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 only get you so far. Half the specifications we end up improving come out of a five-minute phone call about what the part actually does in service. If you are re-sourcing a component, or the material choice on a new design is still open, send us the drawing and tell us the story behind it.
