Effects of processing conditions on porosity features of polyurea cross-linked silica aerogels synthesized by ambient pressure drying

Lightweight porous polyurea-modified silica aerogels have been synthesized through an ambient pressure drying technique. The silica wet gels were prepared by copolymerization and gelation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxylsilane (APTES) during which amine functional groups were introduced into the skeletal framework. The amine sites then became anchors for cross-linking the silica particles of preformed skeleton by attachment of hexamethylene diisocyanate (HDI), resulting in a conformal polyurea coating on the surface of silica aerogels. The effects of APTES/TEOS volume ratio and HDI concentrations on the evolution of microstructure and pore characteristics of silica aerogels were investigated in detail. The various APTES/TEOS volume ratio did not resulted in a significant change of the density, while pores size in the network structure became larger when the APTES/TEOS volume ratio increased from 10% to 30%. The amount of polyurea cross-linked into the aerogels appeared to vary regularly with the increase of HDI concentration from 0 to 20%. More polyurea was attained when silica wet gels were immersed into the solution of a larger HDI concentration, which rendered the interparticle neck region of silica aerogels wider. The resulting conformal coatings of polyurea reinforced the inorganic silica networks while inevitably occupied some voids at the same time. The investigation provides data for the synthesis of tailor made polyurea-modified silica aerogels under ambient pressure drying method.