Chemical additives allow and promote the proper performance of almost all the ceramic production processes.

Their importance has been exponentially raising with the coming of rapid firing cycles that have increased the companies’ productivity, leading to a cost containment.

Nevertheless, this new kind of process has reduced the times of technical intervention in order to get the right rheology and the desired effect.

Here’s an overview of the most important additives involved in the ceramic production processes. 

CHEMICAL ADDITIVES FOR CERAMIC BODY

• Deflocculant
• Binders

CHEMICAL ADDITIVERS FOR GLAZES

• Deflocculant
• Suspending agents
• Binders
• Others

DEFLOCCULANT

The main purpose of deflocculant agents is to decrease the most important rheological parameters within the suspension, such as viscosity and flow limit.

Talking about glazes, the product’s target is to obtain a suspension with the correct rheology that allow a proper applications, regardless of any conditions (density, type of raw material, time between grinding and glazing process).

Talking about ceramic body, the most evident result is to make slurries with low water content, high density and appropriate viscosity. Reaching these targets, the slurries can be used shortly after the grinding process, avoiding long waiting times in order to get sedimentation.

A high density grinding process also offers four key advantages:

• 1. Decreasing of grinding times
• 2. Greater particle size’s uniformity
• 3. Increasing of productivity
• 4. Reduction of energy consumption

For ease of understanding:

1. We need to simplify the complexity of a suspension by taking in consideration the main two aspects: flow limit and viscosity
2. We will consider only the most popular applications 

GLAZING PROCESS BY BELL

In order to obtain a good performance it is important to reach a high viscosity and a very low flow limit within the suspension.

High viscosity, together with high density, creates in fact a stable application: the glaze does not tend to absorb the air bubbles generated by stirrers and pumps.

Moreover, the low flow limit makes the glaze’s rheological behaviour constant, promoting the expulsion of the air bubbles previously absorbed.

GLAZING PROCESS BY DISC

It is an outdated system. 

With this kind of application the glaze suspension moves among a number of discs that, due to their high speed, promote the formation of drops of a variety of dimensions that come together on the tile’s surface.

GLAZING PROCESS BY AIRBRUSH

It is the grazing process par excellence. 

The current spray cabins are made in order to get the right nebulization of all glazes. To do this, the glaze’s rheology must be characterized by very low viscosity and negligible flow limit.

However, it is not possible to work only on the glaze’s rheology in order to achieve a perfect application.

WHICH ARE THE ADDITIVES THAT CAN HELP IT?

From a chemical perspective, the most used products involved in the ceramic glazing process belong to two big families: polyphosphates and polyacrylic acid derivatives.

They provide for a deflocculant action that is shown through a variety of effects.

Following here the most important:

• Complexation of Ca, Mg and other flocculant ions 
• Exchange with deflocculant ions
• Increased repulsion between particles

The current products on sale can be both powdered and in water solution.

The former is suited for application in the milling process, the others are more appropriate to rightly adjust the glaze’s viscosity along the glazing line.

The dosage of these additives has to be controlled very carefully in order to avoid undesirable effects, such as an excess of viscosity, very high flow limit, rapid sedimentation phenomena (glaze’s cementation).
In any case, before proceeding with the application, laboratory test are absolutely required. 

Which are the most important actions of a deflocculant agent in a glaze’s suspension?

• 1. Increasing of the drying times (slowing down of water absorption’s times)
• 2. Uniform application of the glaze
• 3. Persistent sedimentation of the glaze, when it is not used for long times

SUSPENDING AGENTS

Suspending agents are able to reduce the time of sedimentation of the solid particles in suspension. This kind of property is related to many aspects.

Following here the most important:

• 1. Grain size and specific weight of solid part in suspension
• 2. Suspension’s density and rheological behaviour 
• 3. Presence of colloid substance

From a rheological perspective, the sedimentation slows down when the flow limit increases.

The suspending effect can be produced by many actions:

• 1. Specific salts that increase the total ionic charge
• 2. Addition of small quantity of flocculant ions
• 3. Addition of water-soluble polymers with high molecular weight
• 4. Addition of colloidal products

BINDERS

Binders for wet and dry application shall ensure the following functions:

• Providing the glaze with a good binding power with regard to ceramic bodies or layer of glazes previously applied
• Giving uniformity and cohesion to the glaze
• Slowing down of water evaporation: a very speed evaporation rate can lead to fissures, cracks or, more generally, superficial defects

CMC used within binders can cause, inter alia, two important consequences:

• LOW VISCOSITY → they provide a mild fluidifying action, decreasing to a lesser extent the drying times
• HIGH VISCOSITY → they increase the anti settling power and further slow down drying times

The product’s choice often depends on the personal observations and tests of the technician.  No products are clearly out-performing compared to other: it is important to be assessed on a case-by-case basis and work on tailor-made solutions.

DEFOAMER & LEVELING AGENTS

These are auxiliaries with basically two functions:

• 1. Removal of foam or air bubbles 
• 2. Improvement of the surface aspect and removal of superficial defects that clearly appear shortly after application and during the glaze’s drying phase 

Defoamers and levelling agents, depending on the product features, help the water’s penetration within the ceramic body or the underneath layers of glaze.

Using these products, you must take in consideration a few rules:

• They act on the surface tension with very low rates so it is useless (but not harmful) to insist on adding more additive when the product do not yield the expected results
• They usually do not affect the glaze’s rheology when use properly and in the right quantities suggested by the producers
• They are water-soluble: that’s way it is important to clean carefully all application booths after every production cycle