Importance

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Abstract

A cation exchange membrane of a fluorinated polymer used for an electrolysis of an alkali metal chloride is obtained by treating a cation exchange membrane having ion exchange groups of --COOM (M is hydrogen atom or an alkali metal atom) or groups being convertible to said groups and having an ion exchange capacity (A.sub.R) of 0.5 to 2.0 meq/g dry polymer in a hydrolyzed form, to give a weight gain percent (.DELTA.W.sub.R) in the range of .DELTA.W.sub.R =1.6e.sup.1.7A R as the upper side; .DELTA.W.sub.R =0.8e.sup.1.7A R as the lower side; A.sub.R =0.5 as the left side and A.sub.R =2.0 as the right side shown in FIG. 1 (log scale on ordinate) in an aqueous medium of a base or a salt at a concentration of 1 to 20 wt. % at the temperature of 0.degree. to 60.degree. C. after the hydrolysis or at the time of the hydrolysis.

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First Claim