PTFE (Teflon) O-rings and seals — where they work and where they don't

PTFE — better known by the trade name Teflon — is the odd one out among sealing materials. It sits next to rubber elastomers in material tables, but it is not rubber: it is a rigid plastic. Chemically it withstands almost everything and works from -200 °C to +260 °C, which no elastomer can match. At the same time it has zero elasticity — and that is exactly what decides where a PTFE O-ring works brilliantly and where it fails right after installation. This article covers both sides: where PTFE is the right call, where it is not, and what to use instead.

PTFE is a plastic, not an elastomer

Polytetrafluoroethylene (PTFE) is a rigid polymer. Catalogues quote a hardness of around 55 to 65 for it, but that is not rubber softness — it is a different characteristic than for elastomers. The key difference is the behaviour under load: a rubber O-ring is compressed and pushes back against the mating surfaces — that is how it seals. PTFE compresses and stays compressed. We speak of zero compression set recovery — the material has no memory. Add creep (cold flow): under sustained load PTFE slowly flows and escapes from under the contact pressure. A seal made of pure PTFE therefore stops resealing after thermal cycles or after the first movement of the joint.

Almost universal chemical resistance

This is why PTFE is on the table at all. In a compatibility chart it covers practically all chemicals and all solvents, and it handles steam as well as cryogenic temperatures. The exceptions are exotic in everyday practice: molten alkali metals and fluorine gas. So if you are sealing aggressive acids, ketones, esters or chlorinated solvents that ordinary elastomers cannot take, PTFE will almost always pass chemically.

The temperature range is the widest of all fifteen common sealing materials: continuously from -200 °C to +260 °C, short-term up to +300 °C. Downwards it has no competition — the best elastomer for the cold, silicone, stops at -60 °C; PTFE keeps working in cryogenic service. On top of that it carries FDA, USP Class VI and NSF certifications, so it is at home in food and pharma. The standard colour is white; filled grades — carbon-filled black and bronze-filled — improve creep and wear resistance.

Zero elasticity — what it means for the design

An O-ring seals thanks to preload: it is squeezed and springs back against the surfaces. PTFE will not do that job — once compressed, it does not recover. In practice this means three things. First, a pure PTFE O-ring in a classic groove seals only once, at first assembly; replace it after every disassembly. Second, during thermal cycling (metal parts expanding and contracting) PTFE does not follow the changing gap — a leak develops. Third, a plain PTFE O-ring is unsuitable as a dynamic seal on pistons and rods.

Designers solve this in two ways. The first is spring-energized seals: a PTFE profile with a metal spring inside that permanently supplies the contact force the material cannot generate itself. The second is filled PTFE compounds (carbon, bronze), which slow creep down and improve wear — which is also why they are used as guide and sliding elements.

PTFE as a back-up ring — the most common right use

At high pressure and with a larger sealing gap, a rubber O-ring risks extrusion — the material is forced into the clearance and torn away, producing the typical nibbled edge. The fix is a harder elastomer, or a PTFE back-up ring fitted behind the O-ring on the low-pressure side. The division of labour is ideal: the elastomer (NBR or FKM), which has the elasticity, does the sealing, and the rigid PTFE merely bridges the gap so the rubber has nowhere to escape. Here PTFE never needs the one thing it lacks — springiness.

Encapsulated O-rings: FEP/PFA jacket, elastomer core

The second way to get Teflon's chemical resistance into a springy seal is the encapsulated O-ring. The core is an elastomer — typically silicone (VMQ) or FKM — surrounded by a seamless jacket of FEP or PFA, fluoropolymers from the PTFE family. The core supplies the elasticity and memory, the jacket a chemical barrier close to pure PTFE. The silicone core itself works from -60 °C to +200 °C, FKM from -20 °C to +200 °C; always confirm the rating of a specific ring in the manufacturer's datasheet — it is set by the core and jacket combination.

Encapsulated rings have their own rules: the jacket is harder than rubber, so they need more installation force and smooth, radiused edges — a sharp edge damages the jacket and the chemical barrier is gone. They are intended primarily for static and slow applications, not fast dynamic motion.

PTFE vs FFKM: when to pay for the perfluoroelastomer

If you need Teflon's chemical resistance but the seal has to spring back, there is an elastomer answer: FFKM (perfluoroelastomer, for example Kalrez). Chemically it is practically on par with PTFE — it withstands virtually all chemicals, the exceptions again being molten alkali metals and fluorine — and on top of that it is a fully-fledged rubber with hardnesses of 65 to 90 Shore A. It handles +260 °C continuously and up to +325 °C short-term; downwards, however, it stops at -20 °C, while PTFE goes all the way to -200 °C.

The difference is the price. Taking NBR as a baseline of 1.0, PTFE comes out at roughly 8× — but FFKM at 50×, that is 20 to 50 times more expensive than FKM. The decision is therefore simple: a static seal, cryogenics, or a spot where a back-up ring or a spring-energized solution fits → PTFE. A dynamic seal, or a joint that must reseal repeatedly under extreme chemistry and temperature → FFKM, and only where the application genuinely demands it. In between sit encapsulated rings as the price compromise.

Frequently asked questions

Does a PTFE O-ring belong in hydraulics?

As the primary seal, no — dynamic motion and zero springiness rule each other out. As a back-up ring against extrusion at high pressures, it is the standard. Leave the sealing itself to NBR (mineral oils up to +100 °C) or to FKM at higher temperatures.

Can a Teflon seal handle steam?

Yes — steam is on PTFE's compatible-media list and the +260 °C ceiling leaves a reserve. Watch out for pressure and temperature cycling, though — that is exactly where the material's missing memory shows. For cycled steam, consider EPDM (continuous to +130 °C) or FFKM.

Why doesn't my PTFE ring seal after disassembly?

Because it stayed in its compressed shape — PTFE has no elastic memory. That is not a material defect, it is a property. Budget a new part for every disassembly; for a joint that is opened often, consider an encapsulated ring or FFKM.

How to decide

The process is the same as for any material: medium first, then temperature, then pressure and motion. PTFE wins where the chemistry or temperature is beyond what elastomers take and the seal is static — ideally as a back-up ring, a spring-energized profile or an encapsulated variant. Verify the specific material–medium pair in our chemical compatibility table, find the size in the dimension converter, and the configurator will show you straight away what is in stock. If a seal keeps failing and you do not know why, walk through the failure analysis — a nibbled edge or permanent deformation gives the cause away. And if you are still unsure after that, write to Filip.