SHRINKAGE OF SILICA-GELS AGED IN TEOS

A model has been developed for predicting the shrinkage that occurs du ring drying, given knowledge of the initial pore size and modulus of t he gel, The model is in good agreement with experimental results for s ilica gels given a variety of aging treatments. The initial pore size (used to predict the capillary pressure that drives shrinkage) can be found by making an aerogel and performing nitrogen desorption, or by c alculating the effective pore size from the measured permeability; it is shown that those methods give similar results. Thus the drying shri nkage can be predicted using data obtained from the wet gel (viz. perm eability and modulus). At the moment that the liquid/vapor meniscus en ters a pore, the walls will be covered with a film of adsorbed liquid, which reduces the radius of the meniscus below the radius of the pore , and thereby increases the capillary pressure. The film tkickness for the solvents used is not known so one assumes a thickness of 1 nm. If the presence of the layer is ignored, the theory underestimates the d rying shrinkage.