NBR, FKM, PTFE, polyurethane: which material for your seal?
The right profile is not enough: the material must also suit the fluid, temperature and loads. Here is an overview of the most used materials and when to choose them.
Two seals with the same profile and the same dimensions can behave completely differently if they are made of different materials. The material determines fluid compatibility, temperature range, resistance to wear and extrusion, friction and, ultimately, the life of the seal. Let us look at the materials you will most often find in our catalogue and when to choose them.
NBR – nitrile rubber
It is the most widespread elastomer in hydraulics. It resists mineral oils, greases and many fuels well, has excellent elasticity and a moderate cost. It works well at medium temperatures: many of our NBR products indicate a range of about −30 °C / +110 °C, but the exact value depends on the compound and must be checked on the product page. It is not suitable for phosphate ester fluids, many solvents or prolonged ozone exposure.
FKM – fluoroelastomer (Viton)
Chosen when higher temperatures or greater chemical resistance are needed: synthetic oils, fuels, many acids and solvents. It is more expensive than NBR and less elastic at low temperatures, so it is not always the best solution for machines working in the cold. It is widely used for O-rings, oil seals and static seals in hot environments.
PU – polyurethane
Polyurethane is the reference material for dynamic seals under heavy stress: it offers excellent abrasion resistance, tear and extrusion resistance. That is why it is very common in rod seals, wipers and compact seals for mobile machines. Its limits are the maximum temperature, generally lower than that of FKM, and sensitivity to hot water and steam (hydrolysis) for some compounds.
PTFE – polytetrafluoroethylene
PTFE is not an elastomer but a thermoplastic polymer. It has an extremely low coefficient of friction, is practically chemically inert and withstands a very wide temperature range. On its own, however, it is not elastic: in dynamic seals it is paired with an elastomer ring (O-ring or square ring) that presses it against the surface. PTFE is often filled with bronze, glass, carbon, graphite or molybdenum disulphide to improve wear resistance and resistance to creep under load.
EPDM
Ideal for water, steam, glycols, many detergents and special hydraulic fluids, as well as outdoor applications thanks to good weathering resistance. Be careful though: it is not compatible with mineral oils and greases, which make it swell quickly.
Silicone
It offers a very wide temperature range and, in FDA-compliant grades, is suitable for food contact. However, its mechanical and abrasion resistance is low, so it is mainly suited to static or lightly dynamic seals.
Materials for guides and rigid components
For guide rings and back-up rings, rigid materials such as fabric-reinforced phenolic resins, polyester, POM and nylon are used. They do not seal, but they must carry radial loads and keep stable dimensions over time.
How to choose in practice
- Start from the fluid: the material must be compatible, otherwise it swells, hardens or crumbles.
- Check the real minimum and maximum temperature, including peaks.
- Consider the mechanical stresses: pressure, speed, dirt, side loads.
- Evaluate the friction required: for precise or fast movements PTFE is often the best choice.
On our website the material of every product is clearly indicated. If you use special fluids or work at extreme temperatures, write to us before ordering: we will help you choose the most suitable compound.
Frequently asked questions
Can I replace an NBR seal with an FKM one?
Often yes, if profile and dimensions are the same and the application does not work at very low temperatures. FKM costs more, so it is worth using it where it is really needed.
How can I recognise the material of an old seal?
Colour is not a reliable indicator. If there are no markings, refer to the machine data or ask us for advice, stating fluid and temperatures.
Do biodegradable fluids require special materials?
Some do: with ester-based fluids it is important to check material compatibility before changing the oil.
