CHEMICALS FOR CERAMIC PRODUCTION: A FOCUS ON DEFLOCCULANTS AND BINDERS
As we know (check here one of ours previous post about additives ) most of the ceramic production processes depend on suspensions or more generally on chemical dispersions of ceramic powder in a liquid, mostly water.
In order to provide the suspension with the right features in terms of stability and workability according to the different applications involved, it is necessary to add small quantities of appropriate substances (additives) able to modify the rheology of the system, interacting with the solid particles.
Even if ceramic suspensions can be very complex, we can simplify pointing out three main families / components:
- CERAMIC GRIT
The choice of the right additive, and of the quantity to be added to the system, depends on the nature of the above-mentioned components.
For example, additives required to stabilize clays in water can be inappropriate for ossidic powders in organic mediums.
The first classification of an additive can start from its primary effect on the ceramic system to which is added. Of course, an additive can also produce side effects beside its main purpose – requested or not – and therefore it can be included in more than one category at the same time.
The most popular categories of additives are DEFLOCCULANTS and BINDERS.
The first are used to reduce the agglomeration phenomena between particles and so to decrease the system’s viscosity. On the other side, binders are intended to create a net between particles so to provide a certain degree of elasticity.
In everyday language, when we talk about deflocculant we usually refer to a substance able to decrease the viscosity of a system. It should also be said, however, that this is only a consequence of the more important reduction of the agglomeration between particles.
Beside deflocculant and binders there are many other additives that can be used in order to improve both production processes and product quality. Here is a short selection:
► Surfactants: they reduce the systems’ surface tension (in order to enhance the surface’s wettability)
► Plasticizer: they are added to modify the viscoelastic properties
► Antifoaming and foaming agents: they prevent or promote, as needed, the formation of bubbles in the system
► Lubricants: special surfactants that decrease the friction between the solid elements
► Bactericide and fungicide: they slow down or eliminate degradation phenomena of the organic substances included in the system
In the ceramic field, words such as dispersant, deflocculant and fluidifying agent are often used with the same meaning.
Nevertheless it’s important here to underline their difference:
Dispersant ► with dispersant we intend the liquid phase in which the solid particles are suspended
Deflocculant ► with deflocculant we intend any kind of substance that, added in small quantity, reduces the particles’ agglomeration phenomena
Fluidifying agent ► with fluidifying agent we intend any kind of substance able to decrease the viscosity (regardless of how this happens). This definition is quite ambiguous because it is obvious, for example, that a fluidization can be done adding both dispersant or deflocculant agents (even though the two actions are very different).
Dispersants dilute the system and so the viscosity decreases thanks to the reduction of the volumetric content of the suspended solid particles and of the interaction between themselves.
Deflocculants restrict agglomeration phenomena, unchanging the concentration of suspended solids: in this case the viscosity’s decrease is due to the reduction of agglomerations’ size.
Even if water is the most common vehicle used in ceramic production processes, many applications require different mediums such as polyglycols or mix of methyl ethyl ketone or trichloroethylene / ethanol.
Furthermore, as has already been said, the quantity of deflocculant to be added is essential to evaluate its effectiveness.
Another important category of additives is well represented by binders (or flocculant agents).
As the name suggests, their function is to bind the ceramic particles so to create a reticular effect in the system. This structuring process, that can be considered a sort of flocculation between particles, can be used for many purposes: to increase the viscosity (in such cases we are talking about of thickener), to modify the rheology or reduce sedimentation phenomena.
Some binders are made of colloidal particles that can be both inorganic (such as clay minerals particles) and organic. The most used, however, are polymers and salts of different nature.
Colloidal clay particles create agglomerations that form a reticulation, binding together the biggest ceramic particles.
Polymeric binders produce the same process: vinyl, cellulosic and glycolic binders are among the most common. These are mostly non-ionic binders with functional groups along the chain.
All the others are anionic binders with functional groups that became electrically charged once dispersed in the liquid and ionized.
SUMMARY & CONCLUSION
The colloidal stability of all systems used in ceramic production processes is essential to monitor and handle their rheology.
To do this, it is often necessary to add the system with appropriate deflocculants able to resist the attractive forces within the suspension, reducing their agglomeration.
The choice of the right deflocculant depends both on the vehicle and the nature of the solid particles that have to be stabilised.
The correct/perfect quantity must provide the system with the best stabilisation as possible without over-deflocculate the system.
Beside deflocculant, some systems require also binders that usually increase the viscosity due to other flocculation phenomena that have to be monitored, taking appropriate actions on the chemical and physical condition of the system.
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