Silicone elastomers dimethyl

Dimethyl Silicone Elastomers They are having linear polymers of very high Molecular weight within the range of 30,000-7,00,000. In order to prepare these polymers, dimethyl dichlorosilane is made to dissolve in ether and the solution is then mixed with excess of water. 

Compared to dimethyl silicone elastomers, vinyl silicone elastomers have much reduced compression set. A further point is that although silicone elastomers are mostly reinforced with silica, special purpose electrically conducting elastomers are filled with carbon black. Carbon black does not inhibit cure by less active peroxides and may be used with vinyl silicone elastomers. This filler does, however, inhibit vulcanization by benzoyl peroxide (and its derivatives) and therefore cannot be added to dimethyl silicone gums. 

In these elastomers, 5—15% of the methyl groups of polydimethylsiloxane are replaced by phenyl groups to give materials with especially good low temperature properties. The relatively large phenyl groups reduce intermolecular forces and retard the tendency of the polymer to stiffen as the temperature is lowered. Thus dimethyl silicone elastomers lose their elasticity at about —50°C whereas phenyl silicones retain their elasticity at temperatures as low as —lOO C. 

Silicones are generally very fire resistant, depending on their formulation. Certain silicone elastomers used as encapsulants and cable insulation have excellent values of limiting oxygen index (LOI) or UL rating. A specially processed dimethyl silicone fluid, Dow Corning 561, used as a transformer coolant is far more fire resistant than mineral oil. 

Silicone rubbers have emerged during the last 30 years as highly versatile elastomeric materials available in a variety of forms and used in almost every major industry. Based upon high molecular weight polymers of dimethyl siloxane they exhibit the properties and performance characteristics traditionally expected of silicone materials. Industry generally regards silicone elastomers as two distinct product types ... 

Synonyms Dimethyl silicones and siloxanes, reaction prods, with silica Uses Rheology control agent, reinforcing agent, free-flow agent for adhesives. coatings, powd. coatings, defoamers, elastomers, inks, lubricants. liq. resins, sealants, silicones, toners thixotrope for vinyl ester resins... 

Cold grades, that is, one-component room temperature vulcanizable elastomer grades (RTVs) dominate in the case of liquid silicone rubbers. In these cases, the rubbers are branched poly(dimethyl siloxanes) with silanol end groups that can be cross-linked with tetrabutyl titanate or methyl triacetoxy silane. The cross-linking starts on contact with the humidity in the air, whereby in the case of methyl triacetoxy silane, for example, acetic acid is liberated and the methyl trihydroxysilane produced reacts with the silanol groups of the polymer ... 

The basic elastomer for silicone rubber is a linear polysiloxane, buUt from dimethyl sUoxane units with a small amotmt methylvinyl sUoxane groups. The ends of the chains are formed from trimethyl sUoxane- or dimethylvinyl sUoxane groups. The structure of the basic elastomer is shown in Fig. 24.1. Peroxides are usuaUy used for the crosslinking of silicone rubbers (for example dicumyl peroxide or 2,4-dichlorobenzoyl peroxide). 

Inorganic Elastomers The major commercial inorganic elastomer is poly(dimethyl siloxane), known widely as silicone rubber (see Table 9.4). This specialty elastomer has the lowest known glass transition temperature, Tg = -DO C (148) it also serves as a high-temperature elastomer. A common application of this elastomer is as a caulking material. It cross-links on exposure to air. 

Dimethyl silicone polymers are, because of their relatively low polarities, relatively compatible with hydrocarbons and easily swollen by fuels. Substitution for the methyl functionality by polar cyanoethyl and trifiuoropropyl groups imparts significant polarity to these elastomers and makes them resistant to swelling by nonpolar substances these specialized polymers can be compounded into very useful adhesives and sealants for applications requiring fuel resistance. 

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