Method for producing ethylene-tetrafluoroethylene copolymer powder

1. A method for producing an ethylene/tetrafluoroethylene-containing copolymer powder from a slurry having ethylene/tetrafluoroethylene-containing copolymer microparticles dispersed in a fluorinated organic solvent, by means of a centrifugal thin-film evaporator having a cylindrical heat-conducting barrel, a rotary stirring portion wherein a rotary shaft is provided with stirring blades for rotation in the heat-conducting barrel so that the front edges of the stirring blades skim over the inside wall surface of the heat-conducting barrel, and a heating portion for heating the heat-conducting barrel, wherein

the fluorinated organic solvent is at least one member selected from the group consisting of a perfluorocarbon, a hydrofluorocarbon, a hydrochlorofluorocarbon and a hydrofluoroether,

the centrifugal thin-film evaporator has an internal temperature during the method of 30 to 180° C., and

the slurry is fed into the heat-conducting barrel at a feeding linear velocity exceeding 0.1 m/sec.

2. The method according to claim 1 , wherein

the angle θ between the feeding direction of the slurry into the heat-conducting barrel and the tangential direction of the inside wall surface at the intersection of an extended line in the feeding direction and the inside wall surface of the heat-conducting barrel, is at most 30°.

3. The method according to claim 1 , wherein

the concentration of the microparticles in the slurry is from 1 to 15 mass %.

4. The method according to claim 1 , wherein

the content of the fluorinated organic solvent in the ethylene/tetrafluoroethylene-containing powder withdrawn from the centrifugal thin-film evaporator, is from 0.0001 to 50 mass %.

5. The method according to claim 1 , wherein

the slurry is a slurry obtained by solution polymerization of monomers comprising ethylene and tetrafluoroethylene.

6. The method according to claim 1 , wherein

the average particle size of the ethylene-tetrafluoroethylene powder withdrawn from the centrifugal thin-film evaporator, is from 10 to 500 μm.

7. The method according to claim 1 , wherein

the bulk density of the ethylene/tetrafluoroethylene-containing copolymer powder withdrawn from the centrifugal thin-film evaporator, is from 0.4 to 1.8 g/mL.

8. The method according to claim 1 , wherein

the fluorinated organic solvent is CF 3 (CF 2 ) n CF 2 H, CHClFCF 2 CF 2 Cl, or a mixture thereof,

wherein n is an integer of from 4 to 18.

9. The method according to claim 1 , wherein

in the ethylene/tetrafluoroethylene-containing copolymer, the molar ratio of repeating units derived from ethylene to repeating units derived from tetrafluoroethylene is from 30/70 to 60/40.

10. The method according to claim 1 , wherein

the ethylene/tetrafluoroethylene-containing copolymer has repeating units derived from another monomer other than ethylene and tetrafluoroethylene.

11. The method according to claim 10 , wherein said another monomer is a monomer represented by formula (1):

CH 2 ═CR 1 —(CF 2 ) a R 2   (1)

where, in the formula (1), each of R 1 and R 2 which are independent of each other, is a hydrogen atom or a fluorine atom, and a is an integer of from 1 to 12.

12. The method according to claim 1 , wherein volatile components are present in the powder of the ethylene/tetrafluoroethylene-containing copolymer, and the volatile components are present in an amount not more than 3.2 mass %, relative to the mass of the powder.

13. The method according to claim 12 , wherein

the average particle size of the ethylene-tetrafluoroethylene powder withdrawn from the centrifugal thin-film evaporator, is from 20 to 400 μm.

14. The method according to claim 12 , wherein

the average particle size of the ethylene-tetrafluoroethylene powder withdrawn from the centrifugal thin-film evaporator, is from 20 to 300 μm.

15. A method, comprising:

feeding a slurry into a heat-conducting barrel, present in a centrifugal thin-film evaporator, at a feeding linear velocity exceeding 0.1 m/sec, said slurry comprising microparticles of an ethylene/tetrafluoroethylene-containing copolymer dispersed in a fluorinated organic solvent; and

recovering a powder of said ethylene/tetrafluoroethylene-containing copolymer from said centrifugal thin-film evaporator, wherein

said heat-conducting barrel is present in a centrifugal thin-film evaporator having a cylindrical heat-conducting barrel, a rotary stirring portion wherein a rotary shaft is provided with stirring blades for rotation in the heat-conducting barrel so that the front edges of the stirring blades skim over the inside wall surface of the heat-conducting barrel, and a heating portion for heating the heat-conducting barrel, wherein

the fluorinated organic solvent is at least one member selected from the group consisting of a perfluorocarbon, a hydrofluorocarbon, a hydrochlorofluorocarbon and a hydrofluoroether, and

the centrifugal thin-film evaporator has an internal temperature during the method of 30 to 180° C.