What is Defoamers in paints, inks?

Defoamers are chemical additives that reduce and/or prevent the formation of foam in industrial-process liquids. For this article, we’ll discuss defoamers in relation to paints, inks, adhesives and even construction products. The terms anti-foam agent and defoamer are often used interchangeably. However, anti-foam agents more accurately refer to materials that inhibit the generation and formation of bubbles. Dependent upon the application and performance requirements, they consist of polydimethylsiloxanes and other silicones, insoluble oils, stearates and glycols, as well as inorganics, such as silicates and talc.
Foam is a coarse dispersion of a gas in a liquid, where the volume fraction of gas is greater than that of the liquid. The bubbles will migrate to the surface because their density is less than that of the liquid. As the bubbles coalesce and collect at the air/surface interface, the bubble walls thin and break. Defoamers accelerate the process and break the smaller bubbles as well. Generally a defoamer is insoluble in the foaming medium and has surface active properties. An essential feature of a defoamer is the ability to spread rapidly on foamy surfaces.
In industrial processes, foams pose serious problems. They cause defects on surface coatings. They prevent the efficient filling of containers. Some of the sources of foam formation include:

• Inclusion of air through agitation during production, filling and mixing of 2-pack systems. Often high viscosity (epoxies, adhesives).
• Air inclusion on pigment surface, resulting in poor wetting of pigments.
• Application: Roller, spraying and brushing.
• Filtration through a sieve or anything with air on the surface.
• Generation or liberation of gases during the chemical curing processes, e.g. polyisocyanates.
• Introduction of air through substrate wetting (wood coatings and other highly porous substrates).

There are many aspects of defoamers that can be discussed in a technical article, but there is limited space to do so. It is suggested that the following are considered, if you require a more thorough understanding of the subject of foam generation, defoamer composition, mechanisms, etc. (See further resources below.)

1. Foam stabilization
2. Composition and mode of action of defoamers
3. Test methods
4. Paints
5. Inks
6. Adhesives
7. Construction products
8. Selection guidelines
9. Process
10. Application
11. Coatings defects

For this article, the focus will be on the mechanical dissipation of foam as well as the chemical elimination of foam. In the former, there are several effective methods:

• Bubble consolidation is achieved with lower-shear mixing, which will put the microfoam in contact, especially at the surface (vortex), and foam will consolidate to larger bubbles that break more easily.
• Vibration using a rotosieve to reduce the surface tension at the air/liquid interface in processes such as filtration of latexes.
• Using a very fine mist of water spray to disrupt and break the bubble walls.
• Vacuuming, such as with a RotoVap or Ross Mill homogenizer.

Chemical defoamers similarly have analogous methods, which are in the form of composition rather than processes:

• In process, they can either consolidate small foam to larger bubbles, which can then break at the air/liquid interface or break bubbles during mixing and shear. During application, they
  disrupt the bubble wall or cause increased drainage.

In many cases, the choice of a chemical defoamer is a balance between highest efficiency and lowest undesired side effects, coupled with a preferred mechanical process, if available.
There are many considerations in the choice of a defoamer. For paints, persistency over a long shelf-life is required, but the defoamer (often mineral-oil-based) cannot affect color uniformity, such as rub-up. For waterborne flexographic inks, mineral oil is not a consideration, as it causes roller and plate swelling. Therefore, the preferred defoamers are based on glycols and polyglycols (ethers). Silicone defoamers are used in solvent-borne and high-viscosity systems and are used more in the grind stage of pigment dispersion, rather than in the letdown.
Choosing the correct test method that includes the appropriate substrate and perseverance upon storage is key. As an example, if formulating a waterborne polyurethane or acrylic-urethane, a strong defoamer may be required, because application to a porous wood substrate will liberate microfoam, and the coating surface energy is particularly high.
If a film is cast over a sealed paper chart such as a Leneta 2C card, the film may show severe crawling and fisheyes. The conclusion might be that there was an overdose of defoamer. However, testing on a Leneta R1A birch plywood blank would show a uniform coating void of film defects and bubbles.
Heat aging is a quick way to evaluate long-term storage stability. It is usually done at intervals of one week for a total of four at 50°C. The other critical test is compatibility to ensure that the defoamer doesn’t interact or react with a component of the coating and cause seeding.
In summary, defoamers are essential ingredients in a paint formulation that avoids foam formation and stabilizes in waterborne as well as solvent-borne paints. Proper selection of defoamer quality and quantity is essential for optimal defoamer performance. Defoamers compete with foam-stabilizing ingredients in order to be effective in a coating system. The proper defoaming results in paint quality properties, and it is important that test methodologies are appropriate for the specific application or process.
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