Process for producing a porous diffusion membrane

A process for producing a polytetrafluoroethylene porous membrane for isotope separation having an average pore diameter under 1,000A is provided, using a combination of pore forming agent characteristics. The permeability and the separation efficiency can be increased under the combination of five conditions, namely, the solubility parameter, viscosity, mixing ratio, working temperature and working speed of the pore forming agent. The solubility parameter is between 4 and 14, preferably 5 and 12 (casup(l)/cc)sup(1/2); the viscosity is between 0.2 and 3,000, preferably 0.5 and 200 CS at the working temperature. A liquid pore forming agent is mixed with a solvent having a lower boiling point than the agent. This solution is mixed with polytetrafluoroethylene powders. Their mixing ratio is between 10 and 80% by volume, preferably 20 and 50%. After evaporation of the solvent, the agents mixed with the powders are rolled at a temperature between 300 and 3000C, preferably 35 and 2300C at a rolling speed between 0.1 and 50 m/min, preferably 0.13 and 10 m/min. to remove the agent by squeezing. In one example, 1kg of polytetrafluoroethylene was impregnated in vacuum with a solution of 40% by volume of a blended pore forming agent dissolved in 3 liters of solvent. After impregnation, the polytetrafluoroethylene was compressed with N2 under 50kg/cm2 for 1 hour. After evaporation of the solvent, the polytetrafluoroethylene was molded under 30kg/cm2 with a die at 600C, extruded at 600C into a plate like form, and rolled through a 0.2mm gap at 550C. Furthermore it was rolled through a 60μ gap to form a thin film. (Iwakiri, K.)