Ultem PEI Comparisons — vs PEEK, Polycarbonate & PSU
Specifying Ultem (polyetherimide, PEI) requires understanding how it stacks up against the three most frequently substituted alternatives: PEEK (the higher-tier high-performance option), polycarbonate (the lower-cost transparent option), and polysulfone/PPSU (the closest-comparable amber plastic in the same price tier). Each comparison surfaces distinct trade-offs across temperature, flame performance, chemical resistance, cost, and machinability.
At a glance:
- Ultem vs PEEK: Ultem is 30–50% lower cost; PEEK wins on temperature (480°F vs 340°F), chemical resistance, and fracture toughness
- Ultem vs Polycarbonate: Ultem wins on temperature, flame/smoke, and autoclave; PC wins on optical clarity, impact, and cost
- Ultem vs PSU / PPSU: Closest comparison in price and properties; Ultem wins on stiffness and temperature; PPSU edges Ultem on autoclave impact toughness
- All four materials are available as sheet, rod, and tube
- The most frequent engineering question: can Ultem replace PEEK to save cost? Answer depends on temperature and chemical environment.
Ultem vs PEEK
PEEK (polyether ether ketone) and Ultem are the two most-specified high-performance thermoplastics in aerospace, medical, and semiconductor applications. They are often evaluated side-by-side, and the question of substitution arises regularly when budgets are constrained.
Where Ultem Wins
Cost. Ultem 1000 rod and sheet typically prices 30–50% below comparable PEEK stock. For large components, high quantities, or prototype work where material cost is significant, the savings can be substantial.
Inherent flammability. Both carry UL94 V-0, but Ultem's NBS smoke density numbers are lower than PEEK's in standardized aircraft interior testing. For FAR 25.853 aircraft cabin applications, Ultem is the material of record across most commercial aircraft fleets. PEEK also passes FAR 25.853 but is generally not used in cabin interiors due to cost.
Dielectric performance. Ultem's dielectric constant (3.15 at 1 MHz) is slightly lower than PEEK's (~3.2–3.3), with a lower dissipation factor. For high-frequency signal applications, Ultem provides marginally better electrical isolation efficiency.
Where PEEK Wins
Continuous-use temperature. PEEK is rated to 480°F (250°C) continuous; Ultem reaches 340°F (171°C). For any application operating above 340°F—turbine-adjacent aerospace hardware, industrial sterilizers above 300°F, semiconductor thermal processes above 170°C—Ultem cannot substitute.
Chemical resistance. PEEK's semicrystalline structure provides broad solvent resistance, including to chlorinated solvents, ketones, and concentrated acids, where Ultem is attacked. In any solvent-exposure environment, verify Ultem's compatibility before specifying.
Fracture toughness. PEEK's notched Izod impact (~1.3 ft-lb/in) exceeds Ultem's (~1.0 ft-lb/in), and PEEK's elongation at break (~35%) far exceeds Ultem 1000 (~60% in some tests, but much lower in practice for thick sections). In impact-prone or cyclically stressed applications, PEEK is more forgiving.
Fatigue resistance. PEEK's semicrystalline structure provides better fatigue life under cyclic mechanical loading. For dynamically loaded parts (vibrating fixtures, cyclic-pressure seals), PEEK is preferred.
For the full head-to-head property and application breakdown, see the dedicated PEEK vs Ultem PEI comparison.
Ultem vs Polycarbonate / Lexan
Polycarbonate and Ultem are both amorphous thermoplastics with transparent natural color, which leads to frequent comparisons. The substitution question is almost always about whether Ultem's higher performance justifies its higher price over polycarbonate.
Where Ultem Wins
Temperature. Ultem's 340°F continuous-use temperature is 100°F above polycarbonate's 240°F limit. Any application operating above 240°F, or requiring brief excursions above 260°F, eliminates polycarbonate.
Flammability. PC in its standard grades is only UL94 V-2 or HB. PC-FR grades with halogenated or phosphorus additives can achieve V-0, but at cost, mechanical property compromise, and regulatory risk. Ultem is V-0 inherently and achieves FAR 25.853 compliance without additives—disqualifying polycarbonate from most aircraft interior applications.
Autoclave sterilization. Polycarbonate distorts and degrades in as few as 10–50 steam autoclave cycles. Ultem is rated to 2,000+. For any application requiring repeated steam sterilization, Lexan is not a viable option.
Chemical resistance to specific agents. While neither Ultem nor PC have broad solvent resistance, Ultem handles dilute alkaline solutions (common in medical cleaning protocols) better than standard PC.
Where Polycarbonate Wins
Optical clarity. Standard PC achieves 88% light transmission. Ultem 1000 is transparent amber. If true optical transparency for glazing, guards, or light transmission is required, PC is the default choice.
Impact strength. PC's notched Izod impact of 12–17 ft-lb/in makes it nearly unbreakable under normal conditions. Ultem at ~1.0 ft-lb/in is brittle by comparison.
Cost. PC sheet and rod are substantially less expensive than Ultem. For applications where Ultem's temperature, flame, or sterilization properties are not required, PC is the cost-correct choice.
Ultem vs Polysulfone / PPSU
Polysulfone (PSU, sold as Udel) and polyphenylsulfone (PPSU, sold as Radel) are the closest comparison to Ultem in the high-performance thermoplastic market. Both are amorphous, naturally amber or translucent, autoclavable, and positioned in the same market tier. The choice between them is nuanced.
Temperature and Mechanical Properties
Ultem's continuous-use temperature (340°F) slightly exceeds PPSU (350°F) and is well above standard PSU (300°F). At elevated temperature, Ultem also has higher flexural modulus and tensile strength than either PSU or PPSU, making it the better structural option under sustained load above 250°F.
Autoclave and Impact Trade-off
PPSU (Radel) is rated to 1,000+ autoclave cycles; Ultem to 2,000+. Both work for medical reuse. PPSU's higher notched Izod impact strength makes it more forgiving in drop-prone or handling-abuse environments. PSU and PPSU typically price at or below Ultem 1000; for applications where Ultem's stiffness advantage and higher cycle count are not required, polysulfone is the cost-optimal choice.
Summary: Which Material to Specify
| Condition | Specify |
|---|---|
| Service temp > 340°F | PEEK |
| FAR 25.853 aircraft interior | Ultem (first choice) |
| 2,000+ autoclave cycles required | Ultem |
| Maximum impact toughness | Polycarbonate (if temp permits) or PPSU |
| Optical clarity required | Polycarbonate |
| Cost-driven, temp < 300°F, medical | PSU / PPSU |
| Broadest chemical resistance | PEEK |
| Balanced cost and high-performance specs | Ultem 1000 |
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