Addition-cure, room temperature vulcanizing silicone elastomer based syntactic foams with glass and ceramic microballoons

Syntactic foams based on addition cure, room temperature vulcanizing silicone elastomer with two types of microballoons, viz. glass and ceramic, were processed and characterized. The effect of microballoon concentration (varied from 0 to 30 wt%) on the physical, mechanical, thermal, thermo-physical and ablative properties of the foams was investigated. Syntactic foams with glass microballoon (GM) possessed lower density and higher void content compared to those with ceramic microballoon (CM). The specific tensile strength and elastic modulus showed significant improvement; particularly for 20% GM filled system due to good interfacial bonding with silicone matrix. For CM syntactic foam, most of the mechanical properties showed a decline with increase in CM content because of poor interfacial properties. The trend in mechanical properties was corroborated by the microstructure evaluation through scanning electron microscopic (SEM) analyses. The thermo-physical properties such as specific heat, thermal conductivity, thermal diffusivity and co-efficient of thermal expansion showed variations with the type and content of microballoon. Thermo-gravimetric analysis (TGA) showed accelerated thermal decomposition for GM filled system; whereas, CM filled system preserved the thermal stability of silicone along with increase in char residue. Ablative evaluation showed high heat of ablation for GM filled system compared to CM based syntactic foam. The study reveals that syntactic foams based on silicone elastomer with low density and enhanced mechanical properties, suitable for critical weight-sensitive applications, can be formulated with microballoon inclusion. Selection of the appropriate type and concentration of microballoon is crucial in realizing the syntactic foams useful for the intended application.