FIELD

[0001] The present disclosure relates to methods that may be used to acidify and purify aqueous dispersions of fluorinated ionomer salts. The methods include the use of ultrafdtration of acidified dispersions in the presence of a protic acid. The present disclosure also relates to highly- purified aqueous fluorinated ionomers and dispersions.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides methods comprising hydrolyzing a polymer comprising pendent sulfonyl fluoride groups with a base having the formula XOH to form a fluorinated ionomer salt comprising SO3" X ⁺ salt groups, wherein X ⁺ is the counter cation of the base XOH; acidifying the aqueous dispersion of the fluorinated ionomer salt with a first acid comprising a first acid anion and having a pKa of less than 1 to form an acidified dispersion having a pH of no greater than 1 to form the fluorinated ionomer by converting at least 99 mole% of the salt groups to SO3" H ⁺ acid groups; and performing a first ultrafiltration on the acidified dispersion using a membrane having a molecular weight cut-off (MWCO) of at least 4000 Daltons to separate the acidified dispersion into a permeate and a first retentate, wherein the first retentate comprises no greater than 100 ppm of the counter cations of the base, based on the total weight of the fluorinated ionomer in the first retentate, as measured by inductively coupled plasma - optical emission spectrometry conducted according to DIN EN ISO 11885:2009-09.

[0003] In some embodiments, the aqueous dispersion of the fluorinated ionomer is acidified while performing the first ultrafiltration.

DETAILED DESCRIPTION

[0004] Fluorinated ionomers, e.g., copolymers of tetrafluoroethylene (TFE) and monomers having pendent acid groups (e.g., pendent sulfonic acid or carboxylic acid groups) are known. Fluorinated ionomers have been used for making ionomer membranes for electrochemical cells such as fuel cells, water electrolyzers and NaCl/HCl-electrolyzers.

[0005] Often, when fluorinated ionomers are prepared the pendent acid groups are present in their salt form, i.e., the hydrogen cations of the sulfonate groups or carboxylate groups are replaced with an associated cation other than hydrogen. For example, fluorinated ionomers comprising sulfonic acids groups may be prepared by first generating a copolymer of tetrafluoroethylene and a monomer having pendent sulfonyl fluoride groups (also referred to as “sulfonyl fluoride polymer” or “precursor polymer”), which may be post-fluorinated. This sulfonyl fluoride polymer is subsequently hydrolyzed with a base having the formula XOH, where X ⁺ is the counter cation of the base, e.g., KOH, LiOH or NaOH. The sulfonate groups in the resulting “fluorinated ionomer salt” are present as salts where the counter cation is X ⁺ , e.g., K ⁺ , Li ⁺ , Na ⁺ or NR4 ⁺ . The sulfonate groups of the fluorinated ionomer salt are then acidified and converted into sulfonic acid groups to produce a perfluorosulfonic acid polymer, also referred to as “PFSA ionomer” or a fluorosulfonic acid polymer in case of e.g., vinylidene fluoride containing ionomers. [0006] Generally, some steps may be performed with an aqueous dispersion of the ionomer. 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 containing articles, while the presence of low molecular weight fluorinated ions may be the subject of tightening regulations. Also, non-ionic fluorinated species, e.g., fluorinated ethers, might create concerns.

[0007] 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 base-hydrolyze the acid groups, e.g., Na ⁺ , K ⁺ , and Li ⁺ . 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. Some impurities might be generated during post-fluorination or work-up steps.

[0008] Many common purification methods rely on ion exchange resins in order to achieve the desired reduction in ion content. For example, some methods require one or more ion exchange steps alone or in addition to one or more ultrafiltration steps.

[0009] International Patent Application WO 2020/094563 Al (“Dispersible Ionomer Powder and Method of Making Same”) describes contacting aqueous dispersions of fluorinated ionomers or ionomer salts with both anion and cation exchange resins.

[0010] United States Patent US 9,406,958 B2 (“Electrolyte Emulsion and Process for Producing Same”), exemplifies a process in which a fluorinated ionomer salt is first ultrafiltered at a pH of 8 using a purified water wash solution. The ionomer salt is then converted into an acidtype aqueous dispersion through contact with an acid-type ion exchange resin in a subsequent step. [0011] Japanese Patent JP 5978762 B (“Method for Producing Aqueous Dispersion of Fluorine-Containing Polymer and Purified Aqueous Dispersion of Fluorine-Containing Polymer”) exemplifies a purification and conversion process of a salt dispersion into the acid form by first treating an ionomer salt dispersion at pH 8 by ultrafiltration, followed by a cation exchange with an ion exchange resin (pH <7) and finally an additional ultrafiltration step with an acidified dispersion.

[0012] The ion exchange steps of these methods may introduce alternate ions that themselves can create their own concerns. For example, performing ultrafiltration on the salt form of an ionomer dispersion followed by cation-exchange process may introduce cations from the ion exchange resin and may generate HF by converting fluoride salts into HF. In addition, the use of these ion exchange processes requires additional steps and costs such as regenerating the ion exchange resins. Additionally, significant yield losses might occur due to adsorptions of the ionomer polymer on the ion-exchange resin.

[0013] Japanese Patent JP 5119854 B2 (“Fluoropolymer Dispersion and Method for Producing Fluoropolymer Dispersion”) describes a process in which a solution containing a fluorinated ionomer salt prepared at a pH of 10 is ultrafiltered using a purified water wash solution. An acidtype aqueous dispersion is then obtained by contacting the ultrafiltered material with an acidified ion exchange resin in a subsequent step. JP 5119854 B2 teaches that, if the ultrafiltration is conducted in the presence of an acid, impurities such as alkali metal cations cannot be removed by ultrafiltration and may remain in the fluoropolymer dispersion.

[0014] Thus, 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. In addition, there is a desire to simplify and improve the efficiency of the various process steps required to produce purified ionomers from fluorinated ionomer salts having high conversions of the salt to the acid form and to establish an economically/ecologically sound process.

[0015] Surprisingly, the present inventors discovered methods that allow for high conversions of the salts to their acid form while purifying the acidified aqueous dispersions of fluorinated ionomer salts, eliminating the need for separate ion exchange steps such as contacting the aqueous solutions with an ion exchange resin. In the methods of the present disclosure, ultrafiltration of the aqueous fluoropolymer dispersion is conducted at low pH by pre-acidifying the dispersion, using an aqueous solution of a protic acid as the wash solution, or both. The ultrafiltration process uses permeable (porous) membranes to separate the components of the aqueous dispersion based on molecular size.

[0016] Prior to or during the ultrafiltration process of the present invention, the acid groups of the fluorinated ionomer salt (e.g., sulfonate salt groups) are acidified by the protic acid, and the resulting acid form of the ionomer (e.g., a perfluorosulfonic acid polymer) is retained in the retentate. Concurrently, low molecular weight materials pass through the ultrafiltration membrane into the permeate. Such low molecular weight materials may include, e.g., unreacted monomers, short-chain oligomers, surfactants, and fluorinated non-ionic ethers.

[0017] In some embodiments, at least 99 mole% of the sulfonate salt groups are acidified. In some embodiments, at least 99.5 or even at least 99.9 mole% of the sulfonate salt groups are acidified. Upon such acidification, the counter cations of the salt groups become an impurity in the acidified aqueous dispersion. In the methods of the present disclosure these counter cations are removed during the ultrafiltration process.

[0018] These methods can also remove certain other ions, referred to as “target ions,” from the retentate, resulting in a purified dispersion of the fluorinated ionomer. 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 ⁺ and the counterion of the protic acid(s) used in the process. 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, excluding H ⁺ and the counterion of the protic acid(s). In some embodiments, 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 unpurified aqueous dispersions, e.g., ionomer having a molecular weight of less than 10,000 Daltons. Traces of non-ionic fluorinated materials may also be removed.

[0019] Generally, the target ions include the cationic counter ions associated with the acid groups of the fluorinated ionomer salt (i.e., the X ⁺ counter ions used when base hydrolyzing the sulfonyl groups), as well as other cations and anions. Examples of counter ions that may be present include alkali metals such as Li ⁺ , Na ⁺ , K ⁺ , and NRq ⁺ . Other cationic target ions that may be present include cations according to the formula M ⁿ⁺ , wherein M 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", SOq^", C20q2" and HCOO". In some embodiments, the target ions present in the aqueous ionomer dispersion include low molecular weight fluorinated anions such as fluorinated carboxylic acids according to the formula RfCOO", and fluorinated sulfonic acids according to the formula RfSC>3", 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, e.g., 6 to 14 carbon atoms. In some embodiments, the alkyl or alkylene group may be interrupted by 1 or more oxygen atoms.

[0020] In some embodiments, the fluorinated ionomer salt comprises repeat units of tetrafluoroethylene and a fluoro-olefin comprising SC>3" groups, and their associated counter ions. In some embodiments, the fluoropolymer comprises (i) divalent units derived from tetrafluoroethene (TFE) and 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 _e F2 _e may be linear or branched and preferably is linear. 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.

[0021] In some embodiments, the counter cation is an alkali metal ion, e.g., Li ⁺ , Na ⁺ , K ⁺ , or NR4 ⁺ . 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, Li2CC>3, NaOH or KOH) in water and temperatures above 130 °C, e.g., above 160 °C, or even above 200 °C. In some embodiments, the ionomer in the SO2F-form is postfluorinated with e.g., 10 % F2 in N2 at 50 to 200 °C.

[0022] 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%).

[0023] 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 %).

[0024] 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, e.g., MV31, MA3, or PMVE . 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).

[0025] Although not necessary and less preferred, the fluoropolymers may also include units derived from other optional comonomers (e.g., HFP, VDF) 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.

[0026] The inventive process comprises the addition of a strong protic acid, i.e., a protic acid with a pKa of less than 1, to the ionomer salt dispersion. Preferably, the protic acid has a pKa of less than 0. By addition of the strong acid, the pH of the ionomer dispersion which is treated by ultrafiltration can be adjusted to no greater than 1, preferably no greater than 0.

[0027] In some embodiments, the protic acid is an inorganic acid. In some embodiments, the protic acid is selected such that the counter anion can be evaporated during drying of the ionomer, e.g., HNO3 or a fluorinated sulfonic acid with 1 to 3 carbon atoms.

[0028] Ultrafiltration of the acidified aqueous dispersion can be conducted using any known means and membranes. Ultrafiltration membranes are typically defined by their molecular weight cut-off (MWCO), which describes the general upper limit on the size of the molecules capable of passing through the membrane. The MWCO should be greater than the maximum molecular weight (MW) of the ions to be removed. However, the flux rate through the membrane will increase as the MWCO increases, such that a higher value of MWCO can lead to shorter separation times.

[0029] In some embodiments of the present disclosure, 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 fluorinated ionomer 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 ultrafiltration step.

[0030] During the ultrafiltration process, a permeate stream with the ions and impurities is removed from the system. Usually, the starting concentration of the acidified ionomer dispersion is from 5 to 30 wt.% (based on solid ionomer), preferably from 10 to 20 wt.%. The temperature of the process is typically from 10 to 50 °C.

[0031] Some up-concentration of the ionomer dispersion in the retentate may occur as permeate is removed. Usually, the permeate removed from the dispersion will be replenished using a wash solution containing a protic acid to control the solids content of the dispersion and to keep the pH at a low level. For convenience, the same protic acid may be used throughout the process, but this is not necessary. The ultrafiltration process can be run batchwise or continuously.

[0032] Generally, this acid ultrafiltration process is continued until the desired level of acidification of the acid groups of the ionomer is achieved. At any point after the desired level of acidification of the acid groups of the ionomer is achieved, the ultrafiltration process can be continued with deionized water as the wash solution to continue reducing the concentration of ions in the retentate, and to reduce or eliminate the counterions of the protic acid(s). Alternatively, in some embodiments, the acid ultrafiltration process can be continued until the desired reduction in ion levels is reached. Then, the counter anion of the acid used in the acid treatment process can be reduced to its desired level by performing ultrafiltration using only deionized water as the wash solution. In any case, these steps can be performed batchwise or in a continuous process.

[0033] In some embodiments, the concentration of each counter cation in the retentate of the ultrafiltered dispersion is reduced to no greater than 100 ppm by weight based on the weight of the ionomer in the retentate. In some embodiments, the total concentration of all counter cations in the retentate is no greater than 50 ppm, or even no greater than 20 ppm by weight based on the weight of the ionomer in the retentate.

[0034] Other target cations present in the unpurified aqueous dispersion are also removed in the ultrafiltration step. For example, the unpurified aqueous dispersion may comprise one or more target cations according to the formula M ⁿ⁺ , wherein M 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, the concentration of each such target cation in the retentate of the ultrafiltered dispersion is no greater than 5 ppm by weight based on the weight of the ionomer in the retentate. In some embodiments, the total concentration of all such target cations in the retentate of the ultrafiltered dispersion is no greater than 10 ppm, e.g., no greater than 5 ppm, or even no greater than 2 ppm by weight based on the weight of the ionomer in the retentate. [0035] In some embodiments, the total concentration of all counter cations and target cations according to formula M ⁿ⁺ (as described above) in the retentate is no greater than 30 ppm by weight based on the weight of the ionomer, e.g., no greater than 20, or even no greater than 10 ppm by weight based on the weight of the ionomer in the retentate.

[0036] In some embodiments, the unpurified aqueous dispersion comprises one or more target anions selected from the group consisting of F", HCOO", C20q2", S0q2", RfCOO", and RfSC>3"; 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 to 4 oxygen atoms. In some embodiments, one or more of the anions may be present in their protonated form. In some embodiments, the retentate comprises no greater than 300 ppm by weight of each target anion, based on the weight of the ionomer in the retentate. In some embodiments, the purified aqueous dispersion comprises no greater than 200 ppm of each target anion, based on the weight of the ionomer in the retentate. The counter anion of the protic acid(s) used to acidify the dispersion (e.g., NO3") is excluded from this consideration.

[0037] In some embodiments, the unpurified aqueous dispersion comprises €2042". For example, when hydrolysis temperatures of the SC^F-polymer of greater than 150 °C, e.g., greater than 200 °C, are used, C20q2" can be generated. Generally, the higher the hydrolysis temperature, the more C20q2" 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 C^Oq^". based on the total weight of the ionomer. In some embodiments, the retentate comprises no greater than 300 ppm by weight of C20q2", based on the weight of the ionomer in the retentate. In some embodiments, the retentate comprises no greater than 200 ppm, or even no greater than 150 ppm by weight of based on the weight of the ionomer in the retentate.

[0038] In some embodiments, the unpurified aqueous dispersion comprises SOq^" and the retentate comprises no greater than 300 ppm, e.g., no greater than 200 ppm or even no greater than 150 ppm by weight of SOq^" based on the weight of the ionomer in the retentate.

[0039] In some embodiments, the unpurified aqueous dispersion comprises F" and the retentate comprises no greater than 1 ppm, e.g., no greater than 0.5 ppm or even no greater than 0.1 ppm by weight of F" based on the weight of the ionomer in the retentate.

[0040] In some embodiments, the target anions present in the unpurified aqueous dispersion include at least one anion according to the formula RfCOO", wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 6 to 14 carbon atoms. In some embodiments, the retentate comprises no greater than 25 ppb, e.g., no greater than 10 ppb or even no greater than 5 ppb, of each such target anion according to the formula RfCOO", by weight based on the weight of the ionomer in the retentate. In some embodiments, the total concentration of all target anions according to the formula RfCOO", wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 1 to 20 carbon atoms, is no greater than 25 ppb, e.g., no greater than 10 ppb by weight based on the weight of the ionomer in the retentate. [0041] In some embodiments, the target anions in the unpurified aqueous dispersion include at least one anion according to the formula RfSC>3", wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 6 to 14 carbon atoms. In some embodiments, the retentate comprises no greater than 25 ppb, e.g., no greater than 10 ppb or even no greater than 5 ppb, of each such target anion according to the formula RfSC>3", by weight based on the weight of the ionomer in the retentate. In some embodiments, the total concentration of all target anions according to the formula RfSC>3", wherein Rf is a fluorinated (in some embodiments, a perfluorinated) alkyl or alkylene having 1 to 20 carbon atoms is no greater than 25 ppb, e.g., no greater than 10 ppb by weight based on the weight of the ionomer in the retentate.

[0042] In some embodiments, the ultrafiltration can be continued by adding pure water to replenish the permeate; this is usually done at the end of the acid ultrafiltration steps when the concentration of the target ions is below the desired target levels. By doing this water ultrafiltration step, the concentration of the added anion from the acid is reduced/eliminated. In some embodiments, the dispersion can also be upconcentrated to, e.g., 30 wt.% to use the purified dispersion for coating applications.

[0043] In some embodiments, if a dried ionomer product is required, the ultrafiltration step with pure water can be avoided by appropriate selection of the protic acid. For example, if the anion of the protic acid is volatile at the drying conditions, the anion from the acid is removed during drying. Acids with volatile anions include HNO3.

[0044] Generally, the acid ultrafiltration step generates significant quantities of permeate containing the removed ions such as F", C20q2", SOq^", HCOO", RfCOO", RfSO3" and cationic species, but at very low concentrations, e.g., at concentrations of less than 1 wt.%. Also, there may be anions in the permeate from the added acid. Handling and disposal of such a permeate may be undesirable. Therefore, in some embodiments, the permeate may be purified and reused in the ultrafiltration process providing a closed loop system and further achieving a highly concentrated form of the removed contaminants for further treatment. [0045] The purification of the permeate may comprise treatment with a weakly basic anion exchange resin or trialkyl amine extractions; treatment with CaCC>3, BaCC>3, Ca(OH)2; and/or treatment by a final Reverse Osmosis-(RO)-step. The purified water may then be reused as a wash solution, e.g., it may be acidified and reused as the wash solution in the acid ultrafiltration step. [0046] Alternatively, the pH of the permeate may be adjusted to a pH >7, preferably >9, followed by a distillation/evaporation step. This option is beneficial as nearly all the water is recovered. Also, as the contaminants removed by the processes described above are present in high concentrations, it is more practical and economical to incinerate or otherwise destroy them. The pure, recovered water may be used as a wash solution. In some embodiments, it is acidified with a strong protic acid and reused as the wash solution in the acid ultrafiltration step.

[0047] In some embodiments the purified ionomer 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 ionomer 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. In some embodiments, the drying 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.

[0048] 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.

[0049] In some embodiments, the purified ionomer 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.

[0050] In another embodiment, the purified ionomer 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 purified ionomer dispersion and the foam is freeze-dried. A foam can be created, for example, by subjecting the aqueous dispersion to ultrasound irradiation, by injecting a gas into the dispersion, for example CC>2, or by mechanical forces, for example by subjecting the aqueous dispersion to one or more high speed agitator. A combination of the above steps may be used, also.

[0051] In another embodiment, the purified aqueous ionomer 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.

[0052] In some embodiments, the dried ionomer 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 ionomer, for example at concentrations of at least 10 % by weight of the ionomer, at least 30%, or even at least 40% by weight of the ionomer or more. Of course, lower concentrations may be suitable for some applications. For example, the solution may be a 15 wt.% dispersion of the ionomer in n- propanol/water (60/40), having a solution viscosity below 500 mPa*s at shear rates of both 1 sec" and 1000 sec"l.

[0053] The purified ionomer 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, in an electrochemical cell, for example in a chlor-alkali membrane cell, and in water electrolysis, e.g., for H2 generation. The ionomer compositions are particularly suitable for making thin membranes, i.e., membranes having a thickness of less than 50 micrometers, 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) might be added to the 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.

[0054] 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. [0055] In some embodiments, the additives include 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 chloride, bromide, hydroxide, nitrate, sulfonate, acetate, phosphate, and carbonate. 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.

[0056] Manganese cations may be in any suitable oxidation state, including Mn^ ^-1- , Mn^ ⁺ , and Mn4+, but are most typically Mn^ ^-1- . Ruthenium cations may be in any suitable oxidation state, including RiP ⁺ and Ru^ ⁺ . but are most typically Ri ⁺ . 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 ionomer, 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. [0057] In some embodiments, the total concentration of the Ce and Mn ions is at least 500 ppm, based on the weight of the ionomer 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 ionomer in the dispersion (or redispersion)

[0058] 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).

[0059] Test Methods. The following procedures were used in the Examples and are referenced in the claims.

[0060] ICP-OES Procedure 1: 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.

[0061] ICP-OES Procedure 2: Cation concentrations (e.g., metals) were determined as follows. Each dispersion sample was diluted 20X using 3% HNO3 solution (or 200X if the anticipated level of Li ⁺ cations was expected to be greater than 400 ppm based on weight of the ionomer). After dilution, the dispersions were filtered using a 5 micron Acrodisc® syringe filter with Supor® polyethersulfone membrane (Pall Corporation, Port Washington, NY). The metal content was determined from the diluted samples using a Perkin Elmer Optima 8000 ICP-OES with Sc as an internal standard.

[0062] IC Procedure 1: Inorganic anion concentrations were measured as follows. The aqueous ionomer 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).

[0063] IC Procedure 2: Aqueous ionomer dispersions in the range of 10 - 20 wt.% were diluted with water to prepare diluted dispersions at a weight ratio of 1 part ionomer dispersion to 100 - 200 parts water to obtain anion concentrations in the calibrated detection range, if present. One to two blank samples were run between dispersion samples to ensure no anion carryover from one sample to the next. Stock standard solutions containing 10, 1, 0.5, 0.1 and 0.05 ppm Br, Cl, F, NO2", NO3", SOZ^", and POz^" were prepared by 10-, 100-, 200-, 1000- and 2000-fold dilution, respectively, of Metrohm Custom Anions Mix 3 (REAIC1035, Metrohm, Riverview, FL). A stock 1000 ppm solution of oxalic acid was prepared from oxalic acid dihydrate (0230-01, J.T. Baker, Phillipsburg, NJ, USA) and 18 MOhm DI water (Bamstead GenPure System, Thermo Scientific, Waltham, MA, USA). Stock standard solutions of oxalic acid were prepared by serial dilution of the 1000 ppm standard to prepare concentrations of 1, 0.1, and 0.06 ppm. The stock standard solutions and samples were then injected at 50 pL into a Dionex ICS-2000 (Dionex, Sunnyvale, CA) equipped with an AG 18 guard column, lonPac AS 18 column heated to 30 °C, sequentially suppressed conductivity detector using a Dionex AERS 500 4mm (75 mA) suppressor, and Dionex EGC III KOH eluent generator. The data was collected at a rate of 5.00 Hz with an eluent flow rate of ImL/min with an eluent multistep gradient following 12 mM ramped to 30 mM KOH over the first 10 minutes, 10 minute hold at 30 mM KOH, and ramp back to 12 mM KOH over 3 minutes ending with a 1 minute hold. Data analysis was conducted using Chromeleon 7.2.8 Chromatography Data System (Thermo Fisher Scientific, Waltham, MA).

[0064] ISE Procedure: Free Fluoride of an aqueous dispersion was determined using an Orion Star A214 pH/ISE meter equipped with a fluoride combination electrode Orion 9609BNWP (Thermo Fisher Scientific, Waltham, MA). A calibration curve was constructed via direct calibration of 2 fluoride standards: 1 ppm F" and 10 ppm F" ISE standards. Equal parts by volume of dispersion were combined with a 50:50 solution of TISAB II buffer to DI water. (All calibration standards and buffer are available from Thermo Fisher Scientific, Waltham, MA). The aqueous dispersion with buffer was measured and total fluoride content calculated based on solid content of the dispersion. [0065] LC-MS Procedure. Organic ion concentrations were determined using liquid chromatography-mass spectroscopy as follows. One gram of each dispersion was mixed with 4.5 mL of a 2 ng/mL methanol solution of ' ■’C-labcled perfluorinated carboxylic and sulfonic acids (MPFAC-C-ES, available from Wellington Laboratories (Guelph, Ontario, Canada)). The solution was shaken for 24 hours at room temperature, after which it was centrifuged, with the resulting supernatant being collected for analysis. If further dilution of extracts is 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.

[0066] Samples were analyzed for perfluorobutanesulfonic acid (PFBS) using an Agilent 1290 Infinity HPLC coupled with an Agilent 6495 LC-QQQ triple quadrupole mass spectrometer, both of which are available from Agilent Technologies, Inc (Santa Clara, CA). The separations were performed using an Agilent Poroshell 120 EC-C8 column (2.1 x 50 mm, 2.7 pm). 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.

[0067] 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.

[0068] For the detection of PFBS, the following MRM transitions were used:

(i) 299 Daltons to 99 Daltons; and

(ii) 299 Daltons to 80 Daltons.

[0069] For the detection of M3PFBS, the following MRM transition was used:

302 Daltons to 80 Daltons.

[0070] Examples

Table 1 - Summary of materials

[0071] Comparative Example 1 : Ultrafiltration of PFSA-Li Dispersion without acid treatment. [0072] A PFSA-Li EW 725 dispersion was prepared as described in Example 1 of W02020/183306 to produce a sulfonyl fluoride copolymer with 22.8 mol% units derived from MV4S and an MFI (265 °C/5 kg) of 25 g/ 10 minutes. The sulfonyl fluoride copolymer was hydrolyzed with LiOH at a mole ratio of LiOH to SO2F of 4.0 to 1 (but without the use of lithium carbonate) at a temperature above 180°C. The Li level of the hydrolyzed dispersion was 3.1 wt.% based on the weight of the fluoropolymer.

[0073] Ultrafiltration was run using a ceramic membrane with a MWCO of 15,000 Daltons from TAMI Industries. 39 kg PFSA-Li dispersion with 10 wt.% solid content was fed into the feed container. Cross-flow filtration was run at a feed rate of 2750 1/h, feed pressure of 310 kPa (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 m/s. Permeating water was replaced by fresh water after each filtration run (discontinuous approach). Filtration steps were as follows:

• Filtration until feed reaches ca. 16 wt.% solid content (1st run)

• Dilution of feed down to 10 wt.% solid content

• Filtration until feed reaches ca. 16 wt.% solid content (2nd run)

• Dilution of feed down to 10 wt.% solid content

• Filtration until feed reaches ca. 16 wt.% solid content (3rd run)

[0074] The obtained results are summarized in Table 2. After the first UF run, the Li level in the dispersion was reduced by about 50% (from 31,000 ppm Li to 15,465 ppm relative to weight of the ionomer). The level of Li expected to be associated with the ionomer itself is 9305 ppm. After the third UF run, ICP analytics revealed a Li level of 12585 ppm relative to weight of the ionomer, meaning a further reduction of only 9% in the following two UF runs. Oxalic acid was reduced by 85.6% after three UF runs, whereas sulfate was reduced by 83.5%. However, the concentrations of each of these ions were still greater than 500 ppm based on the weight of the ionomer.

Table 2: Comparative Example 1, ion concentrations relative to weight of the ionomer.

[0075] Example 1 : Ultrafiltration of PFSA-Li Dispersion after acid treatment step.

[0076] Approximately 29 kg of the 16 wt.% dispersion obtained from Comparative Example 1 after the third ultrafiltration run were diluted with 20 L of 10 wt.% of HNO3 (pKa < 1) and rolled for 2 hours. The obtained dispersion had a pH of about 0.5 and an ionomer mass content of about 7.1 wt.%. The acidified dispersion was ultrafiltered using the 15 kDa ceramic membrane as used in Comparative Example 1. Cross-flow filtration was run at a dispersion feed rate of 2800 liters per hour, feed pressure of 340 kPa (3.4 bar), retentate pressure of 200 kPa (2.0 bar) and a permeate pressure of 50 kPa (0.5 bar). The ultrafiltration was performed without dilution (i.e., with no added wash solution) until an approximately 20 wt.% ionomer solid content dispersion was obtained. The lithium level was reduced by 71% from an initial level of 12585 ppm down to 3595 ppm relative to the weight of the ionomer using this post-acid treatment ultrafiltration step.

[0077] Example 2: Ultrafiltration of PFSA-Li Dispersion simultaneously with acid treatment. [0078] 750EW PFSA in acidic (H+) form was prepared from a sulfonyl fluoride copolymer with 21.3 mol% units derived from MV4S and an MFI (265/5) of 25 g/10 minutes. The sulfonyl fluoride copolymer was prepared according the methods described in Example 1 of

W02020/183306, except the sulfonyl fluoride copolymer was hydrolyzed with LiOH at a mole ratio of Li OH to SO2F of 3.0 to 1 and without the use of lithium carbonate. After cation exchange to convert the ionomer to its acidic form, the dispersion was oven dried at 60 °C for 50 hours to form a friable ionomer containing 6.88 wt.% moisture. The Li content of the ionomer was determined by ICP-OES Procedure 1 to be 2.7 ppm based on the weight of the polymer. This initial Li concentration is noted in Table 3 under “Pre -Ultrafiltration”.

[0079] A 500 g aqueous dispersion containing 20 wt.% solids was prepared by combining 107.4 g of this dried ionomer with 392.6 g of Ultrapure H2O in a plastic bottle and mixing on a roller at 60 rpm for 16 hours (“Dispersion A”). Accounting for the moisture in the ionomer, this resulted in a 20 wt.% solids dispersion. Dispersion A was analyzed by the ISE Procedure for fluoride ions.

[0080] A separate 500 g dispersion containing 10 wt.% solids was prepared from 53.45 g of the same dried ionomer in the same manner (“Dispersion B”). Dispersion B was concentrated by ultrafiltration using a tangential flow filtration setup comprising a hollow fiber filtration module (S02-S050-05-N, PS/50 kDa, Spectrum, Inc., Rancho Dominguez, CA) that had been washed with 6 gallons Ultrapure H2O to collect 200 g permeate that was analyzed by IC Procedure 2 for oxalate and sulfate ions and by ISE Procedure for fluoride ions. The results are shown in Table 3 under “Pre-Ultrafiltration”.

[0081] Next, 496.26 g of Dispersion A were spiked with 2.4 g lithium hydroxide monohydrate to target a dispersion with 4000 ppm Li ⁺ based on the weight of the ionomer. The sample was diluted to 10 wt.% polymer upon addition of 496.25 g 10 wt.% HNO3 (aq). The diluted, acidified dispersion was agitated for 15 minutes in a bottle on a roller. This material was sampled for Li ⁺ analysis by ICP-OES Procedure 2 and shown in Table 3 under “Pre-ultrafiltration, Li ⁺ Spiked”. [0082] Ultrafiltration was the performed using a tangential flow filtration setup comprising a hollow fiber filtration module (S02-S050-05-N, PS/50 kDa, Spectrum, Inc., Rancho Dominguez, CA) that had been pre-washed with 0.1 M KOH at room temperature followed by 20% aqueous ethanol and flushed thoroughly with Ultrapure H2O. Ultrafiltration by batch mode was conducted until the retentate contained approximately 17 wt.% solids, by removal of 409 g permeate. The retentate was diluted with 409 g of a 10 wt.% HNO3 (aq) wash solution, mixed by circulating in the ultrafiltration setup without backpressure, and sampled for ICP measurement. The batch cycling of concentration from 10 to 17 wt.% and dilution back to 10 wt.% with 10% HNO3 (aq) was conducted a total of seven times, sampling after each dilution.

[0083] The final dispersion in 10% HNO3 (aq) solvent was further washed with 7646.09 g Ultrapure H2O by ultrafiltration using a continuous ultrafiltration method to reduce the concentration of nitrate ions, i.e., the counter anions of the protic acid used in the acid ultrafiltration steps. The nitrate ion concentration in the dispersion after the Ultrapure H2O ultrafiltration step was 84 ppm HNO3 based on the weight of the fluoropolymer.

[0084] Results of IC and ICP are shown in Table 3 as determined by IC Procedure 2 and ICP- OES Procedure 2, except as noted above.

Table 3: Ion concentrations for Example 2.

[0085] The dried ionomer used to prepare the Pre-ultrafiltration sample in Example 2 was used to prepare 50 g of aqueous dispersion with 19.6 wt.% solids. This sample and the retentate of the Post-H2O Wash sample in Table 3 with fluoropolymer solids content of 10.6 wt.% were analyzed for perfluorobutanesulfonic acid (PFBS) according to the LC-MS procedure. The average PFBS content was 35.85 ppb before ultrafdtration and 0.80 ppb after the final water wash representing an 97.8% decrease in PFBS. The PFBS results are based on the weight of the fluoropolymer.

[0086] Example 3: Ultrafiltration of PFSA-Li Dispersion simultaneously with acid treatment. [0087] A tangential flow filtration setup comprising a hollow fiber filtration module (S02- S050-05-N, PS/50 kDa, Spectrum, Inc., Rancho Dominguez, CA) was utilized for ultrafiltration by diafiltration method. A 500 g dispersion containing 10 wt.% 750 EW PFSA in acidic form (H+) was prepared similarly to Example 2 except 53.7 g of the dried ionomer used in Example 2 was mixed with 446.3 g of DI water. The 500 g dispersion was spiked with 1.2089 g LiOH«H2O(s) and rolled for 15 minutes to ensure complete dissolution and mixing.

[0088] An initial 5 g sample was collected as a Pre-ultrafiltration sample. Ultrafiltration was performed with 445.75 g a 10 wt.% HNO3 (aq) (nitric acid wash solution), collecting 404.94 g permeate and a 4.99 g treated-retentate sample (post-cycle 1). Further diafiltration was conducted with 880.15 g nitric acid solution and led to the collection 841.2 g permeate and a post-treatment collection of a 5.03 g sample (post-cycle 2). Further diafiltration was conducted with 869.95 g nitric acid solution and led to the collection of 879.91 g permeate and a post-treatment collection of 4.99 g (post-cycle 3). Further diafiltration was conducted with 860.01 g nitric acid solution and led to the collection of 835.34 g permeate and a post-treatment collection of 4.99 g (post-cycle 4). Further diafiltration was conducted with 860.01 g nitric acid solution and led to the collection of 834.82 g permeate and a post-treatment collection of 5.01 g (post-cycle 5). Further diafiltration was conducted with 840.01 g nitric acid solution and led to the collection of 823.01 g permeate and a post-treatment collection of 5.02 g (post-cycle 6). Analysis of retentate samples by ICP Method 2 showed a reduction of Li ⁺ down to levels of 0.8 ppm based on weight of fluoropolymer. The concentrations of Na, K and Al cations were also reduced. In addition to Al ⁺² , the ICP method was used to detect other target cations of the formula M ^11-1- ; however, the concentrations of the Cr, Cu, Fe, Ni, and Ti cations were all below their respective detection limits in both the Pre- ultrafiltration sample and in the Post-cycle 11 sample. The detection limits for these cations ranged from 0.01 to 0.008 ppm. Table 4 - Example 3 Ultrafiltered retentate values from ICP Method 2 (Lv ).