Residue reduction in perfluoroalkoxy alkane (PFA) dispersions

Abstract

A process includes contacting a perfluoroalkoxy alkane (PFA) dispersion having a first cumulative concentration of linear C9-C14 perfluoroalkyl carboxylic acids to an ion exchange resin to remove at least 95% of the linear C9-C14 perfluoroalkyl carboxylic acids from the PFA dispersion. A PFA dispersion includes particles of perfluoroalkoxy alkane dispersed in an aqueous liquid. The particles have a raw dispersion particle size of less than 180 nm. The PFA dispersion has a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-billion or less.

Description

RESIDUE REDUCTION IN PERFLUOROALKOXY ALKANE (PFA) DISPERSIONSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application No. 63 / 563,028 filed March 8, 2024, the disclosure of which is incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure is related to the field of fluoropolymers. More specifically, the present disclosure is related to perfluoroalkoxy alkane (PFA) dispersions and composition having reduced residue levels and processes of forming the same.BACKGROUND

[0003] Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. One method of applying fluoropolymers to substrates is by dispersion coating, i.e. , applying the dispersion in dispersion form to the substrate with the subsequent application of heat for drying and coalescence.

[0004] Dispersion coating processes typically employ fluoropolymer dispersions in a more concentrated form than the as-polymerized dispersion. Thus, dispersions are often concentrated, such as, for example, by the method taught in U.S. Patent No. 3,037,953, which includes the addition of a nonionic surfactant to the as-polymerized dispersion, heating to above the cloud point, and removing the clear upper supernate that forms above the concentrated dispersion. In addition, to inhibit the growth of bacteria, basic compounds such as, for example, ammonium hydroxide or sodium hydroxide, are added to increases the pH of the dispersion sufficiently such that bacteria do not grow. Addition of base is typically done prior to concentrating the dispersion, i.e., prior to or together with the addition of nonionic surfactant to the dispersion.

[0005] The concentrated dispersions used for dispersion coating thus may contain a significant quantity of nonionic surfactant, e.g., 6-8 wt % percent based on the weight of fluoropolymer solids in the dispersion. Dispersion coating processes using concentrated dispersions include the steps of applying concentrated dispersion to a substrate by common techniques such as spraying, roller or curtain coating, drying the substrate to remove volatile components (primarily water and nonionic surfactant), and baking the substrate. When baking temperatures are high enough, the primary dispersion particles fuse and become a coherent mass. Baking at high temperatures to fuse particles of non-melt-processible fluoropolymer is often referred to as sintering.

[0006] As described in U.S. Patent No. 2,559,752, fluorosurfactants are used as non-telogenic dispersing agents in the manufacture of aqueous fluoropolymer dispersions and thus, unless removed, fluorosurfactants are normally present in aqueous fluoropolymer dispersions. Due to environmental concerns, it is frequently desirable to reduce the fluorosurfactant content and content of other fluorinated residues of fluoropolymer dispersions.

[0007] Other fluorinated residuals of manufactured fluoropolymers include certain impurities from the manufacturing process, including perfluoroalkyl carboxylic acids (PFCAs). For example, a conventional process of aqueous emulsion polymerization to form a dispersion of a copolymer of tetrafluoroethylene and perfluoroalkylvinyl ether (PFA) dispersion produces significant levels of PFCAs in the dispersion.

[0008] A PFA is a copolymer of tetrafluoroethylene (TFE) and perfluoroalkylvinyl ether (PAVE). Short chain PAVEs include perfluoromethylvinyl ether (PMVE), perfluoroethylvinyl ether (PEVE), and perfluoropropylvinyl ether (PPVE).

[0009] Certain PFCA levels, particularly the levels of the linear (i.e., unbranched) perfluoroalkyl carboxylic acids C8-C14, are being subject to increasing regulatory control. The linear C8 PFCA is n-perfluorooctanoic acid (CsFizCOOH; PFOA; C8). The linear C9-C14 perfluoroalkyl carboxylic acids include n-perfluorononanoic acid (C9F19COOH; PFNA; C9), n-perfluorodecanoic acid (C10F21COOH; PFDA; C10), n-perfluoroundecanoic acid (C11F23COOH; PFUDA; C11), n-perfluorododecanoic acid (C12F25COOH; PFDoA; C12), n-perfluorotridecanoic acid (C13F27COOH; PFTrDA; C13), and n-perfluorotetradecanoic acid (C14F29COOH; PFTeDA; C14).

[0010] As a part of environmental stewardship and high-quality manufacturing processes, technology has been developed for the reduction of residual PFCAs in polymer products including efficient, cost effective, practical processes for residuals reduction in aqueous workstreams.SUMMARY

[0011] In some embodiments, a process produces an aqueous PFA fluoropolymer dispersion with low linear C9-C14 perfluoroalkyl carboxylic acid content by first polymerizing at least one fluoromonomer in an aqueous medium in the presence of a fluorosurfactant and a nucleant to produce the aqueous PFA fluoropolymer dispersion having an initial linear C9-C14 perfluoroalkyl carboxylic acid content and fluoropolymer particles having a raw dispersion particle size of less than 180 nm. The process also includes modifying the fluorosurfactant dispersion by adding a nonionic surfactant to the fluorosurfactant dispersion, contacting the modified fluoropolymer dispersion with an ion exchange resin to remove fluorinated surfactant and to reduce the linear C9-C14 perfluoroalkyl carboxylic acid content to a predetermined level, and separating the anion exchange resin from the dispersion after the linear C9-C14 perfluoroalkyl carboxylic acid content has been reduced. In some embodiments, the process further includes concentrating the PFA dispersion to a predetermined final solids percent.

[0012] In one embodiment, a perfluoroalkoxy alkane (PFA) dispersion comprises particles of perfluoroalkoxy alkane dispersed in an aqueous liquid. The particles have a raw dispersion particle size of less than 180 nm. The PFA dispersion has a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-bil lion or less.

[0013] In one embodiment of the dispersion, the perfluoroalkane is a copolymer of tetrafluoroethylene and a comonomer selected from the group consisting of perfluoromethylvinyl ether, perfluoroethylvinyl ether, perfluoropropylvinyl ether, and a combination thereof.

[0014] In one embodiment of the dispersion, the comonomer is perfluoromethylvinyl ether.

[0015] In another embodiment of the dispersion, the comonomer is perfluoroethylvinyl ether.

[0016] In another embodiment of the dispersion, the comonomer is perfluoropropylvinyl ether.

[0017] In another embodiment of the dispersion, the raw dispersion particle size is less than 170 nm.

[0018] In another embodiment of the dispersion, the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 200 parts-per-billion or less.

[0019] In another embodiment of the dispersion, the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 100 parts-per-billion or less.

[0020] In another embodiment of the dispersion, the solids content is at least 40 wt%.

[0021] In another embodiment, a process comprises contacting a perfluoroalkoxy alkane (PFA) dispersion having a first cumulative concentration of linear C9-C14 perfluoroalkyl carboxylic acids to an ion exchange resin to remove at least 95% of the linear C9-C14 perfluoroalkyl carboxylic acids from the PFA dispersion.

[0022] In one embodiment of the process, the ion exchange resin comprises counterions selected from the group consisting of hydroxyl, proton, and chloride and has a resin functionality selected from the group consisting of Type I, Type II, tributylammonium, and quaternary ammonium.

[0023] In another embodiment of the process, the ion exchange resin is a strong base anion resin.

[0024] In another embodiment of the process, the ion exchange resin is polystyrenic gel Type II ion exchange resin.

[0025] In another embodiment, the process further comprises selecting a concentration of a nucleant to form the PFA dispersion by dispersion polymerization to have a raw dispersion particle size of less than 180 nm.

[0026] In another embodiment of the process, the raw dispersion particle size is less than 170 nm.

[0027] In another embodiment, the process further comprises forming the PFA dispersion having a raw dispersion particle size of less than 180 nm with the selected concentration of the nucleant.

[0028] In another embodiment of the process, the nucleant is a perfluoropolyether carboxylic acid.

[0029] In another embodiment of the process, the PFA dispersion comprises particles of perfluoroalkoxy alkane dispersed in an aqueous liquid.

[0030] In another embodiment of the process, the perfluoroalkoxy alkane is a copolymer of tetrafluoroethylene and a comonomer selected from the group consisting of perfluoromethylvinyl ether, perfluoroethylvinyl ether, perfluoropropylvinyl ether, and a combination thereof.

[0031] In another embodiment of the process, the comonomer is perfluoromethylvinyl ether.

[0032] In another embodiment of the process, the comonomer is perfluoroethylvinyl ether.

[0033] In another embodiment of the process, the comonomer is perfluoropropylvinyl ether.

[0034] In another embodiment of the process, the PFA dispersion has a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-bil lion or less.

[0035] In another embodiment of the process, the process further comprises separating the PFA dispersion from the ion exchange resin.

[0036] In another embodiment of the process, the process further comprises concentrating the PFA dispersion to a final solids content of at least 40 percent, by weight.

[0037] In another embodiment of the process, the total concentration of linear COCI 4 perfluoroalkyl carboxylic acids is about 200 parts-per-billion or less.

[0038] In another embodiment of the process, the total concentration of linear COCI 4 perfluoroalkyl carboxylic acids is about 100 parts-per-billion or less.DETAILED DESCRIPTION

[0039] As discussed above, dispersion coating processes typically employ fluoropolymer dispersions in a more concentrated form than the as-polymerized dispersion, i.e., the concentrated dispersions have a final fluoropolymer solids content of about 35 to about 70 wt %. These concentrated dispersions contain surfactants and a significant quantity of nonionic surfactant in the range of 2-11 wt %, typically 6-8 wt % based on the weight of fluoropolymer in the dispersion. These concentrated dispersions also contain a significant quantity of fluorinated residues, including linear C9-C14 perfluoroalkyl carboxylic acids.

[0040] Provided are PFA compositions and PFA dispersions having reduced levels of linear C9-C14 perfluoroalkyl carboxylic acids and methods of forming such compositions and dispersions.

[0041] It has been surprisingly found that linear C9-C14 perfluoroalkyl carboxylic acids can more easily be removed by ion exchange from certain PFA dispersions having a lower raw dispersion particle size (RDPS).

[0042] In some embodiments, a perfluoroalkoxy alkane (PFA) dispersion includes particles of perfluoroalkoxy alkane dispersed in an aqueous liquid. The particles have a raw dispersion particle size (RDPS) of less than 180 nm. The PFA dispersion has a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-bil lion or less (ppb).

[0043] In some embodiments, the perfluoroalkylvinyl ether (PAVE) of the PFA is perfluoromethylvinyl ether (PMVE), perfluoroethylvinyl ether (PEVE), perfluoropropylvinyl ether (PPVE), and a combination thereof. In some embodiments, the comonomer includes PMVE. In some embodiments, the comonomer includes PEVE. In some embodiments, the comonomer includes PPVE. Appropriate amounts of PAVE include, but are not limited to, up to about 12 wt%, alternatively about 0.1 to about 12 wt%, alternatively about 1 to about 10 wt%, alternatively about 2 to about 8 wt%, alternatively about 3 to about 5 wt%, or any value, range, or sub-range therebetween, based on the total weight of the PFA polymer.

[0044] An appropriate raw dispersion particle size (RDPS) may include, but is not limited to, less than 190 nm, alternatively 189 nm to about 150 nm, alternatively less than 180 nm, alternatively 179 nm to about 150 nm, alternatively 175 nm to about 150 nm, alternatively about 170 nm to about 150 nm, alternatively about 170 nm to about 160 nm, alternatively about 160 nm to about 150 nm, or any value, range, or subrange therebetween.

[0045] An appropriate total concentration of linear C9-C14 perfluoroalkyl carboxylic acids may include, but is not limited to, about 500 ppb or less, about 200 ppb or less, alternatively about 150 ppb or less, alternatively about 100 ppb or less, alternatively about 75 ppb or less, alternatively about 50 ppb or less, or any value, range, or subrange therebetween.

[0046] An appropriate solids content may include, but is not limited to, at least 20 wt%, alternatively at least 30 wt%, alternatively about 20 wt% to about 70 wt%, alternatively about 30 wt% to about 60 wt%, alternatively about 20 wt% to about 60 wt%, alternatively about 40 wt% to about 60 wt%, or any value, range, or sub-range therebetween.

[0047] Aqueous fluoropolymer dispersions are useful as coating or impregnating compositions and to make cast films.

[0048] In some embodiments, a process produces an aqueous PFA fluoropolymer dispersion with a low linear C9-C14 perfluoroalkyl carboxylic acid content by first polymerizing at least one fluoromonomer in an aqueous medium in the presence of at least one fluorosurfactant and a nucleant in a predetermined amount to produce the aqueous PFA fluoropolymer dispersion having an initial linear C9-C14 perfluoroalkyl carboxylic acid content and fluoropolymer particles having a raw dispersion particle size (RDPS) of less than 180 nm.

[0049] In some embodiments, the polymerization occurs in an aqueous solution including a nucleant and one or more surfactants. In some embodiments, the concentration of the nucleant is selected to form the PFA dispersion by dispersion polymerization to have a predetermined raw dispersion particle size.

[0050] In some embodiments, the dispersion polymerization occurs by a process as described in U.S. Patent No. 8,519,072 or in U.S. Patent No. 9,732,212, which are incorporated by reference herein.

[0051] In some embodiments, the RDPS is achieved, selected, or controlled by a process as described in U.S. Patent No. 9,732,212.

[0052] In some embodiments, the dispersion polymerization includes copolymerization of tetrafluoroethylene (TFE) and perfluoroalkylvinyl ether (PAVE) in an aqueous dispersion including a nucleant and a fluorosurfactant.

[0053] Appropriate nucleants may include, but are not limited to, water-soluble hydrocarbon-containing surfactants, nonionic hydrocarbon-containing surfactants, cationic hydrocarbon-containing surfactants, or perfluoropolyethers.

[0054] Appropriate nonionic surfactant nucleants may include, but are not limited to, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters, sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, glycerol esters, their derivatives, and the like. More specifically, appropriate polyoxyethylene alkyl ethers may include, but are not limited to, polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene behenyl ether, and the like. More specifically, appropriate polyoxyethylene alkyl phenyl ethers may include, but are not limited to, polyoxyethylene nonyl phenyl ether, polyoxyethylene octyl phenyl ether and the like. More specifically, appropriate polyoxyethylene alkyl esters may include, but are not limited to, polyethylene glycol monolaurylate, polyethylene glycol monooleate, polyethylene glycol monostearate and the like. More specifically, appropriate sorbitan alkyl esters may include, but are not limited to, polyoxyethylene sorbitan monolaurylate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate, and the like. More specifically, appropriate polyoxyethylene sorbitan alkyl esters may include, but are not limited to, polyoxyethylene sorbitan monolaurylate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, and the like. More specifically, appropriate glycerol esters may include, but are not limited to, glycerol monomyristate, glycerol monostearate, glycerol monooleate, and the like. More specifically, appropriate derivatives may include, but are not limited to,polyoxyethylene alkyl amine, polyoxyethylene alkyl phenyl-formaldehyde condensate, polyoxyethylene alkyl ether phosphate, and the like.

[0055] Particularly preferable nonionic surfactant nucleants include polyoxyethylene alkyl ethers and polyoxyethylene alkyl esters. Particularly preferable nonionic surfactant nucleants have a hydrophilic-lipophilic balance (HLB) value of 10 to 18. Particularly preferable nonionic surfactant nucleants include polyoxyethylene lauryl ether (5 to 20 ethylene oxide units), polyethylene glycol monostearate 10 to 55 ethylene oxide units), and polyethylene glycol monooleate (6 to 10 ethylene oxide units).

[0056] Appropriate perfluoropolyether nucleants may include, but are not limited to, perfluoropolyether carboxylic acids, such as, for example, Krytox™ 157FSH (The Chemours Company FC, LLC, Wilmington, DE) nucleant, which has the chemical structure C3H7-O-(CF(CF3)-CF2-O)n-CF(CF3)-COOH.

[0057] In some embodiments, the surfactants include a polymeric fluorosurfactant or a non-polymeric fluorosurfactant.

[0058] Appropriate fluorosurfactants may include, but are not limited to, a non- telogenic, anionic dispersing agent, soluble in water and including an anionic hydrophilic group and a hydrophobic portion. In some embodiments, the hydrophobic portion is an aliphatic fluoroalkyl group containing at least four carbon atoms, all except at most one of which, and that one the closest to the solubilizing group, bearing at least two fluorine atoms, the terminal carbon atom bearing in addition an atom consisting of hydrogen or fluorine. These fluorosurfactants are used as a polymerization aid for dispersing and because they do not chain transfer, they do not cause formation of polymer with undesirable short chain length. An extensive list of suitable fluorosurfactants is disclosed in U.S. Patent No. 2,559,752. Preferably, the fluorosurfactant is a perfluorinated carboxylic acid having 6-10 carbon atoms and is typically used in salt form. Specific suitable fluorosurfactants include ammonium perfluorocarboxylates, such as, for example, ammonium perfluorocaprylate or ammonium perfluorooctanoate. In some embodiments, the fluorosurfactant is 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propionic acid. The fluorosurfactants are usually present in the amount of 0.02 to 1 wt % with respect to the amount of polymer formed.

[0059] In some embodiments, the polymerization conditions are selected to provide fluoropolymer particles having a predetermined raw dispersion particle size (RDPS). In some embodiments, the predetermined raw dispersion particle size is less than 180 nm. In some embodiments, an increased concentration of nucleant provides the predetermined RDPS. In some embodiments, a decreased reaction time provides the predetermined RDPS.

[0060] In some embodiments, a process includes modifying the aqueous PFA fluoropolymer dispersion by adding nonionic surfactant to the aqueous PFA fluoropolymer dispersion after producing the aqueous PFA fluoropolymer dispersion.

[0061] Appropriate nonionic surfactants may include, but are not limited to, Tergitol™ TMN-10 or Tergitol™ TMN-6 surfactant (The Dow Chemical Company, Midland, Ml). Other additives may be added during the modifying, including, but not limited to, ethylenediaminetetraacetic acid (EDTA); a base, such as, for example, ammonium hydroxide, to adjust the pH; a viscosity reducer and / or ion replacer, such as, for example, ammonium sulfate; ammonium fluoride; and / or water.

[0062] In some embodiments, a process includes contacting the modified aqueous PFA fluoropolymer dispersion containing linear C9-C14 perfluoroalkyl carboxylic acid with an ion exchange resin to reduce linear C9-C14 perfluoroalkyl carboxylic acid content to a predetermined level or by a predetermined percentage.

[0063] Appropriate ion exchange resin counterions may include, but are not limited to, hydroxyl, proton, or chloride counterions. In some embodiments, the ion exchange resin is a strong base anion resin. In some embodiments, the ion exchange resin is a polystyrenic gel Type II resin.

[0064] Appropriate ion exchange resin functionalities may include, but are not limited to, Type I, Type II, tributylammonium, or quaternary ammonium.

[0065] Appropriate ion exchange resin backbones may include, but are not limited to, divinylbenzene-crosslinked polystyrene or divinylbenzene-crosslinked polyacrylate.

[0066] Appropriate ion exchange resin porosities may include, but are not limited, porous, microporous, hyperporous, or gel.

[0067] Appropriate commercial ion exchange resins may include, but are not limited to, Purofine™ PFA300, Purofine™ PFA300OH, Purofine™ PFA694, Purofine™ PFA694E, Purolite™ A501 P, Purolite™ A850, or Purolite™ A860 ion exchange resins (Purolite Company, King of Prussia, PA); CalRes 2301 or CalRes2304 (Calgon Carbon Corporation, Pittsburgh, PA); AmberLite™ PSR2 Plus or AmberLite™ IRA458 ion exchange resins (E. I. du Pont de Nemours, Inc., Wilmington, DE); DIAION™ SAF12A, DIAION™ SAF11AL, DIAION™ HPA25M, DIAION™ HPA512L, or DIAION™ PA312 ion exchange resins (Mitsubishi Chemical Corporation, Tokyo, Japan); or Lewatit™ TP106, Lewatit™ TP108, or Lewatit™ MonoPlus TP109 ion exchange resins (Lanxess Deutschland GmbH, Cologne, Germany).

[0068] Any of a variety of techniques that bring the dispersion in contact with the anion exchange resin and then separate the dispersion from the anion exchange resin can be used for carrying out the ion exchange process. For example, the process can be carried out by addition of ion exchange resin bead to the dispersion in a stirred tank, in which a slurry of the dispersion and resin is formed, followed by separation of the dispersion from the anion exchange resin beads by filtration. Another suitable method is to pass the dispersion through a fixed bed of anion exchange resin instead of using a stirred tank. Flow can be upward or downward through the bed and no separate separation step is needed beyond the flowing since the resin remains in the fixed bed.

[0069] In some embodiments, a process includes concentrating an aqueous PFA fluoropolymer dispersion having a reduced linear C9-C14 perfluoroalkyl carboxylic acid content to a predetermined solids percent.

[0070] In some embodiments, the concentrating is by a method taught in U.S. Patent No. 3,037,953, which is incorporated by reference herein. In some embodiments, the concentrating includes adding a nonionic surfactant to the as polymerized dispersion, heating to above the cloud point, and removing the clear upper supernatant layer forming above the concentrated dispersion. In some embodiments, the heating is to about 60 to about 70°C for about 2 to about 4 hours.TEST METHODSRaw Dispersion Particle Size (RDPS) Measurements

[0071] After dispersion polymerization of the PFA dispersions of Comparative Example 1 (CE1), Inventive Example 1 (IE1), Inventive Example 2 (IE2), and Inventive Example 3 (IE3), their RDPS was measured using a Malvern Zetasizer Advance Blue (Malvern Panalytical Inc., Westborough, MA) particle size analyzer.

[0072] After dispersion polymerization of the PFA dispersions of Comparative Example 2 (CE2), Inventive Example 4 (IE4), and Inventive Example 5 (IE5), their RDPS was measured using a Nanotrac Wave II (Microtrac MRB, York, PA) particle size analyzer.

[0073] Both particle size analyzers operate on the similar principle of a dynamic light scattering (DLS) technique and are expected to provide similar results. One was chosen over the other merely for convenience of proximity to the sample. The Zetasizer particle size analyzer uses the Stokes- Einstein relationship to determine the RDPS from the DLS data, whereas the Nanotrac particle size analyzer uses Mie calculation theory to determine the RDPS from the DLS data.Melt Flow Rate (MFR) Measurements

[0074] Melt flow rate was measured according to ASTM D-1238 using a 5-kg weight on the molten polymer at a melt temperature which is standard for the specific copolymer.Percent Solids Measurements

[0075] The reported weight percent of fluoropolymer solids in aqueous dispersions was measured using an MB45 Moisture Analyzer (Ohaus Corporation, Parsippany, NJ). The measurement was carried out as follows. A clean glass fiber pad was placed on the instrument balance and tared. A two-gram dispersion sample was pipetted on the glass fiber pad. The drying process was started by pushing the start button. The integral halogen dryer was programed to reach 175°C. During the drying process, water vaporized, and upon completion, the results were displayed as wt% solids. Typical drying times were about 5 minutes.Perfluoroal kyl Carboxylic Acid (PFCA) Measurements

[0076] The individual concentrations of linear C4-C14 perfluoroalkyl carboxylic acids in a PFA dispersion were measured after first extracting the perfluoroalkyl carboxylic acids from the fluoropolymer dispersion. After extraction, the sample extract can be interrogated using a variety of analytical techniques. Liquid chromatography (LC) with tandem mass spectrometry was then used to measure the concentrations of linear C4-C14 perfluoroalkyl carboxylic acids. A targeted LC method utilizing triple quadrupole detection (LC / QqQ) separated analytes based on their unique retention time and distinct Multiple Reaction Monitoring (MRM) transition, allowing discrete analytes to be measure at concentrations as low as parts-per-trillion.EXAMPLESSynthesis of Perfluoroal koxy Alkane (PFA) Dispersion: Comparative Examples

[0077] Two Comparative Examples (CE1 and CE2) were polymerized under similar conditions by a dispersion polymerization process, as disclosed in U.S. Patent No. 8,519,072 or in U.S. Patent No. 9,732,212, which are incorporated by reference herein. The vinyl ether was perfluoropropylvinyl ether (PPVE).

[0078] After polymerization of the dispersion of CE1 , the RDPS of CE1 was measured to be about 192 nm. The percent solids of CE1 was determined to be about 27.1 wt%, and the melt flow rate (MFR) was about 2 g / 10 min. The individual concentrations of linear C4-C14 PFCAs in the dispersion were measured. Table 1 shows the measured individual concentrations in parts-per-billion (ppb) of C8-C14 PFCAs and the summed total amount of measured linear C9-C14 PFCAs in CE1 prior to ion exchange (OX).

[0079] After synthesis of the dispersion of CE2, the RDPS of CE2 was measured to be about 190-200 nm. The percent solids of CE2 was determined to be about 33.8 wt%, and the MFR was about 2 g / 10 min. The individual concentrations of linear C4- C14 PFCAs in the dispersion were measured. Table 2 shows the measured individual concentrations ppb of C8-C14 PFCAs and the summed total amount of measured linear C9-C14 PFCAs in CE2 prior to ion exchange (OX). The measureddifferences between CE1 and CE2 show the potential variability between different batches.Synthesis of Perfluoroal koxy Alkane (PFA) Dispersions: Inventive Examples

[0080] Five Inventive Examples having a smaller RDPS than that of CE1 and CE2 were synthesized. The vinyl ether for Inventive Example 1 (IE1) and Inventive Example 4 was perfluoropropylvinyl ether (PPVE). The vinyl ether for Inventive Example 2 (IE2), Inventive Example 3 (IE3), and Inventive Example 5 (IE5) was perfluoroethylvinyl ether (PEVE).

[0081] The Inventive Examples were synthesized by the same dispersion polymerization process as for the Comparative Examples, except that a higher initial concentration of the nucleant was provided in the reaction mixture to give a lower raw dispersion particle size (RDPS) than for the Comparative Examples.

[0082] Inventive Example 1 had an RDPS of about 169 nm. The percent solids was determined to be about 19.8 wt%. The MFR was about 2 g / 10 min.

[0083] Inventive Example 2 had an RDPS of about 164 nm. The percent solids was determined to be about 27.2 wt%. The MFR was about 7 g / 10 min.

[0084] Inventive Example 3 had an RDPS of about 151 nm. The percent solids was determined to be about 21.4 wt%. The MFR was about 16 g / 10 min.

[0085] Inventive Example 4 had an RDPS of about 160-170 nm. The percent solids was determined to be about 34.2 wt%. The MFR was about 2 g / 10 min.

[0086] Inventive Example 5 had an RDPS of about 150-170 nm. The percent solids was determined to be about 35.1 wt%. The MFR was about 7 g / 10 min.

[0087] The individual concentrations of linear C4-C14 PFCAs in the dispersions were measured. Table 1 shows the measured individual concentrations in parts-per- billion (ppb) of C8-C14 PFCAs and the summed total amount of measured linear C9- C14 PFCAs for CE1 , IE1, IE2, and IE3 prior to ion exchange (OX). Table 2 shows the measured individual concentrations in parts-per-billion (ppb) of C8-C14 PFCAs and the summed total amount of measured linear C9-C14 PFCAs for CE2, IE4, and IE5 prior to ion exchange (OX).First Scale Removal of PFCA from PFA Dispersions by Ion Exchange

[0088] Comparative Example 1 and Inventive Examples 1, 2, and 3 were applied to a Purofine™ PFA300OH ion exchange resin on a column at a first scale. The individual concentrations of linear C4-C14 PFCAs in the dispersion were measured after a first pass through the column (1X) and again after a second pass through the column (2X). Table 1 shows the measured individual concentrations in ppb of C8- C14 PFCAs and the summed total amount of measured linear C9-C14 PFCAs.Table 1

[0089] As shown in Table 1 , PFCAs were removed much more efficiently from the Inventive Examples than Comparative Example 1. Only about 50% of the linear COCI 4 PFCAs was removed after a first pass through the ion exchange column, and only about 60% was removed after a second pass. In contrast, about 97 to 99% of the linear C9-C14 PFCAs was removed after a first pass through the ion exchange column for the Inventive Examples. Although Table 1 shows that a second pass removed a negligible amount further PFCAs from the Inventive Examples, significant amounts of further PFCAs were removed in a second pass at the first scale in other inventive examples not shown.Second Scale Removal of PFCA from PFA Dispersions by Ion Exchange

[0090] Comparative Example 2 and Inventive Examples 4 and 5 were applied to a Purofine™ PFA300OH ion exchange resin on a column at a second scale larger than the first scale. The individual concentrations of linear C4-C14 PFCAs in the dispersion were measured after a first pass through the column (1X) and again after a second pass through the same column (2X). Table 2 shows the measured individual concentrations in ppb of C8-C14 PFCAs and the summed total amount of measured linear C9-C14 PFCAs. The concentrations for IE5 prior to ion exchange and after one ion exchange are the averages of three measurements. The concentrations for IE5 after two ion exchange are the averages of two measurements.Table 2

[0091] As shown in Table 2, PFCAs were removed much more efficiently from the Inventive Examples. Only about 80% of the linear C9-C14 PFCAs was removed after a first pass through the ion exchange column, and only about 85% was removed after a second pass. In contrast, about 98 and about 99% of the linear C9-C14 PFCAs was removed after a first pass through the ion exchange column for the Inventive Examples. About 99% of the linear C9-C14 PFCAs was removed after the second pass through the ion exchange column for the Inventive Examples.Concentration of PFA Dispersions

[0092] After ion exchange, Comparative Example 2 and Inventive Examples 4 and 5 were concentrated (C) to about 60 wt% solids and the individual concentrations of PFCAs were measured. The results are shown in Table 3 along with the 2X concentrations for comparison. The concentrations for IE5 are an average of three measurements.Table 3

[0093] Table 3 shows that the concentrations of the PFCAs roughly doubled upon roughly doubling the percent solids, indicating that the PFCAs were likely associated with the PFA particles rather than the liquid in the dispersions.Ion Exchange Resins

[0094] In addition to the Purofine™ PFA300OH ion exchange resin used in the above-described Examples, additional ion exchange resins were tested for ion exchange effectiveness for removal of PFCAs from PFA dispersions. CalRes 2301 , AmberLite™ PSR2 Plus, DIAION™ SAF12A, and DIAION™ PA312 commercial ion exchange resins all had a similar effectiveness to the Purofine™ PFA300OH ion exchange resin.Further Evaluations

[0095] Several variations were performed to determine their effect on the ability to provide PFA dispersions with reduced PFCA levels. The Comparative Examples had a pH of 9 prior to ion exchange, whereas the Inventive Examples had a pH of 3. To rule out a role of pH, the pH of an Inventive Example after polymerization but prior toion exchange was adjusted to a pH of 9 with a negligible effect on the ion exchange effectiveness.

[0096] The Comparative Examples included more surfactant during polymerization than the Inventive Examples. To rule out a role of surfactant concentration, surfactant was added to an Inventive Example after polymerization but prior to ion exchange with a negligible effect on the ion exchange effectiveness.

[0097] In a first scale, a series ion exchange through two ion exchange columns produced similar PFCA removal to running a sample twice through the same ion exchange column.

[0098] Column packing and flowrate during ion exchange were independently varied and shown to have a negligible effect on ion exchange effectiveness.

[0099] Without wishing to be bound by theory, it is believed that the smaller RDPS of the PFA dispersions of the Inventive Examples permits the greater effectiveness of ion exchange removal of PFCAs from the Inventive Examples than the Comparative Examples.

[0100] All above-mentioned references are hereby incorporated by reference herein.

[0101] While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof, without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A perfluoroalkoxy alkane (PFA) dispersion comprising particles of perfluoroalkoxy alkane dispersed in an aqueous liquid, the particles having a raw dispersion particle size of less than 180 nm, the PFA dispersion having a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-bill ion or less.

2. The PFA dispersion of claim 1, wherein the perfluoroalkane is a copolymer of tetrafluoroethylene and a comonomer selected from the group consisting of perfluoromethylvinyl ether, perfluoroethylvinyl ether, perfluoropropylvinyl ether, and a combination thereof.

3. The PFA dispersion of claim 2, wherein the comonomer is perfluoromethylvinyl ether.

4. The PFA dispersion of claim 2, wherein the comonomer is perfluoroethylvinyl ether.

5. The PFA dispersion of claim 2, wherein the comonomer is perfluoropropylvinyl ether.

6. The PFA dispersion of claim 1, wherein the raw dispersion particle size is less than 170 nm.

7. The PFA dispersion of claim 1, wherein the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 200 parts-per-billion or less.

8. The PFA dispersion of claim 7, wherein the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 100 parts-per-billion or less.

9. The PFA dispersion of claim 1 , wherein the solids content is at least 40 wt%.

10. A process comprising: contacting a perfluoroalkoxy alkane (PFA) dispersion having a first cumulative concentration of linear C9-C14 perfluoroalkyl carboxylic acids with an ion exchange resin to remove at least 95% of the linear C9-C14 perfluoroalkyl carboxylic acids from the PFA dispersion.

11. The process of claim 10, wherein the ion exchange resin comprises counterions selected from the group consisting of hydroxyl, proton, and chloride and has a resin functionality selected from the group consisting of Type I, Type II, tributylammonium, and quaternary ammonium.

12. The process of claim 10, wherein the ion exchange resin is a strong base anion resin.

13. The process of claim 10, wherein the ion exchange resin is a polystyrenic gel Type II ion exchange resin.

14. The process of claim 10 further comprising selecting a concentration of a nucleant to form the PFA dispersion by dispersion polymerization to have a raw dispersion particle size of less than 180 nm.

15. The process of claim 14, wherein the raw dispersion particle size is less than 170 nm.

16. The process of claim 14 further comprising forming the PFA dispersion having a raw dispersion particle size of less than 180 nm with the selected concentration of the nucleant.

17. The process of claim 14, wherein the nucleant is a perfluoropolyether carboxylic acid.

18. The process of claim 10, wherein the PFA dispersion comprises particles of perfluoroalkoxy alkane dispersed in an aqueous liquid.

19. The process of claim 18, wherein the perfluoroalkoxy alkane is a copolymer of tetrafluoroethylene and a comonomer selected from the group consisting of perfluoromethylvinyl ether, perfluoroethylvinyl ether, perfluoropropylvinyl ether, and a combination thereof.

20. The process of claim 19, wherein the comonomer is perfluoromethylvinyl ether.

21. The process of claim 19, wherein the comonomer is perfluoroethylvinyl ether.

22. The process of claim 19, wherein the comonomer is perfluoropropylvinyl ether.

23. The process of claim 10, wherein the PFA dispersion has a solids content of at least 20 wt% and a total concentration of linear C9-C14 perfluoroalkyl carboxylic acids of about 500 parts-per-billion or less.

24. The process of claim 23, wherein the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 200 parts-per-billion or less.

25. The process of claim 10 further comprising separating the PFA dispersion from the ion exchange resin.

26. The process of claim 10 further comprising concentrating the PFA dispersion to a final solids content of at least 40 percent, by weight.

27. The process of claim 26, wherein the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 200 parts-per-billion or less.

28. The process of claim 27, wherein the total concentration of linear C9-C14 perfluoroalkyl carboxylic acids is about 100 parts-per-billion or less.