EP1433214A1 - Water permeable sheet and uses thereof - Google Patents

Abstract

There is described a water permeable sheet suitable for use as a recuparator membrane in a fuel cell. The sheet is substantially impermeable to gas, and has the following properties: (a) a water vapour transmission rate (WVTR) of at least 200 g² / m² / day at 25°C and 75% relative humidity; and (b) a mean dimensional stability (over 100 cycles as defined herein) for a wet or dry sheet in any direction parallel to the sheet surface of less than about 15% change in linear dimension. The sheet may be a laminate of a water permeable first layer (such as cellulose) bonded to a supporting porous layer comprising either a matrix of water insoluble fibres (such as a non-woven glass fibre mat) or a porous plastic film (such as polyethylene); or may be an impregnate of a matrix of water insoluble fibres (such as non woven glass fibre mat) impregnated with a water permeable material (such as cellulose). Laminated sheets of the invention may comprise additives (such as urethane / ethylene oxide co-polymer and/or silica) in the laminating adhesive to improve the WVTR of the laminate without significantly reducing its de-lamination strength. Optionally other additives may be added to the sheet such as anti-microbial agents (such as PHMB, titanium dioxide and silver chloride mixture and/or silver nitrate) and heat conductors (such as metal or carbon black powders). Such highly water permeable, gas impermeable and dimensionally stable sheets may be used as recuparator membranes to manage heat, water and/or humidity levels in the reactant streams of a fuel cell.

Description

WATER PERMEABLE SHEET AND USES THEREOF

The present invention relates to water-permeable sheets with improved mechanical properties, which are suitable for use in for example fuel cells for the management of water, humidity and/or heat levels therein.

Fuel cells convert fuel and oxidant to electricity and reaction product. Fluid fuel (for example, hydrogen or methanol) is directed to an anode where it is oxidised by a catalyst to produce electrons (NB a methanol fuel can be used in a fuel cell either as a source of the hydrogen reactant or a reactant itself which directly oxidised at the anode). Fluid oxidant (such as oxygen in the air) is directed to the cathode and is reduced at the cathode by a catalyst. To prevent combustion, the fuel and oxidant fluids are separated by a suitable barrier membrane which is electrically non-conductive yet can exchange ions. Typically a catalyst is located between the barrier membrane and electrode to induce an electrochemical reaction. Water absorbed by the barrier membrane can also help protons migrate from the anode to the cathode. Electrons produced at the anode induce an electrical current through an external circuit from the anode to the cathode.

There are many types of fuel cell and non limiting examples of several common types of fuel cell are listed below. In each case the fuel is oxidised at the anode optionally in the presence of a catalyst such as platinum and the oxidant is reduced at the cathode to liberate heat and electrons are liberated at the anode and accepted at the cathode which are available to do work in an external circuit.

Polymer electrolyte membrane (PEM) fuel cells comprise a solid polymer electrolyte (such as solid organic polyperflurosulphonic acid polymer) disposed between two porous electrically conductive electrode layers. PEM cells typically operate at between 60°C and 100°C. The electrochemical reactions of a PEM cell are believed to be as follows: Anode reaction : 2H₂ → 4H⁺+ 4e^" Cathode reaction: O? +4H⁺ +4e → 2H?O + ΔHt

Cell: 2H₂ + O₂ → 2H₂O+ ΔH| (1.23 V)

Alkaline fuel cells (AFC) comprise an electrolyte of an aqueous solution of potassium hydroxide soaked in a matrix and typically operate between 90°C and 100°C. The electrochemical reactions of an AFC are believed to be as follows:

Anode reaction: 2H₂ + 4(OH^") → 4H₂O + 4e^"

Cathode reaction: OP + 2H_?O +4e → 4(OH") + ΔHt

Cell: 2H₂ + O₂ → 2H₂O + ΔH| (voltage) Phosphoric acid fuel cells (PAFC) comprise an electrolyte of liquid phosphoric acid soaked in a matrix and typically operate between 175°C and 200°C. The electrochemical reactions of a PAFC are believed to be as for a PEM cell.

Molten carbonate fuel cells (MCFC) comprise an electrolyte of a liquid solution of lithium, sodium and/or potassium carbonates soaked in a matrix and typically operate between 600°C and 1000°C. The electrochemical reactions of an MCFC are believed to be as follows (where carbon dioxide is produced at the anode and consumed at the cathode):

Anode reaction: 2H₂ + 2CO₃ ^2" → 2H₂O + 2CO₂ + 4e^" Cathode reaction: O + 2CO_? +4e → 2CO₃ ^2'+ ΔHt

Cell: 2H₂ + O₂ + 2CO₂ → 2H₂O + 2CO₂+ ΔHt (voltage)

Solid oxide fuel cells (SOFC) comprise an electrolyte of a solid zirconium oxide with a small amount of yttrium oxide and typically operate between 600°C and 1000°C. The electrochemical reactions of a SOFC are believed to be as follows:

Anode reaction: 2H₂ + 2O^2" → 2H₂O + 4e^"

Cathode reaction: O₉ +4e → 2O^2' + ΔHt

Cell: 2H₂ + O₂ → 2H₂O + ΔHt (voltage)

Thus it can be seen that in all the common types of fuel cell, water is produced as a by-product either at the anode or the cathode. Thus in theory all fuel cell types which produce water (and not just those listed above) have need of an improved means to manage water within the cell. However it will be appreciated that due to other factors, such as operating conditions within the cell, of the types described above PEM cells have a particular suitability for improved methods of water management.

Because in each of the fuel cell types above water is produced at one of the electrodes (e.g. at the cathode for a PEM fuel cell) the relevant reactant fluid (fuel or oxidant) will absorb water (e.g. the air leaving a PEM cell will become more humid). Thus the fresh reactant entering the fuel cell inlet is significantly drier than that leaving the outlet. If the reactant entering the fuel cell is not adequately humidified, water may be absorbed from the ion-exchange membrane nearest the inlet. Furthermore if the reactant stream becomes saturated with water vapour, additional water may be carried as droplets. Liquid water in the reactant fluid can flood the electrode and obstruct the catalyst. Flooding is more likely to occur closest to the outlet, where the reactant has had the most opportunity to accumulate product water. It is also desirable to keep the ion-exchange membrane moist to enhance ionic conductivity; reduce physical degradation and/or prevent cracking which may result in structural failure and leaks. For this reason, it is preferred that reactant fluid is sufficiently humidified to hydrate the ion-exchange membrane. It is desired that a balance is achieved between the humidities of each of the fluid streams as they enter and leave the cell. If several fuel cells are arranged in as a stack in series it is also possible that a reactant fluid which contains unused reactant may be exhausted from one cell and used as the input stream for that reactant in the next cell. So for similar reasons it may also be necessary to de-humidfy an outlet stream whether or not the water is recycled into another fluid stream in the cell or cell- stack.

Thus management of water is important to achieve optimum performance in a fuel cell and various water management systems have been proposed to achieved this such as the recuparator for a PEM cell described in US 6,106,964 the contents of which are hereby incorporated by reference.

US 6,106,964 describes a recuparator with a water permeable membrane to extract product water from the exhaust oxidant. The water can then optionally be recycled as necessary to humidify the input reactants. It is well known that cellulose film is a good barrier to gases such as oxygen and hydrogen (especially when dry) yet readily allows water to pass through the film. Cellulose film (such as that generally available from UCB Films under the registered trade mark Cellophane®) is described in this reference as a suitable material for use as the water permeable membrane.

The properties desired in a recuparator will be described, for illustrative purposes only, with reference to a specific example of the polymer electrolyte membrane (PEM) fuel cell, described above. In this cell, two graphite electrodes sandwich an electrically conductive PEM the surfaces of which are coated with a platinum based catalyst. The reactive components are gaseous hydrogen as fuel and gaseous air (containing oxygen) as the oxidant. The gases are kept separate to prevent combustion and passed over the catalyst coatings through channels in the electrodes, hydrogen being directed to the anode and air to the cathode. The PEM acts as a gas barrier to prevent the hydrogen and air from contacting each other, whilst allowing protons liberated at the anode to diffuse through the PEM to react with the air at the cathode to give water and heat. Thus the air exhausted from the cell is hot and humid. For the reasons given above, it is desirable to de-humidify the air exhaust. Yet the other gas streams (both inlet gases and the unused hydrogen exhausted) are much drier and colder then the air exhausted and for the reasons given above to is also desirable to humidify these other gas streams (especially the air inlet).

An ideal recuparator membrane for this PEM cell allows water to pass through the membrane from the humid exhausted air to humidify any of the other gas streams (such as either or both of the dry inlet gases or the dry hydrogen exhaust) as well as de-humidify the exhausted air. Yet it is important that the membrane whilst being sufficiently highly water permeable to transfer the water across also acts as a sufficiently good gas barrier to prevent substantial mixing of the different gas streams. For example if the exhaust air is used to humidify the hydrogen gas it is clearly important that the hydrogen and air do not mix as then they may react directly (burn) which as well as being dangerous does not generate an electric current. Mixing of the inlet air with the exhaust air through the recuparator membrane would also be undesirable as the air could then bypass the channels adjacent the PEM which would reduce or stop the catalytic cathodic reduction of the air and hence the current output from the cell.

It will be appreciated that for different types of fuel-cells the specific gas streams to be humidified and/or de-humidfied may vary but the general principles for recuparator membranes are the same.

Optionally it would also be useful if a recuparator membrane could additionally reduce heat loss from the fuel cell and/or improve efficiency of the fuel cell by transferring heat from the hot exhaust stream across the recuparator membrane to the cold inlet stream.

However present cellulose films are unsatisfactory as a water permeable membrane in a fuel cell recuparator for various reasons. For example they do not have sufficiently robust physical or mechanical properties to be used in a fuel cell (for example as described in US 6,106,964) Conventional cellulose film readily absorbs water, to swell significantly when wet. However large dimensional changes in the recuperator membrane are unacceptable as for example this could lead to tears or leaks across the membrane or block the narrow channels within the fuel cell as the membrane cycles between its wet and dry states during use. Pure cellulose film is also not a very efficient conductor of heat.

It is an object of the present invention to provide a sheet which has improved properties and/or solves some and/or all of the problems identified herein with prior art recuparator membranes.

As used herein the term sheet encompasses as the context dictates for example all membranes, films, laminates, webs, vellums, pellicles, skins, matrices, mats, veils, weaves, coatings, additives, impregnates, composites, and similar terms, mixtures and/or combinations thereof whether synthetic or natural which may be suitable for the uses described herein.

The applicant has developed sheets with surprisingly improved mechanical properties which for example can be used advantageously in fuel cell recuperators or any other use where for example it is desirable to have a dimensionally stable, highly water permeable sheet, which also acts as a good gas barrier between streams of fuel and oxidant gases.

Therefore broadly in accordance with the present invention there are provided sheets, apparatus methods and/or uses as described herein and/or in the independent claims herein. Further preferred features of the invention are described herein and/or in the dependent claims herein. One aspect of the present invention broadly comprises a water permeable sheet; having:

(a) a water vapour transmission rate of at least 200 g² / m² / day at 25°C and 75% relative humidity; and

(b) a mean dimensional stability (over 100 cycles as defined herein) for a wet or dry sheet in any direction parallel to the sheet surface of less than about a 15% change in linear dimension.

Conventionally the longitudinal direction (LD), parallel to the longest axis of a sheet, is the direction in which a sheet web passes through a production machine, and thus is also known as the machine direction (MD). For example, a sheet web may be stretched in the forward web direction (i.e. MD). Conventionally the transverse direction (TD) is normal to the LD and parallel to the sheet surface. The sheet web may also be stretched sideways (i.e. "TD") by a suitable process such as well known bubble and/or stenter processes Preferred sheets are those oriented in both MD and TD and such orientation may be simultaneous and/or sequential. The terms MD and TD may also denote the direction on the sheet in which (optionally anisotropic) properties are measured to give values as specified herein.

Conventionally if orientation is used it would be applied separately to a polymeric film which is a component of a sheet of the invention (such as a cellulose film) not to a composite sheet (e.g. laminate or impregnate of cellulose with a fibre matrix). However these techniques may be used to 'hold' the composite sheet to ensure no shrinkage.

A sheet of the present invention may have a dimensional stability, tensile strength and/or change in size after swelling after water absorption which is substantially the same in the MD and TD, preferably which is substantially anisotropic (measured in any direction parallel to the sheet surface).

These properties are desirable because in preferred fuel cell recuparators, sheets of the present invention may be located within channels which may be substantially long and thin. It is more preferred that the MD of a sheet of the invention is aligned substantially parallel with the direction of such channels which reduces the possibility that the channel will be blocked to fluid flow during under the conditions of use of the recuparator.

Preferably a sheet of the present invention exhibits at least one of the following mean dimensional changes (after being cycled for at least 100 times from a dry sheet at 20°C to a water saturated sheet at 70°C): wet TD of less than about 3.0 %, more preferably about 1.5%; dry TD of less than about 13.5 %, more preferably about 9.5%; wet MD of less than about 2.5%, more preferably about 0.9 %; and/or dry MD of less than about 6.5 %,more preferably about 2.5%. Preferably a sheet of the present invention has a mean thickness less than about 300 microns, more preferably less than 200 microns.

Preferably a sheet of the present invention has a tensile strength when wet of at least 100 N / (mm)² in the TD and/or MD.

Sheets of the invention may exhibit any suitable structure which provides the water permeability and mechanical properties specified herein. The sheet may be a single layer or a multi-layer film comprising any suitable combination of the layer(s) described herein e.g. as surface layer(s) on one or both outer surfaces of the sheet and/or as one or more layer(s) sandwiched in the core of the sheet. As appropriate each layer may be a coating, a self supporting coherent film or a fibrous or porous layer (optionally impregnated).

Optionally sheets of the invention will have one or more gas barrier / water permeable layer(s) (such as cellulose or PLA films or coatings) and one or more other layer(s) to provided mechanical strength to the overall sheet (such as: a glass fibre mat and/or a porous thermoplastic film - e.g. polyethylene film which is mechanically perforated or formed by co-extrusion with solvent to create pores in the film). The gas barrier / water permeable layer and the mechanical strengthening layer can also be one and the same (e.g. where the sheet is a fibrous mat impregnated with cellulose, or is a mechanically strong cellulose film - e.g. made by the NMMO process)

Thus sheets of the invention may comprise a coherent web of water permeable film, an (optionally non-woven) fibrous web; a coated web, a laminated web, a fibre impregnated web and/or any suitable mixtures and/or combinations thereof.

One embodiment of a sheet of the invention comprises a self supporting sheet of water permeable material which has the desired mechanical properties herein due to the treatment and selection of the water permeable material (such as for example the strong cellulosic film made by the NMMO process). In another embodiment of a sheet of the invention the water permeable material may comprise a coating on (and/or a film laminated to) a perforated and/or porous base film which provides the desired mechanical properties to the total sheet (such as a PLA or cellulose film laminated to a porous polyethylene film). For convenience both these embodiments are referred to herein as "coherent sheets" as they do not comprise fibres, fibrous mat and/or woven sheet components.

It has been found that sheets of the present invention may also advantageously comprises a mixture of fibres and water permeable material to form a water permeable sheets with the desired improved mechanical properties. It is preferred that the fibres and water permeable component are made from different materials. In a yet other embodiment of a sheet of the invention suitable fibres (e.g. glass fibres) may be added to a self supporting sheet of water permeable material (e.g. cellulose film) and dispersed homogeneously therein to improve its mechanical properties For convenience this arrangement is referred to herein as a "fibrous sheet". In a still other embodiment of a sheet of the present invention a matrix of fibres (such as a woven sheet or non woven fibre mat - e.g. glass fibre mat) is impregnated with the water permeable material (e.g. cellulose). For convenience this arrangement is referred to herein as an "impregnated sheet".

In an addition embodiment of a sheet of the present invention fibrous and /or impregnated sheets may be attached (by lamination and/or coating) to one or more layers and/or coats of material and/or film on one of both sides to improve the desired properties herein (such as water permeability : gas barrier and/or mechanical strength) and/or for other reasons. For convenience this arrangement is referred to herein as a "composite sheet" as it contains elements of different embodiments of the invention.

Suitable materials for use as a water permeable component of the sheets of the present invention comprise one or more: polyvinyl alcohols (PVA), polyurethanes, Goretex®, polysulphones, grafted polypropylene, polyamides, biopolymers, micro-perforated polyolefins (for example Tylex®), moisture permeable voided films; mixtures thereof and/or combinations thereof having the desired properties herein. Such materials if film forming may optionally comprise holes to achieve the desired water permeability (e.g. perforations made by mechanical or other means such as laser, or chemical etching). Materials which are inherently porous and have the desired properties or good water permeability and good gas barrier without further treatment are especially preferred (for example cellulose and cellulose derivatives).

Types of film-forming and/or impregnating biopolymers that may be used (after where necessary suitable modification) are described below.

The biopolymers (e.g. biopolymeric films) which may be used in present invention may be obtained and/or obtainable from a biological (preferably plant and/or microbial) source and may comprise those organic polymers which comprise substantially carbon, oxygen and hydrogen. Conveniently biopolymers may be selected from carbohydrates; polysaccharides (such as starch, cellulose, glycogen, hemi-cellulose, chitin, fructan inulin; lignin and/or pectic substances); gums; proteins, optionally cereal, vegetable and/or animal proteins (such as gluten [e.g. from wheat], whey protein, and/or gelatin); colloids (such as hydro-colloids, for example natural hydrocolloids, e.g. gums); other polyorganic acids (such as polylactic acid and/or polygalactic acid) effective mixtures thereof; and/or effective modified derivatives thereof.

Further details of each of these biopolymers are given below.

Starch may comprises native and/or modified starch obtained and/or obtainable from one or more plant(s); may be a starch, starch-ether, starch-ester and/or oxidised starch obtained and/or obtainable from one or more root(s), tuber(s) and/or cereal(s) such as those obtained and/or obtainable from potato, waxy maize, tapioca and/or rice.

Gluten may comprise a mixture of two proteins, gliadin and glutenin whose amino acid composition may vary although glutamic acid and proline usually predominate.

Gums are natural hydro-colloids which may be obtained from plants and are typically insoluble in organic solvents but form gelatinous or sticky solutions with water. Gum resins are mixtures of gums and natural resins.

As used herein the term carbohydrate will be understood to comprise those compounds of formula C_x(H₂O)_y. which may be optionally substituted. Carbohydrates may be divided into saccharides (also referred to herein as sugars) which typically may be of low molecular weight and/or sweet taste and/or polysaccharides which typically may be of high molecular weight and/or high complexity.

Polysaccharides comprise any carbohydrates comprising one or more monosaccharide (simple sugar) units. Homopolysaccharides comprise only one type of monosaccharide and heteropolysaccharides comprise two or more different types of sugar. Long chain polysaccharides may have molecular weights of up to several million daltons and are often highly branched, examples of these polysaccharides comprise starch, glycogen and cellulose. Polysaccharides also include the more simple disaccharide sugars, trisaccharide sugars and/or dextrins (e.g. maltodextrin and/or cyclodextrin).

Polysaccharides may comprise a polymer of at least twenty or more monosaccharide units and more preferably have a molecular weight (M_w) of above about 5000 daltons. Less complex polysaccharides comprise disaccharide sugars, trisaccharide sugars, maltodextrins and/or cyclodextrins.

Complex polysaccharides which may be used as biopolymers to form or comprise films of present invention comprise one or more of the following:

Starch (which occurs widely in plants) may comprise various proportions of two polymers derived from glucose: amylose (comprising linear chains comprising from about 100 to about 1000 linked glucose molecules) and amylopectin (comprising highly branched chains of glucose molecules). Glycogen (also known as animal starch ) comprises a highly branched polymer of glucose which can occur in animal tissues.

Cellulose comprises a long unbranched chain of glucose units.

Chitin comprises chains of N-acetyl-D-glucosamine (a derivative of glucose) and is structurally very similar to cellulose. Fructans comprise polysaccharides derived from fructose which may be stored in certain plants. Inulin comprises a polysaccharide made from fructose which may be stored in the roots or tubers of many plants.

Lignin comprises a complex organic polymer that may be deposited within the cellulose of plant cell walls to provide rigidity.

Pectic substances such as pectin comprise polysaccharides made up primarily of sugar acids which may be important constituents of plant cell walls. Normally they exist in an insoluble form, but may change into a soluble form (e.g. during ripening of a plant).

Polylactic and/or polygalactic polymers and the like comprise those polymeric chains and/or cross-linked polymeric networks which are obtained from, obtainable from and/or comprise: polylactic acid; polygalactic acid and/or similar polymers and which may be made synthetically and/or sourced naturally.

Other types of polysaccharide derivatives one or more of which may also be used in the present invention may comprise any effective derivative of any suitable polysaccharide (such as those described herein) for example those derivatives selected from, amino derivatives, ester derivatives (such as phosphate esters) ether derivatives; and/or oxidised derivatives (e.g. acids).

Preferred biopolymer films used in the present invention are those formed from a biopolymer selected from cellulose, cellulose derivatives (such as cellulose acetate) and/or polylactic acid.

Preferably the cellulose used in the present invention to form a film and/or impregnate a matrix is cellulose formed by precipitation and/or regeneration of cellulose from a suitable fluid.

The cellulose film / impregnated matrix may be formed by any suitable process. For example chemical regeneration and coagulation (osmotic dehydration) are used in the well known viscose process in which the viscose fluid comprises sodium cellulose xanthate in caustic soda. A cellulose film is cast in situ on to the matrix by treatment with viscose and then regenerated using with sulphuric acid thus forming the regenerated cellulose. Other known methods for forming cellulose use others methods such as coagulation, solvent removal and/or formation of a cellulose complex in fluids and/or solvents such as: N-methyl morpholine-N-oxide (NMMO); N- methyl pyrrolidone (NMP) with anhydrous lithium chloride (LiCI); dimethyl acetamide (DMA or DemAc) and/or cuprammonium. Films made by any of these methods are also useful to make sheets of this method.

If an impregnated sheet of the invention is desired, a fibre matrix (e.g. glass fibre mat) can be added to the cellulose containing fluid in any of the above processes (e.g. viscose) and cellulose can be formed in situ in the normal manner to produce a fibre matrix impregnated with cellulose. If a fibrous sheet of the invention is desired, the fibres (e.g. glass fibres) can also be added to the cellulose containing fluid in any of the above processes to form in the normal manner a cellulose film impregnated with strengthening fibres.

Usefully sheets of the invention may be optionally softened using any suitable conventional softening agent.

Conveniently films used in the present invention substantially comprise cellulose from a wood source, most preferably at least 90% of the cellulosic material is from a wood source.

The sheets of the invention may include one or more plasticisers (preferably non-migratory), but, typically, such plasticisers are not included. Generally the membrane is a porous layer that does not include wettable material coatings, metal coatings or fillers such as, for example, inorganic particles.

Preferably the biopolymeric film, especially if a cellulosic film prepared by the NMMO process, is oriented in the TD and/or MD, optionally at a stretch ratio of 7 to 1.

As stated above, preferred sheets of the invention comprises fibres either dispersed within the sheet and/or as a fibre matrix.

Conveniently the fibres and/or fibre matrix comprise tough materials such as fibres of inorganic minerals and/or some or more of the following materials: poly vinyl acetates (PVAc), polyvinyl alcohols (PvOH); PVAc and/or PVOH binders; cellulose (for example those cellulose fibres available commercially from Courtalds under the trade marks Tencel® and/or Lyocel®); rayon, glass, polyester, carbon (e.g. carbon fibre composites and/or exfoliating graphite), metal coated carbon, aramid, quartz, silicon carbide, polyamides, polysulphones; copolymers of acrylonitrile butadiene and styrene (ABS polymers), poly vinyl chloride (PVC), alumina, high performance textile fibres; paper; epoxy composite; rockwool, optionally cross-linked stryene / acrylic binders, wetting surfactants and/or any suitable mixtures and/or combinations thereof.

However the laminates of the present invention must be distinguished from conventional battery separator where a wicking layer is laminated to a cellulose film to improve the flow of electrolyte through the separator. Battery wicking layers are generally formed of one or more non-woven layers of cellulose, rayon, PVOH, PVAc, polyamide and/or polysulphone fibres and mixtures thereof. Such wicking layers (more fully described in US 6,159,634 - Duracell) perform a different function to the more rigid fibre laminates of the present invention which are designed to impart greater dimensional stability to the laminated sheet. A conventional laminated battery separator will not work satisfactorily as a fuel cell recuparator membrane as it will swell on wetting and block the fuel cell channels. Therefore more conveniently the fibres and/or fibre matrix especially in a laminate of the present invention comprise one or more of the following materials: glass, polyester, carbon (e.g. carbon fibre composites and/or exfoliating graphite), metal coated carbon, aramid, quartz, silicon carbide, ABS, PVC, alumina, high performance textile fibres; paper; epoxy composite; rockwool, and/or any suitable mixtures and/or combinations thereof.

The fibres and/or fibre matrix may optionally comprise other materials such as binders (e.g. cross-linked stryene / acrylic binders) and/or wetting surfactants.

The fibre matrix which may be used in certain embodiments of sheets of the present invention comprises any matrix sheet which is designed to improve the physical properties of the membrane but preferably it is anisotropic; i.e. has the same mechanical properties in all directions parallel to the sheet surface. This is particularly preferred as if the sheet could stretch in differently in different directions this could cause creasing of the water permeable membrane use in association with the matrix. It is also preferred that the matrix does not exhibit creep under the range of temperature and humidity experienced by the sheet in the recuparator.

It is preferred that where present a fibre matrix is non woven so that it has substantially anistropic dimensional stability (i.e. so the sheet does not deform with the weave of the woven fabric). More preferred fibre matrices comprise a non woven mat (higher density of fibre) or veil (lower density of fibre).

Preferably the fibres are inert and substantially free of any additives which may react with the fuel and/or oxidant in the recuperator during its conditions of use. Conveniently the fibres comprise, mineral, glass and/or carbon fibres.

The fibres may have a mean length of from about 3 to about 20 mm, preferably from about 5 mm to about 15 mm, more preferably about 12 mm. The mean fibre diameter may be from about 1 micron to about 50 microns, preferably from about 5 microns to about 20 microns, more preferably from about 10 microns to about 15 microns.

Preferably a fibre matrix of the present invention has an areal weight from about 5 to about 50 g/m², preferably from about 10 to about 30 g/m², more preferably about 20 g/m².

Preferably a fibre matrix of the present invention has a mean thickness less than about 200 microns, more preferably about 160 microns.

Preferably a fibre matrix of the present invention has a tensile strength (optionally in the MD, more optionally in both the MD and TD) from about 1 to about 100 N / 15 mm.min, preferably from about 5 to about 50 N / 15 mm.min, more preferably about 15 N / 15 mm.min. The fibre matrix (before impregnation) may comprise up to about 95 % by weight of fibres, preferably from about 80 % to about 95%, more preferably from about 85% to about 92% by weight of fibres.

Optionally the (un-impregnated) fibre matrix comprises from about 5% to about 20%, more optionally from about 8% to about 15% by weight of a suitable binder.

Preferably a fibre matrix of the present invention has a mean thickness less than about 200 microns, more preferably about 160 microns.

More conveniently the fibre matrix comprises a non woven mat and/or veil of (optionally corrosion resistant) glass fibres and/or carbon fibres.

Most conveniently the fibre matrix comprises the glass fibre veil available from Technical Fibre Products Limited under the designation Optimat Surface One Glass Veil Prov. S2 0585/00.

Most preferred sheets of the present invention comprise: a) a laminate of a cellulose film adhered (preferably with a lamination adhesive) to a layer of glass fibre matrix; b) a coating of cellulose on a porous polyethylene film ; and/or c) a glass fibre matrix layer impregnated with cellulose.

As necessary for a sheet of the present invention (for example a laminate of a fibre matrix to a water permeable sheet) any component layers may be attached to each other by any suitable means, for example an adhesive. However other bonding methods may also be used to attached layers, as well as or instead of adhesive, such as radio frequency heating, laser welding etc.

The adhesive may comprise any material that is substantially inert to the fuel and/or oxidant, which are substantially water insoluble under the conditions of use (high humidity at up to 80°C) and which can form a physical and/or chemical bond between the water permeable layer and fibre matrix sufficient that the laminate separator forms an integrated unit without significantly decreasing the water permeability of the laminate.

Examples of adhesives suitable for this use comprise: polyurethanes, blends, copolymers and/or mixtures thereof such as the adhesive available under the trade designation Anglo Surestick 3353 (17% solids). The amount of material in the adhesion layer is preferably less than about 6 gm^"2, more preferably from about 1 gm^"2 to about 5 gm^"2, and most preferably about 3 gm^"2. However the applicant has discovered that conventional polyurethane adhesives have a low water vapour permeability (also referred to herein as WVP) which reduces the overall WVP of the laminated sheet of the present invention. A preferred object of the present invention is to improve the WVP of the laminating adhesive without significantly reducing the strength of the lamination bond.

Therefore in a preferred embodiment of the invention there is provided a laminated sheet of the invention as described herein where the laminating adhesive comprises an effective amount of a WVP improving additive and the de-lamination strength of the sheet is substantially unchanged compared to a laminated sheet prepared without said additive.

Preferably the laminating adhesive comprises a conventional polyurethane blended with an effective amount of a WVP improving additive selected from: (i) urethane / ethylene oxide co-polymers, (ii) hydrophilic polyurethanes the reaction product of polyisocyanates; polyols containing at least two isocyanate reactive groups; and optionally an active hydrogen-containing chain extender; and/or (iii) porous silica.

Preferably additives which may be used with laminating adhesives to improve the WVP (breathablity) of the laminate herein may comprise those coatings claimed and described in the applicant's patent application WO98/41314, the contents of which are hereby incorporated by reference.

The laminating adhesive blend can be applied to the sheets to be laminated by coating from a solution, lamination, extrusion coating and/or in-situ polymerisation onto either or both surfaces of the sheets to be laminated.

It is preferred that the sheets of the invention have anti-microbial (such as anti-fungal and/or anti- bacterial) properties. Wherever water is transported or stored there are many opportunities, for microbes such as legionella to grow and accumulate. The hot humid environment inside a fuel cell is particular vulnerable to such microbial growth. As well as being inherently undesirable for health reasons the growth of micro-organisms can seriously impair the efficiency of a fuel cell by blocking or reducing transport of material across the various ion exchange and/or recuparator membranes and/or by blocking or restricting fluid flow through the various channels within the fuel cell.

However any anti-microbial agent which must be used by be able to survive the conditions within the fuel cell and also must not require UV light to be active. Optionally a sheet of the present invention comprises an effective amount of a suitable antimicrobial agent, more optionally incorporated with the cellulose component thereof.

The anti-microbial agent can be added by immersion of a sheet of the invention (and/or by passing a sheet web through) in an immersion bath so the agent is absorbed directly. Alternatively the agent can be adding to a solvent or dispersing medium from which a component (e.g. cellulose) is to be formed either as a film for later lamination (as described herein) or for impregnation in a fibre matrix (also as described herein).

In one embodiment of the present invention the process for the production of regenerated cellulose to which the present invention can be applied can be a process in which the cellulose is first converted into a cellulose derivative, for example using the cellulose xanthate process, and the cellulose is then regenerated by treating the derivative with a suitable reagent, or in which the cellulose is dissolved in a solvent, for example a tertiary amine N-oxide, and then coagulated by solvent removal by immersion in a non-solvent or a mixture thereof.

The water content of the regenerated cellulose which is treated with anti-microbial agent will usually be from 45 to 85 %, preferably from 50 to 82 %, and more preferably from 75 to 80 % by weight.

If the anti-microbial agent comprises silver the so-called cuprammonium process for cellulose regeneration is generally not preferred due to the possible presence of copper which may have adverse effects on the anti-microbial and other properties of the resultant silver impregnated sheet.

Therefore a still further aspect of the invention provides a sheet of the present invention which comprises an anti-microbially effect amount of an anti-microbial agent which is not activated by UV radiation. Preferred agents are selected from: poly(hexamethylenebiguanide)hydrochloride (PHMB); a mixture of titanium dioxide and silver chloride; and/or silver nitrate (e.g. from aqueous solution)

A yet further aspect of the present invention provides use of an anti-microbial agent which is not activated by UV radiation (preferably the agent is selected from: PHMB, titanium dioxide and silver chloride mixture and/or silver nitrate) to inhibit microbial growth in a fuel cell comprising a recuparator as described herein.

Details of preferred anti-microbial agents together with experimental evidence illustrating the utility in the sheets of the present invention (or components thereof) are given in the Examples herein with as a comparison a further anti-microbial agent which does not have an anti-microbial effect in sheets of the present invention. Anti-microbial cellulose films can be used as described herein to prepare a recuparator sheet of the present invention which is less susceptible to antimicrobial growth during the conditions of use within the fuel cell.

The use of PHMB as an anti-microbial in aqueous solution for various conventional uses is well known and for example is described in WO 95/15682; WO 95/15683; WO 97/34834; WO 98/00368; WO 98/32421 and WO 02/03899. However its use in fuel cells has not been described. Without wishing to be bound by any mechanism poly(hexamethylenebiguanide) hydrochloride (also referred to herein as "PHMB") is a cationic polymer and it is believed that its positively charged groups are strongly attracted to the negative charges in cellulose. The cations attack and rupture negatively charged bacterial cell walls to achieve an anti-microbial effect. This liberates low molecular weight cellular material and prevents the uptake of amino acids essential to further cellular growth. Bacterial resistance to the additive is unlikely to occur. PHMB can be used to reduce the effects of bacterial action such as odour regeneration, degradation of the fabric and transfer of organisms. (See Samples 1 to 4 below)

A composite of titanium dioxide, which contains a sparing soluble silver chloride can be used as the anti-microbial agent (such as a 10% aqueous dispersion of the TiO₂/AgCI composite available commercially under the trade designation JM Acticare). Without wishing to be bound by any mechanism it is believed that in an aqueous medium, an equilibrium develops between the silver chloride and silver ions. Micro-organisms take up the silver ions, which bind to thiol groups in cellular enzymes, inhibiting their activity and preventing micro-organism reproduction. This alters the equilibrium so that more ions are released to kill yet more micro-organisms, (see Samples 5 to 7 below)

Cellulose forming fluid (e.g. viscose) may be may be treated with aqueous silver nitrate solution (optionally for a period of from about 5 to about 10 seconds) preferably before the cellulose has been fully dried to make a sheet of the present invention with an anti-microbial effect. The sheet may be un-dried between the cellulose film formation or matrix impregnation step and the treatment with aqueous silver nitrate solution and then the sheet may be subsequently dried. If the cellulose is treated (with aqueous silver nitrate solution) when wet then levels of silver impregnation under otherwise similar impregnation conditions are achievable in periods of the order of 10 seconds compared with 30 minutes or more when the cellulose is treated when dry.

Silver impregnation of wet regenerated cellulose is preferably effected using an aqueous solution of silver nitrate containing from about 0.05% to about 2.0 % by weight and more preferably from about 0.1 % to about 0.4% by weight of silver nitrate. In addition, the silver nitrate solution can also contain from about 0.05% to about 2.0% by weight and more preferably from about 0.1% to about 0.4 % by weight of sodium acetate or potassium acetate. The silver impregnation process is preferably effected at a temperature of from 40 to 100°C, more preferably at a temperature of from 70 to 90°C. Impregnation of a film with silver can be effected by a variety of means but it is preferably effected by immersing not fully dried regenerated cellulose from a regeneration process in an aqueous silver nitrate solution. In general a period of exposure of the wet cellulose to the solution of not more than 30 seconds results in good levels of uptake of silver by the cellulose. Preferred exposure times are from 5 to 10 seconds. This is in marked contrast with other processes (such as those described in US 3,013,099) in which periods of 30 to 60 minutes are required in order to achieve comparable levels of silver uptake starting from dried film. This wet impregnation process is thus preferred to dry impregnation to impart anti-microbial activity to sheets of the present invention.

It is a subsidiary object of the invention to reduce heat loss from the exhaust stream of the fuel cell. This can be achieved by improving the heat conduction across a recuparator of the present invention.

Therefore in a still yet more further aspect of the present invention there is provided a sheet as described herein which further comprises a heat conducting additive therein (such as metal or carbon black). The additive is added to improve the heat transfer properties across the recuparator membrane. Preferably the additive is in powder form so that the pores of the sheet are not blocked and the water permeability of the sheet is not significantly adversely effected.

Another aspect of the invention provides a sheet obtained and/or obtainable by any method as described and/or claimed herein.

A further aspect of the present invention broadly provides a water management system for a fuel cell which comprises at least one sheet of the present invention. Preferred water management systems comprise a fuel cell recuparator. More preferably the recuparator is a separate unit located apart from the fuel cell electrodes and thus less subject to the more extreme temperatures and other conditions present in the heart of a fuel cell. This allows the use of sheets of the present invention with non PEM fuel cell types which might otherwise exhibit conditions which are incompatible with the preferred materials used for sheets of the present invention (e.g. a cellulose glass fibre laminate or impregnate). It is also possible that a single recuparator can be used to service multiple fuel cells connected as a stack either in series or in parallel. Thus each individual fuel cell element within a stack does not necessarily require its own separate recuparator. Suitable fuel cell water management systems and recuparators are described and claimed in US 6,106,964.

A yet further aspect of the present invention provides a fuel cell comprising at least one water management system and/or sheet of the present invention. A yet other aspect of the present invention provides a vehicle which comprises at least one fuel cell of the invention.

A still other aspect of the present invention comprises a power source comprising at least one fuel cell of the present invention.

Figures

The invention is illustrated by the following non-limiting figures in plot various data given in the

Tables herein: Figure 1 is a plot of water vapour pressure (WVP) results.

Figure 2 shows the drop in WVP relative to a known cellulose film denoted by the label "Comp A"

(that available commercially from UCB Films under the registered trademark Cellophane ® 350

POO).

Figures 3 & 4 are plot of the variation of dimensional stability cycle by cycle (respectively MD & TD) for Examples 1 to 4.

Figures 5 to 8 shows the overall change in dimensional stability for respectively Examples 1 to 4 at different conditions.

Examples The invention will now be illustrated by the following non-limiting examples where:

Example 1 is a laminate of a conventional cellulose film web (regenerated from viscose) adhered to glass fibre veil and made using a cellulose web speed of 10mmin^"1 with no nip.

Example 2 is a laminate of a conventional cellulose film web (regenerated from viscose) adhered to a glass fibre veil and made using a cellulose web speed of 10mmin^"1 with a nip roller. Example 3 is a laminate of a cellulose film to a glass fibre veil made using a cellulose web speed of 60mmin^"1 with no nip.

Example 4 is a glass fibre veil impregnated with cellulose regenerated in situ from a 20% (w/w) cellulose dispersion in NMMO at 20°C, substance = 136.26 gm^"2.

Example 5 is glass fibre veil impregnated with cellulose regenerated in situ from a 20% (w/w) cellulose dispersion in NMMO at 80°C, substance = 129.68 gm^"2.

Examples 6 to 14 are various laminates of cellulose film to glass fibre mat prepared to show the effect of different additives to the laminating adhesive.

Example 15 is a laminate of cellulose film to a porous polyethylene film.

Comp A is a conventional regenerated cellulose film of 25 microns thickness made by the viscose process and available from UCB Films under the trade mark Cellophane® 350 P00.

Comp B is a conventional cellulose film of 11 microns thickness made by the NMMO process.

Samples 1 to 10 and Comps V, W, X, Y & Z are various cellulose films (and controls) prepared to test the effect of adding various anti-microbial agents to the film. Examples 1 to 3 - Laminated recuperator membranes Laminated membranes of the invention may be prepared as described below

A regenerated cellulose film was prepared by the known method of regeneration from a dope bath comprising 13% cellulose in N-methyl-morpholine-N-oxide (also known as NMMO) and as described in for example US 4,226,221 and elsewhere. The film was extruded through a slot die under the following conditions to obtain a insulating sheet of the invention having the properties described herein: die gap = 50 microns; dope flow rate 112 litres per hour; MD draw ratio 0.51 ; and TD draw ratio 3.0. This gave a cellulose film (less than 25 microns thick) with a tensile strength in the TD of 122 N/mm².

Three cellulose film / adhesive / glass fibre veil laminate samples were prepared as described herein.

The cellulose film was that available from UCB Films under the registered trade mark Cellophane® 350P00 coated (in a bath) with 0.35% of the anchor resin available under the trade designation PT788.

The adhesive was that available under the trade designation Anglo Surestick 3353 (17% solids) which had been diluted with ethyl acetate (at a weight ratio of 70:30 adhesive to solvent).

The glass fibre veil was obtained from Technical Fibre Products Limited (located in Kendal Cumbria, U.K.) under their designation Optimat Surface One Glass Veil Prov. S2 0585/00. The veil was a wet laid non woven C-glass veil made from 12 mm long fibres of corrosion resistant glass and mean fibre diameter of 11 to 13 microns. The veil contained between 8% to 12% (w/w) of a cross-linked stryene acrylic binder (which also contained surfactant). The veil had an areal weight of 20 g / m², thickness of 0.16 mm and a MD tensile strength of 15N / 15 mm.min.

The adhesive was applied to between the cellulose film and veil using a reverse gravure kiss-coat method and an optional nip roller as described below. Each example laminate was produced with the film web run at the speeds given below.

Example 1 was prepared from a cellulose film web run at 10mmin^"1 with no nip roller. Example 2 was prepared from a cellulose film web run at 10mmin^"1 with a rubber roller placed immediately behind the film/glass fibre contact point in order to form a nip. This was applied with minimal pressure, which was still enough to drive the adhesive right through the mesh and out onto the roller. Example 3 was prepared from a cellulose film web run at 60mmin^"1 with no nip roller.

Example 4 & 5 - Impregnated recuperator membranes

Sheets of the invention prepared by impregnation of a glass fibre mat with cellulose were prepared as described below.

The two Examples (4 & 5) were prepared by passing the glass fibre veil S2 0585/00 (as described in Examples 1 to 3 above) through a regeneration bath of 20% cellulose (w/w) in NMMO. The cellulose was regenerated in the normal manner to leave a veil impregnated with regenerated cellulose. Example 4 was prepared with the regeneration bath at 20°C and Example 5 with the bath at 80°C.

Results The sheets of the invention exemplified herein were tested for thickness, water permeability (as measured by WVTR), dimensional stability (including wet / dry cycling), and/or examined under the microscope. The results obtained are discussed below.

Thickness

Table 1

Water vapour transmission rate (WVTR)

WVTR was measured at 25°C and 75% relative humidity (RH) in units of g / 24 hours / m²; as described in standard method DIN 53122.

The impregnated membrane Example 4 was less permeable to water vapour than the unmodified cellulose film - Comp A. The laminated membranes formed from un-nipped films (Examples 1 & 3), also had lower water vapour transmission rates (WVTR) than Comp A. However, the laminated membrane from nipped film (Example 2) showed a better water transmission than membrane from unipped film (Examples 1 & 3).

Without wishing to be bound by any mechanism one explanation for this difference is due to a reduction in adhesive coat-weight resulting from squeezing of excess adhesive out of the laminate. Alternatively, these results may be due to better distribution of adhesive throughout the mesh in Example 2 leading to a more porous adhesive lattice relative to membranes from Examples 1 & 3 where the adhesive probably sits in a more dense, more concentrated layer at the interface.

The use of a lower adhesive coat-weight may also be able to improve WVTR of the laminate membrane although this may lead to a poorer mesh-cellulose bond which is undesired.

Dimensional Stability (Cycle by cycle)

Linear dimension (in MD or TD) of samples of membranes of invention were measured when wet (i.e. saturated with water at 60°C) and dry (at room temperature). Examples 1 to 4 herein were wet / dry cycled for 100 cycles, each cycle consisting of 1 minute total immersion in water followed by 10 minutes drying in an oven at 60°C. The cycle by cycle dimensional change for jwet rdry each 'nth' cycle was calculated as follows (see Table 4): Wet dimensional change = rdry rdry and dry dimensional change = The mean L^m and ifi values can be calculated over the 100 cycles and converted into a percentage to give an overall wet and dry dimensional stability for the membrane.

The dimensional stability data is given in the Tables 3 to 5 below as percentage changes where lower values denote sheets with the more desired higher dimensional stabilities.

Table 4 - Cycle by cycle dimensional stability (over 3 wet / dry cycles) shown in Figures 5 to 8 (as % change)

Table 5 - Relative to initial dimensional stability (Dry 1) over 3 wet / dry cycles (as % change)

For both Examples 1 & 2 the cellulose film remained joined to the glass fibre veil for the 100 cycles, although the Examples showed some wrinkling after ten cycles. In both cases a definite degree of sag could be seen after 40 cycles.

It is believed that in the Examples herein the main dimensional instability lies in the TD. This is demonstrated by Figure 8 which indicates that Example 4 (the impregnated membrane) is a particularly dimensionally stable membrane in the MD but less stable in the TD. This could be related to the properties of the particular NMMO cellulose film used which was oriented in the MD (and therefore will exhibit less dimensional change in the MD on cycling from wet to dry). To counter this the cellulose film may also be oriented in the TD as well to also reduce TD dimensional change during cycling. Ideally, a static situation would probably be the optimum. A TD dimensional change was also evident in the laminated membranes (Example 1 to 3) where it was observed that wrinkles tended to run along the MD. Clearly, if a cellulose film with a higher dimensional stability in the TD was used then this wrinkled effect would be less.

Another effect observed was the good dimensional stability performance of Example 3 which may be due to a difference in adhesive lay down or optimisation in (the faster) machine speed. It was also noted that laminated membrane from nipped film (Example 1) always out-performed that from un-nipped film (Example 2) in terms of dimensional stability. Without wishing to be bound by any mechanism this could be due to the adhesive being distributed throughout the mesh, which when it sets will add rigidity to the laminate.

Microscopy:

Microscopy of samples taken from each of Examples 1 to 5 using a Zeiss Compound microscope with cross-polarised light showed no black areas usually associated with holes in any of the samples. However of the impregnated membranes, the sample from Example 5 had a more "grainy" appearance when compared to that from Example 4.

Examples 6 to 13

The applicant wished to improve the WVP of laminated films of the invention where the cellulose film (350P00) was anchored to the glass fibre veil using a polyurethane (PU) adhesive (Anglo Surestick 3353) whilst minimising any reduction in bond strength. This was achieved by the addition of various additives to the PU adhesive.

Additive 1 was an urethane / ethylene oxide co-polymer (for example obtainable as described in

Example 1.2.1 of US 6294092);

Additive 2 was a porous silica of mean particle size 700 microns; and

Additive 3 was a porous silica of mean particle size 100 microns.

The examples were prepared as analogously to Example 2 herein except the cellulose film used was a 11 micron thick film made by the NMMO process (rather than a 25 micron thick viscose film 350P00). The thinner film presented less of a water barrier. For Examples 6 to 10 the cellulose was also stretched in a stenter before lamination so that the film would shrink less in water and thus be less of a de-laminating force. The adhesive was used had a slightly lower coat weight (2-4 g/m²) than Example 2 but apart of the amount of additive indicated was otherwise identical. In Table 6 "%" indicates percentage of additive added by weight of total adhesive; "WVP" is water vapour permeability measured conventionally (in units of g/m²/day) and "Delam" is the strength (in N/cm) required to de-laminate the cellulose layer from the fibre matrix; NM indicates not measured, and NA denotes not applicable. Comp B refers to the un-laminated 11 micron thick NMMO cellulose film. Examples 6 and 11 are laminated films made without the corresponding adhesive additives (respectively using stentered and unstentered NMMO cellulose) .

Table 6

Example Additive % WVP (o/m²/dav) Delam (N/cm)

PU /EO copolymer additive

6 none NA 280 85

7 1 15 330 55

8 1 30 305 45

9 1 45 360 35

10 1 60 370 20

Silica additives

11 none NA 330 NM

12 2 2 370 NM

13 2 2 450 NM

14 3 5 360 NM

15 3 5 340 NM

Cellulose films

Comp B NA NA 555 NA

Comp A NA NA 425 NA

Example 14 - Porous polyethylene film coated with cellulose

A 14gm^"2 coating of cello was coated in a suitable conventional manner onto 200 micron thick un- ribbed porous polyethylene (PE) film (available under the trade designation Entek). The pores in the base film had been created by co-extruding the polyethylene with oil to form the film and then extracting the oil from the film. The water vapour permeability of this coated film was measured and found to be 67.02 % of that of un-coated cellulose film (Comp A). The coated film had satisfactory WVP for use as a fuel cell recuparator membrane but improved mechanical properties compared Comp A . Samples 1 to 4 and Comp Z (PHMB anti-microbial)

Several samples (Samples 1 to 4) of A4 sheets of cellulose film with PHMB (suitable for later use to make laminated sheets of the invention) were evaluated for antibacterial activity using the AATCC Test Method 100 described below. The PHMB used as the anti-microbial agent was a 20% aqueous dispersion of PHMB available commercially from Avecia Biocides under the trade mark Reputex 20. The Reputex 20 was added as a component to an immersion bath for cellulose film at a suitable concentration to achieve various mass concentrations of PHMB in the softened film (0.25%, 0.5% ,1.0%) and in the unsoftened film (1.0%). A control sample of cellulose film without PHMB (Sample Z) was tested at the same time (see below)

AATCC Test 100 (1998) is a quantitative test, which counts the growth or survival a bacterial population on a material, and therefore anti-bacterial efficacy. A shaken overnight culture of Staphylococcus aureus in nutrient broth was diluted to approx. 1x10⁵ cells/ml in 25% nutrient broth in sterile physiological saline solution. This culture was used to inoculate the film samples. Two pieces were taken from each sample of film, and inoculated with 1 ml of the suspension of Staphylococcus aureus. After inoculation, one piece was immediately neutralised with 100ml of neutraliser solution (CEN standard neutraliser solution), and shaken vigorously for 60 seconds. Surviving cells were counted by a 'serial dilution pour plate' technique on nutrient agar plates, using saline solution as diluent. The other inoculated sample was incubated at 370c for 24 hours. The neutralising and counting technique were carried out as above. The results are given in the Table below, where Samples 1 to 3 and the control (sample Z) were softened films and sample 5 was unsoftened. The bacterial count was measured in units of "cfu/ml" i.e. colony forming unit per millilitre of inoculum.

Table 7

Sample Reputex 20 O time 24 hours Log reduction

(%) cfu/ml cfu/ml Control (samplel) z 0 3.2 x10^s 5.0 x10⁶ NA

1 0.25 <10² 4.5

2 0.5 <10² 4.5

3 1.0 <10² 4.5

4 1.0 <10² 0.7

The results show Staphylococcus aureus grew from an initial count of 10⁵ to approx. 10⁶ after 24 hours incubation on the untreated film. On the four treated films (Samples 1 to 4), there were no viable organisms after 24 hours incubation which indicates a strong antibacterial effect. Samples 5 to 7 and Comp X & Y (JM Acticare anti-microbial)

JM Acticare includes a composite of titanium dioxide, which contains a sparing soluble silver chloride. The JM Acticare anti-microbial agent was injected into viscose from which a cellulose film was regenerated as described herein. In fact the other method (impregnation of a glass fibre mat) could equally have been used. The concentration of the JM Acticare injected into the viscose was selected to achieve various mass concentrations of agent with respect to the cellulose in the final film or mat (0.1 %, 0.5% ,1.0%) (see below). These cellulose film samples (Samples 5 to 7) containing JM Acticare prepared as above were evaluated for antibacterial activity against an E.coli and S.aureus challenge inoculum as follows. Two gram samples of each cellulose film were place in 20ml sterile distilled water in a 100ml sterile container and shaken for 24 hours at 30 strokes/minute at a temperature of 40°C. After this period the temperature of the samples was adjusted to 20°C. The pooled inoculum of E.coli and S.aureus was added to the test samples at a level of 1.4x10⁶ cfu ml^"1. Samples were taken and total viable counts (TVCs) were determined at 0, 6, 24 and 48 hours and 7 days to estimate the antibacterial activity of the cellulose film samples. The % denotes the amount of the JM Acticare antimicrobial agent added to the base film. Two control samples were also tested at the same time. Sample X was a cellulose film without JM Acticare, Sample Y was a bacterial control to determine the effect of the test on the organisms without base film. The results are given as follows:

Table 8

Sample % Baseline 0 hours 6 hours 24 hours 48 hours 7 days

X 0% 1.40 x 10⁶ 4.48 x 10^s 4.42 x 10^s 6.24 x 10⁴ 9.60 x 10⁴ 1.39 x 10^s

5 0.1 % 1.40 x 10° 1.00 x 10² 1.00 x 10° 1.00 x 10° 1.00 x 10° 1 .00 x 10°

6 0.5% 1.40 x 10⁶ 1.00 x 10¹ 1.00 x 10° 1.00 x 10° 1.00 x 10° 1.00 x 10°

7 1.0% 1.40 x 10⁶ 3.00 x 10² 1.00 x 10° 1.00 x 10° 1.00 x 10° 1.00 x 10°

Y No film 1.40 x 10° 4.32 x 10⁴ 7.76 x 10³ 4.12 x 10³ 1.20 x 10^s 8.70 x 10²

The base film sample (Sample X) demonstrated no apparent antibacterial activity. The results from the bacterial control (Sample Y) indicated that there was some loss of viability in distilled water. However, a bacterial count of 1x10³ cfu/ml was still present after seven days. All of the samples containing JM Acticare achieved a 100% reduction in count (i.e. total elimination of the challenge organisms) within 6 hours contact. The JM Acticare therefore was shown to add substantial antibacterial activity to the cellulose film. Samples 8 to 10 and Comp W

The following descriptions of a method of impregnating cellulose with silver for use in a fuel cell recuparator of the invention are described by way of illustration only.

2

An unsoftened regenerated cellulose film weighing 35g/m was immersed in an aqueous solution of silver nitrate (1 % AgNO₃ by weight) containing 1 % by weight of sodium acetate at a temperature of 70°C for a period of 30 minutes. Thereafter the film was washed in water and dried. The silver content of the resulting impregnated film (Sample 8), which was brown in colour, was 2.49% by weight of the un-impregnated film, which is less than 10 ppm silver. This procedure was repeated except that the film was immersed in the silver nitrate solution for a period of 60 minutes. The silver content of this film (Sample 9), which was brown in colour, was 2.53 % by weight.

A wet web of regenerated cellulose was produced using the cellulose xanthate process, the web having been produced by casting the cellulose xanthate into aqueous sulphuric acid, followed by washing with water, aqueous sodium hydroxide solution and aqueous sodium hypochlorite solution to remove inorganic materials, such as elemental sulphur and thiocarbonates, but without drying to form a finished cellulose film. A sample of this web, which if dried would

2 produce a film weighing 35g/m , was immersed in a bath of silver nitrate and sodium acetate as described in above at a temperature of 85°C for a period of 10 seconds. The web was then removed from the bath, washed with water, and dried to produce a regenerated cellulose film (Sample 10) which was yellow/brown in colour and had a silver content of 2.67 % by weight.

Immersion of the dry film in the silver nitrate solution used in the wet process above for a period of approximately 10 seconds resulted in no change in the appearance of the film (Comp W), indicating that there had been little significant take-up of silver. This film (Comp W) would not have a significant anti-microbial effect.

Thus a process where the film was impregnated with silver nitrate whilst still wet produced a film (Sample 10) with a higher silver content than that produced by impregnating the dry film under substantially identical conditions (Comp W) but in a period of one two hundredth of the time. This wet impregnation process is preferred to impart anti-microbial activity to sheets of the present invention.

Comp V

The applicant has found that not all anti-microbial agents provide an anti-microbial effect in a sheet of the present invention. As a comparison a proprietary silver salt of zirconium phosphate was used as the anti-microbial agent (a 20% aqueous dispersion available commercially from Milliken Chemicals under the trade designation Alphasan RC 5000). Alphasan RC 5000 is a widely used multi-use antibacterial agent based on a proprietary silver salt of zirconium phosphate. The Alphasan RC 5000 anti- microbial agent was injected into viscose from which a cellulose film was regenerated as described herein. The concentration of this anti-microbial injected into the viscose was selected to achieve various mass concentrations of agent in the final film (0.5% , 1%, 3% ,5% ,& 10%). Analysis of these samples did not detected silver on the surface the film. Without wishing to be bound by any mechanism this may be due to the formation of silver sulphide, which be consistent with a yellowing or browning of these samples. The films showed no anti-microbial properties.

Further embodiments of the invention

From these results the applicant has deduced that improvements in WVTR and/or TD stability can be addressed in the following manner. These embodiments are also to be considered as within the scope of the present invention.

Lamination using thinner, more dimensionally stable NMMO cellulose film, anchored to the glass fibre veil may improve the sheet properties. The thinner NMMO cellulose film will have a lower WVTR than Comp A and at the same time be more dimensionally stable, which will result in less force on the mesh, reducing the wrinkling to produce a laminated membrane of the invention with a higher WVTR than the Examples herein (thinner sheet means lower barrier i.e. higher WVTR).

Use of an different lamination adhesive may improved WVTR such as a two-part laminate adhesive based on the Anglo Surestick 3353 but with the addition of an isocyanate based curer. It is believed this will increase the bond strength of the cellulose film to the glass fibre veil although decrease the WVTR of the resultant sheet compare to the Examples herein. However a suitable material such as PEG can be added to the adhesive to increase the WVTR if desired.

Use of a glass fibre veil of improved strength may improve dimensional stability of the sheet. A multi-layer sheet, such as three layer sandwich (for example a cellulose to fibre veil to cellulose, adhesive laminate or a fibre veil to cellulose to fibre veil, adhesive laminate) may also improve dimensional stability to a sheet of the present invention although may also decrease WVTR, but to an acceptable extent.

Claims

1. A water permeable sheet substantially impermeable to gases, the sheet comprising:

(a) a laminate of a substantially water permeable first layer bonded to a supporting porous layer comprising a matrix of water insoluble fibres of inorganic mineral and/or a porous plastic film, optionally polyethylene or polypropylene; and/or

(b) a matrix of water insoluble fibres impregnated with a water-permeable material.

2. A water permeable sheet; having; (a) a water vapour transmission rate of at least 200 g² / m² / day at 25°C and 75% relative humidity; and

(b) a mean dimensional stability (over 100 cycles as defined herein) for a wet or dry sheet in any direction parallel to the sheet surface of less than about a 15% change in linear dimension.

3. A water permeable sheet as claimed in any preceding claim, which comprises a laminate of a substantially water permeable first layer bonded to a supporting porous layer comprising a matrix of water insoluble fibres.

4. A water permeable sheet as claimed in any preceding claim which comprises a matrix of water insoluble fibres impregnated with a water-permeable material.

5. A sheet as claimed in any preceding claim, in which the water-permeable material, sheet and/or layer comprises polyvinyl alcohols (PVOH), polyurethanes, polysulphones, grafted polypropylene, polyamides, biopolymers, micro-perforated polyolefins, moisture permeable voided films; mixtures thereof and/or combinations thereof.

6. A sheet as claimed in the immediately preceding claim, in which the biopolymer is selected from cellulose, polylactic acid and/or suitable derivatives thereof.

7. A sheet as claimed in the immediately preceding claim, in which the cellulose was prepared by a regeneration method.

8. A sheet as claimed in the immediately preceding claim, in which the cellulose was prepared by regeneration from a bath of an N- amine oxide fluid.

9. A sheet as claimed in any preceding claim, in which the fibres comprises one or more of the following materials: inorganic minerals, poly vinyl acetates (PVAc), polyvinyl alcohols (PvOH), cellulose, rayon, glass, polyester, carbon (optionally metal coated), aramid, quartz, silicon carbide, polyamides, polysulphones; copolymers of acrylonitrile butadiene and styrene (ABS polymers), poly vinyl chloride (PVC), alumina, high performance textile fibres; paper; epoxy composite; rockwool and/or any suitable mixtures and/or combinations thereof.

10. A sheet as claimed in any preceding claim, in which the fibres comprise one or more of the following materials: glass, polyester, carbon, aramid, quartz, silicon carbide, ABS, PVC, alumina, epoxy composite; rockwool, and/or any suitable mixtures and/or combinations thereof.

11. A sheet as claimed in the immediately preceding claim, in which the fibres substantially comprise glass and/or carbon.

12. A sheet as claimed in any preceding claim which further comprises an anti-microbial agent other than silver salts of zirconium phosphate.

13. A sheet as claimed in the immediately preceding claim, in which the anti-microbial agent is selected from PHMB, silver nitrate and/or a mixture of titanium dioxide and silver chloride.

14. A sheet as claimed in the preceding claim which comprises cellulose, in which the antimicrobial agent is silver nitrate impregnated into the cellulose when wet from an aqueous solution.

15. A sheet as claimed in any preceding claim which further comprises an heat conducting additive.

16. A sheet as claimed in the immediately preceding claim, in which the heat conductive additive is selected from metal or carbon black powder.

17. A laminated sheet as claimed in any preceding claim, which further comprises an adhesive layer between a water-permeable and the supporting layers.

18. A laminated sheet as claimed in the preceding claim, in which the adhesive layer comprises a polyurethane adhesive with optionally an isocyanate based curing agent and optionally diluted with ethyl acetate.

19. A laminated sheet as claimed in either of the two preceding claims, in which the laminating adhesive comprises an additive to improve the WVTR of the laminate without significantly reducing its de-lamination strength.

20. A laminated sheet as claimed in the immediately preceding claim, in which the adhesive additive is selected from a urethane / ethylene oxide co-polymer, from silica and optionally from those coating compositions described in US 6,294,092.

21. A laminated sheet as claimed in either of the two preceding claims, in which the adhesive additive is present in an amount from about 10% to about 70% by weight of the total adhesive.

22. A method of making a laminated sheet as claimed in any preceding claim comprising the steps of

(a) preparing a web of a water-permeable film; and

(b) bonding said web to a porous support layer optionally a non woven fibrous layer;

(c) optionally by applying an adhesive layer as described in any of claims 17 to 21 ; to form a multi-layered web comprising at least one water permeable layer and at least one porous fibrous layer.

23. A method as claimed in the immediately preceding claim, in which the bonding step comprises applying an adhesive to the web and/or porous layer on the adjacent surface(s) thereof.

24. A method of making an impregnated sheet as claimed in any preceding claim, comprising the steps of:

(a) immersing a porous fibrous matrix in a bath containing a water permeable material dispersed therein; and (b) regenerating said water permeable material within said matrix; to form a porous fibrous matrix impregnated with a water permeable material.

25. A method as claimed in the immediately preceding claim, in which the bath comprises cellulose dispersed in a non solvating fluid.

26. A sheet obtained and/or obtainable by a method as claimed any of claims 22 to 25.

27. A water management means for a fuel cell comprising at least one sheet as claimed in any of claims 1 to 21.

28. A water management means as claimed in the immediately preceding claim, which is a fuel cell recuparator optionally as claimed in US 6,106,964.

29. A water management means as claimed in either of the two preceding claims, in which the at least one sheet as claimed in any of claims 1 to 21 , 26 and/or 38, is located in fluid connection with a conduit which carries reactant into or out of a fuel cell, the sheet acting as a water permeable membrane and a substantial gas barrier between the interior and exterior of the conduit.

30. A water management means as claimed in any of the three preceding claims, in which the at least one sheet as claimed in any of claims 1 to 21 , 26 and/or 38, is located in fluid connection between the oxidant outlet and oxidant inlet of a fuel cell to allow water to pass there between but to prevent substantial diffusion of gases there-between.

31. A fuel cell comprising at least one water management means as claimed in any of claims 27 to 30.

32. A power source comprising at least one fuel cell as claimed in the immediately preceding claim.

33. A vehicle comprising at least one fuel cell as claimed in claim 31.

34. Use of a sheet as claimed in any of in any of claims 1 to 21 , 26 and/or 38, in a fuel cell.

35. Use of a sheet as claimed in any of in any of claims 1 to 21 , 26 and/or 38, for the purpose of managing water in a fuel cell.

36. Use of a sheet as claimed in any of in any of claims 1 to 21 , 26 and/or 38, in the manufacture of a fuel cell and/or recuparator therefor.

37. A method of manufacturing a fuel cell recuparator, fuel cell, power source and/or vehicle comprising the step of

(a) locating a sheet as claimed in any of in any of claims 1 to 21 , 26 and/or 38, in fluid connection with any conduit which carries reactant into or out of a fuel cell.

38. A sheet as described herein with reference independently to any of the Figures and/or Examples herein.