PEI vs PEEK: Which High‑Temperature Plastic Should You Choose?
| May 27, 2026
1. Introduction PEI vs PEEK
When engineers need a polymer that can withstand sustained heat, resist chemicals, and maintain mechanical integrity, two materials often top the list: PEI (polyetherimide) and PEEK (polyetheretherketone). Both are high‑performance engineering thermoplastics used in aerospace, medical, oil & gas, electronics, and automotive industries. But they are not interchangeable.
PEI, best known under the brand name Ultem™ from SABIC (and also available as Valiant PEI from domestic suppliers), offers an excellent balance of cost, flame retardancy, and thermal stability up to 170°C. PEEK, on the other hand, is the go‑to choice for extreme environments – continuous use up to 260°C and outstanding chemical resistance – but at a significantly higher price.
In this guide, we will compare the two materials side by side, look at real applications, and help you decide which one is right for your project. We will also show how CISKO NEW MATERIAL can supply both (and provide expert guidance).
2. At‑a‑Glance Comparison Table
| Property | PEI (Ultem / Valiant) | PEEK (Victrex, Solvay, etc.) |
|---|---|---|
| Continuous use temperature (RTI) | 170°C | 260°C |
| Glass transition temperature (Tg) | 217°C | 143°C |
| Melting point | Amorphous – no distinct melt | 343°C (semi‑crystalline) |
| Tensile strength (unfilled) | 110 MPa | 100–110 MPa |
| Tensile strength (30% glass filled) | 175 MPa (Ultem 2300) | 170–190 MPa |
| Flexural modulus (unfilled) | 3,300 MPa | 3,800 MPa |
| Flexural modulus (30% glass) | 9,400 MPa | 12,000 MPa |
| Chemical resistance | Good (amorphous) – resists many solvents, but susceptible to some organics | Excellent – resists virtually all organic solvents, strong acids, and bases |
| Flame retardancy | Inherent UL94 V0 (0.75mm) and 5VA (3mm); low smoke | V0 achievable with additives; inherently more difficult to burn but usually requires modifiers |
| Dielectric strength | 28 kV/mm | 19–24 kV/mm |
| Moisture absorption (24h) | 0.25% | 0.10% |
| Transparency options | Transparent amber (Ultem 1000, 1285) | Opaque only (semi‑crystalline) |
| Relative cost (per kg) | $$ (mid‑range) | $$$$ (2–3x PEI) |
Key takeaway: PEI is often sufficient for temperatures up to 170°C and many chemical exposures, at a much lower cost. PEEK only becomes necessary when the application exceeds PEI’s thermal or chemical limits.
3. When to Choose PEI (Ultem / Valiant)
PEI is the smarter choice for the majority of high‑temperature engineering applications that do not require continuous exposure above 170°C or aggressive chemical attack. Here are the key reasons to pick PEI.
3.1 Lower Cost (Typically 30–50% Less than PEEK)
Price is often the deciding factor. Unfilled PEI (e.g., Ultem 1000) typically costs between 25–40 per kg, while unfilled PEEK can range from 80–120 per kg. For glass‑filled grades, the difference is similar.
Example: A project requiring 1,000 kg of material per year:
- PEI cost: 30/kg=30,000
- PEEK cost: 90/kg=90,000
- Savings with PEI: $60,000 per year.
For many buyers, this difference is enough to design within PEI’s limits.
3.2 Transparent Amber Option (Ultem 1000, 1285)
PEI is naturally transparent amber (for unfilled grades). This is a unique advantage for applications where visual inspection of internal components or fluid flow is needed. PEEK is opaque and cannot be made transparent.
- Ultem 1000 – general transparent grade.
- Ultem 1285 – transparent, FDA compliant for food contact.
3.3 Excellent Electrical Properties at High Frequencies (5G)
PEI has a low and stable dielectric constant (Dk) and dissipation factor (Df) across a wide frequency range, including 5G bands (1–10 GHz). This makes it ideal for:
- Antenna housings
- RF connectors
- PCB substrates and interposers
| Frequency | Dielectric Constant (PEI) |
|---|---|
| 1.1 GHz | 3.01 |
| 5 GHz | 3.02 |
| 10 GHz | 3.02 |
PEEK also has good electrical properties, but its Dk is slightly higher and less stable at high frequencies, and its higher cost is rarely justified for 5G components.
3.4 Sufficient for Most Continuous Use up to 170°C
The Relative Thermal Index (RTI) for PEI is 170°C for both electrical and mechanical properties. This means it can be used continuously for 100,000 hours (about 11 years) at that temperature without losing more than 50% of its initial properties.
Many real‑world applications – aircraft interiors (FAR 25.853), under‑hood automotive (150°C max), medical sterilisation (134°C autoclave), and electronics – operate well below 170°C. For these, PEI is more than adequate.
3.5 Inherent Flame Retardancy (UL94 V0 without Additives)
PEI is inherently flame retardant due to its aromatic structure. It achieves UL94 V0 at 0.75 mm (Ultem 1000) and even 0.25 mm for glass‑filled grades (Ultem 2300). It also passes FAR 25.853 for aircraft interiors with low smoke and toxicity.
PEEK on its own is not easily flame retardant; it requires additives that can degrade mechanical properties or increase cost.
3.6 Domestic, Cost‑Effective Alternative (Valiant PEI)
Through our partner Valiant, CISKO offers a domestic (Chinese) PEI that is fully equivalent to Ultem for many applications, at a lower price point. Valiant PEI grades (VAT002, VAT002HF, G10–G30, powders) are REACH and RoHS compliant, and we have successfully qualified them in automotive and industrial components.
4. When to Choose PEEK
PEEK is an exceptional material, but it is also overkill for many applications. Only choose PEEK when the operating environment exceeds PEI’s capabilities.
4.1 Continuous Service Above 200°C
PEI’s RTI is 170°C; above that, it will soften and creep. PEEK, with a Tg of 143°C (where it becomes rubbery) but a melting point of 343°C, maintains useful mechanical properties up to 260°C (RTI). If your part will see sustained temperatures of 200–260°C, PEEK is necessary.
Example applications:
- Downhole oil & gas seals (well temperatures 200–250°C)
- Turbine engine components
- High‑temperature bearings
4.2 Extreme Chemical Exposure
PEI resists many chemicals but can be attacked by strong bases, aromatic solvents (e.g., toluene), and some acids. PEEK, on the other hand, is virtually inert to all organic solvents, strong acids (except concentrated sulfuric), and bases. It is the material of choice for:
- Chemical pump housings and impellers
- Semiconductor wet processing equipment
- Medical implants (long‑term biocompatibility)
4.3 Wear Resistance Against Metals (Bearing Applications)
While PEI can be compounded with PTFE or graphite (e.g., Ultem 4000 series) to improve wear, PEEK naturally has better sliding wear properties and a lower coefficient of friction. For non‑lubricated bearings against steel, PEEK outperforms PEI.
Typical bearing wear rates (against steel, PV = 2000 psi·fpm):
- Unfilled PEEK: K-factor ~ 5–10 x 10⁻¹⁰
- PTFE‑filled PEEK: even lower
- PTFE‑filled PEI (Ultem 4001): K-factor ~ 27–72 x 10⁻¹⁰
If your application involves continuous sliding contact without lubrication, PEEK is often the better choice.
4.4 Sterilisation by Steam (Autoclave) – Both are good, but PEEK is superior
Both PEI and PEEK can withstand steam autoclaving (134°C). However, PEEK can tolerate many more cycles without hydrolysis or loss of mechanicals. For medical devices that require thousands of autoclave cycles, PEEK is preferred. For occasional sterilisation, PEI (especially Ultem 1285) is sufficient.
5. Cost‑Benefit Analysis for Real Projects
5.1 Example 1: Aerospace Interior Clip
Requirement: A small clip to hold cabin panels. Must pass FAR 25.853 vertical burn, operate at 80°C ambient (spikes to 120°C), have good impact strength, and be lightweight.
| Material | Suitability | Cost per part (moulded) |
|---|---|---|
| PEI (Ultem 1000) | ✅ Passes burn test, Tg 217°C, cheap to mould | $0.20 |
| PEEK (unfilled) | ✅ Over‑specified, also passes but much more expensive | $0.70 |
Conclusion: PEI saves 0.50perpart.For500,000parts/year,thatis∗∗250,000 annual savings**. PEI is the clear winner.
5.2 Example 2: Downhole Seal for Oil & Gas
Requirement: Seal that must operate at 230°C continuous, exposed to sour gas (H₂S) and crude oil for 2 years. Must maintain elasticity and sealing force.
| Material | Suitability | Cost per kg |
|---|---|---|
| PEI | ❌ RTI 170°C – will soften and creep at 230°C | $35 |
| PEEK (unfilled) | ✅ RTI 260°C, chemically inert, excellent creep resistance | $110 |
Conclusion: PEEK is mandatory. No alternative.
5.3 Example 3: 5G Antenna Housing (Outdoor)
Requirement: UV resistance, low dielectric loss at 3.5 GHz, operating temperature range -40°C to +85°C, UL94 V0.
| Material | Suitability | Cost per kg |
|---|---|---|
| PEI (Ultem 1000) | ✅ UV stable (F1 rating), Dk 3.0, V0 at 0.75mm | $28 |
| PEEK | ✅ but UV degrades faster without additives; Dk ~3.2, higher loss | $90 |
Conclusion: PEI is the smart, cost‑effective choice.
6. CISKO’s Portfolio
At CISKO NEW MATERIAL, we supply high‑performance engineering plastics for demanding applications. Our portfolio includes:
PEI (Polyetherimide)
- SABIC Ultem™ – Full range: 1000, 1010, 1285, 2100–2400, 4000 series, 3452, etc.
- Valiant PEI – Domestic, cost‑effective grades: VAT002, VAT002HF, G10, G20, G30, micron powders (P50–P350)
- Advantages: Lower cost, transparent options, UL94 V0, REACH/RoHS
PEEK (upon request)
We are not a direct PEEK distributor, but we can source genuine PEEK (Victrex, Solvay, Evonik) for customers who require it. Contact us with your specifications, and we will provide a quote.
Technical Support
Our engineering team can help you select the right material based on your thermal, chemical, mechanical, and budget requirements. We provide:
- Free material selection consulting
- Sample quantities for testing
- Full documentation (TDS, COA, UL, REACH, RoHS)
7. Conclusion: Don’t Over‑Specify
Both PEI and PEEK are outstanding high‑temperature plastics, but they serve different needs. PEI is often the more economical and perfectly adequate choice for applications up to 170°C, for flame‑retardant parts, for transparent components, and for high‑frequency electronics. PEEK is necessary only when sustained temperatures exceed 200°C, aggressive chemicals are present, or extreme wear resistance is required.
Rule of thumb: If you think you need PEEK, first check whether PEI can do the job at half the cost. In many cases, it can.
Let our engineers guide you. We will help you compare properties, run cost models, and even provide samples for testing.
📧 Contact us for a free material selection sheet: info@pp-pei.com
📞 Call: +86 13538114678
🔗 Browse PEI products: Ultem PEI | Valiant PEI
Or request a custom quote for PEEK if your application truly demands it.
