FKM vs PTFE O-Rings: When to Choose Each

FKM (fluorocarbon rubber, commonly called Viton) and PTFE (polytetrafluoroethylene, commonly called Teflon) are both premium materials chosen for harsh environments. They overlap in temperature range and chemical resistance, yet they are fundamentally different: FKM is an elastomer; PTFE is a rigid thermoplastic. Choosing the wrong one leads to immediate seal failure. This guide explains the critical differences and provides a decision framework for selecting the right material.

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Key Differences at a Glance

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Temperature: PTFE Wins on Paper

PTFE operates from -200°C (cryogenic) to +260°C continuously — a wider range than any elastomer. FKM tops out at +200°C and becomes stiff below -20°C.

However, temperature alone does not determine the right choice. PTFE's lack of elasticity means it cannot seal dynamically at any temperature without an energizer (spring or elastomer core). FKM maintains its rubber-like recovery across its entire range, making it the correct choice for dynamic seals up to +200°C.

Choose PTFE when: temperatures exceed +200°C, drop below -40°C, or involve cryogenic fluids. Choose FKM when: the application is dynamic or requires elastic recovery between -20°C and +200°C.

Chemical Resistance: PTFE Is Nearly Universal

PTFE resists virtually every industrial chemical: concentrated sulfuric acid, hydrofluoric acid, strong alkalis, chlorinated solvents, aromatic hydrocarbons, and oxidising agents. Its only significant weaknesses are molten alkali metals and fluorine gas at high temperature.

FKM also has excellent chemical resistance — superior to NBR and EPDM — but it is attacked by:

• Steam and hot water above 100°C
• Polar solvents (ketones, esters, acetone)
• Strong bases (concentrated caustic)
• Low-molecular-weight amines

Choose PTFE when: the media includes strong acids, caustics, ketones, or solvents that attack FKM. Choose FKM when: the media is petroleum-based, aromatic fuels, or moderate chemicals that do not include the FKM-incompatible families listed above.

Elasticity and Sealing Mechanism

This is the decisive difference. FKM behaves like rubber: it compresses, deforms to fill surface imperfections, and springs back when pressure is removed. This elastic recovery is what makes an O-ring seal work.

PTFE has no elastic recovery. When compressed, it cold-flows (creeps) under load and does not return to its original shape. A PTFE O-ring relies entirely on the groove geometry and external compression to maintain sealing force. If the compression is lost — due to thermal cycling, vibration, or load relaxation — the seal leaks.

Choose FKM for: all dynamic applications (reciprocating, rotary, oscillating) and any static seal where thermal cycling or vibration is present. Choose PTFE for: static seals only — flanges, valve seats, fitting faces, and cryogenic joints where no motion occurs.

Pressure and Extrusion

FKM O-rings extrude into clearance gaps under high pressure unless the groove clearance is tight or backup rings are used. PTFE is much harder (Shore D vs Shore A) and resists extrusion better at the same pressure.

However, because PTFE cannot recover, a small clearance gap that opens under pressure will cause leakage regardless of extrusion resistance. In practice, both materials need controlled clearance for high-pressure service, and both benefit from backup rings.

Cost Comparison

PTFE is roughly double the cost of FKM for equivalent sizes. The price gap widens for very large diameters because PTFE is machined or sintered rather than molded in simple compression presses.

For most applications where FKM is chemically compatible, it is the more cost-effective choice. PTFE is justified only when its chemical or temperature advantages are essential.

Application Selection Matrix

When to Upgrade from FKM to PTFE

• The temperature exceeds +200°C continuously
• The chemical is a ketone, strong caustic, or chlorinated solvent that attacks FKM
• The application is static and requires FDA compliance with aggressive CIP chemicals
• The seal must survive cryogenic temperatures below -40°C
• The fluid contains strong oxidisers that degrade FKM

When NOT to Use PTFE

• Any application with shaft rotation or reciprocating motion (unless spring-energized)
• Seals subject to thermal cycling where load relaxation would cause loss of compression
• Low-pressure pneumatic seals that rely on rubber conformability to seal rough surfaces
• Applications requiring quick assembly where precise groove compression is difficult to guarantee

Spring-Energized PTFE Seals

For dynamic applications where PTFE's chemical resistance is needed but elasticity is also required, spring-energized PTFE seals combine a PTFE jacket with a stainless-steel canted coil spring. The spring provides the sealing force that PTFE lacks. These are used for:

• Reciprocating chemical pumps
• High-vacuum rotary feedthroughs
• Cryogenic valve stems
• Food & pharma mixers with aggressive CIP chemicals

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Frequently Asked Questions

Can PTFE replace FKM in the same groove? Yes, dimensionally — a PTFE O-ring to AS568 or ISO 3601 dimensions fits the same groove. However, the groove design must be reviewed because PTFE requires more precise compression control and is less forgiving of clearance gaps.

Is PTFE better than FKM for food-grade applications? Both are FDA compliant. PTFE is preferred for aggressive chemical CIP regimes. FKM is preferred for high-temperature food processing where some elasticity is needed.

Which lasts longer — FKM or PTFE? In a correctly specified static application, both can last many years. In a dynamic application, PTFE will fail quickly because it has no elastic recovery. Match the material to the application rather than assuming one is universally superior.

What is a spring-energized PTFE seal? It is a PTFE jacket with an internal stainless-steel spring that provides the sealing force PTFE lacks. These are used for dynamic and high-vacuum applications where standard PTFE O-rings are unsuitable.

Can I use PTFE backup rings with FKM O-rings? Yes. PTFE backup rings are commonly used with FKM O-rings in high-pressure applications to prevent extrusion. The materials are fully compatible.

Why does PTFE cold-flow? PTFE is a thermoplastic with no cross-linked molecular structure. Under sustained mechanical load, the polymer chains slide past each other, causing permanent deformation. Proper groove design limits this effect.