possible to prepare superhydrophobic engineering materials with new potential applications in industry in the future. First, we prepared a PPS coating. A typical scanning electron microscopy (SEM) image is shown in Figure 1a. It is clear that no obvious micrometer-scale or nanometer-scale structure is observed for the pure PPS coating surface, and the WCA on the surface is about 96° (Fig. 2a). Second, a PTFE/ PPS composite coating was produced, with a micrometer-scale structure surface (Fig. 1b). The WCA on it is about 128° (Fig. 2b), indicating that the PTFE/PPS coating is more hydrophobic than the pure PPS coating because of the existence of fluorine groups (–C–F2). PTFE/PPS coatings with holes can be constructed by adding (NH4)2CO3 solution to PTFE/PPS coatings. The holes with a porous network micrometer-scale structure on the surface can be clearly observed after decomposition and volatilization of (NH4)2CO3. A relatively high WCA of about 140° (Fig. 2c) is obtained because the air that is trapped between the droplets and the porous micrometerscale structure of the coating surface minimizes the real contact area. Finally, a new method has been developed for fabricating superhydrophobic coatings on various metal substrate surfaces, such as copper and aluminum, by using PTFE, PPS, (NH4)2CO3, and surfactant solutions. The WCAs of these coatings increased from 128° to 165° (Fig. 2b–e). The PTFE/ PPS superhydrophobic coating was created by adding 10 wt % nonionic surfactant (octylphenol poly(oxyethylene C O M M U N IC A TI O N

Subfields: Biomedical Engineering, Mechanics of Materials, Surfaces, Coatings and Films

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