PTFE O-rings: universally resistant across the widest temperature range

What are PTFE O-rings?

PTFE O-rings are made from solid polytetrafluoroethylene, a semi-crystalline fluoroplastic. It is the same material that became known under the brand name Teflon.

How to read the graphic: The grey chain of carbon atoms (C) forms the backbone of the polymer. Above and below each C sit fluorine atoms (F). The green frame represents the continuous fluorine protective shell: the fluorine atoms completely envelop the carbon chain and shield it from the outside.

This is exactly what makes PTFE so stable. The bond between carbon and fluorine is one of the strongest single bonds in organic chemistry. Because the chain is completely enclosed by fluorine, attacking media find no point of attack, unlike materials with exposed hydrogen atoms.

In practice this means: PTFE is chemically almost universally resistant, against acids, alkalis, solvents and hot steam, and across a very wide temperature range. The price for this is the lack of rubber elasticity. PTFE is a thermoplastic and tends to cold flow, which is why it is used mainly as a flange seal or as an encapsulated O-ring.

Unlike elastomers, PTFE is a thermoplastic and therefore not rubber-elastic. The ring consists entirely of a single material, without a core or jacket. This makes it extremely chemically resistant, but deprives it of the restoring force of a rubber.

The most important properties

• Almost universal media resistance: resistant to practically all acids, alkalis and solvents. Check specific media in the media resistance tool.
• Largest temperature range: −250 to +250 °C, the widest of all sealing materials.
• Very low friction: pronounced non-stick and sliding properties.
• Not elastic: thermoplastic, without rubber recovery.
• Hardness: 60 Shore D, PTFE is measured on the harder Shore D scale.
• Approvals: FDA. Shelf life: practically unlimited.

PTFE is the material of choice when chemical resistance and an extreme temperature range matter and a static flange seal is sufficient.

Temperature ranges compared

No other sealing material covers such a wide temperature range as PTFE: from −250 to +250 °C. For cryogenic applications, PTFE is therefore often the only choice. The graphic places PTFE among the other materials.

Continuous service temperatures of typical sealing materials as a standard compound, NH materials in coral. Values are guide figures depending on the compound; the relevant data sheet is authoritative.

Price level compared

PTFE is in the mid price range, cheaper than FEP-encapsulated and well below FFKM. The graphic shows the relative price level, based on NBR as the cheapest baseline equal to 1.

Cost factor per material as a guide figure, relative to NBR equal to 1, NH high-performance materials in coral. The values provide rough orientation. The actual price depends on dimension, compound and quantity.

What is PTFE resistant to?

PTFE is one of the most chemically resistant materials of all. It is fully fluorinated and inert to almost all media:

• Acids and alkalis, including concentrated
• Solvents, ketones and esters
• Oils, greases and fuels
• oxidizing and aggressive media

Only very few media attack PTFE, such as elemental fluorine and molten alkali metals, similar to FFKM.

Special characteristics: cold flow and lack of elasticity

Two properties fundamentally distinguish PTFE from elastomers:

Lack of elasticity: PTFE does not spring back. The sealing effect is created by deformation during bolting, not by an elastic preload as with rubber.

Cold flow: Under sustained pressure, PTFE deforms slowly and permanently (creep, also called cold flow). The design must account for this, for example through limited compression and a closed groove.

PTFE, FEP and FFKM compared

Three materials offer almost universal resistance but differ in elasticity, temperature and price:

PTFE scores with temperature range and resistance, FEP adds a springy core, FFKM offers full elasticity.

Technical parameters at a glance

The following values are taken from the PTFE data sheet and tested to ASTM.

Hardness in Shore D per ASTM D2240, tensile strength and elongation at break per ASTM D412, density per ASTM D1817. Compression set and TR-10 are not as meaningful for PTFE as they are for elastomers and are provided on request. The data sheet is authoritative.

Installation and design

PTFE O-rings are not stretchable and are used exclusively as static flange seals:

• Installation in a closed groove, not as a piston or rod seal.
• The ring cannot be stretched over components. The groove must allow insertion without stretching, often designed as a split or open groove.
• Limit compression to keep cold flow low.
• Determine groove dimensions and tolerances with the O-ring groove calculation and the O-ring tolerances.

Typical applications

Are there alternatives to PTFE?

If the sealing point needs elasticity or a dynamic function, PTFE is not suitable. In that case, FFKM (ECOLAST) for full elasticity or FEP-encapsulated O-rings for a springy core are the right choice. If it is only about oils and fuels in the medium temperature range, FKM is the economical solution.

Frequently asked questions