Defoaming liquids

Many similar hydrocarbon duids such as kerosene and other paraffinic and naphthenic mineral oils and vegetable oils such as linseed oil [8001-26-17, com oil, soybean oil [8001-22-7] peanut oil, tall oil [8000-26-4] and castor oil are used as defoamers. Liquid fatty alcohols, acids and esters from other sources and poly(alkylene oxide) derivatives of oils such as ethoxylated rosin oil [68140-17-0] are also used. Organic phosphates (6), such as tributyl phosphate, are valuable defoamers and have particular utiHty in latex paint appHcations. Another important class of hydrocarbon-based defoamer is the acetylenic glycols (7), such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol which are widely used in water-based coatings, agricultural chemicals, and other areas where excellent wetting is needed. 

Acoustic transducers operating at 10 and (or) 20 kHz are capable of defoaming liquids provided the acoustic source is placed above the liquid surface upon which the foam is being generated. 

Defoamers (qv) are available in several forms, composed of many different materials. Historically, paste and soHd defoamers were used extensively. Composed of fatty acids, fatty amides, fatty alcohols, emulsifiers (and mineral oil [8012-95-1] in the high soflds paste emulsions), these defoamers required emulsification (brick) or dilution (paste) before use. Liquid defoamers have become the preferred form, insofar as concern about handling and ovemse have been overcome. 

Liquid-Phase Components. It is usual to classify organic Hquids by the nature of the polar or hydrophilic functional group, ie, alcohol, acid, ester, phosphate, etc. Because lowering of surface tension is a key defoamer property and since this effect is a function of the nonpolar portion of the Hquid-phase component, it is preferable to classify by the hydrophobic, nonpolar portion. This approach identifies four Hquid phase component classes hydrocarbons, polyethers, siHcones, and duorocarbons. 

The addition of defoamers can restore some of this lost strength, but only to a certain point. The defoaming mechanism usually relies upon diffusion of gas bubbles together in the liquid adhesive to form larger bubbles, which, in turn, rise to the surface and break. Diffusion generally decreases with increasing viscosity and stops when the gel point is reached, i.e., when the viscosity approaches infinity, accordingly ... 

Defoamer formulations currently contain numerous ingredients to meet the diverse requirements for which they are formulated. Various classification approaches are possible, including classification by application, physical form of the defoamer, and the chemical type of the defoamer. In general, defoamers contain a variety of active ingredients, both in solid and in liquid states, and a number of ancillary agents such as emulsifiers, spreading agents, thickeners, preservatives, carrier oils, compatibilizers, solvents, and water. 

Because lowering the surface tension is the most important physical property of a defoamer, it is reasonable to classify the defoamer by the hydrophobic operation of the molecule. In contrast, the classification of organic molecules by functional groups are often polar and hydrophilic (i.e., alcohol, acid, and salt are common in basic organic chemistry). Four classes of defoamers are known as liquid phase components ... 

Often, dispersed solids are active in defoaming in suitable formulations. Some liquid defoamers are believed to be active only in the presence of a solid. It is believed that a surface-active agent present in the system will carry the solid particles in the region of the interface and the solid will cause a destabilization of the foam. 

For example, a synergistic defoaming occurs when hydrophobic solid particles are used in conjunction with a liquid that is insoluble in the foamy solution [652]. Mechanisms for film rupture by either the solid or the liquid alone have been elucidated, along with explanations for the poor effectiveness, which are observed with many foam systems for these single-component defoamers. 

Several factors contribute to the dual nature of silicone defoamers. For example, soluble silicones can concentrate at the air-oil interface to stabilize bubbles, while dispersed drops of silicone can accelerate the coalescence process by rapidly spreading at the gas-liquid interface of a bubble, causing film thinning by surface transport [1163]. 

The defoamer is dispersed in fine droplets in the liquid. From the droplets, the molecules may enter the surface of the foam. The tensions created by this spreading result in the eventual rupture of the film. 

The above statements are adequate for liquid defoamers that are insoluble in the bulk. Experience has proven, however, that certain dispersed hydrophobic solids can greatly enhance the effectiveness of defoaming. A strong correlation between the effectiveness of a defoamer and the contact angle for silicone-treated silica in hydrocarbons has been established [300]. It is believed that the dewetting process of the hydrophobic silica causes the collapse of a foam by the direct mechanical shock occurring by this process. 

Liquid raw materials such as polymer emulsions, defoamers, pigment dispersions, dye solutions, dispersing aids and emulsifiers are all products that can themselves become infected with micro-organisms if not produced from non-contaminated ingredients, under good manufacturing conditions and with an effective preservative. 

Surface wave, 17 422. See also S-wave Surfactant adsorption, 24 119, 133-144 at the air/liquid and liquid/liquid interfaces, 24 133-138 approaches for treating, 24 134 measurement of, 24 139 at the solid/liquid interface, 24 138-144 Surfactant blends, in oil displacement efficiency, 13 628-629 Surfactant-defoamers surface tension, <5 244t Surfactant-enhanced alkaline flooding,... 

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These agents may operate via a number of mechanisms, but the most common ones appear to he those of entry and/or spreading. The defoamer must first of all he insoluble in the foaming liquid for these mechanisms to function. Second, the surface tension of the defoamer must be as low as possible. The interfacial tension between defoamer and foamer should be low. but not so low that emulsification of the defoamer may occur. Third, the defoamer should be dispersible in the foaming liquid. It was first shown in I fM8 that thermodynamically the entry of the defuamcr droplet into a bubble surface occurs when the entering coefficient has a positive value. The physics of bubbles is described in entry on Foam. 

The more eflicienl defoaming mechanism of spreading involves transpon of underlying liquid so that the liquid is replaced by a film of deloumcr that docs not support foam. A drop of oleic add added to water spreads at a velocity of 30 miles (48.2 kilometers) per hour. The mechanical shock to a film by such a defoamer may be considerable. In addition lo the foam-destroying aspect, spreading is also of value as a defoamer-dispersion method, particularly in viscous or poorly stirred systems. 

In many applications we want to prevent the formation of foam or get rid of already existing foam (see, for example, Table 11.1). Usually chemicals are added to achieve this (see Ref. [564] for an introduction). We distinguish between antifoamers and defoamers. An-tifoamers are added to the liquid prior to foam formation and act to prevent or inhibit foam formation. Defoamers or foam breakers are added to eliminate existing foam. They can only reach the outer surface of a foam. 

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