Perfluoroelastomers

They show good to excellent resistance to highly aromatic solvents, polar solvents, water and salt solutions, aqueous acids, dilute alkaline solutions, oxidative environments, amines, and methyl alcohol. Care must be taken in choice of proper gum and compound. Hexafluoropropylene-containing polymers are not recommended for use in contact with ammonia, strong caustic (50% sodium hydroxide above 70°C), and certain polar solvents such as methyl ethyl ketone and low molecular weight esters. However, perfluoroelastomers can withstand these fluids. Propylene-containing fluorocarbon polymers can tolerate strong caustic. 

Septums made of silicone or perfluoroelastomer can be used. This type of injector is... 

The first commercial fluoroelastomer, Kel-F, was developed by the M. W. Kellog Company in the early to mid-1950s and is a copolymer of vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE). Another fluorocarbon elastomer, Viton A, is a copolymer of VDF and hexafluoropropylene (HFP) developed by du Pont was made available commercially in 1955. The products developed thereafter can be divided into two classes VDF-based fluoroelastomers and tetrafluoroethylene (TFE)-based fluoroelastomers (perfluoroelastomers).72 The current products are mostly based on copolymers of VDF and HFP, VDF and MVE, or terpolymers of VDF with HFP and TFE. In the combination of VDF and HFP, the proportion of HFP has to be 19 to 20 mol% or higher to obtain amorphous elastomeric product.73 The ratio of VDF/HFP/TFE has also to be within a certain region to yield elastomers as shown in a triangular diagram (Figure 2.2).74... 

Perfluoroelastomers represent a special subgroup of fluorocarbon elastomers. They are essentially rubbery derivatives of PTFE and exhibit exceptional properties, such as unequaled chemical inertness and thermal stability. Currently, there are two types of known commercial perfluoroelastomers, KALREZ and PERLAST . These have ASTM designation FFKM. 

Perfluoroelastomers, i.e., elastomers based on perfluoromethyl vinyl ether (PMVE) and TFE, exhibit a virtually unmatched resistance to a broad class of chemicals except fluorinated solvents. On the other hand, they are adversely affected by hydraulic fluid, diethyl amine, and fumed nitric acid resulting in swelling of the elastomer by 41, 61, and 90%, respectively.10... 

Perfluoroelastomers (FFKM), such as KALREZ, are particularly suited for extreme service conditions. They are resistant to more than 1,500 chemical substances, including ethers, ketones, esters, aromatic and chlorinated solvents, oxidizers, oils, fuels, acids, and alkali and are capable of service at temperatures up to 316°C (600°F).55 Because of the retention of resilience, low compression set, and good creep resistance, they perform extremely well as static or dynamic seals under conditions where other materials, such as metals, FKM, PIPE and other elastomers, fail. Parts from FFKM have very low outgassing characteristics and can be made from formulations, which comply with FDA regulations.56 Primary areas of application of perfluoroelastomers are paint and coating operations, oil and gas recovery,... 

Since perfluoroelastomer parts are primarily used in fluid sealing environments, it is essential to pay attention to seal design parameters, especially as they relate to the mechanical properties of the elastomeric material being used. The sealing performance depends on the stability of the material in the fluid, its mechanical properties, mechanical design, and installation of the seal.57... 

Perfluoroelastomers, such as Kalrez (copolymer of TEE and PMVE), can maintain their thermal stability to temperatures as high as 300°C (572°F) or even higher, with a maximum continuous service temperature of 315°C (599°F). Moreover, instead of hardening, the elastomer becomes more elastic with aging.13... 

Polyurethane-based FTPEs are produced by reacting fluorinated polyether diols with aromatic disocyanates. The resulting block copolymers contain fluorinated polyether soft segments.68 Another possible method of preparation of fluorinated TPE is dynamic vulcanization. Examples are a blend of a perfluoroplastic and a perfluoroelastomer containing curing sites or a combination of VDF-based fluo-roelastomers and thermoplastics, such as polyamides, polybutylene terephtalate, and polyphenylene sulhde.69 70... 

Perfluoroelastomers prepared by Bish [3] and Hung [4] containing roughly 3 wt% of the termonomer perfluoro(8-cyano-5-methyl-3,6-dioxa-l-octene),... 

The thermal stability of fluorocarbon elastomers also depends on their molecular structure. Fully fluorinated copolymers, such as copolymer of TFE and PMVE (Kal-rez), are thermally stable up to temperatures exceeding 300°C (572°E). Moreover, with heat aging this perfluoroelastomer becomes more elastic rather than embrittled. Eluorocarbon elastomers containing hydrogen in their structures (e.g., Viton, Dyneon, and DAI-EL EKM) exhibit a considerably lower thermal stability than the perfluori-nated elastomer. Eor example, the long-term maximum service temperature for FKM... 

Perfluoroelastomers (ASTM designation FFKM) are essentially copolymers of two perfluorinated monomers, TFE and PMVE with a CSM, which is essential for cross-linking. Perfluoroelastomers can be cured by ionizing radiation without any additives. The advantage of radiation cured FFKM is the absence of any additives, so that the product is very pure. The disadvantage is the relatively low upper-use temperature of the cured material, typically 150°C, which limits the material to special sealing applications only [32]. 

In the aerospace industry, too, there is an increasing demand for high-performance electrical insulation with a high degree of tire safety and light weight. Here the trend is for foamed fluoroplastics. FEP is attractive for this purpose. Fuel-tank seals are made predominantly from special perfluoroelastomers. 

Kalrez perfluoroelastomer Kalrez is a registered trademark of DuPont... 

The longer the polymer chain is, the better is the resistance to compression set because of the improved ability to store energy. In Figure 8.3, the sealing compounds are NBR (nitrile rubber), EPDM (ethylene-propylene diene rubber), FVMQ (fluoro-vinyl-methyl [fluorosilicone]), VMQ (vinyl-methyl silicone), FKM (fluroelastomers), and FFKM (perfluoroelastomers). 

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