HNBR is nitrile rubber whose butadiene double bonds have been mostly hydrogenated. That makes it far more stable in heat and in aggressive oilfield fluids. Three polymer features define a grade, and a fourth, the cure system, decides much of the final performance.
1. Acrylonitrile (ACN) content
ACN controls oil resistance and low-temperature flexibility.
- Low ACN (about 17 to 25 %): better cold flexibility, lower oil resistance.
- Medium ACN (about 33 to 36 %): balanced.
- High ACN (about 43 to 50 %): best oil, fuel and gas resistance, poor cold flexibility.
Estimate ACN from the nitrile band intensity in IR against a calibration curve, from total nitrogen measured by elemental analysis, and from the glass transition.
2. Residual double bonds (RDB)
Fully saturated HNBR has less than about 1 % residual double bonds, partially hydrogenated grades keep several per cent for sulfur or faster peroxide cure. Residual unsaturation is estimated by infrared (the trans double bond band near 970 cm⁻¹) and by iodine value in specialist labs.
3. Polymer architecture
Mooney viscosity, molecular weight distribution and carboxylated or special types change processing and properties. These are rarely recoverable from the part alone.
4. Peroxide cure with coagents
Most HNBR seals are cured by peroxide with a coagent. The peroxide is gone after cure. The coagent leaves reacted residues. The cure state is best judged by compression set, swelling and the cure curve of a trial mix.
Fillers
Reinforcing carbon black is typical, silica is used in some grades, zinc oxide and magnesium oxide serve as acid acceptors and cure aids, and a plasticiser is added for low-temperature flexibility. Look at GC-MS of the extract for plasticiser type.
What to match for oilfield duty
- ACN and hydrogenation level.
- Filler type and loading, for rapid gas decompression resistance.
- Peroxide and coagent system, for compression set at temperature.
- Plasticiser, because it migrates in service.
Mixing and dispersion strongly influence these properties. See the insights section on internal mixing of peroxide compounds.
