Silicone Antifoams

Two theories exist to describe how antifoams work. Both are based on the fact that pure liquids do not foam. The presence of some stabilizing material is necessary in the surface layer of foam films to allow them to remain stable. These theories are also based on the premise that antifoam-ing agents either replace or modify these stabilizing materials. The first theory states that the antifoam disperses in the form of fine drops into the liquid film between the bubbles and spreads as a thick duplex film, ie, a film that is thick enough to have two definite surfaces. The tension created by the spreading duplex film leads to the rupture of the original liquid film (bubble wall), destroying the foam.

The second theory is similar to the first and states that the antifoam produces a less cohesive, mixed monolayer on the surface of the liquid film. Because this monolayer is of less coherence than the original film stabilizing monolayer, it will cause the destabilization of the bubble wall (3). In either case, it is important to note that antifoams must be surface active, that they must be able to spread to the entire foaming surface of the process, and that they must remain insoluble to continue to be active.

Since the discovery of polydimethylsiloxane in 1943 by scientists of what is now Dow Corning Corp., there have been many applications found for this unique substance. Applied as antifoams since shortly after their discovery, these silicone-based products are able to provide benefits that most organic defoamers cannot (4). Due to the inert nature of the polydimethylsiloxane molecule, silicone-based antifoams will not react with most process media. This inert nature also allows the antifoam to remain effective longer than reactive organic material. Silicones exhibit a low surface tension (<20 dynes/cm) (5), allowing them to spread quickly and evenly over the foaming surface, an important requirement for an antifoam already mentioned.

Both of these attributes contribute to the most important benefit of using silicone antifoams: their efficiency. Silicone-based antifoams are effective usually in amounts less than 100 parts active silicone per million parts foaming media in industrial applications and in as little as 510 ppm in food-grade systems.

Silicone antifoams are available as three basic types of product. First, the polydimethylsiloxane, or silicone oil, can be used as an effective antifoam for many nonaqueous-type processes. Second, the silicone oil can be compounded with hydrophobic silica to form a product that can be useful in both aqueous and nonaqueous systems. It has been theorized that the addition of this silica gives an added benefit of physically disrupting the foam interlayer. The third and most effective product for aqueous systems is an emulsion of the silicone—silica compound. An emulsion is the most effective carrier of a compound in a water environment, allowing for ease in dilution and dispersion.

Silicones can be used in many processes. There are silicone-based antifoams that are permissible for use in direct contact with food, as per the Food and Drug Administration's regulation #173.340, which allows for up to 10 parts of active silicone per million parts of food in the final product. Certain silicones also meet the U.S. Department of Agriculture's specifications for federally inspected foods.

Many are also kosher approved. Silicone antifoams usually will not impart any taste or smell to a product. Because these antifoams are essentially inert and are used in such low amounts, they rarely affect process reactions.

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  • raffaella
    Why pure liquids do not foam?
    8 years ago

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