FIELD OF THE INVENTION

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.

BACKGROUND TO THE INVENTION

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 salt 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, poly- trifluoroethylene, 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 dyne/cm into a membrane having a three-dimensional network structure of intercommunica¬ ting 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 sec/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/chlorotrifluoro- ethylene copolymer (Halar) to the chemically adverse environment of a lithium battery is described in Patent Specification No. US-A-4405694 (Goebel et al) but only in the context of an insulating sleeve of non-porous material for a conductive jumper element.

Our Patent Specification No. EP-A-0188114 describes and claims a polymeric film which comprises a halopolymer in which the repeating units are ~( ^c _n ^H 2 _n ^~ and ~( ^c _m ^X 2 _m )~ ⁿ 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%.

Our co-pending British patent application No. 8813932.4 describes and claims the production of 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 was disclosed 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. The above application disclosed 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 comprised a copolymer, for example one which comprises ethylene and tetrafluoroethylene as the monomer units, although chloroethylenes and

fluorochloroethylenes were also used as the monomer units. In another form, the film comprised a copolymer that comprised longer chain monomer units such as propylene, butylene and halogenated analogues thereof. Particularly preferred halopolymers for use in the invention were those sold under the trade marks Tefzel and Halar.

The term "film" is used to denote a non-fibrous self- supporting 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 D- 2873-70. The porosimetry of the films may advantageously be above 55%, and preferably equal to or above about 60 to 70% measured by density.

A significant advantage of the films of British Patent No. 8813932.4 which flowed from the use of the extractable polymer was their high surface porosity. When highly filled polymers were melt processed, there was a tendency for the resultant product to have a polymer rich skin. For most conventional uses this was an advantage since it allowed 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 was a positive disadvantage. The surface skin

impedes access of the extracting liquid to the filler particles so that their rate of dissolution was reduced and complete dissolution might not have been possible. A further problem is that the membranes were inhomogeneous and their properties are determined by the relatively non-porous surface layer. These difficulties were reduced or overcome by the extractable polymer which during melt processing of the highly filled fluorocarbon polymer to form a film was incompatible with the halopolymer and migrated to the major surfaces of the film and prevented the formation of a skin of homogeneous halopolymer. When the extractable salt and polymer were removed by immersing the film in a solvent therefor e.g. an aqueous acid or alkali depending on the nature of the salt to be removed a highly porous surface was produced which communicated 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 were 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

2 the same conditions was 55-60 ohms cm .

The above British application further provided an electrochemical cell in which the separator comprises a polymeric film which is undrawn after formation of its pore structure and which comprises a halopolymer as aforesaid. The cell could comprise a container having two electrically isolated terminals, the container having therein an anode connected to one terminal, a cathode connected to the other terminal, a fluid electrolyte, an ionizable solute dissolved in the electrolyte and a separator positioned between and in contact with the anode and the cathode.

The above mentioned British application 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 additives, 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 additive to render the film porous and extracting at least some of said other additive to impart surface porosity to the film, wherein the resulting film has a porosity of more than 50% by volume.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative route to films of high porosity or larger pore size or a route to films of even higher porosity than those disclosed in British Patent Application No. 8813932.4.

It has now surprisingly been found that the molecular weight of the extractable polymer influences the mean

pore size, surface porosity and tortuosity factor (the ratio between pore length and membrane thickness) of the polymeric film produced and enables the production of films of novel structure.

The invention provides a method of making a polymeric film having a volume porosity of not less than 20% (and preferably not less than 50%) which comprises providing a film of a polymer composition, the composition comprising (a) a halopolymer in which the repeat units are -(Cn H2n)- and -(Cm X2m)- where each

X independently represents fluorine or chlorine and the values of n and m are greater than one ana less than six, and (b) at least one extractable salt and at least one extractable polymer which is substantiaaly insoluble in the halopolymer, and subsequently extracting at least some of the extractable component so as to render the film porous, wherein the extractable component is a material of molecular weight less than 1 million. The above molecular weight is a weight average molecular weight derived e.g. by rhe-ological measurements, as are other molecular weights quoted hereinafter.

The invention provides a polymeric film which comprises a halopolymer in which the repeating units are -(CnH_2n)- 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 being the result of firstly melt processing a halopolymer, an extractable salt and an extractable polymer and secondly extracting at least some of the extractable salt to render the film porous and extracting at least some of the polymer to impart surface porosity to the film wherein the extractable polymer is of molecular weight less than 1 million. The invention further provides an electrochemical

cell, particularly but not exclusively a cell having a lithium anode and a polarising electrolyte, having a separator which is a film as aforesaid.

The use of a lower molecular weight polyalkylene oxide halopolymer or copolymer which is solid at room temperature and is of molecular weight less than 1 million and particularly of molecular weight 100,000-500,000, especially about 300,000 as the extractable polymer in a melt processed mixture comprising more than 150 parts of extractable salt and not more than 80 parts by weight of extractable polymer per 100 parts by weight of the halopolymer provides for further novel film structures to be produced.

Thus the invention provides a polymeric film which comprises a halopolymer in which the repeating units are -(Cn, H2~n)- and -(Cm X~2m)- 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 not less than 30% by volume (preferably not less than 55% by volume and especially 60 - 70% by volume) and

(i) a mean pore size greater than 0.7 urn,

(preferably above 1.0 um and especially about

1.5 um); and/or

(ii) a surface porosity of more than 30%,

(preferably more than 35% and especially about 50%), and/or

(iii) a tortuosity factor of not more than 2.5, especially about 1.5.

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, 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 150 C 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 (PVF ₂ ) 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 when fabricated into 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 is microporous and comprises a halopolymer as aforesaid.

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 ~( ^c _n ^H 2 _n ^~ and ~(C ^x _2m )- 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, being incompatible therewith when both are molten and being a polyalkylene oxide of molecular weight less than 1 million;

(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 -(C H_ )- and ~( ^c _m ^X 2 _m ^~ ^{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 extractable polymer being a polyalkylene oxide of molecular weight less than 1 million.

The invention further provides an electrochemical cell in which the separator comprises a polymeric film which is undrawn after formation of its pore structure and which comprises a halopolymer in which the repeat ⁱ ng units are ~ ^(c _n ^H 2 ^~ ^and ""^ ^C m ^X ₂ m^~ ^{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 60% by volume (e.g. 70-80% 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 cm 3cm-2min-1 at 20 psi, and preferably at least 900 cm cm ² min

In a further aspect of the invention it has been found that certain highly filled compositions which contain a blend of polymer when extruded into tape and reduced in thickness by passage through pressure rollers located downstream of the extruder die are liable to breakage.

This problem has been solved, according to a further aspect of the invention, by the presence of an anti-oxidant in the mixture to be extruded.

DESCRIPTION OF PREFERRED FEATURES

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 compared to that in EP-A-0188114 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 nominal upper limit of particle size of more than 6 microns, typically to a nominal upper limit of 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 although from a

manufacturing standpoint 60 microns is preferred with particles of this size. 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-0188114, and the mean pore size has increased from 0.1 to 0.5 microns measured 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 advantageously be present in an amount of not more than 80 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. The molecular weight of the e.g. polyethylene oxide homopolymer may be in the range of from 20,000 -5 million and a material molecular weight about 4 million (Polyox WSR 301 - Trade Mark) has been used with satisfactory results. For the production of high porosity materials it may be desirable to use materials that are solid at room temperature but are of lower molecular weight e.g. Polyox WSRN .750 (molecular weight 300,000) and Polyox N 10 (molecular weight 100,000).

It has also been found advantageous to add a process aid or plasticiser to the composition in an amount of 1-3% by weight of the total weight of the formulation. 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 enhancers. 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 together with the finding that they are substantially completely removed from the film during the extraction of the salt and extractible polymer. Other process aids or platicisers that might be used include high temperature plasticisers, e.g. phosphate plasticisers, such as Reofos 95 (Ciba Geigy), or tritolyl phosphate.

In grades of the film that have levels of filler at least equal to those disclosed in our co-pending British patent application No. 8813932.4 it has been found desirable to incorporate an anti-oxidant in order to minimise breakage of the tape. These anti-oxidants could be of the hindered amine type but are preferably phenolic anti-oxidants, in particular hindered phenolic anti-oxidants. We have found that particularly satisfactory results are obtained with butylated hydroxy toluene (BHT) . The proportion of anti-oxidant required will vary dependant upon the precise nature of the anti-oxidant and the composition used, but we have found that amounts of at least 0.5% by weight are desirable, and particularly good results have been obtained when using 1-2% and especially about 2% by weight of butylated hydroxy toluene. Greater amounts may be added, but these increase the cost without any compensating production or performance advantage. The anti-oxidant used will, of course, preferably be selected so that it is readily

and substantially completely removed under the conditions of the subsequent extraction step.

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 150 micrometres, advantageously less than 75, and typically 50-70 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 Reference Example and Examples.

Reference Example

Ethylene/tetrafluoroethylene copolymer (Tefzel 210), lithium carbonate and polyethylene oxide (Polyox WSR 301 - Trade Mark) were very thoroughly compounded using a twin screw compounding 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

100-175°C to produce film having a thickness of approximately 50 micrometers. This thinned film was then treated with a 14% solution of HCl containing

Teepol as wetting agent at room

temperature (c. 23 C) 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 found as indicated in the attached Table. Airflow through the membrane was at a pressure difference of 20 psi.

Parts Parts Air Pore Size sz Pts by wght

Run um Salt TAIC PEO Flow* Min Nom Max

A 6 100 0 22 <1 0.109 0.128 0.294

B 6 167 5.8 22 2.8 0.109 0.128 0.264

C 6 167 6 33 5 0.112 0.180 0.475

D 15 167 6 33 13.3 0.243 0.416 0.950

E 15 200 6.7 33 22 0.271 0.465 0.822

F 25 167 6 33 16 0.300 0.473 1.133

G 25 200 6.7 33 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

- 22 - where k represents the specific conductivity of the electrolyte in :hms -1cm-1, Ro is the resistance in ohms of the separator in the electrolyte, A is the area of the separator in sq.cms., P is the porosity and L is the thickness of the membrane. The results show an increase in pore size and in surface porosity on reduction of the molecular weight of from 4 million (Polyox WSR 301) to 300,000 (Polyox WSRN 750). A marked increase in pore size, surface porosity, airflow and a marked decrease in tortuosity factor is noted where the molecular weight of the Polyox was reduced to about 100,000 (Polyox N 10). Since ionic conductivity of a separator membrane in a given electrolyte depends upon the porosity and pore structure of the membrane, these differences should result in improved electrical performance.

Example 2

Test cells of the same construction were fabricated using as separator the materials of samples 1.1 and

1.5 respectively, 1.8 M thionyl chloride as electrode, a porous carbon cathode and a lithium foil anode. The

2 two cells were discharged at a rate of 1 mA/cm and had operating voltages of 2.60 volts and 3.30 volts respectively. The higher operating voltage of the second cell is believed to have been the result of the increased porosity and more open pore structure of the cell of sample 1.5.

- 21 -

Example 1

The procedure of the Reference Example was repeated using the grades of polyethylene oxide (Polyox) and the amounts of lithium carbonate and Polyox per 100 parts of Tefzel indicated in the attached table. Formulation 1.1 in the table below contained no triallyl isocyanurate, formulation 1.2 contained 6 parts by weight per 100 parts by weight of Tefzel and formulations 1.3-1.5 contained 7 parts by weight. The wetting agent was altered from Teepol which is a phosphate based anionic wetting agent to Triton X which is a non-ionic wetting agent and is a modified form of polyethylene oxide. The extruded film was rolled to the indicated thicknesses and treated as described. Airflow through the resulting membrane was measured in litres per minute at a pressure difference across the membrane of 20 psi. Volume porosity was measured by density. Surface porosity was measured by taking scanning electron micrographs of the separator surface, enhancing the micrographs to clearly define the porous regions, measuring the area of the porous regions using an image analyser and calculating the percentage of the total area constituted by the area of the porous regions. A tortuosity factor was calculated by measuring the resistance of an electrolyte, measuring the porosity, thickness and area of a piece of the membrane used as separator in a test cell, and measuring the specific conductivity of the electrolyte and the resistance of the separator in the electrolyte. Tortuosity was then calculated according to the formula:

T 1/2

SAMPLE NO. PARTS PARTS GRADE OF THICKNESS MEAN PORE AIR FLOW VOLUME SURFACE TORTUOS

3 -2 -1 SALT PEO PEO {μn) SIZE pm CM CM MM POROSITY POROSITY FACTOR % %

1.1 100 22 301 40-50 0.09-0.15 50-100 50 20 4.0

.1.2 168 27 301 40-50 0.15-0.30 200-800 60 25 3.0

1.3 200 35 301 60 0.40-0.70 1000-1500 66 30 2.0

1.4 200 35 N750 60 0.70-1.0 1000-1500 66 35-50 2.0

1.5 200 35 N10 60 1.50-3.0 5500-12500 66 52 1.5

Example 3

The effect of anti-oxidant was investigated on a mixture for extrusion that had the following composition (parts by weight):

Tefzel 210 - 100

Lithium carbonate - 130

Polyox N750 - 17.5 Triallyl isocyanurate - (to give 2% by weight) A mixture of equal weights of the Tefzel (pellets) and lithium carbonate (powder, average size 25 microns) together with the Polyox and anti-oxidant (if present) were mixed together in a Papenmeier blade mixer at 1500 rpm for a period of 2 minutes at ambient temperatures. The resulting mixture was passed through a twin screw compounder at 250-260 C so that the Tefzel melted and the other ingredients became homogeneously dispersed therein. The product emerged as strands which were converted into chips. The resulting chips were mixed with further lithium carbonate powder to give a composition as indicated above using the same mixer and conditions as before. The two stage mixing assists dispersion of the relatively high proportion of lithium carbonate in the Tefzel and prevents agglomeration of the lithium carbonate particles. The resulting mixture was extruded at a die temperature of 265 C using a 32 mm single screw extruder having a 7 inch "coathanger" die to give a flat tape 7 inches wide and typically about

500 micrometers thick. The resulting tape was rolled by means of nip rollers maintained at 120 C. and having a pressure of 60 psi at a rotational speed such as to reduce the film to give a tape 110 micrometers thick . This tape was spooled onto a take-up device. In a separate operation the tape was then passed from the take up device through a bath of dilute

hydrochloric acid at ambient temperatures for a dwell time of 5 to 10 minutes, passed to a wash tank and dried.

The effect of varying the proportion of anti-oxidant on the length of tape that could be extruded without breakage was as follows:

Level of anti-oxidant Approximate Extrusion Length

0 130

1% 250

2% 500

I The above %ages for anti-oxidant arebased on the total material including filler.

Samples of the above three mixtures were stored for periods up to 480 hours at room temperature and the effect of the presence of anti-oxidant on melt flow time was measured. In the mixture containing anti-oxidant, the melt flow time increased progressively and rapidly with storage time, whereas this effect was not observed with the mixtures containing anti-oxidant, where the melt flow time was substantially level even on prolonged storage. It is therefore believed that the action of the anti-oxidant is to protect the Polyox during mixing and during the subsequent extrusion stages so that partly decomposed Polyox does not build up on the extrusion die and give rise to weakness in the resulting tape which is believed to be the main source of tape breakage. Although there was a slight decrease in pore size as anti-oxidant was added, the tape was nevertheless microporous and useful as a cell separator in lithium/thionylchloride cells.