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PERFECTING THE ART OF PERFLUOROELASTOMERS
Part three of a special three part series on fluorine-based materials
Go to part one of the series
Go to part two of the series
by Rick Hudson
Over the course of our last two issues, I have been taking an in-depth look at fluorine-based elastomeric materials. In our May/June issue, I provided some background information on fluorocarbon elastomers, most notably Viton®. In our July/August edition, I then moved on to base-resistant compounds, such as Aflas® and Viton ETP. In this issue, I’d like to conclude the series by focusing on the advantages offered by perfluoroelastomers. As you’ll see, perfluoroelastomers provide a truly remarkable level of chemical resistance, making them ideal for a wide variety of very harsh applications.
Designated under ASTM D1418 as FFKM, most of today’s commercial perfluoroelastomers are terpolymers of tetrafluoroethylene (TFE), perfluoromethylvinyl ether (PMVE), and a cure site monomer (CSM). The fully-fluorinated monomers contained in perfluoroelastomers are the reason they exhibit superior chemical resistance. As previously noted in relation to fluorocarbon elastomers, the bonds between carbon and fluorine atoms are extremely strong, making the chemical structure virtually unbreakable. Also, polymers with high levels of fluorine (as opposed to hydrogen) have proven to be more stable and less reactive. Finally, perfluoroelastomers also enjoy immunity from chemical attack due to saturation along the polymer’s backbone. There are no double bonds to be attacked by degradants such as oxygen, ozone, UV light, or harsh chemicals.
PIONEERING
THE POLYMER Modern
perfluoroelastomers can trace their lineage back to the late 60s, when chemists
at DuPont pioneered what came to be known as Kalrez®. In
so doing, they combined the chemical resistance of Teflon® and the elasticity
of Viton into a fully-fluorinated polymer that could be cross-linked. Differences
in perfluoroelastomer performance are often due to the manner in which the
material is cross-linked. In the early days, perfluoroelastomer compounds
made use of bisphenol cross-links (like those still seen in current copolymer
fluoroelastomers). Bisphenol curing works fine for fluoroelastomers, but
it became clear that these bisphenol cross-links were causing perfluoroelastomers
to undergo a high degree of compression set. As a result, in the mid-80s
DuPont developed compound 4079. This new perfluoroelastomer formulation utilized
high temperature triazine cross-links. Compression set was reduced, and,
as an added bonus, thermal properties were enhanced (allowing the material
to stay resilient even in temperatures approaching 600° F, 315° C).
Because of the presence of aggressive chemicals and the need to exclude microcontaminants, seals used in the manufacturing of integrated circuits (ICs) must withstand harsh fluids while resisting extraction. Perfluoroelastomers like Kalrez (which is resistant to over 1,600 solvents, chemicals, and plasmas) have found wide use within the semiconductor industry. Kalrez seals are also common in the oil exploration and refining industries, as well as in chemical processing and transportation seals. Be aware that Kalrez’s vulnerability to compression set generally increases as temperatures increase. Despite its overall chemical resistance, Kalrez can swell when in contact with uranium hexafluoride, fully halogenated Freon®, and some fluorinated solvents. Kalrez should not be exposed to molten or gaseous alkali metals.
PRODUCING THE PARTS As instrumental as they were to the development and acceptance of perfluoroelastomer compounds, the DuPont personnel were not the only ones on the case. At about the same time that DuPont was finding new success with triazine cross-links, another company was experimenting with peroxide cure systems. Greene, Tweed & Company started producing Chemraz® parts based on an imported, peroxide cross-linked perfluoroelastomer.
Chief among Chemraz’s virtues is its outstanding overall chemical compatibility. Chemraz parts are resistant to almost every chemical compound, including fuels, ketones, esters, alkalines, alcohols, aldehydes, and both organic and inorganic acids. Chemraz also has very good resistance to compression set, and it offers outstanding steam resistance. Chemraz has an upper temperature limit of about 450° F (232° C). As with Kalrez, Chemraz has found a place in the demanding semiconductor industry. Greene, Tweed prepares Chemraz compounds in a state-of-the-art clean room to ensure purity from the very beginning.
PERFECTING THE PROCESS Not to be outdone, International Seal Company (ISC) launched their own perfluoroelastomer program in 1996. They developed their compound – Aegis® - with an eye toward providing a cost-competitive (yet still high performance) alternative to Kalrez and Chemraz. The 1998 merger of ISC with Freudenberg-NOK provided significant improvements in both the technology and the processing of perfluoroelastomers. Some of ISC’s perfluoroelastomer compounds are cross-linked using peroxide; some are not.
Last year, ISC introduced five new Aegis compounds specifically tailored to the semiconductor industry, and each of these "SC" perfluoroelastomers is environment-specific. For example, SC1001 is a low compression set compound intended primarily for wet chemical applications. SC1011 is used mainly in dry process, vacuum, and plasma environments. SC1090 is the cleanest grade and is useful for both wet chemical and plasma projects. SC1070 is a high temperature formulation (viable up to 572° F, 300° C) intended for low-pressure chemical vapor deposition and furnace applications. With high temperature resistance comparable to SC1070, SC1071 is suited for aggressive plasma environments. New Aegis compounds are also being developed in response to industry needs. As a matter of fact, ISC offers a wide range of seal materials (produced in a class 100/1000 clean room) specifically designed to provide both the high purity and the extraordinary chemical resistance demanded by the semiconductor industry. Aegis seals are also commonly used in chemical and petroleum processing, analytical instruments, automotive systems (fuel and oil), and spray painting systems. The main advantages of Aegis compounds over Kalrez and Chemraz: less compression set and higher strength at a lower cost.
So there you have it. I sincerely hope you’ve found our series on fluorine-based materials helpful. If so, let me know! My e-mail address is rick@rlhudson.com. I look forward to hearing from you. And keep watching this space! Our next issue will be devoted entirely to polyurethane, and I’ll be bringing you the latest information on the exciting new advances in polyurethane O-ring technology. The rumor around here is that my favorite superhero, THANE, will also be making a very special appearance. I can’t wait to see what that’s all about! Until then, please feel free to call us at 1-800-722-6766 if we can be of further technical assistance.
Aegis® is a registered trademark of International Seal Company. Aflas® is a registered trademark of Asahi Glass. Chemraz® is a registered trademark of Greene, Tweed & Company. Freon® and Teflon® are registered trademarks of E.I. du Pont de Nemours & Company. Kalrez® and Viton® are registered trademarks of DuPont Dow Elastomers.
