Polydimethylsiloxane Based Room Temperature Curable, Low Outgassing and Low Density Thermal Protection System

Abstract

Thermal protection systems based on hollow particle filled composites emerge as promising material for weight sensitive aerospace applications. The present paper depicts the development of a low outgassing, low density silicone syntactic foam based thermal protection system consisting of hydroxy terminated polydimethylsiloxane (PDMS), micro cells and other additives. The addition of microballoons increases the mechanical strength, while the density could be brought down considerably upto 0.5 g/cc, rendering it suitable for application on metallic and CFRP substrates. Incorporation of a silane-coupling agent was found to improve the adhesion considerably. The system was conferred the consistency for spray coating by suitable rheological modifiers. Though there is a marginal decrease in specific heat, this is offset by a considerable reduction in thermal conductivity to keep the thermal diffusivity on low profile. Mass-loss at 520 K was observed to be less than 1%. The optical properties of the system include IR emissivity in the range of 0.78 and solar absorptivity of 0.17. The system was developed as a room temperature curing, sprayable system and evaluated as a low density thermal protection system for space vehicles for regions intended to experience temperatures up to 650 K. Functional evaluation of the developed system as a topcoat was evaluated by KHS and wind tunnel tests under the test condition for nose cone region of PSLV. During the entire heating simulation, the specimen surface was intact and was stable. The base polymer, PDMS was further subjected to pre-treatment process prior to the compounding to remove the low molecular weight fractions for reducing the outgassing characteristics. The system was further formulated with the same filler formulation so to achieve the low outgassing characteristics while retaining other thermo-physical characteristics. This promising system can find potential application as TPS material for futuristic manned missions.