SHRINKAGE DURING DRYING OF SILICA-GEL

A model is provided for predicting gel shrinkage during drying by combining the empirical observations that: (1) the bulk modulus of a gel increases with density, rho, according to K-p = K-0 (rho/rho(y))(m) (2.5 less than or equal to m less than or equal to 4); and (2) the variation in pore radius, r, is approximately proportional to pore volume (contrary to the dependence, r alpha rho(-1/3), conventionally assumed). No allowance is made for viscoelastic relaxation, so the model applies only for drying from an inert solvent. The model may be used to guide processing efforts to yield either aerogel-like materials with high porosity or xerogels with high density and small pore size for adsorbents, membranes, etc. The extent of shrinkage is governed by two dimensionless parameters, P and m. The quantity P = A(s) gamma cos(theta)m rho(y)/K-0 (where A(s) is specific surface area, gamma is surface tension, and theta is the contact angle) represents the relative magnitudes of capillary pressure and gel stiffness, and m describes the variation of stiffness with density. For P values less than 1, the shrinkage upon drying is less than 10%, and is reversible. For shrinkages greater than similar or equal to 50%, the density increase is irreversible, and is proportional to P-1/(m-1). Predicted shrinkage is compared with experimental results for silica gels dried from different surface tension pore fluids (2 < gamma < 68 dyn/cm), initial wet gel density (0.06 < rho rho(0) < 0.15 g/cm(3)), and gel stiffness (0.3 < K-0 < 1MPa).