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PEEK vs PEI (ULTEM): Thermal Performance, Strength & Cost Comparison

Comprehensive Property Comparison

Property		PEEK	PEI (ULTEM)	Advantage
Thermal	Continuous service temp	250°C	170°C	PEEK (+80°C)
	Glass transition (Tg)	143°C	217°C	PEI
	Melting point	343°C	Amorphous	PEEK
	HDT (1.8 MPa)	152°C	200°C	PEI (+48°C)
Mechanical	Tensile strength	98 MPa	105 MPa	PEI
	Flexural modulus	3.6 GPa	3.3 GPa	PEEK
	Impact (notched)	7.5 kJ/m²	5 kJ/m²	PEEK
	Wear resistance	Excellent	Fair	PEEK
Chemical	Acid resistance	Good	Fair	PEEK
	Base resistance	Good	Fair	PEEK
	Solvent resistance	Good	Fair	PEEK
Fire Safety	UL94 rating	V-0 (1.45mm)	V-0 (0.4mm)	PEI (thinner)
Fire Safety	LOI	35%	47%	PEI
Physical	Density	1.32 g/cm³	1.27 g/cm³	PEI (lighter)
	Water absorption 24h	0.06%	0.25%	PEEK
	Transparency	Opaque	Amber-transparent	PEI
Cost	Price (USD/kg)	$85–110	$20–35	PEI (3–4× cheaper)

When to Choose PEEK

PEEK is the clear winner when your application requires:

Higher Continuous Temperature

PEEK operates at 250°C continuously — 80°C higher than PEI. For applications in hot environments (downhole tools, high-temp chemical processing, semiconductor processing), PEEK is essential.

Superior Chemical Resistance

PEEK resists a broader range of chemicals than PEI, which is attacked by some polar solvents (acetone, MEK, chlorinated solvents). For chemical processing environments, PEEK is the safer choice.

Better Wear and Friction Properties

PEEK‘s excellent wear resistance makes it the preferred bearing and wear material. PEI has poor wear characteristics and is not recommended for sliding or rubbing applications.

Lower Moisture Absorption

PEEK absorbs only 0.06% water at 24h versus PEI‘s 0.25%. For applications requiring maximum dimensional stability in humid conditions, PEEK is superior.

Medical Implant Compliance

PEEK meets USP Class VI and ISO 10993 for implantable devices. PEI is not typically used for implants.

When to Choose PEI

PEI is the better choice when:

Cost is a Primary Factor

At 25–35% of PEEK‘s price, PEI delivers comparable mechanical strength and inherent flame retardancy at a fraction of the cost. For non-extreme-temperature applications, PEI offers better value.

Fire Safety at Minimum Thickness

PEI achieves UL94 V-0 at 0.4mm — PEEK requires 1.45mm. For thin-wall components requiring fire certification, PEI is superior.

Transparency is Required

PEI‘s amber transparency is useful for sight glasses, inspection windows, and optical applications where PEEK‘s opacity is a limitation.

Higher HDT Under Load

PEI‘s HDT of 200°C (vs PEEK‘s 152°C) means PEI maintains stiffness at higher temperatures under load — counterintuitively, PEI is stiffer at 150°C than PEEK due to its higher Tg.

Aerospace Interior Applications

PEI‘s inherent FST compliance, surface finish quality, and lower cost make it the preferred material for aircraft cabin interiors.

Application Selection Matrix

Application	Recommended	Reason
Aircraft interior panels	PEI	FST + cost + surface
Downhole seal	PEEK	Temperature + chemicals
Semiconductor wafer carrier	PEEK	Purity + wear
Medical implant	PEEK	Biocompatibility
Electrical connector (high freq)	PEI	Dielectric + cost
Sight glass	PEI	Transparency
Bearing bush	PEEK	Wear resistance
Thin-wall fire-rated part	PEI	V-0 at 0.4mm

For broader comparisons, see our PEEK vs PTFE guide and Engineering Plastics Overview.

FAQ

Q1: Is PEEK stronger than PEI?

PEI actually has a slightly higher tensile strength (105 MPa vs 98 MPa for PEEK). However, PEEK has higher impact toughness and much better wear resistance. PEEK also maintains its properties at much higher temperatures (250°C vs 170°C continuous service).

Q2: Which is more expensive: PEEK or PEI?

PEEK is significantly more expensive, typically 3–4 times the cost of PEI. PEEK costs $85–110/kg while PEI costs $20–35/kg. PEI offers similar mechanical strength and flame retardancy at a fraction of the cost when temperature requirements are below 170°C.

Q3: Does PEI have better fire resistance than PEEK?

PEI has superior fire resistance metrics. PEI achieves UL94 V-0 at 0.4mm thickness (vs 1.45mm for PEEK) and has a higher limiting oxygen index (47% vs 35%). For thin-wall fire-rated components, PEI is the better choice.

Q4: Can PEI replace PEEK in my application?

PEI can replace PEEK if: your continuous service temperature is below 170°C, you don’t need wear resistance, chemical exposure is mild, and the component is not an implant. PEI offers 60–70% cost savings while maintaining comparable strength and flame retardancy.

Choosing between PEEK and PEI? Contact NAGOMER for expert material advice, or browse our PEEK products and PEI sheets.

NAGOMER — Professional Engineering Plastics Solutions

www.nagomer.com | info@nagomer.com