FFKM Temperature Range Explained: Continuous, Peak and Low-Temperature Limits

FFKM is a family of perfluoroelastomer compounds rather than one standardized formulation.

As a broad screening guide, commercial FFKM compounds may fall into the following ranges:

These figures are not universal specifications.

Current commercial examples demonstrate the variation between compounds. DuPont publishes a maximum service temperature of 327°C and a lowest service temperature of −18°C for one Kalrez 7075 formulation. Trelleborg lists one low-temperature FFKM at −40°C to 250°C, while a different high-temperature compound is listed at −10°C to 300°C.

The difference between these grades illustrates an important tradeoff:

Improving low-temperature flexibility may reduce maximum high-temperature capability, while optimizing a compound for extreme heat may limit its performance below freezing.

The continuous service temperature is the highest temperature at which a specific compound is expected to maintain useful sealing performance during prolonged exposure under defined conditions.

It is not simply the temperature at which the polymer avoids melting or immediate destruction.

To provide long-term sealing, the material must retain sufficient:

• Elastic recovery
• Sealing force
• Tensile strength
• Compression-set resistance
• Dimensional stability
• Chemical resistance

For example, DuPont suggests a maximum continuous service temperature of 316°C (600°F) for Kalrez 4079AMS. The same product information states that brief excursions above this temperature may be possible, which clearly separates its continuous rating from short-term peak exposure.

Greene Tweed similarly publishes a maximum operating temperature of 316°C for Chemraz 555, a specific FFKM formulation intended for demanding chemical-processing, energy and semiconductor applications.

What does “continuous” actually mean?

The term does not necessarily mean that a seal will provide unlimited service life at the published maximum temperature.

Seal life near the upper temperature limit may depend on:

• Total exposure time
• Chemical media
• Pressure
• Compression
• Temperature cycling
• Seal geometry
• Oxygen exposure
• Dynamic movement
• Required leakage rate

An FFKM seal operating continuously at 300°C may have a much shorter useful life than the same compound operating at 230°C.

Published maximum temperatures should therefore be treated as material-selection boundaries, not guaranteed service-life values.

A peak temperature—also called a short-term excursion temperature—is the highest temperature a compound may tolerate for a limited period.

It is normally higher than the recommended continuous service temperature.

For example, one low-temperature Trelleborg FFKM compound is published with:

• Continuous service range: −40°C to 240°C
• Maximum excursion temperature: 250°C

The manufacturer also states that the final maximum and minimum temperatures must be agreed according to the specific application.

Another Trelleborg high-temperature grade is listed for continuous service up to 300°C, with published excursion capability up to 325°C.

How long is a “short excursion”?

There is no universal duration that applies to all FFKM compounds.

An excursion could mean:

• A brief startup event
• A cleaning cycle
• A temporary process upset
• Several minutes at elevated temperature
• A controlled thermal-processing step

The acceptable duration and frequency must be confirmed with the material supplier.

A seal exposed to one short peak may survive, while repeated peaks during every production cycle may cause accelerated aging.

Information required for peak-temperature evaluation

Provide:

• Peak temperature
• Duration of each peak
• Number of peaks per day or production cycle
• Heating rate
• Cooling rate
• Pressure during the peak
• Chemical media present
• Required service life

Repeated excursions should be treated as part of the normal operating cycle rather than as rare emergencies.

The minimum service temperature is the lowest temperature at which a seal is expected to retain enough flexibility and recovery to maintain contact with the sealing surfaces.

At low temperatures, FFKM does not normally fail through melting or chemical decomposition. Instead, it gradually becomes harder and less elastic.

This can cause:

• Loss of sealing force
• Leakage during startup
• Slow recovery after compression
• Failure to follow shaft movement
• Leakage during pressure changes
• Cracking during installation or movement

The minimum service temperature varies substantially between compounds.

For example:

These supplier-specific examples show why buyers should request the exact compound designation instead of specifying only “FFKM.”

Several test values may be used to describe low-temperature elastomer performance.

Glass transition temperature

Glass transition temperature, commonly written as Tg, indicates the temperature region where an elastomer changes from a flexible rubber-like state toward a harder, glass-like state.

Tg is useful for comparing materials, but it does not directly guarantee that a seal will remain leak-free at that temperature.

Actual sealing performance also depends on:

• Compression
• Pressure
• Seal cross-section
• Rate of temperature change
• Hardware movement
• Surface condition
• Required response speed

TR10

TR10 is the temperature at which a previously stretched elastomer specimen has retracted by 10% under a defined test method.

It can be useful for comparing low-temperature recovery between compounds.

For one low-temperature FFKM, Trelleborg publishes a TR10 value of −28°C while listing a continuous service range extending to −40°C. This demonstrates that service-range values and laboratory transition values measure different aspects of performance.

Low-temperature leakage testing

For critical equipment, an actual leakage or pressure-retention test is more useful than relying only on Tg or TR10.

Testing should reproduce:

• Minimum operating temperature
• Actual seal size
• Real groove dimensions
• System pressure
• Pressure cycling
• Expected movement
• Startup conditions

A static flange and a reciprocating rod may require different minimum temperature limits even when they use the same FFKM compound.

A material may work successfully in a static seal at a temperature where it cannot respond quickly enough in a dynamic application.

Static sealing

Static applications include:

• Flanges
• Valve bodies
• Chamber lids
• Pipe connections
• Instrument housings

Once compressed, the seal may remain in continuous contact with the hardware. This can allow useful sealing at relatively low temperatures if the joint does not move.

Dynamic sealing

Dynamic applications include:

• Reciprocating rods
• Rotating shafts
• Moving valve stems
• Pump components
• Oscillating equipment

The seal must continuously deform and recover. As temperature decreases, increased hardness and slower elastic response can cause leakage or accelerated wear.

For dynamic applications, evaluate:

• Movement speed
• Stroke
• Friction
• Lubrication
• Surface finish
• Startup temperature
• Breakaway force

Do not automatically use a published static minimum as the dynamic operating limit.

A maximum temperature measured in air does not automatically apply when the seal is exposed to chemicals.

Seal performance depends on the combined effects of:

• Temperature
• Chemical stability
• Mechanical properties
• Gland design

DuPont’s FFKM seal-design guidance explicitly identifies chemical stability, temperature stability, inherent mechanical properties and seal design as connected factors in overall sealing performance.

Chemical media can reduce usable temperature

A compound rated for 300°C in dry air may have a lower recommended limit in:

• Steam
• Hot water
• Amines
• Strong oxidizing acids
• Reactive process gases
• Solvent mixtures
• Aggressive cleaning chemicals

Heat accelerates many chemical interactions. A fluid that produces minimal swelling at room temperature may cause excessive volume change, softening or loss of strength at elevated temperatures.

Always check compatibility at the actual maximum chemical-service temperature.

Steam resistance should be evaluated separately from dry-heat resistance.

Steam combines:

• High temperature
• Water
• Pressure
• Thermal cycling
• Possible cleaning chemicals

A compound developed for extreme dry heat is not automatically the best material for steam-in-place or hot-water service.

Commercial FFKM portfolios include specific compounds for hot water and steam. Trelleborg, for example, lists separate grades for general chemical processing, ultra-low temperatures, high temperatures and high-temperature steam environments.

For steam applications, provide:

• Saturated or superheated steam
• Steam pressure
• Continuous temperature
• Peak temperature
• Cycle duration
• Number of cycles
• Heating and cooling rate
• Cleaning chemicals

A statement such as “the system reaches 250°C” is not enough to determine suitability.

A seal may tolerate a stable temperature but perform poorly when repeatedly cycled between hot and cold conditions.

Temperature cycling causes:

• Expansion and contraction
• Changes in seal squeeze
• Changes in hardness
• Stress relaxation
• Movement between seal and gland
• Temporary loss of contact pressure

DuPont notes that newer FFKM grades can provide improved resistance to temperature cycling and that long-term sealing performance is connected to compression behavior and retained sealing force.

Important cycling information

When requesting a material recommendation, specify:

• Minimum cycle temperature
• Maximum cycle temperature
• Heating time
• Cooling time
• Dwell time
• Number of cycles
• Pressure during each stage
• Process media during each stage

A pharmaceutical seal exposed to repeated SIP cycles may require a different compound from a static seal operating continuously at the same maximum temperature.

FFKM expands as temperature increases.

If the groove does not provide sufficient space for thermal expansion and expected chemical swelling, the seal may become over-compressed.

Possible consequences include:

• Excessive contact stress
• Extrusion
• Difficult movement
• Accelerated wear
• Seal damage
• Permanent deformation

FFKM should not always be installed as a direct replacement for another elastomer without reviewing:

• Groove width
• Groove depth
• Percentage squeeze
• Groove fill
• O-ring stretch
• Hardware tolerances
• Extrusion clearance

DuPont recommends considering both application conditions and elastomer properties when designing an FFKM gland; failure to account for these factors may cause premature seal failure.

Compression set describes the permanent deformation remaining after an elastomer has been compressed for a specified time and temperature.

At elevated temperatures, excessive compression set can reduce the seal’s ability to maintain contact pressure.

Possible symptoms include:

• Leakage after shutdown
• Leakage following a cooling cycle
• Failure during vacuum operation
• Reduced maintenance intervals
• Permanent flattening of the O-ring

When comparing compounds, check the compression-set test conditions:

• Test temperature
• Test duration
• O-ring or test specimen
• Percentage compression
• Recovery time
• Test standard

A value measured for 70 hours at 200°C cannot be directly compared with one measured for 1,000 hours at 250°C.

For continuous high-temperature service, long-term seal-force retention may be more useful than a single short compression-set result.

Pressure and temperature should not be evaluated separately.

At high temperatures:

• The elastomer becomes softer
• Thermal expansion increases groove fill
• Extrusion risk may increase
• Chemical reaction rates can increase

At low temperatures:

• The material becomes harder
• Elastic recovery slows
• Low-pressure sealing may become more difficult
• Rapid pressure changes may cause temporary leakage

High-pressure gas systems also require evaluation of rapid gas decompression. A compound with suitable temperature capability may still experience internal cracking or blistering if it has not been designed and tested for rapid decompression.

Before selecting a compound, separate the application temperature into the following categories.

1. Minimum storage temperature

The lowest temperature experienced while the equipment or replacement seal is stored.

2. Minimum startup temperature

The lowest temperature at which the equipment must start and seal immediately.

3. Normal operating temperature

The temperature present during most of the process.

4. Maximum continuous temperature

The highest temperature maintained for extended periods.

5. Peak excursion temperature

The highest short-duration temperature.

6. Cleaning temperature

The temperature during CIP, solvent flushing or equipment cleaning.

7. Sterilization temperature

The temperature and pressure during SIP or autoclave exposure.

8. Shutdown temperature

The temperature present when pressure is removed or equipment stops moving.

Each value may affect compound selection differently.

Using 325°C as the universal FFKM limit

Only selected compounds are published for operation or excursions near 325°C.

Confusing maximum service temperature with continuous temperature

A maximum or excursion value may apply only for a limited duration.

Ignoring minimum startup temperature

Equipment may leak during a cold startup even if it seals correctly after warming.

Selecting by Tg alone

Glass transition is a material indicator, not a complete leakage-performance rating.

Ignoring process chemicals

The maximum dry-air temperature may be higher than the allowable temperature in steam, acids, solvents or amines.

Treating repeated peaks as rare excursions

A temperature reached during every process cycle should be considered part of normal service.

Replacing another elastomer without reviewing the groove

Thermal expansion and compression behavior may differ.

Selecting the highest-temperature compound for every application

A compound optimized for extreme heat may provide less low-temperature flexibility or may not be ideal for steam or specific chemicals.

Provide the following information to the seal manufacturer:

For critical equipment, consider testing the proposed compound under representative conditions.

Testing may include:

• High-temperature air aging
• Chemical immersion at operating temperature
• Compression-set testing
• Seal-force retention testing
• Thermal cycling
• Low-temperature leakage testing
• Pressure cycling
• Steam-cycle testing
• Dynamic wear testing
• Equipment-level trials

The test should use the actual compound—not only a generic FFKM sample.

Where possible, reproduce:

• Real chemical concentration
• Actual seal dimensions
• Real gland geometry
• Normal compression
• Maximum pressure
• Temperature-cycle duration
• Expected service interval

The FFKM temperature range cannot be represented accurately by one minimum and one maximum number.

Depending on the specific formulation, commercial FFKM compounds may provide:

• Continuous high-temperature capability above 300°C
• Brief excursion capability around or above 325°C
• Low-temperature performance down to approximately −40°C
• Specialized resistance to steam, amines, plasma or high-pressure gas

However, these capabilities are not normally available in the same degree from every compound.

When selecting an FFKM seal, distinguish between:

• Continuous service temperature
• Short-term peak temperature
• Minimum startup temperature
• Static and dynamic low-temperature performance
• Dry-air and chemical-service limits
• Material ratings and actual seal-system limits

The correct compound should be selected using the complete temperature cycle together with chemical media, pressure, movement, groove design and required service life.

For applications close to the published limits, a specific compound review and representative testing are strongly recommended.