Development of an innovative mercury intrusion technique to examine defects plugging after CVD treatment of NF composite membranes

A novel method has been developed to reveal the possible existence of pore structure defects on the thin and ultra-thin separating layers of composite inorganic membranes. In this work the method is applied to sol–gel and CVD treated microporous membranes and involves a modified mercury porosimetry technique in which the only surface exposed for mercury intrusion is the one of the nanofiltration (NF) layer. The validity of the test method was verified by comparison with the outcome of gas phase permeability experiments. Defects of size above 5 nm are accurately and reliably detected. The size range from 3.6 nm to 5 nm, although “traceable” with normal mercury intrusion up to 60,000 psig, remains somewhat unexploited with the new technique. This is due to interferences induced by the elasticity properties (compression modulus and strength) of the material applied as a “plug” to membrane support. Continuity imperfections on the NF layer of a commercially available membrane were detected by application of the modified mercury intrusion technique. The area examined was about ∼7% of the total NF layer area exposed by the tube and the size of revealed defects was in the range between 20 nm and 100 nm. Furthermore sections of the defects bearing membrane were modified by chemical vapor deposition of SiO₂ to examine the possibility of mending the existing cracks. Significant reduction of the defects size has been observed with the novel mercury intrusion technique.