1. Field of the Invention
The present invention relates to a cation exchange membrane of a fluorinated polymer for an electrolysis. More particularly, it relates to a cation exchange membrane of a fluorinated polymer which imparts high current efficiency and low electric resistance in an electrolysis of an aqueous solution of an alkali metal chloride.
2. Description of Prior Art
It has been proposed to use a cation exchange membrane of a fluorinated copolymer produced by copolymerizing a fluorinated olefin such as CF.sub.2 .dbd.CF.sub.2 and a fluorovinyl ether having sulfonic acid group or a functional group being convertible to sulfonic group such as CF.sub.2 .dbd.CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF.sub.2 SO.sub.2 F and hydrolyzing the fluorinated copolymer to convert --SO.sub.2 F groups into --SO.sub.3 H groups, as a membrane for an electrolysis of an alkali metal chloride. (U.S. Pat. No. 4,025,405)
When the cation exchange membrane of the fluorinated copolymer is used as a membrane for an electrolysis of an alkali metal chloride, excellent acid resistance, chlorine resistance and alkali resistance are found, however it is difficult to maintain high current efficiency in a case of increasing a concentration of an alkali metal hydroxide as the product.
It has been proposed, in order to overcome such disadvantage, to improve the characteristics by converting sulfonic acid groups as the ion exchange groups of the cation exchange membrane of a fluorinated polymer into carboxylic acid groups by treating with a reducing agent or an oxidizing agent. (U.S. Pat. No. 4,151,053 and U.S. Pat. No. 4,200,711). The resulting modified cation exchange membrane has an ion exchange capacity of only upto about 0.9 meq./g. polymer because of high polarity of sulfonic acid group. It is difficult to obtain a cation exchange membrane having greater ion exchange capacity.
When sulfonic acid groups which provide small ion exchange capacity are converted into carboxylic acid groups, an electric resistance is higher. Therefore, only thin surface layer in a cathode side should be converted into carboxylic acid groups. When carboxylic acid groups contact with an aqueous solution of an alkali metal hydroxide having high concentration at high temperature for a long time, carboxylic acid groups are gradually decomposed. When the modified cation exchange membrane is continuously used, the desired characteristics are deteriorated. It is necessary to treat again the membrane so as to convert sulfonic acid groups into carboxylic acid groups. In order to convert sulfonic acid groups into carboxylic acid groups in the thin layer of the membrane so as to prevent the increase of the electric resistance it is necessary to take care of the operations and it is not easy to control the operations in an industrial work.
On the other hand, it has been proposed to use a cation exchange membrane of a fluorinated copolymer of a fluorinated olefin such as CF.sub.2 .dbd.CF.sub.2 and a fluorovinyl ether having carboxylic acid group or a functional group being convertible to carboxylic acid group such as CF.sub.2 .dbd.CFO(CF.sub.2).sub.n COOM wherein n is 1 to 12 and M represents hydrogen atom or an alkali metal atom, as a membrane for an electrolysis of an alkali metal chloride. (U.S. Pat. No. 4,065,366 and U.S. Pat. No. 4,202,743).
When such cation exchange membrane is used, a degree of dissociation of carboxylic acid groups is lowered to rise a cell voltage in an operation at low pH with an addition of an acid such as hydrochloric acid so as to control a concentration of oxygen in chlorine gas formed in an anode compartment.