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Our current understanding regarding the evaluation of the thermal endurance and UV radiation stability of two different types of commercially available aromatic polyimides at elevated temperatures is presented. Such materials are typically used as thermal control foils. To characterize the thermal endurance a high vacuum thermal ageing campaign was performed at 350 'C up to 4452 h. During that test the mass loss was found to be higher for the Kapton HN (DuPont, USA)-film than for the Upilex S (UBE Industries, Japan)-film material. Further UV/VIS/NIR transmission and thermo-optical measurements revealed the degradation behaviour of both films versus time at 350 'C. It was found that in general the Upilex S-film degrades at a slower rate. To screen the radiation stability an environmental UV test was performed with samples being simultaneously at 100, 230 and 350 'C. After the test it was noted that the Kapton HN-film shows for all three temperatures a higher mass loss than the Upilex S-film. A non-linear correlation of the activation energies for the mass loss of the Kapton-HN film as a function of temperature was also found. Furthermore, a higher loss of transmittance (UVIVIS) was found for the Kapton HN-film and a higher degradation of the a/E ratio when compared with the Upilex-S-film. For the latter material it seems that this degradation is temperature activated.

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