If you're comparing PEEK and PEI (Ultem) for a part, you've probably already figured out they're not interchangeable. What you might not know is how differently they behave once the temperature climbs past 150°C — and that's where most of the expensive mistakes happen.
I've seen intelligent engineers spec PEI for a 180°C application because the datasheet HDT looked fine at 200°C. The part softened in service. We replaced it with PEEK and the problem went away. Here's what I wish they'd known going in.
The crystalline vs amorphous thing is not academic
PEEK is semi-crystalline. PEI is amorphous. This single difference at the molecular level explains most of what happens in the real world.
PEEK's polymer chains pack into ordered crystalline regions — typically 30-35% crystallinity in a well-processed part. Those crystals act like physical crosslinks. Above PEEK's glass transition (143°C), the amorphous regions soften but the crystals hold their structure and the part retains useful load-bearing capacity all the way to the melt point at 343°C.
PEI has none of that. Its bulky imide rings prevent crystallization entirely. Above its Tg of 217°C, the entire material softens progressively and loses structural capacity. Despite PEI having a higher Tg than PEEK, PEEK dominates above 200°C because those crystals are doing the work while PEI is just... softening.
| Property | PEEK | PEI |
|---|---|---|
| Structure | Semi-crystalline | Amorphous |
| Tm | 343°C | None |
| Tg | 143°C | 217°C |
| Continuous use (UL746B) | 260°C | 170°C |
That 90°C gap in continuous service temperature separates entirely different application classes. Below 170°C, both work. Above that, you're choosing PEEK.
Heat performance
PEEK holds about 70% of its room-temperature tensile strength at 150°C. PEI holds roughly 50%. At 200°C, PEI is done — essentially no structural capacity. PEEK still carries 40-50%.
For short-term heat, glass-filled PEEK (30% GF) reaches an HDT of 315°C at 1.8 MPa. PEI's glass-filled HDT tops out around 210°C. The numbers paint a clear picture: for sustained elevated temperature, PEEK wins by a wide margin.
At the cold end, both handle cryogenic temperatures, but PEI gets brittle below about -40°C while PEEK stays usable to -60°C. If you're thermal cycling from cryo to hot, PEEK survives more cycles before microcracking.
Thermal expansion favors PEI if your temperature range stays moderate. PEI's CTE is ~56 μm/m·°C and stays linear across its range. PEEK jumps from ~47 below Tg to ~140 above it. For precision parts that can't shift dimension across temperature, PEI's predictability helps — but only within its 170°C ceiling.
How they handle load
At room temperature, PEI slightly edges PEEK on tensile (105 vs 95-100 MPa). Their moduli are close: 3.6 GPa for PEEK, 3.5 for PEI. The difference is toughness.
PEEK stretches 30-50% before breaking. PEI stretches about 6%. In practice this means PEI cracks where PEEK yields. For snap-fits, clips, or anything that sees impact, PEI will let you down.
Glass or carbon fiber fillers push PEEK's modulus past 20 GPa and flexural strength over 300 MPa — numbers PEI can't reach with any filler.
Wear and friction
Unfilled PEEK has decent wear resistance on its own. Unfilled PEI wears faster. With fillers (PTFE, carbon fiber, graphite), both improve substantially, but PEI wear grades are more prone to edge chipping because of that lower base toughness.
For dynamic seals, bearings, or any sliding wear application, PEEK is the safer bet. PEI works for lower loads where cost matters more.
Chemical compatibility
Here's where PEEK really separates itself. It shrugs off virtually everything except concentrated sulfuric and nitric acids — steam, hot water, hydraulic fluid, brake fluid, Skydrol, MEK, toluene, you name it. PEI's chemical range is narrower. It's fine with water, dilute acids, alcohols, and aliphatic hydrocarbons. But chlorinated solvents, esters, ketones, and many aromatics will attack it, especially under stress.
If your part touches any organic solvent or aggressive industrial chemical, go with PEEK. A quick compatibility test beats a field failure.
The money side
PEI costs roughly 40-60% less than PEEK per kilogram. That's the headline number that keeps PEI in the conversation. But raw material price isn't the whole story. A PEI part that fails in a 200°C environment costs infinitely more than a PEEK part that doesn't. The material cost delta is noise compared to a field failure.
PEEK is easier to machine than PEI in some ways — it's more forgiving on tool engagement and doesn't chip as easily at corners. PEI's higher stiffness and lower elongation mean you need sharper tools and lighter cuts. The machining cost difference is usually smaller than the material price difference.
How to decide
- Below 170°C, chemically mild, cost-sensitive, no impact loads? PEI.
- Above 170°C, any chemicals, impact loading, dynamic wear? PEEK. Every time.
- Precision across temperature? PEI (if within range). If outside PEI's range, PEEK with CTE factored in.
- Not sure about chemicals? PEEK.
Send us your requirements if you want a recommendation specific to your application. We stock both and machine them daily — I can usually tell you which one makes sense after a 5-minute look at your drawing and operating conditions.