Vespel

  Vespel is the trademark of a durable high-performance polyimide-based polymer (or plastic) manufactured by DuPont ^[1].

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Characteristics and applications

This rather expensive polymer is mostly used in aerospace, semiconductor and transportation technology. It combines heat resistance, lubricity, dimensional stability, chemical resistance, and creep resistance, to be used in hostile and extreme environmental conditions.

Unlike most plastics, it does not produce significant outgassing even at high temperatures, which makes it useful for lightweight heat shields and crucible support. It also performs well in vacuum applications, down to extremely low cryogenic temperatures. However, Vespel tends to absorb a small amount of water, resulting in longer pump time while placed in a vacuum.

Although there are polymers superseding polyimide in all of these properties, the combination of them is the main advantage of Vespel.

Magnetic properties

Another interesting property makes vespel a material of choice in designing high-resolution NMR probes for NMR spectroscopy: its volume magnetic susceptibility is a very close match to that of water at room temperature. For example, in SI units, volume magnetic susceptibility of water at 20 °C is –9.03×10^{-6 [2]}, whereas volume susceptibility of vespel SP-1 at 21.8 °C was measured to be −(9.02±0.25)×10^{-6 [3]}. Negative values indicate that both water and vespel are diamagnetic. Matching volume magnetic susceptibilities of materials surrounding NMR sample to that of the solvent can reduce susceptibility broadening of magnetic resonance lines.

Processing for manufacturing applications

Vespel can be processed by Direct-Forming (DF parts) and Isostatic Molding (basic shapes - plates, rods & tubes). For prototype quantities, basic shapes are typically used for cost efficiency since tooling is quite expensive for DF parts. For large scale CNC production, DF parts are often used to reduce per part costs, at the expense of material properties which are inferior to those of isostatically produced basic shapes.

Types

For different applications, special formulations are blended / compounded. Some examples of standard polyimide compounds are:

virgin polyimide

provides operating temperatures from cryogenic to 300°C (570°F), high plasma resistance, as well as a UL rating for minimal electrical and thermal conductivity. This is the unfilled base polyimide resin. It also provides high physical strength and maximal elongation, and the best electrical and thermal insulation values. Examples of products under this category are Vespel SP-1 and Meldin 7001.

15% graphite by weight

added to the base resin for increased wear resistance and reduced friction in applications such as bearings, thrust washers, bushings, seal rings, slide blocks and other wear applications. This compound has the best mechanical properties of the graphite-filled grades, but lower than the virgin grade. Examples of products under this category are Vespel SP-21 and Meldin 7021.

40% graphite by weight

for enhanced wear resistance, lower friction, improved dimensional stability (low coefficient of thermal expansion), and stability against oxidation. Examples: Vespel SP-22 and Meldin 7022.

10% PTFE and 15% graphite by weight

added to the base resin for the lowest coefficient of friction over a wide range of operating conditions. It also has excellent wear resistance up to 149°C (300°F). Typical applications include sliding or linear bearings as well as many wear and friction uses listed above. Trade names are Vespel SP-211 and Meldin 7211.

15% moly-filled (molybdenum disulfide solid lubricant)

for wear and friction resistance in vacuum and other moisture-free environments where graphite actually becomes abrasive. Typical applications include seals, bushings, bearings, gears, and other wear surfaces in outer space, ultra-high vacuum or dry gas applications. Typical trade names found for this grade are Vespel SP-3 and Meldin 7003.

Material properties

Lower-cost alternatives

Vespel is a polyimide material and for many years was the only option for high-end polyimide parts. It is a proven material in critical applications. It is also out of reach for many applications due to its high cost. Recently, other polyimide materials such as Meldin and Plavis have brought additional options in the polyimide market and may help reduce the overall market cost of polyimide shapes.

See also

• Plastics
• polyimide

References