Polyimide (PI) vs PEEK: Selecting High-Temperature Engineering Plastic
When the application runs hot, engineers eventually face a classic question: polyimide (PI) or polyetheretherketone (PEEK)? Both are ultra-performance engineering plastics that outperform metal in specific roles, yet they are built for different strengths. This guide gives a verified side-by-side comparison — thermal, mechanical, chemical, processing, and cost — so you can select the right material for your operating conditions instead of defaulting to the "highest number wins" approach.
1. The Short Version
- Choose PI when continuous operating temperature exceeds ~260–300 °C, when you need extreme radiation resistance, or when outgassing in vacuum must be near zero (aerospace, spacecraft, semiconductor).
- Choose PEEK when you need toughness under dynamic load, broad chemical resistance including hydrolysis/steam, easy processing via injection moulding or extrusion, and a favourable cost in volume — typically below ~260 °C continuous use.
There is no absolute winner. The right answer depends on temperature profile, mechanical loading, chemical environment, and production volume.
2. Thermal Performance
This is the axis where the two materials diverge most clearly.
| Thermal property | PEEK | Polyimide (PI) |
|---|---|---|
| --- | --- | --- |
| Glass transition temperature | ~143 °C (semi-crystalline) | ~270–410 °C depending on grade |
| Melting point | ~343 °C (melts) | No true melting point; decomposition ~500 °C |
| Continuous use temperature | 250–260 °C (to ~300 °C short-term) | Up to ~300–400 °C (grade-dependent) |
| Short-term temperature tolerance | ~300–310 °C | ~400–500 °C |
Sources: Victrex® PEEK datasheets (ISO 11357); DuPont™ Vespel® PI datasheets; industry PI technical literature.
PI's lack of a melting point and higher glass transition let it sit at temperatures where PEEK would soften or creep. Vespel® SP-1, for example, is rated by DuPont for max continuous operating temperature of ~260 °C (500 °F) for standard grades, with high-temperature polyimide grades extending well beyond 300 °C. PEEK's ~143 °C Tg means unreinforced modulus drops noticeably above ~150 °C, though it still performs to 260 °C in many applications.
Key insight: continuous-use rating is not the same as thermal-cycle fatigue tolerance. Under fluctuating temperatures, PEEK often shows better fatigue resistance than PI, even though PI has the higher ceiling.
3. Mechanical Properties
| Property (23 °C) | PEEK | Polyimide (PI) |
|---|---|---|
| --- | --- | --- |
| Tensile strength | ~90–115 MPa (unreinforced) | ~65–110 MPa typical (100+ MPa some grades) |
| Elongation at break | ~25–150 % — tough and ductile | <50 %, often more brittle |
| Flexural modulus | ~3.6–4.2 GPa | ~3.2–5.6 GPa (grade-dependent) |
| Density | ~1.30–1.43 g/cm³ | ~1.43–1.73 g/cm³ |
| Impact strength | High (unnotched Izod: no break) | Moderate; notch-sensitive |
Sources: Victrex PEEK datasheets; DuPont™ Vespel® SP-1/SP-21 datasheets (ASTM D-638, D-790).
PEEK's defining mechanical advantage is toughness. It retains a large share of its strength at temperature, absorbs impact without brittle failure, and resists fatigue under repeated dynamic loading — ideal for gears, bearings, valve parts, and seal back-up rings. PI is stiffer when hot and retains excellent dimensional stability at very high temperatures, but it is inherently more brittle and costly to toughen, and its elongation at break is generally lower.
4. Chemical Resistance
| Environment | PEEK | Polyimide (PI) |
|---|---|---|
| --- | --- | --- |
| Fuels, oils, hydraulic fluids | Excellent | Good |
| Most acids / bases | Excellent (except concentrated sulfuric acid) | Good, but poor vs. strong alkalis and oxidisers |
| Steam / hot water (hydrolysis) | Good | Poor — hydrolyses in steam/hot water |
| Common solvents | Excellent | Good |
Sources: Victrex and Evonik PEEK literature; industry PI chemical-resistance guidance.
This is a pivotal differentiator for many industrial applications. PEEK is the stronger choice in hot, wet, or chemically aggressive service — steam, hydrolysis, and broad solvent resistance. PI is the stronger choice where the priority is extreme heat plus radiation in a dry environment (aerospace, vacuum), where its hydrolysis sensitivity matters less.
5. Processing and Manufacturing
| Aspect | PEEK | Polyimide (PI) |
|---|---|---|
| --- | --- | --- |
| Processability | Thermoplastic — injection moulding, extrusion, compression, CNC machining | Difficult — does not readily melt; sintering / hot-pressing / compression moulding |
| Tooling / equipment | Standard high-temperature moulding; machining from stock | Specialised; higher process complexity and cost |
| Post-processing | Machining clean, no required post-cure | Some grades need imidisation / post-cure cycles |
| Suitable for high volume | Yes (injection moulding) | Limited (near-net-shape / machining) |
Source: Victrex processing guides; QUANDA / industry PI processing notes.
If your volume is high or your geometry complex, PEEK's melt-processability gives it a decisive practical and cost advantage. PI is more of a "near-net-shape + machining" material, and its process difficulty can drive up total cost in mass production.
6. Electrical, Radiation, and Other Properties
- Electrical: Both are good insulators. PI offers a very low dielectric constant/loss at high frequency and is the classic film dielectric (e.g., in flex circuits and semiconductor insulation); PEEK has excellent but generally moderate dielectric properties (Dk ≈ 3.2, low loss).
- Radiation resistance: PI is outstanding, tolerating high gamma/X-ray doses and deep-space radiation. PEEK shows moderate radiation resistance and can degrade under prolonged high-dose exposure.
- Outgassing: PI has near-zero outgassing in vacuum — preferred in spacecraft. PEEK is also low-outgassing but generally not rated as highly.
- Biocompatibility: PEEK is medical-grade and used in human implants (e.g., spinal and joint devices); PI is generally not used for in-vivo implantation.
7. Selection Decision Matrix
| Decision factor | Choose PEEK | Choose PI |
|---|---|---|
| --- | --- | --- |
| Continuous operating temp | ≤ ~260 °C | > ~260–300 °C |
| Dynamic load / fatigue | ✔ (gears, bearings, seals) | Static / low-cycle preferred |
| Toughness priority | ✔ | Stiffness priority |
| Hot/wet service, steam, hydrolysis | ✔ | Avoid |
| Broad chemical resistance | ✔ | Selective |
| Broad chemical + hydrolysis | ✔ | prefer PEEK |
| Extreme radiation / vacuum, low outgassing | — | ✔ |
| High-frequency/ultra-low-loss electrical | — | ✔ better |
| Large-volume injection moulding / extrusion | ✔ | Limited |
| Medical implant / biocompatibility | ✔ | No |
| Cost sensitivity / volume | ✔ (favourable) | High cost |
8. Common Selection Mistakes to Avoid
- Temperature-only thinking. "Continuous use temperature" ≠ thermal-cycle fatigue tolerance. A material that survives steady heat may fail under repeated thermal cycles.
- Ignoring chemical environment. Short-term immersion tests don't capture long-term degradation in hot, humid, or reactive media. Check hydrolysis and specific-media compatibility for your service life.
- Underestimating processing cost. PI's raw material and process complexity can inflate total cost in volume production — compare life-cycle cost, not just unit price.
- Over-simplifying "which is strongest." PEEK is tougher; PI is hotter-rated. Match the material to the real load and temperature envelope.
Frequently Asked Questions
- Can PEEK be used above 300 °C? Not continuously in most grades — it softens/creeps near and above 260 °C under load. For sustained 300 °C+, PI is the safer choice.
- Is PI injection-mouldable like PEEK? Generally no — most PI does not melt cleanly and requires sintering, hot-pressing, or compression moulding, plus often post-cure.
- Which is better for a wet, hot seal — PEEK or PI? For steam/hydrolysis service, PEEK is the stronger choice; PI hydrolyses in hot water/steam.
Related Topics
- PEEK Properties: A Complete Engineering Guide
- PTFE Seals for Hydrogen Electrolyzers: Material Selection Guide
- How to Choose a PEEK Parts Manufacturer
Dalian Luyang Technology Development Co., Ltd. (www.newmaterialchina.com) manufactures PEEK, PI (LY-601), and PTFE precision parts, plus CFR-PEEK (LY-501) and machined/components for aerospace, medical, semiconductor, oil & gas, and hydrogen energy. ISO 9001 & ISO 13485, established 2006. Contact: liyulong@luyangkeji.com · WhatsApp +86 15989781752. Educational reference — always verify selected grades against official datasheets.