DuPont Vespel® Polyimide
Vespel® is the DuPont trade name for a group of materials used to make a wide variety of components. These include fasteners such as screws, flat washers, hex nuts, hex head cap screws, etc. Vespel® is used to make custom parts such as valve seals, thrust washers, pintle tips and actuator guide bushings. It is a high performance plastic used principally in the semiconductor, aerospace and in transportation applications. In this blog we will focus on the use of Vespel® in the aerospace industry. In particular, we will discuss the use of the SP1 and SP21 grades of the material in aerospace seats and seals for valves.
Metals are considered stiff materials with a relatively high modulus. Metal to metal valve seals can prevent the two mating parts from conforming to the very small irregularities between them. Therefore, create leak paths can occur. Also, metal components that are required to slide together may also cause gall especially in instances where liquid lubricants cannot be used. Debris from galling can cause contamination and therefore the system will not preform properly. Excessive galling could lead the valve to seize and fail.
High performance engineering thermoplastics have a moderate modulus at room temperature. Therefore, at room temperature components made from these thermoplastics such as nylon, PCTFE and PEEK conform to minor irregularities in the metal mating surface. These plastics also can have low friction when sliding against a metal surface. The valve will run smoothly with an extended life.
However, when performing in very high temperature applications, these plastics will become soft and show accelerated creep and stress relaxation. Over time, they can also degrade and become brittle. Thermoplastics may also have high rates of thermal expansion quite different from the mating metal part(s). This can cause leak paths in the valve and potential valve failure.
Also, most of these same thermoplastics do not perform well in extremely low temperatures. They can become stiff and brittle. When cryogenic temperatures are combined with compressive loads as in valves, the plastic seal may even crack and therefore the valve eventually will fail.
Vespel® SP1 is the unfilled resin. It exhibits:
- excellent electrical insulating characteristics
- resistant to stress relaxation
- superior thermal insulation
- thermal stability
- continuous service temperature – cryogenic temperatures to 260 ° C (500 ° F)
- creep resistant
- lightweight when compared to metals
- excellent wear and friction performance
- impact resistant
- maintains more of its apparent modulus over time
- does not require external lubrication
- low and consistent thermal expansion and contraction rates.
SP21 is the 15% graphite-filled grade of Vespel®. It has all the properties of the SP1 unfilled grade and:
- a lower coefficient of friction improving its dimensional stability
- is compatible with liquid oxygen systems
- high auto ignition temperature
DuPont® Vespel® SP1 is a thermoset polyimide. Its maintains significant modulus (stiffness) even at temperatures as high as 260° C (500° F) with short term temperatures reaching 900° F. It maintains much of its apparent modulus over time even in applications where high pressure and high temperatures are involved. This make it an excellent choice for valve seats and seals in aerospace applications. It will also perform well in cryrogenic temperatures.
Uses for Vespel ® SP21:
Some aerospace valves must operate high in the atmosphere, in polar climates or in space. Therefore the seals are exposed to extremely cold temperatures. PCTFE, which is a common valve seat material, will become brittle at temperatures below 150 kelvin. The compressive strength of the Vespel® SP21 grade in particular remains consistent when cooled to cryogenic temperatures. Therefore, the seals retain their ductility and are able to maintain a high pressure seal without showing brittle fracture.
Other grades of Vespel® have been approved by NASA for different high performance spacecraft valve uses including the Ares I Upper Stage J-2X engine.
It is important to evaluate the conditions the Vespel® parts must perform in such as the topics covered above but also what chemicals are present and any other factors. This will help you decide which of the many grades of Vespel® would work best in your application.
Note: Do not use Vespel® in medical applications in which the material is implanted in the human body or comes into contact with internal body fluids or tissue.
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