Porous membrane structure and method

A method of treating a membrane, said method comprising the steps of:

• providing a membrane with a node and fibril structure of which surfaces of the nodes and fibrils define a plurality of pores of pores extending through the membrane;
• providing a water-based dispersion of oleophobic fluoropolymer solids, the dispersion has surface tension and relative contact angle properties such that the dispersion is incapable of wetting pores in the membrane;
• diluting the dispersion with a water-miscible liquid wetting agent selected from the group consisting of ethanol, isopropyl alcohol, methanol, n-propanol, n-butanol, N-N-dimethylformamide, methyl ethyl ketone and water soluble e- and p- series glycol ethers to provide a dilution dispersion that has surface tension and relative contact angle properties such that the resultant dispersion is capable of wetting pores in the membrane so solids in the dispersion are carried into the pores in the membrane;
• wetting surfaces of the nodes and fibrils of the membrane with the resultant dispersion;
• removing the wetting agent and other fugitive material from the membrane;
• and
• thermally coalescing the oleophobic fluropolymer solids onto surfaces of the nodes and fibrils without completely blocking pores in the membrane by heating the wetted membrane.