Investigation of the Degradation of Thermal Control Materials by Thermal Analysis

Abstract

Thermal analysis is the name of a collection of techniques that investigates material properties as a function of temperature. Four main techniques which all look at specific material properties are differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechnical analysis (TMA) and dynamic mechanical analysis (DMA). The commercially available polyimide foils Kapton HN and Upilex S are often used as external thermal control material on spacecraft. For missions to the inner part of the solar system, these materials need to withstand an elevated service temperature in combination with high intensity solar radiation, such as UV and protons. The thermal stability of Kapton and Upilex has been investigated at around 250³ C for a mission to Venus and at 350³C for a mission to Mercury. Samples were aged at representative temperature levels, also in combination with high intensity UV and proton radiation. It was found that TMA and DMA revealed, in good detail, the environmental effects of the behavior of the materials. Both foils are known to be thermally stable materials. Assessment by TGA and kinetic modeling indicates a better stability of Upilex at 350 ³C. The DMA experiments of foils aged at 350³C for up to 3580 h prove that the glass transition of Kapton is largely affected and almost completely disappears. Upilex is also affected but remains more stable. The TMA assessment revealed annealing of residual stresses at elevated temperature that cause (partial) shrinking of the foils. Ageing at around 250³C causes such annealing besides other more subtle changes. The added irradiation to high intensity UV and protons does not cause significant changes compared to thermal ageing alone. The DMA results are in good agreement with the TMA. The behavior of Upilex is deemed to be somewhat less affected by the environmental testing than Kapton.