Aqueous dispersion of polytetrafluoroethylene and process for its production

1. An aqueous dispersion of polytetrafluoroethylene comprising:

from 55 to 70 mass % of polytetrafluoroethylene fine particles having an average particle diameter of from 0.1 to 0.5 μm,

from 0.0001 to 0.02 mass %, based on the mass of polytetrafluoroethylene, of a fluorine-containing carboxylic acid salt represented by the formula (1),

from 1 to 20 mass %, based on the mass of polytetrafluoroethylene, of a nonionic surfactant represented by the formula (2) and/or the formula (3), and

from 0.001 to 0.10 mass %, based on the mass of polytetrafluoroethylene, of a carboxylic acid salt represented by the formula (4);

wherein formulas (1)-(4) are as follows:

formula (1):

R 1 —COOX (wherein R 1 is a C 5-9 alkyl group having fluorine atoms substituted for from 90 to 100% of the hydrogen atoms (which may contain from 1 to 2 etheric oxygen atoms), and X is an ammonium ion);

formula (2): R 2 —O-A-H (wherein R 2 is a C 8-18 alkyl group, and A is a polyoxyalkylene chain consisting of from 5 to 20 oxyethylene groups and from 0 to 2 oxypropylene groups);

formula (3): R 3 —C 6 H 4 —O—B—H (wherein R 3 is a C 4-12 alkyl group, and B is a polyoxyethylene chain consisting of from 5 to 20 oxyethylene groups); and

formula (4): R 4 —COOY (wherein R 4 is a C 6-16 alkyl group, alkenyl group or aryl group having fluorine atoms substituted for at most 20% of the hydrogen atoms, and Y is a cationic group represented by the formula {HO(CH 2 ) n } x N + H 4-x wherein n is an integer of from 2 to 4, and x is an integer of from 0 to 4).

2. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein

the fluorine-containing carboxylic acid salt represented by the formula (1) is contained in an amount of from 0.001 to 0.02 mass %, based on the mass of polytetrafluoroethylene,

the nonionic surfactant represented by the formula (2) and/or the formula (3) is contained in an amount of from 2 to 12 mass %, based on the mass of polytetrafluoroethylene, and

the carboxylic acid salt represented by the formula (4) is contained in an amount of from 0.005 to 0.08 mass %, based on the mass of polytetrafluoroethylene.

3. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein the fluorine-containing carboxylic acid salt represented by the formula (1) is at least one member selected from the group consisting of C 7 F 15 COONH 4 , HC 7 F 14 COONH 4 , C 6 F 13 COONH 4 , HC 6 F 12 COONH 4 , C 8 F 17 COONH 4 , C 4 F 9 OC 2 F 4 OCF 2 COONH 4 and C 2 F 5 OC 2 F 4 OCF 2 COONH 4 .

4. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein in the nonionic surfactant represented by the formula (2), R 2 is a C 10-16 alkyl group, and A has from 7 to 12 oxyethylene groups.

5. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein in the nonionic surfactant represented by the formula (3), R 3 is a C 6-10 alkyl group, and B has from 6 to 16 oxyethylene groups.

6. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein a carboxylic acid represented by R 4 —COOH which corresponds to the carboxylic acid salt represented by the formula (4) has a pKa of from 4.1 to 6.

7. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein the carboxylic acid salt represented by the formula (4) is at least one member selected from the group consisting of ammonium laurate, ethanolamine laurate, ammonium cinnamate, ammonium p-t-butyl benzoate, ammonium fluorophenyl acetate, ammonium p-fluorobenzoate and ammonium natural coconut oil fatty acid.

8. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein the polytetrafluoroethylene fine particles having an average particle diameter of from 0.1 to 0.5 μm are contained in an amount of from 58 to 68 mass %.

9. The aqueous dispersion of polytetrafluoroethylene according to claim 1 , wherein the fluorine-containing carboxylic acid salt represented by the formula (1) is C 7 F 15 COONH 4 .

10. The aqueous dispersion of polytetrafluoroethylene of claim 1 that is produced by a process comprising:

dissolving from 0.001 to 0.10 mass %, based on the mass of polytetrafluoroethylene, of a carboxylic acid salt represented by the formula (4) in a low concentration aqueous dispersion of polytetrafluoroethylene comprising from 1 to 40 mass % of polytetrafluoroethylene fine particles having an average particle size of from 0.1 to 0.5 μm, from 0.000 1 to 0.02 mass %, based on the mass of polytetrafluoroethylene, of a fluorine-containing carboxylic acid salt represented by the formula (1), and from 1 to 20 mass %, based on the mass of polytetrafluoroethylene, of a nonionic surfactant represented by the formula (2) and/or the formula (3), and

then concentrating the low concentration aqueous dispersion of polytetrafluoroethylene to give a high concentration aqueous dispersion of polytetrafluoroethylene containing polytetrafluoroethylene at a concentration of from 60 to 75 mass %.