Silicone rubbers, peroxide vulcanization

The polysiloxane from experiment b) is soluble in toluene. It can be converted by hot vulcanization into an insoluble silicone rubber. Using a small blender, 10 g of the polymer are kneaded with 10 g of quartz powder or 7.5 g of ground kieselguhr,and 0.6 g of dibenzoyl peroxide paste (50% in silicone oil).To work the additives into the silicone rubber without a mechanical blender is very tedious and difficult to achieve completely. 

The most important crosslinking agent for pressureless hot air vulcanization of silicone rubber is DClBP(Bis-2,4-dichloro-benzoyl-peroxide) (Fig. 14). It generates decomposition products, which are removed in a post-curing process with air blowing. This product provides the advantage of a fast cure rate and bubble-free cured transparent rubber. 

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. 

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of Cr Uses Catalyst, crosslinking agent used for curing heat-vulcanized silicone rubbers, food-contact rubber articles Manuf./Distrib. Gelest http //www.geiest.com Trade Name Synonyms Aztec DCLBP-50-PSI [Aztec Peroxides] Perkadox PD t[Flexsys http //www. fiexsys. com]... 

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. 

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown [93] that the revulcanization process of devulcanized SBR was essentially different from that of the virgin SBR. The induction period is shorter or absent for re vulcanization of the devulcanized SBR. This is also true for other unhlled and carbon-black-filled rubbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur-containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in the case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules, resulting in crosshnking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber [93]. 

Organofunctional polysiloxanes with silanol groups can be cross-linked at low temperatures with tetrabutyl titanium, methyl triacetoxy-silane, and similar compounds and are thus used as cold vulcanization materials. In contrast, hot vulcanizable silicones possess one vinyl group in about 500-1000 monomeric units and can be cross-linked with peroxides. All silicon rubbers are filled with highly dispersed silica since the unfilled rubber is practically a useless elastomer. 

Peroxide Vulcanization of Silicone Rubbers. Silicone rubbers (high-molecular-... 

Silicone rubber is obtained from polydimethylsiloxane (PDMS) in two ways (1) a high-temperature vulcanization process using a crosslinking reaction in the presence of peroxides and at high temperatures and (2) a room-temperature vulcanization process by using a crosslinking reaction of linear silicones with functional groups in the presence of Pt-based catalysts [99],... 

The radical crosslinking method involves heating the polysiloxane with dicumyl peroxide, ditertiary butyl peroxide, benzoyl peroxide, or bis-2,4-dichlorobenzoyl peroxide. The peroxide radical abstracts hydrogen from the polymer chain and creates a radical site on the interior of the chain. Two such sites interact to randomly form the crosslink. The major disadvantage of this technique is its commercial inefficiency. Obviously, vulcanization can only be carried out in a mould to produce the final silicon rubber product. 

Silicone, natural, and synthetic rubbers have been used for the fabrication of implants. Natural rubber is made mostly from the latex of the Hevea brasiliensis tree and the chemical formula is the same as that of cw-1,4 polyisoprene. Natural rubber was found to be compatible with blood in its pure form. Also, cross-linking by x-ray and organic peroxides produces rubber with superior blood compatibility compared with rubbers made by the conventional sulfur vulcanization. 

Peroxides are vulcanizing agents for elastomers, which contain no sites for attack by other types of vulcanizing agents. They are useful for ethylene-propylene rubber (EPR), ethylene-vinyl acetate copolymers (EAM), certain millable urethane rubbers, and silicone mbbers. They generally are not useful for vulcanizing butyl rubber (poly[isobutylene-co-isoprene]) because of a tendency toward chain scission, rather than crosslinking, when the polymer is subjected to the action of peroxide. 

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