Studies on the thermal properties of silicone polymer based thermal protection systems for space applications

Abstract

Thermal decomposition, thermal conductivity, specific heat and flammability of polydimethyl siloxane (PDMS) resin and three PDMS-based systems with three different types of additives were studied using various thermo-analytical techniques. The additives were found to modify the thermal properties of PDMS to different extents. Among the three systems, highest specific heat and lowest thermal conductivity, desired for thermal protection application, were obtained for System I. The thermal stability of the three systems is in the order System I > System II > System III. Even though System III showed the lowest specific heat and highest thermal conductivity, due to its very high LOI among all other systems, the system finds application as a flame retardant. Thermogravimetric–mass spectroscopic analysis was carried out to find out the decomposition products. The major decomposition product was identified as hexamethylcyclotrisiloxane in all the systems. The decomposition kinetics of these systems was investigated by thermogravimetry using multiple heating rate methods, viz. Kissinger and Flynn–Wall–Ozawa. The kinetic parameters, viz. activation energy and pre-exponential factor, were derived using the two methods for all the systems. From the kinetic study, it is confirmed that the activation energy for the decomposition of the neat PDMS decreased as decomposition progressed, whereas the activation energy increased for the silicone composite systems.