Most anti-rain products, from the cheap Turtle Wax to the expensive RainX, are a composition based on silicone, wax and white spirit.
The more expensive, the higher the quality of the base and, accordingly, it forms a thinner and more durable film on the glass. The film formed after applying the product adheres to the glass due to adhesion, that is, simply put, it sticks, just like wax on a car body.
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The advantages are ease of application, safety in handling and the ability to easily remove the composition with anti-silicone. The downside is that it lasts until it is washed off with wipers or in a car wash with chemicals and a sponge.
Permanent polyfluoroalkylsilane coating Aquapel from the States works fundamentally differently.
The main component of this product is a polytetrafluorocarbon “tail”, popularly called Teflon, with a halogen-substituted silicon atom attached to it. Under the influence of a catalyst, which can be either a special additive or air moisture (there is no unambiguous information on Aquapel, this is a trade secret of the manufacturer), a silicon atom with a sewn Teflon “tail” forms a covalent siloxane bond with a silicon atom included in the structure of the glass surface (the first reaction in the diagram above).
According to the reaction: (glass) - Si-OH + Cl-SiX2-(CF2) n -> (glass) - Si-O{HCl}-SiX2-(CF2) n -> (glass) - Si-O-SiX2-(CF2) n + HCl^
Where X is one of the substituents (chlorine, as in the diagram above, or a hydroxy group).
Further, if substituent X is not a hydroxy group, then the formation of cross-links between the attached molecules is possible (the second reaction in the diagram), but this is no longer so important from a practical point of view.
As a result, the result is a glass surface to which polytetrofluorocarbon “tails” are covalently attached, that is, glass coated with Teflon fully chemically bonded to the surface.
By the way, this is exactly how the oleophobic coating is formed on the screens of modern smartphones - a nanometer-thick film that repels fats from the fingers from the touch screen. In principle, if you can protect your phone’s speaker from the substances described below, you can also renew the worn-out oleophobic coating on your phone’s screen during the process of coating your car windows. But before application, you need abrasive polishing to remove grease and silicone, then apply a new coating - exactly like with car windows.
From practical points - the coating is resistant to organic solvents, it can only be removed mechanically along with the top layer of glass (road sandblasting or abrasive polishing). When applying, it requires absolutely clean glass, since silicone residues will act as a barrier to the above-described reaction.
When applying, it is extremely important to avoid contact with drops of water (remember what was written above about the catalyst?). When water gets in, the product immediately “rolls up” around the perimeter of the drop and firmly adheres to the glass in a thick layer. The result is a white translucent stain that can only be removed by abrasive polishing. It will not be possible to wash off or dissolve the stain.
Regarding the lower efficiency of polyfluoroalkylsilane rain guards at low speeds compared to freshly applied silicone rain guards, even despite the larger contact angle - yes, this is true.
The surface of glass treated with silicone anti-rain is quite flat at the molecular level. Molecules of polyalkylsiloxanes (silicone) form a flat structure on the surface of the glass - you can visualize it by pouring a box of pencils onto the floor and distributing them in a more or less even layer a couple of centimeters thick.
But the surface of “Teflon” glass is a “forest” of Teflon “tails” located perpendicular to the plane of the glass (the second picture in the reaction diagram demonstrates this quite clearly). As a result, small drops can “stick” to the surface if the oncoming air flow is insufficient.