Kapton is a polyimide film developed by DuPont which can remain stable in a wide range of temperatures, from -273 to +400 °C (0 – 673 K).[1] Kapton is used in, among other things, flexible printed circuits (flexible electronics) and thermal micrometeoroid garments, the outside layer of space suits.
The chemical name for Kapton K and HN is poly(4,4'-oxydiphenylene-pyromellitimide). It is produced from the condensation of pyromellitic dianhydride and 4,4'-oxydiphenylamine. Kapton synthesis is an example of the use of a dianhydride in step polymerization. The intermediate polymer, known as a "poly(amic acid)," is soluble because of strong hydrogen bonds to the polar solvents usually employed in the reaction. The ring closure is carried out at high temperatures (200–300 °C, 473-573 K).
Kapton-insulated wiring has been widely used in electrical wiring of civil and military aircraft because of it is lighter than other insulators and has good insulating and temperature characteristics. For these reasons, it is the material of which the sunshield of the James Webb Space Telescope will be made.[2] However, it was soon found to have very poor resistance to mechanical wear, mainly abrasion within cable harnesses due to aircraft movement. Many aircraft models have had to undergo extensive rewiring modifications, sometimes completely replacing all the Kapton-insulated wiring, because of short circuits caused by the faulty insulation. Kapton, according to the F.A.A., is the only wire insulating material shown to dry arc-track at 10,000 °C and has been shown to be the only material to ignite insulation blankets.
According to a NASA internal report, space shuttle "wires were coated with an insulator known as Kapton that tended to break down over time, causing short circuits and, potentially, fires."[3] The NASA Jet Propulsion Laboratory has considered Kapton as a good plastic support for solar sails because of its long duration in the space environment.[4]
Kapton is also commonly used as a material for windows of all kinds at X-ray sources (synchrotron beam-lines and X-ray tubes) and X-ray detectors. Its high mechanical and thermal stability as well as its high transmittance to X-rays make it the preferred material. It is also relatively insensitive to radiation damage.[5] Another prominent material for these purposes is beryllium.
The thermal conductivity of Kapton in temperatures from 0.5 to 5 kelvin is rather high κ = 4.638×10−3 T0.5678 W·m−1·K−1.[6] This together with its good dielectric qualities and its availability as thin sheets have made it a favorite material in cryogenics (devices working in temperatures down to absolute zero, -273 °C or 0 K).
Kapton is also regularly used as an insulator in ultra-high vacuum environments as it has a low outgassing rate.
References
- ^ X.-F. Navick et al. (2004). "Fabrication of ultra-low radioactivity detector holders for Edelweiss-II". NIM A 520: 189–192. doi:.
- ^ Nasa description of the JWST sunshield
- ^ High Tech in the 1970s, Shuttles Feel Their Age
- ^ J. L. Wright, Space Sailing, Gordon and Breach, 1992
- ^ Janez Megusar (1997). "Low temperature fast-neutron and gamma irradiation of Kapton polyimide films". Journal of Nuclear Materials 245: 185–190. doi:.
- ^ Jason Lawrence, A. B. Patel and J. G. Brisson (2000). "The thermal conductivity of Kapton HN between 0.5 and 5 K". Cryogenics 40: 203–207. doi:.
External links
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