Subcritically dried resorcinol-formaldehyde aerogels from a base-acid catalyzed synthesis route

Abstract

Resorcinol-formaldehyde (RF) aerogels are prepared from a newly developed base-acid catalyzed sol-gel synthesis route. RF-gels synthesized are nanostructured and can be dried under ambient conditions without significant shrinkage. The sol-gel reaction of resorcinol (R) and formaldehyde (F) in water (W) (R/F = 0.7, R/W = 0.04) is initially catalyzed by Na2CO3 with a molar ratio of resorcinol to catalyst (R/C) of 100. After a specified time after starting the reaction (1-3 h), citric acid (1 M, 4–20 vol.-%) is added which initiates the acid catalyzed step and causes a spontaneous gelation. The time of change in pH value by adding acid is of great importance and influences the structure and properties of the material. RF-aerogels consisting of small particles much smaller than 100 nm are prepared by applying a long base catalyzed step. Particle size, density, thermal conductivity and surface area are basically independent of the amount of citric acid, but this parameter influences the mechanical resistance. A theoretical model is used to describe the time dependence of catalyst change in the sol-gel process.