Polydimethylsiloxane Oils

Silicone Fluids. Sihcone fluids are used in a wide variety of appHcations, including damping fluids, dielectric fluids, poHshes, cosmetic and personal care additives, textile finishes, hydraiflic fluids, paint additives, and heat-transfer oils. Polydimethylsiloxane oils are manufactured by the equihbrium polymerisation of cycHc or linear dimethyl silicone precursors. Trifunctional organosilane end groups, typically trimethylsilyl (M), are used, and the ratio of end group to chain units (D), ie, M/D, controls the ultimate average molecular weight and viscosity (112). Low viscosity fluids,... 

If a material other than a hydrocarbon is used as the base material of an ointment-like system, the ointment bears the name of its principal ingredient. There are silicone ointments that contain polydimethylsiloxane oil in large proportion. These reportedly act as excellent water barriers and superior emollients. Some are actually used to protect skin from the undesirable effects of long immersion in water. 

Polydimethylsiloxane oils, properties and applications of, 22 573-575, 576 Polydimethylsiloxane diol, in silanol condensation, 22 566 Polydimethylsiloxanes, 10 3 Polydioxanone, bioresorbable polymer,... 

If polydimethylsiloxane oils are emulsified together with fine particulate solids, e.g. fumed silicas, very effective defoaming agents are obtained for aqueous media e.g. for the production of aqueous organic polymer dispersions and for dyeing processes in the textile industry. 

For surfactants with other types of hydrophobic groups, the CMC values are less readily available because far fewer measurements have been made, especially on pure compounds. However, the oil/water interfacial tensions are 56 and 46 dyn/cm for fluorocarbon and polydimethylsiloxane oils respectively, compared to the value of 52 dyn/cm for hydrocarbon/water. Hence from the sizes of the hydrophobic groups (see Table 21.1 below), the magnitude of the hydrophobic effect, and hence their CMCs can be estimated. This clearly shows that the well-known lower CMCs for fluorocarbon surfactants compared to normal derivatives arise from the much larger size of fluorocarbons, rather than any magic structuring of water ... 

Silica hydrosol of 3-5 nm particles emulsified with mineral oil was water-stripped and reacted with polydimethylsiloxane oil to obtain a hydrophobic antifoam agent (463). 

The theory of filling submicroscopic surface irregularitiu with colloidal silica, as on painted surfaces, has been described by Rusher and Yates (579). To reduce soiling of a hydrophilic surface such as upholstery fabric by making it both hydrophobic and soil resistant, a polydimethylsiloxane oil is incorporated with colloidal silica... 

In contradiction of this expectation, Denkov and cowoikers [34-36] have shown that p for entry of oil drops into the air-water surface of surfactant solutions is usually essentially independent of the equatorial radii of those drops for submicellar and relatively dilute micellar solutions (where concentrations are <10 x CMC). Systems included dodecane (and other oils) in aqueous salt solutions of sodium dodecylben-zene sulfonate and polydimethylsiloxane oil in sodium dodecyl polyoxyethylene sulfate solutions. Experimental results [36] for the critical applied capillary pressures, p , as a function of equatorial drop radius for the latter system, are presented in Figure 3.7 to exemplify typical behavior. For relatively low surfactant concentrations, pf" is seen to be essentially constant. An exception concerns an extremely high surfactant concentration of 200 x CMC where p became strongly dependent... 

Polydimethylsiloxane oils are characterized by low air-oil surface tensions of -21 mN m at ambient temperatures. Values of the air-oil, surface tensions for typical oils in pure water are shown in Table 3.2. The oil-water surface tension against pure water is significantly lower than that of... 

Entry and Spreading Coefficients for Some Polydimethylsiloxane Oils on Pure Water... 

Spreading Pressures, Entry, and Spreading Coefficients for Polydimethylsiloxane Oils on Aqueous Micellar Surfactant Solutions... 

FIGURE 3.21 Radial spreading rate of polydimethylsiloxane oils as function of shear viscosity. Substrate 14.8 mM aqueous tetradecyltrimethylammonium bromide solution. (Reprinted from Colloids Surf A, 122, Bergeron, V., Cooper, P., Fischer, C., Giermanska-Kahn, J., Langevin, D., Pouchelon, A., 103. Copyright 1997, with permission from Elsevier.)... 

Plateau border with trapped drops of polydimethylsiloxane oil... 

Comparison of Applied Critical Capillary Pressures Entry Spreadings and Bridging Coefficients with Foam Behavior for Neat Polydimethylsiloxane Oils with Various Surfactant Solutions... 

FIGURE 6.2 Deactivation of antifoam effect of polydimethylsiloxane oil alone. Polydimethylsiloxane, of viscosity 50 mPa s, was added neat to solution of 5 x 10" M sodium nonyl benzene sulfonate in 3 x 10 M NaCl. (a) Volume of air in foam generated by shaking 0.025 dm of a surfactant solution containing 0.63 g dm" of the oil in a 0.1 dm measuring cylinder—each cycle consisting of 30 shakes, (b) Effect of several shake cycles on drop size distribution of the oil (by Malvern Sizer). (After Yorke, J.W.H., unpublished work.)... 

The polydimethylsiloxane oils used for antifoams usually spread on the air-water surfaces of surfactant solutions (see Section 3.6.2). At equilibrium, this process produces either complete wetting and duplex films for which 5 = 0 or pseudo-partial wetting and oil films in contact with lenses of bulk oil for which 5 0 (see Section 3.6.2.1). It has been shown by Racz et al. [3], and later confirmed by Denkov et al. [6, 7, 21], that deactivation of hydrophobed silica-polydimethylsiloxane antifoams correlates with the disappearance of this spread oil film. These studies used solutions of both anionic (SDS [3] and AOT [6,7]) and non-ionic surfactants (alkyl glu-copyranoside) [21]. Loss of the spread layer during deactivation is accompanied by an increase in surface tension to that of the pure surfactant solution [6]. It has also been directly observed using ellipsometry [21]. This finding is key to the understanding of deactivation because the presence of a spread layer of polydimethylsiloxane at the air-water surface is a clear indicator that oil has emerged into that surface. 

Not only must coalescence be involved in the antifoam deactivation process but it must obviously also be involved in the reactivation of deactivated antifoam dispersion by addition of fresh polydimethylsiloxane oil. Here the effectiveness of the added oil has been shown to decrease with decreasing drop size, being most effective if neat oil is simply added to the deactivated dispersion. It is obvious that this effect must concern the efficiency of the coalescence of neat oil drops with silica-rich, but... 

There is little experimental evidence in the scientific literature concerning the existence of this supposed optimum. Indeed there have apparently been few published studies of the effect of oil viscosity on both the effectiveness and deactivation rates of hydrophobed silica-polydimethylsiloxane antifoams. Evidence that increase in polydimethylsiloxane oil viscosity can reduce the rate of deactivation of hydrophobed silica-oil antifoams is, for example, presented by Racz et al. [3]. These authors repeatedly pulled films, using a film frame, from surfactant solution upon which antifoam had been spread. The time for film rupture was measured. After several hundred films had been pulled, the film rupture time increased dramatically indicating partial antifoam deactivation. Increase in the viscosity of the oil in a hydrophobed silica-oil antifoam, from 200 to 1000 cSt, increased the number of films that could be pulled by more than 50%, implying a decrease in rate of deactivation [3]. 

It is, however, obvious from Figure 6.10 that increasing the oil viscosity from 5 to 1000 mPa s produces a decrease in rate of deactivation. It has been shown (see Section 3.6.2.2) [4, 41] that the spreading rate of polydimethylsiloxane oils on water... 

Mixtures of polydimethylsiloxanes oils and hydrophobed silicas have formed the basis of many patents concerned with the control of detergent foam in front-loading drum-type textile washing machines. It is likely that this type of antifoam is actually included in many commercial detergent powders designed for that application. 

It is clear from the figure that increasing the viscosity, and therefore the molecular weight, of polydimethylsiloxane oils decreases the solubility and slightly increases the persistence of antifoam effects up to higher concentrations of hexane. 

Chem. Descrip. Polydimethylsiloxane oil compd. with mineral filler Ionic Nature Nonionic CAS 9016-00-6... 

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