Foamable fluoropolymer composition

1. A foamable composition for extruded foam insulation on a conductor, said insulation containing cells of uniform size concentrated towards the center of said insulation, whereby said insulation has a skin on its interior and exterior surfaces, said composition comprising: a) a partially-crystalline melt-processible fluorinated tetrafluoroethylene/hexafluoropropylene copolymer being substantially free of alkali metal ion, having less than about 20 unstable end groups per 10 6 carbon atoms, and having a melt flow rate of from about 25 to 35 g/10 min, and b) a foam nucleating package providing said uniform foam cell size wherein the foam cell size of 90% of the foamed cells is 50 micrometers or less, and said foam nucleating package ranges from 0.1-10 wt % of the combined weight of said tetrafluoroethylene/hexafluoropropylene copolymer and said foam nucleating package, the extrusion of said foam insulation exceeding 500 ft/min, and wherein said foam nucleating package comprises i) boron nitride, ii) calcium tetraborate, and iii) a foam nucleating agent of the formula

[Z(CF 2 ) x (CF 2 CFX) p (R′) y (CH 2 ) z RO 3 ] n M, wherein:

Z is selected from CCl 3 , CCl 2 H, H, F, Cl or Br;

each X, independently, is selected from H, F or Cl;

R is sulfur or phosphorus;

M is H or a metallic, ammonium, substituted ammonium or quaternary ammonium cation;

x is an integer and is 0 to 10;

p is an integer and is 0 to 6;

y is 0 or 1;

z is an integer and is 0 to 10; x+y+z+p is a positive integer or, if x+y+z+p is 0, Z is CCl 3 or CCl 2 H;

n is the valence of M; and

R′ is selected from a C 5-6 perfluorinated alicyclic ring diradical; a C 1-16 perfluorinated aliphatic polyether diradical with repeat units selected from [CF 2 O], [CF 2 CF 2 O], and [CF 2 CF(CF 3 )O]; and a substituted or unsubstituted aromatic diradical, in which Z is H.

2. The composition according to claim 1 , wherein said tetrafluoroethylene/hexafluoropropylene copolymer is made by polymerizing monomers comprising tetrafluoroethylene, hexafluoropropylene and perfluoro(alkyl vinyl ether).

3. The composition of claim 1 wherein the tetrafluoroethylene/hexafluoropropylene copolymer is melt fluorinated.

4. The composition according to claim 1 , wherein said tetrafluoroethylene/hexafluoropropylene copolymer is polymerized and isolated without the use of alkali metal ion containing reagents.

5. The composition according to claim 1 , comprising a tensile strength of at least 2000 psi (13.8 MPa) and an elongation of at least 200%.

6. The foamable composition according to claim 1 wherein the extrusion of said foam insulation is at a rate of at least 1000 ft/min.

7. The foamable composition according to claim 1 wherein said tetrafluoroethylene/hexafluoropropylene copolymer contains 3 to 20 wt % of said hexafluoropropylene.

8. The foamable composition according to claim 7 wherein the extrusion of said foam insulation is at a rate of at least 1000 ft/min.

9. The foamable composition according to claim 1 wherein said foam nucleating agent of said foam nucleating package comprises the barium salt of F(CF 2 ) n CH 2 CH 2 SO 3 H, wherein n is selected from 6, 8, 10 and 12.

10. The foamable composition according to claim 1 wherein said foam nucleating package comprises 91.1±0.5 weight percent boron nitride, 2.5±0.2 weight percent calcium tetraborate, and 6.4±0.2 weight percent weight percent of the barium salt of F(CF 2 ) n CH 2 CH 2 SO 3 H, wherein n is selected from 6, 8, 10 and 12.