Base-acid catalyzed resorcinol-formaldehyde aerogels

Abstract

Resorcinol-formaldehyde (RF) aerogels were synthesized via a newly developed two-step method. This synthesis combines the base and acid catalyzed routes of the polymerization of resorcinol with formaldehyde, which are well known in single application. For the first step a standard RF-recipe was used with the molar ratio of resorcinol to water (R/W) of 0.04 and of resorcinol to formaldehyde (R/F) of 0.7. In the base catalyzed step sodium carbonate as the catalyst was used and the molar ratio of resorcinol to catalyst (R/C) was varied between 10 and 1500. After a certain time while the RF solution gels, 1 M citric acid was added to initiate the acid catalyzed step. By adding citric acid to the base catalyzed solution, the gels formed could be dried under ambient conditions without large shrinkage. Indeed, in the base catalyzed one-step route with R/C ratios < 1000, heavy shrinkage is observed about more than 50 vol%. Small RF-particles construct the three-dimensional gel structure and therefore the particle chains do not withstand the capillary forces during the drying leading to high shrinkage. With the new method we still obtain small particles, but with an additional coating of a thin layer of RF in the acid catalyzed step. This causes a reinforcing effect allowing the subcritical drying. However, this effect depends on the time of changing the catalyst after starting the polymerization. Aerogels prepared by the new two-step route were characterized by density, thermal conductivity and specific surface and the structure was investigated by scanning electron microscopy.