Extrusion of a Foamable Fluoropolymer

Abstract

A process for making an extruded foamable composition is disclosed where the foamable composition includes a partially-crystalline melt processible perfluoropolymer and a foam nucleating package. The process makes a foamed product having uniform foam cell size at high speeds without loss of product quality.

1 . An extrusion process comprising: extruding a foamable composition at a line speed of 1,000 feet per minute or greater on to a conductor to form a layer of foamed composition on said conductor, said layer having foam cells concentrated towards the center of said layer, whereby said layer has a skin on its interior and exterior surfaces, said skin extending from the surface of said layer into the interior of said layer at least about 5% of the shortest distance between the inner and outer surfaces of said layer, said skin having less than about 50% of the total void content of said layer, and said layer comprising foamed cells wherein 90% of the foamed cells are 50 micrometers in diameter or less, wherein said foamable composition comprises:
 i) a partially-crystalline melt-processible fluorinated perfluoropolymer, wherein said perfluoropolymer contains less than 50 ppm alkali metal ion and no greater than 50 —CF 2 CH 2 OH, —CONH 2 , —COF and —COOH endgroups per million carbon atoms, and   ii) a foam nucleating agent ranging from 0.1-10 wt % of the combined weight of said perfluoropolymer and said foam nucleating agent.   
     
     
         2 . The process according to  claim 1 , wherein said foam nucleating agent has a foam nucleating effective amount of at least one thermally stable compound which is selected from the group consisting of sulfonic and phosphonic acids and salts thereof. 
     
     
         3 . The process according to  claim 1 , wherein said perfluoropolymer has a melt flow rate of from about 25 to 35 g/10 min. 
     
     
         4 . The process according to  claim 1 , wherein said perfluoropolymer is made by polymerizing monomers selected from the group consisting of tetrafluoroethylene, hexafluoropropylene and perfluoro(alkyl vinyl ether). 
     
     
         5 . The process according to  claim 1 , wherein said perfluoropolymer is polymerized and isolated without the use of metal-ion containing reagents. 
     
     
         6 . The process according to  claim 1 , wherein said foamed composition has a tensile strength of at least 2000 psi and an elongation of at least 200%. 
     
     
         7 . The process according to  claim 1 , wherein said perfluoropolymer comprises from about 0.5 to 10 by weight of at least one perfluoro(alkyl vinyl ether). 
     
     
         8 . The process according to  claim 1 , wherein said layer of foamed composition on said conductor has a capacitance coefficient of variation (COV) of no greater than 1%. 
     
     
         9 . The process according to  claim 1 , wherein said layer of foamed composition on said conductor has a void content of 10-50%. 
     
     
         10 . The process according to  claim 1 , wherein said layer of foamed composition on said conductor has a spark resistance of less than 1 spark per 1000 feet (300 m) when tested at 1.5 kV. 
     
     
         11 . An extrusion foaming process comprising extruding a foamable composition at a line speed of 1,000 feet per minute or greater on to a conductor to form a layer of foamed composition on said conductor, said layer having foam cells concentrated towards the center of said layer, whereby said layer has a skin on its interior and exterior surfaces, said skin extending from the surface of said layer into the interior of said layer at least about 5% of the shortest distance between the inner and outer surfaces of said layer, said skin having less than about 50% of the total void content of said layer, and said layer comprising foamed cells wherein 90% of the foamed cells are 50 micrometers in diameter or less, wherein said foamable composition comprises:
 a) a partially-crystalline melt-processible fluorinated perfluoropolymer, wherein said perfluoropolymer contains less than 50 ppm alkali metal ion and is fluorinated in the melt and contains no greater than 50 —CF 2 CH 2 OH, —CONH 2 , —COF and —COOH endgroups per million carbon atoms, and   b) a foam nucleating package comprising:   i) boron nitride,   ii) a synergistic amount of at least one inorganic salt that is thermally stable at the fluoropolymer extrusion temperature, and consists of a metal cation and a polyatomic anion, and satisfies the relationship: 0.36[14−pKa]−0.52≧[r−0.2q]2≧0.11[14−pKa]−0.28, where r=crystal ionic radius of the cation, in Angstroms, q=valence of the cation, pKa=−log of Ka for the following reaction: HA −(n-1) <=>H + +A −n , where A is the salt anion, H is hydrogen, and n=absolute value of the valence of the anion; 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 selected from sulfur or phosphorus; M is selected from 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 case, Z is H; and said foam nucleating package ranges from 0.1-5 wt % of the combined weight of said perfluoropolymer and said foam nucleating package.   
     
     
         12 . The process according to  claim 11 , wherein said perfluoropolymer is made by polymerizing monomers selected from the group consisting of tetrafluoroethylene, hexafluoropropylene and perfluoro(alkyl vinyl ether). 
     
     
         13 . The process according to  claim 11 , wherein said perfluoropolymer is made by polymerizing monomers selected from the group consisting of tetrafluoroethylene and perfluoro(alkyl vinyl ether). 
     
     
         14 . The process according to  claim 11 , wherein said at least one inorganic salt of the foam nucleating package comprises calcium tetraborate. 
     
     
         15 . The process according to  claim 11 , wherein said foam nucleating agent of the foam nucleating package comprises the barium salt of F(CF 2 ) n CH 2 CH 2 SO 3 H, wherein n is 6, 8, 10 and 12. 
     
     
         16 . The process according to  claim 11 , 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 of barium salt of F(CF 2 ) n CH 2 CH 2 SO 3 H, wherein n is 6, 8, 10 and 12.