Aerogel-Aerogel-Composites from Resorcinol-Formaldehyde-Aerogel and Silica-Aerogel-Granulate

Abstract

Aerogels are highly porous (up to 99%), nanostructured materials with low density (0.01-0.3 g/cm³), high surface area (200-1000 m²/g) and low thermal conductivity (< 0.03 W/m.K). They are prepared via a sol-gel process and are obtained after a suitable drying of the gel, e.g. under supercritical (CO2) or ambient conditions depending on the materials chemistry. Due to the excellent properties aerogels are suitable to be used as ultralight thermal insulation material for example in building construction, in vehicles or elsewhere. The challenge is to produce large plates or a similar shape of aerogel material adapted to the application. Mostly, aerogels are very sensitive concerning inhomogeneous stresses during all steps of preparation and tend to break due to the high porosity. In our study we fixed silica aerogel (SiO2) granulate fabricated by industry with resorcinol-formaldehyde (RF) aerogel which is an organic aerogel synthesized via a sol-gel reaction of resorcinol (R, 1,3-dihydroxybenzol) and formaldehyde (F) in deionized water (W) with Na2CO3 as the catalyst (C). The composite aerogel is prepared by mixing the hydrophobic silica aerogel granulate into the already highly viscous RF-solution. The mass can then be cast in any mold for gelation and aging and the gel can finally be dried under ambient conditions. The molar ratios used for RF were R/F = 0.7, R/W = 0.04 and R/C = 1500 leading to a less sensitive RF-aerogel. The silica aerogel granulate grain size was between 0.5 and 4 mm in which the choice of size influences the mechanical properties. Properties like density, surface area and thermal conductivity are dependent of the content of silica. A content of 60 vol.-% silica was practicable and we obtained a density of 0.215 g/cm³, a surface area of 306 m²/g and a thermal conductivity at room temperature of 0.023 W/m.K.