WO2008095509A1 - Reinforced ion-exchange membrane comprised of a support, and laminated thereon, a polymeric film - Google Patents

Reinforced ion-exchange membrane comprised of a support, and laminated thereon, a polymeric film Download PDF

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Publication number
WO2008095509A1
WO2008095509A1 PCT/EP2007/000957 EP2007000957W WO2008095509A1 WO 2008095509 A1 WO2008095509 A1 WO 2008095509A1 EP 2007000957 W EP2007000957 W EP 2007000957W WO 2008095509 A1 WO2008095509 A1 WO 2008095509A1
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WO
WIPO (PCT)
Prior art keywords
polymeric film
exchange membrane
ion
reinforced ion
reinforced
Prior art date
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PCT/EP2007/000957
Other languages
French (fr)
Inventor
Erik Middelman
Jörg Henning Balster
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Redstack B.V.
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Publication date
Application filed by Redstack B.V. filed Critical Redstack B.V.
Priority to EP07703267A priority Critical patent/EP2125173A1/en
Priority to PCT/EP2007/000957 priority patent/WO2008095509A1/en
Publication of WO2008095509A1 publication Critical patent/WO2008095509A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports
    • B01D69/107—Organic support material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/10—Supported membranes; Membrane supports
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/422—Electrodialysis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/422—Electrodialysis
    • B01D61/423—Electrodialysis comprising multiple electrodialysis steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/44—Ion-selective electrodialysis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/44—Ion-selective electrodialysis
    • B01D61/54—Controlling or regulating
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
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    • B01D71/06—Organic material
    • B01D71/52—Polyethers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01D71/06—Organic material
    • B01D71/52—Polyethers
    • B01D71/522—Aromatic polyethers
    • B01D71/5222—Polyetherketone, polyetheretherketone, or polyaryletherketone
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    • B01D71/68—Polysulfones; Polyethersulfones
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    • B01D71/82—Macromolecular material not specifically provided for in a single one of groups B01D71/08 - B01D71/74 characterised by the presence of specified groups, e.g. introduced by chemical after-treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/02—Layered products comprising a layer of synthetic resin in the form of fibres or filaments
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/06—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B27/08—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material of synthetic resin
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/12—Layered products comprising a layer of synthetic resin next to a fibrous or filamentary layer
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    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/32—Layered products comprising a layer of synthetic resin comprising polyolefins
    • 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
    • C08J5/2275—Heterogeneous membranes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
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    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1025—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having only carbon and oxygen, e.g. polyethers, sulfonated polyetheretherketones [S-PEEK], sulfonated polysaccharides, sulfonated celluloses or sulfonated polyesters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1027—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having carbon, oxygen and other atoms, e.g. sulfonated polyethersulfones [S-PES]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1032—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having sulfur, e.g. sulfonated-polyethersulfones [S-PES]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1041—Polymer electrolyte composites, mixtures or blends
    • H01M8/1044—Mixtures of polymers, of which at least one is ionically conductive
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1041—Polymer electrolyte composites, mixtures or blends
    • H01M8/1053—Polymer electrolyte composites, mixtures or blends consisting of layers of polymers with at least one layer being ionically conductive
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
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    • H01M8/1058—Polymeric electrolyte materials characterised by a porous support having no ion-conducting properties
    • H01M8/106—Polymeric electrolyte materials characterised by a porous support having no ion-conducting properties characterised by the chemical composition of the porous support
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
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    • H01M8/1058—Polymeric electrolyte materials characterised by a porous support having no ion-conducting properties
    • H01M8/1062—Polymeric electrolyte materials characterised by a porous support having no ion-conducting properties characterised by the physical properties of the porous support, e.g. its porosity or thickness
    • H—ELECTRICITY
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
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    • H01M8/1069—Polymeric electrolyte materials characterised by the manufacturing processes
    • H01M8/1081—Polymeric electrolyte materials characterised by the manufacturing processes starting from solutions, dispersions or slurries exclusively of polymers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/26—Electrical properties
    • 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
    • C08J2371/00—Characterised by the use of polyethers obtained by reactions forming an ether link in the main chain; Derivatives of such polymers
    • C08J2371/08—Polyethers derived from hydroxy compounds or from their metallic derivatives
    • C08J2371/10—Polyethers derived from hydroxy compounds or from their metallic derivatives from phenols
    • C08J2371/12—Polyphenylene oxides
    • 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
    • C08J2381/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing sulfur with or without nitrogen, oxygen, or carbon only; Polysulfones; Derivatives of such polymers
    • C08J2381/06—Polysulfones; Polyethersulfones
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    • H01M8/02—Details
    • H01M8/0289—Means for holding the electrolyte
    • 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
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    • 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
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    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a reinforced ion- exchange membrane comprised of a porous support matrix and, laminated thereon, a polymeric film. More specifically, the present invention relates to a reinforced cationic or anionic exchange membrane. The present invention further relates to a reinforced ion-exchange membrane for use in (reverse) electrodialysis and a method for preparing said reinforced ion-exchange membrane.
  • a membrane is defined as a permselective barrier separating two phases. Certain compounds can pass through the membrane under the influence of a driving force while others are excluded from passing. Because of this, such a membrane has a capacity of selectively transporting (transferring, separating) compounds from one phase (feed) to another (permeate) .
  • a schematic presentation of this process is shown in figure 1.
  • the membranes or barriers according to the present invention are charged polymeric membranes capable of transporting (transferring, separating) charged particles, such as ions or other charged compounds or particles, through an electric potential difference between two phases (solutions) .
  • Such membranes are also designated as ion- exchange membranes, although not specifically limited to ion separations whereby ions are small inorganic charged compounds such as Na + or Cl " .
  • Ion-exchange membranes can generally be divided in two different types depending on the charge of the membrane, i.e., cation-exchange membranes (negative charge, cem) and anion-exchange membranes (positive charge, aem) .
  • Cation-exchange membranes (cem) generally have negatively charged moieties fixed thereon.
  • anion-exchange membranes (aem) generally have positively charged moieties fixed thereon.
  • cation-exchange membranes commonly use charge providing sulfonic acids moieties, i.e., compounds comprising one or more -SO 3 " groups, or carboxylic acids moieties, i.e., compounds comprising one or more -COO " groups, fixed (adhered, cross-linked, connected, bonded) to their basic polymeric films.
  • Cation-exchange membranes are used for various electrically driven membrane based processes. For example, they may serve in electrodialysis as separators between concentrate and diluate compartments, in fuel cells, as proton conductors, and as a cation permeable layer of bipolar membranes selectively permeable for protons and water while retaining anions .
  • anion-exchange membranes commonly use charge providing quaternary ammonium moieties, i.e., compounds comprising one or more -R 3 N + groups, fixed (adhered, cross-linked, connected, bonded) to their basic polymeric film, wherein R represents an alkyl or aryl, linear or branched, substituted or not substituted, of C 1 to C 20 carbon atoms chains such as -C 2 H 5 or -C 4 H 9 .
  • These positively and negatively charged moieties represent strong acids and bases, respectively, and are substantially dissociated, i.e., charge providing, over a broad pH range for example pHs of 2 to 12.
  • Ion-exchange membranes can be used for electrodialysis . Electrodialysis involves a selective transport (transfer, separation) , against the concentration gradient, of ions from one, for example, aqueous solution to another, for example, aqueous solution, under the influence of an electrical potential gradient as the driving force.
  • an ion-exchange membrane When placed in an electrolyte solution, the affinity of an ion-exchange membrane for positively and negatively charged ions in the electrolyte solution is different depending on the charge of the moieties fixed thereon.
  • a cationic ion-exchange membrane i.e., a membrane with negatively charged moieties
  • negatively charged particles such as anions are excluded (stopped, prevented from passing) by cation-exchange membranes because their electrical charge being the same as the charge of the fixed moieties on the membrane. This despite an electric potential difference over the cation- exchange membrane .
  • FIG 3 A specific embodiment of an electrodialysis process is provided in figure 3.
  • an aqueous salt solution is fed to an electrodialysis stack consisting of a series of cation and anion exchange membranes between two working electrodes.
  • the positively charged cations migrate towards the cathode and the negatively charged anions in the opposite direction towards the anode .
  • the cations pass easily through the negatively charged cation-exchange membranes, in contrast with the excluded anions. As shown in figure 3, this results in the separation of the feed solution into a solution enriched in ions (the concentrate) and a solution depleted of ions (the diluate) .
  • Electrodialysis can also, for example, be used in desalination of water, desalination in food and pharmaceutical industry, separation of amino acids and production of salts .
  • a special form of electrodialysis is reverse electrodialysis.
  • a reverse electrodialysis cell is comprised of monopolar ion-exchange membranes arranged in the same order as in conventional electrodialysis.
  • the potential drop is used, which develops when a membrane separates two electrolyte solutions differing in concentrations.
  • the compartments contain therefore alternately high and low concentrations of the electrolyte.
  • the generated current can be taken from this process, i.e., the generation of electric power.
  • the generation of electric power depends on the number of cells.
  • the efficiency of the process also depends on the membrane resistance and their permselectivities .
  • the efficacy or usefulness of ion-exchange membranes is determined by several parameters such as a) a high permselectivity (ion selectivity) b) a low electrical resistance c) a good mechanical strength, and d) a high chemical stability.
  • the properties of ion-exchange membranes are also influenced by the basic polymer film and the type and concentration of the fixed charged moieties.
  • the basic polymer film determines to a large extent the mechanical, chemical and thermal stability of the membrane.
  • such basic polymeric films are made of hydrophobic polymers such as polystyrene, polyethylene or polysulfone .
  • these polymers are largely insoluble in water and show a low degree of swelling, through the introduction of charged moieties ionic groups, they may become water soluble. Thus, the basic polymers are often crosslinked.
  • the swelling (when placed in an aqueous environment) and the chemical and thermal stability are also influenced by the degree of crosslinking.
  • the type and the concentration of the fixed charged moieties influence the permselectivity and the electrical resistance of the membrane, but they also have a significant effect on the swelling and the mechanical properties of the membrane.
  • Another important aspect is the relation between the mechanical strength of the ion-exchange membrane and its thickness. For example, when the thickness of the membrane is increased also the mechanical strength is increased. However, the electric resistance will be undesirably increased.
  • reinforced membranes have been proposed such as for example in WO 95/16730. These membranes are comprised of a support matrix providing the required mechanical strength while the selective ion-exchange is provided by the polymeric film.
  • these reinforced ion-exchange membranes still require a relatively thick ion-exchange membrane or film, for example in the range of 100 to 250 micrometers, to provide the required ion selectivity. Inherently, this increased thickness does not only increases the electric resistance of the reinforced membrane, but also its costs.
  • Another objective of the present invention is to provide a reinforced ion-exchange membrane wherein mechanical strength is substantially provided by the reinforcement and the ion-selectivity is provided by a relatively thin ion- exchange membrane as compared to the prior art .
  • a reinforced ion-exchange membrane comprising a porous support matrix and, laminated on at least on side thereof, a polymeric film, wherein the porous support matrix has no electric resistance as measured in the six compartment configuration and wherein the polymeric film comprises a polymer functionalized with charged moieties in a degree of at least 50%, such as 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% and 100%.
  • the polymeric film comprises a polymer functionalized with charged moieties in a degree of at least 50%, such as 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% and 100%.
  • Preferred is a degree of functionalization between 55% to 85%, such as 60%, 65%, 70%, 75% and 80%.
  • the electrical resistance of the porous support matrix according to the present invention is measured in the six compartment configuration.
  • This measurement is performed with a six compartment cell with a four electrode arrangement, as shown in figure 4, under direct current using a 0.5 M NaCl solution.
  • the porous support matrix is first equilibrated in the measurement solution for more than 24 hours.
  • the measurement solution is added in the compartments 3 and 4, next to the porous support matrix to be investigated, with calomel reference electrodes.
  • an NaCl solution with the same concentration is used as in the measurement compartments to reduce the influence of the electrode reactions at the working electrodes.
  • a current is applied through the working electrodes in the compartments 1 and 6 whereas the electrical potential is measured close to the surface of the porous support matrix in the compartments 3 and 4 with the calomel reference electrodes .
  • the area resistance between the calomel reference electrodes is determined from the current-voltage curve (i-v curve) and is measured with and without the porous support matrix.
  • the difference of the two resistances is considered as the porous support electric resistance (R) . This difference is substantially zero for the porous support matrices according to the present invention.
  • the degree of functionalization (SD) of the polymer comprised in the polymeric film according to the present invention can be measured using the equation 1:
  • M Wff is the molecular weight of the functional group including the counter ion
  • M W(P the molecular weight of the non-functional polymer repeat unit
  • IEC the ion echange capacity
  • the IEC of the polymeric films is determined by titration as decribed by Wilhelm et al . , Journal of Membrane Science 199 (2002), 167-176. Briefly, in case of an illustrative cationic polymeric film, the polymeric film is first brought in the H + form by immersion into IM HCl solution for 24 h (the solution is replaced in between for total ion exchange) .
  • the membrane is soaked in water and is then brought into the Na + form by immersing in 2 M NaCl solution. To ensure a complete exchange, the NaCl solution is renewed several times. The collected solutions are combined and titrated with NaOH to determine the proton content .
  • the IEC is calculated as the ratio of total charge by the dry weight of the polymeric film.
  • the present invention relates to a reinforced ion-exchange membrane, wherein the polymeric film comprises a blend of a functionalized polymer and a non-functionalized polymer.
  • the functionalized polymer is a sulphonated poly (ether ether ketone) functionalized polymer.
  • non-functionalized polymer is used herein to encompass any polymer which is not functionalized with charged moieties providing ionic selectivity to the reinforced ion-exchange membrane according to the present invention.
  • the non- functionalized polymer is poly (ethersulphon) non- functionalized polymer.
  • the reinforced ion-exchange membrane according to the present invention comprises a polymeric film comprising 50% to 90%, such as 55%, 60%, 65%, 70%, 75%, 80% and 85% by weight, functionalized polymer and 50% to 10%, such as 45%, 40%, 35%, 30%, 25%, 20% and 15%, by weight non- functionalized polymer.
  • the reinforced ion-exchange membrane according to the present invention comprises a polymeric film comprising 60% to 85%, such as 65%, 70%, 75% and 80%, by weight functionalized polymer and 40% to 15%, such as 35%, 30%, 25% and 20% by weight, non-functionalized polymer.
  • the reinforced ion-exchange membranes according to the present invention preferably comprise charged moieties selected from the group consisting of sulfonic acids, carboxylic acids, and ammonium moieties providing a charge to the polymeric film over a broad pH range. More preferably, the charged moieties are sulfonic acids.
  • porous support matrix can be any nonconducting porous support matrix
  • porous support matrices comprised of a non-woven fabric comprised of a material selected from the group consisting of glass, silica, polyolefins, polypropylene, polyethylene, polyurethane, polystyrene, polybutylene and co-polymers thereof. More preferred are non-woven fabrics comprised of polypropylene and polyethylene.
  • the reinforced ion-exchange membrane according to the present invention comprises a porous support matrix, as defined above, with a thickness of between 30 to 150 micrometers and, laminated thereon, a polymeric film, as defined above, with a thickness of between 5 to 15 micrometers .
  • a reinforced ion-exchange membrane comprising a porous support matrix, as defined above, with a thickness of between 40 to 130 micrometers and, laminated thereon, a polymeric film, as defined above, with a thickness of between 8 to 11 micrometers .
  • the present invention also relates to a method for preparing a reinforced ion-exchange membrane comprising: a) providing a polymeric film according to the present invention and a porous support matrix according to the present invention; b) covering the polymeric film with a solvent for said polymeric film or a solution of a monomer of which said polymeric film is comprised; c) pressing said porous support matrix onto the polymeric film; and d) removing said solvent or solution to provide a reinforced ion-exchange membrane according to the present invention.
  • the reinforced ion-exchange membranes according to the present invention a particularly useful when used for (reverse) electrodialysis .
  • the present invention also relates to the use of a present reinforced ion-exchange membrane for electrodialysis or reverse electrodialysis applications .
  • Particular examples of applications of electrodialysis or reverse electrodialysis applications are, but not limited thereto, desalination of water, desalination in food and pharmaceutical industry, separation of amino acids, production of salt, fuel cells, electric power generation and immediate and sustained release carriers for pharmaceuticals .
  • Fig. 1 is a schematic drawing of a membrane separation
  • Fig. 2 is a schematic presentation of a cation-exchange membrane next to an electrolyte solution
  • Fig. 3 shows the principle of electrodialysis
  • Fig. 4 shows a six compartment cell for the determination of electric resistance
  • Fig. 5 shows a chronopotentiometric curve of a CMX cation exchange membrane
  • Fig. 6 shows the experimental setup of an impedance measurement
  • ' Fig. 7 shows the conductivity and water content of various
  • S-PEEK/PES blends with a degree of 61% and 83% sulphonation (SD) ;
  • Fig. 8 shows the permselectivity (ion selectivity) versus the thickness of two S-PEEK/PES blends, i.e., 60/40 and 80/20, respectively, with a degree of sulphonation (SD) of 80%.
  • Example 1 a polymeric film suitable for a reinforced cation-exchange membrane according to the present invention
  • Sulphonated poly (ether ether ketone), S-PEEK, was chosen as the functionalized polymer in the cation exchange polymeric film. This material is blended with the non- functionalized polymer poly (ethersulphone) , PES, to improve membrane properties.
  • PES non-functionalized polymer poly
  • PEEK 450PF from Victrex As precursor polymer PEEK 450PF from Victrex was used. PEEK is an amorphous polymer, with low water absorption and high solvent resistance. The polymer was dried for more than 24 hours in a vacuum oven, at 100 0 C.
  • reaction vessel was immersed in an ice bath to stop the reaction.
  • the sulphonated polymer (S-PEEK) was precipitated in demineralized water of maximum 5 0 C and washed until the pH was 7. Subsequently, the polymer was dried in air at room temperature and in a vacuum oven at 100 0 C.
  • the polymer solution was filtered over a 40 micrometer metal filter.
  • the films were prepared by the evaporation technique. Briefly, the solutions were cast on glass plates with different casting knifes. The films were dried in an N 2 atmosphere at 40-80 0 C for 1 week.
  • the polymeric films obtained were washed with water and placed in a vacuum oven at 3O 0 C for one week.
  • the dry polymeric films were stored in 0.5 M NaCl solution and their thickness in wet state (d wet ) was measured.
  • Non-woven as polymeric film support Two different non-woven fabrics from Freudenberg (Viledon) , made from polypropylene/polyethylene, were used as polymeric film support matrix:
  • Viledon non-woven thickness 120 micrometers 2. Viledon non-woven, thickness 45 micrometers
  • the prepared polymeric films were characterized by measurements of the ion exchange capacity (IEC) , water uptake (w) , electrical resistance (R) , and the permselectivity (S) .
  • IEC ion exchange capacity
  • w water uptake
  • R electrical resistance
  • S permselectivity
  • the IEC of the membranes was determined by titration. For this, the polymeric film was first brought in the H + form by immersion into 1 M HCl solution for 24 hours (the solution was replaced in between for total ion exchange) .
  • the polymeric film was soaked in water and was then brought into the Na + form by immersing in 2 M NaCl solution. To ensure complete exchange, the NaCl solution was renewed several times .
  • the collected solutions were combined and titrated with NaOH to determine the proton content.
  • the IEC was calculated as the ratio of total charge by the dry weight of the polymeric film .
  • the degree of sulphonation was calculated according to equation 1 :
  • M w,p is the molecular weight of the polymer non-functional repeat unit and M W/f is the molecular weight of the functional group including the counter ion (-SO 3 Na) .
  • the water uptake of the polymeric films was measured in different ionic forms following the weighing procedure.
  • the polymeric film was first brought into the desired ionic form (H + , Na + or Ca 2+ and the weight of the wet (m ⁇ ,.) and dry (m dry ) sample was measured.
  • the water uptake of the polymeric film is given by equation 2:
  • the charge density (x) of the polymeric film can be calculated by equation 3:
  • the permselectivity was determined by static membrane potential measurement.
  • the test system consisted of two cells separated by the polymeric film sample. Two calomel reference electrodes (Schott B2810) were placed into the solutions on either side of the polymeric film and were used to measure the potential difference across the polymeric film.
  • the permselectivity (S) of the polymeric film is given by the ratio of the measured potential difference ( ⁇ V raeas ) and the potential difference calculated by equation 4 for a 100% permselective polymeric film ( ⁇ V calc ) :
  • the electrical resistance of the polymeric films was measured in a six compartment cell with a four electrode arrangement (figure 4) under direct current using a 0.5M NaCl solution.
  • the polymeric film was first equilibrated in the measurement solution for more than 24 hours.
  • the measurement solution was added in the compartments 3 and 4, next to the investigated polymeric film with the calomel reference electrodes.
  • a NaCl solution with the same concentration was used as in the measurement compartments to reduce the influence of the electrode reactions at the working electrodes.
  • the current was applied through the working electrodes in the compartments 1 and 6, whereas the electrical potential was measured close to the polymeric film surface in the compartments 3 and 4 with the calomel reference electrodes.
  • the area resistance between the calomel reference electrodes was determined from the current-voltage curve (i-v curve) and was measured with and without the polymeric film. The difference of the two resistances was considered as the polymeric film area resistance (R) .
  • the specific conductivity was then calculated using equation 5:
  • the cell has a Teflon interior with two circular gold or stainless steel electrodes, surface area of 0.28 cm 2 . Both electrodes are connected with two wires, one for carrying the current and one for acting as a potential probe.
  • the cell was connected to a frequency response analyzer (Solatron 1255) .
  • the polymeric film sample was sandwiched between two gold electrodes. The impedance spectrum was measured in the frequency range of 100 Hz to 0.2 MHz with a potential of 0.01 V at 25 0 C and a 100% relative humidity.
  • the resistance value associated with the polymeric film conductivity was determined from the high frequency intercept of the impedance with the real axis. The conductivity was calculated using the equation 5.
  • Another way to measure the transport number and therefore the selectivity of the polymeric films under an applied electric field is chronopotentiometry . This is an electrochemical characterization method that measures the electric potential response of a system to an imposed current.
  • Chronopotentiometric measurements are active measurements performed in the same configuration as the resistance measurements (see figure 4) in a 0.1 M NaCl solution. A constant current density is applied and the voltage drop between the electrode and a reference electrode is measured as a function of time.
  • a typical example of a chronopotentiometric curve, measured with a CMX membrane in a 0.1 M NaCl solution when applying a current density above the limiting current density of the system, can be seen in figure 6.
  • the chronopotentiometric curve has four distinct regions. The experiment is started without applying a current. Because the solutions on either side of the polymeric film are equal, the voltage drop remains zero. A fixed current density, which is higher than the limiting current density of the system, is applied at point A.
  • i is the current density
  • C 0 is the concentration of the solution
  • z is the valence of the ion
  • F is the Faraday constant
  • D is the diffusion coefficient
  • t + and t are the diffusion coefficient
  • This equation is called Sand equation. This equation was used to calculate the transport number and therefore the selectivity of the membrane under an applied current density.
  • the S-PEEK/PES blends show a constant permselectivity with decreasing film thickness. Only the thinnest film of approximately 9 micrometers shows a slightly reduced permselectivity. Therefore, the S-PEEK/PES blends provide a suitable polymeric film for a reinforced cation-exchange membrane .
  • polymeric films with various thicknesses were prepared from made from the different S-PEEK solutions (see table 2) .
  • Table 2 Measured properties of the prepared S-PEEK based polymeric films.
  • the relation between the thickness of the polymeric films obtained and the ion selectivity is shown in figure 8.
  • the S-PEEK/PES blends show very high permselectivities even with very thin polymeric films.
  • the 9 and 10 micrometers layers of both S-PEEK/PES blends lose less than 2% of selectivity compared to the thick ones.
  • the thin S-PEEK/PES 60/40 film (9 micrometers) was pressed onto the support material (non-woven comprised of polypropylene and polyethylene) and investigated.
  • the S-PEEK/PES solution (60% S-PEEK) was casted directly over the nonwoven support material (Viledon, 0.12 mm thick polyester, obtained by Freudenberg, UK) and dried as described in the experimental section.
  • the S-PEEK/PES membranes were prepared as described in the experimental section. After drying, the surface of the membrane is covered with a thin layer of solvent (NMP) and the support is pressed onto this membrane.
  • NMP solvent
  • S-PEEK/PES membranes were prepared as described in the experimental section. After drying, the surface of the membrane is covered with a thin layer of S-PEEK solution (in NMP) and the support is pressed onto this membrane.
  • the first method does not result in selective membranes.
  • the S-PEEK/PES solution sinks inside the support, without forming a selective layer. Therefore this method is not suited to prepare the reinforced ion-exchange membranes according to the present invention.
  • the other two methods i.e., layering a polymeric film onto the porous support matrix yield the supported ion- exchange membranes according to the present invention.
  • H 2 SO 4 13 euro/L from Merck (95-97%)
  • For the sulphonation of 1 kg PEEK 16.7 liters H 2 SO 4 is needed, resulting in 286.7 euroC/kg S-PEEK. Because the S-PEEK has to be washed with water and the wastewater has do be disposed, a total cost is assumed of 500 euro/kg for the production of S-PEEK.
  • the averaged density of the S-PEEK/PES blend is 1306 kg/m 3 . Therefore one square meter of a 10 micrometers S-PEEK/PES (60/40) polymeric film would cost about 5 euro.
  • the properties, i.e., mechanical strength and ion- slectivity, of the S-PEEK/PES blends based reinforced ion- exchange membranes and the production cost are very advantegous compared to reinforced ion-exchange memebranes according to the prior art.
  • Thin S-PEEK/PES films with high selectivity and low resistance can be produced.
  • the relative high ratio of PES provides high selectivities and decreases the production costs of the membranes.
  • the tested Viledon support (non-woven comprised of polypropylene and polyethylene) can be advantegously used as porous polymeric support.

Abstract

The present invention relates to a reinforced ion- exchange membrane comprised of a porous support matrix and, laminated thereon, an polymeric film. Specifically, the present invention relates to a reinforced ion-exchange membrane comprising a porous support matrix and, laminated on at least on side thereof, a polymeric film, wherein the porous support matrix has no electric resistance as measured in the six compartment configuration and wherein the polymeric film comprises a polymer functionalized with charged moieties in a degree of at least 50%.

Description

REINFORCED ION-EXCHANGE MEMBRANE COMPRISED OF A SUPPORT, AND LAMINATED THEREON, A POLYMERIC FILM
description
The present invention relates to a reinforced ion- exchange membrane comprised of a porous support matrix and, laminated thereon, a polymeric film. More specifically, the present invention relates to a reinforced cationic or anionic exchange membrane. The present invention further relates to a reinforced ion-exchange membrane for use in (reverse) electrodialysis and a method for preparing said reinforced ion-exchange membrane.
A membrane is defined as a permselective barrier separating two phases. Certain compounds can pass through the membrane under the influence of a driving force while others are excluded from passing. Because of this, such a membrane has a capacity of selectively transporting (transferring, separating) compounds from one phase (feed) to another (permeate) . A schematic presentation of this process is shown in figure 1.
A number of different types of these membranes are known and can be classified based on various criteria such as morphology, for example porous, non-porous or continues; symmetric, asymmetric; neutral or charged; materials used such as glass fibre, polymers, organic and inorganic compounds; and suitable driving forces such as a pressure difference, a concentration difference or a electrical potential difference. The membranes or barriers according to the present invention are charged polymeric membranes capable of transporting (transferring, separating) charged particles, such as ions or other charged compounds or particles, through an electric potential difference between two phases (solutions) . Such membranes are also designated as ion- exchange membranes, although not specifically limited to ion separations whereby ions are small inorganic charged compounds such as Na+ or Cl".
Ion-exchange membranes can generally be divided in two different types depending on the charge of the membrane, i.e., cation-exchange membranes (negative charge, cem) and anion-exchange membranes (positive charge, aem) . Cation-exchange membranes (cem) generally have negatively charged moieties fixed thereon. In contrast, anion-exchange membranes (aem) generally have positively charged moieties fixed thereon.
In general, cation-exchange membranes commonly use charge providing sulfonic acids moieties, i.e., compounds comprising one or more -SO3 " groups, or carboxylic acids moieties, i.e., compounds comprising one or more -COO" groups, fixed (adhered, cross-linked, connected, bonded) to their basic polymeric films. Cation-exchange membranes are used for various electrically driven membrane based processes. For example, they may serve in electrodialysis as separators between concentrate and diluate compartments, in fuel cells, as proton conductors, and as a cation permeable layer of bipolar membranes selectively permeable for protons and water while retaining anions .
In general, anion-exchange membranes commonly use charge providing quaternary ammonium moieties, i.e., compounds comprising one or more -R3N+ groups, fixed (adhered, cross-linked, connected, bonded) to their basic polymeric film, wherein R represents an alkyl or aryl, linear or branched, substituted or not substituted, of C1 to C20 carbon atoms chains such as -C2H5 or -C4H9. These positively and negatively charged moieties represent strong acids and bases, respectively, and are substantially dissociated, i.e., charge providing, over a broad pH range for example pHs of 2 to 12. Ion-exchange membranes can be used for electrodialysis . Electrodialysis involves a selective transport (transfer, separation) , against the concentration gradient, of ions from one, for example, aqueous solution to another, for example, aqueous solution, under the influence of an electrical potential gradient as the driving force.
When placed in an electrolyte solution, the affinity of an ion-exchange membrane for positively and negatively charged ions in the electrolyte solution is different depending on the charge of the moieties fixed thereon. For example, in case of a cationic ion-exchange membrane, i.e., a membrane with negatively charged moieties, negatively charged particles such as anions are excluded (stopped, prevented from passing) by cation-exchange membranes because their electrical charge being the same as the charge of the fixed moieties on the membrane. This despite an electric potential difference over the cation- exchange membrane .
In contrast, cations, being not excluded (repelled) because of the charge of the membrane, are allowed to pass under the influence of an electrical potential difference. A schematic representation of this process is provided in figure 2.
A specific embodiment of an electrodialysis process is provided in figure 3. In figure 3, an aqueous salt solution is fed to an electrodialysis stack consisting of a series of cation and anion exchange membranes between two working electrodes. When an electric field is applied, the positively charged cations migrate towards the cathode and the negatively charged anions in the opposite direction towards the anode . The cations pass easily through the negatively charged cation-exchange membranes, in contrast with the excluded anions. As shown in figure 3, this results in the separation of the feed solution into a solution enriched in ions (the concentrate) and a solution depleted of ions (the diluate) .
The production of potable water from brackish water is an important application of electrodialysis . Electrodialysis can also, for example, be used in desalination of water, desalination in food and pharmaceutical industry, separation of amino acids and production of salts .
A special form of electrodialysis is reverse electrodialysis. A reverse electrodialysis cell is comprised of monopolar ion-exchange membranes arranged in the same order as in conventional electrodialysis. For reverse electrodialysis, the potential drop is used, which develops when a membrane separates two electrolyte solutions differing in concentrations.
The compartments contain therefore alternately high and low concentrations of the electrolyte. The generated current can be taken from this process, i.e., the generation of electric power. The generation of electric power depends on the number of cells. The efficiency of the process also depends on the membrane resistance and their permselectivities .
The efficacy or usefulness of ion-exchange membranes is determined by several parameters such as a) a high permselectivity (ion selectivity) b) a low electrical resistance c) a good mechanical strength, and d) a high chemical stability.
It is often difficult to optimize the properties of ion-exchange membranes because the parameters determining the different properties often act contrary. For instance, a high degree of crosslinking can provide the desired mechanical strength but increases the electrical resistance.
The properties of ion-exchange membranes are also influenced by the basic polymer film and the type and concentration of the fixed charged moieties.
The basic polymer film determines to a large extent the mechanical, chemical and thermal stability of the membrane. In general, such basic polymeric films are made of hydrophobic polymers such as polystyrene, polyethylene or polysulfone .
Although these polymers are largely insoluble in water and show a low degree of swelling, through the introduction of charged moieties ionic groups, they may become water soluble. Thus, the basic polymers are often crosslinked.
In addition to the already mentioned mechanical strength and electric resistance, the swelling (when placed in an aqueous environment) and the chemical and thermal stability are also influenced by the degree of crosslinking. The type and the concentration of the fixed charged moieties influence the permselectivity and the electrical resistance of the membrane, but they also have a significant effect on the swelling and the mechanical properties of the membrane.
Another important aspect is the relation between the mechanical strength of the ion-exchange membrane and its thickness. For example, when the thickness of the membrane is increased also the mechanical strength is increased. However, the electric resistance will be undesirably increased.
In order to solve the above problem with mechanical strength, reinforced membranes have been proposed such as for example in WO 95/16730. These membranes are comprised of a support matrix providing the required mechanical strength while the selective ion-exchange is provided by the polymeric film.
However, these reinforced ion-exchange membranes according to the prior art still require a relatively thick ion-exchange membrane or film, for example in the range of 100 to 250 micrometers, to provide the required ion selectivity. Inherently, this increased thickness does not only increases the electric resistance of the reinforced membrane, but also its costs.
Therefore, it is an objective of the present invention to provide a reinforced ion-exchange membrane, which provides an improved balance between a high permselectivity, a low electrical resistance, a good mechanical strength, and a high chemical stability as compared to the prior art.
Another objective of the present invention is to provide a reinforced ion-exchange membrane wherein mechanical strength is substantially provided by the reinforcement and the ion-selectivity is provided by a relatively thin ion- exchange membrane as compared to the prior art .
Further, it is an objective of the present invention to provide a reinforced ion-exchange membrane at relatively low costs. These and other objectives are met by a reinforced ion-exchange membrane as defined in the appended claims.
Specifically, these and other objectives are met by a reinforced ion-exchange membrane comprising a porous support matrix and, laminated on at least on side thereof, a polymeric film, wherein the porous support matrix has no electric resistance as measured in the six compartment configuration and wherein the polymeric film comprises a polymer functionalized with charged moieties in a degree of at least 50%, such as 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% and 100%. Preferred is a degree of functionalization between 55% to 85%, such as 60%, 65%, 70%, 75% and 80%. The electrical resistance of the porous support matrix according to the present invention is measured in the six compartment configuration. This measurement is performed with a six compartment cell with a four electrode arrangement, as shown in figure 4, under direct current using a 0.5 M NaCl solution. Briefly, the porous support matrix is first equilibrated in the measurement solution for more than 24 hours. The measurement solution is added in the compartments 3 and 4, next to the porous support matrix to be investigated, with calomel reference electrodes. In the compartments 2 and 5, an NaCl solution with the same concentration is used as in the measurement compartments to reduce the influence of the electrode reactions at the working electrodes.
A current is applied through the working electrodes in the compartments 1 and 6 whereas the electrical potential is measured close to the surface of the porous support matrix in the compartments 3 and 4 with the calomel reference electrodes .
The area resistance between the calomel reference electrodes is determined from the current-voltage curve (i-v curve) and is measured with and without the porous support matrix. The difference of the two resistances is considered as the porous support electric resistance (R) . This difference is substantially zero for the porous support matrices according to the present invention.
The degree of functionalization (SD) of the polymer comprised in the polymeric film according to the present invention can be measured using the equation 1:
Figure imgf000009_0001
wherein MWff is the molecular weight of the functional group including the counter ion; MW(P the molecular weight of the non-functional polymer repeat unit; and IEC the ion echange capacity.
The IEC of the polymeric films is determined by titration as decribed by Wilhelm et al . , Journal of Membrane Science 199 (2002), 167-176. Briefly, in case of an illustrative cationic polymeric film, the polymeric film is first brought in the H+ form by immersion into IM HCl solution for 24 h (the solution is replaced in between for total ion exchange) .
To remove absorbed acid, the membrane is soaked in water and is then brought into the Na+ form by immersing in 2 M NaCl solution. To ensure a complete exchange, the NaCl solution is renewed several times. The collected solutions are combined and titrated with NaOH to determine the proton content .
The IEC is calculated as the ratio of total charge by the dry weight of the polymeric film.
According to a preferred embodiment, the present invention relates to a reinforced ion-exchange membrane, wherein the polymeric film comprises a blend of a functionalized polymer and a non-functionalized polymer. In a particularly preferred embodiment, the functionalized polymer is a sulphonated poly (ether ether ketone) functionalized polymer.
The term "non-functionalized polymer" is used herein to encompass any polymer which is not functionalized with charged moieties providing ionic selectivity to the reinforced ion-exchange membrane according to the present invention. In a particulary preferred embodiment, the non- functionalized polymer is poly (ethersulphon) non- functionalized polymer. Preferably, the reinforced ion-exchange membrane according to the present invention comprises a polymeric film comprising 50% to 90%, such as 55%, 60%, 65%, 70%, 75%, 80% and 85% by weight, functionalized polymer and 50% to 10%, such as 45%, 40%, 35%, 30%, 25%, 20% and 15%, by weight non- functionalized polymer.
More preferably, the reinforced ion-exchange membrane according to the present invention comprises a polymeric film comprising 60% to 85%, such as 65%, 70%, 75% and 80%, by weight functionalized polymer and 40% to 15%, such as 35%, 30%, 25% and 20% by weight, non-functionalized polymer.
The reinforced ion-exchange membranes according to the present invention preferably comprise charged moieties selected from the group consisting of sulfonic acids, carboxylic acids, and ammonium moieties providing a charge to the polymeric film over a broad pH range. More preferably, the charged moieties are sulfonic acids.
Although the porous support matrix can be any nonconducting porous support matrix, preferred are porous support matrices comprised of a non-woven fabric comprised of a material selected from the group consisting of glass, silica, polyolefins, polypropylene, polyethylene, polyurethane, polystyrene, polybutylene and co-polymers thereof. More preferred are non-woven fabrics comprised of polypropylene and polyethylene.
Advantageously, the reinforced ion-exchange membrane according to the present invention comprises a porous support matrix, as defined above, with a thickness of between 30 to 150 micrometers and, laminated thereon, a polymeric film, as defined above, with a thickness of between 5 to 15 micrometers .
More preferred is a reinforced ion-exchange membrane according to the present invention, comprising a porous support matrix, as defined above, with a thickness of between 40 to 130 micrometers and, laminated thereon, a polymeric film, as defined above, with a thickness of between 8 to 11 micrometers . The present invention also relates to a method for preparing a reinforced ion-exchange membrane comprising: a) providing a polymeric film according to the present invention and a porous support matrix according to the present invention; b) covering the polymeric film with a solvent for said polymeric film or a solution of a monomer of which said polymeric film is comprised; c) pressing said porous support matrix onto the polymeric film; and d) removing said solvent or solution to provide a reinforced ion-exchange membrane according to the present invention.
The reinforced ion-exchange membranes according to the present invention a particularly useful when used for (reverse) electrodialysis . Hence, the present invention also relates to the use of a present reinforced ion-exchange membrane for electrodialysis or reverse electrodialysis applications . Particular examples of applications of electrodialysis or reverse electrodialysis applications are, but not limited thereto, desalination of water, desalination in food and pharmaceutical industry, separation of amino acids, production of salt, fuel cells, electric power generation and immediate and sustained release carriers for pharmaceuticals .
The present invention will be further illustrated using the following examples of preferred embodiments of the present invention. In the examples, reference is made to and principles are demonstrated by the appended figures, wherein:
Fig. 1 is a schematic drawing of a membrane separation; Fig. 2 is a schematic presentation of a cation-exchange membrane next to an electrolyte solution; Fig. 3 shows the principle of electrodialysis; Fig. 4 shows a six compartment cell for the determination of electric resistance;
Fig. 5 shows a chronopotentiometric curve of a CMX cation exchange membrane;
Fig. 6 shows the experimental setup of an impedance measurement; ' Fig. 7 shows the conductivity and water content of various
S-PEEK/PES blends with a degree of 61% and 83% sulphonation (SD) ;
Fig. 8 shows the permselectivity (ion selectivity) versus the thickness of two S-PEEK/PES blends, i.e., 60/40 and 80/20, respectively, with a degree of sulphonation (SD) of 80%.
EXAMPLES Example 1 : a polymeric film suitable for a reinforced cation-exchange membrane according to the present invention
Introduction
Sulphonated poly (ether ether ketone), S-PEEK, was chosen as the functionalized polymer in the cation exchange polymeric film. This material is blended with the non- functionalized polymer poly (ethersulphone) , PES, to improve membrane properties. The respective chemical representations of these polymers are provided below.
Figure imgf000013_0001
repeat units of a sulphonated S-PEEK polymer
Figure imgf000013_0002
repeat units of PES
Materials and methods
Preparation of S-PEEK
As precursor polymer PEEK 450PF from Victrex was used. PEEK is an amorphous polymer, with low water absorption and high solvent resistance. The polymer was dried for more than 24 hours in a vacuum oven, at 1000C.
One liter of concentrated sulphuric acid (95-98 wt%, extra pure) was placed in a reaction vessel at 25°C and 60 grams polymer was added under stirring. The reaction mixture was stirred for several hours, at controlled temperature, to achieve the desired sulphonation.
Then, the reaction vessel was immersed in an ice bath to stop the reaction. The sulphonated polymer (S-PEEK) was precipitated in demineralized water of maximum 50C and washed until the pH was 7. Subsequently, the polymer was dried in air at room temperature and in a vacuum oven at 1000C.
Preparation of S-PEEK/PES polymeric films To prepare films of S-PEEK and S-PEEK/PES blends, the polymers were added in the desired amount to the solvent N-methyl pyrolidinone (NMP, C5H9NO) . The following polymer solutions of 20wt% polymer in NMP, as solvent, were prepared:
1. 100% S-PEEK (62% sulphonated)
2. 100% S-PEEK (80% sulphonated)
3. 80% S-PEEK (80% sulphonated) with 20% PES
4. 60% S-PEEK (80% sulphonated) with 40% PES
After a minimum of 24 hours of stirring, the polymer solution was filtered over a 40 micrometer metal filter. The films were prepared by the evaporation technique. Briefly, the solutions were cast on glass plates with different casting knifes. The films were dried in an N2 atmosphere at 40-800C for 1 week.
Since NMP was still present in the films after one week, the polymeric films obtained were washed with water and placed in a vacuum oven at 3O0C for one week. The dry polymeric films were stored in 0.5 M NaCl solution and their thickness in wet state (dwet) was measured.
Non-woven as polymeric film support Two different non-woven fabrics from Freudenberg (Viledon) , made from polypropylene/polyethylene, were used as polymeric film support matrix:
1. Viledon non-woven, thickness 120 micrometers 2. Viledon non-woven, thickness 45 micrometers
polymeric film characterisation
The prepared polymeric films were characterized by measurements of the ion exchange capacity (IEC) , water uptake (w) , electrical resistance (R) , and the permselectivity (S) .
These properties were used to calculate the degree of sulphonation (SD) and the specific conductivity (Cond) of the polymeric film. The IEC of the membranes was determined by titration. For this, the polymeric film was first brought in the H+ form by immersion into 1 M HCl solution for 24 hours (the solution was replaced in between for total ion exchange) .
To remove absorbed acid, the polymeric film was soaked in water and was then brought into the Na+ form by immersing in 2 M NaCl solution. To ensure complete exchange, the NaCl solution was renewed several times .
The collected solutions were combined and titrated with NaOH to determine the proton content. The IEC was calculated as the ratio of total charge by the dry weight of the polymeric film . The degree of sulphonation was calculated according to equation 1 :
SD: M-ffiC l-MWJIEC wherein Mw,p is the molecular weight of the polymer non-functional repeat unit and MW/f is the molecular weight of the functional group including the counter ion (-SO3Na) .
The water uptake of the polymeric films was measured in different ionic forms following the weighing procedure. The polymeric film was first brought into the desired ionic form (H+, Na+ or Ca2+ and the weight of the wet (m^,.) and dry (mdry) sample was measured. The water uptake of the polymeric film is given by equation 2:
The charge density (x) of the polymeric film can be calculated by equation 3:
Figure imgf000016_0001
w
The permselectivity was determined by static membrane potential measurement. The test system consisted of two cells separated by the polymeric film sample. Two calomel reference electrodes (Schott B2810) were placed into the solutions on either side of the polymeric film and were used to measure the potential difference across the polymeric film.
On one side of the polymeric film, a 0.1 M NaCl solution flowed through the cell, on the other side a 0.5 M NaCl solution (25°C) . The permselectivity (S) of the polymeric film is given by the ratio of the measured potential difference (ΔVraeas) and the potential difference calculated by equation 4 for a 100% permselective polymeric film (ΔVcalc) :
Figure imgf000017_0001
In case of a 0.1 and 0.5 M NaCl solution, the calculated potential difference using the Nernst equation is 37.9 mV.
The electrical resistance of the polymeric films was measured in a six compartment cell with a four electrode arrangement (figure 4) under direct current using a 0.5M NaCl solution.
The polymeric film was first equilibrated in the measurement solution for more than 24 hours. The measurement solution was added in the compartments 3 and 4, next to the investigated polymeric film with the calomel reference electrodes. In the compartments 2 and 5, a NaCl solution with the same concentration was used as in the measurement compartments to reduce the influence of the electrode reactions at the working electrodes. The current was applied through the working electrodes in the compartments 1 and 6, whereas the electrical potential was measured close to the polymeric film surface in the compartments 3 and 4 with the calomel reference electrodes. The area resistance between the calomel reference electrodes was determined from the current-voltage curve (i-v curve) and was measured with and without the polymeric film. The difference of the two resistances was considered as the polymeric film area resistance (R) . The specific conductivity was then calculated using equation 5:
Cond=i^
Figure imgf000017_0002
AC impedance spectroscopy was used as a second method to measure the electrical resistance of the membranes and can be used to calculate the proton conductivity of the membranes. The measurements were performed by impedance spectroscopy in a home made cell as shown in figure 5.
The cell has a Teflon interior with two circular gold or stainless steel electrodes, surface area of 0.28 cm2. Both electrodes are connected with two wires, one for carrying the current and one for acting as a potential probe. The cell was connected to a frequency response analyzer (Solatron 1255) . The polymeric film sample was sandwiched between two gold electrodes. The impedance spectrum was measured in the frequency range of 100 Hz to 0.2 MHz with a potential of 0.01 V at 250C and a 100% relative humidity.
The resistance value associated with the polymeric film conductivity was determined from the high frequency intercept of the impedance with the real axis. The conductivity was calculated using the equation 5. Another way to measure the transport number and therefore the selectivity of the polymeric films under an applied electric field is chronopotentiometry . This is an electrochemical characterization method that measures the electric potential response of a system to an imposed current.
Investigation of kinetic effects, adsorption and transport phenomena near electrode surfaces can be made with this technique. Chronopotentiometric measurements are active measurements performed in the same configuration as the resistance measurements (see figure 4) in a 0.1 M NaCl solution. A constant current density is applied and the voltage drop between the electrode and a reference electrode is measured as a function of time. A typical example of a chronopotentiometric curve, measured with a CMX membrane in a 0.1 M NaCl solution when applying a current density above the limiting current density of the system, can be seen in figure 6. The chronopotentiometric curve has four distinct regions. The experiment is started without applying a current. Because the solutions on either side of the polymeric film are equal, the voltage drop remains zero. A fixed current density, which is higher than the limiting current density of the system, is applied at point A.
At this point an instantaneous increase in voltage drop occurs, which is the first part of the curve. The reason of this increase is the initial ohmic resistance of the system consisting of solution and membrane between the tips of the voltage measuring capillaries.
In the second part of the curve, there is a slight increase in voltage drop in time. At a certain time this is followed by a strong increase in voltage drop, which is the third part of the curve. The point at which this strong increase occurs is the transition time (t) which can be determined by the intersection of the tangents to second and third part of the curve. The fourth part of the curve is reached where the voltage drop levels off. When an electric current is applied to the ion exchange membrane system, concentration polarization phenomena arise as a result of concentration gradients formed near to the membrane surface.
The transient process occurring near the membrane until a steady state is reached can be studied by measuring the voltage drop across the membrane as a function of time. When the concentration reaches zero at the membrane surface, the voltage will rapidly increase. The time taken for voltage transition to occur after a constant current is applied is called the transition time (t) , which is given by equation 6:
Figure imgf000020_0001
wherein i is the current density, C0 is the concentration of the solution, z is the valence of the ion, F is the Faraday constant, D is the diffusion coefficient, t+ and t are
transport numbers of the ion in the membrane and the solution phases respectively.
This equation is called Sand equation. This equation was used to calculate the transport number and therefore the selectivity of the membrane under an applied current density.
Results and discussion
Effect of thickness decrease on the properties of the ion exchange layers
In the first series of experiments, S-PEEK/PES blends (60% S-PEEK SD80%, 40% PES) polymeric films with different thicknesses were prepared and the permselectivities (S) , the electrical resistances (R) and conductivities (Cond) were investigated. Table 1 shows the measured and calculated results. Table 1: Measured properties of the prepared ion exchange films.
Figure imgf000021_0001
The S-PEEK/PES blends show a constant permselectivity with decreasing film thickness. Only the thinnest film of approximately 9 micrometers shows a slightly reduced permselectivity. Therefore, the S-PEEK/PES blends provide a suitable polymeric film for a reinforced cation-exchange membrane .
Effect of blend ratio on the properties of the prepared thin ion-exchange layers
In order to find the optimum cation exchange layer considering resistance and permselectivity, polymeric films with various thicknesses were prepared from made from the different S-PEEK solutions (see table 2) .
Table 2: Measured properties of the prepared S-PEEK based polymeric films.
Figure imgf000021_0002
Figure imgf000022_0001
Also the influence of weight ratio between S-PEEK and
PES on the conductivity and the water uptake was measured .
The results thereof are shown in figure 7.
The relation between the thickness of the polymeric films obtained and the ion selectivity is shown in figure 8. The S-PEEK/PES blends show very high permselectivities even with very thin polymeric films. The 9 and 10 micrometers layers of both S-PEEK/PES blends lose less than 2% of selectivity compared to the thick ones.
If layers of pure S-PEEK are used, this picture changes. Both S-PEEK films show lower selectivities compared to the S-PEEK/PES blend with an increased loss of selectivity with decreasing thickness.
Focussing on the polymeric film conductivity, one can see that the conductivity is varying. This is a result of the calculation of the conductivity out of the electrical resistance and the wet polymeric film thickness. The measurements of both parameters yield errors leading to a larger error in the calculated value.
To have a better insight, impedance spectroscopy measurements have been performed, showing constant conductivities for the polymeric films with decreasing thickness, taking the error of the film thickness into account .
Reinforced ion exchange membranes
To investigate the influence of the chosen support material on the resistance of the prepared polymeric films, the thin S-PEEK/PES 60/40 film (9 micrometers) was pressed onto the support material (non-woven comprised of polypropylene and polyethylene) and investigated.
Table 3 Reinforced cation-exchange membranes
Figure imgf000023_0001
*) as measured in the six compartment configuration
Measuring the resistance of the support materials in the six compartment configuration without the polymeric film did not result in an electrical resistance, because the solution can pass freely through the support material, the current transport is not affected. Because we subtract the solution resistance from the resistance measured with the sample, no extra resistance can be found. Using impedance spectroscopy as tool for the measurement the electrical resistance of the 0.5 M NaCl solution in the support can be measured. Because the 120 micrometers support is approximately 2.7 times thicker than the 45 micrometers support, the measured value is inherently 2.7 times higher.
It is important to note, that the resistances measured with impedance spectroscopy for the reinforced cation-exchange membranes are all lower than the ones measured with the six compartment cell. This is due to the transport of protons instead of Na+ ions in this kind of measurement. Focussing on the measured resistance of the
S-PEEK/PES film with the support in the six compartment cell, one can see that the resistance increases about 0.6 to 0.7 Ωcm2. This increase can be caused by the loss of conductive membrane area when the support is applied (similar to the increase of resistance when a spacer is used in an electrodialysis cell).
Where the membrane layer is in contact with the support material, no ions can pass, reducing the effective membrane area. This reduction is depending on the porosity of the support material.
It can be concluded that the use of both support materials results in a low resistance increase, providing very efficient reinforced ion exchange membranes with a high mechanical strength and a high permselectivity or ion selectivity.
Effect of the preparation methods on the membrane properties of reinforced ion-exchange membranes
Three methods for the preparation of reinforced ion exchange membranes were investigated.
1. The S-PEEK/PES solution (60% S-PEEK) was casted directly over the nonwoven support material (Viledon, 0.12 mm thick polyester, obtained by Freudenberg, UK) and dried as described in the experimental section.
2. The S-PEEK/PES membranes were prepared as described in the experimental section. After drying, the surface of the membrane is covered with a thin layer of solvent (NMP) and the support is pressed onto this membrane.
3. S-PEEK/PES membranes were prepared as described in the experimental section. After drying, the surface of the membrane is covered with a thin layer of S-PEEK solution (in NMP) and the support is pressed onto this membrane.
The first method does not result in selective membranes. The S-PEEK/PES solution sinks inside the support, without forming a selective layer. Therefore this method is not suited to prepare the reinforced ion-exchange membranes according to the present invention.
The other two methods, i.e., layering a polymeric film onto the porous support matrix yield the supported ion- exchange membranes according to the present invention.
Preliminary cost calculation
In order to have a small impression of the production costs of the reinforced ion-exchange membranes according to the present invention, a preliminary calculation (2006) can be done, estimating the costs for the thin (10 micrometers) S-PEEK/PES (60/40) polymeric film.
Materials:
S-PEEK: 85 euro/kg from Victrex (for charges over 720 kg) PES: 30 euro/kg
NMP: 17 euro/L from Acros (99%)
H2SO4: 13 euro/L from Merck (95-97%) For the sulphonation of 1 kg PEEK, 16.7 liters H2SO4 is needed, resulting in 286.7 euroC/kg S-PEEK. Because the S-PEEK has to be washed with water and the wastewater has do be disposed, a total cost is assumed of 500 euro/kg for the production of S-PEEK.
For the production of S-PEEK/PES blends (60/40) this results in polymer costs of 312 euro/kg.
Because a 20wt% solution of polymer is used in NMP, 4 kg NMP is needed for 1 kg of polymer resulting 70 euro/kg polymer. Therefore the total costs per 1 kg polymer dissolved in NMP are 382 euro/kg.
The averaged density of the S-PEEK/PES blend is 1306 kg/m3. Therefore one square meter of a 10 micrometers S-PEEK/PES (60/40) polymeric film would cost about 5 euro.
Conclusions
The properties, i.e., mechanical strength and ion- slectivity, of the S-PEEK/PES blends based reinforced ion- exchange membranes and the production cost are very advantegous compared to reinforced ion-exchange memebranes according to the prior art.
Thin S-PEEK/PES films with high selectivity and low resistance can be produced. The relative high ratio of PES provides high selectivities and decreases the production costs of the membranes.
The tested Viledon support (non-woven comprised of polypropylene and polyethylene) can be advantegously used as porous polymeric support.

Claims

CIAIMS
1. Reinforced ion-exchange membrane comprising a porous support matrix and, laminated on at least one side thereof, a polymeric film, wherein the porous support matrix has no electric resistance as measured in the six compartment configuration and wherein the polymeric film comprises a polymer functionalized with charged moieties in a degree of at least 50%.
2. Reinforced ion-exchange membrane according to claim
1, wherein the polymeric film comprises a blend of a functionalized polymer and an non-functionalized polymer.
3. Reinforced ion-exchange membrane according to claim
2, wherein the polymeric film comprises 50% to 90% functionalized polymer and 50% to 10% non-functionalized polymer .
4. Reinforced ion-exchange membrane according to claim
3, wherein the polymeric film comprises 60% to 85% functionalized polymer and 40% to 15% non-functionalized polymer .
5. Reinforced ion-exchange membrane according to any of the claims 1 to 4, wherein the polymeric film comprises a polymer functionalized with charged moieties in a degree of 55% to 85%.
6. Reinforced ion-exchange membrane according to any of the claims 1 to 5, wherein the charged moieties are selected from the group consisting of sulfonic acids, carboxylic acids, and ammonium moieties.
7. Reinforced ion-exchange membrane according to claim 6, wherein the charged moieties are sulfonic acids.
8. Reinforced ion-exchange membrane according to any of the claims 1 to 7, wherein the polymeric film comprises sulphonated poly (ether ether ketone) functionalized polymer.
9. Reinforced ion-exchange membrane according to any of the claims 1 to 8, wherein the polymeric film comprises poly (ethersulphon) non-functionalized polymer.
10. Reinforced ion-exchange membrane according to any of the claims 1 to 9, wherein the porous support matrix is a non-woven comprised of a material selected from the group consisting of glass, silica, polyolefins, polypropylene, polyethylene, polyurethane, polystyrene, polybutylene and copolymers thereof.
11. Reinforced ion-exchange membrane according to claim 10, wherein the porous support matrix is a non-woven comprised of polypropylene and polyethylene.
12. Reinforced ion-exchange membrane according to any of the claims 1 to 11, comprising a porous support matrix with a thickness of between 30 to 150 micrometers and, laminated thereon, a polymeric film with a thickness of between 5 to 15 micrometers.
13. Reinforced ion-exchange membrane according to claim 12, comprising a porous support matrix with a thickness of between 40 to 130 micrometers and, laminated thereon, a polymeric film with a thickness of between 8 to 11 micrometers .
14. Method for preparing a reinforced ion-exchange membrane comprising: a) providing a polymeric film and a porous support matrix as defined in any of the claims 1 to 13; b) covering the polymeric film with a solvent for said polymeric film or a solution of a monomer of which said polymeric film is comprised; c) pressing said porous support matrix onto the polymeric film; and d) removing said solvent or solution to provide a reinforced ion-exchange membrane.
15. Use of a reinforced ion-exchange membrane according to any of the claims 1 to 13 for electrodialysis or reverse electrodialysis applications.
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