Process for producing a porous diffusion membrane

A process for producing a polytetrafluoroethylene (PTFE) which is used for gaseous diffusion barrier having an average pore diameter of less than 1,000A is provided for separation of uranium isotopes, using at least two kinds of pore forming agents. The process comprises mixing a blended liquid pore forming agent of at least two different solubility parameters with a PTFE resin fine powder, and tightly contacting a molded thin film of the resin with a porous support having an average pore diameter between 0.1μ and 300μ. The molding is carried out with compressive and shearing stresses. Under these conditions, the permeability and the separation factor increase simultaneously. The blended pore forming agents may be a mixture of hydrocarbons, silicon oils, surface active agents or the like. The porous support may be a sintered porous material, expanded material or a fibrous porous material. In one example, 1kg of PTFE was impregnated under 0.1mmHg with a 3 liter solution containing 40% by volume of pore forming agents of different kinds of solubility parameter. The impregnated resin was compressed with N2 at 50kg/cm2 for 1 hour. Thereafter, the solvent was evaporated. The resin was pressed in a 100mm x 30mm die at 30kg/cm2 and a temperature of 600C. It was then extruded at 600C into a plate like form, rolled through a roll gap of 0.2mm at 550C in a direction perpendicular to the extrusion, and then rolled through a 0.1μ gap together with a porous nickel support of 1mm thickness having a porosity of 45% and an average pore diameter of 30μ to squeeze a portion of pore forming agent into the nickel support. (Iwakiri, K.)