Antifoams / Defoamers
Antifoaming agents vary greatly in effect depending on the kinds of foaming liquids. An antifoaming agent may be effective in some liquids but ineffective in others.
Therefore, various antifoaming agents have been developed and are used according to the kinds of the foaming liquids. Some of the antifoaming agents commonly used are grouped by their constituents into silicone, surfactant, paraffin, and mineral oil types.
Silicone resin (dimethyl polysiloxane)
Antifoaming agents containing silicone resin, which are emulsion-dispersed in water with an emulsifier (a surfactant), are called emulsion-type antifoaming agents. Some antifoaming agents, in which a fine powder of silicon dioxide is added to promote their antifoaming effect, are also available and are applied in the textile or food processing industries, and to the various kinds of industrial and municipal wastewater treatment. This type of antifoaming agent shows a constant effect on a wide range of foaming liquids, and thus can be applied to various fields.
On the other hand, in papermaking and metal-finishing, where the treated wastewater is reused, the antifoaming agents are not used much as they can produce an occasional undesirable effect on the finished product.
This type of antifoaming agent is mostly emulsion-dispersed surfactants of lower HLB values with emulsifiers, and is used for textiles, metal-finishing, papermaking, and wastewater treatment.
Some of them have constituents comprising reacting fatty acid or higher alcohol with polyethyleneoxide or polypropyleneoxide.
These types of compounds easily disperse in water as emulsions, and are widely applied to paper-board making, paint, synthetic rubber, and synthetic resin industries. (Emulsion-dispersed in water like this is called a self-emulsification type.)
The self-emulsification type is more durable in effect than the emulsion type and is often used for paper-board making which requires a long lasting antifoaming effect. This type, however, has a fault that an excessive addition will exert an unfavorable effect on paper quality (sizing degree).
This type is made by emulsion-dispersing paraffin wax or its modified products with emulsifiers, and is used in the same application fields as the surfactant type emulsion antifoaming agents.
Both the surfactant- and paraffin-emulsion types hardly affect paper quality, and thus are applied to the manufacturing process of wood-free paper.
Mineral oil is mixed with metal soap, silicone oil, or silicon dioxide in order to enhance the antifoaming effect. Various surfactants are added to provide an easier diffusion of the mineral oil over the surface of the foaming liquid or to give homogeneous dispersion of the metal soap in the mineral oil.
These antifoaming agents are insoluble or undispersible in water and are mainly used in the pulping process, where the emulsion or the self-emulsifying type cannot be applied.
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