Silicone vs EPDM: Selecting the Right Elastomer for High-Temperature Seals

Silicone or EPDM: which should you use?

First, is there oil?

If the seal touches mineral oil, fuel or grease, this comparison is over before it starts. Both compounds swell in petroleum media — EPDM badly, silicone significantly — and a swollen seal grows, softens and extrudes out of its groove.

The answer there is nitrile up to about +120 °C, AEM to about +160 °C continuous, or a fluoroelastomer above that. It is worth saying plainly, because silicone’s temperature range makes it look like the obvious choice for a hot engine-bay part — and a hot engine-bay part is usually a hot oily part.

Ask what the seal touches before you ask how hot it gets. Temperature narrows the field; the fluid decides it.

EPDM is two materials, and the cure system decides which one you get

Almost every datasheet gives EPDM a single temperature range. The range depends on how the compound was crosslinked, and the two options are not close.

Sulphur-cured EPDM is the standard, economical grade, and it is good to around +120 °C. Peroxide-cured EPDM reaches around +150 °C — and in steam the gap is wider than the headline numbers suggest: peroxide-cured EPDM resists steam to around +135 °C, while sulphur-cured EPDM is far lower.

The reason is the crosslink itself. Peroxide curing forms carbon–carbon bonds, which carry more energy than the sulphur bridges of a conventional cure — so a peroxide-cured compound holds up better to heat and oxidation, and it also gives a lower compression set, which is what a seal actually needs over years of service.

So “EPDM” on a drawing is an incomplete specification for anything hot. Say which cure system, or say the temperature and the medium and let the manufacturer choose.

Steam is not the same as heat

A compound’s dry-heat rating does not tell you how it behaves in steam. Steam is hot water under pressure, and it attacks elastomers chemically as well as thermally — so a material can hold up to +200 °C in dry air and be a poor choice at +130 °C in a steam line.

EPDM is the steam compound. Peroxide-cured grades handle steam to around +135 °C, and EPDM is the standard answer for hot water and steam service across the process industries. Silicone is rated far higher in dry heat, and it is not the first choice in pressurised steam, though certain grades are used for steam sterilising duty at around +150 °C.

So the dividing line is not a temperature. It is whether the heat arrives wet or dry. Wet and hot: EPDM. Dry and very hot: silicone. And if it is wet, hot and oily, it is neither.

Silicone and EPDM, side by side

Read the verdict column and the pattern is clear: silicone wins on temperature and loses on nearly everything else. It is the right answer when the temperature demands it — and an expensive, easily torn answer when it does not.

What silicone is not good at

Silicone has a reputation as the premium elastomer, and on temperature it earns it. Everywhere else it is a compromise, and the compromises are worth knowing before it is specified by default.

It tears. Silicone’s tear strength is low compared with almost every other sealing compound, which matters on thin sections, on parts that are stretched over something during assembly, and on anything with a sharp corner in the tooling. A silicone part that survives its duty perfectly can be destroyed by a fitter’s thumb.

It abrades. That rules it out of most dynamic duty — reciprocating seals, anything sliding in a bore — where EPDM or nitrile would last far longer.

And it costs. Silicone is materially more expensive than EPDM, and on a part where both would work the cost difference is real at volume.

Specify silicone when the temperature or the cold demands it. On any other basis, EPDM is usually the better engineering answer as well as the cheaper one.

How to specify either one so it arrives right

Name the material, the temperature, the medium and the hardness — and for EPDM, the cure system. A drawing that says only “EPDM 70 Shore A” leaves the most important decision to whoever quotes it cheapest.

ASTM D2000 does this compactly, because its Type letter is the heat-ageing temperature and its Class letter is the oil-swell requirement. EPDM compounds typically fall in the AA and CA classes and silicones in the FE and GE classes — and both sit in Class A, no oil-swell requirement, which is the standard’s way of saying what section 03 says in a sentence.

For silicone, ask about post-cure. A post-cure drives off residual volatiles and is what lets a silicone part reach its published compression-set figures. A silicone seal that has not been post-cured will not behave like the datasheet, and on a long-life static seal that is the difference between working and slowly leaking.

One thing this guide does not cover

Silicone is the elastomer most associated with food, pharmaceutical and medical work, and anyone reading a silicone article on a manufacturer’s website may reasonably wonder whether that is on offer here. It is not.

We hold no food-contact, FDA, pharmaceutical or medical device approval of any kind, and nothing in this guide should be read as a claim to one. Where a seal contacts food, drinking water, a pharmaceutical product or a patient, that approval has to come from a supplier who holds it.

What we hold is ISO 9001:2015 — certificate 09111526A, Integral Certification Ltd, valid to 21 February 2027, scope: rubber moulded components. That is a quality-management certification, not a product approval. We are upgrading to IATF 16949, targeted by the end of 2026. Our silicone work is industrial: thermal seals, gaskets and mouldings for equipment.

We test tensile and elongation to ASTM D412 and compression set to ASTM D395 Method B in our own lab, and those results ship with every consignment.

Five mistakes when choosing between silicone and EPDM

Reaching for silicone because the part is hot

A hot part in an engine bay is usually a hot oily part, and both of these compounds swell in oil. Ask what it touches before you ask how hot it gets.

Specifying “EPDM” for a hot or steam duty

Sulphur-cured EPDM reaches about +120 °C; peroxide-cured reaches about +150 °C and holds steam far better. Name the cure system or the duty.

Reading a dry-heat rating as a steam rating

Steam attacks chemically as well as thermally. A compound comfortable at +200 °C in air can be a poor choice at +130 °C in a steam line.

Using silicone where anything moves or stretches

Its tear and abrasion resistance are low. Thin sections, assembly stretch and sliding duty all favour EPDM or nitrile.

Assuming silicone means food or medical grade

Those are approvals, not a material. The compound alone certifies nothing, and the approval has to be held by the manufacturer.