ASTM D 1418 Designation: FEPM
ASTM D 2000, SAE J200 Type / Class: HK
STANDARD COLOR: Black
RELATIVE COST: High
GENERAL TEMPERATURE RANGE: 0° to +400° F
The FEPM designation was originally directed at copolymers of tetrafluoroethylene (TFE) and propylene (P) such as is shown in Figure 53. TFE/P provides a unique combination of chemical, heat, and electrical resistance. Chemically, TFE/P resists both acids and bases, as well as steam, amine-based corrosion inhibitors, hydraulic fluids, alcohol, and petroleum fluids. TFE/P is also resistant to ozone and weather. TFE/P typically retains its remarkable chemical resistance even in high temperatures (short exposures up to 450° F, 232° C), and tests have shown that electrical resistance actually improves with heat exposure. Nor do physical properties suffer; tensile strength typically approaches 2,500 psi.
The first TFE/P compound to be commercially marketed was Aflas® (a product of Asahi Glass). In a sense, Aflas defined the initial boundaries for base-resistant materials. Different grades of Aflas have different molecular weights. Most molded and extruded products are made from Aflas 150P, which has a molecular weight of about 130,000. In comparison, Aflas 100H has a molecular weight of 200,000 and is typically used where high pressures are to be sealed, such as in oil field applications. TFE/P compounds are also widely used in the chemical processing, automotive, and aerospace industries. As shown in Table 12, TFE/P compounds are not as good as standard FKM-A (e.g. Viton A) compounds in terms of hydrocarbon resistance, but TFE/P surpasses FKM-A in resistance to strong bases, amines, and polar solvents.
Though it was the first, Aflas is not the only base-resistant fluoroelastomer on the market. DuPont Dow also offers a wide range of excellent materials in this area. Despite being marketed under the same trade name (Viton) as their FKM “cousins,” these base-resistant types more closely resemble the FEPM formulations previously discussed. Viton VTR-7480 is a copolymer of tetrafluoroethylene and propylene. Its chemical and processing properties are analogous to those of Aflas 150P, making Viton VTR-7480 suitable for both molded and extruded goods. Viton VTR-7512 is also a TFE/P copolymer, in this case similar to Aflas 100H. With its higher molecular weight (relative to Viton VTR-7480), Viton VTR-7512 is more extrusion resistant and good in higher pressures.
The DuPont engineers continue to expand and refine their line of fluoroelastomers in response to the needs of industry. For example, automotive powertrain applications are making greater demands on seal compounds. Increasingly aggressive lubricants and higher temperatures are testing even the best materials.
In response, DuPont developed two new base-resistant Viton materials. These latest additions were unveiled at the Society of Automotive Engineers 2000 World Congress in Detroit. The first of these, Viton TBR-501C, is a terpolymer combining TFE/P with a low level of vinylidene fluoride (VF2). The “TBR” in the name refers to its “total base resistance.” It is completely resistant to highly basic lubricant additives. Viton TBR-501C also has excellent high temperature resistance and outstanding processibility.
The other new addition, Viton IBR-401C, is a terpolymer combining TFE/P with a higher level of VF2 (roughly three times as much as is in Viton TBR-501C). The “IBR” refers to its “intermediate base resistance.” It is not as base-resistant as Viton TBR-501C, but it still offers a good balance of base and hydrocarbon resistance, as well as excellent processibility.
In both the IBR and the TBR types, presence of the VF2 does three things. First, it improves the materials’ low temperature properties. Second, it also improves the materials’ resistance to swelling in hydrocarbon oils, though this improvement comes at the expense of base resistance. That’s why Viton IBR-401C has less base resistance than Viton TBR-501C; the IBR material has a greater VF2 content. Third, the VF2 allows both these materials to be bisphenol cured rather than peroxide cured (the norm for TFE/P copolymers). Bisphenol curing makes for easier processing and lower scrap rates.
You may encounter instances in which none of the materials already discussed quite meet the needs of an application, especially if you require good low temperature properties in partnership with both base resistance and low swell in hydrocarbons. Viton specialty type GFLT may work, but you might also consider Viton ETP (also known as Viton Extreme). The “ETP” is short for ethylene, tetrafluoroethylene, and perfluoromethyl vinyl ether (PMVE). The Viton ETP formulations (such as ETP-500 and ETP-900) offer the most comprehensive fluids resistance (including resistance to strong bases, amines, and polar solvents) of all the Viton types. They also combine outstanding resistance to hydrocarbons (including fuels) and high temperature resistance (equal to standard FKM compounds) with good low temperature flexibility and good processibility. As you can see in Table 12, Viton ETP surpasses both standard FKM-A and TFE/P in resistance to hydrocarbons, strong bases, amines, and polar solvents.
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“TFE/P provides a unique combination of chemical, heat, and electrical resistance.”