WO2022224105A1

WO2022224105A1 - Purifying aqueous ionomer dispersions

Info

Publication number: WO2022224105A1
Application number: PCT/IB2022/053538
Authority: WO
Inventors: Stefan Huber; Lisa P. Chen; Gregg D. Dahlke; Denis Duchesne; Klaus Hintzer; Matthew J. LINDELL; Mark W. Muggli; Arne Thaler; Michael A. Yandrasits; Fee ZENTIS
Original assignee: 3M Innovative Properties Company
Priority date: 2021-04-22
Filing date: 2022-04-14
Publication date: 2022-10-27

Abstract

Method of producing purified fluoropolymers comprising repeat units of tetrafluoroethylene and a fluoro-olefin comprising SO₃- groups are described. In a first step, an unpurified aqueous dispersion of the fluoropolymer, including the counter-cations of the SO₃- groups, is subjected to cation exchange. This cation-exchanged dispersion is then subjected to ultrafiltration. The resulting purified dispersions and fluoropolymers contain low levels of both cations and anions. Articles prepared from such purified fluoropolymers are also described.

Description

PURIFYING AQUEOUS IONOMER DISPERSIONS

FIEUD

[0001] The present disclosure relates to methods of purifying aqueous ionomer dispersions. The methods include performing cation exchange followed by ultrafiltration. The present disclosure also relates to highly-purified aqueous ionomer dispersions and ionomers.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides methods of producing a purified fluoropolymer, including those described in claim 1-21.

[0003] In another aspect, the present disclosure provides articles made from such purified fluoropolymers, including those described in claim 22-23.

[0004] The details of one or more embodiments of the invention are also set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAIUED DESCRIPTION

[0005] Fluorinated ionomers, e.g., copolymers of tetrafluoroethylene (TFE) and monomers having sulfonic acid pendant groups are known for making ionomer membranes for electrochemical cells such as fuel cells or used in electrolysis applications, e.g., NaCl, HC1 or water electrolysis. Typically, such fluorinated ionomers are prepared by first generating a copolymer of TFE and a monomer having pendent sulfonyl fluoride groups (also referred to as “sulfonyl fluoride polymer” or “precursor polymer”), post fluorinating the dry polymer and subsequently hydrolyzing the sulfonyl groups. The resulting sulfonate groups are then converted into sulfonic acid groups to produce a perfluorosulfonic acid polymer, also referred to as “PFSA ionomer.”

[0006] Generally, most of these steps are performed with an aqueous dispersion of the fluoropolymer.

The presence of certain ions in the aqueous dispersions or finished ionomers can raise a variety of concerns. For example, the presence of certain ions may be undesirable in electrochemical applications, the presence of other ions may contribute to defects in ionomer applications, while the presence of low molecular weight fluorinated ions may be the subject of tightening regulations.

[0007] For example, certain ions may be present as a result of the raw materials used, e.g., the surfactants or other additives used in the polymerization step, or counter ions used to hydrolyze the sulfonyl groups, e.g., Na, K, Li, and NR4. Other ions may be present as contaminants from processing equipment, e.g., Ni and Fe. Other ions such as Ce and Mn, may be present in recycled materials.

[0008] Although numerous approaches for purifying aqueous fluoropolymer dispersions have been attempted, there is an ongoing need to achieve even lower levels of certain ions and to reduce the costs and challenges of removing them. For example, prior art purification methods often conclude with an ion exchange step. However, this step introduces alternate ions that themselves may create their own concerns. For example, performing ultrafiltration of the salt form of an ionomer followed by cation- exchange process might introduce cations and HF. Other methods require multiple ultrafiltration steps separated by one or more ion exchange steps in order to achieve the desired reduction in ion content. In addition, some processes may adversely affect the fluoropolymers, generating additional undesired ions such as short chain perfluorinated materials. Surprisingly, the present inventors have discovered that performing cation exchange before ultrafiltration can generate highly purified dispersions without the need for a subsequent ion exchange step or additional ultrafiltration steps.

[0009] Generally, the methods of the present disclosure involve two steps. First, an unpurified aqueous dispersion of a fluoropolymer comprising repeat units of tetrafluoroethylene and a fhioro-olefm comprising SO3 groups with their associated counter ions is subjected to cation exchange to form a cation-exchanged aqueous dispersion of a fluoropolymer comprising sulfonic acid groups. Next, this cation-exchanged aqueous dispersion is subjected to ultrafiltration to form a purified aqueous dispersion comprising a purified fluoropolymer. Optionally the process might be designed as a closed loop to reduce water consumption and gain back valuable materials.

[0010] These methods are used to remove certain ions, referred to as “target ions.” Generally, the target ions are all ions present in the aqueous fluoropolymer dispersion that have a theoretical molecular weight (MW) of less than 150 Daltons, excluding H⁺. In some embodiments, the target ions include all ions present in the aqueous fluoropolymer dispersion having a theoretical molecular weight of less than 500, or even less than 1000 Daltons. In some embodiments, even higher molecular weight ions may also be removed, e.g., less than 10,000 Daltons. For example, low molecular weight ionomer species may be present in the aqueous dispersions, e.g., ionomer having a molecular weight of less than 10,000 Daltons. Additionally inert compounds such as fluorinated ethers are also removed.

[0011] Generally, the target ions include the cationic counter ions associated with the SO3 groups of the fluoropolymer (except for H⁺), other cations, and anions. Examples of suitable counterions that may be present include alkali metals such as Li⁺, Na⁺, K⁺and NRq⁺. including an excess of such cations.

Other cationic target ions that may be present include cations according to the formula Xⁿ⁺, wherein X is selected from the group consisting of Al, Ce, Cu, Cr, Fe, Mn, Ni, Ti and combinations thereof, and n is the valence of the cation. Anionic target ions that may be present include F , Cl ,

and

HCOO . In some embodiments, the target ions present in the aqueous fluoropolymer dispersion include low molecular weight fluorinated anions such as fluorinated carboxylates according to the formula RfCOO , and fluorinated sulfonates according to the formula RfS03 , where Rf is a linear or branched, fluorinated (in some embodiments, perfluorinated) alkyl or alkylene group having 1-40 carbons atoms, e.g., 1-20 carbon atoms, and Rf may be interrupted by 1 or 2 oxygen atoms.

[0012] Generally, the fluoropolymer comprises repeat units of tetrafluoroethylene and a fluoro-olefm comprising SO3 groups, and their associated counter ions. In some embodiments, the fluoropolymer comprises (i) divalent units derived from tetrafluoroethene (TFE) represented by formula -[CF2-CF2]- and (ii) divalent units represented by formula (I),

wherein a represents 0 or 1, b is an integer from 2 to 8, c is an integer from 0 to 2, e is an integer from 1 to 8, and X represents a counter cation other than a hydrogen cation. C_eF2_e may be linear or branched and preferably is linear. C_bF₂b may be linear or branched and preferably is branched. In some embodiments, b is a number from 2 to 6 or 2 to 4. In some embodiments, a is 0. In some embodiments both a and c are 0, and e is 3 to 8, preferably 3 to 6, more preferably 3 to 4, and most preferably 4.

[0013] In some embodiments, the counter cation is an alkali metal ion, e.g., Li⁺, Na+ or K+, or an ammonium cation NR4⁺ (Ri selected from H and/or an alkylgroup with 1-6 carbon atoms and/or a combination of those). In some embodiments, the counter cation is Li+. In some embodiments, the ionomer in the salt form is obtained by hydrolyzing the SO2F form of the fluoropolymer by reaction with a base (e.g., LiOH, L12CO3, or NaOH) in water and at temperatures above 130 °C, e.g., above 160 °C, or even above 200 °C.

[0014] In some embodiments, the fluoropolymer contains at least 60 mole % of divalent units represented by formula -[CF2-CF2]-, based on the polymer (100 mole %). In some embodiments, the fluoropolymer comprises at least 65, 70, 75, 80, or 90 mole % of divalent units represented by formula - [CF2-CF2]-, based on the polymer (100%).

[0015] Generally, the fluoropolymer contains from 10 to 40 mole % of divalent units represented by formula (I) based on the polymer (100 mole %). In some embodiments, the fluoropolymer comprises from 15 to 25 mole % of units represented by formula (I) based on the total units of the polymer (100 mole %).

[0016] In some embodiments, the fluoropolymer may include one or more additional comonomers.

For example, in some embodiments, the fluoropolymer may include divalent units independently represented by formula (II):

wherein m’ is 0 or 1, and Rf_| is a linear or branched perfluoroalkyl group having from 1 to 12 carbon atoms that can be interrupted once or more than once by an ether oxygen atom, in which case Rf_| is a perfluoroalkoxyalkyl group. Typically, such fluoropolymers contain from 0.05 to 15 mole % units represented by formula (II) based on the polymer (100 mole %). In some embodiments, the fluoropolymer comprises from 3 to 15 mole %, preferably from 5 to 10 mole% of units represented by formula (II). [0017] Although not necessary and less preferred, the fluoropolymers may also include units derived from other optional comonomers, in addition to or as alternatives to the optional comonomers according to formula (II). Such other optional comonomers include, for example, fluorinated olefins, non- fluorinated olefins and modifiers and cross-linkers, typically bisolefins. Typically, the fluoropolymers contain less than 10 mol% of units derived from optional comonomers and, preferably less than 5 mol% and more preferably 0 mol% the total moles of comonomer units of the fluoropolymer.

[0018] In some embodiments, it may be desirable to post-fluorinate the ionomers to reduce the carboxylic end-groups below 300 1/lMio C-Atoms, more preferably below 200 1/lMio C-Atoms by common technologies, e.g., fluorination with F2/N2 at temperatures above 50°C.

[0019] In the first step, cation exchange is performed on an unpurified aqueous fluoropolymer dispersion. As used herein, an unpurified aqueous dispersion is a dispersion that has not been processed to remove ions, e.g., it has not been subjected to ultrafiltration or anion exchange. However, in some embodiments, the unpurified aqueous fluoropolymer dispersion may have been treated to remove undissolved species like precipitated salts. Although not limited, generally, in some embodiments, the concentration of the fluoropolymer in the aqueous dispersion is between 5-30 wt.% , e.g., 10-25 wt.%. [0020] Any suitable cation exchange process may be used. In some embodiments, performing cation exchange comprises contacting the unpurified aqueous dispersion with a cation exchange membrane. In some embodiments, performing cation exchange comprises contacting the unpurified aqueous dispersion with a cation exchange resin, e.g., beads.

[0021] Useful cation exchange resins include polymers (typically cross-linked) that have a plurality of pendant anionic or acidic groups such as, for example, polysulfonates or polysulfonic acids, polycarboxylates or polycarboxylic acids. Examples of useful sulfonic acid cation exchange resins include sulfonated styrene-divinylbenzene copolymers, sulfonated crosslinked styrene polymers, phenol- formaldehyde-sulfonic acid resins, and benzene -formaldehyde -sulfonic acid resins. Preferably, the cation exchange resin is an organic acid cation exchange resin, such as carboxylic/sulfonic acid cation exchange resin. Cation exchange resins are available commercially from a variety of sources. Cation exchange resins are commonly supplied commercially in either their acid or their sodium form. If the cation exchange resin is not in the acid form (i.e., protonated form) it may be at least partially or fully converted to the acid form in order to avoid the generally undesired introduction of other cations into the dispersion. This conversion to the acid form may be accomplished by means well known in the art, for example by treatment with any adequately strong acid (e.g., HC1, H2SO4 and HNO3).

[0022] The ion-exchange process may be carried out in a batch process, or in a continuous mode using fixed bed columns, for example a column with a length to diameter ratio of 20: 1 to 1:1, preferably 10: 1 to 2: 1, at a flow rate of 0.5 to 10 bed volume/hour, preferably 0.8 to 5 bed volumes/h. The process can be monitored on-line or off-line by pH-meter, metal-analyzer (for example if the hydrolyzation is carried out using Li-salts, the lithium content can be measured) or other techniques (e.g., conductivity etc.). The ion- exchange resin can be a multimodal or a monomodal resin in respect to functionalization (multifunctional resins) and/or particle size, and it can have a broad or narrow particle size distribution. Commercially available cation exchange resins include but are not limited to those available under the trade designation LEWATIT MONO PLUS S 100, AMBERLITE IR-120(Plus) or PUROLITE 150C TLH.

[0023] In this first step, at least the counter cations (excluding H⁺) are removed. In some embodiments, the concentration of each counter ion (other than hydrogen cations) in the cation- exchanged aqueous dispersion is no greater than 200 ppm based on the weight of the fluoropolymer.

Thus, an aqueous dispersion containing 20 wt.% of the fluoropolymer would contain no greater than 40 ppm of each counter ion based on the total weight of the dispersion.

[0024] In some embodiments, the concentration of each counter ion in the cation-exchanged dispersion is no greater than 70 ppm by weight based on the weight of the fluoropolymer. In some embodiments, the total concentration of all counter cations in the cation-exchanged dispersion is no greater than 200 ppm, e.g., no greater than 70 ppm, or even no greater than 20 ppm by weight based on the weight of the fluoropolymer.

[0025] Other target cations present in the unpurified aqueous dispersion are also removed in the first step. For example, the unpurified aqueous dispersion may comprise one or more target cations according to the formula Xⁿ⁺, wherein X is selected from the group consisting of Al, Ce, Cu, Cr, Fe, Mn, Ni, Ti and combinations thereof, and n is the valence of the cation. In some embodiments, after the first step, the concentration of each target cation in the cation-exchanged dispersion is no greater than 20 ppm by weight based on the weight of the fluoropolymer.

[0026] In some embodiments, the total concentration of all counter cations and target cations in the cation-exchanged dispersion is no greater than 200 ppm by weight based on the weight of the fluoropolymer, e.g., no greater than 75 or even no greater than 50 ppm by mass based on the weight of the fluoropolymer.

[0027] Following the cation exchange of step (i), the cation-exchanged aqueous dispersion contains one or more target anions. In step (ii), ultrafiltration is performed on this cation-exchanged aqueous dispersion to reduce the concentration of such anions. Optionally, the cation exchanged dispersion after step (i) is dried to reduce volatile substances, e.g., HF, inerts, etc., redispersed with fresh deionized water (pH <7) and then ultrafdtered in step (ii).

[0028] For example, in some embodiments, the cation-exchanged dispersion comprises one or more target anions selected from the group consisting of HCOO

F , RfCOO , and RfS03 ; wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 1 to 20 carbon atoms, and Rf may be interrupted by 1 or 2 oxygen atoms, including also the partly dissociated form of those. In some embodiments, other target anions may be present such as, e.g., Cl , NO2 , NO3 . In some embodiments, one or more of the anions may be present in their protonated form. As used here, the term anion refers to both the anions and the anions in their protonated form. In some embodiments, step (ii) is performed until the purified aqueous dispersion comprises no greater 375 ppm by weight of each target anion, based on the weight of the fluoropolymer. In some embodiments, step (ii) is performed until the purified aqueous dispersion comprises no greater 300 ppm of each target anion, or even not greater than 150 or even 50 ppm, based on the weight of the fluoropolymer.

[0029] In some embodiments, the cation-exchanged dispersion comprises C^Oq^ . e.g., when hydrolysis temperatures of greater than 150 °C, e.g., greater than 200 °C, are used, C2O42 , can be generated. Generally, the higher the hydrolysis temperature, the more C

will be generated. In some embodiments, the unpurified aqueous dispersion contains greater than 500 ppm, greater than 1000 ppm, or even greater than 5000 ppm by weight of

based on the total weight of the fluoropolymer. In some embodiments, the purified aqueous dispersion comprises no greater 300 ppm by weight of

based on the weight of the fluoropolymer. In some embodiments, the purified aqueous dispersion comprises no greater 150 ppm of C2O42 , by weight based on the weight of the fluoropolymer.

[0030] In some embodiments, the cation-exchanged dispersion comprises F and/or SC^ and the purified aqueous dispersion comprises no greater 300 ppm, e.g., no greater than 150 ppm, by weight of F and/or SOq^ , based on the weight of the fluoropolymer.

[0031] In some embodiments, the concentration of each target anion in the purified aqueous dispersion is no greater than 100 ppm, e.g., no greater than 50 ppm, by weight based on the weight of the fluoropolymer.

[0032] In some embodiments, the target anions include at least one anion having the RfCOO , wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 1 to 20 carbon atoms, wherein the purified aqueous dispersion comprises no greater 500 ppb, preferably no greater than 100 ppb, e.g., no greater than 25 ppb or even no greater than 10 ppb, of each target anion according to the formula RfCOO , by weight based on the weight of the fluoropolymer. In some embodiments, the total concentration of all target anions according to the formula RfCOO , where Rf has 8 to 13 carbon atoms is no greater than 1 ppm, preferably no greater than 500 ppb, preferably no greater than 50 ppb, e.g., no greater than 25 ppb. [0033] In some embodiments, the target anions include at least one anion having the formula RfS03 , wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 1 to 20 carbon atoms, wherein the purified aqueous dispersion comprises no greater 500 ppb, preferably no greater thanlOO ppb, e.g., no greater 25 ppb or even no greater than 10 ppb, of each target anion according to the formula RfSC>3 , by weight based on the weight of the fluoropolymer.

[0034] There are dead-end filtration methods for feeding the aqueous composition perpendicular to the membrane and a cross-flow filtration method (tangential flow filtration (TFF)) for feeding the aqueous composition in parallel with the membrane. The cross-flow ultrafiltration is particularly preferred from the point that clogging of pores of the membrane hardly occurs and the feeding of the process water can be carried out at relatively low pressure. The filtration pressure is represented by an average pressure of an inlet pressure of the membrane (hereinafter also referred to as "Pin") and an outlet pressure of the membrane (hereinafter also referred to as "Pout"). The average filtration pressure may be, in case of the tangential flow filtration (TFF) or cross-flow filtration method, from 0.01 MPa to 1 MPa, preferably from 0.1 MPa to 0.5 MPa. Alternatively, the pressure build-up produced at the inlet of the membranes or membrane packages with the aid of the pumps may be between about 1.0 and 10.0 bar, depending on the delivery capacity and the tube cross-section. The operating temperature in the ultrafiltration circulation as described herein can be chosen within fairly wide limits, in fact between 5 and 90°C, but the ultrafiltration is preferably carried out at somewhat elevated temperatures, that is to say between 20 and 85°C, whereupon the rate of the process can be increased. For example, the operation or process parameters of the ultrafiltration step may include a temperature in the range of from 20 to 50°C or from 20 to 25°C. That is, ambient temperatures are sufficient for the ultrafiltration as described herein. The filtration pressure may be restricted by the circulation flow rate of the process water (in other words, when the circulation flow rate is increased, the filtration pressure has to be set at a low pressure). When the feeding flow rate of the aqueous composition is set by the differential pressure of the closed vessels, the filtration pressure is not affected by the feeding flow rate of the process water and can be set freely. Means which do not substantially give a shearing force to the aqueous composition or water stream, for example, means for applying a pressure to the composition or water stream with clean air or inert gas can be employed. Examples of the inert gas are nitrogen gas and the like. From the viewpoint of cost, clean air is preferable. In case where pressure is applied with the compressed air, a pressure to be applied may be determined so that the filtration pressure becomes an average pressure of the above-mentioned inlet pressure and outlet pressure of the membrane. The flow rate of process water may vary depending on kind and pore size of the ultrafiltration membrane and the aqueous composition or water stream feeding method. Usually, it may be preferable to set a linear velocity at 0.5 to 16 m/sec, more preferably 1 to 7 m/sec in the tangential flow filtration (TFF) or cross-flow filtration. The flow rate of the aqueous composition may also be set by a differential pressure of the feeding vessel of the aqueous composition or water stream and the receiving side vessel. [0035] Similarly, it is preferred that ultrafiltration is carried out in a cross-flow arrangement. A cross flow arrangement of membranes or modules of membranes is also advantageous from the point of view of avoidance of blocking or plugging of the membranes, and further advantageous with regard to continuous flow and effective fdtration. Furthermore, ultrafdtration may be carried out by means of membranes selected from polymeric and ceramic membranes, preferably from ceramic membranes. Ceramic membranes are preferred since they were found to be more chemically stable and more resistant towards higher pressure and loads as experienced in higher through-put arrangements.

[0036] Ultrafdtration as performed in step (ii) with the target to remove/reduce the amount of unwanted species as described above is performed in a diafdtration operation mode, meaning a combination of ultrafdtration and dialysis, in a cross-flow arrangement. Permeate containing a fraction of the unwanted species is removed, and fresh deionized water including purified water from recycled permeate, is added until target concentration of the unwanted species is reached. This diafdtration process can be done batch-wise, means by up-concentrating to a target solid concentration and then diluting again with fresh deionized water, or continuously, by adding fresh deionized water, in which case permeate which contain unwanted species is continuously removed.

[0037] Generally, performing ultrafdtration on the cation-exchanged dispersion to form a purified aqueous dispersion can be conducted using any known means. For example, in some embodiments, the cation-exchanged dispersion can be filtered using a membrane having a molecular weight cut-off (MWCO) large enough to allow all the target ions to pass, while retaining the fluoropolymer. In some embodiments, the MWCO is at least 4000, e.g., at least 10,000, or even at least 15,000 Daltons. In some embodiments, the MWCO is no greater than 50,000, e.g., no greater than 30,000 Daltons. As the fluoropolymer typically has a molecular weight orders of magnitude larger than the target ions, higher MWCO values may be used to enhance the speed and efficiency of the ultrafdtration step.

[0038] As the ultrafdtration of step (ii) does not introduce additional cations, no further purification steps are required. For example, in some embodiments, the total concentration of all counter ions and target cations in the purified aqueous dispersion is no greater than 100 ppm by weight based on the weight of the purified fluoropolymer, e.g., no greater than 75 or even no greater than 50 ppm by weight based on the weight of the fluoropolymer.

[0039] In some embodiments, the cation-exchanged dispersion can be filtered using a diafdtration process, e.g., a batch or continuous diafdtration process. The water removed with the permeate may be replaced by adding a corresponding amount of fresh deionized and/or recycled/purified water (pH <7) to the retentate.

[0040] Although the purified aqueous dispersion of the purified fluoropolymer may be used as-is, in some embodiments, it may be desirable to up-concentrate or coagulate the dispersion. For example, in the methods of the present disclosure the aqueous dispersions may contain 10 to 30% by weight of the fluoropolymer based on the total weight of the dispersion. Some up-concentration may occur in the ultrafdtration step, e.g., if the amount of fresh deionized water added to the retentate is less than the amount removed with the permeate. The ultrafdtration purification process is performed at a solid content of 5-20 %, preferably at a solid content of 10-15 %. The removed amount of permeate is replaced by fresh deionized water so that the solid content is kept within the preferred solid content range, preferably constant. After the target anion concentration is reached, the addition of fresh deionized water is stopped, optionally followed by an up-concentration step towards 10-30 % solid content, preferably towards 15- 25wt% solid content. Optionally process control by conductivity and/or pH-measurement of the permeate can be used.

[0041] In some embodiments, an intermediate step may be performed between the cation exchange step and the ultrafiltration step to reduce some ions by alternative methods, which may improve the efficacy of the subsequent ultrafiltration step. For example, in some embodiments, the levels of SOq^ and F are reduced by passing the aqueous dispersion over CaC03/Ca(0H)2 or hydroxyapatite. Additionally, or alternatively the aqueous dispersion may be treated with a weak anion exchange resin to remove Rf-COOH/ Rf -SO3H. Optionally, the cation exchanged dispersion is first dried in an intermediate step to reduce volatile compounds, redispersed with fresh deionized water and then run through the ultrafiltration purification step as described above.

[0042] Generally, the UF step, but also the ion exchange resin regeneration step, generates significant quantities of permeate and regeneration streams containing the removed ions, such as F

RfCOO , RfS03 and possible fluorinated inert materials (e.g., fluorinated ethers), at concentrations of less than 2 wt.%. Handling and disposal of such streams may be undesirable. Therefore, in preferred embodiments these streams, in particular the permeate stream of the ultrafiltration step, may be purified and reused in the ultrafiltration process providing a closed system. The obtained fraction containing high concentrations of contaminants can be further treated, recycled or disposed. The purification of the water streams can be done by treatment with a weak anion exchange resin or a trialkylamine extraction, treatment with CaC03, BaC03, Ca(OH)2, Hydroxyapatite and/or treatment with a reverse osmosis unit. The purified water may then be reused to replenish the permeate losses of the UF-step. The recovered water may be at a pH<7, but not basic. Alternatively, the combined water streams can be adjusted to pH>7, preferably >9 followed by a distillation/evaporation step. This option is beneficial as nearly all water is recovered. Also, the up-concentrated contaminants are in high concentrations so that they can be destroyed/incinerated in an economic way. A further method of treatment of the upconcentrated fluoroorganic compounds is incineration, which yields fluoride (in form of HF) and carbon dioxide (C02) and which is not desirable in light of the desire to reduce C02 emissions. Consequently, other methods to cope with the non-usable materials are preferred. Pyrolysis-will convert the materials into monomers. The pyrolysis can be done in a fluidized bed reactor as described in W02010/039820 or in an agitated bed reactor as described in WO2021/165923. Gasification of the fluorine containing residuals in presence of low oxygen contents and/or water at temperature of 800-1500°C yields carbon monoxide and hydrogen (syngas), HF and carbon (no C02 is emitted). Electrochemical oxidation where the concentrates are contacted with hydroxy-generating electrode degrade low-molecular weight fluoroorganic compounds into carbon dioxide, water, fluoride ions, and, optionally, cations. All three alternatives (pyrolysis, electrochemical oxidation and gasification) to mitigate the non-usable fluorinated materials avoid C02 emissions and generate monomers or at least the valuable HF. This may have the advantage that essentially all of the fluorine involved may be recovered and be reused again. This basically means the fluorine cycle is closed. Accordingly, the process according to the present disclosure preferably comprises an additional step in which the extracted compounds are further processed by incineration, pyrolysis, gasification, and/or electrochemical oxidation such that fluoride, fluorinated monomers, syngas, hydrogen fluoride and/or carbon are generated.

[0043] In some embodiments the purified aqueous dispersion is dried. In some embodiments, drying is carried out at a temperature and for an effective time to reduce the moisture content of the aqueous PFSA ionomer composition, preferably the cation-exchanged composition, to a moisture content of less than 15 weight %, preferably less than 12 weight %, or even less than 10 weight % based on the weight of the dried composition. The drying preferably is carried out such that the moisture content does not decrease below 2 wt. %, e.g., not below 3 wt.% or below 4 wt. %, based on the weight of the dried composition.

[0044] Preferably the drying process is carried out such that the temperature of the composition does not exceed 220°C or 200°C or even 100°C. In some embodiments, the drying is carried out by a cryo process, i.e., a process comprising freezing the dispersion.

[0045] In some embodiments, the purified aqueous dispersion is spray dried. Typically, the spray drying process leads to particles having a particle size of from about 10 to 300 pm. These sizes may be the D50 value or may be the maximum particle sizes. Using sieves can make sure that particles with diameters in excess of the above sizes are removed from the compositions. The particles are typically substantially spherical which means the particles are spherical or, if not perfectly spherical, their geometric shape can be best approximated by a spherical shape.

[0046] In another embodiment, the purified aqueous dispersion is freeze dried. Freeze-drying results in a flaky material, typically of a size between 5 pm to 1000 pm. The flakes can be crushed into smaller particles. In one embodiment a foam is created from the aqueous composition and the foam is freeze- dried. A foam can be created, for example, by subjecting the aqueous composition to ultrasound irradiation, by injecting a gas into the dispersion, for example CO2, or by mechanical forces, for example by subjecting the aqueous composition to one or more high speed agitator. A combination of the above steps may be used, also.

[0047] In another embodiment, the purified aqueous dispersion is freeze-granulated, e.g., freeze- granulation with liquid nitrogen. Typically, freeze-granulation leads to spherical particles having a particle size between 10 pm and 500 pm. These particle sizes may be the D50 value or may be the maximum particle sizes. Using sieves can make sure that particles with diameters in excess of the above sizes are removed from the compositions. The particles obtained by freeze-granulation are typically porous. They are typically spherical or substantially spherical which means the particles are not perfectly spherical, but their geometric shape can be best approximated by a spherical shape. [0048] In some embodiments, the dried fluoropolymer may be redispersed in water, water- miscible solvents, or combinations thereof, to make liquid compositions. Suitable water-miscible solvents include, e.g., alkyl alcohols, including linear or branched alkyl alcohols having 1 to 5 carbon atoms (i.e., the aqueous solvent may comprise at least one alkyl alcohol according to the formula R-OH, where R is a linear or branched alkyl group having 1 to 5 carbon atoms). In some embodiments, the redispersed compositions have high concentrations of the fluoropolymer, for example at concentrations of at least 10 % by weight of the fluoropolymer, at least 30%, or even at least 40% by weight of the fluoropolymer or more. Of course, lower concentrations may be suitable for some applications. For example, the solution may be a 15 wt.% dispersion of the fluoropolymer in n-propanol/water (60/40), having a solution viscosity below 500 mPa_*s at shear rates of both 1 sec l and 1000 sec l.

[0049] The purified compositions of the present disclosure may be used, for example, in the manufacture of membranes, for example polymer electrolyte membranes for use in fuel cells, electrolyte membranes in an electrochemical cell, for example in a chlor-alkali membrane cell, redox flow batteries or electrolyzers. The fluoropolymer compositions are particularly suitable for making thin membranes, i.e. membranes having a thickness of less than 50 micrometer, preferably less than 30 micrometers, for example between 20 and 40 micrometers, or between 10 to 28 micrometers. The membranes are typically extended sheets and may have a length of greater than 12 cm. Typically, the membrane is cast from a liquid composition or a dispersion and then dried, annealed, or both. The membrane may be cast on a support. Typically, the supporting matrix is electrically non-conductive. Typically, the supporting matrix is composed of a fluoropolymer, which is more typically perfluorinated. Typical matrices include porous polytetrafluoroethylene (PTFE), such as biaxially stretched PTFE webs. In another embodiment fdlers (e.g., fibers like PVDF, polysulfone, etc.) might be added to the PFSA ionomer composition to reinforce the membrane. After forming, the membrane may be annealed, typically at a temperature of 120 °C or higher, more typically 130 °C or higher, most typically 150 °C or higher.

[0050] For making a membrane a purified aqueous dispersion (including redispersions) according to the present disclosure may be used. In some embodiments, additives may be added before the membrane is cast. The additives may be added as solid materials or dissolved or dispersed in a liquid.

[0051] In some embodiments, the additive includes a salt of at least one of cerium, manganese or ruthenium or one or more cerium oxide or zirconium oxide compounds and is added to the PFSA ionomer before membrane formation. The salt of cerium, manganese, or ruthenium may comprise any suitable anion, including carbonate, chloride, bromide, hydroxide, nitrate, sulfonate, acetate, and phosphate. More than one anion may be present. In some embodiments, it may be useful to use anions that generate volatile or soluble acids, for example chloride or nitrate or carbonate.

[0052] Manganese cations may be in any suitable oxidation state, including

but are most typically h^+ Ruthenium cations may be in any suitable oxidation state, including Riri⁺ and Ru^⁺, but are most typically Riri⁺. Cerium cations may be in any suitable oxidation state, including Ce^⁺ and Cc^⁺. The amount of such cations added is typically between 0.001 and 0.5 charge equivalents based on the molar amount of acid functional groups present in the fluoropolymer, more typically between 0.005 and 0.2, more typically between 0.01 and 0.1, and more typically between 0.02 and 0.05.

In some embodiments, the use of Ce or Mn ions are preferred.

[0053] In some embodiments, the total concentration of the Ce and Mn ions is at least 500 ppm, based on the weight of the fluoropolymer in the dispersion (or redispersion). In some embodiments, the total concentration of the Ce and Mn ions is at least 1000, at least 5,000 or even at least 10,000 ppm, based on the weight of the fluoropolymer in the dispersion (or redispersion).

[0054] In some embodiments, the purified compositions of the present disclosure may also be used for making a catalyst ink composition. In some embodiments, the purified compositions of the present disclosure may be used for making a binder for an electrode or a battery (for example, lithium ion batteries).

[0055] Test Methods. The following procedures were used in the Examples and are referenced in the claims.

[0056] ICP-OES Procedure. Cation concentrations (e.g., metals) were determined as follows. A sample was placed in a quartz glass vessel and ashed at 550 °C to remove the organic materials. The residue was dissolved in acid. Metal content was determined from the dissolved sample by inductively coupled plasma - optical emission spectrometry (ICP-OES) using a ICAP 7400 DUO instrument from Thermo Fisher Scientific. Measurements were conducted according to DIN EN ISO 11885:2009-09. [0057] IC Procedure. Inorganic anion concentrations were measured as follows. The aqueous fluoropolymer dispersions were diluted with water to obtain diluted dispersions at a weight ratio of 1 part ionomer to 200 parts water. Anion concentrations were determined on the prepared sample by ion chromatography (IC) according to DIN EN ISO 10304-1:2009 using a DIONEX Thermo ICS 2100 IC instrument (column: AS 15 - Thermo).

[0058] LC-MS Procedure. Organic ion concentrations were determined as follows. Dispersion samples were mixed 1:1 (v/v) with a 2 ng/mL methanol solution of methanol solution of ^C-labeled perfhiorinated carboxylic and sulfonic acids (MPFAC-C-ES, available from Wellington Laboratories (Guelph, Ontario, Canada)). The solution was shaken for 24 hours, after which it was centrifuged, with the resulting supernatant being collected for analysis. If further dilution of extracts were desired (e.g., if concentrations exceed the calibration range or to overcome other issues such ion suppression etc.), the extracts were diluted with additional amounts of the 2 ng/mL solution of MPFAC-C-ES.

[0059] Samples were analyzed for PFBS using an Agilent 1290 Infinity HPLC coupled with an Agilent 6495 triple quadrupole mass spectrometer, both of which are available from Agilent Technologies, Inc (Santa Clara, CA). The separations were performed using a reverse phase C8 column (Eclipse XDB-C8, 3.5 pm, 3.0 X 100 mm), available from Agilent. To prevent background interference from the leaching of certain substances from components of the HPLC, a pre-column (Eclipse XDB-C18 3.0 X 150 mm, 5 pm) was placed between the binary pump and the autosampler. This results in a delayed interference peak that separates chromatographically from the sample analyte peak. Two microliters of the solution were injected into the column, which had been heated to 50 °C. The mobile phase solutions that were used for the separation were A: 6 mM ammonium acetate in water, and B: 6 mM ammonium acetate in 98:2 (v:v) acetonitrile-water. The LC gradient used was as follows: t=0 minutes 90%A and 10%B; t=5 min 100%B; and t=6 min 100%B. The total solvent feed rate was 0.5 mL/minute, and the post time was 3 minutes.

[0060] The mass spectrometer was run in multiple reaction monitoring (MRM) scan mode. Collision energies for the MRMs for perfluorobutane sulfonic acid (PFBS) and the corresponding internal labeled standard (M3PFBS) were optimized using the Agilent optimizer program.

[0061] For the detection of PFBS, the following MRM transitions were used:

(i) 299 Daltons to 99 Daltons; and

(ii) 299 Daltons to 80 Daltons.

[0062] For the detection of M3 PFBS, the following MRM transition was used:

302 Daltons to 80 Daltons.

[0063] Quality control measures, with both the internal standards and additional external standards, were used to ensure the accuracy of the data.

[0064] Example 1 - Purification via cross-flow ultrafiltration in batch mode (batch mode diafiltration).

[0065] A 20 wt.% aqueous ionomer dispersion was obtained by hydrolyzing post-fluorinated sulfonyl fluoride copolymer (745EW ionomer prepared as in Example 1 of W02020/183306, MFI = 18, COOH- end-groups = 178 per 1 million C-Atoms) in the presence of LiOH at a temperature above 180 °C. Metal analysis obtained according to the ICP-OES Procedure revealed the following metal concentration relative to the 20 wt.% dispersion: Li = 3200 ppm, Na = 6 ppm, K = 5 ppm; whereas all other metals evaluated (Fe, Cr, Ni, Cu, Al) were below the detection limit of 0.5 ppm. This dispersion was ion exchanged in a column filled with regenerated PUROLITE C150TLH (from Purolite Ltd.) cation exchange resin in the H+ form at a flow rate of 2 BV/h.

[0066] After the ion exchange process was finished, metal analysis obtained according to the ICP- OES Procedure revealed the following metal concentrations in the dispersion having a solids content of 13.6 wt.% fluoropolymer after ion exchange: Li = 1.6 ppm, Na = 0.6 ppm, K = <0.5 ppm, whereas all other metals evaluated (Fe, Cr, Ni, Cu, Al) were still below detection limit of 0.5 ppm. The ion- exchanged material was dried, and a portion of this material was redispersed in deionized water to obtain a 10 wt.% aqueous dispersion of the fluoropolymer for ultrafiltration.

[0067] Ultrafiltration was run with this ion-exchanged material using a ceramic membrane with a MWCO of 15,000 Daltons rom TAMI Industries. A 40 kg aqueous solution with 10 wt.% solid content was fed into the feed container. Cross-flow filtration was run at a feed rate of 2800 liters/hour, feed pressure of 310 kilopascal (3.1 bar), retentate pressure of 200 kPa (2.0 bar) and a permeate pressure of 50 kPa (0.5 bar). The corresponding feed flow velocity over the membrane was 4.2 meters/second. Permeating water was replaced by fresh deionized water (pH = 6) after each filtration run (discontinuous approach). The following filtration steps were repeated four times:

• Filtration until feed reaches about 16 wt.% solid content • Dilution of feed down to 10 wt.% solid content [0068] The concentrations of inorganic anions were obtained according to the IC Procedure on samples after the first and fourth cycle of this ultrafiltration process and are summarized in Table 1. For comparison, all values were adjusted to correspond to a 20 wt.% fluoropolymer composition. Concentrations of oxalic acid and sulfate in the retentate were both below their respective detection limits (oxalic acid detection limit was 10 ppm; sulfate detection limit was 50 ppm). This corresponds to an oxalic acid reduction of more than 98% and sulfate reduction of more than 85% by performing four diafiltration runs in batch mode.

Table 1: Sulfate and oxalic acid concentrations in the dispersion (adj. to 20 wt.% fluoropolymer). Evaluation of the IC-data was performed at a detection limit of <10ppm for oxalic acid, <50ppm for sulfate.

[0069] Example 2 - Purification via cross-flow ultrafiltration in continuous mode (continuous diafiltration mode).

[0070] A 20 wt.% ionomer dispersion was obtained by hydrolyzing post-fluorinated ionomer crumb (733EW ionomer prepared as in Example 1 of W02020/183306, MFI = 20, COOH-end-groups = 78 per 1 million C-Atoms ) in presence of Li OH at a temperature above 180 °C, similar to Example 1. Metal analysis obtained according to the ICP-OES Procedure revealed the following metal concentration in the dispersion (adjusted to a 20 wt.% fluoropolymer dispersion): Li = 4100 ppm, Na = 5 ppm, K = 3 ppm, A1 = 1.2 ppm whereas all other metals evaluated (Fe, Cr, Ni, Cu) were below detection limit of 0.5 ppm. This dispersion was ion exchanged in a column filled with regenerated PUROLITE C150TLH cation exchange resin in the H+ form until the desired cation concentrations were reached. After the ion exchange process was finished, metal analysis obtained according to the ICP-OES Procedure revealed the following metal concentrations in a dispersion having 12.3 wt.% fluoropolymer after ion exchange: Li = < 0.5 ppm, Na = 0.6 ppm, K = < 0.5 ppm, whereas all other metals evaluated (Fe, Cr, Ni, Cu, Al) were still below detection limit of 0.5 ppm. The ion-exchanged material was dried, and a portion of this material was redispersed in deionized water to obtain a 10 wt.% aqueous dispersion for ultrafiltration purification. [0071] Continuous ultrafiltration experiments using a ceramic membrane with a MWCO of 15,000 Daltons (TAMI Industries) were conducted with the ion-exchanged ionomer dispersion. Fresh DI water (pH = 6) was continuously added to the feed tank in the same rate as permeate was withdrawn in order to keep the solid content constant. After four hours of continuous cross-flow filtration, the level of sulfate, oxalic acid and fluoride as obtained according to the IC Procedure were drastically reduced. These results, adjusted to a 20 wt.% solids composition, are summarized in Table 2. Table 2: Concentrations in the dispersion (adjusted to a 20 wt.% fluoropolymer composition). Evaluation of the IC-data was performed at a lower detection limit compared to example 1 (data evaluation with a detection limit <5ppm for oxalic acid, fluoride, and sulfate).

[0072] The reduction of certain Rf-S0₃ ions was determined according to the LC-MS Procedure. The peak signal intensities divided by the peak area for these ions were measured on samples prepared in Example 2, both before and after ultrafiltration. The signal intensities divided by area in the permeate before ultrafiltration (Si) and the permeate after ultrafiltration (Sf) are shown in Table 3. The reduction factor (Si/Sf) and the percent reduction (100*(Si-Sf)/Si) are also shown in Table 3.

Table 3: Rf-S0₃ reduction in the permeate as achieved with ultrafiltration.

[0073] The reduction in the concentration of F(CF2)4S03 (C4 Sulfonate) ions in the fluoropolymer purified by the ultrafiltration process was quantified according to the LC-MS Procedure. Samples were prepared from Example 2 material, both before and after ultrafiltration. The C4-Sulfonate concentration in the fluoropolymer was reduced from > 400 ppb based on the weight of the fluoropolymer prior to ultrafiltration to 2 ppb after ultrafiltration, a reduction of more than 99.5%.

[0074] Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

Claims

What is Claimed is:

1. A method of producing a purified fluoropolymer, the method comprising

(i) performing cation exchange on an unpurified aqueous dispersion of a fluoropolymer comprising repeat units of tetrafluoroethylene and a fluoro-olefm comprising SO3 groups, and a counter ion according to the formula M⁺, where M is selected from the group consisting of Li⁺, Na⁺, K⁺ and combinations thereof, to form a cation-exchanged aqueous dispersion of a fluoropolymer comprising sulfonic acid groups; and

(ii) performing ultrafiltration on the cation-exchanged dispersion to form a purified aqueous dispersion comprising a purified fluoropolymer.

2. The method of claim 1, wherein M⁺ is Li⁺.

3. The method of claim 1 or 2, wherein the concentration of each counter ion in the cation-exchanged aqueous dispersion is no greater than 200 ppm by weight based on the weight of the fluoropolymer, as measured according to the ICP-OES Procedure.

4. The method according to any one of the preceding claims, wherein the concentration of each counter ion in the cation-exchanged dispersion is no greater than 70 ppm by weight based on the weight of the fluoropolymer, as measured according to the ICP-OES Procedure.

5. The method according to any one of the preceding claims, wherein the unpurified aqueous dispersion further comprises one or more target cations according to the formula Xⁿ⁺, wherein X is selected from the group consisting of Al, Ce, Cu, Cr, Fe, Mn, Ni, Ti and combinations thereof, and n is the valence of the cation, wherein the concentration of each target cation in the cation-exchanged dispersion is no greater than 20 ppm by weight based on the weight of the fluoropolymer, as measured according to the ICP-OES Procedure.

6. The method according to any one of the preceding claims, wherein the total concentration of all counter ions and target cations in the cation-exchanged dispersion is no greater than 100 ppm by weight based on the weight of the fluoropolymer, as measured according to the ICP-OES Procedure.

7. The method according to any one of the preceding claims, wherein performing cation exchange comprises contacting the unpurified aqueous dispersion with beads of a cation exchange resin.

8. The method according to any one of the preceding claims, wherein the cation-exchanged dispersion comprises one or more inorganic target anions selected from the group consisting of C2O42 , SOq^ . and F , and wherein the purified aqueous dispersion comprises no greater 375 ppm of each inorganic target anion, by weight based on the weight of the purified fluoropolymer, as measured according to the IC Procedure.

9. The method of claim 8, wherein cation-exchanged dispersion comprises C2O42 , and the purified aqueous dispersion comprises no greater 300 ppm of

by weight based on the weight of the purified fluoropolymer, as measured according to the IC Procedure.

10. The method of claim 8 or 9, wherein cation-exchanged dispersion comprises SO^ and the purified aqueous dispersion comprises no greater 300 ppm of SO^ , by weight based on the weight of the purified fluoropolymer, as measured according to the IC Procedure.

11. The method according to any one of claims 8 to 10, wherein cation-exchanged dispersion comprises F and the purified aqueous dispersion comprises no greater 300 ppm of F , by weight based on the weight of the purified fluoropolymer, as measured according to the IC Procedure.

12. The method of any one of claims 8 to 11, wherein, the concentration of each inorganic target anion in the purified aqueous dispersion is no greater than 150 ppm by weight based on the weight of the purified fluoropolymer, as measured according to the IC Procedure.

13. The method according to any one of the preceding claims, wherein the cation-exchanged dispersion comprises one or more organic target anions selected from the group consisting of anions having the formula RfCOO , wherein Rf is a fluorinated alkyl or alkylene having 1 to 20 carbon atoms, wherein the purified aqueous dispersion comprises no greater 100 ppb, optionally no greater than 25 ppb, of each target anion according to the formula RfCOO , by weight based on the weight of the purified fluoropolymer, as measured according to the LC-MS Procedure.

14. The method according to any one of the preceding claims, wherein the target anions include at least one anion having the RfS03 , wherein Rf is a fluorinated alkyl or alkylene having 1 to 20 carbon atoms, wherein the purified aqueous dispersion comprises no greater 100 ppb, optionally no greater than 25 ppb, of each target anion according to the formula RfS03 , by weight based on the weight of the purified fluoropolymer, as measured according to the LC-MS Procedure.

15. The method of any one f claims 8 to 13, wherein the total concentration of all counter ions, excluding H⁺, and target cations in the purified aqueous dispersion is no greater than 100 ppm by weight based on the weight of the purified fluoropolymer, as measured according to the ICP-OES Procedure.

16. The method according to any one of the preceding claims, wherein the fluoropolymer further comprises divalent repeat units independently represented by formula (II):

wherein m’ is 0 or 1, and Rf_| is a linear or branched perfluoroalkyl group having from 1 to 12 carbon atoms that can be interrupted once or more than once by an ether oxygen atom.

17. The method according to any one of the preceding claims, further comprising adding Ce ions, Mn ions, or a combination thereof to the purified aqueous dispersion, wherein the total concentration of the Ce and Mn ions is at least 500 ppm, by weight based on the weight of the purified fluoropolymer.

18. The method of any one of claims 1 to 17, further comprising drying the purified aqueous fluoropolymer dispersion to form a dried, purified fluoropolymer comprising no greater than 15 wt.% water based on the total weight of the purified fluoropolymer.

19. The method of claim 18, further comprising redispersing the dried, purified fluoropolymer in an aqueous solvent to form a second aqueous dispersion of the purified fluoropolymer.

20. The method of claim 19, wherein the aqueous solvent comprises at least one alkyl alcohol according to the formula R-OH, where R is a linear or branched alkyl group having 1 to 5 carbon atoms.

21. The method according to claim 19 or 20, further comprising adding Ce or Mn ions to the second aqueous dispersion wherein the total concentration of the Ce and Mn ions is at least 500 ppm, based on the weight of the purified fluoropolymer.

22. A polymer electrolyte membrane comprising the purified fluoropolymer prepared by the method according to any one of the preceding claims.

23. A membrane electrode assembly comprising at least one polymer electrolyte membrane according to claim 22.