During the production and application of paint systems, foam is an undesired side-effect that increases production time, makes it more difficult to fill vessels with the correct amount of paint, and contributes to surface defects such as craters and weak points in the dried film.

Foam is not stable in pure liquids. Foam is only stable in systems that contain surfactant-like substances such as wetting agents or certain surface control additives needed to improve important properties of the paint, see “Dispersing Technology” and “Surface Control”.

What all these surfactants have in common is the fact that they can migrate to the air/liquid interface of the paint, thereby reducing the surface tension.

Foam, trapped air bubbles, can originate at various stages of a coating’s manufacture or use, during pumping, stirring, dispersing or application of the liquid paint.

The air-liquid interface of these bubbles is surrounded by the surfactants present in the paint. In a low viscosity paint, the low density of these bubbles means that they rise to the surface. As the bubbles rise, they can combine to produce larger bubbles, which rise even more quickly to the surface.

These bubbles accumulate and deform as well as deforming the surface of the paint. The air cannot escape because lamella are formed that are stabilized by the presence of surfactants already in the paint. Without surfactants, drainage of the liquid would cause thinning of the lamella until breakage occurred. However, the presence of these surfactants avoids the thinning of the lamella by:

1. Counterflow of liquid due to a surface tension difference, as a result of interface stretching, this is called the Marangoni effect;

2. Repulsion by the surfactants at the interfaces, through steric and electrostatic mechanisms;

These stabilizing effects result in elasticity of the lamella that prevents the lamella from reaching a critical thickness of around 10 nm, which is the criterion for lamella breakage.

To eliminate foam, a defoamer with one or more of the following properties must be used to avoid these stabilizing effects:

1. Foam destruction to eliminate existing foam

2. Foam prevention to avoid formation of foam

3. Air release to assist air bubbles to rise to the surface

The defoamer is active mainly in the stabilized lamella. Therefore it must be insoluble in the paint system and mobile so it can enter in the lamella where it has to spread at the interface and displace the surfactants. The defoamer must have a lower surface tension than the surfactant, leading to an opposite Marangoni effect, i.e. fast thinning and collapse of the lamella.

Possible chemical structures for defoamers are molecules with a low surface tension such as silicon and mineral oils, fatty acids and fluorocarbons. To increase defoaming efficiency, solid particles with a low surface tension can be included, such as hydrophobic silica and metallic soaps.

These materials can be incorporated in carriers such as water and organic solvents in order to provide easier addition and faster distribution of the active substance in the liquid paint. 100% active defoamers are suitable for systems that will be placed under shear stress such as grinding, which ensures the distribution and activity of the defoamer.

The type of defoamer used depends on the type of system with the foam problem. Polysiloxanes, polyacrylates and polyolefins are effective in solvent-based and solvent-free systems because these systems already have low surface tension. Pure polydimethylsiloxanes are very critical in their compatibility, which can cause side effects such as cratering. The best balance between compatibility and incompatibility is achieved by organically modified polysiloxanes. Modification with fluorine gives even lower surface tensions.

A wider range of chemical structures can be used with water-based systems due to the generally higher surface tension of these systems, so mineral oil types and silicons are highly effective.  An important point to consider is the incorporation of the defoamer in the paint system. Since defoamers are not soluble in the system, good distribution of the active substance is necessary. This can be controlled by the mixing speed and time, otherwise craters can form or loss of defoaming efficiency is observed.


Since the performance of a defoamer is difficult to predict due to the variety of raw materials used in a paint formulation and the method of application, evaluation of one’s system is crucial. This is easily done by “stirring or shaking tests” on the system in question to find the best choice of product and optimal dosage.