Materials

Two compounds, chosen properly.

NBR covers most of what a workshop sees. FKM covers the rest — high temperature, and the fluids that dissolve NBR. We keep two because a third would only be there for the catalogue, and getting the selection wrong is the most expensive mistake in this product.

NBR

Nitrile · Buna-N · black

Hardness: Shore A 70 as standard · A90 on request
Temperature: −30 to +100 °C
Good for: Mineral hydraulic oil, grease, diesel, water, air, mild aqueous solutions
Not for: Ester-based and phosphate-ester fluids, ketones, ozone, strong acids, brake fluid
Standards: DIN 3771 · ISO 3601 · AS568 · BS1806
Typical use: Hydraulic fittings, flange joints, end caps, pneumatic components

FKM

Fluorocarbon · Viton® type

Hardness: Shore A 70 · 75 · 90
Temperature: −20 to +200 °C
Good for: High temperature, ester and phosphate-ester fluids, fuels, mineral acids, steam
Not for: Ketones, low-molecular-weight esters, ammonia, hot water and steam above 150 °C
Standards: DIN 3771 · ISO 3601 · AS568 · BS1806
Colour: Black, brown or green

Colour is not a grade. FKM is supplied black, brown or green depending on the pigment the compounder uses. Two rings of the same colour can be different compounds, and two rings of different colours can be the same one. Specify by material, not by eye.

Selection

Start from the fluid, not from the temperature.

Temperature decides which materials survive. The fluid decides which ones are compatible. When the two disagree, the fluid wins — a ring that swells by ten per cent will leak long before it burns.

If your application is…	Use	Because
Mineral hydraulic oil, up to 90 °C	NBR 70	The default. Cheapest compound that works, and the one most equipment was designed around
Mineral oil above 100 °C	FKM	NBR hardens and loses compression set above its limit
Phosphate-ester or biodegradable ester fluid	FKM	Ester fluids swell NBR severely — this is the classic failure
Water, air, mild coolant	NBR 70	Fine up to 100 °C. For continuous hot water or steam, ask us about EPDM
Diesel, kerosene, light fuel oil	NBR 70	Good resistance; FKM also works but is not needed
Brake fluid, ketones, strong solvents	Neither	Ask us. This needs EPDM, FFKM or PTFE, and it is worth a conversation before ordering

If you are unsure, send the fluid name and the operating temperature. We will tell you which compound, and we will tell you if the answer is “neither of ours”. That costs us an order occasionally and saves both of us a return.

Also available

Other compounds, on request.

EPDM, silicone (VMQ) and HNBR are available against your part number but are not stocked as standard lines, because they serve applications our customers rarely bring us — hot water, food contact, refrigeration. Ask, and we will confirm availability before quoting.

Ask about a compound Documents we supply