Antifoaming mechanisms, silicone

Solid-Phase Components. Dispersed sohds are vital ingredients in commercial antifoam formulations. Much of the cmrent theory on antifoaming mechanism ascribes the active defoaming action to this dispersed solid phase with the liquid phase primarily a carrier fluid, active only in the sense that it must be surface-active in order to carry the solid particles into the foam films and cause destabilization. For example, PDMS, despite its considerable effectiveness in nonaqueous systems, shows little foam-inhibiting activity in aqueous surfactant solutions. It is only when compounded with hydrophobic silica [7631-86-9] to give the so-called silicone antifoam compounds that highly effective aqueous defoamers result. The three main solid-phase component classes are hydrocarbons, silicones, and fluorocarbons. 

Although glycol-water formulations are not prone to foaming, mechanical and chemical factors may cause foaming in the system. The use of corrosion inhibitors and the presence of contaminants may enhance the tendency to form foams. For these reasons, antifoaming agents, such as silicones, polyglycols, or oils, are sometimes added. 

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

The prevention of foaming and the destruction of existing foams is often a matter of practical importance for example, polyamides and silicones find use as foam inhibitors in water boilers. Antifoaming agents act against the various factors which promote foam stability (described above) and, therefore, a number of mechanisms may be operative. 

Polysiloxanes (silicones) are one of the most studied classes of polymers. They exhibit a variety of useful properties not common to non-metal-contain-ing macromolecules. They are characterized by combinations of chemical, mechanical, electrical, and other properties that, when taken together, are not found in any other commercially available class of materials. The initial footprints on the moon were made by polysiloxanes. Polysiloxanes are currently sold as high-performance caulks, lubricants, antifoaming agents, window gaskets, O-rings, contact lens, and numerous and variable human biological implants and prosthetics, to mention just a few of their applications. 

Activated dimethicone (activated polymethyl-siloxane) is a mixture of liquid dimethicones containing finely divided silica to enhance the defoaming properties of the silicone. The mechanism by which dispersion of colloidal silica antifoams improves their action is not well understood. 

The rate of fixation of the monochlorotriazinyl dyes is very much slower. The result is that there is abundant opportunity for the normal acid dyeing mechanism to give uniform distribution of the dye molecules before they become permanently anchored by covalent bond formation. It has been found that, for some unknown reason, cationic surface active compounds tend to reduce skitteriness, if a non-ionic compound is also present to prevent the precipitation of the cation-dye complex. In order to prevent foaming which can become troublesome the addition of a silicone antifoaming product has been recommended. 

Zemeth, Z., Racz, G. and Koezo, K., Foam control by silicone polyethers - mechanism of cloud point antifoamers, J. Colloid Interface ScL, 207, 386-394 (1998). 

Hydrophobic antifoams, the second group, attract surfactant from the foam bubbles and lead to film, rupture dispersion of hydrophobed silica particles in silicone oil is believed to occur following this mechanism [49]. Another hypothetical mechanism for a silicone defoamer is proposed by Dippenaar [50], Garrett [51], and Berg [52], in which the hydrophobic particles bridge the foam lamella, dewetting occurs at the surface of the particle, and, consequently, the foam breaks. 

The mechanisms of antifoaming and defoaming are not so well understood. However, these compounds interfere in one or more ways with the stabilization methods discussed earlier. Potential mechanisms are to increase surface tension or to decrease elasticity, bulk- or surface viscosity, or electrostatic repulsion in the lamellae. Defoamers are formulations (multicomponent products) and typically contain various oils (e.g. silicone oUs) and hydrophobic particles making the possible interactions quite versatile. Some of the basic requirements of a defoamer are limited solubility in aqueous phase so that it goes to the interface, surface tension should be below the value of foam liquid to enhance entry and spreading at air-hquid interphase, and low interfacial tension with foaming liquid to enhance spreading at air-hquid interface. Formation and use of oil lenses is an often used route to foam destruction. 

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