Aqueous polymerization of fluorinated monomer using polymerization agent comprising fluoropolyether acid or salt and short chain fluorosurfactant

Abstract

A process includes polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and polymerization agent to form an aqueous dispersion of particles of fluoropolymer. The polymerization agent includes fluoropolyether acid or salt having an Mn of at least about 800 g/mol and fluorosurfactant having the formula: [R1—On-L-A−] Y+. R1 is a linear or branched partially or fully fluorinated aliphatic group which may contain ether linkages; n is 1; L is a linear or branched alkylene group which may be nonfluorinated, partially fluorinated, or fully fluorinated and may contain ether linkages; A− is a carboxylate; and Y+ is hydrogen, ammonium, or alkali metal cation. The chain length of R1—On-L- is not greater than 6 atoms. The polymerization agent includes a minor amount of fluoropolyether acid or salt thereof and a major amount of fluorosurfactant. The polymerizing produces less than about 3 wt % undispersed fluoropolymer based on the total weight of fluoropolymer produced.

What is claimed is: 
     
         1 . A process comprising polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and polymerization agent to form an aqueous dispersion of particles of fluoropolymer, said polymerization agent comprising:
 fluoropolyether acid or salt thereof having a number average molecular weight of at least about 800 g/mol; and   fluorosurfactant having the formula:
   [R 1 —O n -L-A − ]Y + 
 
   wherein:
 R 1  is a linear or branched partially or fully fluorinated aliphatic group which may contain ether linkages; 
 n is 0 or 1; 
 L is a linear or branched alkylene group which may be nonfluorinated, partially fluorinated or fully fluorinated and which may contain ether linkages; 
 A −  is an anionic group selected from the group consisting of carboxylate, sulfonate, and sulfonamide anion; and 
 Y +  is hydrogen, ammonium or alkali metal cation; 
 with the proviso that the chain length of R 1 —O n -L- is not greater than 6 atoms, 
 wherein said fluorosurfactant comprises at least about 65 wt % of said polymerization agent; and 
 wherein said polymerizing produces less than about 10 wt % undispersed fluoropolymer based on the total weight of fluoropolymer produced. 
   
     
     
         2 . The process of  claim 1  wherein said chain length of R 1 —O n -L- is 3 to 6 atoms. 
     
     
         3 . The process of  claim 1  wherein said chain length of R 1 —O n -L- is 4 to 6 atoms. 
     
     
         4 . The process of  claim 1  wherein said chain length of R 1 —O n -L- is 3 to 5 atoms. 
     
     
         5 . The process of  claim 1  wherein said chain length of R 1 —O n -L- is 4 to 5 atoms. 
     
     
         6 . The process of  claim 1  wherein when said polymerization agent comprises a fluorosurfactant to fluoropolyether weight ratio of 5:1, said polymerization agent has a surface tension in water at a concentration of 6000 ppm at 23° C. of at least about 30% less than the surface tension of the fluorosurfactant alone in water at 23° C. at a concentration of 6000 ppm. 
     
     
         7 . The process of  claim 1  wherein n is 1. 
     
     
         8 . The process of  claim 7  wherein:
 R 1  is a linear or branched partially or fully fluorinated alkyl group having 1 to 3 carbon atoms which may contain ether linkages; and 
 L is an alkylene group selected from —CX(R 2 )—, wherein R 2  is fluorine or perfluoromethyl and X is hydrogen or fluorine, and —CZ 1 Z 2 CZ 3 Z 4 —, wherein Z 1 , Z 2 , Z 3 , and Z 4  are independently selected from the group consisting of hydrogen and fluorine. 
 
     
     
         9 . The process of  claim 8  wherein:
 L is an alkylene group selected from the group consisting of —CF(CF 3 )—, —CF 2 CF 2 —, —CHFCF 2 —, and —CF 2 CHF—. 
 
     
     
         10 . The process of  claim 7  wherein R 1  is a linear partially or fully fluorinated alkyl group having 2 to 3 carbon atoms. 
     
     
         11 . The process of  claim 1  wherein R 1  is fully fluorinated. 
     
     
         12 . The process of  claim 7  wherein:
 R 1  is CF 3 CF 2 CF 2 —; 
 L is —CF(CF 3 )—; and 
 A −  is carboxylate; and 
 Y +  is hydrogen or ammonium. 
 
     
     
         13 . The process of  claim 1  wherein:
 n is 0 and R 1  and L collectively comprise a perfluoroalkyl group having 4-6 carbons; and 
 A −  is sulfonate or sulfonamide anion. 
 
     
     
         14 . The process of  claim 1  wherein said aqueous medium contains less than about 300 ppm of perfluoroalkane carboxylic acid or salt fluorosurfactant having 8 or more carbon atoms based on the weight of water in said aqueous medium. 
     
     
         15 . The process of  claim 1  wherein said fluoropolyether acid or salt thereof having a number average molecular weight of at least about 800 g/mol comprises acid groups selected from carboxylic acid, sulfonic acid, sulfonamide, phosphonic acid. 
     
     
         16 . The process of  claim 1  wherein said fluoropolyether acid or salt thereof has a number average molecular weight of about 800 to about 3500 g/mol. 
     
     
         17 . The process of  claim 1  wherein said fluoropolyether acid or salt thereof has a number average molecular weight of about 1000 to about 2500 g/mol. 
     
     
         18 . The process of  claim 1  wherein said polymerization agent is present in said aqueous medium in an amount of about 5 ppm to about 10000 ppm based on the weight of water in said aqueous medium. 
     
     
         19 . The process of  claim 1  wherein said polymerization agent is present in said aqueous medium in an amount of about 5 ppm to about 3000 ppm based on the weight of water in said aqueous medium. 
     
     
         20 . The process of  claim 1  wherein said aqueous dispersion of particles of fluoropolymer formed by said process has a fluoropolymer solids content of at least about 10 wt %. 
     
     
         21 . The process of  claim 1  wherein said aqueous dispersion of particles of fluoropolymer formed by said process has a fluoropolymer solids content of about 20 wt % to about 65 wt %. 
     
     
         22 . The process of  claim 1  wherein said at least one fluorinated monomer is selected from the group consisting of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), trifluoroethylene, hexafluoroisobutylene, perfluoroalkyl ethylenes, fluorovinyl ethers, vinyl fluoride (VF), vinylidene fluoride (VF2), perfluoro-2,2-dimethyl-1,3-dioxole (PDD), perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), perfluoro(allyl vinyl ether) and perfluoro(butenyl vinyl ether). 
     
     
         23 . The process of  claim 1  wherein said particles of fluoropolymer produced comprise PTFE or modified PTFE having a comonomer content of no greater than about 1 wt %. 
     
     
         24 . The process of  claim 23  wherein said PTFE or modified PTFE has a melt creep viscosity of at least about 10 8  Pa·s. 
     
     
         25 . The process of  claim 1  wherein said particles of fluoropolymer produced comprise a melt-processible copolymer comprising at least about 60-98 wt % tetrafluoroethylene units and about 2-40 wt % of at least one other monomer. 
     
     
         26 . The process of  claim 1  wherein said aqueous medium is substantially free of perfluoropolyether oil. 
     
     
         27 . The process of  claim 1  wherein said polymerization medium is substantially free of fluoropolymer seed at polymerization kick-off. 
     
     
         28 . The process of  claim 1  wherein said polymerizing produces less than about 3 wt % undispersed fluoropolymer based on the total weight of fluoropolymer produced.