Elastomers and PTFE are not the only seal materials. Hard thermoplastics supply strength, creep resistance and wear performance, usually in the supporting role.
| Polymer | Continuous temperature (approx) | Strengths | Limits |
|---|---|---|---|
| PEEK | to 250 degrees C | Strong, stiff, chemical and hydrolysis resistant, low creep | Cost, abrasive if filled with glass |
| Polyimide (PI) | to 300 degrees C | Highest temperature, low wear, dimensionally stable | Brittle, strong alkali and hot water attack, costly |
| PPS | to 220 degrees C | Good chemical range, rigid | Brittle |
| UHMWPE | to 80 degrees C | Excellent wear, toughness, low friction | Low temperature limit, creeps |
| Nylon (PA) | to 100 degrees C | Cheap, tough, wear resistant | Absorbs water, hydrolyses in hot water or glycol |
| POM (acetal) | to 90 degrees C | Dimensionally stable, low friction | Weak in strong acids/alkali |
| PAI (Torlon-type) | to 250 degrees C | Very high strength | Moisture sensitive, cost |
Typical sealing uses
- Back-up rings: PEEK and nylon at high pressure.
- Guide rings and wear bands: filled PTFE, UHMWPE, PEEK, phenolic fabric.
- Valve seats: PEEK, polyimide, nylon, filled PTFE.
- Seal bodies: PEEK jackets for HPHT downhole seals.
- Thrust washers: PI, PAI, PEEK.
Fillers
Glass and carbon fibre raise strength and reduce creep, but increase counterface wear. PTFE and graphite fillers lower friction. Make sure the filled grade suits the shaft.
Why consider them now
Regulatory pressure on fluoropolymers (see the PFAS article) means non-fluorinated options such as PEEK and polyimide are worth qualifying in parallel for back-ups, bearings and seats.
Design pointers
- Check expansion differences with the housing.
- Specify annealed grades for dimensional stability.
- Provide lead-in chamfers; hard polymers can crack when forced over edges.
- Use the supplier's chemical data for your actual fluid.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
