Cation-selective polymer films are widely used in industry, particularly, as ion-exchange membranes for electrochemical applications, such as electrolysis, electrodialysis, fuels cells and secondary batteries.

Typically, such membranes are formed from polymers having a fluorinated polymer backbone and pendant ion-exchange groups, such as sulfonic (-S03-), carboxylic (-C02-) or phosphonic (-PO32-) groups. The negative charges on these groups are usually counter-balanced by cations of, for example, hydrogen and alkali metals (eg Li+, Na+, K+).

Conventionally, such ion-exchange membranes are obtained from precursor polymers having sulfonyl, carboxyl and phosphonyl groups in their non-ionic form (-S02X,-COX or- P02X, wherein X = halogen). The precursor polymer is heated, extruded as a solid sheet, and then treated with an aqueous solution of an alkali metal hydroxide to convert the polymer from its non-ionic form to its salt form (-S03M,-C02M or- P03M2, wherein M = alkali metal ion). The polymer sheet may then be treated with an acid, such as sulfuric acid, to convert the polymer into its acid form (-S03H,-C02H or- P03H2).

Surprisingly, it has now been found that by hydrolysing the precursor polymer in solution rather than solid form, the

ion-exchange properties of the resulting membrane can be improved.

Thus, according to a first aspect of the present invention, there is provided a process for the manufacture of an ion exchange membrane, said process comprising the steps of: a) providing a solution of a precursor polymer, said precursor polymer comprising a polymeric backbone and pendant groups selected from-S02X,-COX and-POX2, wherein X is a halogen; b) hydrolysing the solution of the precursor polymer to convert at least one of the pendant groups into the salt form, and c) casting hydrolysed polymer into a membrane.

Suitable precursor polymers include fluorinated carbon polymers, such as perfluorinated polymers. In one embodiment of the invention, the precursor polymer is prepared from at least two monomers, one of which is a fluorinated vinyl monomer, and the other of which is a fluorinated vinyl monomer comprising at least one pendant group selected from -S02X,-COX and-POX2. Preferably, the pendant group is- S02X. X may be any halogen group, but is preferably, Cl or F, more preferably, F.

Suitable fluorinated vinyl monomers include vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkyl vinyl ether), tetrafluoroethylene and mixtures thereof.

Suitable fluorinated vinyl monomers comprising a-S02X,-COX or-POX2 group may be represented by the general formulae CF2=CFRS02X, CF2=CFRCOX or CF2=CFRPOX2, wherein R is a bifunctional radical comprising from 2 to 8 carbon atoms.

Preferably, R is perfluorinated. R may be branched or unbranched, and preferably includes one or more ether linkages.

In an alternative embodiment, the precursor polymer is prepared by grafting an unsaturated monomer to a polymer backbone. Suitable unsaturated monomers include styrene, trifluorostyrene, alphamethylstyrene, alpha, beta- dimethylstyrene, alpha, beta, beta-trimethylstyrene, ortho- methylstyrene, meta-methylstyrene, para-methylstyrene, divinylbenzene, triallylcyanurate, acrylic acid, methacrylic acid, vinylpyrrolidone, vinylpyridine, vinylacetate, trifluorovinylacetate, and methylvinyltoluene and mixtures thereof.

Suitable polymer backbones may be polymers, copolymers or terpolymers formed from hydrocarbon, halogenated or perhalogenated (in particular fluorinated or perfluorinated) monomers. Preferably the polymer backbone is selected from polyethylene (PE), polytetrafluoroethylene (PTFE), Polyhexafluoropropylene (HFP), tetrafluoroethylene- hexafluoropropylene copolymer (FEP), tetrafluoroethylene- propylene copolymer, tetrafluoroethylene-ethylene copolymer (ETFE), hexafluoropropylene-propylene copolymer, hexafluoropropylene-ethylene copolymer, polyvinylidene fluoride (PVDF), vinylidene fluoride tetrafluoroethylene copolymer (PVDF-TFE), vinylidene fluoride

hexafluoropropylene copolymer (PVDF-HFP or"Kynar-Flex"), polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, polyvinylidene-hexafluoropropylene copolymer, chlorotrifluoroethylene-ethylene copolymer, chlorotrifluoroethylene-propylene copolymer, perfluoroalkoxy copolymer, polychloroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, or perfluoroalkoxy copolymer (PFA). Fluorinated or perfluorinated polymers, copolymers or terpolymers are particularly preferred. A suitable graft polymerisation process is described in WO 00/15679, the content of which is incorporated herein by reference.

Examples of available precursor polymers include the Nation"" range of materials (produced by DuPont), the Flemion"'range of materials (produced by Asahi Glass) and the Aciplex" range of materials (produced by Asahi Kasei).

To form the solution provided in step a) of the present invention, the precursor polymer may be dissolved in a suitable solvent. Suitable solvents include halogenated solvents, preferably, fully halogenated solvents, such as perfluorinated solvents. Preferably, the solvent has a boiling point of above 80 degrees C, more preferably, above 100 degrees C. The solvent may be optionally mixed with water: for example, the concentration of the solvent in water may be 2 to 10 wt %.

An example of a suitable solvent is CF2XCFXO [CF2C (CF3) FOI, [CF2] my wherein X is F, Cl, Br, or I; n is 0,1 or 2; m is 1,2 or 3; and Y is-COOCH3 or-S02F.

Preferred solvents include perfluoromethylcyclohexane, perfluorodimethylcyclobutane, 1,1, 2-trichlorofluoroethane, perfluorooctane, perfluorobenzene, perfluorodecalin, perfluorotetradecahydrophenanthrene, perfluorotrihexylamine, perfluorooctanic acid, perfluorodecanoic acid, perfluorotributylamine, perfluorotrialkylamine, pentafluorophenol, pentafluorobenzoic acid, perfluoroheptane and perfluorooctadecahydronaphthalene.

Suitable solvents are described in US 5,290, 846, US 4,298, 697, US 4,446, 269 and US 4,348, 310.

The precursor polymer may be dissolved in the solvent at an elevated temperature, for example, above 40 degrees C.

Preferably, temperatures in excess of 60 degrees C are employed. In a preferred embodiment, the precursor polymer is dissolved in the solvent at 80 to 140 degrees C, preferably, at 90 to 110 degrees C.

The weight ratio of precursor polymer to solvent employed in step a) may be 0. 1-10 : 70-100, preferably, 0. 5-5 : 85 - 95, for example, 1: 90.

In step b), the solution of the precursor polymer is hydrolysed. This hydrolysis step converts at least one of the-S02X,-COX or-POX2 pendant groups into its corresponding salt form (e. g.-S03M,-C02M or-P03M2).

Preferably, more than 60% of the pendant groups of the precursor polymer are converted to their corresponding salt form. More preferably, 90 to 100 % are converted.

The hydrolysis reaction may be carried out using a hydroxide as a hydrolysing agent. Suitable hydroxides include alkali metal hydroxides, such as sodium or potassium hydroxide. In a preferred embodiment, an aqueous solution of sodium or potassium hydroxide is employed. By hydrolysing the precursor polymer in solution, the hydrolysis reaction is carried out in the homogeneous phase. Preferably, the hydrolysis step is carried out at a temperature above 30 degrees C, for example, between 50 and 100 degrees C. The hydrolysis step may be carried out using a mixture of an aqueous hydroxide and a perfluorinated solvent. In the case of the latter, the temperature of the hydrolysis step should be less than the boiling point of the perfluorinated solvent.

In one embodiment of the invention, the solution of the precursor polymer is hydrolysed to form a solid precipitate of the precursor polymer in its salt form. This solid precipitate may be isolated, and dissolved in a suitable solvent, prior to the casting step (c).

Suitable solvents for dissolving the precipitate include polar solvents, such as water and organic solvents that are miscible with water. Suitable solvents are described in US 6 277 512 and"Swelling study of Perfluorosulphonated lonomer Membranes" (Polymer, 1993, Volume 34, Number 2). Examples of suitable solvents include water, alcohols (e. g. Cl to C4 alcohols), amides, phosphates, ketones and esters. Specific examples include water, methanol, ethanol, 2-propanol, 1- butanol, 1-pentanol, glycerol, ethylene glycol, formamide, N-methylformamide, N, N-dimethylformamide, N, N- diethylformamide, N, N-dimethylacetamide, N, N- diethylacatamide, trimethylphosphate, triethylphosphate, tributylphosphate, hexamethylphosphotramide, dimethylsulphoxide, N-methylpyrrolidinone, cyclohexanone, 2- ethoxyethanol, tetrahydrofuran, propylene carbonate, butyl acetate, dioxane, pyridine, hydrazine and acetonitrile.

Mixtures of solvents may also be used. Additionally, co- solvents, such as glycerol, cyclohexanone and other high- boiling solvents may be employed to aid the dissolving process.

Preferably, the solvent is heated. The temperature to which the solvent is heated will depend on the nature of the solvent. For DMSO, for instance, a temperature of above 80 degrees, for example, between 120 and 150 degrees C may be employed.

In step c), the hydrolysed polymer is cast into a membrane.

Conventional casting techniques may be employed. For example, a solution of the hydrolysed polymer may be filtered, poured into a suitable vessel and left to dry to

form a suitable film. Preferably, the solution is dried at an elevated temperature below the boiling point of the solvent used. The drying step may be carried out at reduced pressure, for example, at a pressure of 200 to 1000 mbar, preferably, 500 to 800 mbar. Alternatively, the drying step may be carried out at atmospheric pressure.

Once cast, the membrane may be heat-treated, for example, at above 80 degrees. For example, the membrane may be heated from 100 to 150 degrees. Alternatively, the membrane may be heated at higher temperatures, for example, at above 200 degrees C.

The process of the present invention may further comprise the step of converting the polymer to its acid form. This conversion step may be carried out by reacting the hydrolysed polymer with an aqueous solution of an acid.

Suitable acids include sulfuric acid, nitric acid, and hydrochloric acid.

In a preferred embodiment, the hydrolysed polymer is converted to its acid form once the polymer has been cast as a membrane (i. e. after step c) ). Once the membrane has been converted into its acid form, the membrane may be washed, for example, with distilled water. The washed membrane may then be dried to constant weight.

According to a second aspect of the present invention, there is provided an ion exchange membrane obtainable by the process as herein described.

The ion exchange membrane of the present invention may have an equivalent weight of 600 to 1500, preferably, 800 to 1300. In a preferred embodiment, the equivalent weight of the ion exchange membrane is 900 to 1100.

The thickness of the ion exchange membrane may be 5 to 300 microns, preferably, 10 to 250 microns, and more preferably, 20 to 180 microns.

The transport number (t+) of the membrane may be above 0.7, preferably, above 0.8, for example, between 0.90 and 0.99.

The permselectivity of the membrane may be at least 70%, preferably, 75 to 99%, for example, 85 to 95%.

The resistance of the membrane may be less than 200 Ohms cm- 1, preferably, less than 140 Ohms cm-1. For example, the resistance of the membrane may be 20 to 100 Ohms cm-1, preferably, 50 to 90 Ohms cm-1, for example, 70 to 85 Ohms cm. The aerial resistance of the membrane may be 0.5 to 3 Ohms cm~2, preferably, 0.8 to 2 Ohms cm-1.

The ion exchange capacity of the membrane may be 0.5 to 1.5 meq/g, preferably, 0.7 to 0.9 meq/g.

The membranes of the present invention may be used in a variety of electrochemical systems. In particular, they may be used as cation exchange membranes in chloro-alkali cells, or in regenerative fuel cells (RFCs), such as those described in US-A-4485154. They may also be used as membranes for Proton Exchange Membrane (PEM) Fuel Cells.

It has been found that for a given permselectivity, the membranes of the present invention exhibit a lower resistance than prior art membranes having the same permselectivity.

These and other aspects of the present invention will now be described with reference to Figure 1, which depicts a block diagram of an apparatus for carrying out steps a) and b) of an embodiment of the process of the present invention.

Figure 1 depicts an apparatus 10 for carrying out steps a) and b) in accordance with an embodiment of the present invention. The apparatus 10 comprises a reactor 12, an extractor 14 and a condenser 16. The extractor 14 contains granules of the precursor polymer (-SO2F) 18.

A mixture of potassium hydroxide (base), perfluorooctadecahydronaphthalene (solvent) and water is heated to reflux in reactor 12. This heating step causes the solvent/water mixture to evaporate and condense in condenser 16. The condensate flows from the condenser 16 into the extractor 14, where it comes into contact with the granules of the precursor polymer (-SOzF) 18, causing the polymer 18 to dissolve. The resulting polymer 18 solution is passed into the reactor 12. The reaction converts the precursor polymer into its salt form, which precipitates out of solution as a solid (not shown). The solid is subsequently removed from the apparatus 10 for further reaction. For example, the precipitated solid may be dissolved in DMF to form a solution, which is subsequently cast into a membrane in step c).

Example 1 7g of 909 EW sulfonyl fluoride precursor polymer (Flemion Asahi Glass Co. Ltd) was dissolved in 20 ml of perfluorooctadecahydronaphthalene. The resulting solution (0.17 wt %) was hydrolysed in an aqueous solution of KOH to produce a solid precipitate.

The solid precipitate was filtered, repeatedly washed with water, and dried at 180 degrees C and 20 Pa to a constant weight (7.1 g).

Example 2 1.6 g of the precipitate of Example 1 was dissolved in 21.4 g of dimethylformamide at 140 degrees C. The solution was filtered and poured into a plurality of petri dishes, each having an inner diameter of 95mm. The solution was then left to dry in a vacuum oven at 200 degrees C at 700 mbar.

The resulting membranes were transparent and flexible, and had a thickness of 120 to 145 microns.

Example 3 In this Example, the resistances of two of the membranes produced in Example 2 were measured in both AC and DC modes.

For both modes, the following experimental procedure was employed.

A sample of the membrane (0.7854 cm2 diameter) was clamped between the two halves of a cell containing 0. 1M KCl solution maintained at 25°C. In each chamber of the cell, a platinised Pt flag and a standard electrode were immersed in the electrolyte solution. The standard electrodes were equipped with Luggin capillaries at the membrane surface. The capillaries were filled with saturated KC1 and stopped with asbestos plugs.

For the DC measurements, the resistance of the membrane was determined by comparing the potential difference between the standard electrodes in the presence and absence of the membrane. A current of 2mA was employed. The measurements were carried out using a V544 Digital Voltmeter (Metronic, Poland) For AC measurements, the resistance of the membrane was determined by measuring the potential difference between the platinised Pt electrodes. A PW 9527 Digital Conductivity Meter (Phillips, Netherlands) was employed. An AC frequency of 80 Hz and 4 kHz were used.

The results are shown in Table 1 below. The results are compared with the resistances observed with conventional membranes, such as Nafion 117.

Table 1 Membrane Alternating Current Direct 4kHz Current 2.6 mAcm-2 Thickness Areal R Specific Areal R Specific (cm) (Q cm~2) R (Q cm 2) R (Q cm~1) (Q cm~1) Nafion 0.0190 4.98 262.14 3.98 209.64 117 R1 0.0158 1.30 82.50 1.27 80.37 R2 0. 0158 1. 37 86. 49 1. 14 72. 05

Example 4 In this Example, the transport number and permselectivity of the membranes of Example 2 were measured. The results are shown in Table 2 below. These results are compared to the corresponding transport number and permselectivity measurements of Nafion 117.

As can be seen from Tables 1 and 2, the membranes of the present invention exhibit a lower resistance than the Nafion 117 membranes, even though the transport number and permselectivity of the membranes are comparable.

Table 2 Membrane Transport Permselectivit Number, t+ (%) Nafion 117 0.92 77.81 R1 0.96 87.92 R2 0.91 75.30

Example 5 In this Example, the ion exchange capacity of the precipitate of Example 1 was determined.

A sample of the precipitate was stirred in an excess of 1M HCl for 72 hours. By treating the precipitate in this manner, the-SO3K groups on the polymer were converted into their- SO3H form. The treated polymer was then washed with deionised water and dried at 120°C under vacuum to a constant weight.

The polymer was then divided between two flasks (0.26g of polymer in each flask).

25 cm3 aqueous NaOH (0.10077 mmol/cm3) was then added to each flask, and the resulting mixture stirred for 48 hours. A portion of the solution was taken and titrated using aqueous HCl (0.1070 mmol/cm3) to determine the change in NaOH concentration.

The calculation of the ion-exchange capacity (IEC) was: IEC + (V HCL-VHC1) X f HcL/G

where V'HCL and Vxcl are the amounts of 0. 1M HC1 consumed in a blank experiment and the determination of IEC respectively, f HCL is the factor of HCL and G is the mass of the dried sample. The measured IEC of the sample was 0.834 meq/g.