j. Soc. Cosmet. Chem., 30, 105-125 (March/April 1979) Antifoams R. D. KULKARNI, E. D. GODDARD and M. R. ROSEN Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, NY 10591. Received August I, 1978. Synopsis A general background on the subject of ANTIFOAMS is presented which includes their classification, preparation and testing. Emphasis is given to silicone-based compositions whose introduction in this field some three decades ago led to major changes in antifoaming technology. Fundamental mechanisms involved in the antifoaming process are outlined and a description is given of some of the mechanisms of foam stabilization. Lastly, a new, experimental antifoam, of "transient character," is described and its possible applications are outlined. Its activity in aqueous foaming systems lasts for a limited period thereafter the full foaming power of the solution is restorod. INTRODUCTION A foam is an aggregate of bubbles. More precisely, it is a dispersion of gas in a liquid where liquid forms the continuous phase. The phenomenon of foaming is primarily governed by the properties of the interfaces involved and foaming is always accompanied by an increase in the interfacial area of the system and hence its total energy. Thus, in a strict thermodynamic sense, foams are basically unstable and are therefore self-destroying. Their formation requires a net input of energy. The ease of their formation or their stability is, among other factors, determined by the gas/liquid interfacial energy. Pure liquids do not foam and therefore, for foaming, at least one more component is required. For persistent foams one needs the presence of a surface active ingredient. In most practical systems the foam stabilizing component is either a soluble surfactant, a finely divided solid, or both. While the mechanism of foam stabilization is not well understood, the role of solids, in particular, is least understood. Nevertheless it is well recognized that foams derive their stability by offering resistance to the external as well as internal stresses that can cause foam destruction. For reviews on the subject of foam stabilization see references 1-7. Depending upon the conditions prevailing in the solutions, a generated foam can span a remarkable range of stabilities, which can vary from milliseconds to almost unlimited duration. In many practical operations foams last long enough to interfere with physical or chemical processing and this has warranted the development of meap,_s for 105

106 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS their rapid destruction. Such needs can be visualized for a wide spectrum of systems ranging from waste water treatment, industrial fermentation, textile or paper process- ing, pharmaceutical manufacture, and various filling operations, to domestic machine dishwashing, laundering and cooking. In general, foam control is achieved either by destroying the existing foam, i.e., defoaming, or by preventing the formation of the foam. This is generally accomplished through mechanical, thermal or chemical means. Of these three techniques, chemical means are by far the most efficient for reasons which will be presented in some detail, and are the most widely used. Accordingly, the present paper will deal exclusively with foam prevention or destruction as achieved by chemical means. We define an antifoam as a chemical compound or mixture of chemical compounds which, when added in small amounts to a foaming system, either reduces or destroys its foaming tendency and so achieves a degree of foam control. Antifoams are, typically, highly surface active compounds having low solubility in the foaming solutions. A large number of fluids have been used in formulating antifoams, and they include naturally occurring oils and fatty materials, petroleum products and silicone oils. Traditionally, fatty acids and alcohols, and oils and fats have been used as foam inhibitors. These antifoams are able to perform very well for specific systems under particular conditions. Their main limitation is their large variation in performance with slight changes in the physical and chemical conditions prevailing in the foaming system. For example, lard oil and similar products can perform excellently as long as the operating temperatures are close to their melting point. However variation in the operating temperature can have a serious effect on their activity.' On the other hand, antifoams based on silicone oil are effective in a large number of systems under a broad range of operating conditions. Their versatility is, in fact, the major reason for their wide acceptance in the field. Accordingly, they will receive chief emphasis in this review. THEORETICAL TREATMENT OF FOAMING AND ANTIFOAMING In describing the essentials of this type of foam control, we will put appropriate emphasis on the fundamentals of foaming also. It is important to point out 1) that the fundamentals of both foaming (1-7) and antifoaming (8-20) are incompletely under- stood and 2) that the two phenomena have largely been treated independently with little attempt to understand the one in relation to the other. Our attempt is to treat foaming and antifoaming together so as to obtain an improved understanding of the total phenomenon. FOAMING A freshly formed foam passes through several different states before rupture and eventual collpase. During its lifetime, the bubble size and size distribution contin- uously change. The shape of the bubbles correspondingly changes from spherical to pentagonal-dodecahedral in the fully drained state. During drainage, the thickness of the lamellae and the foam density decrease in a rather complex manner. In addition to the force of gravity, films will thin in consequence of the well known capillary suction