Synthesis and characterization of super water-repellent and self-cleaning coatings by the sol-gel method

Wettability of solid surfaces with a liquid is a very important phenomenon in our daily life as well as in many industrial processes. The development of a surface with a high water contact angle is one of the subjects of great interest. The contact angle ? is an important parameter in surface science, being a common measure of the hydrophobicity / hydrophilicity of a solid surface. A surface is defined liquid repellent when the contact angle ? is greater than 90°; otherwise it is classified as wettable. In recent years, great interest and many studies have been focused on surfaces with water contact angle higher than 150°, defined as super-hydrophobic [1-4]. Because of the small contact area with water, on a super-hydrophobic surface chemical reactions or bond formation through water are limited. In addition, contaminating particles can be carried away by water droplets, resulting in a self-cleaning surface. Accordingly, several phenomena such as snow sticking, oxidation and current conduction are expected to be inhibited on surfaces like that. Therefore, water-repellent coatings on glass or ceramic substrates have high potentiality for many practical applications, such as eye-glasses, cover glasses for solar-cells, automobile windshields, electronic components, and so on. The wettability is governed by two factors: the chemical nature of the solid surface and its surface topology. Superhydrophobic properties require surfaces with appropriate surface roughness and low surface energy. The contact angle ? between a flat solid surface and a liquid droplet is given by the Young equation, assuming ideal surfaces (smooth, homogeneous, rigid and inert): cos ? = (?SV -?SL) / ?LV ; where ?SL, ?SV, and ?LV denote the surface free energies of solid-liquid, solid-gas and liquid-gas interfaces, respectively. In order to describe the behavior of rough surfaces and porous and inhomogeneous materials, Wenzel and, afterwards, Cassie modified the Young equation. Thanks to a combination of Wenzel and Cassie mode, the practical wetting behavior of most hydrophobic rough surfaces can be explained, although the ratio of their contributions depends on chemical composition, roughness dimensions, and surface morphology. Ideally, according to Young equation, a ? unique contact angle is expected for a given system. In a real system, however, a broad range of contact angles is usually obtained. The upper limit of this range is the advancing contact angle, ?a, the lower is the receding one, ?r. Therefore, a single and "stationary" contact angle does not adequately describe the hydrophobicity of a surface, because this property is determined by the hysteresis value between advancing and receding contact angles [5 and references therein]. In the present work, hydrophobic coatings have been synthesized by the sol-gel technique, using tetraethylorthosilicate (TEOS) and a fluoroalchylsilane (FAS) as precursors. Many parameters relevant to synthesis, deposition and thermal treatments have been varied in order to achieve thin, homogeneous and transparent films and to enhance the hydrophobic surface properties. Pure silicon wafer and silica glass were employed as substrates. Coatings characterization was carried out by Secondary Ion Mass Spectrometry (SIMS), Fourier Transform Infrared Spectroscopy (FTIR) and Atomic Force Microscopy (AFM). The thicknesses were measured by means of a mechanical profiler. The film adhesion was estimated by an empirical scratch test. The hydrophobic properties were tested using deionized water and water-soluble inks, while the self-cleaning behavior was checked using talcum powder. Regarding the surface chemical factor, the analyses revealed that the hydrophobic groups are uniformly distributed along the coatings and are still present after prolonged thermal treatments up to 250°C. As far as the topological factor is concerned, future developments will be focused on further improving the control of the surface roughness. An apparatus for sessile drop contact angle measurements, performed in dynamic mode, has been assembled. The main components of this apparatus are a light diffused source, a mobile sample holder, a motor driven syringe, a video camera and a computer. The proprietary ASTRA software for axisymmetric drop shape analyses, developed in LabVIEW IMAQ environment, was slightly modified to fully automatize the experiment. At the present, contact angles measurements on coatings modified in order to improve the chemical hydrophobic nature showed advancing contact angles around 125° and receding angles around 105°. Preliminary results concerning textured surfaces containing fluoroalchyl groups provided advancing contact angles bigger than 140° (see the reported figure). Moreover, preliminary investigations on the self-cleaning properties of the coatings revealed promising results. Further developments of this work will be the changing of topological surface properties in order to enhance the hydrophobic behavior of the functionalised coatings. Moreover, the synthesis and deposition conditions have to be optimized in order to improve the mechanical resistance.