#73 Dispersants for ceramic bodies: rheological control and slurry management
Contents
- What are dispersants for ceramic bodies?
- Water, solids and viscosity: finding the right operating point
- When viscosity makes slip handling difficult
- Beyond viscosity: thixotropy and behavior under shear
- Different raw materials, different responses
- The role of water and dissolved ions
- Different chemical families and mechanisms
- Dosage also has an optimum range
- From product selection to body characterization
1. What are dispersants for ceramic bodies?
In the wet preparation of ceramic tile bodies, raw materials are dispersed in water and milled to produce a slip with suitable properties for the subsequent processing stages. In this context, dispersants, also referred to as dispersing agents, are additives primarily used to modify the rheological behavior of the suspension. They make it possible to work with relatively high solids contents while maintaining a viscosity compatible with the process.
Their role is not simply to make the body “more fluid”. A ceramic slip is a concentrated suspension of water and mineral particles differing in size, shape, surface properties and composition. The interactions between these particles largely determine the behavior of the suspension.
When these interactions promote the formation of particle aggregates or structures, resistance to flow can increase. A dispersant, by contrast, promotes better particle dispersion in the aqueous phase, modifying particle interactions and therefore the rheological behavior of the slip. This can result in lower viscosity at the same water content.
However, there is no universal relationship between the amount of dispersant added and the response of the body. Ceramic suspensions are complex systems, and dispersant performance depends on the raw materials used, their particle size and mineralogy, water composition, solids concentration and the conditions under which the slip is prepared. Product selection and dosage therefore need to be evaluated for the specific body composition.
2. Water, solids and viscosity: finding the right operating point
A common situation is a slip that reaches a suitable handling viscosity only when its water content is increased.
From an operational perspective, this may work, but it has consequences for the subsequent stages. Much of the water contained in a slip intended for spray drying must subsequently be evaporated in the spray dryer. Reducing the water content of the incoming suspension therefore means, all other conditions being equal, that less water needs to be thermally removed, with a corresponding reduction in energy costs.
A suitable dispersant can make it possible to increase the solids content without raising viscosity beyond the limits that can be handled by the equipment. The optimum operating point must, however, be determined experimentally. Simply aiming for the highest possible density is not necessarily the right approach, as the slip must retain properties compatible with discharge, storage, screening, pumping and spray drying.
The relationship between solids content and rheological behavior is therefore particularly important.
3. When viscosity makes slip handling difficult
Excessive viscosity can cause difficulties at different stages of the process. Adding water is one possible corrective measure, but it is not the only option. Examining the deflocculation curve can help determine whether better flow behavior can be achieved by improving particle dispersion.
This is important because slip viscosity is not determined solely by water content. The rheological behavior of the body is also influenced by the concentration and characteristics of the particles, their interactions, and the type and dosage of dispersant. Two slips with the same density may therefore exhibit different rheological behavior. Similarly, replacing one dispersant with another at the same dosage does not necessarily produce the same result.
4. Beyond viscosity: thixotropy and behavior under shear
A viscosity value measured under a specific set of conditions provides important information, but it does not fully describe the rheological properties of a ceramic slip.
Concentrated ceramic suspensions frequently exhibit non-Newtonian behavior, meaning that their apparent viscosity can vary depending on the applied shear conditions. Phenomena such as thixotropy, dilatancy and yield stress can also be relevant to the industrial handling of the suspension. Thixotropy describes a time-dependent response in which the structure of the suspension changes under shear and recovers, at least partially, when the shear is reduced or removed.
From a production perspective, this means that a slip should not be assessed solely in terms of “how viscous” it is, but also according to how it flows under actual process conditions and how its rheological response changes when those conditions vary. The dispersant therefore becomes a means of controlling the overall rheology of the system.
For its dispersing agents used in ceramic body preparation, for example, Zschimmer & Schwarz indicates the possibility of adjusting parameters such as viscosity, thixotropic behavior, structural viscosity and other flow characteristics.
5. Different raw materials, different responses
One of the factors that makes deflocculation less predictable than it might initially appear is the variability of raw materials.
Clays are not chemically or mineralogically uniform materials. Changes in body composition or in the characteristics of an individual raw material can alter the rheological response of the slip and, consequently, the relative effectiveness of a given deflocculation system.
Common rheological additives can behave differently depending on the clay being processed. The selection of a dispersant should therefore be considered together with the characterization of the body in which it will be used.
In practice, a product that provides a good operating point for one formulation is not necessarily the most suitable solution for another. The same applies when an apparently unchanged formulation shows variations associated with different batches of raw materials.
In these situations, a dispersant can help bring the slip back towards the required rheological conditions. However, before simply increasing the dosage, it is useful to identify which parameter has actually changed. A change in viscosity may be an indication of a broader change within the system.
6. The role of water and dissolved ions
The characteristics of the water used to prepare the slip also affect the balance of the system.
Ionic species in the aqueous phase can modify particle interactions and influence the effectiveness of the dispersant. Calcium ions are one example. In such systems, low-molecular-weight sodium polymethacrylates can perform a dual function: providing electrosteric stabilization of the particles while also sequestering cations that promote flocculation.
This helps explain why the behavior of a ceramic slip cannot be attributed solely to the composition of the solid raw materials. The composition of the liquid phase can also influence the result.
This becomes particularly relevant when the source or characteristics of the process water change, or when recycled water enters the system with a composition that may differ from that of fresh water. When an unexpected change in rheological behavior occurs, the aqueous phase is therefore one of the factors worth investigating.
7. Different chemical families and mechanisms
Several families of dispersants are used in the ceramic industry, including silicates, phosphates and other inorganic salts, as well as organic polymers such as polyacrylates and polymethacrylates. Mixtures of organic and inorganic components are also used.
Their behavior is not necessarily equivalent.
In the case of sodium silicate, its action is associated, among other factors, with changes in pH and the adsorption of negatively charged species onto the surfaces of clay particles, with a significant electrostatic stabilization component. Some polymethacrylates, by contrast, have been described as providing predominantly electrosteric stabilization, combined with the ability to interact with flocculating cations.
These different mechanisms are one reason why commercial formulations may combine several components. However, interactions between different dispersants may be additive, synergistic or less favorable depending on the system. It should therefore not be assumed that simply combining two substances that are individually effective will necessarily improve deflocculation. Studies on dispersant blends confirm the need to evaluate these interactions experimentally.
8. Dosage also has an optimum range
If a certain amount of dispersant clearly reduces viscosity, it may seem logical to increase the dosage further to achieve a greater effect. In practice, however, the relationship is not necessarily linear.
The effect of dosage can be evaluated experimentally by constructing a deflocculation curve, measuring viscosity or other rheological parameters at progressively increasing additive levels while keeping the other test conditions constant. This makes it possible to identify the dosage—or, more accurately, the dosage range—best suited to the specific composition and process conditions. Beyond this range, adding more dispersant may not lead to any further reduction in viscosity and, in some systems, may alter the rheological behavior again. Additive dosage therefore cannot be regarded as a universal value that can be transferred automatically from one composition to another.
In practical terms, for some systems used in the wet milling of ceramic tile bodies, the optimum dosage may be in the range of a few tenths of a percent. This should, however, be regarded only as an order of magnitude.
9. From product selection to body characterization
This is also the approach behind the range of dispersants offered by Zschimmer & Schwarz Ceramco, developed to provide targeted control of the rheological properties of ceramic suspensions.
The FLUICER range includes solutions with different characteristics, allowing the most suitable dispersant to be selected according to the system being treated, the required rheological behavior and the specific process requirements.
The product itself, however, is only one part of the solution. Selecting the appropriate dispersant and dosage becomes meaningful when combined with suspension characterization and an understanding of the actual process conditions. The most appropriate operating point is ultimately identified by considering the product, formulation and application conditions together.
