Testing of Sunscreen Protected Silicone Seals from MISSE-12 and MISSE-13 On-Orbit Flights

In a continued investigation of the effects of space environments, silicone S0383–70 test articles were flown as part of the Materials International Space Station Experiment–12 (MISSE–12) and MISSE–13 Polymers and Composites Experiments–3 (PCE–3) and PCE–4, respectively, on the exterior of the International Space Station (ISS). The effects of simultaneous exposure to space environments (temperature, vacuum pressure, atomic oxygen (AO), and ultraviolet (UV) radiation) on the silicone compound were explored for test articles fabricated from S0383–70 with and without coatings applied to the surface. The following three coatings were evaluated: Braycote 601EF grease (BP Lubricants USA Inc.), Braycote 601EF plus Z–COTE® zinc oxide powder (BASF Corp.) (BZ coating), and Dow Corning 7 grease (Dow Corning Corp.) plus Z–COTE® (DCZ coating). In addition, tests articles fabricated from a reformulated silicone compound of S0383–70 with titanium dioxide (TiO2) were part of the investigation. The Braycote 601EF-coated test articles represented current-use configuration of the S0383–70 material as a docking seal. The other two coatings were sunscreens developed at the NASA Glenn Research Center to prevent damage to the elastomer from UV radiation. Like the sunscreen coatings, the reformulated compound was designed to minimize the effects of UV radiation. The primary objective of this study was to compare the effectiveness of the countermeasures designed to protect silicone seals from the space environment. Physical properties of color, mass, and durometer hardness and the performance property of leak rate were evaluated to determine the effectiveness of these protective measures. Evidence of changes or damage to the surfaces was observed in the photographs and scanning electron microscopy (SEM) images of the test articles, and the change in fluorescence indicated some change at the molecular level. The leak rates measured for test articles fabricated with the baseline S0383–70 compound and the reformulated compound significantly increased to unacceptable levels, indicating the concentration of the TiO2 particles was not great enough to protect the material. However, the leak rates of the test articles with surface coatings were no different than those of the ground control (i.e., unexposed) test articles, indicating the coating provided protection to the underlying material. Overall, the coatings provided protection to the S0383–70 elastomer from the damaging effects of space environments. The coatings were durable and still intact at the end of the mission. The positive results obtained from the MISSE–12 and MISSE–13 test articles will aid in the continued advancement of the BZ and DCZ coatings as viable options for silicone space seals.