FKM fluoroelastomer

Fig. 1. SAE J200 Classification system for ASTM No. 3 oil where in volume swell nr = no requirement. EPDM is ethylene—propylene—diene monomer HR, butyl mbber SBR, styrene—butadiene mbber NR, natural mbber VMQ, methyl vinyl siUcone CR, chloroprene FKM, fluoroelastomer FVMQ, fluorovinyl methyl siUcone ACM, acryUc elastomers HSN, hydrogenated nitrile ECO, epichlorohydrin and NBR, nitrile mbber.

Several commercial seal compounds based on FKM fluoroelastomer have now gained acceptance in this exacting condition. Field experience has been gained with seals in such applications as process ball valves and pig launcher units. 

Two demanding environments are considered explosive decompression and hydrogen sulphide exposure. A brief review of previous data is given for the former, comparing different polymer types. An attempt is also made to establish a compounding baseline to optimise HNBR performance by examining formulation variables and their effects. For the latter, a comparison of HNBR with NBR, FKM fluoroelastomer and TFE/P is made, as well as a brief look at the effect of ACN (acrylonitrile) content on performance. 4 refs. CANADA... 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

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. 

Carbon Filled carbon Fluoroelastomer (FKM) Flexible graphite Flardfacing... 

Coatings and sealants for varied industrial applications are made by dissolving compounds of low-viscosity FKM elastomers, such as VITON C-10, VITON A-35, or FLUOREL 2145, in methyl ethyl ketone, ethyl acetate, methyl isobutyl ketone, amyl acetate, or other related ketones.47 Such products have typical useful storage life of 7 days at 24°C (75°F) and cure within 2 weeks.58 The subject of coatings and sealants based on fluoroelastomers is covered in more detail in Modern Fluoropoly-mers (Scheirs, J., Ed.), Chapter 23 (Ross Jr., E. W. and Hoover, G. S.), John Wiley Sons, New York (1997). 

Currently, seven major manufacturers produce fluorocarbon elastomers, and these are listed in Table 5.1. The main commercially available fluorocarbon elastomers are listed in Table 5.2. In the ASTM D1418, fluorocarbon elastomers have a designation FKM, and in the International Organization of Standardization (ISO) R1629 their designation is FPM. Current worldwide fluoroelastomer demand is 21,000 metric tons, with average annual growth of approximately 7% [5]. 

Fluoroelastomers (ASTM designation FKM) such as copolymers of VDF and HEP typically have a maximum continuous service temperature of 215°C (419°F). Some metal oxides may cause dehydrofluorination at a temperature of 150°C (302°F) or even lower [16]. Copolymers of VDF and CTFE (e.g., Kel-F) have a maximum longterm service temperature of 200°C (392°F). Fluorocarbon elastomers based on copolymers of VDF/HPFP (hydropentafluoropropylene) and on terpolymers of VDF/ HPFP/TFE have lower thermal stability than copolymers of VDF/HFP because they have a lower fluorine content than the latter [17]. A detailed study of thermal stability of fluoroelastomers was performed by Cox, Wright, and Wright [18]. 

DuPont Performance Elastomers, Chemical Resistance Guide, Version 6.0 (August 2006) Overview of Elastomers and Elastomer Materials, ElastomeFs Characteristics, Fluoroelastomer (Viton. FKM) and Nitrile, hydrogenated Nitrile (NBR, HNBR), Available at http /www.dupontelastomers. com/Tech Info/chemical.asp (accessed October 7, 2006). 

Chem. Descrip. Fluoroelastomer polymer with curative Uses Fluoroelastomer for high-temp, chem.-resist. coatings, as substitute for solv.-based fluoroelastomer fabric coatings Features 70% FKM solids emulsion for applic. by spraying, dipping, or silk screening resistant to oils and corrosive chem. 

Fluoro rubber of the polymethylene type having substituent fluoro and perfluoroalkyl or perfluoroalkoxy groups on the polymer chain Fluoroelastomer FKM... 

Properties of Fluoroelastomers. FKM elastomers can have reasonably low glass transition temperatures, in the range of-18 to -40°C. The tensile properties of fluoroelastomers vulcanizates are fairly good but can decrease considerably with increasing temperatures. 

Uses of Fluoroelastomers. FKM elastomers are expensive, but their demand is high because of their unusual stability in very severe environments. They are used in specialty products, e.g., shaft seals of internal combustion engines, and components in aircraft and rockets. Products include seals, gaskets, liners, hoses, protective fabric coatings, diaphragms, roll covers, and cable jacketing. 

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