A DIFFUSION-CONTROLLED PROCEDURE TO CLOSE PORES IN CERAMIC MEMBRANES

A diffusion-controlled caulking treatment using aqueous colloidal sili ca to plug large pores in ceramic membranes is described. Unlike the c onventional slip-casting process, this more controllable process can, in principle, be tailored by proper silica particle size selection to allow minimal deposition in the small desirable pores while enabling t he elimination of large, less selective pores. The 2000 Angstrom Anodi sc(R) membrane has been chosen as a model for the development of the p rocedure. Via the diffusion-controlled process, a majority of the 2000 A pores were closed using 100 to 200 Angstrom silica with negligible deposition on external membrane surfaces. The technique, therefore, is promising for selective closure of large undesirable pores without su rface fouling or closure of desirable 100 Angstrom or smaller pores. T he efficiency of the procedure has been characterized by a number of i ndependent methods. The gas and water permeability of the caulked memb ranes decreased one and two orders of magnitude, respectively. Pore si ze analyses by a variation of the bubble point test and by rejection m easurements of polyethylene glycol (MW 1500 to 12000) have found the a verage pore size of the dried silica-treated membranes to be approxima tely equal to the precursor silica sol particle size used in the treat ment. While this suggests less than close packing in the large pores, the low permeability of the silica-treated membrane demonstrates that the treatment is highly effective. Indeed, given the selective caulkin g capability of this approach, it appears to be an attractive means of healing defects in microporous ceramic membranes.