WO2023276468A1

WO2023276468A1 - Polyolefin microporous membrane and battery separator

Info

Publication number: WO2023276468A1
Application number: PCT/JP2022/020339
Authority: WO
Inventors: 竹田健人; 窪田隆; 李丹
Original assignee: 東レ株式会社
Priority date: 2021-06-30
Filing date: 2022-05-16
Publication date: 2023-01-05
Also published as: KR20240026878A; JPWO2023276468A1

Abstract

The objective of the present invention is to provide a polyolefin microporous membrane which is a thin membrane when used as a separator, has a low shutdown temperature, and has both mechanical strength and insulating properties after melting. The polyolefin microporous membrane is characterized in that: the membrane has a thickness of at most 6 μm; the puncture strength for an equivalent of 5 μm is at least 1.7 N; the shutdown temperature as measured through temperature-rising air permeability is 80-138 °C; and the crystallinity of polypropylene when reaching 169 °C is 3-200 ppm.

Description

Polyolefin Microporous Membrane and Battery Separator

The present invention relates to a polyolefin microporous membrane and a battery separator.

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors. Among them, a microporous film made of polyolefin as a resin material is widely used as a secondary battery separator because it is excellent in chemical resistance, insulation, mechanical strength, etc., and has shutdown characteristics. In addition, batteries must meet safety requirements such as self-discharge characteristics, nail penetration tests, hot box tests, and impact resistance tests to extend life and prevent capacity deterioration. Improvements in insulation, mechanical strength, shutdown characteristics, etc. are required.

From the viewpoint of thermal safety of the battery, in Patent Document 1, the polyolefin laminated microporous membrane consists of at least three layers, the film thickness is in the range of 3 to 25 μm, and the meltdown temperature is in the range of 159 to 200 ° C. has an air permeability in the range of 50 to 300 seconds, a puncture strength in the range of 100 to 550 gf, contains polypropylene only in the inner layer of the three layers, and at least one layer forming the surface layer has a melt flow A separator film has been proposed, characterized in that it contains a resin with a rate of 50-150 g/10 min and a melting point of 120-130°C. Further, in Patent Document 2, a laminated microporous membrane made of polyethylene and polypropylene and having a thickness of 5 to 20 μm, the microporous membrane contains 3 to 50% of polypropylene, and the difference between the shutdown temperature and the membrane rupture temperature is is 33° C. or higher, the shutdown temperature is 140° C. or lower, and the membrane rupture temperature is 150° C. or higher. In Patent Document 3, in order to ensure battery safety at high temperatures, it is a microporous material characterized by containing polymethylpentene having a Tm of 200.0 ° C. or more and an MFR of 80.0 dg / min or less. , a meltdown temperature of 180.0° C. or higher, a shutdown temperature of 131.0° C. or lower, and a 170° C. TD heat shrinkage of 30.0% or lower. ing.

JP 2015-208894 A JP 2011-63025 A JP 2012-530802 A

In recent years, with the improvement of the driving performance of electric vehicles and the shift from 4G to 5G Internet communication, lithium-ion secondary batteries are required to have even higher capacity and higher safety. Therefore, separators are required to be thinner, maintain insulation at high temperatures inside the battery, and improve mechanical strength and shutdown characteristics.

However, it becomes difficult for the separator to maintain its mechanical strength and insulation at high temperatures due to thinning. In addition, for example, the film-forming conditions can be adjusted in order to improve the mechanical strength of the separator, but such a method tends to increase the shutdown temperature. As a result, the internal temperature of the battery rises excessively when abnormal heat is generated, and the insulation at high temperatures cannot be sufficiently maintained. With the separators of Patent Documents 1 to 3, when thinned, it is not possible to achieve a sufficient balance between the maintenance of insulating properties at high temperatures and the mechanical strength.

In view of the above circumstances, the object of the present invention is to provide a polyolefin microporous membrane that is thin, has a low shutdown temperature, and has both high mechanical strength and insulating properties after melting.

The polyolefin microporous membrane of the first aspect of the present invention has a thickness of 6 μm or less, a puncture strength equivalent to 5 μm of 1.7 N or more, and a shutdown temperature of 80° C. or more as measured by the temperature-rising air permeability method. The temperature is 138° C. or lower, and the crystallinity of the polypropylene at 169° C. is 3 ppm or more and 200 ppm or less.

Further, the polyolefin microporous membrane may be a multi-layer microporous membrane consisting of a plurality of layers.

The polyolefin microporous membrane has peaks in the molecular weight ranges of 5.0×10 ⁴ to 1.0×10 ⁵ and 3.0×10 ⁵ to 7.0×10 ⁵ in the GPC chart. You may have

Also, polyethylene having a weight average molecular weight of 4.0×10 ⁵ or more and 1.0×10 ⁶ or less may be contained.

Further, the polyolefin microporous membrane may have a polypropylene concentration of 3.5% by mass or more and 10.0% by mass or less.

In addition, the polyolefin microporous membrane may have a porous layer laminated on at least one side of the polyolefin microporous membrane.

The battery separator of the second aspect of the present invention comprises the polyolefin microporous membrane.

According to the present invention, it is possible to provide a polyolefin microporous membrane that is thin, has a low shutdown temperature, and has both puncture strength and insulation after melting. In particular, polyolefin microporous membranes are suitably used as battery separators.

The present embodiment of the present invention will be described below. It should be noted that the present invention is not limited to the embodiments described below.

The polyolefin microporous membrane of the present invention has a thickness of 6 μm or less, a puncture strength equivalent to 5 μm of 1.7 N or more, and a shutdown temperature measured by a temperature-rising air permeability method of 80° C. or more and 138° C. or less. , the crystallinity of the polypropylene when reaching 169° C. is 3 ppm or more and 200 ppm or less.

The upper limit of the thickness of the polyolefin microporous membrane of the present invention is 6 μm or less. If the film thickness exceeds 6 μm, the high capacity of the battery cannot be achieved. The upper limit of the film thickness is preferably 4.7 μm or less, more preferably 4.5 μm or less. The lower limit of the film thickness is preferably 1 μm or more, more preferably 3.0 μm or more, from the viewpoint of puncture strength and insulation at high temperatures. When the film thickness is within the above preferred range, when the polyolefin microporous film is used as a battery separator, the amount of active material in the electrode can be increased by the amount of the reduced film thickness, resulting in an improvement in battery capacity. . The film thickness can be set within a predetermined range by adjusting the extrusion rate and the heat setting temperature.

The polyolefin microporous membrane of the present invention has a puncture strength converted to a thickness of 5 μm (puncture strength converted to 5 μm) of 1.7 N or higher, and a shutdown temperature of 80° C. or higher and 138° C. or lower. Due to these characteristics, the film is resistant to breakage even when high tension is applied, and has high durability, and when incorporated into a battery, it has excellent self-discharge characteristics. Furthermore, when the battery heats up abnormally, it shuts down more quickly and prevents the temperature from rising. Further, when the shutdown temperature is 80° C. or higher, unnecessary shutdown does not occur in extremely hot regions or seasons, and the possibility of impairing the function as a battery is low, which is preferable. The balance between the puncture strength equivalent to 5 μm and the shutdown temperature can be adjusted within a predetermined range by adjusting the film-forming conditions such as the molecular weight of the polyolefin, the compounding ratio, and the stretching temperature in the manufacturing process. The lower limit of the 5 μm equivalent puncture strength is preferably 1.7 N or more, more preferably 1.9 N or more, from the viewpoints of suppressing the defective rate in the battery process, maintaining the self-discharge characteristics of the battery, and compressing resistance. Although the upper limit is not particularly limited, 3.0N or less is preferable. The shutdown temperature preferably has an upper limit of 137° C. or lower, more preferably 136° C. or lower, from the viewpoint of suppressing abnormal heat generation of the battery more quickly.

In the polyolefin microporous film of the present invention, the crystallinity of polypropylene when heated to 169°C (hereinafter sometimes referred to as the crystallinity of polypropylene when reaching 169°C) is 3 ppm or more and 200 ppm or less. The fact that the crystallinity of polypropylene is 3 ppm or more indicates that the regular structure of polypropylene remains sufficiently when it reaches 169 ° C., and it is difficult to relax even when the temperature rises, so it has excellent shape retention properties after melting and is in an insulating state. can be kept in good condition. In addition, when the crystallinity of polypropylene is 200 ppm or less, the amount of ordered structures does not become excessive, and phase separation between polyolefin and polypropylene at high temperatures is suppressed, holes are less likely to open, and short circuits can be further suppressed. The crystallinity of polypropylene is preferably 10 ppm or more and 170 ppm or less, more preferably 20 ppm or more and 150 ppm or less, from the viewpoint of suppressing short circuits due to film breakage at high temperatures.

Here, the crystallinity of polypropylene when it reaches 169°C can be obtained by differential scanning calorimetry (DSC), which will be described later. Crystallinity can be brought within a given range, for example, by adding polypropylene of a given molecular weight and melting point. In this polyolefin microporous membrane, the crystallinity of the polypropylene when it reaches 169° C. should be adjusted in consideration of the compatibility with other polyolefins to be mixed, and the molecular weight and melting point of the other polyolefins.

In order to satisfy the puncture strength, shutdown temperature, and crystallinity of polypropylene when reaching 169°C, the polyolefin microporous membrane can also be a multilayer microporous membrane consisting of multiple layers.

The polyolefin constituting the polyolefin microporous membrane of the present invention has a molecular weight in the range of 5.0×10 ⁴ to 1.0×10 ⁵ and 3.0 in the GPC chart from the viewpoint of facilitating control of strength and shutdown characteristics. It is preferable that each have a peak in the range of ×10 ⁵ to 7.0×10 ⁵ .

The polyolefin microporous membrane of the present invention preferably contains polyethylene having a weight average molecular weight of 4.0×10 ⁵ or more and 1.0×10 ⁶ or less. More preferably, the weight average molecular weight of polyethylene is 4.0×10 ⁵ or more and 1.0×10 ⁶ or less. By using the polyolefin resin composition within the above range for the polyolefin constituting the polyolefin microporous membrane, the strength is maintained even when formed into a thin film, and the membrane is easily melted, resulting in excellent shutdown characteristics. The weight-average molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by the GPC method.

The polyolefin microporous membrane of the present invention contains polyethylene and isotactic polypropylene, and the concentration of isotactic polypropylene with respect to the total mass of polyethylene and isotactic polypropylene is 3.5% by mass or more and 10.0% by mass or less. is preferred. More preferably, it is 4.0% by mass or more and 6.0% by mass or less. When the lower limit of the polypropylene concentration is within the above preferred range, the polypropylene component remains even after the polyolefin melts, has a sufficient network, and can maintain heat resistance. When the upper limit of the polypropylene concentration is within the above preferred range, a decrease in the puncture strength of the entire film is suppressed, holes are less likely to open, and short circuits can be further suppressed. The polypropylene concentration in the polyolefin microporous film can be determined by infrared spectrometry (IR measurement), which will be described later. The polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane can be controlled by the polypropylene concentration contained in the polyolefin resin raw material forming the polyolefin microporous membrane. The concentration of polypropylene contained with respect to the total weight of polyethylene and isotactic polypropylene in the polyolefin resin raw material is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 2.0% by mass or more6. It is 0% by mass or less, more preferably 3.0% by mass or more and 5.5% by mass or less. Although the lower limit of the porosity of the polyolefin microporous membrane of the present invention is not particularly limited, it is, for example, 20% or more, more preferably 30% or more. Although the lower limit of the porosity is not particularly limited, it is, for example, 70% or less, preferably 60% or less. When the porosity is within the above range, it is possible to increase the retention amount of the electrolytic solution and ensure high ion permeability. Further, when the porosity is within the above range, rate characteristics are improved. Moreover, from the viewpoint of further enhancing ion permeability and rate characteristics, the porosity is preferably 20% or more. The porosity can be set within the above range by adjusting the compounding ratio of the constituent components of the polyolefin, the draw ratio, the heat setting conditions, and the like in the manufacturing process.

The heat shrinkage rate in the machine direction of the polyolefin microporous membrane of the present invention is, for example, 10% or less, preferably 9% or less, and more preferably 8% or less. The heat shrinkage rate in the width direction of the polyolefin microporous membrane at 120° C. for 1 hour is, for example, 10% or less, preferably 9% or less, and more preferably 7% or less. Although the lower limit of the heat shrinkage in the machine direction and the lower limit of the heat shrinkage in the width direction are not particularly limited, they are preferably -2.0% or more. When the upper limits of the heat shrinkage rate in the machine direction and the heat shrinkage rate in the width direction are within the above ranges, the risk of deformation inside the battery and short circuits at the ends can be reduced, and battery safety is improved. . The heat shrinkage of the polyolefin microporous membrane can be controlled within the above range by adjusting the compounding ratio of the constituent components of the polyolefin, the draw ratio, the heat setting conditions, and the like in the manufacturing process.

(Method for producing polyolefin microporous membrane)
The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or a multilayer microporous membrane comprising a plurality of layers. The layer structure is preferably two or more layers, more preferably three layers, and it is particularly preferred that A layer and B layer having different resin compositions are A layer/B layer/A layer or B layer/A layer/B layer. preferable. The polyolefin resin composition A and the polyolefin resin composition B constituting the A layer and the B layer are described below.

(1) Polyolefin resin composition A
The polyolefin resin composition A may contain polyethylene a1 and polyethylene a2.

(Polyethylene a1)
Polyethylene a1 is polyethylene having a weight average molecular weight (Mw) of 7.0×10 ⁵ or more. Polyethylene a1 may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene, but it is preferable to use an ethylene homopolymer. Preferred α-olefin copolymers other than ethylene are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene. The content of α-olefins other than ethylene is preferably 5 mol % or less based on 100 mol % of the α-olefin copolymer. From the viewpoint of uniformity of the pore structure of the polyolefin microporous membrane, it is preferably an ethylene homopolymer.

Polyethylene a1 preferably has a weight-average molecular weight (Mw) of 7.0×10 ⁵ or more and less than 2.0×10 ⁶ from the viewpoint of facilitating control of the strength, stretchability, and melting of the microporous membrane. 0×10 ⁶ or more and 1.8×10 ⁶ or less is more preferable. The melting point of polyethylene a1 is preferably 134° C. or higher and 137° C. or lower, more preferably 134° C. or higher and 136° C. or lower. The content of polyethylene a1 is preferably 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more based on 100% by mass of the polyolefin resin composition A. The upper limit is 95% by mass.

(Polyethylene a2)
Polyethylene a2 has a weight average molecular weight (Mw) of 5.0 × 10 ⁴ or more and less than 7.0 × 10 ⁵ and 3.0 × 10 ⁵ or less from the viewpoint of facilitating control of melting of the microporous membrane. is preferred, and 2.0×10 ⁵ or less is more preferred. Polyethylene a2 is preferably a low melting point component, preferably has a melting point of 130° C. or more and less than 134° C., more preferably 130° C. or more and 133° C. or less, and preferably 130° C. or more and 132° C. or less. More preferred. Polyethylene a2 is preferably at least one selected from the group consisting of high density polyethylene, medium density polyethylene, branched low density polyethylene and linear low density polyethylene, and a small amount of other α-olefin copolymer other than ethylene. It may be a copolymer containing. Preferred α-olefin copolymers other than ethylene are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene. The content of α-olefins other than ethylene is preferably 10 mol % or less based on 100 mol % of the α-olefin copolymer. The content of polyethylene a2 is preferably 50% by mass or less, more preferably 40% by mass or less, and still more preferably 30% by mass or more, relative to 100% by mass of the polyolefin resin composition A.

(2) Polyolefin resin composition B
The polyolefin resin composition B may contain polyethylene b1 and polypropylene.

(Polyethylene b1)
Polyethylene b1 can be the same as polyethylene a1 in the above item. However, the same meaning means polyethylene having the same range of molecular weight and melting point as polyethylene a1.

(polypropylene)
The type of polypropylene is not particularly limited as long as it satisfies the following molecular weights and melting points. The weight average molecular weight (Mw) of polypropylene is preferably 1×10 ⁶ or more, more preferably 1.2×10 ⁶ or more, more preferably 1.2×10 ⁶ from the viewpoint of phase separation and shape retention of the microporous membrane at high temperatures. ˜4×10 ⁶ is even more preferred. The melting point of polypropylene is preferably 155 to 175°C, more preferably 160 to 170°C.

Polypropylene may be a propylene homopolymer, a copolymer of propylene and other α-olefins and/or diolefins (propylene copolymer), or a mixture of two or more selected from these, but propylene is more preferably used alone. Either a random copolymer or a block copolymer can be used as the propylene copolymer. As the α-olefin in the propylene copolymer, α-olefins having 8 or less carbon atoms are preferred. Examples of α-olefins having 8 or less carbon atoms include ethylene, butene-1, pentene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene and combinations thereof. As the diolefin in the propylene copolymer, a diolefin having 4 to 14 carbon atoms is preferable. Examples of diolefins having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene and 1,9-decadiene. The contents of other α-olefins and diolefins in the propylene copolymer are preferably adjusted so that the polypropylene has the above preferred melting point range. The polypropylene content is preferably 10% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 20% by mass or less, relative to 100% by mass of the polyolefin resin composition B.

The above polyolefin resin compositions A and B may contain resin components other than polyethylene a1, a2, b1 and polypropylene, if necessary. As other resin components, for example, a resin that further imparts heat resistance can be contained. In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retardants are added to the extent that they do not impair the effects of the present invention. may be included.

When the layer structure of the polyolefin microporous membrane is A layer/B layer, the thickness ratio of A layer/B layer is preferably 5/95 to 90/10, more preferably 30/70 to 80/20, It is more preferably 35/65 to 75/25. As a result, even a thin film can have high heat resistance while maintaining puncture strength.

In the present embodiment, a microporous membrane may be formed by laminating a porous layer on at least one side of the polyolefin microporous membrane. Although the porous layer is not particularly limited, for example, a porous layer made of resin may be laminated. The resin used here is not particularly limited, and known resins can be used, including acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, and polyimide resins. The porous layer may further contain inorganic particles, and the inorganic particles are not particularly limited, and known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, and the like. is mentioned.

(3) Method for producing a microporous polyolefin membrane The method for producing a microporous polyolefin membrane of the present invention includes the following steps. Details of each step will be described.
(a) Preparation of layer A and layer B solutions (b) Formation of gel sheet (c) First stretching (d) Plasticizer removal and drying (e) Second stretching (f) Heat treatment.

(a) Preparation of solutions for layers A and B Layers A and B are composed of the aforementioned polyolefin resin composition A and polyolefin resin composition B, respectively. A plasticizer is added to the polyolefin resin composition in a twin-screw extruder and melt-kneaded to prepare solutions for the A layer and the B layer, respectively. The polyolefin resin composition preferably contains 10% by mass or more and 30% by mass or less with respect to the entire resin solution. By adjusting the concentration of the polyolefin resin composition within the above range, melt fracture and neck-in at the die exit can be prevented when the polyolefin solution is extruded, and the moldability and appearance of the extrudate are improved.

The solutions of the A layer and the B layer are each fed from an extruder to a single die, where both solutions are extruded in the form of a layered sheet to obtain an extrudate. The extrusion method may be either a flat die method or an inflation method. In either method, the solutions are supplied to separate manifolds and layered at the lip inlet of the multi-layer die (multi-manifold method), or the solutions are pre-layered and fed to the die (block method). can be used. A common method can be applied to the multiple manifold method and the block method. The gap of the multilayer flat die can be set to 0.1 mm or more and 5 mm or less. The extrusion temperature is preferably 140° C. or higher and 250° C. or lower, and the extrusion speed is preferably 0.2 to 15 m/min. The film thickness ratio of the layers can be adjusted by adjusting the extrusion rate of the solution for each layer.

The thickness of each sheet is preferably 5/95 to 90/10, more preferably 30/70 to 80, where the sheet thickness of the solution forming layer A/the sheet thickness of the solution forming layer B is 5/95 to 90/10. /20, more preferably 35/65 to 75/25. A polyolefin microporous membrane having an excellent balance between strength and melting can be obtained while the polypropylene network is maintained within the preferred ranges of the polyolefin composition, the thickness ratio of each sheet, and the stretching conditions described later. In the polyolefin microporous film, when the polypropylene concentration of the polyolefin resin composition B is 10% by mass or more and 30% by mass or less, from the viewpoint of maintaining the polypropylene network, it is 0/100 (single layer structure of B layer). , for example, it is preferable to dispose the polyolefin resin composition B intensively in multiple layers, such as 50/50. As a result, even a thin film can have higher heat resistance while maintaining puncture strength.

(b) Formation of gel-like sheet A gel-like sheet is formed by cooling the obtained extrudate. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but cooling by contacting with a roll cooled with a cooling medium is preferable. Cooling is preferably carried out at a rate of 50° C./min or more until at least the gelation temperature. Cooling is preferably performed to 25° C. or lower. When the cooling rate is within the above range, the crystallinity is kept within a suitable range, and a gel-like sheet suitable for stretching is obtained.

(c) First stretching Next, the gel-like sheet is stretched. After preheating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial stretching or biaxial stretching. The draw ratio (area draw ratio) is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The draw ratios in the longitudinal direction (hereinafter sometimes referred to as MD) and the width direction (hereinafter sometimes referred to as TD) may be the same or different, and the stretch ratio in both MD and TD should be 3 times or more. preferable.

The lower limit of the first stretching temperature is preferably 100°C or higher and 130°C or lower, more preferably 110°C or higher and 120°C or lower. By using the above-described polyolefin compositions A and B and the layer structure, and by setting the stretching temperature within the preferred range described above, film breakage due to stretching of the low-melting-point polyolefin resin is suppressed, and high magnification stretching is possible. As a result, the puncture strength of the polyolefin microporous membrane is improved, and an increase in the shutdown temperature is easily suppressed. In addition, phase separation of polyethylene and polypropylene is suppressed at high temperatures, and shape retention is improved. Therefore, when used as a battery separator, even a thin film is excellent in mechanical strength and battery safety.

(d) Removal of plasticizer Next, the plasticizer contained in the gel-like sheet is removed and dried using a washing solvent. Since the washing solvent and the method for removing the plasticizer using the washing solvent are known, the explanation thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used. After removing the plasticizer, it is dried by a heat drying method or an air drying method. Any method capable of removing the wash solvent may be used, including conventional methods such as heat drying, air drying (moving air), and the like.

(e) Second Stretching A microporous polyolefin membrane can be obtained by stretching the dried sheet in at least one direction after preheating (dry stretching). The second stretching can be performed by a tenter method or the like while heating. The final draw ratio of the second drawing is preferably 1.1 times or more, more preferably 1.4 times or more. By setting the final draw ratio within the above range, the puncture strength can be easily controlled within the desired range. However, when the film is stretched at a high magnification, the shutdown temperature and heat shrinkage increase, so the stretching is preferably 9 times or less. When the polyolefin compositions A and B described above and the layer structure are used, film breakage due to stretching of the polyolefin resin having a low melting point is suppressed, and control is facilitated.

(f) Heat treatment After the second stretching, heat treatment is performed while the width is fixed while being held by clips. The heat treatment is preferably performed at 115.0° C. or higher and 135.0° C. or lower. By setting the heat treatment temperature within the above range, the thermal shrinkage of the polyolefin microporous membrane can be suppressed. A thermal relaxation treatment may be performed during the heat treatment. When thermal relaxation treatment is performed, the relaxation rate can be 5% or more and 30% or less, with the immediately preceding length being 100%. By setting the relaxation rate within the above range, it is possible to reduce the thermal shrinkage rate and suppress fluttering in the step of manufacturing the microporous membrane after the relaxation.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[Measuring method]
(1) Film thickness The thickness of the polyolefin microporous film at five points in the range of 95 mm × 95 mm, upper left, upper right, center, lower left, and lower right, is measured by a contact thickness meter (Lightmatic manufactured by Mitutoyo Co., Ltd., contact pressure 0.01 N). , using a 10.5 mmφ probe), and the average value was taken as the film thickness (μm). If the sample size cannot be 95 mm×95 mm, the sample may be cut out in an arbitrary size and measured at the upper left, upper right, center, lower left, and lower right points.

(2) Metsuke A polyolefin microporous membrane cut into 5 cm squares was prepared, the mass was measured with a precision balance (5 significant digits (0.0000 g)), and the weight was calculated by dividing the mass by 25 cm ² . If the sample size cannot be 5 cm×5 cm, the sample may be cut into an arbitrary size and the measured mass divided by the area.

( ³ ) Porosity A polyolefin microporous membrane is cut into a size of 95 mm × 95 mm, its volume (cm ³ ) and mass (g) are obtained, and the porosity ( %) was calculated by the following formula.
Formula: porosity = ((volume - mass / membrane density) / volume) x 100
Here, the film density was set to 0.99 g/cm ³ . The film thickness measured in (1) above was used to calculate the volume.

(4) Air resistance For the polyolefin microporous membrane, air resistance (sec / 100 cm ³ ) was measured.

(5) Puncture strength Using a needle with a diameter of 1 mm (the tip is 0.5 mmR), the maximum load value S (N) when piercing a polyolefin microporous membrane with a film thickness T (μm) at a speed of 2 mm / sec was measured. . The converted puncture strength with a film thickness of 5 μm was calculated by the following formula.
Formula: Converted puncture strength = S (N) x 5 (μm)/T (μm).

(6) Shutdown temperature (also called SD temperature)
A polyolefin microporous membrane punched into a circle with a diameter of 45 mm was exposed to an atmosphere of 20° C., and the air resistance was measured while the temperature was raised at a rate of 5° C./min. The temperature when reaching ³ was defined as the shutdown temperature and the average value of two measurements was used. The air resistance was measured using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P8117:2009.

(7) Meltdown temperature (also called MD temperature)
A polyolefin microporous membrane punched into a circle with a diameter of 45 mm was exposed to an atmosphere of 20 ° C., and the air resistance was measured while increasing the temperature at a rate of 5 ° C./min. After reaching 100 cm ³ , the temperature was continued to rise, and the temperature at which the air resistance was less than 100,000 seconds/100 cm ³ was defined as the meltdown temperature. The air resistance was measured using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P8117:2009.

(8) Thermal shrinkage The polyolefin microporous membrane was cut into a size of 95 mm × 95 mm, and the length (mm) of the test piece before shrinkage at room temperature (25 ° C.) was measured in both the machine direction and the width direction. After exposing the test piece of the porous membrane to a temperature of 105° C. for 8 hours without applying a load, the test piece was returned to room temperature and the length (mm) after shrinkage in the machine direction and the width direction was measured. Then, the heat shrinkage rate (%) in the machine direction and width direction was obtained using the following equations.
Formula: MD heat shrinkage (%) = (1 - length after shrinkage in machine direction / length before shrinkage in machine direction) x 100
TD thermal shrinkage rate (%) = (1-length after shrinkage in width direction/length before shrinkage in width direction) x 100
(9) Weight Average Molecular Weight The weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the polyolefin resin and polyolefin microporous membrane were obtained by gel permeation chromatography (GPC) using the following measurement conditions.
Measurement conditions Measurement device: Agilent high temperature GPC device PL-GPC220
・ Column: Agilent PL1110-6200 (20 μm MIXED-A) × 2 ・ Column temperature: 160 ° C.
- Solvent (mobile phase): 1,2,4-trichlorobenzene - Solvent flow rate: 1.0 mL / min - Sample concentration: 0.1 mass% (dissolution conditions: 160 ° C. / 3.5 H)
・Injection volume: 500 μL
・ Detector: Agilent differential refractive index detector (RI detector)
• Viscometer: Viscosity detector manufactured by Agilent • Calibration curve: Created by a universal calibration curve method using a monodisperse polystyrene standard sample.
Also, the peak positions on the low molecular weight side and on the high molecular weight side were estimated as follows.
• Low-molecular-weight peak position: the peak position of the Gaussian function on the low-molecular-weight side when the molecular weight distribution is fitted with two Gaussian functions.
-High molecular weight side peak position: molecular weight at the maximum value of the molecular weight distribution.

(10) Melting point and melting peak The melting point of the polyolefin resin and the melting peak of the polyolefin microporous membrane were determined by a scanning differential calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER). The polyolefin resin and the polyolefin microporous membrane were each placed in a sample holder, heated from 30° C. to 230° C. to melt completely, held at 230° C. for 3 minutes, and heated at a rate of 10° C./min for 30 minutes. The temperature was lowered to °C. Using this as the first temperature rise, the same measurement was repeated, and the melting point (Tm) of the polyolefin resin and the heat of fusion of the polyolefin microporous membrane were determined from the endothermic peak at the second temperature rise. A straight line connecting 30° C. and 230° C. was used as a baseline for calculating the heat of fusion. For the polyolefin resin, the peak at the heat of fusion of 70 J/g or more was regarded as the endothermic peak, and for the polyolefin microporous membrane, the peak at the heat of fusion of 0.1 J/g or more was regarded as the endothermic peak.

(11) Layer ratio The layer ratio of the polyolefin microporous membrane was observed using a transmission electron microscope (TEM) under the following measurement conditions.
Measurement conditions and sample preparation: A polyolefin microporous membrane is dyed with ruthenium tetroxide and cross-sectioned with an ultramicrotome.
・Measuring device: transmission electron microscope (JEOL JEM1400Plus type)
・Observation conditions: acceleration voltage of 100 kV
Observation direction: TD/ND.

(12) Crystallinity of polypropylene at 169° C. The crystallinity of polypropylene in the polyolefin microporous membrane at 169° C. was obtained by the following measurement.
Measurement conditions Measurement device: Scanning differential calorimeter (PARKING ELMER PYRIS DIAMOND DSC)
・Sample mass: 6 mg
Atmospheric gas: Nitrogen Starting temperature: 30°C
・Temperature increase rate: 5°C/min ・Achievement temperature: 169°C
・Holding time at reaching temperature: 5 minutes ・Temperature decrease rate: 30°C/minute ・End temperature: 30°C.

In the temperature-lowering process of the above measurement, an exothermic peak due to crystallization of polypropylene is detected in a temperature range of 120° C. or higher when the ordered structure remains. In the present invention, from the exothermic peak detected when the temperature was raised to 169° C. and then cooled, the crystallinity χ for all resins of polypropylene was obtained by the following formula.
Formula: χ = ΔH _PP /ΔH _PP ^f × (isotactic polypropylene concentration)
Here, _ΔHPP represents the enthalpy of crystallization (J/g) in the process of cooling the polypropylene structure. _ΔHPP is the value obtained by dividing the area surrounded by the DSC curve and the line connecting the start temperature and end temperature of the exothermic peak due to crystallization of polypropylene on the DSC curve in the temperature-lowering process by the mass of the measurement sample. Also, the isotactic polypropylene concentration is a value obtained from IR measurement described later. For example, if the polypropylene is isotactic polypropylene, ΔH _PP ^f represents the complete melting enthalpy (J/g) of polypropylene. ΔH _PP ^f was calculated using 170 J/g.

(13) Measurement of polypropylene concentration for the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane The isotactic polypropylene concentration for the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane is measured by IR. was obtained from the peak intensity ratio of 1462 cm ^-1 derived from polyethylene and 1376 cm ^-1 derived from isotactic polypropylene. Measurement conditions are as follows.
・Measurement device: FT-IR device (FT/IR-6600 manufactured by JASCO Corporation)
・Measurement temperature: 25°C
・Aperture: X=300 μm, Y=300 μm
・Number of integration times: 16 times ・Resolution: 4 cm ⁻¹
Formula: isotactic polypropylene concentration (%)=(1462 cm ⁻¹ peak height×conversion factor 1)/(1376 cm ⁻¹ peak height×conversion factor 2)×100. Here, the conversion factor 1 is 20 and the conversion factor 2 is 10.

(14) Hot-box characteristics Battery safety was evaluated by the following hot-box characteristics.

Battery Production In a lithium-ion secondary battery, a positive electrode and a negative electrode are laminated with a separator interposed therebetween, and the separator contains an electrolytic solution (electrolyte). Lithium cobalt composite oxide LiCoO ₂ was used as the positive electrode active material, graphite was used as the negative electrode active material, and 1 mol/L LiPF ₆ prepared in a mixed solvent of DC/dimethyl carbonate (DMC) was used as the electrolyte. A battery is assembled by laminating a positive electrode, a separator made of a microporous film, and a negative electrode, then preparing a wound electrode body by a conventional method, inserting it into a battery can, impregnating it with an electrolytic solution, and then a positive electrode terminal equipped with a safety valve. The battery lid, which also serves as a battery, was crimped through a gasket.

Hot box test The assembled battery was charged at a current value of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V, and then discharged at a current of 0.2C to a final voltage of 3.0V. rice field. Next, after constant current charging to 4.2 V at a current value of 0.2 C, constant voltage charging to 4.2 V was performed as pretreatment. The pretreated battery was placed in an oven, heated from room temperature at a rate of 5° C./min, and left at 150° C. for 30 minutes. A charge voltage drop of 50% or more within 15 minutes after reaching 150° C. was rated unacceptable, a charge voltage drop of 20 to 50% was rated acceptable, and a charge voltage drop of 20% or less was rated excellent.

(15) Self-discharge characteristics Self-discharge characteristics (K value) were evaluated by the following method. After constant current charging to a battery voltage of 3.85 V at a current value of the test secondary battery 0.5 C assembled in the following (Evaluation battery production method), until the battery voltage reaches 0.05 C at 3.85 V Constant voltage charging was performed. The open-circuit voltage was measured after the battery was left for 24 hours, and this value was defined as V1. With respect to this battery, the open circuit voltage was measured after being left for another 24 hours, that is, after being left for a total of 48 hours after charging, and this value was defined as V2. The K value was calculated from the obtained V1 and V2 values by the following formula.
Formula: K value = (V1-V2)/24.

[Example 1]
(1) Preparation of layer A solution 90% by mass of polyethylene having a weight average molecular weight of 1.5×10 ⁶ and a melting point of 136.0° C. and 10 mass of polyethylene having a weight average molecular weight of 1.0×10 ⁵ and a melting point of 132.0° C. % was melt-kneaded with liquid paraffin with a twin-screw extruder so that the resin concentration was 17% by mass, and a polyolefin solution A was prepared.

(2) Preparation of Layer B Solution 70% by mass of polyethylene having a weight average molecular weight of 1.5×10 ⁶ and a melting point of 136.0° C. and 30% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 20% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Molding of gel-like sheet Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of A layer solution/B layer solution/A layer solution is 25/50. /25. The extruded compact was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet.

(4) First Stretching, Removal of Film-Forming Solvent, and Drying The gel-like sheet was simultaneously stretched 5 times in both MD and TD at 110.0° C. using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried to obtain a dried microporous membrane.

(5) Second stretching and heat treatment After that, after preheating at 128.0 ° C. and stretching 1.6 times in TD with a tenter stretching machine, TD was relaxed by 15.0% and held in a tenter. It was heat-set at 128.0° C. while heating to obtain a polyolefin microporous membrane. Table 1 shows the characteristics of the obtained polyolefin microporous membrane.

[Example 2]
The polyolefin solutions A and B were drawn in the same manner as in Example 1 except that the polyolefin solutions A and B were extruded so that the layer thickness ratio of B layer solution/A layer solution/B layer solution was 25/50/25. A membrane was obtained.

[Example 3]
(1) Preparation of layer A solution 70% by mass of polyethylene having a weight average molecular weight of 1.5×10 ⁶ and a melting point of 135.0° C. and 30 mass of polyethylene having a weight average molecular weight of 1.0×10 ⁵ and a melting point of 132.0° C. % was melt-kneaded with liquid paraffin using a twin-screw extruder to give a resin concentration of 20% by mass, to prepare a polyolefin solution A.

(2) Preparation of Layer B Solution 85% by mass of polyethylene having a weight average molecular weight of 1.5×10 ⁶ and a melting point of 135.0° C. and 15% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 20% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Gel-like sheet molding Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of A layer solution/B layer solution/A layer solution is 20/60. /20 was extruded. The extruded compact was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet.

(4) First stretching, removal of film-forming solvent, drying The gel sheet was stretched, liquid paraffin was removed, and dried in the same manner as in Example 1, except that the stretching temperature was 112.5 ° C., and after drying, the A microporous membrane was obtained.

(5) Second stretching, heat treatment After preheating at 127.0°C and stretching 1.5 times in TD with a tenter stretching machine, 15.0% relaxation is applied to TD and 127 while holding in the tenter. It was heat-set at 0°C to obtain a polyolefin microporous membrane.

[Example 4]
Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T die, and extruded so that the layer thickness ratio of B layer solution/A layer solution/B layer solution is 30/40/30, A polyolefin microporous membrane was obtained in the same manner as in Example 3, except that the first stretching temperature was 113.5°C, and the second stretching temperature and heat setting temperature were 126.0°C.

[Example 5]
A polyolefin microporous membrane was produced in the same manner as in Example 4 except that the first stretching temperature was 114.5°C, the second stretching temperature and heat setting temperature were 126.0°C, and the relaxation rate was 10.0%. Obtained.

[Example 6]
A polyolefin microporous membrane was obtained in the same manner as in Example 5, except that the first stretching temperature was set to 115.0°C.

[Example 7]
The layer B solution/layer A solution/layer B solution was extruded so that the layer thickness ratio was 25/50/25, and the first stretching temperature was 115.5° C. and the relaxation rate was 15.0%. A polyolefin microporous membrane was obtained in the same manner as in Example 5 except for the above.

[Example 8]
(1) Preparation of layer A solution 85% by mass of polyethylene having a weight average molecular weight of 7.5×10 ⁵ and a melting point of 136.0° C. and 15 mass of polyethylene having a weight average molecular weight of 1.0×10 ⁵ and a melting point of 132.0° C. % was melt-kneaded with liquid paraffin with a twin-screw extruder so that the resin concentration was 25% by mass, to prepare a polyolefin solution A.

(2) Preparation of Layer B Solution 90% by mass of polyethylene having a weight average molecular weight of 7.5×10 ⁵ and a melting point of 136.0° C. and 10% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 15% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of B layer solution/A layer solution/B layer solution was 15/70. /15 was extruded. The extruded compact was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet.

(4) First stretching, removal of film-forming solvent, drying The gel-like sheet was stretched, liquid paraffin was removed, and dried in the same manner as in Example 1, except that the stretching temperature was set to 112.0 ° C., and after drying, the A microporous membrane was obtained.

(5) Second stretching and heat treatment After preheating at 130.0°C and stretching to 1.8 times in TD with a tenter stretching machine, TD was relaxed by 15.0% and held in a tenter to 130 degrees. It was heat-set at 0°C to obtain a polyolefin microporous membrane.

[Example 9]
The proportions of polyethylene and polypropylene in the polyolefin solution B are set to 90% and 10%, respectively, and the layer A solution/layer B solution/layer A solution is extruded so that the layer thickness ratio is 30/40/30, and the second A microporous polyolefin membrane was obtained in the same manner as in Example 5, except that the draw ratio was 1.8 and the relaxation rate was 15.0.

[Example 10]
A polyolefin microporous membrane was obtained in the same manner as in Example 8 except that the first stretching temperature was 113.0°C and the second stretching temperature was 125.0°C.

[Example 11]
A layer solution/B layer solution/A layer solution was extruded so that the layer thickness ratio was 25/50/25. A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the content was 0%.

[Example 12]
The layer A solution/layer B solution/layer A solution was extruded to a layer thickness ratio of 35/30/35, the first stretching temperature was 112.0°C, and the second stretching relaxation rate was 10.0°C. A polyolefin microporous membrane was obtained in the same manner as in Example 8, except that the content was 0%.

[Comparative Example 1]
(1) Preparation of Layer A Solution 40% by mass of polyethylene having a weight average molecular weight of 2.0×10 ⁶ and a melting point of 133.0° C. and 60% by mass of polyethylene having a weight average molecular weight of 3.0×10 ⁵ and a melting point of 136.0° C. was melt-kneaded with liquid paraffin with a twin-screw extruder so that the resin concentration was 25% by mass, to prepare a polyolefin solution A.

(2) Preparation of Layer B Solution 80% by mass of polyethylene having a weight average molecular weight of 3.0×10 ⁵ and a melting point of 135.0° C. and 20% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 25% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Gel-like sheet molding Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of B layer solution/A layer solution/B layer solution is 10/80. /10. The extruded compact was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet. At this time, the ejection was adjusted so that the thickness after stretching was about 4.0 μm by stretching, which will be described later.

(4) First Stretching, Removal of Film-Forming Solvent, and Drying The gel-like sheet was simultaneously stretched 5 times in both MD and TD at 115.0° C. using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried to obtain a dried microporous membrane.

(5) Second stretching and heat treatment After that, after preheating at 125.5 ° C. and stretching 1.5 times in TD with a tenter stretching machine, TD was relaxed by 15.0% and held in a tenter. It was heat-set at 125.5° C. while heating to obtain a polyolefin microporous membrane.

[Comparative Example 2]
(1) Preparation of layer A solution 40% by mass of polyethylene having a weight average molecular weight of 1.6 × 10 ⁶ and a melting point of 134.0°C and 60% by weight of polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ and a melting point of 135.0°C % was melt-kneaded with liquid paraffin using a twin-screw extruder to give a resin concentration of 28.5% by mass, to prepare a polyolefin solution A.

(2) Preparation of Layer B Solution 70% by mass of polyethylene having a weight average molecular weight of 3.0×10 ⁵ and a melting point of 135.0° C. and 30% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 25% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Molding of gel-like sheet Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of A layer solution/B layer solution/A layer solution is 40/20. /40 was extruded. The extruded extrudate was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet.

(5) Second stretching and heat treatment After that, after preheating at 128.0 ° C. and stretching 1.6 times in TD with a tenter stretching machine, TD was relaxed by 15.0% and held in a tenter. It was heat-set at 128.0° C. while heating to obtain a polyolefin microporous membrane.

[Comparative Example 3]
(1) Preparation of Layer A Solution 30% by mass of polyethylene having a weight average molecular weight of 2.0×10 ⁶ and a melting point of 133.0° C. and 70% by mass of polyethylene having a weight average molecular weight of 3.0×10 ⁵ and a melting point of 135.0° C. was melt-kneaded with liquid paraffin with a twin-screw extruder so that the resin concentration was 28.5% by mass, to prepare a polyolefin solution A.

(2) Preparation of Layer B Solution 50% by mass of polyethylene having a weight average molecular weight of 3.0×10 ⁵ and a melting point of 135.0° C. and 50% by mass of polypropylene having a weight average molecular weight of 2.0×10 ⁶ were combined to a resin concentration of 30% by mass. % by melt-kneading liquid paraffin with a twin-screw extruder to prepare a polyolefin solution B.

(3) Molding of gel-like sheet Polyolefin solutions A and B are supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of A layer solution/B layer solution/A layer solution is 35/30. /35 was extruded. The extruded compact was cooled while being taken up by a cooling roll controlled at 25° C. to form a gel-like sheet.

(4) First Stretching, Removal of Film-Forming Solvent, and Drying The gel-like sheet was simultaneously stretched 5 times in both MD and TD at 114.0° C. using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried to obtain a dried microporous membrane.

(5) Second stretching and heat treatment After that, after preheating at 125.0 ° C. and stretching 1.5 times in TD with a tenter stretching machine, TD was relaxed by 10.0% and held in a tenter. It was heat-set at 125° C. while heating to obtain a polyolefin microporous membrane.

[Comparative Example 4]
Except that a gel-like sheet was formed only with the polyolefin solution A of Comparative Example 1, the first stretching temperature was 114.0°C, the second stretching and heat setting temperature was 130.0°C, and the relaxation rate was 20%. A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1.

[Reference example 1]
A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the extrusion discharge was adjusted so that the polyolefin microporous membrane had a thickness of about 9.0 μm.

[result]
Table 2 shows the physical property measurement results of the obtained polyolefin microporous membrane. The polyolefin microporous membranes obtained in the examples are thinner than the comparative examples, but have a lower shutdown temperature, and have both puncture strength and insulation properties after melting. Excellent battery safety and self-discharge characteristics.

When the polyolefin microporous membrane of the present invention is used as a battery separator, it is possible to provide a polyolefin microporous membrane that is safe even when the battery is in a high temperature state even if it is a thin film.

Claims

The film thickness is 6 μm or less, the 5 μm equivalent puncture strength is 1.7 N or more, the shutdown temperature measured by the temperature-rising air permeability method is 80° C. or more and 138° C. or less, and the polypropylene crystals when reaching 169° C. A polyolefin microporous membrane having a degree of 3 ppm or more and 200 ppm or less.
The polyolefin microporous membrane according to claim 1, which is a multilayer microporous membrane consisting of a plurality of layers.
3. The composition according to claim 1 or 2, which has peaks in a molecular weight range of 5.0×10 4 to 1.0×10 5 and a molecular weight range of 3.0×10 5 to 7.0×10 5 on a GPC chart. Polyolefin microporous membrane.
4. The polyolefin microporous membrane according to any one of claims 1 to 3, comprising polyethylene having a weight average molecular weight of 4.0×10 5 or more and 1.0×10 6 or less.
Any one of claims 1 to 4, comprising polyethylene and isotactic polypropylene, and having an isotactic polypropylene concentration of 3.5% by mass or more and 10.0% by mass or less with respect to the total mass of polyethylene and isotactic polypropylene. The polyolefin microporous membrane described.
The polyolefin microporous membrane according to any one of claims 1 to 5, which has a porous layer on at least one side.
A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 6.