Fluorocarbon elastomer

Fluorocarbon elastomers represent the largest group of fluoroelastomers. They have carbon-to-carbon linkages in the polymer backbone and a varied 

Since fluorocarbon elastomers being discussed here contain hydrogen in their molecules, they have the tendency to cross-link in addition to scission, common in fluoropolymers when exposed to radiation. The cross-linking predominates, but there is still a significant degree of chain scission.  

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]. 

Perfluoroelastomers represent a special subgroup of fluorocarbon elastomers. They are essentially rubbery derivatives of polytetrafluoroethylene (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. 

An alternating copolymer of TFE and propylene (TEE/P) and a terpolymer TFE/P/VDF are fluorocarbon elastomers commercially available under the trademark APE AS. They are characterized by improved low-temperature and electrical properties and steam resistance when compared with FKM and are comparable to PPKM in chemical resistance at lower cost (details in next section). TFE/P has the ASTM D1418 and the ISO 1629 designations FEPM, and in ASTM D2000/SAE J200 it is classified as Type/Class HK. 

Fluoiocaibon elastomeis aie synthetic, noncrystaUine polymers that exhibit elastomeric properties when cross-linked. They are designed for demanding service appHcations in hostile environments characterized by broad temperature ranges and/or contact with chemicals, oils, or fuels. 

Mihtary interest in the development of fuel and thermal resistant elastomers for low temperature service created a need for fluorinated elastomers. In the early 1950s, the M. W. Kellogg Co. in a joint project with the U.S. Army Quartermaster Corps, and 3M in a joint project with the U.S. Air Force, developed two commercial fluorocarbon elastomers. The copolymers of vinyUdene fluoride, CF2=CH2, and chlorotrifluoroethylene, CF2=CFC1, became available from Kellogg in 1955 under the trademark of Kel-F (1-3) (see Fluorine compounds, ORGANic-POLYcm.OROTRiFLUOROETHYLENE Poly(vinylidene) fluoride). In 1956, 3M introduced a polymer based on poly(l,l-dihydroperfluorobutyl acrylate) trademarked 3M Brand Fluorombber 1F4 (4). The poor balance of acid, steam, and heat resistance of the latter elastomer limited its commercial use. 

In the late 1950s, the copolymers of vinyUdene fluoride and hexafluoropropylene, CF2=CFCF3, were developed on a commercial scale by 3M (Fluorel) and by Du Pont (Viton) (5—8). In the 1960s, terpolymers of vinyUdene fluoride, hexafluoropropylene, and tetrafluoroethylene, CF2=CF2, were developed (9) and were commercialized by Du Pont as Viton B. At about the same time, Montedison developed copolymers of vinyUdene fluoride and 1-hydropentafluoropropylene as well as terpolymers of these monomers with tetrafluoroethylene, marketed as Tecnoflon polymers (10,11). 

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). 

Bromine- and iodine-containing fluoroolefins have been copolymerized with the above monomers in order to allow peroxide cure (14—21). The peroxide cure system does not requite dehydrofluorination of the polymer backbone, resulting in an elastomer that shows improved properties after heat and fluid aging. 

Name Chemical Name Vulcanising Agent Stretching Crystallisation Gum Tensile Strength 

Natural Rubber Cl s-1,4- polyisoprene ( 99%) Sulfur Good Good 

Styrene- butadiene rubber Polybutadiene-co- styrene Sulfur Poor Poor 

Butadiene rubber Polybutadine cis-1, 4 97%) Sulfur Fair Fair 

Isoprene rubber C s-1,4 Polyisoprene 97%) Sulfur Good Good 

As in neoprene, except that the exposure time should be 10—15 min, since this is a non-acid resis- 

For optimum bond strength, use the procedure described above under Ethylene—Chlorotiifluoro-ethylene Copolymer (Section 1.2.1).  

A simple solvent wipe with methanol may be used 

These rubbers have become the second largest in volume of the fluoroelastomer types. The non-sili-cone-containing fluoroelastomers are discussed below. FluorosiUcone rubbers retain most of the useful qualities of the regular silicone rubbers and have improved resistance to many fluids, except for ketones and phosphate esters. They are most useful when low-temperature resistance is required, in addition to fluid resistance. An effective treatment is a solvent wipe with methyl, ethyl, or isopropyl alcohol, or with toluene. 

Wipe or spray with, or immerse in 1,1-tri-chloroethane or Freon TMC, acetone, MEK, toluene, ethyl, or isopropyl alcohol 

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Uses. Vinyhdene fluoride is used for the manufacture of PVDF and for copolymerization with many fluorinated monomers. One commercially significant use is the manufacture of high performance fluoroelastomers that include copolymers of VDF with hexafluoropropylene (HFP) (62) or chlorotrifluoroethylene (CTFE) (63) and terpolymers with HEP and tetrafluoroethylene (TEE) (64) (see Elastomers, synthetic-fluorocarbon elastomers). There is intense commercial interest in thermoplastic copolymers of VDE with HEP (65,66), CTEE (67), or TEE (68). Less common are copolymers with trifluoroethene (69), 3,3,3-trifluoro-2-trifluoromethylpropene (70), or hexafluoroacetone (71). Thermoplastic terpolymers of VDE, HEP, and TEE are also of interest as coatings and film. A thermoplastic elastomer that has an elastomeric VDE copolymer chain as backbone and a grafted PVDE side chain has been developed (72). 

Low Temperature Properties. The property of solvent resistance makes fluorosihcone elastomers usefiil where alternative fluorocarbon elastomers cannot function. The abiHty to retract to 10% of their original extension after a 100% elongation at low temperature is an important test result. Eluorosihcones can typically pass this test down to —59°C. The brittle point is approximately —68°C. 

Extmsion techniques are used to make tubes, rods, gaskets, preforms, etc. Standard mbber equipment may be used to extmde fluorosihcone elastomers. The green strength of fluorosihcones is less than that of typical fluorocarbon elastomers, and this should be considered when designing the feed system. 

Fluorocarbon Elastomers. These elastomers were developed by both the Du Pont and 3M companies during the 1950s. They are the most resistant elastomers to heat, chemicals, and solvents known, but they are also the most expensive, ie, between 22 and 35 per kg. The most common types are copolymers of vinyHdene fluoride and hexafluoropropene, thus ... 

Fig. 3. Heat and oil resistance of CSM compared to other elastomers by ASTM D2000. A—K iadicate grades of CSM. The other ASTM designations are as follows AM, acryhc elastomers CR, chloroprene mbber EPDM, ethylene—propjiene—diene mbber FKM, fluorocarbon elastomers FQ, fluorosiUcones ...

Copolymers of propylene and tetrafluoroethylene, which are sold under the Aflas trademark by 3M, have been added to the fluorocarbon elastomer family (21—26). Also 3M has introduced an incorporated cure copolymer of vinyUdene fluoride, tetrafluoroethylene and propylene under the trademark Fluorel 11 (27). These two polymers (Aflas and Fluorel 11) do not contain hexafluoropropylene. The substitution of hexafluoropropylene with propylene is the main reason why these polymers show excellent resistance toward high pH environments (28). Table 1 Hsts the principal commercial fluorocarbon elastomers in 1993. 

Table 2 summarizes general characteristics of vulcanizates prepared from commercially available fluorocarbon elastomer gumstocks. ... 

Table 2. Fluorocarbon Elastomers Physical Property Ranges ...

Fig. 2. Tensile strength retention, continuous service, for fluorocarbon elastomers. Compound I (see Table 4).

Chemical Resistance. Fluorocarbon elastomer compounds show excellent resistance to automotive fuels and oils, hydrocarbon solvents, aircraft fuels and oils, hydrauHc fluids, and certain chlorinated solvents, and may be used without reservation. 

Compression Set Resistant. One property of fluorocarbon elastomers that makes them uniquely valuable to the sealing industry is their extreme resistance to compression set. Figure 4 plots compression set vs time for compounds prepared especially for compression set resistance (O-ring grades). 

Fig. 4. Compression-sets of fluorocarbon elastomers at 200°C, 3.5 mm O-rings A, Compound 1 (see Table 4) B, Compound 11 (see Table 4).

The polymei latex is then coagulated by addition of salt oi acid, a combination of both, oi by a fiee2e—thaw process. The cmmb is washed, dewatered, and dried. Since most fluorocarbon elastomer gums are sold with incorporated cure systems, the final step in the process involves incorporation of the curatives. This can be done on a two-roU mill, in an internal mixer, or in a mixing extmder. 

The manufacture of the majority of fluorocarbon elastomer gums includes the addition of an incorporated cure system comprising an organic onium cure accelerator, such as triphenylbenzylphosphonium chloride [1100-88-5] and a bisphenol cross-linking agent, such as... 

Compounding. Owing to the number of ingredients required in a conventional mbber recipe, fluorocarbon elastomer compounding seems simple compared to typical hydrocarbon elastomer recipes. However, the apparent simplicity of such formulations makes a selection of appropriate... 

Ring S. In O-ring appHcations, the primary consideration is resistance to compression set. A fluorocarbon elastomer gum is chosen for O-ring apphcations based on its gum viscosity, cross-link density, cure system, and chemical resistance so that the best combination of processibiUty and use performance is obtained. Sample formulations for such uses are given in Table 4. 

Table 6. Fluorocarbon Elastomer Molded Goods Compound...

Extruded Articles. In extmded article compounding, the most important parameters are scorch safety and flow characteristics (53). The bisphenol cure system again offers the best scorch resistance of the available fluorocarbon elastomer cure systems. Good flow characteristics can be achieved through proper selection of gum viscosities. Also, the addition of process aids to the formulation can enhance the flow characteristics. Typical formulations for extmsion grade fluorocarbon elastomers are given iu Table 7. 

Table 7. Fluorocarbon Elastomer Extrusion Grade Compound...

Internal mixing is widely used with fluorocarbon elastomers. Gumstocks and compounds that are particularly successful fall in the viscosity ranges discussed earlier, and use both incorporated bisphenol-type and peroxide cure systems. A typical internal mix cycle mns 6—8 min with a drop temperature of 90—120°C. The typical formulations in Tables 4 and 7 are readily mixed in an internal mixer. 

Transfer mol ding minimises preforming, and is usually used for the production of very small parts however, this technique may generate excessive amounts of scrap material. Flow requirements can be quite high, but fluorocarbon elastomers are available that are effective iu this appHcation. 

Injection molding is finding expanded usage in the mbbei industry. Fluorocarbon elastomers can be successfully molded via this technique. 

All types of mol ding may be carried out at 150 to 200°C. This allows mol ding times of five minutes or less for most fluorocarbon elastomer parts, but this time is dependent on part size. 

Extrusion. Extmsion techniques are used in the preparation of tubing, hose, O-ring cord, preforms and shaped gaskets. Typical extmsion conditions are 70 to 85°C for the barrel temperature and 95 to 110°C for the head temperature. The extmded forms are normally cured in a steam autoclave at 150 to 165°C. Some special grades of peroxide curable fluorocarbon elastomers can be hot air vulcanized. 

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