Vinyl groups silicone rubber

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

VMQ Silicone rubbers having both methyl and vinyl substituent groups on the polymer chain. 

FVMQ silicone rubber having fluorine, vinyl and methyl substituent groups on the... 

Hydrosilylation of unsaturated organosilicon compounds has also found several applications in molecular and polymer organosilicon chemistry. In particular, the addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane, catalyzed exclusively by Pt compounds and providing an activated cure for silicon rubber [10], has been of great practical importance. Hydrosilylation of the vinyl group at silicon seems to be effective synthetic method for preparation of oligomers and polymers with a linear or cyclolinear stmcture (polyhydrosilylation), and can occur either via the addition of dihydro-carbosilanes and -siloxanes to divinyl-silanes and -siloxanes [25, 26] or by intermolecular hydrosilylation [4] (eq. (1)). 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

Dimethyl silicone rubbers show a high compression set. (For example, normal cured compounds have a compression set of 20-50% after 24 h at 150°C.) Substantially reduced compression set values may be obtained by using a polymer containing small amounts of methylvinylsiloxane. Rubbers containing vinyl groups can be cross-linked by weaker peroxide catalysts. Where there is a high vinyl content (4-5% molar), it is also possible to vulcanize with sulfur. 

Poly(Dimethyl Siloxane) Sihcone Rubber, Usually Copolymer with Vinyl groups (VMQ) Poly(Dimethyl Siloxane) Copolymer with Phenyl-Bearing Siloxane and Vinyl Groups (PVMQ) Room Temperature Vulcanizing Silicone Polysulfide (ET and EOT) Polyurethane (AU and EU)... 

MethylvinyUluorosilicone Silicone rubbers containing pendant vinyl, methyl, and fluorine groups. Can be additionally cross-linked via vinyl groups. Has good resistance to petroleum products at elevated temperatures. 

Organofunctional polysiloxanes with silanol groups can be cold cured with methyl triacetoxysilane, tetrabutyl titanate, etc. On the other hand, organofunctional polysiloxanes with about 0.2% vinyl groups are hot cured with peroxides. All these silicon rubbers are filled with highly dispersed silica since the unfilled rubber is practically useless as an elastomer. 

None of these rubbers has carbon-carbon double bonds. Consequently, they have relatively good aging properties, but, on the other hand, they cannot be vulcanized by the classical sulfur process. For this reason, some of these rubbers are cross-linked with the aid of peroxides, and, in this case, by polymerization of vinyl groups in the case of some silicone rubbers or by free radical transfer reactions in the case of ethylene/vinyl acetate or acrylic rubbers. Other speciality elastomers are cross-linked by reaction with diamines, for example, in the cases of acrylic, epichlorohydrin and fluorine rubbers. 

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