PEI / Ultem

Overview

PEI (polyetherimide) is the amorphous high-performance thermoplastic that sits between engineering plastics (PC, PSU) and the semi-crystalline high-temperature thermoplastics (PEEK, PPS). Sabic’s Ultem is synonymous with the polymer — much like Delrin for POM or Lexan for PC. The defining property mix:

1. Glass transition at ~217°C — the highest of any commercial amorphous thermoplastic. Continuous service temperature of 170°C means PEI bridges the gap between PC (~115°C) and PEEK (~250°C) without the cost or processing challenge of PEEK.
2. Inherent UL94 V-0 with no flame retardant additives — and an oxygen index of ~47%, among the highest of any thermoplastic. This is a chemistry property, not a compounding choice.
3. Highest dielectric strength of common thermoplastics (~830 V/mil). PEI is the default specification for high-voltage electrical insulators where temperature, dielectric strength, and flame behavior all matter.
4. Translucent amber color, even unfilled. PEI is one of only a few high-performance plastics that’s semi-transparent — useful for optical inspection of parts and a recognizable visual signature.
5. Hydrolytic stability through hundreds of steam autoclave cycles. The default material for autoclavable medical instruments at 135°C / 0.2 MPa repeated sterilization.
6. Gamma- and e-beam-radiation tolerant to ~50 kGy without significant property loss — significant for medical and aerospace applications.

The selection drivers for PEI versus competing materials:

• vs PC — pick PEI when continuous service is above 115°C, flame rating is critical, or autoclave sterilization is required. PC wins on cost and impact resistance.
• vs PSU — PEI is stiffer and stronger; PSU is tougher and cheaper. PEI typically wins on temperature; PSU wins on impact.
• vs PEEK — PEI is cheaper, easier to machine, and semi-transparent. PEEK wins on continuous-service temperature, chemical resistance, and semi-crystalline mechanical properties retention to 250°C.
• vs PPS — PEI is more dimensionally stable (amorphous, lower CTE), has higher dielectric strength, and is autoclavable. PPS wins on chemical resistance and continuous-service temperature for semi-crystalline applications.

Machining notes

PEI is one of the more forgiving high-performance plastics to machine — amorphous structure means no semi-crystalline transition behavior to manage, and the high Tg (217°C) means frictional heat doesn’t soften the cut surface at typical machining temperatures.

Practical recipe for unfilled Ultem 1000 stock:

• Sharp carbide tooling, polished edges
• Speed: 300–800 SFM
• Feed: 0.005–0.015 in/rev
• Coolant: recommended for production, air-blast OK for light cuts
• No annealing required for unfilled — stock shapes are pre-stress-relieved

Unlike PC, residual machining stress in PEI does not cause ESC in most common environments — IPA wipe-down, water-based cleaners, and typical shop chemistries don’t trigger crazing. This is one of PEI’s quiet practical advantages: it doesn’t require the post-machining anneal that PC needs for solvent-exposed service.

Glass-filled Ultem 2300 is abrasive but manageable with carbide. Expect 40–60% reduction in tool life versus unfilled. Carbon-filled Ultem 7800 is significantly more abrasive — PCD tooling for production runs.

Bonds well with structural epoxies (3M Scotch-Weld DP-460, Loctite EA E-20HP), cyanoacrylates, and UV-cure adhesives. Surface preparation (light abrasion, IPA clean) is usually sufficient — no plasma treatment required. This is a practical advantage over PEEK, which needs surface activation for most adhesives.

Process caveats and FDM

PEI’s FDM story is more developed than nearly any other high-performance thermoplastic. Ultem 9085 and Ultem 1010 are mature industrial FDM materials with established print parameters, well-documented mechanical properties, and (in 9085’s case) FAR 25.853 aerospace flame certification for aircraft interior parts.

Practical FDM requirements:

• All-metal hotend rated 350–380°C
• Heated chamber 90°C minimum (130°C+ preferred for 1010)
• Bed temperature 130–160°C
• Filament drying mandatory — 150°C for 4+ hours immediately before printing
• Stratasys Fortus, AON3D, and similar industrial machines

Ultem 9085 is the FDM grade qualified for aircraft interior parts — ducting, brackets, structural components on Boeing and Airbus airframes. The combination of FAR 25.853 compliance, autoclavability, and ability to print complex geometries makes it nearly unique. Cost per kilogram of filament is dramatically higher than pellet form (often $200–400/kg), reflecting the certification overhead.

Ultem 1010 has higher mechanical and thermal performance, NSF 51 food- contact certification, and is used for FDM end-use parts where heat tolerance and chemical resistance matter.

The PEI vs PEEK decision

This is the single most common high-performance plastic selection question once you’ve decided you’re outside engineering-plastic territory.

Pick PEI / Ultem when:

• Continuous service is 100–170°C (above PC but below PEEK’s sweet spot)
• Cost matters — PEI runs ~25–50% the price of PEEK
• Transparency is useful for inspection
• Inherent V-0 with no FR additives is a selection driver
• Aerospace FDM (Ultem 9085) is the part-making approach

Pick PEEK when:

• Continuous service exceeds 200°C
• Chemical resistance to concentrated solvents, fuels, or hot caustic is critical
• Semi-crystalline mechanical-property retention to elevated temperature is required
• Tribological performance (bearings, seals at temperature) is the driver

The practical default in machined stock is PEI — Ultem 1000 covers more applications at lower cost than PEEK and is easier to source.

Failure modes worth designing around

Chlorinated solvent attack is the dominant chemical failure mode. Methylene chloride is the documented ESC agent. Partial swelling occurs in DMSO and dimethylacetamide at elevated temperature. Most other common chemistries — IPA, ethanol, water-based cleaners, fuels, oils, typical acids and bases — are fully tolerated.

Concentrated hot caustic (NaOH, KOH >60°C) hydrolyzes the imide linkage. Not for hot caustic CIP service. For aggressive base chemistry, PEEK or fluoropolymers are the right choices.

Notch sensitivity — unfilled PEI’s notched Izod is moderate (~1.0 ft·lb/in). Sharp internal corners are crack initiation sites in impact-loaded service. Radius generously, especially in glass-filled grades where elongation drops to 3–6%.

UV yellowing in unstabilized PEI occurs within months of direct sunlight. The amber color masks early yellowing but elongation drops detectably within a year. Use UV-stabilized formulations or carbon-filled grades for outdoor structural service.

Long-term hot-water immersion — PEI tolerates repeated steam autoclave cycles (well-documented to hundreds of cycles), but continuous immersion in hot water for months degrades the material over time. Cyclic exposure is fine; continuous boiling-water immersion is not.

Variant selection guidance

• Ultem 1000 (unfilled) — the default. Machined stock, general engineering use, semi-transparent applications.
• Ultem 2300 (GF30) — structural grade. Stiffness and dimensional stability matter more than impact or ductility.
• Ultem HU1000 / HU2000 (medical) — USP Class VI biocompatible. Steam autoclavable hundreds of cycles. Premium pricing.
• Ultem 9085 (FDM aerospace) — FAR 25.853 flame-rated. Aircraft interior parts, ducting, brackets.
• Ultem 1010 (FDM high-strength) — NSF 51 food-contact, higher thermal and mechanical performance than 9085. Food, medical, general FDM end-use parts.
• Ultem 7800 (CF30) — maximum stiffness-to-weight. Semiconductor fixturing, aerospace structural. PCD tooling for production machining.

Applications by industry

• Medical and surgical — sterilizable instrument handles, surgical tool housings, sterilization trays, orthopedic instrumentation. The combination of steam autoclave tolerance, gamma/e-beam compatibility, and USP Class VI grades makes PEI a default specification.
• Aerospace — Ultem 9085 for FDM-printed aircraft interior parts (ducting, brackets, interior panels). Ultem 1000 for thermoformed interior trim. Compliant with FAR 25.853 flammability requirements.
• Semiconductor — chip burn-in sockets, wafer-handling tools, test fixturing. Combination of dimensional stability, dielectric strength, and chemical resistance to clean-room chemistries.
• Electrical and electronic — high-voltage insulators, connector housings, breakers, switchgear. The dielectric strength (~830 V/mil) is among the highest of any thermoplastic.
• Pharmaceutical — production equipment components, filling machinery, autoclavable fixtures. IPA wipe-down resistance is a practical advantage over PC.
• Food processing — Ultem 1010 (NSF 51) is used for FDM food- contact end-use parts; unfilled Ultem 1000 covers heated food- handling components.
• Chemical processing — moderate-temperature pump components, valves, fittings. PEEK or PPS is preferred above 200°C continuous.