Evaluation of thermal control materials degradation in simulated space environment

Indian Space Research Organisation (ISRO) has been actively pursuing research and development of a host of materials to be used for its satellite and launch vehicle programs. A variety of Thermal control materials have been developed in-house for use in 1015 years in the I:EO and GEO orbits. The present study was carried out [1] to evaluate the effect of synergistic radiation on the thermal control materials and verify the assumptions of BOL and EOL values of materials including White paints, second-surface mirrors, aluminized films of polyimide and polyester, White anodisation and Germanium tapes. The space environment simulation using UV, protons and electrons was carried out at ONERA using a combined radiation test facility. A long term test extending to over three months in vacuum was performed to simulate a three years exposure on the N-S panels in the geostationary orbit for a three axis stabilized spacecraft. Reflectance spectra were measured in-situ in the solar range (250-2500 nm) enabling the evaluation of initial air to vacuum transition effects and the final transition ( nitrogen and air exposure effects). This is in addition to the investigation of the UV and particulate radiation induced degradation over three years, measured by steps of 0.5 year. Little spectral water desorption effect occurred during initial transition whereas large bleaching of degradation happened (e.g. in white paint) when the first nitrogen inlet was permitted during return to ambient atmospheric pressure. This bleaching effect increased on exposure to air and continued for one week. Specific spectral bleaching of degradations were observed in the infrared. on some samples where UV provoked bleaching of previous step particles degradation. The solar absorptance values were deduced from the reflectance data. The degradation has been observed most in white paints and white anodisation while the second surface mirrors and aluminium paint have been quite stable. Empiric degradation models have been applied on the most degraded materials to extrapolate the degradation for long duration exposure. To complete the thermo-optical properties investigations, infrared emissivity measurements were performed in air at the beginning and end of test showing a good stability, except in the case of polyimide and FEP based thermal control materials.