July 2017

Low Temperature Elastomers

Low temperatures play havoc with elastomeric gasket materials, as NASA will testify. (Details of how seal failure caused the Challenger disaster are available on the NASA website.) The issue is that at low temperatures gasket materials like nitrile rubber and neoprene become stiffer and less able to fill gaps. The TR10 number, derived from ASTM D1392, shows the temperature at which this stiffening effects sealing performance. The problem for people buying gasket material is knowing how low temperatures can go and which low temperature elastomer is best.

Global Low Temperature

Military Standard MIL_HDBK_310_1851 “Global Climatic Data For Developing Military Products”, tells equipment developers what conditions to design for. This notes that the lowest temperature ever recorded is -68°C (-90°F), in the USSR. Statistically, the lowest temperature to be expected in the coldest regions of the world is -69°C (-92°F).

Low Temperatures in the USA

Weather.com tells us the lowest temperature experienced in the U.S. is -62°C (-80°F), in Prospect Creek Alaska. Closer to home, the lowest in the contiguous 48 is -57°C (-70°F), measured at Rogers Pass, Montana. For those in the Midwest, record lows in Illinois, Indiana and Ohio are in the -34°C (-30°F) range, while Michigan, Wisconsin, and Iowa have records ranging from -43°C to -48°C (-45°F to -55°F).

Of course, this is without the effects of wind chill. Air flowing over surfaces takes heat away, making the temperature appear lower than it actually is. And the harder the wind blows, the greater the cooling effect.

Implications for Gasket Material Selection

When selecting gasket material for outdoor applications it’s essential to determine the lowest possible temperature. Not doing so risks leaks during periods of extreme cold, which is never a good time to be replacing a failed gasket!

Most nitrile rubber, EPDM and neoprene gasket materials work down to around -46°C (-50°F). HNBR is limited to around -40°C (-40°F) while silicone will endure temperatures down to around -59°C (-75°F). (values vary for individual material grades.)

If a gasket might be exposed to low temperatures material should be selected to suit. Specialists at Hennig Gasket will be happy to advise.

What is TR-10 (temperature of retraction) for Gasket Material?

Elastomeric gaskets, like those made from nitrile rubber and neoprene, work because they are flexible. Compressed in a flange, the material fills the gap between joint faces, regardless of how each side moves. When a joint gets cold the gap often grows wider, and the gasket material is expected to recover enough to maintain a good seal.

When elastomers become very cold they lose this ability. When choosing gasket material for an application that could see low temperatures it’s important to check it has sufficient low temperature flexibility. This is indicated by their TR10 number.

Low-temperature Behavior

Nitrile rubber and neoprene gaskets are flexible because they are made from entangled chains of molecules that move over one another. When temperatures drop, contraction limits the movement of these molecules and the material gets stiffer. Eventually, there’s no room for any movement and the material becomes brittle.

The “freezing” temperature of an elastomer is termed it’s Glass Transition Temperature (Tg). Strictly speaking, freezing happens over a range, and the brittle point is several degrees lower than the Tg. What matters for gaskets though is when the material loses any ability to recover.

The Temperature of Retraction

ASTM D1392 advises that “..retraction rate is believed to correlate with low-temperature flexibility of … rubbers.” Retraction is measured by first stretching and then freezing an elastomer. It’s then gradually warmed up, and as the molecular chains regain some freedom to move it starts to shrink back.

This recovery is measured against temperature and indicates when the elastomer transitions into rubber-like behavior. Experience shows this happens when it recovers by around 10% of it’s elongated length. The point at which this happens is the temperature of 10% retraction or TR10.

Gasket Material for Low-temperature Applications

If an elastomeric gasket will get cold, two things are needed: the lowest anticipated temperature, and the TR10 value for the candidate materials. If TR10 is not available use Tg as a substitute. For reliable in-service performance, ensure these values are lower than the worst conditions the gasket will experience.

Weather Resistant Gasket Material Selection

Weather is hard on gasket material, thus identifying a weather resistant gasket material is important. Ultraviolet (UV) rays in sunlight attack bonds between carbon atoms in many rubbers. Ozone, (O3) often found in polluted industrial areas and around electrical enclosures, does the same. Temperature extremes, especially cold, affect material performance, as can moisture.

Open vs Closed-Cell Gasket Material

Open cells will absorb moisture. If temperatures drop to freezing the subsequent expansion could quickly destroy the material. Always choose closed cell material for a gasket that’s likely to get wet.

Materials to Avoid in Outdoor Applications

Inexpensive nitrile rubber/NBR, often selected for its good oil and solvent resistance, is not a good choice for weather resistant gasket material. The same goes for SBR. Nitrile gasket material is poor with both O3 and UV light, although water resistance is quite good. Its low-temperature limit is around -30F (-34C).

Better Weather Resistant Gasket Materials

Neoprene is a popular choice for gasket material but is not recommended for outdoor use. While it can handle a wide temperature range and offers moderate weather resistance, O3 resistance is poor.

Best Weather Resistant Gasket Materials

A little more expensive than Neoprene, EPDM is good for outdoor gasket applications not involving exposure to oils or solvents. EPDM provides a combination of good O3, UV, and general weather resistance.

Like EPDM, silicone gasket material resists attack by O3, UV, and general weather resistance, but also has excellent fungal and biological resistance. (Sometimes relevant in humid climates.) It has a wide temperature range but poor abrasion resistance is a limitation in applications with a significant joint movement or frequent opening. Silicone also suffers from high gas permeability and is not recommended for use with ketones, petroleum or chlorinated solvents.

For superior outdoor performance, fluorosilicone materials stand out. These have excellent resistance to O3, UV, and weather plus a very wide temperature range. They are also good with fuels. Their main limitations are poor abrasive performance and high price.

Gasket Material Selection Advice

Always consider the application environment when choosing weather resistant gasket material. Outdoor locations expose a gasket to UV light, weathering, and possibly O3. Extremely low temperatures can also be a problem. Hennig Gasket material specialists can provide further advice.