Viton is a DuPont trade name for one of the more exotic gasket materials. It’s actually a form of synthetic rubber known as a fluoroelastomer and referred to in ASTM D1418 terminology as “FKM”. Whether FKM or Viton, gaskets made from this material have an impressive range of properties. It’s downside is that the material itself is expensive. That’s why custom Viton gaskets are held in reserve for the most challenging sealing applications.
Most synthetic rubbers, like SBR, are composed of long chains of carbon and hydrogen atoms. These have a limited temperature range and tend to swell when exposed to oils. They also break down slowly when exposed to UV, as is found in sunlight.
Adding fluorine dramatically changes these properties. Bonding tightly to the carbon atoms, the fluorine prevents other compounds from breaking down the chains, but still provides excellent flexibility.
Gasket materials need good compression set resistance, (the ability to spring back or recover when a load is taken off,) a wide temperature range, and good chemical resistance. FKM performs exceptionally well in regard to all of these.
The ASTM D2000 standard for classifying elastomers grades FKM as HK. The “H” indicates it’s performance deteriorates only slightly after prolonged exposure to temperatures of 250°C (480°F), (and it still functions at 300°C (570°F).) At the other end of the temperature scale, FKM can still provide effective sealing at -40°C (-40°F).
The “K” in the ASTM grade describes resistance to swelling. “K” is the lowest possible rating, meaning FKM swells less than almost any other elastomeric material. (For comparison, SBR is typically graded as “AA”.)
As regards other aspects of chemical resistance, FKM withstands ozone, hydrocarbon lubricating oils and typical automotive fuels. Where it performs less well is against strong acids, alkalies and ketones.
Good Applications for FKM
FKM is used extensively in the aerospace industry, to an extent in automotive applications, and more prosaically, around freezer doors. It’s UV resistance also makes it good choice in situations of prolonged exposure to sunlight.
One consideration when comparing FKM gaskets with less costly alternatives is the cost of gasket replacement. As FKM holds up so much better to harsh conditions it often lasts several times longer than other gasket materials. Thus FKM can pay for itself by reducing maintenance costs.
The names Viton and FKM both describe a family of fluoroelastomers, rather than just one material. There are variations in properties between the different grades. Before ordering Viton gasket material consult a product specialist to determine which grade might be most appropriate for your application.