Selecting the right seal material for elevated-temperature service means looking past headline temperature ratings. A compound advertised for +230°C may survive that peak for a few hours but lose half its sealing force through compression set within weeks. The real engineering decision balances continuous operating temperature, peak excursions, oxidation stability, chemical environment, required life, and seal geometry.
Quick answer: For continuous service to +200°C in dry oil or air, FKM is the standard elastomeric choice. For +200°C to +260°C combined with aggressive chemicals, specify FFKM. For +260°C and above, or when the chemistry defeats every elastomer, move to PTFE — solid for static, spring-energized for dynamic or cycled service. AFLAS o-rings are the best elastomeric option for steam and amines to +200°C. VMQ silicone handles dry air to +230°C but fails in hydrocarbons. Above +300°C, flexible graphite and metal seals are the only practical options. Always design for the continuous temperature, not the peak.
Temperature Thresholds by Material
The table below lists practical continuous and short-term peak limits for common high-temperature seal materials in dry air or inert gas. Hot fluids, steam, oxidizers, and aromatic hydrocarbons reduce these limits.
| Material | Polymer Family | Continuous Max (°C) | Peak (°C) | Low-Temp Limit (°C) | Key Heat-Related Limitation |
|---|---|---|---|---|---|
| VMQ (Silicone) | Polysiloxane | +200 | +230 | −60 | Low strength; poor in oils/fuels |
| FKM (Fluorocarbon) | Fluorocarbon elastomer | +200 | +230 | −20 | Dehydrofluorination in steam/amine >+150°C |
| AFLAS (FEPM) | TFE/propylene copolymer | +200 | +230 | −5 | Poor in aromatics; moderate compression set |
| HNBR | Hydrogenated nitrile | +150 | +175 | −30 | Oxidative degradation above +150°C |
| FFKM (Perfluoroelastomer) | Perfluorinated elastomer | +260–+325 | +300–+350 | −15 to −40 | Cost; moderate mechanical strength |
| PTFE | Fluoropolymer | +260 | +280 | −270 | No elastic recovery; requires energization |
| Filled PTFE | PTFE composite | +260 | +280 | −200 | Filler limits chemical purity |
| Flexible graphite | Laminated carbon | +450 (oxidizing) / +3,000 (inert) | +3,000 | −200 | Fragile; needs compression retention |
| PEEK | High-performance thermoplastic | +250 | +300 | −60 | No elastic recovery; machined |
| Metal (316L/Inconel) | Metal seal | +450 to +800+ | +1,000+ | −270 | High bolt load; no elasticity |
How to read these numbers: Continuous maximum is the temperature at which the material retains acceptable sealing force and physical properties for thousands of hours. Peak is survival for minutes to a few hours — acceptable for startup, upset, or intermittent exposure, not for normal operation.
Continuous vs Peak Temperature
Every high-temperature seal application has three temperatures, and only one should drive material selection:
- Continuous operating temperature — the steady-state temperature the seal sees for most of its life. Design for this.
- Peak temperature — short excursions during startup, shutdown, or process upset. Verify the peak is within the material's short-term survival limit.
- Ambient or cold-start temperature — the lower extreme, which determines whether the seal remains flexible enough to seal when cold.
A common mistake is selecting a material based on peak temperature. A standard FKM O-ring can survive +230°C for a few hours, but at continuous +230°C its compression set exceeds 50% within days. An FFKM grade rated for +260°C continuous may handle the same +230°C peak for thousands of hours with compression set under 30%.
| Temperature Regime | Design Rule |
|---|---|
| Continuous | Use ≤ material continuous max with 10–20°C margin for long life |
| Peak < 5% of operating time | Peak must be below short-term limit |
| Peak > 10% of operating time | Treat peak as continuous |
| Thermal cycling | Prioritize low compression set and stable backbone (FFKM or PTFE) |
Oxidation & Thermal Aging
At high temperature, oxygen attacks the polymer backbone. The mechanism and rate depend on the material:
- NBR, HNBR, EPDM undergo oxidative crosslinking, hardening, and cracking. NBR is generally limited to +120°C continuous; HNBR and EPDM to +150°C.
- FKM resists oxidation well because the C–F bond shields the backbone. In dry air or oil it ages gracefully to +200°C, but steam or amines trigger dehydrofluorination above +150°C.
- VMQ silicone degrades by backbone scission, softening and losing strength. It performs well in dry heat to +230°C but has poor hydrocarbon resistance.
- FFKM has an all-fluorinated backbone with no C–H bonds, resisting oxidation and chemical attack to +300°C and above.
- PTFE is thermally stable to +260°C. Above +327°C it depolymerizes and releases toxic fluorocarbon products; it must never contact open flames or surfaces above +280°C.
- Flexible graphite resists oxidation in air to about +450°C. In inert or reducing atmospheres it survives up to +3,000°C, limited mainly by the hardware.
For long-life seals in oxidizing environments, add a 10–20°C margin below the material's rated continuous maximum.
Compression Set at High Temperature
Compression set is the permanent deformation remaining after a seal is compressed and released. It is the dominant failure mode in high-temperature elastomeric seals. An O-ring with 50% compression set has lost half its cross-sectional height; in a standard gland, effective squeeze drops close to zero and the seal leaks at low pressure.
Typical 70-hour compression set values per ASTM D395 Method B:
| Material | +100°C | +150°C | +200°C | +225°C | +250°C |
|---|---|---|---|---|---|
| NBR 70 | 25–40% | 50–70% | — | — | — |
| HNBR 70 | 15–25% | 35–50% | 60–80% | — | — |
| FKM 75 | 10–18% | 18–30% | 30–45% | 45–60% | — |
| VMQ 70 | 15–25% | 25–40% | 40–55% | 55–70% | — |
| AFLAS 75 | 15–25% | 25–40% | 35–50% | 50–65% | — |
| FFKM 75 | 8–15% | 12–20% | 18–30% | 25–40% | 35–50% |
PTFE does not exhibit compression set in the elastomeric sense because it is not crosslinked — it cold-flows under sustained load. That is why solid PTFE seals need either a rigid groove that maintains compression or a spring energizer that provides continuous seating force.
At temperatures where FKM compression set exceeds 40%, FFKM becomes the technically correct choice even if the chemistry does not strictly require it.
Material Selection Matrix by Temperature Range and Media
+100°C to +150°C
| Application | Recommended Material | Notes |
|---|---|---|
| Mineral oil hydraulics to +120°C | FKM 75 | Standard upgrade from NBR |
| Automotive engine oil to +150°C | FKM 80–90 | High-fluorine grade for long life |
| Steam to +150°C | EPDM 80 | Saturated steam; degrades above +150°C |
| Hot water to +120°C | EPDM 70–80 | Not for oil contact |
| Dry air/pneumatics to +120°C | VMQ 70 | Wide temperature swing |
| General industrial to +150°C | HNBR 80–90 | Cost-effective FKM alternative |
+150°C to +200°C
Material choice becomes critical. NBR is out; HNBR is at its limit.
| Application | Recommended Material | Critical Consideration |
|---|---|---|
| Dry heat, air, inert gas | FKM 75–90 | Avoid steam and amines |
| Dry heat with wide swing | VMQ 70–80 | Poor in hydrocarbons |
| Steam to +200°C | AFLAS 75–80 | Best elastomeric steam resistance |
| Hot amines to +200°C | AFLAS 75–80 | FKM dehydrofluorinates |
| Oil/fuel to +200°C | FKM 80–90 | High-fluorine grade for longest life |
| Mixed steam + hydrocarbon | AFLAS 75–80 | Unique mixed-media capability |
Important: FKM and AFLAS share the same +200°C dry-heat ceiling but are not interchangeable. FKM fails in steam and amines; AFLAS swells excessively in aromatic hydrocarbons.
+200°C to +260°C
Only FFKM and PTFE provide reliable long-term service.
| Application | Recommended Material | Critical Consideration |
|---|---|---|
| Continuous dry heat to +260°C | FFKM 75–90 | Highest-performing elastomer |
| Static seal, aggressive chemical + heat | PTFE | Chemically inert; needs energization |
| Dynamic seal, heat + chemical | FFKM 75–80 | Only elastomeric dynamic option |
| Static flange, intermittent +260°C | FEP encapsulated | FEP layer limits to +205°C |
| Aerospace engine peripherals | FFKM low-temp grade | Cold-start + hot-operation balance |
Above +260°C
No elastomer is reliable long-term. PTFE, graphite, and metal seals are required.
| Temperature | Material Options | Seal Geometry |
|---|---|---|
| +260°C to +300°C | PTFE with spring energizer | Spring-energized seal |
| +260°C to +300°C | PEEK with metal spring | Metal-spring-energized PEEK |
| +300°C to +450°C | Flexible graphite, metal O-rings | Solid metal or graphite seals |
| Above +450°C | Metal C-rings, E-rings, welded metal seals | Custom metal seal design |
For detailed spring-energized seal design, see our spring-energized seals engineering guide.
Filled PTFE & Graphite Options
Virgin PTFE has the broadest chemical resistance but the highest cold-flow rate. Fillers reduce creep and improve wear at the cost of some chemical purity.
| Filler | Content | Creep Reduction | Wear Improvement | Best Use |
|---|---|---|---|---|
| Virgin (unfilled) | 0% | Baseline | Baseline | Maximum chemical resistance; pharma/semiconductor |
| Glass fiber | 15–25% | ~50% | 2–3× | General industrial static seals |
| Carbon/graphite | 15–25% | ~40% | 3–5× | Dry-running rotary; self-lubricating |
| Bronze | 25–40% | ~60% | 5–10× | Heavy-load hydraulics; non-corrosive media |
| Carbon fiber | 10–15% | ~45% | 4–6× | High-performance dynamic seals |
| PEEK | 10–20% | ~40% | 3–4× | High-temp; broad chemical resistance |
| Graphite (natural) | 10–15% | ~35% | 3× | Food/pharma FDA-compliant dry service |
Flexible graphite is made from exfoliated graphite flakes compressed into rings or sheets. It is chemically inert, thermally stable, and conformable under load. In oxidizing atmospheres it is generally limited to +450°C; in steam, inert gas, or reducing environments it can exceed +3,000°C. Flexible graphite is fragile and requires a metal substrate or retaining geometry to prevent blowout. It is widely used in valve stem packing, flange gaskets, and high-temperature exhaust joints.
For aggressive chemistry where PTFE is the only option but creep is a concern, specify a filled grade or a spring-energized PTFE seal rather than relying on solid PTFE alone.
High-Temperature Groove Design
High-temperature seals need grooves that account for thermal expansion, compression set, and material-specific behavior.
Elastomeric O-Rings at High Temperature
| Parameter | Typical Value | Rationale |
|---|---|---|
| Compression rate (static) | 18–25% | Higher end compensates for set |
| Groove fill rate | 65–80% | Allows thermal expansion without overfill |
| Corner radius | ≥ 0.20 mm | Reduces notch sensitivity |
| Surface finish (static) | Ra ≤ 0.8 μm | Better sealing with harder compounds |
| Hardness | 80–90 Shore A | Resists extrusion and set at temperature |
At high temperature, metal housings expand more than elastomers. Verify that thermal expansion does not over-compress the seal or open the clearance gap enough to cause extrusion.
Solid PTFE Seals
| Parameter | Solid PTFE | Elastomeric O-Ring |
|---|---|---|
| Compression rate (static) | 20–30% | 15–22% |
| Groove fill rate | 70–80% | 65–85% |
| Mating surface Ra | ≤ 0.4 μm | ≤ 0.8 μm |
| Corner radius | ≥ 0.25 mm | ≥ 0.10 mm |
| Thermal cycling tolerance | Poor | Good |
Solid PTFE cannot conform to rough surfaces and cannot recover after thermal cycling. Use it only for static flanges at essentially constant temperature, or plan for periodic re-torquing.
Spring-Energized PTFE Seals
Spring-energized seals require manufacturer-specific groove geometry. General guidelines:
- Groove width must allow lip deflection and spring travel.
- Groove depth is based on uncompressed jacket height plus spring deflection range.
- Lead-in chamfer should be 20–30° over 2–3 mm to prevent jacket damage during assembly.
- Surface finish on dynamic contact surfaces: Ra 0.10–0.25 μm.
When to Use Metal Seals
Metal seals become necessary when:
- Continuous temperature exceeds +300°C in oxidizing service.
- The application requires zero permeation (ultra-high vacuum, helium leak testing).
- Pressure is very high and elastomer extrusion cannot be controlled.
- The process fluid attacks every polymer option.
| Metal Seal Type | Temperature Range | Pressure | Notes |
|---|---|---|---|
| Metal C-ring | −270°C to +450°C | High | Stainless steel or Inconel; moderate springback |
| Metal E-ring | −270°C to +650°C | Very high | Higher springback than C-ring |
| Metal O-ring | −270°C to +800°C+ | Very high | Hollow or solid; plated for sealing |
| Welded metal bellows | Cryogenic to +800°C+ | High | Zero external leakage; high cost |
Metal seals require high bolt load and precision surface finishes. They are not drop-in replacements for O-ring grooves.
Thermal Cycling and Heat-Soak Effects
High temperature is not just a peak value — the rate and frequency of temperature change often determine seal life more than the maximum temperature. Thermal cycling causes two problems:
- Differential thermal expansion: Metal housings, the O-ring, and any backup rings expand and contract at different rates. A seal that fits at +20°C may be over-compressed at +200°C and under-compressed after cool-down.
- Compression set acceleration: Each hot cycle adds to the permanent deformation accumulated by the elastomer. After repeated heat-soak cycles, an O-ring may no longer recover enough to seal during the cool-down phase.
Design mitigations:
- Select groove dimensions at the maximum operating temperature, not room temperature
- Allow extra groove volume (target 75–80% fill at max temp) for thermal expansion
- Use lower compression set materials (FFKM instead of FKM) when cycling is frequent
- Avoid clamping the O-ring between materials with very different expansion coefficients without analyzing the resulting squeeze change
- For systems with long cool-down periods, verify the seal can seal at the minimum as well as maximum temperature
High-Temperature O-Ring Selection Checklist
Use this checklist before finalizing a material for a hot application:
- [ ] Continuous operating temperature verified (not just peak)
- [ ] Fluid chemistry confirmed at maximum temperature
- [ ] Compression set at end-of-life temperature and time estimated
- [ ] Thermal cycle count and amplitude estimated
- [ ] Groove fill rate at max temperature checked (≤ 85%)
- [ ] Backup rings considered if pressure > 100 bar
- [ ] Metal or graphite seal options evaluated if > +260°C continuous
- [ ] Cost-per-hour life compared across candidate materials
- [ ] Supplier batch test data requested for FFKM/high-temp compounds
For temperatures above +260°C, review our metal O-rings page and spring-energized PTFE seals as alternatives to elastomeric O-rings.
FAQ
Q1: What is the highest continuous temperature an elastomeric seal can handle?
Standard FFKM high-temperature grades are rated for continuous service up to +325°C, with specialty grades approaching +327°C. Above that, no elastomer retains adequate sealing force long-term; PTFE, flexible graphite, or metal seals are required.
Q2: Can FKM seals run continuously at +200°C?
Yes, in dry heat, oil, or inert gas. Standard FKM compounds maintain acceptable properties for hundreds to thousands of hours at +200°C in hydrocarbon service. The limitation is chemistry: FKM degrades rapidly in steam or amines above +150°C. For continuous +200°C service life beyond 2,000 hours, FFKM is the safer choice.
Q3: Why does silicone fail in high-temperature oil service?
VMQ silicone has excellent oxidation resistance in dry air to +230°C, but it swells 50–100% and loses strength in hydrocarbon oils within days. VMQ is correct for dry heat, oven doors, and exhaust systems with no oil contact; it is incorrect for hydraulic systems or fuel systems regardless of temperature.
Q4: What causes seal hardening and cracking at high temperature?
Hardening and cracking indicate thermal-oxidative degradation. NBR and HNBR crosslink further under heat, becoming hard and brittle. FKM and FFKM resist this because fluorine protects the backbone. VMQ degrades by chain scission and tends to soften rather than harden.
Q5: How does compression set lead to leakage?
Compression set is permanent deformation. A compressed seal generates contact stress against the mating surface; at high temperature, polymer chains relax and crosslinks break. When pressure drops or temperature cycles, the seal does not spring back. Effective squeeze falls below the sealing threshold and leakage begins.
Q6: When should I choose PTFE over FFKM?
Choose PTFE when temperature exceeds FFKM's practical limit (>+300°C), when the chemistry attacks FFKM, when cryogenic service is required, or when cost excludes FFKM and elastic recovery is not needed. Choose FFKM when thermal cycling, vibration, or imperfect surface finish requires elastic recovery. For dynamic PTFE applications, use spring-energized seals.
Q7: What is the practical upper limit for flexible graphite seals?
In oxidizing air or flue gas, flexible graphite is generally limited to +450°C. In steam, nitrogen, hydrogen, or reducing atmospheres, it can exceed +3,000°C. The limit is usually set by the containing hardware and oxidation inhibitors, not by the graphite itself.
Q8: How do I know if I need a metal seal instead of an elastomer or PTFE?
Specify metal seals when continuous temperature is above +300°C in oxidizing service, when zero permeation is required, when pressure is too high for polymer extrusion control, or when the process fluid destroys all polymer options. Metal seals are custom-engineered components, not standard catalog items.
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Need help selecting a high-temperature seal material? Use our material selector tool or request a quote with your operating temperature (continuous and peak), contacting fluids, required life, and seal geometry. We supply FKM, FFKM, PTFE, AFLAS, VMQ, and spring-energized PTFE seals in standard and custom sizes, with material certification and compression-set data available on request. For application engineering support, contact our team.