Ask ten engineers to specify an elastomer and you will get eleven opinions. That is not a failure of the industry; it is a reflection of how much a rubber compound actually does. Elastomers seal, damp, cushion, insulate, transmit torque and swallow tolerances that machined parts cannot. Getting the specification right is less about picking a "best" material and more about ranking the compromises in the order that your application demands.
This is the sequence we use when we sit down with a customer's drawing and a blank line for "material".
Start with the environment, not the material
Before anyone types "Viton" or "silicone" into a specification block, we ask three questions.
First, what is the operating envelope? Not just the nameplate temperature, but the excursions. A shaft seal that lives at 90 °C in normal service but sees 150 °C during a cold start does not want an NBR. A gasket that is nominally room-temperature but sits next to a hot exhaust manifold is thermally cycled far harder than the datasheet suggests.
Second, what is the fluid or media? "Oil" is not a specification — mineral hydraulic oil, synthetic ester turbine oil and biodiesel-blended fuel oil each attack different compounds. Where a component sees more than one media, the worst offender sets the material.
Third, is the duty static or dynamic? A static face seal tolerates a wider hardness range and a broader compression set spec than a rotating shaft seal, where microscopic wear allowances dominate the choice.
Only after those three answers do we begin narrowing families.
The workhorses, and where each earns its living
**Nitrile (NBR)** remains the default oil-resistant elastomer, and there is a reason it dominates automotive and hydraulic markets: it is cheap, mouldable, chemically compatible with the fluids most machinery encounters, and its mechanical properties are well characterised across a wide hardness range. Its ceiling is roughly 100–120 °C in continuous service. Above that, or in the presence of ozone, it embrittles.
**EPDM** is the outdoor and hot-water specialist. It shrugs off ozone, UV and weathering, handles brake fluid and glycol coolants, and takes steam without complaint. It is entirely wrong for anything petroleum-based — mineral oil will swell an EPDM seal to failure in weeks.
**Silicone (VMQ)** is the compound engineers reach for when the temperature range is wider than any other family can span: a well-formulated silicone runs from −60 °C to +200 °C and above. It is also the default for skin-contact, food-contact and implantable medical applications because it is inherently biocompatible. Its weaknesses are mechanical: silicone tears easily and does not tolerate dynamic sealing against a moving shaft.
**FKM (Viton and equivalents)** takes over when NBR runs out of heat or fluid resistance. Continuous service to 200 °C, excellent aggressive-fluid resistance, and the mechanical toughness NBR is known for. It costs several times more per kilogram than nitrile, so it is specified where the application demands it — not as a default upgrade.
When you need the specialist grades
**FFKM** — perfluoroelastomer — exists for one reason: nothing else survives chemical processing at 300 °C. It is by far the most expensive family in common use, and it is specified for semiconductor plasma seals, hot chemistry vessels and aerospace bleed-air components where a leak is an entire vehicle grounded. If your application does not need it, you should not be paying for it.
**FVMQ (fluorosilicone)** is silicone with fuel resistance grafted on, and it is the reason aircraft fuel systems can carry silicone-family low-temperature performance into a service environment that would swell an ordinary VMQ. It is the compound family behind AMS-R-25988 (covered in more detail in a separate post).
**HNBR** is the answer engineers arrived at when they wanted nitrile's toughness at higher temperatures than nitrile can survive. Hydrogenated NBR keeps the oil resistance and adds around 30 °C of thermal headroom, at roughly two-to-three times the cost.
The question we always ask
If a component is being re-sourced from another supplier, the single most useful piece of information is not the incoming drawing — it is the failure mode of the part it is replacing. A seal that hardened and cracked tells you the compound was heat-limited or ozone-attacked. A seal that swelled and extruded tells you the fluid was wrong. A seal that lost its squeeze without visible damage tells you compression set was the culprit. Each failure pattern points at a different family of substitutes, and picking blindly from a datasheet without that context is guessing.
A word on hardness
Shore A hardness is the number that gets thrown around, but it is a comparatively blunt instrument. Two 70-Shore compounds from different suppliers can behave very differently under compression set, tear strength and long-term ageing. When the application is critical, we specify hardness plus a named compound, not hardness alone.
Send us the drawing, and the story
Datasheets are a starting point, not an answer. Half the specifications we improve on come from a five-minute conversation about what the part actually does in service — not from a longer study of the datasheet. If you have a component you are re-sourcing or a new design where the material choice is open, send us the drawing and the story. That is where good material selection starts.
