KR20160014616A - Multilayer, microporous polyolefin membrane, and production method thereof - Google Patents

Multilayer, microporous polyolefin membrane, and production method thereof Download PDF

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KR20160014616A
KR20160014616A KR1020157033887A KR20157033887A KR20160014616A KR 20160014616 A KR20160014616 A KR 20160014616A KR 1020157033887 A KR1020157033887 A KR 1020157033887A KR 20157033887 A KR20157033887 A KR 20157033887A KR 20160014616 A KR20160014616 A KR 20160014616A
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polyolefin
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polyethylene
microporous membrane
polyolefin resin
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KR1020157033887A
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Korean (ko)
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타케시 이시하라
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도레이 배터리 세퍼레이터 필름 주식회사
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Priority to JP2013115004 priority
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Priority to PCT/JP2014/064248 priority patent/WO2014192862A1/en
Publication of KR20160014616A publication Critical patent/KR20160014616A/en

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    • CCHEMISTRY; METALLURGY
    • C08ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08JWORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G
    • C08J9/00Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/26Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a solid phase from a macromolecular composition or article, e.g. leaching out
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B32LAYERED PRODUCTS
    • B32BLAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00Layered products comprising a layer of synthetic resin
    • B32B27/32Layered products comprising a layer of synthetic resin comprising polyolefins
    • CCHEMISTRY; METALLURGY
    • C08ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08JWORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G
    • C08J5/00Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/18Manufacture of films or sheets
    • CCHEMISTRY; METALLURGY
    • C08ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08LCOMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L23/00Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
    • C08L23/02Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
    • C08L23/04Homopolymers or copolymers of ethene
    • C08L23/06Polyethene
    • CCHEMISTRY; METALLURGY
    • C08ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08LCOMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L23/00Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
    • C08L23/02Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
    • C08L23/10Homopolymers or copolymers of propene
    • C08L23/12Polypropene
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2/00Constructional details or processes of manufacture of the non-active parts
    • H01M2/14Separators; Membranes; Diaphragms; Spacing elements
    • H01M2/145Manufacturing processes
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2/00Constructional details or processes of manufacture of the non-active parts
    • H01M2/14Separators; Membranes; Diaphragms; Spacing elements
    • H01M2/16Separators; Membranes; Diaphragms; Spacing elements characterised by the material
    • H01M2/164Separators; Membranes; Diaphragms; Spacing elements characterised by the material comprising non-fibrous material
    • H01M2/1653Organic non-fibrous material
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2/00Constructional details or processes of manufacture of the non-active parts
    • H01M2/20Current conducting connections for cells
    • H01M2/34Current conducting connections for cells with provision for preventing undesired use or discharge, e.g. complete cut of current
    • H01M2/348Current conducting connections for cells with provision for preventing undesired use or discharge, e.g. complete cut of current in response to temperature

Abstract

A polyolefin microporous membrane excellent in oxidation resistance, electrolyte liquidity and shutdown characteristics, and excellent in permeability and strength balance is provided. Poly comprising a polypropylene and a polyethylene, a liquid-state electrolyte is less than 20 seconds, and the shutdown temperature is not more than 132 ℃, a gas transmission rate of the film in terms of a thickness of 20㎛ 700sec / 100cm 3 or less, in terms of the layer thickness is 20㎛ A polyolefin microporous membrane having a piercing strength of 2,000 mN or more and a polypropylene distribution uniform in the in-plane direction.

Description

TECHNICAL FIELD [0001] The present invention relates to a microporous polyolefin microporous membrane and a method for producing the microporous membrane,

The present invention relates to a polyolefin microporous membrane and a method for producing the same, and more particularly to a polyolefin microporous membrane useful as a battery separator and a method for producing the same.

BACKGROUND ART Polyolefin microporous membranes are used for various purposes such as battery separators, diaphragms for electrolytic capacitors, various filters, breathable waterproofing medical treatment, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When the polyolefin microporous membrane is used as a separator for a battery, particularly as a separator for a lithium ion battery, its performance is closely related to battery characteristics, battery productivity and battery safety. Therefore, excellent permeability, mechanical properties, heat shrinkage resistance, shutdown characteristics, meltdown characteristics and the like are required. For example, when a separator for a battery having a low mechanical strength is used, the voltage of the battery may be lowered due to short-circuiting of the electrode. Further, it is known that the lithium ion battery is deteriorated in battery performance when it is continuously used while being charged in a state close to full charge, and oxidative deterioration of the separator is also one of the causes, so that improvement of the separator has been demanded.

Up to now, improvements in raw material composition, stretching conditions, heat treatment conditions and the like have been studied as methods for improving the physical properties of the polyolefin microporous membrane, and it has been proposed to mix polypropylene as a means for improving heat resistance (see, for example, 2002-105235, and JP-A-2003-183432). In particular, recently, properties related to battery productivity such as permeability, mechanical properties, heat shrinkability, and electrolyte liquor property (electrolyte pourability) and characteristics relating to battery life such as oxidation resistance have also been emphasized.

For example, in Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. 11-269290), microscopic irregularities are formed on the surface of a polyolefin microporous membrane by adding a specific amount of polypropylene to ultrahigh molecular weight polyethylene or a composition thereof Discloses a polyolefin microporous membrane which is excellent in permeability and mechanical strength and improved in moldability and improved permeability and retention of an electrolyte solution. Patent Document 2 (Japanese Unexamined Patent Application Publication No. 11-114484) discloses a polyolefin multi-layered microporous membrane suitable as a separator capable of achieving both oxidation resistance and cycle characteristics. The polyolefin multilayer microporous membrane comprises 5 to 50% by weight of a polypropylene component and 50 to 95% Wherein the polyethylene component comprises ultrahigh molecular weight polyethylene and the temperature difference between the melting point (Tme) of the polyethylene component and the melting point (Tmp) of the polypropylene component is -20 ° C <Tmp-Tme <23 ° C , And a bubble point of 400 to 600 kPa. In Patent Document 3 (Japanese Unexamined Patent Publication (Kokai) No. 2004-152614), when a polyolefin such as a specific polypropylene is added to polyethylene and blended to produce a film, when the polyolefin segregates on the surface and the content of polyethylene in the vicinity of the surface decreases And it is disclosed that the microporous membrane of such a surface can suppress the generation of gas and the lowering of the discharge capacity at the time of high temperature storage. The microporous membrane is a monolayer containing polyethylene in an amount of 50% by weight or more. The polypropylene has a polyethylene content in the vicinity of the surface of at least one surface of the film smaller than the average value of the entire film, a polypropylene having a viscosity average molecular weight of 200,000 or more, By weight of a low-molecular-weight polypropylene is 5 to 20% by weight of the entire film constituting material. Further, in Patent Document 4 (Japanese Patent Laid-Open Publication No. 06-062500), it has been reported that a sufficient safety function and strength are obtained even when the polyolefin microporous membrane and the polyethylene microporous membrane, which consist essentially of polyethylene and polypropylene, have. The polyolefin microporous membrane of Patent Document 4 is characterized in that strength and safety (hole closure temperature and rupture temperature) are secured by laminating a layer made of polypropylene and ultrahigh molecular weight polyethylene and a layer made of polyethylene. The combination of polypropylene and ultrahigh molecular weight polyethylene has heat resistance and strength to prevent the hole closure temperature from rising in the polyethylene layer. In Patent Document 5 (Japanese Unexamined Patent Publication (Kokai) No. 5-234578), a polymer having a specific molecular weight distribution and polypropylene having a weight average molecular weight in a specific range is used as a polymer component, and an inorganic fine powder, an organic liquid Is used as a raw material for producing a film, it is possible to provide a separator for a battery using an organic electrolytic solution, which is excellent in mechanical properties and safety even when the ratio of ultra-high molecular weight portion increases in the molecular weight distribution of polyethylene, I am proposing. The separator contains polyethylene having a weight average molecular weight of 1.0 x 10 &lt; 4 &gt; to 1.0 x 10 &lt; 6 &gt; and a polyethylene having a weight average molecular weight of 1.0 x 10 & Wherein the polyolefin microporous membrane has a thickness of 10 to 500 占 퐉 and a porosity of 40 to 85% by weight based on the total weight of polyethylene and polypropylene, and the polyolefin microporous membrane has a thickness of 10 to 500 占 퐉, , The maximum pore diameter is 0.05 to 5 占 퐉, and the difference between the filament temperature and the non-pore temperature (non-pore size) is 28 to 40 占 폚. Patent Document 6 (International Publication No. WO 2007/015416) discloses a polyolefin microporous membrane composed of polyethylene and polypropylene having a viscosity average molecular weight of 100,000 or more, which contains polypropylene in an amount of 4 wt% or more, and polyolefin microporous membrane The polyolefin microporous membrane is characterized in that the terminal vinyl group concentration per 10,000 carbon atoms in the polyolefin constituting the polyolefin microporous membrane is 2 or more. The polyolefin microporous film has both an inner film and a low heat shrinkability, has excellent fuse characteristics, and has a uniform film thickness. Patent Document 1: JP-A-11-269290 Patent Document 2: Japanese Laid-Open Patent Publication No. 2011-111484 Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-152614 Patent Document 4: Japanese Laid-Open Patent Publication No. 2011-063025 Patent Document 5: JP-A-5-234578 Patent Document 6: International Publication No. WO 2007/015416

In order to improve the oxidation resistance by mixing polypropylene, it is necessary to add a considerable amount of polypropylene. However, there is a disadvantage in that the permeability and strength balance of the polyethylene microporous membrane is impaired, in particular, the strength is lowered by increasing the content of polypropylene. Further, there is a problem that a sufficient shutdown temperature can not be obtained. Therefore, it is required to have excellent permeability and strength balance and shutdown characteristics of the polyethylene microporous membrane in order to ensure productivity, safety, and output characteristics of the battery while improving the oxidation resistance of the separator related to battery life have.

Accordingly, an object of the present invention is to provide a polyolefin microporous membrane excellent in oxidation resistance, electrolyte liquidity and shutdown characteristics, and excellent in permeability and strength balance.

In order to solve the above-mentioned problems, the polyolefin microporous membrane of the present invention has the following constitution. In other words,

As the polyolefin microporous membrane made of a first polyolefin resin, including polypropylene and polyethylene, the liquid-state electrolyte is less than 20 seconds, and the shutdown temperature is less than 132 ℃, a gas transmission rate of the film in terms of a thickness of 20㎛ 700sec / 100cm 3 , A puncture strength of not less than 2,000 mN in terms of a film thickness of 20 mu m, and a polypropylene distribution (hereinafter, referred to as a PP distribution) uniform in the in-plane direction.

The method for producing a polyolefin microporous membrane of the present invention has the following constitution. In other words,

(a) A process for preparing a polyolefin solution by melt kneading a polyolefin resin and a film forming solvent, wherein the polyolefin resin comprises an ultra-high molecular weight polyethylene having polyethylene as a main component and a weight average molecular weight of 1.0 x 10 6 or more, 130 ℃ or less processes, including polyethylene and having a weight average molecular weight of greater than less than 3.0 × 10 5 6.0 × 10 4 of polypropylene less than 5% by weight at least 0.5% by weight,

(b) a step of extruding the polyolefin solution at a shear rate of 60 / sec or more to form a formed body,

(c) cooling the obtained extrusion molded product at a cooling rate of 30 DEG C / sec or more to form a gel sheet,

(d) a step of stretching the obtained gel-like sheet in at least one axial direction to prepare a drawn product,

(e) removing the film-forming solvent from the obtained stretched product.

The polyolefin microporous membrane of the present invention is characterized in that the average value of the standardized polypropylene / polyethylene ratio (hereinafter, standardized PP / PE ratio) measured by Raman spectroscopy is 0.5 or more, the standard deviation of the normalized PP / PE ratio is 0.2 or less , And the kurtosis of the normalized PP / PE ratio is 1.0 or less -1.0 or more.

The polyolefin microporous membrane of the present invention preferably has a weight average molecular weight of 6.0 x 10 &lt; 4 &gt; and less than 3.0 x 10 &lt; 5 &gt;

In the polyolefin microporous membrane of the present invention, it is preferable that the first polyolefin resin contains 0.5 wt% or more and less than 5.0 wt% of polypropylene.

In the polyolefin microporous membrane of the present invention, it is preferable that the first polyolefin resin contains 1.0 wt% or more and 50.0 wt% or less of polyethylene having a weight average molecular weight of 1.0 x 10 6 or more.

The polyolefin microporous membrane of the present invention preferably comprises polyethylene having a melting point of 130 캜 or lower.

In the polyolefin microporous membrane of the present invention, it is preferable that the content of polyethylene having a melting point of 130 캜 or less is 10.0% by weight or more and 38.0% by weight or less of the first polyolefin resin.

The polyolefin microporous membrane of the present invention comprises a first microporous layer composed of three or more microporous layers and composed of a first polyolefin resin constituting at least one of the surface layers and a second microporous layer composed of a second polyolefin resin disposed between both surface layers It is preferable that either or both of the first microporous layer and the second microporous layer include polyethylene having a melting point of 130 캜 or lower.

The polyolefin microporous membrane of the present invention preferably has a content of polyethylene having a melting point of 130 캜 or lower of 10.0% by weight or more and 38.0% by weight or less of the first polyolefin resin or the second polyolefin resin.

The polyolefin microporous membrane of the present invention is preferably such that the second polyolefin resin contains 1.0 to 50.0 wt% of polyethylene having a weight average molecular weight of 1.0 x 10 6 or more and does not contain polypropylene.

In the polyolefin microporous membrane of the present invention, it is preferable that the sum T (A) of the thicknesses of both surface layers and the sum T (B) of the thicknesses of the respective layers disposed between both surface layers satisfy the following formula (1).

<Formula 1>

60? T (A) / (T (A) + T (B)) 100

The polyolefin microporous membrane of the present invention preferably has a shutdown temperature of 128 DEG C or lower.

The microporous polyolefin membrane of the present invention is comprising a polypropylene and polyethylene, and the main liquid electrolyte is 20 seconds or less, and the shutdown temperature is less than 132 ℃, the air permeability of the film in terms of thickness to 20㎛ 700sec / 100cm 3 or less, and , A piercing strength of 20 m or more in terms of a membrane thickness of 2,000 mN or more, and a PP distribution is uniform in the in-plane direction, thereby exhibiting excellent oxidation resistance, electrolyte liquidity and shutdown characteristics, and excellent permeability and strength balance.

When the polyolefin microporous membrane of the present invention is used as a separator for a battery, since the polypropylene contributing to oxidation resistance is not present on the surface in contact with the electrode, partial deterioration of the separator caused during charging and discharging of the battery can be suppressed The battery can be made longevity. Further, when the shutdown temperature is lower, the battery reaction can be safely stopped at the time of an abnormal reaction.

Further, the polyolefin microporous membrane of the present invention has excellent air permeability / strength balance, exhibits an electrolyte liquidity equivalent to that of the polyethylene microporous membrane, and exhibits a uniform film thickness distribution. Thus, when the polyolefin microporous membrane of the present invention is used as a battery separator, the productivity of the battery is improved, and the battery can be longevity due to excellent oxidation resistance.

Further, according to the process for producing a polyolefin microporous membrane of the present invention, the polyolefin microporous membrane of the present invention having the above-mentioned characteristics can be efficiently produced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the distribution of normalized PP / PE ratio of the first microporous layer of the polyolefin microporous membrane of the present invention (Example 2). FIG.
2 is a graph showing a two-dimensional distribution diagram of the normalized PP / PE ratio of the first microporous layer of the polyolefin microporous membrane of the present invention (Example 2).
3 is a graph showing a two-dimensional distribution of the normalized PP / PE ratio of the first microporous layer of the polyolefin microporous membrane (Comparative Example 2).

Hereinafter, embodiments for carrying out the present invention will be described in detail. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

The polyolefin microporous membrane of the present invention may be a single layer or a multilayer of two or more layers. Among them, a multilayer microporous membrane composed of three layers of microporous layers is preferable. The polyolefin microporous membrane of the present invention has at least one first microporous layer. In the polyolefin microporous membrane of the present invention, the first microporous layer is composed of polyolefin resin (first polyolefin resin) containing polyethylene as a main component and containing polypropylene. The first microporous layer is at least one surface layer of the polyolefin microporous membrane of the present invention. The layer other than the first microporous layer may be a second microporous layer composed of the second polyolefin resin. In the case where the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous layers, a three-layer structure in which both the surface layer (skin layer) is a first microporous layer and the second microporous layer is disposed between both surface layers .

Hereinafter, the polyolefin resin used in the polyolefin microporous membrane of the present invention will be described.

[1] raw materials

[Polyolefin Resin]

The polyolefin resin constituting the polyolefin microporous membrane of the present invention contains polyethylene (PE) as a main component, the total amount of the polyolefin resin is 100% by weight, the proportion of the polyethylene is preferably 80% by weight or more, more preferably 90% %, Even more preferably at least 95%. The polyolefin resin may be a composition comprising a resin other than the polyolefin. Accordingly, the term &quot; polyolefin resin &quot; may include not only polyolefin but also resins other than polyolefin.

The polyolefin resin is composed of a first polyolefin resin when the polyolefin microporous membrane of the present invention is a single layer microporous membrane.

On the other hand, in the case of the polyolefin resin, when the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous membranes, the first polyolefin resin constituting the first microporous layer and the second polyolefin resin constituting the second microporous layer .

[First Polyolefin Resin]

In the polyolefin microporous membrane of the present invention, the first microporous layer is composed of the first polyolefin resin. The first polyolefin resin includes polypropylene in addition to polyethylene. Each component will be described in detail below.

Polyethylene

(A) polyethylene (hereinafter referred to as "PE (A)") having a Mw (weight average molecular weight) of less than 1.0 × 10 6 , or (b) PE (A) and ultra high molecular weight polyethylene having an Mw of 1.0 × 10 6 or more (UHMwPE ) (Hereinafter referred to as &quot; PE composition (B) &quot;).

The ratio Mw / Mn (molecular weight distribution) of the Mw and the number average molecular weight (Mn) of the PE (A) and the PE composition (B) is not limited, but is preferably within a range of 5 to 300, More preferably in the range of 5 to 25, When the Mw / Mn is in the above-mentioned preferable range, the extrusion of the polyethylene solution is easy, and the polyolefin multilayer microporous membrane obtained is also excellent in strength.

PE (A)

The PE (A) may be any one of high density polyethylene (HDPE), medium density polyethylene (MDPE) and low density polyethylene (LDPE), but HDPE is preferred. PE (A) may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other? -Olefin. Examples of the? -Olefins other than ethylene include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate and styrene.

PE (A) is, for example, about 2.0 × 10 5 ~ about 0.9 × 10 6 of, 1.0 × 10 6 less than the weight-average molecular weight (Mw), a molecular weight distribution in the range of about 2.0 ~ 50.0 (MWD, Mw ranging from Is defined as a value divided by the number average molecular weight Mn), and polyethylene having less than 0.20 terminal unsaturated groups per 10,000 carbon atoms. The Mw of the PE (A) is preferably 1.0 x 10 4 or more and less than 5.0 x 10 5 . Among them, the Mw of HDPE is more preferably 5.0 x 10 4 or more and less than 4.0 x 10 5 . The PE (A) may be made of Mw or two or more species having different densities. Optionally, the PE (A) has an end unsaturation within the range of 0.14 or less, or 0.12 or less, for example, 0.05 to 0.14 (e.g., below the measurement limit) per 10,000 carbon atoms.

The melting point of PE (A) preferably exceeds 130 캜.

PE composition (B)

When the polyethylene is the PE composition (B), the upper limit of the PE (A) is preferably 98.5% by weight, more preferably 94.0% by weight, based on 100% by weight of the total weight of the first polyolefin resin. The lower limit of the PE (A) is preferably 45.0 wt%, more preferably 46.5 wt%.

The content of UHMwPE is preferably 50.0% by weight or less based on 100% by weight of the total weight of the first polyolefin resin. Particularly preferably 45.0% by weight or less. When the content is within the above-mentioned preferable range, the pressure is not increased during molding and the productivity is good. The lower limit of the content is not particularly limited, but is preferably 1.0% by weight, particularly preferably 30.0% by weight, from the viewpoints of maintaining mechanical strength and maintaining a high melt-down temperature. By setting the UHMwPE to 1 wt% or more and 50.0 wt% or less, a polyolefin microporous membrane having an excellent balance of strength and permeability can be obtained.

The Mw of UHMwPE is preferably in the range of 1.0 x 10 6 to 3.0 x 10 6 . By setting the Mw of UHMwPE to 3.0 x 10 &lt; 6 &gt; or less, melt extrusion can be facilitated. UHMwPE may be a homopolymer of ethylene as well as a copolymer containing small amounts of other? -Olefins. Other? -Olefins other than ethylene may be the same as the above.

PE composition (B), as an optional component a Mw of 1.0 × 10 4 ~ 4.0 × 10 6 of polybutene-1, and the Mw was 1.0 × 10 4 ~ 4.0 × 10 6 at least one of an ethylene / α- olefin copolymer . &Lt; / RTI &gt; These optional components are preferably contained in an amount of 40.0% by weight or less based on 100% by weight of the entire first polyolefin resin.

Polypropylene

The content of the polypropylene is preferably less than 5.0% by weight based on 100% by weight of the total weight of the first polyolefin resin. The upper limit of the content of the polypropylene is preferably 3.5% by weight. The lower limit of the content of the polypropylene is preferably 0.5% by weight, more preferably 1.0% by weight. When the content of polypropylene is within the above range, oxidation resistance, film thickness uniformity and strength are improved.

Mw of the polypropylene is preferably greater than 6.0, less than 3.0 × 10 5 × 10 4, and more preferably 6.0 × 10 4 to greater than 1.5 × 10 5 less. The molecular weight distribution (Mw / Mn) of the polypropylene is preferably 1.01 to 100, more preferably 1.1 to 50. The polypropylene may be a single substance or may be a composition comprising two or more polypropylenes.

Although not limited, the melting point of the polypropylene is preferably 150 to 175 ° C, more preferably 150 to 160 ° C.

The polypropylene may be not only a homopolymer but also a block copolymer and / or a random copolymer containing other? -Olefin or diolefin. As other olefins, for example, ethylene or an alpha -olefin having 4 to 8 carbon atoms is preferable. Examples of the? -Olefin having 4 to 8 carbon atoms include 1-butene, 1-hexene, 4-methyl-1-pentene and the like. The carbon number of the diolefin is preferably 4 to 14. Examples of the diolefins having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, and the like. The content of other olefins or diolefins is preferably less than 10 mol% based on 100 mol% of the propylene copolymer.

[Second Polyolefin Resin]

The second polyolefin resin constituting the second microporous layer is as follows.

The second polyolefin resin comprises polyethylene. As the polyethylene, polyethylene described in the first polyolefin resin can be used. (A) polyethylene (PE (A)) having Mw (weight average molecular weight) of less than 1.0 x 10 6 , or (b) PE (A) and ultrahigh molecular weight polyethylene (UHMwPE) having Mw of 1.0 x 10 6 or more (PE composition (B)). It is preferable that the second polyolefin resin does not contain polypropylene.

When the polyethylene is the PE composition (B), the upper limit of the PE (A) is preferably 99.0% by weight, more preferably 95.0% by weight, based on 100% by weight of the total weight of the second polyolefin resin. The lower limit of the PE (A) is preferably 50.0% by weight, more preferably 70.0% by weight.

The content of UHMwPE is preferably set to 100% by weight based on the total weight of the second polyolefin resin and 50% by weight or less based on 100% by weight of the entire polyethylene. Particularly preferably 30.0% by weight or less. When the content is within the above range, pressure rise is suppressed even during molding and productivity is improved. The lower limit of the content is not particularly limited, but is preferably 1.0% by weight, particularly preferably 5.0% by weight, from the standpoint of maintaining mechanical strength and maintaining a high melt-down temperature. By making UHMwPE 1.0 wt% or more and 50.0 wt% or less, it is possible to obtain a polyolefin microporous membrane excellent in strength and permeability balance.

PE composition (B), as an optional component a Mw 1.0 × 10 4 ~ 4.0 × 10 6 of polybutene-1, and the Mw was 1.0 × 10 4 ~ 4.0 × 10 6 at least one of an ethylene / α- olefin copolymer . &Lt; / RTI &gt; The content thereof is preferably 40.0% by weight or less based on 100% by weight of the entire second polyolefin resin.

[Components other than polyethylene and polypropylene in polyolefin resin]

As described above, the first and second polyolefin resins may be polyolefins other than polyethylene, polypropylene, or a composition containing a resin other than polyolefin. Examples of polyolefins other than polyethylene and polypropylene include homopolymers and copolymers of, for example, pentene-1, hexene-1, 4-methylpentene-1, octene and the like in addition to polybutene-1.

Further, when the polyolefin resin contains a heat-resistant resin, the meltdown temperature is improved when the polyolefin microporous membrane is used as a battery separator, so that the high temperature storage characteristics of the battery are further improved.

As the heat resistant resin, those described in WO 2006/137540 can be used. The content of the heat resistant resin is preferably 3 to 20% by weight, more preferably 3 to 15% by weight, based on 100% by weight of the entire polyolefin resin. When the content is in the above-mentioned preferable range, mechanical strength such as sting strength and tensile breaking strength is excellent.

[Low melting point polyethylene (hereinafter referred to as PE (C))]

When the polyolefin microporous membrane of the present invention is a single layer film, it is preferable that a part of the PE (A) in the first polyolefin resin is replaced with polyethylene (PE (C)) having a melting point of 130 캜 or lower.

When the polyolefin microporous membrane of the present invention is a multilayered microporous membrane composed of a plurality of microporous layers, either or both of PE (A) in the first polyolefin resin or PE (A) in the second polyolefin resin is partially PE ). More preferably, either PE (A) in the first polyolefin resin or PE (A) in the second polyolefin resin is partially substituted with PE (C). Among them, it is preferable that PE (C) is contained in the first polyolefin resin. A lower shutdown temperature can be obtained.

The upper limit of the content of PE (C) is 38.0 wt%, more preferably 35.0 wt%, based on 100 wt% of the total weight of the first polyolefin resin or the second polyolefin resin containing PE (C). The lower limit of the content of PE (C) is 10.0% by weight, and more preferably 15.0% by weight. By including PE (C) in an amount of 10.0% by weight or more, a polyolefin microporous membrane having a shutdown temperature of 132 占 폚 or less, excellent oxidation resistance and excellent physical properties can be obtained.

The upper limit of the melting point of PE (C) is 130 占 폚, and more preferably 128 占 폚. The lower limit of the melting point of PE (C) is 110 占 폚, and more preferably 115 占 폚. The upper limit of the Mw of the PE (C) is preferably 4.0 x 10 5 , more preferably 3.5 x 10 5 . The lower limit of the Mw of the PE (C) is preferably 5.0 x 10 3 , more preferably 6.0 x 10 3 . The MWD of PE (C) is preferably about 1 to about 50, more preferably about 2.0 to about 30.

When the polyolefin microporous membrane of the present invention is composed of three or more microporous layers, it may include a third microporous layer or more microporous layers. When the polyolefin microporous membrane of the present invention is composed of three microporous layers, the third microporous layer is located on the surface layer opposite to the first microporous layer. The resin constituting the third microporous layer is not particularly limited, but it may be composed of the first polyolefin resin or the second polyolefin resin, but preferably does not contain polypropylene.

[2] Production method of polyolefin microporous membrane

Next, a method for producing the polyolefin microporous membrane of the present invention will be described. Further, the method for producing the polyolefin microporous membrane of the present invention is not limited thereto.

The method for producing a polyolefin microporous membrane of the present invention comprises:

(a) a step of preparing a polyolefin solution by melt kneading a polyolefin resin and a film forming solvent,

Wherein the polyolefin resin comprises polyethylene as a main component,

Ultrahigh molecular weight polyethylene having a weight average molecular weight of 1.0 x 10 6 or more,

Polyethylene having a melting point of 130 캜 or lower, and

A weight average molecular weight of 6.0 × 10 4 3.0 × 10 5 large and less than the polypropylene comprising the steps of: containing less than 5% by weight at least 0.5% by weight,

(b) extruding the polyolefin solution at a shear rate of 60 / sec or more to form a formed body,

(c) cooling the obtained extrusion molded product at a cooling rate of 30 DEG C / sec or more to form a gel sheet,

(d) stretching the obtained gel-like sheet in at least one axial direction to prepare a drawn product,

(e) removing the film-forming solvent from the obtained stretched product.

The production method in the case where the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous layers can be roughly classified into four types according to the lamination method,

(2-1) First Production Method

A first production method for producing a polyolefin multilayer microporous membrane comprises the steps of (i) melt kneading a first polyolefin resin and a film forming solvent to prepare a first polyolefin solution, (ii) preparing a second polyolefin resin and a film forming solvent (Iii) extruding the first and second polyolefin solutions simultaneously on one die, and (iv) cooling the resulting extrudate to form a gel-like sheet. (Vi) a step of removing (cleaning) the film-forming solvent from the stretched product; and (vii) a step of removing the gel-like sheet from the stretched product And then drying the film after the drying. (viii) a step of re-stretching the dried film in at least one axial direction (second stretching step), and (ix) a step of heat-treating after the steps (i) to (vii). If necessary, any one of the heat fixation treatment step, the heat roll treatment step and the thermal solvent treatment step may be provided before the step (vi) for removing a film forming solvent. After the steps (i) to (ix), a drying step, a heat treatment step, a crosslinking treatment step by ionizing radiation, a hydrophilic treatment step, a surface coating treatment step and the like may be provided. (V) a step of heat-treating the stretched product after the first stretching step may be provided.

(i) preparing a first polyolefin solution

The first polyolefin resin and the film-forming solvent are melt-kneaded to prepare a first polyolefin solution. A suitable film forming solvent is added to the above-mentioned first polyolefin resin and then melt-kneaded to prepare a polyolefin resin solution. As a melt kneading method, for example, a method using a twin-screw extruder described in the specification of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well known, its explanation is omitted. The polyolefin resin concentration of the polyolefin resin solution is 20 to 50% by weight, preferably 25 to 45% by weight, based on 100% by weight of the total of the polyolefin resin and the film forming solvent. When the polyolefin resin concentration of the polyolefin resin solution is within the above range, a decrease in the productivity and a decrease in the moldability of the gel-like sheet are prevented.

As the first polyolefin resin, those described above can be used.

(ii) preparing a second polyolefin solution

The second polyolefin resin and the film-forming solvent are melt-kneaded to prepare a second polyolefin solution. The film-forming solvent used in the second polyolefin solution may be the same as or different from the film-forming solvent used in the first polyolefin solution, but the same is preferable. The other preparation methods may be the same as in the case of preparing the first polyolefin solution.

As the second polyolefin resin, those described above can be used.

(iii) Extrusion

The first and second polyolefin solutions are respectively supplied from an extruder to one die, where the two solutions are combined in layers and extruded into a sheet. In the case of producing a polyolefin multilayer microporous membrane having a structure of three or more layers, the first polyolefin solution forms at least one surface layer (first microporous layer), and the second polyolefin solution forms at least one layer (Preferably in contact with one or both of the two surface layers) to form a layer.

The extrusion method may be either a flat die method or an inflation method. Either method may be used in which the solution is supplied to each manifold and laminated in layers on the lip inlet of the multilayer die (multiple manifold method), or a method of feeding the solution into the die in advance as a layer flow Block method) can be used. The multiple manifold method and the block method are well known in the art and their detailed description is omitted. The gap of the multi-layer flat die is preferably 0.1 to 5 mm. The extrusion temperature is preferably 140 to 250 占 폚, and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness ratio of the first and second microporous layers can be adjusted by controlling the extrusion amounts of the first and second polyolefin solutions.

The ratio (L / D) of the length (L) to the diameter (D) of the screw of the twin screw extruder is preferably in the range of 20 to 100. The bore diameter of the biaxial extruder is preferably 40 to 200 mm. The ratio (Q / Ns) of the input amount of the polyolefin resin solution (Q (kg / h)) to the screw revolution (Ns (rpm)) when the polyolefin resin is put in the twin screw extruder is set to 0.1 to 0.55 kg / . The screw rotation speed (Ns) is preferably 180 rpm or more. The upper limit of the screw rotation speed (Ns) is not particularly limited, but is preferably 500 rpm.

As the extrusion method, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. In the method for producing a polyolefin microporous membrane of the present invention, the polyolefin resin solution containing the first polyolefin resin solution Is not less than 60 / sec. The shear rate from the die is more preferably 150 / sec or more.

(iv) Formation of gel sheet

(iii) is cooled to form a gel-like sheet. As a method of forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. The cooling is preferably carried out until the temperature of the extrusion molded article becomes 40 DEG C or lower. Through cooling, the microphase of the polyolefin separated by the film forming solvent can be immobilized. As the cooling method, there can be used a method of making contact with a coolant such as cold air or cooling water, a method of contacting with a cooling roll, or the like.

In the method for producing a polyolefin microporous membrane of the present invention, the cooling rate of the extrusion-molded article of the polyolefin resin solution containing the first polyolefin resin solution is 30 ° C / sec or more.

By suitably controlling the shear rate and cooling rate from the die, it is easy to make the distribution of the polypropylene uniform in the gel sheet, and the oxidation resistance and the electrolyte liquidity are improved.

(v) First drawing step

The obtained gel-like sheet is stretched in at least one axial direction. Cleavage between the layers of the polyethylene crystalline lamellar occurs by the first stretching, and the polyethylene phase is refined to form a large number of fibrils. The obtained fibrils form a three-dimensional network structure (a three-dimensionally irregularly connected network structure). Since the gel-like sheet contains a solvent for film formation, it can be stretched uniformly. The first stretching can be carried out at a predetermined magnification by heating the gel sheet, followed by a conventional tenter method, a roll method, an inflation method, a rolling method, or a combination of these methods. The first stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either biaxial stretching or continuous stretching may be performed.

Although the stretching magnification depends on the thickness of the gel sheet, it is preferably 2 times or more, and more preferably 3 to 30 times in the uniaxial stretching. In biaxial stretching, it is preferable to set the polyolefin microporous membrane to at least 3 times or more, that is, 9 times or more at an area multiplication rate in any direction, thereby improving the sticking strength of the obtained polyolefin microporous membrane, making it possible to achieve high elasticity and high strength. When the area ratio is within the above preferable range, there is no restriction in terms of the drawing apparatus, drawing operation, and the like. In biaxial stretching, it is preferable to set the magnifications in both directions to the same magnification.

The first stretching temperature is preferably set to a temperature at which the melting point of polyethylene used for preparing the polyolefin solution exceeds about 10 캜. The stretching temperature may be in the range of more than Tcd to less than Tme. Tme and Tcd are the melting point and the crystal dispersion temperature of all the polyethylenes used for preparing the polyolefin solution, respectively. If the stretching temperature is Tme + 10 占 폚 or less, orientation of the molecular chains of the polyolefin in the gel-like sheet tends to be promoted during stretching. On the other hand, if the stretching temperature is Tcd or more, the film breaking due to stretching is suppressed, and high-magnification stretching becomes possible. In one embodiment, the stretching temperature is from about 90 ° C to about 140 ° C, or from about 100 ° C to about 130 ° C. When the polyolefin resin is made of polyethylene of 90 wt% or more, the stretching temperature is usually within a range of 90 to 130 캜, preferably within a range of 100 to 125 캜, and more preferably within a range of 105 to 120 캜.

The Tme of PE (A), ultrahigh molecular weight polyethylene (UHMwPE), the second polyethylene or polyethylene composition (PE composition (B)) is generally from about 130 ° C to about 140 ° C and the Tcd is from about 90 ° C to about 100 ° C. Tcd can be obtained from the temperature characteristic of dynamic viscoelasticity according to ASTM D 4065.

The first stretching can be carried out at different temperatures, and the stretching temperature at the front end and the rear end and the final stretching ratio are set within the above ranges. The polyolefin microporous membrane can be stretched with a temperature distribution in the direction of the film thickness according to the desired physical properties, thereby obtaining a polyolefin microporous membrane having more excellent mechanical strength. As such a method, for example, the method disclosed in Japanese Patent No. 3347854 can be used.

(vi) Removal (cleaning) of a solvent for forming a film

Next, the solvent for film formation remaining in the drawn gel-like sheet (stretched product) is removed by using a cleaning solvent. Since the polyolefin phase is phase-separated from the solvent for forming a film, a porous film is obtained by removing the solvent for forming the film. The cleaning solvent and the method for removing the solvent for forming a film using the cleaning solvent are well known, and therefore, a description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(vii) drying of the membrane

The polyolefin microporous membrane obtained by removing the solvent for film formation is dried by a heat drying method, a blow drying method, or the like.

(viii) Second stretching step

Further, the film after drying can be stretched again in at least one axial direction. The second stretching can be performed by a tenter method or the like as in the first stretching while heating the film. The second stretching may be uniaxial stretching or biaxial stretching.

The second stretching temperature may be approximately equal to or lower than the melting point Tme of all the polyethylenes used for preparing the polyolefin solution. In one embodiment, the second draw temperature is from about Tcd to about Tme. If the second stretching temperature is lower than Tme, the permeability of the resulting polyolefin microporous membrane becomes appropriate, and the unevenness of physical properties such as permeability in the transverse direction (TD direction) tends to be suppressed, while the second stretching temperature If Tcd is more than Tcd, it is possible to prevent the film from being stretched and to stretch uniformly. When the polyolefin resin is made of polyethylene, the stretching temperature is usually within the range of 90 to 140 占 폚, preferably within the range of 100 to 140 占 폚.

The magnification in the uniaxial direction of the second stretching is preferably 1.1 to 1.8 times. For example, in the case of uniaxial stretching, the film is stretched in the MD direction (the film production direction, also referred to as the machine direction or the longitudinal direction) or the TD direction (the same plane direction as the longitudinal direction and also referred to as the transverse direction) 1.8 times. In the case of biaxially stretching, it is set to 1.1 to 1.8 times in the MD direction and in the TD direction, respectively. In the case of biaxial stretching, the respective stretching magnifications in the MD and TD directions may be different from each other in the respective directions as long as they are 1.1 to 1.8 times. When the stretching magnification falls within the above range, permeability, heat shrinkability, electrolyte absorbency and compressibility of the obtained polyolefin microporous membrane tend to be improved. The magnification of the second stretching is more preferably 1.2 to 1.6 times.

The second stretching speed is preferably 3% / sec or more in the elongation axis direction. For example, in the case of uniaxial stretching, it should be 3% / sec or more in the MD direction or TD direction. In the case of biaxially stretching, it should be 3% / sec or more in MD direction and TD direction respectively. The elongation speed (% / sec) in the elongation axis direction indicates the ratio of the length elongated per second at 100% in the direction of the elongation axis before re-stretching in the region where the film (sheet) is re-stretched. When the stretching speed is set to 3% / sec or more, the gas permeability of the obtained polyolefin microporous membrane becomes appropriate, and the unevenness of physical properties such as permeability in the sheet width direction tends to be suppressed. The second stretching speed is preferably 5% / sec or more, and more preferably 10% / sec or more. In the case of biaxial stretching, the stretching speeds in the MD and TD directions may be different from each other in the MD and TD directions as long as the elongation speed is 3% / sec or more. The upper limit of the second stretching speed is not particularly limited, but is preferably 50% / sec or less from the viewpoint of preventing breakage.

(ix) Heat treatment process

The film after the second stretching can be heat-treated. The reticulated structure of the fibril formed by the second stretching is retained, so that a polyolefin microporous membrane having high micropore size and excellent strength can be produced. The heat treatment may employ heat fixing treatment and / or thermal relaxation treatment. The heat fixing treatment is a heat treatment for heating while keeping the dimension of the film unchanged. The heat relaxation treatment is a heat treatment in which the film is heat shrunk in the MD or TD direction during heating. In particular, the crystal of the film is stabilized by the heat fixing treatment. The heat treatment can be carried out by a conventional method such as a tenter method, a roll method or a rolling method. For example, a method disclosed in Japanese Laid-Open Patent Publication No. 2002-256099 can be used as a thermal relaxation treatment method.

The heat treatment is carried out within a temperature range of not lower than the crystalline dispersion temperature of all the polyolefin resins constituting the polyolefin microporous membrane to the melting point or lower. The heat fixation treatment temperature is preferably within the range of the second stretching temperature ± 5 ° C, whereby the physical properties are stabilized. It is more preferable that this temperature is within the range of the second drawing temperature +/- 3 DEG C.

It is preferable to employ an in-line method in which first stretching, solvent removal for film formation, drying, second stretching, and heat treatment are continuously performed on a series of lines. However, if necessary, an off-line method may be employed in which the film after the drying treatment is temporarily wound and then the film is subjected to the second stretching and heat treatment.

(x) Other processes

Before the film-forming solvent is removed from the gel-like sheet subjected to the first stretching, any one of a heat fixing treatment process, a heat roll treatment process and a thermal solvent treatment process can be provided. It is also possible to provide a step of heat-fixing the film after the cleaning or the second stretching step. The stretched gel sheet before and / or after the cleaning, and the method of heat-fixing the film in the second stretching process may be the same as described above.

(2-2) Second Manufacturing Method

A second method for producing a polyolefin multilayer microporous membrane comprises the steps of (i) melt kneading a first polyolefin resin and a film forming solvent to prepare a first polyolefin solution, (ii) preparing a second polyolefin resin and a film- (Iii-2) immediately after the first and second polyolefin solutions are extruded from separate dies, (iv) the obtained extrusion-molded article (laminate) is cooled to obtain a gel-like sheet Is formed. That is, the first manufacturing method differs from the first manufacturing method in that a polyolefin solution is laminated in one die to form an extrusion molded article, whereas the second manufacturing method differs only in that the solution is laminated immediately after being extruded from a separate die, The same method as in the first manufacturing method can be employed.

Since the second method is the same as each step in the first production method except for the step (iii-2), only the step (iii-2) will be described. In step (iii-2), the first and second polyolefin solutions are respectively extruded from the adjacent die connected to each of the plurality of extruders into a sheet, and the temperature of each solution is raised (for example, 100 DEG C or more) And immediately laminated to obtain a laminated extrusion-molded article. The other steps may be the same as the first manufacturing method.

(2-3) Third Manufacturing Method

A third production method for producing a polyolefin multilayer microporous membrane comprises the steps of (i) melt kneading a first polyolefin resin and a film forming solvent to prepare a first polyolefin solution, (ii) preparing a second polyolefin resin and a film forming solvent (Iii-3-1) extruding the first polyolefin solution from one die to form a first extrusion molded product, and (iii-3-2) extruding the second polyolefin solution into another (Iv-3) The obtained first and second extrusion-molded bodies are cooled to form first and second gel-like sheets, (v-3) First and second extruded products are extruded from the die, (Xi-3) stretched first and second stretched products, and (vi) removing the film-forming solvent from the obtained stretched product. That is, the gel-like sheet is subjected to the respective steps until the gel-like sheet is stretched, and then laminated thereafter. In the following steps, the same method as in the first production method may be adopted. Between steps (vi-3) and (vii-3), a step (viii-3) of stretching a gel laminated sheet may be provided. The steps (iii-3-1) and (iii-3-2) are different from the step (iii) in the first production method only in that the first and second polyolefin solutions are not combined in layers. The die used is the same as the die used in the step (iii-2) in the second production method. The step (iv-3) is different from the step (iv) in the first production method only in that the first and second extruded bodies are cooled separately. The step (v-3) differs from the step (v) in the first production method only in that the first and second gel-like sheets are each stretched. On the other hand, the step (xi-3) is a step not included in the first and second production methods in which the first and second stretched products are laminated, but known methods can be used for laminating the stretched products.

(2-4) Fourth Manufacturing Method

A fourth manufacturing method for producing a polyolefin multilayer microporous membrane comprises the steps of (i) melt kneading a first polyolefin resin and a film forming solvent to prepare a first polyolefin solution, (ii) preparing a second polyolefin resin and a film forming solvent (Iii-4-1) extruding the first polyolefin solution from one die, (iii-4-2) extruding the second polyolefin solution from the other die, and (iv) melt- kneading the second polyolefin solution to prepare a second polyolefin solution, (V-4) stretching the first and second gel-like sheets, and (vi-4) stretching each stretched product (vi-4) (Vii-4) drying the obtained first and second polyolefin microporous membranes, (viii-4) stretching at least the second polyolefin microporous membrane, (xi-4) And a second polyolefin microporous membrane. That is, the steps are carried out separately until the porous film is formed, and then laminated to form a multi-layered microporous film. If necessary, the first and second polyolefin microporous membranes (ix-4) may be subjected to a heat treatment process between steps (vii) and (viii-4). In the following processes, the same method as in the first production method may be employed.

Up to step (v-4) can be carried out in the same manner as in the third production method. The step (vi-4) differs from the step (vi) in the first and third manufacturing methods only in that the film-forming solvent is removed in the first and second stretches, respectively. The process (vii-4) differs from the process (vii) in the first and third manufacturing methods only in that the first and second films are respectively dried.

On the other hand, the step (viii-4) is a step not necessarily required in the first to third production methods, but in the fourth production method, at least the second polyolefin microporous film is re-stretched in this step (viii-4). The stretching temperature is preferably not higher than the melting point, more preferably from the crystal dispersion temperature to the melting point. If necessary, the first polyolefin microporous membrane can be stretched. The stretching temperature is preferably not higher than the melting point, more preferably from the crystal dispersion temperature to the melting point. Even when any one of the first and second polyolefin microporous membranes is stretched, the stretching magnification can be the same as the first production method except for stretching the polyolefin microporous membrane that is not laminated.

Although the step (xi-4) is a step not included in the first to third production methods in which the first and second films are laminated, a known method can be used for lamination of the film, similar to lamination of drawn products.

As described above, the production method of the polyolefin microporous membrane of the present invention has been described in four classes according to the lamination method. When these are summarized, the necessary steps are the steps (a) to (e).

Step (a) corresponds to steps (i) and (ii) of the first to fourth manufacturing methods.

The step (b) includes the step (iii) of the first production method, the step (iii-2) of the second production method, the step (iii-3-1) of the third production method, 4-1).

Step (c) is a step (iv) of the first production method, Step (iv-2) of the second production method, Step (iv- .

Step (d) corresponds to step (v) of the first to second manufacturing methods, step (v-3) of the third manufacturing method and step (v-4) of the fourth manufacturing method.

Step (e) corresponds to step (vi) of the first to third manufacturing methods and step (vi-4) of the fourth manufacturing method.

When the polyolefin microporous membrane of the present invention is a single layer, it comprises only the step of producing the first microporous layer among the above-described methods of producing the multilayer microporous membrane. As an example, the polyolefin microporous membrane of the present invention can be obtained by (2-4) (i), (ii), (iii-4-1), (iv- -4) and (vii-4).

[3] Structure, properties and measurement method of polyolefin microporous membrane

The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following properties. Structures, physical properties and methods for measuring the properties are described below.

(1) Standardized PP / PE ratio

In the polyolefin microporous membrane of the present invention, the PP distribution of the first microporous layer is uniform in the in-plane direction. As an example of expressing the uniformity of the PP distribution, the peak PP / PE ratio (PP / PE ratio) of PP to PE obtained by Micro-Raman spectroscopy was set to 1 PE ratio can be expressed by a structure in which the average value / standard deviation / kurtosis of the normalized PP / PE ratio shows a constant value. That is, the polyolefin microporous membrane of the present invention preferably has a structure in which the normalized PP / PE ratio is not less than 0.5 as an average value, not more than 0.2 as a standard deviation, and not more than 1.0 and not more than -1.0 as a parameter indicating the distribution shape.

A method of measuring the PP / PE ratio of the membrane surface according to the brown rice Raman spectroscopy will be described below. According to the brown rice Raman spectroscopy, an area analysis was performed using a 532 nm wavelength laser and a spot diameter of 1 micron to 2 microns and a 20 micron micron field of view in a depth direction. The peak intensity ratio of a frequency of 807 cm -1 (PP) and a frequency of 1127 cm -1 (PE) is measured. Quot; normalized PP / PE ratio &quot; when the maximum value of the intensity ratio in the 20 x 20 micron field of view is 1.

When the average value of the normalized PP / PE ratio is within the above-mentioned preferable range, the portion where the polypropylene concentration is low is small and the portion where the polyethylene is predominant does not increase, and the polyethylene is mainly charged by the oxidation reaction accompanying the charge- It is considered that the deterioration hardly progresses and the cycle characteristics are maintained favorably because the fraction is small.

If the standard deviation of the normalized PP / PE ratio is within the above preferable range, it is considered that the oxidation resistance is hardly deteriorated because the change in the polypropylene concentration is small and the portion having low polypropylene concentration is small.

If the distribution of the polypropylene concentration is within the above-mentioned preferable range, the portion with low polypropylene concentration is small, and the portion where the oxidation resistance is poor in the battery is unlikely to occur, so that the battery performance is good. The presence of a portion having a high polypropylene concentration to some extent is likely to improve the oxidation resistance. From these results, it has been found that the distribution of the proper standardized PP / PE is essential for the improvement of the oxidation resistance of the polyolefin microporous membrane.

Since the polyolefin microporous membrane of the present invention has a uniform PP distribution in the in-plane direction as described above in the first microporous layer, the polyolefin microporous membrane is excellent in oxidation resistance. When the content of polypropylene is less than 5% by weight, the lowering of physical properties by polypropylene is suppressed and the permeability, strength and electrolyte absorbency are excellent, which is preferable. Therefore, when used as a separator for a lithium ion battery, excellent battery productivity, safety, and battery cycle characteristics can be realized.

(2) The air permeability (sec / 100cm 3 / 20㎛)

The upper limit of the gurley value in terms of the film thickness of the polyolefin microporous membrane of the present invention in terms of 20 탆 is preferably 700 sec / 100 cm 3 , more preferably 600 sec / 100 cm 3 , Is 550 sec / 100 cm &lt; 3 & gt ;. Polyolefin microporous lower limit of air permeability is also a film in terms of a thickness of 20㎛ the membrane of the present invention is preferably a 20sec / 100cm 3, more preferably 100sec / 100cm 3. When the air permeability is within this range, when the polyolefin microporous membrane is used as a battery separator, the battery capacity is large, the cycle characteristics of the battery are good, the shutdown is sufficiently performed when the temperature inside the battery rises, The resistance value is hard to rise, and the average electrochemical stability is good. The air permeability was measured in accordance with JIS P 8117, and the value obtained by converting the film thickness to 20 mu m.

(3) Porosity (%) (Porosity)

The porosity of the polyolefin microporous membrane of the present invention is preferably 25 to 80%, more preferably 30 to 50%. When the porosity is within the above range, the permeability and strength when the polyolefin microporous membrane is used as a battery separator are appropriate, and short-circuiting of the electrode is suppressed. The porosity is a value measured by the mass method.

Porosity (%) = 100 x (w2-w1) / w2

w1: actual weight of microporous membrane

w2: weight of equivalent nonporous membrane (of the same polymer) having the same size and thickness

(4) puncture strength (mN / 20 占 퐉)

The striking strength was a value obtained by measuring the maximum load value when the polyolefin microporous membrane was stuck at a speed of 2 mm / sec using a needle having a diameter of 1 mm (0.5 mm R) and converting the film thickness to 20 μm. The penetration strength of the polyolefin microporous membrane of the present invention in terms of a film thickness of 20 占 퐉 is preferably 2,000 mN or more, more preferably 4,000 mN or more, and still more preferably 5,000 mN or more. When the penetration strength is 2,000 mN / 20 mu m or more, shorting of the electrode can be effectively suppressed when the polyolefin microporous membrane is inserted into the battery as a battery separator.

(5) Tensile breaking strength (kPa)

The tensile fracture strength of the polyolefin microporous membrane of the present invention is 60,000 kPa or more, more preferably 80,000 kPa or more, and even more preferably 100,000 kPa or more in any direction in the MD and TD directions. As the tensile breaking strength is 60,000 kPa or more, it is easy to prevent the waviness at the time of manufacturing the battery. The tensile breaking strength was measured according to ASTM D882 using a rectangular test piece having a width of 10 mm.

(6) Tensile elongation at break (%)

The tensile elongation at break of the polyolefin microporous membrane of the present invention is preferably 80% or more, and more preferably 100% or more, in both the MD and TD directions. Thus, it is easy to prevent the corrugated membrane at the time of manufacturing the battery. The tensile elongation at break was measured according to ASTM D882 using a rectangular test piece having a width of 10 mm.

(7) Heat shrinkage (%)

The heat shrinkage ratio of the polyolefin microporous membrane of the present invention after being exposed at a temperature of 105 DEG C for 8 hours is preferably 10% or less, more preferably 8% or less, and still more preferably 6% or less in both the MD and TD directions. When the heat shrinkage ratio is 10% or less, when the polyolefin microporous membrane is used as a separator for a lithium battery, the possibility of short-circuiting of electrodes due to shrinkage of the end of the separator at the time of heat generation is low.

The heat shrinkage percentage is a value obtained by measuring the heat shrinkage ratios of the polyolefin microporous membrane in the MD direction and the TD direction when the polyolefin microporous membrane is exposed at 105 DEG C for 8 hours, three times each, and calculating an average value. The heat shrinkage rate is expressed by the following equation.

Heat shrinkage percentage (%) = 100 x (length before heating - length after heating) / length before heating

(8) Shutdown temperature

The shutdown temperature of the polyolefin microporous membrane of the present invention is 132 占 폚 or lower, more preferably 128 占 폚 or lower, even more preferably 126 占 폚 or lower. The shutdown temperature is also measured according to the method disclosed in WO 2007/052663. According to this method, the polyolefin microporous membrane is exposed in an atmosphere at 30 占 폚, the temperature is raised at 5 占 폚 / min, and the permeability of the membrane is measured therebetween. The shutdown temperature of the polyolefin microporous membrane was defined as the temperature when the permeability (gelling value) of the polyolefin microporous membrane first exceeded 100,000 sec / 100 cm 3 . The air permeability of the polyolefin microporous membrane was measured according to JIS P 8117 using a porosimeter (manufactured by Asahi Seiko Co., Ltd., EGO-1T).

(9) Electrolyte liquid

The electrolyte solution liquidity of the polyolefin microporous membrane of the present invention is 20 seconds or less. More preferably 10 seconds or less, still more preferably 5 seconds or less. Electrolyte liquidity was evaluated by the penetration time of propylene carbonate. A sample of 50 mm x 50 mm is placed on a glass plate, 0.5 ml of propylene carbonate is dropped from about 2 cm above the sample, and the time measurement is started from the completion of dropping. Immediately after the dropping, the propylene carbonate swells up on the membrane with surface tension, but the propylene carbonate that has been dripped infiltrates with the lapse of time. When all of the propylene carbonate on the membrane has permeated, the time measurement is stopped to determine the penetration time. The penetration time is better than 20 seconds, more than 20 seconds, less than 50 seconds slightly better, more than 50 seconds is inappropriate.

(10) Average electrochemical stability (leakage current value) (mAh)

In order to measure the electrochemical stability, a film having a length (MD) of 70 mm and a width (TD) of 60 mm is disposed between a cathode and an anode having the same area as the film. The negative electrode is made of natural graphite, and the positive electrode is made of LiCoO 2 . The electrolyte is prepared by dissolving LiPF 6 as a 1 M solution in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (3/7, V / V). An electrolyte is impregnated in the membrane between the cathode and the anode to complete the cell.

Then, the battery is exposed to an applied voltage of 4.3 V while being exposed to a temperature of 60 캜 for 28 days. The term &quot; electrochemical stability &quot; is defined as the integral current (mAh) flowing between the voltage source and the battery over a period of 28 days. The electrochemical stability is measured for three cells under the same conditions (three cells of the same conditions are prepared from three samples of the same condition of the membrane). The average electrochemical stability (leakage current value) is an average (arithmetic mean) of values of electrochemical stability of the three cells measured.

The electrochemical stability is a film characteristic related to the oxidation resistance of a film when the film is used as a separator in a battery which is exposed to a relatively high temperature during storage or use. The electrochemical stability is in units of mAh and generally lower values are desirable (indicating less total charge loss during storage or overcharging at high temperatures). BACKGROUND ART Automobile batteries such as batteries used for starting electric power vehicles for moving an electric vehicle or a hybrid electric vehicle or supplying electric power to the power means thereof and electric power tool batteries are used for relatively high output and large capacity applications, A slight loss of battery capacity, such as self-discharge loss caused by electrochemical instability of the battery, becomes a serious problem. The average electrochemical stability of the polyolefin microporous membrane of the present invention is preferably 45.0 mAh or less, particularly preferably 35.0 mAh or less. The term &quot; high capacity &quot; battery generally means a battery capable of supplying at least 1 Ah (1Ah), for example, 2.0 Ah to 3.6 Ah.

(11) Thickness of film

The film thickness of the polyolefin microporous membrane of the present invention is preferably 5 to 50 占 퐉, more preferably 5 to 35 占 퐉, and more preferably 10 to 25 占 퐉, for example, when it is used as a battery separator. The method of measuring the film thickness may be either a contact thickness measuring method or a non-contact thickness measuring method. For example, the thickness can be measured with a contact type thickness meter over a width of 10.0 cm at intervals of 1.0 cm in the longitudinal direction, and then the film thickness can be obtained by averaging the values. As the contact type thickness measuring device, for example, a thickness measuring device such as Litematic of Mitutoyo Corporation is suitable.

When the polyolefin microporous membrane of the present invention is composed of three or more microporous layers, the sum T (A) of the thicknesses of both surface layers and the total thickness T (B) of the thicknesses of the respective layers disposed between both surface layers satisfy the following equation It is preferable to satisfy it.

<Formula 1>

60? T (A) / (T (A) + T (B)) 100

By satisfying the relationship of the above formula (1), it is possible to obtain a polyolefin microporous film superior in low shutdown temperature. It is more preferable to satisfy the relationship of the following formula (2).

<Formula 2>

60? T (A) / (T (A) + T (B)) 100 <90

By satisfying the relationship of the formula (2), a balance of strength and permeability is excellent, and a polyolefin microporous film superior in shutdown temperature can be obtained.

Further, it is more preferable to satisfy the relationship of the following formula (3).

<Formula 3>

60? T (A) / (T (A) + T (B)) 100 <85

(12) Appearance

The appearance of the film was evaluated by visual / multi-point film thickness measurement. Good &quot; was determined for the small variation in thickness with the naked eye. &Quot; Good &quot; corresponds to a case where the film thickness variation is less than 5 microns in the film thickness measurement at multiple points.

(13) Melting point

The melting point of the resin was measured according to JIS K 7122 in the following procedure. That is, the resin sample was placed in a sample holder of a scanning differential calorimeter (product of Perkin Elmer, Inc., model DSC-System7), heat treated in a nitrogen atmosphere at 230 占 폚 for 10 minutes, After cooling to 40 캜, it was maintained at 40 캜 for 2 minutes, and then heated to 230 캜 at a rate of 10 캜 / min. And the temperature at which the maximum endothermic amount became the peak (peak temperature) was taken as the melting point.

[4] batteries, etc.

As described above, the polyolefin microporous membrane of the present invention is excellent in oxidation resistance and electrolyte liquidity, is hardly blackened even after repeating charging and discharging as a battery, is excellent in permeability and strength balance, and is thus particularly suitable as a battery separator .

The separator comprising the polyolefin microporous membrane of the present invention can be used for a battery and an electric double layer capacitor. There is no particular limitation on the type of the battery / condenser to be used, but it is particularly suitable for use in a lithium secondary battery / lithium ion capacitor. For the lithium secondary battery / capacitor using the separator made of the polyolefin microporous membrane of the present invention, well-known electrodes and electrolytes can be used. The structure of the lithium secondary battery / capacitor using the separator made of the polyolefin microporous membrane of the present invention may also be a known one.

Example

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. The physical properties of the polyolefin microporous membrane were determined by the method described above.

Example  One

(1) Preparation of the first polyolefin solution

First, based on the total weight of the polyolefin composition (a) Mw is 2.0 × 10 6 of HDPE (Mw / Mn: 8, melting point: 136 ℃) 20% by weight, (b) Mw is 2.5 × 10 5 of HDPE (Mw / 20 weight% of HDPE (Mw / Mn: 2.6, melting point: 123 占 폚) having an Mw of 1.8 占04 , , and (d) 3 wt% of polypropylene having an Mw of 9.7 x 10 4 (Mw / Mn: 2.6, melting point: 155 ° C) was prepared by dry blending. Butylphenylacetate] methane as an antioxidant to 0.2 part by weight per 100 parts by weight of the first polyolefin composition to obtain a first polyolefin composition, To prepare a polyolefin resin.

25 parts by weight of the first polyolefin resin was fed to a steel kneading twin screw extruder and 75 parts by weight of liquid paraffin (50 cSt at 40 캜) was fed from a side feeder to a twin screw extruder. And melt-kneaded at 210 DEG C and 200 rpm to prepare a first polyolefin solution.

(3) Preparation of microporous membrane

The first polyolefin solution was fed from a twin-screw extruder to a die to form an extrusion molded article. The extrudate was passed through a cooling roll controlled at 20 캜 and cooled to form a gel sheet. The shear rate in the die of the extrusion molded article was 205 / sec, and the cooling rate in the cooling roll was 37 ° C / sec. The obtained gel-like sheet was subjected to simultaneous biaxial stretching (first stretching) at a stretching magnification of 5 x 5 times at a temperature of 115 캜 using a tenter stretching machine and wound. Subsequently, a part of the drawn stretched product was collected and fixed to a frame plate (size: 20 cm x 20 cm, made of aluminum (hereinafter the same)), immersed in a washing bath of methylene chloride adjusted to a temperature of 25 캜, For 3 minutes. The washed membrane was air-dried at room temperature. The dried microporous membrane was subjected to a second stretching (re-stretching) at a draw ratio of 1.2 times in the TD direction at 118.3 占 폚 through a batch stretching machine and thereafter subjected to a thermal relaxation treatment to a draw ratio of 1.0 in the TD direction under the same temperature, And then heat-fixed for 10 minutes at a reheat temperature in a state of being mounted on a post-stretching machine to produce a polyolefin microporous film.

Example  2

(1) Preparation of the first polyolefin solution

First, based on the total weight of the polyolefin composition (a) Mw is 2.0 × 10 6 of HDPE (Mw / Mn: 8, melting point: 136 ℃) 20% by weight, (b) Mw is 2.5 × 10 5 of HDPE (Mw / (Mw / Mn: 3.0, melting point: 123 占 폚) having an Mw of 2.4 占04 and a melting point of 130 占 폚) , and (d) 3 wt% of polypropylene having an Mw of 9.7 x 10 4 (Mw / Mn: 2.6, melting point: 155 ° C) was prepared by dry blending. 0.2 parts by weight of tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant per 100 parts by weight of the first polyolefin composition was dry- To prepare a polyolefin resin.

25 parts by weight of the first polyolefin resin was fed to a steel kneading twin screw extruder and 75 parts by weight of liquid paraffin (50 cSt at 40 캜) was fed from a side feeder to the twin screw extruder. And melt-kneaded at 210 DEG C and 200 rpm to prepare a first polyolefin solution.

(2) Preparation of the second polyolefin solution

The second polyolefin solution was prepared in the same manner as the first polyolefin solution except for the following points. (A) 20% by weight of UHMwPE (Mw / Mn: 8.0, melting point: 136 ° C) having an Mw of 2.0 × 10 6 , (b) HDPE having an Mw of 2.5 × 10 5 (Mw / Mn: 8.6, terminal vinyl group concentration: 0.1 units / 10000 carbon atoms, melting point: 134 占 폚) was prepared by dry blending the second polyolefin composition. Butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant was 0.2 part by weight per 100 parts by weight of the second polyolefin composition, To prepare a polyolefin resin. 25 parts by weight of the obtained second polyolefin composition was fed to a steel kneading twin-screw extruder, and 75 parts by weight of liquid paraffin (50 cSt at 40 캜) was fed from a side feeder to a twin-screw extruder. And melt-kneaded at 210 DEG C and 200 rpm to prepare a second polyolefin solution.

(3) Preparation of microporous membrane

The first and second polyolefin solutions were fed from respective twin-screw extruders to a three-layer T-die, and the layer composition was adjusted to a value of 3/3, a first polyolefin solution / a second polyolefin solution / a first polyolefin solution and a layer thickness ratio of 35/30/35 Layer extruded body was formed. This extrudate was passed through a cooling roll controlled at 20 캜 and cooled to form a three-layered gel-laminated sheet. The shear rate in the die of the extrusion molded article was 200 / sec, and the cooling rate in the cooling roll was 36.5 ° C / sec. The obtained gel-like laminated sheet was subjected to simultaneous biaxial stretching (first stretching) at a stretching magnification of 5 x 5 times at a temperature of 115 캜 using a tenter stretching machine and wound. Subsequently, a part of the drawn stretched product was taken out and fixed to a frame (size: 20 cm x 20 cm, made of aluminum (hereinafter the same)), immersed in a washing bath of methylene chloride adjusted to a temperature of 25 캜 and dried at 100 rpm for 3 minutes It was washed while rocking. The washed membrane was air-dried at room temperature. The dried microporous membrane was subjected to a second stretching (re-stretching) at a draw ratio of 1.4 times in the TD direction at 122 占 폚 through a batch stretching machine, then heat relaxed to 1.2 times in the TD direction under the same temperature, And then thermally fixed at a reheat temperature for 10 minutes in a mounted state to produce a polyolefin multilayer microporous membrane.

Example  2 ~ Example  10 and Comparative Example  1 ~ Comparative Example  8

A polyolefin microporous membrane was produced in the same manner as in Example 1 in Comparative Example 1, and in Examples 2 to 10 and Comparative Examples 2 to 8 in the raw materials and conditions shown in Tables 1 and 2, respectively. In addition, &quot; - &quot; in Table 1 and Table 2 indicates that the material does not contain UHMwPE or HDPE2 in the table.

Figure pct00001

Figure pct00002

Table 3 and Table 4 show the physical properties of the polyolefin microporous membranes of Examples 1 to 10 and Comparative Examples 1 to 8. In Table 4, &quot; - &quot; in Comparative Example 7 indicates that the surface could not be measured due to the large irregularities judged by the naked eye.

Figure pct00003

Figure pct00004

As shown in Tables 3 and 4, all of the polyolefin microporous membranes of Examples 1 to 9 are excellent in the electrolyte liquidity and uniform in PP distribution. In addition, the leakage current value is 45 mAh or less and exhibits excellent oxidation resistance. In addition, the shutdown temperature is 132 占 폚 or less, which is more excellent in safety and physical property balance when used in a battery. 1 is a graph showing the distribution of the standardized PP / PE ratio of the surface layer of the polyolefin microporous membrane of Example 2. It can be seen that the normalized PP / PE ratio is concentrated in a narrow range of 0.5 or more. Fig. 2 shows a two-dimensional distribution diagram of the normalized PP / PE ratio of the surface layer of the polyolefin microporous membrane of Example 2, in which a region with a low polypropylene concentration (dark-colored portion) is hardly visible, . 3 shows a two-dimensional distribution diagram of the standardized PP / PE ratio of the surface layer of the polyolefin microporous membrane of Comparative Example 2. The polypropylene is present in a region having a low concentration of polypropylene (dark portion) I do not know.

Industrial use

The present invention provides a polyolefin microporous membrane excellent in oxidation resistance, electrolyte liquidity and shutdown characteristics, and excellent in permeability and strength balance.

The polyolefin multilayer microporous membrane of the present invention has a suitable performance as a capacitor device, a capacitor, a battery, and other non-aqueous electrolytic solution, and contributes to improvement of safety and reliability. In particular, it can be suitably used as a separator for a battery, more specifically as a separator for a lithium ion battery, and it is possible to increase the life span and safety of the battery. As other applications, they are also used as various separation membranes such as a component part of a fuel cell, a humidifying membrane, a filtration membrane and the like, and thus they are industrially applicable in these fields.

Claims (13)

  1. As the polyolefin microporous membrane made of a first polyolefin resin, including polypropylene and polyethylene, the liquid-state electrolyte is less than 20 seconds, and the shutdown temperature is less than 132 ℃, a gas transmission rate of the film in terms of a thickness of 20㎛ 700sec / 100cm 3 And a puncture strength of not less than 2,000 mN in terms of a film thickness of 20 mu m and a polypropylene distribution uniform in the in-plane direction.
  2. The method according to claim 1,
    Wherein the average value of the normalized polypropylene / polyethylene ratios measured by Raman spectroscopy is 0.5 or more, the standard deviation of the normalized polypropylene / polyethylene ratio is 0.2 or less, and the standardized polypropylene / polyethylene ratio is 1.0 or less -1.0 or more.
  3. 3. The method according to claim 1 or 2,
    Wherein the polypropylene has a weight average molecular weight of greater than 6.0 x 10 &lt; 4 &gt; and less than 3.0 x 10 &lt; 5 & gt ;.
  4. 4. The method according to any one of claims 1 to 3,
    Wherein the first polyolefin resin comprises 0.5 wt% or more and less than 5.0 wt% of polypropylene.
  5. 5. The method according to any one of claims 1 to 4,
    Wherein the first polyolefin resin comprises 1.0 wt% or more and 50.0 wt% or less of polyethylene having a weight average molecular weight of 1.0 x 10 6 or more.
  6. 6. The method according to any one of claims 1 to 5,
    A polyolefin microporous membrane comprising polyethylene having a melting point of 130 占 폚 or less.
  7. The method according to claim 6,
    Wherein the content of polyethylene having a melting point of 130 占 폚 or less is 10.0 wt% or more and 38.0 wt% or less of the first polyolefin resin.
  8. 6. The method according to any one of claims 1 to 5,
    A first microporous layer made of a first polyolefin resin and a second microporous layer made of a second polyolefin resin disposed between both surface layers, the first microporous layer being composed of three or more microporous layers and constituting at least one side of the surface layer, Wherein either or both of the porous layer and the second microporous layer comprises polyethylene having a melting point of 130 캜 or lower.
  9. 9. The method of claim 8,
    The content of polyethylene having a melting point of 130 占 폚 or less is 10.0 wt% or more and 38.0 wt% or less of the first polyolefin resin or the second polyolefin resin.
  10. 10. The method according to claim 8 or 9,
    Wherein the second polyolefin resin comprises 1.0 to 50.0 wt% of polyethylene having a weight average molecular weight of 1.0 x 10 6 or more and does not contain polypropylene.
  11. 11. The method according to any one of claims 8 to 10,
    Wherein the sum T (A) of the thicknesses of the both surface layers and the total thickness T (B) of the thicknesses of the respective layers disposed between the both surface layers satisfy the following formula (1).
    <Formula 1>
    60? T (A) / (T (A) + T (B)) 100
  12. 12. The method according to any one of claims 1 to 11,
    Wherein the polyolefin microporous membrane has a shutdown temperature of 128 占 폚 or less.
  13. (a) A process for preparing a polyolefin solution by melt kneading a polyolefin resin and a film forming solvent, wherein the polyolefin resin is a mixture of an ultra high molecular weight polyethylene having polyethylene as a main component and a weight average molecular weight of 1.0 x 10 6 or more, polyethylene with weight average molecular weight of 6.0 × 10 4 3.0 × 10 5 large and less than the polypropylene process comprising less than 5% by weight at least 0.5% by weight,
    (b) a step of extruding the polyolefin solution at a shear rate of 60 / sec or more to form a formed body,
    (c) cooling the obtained extrusion molded product at a cooling rate of 30 DEG C / sec or more to form a gel sheet,
    (d) a step of stretching the obtained gel-like sheet in at least one axial direction to prepare a stretched product, and
    (e) removing the film-forming solvent from the obtained stretched product.
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