FKM compounds are short on ingredients and long on consequences. A typical recipe has the fluoropolymer, a coarse filler, an acid acceptor pair and a cure system. Small changes in any of them move heat ageing, compression set and fluid resistance.
Step 1: confirm that it is fluoroelastomer
Density of about 1.8 to 2.0 g/cm³, strong C–F bands in IR, and fluorine content by elemental analysis. A DSC Tg near −20 °C suggests a standard type. A Tg near −35 to −40 °C points to a low-temperature grade.
Step 2: classify the polymer
- Copolymer (VDF and HFP), terpolymer (adding TFE), or a peroxide-curable type with a cure-site monomer.
- Fluorine content from elemental analysis.
- Low-temperature types using ether-containing monomers, which carry a glass transition several tens of degrees lower.
- Perfluoroelastomer if fluorine content is above about 72 % and the part is very expensive and very resistant.
Differences between a bisphenol-cured and a peroxide-cured FKM are among the biggest property drivers. They are also hard to see after cure.
Step 3: ash, acid acceptors and filler
Ash typically shows calcium and magnesium (from calcium hydroxide and magnesium oxide in bisphenol cures), or zinc oxide or lead oxide in some peroxide cures, plus a coarse filler such as thermal black, barium sulfate or mineral fillers. Because acid acceptors react with fluoride during cure and burn, the ash composition and mass differ from the original recipe. Take that into account.
Step 4: cure system clues
- Calcium hydroxide plus magnesium oxide and quaternary phosphonium traces: bisphenol cure.
- Triallyl isocyanurate (TAIC) or similar coagent residues with a peroxide: peroxide cure.
- Compression set after post-cure and cure behaviour on a rheometer are the best indicators.
Safety
Heating fluoroelastomers above about 300 to 400 °C releases hydrogen fluoride and other corrosive and toxic fumes. TGA, ashing and pyrolysis instruments must be designed for acid gases, vented and scrubbed, and the lab needs training and an HF safety plan.
Matching
Match polymer type and fluorine content first, then the acid acceptor system, filler and post-cure. Compare compression set at the duty temperature and low-temperature behaviour (TR10). For HPLT seals, fluorine content and cold-flexibility are in direct competition, and the match must be judged on tests, not on chemistry alone.
Sources and further reading
- Manufacturer technical literature for typical FKM formulations and cure systems (for example, public datasheets for peroxide- and bisphenol-curable grades).
