FABRICATION OF LOW-DENSITY FOAM SHELLS FROM RESORCINOL-FORMALDEHYDE AEROGEL

Resorcinol-formaldehyde (RF) aerogel chemistry has been used with enca psulation techniques to fabricate low-density, transparent, foam shell s. To accomplish this, the gelation time was reduced from several hour s to several minutes by the addition of acid following base-catalyzed RF particle growth. However, additional ''annealing'' of the gel for a t least 20 h was needed to maximize crosslinking and minimize swelling in exchange solvents. Increasing the molar ratio of formaldehyde to r esorcinol from 2 to 3 also helped to increase crosslinking. Densificat ion of the foam shells due to dehydration during curing was greatly re duced by judicious choice of immiscible oil phases and by saturating t he exterior oil phase during the annealing stage. Shells have been pro duced with diameters of about 2 mm, wall thicknesses ranging from 100 to 200 mu m and foam densities approaching 50 mg/cc. (C) 1997 John Wil ey & Sons, Inc.