GB2219589A

GB2219589A - Microporous halopolymer films

Info

Publication number: GB2219589A
Application number: GB8813932A
Authority: GB
Inventors: Mike George Land Dorling; David John Barker; Robert Hamilton Mcloughlin
Original assignee: Scimat Ltd
Current assignee: Scimat Ltd
Priority date: 1988-06-13
Filing date: 1988-06-13
Publication date: 1989-12-13
Also published as: GB8813932D0; IL90603A0; EP0419528A1; WO1989012659A1; JPH03504988A

Abstract

A microporous polymeric film of high porosity comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six. The film is the result of firstly melt processing a mixture of the halopolymer, more than 150 parts by weight of an extractable salt and not more than 80 parts by weight of an extractable polymer per 100 parts by weight of the halopolymer, the extractable polymer not being completely and homogeneously mixed with the halopolymer and being less viscous than the halopolymer when both are molten so that the surfaces of the film resulting from melt processing are rich in the extractible polymer, and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of said polymer to impart surface porosity to the film. The film has a porosity of more than 50 % by volume and more usually 60-70 %. The film may be used as the separator of an electrochemical cell e.g. a battery having a lithium anode and a thionyl chloride electrolyte. It has further been found that high porosities and good electrical properties can be obtained by using as extractable polymer a material having a molecular weight less than one million e.g. a polyethylene oxide of molecular weight about 100,000.

Description

MICROPOROUS FILMS PLUS PLASTICISER This invention relates to microporous polymer films, to methods for making them, to a polymer composition used in their manufacture and to an electrochemical cell in which they are used.

Patent Specification No. US-A-3859402 (Bintliff) describes the preparation of a thin microporous fluorocarbon polymer sheet material alleged to have a uniform microporosity which was useful in preparing electrodes capable of breathing oxygen from air.

Fluorocarbon polymer particles were mixed with particles of a metallic salt pore former, the resultant mixture was formed into a sheet material and the metallic pore former (which was e.g. calcium formate, sodium chloride or sodium carbonate) was removed e.g. by dipping the sheet into water. The polymer could be polytetrafluorethylene, polytrifluoroethylene, polyvinylfluoride, polyvinylidene fluoride, polytrifluorochloroethylene and copolymers thereof.

Patent Specification No. US-A-4613441 (Kohno et al, assigned to Asahi) describes a process for making a thermoplastic resin having a critical surface tension of not higher than 35 dyn/cm into a membrane having a three-dimensional network structure of intercommunicating pores. The network structure is contrasted with a through-pore structure in which pores extend substantially linearly through the membrane from the front surface to the back surface. The network structure including communicating pores has high porosity combined with long path length compared to a through-pore membrane of the same thickness and the actual pore diameter is much smaller than the diameter of the pores exposed on the surface.An initial porosity is formed in the membrane using finely divided silica which is dissolved in aqueous sodium hydroxide to give a structure having an average pore diameter of 0.05-1 micron and a porosity of 30-70%.

The membrane is then stretched by space drawing in at least one direction to enhance the porosity and at the same time improve mechanical strength. In one example an ethylene/tetrafluoroethylene copolymer (Tefzel 200) is formed into a porous membrane of thickness 75 microns, average pore diameter of 0.55 microns and porosity of 85% with an air permeability of 60 sex./100 cc 100 microns measured by method A of ASTM D-762. However, the above ASTM test is done using mercury porosimetry and does not give a true picture of the interconnection between the pores of the material which governs air flow through it.

The resistance of the ethylene/tetrafluoroethylene copolymer (Tefzel) and the ethylene/chlorotrifluoroethylene copolymer (Halar) to the chemically adverse environment of a lithium battery is described in Patent Specification No. US-A-4405694 (Goebel et al).

Our Patent Specification No. EP-A-0188114 describes and claims a polymeric film which comprises a halopolymer in which the repeating units are -(CnH2n)- and #(CmX2m )- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a porosity of at least 20% by volume. In an example, Tefzel was compounded with lithium carbonate and polyethylene oxide and extruded to give a film that after extraction of the extractable components had a porosity determined according to ASTM D2873-70 of 45%.

It is an object of the invention to provide films of higher levels of porosity than are reported in EP-A 0188114 and high fluid permeability without stretching after removal of the pore-forming material.

It has been found, according to one aspect of the invention that the presence in admixture with the aforesaid halopolymer of an extractable polymer that when molten is incompatible with the halopolymer and has a lower viscosity than the halopolymer enables higher proportions of extractable salt to be incorporated into the extruded film and higher porosities to be obtained.

Thus the invention: provides a polymeric film which comprises a halopolymer as aforesaid, characterised in that: (a) the film is the result of firstly melt processing a mixture of the halopolymer, more than 150 parts by weight of an extractable salt and not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer, said extractable polymer not mixing completely and homogeneously with the halopolymer and being less viscous than the halopolymer when both are molten so that the surfaces of the film resulting from melt processing are rich in the extractable polymer, and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of said polymer to impart surface porosity to the film; and (b) the film has a porosity of more than 50% by volume.

Preferably the film comprises a copolymer, for example one which comprises ethylene and tetrafluoroethylene as the monomer units, although chloroethylenes and fluorochloroethylenes can also be used as the monomer units. In another form, the film may comprise a copolymer that comprises longer chain monomer units such as propylene, butylene and halogenated analogues thereof. Particularly preferred halopolymers for use in the invention are those sold under the trade marks Tefzel and Halar.

The term "film" is used to denote a non-fibrous selfsupporting sheet. A microporous film is a porous film in which the details of pore configuration and/or arrangement are discernible only by microscopic examination. Preferably the pores or open cells in the films are smaller than those which can be seen using an optical microscope, when electron microscopy may be used to resolve details of the pore structure.

Generally the maximum dimension of a substantial number of the pores will be less than 5 micrometers, preferably less than 2 micrometers, measured by mercury instrusion porosimetry according to ASTM D2873-70.

The porosimetry of the films may advantageously be not less than 80% by volume, and preferably 85-90% by volume, measured by mercury intrusion porosimetry, again according to ASTM D-2873-70.

A significant advantage of the films of the invention which flows from the use of the extractable polymer is their high surface porosity. When highly filled polymers are melt processed, there is a tendency for the resultant product to have a polymer rich skin.

For most conventional uses this is an advantage since it allows less expensive polymer compositions to be used by the introduction of relatively coarse filler particles whilst retaining a smooth surface finish in the resulting moulded product. But in the use of highly filled compositions to make a microporous membrane by melt processing followed by removal of the filler, the surface skin of polymer is a positive disadvantage. The surface skin impedes access of the extracting liquid to the filler particles so that their rate of dissolution is reduced and complete dissolution may not be possible. A further problem is that the membranes are inhomogeneous and their properties are determined by the relatively non-porous surface layer.These difficulties are reduced or overcome by the extractable polymer which during melt processing of the highly filled fluorocarbon polymer to form a film is incompatible with the halopolymer and migrates to the major surfaces of the film and prevents the formation of a skin of homogeneous halopolymer. When the extractable salt and polymer are removed by immersing the film in a solvent therefor e.g. an aqueous acid or alkali a highly porous surface is produced which communicates the pore structure in the body of the film with the opposed faces thereof.

The nature of the pore structure at the major surfaces of the membranes of the invention is apparent from the accompanying Figures 1 and 2 which are micrographs of the major surfaces of otherwise similar films made with and without the presence of polyethylene oxide as extractable polymer. The film of Figure 1 is seen to have a large number of pores or voids 10 through its surface, whereas the film of Figure 2 has a lesser number of voids 10 and a large number of regions 12 that appear as shadows in the micrograph and are cavities beneath the surface of the membrane that have not developed into voids through it because they are closed by a thin skin layer of halopolymer.These differences in appearance correspond to performance differences, the membrane of Figure 1 having a resistivity of 12-15 ohms cm2, whereas that of the membrane of Figure 2 measured in the same cell under the same conditions was 55-60 ohms cm2.

Microporous films of the halopolymers defined above have chemical and physical properties which are advantageous for use in a variety of high performance applications, such as battery separators, ion-exchange membranes and electrolysis membranes, as well as for less demanding applications such as in breathable fabrics and in packaging and medical applications.

A significant advantage of the microporous film of the invention is that it can be used in high temperature applications. For example, a film of Tefzel may be used at temperatures up to at least about 1750C without significant change in dimensions or porosity.

The superior high temperature performance of the film of the invention allows it to be used in high temperature applications, for example in high temperature electrochemical cells where previously used microporous films cannot function.

In accordance with the invention, films can be produced that are chemically inert towards reactive metals commonly used as anodes in electrochemical cells, for example metals of Groups I and II of the Periodic Table. This property of the films is surprising in view of firstly the reactivity towards lithium and sodium (at least) of the well known halogenated polymers polyvinylidene fluoride (PVF2) and polytetrafluoroethylene (PTFE) and secondly the high surface area to bulk ratio of the film and consequent high proportion thereof available for contact with the lithium and electrolyte.

The films of the invention can also be chemically inert towards many aggressive liquids found, for example, in electrochemical cells, electrolysis cells and in other applications. Thus the preferred films of the invention are inert towards acids and alkalis as well as towards reactive fluids such as oxyhalides of elements of Group VA and Group VIA of the Periodic Table (as published in the Condensed Chemical Dictionary, 9th Edition, Van Norstrand Reinhold, 1977), for example thionyl chloride, sulphuryl chloride and phosphoryl chloride. The films can therefore be used in many applications where the use of relatively thick and weak non-woven glass fibre mats has previously been unavoidable. The films possess significant advantages when used as separators in fabricating cells. Despite their high porosity the films are suprisingly strong and easy to handle.An example of such an application is as a separator in a lithium/thionyl chloride cell where in a cell of a given standard size, longer lengths of coiled electrode material and separator can be fitted into the available internal dimensions of the cell, permitting lower current densities to be used for a given current output and higher material usage to be obtained.

Accordingly, in another aspect the invention provides an electrochemical cell in which the separator comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, characterised in that:: (a) the film is the result of firstly extruding a mixture of the halopolymer, more than 150 parts by weight of an extractable salt and not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer, said extractable polymer being incompatible with the halopolymer and being less viscous than the halopolymer when both are molten so that it has at least partly migrated to the surfaces of the film during extrusion, and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of said polymer to impart surface porosity to the film; and (b) the film has a porosity of more than 50% by volume.

The invention further provides an electrochemical cell in which the separator comprises an polymeric film which is undrawn after formation of its pore structure and which comprises a halopolymer in which the repeating units are -(CnH2n)- and #(CmX2m )- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a three-dimensional microporous structure including intercommunicating pores that give rise to a high level of tortuosity between the membrane surfaces, having a porosity of not less than 80% by volume (e.g. 85-90% by volume), a thickness of 15 to 200 microns and a highly porous surface such that the airflow through the membrane is at least 200 cm3cm 2min 1 at 20 psi, and preferably at least 900 cm3cm#2min#1.

In a further aspect the invention provides a method of making a polymeric film having a porosity of more than 20% by volume, which comprises: (a) mixing together a first component which is a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, more than 150 parts by weight per 100 parts by weight of the halopolymer of a second component which is an extractable salt and not more than 80 parts by weight per 100 parts by weight of the halopolymer of an extractable polymer, the extractable polymer being less viscous than the halopolymer and incompatible therewith when both are molten; (b) extruding the mixture to form a film in which the extractable polymer has migrated to the surfaces; and (c) extracting at least some of the extractable salt to convert the film into a three-dimensional network structure including communicating pores and extracting at least some of said polymer to increase the number of pores opening through the major surfaces of the film.

The method enables microporous films of halopolymers to be made conveniently. By careful selection of the extractable components, the porosities described above can be obtained.

The invention also provides a polymer composition for extrusion into a film as aforesaid, which comprises: (a) a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six; (b) more than 150 parts by weight of an extractable salt; and (c) not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer.

The invention yet further provides a method for manufacturing a porous film which comprises melt processing into film a mixture of a plastics material and at least two fillers, one of which is incorporated into the body of the film and the other of which migrates preferentially (but not necessarily completely) to the surface of the film, extracting at least some of said one filler to render the film porous and extracting at least some of said other filler to impart surface porosity to the film, wherein the resulting film has a porosity of more than 50% by volume.

The extractable salt may be present in an amount of from 150 to 300 parts per 100 parts by weight of the halopolymer, preferably from 150 to 200 parts. It should be selected according to the end use of the porous film, since at least a small amount of the salt is likely to remain in the film after extraction, and any remaining salt must be chemically compatible with other materials with which the film comes into contact when in use. For example, if the film is to be used as a separator in an electrochemical cell which has a reactive metal anode, the extractable salt should be electrochemically compatible with other cell components. Thus the salt should be of a metal which is at least as electropositive as the metal of the anode. For example, when the film is to be used as a separator in a lithium cell, the salt should be a lithium salt.Preferred lithium salts include in particular the carbonate which has a high decomposition temperature, can withstand the temperatures needed to process fluorocarbons, and is compatible with lithium battery systems, also the chloride, phosphate and aluminate, and less preferably the nitrate, sulphate, trifluoromethyl sulphonate and tetrafluoroborate. In the finished film the effect of the increased amount of the lithium carbonate is to increase the amount of air flow through the membrane (which correlates to separator conductivity). It has been found e.g. when using lithium carbonate that it is advantageous from the standpoint of the porosity of the eventual membrane to grind the lithium carbonate in a fluid energy mill or particle collider, and to grind to a maximum particle size of more than 6 microns, typically to a maximum particle size of 15 or 25 microns.These relatively large sizes still enable relatively high loadings of lithium carbonate to be achieved in the extruded film and enable a finished film of thickness about 50 microns to be produced. Surprisingly, particles of maximum size less than 25 microns can be incorporated into a 50 micron film. The increase in airflow is believed to be the result of an increase in the size of the interconnection holes. A ninefold increase in airflow through the membrane has been achieved in highly porous membranes according to the invention when compared to those of our earlier patent specification No. EP-A-0l88114, and the mean pore size has increased from 0.1 to 0.5 microns mesaured using a Coulter porometer.

It is particularly preferred that the extractable polymer and the salt are selected to be soluble in one solvent. This makes more convenient the extraction of polymer and salt and significantly fewer extractions need be performed. For convenience the polymer and salt will be selected to be soluble in an aqueous solvent such as water or an aqueous acid solution.

Other solvents may, however, be selected. In some applications, the extracting solvent may be a liquid with which the film comes into contact when in use, for example the electrolyte of an electrochemical cell.

The extractable polymer is incompatible with the fluorocarbon polymer (i.e. does not substantially mix therewith when both are molten) and has a lower viscosity when molten than the molten fluorocarbon polymer when both are at the same temperature. It may be present in an amount of not more than 60 parts by weight per part by weight of the halopolymer. It is selected to have a solubility in the extracting solvent that is significantly higher than the solubility of the halopolymer. When water or another aqueous based solvent is selected as the solvent, the extractable polymer may be selected from the following list (which is not exhaustive): alkylene oxide homo- and copolymers; vinyl alcohol homo- and copolymers; vinyl pyrrolidone homo- and copolymers; acrylic acid homo- and copolymers; methacrylic acid homo- and copolymers.

Certain naturally occurring polymers such as polysaccharides may also be used as the extractable polymer component in certain applications.

Particularly preferred materials are ethylene oxide polymers such as that sold under the trade name Polyox. The use of ethylene oxide polymers (PEO) as the extractable polymer is advantageous since they are water soluble and melt processable. It is, however, surprising that polyethylene oxide is not substantially degraded in the high temperature high shear conditions used to extrude ETFE. Degradation of PEO is accelerated in acidic media and trace amounts of HF are given off during extrusion of a fluorocarbon polymer such as Tefzel which would be expected to catalyse the degradation of the PEO. It is believed that the lithium carbonate used as extractable salt also functions as an acid acceptor for HF and thereby enables the PEO to survive long enough to pass through the extruder.

It has also been found advantageous to add a plasticiser to the composition in an amount of 1-5 parts, preferably 1.5-3 parts, per 100 parts by the fluoropolymer. The plasticisers that it has been found advantageous to use are triallyl cyanurate and triallyl isocyanurate which are more commonly used as radiation cross-linking enhancer. The effectiveness of these compounds as plasticisers under the severe processing conditions encountered in the melt processing stage of the film manufacture is a further surprising feature of the invention. Other platicisers that might be used include high temperature phosphate plasticisers such as Reofos 95 (Ciba Geigy), or tritolyl phosphate.

The components of the film may be blended using conventional polymer blending apparatus such as a twin screw extruder or a two-roll mill. The film is preferably formed as a thin strip or sheet, and it may be made in this form by a melt processing technique, for example by extrusion, although blow and compression moulding techniques are examples of alternative techniques that might be used. Melt processing techniques are desirable because they allow films to be made with consistent properties and permit the production of thin films. Furthermore melt processing techniques allow a film to be made continuously. The film may be extruded onto, or coextruded with, another component with which it will be in contact when in use. Once formed, the film may be cut into pieces of suitable size, or it may be formed into a roll for ease of transportation and storage.

The chosen final thickness of the film is dependent on the end use, and factors such as the desired strength, flexibility, barrier properties and so on will generally have to be considered. The materials of the film may be produced to a thickness of less than 75 micrometres, typically 35-60 micrometres.

The method may include the step of deforming the film so as to reduce its thickness prior to extraction of the extractible component. The film may be deformed by up to 25%, up to 50% or up to 80% or more, depending on, for example: the dimensions of the film, the desired nature of the pores, the nature of the halopolymer and the extractible components. The deformation is preferably carried out using rollers, for example nip rollers in line with an extrusion die, although other techniques including stretching of the film may be used. Deformation of the film can increase the efficiency of the extraction step and can also affect the nature of the pores. For example passing the film through nip rollers can affect the tortuosity of the pores.The benefit of deformation prior to extraction of the filler is that the unextracted filler increases the likelihood of local rupturing of the film between individual particles of filler so that when the filler is extracted inter-pore communication is increased. Stretching after the filler has been removed is less advantageous since it increases pore size but does not correspondingly increase pore interconnection.

The invention will now be further described in the accompanying Example.

Example Ethylene/tetrafluoroethylene copolymer (Tefzel 210), lithium carbonate and polyethylene oxide (Polyox WSR 301 - Trade Mark) were very thoroughly compounded using a twin screw extruder to give a homogeneous blend containing 45 parts Tefzel, and lithium carbonate and Polyox in the amounts indicated in the Table below. Where plasticiser is added to the above mixture, it is tumble blended until homogeneously mixed. The compound was further extruded using a single screw extruder to produce a film of thickness 0.1 mm which was rolled using rollers at a temperature in the range 100-1750C to produce film having a thickness of approximately 50 micrometres.This thinned film was then treated with a 14% solution of HC1 containing Tefzel as wetting agent at room temperature (c. 230C) to remove the lithium carbonate and Polyox leaving a microporous web of Tefzel. The excess acid and reaction products were removed by washing with distilled water prior to drying of the film. The porosity and pore size distribution of the resulting film, determined according to ASTM D2873-70, using a Coulter porimeter and was found as indicated in the attached Table. Airflow through the membrane was at a pressure difference of 20 psi.

Parts Parts Air Pore Size Run St ###t bAght PEO Flow* Min Nom Max A 6 45 0 10 < 1 0.109 0.128 0.294 B 6 75 2 10 2.8 0.109 0.128 0.264 C 6 75 2 15 5 0.112 0.180 0.475 D 15 75 2 15 13.3 0.243 0.416 0.950 E 15 90 2 15 22 0.271 0.465 0.822 F 25 75 2 15 16 0.300 0.473 1.133 G 25 90 2 15 19 0.262 0.454 1.191 Salt = lithium carbonate TAIC = triallyl isocyanate * air flow measured in litres per minute Sz = size of the salt particles

Claims

CLAIMS: 1. A polymeric film which comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, characterised in that:: (a) the film is the result of firstly melt processing a mixture of the halopolymer, more than 150 parts by weight of an extractable salt and not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer, said extractable polymer not being completely and homogenously mixed with the halopolymer and being less viscous than the halopolymer when both are molten so that the surfaces of the film resulting from melt processing are rich in the extractible polymer, and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of said polymer to impart surface porosity to the film; and (b) the film has a porosity of more than 50t by volume.
2. A film according to claim 1, wherein the respective monomer units are -(CnH2n)- and -(CmX2m)-.
3. A film according to claim 2, wherein the polymer comprises a copolymer in which the monomer units are -CH2CH2- and either -CF2CF2- or -CF2CFCl.
4. A film according to claim 1, 2 or 3 derived by extrusion of a mixture in which the extractable salt was a lithium salt.
5. A film according to claim 4, in which the extractable salt had an upper limit of particle size greater than six microns.
6. A film according to claim 5, in which the extractable salt had an upper limit of particle size of 15 microns.
7. A film according to claim 5, in which the extractable salt had an upper limit of particle size of 25 microns.
8. A film according to any of claims 4 to 7, in which the salt was lithium carbonate.
9. A film according to any preceding claim, in which the amount of extractable salt extruded with the polymer was from 150 to 200 parts by weight per 100 parts by weight of the halopolymer.
10. A film according to any preceding claim, in which the extractable polymer was a homopolymer or a copolymer of an alkylene oxide.
11. A film according to claim 10, in which the extractable polymer was a polyethylene oxide of molecular weight 50,000-5 million.
12. A film according to any preceding claim, which is the result of extruding a mixture further comprising triallyl cyanurate or triallyl isocyanurate.
13. A film according to claim 12, wherein the mixture extruded comprises 1.5-3 parts by weight of triallyl cyanurate or triallyl isocyanurate per 100 parts by weight of halopolymer.
14. A film according to any preceding claim, having a porosity of above 80%.
15. A film according to claim 14, having a porosity of 85-90%.
16. A film according to any preceding claim, having a thickness of about 50 microns.
17. A polymeric film which is undrawn after formation of its pore structure and which comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a three-dimensional microporous structure including intercommunicating pores that give rise to a high level of tortuosity between the membrane surfaces, having a porosity of not less than 80% by volume, a thickness of 15 to 200 microns and a highly porous surface such that the airflow through the membrane is at least 200 cm3cm-2min'l at 20 psi.
18. A polymeric film which comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a porosity of 85-90% by volume.
19. A method of making a polymeric film having a porosity of more than 20% by volume, which comprises: (a) mixing together a first component which is a halopolymer in which the repeating units are -(CnH2n) and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, more than 150 parts by weight per 100 parts by weight of the halopolymer of a second component which is an extractable salt and not more than 80 parts by weight per 100 parts by weight of the halopolymer of an extractable polymer, the extractable polymer being less viscous than the halopolymer and incompatible therewith when both are molten; (b) melt processing the mixture to form a film in which the extractable polymer has migrated to the surfaces; and (c) extracting at least some of the extractable salt to convert the film into a three-dimensional network structure including communicating pores and extracting at least some of said polymer to increase the number of pores opening through the major surfaces of the film.
20. A method according to claim 19, wherein at least some of the extractable polymer and at least some of the extractable salt are extracted from the polymer composition by means of a single solvent.
21. A method according to claim 19 or 20, in which the film is deformed to reduce its thickness before extraction of the extractable components.
22. A polymer composition which comprises: (a) a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six; (b) more than 150 parts by weight of an extractable salt; and (c) not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer.
23. A polymer composition according to claim 22, further comprising triallyl cyanurate or triallyl isocyanurate.
24. A polymer composition according to claim 23, comprising 1.5-3 parts by weight of triallyl cyanurate or triallyl isocyanurate per 100 parts by weight of halopolymer.
25. An electrochemical cell in which the separator comprises a microporous film, said film comprising a halopolymer in which the repeating units are -(CnH2n)- and -(C in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, the film having a porosity of 85-90% by volume.
26. An electrochemical cell in which the separator comprises a polymeric film which comprises a halopolymer in which the repeating units are -(CnH2n)- and -(CmX2m)- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, characterised in that: : (a) the film is the result of firstly extruding a mixture of the halopolymer, more than 150 parts by weight of an extractable salt and not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer, said extractable polymer being less viscous than the halopolymer and incompatible therewith when both are molten so that the surfaces of the film resulting from extrusion are rich in extractible polymer, and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of said polymer to impart surface porosity to the film; and (b) the film has a porosity of more than 50% by volume.
27. An electrochemical cell according to claim 26, in which the film has a porosity of more than 80% by volume.
28. An electrochemical cell according to claim 27 comprising a lithium anode and a thionyl chloride electrolyte.
29. A method for manufacturing a porous film which comprises melt processing film into a mixture of a plastics material and at least two fillers, one of which is incorporated into the body of the film and the other of which migrates preferentially to the surface of the film, extracting at least some of said one filler to render the film porous and extracting at least some of said other filler to impart surface porosity to the film, wherein the resulting film has a porosity of more than 50% by volume.