PARAMETRIC STUDY OF STRENGTH OF SILICA-GELS

Cylindrical gel bodies were produced by using Si(OC2H5)(4), C2H5OH and H2O with various catalysts. A systematic parametric study of gel stre ngth and microstructure as a function of C2H5OH/H2O molar ratio and ca talyst concentration in sol, catalyst type as well as aging time, was conducted. The strongest gels were obtained by using H2SO4, HNO3 or HC l as catalysts. Gels catalyzed with oxalic acid were weaker. The stren gth of the gels increased with a decrease of C2H5OH/H2O ratio. The xer ogel microstructures of gels catalyzed with H2SO4, HNO3, HCl, or oxali c acid were very similar, indicating that they had similar permeabilit ies during the later stages of drying. However, gels catalyzed with HF showed significantly distinct properties. They were much weaker and h ad larger pores, larger porosity and smaller surface area. Although th e strength of HF-catalyzed gels could be increased by increasing the c atalyst concentration, aging weakened gels considerably because of the dissolution of the network by HF present in the pores.