Process for producing elastic fluorocopolymer and crosslinked fluororubber

1. A process for producing an elastic fluorocopolymer, comprising:

emulsion-polymerizing at least one fluoromonomer (a), a vinyl ester monomer (b), and optionally, at least one hydrocarbon monomer (c), in an aqueous medium to produce an elastic fluorocopolymer latex;

adjusting the pH of the elastic fluorocopolymer latex to from 1 to 9; and

coagulating the elastic fluorocopolymer latex to isolate an elastic fluorocopolymer wherein

the fluoromonomer (a) is at least one selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride and a perfluorovinyl ether represented by CF 2 ═CF—O—R f , wherein R f is a C 1-8 perfluoroalkyl group or a perfluoro(alkoxyalkyl) group),

the vinyl ester monomer (b) is represented by the formula:

CR 1 R 2 ═CR 3 COOCH═CH 2

wherein

each of R 1 and R 2 which are independent of each other, is a hydrogen atom, a C 1-10 alkyl group or a C 1-10 alkoxyalkyl group containing an etheric oxygen atom, and R 3 is a hydrogen atom, a fluorine atom or a methyl group, and

the optional at least one hydrocarbon (c) is at least one selected from the group consisting of ethylene, propylene and a vinyl ether represented by CH 2 ═CH—O—R 4 , wherein R 4 is a C 1-8 alkyl group or an alkoxyalkyl group.

2. The process for producing an elastic fluorocopolymer according to claim 1 , wherein R 2 and R 3 in the vinyl ester monomer (b) are hydrogen atoms.

3. The process for producing an elastic fluorocopolymer according to claim 1 , wherein the vinyl ester monomer (b) is at least one member selected from the group consisting of vinyl methacrylate and vinyl crotonate.

4. The process for producing an elastic fluorocopolymer according to claim 1 , wherein in the elastic fluorocopolymer, a molar ratio of repeating units (l) based on the fluoromonomer (a) to repeating units (n) based on the hydrocarbon monomer (c) is in the range (n)/(l)=30/70 to 70/30.

5. The process for producing an elastic fluorocopolymer according to claim 1 , wherein the fluoromonomer (a) is tetrafluoroethylene, the hydrocarbon monomer (c) is propylene, and in the elastic fluorocopolymer, (a) molar ratio of repeating units (l) based on the fluoromonomer (a) to repeating units (n) based on the hydrocarbon monomer (c) is in the range (n)/(l)=40/60 to 60/40.

6. The process for producing an elastic fluorocopolymer according to claim 1 , wherein in the elastic fluorocopolymer, a molar ratio according to (m)/((l)+(n)) of repeating units (l) based on the fluoromonomer (a), repeating units (m) based on the vinyl ester monomer (b) and repeating units (n) based on the hydrocarbon monomer (c) is in the range (m)/((l)+(n))=0.0001 to 0.1.

7. The process for producing an elastic fluorocopolymer according to claim 1 , wherein the pH is from 1 to 8.

8. The process for producing an elastic fluorocopolymer according to claim 1 , wherein a pH of the emulsion polymerization is from 1 to 8.

9. An elastic fluorocopolymer, which is obtained by the process for producing an elastic fluorocopolymer as defined in claim 1 .

10. A crosslinked fluororubber, which is obtained by cross-linking an elastic fluorocopolymer obtained by the process for producing an elastic fluorocopolymer as defined in claim 1 .

11. The process for producing an elastic fluorocopolymer according to claim 1 , wherein the aqueous medium comprises a water-soluble solvent selected from the group of water-soluble solvents consisting of water, tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, tripropylene glycol and mixtures thereof.

12. The process for producing an elastic fluorocopolymer according to claim 11 , wherein a concentration of the water-soluble organic solvent is in the range of from 1 to 50 parts by mass per 100 parts by mass water.

13. The process for producing an elastic fluorocopolymer according to claim 1 , wherein an emulsifier is an anionic emulsifier selected from the group consisting of a hydrocarbon emulsifier, a fluoroalkyl carboxylate, and a fluorine-containing emulsifier represented by the formula

F(CF 2 ) n O(CF(X)CF 2 O) m CF(X)COOA

wherein

X is a fluorine atom or a C 1-3 perfluoroalkyl group,

A is a hydrogen atom, an alkali metal or NH 4 ,

n is an integer of from 1 to 10, and

m is 0 or an integer of from 1 to 3.

14. The process for producing an elastic fluorocopolymer according to claim 13 , wherein a content of the emulsifier is in the range of from 0.01 to 15 parts by mass per 100 parts by mass of the aqueous emulsion.

15. The process for producing an elastic fluorocopolymer according to claim 1 , wherein a pH of the coagulating is in the range of from 1 to 5.

16. The process for producing an elastic fluorocopolymer according to claim 1 , wherein a pH of the emulsion-polymerizing is in the range of 1 to 5 and a pH of the coagulating is in the range of from 1 to 5.

17. The process for producing an elastic fluorocopolymer according to claim 1 , wherein the coagulating comprises a coagulating method selected from the group consisting of addition of a metal salt, addition of an inorganic acid, mechanical sheering, and freeze-thawing.