FFKM vs FKM, The Complete Comparison

Every question engineers ask about perfluoroelastomer and fluoroelastomer, answered by a specialist with 36 years in the field.

Get a free material assessment Call Andrew: 07767 842601

In one line: FKM is a partially fluorinated rubber rated to around 200C that handles oils, fuels and many chemicals cheaply. FFKM is fully fluorinated, rated to 325C in high temperature grades, resists almost all industrial chemistry including steam, ketones and amines, and costs five to fifty times more. Specify FKM unless the chemistry or the temperature genuinely beats it.

Most of the FFKM enquiries we take are triggered by the same story. A seal that ran happily for years starts failing every few months. The process changed, or a new cleaning cycle went in, and the material specification never got revisited. Sometimes the answer is FFKM. Often it is a different FKM grade, or a hardware fix that costs nothing.

This page lays out the full comparison, the grade families inside each material, the decision checklist we use, and the failure modes a material upgrade will never fix.

FFKM vs FKM Side by Side

Comparison of FFKM perfluoroelastomer and FKM fluoroelastomer properties
Property	FFKM	FKM (Viton)
ASTM D1418 designation	FFKM, perfluoroelastomer	FKM, fluorocarbon rubber
Fluorine content	About 71 to 73 percent, fully fluorinated backbone	About 66 to 70 percent, partially fluorinated
Max continuous temperature	260C standard, 325C high temperature grades	About 200C to 210C, 225C short excursions
Low temperature limit	About -15C to -25C	About -20C, down to -40C in GLT and GFLT grades
Chemical compatibility	Near universal, commonly quoted at 1,600 to 1,800 plus media	Broad with oils, fuels and many chemicals, with known gaps
Steam and hot water	Excellent in steam rated grades	Poor above about 150C, hardens and cracks
Ketones, esters, amines	Resistant	Attacked, swells and softens
Compression set	Very low in the right grade, holds sealing force	Good, but degrades faster at high temperature
Elastic recovery	Rubber like, recovers after load cycling	Rubber like
Availability	Longer lead times, fewer stock sizes	Widely stocked in AS568 and metric sizes
Relative cost	Typically 5 to 50 times FKM, more for speciality grades	Low to moderate

Figures are typical for general purpose compounds. Real limits are grade specific and are confirmed against the compound datasheet before we quote.

Which One Do You Need?

Specify FKM when

Continuous temperature below about 180C to 200C
Oils, fuels, hydraulic fluids, mineral based lubricants
Stable chemistry with no steam or solvent cleaning cycle
Seals already lasting two years or more in service
Cost sensitive duty with easy access for planned changeout
Cold service where a GLT or GFLT grade is needed below -25C

Specify FFKM when

Continuous temperature above 200C, or 325C in high grade
Ketones, esters, ethers, amines, hot acids or strong alkalis
Clean in place and sterilise in place cycles alongside process duty
Semiconductor plasma, wafer fabrication and high purity service
Repeat chemical failures inside 12 to 18 months
Any duty where an unplanned shutdown costs more than the seal set

Grade Matters More Than the Family Name

Choosing FFKM over FKM is only the first decision. Inside each family the grade changes the temperature ceiling, the chemical behaviour and the price by a wide margin.

FKM grade families

Type A general purpose: the default, oils, fuels and many chemicals to about 200C.
GLT and GFLT: low temperature grades reaching about -40C, GFLT with better fluid resistance.
ETP: extreme chemical resistance FKM that closes part of the gap to FFKM at a lower price.
High fluorine types: better solvent and acid resistance than Type A, still short of FFKM.

FFKM grade families

General purpose black: broad chemical resistance to around 260C.
High temperature: continuous service to 325C for steam and hot process duty.
White and food contact: FDA and USP Class VI compounds for pharma and food.
Explosive decompression resistant: for high pressure gas service.
Semiconductor and plasma: low particle, low outgassing compounds.

Brand names including Kalrez, Chemraz, Perlast, Simriz, Isolast and Markez are all FFKM compounds. Trademarks belong to their respective owners. More on Kalrez alternatives and approved vendor lists.

The Cost Argument, Honestly

FFKM is expensive per part and there is no way around that. The question is not what the ring costs, it is what a year of running costs. If an FKM seal is being changed three times a year, each change takes a shift, and the line stops while it happens, the labour and lost output dwarf the material difference. If the FKM seal lasts three years, FFKM is money burnt.

Work it out on your own numbers rather than ours.

Open the seal cost calculator

Six Failure Modes, and Whether FFKM Fixes Them

Chemical attack

Swelling, softening, blistering or a sticky surface. This is the failure a move from FKM to FFKM actually fixes.

Thermal degradation

Hardening, a glazed surface and radial cracks. Fixed by a higher temperature grade, or by cooling the seal chamber.

Explosive decompression

Internal blisters and splits after a rapid gas pressure drop. Needs an ED resistant grade and correct groove fill, not just FFKM.

Extrusion and nibbling

A chewed edge on the low pressure side. A hardware problem: reduce the clearance gap or add a back up ring.

Compression set

A flat, permanently squashed ring. Groove dimensions, over compression or the wrong grade for the temperature.

Installation damage

Cuts, spiral twists and nicks. No material upgrade survives a sharp edge or a missing chamfer.

Diagnosing the mode before changing the material is the whole job. Use the seal troubleshooting guide.

FFKM vs FKM, Questions Answered

What is the difference between FFKM and FKM?

FKM is a partially fluorinated fluoroelastomer, usually 66 to 70 percent fluorine by weight, rated to roughly 200C to 210C continuous. FFKM is fully fluorinated perfluoroelastomer, around 71 to 73 percent fluorine with a perfluorinated backbone and perfluorinated cure sites, rated to 260C as standard and up to about 325C in high temperature grades. The practical difference is that FFKM survives steam, amines, ketones, esters, hot acids and strong bases that attack FKM, and it costs many times more.

Is Viton FKM or FFKM?

Viton is a trade name for FKM fluoroelastomer, owned by its manufacturer. It is not FFKM. Kalrez, Chemraz, Perlast, Simriz, Isolast and Markez are trade names for FFKM. All trademarks belong to their respective owners and we are not affiliated with them.

When should I upgrade from FKM to FFKM?

Upgrade when the process temperature is continuously above about 200C, when the chemistry includes ketones, esters, amines, hot steam, concentrated acids or strong alkalis, when a clean in place or sterilise in place cycle sits alongside solvent duty, or when the same FKM seal is being replaced more often than every 12 to 18 months for chemical reasons. If the failure is mechanical, upgrading the material fixes nothing.

How much more expensive is FFKM than FKM?

Typically five to fifty times the price of an equivalent FKM part, and more again for branded or speciality grades in large cross sections. The comparison that matters is not part price, it is cost per year of uninterrupted running. One avoided unplanned shutdown usually pays for years of FFKM.

What temperature can FKM and FFKM handle?

General purpose FKM runs from about -20C to 200C, with specialist low temperature grades reaching -40C or lower and some grades rated to around 225C for short periods. FFKM runs from about -15C to 260C as standard, with high temperature grades to 325C continuous. FFKM is weaker than FKM at low temperature, so cold service can favour a specialist FKM grade.

Does FFKM resist steam?

Yes, in the correct grade. Steam above roughly 150C attacks standard FKM, causing hardening, cracking and volume change. FFKM grades formulated for steam and amine service hold up through repeated SIP cycles. Not every FFKM grade is a steam grade, so the compound has to be selected, not assumed.

Is FFKM better than FKM in every way?

No. FKM is better at low temperature, better on abrasion and tear in many duties, far cheaper, and available off the shelf in more sizes. FFKM wins on chemical breadth and high temperature only. Where the duty does not demand those, FFKM is wasted money.

What chemicals attack FKM but not FFKM?

Ketones such as acetone and MEK, esters, ethers, low molecular weight amines, hot concentrated caustic, hot steam, some organic acids, and many polar solvent blends. These are common in cleaning cycles and in modern process chemistry, which is why long standing FKM specifications start failing after a process change.

Can FFKM replace PTFE?

Often, and usually for the better. PTFE matches FFKM on chemical breadth but has almost no elastic recovery, so it cold flows and loses sealing force under load cycling. FFKM gives PTFE level chemistry with rubber like recovery, which is why it succeeds in flanges and glands where PTFE keeps relaxing.

What are FKM grades GLT, GFLT and ETP?

They are specialist FKM families. GLT and GFLT are low temperature grades reaching about -40C, with GFLT adding better fluid resistance. ETP is an extreme chemical resistance FKM that closes part of the gap to FFKM at a lower price. In some duties an ETP grade is the correct answer instead of jumping straight to FFKM.

Why did my FFKM seal fail even though it is the best material?

Because most seal failures are not chemical. Explosive decompression in gas service, extrusion through an oversized clearance gap, thermal cycling, compression set from a badly dimensioned groove, and installation damage all destroy FFKM exactly as they destroy FKM. A material upgrade only fixes a material problem.

Is FFKM FDA compliant?

Certain FFKM grades are formulated to FDA, USP Class VI and 3A requirements, and certain FKM grades are too. Compliance is a property of the specific compound with its own documentation, not of the polymer family. Tell us which approval you need and we supply a grade that carries it with the paperwork.

Can I use an FFKM O-ring in an existing FKM groove?

Usually yes, since both are supplied in standard AS568 and metric BS ISO 3601 sizes and the groove design is the same. Check the clearance gap and squeeze against the harder FFKM compound, and check thermal expansion if the temperature is going up at the same time.

How do I choose between FFKM and FKM for my pump seal?

List the actual fluid, the maximum temperature at the seal face rather than in the line, the pressure, any cleaning or flushing chemistry, and how long the seal currently lasts. If FKM is failing chemically or thermally, move to FFKM. If it is failing mechanically, fix the design or the fitting. Send us those five details and we will tell you which it is, at no charge.

Do you supply an alternative to Kalrez?

Yes. We cross reference FFKM compounds on properties rather than on trade name, so you get like for like performance from a competitively priced grade. Send the current part number, grade, fluid and temperature.

Ask Andrew about your duty