Polydimethylsiloxane surface tension

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

Surfactants. Silicone surfactants lower the surface tension and emulsify incompatible ingredients. Copolymers of polydimethylsiloxane and polyoxyalkylene were the originally used surfactants. Surfactants with Si—O—C bonds are hydrolysable, but those with only a Si—C bond are non-hydrolysable. Several non-hydrolysable alkyl pendant silicone copolymers are used as surfactants for specific formulations (Seymour, 1992). 

The most important antifoam chemicals used to deal with non-aqueous foams are polydimethylsiloxanes. These are available in a range of molecular weights from low viscosity fluids of 1000 to several million cSt. Silicone fluids of the correct molecular weight have limited solubility in oil and a surface tension of 21 mN/m, lower than that of hydrocarbon oils. Fluorosilicones exhibit even lower surface tension, but their use is limited as they are considerably more expensive. 

The bulk of today s commercial applications of silicones utilise the intrinsic properties of the polydimethylsiloxanes, such as their low surface tension, thermal stability, good... 

Surface tension vs. temperature for PS and PTHF are shown in Figure 15. The data of Gains and Bender (17) on PS surface tensions (yGps) agree well with ours. However, our value for the surface tension of PTHF (yoPthf) is about 4.5 dynes/cm higher than Roe s value (18). The reason for this discrepancy is not clear. The y0i thf is always smaller than the yoPS by Ay = 3 dynes/cm, which is much smaller than the surface tension difference between the blocks of PS and polydimethylsiloxane. The time-dependent surface tensions of four blends (ST-PS, ST-PTHF, PS-PTHF, and PTHF-PS) were measured. To prepare the blends, the block or homopolymers were added in small amounts (0.3-1 wt %) to the homo-PS or PTHF. The mixture was completely dissolved in benzene, the solutions were quickly frozen by a dry ice-acetone mixture, and the samples were freeze dried. 

Polydimethylsiloxanes have many unique surface applications associated with the lowest surface tension of any non-fluorinated polymer and the high flexibility of the siloxane backbone [14]. 

Polydimethylsiloxanes achieve high concentrations at boundaries between different phases which accounts for applications such as release agents and defoamers. The combination of organic polyalkylene oxide pendants on the inorganic backbone of polydimethylsiloxane adds the ability to control wetting and surface tension. A typical structure is shown below and is alternately referred to as a block, graft, or pendant copolymer system. In earlier technology versions, the propyl link between the alkylene oxide and polysiloxane was replaced by a direct Si-O-C bond which is not hydrolytically stable. 

The precise value of the surface tension is the result of a number of interactions however, the values for high polymers vary from about 20 mN m for highly hydrophobic materials such as polydimethylsiloxane (PDMS) and polyfluorocarbons to values of the order of 45 mN m for polar materials such as polyesters and polyamides. The surface tension of water has a value of 80 and hence most materials are only mildly hydrophilic. 

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