PEEK Properties: A Complete Engineering Guide
Polyetheretherketone (PEEK) is a high-performance semi-crystalline thermoplastic that has become the reference material for engineers who need strength, heat resistance, and chemical stability in a single polymer. Unlike commodity thermoplastics, PEEK retains useful mechanical properties at temperatures where most plastics fail, and it resists nearly every common industrial chemical. This guide compiles the verified technical data an engineer or buyer needs to specify PEEK with confidence — from thermal limits and mechanical strength to the four main processing routes.
1. What Is PEEK?
PEEK is a member of the polyaryletherketone (PAEK) family. Its repeating backbone of aromatic rings linked by ether (-O-) and ketone (-CO-) groups gives it a rigid, thermally stable chain. It is naturally semi-crystalline when processed correctly, which is what gives it its outstanding combination of stiffness, toughness, and solvent resistance.
Key identity values (verified against Victrex® datasheets, ISO methods):
- Density (crystalline): 1.30 g/cm³ (ISO 1183)
- Glass transition temperature (Tg): 143 °C onset / ~150 °C midpoint (ISO 11357-2)
- Melting point (Tm): 343 °C (ISO 11357-3)
- Water absorption (saturation, 23 °C): 0.45 % (ISO 62) — very low, giving excellent dimensional stability
Source: Victrex® PEEK polymer datasheets (VICTREX™ 650G, 450G/381G), ISO 1183 / ISO 11357 / ISO 62.
2. Thermal Properties: the 260 °C Continuous-Use Reference
PEEK's defining advantage is its temperature capability. Its Tg of ~143 °C and melting point of 343 °C define the thermal envelope:
| Thermal property | Typical value | Test method |
|---|---|---|
| --- | --- | --- |
| Continuous service temperature | 250–260 °C (up to ~300 °C short-term) | UL 746 / long-term aging |
| Glass transition temperature | 143 °C (onset), ~150 °C (midpoint) | ISO 11357-2 |
| Melting temperature | 343 °C | ISO 11357-3 |
| Heat deflection temperature (HDT) | 152–160 °C at 1.8 MPa | ISO 75-2 |
| Thermal conductivity | ~0.29 W/(m·K) at 23 °C | ISO 22007-4 |
| CLTE (below Tg) | ~45 ppm/K | ISO 11359-2 |
A widely cited figure is 260 °C continuous use temperature (UL 746). Leading suppliers (Victrex, Evonik VESTAKEEP®) confirm reliable performance to 260 °C (500 °F) under many load conditions, with Evonik noting a service range from -196 °C to +300 °C depending on grade and loading. Important caveat: the maximum allowable temperature depends on the duration and magnitude of mechanical stress — a loaded seal at 260 °C behaves very differently from an unloaded part. Always validate against the specific grade datasheet and your operating stress.
Sources: Victrex PEEK datasheets; Evonik VESTAKEEP® technical literature.
3. Mechanical Properties
PEEK strikes an unusual balance of strength, stiffness, and toughness. Unreinforced grades are ductile and impact-resistant; glass- or carbon-fibre-filled grades trade some ductility for much higher modulus.
Unreinforced PEEK (typical, 23 °C):
- Tensile modulus: ~4,000 MPa (ISO 527-1)
- Tensile stress at yield: 95–100 MPa (ISO 527-2)
- Tensile strain at break: 25–50 % (ISO 527-2) — ductile, tough
- Flexural modulus: ~3,600–3,900 MPa (ISO 178)
- Flexural stress: ~155–165 MPa at 23 °C (ISO 178)
- Notched Izod impact: ~5–7 kJ/m²; unnotched Izod: no break (ISO 180)
- Shore D hardness: ~84
Filled PEEK (e.g., 30 % glass fibre, typical):
- Tensile modulus: ~9,000 MPa
- Tensile strength at break: ~150–155 MPa at 23 °C
- Higher stiffness and lower CTE, but reduced elongation (brittle fracture)
Sources: Victrex™ 650G, 450G/381G and 150GL20 datasheets, ISO 527 / ISO 178 / ISO 180.
PEEK's fatigue resistance and low creep are also notable — which is why it replaces metal in parts subject to repeated dynamic loading, such as valves, bearing cages, and seal back-up rings.
4. Chemical Resistance
PEEK is insoluble in almost all common solvents, acids, and bases at normal temperatures. Its only well-known practical vulnerability is concentrated sulfuric acid, which will attack the polymer. It shows excellent resistance to:
- Fuels, hydraulic fluids, and lubricants
- Most inorganic acids and bases (except concentrated sulfuric / certain strong oxidisers)
- Steam and hot water (hydrolysis resistance is good)
- Common industrial solvents, ketones, and hydrocarbons
Its hydrolysis resistance and steam tolerance make it an outstanding choice for hot, wet, and chemical service where polyimides or nylons would degrade.
Sources: Victrex and Evonik product literature; industry chemical-resistance charts.
5. Processing Methods
PEEK is a true thermoplastic: it melts and can be processed by the full range of thermoplastic routes. The four principal methods:
| Method | Best suited for | Notes |
|---|---|---|
| --- | --- | --- |
| Injection moulding | High-volume, complex 3D shapes | Melt temp ~360–415 °C; mould temp 170–200 °C; dry resin to <0.02 % moisture |
| Extrusion | Rod, tube, film, sheet, wire coating, monofilament | Cylinder temps ~350–400 °C; used to produce stock shapes |
| Compression moulding | Large or simple shapes, thick sections, stock | Lower tooling cost, good for short runs |
| CNC machining | Precision parts, low-to-mid volumes, tight tolerances | Machined from extruded or compression-moulded blanks |
Processing note for buyers: quality PEEK parts depend as much on the process as the material. Correct drying (120–150 °C, 3–5 h, <0.02 % moisture), elevated mould temperatures (170–200 °C) to maintain crystallinity, and stress-relief/annealing all determine final dimensional stability. A part's final properties are highly dependent on part geometry, mould design, and processing conditions — datasheet values are for comparison and validation, not as cut-and-dry design limits.
Sources: Victrex processing guides; Xometry PEEK datasheet.
6. PEEK in Context: PI and PTFE Comparison
Engineers often compare PEEK to its two neighbours in the high-performance family:
- Polyimide (PI): Higher continuous-use ceiling (to ~300 °C and beyond) and superior radiation resistance, but more expensive, harder to process (does not melt easily), and more susceptible to hydrolysis and strong alkalis. PEEK wins on toughness, processability, and broad chemical/hydrolysis resistance.
- PTFE: Unmatched chemical inertness and the lowest coefficient of friction, and a very wide temperature range (-260 to +260 °C), but PTFE is mechanically weak and creeps (cold flows) under sustained load. PEEK is far stronger and more dimensionally stable, making it better for structural and load-bearing parts.
There is no universal "best" material — selection depends on temperature, load, chemical environment, and production volume. See our separate guides on polyimide vs. PEEK and PTFE sealing selection for the full comparison.
7. Specifications to Request
When specifying or purchasing PEEK parts, ask your supplier for:
- Full material datasheet with ISO/ASTM test methods and grade designation
- ISO 9001 quality-system certification (production) and, for medical, ISO 13485
- Traceability from raw resin to finished part
- Dimensional inspection report (tolerance, flatness, concentricity)
- Certification of filling/grade where relevant (e.g., CFR-PEEK for structural/medical)
8. Where PEEK Is Used
- Hydrogen energy / electrolysers: seal back-up rings, insulator frames, electrode-frame components, bipolar-plate housings (PEEK's chemical + electrical + dimensional performance)
- Aerospace: bearings, bushings, brackets, cable clamps
- Medical: implantable and surgical instrument components (ISO 13485 / medical grades)
- Oil & gas: sealing rings, valve seats, downhole components
- Semiconductor: wafer carriers, test sockets, chemical-resistant fixtures
- Robotics: gears, wear pads, precision structural parts
Frequently Asked Questions
- Can PEEK be used continuously at 260 °C? Yes, as a widely cited continuous-use rating — but validate against grade and operating stress. Loaded parts may need lower temperature limits.
- Is PEEK machinable? Yes. Extruded or compression-moulded PEEK blanks are readily CNC-machined to tight tolerances for low-to-mid-volume precision parts.
- What is the difference between unreinforced and CFR-PEEK? Unreinforced PEEK is tough and ductile; carbon-fibre-reinforced (CFR-PEEK) offers higher stiffness, lower CTE, and better wear, at some cost in ductility.
Related Topics
- Polyimide (PI) vs PEEK: Selecting High-Temperature Engineering Plastic
- PTFE Seals for Hydrogen Electrolyzers: Material Selection Guide
- PEEK / PI / PTFE FAQ for Engineers and Buyers
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.