Vespel Polyimide Properties — Thermal, Mechanical & Electrical

Vespel polyimide holds a singular position in the plastics landscape: it is the highest continuous-use-temperature unfilled plastic available in machined stock form, rated at 550°F (288°C) continuous with no melt transition at any temperature. Understanding why requires looking at the polymer chemistry, the sintering process, and what "no melt point" actually means for a load-bearing part. This page covers all three property families — thermal, mechanical, and electrical — for SP-1 (unfilled), SP-21 (15% graphite), and SP-22 (40% graphite).

At a Glance

• Continuous-use temperature: 550°F (288°C) — highest of any unfilled, machinable plastic
• No melt point: sublimes near 500°C without entering a liquid phase
• Tensile strength (SP-1): 12,500 psi; maintained above 400°F unlike PEEK
• Compressive strength (SP-1): 22,000 psi — load-bearing without cold flow
• CTE: ~28 × 10⁻⁶ in/in/°F — low for a plastic; design accordingly
• Volume resistivity (SP-1): >10¹⁶ Ω·cm — excellent insulator through full temperature range
• Density: 1.43 g/cc (SP-1) to 1.65 g/cc (SP-22)

Thermal Properties — The Defining Advantage

Continuous Use Temperature: 550°F (288°C)

Most engineering plastics are limited not by chemistry but by the onset of creep under sustained load. PEEK reaches its continuous limit at 480°F; Torlon PAI at approximately 500°F. Vespel's sintered polyimide structure allows it to sustain mechanical load to 550°F continuously without measurable creep, stress relaxation in seal rings, or loss of interference fit in press-fit bushings.

This 50–70°F margin over its nearest competitors is the primary design reason to accept Vespel's cost premium. In jet engine hot-section components or semiconductor process equipment that runs at 500°F process temperature, no alternative exists.

Short-Term Excursions: 900°F (482°C)

Vespel can survive brief thermal excursions to 900°F without structural failure. "Brief" means seconds to low minutes at temperature — not sustained operation. This capacity is important for aerospace components that must survive an engine fire-zone qualification test or an unplanned over-temperature event without cracking or deforming beyond functional limits.

No Melt Point — What This Means in Practice

Conventional thermoplastics have a melt temperature at which the polymer chains disengage from crystalline order and the material loses load-bearing capacity rapidly. PEEK melts at 343°C (649°F); Nylon at around 220°C (428°F). A bearing running hotter than its melt point fails suddenly.

Vespel is neither a true thermoset nor a thermoplastic in the conventional sense. Its imide backbone is so thermally stable that the material does not enter a liquid phase before it begins to thermally decompose. At approximately 500°C, it sublimes — converting directly from solid to vapor — without ever reaching a viscous or liquid state. For a bearing or seal, this means failure is gradual degradation of surface properties at extreme temperature, not catastrophic softening and loss of geometry.

Cryogenic Performance

Vespel does not become brittle at cryogenic temperatures. Tested down to liquid nitrogen temperatures (−196°C / −321°F), it retains adequate toughness and dimensional stability for rocket propulsion applications, satellite mechanisms, and cryogenic valve seats. Very few polymers cover both ends of this range — from cryogenic to 550°F continuous — in a single material.

Mechanical Properties

Tensile and Compressive Strength

Vespel SP-1's tensile strength of 12,500 psi is modest compared to filled PEEK or Torlon at room temperature. The critical distinction is strength retention at temperature: while PEEK loses 30–40% of tensile strength approaching 480°F, Vespel retains a higher fraction of its room-temperature strength through 550°F. For a bearing or seal, the relevant number is strength at operating temperature, not at room temperature.

Compressive strength is more important for most Vespel applications than tensile strength. SP-1's 22,000 psi compressive strength allows it to support high contact stresses in thrust washers and journal bearings without cold flow — the gradual extrusion of material under sustained compressive load that limits softer plastics in bearing service.

Creep Resistance

Creep resistance is where Vespel separates from all thermoplastic competitors. Under sustained compressive load at 500°F, Vespel SP-1 exhibits creep rates orders of magnitude lower than PEEK or Torlon. This is fundamental to seal ring applications: an O-ring energized Vespel seal ring holds its squeeze over service life; a PEEK ring in the same environment would relax and leak.

Hardness and Brittleness

Vespel is harder and more brittle than PEEK or Torlon. Rockwell E hardness for SP-1 is approximately 45 — high enough that conventional milling with HSS tooling causes rapid tool wear and surface cracking. Sharp carbide or diamond tooling with careful chip load management is required. See the Vespel machining guide for complete tooling recommendations.

Friction and Wear

SP-1 unfilled has a coefficient of friction of approximately 0.35 against steel without lubrication — acceptable for many bearing applications but not ideal for high-PV service. SP-21 and SP-22 use graphite as a solid lubricant, reducing CoF to approximately 0.12–0.18 depending on grade and load. SP-22's 40% graphite loading produces the best wear life at high contact pressure but at the cost of tensile and flexural strength.

For grade selection based on PV (pressure × velocity) loading, see the Vespel grades page.

Electrical Properties

SP-1 Vespel is a class-leading electrical insulator through the full operating temperature range. This is distinct from most filled grades — graphite is electrically conductive, so SP-21 and SP-22 have significantly lower resistivity. If your application requires both electrical insulation and elevated temperature, SP-1 is the only Vespel grade appropriate.

These electrical properties make SP-1 Vespel the first-choice insulator for:

• Semiconductor wafer carrier arms in plasma etch chambers where both RF isolation and process-temperature stability are required
• High-voltage standoffs in power electronics operating above PEEK's temperature limit
• Probe card components in IC test equipment with extended burn-in cycles at elevated temperature

Chemical Resistance

Vespel has good resistance to most solvents, fuels, and process gases at elevated temperature. It resists:

• Hydrocarbon fuels and lubricating oils (including aviation jet fuel JP-4, JP-5, JP-8)
• Dilute acids and bases at moderate temperatures
• Most halogenated cleaning solvents
• High-vacuum and high-purity gas environments (very low outgassing)

Vespel is attacked by concentrated alkalis, steam at very high temperature (above approximately 300°F), and certain strong oxidizing acids. It is not appropriate for prolonged steam service — a common application error when engineers confuse "high-temperature plastic" with "steam-compatible."

Dimensional Stability

The combination of low CTE, negligible moisture absorption (<0.01% water absorption for SP-1), and absence of a melt transition produces dimensional stability across temperature swings that no competing plastic matches. Parts machined to tolerance at room temperature hold that tolerance at 400°F — critical for precision semiconductor fixtures, bearing fits in jet engine assemblies, and vacuum chamber sealing faces.

For design calculations, use a CTE of 28 × 10⁻⁶ in/in/°F for SP-1 and account for the full temperature range from ambient installation temperature to maximum operating temperature. Vespel's CTE is isotropic for rod and tube stock (no fiber reinforcement directionality), simplifying calculations.

For stock availability and pricing on Vespel rod and tube in all three SP grades, see the Vespel line card.

More related guides

Cross-cluster suggestions to help shoppers and engineers explore adjacent topics:

Applications

• Bushings Bearings

Industries

• Aerospace
• Defense Military

Compare to other materials

• Torlon Vs. Vespel
• Vespel Vs. Peek

Frequently asked questions — Polyimide Vespel FAQ