Fluoroelastomers

Fluoroelastomers are for the most part based on the combination of vinylidene fluoride (VDF) with other monomers that disrupt the high crystallinity typical for the PVDF homopolymer. The properties of the resulting elastomeric materials are determined by the short VDF sequences and low or negligible crystallinity. 

Typical Values of Refractive Index of Different Fluoroplastics 

Note ETFE, copolymer of ethylene and tetrafluoroethylene HTE, terpolymer of hexafluoropropyl-ene, tetrafluoroethylene and ethylene FEP, fluorinated ethylene-propylene copolymer PVDF, poly(vinylidene fluoride) THV, terpolymer of tetrafluorioethylene, hexafluoropro-pylene and vinylidene fluoride. 

Sources Dyneon Fluoroplastics, Product Comparison Guide, 99-0504-1501-1, Dyneon LLC, 2003 Dyneon Huroplastics, Product Comparison Guide, 5845 HB 98-0504-1611-8, Dyneon LLC, 2007 (With permission). 

Elastomers based on VDF and TFE-VDF-HFP consist of fine particles 16 to 30 nm in diameter in contrast to PTFE, which has a rod-like microstructure in which the elementary fibrils are approximately 6 nm wide and the molecular chains are all extended [12]. For example, the properties of a VDF/HFP elastomer such as their resilience and flexibility can be related to spherical domains with diameter approximately 25 nm that are interconnected [10]. The diameter of these particles was found to be proportional to the molecnlar weight of the elastomer [12]. 

Fluoroelastomers combine excellent chemical resistance (e.g. oxidizing acids, and alkalis) and high-temperature resistance (i.e., up to 275-300°C for short periods of time) excellent oxidation resistance good resistance to fuels containing up to 30% aromatics mostly poor resistance in solvents or organic media by contrast with fluorinated plastics. 

Viton A is a family of fluoroelastomer dipolymers, that is they are polymerized from two monomers, vinylidene fluoride (VF2) and hexafluoropropylene (HFP). Viton A fluoroelastomers are general purpose types that are suited for general molded goods such as o-rings and v-rings, gaskets, and other simple and complex shapes. 

Viton B is a family of fluoroelastomer terpolymers, that is they are polymerized from three monomers, vinylidene (VF2), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). Viton B fluoroelastomers offer better fluid resistance than A type fluoroelastomer. 

The use of fluoroelastomers has evolved into being very dependent on the automotive industry. Almost 65% of the fluoroelastomers produced are used in automotive applications. The rest are used in chemical and petroleum applications (14%), aerospace (14%), and the energy industry (7%). 

Through emulsion polymerization, the following polymers are examples of some of the FKM polymers sold commercially  

Also FKM elastomers are made from TEE, VF2 and fluorinated vinyl ether as well as a special pentapolymer of TEE, HEP, ethylene, fluorinated vinyl ether and VF2. 

Fluoroelastomer Fluoropolymer elastomer Fluorocarbon elastomer Fluorine rubber 

FKM (official ASTM abbreviation, all containing vinyiidene fluoride as one of the monomers) 

Over the years many fluoroelastomers have been prepared in addition to the materials described earlier in this chapter. These include fluorinated polyurethanes, fluorinated polyepoxides, hexafluoro-acetone/propylene oxide copolymers and polyfluorals. Many of these materials are thermally unstable, a fact which stresses the point that the presence of C—F bonds with their high bond strength is no guarantee of polymer thermal stability. One particular type of fluoroelastomer which is of technical importance, the fluorosilicone rubber family, are however of good thermal stability and are considered together with the silicone rubbers in a later chapter. 

Cooper, J. R., Chapter 4E of Polymer Chemistry of Synthetic Elastomers Part I (Eds. J. P. Kennedy and E. G. M. Tornqvist), Interscience, New York (1968). 

Finlay, J. B., Hallenbeck, A., MacLachlan, J. D. and Pelosi, L. F., Paper presented to International Rubber Conference, Brighton, England, May 1977. (Preprints produced by the Plastics and Rubber Institute.) 

Paper presented to 169th Meeting of the American Chemical Society, Philadelphia (1975). 

Fluoroelastomer-based nanocomposites were prepared using various nanoclays and their different properties were studied [93-98]. 

Maiti and Bhowmick reported exciting results that a polar matrix like fluoroelastomer (Viton B-50) was able to exfoliate unmodified clay (Cloisite NA ) as well as the modified one (Cloisite 20A) [93]. They studied morphology, mechanical, dynamic mechanical and swelling properties of fluoroelastomer nanocomposites. The unmodified-clay-filled systems showed better properties than the modified ones (Table 2.3). 

For example, the increment in maximum stress over the neat polymer is 100% and 53% in the case of the unmodified- and the modified-clay-fiUed samples, respectively. The extraordinary results obtained with the unmodified clays were explained with the help of thermodynamics and surface energetics. They explained it as follows. 

FIGURE 2.11 Plot of oxygen transmission rate versus time for brominated poly(isobutylene-co-isoprene) (BIMS) nanocomposite. 

Source Maiti, M. and Bhowmick, A.K., J. Polym. Sci., Part B Polym. Phys., 44, 162, 2006. Courtesy of Wiley InterScience. 

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Chemicals. Both organic and inorganic fluorine-containing compounds, most of which have highly speciali2ed and valuable properties, are produced from HF. Typically these fluorinated chemicals are relatively complex, sometimes difficult to manufacture, and of high value. These materials include products used as fabric and fiber treatments, herbicide and pharmaceutical intermediates, fluoroelastomers, and fluorinated inert Hquids. Other products include BF, SF, and fluoborates. 

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). 

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

The fluoroelastomers possess good mbber properties with the added advantages of being nonburning, hydrophobic, and solvent- and fuel-resistant. In addition to these, because of flexibiHty down to about —60° C, these polymers have been used in seals, gaskets, and hoses in army tanks, in aviation fuel lines and tanks, as well as in cold-climate oil pipeline appHcations. These polymers have also found appHcation in various types of shock mounts for vibration dampening (14,17). 

Natural mbber comes generally from southeast Asia. Synthetic mbbers are produced from monomers obtained from the cracking and refining of petroleum (qv). The most common monomers are styrene, butadiene, isobutylene, isoprene, ethylene, propylene, and acrylonitrile. There are numerous others for specialty elastomers which include acryUcs, chlorosulfonated polyethylene, chlorinated polyethylene, epichlorohydrin, ethylene—acryUc, ethylene octene mbber, ethylene—propylene mbber, fluoroelastomers, polynorbomene, polysulftdes, siUcone, thermoplastic elastomers, urethanes, and ethylene—vinyl acetate. 

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.

Fluoroelastomers. The fluoroelastomers were introduced to the mbber industry in the late 1950s by the DuPont Company. They were made by modification of Teflon polymers and designed to have exceUent heat and chemical resistance, but remain elastomeric in nature. They were very expensive and have found use in limited appHcations. However, with the increasing demand in the automotive and industrial market for improved reHabUity and longer Hfe, the elastomeric fluoroelastomers have made significant inroads into these appHcations (see Elastomers, synthetic-fluorocarbon ELASTOTffiRS). 

Fluoroelastomers excel compared to aU other elastomers in heat, chemical, flame, weathering, fuel, and o2one resistance. In addition oU, oxygen, and water resistance ate very good. The fluoroelastomers, however, ate attacked by amines and some highly polar solvents. The abrasion resistance and low temperature properties ate adequate for most appHcations. 

Parts made from fluoroelastomers ate used ia appHcations that justify their high cost, usually where the maintenance and replacement costs are high enough to offset the initial cost of the part. These include automotive appHcations such as valve stem seals, fuel injector components, radiator, crankcase and transmission seals, and carburetor needle tips. Numerous seals and gaskets in the marine, oilfield, and chemical processing industries employ fluoroelastomers. In addition, many hoses in the automotive and chemical industry are made entirely of fluoroelastomer compounds or have a veneer of the fluoroelastomer as a barrier exposed to the harsh environment. Seals and gaskets in military appHcations and the binder for flares and missile appHcations ate made with fluoroelastomers. 

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. 

Thiols iateract readily with many mbber-containing materials. For this reason, care should be taken ia the selection of gasket and hose materials. Teflon, Kel-F, Viton, or other suitable fluoroelastomers function as gasket materials. Viton is suitable for hoses. Carbon steel is useful for many thiols, although some thiols become very discolored when carbon steel is utilized. In these cases, the use of stainless steel is very desirable. Isolation from air and water also minimizes color formation. 2-Mercaptoethanol and 1,2-ethanedithiol should be stored ia stainless steel (61). 

The excellent properties of these fluoroelastomers come with a high price tag. Kalrez, for example, is extremely expensive ( 33—44/kg). These materials are used in automotive appHcations (seals, gaskets, fuel hose lines, engine parts, etc), where they can withstand under-hood temperatures. They are also used in equipment for oil and gas production and chemical processing. U.S. consumption in 1988 was 3100 t (76). 

J. P. Auchter, R. Mulach, and K. Tsuchiya, CEH Data Summary, Fluoroelastomers, Stanford Research Institute, Menlo Park, Calif., 1989, p. 

Soft reliners can weaken the strength of the heat-cured resin, because they reduce the thickness of the denture base and allow the diffusion of the monomer or solvent from the reliner into the base. Reflned dentures stain readily and are difficult to clean. A polyphosphazine fluoroelastomer has also been developed in an attempt to overcome the deficiencies of available liners (218). 

Fluoroelastomer (Vitou, Fluorel 2141, Kel-F) 450 Can be used at high temperatures with many fuels, lubricants, hydraulic fluids, solvents, highly resistant to ozone, weathering. Good mechanical properties. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

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. 

In 1955 Barr and Haszeldine, working in Manchester, prepared nitroso-fluoroelastomers of the general type ... 

The rubber has a very low of -68°C, excellent hydrolytic stability and excellent resistance to ozone, solvents and acids. In addition the rubber does not bum even in an oxidising atmosphere. Although its properties are virtually unchanged in the range -75 to + 120°C it does not possess the heat resistance of other fluoroelastomers. This polymer was marketed by Firestone in the mid-1970s as PNF rubber, but in 1983 the Ethyl Corporation obtained exclusive rights to the Firestone patents and the polymer is now marketed as Eypel F. 

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers. 

The most widely used fluoroelastomers are copolymers of VDF and HFP and optionally also TFE. HFP interrupts the crystallimty of otherwise crystaUme PVDF TFE increases the fluonne content for mcreased solvent and heat resistance without raising the glass transition temperature as much as would an equivalent amount of additional HFP The dipolymer is random except that there are no contiguous HFP units and can therefore be represented as follows... 

Figures 10-14 illustrate the reductions obtained in hardness, 50% modulus, 100% modulus, elongation, and tensile strength, respectively, for a fluoroelastomer compound produced under normal production conditions. Table 1 shows the improvement obtained in detail.

Figure 10 Comparison of hardness 3 sigma variability ratings fluoroelastomer compound.

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