Novel superflexible resorcinol-formaldehyde aerogels and combining of them with aramid honeycombs
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esearch Letters
Novel superflexible resorcinol–formaldehyde aerogels and combining of them with aramid honeycombs Marina Schwan, Barbara Milow, and Lorenz Ratke, Institute of Materials Research, German Aerospace Center, DLR, 51170 Cologne, Germany Address all correspondence to Marina Schwan at [email protected] (Received 31 July 2014; accepted 28 October 2014)
Abstract
We report a new insulation composite of aramid honeycombs filled with superflexible resorcinol–formaldehyde aerogels. Aerogels produced via a sol–gel process were dried with supercritical CO2. The aerogels exhibit a high, rubber-like flexibility, due to almost zero shrinkage and networking of nanoparticles and suitably sized macropores. The high porosity of the aerogels in the range of about 95–98% leads to a low thermal conductivity about 0.037 W/mK and low bulk density of 0.05 g/cm3. The filling of light and stiff aramid honeycombs with these flexible aerogels results in a composite with decreased thermal conductivity and modified mechanical properties.
Aerogels are highly open porous, nanostructured solid materials, and are produced via a sol–gel process. First inorganic silica aerogels were reported by Kistler.[1] Pekala developed organic resorcinol–formaldehyde (RF) aerogels in 1989.[2] These materials have many fascinating properties such as high porosity (80–99%), low bulk density (0.01–0.3 g/cm3), low thermal conductivity (0.01–0.05 W/mK), and high surface area (500– 2000 m2/g).[3] Owing to these properties they have many applications: thermal and sound insulation,[4] filler for vacuum isolation panels,[5] pyrolyzed RF aerogels – so-called carbon aerogels – are used in foundries[6] or for batteries or fuel cells.[7] Owing to their high porosity most aerogels are brittle and mechanically not resilient. Various methods have been developed to improve their mechanical properties and increase the flexibility of silica aerogels.[8] We reported recently the synthesis of flexible organic RF aerogels dried at ambient conditions.[9] We modified the route and obtained highly flexible aerogels by supercritical drying (SCD) of wet gels with CO2. These new type of RF aerogels are elastically deformable by more than 50% and are comparable with soft sponges or rubber. Owing to a high stiffness to weight ratio filled and unfilled aluminum, NomexTM (aramid), paper, or thermoplastic honeycombs are often used as sandwich panels and in structural applications as a secondary structure. Aircrafts, vehicles, buildings, and furnitures use honeycombs as a core, insulating, and protective material.[10] Several researches have studied structural and mechanical properties of honeycombs. Gibson and Ashby developed models to analyze them with respect to cell size and shape, loading kind, and directions.[11] Models for filled honeycombs were suggested by Abd El-Sayed et al.[12] and Beblo et al.[13] The great advantage of filled honeycombs is their improved properties. Syntactic foam-filled honeycombs
show an increase in Young’s modulus about 30% and in energy absorption about 48%.[14]
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