Material
Polyimide PI
The top of the polymer temperature ladder, sold mainly as Vespel. Runs to 300 degrees Celsius continuously, wears almost not at all unlubricated, and is bought as finished shapes because it cannot be melt processed.
Overview
Polyimide is where polymers stop behaving like polymers. It does not melt, so it cannot be injection moulded or extruded like PEEK or PEI; parts are made by direct forming and sintering the resin powder, which means you buy semi-finished shapes or finished parts rather than resin. In exchange it operates continuously around 300 degrees Celsius, holds dimensional stability through cryogenic to high temperature swings, and has the best unlubricated wear behaviour of any polymer, which is why it appears as jet engine wear rings, seals and thrust washers running dry in places no grease would survive. It is very expensive and comes from an extremely narrow supply base, so it tends to be specified only where nothing cheaper works.
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Defense applications
Jet engine wear and seal components
Wear rings, seals and bushings in the cooler sections of a gas turbine, running unlubricated at temperatures that would carbonise oil.
Engine wear rings, Bushings and bearings, Seal rings, Vane bushings
Low friction seals and washers
Thrust washers and sliding surfaces where the alternative is a metal part needing lubrication that cannot be maintained in service.
Thrust washers, Back-up rings, Sliding seals, Piston rings
Cryogenic and vacuum hardware
Structural and insulating components stable from cryogenic temperature upward, with very low outgassing for vacuum and space use.
Cryogenic supports, Vacuum feedthrough insulation, Space mechanism components
Electrical insulation at temperature
Insulating hardware and film where operating temperature exceeds what other polymers survive.
High temperature insulators, Wire and cable insulation film, Flexible circuit substrate
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Vespel SP-1 | Unfilled polyimide | The base unfilled grade, with the best electrical properties and elongation in the family. | Stock Shape, Molded Component | |
| Vespel SP-21 | 15% graphite filled polyimide | The general purpose wear grade. Graphite loading gives the low friction unlubricated behaviour the family is specified for. | Stock Shape, Molded Component | |
| Vespel SP-211 | Graphite and PTFE filled polyimide | Lowest coefficient of friction in the family, for start-stop and marginal lubrication conditions. | Stock Shape, Molded Component | |
| Vespel SP-3 | Molybdenum disulphide filled polyimide | Formulated for vacuum and dry nitrogen service where graphite grades perform poorly without moisture. | Stock Shape, Molded Component |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 1.34 to 1.43 | g/cm3 | Grade dependent |
| Continuous service temperature | approximately 300 | degrees C | With short excursions substantially higher |
| Melting behaviour | Does not melt | The defining processing fact: no injection moulding or extrusion is possible | |
| Tensile strength | 70 to 90 | MPa | Unfilled |
| Coefficient of friction, unlubricated | Very low | Graphite filled grades run dry against metal almost indefinitely | |
| Limiting PV | Very high | Pressure-velocity limit far above other polymers; the number bearing designers use | |
| Outgassing | Very low | Suitable for vacuum and space applications | |
| Relative cost | Highest of the engineering polymers | Driven by direct forming and a very narrow supply base |
Advantages
- Highest continuous service temperature of the machinable polymers, around 300 degrees Celsius
- Best unlubricated wear and friction behaviour of any polymer
- Dimensionally stable from cryogenic to high temperature
- Very low outgassing, suitable for vacuum and space
- Excellent electrical insulation at temperature
- Does not creep or cold flow the way lower-temperature polymers do
Limitations
- Cannot be melt processed, so there is no injection moulding or extrusion route
- Supplied only as semi-finished or finished shapes; you cannot buy resin and make it yourself
- Extremely narrow supply base, effectively one dominant producer
- The most expensive engineering polymer in common use
- Absorbs moisture, which causes dimensional change that must be designed around
- Attacked by strong bases and by some hydraulic fluids
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM D6456 | ASTM International | Polyimide moulding and extrusion materials |
| AMS 3644 | SAE International | Polyimide moulded parts and shapes for aerospace |
| ASTM D3702 | ASTM International | Wear rate and coefficient of friction by thrust washer testing |