KR102229797B1 - Multilayer, microporous polyolefin membrane, and production method thereof - Google Patents

Abstract

The present invention provides a polyolefin multilayer microporous membrane having excellent oxidation resistance, electrolyte solution injection properties, and shutdown properties, and excellent balance of permeability and strength.
A polyolefin multilayer microporous membrane obtained by forming a gel sheet using a polyolefin resin containing polypropylene, stretching and washing it in at least one direction, having a first microporous layer comprising polypropylene, and at least one surface layer comprising the above A polyolefin multilayer microporous membrane having a first microporous layer, an electrolyte solution pourability of 20 seconds or less, a PP distribution of the first microporous layer is uniform in an in-plane direction, and a shutdown temperature of 132°C or less.

Description

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

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

Polyolefin multilayer microporous membranes are used in various applications such as battery separators, electrolytic capacitors, various filters, moisture-permeable waterproof medical devices, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When a polyolefin multilayer microporous membrane is used as a battery separator, particularly a lithium ion battery separator, its performance is deeply related to 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, lithium ion batteries are known to deteriorate battery performance if they are continuously used while being charged in a state close to full charge, and oxidation deterioration of the separator is also one of the causes, and thus, improvement of the separator has been required.

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

For example, in Patent Document 1 (Japanese Laid-Open Patent Publication No. 11-269290), microscopic irregularities are created on the surface of a polyolefin microporous membrane by adding a specific amount of polypropylene to an ultra-high molecular weight polyethylene or a composition thereof. Disclosed is a polyolefin microporous membrane having excellent permeability and mechanical strength, improved moldability, and improved permeability and retention of an electrolyte. In addition, in Patent Document 2 (Japanese Laid-Open Patent Publication No. 2011-111484), as a polyolefin multilayer microporous membrane suitable as a separator capable of achieving both oxidation resistance and cycle characteristics, a polypropylene component of 5 to 50% by weight and a polyethylene component of 50 to 95. % By weight, and the polyethylene component contains 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, a polyolefin multilayer microporous membrane having a bubble point of 400 to 600 kPa is disclosed. In Patent Document 3 (Japanese Laid-Open Patent Publication No. 2004-152614), when a film is prepared by adding and blending a polyolefin such as a specific polypropylene to polyethylene, the polyolefin is segregated on the surface and the content of polyethylene in the vicinity of the surface decreases. It is disclosed that the microporous membrane on the surface is capable of suppressing gas generation and reduction in discharge capacity during storage at a high temperature. This microporous membrane is a single layer containing 50% by weight or more of polyethylene, and the content of polyethylene in the vicinity of the surface of the membrane on at least one side is less than the average value of the entire membrane, and the viscosity average molecular weight is 200,000 or more, and the viscosity average molecular weight is 50,000. It is characterized in that the following low molecular weight polypropylenes each contain 5 to 20% by weight of the total material constituting the membrane. In addition, Patent Document 4 (Japanese Laid-Open Patent Publication No. 2011-063025) reports that by laminating a polyolefin microporous membrane and a polyethylene microporous membrane, which are essential for polyethylene and polypropylene, sufficient safety functions and strength are obtained even when thinning. have. The polyolefin microporous membrane of Patent Document 4 is characterized by assuring strength and safety (hole occlusion temperature and rupture 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 maintains heat resistance and strength, and prevents an increase in the temperature of clogging holes in the polyethylene layer. In Patent Document 5 (Japanese Patent 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, inorganic fine powder, and organic liquid By using a mixture consisting of as a raw material for film production, even if the ratio of the ultra-high molecular weight portion in the molecular weight distribution of polyethylene is increased, there is no pressure increase during film forming, excellent mechanical properties, and excellent safety, we propose a battery separator using an organic electrolytic solution. I'm doing it. This separator contains polyethylene containing 10% by weight or more of a portion having a molecular weight of 1.0×106 or more, and 5% by weight or more of a portion having a molecular weight of 1.0×105 or less, and polypropylene having a weight average molecular weight of 1.0×104 to 1.0×106 It is composed of a polyolefin microporous membrane composed of a matrix, and 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 rupture temperature and the no-gap temperature (無孔化度) is 28 ~ 40 ℃. Patent Document 6 (International Publication No. WO2007/015416) is a polyolefin microporous membrane 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 constituting 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 is 2 or more is proposed. It is disclosed that the polyolefin microporous membrane achieves both rupture resistance and low heat shrinkage, has excellent fuse characteristics, and has a uniform film thickness. Patent Document 1: Japanese Patent Laid-Open Publication No. Hei 11-269290 Patent Document 2: Japanese Patent Laid-Open Publication No. 2011-111484 Patent Document 3: Japanese Patent Laid-Open Publication No. 2004-152614 Patent Document 4: Japanese Patent Laid-Open Publication No. 2011-063025 Patent Document 5: Japanese Patent Laid-Open Publication No. 5-234578 Patent Document 6: International Publication No. WO2007/015416

In order to improve oxidation resistance by adding polypropylene, it is necessary to add a significant amount of polypropylene, but there is a disadvantage in that increasing the content of polypropylene impairs the balance of permeability and strength of the polyethylene microporous membrane, and in particular decreases the strength. In addition, there is a problem that a sufficient shutdown temperature is not obtained. Therefore, in order to ensure the productivity, safety and output characteristics of the battery while attempting to improve the oxidation resistance of the separator related to the battery life, it is required to maintain the excellent balance of permeability and strength of the polyethylene microporous membrane, and also the shutdown characteristics. have.

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

In order to solve the above-described problem, the polyolefin multilayer microporous membrane of the present invention has the following configuration. In other words,

It has a first microporous layer containing polypropylene, the electrolyte solution injectability is 20 seconds or less, the shutdown temperature is 132°C or less, at least one surface layer is the first microporous layer, and the polypropylene distribution of the first microporous layer ( Hereinafter, PP distribution) is a polyolefin multilayer microporous membrane uniform in the in-plane direction.

In addition, the method for producing a polyolefin multilayer microporous membrane of the present invention has the following configuration. In other words,

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

(a-1) Polyethylene having a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms and a weight average molecular weight of less than ^{1.0 × 10 6 by infrared spectroscopy, and a weight average molecular weight greater than 6.0 × 10 4} and less than 3.0 × 10 ⁵ A step of preparing a first polyolefin solution by melt-kneading a first polyolefin resin containing polypropylene and a film-forming solvent, and

(a-2) comprising polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms by infrared spectroscopy and a weight average molecular weight of less than ^{1.0×10 6} and ultra-high molecular weight polyethylene having a ^{weight average molecular weight of 1.0×10 6 or more} A process comprising the step of preparing a second polyolefin solution by melt-kneading a second polyolefin resin and a film-forming solvent,

(b) forming a molded article by extruding a polyolefin solution at a shear rate of 60/sec or more, and

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

(d) a step of producing a stretched product by stretching the obtained gel-like sheet in at least one axial direction; and

(e) It is a method for producing a polyolefin multilayer microporous membrane comprising a step of removing the solvent for film formation from the obtained stretched product.

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

In the polyolefin multilayer microporous membrane of the present invention, the weight average molecular weight of the polypropylene is greater than ^{6.0×10 4 and less than 3.0×10 5} , and the content is the total weight of the polyolefin resin of the first microporous layer in the first microporous layer. It is preferable to contain 0.5% by weight or more and less than 5% by weight as 100% by weight.

The polyolefin multilayer microporous membrane of the present invention consists of three or more layers, and includes a second microporous layer disposed between both surface layers, and the second microporous layer has a terminal vinyl group concentration of 0.2 per 10,000 carbon atoms by infrared spectroscopy. It is preferred to include more than two polyethylenes.

It is preferable that the polyolefin multilayer microporous membrane of the present invention has a three-layer structure in which the second microporous layer is disposed between both surface layers made of the first microporous layer.

In the polyolefin multilayer microporous membrane of the present invention, the second microporous layer contains at least 20.0% by weight of polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms as 100% by weight of the total polyolefin of the second microporous layer. It is desirable.

In the polyolefin multilayer microporous membrane of the present invention, the first microporous layer is made of a first polyolefin resin, the first polyolefin resin has a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms, and a weight average molecular weight of 1.0×10 It is preferable to include ^{polyethylene having less than 6} and polypropylene having a weight average molecular weight of ^{greater than 6.0×10 4} and less than 3.0×10 ^5.

In the polyolefin multilayer microporous membrane of the present invention, the first polyolefin resin has a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms, and a weight average molecular weight of 5.0×10 ⁴ or more and less than 5.0×10 ^5. Ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 ⁶ or more and less than 3.0×10 ⁶ to 100% by weight with a total weight of 100% by weight) And the weight average molecular weight is ^{greater than 6.0 × 10 4} and less than 3.0 × 10 ⁵ polypropylene (0.5% by weight or more and 5.0% by weight based on the total weight of the first polyolefin resin as 100% by weight) %).

In the polyolefin multilayer microporous membrane of the present invention, the second microporous layer is made of a second polyolefin resin, the second polyolefin resin has a terminal vinyl group concentration of 0.2 per 10,000 carbon atoms or more, and a weight average molecular weight of 5.0×10 ^{Polyethylene of 4} or more and less than 1.0×10 ⁶ (amount that becomes 20.0% by weight or more and 99.0% by weight or less based on the total weight of the second polyolefin resin as 100% by weight), and the terminal vinyl group concentration is less than 0.2 per 10,000 carbon atoms, by weight High-density polyethylene having an average molecular weight of 5.0×10 ⁴ or more and less than 1.0×10 ⁶ (amount of 0.0% by weight or more and 79.0% by weight or less based on the total weight of the second polyolefin resin as 100% by weight), and a weight average molecular weight of 1.0×10 ⁶ or more It comprises ultra-high molecular weight polyethylene of less than 3.0×10 ⁶ (an amount of 1.0 wt% or more and 50.0 wt% or less based on 100 wt% of the total weight of the second polyolefin resin), and preferably does not contain polypropylene.

The polyolefin multilayer microporous membrane of the present invention has a first microporous layer containing polypropylene, has an electrolyte injectability of 20 seconds or less, a shutdown temperature of 132°C or less, at least one surface layer is the first microporous layer, and the first microporous layer. Since the distribution of PP in the air layer is uniform in the in-plane direction, oxidation resistance, electrolyte solution injection property, and shutdown characteristics are excellent. By lowering the shutdown temperature, it is possible to more safely stop the battery reaction in the battery in the event of an abnormal reaction.

When a polyolefin multilayer microporous membrane is used as a battery separator, if a portion having a high polyethylene concentration partially exists in the polyolefin multilayer microporous membrane, deterioration of the polyolefin multilayer microporous membrane may occur during charging and discharging of the battery. By using the polyolefin multilayer microporous membrane of the present invention, deterioration occurring during charging and discharging of the battery can be suppressed, and the battery life can be extended.

In the polyolefin multilayer microporous membrane of the present invention, a weight average molecular weight of polypropylene having a weight average molecular weight of ^{greater than 6.0 × 10 4} and less than 3.0 × 10 ^{5 is} used as 100% by weight of the total weight of the polyolefin resin in the first microporous layer in the first microporous layer. It is preferable to contain less than 5% by weight. This is because it has an excellent balance of air permeability and strength, and has an electrolyte solution injectability equivalent to that of a polyethylene multilayer microporous membrane. Further, if the content of the specific polypropylene is less than 5% by weight, it is preferable because the film thickness distribution becomes uniform. When the polyolefin multilayer microporous membrane of the present invention is used as a battery separator, the productivity of the battery is improved, and the battery life can be extended due to its excellent oxidation resistance.

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

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

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

The polyolefin multilayer microporous membrane of the present invention has two or more layers, preferably three layers, and has at least one first microporous layer. In the polyolefin multilayer microporous membrane of the present invention, the first microporous layer is composed of polyethylene as a main component, and is composed of a polyolefin resin (first polyolefin resin) containing polypropylene. Further, the first microporous layer is at least one surface layer of the polyolefin multilayer microporous membrane of the present invention. Layers other than the first microporous layer may be a second microporous layer made of a second polyolefin resin. It is preferable that the polyolefin multilayer microporous membrane of the present invention has a three-layer structure in which both surface layers (skin layers) are the first microporous layer, and the second microporous layer is disposed between both surface layers (core layer).

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

[1] Raw materials

[Polyolefin resin]

The first and second polyolefin resins constituting the polyolefin multilayer microporous membrane of the present invention are made of polyethylene (PE) as a main component, and the ratio of polyethylene is preferably 80% by weight or more, with the total polyolefin resin being 100% by weight. Preferably it contains 90% by weight or more. The first and second polyolefin resins may be compositions containing resins other than polyolefins. Thus, the term "polyolefin resin" may include not only polyolefins but also resins other than polyolefins.

[1st polyolefin resin]

In the polyolefin multilayer microporous membrane of the present invention, the first microporous layer is composed of a 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 a Mw (weight average molecular weight) of ^{less than 1.0×10 6} (hereinafter referred to as “PE(A)”), or (b) PE(A) and ultra-high molecular weight polyethylene (UHMwPE) having a ^{Mw of 1.0×10 6 or more (UHMwPE} ) It is preferable that it is a composition (hereinafter, "PE composition (B)") consisting 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 strength of the obtained polyolefin multilayer microporous membrane is also excellent.

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-olefins. Examples of other α-olefins other than ethylene include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene.

PE(A) is a polyethylene having less than 0.20 terminal unsaturated groups per 10,000 carbon atoms. Optionally, PE(A) has terminal unsaturation groups in the range of 0.14 or less or 0.12 or less, such as 0.05 to 0.14 (eg below the measurement limit) per 10,000 carbon atoms. In addition, PE (A) is, for example, a weight average molecular weight (Mw) of less than 1.0 × 10 ^{6 in the range of about 2.0 × 10 5} to about 0.9 × 10 ⁶ , molecular weight distribution in the range of about 2.0 to 50.0 (MWD, Mw Is defined as a value divided by the number average molecular weight Mn). It is preferable that Mw of PE(A) is 1.0×10 ⁴ or more and ^{less than 5.0×10 5.} Among these, 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 different types of Mw or different density.

PE composition (B)

When polyethylene is the PE composition (B), the upper limit of PE (A) is preferably 99.5% by weight, more preferably 98.0% by weight, based on the total weight of the first polyolefin resin as 100% by weight. The lower limit of PE(A) is preferably 45.0% by weight, more preferably 60% by weight.

The content of UHMwPE is preferably 50% by weight or less by making the total weight of the first polyolefin resin 100% by weight. It is particularly preferably 40% by weight or less. When this content is in the above preferable range, no pressure increase is caused during molding, and productivity is also good. In addition, the lower limit of this content is not particularly limited, but from the viewpoint of maintaining mechanical strength and maintaining a high melt down temperature, it is more preferably 0.1% by weight, more preferably 1% by weight, and particularly preferably 25% by weight. . When UHMwPE is 0.1% by weight or more and 50% by weight or less, a polyolefin multilayer microporous membrane excellent in 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. Melt extrusion can be facilitated by setting Mw of UHMwPE to 3.0×10 ^{6 or less.} UHMwPE may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other α-olefins. Other α-olefins other than ethylene may be the same as above.

The PE composition (B) may contain any of polybutene-1 having ^{a Mw of 1.0×10 4} to 4.0×10 ⁶ as an optional component and an ethylene/α-olefin copolymer having a ^{Mw of 1.0×10 4} to 4.0×10 ⁶ It's okay. It is preferable that these optional components are contained in an amount of 40% by weight or less based on the total amount of the first polyolefin resin as 100% by weight.

Polypropylene

The content of polypropylene is preferably less than 5.0% by weight based on the total weight of the first polyolefin resin as 100% by weight. The upper limit of the content of polypropylene is preferably 3.5% by weight. The lower limit of the content of 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 polypropylene, it is more preferable that it is 1.1-50. Polypropylene may be a single substance 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.

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, ethylene or an α-olefin having 4 to 8 carbon atoms is preferable. As a C4-C8 alpha-olefin, 1-butene, 1-hexene, 4-methyl-1-pentene, etc. are mentioned, for example. The number of carbon atoms of the diolefin is preferably 4 to 14. As a C4-C14 diolefin, butadiene, 1, 5-hexadiene, 1, 7-octadiene, 1, 9-decadiene, etc. are mentioned, for example. It is preferable that the content rate of other olefins or diolefins is less than 10 mol% based on 100 mol% of the propylene copolymer.

[2nd polyolefin resin]

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

The second polyolefin resin comprises polyethylene. Polyethylene has (a) Mw (weight average molecular weight) of ^{less than 1.0×10 6} and a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms by infrared spectroscopy (hereinafter, PE(C)), (b) PE (C) and a ^{composition consisting of ultra-high molecular weight polyethylene (UHMwPE) having Mw of 1.0×10 6} or more (hereinafter, PE composition (D)), or (c) PE(A), PE(C) and Mw of 1.0×10 ⁶ It is preferable that it is a composition made of the above-mentioned ultra-high molecular weight polyethylene (UHMwPE) (hereinafter, PE composition (E)). The polyethylene described in the first polyolefin resin can be used. It is preferable that the second polyolefin resin does not contain polypropylene.

When PE(C) is contained in the second polyolefin resin, the shutdown temperature of the polyolefin multilayer microporous membrane of the present invention is lowered. This is considered to be because the mobility of the polyethylene component increases in the layer containing PE(C), and the pores are clogged at a lower temperature.

PE(C) is at least 0.20 per ^{1.0×10 4} carbon atoms, preferably at least 0.30 per ^{1.0×10 4 carbon atoms, more preferably 0.50 or more per 1.0×10 4} carbon atoms, for example, carbon atoms It has a concentration of terminal vinyl groups by infrared spectroscopy in the range of 0.6 to 10.0 per 1.0×10 ^{4 pieces.} Further, the weight average molecular weight of PE(C) is ^{less than 1.0×10 6} , preferably the upper limit of the weight average molecular weight is 0.9×10 ⁶ , and more preferably 8.0×10 ⁵ . The lower limit of Mw of PE(C) is 2.0×10 ⁵ , preferably 3.0×10 ⁵ . The MWD of PE(C) is preferably about 2 to about 50, more preferably about 4 to about 15.

When polyethylene is a PE composition (D) or a PE composition (E), the upper limit of the content of PE (C) is preferably 99.0% by weight, and more preferably 95.0% by weight of the total weight of the second polyolefin resin as 100% by weight. It is weight percent. The lower limit of PE(C) is preferably 20.0% by weight, more preferably 60.0% by weight. By containing 20% by weight or more of PE (C), it is possible to obtain a polyolefin multilayer microporous membrane having good oxidation resistance and excellent physical property balance while maintaining a shutdown temperature of 132°C or less.

The content of UHMwPE is preferably 50.0% by weight or less by making the total weight of the second polyolefin resin 100% by weight. It is particularly preferably 40.0% by weight or less. This is because, when this content is within the above range, an increase in pressure is suppressed even during molding, and productivity is improved. Further, the lower limit of this content is not particularly limited, but it 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. When UHMwPE is 1.0% by weight or more and 50.0% by weight or less, a polyolefin multilayer microporous membrane excellent in balance of strength and air permeability can be obtained.

PE composition (D) or PE composition (E) have an Mw of 1.0 × 10 ⁴ ~ 4.0 × 10 ⁶ as an optional component of polybutene-1 and an Mw of ^{1.0 × 10 4 ~ 4.0 × 10} 6 of ethylene / α- olefin Any of the coalescence may be added. The amount of these added is preferably 40% by weight or less based on the total weight of the second polyolefin resin being 100% by weight.

[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 a composition containing resins other than polyolefins. Examples of polyolefins other than polyethylene and polypropylene include homopolymers and copolymers such as polybutene-1, pentene-1, hexene-1, 4-methylpentene-1, and octene.

In addition, when the polyolefin resin contains a heat-resistant resin, the melt down temperature is improved when the polyolefin multilayer microporous membrane is used as a battery separator, so that the high-temperature storage properties of the battery are further improved.

As the heat-resistant resin, those described in International Publication No. 2006/137540 can be used. The amount of the heat-resistant resin to be added is preferably 3 to 20% by weight, more preferably 3 to 15% by weight, with the total polyolefin resin being 100% by weight. When this content is in the above preferred range, mechanical strength such as puncture strength and tensile breaking strength is excellent.

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

[2] Method for producing polyolefin multilayer microporous membrane

Next, the method of manufacturing the polyolefin multilayer microporous membrane of the present invention will be described. In addition, the method of manufacturing the polyolefin multilayer microporous membrane of the present invention is not limited thereto.

The method for producing a polyolefin multilayer microporous membrane of the present invention,

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

(a-1) Polyethylene having a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms and a weight average molecular weight of less than ^{1.0 × 10 6 by infrared spectroscopy, and a weight average molecular weight greater than 6.0 × 10 4} and less than 3.0 × 10 ⁵ A step of preparing a first polyolefin solution by melt-kneading a first polyolefin resin containing polypropylene and a film-forming solvent, and

(a-2) comprising polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms by infrared spectroscopy and a weight average molecular weight of less than ^{1.0×10 6} and ultra-high molecular weight polyethylene having a ^{weight average molecular weight of 1.0×10 6 or more} A process including a process of preparing a second polyolefin solution by melt-kneading a second polyolefin resin and a film-forming solvent,

(b) forming a molded article by extruding a polyolefin solution at a shear rate of 60/sec or more, and

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

(d) a step of producing a stretched product by stretching the obtained gel-like sheet in at least one axial direction; and

(e) a step of removing the film-forming solvent from the obtained stretched material.

Since the manufacturing method of the polyolefin multilayer microporous membrane of the present invention can be classified into four types according to the lamination method, it will be described below for each classification.

(2-1) The first manufacturing method

The first production method for producing a polyolefin multilayer microporous membrane of the present invention is: (i) melt-kneading a first polyolefin resin and a film-forming solvent to prepare a first polyolefin solution, and (ii) a second polyolefin resin and film formation. The solvent is melt-kneaded to prepare a second polyolefin solution, (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. In addition, (v) a step of producing a stretched product by stretching the gel-like sheet in at least one axial direction (first stretching step), (vi) a step of removing (washing) a film-forming solvent from the stretched product, and (vii) washing. It includes a step of drying the subsequent film. After the steps (i) to (vii), (viii) a step of stretching the dried film again in at least one axial direction (second stretching step) and (ix) a step of heat treatment may be further included. If necessary, one of a heat setting treatment step, a heat roll treatment step, and a heat solvent treatment step may be provided before the film-forming solvent removal step of (vi). In addition, after the steps (i) to (ix), a drying step, a heat treatment step, a crosslinking treatment step by ionizing radiation, a hydrophilic treatment step, a surface coating treatment step, and the like may be provided. Moreover, (v) After the 1st extending|stretching process, you can also provide the process of heat-processing a stretched object.

(i) Preparation of the first polyolefin solution

The first polyolefin resin and the film-forming solvent are melt-kneaded to prepare a first polyolefin solution. After mixing the above-described first polyolefin resin with a suitable film-forming solvent, it is melt-kneaded to prepare a polyolefin resin solution. As the melt-kneading method, for example, a method using a twin screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is known, its description is omitted. However, as for the polyolefin resin concentration in the polyolefin resin solution, the total of the polyolefin resin and the film-forming solvent is 100% by weight, and the polyolefin resin is 20 to 50% by weight, preferably 25 to 45% by weight. When the polyolefin resin concentration in 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 the first polyolefin resin, those described above can be used.

(ii) Preparation of the second polyolefin solution

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

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

(iii) extrusion

Each of the first and second polyolefin solutions is fed from an extruder to one die, where the two solutions are combined in a layered form 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 (the first microporous layer), and the second polyolefin solution is at least one layer between the two surface layers (the second microporous layer). ) To form (preferably to contact one or both of the surface layers), the two solutions are combined in a layered form and extruded into a sheet form.

The extrusion method may be either a flat die method or an inflation method. Either way, a method of supplying a solution to each manifold and laminating in a layered form at the lip inlet of a multilayer die (multiple manifold method), or a method of supplying the solution to the die in a pre-layered flow ( Block method) can be used. Since the multiple manifold method and the block method itself are known, detailed descriptions 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 controlling the amount of each extrusion of the first and second polyolefin solutions, the film thickness ratio of the first and second microporous layers can be adjusted.

The ratio (L/D) of the length (L) and diameter (D) of the screw of the twin screw extruder is preferably in the range of 20 to 100. It is preferable that the inner diameter of the cylinder of the twin screw extruder is 40 to 200 mm. When putting the polyolefin resin into the twin screw extruder, the ratio (Q/Ns) of the input amount of the polyolefin resin solution (Q (kg/h)) to the screw rotation speed (Ns (rpm)) is 0.1 to 0.55 kg/h/rpm It is preferable to do it. It is preferable to set the screw rotation speed (Ns) to 180 rpm or more. The upper limit of the screw rotation speed (Ns) is not particularly limited, but 500 rpm is preferred.

As the extrusion method, for example, a method disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used, but in the method for producing a polyolefin multilayer microporous membrane of the present invention, a polyolefin resin solution containing a first polyolefin resin solution It is characterized in that the shear rate from the die is 60/sec or more. It is more preferable that the shear rate from the die is 150/sec or more.

(iv) Formation of gel-like sheet

The extruded body obtained by (iii) is cooled to form a gel-like sheet. As a method for forming a gel-like sheet, for example, a method disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. It is preferable to perform cooling until the extruded body becomes 40 degreeC or less. Through cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. As the 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 method for producing a polyolefin multilayer microporous 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 and the cooling rate from the die are properly controlled, it is easy to make the distribution of polypropylene in the gel-like sheet uniform, and the oxidation resistance and electrolyte solution injection property are improved.

(v) 1st stretching process

The obtained gel-like sheet is stretched in at least one axial direction. The first stretching causes cleavage between the polyethylene crystal lamellar layers, 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 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 by a conventional tenter method, roll method, inflation method, rolling method, or a combination of these methods. The first stretching may be uniaxial or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching can be performed.

The draw ratio varies depending on the thickness of the gel-like sheet, but in uniaxial stretching, it is preferably set to 2 times or more, more preferably 3 to 30 times. In biaxial stretching, it is preferable to make it at least 3 times or more in any direction, that is, 9 times or more in area magnification, whereby the puncture strength of the obtained polyolefin multilayer microporous membrane is improved, and high elasticity and high strength can be achieved. In addition, if the area magnification is within the above-described preferable range, there are no restrictions in terms of a stretching device, stretching operation, and the like. In addition, in biaxial stretching, it is preferable that the magnification in both directions is the same magnification.

It is preferable that the first stretching temperature is a temperature that exceeds about 10°C of the melting point of the polyethylene used to prepare the polyolefin solution. The stretching temperature may be in the range of greater than Tcd to less than Tme. Tme and Tcd are the melting points and crystal dispersion temperatures of all polyethylenes used in the preparation of the polyolefin solution, respectively. When the stretching temperature is Tme +10°C or less, the orientation of the molecular chains of the polyolefin in the gel-like sheet tends to be promoted during 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 becomes possible. In one embodiment, the stretching temperature is about 90°C to about 140°C, or about 100°C to about 130°C. When the polyolefin resin is made of 90% by weight 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.

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

The first stretching may perform multi-stage stretching at different temperatures, and the stretching temperatures and final stretching ratios at the front and rear ends are within the above ranges, respectively. Depending on the desired physical properties, it can be stretched by placing a temperature distribution in the film thickness direction, thereby obtaining a polyolefin multilayer microporous film having more excellent mechanical strength. As the method, for example, a method disclosed in Japanese Patent No. 3347854 can be used.

(vi) Film formation solvent removal (washing) process

Next, the solvent for film formation 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 is obtained by removing the film-forming solvent. Since the cleaning solvent and the method of removing the solvent for film formation using the same are known, the description will be omitted. For example, a method disclosed in Japanese Patent No. 2132327 or Japanese Unexamined Patent Publication No. 2002-256099 can be used.

(vii) Membrane drying process

The polyolefin multilayer microporous membrane obtained through removal of the film-forming solvent is dried by a heat drying method, an air drying method, or the like.

(viii) 2nd stretching process

Further, the dried film can be stretched again at least in the uniaxial direction. The second stretching can be carried out by heating the film, similarly to the first stretching, through a tenter method or the like. The second stretching may be uniaxial or biaxial stretching.

The second stretching temperature may be substantially equal to or less than the melting point Tme of all polyethylenes used to prepare the polyolefin solution. In one embodiment, the second stretching temperature is about Tcd to about Tme. When the second stretching temperature is Tme or less, the permeability of the obtained polyolefin multilayer microporous membrane becomes appropriate, and nonuniformity in physical properties such as permeability in the transverse direction (width direction: TD direction) tends to be suppressed, while the second stretching temperature When is Tcd or more, the film breakage due to stretching is suppressed, and it becomes possible to uniformly stretch. When the polyolefin resin is made of polyethylene, the stretching temperature is usually within the range of 90 to 140°C, and preferably within the range of 100 to 140°C.

The magnification of the second stretching in the uniaxial direction is preferably 1.1 to 1.8 times. For example, in the case of uniaxial stretching, 1.1~ in the MD direction (membrane production direction, also referred to as machine direction and length direction) or TD direction (membrane direction and the same plane and perpendicular direction, also referred to as transverse direction). Make it 1.8 times. In the case of biaxial stretching, it is 1.1 to 1.8 times in the MD direction and the TD direction, respectively. In the case of biaxial stretching, the stretching ratios in the MD direction and the TD direction may be different from each other in each direction as long as 1.1 to 1.8 times. When the draw ratio was within the above range, it was confirmed that the obtained polyolefin multilayer microporous membrane tends to improve permeability, heat shrinkage resistance, electrolyte absorbency, and compression resistance. It is more preferable that the magnification of the second stretching is 1.2 to 1.6 times.

It is preferable that the 2nd drawing speed is 3%/sec or more in the drawing axial direction. For example, in the case of uniaxial stretching, it is set to 3%/sec or more in the MD direction or the 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 (%/sec) in the stretching axial direction refers to the ratio of the length increased per second by making the length in the stretching axial direction before re-stretching in a region where the film (sheet) is re-stretched as 100%. When this drawing speed is 3%/sec or more, the permeability of the obtained polyolefin multilayer microporous membrane becomes appropriate, and there is a tendency that nonuniformity in physical properties such as permeability in the sheet width direction is suppressed. The second stretching speed is preferably 5%/sec or more, and more preferably 10%/sec or more. In the case of biaxial stretching, as long as the stretching speeds in the MD direction and the TD direction are 3%/sec or more, they may be different from each other in the MD direction and the TD direction, but the same is preferable. There is no particular limitation on the upper limit of the second stretching speed, but it is preferably 50%/sec or less from the viewpoint of preventing breakage.

(ix) heat treatment process

The film after the second stretching can be heat-treated. A network structure made of fibrils formed by the second stretching is retained, and a polyolefin multilayer microporous membrane having large micropores and excellent strength can be produced. Heat treatment may use heat setting treatment and/or heat relaxation treatment. The heat setting treatment is a heat treatment that heats while keeping the film dimensions 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 crystal of the film is stabilized by 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 Laid-Open Patent Publication No. 2002-256099 is exemplified.

The heat treatment is performed within a temperature range of not less than the crystal dispersion temperature to not more than the melting point of all the polyolefin resins constituting the polyolefin multilayer microporous film. The heat setting treatment temperature is preferably in the range of the second stretching temperature ± 5°C, thereby stabilizing the physical properties. It is more preferable that this temperature is in 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, the removal of the film-forming solvent, drying, the second stretching, and the heat treatment are performed continuously on a series of lines. However, if necessary, an off-line method may be employed in which the film after the drying treatment is once wound, and then the film is unwound to perform second stretching and heat treatment.

(x) other processes

Before removing the film-forming solvent from the gel-like sheet subjected to the first stretching, one of a heat setting treatment step, a heat roll treatment step, and a heat solvent treatment step may be provided. Further, it is possible to provide a step of heat-setting the film after washing or during the second stretching step. The method of heat setting treatment of 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) 2nd manufacturing method

The second method of manufacturing a polyolefin multilayer microporous membrane is (i) melt-kneading the first polyolefin resin and the film-forming solvent to prepare a first polyolefin solution, and (ii) melt the second polyolefin resin and the film-forming solvent. A second polyolefin solution was prepared by kneading, (iii-2) the first and second polyolefin solutions were laminated immediately after extruding on separate dies, and (iv) the obtained extruded body (laminate) was cooled to form a gel-like sheet. It is characterized in that to form. In other words, the first manufacturing method differs only in that the solution is laminated immediately after extruding the solution in a separate die, whereas the first manufacturing method laminates a polyolefin solution in one die to form an extruded body. The same method as the first manufacturing method can be employed.

Since the second method is the same as each step in the first manufacturing method except for the step (iii-2), only the step (iii-2) will be described. In step (iii-2), each of the first and second polyolefin solutions is extruded into sheets 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 extrusion molded body. Other processes may be the same as the first manufacturing method.

(2-3) 3rd manufacturing method

A third production method for producing a polyolefin multilayer microporous membrane comprises: (i) melt-kneading a first polyolefin resin and a film-forming solvent to prepare a first polyolefin solution, and (ii) a second polyolefin resin and a film-forming solvent. Melt-kneading to prepare a second polyolefin solution, (iii-3-1) extruding the first polyolefin solution from one die to form a first extruded body, and (iii-3-2) adding the second polyolefin solution to another Extruded from a die to form a second extruded body, (iv-3) the obtained first and second extruded bodies were respectively cooled to form first and second gel-like sheets, and (v-3) the first and second extruded bodies were cooled. (2) Each gel-like sheet is stretched, (xi-3) stretched first and second stretched products are laminated, and (vi) a film-forming solvent is removed from the obtained stretched product. That is, until the gel-like sheet is stretched, it is separately carried out and then laminated, 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 process of the gel-like laminated sheet can also be provided. Steps (iii-3-1) and (iii-3-2) differ from the step (iii) in the first manufacturing method only in that the first and second polyolefin solutions are not combined in a layered form. The die used is the same as the die used in the step (iii-2) in the second manufacturing method. The step (iv-3) differs from the step (iv) in the first manufacturing method only in that the first and second extruded bodies are separately cooled. The step (v-3) differs from the step (v) in the first manufacturing method only in that the first and second gel sheets are respectively stretched. On the other hand, the step (xi-3) is a step that is not present in the first and second manufacturing methods of laminating the first and second stretched products, but a known method can be used for laminating the stretched material.

(2-4) 4th manufacturing method

A fourth production method for producing a polyolefin multilayer microporous membrane comprises: (i) melt-kneading a first polyolefin resin and a film-forming solvent to prepare a first polyolefin solution, and (ii) a second polyolefin resin and a film-forming solvent. Melt-kneaded to prepare a second polyolefin solution, (iii-4-1) the first polyolefin solution was extruded from one die, (iii-4-2) the second polyolefin solution was extruded from another die, and (iv -4) Each obtained extruded body was cooled to form first and second gel-like sheets, (v-4) each of the first and second gel-like sheets were stretched, and (vi-4) each stretched product The solvent for film formation was removed in (vii-4) the obtained first and second polyolefin microporous membranes were dried, (viii-4) at least the second polyolefin microporous membrane was stretched, and (xi-4) the first And a step of laminating a second polyolefin microporous membrane. That is, it is carried out separately until a porous film is obtained, and then laminated to obtain a multilayer microporous film. If necessary, a heat treatment step may be performed on each of the (ix-4) first and second polyolefin microporous membranes between steps (vii) and (viii-4). In addition, the following steps can employ the same method as the first manufacturing method.

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

On the other hand, step (viii-4) is a step that is not necessarily required in the first to third manufacturing methods, but in the fourth manufacturing method, at least the second polyolefin microporous membrane is redrawn in this step (viii-4). The stretching temperature is preferably not more than the melting point, and more preferably the crystal dispersion temperature to the melting point. If necessary, the first polyolefin microporous membrane can also be stretched. The stretching temperature is preferably not more than the melting point, and more preferably the crystal dispersion temperature to the melting point. In the case of stretching any of the first and second polyolefin microporous membranes, the draw ratio may be the same as the first production method except for stretching the unlaminated polyolefin microporous membrane.

In addition, the step (xi-4) is a step that is not present in the first to third manufacturing methods of laminating the first and second films, but the lamination of the film can be performed using a known method similar to lamination of a stretched product.

In the above, the method for manufacturing the polyolefin multilayer microporous membrane of the present invention has been described in four categories according to the lamination method, but if these are summarized, steps (a) to (e) are required.

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

Step (b) is 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 step (iii-) of the fourth manufacturing method. It corresponds to 4-1).

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

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

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

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

(1) Standardized PP/PE ratio

The polyolefin multilayer 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 film surface was set to 1 for the peak intensity ratio (PP/PE ratio) of PP and PE determined by Micro-Raman spectroscopy. If the relative value at the time is the normalized PP/PE ratio, the average value/standard deviation/kurtosis of the normalized PP/PE ratio can be expressed in a structure representing a constant value. That is, it is preferable that the polyolefin multilayer microporous membrane of the present invention 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 as a kurtosis parameter indicating the shape of the distribution.

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

When the average value of the normalized PP/PE ratio is in the above preferred range, there are few parts with a low polypropylene concentration, and the part mainly composed of polyethylene does not increase, so that polyethylene is the main due to the oxidation reaction accompanying charging and discharging in the battery. Since there are few parts, deterioration is difficult to proceed, and it is thought that the cycle characteristics are maintained satisfactorily.

When the standard deviation of the normalized PP/PE ratio is in the above preferred range, it is considered that the change in the polypropylene concentration is small and there are few portions with a low polypropylene concentration, so that the oxidation resistance is also difficult to deteriorate.

In addition, when the distribution of the polypropylene concentration is in the above-described preferred range, there are few portions with a low polypropylene concentration, and portions in the battery with poor oxidation resistance are less likely to occur, and the battery performance is good. It is easy to improve oxidation resistance if there is a portion with a high polypropylene concentration to some 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 multilayer microporous membrane.

The polyolefin multilayer microporous membrane of the present invention has a uniform PP distribution in the in-plane direction as described above in the first microporous layer, and thus has excellent oxidation resistance. In addition, when the content of polypropylene is less than 5% by weight, deterioration of physical properties due to polypropylene is suppressed, and since it is excellent in permeability, strength, and electrolytic solution absorption, it is preferable. Therefore, when used as a separator for lithium ion batteries, excellent battery productivity, safety, and battery cycle characteristics can be realized, respectively.

(2) Air permeability (sec/100 cm 3/20 μm)

The air permeability (Gurley value) obtained by converting the film thickness of the polyolefin multilayer microporous membrane of the present invention into 20 μm is preferably 20 to 600 sec/100 cm 3, more preferably 100 to 500 sec/100 cm 3. If the air permeability is within this range, when the polyolefin multilayer microporous membrane is used as a battery separator, the battery capacity is large, the battery cycle characteristics are good, and shutdown is sufficiently performed when the temperature inside the battery rises, while charging and discharging when used as a battery At the time, the resistance value is difficult to increase, and the average electrochemical stability is good. In addition, the air permeability is measured according to JIS P 8117, and is a value obtained by converting the film thickness to 20 μm.

(3) Public ratio (%) (空孔率)

The porosity of the polyolefin multilayer 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 in the case of using the polyolefin multilayer microporous membrane 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 membranes (of the same polymer) of the same size and thickness

(4) Puncture strength (mN/20㎛)

Prick strength is a value obtained by measuring the maximum load value when piercing the polyolefin multilayer microporous membrane at a speed of 2 mm/sec using a needle having a diameter of 1 mm (0.5 mm R), and converting the film thickness to 20 μm. The prick strength of the polyolefin multilayer microporous membrane of the present invention in terms of 20 μm is preferably 2,000 mN or more, preferably 2,500 mN or more, and more preferably 4,000 mN or more. If the puncture strength is 2,000 mN/20 µm or more, a short circuit of the electrode can be effectively suppressed when the polyolefin multilayer microporous membrane is inserted into the battery as a battery separator.

(5) Tensile breaking strength (kPa)

The tensile breaking strength of the polyolefin multilayer microporous membrane of the present invention is 60,000 kPa or more, more preferably 80,000 kPa or more, and even more preferably 100,000 kPa or more in either direction of the MD direction and the TD direction. When the tensile breaking strength is 60,000 kPa or more, it is easy to prevent rupture during battery production. Tensile breaking strength is a value measured according to ASTM D882 using a rectangular test piece having a width of 10 mm.

(6) Tensile breaking elongation (%)

The tensile elongation at break of the polyolefin multilayer microporous membrane of the present invention is preferably 80% or more, and more preferably 100% or more in either direction in the MD direction and the TD direction. Thereby, it is easy to prevent rupture during battery manufacturing. Tensile elongation at break is a value measured according to ASTM D882 using a rectangular test piece having a width of 10 mm.

(7) Heat shrinkage rate (%)

The heat contraction rate after exposing the polyolefin multilayer microporous membrane of the present invention at a temperature of 105° C. for 8 hours is preferably 15% or less, more preferably 10% or less, and even more preferably 6% or less in both the MD direction and the TD direction. to be. When the thermal contraction rate is 15% or less, when the polyolefin multilayer microporous membrane is used as a separator for a lithium battery, the end of the separator contracts during heat generation, thereby reducing the possibility of a short circuit of the electrode.

The heat shrinkage rate is a value obtained by measuring the heat shrinkage rates in the MD direction and the TD direction three times, respectively, when the polyolefin multilayer microporous membrane is exposed at 105°C for 8 hours, and calculating an average value, respectively. The thermal contraction rate is expressed by the following equation.

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

(8) shutdown temperature

The shutdown temperature of the polyolefin multilayer microporous membrane of the present invention is 135°C or less, more preferably 132°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 multilayer microporous membrane is exposed to an atmosphere of 30° C., the temperature is raised to 5° C./min, and the air permeability of the membrane is measured in the meantime. The shutdown temperature of the polyolefin multilayer microporous membrane was defined as the temperature when the air permeability (Gurley value) of the polyolefin multilayer microporous membrane initially exceeded 100,000 sec/100 cm 3. The air permeability of the polyolefin multilayer 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 solution injection property of the polyolefin multilayer microporous membrane of the present invention is 20 seconds or less. More preferably, 10 seconds or less, and 5 seconds or less are particularly preferable. The electrolyte solution injectability was evaluated as the penetration time of propylene carbonate. A sample of 50 mm x 50 mm is placed on a glass plate, 0.5 ml of propylene carbonate is added dropwise from about 2 cm above the sample, and time measurement is started from the end of the dropping. Immediately after completion of the dropping, propylene carbonate swells on the film due to surface tension, but the dropped propylene carbonate penetrates over time. When all propylene carbonate on the membrane has permeated, the time measurement is stopped and the penetration time is set. A penetration time of 20 seconds or less is considered good, a penetration time greater than 20 seconds and slightly better than 50 seconds, and more than 50 seconds is regarded as inappropriate.

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

To measure the electrochemical stability, a film having a length (MD) of 70 mm and a width (TD) of 60 mm is placed between the anode and the cathode 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 melting LiPF 6} in a 1M 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 negative electrode and the positive electrode.

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

Electrochemical stability is a film property related to the oxidation resistance of a film when the film is used as a separator in a battery that is exposed to relatively high temperatures during storage or use. The electrochemical stability is in mAh, and generally a lower value is preferred (indicating less total charge loss during storage at high temperature or overcharging). Since the battery for automobiles and power tool batteries, such as batteries used for starting power means for moving electric vehicles or hybrid electric vehicles, or for supplying power to the power means, are used for relatively high output and large capacity, A slight loss of battery capacity, such as self-discharge loss due to electrochemical instability, is also an important problem. The average electrochemical stability of the polyolefin multilayer 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 generally means a battery capable of supplying at least 1 amp hour (1Ah), for example 2.0Ah to 3.6Ah.

(11) film thickness

The film thickness of the polyolefin multilayer microporous membrane of the present invention is preferably 5 to 50 µm, more preferably 5 to 35 µm, and still more preferably 10 to 25 µm when used as a battery separator, for example. The measurement method of the film thickness is irrelevant whether it is a contact thickness measurement method or a non-contact thickness measurement method. For example, it can be measured with a contact-type thickness gauge over a width of 10.0 cm at 1.0 cm intervals in the longitudinal direction, and then the average value can be calculated to obtain the film thickness. As the contact-type thickness measuring device, for example, a thickness measuring device such as Litematic manufactured by Mitutoyo Corporation is suitable.

(12) Appearance

The appearance of the film was evaluated by visual/multipoint film thickness measurement. It was set as "good" for the small 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

The melting point of the resin was measured in the following procedure in accordance with JIS K 7122. That is, the resin sample was left standing in a sample holder of a scanning differential calorimeter (manufactured by Perkin Elmer, Inc., DSC-System7 type), heat-treated at 230°C for 10 minutes in a nitrogen atmosphere, and then at 10°C/min. After cooling to 40°C, it was kept at 40°C for 2 minutes, and then heated to 230°C at a rate of 10°C/min. The temperature (peak temperature) at which the maximum endothermic amount became the melting point.

[4] Batteries, etc.

As described above, the polyolefin multilayer microporous membrane of the present invention is excellent in oxidation resistance and electrolyte injectability, it is difficult to cause blackening even after repeated charging and discharging as a battery, and it is particularly suitable as a battery separator because it has excellent balance of permeability and strength. Do.

The separator made of the polyolefin multilayer microporous membrane 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 this, it is particularly suitable for use as a lithium secondary battery/lithium ion capacitor. Known electrodes and electrolytes can be used for the lithium secondary battery/capacitor using the separator made of the polyolefin multilayer microporous membrane of the present invention. Further, the structure of a lithium secondary battery/capacitor using a separator made of a polyolefin multilayer microporous membrane 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 multilayer microporous membrane was determined by the above-described method.

Example One

(1) Preparation of the first polyolefin solution

With respect to the total weight of the first polyolefin composition, (a) Mw of 2.5×10 ⁵ HDPE (Mw/Mn: 8.6, terminal vinyl group concentration 0.1 per/10000 carbon)) 97% by weight, (b) Mw is 9.7 A 1st polyolefin composition containing 3 weight% of x10 ⁴ phosphorus polypropylene (Mw/Mn: 2.6, melting point: 155°C) was prepared by dry blending. As an antioxidant, 0.2 parts by weight of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate] methane is dry blended per 100 parts by weight of the first polyolefin composition, and the first polyolefin is Resin was prepared.

30 parts by weight of the first polyolefin resin was supplied to a strong kneading twin screw extruder, and 70 parts by weight of liquid paraffin (50 cSt at 40°C) was supplied from a side feeder to the twin screw extruder. It melt-kneaded at 210 degreeC and 200 rpm, and prepared the 1st polyolefin solution.

(2) Preparation of the second polyolefin solution

The second polyolefin solution was prepared in the same manner as the method for preparing the first polyolefin solution except for the following points. Based on the total weight of the second polyolefin composition, (a) 20% by weight of UHMwPE (Mw/Mn: 8.0) ^{having an Mw of 2.0×10 6 , (b) HDPE having a Mw of 3.0×10 5} (Mw/Mn: 13.5, A second polyolefin composition containing 80% by weight of a terminal vinyl group concentration of 0.9 per/10000 carbon) was prepared by dry blending. As an antioxidant, 0.2 parts by weight of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate] methane is dry blended per 100 parts by weight of the second polyolefin composition, and the second polyolefin is Resin was prepared. 25 parts by weight of the obtained second polyolefin composition was fed to a strong kneading twin screw extruder, and 75 parts by weight of liquid paraffin (50 cSt at 40°C) was fed from the side feeder to the twin screw extruder. It melt-kneaded at 210 degreeC and 200 rpm, and prepared the 2nd polyolefin solution.

(3) Preparation of microporous membrane

The first and second polyolefin solutions are supplied from each twin screw extruder to a three-layer T-die, and the layer structure is 3 with a first polyolefin solution/second polyolefin solution/first polyolefin solution and a layer thickness ratio of 8.5/83/8.5. A layer of extruded body was formed. This extruded body was passed through a cooling roll controlled at 20°C and cooled to form a three-layer gel-like laminated sheet. In addition, the shear rate in the die of the extruded body was 210/sec, and the cooling rate in the cooling roll was 38°C/sec. The obtained gel-like laminated sheet was subjected to simultaneous biaxial stretching (first stretching) of 5×5 times the stretching ratio at a temperature of 116°C using a tenter stretching machine and wound up. Then, a part of the wound was taken from the stretched material, fixed on a frame plate [size: 20cm×20cm, made of aluminum (the same below)], and immersed in a washing tank of methylene chloride controlled at a temperature of 25°C. Washing was carried out while shaking at 100 rpm for 3 minutes. The washed membrane was air-dried at room temperature. The dried microporous membrane is second stretched (re-stretched) at 126°C in the TD direction at a draw ratio of 1.4 times through a batch drawer, and then heat-set for 10 minutes at the temperature of re-stretching while mounted on the batch drawer. A polyolefin multilayer microporous membrane was produced.

Example 2~ Example 7 and Comparative example 1~ Comparative example 8

Using the raw materials and conditions shown in Tables 1 and 2, a polyolefin multilayer microporous membrane was produced in the same manner as in Example 1. In Examples 2, 7, and Comparative Examples 2 to 4, after the second stretching (re-stretching) was performed at the temperature and magnification shown in the table, heat relaxation treatment was performed in the TD direction, and then the temperature of the re-stretching in a state mounted on a batch stretching machine. Heat-setting treatment was performed for 10 minutes to prepare a polyolefin multilayer microporous membrane. In addition, "-" in Table 1 and Table 2 indicates that UHMwPE or HDPE2 in the table was not included and that thermal relaxation treatment was not performed.

[Table 1]

[Table 2]

Tables 3 and 4 show the physical properties of the polyolefin microporous membranes of Examples 1 to 7 and Comparative Examples 1 to 8. In addition, "-" in Comparative Example 2 in Table 4 indicates that the surface has large irregularities that can be judged by the naked eye, and thus cannot be measured.

[Table 3]

[Table 4]

Referring to Tables 3 and 4, the polyolefin microporous membranes of Examples 1 to 8 all have excellent electrolyte solution injection properties, and have a uniform PP distribution. In addition, the leakage current value is 45 mAh or less, thereby exhibiting excellent oxidation resistance. In addition, the shutdown temperature is 132°C or less, and when used in a battery, the safety is more excellent and the balance of physical properties is excellent. 1 is a graph showing the distribution of the normalized PP/PE ratio of the surface layer of the polyolefin multilayer microporous membrane of Example 1, and it can be seen that the standardized PP/PE ratio is concentrated in a narrow range of 0.5 or more. FIG. 2 shows a two-dimensional distribution of the normalized PP/PE ratio of the surface layer of the polyolefin multilayer microporous membrane of Example 1, and a region with a low polypropylene concentration (a dark part) is hardly visible, and polypropylene is present on average. You can see what you are doing. On the other hand, FIG. 3 shows a two-dimensional distribution of the normalized PP/PE ratio of the surface layer of the polyolefin multilayer microporous membrane of Comparative Example 1, and there are many areas with a low polypropylene concentration (dark part), and polypropylene is on average in the surface layer. You can see that it doesn't exist.

On the other hand, the polyolefin multilayer microporous membrane of Comparative Example 1 ^{contained polypropylene having a weight average molecular weight of 3.0×10 5} or more, the air permeability was deteriorated, the electrolyte solution injection property was remarkably decreased, and the oxidation resistance was not excellent.

The polyolefin microporous membrane of Comparative Example 2 contains 8% by weight of the same polypropylene as the polypropylene used in Examples 1 to 8. The porosity increased and the air permeability decreased, but the strength decreased. The appearance of the film was visually uneven, and it was confirmed that the film was inferior in terms of general physical properties as a battery separator.

The polyolefin multilayer microporous membrane of Comparative Example 3 contains 0.3% by weight of the same polypropylene as the polypropylene used in Examples 1 to 8. Although the dispersibility (standard deviation, kurtosis) of polypropylene is good, it is considered that the polypropylene concentration in the vicinity of the surface is insufficient and the oxidation resistance is not improved.

In Comparative Example 4, since polyethylene having 0.2 or more terminal vinyl groups was not included in the intermediate layer, the balance of oxidation resistance, liquid injectability, air permeability/prick strength was excellent, but the shutdown temperature exceeded 132°C and the shutdown temperature was high.

In Comparative Example 5, the shutdown temperature did not fall sufficiently because the content of ultra-high molecular weight polyethylene (UHMwPE) and PE (C) having ^{Mw of 1.0×10 6 or more in the intermediate layer was small.}

In Comparative Example 8, since PE (C) was contained in the surface layer, oxidation resistance was insufficient.

In Comparative Example 7, using the same resin composition as in Example 1, the shear rate from the T-die was lowered, the permeability was deteriorated, and the electrolyte solution injectability and oxidation resistance were deteriorated.

In Comparative Example 8, using the same resin composition as in Example 1, the cooling rate was lowered, the permeability was deteriorated, and the electrolyte solution injectability and oxidation resistance were deteriorated.

Industrial applicability

From the above, the polyolefin multilayer microporous membrane of the present invention is excellent in oxidation resistance, electrolyte injectability, and shutdown properties, and also has excellent permeability and strength balance, and can increase battery life and safety. This polyolefin multilayer microporous membrane has suitable performance as a storage device for non-aqueous electrolyte solutions such as capacitors, capacitors, and batteries, and can contribute to improvement in safety and reliability. Among them, it can be suitably used as a battery separator, more specifically, a lithium ion battery separator. As other uses, since it is also used as a component part of a fuel cell, various separation membranes such as a humidification membrane and a filtration membrane, there is a possibility of industrial use in the field.

Claims (10)

It has a first microporous layer containing polypropylene, an electrolyte solution injectability of 20 seconds or less, a shutdown temperature of 132°C or less, at least one surface layer is the first microporous layer, and the polypropylene distribution of the first microporous layer is Uniform in the in-plane direction,
The average value of the standardized polypropylene/polyethylene ratio measured by Raman spectroscopy of the first microporous layer is 0.5 or more, the standard deviation of the standardized polypropylene/polyethylene ratio is 0.2 or less, and the kurtosis of the standardized polypropylene/polyethylene ratio is 1.0 or less -1.0 The above polyolefin multilayer microporous membrane. delete The method of claim 1,
The weight average molecular weight of the polypropylene is greater than ^{6.0 × 10 4 and less than 3.0 × 10 5} , and the content is 0.5% by weight or more based on the total weight of the polyolefin resin of the first microporous layer in the first microporous layer as 100% by weight. Polyolefin multilayer microporous membrane of less than 5% by weight. The method of claim 1,
A polyolefin multilayer microporous membrane comprising a second microporous layer disposed between both surface layers, wherein the second microporous layer comprises polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms by infrared spectroscopy. The method of claim 4,
A polyolefin multilayer microporous membrane having a three-layer structure in which the second microporous layer is disposed between both surface layers of the first microporous layer. The method of claim 5,
The second microporous layer is a polyolefin multilayer microporous membrane comprising at least 20% by weight of polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms as 100% by weight of the total polyolefin of the second microporous layer. The method of claim 4,
The first microporous layer is made of a first polyolefin resin, and the first polyolefin resin has a polyethylene having a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms and a weight average molecular weight of less than 1.0×10 ⁶ and a weight average molecular weight. A polyolefin multilayer microporous membrane comprising polypropylene that is greater than 6.0×10 ^{4 and less than 3.0×10 5.} The method of claim 7,
The first polyolefin resin is a high-density polyethylene having a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms and a weight average molecular weight of 5.0×10 ⁴ or more and less than 5.0×10 ⁵ (the total weight of the first polyolefin resin is 100% by weight). 45.0% by weight or more and 99.5% by weight or less), ^{ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 6} or more and less than 3.0×10 ⁶ (0.0% by weight or more and 50.0% by weight based on the total weight of the first polyolefin resin as 100% by weight) Or less) and polypropylene having a weight average molecular weight of ^{greater than 6.0×10 4} and less than 3.0×10 ⁵ (an amount of 0.5% by weight or more and less than 5.0% by weight based on the total weight of the first polyolefin resin being 100% by weight). A polyolefin multilayer microporous membrane obtained by doing so. The method of claim 8,
The second microporous layer is made of a second polyolefin resin, and the second polyolefin resin has a terminal vinyl group concentration of 0.2 per 10,000 carbon atoms or more and a weight average molecular weight of 5.0×10 ⁴ or more and less than 1.0×10 ⁶ (Amount of 20.0% by weight or more and 99.0% by weight or less based on the total weight of the second polyolefin resin as 100% by weight), the terminal vinyl group concentration is less than 0.2 per 10,000 carbon atoms, and the weight average molecular weight is 5.0 × 10 ⁴ or more 1.0 High-density polyethylene less than ×10 ⁶ (an amount of 0.0% by weight or more and 79.0% by weight or less based on the total weight of the second polyolefin resin), ultra-high molecular weight polyethylene having a ^{weight average molecular weight of 1.0 × 10 6} or more and less than 3.0 × 10 ^{6 (} A polyolefin multilayer microporous membrane comprising an amount of 1.0% by weight or more and 50.0% by weight or less based on the total weight of the second polyolefin resin as 100% by weight, and does not contain polypropylene. (a) As a step of preparing a polyolefin solution by melt-kneading a polyolefin resin and a film-forming solvent,
(a-1) Polyethylene having a terminal vinyl group concentration of less than 0.2 per 10,000 carbon atoms and a weight average molecular weight of less than 1.0×10 ⁶ by infrared spectroscopy, and a poly having a weight average molecular weight ^{greater than 6.0×10 4} and less than 3.0×10 ⁵ A step of preparing a first polyolefin solution by melt-kneading a first polyolefin resin containing propylene and a solvent for film formation, and
(a-2) A preparation containing polyethylene having a terminal vinyl group concentration of 0.2 or more per 10,000 carbon atoms and a weight average molecular weight of ^{less than 1.0×10 6} by infrared spectroscopy and ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 ^{6 or more} 2 A process comprising the step of preparing a second polyolefin solution by melt-kneading a polyolefin resin and a film-forming solvent,
(b) forming a molded article by extruding a polyolefin solution at a shear rate of 60/sec or more,
(c) 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 producing a stretched product by stretching the obtained gel-like sheet in at least uniaxial direction, and
(e) A method for producing a polyolefin multilayer microporous membrane comprising a step of removing the solvent for film formation from the obtained stretched product.

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