Polydimethylsiloxanes

Name(s) polydimethylsiloxane, dimethylpolysiloxane, silicone oil, silicone fluid CAS 9016-00-6 

Color transparent, colorless Odor none (slight oily) State liquid up to 1,000,000 daltons 

Moisture content, % traces Water solubility, % traces pH 7 

NFPA flammability 0 NFPA health 0 NFPA instahility 0 

Packaging class not regulated Autoignition temp., °C 418- 490 Flash point, °C 79 to 326 

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Anti-foaming agents polydimethylsiloxanes, fluorosilicones, acrylates. 

Fig. IV-10. Wave-damping behavior of polydimethylsiloxane heptadecamer on water at 25°C at (a) 60 cps and (b) 150 cps. Curve (c) gives the ir-

Another approach is to use the LB film as a template to limit the size of growing colloids such as the Q-state semiconductors that have applications in nonlinear optical devices. Furlong and co-workers have successfully synthesized CdSe [186] and CdS [187] nanoparticles (<5 nm in radius) in Cd arachidate LB films. Finally, as a low-temperature ceramic process, LB films can be converted to oxide layers by UV and ozone treatment examples are polydimethylsiloxane films to make SiO [188] and Cd arachidate to make CdOjt [189]. 

Polydimethylsiloxane [9016-00-6] [63148-62-9], or sikeone, is used at a level of approximately 10 parts per million to control foam in food products. The sikeone disperses itself throughout the kquid film that makes up the foam and causes it to coUapse (15). 

Syltherm XLT is a polydimethylsiloxane intended for Hquid-phase systems which operate at low temperatures. Syltherm 800 is a modified dimethylsiloxane polymer intended for Hquid-phase systems. The recommended maximum fluid temperature is greater than the autoignition temperature. 

The manufacture of polydimethylsiloxane polymers is a multistep process. The hydrolysis of the chlorosilanes obtained from the direction process yields a mixture of cycHc and linear sdanol-stopped oligomers, called hydrolysate (eq. 7) (21). In some cases, chloro-stopped polymers can also be obtained (59). 

The organotin sdanolate can then react with the polydimethylsiloxane diol by either attack on the SiOC bond or by sdanolysis of the SnOC bond (193,194). Other metal catalysts include chelated salts of titanium and tetraalkoxytitanates. Formation of a cross-linked matrix involves a combination of the three steps in equations 24—26. 

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,... 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Silk (qv) suture is made from the threads spun by the silkworm Bombjx mori. The fiber is composed principally of the protein fibroin and has a natural coating composed of sericin gum. The gum is usually removed before braiding the silk yams to make sutures in a range of sizes. Fine silk sutures may be made by simply twisting the gum-coated silk yams to produce the desired diameter. White silk is undyed. Silk is either dyed black with logwood extract or blue with D C Blue No. 9. The suture may be uncoated or coated either with high molecular weight polydimethylsiloxane or with wax. 

The most widely used sUicones are polymers of methyl(hydrogen)sUoxane and of dimethylsiloxane. Polydimethylsiloxane is the basic polymer used in sUicone repeUents. If the polymer is terminated with methyl groups it is inert however, if it is terminated with hydroxyl groups, it can be cross-linked. Continuous, durable coatings result from the use of curable blends of polydimethyl siloxane and polymethyl(hydrogen)sUoxane. The sUicone finish encapsulates individual fibers. 

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. 

Condensation siHcone materials are based on hydroxyl-terminated polydimethylsiloxane and are made in the form of two paste or paste—Hquid catalyst systems. 

Polyisobutylene and similar copolymers appear to "pack" well (density of 0.917 g/cm ) (86) and have fractional free volumes of 0.026 (vs 0.071 for polydimethylsiloxane). The efficient packing in PIB is attributed to the unoccupied volume in the system being largely at the intermolecular interfaces, and thus a polymer chain surface phenomenon. The thicker cross section of PIB chains results in less surface area per carbon atom. 

In principle, A can be any polymer normally regarded as a hard thermoplastic, eg, polystyrene, poly(methyl methacrylate), or polypropylene, and B can be any polymer normally regarded as elastomeric, eg, polyisoprene, polybutadiene, polyisobutylene, or polydimethylsiloxane (Table 2). 

Silicones. Polydimethylsiloxanes, polydiphenylsiloxanes, and polymethylphenylsHoxanes are generally called siUcones (see Silicon COMPOUNDS, silicones). With a repeating unit of alternating siUcon-oxygen, the siloxane chemical backbone stmcture, siUcone possesses excellent thermal stabiUty and flexibility that are superior to most other materials. Polydimethjisiloxane provides a very low glass-transition temperature T material but is suitable for use... 

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. 

Since both Si—O and Si—CHj bonds are thermally stable it is predictable that the polydimethylsiloxanes (dimethylsilicones) will have good thermal stability and this is found to be the case. On the other hand since the Si—O bond is partially ionic (51%) it is relatively easily broken by concentrated acids and alkalis at room temperature. 

A typical condensation system involves the reaction of a silanol-terminated polydimethylsiloxane with a multi-functional organosilicon cross-linking agent such as Si(RO)4 Figure 29.8). Pot life will vary from a few minutes to several hours, depending on the catalysts used and the ambient conditions. Typical catalysts include tin octoate and dibutyl tin dilaurate. 

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