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SILVER, TITANIA AND ANTIBACTERIAL CERAMICS: THE CHEMISTRY POINT OF VIEW

The tile market is now increasingly focused on the development of surfaces able to ensure and emphasize the already well-known ceramic’s features, both in terms of hygiene and cleanliness. Words such as antibacterial, bactericidal, bacteriostatic and sanitizing, in fact, are currently filling the pages of many trade magazines that belong to several and different industrial fields.
Among all the solutions at present under investigation, there are two substances that seem to be the most likely to develop the antibacterial power, highly sought by companies: 
silver and titanium dioxide.
Let’s see them closer.

 

SILVER IONS (Ag+)

Silver is a noble metal with a long-standing tradition. It is a trace element even present in organism’s tissues and, even though in very small quantities, it is also taken by humans through food. Silver is being used in many medical and industrial applications as well as in different fields. It usually does not find application as a pure metal but after specific processes that provide it with the the requested and proper features for sanitary use. In this regards we can definitely talk about colloidal and ionic silver.

 

Even if terms and wording are ambiguous as well as slippery, let’s try to define the main chemical difference between the two categories.

► Silver in ionic form (Ag+) contains monovalent cations, responsible for the biocide action on micro organisms

► Colloidal silver appears in form of nanometric metallic particles (whose dimension can be extremely small such as a few nanometres). Metallic Silver (Ag°) releases Ag+ ions with biocide action when it comes into contact with humidity / water

 

What does the silver antimicrobial action depend on?
It is closely related to the amount of released bioactive ions (silver with positive charge) and to their interaction with bacterial and 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.

 

► They damage both structure and functionality of bacterial cell walls, deposited on the microorganisms’ surface in the form of granule.

► They inhibit the main physiological functions of bacterial cells, interfering with their vital enzymes, blocking their respiration and breaking down their cell membranes

► They inhibit the duplication of bacterial DNA, responsible for pathogens’ proliferations

 

1. Silver ions' particles penetrate into microbe's cell membrane
2. They proceed their action by blocking its breathing
3  They attack the microbe's DNA, preventing its reproduction

 

 

TITANIUM DIOXIDES (TiO2)

Titanium dioxide, when matched with UV radiations, provides the surface with photo-catalytic features.The photo-catalytic process promoted by radiations, generates oxygen radicals and mono-atomic and tri-atomic oxygen molecules that give life to the antibacterial action.
Titanium dioxide is also known for its hydrophilic characteristics through which it is used in many fields, including ceramic. (For example, it significantly reduces the water drops on shower walls as well as on cars’ rear-view mirrors.)

 

 

MEASUREMENTS & CLASSIFICATIONS

On a theoretical level the explanation is quite simple but what is the real and practical treatments’ effectiveness? Which are the problems with which R&D, and therefore ceramic producers, have to face? As we have already pointed out, a surface can be declared antibacterial only if it has the ability to cut down 99.9% of the four main bacterial strains (even if in same cases this upper limit does not seem to be so rigid). A classification has also been formulated in order to identifies and categorize the effectiveness of the antibacterial action:

 

► R0: < 90%
► R1: > 90%
► R2: > 99%
► R3: > 99,9%
► R4: > 99,99%
► R5: > 99,999%


Beyond this caption and yet important categorization, we could generally say that some solutions are effective but also that, even today, new insights and developments are needed to solve several critical points on which R&D departments are currently working. 
Let’s see the most significant.

 

MAIN PROBLEM AREAS

  1. In the first place, the active principles must not undergo structural changes in application: any change may alter the effectiveness of the antibacterial agent, nullifying its effect or even becoming responsible for side effects. Titanium Dioxide, for example, is active at certain temperatures but it loses its full efficacy turning into Rutile, when subjected to higher temperatures. 
    The ceramic firing cycles’ high temperatures are still an important topic if not a critical issue. Ceramic sector is, in fact, experiencing a time gap compared to other industry sectors (such as textiles, plastics or steel) that for several years have been using antibacterial applications according to their different processes and lower temperatures of application.
     
  2. The effectiveness of the antibacterial action is also closely related to the high specific surface area of the active principles in use. At the same time it is important to counteract the nano-powders tendency to aggregation that may affect or lower their antibacterial efficacy. In these cases, especially in the most problematic ones, it is necessary to resort to sonication (use of ultrasound) to speed up the dissolution of solutes.
     
  3. The active principles’ effectiveness is at its highest only when principles remain on the surface, in contact with the air and the outermost part of the tile: they must not sink into the glaze and/or be incorporated by it. This is a very thorny issue as it opens up clear reflections on the different application options that can be used to reach a surface with antibacterial properties.
     
  4. Another issue on which much attention must be paid concerns the materials within which the active substance must be added: glazes, inks, surface treatment products, (etc.), are not independent variables but they must be properly and carefully designed. Subjects such as softening and melting temperature, viscosity and density, interact, for example, with active principles enhancing or, in contrast, reducing and neutralizing their action.

 

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, type of 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. 



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