FEP-encapsulated O-rings: chemical resistance with an elastic core

What are FEP-encapsulated O-rings?

FEP-encapsulated O-rings are composite components. An elastic core sits inside, enclosed on the outside by a seamless jacket of FEP (fluorinated ethylene propylene), a fluoroplastic from the same family as PTFE.

This combination unites two strengths: the core provides restoring force and sealing elasticity, the FEP jacket provides the almost universal chemical resistance. The jacket is closed and seamless, so that the medium comes into contact exclusively with FEP.

The most important properties

• Almost universal media resistance: the FEP jacket withstands the vast majority of acids, alkalis, solvents and oils. Check specific media in the media resistance tool.
• Temperature resistance: −25 to +205 °C (FEP-FKM), with the silicone core −60 to +205 °C (FEP-SIL).
• Springy core: unlike solid PTFE, the elastic core provides restoring force, so the O-ring seals with a defined preload.
• Hardness: 90 Shore A.
• Approvals: FDA.
• Storage life: 15 years with proper storage in accordance with DIN 7716.

FEP-encapsulated O-rings close the gap between solid elastomers and PTFE. They are more resistant than FKM and more elastic than PTFE, though stiffer than a full elastomer.

Temperature ranges compared

The usable temperature range depends on the core. FEP-FKM works up to +205 °C, while FEP-SIL stays flexible down to −60 °C thanks to its core. The chart places FEP among the common materials.

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

Price level compared

FEP-encapsulated O-rings are significantly cheaper than FFKM and still deliver resistance in a similar league. The chart shows the relative price level, based on NBR as the cheapest baseline set to 1.

Cost factor per material as a guide value, based on NBR set to 1, NH high-performance materials in coral. The values are for rough orientation only. The actual price depends on dimension, compound and quantity.

What is FEP resistant to?

The chemical resistance comes solely from the FEP jacket. FEP belongs to the fluoroplastics and, like PTFE, is resistant to almost all media:

• Acids and alkalis, even at higher concentration
• Solvents, ketones and esters
• Oils, greases and fuels
• oxidizing and aggressive media

Since only the jacket contacts the medium, the resistance of the core to the medium is irrelevant. The core determines temperature behaviour and restoring force. As with PTFE and FFKM, the exceptions are only a very few media such as elemental fluorine.

Core and jacket: FEP-FKM or FEP-SIL?

The core makes the difference between the two variants. The FEP jacket, and therefore the chemical resistance, is identical.

FEP-FKM is the standard choice. Choose FEP-SIL when the application extends into the cold range down to −60 °C.

FEP, PTFE and FFKM compared

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

FEP is the elastic compromise, PTFE the rigid material for extreme temperatures, FFKM the fully elastic top-tier material.

Our FEP variants

Both variants with an FDA jacket. All test values are in the data sheets.

Technical properties at a glance

The following values come from the data sheets and are tested to ASTM.

Tensile strength and elongation at break to ASTM D638, hardness to ASTM D2240, density to ASTM D792. Compression set (22 h / 150 °C) and TR-10 on request. The respective data sheet is authoritative.

Installation and design

FEP-encapsulated O-rings are significantly stiffer than full elastomers. This has consequences for installation:

• Use as a flange and rod seal, not as a heavily stretched piston seal.
• The O-ring can only be stretched to a limited extent. The groove should be designed so that it can be fitted without excessive expansion.
• Determine groove dimensions and tolerances with the O-Ring groove calculation and the O-Ring tolerances.

Since the jacket must remain closed, sharp edges and overstretching must be strictly avoided.

Typical applications

Are there alternatives to FEP?

If the resistance of FKM is not enough and full elasticity is not essential, FEP-encapsulated O-rings are an economical choice. If you need full elasticity with the same resistance, FFKM (ECOLAST) is the alternative, though more expensive. For very wide temperature ranges or purely static flange seals, solid PTFE can be suitable. For oils and fuels in the medium temperature range, FKM is often sufficient.

Frequently asked questions