Fluorinated cation exchange membrane and process for preparing the same

In a process for the preparation of a fluorinated cation exchange membrane used for electrolysis comprising treating, with a treating agent selected from the group consisting of an aqueous reducing solution of an inorganic acid, an aqueous reducing solution of an inorganic salt, an aqueous reducing solution of hydrazine, an aqueous reducing solution of an inorganic acid and an inorganic salt, and an aqueous reducing solution of an inorganic salt and hydrazine, one surface of a membrane comprising a fluorocarbon polymer containing pendant groups of the formula (3):

• --OCF.sub.2 CF.sub.2 SO.sub.2 X (3)
• wherein each X independently is fluorine, chlorine, bromine, hydrogen, ammonium, a quaternary ammonium or a metal atom,
• to convert part of the pendant groups of the formula (3) into pendant groups of the formula (1):
• --OCF.sub.2 COOM (1)
• wherein each M independently is hydrogen, ammonium, a quaternary ammonium or a metal atom,
• said fluorinated cation exchange membrane comprising a fluorocarbon polymer containing pendant carboxylic acid and/or carboxylate groups and pendant sulfonic acid and/or sulfonate groups, the proportion of the density of pendant carboxylic acid and/or carboxylate groups relative to the total density of pendant carboxylic acid and/or carboxylate groups and pendant sulfonic acid and/or sulfonate groups being different between one surface and an internal plane in cross-section parallel to the surfaces of the membrane;
• the improvement wherein one surface of a membrane comprising a fluorocarbon polymer containing pendant groups of the formula (3):
• --OCF.sub.2 CF.sub.2 SO.sub.2 X (3)
• wherein each X independently is fluorine, chlorine, bromine, hydrogen, ammonium, a quaternary ammonium or a metal atom,
• is treated with a treating agent selected from the group consisting of an aqueous reducing solution of at least one inorganic acid, an aqueous reducing solution of at least one inorganic salt, an aqueous reducing solution of hydrazine, an aqueous reducing solution of at least one inorganic acid and at least one inorganic salt, and an aqueous reducing solution of at least one inorganic salt and hydrazine, in the presence of at least one reaction controlling agent selected from the group consisting of C.sub.1 -C.sub.12 carboxylic acids, C.sub.1 -C.sub.12 sulfonic acids, C.sub.1 -C.sub.12 alcohols, C.sub.1 -C.sub.12 nitriles and C.sub.2 -C.sub.12 ethers, provided that when the treating agent includes hydrazine, said at least one reaction controlling agent is selected from C.sub.1 -C.sub.12 alcohols, C.sub.1 -C.sub.12 nitriles and C.sub.2 -C.sub.12 ethers, thereby to prepare a fluorinated cation exchange membrane
• which comprises a fluorocarbon polymer containing pendant groups of the formula (1):
• --OCF.sub.2 COOM (1)
• wherein each M independently is hydrogen, ammonium, a quaternary ammonium or a metal atom,
• and pendant groups of the formula (2):
• --OCF.sub.2 CF.sub.2 SO.sub.3 M (2)
• wherein each M independently is as defined above, and
• wherein the proportion (p) represented by the equation (a):
• p=A/(A+B).times.100(%) (a)
• wherein A is the density of pendant groups of the formula (1) and B is the density of pendant groups of the formula (2),
• is at least 20% in one surface of the membrane, and said proportion (p) gradually decreases from the one surface to the other surface or to that plane within the membrane where A reaches zero, said proportion (p) and a thickness (t) in microns between the one surface and the other surface or a plane within the membrane in cross-section parallel to the surfaces of the membrane satisfying the following inequality (b):
• .vertline..DELTA.p/.DELTA.t.vertline.=12(%/.mu.) (b).