Surface and Bulk Degradation of Teflon® FEP Retrieved from the Hubble Space Telescope Solar Arrays

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Teflon® fluorinated ethylene propylene (FEP) is used on the Hubble Space Telescope (HST) as the outer layer of multilayer insulation (MLI) blankets. During space shuttle servicing missions (SM) to HST through thickness cracks were observed in the FEP layers. Material brought to Earth for investigation showed signs of severe degradation. During servicing mission 3B, the 4th servicing mission to HST in March 2002, the second pair of European Space Agency ESA built solar arrays was retrieved and flown back to Earth after 8.25 years in space. Samples of the MLI thermal control material were taken from the solar array drive arm (SADA) and investigated for surface and bulk degradation. The MLI was enveloped around the SADA and thus allowed the examination of FEP degradation in dependence of the orientation of space exposed areas with respect to the Sun. Therefore micrographs of FEP surfaces and fractographs of through thickness cracks were taken and the solar absorptance αS and the normal emittance εN was measured on the entire MLI. Differential scanning calorimetrical analyses as well as electron spectroscopy for chemical analyses were conducted to analyze chemical changes of the bulk polymer and the surface layer, respectively. Tensile properties and mass losses of the space exposed material were evaluated as well. Results of the investigated thermal control material are compared with samples exposed on the ground to thermal cycling, soft X-rays and vacuum ultra-violet radiation as well as to previously reported results on space-exposed FEP. The paper thus gives further insight into the mechanism and processes contributing to the on orbit degradation of Teflon® FEP and correlates the presented data with exposure levels and exposure conditions.