KR102266028B1 - Multilayer, microporous polyolefin membrane, and production method thereof - Google Patents

Abstract

Provided is a polyolefin microporous membrane that is excellent in oxidation resistance, electrolyte injection property and shutdown properties, and also has excellent permeability and strength balance. Containing polypropylene and polyethylene, the electrolyte injection property is 20 seconds or less, the shutdown temperature is 132°C or less, the air permeability converted to the film thickness of 20 µm is 700 sec/100 cm ³ or less, and the film thickness is converted to 20 µm A polyolefin microporous membrane having a puncture strength of 2,000 mN or more and a polypropylene distribution uniform in the in-plane direction.

Description

Polyolefin microporous membrane and manufacturing method thereof {MULTILAYER, MICROPOROUS POLYOLEFIN MEMBRANE, AND PRODUCTION METHOD THEREOF}

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 in various applications such as battery separators, diaphragms for electrolytic capacitors, various filters, moisture-permeable and waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When a polyolefin microporous membrane is used as a battery separator, especially a separator for lithium ion batteries, the performance is closely related with battery characteristics, battery productivity, and battery safety. Therefore, excellent permeability, mechanical properties, heat shrinkage resistance, shutdown properties, meltdown properties, and the like 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 electrode. In addition, it is known that battery performance deteriorates when lithium ion batteries are continuously used while charging in a state close to full charge, and since oxidative deterioration of the separator is also one of the causes, improvement of the separator has been demanded.

As a method of improving the physical properties of polyolefin microporous membranes, improvement of raw material composition, stretching conditions, heat treatment conditions, etc. have been studied so far, and mixing of polypropylene has been proposed as a means of increasing heat resistance (for example, in Japanese Unexamined Patent Publication) 2002-105235, Japanese Patent Laid-Open No. 2003-183432). In particular, in recent years, in addition to permeability, mechanical properties, heat shrinkage resistance, etc., characteristics related to battery productivity, such as electrolyte injection property, and characteristics related to battery life, such as oxidation resistance, have come to be considered as important.

For example, in Patent Document 1 (Japanese Patent Application Laid-Open No. 11-269290), microscopic irregularities are generated on the surface of a polyolefin microporous film by adding a specific amount of polypropylene to ultra-high molecular weight polyethylene or a composition thereof. Disclosed is a polyolefin microporous membrane having excellent permeability and mechanical strength, and improved moldability and permeability and retention of electrolyte. 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 95% by weight of a polyethylene component % by weight, wherein 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 , also discloses 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 film is prepared by adding a polyolefin such as a specific polypropylene to polyethylene and blending it, the polyolefin segregates on the surface and the content of polyethylene in the vicinity of the surface decreases. It is disclosed that such a microporous film on the surface can suppress gas generation or a decrease in discharge capacity during high-temperature storage. This microporous membrane is a single layer containing 50% by weight or more of polyethylene, and the content of polyethylene near the surface of the membrane on at least one side is lower than the average value of the entire membrane, and polypropylene having a viscosity average molecular weight of 200,000 or more and a viscosity average molecular weight of 50,000 It is characterized in that each of the following low molecular weight polypropylenes contains 5 to 20% by weight of the total film constituent material. In addition, in Patent Document 4 (Japanese Patent Application Laid-Open No. 2011-063025), it is reported that sufficient safety function and strength can be obtained even when thinned by laminating a polyolefin microporous membrane containing polyethylene and polypropylene as essential and a polyethylene microporous membrane. have. The polyolefin microporous membrane of Patent Document 4 is characterized by ensuring strength and safety (hole occlusion temperature and membrane breaking temperature) by laminating a layer made of polypropylene and ultra-high molecular weight polyethylene and a layer made of polyethylene. By combining polypropylene and ultra-high molecular weight polyethylene, it has heat resistance and strength, and prevents an increase in the hole occlusion temperature in the polyethylene layer. In Patent Document 5 (Japanese Patent Application Laid-Open No. 5-234578), polyethylene having a specific molecular weight distribution and polypropylene having a weight average molecular weight in a specific range are used as polymer components, and this and inorganic fine powder and organic liquid are used as polymer components. By using a mixture consisting of is proposing This separator contains polyethylene containing 10 wt% or more of a moiety having a molecular weight of 1.0 x 106 or more, and 5 wt% or more of a moiety having a molecular weight of 1.0 x 105 or less, and polypropylene having a weight average molecular weight of 1.0 x 10 4 to 1.0 x 106 It is composed of a polyolefin microporous membrane composed of a matrix, wherein the amount of the polypropylene is 5 to 45% by weight of the total weight of polyethylene and polypropylene, and the polyolefin microporous membrane has a thickness of 10 to 500 μm and a porosity of 40 to 85% , the maximum pore diameter (孔徑) is 0.05 ~ 5㎛, the difference between the film breaking temperature and the non-porous temperature (無孔化度) is 28 ~ 40 ℃. Patent Document 6 (International Publication No. WO 2007/015416) is a polyolefin microporous membrane composed 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 membrane by infrared spectroscopy. A polyolefin microporous membrane characterized in that the concentration of terminal vinyl groups per 10,000 carbon atoms in the polyolefin constituting the polyolefin is 2 or more is proposed. It is disclosed that the polyolefin microporous film has both film breakage resistance and low heat shrinkage properties, and has excellent fuse characteristics and a uniform film thickness. Patent Document 1: Japanese Patent Application Laid-Open No. Hei 11-269290 Patent Document 2: Japanese Patent Application Laid-Open No. 2011-111484 Patent Document 3: Japanese Patent Application Laid-Open No. 2004-152614 Patent Document 4: Japanese Patent Application Laid-Open No. 2011-063025 Patent Document 5: Japanese Patent Laid-Open No. (Hei) 5-234578 Patent Document 6: International Publication No. WO 2007/015416

In order to improve oxidation resistance by blending polypropylene, it is necessary to blend a significant amount of polypropylene. However, increasing the content of polypropylene impairs the balance between permeability and strength of the polyethylene microporous membrane, in particular, the strength decreases. In addition, there is a problem that a sufficient shutdown temperature cannot be obtained. Therefore, in order to ensure the productivity, safety and output characteristics of the battery while improving the oxidation resistance of the separator related to battery life, it is required to have excellent permeability and strength balance of the polyethylene microporous membrane, and also to have shutdown characteristics. have.

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

In order to solve the above-mentioned subject, the polyolefin microporous film of this invention has the following structure. In other words,

A polyolefin microporous film made of a first polyolefin resin containing polypropylene and polyethylene, wherein the electrolyte injection property is 20 seconds or less, the shutdown temperature is 132° C. or less, and the air permeability converted to a film thickness of 20 µm is 700 sec/100 cm ³ It is a polyolefin microporous film characterized in that the puncture strength is 2,000 mN or more when the film thickness is converted to 20 µm, and the polypropylene distribution (hereinafter, PP distribution) is uniform in the in-plane direction.

Moreover, the manufacturing method of the polyolefin microporous film of this invention has the following structure. In other words,

(a) melt-kneading a polyolefin resin and a solvent for film formation to prepare a polyolefin solution, wherein the polyolefin resin contains polyethylene as a main component, and has a weight average molecular weight of 1.0×10 ⁶ or more and ultra-high molecular weight polyethylene with a melting point A process comprising 0.5% by weight or more and less than 5% by weight of polyethylene having a temperature of 130° C. or ^{less and polypropylene having a weight average molecular weight of greater than 6.0 × 10 4} and less than 3.0 × 10 ^{5 ,}

(b) extruding a polyolefin solution at a shear rate of 60/sec or more to form a molded article;

(c) a step of cooling the obtained extruded body at a cooling rate of 30° C./sec or more to form a gel-like sheet;

(d) stretching the obtained gel-like sheet in at least one axial direction to make a stretched product;

(e) a method for producing a polyolefin microporous membrane including a step of removing the film-forming solvent from the obtained stretched product.

In the polyolefin microporous membrane of the present invention, the average value of the standardized polypropylene/polyethylene ratio (hereinafter referred to as the standardized PP/PE ratio) measured by Raman spectroscopy is 0.5 or more, and the standard deviation of the standardized PP/PE ratio is 0.2 or less , it is preferable that the kurtosis of the normalized PP/PE ratio is 1.0 or less and -1.0 or more.

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

As for the polyolefin microporous film of this invention, it is preferable that the said 1st polyolefin resin contains 0.5 weight% or more and less than 5.0 weight% of polypropylene.

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

It is preferable that the polyolefin microporous membrane of this invention consists of polyethylene whose melting|fusing point is 130 degrees C or less.

In the polyolefin microporous membrane of this invention, it is preferable that content of the polyethylene whose said melting|fusing point is 130 degrees C or less is 10.0 weight% or more and 38.0 weight% or less of the said 1st polyolefin resin.

The polyolefin microporous membrane of the present invention is composed of three or more microporous layers, a first microporous layer comprising a first polyolefin resin constituting at least one of the surface layers, and a second microporous layer comprising a second polyolefin resin disposed between both surface layers It has a porous layer, and it is preferable that either or both of the said 1st microporous layer or the said 2nd microporous layer contain polyethylene whose melting|fusing point is 130 degrees C or less.

In the polyolefin microporous membrane of this invention, it is preferable that content of the polyethylene whose melting|fusing point is 130 degrees C or less is 10.0 weight% or more and 38.0 weight% or less of 1st polyolefin resin or 2nd polyolefin resin.

In the polyolefin microporous membrane of the present invention, the second 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, and does not contain polypropylene.}

In the polyolefin microporous film 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 the surface layers satisfy the relationship of the following formula (1).

<Equation 1>

60 ≤ T(A)/(T(A)+T(B))×100

It is preferable that the shutdown temperature of the polyolefin microporous film of this invention is 128 degrees C or less.

The polyolefin microporous membrane of the present invention contains polypropylene and polyethylene, and has an electrolyte 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 of 700 sec/100 cm ³ or less. , the puncture strength in terms of the film thickness of 20 μm is 2,000 mN or more, and the PP distribution is uniform in the in-plane direction, so that the oxidation resistance, electrolyte injection property and shutdown properties are excellent, and the permeability and strength balance are excellent.

When the polyolefin microporous film of the present invention is used as a battery separator, the polypropylene contributing to oxidation resistance is present without being unevenly distributed on the surface in contact with the electrode, so partial deterioration of the separator that occurs during charging and discharging of the battery can be suppressed. Battery life can be extended. In addition, since the shutdown temperature is lower, the battery reaction can be safely stopped at the time of an abnormal reaction.

Moreover, the polyolefin microporous membrane of this invention is excellent in air permeability-strength balance, can show electrolyte solution injection property equivalent to a polyethylene microporous membrane, and shows a uniform film thickness distribution. From this, when the polyolefin microporous film of the present invention is used as a battery separator, the productivity of the battery is improved, and the battery life can be prolonged due to excellent oxidation resistance.

Moreover, according to the manufacturing method of the polyolefin microporous membrane of this invention, the polyolefin microporous membrane of this invention which has the above-mentioned characteristic can be manufactured efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS It is a graph which shows the distribution of the normalized PP/PE ratio of the 1st microporous layer of the polyolefin microporous membrane (Example 2) of this invention.
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 (Example 2) of the present invention.
It is a graph which shows the two-dimensional distribution diagram of the normalized PP/PE ratio of the 1st microporous layer of a polyolefin microporous membrane (Comparative Example 2).

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated in detail. In addition, this invention is not limited to the following embodiment, It can variously deform and implement within the range of the summary.

A single layer may be sufficient as the polyolefin microporous film of this invention, and the multilayer of two or more layers may be sufficient as it. Especially, it is preferable that it is a multilayer microporous film which consists of three microporous layers. The polyolefin microporous membrane of this invention has at least one layer of a 1st microporous layer. In the polyolefin microporous membrane of the present invention, the first microporous layer has polyethylene as a main component and is composed of a polyolefin resin (first polyolefin resin) containing polypropylene. In addition, a 1st microporous layer is at least one surface layer of the polyolefin microporous film of this invention. A layer other than the first microporous layer may be a second microporous layer composed of a second polyolefin resin. When the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous layers, both surface layers (skin layers) are the first microporous layers, and the second microporous layer is disposed between the two surface layers (core layer). It is preferable to have

The polyolefin resin used in the polyolefin microporous film of this invention is demonstrated below.

[1] Raw materials

[Polyolefin resin]

The polyolefin resin constituting the polyolefin microporous membrane of the present invention has polyethylene (PE) as a main component, and the total 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, even more preferably 95% or more. The polyolefin resin may be a composition including a resin other than polyolefin. Therefore, the term "polyolefin resin" may include not only polyolefin but also resins other than polyolefin.

The said polyolefin resin consists of 1st polyolefin resin, when the polyolefin microporous film of this invention is a single-layer microporous film.

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 is a first polyolefin resin constituting the first microporous layer and a second polyolefin resin constituting the second microporous layer. can

[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 is shown in detail below.

polyethylene

Polyethylene is (a) polyethylene having Mw (weight average molecular weight) of ^{less than 1.0×10 6} (hereinafter “PE(A)”), or (b) PE(A) and ultra-high molecular weight polyethylene (UHMwPE ^{) having Mw of 1.0×10 6 or more.} It is preferable that it is a composition (hereinafter, "PE composition (B)") which consists of ).

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 within the range of 5 to 300, and within the range of 5 to 100 It is more preferable that it is, and it is especially preferable that it exists in the range of 5-25. When Mw/Mn is in the above preferred range, extrusion of the polyethylene solution is easy, and the obtained polyolefin multilayer microporous membrane is excellent in strength.

PE(A)

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 ?-olefins. Examples of the ?-olefin 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, in the range of about 2.0×10 ⁵ to about 0.9×10 ⁶ , a weight average molecular weight (Mw) less than 1.0×10 ⁶ , a molecular weight distribution (MWD, Mw) in the range of about 2.0 to 50.0 is defined as the number average molecular weight divided by Mn), and polyethylene having less than 0.20 terminal unsaturated groups per 10,000 carbon atoms. Mw of PE(A) is preferably ^{1.0×10 4} or more and ^{less than 5.0×10 5 .} Among them, Mw of HDPE is more preferably ^{5.0×10 4} or more to ^{less than 4.0×10 5 .} PE(A) may be made of two or more types having different Mw or different densities. Optionally, PE(A) has no more than 0.14, or no more than 0.12, such as in the range of 0.05-0.14 (eg below the limit of measurement) per 10,000 carbon atoms.

It is preferable that the melting point of PE(A) exceeds 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% by weight, with the total weight of the first polyolefin resin being 100% by weight. It is preferable that the lower limit of PE(A) is 45.0 weight%, More preferably, it is 46.5 weight%.

It is preferable that content of UHMwPE sets the 1st polyolefin resin total weight as 100 weight%, and sets it as 50.0 weight% or less. Especially preferably, it is 45.0 weight% or less. If this content is the said preferable range, it does not cause a pressure rise at the time of shaping|molding, and productivity is also favorable. Moreover, the lower limit in particular of this content is although it does not restrict|limit, It is more preferable that it is 1.0 weight% from the point of maintaining mechanical strength and maintaining a high meltdown temperature, It is especially preferable that it is 30.0 weight%. By setting the UHMwPE to 1% by weight or more and 50.0% by weight or less, it is possible to obtain a polyolefin microporous membrane excellent in balance between strength and air permeability.

Mw of UHMwPE is preferably in the range of 1.0×10 ⁶ to 3.0×10 ^{6 .} By setting Mw of UHMwPE to 3.0×10 ⁶ or less, melt extrusion can be facilitated. UHMwPE may be not only a homopolymer of ethylene, but also a copolymer containing a small amount of other α-olefins. Other ?-olefins other than ethylene may be the same as described 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 may include. These optional components are preferably included in an amount of 40.0% by weight or less based on 100% by weight of the total of the first polyolefin resin.

polypropylene

Content of polypropylene makes 100 weight% of 1st polyolefin resin total weight, and it is preferable that it is less than 5.0 weight%. The upper limit of content of polypropylene becomes like this. Preferably it is 3.5 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. It is preferable that it is 1.01-100, and, as for the molecular weight distribution (Mw/Mn) of a polypropylene, it is more preferable that it is 1.1-50. The polypropylene may be a single substance, or a composition containing two or more polypropylenes may be used.

Although not restrictive, it is preferable that melting|fusing point of polypropylene is 150-175 degreeC, More preferably, it is 150-160 degreeC.

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

[Second polyolefin resin]

The aspect of the 2nd polyolefin resin which comprises a 2nd microporous layer is as follows.

The second polyolefin resin comprises polyethylene. Polyethylene as described in the first polyolefin resin can be used. That is, polyethylene is (a) polyethylene (PE(A)) having Mw (weight average molecular weight) of ^{less than 1.0×10 6} , or (b) PE(A) and ultra-high molecular weight polyethylene (UHMwPE) having ^{Mw of 1.0×10 6 or more.} It is preferable that it is a composition (PE composition (B)) which consists of. It is preferable that 2nd polyolefin resin 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% by weight, with the total weight of the second polyolefin resin being 100% by weight. It is preferable that the lower limit of PE(A) is 50.0 weight%, More preferably, it is 70.0 weight%.

It is preferable that content of UHMwPE sets the 2nd polyolefin resin total weight as 100 weight%, and makes the whole polyethylene into 100 weight% and sets it as 50.0 weight% or less. Especially preferably, it is 30.0 weight% or less. It is because a pressure rise is suppressed also at the time of shaping|molding as this content is in the said range, and productivity improves. Further, the lower limit of the content is not particularly limited, but is more preferably 1.0% by weight, particularly preferably 5.0% by weight, from the viewpoint of maintaining mechanical strength and maintaining a high meltdown temperature. By setting the UHMwPE to 1.0% by weight or more and 50.0% by weight or less, it is possible to obtain a polyolefin microporous membrane excellent in balance between strength and air permeability.

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 may include. These content makes the whole 2nd polyolefin resin 100 weight%, and it is preferable that it is 40.0 weight% or less.

[Components other than polyethylene and polypropylene in polyolefin resin]

As described above, the first and second polyolefin resins may be a polyolefin other than polyethylene and polypropylene, or a composition including a resin other than polyolefin. Examples of polyolefins other than polyethylene and polypropylene include homopolymers and copolymers other than polybutene-1, such as pentene-1, hexene-1, 4-methylpentene-1, and octene.

Moreover, when a polyolefin resin contains a heat resistant resin, when a polyolefin microporous film is used as a separator for batteries, since a meltdown temperature will improve, the high temperature storage characteristic of a battery will further improve.

As the heat-resistant resin, those described in WO 2006/137540 can be used. It is preferable that it is 3 to 20 weight% by making the whole polyolefin resin 100 weight%, and, as for content of heat resistant resin, it is more preferable that it is 3 to 15 weight%. If this content rate is the said preferable range, it is excellent in mechanical strength, such as puncture strength and tensile breaking strength.

[Low-melting polyethylene (hereinafter, PE(C))]

When the polyolefin microporous film of the present invention is a single-layer film, it is preferable that a part of PE(A) in the first polyolefin resin is substituted 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 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 (C ) is preferably substituted. More preferably, either PE(A) in the first polyolefin resin or PE(A) in the second polyolefin resin is partially substituted with PE(C). Especially, it is preferable that PE (C) is contained in 1st 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, with the total weight of the first polyolefin resin or the second polyolefin resin containing PE(C) being 100% by weight. The lower limit of the content of PE(C) is 10.0% by weight, more preferably 15.0% by weight. By including 10.0% by weight or more of PE(C), it is possible to obtain a polyolefin microporous membrane having a shutdown temperature of 132° C. or less, good oxidation resistance and excellent balance of physical properties.

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 Mw of PE(C) ^{is preferably 5.0×10 3} , and more preferably 6.0×10 ³ . The MWD of PE(C) is preferably from about 1 to about 50, more preferably from about 2.0 to about 30.

When the polyolefin microporous membrane of the present invention is composed of three or more microporous layers, the third microporous layer or more microporous layers can be included. When the polyolefin microporous membrane of the present invention is constituted by three microporous layers, the third microporous layer is located on the surface layer on the opposite side to the first microporous layer. The resin constituting the third microporous layer is not particularly limited, and may be formed of the first polyolefin resin or the second polyolefin resin, but preferably does not contain polypropylene.

[2] Method for producing polyolefin microporous membrane

Next, the manufacturing method of the polyolefin microporous film of this invention is demonstrated. In addition, the manufacturing method of the polyolefin microporous film of this invention is not limited to this.

The manufacturing method of the polyolefin microporous membrane of this invention,

(a) melt-kneading a polyolefin resin and a solvent for film formation to prepare a polyolefin solution,

Polyolefin resin has polyethylene as a main component,

Ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 ^{6 or more;}

polyethylene having a melting point of 130° C. or less, and

A process comprising 0.5 wt% or more and less than 5 wt% of polypropylene having a weight average molecular weight ^{greater than 6.0×10 4} and less than 3.0×10 ^{5 ;}

(b) extruding the polyolefin solution at a shear rate of 60/sec or more to form a molded article;

(c) cooling the obtained extruded body at a cooling rate of 30° C./sec or more to form a gel-like sheet;

(d) stretching the obtained gel-like sheet in at least one axial direction to form a stretched product;

(e) removing the solvent for film formation from the obtained drawn product.

In the case where the polyolefin microporous membrane of the present invention is a multilayer microporous membrane composed of a plurality of microporous layers, the manufacturing method can be broadly classified into four types according to the lamination method, and will be described for each classification below.

(2-1) First manufacturing method

A first production method for producing a polyolefin multilayer microporous membrane is (i) melt-kneading a first polyolefin resin and a solvent for film formation to prepare a first polyolefin solution, (ii) a second polyolefin resin and a solvent for film formation A second polyolefin solution is prepared by melt-kneading, (iii) the first and second polyolefin solutions are simultaneously extruded in one die, and (iv) the obtained extruded body is cooled to form a gel-like sheet. Further, (v) a step of stretching the gel-like sheet in at least one axial direction to form a stretched product (first stretching step), (vi) a step of removing (washing) the film-forming solvent from the stretched product, and (vii) washing a step of drying the subsequent film. After the steps (i) to (vii), (viii) a step of re-stretching the dried film in at least one axial direction (second stretching step), and (ix) a heat treatment step may be further included. If necessary, before the solvent removal step for film formation of (vi), any one of a heat setting treatment step, a hot roll treatment step, and a heat solvent treatment step may be provided. 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. Moreover, you may provide the process of heat-processing a stretched material after (v) 1st extending|stretching process.

(i) Preparation of the first polyolefin solution

A 1st polyolefin solution is prepared by melt-kneading 1st polyolefin resin and the solvent for film formation. After mix|blending the suitable solvent for film formation with the 1st polyolefin resin mentioned above, it melt-kneads and prepares a polyolefin resin solution. As the melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent Nos. 2132327 and 3347835 can be used. Since the melt-kneading method is known, its description is omitted. However, the polyolefin resin concentration of the polyolefin resin solution makes the total of the polyolefin resin and the solvent for film formation 100 weight%, and the polyolefin resin is 20 to 50 weight%, Preferably it is 25 to 45 weight%. When the polyolefin resin concentration of the polyolefin resin solution is within the above range, a decrease in productivity and a decrease in the moldability of a gel-like sheet are prevented.

As 1st polyolefin resin, the thing as mentioned above can be used.

(ii) Preparation of a second polyolefin solution

The second polyolefin resin and the solvent for film formation are melt-kneaded to prepare a second polyolefin solution. Although the solvent for film formation used for a 2nd polyolefin solution may be the same as or different from the solvent for film formation used for a 1st polyolefin solution, the same thing is preferable. The preparation method other than that may be the same as the case of preparation of the 1st polyolefin solution.

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

(iii) extrusion

The first and second polyolefin solutions are respectively fed from the extruder to one die, where the two solutions are combined in layers and extruded into a sheet. When manufacturing 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 (second microporous layer) between the two surface layers. The two solutions are combined layer by layer and extruded into a sheet to form a co-layer (preferably in contact with one or both of the surface layers).

The extrusion method may be any of a flat die method and an inflation method. Either way, the solution is supplied to each manifold and stacked in layers at the lip inlet of the multi-layer die (multi-manifold method), or the solution is fed to the die in a layered flow in advance ( block method) can be used. Since the multiple manifold method and the block method itself are known, detailed descriptions thereof will be omitted. It is preferable that the gap of the flat die for multilayer is 0.1-5 mm. As for extrusion temperature, 140-250 degreeC is preferable, and, as for an extrusion speed, 0.2-15 m/min is preferable. By adjusting each extrusion amount of the 1st and 2nd polyolefin solution, the film thickness ratio of a 1st and 2nd microporous layer can be adjusted.

As for the ratio (L/D) of the length (L) and diameter (D) of the screw of a twin screw extruder, the range of 20-100 is preferable. It is preferable that the cylinder inner diameter of a twin screw extruder is 40-200mm. When the polyolefin resin is put into the twin-screw extruder, the ratio (Q/Ns) of the input amount (Q (kg/h)) of the polyolefin resin solution to the screw rotation speed (Ns (rpm)) is 0.1 to 0.55 kg/h/rpm It is preferable to 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 Nos. 2132327 and 3347835 can be used, but in the method for producing a polyolefin microporous film of the present invention, a die of a polyolefin resin solution containing the first polyolefin resin solution It is characterized in that the shear rate from 60/sec or more. The shear rate from the die is more preferably 150/sec or more.

(iv) Formation of a gel-like sheet

The extruded body obtained by (iii) is cooled to form a gel-like sheet. As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent Nos. 2132327 and 3347835 can be used. It is preferable to perform cooling until an extruded body becomes 40 degrees C or less. Through cooling, it is possible to immobilize the microphase of the polyolefin separated by the film-forming solvent. As a cooling method, a method of contacting a refrigerant such as cold air or cooling water, a method of contacting a cooling roll, or the like can be used.

In the manufacturing method of the polyolefin microporous film of this invention, the cooling rate of the extrusion molding of the polyolefin resin solution containing the 1st polyolefin resin solution is 30 degreeC/sec or more, It is characterized by the above-mentioned.

When the shear rate from the die and the cooling rate are appropriately controlled, it is easy to uniformly distribute the polypropylene in the gel-like sheet, and the oxidation resistance and the electrolyte injection property are improved.

(v) 1st extending process

The obtained gel-like sheet is stretched in at least one axial direction. Cleavage between polyethylene crystal lamellar layers occurs by the first stretching, and the polyethylene phase is refined to form a plurality of fibrils. The obtained fibrils form a three-dimensional network structure (a three-dimensionally irregularly connected network structure). Since the gel-like sheet contains the solvent for film formation, it can be extended|stretched uniformly. After heating the gel-like sheet, 1st extending|stretching can be performed at predetermined magnification by the normal 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 preferable. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be performed.

Although a draw ratio changes with the thickness of a gel-like sheet|seat, it is preferable to set it as 2 times or more in uniaxial stretching, and it is more preferable to set it as 3 to 30 times. In biaxial stretching, it is preferable to set it to be at least 3 times or more in either direction, that is, 9 times or more in terms of area magnification, whereby the puncture strength of the polyolefin microporous membrane obtained is improved, and high elasticity and high strength are attained. Moreover, if an area magnification is the said preferable range, a restriction|limiting does not arise in points, such as an extending apparatus and extending|stretching operation. In addition, in biaxial stretching, it is preferable to make the magnification into the same magnification in both directions.

It is preferable that 1st extending|stretching temperature makes melting|fusing point of polyethylene used for preparation of a polyolefin solution into temperature or less which exceeds about 10 degreeC. The stretching temperature may range from more than Tcd to less than Tme. Tme and Tcd are the melting point and crystal dispersion temperature of all polyethylenes used for preparing the polyolefin solution, respectively. When extending|stretching temperature is Tme+10 degreeC or less, there exists a tendency for the orientation of the molecular chain of polyolefin in a gelatinous sheet to be accelerated|stimulated during extending|stretching. On the other hand, when the stretching temperature is Tcd or higher, film breakage due to stretching is suppressed, and stretching at a high magnification is 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 consists of 90 wt% or more of polyethylene, the stretching temperature is usually in the range of 90 to 130°C, preferably in the range of 100 to 125°C, and 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 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 may be performed in multiple stages of stretching at different temperatures, and the stretching temperature and the final stretching ratio of the front and rear ends are within the above ranges, respectively. Depending on the desired physical properties, the film can be stretched with 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) Solvent removal (cleaning) process for film formation

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

(vii) film drying process

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

(viii) second stretching step

Moreover, the film|membrane after drying can be stretched again at least uniaxially. The second stretching may 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 substantially equal to or lower than the melting point Tme of all polyethylenes used for preparing the polyolefin solution. In one embodiment, the second draw temperature is about Tcd to about Tme. When the second stretching temperature is Tme or less, the permeability of the resulting polyolefin microporous membrane becomes appropriate, and the variation in physical properties such as permeability in the transverse direction (width direction: TD direction) tends to be suppressed, while the second stretching temperature is If it is Tcd or more, film breakage due to stretching is suppressed, and uniform stretching becomes possible. 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.

It is preferable that the magnification in the uniaxial direction of 2nd extending|stretching shall be 1.1-1.8 times. For example, in the case of uniaxial stretching, 1.1~ in the MD direction (referring to the production direction of the film, also referred to as machine direction or longitudinal direction) or TD direction (referring to a direction coplanar and perpendicular to the longitudinal direction, also referred to as transverse direction) 1.8 times. In the case of biaxial stretching, 1.1 to 1.8 times each in the MD direction and the TD direction. In the case of biaxial stretching, the respective draw ratios in the MD direction and the TD direction may be different from each other in each direction as long as they are 1.1 to 1.8 times. When the draw ratio was within the above range, it was confirmed that the permeability, heat shrinkage resistance, electrolyte solution absorption and compression resistance of the obtained polyolefin microporous membrane were improved. As for the magnification of 2nd extending|stretching, it is more preferable to set it as 1.2-1.6 times.

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

(ix) heat treatment process

The film after the second stretching may be heat-treated. The network structure made of fibrils formed by the second stretching is maintained, and a polyolefin microporous membrane having a large micropore diameter and excellent strength can be manufactured. The heat treatment may use a heat setting treatment and/or a heat relaxation treatment. The heat setting treatment is a heat treatment in which the film is heated while keeping the size of the film unchanged. The thermal relaxation treatment is a heat treatment in which the film is thermally contracted in the MD direction or the TD direction during heating. In particular, the crystals of the film are stabilized by heat setting treatment. The heat treatment may 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, the method disclosed in Unexamined-Japanese-Patent No. 2002-256099 is mentioned.

The heat treatment is carried out within a temperature range from above the crystal dispersion temperature to below the melting point of all polyolefin resins constituting the polyolefin microporous film. The heat setting 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 exists in the range of 2nd extending|stretching temperature +/-3 degreeC.

Although not restrictive, it is preferable to employ|adopt the inline method which performs 1st extending|stretching, the solvent removal for film|membrane formation, drying, 2nd extending|stretching, and heat processing continuously on a series of lines. However, if necessary, an offline method in which the film after the drying treatment is wound once and then unwound to perform the second stretching and heat treatment may be employed.

(x) other processes

Before removing the solvent for film formation from the gel-like sheet|seat which performed 1st extending|stretching, any one of a heat setting treatment process, a hot roll treatment process, and a hot solvent treatment process can be provided. Moreover, the process of heat-setting with respect to the film|membrane after washing|cleaning or in a 2nd extending process can be provided. The method of heat-setting the stretched gel-like sheet before and/or after washing and the film in the second stretching step may be the same as described above.

(2-2) Second manufacturing method

A second method for producing a polyolefin multilayer microporous membrane is (i) melt-kneading a first polyolefin resin and a solvent for film formation to prepare a first polyolefin solution, and (ii) melt the second polyolefin resin and a solvent for film formation kneading to prepare a second polyolefin solution, (iii-2) laminating immediately after extruding the first and second polyolefin solutions with separate dies, (iv) cooling the obtained extruded body (laminated body) to form a gel-like sheet characterized by forming That is, the first manufacturing method differs only in that the polyolefin solution is laminated in one die to form an extruded body, whereas the second manufacturing method is laminated immediately after the solution is extruded in a separate die, and the following process is The same method as the first manufacturing method can be employed.

Since the 2nd method is the same as each process in a 1st manufacturing method except for a process (iii-2), only a process (iii-2) is demonstrated. In step (iii-2), the first and second polyolefin solutions are each extruded into a sheet form from adjacent dies connected to each of the plurality of extruders, and while the temperature of each solution is high (for example, 100° C. or higher) It is laminated immediately to obtain a laminated extruded body. The steps other than this may be the same as those of the first manufacturing method.

(2-3) Third manufacturing method

A third production method for producing a polyolefin multilayer microporous membrane is (i) melt-kneading a first polyolefin resin and a solvent for film formation to prepare a first polyolefin solution, (ii) a second polyolefin resin and a solvent for film formation Melt-kneading to prepare a second polyolefin solution, (iii-3-1) extruding the first polyolefin solution with one die to form a first extrusion molded body, (iii-3-2) separating the second polyolefin solution from another Extrusion with a die to form a second extruded body, (iv-3) cooled the obtained first and second extruded bodies, respectively, to form first and second gel-like sheets, (v-3) first and second extruded bodies Two gel-like sheets are respectively stretched, (xi-3) the stretched first and second stretched articles are laminated, and (vi) the film-forming solvent is removed from the obtained stretched article. That is, each is carried out until the gel-like sheet is stretched, followed by lamination, and the following steps can employ the same method as the first manufacturing method. Between the steps (vi-3) and (vii-3), (viii-3) the stretching step of the gel-like laminate sheet or the like 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 a layered manner. The die to be used is the same as the die used in the step (iii-2) in the second manufacturing method. The process (iv-3) differs from the process (iv) in the 1st manufacturing method only in the point which respectively separately cools a 1st and 2nd extrusion molding body. The step (v-3) differs from the step (v) in the first manufacturing method only in that the first and second gel-like sheets are respectively stretched. In addition, although the process (xi-3) is a process absent in the 1st and 2nd manufacturing method of laminating|stacking 1st and 2nd stretched material, a well-known method can be used for lamination|stacking of a stretched object.

(2-4) 4th manufacturing method

A fourth production method for producing a polyolefin multilayer microporous membrane is (i) melt-kneading a first polyolefin resin and a solvent for film formation to prepare a first polyolefin solution, (ii) a second polyolefin resin and a solvent for film formation Melt-kneading to prepare a second polyolefin solution, (iii-4-1) extruding the first polyolefin solution with one die, (iii-4-2) extruding the second polyolefin solution with another die, (iv -4) Each obtained extruded article is cooled to form first and second gel-like sheets, (v-4) first and second gel-like sheets are respectively stretched, and (vi-4) each stretched stretched product removing the film-forming solvent in (vii-4) drying the obtained first and second polyolefin microporous membranes, (viii-4) stretching at least the second polyolefin microporous membrane, (xi-4) first And it has the process of laminating|stacking a 2nd polyolefin microporous film. That is, it is carried out separately until it is set as a porous film, and it is laminated|stacked after that to set it as a multilayer microporous film. If necessary, between steps (vii) and (viii-4), each of the (ix-4) first and second polyolefin microporous membranes may be subjected to a heat treatment step. In addition, the method similar to the 1st manufacturing method can be employ|adopted for the following process.

It can implement up to a process (v-4) similarly to a 3rd manufacturing method. The process (vi-4) differs from the process (vi) in the 1st and 3rd manufacturing methods only in the point which respectively removes the solvent for film formation from the 1st and 2nd stretched object. The process (vii-4) differs from the process (vii) in the 1st and 3rd manufacturing methods only in that the 1st and 2nd film|membrane are respectively dried.

On the other hand, although the process (viii-4) is a process which is not necessarily required in the 1st - 3rd manufacturing methods, in the 4th manufacturing method, at least the 2nd polyolefin microporous film is re-stretched in this process (viii-4). Melting|fusing point or less is preferable, and, as for extending|stretching temperature, crystal dispersion temperature - melting|fusing point are more preferable. If necessary, the first polyolefin microporous membrane can also be stretched. Melting point or less is preferable and, as for extending|stretching temperature, crystal dispersion temperature - melting|fusing point are more preferable. Also in the case of stretching any one of the first and second polyolefin microporous membranes, the draw ratio may be the same as in the first manufacturing method except for stretching the non-laminated polyolefin microporous membrane.

In addition, although the process (xi-4) is a process not in the 1st - 3 manufacturing methods which say lamination|stacking of the 1st and 2nd film|membrane, a well-known method can be used for lamination|stacking of a film|membrane similarly to lamination|stacking of a stretched material.

As mentioned above, although the manufacturing method of the polyolefin microporous film of this invention was demonstrated into four classifications according to the lamination|stacking method, if these are put together, it becomes a process (a) - (e) as a necessary process.

A process (a) corresponds to process (i) and process (ii) of the 1st - 4th manufacturing method.

Step (b) is step (iii) of the first production method, step (iii-2) of the second production method, step (iii-3-1) of the third production method, and step (iii-) of the fourth production method It corresponds to 4-1).

Step (c) is step (iv) of the first production method, step (iv-2) of the second production method, step (iv-3) of the third production method, and step (iv-4) of the fourth production method corresponds to

The process (d) corresponds to the process (v) of the 1st - 2nd manufacturing method, the process (v-3) of the 3rd manufacturing method, and the process (v-4) of the 4th manufacturing method.

A process (e) corresponds to the process (vi) of the 1st - 3rd manufacturing method, and the process (vi-4) of the 4th manufacturing method.

When the polyolefin microporous membrane of this invention is a single layer, it consists only of the process of manufacturing a 1st microporous layer among the manufacturing methods of the above-mentioned multilayer microporous membrane. As an example, the polyolefin microporous film of this invention (2-4) 4th manufacturing method (i), (ii), (iii-4-1), (iv-4), (v-4), (vi -4) and the steps of (vii-4) are included.

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

The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties. Hereinafter, the structure, physical properties, and a method for measuring the same 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 maximum PP/PE ratio on the membrane surface was set to 1 for the peak intensity ratio (PP/PE ratio) of PP and PE obtained by Micro-Raman spectroscopy. If the relative value at the time is the normalized PP/PE ratio, it can be expressed in a structure in which the average value/standard deviation/kurtosis of the normalized PP/PE ratio is 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 0.5 or more as an average value, 0.2 or less as a standard deviation, and 1.0 or less -1.0 or more with a kurtosis parameter indicating the shape of the distribution.

A method for measuring the PP/PE ratio of the film surface by microscopic Raman spectroscopy will be described below. According to microscopic Raman spectroscopy, using a laser with a wavelength of 532 nm, area analysis was performed with a spot diameter of 1 micron in a field of view of 1 to 2 microns in the depth direction and 20 × 20 microns in the depth direction, and a total of 400 points was obtained. Measure the peak intensity ratio at a frequency of 807 cm ^-1 (PP) and a frequency of 1127 cm ^{-1 (PE).} Let the relative value when the maximum value of the intensity ratio within the field of view of 20 x 20 microns be 1 "standardized PP/PE ratio".

When the average value of the standardized PP/PE ratio is in the above preferred range, there are few parts with a low polypropylene concentration, and the part mainly made of polyethylene does not increase, so that polyethylene is mainly due to the oxidation reaction accompanying charging and discharging in the battery. Since there are few parts, deterioration is hard to advance and it is thought that cycling characteristics are maintained favorably.

If the standard deviation of the standardized PP/PE ratio is in the above preferred range, the change in the polypropylene concentration is small, and since there are few portions where the polypropylene concentration is low, it is considered that the oxidation resistance is also difficult to deteriorate.

In addition, if the distribution of the polypropylene concentration is within the above preferred range, there are few portions with a low polypropylene concentration, and a portion with poor oxidation resistance in the battery is less likely to occur, so that the battery performance is good. It is easy to improve the oxidation resistance if there is a portion having a high polypropylene concentration to a certain extent. From these results, it was found that an appropriate distribution of standardized PP/PE 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 is excellent in oxidation resistance. In addition, when the content of polypropylene is less than 5% by weight, it is preferable because the decrease in physical properties due to polypropylene is suppressed and the permeability, strength and absorption of the electrolyte are excellent. For this reason, when it uses as a separator for lithium ion batteries, respectively, the outstanding battery productivity, safety, and battery cycle characteristics can be implement|achieved.

(2) Breathability (sec/100cm ³ /20㎛)

The upper limit of the air permeability (Gurley value) in terms of the film thickness of the polyolefin microporous membrane of the present invention to 20 μm ^{is preferably 700sec/100cm 3} , more preferably 600sec/100cm ³ , and still more preferably is 550 sec/100 cm ³ . It is preferable that the lower limit of the air permeability which converted the film thickness of the polyolefin microporous membrane of this invention into 20 micrometers is 20 sec/100cm<^3> , More preferably, it is 100 sec/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, and shutdown is sufficiently performed when the temperature inside the battery rises, while charging and discharging when used as a battery It is difficult to increase the resistance value at the time, and the average electrochemical stability is good. In addition, the air permeability is measured according to JIS P 8117, and is a value calculated|required by converting the film thickness into 20 micrometers.

(3) Porosity (%) (空孔率)

It is preferable that the porosity of the polyolefin microporous film of this invention is 25 to 80 %, More preferably, it is 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 circuit of the electrode is suppressed. The porosity is a value measured by the mass method.

Porosity (%) = 100 × (w2-w1)/w2

w1: Actual weight of the microporous membrane

w2: weight of equivalent non-porous membrane (of the same polymer) having the same size and thickness

(4) puncture strength (mN/20㎛)

The puncture strength is a value obtained by measuring the maximum load value when the polyolefin microporous film is pierced 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 into 20 µm. It is preferable that the puncture strength which converted the film thickness of the polyolefin microporous film of this invention into 20 micrometers is 2,000 mN or more, More preferably, it is 4,000 mN or more, More preferably, it is 5,000 mN or more. When a puncture strength is 2,000 mN/20 micrometers or more and a polyolefin microporous film is inserted in a battery as a battery separator, the short circuit of an electrode can be suppressed effectively.

(5) Tensile breaking strength (kPa)

The tensile breaking strength of the polyolefin microporous membrane of the present invention is 60,000 kPa or more, more preferably 80,000 kPa or more, and still more preferably 100,000 kPa or more in both the MD and TD directions. As the tensile breaking strength is 60,000 kPa or more, it is easy to prevent film breakage during battery production. Tensile breaking strength is a value measured according to ASTM D882 using a rectangular test piece with 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 in both the MD direction and the TD direction, and more preferably 100% or more. Thereby, it is easy to prevent the film breakage at the time of battery manufacture. The tensile elongation at break is a value measured according to ASTM D882 using a rectangular test piece with a width of 10 mm.

(7) Heat shrinkage (%)

The thermal contraction rate of the polyolefin microporous film of the present invention after exposure to a temperature of 105° C. for 8 hours is preferably 10% or less in both the MD direction and the TD direction, more preferably 8% or less, and still more preferably 6% or less. When the thermal contraction rate is 10% or less, when the polyolefin microporous film is used as a separator for a lithium battery, the possibility that the end of the separator shrinks during heat generation and short circuit of the electrode occurs is lowered.

Thermal contraction rate is a value calculated|required by measuring the thermal contraction rate of the MD direction and TD direction at the time of exposing a polyolefin microporous film at 105 degreeC for 8 hours each three times, and calculating an average value, respectively. Thermal contraction rate is expressed by the following formula|equation.

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

(8) shutdown temperature

The shutdown temperature of the polyolefin microporous membrane of this invention is 132 degreeC or less, More preferably, it is 128 degrees C or less, More preferably, it is 126 degrees C or less. In addition, the shutdown temperature is measured according to 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, while the air permeability of the membrane is measured. The shutdown temperature of the polyolefin microporous membrane was defined as the temperature when the air permeability (Gurley value) of the polyolefin microporous membrane first ^{exceeded 100,000 sec/100 cm 3 .} The air permeability of the polyolefin microporous membrane is measured according to JIS P 8117 using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T).

(9) electrolyte injectability

The electrolyte injection property of the polyolefin microporous membrane of this invention is 20 second or less. More preferably, 10 seconds or less, even more preferably 5 seconds or less, is particularly preferable. Electrolyte injectability was evaluated by the penetration time of propylene carbonate. A 50 mm x 50 mm sample is mounted on a glass plate, 0.5 ml of propylene carbonate is dripped from about 2 cm of the sample, and time measurement is started from the completion|finish of dripping. Immediately after completion of the dripping, the propylene carbonate swells on the membrane with surface tension, but the dripped propylene carbonate permeates with time. When all of the propylene carbonate on the membrane has permeated, the time measurement is stopped and the permeation time is set as the permeation time. Penetration time makes 20 seconds or less favorable, it is larger than 20 seconds, 50 seconds or less makes slightly good, and what exceeds 50 second makes it unsuitable.

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

To measure the electrochemical stability, a membrane with a length (MD) of 70 mm and a width (TD) of 60 mm is placed between a cathode and an anode having the same area as the membrane. 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). The cell is completed by impregnating the electrolyte into the membrane in the region between the cathode and the anode.

The cell is then exposed to an applied voltage of 4.3 V while exposing it to a temperature of 60° C. for 28 days. The term "electrochemical stability" is defined as the integral current (mAh) flowing between a voltage source and a cell over 28 days. Electrochemical stability was measured for three cells under the same conditions (three cells under the same conditions were prepared from three membrane samples under the same conditions). The average electrochemical stability (leakage current value) is an average (arithmetic mean) of the values of the measured electrochemical stability of three batteries.

Electrochemical stability is a membrane property related to oxidation resistance of a membrane when the membrane is used as a separator in a cell that is exposed to relatively high temperatures during storage or use. Electrochemical stability is in mAh, and lower values are generally preferred (indicating less total charge loss during storage at high temperatures or overcharging). Since batteries for automobiles, such as batteries used for starting a power means for moving an electric vehicle or hybrid electric vehicle, or supplying power to the power means, and batteries for power tools are used for relatively high-output and large-capacity applications, the battery separator Even a slight loss of battery capacity, such as self-discharge loss due to the electrochemical instability of the battery, becomes 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 "large capacity" battery usually means a battery capable of supplying at least 1 Ampere-hour (1 Ah), for example, 2.0 Ah to 3.6 Ah.

(11) film thickness

As for the film thickness of the polyolefin microporous film of this invention, when using as a battery separator, for example, 5-50 micrometers is preferable, 5-35 micrometers is more preferable, 10-25 micrometers is still more preferable. The film thickness measurement method does not matter whether it is a contact type thickness measurement method or a non-contact type thickness measurement method. For example, it can be measured with a contact thickness gauge 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 taking an average value. As the contact-type thickness gauge, for example, a thickness gauge such as Litematic manufactured by 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 thickness of both surface layers and the sum T (B) of the thickness of each layer disposed between the two surface layers is the relationship of the following formula 1 It is desirable to be satisfied.

<Equation 1>

60 ≤ T(A)/(T(A)+T(B))×100

By satisfying the relationship of Formula 1, an excellent polyolefin microporous membrane with a lower shutdown temperature can be obtained. It is more preferable to satisfy the relationship of the following formula (2).

<Equation 2>

60 ≤ T(A)/(T(A)+T(B))×100 < 90

By satisfying the relationship of Formula 2, it is possible to obtain a polyolefin microporous membrane excellent in balance between strength and permeability and excellent in low shutdown temperature.

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

<Equation 3>

60 ≤ T(A)/(T(A)+T(B))×100 < 85

(12) Appearance

The appearance of the film was evaluated by visual/multipoint film thickness measurement. It was set as "good|favorableness" about the thing with small fluctuation|variation in thickness visually. "Good" corresponds to the case where the film thickness fluctuation is less than 5 microns in the film thickness measurement at multiple points.

(13) melting point

Melting|fusing point of resin was measured in the following procedure based on JIS　K　7122. That is, the resin sample is left still in the sample holder of a scanning differential calorimeter (Perkin Elmer, 　 Inc., DSC-System 7 type), in a nitrogen atmosphere, heat-treated at 230 ° C. for 10 minutes, and at 10 ° C./min. After cooling to 40°C, the temperature was maintained at 40°C for 2 minutes, and then heated to 230°C at a rate of 10°C/min. The temperature (peak temperature) which became the maximum amount of heat absorption was made into melting|fusing point.

[4] Batteries, etc.

As described above, the polyolefin microporous membrane of the present invention has excellent oxidation resistance and electrolyte injection property, so blackening is difficult to occur even after repeated charging and discharging as a battery, and excellent permeability and strength balance, so it is particularly suitable as a battery separator. .

The separator made of the polyolefin microporous film of the present invention can be used for batteries and electric double-layer capacitors. Although there is no particular limitation on the type of battery/capacitor using the same, it is particularly suitable for a lithium secondary battery/lithium ion capacitor use. A well-known electrode and electrolyte solution can be used for the lithium secondary battery/capacitor using the separator which consists of a polyolefin microporous film of this invention. In addition, the structure of a lithium secondary battery/capacitor using the separator made of the polyolefin microporous film of the present invention may also be known.

Example

The present invention will be described in more detail by 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 method mentioned above.

Example One

(1) Preparation of 1st polyolefin solution

Based on the total weight of the first polyolefin composition (a) Mw is 2.0 × 10 ⁶ HDPE (Mw / Mn: 8, melting point: 136 ° C.) 20% by weight, (b) Mw is 2.5 × 10 ⁵ HDPE (Mw / Mn: 8.6, terminal vinyl group concentration 0.1 per/10000 carbon, melting point: 134°C) 57% by weight, (c) ^{HDPE having Mw of 1.8×10 4} (Mw/Mn: 2.6, melting point: 123°C) 20% by weight , (d) A first polyolefin composition containing 3 wt% of polypropylene (Mw/Mn: 2.6, melting point: 155°C) having a Mw of 9.7 × 10 ^{4 was prepared by dry blending.} As an antioxidant, tetrakis [methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane was dry-blended in an amount of 0.2 parts by weight per 100 parts by weight of the first polyolefin composition, followed by a first A polyolefin resin was prepared.

25 parts by weight of the first polyolefin resin 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 a side feeder to the twin-screw extruder. Melt-kneading was carried out at 210 degreeC and 200 rpm, and the 1st polyolefin solution was prepared.

(3) Preparation of microporous membrane

The first polyolefin solution was fed from a twin screw extruder to a die to form an extruded body. The extruded body was passed through a cooling roll controlled at 20 DEG C and cooled to form a gel-like sheet. In addition, the shear rate in the die|dye of the extrusion molding body was 205/sec, and the cooling rate in a cooling roll was 37 degreeC/sec. The obtained gel-like sheet was subjected to simultaneous biaxial stretching (first stretching) at a temperature of 115°C at a temperature of 115°C using a tenter stretching machine (first stretching) and wound up. Then, a part was taken from the wound stretched product, fixed to a frame plate [size: 20 cm x 20 cm, made of aluminum (hereinafter the same)], immersed in a washing tank of methylene chloride temperature controlled at 25 ° C. It washed with shaking for 3 minutes. The washed film was air-dried at room temperature. After a second stretching (re-stretching) of the dried microporous membrane at a draw ratio of 1.2 times in the TD direction at 118.3° C. through a batch stretching machine, a heat relaxation treatment is performed in the TD direction at the same temperature to a draw ratio of 1.0 times, and the A polyolefin microporous membrane was produced by heat-setting at a re-stretching temperature for 10 minutes while mounted on a post-batch stretching machine.

Example 2

(1) Preparation of 1st polyolefin solution

Based on the total weight of the first polyolefin composition (a) Mw is 2.0 × 10 ⁶ HDPE (Mw / Mn: 8, melting point: 136 ° C.) 20% by weight, (b) Mw is 2.5 × 10 ⁵ HDPE (Mw / Mn: 8.6, terminal vinyl group concentration of 0.1 per/10000 carbon, melting point: 134°C) 52% by weight, (c) ^{HDPE having an Mw of 2.4×10 4} (Mw/Mn: 3.0, melting point: 123°C) 25% by weight , (d) A first polyolefin composition containing 3 wt% of polypropylene (Mw/Mn: 2.6, melting point: 155°C) having a Mw of 9.7 × 10 ^{4 was prepared by dry blending.} As an antioxidant, 0.2 parts by weight of tetrakis [methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane per 100 parts by weight of the first polyolefin composition was dry blended to obtain a first 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. Melt-kneading was carried out at 210 degreeC and 200 rpm, and the 1st polyolefin solution was prepared.

(2) Preparation of second polyolefin solution

The 2nd polyolefin solution was prepared similarly to the preparation method of the 1st polyolefin solution except the following points. Based on the total weight of the second polyolefin composition (a) Mw is 2.0 × 10 ⁶ UHMwPE (Mw / Mn: 8.0, melting point: 136 ° C.) 20% by weight, (b) Mw is 2.5 × 10 ⁵ HDPE (Mw / Mn:8.6, the terminal vinyl group concentration of 0.1 per/10000 carbon, melting|fusing point:134 degreeC) The 2nd polyolefin composition containing 80 weight% was prepared by dry blending. As an antioxidant, tetrakis [methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane was dry-blended in an amount of 0.2 parts by weight per 100 parts by weight of the second polyolefin composition to obtain a second 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 a side feeder to the twin-screw extruder. Melt-kneading was carried out at 210 degreeC and 200 rpm, and the 2nd polyolefin solution was prepared.

(3) Preparation of microporous membrane

The first and second polyolefin solutions are fed from each twin screw extruder to a three-layer T-die, and the layer composition is 3 of the first polyolefin solution/second polyolefin solution/first polyolefin solution and the layer thickness ratio is 35/30/35. An extrusion of the layer was formed. This extruded body was passed through a cooling roll controlled at 20°C and cooled to form a three-layer gel-laminated sheet. In addition, the shear rate in die|dye of the extrusion molding body was 200/sec, and the cooling rate in a cooling roll was 36.5 degreeC/sec. The obtained gel-laminated sheet was subjected to simultaneous biaxial stretching (first stretching) at a draw ratio of 5x5 at a temperature of 115°C using a tenter stretching machine, and was wound up. Then, a part is taken from the wound stretched product, fixed to a mold plate [size: 20 cm x 20 cm, made of aluminum (hereinafter the same)], immersed in a washing tank of methylene chloride temperature-controlled at 25 ° C., and 3 minutes at 100 rpm It washed while shaking. The washed film was air-dried at room temperature. After a second stretching (re-stretching) of the dried microporous membrane at a draw ratio of 1.4 times in the TD direction at 122 ° C. through a batch stretching machine, heat relaxation to 1.2 times the draw ratio in the TD direction at the same temperature, and then in the batch stretching machine In the attached state, the polyolefin multilayer microporous membrane was produced by heat setting treatment at the re-stretching temperature for 10 minutes.

Example 2~ Example 10 and comparative example 1~ comparative example 8

With the raw materials and conditions shown in Tables 1 and 2, Comparative Example 1 produced a polyolefin microporous membrane in the same manner as in Example 1 and Example 1 and Examples 2 to 10 and Comparative Examples 2 to 8. In addition, "-" 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 addition, in Table 4, "-" in Comparative Example 7 indicates that the surface had large irregularities that could be visually judged and could not be measured.

Referring to Tables 3 and 4, all of the polyolefin microporous membranes of Examples 1 to 9 have excellent electrolyte injection properties and uniform PP distribution. In addition, the leakage current value is 45 mAh or less, indicating excellent oxidation resistance. In addition, the shutdown temperature is 132° C. or less, and when used in a battery, it is more safe and has excellent balance of properties. 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, and it can be seen that the normalized PP/PE ratio is concentrated in a narrow range of 0.5 or more. 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, and the region with a low polypropylene concentration (dark part) is hardly seen, and polypropylene is present on average; it can be seen that there is 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, and there are many regions with low polypropylene concentration (dark part), and polypropylene is present on average in the surface layer You can see what you're not doing.

Industrial Applications

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

The polyolefin multilayer microporous film of the present invention has performance suitable as a power storage device for a non-aqueous electrolyte solution such as a capacitor use, a capacitor use, and a battery use, and can contribute to improvement of safety and reliability. Especially, it can use suitably as a separator for batteries, more specifically, as a separator for lithium ion batteries, and can increase the life span and safety of a battery. For other uses, since it is also used as a component of a fuel cell, as various separation membranes such as a humidification membrane and a filtration membrane, there is a possibility of industrial application in these fields.

Claims (13)

A polyolefin microporous film made of a first polyolefin resin containing polypropylene and polyethylene, wherein the electrolyte injection property is 20 seconds or less, the shutdown temperature is 132° C. or less, and the air permeability converted to a film thickness of 20 µm is 700 sec/100 cm ³ or less, the puncture strength converted to 20 μm in film thickness is 2,000 mN or more, and the polypropylene distribution is uniform in the in-plane direction,
The average value of the standardized polypropylene / polyethylene ratio measured by Raman spectroscopy is 0.5 or more, the standard deviation of the standardized polypropylene / polyethylene ratio is 0.2 or less, and the kurtosis of the normalized polypropylene / polyethylene ratio is 1.0 or less -1.0 or more,
The polyolefin microporous film in which the said 1st polyolefin resin contains 0.5 weight% or more and less than 5.0 weight% of polypropylene. delete According to claim 1,
A polyolefin microporous membrane having a weight average molecular weight of the polypropylene ^{greater than 6.0×10 4} and less than 3.0×10 ^{5 .} delete 4. The method of claim 1 or 3,
The polyolefin microporous film in which the said 1st polyolefin resin contains 1.0 weight% or more and 50.0 weight% or less of polyethylene whose weight average molecular weight is 1.0x10<^{6> or more.} 4. The method of claim 1 or 3,
A polyolefin microporous membrane comprising polyethylene having a melting point of 130°C or less. 7. The method of claim 6,
Polyolefin microporous film whose content of the polyethylene whose melting|fusing point is 130 degrees C or less is 10.0 weight% or more and 38.0 weight% or less of the said 1st polyolefin resin. 4. The method of claim 1 or 3,
It is composed of three or more microporous layers and has 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, the first microporous layer comprising: Polyolefin microporous membrane, characterized in that either or both of the porous layer and the second microporous layer contain polyethylene having a melting point of 130°C or less. 9. The method of claim 8,
Polyolefin microporous film whose content of polyethylene whose melting|fusing point is 130 degrees C or less is 10.0 weight% or more and 38.0 weight% or less of 1st polyolefin resin or 2nd polyolefin resin. 10. The method of claim 9,
The polyolefin microporous film wherein 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.} 11. The method of claim 10,
A polyolefin microporous film characterized in 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 the surface layers satisfy the relationship of the following formula (1).
<Equation 1>
60 ≤ T(A)/(T(A)+T(B))×100 4. The method of claim 1 or 3,
A polyolefin microporous film characterized in that the shutdown temperature is 128°C or less. (a) melt-kneading a polyolefin resin and a solvent for film formation to prepare a polyolefin solution, wherein the polyolefin resin contains 80% by weight or more of polyethylene with respect to 100% by weight of the polyolefin resin and has a weight average molecular weight of 1.0×10 ⁶ or more ultra-high molecular weight polyethylene, a polyethylene having a melting point of 130° C. or ^{less, and a polypropylene having a weight average molecular weight greater than 6.0 × 10 4} and less than 3.0 × 10 ⁵ , a process comprising 0.5% by weight or more and less than 5% by weight;
(b) extruding a polyolefin solution at a shear rate of 60/sec or more to form a molded article;
(c) a step of cooling the obtained extruded body at a cooling rate of 30° C./sec or more to form a gel-like sheet;
(d) stretching the obtained gel-like sheet in at least one axial direction to produce 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 drawn product.

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