Silicone elastomers fluoro

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]. 

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... 

Perfluoropyridine (3.0 g, 17.8 mmol), anhyd KF (3.0 g, 55.9 mmol), and sulfolane (15 mL) were introduced into a Carius tube (ca. 100 mL) and hexafluoropropene (5.0 g, 33.3 mmol) was introduced from a vacuum system. The tube was sealed under vacuum and heated for 12 h at 130 C. It was then cooled in liquid air and opened to a vacuum system. The volatile components were fractionated in the vacuum system to give hexafluoropropene (0.5 g) and higher boiling material (8.0 g). The latter was separated by GC (silicone elastomer on Celite) to give derivatives of hexafluoropropene (2.7 g), perfluoropyridine (0.3 g), and per-fluoro(4-isopropylpyridine) yield 4.8 g (94%). 

The best source of known samples of rubbers is to purchase labelled O rings, as one of these will last for quite a long time if carefully used in the burning tests. Natural rubber has quite a distinctive odour when burned everyone must know the smell of burning rubber, it is easily possible to distinguish between this and nitrile or neoprene for instance. Fluoro and silicone elastomers likewise have their own characteristic combustion properties. 

Some typical elastomers are natural rubber, which is gathered from trees, SBR rubber, which is used a lot in motorcar tires, neoprene, as in wet suits and oil seals, EPDM, a general purpose rubber, butyl, a heat-resistant rubber with the ability to keep the air in car tires, nitrile for oil seals, silicones for heat resistance, fluoro-elastomers for chemical resistance, and last but not least, polyurethanes, which cover a number of the above fields. Table 1.1 shows some of the advantages of castable polyurethanes over conventional rubbers. 

Silicone rubber offers a set of unique properties to the market, which cannot be obtained by other elastomers. The Si-0 backbone provides excellent thermal stability and, with no unsaturation in the backbone, outstanding ozone and oxidative stability. The very low glass transition temperature, combined with the absence of low-temperature crystallization, puts silicones among the materials of choice for low-temperature performance. The fluoro-substituted versions provide solvent, fuel, and oil resistance along with the above-mentioned stability advantages inherent with the silicone backbone. 

Sheets of Class SC (caulking compounds) elastomers, 0.16 cm thick, were prepared by casting the caulks in open molds, 15 cm X 2.5 cm. The bottom of the mold was covered with a thin sheet of Mylar, which had been coated with a silicone and then a fluoro-... 

Elastomers used for connector insulators include polychloroprenes, silicones, fluoro-silicones and heat-setting compounds which enable the special characteristics to be incorporated for example, flame resistance, low toxicity and low smoke. Neoprene is used as spacer material and also for washers where its sealing qualities, good resistance to cracking, rotting, oils and petrol and its good low temperamre flexibility may be beneficial in specific applications. 

Figure 19.11 presents a typical result of the helix test. As mentioned above, the length of the extended helix is a measure of the processability. This test has been performed with a variety of potential gasket materials to achieve a data baseline. The values are listed in Table 19.4. The results show that the processability depends on the type (silicon > fluoro-elastomer FKM) and also on specific material configuration (FKM low viscosity > regular FKM). 

Currently-used elastomers include natural rubber, the identical synthetic cis-polyisoprene, butyl rubber, nitrile rubber, neoprene rubber, ethylene-propylene co-polymer rubber, fluoro rubber, urethane rubber and silicone rubber. 

---