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1. Definition & primary function
Wettability is, in general terms, the property that describes a liquid’s ability to adhere to a solid surface. It can also be seen as the process through which a liquid comes into contact with a solid.
This property plays a key role in determining whether a surface is hydrophobic (water-repellent) or hydrophilic (water-attracting). It is typically assessed using contact angle measurements, which are based on the angle formed where the liquid-vapor interface meets the liquid-solid interface.
In ceramics, this refers to the contact angle between an aqueous suspension and the substrate, or to the solid content within the system.
- Low contact angle values indicate high wettability (hydrophilicity)
- High contact angle values indicate low wettability (hydrophobicity)
Hydrophilicity, which literally means a strong affinity for water, gives a surface maximum wettability.
In contrast, when a material is hydrophobic—literally “afraid of water”—it shows no wettability, and water typically forms separate, isolated droplets on the surface rather than spreading out.
Wetting agents create the right conditions to achieve proper wettability of a substrate. They are mostly organic molecules. In ceramics, they act at both the ceramic body/glaze interface and the glaze/air interface. By modifying the surface tension of the liquid, they help ensure a smooth, even application of the glaze.
LEVELING EFFECT
One of the main effects of wetting agents is their leveling action. By reducing the surface tension of water, they improve how the liquid moves between the solid particles in the suspension, such as glazes or frits.
This action helps break up agglomerates of inorganic particles, promoting even dispersion and improving the flow and spread of the product over the ceramic surface. This function is especially important in bell or vela applications, where uniform distribution of the material is essential for both the appearance and technical performance of the finished product.
FOCUS: SURFACE TENSION
Surface tension is the tendency of a liquid to behave as if its surface were a thin, flexible layer. It happens because of the attractive forces between the liquid’s molecules, which pull them together.
Deep inside the liquid, these forces balance out because each molecule is surrounded by others in every direction. At the surface, things are different: molecules have neighbors only to the sides and below, so they are pulled inward. This creates a force that makes the surface shrink as much as possible. As a result, droplets naturally form a round shape, since a sphere has the smallest surface area for a given volume.
Surface tension is measured in newtons per meter (N/m) and changes depending on the type of liquid and its temperature. Water, for example, is marked by high surface tension because its molecules form hydrogen bonds. This is why some insects can stand or walk on water without sinking.
This property is essential in many every day and industrial processes. It explains how bubbles form, how liquids move through tiny spaces like plant roots or thin tubes, how detergents help water spread and clean better, how inkjet printers work, and even how plants draw water upward to nourish themselves.
2. Explanatory cases
a. Prevetion of spray-nozzle clogging
In spray application processes, poorly wettable granules can lead to the formation of solid clumps inside the suspension, which may eventually clog the nozzles. Sometimes the granules (grit) in the mixture are not fully bonded or evenly dispersed in the water. As a result, they act separately from the liquid phase, which consists of water and additives.
When the granules do not come into direct contact with the water—in other words, when they are not properly wetted—the suspension behaves like a heterogeneous system. This is like what happens when you pour sand into a glass of water: the sand stays apart instead of blending smoothly.
During application, this behavior can cause localized clusters of material to form, especially near the nozzles, increasing the risk of blockages and uneven spraying.
In these situations, adding specific wetting agents allows water to move between the granule particles, helping them spread evenly and flow smoothly within the system.
This action leads to a more stable suspension, which in turn reduces or even eliminates blockages during production. As a result, the process runs more smoothly and continuously, with fewer interruptions and greater overall efficiency.
It is important to note that both the type and amount of wetting agent must be carefully adjusted based on the characteristics of the suspension, the granules, and the application conditions.
In more complex cases, a targeted laboratory study may be needed to fine-tune the formulation and achieve the best possible performance.
b. Managing the TV screen effect
The so-called “television effect” is a visual defect where the glaze thickens along the edges of a tile, resembling the curved border of an old CRT television screen.
This issue occurs when the glaze migrates toward the edges or perimeter of the tile. The movement is driven by surface tension values that are slightly too high. In response to this tension, the glaze naturally tends to pull itself into a rounded shape, moving toward the edges to reduce surface energy, which leads to the characteristic uneven build-up.
There are several ways to prevent this defect, but the most common approaches focus on two key actions.
The first is to increase the drainage rate by using specific chemical additives. The second is to reduce the surface tension of the system with substrate wetting agents.
By carefully lowering the surface tension, the glaze spreads more evenly across the tile, avoiding build-up along the edges. This not only minimizes the television effect but also helps create a smoother, more uniform finish.
c. Post-digital printing applications & water-repellency phenomena
In water-based applications applied after digital printing—such as top glazes, granule slurries, or high amount of glaze for polishing—repellency issues can occur.
These problems are caused by the incompatibility between the non-polar solvents in the inks and the polar, water-based components of the subsequent suspensions.
As a result, the glaze fails to adhere properly to the decorated areas, leading to poor coverage and visible defects that compromise the final appearance.
In these situations, using the right substrate wetting agents helps lower the surface tension of the water-based suspension. This makes it easier for the glaze to spread over areas treated with ink, creating a more compatible interface between the two layers.
With surface tension values brought closer together, the repellency effect is reduced, allowing the glaze to adhere evenly, even on the most challenging areas, and ensuring uniform, consistent coverage across the entire surface.
3. Conclusions
Wetting agents are a key group of additives for quality control in ceramic production. They play multiple roles, from improving leveling to preventing aesthetic and technical defects, as well as solving compatibility issues between water-based and solvent-based systems. Choosing and applying them correctly has a direct impact on both process efficiency and the performance of the final product.
A deep understanding of how these additives work and the mechanism behind their action is essential for developing solutions that are reliable, compatible, and tailored to the evolving needs of the ceramic industry.