Antifoam or defoamer compounds are used to prevent or break up foams. They are insoluble in the stock that produces the foam, and they are surface active; that is they spread across the surface at the air-liquid interface. They have very low viscosity which gives them low surface tension. As they spread over the foam, they lower the surface tension of the bubble causing them to break. There are a wide range of compounds that serve well as antifoams, but most can be categorised as oil-based, silicone-based, or water-based.
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We all know that oil and water do not mix, so oil possesses the first component of an antifoam compound: it is insoluble in water and water-based liquids. Any oil that won't dissolve in the stock will work, and vegetable and mineral oils are often used. Wax is usually added to increase the compound's efficiency. Oil based antifoams are often used to break up surface foams.
Silicone-based antifoams are made of hydrophobic silica; that is silica that will not mix with water. The silica is dispersed in silicone oil (oil based on silica rather than carbon). Silicone-based antifoams work well on surface foams and are also useful for releasing entrained air from the stock. They are also effective for preventing foam formation.
Water-based antifoams are oils and waxes in a water base. The oils and waxes are distributed through the stock by the water base, and they are most useful for releasing entrained air from the stock. Among other uses, they have been used successfully in paper manufacturing and waste water treatment processes.
Some antifoam compounds contain polypropylene glycol and polythene glycol. Delivered as water-based emulsions, as oils, or in water solutions, they are particularly useful when foaming deposits are a problem.
Alkyl acrylate polymers are also used in some antifoam compounds. Usually delivered in a solvent carrier, they are particularly useful in applications when the release of entrained air is more important than breaking up surface foam.
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