Flexibilisation of Resorcinol-Formaldehyde Aerogels

Abstract

Organic resorcinol - formaldehyde (RF) aerogels are usually hard and brittle with Young’s-moduli in the range of 1-2 MPa, strengths about 100 kPa and densities in the range of 0.2 to 0.4 g/cm3. We observed that special conditions of aerogel synthesis lead surprisingly to 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 flexible RF aerogel are prepared with resorcinol (R) to water (W) molar ratio 0.008, the R/F molar ratio is fixed at 1:2. The molar ratio R/C is 50:1. The pH is adjusted in the range 5.4-5.6. After one week in the oven at 80°C the gel is washed in acetone and not dried supercritically but in an oven at 80°C. The flexible RF-aerogels show densities around 0.060 g/cm3; a measurable shrinkage is not observed. Their thermal conductivity 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.