Polyolefin microporous membrane

Abstract

To provide a polyolefin microporous membrane exhibiting excellent output performance, high strength, and a low shrinkage ratio.SOLUTION: A polyolefin microporous membrane is provided in which, for the average pore area S (nm2) of openings on one surface and on the other surface of the microporous membrane, with Sa denoting the smaller average pore area and Sb denoting the larger average pore area, the relationships 400≤Sa≤2000, 400≤Sb≤2000, and 0.9≤Sa / Sb≤1.0 are satisfied.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polyolefin microporous membrane.

Background Art

[0002] Polyolefin microporous membranes are used as filters, separators for fuel cells, separators for capacitors, etc. In particular, they are suitably used as separators for non-aqueous electrolyte secondary batteries such as lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, etc. The reason is that polyolefin microporous membranes have excellent mechanical strength, shutdown characteristics, and ion permeation performance.

[0003] In recent years, since lithium-ion batteries are used in in-vehicle applications, it is necessary to shorten the charging time and improve the acceleration performance. As requirements for batteries, rapid charging (high-current charging) and increased power consumption (high-current discharging) are demanded. Along with this, the requirements for separators for improving output characteristics have become even higher. In addition, as the cruising range of automobiles increases, the capacity of the battery is increasing, and further thinning of the separator is required. However, when the separator is thinned, the strength decreases, so short circuits due to electrodes or foreign matters (decrease in foreign matter resistance) and membrane breakage (decrease in impact resistance) are likely to occur when the battery is impacted, and the performance maintenance characteristics of the battery deteriorate. In addition, charging and discharging at high output generate a large amount of heat in the battery, and short circuits (shorts) of the battery due to shrinkage of the separator are likely to occur. Therefore, coexistence of even higher strength, lower shrinkage rate, and higher output characteristics than before is required. As a method for improving the output characteristics, Patent Document 1 discloses a method of using a blend of polyethylene (PE) and polypropylene (PP) as the resin constituting the polyolefin microporous membrane to increase the porosity, control the number of pores, the average pore diameter, and the tortuosity, and improve the ion conductivity. Patent Document 2 discloses a method of improving the wettability with respect to an electrolytic solution and the retention of the electrolytic solution in a membrane by controlling the pore structure on the surface and inside of a polyolefin microporous membrane, thereby improving battery characteristics such as cycle characteristics. Patent Document 3 discloses a method of improving output characteristics by appropriately controlling the ratio of small-aperture portions to large-aperture portions on the surface of a polyolefin microporous membrane. Patent Document 4 discloses a method of reducing the difference in the opening state on the surface of a polypropylene microporous membrane by controlling the crystallization temperature of the polypropylene microporous membrane, thereby improving the resistance as a separator and the output characteristics.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, for polyolefin microporous membranes as described in Patent Documents 1 and 2, in order to improve the output characteristics, it is necessary to control the pore structure such as increasing the porosity, increasing the number of pores, and increasing the through-holes. Therefore, sufficient strength cannot be achieved, and the shrinkage rate cannot be reduced, which is insufficient for achieving both the output characteristics and the performance maintenance characteristics. For polyolefin microporous membranes as described in Patent Document 3, in order to improve the output characteristics, it is necessary to increase the draw ratio, increase the ratio of small-aperture portions in the pore size distribution on the surface, and decrease the ratio of large-aperture portions. Therefore, the shrinkage rate cannot be sufficiently reduced, which is insufficient for achieving both the output characteristics and the performance maintenance characteristics. As described in Patent Document 4, the polyolefin microporous membrane controls the surface opening diameter and the surface opening diameters of the front and back surfaces by controlling the crystallization temperature of polypropylene in order to improve the output characteristics. However, it is necessary to increase the surface opening diameter in order to reduce the internal resistance, and the compatibility between the performance maintenance characteristics such as short circuit due to dendrites and the output characteristics is insufficient. In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having excellent output characteristics and good performance maintenance characteristics (high strength, low shrinkage rate) at a lower stretching ratio than conventional ones by controlling the surface pore structure.

Means for Solving the Problems

[0006] The present inventors have found that by reducing the difference between the front and back of the pores on the surface of the polyolefin microporous membrane, a polyolefin microporous membrane excellent in output characteristics and strength can be obtained as compared with the prior art. Furthermore, the present inventors have found that the difference between the front and back of the pores on the surface is reduced by cooling the surface not in contact with the cooling roll with a solvent in the casting step of the polyolefin microporous membrane, and have completed the present invention.

[0007] In order to solve the above problems and to achieve the object, the present invention has the following configuration. (1) Regarding the average area S (nm 2 ) of the pores on one surface and the other surface of the microporous membrane, when the average areas of the pores are set as Sa and Sb from the smaller one, a polyolefin microporous membrane satisfying 400 ≦ Sa ≦ 2000, 400 ≦ Sb ≦ 2000, and 0.9 ≦ Sa / Sb ≦ 1.0. (2) Regarding the porosity X (%) of the pores on one surface and the other surface of the microporous membrane, when the porosities are set as Xa and Xb from the smaller one, the polyolefin microporous membrane according to (1), which satisfies 10 ≦ Xa ≦ 20, 10 ≦ Xb ≦ 20, and 0.9 ≦ Xa / Xb ≦ 1.0. (3) The number of pores N (pieces / μm on one surface and the other surface of the microporous membrane 2Regarding (1), when the number of apertures is Na and Nb in ascending order, the polyolefin microporous membrane according to (1) or (2) that satisfies 0.9 ≦ Na / Nb ≦ 1.0, 50 ≦ Na ≦ 200, and 50 ≦ Nb ≦ 200. (4) The weight average molecular weight of the polyolefin microporous membrane is 8.0×10 5 or more, and the polyolefin microporous membrane according to any one of (1) to (3). (5) In the raw material resin of the polyolefin microporous membrane, the content of polyolefin having a weight average molecular weight of 1.0×10 6 or less is less than 40% by mass, and the content of polyolefin having a weight average molecular weight of 1.5×10 6 or more is 30% by mass or more, and the polyolefin microporous membrane according to any one of (1) to (4).

Advantages of the Invention

[0008] According to the present invention, when used as a separator for a battery, it is possible to provide a polyolefin microporous membrane that imparts high output characteristics and high performance maintenance characteristics and contributes to miniaturization and high output of the battery.

Brief Description of the Drawings

[0009]

Figure 1

Modes for Carrying Out the Invention

[0010] Hereinafter, embodiments of the polyolefin microporous membrane according to the present invention will be described.

[0011] (Polyolefin Microporous Membrane) Regarding the polyolefin microporous membrane according to an embodiment of the present invention, for the average area S (nm 2 ) of the apertures on one surface and the other surface of the microporous membrane, when the average areas of the apertures are Sa and Sb in ascending order, the average areas Sa and Sb of the apertures are each 400 nm 2 or more and 2000 nm2 is as follows. When the average area of the apertures is 2000 nm 2 or less, it is possible to suppress short circuits due to dendrite growth, etc., obtain a good number of apertures, increase the ion permeation path, and obtain excellent battery characteristics. Therefore, the average areas Sa and Sb of the apertures are preferably 2000 nm 2 or less, more preferably 1900 nm 2 or less, and even more preferably 1800 nm 2 or less. On the other hand, when the average area of the apertures is 400 nm 2 or more, a good aperture ratio can be obtained, and high ion permeability can be obtained. Therefore, the average areas Sa and Sb of the apertures are preferably 400 nm 2 or more, more preferably 500 nm 2 or more, and even more preferably 600 nm 2 or more. The average area of the apertures can be specifically measured by the method described in the examples.

[0012] Regarding the average area S (nm 2 ) of the apertures on one surface and the other surface of the microporous membrane, when the average areas of the apertures are set as Sa and Sb in ascending order, the ratio of Sa to Sb, that is, the ratio of the average areas of the apertures on the front and back surfaces, is preferably 0.9 or more and 1.0 or less. When the ratio of the average areas of the apertures on the front and back surfaces is 0.9 or more and 1.0 or less, the pore structure becomes uniform in the thickness direction of the microporous membrane, so the tortuosity decreases, and a separator with low resistance can be obtained. Therefore, the ratio of the average areas of the apertures on the front and back surfaces is more preferably 0.93 or more and 1.0 or less, and even more preferably 0.95 or more and 1.0 or less.

[0013] Regarding the porosity X (%) of one surface and the other surface of the polyolefin microporous membrane according to an embodiment of the present invention, when the porosities are defined as Xa and Xb in ascending order, it is preferable that Xa is 10% or more and 20% or less, and Xb is 10% or more and 20% or less. When both porosities Xa and Xb are 10% or more, the balance of permeability, strength, and electrolyte content is improved, the non-uniformity of the battery reaction is eliminated, and as a result, the generation of dendrites is suppressed. Therefore, both porosities Xa and Xb are more preferably 11% or more, and even more preferably 12% or more. Although good output characteristics can be obtained by increasing the porosity, battery performance maintenance characteristics such as a decrease in puncture strength and an increase in shrinkage rate deteriorate. Therefore, both porosities Xa and Xb are preferably 20% or less, more preferably 19% or less, and even more preferably 18% or less.

[0014] Regarding the porosity X (%) of one surface and the other surface of the polyolefin microporous membrane according to an embodiment of the present invention, when the porosities are defined as Xa and Xb in ascending order, the ratio of Xa to Xb, that is, the ratio of the front and back porosities (Xa / Xb), is preferably 0.9 or more and 1.0 or less. The ratio of the front and back porosities is preferably 0.93 or more and 1.0 or less, and even more preferably 0.95 or more and 1.0 or less. When the ratio of the front and back porosities (Xa / Xb) is within the above range, the pore structure becomes uniform in the thickness direction of the microporous membrane, so the tortuosity decreases and a separator with low resistance can be obtained. The method for setting the ratio of the front and back porosities (Xa / Xb) within the above range is not particularly limited. In the present invention, the porosity specifically represents a value measured by the method described in the examples.

[0015] Regarding the number of pores N (pieces / μm 2 ) per unit area of one surface and the other surface of the polyolefin microporous membrane according to an embodiment of the present invention, when the number of pores is defined as Na and Nb in ascending order, Na is 50 pieces / μm 2 or more and 200 pieces / μm 2 or less, and Nb is 50 pieces / μm 2 or more and 200 pieces / μm 2 or less. It is preferable that both the number of pores Na and Nb are 50 pieces / μm2 When the above conditions are met, a good aperture ratio can be obtained, the ion transmission path increases, and excellent battery characteristics can be obtained. Therefore, the number of apertures Na and Nb is 60 apertures / μm 2 or more is more preferable, and 70 apertures / μm 2 or more is more preferable. Although good output characteristics can be obtained by increasing the number of apertures, battery performance maintenance characteristics such as a decrease in puncture strength and an increase in shrinkage rate deteriorate. Therefore, the number of apertures Na and Nb is 200 apertures / μm 2 or less is preferable, 190 apertures / μm 2 or less is more preferable, and 180 apertures / μm 2 or less is even more preferable.

[0016] Regarding the number of apertures N (apertures / μm 2 ) per unit area of one surface and the other surface of the microporous membrane, when the number of apertures is set as Na and Nb in ascending order, the ratio of Na to Nb, that is, the ratio of the number of apertures on the front and back surfaces, is preferably 0.9 or more and 1.0 or less. When the ratio of the number of apertures on the front and back surfaces is 0.9 or more and 1.0 or less, the pore structure becomes uniform in the thickness direction of the microporous membrane, so the tortuosity decreases and a separator with low resistance can be obtained. Therefore, the ratio of the number of apertures on the front and back surfaces is more preferably 0.93 or more and 1.0 or less, and even more preferably 0.95 or more and 1.0 or less. In the present invention, the number of apertures specifically represents the value measured by the method described in the examples.

[0017] The polyolefin microporous membrane according to an embodiment of the present invention preferably has a weight average molecular weight of 8.0×10 5 or more. By setting the weight average molecular weight to 8.0×10 5 or more, the entanglement density in the amorphous region increases, and stress is uniformly applied to the polyethylene resin layer in the stretching process, so a uniform structure with few large pores can be formed and good output characteristics can be obtained. Also, by having a weight average molecular weight of 8.0×10 5 or more, the number of tie molecules increases and high strength is obtained. Therefore, the ion resistance and strength, which are in a trade-off relationship, are improved, and it becomes possible to achieve both battery performance maintenance characteristics and output characteristics. The weight average molecular weight of the polyolefin microporous membrane is 9.0×105 The above is more preferable, and 1.0×10 6 or more and 2.0×10 6 or less is even more preferable.

[0018] The polyolefin microporous membrane according to the embodiment of the present invention preferably has a shrinkage rate in the MD direction and a shrinkage rate in the TD direction of 7.5% or less when held at 105 ° C for 8 hours. More preferably, it is less than 7.0%, and even more preferably, it is less than 6.5%.

[0019] The polyolefin microporous membrane according to the embodiment of the present invention preferably has a puncture strength in terms of basis weight of 45 mN / (g / m 2 ) or more. More preferably, it is 50 mN / (g / m 2 ) or more and 80 mN / (g / m 2 ) or less.

[0020] The polyolefin microporous membrane according to the embodiment of the present invention preferably contains a polyethylene-based resin as a main component. Here, the main component refers to the component having the largest content in terms of mass% among the components constituting the polyolefin microporous membrane. Further, the polyethylene-based resin component in the polyolefin microporous membrane is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 96% by mass or more, and particularly preferably 99% by mass or more. By setting the content of the polyethylene-based resin component in the polyolefin microporous membrane within the above range, the film-forming property and uniformity of the microporous membrane are excellent, and at the same time, the performance balance as a battery separator such as film strength and permeability is excellent. Here, the polyolefin microporous membrane may contain two or more kinds of polyethylene-based resins. In that case, the total amount of the polyethylene-based resins is defined as the amount of the polyethylene-based resin component constituting the polyolefin microporous membrane.

[0021] In the polyolefin microporous membrane according to the embodiment of the present invention, various polyethylene-based resins can be used, such as ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, branched low density polyethylene, linear low density polyethylene, and the like. Further, the polyethylene-based resin may be a homopolymer of ethylene or a copolymer of ethylene and other α-olefins. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. Here, the polyethylene-based resin shall contain ethylene in an amount exceeding 50 mol% with respect to all raw material monomer components.

[0022] The polyolefin microporous membrane according to the embodiment of the present invention has a weight average molecular weight of 1.0×10 6 or less in the raw material resin, and the content of polyethylene is less than 40% by mass, and it is preferable that the content of polyethylene with a weight average molecular weight of 1.5×10 6 or more is 30% by mass or more.

[0023] The narrower the molecular weight distribution of the polyolefin resin used as the raw material, the more the system is unified and the easier it is to obtain uniform micropores. However, the narrower the distribution, the lower the moldability. Therefore, not only polyolefin with a weight average molecular weight of 1.5×10 6 or more, but also polyolefin with a weight average molecular weight of 1.0×10 6 or less is contained to improve the moldability. On the other hand, when the molecular weight distribution increases, the low molecular weight components increase, so the strength decreases and the melting and fusion of fine fibrils in stretching and heat setting are likely to occur. Therefore, the content of polyethylene with a weight average molecular weight of 1.0×10 6 or less is preferably less than 40% by mass, more preferably less than 30% by mass, and even more preferably 1% by mass or more and less than 20% by mass. The higher the content of high molecular weight components, the higher the entanglement density in the amorphous region. Since stress is uniformly applied to the polyethylene resin layer in the stretching process, a uniform structure with few large pores can be formed, and good output characteristics can be obtained. Therefore, the weight average molecular weight of 1.5×10 6The content of polyethylene mentioned above is preferably 40% by mass or more, more preferably 60% by mass or more, and even more preferably 80% by mass or more.

[0024] (Method for manufacturing a polyolefin microporous membrane) Next, the method for manufacturing a polyolefin microporous membrane in the embodiment of the present invention will be described. Examples of the method for manufacturing a polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. From the viewpoint of controlling the structure and physical properties of the film, a wet film-forming method is preferable as the method for manufacturing a polyolefin microporous membrane in the present embodiment.

[0025] Hereinafter, the method for manufacturing a polyolefin microporous membrane by a wet method will be described. Note that the following description is an example of the manufacturing method and is not limited to this method.

[0026] The method for manufacturing a polyolefin microporous membrane in the embodiment of the present invention preferably includes the following steps (1) to (5) in order, may further include the following step (6), and after step (6), or instead of step (6), may further include the following step (7).

[0027] (1) A step of melt-kneading the polyolefin resin and a film-forming solvent to prepare a polyolefin resin composition (2) A step of extruding the polyolefin resin composition and cooling it to form a gel-like sheet (3) A first stretching step of preheating and stretching the gel-like sheet (4) A step of removing the film-forming solvent from the stretched gel-like sheet (5) A step of drying the sheet after removing the film-forming solvent (6) A second stretching step of preheating and stretching the dried sheet (7) A step of heat-treating the dried sheet A polyolefin resin composition is prepared by heating and dissolving a polyolefin resin in a plasticizer (film-forming solvent). The plasticizer is not particularly limited as long as it can uniformly disperse the polyolefin resin, but in order to enable relatively high magnification stretching, the solvent is preferably a liquid at room temperature. Examples of the solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions with boiling points corresponding to these, as well as phthalic acid esters that are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate. In order to obtain a stable gel-like sheet, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0028] The blending ratio of the polyolefin resin and the plasticizer is preferably such that the content of the polyolefin resin is 10 to 50% by mass based on the total mass of the polyolefin resin composition. By setting the content of the polyolefin resin within the above range, the dispersion state of the polyolefin resin and the plasticizer becomes good, and the strength, permeability, and heat resistance of the obtained microporous membrane are excellent. Also, when forming into a sheet, the swell and neck-in amount at the die outlet are appropriate, and the formability and film-forming property of the sheet are also good.

[0029] From the viewpoint of obtaining a uniform kneaded state, the melt-kneading of the polyolefin resin and the plasticizer is preferably carried out in a twin-screw extruder.

[0030] The resin temperature during kneading is preferably 150 °C or higher, more preferably 160 °C or higher, still more preferably 180 °C or higher, and the upper limit is preferably 250 °C or lower, more preferably 240 °C or lower, and still more preferably 230 °C or lower. By setting the temperature of the polyolefin resin composition during kneading within the above range, a decrease in strength due to resin deterioration can be prevented, and the polyolefin resin and the plasticizer can be uniformly melt-kneaded.

[0031] Also, during kneading with a twin-screw extruder, Q / Ns calculated from the ratio of the extrusion mass Q (kg / hr) to the screw rotation speed Ns (rpm) is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.1 or more. This can prevent a decrease in strength due to resin deterioration during kneading. Also, the upper limit is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less. This can apply sufficient shear to the polyolefin resin composition and obtain a uniform dispersion state.

[0032] (2) Gel sheet formation step Supply the melt of the polyolefin resin composition from the extruder to the die and extrude it in the form of a sheet. The extrusion method may be either the T-die method or the inflation method. Also, a plurality of polyolefin resin compositions of the same or different compositions may be supplied from a plurality of extruders to a single multi-manifold type composite T-die and laminated in layers, and then extruded into a sheet having a laminated structure. The extrusion temperature is preferably 140 to 250°C.

[0033] The shear rate applied when extruding the melt from the die is preferably 150 sec -1 or more, more preferably 200 sec -1 or more, even more preferably 250 sec -1 or more, particularly preferably 300 sec -1 or more, most preferably 400 sec -1 or more. By setting the shear rate within the above range, the orientation state of the sheet surface can be controlled, the structural difference between the front and back when forming a microporous membrane can be reduced, and the pore structure can be easily refined. The upper limit of the shear rate is preferably 1000 sec -1 or less from the viewpoint of compatibility with the film quality, and more preferably 800 sec -1 or less. The shear rate can be calculated by the following formula in extrusion using a T-die. γ = 6Q / (Wt 2 ) γ: Shear rate ( / sec) Q: Discharge amount (cm 3 / sec) W: Width of the T-die outlet (cm) t: Slit gap of the die outlet (cm) The resin composition melt-extruded in a sheet form becomes a gel-like sheet by being cooled and solidified. In the cooling process, it is preferable to cool it to 10 to 50 °C before the first stretching process described later. This is because it is preferable that the final cooling temperature is equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching becomes easier in the subsequent stretching. Also, the cooling rate at this time is preferably carried out at a rate of 50 °C / min or more, more preferably 100 °C / min or more, and even more preferably 150 °C / min or more. Generally, when the cooling rate is slow, relatively large crystals are formed, so the higher-order structure of the gel-like sheet becomes coarser, and the gel structure forming it also becomes larger. On the other hand, when the cooling rate is fast, relatively small crystals are formed, so the higher-order structure of the gel-like sheet becomes dense and uniform stretching becomes possible, making it easy to increase the strength of the polyolefin microporous membrane or to refine the pore structure. The gel-like sheet is preferably cooled by a cooling roll or a cooling belt.

[0034] As described above, it is preferable that the gel sheet be cooled at a rate of 50°C / min or more. However, when cooling is performed by bringing a cooling device such as a cooling roll or a cooling belt into contact with one side of the gel sheet, the cooling rate of the surface of the gel sheet that is not in contact with the cooling device becomes slow. As a result of intensive studies by the present inventors on the increase in the difference in the pore area between the front and back surfaces in the conventional polyolefin microporous membrane as described in the above-mentioned patent document, it has been found that this difference in cooling rate has a great influence. Particularly when the thickness of the gel sheet is large, the cooling rate of the surface of the gel sheet that is not in contact with the cooling device becomes extremely slow, the higher-order structure of the gel sheet becomes coarser, and the gel structure forming it also becomes large. In this case, the difference in the higher-order structure becomes large between the surface of the gel sheet that is in contact with the cooling device and the surface that is not in contact with it, and the difference in surface structure parameters such as the pore ratio becomes large between the surface that was in contact with the cooling device and the surface that was not in contact with it when the polyolefin microporous membrane was formed. Therefore, it is preferable to cool the gel sheet by spraying a coolant onto the surface that is not in contact with the cooling device in this forming step. Examples of preferable coolants include water, dry air, a mixture of fine water droplets and air, and the above-mentioned plasticizer. By using water as the coolant, since water has a large specific heat and latent heat of vaporization, it can be cooled efficiently, and since it is easy to separate from the gel sheet and the plasticizer, handling becomes simple.

[0035] It is preferable that the coolant be sprayed so that the flow rate is uniform in the width direction of the gel sheet. If there is variation in the cooling rate of the gel sheet in the width direction, there is a possibility that the strength and output characteristics of the polyolefin microporous membrane will vary in the width direction when the polyolefin microporous membrane is formed. By spraying the coolant so that the flow rate is uniform in the width direction of the gel sheet, a polyolefin microporous membrane having uniform strength and output characteristics in the width direction can be obtained.

[0036] The coolant is preferably sprayed after the position where the gel sheet discharged from the die of the extruder contacts the cooling device, and more preferably sprayed at the position where it contacts the cooling device. Also, it is preferable to spray within 2 seconds after contacting the cooling device. When the gel sheet contacts the cooling device, the surface in contact with the cooling device is rapidly cooled while the non-contact surface is cooled relatively slowly. By spraying the coolant as soon as possible after the gel sheet contacts the cooling device, the difference in cooling rate between the surface in contact with the cooling device and the non-contact surface can be reduced. On the other hand, before the gel sheet contacts the cooling device, the surface of the gel sheet is still in a molten state. Therefore, if the coolant is sprayed before the gel sheet contacts the cooling device, irregularities of 0.1 mm or more may occur on the sheet surface due to the sprayed coolant, and the porosity ratio on the front and back may deteriorate instead. Therefore, by spraying the coolant after the position where the gel sheet contacts the cooling device, the generation of irregularities of 0.1 mm or more on the surface can be suppressed, and a polyolefin microporous membrane with little difference in surface porosity ratio between the front and back and excellent output characteristics can be obtained.

[0037] When a liquid is used as the coolant, the angle formed by the direction in which the coolant spraying device discharges the coolant and the gel sheet at the position where the coolant is sprayed is preferably 90° or less. If the angle formed by the discharge direction of the coolant and the conveyance direction of the gel sheet is large, the coolant is likely to scatter, and the scattered coolant may adhere to the molten gel sheet, resulting in irregularities on the surface. Therefore, by spraying the coolant at an angle of 90° or less between the direction in which the coolant spraying device discharges the coolant and the gel sheet at the position where the coolant is sprayed, a polyolefin microporous membrane with few irregularities of 0.1 mm or more on the surface can be obtained.

[0038] (3) First stretching step Next, the obtained gel sheet is stretched in at least one axial direction, and it is preferable to preheat the gel sheet before stretching. The preheating temperature is preferably 90 to 130°C, more preferably 105°C or higher, still more preferably 110°C or higher, and also more preferably 120°C or lower, still more preferably 117°C or lower. By performing the preheating at the above conditions, a polyolefin microporous membrane that is uniformly stretched and has a uniform fine pore structure can be obtained in the stretching step.

[0039] The preheated gel sheet is preferably stretched at a predetermined magnification by the tenter method, roll method, inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, but simultaneous biaxial stretching is preferred. By performing simultaneous biaxial stretching, it becomes easy to reduce the structural difference between the front and back surfaces of the polyolefin microporous membrane surface and to adjust the standard deviation of the orientation parameter in the film plane to a preferable range.

[0040] The stretching ratio (area stretching ratio) in this step is preferably 16 times or more, more preferably 25 times or more. Also, the stretching ratio is preferably 4 times or more, more preferably 5 times or more, in either the machine longitudinal direction (MD direction) or the machine width direction (TD direction). The stretching ratios in the MD direction and TD direction may be the same or different, and by setting the area stretching ratio within the above range, the mechanical strength and permeability can be enhanced. Also, the area stretching ratio in this step is preferably 100 times or less, more preferably 64 times or less, whereby a polyolefin microporous membrane with excellent membrane strength can be obtained while preventing film breakage.

[0041] The stretching temperature of this process is preferably within the range of the crystal dispersion temperature (TCD) of the polyethylene-based resin to (TCD + 30) °C, more preferably (TCD + 5) °C or higher, particularly preferably (TCD + 10) °C or higher, and more preferably (TCD + 28) °C or lower, particularly preferably (TCD + 26) °C or lower. When the stretching temperature is within the above range, film breakage during stretching is suppressed, and high-magnification stretching can be achieved.

[0042] The crystal dispersion temperature (TCD) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. When using a polyethylene-based resin as the polyolefin resin, ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene resin compositions have a crystal dispersion temperature of about 100 to 110 °C. Therefore, the stretching temperature is preferably 90 to 130 °C, more preferably 105 °C or higher, even more preferably 110 °C or higher, and more preferably 120 °C or lower, even more preferably 117 °C or lower. Through stretching as described above, cleavage occurs between polyethylene-lamellae, the polyethylene-based resin phase is refined, and a large number of fibrils are formed. The fibrils form a three-dimensional irregularly connected network structure.

[0043] The stretching speed in the MD direction of this process is preferably 300% / min or higher, more preferably 700% / min or higher, even more preferably 1000% / min or higher, particularly preferably 1200% / min or higher, and most preferably 1500% / min or higher. Also, it is preferably 5000% / min or lower. By setting the stretching speed in the MD direction within the above range, it becomes easy to make the pore structure fine and uniform. Note that the stretching speed in this process is the value obtained by dividing the elongation from the start of stretching to the end of stretching by the required time when the elongation rate of the microporous membrane before stretching is 0% and the elongation rate per one-fold stretching ratio is 100%.

[0044] The stretching speed in the TD direction of this process is preferably 300% / min or more, more preferably 700% / min or more, still more preferably 1000% / min or more, particularly preferably 1200% / min or more, and most preferably 1500% / min or more. Also, it is preferably 5000% / min or less. By setting the stretching speed in the TD direction within the above range, it becomes easy to make the pore structure fine and uniform.

[0045] (4) Removal step of the solvent for film formation Using a cleaning solvent, the removal (cleaning) of the solvent for film formation is performed. The polyolefin resin phase is phase-separated from the solvent phase for film formation. Therefore, when the solvent for film formation is removed, a porous membrane composed of fibrils forming a fine three-dimensional network structure and having three-dimensionally irregularly communicating pores (voids) is obtained. Since the cleaning solvent and the method for removing the solvent for film formation using the same are well-known, the description is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and JP-A-2002-256099 can be used.

[0046] (5) Drying step The polyolefin microporous membrane from which the solvent for film formation has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably below the crystal dispersion temperature (TCD) of the polyolefin resin, and particularly preferably 5°C or more lower than the TCD. Drying is preferably carried out until the residual cleaning solvent becomes 5 parts by mass or less, more preferably 3 parts by mass or less, with the total mass of the polyolefin microporous membrane being 100 parts by mass (dry mass).

[0047] (6) Second stretching step Next, the dried microporous membrane is stretched at a predetermined area stretching ratio in at least one axial direction. The stretching of the dried film (second stretching) is also called dry stretching. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous stretching or sequential stretching may be used, but sequential stretching is preferred. In the case of sequential stretching, it is preferably stretched in the MD direction first, and then continuously stretched in the TD direction.

[0048] The area stretching ratio in dry stretching is preferably 1.2 times or more, more preferably 1.5 times or more, still more preferably 2.0 times or more, and particularly preferably 3 times or more. By setting the area stretching ratio in dry stretching within the above range, the film strength can be easily controlled within a desired range. Further, by setting the area stretching ratio in dry stretching to 16 times or less, good permeability can be easily obtained. Note that the area stretching ratio described in this step indicates the product of the dry stretching ratios in the MD direction and the TD direction.

[0049] The stretching ratio in the MD direction of dry stretching is preferably 1.2 times or more, more preferably 1.4 times or more, still more preferably 1.6 times or more, and particularly preferably 1.7 times or more, and preferably 4 times or less. By setting the stretching ratio in the MD direction of dry stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within a desired range.

[0050] The stretching ratio in the TD direction of dry stretching is preferably 1.2 times or more, more preferably 1.4 times or more, still more preferably 1.6 times or more, and particularly preferably 1.7 times or more, and preferably 4 times or less. By setting the stretching ratio in the TD direction of dry stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within a desired range.

[0051] When the stretching ratio in the MD direction in this step is S MD and the stretching ratio in the TD direction is S TD , |S MD - S TD | is preferably 0.5 or less, more preferably 0.3 or less, and particularly preferably 0.1 or less. By setting |S MD - S TD | within the above range, a structure in which the fibrils of the polyolefin microporous membrane are uniformly oriented in the plane can be obtained, and it becomes easy to adjust the standard deviation of the orientation parameter and the arithmetic mean roughness (Sa) in the film plane within a desired range. Note that the stretching ratio in this step refers to the stretching ratio of the microporous membrane immediately before being subjected to the next step, based on the microporous membrane immediately before this step.

[0052] In this process, the stretching temperature in the MD direction is preferably 60°C or higher, more preferably 80°C or higher. Also, it is preferably 130°C or lower, more preferably 120°C or lower. By setting the stretching temperature in the MD direction within the above range, the resulting polyolefin microporous membrane has excellent permeability and strength and can be stretched uniformly.

[0053] In this process, the stretching temperature in the TD direction is preferably 80°C or higher, more preferably 100°C or higher. Also, it is preferably 145°C or lower, more preferably 135°C or lower. By setting the stretching temperature in the TD direction within the above range, the resulting polyolefin microporous membrane has excellent permeability and strength and can be stretched uniformly.

[0054] (7) Heat treatment process Also, after the step (6) or instead of the step (6), the dried polyolefin microporous membrane can be heat-treated. By heat treatment, the crystals are stabilized and the lamellae are homogenized. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. Heat setting treatment is a heat treatment in which heating is performed while maintaining the dimensions of the membrane unchanged. Heat relaxation treatment is a heat treatment in which the membrane is thermally shrunk in the MD direction or TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. The relaxation rate in the relaxation treatment is the value obtained by dividing the dimension of the membrane after the relaxation treatment by the dimension of the membrane before the relaxation treatment. The relaxation rates in the MD and TD directions of the membrane are both preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. Also, from the viewpoint of the planarity of the microporous membrane, it is preferably 0.80 or higher, more preferably 0.90 or higher. The heat treatment temperature is preferably within the range of TCD to the melting point of the polyolefin resin. The melting point can be measured by a differential scanning calorimeter (DSC) based on JIS K7121 (1987).

[0055] The polyolefin microporous membrane obtained as described above can be used in various applications such as filters, separators for secondary batteries, separators for fuel cells, and separators for capacitors. (Separator for battery and secondary battery) When used as a separator for a battery, the polyolefin microporous membrane of the present invention is excellent in performance maintenance characteristics and output characteristics, and thus is particularly preferably used as a separator for a secondary battery that requires high energy density, high capacity, and high output.

Example

[0056] The present invention will be described in more detail with reference to examples, but the embodiments of the present invention are not limited to these examples. In addition, the evaluation in this application was performed under an environment of a temperature of 23 ° C and a humidity of 65% unless otherwise specified. The evaluation methods and analysis methods used in the examples are as follows.

[0057] (Measurement method) [Thickness] The film thicknesses at any five points within a 50 mm × 50 mm range of the polyolefin microporous membrane were measured with a contact thickness gauge, "Lightmatic" (registered trademark) VL-50 manufactured by Mitutoyo Corporation (10.5 mmφ super-hard spherical measuring head, measuring load 0.01 N), and the average value was taken as the thickness (μm).

[0058] [Porosity] A sample was cut out from the polyolefin microporous membrane into a 50 mm × 50 mm square, and its volume (cm 3 ) and mass (g) were measured. From these values and the bulk density (g / cm 3 ), the porosity of the polyolefin microporous membrane was calculated by the following formula. The bulk density was calculated assuming a constant value of 0.99 g / cm 3 . In this measurement, samples were cut out from three arbitrary positions of the polyolefin microporous membrane, and the average value of the measured porosities was calculated. Formula: Porosity (%) = [(Volume - Mass / Film density) / Volume] × 100.

[0059] [Thrust strength in terms of basis weight] The thrust strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was 2 mm / sec. Using a force gauge (DS2-20N manufactured by IMADA Co., Ltd.), the maximum load (mN) when a polyolefin microporous membrane was pierced with a needle having a spherical tip (curvature radius R: 0.5 mm) and a diameter of 1.0 mm was measured, and the value obtained from the following formula was defined as the thrust strength in terms of basis weight (mN / (g / m 2 )) Formula: Thrust strength in terms of basis weight (mN / (g / m 2 )) = Maximum load (mN) / Basis weight of polyolefin microporous membrane (g / m 2 ) Note that the basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25°C, and using the following formula. Formula: Basis weight (g / m 2 ) = Mass (g) / (50 (mm) × 50 (mm)) × 10 6 [Heat shrinkage rate (%) at 105°C / 8 h] Three samples were taken from the center part in the width direction of the polyolefin microporous membrane by cutting out 50 mm square test pieces, and the shrinkage rate (heat shrinkage rate) in the MD direction when each was held at 105°C for 8 hours was measured. The average value thereof was defined as the shrinkage rate in the MD direction (MD heat shrinkage rate). Also, the same measurement was performed for the TD direction to obtain the shrinkage rate in the TD direction (TD heat shrinkage rate).

[0060] [Resistance value in terms of 10 μm] The polyolefin microporous membrane was cut into φ19 mm as test pieces in an atmosphere with a humidity of 30% ± 10%. One test piece was placed in a coin cell case (CR2032 standard) together with other materials, and electrolyte was injected. As the other materials and the electrolyte, LiCoO2 was used for the positive electrode, artificial graphite was used for the negative electrode, and a 1 mol / L solution of LiPF6 in a solvent with a volume ratio of EC and EMC of 4:6 was used for the electrolyte. Here, LiPF6, EC, and EMC represent the following respectively. LiPF6: Lithium hexafluorophosphate EC: Ethylene Carbonate EMC: Ethyl Methyl Carbonate Using a vacuum dryer, the electrolyte was vacuum-impregnated into the coin cell material at a gauge pressure of -50 kPa for 1 minute. Then, the coin cell was sealed with a crimper to create an evaluation coin cell. The resistance value of the fabricated coin cell was measured at a frequency of 200 kHz using an impedance analyzer in an atmosphere of 25°C. Since the obtained resistance value includes the resistance other than the polyolefin microporous membrane, the number of polyolefin microporous membranes was changed and the above measurement was performed, and the resistance value per polyolefin microporous membrane (Ω·cm 2 ) was calculated. Then, the resistance value converted to a film thickness of 10 μm was calculated by the following formula. Formula: R = R1 × 10 / T Here, R: Resistance value converted to 10 μm (Ω·cm 2 / 10 μm) R1: Resistance value per polyolefin microporous membrane (Ω·cm 2 ) T: Film thickness of the polyolefin microporous membrane (μm). Also, it is known that the resistance value is generally affected by the porosity, and the higher the porosity, the lower the resistance value. In order to consider the difference in porosity for each sample when evaluating the resistance value, the resistance value converted to a porosity of 45% was calculated by the following formula. Formula: Rp = R × ([Porosity] / 45%) Here, Rp = Resistance value at a porosity of 45% and converted to 10 μm (Ω·cm2 / 10 μm). The resistance value of the polyolefin microporous membrane at a porosity of 45% and converted to 10 μm was evaluated according to the following criteria and used as an index for the output characteristics when used as a separator. ○: Less than 0.6 Ω·cm 2 / 10 μm ×: 0.6 Ω·cm or more 2 / 10 μm.

[0061] [Weight-average molecular weight of polyolefin microporous membrane] The molecular weight distribution of the polyolefin (measurement of weight-average molecular weight, molecular weight distribution, content of a predetermined component, etc.) was measured by high-temperature gel permeation chromatography (GPC). The measurement conditions were as follows.

[0062] Apparatus: High-temperature GPC apparatus (Instrument No. GPC-H-3, manufactured by Tosoh Corporation, HLC-8321GPC / HT) Detector: Differential refractive index detector RI Guard column: Shodex G-HT Column: Shodex HT806M (2 pieces) (φ8.0 mm × 30 cm, manufactured by Showa Denko) Solvent: 1,2,4-Trichlorobenzene (TCB, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (added with 0.1% BHT) Flow rate: 1.0 mL / min Column temperature: 145 °C Sample preparation: 5 mL of the measurement solvent was added to 5 mg of the sample, and the mixture was heated and stirred at 160 to 170 °C for about 60 minutes. After that, the obtained solution was filtered through a metal filter (pore size 0.5 μm).

[0063] Injection volume: 0.300 mL Standard sample: Monodisperse polystyrene manufactured by Tosoh Corporation, dibenzyl manufactured by Tokyo Chemical Industry Data processing: GPC data processing system manufactured by TRC After that, the obtained Mw was converted to polyethylene (PE). The conversion formula is as follows.

[0064] Mw (PE conversion) = Mw (PS conversion measured value) × 0.468.

[0065] [Open pore rate of the surface] A 3mm x 3mm square sample was cut from the polyolefin microporous membrane and platinum was vapor-deposited for 40 seconds at a sputtering current of 20mA using a coating device (JEOL JFC-1600). Secondary electron images of the sample surface were observed using a field-emission scanning electron microscope (JEOL JSM-6701F) at an accelerating voltage of 2kV, a magnification of 10,000x, and a field of view of 12µm x 9.6µm. Images were analyzed using MVTec's HALCON Ver. 13.0 software to identify pores. The images before binarization were taken at an accelerating voltage of 2 kV, a magnification of 10,000 times, a field of view of 11.7 μm × 9.4 μm (1280 pixels × 1024 pixels), and an 8-bit (256 gradations) grayscale image. The image analysis method involved first removing noise from the 256-level grayscale image using a 3 pixel x 3 pixel average, then performing dynamic binarization processing using a threshold of -30 from the 21 pixel x 21 pixel average image to extract dark areas, and counting these as open holes.The surface open hole rate was calculated using the area of the entire image (S_all) and the total area of the open holes (S_open) using the following formula. Formula: Surface porosity X(%)=(S_open / S_all)×100 The porosity ratios X (%) of one surface and the other surface of the microporous membrane are designated Xa and Xb, in ascending order of the porosity ratios of the surfaces.

[0066] [Average surface pore area] The average area of the open pores on the surface was calculated as the arithmetic mean using the number of counted open pores (N_open) according to the following formula. Formula: Average surface area of open pores S (μm 2 )=S_open / N_open The average area S (μm 2 ) and the average surface area of the pores is Sa and Sb in order from the smallest.

[0067] [Number of pores per surface area] The number of pores per surface area was calculated using the following formula. Formula: Number of pores N (pieces / μm) per surface area 2 ) = N_open / S_all Regarding the number of pores N (pieces / μm) per surface area of one surface and the other surface of the microporous membrane 2 ), let the number of pores per surface area be Na and Nb in ascending order.

[0068] [Performance maintenance characteristics of polyolefin microporous membrane] When the polyolefin microporous membrane is used as a separator for a secondary battery, the performance maintenance characteristics are evaluated according to the following criteria, and it is judged as qualified at times A and B. A: All of the following 2 items are 〇 B: Among the following 2 items, there are 1 - 2 △ and 0 × C: Other than A and B above (among the following 2 items, there is 1 or more ×) Item 1 〇: The puncture strength in terms of basis weight is 50 mN / (g / m 2 ) or more △: The puncture strength in terms of basis weight is 45 mN / (g / m 2 ) or more and less than 50 mN / (g / m 2 ) ×: The puncture strength in terms of basis weight is less than 45 mN / (g / m 2 ) Item 2 〇: The MD shrinkage rate and TD shrinkage rate are less than 6.5% △: The MD shrinkage rate and TD shrinkage rate are 6.5% or more and less than 7.5% ×: The MD shrinkage rate and TD shrinkage rate are 7.5% or more.

[0069] (Example 1) Ultra-high molecular weight polyethylene with a weight average molecular weight of 1.5×10 6 was used as the raw material. 75 parts by mass of liquid paraffin was added to 25 parts by mass of ultra-high density polyethylene, and further 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the mass of the ultra-high molecular weight polyethylene and mixed to prepare a polyethylene resin solution. The obtained polyethylene resin solution was fed into a twin-screw extruder, kneaded at 180 °C, supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 15 °C to form a gel sheet. Fig. 1 shows a schematic diagram of the cooling process of the gel sheet according to Example 1. The gel sheet 2 extruded from die 1 was wound and cooled with the cooling roll 3. Here, by spraying a coolant onto the gel sheet 2 at the coolant spraying position 5 using the coolant spraying device 4, the surface of the gel sheet that was not in contact with the cooling roll 3 was cooled. The angle 6 through which the gel sheet was conveyed from the position where it contacted the cooling roll to the coolant spraying position was set to 13°. At this time, 1 second had elapsed since the gel sheet contacted the cooling roll. The angle 9 formed by the direction 7 in which the coolant spraying device discharges the coolant and the conveying direction 8 of the gel sheet at the coolant spraying position 5 was set to 45°. Water at 20 °C was sprayed evenly in the width direction from the coolant spraying device 4 at a flow rate of 12 mL / min per area of 1 cm × 1 cm in the conveying direction. The obtained gel sheet was simultaneously biaxially stretched 5 times in the longitudinal and width directions at 115 °C using a batch-type tenter stretcher, the sheet width was fixed in the tenter stretcher as it was, and it was held at a temperature of 115 °C for 10 seconds.

[0070] Next, the stretched gel sheet was immersed in a methylene chloride bath in a washing tank, and after removing the liquid paraffin, it was dried to obtain a polyolefin microporous membrane. Finally, using an oven, heat fixation was carried out at a temperature of 125 °C for 10 minutes without stretching. [[ID=il]]

[0071] (Example 2) In the manufacturing method described in Example 1, without spraying water on the gel sheet, at a position 13° advanced in the conveying direction after the gel sheet contacted the cooling roll, air at 20 °C was sprayed evenly in the width direction from a gap of 0.5 mm at a distance of 6 cm from the gel sheet at an angle perpendicular to the conveying direction at a speed of 15 m / s to cool the surface not in contact with the cooling roll. A polyolefin microporous membrane was formed in the same manner as the method described in Example 1 except for the above.

[0072] (Comparative Example 1) A polyolefin microporous membrane was formed in the same manner as in Example 1, except that water was not sprayed onto the gel sheet.

[0073] (Comparative Example 2) A polyolefin microporous membrane was formed in the same manner as in Example 1, except that the position where water was sprayed onto the gel sheet was changed to a position 40° advanced in the transport direction after the gel sheet came into contact with the cooling roll.

[0074] (Comparative Example 3) A polyolefin microporous membrane was formed in the same manner as in Example 1, except that the position where air was sprayed onto the gel sheet was changed to a position 40° advanced in the transport direction after the gel sheet came into contact with the cooling roll. Examples and comparative examples are shown in Table 1. In the table, "MD" means the MD direction and "TD" means the TD direction.

[0075]

Table 1

Explanation of Reference Numerals

[0076] 1: Die 2: Gel sheet 3: Cooling roll 4: Coolant spraying device 5: Coolant spraying position 6: Angle (spraying position) through which the gel sheet is transported from the time it comes into contact with the cooling roll until the coolant is sprayed 7: Direction in which the coolant spraying device discharges the coolant 8: Transport direction of the gel sheet at the coolant spraying position ​

Claims

1. For the average pore area S (nm ２ ) of the pores on one surface and the other surface of the microporous membrane, when the average pore areas of the pores are Sa and Sb from the smaller one, a polyolefin microporous membrane satisfying 400 ≤ Sa ≤ 2000, 400 ≤ Sb ≤ 2000, and 0.9 ≤ Sa / Sb ≤ 1.

0.

2. Regarding the porosity X (%) of one surface and the other surface of the microporous membrane, when the porosities are set as Xa and Xb from the smaller one, the polyolefin microporous membrane according to Claim 1, which satisfies 10 ≦ Xa ≦ 20, 10 ≦ Xb ≦ 20, and 0.9 ≦ Xa / Xb ≦ 1.

0.

3. The number of pores N (pieces / μm) on one surface and the other surface of the microporous membrane ２ ), when the number of pores is set as Na and Nb from the smaller one, the polyolefin microporous membrane according to claim 1 or 2, which satisfies 0.9 ≦ Na / Nb ≦ 1.0, 50 ≦ Na ≦ 200, and 50 ≦ Nb ≦ 200.

4. The weight-average molecular weight of the polyolefin microporous membrane is 8.0×10 ５ or more, and the polyolefin microporous membrane according to claim 1 or 2.

5. The weight-average molecular weight in the raw material resin of the polyolefin microporous membrane is 1.0×10 ６ The content of the following polyolefin is less than 40% by mass, and the content of the polyolefin with a weight-average molecular weight of 1.5×10 ６ The polyolefin microporous membrane according to claim 1 or 2, wherein the content of the above polyolefin is 30% by mass or more.

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