Self-cleaning and Antibacterial Surfaces
There are several substances with photocatalytic properties suitable for obtaining self-cleaning surfaces.
In the ceramic sector, the most used is TiO2, Anatase, but there are many other compounds with these properties, for example: ZnO, CeO2, ZrO2, SnO2, CdS, ZnS etc.
In very simple terms, when the photoactive compound is hit by a photon of sufficient energy, a positive charge is created on the surface of the crystal which, reacting with water, leads to the formation of very reactive radicals that destroy the polluting organic molecules.
This action also occurs with the molecules that make up viruses and bacteria, this explains the antibacterial properties associated with these compounds.
In addition to self-cleaning tiles, there are many other products with similar characteristics, for example the plastic fabric of beach chairs contains ZnO to avoid the contamination of fungi and bacteria between users. Many glasses used in buildings in the city have a self-cleaning external surface, the same goes for concrete. Adding Anatse to the concrete makes it possible to obtain building surfaces that do not become dirty due to smog or the growth of moss and lichens.
The application of Anatase in ceramic must be done after the tile has been fired because, if applied to raw glaze, it reacts with it during firing, losing all its properties. It is normally applied by spray, by airless, and re-fired in third-firing. Annealing is necessary in order for the crystal to adhere to the enamel surface. Very often the product is applied to the tiles in a specific cooling zone of the kiln. In this area there is a gap between the oven modules so that the product can be sprayed on the surface of very hot tiles.
The disadvantage of using Anatase is that it requires a substantial light intensity to exert its effect and also the treated surface loses its effectiveness over time due to the removal of the active layer due to wear.
There is the possibility of imparting virucidal and antibacterial properties to the surfaces of the tiles through the action of metal ions. It has been found that the following ions have an antibacterial / virucidal action with the following strength scale: Ag +> Co 2+ ≥ Ni 2+ ≥ Al 3+ ≥ Zn 2+ ≥ Cu 3+ = Fe 3+> Mn 2+ ≥ Sn 2+ ≥ Ba 2+ ≥ Mg 2+ ≥ Ca 2+.
Many surfaces of daily use have antibacterial properties, from the stainless steel surfaces of professional kitchens, to the brass handles, to the silver cutlery up to the sanitary masks soaked in silver or copper salts. These are just a few examples of surfaces in which their sanitizing properties have been voluntarily or involuntarily used.
In the ceramic tile sector, the action of the Silver ion has often been used as a sanitizing agent. Often applied in the form of granules containing silver or to the surface application, on raw enamel, of solutions of salts thereof.
The action of these metal ions is due to their property of binding to the proteins of the bacterial and viral membranes causing their denaturation and inactivation.
The advantage of Silver antibacterial surfaces compared to TiO2 surfaces is due to their mechanism of action that does not require lighting, and to a practically unlimited duration of the sanitizing effect.