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.

Degradation of Hubble Space Telescope Aluminized-Teflon Bi-Stem Thermal Shields

“…Anti-solar-facing surfaces exposed to electron and proton radiation experienced only marginal degradation in space, but solar-facing surfaces experienced severe degradation likely due to the additional radiation exposure (soft X-rays and UV radiation) combined with the increased on-orbit heating for Sun-facing surfaces. These results are consistent with prior studies that show the effect of heating on increased density and crystallinity, and thus increased embrittlement, of radiation exposed FEP [11,14,15].…”

“…These results are consistent with de Groh et al who show pristine FEP increases in density with 200 1 C heating, but that FEP from HST has greater increases in density for the same heat treatment [12]. This is attributed to irradiation-induced scission of bonds in space, which allows for greater mobility and crystallization upon heating than that which occurs with non-irradiated FEP [11,14]. Density measurements of heated irradiated FEP have been found to be an effective tool for providing information on the extent of scission damage, which is particularly helpful for samples that are too small for tensile testing.…”

Mechanical Properties of Teflon® FEP Retrieved from the Hubble Space Telescope

“…All samples were taken directly from the heated ovens into ambient room temperature, receiving similar quick cooling rates. A two week (336 h) exposure was chosen as the typical goal for the heat treatments based on previous testing that indicated the majority of changes occur within the first 72 h of heating [5]. Therefore, heating for two weeks is well beyond the 72 h heating minimum, and the exact time of heating beyond 72 h should not be critical.…”

Characterization, performance and optimization of PVDF as a piezoelectric film for advanced space mirror concepts.

“…The multilayer insulation (MLI) blankets for the HST (made from a 127 µm thick layer of Teflon FEP with 100 nm of vapor deposited silver backing [6]) have very low α s and high ε due to the high reflection of incident solar energy by the silver backing, and good thermal emittance of FEP [8]. With the FEP layer facing away from the Earth the polymer experiences thermal cycling from -100 to > +100 °C (estimates range from 130 °C [63] to 150 °C [8]) every 96 minutes as the satellite passes in and out of the Earth's shadow. If the aluminum side of the MLI blanket, however, is facing the sun then the temperature can reach 200 °C due to the lower emittance from the aluminum surface, as happened when a part of the blanket peeled off and curled over.…”