Process for producing a porous diffusion membrane

A process for producing a polytetrafluoroethylene membrane for isotope separation having an average pore diameter under 1000 A is provided, using a combination of pore forming agent characteristics in conjunction with material molding conditions. The permeability and the separation efficiency can be increased under five conditions, namely, the solubility parameter, viscosity, mixing ratio, working temperature and working speed of the pore forming agents. The solubility parameter is between 4 and 14, preferably 5 and 12. The viscosity is between 0.2 and 3000, preferably 0.5 and 200 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 agent is mixed with the powders at a temperature between 0.1 and 50 m/min, preferably 0.13 and 10 m/min. The film is superimposed onto and incorporated with a support having an average pore diameter between 0.1 μm and 300 μm, preferably 0.2 and 100 μm. In this way, large pores are squeezed into the support, so that a finer pore distribution of films is obtained. In one example, an impregnated polytetrafluoroethylene thin film was made by sequentially pressing, extrusion and rolling. A part of the pore forming agent was squeezed through a roll gap of 1.01 μm by pressing and rolling into a porous nickel support. (Iwakiri, K.)