Modified polytetrafluoroethylene fine powder and its manufacturing method, and electric wire and tube using it

1. A modified polytetrafluoroethylene fine powder which is a fine powder of a non-melt-processable modified polytetrafluoroethylene, wherein the modified polytetrafluoroethylene comprises units derived from tetrafluoroethylene, units derived from hexafluoropropylene, units derived from a perfluoro(alkyl vinyl ether) represented by the formula (I) and units derived from a (perfluoroalkyl)ethylene represented by the formula (II):

CF 2 ═CFO—C n F 2n+1   (I)

wherein n is an integer of from 1 to 6,

CH 2 ═CH—C m F 2m+1   (II)

wherein m is an integer of from 3 to 7, and

wherein the modified polytetrafluoroethylene fine powder has a core-shell structure comprising a core part made of a modified PTFE having no unit derived from the hexafluoropropylene and a shell part made of a modified PTFE having units derived from the hexafluoropropylene.

2. The modified polytetrafluoroethylene fine powder according to claim 1 , which has an average primary particle size of from 0.20 to 0.28 μm.

3. The modified polytetrafluoroethylene fine powder according to claim 1 , wherein the sum of the units derived from hexafluoropropylene, the units derived from a perfluoro(alkyl vinyl ether) and the units derived from a (perfluoroalkyl)ethylene, is at most 0.3 mass %, per a total mass of the fine powder.

4. The modified polytetrafluoroethylene fine powder according to claim 1 , wherein the units derived from hexafluoropropylene is from 0.005 to 0.090 mass %, the units derived from a perfluoro(alkyl vinyl ether) is from 0.01 to 0.20 mass %, and the units derived from a (perfluoroalkyl)ethylene is from 0.001 to 0.010 mass %, per a total mass of the fine powder.

5. The modified polytetrafluoroethylene fine powder according to claim 1 , wherein the core part:shell part representing the mass ratio of the core part to the shell part is from 70:30 to 95:5.

6. A method for manufacturing the modified polytetrafluoroethylene fine powder according to claim 1 , the method comprising a first step of polymerizing tetrafluoroethylene, the perfluoro(alkyl vinyl ether) and the (perfluoroalkyl)ethylene in the presence of an aqueous medium, a polymerization initiator and an emulsifier and a second step of adding hexafluoropropylene so as to be polymerized with tetrafluoroethylene.

7. The method according to claim 6 , wherein the modified polytetrafluoroethylene fine powder has a core-shell structure and wherein in the first step, tetrafluoroethylene is continuously or intermittently supplied to a polymerization reactor, and at a time when from 70 to 95 mass % of tetrafluoroethylene in the total amount to be used in the polymerization reaction is supplied, the second step is initiated by adding hexafluoropropylene.

8. The method according to claim 6 , wherein the emulsifier is at least one fluorinated emulsifier selected from the group consisting of a C 5-8 fluorinated carboxylic acid having an etheric carbon atom and salts thereof.

9. A method for coating an electric wire, the method comprising:

providing a mixture comprising the modified polytetrafluoroethylene fine powder according to claim 1 ; and

coating at least a portion of the electric wire with the mixture thereby producing an electric wire having a coating layer.

10. An electric wire having a coating layer, produced by the method according to claim 9 .

11. A method for producing a tube, the method comprising:

providing a mixture comprising the modified polytetrafluoroethylene fine powder according to claim 1 ; and

molding the mixture thereby producing the tube.

12. A tube, produced by the method according to claim 11 .