FFKM Perfluoroelastomer Seals and O-Rings
The most chemically resistant elastomer made, supplied with the advice to tell you when you do not need it.
In one line: FFKM (perfluoroelastomer) is a fully fluorinated rubber sealing material rated to around 325C with near universal chemical resistance. It is the highest performing, and most expensive, elastomer available, and it is sold under brand names including Kalrez, Chemraz, Perlast, Simriz and Isolast.
FFKM, or perfluoroelastomer, is what you reach for when the chemistry has already beaten FKM, PTFE will not flex enough and the plant cannot afford another unplanned stop. Fully fluorinated, stable to 325C, and resistant to almost everything a process plant can put through a seal.
It is also the most expensive elastomer on the shelf, which is why half the FFKM enquiries we take should never have been FFKM enquiries. We quote what the duty needs. If a cheaper material will do the job for ten years, we will say so.
FFKM vs FKM, In Plain Terms
Chemistry
FKM handles oils, fuels and many chemicals. FFKM handles steam, amines, ketones, esters, hot acids, strong bases and solvent blends that turn FKM to jelly.
Temperature
FKM gives up near 200C. FFKM runs to 260C as standard and 325C in high temperature grades.
Cost
FFKM is many times the price of FKM. It earns that back in one avoided shutdown and nowhere else.
When FKM is the right answer
Steady chemistry, temperature under 180C, no steam cycle. Specify FKM and spend the difference on spares.
Full comparison: FFKM vs FKM, the complete guide, the FFKM chemical compatibility chart and the Kalrez alternative cross reference. Longer read: why your specification needs reviewing.
FFKM Properties at a Glance
| Property | FFKM | FKM (Viton) | PTFE |
|---|---|---|---|
| ASTM D1418 name | Perfluoroelastomer | Fluorocarbon rubber | Fluoropolymer, not an elastomer |
| Max continuous temperature | 260C standard, 325C high grade | About 200C | 260C |
| Low temperature limit | About -15C to -25C | About -20C to -40C | About -200C |
| Chemical resistance | Near universal, 1600+ media | Good with oils and fuels | Near universal |
| Steam and amines | Excellent in the right grade | Poor, hardens and cracks | Excellent |
| Elastic recovery | Good, seals like rubber | Good | Poor, cold flows |
| Relative cost | Highest | Low to moderate | Low to moderate |
Figures are typical for general purpose compounds. Actual limits are grade specific, so send us the duty and we will confirm against the compound datasheet before you buy.
FFKM Brand Names, and What They Actually Are
Every name below is a branded FFKM compound. None of them is a different polymer family, and none of them is automatically the right one for your duty.
| Brand name | Material | Typically specified for |
|---|---|---|
| Kalrez | FFKM | Chemical process, pharmaceutical, semiconductor |
| Chemraz | FFKM | Chemical, semiconductor, food and pharma grades |
| Perlast | FFKM | Semiconductor, pharmaceutical, aggressive process duty |
| Simriz | FFKM | Chemical process and high temperature duty |
| Isolast | FFKM | Oil and gas, chemical, explosive decompression duty |
| Markez | FFKM | Chemical and semiconductor duty |
All trademarks belong to their respective owners. We are not affiliated with them. Send your existing part number and duty and we will cross reference an equivalent compound on properties. More on Kalrez alternatives and approved vendor lists.
FFKM Grades We Supply
General purpose black FFKM
Broad chemical resistance to around 260C. The default where FKM is failing chemically.
High temperature grades
Continuous service to 325C for steam, hot oil and high temperature process duty.
White and food contact grades
FDA and USP Class VI compounds for pharmaceutical, biotech and food processing.
Explosive decompression resistant
Rapid gas decompression grades for oil, gas and high pressure gas duty.
Semiconductor and plasma grades
Low particle, low outgassing compounds for wafer fabrication and plasma etch.
Steam and amine grades
For clean in place, sterilise in place and amine service that hardens FKM.
Kalrez, Chemraz, Perlast, Simriz and Isolast are trademarks of their respective owners. We cross reference equivalent compounds on properties, temperature and chemical compatibility, not on name.
What We Need to Quote FFKM Accurately
- Size: AS568 dash number, metric size, or groove diameter and cross section
- The fluid or fluids, including cleaning and sterilisation chemistry
- Operating temperature and any peak or excursion temperature
- Pressure, and whether the duty involves rapid gas decompression
- Any approval you need: FDA, USP Class VI, 3A, NORSOK
- The existing part number or grade if you are replacing a branded ring
- Quantity and how urgently it is needed on site
Stocked sizes despatch same day when ordered before 2pm, with UK wide delivery and international despatch through our partner network.
Why FFKM Seals Still Fail
Explosive decompression
Gas dissolves into the elastomer under pressure and tears it apart on rapid depressurisation. Blistered, split rings mean you need an ED resistant grade, not a different size.
Extrusion
Clearance gap too large for the pressure. The ring squeezes into the gap and nibbles away. Back up rings or a tighter gap fix it.
Compression set
Groove too shallow, squeeze too high, temperature too long at the top of the range. The ring takes a permanent shape and stops sealing.
Thermal cycling
Repeated hot to cold cycles work the ring against the groove until it fatigues, especially at the low temperature end where FFKM is weakest.
Installation damage
Sharp threads, unchamfered edges and no lubricant nick the ring on assembly. An expensive ring fails as fast as a cheap one.
The wrong diagnosis
If the original failure was mechanical, upgrading to FFKM changes nothing except the invoice.
More detail: why FFKM seals fail in chemical processing.
FFKM Questions and Answers
What is FFKM?
FFKM is perfluoroelastomer. It is a fully fluorinated rubber, so almost every carbon bond in the polymer backbone is protected by fluorine. That is why it resists nearly all industrial chemistry and holds up to around 325C, where FKM gives up near 200C. It is the most chemically resistant elastomer available, and the most expensive.
What does FFKM stand for?
The letters come from the ASTM D1418 naming system. FKM is a fluorocarbon rubber. The extra F in FFKM means fully fluorinated, or perfluoroelastomer. Kalrez, Chemraz, Perlast, Simriz and Isolast are brand names for FFKM materials, not different polymers. All trademarks belong to their respective owners.
What is the temperature range of FFKM?
General purpose FFKM grades run from about minus 15C to 260C. High temperature grades reach 325C continuous, and some speciality compounds go higher for short excursions. Low temperature performance is the weak point, so if you need good sealing below minus 20C, tell us before we quote a grade.
What is the difference between FFKM and FKM?
FKM (often called Viton) is partially fluorinated and handles most oils, fuels and many chemicals to around 200C. FFKM is fully fluorinated and handles steam, amines, ketones, esters, hot acids, strong bases and aggressive cleaning cycles that destroy FKM, at temperatures up to 325C. FFKM costs a lot more, so it is worth specifying only where the chemistry or the temperature genuinely demands it.
Can you supply an alternative to Kalrez?
Yes. We supply FFKM O-rings and seals in equivalent compounds cross referenced to the grade you currently buy. Send the existing part number, the grade, the fluid and the temperature and we will match the compound, not just the size. Cross referencing is done on properties, so you get like for like performance rather than a guess.
What sizes of FFKM O-ring can you supply?
Standard AS568 imperial sizes, metric BS ISO 3601 sizes, and non standard or oversize rings made to your dimensions. If you have a groove but no part number, send the groove diameter and cross section and we will size it properly.
How much does an FFKM O-ring cost?
Far more than FKM, and it varies with size, cross section, grade and quantity. The cost that matters is not the ring, it is the unplanned shutdown a failed ring causes. Where FFKM is overkill we will tell you and quote the cheaper material instead.
Which industries use FFKM seals?
Chemical processing, pharmaceutical and biotech, semiconductor, oil and gas, aerospace, food processing with aggressive clean in place cycles, and any plant running steam sterilisation alongside solvent duty. Anywhere the chemistry changes through the batch is FFKM territory.
Is FFKM FDA compliant?
Certain FFKM grades are formulated to FDA, USP Class VI and 3A requirements, and some carry explosive decompression resistance for gas duty. Compliance is grade specific, so tell us which approval you need and we will supply a compound that carries it with the paperwork.
Why did my FFKM seal still fail?
Usually because the failure was never chemical. Explosive decompression, extrusion through an oversized gap, thermal cycling, compression set from an undersized groove or simple installation damage will all fail FFKM. Upgrading the material fixes nothing if the design or the fitting is the problem.