PEEK vs PTFE vs Polyimide for Sealing: Which Material, When?
Sealing is one of the hardest jobs in engineering: a seal must hold fluid or gas under pressure, survive temperature, resist whatever media it touches, and outlast the machine it protects. When the demands are high, engineers choose among the three "big three" performance fluoropolymer/imide materials — PEEK, PTFE, and polyimide (PI). Each brings a different strength, and there is no universal winner. This guide gives a verified three-way comparison and a practical, condition-driven way to choose the right one for your sealing application.
1. The Three Candidates in One Table
| Property | PEEK | PTFE | Polyimide (PI, e.g., LY-601) |
|---|---|---|---|
| --- | --- | --- | --- |
| Continuous service temperature | ~260 °C | -260 to +260 °C | ~360 °C (dry) |
| Melting point | ~343 °C | ~327 °C | No true melt |
| Tg | ~143 °C | ~130 °C | ~290 °C |
| Tensile strength | ~95–100 MPa | ~15–40 MPa | ~56 MPa |
| Coefficient of friction | ~0.3–0.5 (unreinforced) | ~0.05–0.10 | ~0.28 |
| Chemical resistance | Excellent | Near-total (inert) | Good (poor vs. steam) |
| Hydrolysis (hot wet) | Good | Inert | Poor |
| Creep resistance | Good | Poor (cold flows) | Good |
| Processability | Injection/extrusion/CNC | Compression/skive/calender | Compression/sintering + CNC |
Sources: Victrex® / Evonik PEEK datasheets; PTFE fluoropolymer industry data; LY-601 measured PI data (UL 746B, ISO methods).
2. The Selection Logic: Pressure, Temperature, Media, Life
Four inputs decide the seal material: pressure, temperature, media, and required life.
If temperature is the driver
- Below ~260 °C and you need strength + hydrolysis resistance → PEEK
- Below ~260 °C and you need inertness + low friction → PTFE
- Above ~260–300 °C in dry service → PI (LY-601) (its ~360 °C continuous rating and low wear shine here)
If pressure/load is the driver
- Sustained load with creep resistance → PEEK (or PI) — PTFE creeps and loses sealing stress under sustained bolt load
- High load carrying → PEEK, fibre-filled if stiffness is needed
If media/chemistry is the driver
- Aggressive chemicals, strong bases/acids → PTFE (near-total inertness) or PEEK (if structural)
- Electrolyzer KOH / PEM acidic media → both work; PTFE for gaskets, PEEK for structural/back-up
- Hot water/steam → PEEK (good hydrolysis); avoid PI (hydrolyses)
If wear/friction is the driver
- Lowest friction → PTFE (coefficient ~0.05–0.10)
- Low wear + strength in moving/dynamic parts → PEEK / CFR-PEEK (e.g., LY-205 measured friction ~0.13, very low wear volume)
3. Application-by-Application Conclusions
| Sealing application | First choice | Second choice | Why |
|---|---|---|---|
| --- | --- | --- | --- |
| Electrolyzer primary gasket | PTFE | Filled/ePTFE PTFE | Inertness, low friction, chemical resistance |
| Electrolyzer back-up ring / structural seal | PEEK | PI | Creep resistance + strength under bolt load |
| High-temperature dry seal (>300 °C) | PI (LY-601) | — | ~360 °C continuous, low wear |
| Hot water / steam seal | PEEK | PTFE | Hydrolysis resistance; avoid PI |
| Low-friction / low breakaway seal | PTFE | — | Lowest coefficient of friction |
| Dynamic / wear ring (pump, actuator) | PEEK / CFR-PEEK | PI (very high temp) | Strength + low wear + toughness |
| Media-separating static gasket, extreme chemistry | PTFE | ePTFE | Near-total inertness |
| Compressed/creep-resistant static seal at temp | PEEK | PI (dry high temp) | General creep resistance |
4. The Single Fatal Mistake People Make
Constituing a seal from one material for every condition. The most common specification error is picking "PTFE everywhere" for purity and inertness, then watching gaskets loosen under sustained stack pressure (cold flow), or overpaying for PI in a steam environment where it hydrolyses. Real engineering practice combines materials:
- PTFE gasket for primary sealing + PEEK back-up ring/frame for structural support
- PEEK or PI frame/seal structure + PTFE low-friction contact face in the same assembly
The two-material stack leverages each material's strength and survives conditions a single material cannot.
5. Decision Checklist for Your Seal
Work through these in order:
- [ ] Define peak and continuous temperature; separate loaded vs unloaded
- [ ] Define peak pressure and whether load is sustained (drives creep concern)
- [ ] Define media and check chemical + hydrolysis compatibility
- [ ] Define required life / cycling and whether friction/wear is critical
- [ ] Choose primary and backup material per the table above
- [ ] Consider a two-material stack (PTFE seal + PEEK/PI structural)
- [ ] Get a datasheet and a compression/leakage/creep test for your service conditions
- [ ] Ask the supplier for fugitive-emission compliance where relevant (e.g., TA-Luft/VDI 2440 for gaskets)
6. Summary: The One-Line Guide
- PTFE = inert, low-friction, extreme-chemistry sealing — but weak and creep-prone, so not for structural/load-bearing.
- PEEK = the structural, creep-resistant, hydrolytically stable sealing workhorse to ~260 °C; pair PTFE gaskets with PEEK frames/back-up rings.
- PI (LY-601) = the ultra-high-temperature dry sealing material to ~360 °C where PEEK runs out of headroom.
Frequently Asked Questions
- Which material has the lowest coefficient of friction? PTFE, at roughly 0.05–0.10 — the lowest of any solid — making it ideal for low-breakaway and low-friction seals; PEEK and PI are higher but much stronger.
- PEEK vs PTFE for a seal under sustained pressure? PEEK — it resists creep and holds sealing load under sustained pressure, whereas PTFE cold-flows and loosens. Use PTFE where inertness/friction dominates and load is light.
- When should I use polyimide (PI) for a seal instead of PEEK? If the seal runs in sustained dry heat above ~260–300 °C — PI's ~360 °C continuous rating and low wear justify it. Avoid PI in hot water/steam (it hydrolyses).
Related Topics
- PEEK in Hydrogen / Electrolysis Systems: Seals, Frames & Gaskets
- PTFE Seals for Hydrogen Electrolyzers: Material Selection Guide
- Material Selection Decision Tree: PEEK vs PTFE vs PI vs PSU
Dalian Luyang Technology Development Co., Ltd. (www.newmaterialchina.com) manufactures PEEK, PI, and PTFE precision parts — seals, inserts, machined components, and electrolyser frames — under ISO 9001 and ISO 13485. Established 2006. Contact: liyulong@luyangkeji.com · WhatsApp +86 15989781752. This guide is educational reference material; always verify against your chosen grade's official datasheet.