Thermal contributions to the degradation of Teflon (R) FEP on the Hubble Space Telescope

Metallized Teflon (R) fluorinated ethylene propylene (FEP) thermal control material on the Hubble Space Telescope (HST) is degrading in the space envi ronment. Teflon (R) 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 insulat ion (MLI) covering the telescope was found to be cracked in many locations around the telescope. Teflon (R) FEP retrieved during SM2 was more embrittl ed than the FEP retrieved 2.8 years later from a different location, during the third servicing mission (SM3A). Studies have been conducted to underst and the degradation of FEP on HST, and the difference in the degree of degr adation of FEP from each of the servicing missions. The retrieved SM2 mater ial experienced a higher temperature extreme during thermal cycling (200 de greesC) than the first servicing mission (SMI) and SM3A materials (upper te mperature of 50 degreesC), therefore an investigation on the effects of hea ting FEP was also conducted. Samples of pristine FEP and SM1, SM2 and SM3A retrieved FEP were heated to 200 degreesC and evaluated for changes in prop erties. Heating at 130 degreesC was also investigated because FEP bi-stern thermal shields are expected to cycle to a maximum temperature of 130 degre esC on-orbit. Tensile, density, x-ray diffraction crystallinity and differe ntial scanning calorimetry data were evaluated. It was found that heating p ristine FEP caused an increase in the density and practically no change in tensile properties. However, when as-retrieved space samples were heated, t he density increased and the tensile properties decreased. Upon heating, al l samples experienced an increase in crystallinity, with larger increases i n the space-exposed FER These results indicate that irradiation of FEP in s pace causes chain scission, resulting in embrittlement, and that excessive heating allows increased mobility of space-environment-induced scissioned c hains. Thermal exposure was therefore found to have a major impact on the e xtent of embrittlement of FEP on HST.