Polyolefin microporous membrane

Abstract

To provide a polyolefin microporous film which can impart high safety to abnormal heat generation of a battery when used as a separator for a secondary battery.SOLUTION: A polyolefin microporous film contains a polyethylene-based resin, and an amorphous polymer incompatible with the polyethylene-based resin, and has a melt-down temperature of 155°C or higher and does not have a peak in a range of 155°C to 250°C by a differential scanning calorimeter.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin microporous membrane, and more particularly to a polyolefin microporous membrane that has excellent battery safety when used as a separator for secondary batteries. [Background technology]

[0002] Microporous membranes are used in a variety of fields, including filters such as filtration membranes and dialysis membranes, separators for secondary batteries, and separators for electrolytic capacitors. Among these, polyolefin microporous membranes, which use polyolefin as the resin material, have excellent chemical resistance, insulation properties, and mechanical strength, and have recently been widely used as separators for secondary batteries.

[0003] Rechargeable batteries, such as lithium-ion batteries, are widely used in personal computers and mobile phones due to their high energy density. They are also expected to be used as power sources for electric and hybrid vehicle motors, and as stationary storage batteries.

[0004] In recent years, lithium-ion secondary batteries with high energy density designs tend to have low thermal stability due to the electrode materials used. As a result, there is a growing demand for secondary battery separators to prevent short circuits during abnormal heat generation by improving heat resistance.

[0005] Patent Document 1 relates to a separator for secondary batteries that combines the shutdown properties of the polyethylene microporous membrane with the heat resistance of the polypropylene-containing layer by laminating a microporous membrane containing polyethylene and polypropylene as essential components.

[0006] Patent Document 2 describes a microporous membrane made of a ring-opening polymer of a polycyclic norbornene monomer containing dicyclopentadiene, which can suppress thermal shrinkage during abnormal heat generation when used as a separator for secondary batteries. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-321323 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-139111 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] With respect to Patent Document 1, by including polypropylene, the heat resistance is improved compared to a microporous membrane made of polyethylene alone. However, when the polypropylene crystals melt, a large shrinkage deformation occurs, and there is a possibility that the safety during abnormal heat generation is insufficient for a battery with a high energy density design. Patent Document 2 is a microporous membrane made of a resin with high heat resistance and has excellent heat resistance. However, there are still problems with the basic performance as a separator, such as permeability and membrane strength, and there is concern that a rapid shrinkage deformation may occur in the temperature range where the crystal component melts, and there is a possibility that the safety against abnormal heat generation is insufficient.

[0009] The problem of the present invention is to solve the above. That is, when used as a separator for a battery, it is to provide a polyolefin microporous membrane that can impart high safety against abnormal heat generation of the battery. [Means for Solving the Problems]

[0010] In order to solve the above problems and achieve the object, the present invention has the following configuration. In the following description, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. 〔1〕A polyolefin microporous membrane containing a polyethylene-based resin and an amorphous polymer incompatible with the polyethylene-based resin, having a melt-down temperature of 155°C or higher and having no peak in the range of 155°C to 250°C in the DSC curve obtained by a differential scanning calorimeter (DSC). [2] The polyolefin microporous membrane according to [1], wherein the proportion of components with a molecular weight of 2 million or more in the molecular weight distribution curve of polyethylene measured by gel permeation chromatography is 15% or less. [3] A polyolefin microporous membrane according to [1] or [2], wherein the thermal shrinkage rate after 8 hours at 105°C is 10% or less. [4] A polyolefin microporous membrane according to any one of items [1] to [3] above, wherein the shutdown temperature is 138°C or lower. [5] A polyolefin microporous membrane according to any one of the above items [1] to [4], wherein the meltdown temperature is 165°C or higher. [6] A polyolefin microporous membrane according to any one of the above [1] to [5], wherein the amorphous polymer incompatible with the polyethylene resin is a cyclic olefin polymer. [7] A polyolefin microporous membrane according to any one of the above items [1] to [6], comprising 50 to 99% by mass of polyethylene resin and 1 to 50% by mass of a polymer incompatible with polyethylene resin. [8] A polyolefin microporous membrane according to any one of the above [1] to [7], wherein the first phase made of a polyethylene resin has a continuous structure, and the second phase made of a polymer incompatible with the polyethylene resin has a discontinuous structure, and the second phase has a domain diameter of 10 to 300 nm. [9] Air permeability based on a thickness of 1 μm is 25 seconds / 100 cm 3 The following is a polyolefin microporous membrane as described in any one of the above items [1] to [8].

[10] A polyolefin microporous membrane according to any one of the above items [1] to [9], wherein the puncture strength per 1 μm thickness is 10 gf or more.

[11] A polyolefin microporous membrane according to any one of the above items [1] to

[10] , having a thickness of 20 μm or less. [Effects of the Invention]

[0011] The polyolefin microporous membrane according to the present invention has high heat resistance and shape retention performance at high temperatures. Therefore, when used as a separator for secondary batteries, it is possible to provide a polyolefin microporous membrane that can provide high safety against abnormal heat generation in batteries. [Modes for carrying out the invention]

[0012] The embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below.

[0013] The polyolefin microporous membrane according to an embodiment of the present invention (hereinafter sometimes simply referred to as "microporous membrane") comprises a polyethylene resin and an amorphous polymer incompatible with the polyethylene resin, has a meltdown temperature of 155°C or higher, and does not have a peak in the range of 155°C to 250°C in the DSC curve obtained by differential scanning calorimeter (DSC).

[0014] The meltdown temperature of the microporous membrane can be measured by the method described later. The meltdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is 155°C or higher, preferably 165°C or higher, more preferably 170°C or higher, and even more preferably 180°C or higher. When the meltdown temperature is within the above range, the microporous membrane has excellent heat resistance. That is, when the microporous membrane is used as a separator for a battery, the insulation of the electrodes is maintained when the battery overheats abnormally, resulting in a battery with excellent safety. The upper limit of the meltdown temperature is not particularly limited, but for example, it can be 250°C or lower, and more preferably 220°C or lower. Setting the meltdown temperature within the above range makes it easier to achieve compatibility with shutdown properties. In order to set the meltdown temperature within the above range, it is preferable to set the raw material composition and film formation conditions of the microporous membrane within the range described later. The meltdown temperature is the temperature at which the electrodes are no longer insulated when the polyolefin microporous membrane is heated and the pores are closed by the shutdown phenomenon described later, and further heating is continued.

[0015] The polyolefin microporous membrane according to the embodiment of the present invention does not have a peak in the 155°C to 250°C range in the DSC curve obtained by differential scanning calorimeter (DSC). Because there is no peak in the 155°C to 250°C range, the microporous membrane maintains a stable membrane shape within this temperature range, providing excellent performance in ensuring safety during abnormal heat generation. While the exact relationship between the presence or absence of the peak in the 155°C to 250°C range and safety during abnormal battery heat generation is unclear, it is believed that rapid changes in membrane morphology and performance during phase transitions such as crystal melting can cause ion paths between electrodes. The presence or absence of a peak in the 155°C to 250°C range in differential scanning calorimeter data can be measured and confirmed by the method described later.

[0016] The molecular weight distribution of the polyolefin microporous membrane according to the embodiment of the present invention is preferably such that the ratio of components with a molecular weight of 2 million or more in the polyethylene molecular weight distribution curve obtained by gel permeation chromatography (GPC) is 15% or less, more preferably 10% or less, even more preferably 7% or less, and particularly preferably 5% or less. When the ratio of polyethylene components with a molecular weight of 2 million or more is within the above range, deformation of the microporous membrane at high temperatures is suppressed, and when the microporous membrane is used as a battery separator, a battery with superior safety can be made. There is no particular lower limit set for the ratio of components with a molecular weight of 2 million or more, but a value of 0.001% or more is preferable because it results in good film-forming properties and film strength. In order to make the ratio of components with a molecular weight of 2 million or more in the polyethylene molecular weight distribution curve within the above range, it is preferable to set the raw material composition and film-forming conditions of the microporous membrane within the range described later. Furthermore, the ratio of components with a molecular weight of 2 million or more in the polyethylene molecular weight distribution curve can be measured and calculated by the method described later.

[0017] The polyolefin microporous membrane according to the embodiment of the present invention preferably has a thermal shrinkage rate of 10% or less, more preferably 7% or less, and even more preferably 4% or less when heated at 105°C for 8 hours. There is no particular lower limit for the thermal shrinkage rate after 105°C for 8 hours, but it is preferable to be -0.1% or more in order to ensure good flatness of the microporous membrane. By setting the thermal shrinkage rate after 105°C for 8 hours within the above range, when the microporous membrane is used as a battery separator, it is possible to suppress short circuits at the ends that occur due to separator shrinkage during abnormal battery overheating. The thermal shrinkage rate after 105°C for 8 hours can be set within the above range by adjusting the raw material mixing ratio, stretching ratio, heat setting conditions, etc. during the manufacturing process. Furthermore, the thermal shrinkage rate after 105°C for 8 hours can be calculated by the method described later.

[0018] The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 138°C or lower, more preferably 136°C or lower, and even more preferably 135°C or lower. By controlling the shutdown temperature within the above range, excellent battery safety is achieved when used as a battery separator. The lower limit of the shutdown temperature is not particularly limited, but it is preferably 110°C or higher because it is easier to achieve compatibility with permeability. In order to set the shutdown temperature within the above range, it is preferable to set the raw material composition and film formation conditions of the microporous membrane within the range described later. The shutdown temperature is a temperature measured by the method described later, and is the temperature at which, when the polyolefin microporous membrane is heated, the resin part shrinks and melts, closing the porous structure, and substantially preventing discharge and charging when used as a battery separator.

[0019] In the polyolefin microporous membrane according to embodiments of the present invention, it is preferable that the first phase, made of a polyethylene resin, has a continuous structure, and the second phase, made of a polymer incompatible with the first phase, has a discontinuous structure. Furthermore, the domain diameter of the second phase forming the discontinuous structure is preferably 10 to 300 nm, more preferably 20 nm to 250 nm, even more preferably 20 nm to 150 nm or less, and particularly preferably 20 nm to 100 nm.

[0020] The second phase having the domain diameter described above forms a uniform pore structure, enabling both strength and permeability in the polyolefin microporous membrane, and the finely dispersed second phase provides excellent heat resistance. The phase separation structure in the microporous membrane can be observed and the domain diameter can be calculated using the method described later. The domain diameter of the second phase forming the discontinuous structure can be set to the above range by adjusting the resin components forming the second phase, their mixing ratios in the manufacturing process, kneading conditions, stretching conditions, etc.

[0021] The polyolefin microporous membrane according to the embodiment of the present invention preferably has an air permeability of 25 seconds / 100 cm per 1 μm thickness. 3 Below, more preferably 15 seconds / 100cm 3 More preferably, 12 seconds / 100cm 3 The following is particularly preferable: 10 seconds / 100cm 3 The following applies. While no specific lower limit is set for air permeability based on a thickness of 1 μm, 1 second / 100 cm is chosen to facilitate compatibility with film strength. 3 The above is preferable. By setting the air permeability per 1 μm thickness within the above range, a microporous film with excellent charge-discharge characteristics can be obtained when used as a battery separator. The air permeability per 1 μm thickness can be set within the above range by adjusting the raw material mixing ratio, stretching ratio, heat-fixing conditions, etc., during the manufacturing process.

[0022] The polyolefin microporous membrane according to an embodiment of the present invention preferably has a puncture strength in terms of a thickness of 1 μm of 10 gf or more, more preferably 15 gf or more, still more preferably 20 gf or more, further preferably 25 gf or more, and particularly preferably 30 gf or more. The upper limit of the puncture strength in terms of a thickness of 1 μm is not particularly limited, but it is preferably 100 gf or less, for example, because it becomes easy to control the shutdown temperature within an appropriate range. When the puncture strength in terms of a thickness of 1 μm is within the above range, the microporous membrane is resistant to external shocks when used as a separator for a battery and is excellent in safety. The puncture strength in terms of a thickness of 1 μm can be made within the above range by adjusting the blending ratio of raw materials, the draw ratio, the heat setting conditions, etc. in the manufacturing process.

[0023] The thickness of the polyolefin microporous membrane according to an embodiment of the present invention can be appropriately adjusted according to the application, but it is preferably 20 μm or less, more preferably 15 μm or less, still more preferably 10 μm or less, and particularly preferably 8 μm or less. Also, it is preferably 3 μm or more, more preferably 5 μm or more. By setting the thickness of the polyolefin microporous membrane within the above range, it is possible to achieve both safety and high battery capacity when used as a separator for a secondary battery.

[0024] The porosity of the polyolefin microporous membrane according to an embodiment of the present invention is preferably 35% or more, more preferably 40% or more, still more preferably 45% or more, and particularly preferably 50% or more. Although no upper limit is particularly provided for the porosity, it is preferably 80% or less because it can suppress a decrease in membrane strength. When the porosity is within the above range, the microporous membrane has excellent output characteristics when used as a separator for a secondary battery. The porosity can be made within the above range by adjusting the blending ratio of raw materials, the draw ratio, the heat setting conditions, etc. in the manufacturing process. The resistance value of the polyolefin microporous membrane according to an embodiment of the present invention at 180°C is preferably 100 Ω·cm 2 or more, more preferably 3000 Ω·cm 2 or more, still more preferably 7000 Ω·cm 2In particular, 10,000 Ω·cm is preferred. 2 That concludes the explanation. When the resistance value at 180°C falls within the above range, the microporous film exhibits excellent heat resistance. In other words, when a microporous film is used as a separator for batteries, the insulation of the electrodes is maintained during abnormal heat generation in the battery, resulting in a battery with superior safety. Note that there is no particular upper limit to the resistance value at 180°C, but for example, 100,000 Ω·cm 2 Below ℃, and also 30,000 Ω·cm 2 One characteristic is that the temperature is below ℃. In order to achieve the above range of resistance at 180℃, it is preferable to set the raw material composition and film formation conditions of the microporous film within the range described later. The resistance at 180℃ can be measured by the method described later.

[0025] The following describes the specific configuration of the polyolefin microporous membrane in this embodiment, but it is not necessarily limited to this configuration.

[0026] The polyolefin microporous membrane according to the embodiment of the present invention contains a polyethylene resin, and it is more preferable that the polyethylene resin is the main component. (Hereinafter, the polyethylene resin used in the polyolefin microporous membrane will be referred to as "Resin A.") The main component referred to here is the component that has the highest mass percentage content among the components constituting the polyolefin microporous membrane. Furthermore, if two or more polyethylene resins constitute Resin A, the total content of those polyethylene resins will be considered as the content of Resin A, and it is sufficient that this total content is the highest mass percentage content among the components constituting the polyolefin microporous membrane.

[0027] Examples of resin A include polyethylene-based resins such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, and low-crystalline or amorphous ethylene-α-olefin copolymers. The α-olefin is not particularly limited as long as it can copolymerize with ethylene, and examples include 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-pentene, 1-heptene, vinyl acetate, methyl methacrylate, and styrene. Among the above resins, resin A preferably contains high-density polyethylene and ultra-high molecular weight polyethylene from the viewpoint of achieving both permeability and heat resistance. Resin A may be selected from two or more of the above resins.

[0028] Resin A is high-density polyethylene (density: 0.940 g / m³) in terms of melt extrusion characteristics and stretching characteristics. 3 More than 0.970g / m 3 It is preferable to include the following. The high-density polyethylene used in resin A may be not only an ethylene homopolymer, but also a copolymer containing other α-olefins in order to lower the melting point and crystallinity. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, etc. Furthermore, α-olefins 13 This can be confirmed by measuring with 1C-NMR.

[0029] When using the high-density polyethylene described above as resin A, the weight-average molecular weight of the high-density polyethylene is preferably 1 × 10⁻⁶. 4 That is all. far1×10 5 More preferably 3.0 × 10 5 In particular, 1 × 10 6 That concludes the explanation. Furthermore, the upper limit of the weight-average molecular weight of high-density polyethylene is preferably 3.0 × 10⁻⁶. 6 The following is more preferable: 2.0 × 10 6 More preferably, 1.7 × 10 6The following is true: By setting the weight-average molecular weight of high-density polyethylene within the above range, the manufactured microporous membrane exhibits superior safety in the event of abnormal heat generation when used as a battery separator.

[0030] When using the high-density polyethylene described above as resin A, the melting point of the high-density polyethylene is preferably 125°C or higher, more preferably 130°C or higher. Furthermore, it is preferably 137°C or lower, and more preferably 135°C or lower. By setting the melting point of the high-density polyethylene within this range, it is possible to achieve both a low shutdown temperature and excellent permeability.

[0031] The content of resin A in the polyolefin microporous membrane is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and also preferably 99% by mass or less, more preferably 97% by mass or less, and even more preferably 95% by mass or less. By setting the content of resin A in the polyolefin microporous membrane within the above range, it becomes easier to control the strength, permeability, heat resistance, and shutdown characteristics of the microporous membrane.

[0032] Furthermore, when manufacturing polyolefin microporous films, a film-forming solvent such as liquid paraffin is used as a plasticizer when preparing the polyolefin resin composition. Since this film-forming solvent is removed during the manufacturing process of the microporous film, the composition of the polyolefin resin composition differs from the composition of the polyolefin microporous film. In other words, the content of resin A in the polyolefin microporous film corresponds to the content of resin A relative to the composition of the polyolefin resin composition after removing components that are removed, such as the film-forming solvent.

[0033] In the embodiment of the present invention, the polyolefin microporous membrane is mixed with a resin different from resin A (hereinafter referred to as "resin B") to enhance safety when used as a battery separator. Resin B is incompatible with resin A and forms a phase-separated structure within the microporous membrane. Resin B is also an amorphous resin.

[0034] Examples of amorphous resins used for resin B include cyclic olefin polymers containing alicyclic structures as repeating units, such as cycloalkanes and cycloalkenes, as well as polyvinyl chloride, acrylic resins, polycarbonate, polystyrene, polybutadiene, styrene elastomers, polysulfone, ABS resin, and silicone resin. Among the above resins, resin B is more preferably a cyclic olefin polymer containing alicyclic structures as repeating units. By selecting resin B from the above polymers, a uniform fine domain structure is formed in the microporous membrane, and when the microporous membrane is used as a battery separator, a battery with excellent safety in the event of abnormal heat generation can be made without impairing battery performance such as ion permeability. Furthermore, resin B may be selected from two or more of the above resins.

[0035] The glass transition temperature of the amorphous resin used in resin B is preferably 70°C or higher, more preferably 120°C or higher, even more preferably 150°C or higher, and particularly preferably 170°C or higher. Furthermore, the glass transition temperature is preferably 250°C or lower, more preferably 230°C or lower, and even more preferably 210°C or lower. By setting the glass transition temperature of resin B within the above range, a phase separation structure consisting of resin A phase, resin B phase, and plasticizer phase is uniformly and finely formed when the molten and kneaded resin is cooled, and the resulting microporous film has superior heat resistance. Two or more amorphous resins with different glass transition temperatures may be used for resin B in order to improve the dispersion state in the polyolefin microporous film.

[0036] The content of resin B in the polyolefin microporous film is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Furthermore, it is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. By setting the content of resin B within the above range, the dispersion state of resin B in the microporous film is improved, making it easier to control the balance between heat resistance and permeability. The layer structure of the polyolefin microporous film according to the embodiment of the present invention may be a single layer film or a laminated film consisting of at least two layers having different properties. In the case of a laminated film, it is preferable that at least one layer consists of the aforementioned resins A and B.

[0037] Next, a method for producing a polyolefin microporous membrane according to an embodiment of the present invention will be described. Examples of methods for producing a polyolefin microporous membrane include a dry film formation method and a wet film formation method. In this embodiment, a wet film formation method is preferred from the viewpoint of controlling the structure and physical properties of the membrane.

[0038] The following describes a method for producing polyolefin microporous membranes using a wet process. Note that the following description is just one example of a manufacturing method and is not limited to this method.

[0039] The method for producing a polyolefin microporous membrane in embodiments of the present invention preferably includes the following steps (1) to (5) in order, and may further include the following step (6), and may also include the following step (7) after or in place of step (6).

[0040] (1) A step of melting and kneading the polyolefin resin and film-forming solvent to prepare a polyolefin resin composition. (2) The process of extruding the polyolefin resin composition and cooling it to form a gel sheet. (3) A first stretching step in which the gel sheet is preheated and stretched. (4) Step of removing the film-forming solvent from the stretched gel sheet. (5) A step of drying the sheet after removing the film-forming solvent. (6) A second stretching step in which the dried sheet is preheated and stretched. (7) A step of heat-treating the dried sheet. (1) Preparation process of polyolefin resin composition A polyolefin resin composition is prepared by heating and dissolving resin A and resin B in a plasticizer (film-forming solvent). The plasticizer is not particularly limited as long as it is a solvent that can uniformly disperse resin A and resin B, but it is preferable that the solvent be liquid at room temperature in order to enable relatively high-magnification stretching. Examples of solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate. To obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0041] The blending ratio of resin A and resin B to the plasticizer is preferably such that the total content of resin A and resin B is 10 to 50% by mass relative to the total mass of the polyolefin resin composition. By setting the total content of resin A and resin B within the above range, the dispersion state of resin A / resin B / plasticizer is good during the melt kneading described later, resulting in a microporous film with excellent strength, permeability, and heat resistance. In addition, when forming into a sheet, the amount of swell and neck-in at the nozzle exit is appropriate, resulting in good sheet moldability and film-forming properties.

[0042] The content of resin B in the polyolefin resin composition is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less. Furthermore, the content of resin B is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. By setting the content of resin B in the polyolefin resin composition within the above range, resin B forms a good dispersion state in the polyolefin resin composition, and a polyolefin microporous film with superior strength, permeability, and heat resistance can be obtained.

[0043] The method for uniformly melt-mixing resins A and B with the plasticizer is not particularly limited, but it is preferable to carry it out in a twin-screw extruder. The resin temperature during kneading is preferably 140°C or higher, more preferably 150°C or higher, and even more preferably 160°C or higher. The upper limit is preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 230°C or lower. By keeping the temperature of the polyolefin resin composition during kneading within the above range, a decrease in strength due to resin degradation can be prevented, and resins A and B and the plasticizer can be uniformly melt-mixed.

[0044] Furthermore, during mixing in a twin-screw extruder, the Q / Ns ratio, calculated from the ratio of the extrusion mass Q (kg / hr) to the screw rotation speed Ns (rpm), is preferably 0.01 or higher, more preferably 0.05 or higher, and even more preferably 0.1 or higher. This prevents a decrease in strength due to resin degradation during mixing. The upper limit is preferably 5.0 or lower, more preferably 3.0 or lower, and even more preferably 2.0 or lower. This allows sufficient shear to be applied to the polyolefin resin composition, resulting in a uniform dispersion state.

[0045] (2) Process for forming a gel-like sheet A molten polyolefin resin composition is supplied from an extruder to a die and extruded into a sheet.

[0046] The extrusion method may be either the flat die method or the inflation method. Alternatively, multiple polyolefin resin compositions of the same or different compositions may be supplied from multiple extruders to a single multi-manifold type composite T-die, laminated in layers, and extruded into a laminated sheet. The extrusion temperature is preferably 140 to 250°C, and the extrusion speed is preferably 0.2 to 15 m / min.

[0047] The resin composition, melt-extruded into a sheet, becomes a gel-like sheet upon cooling and solidification. It is preferable to cool the sheet to 10-50°C during the cooling process. This is because it is preferable to keep the final cooling temperature below the crystallization completion temperature, as this refines the higher-order structure, facilitating uniform stretching during subsequent stretching. Furthermore, the cooling rate at this stage is preferably 50°C / min or higher, more preferably 100°C / min or higher, and even more preferably 150°C / min or higher. Generally, a slower cooling rate results in the formation of relatively large crystals, leading to a coarser higher-order structure in the gel-like sheet and a larger gel structure. Conversely, a faster cooling rate results in the formation of relatively small crystals, resulting in a denser higher-order structure in the gel-like sheet, leading to improved film strength and elongation in addition to uniform stretching. Cooling methods include direct contact with cold air, cooling water, or other cooling media, contact with a roll cooled by a refrigerant, or the use of a casting drum.

[0048] (3) First stretching process Next, the obtained gel-like sheet is stretched in at least one axial direction, but it is preferable to preheat the gel-like sheet before stretching. The preheating temperature is preferably 90 to 130°C, more preferably 105°C or higher, even more preferably 110°C or higher, and even more preferably 120°C or lower, and even more preferably 117°C or lower. By performing the preheating under the above conditions, a polyolefin microporous film having a uniformly stretched and uniformly fine pore structure can be obtained during the stretching process.

[0049] The preheated gel sheet is preferably stretched to a predetermined magnification by the tenter method, roll method, inflation method, or a combination thereof. The stretching can be uniaxial or biaxial, but biaxial stretching is preferred. In the case of biaxial stretching, any of the following methods may be used: simultaneous biaxial stretching, sequential biaxial stretching, or multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching).

[0050] The stretching ratio (area stretching ratio) in this process is preferably 16 times or more, and more preferably 25 times or more. Furthermore, the stretching ratio is preferably 4 times or more in both the machine longitudinal direction (MD direction) and the machine width direction (TD direction), and more preferably 5 times or more. The stretching ratios in the MD direction and the TD direction may be the same or different. By setting the area stretching ratio within the above range, mechanical strength and permeability can be increased. In addition, the area stretching ratio in this process is preferably 100 times or less, more preferably 49 times or less, which prevents film breakage and at the same time ensures good dispersion of heat-resistant components within the film surface, resulting in high heat resistance when used as a separator for secondary batteries. Note that the stretching ratio in this process refers to the area stretching ratio of the polyolefin microporous film immediately before being subjected to the next process, based on the polyolefin microporous film immediately before this process.

[0051] The stretching temperature in this process is preferably within the range of the crystalline dispersion temperature (TCD) of the polyethylene resin to (TCD+30)°C, more preferably (TCD+5)°C or higher, particularly preferably (TCD+10)°C or higher, even more preferably (TCD+28)°C or lower, and particularly preferably (TCD+26)°C or lower. When the stretching temperature is within the above range, film breakage due to stretching is suppressed, and high-magnification stretching is possible.

[0052] The temperature of crystal dispersion (TCD) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. When polyethylene resin is used as the polyolefin resin, ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene resin compositions have a temperature of crystal dispersion of about 100 to 110°C, so it is preferable to set the stretching temperature to 90 to 130°C, more preferably 105°C or higher, even more preferably 110°C or higher, even more preferably 120°C or lower, and even more preferably 117°C or lower.

[0053] The stretching described above causes cleavage between the polyethylene lamellae, resulting in the refinement of the polyethylene resin phase and the formation of numerous fibrils. These fibrils form a three-dimensional, irregularly linked network structure.

[0054] (4) Removal of film-forming solvent The film-forming solvent is removed (washed) using a washing solvent. The polyolefin resin phase is phase-separated from the film-forming solvent phase. Therefore, when the film-forming solvent is removed, a porous film is obtained consisting of fibrils that form a fine three-dimensional network structure and has irregularly interconnected pores (voids) in three dimensions. The washing solvent and the method for removing the film-forming solvent using it are well known, so their explanation is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Publication No. 2002-256099 can be used.

[0055] (5) Drying process The polyolefin microporous film, from which the film-forming solvent has been removed, is dried by a heat drying method or an air drying method. The drying temperature is preferably below the crystalline dispersion temperature (TCD) of the polyolefin resin, and is particularly preferably 5°C or more lower than the TCD. Drying is preferably carried out until the remaining washing solvent is 5 parts by mass or less, and more preferably 3 parts by mass or less, with the total mass of the polyolefin microporous film being 100 parts by mass (dry mass).

[0056] (6) Second stretching process The dried polyolefin microporous film may be stretched at least uniaxially (second stretching step). The polyolefin microporous film may be preheated before the second stretching step. The preheating temperature is preferably 90 to 140°C, more preferably 95°C or higher, even more preferably 100°C or higher, and even more preferably 135°C or lower, and even more preferably 130°C or lower. The stretching of the polyolefin microporous film can be carried out by the tenter method, roll method, inflation method, etc., while heating, as described above. Stretching may be uniaxial or biaxial. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, or multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.

[0057] The area stretching ratio in this process is preferably 16.0 times or less, more preferably 4.0 times or less, and even more preferably 2.0 times or less. In the case of biaxial stretching, the stretching ratios in the MD direction and the TD direction may be the same or different. The stretching ratio in this process refers to the stretching ratio of the polyolefin microporous film immediately before being subjected to the next process, based on the polyolefin microporous film immediately before this process.

[0058] (7) Heat treatment process Furthermore, after step (6), or instead of step (6), the dried polyolefin microporous film can be heat-treated. Heat treatment stabilizes the crystals and homogenizes the lamellae. As heat treatment methods, heat setting treatment and / or heat relaxation treatment can be used. Heat setting treatment is a heat treatment in which the film is heated while being held so that its dimensions do not change. Heat relaxation treatment is a heat treatment in which the film is thermally contracted in the MD direction or TD direction while being heated. Heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a heat relaxation treatment method, the method disclosed in Japanese Patent Application Publication No. 2002-256099 can be cited. 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 differential scanning calorimeter (DSC) based on JIS K7121 (1987).

[0059] The polyolefin microporous membrane obtained as described above can be used in a variety of applications, such as filters, separators for secondary batteries, separators for fuel cells, and separators for capacitors. In particular, when used as a battery separator, it offers excellent permeability and high safety, making it more preferable for use as a separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles. [Examples]

[0060] The present invention will be described in further detail below with reference to examples. Unless otherwise specified, measurements in this application are performed under conditions of 23°C and 65% humidity. Furthermore, the present invention is not limited to these examples.

[0061] [Measurement method] [thickness] The film thickness of a polyolefin microporous membrane was measured at five arbitrary points within a 50 mm x 50 mm area using a contact thickness gauge, Mitutoyo Lightmatic VL-50 (10.5 mmφ carbide spherical probe, measuring load 0.01 N), and the average value was defined as the thickness (μm).

[0062] [Porosity] A 50mm x 50mm square sample was cut from the polyolefin microporous membrane, and its volume (cm³) was measured. 3 The volume (g) and mass (g) were measured. These values ​​and the membrane density (g / cm³) were also measured. 3 The porosity of the polyolefin microporous membrane was calculated using the following formula. The membrane density was 0.99 g / cm³. 3 The calculation was performed assuming a constant value. For this measurement, samples were cut from three arbitrary locations on the polyolefin microporous membrane, and the average value of the porosity measured for each sample was calculated. Formula: Porosity (%) = (Volume - Mass / Membrane Density) / Volume × 100.

[0063] [Shutdown temperature, meltdown temperature, resistance value at 180°C] A circular sample with a diameter of 19 mm was cut from a polyolefin microporous membrane, and components for a 2032 type coin cell (top cover, bottom cover, gasket (made of PFA), spacer (cylindrical, 15.5 mm in diameter and 1.0 mm thick), and wave washer) were prepared. All of the above 2032 type coin cell components were purchased from Hosen Co., Ltd. The following describes the procedure for fabricating the evaluation cell, but all of these operations were performed in a dry room with a dew point temperature of -35°C or lower. A sample for measurement and a gasket were placed on the inside bottom of the lower lid of the 2032 type coin cell component, starting from the lower lid side. Next, a solution was prepared by dissolving LiBF4 in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (EC / PC = 50 / 50 [weight ratio]) to a concentration of 1 M (manufactured by Kishida Chemical Co., Ltd.), and adding 0.3% by mass of surfactant F-444 (manufactured by DIC Corporation). 0.1 mL of this solution was then poured into the aforementioned coin cell. Subsequently, a spacer (16 mm in diameter, 1 mm thick) was placed on the sample for measurement in the hollow part of the gasket, and the cell was left to stand at a pressure of -50 kPa for 1 minute twice to impregnate the polyolefin microporous membrane with the electrolyte. After that, a wave washer and an upper lid were placed on the spacer, starting from the spacer side, and the cell was sealed using a coin cell crimping machine (manufactured by Hosen Co., Ltd.) to create an evaluation cell.

[0064] The evaluation cell described above was clamped with a coaxial contact probe placed inside an oven, and its resistance was measured using an LCR meter (HIOKI E.E. CORPORATION) at an amplitude of 50 mV and a frequency of 1 kHz. The coin cell temperature was monitored by attaching a resistance thermometer to the top cover of the cell. The coin cell temperature was raised from room temperature to 50°C and left to stand for 10 minutes, then the resistance was measured while raising the temperature at a rate of 5°C / min to 180°C. The resistance of the evaluation cell was initially 1 kΩcm. 2 The temperature at which it exceeds a certain value is defined as the shutdown temperature of the polyolefin microporous membrane. The heating is then continued from this shutdown temperature until the resistance returns to 1 kΩcm. 2 The temperature at which this occurred was defined as the meltdown temperature. Furthermore, the resistance value at 180°C was read and converted using the sample's measurement area (the area where the separator and spacer are in contact) to obtain the resistance value at 180°C (Ω·cm²). 2 The following was calculated: This measurement involved cutting out any two locations from a polyolefin microporous membrane, performing the above measurement on each location, and calculating the average value.

[0065] [Differential Scanning Calorimetry (DSC)] The crystal melting peak of the polyolefin microporous membrane was measured by differential scanning calorimetry (DSC). A 6.0 mg sample was sealed in an aluminum pan and held at 30 °C for 1 minute in a nitrogen gas atmosphere using a PYRIS Diamond DSC manufactured by PerkinElmer, and then the temperature was raised from 30 °C to 250 °C at a rate of 10 °C / min.

[0066] (Detection of peaks in the range of 155 °C to 250 °C) Using the DSC curve with the horizontal axis: temperature (°C) and the vertical axis: heat flow (mW) obtained from the above DSC measurement, the presence or absence of peaks in the range of 155 °C to 250 °C was determined. When the conditions described in (A) below were satisfied, it was determined that there was a peak, and when not satisfied, it was determined that there was no peak. (A) It has an extreme value between 155 °C and 250 °C, and the area of the peak forming the extreme value is 1 J / g or more.

[0067] In addition, for the area of the peak forming the extreme value shown in (A) above, let the temperature showing the extreme value be Tm, the point closest to Tm among the points where the slope becomes 0 on the lower temperature side than Tm be T1, and the point closest to Tm among the points where the slope becomes 0 on the higher temperature side than Tm be T2. The area is calculated by drawing a baseline between T and T and integrating the area enclosed by the peak forming the extreme value and the baseline. A and T B and integrating the area enclosed by the peak forming the extreme value and the baseline. (1) When T1 ≥ Tm - 15, T A = T1. (2) When T1 < Tm - 15, T A = Tm - 15. (3) When T2 ≤ Tm + 15, T A = T2. (4) When T2 > Tm + 15, T A = Tm + 15.

[0068] [Weight-average molecular weight of polyolefin resin, ratio of polyethylene component with molecular weight of 2 million or more in polyolefin microporous membrane] The weight-average molecular weight (Mw) of the polyolefin resin and the ratio of polyethylene components with a molecular weight of 2 million or more in the polyolefin microporous membrane were determined by gel permeation chromatography (GPC) under the following conditions. The ratio of the peak area corresponding to molecular weights of 2 million or more to the total peak area of ​​the differential molecular weight distribution curve of the polyolefin microporous membrane was calculated, and this was defined as the ratio of components with a molecular weight of 2 million or more in the polyolefin microporous membrane.

[0069] • Measuring device: Waters Corporation GPC-150C • Column: Showa Denko Corporation Shodex UT806M Column temperature: 135℃ • Solvent (mobile phase): o-dichlorobenzene • Solvent flow rate: 1.0 ml / min • Sample concentration: 0.1% by mass (Dissolution conditions: 135°C / 1h) Injection volume: 500 μl • Detector: Differential refractometer (RI detector) manufactured by Waters Corporation • Calibration curve: Created from a calibration curve obtained using monodisperse polystyrene standard samples, with a polyethylene conversion factor (0.46).

[0070] [Heat shrinkage rate after 8 hours at 105℃] A 5cm x 5cm square sample was cut from a polyolefin microporous membrane, and the distance between two pairs of parallel sides was measured and designated as L1 and L2. The distance between the two sides was measured by comparing the midpoints of each side. Next, the sample was placed in an oven heated to 105°C and heated, and removed after 8 hours. The distance at the location where L1 was measured was measured again and designated as L3, and the distance at the location where L2 was measured again and designated as L4. Using these values, the thermal shrinkage rate after 8 hours at 105°C was calculated using the following formula. This measurement was performed at three arbitrary locations within the polyolefin microporous membrane surface, and the average value was calculated as the thermal shrinkage rate (%) after 8 hours at 105°C. Formula: Thermal shrinkage rate (%) after 8 hours at 105℃ = {(L1-L3) / L1+(L2-L4) / +L2)} / 2×100.

[0071] [Domain diameter in polyolefin microporous membranes] Polyolefin microporous membranes were stained with RuO4, and thin sections were prepared using a microtome to observe the cross-section in the thickness direction and the median-scaled diameter (MD) direction. Observation was performed using a transmission electron microscope (JEOL JEM1400Plus) at an acceleration voltage of 100kV and a magnification of 50000. Since the staining differed in the resin A portion, resin B portion, and void portions, observation images with contrasting light and dark areas were obtained. In the above observation images, the domain area was calculated for the domain-like contrast regions originating from resin B, and the diameter was calculated assuming the cross-section of the domain was circular. The domain diameter was calculated by performing the above measurements and calculations for 10 arbitrary domains, and the average value was taken as the domain diameter (nm) in the polyolefin microporous membrane.

[0072] [Air permeability calculated based on a thickness of 1 μm] For polyolefin microporous membranes, in accordance with JIS P-8117:2009, the permeability (seconds / 100cm) was measured using a Wangyan type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) at an atmosphere of 25°C. 3 The thickness was measured using the method described above, and the air permeability (seconds / 100cm²) per 1 μm thickness was calculated using the following formula. 3 ) was calculated.

[0073] Formula: Air permeability (seconds / 100cm²) calculated based on a thickness of 1 μm 3 ) = Air permeability (sec / 100cm 3 ) / Thickness of polyolefin microporous membrane (μm) [Puncture strength calculated based on a thickness of 1 μm] Puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was set to 2 mm / second. Using a force gauge (DS2-20N, manufactured by Imada Corporation), the maximum load (gf) was measured when a 1.0 mm diameter needle with a spherical tip (radius of curvature R: 0.5 mm) was punctured into a polyolefin microporous membrane in a 25°C atmosphere. The thickness value measured by the method described above was used, and the value obtained from the following formula was defined as the puncture strength (gf) per 1 μm thickness.

[0074] Formula: Puncture strength (gf) per 1 μm thickness = Maximum load (gf) / Thickness of polyolefin microporous membrane (μm) [Soldering iron heat resistance test] A soldering iron heat resistance test was conducted to evaluate the safety of polyolefin microporous membranes used as battery separators against abnormal battery overheating. A high-power, compact, temperature-controlled soldering iron, model FX-951, manufactured by Hakko Corporation, was used, and the soldering tip was a Hakko Corporation T12-BC2 (cylindrical tip with a diameter of 2 mm). The soldering iron was held vertically with the tip facing downwards, on a stage that could be raised and lowered in the height direction. Next, a 5cm square was cut from the polyolefin microporous membrane. This cut-out membrane was placed on a donut-shaped stainless steel plate with an outer diameter of 5cm, an inner diameter of 1.5cm, and a thickness of 2mm. An evaluation sample was then prepared by placing the same stainless steel plate on top of the donut-shaped membrane. Care was taken to prevent wrinkles from forming in the cut-out polyolefin microporous membrane, and the center of the polyolefin microporous membrane was aligned with the center of the stainless steel plate.

[0075] The evaluation sample was placed on a 5cm square, 1mm thick stainless steel plate. The position and height of the stage and the position of the evaluation sample were adjusted so that the tip of the soldering iron was positioned 1cm vertically upward from the surface of the polyolefin microporous membrane in the center of the evaluation sample. After setting the soldering iron temperature to 400°C and waiting for more than 3 minutes, the stage holding the soldering iron was lowered vertically until it penetrated the polyolefin microporous membrane of the evaluation sample and the tip of the soldering iron touched the stainless steel plate. It was held in this position for 10 seconds, and then the soldering iron was raised to remove the evaluation sample.

[0076] [Analysis of soldering iron heat resistance test samples] The sample, after the solder heat resistance test described above, was attached to a paper frame measuring 2 mm thick, 7 cm square on the outside, and 5 cm x 3.5 cm on the inside, ensuring that no wrinkles formed. This was then placed on black drawing paper (Daio Paper Co., Ltd. C-55), and the perforations made by the soldering iron were observed using a digital microscope VH-900 (Keyence Corporation). The microscope's magnification and focus were adjusted to obtain an image in which the entire perforation area was clearly visible on the screen. The brightness range of the obtained image was adjusted using the digital microscope to detect the perforation area by brightness extraction, and the area of ​​the perforation region was calculated. Based on the obtained area of ​​the perforation, the safety of use as a battery separator was determined according to the following criteria, with a △ or ○ indicating a pass. 23mm 2 Less than: ○ (Good) 23mm 2 Above, 25mm 2 Less than: △ (Slightly good) 25mm 2 or more: × (defective).

[0077] Furthermore, if the MD and TD directions of the polyolefin microporous film to be measured are unknown in the aforementioned measurement, the polyolefin microporous film is positioned with one direction of the film plane as a reference, and the tensile breaking strength is determined for a total of seven directions from 0° to 90° by shifting the film by 15° increments, using the following method. The direction with the greatest tensile breaking strength is considered the MD direction, and the direction perpendicular to the direction with the greatest tensile breaking strength is considered the TD direction. [Tensile breaking strength] Tensile tests were conducted using a tensile testing machine (Shimadzu Autograph AGS-J type) in accordance with JIS K7127. The strength at which the sample broke was divided by the cross-sectional area of ​​the sample before the test to obtain the tensile breaking strength (MPa). The measurement conditions were: temperature; 23±2℃, sample shape; width 10mm × length 50mm, chuck distance; 20mm, tensile speed; 100mm / min. A paper frame with a width 40 × 60mm and a 20 × 20mm cutout in the center was used as the sample holder. A 10mm wide × 50mm long sample was clamped in the sample holder and chucked at a pressure of 0.4MPa. After that, both ends of the sample holder (the center of the two sides parallel to the sample among the four sides of the frame of the sample holder) were cut and measured. Three measurements were performed, and the average of the three measurements was taken as the tensile breaking strength.

[0078] (Example 1) Resin A has a weight-average molecular weight (Mw) of 1.5 × 10⁻⁶ 6 For resin B, we used high-density polyethylene with a melting point of 135°C, and as resin B, we used an amorphous resin, cyclic olefin copolymer (COC) (COC TOPAS® 6017S-04 manufactured by Polyplastics) with a glass transition temperature of 178°C.

[0079] A mixture of 16% by mass of resin A, 4% by mass of resin B, 0.2% by mass of the antioxidant tetrakis[methylene-3-(3,5-diter-butyl-4-hydroxyphenyl)-propionate]methane, and 79.8% by mass of liquid paraffin was placed in a kneading evaluation test apparatus (Laboplastmill, manufactured by Toyo Seiki Co., Ltd.) and melt-kneaded for 20 minutes at a temperature of 200°C and a cylinder rotation speed of 50 rpm to prepare a polyolefin resin composition. The polyolefin resin composition was sandwiched between "Teflon" (registered trademark) sheets and pressed at 200°C. After removing the sample, it was sandwiched between metal plates heated to 25°C and cooled to form a gel-like sheet.

[0080] The obtained gel-like sheet was cut into an 80 mm square, preheated at 115°C for 300 seconds, and simultaneously biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm / min so that the sheet was expanded five times in one direction and five times in the direction perpendicular to that direction (first stretching). The stretched film was washed in a methylene chloride washing tank to remove liquid paraffin, the washed film was dried in a drying oven adjusted to 20°C, and then heat-set (heat-treated) in an electric oven at 125°C for 10 minutes to obtain a polyolefin microporous film.

[0081] (Example 2) Resin A has a weight-average molecular weight (Mw) of 3.0 × 10⁻⁶ 5 60% by mass of high-density polyethylene with a melting point of 136°C, and a weight-average molecular weight (Mw) of 2.0 × 10⁶ 6 The procedure was carried out in the same manner as in Example 1, except that a mixture consisting of 40% by mass of high-density polyethylene with a melting point of 133°C was used, 20% by mass of resin A, 5% by mass of resin B, 0.2% by mass of the antioxidant tetrakis[methylene-3-(3,5-diter-butyl-4-hydroxyphenyl)-propionate]methane, and 74.8% by mass of liquid paraffin were mixed and placed into a kneading evaluation test apparatus (Laboplastmill manufactured by Toyo Seiki Co., Ltd.), and heat-fixed (heat-treated) in an electric oven at 120°C for 10 minutes, to obtain a polyolefin microporous membrane.

[0082] (Example 3) A polyolefin microporous film was obtained by following the same procedure as in Example 1, except that a polyolefin resin composition was prepared using 18% by mass of resin A and 2% by mass of resin B.

[0083] (Example 4) A polyolefin microporous membrane was obtained by performing the same procedure as in Example 1, except that the gel-like sheet was cut into an 80 mm square, and the sheet was simultaneously biaxially stretched by 7 times in one direction and 7 times in a direction perpendicular to the aforementioned direction.

[0084] (Example 5) A polyolefin microporous film was obtained by following the same procedure as in Example 1, except that a cyclic olefin polymer (COP) (COP ZEONOR® 1600R, manufactured by Nippon Zeon Co., Ltd.) with a glass transition temperature of 163°C was used.

[0085] (Example 6) Resin A has a weight-average molecular weight (Mw) of 1.5 × 10⁻⁶ 6 60% by mass of high-density polyethylene with a melting point of 135°C, and a weight-average molecular weight (Mw) of 9.0 × 10⁻⁶. 4 A polyolefin microporous membrane was obtained by carrying out the procedure in the same manner as in Example 1, except that a mixture consisting of 40% by mass of high-density polyethylene with a melting point of 132°C was used.

[0086] (Example 7) A polyolefin microporous film was obtained by following the same procedure as in Example 1, except that a mixture consisting of 75% by mass of COC (COC TOPAS® 6017S-04 manufactured by Polyplastics) with a glass transition temperature of 178°C and 25% by mass of COC (COC TOPAS® 8007S-04 manufactured by Polyplastics) with a glass transition temperature of 78°C was used as resin B.

[0087] (Comparative Example 1) A mixture of 25% by mass of resin A, 0.2% by mass of the antioxidant tetrakis[methylene-3-(3,5-ditter-butyl-4-hydroxyphenyl)-propionate]methane, and 74.8% by mass of liquid paraffin was placed into a kneading evaluation test apparatus (Laboplastmill, manufactured by Toyo Seiki Co., Ltd.), and the procedure was carried out in the same manner as in Example 2, except that resin B was not used, to obtain a polyolefin microporous membrane.

[0088] (Comparative Example 2) Resin A has a weight-average molecular weight (Mw) of 3.0 × 10⁻⁶ 5 40% by mass of high-density polyethylene with a melting point of 136°C, and a weight-average molecular weight (Mw) of 2.0 × 10⁻⁶. 6 The procedure was carried out in the same manner as in Example 2, except that a mixture consisting of 60% by mass of high-density polyethylene with a melting point of 133°C was used, to obtain a polyolefin microporous membrane.

[0089] (Comparative Example 3) A polyolefin microporous film was obtained by following the same procedure as in Example 1, except that polymethylpentene (TPX® MX002, manufactured by Mitsui Chemicals, Inc.), a crystalline resin, was used as resin B.

[0090] (Comparative Example 4) A polyolefin microporous film consisting of three layers, A / B / A, was fabricated. The A layer was resin A with a weight-average molecular weight of 3.0 × 10⁶ 5 High-density polyethylene with a melting point of 136°C is used as resin B, with a melting point of 160°C and a weight-average molecular weight of 4.0 × 10 5 Using polypropylene, 24% by mass of resin A, 6% by mass of resin B, and 0.2% by mass of tetrakis[methylene-3-(3,5-diter-butyl-4-hydroxyphenyl)-propionate]methane, an antioxidant, were fed into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42). 69.8% by mass of liquid paraffin [35 cst (40℃)] was supplied from the side feeder of the twin-screw extruder, and the mixture was melt-kneaded at 210℃ and 250 rpm to prepare the first polyolefin resin composition.

[0091] Layer B is resin A with a weight-average molecular weight (Mw) of 3.0 × 10 5 Using high-density polyethylene with a melting point of 136°C, 30% by mass of resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]methane, an antioxidant, were fed into a strong-kneading twin-screw extruder (inner diameter 58 mm, L / D = 42). 69.8% by mass of liquid paraffin [35 cst (40°C)] was supplied from the side feeder of the twin-screw extruder, and the mixture was melt-kneaded at 210°C and 250 rpm to prepare a second polyolefin resin composition.

[0092] After removing foreign matter from each twin-screw extruder by passing them through a filter, the first and second polyolefin solutions were extruded into a three-layer T-die (A / B / A) such that the first polyolefin resin composition formed layer A and the second polyolefin resin composition formed layer B. The discharge volume of each layer was adjusted using a gear pump so that the ratio of the layer thicknesses of A / B / A was 1 / 3 / 1. The extruded molded body was cooled while being taken up at a take-up speed of 2 m / min using a cooling roll heated to 30°C to form a gel-like sheet.

[0093] The obtained gel-like sheet was cut into an 80 mm square, preheated at 115°C for 300 seconds, and simultaneously biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm / min so that the sheet was expanded five times in one direction and five times in the direction perpendicular to that direction (first stretching). The stretched film was washed in a methylene chloride washing tank to remove liquid paraffin, the washed film was dried in a drying oven adjusted to 20°C, and then heat-set (heat-treated) in an electric oven at 125°C for 10 minutes to obtain a polyolefin microporous film.

[0094] (Comparative Example 5) The polyolefin microporous membrane obtained in Comparative Example 1 was irradiated with an electron beam at an acceleration voltage of 200 kV and an irradiation dose of 200 kGy to obtain a crosslinked polyolefin microporous membrane.

[0095] (Comparative Example 6) A mixture of 20% by mass of resin A, 0.2% by mass of the antioxidant tetrakis[methylene-3-(3,5-ditter-butyl-4-hydroxyphenyl)-propionate]methane, and 79.8% by mass of liquid paraffin was placed into a mixing evaluation test apparatus (Laboplastmill, manufactured by Toyo Seiki Co., Ltd.), and the procedure was carried out in the same manner as in Example 1, except that resin B was not used, to obtain a polyolefin microporous membrane.

[0096] (evaluation) Table 1 shows the polyolefin resin compositions and deposition conditions used in the production of each polyolefin microporous film, as well as the evaluation results for the obtained polyolefin microporous films. In Table 1, "-" in "Polyolefin Resin Composition" indicates that the component is not present. Furthermore, "Domain Diameter in Polyolefin Microporous Film" refers to "the domain diameter of the second phase, which consists of a polymer incompatible with the polyethylene resin and has a discontinuous structure, where the first phase, consisting of a polyethylene resin, has a continuous structure." "-" indicates that the presence of domains could not be confirmed and therefore measurement was not possible.

[0097] The polyolefin microporous membranes of Examples 1 to 7 contain polyethylene-based resin and amorphous resin incompatible with polyethylene, have a meltdown temperature of 155°C or higher, and do not exhibit a peak in the 155°C to 250°C range in the DSC curve obtained by differential scanning calorimeter (DSC). Therefore, they show good results in soldering iron heat resistance tests and can provide high safety even in the event of abnormal battery overheating when used as a separator for secondary batteries. On the other hand, the polyolefin microporous membranes of Comparative Examples 1 to 6 either do not contain polyethylene-based resin and amorphous resin incompatible with polyethylene, have a meltdown temperature of less than 155°C, or exhibit a peak in the 155°C to 250°C range in the differential scanning calorimeter, resulting in poor results in soldering iron heat resistance tests and insufficient safety when used as a separator for secondary batteries.

[0098] [Table 1]

[0099] [Table 2] [Industrial applicability]

[0100] When the polyolefin microporous membrane of the present invention is used as a battery separator, it can provide high safety against abnormal heat generation in batteries. Therefore, it can be suitably used as a separator for secondary batteries, especially those requiring high capacity.

Claims

1. The material contains 50 to 99% by mass of polyethylene resin and 1 to 50% by mass of an amorphous polymer incompatible with the polyethylene resin, has a meltdown temperature of 155°C or higher, and does not have a peak in the range of 155°C to 250°C in the DSC curve obtained by differential scanning calorimeter (DSC). The first phase, made of a polyethylene resin, has a continuous structure, and the second phase, made of a polymer incompatible with the polyethylene resin, has a discontinuous structure, with the second phase being a polyolefin microporous membrane having domain diameters of 10 to 300 nm.

2. The polyolefin microporous membrane according to claim 1, wherein the proportion of components with a molecular weight of 2 million or more in the molecular weight distribution curve of polyethylene measured by gel permeation chromatography is 15% or less.

3. A polyolefin microporous membrane according to claim 1 or 2, wherein the thermal shrinkage rate after 8 hours at 105°C is 10% or less.

4. A polyolefin microporous membrane according to any one of claims 1 to 3, wherein the shutdown temperature is 138°C or lower.

5. A polyolefin microporous membrane according to any one of claims 1 to 4, wherein the meltdown temperature is 165°C or higher.

6. A polyolefin microporous membrane according to any one of claims 1 to 5, wherein the amorphous polymer incompatible with the polyethylene resin is a cyclic olefin polymer.

7. Air permeability calculated based on a thickness of 1 μm is 25 seconds / 100 cm. ３ The polyolefin microporous membrane according to any one of claims 1 to 6, which is as follows:

8. A polyolefin microporous membrane according to any one of claims 1 to 7, wherein the puncture strength per 1 μm thickness is 10 gf or more.

9. A polyolefin microporous membrane according to any one of claims 1 to 8, wherein the thickness is 20 μm or less.

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