FKM (fluorocarbon rubber, commonly branded as Viton) and PTFE (polytetrafluoroethylene, Teflon) are both premium sealing materials for harsh chemical and high-temperature environments — but they are mechanically opposite materials.
Quick answer: FKM is an elastomer — it compresses, generates contact stress through elastic deformation, and returns to its original shape after load is removed. Use FKM for any dynamic sealing application (reciprocating, rotary, oscillating) and for static applications where its chemical resistance is sufficient. PTFE is a thermoplastic — it cold-flows under load and never recovers. Use PTFE only for static sealing when the process chemistry attacks FKM (ketones, amines, hot caustic, steam) or when temperature exceeds +200°C. Using PTFE in a dynamic position causes immediate leakage.
The Decisive Difference: Elastomer vs Thermoplastic
| Property | FKM (Fluorocarbon Rubber) | PTFE (Polytetrafluoroethylene) |
|---|---|---|
| Material class | Crosslinked elastomer | Semi-crystalline thermoplastic |
| Elastic recovery | Excellent — returns to original shape | None — permanently deforms under load |
| Sealing force source | Elastic compression of crosslinked polymer network | External compressive force maintained by groove geometry |
| Dynamic service | Excellent — designed for motion | Not suitable — PTFE flows away from contact |
| Static service | Excellent | Good when groove maintains compression; poor under thermal cycling |
| Hardness scale | 60–90 Shore A (soft rubber) | 55–65 Shore D (rigid thermoplastic — approximately 6× harder than FKM) |
| Cold flow under load | Negligible at correct compression | Continuous — PTFE creeps under any sustained mechanical load |
| Self-compensation for surface irregularities | Excellent — deforms elastically to fill surface imperfections | Poor — PTFE does not conform to rough surfaces |
Cold flow mechanism in detail: PTFE has no crosslinked molecular network to prevent chain sliding. Under compressive load, PTFE molecular chains slide past each other permanently. In a static groove with rigid metal walls on all sides, cold flow fills the groove profile and creates an adequate static seal — but if compressive load changes (thermal cycling, vibration, retorquing), the cold-flowed PTFE cannot recover. Quantitatively, virgin PTFE at +100°C under 3.5 MPa contact stress loses approximately 8–15% of original thickness in 1,000 hours (ASTM F36 creep relaxation). This is why PTFE flanged connections require retorquing after first heat-up.
FKM Grades: Not All FKM Is the Same
FKM is available in several fluorine-content grades with different chemical resistance profiles:
| FKM Grade | VF₂ Content | HFP/TFE Content | Fluorine Content | Chemical Resistance | Low-Temp Limit | Relative Cost |
|---|---|---|---|---|---|---|
| Type 1 (Dipolymer) | ~65 mol% | 35% HFP | 66% by weight | Good for hydrocarbons, fuels | −20°C | 1× |
| Type 2 (Terpolymer) | 60% VF₂ | 20% HFP + 20% TFE | 68% | Better overall resistance | −20°C | 1.2× |
| Type 3 (Low-temp grade, GFLT) | 45–55% VF₂ | High TFE + perfluorovinyl ether | 68–70% | Broad resistance | −40°C | 1.5–1.8× |
| Type 4 (Perfluoroether FKM, FFPKM) | Perfluoro | — | 73% | Superior; see FFKM article | −25°C | 3–5× |
| Bisphenol cure (standard) | Any of above | — | — | Standard; good amine resistance | Grade-dependent | 1× |
| Peroxide cure | Any of above | — | — | Better steam/acid resistance; lower compression set | Grade-dependent | 1.3× |
For most industrial hydraulic, fuel, and chemical applications, Type 1 or Type 2 FKM (bisphenol or peroxide cure) is appropriate. For alcohol-containing fuels (E85, methanol blends), Type 3 or GFLT grades significantly reduce swell. Always specify FKM type when ordering for chemical-critical applications — generic "FKM" may refer to any of these.
Temperature Comparison
| Temperature Range | FKM Performance | PTFE Performance | Correct Choice |
|---|---|---|---|
| Below −40°C (cryogenic) | Too stiff — loses elastic recovery below −20°C standard grades | Excellent — remains flexible at −270°C | PTFE or spring-energized PTFE |
| −40°C to −20°C | Marginal (standard); GLT/GFLT grades adequate | Excellent | FKM low-temp grade or PTFE |
| −20°C to +120°C | Excellent — standard service range | Good (static) | FKM for dynamic; PTFE for static chemical |
| +120°C to +200°C | Excellent — well within FKM range | Good (static) | FKM for dynamic; either for static |
| +200°C to +260°C | Approaches thermal limit; peroxide FKM better | Excellent (static) | PTFE static; FFKM for dynamic |
| Above +260°C | Fails — degradation and outgassing | Approaches PTFE limit | Metal seals; specialty ceramics |
Steam service — FKM failure mechanism: FKM is not suitable for continuous steam service. The degradation mechanism differs from other chemical attack: the fluorine atoms in the FKM backbone are susceptible to hydrolysis by high-energy water at +100°C and above, particularly at the CF₂ sites adjacent to CH₂ units (in standard VF₂-containing grades). The result is dehydrofluorination (HF elimination), forming C=C double bonds in the polymer chain — the seal hardens and cracks, with HF potentially contaminating the steam condensate. PTFE (static) or EPDM (static steam to +150°C) are the alternatives for steam service.
Chemical Resistance: FKM-Specific Incompatibilities
Both FKM and PTFE have broad chemical resistance, but FKM has specific chemical families that attack it while PTFE has near-universal resistance.
FKM Incompatibilities (Use PTFE Instead for Static Service)
| Chemical Attacking FKM | Effect on FKM | PTFE Behavior | Notes |
|---|---|---|---|
| Ketones (MEK, acetone, cyclohexanone) | 30–80% volume swell within 24–72h; dimensional collapse | < 1% dimensional change — inert | Most common reason to switch to PTFE |
| Esters (ethyl acetate, butyl acetate) | 20–50% swell; significant softening | Inert | Cleaning solvents and process chemicals |
| Primary amines (MEA, aniline, propylamine) | Dehydrofluorination at > 60°C — progressive dissolution | Inert to amines | Amines remove fluorine from FKM backbone |
| Secondary amines (DEA, piperazine) | Slower dehydrofluorination than primary; still incompatible at temperature | Inert | Common in amine scrubbers |
| Hot NaOH (> 5%, > 80°C) | Progressive surface attack; swelling | Resists concentrated caustic | Strong alkali at temperature |
| Steam above +100°C | Hydrolysis; HF generation; cracking | Resists steam at any temperature and pressure | Steam is one of the clearest FKM limitations |
| Concentrated ethanol / methanol | Type 1: 15–40% swell; Type 3 GF: 5–10% | < 1% | Type selection matters for alcohols |
| Fuming HNO₃ (> 70% concentration) | Oxidative surface attack | Resists most oxidizing acids | Limited to extreme concentration |
| Skydrol (phosphate ester) | Significant degradation | Inert | Aerospace hydraulic fluid — EPDM/ECO preferred |
FKM Volume Swell Reference Data (ASTM D471, +70°C / 70h)
| Fluid | FKM Type 1 Swell | FKM Type 3 (GF) Swell | PTFE |
|---|---|---|---|
| IRM 902 reference oil | 3–8% | 2–6% | < 0.5% |
| Diesel fuel (ULSD) | 3–8% | 2–5% | < 0.5% |
| Gasoline (25% aromatics) | 5–12% | 3–8% | < 0.5% |
| E85 (85% ethanol) | 20–40% | 8–18% | < 0.5% |
| MEK (methyl ethyl ketone) | 60–100%+ | 50–90%+ | < 0.5% |
| Acetone | 80–120%+ | 70–100%+ | < 0.5% |
| 10% NaOH (ambient) | 2–5% | 2–5% | < 0.5% |
| 10% NaOH (+80°C) | 8–18% | 5–12% | < 0.5% |
PTFE Incompatibilities (Extreme Conditions Only)
| Chemical | PTFE Behavior | |
|---|---|---|
| Alkali metals (Na, K, Li) | Attacked — dehalogenation; produces dark discoloration | Use metal seals |
| Fluorine gas (F₂) at > +150°C | Slow surface attack | Specialty materials required |
| Fuming fluorosulfuric acid | Marginal resistance | Test application-specifically |
For virtually all industrial chemicals, PTFE provides broader chemical resistance than FKM. The FKM incompatibilities listed above represent the situations where PTFE substitution for static sealing is appropriate.
Static vs Dynamic Service: The Most Important Criterion
For dynamic seals (reciprocating rods/pistons, rotary shafts, oscillating motion):
- The seal must maintain contact with the moving surface throughout the stroke cycle
- Elastic recovery re-establishes contact after each dynamic loading cycle
- PTFE in a dynamic groove: cold-flowed PTFE is displaced by the moving surface rather than recovering contact — leakage begins within hours
- FKM is required for all dynamic sealing applications within its temperature and chemical range
- When PTFE's chemical resistance is needed with dynamic motion → spring-energized PTFE seal
For static seals (flanges, valve seats, threaded fittings, face seals):
- The seal is compressed once at assembly; the groove maintains compression
- Elastic recovery is not needed after initial compression
- PTFE is an acceptable static seal when the chemistry attacks FKM
- FKM is preferred for static seals with thermal cycling — FKM's elastic recovery compensates for dimensional changes through temperature excursions without requiring retorquing
Thermal cycling behavior comparison:
| Thermal Event | FKM Static Seal | PTFE Static Seal |
|---|---|---|
| Single heat-up to service temperature | Maintains contact via elastic recovery | Cold-flows slightly at high temp; may need retorquing |
| Cooldown after heat-up | Recovers elastically; maintains sealing | Cannot recover; compression may reduce |
| 5 thermal cycles (hot process shutdown/startup) | Sealing maintained; no action required | Compression progressively decreases; retorque may be needed |
| 50+ thermal cycles | Sealing maintained throughout service | Significant compression loss — may leak without maintenance |
Extrusion Resistance: PTFE's Hardness Advantage
PTFE's much higher hardness (55–65 Shore D vs 60–90 Shore A for FKM) provides better extrusion resistance at high pressure:
| Pressure Range | FKM Extrusion Risk (70 ShA, 0.15 mm radial clearance) | PTFE Extrusion Risk | Recommended Approach |
|---|---|---|---|
| < 70 bar | Low | Very low — PTFE bridges clearance easily | Either — FKM preferred for dynamic |
| 70–150 bar | Moderate | Low | FKM with tight clearance; PTFE static |
| 150–250 bar | High — backup rings needed | Low–moderate | FKM + PTFE backup rings; PTFE static |
| > 250 bar | Very high — backup rings critical | Moderate (PTFE itself may extrude) | FKM + PEEK backup; or Spring-energized PTFE |
PTFE backup rings with FKM O-rings: The most common use of PTFE in conjunction with FKM is as backup (anti-extrusion) rings on the low-pressure side of an FKM O-ring. The FKM O-ring provides the elastic sealing force; the PTFE backup ring bridges the clearance gap and prevents FKM extrusion. This combination is the industry standard for hydraulic cylinders above 150 bar.
Spring-Energized PTFE Seals: The Hybrid Solution
For applications requiring PTFE's chemical resistance with an active sealing force — dynamic service in aggressive chemistry — spring-energized PTFE seals bridge the gap:
- PTFE jacket: Precision-machined PTFE profile (U-cup, lip, or delta geometry) provides the chemical-resistant process contact surface
- Metal spring (energizer): Canted-coil, cantilever, or helical spring provides continuous outward radial force
- Spring compensates for cold flow: As PTFE creeps under contact load, the spring expands to maintain contact force — sealing force is spring-determined, not elastomer-recovery-determined
Cold flow in spring-energized PTFE seals (quantified):
- Virgin PTFE jacket, +100°C, 3.5 MPa contact stress, 1,000h: 8–15% thickness loss
- 15% glass-filled PTFE jacket, same conditions: 4–8% thickness loss
- 25% carbon-filled PTFE jacket: 3–6% thickness loss
- PEEK jacket (for extreme conditions): 0.5–1.5% thickness loss
Spring selection by application:
| Application | Spring Material | Temperature Range | Notes |
|---|---|---|---|
| General industrial chemical | 316 SS canted coil | −100°C to +260°C | Standard specification |
| Cryogenic (LNG, LOX, LN₂) | Inconel 718 | −270°C to +260°C | Maintains force at cryogenic temp |
| High-temperature process | Inconel 718 or Hastelloy | −100°C to +650°C (spring) | PTFE jacket limits to +260°C |
| Highly corrosive (HCl, HF) | Hastelloy C276 | −100°C to +260°C | Spring resistant to halide acids |
Applications for spring-energized PTFE:
- Reciprocating chemical pumps with ketone, amine, or other FKM-incompatible fluids
- Cryogenic valve stems (LNG, LOX, LN₂) where no elastomeric O-ring survives
- Analytical instrument valves (HPLC, GC) requiring ultra-low friction and solvent resistance
- Semiconductor valve bodies requiring PTFE chemistry with dynamic motion
- High-vacuum rotary feedthroughs requiring near-zero outgassing with motion (ASTM E595 outgassing: PTFE TML < 0.01%)
Spring-energized PTFE seals require custom groove geometry (not a standard O-ring groove) and cost 5–15× more than a standard FKM O-ring. They are the engineering-correct solution when FKM cannot handle the chemistry and PTFE cannot handle the motion.
Cost Comparison
| Item | Relative Cost | Notes |
|---|---|---|
| Standard FKM O-ring (AS568 stock size) | 1× | Standard stocked item |
| Standard PTFE lathe-cut O-ring | 1.5–2.5× | Machining cost; less elastic = tighter groove tolerance |
| Spring-energized PTFE seal | 5–15× | Custom machining + spring assembly |
| FKM + PTFE backup ring set | 1.3–1.8× | FKM seal + PTFE backup ring combined |
| FFKM O-ring | 10–30× | When FKM temperature/chemical limit is exceeded |
Application Selection Matrix
| Application | Correct Material | Rationale |
|---|---|---|
| Hydraulic cylinder rod seal (petroleum oil) | FKM | Dynamic — elastic recovery required |
| High-temp fuel system O-ring (+150°C) | FKM | Dynamic + fuel compatible |
| Chemical reactor flange (ketone service) | PTFE | Static + FKM-incompatible chemistry |
| Cryogenic valve seat (LNG, LOX) | Spring-energized PTFE | Static or dynamic at −270°C; FKM too stiff |
| Semiconductor etch chamber flange | PTFE or FFKM | Static + aggressive plasma chemistry |
| Aerospace engine seal (fuel, +180°C) | FKM (peroxide cure) | Dynamic + fuel + high temp |
| CIP/SIP food equipment flange (caustic) | PTFE or EPDM | Caustic resistance + FDA; static |
| Reciprocating pump (ketone solvent) | Spring-energized PTFE | Dynamic + ketone — no alternative |
| High-pressure hydraulic cylinder (> 150 bar) | FKM + PTFE backup rings | FKM seals; PTFE prevents extrusion |
| Steam trap valve seat (+150°C steam) | PTFE (static) or EPDM | Steam attacks standard FKM |
| Amine scrubber valve (MEA/DEA contact) | PTFE (static) | Amines cause FKM dehydrofluorination |
| HPLC column fitting (solvent gradient) | Spring-energized PTFE | Alternating solvents; minimal dead volume |
FAQ
Q1: Can PTFE replace FKM in the same O-ring groove?
Dimensionally yes — a PTFE lathe-cut O-ring to AS568 or ISO 3601 dimensions fits the same groove. However, the application must be static — PTFE in a dynamic groove will leak immediately. The groove depth must be more tightly controlled for PTFE than FKM (PTFE requires ±0.03 mm tolerance vs ±0.10 mm for FKM) because PTFE cannot self-compensate for dimensional variation. PTFE also requires smoother groove surfaces (Ra ≤ 0.4 µm) than FKM (Ra ≤ 0.8 µm) because PTFE does not elastically conform to surface irregularities.
Q2: Is PTFE better than FKM for food-grade applications?
Both comply with FDA 21 CFR §177.2600 for repeated food contact. PTFE is preferred for static connections with aggressive CIP cleaning chemistry (concentrated caustic, peracetic acid, ketone-based sanitizers) where FKM would degrade. FKM is preferred for static and dynamic seals in hot process equipment where temperature and some elasticity are required. For steam-sterilized food or pharmaceutical equipment, EPDM (not FKM or PTFE) is the standard SIP material.
Q3: Which lasts longer — FKM or PTFE?
In a correctly specified static application, both can last many years. In a dynamic application, PTFE fails immediately or within hours — FKM lasts years. In a high-temperature chemical environment above +200°C, PTFE (static) outlasts FKM. The correct answer: the material matched to the application lasts longest; the wrong material fails quickly regardless of its intrinsic properties.
Q4: Why does PTFE cold-flow and FKM does not?
FKM is a crosslinked elastomer — covalent crosslinks between polymer chains prevent chain sliding under load. Compression causes elastic deformation; crosslinks return chains to original position when load is removed. PTFE is a linear thermoplastic with no crosslinks — chains are held only by intermolecular van der Waals forces, which are overcome by sustained mechanical stress, causing chain sliding (cold flow). This is an intrinsic property of all uncrosslinked thermoplastics and cannot be eliminated by compounding.
Q5: Can I use PTFE O-rings in high-pressure hydraulic service?
PTFE lathe-cut O-rings in a standard hydraulic groove provide poor high-pressure static sealing because PTFE cold-flows into the clearance gap at high pressure. PTFE's correct role in hydraulic service is as backup (anti-extrusion) rings on the low-pressure side of FKM O-rings, not as the primary seal. For chemical-resistant dynamic hydraulic sealing, spring-energized PTFE seals with proper groove design are the correct solution.
Q6: What temperature can spring-energized PTFE seals handle?
The PTFE jacket limits service to +260°C continuous. The spring material determines cryogenic performance: 316 SS canted coil springs to −100°C; Inconel 718 springs extend service to −270°C (absolute cryogenic). For temperatures above +260°C with dynamic motion, FFKM O-rings (to +310°C) or custom metal seals are required — there is no PTFE-based solution above +260°C.
Q7: Does FKM harden in steam?
Yes. FKM degrades in steam above +100°C through two mechanisms: (1) hydrolysis at the CF₂ sites adjacent to CH₂ units in VF₂-containing grades, and (2) dehydrofluorination (HF elimination) accelerated by alkaline steam condensate. The result is a progressive increase in crosslink density (hardening), dimensional change, and HF emission into the steam condensate. Peroxide-cured FKM has better steam resistance than bisphenol-cured FKM, but neither is suitable for continuous steam service. For steam, use EPDM (peroxide cure, to +150°C) or PTFE (static, any pressure) or AFLAS (to +200°C in steam).
Q8: When should I use FFKM instead of FKM?
Move to FFKM (perfluoroelastomer) when: (1) service temperature exceeds +200°C with dynamic motion; (2) the fluid is a ketone, amine, or other FKM-incompatible chemistry that also requires dynamic service (FKM fails, PTFE cold-flows, only FFKM provides both elastic recovery and universal chemical resistance); (3) the application is semiconductor process chamber service where plasma and etchant chemistry requires ultra-low outgassing plus elastic sealing; (4) downhole oilfield service above +200°C with H₂S and mixed hydrocarbons. FFKM costs 10–30× more than standard FKM — confirm that FKM is actually insufficient before specifying FFKM.
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Need FKM or PTFE seals for chemical or high-temperature service? Contact our engineering team with your fluid, temperature, pressure, and dynamic vs static requirement — we provide material recommendations with compatibility data and supply FKM O-rings, PTFE lathe-cut rings, and spring-energized PTFE seals from 1-piece MOQ with 3–5 business day delivery on stocked items.