#19 Ceramic glaze application: methods, rheology & synthetic chemicals
- 1. Introduction
- 2. To each scenarios a specific set of chemicals
a) Vela and bell applications
b) Airless spray application
- 3. Microorganisms and chemichal degradations
- 4. Standard preventive and corrective actions
- 5. A new scenario: the synthetic chemicals
Among the several criticality that ceramic producers must face during production, the glaze and engobes microbiological degradation is one of the most frequent and difficult to solve. These two semi-finished products are usually grinded in the grinding mills where CMC and DISPERSANTS are added to ensure a proper development of the process and, at the same time, to provide glaze or engobe with the right features. However, sometimes both dispersants and CMC can be attacked by bacteria becoming a source of nourishment: bacteria produce a degradation of the glaze, reducing or nullifying the action of all chemicals.
2. To each scenario a different set of chemicals
Let’s step back by making a list of the most important application systems, since the chemicals involved change, as usual, according not only to the glaze’s features but also to the parameters of the production line, such as the work density, the rheology of the products or the application system.
VELA and/or BELL APPLICATIONS
Just to make an example, a glaze applied by means of vela or bell systems should be usually mixed with chemicals able to reach a high-density operation (such as 1700 /1850 g/l at 25°C) as well as a high level of cohesion of the watery system so to ensure an homogeneous and compact discharge of the glaze on the raw ceramic support. This usually means to work with a high fluidization level, with the use of CMC marked by a medium or high viscosity value and with dosages developed from time to time to promote a glaze’s drying process that is eligible to a multiple application: an application of engobe and glaze by means of two application machines (bells or vela). The engobe applied by the first machine must dry on the support right before the tile reaches the second machine that discharge the glaze: this is very important to ensure the best integration between the two layers.
AIRLESS SPRAY APPLICATIONS
On the other hand, medium-density glaze applications by means of airless spray systems (such as 1450 g/l at 25°C), require during the grinding stage a different kind of approach: the goal is to reach lower values of density and viscosity. In this case, to avoid sedimentation phenomena as well as lack of cohesion, the fluidization shouldn’t be too strong. At the same time the chosen CMC should be marked by a viscosity value able to ensure a good leveling according to the glaze (1) and to the tile’s temperature (2).
What does that mean?
(1) THE GLAZE
Each CMC can sometimes have very different properties, and this is the reason why it is very important to choose them according to the features of the glaze involved in the production process. Just to make an example, in case of a very plastic glaze (intrinsically characterized by high values of viscosity and cohesion), it would be better to use low-viscosity CMC or to decrease the dosage. On the other hand, a low-plastic glaze (that is poor in clay) would require CMC with a higher viscosity value to ensure a good cohesion and a proper drying process on the tile.
(2) THE TEMPERATURE
In general, both the selected CMC and the dosage can affect the drainage and evaporation times of the water of the glaze suspension, by increasing or decreasing them as needed. In this regard, the temperature of the ceramic support on which the glaze is discharged (operating temperature) is a very important parameter that must be considered when choosing the CMC.
In case of airless spray system applications, together with CMC, it is often necessary to use other kind of chemicals able to ensure a proper application such as, for example, a good leveling of the glaze. In case of glaze applications after the digital printing stage, instead, it is useful the use of compatibilizers to match the different chemical nature of solvent-based inks and water-based glazes.
In general, binders are always needed, both during the grinding process and along the glazing line. They ensure the proper values of cohesion, the appropriate rheology values of the glaze and a reasonable binding power between the glaze’s particles after application on the ceramic support. These parameters are essential for avoiding the so called “dusting phenomenon” due to the lack of cohesion between the glaze’s inorganic particles after the drying process. Among all chemicals, we must point out some glue able to increase the binding power of the glaze, increasing at the same time the drying times, therefore improving the leveling on the support and avoiding the formation of non-standard drop of glaze (too big) that would compromise the tile.
3. Microorganisms & binders' degradation
If on the one hand the density as well as the viscosity and rheology values of the glaze can be somehow managed, on the other hand, the presence of microorganisms (and more specifically of bacteria) is much more difficult to handle. Especially during the production process where, due to the presence of water, bacteria find nourishment in the binders based on CMC and modified starches. To make things worse, we must underline that we usually discover bacteria only when we smell unpleasant odors deriving from metabolites, that is when the degradation is already underway.
Bacteria not only produce bad odors, but they usually also affect the action of binders, by decreasing or even nullifying their power. Moreover, the metabolic waste inside the glaze may often lead to application problems and to technical or aesthetical defects of the ceramic tile that can be visible even after the firing process (such as, for example, pinholes).
4. Preventive & corrective actions (standard)
How to prevent and dealing with the problem? Let’s see the main important actions.
One of the less invasive actions is to add the glaze, along the glazing line, with an additional amount of binder, just before application. However, even if this expedient is usually useful to restore the features of the suspension, it is not able - sometimes - to avoid post-firing defects. In addition, this kind of intervention must happen rapidly, since the bacteria’s degradation action occurs very quickly.
A second action that can be put in place to preserve the binders’ properties, it is the addition of a huge amount of biocide (Kill-dose) to kill and eliminate ALL the microorganisms and therefore to restore the chemical’s binding power. Despite this activity can extend the glaze’s life, sometimes it cannot be strong enough to avoid the defects promoted by the degraded components that are now inside the glaze.
The third action - unlike the previous two - can be considered as a preventive action since it occurs when bacteria do not have attacked the glaze yet. The solution consists in adding the glaze, during and/ or after the grinding stage, with a proper mix of biocides/preservatives. A solution that, however, is sometimes difficult to put in practice because it is almost impossible to 100% check the healthiness of all the production area of the plant.
5. The new scenario: the synthetic chemicals
If this is the scenario, it would be important to find a different way to face the chemical’s power reduction by finding a solution able to solve the problem at its root. This means thinking about and so developing different category of chemicals that cannot be attacked and affected by microorganisms. In this regard, R&D labs have been working on this topic for some time and, even if they are still searching for definitive solutions, some interesting results seem to be already very promising. This new approach based on FULLY SYNTHETIC CHEMICALS, of course, cannot eliminate the presence of bacteria inside watery systems (such as glazes) but it can reduce and contain their proliferation and stop additives’ chemical alterations.
What is the main feature of these new products and why do they not degrade?
Let’s start from an important introduction.
In general, all products of natural origin (or natural-modified origin such as CMC) are usually a nourishment for microorganisms because of their molecular nature that can be easily degraded through many actions of biological nature. The use of SYNTHETIC MOLECULES (the word “synthetic” refers, in this case, to single monomers and/or oligomers made to properly react to obtain new molecules that do not exist in nature) makes bacteria - that can affect and destroy the molecules of natural origin - uncapable of finding the passkey that let them proceed on their degradation process.
Here’s a very simple example to simplify and clarify the difference between the two types of molecules. Think of the polymer (or of the natural molecule) as it was a wool yarn. Bacteria have scissors (their mechanism of action) that cut the yarn in a lot of small pieces that therefore becomes food. Think, instead, of the synthetic molecule as it was a barbed wire that hampers bacteria (the scissors), remaining safe and functional.