PEEK vs Ultem® Material Matchup

If you are choosing between Ultem® (PEI) and PEEK, there is likely a demanding thermal or electrical requirement (or perhaps both) in your application. In this article, we will briefly compare key characteristics of these two materials to assist in the material selection process.

Elevated Temperature Performance

A key difference between PEEK and Ultem® is their different morphological structures. Ultem® is amorphous while PEEK is semi-crystalline (and contains both amorphous and crystalline regions). This may sound like a useless factoid, but it has strong implications for performance at elevated temperatures, especially above each material’s glass transition temperature.

PEEK exhibits a glass transition temperature (Tg) of around 289F and Ultem experiences its Tg around 420F, yet PEEK is used continuously in applications seeing 480F (and at even higher temperatures for short periods), while Ultem’s max continuous operating temperature is commonly reported as 340F. This is because at a material’s Tg, the amorphous regions soften or rubberize and the crystalline regions remain unaffected. As a result, the maximum usable service temperature of amorphous materials is limited accordingly. This softening behavior is depicted graphically in Figure 1. below.


For higher strength performance and increased dimensional stability, fiber-reinforced versions of both materials are standardly available. The effects of temperature on flexural modulus for 30% glass and carbon fiber-filled PEEK are depicted graphically in Figure 2. Note that while fiber reinforcement provides improved mechanical performance, it has a negligible effect on Tg. Table 1 shows example material properties that are significantly affected by the presence of fillers for each of these material grades.




PEEK 450G PEEK 450GL30 PEEK 450CA30 Ultem® 1000 PEI Ultem® 2300
Tensile Strength 14,000 psi 25,000 psi 38,000 psi 15,000 psi 24,000 psi
Elongation at Break 45% 2% 2% 60% 2%
Flexural Modulus 550,000 psi 1,600,000 psi 3,500,000 psi 500,000 psi 1,200,000 psi
Coefficient of
Thermal Expansion
2.5 x 10^-5
(flow direction)
1.0 x 10^-5
​(flow direction)
0.3 x 10^-5
​(flow direction)
2.8 x 10^-5
​(flow direction)
1.1 x 10^-5
​(flow direction)

Source: VICTREX® and SABIC
Values were rounded for readability. Tested values for purchased materials may vary. 

Electrical Property and Environmental Influences Comparison

Due to their electrical properties, both materials are widely used for electrical components including semiconductor test sockets and electrical connectors. Ultem® boasts (or could if it was able to talk) the highest dielectric strength of any commercially available thermoplastic at 830 V/mil (per ASTM D149). PEEK’s reported dielectric strength is 480 V/mil.

For applications requiring lower levels of electrical resistivity, electro-static dissipative and anti-static material grades are offered.

Both materials are resistant to steam and are suitable for use in reusable medical components subject to repeating autoclave cycles. PEEK has gained widespread popularity in downhole oil and gas applications due to its resistance to harsh chemicals including H2S. While Ultem® is also resistant to a broad spectrum of chemicals, its brittle nature makes it more prone to environmental stress cracking. Ultem®'s compatibility with aromatic and chlorinated hydrocarbons, ketones, esters, and strong bases, such as sodium hydroxide, is comparatively limited.

Ultem® has been found to experience a negligible change in mechanical properties following prolonged exposure to UV radiation. PEEK is affected by outdoor weathering, however, data suggest the effects on mechanical properties are minimal for at least a year and reducible by inclusion of a color pigment (Table 2).


Source: McKeen

Heiland et al. studied the radiation resistance of various high performance engineering plastics and found PEEK (and other polyaryletherketones) to exhibit the highest levels of radiation resistance followed by polyimides, including Ultem®.

Other Key Characteristics: Wear Performance, Flammability, Industry Compliances, and Relative Cost

For wear applications, PEEK is generally preferred due to much higher limiting PV capabilities and a significantly lower wear rate. Specialty bearing grades of PEEK containing filler packages with various loadings of PTFE, carbon-fiber, and graphite powder are also available to reduce friction and improve wear life.

Both materials are flame-retardant and hold UL 94 V-0 flammability ratings from thin cross-sections. Ultem® has also been rated 5VA at 1.6mm and exhibits a higher Limiting Oxygen Index (LOI) of 47%.

Standard PEEK and Ultem® grades are FDA compliant. Both are also readily available in USP Class VI compliant grades for medical applications. These materials can meet many other industry compliances, but the full list is outside the scope of this article.

It is also worth noting that natural Ultem® is translucent amber while natural PEEK is an opaque tan color. Both materials are available in natural and black color in rod and plate. Natural-colored PEEK tube is also readily available.

Lastly, PEEK stock shapes cost roughly three times the price of their Ultem® counterparts. For seals or rings (especially of larger diameters), this price differential is often mitigated due to the widespread availability of PEEK tube stock, however.

Hopefully, you found the above helpful in differentiating between Ultem® and PEEK. While both materials offer high performance at elevated temperatures and even share some common applications, their characteristics are often distinct enough to render one candidate more suitable for a particular end use.

The preceding information is intended to be a quick comparison of key material properties for screening purposes during material selection procedures and not a comprehensive discussion of each material. 

About the author

Dave Seiler is Business Development Manager at Curbell Plastics who focuses on high-performance engineering plastics. He has literally been involved with plastics his whole life (his first word was “Kynar”) and spends much of his free time reading academic literature on a diverse base of polymer materials. Yeah, he’s that guy. Dave is ready to take your call, set up a webinar, or travel to your facility to discuss your unique application needs and help identify cost-effective engineered plastic candidate materials. Contact Dave.

Figure 1:
Quadrant High Performance Products and Application Guide. Quadrant, 2017, pp. 9-10.

Figure 2:
Victrex Material Properties Guide. Victrex, p. 9

Table 1:
“Datasheet Downloads – Victrex.” Victrex, Victrex, 2020,

“Sabic – Ultem Resin.” Sabic, Sabic, 2020,

Table 2:
McKeen, Laurence. The Effect of UV Light and Weather on Plastics and Elastomers. 3rd ed., Elsevier, Inc., 2013, p. 395

Other Reference:
Heiland, Kirstin, et al. 

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