Cryogenic sealing is one of the most unforgiving disciplines in O-ring engineering. At −162°C the methane inside an LNG loading arm is a liquid; at −196°C liquid nitrogen boils off through the smallest leak path; at −183°C liquid oxygen turns organic contamination into an ignition hazard; and at −253°C liquid hydrogen squeezes through pores and permeates materials that stop heavier gases. The elastomer that seals perfectly at room temperature can become glassy, brittle, and permeable within seconds of cold exposure.
Quick answer: For continuous service below approximately −60°C, no standard elastomer O-ring is reliable. In the −60°C to −40°C range, choose VMQ (silicone) or low-temperature FKM GLT/GFLT for static, low-pressure seals. From −40°C up to ambient, low-ACN NBR, HNBR LT, and low-temp FKM are viable in oil and fuel systems. For true cryogenic service — LNG (−162°C), liquid nitrogen (−196°C), liquid oxygen (−183°C), or liquid hydrogen (−253°C) — the correct choice is PTFE, almost always in a spring-energized seal design. FFKM low-temperature grades bridge the gap down to about −25°C to −40°C when both chemical resistance and limited cold flexibility are required. Groove design must account for thermal contraction, gland fill, and squeeze loss across the full temperature swing.
Cryogenic Temperature Ranges
For O-ring selection, split the low-temperature spectrum into three zones. Each zone has different material options.
| Zone | Typical Temperatures | Representative Fluids | Elastomer Options | Polymer/Metal Options |
|---|---|---|---|---|
| Cold / Arctic | −40°C to −60°C | Cold climates, refrigerated gases, LP gas | VMQ, FKM GLT/GFLT, low-ACN NBR, HNBR LT, FFKM LT | Standard hardware |
| Deep Cold | −60°C to −150°C | Cold-soak vents, cryogenic pre-cooling | None reliably | FEP encapsulated (limited), PTFE |
| Cryogenic | −150°C to −270°C | LNG (−162°C), LOX (−183°C), LN₂ (−196°C), LH₂ (−253°C), LHe (−269°C) | None | PTFE, spring-energized PTFE, metal seals |
If your minimum service temperature is −60°C or warmer, an elastomer O-ring can still work if the compound and groove are correct. Below −60°C, elastomers pass through their glass-transition region and lose the elastic recovery that makes an O-ring seal.
Material Selection by Temperature
Elastomers and Polymers for Cold Service
| Material | Practical Min Temp (°C) | TR10 (°C) | Tg, DSC (°C) | Best Media | Notes |
|---|---|---|---|---|---|
| VMQ (Silicone) | −60 | −55 to −60 | −60 to −120 | Air, water, inert gas, food/medical | Best cold flexibility of commercial elastomers; poor tear and hydrocarbon resistance |
| FKM GLT | −30 to −40 | −24 to −32 | −30 to −35 | Oils, fuels, hydraulic fluids | Standard low-temp FKM; retains FKM chemical resistance |
| FKM GFLT | −40 to −50 | −35 to −40 | −35 to −45 | Oils, fuels, aggressive hydrocarbons | Improved low-temp grade; higher fluorine content |
| Low-ACN NBR | −40 to −50 | −35 to −45 | −50 to −60 | Mineral oils, aliphatic fuels, water/glycol | Lower ACN = better cold, worse oil swell |
| HNBR LT | −35 to −45 | −25 to −37 | −35 to −45 | Oil, sour gas, ozone, steam | Better heat and oil aging than NBR |
| FFKM LT | −25 to −40 | −20 to −30 | −15 to −30 | Virtually all chemicals | High cost; used when chemistry + low temp overlap |
| PTFE | −200 | N/A | ~−117 (amorphous) | All chemicals except molten alkalis | Non-elastic; requires spring energization or rigid gland |
VMQ is the default for static seals in clean, cold gas service, but it tears easily, permeates gases, and swells in hydrocarbons.
FKM GLT and GFLT extend FKM's useful range to −30°C to −50°C while keeping oil and fuel resistance. GFLT trades a few degrees of cold flexibility for better chemical resistance.
Low-ACN NBR trades oil resistance for cold flexibility and works down to −40°C to −50°C in static service. HNBR LT adds hydrogen saturation for better heat, ozone, and mechanical properties at similar cold limits.
FFKM low-temperature grades are the only elastomeric option when amines, ketones, strong acids, or oxidizers are present at temperatures that would destroy FKM, VMQ, or NBR. They are not cryogenic seals and stop near −25°C to −40°C.
PTFE is the only true cryogenic O-ring material. It is chemically inert from near −200°C to +260°C, but it has no elastic recovery and cold-flows under load. Solid PTFE works in rigid static grooves; cycling, pressure variation, or motion needs a spring-energized PTFE seal. The metal spring supplies the seating force PTFE cannot generate on its own.
Glass Transition & TR10: When Rubber Turns to Glass
An elastomer seals because its polymer chains move under stress and spring back. As temperature drops, chain mobility decreases. At the glass-transition temperature (Tg), the polymer changes from rubbery to glassy and loses nearly all elastic recovery. Tg is measured by DSC (ASTM D7426 / ISO 22768).
TR10 is the temperature at which a stretched, frozen specimen retracts 10% during warming (ASTM D1329 / ISO 2921). It is more useful than Tg because it reflects the temperature at which the material begins to regain useful elasticity. Practical rules of thumb:
- For static seals, the minimum is roughly 10°C below TR10.
- For dynamic seals, do not operate below TR10.
| Material | Tg (DSC, °C) | TR10 (°C) | Practical Static Min (°C) | Practical Dynamic Min (°C) |
|---|---|---|---|---|
| Standard FKM | −20 to −16 | −17 to −20 | −25 to −30 | −20 |
| FKM GLT | −30 to −35 | −24 to −32 | −35 to −40 | −30 |
| FKM GFLT | −35 to −45 | −35 to −40 | −45 to −50 | −40 |
| VMQ | −60 to −120 | −55 to −65 | −65 to −75 | −60 |
| Low-ACN NBR | −50 to −60 | −35 to −45 | −45 to −55 | −40 |
| HNBR LT | −35 to −45 | −25 to −37 | −40 to −45 | −35 |
| FFKM LT | −15 to −30 | −20 to −30 | −25 to −40 | −25 |
If the seal must move or energize itself at startup, TR10 — not the catalog minimum — is the real limit.
Gas Permeation & Embrittlement
Gas permeation matters more in cryogenic service than many designers expect. Permeation coefficients drop as temperature falls, but cryogenic systems cycle: the seal warms during shutdown, absorbs gas, and re-cools. If pressure is released quickly, the gas blisters and cracks the elastomer — explosive decompression (AED/RGD). Cold, stiff seals are especially vulnerable because they cannot deform to relieve internal gas pockets.
Small-molecule gases are the worst offenders. Hydrogen and helium permeate materials that stop nitrogen or methane. In liquid hydrogen service, even PTFE has measurable hydrogen permeation; the design emphasis shifts to minimizing leak paths rather than expecting zero permeation.
Relative Gas Permeability (Nitrogen at ~25°C)
| Material | N₂ Permeability Coefficient (×10⁻⁶ (cm·cm³)/(cm²·s·bar)) | Trend |
|---|---|---|
| VMQ | ~150 | Very high — poor gas barrier |
| FVMQ | ~40 | High, but better than VMQ |
| NR / SBR | ~4–12 | Moderate |
| NBR (medium ACN) | ~0.5 | Low |
| HNBR | ~0.3–0.6 | Low |
| FKM | ~0.25 | Very low |
| FFKM | ~0.2–0.5 | Very low |
| PTFE | <0.1–0.5 | Low, but gas-dependent |
Absorbed gases can also embrittle elastomers by escaping on warm-up and leaving microcracks, which is why cryogenic gas seals are often replaced after a defined number of cycles.
Compression Set at Low Temperature
Compression set is usually treated as a high-temperature failure mode, but it is equally important in the cold. When an O-ring is compressed at low temperature, the polymer chains are less mobile and may not spring back fully when the load is removed. After warm-up, a high residual set means the seal no longer fills the groove and leaks at low pressure.
Typical Residual Compression Set After Cold Exposure
| Material | Residual Set After Cold Soak & Return to Ambient | Interpretation |
|---|---|---|
| VMQ | 35–55% | Acceptable for static cold service; poorer after repeated cycling |
| FKM GLT | 40–60% | Use higher initial squeeze; verify at actual minimum temperature |
| FKM GFLT | 35–55% | Better than GLT in some compounds; request compound data |
| Low-ACN NBR | 50–70% | Higher set; replace after limited cold cycles |
| HNBR LT | 40–60% | Better aging than NBR; still needs squeeze verification |
| FFKM LT | 30–50% | Lowest set among elastomers at low temperature; high cost |
| PTFE | N/A (cold flow) | Design for creep with spring energizer or metal backup |
A 40% residual set means the O-ring has lost nearly half its cross-sectional height; in a gland designed for 20% squeeze, the effective squeeze can drop below zero.
Groove Design for Cryogenics
The two biggest differences from room-temperature design are thermal contraction and stiffness increase. If the groove does not maintain squeeze across the full range, the seal opens a leak path.
Thermal Contraction from +20°C to −196°C
| Material | Linear CTE (×10⁻⁶/°C) | Approximate Contraction to −196°C |
|---|---|---|
| 316L stainless steel | ~16 | 0.3% |
| Aluminum | ~23 | 0.5% |
| FKM | ~160–200 | 3–4% |
| VMQ | ~200–250 | 4–5% |
| NBR / HNBR | ~200 | 4% |
| PTFE | ~100–150 | 2% |
Because the seal contracts more than the metal, a squeeze that looks generous at assembly can disappear at cryogenic temperature. For elastomer O-rings in cold service, design for 20–30% squeeze at ambient for static seals and verify that residual squeeze at minimum temperature remains above 10–15%. For dynamic seals, keep squeeze in the 15–22% range to limit friction when the material warms up.
Gland Fill and Squeeze Targets
| Parameter | Cold Elastomer Seals (−40°C to −60°C) | Solid PTFE Static Seals | Spring-Energized PTFE |
|---|---|---|---|
| Ambient squeeze | 20–30% | 25–30% | Set by spring preload |
| Gland fill | 60–75% | 70–85% | Per seal supplier groove |
| Groove corner radius | ≥0.25 mm | ≥0.5 mm | ≥0.5 mm |
| Surface finish (sealing face) | Ra ≤0.8 μm | Ra ≤0.4 μm | Ra ≤0.4 μm static; ≤0.2 μm dynamic |
| Backup ring | PTFE or PU if pressure >500 psi | Metal backup to limit cold flow | Integral in design |
Aim for the lower half of the standard 65–85% fill range in cold service; overfilled grooves trap the seal, and sharp edges nick cold-stiffened rubber.
Special Considerations by Fluid
LNG (−162°C)
LNG is cold, flammable, and handled in cyclic loading/unloading. Elastomer O-rings are not suitable for liquid LNG contact; spring-energized PTFE with bronze or carbon fillers is the standard solution. Hardware must be austenitic stainless steel or aluminum, and pre-cool rates should be controlled to avoid thermal shock cracking of PTFE. See our LNG & cryogenic sealing application page.
Liquid Nitrogen (−196°C) and Cold Gases
Liquid nitrogen is inert, so temperature and thermal shock matter more than chemical compatibility. Solid PTFE works for static face seals; spring-energized PTFE is preferred for cycling valves and transfer lines. Vent slowly.
Liquid Oxygen (−183°C) — Cleanliness Is Safety
LOX is a powerful oxidizer. Organic contamination can ignite in oxygen-enriched conditions. Specify virgin or carbon-filled PTFE, clean to oxygen-service standards, avoid hydrocarbons, and use Inconel or Monel springs where sparking is a concern.
Liquid Hydrogen (−253°C)
Hydrogen leaks through seals that stop helium or nitrogen. At −253°C, only PTFE or metal seals are viable. Minimize permeation paths, qualify with helium mass-spectrometer leak testing, and avoid elastomers entirely.
Vacuum and Cryogenics
Cryogenic vacuum systems face outgassing and permeation. Spring-energized PTFE with no elastomeric core is the standard for high-vacuum and ultra-high-vacuum cryogenic seals. Pre-bake components before assembly.
FAQ
Q1: Can I use a standard FKM or NBR O-ring for LNG service?
No. At −162°C both materials are glassy and brittle and will leak immediately. LNG requires PTFE or metal seals.
Q2: What is the lowest temperature a rubber O-ring can seal?
Approximately −60°C for continuous static service with special silicone; closer to −40°C for dynamic seals. Below −60°C, PTFE or spring-energized PTFE is required.
Q3: What is TR10, and why does it matter?
TR10 is the temperature at which a stretched, frozen rubber specimen retracts 10% on warming. It indicates when the material regains useful elasticity — a better guide than the catalog minimum for real sealing performance.
Q4: Why do cryogenic seals need more squeeze?
Because the seal contracts more than the metal and becomes stiffer, initial squeeze must leave residual compression at the minimum temperature. A 20–30% ambient squeeze is common for static cryogenic elastomer seals.
Q5: Is PTFE safe for liquid oxygen?
Yes, PTFE is chemically compatible with liquid oxygen and has a very high limiting oxygen index. For flowing or high-velocity oxygen, use anti-static (carbon-filled) PTFE and clean all parts to oxygen-service standards.
Q6: When should I choose a spring-energized PTFE seal instead of a solid PTFE O-ring?
Use spring-energized PTFE whenever the seal will experience thermal cycling, pressure variation, dynamic motion, or a large temperature drop. The metal spring compensates for PTFE cold flow and thermal contraction, maintaining contact force that a solid PTFE ring cannot provide.
Q7: Why is hydrogen so difficult to seal at cryogenic temperatures?
Hydrogen has the smallest molecular diameter and the highest diffusivity. It permeates through elastomers and some plastics. At −253°C, only PTFE or metal seals are practical, and the design must minimize all leak paths and be qualified with sensitive leak testing.
Q8: What groove fill should I target for a cryogenic O-ring?
Aim for roughly 60–75% gland fill at assembly. This leaves room for thermal contraction, re-expansion during warm-up, and fluid swell. Overfilled grooves can trap the seal and cause extrusion; underfilled grooves allow rolling or loss of contact.
Closing
Cryogenic O-ring selection is not about finding a miracle rubber that works at −200°C — no such elastomer exists. It is about matching the material and seal geometry to the real temperature, fluid, pressure, and cycle profile. Use elastomers only where they remain above their glass transition and TR10 limits, and move to PTFE or spring-energized designs for true cryogenic service.
If you are specifying seals for LNG, liquid nitrogen, oxygen, or hydrogen, start with the right material and verify the groove design across the full thermal envelope. Use our material selector tool to compare compounds, or request a quote and our engineering team will review your temperature range, fluid compatibility, and groove geometry. For complex cryogenic designs, contact us directly.