Anti-Stain (Anti-Contamination)
Fluorine compounds are a group of substances with the lowest surface tension among solid materials, with surface tension as low as 6–30 mN/m. Liquid contaminants, such as fingerprints, skin oil, and water stains, adhere to surfaces due to intermolecular forces between the contaminant and the surface molecules of the material.
By applying a fluorine-based coating to the material's surface, the fluorine compound can significantly reduce the surface tension of the material, thereby reducing the adhesion of dirt.
Fluorine-based coatings used to reduce fingerprint adhesion and prevent stains generally come in two types:
This type of coating contains organofluorine groups and uses silane coupling agents. After application, the silane reacts with inorganic materials such as glass and metals, forming a monomolecular fluorine layer on the surface.
Since the coating material reacts with inorganic materials through silane coupling, it offers excellent adhesion and wear resistance. Furthermore, because the monolayer is only about 10–20 nm thick, it is invisible to the naked eye, does not damage the appearance or optical properties of the coated substrate, and does not cause scratching.
Even if the coating material is worn off or aged, this delamination occurs at the molecular level and cannot be seen with the naked eye. Therefore, unlike regular coatings, there will be no visible peeling that could degrade the appearance, preserving the aesthetic value of the coated object. For these reasons, it is considered the most suitable coating for preventing fingerprints.
However, if the surface of the material, such as typical plastics, does not contain hydroxyl (-OH) or carboxyl (-COOH) groups, no coupling reaction will occur, resulting in poor adhesion. In such cases, pre-treatment with Polysilazane or Tetramethoxysilane is necessary to form an inorganic glass-like (silicon dioxide) layer on the surface before applying the coating.
Like the silane coupling type, this is a reaction-type anti-stain coating that contains PFPE (perfluoropolyether) groups and reacts with metal surfaces to adhere to them. The silane coupling type described above also reacts with metals and adheres to them, but this coating provides even stronger adhesion and creates a durable, wear-resistant, and stain-resistant surface.
Similar to the silane coupling type, this coating has a thickness of 10–20 nm, making it invisible to the naked eye. It does not alter the appearance of the material while offering excellent anti-stain properties.
For example, in a bathroom setting, a faucet with the coating applied on the right half showed significantly better stain resistance. After a month of use, dirt that could typically not be removed by simply wiping (such as water spots and cleaning residue) was easily removed from the coated surface.
This type of coating can also be applied to resin-molded metal molds as a releasing agent. It not only reduces the surface tension of the mold but, with a dynamic friction coefficient below 0.09, ensures excellent release properties. Additionally, because the coating is only 10–20 nm thick, it faithfully reproduces fine structures from the nano to micrometer scale, such as those required for nanoimprint technology. Since the release coating does not stick to the workpieces, no cleaning is necessary after demolding, significantly improving production efficiency.
This system comes in two forms: one that can be added to free radical polymerizable UV-curable resins as an additive, and another that is already mixed into UV-curable resins or hardeners and can be used directly. Both systems cure under UV light. The key feature of this coating is its fast curing speed, making it suitable for high-speed processing of plastic films in a roll-to-roll manner.
The curing reaction is a crosslinking process caused by free radicals, and polymerization is inhibited by oxygen. Therefore, it is essential to shield the oxygen during exposure, either by using nitrogen or by increasing the thickness of the film under normal conditions. After application, the surface usually appears glossy.
Although the static contact angle of this coating is similar to other types, its surface structure is not fully covered with fluorine compounds, so its fingerprint resistance is not as effective as that of the reaction-type coatings mentioned above.
