Process for preparing porous membranes

A process is provided for preparing a polytetrafluorethylene (PTFE) membrane having a mean pore size of 1000 A or less and utilized for the separation of uranium isotopes or other gaseous mixtures. The process comprises irradiating PTFE powder with ionizing radiations (104 to 5x105rad), mixing the powder with a liquid pore-forming agent, shaping the mixture into a film by press molding or calendering, and removing the pore-forming agent from the film. The product has a high efficiency of separation and a high permeability constant. In an example, PTFE powder was irradiated with varied doses of Co-60 gamma rays and blended with 38.5% by volume of white oil. Each mixture was pressed and calendered to give a film of 70 micron in thickness. The white oil was extracted with trichlorethylene. Radiation dose (rad), pressure of calendering (kg/6 cm2), permeability constant for Ar isotopes (mole/cm3.min.cm Hg), efficiency of separation for Ar isotopes (%), permeability constant for UF2 isotopes and efficiency of separation for UF2 isotopes were, respectively: 1, 235, 7.6x10-6, 80.0, 2.3x10-6, 48.0, 1x104, 160, 11.2x10-6, 84.0, 3.7x10-6, 58.0, 1x106, 140, 13.0x10-8, 83.0, 4.3x10-6, 55.0. (Kaichi, S.)