Polyolefin microporous membranes, battery separators, and secondary batteries

Abstract

Provided are: a microporous polyolefin film which contains a polyethylene-based resin as a main component, has an arithmetic mean roughness Sa(A) of 1.0 μm or less when measured by a scanning white-light interference microscopic method after a thermal treatment at 160°C, and has a piercing strength of 500 mN / (g / m2) or more in terms of basis weight; and a separator for batteries and a secondary battery, in each of which the microporous polyolefin film is used.

Description

[Technical Field]

[0001] This invention relates to separation membranes used for the separation and selective permeation of substances, and to polyolefin microporous membranes widely used as isolation materials in electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. Furthermore, this invention relates to battery separators and secondary batteries. [Background technology]

[0002] Polyolefin microporous membranes are mainly used as filters, fuel cell separators, and capacitor separators. They are particularly well-suited as separators for non-aqueous electrolyte secondary batteries, such as lithium-ion batteries widely used in notebook computers and mobile phones. This is because polyolefin microporous membranes possess excellent mechanical strength, shutdown temperature, and ion permeability.

[0003] Furthermore, the separator is required to have a function that ensures safety in the event of abnormal battery overheating. The shutdown temperature mentioned above is the function of interrupting the current by melting the polyolefin microporous membrane and clogging the pores, and a lower shutdown temperature is preferable. On the other hand, even after shutdown, the temperature inside the battery continues to rise for a certain period of time, but if the temperature is higher than the shutdown temperature, the separator may perforate and insulation may not be maintained, resulting in a meltdown phenomenon, and a higher temperature at which this meltdown occurs (meltdown temperature) is preferable.

[0004] In recent years, with the miniaturization of electronic devices and the expansion into in-vehicle applications as the main focus, the capacity of lithium-ion secondary batteries has been increasing. Along with the increase in battery capacity, the thermal stability of electrode materials used tends to decline. For separators used in such high-capacity batteries, laminated films in which a heat-resistant porous layer composed of inorganic particles, a binder, etc. is provided on a polyolefin microporous membrane are currently widely used in order to enhance heat resistance. On the other hand, while the laminated film prevents the thermal shrinkage of the separator and has a deterrent effect in cases where the electrodes are exposed at the battery ends, melt-down may occur due to pinholes in local parts of the separator, and further improvement in the melt-down temperature is required even for laminated films.

[0005] For example, Patent Document 1 describes a separator for a secondary battery that combines the shut-down characteristics of a polyethylene microporous membrane and the heat resistance of a polypropylene-containing layer by laminating a microporous membrane having polyethylene and polypropylene as essential components with a polyethylene microporous membrane.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Although the microporous membrane described in Patent Document 1 has improved heat resistance by blending polypropylene, which has a higher melting point than polyethylene, other basic properties as a separator, such as film strength, may be impaired due to the phase separation structure of polyethylene and polypropylene. Also, the heat resistance when a heat-resistant porous layer is provided on the microporous membrane is not mentioned.

[0008] The object of the present invention is to solve the above problems. That is, when a heat-resistant porous layer is provided and used as a separator for a battery as a laminated film, it is possible to impart high safety against abnormal heat generation of the battery, and by having excellent film strength, it is possible to reduce the resistance and increase the capacity of the battery. An object of the present invention is to provide a polyolefin microporous film.

Means for Solving the Problems

[0009] 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 film mainly composed of a polyethylene-based resin, having an arithmetic mean roughness Sa(A) measured by a scanning white light interference microscope method after heat treatment at 160°C of 1.0 μm or less, and a puncture strength in terms of basis weight of 500 mN / (g / m , , , , , , , 2 , ) or more. 〔2〕 The sum of the maximum shrinkage forces in terms of basis weight in the longitudinal and width directions measured by thermomechanical analysis is 12.0 mN / (g / m 2 ) or less, the polyolefin microporous film according to the above 〔1〕. 〔3〕[[ID=二十二]] When the arithmetic mean roughness before the heat treatment at 160°C is defined as Sa(B), the polyolefin microporous film according to the above 〔1〕 or 〔2〕, wherein Sa(A) / Sa(B) is 20 or less. 〔4〕 The polyolefin microporous film according to any one of the above 〔1〕 to 〔3〕, wherein the Sa(B) is 0.05 μm or more. 〔5〕 The polyolefin microporous film according to any one of the above 〔1〕 to 〔, having a surface porosity of 15% or more. 〔6〕 A polyolefin microporous membrane according to any one of items [1] to [5] above, wherein, in the differential molecular weight distribution curve of the polyethylene resin measured by gel permeation chromatography (GPC), the area ratio of polyethylene resin components with a molecular weight of 50,000 or less to the peak area of ​​all molecular weight components is 10% or more, and the area ratio of polyethylene resin components with a molecular weight of 1,000,000 or more is 10% or more. [7] A battery separator using a polyolefin microporous membrane as described in any one of items [1] to [6]. [8] A secondary battery using the battery separator described in [7]. [Effects of the Invention]

[0010] According to the present invention, when a heat-resistant porous layer is provided and used as a separator for batteries, it is possible to provide high safety against abnormal heat generation in batteries, and by having excellent permeability and film strength, it is possible to provide a polyolefin microporous membrane that enables lower resistance and higher capacity in batteries. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below.

[0012] [Polyolefin microporous membrane] The polyolefin microporous membrane according to the embodiment of the present invention is mainly composed of a polyethylene resin, has an arithmetic mean roughness Sa(A) of 1.0 μm or less measured by scanning white light interference microscopy after heat treatment at 160°C, and has a puncture strength of 500 mN / (g / m²) 2 That's all.

[0013] The polyolefin microporous film according to the embodiment of the present invention has an arithmetic mean roughness Sa(A) of 1.0 μm or less, measured by scanning white light interference microscopy after heat treatment at 160°C, preferably 0.5 μm or less, more preferably 0.2 μm or less, even more preferably 0.1 μm or less, and particularly preferably 0.06 μm or less. By setting the Sa(A) of the polyolefin microporous film after heat treatment at 160°C within the above range, the film shape retention performance, i.e., meltdown characteristics, when a heat-resistant porous layer is provided to form a laminated film is excellent, and the laminated film is safe when used as a separator for batteries. From the above viewpoint, the lower limit of Sa(A) after heat treatment at 160°C is not particularly limited, but from the viewpoint of compatibility with productivity, it is, for example, 0.01 μm or more. Sa(A) and Sa(B), described below, can be measured specifically by the method described in the examples. In order to achieve the above range for Sa(A) after heat treatment at 160°C, it is preferable to set the raw material composition and film formation conditions of the microporous film within the range described later.

[0014] In the polyolefin microporous film according to the embodiment of the present invention, it is preferable that Sa(A) / Sa(B) is 20 or less, where Sa(B) is the arithmetic mean roughness before heat treatment at 160°C, as measured by scanning white light interference microscopy. More preferably, Sa(A) / Sa(B) is 10 or less, even more preferably 5 or less, particularly preferably 2 or less, and most preferably 1 or less. By setting Sa(A) / Sa(B) within the above range, the film shape retention performance at high temperatures is excellent, and when a heat-resistant porous layer is provided on the polyolefin microporous film to form a laminated film and used as a separator for a battery, the adhesion with the heat-resistant porous layer is excellent, resulting in a battery with superior safety. From the above viewpoint, the lower limit of Sa(A) / Sa(B) is not particularly limited, but from the viewpoint of compatibility with productivity, it is for example 0.1 or more. In order to keep Sa(A) / Sa(B) within the above range, it is preferable to keep the raw material composition and film formation conditions of the microporous membrane within the range described later.

[0015] The polyolefin microporous membrane according to an embodiment of the present invention preferably has Sa(B) of 0.05 μm or more, more preferably 0.06 μm or more, still more preferably 0.08 μm or more, and particularly preferably 0.10 μm or more. By setting Sa(B) of the polyolefin microporous membrane within the above range, when a heat-resistant porous layer is provided on the polyolefin microporous membrane to form a laminated film and used as a separator for a battery, excellent adhesion to the heat-resistant porous layer can be obtained, and high safety can be imparted. From the above viewpoint, the upper limit of Sa(B) is not particularly limited, but from the viewpoint of compatibility with the strength and film-forming property of the microporous membrane, it is, for example, 0.5 μm or less. In order to set Sa(B) within the above range, it is preferable that the raw material composition and film-forming conditions of the polyolefin microporous membrane are within the ranges described later.

[0016] The polyolefin microporous membrane according to an embodiment of the present invention has a puncture strength in terms of basis weight of 500 mN / (g / m 2 ) or more, preferably 600 mN / (g / m 2 ) or more, more preferably 800 mN / (g / m 2 ) or more, and still more preferably 1000 mN / (g / m 2 ) or more. When the puncture strength in terms of basis weight is within the above range, when the microporous membrane is used as a separator for a battery, it is possible to increase the capacity by thinning the separator, and the resistance to impact from outside the battery and foreign matter inside the battery is enhanced, and the safety is excellent. The upper limit of the puncture strength in terms of basis weight is not particularly limited, but since it becomes easy to control Sa(A) after heat treatment at 160° C. within an appropriate range, for example, it is preferably 2000 mN / (g / m 2 ) or less.

[0017] Here, the puncture strength in terms of basis weight refers to the puncture strength L2 calculated by the formula: L2 = L1 / T1, where the puncture strength is L1 (mN) in a polyolefin microporous membrane with a basis weight of T1 (g / m 2 ).

[0018] Specifically, the puncture strength in terms of basis weight can be measured by the method described in the examples. In order to achieve the puncture strength calculated based on basis weight 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.

[0019] The polyolefin microporous film according to the embodiment of the present invention preferably has a surface porosity of 15% or more, more preferably 17% or more, even more preferably 20% or more, and particularly preferably 22% or more. By setting the surface porosity of the polyolefin microporous film within the above range, in addition to being able to reduce resistance when used as a battery separator, when a heat-resistant porous layer is provided on the polyolefin microporous film to form a laminated film and used as a battery separator, excellent adhesion with the heat-resistant porous layer can be provided, thus giving high safety. From the above viewpoint, there is no particular upper limit to the surface porosity, but from the viewpoint of achieving compatibility with the strength of the microporous film, it is for example 50% or less. The surface porosity can be evaluated and calculated using the method described below. In order to achieve the above-mentioned surface porosity, it is preferable to set the raw material composition and film formation conditions of the microporous film within the range described later.

[0020] The polyolefin microporous membrane according to the embodiment of the present invention has a maximum shrinkage force in the longitudinal and width directions, measured by thermomechanical analysis (TMA), which is 12.0 mN / (g / m²). 2 Preferably, it is less than or equal to 10 mN / (g / m³). 2 ) More preferably 8 mN / (g / m 2 ) or less. By setting the sum of the maximum shrinkage forces in the longitudinal and width directions (based on basis weight), as measured by TMA, of the polyolefin microporous membrane to the above range, when used as a battery separator, thermal shrinkage of the separator when the battery overheats can be suppressed, providing high safety. From the above viewpoint, the lower limit of the sum of the maximum shrinkage forces in the longitudinal and width directions (based on basis weight) is not particularly limited, but from the viewpoint of compatibility with the strength and permeability of the microporous membrane, for example, 1 mN / (g / m 2 That's all. The sum of the maximum shrinkage forces in the longitudinal and widthwise directions, converted to basis weight, can be measured specifically by the method described in the examples. In order to ensure that the sum of the maximum shrinkage forces in the longitudinal and widthwise directions, converted to basis weight, falls 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.

[0021] In the polyolefin microporous membrane according to the embodiment of the present invention, it is preferable that, in the differential molecular weight distribution curve of the polyethylene resin measured by the gel permeation chromatography (GPC) method described later, the area ratio of polyethylene resin components with a molecular weight of 50,000 or less to the peak area of ​​all molecular weight components is 10% or more, and the area ratio of polyethylene resin components with a molecular weight of 1,000,000 or more is 10% or more. The area ratio of polyethylene resin components with a molecular weight of 50,000 or less is more preferably 15% or more, and even more preferably 20% or more. Furthermore, the area ratio of polyethylene resin components with a molecular weight of 50,000 or less is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. The area ratio of polyethylene resin components with a molecular weight of 1,000,000 or more is more preferably 15% or more, and even more preferably 20% or more. Furthermore, the area ratio of polyethylene resin components with a molecular weight of 1,000,000 or more is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. By setting the amounts of polyethylene-based resin components with a molecular weight of 50,000 or less and polyethylene-based resin components with a molecular weight of 1,000,000 or more in the polyolefin microporous membrane to the above range, it is possible to increase the strength of the polyolefin microporous membrane while suppressing its thermal shrinkage, and when used as a separator for batteries, it exhibits excellent meltdown characteristics. In order to keep the amounts of polyethylene resin components with a molecular weight of 50,000 or less and polyethylene resin components with a molecular weight of 1,000,000 or more in the polyolefin microporous membrane within the above range, it is preferable to keep the raw material composition and mixing conditions of the microporous membrane within the range described later.

[0022] In the polyolefin microporous membrane according to the embodiment of the present invention, the area ratio of polyethylene resin components with a molecular weight of 2 million or more is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less, in the molecular weight distribution of polyethylene resin measured by the GPC method. Furthermore, the area ratio of polyethylene resin components with a molecular weight of 2 million or more is preferably 1% or more, and more preferably 3% or more. By setting the amount of polyethylene resin components with a molecular weight of 2 million or more in the polyolefin microporous membrane within the above range, it is possible to increase the strength of the polyolefin microporous membrane while suppressing its thermal shrinkage, and when used as a separator for batteries, it exhibits excellent meltdown characteristics.

[0023] The polyolefin microporous membrane according to the embodiment of the present invention preferably has an air permeability of 30 seconds / 100 cm in terms of thickness. 3 / μm or less, more preferably 20 seconds / 100cm 3 / μm or less, more preferably 15 seconds / 100cm 3 It is less than / μm. There is no specific lower limit set for air permeability when calculated based on thickness, but 1 second / 100cm is chosen because it makes it easier to achieve compatibility with film strength. 3 It is preferable that the permeability in terms of thickness is 1 / μm or greater. By setting the air permeability in terms of thickness to the above range, a microporous film with excellent charge-discharge characteristics can be obtained when the microporous film is used as a separator for batteries. The air permeability, calculated based on thickness, can be achieved within the above range by adjusting the raw material mixing ratio, stretching ratio, and heat-fixing conditions during the manufacturing process.

[0024] The thickness of the polyolefin microporous membrane according to the embodiment of the present invention can be appropriately adjusted depending on the application, but is preferably 20 μm or less, more preferably 15 μm or less, even more preferably 10 μm or less, and particularly preferably 8 μm or less. It is also preferably 2 μm or more. By setting the thickness of the polyolefin microporous membrane within the above range, both safety and high battery capacity can be achieved when used as a battery separator. The thickness can be set to the above range by appropriately adjusting the film formation conditions, such as the extrusion conditions.

[0025] The porosity of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. There is no particular upper limit to the porosity, but it is preferably 80% or less in order to suppress a decrease in membrane strength. With a porosity within the above range, the output characteristics are excellent when the microporous membrane is used as a separator for secondary batteries. The porosity can be set to the above range by adjusting the raw material formulation, stretching ratio, heat setting conditions, etc., during the manufacturing process.

[0026] The polyolefin microporous membrane according to the embodiment of the present invention preferably has a thermal shrinkage rate in the MD direction and TD direction of 15% or less, more preferably 10% or less, and even more preferably 8% or less after being stored at 120°C for 1 hour. If the thermal shrinkage rate of the polyolefin microporous membrane is within the above range, it provides excellent safety in the event of abnormal heat generation when the polyolefin microporous membrane is used as a separator for batteries. Furthermore, although there is no particular lower limit set for the thermal shrinkage rate in the MD direction and TD direction after being stored at 120°C for 1 hour, it is preferable that it be 0% or more in order to suppress the occurrence of wrinkles and sagging and maintain the quality of the film in a desirable state. In order to achieve the thermal shrinkage rate within the above range, it is preferable to keep the raw material composition, mixing conditions, and film formation conditions of the microporous membrane within the range described later.

[0027] The following describes the specific configuration of the polyolefin microporous membrane in this embodiment, but it is not necessarily limited to this configuration.

[0028] The polyolefin microporous membrane according to the embodiment of the present invention is mainly composed of polyethylene resin. The main component referred to here is the component that has the highest mass percentage content among the components constituting the polyolefin microporous membrane. The polyethylene 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 resin component in the polyolefin microporous membrane within the above range, the microporous membrane exhibits excellent film-forming properties and uniformity, while simultaneously providing an excellent balance of performance as a battery separator, such as film strength and permeability. The polyolefin microporous membrane may contain two or more types of polyethylene resin; in this case, the total amount of polyethylene resins should be considered the amount of polyethylene resin component constituting the polyolefin microporous membrane. The polyethylene resin content in the polyolefin microporous membrane can be measured by the method described later.

[0029] The polyolefin microporous membrane according to the embodiments of the present invention can use various polyethylene resins, including ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene. The polyethylene resin may be an ethylene homopolymer or a copolymer of ethylene and another α-olefin. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Here, the polyethylene resin contains more than 50 mol% of ethylene relative to the total raw material monomer components.

[0030] The polyolefin microporous membrane according to the embodiment of the present invention preferably contains ultra-high molecular weight polyethylene (hereinafter described as resin A) among the polyethylenes described above, and more preferably contains resin A and high-density polyethylene (hereinafter described as resin B).

[0031] The ultra-high molecular weight polyethylene used as resin A preferably has a weight-average molecular weight (Mw) of 800,000 or more, more preferably 900,000 or more, and even more preferably 1,000,000 or more. Furthermore, it is preferable that the weight-average molecular weight (Mw) be 2,500,000 or less, more preferably 2,000,000 or less, and even more preferably 1,400,000 or less. By setting the weight-average molecular weight of resin A within the above range, the thermal shrinkage of the polyolefin microporous membrane is suppressed, and when used as a battery separator, it offers superior safety in the event of abnormal heat generation.

[0032] The melting point of resin A is preferably 133°C or higher, and more preferably 135°C or higher. By setting the melting point of resin A within the above range, a microporous film with excellent permeability and strength is obtained. The melting point can be measured by the DSC method described later.

[0033] The content of resin A in the polyolefin microporous membrane is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more. Furthermore, it is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less. By setting the content of resin A in the polyolefin microporous membrane within the above range, when the polyolefin microporous membrane is used as a battery separator, it is possible to provide excellent battery safety by exhibiting superior membrane strength and suppressing thermal shrinkage.

[0034] High-density polyethylene (density: 0.940 g / m³) is used as resin B. 3 More than 0.970g / m 3The weight-average molecular weight (Mw) of the following is preferably 10,000 or more, more preferably 20,000 or more, and even more preferably 50,000 or more. Furthermore, the weight-average molecular weight (Mw) is preferably 200,000 or less, more preferably 150,000 or less, and even more preferably 100,000 or less. By setting the weight-average molecular weight of resin B within the above range, when a polyolefin microporous membrane is used as a battery separator, thermal shrinkage is suppressed, thereby providing excellent battery safety.

[0035] The melting point of resin B is more preferably 128°C or higher, and even more preferably 130°C or higher. It is also preferably 135°C or lower, and more preferably 134°C or lower. By setting the melting point of resin B within the above range, when a polyolefin microporous membrane is used as a battery separator, it is possible to provide excellent battery safety by simultaneously exhibiting superior shutdown performance and suppressing thermal shrinkage.

[0036] The heat of fusion (ΔH) of resin B, as measured by differential scanning calorimetry (DSC), is preferably 200 J / g or more, more preferably 210 J / g or more, and even more preferably 220 J / g or more. By setting the heat of fusion (ΔH) of resin B within the above range, when a polyolefin microporous film is used as a battery separator, the film strength can be increased while suppressing deformation of the film due to crystal melting, thereby providing excellent battery safety. From the above viewpoint, there is no particular upper limit set for the heat of fusion (ΔH) of resin B, but from the viewpoint of film-forming properties, it is preferably 280 J / g or less.

[0037] In the temperature distribution curve of the heat of crystalline melting of resin B, measured by differential scanning calorimetry (DSC), the full width at half maximum of the crystal melting peak is preferably 6°C or less, more preferably 5°C or less. It is also preferably 1°C or more, and more preferably 2°C or more. By setting it within this range, deformation of the polyolefin microporous film due to crystal melting is suppressed, resulting in superior safety when used as a separator for batteries.

[0038] The content of resin B in the polyolefin microporous membrane is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more. Furthermore, it is preferably 70% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. By setting the content of resin B in the polyolefin microporous membrane within the above range, when the polyolefin microporous membrane is used as a battery separator, it is possible to provide excellent battery safety by exhibiting superior membrane strength and suppressing thermal shrinkage.

[0039] The polyolefin microporous membrane according to the embodiment of the present invention may contain resins other than polyethylene resins, and for example, the addition of a polypropylene resin is preferable from the viewpoint of improving the heat resistance of the microporous membrane. In addition to homopolypropylene, block copolymers and random copolymers can also be used as types of polypropylene resins. Block copolymers and random copolymers may contain copolymer components with α-olefins other than propylene, and examples of α-olefins include ethylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. Here, the polypropylene resin contains more than 50 mol% of propylene relative to the total raw material monomer components. The amount of polypropylene resin added is preferably 20% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, based on the total mass of the polyolefin microporous membrane. By keeping it within the above range, a polyolefin microporous membrane with excellent productivity, quality, and strength is obtained.

[0040] The polyolefin microporous membrane may contain resin components other than polyethylene-based resins and polypropylene-based resins, as needed. Furthermore, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, blocking inhibitors, fillers, crystal nucleating agents, and crystallization retarders may be included, to the extent that they do not impair the effects of the present invention.

[0041] The polyolefin microporous film of the present invention is preferably used as a laminated film having one or more heat-resistant porous layers on at least one surface. The heat-resistant porous layer is not particularly limited, but preferably contains, for example, a binder made of resin and inorganic particles. Examples of binder components include acrylic resin, polyvinylidene fluoride resin, polyamide-imide resin, polyamide resin, aromatic polyamide resin, polyimide resin, polyvinyl alcohol resin, and cellulose ether resin. Examples of inorganic particles constituting the heat-resistant porous layer include alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, zeolite, glass filler, kaolin, talc, mica, titanium dioxide, calcium fluoride, and lithium fluoride.

[0042] The average particle size of inorganic particles is preferably 0.3 μm or more and 1.8 μm or less, and more preferably 0.5 μm or more and 1.5 μm or less. The average particle size can be measured using a laser diffraction or dynamic light scattering measuring device. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe can be measured with a particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.), and the average particle size can be taken as the value of the particle diameter (D50) when 50% of the smallest particles have been accumulated by volume. The shape of the particles can be spherical, approximately spherical, plate-like, or needle-like, but is not particularly limited.

[0043] In addition to the above raw materials, the heat-resistant porous layer may also contain components for adjusting wettability, such as surfactants, to improve coating properties.

[0044] The proportion of inorganic particles in the heat-resistant porous layer is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 95% by mass or more. It is also preferable that it be 99% by mass or less. By setting the proportion of inorganic particles in the heat-resistant porous layer within the above range, both heat resistance and permeability of the laminated film can be achieved.

[0045] The thickness of the heat-resistant porous layer is preferably 0.5 to 5 μm, and more preferably 1 to 4 μm, from the viewpoint of achieving both heat resistance when used as a battery separator and high battery capacity. The method for forming the heat-resistant porous layer is not particularly limited, but examples include the reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, blade coating method, and die coating method. Furthermore, after coating a polyolefin microporous film with the above method, the heat-resistant porous layer can be formed by drying the solvent under conditions of a drying temperature of 40 to 100°C and a drying time of 3 to 120 seconds.

[0046] The solid content concentration of the coating solution for forming the heat-resistant porous layer is not particularly limited as long as it can be applied uniformly, but it is preferably 20% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 80% by mass or less. The solvent used in the coating solution is not particularly limited as long as it can uniformly disperse the binder and inorganic particles mentioned above, but examples include water, alcohols, and acetone.

[0047] [Method for manufacturing polyolefin microporous membranes] 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.

[0048] 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. 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).

[0049] (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.

[0050] (1) Preparation of polyolefin resin composition 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 is a solvent that can uniformly disperse the polyolefin resin, 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.

[0051] The blending ratio of polyolefin resin to plasticizer is preferably such that the polyolefin resin content is 10 to 50% by mass relative to the total mass of the polyolefin resin composition. By setting the polyolefin resin content within this range, the dispersion state of the polyolefin resin and plasticizer is improved, resulting in excellent strength, permeability, and heat resistance of the resulting microporous film. Furthermore, 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.

[0052] The melt-mixing of polyolefin resin and plasticizer is preferably carried out in a twin-screw extruder from the viewpoint of obtaining a uniform mixed state. Furthermore, adjusting the distance between the screw tip and the vent hole, the distance between the vent holes, and the screw configuration of the twin-screw extruder is preferable from the viewpoint of suppressing torque fluctuations during extrusion and uniformly dispersing polyolefin resin and plasticizer consisting of multiple types. Specifically, when the outermost diameter of the screw is D, it is preferable to use at least one screw piece in which the distance between the screw tip and the vent hole is 1.0D to 15.0D, and the length in the raw material conveying direction between them is 0.2D to 0.9D, and not to use two or more screw pieces in which the length in the raw material conveying direction is 1.5D or more.

[0053] The resin temperature during kneading is preferably 150°C or higher, more preferably 160°C or higher, and even more preferably 180°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 the polyolefin resin and plasticizer can be uniformly melt-kneaded.

[0054] 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.

[0055] (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. The extrusion method may be either a flat die method or an 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.

[0056] 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.

[0057] The neck-in ratio of the sheet-shaped melt-extruded resin is preferably 70% or more, more preferably 80% or more, and even more preferably 85% or more. The neck-in ratio is a value calculated by the formula: A / B × 100, where A is the width of the gel-like sheet after cooling and solidification, and B is the width of the die discharge port. By setting the neck-in ratio within the above range, the molecular chain orientation of the resin during melt extrusion is suppressed, and crystallization is delayed, resulting in a rougher surface of the microporous film. A microporous film with a high degree of roughness has excellent adhesion to the heat-resistant porous layer, and when used as a battery separator, it can result in a battery with superior safety. From the above viewpoint, there is no particular upper limit set for the neck-in ratio, but from the viewpoint of film formation stability, it is preferably 99% or less. The neck-in ratio can be set within the above range by adjusting the formulation of the polyolefin resin composition, adjusting the resin temperature during extrusion, adjusting the distance between the casting drum and the die lip, or by assisting the adhesion between the casting drum and the gel-like sheet with an air knife or air chamber.

[0058] (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.

[0059] 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).

[0060] 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 improved. In addition, the area stretching ratio in this process is preferably 100 times or less, more preferably 64 times or less, which prevents film breakage and allows for the production of a polyolefin microporous film with excellent film strength. 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.

[0061] 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.

[0062] The temperature of crystal dispersion (TCD) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. When polyethylene-based resins are used as polyolefin resins, ultra-high molecular weight polyethylene, polyethylenes other than ultra-high molecular weight polyethylene, and polyethylene resin compositions have a temperature of crystal dispersion of about 100 to 110°C. Therefore, 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.

[0063] The stretching described above causes cleavage between the polyethylene lamellars, 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.

[0064] (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 Publication No. 2002-256099 can be used.

[0065] (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).

[0066] (6) Second stretching process The dried polyolefin microporous membrane may be stretched in at least one axial direction (second stretching step). The polyolefin microporous membrane 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 150°C or lower, and even more preferably 140°C or lower. The stretching of the polyolefin microporous membrane 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.

[0067] The area stretching ratio in this process is preferably 4.0 times or less, more preferably 2.0 times or less, even more preferably 1.7 times or less, and particularly preferably 1.5 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.

[0068] (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 for the heat treatment method, a heat-setting treatment and / or a heat-relaxation treatment can be used. A heat-setting treatment is a heat treatment in which the film is heated while being held so that its dimensions do not change. A heat-relaxation treatment is a heat treatment in which the film is thermally contracted in the MD direction or TD direction while being heated. 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 dimensions of the film after the relaxation treatment by the dimensions of the film before the relaxation treatment, and the relaxation rate of the film in both the MD and TD directions is 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 film, it is preferably 0.80 or more, and more preferably 0.90 or more. 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 using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (1987).

[0069] 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.

[0070] [Battery separators and secondary batteries] The present invention also relates to a battery separator using the polyolefin microporous membrane described above, and more preferably to a battery separator using a laminated film having one or more heat-resistant porous layers on at least one surface of the polyolefin microporous membrane. When a heat-resistant porous layer is provided and used as a battery separator, it is possible to provide high safety against abnormal heat generation in the battery, and the excellent permeability and film strength make it possible to reduce the resistance and increase the capacity of the battery. Details of the heat-resistant porous layer are as described above. The present invention also relates to a secondary battery using the above-mentioned battery separator. [Examples]

[0071] 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. Unless otherwise specified, evaluations in this application were performed under conditions of 23°C and 65% humidity. The evaluation and analysis methods used in the examples are as follows.

[0072] [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).

[0073] [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

[0074] [Air permeability] For polyolefin microporous membranes, in accordance with JIS P-8117:2009, the permeability (seconds / 100cm) was measured using a Wang Ren type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) at an atmosphere of 25°C. 3 The following was measured: In addition, the air permeability in terms of thickness was calculated by dividing by the thickness (μm) of the polyolefin microporous membrane measured by the method described above.

[0075] [Puncture strength converted to basis weight] 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 Co., Ltd.), the maximum load (mN) 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 value obtained from the following formula was converted to puncture strength (mN / (g / m²)) in basis weight. 2 )) Formula: Puncture strength converted to basis weight (mN / (g / m) 2 )) = Maximum load (mN) / Basis weight of polyolefin microporous membrane (g / m 2 ) The basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm x 50 mm square sample from the polyolefin microporous membrane, measuring its mass (g) at room temperature (25°C), and using the following formula. Formula: Basis weight (g / m 2 )=mass(g) / (50(mm)×50(mm))×10 6

[0076] [Gel Permeation Chromatography (GPC)] The weight-average molecular weight (Mw) of the polyolefin resin, the area ratio of polyethylene components with a molecular weight of 50,000 or less in the polyolefin microporous membrane to the peak area of ​​all molecular weight components, the area ratio of polyethylene components with a molecular weight of 1 million or more to the peak area of ​​all molecular weight components, and the area ratio of polyethylene components with a molecular weight of 2 million or more to the peak area of ​​all molecular weight components were determined by the GPC method under the following conditions. In the differential molecular weight distribution curve obtained by the GPC method, the area ratio of each molecular weight component was determined from the ratio of the area of ​​each molecular weight region to the peak area of ​​all molecular weight components. • 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).

[0077] [Differential Scanning Calorimetry (DSC)] A 6.0 mg sample was sealed in an aluminum pan, and using a Parking Elmer PYRIS Diamond DSC, the temperature was increased from 30°C to 230°C at a rate of 10°C / min (first heating), held at 230°C for 5 minutes, cooled at a rate of 10°C / min, and then heated again from 30°C to 230°C at a rate of 10°C / min (second heating). The heat of fusion of the raw material polyolefin resin, the full width at half maximum of the crystalline melting peak, and the melting point were calculated from the crystalline melting peak obtained by drawing a baseline between 60°C and 200°C in the temperature distribution curve of the endothermic amount measured during the second heating step of the DSC measurement described above. The melting point was defined as the temperature at which the maximum endothermic amount was observed, and the heat of fusion was calculated from the area of ​​the crystalline melting peak. Furthermore, the polyethylene resin content of the polyolefin microporous membrane was calculated using the following formula from the area of ​​the crystal melting peak (ΔH1) obtained by drawing a baseline between 60°C and 155°C, and the area of ​​the crystal melting peak (ΔH2) obtained by drawing a baseline between 155°C and 200°C, in the temperature distribution curve of the endothermic amount measured during the second heating step of the DSC measurement described above: Polyethylene resin content of polyolefin microporous membrane (%) = 100 × ΔH1 / (ΔH1 + ΔH2)

[0078] [Method for measuring Sa(A) (Preparation of evaluation samples)] A polyolefin microporous membrane was cut into 15 mm squares and attached to the center of a 20 mm long piece of polyimide tape (API-114AFR, manufactured by Chuko Kasei Co., Ltd., tape width 19 mm), ensuring that no wrinkles formed. The polyolefin microporous membrane attached to the polyimide tape was placed with the polyimide tape side down on a 5 cm square, 2 mm thick aluminum plate, and the four sides were secured to the aluminum plate with the polyimide tape (API-114AFR, manufactured by Chuko Kasei Co., Ltd., tape width 19 mm) to prevent wrinkles. When securing the four sides, the polyimide tape was applied so that approximately 2 mm of the outer circumference of the polyolefin microporous membrane was secured. The above sample was placed in an oven with a chamber temperature of 160°C and removed after 15 minutes. In addition to the first evaluation sample prepared, a sample was also prepared with the polyolefin microporous membrane facing the opposite side from when the tape was first attached, resulting in a total of two evaluation samples.

[0079] [Method for measuring Sa(A)] The samples prepared using the method described above were attached to aluminum plates, and their surface roughness was measured using a Hitachi High-Tech Science VS-1540 scanning white light interference microscope. The measurements were carried out under the conditions shown below, and the three-dimensional surface roughness parameter Sa(A) was calculated in accordance with ISO 25178. Four measurements were performed on each evaluation sample under the same conditions for the two prepared samples, and the obtained values ​​were averaged. • Objective lens: 5x • Lens barrel: 0.5x • Wavelength filter: 530 white • Camera: High resolution • Measurement mode: Wave • Cut-off: None

[0080] [Method for measuring Sa(B)] A 5cm x 5cm polyolefin microporous membrane was cut and attached to a metal frame with outer dimensions of 6cm x 6cm and inner dimensions of 3cm x 3cm, ensuring that no wrinkles formed. Using a Hitachi High-Tech Science VS-1540 scanning white light interference microscope, Sa(B) was calculated in the same manner as described above. Under the same conditions, measurements were taken at 4 points on each surface of the polyolefin microporous membrane, for a total of 8 points, and the measured values ​​were averaged.

[0081] [Surface porosity] A Pt-deposited polyolefin microporous film was observed using a scanning electron microscope (JEOL Ltd. JSM-6701F) with an acceleration voltage of 2kV, a working distance (WD) of 8μm, and a magnification of 10,000x in SEI mode to obtain a secondary electron image of the surface. The image used for binarization was an 8-bit (256-level) grayscale image with an observation area of ​​11.7μm × 9.4μm (1280 pixels × 1024 pixels). By performing binarization using MVTec Software's HALCON 13, the area ratio of open regions (surface porosity) on the microporous film surface was extracted. The image processing method involved denoising the surface SEM image by averaging 3x3 pixels, then dynamically binarizing the resulting 21x21 pixel averaged image with a threshold of -30 tones to extract dark areas. All independent dark areas were counted as holes, and the area of ​​all counted holes was summed up to calculate the area of ​​open areas within the SEM observation region (11.7 μm × 9.4 μm = 110 μm). 2 The surface porosity was defined as the ratio of the open area to the total surface area. The above measurement was performed by taking 5 measurements on each surface of the polyolefin microporous film, for a total of 10 measurements, and averaging them.

[0082] [Sum of maximum contraction force converted to basis weight in the longitudinal and widthwise directions] A thermomechanical analyzer (Seiko Electronics Industry Co., Ltd., TMA / SS6100) was used to set the test specimen so that the sample dimensions of the measurement section were 10 mm in length and 3 mm in width. After applying an initial load of 9.8 mN to the sample, the deformation of the sample was kept constant, and the temperature was increased from room temperature (23°C) to 160°C at a heating rate of 5°C / min. The change in the shrinkage force of the sample in response to temperature changes was measured by detecting the load on the probe. The value at which the shrinkage force was greatest between room temperature and 160°C was defined as the maximum shrinkage force. For the maximum shrinkage force in the longitudinal direction, the above measurement was performed by sampling so that the length direction of the test specimen was the longitudinal direction of the polyolefin microporous membrane, and the average value of three measurements was calculated using the same procedure. For the maximum shrinkage force in the width direction, the above measurement was performed by sampling so that the length direction of the test specimen was the width direction of the polyolefin microporous membrane, and the average value of three measurements was calculated using the same procedure. From the maximum shrinkage forces in the longitudinal and width directions obtained by the above method, the sum of the maximum shrinkage forces in the longitudinal and width directions, converted to basis weight, was calculated using the following formula. Formula: Sum of maximum shrinkage force converted to basis weight in the longitudinal and widthwise directions (mN / (g / m) 2 )) = (Maximum contraction force in the longitudinal direction (mN) + Maximum contraction force in the width direction (mN)) / Basis weight of polyolefin microporous membrane (g / m 2 )

[0083] [Thermal shrinkage rate] Square samples measuring 100 mm in both the MD and TD directions were cut from the polyolefin microporous membrane. Next, the sample was placed in an oven heated to 120°C and heated. After 1 hour, it was removed, and the sample length in the MD and TD directions was measured. The length in the MD direction after being placed in the oven was L MD (mm), length in the TD direction is L TD The thermal shrinkage rate after storage at 120°C for 1 hour was calculated using the following formula, assuming a dimension of (mm). This measurement was performed at three arbitrary locations within the surface of the polyolefin microporous membrane, and the average value was calculated. Equation 1: Thermal shrinkage rate (%) after storage at 120°C for 1 hour in the MD direction = {(100-L MD ) / 100)×100 Equation 2: Thermal shrinkage rate (%) after storage at 120°C for 1 hour in the TD direction = {(100-L TD ) / 100)×100

[0084] [Meltdown temperature of polyolefin microporous membranes] 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.

[0085] 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 [mass ratio]) to a concentration of 1 mol / L (manufactured by Kishida Chemical Co., Ltd.), and adding 0.3 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 was placed on top of the sample for measurement in the hollow part of the gasket, and the cell was left to stand for 1 minute under a gauge pressure of -50 kPa twice to impregnate the polyolefin microporous membrane with the electrolyte. After that, a wave washer and an upper lid were placed on top of 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.

[0086] 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. For this measurement, two arbitrary locations were cut from the polyolefin microporous membrane, and the above measurement was performed on each, and the average value was calculated. Furthermore, based on the measured meltdown temperature and the resistance value at 180°C, the safety of use as a battery separator was determined according to the following criteria, with A, B, and C being considered acceptable. A: Meltdown temperature is 180°C or higher, and resistance at 180°C is 10kΩcm. 2 That's all. B: Meltdown temperature 180°C or higher, and resistance value at 180°C is 1 kΩcm 2 The above 10kΩcm 2 less than C: Meltdown temperature is between 170°C and 180°C D: Meltdown temperature between 160°C and 170°C E: Meltdown temperature below 160°C

[0087] [Meltdown temperature of multilayer films] Acrylic emulsion (Showa Denko K.K., "Polysol" (registered trademark) AT-731, 47% non-volatile content), alumina particles with an average particle size of 0.5 μm, and deionized water were mixed in a mass ratio of 2:55:43. This mixture was placed in a polypropylene container with zirconium oxide beads and dispersed for 12 hours using a paint shaker (Toyo Seiki Seisakusho Co., Ltd.). The mixture was then filtered through a filter with a filtration limit of 5 μm to obtain a coating solution. The coating solution was applied to a polyolefin microporous membrane using a wire bar and dried in a hot air oven set to 50°C for 1 minute to obtain a laminated film with a heat-resistant porous layer. The wire bar was selected and applied so that the coating thickness after drying of the heat-resistant porous layer was 3 μm. The meltdown temperature of the laminated film obtained using the above procedure was measured according to the method described above.

[0088] [Example 1] For the polyolefin raw material, resin A has Mw of 1.0 × 10 6 70% by mass of ultra-high molecular weight polyethylene with a melting point of 136°C, and resin B with Mw of 6 × 10 430% by mass of high-density polyethylene with a melting point of 132°C, a ΔH of 220 J / g, and a full width at half maximum of the crystal melting peak of 4°C was used. 75% by mass of liquid paraffin was added to 25% by mass of the above polyolefin raw material, and then 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 polyolefin resin composition. The obtained polyolefin resin composition was fed into a twin-screw extruder with the screw configuration shown in Table 1 and kneaded at 180°C to prepare a polyethylene solution. The obtained polyethylene solution was supplied to a T-die heated to 200°C, and the extruded material was cooled in a casting drum controlled to 35°C to form a gel-like sheet. During the formation of the gel-like sheet, the sheet transport direction was defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction within the film surface was defined as the width direction. At this time, the distance between the apex of the casting drum and the die lip was adjusted to achieve a sheet neck-in ratio of 85%. The obtained gel-like sheet was cut into 80 mm squares and set in a batch-type biaxial stretcher. After preheating at 115°C for 300 seconds, the sheet was simultaneously biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm / min so that the sheet was eight times larger in both the longitudinal and width directions. The stretched film was washed in a methylene chloride washing tank to remove liquid paraffin, and the washed film was dried in a drying oven adjusted to 20°C. A polyolefin microporous film was obtained by heat setting treatment (heat treatment) in an electric oven at 125°C for 10 minutes.

[0089] [Example 2] For the polyolefin raw material, resin A has Mw of 1.0 × 10 6 , 90% by mass of ultra-high molecular weight polyethylene with a melting point of 136°C, and resin B with Mw of 6 × 10 4 The procedure was carried out in the same manner as in Example 1, except that 10% by mass of high-density polyethylene with a melting point of 132°C, a ΔH of 220 J / g, and a full width at half maximum of the crystal melting peak of 4°C was used.

[0090] [Example 3] For the polyolefin raw material, resin A has Mw of 1.5 × 10 6 80% by mass of ultra-high molecular weight polyethylene with a melting point of 136°C, and resin B with Mw of 6 × 10 4 The procedure was carried out in the same manner as in Example 1, except that 20% by mass of high-density polyethylene with a melting point of 132°C, a ΔH of 220 J / g, and a full width at half maximum of the crystal melting peak of 4°C was used.

[0091] [Example 4] The procedure was carried out in the same manner as in Example 1, except that the distance between the apex of the casting drum and the die lip was adjusted during gel sheet formation to adjust the sheet neck-in ratio to 91%.

[0092] [Example 5] The gel sheet was simultaneously biaxially stretched to 7 times its length and 7 times its width. After removing the liquid paraffin and washing the film, it was dried in a drying oven adjusted to 100°C for 10 minutes. It was then cut into 80 mm squares, set in a batch-type biaxial stretcher, preheated at 130°C for 10 minutes, and heat-set. The process was carried out in the same manner as in Example 1, except that it was stretched to 2.0 times its width at a stretching temperature of 130°C and a stretching speed of 1000 mm / min, and then heat-relaxed to 0.90 times its width.

[0093] [Comparative Example 1] For the polyolefin raw material, resin A has Mw of 2.5 × 10 6 , 40% by mass of ultra-high molecular weight polyethylene with a melting point of 133°C, and resin B with Mw of 3.5 × 10 5 The procedure was carried out in the same manner as in Example 1, except that 60% by mass of high-density polyethylene with a melting point of 135°C, a ΔH of 190 J / g, and a full width at half maximum of the crystal melting peak of 6°C was used, a 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 7 times larger in the longitudinal direction and 7 times larger in the width direction.

[0094] [Comparative Example 2] For the polyolefin raw material, resin A has Mw of 1.0 × 10 6 The procedure was carried out in the same manner as in Example 1, except that 100% by mass of ultra-high molecular weight polyethylene with a melting point of 136°C was used.

[0095] [Comparative Example 3] For the polyolefin raw material, resin B has a Mw of 6.0 × 10 4 In addition, 25% by mass of high-density polyethylene (resin B) with a melting point of 132°C, a ΔH of 220 J / g, and a full width at half maximum of the crystal melting peak of 4°C was used, and separately from resin A and resin B, Mw was 1.0 × 10 6 The procedure was carried out in the same manner as in Example 1, except that 5% by mass of polypropylene (PP) was used.

[0096] [Comparative Examples 4-6] As shown in Table 3, the procedure was carried out in the same manner as in Example 1, except that the distance between the screw tip and the vent hole and the configuration of the extrusion screw were changed.

[0097] [Comparative Example 7] For the polyolefin raw material, resin A has Mw of 1.0 × 10 6 , 50% by mass of ultra-high molecular weight polyethylene with a melting point of 136°C, and resin B with Mw of 6 × 10 4 The procedure was carried out in the same manner as in Example 1, except that 50% by mass of high-density polyethylene with a melting point of 132°C, a ΔH of 220 J / g, and a full width at half maximum of the crystal melting peak of 4°C was used, a gel-like sheet was cut into 80 mm squares, set in a batch-type biaxial stretcher, preheated at 110°C for 300 seconds, simultaneously biaxial stretched at a stretching temperature of 110°C, the liquid paraffin was removed, the washed film was dried in a drying oven adjusted to 20°C, and then heat-set (heat-treated) in an electric oven at 120°C for 10 minutes.

[0098] [Comparative Example 8] The procedure was carried out in the same manner as in Example 1, except that the distance between the apex of the casting drum and the die lip was adjusted during gel-like sheet formation to adjust the sheet's neck-in ratio to 69%.

[0099] Examples and comparative examples are shown in Tables 1 to 4.

[0100] [Table 1]

[0101] [Table 2]

[0102] [Table 3]

[0103] [Table 4]

[0104] Examples 1-5 are primarily composed of polyethylene, and after heat treatment at 160°C, the arithmetic mean roughness Sa(A), measured by scanning white light interference microscopy, is 1.0 μm or less, and the puncture strength (based on basis weight) is 500 mN / (g / m²). 2 ) and above. Therefore, good results were obtained in the evaluation of meltdown characteristics after the heat-resistant porous layer was provided. On the other hand, comparative examples 1, 2, 7, and 8 did not meet the above range in Sa(A) or puncture strength converted to basis weight, and the meltdown characteristics after the heat-resistant porous layer was provided were poor. In addition, comparative examples 3 to 6 did not yield uniform and high-quality samples, and in particular comparative examples 4 and 5 did not yield samples worthy of evaluation as microporous membranes. Furthermore, in comparative examples 3 and 6, due to the unevenness of the polyolefin microporous membrane, it was not possible to uniformly form a heat-resistant porous layer on the polyolefin microporous membrane, and the meltdown characteristics were poor. [Industrial applicability]

[0105] When the polyolefin microporous membrane of the present invention is used as a battery separator in a laminated film with a heat-resistant porous layer, it can provide high safety against abnormal heat generation in batteries and has excellent film strength, making it particularly suitable for use as a separator in secondary batteries where high capacity is required.

[0106] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2021-160022 filed on September 29, 2021, the contents of which are incorporated herein by reference.

Claims

1. The polyethylene resin is the main component, and in the differential molecular weight distribution curve of the polyethylene resin measured by gel permeation chromatography (GPC), the area ratio of polyethylene resin components with a molecular weight of 50,000 or less to the peak area of ​​all molecular weight components is 10% or more, and the area ratio of polyethylene resin components with a molecular weight of 1,000,000 or more is 10% or more, the arithmetic mean roughness Sa(A) measured by scanning white light interference microscopy after heat treatment at 160°C is 1.0 μm or less, and the puncture strength on a basis weight basis is 500 mN / (g / m). ２ A polyolefin microporous membrane having a surface area of ​​) or greater.

2. The sum of the maximum shrinkage force in the longitudinal and width directions, converted to basis weight, as measured by thermomechanical analysis, is 12.0 mN / (g / m). ２ The polyolefin microporous membrane according to claim 1, wherein the following conditions apply.

3. The polyolefin microporous film according to claim 1 or 2, wherein Sa(A) / Sa(B) is 20 or less, when Sa(B) is the arithmetic mean roughness before the heat treatment at 160°C.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the Sa(B) is 0.05 μm or larger.

5. A polyolefin microporous membrane according to claim 1 or 2, wherein the surface porosity is 15% or more.

6. A battery separator using a polyolefin microporous membrane according to claim 1 or 2.

7. A secondary battery using the battery separator described in claim 6.

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