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BUILDING THE BASIS FOR BASE RESISTANCE
Part two of a special three-part series on fluorine-based materials
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by Rick Hudson
In our last issue, I began a three-part series on fluorine-based materials. In that first installment, I surveyed the general history of fluorocarbon elastomers (FKM), then focused more specifically on the benefits offered by several of the most common FKM formulations (such as Viton® A, Viton B, and Viton GF). In all honesty, I had not anticipated the response I got as a result. I want to thank each of you who took the time to call or e-mail me regarding your Viton success stories. I like nothing better than to hear about applications that redefine what’s possible.
Though standard FKM compounds offer great advantages (including excellent resistance to hydrocarbon fluids and very good low temperature flexibility), their limited base resistance makes them unsuited for some environments. The hexafluoropropylene (HFP) and vinylidene fluoride (VF2) sequences found in standard FKM formulations can be compromised (dehydrofluorinated) by exposure to strong bases and amines. For example, the amine-based corrosion inhibitors used in many oil field applications can cause unwanted crosslinking in standard fluorocarbon seals. Exposure increases seal hardness, and tensile strength and elongation decrease. Surface cracks may form and develop into leak paths. The effects of chemical degradation are exacerbated when exposure occurs in high temperature situations. The need to overcome these problems served as the basis for the development of base-resistant materials.
You may recall that one of the FKM types I profiled last time, Viton GFLT, has no HFP-VF2 linkages in its backbone (the HFP having been replaced by perfluoromethyl vinyl ether – PMVE – in order to improve low temperature flexibility). The PMVE-VF2 linkages can be dehydrofluorinated by bases or amines, but not as readily as HFP-VF2 linkages. In addition, Viton GFLT has less VF2, so the number of PMVE-VF2 linkages is lower. Bottom line: Viton GFLT offers a higher degree of base resistance than standard (HFP-VF2) FKM materials.
There are also other materials on the market that offer varying degrees of base resistance. As distinguished from FKM compounds, base-resistant fluoroelastomers are designated under ASTM D1418 as FEPM. You may already be familiar with some of their trade names (such as Aflas® and Viton ETP), but you may not realize just how much these base-resistant compounds have to offer.
DEFINING THE BOUNDARIES The FEPM designation was originally directed at copolymers of tetrafluoroethylene (TFE) and propylene (P). 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 (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. There are varying grades of Aflas, each with their own molecular weight. 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 the chart, 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.
TESTING THE LIMITS Though it was the first, Aflas is not the only base-resistant fluoroelastomer on the market. On the contrary, DuPont Dow 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. As a Viton distributor, I’d like to call a few of these to your attention.
Viton VTR-7480 is a copolymer of tetrafluoro-ethylene 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. Because of its increased molecular weight (relative to Viton VTR-7480), Viton VTR-7512 is more extrusion resistant and therefore good at handling higher pressures.
But wait – there’s more. One of the reasons I enjoy being a DuPont distributor is that their material engineers are never satisfied to "run in place." They continue to expand DuPont’s line of fluoroelastomer compounds in response to the ever-changing needs of industry. For example, automotive powertrain applications are making greater and greater demands on seal compounds. Increasingly aggressive lubricants and higher temperatures are testing the limits of even the best materials.
In response, DuPont developed two new base-resistant Viton materials. These latest additions were unveiled in March during 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 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 processing characteristics.
The other new addition, Viton IBR-401C, is a ter-polymer 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." In other words, 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.
It may help you to know that, in both the IBR and the TBR types, presence of the VF2 does three things. First, it improves the low temperature properties of the materials. 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. And third, the VF2 allows both these materials to be bisphenol cured rather than peroxide cured (which is the norm for TFE/P copolymers). Bisphenol curing makes for easier processing and lower scrap rates.
GOING TO EXTREMES Before moving on, there’s just one other product I’d like to mention, namely Viton ETP Extreme. 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.
The "ETP" is short for ethylene, tetrafluoro-ethylene, 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. Now that’s what I call an "extreme"-ly impressive combination of properties! As you can see in the chart, Viton ETP surpasses both standard FKM-A and TFE/P in resistance to hydrocarbons, strong bases, amines, and polar solvents.
I hope you’ve found this look at base-resistant fluoroelastomers helpful. In our next issue, I’ll conclude our series on fluorine-based materials by spotlighting perfluoroelastomers (such as Aegis®, Chemraz®, and Kalrez®). Until then, please feel free to call us at 1-800-722-6766 for more information on base-resistant compounds.
