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

Abstract

The present invention addresses the problem of providing a polyolefin microporous membrane which demonstrates excellent dendrite resistance and output properties when used as a separator for batteries and demonstrates excellent filtration accuracy and high permeability when used for liquid filters.　The present invention is a polyolefin microporous membrane which satisfies the requirement that the value represented by the formula: X2 / X1 is 15 or more and has a thickness of 30 μm or less, in which, in the formula, Ymax represents a maximum value of dV / d(LogD) in a pore diameter range of 0.01 μm to 10 μm, and X1 and X2 represent pore diameters satisfying the formula: Ymax / 2 in a pore diameter range of 0.01 μm to 10 μm and are arranged in order from the smaller to the larger in a pore diameter distribution determined by mercury intrusion porosimetry in which pore diameter is expressed with the X axis and dV / d(LogD) is expressed with the Y axis.

Description

[Technical Field]

[0001] The present invention relates to a microporous polyolefin membrane, a separator for a secondary battery, a liquid filter, a secondary battery, and a filtration unit. [Background technology]

[0002] Polyolefin microporous membranes are used as filters, fuel cell separators, capacitor separators, etc. They are particularly suitable for use as separators for lithium-ion secondary batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, etc. Furthermore, due to their uniform fine pore structure and excellent solvent and chemical resistance, polyolefin microporous membranes are widely used in various filter applications, such as water treatment membranes, ultrafiltration membranes, microfiltration membranes, and breathable waterproof clothing.

[0003] In recent years, development of lithium-ion secondary batteries, primarily for automotive applications, has been progressing with the aim of increasing battery size and achieving higher energy density, capacity, and power output. As a result, some batteries are prone to dendrite formation due to lithium precipitation, and separators are increasingly required to be 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. However, reducing the pore size of microporous membranes can result in reduced permeability, which can lead to reduced power output characteristics.

[0004] Liquid filter applications require both high-precision separation and high permeability. For example, in semiconductor manufacturing processes, as the wiring pitch of semiconductors becomes finer, the upper limit of the permissible size of foreign particles during the process is decreasing, and filters used to filter liquids used in semiconductor manufacturing are required to be able to capture even smaller foreign particles. On the other hand, from the perspective of processing capacity, a decrease in permeability is undesirable.

[0005] Patent Document 1 describes a liquid filter substrate that has both high liquid permeability and high particle capture ability under high pressure, and discloses a polyolefin microporous membrane for liquid filters that has a mean flow pore size measured by a half-dry method using gas-liquid phase displacement and a mean flow pore size dLLP measured by a half-dry method using liquid-liquid phase displacement within a specified range.

[0006] Patent Document 2 describes a polyolefin microporous membrane with a small pore size and excellent air permeability, and discloses a laminated polyolefin microporous membrane with an air resistance of 10 to 200 sec / 100 ml and a bubble point pore size of 5 to 35 nm.

[0007] Patent Document 3 describes a microporous membrane formed by laminating layers of different resin compositions made of PE and PP, in which the resin compositions and pore structures of the surface and inner layers are different, and thus the polyolefin microporous membrane has an excellent balance of permeability, mechanical strength, meltdown properties, electrolyte absorption, and electrolyte retention when used as a battery separator. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-167198 [Patent Document 2] International Publication No. 2018 / 168871 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-255307 Summary of the Invention [Problem to be solved by the invention]

[0009] Patent Document 1 is a technology that can achieve both particle removal performance and permeability during filtration under high-pressure conditions by suppressing deformation of the pore structure under high pressure, but it cannot improve the trade-off between small pore size and high permeability.

[0010] Patent Documents 2 and 3 describe technologies that can improve the trade-off between small pore size and high permeability by separating functions through lamination, but the small pore layer is formulated as a blend of PE and PP, and in designs that blend different resins like this, the phase separation of the resins causes the pore structure to become non-uniform, so there is room for improvement in the balance between small pore size and high permeability. [Means for solving the problem]

[0011] In order to solve the above problems and achieve the object, the present invention has the following configuration: In the following description, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. [1] In the pore size distribution measured by mercury intrusion, the pore size is on the X axis and dV / d(LogD) on the Y axis. The maximum value of dV / d(LogD) in the pore size range of 0.01 μm to 10 μm is shown as Y. max The pore diameter is set to Y within the range of 0.01 μm to 10 μm. max / 2, where X1 and X2 are pore sizes from the smallest to the largest, and X2 / X1 is 15 or more, and the thickness is 30 μm or less. Here, V: cumulative pore volume (cm 3 / g) D: Pore diameter (μm). [2] The polyolefin microporous membrane according to [1] above, which has a bubble point pressure of 1900 kPa or more. [3] The polyolefin microporous membrane according to [1] or [2] above, which contains 90% by mass or more of polyethylene. [4] The microporous polyolefin film according to any one of [1] to [3] above, which has an air resistance of 300 seconds or less in terms of a thickness of 10 μm. [5] The microporous polyolefin film according to any one of the above [1] to [4], which is used as a separator for a secondary battery. [6] The polyolefin microporous membrane according to any one of the above [1] to [4], which is used as a liquid filter. [7] A separator for a secondary battery, which uses the polyolefin microporous membrane according to any one of [1] to [4] above. [8] A liquid filter using the polyolefin microporous membrane according to any one of [1] to [4] above. [9] A secondary battery using the separator for secondary batteries according to [7] above.

[10] A filtration unit using the liquid filter described in [8] above. [Effects of the Invention]

[0012] The polyolefin microporous membrane of the present invention, when used as a battery separator, has excellent dendrite resistance and output characteristics, and is therefore suitable for use as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as for electric vehicles, etc. Furthermore, when used as a liquid filter, it has excellent filtration accuracy and high permeability, and is therefore suitable for use as a high-precision liquid filter that requires the removal of minute foreign matter, such as in semiconductor processes. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these.

[0014] The polyolefin microporous membrane of the present invention contains a polyethylene resin as a main component. The main component here refers to the component that constitutes the polyolefin microporous membrane with the highest content, expressed as % by mass. The polyethylene resin content in the polyolefin microporous membrane is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more. Setting the polyethylene resin content in the polyolefin microporous membrane within the above range allows for a polyolefin microporous membrane with an excellent pore structure uniformity and excellent filtration accuracy and high permeability when used as a liquid filter. The polyethylene resin content in the polyolefin microporous membrane can be measured by the method described below.

[0015] As the polyethylene-based resin, various polyethylenes can be used, such as ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene.

[0016] The polyethylene resin may be a homopolymer of ethylene or a copolymer of ethylene with another α-olefin, such as propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, or styrene.

[0017] The polyethylene resin may contain two or more types of polyethylene.

[0018] The polyolefin microporous membrane may contain a small amount of polyolefin other than polyethylene as long as the effects of the present invention are not impaired. For example, if the polyolefin microporous membrane contains polypropylene, the meltdown temperature may be improved when used as a battery separator. Furthermore, the filtration accuracy may be excellent when used as a liquid filter.

[0019] The polypropylene may be a homopolymer, a block copolymer, or a random copolymer. The block copolymer or random copolymer may contain a copolymer component with an α-ethylene other than propylene, and the other α-ethylene is preferably ethylene.

[0020] However, when the polypropylene is contained, the mechanical strength and permeability tend to decrease compared to when polyethylene is used alone, so the content of the polypropylene in the polyolefin microporous membrane is preferably 0 to 20% by mass, more preferably 0 to 10% by mass, even more preferably 0 to 5% by mass, and most preferably 0% by mass.

[0021] The polyolefin microporous membrane may be a single-layer membrane, but is preferably a laminated membrane having an A layer mainly composed of a polyethylene-based resin and a B layer mainly composed of a polyethylene-based resin with different properties. Generally, when the pore size of a porous membrane is reduced, the fluid flow path becomes narrower, resulting in increased pressure loss and reduced permeability. A preferred embodiment of the polyolefin microporous membrane of the present invention is a multilayer polyolefin microporous membrane that combines small pore size and high permeability, comprising an A layer with a small pore size and a B layer with high permeability, produced by using a polyethylene-based resin with a high viscosity average molecular weight (Mv) as the main component for the A layer and a polyethylene-based resin with a low viscosity average molecular weight (Mv) as the main component for the B layer, and produced under the production conditions described below.

[0022] The resin constituting Layer A contains a polyethylene resin as a main component. The polyethylene resin component in Layer A is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more. By setting the content of the polyethylene resin component in Layer A within the above range, the uniformity of the pore structure is excellent, resulting in a polyolefin microporous membrane that exhibits excellent dendrite resistance and output characteristics when used as a battery separator for a secondary battery, and excellent filtration accuracy and high permeability when used as a liquid filter.

[0023] The resin constituting Layer B contains a polyethylene resin as a main component. The polyethylene resin component in Layer B is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more. By setting the content of the polyethylene resin component in Layer B within the above range, a polyolefin microporous membrane can be obtained that has excellent pore structure uniformity, excellent dendrite resistance and output characteristics when used as a battery separator for a secondary battery, and excellent filtration accuracy and high permeability when used as a liquid filter.

[0024] The viscosity average molecular weight (Mva) of the polyethylene resin contained in layer A is 150 × 10 4 It is preferable that the ratio is 200×10 or more, and more preferably 200×10 4 More preferably, 250 × 10 4 More preferably, 300×10 4 The larger Mva is, the better. However, if it is too large, the film forming property decreases. 4 The following is preferable: 450 x 10 4 The following is more preferred. By setting the viscosity average molecular weight of the polyethylene resin contained in Layer A within the above range, it is possible to achieve a uniform and fine pore structure, resulting in a polyolefin microporous membrane that exhibits excellent dendrite resistance and output characteristics when used as a battery separator for a secondary battery, and excellent filtration accuracy and high permeability when used as a liquid filter. In addition, it becomes easier to control the layer ratio of Layer A to Layer B, which will be described later, within a preferred range. The Mva value can be adjusted by the raw material composition and kneading conditions of Layer A.

[0025] The viscosity average molecular weight (Mvb) of the polyethylene resin contained in layer B is 125 × 10 4 It is preferable that the value is equal to or less than 100×10 4 or less, more preferably 50 × 10 4 or less, more preferably 40 × 10 4The smaller Mvb is, the better. However, if it is too small, the film-forming property decreases. 4 More than 5×10 is preferable. 4 The above is more preferable. By setting the viscosity average molecular weight of the polyethylene resin contained in Layer B within the above range, a polyolefin microporous membrane with excellent permeability can be obtained, and it becomes easier to control the layer ratio of Layer A to Layer B described above within a preferred range. Mvb can be adjusted by the raw material composition and kneading conditions of Layer B.

[0026] In the polyolefin microporous membrane of the present invention, when the viscosity average molecular weight of the resin constituting Layer A is Mva and the viscosity average molecular weight of the resin constituting Layer B is Mvb, the relationship between Mva and Mvb preferably satisfies the following formula: Mva-Mvb ≥ 100 × 10 4 The value of (Mva-Mvb) is preferably 150×10 4 More preferably, 200 × 10 4 More preferably, 250 × 10 4 From the viewpoint of achieving both small pore size and high flow rate, the larger the value of (Mva - Mvb), the better. However, if it is too large, the molecular weight difference and viscosity difference between the resin constituting layer A and the resin constituting layer B become too large, and film formability in co-extrusion decreases. Therefore, the value of (Mva - Mvb) is set to 500 × 10 4 Less than 400 x 10 is preferable. 4 Less than 300×10 is preferable. 4 The following is particularly preferred: To set the value of (Mva - Mvb) within the above range, it is preferred that the raw material compositions and molecular weights of the raw materials of layers A and B be within the above ranges.

[0027] The melting point of the polyethylene resin contained in Layer A is preferably 136°C or lower, more preferably 133°C or lower, even more preferably 130°C or lower, and most preferably 129°C or lower. By setting the melting point within this range, it is possible to make the pore structure uniform and fine, and to obtain a polyolefin microporous membrane that has excellent dendrite resistance and output characteristics when used as a battery separator for a secondary battery, and has excellent filtration accuracy and high permeability when used as a liquid filter.

[0028] The melting point of the polyethylene resin contained in Layer B is preferably 129° C. or higher, more preferably 132° C. or higher, even more preferably 134° C. or higher, and most preferably 135° C. or higher. By setting the melting point within the above range, a polyolefin microporous membrane with excellent permeability can be obtained.

[0029] The polyolefin microporous membrane of the present invention may contain various additives, such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, and fillers, within limits that do not impair the effects of the present invention. By appropriately selecting the type and amount of antioxidant or heat stabilizer, the properties of the microporous membrane can be adjusted or enhanced. In particular, it is preferable to add an antioxidant to suppress oxidative degradation of the polyethylene resin due to its thermal history. As the antioxidant, it is preferable to use one or more selected from the group consisting of 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7).

[0030] Examples of the layer structure of the laminated membrane include layer A / layer B, layer A / layer B / layer A, and layer B / layer A / layer B, with layer A / layer B / layer A / layer B being preferred. The layer structure described above allows for the production of a polyolefin microporous membrane that exhibits excellent dendrite resistance and output characteristics when used as a battery separator for a secondary battery, and excellent filtration accuracy and permeability when used as a liquid filter.

[0031] In the polyolefin microporous membrane of the present invention, the total thickness of Layer A is preferably 0.5 to 15.0 μm. By having the thickness of Layer A be 15.0 μm or less, more preferably 5.0 μm or less, even more preferably 3.0 μm or less, and even more preferably 2.0 μm or less, a decrease in permeability can be suppressed. By having the thickness of Layer A be 0.5 μm or more, a decrease in filtration accuracy due to an increase in pore size in areas where Layer A is locally too thin due to uneven lamination or the like can be suppressed. In the polyolefin microporous membrane of the present invention, by having the raw material composition and molecular weights of the raw materials constituting Layer A within the above-mentioned ranges, the pore size can be effectively reduced even when the proportion of the total thickness of Layer A is reduced.

[0032] In the polyolefin microporous membrane of the present invention, the ratio of the total thickness of Layer A to the total layer thickness is preferably 50% or less. A thickness ratio of Layer A of 50% or less, more preferably 40% or less, even more preferably 30% or less, and even more preferably 20% or less can prevent deterioration in permeability, prevent a decrease in output characteristics when used as a battery separator, and prevent a decrease in flow rate when used as a liquid filter. From the viewpoint of permeability, a lower thickness ratio of Layer A is preferable; however, if it is too low, the filtration life and lamination accuracy may decrease, so the lower limit is about 5%. To achieve the thickness ratio of Layer A in the above range, the raw material composition and molecular weight of the raw materials for the polyolefin microporous membrane are set within the above-mentioned ranges, and it is also preferable that the extrusion conditions during polyolefin microporous membrane production be set within the ranges described below to obtain a high-quality polyolefin microporous membrane with excellent lamination accuracy.

[0033] The polyolefin microporous membrane of the present invention preferably has a thickness of 3 μm or more and 30 μm or less. A thickness of 30 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, even more preferably 10 μm or less, and even more preferably 8 μm or less, results in excellent permeability. Furthermore, a thickness of 3 μm or more, more preferably 5 μm or more, results in excellent handleability. The thickness of the polyolefin microporous membrane can be adjusted by membrane-forming conditions.

[0034] The polyolefin microporous membrane of the present invention has a pore size distribution measured by mercury intrusion porosimetry, with pore size on the X axis and dV / d(LogD) on the Y axis, such that the maximum value of dV / d(LogD) in the pore size range of 0.01 μm to 10 μm is represented by Y. max The pore diameter is set to Y within the range of 0.01 μm to 10 μm. max When the pore sizes that satisfy the above condition are designated as X1 and X2 in order from the smallest, X2 / X1 is 15 or more. Here, V: cumulative pore volume (cm 3 / g) D: Pore diameter (μm).

[0035] An X2 / X1 ratio of 15 or more, preferably 20 or more, more preferably 30 or more, and even more preferably 50 or more, enables the production of a polyolefin microporous membrane that exhibits excellent dendrite resistance and output characteristics when used as a battery separator, and excellent filtration accuracy and permeability when used as a liquid filter. From the above viewpoints, the upper limit of X2 / X1 is not particularly limited, but from the viewpoint of compatibility with productivity, X2 / X1 is preferably 500 or less, more preferably 100 or less. X2 / X1 can be controlled within the above range by controlling the layer structure, raw material composition, and molecular weights of the raw materials of the polyolefin microporous membrane within the above-mentioned ranges, and / or controlling the membrane-forming conditions within the ranges described below.

[0036] Regarding the relationship between the layer structure and X2 / X1, a design in which Layer A, which has a fine pore structure, is laminated with Layer B, which has a coarse pore structure, allows fine pores and coarse pores to coexist within the polyolefin microporous membrane, which tends to increase X2 / X1.

[0037] Regarding the relationship between raw material composition and X2 / X1, increasing the viscosity-average molecular weight (Mv) of the polyethylene resin used in the polyolefin microporous membrane tends to decrease X1 and X2. This is thought to be due to the increased number of entanglement points between polyethylene molecular chains, resulting in finer pores. Furthermore, decreasing the Mv of the polyethylene resin tends to increase the pore size and increase X1 and X2. Therefore, in the aforementioned layer structure, by designing the polyethylene resin used in Layer A to have a high Mv and the polyethylene resin used in Layer B to have a low Mv, X2 / X1 tends to increase.

[0038] Regarding the relationship between film-forming conditions and X2 / X1, when the stretching temperature is low, stretching occurs while the entanglement of polyethylene molecular chains is constrained, which tends to result in finer pores and lower X1 and X2. When the stretching temperature is high, stretching occurs while the entanglement of polyethylene molecular chains is easily unwound, which tends to result in coarser pores and higher X1 and X2. Furthermore, when the Mv of the polyolefin resin used in the polyolefin microporous membrane is low, the pore refinement that occurs when the stretching temperature is low is suppressed. Therefore, in a design in which an A layer made of a polyethylene resin with a high Mv and a B layer made of a polyethylene resin with a low Mv are laminated, lowering the stretching temperature tends to increase X2 / X1.

[0039] By adjusting the layer structure, raw material composition, and film formation conditions described above, it becomes easy to control X2 / X1 within a target range. The specific control range is preferably set to the range described below.

[0040] X2 / X1 indicates the spread of the pore size distribution in the polyolefin microporous membrane, and a large X2 / X1 ratio indicates that fine pores and coarse pores are widely distributed in the polyolefin microporous membrane. Widely distributing fine pores and coarse pores in the polyolefin microporous membrane is believed to result in a polyolefin microporous membrane with excellent battery separator and liquid filter applications due to the effect of functional separation, with the fine pores providing dendrite resistance (as a battery separator) and filtration performance (as a liquid filter) and the coarse pores improving power characteristics (as a battery separator) and liquid permeability (as a liquid filter). Pores with a pore size less than 0.01 μm significantly deteriorate power characteristics and liquid permeability, while pores with a pore size greater than 10 μm significantly deteriorate dendrite resistance and filtration performance, so it is preferable to have few of either.

[0041] The polyolefin microporous membrane of the present invention preferably has a bubble point pressure of 1900 kPa or more, as determined by the measurement method described below. The bubble point pressure represents the pressure at which air first penetrates when air pressure is applied from one side of a polyolefin microporous membrane impregnated with a liquid. Because the air first penetrates the part of the polyolefin microporous membrane with the largest pores in the in-plane direction, a high bubble point pressure indicates that the polyolefin microporous membrane has a small pore size. When the polyolefin microporous membrane has a bubble point pressure of 1900 kPa or more, more preferably 2100 kPa or more, even more preferably 2300 kPa or more, and even more preferably 2500 kPa or more, it exhibits excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter. From the viewpoint of permeability, the bubble point pressure is preferably 4000 kPa or less. To achieve a bubble point pressure within the above range, it is preferable that the raw material composition and molecular weight of the polyolefin microporous membrane be within the above-mentioned ranges, and / or the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0042] The polyolefin microporous membrane of the present invention has an air resistance of 300 seconds / 100 cm converted into a thickness of 10 μm. 3 It is preferable that the air resistance converted into a thickness of 10 μm is 300 sec / 100 cm or less. 3 Less than 280 seconds / 100cm, preferably less than 280 seconds / 100cm 3 Less than 250 seconds / 100 cm, more preferably 3 Below 200 seconds / 100 cm, particularly preferably 3 By keeping the air permeability resistance at 10 μm thickness or less, it is possible to prevent a decrease in output characteristics when used as a battery separator and to prevent a decrease in flow rate when used as a liquid filter. 3 More than 100 seconds / 100cm is preferable. 3 To achieve an air permeation resistance within the above range, it is preferable that the raw material composition and molecular weight of the raw materials for the polyolefin microporous membrane be within the above-mentioned ranges, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0043] The polyolefin microporous membrane of the present invention preferably has a porosity of 35% or more. A porosity of 35% or more, more preferably 40% or more, even more preferably 45% or more, and even more preferably 50% or more can prevent a decrease in output characteristics when used as a battery separator and prevent a decrease in flow rate when used as a liquid filter. From the viewpoint of handleability, the porosity is preferably 70% or less. To achieve the porosity in the above range, it is preferable that the raw material composition and molecular weight of the raw materials for the polyolefin microporous membrane be within the above-mentioned ranges, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0044] The process for producing a polyolefin microporous membrane preferably comprises the following steps (a) to (e): Hereinafter, examples of the method for producing a polyolefin microporous membrane using the above-mentioned raw materials will be described, but the method is not necessarily limited thereto. (a) Polyolefin raw materials, plasticizers, and additives are kneaded and dissolved to prepare a polyolefin solution. (b) The melt is extruded, formed into a sheet, and cooled to solidify. (c) The obtained sheet is stretched by a roll method or a tenter method. (d) The plasticizer is extracted from the resulting stretched film and the film is dried. (e) Then, heat treatment / re-stretching is carried out.

[0045] Each step will be described below.

[0046] (a) Preparation of polyolefin solution A polyolefin solution is prepared by heating and dissolving a polyolefin resin in a plasticizer. The plasticizer is not particularly limited as long as it can sufficiently dissolve polyethylene. However, to enable relatively high-magnification stretching, it is preferable that the plasticizer be liquid at room temperature. Examples of plasticizers 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 plasticizer content, it is preferable to use a nonvolatile liquid solvent such as liquid paraffin. A solvent that is miscible with polyethylene in the melt-kneaded state but solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, using a solid solvent alone may result in uneven stretching.

[0047] When the polyolefin microporous membrane of the present invention is a laminate membrane, in preparing the polyolefin solution, the proportion of polyolefin resin relative to the total of the polyolefin resin and plasticizer in Layer A (100% by mass) (hereinafter referred to as the resin concentration of the polyolefin solution) is preferably 5% by mass or more. A resin concentration of the polyolefin solution in Layer A of 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and even more preferably 20% by mass or more facilitates reducing the pore size. To maintain sheet formability and prevent a decrease in membrane formability, the resin concentration of the polyolefin solution in Layer A is preferably 40% by mass or less, more preferably 35% by mass or less.

[0048] The resin concentration of the polyolefin solution for Layer B is preferably 35% by mass or less. By setting the resin concentration of the polyolefin solution for Layer B to 35% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less, and even more preferably 20% by mass or less, it becomes easier to improve permeability. In order to prevent deterioration of the sheet formability and film formability, the resin concentration of the polyolefin solution for Layer B is preferably 5% by mass or more, more preferably 10% by mass or more.

[0049] The viscosity of the plasticizer is 20 to 200 cSt (20 × 10 -6 ~200×10 -6 m 2 / s). If the viscosity at 40°C is 20 cSt or more, the sheet extruded from the die is less likely to be non-uniform. On the other hand, if the viscosity is 200 cSt or less, the plasticizer can be easily removed. The viscosity of the plasticizer is measured at 40°C using an Ubbelohde viscometer.

[0050] The method for uniformly melt-kneading the polyolefin solution is not particularly limited, but when a high-concentration polyolefin solution is to be prepared, it is preferable to carry out the melt-kneading in a twin-screw extruder. If necessary, various additives such as antioxidants may be added within a range that does not impair the effects of the present invention. Addition of an antioxidant is particularly preferable to prevent oxidation of polyethylene.

[0051] In the extruder, the polyolefin solution is preferably mixed uniformly at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (Tm + 10)°C to (Tm + 120)°C, where Tm (°C) is the melting point of the polyolefin resin. It is more preferably (Tm + 20)°C to (Tm + 100)°C. Here, the melting point refers to the value measured by DSC according to JIS K7121 (1987) (hereinafter the same). Setting the melt-kneading temperature to (Tm + 10)°C or higher prevents unmelted material from remaining in the extrudate extruded from the die, thereby preventing membrane rupture and other problems during the subsequent stretching process. Setting the melt-kneading temperature to (Tm + 120)°C or lower prevents degradation due to thermal decomposition of the polyolefin and suppresses deterioration of the physical properties of the resulting microporous membrane, such as strength and porosity. It also prevents the appearance from being deteriorated by decomposition products being deposited on chill rolls or rolls in the stretching process and adhering to the sheet. For example, when the polyolefin resin is polyethylene, the polyethylene composition has a melting point of approximately 130 to 140°C, so the melt-kneading temperature is preferably in the range of 140 to 250°C. It is more preferably 160 to 230°C, even more preferably 170 to 200°C, and even more preferably 180 to 200°C.

[0052] Furthermore, when a laminated film is produced as the polyolefin microporous film of the present invention, the resin compositions of the respective layers can be melt-kneaded in a twin-screw extruder, and the resulting mixture can be supplied to a multi-manifold composite T-die for co-extrusion.

[0053] (b) Formation of extrudate and gel-like sheet The resulting extrudate is then cooled to obtain a gel-like sheet, which solidifies the polyethylene microphase separated by the solvent. The cooling step is preferably performed to 10 to 50°C. This is because the final cooling temperature is preferably below the crystallization end temperature. By reducing the high-order structure, uniform stretching becomes easier in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30°C / min or more until the temperature is at least below the gelation temperature. A cooling rate of less than 30°C / min increases the crystallinity, making it difficult to obtain a gel-like sheet suitable for stretching. Generally, a slow cooling rate results in the formation of relatively large crystals, which results in a coarse high-order structure in the gel-like sheet and a large gel structure. In contrast, a fast cooling rate results in the formation of relatively small crystals, which results in a dense high-order structure in the gel-like sheet, which leads to uniform stretching and improved film strength and elongation.

[0054] The cooling method may be a method of directly contacting the material with cold air, cooling water or other cooling medium, a method of contacting the material with a roll cooled with a cooling medium, or a method using a casting drum or the like.

[0055] (c) Stretching process The gel-like sheet thus obtained is stretched by any of the inflation method, simultaneous biaxial stretching method, and sequential biaxial stretching method, but among these, it is preferable to employ the simultaneous biaxial stretching method or sequential biaxial stretching method in terms of film formation stability, thickness uniformity, and control of high film rigidity and dimensional stability.

[0056] Examples of combinations of stretching devices and stretching methods that can be used include MD (machine direction) uniaxial stretching using a roll stretching machine, TD (transverse direction) uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter.

[0057] The stretching temperature is preferably set to the melting point of the gel-like sheet + 10°C or less, more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the gel-like sheet + 5°C). Specifically, since a polyethylene composition has a crystal dispersion temperature of approximately 90 to 100°C, the stretching temperature is preferably 90 to 125°C, more preferably 90 to 120°C, even more preferably 90 to 110°C, and particularly preferably 95 to 105°C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it can be determined by NMR. A stretching temperature of 90°C or higher can generate sufficient pores, improve the uniformity of the membrane thickness, increase the porosity, and improve permeability. Furthermore, a stretching temperature of 125°C or less can prevent the sheet from melting, prevent a decrease in permeability due to pore blockage, and prevent the pore size from becoming too large.

[0058] The stretching ratio varies depending on the thickness of the gel-like sheet, but is preferably 3 times or more in both MD and TD from the viewpoint of membrane thickness uniformity. The areal stretching ratio is preferably 9 times or more. By setting the areal stretching ratio to 9 times or more, more preferably 16 times or more, and even more preferably 25 times or more, orientation progresses, the crystallinity increases, making it easier to obtain a uniform membrane, and a microporous membrane with excellent melting point and strength can be obtained. The areal stretching ratio is preferably 100 times or less. Setting the areal stretching ratio to 100 times or less can prevent frequent breakage during the production of the microporous membrane, thereby preventing a decrease in productivity.

[0059] Stretching causes cleavage of the higher-order structure formed in the gel sheet, resulting in a finer crystalline phase and the formation of numerous fibrils. The fibrils form a network structure in which the fibrils are connected irregularly in three dimensions. Stretching not only improves mechanical strength, but also cleaves the fibrils, making them porous and reducing the pore size, resulting in a polyolefin microporous membrane suitable for battery separators and liquid filters. Furthermore, by stretching before removing the plasticizer, the polyolefin is in a sufficiently plasticized and softened state, which facilitates smooth cleavage of the higher-order structure and allows for uniform refinement of the crystalline phase. Furthermore, because cleavage is easy, strain is less likely to remain during stretching, resulting in a lower thermal shrinkage rate compared to stretching after removing the plasticizer.

[0060] (d) Washing and drying process Next, in the washing step, the plasticizer remaining in the gel-like sheet is removed using a washing solvent. Because the polyethylene phase and the solvent phase are separated in the microstructure of the gel-like sheet, removal of the plasticizer yields a microporous membrane.

[0061] Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain fluorocarbons such as trifluoroethane. These cleaning solvents have a low surface tension, specifically, 24 mN / m or less at 25°C. By using a cleaning solvent with a low surface tension, the shrinkage of the network structure that forms the micropores during drying after cleaning is suppressed by the surface tension at the gas-liquid interface, resulting in a microporous membrane with porosity and permeability. These cleaning solvents can be selected appropriately depending on the plasticizer and can be used alone or in combination.

[0062] Washing can be performed by immersing the gel-like sheet in a washing solvent for extraction, showering the gel-like sheet with the washing solvent, or a combination of these. The amount of washing solvent used varies depending on the washing method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet. The washing temperature may be 15 to 30°C, and may be heated to 80°C or less as needed. When washing by immersion, the longer the time the gel-like sheet is immersed in the washing solvent, the better, from the viewpoints of enhancing the washing effect of the solvent, preventing the resulting microporous membrane from having nonuniform properties in the TD and / or MD, and improving the mechanical and electrical properties of the microporous membrane. The above-described washing is preferably performed until the residual solvent in the washed gel-like sheet, i.e., the microporous membrane, is less than 1% by mass.

[0063] After the washing step, the solvent in the microporous membrane is removed by drying in a drying step. The drying method is not particularly limited, and methods using metal heating rolls or hot air can be selected. The drying temperature is preferably 40 to 100°C, more preferably 40 to 80°C. If the drying is insufficient, the porosity of the microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

[0064] (e) Heat treatment / re-stretching process The dried microporous membrane may be stretched (restretched). Restretching can be carried out by a tenter method or the like while heating the microporous membrane, similar to the stretching described above. Restretching may be uniaxial or biaxial. Multistage stretching is carried out by combining simultaneous biaxial and / or sequential stretching.

[0065] Examples of combinations of stretching apparatuses and stretching methods used in re-stretching include MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter or a combination of a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter.

[0066] The heat treatment / re-stretching temperature is preferably equal to or lower than the melting point of the polyethylene composition, and more preferably within the range of (Tcd-20°C) to the melting point. Specifically, the temperature is preferably 70 to 135°C, more preferably 80 to 125°C, and even more preferably 90 to 120°C. If the heat treatment / re-stretching temperature is too high, the pore size may become too large.

[0067] The stretching ratio in the re-stretching is preferably 0.8 times or more and 1.6 times or less, based on the film area before re-stretching. A stretching ratio in the re-stretching of 0.8 times or more, more preferably 0.9 times or more, and even more preferably 1.0 times or more, results in better permeability. Furthermore, a stretching ratio in the re-stretching of 1.6 times or less, more preferably 1.5 times or less, and even more preferably 1.4 times or less can prevent the pore size from becoming too large. A stretching ratio of less than 1.0 times indicates that the film is relaxed.

[0068] (f) Other processes Furthermore, depending on other applications, the microporous membrane can be subjected to hydrophilization treatment, such as monomer grafting, surfactant treatment, or corona discharge.

[0069] In the surfactant treatment, any of nonionic, cationic, anionic, and amphoteric surfactants can be used, but nonionic surfactants are preferred. The multi-layer, microporous membrane is immersed in a solution prepared by dissolving the surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the multi-layer, microporous membrane by a doctor blade method.

[0070] The polyolefin microporous membrane can also be crosslinked by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. The crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane. The monomer grafting is preferably performed after the crosslinking treatment.

[0071] The polyolefin microporous membrane of the present invention may be formed into a multilayer polyolefin porous membrane by laminating a porous layer other than polyolefin by coating or vapor deposition for the purpose of imparting functions such as meltdown properties, heat resistance, and adhesiveness.

[0072] The other porous layer is not particularly limited, but for example, a porous layer such as an inorganic particle layer containing a binder and inorganic particles may be laminated.

[0073] The binder component constituting the inorganic particle layer is not particularly limited, and known components can be used, such as acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, and polyimide resin.

[0074] The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used, for example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, etc. Furthermore, the multilayer polyolefin porous membrane may be one in which the porous binder resin is laminated on at least one surface of a polyolefin microporous membrane.

[0075] The polyolefin microporous membrane of the present invention can be used in a variety of applications, including filters, fuel cell separators, and capacitor separators. When used as a battery separator, it exhibits excellent dendrite resistance and output characteristics, making it particularly suitable for use as a battery separator for secondary batteries, such as those required for electric vehicles, which require high energy density, high capacity, and high output. Furthermore, when used as a liquid filter, it exhibits excellent filtration accuracy and high permeability, making it suitable for use 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, or pleated filtration units, with pleated filtration units being preferred due to the increased 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 the reinforcing membrane and the membrane with a heated roll or the like, the membrane is pleated with peaks and valleys and incorporated into the filtration unit for use. [Example]

[0076] The present invention will be described in more detail with reference to examples. However, the embodiments of the present invention are not limited to these examples. Evaluations in this application were performed in an environment of 23°C and 65% humidity unless otherwise specified. The evaluation and analysis methods used in the examples are as follows.

[0077] [Film thickness] The film thickness of five randomly selected points within a 50 mm × 50 mm area of ​​the polyolefin microporous film was measured 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 film thickness (μm).

[0078] [Porosity] A 50mm x 50mm square sample was cut from the polyolefin microporous membrane, its mass (g) was measured, and its volume (cm) was calculated by measuring the membrane thickness using the method described above. 3 In addition to the mass and volume mentioned above, the film density (g / cm 3 The porosity of the polyolefin microporous membrane was calculated from the value of 0.99 g / cm using the following formula. 3 In this measurement, samples were cut out from three randomly selected positions on the polyolefin microporous membrane, and the average porosity measured at each was calculated. Porosity (%) = [(volume - mass / membrane density) / volume] x 100 ... (formula).

[0079] [Air resistance] For a microporous membrane with a thickness of T1 (μm), the air permeability resistance P1 (seconds / 100 cm) was measured using an Oken air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 (2009). 3 ) was measured. In addition, the air permeability resistance P2 (10 μm equivalent) (seconds / 100 cm) per 10 μm thickness was calculated using the formula: P2 = (P1 × 10) / T1. 3 / 10 μm) was calculated.

[0080] [Polyolefin viscosity average molecular weight (Mv)] The viscosity average molecular weight Mv was determined by the following method in accordance with ISO1628-3 (2010). First, 20 mg of polyolefin resin was weighed out and purged with nitrogen. Then, 20 mL of decalin was added and the mixture was stirred at 150°C for 2 hours to dissolve the polyolefin resin. The solution was placed in a thermostatic bath at 135°C and the drop time (ts) between the gauge lines was measured using a Cannon-Fenske viscometer (Shibata Scientific Instruments Co., Ltd.: Product No. -100). The drop time (ts) between the gauge lines was also measured for samples with resin amounts of 10 mg, 5 mg, 2 mg, and 0 mg. The reduced viscosity (ηsp / C) of the polyethylene resin composition was determined according to the following formula: ηsp / C=(ts / tb-1) / 0.1 (unit: dL / g) The relationship between the concentration (C) (unit: g / dL) and the reduced viscosity (ηsp / C) of the polyethylene resin composition was plotted respectively, and an approximate linear equation was derived by the least squares method, and the intrinsic viscosity ([η]) was obtained by extrapolating to a concentration of 0. Next, the viscosity average molecular weight (Mv) was calculated from the value of the intrinsic viscosity [η] using the following equation. Mv=(5.34×10 4 )×[η] 1.49 …(Equation).

[0081] [Pore size distribution of the polyolefin microporous membrane by mercury intrusion method] Using a pore size distribution measuring device (Autopore V9620 manufactured by Micromeritics), the pore distribution was determined in the range of pore diameters from 0.004 to 200 μm. The pore diameter was calculated using the following equation (Washburn's equation). PD = -4σcosθ …(Equation) Here, P: Pressure (kPa) σ: Surface tension of mercury (480 dynes / cm) D: Pore diameter (μm) θ: Contact angle between mercury and the sample (140°).

[0082] The method for calculating X2 / X1 from the pore size distribution curve (X-axis: pore size, Y-axis: dV / d(LogD)) obtained by the mercury intrusion method is shown in the following I to III. I. Read the maximum value of dV / d(LogD) (Y max ) and the value of the X-axis at this time (X max ) in the range where the X-axis is from 0.01 μm to 10 μm. II. In the range where the X-axis of the above pore size distribution curve is from 0.01 μm to 10 μm, for the intersection point where Y = Y max / 2, X(Y=Y max (Except when there are three or more Xs that satisfy / 2) let Small that from X1 、X be X2. III. Calculate X2 / X1 from X1 and X2 (X1 < X2) obtained by the above method.

[0083] [Bubble point] The bubble point of the polyolefin microporous membrane was determined using a Perm Porometer (CFP-1500A, manufactured by PMI). Galwick (propylene, 1,1,2,3,3,3-hexahydrofluoric acid oxide, surface tension: 15.9 dynes / cm) was used as the impregnation liquid for the polyolefin microporous membrane, and the pressure at which the flow rate reached 20 cc / min or more was defined as the bubble point pressure (kPa).

[0084] [Differential scanning calorimetry (DSC)] A 6.0 mg sample was sealed in an aluminum pan and heated using a differential scanning calorimeter (Parking Elmer PYRIS Diamond DSC) from 30 ° C to 230 ° C at a rate of 10 ° C / min for the first time, held at 230 ° C for 5 minutes, cooled to 30 ° C at a rate of 10 ° C / min, and heated from 30 ° C to 230 ° C at a rate of 10 ° C / min for the second time. The melting point of the raw material polyolefin resin was 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 described above. The melting point was the temperature at which the maximum endotherm was measured. The polyethylene resin content in the polyolefin microporous membrane was calculated from the area of ​​the crystalline melting peak (ΔH1) obtained by drawing a baseline between 60°C and 155°C and the area of ​​the crystalline melting peak (ΔH2) obtained by drawing a baseline between 155°C and 200°C in the temperature distribution curve of the endotherm measured during the second heating run in the DSC measurement described above, using the following formula: Polyethylene resin content (%) of polyolefin microporous membrane = 100 × ΔH1 / (ΔH1+ΔH2) ... (formula).

[0085] [Example 1] (First polyolefin solution) The polyolefin raw material for the A layer is a polymer with a viscosity average molecular weight (Mv) of 3.0 × 10 6100 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C) was used. To this was added 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 as antioxidants to obtain a polyolefin mixture.

[0086] 15 parts by mass of the obtained polyolefin mixture was fed into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and a polyolefin having a viscosity of 35 cSt (35 × 10) at 40 °C was extracted from the side feeder of the twin-screw extruder. -6 m 2 85 parts by mass of liquid paraffin (flow rate: 1 / s) was supplied to the mixture, and the mixture was melt-kneaded at 210°C and 200 rpm to prepare a first polyolefin solution.

[0087] (Second polyolefin solution) The polyolefin raw material for the B layer is a polymer with a viscosity average molecular weight (Mv) of 3.7 x 10 5 100 parts by mass of high-density polyethylene (melting point 135°C) was used, 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 to obtain a polyolefin mixture.

[0088] 25 parts by mass of the obtained polyolefin mixture was fed into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and a polyolefin having a viscosity of 35 cSt (35 × 10) at 40 °C was extracted from the side feeder of the twin-screw extruder. -6 m 2 75 parts by mass of liquid paraffin (1 / s) was supplied and melt-kneaded under conditions of 210°C and 200 rpm to prepare a second polyolefin solution.

[0089] (gel sheet) The first and second polyolefin solutions were passed through filters from each twin-screw extruder to remove foreign matter, then fed to a three-layer T-die and extruded in a layer configuration of second polyolefin solution / first polyolefin solution / second polyolefin solution (Layer B / Layer A / Layer B) with the extrusion rate ratio of each layer adjusted to 1 / 2 / 1 (Layer B / Layer A / Layer B). The extrudate was cooled while being taken up at a take-up speed of 1.5 m / min on a cooling roll adjusted to 30°C, to form a gel-like three-layer sheet.

[0090] (Stretching) The above gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD and TD directions at 100° C. using a stretching machine.

[0091] (Washing and drying) The stretched gel-like three-layer sheet was immersed in a methylene chloride bath adjusted to 25° C. to thoroughly remove the liquid paraffin, and then air-dried at room temperature.

[0092] (heat setting treatment) The obtained dried film was subjected to heat setting treatment at 100°C for 3 minutes.

[0093] The thickness of the obtained polyolefin porous membrane was 29 μm. Table 1 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0094] [Example 2] (First polyolefin solution) The polyolefin raw material for the A layer is a polymer with a viscosity average molecular weight (Mv) of 3.5 x 10 6 100 parts by mass of ultra-high molecular weight polyethylene (melting point 135° C.) was used. The same antioxidant as in Example 1 was added thereto in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0095] 20 parts by mass of the obtained polyolefin-polyolefin mixture was treated in the same manner as in Example 1 to obtain a polyolefin-polyolefin mixture having a viscosity of 35 cSt (35 × 10 -6 m 2 The mixture was melt-kneaded with 80 parts by mass of liquid paraffin (100 parts by mass / s) to prepare a first polyolefin solution.

[0096] (Second polyolefin solution) The same second polyolefin solution as used in Example 1 was used as the second polyolefin solution.

[0097] (gel sheet) A gel-like three-layer sheet was formed in the same manner as in Example 1, except that the first and second polyolefin solutions were used and the take-up speed with the cooling roll was set to 4 m / min, and the thickness of the polyolefin microporous membrane was adjusted to 11 μm.

[0098] (Stretching, washing and drying, heat setting) Using the above three-layer gel sheet, a polyolefin porous membrane was obtained in the same manner as in Example 1.

[0099] [Example 3] A polyolefin porous membrane was obtained in the same manner as in Example 1, except that the ratio of the discharge amounts of the layers in forming the gel-like sheet was 2 / 1 / 2 (layer B / layer A / layer B).

[0100] [Example 4] (First polyolefin solution) The same first polyolefin solution as used in Example 1 was used as the first polyolefin solution.

[0101] (Second polyolefin solution) The polyolefin raw material for the B layer is a polymer with a viscosity average molecular weight (Mv) of 2.0 x 10 6 30 parts by mass of high density polyethylene (melting point 133°C) and Mv of 3.7 x 10 5 70 parts by mass of high-density polyethylene (melting point 135°C) was used. To this, 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 to obtain a polyolefin mixture.

[0102] 30 parts by mass of the obtained polyolefin mixture was treated in the same manner as in Example 1 to obtain a polyolefin mixture having a viscosity of 35 cSt (35 × 10 -6 m 2 The mixture was melt-kneaded with 70 parts by mass of liquid paraffin (100 parts by mass / s) to prepare a first polyolefin solution.

[0103] (Gel-like sheet, stretching, washing and drying, heat setting treatment) Using the first and second polyolefin solutions, the same procedures as in Example 1 were carried out to obtain a porous polyolefin membrane.

[0104] [Example 5] (First polyolefin solution) The polyolefin raw material for the A layer is a polymer with a viscosity average molecular weight (Mv) of 3.0 × 10 6 70 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C), and Mv of 9.0 x 10 4 30 parts by mass of high density polyethylene (melting point 132°C) was used. To these, the same antioxidant as in Example 1 was added in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0105] 23 parts by mass of the obtained polyolefin mixture was treated in the same manner as in Example 1 to obtain a polyolefin mixture having a viscosity of 35 cSt (35 × 10 -6 m 2 The mixture was melt-kneaded with 77 parts by mass of liquid paraffin (100 parts by mass / s) to prepare a first polyolefin solution.

[0106] (Second polyolefin solution) The same second polyolefin solution as used in Example 1 was used as the second polyolefin solution.

[0107] (gel sheet) The first and second polyolefin solutions were passed through filters from each twin-screw extruder to remove foreign matter, and then fed to a three-layer T-die and extruded in a layer configuration of second polyolefin solution / first polyolefin solution / second polyolefin solution (Layer B / Layer A / Layer B) with the extrusion rate ratio of each layer adjusted to 2 / 1 / 2 (Layer B / Layer A / Layer B). The extrudate was cooled while being taken up at a take-up speed of 4 m / min on a cooling roll adjusted to 30°C, to form a gel-like three-layer sheet.

[0108] (Stretching) The above gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD and TD directions at 100° C. using a stretching machine.

[0109] (Washing and drying) The stretched gel-like three-layer sheet was immersed in a methylene chloride bath adjusted to 25°C to thoroughly remove the liquid paraffin, and then air-dried at room temperature.

[0110] (heat setting treatment) The obtained dried film was subjected to heat setting treatment at 100°C for 3 minutes.

[0111] [Example 6] A polyolefin porous membrane was obtained in the same manner as in Example 5, except that re-stretching was carried out in the TD direction to a stretching ratio of 1.3 times during the heat setting treatment at 100°C for 3 minutes.

[0112] [Comparative Example 1] A polyolefin porous membrane was obtained in the same manner as in Example 1, except that the second polyolefin solution had the same composition as the first polyolefin solution, the take-up speed on the cooling roll was set to 4 m / min, and the thickness of the polyolefin microporous membrane was adjusted to 11 μm.

[0113] Comparative Example 2 A polyolefin porous membrane was obtained in the same manner as in Example 1, except that the first polyolefin solution had the same composition as the second polyolefin solution, the take-up speed on the cooling roll was set to 4 m / min, and the thickness of the polyolefin microporous membrane was adjusted to 10 μm.

[0114] Comparative Example 3 (First polyolefin solution) The polyolefin raw material for the A layer is a polymer with a viscosity average molecular weight (Mv) of 2.0 × 10 6 30 parts by mass of high density polyethylene (melting point 133°C) and a viscosity average molecular weight (Mv) of 3.7 x 10 5 70 parts by mass of high density polyethylene (melting point 135° C.) was used. To these, the same antioxidant as in Example 1 was added in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0115] 30 parts by mass of the obtained polyolefin mixture was treated in the same manner as in Example 1 to obtain a polyolefin mixture having a viscosity of 35 cSt (35 × 10 -6 m 2 The mixture was melt-kneaded with 70 parts by mass of liquid paraffin (100 parts by mass / s) to prepare a first polyolefin solution.

[0116] (Second polyolefin solution) The polyolefin raw material for the B layer is a polymer with a viscosity average molecular weight (Mv) of 3.7 x 10 5 50 parts by mass of high density polyethylene (melting point 135°C), viscosity average molecular weight (Mv) of 2.0 x 10 6 50 parts by mass of polypropylene (melting point 163°C) was used. To these, the same antioxidant as in Example 1 was added in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0117] 30 parts by mass of the obtained polyolefin mixture was treated in the same manner as in Example 1 to obtain a polyolefin copolymer having a viscosity of 35 cSt (35 × 10 -6 m 2 / s) and 70 parts by mass of liquid paraffin to prepare a second polyolefin solution.

[0118] (gel sheet) The first and second polyolefin solutions were passed through filters from each twin-screw extruder to remove foreign matter, and then fed to a three-layer T-die and extruded in a layer configuration of second polyolefin solution / first polyolefin solution / second polyolefin solution (Layer A / Layer B / Layer A) with the extrusion rate ratio of each layer adjusted to 2 / 1 / 2 (Layer A / Layer B / Layer A). The extrudate was cooled while being taken up at a take-up speed of 4 m / min on a cooling roll adjusted to 30°C, to form a gel-like three-layer sheet.

[0119] (Stretching) The above gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD and TD directions at 110° C. using a stretching machine.

[0120] (Washing and drying) The stretched gel-like three-layer sheet was immersed in a methylene chloride bath adjusted to 25°C to thoroughly remove the liquid paraffin, and then air-dried at room temperature.

[0121] (heat setting treatment) The obtained dried film was subjected to heat setting treatment at 110°C for 3 minutes.

[0122] The thickness of the resulting polyolefin porous membrane was 10 μm.

[0123] Comparative Example 4 A porous polyolefin membrane was obtained in the same manner as in Example 5, except that the second polyolefin solution had the same composition as the first polyolefin solution.

[0124] Comparative Example 5 A polyolefin porous membrane was obtained in the same manner as in Example 5, except that the first polyolefin solution had the same composition as the second polyolefin solution.

[0125] [evaluation] The polyolefin microporous membranes of Examples 1 to 6 had an X2 / X1 ratio of 15 or more and a thickness of 30 μm or less, and therefore exhibited excellent dendrite resistance and output characteristics when used as battery separators and excellent filtration accuracy and permeability when used as liquid filters. On the other hand, the polyolefin microporous membranes of Comparative Examples 1 to 3 had an X2 / X1 ratio of less than 15.

[0126] [Table 1] [Industrial Applicability]

[0127] The polyolefin microporous membrane of the present invention, when used as a battery separator, has excellent dendrite resistance and output characteristics, and is therefore suitable for use as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as for electric vehicles, etc. Furthermore, when used as a liquid filter, it has excellent filtration accuracy and high permeability, and is therefore suitable for use as a high-precision liquid filter that requires the removal of minute foreign matter, such as in semiconductor processes.

Claims

1. In a pore size distribution measured by mercury intrusion porosimetry, with pore size on the X axis and dV / d (LogD) on the Y axis, the maximum value of dV / d (LogD) in the range of X of the pore size distribution from 0.01 μm to 10 μm is defined as Y. ｍａｘ and X of the pore size distribution is in the range of 0.01 μm to 10 μm, and Y=Y ｍａｘ / 2 (excluding the case where there are three or more Xs that satisfy Y = Y max / 2), １ , X ２ When X ２ / X １ A polyolefin microporous membrane having a molecular weight of 15 or more and a thickness of 30 μm or less. Here, V: cumulative pore volume (cm ３ / g) D: Pore diameter (μm).

2. The microporous polyolefin membrane according to claim 1, having a bubble point pressure of 1900 kPa or more.

3. The polyolefin microporous membrane according to claim 1 or 2, comprising 90% by mass or more of polyethylene.

4. Air resistance converted to a thickness of 10 μm is 300 seconds / 100 cm ３ The polyolefin microporous membrane according to claim 1 or 2, wherein:

5. The polyolefin microporous membrane according to claim 1 or 2, which is used as a separator for a secondary battery.

6. The microporous polyolefin membrane according to claim 1 or 2, which is used as a liquid filter.

7. A separator for a secondary battery, comprising the polyolefin microporous membrane according to claim 1 or 2.

8. A liquid filter using the polyolefin microporous membrane according to claim 1 or 2.

9. A secondary battery using the separator for secondary batteries according to claim 7.

10. A filtration unit using the liquid filter according to claim 8.

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