Virtually all the components in the coating formulation can influence foaming behavior positively or negatively. The coating substrate and method of application also have an impact on foaming behavior. A coating cannot be “generally” defoamed: For example, the coating surface may be fine for a spray application, but when using the same coating system on a curtain coater there may be issues with foam.
As we are focusing on coatings, we only deal with liquid foams and these are defined as a fine distribution of a gas (generally air) in a liquid. A characteristic feature of this type of foam compared with other physical states is the extremely large interface between the gas and the liquid – a lamella that separates the gas bubbles from each other. For energetic reasons, each liquid endeavors to keep its surface as small as possible, i.e. a foam is always a higher energy state than the liquid phase and is only made possible as a result of foam-stabilizing effects. As soon as they are produced, the gas bubbles in the liquid phase rise to the surface. According to Stokes' law, the ascent rate v is dependent upon the radius r of the bubbles and the viscosity η of the liquid.
When the gas bubble has reached the surface, liquid drains out of the foam lamella, i.e. the thin liquid film around the gas bubble (drainage effect): The lamella becomes increasingly thinner and below a thickness of approx. 10 nm the lamella tears and the foam bubble bursts open. If the processes take place as described, there is no problem with foam as no stable foam bubbles are produced; this is the case, for example, in pure liquids: pure liquids therefore do not foam.
For stable foam bubbles to produce, foam-stabilizing substances must be present in the liquid phase. In general, these are surface-active substances (surfactants), which are characterized by the fact that they contain hydrophobic and hydrophilic chemical groups in the molecule. This structure makes it possible for them to orientate at the liquid/gaseous interface, reduce the interfacial tension and thereby create the requirements for stable foam. Every coating formulation (whether aqueous, solvent-free or using organic solvents) contains a multitude of these types of surface-active substances of the most diverse chemistry and origin. For this reason, foam formation should be expected, in principle, in every coating system.
If you observe the life history of a foam you will establish that it loses liquid over time due to the described drainage effect, thereby changing its structure. Shortly after being produced, foam is characterized by still containing a relatively high level of liquid; this type of foam is therefore referred to as “wet foam” or “spherical foam” because the gas bubbles are still spherical and barely deform against each other. The liquid then drains out of the foam lamellae (drainage effect), the lamellae become thinner, the gas bubbles move closer together, deform against each other and become polyhedra. This foam is now called “dry” or “polyhedral foam”.
As the foam lamellae become increasingly thinner, this drainage effect would cause the foam to collapse if there were no opposing effects. One of these effects comes from the chemical structure of the foam-stabilizing substances, the surfactants. In aqueous systems, the hydrophilic groups are ionically constructed. The two interfaces of a foam lamella, which are covered with surfactants, come increasingly closer when the liquid drains away until eventually they interact across the lamella. The like electrical charges repel each other, further drying and therefore collapse of the foam is prevented by the electrostatic repulsion between the surfactant molecules.
Another stabilizing effect results from the elasticity of the foam lamellae. If the lamellae are stretched slightly, this stretching causes a drop in the surfactant concentration in the interfaces of the stretched region as the same number of surfactant molecules is then distributed over a larger surface. A reduction in surfactant concentration at the surface, however, causes an increase in surface tension, which causes the lamella to return to an energetically favorable state. This difference in surface tension (γ) is compensated for by the system by a mixture of water and surfactant being discharged from the body and surface of the lamella into the previously stretched region of the lamella. This special foam-stabilizing effect is called the Gibbs-Marangoni effect.