In this field, enamel-based coatings are already known, but these do not guarantee proper anti-adherence, and need high vitrification temperatures (above 540° C.). Sol-gel coatings, especially those obtained by polymerisation of silanes, are also known. But they do not deposit significant thickness and generally become faience for thicknesses of the order of 10 microns. Also, their cohesion is achieved only if high enough temperatures are applied over long periods, for example temperatures above 400° C., for at least thirty minutes. On the contrary, this type of coating allows a range of use in temperatures greater than those of PTFE-based coatings, up to 600° C., as opposed to 300° C. maximum for PTFE-based coatings. But this happens to the detriment of anti-adherence.
The disadvantage of such coatings is that they lose their hydrophobic properties after aggression likely to degrade the surface of the coating, and especially chemical aggression such as that caused by dishwasher detergents, or aggression of mechanical type such as that caused by excessive rubbing of the coating by means of an abrasive buffer, or even exposure of the order of a few minutes at extreme temperature.
To rectify these problems, the applicant has developed an anti-stick coating obtained via sol-gel polymerisation of a composition based on at least one metallic alcoxide and a colloidal metallic oxide. The applicant has discovered surprisingly that when a very small quantity of silicon oil is added to such a composition, the coating formed by sol-gel from this composition not only resists high temperature, but is also capable of supporting contact of a few minutes with the flame of a Bünsen burner (the temperature of which is above 600° C.), and then rapidly regaining its hydrophobic character.
It is known to the person skilled in the art to use silicon oil for making sol-gel coatings from a composition based on alcoxysilane and colloidal silica.
US patent application US 2006/0147829 describes a process for making superhydrophobic coatings for self-cleaning surfaces, having a contact angle with water greater than 130°. This process comprises a preparation step of nanoparticles (especially silica nanoparticles) preferably of a size varying from 1 nm to 100 nm, and a processing step of these particles with a hydrophobic agent and an additive, the hydrophobic agent preferably being based on silicon, and the additive being selected from mineral and organic bases. These nanoparticles can be prepared by a humid synthesis process, and in particular by sol-gel method, from precursors comprising water, solvent, especially an alcohol, and a metallic alcoxide such as for example tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). After forming of the nanoparticles, they are mixed directly with the hydrophobic agent and the additive, and left to react at a temperature of between 0 and 100° C. The resulting material is applied to a substrate (especially glass, plastic, metal, ceramic, or polymer or composite) according to any technique for producing a coating from a liquid (by centrifuge, soaking, or by coating by brush or roller). Then, after being applied to the substrate the coating is dried at a temperature between ambient temperature and 200° C. The resulting coating has a hydrophobic surface forming a contact angle with water of over 130°, or even over 150°.
However, there is no mention in US 2006/0147829 of the antiadhesive character of the resulting coating, nor of its capacity to resist high temperatures and its aptitude for revealing a hydrophobic character after aggression of the surface of the coating. Also, US 2006/0147829 does not indicate that the sol-gel coating is in the form of a continuous film having a thickness of at least 10 μm.