A general-purpose FKM is chosen for heat and chemicals. Its weak point is the cold. For cold-climate pipelines, LNG-adjacent equipment and winter start-up, the question becomes: which compound still seals at the low end of the window?
Why FKM stiffens
The glass transition temperature (Tg) of a rubber marks where it changes from rubbery to stiff. A typical general-purpose FKM has a Tg a little below -15 °C. Seals begin to lose useful recovery near that point, so the practical sealing limit sits a few degrees above it, often around -20 °C. Chemistry changes (more flexible monomers, different fluorine content, special polymer types) move Tg lower, at a price in fluid resistance or cost.
The approaches in plain terms
- Low-temperature FKM types built from different monomers, giving a lower Tg while keeping much of the heat and fluid resistance.
- Blends and compounding choices such as plasticiser type. Take care: a plasticiser that helps in the cold can extract in service.
- A different polymer when the fluid allows it. HNBR with lower acrylonitrile content is more flexible in the cold. Ask about its limits in sour or amine service.
What the cold-end numbers mean
- Tg by DSC tells you where the transition sits. It is not the sealing limit.
- TR10 (ASTM D1329) is the temperature at which a stretched, frozen sample has retracted by 10 percent. It is closer to the practical limit.
- Compression set at low temperature (ASTM D1299 or ISO 815-2) shows whether a seal springs back after being held cold.
- Brittleness temperature (ASTM D2137) is a failure limit, not a sealing limit.
Questions to ask
- Which low-temperature test, at which temperature, and what was the result?
- Is compression set reported after both heat ageing and cold holding?
- What is the effect on fluid swelling and heat ageing compared with a standard grade?
- Does the lot record report the same tests, so you can see consistency?
Our LT and ULT grades are written for this duty. FKM Low-Temp 90P (FKM-LT90P-01) and FKM Ultra-Low-Temp 90P (FKM-ULT90P-01) on the compounds page show how the targets, limits and test plan are laid out. State your own cold and hot ends and we will propose candidates.
