Foam means a two-phase stable dispersion in which gas molecules becomes enclosed by liquid.

Pure liquids do not foam. When shaking pure liquid, the gas absorbed tends to form spherical bubbles since this involves the least amount of surface energy. However, the bubbles will rise toward the surface and collapse immediately due to the absence of any stabilization.

There are no surface-active agents present in pure liquids.
As the air in the bubble is released, the liquid quickly rushes into the space vacated by the air; hence, a stable foam is not achieved in pure liquids.

One of the primary reasons for foam formation is the presence of other components that reduce the surface tension.

When does foaming occur?

One of the primary reasons for foaming is the presence of additives that reduce surface tension in the coating formulation. These additives are surfactants and wetting agents, emulsifying agents, dispersing agents, levelling agents and other additives. In paints, coatings and inks, the air gets incorporated into a system resulting in foaming at various stages like manufacturing, packaging, handling & transport, application, drying. 

How to avoid Foam

Controlling foam is one of the main challenges for both the manufacturers and the applicators to ensure the smooth and uniform appearance of the paint film. Foam control agents are added to coating and ink formulations to prevent air entrapment and foaming. 

Foam control agents are classified into:

• Anti-foam additives  
• De-foamers and Deaerators or Air-release agents

Antifoams/Anti-foaming Agents are added in paints to reduce:

• Foam problems during manufacturing.
• Aeration during can filling.
• Air bubble formation during application. 

An anti-foam agent should not cause pigment flocculation or emulsion instability, possess poor compatibility resulting in defects in the wet or dry film and affect tint colour appearance.

Defoamers: Defoamers destroy foam after it has been formed.

Defoamers are low surface tension liquids that can enter the foam lamella. It acts as a carrier medium to transport hydrophobic particles into the foam lamella, resulting in the collapse of the foam lamella. 

In emulsion paints, foaming can lead to the undesirable formation of surface defects: craters and pores in the coating film. Defoamers reduce the surface tension of the liquid to such an extent that the air bubbles in the foam collapse. 

For aqueous coatings, two distinct properties of defoamers are important: defoaming itself and deaerating.

• To destroy macroscopic foam formed in the coating after application – defoaming
• To remove air trapped during the coating process – deaerating.