November 2016

How Does Foam Seal?

Foam gaskets for HVAC systems and enclosure panels and doors are cut from elastomeric materials with a cellular structure that looks like sponge. Although compressible, sponges tend to absorb water, so we’re sometimes asked how foam can make a good sealing material.

Open or Closed?

Foam is made by trapping bubbles in the material as it solidifies. These form during the chemical reaction that creates the elastomeric material. Foam manufacturers control the size of these bubbles by managing the reaction process and using chemical additives.

An important property to consider in foam gasket material is whether the cellular structure is open or closed. When the structure is closed bubbles are closed off from one another and there’s no way for air or any other fluid, to move between them. In contrast, in an open structure most of the cell walls are missing, which lets fluid travel through readily.

Sponges are generally open because their purpose is to absorb water. Foam with a closed structure doesn’t have this ability, and that makes it fluid-resistant.

Open cell foam can be used as gasket material provided complete impermeability isn’t needed. Compressing it to less than 50% of its free thickness leaves very little space for liquid to pass through. This allows it to be used where light water exposure is possible.

Other Pluses and Minuses

A cellular structure makes elastomeric gasket materials less dense, which is beneficial when weight is a concern. They also tend to be soft and compress easily, although closed cell material is firmer because the air inside the cells has nowhere to go. While compressed, the air slowly permeates through the cell walls, and recovery is slower when the load is taken off.


Flexible cellular materials are covered by ASTM D1056. This standard uses a three character code to describe foam materials. The first character is either a “1”, meaning an open structure, or a “2”, meaning closed. When ordering a foam gasket material it’s important to be clear which is needed. If in doubt, speak with a Hennig Gasket material specialist.

Does Your Application Need a Viton Gasket?

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.

Chemistry Lesson

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.

Key Properties

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.