AFLAS and FKM occupy adjacent positions in the high-performance elastomer tier, but they are engineered for different chemical environments. Understanding which environments favor each material — and why — prevents the common error of specifying FKM based on temperature rating alone, then discovering it fails in steam or amine service.
Definition Block
FKM (fluorocarbon rubber): A family of fluorinated elastomers based primarily on vinylidene fluoride (VF2) copolymers. The most common types are: Type 1 (VF2/HFP, ~65–66% fluorine), Type 2 (VF2/HFP/TFE, ~66–68% fluorine), and GF-type (VF2/PMVE/TFE, ~70%+ fluorine). Commercially known under trade names including Viton (Chemours), Tecnoflon (Solvay), and Dai-El (Daikin). Excellent resistance to hydrocarbons, oils, fuels, and many acids. Service range approximately −20°C to +200°C (dry heat). Critical weakness: vinylidene fluoride (VF2) units in the backbone are susceptible to dehydrofluorination by bases, amines, and hot water above ~150°C.
AFLAS (tetrafluoroethylene/propylene rubber, also designated FEPM): A copolymer of tetrafluoroethylene (TFE) and propylene (P), developed by Asahi Glass and sold under the trade name AFLAS. Contains no vinylidene fluoride — a critical structural difference from FKM. Fluorine content approximately 54% by weight, but the absence of VF2 makes it fundamentally different in chemical behavior from fluorocarbon elastomers of similar fluorine content. Service range approximately −5°C to +200°C (standard grades). Critical strength: resistant to steam, hot water, amines, strong bases, and sour gas environments that cause VF2-driven FKM degradation.
The Decisive Structural Difference
Both FKM and AFLAS are fluorinated elastomers, but the presence or absence of vinylidene fluoride (VF2) in the polymer backbone determines how they respond to two specific chemical environments: hot water/steam and amines.
VF2 Dehydrofluorination: The FKM Failure Mechanism
Vinylidene fluoride (VF2) in FKM creates a structural vulnerability at elevated temperature:
- VF2 monomer contributes segments with the structure —CH₂—CF₂— in the polymer backbone
- The hydrogen atoms in the CH₂ group, positioned adjacent to the highly electronegative CF₂ group, are significantly more acidic (more abstractable) than hydrogen in non-fluorinated polymers
- Nucleophilic species — water at high temperature (above ~150°C), primary amines (R-NH₂), secondary amines (R₂NH), hydroxide ions (OH⁻) — abstract these activated hydrogens
- Abstraction initiates dehydrofluorination: HF is eliminated from adjacent CH₂—CF₂ units, creating a C=C double bond in the backbone
- This double bond is highly reactive: further oxidation, hydrolysis, or crosslinking reactions proceed at the C=C site
- Progressive backbone scission or additional crosslinking causes hardening, cracking, and loss of elastic recovery
Rate dependence: Dehydrofluorination of FKM is strongly temperature-dependent. Below +120°C in water, the reaction rate is slow — FKM performs adequately in water at moderate temperature. Above +150°C in steam, the reaction rate increases dramatically — FKM begins to fail within hundreds of hours of continuous exposure. At +200°C in saturated steam, FKM failure occurs within tens of hours.
Amine attack rate: Primary and secondary amines are much stronger nucleophiles than water. Even at ambient temperature, concentrated primary amines (e.g., ethylenediamine, diethanolamine) attack FKM. Dilute amine solutions (ppm-level boiler treatment chemicals) attack FKM significantly above +100°C.
AFLAS: No VF2, No Dehydrofluorination
AFLAS (TFE/P) contains no hydrogen atoms adjacent to fluorine atoms in the TFE segments — the TFE units are —CF₂—CF₂—, fully fluorinated with no activated hydrogens. The propylene segments contribute polymer flexibility through their own backbone chemistry but do not provide the nucleophilic attack sites present in VF2.
The result: AFLAS is inert to the dehydrofluorination mechanism. Hot water, steam, amines, and strong bases that attack FKM's VF2 sites find no equivalent reactive site in AFLAS.
Temperature Range Comparison
| Parameter | FKM (Type 1, standard) | FKM (Type 2 / GF-grade) | AFLAS |
|---|---|---|---|
| Continuous service (dry heat) | −20°C to +200°C | −15°C to +200°C (GFLT grade) | −5°C to +200°C |
| Continuous service (saturated steam) | Not recommended above ~150°C | Not recommended above ~160°C | Up to +200°C (21 bar steam) |
| Hot water continuous (>+100°C) | Limited; degradation accelerates >120°C | Limited; degradation accelerates >130°C | Excellent to +200°C |
| Low-temperature limit | −20°C (standard) | −25°C (GFLT grade) | −5°C (standard); verify any sub-zero grade with supplier |
| Short-term peak (dry heat) | +230°C | +230°C | +230°C |
| Ozone resistance | Excellent | Excellent | Excellent |
| UV resistance | Excellent | Excellent | Good |
Important nuance on temperature ratings: Both FKM and AFLAS carry similar maximum temperature ratings in dry heat. The critical difference emerges in steam and wet-chemical service. An FKM compound rated to +200°C in oil service may show significant degradation in saturated steam at +150°C within 1,000 hours. The temperature rating on a compound datasheet alone is insufficient to evaluate steam suitability — the service medium must be considered.
Quantified steam immersion data (indicative, actual values vary by compound formulation):
| Condition | FKM Type 1 Result | AFLAS Result |
|---|---|---|
| Water, +100°C, 1,000h | Acceptable (< 5% hardness change) | Excellent |
| Saturated steam, +121°C, 1,000h | Marginal (5–15% hardness increase) | Excellent |
| Saturated steam, +150°C, 500h | Limited (>15% hardness increase, cracking) | Good (< 5% change) |
| Saturated steam, +177°C, 200h | Failed (severe hardening, cracking) | Acceptable (5–10% change) |
| Saturated steam, +200°C, 100h | Catastrophic failure | Marginal (10–20% change) |
Chemical Resistance: Where Each Material Wins
Environments Where FKM Outperforms AFLAS
| Chemical/Environment | FKM Performance | AFLAS Performance | Notes |
|---|---|---|---|
| Mineral oils and hydraulic fluids | Excellent | Good | FKM preferred for standard hydraulic service |
| Petroleum fuels (gasoline, diesel, jet fuel) | Excellent | Limited | FKM is the standard for fuel systems |
| Aromatic hydrocarbons (toluene, xylene) | Excellent | Poor | AFLAS swells significantly in aromatics |
| Chlorinated solvents | Good to excellent | Limited | FKM generally better |
| Aliphatic mineral acids (H₂SO₄, HNO₃ dilute) | Good | Moderate | FKM performs better in mineral acids |
| High-temperature oil (above +150°C, dry) | Excellent | Good | FKM has better track record in oil/fuel service |
In practice: When the primary process fluid is a hydrocarbon — fuel, oil, solvent, or most industrial chemicals — FKM is almost always the correct choice. AFLAS offers no meaningful advantage in hydrocarbon service and costs significantly more.
Environments Where AFLAS Outperforms FKM
| Chemical/Environment | FKM Performance | AFLAS Performance | Notes |
|---|---|---|---|
| Saturated steam (above +150°C) | Poor to limited | Excellent | VF2 hydrolysis makes FKM unreliable in steam |
| Hot water (above +120°C continuous) | Limited | Excellent | Same dehydrofluorination mechanism |
| Primary amines (RNH₂) | Poor | Excellent | Amines attack VF2 in FKM; AFLAS has no VF2 |
| Secondary amines (boiler water treatment) | Poor to limited | Excellent | Morpholine, cyclohexylamine, filming amines |
| Strong bases (NaOH, KOH above +80°C) | Limited | Excellent | Caustics attack VF2 segments |
| Sour gas (H₂S + water) | Good (Type 2 FKM) | Excellent | AFLAS superior in wet H₂S + amine combined service |
| Phosphate ester hydraulic fluids | Limited | Excellent | Well-documented advantage for AFLAS |
| Geothermal fluids (hot brine + H₂S + CO₂) | Limited | Excellent | Mixed sour/wet chemistry |
| Ammonium compounds | Poor | Good | Ammonia attacks FKM; AFLAS tolerates dilute ammonia |
| MEK and ketones | Poor | Poor | Neither material handles ketones — PTFE or FFKM needed |
Amine Chemistry Detail
Amines are especially damaging to FKM because primary and secondary amines (R-NH₂, R₂NH) are strong nucleophiles — they react directly with the VF2 segments in the polymer backbone through the same dehydrofluorination mechanism as water, but more aggressively and at lower temperatures:
- Morpholine (used in boiler water treatment): attacks FKM at concentrations above 100 ppm at +100°C; in steam condensate return lines, concentrations are sufficient to degrade FKM seals within 500–2,000 hours
- Cyclohexylamine (filming amine in steam systems): similar to morpholine in aggressiveness toward FKM
- Ethanolamine and diethanolamine (MEA, DEA): used in gas sweetening absorbers; primary amines that attack FKM aggressively at operating temperatures of +40–80°C at concentrations of 20–50% by volume
- Corrosion inhibitor packages: many oilfield corrosion inhibitors are amine-based (imidazolines, quaternary amines); presence in produced water or injection fluids alongside FKM seals should trigger material review
AFLAS, with no VF2 content, is inert to all these amine attack mechanisms — it is the standard material for steam systems using filming amine treatment and for oilfield equipment handling amine-based corrosion inhibitors.
Oilfield and Sour Gas Applications
In oil and gas production environments, the chemical mix is rarely simple hydrocarbons alone. A typical well stream may contain:
- C1–C4 hydrocarbons (methane through butane)
- H₂S at 5–30% by volume in sour fields
- CO₂ at 1–15% by volume
- Produced water (saline brine, pH 4–7)
- Amine corrosion inhibitors in the completion/treatment fluid
- Temperatures from −10°C surface to +200°C downhole
NACE MR0175 / ISO 15156 guidance: These standards define material requirements for equipment in H₂S-containing petroleum environments. For elastomers, ISO 15156-3 Annex B provides qualification criteria. Both FKM and AFLAS have established qualification histories in sour service — the deciding factor is the combined chemical environment, not H₂S concentration alone.
FKM (standard Type 1) performs well with the hydrocarbon fraction and tolerable in moderate dry H₂S. However, when amines are present alongside H₂S and water at elevated temperature — a common oilfield scenario — FKM degrades significantly faster. Type 2 FKM (high-fluorine grade) shows better wet H₂S resistance than Type 1 but still contains VF2 and remains vulnerable to amine attack.
AFLAS handles all three (H₂S, amines, hot water) simultaneously — which is why AFLAS is specified in: downhole completion equipment, geothermal wellhead seals, steam-assisted oil recovery (SAGD) valve seals, chemical injection equipment handling amine corrosion inhibitors, and gas sweetening absorber internals.
H₂S concentration thresholds (indicative):
| H₂S Condition | FKM Type 1 | FKM Type 2 | AFLAS |
|---|---|---|---|
| Dry H₂S, <200°C | Good | Excellent | Good |
| Wet H₂S (H₂S + water), <150°C | Marginal | Good | Excellent |
| Wet H₂S + amines, any temperature | Poor | Limited | Excellent |
| Sour gas + CO₂ + brine + amines | Poor | Marginal | Excellent |
Hardness and Compound Options
| Property | FKM | AFLAS |
|---|---|---|
| Standard hardness range | 50–90 Shore A | 70–90 Shore A |
| Common stock hardness | 70–80 Shore A | 70 Shore A (limited) |
| Low-temperature grades available | Yes (Type 2/GF-grade, −25°C) | Very limited; verify with supplier |
| High-fluorine grades available | Yes (up to 71%+ F) | AFLAS is a single polymer type |
| Available compound variants | Many (cure system, fluorine content, filler) | Limited |
| Color (standard) | Black or brown (AMS-grade) | Black |
AFLAS hardness options are more limited than FKM. AFLAS is not available in soft grades (40–60 Shore A) that FKM offers — the minimum practical AFLAS Shore A is approximately 65–70. For applications requiring low-durometer seals in aggressive wet/steam environments, FFKM low-temperature grades or specialty EPDM may need to be evaluated.
Physical Property Comparison
| Property | FKM 75A (typical) | AFLAS 70A (typical) |
|---|---|---|
| Tensile strength | 10–18 MPa | 8–15 MPa |
| Elongation at break | 150–300% | 150–250% |
| Compression set (ASTM D395 B, 200°C/70h) | 20–40% | 25–45% |
| Specific gravity | 1.80–1.85 g/cm³ | 1.55–1.60 g/cm³ |
| Hardness stability in hot water (+150°C, 1,000h) | Poor (>20% change) | Good (<5% change) |
| Swell in toluene (ASTM D471) | Low (2–6%) | High (20–50%) — not suitable |
| Low-temperature brittle point | −30 to −40°C | −10 to −20°C |
Cost and Supply Chain
| Factor | FKM | AFLAS |
|---|---|---|
| Relative cost (vs NBR) | 5–15× | 15–40× |
| Relative cost (vs FKM) | Baseline | 3–6× more expensive |
| Stock availability | Very high (full AS568 range) | Limited (mainly custom or specialty stock) |
| Lead time (standard sizes) | 3–7 days from stock | 7–21 days (often made to order) |
| Compound suppliers | Multiple (Chemours, Solvay, Daikin, etc.) | Limited (AFLAS is Asahi's trade name; TFE/P alternatives exist but supply chain is narrower) |
| MOQ (standard sizes) | 1–50 pieces | 50–200 pieces (custom) |
AFLAS should be specified because the chemistry demands it — not as a general upgrade. When FKM technically works, the 3–6× cost premium and longer lead time for AFLAS are not justified.
Application Selection Matrix
| Application | Better Choice | Why |
|---|---|---|
| Automotive fuel systems | FKM | Better hydrocarbon and aromatic fuel resistance |
| Steam valves, boiler systems | AFLAS | FKM hydrolyzes in sustained steam above +150°C |
| Steam + filming amine boiler water treatment | AFLAS | Amines attack FKM; EPDM also considered |
| Sour gas sealing (H₂S + hydrocarbon, dry) | FKM Type 2 or AFLAS | FKM Type 2 acceptable in dry service; AFLAS for wet |
| Sour gas + water + amines combined | AFLAS | Multiple FKM failure mechanisms present |
| Aerospace fuel or oil systems | FKM | Strong track record in military/civil aviation; cost |
| Phosphate ester hydraulic fluid | AFLAS | AFLAS is the standard recommendation for phosphate ester |
| Aromatic solvent handling | FKM | AFLAS swells significantly in aromatics |
| Geothermal and hot brine | AFLAS | Designed for this exact chemistry |
| Downhole oilfield (amines + H₂S + heat) | AFLAS | Multiple failure mechanisms mitigated |
| Gas sweetening absorber seals (MEA/DEA) | AFLAS | Primary amine concentrations destroy FKM |
| SAGD (steam-assisted gravity drainage) | AFLAS | High-temperature steam + bitumen chemistry |
| General industrial oil service | FKM | Better economics where FKM is technically adequate |
| Chemical plant — ketone service | Neither (PTFE/FFKM) | Ketones attack both materials |
Cost-Benefit Analysis Framework
Given AFLAS's 3–6× cost premium over FKM, the decision to specify AFLAS requires quantified justification:
- Document the failure mode: Confirm that the current FKM failure is chemical degradation from steam, water, or amines — not mechanical wear, extrusion, or installation damage. Only chemical degradation caused by VF2 attack is solved by switching to AFLAS.
- Quantify the replacement cost: If FKM seals are replaced every 3 months at $2/seal (labor + material + downtime cost = $50/event), annual cost = $200/seal point. If AFLAS at $10/seal lasts 24 months, annual cost = $60/seal point. AFLAS is 70% cheaper annually despite 5× higher unit price.
- Consider the downtime cost: For a steam valve in a critical production system, an unplanned maintenance event may cost $10,000–$100,000 in lost production. Even a single unplanned failure event justifies AFLAS at virtually any unit price.
- Verify no simpler solution: Before switching to AFLAS, confirm whether the FKM failure can be addressed by: (a) reducing steam temperature or eliminating steam contact; (b) switching to EPDM (acceptable for steam and some amines, incompatible with hydrocarbons but cheaper than AFLAS for pure steam service).
Procurement Notes
AFLAS is available from stock in limited standard sizes (primarily common AS568 cross-sections and IDs used in the oilfield and industrial markets). Custom sizes are made to order. MOQ is 1 piece for custom configurations using cord-splice (for large-ID AFLAS O-rings) where available; standard molded sizes have MOQ 50–200 pieces. Lead time is 10–21 business days for custom AFLAS, versus 3–7 days for standard FKM sizes from stock. Material certification (CoC, compound data) is provided with each shipment.
FAQ
Q1: Is AFLAS better than FKM?
Not in every environment. AFLAS is better in steam, hot water, amines, strong bases, sour gas with combined water/amine contamination, and phosphate ester hydraulic fluids. FKM is better — and significantly more economical — in petroleum fuels, oils, aromatic hydrocarbons, and most standard industrial chemical applications. The choice must be driven by the specific media, not a general performance ranking.
Q2: Why does FKM fail in steam service?
Standard FKM contains vinylidene fluoride (VF2) repeat units in its polymer backbone with the structure —CH₂—CF₂—. At temperatures above approximately +150°C in the presence of water, the hydrogen atoms adjacent to the CF₂ groups are nucleophilically attacked by water molecules — a dehydrofluorination reaction that eliminates HF and creates reactive C=C bonds in the backbone. These bonds undergo further reactions (crosslinking or chain scission) that progressively harden and crack the elastomer. AFLAS contains no VF2 and is not subject to this mechanism.
Q3: Can AFLAS replace FKM in fuel service?
Generally no. AFLAS has limited resistance to aromatic hydrocarbons (toluene, xylene) and does not match FKM's performance in fuel systems — AFLAS swell in toluene is 20–50%, making it unsuitable for high-aromatic fuel environments. FKM is the preferred material for automotive, aviation, and industrial fuel sealing. Use AFLAS when the service is steam, wet amines, or mixed oilfield chemistry — not as a general upgrade to FKM in hydrocarbon-dominated applications.
Q4: Is AFLAS a good material for sour gas?
Yes. AFLAS is widely specified for sour gas (H₂S) sealing, particularly in oilfield applications where H₂S co-exists with amines, produced water, and elevated temperature. The advantage over FKM is most pronounced when water and amines are present alongside H₂S — dry H₂S alone is handled adequately by FKM Type 2 (high-fluorine grade). Consult NACE MR0175 / ISO 15156 for specific guidance on elastomer qualification in sour service.
Q5: Why does AFLAS have a higher minimum service temperature than FKM?
AFLAS's propylene (P) component introduces flexibility but also limits low-temperature performance. The glass transition temperature of standard AFLAS grades produces a −5°C low-temperature limit (TR10 approximately −5°C), versus −20°C for standard FKM. When both steam resistance and low-temperature flexibility below −5°C are required simultaneously, standard AFLAS does not cover the full operating range. Options include: FFKM low-temperature grades, or evaluating whether a different seal geometry can use EPDM (excellent steam resistance, better low-temperature to −40°C, but incompatible with hydrocarbons). Any AFLAS sub-zero grade must be verified with the compound supplier.
Q6: What is the price difference between AFLAS and FKM?
AFLAS typically costs 3–6× more per piece than equivalent-size standard FKM O-rings in the same hardness. The premium reflects: AFLAS monomer cost (higher than FKM monomer), narrower supply base (fewer compound suppliers), and lower production volume (less manufacturing scale economy). AFLAS should be specified only when the chemistry justifies the premium. In applications where FKM is technically adequate, the cost difference cannot be recovered through performance.
Q7: Is EPDM a cheaper alternative to AFLAS for steam service?
For pure steam service without hydrocarbon contact, EPDM is a viable and significantly cheaper alternative to AFLAS. EPDM has excellent steam and water resistance, operates to +150°C in steam, and costs 3–5× less than AFLAS. The critical limitation: EPDM is completely incompatible with petroleum oils and hydrocarbons — it swells dramatically in mineral oil. If the steam system has any possibility of oil contamination (lubricated valve stems, oil-wetted packing near steam joints, maintenance with oil-contaminated tools), EPDM is not safe. AFLAS is the correct choice when the service involves both steam and hydrocarbon contact in the same seal. If the service is exclusively steam with no hydrocarbon risk, EPDM is the more economical specification.
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Experiencing FKM failure in steam or amine service? Contact our engineering team with your fluid chemistry (steam temperature/pressure, amine type and concentration), operating temperature, and current O-ring failure description — we confirm whether AFLAS addresses the root cause, identify the correct compound grade, and supply AFLAS O-rings in AS568/ISO 3601 sizes with material CoC, with 10–21 day lead time for custom sizes and MOQ 1 piece for cord-splice configurations.