Silicone saturated elastomers

The industrial relevance of peroxide-curing of elastomers is by far larger for main-chain saturated elastomers, such as silicone elastomers, acrylic elastomers, fluoro elastomers... 

Saturated elastomers are often cured by peroxide, often aided by catalysts. These include chlorinated polyethylene, fluorocarbon, acrylic ester, epichlorohydrin, polysulfide, polyurethane, and silicone. The peroxide radical abstracts an unstable hydrogen from the polymer, leaving a polymer radical, and then polymer radicals couple to produce C-C cross-links. 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

A second complication can come from materials that helium can easily permeate such as the elastomers used in O-ring connections. Helium readings from these materials can distract you from other true leak readings. Once an elastomer has become saturated with helium, it slowly desorbs the gas, which creates a large background noise. This background noise interferes with a helium leak detector s sensitivity and performance. All elastomers readily absorb, and slowly desorb, helium. Thus the fewer elastomers used within a system, the less of a problem there will be. Silicone tends to absorb helium the most, followed by nat-... 

Most compounders use a combination of physical and chemical antiozonants and achieve excellent protection in this way. For more severe ozone-resistance problems, there are, of course, a number of specialty elastomers that are saturated and therefore completely ozone-resistant ethylene/propylene rubber, chlorinated and chlorosulfonated polyethylene, ethylene/vinyl acetate, ethylene/acrylic esters, butyl rubber, SEES, plasticized PVC, butyl acrylate copolymers, polyepichlorohydrin and copolymers, polyetherester block copolymer, polyurethane, and silicone. 

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