They are almost exact opposites
Nitrile — NBR, sometimes called Buna-N — is built for oil. Its acrylonitrile content gives it excellent resistance to petroleum oils, diesel, hydraulic fluid and grease. EPDM is built for everything oil is not: water, steam, sunlight, ozone, weather.
Neither tolerates the other domain. Put EPDM in mineral oil and it swells dramatically, softens and loses all sealing force. Put NBR outdoors in sunlight and it cracks along the flex lines within a season.
How the mistake usually happens
Rarely through ignorance. Usually through drift. A maintenance store orders "rubber gaskets, 70 shore, black" because that is what the last box said. Nobody records which polymer it was. Two suppliers later the compound has quietly changed, and a batch of EPDM lands in an oil application. The gaskets look right, fit right, and fail in six weeks.
The other route is a drawing that specifies only hardness and colour. Hardness tells you nothing about chemical resistance. A 70 shore nitrile and a 70 shore EPDM are equally hard and completely different materials.
Specify the polymer, always
A rubber specification needs polymer, hardness and, where it matters, temperature range: "NBR, 70 ± 5 Shore A, −30 to +100 °C" is a specification. "Black rubber, 70 shore" is a lottery ticket.
Telling them apart when nobody wrote it down
If you have an unlabelled part and need to know what it is, a few things help. None is conclusive alone, but together they are usually decisive.
- The oil soak. Cut a small piece, weigh it, leave it in engine oil at room temperature for 24 hours and weigh again. EPDM will gain significant weight and feel spongy. Nitrile will barely change.
- The stretch-and-look. Stretch an old part and look at the surface under good light. Fine cracks perpendicular to the stretch suggest ozone attack, which points to nitrile used outdoors.
- Where it came from. A part off a hydraulic or fuel system is almost certainly nitrile. A part off a water, steam, roof or outdoor assembly is very likely EPDM.
- Send it to us. We can usually identify the family from hardness, density, surface and swell behaviour, and reproduce it correctly.
When neither is right
Some duties genuinely need both oil and weather resistance — outdoor equipment with an oil leak path, for instance. That is what neoprene is for: worse than nitrile on oil, worse than EPDM on weather, but acceptable at both, which neither of the others manages.
And if the application involves solvents, acids or sustained heat above 140 °C, both are out and the answer is Viton. It costs several times as much per part, which stops being relevant the first time an unplanned shutdown is avoided.
The material selector on this site walks through media, temperature and duty and ranks all eight compounds we mould, with the reasoning shown for each.
Have a part that fits this description?
Send the dimensions and the operating conditions. We will tell you what compound it should be in and quote it — including saying so if it is not a job for us.