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.
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.