Causes of foam formation

Water-based paints are not pure liquids; they are colloidal dispersions of particles (pigments, resins) in a carrier (water). One of the key factors in the stability of air bubbles in paints is surfactants, as it is they that hold all the different ingredients together and prevent them from separating or settling.

Surfactants are a component that prevents phase separation in emulsions. They improve dispersion and prevent settling by wetting the pigment surface, and improve the overall compatibility of the various ingredients in an aqueous paint formulation. The downside of these surfactants is that they also very effectively stabilise the air in the paint mixture, making it difficult for the air to come together, rise to the surface and escape. This is due to the specific properties of surfactants. Surfactants are substances that are both hydrophobic and hydrophilic. The hydrophobic part of the surfactant surrounds the air bubbles, stabilising them in the paint and preventing them from coalescing and rising to the surface to escape.

Defoamer composition

Formulations have three main components:

1. the carrier;

2. the emulsifier and wetting agent;

3. a third component or active ingredient.  

The carrier can act as its own defoamer. In defoamer formulations, its main function is to transport the hydrophobic components to the laminar interface, where they can do their job. Examples of carriers are mineral and silicone oils.

Emulsifiers or wetting agents are used to disperse the defoamer in the liquid that needs to be defoamed. They control how difficult it is to disperse the defoamer and also have an effect on the effectiveness, persistence and compatibility of the defoamer.

A third component is added to improve the performance, stability or compatibility of the defoamer.

Water-repellent particles, fatty acids, coupling agents and patented components such as defoaming molecules belong to this category.

How defoamers work

It is thanks to defoamers that air bubbles can be removed from a system, overcoming the stabilising effect of surfactants. To do this, the defoamer is formulated to be incompatible with the layer of surfactant at the interface between the bubble wall and the liquid surface or adjacent bubble walls.

The defoamer needs to be a liquid with low surface tension, controlled insolubility or incompatibility, a positive entry coefficient to enter the film layer, and a positive spreading coefficient to spread throughout the film layer. The defoamer also needs to be dispersed as hydrophobic droplets in the formulation. Once the defoamer droplets have entered and distributed throughout the film area, they destabilise the surfactant, causing water to flow out of the film, which leads to the thinning of the bubble wall and the bubble’s eventual collapse.

Therefore, in addition to the surface tension of the defoamer, an important performance characteristic of an effective defoamer is its ability to be dispersed in the liquid to be defoamed and form an effective droplet size.

The demand for low-VOC coating defoamers

With the introduction of regulations relating to VOC emissions and the demand from consumers for more environmentally friendly products, paints have changed dramatically over the past decade, and today architectural paint formulations have near-zero VOC content. The emulsions used are softer polymers that form a film without the need for film-forming aids and gain their hardness through post-application cross-linking. Solvents are no longer needed to extend open time and maintain wet edges during application. All paint components are expected to be zero or near-zero VOC, with water being the only volatile liquid component in the formulation. The use of surfactants becomes more important in stabilising these formulations, and the use of environmentally friendly surfactants has changed the situation with regard to foam generation and stability. In architectural coatings, single-coat finishes are now expected, and one way to achieve this is to formulate coatings with higher viscosities than in the past, both in the low-shear and high-shear ranges. These factors all have an impact on the defoaming requirements of modern coatings.

Solvents can no longer be present to act as temporary defoamers in the formulation, especially during production. Higher viscosity means that bubbles rise more slowly and with greater difficulty from the liquid. Increasing the amount of surfactant used means that the foam is more stable throughout the coating. Defoamers must now be formulated to counteract all these effects. The amount of defoamer in the coating can be reduced, meeting the urgent need to reduce the VOC content of coatings. The more difficult problem of defoaming low-VOC coatings, especially those with near-zero VOC formulations, is solved by using molecular defoamers.