#12A Ceramic glaze applications & chemicals
- 1. Introduction: a glaze without chemicals
- 2. Application process and glaze's proper behavior
- 3. Additives for grinding process: dispersants and binders
b) Grinding & following airless application
c) Grinding & followiing bell or vela applicatio
- 4. Towards the glazing line
a) Possible scenarios
b) The (so called) binders for glaze
c) Leveling and defoamers
- Leveling agents (as surfactants)
- Leveling agents (as wetting agents)
d) Fluidifying and suspending agents
The ceramic glaze application is one of the most delicate steps of the ceramic production process. To get high performance, the glaze must be marked by specific and always different rheological features, according to the application conditions of each production line. The proper formulation of the glaze suspension is basically the precondition for a correct application and a result without technical or aesthetic defects.
That said, let’s make an absurd assumption: what would happen to a glaze if we did not add any chemicals during the grinding stage? What would happen is we used only water?
- It would not be properly cohesive (COHESION)
- It would show sedimentation phenomena (STABILITY)
- It would not level properly on ceramic substrate (LEVELING)
These are the main consequences. Without making a list of all the other possible consequences, we can surely declare that, without the help of proper chemicals, the glaze does not work, and it cannot produce any performing tile.
Glazes are mainly made up of inorganic raw materials and ceramic frits ground in water. The glaze consists of a mix of elements whose features are not enough to produce a cohesive and stable suspension.
2. Application process & glaze's proper behavior
Let's now think, instead, to a good glaze application by means of airless spray system. In this case, the glaze, after being sprayed onto the surface of the ceramic support, levels properly and then dries evenly. These three apparently simple steps, however, are not obvious or trivial and they are not inherently part of the glaze’s nature. The proper glaze behavior is only possible thanks to the calibrated use of chemical components of organic and inorganic nature. Without chemicals the glaze would tend to sediment inside the circuit; its nebulization would be inadequate and the glaze - not cohesive and bound - would not be properly leveled and uniformly discharged on the ceramic support. Not to mention the correct drying times or possible repellent phenomena promoted by the contact between glaze (water-based) and inks (solvent-based).
Chemicals can be added during the grinding phase (that is the preparatory stage) and/or directly along the glazing line, that is when the production process is already taking place.
- GRINDING PHASE (preparatory stage)
- GLAZING LINE (application stage)
3. Chemicals for grinding
DISPERSANT and BINDERS, in terms of formulation, can be significantly different depending on the kind of glaze to which they must be added, on the application system and on the parameters of the production line.
What do we mean with the word DISPERSANT?
This word usually refers to any substance capable of lowering the viscosity of a suspension, regardless of the mechanism of action that develops. More specifically, in ceramic, glaze dispersants are chemicals able to keep separate the watery suspended particles to avoid - or limit - possible interactions.
The actions provided by all chemicals added during the grinding process must consider the application system that will be used along the glazing line. What does that mean? It simply means that the grinding parameters used for glazes applied by means airless spray systems are different from those applied by bell or vela systems.
- GRINDING FOR SUBSEQUENT AIRLESS APPLICATION
In this scenario we need to make a difference between high-density and low-density applications.
A high-density airless application (marked by low water content and high solid content) usually requires the use of DISPERSANTS able to strongly reduce the viscosity value of the suspension. At the same time, the use of BINDERS capable of providing cohesion in the suspension is very recommended. The lack of binders, in fact, could compromise the bonding of the glaze on the ceramic support and at same time could lead to an improper leveling process.
A low-density airless application, instead, (used, for example, for top glaze applications) requires dispersants marked by a less powerful action or – as an alternative - a lower dose. A very high fluidization, in fact, could deconstructe the system, leading, for example, to sedimentation phenomena.
- GRINDING FOR SUBSEQUENT BELL OR VELA APPLICATION
In this case, during the grinding stage it would be important to use very noble rheological and cohesive additives, that is chemicals with a very low number of anions and cations that may be responsible for glaze defects after firing, such as pinholes. In addition, when applied by vela or bell systems -unlike spray application systems - the glaze must be marked by good viscosity levels. In fact, since it has not to be sprayed it must be bound and quite cohesive, to remain compact during discharge. From the machine to the ceramic support.
Speaking of chemicals to be used for glaze grinding, even with these few examples, it’s clear that we can find some general rules but it’s impossible to talk about standard formulations because scenarios and parameters are always different. We can also add that a proper additivation during the grinding stage results not only in a good performance in the following stage along the glazing line, but it also promotes significant benefits already during the grinding step. Just think, for example, of the benefits that you can get by reducing the grinding time thanks to the use of specific chemicals that make the glaze cohesive, according to the needs. In general, the grinding process is faster and without problems when the glaze is more cohesive. And this of course is a great benefit in terms of industrial productivity.
4. Towards the glazing line
Once it has been grinded, the glaze can be applied by means of spray, bell, and vela applications.
The glaze application can also be developed in several and different ways such as:
- Application at the beginning and/or at the end of the line
- Application on large, medium or small tile size
- Application before or after digital decoration
- Application on the decoration or on the grits
In all this cases, chemicals play a fundamental role. So, let's see the most important families of chemicals involved in this stage of the ceramic production process.
- THE (SO CALLED) BINDERS FOR GLAZES
Binders, used for both wet and dry applications, perform several functions.
They provide the glaze with a proper cohesive power towards the ceramic support (or towards the layers of glaze previously applied). The adhesive power acts both with respect to the support and to each layer of glaze that are applied one at the top of the other. From a chemical point of view, what is the origin of the binding power? The binding power is provided by the presence of organic molecules made of functional groups capable of binding together the inorganic particles of the glaze and of binding themselves together, when they lose water thanks to the evaporation process. Because to their ability to create a reticulation, binders can also significantly limit the dusting phenomena, that is the glaze that – when is unbound – returns to dust (or powder) after the evaporation.
Binders are also able to provide the glaze with homogeneity and cohesion and, at the same time to slow down the water’s evaporation and drainage process. Water that is in suspension to allow the application system to promote a good and homogeneous leveling, avoiding surface defects such as small air bubbles or pinholes. The binding power, in fact, also depends on the kind of action that the chemical promotes on drying and drainage times.
What does that mean?
It means that the proper drying/draining times of the different layers that make up the ceramic body - engobe, glaze, ink, grit, covering glaze, etc. - are fundamental to ensure that each layer is applied only when the previous one has reached the right drainage value. This is especially true in cases of glaze application on engobes. If a glaze is too wet when the tile arrives under the printheads of the digital printing machine, problems may arise due to the water vapor that could affect the printing system.
Even if it’s not one of their most important actions, binders are also capable of slowing down or limiting possible glaze sedimentation phenomena inside the tanks.
- LEVELING AGENTS & DEFOAMERS
- a) LEVELING AGENTS (SURFACTANTS)
Talking about SURFACTANTS means talking about high performing soluble products that, by definition, can lower the surface tension of the system thanks to their molecular structure: Hydrophilic Head + Hydrophobic Tail.When the surfactant is added to the glaze suspension, the following happens on the surface: the tail, due to its hydrophobic property, remains outside the water (exposed to the air) while the hydrophilic head, that is in water, separates the surface molecules of the water by lowering its tension. The reduction of glaze’s surface tension produces its better application on the tile and therefore a better leveling. Since surfactants are completely soluble, even if they do not normally lead to surface defects such as holes or pinholes (especially thanks to their low dosage), they can be highly foaming and difficult to control.
How dosurfactanst produce foam?
What is the structure of a foam bubble?
Foam bubbles generated by surfactants are nothing more than spherical films of water molecules held together by the surfactants themselves. In ceramics, when the glaze suspension is set in motion inside the tank, air enters into circulation within the glaze (especially at high levels of agitation) and foam bubbles rise to the surface.
This is the reason why surfactants are not frequently used in the ceramic production process and are preferably replaced with more easily manageable WETTING AGENTS.
- b) LEVELING AGENTS (WETTING AGENTS)
In some cases, it is possible to use PARTIALLY HYDROSOLUBLE LEVELING AGENTS which, on the one hand, do not possess any antifoaming properties, but on the other hand, can wet the inorganic glaze particles by lowering the water’s surface tension and therefore to encourage a de-agglomeration of the particles, thus facilitating their homogeneous dispersion within the system. The decrease of water’s high surface tension lets the water move between the suspended and agglomerated solid particles, producing and creating a distance between one and the other. The de-agglomeration, together with the lower glaze’s surface tension, turns in the leveling effect. This is the case, for example, with wetting/leveling agents used for bell or vela applications.
In other words: wetting agents are organic molecules that act both in the inter-phase between the ceramic/glaze substrate and in the glaze/air inter-phase: the leveling action they promote is specifically produced by decreasing the surface tension of water in the glaze/air inter-phase.
- c) DEFOAMERS
DEFOAMERS are chemical agents able to prevent and/or remove air bubbles that can sometimes be trapped inside the glaze or on its surface. They can added both inside the stirring tank and after application on the glaze surface already discharged on the ceramic support. How does the defoamer work?
The defoamer, being insoluble in water, can destabilize the foaming system by acting on the foam’s surface tension nad by breaking the surfactants’ hydrophobic tails and therefore causing the explosion of the bubbles.
To better understand let's make an example from the world of cooking. Let's start to beat it. If we beat only the egg white, the foam appears. Instead, if we beat the entire egg (egg-white + yolk) no foaming phenomena will be produced. Why?
Egg yolk contains very high percentages of fats (particularly triglycerides) that act as antifoams.
When applied to not very plastic glazes (with a low clay content) or hard glazes (such as glazes containing only frits), defoamers tend to remain micro-dispersed in water, sometimes causing defects such as holes or pinholes. The use of DEFOAMERS to counteract the foam promoted by the surfactant can therefore partially or totally solve the problem, but the balance is sometimes rather unstable. On the one hand, we fight against the surfactant’s foaminess by using a defoamer, and on the other hand, it is simultaneously mandatory to contrast the action of the latter which, in high dosage, can produce depressions and holes due to its water insolubility. If this is the scenario, it does not mean that in ceramics surfactants and defoamers cannot be used in combination. This is what frequently happens, but the combo must be carefully checked until a perfect balance is achieved with a tailored solution.
- DISPERSANT & SUSPENDING AGENTS
Suspending and thinning additives are true glaze correctives.
The glaze fluidization occurs when it is highly structured and therefore when a proper leveling on the ceramic support is difficult. Dispersants, in this perspective, are involved to change the structure of the aqueous system, to deconstruct it.
Just like dispersants used in the grinding step, they succeed in lowering the suspension’s viscosity by keeping separate the solid particles suspended within the liquid phase. This action leads to the right viscosity value required by the type of application chosen, allowing for proper leveling of the glaze.
On the other hand, suspending agents act on the possible sedimentation phenomena of the glaze that may occur inside the application tools, such as, for example, the stirring tanks. As the name clearly explains, they keep the glaze’s solid parts in suspension within the system.
How do suspending agents act?
IN THE SIMPLEST POSSIBLE TERMS, they can act on a steric level, by producing a grid in the system, thanks to the use of polymers that prevent and stop the particles’ agglomeration process, counteracting the gravitational force that would bring these solid parts to fall in the lowest part of the container. Suspending agents can also act on a ionic charge level by increasing some ions that have the possibility to saturate the water, thus giving structure to the system.
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