Synthesis and Properties of Flexible Resorcinol-Formaldehyde Aerogels

Abstract

Organic resorcinol - formaldehyde (RF) aerogels dried under ambient conditions with a large resorcinol (R) to catalyst (C) ratio are usually hard and brittle with strengths about 100 kPa and Young’s-moduli in the range of 1-2 MPa and densities in the range of 0.2 to 0.4 g/cm3. We observe that special conditions of aerogel preparation yield surprisingly flexible aerogels. They can be bent easily (see figure 1), similar to the well-known flexible silica aerogels. They have a low density, a very low elastic modulus of around 70 kPa and are elastically deformable by more than 40% in an almost reversible manner. The procedure of their preparation is simple: The resorcinol is dissolved in deionized water with a very large amount of water (W), close to the gelation limit (molar ratio R/W ≈ 0.008). The R/F molar ratio is fixed at 1:2. The molar ratio R/C is 50:1 usually requiring supercritical drying. The pH is adjusted to be in the range 5.4-5.6. The solution is placed in an oven at 80°C for one week. The gel is washed in acetone and not dried supercritical but in an oven at 80°C. The density of flexible RF-aerogels is about 0.060 g/cm3, a shrinkage is not observed. The thermal conductivity of aerogel is 0.046 W/(mK). The SEM picture in Figure 2 shows the continuous network of particles with sizes of around 1 µm and pores of about 10 µm. The effects of various sol-gel parameters on the flexibility, such as R/W, R/C molar ratios, pH of initial solution, gelation temperature, have been investigated. The aerogels of different densities, microstructures and particle size were obtained by varying the R/W molar ratio. It has been observed that the pH plays a very important role in synthesis of flexible resorcinol-formaldehyde aerogels. The differences between hard and flexible RF aerogels were studied by recording 13C NMR spectra and TGA-FT-IR curves.