CA1322425C - Microporous films - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The invention provides a method of making a polymeric film having a porosity of not less than 20% which comprises providing a film of a polymer composition, the composition comprising: (a) a halopolymer in which the repeat units are -(CnH2n)- and -(CmX2m)- where each X independently represents fluorine or chlorine and the values of n and m are greater than one or less than six; (b) an extractable salt and an extractable polymer which are substantially insoluble in the halopolymer; and subsequently extracting at least some of the extractable component so as to render the film porous.

Description

13~24~

The present invention relates to microporous polymer :Eilms .

In particular, the present invention relates to a method of making a polymeric film having a porosity of not less than 20~ and a polymer composition Eor use in the process.

This appllcation is a divisional application of copending Application No . 497, 808 filed December 19, 1985.

Microporous films are used in a wide range of appllcations, generally to provide a selective barrier. For example they may be used as battery separators, ion-exchange membranes, electrolysis membranes as well as in breathable 1~ fabrics and medical and packaging applications. Commonly used polymeric films comprise polyolefins such as polyethylene and polypropylene which can conveniently ba made porous by extraction of a soluble component. Such ~ilms are chemically inert towards many acids and alkalis and towards many reactive metals.
2U However, there exist a number of solvents with which a polyolefin film cannot be used because of chemical incompatibility.
Furthermore, the maximum operating temperature of polyolefin films is about 120C, and their use in h~gh temperature applications is therefore not possible.
2~
The present invention provides a microporous polymeric film which comprises a halopolymer. Such films can be suitable for use in high temperature applications and can be substantially inert to certain chemically aggre~sive sub~tances, for example 3~ alkal1 and alkaline earth me~als.
:::
In one aspect, the invention provides a microporous polymeric film comprising a halopolymer in which the repeat units are ~(CnH2n)~ and -(CmX2m)-, where each X independently 3 r represents fluorine or chlorine and the values of n and m are ...~.

"

~3224~

greater than one and l~ss than six, the film having a porosity of no-t less than 20~ by volume.

Preferably, the film comprises a copolymer, for example one which comprises ethylene and tetrafluoroethylene as the :l.u .

~: 3U
:

: :
~ 3~
': ~ , ~ - la -' ' ' :

~224~

monomer unit~, although chloroethylene~ and fluorochloro-ethylenes can however also be used as the monomer units. In another embodiment, the film may comprlse a copolymer that compriqes longer chain monomer unitq such a~ propylene, buty-lene and halogenated analogues thereof. Paricularly pre-ferred halopolymers for use in'the invention are those qold uncler the Trade Marlcs Tefzel (ethylene~tetrafluoroethylene) and l~alar (ethy]ene~chlorotrlfluoroethylene).
The term "film" is used to dcnote a non-~lbrous self-supporting sheet. A mlcroporous film is a porous film in which the details o~ pore configuratlon and/or arrangement are discernable only by microscopic examination. Preferably the pore~ or open cells in the film~ are smaller than those which can be seen u~ing an optlcal microsoope, when electron microscopy technique3 may be used to resolve detailq o~ the pore structure. Generally, the maximum dlmension of a substantial number of the pores wlll be less than 5 microme-ter~, preferably less than 2 micrometers, measured by mercury intrusion porosimetry acoordlng to ASTM D 2873-70.
The poroslty o~ the films may advantageously be not le~
than 30%, preferably 'not les~ than 40~ by volume, for example from 40 to 50~ by volume 9 measured by mercury intrusion poro-simetry, again according to ASTM D2873-70.
Microporous films of the halopolymers defined~above have chemlcal and physical propertie3 whlch are advantageous for use in a variety o~ high perfcrmance applicationq, such aq battery qeparator , lon-exchange membranes and electroly~l~
membranes, as well as for less demanding applications ~uch aA
in brsathable fabricst and in packaging and medical appllca-tion~.
A signlficant advantage of the microporous film of the invention, is that it can be used in high temperature appll-cations. For example a film formed from Tefzel may be used .

', .

32242~
- 3 ~

at temperatures up to at lea~qt about l75C without signifi-cant change in dimen~ionq or poroslty. The superior high temperature per~ormance of the ~ilm of the lnvention allows them to be u~ed in high temperature applications, for example, hlgn temperature eleotroohemlcal cells where pre-viously used mlcroporous ~llms cannot functlon.
Ir- accor~ance w1th the inventlon, fllm3 can be produced which are chemlcally inert towards reactive metal~ commonly u9ed a9 anodes in eleatrochemical cell~, 9uch a3 for example metals o~ Croups I and II of the periodic table. This pro-perty of the f`ilms is surprising in view of the reactivity, towards llthium and sodium (at least), of the well-known halogenated polymers polyvinylldene fluoride ~PVF2) and poly-tetrafluoroethylene (PTFE).
The fllms of the lnventlon can alqo be chemically inert towards many aggressive liquids found for example in electrochemical cells, electrolysls cells and in other appll-cation~. Thus, the preferred film~ of the invention are inert towards acids and alkali~, as well as towards reactive fluids ~uch as oxyhalides of elements o~ Group VA and Group VIA of the periodic table (as publiqhed in The Condensed Chemical Dictionary, 9th Edition, Van No~trand Reinhold, 1977), for example thionyl chloride, sulphuryl chloride and phosphoryl chloride. The films can therefore be used in many application~ where the u~e of cumbersome non-woven glass fibre mats ha~ previously been unavoidable, and a ~ignificant saving~in size and weight may also;thereby by obtained. An example of ~uch an applicatlon is as~a ~eparator in a lithiam/thionyl chloride ce~ Accordingly, ~n another aspeot, the invent~on provides an eleotrochem~cal cell in which~the separator~comprise~ a mi¢roporou~ film which compri3e~ a halopolymer i~n which the repeat~ unit~ are ~(Cn~ H2n)~ and -~(Cm X2m)-, where each X independently represent~ fluorine or ohlorine and the value~ of n and m are greater than one and le93 than six.

: :

~322~2~

In a further aspect, the invention provide3 a method of making a polymerlc ~ilm havlng a porosity of not les~ than 20~ which oompr1ses providing a fllm of a polymer compo~ition comprising:
~a) a halopolymer in which the` repeat unlt~ are ~(Cn H2n)- and ~(Cm X2~n)-, where eaoh X lndependently represent~ fluorine or chlorine and the value~ of n and m are greater than one and less than 9 i X;
tb) at lea~t one extractable component whlch is sub~tantially in~oluble in the halopolymer;
and subsequently extracting at least some of the extractable component ~o as to render the film mlcroporous.
The method enables mlcroporou~ fllms of halopolymers to be made conveniently. By careful selection of the extrac-table components, a poro~ity of not less than 30~ by volume, for example not less than 40~ can be achieved. Preferably the polymer composition comprlses an extractable ~alt and an extractable polymer. The extractable salt may be preqent in an amount of from 10 to 150 parts per 100 part~ of the halo-polymer, preferably from 50 to 120 parts, espe¢ially from 95 to 105 parts. The extractable polymer may be pre~ent in an amount of not more than 60 parts per 100 parts of the halopo-lymer, preferably from 5 to 30 parts, especially from 20 to 25 part~. The proportion~ o~ the component~ of the com-position may be varied, depending on uch ~actors a~ the desired poro~ity of the film, the deqired porosity profile through the thickneq~ of thQ film, the ~ize of the pore~, the nature and chemical oompatibilitieq of the co~ponents.
A high poro~ity can be achieved using a combinatlon of extractable polymer and salt, and although the rea~on~ for this are not fully understood, it would appear that the extractable polymer make~ ea~ier the extractlon o~ the salt `

~3224~

from the halopolymer matrix, poss1bly by acting as a wetting agent. Furthermore, as shown by scanning electron micrographs of a film produced by the preferred method, the presence of the extractable polymer appears to increase the qurface porosity of the film compared with the poroqity of film~ prepared by extraction of a salt alone, in which latter filmq the surface porosity limits performance significantly.
Preferably the method includes the step of deformlng the film so as to reduce lts th1cknes~ prlor to extractlon o~ the extractable component. The film may be deformed by up to 25~, up to 50~, up to 85~ or by more, depending on for example the dimens10ns of the film, the desired nature of the pores, the nature of the halopolymer and the extraotable com-ponent~. The deformation is preferably carried out using rollers, for example nip rollers ln line with an extrusion die, althou~h other technique~ inoluding stretching of the film may also be uqed. Deformation of the film can increase the efflclency of the extraction ~tep and can ~lso affect the nature of the pores; for example passing the film though nip rollers can affect the tortuosity of the pores.
In another aspect, the invention provides a polymer com-position which comprises:
a) a halopolymer in which the repeat units are -(Cn H2n)-and ~(Cm X2m)-, where each X independently repre~ents fluorine or chlorine and the values of n and m are greater than one and less than six;
~ ~ b) an extraotable salt;
;~ ~ c)~ an extractable polymer ~ ~
For~ome appllcations, it wil1 be~acceptable for some of the extractable polymer or the salt or both to remain in the .
halopolymer matrix after extraction~ ~eneralIy however, it is preferred that sub~tantially al~ of the polymer and salt :: :

~322~2~

are extracted from the matrix, sinoe the porosity is thereby maximised.
It is particularly preferred that the extractable polymer and the salt are selected to be soluble in one solvent.
This maltes more convenient the extractlon of polymer and salt since significantly fewer extractionq need be performed. For convenience, the polymer and salt will be selected to be soluble in an aqueous solvent, ~uch as water or an aqueou~
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, such as for example the electrolyte of an electrochemical cell.
The extractable polymer ls selected to have a solubillty 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 extrac-table 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 naturaIly occurring polymers~such as poly-saccharides may also be used aa the extractable polymer com~
pone~nt in certain applications.
ParticularLy preferred materials are ethylene oxide ~ .
polymers, such as that sold under the Trade Mark Polyox. The use of ethylene oxide polymers as the extractable polymer is advantageous since~ they are water soluble and melt-prooessable.
The extra'ctable salt ~hould be selected according to the end use of the porous f~lm, qince at least a small amount o~

, .

~32~2~

the salt i~ likely to remain in the film after extraction, and any remaining salt must be chemically compatible with other materials with which the film comeq into contact when in use. For example, if the film is to be used as a separa-tor in an electrochemical cell which has a reactive metal anode, the extractable salt should be electrochemically com-patible with other cell components. Thu~ the salt should be of a metal which is at least as electropo9itive as the metal of the anode; for example when the fllm 1s to be used a~ a separator in a llthium cell, the salt should be a lithlum salt; preferred lithium salts include the carbonate, chloride, phosphate and aluminate, although other lithium salts may be used including the nitrate, sulphate, trifluoro-methylsulphonate and tetrafluoroborate.
It will be under~tood that in some clrcumstances, it will be appropriate to add other components to the polymer compo~ition such as antioxidants, UV ~tabilisers, processing aids, dispersal aids, cross-linking agents, prorad~ and~or antirad~ and 90 on.
The components of the film may be blended using conven-tional polymer blending apparatus 3uch as a twin-~crew extruder or a two-roll mill. The fllm i~ preferably formed as a thin strip of ~heet, and it may be made in this form by a melt proce~ing technique, for example by extrusion 9 although blow and compres~ion moulding technlque~ are example~ of alternative techniques whioh may be u~ed. Melt processing technique~ are preferred :since~ they allow films to be made with con~i~tent propertie~ and permit the production o~ thin film~. Furthermore, melt procss~ing teohniques alIow a fllm to be made continuou~ly. The film may be extruded onto, or coextruded with, another component with whlch it will be in contact when in use. Once formed, the film may be cut into pieces of suitable ~ize,~ or it may b:e formed into a roll, for ease of transportatlon and storage.

:: :

132~2~

It is particularly surprising that the defined halopoly-mers can be blended with the preferred extractable polymers mentioned above using melt proce~sing technique~, in view of the wide dirference between the melting points of the two clas~es o~ materials. In 30me`case~, care may be nece~sary in selecting (luantities and mixing conditions to avoid degra-datlon of the extractable polymer and halopolymer.
For certain appllcations, lt may be advantageous to cross-link the polymer of the film. This may be effected by irradiation, for example by electrons, gamma or radiation, or by use o~ a chemical cross-linklng agent.
The chosen final thlckness of the film is dependent on the end use, and factors such as the desired strength, flexi~
bility, barrier properties and so on will generally have to be considered. The materials of the film may be produced to a thickne~s of les~ than 75 micrometre9, preferably by less than 50 micrometres. Most preferred films for use as a bat-tery separator have a thlckness of le~s than 35 micrometres, for example between 20 and 30 micrometres.
In yet another aspect, the lnvention provides a method of making an electrode separator suitable for use in an electrochemical cell which comprises a reactive metal anode, the method compris~ing extruding onto, or coextrudi~ng with, another~component of;the cell a film of a polymer composition which comprises a halopolymer in which the~repeat units are ~~Cn H2n~~ and ~~Cm~ Xzm)-a where~eaoh X lndependently repre-~ents fluorine or chlor~ine, and the values of n and m are greater than one and le~s than six. Preferably the extruded film w~ll contain an extractable oomponent which can be extracted.
The microporous film of the invention may be used to protect sensitive ma~terlal for greater convenience during handling. For example it may be used to protect sensitive : ~ :

.
:

.
, . . : , . , " " ' - ` ' ' :' , . . ,, ,. : -.

-- - ,` 1 3 2 ~

electrode materlal such as reactlve metal anodes, for example ll-tl-lum anodes, as dlsclosed In EP-A-143562.

The Inventlon wlll now be descrlbed wlth reference to examples, and to the accompanylng drawlngs In whlch:-Flgure 1 Is a grapl~ showlng how the ultImate elonga-tlon of certaln polymerlc materlals Is affected by exposure to a solu-tlon of LIAIC14 In SOC/2; and Flgure 2 shows a test cell as used to measure the elec-trlcal reslstance of a cell separator.

Exam~le l Non-porous samples of ethylene/tetrafluoroethylene copolymer (T~FZEL210), ethylene/chlorotrlfluoroethylene copolymer (H~AR300), polyvlnylldene fluorlde tKY~R 460 - Trade Mark) and polyethylene (G~F9606 Trade Mark~ were malntalned In a 1.9 M
solutlon of LIAIC14 In SOC12 under reflux, and the effect on the ultlmate elongatlon of the samples was monl~ored. The results are shown In Flgure 1, expressed In terms of the percentage of the Inltlal ultlmate elongatlon whlch was retalned after the exposures. The measurements were made at room temperature.

Samples of TEFZEL210 and H~AR300 were also malntalned In a 1.9 M solutlon of LIAICI~ In thlonyl chlorlde ~SOC12~ at room temperature. The ef~ect of the treatment on the mechanlcal prop-ertles of the samples Is shown In Table 1, the results belng expressed as In Flyure 1.
Table 1:
(a~ 8 we~ks at room temperature:

_ 9 _ . ..

'' '' ~ ' .

~322~2~

% Retalned % Retalned Tenslle Strength Ult. Elongatlon -I-lALAn300 75 ' :
; ~ - 9a ~
' ~3224~.~

(b) 32 hour~ under reflux at 850C:

gRetairled ~ Retained Tensile Strength Ult. Elongatlon Tefzel 210 90 77 ~lalar 300 61 57 ~ .
The stability of polytetrafluoroethylene (Zitex G110 -Trade Mark), Kynar 460, Hala~ 300 and Tefzel 210 towards lithium metal was evaluated at elevated temper~ture.
Samples of the four polymerio materlals were drled by placing them in a vacuum oven at 150QC for 8 hour~ The samples were then maintalned in contact under pressure with lithium in a dry envlronment at 850C for 16 days. The following results were obtained on inspectlon:
Tefzel 210: No change in the appearance of lithium or of Tefzel.
Halar 300: Lithium blackened over approximately 80~ of the surface whioh oontacted the polymer sampleO : ~
Kynar 460: Lithium blackened over approximately 95~ of the ~urface whlch contaoted the-polymer sample.
Zitex G110: Lithium blackened over the entire surface ~; whioh contacted the polymer sample, and bonded weakly to the sample.

.

::
.

' ' -` ~322~2~

Example 3 Ethylene/tetrafluoroethylene copolymer (Tefzel 210), lithium carbonate and polyethylene oxide (Polyox WSR 301 - Trade Mark) were compounded using a Baker Perkln3 twin screw extruder to glve a blend containing 100 parts Tefzel, 100 parts lithium carbonate and 22 parts Polyox. The compound was further extruded u~ing a Leiqtritz slngle ~crew extruder to produce a film of thiclcness 0.2 mm which wa~ rolled using roller~s at a temperature in the range 140-220C to produce film having a thickne3s of approximately 27 micrometre~.
This thinned film wa~ then treated with concentrated HCl at room temperature (c. 23C) to remove the lithium carbonate and Polyox leaving a microporous web of Tefzel. The excess acid and reaction products were removed by washlng with distilled water prior to drying of the film. The poroslty of the resulting film, determined acoord~ng to ASTM D2873-70, wa~ found to be 45~.
Example 4:
The electrical resistance of a porou~ Tefzel film, pre-pared by the method described in Example 3, waq measured in three different electrolytes u3ing a test cell a3 shown in Figure 2 of the accompanying drawlngs. As shown in Figure 2, the ¢ell compri~es a container 1 inoorporating ele¢trodes 2 of stainles~ steel connected to a C conductance bridge 3.
sample ~ to be te3ted is ~ealed into the te~t chamber 5 by sealing gaskets 6, and the neceQqary oonductive liquid is introduced via feed porS~ 7.
The result~ are giYen in Table 3:

' "

., , 132~2~

Table 3:

Electrolyte Resistance at 30~C
(ohm cm2) 1.9 Molar Li~lCll~ in SOCl2 4 l.0 Molar LiC104 in 50/50 7 Propylene carbonate 1,2-dlmethyoxyethane 30% ww K0~l in water 0.45 Example 5:
Tefzel 210 and lithium chloride in the ratio 40:60 were blended using a Leistritz twin screw extruder. The resulting blend was then re-extruded to produoe a web having a width 40 cm and thickness 0.5 mm. The thickness of the web was then reduced to 0.15 mm by passing it ~hrough the nips of a calen-dar roller stack. The web was 30aked in water to leach out the lithium chloride, and then dried thoroughly. The poro-sity of the film was found to be ll8~ by volume.
The film was used as a ~eparator in a t~est cell as described above, in whioh the electrolyte was 1.9 M LiAlC14 in SOCl2. The resistance of the film;was measured u~lng a conduotivity bridge, and found to bé 16i ohm cm2 at room tem-perature.

~, ; :' :
:

, ,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a polymeric film having a porosity of not less than 20% which comprises providing a film of a polymer composition, the composition comprising:

(a) a halopolymer in which the repeat units are -(CnH2n)- and -(CmX2m)- where each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six;

(b) an extractable salt and an extractable polymer which are substantially insoluble in the halopolymer; and subsequently extracting at least some of the extractable component so as to render the film porous.

2. A method as claimed in Claim 1, in which the extractable salt is present in an amount of from 10 to 150 parts by weight per 100 parts by weight of the halopolymer.

3. A method as claimed in Claim 2, in which the extractable polymer is present in an amount of not more than 60 parts by weight per 100 parts by weight of the halopolymer.

4. A method as claimed in Claim 3, in which the extractable polymer is a homo- or copolymer of an alkylene oxide.

5. A method as claimed in Claim 1, in which the extractable salt is a lithium salt.

6. A method as claimed in Claim 5, in which 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.

7. A method as claimed in Claim 1, in which the film is deformed so as to reduce its thickness prior to extraction of the extractable component.

8. A method as claimed in Claim 1, which includes the step of forming the film from the said polymer composition.

9. A polymer composition which comprises:

(a) a halopolymer in which the repeat units are -(CnH2n)- and are -(CmX2m)-, where each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six;

(b) an extractable salt; and (c) an extractable polymer.