Process for preparing aqueous fluoropolymer dispersions and the concentrated aqueous fluoropolymer dispersions produced by such process

Abstract

Fluorine-containing emulsifiers can be removed from fluoropolymer dispersions by multiple cycles of thermal concentration. After repeated cycles of dilution and extraction the fluorine-containing emulsifier can be reduced to a very low level. The resulting dispersion can be formulated into an aqueous dispersion that can be used for coating applications, such as seal, packing, fabric and metal coating.

1 . A process for preparing a concentrated aqueous dispersion of PTFE, which comprises: 
 (1) adding a non-fluorinated emulsifier to a starting aqueous dispersion of PTFE, wherein said starting aqueous dispersion of PTFE comprises a fluorinated surfactant, to obtain a stabilized dispersion;    (2) heating said stabilized dispersion to a temperature of 40 to 100° C., to obtain a heated dispersion;    (3) maintaining said heated dispersion at said temperature until it separates into an upper supernatant layer that is rich in said fluorinated surfactant and a lower concentrated dispersion of PTFE layer with reduced level of said fluorinated surfactant;    (4) isolating said lower concentrated dispersion of PTFE from said supernatant upon cooling, to obtain an isolated concentrated dispersion of PTFE; and    (5) diluting said isloated concentrated dispersion of PTFE with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 20 to 50 wt. %, based on the total weight of said concentrated PTFE dispersion.    
   
   
       2 . The process of  claim 1 , further comprising: 
 (6) subjecting said concentrated aqueous PTFE dispersion to a repetition of steps (1) to (5).    
   
   
       3 . The process of  claim 2 , wherein said repetition of steps (1) to (6) is carried out from 1 to 4 times.  
   
   
       4 . The process of  claim 1 , further comprising: 
 (7) collecting said upper supernatant layer for surfactant recycling.    
   
   
       5 . The process of  claim 1 , wherein said lower concentrated dispersion of PTFE is diluted with purified water to obtain a concentrated aqueous PTFE dispersion having a PTFE concentration of 30 to 36 wt. %, based on the total weight of said concentrated PTFE dispersion.  
   
   
       6 . The process of  claim 1 , which further comprises: 
 (1a) adding a base to said stabilized dispersion, to obtain a basified dispersion, prior to said heating (2).    
   
   
       7 . The process of  claim 6 , wherein said basified dispersion has a pH>9.  
   
   
       8 . The process of  claim 6 , wherein said stabilized dispersion is basified by adding a 30 wt. % aqueous solution of NH 4 OH in an amount of 0.1 to 1 wt. %, based on the total weight of said stabilized dispersion.  
   
   
       9 . The process of  claim 6 , wherein said stabilized dispersion is basified by adding a 30 wt. % aqueous solution of NH 4 OH in an amount of 0.1 to 0.5 wt. %, based on the total weight of said stabilized dispersion.  
   
   
       10 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <400 ppm.  
   
   
       11 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <300 ppm.  
   
   
       12 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <200 ppm.  
   
   
       13 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <100 ppm.  
   
   
       14 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <50 ppm.  
   
   
       15 . The process of  claim 1 , wherein said concentrated aqueous PTFE dispersion comprises said fluorinated surfactant in an amount of <30 ppm.  
   
   
       16 . The process of  claim 1 , wherein said PTFE contains at least one comonomer selected from the group consisting of hexafluoropropene, perfluoro-(n-propyl-vinyl)-ether, perfluorohexyl vinylether, chloro-trifluoroehtylene, and mixtures thereof.  
   
   
       17 . The process of  claim 1 , wherein said PTFE is a homopolymer of tetrafluoroethylene.  
   
   
       18 . The process of  claim 1 , wherein said PTFE is a homopolymer of TFE with a SSG of 1.30 to 2.30.  
   
   
       19 . The process of  claim 1 , wherein said PTFE has a SSG of 1.90 to 2.30.  
   
   
       20 . The process of  claim 1 , wherein said starting aqueous dispersion of PTFE comprises at least one fluorinated surfactant selected from the group consisting of straight chain or branched chain C 4 -C 12 -perfluorocarboxylic acid salts, straight chain or branched chain C 4 -C 12 -hydrofluorocarboxylic acid salts, straight chain or branched chain C 4 -C 12 -perfluoroalkane sulfonic acid salts, and straight chain or branched chain C 4 -C 12 -hydrofluoroalkane sulfonic acid salts, straight chain or branched chain C 4 -C 12 -perfluoroalkane phosphonic acid salts, and straight chain or branched chain C 4 -C 12 -hydrofluoroalkane phosphonic acid salts and mixtures thereof  
   
   
       21 . The process of  claim 1 , wherein said starting aqueous dispersion of PTFE comprises ammonium perfluorooctanoate.  
   
   
       22 . The process of  claim 1 , wherein said starting aqueous dispersion of PTFE comprises a homopolymer of TFE in an amount of between 5 and 40 wt. %, based on the total weight of said starting dispersion.  
   
   
       23 . The process of  claim 1 , wherein said starting aqueous dispersion of PTFE comprises a homopolymer of TFE in an amount of 30 to 35 wt. %, based on the total weight of said starting dispersion.  
   
   
       24 . The process of  claim 1 , wherein said starting dispersion of PTFE comprises PTFE having a primary particle size between 0.1 to 0.5 microns.  
   
   
       25 . The process of  claim 1 , wherein said concentrated PTFE dispersion comprises a homopolymer of TFE in an amount of between 30 and 80 wt. %, based on the total weight of said concentrated PTFE dispersion.  
   
   
       26 . The process of  claim 1 , wherein said concentrated PTFE dispersion comprises PTFE having a primary particle size between 0.1 to 0.5 microns after thermal concentration.  
   
   
       27 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier.  
   
   
       28 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one anionic emulsifier.  
   
   
       29 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one cationic emulsifier.  
   
   
       30 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier and at least one anionic emulsifier.  
   
   
       31 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier and at least one cationic emulsifier.  
   
   
       32 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises a mixture of at least one nonionic emulsifier, at least one cationic emulsifier, and at least one anionic emulsifier.  
   
   
       33 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one nonionic surfactant which has a cloud point range from 15 to 90° C.  
   
   
       34 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one cationic surfactant which has a cloud point range from 15 to 90° C.  
   
   
       35 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one anionic surfactant which has a cloud point range from 15 to 90° C.  
   
   
       36 . The process of  claim 1 , wherein said non-fluorinated emulsifier is added to said starting aqueous dispersion of PTFE in an amount of 5 to 25 wt. %, based on the total weight of PTFE in said starting aqueous dispersion of PTFE.  
   
   
       37 . The process of  claim 1 , further comprising: 
 (1b) adding an electrolyte to the stabilized dispersion, to obtain an electrolyte-containing dispersion, prior to said heating (2).    
   
   
       38 . The process of  claim 1 , wherein said electrolyte is a base.  
   
   
       39 . The process of  claim 1 , wherein the preferred base is ammonium hydroxide (30% aqueous solution).  
   
   
       40 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one anionic emulsifier selected from the group consisting of sodium lauryl sulfate, ammonium lauryl sulfate, a carboxylic acid salt, a sulfuric acid ester salt, a sulfated polyoxyethylenated straight-chain alcohol having a structure of R(OC 2 H 4 ) x SO 4   − M + , wherein R is a straight chain or branched chain C 4 -C 20 -alkyl group, a sulfated triglyceride oil, and mixtures thereof.  
   
   
       41 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one cationic emulsifier selected from the group consisting of Ethomeen T/12, Ethomeen T/15, Ethomeen 18/15, Ethomeen 18/25, Ethomeen 18/60, a long-chain amine and a salt thereof, a synthetic C 12  -C 18  primary, secondary, or tertiary amine; a diamine and polyamine and a salt thereof having a structure of (RCONHCH 2 CH 2 ) 2 NH, wherein R is a straight chain or branched chain C 4 -C 20 -alkyl group; a polyoxyethylenated (POE) long-chain amine having a structure of RN[(CH 2 CH 2 O) x H] 2 , wherein R is a straight chain or branched chain C 4 -C 20 -alkyl group; a quaternized polyoxyethylenated long-chain amine having a structure of RN(CH 3 )[(C 2 H 4 O) x H] 2    + Cl − , wherein R is a straight chain or branched chain C 4 -C 20 -alkyl group; an amine oxide, and mixtures thereof.  
   
   
       42 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier has the following formula (1) or (2):  
       R—O-A-H   (1)  
     wherein R is a straight chain or a branched chain C 8-18 -alkyl group and A is a polyoxyalkylene chain containing 5 to 20 oxyethylene groups and from 0 to 2 oxypropylene groups;  
       R′—C 6 H 4 —O-A′-H   (2)  
     where R′ is a straight chain or a branched chain C 4-12 -alkyl group and A′ is a polyoxyethylene chain containing 5 to 20 oxyethylene groups.  
   
   
       43 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier which has the following formula:  
       R″—(C 2 H 4 O) m —OH,  
     where R″ is a primary or secondary C 11-18 -alkyl group or is  
     
       
         
         
             
             
         
       
     
     where R 1  is a primary, secondary, or tertiary C 8-12 -alkyl group, and m is a number from 8 to 14.  
   
   
       44 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one nonionic emulsifier selected from the group consisting of a polyoxyethylene, carboxylic esters, an anhydrosorbitol esters, a natural ethoxylated fat, a natural ethoxylated oils, a a natural ethoxylated wax, a glycol ester of a fatty acid, an alkyl polyglycoside, a carboxylic amides, a fatty acid glucamide, a polyalkylene oxide block polymer, a poly(oxyethylene-co-oxypropylene) nonionic surfactant, and mixtures thereof.  
   
   
       45 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one emulsifier selected from the group consisting of Triton N-11, Synperonic OP11EO, Synperonic 91/6, Tergitol 15-S-12, Tergitol 15-S-9, Tergitol 15-S-7, Newcol 1310, Dispanol TOC, and mixtures thereof.  
   
   
       46 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises at least one emulsifier selected from the group consisting of Newcol 1308FA(90), Ethox 4031 Mod 38, Ethox 4031 Mod 64, Synperonic OP10EO, Triton X-100, and mixtures thereof.  
   
   
       47 . The process of  claim 1 , wherein said non-fluorinated emulsifier comprises Ethox4031.  
   
   
       48 . The process of  claim 1 , wherein said starting aqueous dispersion of PTFE is a raw autoclave dispersion.  
   
   
       49 . A concentrated aqueous dispersion of PTFE, which is prepared by the process of  claim 1 .  
   
   
       50 . The concentrated aqueous dispersion of PTFE of  claim 49 , which is formulated into an aqueous dispersion for coating applications such as seal, packing, fabric and metal coating.  
   
   
       51 . A method of forming a coated substrate, said method comprising: 
 (a) forming a coating layer on a substrate,    wherein said coating layer comprises a dispersion of PTFE, and wherein said dispersion of PTFE has been prepared from a concentrated aqueous dispersion of PTFE prepared by the process of  claim 1 .    
   
   
       52 . The method of  claim 51 , wherein said substrate comprises at least one material selected from the group consisting of fabric, metal, plastic, wood, and combinations thereof.