Polymer membrane
FIELD OF THE INVENTION 
     The present invention relates to polymer membranes. The invention relates in particular to a composite polymer membrane, a method of making a composite polymer membrane, and a method of crosslinking the material of a microporous polymer matrix. 
     BACKGROUND OF THE INVENTION 
     A composite polymer membrane, which comprises a microporous matrix formed from a first polymeric material and a second polymeric material within the pores of the matrix, can be used as a selective barrier. For example, such a membrane may be used to separate components of a liquid. By use of an ion exchange material as the second polymeric material, such a membrane may be used as a selective barrier for ions in solution; for example, such a membrane may be used as an electrode separator in an electrochemical device, in which ionic conduction between electrodes of the device is required and in which a barrier is required to prevent migration of certain dissolved species (such as ions) and of electrode material in particulate form. The barrier is also required in secondary cells on recharging, when loosely attached material is deposited on the anode, often in the form of dendrites, which is required not to make contact with the cathode. A composite polymer membrane can be used to separate materials by other mechanisms, for example by ultrafiltration. 
     A suitable composite polymer membrane for use as an electrode separator in an electrochemical device is disclosed in our U.S. Pat. No. 5,256,503 the disclosure of which is incorporated herein by reference thereto. The membrane comprises a porous matrix of polyethylene which has been formed from a blend of polyethylene and polyethylene oxide ;by removal of the polyethylene oxide. 
     The pores in the matrix contain polyacrylic acid which preferably has been polymerized in situ in the pores, and which blocks, and preferably fills, the pores of the matrix. The acrylic acid is supplied to the pores of the matrix in solution with a crosslinking agent and a photoinitiator. Polymerization and crosslinking of the acrylic acid are initiated by exposure to ultraviolet radiation. 
     The polymerization and crosslinking reactions of the acrylic acid in the composite membrane disclosed in U.S. Pat. No. 5,256,503 compete for the acrylic acid, and by selecting the relative rates of the competing reactions, it is possible to control the degrees of crosslinking and polymerization of the acrylic acid. It has been found possible to vary the properties of the known membrane by appropriate selection of the crosslink density of the acrylic acid impregnant, for example to provide a membrane with a low resistance and relatively weak barrier performance using a low crosslink density, and to provide a membrane with relatively high resistance and good barrier performance using a high crosslink density. The degree of crosslinking can be selected by for example providing the impregnant with an appropriate quantity of crosslinking agent for reaction. However, a remaining difficulty lies in producing a membrane by the technique disclosed in U.S. Pat. No. 5,256,503 which has a good barrier performance towards certain dissolved species (such as silver or mercury ions). 
     Another approach to producing a composite polymer membrane is the subject of U.S. Pat. No. 3,427,206, and involves grafting a polymer of an ethylenically unsaturated carboxylic acid such as acrylic acid onto a non-porous sheet of a material such as polyethylene. This approach can be contrasted to that discussed above, in that ion migration takes place through the bulk polymer of the sheet to which acrylic acid has been grafted, rather than through regions of polyacrylic acid contained within pores in a microporous sheets. 
     Membranes which are formed by grafting an ion exchange material onto the polymeric material of a continuous sheet may be crosslinked, generally before the ion exchange material is supplied. It has been found that crosslinking of such membranes has effects which are similar to those in the membranes disclosed in WO-A-87/06395, affecting ionic conductivity through the membrane and barrier performance. This can be understood since crosslinks are provided in both systems effectively between molecules of ion exchange material, the material in the first system described above being bulk material, whereas that in the second system described above is grafted onto the polymeric material of a continuous sheet. 
     Polymer membranes of the type disclosed in WO-A-87/06395 which comprises a porous matrix of a first polymeric material, with a second polymeric material within its pores, have been found to have advantages compared with membranes which comprise a continuous sheet of a first polymeric material with a second material grafted to the material of the sheet. For example, the ionic conductivity through a filled microporous sheet has been found to be higher than that through a grafted sheet. 
     SUMMARY OF THE INVENTION 
     According to the present invention, it has been found that the performance of certain composite polymer membranes can be improved in significant respects by crosslinking the polymeric material of a microporous matrix, whose pores are blocked by a second material through which conduction takes place, for example by an ion exchange mechanism, or by a mechanism involving ultrafiltration. 
     Accordingly, in one aspect, the invention provides a composite polymer membrane comprising a first polymeric material which defines a porous matrix, and a second polymeric material which at least partially fills, and thereby blocks, the pores of the matrix, at least the first material, and preferably each of the first and second materials, being crosslinked. 
     In another aspect, the invention provides a method of making a composite polymer membrane, which comprises: 
     (a) providing a porous matrix defined by a first polymeric material, which is impregnated with a first reagant which, on exposure to an appropriate treatment, leads to crosslinking of the first polymeric material; 
     (b) causing the first polymeric material and the first reagant to react so as to crosslink the first polymeric material; and 
     (c) impregnating the porous matrix with a curable second material. 
     Preferably, the second material is impregnated into the matrix as a component of a mixture, together with a second reagant which, on exposure to an appropriate treatment, leads to curing of the second material. Curing may involve polymerization or crosslinking or, most preferably, both, and the second reagant leads to crosslinking of the second material. It is preferred that the method includes the steps of impregnating the matrix with a mixture of a second material which is polymerizable and a second reagant which, on exposure to an appropriate treatment, leads to crosslinking of the polymerizable material, subsequently causing the second material to polymerize and to react with the second reagant. 
     It has been found, surprisingly, that crosslinking the first polymeric material of the membrane can influence significantly the transport characteristics of species through the membrane, for example the barrier performance of the membrane towards dissolved species such as certain ions compared with the performance of a membrane in which the performance of a membrane in which the first polymeric material is not crosslinked, as disclosed in WO-A-87/06395. This result is achieved despite the fact that ion migration takes place through regions of the membrane occupied by the second polymeric material rather than the first polymeric material. Particularly significantly, it has been found that such improved barrier performance is obtained without any significant increase in resistance of the membrane to conduction through it of species, for example ions, as had been encountered using the technique disclosed in WO-A-87/06395, for example, as measured as the internal resistance of an electrochemical device in which the membrane is incorporated as an electrode separator. Furthermore, compared with membranes produced by the technique disclosed in U.S. Pat. No. 3,427,206, the membranes of the present invention retain the advantage of those disclosed in WO-A-87/06395 of low resistance to ionic transfer. The invention therefore allows the barrier properties towards certain ionic species and resistance So ionic transfer to be optimised. This in turn can allow the design of an electrochemical cell with a long shelf life. 
    
    
     DETAILED DESCRIPTION 
     The first polymeric material of the membrane will generally function to provide a support for the second polymeric material. The first polymeric material will therefore generally be an inert material which has suitable physical properties for it to-function as such a support. Examples of suitable materials include polyolefins, especially polyethylene, although other materials such as polyvinyl chloride, polyamides, polyesters, polysulphones and polytetrafluoroethylene may be used. 
     The matrix provided by the first polymeric material is preferably formed by removal, especially by liquid extraction, of a removable material dispersed therein. Liquid extraction of polyethylene oxide from an article comprising a mixture of polyethylene and polyethylene oxide is a preferred example; other possibilities include the extraction of a soluble salt such as lithium carbonate from a mixture of polyethylene and the salt, for example using an acid such as nitric acid or hydrochloric acid, (as disclosed in EP-A-219190), extraction of polyvinylacetate from a sheet provided by a mixture of polyvinylacetate and polyvinyl chloride, and extraction of ethylene/vinylacetate copolymer from a mixture of ethylene/vinylacetate polymer and polypropylene. 
     An advantage of forming the microporous matrix using an extract method is that the pores of the resulting structure are highly tortuous, more so than those of pores produced by stretching or perforation techniques. It is thought that a highly tortuous structure helps to retain volatile polymerizable materials such as acrylic acids in the pores of the structure until polymerization is effected. 
     However, the matrix of the membrane should be microporous in the sense that details of its structure are discernable only by microscopic examination. It is particularly preferred that the structure of the pores is so fine that it can only be discerned by use of election microscopy techniques, which can resolve details of structure below 5 micrometers. 
     It will be understood that when the term microporous is used to describe the matrix defined by the first polymeric material, the term is applicable to a component of a membrane which consists of interpenetrating phases, as might be the case for a membrane derived from a mixture of two components. 
     Preferably, the volume of the pores in the matrix defined by the first polymeric material is at least about 10%, or preferably at least about 30% in order to achieve satisfactory ionic conductivity. A pore volume of at least 70% or even higher can be preferred for many applications. 
     The thickness of the membrane is preferably less than about 250 μm, more preferably less than about 200 μm especially less than about 100 μm. It is also preferred that material of the membrane which is to react by crosslinking is less than about 200 μm from the or a surface of the membrane which is exposed to the medium by which the crosslinking reaction is initiated, more preferably less than about 150 μm from that surface, especially less than about 100 μm from that surface. 
     The second material of the membrane is selected according to the intended end use of the membrane. The second material may be supplied to the membrane as a monomer and, for many applications, will then be polymerized in situ, after it has been impregnated into the membrane. Preferred materials which may be used include a vinyl monomers capable of reacting with an acid or a base to form a salt, either directly or indirectly after appropriate work up, perhaps involving for example hydrolysis or sulphation. Especially preferred materials include ethylenically unsaturated acids and their esters such as acrylic acid, methacrylic acid, ethyl acrylate and methyl acrylate, maleic acid, maleic anhydride, acrylamide, monomethyl acrylamide and N,N,-dimethyl acrylamide, vinyl acetate, and vinyl pyridines such as 2-vinylpyridine, 2-methyl-5-vinylpyridine and 2-vinyl-5-ethylpyridine. It is particularly preferred to supply the second material to the pores of the matrix defined by the first material in monomer form for subsequent polymerization in situ since it allows certain second materials to be introduced in liquid form to be converted subsequently into a solid form by polymerization. It is envisaged that the second material might be supplied to the matrix in solution in polymeric form. The solvent may then be removed, and the resulting polymeric impregnant may then be crosslinked if desired. 
     The second material, together with an appropriate reagant as an initiator or a crosslinking agent or both when present, may be introduced into the pores of the matrix defined by the first polymeric material in solution. It is, however, desirable to use a solution of the second material which polymerizes and is crosslinked at an acceptable rate without an unacceptable phase separation of the solution. For example, a solution consisting of the following could be used (percentages given by weight): 
     
         ______________________________________                                    
acrylic acid (reactive monomer)                                           
                         60-95%                                           
crosslinking agent       0.5-25%                                          
photoinitiator           0.1-5%                                           
water up to about        60%                                              
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     It is envisaged that the second material might be supplied to the matrix in solution in polymeric form. The solvent may then be removed, and the resulting polymeric impregnant may then be crosslinked if desired. Other components may be added to the solution such as a surfactant for compatibility or a base to neutralize (at least partially) the acrylic acid. 
     When the second material is polymerized in situ, and is crosslinked using a crosslinking agent, polymerization and crosslinking reactions can compete for monomeric second material. The density of crosslinks can be adjusted to suit a particular application by selecting a crosslinking agent with an appropriate activity towards the monomeric second material compared with the reactivity of that material in polymerization reaction. More preferably, the crosslink density is adjusted mixture of the second material and the crosslinking agent in appropriate proportions. The concentration of reaction initiator such as a photo initiator can affect the relative rates of competing reactions and, therefore, also the crosslink density. Examples of suitable crosslinking agents include triallyl isocyanurate (TAIC), triallyl cyanurate (TAC), 1,5-hexadiene-3-ol, 2,5-dimethyl-1,5-hexadiene, 1,5-hexadiene, 1,7-octadiene, 3,7-dimethyl-2,6-octadiene-1-ol, and certain diacrylates such as polyethylene glycol diacrylate and dimethacrylate. A particularly preferred crosslinking agent for the second material is divinylbenzene. 
     Generally, the pores of the matrix will the blocked over substantially the entire area of the membrane. For optimum barrier properties, it is preferred that at least 20% of the volume of the pores which are blocked be filled by the second polymeric material, more preferably at least about 75%, especially at least about 95%. It is particularly preferred that the pores be substantially filled with the second polymeric material, it being understood that this does not require the exposed surface of the second material in the pores to be exactly in line with the main surface of the membrane, and the degree of filling may vary somewhat depending on the Shape, size, and tortuosity of the pores. 
     The reagant which is supplied for crosslinking the first polymeric material may be provided at the the at which the matrix is formed from that material, for example by forming the matrix from a blend of the first material and the first reagant. 
     Preferably, one or both of the steps of crosslinking the two component materials of the membrane is initiated by irradiation. Suitable radiation might include, for example, gamma radiation such as from a Co 60  source or electron bombardment, but it is strongly preferred that ultraviolet radiation be used because of its low cost and of the convenience with which it can be used. When irradiation is used to initiate crosslinking of one or both of the materials, appropriate initiators will be used to initiate the reaction. When ultraviolet radiation is used to initiate crosslinking, a suitable initiator might be acetophenone or benzophenone. 
     The use of ultraviolet radiation to initiate crosslinking has the advantage that it can easily be carried out as a continuous process without having to erect elaborate shields for safety reasons. Furthermore, the use of ultraviolet radiation to initiate crosslinking has been found in many cases to result in a faster reaction than when other radiation sources are used. Moreover, the use of ultraviolet reaction has the advantage that it allows the crosslinking reaction to be controlled accurately to give a desired degree of crosslinking. 
     When ultraviolet radiation is used to initiate crosslinking of a polyolefin microporous matrix, a photoinitiator is selected which is capable of abstracting a hydrogen atom from the polyolefin molecules. Suitable photoinitiators will generally be aromatic compounds which contain carbonyl groups, such as acetophenone, benzophenone, 4-chlorobenzophenone and 4,4&#39;-dimethyl benzophenone. 
     Preferably, the reagant which is supplied for crosslinking the first polymeric material is provided in solution. More preferably, it is provided in solution in a solvent which neither dissolves the first polymeric material nor causes it to swell. 
     Accordingly, in a further aspect, the invention provides a method of a method of crosslinking the polymeric material of a microporous matrix, the material comprising a polyolefin which comprises: 
     (a) impregnating the matrix with a solution of a reagant which, on exposure to an appropriate treatment leads to crosslinking of the material; 
     (b) removing the solvent in which the reagant is dissolved by volatilizing it; and 
     (c) treating the reagant to cause crosslinking of the material. 
     The degree of crosslinking of a polymeric material can be expressed in terms of its gel content, as measured according to ASTM D2765-84. It has been found that the barrier performance of a composite polymer membrane is dependent on the degree of crosslinking of the first polymeric material; preferably, that material is crosslinked to give a gel content of at least about 50% more preferably at least about 60% for optimum barrier properties. 
     Suitable solvents for the reagant including acetone, toluene, methanol, butanone and 2-methoxyethanol. Preferably, the solvent in which the reagant is provided is inert towards the crosslinking agent, in particular having no abstractable hydrogen atoms. 
     For many applications, it can be preferred that the membrane include a layer of a vinyl monomer capable of reacting with an acid or a base to form a salt directly or indirectly, which is graft polymerized with the first polymeric material of the porous matrix. The provision of a layer of this kind has the advantage that the membrane can be wet more easily by an aqueous liquid, in particular an aqueous electrolyte, than is possible otherwise. This has the advantage that a membrane can be wet by such an electrolyte more quickly and more thoroughly so that the resistance to ionic migration through the membrane becomes stable quickly and at an advantageously low value. The vinyl monomer may comprise an ethylenically unsaturated carboxylic acid or an ester thereof. Preferably, the second polymeric material in the pores of the matrix and the vinyl monomer on the surface of the matrix are derived from the same monomer material. 
     In yet another aspect, the invention provides an electrochemical device which comprises an anode, a cathode, a liquid electrolyte, and an electrode separator comprising a composite polymer membrane of the type discussed above. 
     Other uses for the composite membrane of the invention include those in which filtration, separation, or concentration are to be achieved. 
     Further information concerning the methods of the invention and materials on which they can be practiced is given below: 
     Formation of Microporous Matrix 
     The microporous matrix on which the invention is practiced in the following examples comprises polyethylene and was formed using the technique disclosed in EP-A-219190. Briefly, that technique involves mixing the polyethylene with particles of a soluble particulate salt and a plasticizer, forming a film from the mixture, stretching the film and removing the salt. 
     A suitable matrix has a porosity of 50 to 53%, a thickness of 40 μm, and an air flow rate of 60 cm 3 .min -1 . cm -2  at 4200 kg.m -2 . 
     Crosslinking of Microporous Matrix 
     (a) Batch process 
     Samples of the microporous matrix described, each 7 cm by 7 cm, were impregnated by immersion in solution. Formulations of eight solutions are set out below in Table 1 (proportions given by weight): 
     
                       TABLE 1                                                     
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Sample Number                                                             
          1     2      3    4    5    6    7    8                         
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Toluene (SPR                                                              
          --    --     97.0 --   99.5 --   91.5 89.0                      
Grade, supplied                                                           
by BDH)                                                                   
Acetone (SPR                                                              
          --    97.0    3.0 99.5 --   91.5 --   --                        
Grade supplied                                                            
by BDH)                                                                   
TAIC      --    --     --   --   --    5.0  5.0 --                        
(PERKALINK                                                                
301 supplied by                                                           
Akzo Chemie)                                                              
(triallyl cyan-                                                           
urate cross-                                                              
linking agent)                                                            
Divinyl   --    --     --   --   --   --   --    8.0                      
benzene (55%                                                              
mixture of                                                                
isomers                                                                   
supplied by                                                               
Aldrich)                                                                  
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     The impregnated matrices were then irradiated on one side for either five or ten minutes under a 15.4 cm 500 watt parallel beam, medium pressure mercury vapor lamp (Hanovia Type UVS 500). The distance between the lamp and the matrix was about 30 cm. After irradiation, the samples were washed in acetone and then dried in an air oven at 60° C. The gel content of each of the irradiated matrices as determined according ASTM D2765-84, and are set out in Table 2 below: 
     
                       TABLE 2                                                     
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                 Gel content                                              
                 (irradiation time)                                       
SAMPLE NUMBER      5 mins  10 mins                                        
______________________________________                                    
1                  0.0      0.0                                           
2                  17.0    19.3                                           
3                  7.3     16.1                                           
4                  17.9    30.1                                           
5                  13.0    24.7                                           
6                  4.9      8.7                                           
7                  4.7     10.6                                           
8                  --      11.1                                           
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     (b) Continuous process 
     A continuous length of the microporous matrix was passed through a transfer roll system which impregnated the matrix with a solution of 0.5% by weight benzophenone in acetone. The matrix then passed continuously between two parallel water cooled medium pressure mercury vapor UV lamps (manufactured by Ultraviolet Products Ltd), which irradiated the film simultaneously on both sides. Each lamp and the matrix could be adjusted between 15 cm and 30 cm, the lamps producing a parallel beam approximately 10 cm wide. The speed with which the matrix passes through the transfer roll system and the UV lamps could be adjusted up to 5 meters.min -1 . After passing through the lamps, the matrix was collected on a spool. 
     Using this continuous irradiation procedure, lengths of the matrix were crosslinked under a variety of conditions set out in Table 3 below. Also shown in Table 3 are the gel contents obtained with each set of conditions, measured according to ASTM D2765-84: 
     
                       TABLE 3                                                     
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Film speed                                                                
         Lamp distance                                                    
                      Lamp power  Gel content                             
(m · min.sup.-1)                                                 
         to film (cm) (W · cm.sup.-1)                            
                                  (%)                                     
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1.2      30           120         38.4                                    
1.2      30           80          30.2                                    
1.2      30           60          22.2                                    
2.0      30           60          23.1                                    
2.0        22.5       60          32.8                                    
2.0      19           60          47.5                                    
1.0      15           120         74.8                                    
1.6      15           120         66.4                                    
2.0      15           120         72.3                                    
2.0      15           80          63.6                                    
2.0      15           60          60.1                                    
3.0      15           60          29.1                                    
4.0      15           60          22.1                                    
5.0      15           60          16.7                                    
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     (c) Effect of Crosslinking 
     The physical properties of the polyethylene microporous matrix were compared before and after crosslinking. The results of the comparison are set out below in table 4: 
     
                       TABLE 4                                                     
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                Uncrosslinked                                             
                          Crosslinked                                     
______________________________________                                    
Gel Content       0           63                                          
Thickness (μm) 41.5        40.5                                        
Air Flow          48          44                                          
(cm.sup.3 · min.sup.-1 · cm.sup.-2)                     
Porosity (%)      51.8        50.7                                        
Transverse Tensile                                                        
                  210 ± 12 248 ± 3                                  
Strength (N · m.sup.-1 at 24° C.)                         
Transverse Elongation (%)                                                 
                  500 ± 60 115 ± 30                                 
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     Concentration of Photoinitiator 
     The optimum concentration of photoinitiator for crosslinking the material of the matrix was determined by preparing acetone solutions containing between 0.25% and 5.0% by weight benzophenone. These solutions were then used to prepare crosslinked matrices using the batch process described above. In this study, each side of each of the matrices was irradiated for five minutes. The gel contents of the resulting matrix&#39;s were measured and are set out in Table 5 below and in the accompanying FIG. 1: 
     
                       TABLE 5                                                     
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Benzophenone                                                              
Concentration (%)                                                         
                Gel Content (%)                                           
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0               0                                                         
0.1             16.6                                                      
0.25            37.6                                                      
0.50            53.3                                                      
0.75            55.9                                                      
1.0             52.9                                                      
1.5             54.5                                                      
2.0             45.9                                                      
3.0             29.2                                                      
5.0             19.1                                                      
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     Effect of Irradiation Time 
     The effect of irradiation time on the degree of crosslinking (as measure by gel content) was determined by preparing an acetone solution containing 0.5% by weight benzophenone. This solution was then used to prepare crosslinked matrices according to the procedure described above. Table 6 below and the accompanying FIG. 2 show the effect of irradiation time on the degree of crosslinking: 
     
                       TABLE 6                                                     
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Irradiation time                                                          
(minutes each side)                                                       
                Gel Content (%)                                           
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1                2.3                                                      
2               13.2                                                      
4               40.3                                                      
8               59.8                                                      
10              57.5                                                      
15              62.5                                                      
20              63.2                                                      
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     Production of Electrode Separator 
     (a) Two-stage process 
     An electrode separator was prepared from a microporous polyethylene matrix which had been crosslinked to a gel content of 58 to 63% using the continuous process described above (film fee 2.0 meters.min -1 ), distance from lamp to film 15 cm, lamp power 80 W cm -1 , distance from lamp to film 15 cm, lamp power 80 W.cm -1 . The matrix was impregnated continuously at a rate of 2 meters.min -1  on a transfer roll system with a solution having the following composition (all amounts percent by weight): 
     
         ______________________________________                                    
Acrylic acid            53.6%                                             
Divinyl benzene          4.4%                                             
(55% active-Aldrich)                                                      
Benzildimethylketal      2.2%                                             
(IRAGACURE photoinitiator 651 ™)                                       
Nonionic surfactant      6.0%                                             
(LUTENSOL ON 70 ™)                                                     
Potassium hydroxide pellets                                               
                        12.3%                                             
Water                   21.5%                                             
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     Once impregnated with this solution, the matrix was passed between two parallel water cooled medium pressure mercury vapor UV lamps to cause the acrylic acid within the pores of the matrix to polymerize and to react with the divinyl benzene. The distance between each of the lamps and the matrix was about 30 cm, and the power of each lamp was about 60 W.cm -1 . The matrix was then passed through a tank containing 5% by weight potassium hydroxide at 65° C. in order to convert the polymerized acrylic acid into its potassium salt form. Finally, the matrix was passed through a series of tanks containing deionized water, and dried over heated rollers at 80° C. 
     The electrode separator so produced was strong and flexible, and had a thickness of 40 μm. Table 7 compares the characteristics of the crosslinked separator with a separator prepared using a non-crosslinked matrix. These characteristics were determined according to the methods described in &#34;Characteristics of Separators for Alkaline Silver -Oxide Secondary Batteries&#34; AD 447301 US Air Force Manual: 
     
                       TABLE 7                                                     
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               Non-crosslinked                                            
                         Crosslinked                                      
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Gel Content (%)  0           58-63                                        
Thickness (μm)                                                         
                 41          40                                           
Moisture Content (%)                                                      
                 17.6        18.3                                         
Resistance       0.210       0.286                                        
(ohm · cm.sup.2 at 24° C.)                                
Swelling (%)                                                              
Length           -1.5        -1.7                                         
Width            +8.0        +9.8                                         
Thickness        +73.2       +56.1                                        
Volume           +84.2       +69.0                                        
Electrolyte      118.9       96.4                                         
Absorbtion (%)                                                            
Acrylic Acid Content                                                      
                 42.1        42.5                                         
(% dry H.sup.+  form)                                                     
Stability in 40% w/w                                                      
                 2.1         2.3                                          
KOH at 71° (% weight                                               
loss after 28 days)                                                       
Transverse Tensile                                                        
                 283 ± 20 272 ± 30                                  
Strength (N · m.sup.-1 at                                        
24° C.)                                                            
Transverse Elongation                                                     
                 520 ± 100                                             
                             209 ± 50                                  
(% at 24° C.)                                                      
Silver Barrier   5           3                                            
(layers penetrated)                                                       
Silver Barrier (Ag.sup.+  in                                              
                 27          11                                           
third layer)                                                              
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     The barrier characteristics to silver ions were determined in two ways as follows: five layers of the electrode separator, each 12 mm by 30 mm, were soaked with a non-woven polypropylene absorber (300 μm thick) of the same size in 40% by weight potassium hydroxide at 60° C. for 24 hours. The layers were then assembled in a laminated construction as shown in the accompanying drawing as a stack on a flat surface with polypropylene absorber at the bottom. A glass tube, 6 mm internal diameter, was clamped on top of the six layer stack, using a rubber O-ring to make a liquid tight seal between the top separator and the tube. An amount of 4 cc of 40% by weight potassium hydroxide was then supplied to the tube, which was then sealed and placed in an air oven at 60° C. for 2 hours. Silver oxide (Ag 2  O) was then added to cover the top surface of the stack completely. The tube was then resealed, and the assembly returned to the oven at 60° C. for 24 hours. The potassium hydroxide and excess solid were then carefully removed and the tube was disassembled. The five separators were then separated, washed with deionized water, and allowed to dry in air. Each layer was visually inspected to determine the number of separator layers which had been penetrated by silver (which appeared as brown coloration). A smaller number of penetrated layers indicated better barrier performance. 
     The barrier characteristics to silver ions were also determined by measuring the content of the ions of the third of the separator layers from the stack referred to above, using atomic absorption techniques. 
     (b) Continuous process 
     An electrode separator was produced in a continuous process using two pairs of ultraviolet lamps, a first pair to initiate crosslinking of the polyethylene of the matrix, and the second pair to initiate crosslinking and polymerization of acrylic acid impregnant. It has been found that the properties of electrode separators made using the batch and continuous process follow one another closely. 
     COMPARATIVE EXAMPLE 
     An electrode separator was produced using the microporous polyethylene matrix and the two stage process referred to above. The composition of the impregnant solution was as follows: 
     
         ______________________________________                                    
Acrylic Acid        50.8%                                                 
Divinylbenzene       8.3%                                                 
Benzildimethylketal  2.2%                                                 
Nonionic Surfactant 10.0%                                                 
Potassium hydroxide pellets                                               
                    11.7%                                                 
Water               17.0%                                                 
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     Compared with the products prepared by the two-stage process described above, the product prepared in this example differs in that the mole ration of divinyl benzene crosslinking agent to acrylic acid is approximately doubled; as a result, the crosslink density in the acrylic acid will also be increased significantly. 
     The electrode separator so produced had the following properties: 
     
                       TABLE 8                                                     
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Gel Content (%)      0                                                    
Thickness (micrometers)                                                   
                     45                                                   
Moisture content (%) 15.7                                                 
Resistance at 24° C. (ohm · cm.sup.2                      
                     0.410                                                
in 40% w/w KOH)                                                           
Swelling (%)                                                              
Length               -0.6                                                 
Width                +5.4                                                 
Thickness            +30.0                                                
Volume               +36.2                                                
Electrolyte Absorption (%)                                                
                     74.5                                                 
Acrylic Acid Content 32.3                                                 
(% dry H.sup.+  form)                                                     
Stability in 40% w/w KOH                                                  
                     1.4                                                  
at 71° C. (% weight loss                                           
after 28 days)                                                            
Silver Barrier (layers                                                    
                     4 to 5                                               
penetrated)                                                               
Silver Barrier (Ag.sup.+  in 3rd                                          
                     20                                                   
layer)                                                                    
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     From a comparison of the two sets of data in Table 7 with the date in Table 8, it can be seen that crosslinking of the polyethylene matrix has the advantageous combined result of providing a highly effective barrier to silver ions, comparable with that obtained in the comparative example in which the concentration of crosslinking agent (and therefore also the crosslink density) in the acrylic acid is doubled. However, in addition to this barrier property, the resistance to ionic conduction through the membrane by an ion exchange mechanism is maintained at a level significantly lower thorn is obtained by increasing the crosslink density in the acrylic acid. 
     Grafted Surface Layer 
     A length of microporous polyethylene film of thickness 30 μm was coated with a photoinitiator by passing the film through a solution consisting of 0.5% by weight benzophenone in acetone, and then allowing the solvent to evaporate. This sensitized film was then passed through the a solution of acrylic acid having the following composition, using a transfer roll system at a speed of 2 m.min -1  : 
     
         ______________________________________                                    
Acrylic acid                25%                                           
Potassium hydroxide pellets (85% assay)                                   
                          11.5%                                           
Copper sulphate (CuSO.sub.4.5H.sub.2 O)                                   
                           0.1%                                           
Non-ionic surfactant (LUTENSOL ON70 ™)                                 
                           3.0%                                           
Water                     60.4%                                           
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     In this solution, half of the acrylic acid monomer is converted potassium acrylate by the potassium hydroxide and the copper sulphate is present as a homopolymerization inhibitor. Other such homopolymerization inhibitors can be used, such as the iron (II) salts or the copper (II) salts, preferably iron (II) sulphate. 
     The grafting and crosslinking reactions were initiated simultaneously by passing the impregnated film between two mercury vapor lamps at a speed of 2m.min -1 . The distance between each lamp and the film was 15 cm, and the power of the lamps was varied to determine the effect on the graft level and the degree of crosslinking. The grafted film was then washed in dilute hydrochloric acid, and then deionized water, in order to remove traces of unreacted monomer, residual copper ions, etc, and then dried in air at 60° C. The gel content of the film was determined by refluxing in toluene whilst the acrylic acid content (that is, the graft level) was calculated by an ion exchange method using potassium hydroxide.. The results for each ultraviolet lamp power setting are set out below in Table 9: 
     
                       TABLE 9                                                     
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Power (W · cm.sup.-1)                                            
                  % Gel   % Graft                                         
______________________________________                                    
60                51.8    0.7                                             
80                62.4    1.7                                             
120               78.9    4.0                                             
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     An electrode separator was prepared from a polyethylene film of thickness 30 μm, to which a quantity of acrylic acid had been co-polymerized in the manner described above. This film was impregnated continuously at a speed of 2m.min -1  on a transfer roll system with a solution of the following composition: 
     
         ______________________________________                                    
Acrylic acid              53.6%                                           
Divinyl benzene            4.4%                                           
Benzildimethylketal (IRAGACURE 651 ™)                                  
                           2.2%                                           
Non-ionic surfactant (LUTENSOL ON70 ™)                                 
                           6.0%                                           
Potassium hydroxide pellets                                               
                          12.3%                                           
Water                     21.5%                                           
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     The impregnated film was then passed between two mercury vapor lamps positioned 30 cm from the film, whose power output had been set at 60 W.cm -1 . After the curing step, the film was washed in 5% potassium hydroxide at 65° C., and then washed again in deionized water. The film was then dried over heated rollers at 80° C. A length of polyethylene film thickness 30 μm which had not been crosslinked or subjected to the grafting treatment was also converted into an electrode separator membrane in the manner described above. 
     The characteristics of these two types of separator are set out in Table 10 below: 
     
                       TABLE 10                                                    
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                 Ungrafted                                                
                         Grafted                                          
______________________________________                                    
Gel content (%)    0         78.9                                         
Graft level (%)    0         4.0                                          
Thickness (μm)  28.0      28.5                                         
Resistance (ohm · cm.sup.2)                                      
                   207       293                                          
Swelling (%)                                                              
Length             -1.5      -1.0                                         
Width              +5.0      +6.7                                         
Thickness          +82.1     +58.9                                        
Volume             +88.4     +67.8                                        
Electrolyte absorption (%)                                                
                   110.0     90.6                                         
Acrylic acid content                                                      
                   37.1      34.3                                         
(% dry H.sup.+  form)                                                     
Transverse tensile 258       278                                          
strength (N · m.sup.-1 at 24° C.)                         
Transverse elongation                                                     
                   181       183                                          
(% at 24° C.)                                                      
Silver barrier (layers penetrated)                                        
                   7 to 8    4 to 5                                       
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     It is found that the separator which is provided with the layer of grafted polyacrylic acid is wetted by an aqueous electrolyte more quickly than the separator without that feature. Quicker wetting with electrolyte has the advantage that the characteristic internal resistance of a cell in which the separator is used is reached more quickly, allowing the cell to be tested more quickly prior to use.