DEFINITION

Rheology is the science that deals with the flow and the strain rate of matter.

It’s a branch of physics that studies the origin, nature and deformation’s characteristics of a matter under the influence of external forces, with particular regard to non-Newtonian liquids***. 

Its main goal is to define the correlations between causes (forces) and effects (deformations and flows) identifying all mechanisms that are the basis of the different rheological behaviours on a microscopic and molecular scale.

***MOST SIGNIFICANT DIFFERENCE BETWEEN NEWTONIAN AND NON-NEWTONIAN LIQUIDS

• Newtonian fluid: fluid with a constant and unchanged viscosity, regardless of the strain rate
• Non-Newtonian Fluid: fluids whose viscosity changes with the strain rate (the relative flow velocity)

RHEOLOGY AND CERAMIC PRODUCTION

In ceramic industry, rheology studies and analyzes all physical characteristics of aqueous suspensions used in manufacturing processes.

Following here the most significant:

• CERAMIC BODIES
• GLAZES
• ENGOBES
• DIGITAL INKS
• GLASS GRIT SUSPENSIONS

Analysis and possible adjustments of the slurries’ rheological features are imperative in order to reach good application results.

A proper rheological study of a ceramic body, for example, helps to provide the system with the right stability and makes the slurry easily manageable, both during the handling and the spray application.

HOW TO STUDY CERAMIC SUSPENSIONS’ RHEOLOGY

Ceramic suspensions must be analyzed with rheometers able to collect all information about the fluid’s flow.

These properties are carefully examined so to check the proper conditions of the suspension to be used in the production process: in most cases it is necessary to modify some rheology’s values in order to get the right suspension for the right process.

It is obvious that the same suspension must have different features according to the application requirements of the any single ceramic manufacturer.

Following here the most important rheological parameters:

• VISCOSITY ► the viscosity of a fluid is a measure of its resistance to deformation at a given rate
• FLOW LIMIT ► it is the value of the minimum force required (shear stress) to run a fluid 
• RHEOLOGICAL BEHAVIOUR ► plastic, pseudo-plastic, newtonian

Data collected by rheometer are displayed using a two-dimensional Cartesian graph: while the vertical axis shows the SHEAR STRESS, the horizontal axis expresses the VELOCITY GRADIENT*.

*VELOCITY GRADIENT is a physical quantity related to the fluid’s movement speed.

VISCOSITY’s value is displayed on the diagram by the slope of the curve and it is the result of the relation between shear stress and velocity gradient.

The greater the curve, the higher is the viscosity.

RHEOLOGICAL BEHAVIOURS IN CERAMIC PRODUCTION

Following here the most representative ceramic suspension’s behaviours:

1. NEWTONIAN BEHAVIOUR
Linear-rate rheological behaviour. The viscosity value is stationary with changes of shear stress or/and the applied velocity gradient.
Oil and water are the most significant examples: these fluids do not change their viscosity whether they are quickly or slowly stirred.

The Newtonian behaviour is well represented in ceramic by digital inks that must be totally Newtonian in order to be correctly applied by digital print heads.

If the ink’s rheological behaviour were plastic or pseudo-plastic, the piezoelectric impulse system (that creates ink drops) would not be effective, giving back a lower-definition printing or, at worst, not allowing the formation of ink-drops. Print heads basically would not be able to spray the drops.

For all these reasons digital inks producers have to take in consideration the idea of providing the inks with the most Newtonian behaviour as possible.

2. PLASTIC BEHAVIOUR
Behaviour marked by a specific flow limit, i.e. by a shear stress below which the fluid acts as a solid.
In other words: when the fluid is in a static position it is extremely compact. When a movement is impressed, the fluid softens becoming more liquid. Pastry cream is the typical example for this category.

Speaking of pseudo-plastic suspensions means mostly talking about slurries for ceramic bodies and water suspensions of glass grits.

These fluids must keep the grit in suspension during their static position, behaving as a solid. On the other side, the suspension in application has to show a de-structuring process behaving as a pseudo-plastic fluid: viscosity decreases as the momentum (or the velocity gradient) increases.

During spray application the fluid is subjected to a very high speed while the viscosity is very low, allowing a proper nebulisation.

Chemicals are needed in order to reach this rheological behaviour:

• Suspending action that ensures the flow limit
• Binding action the provides the system with cohesion and homogeneity 
• Pseudo plasticity

Without the right auxiliaries the mixture, like sand in water, would present sedimentation phenomena precluding a positive result in application.

What about slurries for ceramic bodies?

All clays show a strong plastic behaviour. It is important to add chemicals able to drastically decrease (or remove) both the flow limit and the very high viscosity so to bring the gel system into a fluid system.

3. PSEUDO-PLASTIC BEHAVIOUR 
Behaviour marked by a decrease in viscosity with an increase of the shear stress or velocity gradient. Unlike the plastic behaviour, in this case there is no flow limit and so there is no initial force. The lack of flow limit makes the fluid behave as a liquid when is perfectly still. For this reason, a very low force is needed (almost zero) to provide it with a movement.
Typical example: self-levelling resins for floor applications.
This category may easily present sedimentation phenomena.
 

All glazes for airless applications can be included in this group.
They are nothing but water suspensions of extremely fine vitreous or natural particles.
The chemical and physical features of the particles as well as their size are such that the system does not required a flow limit or, at the most, it needs a very low one.
At rest, the suspension shows in fact a gradual and slow sedimentation.
The lack of flow limit finally ensures a good and proper levelling in application.

The rheological curves of pseudo-plastic and plastic fluids are actually very similar, differing each other in only one aspect: pseudo-plastic fluids do not need flow limit. 

4. DILATING BEHAVIOUR
Rheological behaviour marked by an increase in viscosity with the increase of the velocity gradient. 
Pizza dough, for example, tends to be more compact while is processed and so more viscous. On the other side, at rest it tends to drip, showing a lower viscosity.
Dilating behaviours are rare in ceramic field and they must be avoided, being unfit for ceramic production processes.

In ceramic production, dilating rheological behaviour can cause both application problems and defective product. For this reason, in most cases, it should be avoided.
These behaviours are opposite to the pseudo-plastic ones: as the velocity of movement gets faster, viscosity gradually increases.
This is way spray applications can be difficult or even impossible.

For a better understanding dilating behaviour, we can have a look in the world of motorcycling or extreme sports.
Today exist in commerce protective suits made with materials that have dilating technical features: at rest they are soft, allowing the wearer to easily move. In case of impact (so in conjunction with an increase of the velocity of movement) they immediately harden, protecting from possible contusions.

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THIXOTROPY is a typical time-dependent rheological phenomena that often occurs in ceramic processes: it comes as a progressive decrease in viscosity under the action of a constant velocity gradient.
Barbottina for ceramic bodies usually shows this kind of behaviour.