Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/28—Treatment by wave energy or particle radiation
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F14/00—Homopolymers and copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen
  • C08F14/18—Monomers containing fluorine
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F214/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen
  • C08F214/18—Monomers containing fluorine
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
  • C08J5/20—Manufacture of shaped structures of ion-exchange resins
  • C08J5/22—Films, membranes or diaphragms
  • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
  • C08J5/2218—Synthetic macromolecular compounds
  • C08J5/2231—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds
  • C08J5/2243—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds obtained by introduction of active groups capable of ion-exchange into compounds of the type C08J5/2231
  • C08J5/225—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds obtained by introduction of active groups capable of ion-exchange into compounds of the type C08J5/2231 containing fluorine
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2327/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers
  • C08J2327/02—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers not modified by chemical after-treatment
  • C08J2327/12—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers not modified by chemical after-treatment containing fluorine atoms
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• This invention relates to a method of making a crosslinked polymer electrolyte, typically in the form of a membrane for use as a polymer electrolyte membrane in an electrolytic cell such as a fuel cell, by application of ultra violet radiation to a highly fluorinated fluoropolymer comprising: a backbone derived in part from tetrafluoro- ethylene monomer, first pendent groups which include a group according to the formula
• the pendant substantially fluorinated carbon groups which have no ion exchange functionality may comprise Br.
• U.S. Patent Pub. No. US 2003/0181615 Al purportedly discloses polymers of certain fluorosulfonated fluoromonomers, certain brominated fluoromonomers, and no tetrafluoroethylene (TFE) monomer. ('615 at para. 234 and at para. 64-68).
• TFE tetrafluoroethylene
• U.S. Patent No. 5,260,351 purportedly discloses perfluoroelastomers cured by radiation in the absence of curing agents.
• the reference purportedly relates to curing of fully fluorinated polymers, such as those prepared from tetrafluoroethylene, a perfluoralkyl perfluorovinyl ether, and cure site or crosslinking units providing at least one of nitrile, perfluorophenyl, bromine or iodine in the resulting terpolymer.
• the present invention provides a method of making a crosslinked polymer comprising the steps of: a) providing a highly fluorinated fluoropolymer comprising: a backbone derived in part from tetrafluoroethylene monomer, first pendent groups which include a group according to the formula -SO2X, where X is F, CI, Br,
• the method may additionally comprise, prior to said step b), the step of: c) forming the fluoropolymer into a membrane.
• the membrane has a thickness of 90 microns or less, more typically 60 or less, and most typically 30 microns or less.
• the highly fluorinated fluoropolymer is perfluorinated.
• the first pendent groups are groups according to the formula -0-(CF2)_i-SO2X, and typically X is OH.
• the present invention provides crosslinked polymers and polymer electrolyte membranes made according to any of the methods of the present invention.
• What has not been described in the art, and is provided by the present invention is a method of crosslinking highly fluorinated fluoropolymer comprising a backbone derived in part from tetrafluoroethylene monomer, first pendent groups which include a group according to the formula -SO2X, where X is F, CI, Br, OH or -O"M + , where M + is a monovalent cation, and second pendent groups which include Br, CI or I, which is typically a membrane for use as a polymer electrolyte membrane, using ultraviolet radiation.
• ultraviolet radiation means electromagnetic radiation in the range of wavelengths beginning at 400 nm, or more typically 380nm, or more typically 300nm, and extending downward, typically to 180 nm;
• Equivalent weight (EW) of a polymer means the weight of polymer which will neutralize one equivalent of base;
• hydrolysis product (HP) of a polymer means the number of equivalents (moles) of water absorbed by a membrane per equivalent of sulfonic acid groups present in the membrane multiplied by the equivalent weight of the polymer;
• highly fluorinated means containing fluorine in an amount of 40 wt% or more, typically 50 wt% or more and more typically 60 wt% or more.
• the present invention provides a method of making a crosslinked polymer.
• the polymer to be crosslinked comprises: a backbone derived in part from tetrafluoro- ethylene (TFE) monomer, first pendent groups which include a group according to the formula -SO2X, where X is F, CI, Br, OH or -O"M + , where M + is a monovalent cation, and second pendent groups which include Br, CI or I.
• TFE tetrafluoro- ethylene
• first pendent groups which include a group according to the formula -SO2X, where X is F, CI, Br, OH or -O"M + , where M + is a monovalent cation
• second pendent groups which include Br, CI or I.
• PEM's polymer electrolyte membranes
• PEM's manufactured from the crosslinked polymer according to the present invention may be used in the fabrication of membrane electrode assemblies (MEA's) for use in fuel cells.
• An MEA is the central element of a proton exchange membrane fuel cell, such as a hydrogen fuel cell.
• Fuel cells are electrochemical cells which produce usable electricity by the catalyzed combination of a fuel such as hydrogen and an oxidant such as oxygen.
• Typical MEA's comprise a polymer electrolyte membrane (PEM) (also known as an ion conductive membrane (ICM)), which functions as a solid electrolyte.
• PEM polymer electrolyte membrane
• ICM ion conductive membrane
• Each electrode layer includes electrochemical catalysts, typically including platinum metal.
• GDL's Gas diffusion layers facilitate gas transport to and from the anode and cathode electrode materials and conduct electrical current.
• the GDL may also be called a fluid transport layer (FTL) or a diffuser/current collector (DCC).
• FTL fluid transport layer
• DCC diffuser/current collector
• the anode and cathode electrode layers may be applied to GDL's in the form of a catalyst ink, and the resulting coated GDL's sandwiched with a PEM to form a five-layer MEA. Alternately, the anode and cathode electrode layers may be applied to opposite sides of the PEM in the form of a catalyst ink, and the resulting catalyst-coated membrane (CCM) sandwiched with two GDL's to form a five-layer MEA.
• CCM catalyst-coated membrane
• the five layers of a five-layer MEA are, in order: anode GDL, anode electrode layer, PEM, cathode electrode layer, and cathode GDL.
• protons are formed at the anode via hydrogen oxidation and transported across the PEM to the cathode to react with oxygen, causing electrical current to flow in an external circuit connecting the electrodes.
• the PEM forms a durable, non-porous, electrically non-conductive mechanical barrier between the reactant gases, yet it also passes H + ions readily.
• the polymer to be crosslinked comprises a backbone, which may be branched or unbranched but is typically unbranched.
• the backbone is highly fluorinated and more typically perfluorinated.
• first side groups R may be added to the backbone by grafting. Typically, first side groups R are highly fluorinated and more typically perfluorinated. R may be aromatic or non-aromatic. Typically, R is
• R! is a branched or unbranched perfluoroalkyl, perfluoroalkoxy, or perfluoropolyether group comprising 1-15 carbon atoms and 0-4 oxygen atoms.
• R! is typically -O-R ⁇ - wherein R ⁇ is a branched or unbranched perfluoroalkyl, perfluoroalkoxy, or perfluoropolyether group comprising 1-15 carbon atoms and 0-4 oxygen atoms, R! is more typically -O-R ⁇ - wherein R- is a perfluoroalkyl group comprising 1-15 carbon atoms. Examples of R!
• n i, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15
• (-CF2CF(CF3» n where n is 1, 2, 3, 4, or 5 (-CF(CF 3 )CF2-) n where n is 1, 2, 3, 4, or 5(-CF 2 CF(CF3)-) n -CF 2 - where n is 1, 2, 3 or 4 (-O-CF2CF2-) n where n is 1, 2, 3, 4, 5, 6 or 7 (-0-CF2CF2CF2-) n where n is 1, 2, 3, 4, or 5 (-0-CF2CF2CF2CF2-) n where n is 1, 2 or 3 (_0-CF 2 CF(CF 3 )-) n where n is 1, 2, 3, 4, or 5 (-0-CF 2 CF(CF 2 CF3)-) n where n is 1, 2 or 3 (-0-CF(CF3)CF 2 -)n where n is 1 , 2, 3, 4 or 5 (-0-CF(CF3)CF 2 -)n where n is
• the fluoromonomer providing first side group R may be synthesized by any suitable means, including methods disclosed in U.S. Pat. No. 6,624,328.
• the fluoropolymer includes second pendant groups Q containing Br, CI or I, typically Br.
• second pendant groups Q may be added to the backbone by grafting.
• second pendant groups Q are highly fluorinated and more typically perfluorinated, other than at the bromine position.
• Q is -R ⁇ -Br, where R* is as described above.
• Q is Br, CI or I, typically Br.
• the overall CI, Br or I content is typically 0.05-5 wt% and more typically 0.1 -2 wt%.
• the polymer to be crosslinked may be made by any suitable method, including emulsion polymerization, extrusion polymerization, polymerization in supercritical carbon dioxide, solution or suspension polymerization, and the like, which may be batchwise or continuous.
• chain transfer agents may be used during polymerization to provide a polymer with CI, Br or I end groups. Where such end groups are present, they may be considered pendant groups for the purposes of the present invention.
• chain transfer agents include those having the formula RX n , wherein R is an n-valent alkyl group containing 1-12 carbon atoms, which may be fluorinated or unfluorinated, and wherein X's are independently selected from CI, Br or I. Additional chain transfer agents are exemplified in U.S. Patent Nos. 4,000,356 and 6,380,337.
• the polymerization can be performed in the presence of Br or I" salts in order to introduce terminal Br or I endgroups, as described in EP 407 937.
• the acid-functional pendent groups typically are present in an amount sufficient to result in an hydration product (HP) of greater than 15,000, more typically greater than 18,000, more typically greater than 22,000, and most typically greater than 25,000. In general, higher HP correlates with higher ionic conductance.
• the acid-functional pendent groups typically are present in an amount sufficient to result in an equivalent weight (EW) of less than 1200, more typically less than 1100, and more typically less than 1000, and more typically less than 900.
• EW equivalent weight
• the polymer is brought into contact with a crosslinking agent prior to crosslinking.
• crosslinking agent may be used, such that it will react with at least two radicals generated by removal of CI, Br or I from a pendent group.
• Crosslinking agents which may be useful in the practice of the present invention may include polyaromatic species or polyvinyl species, which may be non-fluorinated or fluorinated to a low level but which are more typically highly fluorinated and more typically perfluorinated.
• Examples of crosslinking agents useful in the practice of the present invention include: trimethylol propyl triacrylate (TMPTA), diphenyl ethers, diphenoxy alkanes, diphenoxy ethers, diphenoxy polyethers, di-, tri- and tetraallyl species, and the like.
• the crosslinking agent and polymer may be mixed by any suitable method, including mixing in solution or suspension, kneading, milling, or the like.
• the crosslinking agent may be added to a formed membrane, e.g. by immersion of the membrane in a solution of the crosslinking agent.
• the crosslinking agent may be added in any suitable amount relative to the number of first pendent groups. If an excess of crosslinking agent is added, the excess may be removed after crosslinking. Alternately, if the crosslinking agent is added in a less than an equimolar amount, it is expected that only a portion of the crosslinks formed will be through molecules of the crosslinking agent.
• the polymer or polymer/crosslinking agent blend is formed into a membrane prior to crosslinking.
• Any suitable method of forming the membrane may be used.
• the polymer is typically cast from a suspension or solution. Any suitable casting method may be used, including bar coating, spray coating, slit coating, brush coating, and the like.
• the membrane may be formed from neat polymer in a melt process such as extrusion. After forming, the membrane may be annealed.
• the membrane has a thickness of 90 microns or less, more typically 60 microns or less, and most typically 30 microns or less. A thinner membrane may provide less resistance to the passage of ions. In fuel cell use, this results in cooler operation and greater output of usable energy.
• the polymer may be imbibed into a porous supporting matrix prior to crosslinking, typically in the form of a thin membrane having a thickness of 90 microns or less, more typically 60 microns or less, and most typically 30 microns or less.
• a porous supporting matrix typically in the form of a thin membrane having a thickness of 90 microns or less, more typically 60 microns or less, and most typically 30 microns or less.
• Any suitable method of imbibing the polymer into the pores of the supporting matrix may be used, including overpressure, vacuum, wicking, immersion, and the like.
• the polymer becomes embedded in the matrix upon reaction of the amidine groups.
• Any suitable supporting matrix may be used.
• the supporting matrix is electrically non-conductive.
• 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. Additional embodiments may be found in U.S. Pats. Nos. RE37,307, RE37,656, RE37/701, and 6,254,978. An effective amount of one or more photoinitiators may be added to the polymer or the membrane at any suitable stage, such as during membrane formation.
• PTFE porous polytetrafluoroethylene
• UV photoinitiator including benzylketals such as IrgacureTM 651, alpha hydroxyketones such as IrgacureTM 2959, BAPO-type photoinitiators such as IrgacureTM 819, diaryliodonium salts, triarylsulfonium salts, azo compounds, peroxides, and the like.
• benzylketals such as IrgacureTM 651
• alpha hydroxyketones such as IrgacureTM 2959
• BAPO-type photoinitiators such as IrgacureTM 819, diaryliodonium salts, triarylsulfonium salts, azo compounds, peroxides, and the like.
• Such initiators include benzophenone and its derivatives; benzoin, ⁇ - methylbenzoin, ⁇ - phenylbenzoin, ⁇ -allylbenzoin, ⁇ -benzylbenzoin; benzoin ethers such as benzil dimethyl ketal ((commercially available under the trade designation "IRGACURE 651" from Ciba-Geigy of Ardsley, N.Y.), benzoin methyl ether, benzoin ethyl ether, benzoin n-butyl ether; acetophenone and its derivatives such as 2-hydroxy- 2-methyl-l-phenyl-l -propanone (commercially available under the trade designation "DAROCUR 1173" from Ciba-Geigy of Ardsley, N.Y.) and 1-hydroxycyclohexyl phenyl ketone (HCPK) (commercially available under the trade designation "IRGACURE 184", also from Ciba-Geigy Corporation); 2-mef
• photoinitiators include pivaloin ethyl ether, anisoin ethyl ether; anthraquinones such as anthraquinone, 2- methylanthraquinone, 2-ethyl anthraquinone, 2-t-butyl anthraquinone, 1- chloroanthraquinone, 2-bromoanthraquinone, 2-nitroanthraquinone, anthraquinone- 1 -carboxaldehyde, anthraquinone-2-thiol, 4- cyclohexylanfhraquinone, 1 ,4-dimethylanthraquinone, 1- methoxyanthraquinone, benzathraquinonehalomethyl triazines; onium salts, for example, diazonium salts such as phenyldiazoniumhexafluorophosphate and the like; diaryliodonium
• the step of crosslinking comprises the step of exposing the fluoropolymer to ultraviolet radiation so as to result in the formation of crosslinks.
• Any suitable apparatus and UV source may be used, including arc lamps, microwave powered lamps, mercury lamps, gallium lamps, lasers, sun lamps, and the like.
• the UV radiation is at wavelengths between 300 and 180 nm, and more typically between 280 and 200 nm.
• the UV radiation is in a dose of 1 mJ/cm ⁇ or more, more typically 5 mJ/cm ⁇ or more, more typically 10 mJ/cm2 or more, and most typically 50 mJ/cm ⁇ or more.
• a continuous process of exposure may be used to treat roll good membranes.
• polymers and membranes made according to the method of the present invention may differ in chemical structure from those made by other methods, in the structure of crosslinks, the placement of crosslinks, the placement of acid-functional groups, the presence or absence of crosslinks on pendent groups or of acid-functional groups on crosslinks, and the like.
• This invention is useful in the manufacture of strengthened polymer electrolyte membranes for use in electrolytic cells such as fuel cells. Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.
• TFE tetrafluoroethylene
• CF2 CF-0-(CF2)4-SO2F
• MV4S CF-0-(CF 2 )2-Br
• 130 g MV4S was preemulsified in water with 15 g APFO emulsifier (ammonium perfluorooctanoate, C7F15COONH4) under high shear (24,000 ⁇ m), using an ULTRA-TURRAX® Model T 25 disperser S25KV-25F (IKA-Werke GmbH & Co. KG, Staufen, Germany) for 2 min.
• APFO emulsifier ammonium perfluorooctanoate, C7F15COONH4
• S25KV-25F IKA-Werke GmbH & Co. KG, Staufen, Germany
• the kettle was further charged with 6 g MV2Br and 178 g gaseous tetrafluoroethylene (TFE) to 8 bar absolute reaction pressure.
• TFE gaseous tetrafluoroethylene
• the polymerization was initiated by addition of 15 g sodium disulfite and 40 g ammonium peroxodisulfate.
• the reaction temperature was maintained at 50 °C.
• Reaction pressure was maintained at 8.0 bar absolute by feeding additional TFE into the gas phase.
• a second portion of MV4S- preemulsion was prepared in the same manner and proportions described above, using 427 g MV4S. The second preemulsion portion was fed into the liquid phase during the course of the reaction continuously.
• the resulting polymer electrolyte is a perfluorinated polymer with acidic side chains according to the formula: -O-(CF2)4-SO3H and side chains according to the formula -0-(CF2)2-Br.
• the resulting mixture was an acid dispersion at 18 to 19% polymer solids.
• This dispersion was concentrated in vacu to about 38% solids and then mixed with n-propanol to give the desired 20 % solids dispersion in a water/n-propanol solvent mixture of about 40% water/60% n-propanol. This base dispersion was used to cast membranes.

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