Aromatic polyimide composite separating membrane

A process for producing an aromatic polyimide composite separating membrane comprising:

• (A) preparing a substrate consisting of an aromatic polyimide porous membrane (1) by forming a dope solution of an aromatic polymeric material consisting essentially of at least one member selected from the group consisting of aromatic polyamides and aromatic polyamic acids and dissolved in a concentration of from 3% to 30% by weight in a solvent consisting essentially of a mixture of 100 parts by weight of at least one polar organic liquid compound capable of dissolving therein at least 5% by weight of said aromatic polymeric material and 5 to 100 parts by weight of at least one additional aromatic compound incapable of dissolving therein an amount of more then 3% by weight of said aromatic polymeric material, compatible with said polar organic liquid compound and having a poorer evaporating rate than that of said polar organic liquid compound, (2) by forming a thin layer of said dope solution, and (3) by drying said thin dope solution layer and then heating said dried layer at an elevated temperature, whereby said thin dope solution layer is converted to a thin solid porous membrane, and where said aromatic polymeric material in said dope solution contains an aromatic polyamic acid, said heating procedure is carried out under conditions adequate to convert said aromatic polyamic acid into the corresponding aromatic polyimide;
• (B) coating at least one surface of said aromatic polyimide porous membrane substrate with a dilute dope solution containing 0.01 to 5 parts by weight of an aromatic polymeric material consisting essentially of at least one polymer selected from the group consisting essentially of soluble aromatic polyimides and aromatic polyamic acids dissolved in a solvent consisting essentially of 100 parts by weight of a base component consisting of at least one organic liquid compound incapable of dissolving therein more than 3% by weight of said aromatic polymeric material in said porous membrane substrate and 0 to 15 parts by weight of an additional component consisting of at least one organic polar liquid compound capable of dissolving therein more than 5% by weight of said aromatic polymeric material and compatible with said base component, to form a coating layer of said dilute dope solution, and
• (C) evaporating said solvent from said coating layer of said dope solution.