CA2179369C

CA2179369C - Method of producing microporous thermoplastic resin membrane

Info

Publication number: CA2179369C
Application number: CA002179369A
Authority: CA
Inventors: Koichi Kono; Kotaro Takita
Original assignee: Tonen Chemical Corp
Current assignee: Toray Battery Separator Film Co Ltd
Priority date: 1995-06-19
Filing date: 1996-06-18
Publication date: 2003-09-16
Anticipated expiration: 2016-06-18
Also published as: JP3493079B2; CA2179369A1; DE69625413D1; KR100426092B1; EP0754488B1; US5853633A; EP0754488A1; KR970001425A; JPH093238A; DE69625413T2

Abstract

A method of producing a microporous thermoplastic resin membrane of excellent permeability having uniform and fine micropores without filling stress concentrators to the thermoplastic resin is provided.
Namely, there is provided a method of producing a microporous thermoplastic resin membrane, in which a thermoplastic resin foam incorporating fine cells having a ratio B/A between a cell size (A) and a wall thickness (B) of the cells of less than 0.5 and having a void ratio of not less than 50 % is subjected to plastic deformation for the boundaries of the cells themselves to such as extent exceeding the shape deformation of the cells, thereby breaking the boundaries of the cells.
Also there is provided a microporous thermoplastic resin membrane of excellent permeability having uniform and fine micropores produced by the method as described above.

Description

1'Irfl_,E OF'I'HE INVENTION
Method of Producing Microporous Thermoplastic Resira Membrane RAC..'KGROUNI:) OF 'THE INVENrI'ION
I . Field of tire Invention 'I'I~e present invention concerns a tnetlrod of producing a a~icroporous tlnerrnoplastic resin membrane and, more in particular, it relates to a method of producing a microporous membrane formed by hreaking cells of a predetermined shape formed in a molded product without filling stress concentrators.

2.. 'I'ecl~nology Review Microporous meml~raoes are widely used in variots applications such as battery separators, electrolytic capacitor separators. various filters, moisture-permeable waterproof clothes, electrolyte thin film, liquid crystal thin film, liquid separation membranes, and materials for gas separation membranes,and filters for medical purposes.
Methods of producing the microporous membranes inc(r.lde, for example, a rnixing/extraction process comprising the steps of mixing ','6042-3 a polyolefin with a pore-forming agent comprising fine powder of different polymers and inorganic material into a state of micro-dispersion and subsequently extracting the pore-forming agent, and a stretching t:~rocess comprising tine stelos of imparting strain such as stretching to a molded polyolefin article containing different solids in a state of r~~ic:ro-dispersion, to break the interfaces between the polyolefin and the different solids, thereby forming pores. I~owever, since stress concentrators such as different polymers and different solids are filled in the polyolefin in these processes, they involve problems in that the fillers may give undesired effects depending on the use of the resultant rt~icroporous rnernbrane. Furthermore, the fillers are dispersed inhornogeneously, resulting in non-uniform micropores.
SUMMARY OF THE INVENrhION
An object of the present invention is to provide a method of producing microporous thermoplastic membrane with excellent a resin permeabilityhaving uniform micropores, without filling stress concentrators in a thermoplastic resin.
Another object of the present invention is to provide a thermoplastic resin foam for producing a microporous membrane with ?6042-3 excellent permeability having uniform and microporous rnernbranes.
A further object of the present invention is to provide a microporous thermoplastic resin membrane having uniform and rnicroporous membranes and excellent permeability formed by the production method as described above.
In order to attain the foregoing objects, the present inventors et. al have made au earnest study and, as a result, have found that a rnicroporous tnernbrane with excellent permeability can be obtained without using stress concentrators, by forming fine cells of a predetermined shape in a thermoplastic resin molded product and tnreaking the interfaces of the cells.
Nar~~ely, the present invention provides a method of producing a tnicroporous thermoplastic resin membrane in which a thermoplastic resin foam containing fine cells having a ratio (B/A) between a cell size (A) and a wall thickness (B) of the cells of less than 0.5 acrd having a void ratio not less than about 50 % is subjected to plastic deformation to the boundaries of the cells themselves to such an extent as to exceed the shape deformation of the cells, thereby breaking the boundaries of the cells.
Also, the present invention provides a microporous thermoplastic resin rnernbrane with excellent permeability having uniform microhores, cv(rich is obtained by subjecting the porous thermoplastic resin foam hawing the specified cell structure as described above to plastic deformation of the boundaries of the cells themselves to such an extent as to exceed the shape deformation of the cells, thereby breaking the boundaries of the cells.
L)ETAILED DESCRII''TION OF THE PREFERRED EMBODIMENTS
The thermoplastic resin in the present invention includes polyolefins such as crystalline hornopolymers or copolymers formed by polymerizing, for example, ethylene, propylene, l-butene, 4-methylpentene and 1-hexerre, and a blend of them, vinyl chloride resin, vinyl acetate resin, polystyrene, fluororesin, polyamide resin, polyacetal resin, polycarbonate, thermoplastic polyimide, thermoplastic polyurethane, polyphenyler~e sulfide and polyvinyl alcohol. Among them, a polyolefin such as polyethylene or polypropylene is preferred in view of the moldability and the economical point. In particular, a polyolefin containing high molecular weight components is preferred, anti a high density polyethylene containing ~n ultrahigh molecular ?6042-3 weight high density polyethylene is particularly preferred in view of the strength and aimed micropores to be formed.
The thermoplastic resin foam in the present invention incorporates fine cells therein and the ratio B/A between the cell size (A) and the wall thickness (B) of the cells is less than 0.5, preferably less than 0.3, more preferably less than 0.2_5. I-~ur-ther, the void ratio of the foam is not less than about 50%, preferably, not less than about 70%, more preferably not less than about 75%. Since the foam of the present invention has fine cells and thin walls, it causes flex deformation to result in depression of fine cells when stress exerts to the foam so as to cause plastic deformation, and thin walls themselves cause plastic deformation when a further stress exerts, to form rnicropores. If the ratio B/A between the size of the cells (A) and the wall thickness (B) of the cell is not less than 0.5, tl~e cells are only deformed and enlarged, failing t.o obtain the aimed to microporous membrane of a fine structure. On the other hand, if the void ratio is less than about 50%, no satisfactory rnicroporous rnernbranes can be.obtained as well.
In this specification, "cell" means the minimum unit of the cell structure constituting a thermoplastic resin foam, which is a small individual space covered by walls.

The fine cells which constitute the thermoplastic resin foam of the present invention preferably have such a size as not lamer than the thickness of the foam and not greater tlrarr 1/10 thickness of the foam.
It is preferred that the diameter of the cell is from about 0.1 ,u m to 10 ~,c 117 arid the thickness of the cell wall is preferably about from about 0.01 ,u rn to 10 ,u m.
T'he thermoplastic resin foam having such fine cells can be formed by any of proclcrction methods, for example, by the following mc;thods:
~ 1 ) A method of for-ming a low density gel from the thermoplastic resin in the form of a solution, by temperature change or phase conversion due to contact with a poor solvent.
(2) A method of forming a foam by mixing a thermoplastic resin with an organic decomposable foaming agent and decomposing the organic c)ecomposable foaming agent by temperature change.
(3) A rnetlrod of physically introducing air, inert gas, low boiling point substance or the like to a thermoplastic resin in a molten state or in the form of a solution, growing the cells by pressure change upon extrusion of the resin and fixing them.
(4) A method of sintering the powder of a thermoplastic resin.
The condition for the. production of the foam is controlled in the production step such that the foam incorporates fine cells at a ratio B/A between the void size (A) and the wall thickness (B) of the cell and the cell ratio is not less than about 50 %.
Among the methods described above, it is preferred that the foam of the present invention is formed by using the method ( 1 ) or (2) since relatively fine cells can be formed stably.
According to the method ( l), a gel-like product is formed by dissolving under heating a thermoplastic resin in an organic solvent, extruding the solution from a die followed by cooling, or by conversion of phase due to contact with a poor solvent. 'hhe solvent is removed from the gel-like product to form fine cells. A method of producing from a composition containing an ultrahigh molecular weight polyolefin is particularly preferred in view of the strength of the microporous membrane obtained. The production process from the polyolefin composition containing the ultrahigh molecular weight polyolefin includes the following method.
The polyolefin is at first dissolved in a solvent under heating to prepare a solution. There is no particular restriction for the solvent so long as it can dissolve the polyolefin effectively. Examples of the solvent include aliphatic or cyclic hydrocarbons such as nonane, _7_ decane, undecane, dodecane and liquid paraffin or fractions of mineral oils haVIIlg bolllllg polrlts COr'1"eSpOr7drrlg to there. Non-volatile solvent such as liduid paraffin is desirable fc>r obtaining a stable gel-like product.
Heat dissolution of the polyolefin is carried out under stirring at a temperature at which it is completely dissolved in the solvent. The temperature varies depending on the polymer and the solvent used and it generally ranges from about 140°C to 250°C in the case of polyethylene c:ornposition. The concentration of the polyolefin solution is from 10 %
by weight to 50 % by weight, preferably from 10 % by weight to 40%
by weight. When the concentration is less than 10% by weight and most preferably form 15%~ to 35% by weight, a large amount of a solvent has to be used uneconomically, and swelling and neck-in are likely to take place at the exit of a die making it difficult to form a sheet in the sheet forming process. C)rr the other hand, if the concentration exceeds SO% by weight, it is difficult to prepare a homogeneous solution. It is desirable to add an antioxidant to the solution to protect the polyolefin from oxidative degradation ulaon dissolution under heating.
Subsequently, the polyolefir~ solution is extruded through the die and formed into a gel-like sheet by cooling. Usually, a sheet die having _g_ ,6042-3 a rectangular orifice is used, but a double-walled cylindrical hollow die, an inflation die or the like rnay also be used. When tile sheet die is used, a die gap is usually from 0.1 mm to S mm, and heated at 140°C -250°C
in the extrusion process. In this case, an extrusion speed is usually from 20 crn/min - 30 cm/min to 2 m/min - 3 rn/min.
A solution thus extruded through the die is formed into a gel-like product by cooling. Cooling is preferably conducted at least to a gelation temperature or lower at a speed of about 50°C/minute or more.
Generally, if the cooling speed is high, the crystallization degree of the resultant gel-like product is increased, and the size of psuede-cell units which form the structure is decreased. On the contrary, if tire cooling speed is low, rough cell units are formed. If the cooling speed is less than about SO°C/min, the degree of crystallization is increased, making it diffi<:ult to obtain a gel-like product suitable to stretching. As the cooling method, a method of bringing the solution into direct contact with cola blow, cooling water or like other cooling rnediurn, a method of bringing the solution into contact with a roll cooled by a cooling medium may be used. The solution extruded from the die may be drawn at a draw ratio of from I to 10, _g_ %6042-3 loreferably, from 1 to 5 before or during cooling. if the draw ratio is more than 10, neck-in is likely to take place, and the breakdown of the sheet upon stretching tends to occur undesirably.
rI'his method can produce a cell-enriched foam in which a resin forms sufficiently thin walls, the ratio B/A between the size of the cells (A) and the wall thickness (B) of the cell is less than 0.5 and the void ratio is not less than about SO ~o.
In the method (2) of using the foarnrng agent, a decomposable foaming agent is used. As tine example of the foaming agent, azodicarbonamide, metal azodicarboxylate, dinitrosopentamethylene tetramine, hydrazodicarbonamide, p-toluene sulfonyl semicarbazide, and s-trihydrazinotriazine can be mentioned.
The addition amount of the foaming agent is generally about from 0.1 parts by weight to 10 loarts by weight based on 100 parts by weight of the thermoplastic resin. A foaming aid or a cross-lit~kir~g aid for controlling the cell size can optionally be added in order to control the decomposing behavior of the foaming agent. The addition amount of the foaming aid or the cross-linking aid is generally about from 0.5 parts to 50 parts by weight based on 100 parts by weight of the thermoplastic resin.

?6042-3 The method of forrnir~g cells by adding the foaming agent to the thermoplastic resin includes, for exanople, a method of heating the thermoplastic resin under pressure irr a die to decompose the foarrning agent amt then reducing the pressure to expand the resin, a method of molding the thermoplastic resin in a die, taking it out and decomposing it by heating again to expand it, in addition to the method of conducting extrusion at a die temperature not lower than the decomposing temperature of the foaming agent. In order to maintain the shape of the cells in the thermoplastic resin foam, it is preferred that the resin is subjected to cross-linking. T'he method of cross-linking ir~clmles a rnetlrod of rrsirrg a chemical cross-linking agent such as an organic peroxide and a method of irradiating radiation rays such as electron rays.
rhlre cells obtained according to each of the methods described above can assume various forms depending on the forming methods and molding conditions, and may be any of a closed or opening type in which cells are independent of or connected with each other, or a mixture thereof, the closed type being particularly preferred.
The boundary of the cells may be in any of planar, columnar or fi brous shape, with the planar shape being particularly preferred.

"6042-3 Further, the microstructure constituting the boundary of the cells may be any of a high molecular lamella crystal or a stacked thereof growing one-dimensionally either in a fibrous or columnar shape, growing two-dirnensiorrally as a planar shape, or further growing tloree-dimensionally in a spherical shape.
In the present invention, it is necessary to break the boundary of the cells of the foam. The boundary of the cells themselves are plastically deformed by exerting a tensile stress or a compression stress and then exerting a tensile stress on the foam at such an extent as exceeding the shape deformation of the cells. Specifically, the foam is subjected to stretching or calendering, followed by predetermined stretching.
Orientation is accomplished by an ordinary method such as a tenter method, a roll method, an inflation method, a calendering method or a CoIIlblllatlor7 thereof at a pre<teterrnined factor.
In the method ( 1 ), stretching is carried out at least monoaxially, preferably, biaxially. Stretching may be applied either simultaneously or sequentially, with simultaneous biaxial stretching being particularly desirable. The stretching temperature is lower than the melting point of the thermoplastic resin plus 10°C or, preferably, ranges from the crystalline dispersion temperature to lower tlrao the c:.rystalline melting ?6042-3 point. In a case of polyethylene, for example, it is from 90 °C to 140°C, preferably, from 100 °C to 130°C. If the stretching temperature is higher than tire melting point plus 10°C, the molecular chain orientation by stretching does not take place because the resin n pelts. If the stretching temperature is lower than the crystal dispersion temperature, the rnernbrane tends to break during stretching on account of the insufficient softening of the resin, and cannot be stretched at a high draw ratio.
The draw ratio varies depending on the thickness of the original foam. The linear draw ratio in one axial direction is greater than twice, preferably :~ tunes to 30 times, and the area( draw ratio is greater than 10 tin nes, preferably 20 times to 400 times. With an area( draw ratio smaller than 10 times, the resulting rnicroporous rn~embrane lacks high modulus and high strength on accouW of insufficient stretching. On the other hand, with an area( draw ratio in excess of 400 times, difficulties exist in the stretching device and operation.
I-~ibuillation is taken place in the boundary of cells on account of plastic deformation by this stretching treatment, resulting in micropores having effective permeability.
The thus obtained product is washed with a solvent to remove "6042-3 tlrc residual resin solvent medium. 'I'lrc sc>Ivent usable for the washing may be a highly volatile solvent including hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as rnethylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as trifluoroethane, and ethers such as diethyl ether and dioxane. 'hhe solvents may be used individually or in conobination, being selected depending on the type of the solvent used to dissolve the polyolefin. 'hhe washing method includes an extraction method by immersing the product with a solvent for extraction, a method of spraying a solvent, or a combination thereof.
In the method (2), a rnulti-stage stretching method is preferred in which stretching is carried out at high temperature after cold stretching.
The cold stretching is preferably carried out at a temper°ature of -100°C
or lower at a draw ratio of 1.1 tunes to 2 tunes, which causes breakage along inner voids such as crazing or shearing bands. Liquid nitrogen or the like can be used as the cooling medium.
Subsequently, the product is preferably subjected to a heat treatment at a temperature ranging from a crystal relaxation temperature (90°C in the case of polyethylene) to the melting point minus 10°C, then stretched at a draw ratio of from 1.5 times to 5 times at the same ?6042-3 temperature range, and further, subjected to heat treatment at the same temperature range.
Expansion of microporous breakage formed by the cold stretching is taken place by the stretching treatment, wlriclr can forth rnicropores having effective permeability.
The thickness of thus obtained microporous membrane of the present invention is from 2 icc m to 200 icc m, preferably, from 5 ~ rn to ~0 ,u m.
( 1='unction) In the present invention, a molded product having fine cells in a thermoplastic resin is formed, and subjected to plastic deformation for the boundaries of the cells themselves at such an extent as exceeding the shape deformation of the cells to break the boundaries of the cells thereby producing a microporous membrane. The rnicroporous membrane thus formed has excellent permeability and appropriate strength without containing stress concentrators.
'the reason why such an effect can be obtained is not always apparent, but it is supposed deformation stresses tend to concentrate on the boundaries of the cells by using cells as a stxrtrng structure. tending to cause finer fibrillaltion thereby forming rnicropores.

'.'.6042-3 EX A M PI_,ES
'The present invention will be explained by the following examples.
The test methods used in the examples are as follows:
( 1 ) Membrane thickness: Measured for a cross section by a scanning electron microscope.
(2) Tensile break strength: Measured according to ASTM D882.
(3) Air permeability: Measured according to JIS P8I 1.7.
(4) Void ratio : Measured by gravimetric method (unit: %) (5) Porosity : Measured by gravimetric method (unit: %) Example I
A mixed liquid of a polyethylene composition was prepared by mixing a resin material (Mw/Mn=18.2) consisting of 3 parts by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 2.5 x 106 and 14 parts by weight of polyethylene (PE) havirng a weight average molecular weight (Mw) of 6.8 x 10~ with 83 parts by weight of liquid paraffin (Kinematic Viscositv;64 cSt/40° ). rI'hen, 0.375 parts by weight of an antioxidant was added to and mixed with 100 parts by weight of the mixed liquid. rI'he mixed lidui<.l was filled into an autoclave equipped with a stirrer and stirred for 90 min. at 200 °C to obtain a homogeneous solution.
'1'l~e solution was extruded from an extruder of 45 mm in diameter through a T-die at 200 °C, brought into contact with a cooling roll at "C to form a gel-like sheet of 1.8 mm thickness.
'1'lre gel-like sheet was washed again witlo p-xylene alter waslirrg wino t-hexane, and sub ,jected to a freeze-dry treatment. T'he cross section of the sheet was observed by a scanning electron microscope to find that open foams with polyethylene composition constituting the boundary of the cells were formed. The size of the cells (A) was 1.81 iec rn, the wall thickness (B) of the cell was 0.18 ~.e m, the ratio B/A was 0.099 and the void ratio was 83~~0.
When an ultra-thin slice of a sample embedded in an epoxy resin was observed by using a tratrsrnission electron microscope, it was found that the microstructure at the boundary of the cells comprised lamellas stacked in the direction of the thickness, which grew two dimensionally in a planar shape.
Subsequently, the gel-like sheet was subjected to biaxial stretching simultaneously at 5 times X 5 tunes at a temperature of 1 15°C and at a draw speed of 5 m/rnin. The resultant stretched membrane was washed with rnethylene chloride to extract arid remove residual liquid paraffin, and then dried and heat set to obtain a fine polyethylene porous membrane of 25 ,cc rn thickness. The fine polyethylene porous membrane had an air permeability of 570 sec/100 ml, a porosity of 45% and a tensile strength at break of 1 120 Kg/crn 2 . When the surface of the membrane was observed by using a scanning electron microscope, it was found that fine interconnected porous structure was formed on account of connection of the fine fibrous products.
Example 2 A 100 ic.c m-thick sheet was prepared by adding 5 parts by weight of azodicarbonamide (manufactured by Eiwa Chemical Co,.) as a foaming agent, 1.0 part by weight of trimethylol propane trimethacrylate as a cr-OSS-llllk111g aid, (manufactured by Shin Nakarnura Chemical Co,.) and 1.0 part by weight of an antioxidant based on 100 parts by weight of a resin component prepared by blending 90 parts by weight of a high density polyethylene (HDPE) having a density of 0.955 g/cm 3 and a rne(t index (MI, 190°C, under a load of 2.16 Kg) of 9g/IOrnin. and 10 parts by weight of polybutene-1 (PB-I) (M8340, manufactured by Mitsui 1'etrochernical Industries Ltd.) having a melt index (MI, 190°C, under load of 2.16 kg) of 4g/1 ()miry, mixing them for 2 miss. at _~0°C at rpro by using a Henschel mixer, then feeding there into an extruder of 50 ITyrTI ~ , length/diarneter ( (_./h) - 28 equipped with a rI'--die and extruding the sheet at an extrusion temperature of 150°C.
Then, the sheet was cross-linked by irradiation of electron rays of ~Sf) KV at a dose rate of BMrad. Then, the sheet was placed in an air oven at 250°C for 1.0 min to decompose the foaming agent and cause to (,low by about 5 times. rI'he sheet had an apparent density after blowing «f 0.19 g/cm 3 When the cross section of the sheet was observed by using a scanning electron microscope, it was found that closed foams in which the high molecular composition constituted boundary of the cells were formed. When an ultra-thin slice of a sample embedded iry an epoxy resin was observed by using a transmission electron microscope, it was found that the boundary of the cells cornprisf:d spherical crystals of microstructure. T'he sheet had a cell size (A) of 28.2 ,u m, the wa(1 thickness (B) of a cell of 3.31 ,u,rn, a ratio B/A of 0.1 17 and a void ratio of 80.1 %.
rI'hen, the sheet was subjected to a monoaxial stretching treatment at I.5 times in liquid nitrogen in order to form crazing in the cell wall, and subjected to a heat treatment again at 1 10°C for 1 S reins. Further, the stretched s)reet was srrhjectecl to rt~onoaxial stretching by twice, and sub-jected to heat treatment at an identical temperature for 1 ~ min. to obtain a microporous polyethylene memhr-a.ne of 1 SO ,u re thickness.
When the surface was observed by a scanning-type electron microscopes, it was found that a pore interconnected structure was formed on account of connection of fibrous products with each other.
The microporous polyethylene membrane had an air permeability c>f 970 sec/100 nU, a porosity of 79% and a tensile strength at break of 125 kg/cm 2 C'.omparative Example 1 A sheet-like product was prepared by feeding 60 parts by weight of lrigh density polyethylene (HL~PE) (density: 0.955 g/cc, MI: 0.1 g/10 rnin),40 parts by weight of calcium carbonate having an average grain ?6042-3 size of 0.0_5 ,u m, and 0.1 part by weight of a fluorosurfactant rllto an extruder of 50 m ~ in diameter and at L/D = 28g, equipped with a T-die by using a Henschel mixer, and extruding them at 200°C. When the product was observed by a scanning electron microscope, it was found that the dispersibility was poor. The sheet was sequentially sub-jected to a biaxial stretching treatment in the machine direction by three times at 110°C arid in transverse direction by twice at 120°C to foam a noicroporous rnernbrane with resultant pin holes. When the surface of the sheet-like product was observed by a scanning electron microscope, it was found that crack-like voids were formed irrhomogeneously at the interface with calcium carbonate which propagated locally in a large area to form pin holes.
C'.omparative Example 2 The sheet obtained in Comparative Example 1 was subjected to an acid-treatment to remove calcium carbonate. When the cross section of the sheet was observed by a scanning electron microscope, dispersibility of cells was poor, in which the cell size (A) was 0.05 ,u m, a wall thickness (B) the cells was 0.029 ,urn, a ratio B/A was 0.58 and a ?6042-3 void ratio was 40%. 1'he sheet-like product was subjected to seduentially biaxial stretching in machine direction by twice at 1 10°C, and in transverse direction by 1.5 times at 120°C to form a rnicroporous lnernbrane with resultant pin holes. When the, surface of the sheet was observed by using a scanning electron microscope, it was found that pores were increased inhomogeneously and large cracks were formed locally to form pin holes.
(Effect of the Invention) As described above specifically, the microporous membrane of the present invention is formed by breaking the boundaries of cell due to plastic deformation of fine cells without filling stress concentrators, in which fine cells are dispersed uniformly, arid which has excellent permeability and appropriate strength with no effects of fillers.
Accordingly the rnicroporous rnetnbrane is suitable to various appllcatlorl llSes sUCh aS battery separators, electrolytic capacitor separators, micro-filtration rnemlaranes, ultrafiltration rnernbranes, various filters, moisture-permeable and waterproof clothes.

%6042-3

Claims

1. A method of producing a microporous thermoplastic resin membrane, which comprises:
subjecting a thermoplastic resin foam containing microfine cells having a B/A ratio of a wall thickness (B) to a void size (A) of the cells of less than 0.5 and having a void ratio of not less than 50%, to plastic deformation of boundaries of the cells themselves to such an extent as to exceed shape deformation of the cells, thereby breaking the boundaries of the cells.

2. The method as defined in claim 1, wherein the B/A
ratio is less than 0.3 and the void ratio is not less than 70%.

3. The method as defined in claim 1 or 2, wherein the thermoplastic resin foam is obtained by cooling a solution of the thermoplastic resin into a low density gel.

4. The method as defined in claim 1 or 2, wherein the thermoplastic resin foam is obtained by mixing the thermoplastic resin with an organic decomposable foaming agent and then decomposing the organic decomposable foaming agent to blow the organic foaming agent by a temperature change.

5. The method as defined in any one of claims 1 to 4, wherein the thermoplastic resin foam is formed of high density polyethylene.

6. The method as defined in any one of claims 1 to 5, wherein the plastic deformation is carried out by stretching or calendering, followed by predetermined stretching.

7. A microporous thermoplastic resin membrane obtained by the production method of any one of claims 1 to 6.

8. A method for producing a microporous thermoplastic resin membrane having a thickness of from 2 µm to 200 µm, which comprises:
(I) providing a sheet of a thermoplastic resin foam having microfine cells having a ratio of a void size (A) to a wall thickness (B) of cells of less than 0.5 and a void ratio of not less than 50%, and (II) subjecting the sheet to plastic deformation of boundaries of the cells by exerting a tensile stress or a combination of a compression stress and a subsequent tensile stress on the sheet to such an extent as to exceed shape deformation of the cells, thereby breaking the boundaries of the cells.

9. A method as defined in claim 8, wherein the step (II) comprises stretching the sheet monoaxially or biaxially at a temperature in the range from the crystalline dispersion temperature to lower than the crystalline melting point plus 10°C of the thermoplastic resin at an areal draw ratio of 10 to 400 times.

10. A method as defined in claim 8, wherein the step (II) is conducted by a multi-stage stretching method which comprises:
a cold stretching of the sheet at a temperature of -100°C or lower at a draw ratio of 1.1 to 2 times, thereby causing breakage along inner voids, a heat treatment of the cold stretched sheet at a temperature from a crystal relaxation temperature to the melting point minus 10°C, a stretching of the heat treated sheet at a draw ratio of from 1.5 times to 5 times in the same temperature range as the heat treatment, and a further heat treatment of the stretched sheet in the same temperature range.

11. A method as defined in claim 8, 9 or 10, wherein the thermoplastic resin is polyethylene.