WO2014192862A1

WO2014192862A1 - Multilayer, microporous polyolefin membrane, and production method thereof

Info

Publication number: WO2014192862A1
Application number: PCT/JP2014/064248
Authority: WO
Inventors: 石原毅
Original assignee: 東レバッテリーセパレータフィルム株式会社
Priority date: 2013-05-31
Filing date: 2014-05-29
Publication date: 2014-12-04
Also published as: CN105246957A; CN105246957B; JPWO2014192862A1; JP6443333B2; KR102266028B1; KR20160014616A

Abstract

Provided is a microporous polyolefin membrane which has excellent oxidation resistance, electrolyte injection performance and shutdown characteristics, and which further has excellent permeability and strength balance. This microporous polyolefin membrane contains polypropylene and polyethylene. The electrolyte injection performance is 20 seconds or less, the shutdown temperature is 132°C or less, the air permeability when converting the membrane thickness to 20μm is 700s/100cm³ or less, the puncture strength when converting the membrane thickness to 20μm is 2,000mN or greater, and the polypropylene distribution is uniform in the in-plane direction.

Description

Polyolefin microporous membrane and method for producing the same

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.

Polyolefin microporous membranes are used in various applications such as battery separators, electrolytic capacitor membranes, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When the polyolefin microporous membrane is used as a battery separator, particularly a lithium ion battery separator, the performance is deeply related to battery characteristics, battery productivity, and battery safety. Therefore, excellent permeability, mechanical characteristics, heat shrinkage resistance, shutdown characteristics, meltdown characteristics, etc. are required. For example, when a battery separator with low mechanical strength is used, the voltage of the battery may decrease due to a short circuit of the electrodes. In addition, lithium ion batteries are known to deteriorate battery performance if they continue to be used while being charged almost fully charged, and oxidative degradation of the separators contributes to this, so improvements in separators are required. I came.

So far, as a method for improving the physical properties of the polyolefin microporous membrane, improvements in the raw material composition, stretching conditions, heat treatment conditions, etc. have been studied, and it has been proposed to mix polypropylene as a means for improving heat resistance (for example, JP 2002-105235 A, JP 2003-183432 A). In particular, in recent years, in addition to permeability, mechanical characteristics, heat shrinkage resistance, etc., characteristics related to battery productivity such as electrolyte injection properties and characteristics related to battery life such as oxidation resistance have come to be emphasized. ing.

For example, in Patent Document 1 (Japanese Patent Laid-Open No. 11-269290), by adding a specific amount of polypropylene to ultrahigh molecular weight polyethylene or a composition thereof, microscopic irregularities are generated on the surface of the microporous polyolefin membrane, and transmission Discloses a microporous polyolefin membrane that has excellent properties and mechanical strength, improved moldability, and improved electrolyte permeability and retention. Further, in Patent Document 2 (Japanese Patent Application Laid-Open No. 2011-111484), as a polyolefin multilayer microporous film suitable as a separator capable of achieving both oxidation resistance and cycle characteristics, 5 to 50% by weight of a polypropylene component and 50 to 50% of a polyethylene component are used. 95% by weight, the polyethylene component includes ultra high 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 polyolefin multilayer microporous membrane having a bubble point of 400 to 600 kPa.

In Patent Document 3 (Japanese Patent Application Laid-Open No. 2004-152614), when a polyolefin such as specific polypropylene is added to polyethylene and blended to form a film, the polyolefin is segregated on the surface, and the polyethylene content in the vicinity of the surface may decrease. It is disclosed that such a microporous film on the surface can suppress gas generation and discharge capacity reduction during high-temperature storage. This microporous membrane is a single layer containing 50% by weight or more of polyethylene, and the polyethylene content in the vicinity of the surface of at least one side of the membrane is less than the average value of the entire membrane, and the viscosity average molecular weight is 200,000 or more. It is characterized by containing 5 to 20% by weight of polypropylene and a low molecular weight polypropylene having a viscosity average molecular weight of 50,000 or less, respectively, based on the whole film constituting material.

Patent Document 4 (Japanese Patent Application Laid-Open No. 2011-063025) discloses a sufficient safety function and strength even when the film thickness is reduced by laminating a polyolefin microporous film essentially comprising polyethylene and polypropylene and a polyethylene microporous film. Has been reported to be obtained. The polyolefin microporous membrane of Patent Document 4 has ensured strength and safety (pore blocking temperature and membrane breaking temperature) by laminating a layer made of polypropylene and ultrahigh molecular weight polyethylene and a layer made of polyethylene. It is a feature. The combination of polypropylene and ultra-high molecular weight polyethylene maintains heat resistance and strength, and the polyethylene layer prevents the pore closing temperature from rising.

In Patent Document 5 (Japanese Patent Application Laid-Open No. 5-234578), a polyethylene component having a specific molecular weight distribution and a polypropylene component having a specific range of weight average molecular weight are used as polymer components, and a mixture comprising the inorganic fine powder and organic liquid is produced. By using it as a raw material for the membrane, an organic electrolyte with excellent mechanical properties and safety can be obtained, even if the proportion of the ultra-high molecular weight portion in the molecular weight distribution of polyethylene is increased, there is no pressure increase during film molding. A battery separator to be used is proposed. This separator has a polyethylene having a molecular weight of 1.0 × 10 ⁶ or more and a weight average molecular weight of 1.0 × 10 ⁴ and a polyethylene containing 10% by weight or more and a molecular weight of 1.0 × 10 ⁵ or less. Composed of a polyolefin microporous membrane composed of a matrix containing -1.0 × 10 ⁶ polypropylene, the amount of polypropylene being 5 to 45% by weight of the total weight of polyethylene and polypropylene, The thickness is 10 to 500 μm, the porosity is 40 to 85%, the maximum pore diameter is 0.05 to 5 μm, and the difference between the film breaking temperature and the non-porous temperature is 28 to 40 ° C.

Patent Document 6 (International Publication No. WO2007 / 015416) is a polyolefin microporous film made of polyethylene and polypropylene having a viscosity average molecular weight of 100,000 or more, containing 4 wt% or more of the polypropylene, and a polyolefin microporous film by infrared spectroscopy. A polyolefin microporous membrane is proposed in which the concentration of terminal vinyl groups per 10,000 carbon atoms in the polyolefin constituting the porous membrane is 2 or more. It is disclosed that the polyolefin microporous film achieves both a tear resistance and a low heat shrinkage, and has excellent fuse characteristics and a uniform film thickness.
Japanese Patent Laid-Open No. 11-269290 JP 2011-111484 A JP 2004-152614 A JP 2011-063025 A JP-A-5-234578 International Publication WO2007 / 015416

In order to improve the oxidation resistance by blending polypropylene, it is necessary to blend a considerable amount of polypropylene, but increasing the polypropylene content impairs the permeability and strength balance of the polyethylene microporous membrane, especially the strength decreases. There is a drawback of doing. Furthermore, there is a problem that a sufficient shutdown temperature cannot be obtained. Therefore, in order to improve the oxidation resistance of the separator related to the battery life and to ensure the productivity, safety and output characteristics of the battery, the excellent permeability and strength balance of the polyethylene microporous membrane, as well as the shutdown characteristics Is required to hold.
Accordingly, an object of the present invention is to provide a microporous polyolefin membrane that is excellent in oxidation resistance, electrolytic solution pouring property, and shutdown characteristics, and further excellent in permeability and strength balance.

In order to solve the above-described problems, the polyolefin microporous membrane of the present invention has the following configuration. That is,
A polyolefin microporous membrane comprising a first polyolefin resin containing polypropylene and polyethylene, having an electrolyte solution injection property of 20 seconds or less, a shutdown temperature of 132 ° C. or less, and an air permeability converted to a film thickness of 20 μm A polyolefin having a degree of 700 sec / 100 cm ³ or less, a puncture strength converted to a film thickness of 20 μm and 2,000 mN or more, and a polypropylene distribution (hereinafter referred to as PP distribution) being uniform in the in-plane direction A microporous membrane.

Moreover, the manufacturing method of the polyolefin microporous film of this invention has the following structure. That is,
(A) A step of preparing a polyolefin solution by melt-kneading a polyolefin resin and a film-forming solvent, wherein the polyolefin resin contains polyethylene as a main component and has a weight average molecular weight of 1.0 × 10 ⁶ or more. A molecular weight polyethylene, a polyethylene having a melting point of 130 ° C. or less, and a polypropylene having a weight average molecular weight of more than 6.0 × 10 ^{4 and} less than 3.0 × 10 ⁵ in an amount of 0.5% by weight or more and less than 5% by weight. Including steps,
(B) a step of extruding a polyolefin solution to form a molded body at a shear rate of 60 / sec or more,
(C) a step of cooling the obtained extruded product at a cooling rate of 30 ° C./sec or more to form a gel-like sheet,
(D) a step of stretching the obtained gel sheet in at least a uniaxial direction to create a stretched product,
(E) A method for producing a polyolefin microporous membrane comprising a step of removing the film-forming solvent from the obtained stretched product.

The polyolefin microporous membrane of the present invention was measured by Raman spectroscopy.
The average value of the normalized polypropylene / polyethylene ratio (hereinafter, normalized PP / PE ratio) 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 It is preferably 1.0 or less and -1.0 or more.

In the polyolefin microporous membrane of the present invention, the polypropylene preferably has a weight average molecular weight of more than 6.0 × 10 ^{4 and} less than 3.0 × 10 ⁵ .

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

In the polyolefin microporous membrane of the present invention, the first polyolefin resin preferably contains 1.0% by weight or more and 50.0% by weight or less of polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more.

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

In the polyolefin microporous membrane of the present invention, the content of polyethylene having a melting point of 130 ° C. or less is preferably 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 is composed of three or more microporous layers,
A first microporous layer made of a first polyolefin resin constituting at least one of the surface layers, and a second microporous layer made of a second polyolefin resin disposed between both surface layers, It is preferable that either one or both of the microporous layer and the second microporous layer contain polyethylene having a melting point of 130 ° C. or lower.

In the polyolefin microporous membrane of the present invention, the content of polyethylene having a melting point of 130 ° C. or less is preferably 10.0% by weight or more and 38.0% by weight or less of the first polyolefin resin or the second polyolefin resin. .

In the polyolefin microporous membrane of the present invention, the second polyolefin resin contains 1.0% by weight or more and 50.0% by weight or less of polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more, and does not contain polypropylene. Is preferred.

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 total thickness T (B) of each layer disposed between both surface layers satisfy the relationship of Formula 1.
60 ≦ T (A) / (T (A) + T (B)) × 100 (Equation 1)

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

The polyolefin microporous membrane of the present invention comprises polypropylene and polyethylene, has an electrolyte solution pouring property of 20 seconds or less, a shutdown temperature of 132 ° C. or less, and an air permeability of 700 μm / thickness converted to a film thickness of 20 μm. 100cm ³ or less, puncture strength converted to a film thickness of 20 μm is 2,000 mN or more, and the PP distribution is uniform in the in-plane direction, resulting in oxidation resistance, electrolyte solution pouring property, and shutdown characteristics. Excellent and further excellent in permeability and strength balance.

When the polyolefin microporous membrane of the present invention is used for a battery separator, the polypropylene that contributes to oxidation resistance exists without being unevenly distributed on the surface in contact with the electrode. The battery life can be extended. Further, since the shutdown temperature is lower, the battery reaction can be safely stopped during an abnormal reaction.

Furthermore, the polyolefin microporous membrane of the present invention is excellent in air permeability and strength balance, can exhibit the same electrolyte solution pouring property as the polyethylene microporous membrane, and exhibits a uniform film thickness distribution. For this reason, 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 extended in terms of excellent oxidation resistance.

Further, according to the method 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.

It is a graph which shows the distribution map of normalized PP / PE ratio of the 1st microporous layer of the polyolefin microporous film (Example 2) of this invention. It is a graph which shows the two-dimensional distribution map of normalized PP / PE ratio of the 1st microporous layer of the polyolefin microporous film (Example 2) of this invention. It is a graph which shows the two-dimensional distribution map of normalized PP / PE ratio of the 1st microporous layer of a polyolefin microporous film (comparative example 2).

Hereinafter, embodiments for carrying out the present invention will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyolefin microporous membrane of the present invention may be a single layer or a multilayer of two or more layers. Among these, a multilayer microporous film composed of three 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 a polyolefin resin (first polyolefin resin) containing polyethylene as a main component and containing polypropylene. Furthermore, the first microporous layer is at least one surface layer of the polyolefin microporous membrane of the present invention. The layers other than the first microporous layer may be a second microporous layer composed of the second polyolefin resin. When the polyolefin microporous membrane of the present invention is a multilayer microporous membrane comprising a plurality of microporous layers, both surface layers (skin layers) are first microporous layers, and a second layer is formed between both surface layers (core layers). It preferably has a three-layer structure in which a microporous layer is arranged.

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

[1] Raw material [Polyolefin resin]
The polyolefin resin constituting the polyolefin microporous membrane of the present invention is composed mainly of polyethylene (PE), and the entire polyolefin resin is 100% by weight, and the proportion of polyethylene is preferably 80% by weight or more, more preferably 90% by weight or more. More preferably, it contains 95% or more. The polyolefin resin may be a composition containing a resin other than polyolefin. Therefore, the term “polyolefin resin” may include not only polyolefin but also resin 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, when the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous membranes, the polyolefin resin includes the first polyolefin resin and the second microporous membrane constituting the first microporous layer. The second polyolefin resin constituting the layer may be used.

[First polyolefin resin]
In the polyolefin microporous membrane of the present invention, the first microporous layer is composed of a first polyolefin resin. The first polyolefin resin contains polypropylene in addition to polyethylene. Details of each component are shown below.

Polyethylene Polyethylene (a) Mw (weight average molecular weight) is less than 1.0 × 10 ⁶ polyethylene (hereinafter “PE (A)”) or (b) PE (A) and Mw is 1.0 ×. A composition comprising 10 ⁶ or more ultrahigh molecular weight polyethylene (UHMwPE) (hereinafter referred to as “PE composition (B)”) is preferred.

The ratio Mw / Mn (molecular weight distribution) of Mw and number average molecular weight (Mn) of PE (A) and PE composition (B) is not limited, but is preferably in the range of 5 to 300. Is more preferable, and a range of 5 to 25 is particularly preferable. When Mw / Mn is within the above preferred range, the polyethylene solution can be easily extruded, and the resulting polyolefin multilayer microporous membrane is excellent in strength.

PE (A)
PE (A) may be any 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 α-olefin. Examples of α-olefins other than ethylene include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like.

PE (A) has a weight average molecular weight (Mw) of less than 1.0 × 10 ⁶ , for example in the range of about 2.0 × 10 ⁵ to about 0.9 × 10 ⁶ , about 2.0 to 50.0. A molecular weight distribution within the range (MWD, defined as Mw divided by number average molecular weight Mn) and polyethylene having less than 0.20 terminal unsaturated groups per 10,000 carbon atoms may be used. The Mw of PE (A) is preferably 1.0 × 10 ⁴ or more and less than 5.0 × 10 ⁵ . In particular, the Mw of HDPE is more preferably 5.0 × 10 ⁴ or more and less than 4.0 × 10 ⁵ . PE (A) may be made of two or more types having different Mw or densities. Optionally, PE (A) has a terminal unsaturation of less than or equal to 0.14 per 10,000 carbon atoms, or less than or equal to 0.12, for example in the range of 0.05 to 0.14 (eg below the measurement limit). Has a group.
The melting point of PE (A) is preferably higher than 130 ° C.

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

The content of UHMwPE is preferably 50.0% by weight or less based on 100% by weight of the entire first polyolefin resin. Especially preferably, it is 45.0 weight% or less. When the content is within the above preferable range, the pressure does not increase during molding, and the productivity is good. Further, the lower limit of the content is not particularly limited, but is more preferably 1.0% by weight and particularly preferably 30.0% by weight from the viewpoint of maintaining mechanical strength and maintaining a high meltdown temperature. By setting UHMwPE to 1 wt% or more and 50.0 wt% or less, a polyolefin microporous membrane having an excellent balance of strength and air permeability can be obtained.

The Mw of UHMwPE is preferably in the range of 1.0 × 10 ⁶ to 3.0 × 10 ⁶ . By making the Mw of UHMwPE to be 3.0 × 10 ⁶ or less, melt extrusion can be facilitated. UHMwPE is not limited to a homopolymer of ethylene but may be a copolymer containing a small amount of other α-olefin. Other α-olefins other than ethylene may be the same as described above.

PE composition (B), polybutene-1 of the Mw as optional ingredients ^{1.0 × 10 4 ~ 4.0 × 10} 6 , and Mw of ^{1.0 × 10 4 ~ 4.0 × 10} 6 ethylene / Any of α-olefin copolymers may be included. 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 polypropylene is preferably less than 5.0% by weight, with the total weight of the first polyolefin resin being 100% by weight. The upper limit of the polypropylene content is preferably 3.5% by weight. The lower limit of the polypropylene content 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.

The Mw of polypropylene is preferably larger than 6.0 × 10 ^{4 and} smaller than 3.0 × 10 ^5, more preferably larger than 6.0 × 10 ^{4 and} smaller than 1.5 × 10 ⁵ . The molecular weight distribution (Mw / Mn) of polypropylene is preferably 1.01 to 100, and more preferably 1.1 to 50. The polypropylene may be a single material or a composition containing two or more types of polypropylene.

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

As the polypropylene, not only a homopolymer but also a block copolymer and / or a random copolymer containing other α-olefin or diolefin may be used. Other olefins are preferably ethylene or α-olefins having 4 to 8 carbon atoms. Examples of the α-olefin having 4 to 8 carbon atoms include 1-butene, 1-hexene, 4-methyl-1-pentene and the like. The diolefin preferably has 4 to 14 carbon atoms. Examples of the diolefin 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% with respect to 100 mol% of the propylene copolymer.

[Second polyolefin resin]
The aspect of the 2nd polyolefin resin which constitutes the 2nd microporous layer is as follows.

The second polyolefin resin includes polyethylene. As the polyethylene, the polyethylene described in the first polyolefin resin can be used. That is, polyethylene has (a) Mw (weight average molecular weight) less than 1.0 × 10 ⁶ polyethylene (PE (A)) or (b) PE (A), and Mw is 1.0 × 10 ⁶ or more. It is preferable that it is a composition (PE composition (B)) consisting of the ultra high molecular weight polyethylene (UHMwPE). The second polyolefin resin preferably does not contain polypropylene.

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

The content of UHMwPE is preferably 50.0% by weight or less based on 100% by weight of the entire second polyolefin resin and 100% by weight of the entire polyethylene. Especially preferably, it is 30.0 weight% or less. This is because when the content is within the above range, an increase in pressure is suppressed even during molding, and productivity is improved. The lower limit of the content is not particularly limited, but is more preferably 1.0% by weight and particularly preferably 5.0% by weight from the viewpoint of maintaining mechanical strength and maintaining a high meltdown temperature. By setting UHMwPE to 1.0 wt% or more and 50.0 wt% or less, a polyolefin microporous film having an excellent balance of strength and air permeability can be obtained.

PE composition (B), polybutene-1 of the Mw as optional ingredients ^{1.0 × 10 4 ~ 4.0 × 10} 6 , and Mw of ^{1.0 × 10 4 ~ 4.0 × 10} 6 ethylene / Any of α-olefin copolymers may be included. These contents are 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 and polypropylene, or compositions containing resins other than polyolefins. Examples of polyolefins other than polyethylene and polypropylene include, besides polybutene-1, homopolymers and copolymers such as pentene-1, hexene-1, 4-methylpentene-1, and octene.

In addition, 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 International Publication WO2006 / 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 within the above preferable range, mechanical strength such as puncture strength and tensile rupture strength is excellent.

[Low-melting polyethylene (hereinafter referred to as PE (C))]
When the polyolefin microporous membrane of the present invention is a single layer membrane, it is preferable that a part of PE (A) in the first polyolefin resin is replaced with polyethylene (PE (C)) having a melting point of 130 ° C. or less. .
When the polyolefin microporous membrane of the present invention is a multilayer microporous membrane comprising a plurality of microporous layers, either PE (A) in the first polyolefin resin or PE (A) in the second polyolefin resin , Or both are preferably partially replaced by PE (C). More preferably, either PE (A) in the first polyolefin resin or PE (A) in the second polyolefin resin is partially replaced with PE (C). Especially, it is preferable that PE (C) is contained in the first polyolefin resin. This is because a lower shutdown temperature can be obtained.
The upper limit of the content of PE (C) is 38.0% by weight, more preferably 35.0% by weight, based on the total weight of the first polyolefin resin or the second polyolefin resin containing PE (C) as 100% by weight. It is. The lower limit of the content of PE (C) is 10.0% by weight, more preferably 15.0% by weight. By containing 10.0% by weight or more of PE (C), a polyolefin microporous membrane having good oxidation resistance and excellent physical property balance can be obtained while maintaining a shutdown temperature of 132 ° C. or lower.

The upper limit of the melting point of PE (C) is 130 ° C, more preferably 128 ° C. The lower limit of the melting point of PE (C) is 110 ° C, more preferably 115 ° C. The upper limit of Mw of PE (C) is preferably 4.0 × 10 ⁵ , more preferably 3.5 × 10 ⁵ . The lower limit of the Mw of PE (C) is preferably 5.0 × 10 ³ , more preferably 6.0 × 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 may be composed of the first polyolefin resin or the second polyolefin resin, but preferably does not contain polypropylene.

[2] Manufacturing method of polyolefin microporous membrane Next, the manufacturing method of the polyolefin microporous membrane of the present invention is explained. In addition, the manufacturing method of the polyolefin microporous film of this invention is not limited to this.

The method for producing a polyolefin microporous membrane of the present invention includes:
(A) a step of melt-kneading a polyolefin resin and a film-forming solvent to prepare a polyolefin solution,
The polyolefin resin is composed mainly of polyethylene,
Ultra high molecular weight polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more,
Including a polyethylene having a melting point of 130 ° C. or less and a polypropylene having a weight average molecular weight of greater than 6.0 × 10 ^{4 and} less than 3.0 × 10 ⁵ , in a range of 0.5 wt% or more and less than 5 wt%;
(B) Extruding the polyolefin solution at a shear rate of 60 / sec or more to form a molded body;
(C) a step of cooling the obtained extruded product at a cooling rate of 30 ° C./sec or more to form a gel sheet;
(D) a step of stretching the obtained gel-like sheet in at least a uniaxial direction to create a stretched 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 comprising a plurality of microporous layers can be roughly classified into four types according to the laminating method.
(2-1) First Production Method A first production method for producing a polyolefin multilayer microporous membrane is as follows: (i) a first polyolefin resin and a film-forming solvent are melt-kneaded to obtain a first polyolefin solution. And (ii) preparing a second polyolefin solution by melt-kneading the second polyolefin resin and a film-forming solvent, and (iii) extruding the first and second polyolefin solutions simultaneously from one die, (Iv) The obtained extruded product is cooled to form a gel sheet. And (v) a step of creating a stretched product by stretching the gel-like sheet at least in a uniaxial direction (first stretching step), (vi) a step of removing (washing) the film-forming solvent from the stretched product, and ( vii) The process of drying the film | membrane after washing | cleaning is included. After the steps (i) to (vii), the method may further include (viii) a step of stretching the dried film at least in a uniaxial direction (second stretching step), and (ix) a step of heat treatment. If necessary, any one of the heat setting treatment step, the heat roll treatment step, and the heat solvent treatment step may be provided before the film forming solvent removal step (vi). Further, after the steps (i) to (ix), a drying step, a heat treatment step, a crosslinking treatment step by ionizing radiation, a hydrophilization treatment step, a surface coating treatment step, and the like may be provided. Further, (v) a step of heat-treating the stretched product may be provided after the first stretching step.

(I) Preparation of first polyolefin solution The first polyolefin resin and a film-forming solvent are melt-kneaded to prepare a first polyolefin solution. A suitable film forming solvent is blended with the first polyolefin resin described above, 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 specifications of Japanese Patent Nos. 2132327 and 3347835 can be used. Since the melt-kneading method is well-known, description is abbreviate | omitted. However, the polyolefin resin concentration of the polyolefin resin solution is 20 to 50% by weight, preferably 25 to 45% by weight of the polyolefin resin, where the total of the polyolefin resin and the solvent for film formation is 100% by weight. When the polyolefin resin concentration of the polyolefin resin solution is within the above range, a decrease in productivity and a decrease in moldability of the gel-like sheet are prevented.

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

(Ii) Preparation of second polyolefin solution A second polyolefin resin and a film-forming solvent are melt-kneaded to prepare a second polyolefin solution. The film forming solvent used for the second polyolefin solution may be the same as or different from the film forming solvent used for the first polyolefin solution, but is preferably the same. The other preparation methods may be the same as in the preparation of the first polyolefin solution.
As the second polyolefin resin, those described above can be used.

(Iii) Extrusion The first and second polyolefin solutions are each fed from an extruder to a die where they are combined in layers and extruded into sheets. When 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 is at least between both surface layers. Both solutions are combined in layers and extruded into sheets to form a single layer (second microporous layer) (preferably in contact with one or both of the surface layers).

The extrusion method may be either a flat die method or an inflation method. In either method, the solution is supplied to separate manifolds and stacked in layers at the lip inlet of a multilayer die (multiple manifold method), or the solution is supplied to the die in a layered flow in advance (block method) Can be used. Since the multi-manifold method and the block method itself are known, a detailed description thereof will be omitted. The gap of the multilayer flat die is preferably 0.1 to 5 mm. The extrusion temperature is preferably 140 to 250 ° C., and the extrusion speed is preferably 0.2 to 15 m / min. By adjusting the extrusion amounts of the first and second polyolefin solutions, the film thickness ratio of the first and second microporous layers can be adjusted.

The screw length (L) to diameter (D) ratio (L / D) of the twin screw extruder is preferably in the range of 20-100. The inner diameter of the twin screw extruder is preferably 40 to 200 mm. When the polyolefin resin is put into the twin screw extruder, the ratio Q / Ns of the amount Q (kg / h) of the polyolefin resin solution to the screw rotation speed Ns (rpm) is set to 0.1 to 0.55 kg / h / rpm. Is preferred. 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 500 rpm is preferable.

As the extrusion method, for example, the methods disclosed in Japanese Patent Nos. 2132327 and 3347835 can be used. In the method for producing a microporous polyolefin membrane of the present invention, a polyolefin resin containing the first polyolefin resin solution is used. The shear rate of the solution from the die is 60 / sec or more. The shear rate from the die is more preferably 150 / sec or more.

(Iv) Formation of gel-like sheet The extruded product obtained in (iii) is cooled to form a gel-like sheet. As a method for forming a gel-like sheet, for example, methods disclosed in Japanese Patent Nos. 2132327 and 3347835 can be used. Cooling is preferably performed until the extruded product reaches 40 ° C. or lower. By cooling, the polyolefin microphase separated by the film-forming solvent can be immobilized. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used.

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

If the shear rate from the die and the cooling rate are appropriately controlled, it is easy to make the distribution of polypropylene uniform in the gel sheet, and the oxidation resistance and the electrolyte solution pouring property become good.

(V) First stretching step The obtained gel-like sheet is stretched in at least a uniaxial direction. The first stretching causes cleavage between the polyethylene crystal lamella layers, the polyethylene phase is refined, and a large number of fibrils are formed. The obtained fibrils form a three-dimensional network structure (a network structure that is irregularly connected three-dimensionally). Since the gel-like sheet contains a film-forming solvent, it can be stretched uniformly. The first stretching can be performed at a predetermined magnification by heating the gel-like sheet and then using a normal tenter method, roll method, inflation method, 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 simultaneous biaxial stretching or sequential stretching may be performed.

The draw ratio varies depending on the thickness of the gel-like sheet, but it is preferably 2 times or more, more preferably 3 to 30 times in uniaxial stretching. In biaxial stretching, it is preferable to make at least 3 times or more in any direction, that is, 9 times or more in area magnification, thereby improving the puncture strength of the obtained polyolefin microporous membrane, and increasing the elasticity and strength. When the area magnification is within the above preferable range, there are no restrictions in terms of stretching apparatus, stretching operation, and the like. In biaxial stretching, the magnification in both directions is preferably set to the same magnification.

The first stretching temperature is preferably not more than about 10 ° C. above the melting point of polyethylene used for preparing the polyolefin solution. The stretching temperature may be in the range of more than Tcd to less than Tme. Tme and Tcd are the melting point and crystal dispersion temperature of all polyethylene used for preparing the polyolefin solution, respectively. When the stretching temperature is Tme + 10 ° C. or lower, the orientation of the molecular chains of the polyolefin in the gel sheet tends to be promoted during stretching. On the other hand, when the stretching temperature is Tcd or more, film breakage due to stretching is suppressed, and stretching at a high magnification 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 composed of polyethylene of 90% by weight or more, the stretching temperature is usually in the range of 90 to 130 ° C, preferably in the range of 100 to 125 ° C, more preferably in the range of 105 to 120 ° C.

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

The first stretching may be performed in multiple stages at different temperatures, and the stretching temperature and the final stretching ratio in the former stage and the latter stage are within the above ranges, respectively. Depending on the desired physical properties, the film may be stretched by providing a temperature distribution in the film thickness direction, whereby a polyolefin microporous film having further excellent mechanical strength can be obtained. As the method, for example, the method disclosed in Japanese Patent No. 3347854 can be used.

(Vi) Film forming solvent removal (cleaning) step Next, the film forming solvent remaining in the stretched gel-like sheet (stretched product) is removed using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent, a porous film can be obtained by removing the film-forming solvent. Since the cleaning solvent and the method for removing the film-forming solvent using the same are known, the description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Laid-Open No. 2002-256099 can be used.

(Vii) Membrane drying step The polyolefin microporous membrane obtained by removing the film-forming solvent is dried by a heat drying method, an air drying method or the like.

(Viii) Second stretching step Furthermore, the dried film may be stretched again in at least a uniaxial direction. The second stretching can be performed by a tenter method or the like, similar to the first stretching, while heating the film. The second stretching may be uniaxial stretching or biaxial stretching.

The second stretching temperature may be substantially the same or lower than the melting point Tme of all polyethylene used for preparing the polyolefin solution. In one embodiment, the second stretching temperature is from about Tcd to about Tme. When the second stretching temperature is Tme or less, the resulting polyolefin microporous membrane has appropriate permeability and tends to suppress variations in physical properties such as permeability in the lateral direction (width direction: TD direction). When the second stretching temperature is Tcd or higher, film breakage due to stretching is suppressed, and uniform stretching can be achieved. When the polyolefin resin is made of polyethylene, the stretching temperature is usually in the range of 90 to 140 ° C, preferably in the range of 100 to 140 ° C.

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, in the MD direction (referred to as the film production direction, also referred to as the machine direction or the longitudinal direction) or the TD direction (referred to as the same direction as the longitudinal direction and perpendicular to the transverse direction). 1 to 1.8 times. In the case of biaxial stretching, it is 1.1 to 1.8 times in the MD direction and TD direction, respectively. In the case of biaxial stretching, each stretching ratio in the MD direction and TD direction may be different from each other as long as it is 1.1 to 1.8 times. When the draw ratio was within the above range, the resulting polyolefin microporous membrane had a tendency to improve the permeability, heat shrinkage resistance, electrolyte absorption and compression resistance. The magnification of the second stretching is more preferably 1.2 to 1.6 times.

The second stretching speed is preferably 3% / second or more in the stretching axis direction. For example, in the case of uniaxial stretching, it is 3% / second or more in the MD direction or TD direction. In the case of biaxial stretching, it is 3% / second or more in the MD direction and TD direction, respectively. The stretching speed (% / second) in the stretching axis direction is the ratio of the length stretched per second with the length in the stretching axis direction before re-stretching being 100% in the region where the film (sheet) is re-stretched. Represents. When the stretching speed is 3% / second or more, the resulting polyolefin microporous membrane has an appropriate gas permeability and tends to suppress variations in physical properties such as permeability in the sheet width direction. The speed of the second stretching is preferably 5% / second or more, and more preferably 10% / second or more. In the case of biaxial stretching, each stretching speed in the MD direction and the TD direction may be different from each other in the MD direction and the TD direction as long as it is 3% / second or more, but is preferably the same. Although there is no restriction | limiting in particular in the upper limit of the speed | rate of 2nd extending | stretching, It is preferable that it is 50% / second or less from a viewpoint of fracture | rupture prevention.

(Ix) Heat treatment step The film after the second stretching may be heat treated. A polyolefin microporous membrane that retains a network composed of fibrils formed by the second stretching, has a large pore diameter, and is excellent in strength can be produced. For the heat treatment, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the dimensions of the film unchanged. The thermal relaxation treatment is a heat treatment that heat-shrinks the film in the MD direction or the TD direction during heating. In particular, the crystal of the film is stabilized by the heat setting treatment. The heat treatment can be performed by a conventional method such as a tenter method, a roll method, or a rolling method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be given.

The heat treatment is performed within a temperature range from the crystal dispersion temperature to the melting point of all polyolefin resins constituting the polyolefin microporous membrane. The heat setting treatment temperature is preferably within the range of the second stretching temperature ± 5 ° C., thereby stabilizing the physical properties. This temperature is more preferably within the range of the second stretching temperature ± 3 ° C.

Although not limited, it is preferable to employ an in-line method in which the 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 system in which the dried film is wound once and then unwound to perform second stretching and heat treatment may be adopted.

(X) Other steps Before removing the film-forming solvent from the gel-like sheet subjected to the first stretching, any of a heat setting treatment step, a heat roll treatment step and a heat solvent treatment step may be provided. Moreover, you may provide the process which heat-sets with respect to the film | membrane after a washing | cleaning or a 2nd extending | stretching process. The method for heat-setting the stretched gel-like sheet before and / or after washing and the film during the second stretching step may be the same as described above.

(2-2) Second Manufacturing Method The second method for manufacturing a polyolefin multilayer microporous membrane is as follows: (i) preparing a first polyolefin solution by melting and kneading a first polyolefin resin and a film-forming solvent. And (ii) preparing a second polyolefin solution by melting and kneading the second polyolefin resin and a film-forming solvent, and (iii-2) extruding the first and second polyolefin solutions from separate dies. Immediately after lamination, (iv) the obtained extruded product (laminated product) is cooled to form a gel-like sheet. In other words, the first manufacturing method forms an extruded body by laminating a polyolefin solution in one die, whereas the second manufacturing method is a method in which the solution is laminated immediately after being extruded from a separate die. However, the following steps can employ the same method as the first manufacturing method.

Since the second method is the same as each step in the first manufacturing method except for step (iii-2), only step (iii-2) will be described. In step (iii-2), the first and second polyolefin solutions are extruded into sheets from adjacent dies connected to each of the plurality of extruders, and the temperature of each solution is high (eg, 100 ° C. or higher). Laminate immediately to obtain a laminated extruded product. The other steps may be the same as in the first manufacturing method.

(2-3) Third Production Method A third production method for producing a polyolefin multilayer microporous membrane is as follows: (i) a first polyolefin resin and a film-forming solvent are melt-kneaded to obtain a first polyolefin solution. (Ii) a second polyolefin resin and a film-forming solvent are melt-kneaded to prepare a second polyolefin solution, and (iii-3-1) the first polyolefin solution is extruded from one die. Forming a first extrudate, and (iii-3-2) extruding the second polyolefin solution from another die to form a second extrudate, and (iv-3) the resulting first and The second extrudates were cooled to form first and second gel sheets, respectively (v-3) the first and second gel sheets were stretched, and (xi-3) stretched Laminating the first and second stretched materials, (vi ) The film-forming solvent is removed from the obtained stretched product. That is, it is performed separately until the gel-like sheet is stretched and then laminated. The following steps can employ the same method as the first manufacturing method. Between the steps (vi-3) and (vii-3), (viii-3) a step of stretching the gel-like laminated sheet may be provided. Steps (iii-3-1) and (iii-3-2) differ from 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 step (iii-2) in the second manufacturing method. Step (iv-3) differs from step (iv) in the first production method only in that the first and second extrudates are separately cooled. Step (v-3) differs from step (v) in the first production method only in that the first and second gel sheets are each stretched. On the other hand, the step (xi-3) is a step that is not in the first and second production methods of laminating the first and second stretched products, but a known method may be used for laminating the stretched products. .

(2-4) Fourth Production Method A fourth production method for producing a polyolefin multilayer microporous membrane is as follows: (i) a first polyolefin resin and a film-forming solvent are melt-kneaded to obtain a first polyolefin solution. And (ii) preparing a second polyolefin solution by melt-kneading the 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 another die, and (iv-4) cooling each of the obtained extrudates to form first and second gel sheets, v-4) stretching the first and second gel sheets, respectively (vi-4) removing the film-forming solvent from each stretched product, and (vii-4) the first and second obtained Dry the microporous polyolefin membrane of ( viii-4) a step of stretching at least the second polyolefin microporous membrane and (xi-4) laminating the first and second polyolefin microporous membranes. That is, the process is performed separately until the porous film is formed, and then laminated to form a multilayer microporous film. If necessary, a heat treatment step may be performed on each of the first and second polyolefin microporous membranes (ix-4) between steps (vii) and (viii-4). Moreover, the following processes can take the same method as a 1st manufacturing method.

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

On the other hand, step (viii-4) is not necessarily required in the first to third production methods, but in the fourth production method, at least the second polyolefin microporous membrane is regenerated in this step (viii-4). Stretch. The stretching temperature is preferably below the melting point, more preferably from the crystal dispersion temperature to the melting point. If necessary, the first polyolefin microporous membrane may also be stretched. The stretching temperature is preferably below the melting point, more preferably from the crystal dispersion temperature to the melting point. In either case of stretching the first and second polyolefin microporous membranes, the stretch ratio may be the same as in the first production method except that the non-laminated polyolefin microporous membrane is stretched.

The step (xi-4) is a step that is not in the first to third manufacturing methods of laminating the first and second films, but the laminating of the film is a known method as in the case of laminating the stretched product. May be used.

As mentioned above, although the manufacturing method of the polyolefin microporous film of this invention was demonstrated by four classification according to the lamination | stacking method, when these are put together, it will become process (a)-(e) as a required process.

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 manufacturing method, the step (iii-2) of the second manufacturing method, the step (iii-3-1) of the third manufacturing method, and the fourth manufacturing. This corresponds to step (iii-4-1) of the method.
Step (c) includes steps (iv) of the first manufacturing method, steps (iv-2) of the second manufacturing method, steps (iv-3) of the third manufacturing method, and steps of the fourth manufacturing method. This corresponds to the step (iv-4).
Step (d) corresponds to step (v) of the first and 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 in the above-mentioned multilayer microporous membrane production method. As an example, the polyolefin microporous membrane of the present invention is (2-4) (i), (ii), (iii-4-1), (iv-4), (v-4) of the fourth production method. , (Vi-4) and (vii-4).

[3] Structure, physical properties and measurement method of polyolefin microporous membrane The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties. Hereinafter, the structure, physical properties, and measurement methods thereof will be described.

(1) Normalized PP / PE ratio The polyolefin microporous membrane of the present invention has a structure in which 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 relative value when the maximum PP / PE ratio on the film surface is set to 1 with respect to the peak intensity ratio of PP and PE (PP / PE ratio) obtained by microscopic Raman spectroscopy. If the normalized PP / PE ratio is used, it can be expressed as a structure in which the average value / standard deviation / kurtosis of the normalized PP / PE ratio shows a constant value. That is, in the polyolefin microporous membrane of the present invention, the normalized PP / PE ratio has an average value of 0.5 or more, a standard deviation of 0.2 or less, and a kurtosis of 1.0 or less as a parameter indicating the shape of the distribution. It preferably has a structure of 1.0 or more.

A method for measuring the PP / PE ratio on the film surface by micro-Raman spectroscopy will be described below. By microscopic Raman spectroscopy, area analysis was performed with a 1 micron spot diameter in a depth direction of 1 to 2 microns and a 20 × 20 micron field using a wavelength of 532 nm laser, and a total of 400 points of frequency 807 cm ⁻¹ (PP), The peak intensity ratio of 1127 cm ⁻¹ (PE) is measured. The relative value when the maximum value of the intensity ratio in the 20 × 20 micron visual field is 1 is defined as “standardized PP / PE ratio”.

When the average value of the standardized PP / PE ratio is within the above-mentioned preferable range, there are few portions with low polypropylene concentration, polyethylene does not increase, and polyethylene is not oxidized by the oxidation reaction accompanying charging / discharging in the battery. Since there are few main parts, deterioration is hard to progress and it is thought that cycling characteristics are maintained favorable.

When the standard deviation of the normalized PP / PE ratio is within the above-mentioned preferable range, the change in polypropylene concentration is small, and there are few places with low polypropylene concentration.

In addition, when the distribution of the polypropylene concentration is within the above-mentioned preferable range, there are few places where the polypropylene concentration is low, a portion having poor oxidation resistance in the battery hardly occurs, and the battery performance is good. The presence of a portion having a high polypropylene concentration to some extent tends to improve the oxidation resistance. From these results, it was found that an appropriate normalized PP / PE distribution is essential for improving 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, it has excellent oxidation resistance. Furthermore, when the content of polypropylene is as low as less than 5% by weight, it is preferable because deterioration of physical properties due to polypropylene is suppressed and the permeability, strength, and electrolyte absorption are excellent. Therefore, when used as a separator for a lithium ion battery, excellent battery productivity, safety, and battery cycle characteristics can be realized.

(2) air permeability (sec / 100cm ³ / 20μm)
The upper limit of the air permeability (Gurley value) when the film thickness of the polyolefin microporous membrane of the present invention is converted to 20 μm is preferably 700 seconds / 100 cm ³ , more preferably 600 seconds / 100 cm ³ , and still more preferably. 550 seconds / 100 cm ³ . The lower limit of the air permeability when the thickness of the polyolefin microporous membrane of the present invention is converted to 20 μm is preferably 20 seconds / 100 cm ³ , more preferably 100 seconds / 100 cm 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 battery cycle characteristics are good, and the battery is shut down sufficiently when the temperature inside the battery rises. When used for the above, the resistance value is hardly increased during charge and discharge, and the average electrochemical stability is good. The air permeability is a value obtained by measuring according to JIS P 8117 and converting the film thickness to 20 μm.

(3) 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 the short circuit of the electrode is suppressed. The porosity is a value measured by a mass method.
Porosity (%) = 100 × (w2−w1) / w2
w1: Actual weight of microporous membrane w2: Weight of equivalent non-porous membrane (of the same polymer) having the same size and thickness

(4) Puncture strength (mN / 20 μm)
The puncture strength is a value obtained by measuring the maximum load value when a polyolefin microporous membrane is pierced at a speed of 2 mm / sec using a needle having a diameter of 1 mm (0.5 mmR) and converting the film thickness to 20 μm. is there. The puncture strength when the film thickness of the polyolefin microporous membrane of the present invention is converted to 20 μm is preferably 2,000 mN or more, more preferably 4,000 mN or more, and further preferably 5,000 mN or more. When the puncture strength is 2,000 mN / 20 μm or more, short-circuiting of electrodes can be effectively suppressed when a polyolefin microporous membrane is incorporated in a battery as a battery separator.

(5) Tensile strength at break (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 further preferably 100,000 kPa or more in both the MD direction and the TD direction. When the tensile breaking strength is 60,000 kPa or more, it is easy to prevent film breakage during battery production. The tensile strength at break is a value measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.

(6) Tensile elongation at break (%)
The tensile fracture elongation of the polyolefin microporous membrane of the present invention is preferably 80% or more, more preferably 100% or more in both the MD direction and the TD direction. Thereby, it is easy to prevent film breakage during battery production. The tensile elongation at break is a value measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.

(7) Thermal shrinkage (%)
The thermal shrinkage after exposure for 8 hours at 105 ° C. of the polyolefin microporous membrane of the present invention is preferably 10% or less in both MD and TD directions, more preferably 8% or less, and even more preferably 6% or less. It is. When the heat shrinkage rate is 10% or less, when the polyolefin microporous membrane is used as a lithium battery separator, the end of the separator shrinks during heat generation, and the possibility of short-circuiting of the electrode is reduced.

The heat shrinkage is a value obtained by measuring the heat shrinkage in the MD direction and the TD direction three times each when the polyolefin microporous membrane is exposed at 105 ° C. for 8 hours, and calculating the average value. The thermal contraction rate is expressed by the following formula.
Thermal shrinkage (%) = 100 × (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 ° C or lower, more preferably 128 ° C or lower, and still more preferably 126 ° C or lower. The shutdown temperature is measured by the method disclosed in International Publication No. 2007/052663. According to this method, the polyolefin microporous membrane is exposed to an atmosphere of 30 ° C., and the temperature is raised at 5 ° C./min, during which the air permeability of the membrane is measured. The shutdown temperature of the polyolefin microporous membrane was defined as the temperature at which the air permeability (Gurley value) of the polyolefin microporous membrane first exceeded 100,000 seconds / 100 cm ³ . The air permeability of the microporous polyolefin membrane is measured according to JIS P 8117 using an air permeability meter (Asahi Seiko Co., Ltd., EGO-1T).

(9) Electrolytic solution pouring property The electrolytic solution pouring property of the polyolefin microporous membrane of the present invention is 20 seconds or less. More preferably, it is 10 seconds or less, and further preferably 5 seconds or less. The electrolyte solution pouring property was evaluated by the penetration time of propylene carbonate. A sample of 50 mm × 50 mm is placed on a glass plate, 0.5 ml of propylene carbonate is dropped from about 2 cm above the sample, and time measurement is started from the end of dropping. Immediately after the completion of dropping, propylene carbonate rises on the film due to surface tension, but the dropped propylene carbonate penetrates with the passage of time. When all the propylene carbonate on the membrane has permeated, the time measurement is stopped and the permeation time is taken. Penetration time of 20 seconds or less is good, 20 seconds or more and 50 seconds or less are good, and those exceeding 50 seconds are unsuitable.

(10) Average electrochemical stability (leakage current value) (mAh)
To measure electrochemical stability, a film having a length of 70 mm (MD) and a width of 60 mm (TD) is placed between a negative electrode and a positive electrode 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 1M solution in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (3/7, V / V). An electrolyte is impregnated in the film in the region between the negative electrode and the positive electrode to complete the battery.

Next, the battery is exposed to an applied voltage of 4.3 V while being exposed to a temperature of 60 ° C. for 28 days. The term “electrochemical stability” is defined as the integrated current (mAh) flowing between the voltage source and the battery over 28 days. Electrochemical stability is measured on three batteries under the same conditions (three batteries of the same condition are made from three film samples of the same condition). The average electrochemical stability (leakage current value) is the average (arithmetic average) of the measured electrochemical stability values of the three batteries.

Electrochemical stability is a film property related to the oxidation resistance of the film when the film is used as a separator in a battery that is exposed to relatively high temperatures during storage or use. Electrochemical stability is in mAh, and generally lower values are desirable (representing less total charge loss during storage or overcharge at high temperatures). Batteries for automobiles, such as batteries used for starting or feeding power to electric vehicles and hybrid electric vehicles, and power tool batteries are used for relatively high output and large capacity applications. Therefore, even a slight loss of battery capacity such as a self-discharge loss due to the electrochemical instability of the battery separator is an important 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 “high capacity” battery usually refers to a battery that can be supplied for 1 amp hour (1 Ah) or more, for example, 2.0 Ah to 3.6 Ah.

(11) Film thickness The film thickness of the polyolefin microporous film of the present invention is preferably 5 to 50 μm, more preferably 5 to 35 μm, even more preferably 10 to 25 μm, for example, when used as a battery separator. The method for measuring the film thickness may be a contact thickness measurement method or a non-contact thickness measurement method. For example, it can be measured with a contact-type thickness meter over a width of 10.0 cm at intervals of 1.0 cm in the vertical direction, and then the average value can be obtained to obtain the film thickness. As the contact-type thickness gauge, for example, a thickness gauge such as Mitutoyo Corporation Lightmatic 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 each layer disposed between both surface layers Preferably satisfies the relationship of Formula 1.
60 ≦ T (A) / (T (A) + T (B)) × 100 (Equation 1)
By satisfy | filling the relationship of Formula 1, the polyolefin microporous film excellent in the lower shutdown temperature can be obtained. It is more preferable to satisfy the relationship of Formula 2.
60 ≦ T (A) / (T (A) + T (B)) × 100 <90 (Formula 2)
By satisfying the relationship of Formula 2, it is possible to obtain a polyolefin microporous film that has an excellent balance between strength and permeability and that is excellent at a low shutdown temperature.
Moreover, it is more preferable to satisfy the relationship of Formula 3.
60 ≦ T (A) / (T (A) + T (B)) × 100 <85 (Equation 3)

(12) Appearance The appearance of the film was evaluated by visual observation / multipoint film thickness measurement. A sample having a small variation in thickness by visual observation was evaluated as “good”. “Good” 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 in accordance with JIS K 7122 according to the following procedure. That is, the resin sample was left in a sample holder of a scanning differential calorimeter (Perkin Elmer, Inc., DSC-System7 type), heat-treated at 230 ° C. for 10 minutes in a nitrogen atmosphere, and heated at 10 ° C./minute for 40 minutes. After cooling to 0 ° C., it was held at 40 ° C. for 2 minutes and then heated to 230 ° C. at a rate of 10 ° C./min. The temperature at which the maximum endotherm was reached (peak temperature) was taken as the melting point.

[4] Battery, etc. As described above, the polyolefin microporous membrane of the present invention is excellent in oxidation resistance and electrolyte solution pouring, and is less likely to be blackened after repeated charge and discharge as a battery. Since it is excellent in strength balance, it is particularly suitable as a battery separator.

The separator comprising the polyolefin microporous membrane of the present invention can be used for batteries and electric double layer capacitors. Although there is no restriction | limiting in particular in the kind of battery / capacitor using this, It is suitable for a lithium secondary battery / lithium ion capacitor use especially. For the lithium secondary battery / capacitor using the separator made of the polyolefin microporous membrane of the present invention, a known electrode and electrolyte may 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.

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. In addition, each physical property of the polyolefin microporous film was calculated | required by the above-mentioned method.

Example 1
(1) Preparation of first polyolefin solution (a) HDPE having a Mw of 2.0 × 10 ⁶ (Mw / Mn: 8, melting point: 136 ° C.) 20% by weight based on the total weight of the first polyolefin composition ( b) HDPE having a Mw of 2.5 × 10 ⁵ (Mw / Mn: 8.6, per terminal vinyl group concentration of 0.1 / 10,000 carbon, melting point: 134 ° C.) 57% by weight, (c) Mw of 1. 8 × 10 ⁴ HDPE (Mw / Mn: 2.6, melting point: 123 ° C.) 20% by weight, (d) polypropylene having Mw of 9.7 × 10 ⁴ (Mw / Mn: 2.6, melting point: 155 ° C.) ) A first polyolefin composition containing 3% by weight was prepared by dry blending. Tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant is dry blended at 0.2 parts by weight per 100 parts by weight of the first polyolefin composition, A polyolefin resin was prepared.

25 parts by weight of the first polyolefin resin was supplied to the strong kneading twin-screw extruder, and 75 parts by weight of liquid paraffin (50 cSt at 40 ° C.) was supplied from the side feeder to the twin-screw extruder. A first polyolefin solution was prepared by melt-kneading at 210 ° C. and 200 rpm.

(3) Production of microporous membrane The first polyolefin solution was supplied from a twin-screw extruder to a die to form an extruded product. The extruded product was cooled by passing through a cooling roll controlled at 20 ° C. to form a gel-like sheet. In addition, the shear rate in the die | dye of an extrusion molded object was 205 / sec, and the cooling rate with a cooling roll was 37 degrees C / sec. The obtained gel-like sheet was subjected to simultaneous biaxial stretching (first stretching) at a stretching ratio of 5 × 5 at a temperature of 115 ° C. using a tenter stretching machine and wound up. Then, a part was taken from the wound stretched product, fixed to a frame plate [size: 20 cm × 20 cm, made of aluminum (hereinafter the same)], dipped in a methylene chloride washing tank adjusted to 25 ° C., and 100 rpm And washed with rocking for 3 minutes. The washed membrane was air dried at room temperature. The dried microporous membrane was subjected to a second stretching (restretching) at a stretching ratio of 1.2 times in the TD direction at 118.3 ° C. by a batch stretching machine, and then a stretching ratio of 1.0 in the TD direction at the same temperature. A polyolefin microporous membrane was prepared by heat-releasing treatment at a re-stretching temperature for 10 minutes while the sample was heat-relaxed twice and then attached to a batch stretching machine.

Example 2
(1) Preparation of first polyolefin solution (a) HDPE having a Mw of 2.0 × 10 ⁶ (Mw / Mn: 8, melting point: 136 ° C.) 20% by weight based on the total weight of the first polyolefin composition ( b) HDPE having Mw of 2.5 × 10 ⁵ (Mw / Mn: 8.6, terminal vinyl group concentration of 0.1 / 10,000 carbon, melting point: 134 ° C.) 52% by weight, (c) Mw of 2. 4 × 10 ⁴ HDPE (Mw / Mn: 3.0, melting point: 123 ° C.) 25% by weight, (d) Polypropylene with Mw of 9.7 × 10 ⁴ (Mw / Mn: 2.6, melting point: 155 ° C.) ) A first polyolefin composition containing 3% by weight was prepared by dry blending. Tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant is dry blended at 0.2 parts by weight per 100 parts by weight of the first polyolefin composition, A polyolefin resin was prepared.

(2) Preparation of second polyolefin solution The second polyolefin solution was prepared in the same manner as the preparation method of the first polyolefin solution except the following points. (A) 20% by weight of UHMwPE (Mw / Mn: 8.0, melting point: 136 ° C.) having a Mw of 2.0 × 10 ⁶ with respect to the total weight of the second polyolefin composition, (b) Mw of 2.5 × A second polyolefin composition containing 10 ⁵ HDPE (Mw / Mn: 8.6, terminal vinyl group concentration 0.1 / 10,000 carbon, melting point: 134 ° C.) 80% by weight was prepared by dry blending. Tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant is dry blended at 0.2 parts by weight per 100 parts by weight of the second polyolefin composition, A polyolefin resin was prepared. 25 parts by weight of the obtained second polyolefin composition was supplied to a strong kneading twin-screw extruder, and 75 parts by weight of liquid paraffin (50 cSt at 40 ° C.) was supplied from the side feeder to the twin-screw extruder. A second polyolefin solution was prepared by melt-kneading at 210 ° C. and 200 rpm.

(3) Production of microporous membrane The first and second polyolefin solutions are supplied from the respective twin-screw extruders to the three-layer T-die, and the layer constitution is first polyolefin solution / second polyolefin solution / first. A three-layer extruded product having a layer thickness ratio of 35/30/35 was formed from the polyolefin solution. The extruded product was cooled by passing through a cooling roll controlled at 20 ° C. to form a three-layer gel-like laminated sheet. In addition, the shear rate in the die | dye of an extrusion molding was 200 / sec, and the cooling rate with a cooling roll was 36.5 degreeC / sec. The obtained gel-like laminated sheet was subjected to simultaneous biaxial stretching (first stretching) at a stretching ratio of 5 × 5 at a temperature of 115 ° C. using a tenter stretching machine and wound up. Then, a part was taken from the wound stretched product, fixed to a frame plate [size: 20 cm × 20 cm, made of aluminum (hereinafter the same)], dipped in a methylene chloride washing tank adjusted to 25 ° C., and 100 rpm And washed with rocking for 3 minutes. The washed membrane was air dried at room temperature. The dried microporous membrane is subjected to a second stretching (restretching) at a stretching ratio of 1.4 times in the TD direction at 122 ° C. with a batch stretching machine, and then to a stretching ratio of 1.2 times in the TD direction at the same temperature. The polyolefin multilayer microporous membrane was produced by heat-releasing for 10 minutes at the re-stretching temperature with the heat relaxed and then attached to the batch stretching machine.

Examples 2 to 10 and Comparative Examples 1 to 8
A polyolefin microporous membrane was prepared in the same manner as in Example 1 for Comparative Example 1 and in Examples 2 to 10 and Comparative Examples 2 to 8 in the same manner as in Example 2 using the raw materials and conditions shown in Tables 1 and 2. In Tables 1 and 2, “-” indicates that UHMwPE or HDPE2 in the table is not included.

Tables 3 and 4 show the physical properties of the polyolefin microporous membranes of Examples 1 to 10 and Comparative Examples 1 to 8. In Table 4, “-” in Comparative Example 7 indicates that the surface has large irregularities that can be visually judged, and measurement was not possible.

Table 3 and Table 4 show that the polyolefin microporous membranes of Examples 1 to 9 are all excellent in electrolyte solution pouring property and have a uniform PP distribution. Furthermore, the leakage current value is 45 mAh or less, and excellent oxidation resistance is exhibited. In addition, the shutdown temperature is 132 ° C. or lower, and the balance between safety and physical properties is better when used in a battery. FIG. 1 is a graph showing the distribution of the normalized PP / PE ratio of the surface layer of the polyolefin microporous membrane of Example 2, which is concentrated in a narrow range where the normalized PP / PE ratio is 0.5 or more. I understand that 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 almost no region where the polypropylene concentration is low (the dark portion) is observed, and the polypropylene is an average. It can be seen that it exists. On the other hand, FIG. 3 shows a two-dimensional distribution diagram of the normalized PP / PE ratio of the surface layer of the polyolefin microporous membrane of Comparative Example 2, where there are many regions where the polypropylene concentration is low (dark portions), and polypropylene is the surface layer. It can be seen that it does not exist on average.

Industrial application fields

The present invention provides a polyolefin microporous membrane that is excellent in oxidation resistance, electrolytic solution pouring property, and shutdown property, and further excellent in permeability and strength balance.
The polyolefin multilayer microporous membrane of the present invention has suitable performance as an electricity storage device for non-aqueous electrolyte solutions for capacitor use, capacitor use, battery use, etc., and contributes to improvement in safety and reliability. Can do. Among them, it can be suitably used as a battery separator, more specifically as a lithium ion battery separator, and can extend the life and safety of the battery. As other uses, it is also used as various separation membranes such as one component of a fuel cell, a humidification membrane, and a filtration membrane, and thus has industrial applicability in those fields.

Claims

A polyolefin microporous membrane comprising a first polyolefin resin containing polypropylene and polyethylene, having an electrolyte solution injection property of 20 seconds or less, a shutdown temperature of 132 ° C. or less, and an air permeability converted to a film thickness of 20 μm A polyolefin microporous membrane having a degree of 700 sec / 100 cm 3 or less, a puncture strength converted to a film thickness of 20 μm or more and 2,000 mN, and a uniform polypropylene distribution in the in-plane direction.
The average value of the normalized polypropylene / polyethylene ratio 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 kurtosis of the normalized polypropylene / polyethylene ratio is 1.0. 2. The polyolefin microporous membrane according to claim 1, which is at least −1.0 or more.
3. The polyolefin microporous membrane according to claim 1, wherein the polypropylene has a weight average molecular weight of more than 6.0 × 10 4 and less than 3.0 × 10 5 .
The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the first polyolefin resin contains 0.5 wt% or more and less than 5.0 wt% of polypropylene.
The polyolefin fine particle according to any one of claims 1 to 4, wherein the first polyolefin resin contains 1.0% by weight or more and 50.0% by weight or less of polyethylene having a weight average molecular weight of 1.0 × 10 6 or more. Porous membrane.
The polyolefin microporous membrane according to any one of claims 1 to 5, comprising polyethylene having a melting point of 130 ° C or lower.
The polyolefin microporous membrane according to claim 6, wherein the content of polyethylene having a melting point of 130 ° C or lower is 10.0% by weight or more and 38.0% by weight or less of the first polyolefin resin.
The first microporous layer is composed of three or more microporous layers, and comprises at least one of the surface layers. The first microporous layer is composed of the first polyolefin resin and the second microporous layer is disposed between the surface layers. 6. The method according to claim 1, further comprising a porous layer, wherein one or both of the first microporous layer and the second microporous layer includes polyethylene having a melting point of 130 ° C. or less. 2. The polyolefin microporous membrane according to item 1.
The polyolefin microporous membrane according to claim 8, wherein the content of polyethylene having a melting point of 130 ° C or lower is 10.0% by weight or more and 38.0% by weight or less of the first polyolefin resin or the second polyolefin resin.
10. The second polyolefin resin according to claim 8 or 9, wherein the second polyolefin resin contains 1.0% by weight or more and 1.0% by weight or less of polyethylene having a weight average molecular weight of 1.0 × 10 6 or more, and does not contain polypropylene. Polyolefin microporous membrane.
11. The sum of thicknesses T (A) of both surface layers and the total thickness T (B) of each layer disposed between both surface layers satisfy the relationship of Formula 1. The polyolefin microporous membrane according to Item.
60 ≦ T (A) / (T (A) + T (B)) × 100 (Equation 1)
The polyolefin microporous membrane according to any one of claims 1 to 11, wherein the shutdown temperature is 128 ° C or lower.
(A) A step of preparing a polyolefin solution by melt-kneading a polyolefin resin and a film-forming solvent, wherein the polyolefin resin contains polyethylene as a main component and has a weight average molecular weight of 1.0 × 10 6 or more. A molecular weight polyethylene, a polyethylene having a melting point of 130 ° C. or less, and a polypropylene having a weight average molecular weight of more than 6.0 × 10 4 and less than 3.0 × 10 5 in an amount of 0.5% by weight or more and less than 5% by weight. Including steps,
(B) a step of extruding a polyolefin solution to form a molded body at a shear rate of 60 / sec or more,
(C) a step of cooling the obtained extruded product at a cooling rate of 30 ° C./sec or more to form a gel-like sheet,
(D) stretching the obtained gel sheet at least in a uniaxial direction to create a stretched product, and
(E) A method for producing a polyolefin microporous membrane comprising a step of removing the film-forming solvent from the obtained stretched product.