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

Abstract

Provided is a polyolefin microporous membrane that, when used as a separator for a battery, imparts high output characteristics and high safety and can contribute to miniaturization and high output of the battery, and that, when used as a liquid filtration filter, can eliminate the trade-off between a smaller pore diameter and higher permeability. 【Solution means】Regarding the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, when Sa of the surface with the larger arithmetic mean roughness is defined as Sa1 and Sa of the surface with the smaller arithmetic mean roughness is defined as Sa2, the following formulas 1 and 2 are satisfied. When the median pore diameter based on the surface area determined by mercury intrusion porosimetry measurement is Ds and the median pore diameter based on the pore volume is Dv, the following formula 3 is satisfied. The polyolefin microporous membrane has a puncture strength per unit basis weight of 700 mN / (g / m 2 ) or more. Formula 1: 10 ≦ (Sa1 + Sa2) / 2 ≦ 80 Formula 2: Sa1 / Sa2 ≦ 1.4 Formula 3: 1.0 ≦ Dv / Ds ≦ 1.2

Description

Technical Field

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

Background Art

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

[0003] In addition, due to its uniform fine pore structure and excellent solvent resistance and chemical resistance, polyolefin microporous membranes are also widely used in various filter applications such as water treatment membranes, ultrafiltration membranes, microfiltration membranes, and moisture-permeable and waterproof clothing.

[0004] Lithium-ion secondary batteries are in increasing demand for miniaturization and high output from the perspective of convenience in various applications. Along with this, the separator is required to be thinner and have lower resistance. Since the strength of the separator decreases due to thinning and lower resistance, short circuits (resistance to foreign substances) caused by electrodes or foreign objects and membrane breakage (decrease in impact resistance) are likely to occur when the battery is subjected to impact, resulting in a decrease in battery safety. Therefore, it is required to design the resin part constituting the microporous membrane to have higher strength than before.

[0005] In addition, in small, high-output lithium-ion secondary batteries, lithium precipitation (dendrites) is likely to occur, so the demand for dendrite resistance in separators is also increasing. Dendrites in lithium-ion secondary batteries are needle-like crystals that form near the interface between the negative electrode and the separator during charge and discharge, and if they grow and penetrate the separator, they may cause a short circuit. Therefore, in batteries where dendrites are likely to occur, separators with small pore diameters that are difficult for dendrites to penetrate are required. On the other hand, when the pore diameter of the microporous membrane is reduced, the permeability may decrease and the resistance may deteriorate.

[0006] Furthermore, the separator is also required to have a function of ensuring safety when the battery overheats abnormally. The above-mentioned shutdown is a function of cutting off the current by melting the polyolefin microporous membrane and clogging the pores when the battery overheats abnormally. Especially in high-output designed batteries, since the risk of abnormal heat generation is high, it is preferable to shut down at a lower temperature. On the other hand, in order to lower the shutdown temperature of the polyolefin microporous membrane, generally a design that reduces the crystal orientation and crystallinity of the polyolefin is adopted, and in some cases, the strength and permeability may decrease.

[0007] Regarding the application of liquid filtration filters, for example, when used for foreign matter removal in semiconductor manufacturing processes, it is required to be able to efficiently collect foreign matter with a smaller diameter. On the other hand, from the perspective of processing capacity, it is not preferable for the permeability to deteriorate.

[0008] As described above, with the miniaturization and high output of lithium-ion secondary batteries, the separator is required to achieve both low resistance and high safety at a higher level, which are conflicting performances. For example, Patent Document 1 discloses a laminated porous polyolefin film that has excellent safety and output characteristics when used as a separator by setting the relationship between the puncture strength, porosity, melting point, and shutdown temperature within a predetermined range. Further, Patent Document 2 describes a polyolefin microporous film that can ensure safety while having excellent cycle characteristics and output characteristics by defining the lithium ion diffusion coefficient and shutdown temperature in the film thickness direction. Additionally, Patent Document 3 describes a porous polyolefin film with excellent output characteristics, safety, and process transportability by defining the porosity, puncture strength, and tensile elastic modulus in the longitudinal direction.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] The microporous film described in Patent Document 1 improves the shutdown characteristics by a laminated design with a low melting point layer (shutdown layer). However, when the microporous film is made thinner, there may be a case where due to insufficient basis weight of the shutdown layer, even if it is melted and closed, it cannot be completely insulated and safety cannot be ensured. Also, in the microporous film described in Patent Document 2, due to the increase in pore size and the decrease in strength caused by blending inorganic particles, it may not be possible to ensure the high level of safety required in recent years. Further, the microporous film described in Patent Document 3 has a high dry stretching ratio and high strength. However, usually, when the dry stretching ratio is high, the microporous film has an increased pore size and an increased shutdown temperature, which is a problem.

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

Means for Solving the Problems

[0012] 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〕Regarding the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, when Sa of the surface with the larger arithmetic mean roughness is Sa1 and Sa of the surface with the smaller arithmetic mean roughness is Sa2, the following formulas 1 and 2 are satisfied, and when the median pore diameter based on the surface area obtained by mercury intrusion porosimeter measurement is Ds and the median pore diameter based on the pore volume is Dv, the following formula 3 is satisfied, and the puncture strength in terms of unit basis weight is 700 mN / (g / m 2 ) or more polyolefin microporous membrane.

[0013] Formula 1: 10 ≦ (Sa1 + Sa2) / 2 ≦ 80 Formula 2: Sa1 / Sa2 ≦ 1.4 Formula 3: 1.0 ≦ Dv / Ds ≦ 1.2 〔2〕In the three-dimensional structure of 4 μm square obtained by focused ion beam scanning electron microscope (FIB-SEM) measurement of the microporous membrane, the number of resin parts having a volume of 1.0×10 8 nm 3 or more is 10 or less, the polyolefin microporous membrane according to 〔1〕. 〔3〕In the three-dimensional structure of 4 μm square obtained by focused ion beam scanning electron microscope (FIB-SEM) measurement of the microporous membrane, the number of resin parts having a volume of 1.0×10 8 nm 3 or more is 10 or less, the polyolefin microporous membrane. 〔4〕In the 4-μm square three-dimensional structure obtained by FIB-SEM measurement of the microporous membrane, the average volume of the resin part is 1.3×10 7 nm 3 or less, the polyolefin microporous membrane according to [1] or [3].

[0014] 〔5〕The polyolefin microporous membrane according to any one of [1] to [4], wherein the average pore diameter measured by a porometer is 35 nm or less. 〔6〕The polyolefin microporous membrane according to any one of [1] to [5], wherein the shutdown temperature is 140 °C or less. 〔7〕The polyolefin microporous membrane according to any one of [1] to [6], wherein the standard deviation of the orientation parameter values measured at 12 points at 15° intervals in any direction in the plane of the microporous membrane by Raman spectroscopy is 0.3 or less. 〔8〕The polyolefin microporous membrane according to any one of [1] to [7], wherein the melting point measured by differential scanning calorimetry (DSC) is 134 °C or less. 〔9〕In the differential molecular weight distribution curve measured by gel permeation chromatography (GPC), when the largest molecular weight having a height of 20% with respect to the maximum value in the range of molecular weight from 10,000 to 10 million is M1 and the smallest one is M2, the polyolefin microporous membrane according to any one of [1] to [8] that satisfies the following formula 4.

[0015] Formula 4: M1 / M2 ≧ 500 〔10〕In the differential molecular weight distribution curve measured by gel permeation chromatography (GPC), the area ratio of the 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 the molecular weight of 1 million or more is 10% or more, the polyolefin microporous membrane according to any one of [1] to [9]. 〔11〕A separator for a battery using the polyolefin microporous membrane according to any one of [1] to

[10] . 〔12〕A secondary battery using the battery separator according to

[11] . 〔13〕The standard deviation of the orientation parameter values measured at 12 points at 15° intervals for any direction in the plane of the microporous membrane by Raman spectroscopy is 0.3 or less, and the puncture strength in terms of unit basis weight is 700 mN / (g / m 2 ) or more for the polyolefin microporous membrane. 〔14〕A liquid filtration filter using the polyolefin microporous membrane according to any one of 〔1〕to 〔10〕. 〔15〕A filtration unit using the liquid filtration filter according to 〔14〕.

Advantages of the Invention

[0016] According to the present invention, when used as a battery separator, it is possible to provide a polyolefin microporous membrane that imparts high output characteristics and high safety and contributes to the miniaturization and high output of the battery. In addition, when used for liquid filtration filter applications, it is excellent in filtration accuracy and permeability, so it can be suitably used as a high-precision filter that requires removal of minute foreign matters such as in semiconductor processes.

Embodiments for Carrying Out the Invention

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

[0018] 〔Polyolefin Microporous Membrane〕 As one aspect of the polyolefin microporous membrane according to the embodiment of the present invention, regarding the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, when Sa of the surface with the larger arithmetic mean roughness is Sa1 and Sa of the surface with the smaller arithmetic mean roughness is Sa2, the following formulas 1 and 2 are satisfied, and when the median pore diameter based on the surface area obtained by mercury intrusion porosimetry measurement is Ds and the median pore diameter based on the pore volume is Dv, formula 3 is satisfied, and the puncture strength in terms of unit basis weight is 700 mN / (g / m 2 ) or more for the polyolefin microporous membrane.

[0019] Formula 1: 10 ≦ (Sa1 + Sa2) / 2 ≦ 80 Formula 2: Sa1 / Sa2 ≤ 1.4 Formula 3: 1.0 ≤ D V / D S ≤ 1.2 As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, regarding the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, when Sa of the surface with the larger arithmetic mean roughness is defined as Sa1 and Sa of the surface with the smaller arithmetic mean roughness is defined as Sa2, a value obtained by (Sa1 + Sa2) / 2, that is, a polyolefin microporous membrane in which the average value of the arithmetic mean roughness on the front and back is 10 nm or more and 80 nm or less can be mentioned. Preferably it is 10 nm or more and 70 nm or less, more preferably 10 nm or more and 60 nm or less, still more preferably 10 nm or more and 50 nm or less, and particularly preferably 10 nm or more and 45 nm or less. By setting (Sa1 + Sa2) / 2 within the above range, the microporous membrane becomes smooth macroscopically, and the pore structure becomes uniform and fine microscopically, so that it has excellent shutdown performance and dendrite resistance, and can be a separator with low resistance. In addition, it can be a polyolefin microporous membrane excellent in filtration accuracy and permeability when used as a liquid filtration filter. Specifically, the arithmetic mean roughness Sa can be measured by the method described in the examples. As a method for setting the value of (Sa1 + Sa2) / 2 within the above range, controlling the molecular weight distribution of the microporous membrane, or setting the raw material composition and film-forming conditions within the ranges described later can be mentioned.

[0020] As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, there is a polyolefin microporous membrane in which the value obtained by Sa1 / Sa2 is 1.4 or less. Preferably it is 1.35 or less, more preferably 1.3 or less, still more preferably 1.2 or less, and particularly preferably 1.1 or less. By setting Sa1 / Sa2 within the above range, the structural difference between the front and back of the microporous membrane can be reduced, and the separator can be made to have a low resistance. In addition, the permeability can be improved when used as a liquid filtration filter. From the above description, the lower limit of Sa1 / Sa2 is not particularly limited, but from the viewpoint of compatibility with the productivity of the microporous membrane, it is, for example, 1.001 or more. As a method for setting the value of Sa1 / Sa2 within the above range, controlling the molecular weight distribution of the microporous membrane, or setting the raw material composition and film-forming conditions within the ranges described later can be mentioned.

[0021] As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the median pore diameter based on the surface area obtained by mercury intrusion porosimeter measurement is D S , and the median pore diameter based on the pore volume is D V . When V / D S is 1.0 or more and 1.2 or less, there is a polyolefin microporous membrane. Preferably it is 1.0 or more and 1.15 or less, more preferably 1.0 or more and 1.1 or less, still more preferably 1.0 or more and 1.05 or less. By setting D V / D S within the above range, large pores are reduced, resulting in a microporous membrane having excellent shutdown performance and dendrite resistance. In addition, the filtration accuracy can be improved when used as a liquid filtration filter. As a method for setting D V / D S within the above range, controlling the molecular weight distribution of the microporous membrane, or setting the raw material composition and film-forming conditions within the ranges described later can be mentioned.

[0022] As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, the puncture strength in terms of unit basis weight is 700 mN / (g / m 2) The polyolefin microporous membrane described above can be mentioned. Preferably, it is 800 mN / (g / m 2 ) or more, more preferably 900 mN / (g / m 2 ) or more, still 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. Also, when used as a liquid filtration filter, it is possible to suppress the deformation of the pore structure due to pressurization during filtration and improve the filtration accuracy. From the above description, the upper limit of the puncture strength in terms of unit basis weight is not particularly limited, but from the perspective of compatibility with thermal shrinkage, for example, it is 3000 mN / (g / m 2 ) or more. In order to set the value of the puncture strength in terms of unit basis weight within the above range, it is preferable that the raw material composition and film-forming conditions of the microporous membrane are within the ranges described later.

[0023] The polyolefin microporous membrane according to an embodiment of the present invention preferably has an average pore diameter of 35 nm or less, more preferably 32 nm or less, still more preferably 28 nm or less, and particularly preferably 25 nm or less. By setting the average pore diameter within the above range, a microporous membrane excellent in strength and dendrite resistance can be obtained. Also, the lower limit of the average pore diameter is not particularly limited, but from the perspective of compatibility with the permeability of the microporous membrane, it is preferably 14 nm or more, and 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 less, more preferably 139°C or less, still more preferably 138°C or less, and particularly preferably 137°C or less. By setting the shutdown temperature within the above range, since the low-temperature shutdown performance is excellent, the safety of the separator can be enhanced. From the above perspective, the lower limit of the shutdown temperature is not particularly limited, but from the perspective of compatibility with the permeability and strength of the microporous membrane, it is preferably 125°C or more.

[0025] As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, there is provided a polyolefin microporous membrane in which the standard deviation of the orientation parameter measured at 12 points at 15° intervals in any direction within the plane of the microporous membrane by Raman spectroscopy is 0.30 or less. The orientation parameter of the polyolefin microporous membrane by Raman spectroscopy can be calculated by the following formula. I = I 1130 / I 1063 I: Orientation parameter I 1130 : Peak intensity at 1130 cm -1 I 1063 : Peak intensity at 1060 cm -1 The 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 part) constituting the microporous membrane are uniformly oriented within the film plane, so that the pore structure is uniformized, and when used as a separator for a battery, a microporous membrane excellent in shutdown characteristics, dendrite resistance, and permeability can be obtained. Further, when used as a filtration filter for liquids, a polyolefin microporous membrane excellent in filtration accuracy and permeability can be obtained. From the above viewpoints, the lower limit of the standard deviation of the orientation parameter is not particularly limited, but it is preferably 0.01 or more from the viewpoint of compatibility with productivity. In order to set the standard deviation of the orientation parameter within the above range, it is preferable that the raw material composition and film-forming conditions of the microporous membrane are within the ranges described later.

[0026] The polyolefin microporous membrane according to an embodiment of the present invention preferably has a melting point measured by the DSC method of 134°C or lower, more preferably 133°C or lower, and even more preferably 132°C or lower. By having the melting point within the above range, in addition to excellent shutdown characteristics, it is possible to make the pore structure of the microporous membrane 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 compatibility with the permeability and strength of the microporous membrane. As a method for setting the melting point of the polyolefin microporous membrane within the above range, controlling the molecular weight distribution of the microporous membrane or setting the raw material composition within the range described later can be mentioned.

[0027] In the differential molecular weight distribution curve measured by the GPC method for the polyolefin microporous membrane according to an embodiment of the present invention, when the largest molecular weight having a height of 20% with respect to the maximum value in the molecular weight range of 10,000 to 10,000,000 is M1 and the smallest is M2, it is preferable that M1 / M2 is 500 or more. M1 / M2 is more preferably 600 or more, even more preferably 700 or more, and particularly preferably 800 or more and 1000 or less. By setting M1 / M2 within the above range, it becomes possible to achieve both the shutdown characteristics and strength of the polyolefin microporous membrane, adjust the standard deviation of the orientation parameter in the plane of the above-mentioned polyolefin microporous membrane to a preferable range, and in the three-dimensional structure of 4 μm square obtained by focused ion beam scanning electron microscope (FIB-SEM) measurement of the microporous membrane, 1.0×10 8 nm 3 The number of resin portions having a volume of the above or more can be easily adjusted to a preferable range. In order to set M1 / M2 of the polyolefin microporous membrane within the above range, it is preferable to set the raw material composition and kneading conditions of the microporous membrane within the range described later.

[0028] The polyolefin microporous membrane according to an embodiment of the present invention preferably has an area ratio of 10% or more with respect to the peak area of all molecular weight components and an area ratio of 10% or more with respect to the molecular weight of 1,000,000 or more in the differential molecular weight distribution curve measured by the gel permeation chromatography (GPC) method described later. The area ratio with respect to the molecular weight of 50,000 or less is more preferably 15% or more, and still more preferably 20% or more. Further, the area ratio with respect to the molecular weight of 50,000 or less is preferably 35% or less, more preferably 30% or less, and still more preferably 25% or less. The area ratio with respect to the molecular weight of 1,000,000 or more is more preferably 15% or more, and still more preferably 20% or more. Further, the area ratio with respect to the molecular weight of 1,000,000 or more is preferably 35% or less, more preferably 30% or less, and still more preferably 25% or less. By setting the amounts of the molecular weight of 50,000 or less and the molecular weight of 1,000,000 or more in the polyolefin microporous membrane within the above ranges, it is possible to achieve both the shutdown characteristics 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 described above to a preferable range.

[0029] As one aspect of the polyolefin microporous membrane according to an embodiment of the present invention, in the three-dimensional structure of 4 μm square obtained by FIB-SEM measurement of the microporous membrane, 1.0×10 8 nm 3 There is mentioned a polyolefin microporous membrane having 10 or less resin portions having a volume of the above or more. More preferably, it is 8 or less, still more preferably 5 or less, and particularly preferably 1 or less. By setting the number of resin portions having a volume of 1.0×10 8 nm 3 or more within the above range, the ionic conductivity is improved when used as a separator for a battery, and since it has a uniform structure, it is excellent in shutdown performance and dendrite resistance. Further, it is possible to obtain a polyolefin microporous membrane excellent in filtration accuracy and permeability when used as a liquid filtration filter. In the three-dimensional structure of 4 μm square by FIB-SEM, 1.0×10 8 nm 3The method for measuring the number of resin portions having the above volume can specifically be measured by the method described in the examples. 1.0×10 8 nm 3 In order to make the number of resin portions having the above volume within the above range, it is preferable that the raw material composition and film-forming conditions of the microporous membrane be within the ranges described later.

[0030] The polyolefin microporous membrane according to an embodiment of the present invention has an average volume per resin portion observed in a 4-μm square three-dimensional structure obtained by FIB-SEM measurement of the microporous membrane of 1.3×10 7 nm 3 or less, more preferably 1.2×10 7 nm 3 or less, still more preferably 1.0×10 7 nm 3 or less. By setting the average volume per resin portion within the above range, the ion conductivity is improved when used as a separator for a battery. Further, a polyolefin microporous membrane excellent in filtration accuracy and permeability can be obtained when used as a filtration filter for liquids. From the above viewpoints, the lower limit of the average volume of the resin portion is not particularly limited, but from the viewpoint of compatibility with the permeability and heat shrinkage rate of the microporous membrane, it is preferably 1.0×10 6 nm or more. In order to make the average volume of the resin portion within the above range, it is preferable that the raw material composition and film-forming conditions of the microporous membrane be within the ranges described later.

[0031] The polyolefin microporous membrane according to an embodiment of the present invention preferably has an air permeability in terms of thickness of 30 seconds / 100 cm 3 / μm or less, more preferably 20 seconds / 100 cm 3 / μm or less, still more preferably 15 seconds / 100 cm 3 / μm or less. The lower limit of the air permeability in terms of thickness is not particularly provided, but since it becomes easy to balance with the film strength, 1 second / 100 cm 3It is preferably at least / μm. By setting the air permeability in terms of thickness within the above range, when the microporous membrane is used as a separator for a battery, a microporous membrane with excellent charge-discharge characteristics can be obtained. The air permeability in terms of thickness can be set within the above range by adjusting the blending ratio of raw materials, the drawing ratio, the heat setting conditions, etc. in the manufacturing process.

[0032] The thickness of the polyolefin microporous membrane according to the embodiment of the present invention can be appropriately adjusted according to the application, but is preferably 20 μm or less, more preferably 15 μm or less, and still more preferably 12 μm or less. Also, it is 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, when used as a separator for a battery, both safety and high battery capacity can be achieved. The thickness can be set within the above range by appropriately adjusting the film-forming conditions such as the extrusion conditions.

[0033] 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 still more preferably 40% or more. Although no upper limit is particularly provided for the porosity, it is preferably 80% or less because a decrease in film strength can be suppressed. When the porosity is within the above range, the output characteristics are excellent when the microporous membrane is used as a separator for a secondary battery. The porosity can be set within the above range by adjusting the raw material formulation, the drawing ratio, the heat setting conditions, etc. in the manufacturing process.

[0034] Hereinafter, the specific configuration of the polyolefin microporous membrane in the present invention will be described, but it 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. Here, the main component refers to the component having the largest content in terms of mass% among the components constituting the polyolefin microporous membrane. Further, the polyethylene-based resin component in the polyolefin microporous membrane is preferably 80% by mass or more, more preferably 90% by mass or more, still 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 range, the film-forming property and uniformity of the microporous membrane are excellent, and at the same time, the performance balance as a battery separator such as film strength and permeability is excellent. Here, the polyolefin microporous membrane may contain two or more kinds of polyethylene-based resins. In that case, the total amount of the polyethylene-based resins is defined as the amount of the polyethylene-based resin component constituting the polyolefin microporous membrane.

[0036] Various polyethylene-based resins can be used for the polyolefin microporous membrane according to an embodiment of the present invention, and examples include ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, branched low density polyethylene, linear low density polyethylene, and the like. Further, the polyethylene-based resin may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. Here, the polyethylene-based resin contains ethylene in an amount exceeding 50 mol% with respect to all raw material monomer components.

[0037] The polyolefin microporous membrane according to an embodiment of the present invention preferably contains ultra-high molecular weight polyethylene (hereinafter described as resin A) among the above-mentioned polyethylenes, and more preferably contains resin A and high density polyethylene (hereinafter described 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, still more preferably 1,000,000 or more, still more preferably 2,000,000 or more, and particularly preferably 2,400,000 or more. Also, 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, it becomes easy to increase the strength of the polyolefin microporous membrane, reduce the arithmetic mean roughness (Sa) by making the pore structure uniform and finer, and reduce the average volume of the resin portion.

[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. Also, it is preferably 125°C or higher, 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 finer, and the shutdown characteristics also become good. Also, the structural difference between both surfaces of the polyolefin microporous membrane is reduced, and it becomes easier to adjust the value of Sa1 / Sa2 to an appropriate range. Although the relationship between the melting point of resin A and the structural difference between both surfaces of the polyolefin microporous membrane has not been clarified, it is presumed that by controlling the melting point of resin A, that is, the crystallization behavior when the resin cools and solidifies, the structural difference caused by the cooling rate difference between the front and back is reduced in the process of forming the gel-like sheet described later. When using 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 lower as resin A, it becomes easy to make the pore structure of the polyolefin microporous membrane uniform and finer, and also reduces the structural difference between both surfaces of the polyolefin microporous membrane, making it easier to adjust the value of Sa1 / Sa2 to an appropriate range.

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

[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 becomes easier to increase the membrane strength and to refine the pore structure.

[0042] The high-density polyethylene (density: 0.940 g / m 3 or more and 0.970 g / m 3 or less) used as Resin B preferably has a weight-average molecular weight (Mw) of 10,000 or more, more preferably 20,000 or more, and even more preferably 50,000 or more. Also, 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, a structure in which fibrils are evenly oriented in the film plane after stretching can be obtained, and it becomes easy to adjust the value of D V / D S and the number of resin portions having a volume of 1.0×10 8 nm 3 or more to a preferable range.

[0043] The melting point of Resin B is more preferably 128°C or more, and even more preferably 130°C or more. Also, it is preferably 135°C or less, and more preferably 134°C or less. By setting the melting point of Resin B within the above range, in addition to the pore structure being refined, the shutdown characteristics are excellent.

[0044] The heat of crystal 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 crystal fusion (ΔH) of resin B within the above range, the film strength can be increased while suppressing the increase in the shutdown temperature. Although no upper limit is particularly provided for the heat of crystal fusion (ΔH) of resin B from the above viewpoints, it is preferably 280 J / g or less from the viewpoint of film-forming properties.

[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. Also, 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, it becomes easy to adjust the number of resin portions having a volume of 1.0×10 8 nm 3 or more to a preferable range, suppress excessive orientation of crystals during stretching, and achieve good shutdown characteristics.

[0046] The polyolefin microporous membrane according to the embodiment of the present invention may contain a resin other than the polyethylene-based resin. For example, adding a polypropylene-based resin is preferable from the viewpoint of improving the heat resistance of the microporous membrane. As the type of the polypropylene-based resin, in addition to homopolypropylene, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer can contain a copolymer component with an α-olefin other than propylene. Examples of the α-olefin include ethylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. Here, the polypropylene-based resin is defined as containing propylene in an amount exceeding 50 mol% based on all raw material monomer components.

[0047] The addition amount of the polypropylene-based resin 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 the above range, a polyolefin microporous membrane excellent in productivity, quality, and strength can be obtained.

[0048] The polyolefin microporous membrane can contain, if necessary, resin components other than the polyethylene-based resin and the polypropylene-based resin. Further, within a range not impairing the effects of the present invention, various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent, a filler, a crystal nucleating agent, and a crystallization retarder may be contained.

[0049] [Manufacturing method of polyolefin microporous membrane] Next, the manufacturing method of the polyolefin microporous membrane in the embodiment of the present invention will be described. Examples of the manufacturing method of the polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. From the viewpoint of controlling the structure and physical properties of the film, the wet film-forming method is preferable as the manufacturing method of the polyolefin microporous membrane in the present embodiment.

[0050] Hereinafter, the manufacturing method of the polyolefin microporous membrane by wet method will be described. Note that the following description is an example of the manufacturing method and is not limited to this method.

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

[0052] (1) A step of melt-kneading the polyolefin resin and the film-forming solvent to prepare a polyolefin resin composition (2) A step of extruding the polyolefin resin composition and cooling it to form a gel sheet (3) A first stretching step of preheating and stretching the gel sheet (4) Step of removing the film-forming solvent from the extended gel-like sheet (5) Step of drying the sheet after removing the film-forming solvent (6) Second stretching step of preheating and stretching the dried sheet (7) Step of heat-treating the dried sheet.

[0053] (1) Preparation step 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 capable of uniformly dispersing the polyolefin resin, but in order to enable stretching at a relatively high magnification, it is preferable that the solvent is liquid at room temperature. Examples of the solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding thereto, and phthalic acid esters such as dibutyl phthalate and dioctyl phthalate that are liquid at room temperature. In order to obtain a stable gel-like sheet, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0054] The blending ratio of the polyolefin resin and the plasticizer is preferably such that the content of the polyolefin resin is 10 to 50% by mass based on the total mass of the polyolefin resin composition. By setting the content of the polyolefin resin within the above range, the dispersion state of the polyolefin resin and the plasticizer becomes good, and the strength, permeability, and heat resistance of the obtained microporous membrane are excellent. In addition, when forming into a sheet shape, the swelling and neck-in amount at the outlet of the die become appropriate, and the formability and film-forming property of the sheet are also good.

[0055] The melt-kneading of the polyolefin resin and the plasticizer is preferably carried out in a twin-screw extruder from the viewpoint of obtaining a uniform kneaded state.

[0056] The resin temperature during kneading is preferably 150°C or higher, more preferably 160°C or higher, still more preferably 180°C or higher, and the upper limit is preferably 250°C or lower, more preferably 240°C or lower, and still more preferably 230°C or lower. By setting the temperature of the polyolefin resin composition during kneading within the above range, it is possible to prevent a decrease in strength due to resin deterioration, and the polyolefin resin and the plasticizer can be uniformly melt-kneaded.

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

[0058] (2) Gel sheet formation step The melt of the polyolefin resin composition is supplied from the extruder to the die and extruded into a sheet. The extrusion method may be either the T-die method or the inflation method. Also, a plurality of polyolefin resin compositions of the same or different compositions may be supplied from a plurality of extruders to a single multi-manifold type composite T-die and laminated in layers, and then extruded into a sheet having a laminated structure. The extrusion temperature is preferably 140 to 250°C.

[0059] The shear rate applied when extruding the melt from the die is preferably 150 sec -1 or higher, more preferably 200 sec -1 or higher, still more preferably 250 sec -1 or higher, particularly preferably 300 sec -1The above is the case. By setting the shear rate within the above range, it is possible to control the orientation state of the sheet surface, reduce the structural difference between the front and back surfaces when forming a microporous membrane, and facilitate the refinement of the pore structure. The upper limit of the shear rate is preferably 1000 sec -1 or less, more preferably 800 sec -1 or less. The shear rate can be calculated by the following formula in extrusion using a T-die. γ = 6Q / (Wt 2 ) γ: Shear rate ( / sec) Q: Discharge rate (cm 3 / sec) W: Width of the T-die discharge port (cm) t: Slit gap of the T-die discharge port (cm) The resin composition melt-extruded in sheet form becomes a gel sheet by cooling and solidifying. In the cooling process, it is preferable to cool to 10 to 50 °C before the first stretching process described later. This is because it is preferable to set the final cooling temperature below the crystallization end temperature. By making the higher-order structure finer, uniform stretching becomes easier in subsequent stretching. Also, the cooling rate at this time is preferably carried out at a rate of 50 °C / min or more, more preferably 100 °C / min or more, and even more preferably 150 °C / min or more. Generally, when the cooling rate is slow, relatively large crystals are formed, so the higher-order structure of the gel sheet becomes coarser, and the gel structure forming it also becomes larger. On the other hand, when the cooling rate is fast, relatively small crystals are formed, so the higher-order structure of the gel sheet becomes dense and uniform stretching becomes possible, which facilitates increasing the strength of the polyolefin microporous membrane or refining the pore structure.

[0060] (3) First stretching process Next, the obtained gel sheet is stretched in at least one axial direction, and it is preferable to preheat the gel sheet before stretching. The preheating temperature is preferably 90 to 130°C, more preferably 105°C or higher, still more preferably 110°C or higher, and also more preferably 120°C or lower, still more preferably 118°C or lower. By performing the preheating temperature under the above conditions, a polyolefin microporous membrane having a uniform pore structure that is uniformly stretched in the stretching step can be obtained.

[0061] The preheated gel sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, but simultaneous biaxial stretching is preferred. By performing simultaneous biaxial stretching, it becomes easy to adjust the resin portion having a volume of 1.0×10 8 nm 3 or more to 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. Also, the stretching ratio is preferably 4 times or more, more preferably 5 times or more, in either the machine longitudinal direction (MD direction) or the machine width direction (TD direction). The stretching ratios in the MD direction and the TD direction may be the same or different, and by setting the area stretching ratio within the above range, the mechanical strength and permeability can be enhanced. Also, the area stretching ratio in this step is preferably 100 times or less, more preferably 64 times or less, whereby a polyolefin microporous membrane that prevents film breakage and has excellent film strength can be obtained.

[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. Also, it is preferably 130°C or lower, more preferably 120°C or lower, and even more preferably 118°C or lower. By stretching as described above, cracks occur between the polyethylene lamellae, the polyethylene resin phase is refined, and a large number of fibrils are formed. The fibrils form a three-dimensionally irregularly connected network structure.

[0064] The stretching rate in the MD direction in this step is preferably 300% / min or higher, more preferably 700% / min or higher, even more preferably 1000% / min or higher, particularly preferably 1200% / min or higher, and most preferably 1500% / min or higher. Also, it is preferably 5000% / min or lower. By setting the stretching rate in the MD direction within the above range, it becomes easy to make the pore structure fine and uniform. Note that the stretching rate in this step is the value obtained by dividing the stretching rate from the start of stretching to the end of stretching by the required time when the stretching rate of the microporous membrane before stretching is 0% and the stretching ratio is 100% per one-fold stretching ratio.

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

[0066] (4) Removal step of the solvent for film formation Using a cleaning solvent, the solvent for film formation is removed (cleaned). The polyolefin resin phase is phase-separated from the solvent phase for film formation. Therefore, when the solvent for film formation is removed, a porous membrane composed of fibrils that form a fine three-dimensional network structure and having pores (voids) that communicate irregularly three-dimensionally is obtained. Since the cleaning solvent and the method for removing the solvent for film formation using the same are known, the description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

[0067] (5) Drying step The polyolefin microporous membrane from which the solvent for film formation has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably 100°C or lower, more preferably 95°C or lower. Drying is preferably carried out until the residual cleaning solvent becomes 5 parts by mass or less, more preferably 3 parts by mass or less, with the total mass of the polyolefin microporous membrane being 100 parts by mass (dry mass).

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

[0069] The areal stretch ratio of the dry stretching is preferably 1.2 times or more, more preferably 1.5 times or more, still 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 areal stretch ratio of the dry stretching within the above range, the structure is made uniform, the value of Sa1 / Sa2 is adjusted to an appropriate range, 1.0×10 8 nm 3It becomes easy to control the number of resin parts having the above volume within a preferable range. Also, by setting the area draw ratio in dry drawing to 16 times or less, the film-forming property becomes stable. Note that the area draw ratio described in this step indicates the product of the dry draw ratios in the MD direction and the TD direction.

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

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

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

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

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

[0075] (7) Heat treatment process Also, after the step (6) or in place of the step (6), the dried polyolefin microporous membrane can be heat-treated. By heat treatment, the crystals are stabilized and the lamellae are homogenized. As the heat treatment method, heat fixation treatment and / or heat relaxation treatment can be used. Heat fixation treatment is a heat treatment in which heating is performed while holding the dimensions of the membrane unchanged. Heat relaxation treatment is a heat treatment in which the membrane is thermally shrunk in the MD direction or TD direction during heating. The heat fixation 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 membrane after the relaxation treatment by the dimensions of the membrane before the relaxation treatment. The relaxation rates in the MD and TD directions of the membrane are both preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. Also, from the viewpoint of the flatness of the microporous membrane, it is preferably 0.80 or higher, more preferably 0.90 or higher. The heat treatment temperature is preferably within the range of 100 to 140°C of the polyolefin resin.

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

[0077] [Separator for Battery and Secondary Battery] As one aspect of the present invention, there are provided a separator for a battery using the polyolefin microporous membrane of the present invention, and a secondary battery using such a separator for a battery. Since the separator for a battery of the present invention is excellent in safety and output characteristics, it can be preferably used particularly for secondary batteries that require higher energy density, higher capacity, and higher output.

[0078] [Liquid Filter and Filtration Unit] As one aspect of the present invention, there are provided a liquid filter using the polyolefin microporous membrane of the present invention, and a filtration unit using such a liquid filter. Since it is excellent in filtration accuracy and high permeability when used for liquid filter applications, it can be preferably used as a liquid filter for semiconductor resists that require high-precision filtration. The polyolefin microporous membrane of the present invention can be used as a liquid filter for a filtration unit such as a sheet shape, a tubular shape, or a pleated shape, and since the filtration area can be increased, it is preferably a pleated filtration unit. When incorporating it into a pleated filtration unit, it is preferable to laminate a reinforcing membrane made of a mesh or a porous body using a resin material on at least one side of the polyolefin microporous membrane of the present invention. After laminating with the reinforcing membrane, make folds at the valleys and weave them into a pleated shape, and then it can be incorporated into a filtration unit for use. [Examples]

[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. In addition, the evaluation in this application was carried out in an environment of temperature 23°C and humidity 65% unless otherwise specified. The evaluation methods and analysis methods used in the examples are as follows.

[0080] [Measurement Method] [Thickness] The film thickness at any five points within a 50 mm × 50 mm range of the polyolefin microporous membrane was measured using a contact thickness gauge, Mitutoyo Corporation's "Lite-Matic" (registered trademark) VL-50 (a super-hard spherical probe with a diameter of over 10.5 mm and a measurement load of 0.01 N), and the average value was taken as the thickness (μm).

[0081] [Porosity] Samples were cut from the polyolefin microporous membrane into 50 mm × 50 mm square shapes, and their volume (cm 3 ) and mass (g) were measured. From these values and the film density (g / cm 3 ), the porosity of the polyolefin microporous membrane was calculated using the following formula. The film density was calculated assuming a constant value of 0.99 g / cm 3 . This measurement was performed by cutting samples from three arbitrary positions on the polyolefin microporous membrane and calculating the average value of the measured porosities. Formula: Porosity (%) = [(Volume - Mass / Film density) / Volume] × 100.

[0082] [Air permeability] For the polyolefin microporous membrane, in accordance with JIS P-8117:2009, the air permeability (seconds / 100 cm 3 ) was measured using the Oka Research air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in an atmosphere of 25°C. Also, by dividing by the thickness (μm) of the polyolefin microporous membrane measured by the method described above, the air permeability in terms of thickness was calculated.

[0083] [Puncture strength in unit basis weight conversion] The 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) when a polyolefin microporous membrane was punctured with a needle having a spherical tip (curvature radius R: 0.5 mm) and a diameter of 1.0 mm was measured, and the value obtained from the following formula was taken as the puncture strength in unit basis weight conversion (mN / (g / m 2 ). Formula: Puncture strength in unit basis weight conversion (mN / (g / m 2)) = Maximum load (mN) / Basis weight of the polyolefin microporous membrane (g / m 2 ) Note that the basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25 °C, and using the following formula. Formula: Basis weight (g / m 2 ) = Mass (g) / (50 (mm) × 50 (mm)) × 10 6 。

[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 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. · Sample preparation: 5 mL of the measurement solvent was added to 5 mg of the sample, and after heating and stirring at 160 to 170 °C for 60 minutes, the resulting solution was filtered through a metal filter (pore size 0.5 µm).

[0085] · Measuring device: High-temperature GPC device, 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, Ltd., with 0.1 wt% BHT added) · Solvent flow rate: 1.0 ml / min · Injection volume: 0.300 mL · Detector: Differential refractive index detector (RI detector) · Standard sample: Monodisperse polystyrene manufactured by Tosoh Corporation, dibenzyl manufactured by Tokyo Chemical Industry Co., Ltd. Thereafter, the obtained Mw was converted to polyethylene (PE). The conversion formula is as follows.

[0086] Mw (in terms of PE) = Mw (measured value in terms of PS) × 0.468.

[0087] [Differential Scanning Calorimetry (DSC)] The melting point of the polyolefin resin, the heat of crystal melting, and the melting point of the polyolefin microporous membrane were determined by the DSC method. 6.0 mg of the sample was sealed in an aluminum pan, and using a PYRIS Diamond DSC manufactured by PerkinElmer, after heating from 30 °C to 230 °C at a rate of 10 °C / min (the first heating), it was 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 (the second heating). The heat of crystal melting and the melting point were calculated from the crystal 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 of the above-described DSC measurement. The melting point was taken as the temperature at the point showing the maximum value of the endothermic amount, and the heat of crystal melting was calculated from the area of the crystal melting peak.

[0088] [Arithmetic mean roughness Sa] The surface roughness was measured using a scanning white light interference microscope VS-1540 manufactured by Hitachi High-Technologies Corporation. VS-Measure (Version 10.0.4) was used as the measurement software, and VS-Viewer (Version 10.0.3) was used as the analysis software. A 50 mm × 50 mm square film was cut from the polyolefin microporous membrane and attached to a circular frame with an inner diameter of 4 cm so that wrinkles and slack did not occur, and this was used as the measurement sample. After measurement and data processing according to the conditions shown below, the arithmetic mean roughness Sa was calculated in accordance with ISO25178. Measurement was performed on both surfaces of each sample, and the value on the surface with a larger Sa was designated as Sa1, and the value on the surface with a smaller Sa was designated as Sa2. In addition, four-point measurements were performed on each surface and the average value was calculated. [Measurement conditions] · Objective lens: 50× · Tube lens: 0.5× · Wavelength filter: 530white · Camera: High pixel · Measurement mode: Wave · Measurement device: Piezo · Field of view size: 1024×1024 (pixel) · Effective pixel number: 50% · Cutoff: None <Data processing conditions> · Surface correction (approximate surface shape): Fourth order · Completion: Complete completion · Filter: Smoothing 9×9 (pixel).

[0089] [Standard deviation of orientation parameter] The orientation parameter of the polyolefin microporous membrane was measured and calculated by Raman spectroscopy. The laser was perpendicularly incident from the normal direction of the film surface (XY plane), and polarized light was used with a polarizer. The measurement sample was rotated, and Raman spectra in 12 directions (0° to 165°) at 15° intervals were obtained with an arbitrary direction in the film surface set as 0°. For the obtained Raman spectra, a straight line was drawn between two points at 800 cm -1 and 1350 cm -1 and corrected with this as the baseline. After that, the maximum value of the intensity between 1045 and 1075 cm -1 was taken as the peak intensity (I1060) at 1060 cm -1 , the maximum value of the intensity between 1115 and 1145 cm -1 was taken as the peak intensity (I1130) at 1130 cm -1 . The peak intensity ratio (I1130 / I1060) between 1130 cm -1 and 1060 cm -1 was used as the orientation parameter. The standard deviation was obtained from the measured values at 12 points (0° to 165°) at 15° intervals with an arbitrary direction in the film surface set as 0°. · Equipment: Micro Raman spectroscopy system ("inVia" manufactured by Renishaw) · 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 diameter based on surface area (Ds), median pore diameter based on pore volume (Dv)] A test piece was cut out in a rectangular shape from an arbitrary position of the polyolefin microporous membrane so that the weight was 0.20 ± 0.02 g, and the area and weight of the test piece were measured. From the area of the test piece and the film thickness measured by the above-described method, the volume of the polyolefin microporous membrane was calculated. The cut-out sample was wound around a sample core so that no bubbles entered the purified water, 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 purified water in the chamber and the pressure was increased step by step, and the measurement was performed up to a maximum pressure of 13.7 MPaG. The relationship between pressure and pore diameter, surface area, and pore volume is as shown in the following formula, and in the pure water intrusion porosimeter, the surface area is calculated by a model assuming a cylindrical pore shape. Regarding the pore size distribution obtained from the relationship between the pore diameter calculated from the pressure at each pressure increase step and the surface area (integrated value) of the pores into which pure water was intruded at each pressure increase step, when the surface area (integrated value) at a maximum pressure of 13.7 MPaG was taken as 100%, the pore diameter at the point where the surface area (integrated value) became 50% was defined as the median value based on the surface area. The median pore diameter based on the pore volume was calculated in the same manner as the above-described method for calculating the median value based on the surface area, with the surface area part replaced by the pore volume. D = -4γcosθ / P Dv = -4γcosθ / 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: Contact angle between polyethylene and water (assumed to be 110°) Pv 50 : When the integrated pore volume at the time of increasing the pressure to 13.7 MPaG is taken as 100%, the pressure at the point where the integrated pore volume is 50% Ps 50 : When the integrated surface area at the time of increasing the pressure to 13.7 MPaG is taken as 100%, the pressure at the point where the integrated surface area is 50% ΔV: Volume of pure water (pore volume) injected when pressure is increased in one step ΔS: Surface area of pores injected when pressure is increased in one step

[0091] [Shutdown temperature] While heating the polyolefin microporous membrane at a heating rate of 5°C / min, the air permeability resistance was measured using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). When the air permeability resistance reached the detection limit of 99999 seconds / 100 cm 3 the temperature reached when it reached Air was determined and taken as the shutdown temperature (°C).

[0092] The measurement cell is composed of an aluminum block and has a thermocouple directly below the polyolefin microporous membrane. The sample was cut into a 5 cm × 5 cm square and the temperature was measured while being fixed around with an O-ring.

[0093] [Average pore diameter] Using a palm porometer (manufactured by PMI, CFP-1500A), the average pore diameter of the polyolefin microporous membrane was determined. GALWICK (surface tension: 15.9 dynes / cm) was used as the impregnating liquid for the polyolefin microporous membrane, and measurements were taken in the order of Dry-up and Wet-up. For the average pore diameter (nm), measurements were performed based on ASTM E1294-89 (1999) (half-dry method). The pore diameter was converted from the pressure (KPa) at the point where the curve showing half the slope of the pressure-flow rate curve in the Dry-up measurement intersects the curve of the Wet-up measurement. The following mathematical formula was used for the conversion of 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 impregnating liquid, "P (KPa)" is the pressure, and "C" is a constant taken as 2860.)

[0095] [Method for obtaining images of the microporous membrane by FIB-SEM (focused ion beam scanning electron microscope)] Continuous images were measured by FIB-SEM under the following conditions. · Sample preparation: After embedding the polyolefin microporous membrane with an epoxy resin, electron staining was performed using OsO4 and the sample was used for measurement. · Observation device: Helios G4 manufactured by Thermo Fisher Scientific · Observation conditions: Acceleration voltage 1 kV · Sample tilt: 52° · Pixel size: Image horizontal direction: 4.5 nm, Image vertical direction: 5.7 nm (after tilt correction) · Slice interval in FIB: 10 nm · In order to perform position confirmation and alignment between each image, a mark with Pt deposited on the film side was made. · Tilt correction: Since the FIB-SEM observation is performed at an angle of 52°, the SEM image is observed with a contraction in the vertical direction. Therefore, in order to make the image in the vertical direction the same as the image observed from the front, it is necessary to multiply by 1.27 ( = / sin52°). The three-dimensional image described later was created using the tilted-corrected image. · Measurement size: Sequential FIB processing was performed on an area of 4 μm × 4 μm or more on the film cross-section, and slicing was performed until the depth reached 4 μm or more. Image acquisition was performed on an area where a volume of 4 μm × 4 μm × 4 μm (401 captured images) could be secured.

[0096] [Method for creating three-dimensional image] For the FIB-SEM images collected by the method described above, a three-dimensional image was created using the following processing with the image processing and analysis software Avizo manufactured by Thermo Fisher Scientific. · When importing the image into the software, tilt correction was performed so that the vertical direction of the image was multiplied by 1.27. · After performing image alignment processing based on the Pt marking deposited on the film side, image cropping processing was performed so that the volume became 4 μm × 4 μm × 4 μm. · Since it is necessary for the voxels to be isotropic in order to perform the image analysis described later, resampling processing was performed to make the voxel size 5 nm × 5 nm × 5 nm. · Among the microporous membranes, after performing binarization processing on the portion where the embedded resin is electron-stained (i.e., the portion corresponding to the pore part of the microporous membrane) and the resin part constituting the microporous membrane, three-dimensional stereoscopic imaging (three-dimensional data conversion) was performed based on the binarization information, and the porosity of the entire three-dimensional structure was calculated. (Note that the calculation of porosity is performed using the function of Volome Fraction. As an operation inside the software, it is calculated by counting the number of voxels assigned to the target, i.e., the binarized pore part, and dividing it by the total number of voxels in the analysis region.) [1.0×10 8 nm 3 or more, the number of resin parts having such a volume, and the average volume of the resin parts Regarding the three-dimensional data created by the above method, image analysis was performed using the image processing and analysis software Avizo manufactured by Thermo Fisher Scientific through the following processes. · For each of the binarized portion where the embedded resin is electron-stained (pore part) and the resin part (film part) constituting the microporous membrane, a segmentation process was performed by Watershed processing. At this time, the parameter (maker extent) of the segmentation process was set to 1 for the pore part and 2 for the film part for the processing. · Pore Network Modeling (PNM) analysis was performed on the segmented resin parts to calculate the volume of each segmented resin part, and the average volume of the resin parts was calculated by averaging all the individual resin part volumes. Also, an analysis filter was applied to the obtained data to create a stereoscopic image in which only the film parts having a volume equal to or greater than the specified threshold value are displayed, and the number thereof was measured. The threshold value here was 1.0×10 8 nm 3 was set.

[0097] [Cyclic test method and removal of samples after cyclic test] The cycle test and the extraction of evaluation samples from the battery after the test were carried out by the method described below. In addition, regarding the operations described in the items of (Production of the positive electrode) to (Production of the battery) and (Extraction of the microporous membrane from the battery after the cycle test) in the following procedures, they were carried out in a dry room with a dew point of -30°C or lower.

[0098] (Production of the positive electrode) As the positive electrode active material, 94 parts by mass of nickel-cobalt-manganese composite oxide NMC (Ni:Co:Mn = 6:2:2 (element ratio)), 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 at a ratio, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was uniformly coated on both sides of an aluminum foil with a thickness of 20 μm serving as the positive electrode current collector, dried, and then compression molded by a roll press machine. At this time, the coating amount per side of the positive electrode mixture was 170 g / m 2 , and the density was made to be 3.0 g / cm 3 .

[0099] (Production of the negative electrode) As the negative electrode active material, 97 parts by mass of artificial graphite, 1 part by mass of carboxymethyl cellulose as the resin binder, and 2 parts by mass of styrene-butadiene copolymer latex were mixed at a ratio, and these were dispersed in purified water to prepare a slurry. This slurry was uniformly coated on both sides of a copper foil with a thickness of 10 μm serving as the negative electrode current collector, dried, and then compression molded by a roll press machine. At this time, the coating amount per side of the negative electrode mixture was 100 g / m 2 , and the density was made to be 1.5 g / cm 3 .

[0100] (Non-aqueous electrolyte) LiPF6 was dissolved as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate:dimethyl carbonate = 3:3:4 (volume ratio) to a concentration of 1.0 mol / L. 1 part by mass of vinylene carbonate was added to 100 parts by mass of this solution to prepare a non-aqueous electrolyte.

[0101] (Production of the battery) Using the above-mentioned positive electrode, the microporous membrane of the present embodiment, and the above-mentioned negative electrode, a flat wound electrode body was fabricated. This flat wound electrode body was sandwiched between aluminum laminate films, sealed with a part of the opening left, dried in a vacuum oven at 80 °C for 6 hours, then the above-mentioned non-aqueous electrolyte was injected, sealed with a vacuum sealer, and a laminate secondary battery with a size of 32 mm × 32 mm and a battery capacity of 300 mAh was fabricated.

[0102] (Pre-charge and discharge treatment) The battery fabricated above was charged at a constant current with a current value of 60 mA until the battery voltage reached 4.2 V, then charged at a constant voltage of 4.2 V until the current value reached 15 mA, rested for 30 minutes, discharged at a constant current with a current value of 60 mA until the battery voltage reached 2.5 V, and rested for 30 minutes. The above charge and discharge were performed for 3 cycles to obtain a battery for cycle test.

[0103] (Cycle test) Using the above laminate secondary battery, it was charged at a constant current with a current value of 900 mA (3C) until the battery voltage reached 4.2 V, then charged at a constant voltage of 4.2 V until the current value reached 15 mA (0.05C), rested for 30 minutes, discharged at a constant current with a current value of 300 mA (1C) until the battery voltage reached 2.5 V, and rested for 30 minutes. The above charge and discharge were repeated 800 cycles.

[0104] (Removal of microporous membrane from the battery after cycle test) The battery cell that had undergone the above cycle test was unsealed and only the microporous membrane was taken out. The taken-out microporous membrane was immersed in 50 mL of dimethyl carbonate for 5 minutes, then rinsed with 50 mL of new dimethyl carbonate, and air-dried sufficiently to obtain a sample for evaluation.

[0105] [Resistance of polyolefin microporous membrane converted to 10 μm thickness](Ω·cm 2 / 10μm) As measurement samples, 12 circular samples with a diameter of 19 mm were cut out and vacuum-dried together with the components of a CR2032 coin cell (upper lid, lower lid, gasket (made of PP), spacer (cylindrical with a diameter of 15.5 mm and a thickness of 1.0 mm), wave washer) purchased from Takizawa Co., Ltd. to remove moisture. In a dry room with an outdoor temperature of -35°C or lower, three measurement samples and a PP gasket were placed in this order on the inner bottom of the lower lid of the above coin cell component. Then, 0.15 mL of an electrolyte solution (manufactured by Kishida Chemical Co., Ltd.) in which LiPF6 was dissolved to a concentration of 1 M in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 4:6 [volume ratio]) was injected. After placing the spacer on the polyolefin microporous membrane, it was left standing for 10 minutes at a pressure of about -50 kPa to impregnate the measurement samples with the electrolyte solution. Then, a wave washer and an upper lid were put on the cell, and it was sealed with a coin cell crimper (manufactured by Takizawa Co., Ltd.) to obtain a sample cell.

[0106] The obtained sample cell was placed in a high-temperature bath thermostat at 25°C and left standing for 2 hours. Then, using an impedance analyzer (manufactured by Hioki E.E. Corporation), the cell resistance (the real value when the imaginary-axis value is 0) was measured at an amplitude of 20 mV and a frequency of 200 kHz. The above measurement was also performed on cells separately prepared with 4 and 5 polyolefin microporous membranes. Based on the slope (resistance per polyolefin microporous membrane) Z obtained by plotting the cell resistance against the number of polyolefin microporous membranes placed in the cell and performing linear approximation, the resistance converted to a thickness of 10 μm of the polyolefin microporous membrane was calculated according to the following formula.

[0107] Resistance converted to a thickness of 10 μm (Ω·cm 2 / 10 μm) = Z × area of the spacer × 10 / thickness of the polyolefin microporous membrane The area of the spacer was taken as 2.01 cm 2

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

[0109] [Safety of Polyolefin Microporous Membrane] When the polyolefin microporous membrane is used as a separator for a secondary battery, its safety was evaluated according to the following criteria, and it was judged to be qualified for both A and B. A: All of the following 3 items are ○ B: Among the following 3 items, there are 1 to 3 △ and 0 × C: Other than A and B above (among the following 3 items, there is 1 or more ×) Item 1 ○: The puncture strength in terms of basis weight is 900 mN or more △: The puncture strength in terms of basis weight is 700 mN or more and less than 900 mN ×: The puncture strength in terms of basis weight is less than 700 mN Item 2 ○: The average pore diameter is 30 nm or less △: The average pore diameter exceeds 30 nm and is 35 nm or less ×: The average pore diameter exceeds 35 nm Item 3 ○: The shutdown temperature is 137°C or less △: The shutdown temperature exceeds 137°C and is 140°C or less ×: The shutdown temperature exceeds 140°C

[0110] [Example 1] To the polyolefin raw material, 70% by mass of ultra-high molecular weight polyethylene with an Mw of 1.2×10 6 and a melting point of 134°C as resin A, and an Mw of 6.0×10 4, 30% by mass of high-density polyethylene with a melting point of 132 °C and 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 further, 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the mass of the ultra-high molecular weight polyethylene and mixed to prepare a polyolefin resin composition. The obtained polyolefin resin composition was put into a twin-screw extruder and kneaded at 180 °C to prepare a polyolefin solution. The obtained polyolefin solution was supplied to a T-die adjusted to 200 °C, and melt-extruded under the condition that the shear rate at the die discharge part was 350 sec -1 , and cooled with a casting drum controlled at 35 °C by melt-extrusion under the condition that Q / Ns was 1.8 to form a gel sheet. The gel sheet was first stretched at 118 °C in both the MD direction and the TD direction with a stretching ratio of 5 times and a stretching speed of 1000% / min in both MD and TD using a simultaneous biaxial tenter stretching machine. After the wet-stretched gel sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then air-dried. Subsequently, as the second stretching, the above microporous membrane was stretched in the MD direction with a stretching ratio of 1.7 times on a roll set at 100 °C using a roll-type stretching machine, and then stretched 1.7 times in the TD direction at 130 °C using a tenter-type stretching machine to obtain a polyolefin microporous membrane. The film-forming conditions of the polyolefin microporous membrane and the like are shown in Table 1, and the evaluation results of the obtained polyolefin microporous membrane and the like are shown in Table 2.

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

[0112] Examples and comparative examples are shown in Tables 1 to 5. Table 5 shows the various analysis results after the cycle test was carried out on the polyolefin microporous membranes of Example 1 and Comparative Example 8 by the above method.

[0113]

Table 1

[0114]

Table 2

[0115]

Table 3

[0116]

Table 4

[0117]

Table 5

Claims

1. With respect to the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, the Sa of the surface having the larger arithmetic mean roughness is defined as 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, and in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC), when the largest molecular weight having a height of 20% of the maximum value in a molecular weight range of 10,000 to 10,000,000 is taken as M1 and the smallest molecular weight is taken as M2, the polyolefin microporous membrane satisfies the following formula 4: Formula 1: 10≦(Sa １ +Sa ２ ) / 2≦80 Formula 2: Sa １ / Sa ２ ≦1.4 Formula 3: 1.0≦Dv / Ds≦1.2 Formula 4: M 1 / M 2 ≧500

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), 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 FIB-SEM measurement of the microporous membrane, the average volume of the resin portion is 1.3 × 10 ７ nm ３ 2. The microporous polyolefin membrane of claim 1, wherein:

4. 2. The microporous polyolefin membrane of claim 1, having an average pore size measured by a porometer of 35 nm or less.

5. The microporous polyolefin membrane of claim 1, having a shutdown temperature of 140° C. or less.

6. 2. The polyolefin microporous membrane according to claim 1, wherein the standard deviation of 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.

7. 2. The microporous polyolefin membrane of claim 1, having a melting point of 134° C. or less as measured by differential scanning calorimetry (DSC).

8. A polyolefin microporous membrane as described in claim 1, in which, in a differential molecular weight distribution curve measured by gel permeation chromatography (GPC), the area ratio of peak areas of molecular weight components with molecular weights of 50,000 or less to the total molecular weight components is 10% or more, and the area ratio of peak areas of molecular weights of 1,000,000 or more is 10% or more.

9. A battery separator using the microporous polyolefin membrane according to claim 1.

10. A secondary battery using the battery separator according to claim 9.

11. A liquid filter comprising the polyolefin microporous membrane according to claim 1.

12. A filtration unit using the liquid filter according to claim 11.

Images