Polyolefin microporous membrane, separator for batteries, secondary battery, filtration filter for liquid, and filtration unit

Abstract

[Problem] The present invention provides a polyolefin microporous membrane which is capable of providing high output characteristics and high safety and is able to contribute to size reduction and output enhancement of a battery if used as a separator for batteries, and which is capable of eliminating trade-off between a smaller pore diameter and a higher permeability if used as a filtration filter for a liquid. [Solution] Disclosed is a polyolefin microporous film which satisfies formula 1 and formula 2, where among the arithmetic average roughnesses Sa (nm) of one surface and the other surface of the microporous film, Sa1 is the Sa of a surface having a larger arithmetic average roughness and Sa2 is the Sa of a surface having a smaller arithmetic average roughness. The polyolefin microporous film satisfies formula 3, where Ds is the median value of pore diameters based on the surface area and Dv is the median value of pore diameters based on the pore volume as determined by pure water intrusion porosimetry, and the polyolefin microporous film has a basis weight-converted puncture strength of 700 mN / (g / m2) or more.　Formula 1: 10 ≤ (Sa1 + Sa2) / 2 ≤ 80 Formula 2: Sa1 / Sa2 ≤ 1.4 Formula 3: 1.0 ≤ Dv / Ds ≤ 1.2

Description

Polyolefin microporous membranes, battery separators, secondary batteries, liquid filters and filtration units

[0001] The present invention relates to a polyolefin microporous membrane that is widely used as a separation membrane for substance separation, selective permeation, etc., and as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. The present invention also relates to a battery separator, a secondary battery, a liquid filter, and a filtration unit.

[0002] Polyolefin microporous membranes are primarily used as filters, fuel cell separators, and capacitor separators. They are particularly suitable for use as separators for nonaqueous electrolyte secondary batteries such as lithium ion batteries, which are widely used in notebook personal computers, mobile phones, etc. This is due to the excellent mechanical strength, shutdown temperature, and ion permeability of polyolefin microporous membranes.

[0003] Furthermore, due to their uniform fine pore structure and excellent solvent and chemical resistance, polyolefin microporous membranes are also widely used in a variety of filter applications, including water treatment membranes, ultrafiltration membranes, microfiltration membranes, and breathable waterproof clothing.

[0004] For lithium-ion secondary batteries, there is a growing need for miniaturization and higher output for convenience in various applications, and this has led to a demand for thinner separators with lower resistance. Because thinner separators with lower resistance reduce their strength, they are more susceptible to short circuits caused by electrodes or foreign objects (foreign object resistance) and membrane rupture (reduced impact resistance) when the battery is subjected to impact, reducing battery safety. Therefore, there is a demand for designs that make the resin portion of the microporous membrane stronger than conventional ones.

[0005] Furthermore, because lithium deposition (dendrites) is prone to occur in small, high-power lithium-ion secondary batteries, there is a growing demand for separators that are dendrite-resistant. Dendrites in lithium-ion secondary batteries are needle-shaped crystals that form near the interface between the negative electrode and separator during charging and discharging. If they grow and penetrate the separator, they can cause a short circuit. Therefore, batteries prone to dendrite formation require separators with small pores that are less likely to be penetrated by dendrites. On the other hand, reducing the pore size of a microporous membrane can reduce permeability and worsen resistance.

[0006] Furthermore, separators are also required to have the function of ensuring safety in the event of abnormal battery heat generation. The shutdown function described above refers to the function of the polyolefin microporous membrane melting and clogging the pores when the battery abnormally heats up, thereby interrupting the current. Since the risk of abnormal heat generation is particularly high in high-power batteries, it is preferable to shut down at a lower temperature. However, lowering the shutdown temperature of a polyolefin microporous membrane generally requires a design that reduces the crystalline orientation and crystallinity of the polyolefin, which can result in reduced strength and permeability.

[0007] For liquid filtration applications, such as removing foreign matter in semiconductor manufacturing processes, there is a demand for filters that can efficiently capture smaller particles. However, from the perspective of processing capacity, a decrease in permeability is undesirable.

[0008] As described above, as lithium-ion secondary batteries become smaller and their output power increases, separators are required to achieve a higher level of both low resistance and high safety, which are often contradictory properties. For example, Patent Document 1 discloses a laminated porous polyolefin film that exhibits excellent safety and power output characteristics when used as a separator by controlling the relationship between pin puncture strength, porosity, melting point, and shutdown temperature within a predetermined range. Patent Document 2 also describes a polyolefin microporous membrane that can ensure safety while maintaining excellent cycle and power output characteristics by defining the lithium ion diffusion coefficient in the thickness direction and the shutdown temperature. Patent Document 3 also describes a porous polyolefin film that exhibits excellent power output characteristics, safety, and process transportability by defining the porosity, pin puncture strength, and longitudinal tensile modulus.

[0009] JP 2019-143142 A JP 2020-084084 A JP 2021-038379 A

[0010] The microporous membrane described in Patent Document 1 has improved shutdown properties through a layer design that includes a layer with a low melting point (shutdown layer), but when the microporous membrane is made thinner, the basis weight of the shutdown layer is insufficient, and even when the pores are melted and closed, complete insulation may not be achieved, making it impossible to ensure safety. Furthermore, the microporous membrane described in Patent Document 2 may not be able to ensure the high level of safety that has been required in recent years due to the large pore size and reduced strength caused by the blend of inorganic particles. Furthermore, the microporous membrane described in Patent Document 3 has a high dry stretch ratio and high strength, but typically, when the dry stretch ratio is high, the pore size of the microporous membrane increases and the shutdown temperature rises, which is a problem.

[0011] As described above, an object of the present invention is to provide a polyolefin microporous membrane that, when used as a battery separator, can impart high output characteristics and high safety and contribute to smaller batteries with higher output, and that, when used as a liquid filter, can eliminate the trade-off between small pore size and high permeability.

[0012] In order to solve the above problems and achieve the object, the present invention has the following configurations. In the following description, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. [1] With respect to the arithmetic mean roughness Sa (nm) of one surface and the other surface of a microporous membrane, Sa of the surface with the larger arithmetic mean roughness is Sa １ The surface with the smaller arithmetic mean roughness is Sa. ２ Formula 1 and Formula 2 are satisfied when Ds is the median pore diameter based on the surface area determined by pure water intrusion porosimetry measurement, and Dv is the median pore diameter based on the pore volume, and Formula 3 is satisfied when Ds is the median pore diameter based on the pore volume determined by pure water intrusion porosimetry measurement, and the pin puncture strength converted into unit basis weight is 700 mN / (g / m ２ ) or more.

[0013] Formula 1: 10≦(Sa １ +Sa ２ ) / 2≦80 Formula 2: Sa １ / Sa ２ ≦1.4 Equation 3: 1.0≦Dv / Ds≦1.2 [2] In a three-dimensional structure of 4 μm square obtained by measuring the microporous membrane with a focused ion beam scanning electron microscope (FIB-SEM), ８ nm ３ [3] The polyolefin microporous membrane according to [1], wherein the number of resin portions having a volume of 1.0 × 10 or more is 10 or less in a three-dimensional structure of a 4 μm square obtained by focused ion beam scanning electron microscope (FIB-SEM) measurement of the microporous membrane. ８ nm ３ [4] A polyolefin microporous membrane having 10 or fewer resin portions each having a volume of 1.3 × 10 or more in a 4 μm square three-dimensional structure obtained by FIB-SEM measurement of the microporous membrane. ７ nm ３ The polyolefin microporous membrane according to [1] or [3], which is:

[0014] [5] The polyolefin microporous membrane of any one of [1] to [4], having an average pore size measured by a porometer of 35 nm or less. [6] The polyolefin microporous membrane of any one of [1] to [5], having a shutdown temperature of 140°C or less. [7] The polyolefin microporous membrane of any one of [1] to [6], having an orientation parameter value measured by Raman spectroscopy at a total of 12 points at 15° intervals in any direction in the plane of the microporous membrane, with a standard deviation of 0.3 or less. [8] The polyolefin microporous membrane of any one of [1] to [7], having a melting point measured by differential scanning calorimetry (DSC) of 134°C or less. [9] The polyolefin microporous membrane of any one of [1] to [7], having a molecular weight that is 20% higher than the maximum value in the molecular weight range of 10,000 to 10,000,000 in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC), with the largest molecular weight being 20% ​​higher than the maximum value in the molecular weight range of 10,000 to 10,000,000. １ , the smallest one is M ２ The polyolefin microporous membrane according to any one of [1] to [8], which satisfies the following formula 4 when

[0015] Formula 4: M １ / M ２ ≧500

[10] The polyolefin microporous membrane of any one of [1] to [9], wherein, in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC), the area ratio of the peak area of ​​a molecular weight of 50,000 or less to the total molecular weight components is 10% or more, and the area ratio of the peak area of ​​a molecular weight of 1,000,000 or more is 10% or more.

[11] A battery separator using the polyolefin microporous membrane of any one of [1] to

[10] .

[12] A secondary battery using the battery separator of

[11] .

[13] A polyolefin microporous membrane having a standard deviation of orientation parameter values ​​measured by Raman spectroscopy at 12 points in total at 15° intervals in any direction in the microporous membrane plane of 0.3 or less, and a pin puncture strength converted into unit basis weight of 700 mN / (g / m ２

[14] A liquid filter using the polyolefin microporous membrane according to any one of [1] to

[10] .

[15] A filtration unit using the liquid filter according to

[14] .

[0016] The present invention provides a polyolefin microporous membrane that, when used as a battery separator, imparts high output characteristics and high safety, contributing to the miniaturization and high output of batteries. Furthermore, when used as a liquid filtration filter, the membrane exhibits excellent filtration accuracy and permeability, making it suitable for use as a high-precision filter for semiconductor processes and other applications where the removal of minute foreign matter is required.

[0017] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments described below.

[0018] [Polyolefin microporous membrane] In one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane is such that Sa of the surface with the larger arithmetic mean roughness is Sa １ The surface with the smaller arithmetic mean roughness is Sa. ２ When the median pore diameter based on the surface area determined by pure water intrusion porosimetry is Ds and the median pore diameter based on the pore volume is Dv, the formula 3 is satisfied, and the pin puncture strength in terms of unit basis weight is 700 mN / (g / m ２ ) or more.

[0019] Formula 1: 10≦(Sa １ +Sa ２ ) / 2≦80 Formula 2: Sa １ / Sa ２ ≦1.4 Formula 3: 1.0≦D Ｖ / D Ｓ In one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane is such that Sa of the surface with the larger arithmetic mean roughness is Sa １ The surface with the smaller arithmetic mean roughness is Sa. ２ In this case, (Sa １ +Sa ２) / 2, i.e., the average value of the arithmetic mean roughness on the front and back surfaces, is 10 nm or more and 80 nm or less. It is preferably 10 nm or more and 70 nm or less, more preferably 10 nm or more and 60 nm or less, even more preferably 10 nm or more and 50 nm or less, and particularly preferably 10 nm or more and 45 nm or less. (Sa １ +Sa ２ By setting the value of (Sa) / 2 within the above range, the microporous membrane becomes smooth macroscopically and the pore structure becomes uniform and fine microscopically, resulting in a separator with excellent shutdown performance and dendrite resistance and low resistance. Furthermore, when used as a liquid filter, the polyolefin microporous membrane can be made to have excellent filtration accuracy and permeability. Specifically, the arithmetic mean roughness Sa can be measured by the method described in the Examples. (Sa １ +Sa ２ Methods for adjusting the value of (x) / 2 to be within the above range include controlling the molecular weight distribution of the microporous membrane and adjusting the raw material composition and membrane production conditions to be within the ranges described below.

[0020] One aspect of the polyolefin microporous membrane according to an embodiment of the present invention is １ / Sa ２ The value of Sa is preferably 1.35 or less, more preferably 1.3 or less, even more preferably 1.2 or less, and particularly preferably 1.1 or less. １ / Sa ２ By setting Sa in the above range, the difference in structure between the front and back of the microporous membrane is reduced, and the separator resistance can be reduced. In addition, when used as a liquid filtration filter, the permeability can be improved. １ / Sa ２ The lower limit of Sa is not particularly limited, but is, for example, 1.001 or more from the viewpoint of compatibility with the productivity of the microporous membrane. １ / Sa ２ The value of can be adjusted to fall within the above range by controlling the molecular weight distribution of the microporous membrane, or by adjusting the raw material composition and membrane production conditions to fall within the ranges described below.

[0021] In one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the median pore diameter based on the surface area determined by pure water intrusion porosimetry is D Ｓ , the median pore diameter based on pore volume is D Ｖ In this case, D Ｖ / D Ｓ Examples of suitable polyolefin microporous membranes include those having a D of 1.0 or more and 1.2 or less. D is preferably 1.0 or more and 1.15 or less, more preferably 1.0 or more and 1.1 or less, and even more preferably 1.0 or more and 1.05 or less. Ｖ / D Ｓ By setting the D in the above range, coarse pores are reduced, resulting in a microporous membrane with excellent shutdown performance and dendrite resistance. Furthermore, when used as a liquid filter, the filtration accuracy can be improved. Ｖ / D Ｓ Methods for adjusting the molecular weight distribution of the microporous membrane to fall within the above range include controlling the raw material composition and membrane production conditions to fall within the ranges described below.

[0022] In one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the pin puncture strength in terms of unit area weight is 700 mN / (g / m ２ ) or more. ２ ) or more, and more preferably 900 mN / (g / m ２ ) or more, and more preferably 1000 mN / (g / m ２ ) or more. By setting the puncture strength in terms of unit basis weight within the above range, a separator with excellent safety can be obtained. Furthermore, when used as a liquid filtration filter, deformation of the pore structure due to pressure applied during filtration can be suppressed, and filtration accuracy can be improved. From the above, there is no particular restriction on the upper limit of the puncture strength in terms of unit basis weight, but from the viewpoint of compatibility with heat shrinkability, it is preferable to set the upper limit to, for example, 3000 mN / (g / m ２ In order to set the pin puncture strength value in terms of unit area weight within the above range, it is preferable that the raw material composition of the microporous membrane and the membrane-forming conditions be within the ranges described below.

[0023] The polyolefin microporous membrane according to an embodiment of the present invention preferably has an average pore size of 35 nm or less, more preferably 32 nm or less, even more preferably 28 nm or less, and particularly preferably 25 nm or less. By setting the average pore size within the above range, a microporous membrane with excellent strength and dendrite resistance can be obtained. Furthermore, although there are no particular restrictions on the lower limit of the average pore size, from the viewpoint of achieving compatibility with the permeability of the microporous membrane, it is preferably 14 nm or more, more preferably 18 nm or more.

[0024] The polyolefin microporous membrane according to an embodiment of the present invention preferably has a shutdown temperature of 140°C or lower, more preferably 139°C or lower, even more preferably 138°C or lower, and particularly preferably 137°C or lower. Setting the shutdown temperature within the above range results in excellent low-temperature shutdown performance, thereby enhancing the safety of the separator. From the above viewpoints, the lower limit of the shutdown temperature is not particularly limited, but from the viewpoint of achieving both the permeability and strength of the microporous membrane, it is preferably 125°C or higher.

[0025] One aspect of the polyolefin microporous membrane according to an embodiment of the present invention is a polyolefin microporous membrane having a standard deviation of 0.30 or less in orientation parameter measured by Raman spectroscopy at a total of 12 points at 15° intervals in any direction in the plane of the microporous membrane. The orientation parameter of a polyolefin microporous membrane measured by Raman spectroscopy can be calculated by the following formula: I=I １１３０ / I １０６３ I: Orientation parameter I １１３０ : 1130cm －１ Peak intensity at I １０６３ : 1060cm －１The standard deviation of the orientation parameter is preferably 0.25 or less, more preferably 0.20 or less, and particularly preferably 0.15 or less. By setting the standard deviation of the orientation parameter within the above range, the fibrils (resin portion) constituting the microporous membrane are uniformly oriented in the film plane, resulting in a uniform pore structure, and a microporous membrane having excellent shutdown characteristics, dendrite resistance, and permeability when used as a battery separator. Furthermore, a polyolefin microporous membrane having excellent filtration accuracy and permeability when used as a liquid filter can be obtained. From the above viewpoints, the lower limit of the standard deviation of the orientation parameter is not particularly limited, but from the viewpoint of compatibility with productivity, it is preferably 0.01 or more. To set the standard deviation of the orientation parameter within the above range, it is preferable that the raw material composition of the microporous membrane and the membrane-forming conditions be within the ranges described below.

[0026] The polyolefin microporous membrane according to an embodiment of the present invention preferably has a melting point measured by DSC of 134°C or lower, more preferably 133°C or lower, and even more preferably 132°C or lower. A melting point within the above range not only provides excellent shutdown properties, but also enables the pore structure of the microporous membrane to be uniform and fine. The lower limit of the melting point is preferably 125°C or higher, more preferably 127°C or higher, and even more preferably 129°C or higher, from the viewpoint of achieving both the permeability and strength of the microporous membrane. Methods for adjusting the melting point of the polyolefin microporous membrane within the above range include controlling the molecular weight distribution of the microporous membrane and adjusting the raw material composition to the range described below.

[0027] In the polyolefin microporous membrane according to an embodiment of the present invention, the largest molecular weight having a height of 20% of the maximum value in the molecular weight range of 10,000 to 10,000,000 in a differential molecular weight distribution curve measured by a GPC method is defined as M １ , the smallest one is M ２ In this case, M １ / M ２ is preferably 500 or more. １ / M ２is more preferably 600 or more, even more preferably 700 or more, and particularly preferably 800 or more and 1000 or less. １ / M ２ By setting the value in the above range, the shutdown property and strength of the polyolefin microporous membrane can be both achieved, the standard deviation of the orientation parameter in the plane of the polyolefin microporous membrane can be adjusted to a preferred range, and the microporous membrane can have a standard deviation of 1.0 × 10 in a 4 μm square three-dimensional structure obtained by focused ion beam scanning electron microscope (FIB-SEM) measurement. ８ nm ３ It becomes easy to adjust the number of resin portions having a volume equal to or larger than this range within a preferred range. １ / M ２ In order to achieve the above range, it is preferable that the raw material composition and kneading conditions for the microporous membrane be within the ranges described below.

[0028] In the polyolefin microporous membrane according to an embodiment of the present invention, in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC) described below, the area ratio of molecular weights of 50,000 or less to the peak area of ​​all molecular weight components is preferably 10% or more, and the area ratio of molecular weights of 1,000,000 or more is preferably 10% or more. The area ratio of molecular weights of 50,000 or less is more preferably 15% or more, and even more preferably 20% or more. The area ratio of molecular weights 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 molecular weights of 1,000,000 or more is more preferably 15% or more, and even more preferably 20% or more. The area ratio of molecular weights 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 amount of molecular weight of 50,000 or less and molecular weight of 1,000,000 or more in the polyolefin microporous membrane within the above ranges, it becomes possible to achieve both shutdown properties and strength of the polyolefin microporous membrane, and it becomes easy to adjust the standard deviation of the orientation parameter in the plane of the polyolefin microporous membrane within a preferred range.

[0029] In one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the microporous membrane has a three-dimensional structure of 4 μm square obtained by FIB-SEM measurement, and the microporous membrane has a surface roughness of 1.0 × 10 ８ nm ３ Examples of suitable polyolefin microporous membranes include a polyolefin microporous membrane having 10 or fewer resin moieties with a volume of 1.0×10 or more. More preferably, the number is 8 or fewer, even more preferably 5 or fewer, and particularly preferably 1 or fewer. ８ nm ３ By setting the number of resin portions having a volume of 1.0 × 10 or more within the above range, when used as a battery separator, the ion conductivity is improved, and the uniform structure provides excellent shutdown performance and dendrite resistance. Furthermore, when used as a liquid filter, a polyolefin microporous membrane with excellent filtration accuracy and permeability can be obtained. A 1.0 × 10 area measurement within a 4 μm square three-dimensional structure by FIB-SEM was performed. ８ nm ３ The number of resin portions having a volume of 1.0×10 or more can be measured by the method described in the Examples. ８ nm ３ In order to set the number of resin portions having the above volume within the above range, it is preferable that the raw material composition of the microporous membrane and the membrane-forming conditions be within the ranges described below.

[0030] The polyolefin microporous membrane according to an embodiment of the present invention has an average volume per resin portion of 1.3 × 10, as observed in a 4 μm square three-dimensional structure obtained by FIB-SEM measurement of the microporous membrane. ７ nm ３ It is preferably equal to or less than 1.2 × 10 ７ nm ３ More preferably, 1.0 × 10 ７ nm ３ By setting the average volume per resin portion within this range, ionic conductivity is improved when the microporous membrane is used as a battery separator. Furthermore, when the microporous membrane is used as a liquid filter, it can have excellent filtration accuracy and permeability. From the above viewpoints, the lower limit of the average volume of the resin portion is not particularly limited, but from the viewpoint of achieving compatibility with the permeability and heat shrinkage rate of the microporous membrane, it is preferable that the lower limit be 1.0 × 10６ In order to set the average volume of the resin portion within the above range, it is preferable that the raw material composition of the microporous membrane and the membrane-forming conditions be within the ranges described below.

[0031] The polyolefin microporous membrane according to an embodiment of the present invention preferably has an air permeability calculated as a thickness of 30 seconds / 100 cm ３ / μm or less, more preferably 20 seconds / 100cm ３ / μm or less, more preferably 15 seconds / 100cm ３ Although there is no particular lower limit for the air permeability converted into thickness, it is set to 1 sec / 100 cm because this makes it easier to achieve compatibility with the membrane strength. ３ / μm or more. By setting the air permeability in terms of thickness within the above range, a microporous membrane with excellent charge / discharge characteristics can be obtained when used as a battery separator. The air permeability in terms of thickness can be set within the above range by adjusting the blending ratio of raw materials, the stretching ratio, the heat setting conditions, etc. during the production process.

[0032] The thickness of the polyolefin microporous membrane according to an embodiment of the present invention can be adjusted appropriately depending on the application, but is preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 12 μm or less. It is also preferably 2 μm or more, more preferably 5 μm or more, and particularly preferably 7 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 within the above range by appropriately adjusting the membrane-forming conditions, such as the extrusion conditions.

[0033] The porosity of the polyolefin microporous membrane according to an 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 a porosity of 80% or less is preferred because it can prevent a decrease in membrane strength. A porosity within this range ensures excellent output characteristics when used as a secondary battery separator. The porosity can be adjusted to the above range by adjusting the raw material formulation, stretch ratio, heat setting conditions, etc. during the production process.

[0034] Specific configurations of the polyolefin microporous membrane of the present invention will be described below, but the present invention is not necessarily limited to the embodiments described below.

[0035] The polyolefin microporous membrane according to an embodiment of the present invention preferably contains a polyethylene-based resin as a main component. The term "main component" as used herein refers to the component that constitutes the polyolefin microporous membrane with the highest content, expressed as % by mass. The polyethylene-based resin component in the polyolefin microporous membrane is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 96% by mass or more, and particularly preferably 99% by mass or more. By setting the content of the components in the polyolefin microporous membrane within the above ranges, the microporous membrane exhibits excellent film formability and uniformity, while also exhibiting an excellent balance of performance characteristics, such as membrane strength and permeability, as a battery separator. The polyolefin microporous membrane may contain two or more polyethylene-based resins, and in this case, the total amount of the polyethylene-based resins refers to the amount of the polyethylene-based resin components constituting the polyolefin microporous membrane.

[0036] Various polyethylene-based resins can be used for the polyolefin microporous membrane according to an embodiment of the present invention, including ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene. The polyethylene-based 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. The polyethylene-based resin contains more than 50 mol% of ethylene relative to all raw monomer components.

[0037] The polyolefin microporous membrane according to an embodiment of the present invention preferably contains, among the above-mentioned polyethylenes, ultra-high molecular weight polyethylene (hereinafter referred to as resin A), and more preferably contains resin A and high-density polyethylene (hereinafter referred to as resin B).

[0038] 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, even more preferably 1,000,000 or more, still more preferably 2,000,000 or more, and particularly preferably 2,400,000 or more. Furthermore, the weight-average molecular weight (Mw) is preferably 4,000,000 or less, more preferably 3,500,000 or less. By setting the weight-average molecular weight of resin A within the above range, the strength of the polyolefin microporous membrane can be increased, the pore structure can be made fine and uniform, thereby reducing the arithmetic mean roughness (Sa), and the average volume of the resin portion can be easily reduced.

[0039] The melting point of resin A is preferably 135°C or lower, more preferably 134°C or lower, and particularly preferably 130°C or lower. It is also preferably 125°C or higher, and more preferably 127°C or higher. By setting the melting point of resin A within the above range, the pore structure of the polyolefin microporous membrane becomes uniform and fine, and the shutdown property is also improved. Furthermore, the structural difference between both surfaces of the polyolefin microporous membrane is reduced, and Sa １ / Sa ２ The value of Sa can be easily adjusted to an appropriate range. Although the relationship between the melting point of resin A and the structural difference on both surfaces of the polyolefin microporous membrane has not been clarified, it is presumed that controlling the melting point of resin A, i.e., the crystallization behavior when the resin is cooled and solidified, reduces the structural difference caused by the difference in cooling rate between the front and back in the process of forming a gel-like sheet, which will be described later. When a polyolefin resin having a weight-average molecular weight (Mw) of 2,510,000 or more and 4,000,000 or less and a melting point of 130°C or less is used as resin A, the pore structure of the polyolefin microporous membrane is easily made uniform and fine, and the structural difference on both surfaces of the polyolefin microporous membrane is reduced, thereby improving Sa. １ / Sa ２ This makes it easier to adjust the value to an appropriate range.

[0040] The melting point of resin A can be set within the above range by controlling the molecular structure (monomer type) constituting resin A or adjusting the number and length of side chains. For example, in the case of a polyethylene-based resin, a method can be mentioned in which ethylene is copolymerized with another α-olefin and the copolymerization rate or the type of α-olefin used is controlled.

[0041] 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. Also, 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, it is easy to increase the membrane strength and make the pore structure finer.

[0042] High-density polyethylene (density: 0.940 g / m) used as resin B ３ 0.970g / m or more ３ The weight average molecular weight (Mw) of Resin B is preferably 10,000 or more, more preferably 20,000 or more, and even more preferably 50,000 or more. The weight average molecular weight (Mw) of Resin B 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, a structure in which fibrils are uniformly oriented in the film plane after stretching is obtained, and D Ｖ / D Ｓ The value of 1.0 x 10 ８ nm ３ This makes it easy to adjust the number of resin portions having a volume above this range to fall within a preferred range.

[0043] 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, the pore structure becomes finer and the shutdown characteristics are excellent.

[0044] The heat of crystalline fusion (ΔH) of resin B 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 crystalline fusion (ΔH) of resin B within the above range, it is possible to increase film strength while suppressing an increase in shutdown temperature. From the above viewpoint, there is no particular upper limit for the heat of crystalline fusion (ΔH) of resin B, but from the viewpoint of film formability, it is preferably 280 J / g or less.

[0045] 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. It is also 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, 1.0 × 10 ８ nm ３ It becomes easy to adjust the number of resin portions having the above volume within a preferred range, and excessive orientation of crystals during stretching is suppressed, resulting in good shutdown characteristics.

[0046] The polyolefin microporous membrane according to an embodiment of the present invention may contain a resin other than a polyethylene-based resin; for example, the addition of a polypropylene-based resin is preferred 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 polypropylene-based resins. Block copolymers and random copolymers can 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-based resin refers to a resin containing more than 50 mol% of propylene relative to all raw material monomer components.

[0047] 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 setting it within this range, a polyolefin microporous membrane with excellent productivity, quality, and strength can be obtained.

[0048] The polyolefin microporous membrane may contain, as needed, resin components other than the polyethylene resin and the polypropylene resin, and may also contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders, provided that the effects of the present invention are not impaired.

[0049] [Method for producing polyolefin microporous membrane] Next, a method for producing a polyolefin microporous membrane in an embodiment of the present invention will be described. Examples of methods for producing a polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. In this embodiment, a wet film-forming method is preferred as a method for producing a polyolefin microporous membrane from the viewpoint of controlling the structure and physical properties of the membrane.

[0050] A wet method for producing a polyolefin microporous membrane will be described below, but the following description is merely an example of the production method and is not intended to limit the scope of the invention.

[0051] The method for producing a polyolefin microporous membrane in an embodiment of the present invention preferably includes the following steps (1) to (5) in order, more preferably includes the following step (6), and may further include the following step (7) after or instead of step (6).

[0052] (1) A process of melt-kneading the polyolefin resin and the membrane-forming solvent to prepare a polyolefin resin composition; (2) A process of extruding the polyolefin resin composition and cooling it to form a gel-like sheet; (3) A first stretching process of preheating and stretching the gel-like sheet; (4) A process of removing the membrane-forming solvent from the stretched gel-like sheet; (5) A process of drying the sheet from which the membrane-forming solvent has been removed; (6) A second stretching process of preheating and stretching the dried sheet; and (7) A process of heat-treating the dried sheet.

[0053] (1) Preparation of Polyolefin Resin Composition: A polyolefin resin composition is prepared by heating and dissolving a polyolefin resin in a plasticizer (membrane-forming solvent). The plasticizer is not particularly limited as long as it can uniformly disperse the polyolefin resin. However, to enable relatively high stretching ratios, the solvent is preferably liquid at room temperature. Examples of solvents include aliphatic, cycloaliphatic, 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, a nonvolatile liquid solvent such as liquid paraffin is preferably used.

[0054] The blending ratio of the polyolefin resin and the plasticizer is preferably 10 to 50% by mass of the polyolefin resin relative to the total mass of the polyolefin resin composition. By setting the polyolefin resin content within this range, the polyolefin resin and the plasticizer are well dispersed, and the resulting microporous membrane has excellent strength, permeability, and heat resistance. Furthermore, when formed into a sheet, the amount of swelling and neck-in at the outlet of the die is appropriate, and the sheet formability and film formability are also good.

[0055] The melt-kneading of the polyolefin resin and the plasticizer is preferably carried out in a twin-screw extruder in order to obtain a uniformly kneaded state.

[0056] 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, with the upper limit being 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, it is possible to prevent a decrease in strength due to resin degradation, and to melt-knead the polyolefin resin and plasticizer uniformly.

[0057] Furthermore, during kneading with a twin-screw extruder, Q / Ns, calculated from the ratio of the extrusion mass Q (kg / hr) to the screw rotation speed Ns (rpm), is preferably 0.01 or more, more preferably 0.05 or more, even more preferably 0.1 or more, and particularly preferably 0.3 or more. This makes it possible to prevent a decrease in strength due to resin deterioration during kneading. The upper limit is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less. This makes it possible to apply sufficient shear to the polyolefin resin composition and obtain a uniformly dispersed state.

[0058] (2) Gel-like Sheet Formation Process A melt of a polyolefin resin composition is fed from an extruder to a die and extruded into a sheet. The extrusion method may be either a T-die method or an inflation method. Alternatively, multiple polyolefin resin compositions of the same or different compositions may be fed from multiple extruders to a single multi-manifold composite T-die, laminated into layers, and extruded into a laminated sheet. The extrusion temperature is preferably 140 to 250°C.

[0059] The shear rate applied when the molten material is extruded from the die is 150 sec. －１ It is preferable that the time is 200 seconds or more. －１ More preferably, it is 250 seconds or more. －１ More than 300 seconds, especially preferred －１ By controlling the shear rate within the above range, the orientation state of the sheet surface can be controlled, and the difference in structure between the front and back sides of the microporous membrane can be reduced, and the pore structure can be easily refined. The upper limit of the shear rate is set to 1000 sec from the viewpoint of compatibility with the quality of the film.－１ It is preferable that the time is less than 800 seconds, and more preferably less than 800 seconds. －１ The shear rate can be calculated using the following formula in extrusion using a T-die: γ = 6Q / (Wt ２ ) γ: shear rate ( / sec) Q: discharge amount (cm ３ / sec) W: width of T-die outlet (cm) t: slit gap of T-die outlet (cm) The melt-extruded resin composition in a sheet form is cooled and solidified to form a gel-like sheet. In the cooling step, it is preferable to cool to 10 to 50°C before the first stretching step described below. This is because it is preferable to set the final cooling temperature below the crystallization end temperature, and by refining the high-order structure, uniform stretching can be easily performed in the subsequent stretching. In addition, the cooling rate at this time is preferably 50°C / min or more, more preferably 100°C / min or more, and even more preferably 150°C / min or more. In general, when the cooling rate is slow, relatively large crystals are formed, which results in a coarse high-order structure of the gel-like sheet and a large gel structure. In contrast, when the cooling rate is fast, relatively small crystals are formed, which results in a dense high-order structure of the gel-like sheet and enables uniform stretching, making it easier to increase the strength of the polyolefin microporous membrane and refine the pore structure.

[0060] (3) First Stretching Step Next, the obtained gel-like sheet is stretched at least uniaxially, 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 more preferably 120°C or lower, even more preferably 118°C or lower. By preheating within the above conditions, a polyolefin microporous membrane can be uniformly stretched in the stretching step and have a uniform, fine pore structure.

[0061] The preheated gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial or biaxial, with biaxial stretching being preferred. In the case of biaxial stretching, simultaneous biaxial stretching, sequential biaxial stretching, or multistage stretching (e.g., a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, with simultaneous biaxial stretching being preferred. Simultaneous biaxial stretching can improve the arithmetic mean roughness (Sa) of the polyolefin microporous membrane surface, the standard deviation of the orientation parameter in the film plane, and the 1.0 × 10 of the polyolefin microporous membrane surface. ８ nm ３ It becomes easy to adjust the resin portion having a volume equal to or larger than this range to fall within a preferred range.

[0062] The stretching ratio (area stretching ratio) in this step is preferably 16 times or more, more preferably 25 times or more. The stretching ratio is preferably 4 times or more, more preferably 5 times or more, in both the machine longitudinal direction (MD direction) and the machine width direction (TD direction). The stretching ratios in the MD direction and the TD direction may be the same or different, and by setting the area stretching ratio within the above range, mechanical strength and permeability can be improved. The area stretching ratio in this step is preferably 100 times or less, more preferably 64 times or less, which prevents membrane rupture and simultaneously provides a polyolefin microporous membrane with excellent membrane strength.

[0063] The stretching temperature in this step is preferably 90°C or higher, more preferably 105°C or higher, and even more preferably 110°C or higher. It is also preferably 130°C or lower, more preferably 120°C or lower, and even more preferably 118°C or lower. The stretching described above causes cleavage between polyethylene lamellae, resulting in the polyethylene resin phase becoming finer and the formation of numerous fibrils. The fibrils form a network structure in which they are three-dimensionally connected irregularly.

[0064] The stretching speed in the MD direction in this step is preferably 300% / min or more, more preferably 700% / min or more, even more preferably 1000% / min or more, particularly preferably 1200% / min or more, and most preferably 1500% / min or more. It is also preferably 5000% / min or less. Setting the MD stretching speed within the above range facilitates achieving a fine and uniform pore structure. The stretching speed in this step is the value obtained by dividing the stretching ratio from the start to the completion of stretching by the required time, assuming that the stretching ratio of the microporous membrane before stretching is 0% and that the stretching ratio per stretch ratio is 100%.

[0065] The stretching speed in the TD direction in this step is preferably 300% or more, more preferably 700% / min or more, even more preferably 1000% / min or more, particularly preferably 1200% / min or more, and most preferably 1500% / min or more. Also, it is preferably 5000% / min or less. By setting the stretching speed in the TD direction within the above range, it becomes easy to make the pore structure fine and uniform.

[0066] (4) Membrane-forming solvent removal process: The membrane-forming solvent is removed (washed) using a wash solvent. The polyolefin resin phase is phase-separated from the membrane-forming solvent phase. Therefore, when the membrane-forming solvent is removed, a porous membrane is obtained, consisting of fibrils that form a fine three-dimensional network structure and having three-dimensionally irregularly interconnected pores (voids). Wash solvents and methods for removing the membrane-forming solvent using them are known, so a detailed description is omitted. For example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Laid-Open No. 2002-256099 can be used.

[0067] (5) Drying step The polyolefin microporous membrane from which the membrane-forming solvent has been removed is dried by heat drying or air drying. The drying temperature is preferably 100°C or lower, more preferably 95°C or lower. Drying is preferably performed until the amount of remaining washing solvent is 5 parts by mass or less, more preferably 3 parts by mass or less, per 100 parts by mass (dry mass) of the total mass of the polyolefin microporous membrane.

[0068] (6) Second Stretching Step Next, the dried microporous membrane is stretched at least uniaxially at a predetermined areal stretch ratio. Stretching the dried film (second stretching) is also called dry stretching. Stretching may be uniaxial or biaxial, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous stretching or sequential stretching may be used, but sequential stretching is preferred. In the case of sequential stretching, it is preferable to stretch the membrane in the MD direction and then continuously stretch it in the TD direction.

[0069] The area stretching ratio in dry stretching is preferably 1.2 times or more, more preferably 1.5 times or more, even more preferably 2.0 times or more, particularly preferably 2.5 times or more, and most preferably 3.0 times or more. By setting the area stretching ratio in dry stretching within the above range, the structure becomes uniform, and Sa １ / Sa ２ Adjust the value to an appropriate range, 1.0 x 10 ８ nm ３ It becomes easy to control the number of resin portions having a volume equal to or larger than this range within a preferred range. Furthermore, film formability is stabilized by setting the areal stretching ratio in dry stretching to 16 times or less. Note that the areal stretching ratio described in this process refers to the product of the dry stretching ratios in the MD and TD directions.

[0070] The stretching ratio in the MD direction in dry stretching is preferably 1.2 or more, more preferably 1.4 or more, even more preferably 1.6 or more, particularly preferably 1.7 or more, and preferably 4 or less. By setting the stretching ratio in the MD direction in dry stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within desired ranges, and the structure can be made uniform.

[0071] The stretch ratio in the TD direction in dry stretching is preferably 1.2 or more, more preferably 1.4 or more, even more preferably 1.6 or more, particularly preferably 1.7 or more, and preferably 4 or less. By setting the stretch ratio in the TD direction in dry stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within desired ranges, and the structure can be made uniform.

[0072] The stretching ratio in the MD direction in this process is S ＭＤ , the stretching ratio in the TD direction is S ＴＤ In this case, |S ＭＤ -S ＴＤ is preferably 0.5 or less, more preferably 0.3 or less, and particularly preferably 0.1 or less. ＭＤ -S ＴＤ By setting | within the above range, a structure in which fibrils of the polyolefin microporous membrane are uniformly oriented in the plane can be obtained, and the standard deviation of the orientation parameter in the film plane and the arithmetic mean roughness (Sa) can be reduced to 1.0 × 10 ８ nm ３ It is easy to adjust the number of resin portions having a volume above this range within a desired range. Note that the stretch ratio in this step refers to the stretch ratio of the microporous membrane immediately before being subjected to the next step, based on the microporous membrane immediately before this step.

[0073] The MD stretching temperature in this step is preferably 60° C. or higher, more preferably 80° C. or higher, and preferably 130° C. or lower, more preferably 120° C. or lower. By setting the MD stretching temperature within the above range, the obtained polyolefin microporous membrane has excellent permeability and strength and can be stretched uniformly.

[0074] The TD stretching temperature in this step is preferably 80° C. or higher, more preferably 100° C. or higher, and preferably 145° C. or lower, more preferably 135° C. or lower. By setting the TD stretching temperature within the above range, the resulting polyolefin microporous membrane has excellent permeability and strength and can be stretched uniformly.

[0075] (7) Heat Treatment Step After or instead of step (6), the dried polyolefin microporous membrane can be heat-treated. Heat treatment stabilizes the crystals and homogenizes the lamellae. Heat setting and / or heat-relaxing can be used as the heat treatment method. Heat setting is a heat treatment in which the membrane is heated while maintaining its dimensions. Heat-relaxing is a heat treatment in which the membrane is thermally shrunk in the MD and TD directions during heating. Heat setting is preferably performed using a tenter or roll method. The relaxation rate in the relaxation treatment is the value obtained by dividing the membrane dimensions after the relaxation treatment by the membrane dimensions before the relaxation treatment. The relaxation rates in both the MD and TD directions of the membrane are preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. From the viewpoint of the planarity of the microporous membrane, the rate is preferably 0.80 or more, more preferably 0.90 or more. The heat treatment temperature is preferably within the range of 100 to 140°C for polyolefin resins.

[0076] The polyolefin microporous membrane obtained as described above can be used for various purposes such as filters, separators for secondary batteries, separators for fuel cells, and separators for capacitors.

[0077] [Battery separator and secondary battery] One embodiment of the present invention is a battery separator using the polyolefin microporous membrane of the present invention, and a secondary battery using such a battery separator. The battery separator of the present invention has excellent safety and output characteristics, and is therefore particularly suitable for use in secondary batteries that require high energy density, high capacity, and high output.

[0078] [Liquid Filter and Filtration Unit] One embodiment of the present invention includes a liquid filter using the polyolefin microporous membrane of the present invention, and a filtration unit using such a liquid filter. Because the membrane exhibits excellent filtration accuracy and high permeability when used as a liquid filter, it is preferably used as a liquid filter for semiconductor resists, which require high-precision filtration. The polyolefin microporous membrane of the present invention can be used as a liquid filter for sheet-shaped, tubular, pleated, or other filtration units, and is preferably a pleated filtration unit because it can achieve a large filtration area. When incorporated into a pleated filtration unit, it is preferable to laminate a reinforcing membrane made of a mesh or porous material using a resin material on at least one side of the polyolefin microporous membrane of the present invention. After bonding to the reinforcing membrane, the membrane is pleated with peak and valley folds and incorporated into a filtration unit for use.

[0079] 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. Evaluations in this application were performed in an environment of a temperature of 23°C and a humidity of 65%, unless otherwise specified. The evaluation and analysis methods used in the examples are as follows.

[0080] [Measurement method] [Thickness] The thickness of a polyolefin microporous membrane was measured at any five points within a 50 mm × 50 mm area using a contact thickness meter, Litematic (registered trademark) VL-50 (10.5 mmφ superhard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation, and the average value was taken as the thickness (μm).

[0081] [Porosity] A 50 mm x 50 mm square sample was cut out from a polyolefin microporous membrane, and its volume (cm ３ The film density (g / cm) and mass (g) were measured. ３ The porosity of the polyolefin microporous membrane was calculated from the above data using the following formula: The membrane density was 0.99 g / cm ３The calculation was performed assuming a constant value of porosity. For this measurement, samples were cut out from three randomly selected positions on the polyolefin microporous membrane, and the average porosity measured at each position was calculated. Formula: Porosity (%) = [(volume - mass / membrane density) / volume] x 100.

[0082] [Air permeability] The air permeability (sec / 100 cm) of a polyolefin microporous membrane was measured in accordance with JIS P-8117:2009 using an Oken air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in an atmosphere of 25°C. ３ The air permeability was calculated in terms of thickness by dividing the measured value by the thickness (μm) of the polyolefin microporous membrane.

[0083] [Puncture Strength in Basis Weight Conversion] The puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was 2 mm / sec. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), the maximum load (mN) when a polyolefin microporous membrane was punctured with a 1.0 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm) was measured, and the value calculated from the following formula was used to represent the puncture strength in basis weight conversion (mN / (g / m ２ Formula: Puncture strength converted into unit 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 the mass (g) at room temperature of 25°C, and using the following formula: ２ ) = mass (g) / (50 (mm) x 50 (mm)) x 10 ６ .

[0084] [Gel Permeation Chromatography (GPC)] The weight-average molecular weight (Mw) of the polyolefin resin and the molecular weight distribution of the polyolefin microporous membrane were determined by GPC under the following conditions. In the differential molecular weight distribution curve obtained by GPC, 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. Sample preparation: 5 mL of measurement solvent was added to 5 mg of sample, and the mixture was heated and stirred at 160 to 170°C for 60 minutes, and the resulting solution was filtered through a metal filter (pore size 0.5 μm).

[0085] Measurement equipment: High-temperature GPC equipment, HLC-8321GPC / HT manufactured by Tosoh Corporation Guard column: "Shodex" (registered trademark) HT-G manufactured by Showa Denko K.K. Column: Two "Shodex" (registered trademark) UT806M manufactured by Showa Denko K.K. Column temperature: 145°C Solvent (mobile phase): 1,2,4-trichlorobenzene (manufactured by Fujifilm Wako Pure Chemical Industries, 0.1 wt% BHT added) Solvent flow rate: 1.0 ml / min Injection amount: 0.300 mL Detector: Differential refractive index detector (RI detector) Standard samples: Monodisperse polystyrene manufactured by Tosoh Corporation, dibenzyl manufactured by Tokyo Chemical Industry Co., Ltd. The obtained Mw was then converted to polyethylene (PE). The conversion formula is as follows:

[0086] Mw (PE equivalent) = Mw (measured value converted into PS equivalent) x 0.468.

[0087] [Differential Scanning Calorimetry (DSC)] The melting point and heat of crystalline fusion of the polyolefin resin and the melting point of the polyolefin microporous membrane were determined by DSC. 6.0 mg of sample was sealed in an aluminum pan, and using a Parking Elmer PYRIS Diamond DSC, the temperature was raised from 30 ° C. to 230 ° C. at 10 ° C. / min (first heating), then held at 230 ° C. for 5 minutes, cooled at a rate of 10 ° C. / min, and again heated at a heating rate of 10 ° C. / min from 30 ° C. to 230 ° C. (second heating). The heat of crystalline fusion and 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 endotherm measured during the second heating of the DSC measurement. The melting point was the temperature at which the maximum endotherm was observed, and the heat of crystalline fusion was calculated from the area of ​​the crystalline melting peak.

[0088] [Arithmetic mean roughness Sa] Surface roughness was measured using a scanning white light interference microscope VS-1540 manufactured by Hitachi High-Tech Science Corporation. VS-Measure (Version 10.0.4) was used as measurement software, and VS-Viewer (Version 10.0.3) was used as analysis software. A 50 mm x 50 mm square film was cut from a polyolefin microporous membrane and attached to a circular frame with an inner diameter of 4 cm so as to prevent wrinkles or slackness, to serve as a measurement sample. Measurements and data processing were performed under the conditions shown below, and the arithmetic mean roughness Sa was calculated in accordance with ISO 25178. Measurements were performed on both surfaces of each sample, and the value on the surface with the largest Sa was taken as Sa. １ , the value on the small surface is Sa ２ Measurements were taken at four points on each surface and the average value was calculated. <Measurement conditions> - Objective lens: 50x - Lens tube lens: 0.5x - Wavelength filter: 530 white - Camera: high pixel - Measurement mode: Wave - Measurement device: Piezo - Field of view size: 1024 x 1024 (pixels) - Number of effective pixels: 50% - Cutoff: None <Data processing conditions> - Surface correction (approximate surface shape): 4th order - Interpolation: full complement - Filter: smoothing 9 x 9 (pixels).

[0089] [Standard deviation of orientation parameter] The orientation parameter of the polyolefin microporous membrane was measured and calculated by Raman spectroscopy. A laser was incident perpendicularly from the normal direction to the film surface (XY plane), and polarized using a polarizer. The measurement sample was rotated, and a Raman spectrum was obtained for each direction at 12 points (0° to 165°) in 15° increments, with any direction in the film plane being set as 0°. The obtained Raman spectrum was measured at 800 cm －１ and 1350 cm －１ After drawing a straight line between the two points and correcting it as the baseline, －１ The maximum strength between 1060 cm －１ Peak intensity (I1060), 1115-1145 cm －１ The maximum strength between 1130 cm －１ The peak intensity (I1130) is 1130cm －１ and 1060 cm －１ The peak intensity ratio (I1130 / I1060) was used as the orientation parameter. An arbitrary direction in the film plane was set as 0°, and measurements were taken at 12 points (0° to 165°) at 15° intervals, and the standard deviation was calculated from the values. - Equipment: Microscopic Raman spectroscopy system (Renishaw "inVia") - 180° backscattering configuration - Spectral length: 250 mm - Diffraction grating: 3000 lines / mm - Excitation laser: 532 nm - 50x objective lens (N.A. = 0.75) - Spot size (spatial resolution): 5 μm.

[0090] [Median pore size based on surface area (Ds) and median pore size based on pore volume (Dv)] A rectangular test piece weighing 0.20±0.02 g was cut from any position on the polyolefin microporous membrane, and the area and weight of the test piece were measured. The volume of the polyolefin microporous membrane was calculated from the area of ​​the test piece and the membrane thickness measured by the method described above. The cut-off sample was wound around a sample core in purified water to prevent air bubbles from being trapped, and placed in the measurement chamber of a pure water intrusion porosimeter (model: WIP-3K-A-1) manufactured by POROUS MATERIALS, INC. Gas was introduced into the pure water in the chamber, and the pressure was increased stepwise until a maximum pressure of 13.7 MPaG was measured. The relationship between pressure, pore size, surface area, and pore volume is expressed by the following equation, and the surface area is calculated using a model in which the pore shape in the pure water intrusion porosimeter is assumed to be cylindrical. Regarding the pore size distribution obtained from the relationship between the pore size calculated from the pressure at each pressure increase stage and the surface area (integral value) of the pores into which pure water was injected at each pressure increase stage, when the surface area (integral value) at the maximum pressure of 13.7 MPaG was taken as 100%, the pore size at the point where the surface area (integral value) was 50% was taken as the median value based on surface area. The median pore size based on pore volume was calculated in the same manner as in the calculation method for the median based on surface area described above, except that the surface area was replaced with pore volume. D = -4γ cos θ / P Dv = -4γ cos θ / Pv ５０ Ds=-4γcosθ / Ps ５０ ΔS=4×ΔV / D D: pore diameter Dv: median pore diameter based on pore volume Ds: median pore diameter based on surface area γ: surface tension of water (assumed to be 72.75 mN / m) θ: polyethylene-water contact angle (assumed to be 110°) Pv ５０ Ps: Pressure at which the cumulative pore volume is 50% when the cumulative pore volume (injection amount) when the pressure is increased to 13.7 MPaG is 100% ５０ ΔV: the pressure at which the integrated surface area is 50% when the integrated surface area when the pressure is increased to 13.7 MPaG is taken as 100%. ΔV: the volume of pure water (pore volume) injected when the pressure is increased by one stage. ΔS: the surface area of ​​the pores injected when the pressure is increased by one stage.

[0091] [Shutdown Temperature] The polyolefin microporous membrane was heated at a temperature increase rate of 5°C / min while measuring the air permeability resistance with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). When the air permeability resistance reached the detection limit of 99,999 seconds / 100 cm ３ The temperature reached by the air was determined and designated as the shutdown temperature (°C).

[0092] The measurement cell was made of an aluminum block and had a structure in which a thermocouple was placed directly under the polyolefin microporous membrane. A sample was cut into a 5 cm x 5 cm square and heated while being fixed with an O-ring around the periphery.

[0093] [Average Pore Diameter] The average pore diameter of the polyolefin microporous membrane was determined using a perm porometer (CFP-1500A, manufactured by PMI). Galwick (surface tension: 15.9 dynes / cm) was used as the impregnation liquid for the polyolefin microporous membrane, and measurements were performed in the dry-up and wet-up states. The average pore diameter (nm) was measured based on ASTM E1294-89 (1999) (half-dry method), and the pore diameter was calculated from the pressure (KPa) at the point where the curve showing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The following equation was used to convert pressure and pore diameter:

[0094] d = C γ / P (In the above formula, "d (nm)" is the average pore diameter of the microporous membrane, "γ (dynes / cm)" is the surface tension of the impregnation liquid, "P (KPa)" is the pressure, and "C" is a constant which is set to 2860.)

[0095] [Method for Obtaining Images of Microporous Membranes by FIB-SEM (Focused Ion Beam Scanning Electron Microscope)] Sequential images were obtained by FIB-SEM under the following conditions: Sample preparation: After embedding a polyolefin microporous membrane in an epoxy resin, OsO ４Electron staining was performed using a Helios G4 microscope manufactured by Thermo Fisher Scientific. Observation conditions: accelerating voltage 1 kV. Sample tilt: 52°. Pixel size: horizontal image direction: 4.5 nm, vertical image direction: 5.7 nm (after tilt correction). FIB slice spacing: 10 nm. To confirm the position and align each image, Pt marks were deposited on the side of the film. Tilt correction: Since FIB-SEM observation was performed at a 52° angle, the SEM image was observed compressed in the vertical direction. Therefore, in order to obtain an image observed from the front in the vertical direction, a magnification of 1.27 (= / sin 52°) was required. The tilt-corrected image was used to create the three-dimensional image described below. Measurement size: FIB processing was performed sequentially on an area of ​​4 μm × 4 μm or more on the cross section of the film, and the film was sliced ​​until the depth was 4 μm or more, and images were obtained of an area where a volume of 4 μm × 4 μm × 4 μm (401 captured images) could be secured.

[0096] [Method for Creating Three-Dimensional Images] Three-dimensional images were created from the FIB-SEM images collected using the method described above using Avizo, an image processing and analysis software manufactured by Thermo Fisher Scientific, using the following process. When importing the images into the software, tilt correction was performed so that the image was magnified by 1.27 times in the vertical direction. After aligning the images using the Pt markings deposited on the side of the film as a reference, the images were cropped to a volume of 4 μm × 4 μm × 4 μm. Since voxels must be isotropic to perform the image analysis described below, the voxels were resampled to a size of 5 nm × 5 nm × 5 nm. The microporous membrane was subjected to binarization of the electronically stained portion of the embedding resin (i.e., the portion corresponding to the pores of the microporous membrane) and the resin portion that constituted the microporous membrane. After that, a three-dimensional stereoscopic image (three-dimensional data) was created based on the binarized information, and the porosity of the entire three dimensions was calculated. (The porosity is calculated using a function called Volume Fraction. The software works by counting the number of voxels that are binarized and assigned to voids, and then dividing this by the total number of voxels in the analysis area.) [1.0 x 10 ８ nm ３Number of resin portions having a volume of ≥ 1000, average volume of resin portions] The three-dimensional data created by the above method was subjected to image analysis using the image processing analysis software Avizo manufactured by Thermo Fisher Scientific, using the following process. The binarized embedded resin was subjected to a watershed process to separate the electronically stained portions (pore portions) and the resin portions (film portions) constituting the microporous membrane. The parameters of the separation process (maker extent) were 1 for the pore portions and 2 for the film portions. A PNM (Pore Network Modeling) analysis was performed on the separated resin portions, and the individual volumes of the separated resin portions were calculated. The average volume of the resin portions was calculated by averaging all of the individual resin portion volumes. Furthermore, an analysis filter was applied to the obtained data to create a three-dimensional image in which only film portions having a volume equal to or greater than a specified threshold were displayed, and the number of these portions was counted. The threshold here is 1.0 x 10 ８ nm ３ It was decided.

[0097] [Cycle test method and removal of samples after cycle test] A cycle test and removal of evaluation samples from the battery after the test were performed according to the method described below. Note that the operations described in the sections (Fabrication of positive electrode) to (Fabrication of battery) and (Removal of microporous membrane from battery after cycle test) in the procedure shown below were performed in a dry room with a dew point of −30° C. or lower.

[0098] (Preparation of Positive Electrode) 94 parts by mass of nickel-cobalt-manganese composite oxide NMC (Ni:Co:Mn = 6:2:2 (element ratio)) as the positive electrode active material, 3 parts by mass of carbon black as the conductive material, and 3 parts by mass of polyvinylidene fluoride (PVDF) as the resin binder were mixed in a ratio of 100 parts by mass, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was uniformly applied to both sides of a 20 μm thick aluminum foil that served as a positive electrode current collector, dried, and then compression molded using a roll press. At this time, the amount of the positive electrode mixture applied to one side was 170 g / m ２ , density is 3.0 g / cm ３ It was made so that

[0099] (Preparation of Negative Electrode) 97 parts by mass of artificial graphite was used as the negative electrode active material, and 1 part by mass of carboxymethyl cellulose and 2 parts by mass of styrene-butadiene copolymer latex were mixed as the resin binder in a ratio of 1 part by mass and 2 parts by mass, and these were dispersed in purified water to prepare a slurry. This slurry was uniformly applied to both sides of a 10 μm-thick copper foil that served as a negative electrode current collector, dried, and then compression-molded using a roll press. At this time, the amount of the negative electrode mixture applied to one side was 100 g / m ２ , density is 1.5 g / cm ３ It was made so that

[0100] (Non-aqueous electrolyte) A mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 3:3:4 (volume ratio) was used, and LiPF 6 was added as a solute. ６ was dissolved to a concentration of 1.0 mol / L. One part by mass of vinylene carbonate was added to 100 parts by mass of this solution to prepare a non-aqueous electrolyte solution.

[0101] (Battery Fabrication) A flat wound electrode body was fabricated using the above positive electrode, the microporous membrane of this embodiment, and the above negative electrode. This flat wound electrode body was sandwiched between aluminum laminate films and sealed except for a partial opening. After drying in a vacuum oven at 80°C for 6 hours, the above nonaqueous electrolyte solution was poured into the electrode body and the electrode body was sealed with a vacuum sealer to fabricate a laminated secondary battery having a size of 32 mm x 32 mm and a battery capacity of 300 mAh.

[0102] (Preliminary charge / discharge treatment) The battery prepared above was charged at a constant current of 60 mA up to a battery voltage of 4.2 V, then charged at a constant voltage of 4.2 V up to a current of 15 mA, and after a 30-minute break, discharged at a constant current of 60 mA down to a battery voltage of 2.5 V, followed by a 30-minute break. Three cycles of the above charge / discharge were carried out to prepare a battery for cycle testing.

[0103] (Cycle Test) The laminated secondary battery was charged at a constant current of 900 mA (3 C) up to a battery voltage of 4.2 V, then charged at a constant voltage of 4.2 V up to a current of 15 mA (0.05 C), and after a 30-minute break, discharged at a constant current of 300 mA (1 C) down to a battery voltage of 2.5 V, followed by a 30-minute break. The above charge / discharge cycle was repeated 800 times.

[0104] (Removal of microporous membrane from battery after cycle test) The battery cell subjected to the cycle test was opened, and only the microporous membrane was removed. The removed microporous membrane was immersed in 50 mL of dimethyl carbonate for 5 minutes, rinsed with 50 mL of fresh dimethyl carbonate, and thoroughly air-dried to obtain a sample for evaluation.

[0105] [10 μm thickness converted resistance of polyolefin microporous membrane] (Ω cm ２ Twelve circular samples with a diameter of 19 mm were cut out as measurement samples, and were vacuum dried together with the components of a CR2032 coin cell purchased from Hosen Co., Ltd. (top cover, bottom cover, gasket (made of PP), spacer (cylindrical, 15.5 mm diameter, 1.0 mm thick), and wave washer) to remove moisture. In a dry room with an outdoor temperature of -35°C or less, three measurement samples and a PP gasket were placed in this order on the inside bottom of the bottom cover of the coin cell component, and LiPF was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 4:6 [volume ratio]) to a concentration of 1 M. ６ 0.15 mL of an electrolyte solution (manufactured by Kishida Chemical Co., Ltd.) containing the above dissolved material was injected. After placing a spacer on the polyolefin microporous membrane, the membrane was left standing for 10 minutes at a pressure of approximately -50 kPa, allowing the measurement sample to be impregnated with the electrolyte. The cell was then covered with a wave washer and a top lid, and sealed with a coin cell crimper (manufactured by Hosen Co., Ltd.) to obtain a sample cell.

[0106] The obtained sample cell was placed in a high-temperature thermostatic bath at 25°C and allowed to stand for 2 hours, after which the cell resistance (real value when the imaginary axis value is 0) was measured using an impedance analyzer (manufactured by Hioki E.E. Corp.) at an amplitude of 20 mV and a frequency of 200 kHz. The same measurement was also performed on cells separately prepared with 4 and 5 polyolefin microporous membranes. The cell resistance was plotted against the number of polyolefin microporous membranes placed in the cell, and the slope (resistance per polyolefin microporous membrane) Z obtained by linear approximation was used to calculate the 10 μm thickness equivalent resistance of the polyolefin microporous membrane according to the following formula:

[0107] 10 μm thickness equivalent resistance (Ω・cm ２ / 10 μm) = Z × spacer area × 10 / polyolefin microporous membrane thickness The area of ​​the spacer is 2.01 cm ２ It was decided.

[0108] The resistance of the polyolefin microporous membrane converted into a 10 μm thickness was evaluated according to the following criteria and used as an index of output characteristics when used as a separator. ２ / Less than 10 μm ×: 1.0 Ω cm ２ / 10 μm or more.

[0109] [Safety of polyolefin microporous membrane] The safety of polyolefin microporous membranes used as secondary battery separators was evaluated according to the following criteria, and a rating of A or B was considered acceptable. A: All three of the following items were ◯ B: Of the three items below, 1 to 3 △ and 0 × C: Other than A and B (one or more × of the three items below) Item 1 ◯: Puncture strength calculated as basis weight was 900 mN or more △: Puncture strength calculated as basis weight was 700 mN or more but less than 900 mN ×: Puncture strength calculated as basis weight was less than 700 mN Item 2 ◯: Average pore size was 30 nm or less △: Average pore size was more than 30 nm but not more than 35 nm ×: Average pore size exceeded 35 nm Item 3 ◯: Shutdown temperature was 137°C or less △: Shutdown temperature was more than 137°C but not more than 140°C ×: Shutdown temperature exceeded 140°C.

[0110] [Example 1] A polyolefin raw material was prepared by adding a resin A having a Mw of 1.2 × 10 ６70% by mass of ultra-high molecular weight polyethylene having a melting point of 134°C, and resin B having a Mw of 6.0 × 10 ４ 30% by mass of high-density polyethylene with a melting point of 132°C and a ΔH of 220 J / g was used. 75% by mass of liquid paraffin was added to 25% by mass of the above polyolefin raw material, and 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 charged into a twin-screw extruder and kneaded at 180°C to prepare a polyolefin solution. The obtained polyolefin solution was fed into a T-die whose temperature was adjusted to 200°C, and the shear rate at the outlet of the die was 350 sec. －１ The microporous membrane was melt-extruded under conditions such that the Q / Ns ratio was 1.8, and cooled on a casting drum controlled at 35°C to form a gel-like sheet. The gel-like sheet was subjected to a first stretching using a simultaneous biaxial tenter stretching machine at 118°C to a stretching ratio of 5x in both the MD and TD directions, and a stretching speed of 1000% / min in both the MD and TD directions. The wet-stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin, and then air-dried. Subsequently, as a second stretching, the microporous membrane was stretched in the MD direction on a roll set at 100°C using a roll-type stretching machine to a stretching ratio of 1.7x, and then stretched in the TD direction at 130°C using a tenter-type stretching machine to obtain a polyolefin microporous membrane. The film-forming conditions for the polyolefin microporous membrane are shown in Table 1, and the evaluation results of the obtained polyolefin microporous membrane are shown in Table 2.

[0111] Examples 2 to 4, Comparative Examples 1 to 8 The same procedures as in Example 1 were carried out except that the raw materials and film-forming conditions were changed as shown in the table.

[0112] Examples and Comparative Examples are shown in Tables 1 to 5. Table 5 shows the results of various analyses of the polyolefin microporous membranes of Example 1 and Comparative Example 8 after the cycle tests were carried out by the above-mentioned methods.

[0113]

[0114]

[0115]

[0116]

[0117]

Claims

1. For the arithmetic mean roughness Sa (nm) of one surface and the other surface of a microporous membrane, the Sa of the surface with the larger arithmetic mean roughness is Sa １ The surface with the smaller arithmetic mean roughness is Sa. ２ When the median pore diameter based on the surface area determined by pure water intrusion porosimeter measurement is Ds and the median pore diameter based on the pore volume is Dv, the formula 3 is satisfied, and the puncture strength converted into unit basis weight is 700 mN / (g / m ２ ) or more. １ +Sa ２ ) / 2 ≦ 80 Formula 2: Sa １ / Sa ２ ≦1.4 Formula 3: 1.0≦Dv / Ds≦1.2 2. In the three-dimensional structure of a 4 μm square obtained by measuring the microporous membrane with a focused ion beam scanning electron microscope (FIB-SEM), the ８ nm ３ The microporous polyolefin membrane according to claim 1 , wherein the number of resin portions having a volume of at least 10 is 10 or less.

3. In the three-dimensional structure of a 4 μm square obtained by measuring the microporous membrane with a focused ion beam scanning electron microscope (FIB-SEM), the ８ nm ３ The microporous polyolefin membrane has 10 or less resin portions each having a volume of at least 100 μm.

4. In the three-dimensional structure of a 4 μm square obtained by FIB-SEM measurement of the microporous membrane, the average volume of the resin portion is 1.3 × 10 ７ nm ３ The microporous polyolefin membrane according to claim 1 or 3, wherein:

5. The microporous polyolefin membrane according to claim 1 or 3, having an average pore size measured by a porometer of 35 nm or less.

6. The microporous polyolefin membrane according to claim 1 or 3, having a shutdown temperature of 140° C. or less.

7. The microporous polyolefin membrane according to claim 1 or 3, in which the standard deviation of the orientation parameter values ​​measured by Raman spectroscopy at a total of 12 points at 15° intervals in any direction in the microporous membrane plane is 0.30 or less.

8. The microporous polyolefin membrane according to claim 1 or 3, which has a melting point of 134° C. or lower as measured by differential scanning calorimetry (DSC).

9. In the differential molecular weight distribution curve measured by gel permeation chromatography (GPC), the largest molecular weight that is 20% higher than the maximum value in the molecular weight range of 10,000 to 10 million is defined as M １ , the smallest one is M ２ The polyolefin microporous membrane according to claim 1 or 3, which satisfies the following formula 4 when M is expressed as １ / M ２ ≧500 10. The microporous polyolefin membrane according to claim 1 or 3, in which, in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC), the area ratio of the peak area of ​​a molecular weight of 50,000 or less to the total molecular weight components is 10% or more, and the area ratio of the peak area of ​​a molecular weight of 1,000,000 or more is 10% or more.

11. A battery separator using the microporous polyolefin membrane according to claim 1 or 3.

12. A secondary battery using the battery separator according to claim 11.

13. The standard deviation of the orientation parameter values ​​measured at 12 points at 15° intervals in any direction within the microporous membrane plane by Raman spectroscopy is 0.3 or less, and the pin puncture strength converted into unit basis weight is 700 mN / (g / m ２ ) or more.

14. A liquid filter using the polyolefin microporous membrane according to claim 1 or 3.

15. A filtration unit using the liquid filter according to claim 14.

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