Effect of Space Exposure on the Tensile Properties of MISSE Teflon Flight Samples

Materials on the exterior of spacecraft in low Earth orbit (LEO) are subject to extremely harsh environmental conditions including various forms of radiation, temperature extremes and thermal cycling, impacts from micrometeoroids and orbital debris, on-orbit contamination, and atomic oxygen (AO) exposure. Radiation combined with thermal exposure can cause embrittlement of thin film polymers, such as the Teflon fluorinated ethylene propylene (FEP) insulation on the exterior of the Hubble Space Telescope. To better understand the effect of space exposure on the mechanical property degradation of Teflon FEP and other thin film spacecraft polymers, 116 tensile samples were exposed to the space environment on the exterior of the International Space Station (ISS) and returned to Earth for mechanical testing. The samples were flown in either zenith or wake orientations as part of three Materials International Space Station Experiment (MISSE) mission experiments. These experiments are the Polymers Experiment with 30 zenith tensile samples flown as part of the MISSE-8 mission, the Polymers and Composites Experiment-1 (PCE-1) with 24 zenith
and 38 wake tensile samples flown as part of the MISSE-9 mission, and the PCE-4 with 24 zenith samples flown as part of the MISSE-13 mission. Two of the MISSE-9 wake samples broke while on-orbit. Post-flight tensile testing was successfully completed on 114 flight and 116 control samples. All Teflon FEP samples were embrittled due to the space exposure. The zenith FEP samples were consistently more embrittled than the wake samples for the same exposure duration. The metallized Teflon samples were more embrittled than the clear Teflon samples for the same exposures. The slightly longer MISSE-9 wake exposure (0.54 years) caused more embrittlement of the Teflon FEP samples than the MISSE-13 wake exposure (0.44 years). A multi-layered stack of 2 mil Teflon FEP indicated that deeper penetrating radiation can cause embrittlement, but less penetrating radiation causes the majority of the damage. Teflon with a black carbon back-surface coating showed that a higher on-orbit temperature causes a synergistic effect that increases the radiation-induced embrittlement of Teflon FEP in LEO. In addition, metallized Polyimide Colorless (CP1) thin film solar sail materials had no significant embrittlement after 0.44 years of LEO wake exposure during the MISSE-13 mission.