Fluorinated elastomers, properties

Fluorinated elastomers inclnde several types, e.g., fluorocarbons and flnorosilicones. The elastomers can be nsed to 315°C, do not bnm, are unaffected by most chemicals, and have excellent electrical properties. In thermal stability and aging, only the silicones are better. Physical properties are good bnt the cost is high. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

The fluorine atom confers chemical inertness, but some hydrogen atoms must be in the chain to maintain mbbery properties. Some fluorinated siHcones are also available where superior low temperature properties are requked (see Elastomers, synttietic— fluorocarbonelastomers). 

In the 1960s and 1970s, additional elastomers were developed by Du Pont under the Viton and Kalrez trademarks for improved low temperature and chemical resistance properties using perfluoro(methyl vinyl ether), CF2=CFOCF3, as a comonomer with vinyUdene fluoride and/or tetrafluoroethylene (12,13) (see Fluorine compounds, organic-tethafluoroethylene polypous and copolyp rs). 

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C—H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at 1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer. 

Elastomers containing fluorine, also called fluorelastomers. These combine the good physical properties of organic elastomers with the thermal stability of inorganic materials. Trade names are Viton, Fluorel, Kel-F and Technoflon. 

Functional perfluoropolyethers11 (Figure 14.7) can also be prepared by direct fluorination in high yields. Difunctional perfluoropolyethers based on fluorinated polyethylene glycol) are of particular interest as possible precursors for elastomers, which should have outstanding high-temperature and low-temperature properties. 

Today it is claimed that the surface fluorination of polymers using F2 gas mixtures enhances a wide range of properties, e.g., low permeability to nonpolar liquids4 improved permselectivity,5-6 excellent wettability and adhesion,7 low friction coefficient (especially for elastomers),8 and chemical inertness.9 Obviously, these properties depend on the chemical composition ofthe fluorinated layer, which in turn is determined by the chemical structure ofthe base polymer, the composition of the F2 gas mixture, and the fluorination parameters. 

Its average molecular weight is 2000 400 with functionality 2.0. The cost of these fluorinated polyols is high. However, PU elastomers with unique chemical resistance and other exceptional properties are produced. 

This structure is similar to that of the copolymer TFE and ethylene, except that the random onentation of the methyl group from nonstereospeciftc free radical copo-lymenzation of propylene affords a noncrystalline structure [35] The relatively low fluorine content (54%) of these elastomers compared with VDF-based elas tomers (66-69 5%) makes them significantly less resistant to swelling by hydrocarbons Because of strict alternation, these elastomers have a relatively high glass transition temperature ( 2 °CJ and consequently limited low temperature properties Furthermore, they must be polymerized with a cure site monomer or receive a postpolymerization treatment to adequately activate them for vulcanization [36] To counteract the limited cure response, low-temperature flexibility, and hydrocarbon resistance, these polymers have also been modified with substantial amounts (ca 35 wt %) of VDF [37, 38] This provides some improvements but inevitably decreases the resistance to bases and polar solvents... 

The introduction of fluorine into the elastomeric macromolecule generally produces materials exhibiting an improved retention of properties at high temperatures, an improved flexibility at low temperatures, and an improved resistance to solvents. Essentially, there are two groups of fluoroelastomers fluoro-inorganic elastomers and fluorocarbon (or fluorohydrocarbon) elastomers. 

Elastomers based on fluorinated polyurethanes exhibit good mechanical properties and resistance to solvents, chemicals, cold, and heat.54 Formulations for fluoroure-thane elastomers are frequently modified with siloxanes to optimize certain properties.55 FEVE-based polyols are used to manufacture elastomeric automobile bumpers and interior trim components.56... 

Takakura et al. [82] investigated on the structure and the mechanical properties of fluorinated segmented poly(urethaneurea)s, showed that their structures were highly ordered and such polymers behaved as elastomers. 

Kalrez (perfluoro-elastomer) All chemicals Alkali metals and fluorine Typical color black. Temperature range -37 to 260°C. Firm compression. Density 2.02. Chemically inert properties similar to Teflon, but mechanically similar to Viton. Very expensive. 21.50... 

Navarrini [5] prepared tetrafluoroethylene/fluorovinyl ether co- and terpo-lymers, (IV), and (V), respectively, that behaved as both plastomers having good thermal and mechanical properties at high temperatures and elastomers with improved properties at low temperatures. Tetrafluoroethylene and non-fluorinated vinyl ether co- and terpolymers were also prepared in the investigation. 

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