AGING AND DRYING OF GELS

This review describes the chemical and physical processes occurring during the aging and drying of an inorganic gel. The chemical reactions that lead to gelation are shown to continue long after the solution gels, causing changes in the composition, structure, and properties of the gel. The formation of new crosslinks produces shrinkage (syneresis) and increases the modulus and viscosity of the gel, so that aging reduces the subsequent shrinkage during drying. If the exterior surface shrinks much faster than the interior, the differential strain can cause warping and cracking; very slow drying is generally required to prevent such catastrophes. The theory of drying is reviewed and shown to account for the observed behavior. Several strategies are described that minimize the tendency of the gel to crack.