Rubber is a rubbery solid only above a temperature called the glass transition temperature (Tg). Below Tg the polymer chains are frozen and the part behaves like glass: stiff, brittle, no longer sealing. Differential scanning calorimetry (DSC) measures Tg by tracking the heat needed to warm a small sample.
Typical Tg ranges
| Polymer | Approximate Tg |
|---|---|
| Silicone (VMQ) | about −120 °C |
| Polybutadiene (BR) | about −100 °C |
| Natural rubber | about −65 °C |
| EPDM | about −50 to −55 °C |
| SBR | about −50 to −60 °C, rising with styrene |
| Polychloroprene | about −45 °C |
| NBR and HNBR | roughly −45 to −15 °C, rising with acrylonitrile content |
| Acrylate rubber (ACM) | roughly −40 to −15 °C depending on grade |
| FKM (standard VDF/HFP) | about −20 to −15 °C |
| Low-temperature FKM | about −40 °C and below, with special monomers |
| FEPM (TFE/propylene) | close to 0 °C |
These are indicative. Grade and composition move the number by several degrees.
What DSC adds to identification
Tg is a quick fingerprint. A nitrile or HNBR with Tg near −35 °C is probably a mid-ACN grade, and a result close to −15 °C points to a high-ACN type. A fluoroelastomer with a Tg around −17 °C is a standard type, while one near −35 °C is a low-temperature grade. When the IR spectrum says "fluoropolymer" and DSC says "−40 °C", you know to look for a low-temperature or perfluoroether-containing family.
Plasticisers lower the apparent Tg. A compound with 15 phr of plasticiser will show a lower Tg than the raw polymer, so measure both the cured part and the extracted rubber.
Why HPLT needs more than Tg
A low Tg is necessary but not sufficient. Seals fail in the cold because of stiffening, loss of recovery and low-temperature compression set, which depend on the network and fillers as well as the polymer. Tg should always be followed by tests such as TR10 and TR70 retraction and low-temperature compression set. Pascalor LT and ULT grades are specified on those, not on Tg alone.
Running DSC well
- Use a small, representative sample, about 5 to 15 mg.
- Run a heating cycle that erases thermal history, then a controlled second heating, and report the midpoint of the transition from the second run.
- Quote the heating rate. A different rate shifts the number.
Sources and further reading
- ASTM D3418 and ISO 11357-2, determination of glass transition temperature by DSC.
