FFKM vs Fluorocarbon (FKM) O-Rings

FFKM (perfluoroelastomer) is chemically close to PTFE while still sealing like rubber. Against standard Viton® (FKM) the trade is simple: broader chemical and temperature capability against a large step in cost. On the Parker O-Ring Handbook (ORD 5700) ranges, standard FKM runs -26 C to 204 C (-15 F to 400 F); the FFKM class runs -26 C to 320 C (-15 F to 608 F), and named brand grade sheets run to 327 C (621 F). FKM falls short mainly with ketones, amines, steam, high vacuum or heat above the FKM ceiling.

FFKM and Viton compared

Which material for which application?

Where FFKM is the usual callout

The media defeats FKM (hot amines, ketones, mixed aggressive streams, steam with oil present), the temperature sits above the FKM ceiling, or the process (semiconductor, pharmaceutical) cannot tolerate a seal failure or contamination. The FFKM guide covers the grade families and the branded vs generic FFKM guide the brands.

Where Viton is the usual callout

Everywhere FKM chemistry survives, it is the economical answer, it is widely available, and any qualified molder can make it. Before you send an FFKM request, check the media list against the Viton guide and the chemical compatibility guide; a specialty FKM grade often covers the one chemical that seemed to force the upgrade.

The options between the two

Aflas (FEPM) covers sour gas, amines and steam where FKM fails but FFKM is more than the job needs; the Parker handbook rates it -9 C to 232 C (16 F to 450 F), and the grade sheet governs. A PTFE-encapsulated O-ring gives PTFE inertness in static service at a fraction of FFKM cost, with no elastic memory. And specialty FKM grades (low-temperature, steam-resistant, base-resistant) close many of the gaps.

What the print might say

How each material fails, and what it looks like

A Viton ring in a ketone or an amine comes out swollen and soft; in steam it hardens and cracks; run too cold it leaks at start-up. An FFKM ring in the wrong service is rarer, and the recognizable case is a fluorinated refrigerant, where it swells like any other elastomer in the wrong medium. Both take a flat-sided set when over-squeezed at temperature, FFKM later than FKM; the compression set guide covers it. Neither cracks from ozone.

Gland and installation notes for the pair

Both materials use the same AS568 sizes and the same gland rules from the Parker handbook, and an FFKM ring goes into a groove designed for Viton without change. The practical difference is cost of error: an FFKM ring damaged on a sharp edge or a thread during assembly is an expensive mistake, so chamfer the gland entry, tape the threads and lubricate with a compatible grease. FFKM's better compression set at temperature means it keeps its squeeze longer in hot static service, which is why it is often chosen for seals that cannot be revisited between overhauls. The squeeze guide has the numbers.

Kalrez is a trademark of Qnity Electronics; Chemraz® is a trademark of Greene Tweed; Parofluor® is a trademark of Parker Hannifin; Aflas is a trademark of AGC; Viton is a trademark of The Chemours Company. No affiliation or endorsement is implied.

Sources

1. Parker O-Ring Handbook ORD 5700, service temperature table, page 7-2 (FFKM -26 C to 320 C), read 2026-09-25
2. Parker O-Ring Handbook ORD 5700, section 2.2, read 2026-09-14
3. Parker O-Ring Handbook ORD 5700, sections 2.2.10 (FFKM) and 2.2.14 (FEPM), read 2026-09-14
4. Kalrez CPI Product Selector Guide KZE-A40126, Qnity Electronics, November 2022, read 2026-09-14
5. 21 CFR 177.2600
6. AMS 7257
7. AMS7259
8. AMS7276
9. AMS7280
10. AS568
11. ASTM D1418
12. USP Class VI

Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Spec values in this guide are drawn from the sources named in the sentence where each appears; the compound datasheet and your application conditions govern the final choice.