Insights into the damage mechanism of Teflon (R) FEP from the Hubble SpaceTelescope

Metallized Teflon(R) FEP (fluorinated ethylene propylene) thermal control m aterial on the Hubble Space Telescope (HST) has been found to be degrading in the space environment. Teflon(R) FEP thermal control blankets (space-fac ing FEP) retrieved during the first servicing mission (SM1) were found to b e embrittled on solar-facing surfaces and contained microscopic cracks. Dur ing the second servicing mission (SM2) astronauts noticed that the FEP oute r layer of the multi-layer insulation (MLI) covering the telescope was crac ked in many locations around the telescope. Large cracks were observed on t he light shield, forward shell and equipment bays. A tightly curled piece o f cracked FEP from the light shield was retrieved during SM2 and was severe ly embrittled, as witnessed by ground testing. A failure review board was o rganized to determine the mechanism causing the MLI degradation. Density, x -ray crystallinity and solid-state nuclear magnetic resonance (NMR) analyse s of the FEP retrieved during SM1 were inconsistent with results of FEP ret rieved during SM2. Because the retrieved SM2 material was curled while in s pace, it experienced a higher temperature extreme during thermal cycling, e stimated at 200 degrees C, than the SM1 material, estimated at 50 degrees C , An investigation on the effects of heating pristine FEP and FEP retrieved from the HST was therefore conducted. Samples of pristine, SMI and SM2 FEP were heated to 200 degrees C and evaluated for changes in density and morp hology. Elevated temperature exposure was found to have a major impact on t he density of the retrieved materials. The characterization of the polymer morphology of the as-received and heated FEP by NMR provided results that w ere consistent with the density results. Differential scanning calorimetry (DSC) was conducted on pristine, SM1 and SM2 FEP DSC results provided evide nce of chain scission and increased crystallinity in the space exposed FEP, which supported the density and NMR results. Samples exposed to simulated solar hare x-rays, thermal cycling and long-term thermal exposure provided information on the environmental contributions to degradation. These findin gs have provided insight into the damage mechanisms of FEP in the space env ironment.