ELECTRODE SEPARATOR

This invention relates to method of making an electrode separator for use in an electrochemical device, and to a method of making an electrochemical device.

Electrochemical devices, for example primary and secondary batteries include an anode, a cathode and an electrolyte. An electrode separator can be located between the anode and the cathode to prevent contact between them and to provide a barrier to migration of electrode materials. Commonly used separators are porous to allow impregnation by liquid electrolyte. The electrolyte can provide a pathway for ion migration between the electrodes.

Preferred materials for use in the manufacture of electrode separators should have minimal adverse impact on the performance of the electrochemical device. Adverse impact on the internal resistence of the device can be minimised by ensuring that the separator is fully wetted by the electrolyte. Also, the porosity of the separator should be as high as possible consistent with providing the required barrier between opposed electrode surfaces.

The separator should also be capable of withstanding significant levels of physical maltreatment. This can apply in particular during assembly of an electrochemical device when a laminate of the separator with electrode materials can be subjected to process steps such as compressive rolling and winding. The material of the separator should also be chemically stable towards materials encountered in the device when in use, whether present for electrochemical reaction or as products of such a reaction.

The requirement for increasing capacity of electrochemical devices means that it is preferable for the thickness of an electrode separator material to be minimised. The thickness of an electrode separator can be reduced by a compressive rolling step (calendering). However, such a process step can result in a reduction of the porosity of the separator and a consequent increase in the internal resistence of an electrochemical device in which the separator is used. Furthermore, permanent compression of a compressibly fabric material (which might be used as the substrate for an electrode separator) can reduce the ability of

the separator to accommodate variations in the spacing between opposed electrodes. If a space remains between the surface of an electrode and the opposing surface of the electrode separator (with its impregnating electrolyte), this can give rise to inefficiencies in discharge (and recharging if the device is rechargeable) of the device.

WO-93/01622 discloses a commercially successful technique for making an electrode separator using non-woven polyolefin fabrics, especially polypropylene. The technique involves impregnating the fabric with a solution of a vinyl monomer such as acrylic acid. The impregnated fabric is then exposed to ultraviolet radiation. The solution of the vinyl monomer includes a photo initiator so that the exposure to ultraviolet radiation causes a graft reaction between the monomer and the polyolefin surface of the fibres to proceed. The grafted vinyl monomer on the surface of the polyolefin fibres makes the fabric wettable by aqueous electrolyte.

The non- woven fabric that is used in the disclosed technique can have a thickness of about 200 μm. The fabric can be calendered to reduce its thickness. This calendering step can be performed prior to the impregnation of the fabric with the vinyl monomer solution.

The present invention provides a method of making an electrode separator which includes a step of reducing its thickness by application of heat and pressure after a vinyl monomer has been grafted to the fibre surfaces of the fabric, so that the reduction in thickness is at least partially retained until the fabric is subsequently exposed to an aqueous solution (for example an electrolyte solution).

Accordingly, in one aspect, the invention provides a method of making an electrode separator for use in an electrochemical device which comprises a non-woven fabric whose fibres comprise a polyolefin, having groups bonded to them by a graft polymerisation reaction between the fibre surfaces and a vinyl monomer which is capable of reacting with an acid or a base to form a salt directly or indirectly, which includes the step of reducing the thickness of the fabric by application to it of heat and pressure under such conditions that (a) the reduction in thickness can be at least partially retained until the fabric is exposed to water or an aqueous solution, and (b) the reduction in thickness is then at least

partially recovered when the fabric is exposed to water or an aqueous solution. The method of the invention has the advantage that the thickness of the electrode separator is reduced while the separator is assembled with other components of the device, in particular when formed as a laminate with layers of the electrode material. The formation of such a laminate can include a step in which the component layers are rolled. The reduced thickness of the electrode separator resulting from the application of heat and pressure can help to minimise the space occupied by the separator in the assembled device. This can be important in order to obtain high capacity devices.

The possibility with the separator of the invention of then recovering at least part of the reduction in thickness of the separator on exposure to an aqueous solution (especially an aqueous electrolyte solution) gives rise to further advantages. The ability of the separator material to expand in thickness means that the separator is able to fill spaces between opposing electrode surfaces, so as to accommodate at least partially variations in the size of the gaps between those electrode surfaces. This has the advantage that gaps between an electrode surface and the electrode separator (with impregnating electrolyte) can be eliminated. Furthermore, recovery (at least partially) of the reduction in thickness of the electrode separator material can help to increase porosity of the separator, subject to available space between the electrodes. This can help to minimise internal resistence of the assembled device. It also increases the volume of electrolyte contained in the separator in the assembled device. The control over the thickness of the electrode separator material that is available from the present invention therefore provides advantages in terms of both assembly of the device and performance when subsequently in use.

The method of the invention can be facilitated by the tackiness that can be associated with certain vinyl monomers, especially when exposed to conditions of increased heat and pressure. The tackiness should be such that the reduction in thickness can be recovered on exposure to an aqueous electrolyte, as a result of interaction between the electrolyte and the vinyl monomer. The interaction can be physical or chemical or both. Preferably, the interaction involves softening of the vinyl monomer, possibly in combination with causing it to swell. It is an advantage of the use of a monomer which is grafted to the surfaces of the

fabric fibres that the monomer does not dissolve freely in the electrolyte but is retained, bonded to the surfaces of the fibres.

The vinyl monomer should be capable of reacting with an acid or a base to form a salt, either directly or indirectly after appropriate workup (for example, involving hydrolysis or sulphonation. This can facilitate wetting of the electrode separator by aqueous electrolyte. Preferred vinyl monomers include ethylenically unsaturated carboxylic acids and esters thereof, for example acrylic acid and methyl acrylate. Other vinyl monomers which might be used can include acrylamide, substituted amides such as N,N-dimethyl acrylamide vinylpyridine and vinylpyrrolidone.

The conditions under which the fabric is exposed to heat and pressure can affect the retention and subsequent recovery of the reduced thickness. It will be appreciated that the reduced thickness will not be recovered if and to the extent that a permanent set is imparted to the fabric fibres as a result of the applied pressure, for example involving folding or creasing or crushing of the fibres. This can limit the pressure that can be applied to the fabric.

Furthermore, the temperature to which the fabric is exposed would often be required to be such that the material of the fabric fibres does not soften to the extent that a permanent set is imparted to the fibres when pressure is applied. It can therefore be preferred that the temperature of the fabric when the pressure is applied to it is less than the temperature at which the material of the fibres softens. Preferably, the difference between the temperature at which the pressure is applied to the fabric and the temperature at which the material of the fibre softens (the Vicat softening point) is at least about 5 ⁰ C, more preferably at least about 1O ⁰ C, especially at least about 25°C, for example at least about 35°C. Preferably, the difference is not more than about 80°, more preferably not more than about 70°, especially not more than about 60°, for example not more than about 40°. The Vicat softening point of a polymeric material can be measured by test method BS2782. For example, the Vicat softening points of polyethylene can range from 77 to 124 ⁰ C, and polypropylene from 145 to 150°C.

Preferably, the temperature of the fabric when the pressure is applied to it (which can be generated by passing the fabric around heated rollers) is greater than 45 ⁰ C, more preferably at least about 60°C, especially at least about 75 ⁰ C (for example about 9O ⁰ C in the case of a fabric in which the fibres are made from polypropylene).

Preferably, the non- woven fabric which is used in the method of the invention has not been subjected to a compression step to cause its thickness to reduce prior to the graft polymerisation reaction between the surfaces of the fabric fibres and the vinyl monomer. This can help to minimise non-recovery of the reduction in thickness of the fabric resulting from the application of heat and pressure after the graft polymerisation reaction. It will be under- stood that, while it can be preferred for there to have been no compression step before the graft polymerisation reaction, it is envisaged that the fibres of certain non- woven fabrics might be welded locally so that the fabric has a stable structure. An example of a fabric type having this feature is a pin-bonded fabric.

Preferably, the ratio of the thickness of the fabric after the application of pressure to its thickness before the application of pressure is not more than about 0.8, more preferably not more than about 0.5, especially not more than about 0.4, for example not more than about 0.3. The available compression will depend on factors such as the fabric construction, the fabric material, the vinyl monomer and so on.

Preferably, the ratio of the thickness of the fabric after exposure to 30% aqueous potassium hydroxide solution at 23 ⁰ C for 60 minutes to the thickness of the fabric after the compression step before the said exposure is at least about 1.1, more preferably at least about 1.2, especially at least about 1.25, for example at least about 1.3. It has an advantage of the present invention that, by appropriate selection of the condition under which heat and pressure are applied to the fabric, much of the applied reduction in thickness can be recovered when the fabric is exposed to an aqueous solution.

The article of the invention can be a woven or a non-woven fabric formed from fibres whose surfaces are provided by a polymeric material. The non-woven fabric can be made by processes in which fibres are laid down to form a fabric directly after extrusion,

generally with a spinning step. Such fabrics are sometimes referred to as spun bonded fabrics. Fabrics can be made by processes which include a step of exposure to air (possibly heated) from a blower, directly after extrusion. Such fabrics are sometimes referred to as melt blown fabrics. Other techniques to form non- woven fabrics can include wet or dry laying of fibres. The fibres of fabrics made directly by extrusion (for example spun bonded fabrics and melt blown fabrics) or by wet or dry laying, can be bonded to one another so that the fabric has integrity and therefore so that it has the mechanical properties required for satisfactory performance. The fibres can be bonded together by techniques which involve the application or heat or pressure or both. Bonds between fibres can be formed by incorporating a material into a fabric which softens when heated. For example, polyethylene can be incorporated into a fabric, either as fibres consisting essentially of polyethylene or as bicomponent fibres consisting of a polypropylene core and a polyethylene sheath. The polyethylene in the fabric can provide the necessary bonds as a result of heating the fabric to a temperature that is greater than the softening point of the polyethylene.

The fabric on which the separator of the invention is based can be a laminate of non- woven fabrics. For example, a laminate can include a first fabric which is formed directly by extrusion, especially with a spinning step, in which the fibres are laid down from the extruder die without exposure to a stream of a gas (a spun bonded fabric), and a second fabric which is formed directly by extrusion, in which the fibres are exposed to a stream of a gas (possibly heated) in the step of laying the fibres down (a melt blown fabric). The fibres of a melt blown fabric can be finer than those which are not exposed to such a gas. This can have advantages because of the combination of high porosity and effective barrier properties that is available. Such a fabric has also been found to be effective in trapping ammonia impurities trapped in electrode materials, which can lead to self-discharge of an electrochemical cell in which the separator is incorporated. The use of such a fabric in combination with one or more spun bonded fabrics can have the advantage of combining these high porosity and effective barrier properties with physical properties which makes the fabric able to withstand the harsh physical treatment which might be encountered during assembly in an electrochemical device and subsequent use. The fabric on which the separator is based can be a laminate of three or more layers. Outer layers can preferably be

spun bonded fabrics. One or more inner layers can be melt blown fabrics. The incorporation of melt blown inner layers has the advantage that such layers are relatively compressible, and that a reduction in thickness can be recovered, at least partially. Component non- woven fabrics of a laminate can be bonded to one another. For example, they can be bonded by the formation of localised welds.

Preferably, the mean thickness of the fibres of non- woven fabric (which might be measured as a mean diameter, especially when the fibres have an approximately circular cross- section) is not more than about 30 μm, more preferably not more than about 20 μm. It can be smaller, especially when the fibres are formed by a spinning technique, for example not more than about 8 μm, preferably not more than about 5 μm. The thickness of the fibres of the fabric will often be at least about 1.0 μm, and can be about 5 μm or more, for example at least about 10 μm, for example about 12 μm, when the fabric is formed by a technique such as wet or dry laying.

The polymeric material on which the article of the invention is based should be capable of reacting with a vinyl monomer when the vinyl monomer and the polymeric material are exposed to ultraviolet radiation, with a photoinitiator or such other additional components as might be necessary. Examples of polymeric materials on which the article of the invention is based include polyamides, polyesters, polyethers, polyimides, polycarbonates, and halogenated polymers such as polyvinyl chloride, fluorinated ethylene propylene (FEP) polymers, and polyvinylidene fluoride. Preferably, the polymeric material comprises a polyolefin. Suitable polyolefins include polyethylene and polypropylene. The polymeric material can comprise more than one material as a mixture. For example, mixtures of polyolefins might be used. It can be particularly preferred to use mixtures of polyethylene and polypropylene. The fact that the technique of the invention can be used to treat polymeric articles having surfaces provided wholly or in part by polypropylene represents a significant advantage.

The polymeric material on which the article of the invention is based can comprise a mixture of two or materials. The article can be formed from two or more polymeric materials, for example in which the article comprises a first polymeric material on a first

region and a second polymeric material in a second region. For example, an article can be formed as a fabric from fibres which comprise two or more materials. Fibres can be formed by co-extrusion, for example in which the core of a fibre is provided by a first material, with a sheath of a second material around the core. Alternatively, fibres can be formed from first and second materials with the two materials arranged contiguously side- by-side.

Preferably, the material of at least some of the fibres from which a polymeric fabric is formed, for example at least about 40% by weight, preferably at least about 60%, more preferably at least about 80%, is substantially homogeneous throughout the thickness of the fibres. It can be preferred for many applications for the material of substantially all of the fibres to be substantially homogeneous throughout their thickness, so that those fibres are formed only from polypropylene or another suitable material (with appropriate additives where necessary).

For many application, the thickness of the separator after the application of heat and pressure (but before exposure to electrolyte) will preferably be at least about 25 μm, more preferably at least about 50 μm; preferably, the thickness is not more than about 400 μm, more preferably not more than about 250 μm, especially not more than about 150 μm, for example not more than about 100 μm. The thickness is measured using test method DIN 53105 which involves dropping a 2.0 kg weight on to a sample of the sheet of area 2.0 cm ² at a speed of 2.0 mm.s ^'1 .

Preferably, the basis weight of the separator of the invention is at least about 20 g.m ^"2 , more preferably at least about 30 g.m ^"2 , for example at least about 40g.m ^"2 . Preferably, the basis weight is not more than about 100 g.m ^'2 , more preferably not more than about 80 g.m ^'2 .

The solvent for the vinyl monomer should not evaporate to a significant degree in the irradiation step of the method. This has been found to confer the advantages of providing greater uniformity of properties of the resulting article, throughout the thickness of the article. Thus there is greater uniformity in the degree of grafting throughout the thickness of the article, leading to improved ion exchange properties through the article. It is

believed that this might arise at least in part because of the transparency of the article which is retained as a result of the retention of the solvent in the pores of the fabric. It has also been found that the degree or adverse effects or both of homopolymerisation of the vinyl monomer can be reduced by selection of an appropriate solvent.

Suitable solvents for use in the method of the invention will generally be transparent to ultraviolet radiation, have no atoms which are abstractable when exposed to radiation, have a high specific heat and a high latent heat of vaporisation, and will not react adversely with the material of the porous article. Preferred solvents will have a boiling point which is greater than about 50 ⁰ C, preferably greater than about 7O ⁰ C. It is also preferred that the boiling point of the solvent be no higher than a temperature at which the porous article might be damaged during the course of the irradiation step of the method. For example, the boiling point of the solvent might be selected to be less than the temperature at which the material of the article melts or softens. Particularly preferred solvents have a latent heat of vaporisation which is greater than about 1000 J.g ^"1 , preferably greater than about 1500 J.g ^"1 , more preferably greater than about 2000 J.g ^'1 , and/or a specific heat capacity which is greater than about 2.0 J.g-'.K ^"1 , preferably greater than about 3.0 J.g ^"1 . K ^"1 , more preferably greater than about 4.0 J.g ^' ^K ^"1 . A value of specific heat capacity, or of latent heat of vaporisation, within these ranges has the advantage that the solvent in the reaction has an enhanced ability to dissipate heat without evaporating to a significant degree, giving rise to the advantages referred to above. A particularly significant further advantage is that the formation of product from the homopolymerisation reaction of the vinyl monomer is restricted, and any such product which is formed is retained in solution rather than being deposited in the pores within the article. This allows the product to be removed easily from the article by washing. The control over the formation of the homopolymerisation product can be achieved without use of inhibiting agents, which can cause contamination problems when the article is in use in certain applications.

Water (including water based solutions) is a particularly preferred solvent.

The impregnation solution can include additional components to optimise reaction conditions such as surfactants to ensure that the solution fully impregnates the non- woven

fabric, an appropriate mixture of solvents to ensure homogeneity of the solution, and so on. When the solution comprises a mixture of solvents, it will generally be preferred for at least one of the solvents to have one or more of the features discussed above (boiling point, latent heat of vaporisation, specific heat capacity etc); preferably, that solvent will be present in an amount of at least about 70% by weight (based on the total weight of the solvents), more preferably at least about 80%, for example about 85% or more. Mixtures of solvents which can be used can comprise mixtures of water with one or more miscible organic solvents. Examples of suitable organic solvents include 2-propanol, ethanol, 2-methoxy ethanol, 2-butanone, ethylene glycols etc.

The ultraviolet radiation initiated reaction can be completed surprisingly quickly, for example by exposing the impregnated article to radiation for as little as 15 seconds, even as little as 5 or 10 seconds, and it has been found that the article after reaction contains a significant amount of grafted monomer, which can be sufficient for the article to be rendered wettable by aqueous solutions such as might be found in certain electrochemical devices. This is to be contrasted with techniques in which graft reactions are initiated using, for example, electron bombardment, either of impregnated article or of article prior to exposure to monomer solution, where reaction times of many minutes, even as long as 50 minutes, can be required in order to obtain a significant degree of grafting, and even after reaction times of this order, the degree of grafting reaction can be too low for many applications. Such prior techniques do not therefore lend themselves to continuous processing in the manner of the present invention.

Techniques by which exposure of the impregnated article to oxygen can be restricted include, for example, carrying out the ultraviolet irradiation step in an inert atmosphere, for example in an atmosphere of argon or nitrogen, or sealing the impregnated article between sheets of material which are impervious to oxygen, but are transparent to ultraviolet radiation of appropriate wavelength for initiating the graft reaction.

Preferably, the impregnation solution includes an initiator for the polymerisation reaction. Preferably, the initiator initiates the reaction by abstracting an atomic species from one of the reacting materials, for example by abstracting a hydrogen atom from the polymer of the

porous article to create a polymer radical. Following such abstraction, the polymer radical, in contact with the first vinyl monomer in solution, can initiate the formation of a grafted branch. When an atom is abstracted from the polymer of the article, the activated polymer molecule can react either with another polymer molecule so that the polymer of the article becomes cross-linked, or with the vinyl monomer in a graft reaction. An example of a suitable initiator is benzophenone. The mole ratio of the first vinyl monomer to the initiator is preferably at least about 50, more preferably at least about 100, especially at least about 175; the ratio is preferably less than about 1500, more preferably less than about 1000, especially less than about 500, more especially less than about 350; for example the ratio can be about 250.

In another aspect, the invention provides an electrochemical device which comprises an anode, a cathode, an electrolyte, and an electrode separator which has been made by a method as discussed above. The device can be an electrochemical cell, especially a rechargeable electrochemical cell. Examples of cell types in which the separator might be used include lithium polymer cells, lithium ion cells, nickel metal hydride cells, and nickel cadmium cells.

In a further aspect, the invention provides a method of making an electrochemical device which comprises an anode, a cathode, and an electrolyte, which includes the step of exposing an electrode separator made by a method as discussed above to the electrolyte.

Measurement of ion exchange capacity

A sample of membrane about 0.5 g is converted into the acid (H ⁺ ) form by immersion in 1.0 M hydrochloric acid at 60 ⁰ C for 2 hours. The sample is washed in distilled water until the washing water shows a pH in the range of about 6 to 7. The sample is then dried to constant weight (W ₁ ) at 70°C.

The dried sample is placed in a 100 ml polyethylene bottle to which is added accurately 10 ml of approximately 0.1 M potassium hydroxide. Additional distilled water can be added to immerse the sample fully. A further 10 ml of potassium hydroxide is added to a

second polyethylene bottle, together with the same amount of distilled water as that added to the bottle containing the sample. Both bottles are stored at 60°C for at least two hours.

After being allowed to cool, the contents of each bottle are transferred to glass conical flasks, and the amount of potassium hydroxide in each is determined by titration with standardised 0.1 M hydrochloric acid, using a phenolphthalein indicator.

The ion exchange capacity, measured in milliequivalents per gram, of the membrane in the dry acid (H ⁺ ) form is calculated according to the equation:

where tj is the titration value of HCl from bottle with the sample, X ₂ is the titration value of HCl from bottle without the sample, and W ₁ is the weight of the dried membrane in acid (H ⁺ ) form.

Measurement of recovery of thickness on contact with electrolyte

A sample with dimension 300 mm x 40 mm was cut from the grafted and compressed nonwoven fabric with the longest dimension being in the cross machine direction. Six thickness measurements were taken across the width of the sample using the test method described previously. The sample was then fully immersed in a solution of 30% w/w potassium hydroxide for one minute at a temperature of 23 °C. The sample was then removed from the solution; washed with deionized water to remove adhering potassium hydroxide and air-dried at 23 ⁰ C. Finally, the thickness of the sample was remeasured at exactly the same positions as the initial measurements. The ratio of the thickness of the fabric after exposure to 30% w/w aqueous potassium hydroxide to the thickness of the fabric after the compression step and before that exposure is given by the equation:

Recovery ratio = 1 _P£ jυ _BE

where T^ is the thickness after exposure to the potassium hydroxide solution and T _BE is the thickness after the compression step and before that exposure.

The above procedure was repeated with immersion times in 30% w/w potassium hydroxide of 5, 10 and 60 minutes.

For measurements on compressed samples that had not undergone the grafting process a small amount of surfactant was added to the potassium hydroxide solution to allow the fabric to be fully wetted out by the immersion solution.

Measurement of recovery of thickness in ambient air at 70°C

A sample was prepared and the thickness before exposure to elevated temperature measured according to procedure described above. The sample was then placed in an air oven at a temperature of 70°C for 60 minutes. The sample was removed, allowed to cool to room temperature, and the thickness remeasured at exactly the same positions as the initial measurements. The ratio of the thickness of the fabric after exposure to elevated temperature to the thickness of the fabric after the compression step and before the said exposure is given by the equation:

Recovery ratio = T^/T^

where T^ is the thickness after exposure to elevated temperature and T _BE is the thickness after the compression step and before that exposure.

EXAMPLE 1

A non- woven polypropylene fabric made from a thermally bonded laminated construction of spunbond/meltblown/spunbond (SMS) was selected. The spunbond component had a mean fibre diameter of 15 to 20 μm and a basis weight of 15 g.m ^"2 , and the meltblown component had a mean fibre diameter of 3 μm and a basis weight of 15 g.m ^"2 . The total basis weight of the fabric was 45 g.m ^"2 and the thickness was 228 μm. The material was supplied by Pegas A S of Znojmo, Czech Republic.

A continuous strip of the non- woven fabric was impregnated with a solution of acrylic acid by passing the fabric around rollers located in a chamber with an atmosphere of nitrogen, so that the fabric passed through the solution. The composition of the impregnation solution was as follows:

The impregnated fabric, still in a nitrogen atmosphere, was passed between four medium pressure mercury lamps, positioned parallel to one another, two each side of the chamber, the chamber at that point being provided by quartz windows. Each of the lamps had a power output of 79 W.cm ^"1 , and produced a parallel beam having a width of 10 cm. The total exposure time of the fabric to the radiation was about 9 seconds.

The fabric was then washed in de-ionized water to remove unreacted components, and dried on two rollers heated to 90°C. The dried fabric had a moisture content (measured by drying to constant weight at 7O ⁰ C) of 6% by weight,

The treated material was hydrophilic and had an ion exchange capacity of 0.53 meg.g ^'1 and a thickness of 243 μm.

Continuous lengths of the treated fabric were compressed so that the thickness was reduced from 243 μm to 120 μm and 100 μm respectively (so that the ratios of the thickness before compression to the thickness after compression were 0.49 and 0.41) using a three-roll calender stack (roll diameter 300 mm). All rolls were at 90 °C. The fabric was pre-heated to 90°C by contacting roll 1 for 6.7 seconds and roll 2 for 7.7 seconds before being compressed at the nip between roll 2 and roll 3. The compressed fabric was drawn away from the nip and passed over a cooling roller before being collected. Sufficient pressure was supplied to the calendering rolls to give the required thickness.

The thickness recovery in 30% w/w potassium hydroxide at 23 °C and in ambient air at 70°C for the compressed grafted samples were measured according to the procedure described above. The results are given in the results table below.

COMPARATIVE EXAMPLE 1

A laminated fabric as described in example 1 but with a meltblown layer of 20 g.m ^"2 and a total basis weight of 50 g.m ^"2 was selected. The thickness of the fabric was 273 μm. The untreated fabric was compressed to 115 μm using the procedure described in Example 1. The compressed fabric was then grafted using the procedure of Example 1.

The ion exchange capacity of the grafted fabric was 1.13 meq.g ^'1 and its thickness had increased to 137 μm.

The thickness recovery in 30% w/w potassium hydroxide at 23 °C and in ambient air at 70 ⁰ C for the compressed samples before and after grafting were measured according to the procedure described above. The results are given in the results table below.

The data from the examples demonstrates that fabric samples treated according to the method of the present invention have a recovery ratio of greater than 1.10 when contacted

with 30% w/w potassium hydroxide at 23 °C for less than 60 minutes. In the comparative samples (where the fabric was compressed first and then grafted, or alternatively compressed and not grafted) the recovery ratio was significantly less than 1.10. The data in the table also that the no, or very little recovery occurs when the treated fabric is stored at 70°C for 60 minutes.

EXAMPLE 2

The laminated fabric of Comparative Example 1 was grafted using the procedure of Example 1 with the difference that the total exposure time of the fabric to radiation was about 11 seconds.

The treated fabric was hydrophilic and had an ion exchange capacity of 0.65meq.g ^"1 and a thickness of 261 μm.

Continuous lengths of the treated fabric were compressed so that the thickness was reduced from 261 μm to 120 μm (so that the ratio of the thickness of the fabric before compression to the thickness after compression was 0.46). The thickness was reduced using the procedure of Example 1 with the following differences, except that the temperatures of the calendering rolls varying between 50°C and 14O ⁰ C, and that the contact time of the fabric on roll 1 was 8.0 seconds and on roll 2 was 9.2 seconds.

The thickness recovery for the compressed grafted samples after 60 minutes immersion in 30% w/w potassium hydroxide for at 23 ⁰ C and in ambient air at 70 ⁰ C is given in the results table below. The results are also depicted graphically in Figure 1.

These results show that the calender roll temperature, and therefore the temperature at which pressure is applied to reduce the grafted fabric thickness, should be at least about 8O ⁰ C. Preferably, the temperature is not more than about 130°C, more preferably not more than about 110°C. At a calender roll temperature lower than 80°C the recovery in thickness in ambient air at 70°C is significant and shows that the reduced thickness of the fabric might not always be maintained during storage prior to immersion of the fabric in water or an aqueous solution. At a calender roll temperature greater than 130 ⁰ C the recovery in fabric thickness on contact with the electrolyte is too small to allow the fabric to be used satisfactorily as a battery separator.