Nitrile rubber (NBR, also sold as Buna-N) is the standard elastomer for oil and fuel seals. It is a copolymer of acrylonitrile and butadiene, and it is the default material for O-rings, lip seals, gaskets and hose in contact with mineral oil, grease and petroleum-based hydraulic fluid. Compounds are available from 20 to 95 Shore A, with tensile strength of 3.4 to 17.2 MPa across the range. Standard compounds work down to about -40 to -35 C, and published upper limits range from 99 to 135 C.
Nitrile rubber properties
NBR is a family of compounds, not one material, so the figures are ranges. Where published supplier data agrees the table says so, and where it does not it gives the spread.
| Property |
Nitrile rubber (NBR) |
Agreement |
| Hardness, range of available compounds | 20 to 95 Shore A | Sources agree |
| Tensile strength, range across compounds | 3.4 to 17.2 MPa | Sources agree |
| Minimum service temperature, standard compounds | -40 to -35 C | Published figures vary (5 sources) |
| Maximum service temperature, standard compounds | 99 to 135 C | Published figures vary (5 sources) |
| Compression set | Good | Sources agree |
| Abrasion and tear resistance | Good | Sources agree |
Temperature range
The lower limit is consistent across suppliers at about -40 to -35 C for standard compounds. The upper limit is not: published figures run from 99 to 135 C. The difference is mostly between continuous and intermittent service, and between general purpose and heat-resistant compounds. Treat the lower end of that range as the continuous limit unless the compound's own datasheet says otherwise. Above it NBR hardens and cracks, and a hydrogenated nitrile (HNBR) or a fluorocarbon (FKM) is the usual step up.
Oil, fuel and chemical resistance
Resistance ratings below are the ones supplier compatibility data agrees on. They are a first screen, not a guarantee: check the specific fluid, concentration and temperature against the compound supplier's chart before specifying.
| Fluid or exposure |
Resistance |
| Mineral oils | Good |
| Petroleum-based hydraulic fluids | Good |
| Fuels (aliphatic hydrocarbons) | Good |
| Animal and vegetable oils | Good |
| Water | Good |
| Dilute acids and alkalis | Good |
| Ozone, sunlight and weather | Poor |
The agreed weak point is weathering. NBR is attacked by ozone and sunlight, so a nitrile seal stored or used in the open cracks at the surface. One detailed supplier compatibility chart also rates NBR as poor against ketones, ethers, amines, halogenated solvents, phosphate ester fluids and non-petroleum brake fluids. Treat those as fluids to check before use.
How acrylonitrile content changes the compound
The proportion of acrylonitrile in the polymer is the main variable between nitrile compounds. More acrylonitrile gives better resistance to petroleum oils and hydrocarbon fuels, and worse flexibility at low temperature. Less acrylonitrile does the opposite. That is why a fuel-resistant nitrile and a low-temperature nitrile are different compounds, and why no single figure describes all NBR.
Nitrile for O-rings and seals
NBR is the lowest cost elastomer that handles oil, with good compression set, tear and abrasion resistance, which is why it is the default for hydraulic and pneumatic seals. Hardness is chosen to suit pressure and gap, as explained on the Shore hardness chart. For sizes see O-ring sizes BS 1806, and for choosing between NBR, FKM, EPDM and the others see O-ring material selection.
References
- ISO 48-4 - Rubber, indentation hardness by durometer method (Shore hardness)
- ISO 37 - Rubber, determination of tensile stress-strain properties
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