Hydrogenated nitrile butadiene rubber (HNBR) starts as nitrile rubber (NBR) and has most of its double bonds saturated with hydrogen. The saturated backbone gives much better heat, ozone and oxidation resistance than NBR, while keeping the oil resistance and toughness that nitrile is known for. HNBR is stronger and tougher than FKM in tension, tear and wear, which is why it is common in dynamic seals, stators, packers, belts and cable parts.
The three choices that define a grade
Acrylonitrile (ACN) content. More ACN means better resistance to oil, fuel and polar fluids, but poorer low-temperature flexibility. Commercial grades range from about 18% to 49% ACN. Low-ACN grades suit cold climates and moderate oils. High-ACN grades suit oils with aromatics and higher swelling.
Degree of hydrogenation. Described as residual double bonds (RDB, in percent) or iodine value. A very low RDB, below about 1%, gives the best heat and ozone resistance and is almost always peroxide cured. A grade with more residual unsaturation, say several percent or an iodine value of 20 or more, can be sulfur cured, which helps adhesion to metal and textile and some dynamic properties.
Mooney viscosity. Higher Mooney grades tolerate more filler and plasticiser and give stronger vulcanisates. Lower-Mooney grades process more easily.
Cure systems
Peroxide cure is the standard for best heat resistance and compression set. Dicumyl peroxide, a bis(tert-butylperoxyisopropyl)benzene or a similar dialkyl peroxide is used, together with a coagent. Zinc diacrylate or zinc dimethacrylate can be added to give very high hardness and strength by forming ionic crosslinks along with covalent ones. This route is used for high-modulus, extrusion-resistant seals.
Sulfur cure is possible for lower-saturation grades. It gives improved adhesion and dynamic behaviour, with lower heat resistance than peroxide.
Compounding notes
- Antioxidants matter more in HNBR than in fluoro-rubbers, but they can interfere with peroxide cure. Amine antioxidants of the diphenylamine type and mercaptobenzimidazoles are the usual choices.
- A low-volatility plasticiser such as TOTM helps flexibility while limiting loss at temperature.
- Fillers range from MT black (low reinforcement) to reinforcing blacks, plus silica and mineral fillers for specific needs.
- Carbon-black loading drives hardness. Many compounds reach 70 to 90 Shore A with 30 to 70 phr.
Where HNBR fits
HNBR is a good choice for oil, grease, many fuels (not heavily aromatic), water and steam to moderate temperature, and for dynamic sealing that needs wear and tear resistance. It is not as good as FKM in aggressive fuels or at the highest temperatures, and it does not match FEPM for steam and strong bases. Typical service is up to about 150 °C continuous, with grade-dependent limits, and it can go lower than FKM in cold service when low-ACN grades are used.
Sources and further reading
- Arlanxeo Therban and Zeon Zetpol grade information.
- P. A. Ciullo and N. Hewitt, The Rubber Formulary, 1999.