JP7758155B2 - Polyolefin microporous membrane, secondary battery separator, liquid filter, secondary battery, and filtration unit - Google Patents

Description

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

Polyolefin microporous membranes are used as filters, fuel cell separators, capacitor separators, and more. They are particularly well suited for use as separators for lithium-ion secondary batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, and other devices. Furthermore, due to their uniform, fine pore structure and excellent solvent and chemical resistance, polyolefin microporous membranes are widely used in a variety of filter applications, including water treatment membranes, ultrafiltration membranes, microfiltration membranes, and breathable, waterproof clothing.

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. On the other hand, reducing the pore size of microporous membranes can reduce permeability, potentially resulting in reduced power output characteristics.

Liquid filter applications require both high-precision separation ability and high permeability. For example, in semiconductor manufacturing processes, as semiconductor wiring pitches become 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. However, from the perspective of processing capacity, a decrease in permeability is undesirable.

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

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.

Patent document 3 describes a microporous membrane made of laminated layers of different resin compositions, consisting of PE and PP. The surface and inner layers have different resin compositions and pore structures, resulting in a polyolefin microporous membrane that, when used as a battery separator, has an excellent balance of permeability, mechanical strength, meltdown properties, electrolyte absorption, and electrolyte retention.

Japanese Patent Application Laid-Open No. 2018-167198 International Publication No. 2018/168871 Japanese Patent Application Laid-Open No. 2008-255307

Patent Document 1 describes 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 does not improve the trade-off between small pore size and high permeability.

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. 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.

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 a pore size distribution measured by mercury porosimetry, where the pore size is on the X axis and dV/d(LogD) is on the Y axis, the maximum value of dV/d(LogD) in the pore size range of 0.01 μm to 10 μm is defined as _Ymax , and pore sizes that satisfy _Ymax /2 within the pore size range of 0.01 μm to 10 μm are defined as _X1 and _X2 , in order from the smallest to the largest, the polyolefin microporous membrane has an _X2 / _X1 ratio 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 polyolefin microporous membrane according to [1] above, having 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 polyolefin microporous membrane according to any one of [1] to [3] above, having an air resistance of 300 seconds or less in terms of a thickness of 10 μm.
[5]
The polyolefin microporous membrane according to any one of [1] to [4] above, which is used as a separator for a secondary battery.
[6]
The polyolefin microporous membrane according to any one of [1] to [4] above, which is used as a liquid filtration filter.
[7]
A separator for a secondary battery, using 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.

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

The following describes embodiments of the present invention. However, the present invention is not limited to these.

The polyolefin microporous membrane of the present invention contains a polyethylene-based resin as a major component. The term "major component" as used herein refers to the component that is most abundant, expressed as % by mass, among the components constituting the polyolefin microporous membrane. The polyethylene-based resin preferably accounts for 90% by mass or more of the polyolefin microporous membrane, 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-based resin in the polyolefin microporous membrane within the above range, a polyolefin microporous membrane with an excellent pore structure uniformity can be obtained, and when used as a liquid filter, the resulting polyolefin microporous membrane exhibits excellent filtration accuracy and high permeability. The content of the polyethylene-based resin in the polyolefin microporous membrane can be measured by the method described below.

Various polyethylenes can be used as the polyethylene-based resin, including, for example, ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene.

The polyethylene resin may be a homopolymer of ethylene or a copolymer of ethylene with another α-olefin. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene.

Furthermore, the polyethylene resin may contain two or more types of polyethylene.

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 membrane may exhibit excellent filtration accuracy when used as a liquid filter.

The polypropylene may be a homopolymer, block copolymer, or random copolymer. Block copolymers and random copolymers may contain copolymer components with α-ethylenes other than propylene, with ethylene being preferred as the other α-ethylene.

However, since the inclusion of polypropylene tends to result in reduced mechanical strength and permeability compared to when polyethylene is used alone, the content of polypropylene in the polyolefin microporous film 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.

The polyolefin microporous membrane may be a single-layer membrane, but is preferably a laminated membrane having an A layer primarily composed of a polyethylene-based resin and a B layer primarily composed of a polyethylene-based resin with different properties. Generally, reducing the pore size of a porous membrane narrows the fluid flow path, 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, where the A layer is primarily composed of a polyethylene-based resin with a high viscosity average molecular weight (Mv) and the B layer is primarily composed of a polyethylene-based resin with a low viscosity average molecular weight (Mv), and is produced under the manufacturing conditions described below.

The resin constituting Layer A contains a polyethylene resin as its primary 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 ensuring that the content of the polyethylene resin component in Layer A falls 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.

The resin constituting Layer B contains a polyethylene resin as its primary 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 ensuring that the content of the polyethylene resin component in Layer B falls within the above range, a polyolefin microporous membrane can be obtained that has an 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.

The viscosity average molecular weight (Mva) of the polyethylene resin contained in Layer A is preferably 150 x 10 ⁴ or more, more preferably 200 x 10 ⁴ or more, even more preferably 250 x 10 ⁴ or more, and even more preferably 300 x 10 ⁴ or more. A larger Mva is preferable, but if it is too large, film formability decreases, so it is preferably 500 x 10 ⁴ or less, more preferably 450 x 10 ⁴ or less. 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, and to obtain 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 is easy to control the layer ratio of Layer A to Layer B, described below, within a preferred range. The Mva value can be adjusted by the raw material composition and kneading conditions of Layer A.

The viscosity average molecular weight (Mvb) of the polyethylene resin contained in Layer B is preferably 125 x 10 ⁴ or less, more preferably 100 x 10 ⁴ or less, even more preferably 50 x 10 ⁴ or less, and even more preferably 40 x 10 ⁴ or less. The smaller the Mvb, the better; however, if it is too small, film formability decreases; therefore, it is preferably 1 x 10 ⁴ or more, more preferably 5 x 10 ⁴ or more. By ensuring that the viscosity average molecular weight of the polyethylene resin contained in Layer B falls within the above range, a polyolefin microporous membrane with excellent permeability can be obtained, and it also becomes easier to control the layer ratio of Layer A to Layer B within the preferred range. Mvb can be adjusted by the raw material composition and kneading conditions of Layer B.

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 more preferably 150 x 10 ⁴ or more, even more preferably 200 x 10 ⁴ or more, and even more preferably 250 x 10 ⁴ or more. From the viewpoint of achieving both a small pore size and a high flow rate, the larger the value of (Mva - Mvb), the better. However, if the value is too large, the molecular weight difference and viscosity difference between the resin constituting Layer A and the resin constituting Layer B will become too large, resulting in a decrease in film formability in co-extrusion. Therefore, the value of (Mva - Mvb) is preferably 500 x 10 ⁴ or less, more preferably 400 x 10 ⁴ or less, and particularly preferably 300 x 10 ⁴ or less. To set the value of (Mva - Mvb) within the above range, it is preferable that the raw material compositions and molecular weights of the raw materials of Layer A and Layer B be within the ranges described above.

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 keeping the melting point within this 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 secondary batteries, and excellent filtration accuracy and high permeability when used as a liquid filter.

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 keeping the melting point within this range, a polyolefin microporous membrane with excellent permeability can be obtained.

The polyolefin microporous membrane of the present invention may contain various additives, such as antioxidants, heat stabilizers, antistatic agents, UV absorbers, antiblocking agents, and fillers, as long as the effects of the present invention are not impaired. 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, for example, 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 (for example, "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7).

The layer structure of the laminated membrane can be A layer/B layer, A layer/B layer/A layer, or B layer/A layer/B layer, with A layer/B layer/A layer and B layer/A layer/B layer structures being preferred. By using such a layer structure, a polyolefin microporous membrane can be obtained 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.

The polyolefin microporous membrane of the present invention preferably has a total thickness of layer A of 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. By having the raw material composition and molecular weight of the raw materials constituting layer A be within the above-mentioned ranges, the polyolefin microporous membrane of the present invention can effectively reduce the pore size even when the ratio of the total thickness of layer A to the total thickness is reduced.

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 suppress deterioration in permeability, suppress a decrease in output characteristics when used as a battery separator, and suppress a decrease in flow rate when used as a liquid filter. From the perspective of permeability, a lower thickness ratio of Layer A is preferable; however, if it is too low, the filtration life and lamination accuracy may be reduced, so the lower limit is approximately 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 preferably within the above-described ranges, and further, in order to obtain a high-quality polyolefin microporous membrane with excellent lamination accuracy, it is preferable to set the extrusion conditions during polyolefin microporous membrane production within the ranges described below.

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 the membrane-forming conditions.

In the pore size distribution of the polyolefin microporous membrane of the present invention, measured by mercury 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 pore size range of 0.01 μm to 10 μm is defined as _Ymax , and pore sizes that satisfy _Ymax /2 within the pore size range of 0.01 μm to 10 μm are defined as _X1 and _X2 , from the smallest to the largest, such that _X2 / _X1 is 15 or greater.
Here,
V: cumulative pore volume (cm ³ /g)
D: Pore diameter (μm).

When _X2 / _X1 is 15 or more, preferably 20 or more, more preferably 30 or more, and even more preferably 50 or more, a polyolefin microporous membrane can be obtained that has 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 kept within the above range by setting 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 by setting the membrane-forming conditions within the ranges described below.

Regarding the relationship between the layer structure and _X2 / _X1 , when a design is adopted in which Layer A having a fine pore structure and Layer B having a coarse pore structure are laminated, fine pores and coarse pores can coexist in the polyolefin microporous membrane, and therefore _X2 / _X1 tends to be high.

Furthermore, regarding the relationship between the 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 because the number of entanglement points between polyethylene molecular chains increases, resulting in finer pores. Furthermore, decreasing the Mv of the polyethylene resin tends to increase the pore size and increase _X1 and _X2 . Therefore, with regard to the above-mentioned 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.

Regarding the relationship between the film-forming conditions and _X2 / _X1 , when the stretching temperature is lowered, stretching occurs in a state in which the entanglement of polyethylene molecular chains is constrained, so the pores tend to become finer and _X1 and _X2 tend to become lower. When the stretching temperature is higher, stretching occurs in a state in which the entanglement of polyethylene molecular chains is easily unwound, so the pores tend to become coarser and _X1 and _X2 tend to become higher. Note that when the Mv of the polyolefin resin used in the polyolefin microporous membrane is lowered, the pores are prevented from becoming finer when the stretching temperature is lowered, so 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 .

By adjusting the layer structure, raw material composition, and film-forming conditions, it becomes easy to control X ₂ /X ₁ within a target range. The specific control range is preferably set to the range described below.

_X2 / _X1 indicates the spread of the pore size distribution in the polyolefin microporous membrane, and a large _X2 / _X1 means that fine pores and coarse pores are widely distributed in the polyolefin microporous membrane.By widely distributing fine pores and coarse pores in the polyolefin microporous membrane, the fine pores are responsible for dendrite resistance (as a battery separator) and filtration performance (as a liquid filter), and the coarse pores are responsible for improving power characteristics (as a battery separator) and improving liquid permeability (as a liquid filter), thereby achieving functional separation, and it is believed that a polyolefin microporous membrane excellent for use as a battery separator and liquid filter can be obtained.Note that pores with a pore size of less than 0.01 μm significantly deteriorate power characteristics and liquid permeability, and pores with a pore size of more than 10 μm significantly deteriorate dendrite resistance and filtration performance, so it is preferable that both are small.

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 portion 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 bubble point pressure of the polyolefin microporous membrane is 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, the membrane 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.

The polyolefin microporous membrane of the present invention preferably has an air resistance, calculated as a 10 μm thickness, of 300 sec/100 cm3 or less. An air resistance, calculated as a 10 μm thickness, of 300 sec/100 cm3 ^{or less} , more preferably 280 sec/100 cm3 ^or less, even more preferably 250 sec/100 cm3 or ^less , and particularly preferably 200 sec/100 cm3 ^or less can suppress a decrease in output characteristics when used as a battery separator and suppress a decrease in flow rate when used as a liquid filter. From the viewpoint of membrane strength, the air resistance, calculated as a 10 μm thickness, is preferably 10 sec/100 cm3 or ^more , more preferably 100 sec/100 cm3 or ^more . To achieve the air 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.

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 a porosity 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 polyolefin microporous membrane production be within the ranges described below.

The process for producing a polyolefin microporous membrane preferably comprises the following steps (a) to (e): Hereinafter, examples of methods for producing a polyolefin microporous membrane using the above raw materials will be described, but the present invention is not necessarily limited to these.
(a) A polyolefin solution is prepared by kneading and dissolving a polyolefin raw material, a plasticizer, and an additive.
(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.

Each process is explained below.

(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 stretching ratios, the plasticizer is preferably 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, a nonvolatile liquid solvent such as liquid paraffin is preferably used. 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.

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 100% by mass of the polyolefin resin and plasticizer in Layer A (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.

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. To prevent deterioration of the sheet moldability and film-forming properties, the resin concentration of the polyolefin solution for Layer B is preferably 5% by mass or more, more preferably 10% by mass or more.

The viscosity of the plasticizer is preferably 20 to 200 cSt (20 x 10 ^-6 to 200 x 10 ^-6 m ² /s) at 40°C. If the viscosity at 40°C is 20 cSt or more, the sheet extruded from the die from the polyolefin solution 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.

Although there are no particular limitations on the method for uniformly melt-kneading the polyolefin solution, it is preferable to use a twin-screw extruder when preparing a highly concentrated polyolefin solution. If necessary, various additives such as antioxidants may be added as long as they do not impair the effects of the present invention. Adding an antioxidant is particularly preferable to prevent oxidation of polyethylene.

In the extruder, the polyolefin solution should be uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-mixing temperature varies depending on the polyolefin resin used, but is preferably between (Tm + 10)°C and (Tm + 120)°C, where Tm (°C) is the melting point of the polyolefin resin. It is more preferably between (Tm + 20)°C and (Tm + 100)°C. Here, the melting point refers to the value measured by DSC in accordance with JIS K7121 (1987) (the same applies below). A melt-mixing temperature of (Tm + 10)°C or higher prevents unmelted material from remaining in the extrudate extruded from the die, preventing membrane rupture during the subsequent stretching process. Furthermore, a melt-mixing temperature of (Tm + 120)°C or lower prevents degradation due to thermal decomposition of the polyolefin and suppresses deterioration of the resulting microporous membrane's physical properties, such as strength and porosity. It also prevents the appearance from being deteriorated due to 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. More preferably, it is 160 to 230°C, even more preferably 170 to 200°C, and even more preferably 180 to 200°C.

In addition, when producing a laminated film as the polyolefin microporous film of the present invention, the resin compositions of each layer can be melt-kneaded in a twin-screw extruder, and then fed into a multi-manifold type composite T-die for co-extrusion.

(b) Formation of Extrudate and Gel-Like Sheet Next, the resulting extrudate is cooled to obtain a gel-like sheet, which can solidify 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 not only allows for uniform stretching but also improves the strength and elongation of the film.

Cooling methods include direct contact with cold air, cooling water, or other cooling media, contact with a roll cooled with a refrigerant, or using a casting drum, etc.

(c) Stretching Step The gel-like sheet obtained is stretched. The stretching method can be any of inflation, simultaneous biaxial stretching, and sequential biaxial stretching, but among these, it is preferable to adopt simultaneous biaxial stretching or sequential biaxial stretching in terms of film formation stability, thickness uniformity, and control of high film rigidity and dimensional stability.

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.

The stretching temperature is preferably set to a temperature no higher than the melting point of the gel-like sheet + 10°C, 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 polyethylene compositions have 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 in accordance with ASTM D 4065. It can also be determined by NMR. A stretching temperature of 90°C or higher can generate sufficient pores, improve membrane thickness uniformity, increase porosity, and improve permeability. Furthermore, a stretching temperature of 125°C or lower can prevent the sheet from melting, prevent a decrease in permeability due to pore blockage, and prevent excessive pore size.

The stretching ratio varies depending on the thickness of the gel-like sheet, but from the perspective of membrane thickness uniformity, it is preferable to stretch the film at 3 times or more in both MD and TD. An areal stretching ratio of 9 times or more is preferred. 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, increasing crystallinity, making it easier to obtain a uniform membrane, and producing a microporous membrane with excellent melting point and strength. An areal stretching ratio of 100 times or less is preferred. 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.

Stretching causes the higher-order structure formed in the gel sheet to cleave, resulting in a finer crystalline phase and the formation of numerous fibrils. The fibrils form a network structure with irregular three-dimensional connections. Stretching not only improves mechanical strength, but also causes the fibrils to cleave, making the fibrils more 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.

(d) Washing and drying step: In the washing step, the plasticizer remaining in the gel-like sheet is removed using a washing solvent. Since the polyethylene phase and the solvent phase are separated in the microstructure of the gel-like sheet, a microporous membrane is obtained by removing the plasticizer.

Examples of cleaning solvents 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 low surface tension, the network structure that forms the micropores is prevented from shrinking during drying after cleaning due to the surface tension at the air-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.

Washing can be performed by immersing the gel-like sheet in a cleaning solvent for extraction, showering the gel-like sheet with the cleaning solvent, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but is generally 300 parts by mass or more per 100 parts by mass of the gel-like sheet. The washing temperature can be 15 to 30°C, and can 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 cleaning solvent, the better, from the perspectives of enhancing the cleaning effect of the solvent, preventing unevenness in the microporous membrane properties in the TD and/or MD directions of the resulting microporous membrane, and improving the mechanical and electrical properties of the microporous membrane. The above-mentioned 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.

After the washing process, the solvent in the microporous membrane is removed by drying in the drying process. There are no particular limitations on the drying method, and methods using metal heating rolls or hot air can be selected. The drying temperature is preferably 40 to 100°C, and more preferably 40 to 80°C. If drying is insufficient, the porosity of the microporous membrane will decrease during subsequent heat treatment, resulting in poor permeability.

(e) Heat treatment/re-stretching step The dried microporous membrane may be stretched (re-stretched). Re-stretching can be carried out by a tenter method or the like while heating the microporous membrane, similar to the stretching described above. Re-stretching may be uniaxial or biaxial stretching. Multi-stage stretching is carried out by combining simultaneous biaxial and/or sequential stretching.

Combinations of stretching devices 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.

The heat treatment/re-stretching temperature is preferably set to a temperature below the melting point of the polyethylene composition, and more preferably within the range of (Tcd - 20°C) to the melting point. Specifically, it 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.

The stretching ratio for re-stretching is preferably 0.8 to 1.6 times the area of the film before re-stretching. A stretching ratio for re-stretching of 0.8 times or more, more preferably 0.9 times or more, and even more preferably 1.0 times or more, will result in better permeability. Furthermore, a stretching ratio for re-stretching of 1.6 times or less, more preferably 1.5 times or less, and even more preferably 1.4 times or less will prevent the pore size from becoming too large. A stretching ratio of less than 1.0 times indicates that the film is relaxed.

(f) Other Steps Depending on the intended use, the microporous membrane may be subjected to a hydrophilization treatment such as monomer grafting, surfactant treatment, or corona discharge.

For 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 the doctor blade method.

The polyolefin microporous membrane can also be crosslinked by irradiation with ionizing radiation such as alpha rays, beta rays, gamma rays, or electron beams. When irradiating with electron beams, 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.

The polyolefin microporous membrane of the present invention may be laminated with porous layers other than polyolefins by coating, vapor deposition, etc. to form a multilayer polyolefin porous membrane in order to impart functions such as meltdown properties, heat resistance, and adhesiveness.

Other porous layers are not particularly limited, but may include, for example, a porous layer such as an inorganic particle layer containing a binder and inorganic particles.

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, polyamide-imide resin, polyamide resin, aromatic polyamide resin, and polyimide resin.

The inorganic particles that make up the inorganic particle layer are not particularly limited, and known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon. 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.

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, pleated, or other filtration units. Pleated filtration units are preferred because of 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 membrane together using a heated roll or the like, the membrane is pleated by creating peak and valley folds and then incorporated into the filtration unit for use.

The present invention will be explained in more detail using 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.

[Film thickness]
The thickness of five points randomly selected within a 50 mm × 50 mm area of the polyolefin microporous membrane 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 thickness (μm).

[Porosity]
A 50 mm x 50 mm square sample was cut from the polyolefin microporous membrane, its mass (g) was measured, and its membrane thickness was measured using the method described above to calculate its volume (cm ³ ). The porosity of the polyolefin microporous membrane was calculated using the following formula from the mass and volume described above, as well as the membrane density (g/cm ³ ). The calculation was performed assuming that the membrane density was a constant value of 0.99 g/cm ³ . For this measurement, samples were cut from three randomly selected positions on the polyolefin microporous membrane, and the average porosity measured for each was calculated.
Porosity (%)=[(volume−mass/membrane density)/volume]×100 (formula).

[Air permeability resistance]
The air resistance P ₁ (seconds/100 cm ³ ) of a microporous membrane having a thickness T ₁ (μm) was measured using an Oken air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117 (2009). The air resistance P ₂ (converted to 10 μm) (seconds/100 cm ³ /10 μm) per 10 μm thickness was calculated using the formula: P ₂ = (P ₁ × 10)/T ₁ .

[Viscosity average molecular weight (Mv) of polyolefin]
The viscosity average molecular weight Mv was determined in accordance with ISO 1628-3 (2010) by the method shown below. First, 20 mg of polyolefin resin was weighed out and purged with nitrogen, after which 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 marked lines was measured using a Cannon-Fenske viscometer (manufactured by Shibata Scientific Instruments Co., Ltd.: product number -100). The drop time (ts) between the marked lines was also measured for samples in which the resin amount was changed to 10 mg, 5 mg, 2 mg, or 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, an approximate linear equation was derived by the least squares method, and the intrinsic viscosity ([η]) was determined 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 (formula).

[Pore size distribution of polyolefin microporous membrane measured by mercury porosimetry]
The pore size distribution was determined using a pore size distribution measuring device (Autopore V9620 manufactured by Micromeritics) in the pore diameter range of 0.004 to 200 μm. The pore size was calculated using the following formula (Washburn formula).
PD=-4σcosθ...(formula)
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°).

Methods for calculating X ₂ /X ₁ from a pore size distribution curve (X axis: pore size, Y axis: dV/d(LogD)) obtained by mercury intrusion porosimetry are shown in I to III below.
I. Read the maximum value (Y _max ) of dV/d (LogD) in the X-axis range of 0.01 μm to 10 μm and the X-axis value (X _max ) at this time.
II. In the range of 0.01 μm to 10 μm on the X axis of the pore size distribution curve, the Xs at which the curve intersects with Y=Y _max /2 (excluding cases where there are three or more Xs that satisfy Y=Y _max /2) are designated X ₁ and X ₂ , in order from the smallest .
III. Calculate X ₂ /X ₁ from X ₁ and X ₂ (X ₁ <X ₂ ) obtained by the above method.

[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).

Differential Scanning Calorimetry (DSC)
A 6.0 mg sample was sealed in an aluminum pan, and using a differential scanning calorimeter (Parking Elmer PYRIS Diamond DSC), the first temperature increase was performed from 30 ° C. to 230 ° C. at 10 ° C./min, held at 230 ° C. for 5 minutes, cooled to 30 ° C. at a rate of 10 ° C./min, and then heated a second time from 30 ° C. to 230 ° C. at a heating rate of 10 ° C./min. 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 observed. 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, using the following formula:
Polyethylene resin content (%) of polyolefin microporous membrane=100×ΔH1/(ΔH1+ΔH2) (formula).

[Example 1]
(First Polyolefin Solution)
As the polyolefin raw material for layer A, 100 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C) having a viscosity average molecular weight (Mv) of 3.0 × ¹⁰⁶ 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.

15 parts by mass of the obtained polyolefin mixture was charged into a strong mixing type twin-screw extruder (inner diameter 58 mm, L/D = 42), and 85 parts by mass of liquid paraffin having a viscosity of 35 cSt (35 × 10 ^-6 m ² /s) at 40°C was supplied from the side feeder of the twin-screw extruder. The mixture was melt-kneaded under conditions of 210°C and 200 rpm to prepare a first polyolefin solution.

(Second Polyolefin Solution)
The polyolefin raw material for layer B was 100 parts by mass of high-density polyethylene (melting point 135°C) having a viscosity average molecular weight (Mv) of 3.7 x ^105. 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.

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

(Gel-like 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 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.

(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.

(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.

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

The thickness of the resulting polyolefin porous membrane was 29 μm. The blending ratio of each component, manufacturing conditions, evaluation results, etc. are shown in Table 1.

[Example 2]
(First Polyolefin Solution)
As the polyolefin raw material for layer A, 100 parts by mass of ultra-high molecular weight polyethylene (melting point 135°C) having a viscosity average molecular weight (Mv) of 3.5 × ¹⁰ was used. To this was added the same antioxidant as in Example 1 in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

20 parts by mass of the obtained polyolefin-polyolefin mixture was melt-kneaded with 80 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 ⁻⁶ m ² /s) at 40° C. in the same manner as in Example 1 to prepare a first polyolefin solution.

(Second Polyolefin Solution)
The same second polyolefin solution as used in Example 1 was used as the second polyolefin solution.

(Gel-like sheet)
A three-layer gel 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 a cooling roll was adjusted to 4 m/min so that the thickness of the polyolefin microporous membrane would be 11 μm.

(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.

[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).

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

(Second Polyolefin Solution)
The polyolefin raw materials used for layer B were 30 parts by mass of high-density polyethylene (melting point 133°C) having a viscosity average molecular weight (Mv) of 2.0 x ¹⁰⁶ and 70 parts by mass of high-density polyethylene (melting point 135°C) having an Mv of 3.7 x ^105. To these were 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.

30 parts by mass of the resulting polyolefin mixture was melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 ⁻⁶ m ² /s) at 40° C. in the same manner as in Example 1 to prepare a first polyolefin solution.

(Gel-like sheet, stretching, washing and drying, heat setting treatment)
Using the first and second polyolefin solutions, a polyolefin porous membrane was obtained in the same manner as in Example 1.

[Example 5]
(First Polyolefin Solution)
The polyolefin raw materials for layer A were 70 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C) with a viscosity average molecular weight (Mv) of 3.0 x ¹⁰⁶ and 30 parts by mass of high-density polyethylene (melting point 132°C) with an Mv of 9.0 x ^104. To these were added the same antioxidant as in Example 1 in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

23 parts by mass of the obtained polyolefin mixture was melt-kneaded with 77 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 ⁻⁶ m ² /s) at 40° C. in the same manner as in Example 1 to prepare a first polyolefin solution.

(Second Polyolefin Solution)
The same second polyolefin solution as used in Example 1 was used as the second polyolefin solution.

(Gel-like 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 structure 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.

(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.

(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.

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

[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.

[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 with the cooling roll was 4 m/min, and the thickness of the polyolefin microporous membrane was adjusted to 11 μm.

[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 with the cooling roll was 4 m/min, and the thickness of the polyolefin microporous membrane was adjusted to 10 μm.

[Comparative Example 3]
(First Polyolefin Solution)
The polyolefin raw materials for layer A were 30 parts by mass of high-density polyethylene (melting point 133°C) having a viscosity average molecular weight (Mv) of 2.0 x ¹⁰⁶ and 70 parts by mass of high-density polyethylene (melting point 135°C) having a viscosity average molecular weight (Mv) of 3.7 x ^105. To these were added the same antioxidant as in Example 1 in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

30 parts by mass of the resulting polyolefin mixture was melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 ⁻⁶ m ² /s) at 40° C. in the same manner as in Example 1 to prepare a first polyolefin solution.

(Second Polyolefin Solution)
The polyolefin raw materials for layer B were 50 parts by mass of high-density polyethylene (melting point 135°C) with a viscosity average molecular weight (Mv) of 3.7 x ^{105 and} 50 parts by mass of polypropylene (melting point 163°C) with a viscosity average molecular weight (Mv) of 2.0 x ^106. To these were added the same antioxidant as in Example 1 in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

30 parts by mass of the obtained polyolefin mixture was melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 ⁻⁶ m ² /s) at 40° C. in the same manner as in Example 1 to prepare a second polyolefin solution.

(Gel-like 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 structure 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.

(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.

(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.

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

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

[Comparative Example 4]
A polyolefin porous 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.

[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.

[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 were polyolefin microporous membranes that 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.

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

Claims (10)

In a pore size distribution obtained by mercury porosimetry, where the X axis represents pore size and the Y axis represents dV/d(LogD), the maximum value of dV/d(LogD) in the range of X in the pore size distribution of 0.01 μm to 10 μm is defined as Ymax, and X1 and X2, in order from the _smallest to the largest, satisfy Y= Ymax _/ 2 within the range of X in the pore size distribution of _0.01 μm to 10 μm (excluding the case where there are three or more Xs that satisfy Y=Ymax/2) _{, have X2} / _X1 of 15 or more and a thickness of 30 μm or less.
Here,
V: cumulative pore volume (cm ³ /g)
D: Pore diameter (μm). The microporous polyolefin membrane according to claim 1, having a bubble point pressure of 1900 kPa or more. The polyolefin microporous membrane according to claim 1 or 2, containing 90% by mass or more of polyethylene. The polyolefin microporous membrane according to claim 1 or 2, which has an air resistance per 10 μm thickness of 300 seconds/100 cm³ or ^less . The polyolefin microporous membrane according to claim 1 or 2, used as a separator for a secondary battery. The polyolefin microporous membrane according to claim 1 or 2, used as a liquid filtration filter. A separator for a secondary battery using the polyolefin microporous membrane described in claim 1 or 2. A liquid filtration filter using the polyolefin microporous membrane described in claim 1 or 2. A secondary battery using the secondary battery separator described in claim 7. A filtration unit using the liquid filter described in claim 8.