Stabilized fluoropolymer and method for producing same

1. A method for producing a stabilized fluoropolymer which comprises producing said stabilized fluoropolymer by subjecting a treatment target substance containing a sulfonic-acid-derived-group-containing fluoropolymer to a fluorination treatment,

wherein said sulfonic-acid-derived-group-containing fluoropolymer is a fluoropolymer containing —SO 3 M (in which M represents H, NR 1 R 2 R 3 R 4 or M 1 1/L ; R 1 , R 2 , R 3 and R 4 are the same or different and each represents H or an alkyl group containing 1 to 4 carbon atoms; and M 1 represents an L-valent metal), and

said treatment target substance has a moisture content of not higher than 500 ppm by mass,

said method comprising the steps of

producing said sulfonic-acid-derived-group-containing fluoropolymer by emulsion polymerization,

drying said sulfonic-acid-derived-group-containing fluoropolymer to produce said treatment target substance, and

fluorinating said treatment target substance,

wherein the sulfonic-acid-derived-group-containing fluoropolymer is a copolymer which is at least binary comprising

an acid-derived group-containing perhalovinyl ether represented by the general formula (I):

CF 2 ═CF—O—(CFY 2 ) m -A  (I)

(wherein Y 2 represents F, Cl, Br or I; m represents an integer of 1 to 5; when m is an integer of 2 to 5, m atoms of Y 2 are the same or different; A represents —SO 2 X or —COZ; X represents F, Cl, Br, I or —NR 5 R 6 and Z represents —NR 7 R 8 or —OR 9 ; R 5 , R 6 , R 7 and R 8 are the same or different and each represents H, an alkali metal element, an alkyl group or a sulfonyl-containing group and R 9 represents an alkyl group containing 1 to 4 carbon atoms) and

a copolymerizable monomer with said acid-derived group-containing perhalovinyl ether,

said copolymerizable monomer is an “other vinyl ether” other than said acid-derived group-containing perhalovinyl ether and an ethylenic monomer, and

said copolymer comprises 5 to 40 mole percent of an acid-derived group-containing perhalovinyl ether unit derived from said acid-derived group-containing perhalovinyl ether, 60 to 95 mole percent of an ethylenic monomer unit derived from said ethylenic monomer and 0 to 5 mole percent of an “other vinyl ether unit” derived from said “other vinyl ether”.

2. The method for producing a stabilized fluoropolymer according to claim 1 ,

wherein the sulfonic-acid-derived-group-containing fluoropolymer further contains —SO 2 X and/or —COZ (wherein X represents F, Cl, Br, I or —NR 5 R 6 and Z represents —NR 7 R 8 or —OR 9 ; R 5 , R 6 , R 7 and R 8 are the same or different and each represents H, an alkali metal element, an alkyl group or a sulfonyl-containing group and R 9 represents an alkyl group containing 1 to 4 carbon atoms).

3. The method for producing a stabilized fluoropolymer according to claim 1 ,

wherein the sulfonic-acid-derived-group-containing fluoropolymer further contains —COOH at the polymer chain terminus or termini.

4. The method for producing a stabilized fluoropolymer according to claim 1 ,

wherein the fluorination treatment is carried out using a gaseous fluorinating agent comprising a fluorine source,

said fluorine source is at least one species selected from the group consisting of F 2 , SF 4 , IF 5 , NF 3 , PF 5 , ClF and ClF 3 and

said fluorine source amounts to not less than 1% by volume of said gaseous fluorinating agent.

5. The method for producing a stabilized fluoropolymer according to claim 4 ,

wherein the fluorine source is F 2 .

6. The method for producing a stabilized fluoropolymer according to claim 1 ,

wherein Y 2 is F and m is 2.