Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

The elastic properties and/or flexibility of polymer reinforced silica aerogels having methyltrimethoxysilane (MTMS) and bis(trimethoxysilylpropyl)amine (BTMSPA) making up the silica structure are examined. The dipropylamine spacer from BTMSPA is used both to provide a flexible linking group in the silica structure, and as a reactive site via its secondary amine for reaction with a tri-isocyanate, Desmodur N3300A. The tri-isocyanate provides an extended degree of branching or reinforcement, resulting in increased compressive strength of the aerogel monoliths while the overall flexibility arising from the underlying silica structure is maintained. The compressive moduli of the reinforced aerogel monoliths in this study range from 0.001 to 158 MPa. Interestingly, formulations across this entire range of modulus recover nearly all of their length after two compressions to 25% strain. Differences in pore structure of the aerogels due to processing conditions and solvent are also discussed.

This study evaluated the effectiveness of dimethyldiethoxysilane (DMDES) precursor in improving the elastic recovery behavior of silica aerogels reinforced with epoxy through amine sites on the silica surface. In the study, two aminosilanes -3-aminopropyltriethoxysilane (APTES) and bis(trimethoxysilylpropyl)amine (BTMSPA)were considered as reactive sites for cross-linking with epoxy. Because of the way the samples were formulated, BTMSPA offered half the number of amine sites compared to APTES at the same level of substitution. Replacing tetraethoxysilane (TEOS) with at least 15 mol% DMDES reduced the number of silicon-oxygen bonds in the aerogel networks and resulted in improved elastic recovery, but up to an order of magnitude lower compressive modulus.BTMSPA aerogels demonstrated strong elastic response without DMDES, with some samples showing near complete recovery. However, these aerogels offered lower modulus than APTES aerogels.