Vespel Grades: SP-1 vs SP-21 vs SP-22 — Selection Guide
Vespel SP-series grades differ in one fundamental variable: graphite filler content. SP-1 contains no filler; SP-21 is 15% graphite by weight; SP-22 is 40% graphite. Everything else — base resin, sintering process, operating temperature range — is the same. The graphite changes friction coefficient, wear rate, PV (pressure × velocity) capacity, and electrical conductivity, which means grade selection is almost always driven by tribological and electrical requirements rather than thermal ones. This guide provides the comparison data and a grade picker by application environment.
At a Glance
- All three grades share the same 550°F continuous / 900°F short-term thermal limit
- SP-1: best electrical insulation, cleanest for vacuum/semiconductor, highest tensile strength
- SP-21: optimal wear-to-strength trade-off for most aerospace and industrial sliding applications
- SP-22: maximum PV and load-bearing; use when contact pressure × velocity exceeds SP-21's limit
- Graphite-filled grades (SP-21, SP-22) are electrically conductive — never substitute for SP-1 in insulating applications
- Vespel TP thermoplastic grades are a separate product family, not covered by this guide
Grade-by-Grade Overview
Vespel SP-1 — Unfilled Polyimide
SP-1 is the baseline sintered polyimide grade. It is amber/gold in color and contains no additives. Select SP-1 when:
- Electrical insulation is required (volume resistivity >10¹⁶ Ω·cm, dielectric strength ~500 V/mil)
- Ultra-high vacuum compatibility is needed — graphite-free, very low outgassing
- FDA or cleanroom contact is required — no filler particles to generate contamination
- Highest tensile and flexural strength within the Vespel family are needed
SP-1 has the highest friction coefficient of the three grades (CoF ~0.35 dry vs. steel), making it a less ideal choice for high-PV bearing surfaces without a supplemental lubricant film. For moderate-load bushings with a grease or film lubrication scheme, SP-1 is workable. For dry-running high-speed applications, SP-21 or SP-22 is preferred.
Vespel SP-21 — 15% Graphite
SP-21 is the most widely stocked Vespel grade and the standard choice for jet engine wear pads, fuel system backup rings, and industrial dry-running bearings. The 15% graphite reduces CoF to approximately 0.18 against steel and drops the wear factor by roughly 8× compared to SP-1, while preserving enough tensile and compressive strength for structural loading.
The graphite in SP-21 also increases thermal conductivity from SP-1's 1.7 BTU·in/hr·ft²·°F to approximately 2.4, which improves heat dissipation at the bearing interface — an important factor in high-speed sliding applications.
SP-21's PV limit of approximately 25,000 psi·ft/min covers the majority of aerospace bearing and seal applications. If your calculated PV exceeds this value, evaluate SP-22.
Vespel SP-22 — 40% Graphite
SP-22 maximizes graphite loading to the point where the graphite network becomes the dominant tribological surface. CoF drops to ~0.12 against steel, wear factor improves to roughly 50 × 10⁻¹⁰ in³/ft·lb·hr, and the PV limit reaches approximately 40,000 psi·ft/min. This makes SP-22 the choice for heavily loaded thrust washers, high-pressure valve seats, and bearing faces in downhole tools that run at high contact pressure.
The trade-off is mechanical: 40% graphite filler reduces tensile strength to approximately 8,000 psi and flexural strength to ~11,000 psi. SP-22 should not be used where tensile loading occurs, and thin-section machining is more prone to breakage than SP-21 or SP-1.
SP-22's dark gray color (from the high graphite content) also makes visual inspection of crack initiation difficult — relevant for NDE inspection planning on critical flight hardware.
Full Grade Comparison Table
PV Selection Guide
The PV limit (pressure in psi multiplied by sliding velocity in ft/min) is the primary bearing selection criterion for Vespel. Calculate the PV for your application:
[ PV = P_ \times V_ ]
Where:
- ( P_ ) = bearing load ÷ projected bearing area (psi)
- ( V_ ) = surface velocity at the bearing interface (ft/min)
Match to grade:
| Calculated PV (psi·ft/min) | Recommended Grade |
|---|---|
| ≤10,000 | SP-1 (if insulation needed) or SP-21 |
| 10,001 – 25,000 | SP-21 |
| 25,001 – 40,000 | SP-22 |
| >40,000 | Consult DuPont/IST — may require direct-formed shapes |
PV limits are empirically determined at a specific temperature (typically room temperature to 300°F). At elevated temperature, both friction coefficient and wear rate increase. For applications above 400°F, apply a conservative safety factor of 0.6–0.7 to the published PV limit and validate with a wear test before committing to a design.
Vespel TP Grades — A Brief Note
DuPont also produces Vespel TP (thermoplastic polyimide) grades, which use a modified polyimide chemistry that can be injection-molded and extruded. TP grades are fundamentally different from SP-series:
- Lower continuous use temperature — typically 250–280°C continuous depending on TP grade
- Conventional processing — injection molding, extrusion
- Lower cost per part at high volume versus direct-formed SP-series
- Different mechanical and chemical properties — consult the TP-grade datasheet separately
TP grades are not interchangeable with SP-series in aerospace OEM specifications. If a drawing calls for "Vespel SP-21," a TP grade cannot be substituted without engineering re-qualification.
Grade Selection Quick Reference
| Application | Grade | Reason |
|---|---|---|
| Semiconductor wafer carrier | SP-1 | No outgassing, electrical insulation |
| Plasma etch chamber spacer | SP-1 | Dielectric, chemical resistance |
| Jet engine vane wear pad | SP-21 | Dry sliding, moderate-to-high PV |
| Thrust washer, high load | SP-22 | Maximum PV and compressive strength |
| Fuel backup ring | SP-21 | Wear + dimensional stability |
| Cryogenic bushing | SP-1 | Toughness at low temperature |
| Dry vacuum pump bushing | SP-21 | Solid lubrication without contamination |
| Probe card standoff | SP-1 | Electrical isolation, dimensional stability |
For full properties supporting this table, see the Vespel material properties page. For specific applications by industry, see Vespel applications.
Order Vespel SP-1, SP-21, or SP-22 rod and tube
Request a Quote →Contact the materials team or visit the Vespel line card to confirm grade availability and request a quote for your specific quantity and diameter.
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