After no-clean clothes and non-reflective glass, self-cleaning coatings are born

With the development of material science, coatings are also moving towards functional materials. Just imagine that we can use non-wet paper, no-clean clothes, and non-reflective curtain wall glass, which will add a lot of convenience to life.

Coatings are materials with the main purpose of decorating and protecting different surfaces, and the current market demand puts forward higher requirements for coatings. In addition to decoration and protection, coatings must have more functions, such as anti-microbial adhesion, easy cleaning, self-healing and other functions. The preparation of these functional coatings requires the manipulation not only of their physical properties but also of surface chemistry.

Research by Alexander Santiago of the Faculty of Polymer Sciences of the University of the Basque Country in Spain has contributed to the development of three functional coatings: one is anti-microbial adhesion coatings (such as marine paints); the other is self-cleaning coatings (such as hydrophobic coatings); and anti-reflective coatings (such as mobile phone screens or coatings for eyeglass lenses). Alexander's hydrophobic coating material is a copolymer that spontaneously microphase-separates. In other words, the urethane portion of the copolymer gives some adhesion to the substrate, while the larger proportion of the siloxane portion remains on the surface roughening it, so it remains hydrophobic and prevents organics from sticking. After various tests, it was found that the hydrophobicity of this system depends largely on the surface roughness rather than the surface oxygen concentration. The anti-microbial adhesion ability of the membrane can be determined by measuring the amount of protein adsorption, and the results show that it is difficult for microorganisms to stick to the surface of the phase separation membrane.

In order to obtain a material with self-cleaning ability, a hydrophobic inorganic nanoparticle synthesized in advance is inserted into the acrylic polymer. In short, these nanoparticles are silicon nanoparticles attached to an organic coating. The best results were obtained by spraying acrylic films on top of these nanoparticles, which resulted in a highly resilient self-cleaning superhydrophobic surface. "This method is efficient and low-cost because the silicon we use is cheaper than other materials commonly used in the market," the researchers said.

In order to achieve anti-reflection properties, the film needs to have a substrate with a lower refractive index, which can be achieved by increasing the porosity. However, the presence of pores reduces the mechanical properties of the anti-reflective surface, which impairs processability. The researchers studied the porosity-toughness relationship with the corresponding refractive index, and the results show that this aspect can be improved.

As the researchers say, there is still much work to be done. For example, nanoparticles often do not fit completely into the voids, and the surface to which the nanoparticles end up attached may not be as expected. For the copolymer's anti-fouling properties, the researchers considered the early experimental results to be promising and could be extrapolated. But for now, the research is still in the laboratory stage. The anti-reflection coating is still in the early stage of research, and many problems need to be solved and improved.