Thermal Contributions to the Degradation of Teflon® FEP on the Hubble Space Telescope

Abstract

Metallized Teflon® fluorinated ethylene propylene (FEP) thermal control material on the Hubble Space Telescope (HST) is degrading in the space environment. Teflon® FEP insulation was retrieved during servicing missions, which occurred in 1993, 1997 and 1999. During the second servicing mission (SM2), the 5 mil aluminized-FEP (Al-FEP) outer layer of multilayer insulation (MLI) covering the telescope was found to be cracked in many locations around the telescope. Teflon® FEP retrieved during SM2 was more embrittled than the FEP retrieved 2.8 years later from a different location, during the third servicing mission (SM3A). Studies have been conducted to understand the degradation of FEP on HST, and the difference in the degree of degradation of FEP from each of the servicing missions. The retrieved SM2 material experienced a higher temperature extreme during thermal cycling (200 °C) than the first servicing mission (SM1) and SM3A materials (upper temperature of 50 °C), therefore an investigation on the effects of heating FEP was also conducted. Samples of pristine FEP and SM1, SM2 and SM3A retrieved FEP were heated to 200 °C and evaluated for changes in properties. Heating at 130 °C was also investigated because FEP bi-stem thermal shields are expected to cycle to a maximum temperature of 130 °C on-orbit. Tensile, density, x-ray diffraction crystallinity and differential scanning calorimetry data were evaluated. It was found that heating pristine FEP caused an increase in the density and practically no change in tensile properties. However, when as-retrieved space samples were heated, the density increased and the tensile properties decreased. Upon heating, all samples experienced an increase in crystallinity, with larger increases in the space-exposed FEP. These results indicate that irradiation of FEP in space causes chain scission, resulting in embrittlement, and that excessive heating allows increased mobility of space-environment-induced scissioned chains. Thermal exposure was therefore found to have a major impact on the extent of embrittlement of FEP on HST.