Silicone surfactants foam formation

Snow, S. A. Stevens, R. E. The Science of Silicone Surfactant Application in the Formation of Polyurethane Foam. In Silicone Surfactants-, Hill, R. M., Ed. Surfactant Science Series Dekker New York, 1999 Vol. 86, Chapter 5, pp 137-158. 

The unique surface characteristics of polysiloxanes mean that they are extensively used as surfactants. Silicone surfactants have been thoroughly studied and described in numerous articles. For an extensive, in-depth discussion of this subject, a recent chapter by Hill,476 and his introductory chapter in the monograph he later edited,477 are excellent references. In the latter monograph, many aspects of silicone surfactants are described in 12 chapters. In the introduction, Hill discusses the chemistry of silicone surfactants, surface activity, aggregation behavior of silicone surfactants in various media, and their key applications in polyurethane foam manufacture, in textile and fiber industry, in personal care, and in paint and coating industries. All this information (with 200 cited references) provides a broad background for the discussion of more specific issues covered in other chapters. Thus, surfactants based on silicone polyether co-polymers are surveyed.478 Novel siloxane surfactant structures,479 surface activity and aggregation phenomena,480 silicone surfactants application in the formation of polyurethane foam,481 foam control and... 

Several other experimental findings support the existence of a microceliular structure in oligomeric foams. Thus, Oween and Denis ) observed an anomalous pattern (in the expression of the authors) for certain types of silicone surfactants the liquid foam system consists of gas bubbles the sizes of which differ by several orders of magnitude. The possibility of formation of very small gas bubbles after a marked reduction of the surface tension coefficient in poly-lurethane formulations has been reported by Dubyaga and Tarakanov ). 

Silicone organic hybrids play a critical role as surfactants in the formation of polyurethane foam (5). These surfactants are... 

FIGURE 5.64 Consecutive cycles of exhaustion and reactivation of mixed oU-silica compound in 10mM solution of the anionic surfactant sodium dioctyl-sulfosuccinate (AOT). An initially active antifoam (defoaming time 5 s) gradually loses its activity with the number of foam formation/destruction cycles in a standard shake test. The introduction of silicone oil results in a perfect restoration of the antifoam activity. Five exhaustion curves (indicated by roman numbers the symbols indicate the experimentally measured defoaming time) and the corresponding four reactivation events (the vertical dashed lines) are shown. (Adapted from Denkov et ah, Langmuir, 16, 2525, 2000.)... 

The real challenge in polyurethane foam formation is to control the chemical and physiochemical processes up to the point where the material finally sets. The sequence and the rate of the chemical reactions are predominately a function of the catalyst and the reactivity of the basic raw materials, polyol and isocyanate. The physiochemical contribution to the overall stability and processability of a system is provided by the silicone surfactants. Optimum foaming results will be achieved only if the correct relationship between chemistry and physics exists [4]. 

One of the technically and commercially most interesting applications of silicone surfactants is their use in the production of polyurethane (PU) foams (Fig. 13). These foams are formed by the reaction of polyols and isocyanates. The finished foams typically have cell sizes in the millimeter range and below with densities mostly less than 50 kg/m, thus forming systems with very large surfaces. The process of foam formation is complex and consists of different phases, which require a variety of properties of the used surfactants. The flexibility of silicone chemistry, especially the broad variety of silicone polyether chemistry, is particularly suited to meet these different requirements. 

The formation of gas bubbles in the liquid phase of the reaction mixture is governed by the presence of nucleation sites. The silicone surfactants allow the development of a large number of nucleation sites, which are needed in order to get a fine and regular distribution of foam cells. Thermodynamically, a small number of large bubbles would be more favorable because of the lower... 

Inverse structures have been published for use in polyurethane foam formation [43]. Monofunctional siloxane chains are attached to a multifunctional polyether backbone. However, significant advantages over classical silicone surfactant structures could not demonstrated. A major drawback of this approach is the difficult synthesis of monofunctional siloxanes. 

Disperse the titanium dioxide 2190 ( Ex Kronos) in Orotan 165 (Rohm Haas, anionic dispersant) and Disperse Ayd W22 (Daniel Products, surfactant), with water as carrier. Antifoam (Byk 035 from Byk Chemie) and flash rust inhibitor (Emadox NA from Labema) are added. When dispersed, add the pigment paste to the let down mix. Add butyl benzyl phthalate plasticiser and Dehydran 1293 (modified polysiloxane defoamer from Henkel, to reduce foam formation on application) and mix well. Adjust viscosity with water. Neocryl A633 is a 42%nvc acrylic/styrene dispersion from Zeneca. Silwet L-77 (Osi) is a nonionic silicone surfactant to improve wetting. 

Various types of equipment and techniques have been employed to suppress foam formation in biological and process equipment. These include both chemical and mechanical methods. Chemical methods include various defoaming chemicals (silicone oils, non-ionic surfactants, etc.) Mechanical methods employ sprays, wire mesh elements, heat, live steam injection, air or steam-operated ejectors, sonic horns, vacuums, centrifuges, and the use of large re-... 

The possibility to use Cm as a parameter characterising foam inhibition has been demonstrated for the first time in [60]. It was shown that the increase in the concentration of silicon oil Caf (antifoam) led to increase in Cm- That is why it was proposed to used the ratio Caf/Cm as a quantitative measure of the defoaming ability. However, it should be noted that the silicon oil concentrations at which inhibition of black spot formation was observed, were very low (10 5-10 9 %). For that reason it is difficult to conclude definitely whether the system was a real solution or represented a diluted emulsion of the antifoam in the surfactant solution. 

More convenient objects for the study of Cm(Caf) dependence have been found to be the low molecular compounds with relatively good solubility (compared to the silicon oil), for example, fatty alcohols, acids and hydrocarbons [48,55]. The experiments commented below were performed with these antifoams. First of all the Cm of films in the absence of an antifoam was determined by gradually increasing the surfactant concentration. Then small doses of the antifoam were introduced in a solution with surfactant concentration slightly above Cm, until the formation of black spots became impossible. At the same time the concentration of antifoam saturation in the solution was fixed. Table 9.2 presents the antifoam concentrations at which black spot formation in microscopic foam films is inhibited. 

The formation of foam during the automatic dishwashing cycle is detrimental to the mechanical washing efficiency of the machine. To prevent foaming, LADD products often contain defoamers. The most commonly used hypochlorite-stable defoamers in LADDs are anionic surfactants such as alkyl phosphate esters and ethoxylated esters [41-43] and silicone oils [44], The structures of the first two types are shown in Figure 9.5. Also described in the patent literature is the use of aliphatic alcohols or acids as defoamers [45],... 

It has been mentioned above that foams can unfavourably affect the refining processes. Formation processes of non-aqueous foams are not well enough studied. In a vast review [264], comparison between aqueous and non-aqueous foams has been made. The stabilisation of non-aqueous foams (e.g. on a hydrocarbon basis) seems to be impossible with usual hydrocarbon surfactants because of the weak liquid-gas interfacial tension lowering gradients. Fluorinated or silicone-type surfactants can be used as eventually better stabilisers. These recommendations are, in our opinion, only applicable to the production of the so-called hardening foams (polystyrene foams, polyurethane foams etc.). 

Alternative Components Besides the surfactant, the antifoam is sometimes added into the aqueous phase in the case of strong agitation, because the foaming problem will disturb the formation of microspheres. When the stirring speed increases, more air is entrained and foam forms. So antifoams of silicon and nonsilicon constituents are used to increase the rate at which air bubbles are dissipated. 

Foam breakers may include inorganic ions such as calcium, which counteract the effects of electrostatic stabilization or reduce the solubility of many ionic surfactants, organic or silicone materials that act by spreading on the interface and displacing the stabilizing surfactant species, or materials that directly interfere with micelle formation. 

The hydrophobic interaction also occurs in other solvents. Solvophobic interactions have been exploited in nonaqueous media using surfactants based around silicone oils or fluorocarbons. The expulsion of both of these molecular groups by bulk liquids leads to adsorption at air-solution interfaces and to the formation of stable, nonaqueous foams, that are used in the production of both rigid and flexible isocyanate-based polymeric foams for insulation and cushioning applications [3]. 

Silicone polyethers are important nonionic surfactants that are also used in cosmetics and household chemistry. However, their most important application is the manufacture of polyurethane foams, both rigid and flexible ones. There are no substitutes for them and their role eonsists in the facilitation of mixing of foam components. They prevent from the formation of large bubbles, facilitate the control of fluidity of liquid mixture (that expands due to the bubble growth), and they enable accurate control of time and degree of foam opening. 

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