Before a single gram goes into a furnace, three quick measurements can tell you a lot. They also guard against gross mistakes later.
Density first
Density separates fluoroelastomers from hydrocarbon rubbers almost on its own.
| Class | Typical compound density, g/cm³ |
|---|---|
| Silicone (VMQ) | 1.1 to 1.5 |
| EPDM, NR, SBR (black-filled) | 1.0 to 1.2 |
| NBR and HNBR | 1.1 to 1.4 |
| Chloroprene | 1.3 to 1.5 |
| FKM | 1.8 to 2.0 |
| FEPM | 1.5 to 1.7 |
| Perfluoroelastomer (FFKM) | 2.0 to 2.2 |
A mid-grey rubber ring with a density near 1.85 is almost certainly fluoroelastomer. If it measures 1.45 it might be HNBR with heavy mineral filler, or a chloroprene.
Hardness and what it needs
Typical filler loadings for a given hardness differ by polymer. A 70 Shore A HNBR may carry about 40 to 50 phr of reinforcing black, while a 75 Shore A FKM can reach that hardness with 30 phr of coarse thermal black. If your part is very hard with a low filler measurement, look for high crosslink density, a plastic phase or resin reinforcement.
Modulus against hardness
Plot modulus at 100 % against hardness. Compounds on the same polymer and filler family fall near one line. A point well above the line suggests more crosslinking or a more reinforcing filler. A point below suggests a plasticiser or low cure.
A decision flow
- Density in the 1.8 to 2.0 band: start with fluoroelastomer and verify by fluorine analysis and IR.
- Density 1.0 to 1.4 with a nitrile band in IR: NBR or HNBR; check hydrogenation by double-bond bands.
- Density near 1.0 to 1.2 with no nitrile and strong CH bands: EPDM, NR or SBR, separated by IR and DSC.
- Silicone confirmed by Si–O bands.
These shortcuts do not replace analysis. They tell you what to expect and flag surprises.
