#07 CERAMIC SURFACES & ANTIBACTERIAL PROPERTIES
- 1. Introduction
- 2. Reference standard: ISO22196/2011
- 3. Titanium Dioxide
a) Antibacterial properties
b) Destruction of polluting organic molecules
c) Self-cleaning surface
- 4. Silver
- 5. Main critical issues
a) No structural changes of the active principle
b) High value of specific surface of the active principle
c) Active principle on the surface
- 6. Appllication systems
a) Pre-firing applications
b) Post-firing applications
- 7. Dosage and treatment's duration
- 8. Universal product or single projects?
Antibacterial protections, already used in several fields of industrial production, had become in more recent times a central issue of ceramic tile’s production.
Market interest, also pushed by the recent events, has speeded up the research within the labs in order to boost a complex and slippery topic. All producers are then showing a strong commitment to increasingly offer cutting edge products that can be especially used in hospitals, clinics, medical surgeries or in high traffic public facilities such as airport or train stations.
2. Reference standard: ISO 22196/2011
From a regulatory point of view (the normative reference is ISO 22196/2011 that concerns both plastic and non-porous surfaces), a tile can be defined as antibacterial when it is able to eliminate the presence of microbes on its surface, regardless of the level and the power of the action promoted by the surface.
In legislation, it does not currently exist a standard that determinates in advance the minimum level of antibacterial action that ceramic tiles must have in order to be defined as antibacterial.
However, a rating system has been conventionally adopted. These parameters, though they do not have force of law, they identify categories, helping to create different classes of antibacterial action.
R0 → R1 = 0% → 90%
R1 → R2 = 90% → 99%
R2 → R3 = 99% → 99,9%
R3 → R4 = 99,9% → 99,99%
R4 → R5 = 99,99% → 99,999%
Each producer tries to get a proper “R” value according to the intend use of the ceramic material.
3. Titanium dioxide (TiO2)
Over the last decade there have been a number of ceramic tile collections treated by means of ANATASE-based products (a meta-stable mineral form of TITANIUM DIOXIDE).
Which are the main features of this material?
A) ANTIBACTERIAL PROPERTIES
Titanium Dioxides, during the photo-catalytic process promoted by UV rays, releases on the surface free radicals able to promote an antibacterial action that prevents the proliferation of microorganisms.
How does it do it?
The radicals’ oxidizing power causes a damage to the outer cell wall of all those microorganisms which are in contact with the photo-catalytic surface: cytoplasmic membranes are attacked and intercellular fluids come out, resulting in the bacteria dying.
B) DESTRUCTION OF POLLUTING ORGANIC MOLECULES
Titanium Dioxide also promotes a catalytic action towards some COV pollutants, therefore improving the air quality.
The chemical and physical process is quite complex and we will just say that the pollutant organic molecules, encountering the surface, take part of the interaction between light and photocatalytic Titania, being “assaulted” until they become completely harmless.
C) SELF-CLEANING SURFACE
Titanium Dioxide, in the presence of sunlight, has hydrophilic features that produce a sort of barrier against superficial soiling.
Along with Titanium Dioxide, the are in nature other metals able to provide surfaces with antibacterial properties.
Building on the experience, silver (a metal that has a long tradition in this field) seems to be the most effective, despite the heterogeneity of the results.
WHAT IS THE CAUSE OF THE SILVER’S ANTIBACTERIAL ACTION?
It is strictly related to the amount of released bioactive ions (ionic silver with positive charge) and to their interaction with pathogenic cell membranes.
What kind of action and impact do positive ions (Ag+) produce?
Silver basically acts by contact: the metal ions come together with the sulphydryl groups within the proteins of micro bacterial organisms, denaturing them up to damage their cellular wall.
- 1. Silver ions damage both structure and functionality of bacteria’s cellular wall, depositing in the form of granules on the microorganisms surface
- 2. They inhibit their most important physiologic functions, interfering with their vital enzymes, blocking their respiration and decomposing their cellular membranes
- 3. They finally inhibit the duplication of the bacterial DNA (fundamental for the proliferation of pathogenic microorganisms)
The silver must be in its active forms in the interface ceramic glass / air (the external part of the tile) in order to be effective.
5. Main critical issues
- 1.The active principles in use must not undergo any structural change in order to be effective. Any modification could alter the results and sometimes may lead to side effects In this regards the issue of firing cycles’ high temperature is fundamental.
- 2. The effectiveness is also related to the high specific area that active principles must have. Active principles, in fact, should not aggregate and consequently it is important to counteract the nano powders’ aggregative behavior in order to reach the widest specific area as possible. The formation of aggregates can negatively affect or reduce their effectiveness. In the hardest situations a sonication process must be used (by means of ultrasounds) in order to brake down the particles.
- 3. Active principles must be on the surface, in contact with the air (in the external part of the tiles): they must not sink and included inside the glaze. Even this is a tricky issue that offers food for thought regarding the kind of application that should be taken into consideration in order to reach the best result.
6. Application systems
Z&S Ceramco has currently taken two different paths.
- 1. PRE-FIRING APPLICATIONS
Pre-firing applications are – today – the most complex as well as the most susceptible in terms of results.
The addition of the active principle inside and/or over the glaze ensures its homogeneous distribution, both on the surface and in the lower layers of glaze. If this is definitely true, it should be recalled at the same time the firing cycle action.
During the glaze’s melting process, in fact, the silver-based active principle that is on the surface is partially or totally included in the lowest layers of the glaze, depowering or sometimes eliminating the antibacterial effect.
The silver’s melting temperature should be also considered.
As we know, ceramic tiles are usually fired inside kilns at a temperature that is about 1.200°C. Instead, Silver in its pure form reaches its melting point at 900°C.
Thanks to these two simple data, it is clear that silver’s superficial layer undergoes a partial evaporation that leads to a significant reduction of the active principle on the surface (especially when compared to the amount of silver initially applied).
Given the current scenario, this kind of application can lead to very different results according to the operating conditions. It is important to check from time to time the process parameters in order to verify the necessary conditions required for a proper and good anti-bacterial application.
On the other side, active principles tend to promote an incredibly high activity so that the ceramic surface shows in several cases, an acceptable or good functionalization level.
Sometimes, in order to emphasize the result, it should be recommended to use a double treatment: a pre-firing application (silver in the glaze) and a post-firing application (silver on the surface).
- 2. POST-FIRING APPLICATIONS
The post-firing application of the silver’s active principle certainly ensure a far superior result, providing the surface with higher antibacterial values.
HOW IS THE APPLICATION PROCESS?
The silver has to be added within protective sealants products (such as Polishield) normally used after the lapping process. These products, once they have been functionalized with silver, can be applied by means of the common machines currently in use (brush machines or polishing machines).
The product that simultaneously carries out an anti-bacterial actions as well as a protection from dirt and chemicals, can be used both with lapped and natural ceramic surfaces.
With lapped surfaces, the anti-bacterial action promoted by the product can significantly range according to distribution of the superficial porosity and to the micro-asperity generated by the polishing process.
A very porous surface is able, for example, to absorb a greater amount of product, helping the distribution on wider superficial area. For the same reason, a polished and low-porous surface may partially reduce the quantity of the applied product, therefore decreasing the anti-bacterial action. However, for this second scenario there is a new high cross-link product range able to cover and seal ceramics with a nano-metric superficial layer, providing the surface with a very good protection action.
7. Dosage & treatments duration
The dosage is not predictable. It depends on the surface’s features as well as on the kind of application and the effectiveness of the treatment itself. In general, you try not to increase the production costs, ranging from 0,30€ to 1,00€ m2.
It is also not possible to ensure ore precisely determine the duration of each treatment.
It can change from product to product and the result can be extremely different, according to the medium application system and/or to the active principle. However there is a topic that is always valid: it is necessary to constantly clean the surface over time in order to renew the nano-particles’ sanitizing action.
8. Universal product or individual projects?
In this context, it is clear that it is impossible to solve all problems with a universal product that can correctly work across the board. Several variables are involved such as raw materials in use, structure of the production line, ceramic product that has to be functionalized, etc...
A natural finished product may require, for example, a different application process than a polished or lapped ones. Furthermore, a certain production line could be able to support application modes more easily than others.
It is therefore necessary to proceed with cross-section evaluations according to partners’ production target: a single application can solve some specific problems leaving others unsolved. The task of a solution provider is exactly this: building a common way at customer’s side, taking into account details, specific features as well as opportunities.
Back to How To