Material
PEI (Polyetherimide) PEI
Sold almost universally as ULTEM. Amber, transparent, flame retardant and dimensionally stable, and the default for avionics housings and connectors where fire performance is the governing requirement.
Overview
Polyetherimide is an amorphous high-performance thermoplastic best known by the ULTEM trade name. It combines high strength and stiffness with inherently excellent flame, smoke and toxicity performance, which is why it dominates aircraft interior and avionics hardware where those properties are regulated rather than merely desirable. Being amorphous, it is transparent, easy to mould into complex thin-walled shapes, and dimensionally very stable, but it is more susceptible to stress cracking in aggressive solvents than semi-crystalline PEEK. It is substantially cheaper than PEEK and polyimide, which is why it wins the large volume enclosure and connector work rather than the extreme-temperature applications.
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Defense applications
Avionics enclosures and housings
Electronics boxes and housings where flame, smoke and toxicity performance is regulated and where a metal enclosure would add weight without adding function.
Avionics boxes, Line replaceable unit housings, Display bezels, Equipment trays
Electrical connectors and insulators
Connector bodies, terminal blocks and insulating hardware needing dimensional stability, dielectric strength and flame resistance together.
Connector bodies, Terminal blocks, Circuit board standoffs, Bus bar insulation
Aircraft interior components
Panels, ducting, brackets and fittings where cabin fire regulations effectively mandate the material class.
Interior panels, Air ducting, Seat components, Latches and fittings
Sterilisable and fluid-handling hardware
Components exposed to repeated sterilisation or to hydraulic and fuel system fluids.
Fluid handling components, Sterilisable hardware, Medical and field equipment
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Unfilled PEI (ULTEM 1000) | Amorphous polyetherimide | The base transparent amber grade, the reference material for enclosures and insulators. | Polymer Resin, Stock Shape, Molded Component | |
| 30% Glass Filled PEI (ULTEM 2300) | Glass fiber reinforced polyetherimide | Roughly triples stiffness and cuts thermal expansion, at the cost of transparency and some toughness. | Polymer Resin, Stock Shape, Molded Component | |
| ULTEM 9085 | PEI blend for additive manufacturing | Formulated for fused deposition additive manufacturing with certified flame, smoke and toxicity performance, which is why it dominates printed aircraft interior parts. | Polymer Resin |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 1.27 | g/cm3 | Unfilled |
| Tensile strength | 105 to 115 | MPa | Around 16 ksi, higher than unfilled PEEK |
| Glass transition temperature | 217 | degrees C | High for an amorphous polymer |
| Continuous service temperature | approximately 170 | degrees C | Below PEEK, well above commodity plastics |
| Flammability | UL 94 V-0 unfilled | Inherently flame retardant with very low smoke generation | |
| Optical transmission | Transparent, amber | Amorphous, so it can be moulded into transparent parts | |
| Dielectric strength | High and stable | Stable across frequency and temperature | |
| Relative cost | Roughly a third to a half of PEEK |
Advantages
- Outstanding flame, smoke and toxicity performance without additives
- Transparent and easily moulded into complex thin-walled shapes
- High dimensional stability and low creep
- Excellent and stable dielectric properties
- Substantially cheaper than PEEK or polyimide
- Sterilisable and hydrolytically stable
Limitations
- Amorphous, so it is prone to stress cracking in aggressive solvents where PEEK is not
- Lower continuous service temperature than PEEK or polyimide
- Notch sensitive, so sharp internal corners must be avoided in design
- Poor ultraviolet resistance without protection
- Principal resin producer is foreign-owned, which some programs weigh
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM D5205 | ASTM International | Polyetherimide moulding and extrusion materials |
| UL 94 V-0 | Underwriters Laboratories | Flammability classification met by unfilled PEI |
| FAR 25.853 | Federal Aviation Regulations | Cabin interior flammability requirements that drive PEI selection |