Rubber Material Engineering Guide
Fluorocarbon Rubber (FKM / Viton™): Properties, Grades, Applications & Selection Guide
Fluorocarbon rubber (FKM) is a family of fluoroelastomers used for seals, gaskets, O-rings, diaphragms, hoses and custom molded components exposed to heat, fuels, oils and demanding chemical media. FKM is valued for heat resistance, low gas permeability, weathering resistance and broad fluid compatibility, but performance varies significantly by polymer family, fluorine level, cure system and compound. Viton™ is a Chemours trademark for specific fluoroelastomers; it is not the generic name for every FKM material.
Material Fundamentals
What Is FKM Rubber, and Is FKM the Same as Viton™?
FKM is the generic abbreviation for a family of fluorinated elastomers. Viton™ is a registered Chemours trademark covering particular fluoroelastomer product families. A part specified as FKM therefore does not automatically contain Viton™ polymer, and the word Viton™ should not be used as a generic substitute for every fluorocarbon rubber.
This distinction matters in purchasing and engineering. Two compounds can both be described as FKM yet differ in polymer chemistry, fluorine level, cure system, fillers, hardness, low-temperature flexibility, compression set and resistance to a specific fluid. The application should be matched to a defined compound specification rather than to the three-letter polymer abbreviation alone.
FKM is often considered when
- Hydrocarbon fuels, mineral oils or lubricants are present.
- Continuous heat is beyond the practical range of many general-purpose rubbers.
- Low gas permeability or strong ozone and weathering resistance is useful.
- Chemical exposure makes NBR, EPDM or silicone unsuitable.
FKM needs another look when
- Low-temperature sealing or dynamic flexibility is critical.
- Hot water, steam, amines, ammonia, strong alkalis or polar fluids are present.
- The part sees rapid gas decompression or severe dynamic abrasion.
- A branded polymer, approval or industry qualification is mandatory.
Performance Profile
What Are the Key Properties of FKM Rubber?
The reason engineers select FKM is usually a combination of heat resistance, hydrocarbon-fluid resistance, low permeability and long-term sealing performance. These are family-level strengths, not a substitute for compound data.
Heat Resistance
Many FKM compounds are designed for service temperatures above the practical range of general NBR and EPDM compounds. Actual aging life still depends on temperature, time, fluid and formulation.
Fuel & Oil Resistance
FKM is widely used with hydrocarbon fuels, mineral oils, lubricants and many synthetic fluids. Fuel chemistry and alcohol content can change swelling and permeation behavior.
Low Permeability
Low gas and fuel permeation is one reason FKM is used in demanding sealing and fuel-system applications. Permeation should still be tested for the actual compound and medium.
Ozone & Weathering
FKM generally provides strong resistance to ozone, oxygen and outdoor aging, useful where heat and environmental exposure occur together.
Compression Set
Properly selected FKM compounds can retain sealing force at elevated temperature, but cure system, post-cure, geometry and continuous compression materially affect the result.
Low-Temperature Limits
General-purpose FKM is not automatically a low-temperature elastomer. Specialty low-temperature families exist, so minimum functional temperature must be defined separately.
Polymer & Grade Selection
How Do FKM Grades, Polymer Chemistry and Fluorine Level Differ?
FKM is a material family rather than one recipe. Changes in monomer composition and fluorine level shift the balance among fuel resistance, chemical resistance, low-temperature flexibility and processing.
As a brand-specific example, Chemours groups major Viton™ fluoroelastomers into A, B, F and ETP families. That naming should not be treated as a universal grade system for all FKM suppliers. For an RFQ, the safest approach is to state the required performance or an exact approved material specification.
| FKM Design Variable | Why It Changes Performance | What the Buyer Should Define |
|---|---|---|
| Polymer family / monomer composition | Changes chemical resistance, low-temperature behavior, cure options and processing | Actual media, minimum temperature and required specification |
| Fluorine level | Higher fluorine content can improve resistance to many hydrocarbon fluids, often with a trade-off in low-temperature flexibility | Fuel/solvent composition and minimum sealing temperature |
| Low-temperature specialty grade | Designed to retain flexibility below the range of conventional general-purpose FKM | Static vs. dynamic function and minimum functional temperature |
| Base / amine-resistant specialty FKM | Special polymer systems can address media that are difficult for conventional FKM | Exact chemical, concentration, water content and temperature |
| Compound formulation | Fillers, curatives, process aids and other ingredients affect hardness, compression set, shrinkage and finished-part properties | Physical-property limits, color, compliance and batch-control requirements |
Do not specify by fluorine percentage alone
Fluorine level is useful engineering context, but it does not fully define a finished compound. Cure system, formulation, post-cure and the exact test medium also affect performance.
What about FFKM?
FFKM is perfluoroelastomer, a separate higher-performance material family. It should not be presented as simply a premium grade of ordinary FKM. Use it when the application genuinely requires its chemical or thermal capability and validate the exact compound.
Crosslinking
Bisphenol-Cured vs. Peroxide-Cured FKM: What Changes?
FKM cure chemistry is part of the material specification. Bisphenol and peroxide systems are both used in commercial FKM, and neither is universally superior.
| Topic | Bisphenol-Cured FKM | Peroxide-Cured FKM |
|---|---|---|
| Typical role | Widely used for many molded seals and general high-temperature FKM applications | Common in selected specialty FKM families and compounds where their polymer chemistry or media requirements favor peroxide cure |
| Compression set | Can provide strong high-temperature compression-set performance with the correct compound and post-cure | Can also provide strong sealing performance; results are compound- and condition-specific |
| Media resistance | Must be checked against the actual fluid; conventional systems are not universal for bases, amines or steam | Used in some specialty formulations targeting difficult media, but peroxide cure alone does not guarantee compatibility |
| Processing | Cure rate, mold release, scorch safety and post-cure depend on the complete recipe | Coagent selection, cure state and processing controls are important to finished properties |
| Selection rule | Specify end-use performance first. Let polymer and compound selection determine the suitable cure system unless the project explicitly requires one. | |
Hardness & Geometry
What FKM Rubber Hardness Should You Choose?
FKM is available in multiple Shore A grades, but hardness should follow sealing geometry, pressure, assembly force, extrusion gap and movement. There is no single correct hardness for all FKM parts.
Lower Hardness
Can improve conformity to mating surfaces and reduce compression force, but may need greater protection against extrusion or excessive deformation.
Medium Hardness
Often considered for general seals and molded parts where flexibility, handling and load support must be balanced. The exact Shore A target remains application-specific.
Higher Hardness
Can improve resistance to deformation and extrusion under pressure, but increases assembly force and can reduce conformity to imperfect surfaces.
Shore hardness is a compound property, not a polymer identity. State both the target hardness and permitted tolerance if hardness is functionally important.
Thermal Limits
What Temperature Range Can FKM Rubber Handle?
A useful engineering starting point for many general-purpose FKM compounds is roughly -20/-25°C to +200°C, but this is not a universal guarantee. Specialty FKM compounds can shift the low-temperature boundary, while high-temperature life depends strongly on time, fluid and sealing stress.
Published supplier ranges should be treated as screening data. A seal that only survives storage at a temperature has a different requirement from a dynamic seal that must flex, recover and maintain pressure at that same temperature.
Low Temperature
Conventional FKM can lose flexibility earlier than EPDM, silicone or specialty low-temperature elastomers. State whether the application is static or dynamic and define the minimum functional temperature.
Continuous Heat
Long exposure can cause hardening, loss of elongation and compression set. Use compound-specific heat-aging data at the required service time.
Short Heat Peaks
Short excursions above a continuous rating are not the same as continuous service. State peak temperature, duration and frequency.
Heat + Fluid
Fuel, oil, steam, acid or solvent can accelerate property change. Temperature and media must be evaluated together rather than as independent specifications.
Fluid Compatibility
Which Fuels, Oils and Chemicals Is FKM Compatible With?
FKM is widely selected for hydrocarbon fuels, mineral oils, lubricants, aromatics and many industrial chemicals, but its chemical resistance is broad rather than universal. Polymer family and fluorine level can change swelling, permeation and property retention.
| Media Group | General FKM Screening | Engineering Note |
|---|---|---|
| Mineral oils & lubricants | Strong starting point | Check additive package, temperature and exposure duration. |
| Hydrocarbon fuels & aromatics | Strong starting point | Fuel chemistry, alcohol/oxygenate content and permeation target can require a specific FKM family. |
| Many organic solvents | Compound-specific | Do not generalize across all solvents; test the actual formulation and temperature. |
| Acids | Often suitable / verify | Acid type, concentration, water content and heat can change compatibility. |
| Hot water & steam | Grade-sensitive | Standard FKM is not automatically a steam material; specialty compounds may be required. |
| Amines, ammonia & strong alkalis | High caution | Conventional FKM may be unsuitable. Base-resistant specialty fluoroelastomers should be evaluated if FKM is desired. |
| Ketones / selected polar solvents | High caution | Conventional FKM can perform poorly; specialty chemistry may behave differently. |
| Glycol-based brake fluids | Usually not first choice | EPDM is commonly evaluated for glycol-based brake-fluid service; validate the complete system. |
| Ozone / weather / oxygen | Strong starting point | Environmental resistance is generally a family strength, but mechanical and media requirements still govern. |
Material Selection
FKM vs. NBR, HNBR, EPDM, Silicone, FVMQ and FFKM: Which Should You Use?
FKM is often chosen when heat and hydrocarbon-fluid resistance are required together. It is not automatically the best choice when low-temperature flexibility, hot water, steam, glycol fluids, abrasion or extreme chemical resistance dominates.
| Material | Typical Selection Strength | Watch Point | When to Compare Against FKM |
|---|---|---|---|
| FKM | Heat + fuels/oils + low permeability + broad chemical resistance | Low temperature and certain polar/basic/steam environments are grade-sensitive | Baseline for demanding hydrocarbon sealing |
| NBR | Economical oil resistance and general sealing | Lower heat, ozone and weathering capability than FKM | When service temperature and chemistry do not justify FKM |
| HNBR | Improved heat, ozone and mechanical performance over NBR | Chemical envelope still differs from FKM | When strength, fatigue or oil service matters but FKM may be excessive |
| EPDM | Water, weathering, ozone and many glycol-based fluids | Generally unsuitable for hydrocarbon oils and fuels | When water/steam/glycol service dominates |
| VMQ Silicone | Broad temperature flexibility, weathering, electrical and clean-contact options | Standard VMQ is not the first choice for fuels, abrasion or many petroleum oils | When low temperature, electrical or clean-contact requirements dominate |
| FVMQ | Silicone-like low-temperature behavior with improved fuel/oil resistance | Mechanical and chemical envelope differs from FKM | When fuel resistance and low-temperature flexibility are both critical |
| FFKM | Extreme chemical and high-temperature sealing capability with selected compounds | Much higher material cost and application-specific qualification | When the chemical environment exceeds suitable FKM capability |
Industrial Uses
Where Is FKM Rubber Used?
FKM is most valuable where the part must seal or survive a combination of heat, hydrocarbon fluids, chemicals, vacuum or low-permeation requirements.
O-Rings, Seals & Gaskets
Static and dynamic sealing components for pumps, valves, gearboxes, engines and industrial equipment exposed to compatible oils, fuels and chemicals.
Fuel-System Components
Seals, grommets, diaphragms and hose layers where fuel resistance and permeation control are important. Modern fuel blends require compound-specific validation.
Engine & Powertrain Parts
Oil seals, connector seals, gaskets and molded parts operating near lubricants and elevated under-hood temperatures.
Pumps, Valves & Chemical Equipment
Sealing parts for compatible process media where heat and chemical exposure exceed the capability of general-purpose elastomers.
Oil & Gas Equipment
Selected FKM compounds are used for high-pressure hydrocarbon and gas service. Rapid-gas-decompression resistance and project-specific qualification must be confirmed where required.
Hoses, Diaphragms & Custom Moldings
Extruded or molded components, thin fuel-resistant layers, bonded parts and complex geometries can use FKM when processing and service conditions are correctly specified.
Failure Analysis
Why Do FKM Seals Swell, Harden, Crack or Leak?
An FKM part can fail even when the polymer family appears compatible. Fluid chemistry, temperature, pressure, low-temperature stiffness, cure state, compression, surface finish and installation all interact.
| Observed Problem | Possible Mechanisms | What to Check |
|---|---|---|
| Swelling / softening | Incompatible fluid, fuel blend or solvent; excessive temperature | Exact media composition, volume change, hardness change and exposure cycle |
| Hardening / cracking | Thermal aging, oxidation, chemical attack or excessive service time | Continuous vs. peak temperature, heat-aging data and elongation retention |
| Permanent flattening / leak | Compression set, insufficient cure/post-cure, excessive squeeze or thermal aging | Compression geometry, cure records and compression-set requirement |
| Extrusion / nibbling | Pressure, clearance gap, hardness or unsupported seal geometry | Maximum pressure, gap size, pressure cycling and backup-ring strategy |
| Cold leakage | Compound becomes too stiff to follow sealing surfaces at minimum temperature | Low-temperature compound family, static/dynamic function and actual minimum temperature |
| Blistering after gas service | Rapid gas decompression in high-pressure gas exposure | Gas composition, pressure, decompression rate and RGD-qualified compound requirement |
| Bond separation | Adhesive/substrate incompatibility, contamination, thermal or chemical attack | Metal preparation, bonding system, cure process and service-fluid exposure |
Custom Manufacturing
How Are Custom FKM Rubber Parts Manufactured?
FKM can be compression, transfer or injection molded and can also be extruded or bonded to substrates with suitable formulations and process controls. The selected compound and cure system affect flow, scorch safety, shrinkage, post-cure and dimensional stability.
- Review the application. Confirm media, temperature, pressure/vacuum, movement, service life and failure risk.
- Select the compound. Define FKM family, hardness, cure system, color and required performance or compliance.
- Review geometry and tooling. Identify parting lines, gates, flash, undercuts, bonded inserts, sealing surfaces and measurement datums.
- Develop samples. Mold or extrude samples using a controlled cure and any required post-cure.
- Validate function. Check dimensions, hardness, visual quality and project-specific physical or media tests.
- Control production. Maintain compound traceability, cure conditions, post-cure, inspection criteria and batch consistency.
Compression Molding
Suitable for many FKM gaskets, diaphragms, molded seals and lower-to-medium volume custom parts where geometry and tooling fit the process.
Transfer / Injection Molding
Useful for repeatable production of complex molded parts when compound flow, gating, cure behavior and tooling economics are properly controlled.
Extrusion
Selected FKM compounds can be extruded into tubing, profiles or thin layers. Extrusion smoothness, dimensional stability and cure system require process-specific development.
Rubber-to-Metal Bonding
FKM can be bonded to selected metal substrates using a validated pretreatment and adhesive system. Bond durability must be checked in the actual service environment.
Deflashing & Inspection
Trimming, cryogenic deflashing or other finishing methods should be matched to geometry and critical sealing surfaces. Flash criteria must be defined separately from dimensions.
Post-Cure
Many FKM formulations use controlled oven post-cure. The required schedule is compound-specific and must be included when dimensional and physical-property validation is performed.
Dimensions & Tooling
What Tolerances Can Be Achieved on FKM Rubber Parts?
FKM part tolerances depend on geometry, molding process, tool design, compound shrinkage, post-cure and measurement method. A tight metal-machining tolerance should not be copied onto a rubber part without functional justification.
ISO 3302-1 can provide a framework for dimensional tolerances on molded, extruded and calendered solid rubber products when it is appropriate to the part. It specifically does not apply to precision toroidal sealing rings, so O-rings should follow the drawing and an applicable O-ring specification such as ISO 3601 when required.
Identify Critical Sealing Dimensions
Separate functional diameters, squeeze height, wall thickness, groove interfaces and bonded locations from noncritical dimensions.
Allow for Compound Shrinkage
FKM shrinkage is influenced by compound, cure, post-cure and tool conditions. Final tooling should be based on the selected production compound.
Define Flash Separately
Parting line, flash extension, gate vestige and trimming acceptance are not the same as dimensional tolerance and should be controlled independently.
Agree the Measurement Method
Rubber deforms under measurement force. Define conditioning, fixture, datum and method for soft or thin features before resolving a dimensional dispute.
Validation
Which Tests Should Be Specified for FKM Rubber?
Test the property that protects the real failure mode. Hardness alone cannot prove that an FKM seal will survive fuel, heat, compression or pressure cycling.
| Test Area | Relevant Standard / Method | What It Helps Verify |
|---|---|---|
| Shore hardness | ISO 48-4:2018 or agreed equivalent | Indentation hardness of the specified compound |
| Tensile / elongation | ISO 37:2024 or agreed equivalent | Strength and elongation before and, when required, after aging |
| Compression set | ISO 815-1:2019 or agreed equivalent | Retention of elastic recovery after prolonged compression at defined temperature |
| Fluid immersion | ISO 1817:2024 or customer-defined method | Volume, mass, hardness and mechanical-property changes after liquid exposure |
| Heat aging | Customer specification or applicable rubber-aging method | Change in hardness, tensile strength, elongation and appearance after thermal exposure |
| Low-temperature behavior | Application-specific low-temperature test | Whether the seal remains flexible and functional at the required minimum temperature |
| Rapid gas decompression | Project-specified oil & gas qualification where applicable | Resistance to blistering or internal damage after high-pressure gas decompression |
| Dimensions / visual quality | Approved drawing and inspection plan | CTQ dimensions, flash, surface defects, parting lines and batch consistency |
Test edition, specimen geometry, exposure time, temperature, acceptance limits and sampling plan should be stated in the project specification. Standards listed here are not automatic certifications of a supplied part.
Regulatory & Specification Boundary
Does FKM Automatically Meet FDA, NSF, Aerospace or Oil & Gas Requirements?
No. FKM describes a polymer family, not a compliance status. A generic black FKM compound cannot be assumed to meet food-contact, drinking-water, medical, aerospace, automotive, oil-and-gas or customer-specific requirements.
- State the exact regulation, standard, customer specification or approval number required.
- Confirm whether compliance applies to raw polymer, finished compound, finished part or complete assembly.
- Confirm color and formulation because fillers, curatives and process aids can affect compliance.
- Request the required document type: declaration, compound data, test report, material certificate or third-party approval.
- For RoHS, REACH or other chemical-substance requirements, confirm the exact compound and requested reporting scope.
- If Viton™ branding or a specific polymer source is mandatory, state it explicitly in the RFQ and purchase specification.
RFQ Preparation
What Information Should You Send for an FKM Rubber RFQ?
A useful FKM RFQ describes the failure environment as clearly as the geometry. This lets material selection and manufacturing review happen before tooling or price is locked.
| RFQ Item | Information to Provide | Why It Matters |
|---|---|---|
| Part definition | 2D dimensions, 3D file or physical sample | Defines geometry, tooling, CTQ dimensions and manufacturing process |
| Function | Static seal, dynamic seal, diaphragm, hose, bonded part, protective component, etc. | Changes mechanical and compound priorities |
| Contact media | Exact fuel, oil, chemical, gas or mixture; concentration and additives if known | Drives FKM family and compatibility review |
| Temperature | Minimum, continuous and peak values plus peak duration | Separates low-temperature and heat-aging requirements |
| Pressure / vacuum | Normal, maximum and cycling conditions | Affects extrusion, permeation and decompression risk |
| Hardness | Target Shore A and tolerance if specified | Affects compression, deformation and pressure capability |
| Material requirement | Generic FKM, approved compound, branded polymer or customer specification | Prevents substitution or traceability misunderstandings |
| Cure / post-cure | Only if explicitly specified by the project | Can affect chemistry, dimensions and final properties |
| Compliance | Exact regulation, approval, test report or customer standard | A generic FKM designation does not prove compliance |
| Quantity | Prototype quantity, order quantity and expected annual usage | Supports process and tooling selection; MOQ and lead time are then confirmed |
| Existing failure | Photos, returned parts and known swelling/cracking/leak conditions | Helps identify material, geometry or process risk before redesign |
FKM FAQ
Frequently Asked Questions About FKM Rubber
Is FKM the same as Viton™?
No. FKM is a generic fluoroelastomer material-family designation. Viton™ is a Chemours trademark for specific fluoroelastomer product families. If a project requires genuine Viton™ polymer or an approved branded compound, specify that requirement explicitly.
Is all FKM rubber black?
No. Black is common, but color depends on the selected compound and formulation. Color must not be used to identify FKM chemistry or compliance; specify the required color and material separately.
What temperature can FKM rubber withstand?
Many general-purpose FKM compounds are catalogued around -20/-25°C to +200°C as an engineering starting point, but specialty low-temperature and high-temperature compounds differ. Final limits must be confirmed for the selected compound, media, time and function.
Is FKM resistant to gasoline and diesel fuel?
FKM is widely used in hydrocarbon-fuel service, but modern fuels can contain alcohols, oxygenates, biodiesel components and additives. Fuel composition, permeation target and temperature should be checked against the exact compound.
Is FKM resistant to engine oil and hydraulic oil?
FKM is a strong starting material for many mineral and synthetic lubricants, but oil chemistry and additives matter. Hydraulic fluids based on other chemistries may require a different elastomer, so identify the exact fluid.
Can FKM be used with steam or hot water?
Do not assume standard FKM is suitable for steam. Steam and hot-water resistance varies significantly by polymer and compound; specialty FKM may be available, while EPDM or another elastomer can be a better starting point in some water/steam systems.
Is FKM resistant to acids, amines and alkalis?
Resistance depends on the exact chemistry. Many acids can be compatible with selected FKM compounds, while conventional FKM requires caution with amines, ammonia and strong bases. Specialty base-resistant fluoroelastomers exist but must be validated for the actual medium.
What does fluorine content change in FKM?
Within comparable fluoroelastomer families, increasing fluorine level can improve resistance to many hydrocarbon fluids, but can reduce low-temperature flexibility. It is one selection variable, not a complete compound specification.
Bisphenol-cured or peroxide-cured FKM: which is better?
Neither is universally better. Cure system should match the polymer family, media, compression-set target, processing and application requirements. Some specialty FKM formulations use peroxide cure, while many general-purpose FKM seals use bisphenol systems.
FKM or NBR: which should I choose?
NBR is often more economical for moderate-temperature oil service. FKM becomes attractive when higher heat resistance, broader hydrocarbon/chemical resistance, lower permeability or improved weathering performance is required.
FKM or EPDM: which should I choose?
FKM is generally the stronger starting point for hydrocarbon fuels and oils. EPDM is often the stronger starting point for water, weathering and glycol-based fluids. The actual chemical and temperature decide the material.
FKM or FVMQ: which should I choose?
FKM typically provides stronger high-temperature hydrocarbon and broad chemical sealing performance. FVMQ can be attractive when fuel/oil resistance must be combined with silicone-like low-temperature flexibility. Compare the actual fluid and mechanical requirements.
What is the difference between FKM and FFKM?
FKM and FFKM are different fluoroelastomer families. FFKM is used when the required chemical or thermal envelope exceeds suitable FKM capability, but it carries a substantially different material-cost and qualification profile.
Does black FKM automatically meet FDA, NSF or other approvals?
No. Color and polymer family do not prove compliance. The exact compound and finished-part requirement must be checked against the stated regulation, customer specification or approval.
What information is needed to quote a custom FKM part?
Send the part dimensions or 3D data, application, exact contact media, minimum/continuous/peak temperatures, pressure or vacuum, movement, hardness if specified, compliance requirements and quantity. A physical sample and failure photos are also useful when replacing an existing part.
Custom FKM Manufacturing
Have an FKM seal, gasket, O-ring, diaphragm or molded part to develop?
Send the part geometry, operating temperature, contact media, pressure, hardness or material requirement and expected quantity. We can review the information needed to confirm compound selection, tooling and manufacturing feasibility.
