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

Abstract

The present invention addresses the problem of providing a polyolefin microporous membrane which exhibits excellent dendrite resistance and output properties when used as a battery separator, and exhibits excellent filtration accuracy and high permeability when used for a liquid filter.　The polyolefin microporous membrane according to the present invention satisfies the following formula 1, formula 2, and formula 3 when the bubble point pore diameter determined by the porometer method is A (nm) and the water permeability under a pressure of 90 kPa is B (mL / min / cm2). Formula 1: A-20×B≤14 Formula 2: A≤30 Formula 3: 0.05≤B≤1.0

Description

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

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

[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 liquid filter applications, such as water treatment membranes, ultrafiltration membranes, and microfiltration membranes.

[0003] In recent years, development of lithium-ion secondary batteries, primarily for automotive applications, has been progressing toward larger batteries with 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 dendrites grow and penetrate the separator, they can cause short circuits. Therefore, batteries prone to dendrite formation require separators with small pores that are less susceptible to dendrite penetration. On the other hand, reducing the pore size of microporous membranes can result in reduced permeability, which can lead to reduced power output characteristics.

[0004] For example, in the semiconductor manufacturing process, as the wiring pitch of semiconductors becomes finer, the upper limit of the size of foreign particles allowed during the process is decreasing, and filters used to filter the liquids used in semiconductor manufacturing are required to be able to capture smaller foreign particles.In addition, to increase the filter's processing capacity, high liquid permeability is required.

[0005] Patent Document 1 describes a liquid filter substrate that has both high liquid permeability and high particle collection 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 predetermined range.

[0006] Patent Document 2 describes a polyolefin microporous membrane having a small pore size and excellent air permeability, and discloses a laminated polyolefin microporous membrane having 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 polyethylene (PE) and polypropylene (PP). The resin compositions and pore structures of the surface and inner layers are different, and the polyolefin microporous membrane is therefore described as having an excellent balance of permeability, mechanical strength, meltdown properties, electrolyte absorption, and electrolyte retention when used as a battery separator.

[0008] JP 2018-167198 A International Publication No. 2018 / 168871 JP 2008-255307 A

[0009] 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.

[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 when the membrane has a layer made of a blend of PE and PP, the pore structure becomes non-uniform due to phase separation of each resin, so there is room for improvement in the balance between small pore size and high permeability. Designs in which each layer is made of a single resin, either PE or PP, have also been considered, but the resin design and membrane-forming conditions have not been optimized, and the pore size has not been sufficiently small.

[0011] An object of the present invention is to provide a polyolefin microporous membrane that exhibits excellent dendrite resistance and output characteristics when used as a battery separator, and exhibits excellent filtration accuracy and high permeability when used as a liquid filter.

[0012] In order to solve the above problems and achieve the object, the present invention and its preferred embodiments have the following configurations. In the present invention, "or more" means the same as or larger than the numerical value indicated therein. Also, "or less" means the same as or smaller than the numerical value indicated therein. [1] The bubble point pore diameter determined by the porometer method is A (nm), and the water permeability under a pressure of 90 kPa is B (mL / min / cm ２ ), a polyolefin microporous film that satisfies the following formulas 1, 2, and 3. Formula 1: A - 20 × B ≦ 14 Formula 2: A ≦ 30 Formula 3: 0.05 ≦ B ≦ 1.0 [2] A polyolefin microporous film that has a pin puncture strength converted into a unit basis weight of 600 mN / (g / m ２ [3] The polyolefin microporous membrane according to [1] above, having a pore volume of 0.6 cm or less as determined by a nitrogen gas adsorption method. ３ [4] The polyolefin microporous membrane according to any one of [1] to [3] above, having a porosity of 40% or more. [5] The polyolefin microporous membrane according to [1] above, having an air resistance of 300 sec / 100 cm ３ [6] The polyolefin microporous film according to any one of [1] to [4] above, having a thickness of 15 μm or less. [7] The polyolefin microporous film according to any one of [1] to [5] above, having a thickness of 15 μm or less. [8] The polyolefin microporous film according to any one of [1] to [5] above, having a thickness of 15 μm or less. [9] The polyolefin microporous film according to any one of [1] to [4] above, having a thickness of 15 μm or less. ２ 2.5g / m or more ２The polyolefin microporous membrane according to any one of [1] to [6] above, comprising the following: [8] The polyolefin microporous membrane according to any one of [1] to [7] above, having a laminate structure of two or more layers each composed mainly of different resins. [9] The polyolefin microporous membrane according to any one of [1] to [8] above, having at least one layer containing 90% by mass or more of a polypropylene-based resin.

[10] A battery separator comprising the polyolefin microporous membrane according to any one of [1] to [9] above.

[11] A battery comprising the battery separator according to

[10] above.

[12] A liquid filter comprising the polyolefin microporous membrane according to any one of [1] to [9] above.

[13] A filtration unit comprising the liquid filter according to

[12] above.

[0013] 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 electric vehicles and other devices that require high energy density, high capacity, and high output. Furthermore, when used as a liquid filter, the membrane has excellent filtration accuracy and processing capacity, and is therefore suitable for use as a high-precision liquid filter that requires the removal of minute foreign matter in semiconductor processes and other applications.

[0014] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these.

[0015] [Polyolefin microporous membrane] The polyolefin microporous membrane of the present invention has a bubble point pore size determined by a porometer method of A (nm) and a water permeability under a pressure of 90 kPa of B (mL / min / cm ２), A-20×B is 14 or less. Usually, there is a trade-off between the pore density and permeability of a microporous membrane. However, after extensive studies, the present inventors have found that by setting the value of A-20×B within a specific range, it is possible to overcome the trade-off and achieve both pore density and permeability at a high level. By setting A-20×B to 14 or less, preferably 13 or less, more preferably 12 or less, and even more preferably 11 or less, 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 processing capacity when used as a liquid filter. Although not particularly limited, from the viewpoint of further improving productivity, A-20×B is preferably 1 or more, more preferably 5 or more. There are no particular limitations on the method for setting the value of A-20×B within the range of the present invention, and examples include methods of controlling the layer structure, raw material composition, and membrane-forming conditions of the polyolefin microporous membrane. For example, when a polyolefin microporous membrane is laminated, if the raw material composition of all layers is made of a polyethylene resin as the main component, the bubble point pore size (A) tends to be larger, and if the raw material composition of at least one layer is made of a polypropylene resin as the main component, the bubble point pore size (A) tends to be smaller.In addition, the higher the proportion of layers made of a polyethylene resin as the main component in the layer structure, the greater the water permeability (B) tends to be, and the higher the proportion of layers made of a polypropylene resin as the main component, the greater the water permeability (B) tends to be.In addition, the more polyethylene resin is blended as raw material, the greater the bubble point pore size (A) tends to be, and the more polypropylene resin is blended as raw material, the greater the bubble point pore size (A) tends to be, and the more polypropylene resin is blended as raw material, the greater the water permeability (B) tends to be, and the more polypropylene resin is blended as raw material, the greater the water permeability (B) tends to be, and the more As a film-forming condition, slow cooling of the melt-extruded polyolefin resin composition tends to increase the bubble point pore diameter (A), while rapid cooling tends to decrease the bubble point pore diameter (A). As a film-forming condition, increasing the stretching temperature of the gel sheet tends to increase the water permeation amount (B), while decreasing the stretching temperature of the gel sheet tends to decrease the water permeation amount (B).By controlling the above conditions, it is possible to keep the value of A-20×B within the above range. Preferred conditions for the layer structure, raw material composition, and film formation conditions will be described later.

[0016] The bubble point pore size (A) of the polyolefin microporous membrane is 30.0 nm or less. By setting A to 30.0 nm or less, preferably 25.0 nm or less, more preferably 20.0 nm or less, even more preferably 18.0 nm or less, and even more preferably 16.0 nm or less, the polyolefin microporous membrane can have excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter. Although not particularly limited, 14.2 nm is the substantial lower limit of A in this evaluation method. The method for setting the value of A within the above range is not particularly limited, and examples include methods of controlling the layer structure, raw material composition, and membrane-forming conditions of the polyolefin microporous membrane.

[0017] The water permeability (B) of the polyolefin microporous membrane is 0.05 mL / min / cm ２ The flow rate B is 0.05 mL / min / cm ２ or more, preferably 0.1 mL / min / cm ２ More preferably, 0.2 mL / min / cm ２ More preferably, 0.25 mL / min / cm ２ More preferably, 0.3 mL / min / cm ２ By satisfying the above conditions, a polyolefin microporous membrane can be obtained that has excellent output characteristics when used as a battery separator and has excellent processing capacity when used as a liquid filter. ２ The flow rate B is 1.0 mL / min / cm or less. ２ Less than 0.9 mL / min / cm ２ Less than or equal to 0.8 mL / min / cm ２By satisfying the following conditions, a polyolefin microporous membrane having excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter can be obtained. The method for achieving B within the range of the present invention is not particularly limited, and examples include methods of controlling the layer structure, raw material composition, and membrane-forming conditions of the polyolefin microporous membrane.

[0018] The polyolefin microporous membrane has a puncture strength of 50 mN / (g / m ２ The puncture strength is preferably 50 mN / (g / m ２ ) or more, more preferably 100 mN / (g / m ２ ) or more, the processability of the polyolefin microporous membrane is improved, and a polyolefin microporous membrane that is excellent in dendrite resistance when used as a battery separator and excellent in filtration accuracy when used as a liquid filter is easily obtained. ２ The puncture strength is preferably 600 mN / (g / m ２ ) or less, more preferably 500 mN / (g / m ２ ) or less, more preferably 400 mN / (g / m ２ ) or less, more preferably 350 mN / (g / m ２ ) or less, a polyolefin microporous membrane that is excellent in output characteristics when used as a battery separator and excellent in processing capacity when used as a liquid filter can be easily obtained.

[0019] The pore volume of the polyolefin microporous membrane determined by nitrogen gas adsorption method is 0.6 cm ３ Since the upper limit of pore size detection by nitrogen gas adsorption is about 100 nm, a small pore volume indicates that there are many pores having a diameter equal to or larger than the upper limit of detection by nitrogen gas adsorption. The pore volume is preferably 0.6 cm ３ / g or less, more preferably 0.5 cm ３ / g or less, more preferably 0.45 cm ３By setting the pore volume to 0.05 cm or less, large pores with excellent permeability are sufficiently formed, resulting in a polyolefin microporous membrane that has excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. ３ The pore volume is preferably 0.05 cm ３ / g or more, more preferably 0.1 cm ３ / g or more, more preferably 0.2 cm ３ / g or more, fine pores are sufficiently formed, resulting in a polyolefin microporous membrane that has excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter.

[0020] The porosity of the polyolefin microporous membrane is preferably 40% or more. By setting the porosity to preferably 40% or more, more preferably 45% or more, and even more preferably 50% or more, the polyolefin microporous membrane can have excellent output characteristics when used as a battery separator and excellent processing ability when used as a liquid filter. Furthermore, the porosity is preferably 90% or less from the viewpoint of handleability during processing.

[0021] The air resistance of the polyolefin microporous membrane is 300 seconds / 100 cm ３ Preferably, the air resistance is 300 seconds / 100 cm or less. ３ Less than 250 seconds / 100 cm, more preferably ３ More preferably, 200 seconds / 100 cm or less ３ More preferably, 150 seconds / 100 cm or less ３ By satisfying the following conditions, a polyolefin microporous membrane can be obtained that exhibits excellent output characteristics when used as a battery separator and excellent processing ability when used as a liquid filter. ３ More than 10 seconds / 100 cm is preferable. ３ The above is more preferable.

[0022] The thickness of the polyolefin microporous membrane is preferably 15 μm or less. By setting the thickness to preferably 15 μm or less, more preferably 12 μm or less, even more preferably 10 μm or less, and even more preferably 8 μm or less, the polyolefin microporous membrane can have excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. Furthermore, the thickness is preferably 1 μm or more. By setting the thickness to preferably 1 μm or more, more preferably 3 μm or more, the membrane can have excellent handleability during processing. The thickness can be adjusted by the membrane-forming conditions.

[0023] The polyolefin microporous membrane preferably contains a polyethylene resin as a main component. The polyethylene resin preferably accounts for 50% by mass or more of the polyolefin microporous membrane. By adjusting the polyethylene resin content to preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, a polyolefin microporous membrane having excellent membrane formability, high porosity, and excellent permeability can be obtained.

[0024] The polyethylene resin may be an ultra-high molecular weight polyethylene, a high-density polyethylene, a medium-density polyethylene, a low-density polyethylene, or a copolymer of ethylene and an α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Among these, ultra-high molecular weight polyethylene or high-density polyethylene is preferred.

[0025] The polyolefin microporous membrane preferably contains a polypropylene-based resin. The polyolefin microporous membrane preferably contains 5% by mass or more of the polypropylene-based resin. By making the polypropylene-based resin preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, the membrane exhibits excellent heat resistance and dendrite resistance when used as a battery separator, and excellent filtration accuracy when used as a liquid filter. The polypropylene-based resin content is preferably 50% by mass or less. By making the polypropylene-based resin preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, the membrane exhibits excellent output characteristics when used as a battery separator, and excellent processing capacity when used as a liquid filter.

[0026] The polypropylene resins may be propylene homopolymers or block copolymers or random copolymers. The block copolymers and random copolymers may contain copolymer components with other α-olefins than propylene. Examples of such other α-olefins include butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene-1.

[0027] The polypropylene resin in the polyolefin microporous film has a basis weight of 0.1 g / m ２ Preferably, the basis weight is 0.1 g / m or more. ２ More preferably, 0.3 g / m ２ More preferably, 0.6 g / m ２ By setting the weight to the above, when used as a battery separator, the heat resistance and dendrite resistance are excellent, and when used as a liquid filter, the filtration accuracy is excellent. ２ Preferably, the basis weight is 2.5 g / m or less. ２ or less, more preferably 2.0 g / m ２ More preferably, 1.5 g / m or less ２By satisfying the following conditions, the battery separator has excellent output characteristics when used as a battery separator, and the liquid filter has excellent processing capacity when used as a liquid filter.

[0028] Specific configurations of the polyolefin microporous membrane will be described below, but the membrane is not necessarily limited to the embodiments described below.

[0029] The polyolefin microporous membrane preferably has a laminated structure of two or more layers each composed of a different resin as a main component. Among these, it is preferable to have an A layer composed mainly of a polyethylene-based resin and a B layer composed mainly of a polypropylene-based resin. The "main component" here refers to the component with the highest content, expressed as mass%, among the components constituting each layer of the polyolefin microporous membrane. Generally, reducing the pore size of a microporous membrane inhibits the movement of ions and fluids, thereby reducing permeability. A preferred embodiment of the polyolefin microporous membrane of the present invention has an A layer composed mainly of a polyethylene-based resin and a B layer composed mainly of a polypropylene-based resin. By producing the membrane under the manufacturing conditions described below, a laminated structure of the A layer having high permeability and the B layer having a fine pore structure is formed, thereby achieving an unprecedented level of compatibility between the contradictory properties of small pore size and high permeability.

[0030] The layer structure of the polyolefin microporous membrane may be layer A / layer B, layer A / layer B / layer A, layer B / layer A / layer B, or layer A / layer B / layer A / layer B / layer A / layer A, but it is preferable to arrange the layers so that layer B does not appear on both surfaces of the polyolefin microporous membrane. Such a layer structure provides excellent membrane formability and membrane uniformity, and allows layer B to be designed to be thin, facilitating the production of a polyolefin microporous membrane with excellent permeability.

[0031] The polyethylene resin content in layer A of the polyolefin microporous membrane is preferably 90% by mass or more. By making the polyethylene resin content preferably 90% by mass or more, more preferably 95% by mass or more, a polyolefin microporous membrane with excellent permeability is obtained.

[0032] Here, the layer A may contain two or more polyethylene resins, and in that case, the total amount of the polyethylene resins is defined as the amount of polyethylene resin components constituting the polyolefin microporous membrane.

[0033] The polyethylene resin used in Layer A may be an ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, low-density polyethylene, or a copolymer of ethylene and an α-olefin. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Among these, ultra-high molecular weight polyethylene or high-density polyethylene is preferred.

[0034] The weight-average molecular weight (Mw) of the polyethylene resin used in the A layer is preferably 500,000 or more. By setting the Mw of the polyethylene resin used in the A layer to preferably 500,000 or more, more preferably 700,000 or more, and even more preferably 1,000,000 or more, the polyolefin microporous membrane has excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter. Furthermore, the Mw of the polyethylene resin used in the A layer is preferably 3,000,000 or less. By setting the Mw of the polyethylene resin used in the A layer to preferably 3,000,000 or less, more preferably 2,000,000 or less, the polyolefin microporous membrane has improved formability, excellent output characteristics when used as a battery separator, and excellent processing capacity when used as a liquid filter.

[0035] The melting point of the polyethylene resin used in the A layer is preferably 130° C. or higher. By setting the melting point to preferably 130° C. or higher, more preferably 133° C. or higher, and even more preferably 135° C. or higher, the resin will have excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. From the viewpoint of improving the output characteristics and processing capacity described above, the higher the melting point, the better, but the practical upper limit is 142° C.

[0036] The layer A may contain a polypropylene-based resin. The content of the polypropylene-based resin in the layer A is preferably 10% by mass or less. By controlling the content of the polypropylene-based resin to preferably 10% by mass or less, more preferably 5% by mass or less, a polyolefin microporous membrane with excellent permeability can be obtained.

[0037] The polypropylene resin in Layer A may be a propylene homopolymer, or may be a block copolymer or a random copolymer. The block copolymer or random copolymer may contain a copolymer component with an α-olefin other than propylene. Examples of such an α-olefin include butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene-1.

[0038] The A layer can contain resin components other than polyethylene resin and polypropylene resin as needed.In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, crystallization retarders, etc. may be contained within the range that does not impair the effects of the present invention.However, the crystal nucleating agent may remain in the polyolefin microporous membrane or may form defects or coarse pores.In particular, when used as a liquid filter, there is a risk of foreign matter being mixed into the filtrate.Therefore, the amount of the crystal nucleating agent added to 100 parts by mass of the polyolefin raw material used in the A layer of the polyolefin microporous membrane is preferably 0.1 parts by mass or less, more preferably 0.01 parts by mass or less.Examples of the crystal nucleating agent for polyolefin include phosphate metal salt-based nucleating agents, sorbitol-based nucleating agents, and carboxylate metal salt-based nucleating agents.

[0039] The total thickness of Layer B of the polyolefin microporous membrane is preferably 5.0 μm or less. By setting the thickness to preferably 5.0 μm or less, more preferably 3.0 μm or less, even more preferably 2.0 μm or less, and even more preferably 1.5 μm or less, the permeability of the polyolefin microporous membrane is improved. Furthermore, the thickness is preferably 0.1 μm or more. By setting the thickness to preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 0.5 μm or more, dendrite resistance and filtration accuracy are likely to be maintained even when thin areas occur locally in Layer B due to thickness unevenness or the like.

[0040] The ratio of the total thickness of Layer B to the total layer thickness of the polyolefin microporous membrane is preferably 35% or less. By setting this ratio to preferably 35% or less, more preferably 25% or less, even more preferably 20% or less, and even more preferably 15% or less, the permeability of the polyolefin microporous membrane is improved. Furthermore, this ratio is preferably 1% or more. By setting this ratio to preferably 1% or more, more preferably 3% or more, and even more preferably 5% or more, dendrite resistance and filtration accuracy are likely to be maintained even when thin areas occur locally in Layer B due to thickness unevenness or the like.

[0041] To achieve the preferred ratio, it is preferable to set the raw material composition and molecular weight of the raw materials for the polyolefin microporous membrane within the above-described ranges, and also set the extrusion conditions during production of the polyolefin microporous membrane within the ranges described below in order to obtain a high-quality polyolefin microporous membrane with excellent lamination precision.

[0042] Layer B may contain a polyethylene resin. The polyethylene resin content in Layer B is preferably 10% by mass or less. By setting the polyethylene resin content to preferably 10% by mass or less, more preferably 5% by mass or less, a polyolefin microporous membrane having fine pores can be obtained.

[0043] The polyethylene resin contained in Layer B may be an ultra-high molecular weight polyethylene, a high-density polyethylene, a medium-density polyethylene, a low-density polyethylene, or a copolymer of ethylene and an α-olefin. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Among these, ultra-high molecular weight polyethylene or high-density polyethylene is preferred.

[0044] The weight-average molecular weight (Mw) of the polyethylene resin used in the B layer is preferably 1 million or more. By setting the Mw of the polyethylene resin used in the B layer to preferably 1 million or more, more preferably 1.2 million or more, and even more preferably 1.5 million or more, the polyolefin microporous membrane exhibits excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter. The Mw of the polyethylene resin used in the B layer is preferably 5 million or less. By setting the Mw of the polyethylene resin used in the B layer to preferably 5 million or less, more preferably 4 million or less, the polyolefin microporous membrane exhibits improved formability, excellent output characteristics when used as a battery separator, and excellent processing capacity when used as a liquid filter.

[0045] The polypropylene resin preferably accounts for 90% by mass or more of the layer B. By making the polypropylene resin content preferably 90% by mass or more, more preferably 95% by mass or more, a polyolefin microporous membrane having a fine pore structure is obtained.

[0046] Here, Layer B may contain two or more polypropylene resins, in which case the total amount of the polypropylene resins is defined as the amount of the polypropylene resin components constituting the polyolefin microporous membrane.

[0047] The polypropylene resin used in Layer B may be a propylene homopolymer, or may be a block copolymer or a random copolymer. The block copolymer or random copolymer may contain a copolymer component with an α-olefin other than propylene. Examples of such an α-olefin include butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene-1.

[0048] The melting point of the polypropylene resin used in Layer B is preferably 155° C. or higher. By setting the melting point to preferably 155° C. or higher, more preferably 160° C. or higher, and even more preferably 163° C. or higher, the polypropylene resin will have excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. From the viewpoint of improving the output characteristics and processing capacity described above, the higher the melting point, the better, but the practical upper limit is 175° C.

[0049] Layer B may contain resin components other than polyethylene resins and polypropylene resins as needed.In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, UV absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be added within the range that does not impair the effects of the present invention.However, since the crystal nucleating agent remains in the polyolefin microporous membrane and may be mixed into the filtrate, especially when used as a liquid filter, the amount of crystal nucleating agent added to 100 parts by mass of the polyolefin raw material used in layer B is preferably 0.1 parts by mass or less, more preferably 0.01 parts by mass or less.Examples of crystal nucleating agents for polyolefins include phosphate metal salt-based nucleating agents, sorbitol-based nucleating agents, and carboxylate metal salt-based nucleating agents.

[0050] By setting the raw material compositions and molecular weights of the raw materials constituting the A layer and the B layer within the above-mentioned ranges and setting the film-forming conditions within the ranges described below, a high-quality polyolefin microporous membrane with excellent lamination precision can be obtained, and since the B layer can be laminated thinly and uniformly, it is possible to achieve both a small pore size and high permeability.

[0051] [Method for producing polyolefin microporous membrane] Next, the method for producing the polyolefin microporous membrane will be described. Examples of the method for producing the polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. The wet film-forming method is preferred as the method for producing the polyolefin microporous membrane from the viewpoint of controlling the structure and physical properties of the membrane.

[0052] A wet method for producing the polyolefin microporous membrane will be described below. Note that the following description is merely an example and is not intended to limit the present invention to the following production method.

[0053] The method for producing the polyolefin microporous membrane preferably includes the following steps (1) to (5) in order, and may subsequently include the following steps (6) and (7).

[0054] (1) A preparation step of melt-kneading a polyolefin resin and a plasticizer (membrane-forming solvent) to prepare a polyolefin resin composition, (2) a gel-like sheet formation step of extruding the polyolefin resin composition and cooling it to form a gel-like sheet, (3) a first stretching step of preheating and stretching the gel-like sheet, (4) a removal step of removing the plasticizer from the stretched gel-like sheet to form a sheet, (5) a drying step of drying the sheet, (6) a second stretching step of preheating and stretching the dried sheet, and (7) a heat treatment step of heat-treating the stretched sheet. (1) Preparation Step: A polyolefin resin composition is prepared by heating and dissolving a polyolefin resin in a plasticizer. The plasticizer is not particularly limited as long as it can uniformly disperse the polyolefin resin, but it is preferable that the plasticizer be liquid at room temperature to enable relatively high stretching ratios. Examples of the plasticizer include aliphatic, cycloaliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, mineral oil fractions having boiling points corresponding to these hydrocarbons, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. However, in order to obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0055] The blending ratio of the polyolefin resin and the plasticizer is preferably 10% by mass or more and 50% by mass or less of the total mass of the polyolefin resin composition. By setting the content of the polyolefin resin within this range, the dispersion state of the polyolefin resin and the plasticizer becomes good, and the strength, permeability, and heat resistance of the obtained polyolefin microporous membrane become excellent. In addition, when forming into a sheet, the amount of swelling and neck-in at the outlet of the die becomes appropriate, and the sheet formability and film formability are also good.

[0056] When the polyolefin microporous membrane is a laminate membrane, each layer is kneaded in a separate extruder to prepare a polyolefin resin composition, which is then fed to the next step. In preparing the polyolefin resin composition, where Xa (mass%) represents the content of polyolefin resin in Layer A relative to the total mass of the polyolefin resin composition, and Xb (mass%) represents the content of polyolefin resin in Layer B relative to the total mass of the polyolefin resin composition, Xb-Xa is preferably 0 mass% or greater. By setting Xb-Xa to preferably 0 mass% or greater, more preferably 5 mass% or greater, and even more preferably 10 mass% or greater, the membrane exhibits excellent dendrite resistance when used as a battery separator and excellent filtration accuracy when used as a liquid filter. Furthermore, Xb-Xa is preferably 30 mass% or less. Setting Xb-Xa to preferably 30 mass% or less allows for a high-quality laminate membrane with a consistent lamination ratio, excellent output characteristics when used as a battery separator, and excellent processing capacity when used as a liquid filter.

[0057] The melt-kneading of the polyolefin resin and the plasticizer is preferably carried out in a twin-screw extruder from the viewpoint of obtaining a uniformly kneaded state. The resin temperature during kneading is preferably 150°C or higher. By setting the resin temperature to preferably 150°C or higher, more preferably 160°C or higher, and even more preferably 180°C or higher, the polyolefin resin and the plasticizer can be uniformly melt-kneaded. Furthermore, the resin temperature is preferably 250°C or lower. By setting the resin temperature to preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 230°C or lower, a decrease in strength due to resin degradation can be prevented.

[0058] (2) Gel-like sheet forming step: The melt of the polyolefin resin composition is fed from an extruder to a die and extruded into a sheet. The extrusion method may be either a T-die method or an inflation method. Alternatively, multiple polyolefin resin compositions of the same or different compositions may be fed from multiple extruders to a single multi-manifold composite T-die, laminated into layers, and extruded into a laminated sheet.

[0059] The temperature of the polyolefin resin composition when extruded from the die is preferably 220° C. or lower. By setting the temperature to preferably 220° C. or lower, more preferably 200° C. or lower, and even more preferably 180° C. or lower, crystallization is quickly completed in the cooling and solidification step described below, making it easier to refine the structure. Furthermore, from the viewpoint of extrusion stability, the temperature is preferably 140° C. or higher.

[0060] The polyolefin resin composition melt-extruded into a sheet is cooled and solidified to form a gel-like sheet. In the cooling step, it is preferable to cool to 10 to 50°C before the first stretching step described below. This is because it is preferable to set the final cooling temperature below the crystallization end temperature, which refines the high-order structure, making it easier to achieve uniform stretching in the subsequent stretching. Furthermore, the cooling rate at this time is preferably 50°C / min or higher. 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. Therefore, by setting the cooling rate to 50°C / min or higher, more preferably 100°C / min or higher, and even more preferably 150°C / min or higher, the high-order structure of the gel-like sheet becomes denser, enabling uniform stretching, thereby increasing the strength of the polyolefin microporous membrane and refining the pore structure.

[0061] The cooling method may be, for example, direct contact with cold air, cooling water or other cooling medium, contact with a roll cooled with a cooling medium, or use of a casting drum.

[0062] The thickness of the gel-like sheet is preferably 0.9 mm or less. By setting the thickness to 0.9 mm or less, more preferably 0.7 mm or less, and even more preferably 0.5 mm or less, the melt-extruded polyolefin resin composition as a whole is rapidly cooled and solidified, and therefore, when the polyolefin microporous membrane is formed into a laminate, the pore structure can be made fine all the way to the inner layer. Furthermore, from the viewpoint of membrane production stability, the thickness is preferably 0.01 mm or more.

[0063] (3) First Stretching Step: Next, the gel-like sheet is stretched at least uniaxially. It is preferable to preheat the gel-like sheet before stretching. The preheating temperature is preferably 90°C or higher. By setting the preheating temperature to preferably 90°C or higher, more preferably 110°C or higher, even more preferably 115°C or higher, and even more preferably 121°C or higher, the sheet exhibits excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. Furthermore, the preheating temperature is preferably 140°C or lower. By setting the preheating temperature to preferably 140°C or lower, more preferably 130°C or lower, even more preferably 127°C or lower, and even more preferably 125°C or lower, the sheet is uniformly stretched in the stretching step, resulting in excellent film formation stability, excellent dendrite resistance when used as a battery separator, and excellent filtration accuracy when used as a liquid filter.

[0064] The preheated gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial or biaxial, with biaxial stretching being preferred. In the case of biaxial stretching, simultaneous biaxial stretching, sequential biaxial stretching, or multistage stretching (e.g., a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, with simultaneous biaxial stretching being preferred because it results in a more uniform pore structure.

[0065] The stretching ratio (area stretching ratio) in this step is preferably 16 times or more, more preferably 25 times or more. The stretching ratio is preferably 4 times or more, more preferably 5 times or more, in both the machine longitudinal direction (MD direction) and the machine width direction (TD direction). The stretching ratios in the MD direction and the TD direction may be the same or different, and by setting the areal stretching ratio within the above range, mechanical strength and permeability can be improved. The areal stretching ratio is preferably 100 times or less. By setting the areal stretching ratio to preferably 100 times or less, more preferably 64 times or less, excellent film formation stability can be achieved.

[0066] The stretching temperature in this step is preferably 90°C or higher. By setting the stretching temperature to preferably 90°C or higher, more preferably 110°C or higher, even more preferably 115°C or higher, and even more preferably 121°C or higher, the film will have excellent output characteristics when used as a battery separator and excellent processing capacity when used as a liquid filter. The stretching temperature is also preferably 140°C or lower. By setting the stretching temperature to preferably 140°C or lower, more preferably 130°C or lower, even more preferably 127°C or lower, and even more preferably 125°C or lower, the film will be uniformly stretched in this step, resulting in excellent film formation stability, excellent dendrite resistance when used as a battery separator, and excellent filtration accuracy when used as a liquid filter.

[0067] (4) Removal Step: The plasticizer is removed (washed) using a washing solvent. Because the polyolefin resin phase is phase-separated from the plasticizer phase, removing the plasticizer results in a porous sheet consisting of fibrils that form a fine three-dimensional network structure and having three-dimensionally irregularly interconnected pores (voids). Washing solvents and methods for removing plasticizers using the same can be known methods, such as those disclosed in Japanese Patent No. 2132327 and Japanese Patent Laid-Open No. 2002-256099.

[0068] (5) Drying step: The sheet from which the plasticizer has been removed is dried by heat drying or air drying. The drying temperature is preferably 100°C or lower, more preferably 95°C or lower. Drying is preferably performed until the amount of remaining washing solvent is 5 parts by mass or less, more preferably 3 parts by mass or less, per 100 parts by mass (dry mass) of the total mass of the polyolefin microporous membrane.

[0069] (6) Second Stretching Step: The sheet that has been through the drying step may be stretched at least uniaxially at a predetermined areal stretch ratio. Stretching after the drying step (second stretching) is also called dry stretching. Dry stretching may be uniaxial or biaxial, and in the case of biaxial stretching, it may be either simultaneous or sequential stretching. In the case of sequential stretching, it is preferable to stretch the sheet in the MD direction and then continuously stretch it in the TD direction.

[0070] The area stretching ratio of the dry stretching is preferably 4 times or less. By setting the area stretching ratio to preferably 4 times or less, more preferably 2 times or less, and even more preferably 1.5 times or less, the pore structure becomes finer and the film formability becomes stable.

[0071] The dry stretching temperature in this step is preferably 80° C. or higher, more preferably 100° C. or higher. The dry stretching temperature is preferably 130° C. or lower, more preferably 120° C. or lower. By setting the dry stretching temperature within the above range, the obtained polyolefin microporous membrane has excellent permeability and can be stretched uniformly.

[0072] (7) Heat Treatment Step: After the second stretching step, or instead of the second stretching step, the sheet that has undergone the drying step or the second stretching step may be heat-treated. Heat-setting and / or heat-relaxing can be used as the heat-treatment method. Heat-setting is a heat treatment in which the membrane, such as the sheet, is heated while maintaining its dimensions. Heat-relaxing is a heat treatment in which the membrane, such as the sheet, is thermally shrunk in the MD or TD direction during heating. Heat-setting is preferably performed using a tenter or roll method. The relaxation rate in the heat-relaxing treatment is the value obtained by dividing the membrane dimensions after the heat-relaxing treatment by the membrane dimensions before the relaxation treatment. The relaxation rates in both the MD and TD directions of the membrane are preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. Furthermore, from the viewpoint of the planarity of the microporous membrane, the relaxation rates are preferably 0.80 or more, more preferably 0.90 or more. The temperature in the heat treatment step is preferably in the range of 90 to 140°C, more preferably in the range of 100 to 120°C.

[0073] The polyolefin microporous membrane can be used for various applications, such as 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 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. After bonding the reinforcing membrane and the membrane using a heated roll or the like, the membrane is pleated by creating peak and valley folds and then incorporated into the filtration unit.

[0074] 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. The evaluations in this application were carried out in an environment of 23°C and 65% humidity unless otherwise specified. The evaluation methods and analysis methods used in the examples are as follows.

[0075] [Film Thickness] The film thickness of a polyolefin microporous film was measured at any five points within a 50 mm × 50 mm area using a contact thickness meter ("Litematic" (registered trademark) VL-50 (10.5 mmφ superhard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation), and the average film thickness (μm) of the five points was calculated.

[0076] [Porosity] A 50 mm x 50 mm square sample was cut out from a polyolefin microporous membrane, and its volume (cm ３ The film density (g / cm) and mass (g) were measured. ３ The porosity of the polyolefin microporous membrane was calculated from the above data using the following formula: The membrane density was 0.99 g / cm ３ The calculation was performed assuming a constant value of 0.01. For this measurement, samples were cut out from three randomly selected positions on the polyolefin microporous membrane, and the average porosity measured at each position was calculated. Formula: Porosity (%) = [(volume - mass / membrane density) / volume] x 100.

[0077] [Air resistance] The air resistance (seconds / 100 cm) of a polyolefin microporous membrane was measured in accordance with JIS P-8117:2009 using an Oken air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in an atmosphere of 25°C. ３ ) was measured.

[0078] [Puncture Strength in Basis Weight Conversion] The puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was 2 mm / sec. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), the maximum load (mN) when a polyolefin microporous membrane was punctured with a 1.0 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm) was measured, and the value calculated from the following formula was used to represent the puncture strength in basis weight conversion (mN / (g / m ２Formula: Puncture strength converted into unit basis weight (mN / (g / m ２ )) = Maximum load (mN) / Basis weight of polyolefin microporous membrane (g / m ２ ).

[0079] The basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25°C, and using the following formula: ２ ) = mass (g) / (50 (mm) x 50 (mm)) x 10 ６ .

[0080] [Gel Permeation Chromatography (GPC)] The molecular weight distribution and average molecular weight of polyolefin resin and polyolefin microporous membrane were determined by GPC under the following conditions. In the differential molecular weight distribution curve obtained by GPC, the weight ratio of each molecular weight component was calculated from the ratio of the area of ​​each molecular weight region to the peak area of ​​all molecular weight components. Note that the molecular weight value obtained by this measurement method is a relative value based on polystyrene. Sample preparation: 5 mL of measurement solvent was added to 5 mg of sample, and the mixture was heated and stirred at 160-170°C for 60 minutes. The resulting solution was then filtered through a metal filter (pore size 0.5 μm). Measurement apparatus: high-temperature GPC apparatus (HLC-8321GPC / HT manufactured by Tosoh Corporation). Guard column: "Shodex" (registered trademark) HT-G manufactured by Showa Denko K.K. Column: two "Shodex" (registered trademark) UT806M manufactured by Showa Denko K.K. Column temperature: 145°C. Solvent (mobile phase): 1,2,4-trichlorobenzene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., with 0.1 wt% BHT added). Solvent flow rate: 1.0 ml / min. Injection amount: 0.300 mL. Detector: differential refractive index detector (RI detector). Standard samples: monodisperse polystyrene manufactured by Tosoh Corporation, dibenzyl manufactured by Tokyo Chemical Industry Co., Ltd.

[0081] [Differential Scanning Calorimetry (DSC)] The melting point of the polyolefin resin was determined by DSC. 6.0 mg of the sample was sealed in an aluminum pan, and using a DSC (PYRIS Diamond manufactured by Parking Elmer), the sample was heated from 30 ° C. to 230 ° C. at a rate of 10 ° C. / min (first heating), then held at 230 ° C. for 5 minutes, cooled at a rate of 10 ° C. / min, and heated again at a rate of 10 ° C. / min from 30 ° C. to 230 ° C. (second heating). The melting point 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. The melting point was the temperature at which the maximum endotherm was observed.

[0082] [Bubble Point Pore Diameter] The bubble point pore diameter of a 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 impregnating liquid for the polyolefin microporous membrane, and the pressure at which the flow rate reached 20 cc / min or more was taken as the bubble point pressure (kPa). The following equation was used to convert the bubble point pressure to the bubble point pore diameter. Note that the measurement was performed with an upper pressure limit of 3200 kPa. For samples in which a bubble point was not detected even when the upper pressure limit was reached, the bubble point pressure was set to 3200 kPa and the bubble point pore diameter was set to 14.2 nm. Formula: d = C γ / P (In the above formula, "d (nm)" is the bubble point pore diameter of the microporous membrane, "γ (dynes / cm)" is the surface tension of the impregnation liquid, "P (kPa)" is the bubble point pressure, and "C" is a constant which is set to 2860.)

[0083] [Water permeability] The water permeability was measured using a filtration device. A resin perforated plate was used as a support, and one PTFE porous membrane (Merck, Omnipore membrane, model number: JAWP01300) was placed on top of it, and one polyolefin microporous membrane cut into a circle with a diameter of 47 mm was placed on top of that to use as a filter material. The polyolefin microporous membrane was entirely wetted with 0.5 mL of ethanol, and the filter material was set in the filtration device. 100 mL of pure water was poured into the filtration device, and a differential pressure of 90 kPa was applied to filter the pure water. The test started when the filtrate began to drip, and ended when the total amount of filtrate reached 95 mL or more and the filtrate no longer dripped for 5 seconds or more. The test time T (min) from the start of the test to the end of the test and the above-mentioned filtration area (cm ２ The water permeability was calculated from the measured values ​​of the water permeability and the water content by the following formula. The room temperature and the pure water temperature were set to 24±1°C when the above measurements were carried out. Formula: Water permeability (mL / min / cm ２ )=100(mL) / T(min) / 13.8(cm ２ ).

[0084] [Measurement of pore volume by nitrogen gas adsorption method] Pore volume measurement by nitrogen gas adsorption method used a measuring instrument, BELSORP18PLUS-HT manufactured by BEL Japan Co., Ltd. As a measurement sample, about 0.1 g of a polyolefin microporous membrane was placed in a glass cell and dried under reduced pressure for about 5 hours before use. The nitrogen gas adsorption / desorption isotherm of each sample was measured under the conditions shown below, and the pore volume (cm) was calculated from the isotherm during the desorption process based on the BJH method. ３ / g) was calculated. Measurement temperature: 77 K (liquid nitrogen temperature) Saturated vapor pressure (P ０ ): 101.3 kPa measured relative pressure (P / P ０ ): about 0.001 to about 0.99 Equilibrium setting time: 180 seconds.

[0085] [Proportion and basis weight of polypropylene resin in polyolefin microporous membrane] The polypropylene resin content in the polyolefin microporous membrane was confirmed by IR spectroscopy. Four polyolefin microporous membranes were stacked to prepare a measurement sample, and the IR spectrum was measured under the following conditions. －１From the peak intensity, the basis weight of the polypropylene resin in the polyolefin microporous membrane was calculated using the following formula A, and the ratio of the polypropylene resin in the polyolefin microporous membrane was calculated using the following formula B. Measurement device: FT-IR device (FT / IR-6600 manufactured by JASCO Corporation) Measurement temperature: 25°C Aperture: X = 300 μm, Y = 300 μm Number of times of accumulation: 16 Resolution: 4 cm －１ Formula A: Basis weight of polypropylene resin (g / m ２ )=2.5×(1376cm －１ Peak height) Here, the constant 2.5 is 1376 cm －１ Formula B, which is a coefficient for converting the peak height into the basis weight of polypropylene: Proportion of polypropylene-based resin (mass%)=basis weight of polypropylene-based resin / basis weight of polyolefin microporous membrane×100.

[0086] The basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25°C, and using the following formula: ２ ) = mass (g) / (50 (mm) x 50 (mm)) x 10 ６ .

[0087] When the polyolefin microporous film is a laminated film and the ratio of the polypropylene-based resin in each layer is to be measured, a peeling starting point between each layer is created using a commercially available adhesive tape, and the layers are carefully peeled off to separate them. The above-mentioned IR spectroscopy is then performed on each separated layer to determine the polypropylene-based resin concentration in each layer. ２ The above peeled samples were collected and the film thickness was measured at 10 random points, and if the thickness unevenness (%) calculated using the following formula was 40% or less, it was determined that peeling had occurred between the layers: Formula: Thickness unevenness (%) = (maximum thickness measured at 10 points - minimum thickness measured at 10 points) / average thickness measured at 10 points x 100.

[0088] [Evaluation of Filtration Accuracy and Permeability of Polyolefin Microporous Membrane] The filtration accuracy and permeability of the polyolefin microporous membrane were evaluated by a filtration test of an aqueous polyethylene glycol solution. A polyolefin microporous membrane was placed in a cylindrical stainless steel liquid permeation cell with an inner diameter of 39 mm. The polyolefin microporous membrane was wetted with 0.5 mL of ethanol, and then 50 mL of a 0.05 mass% aqueous polyethylene glycol solution was placed in the liquid permeation cell. The solution was filtered through the polyolefin microporous membrane at a differential pressure of 500 kPa to collect approximately 5 g of filtrate. The time required for 5.0 g of filtrate to permeate was measured to determine the unit time (min) and unit area (cm). ２ ) permeation rate (g / min / cm ２ ) was calculated. The polyethylene glycol used in the aqueous solution was 25% by mass of Mw 400 (PSS-PEG400 manufactured by Agilent) and 75% by weight of Mw 100,000 (polyethylene oxide manufactured by Sigma-Aldrich, average Mv 100,000 (nominal), powder). The filtrate collected in the above filtration test was subjected to GPC (gel permeation chromatography) to measure the molecular weight distribution of the polyethylene glycol contained in the filtrate. The analysis conditions are as follows. Detector: differential refractive index detector RI (RI-504 manufactured by Showa Denko, sensitivity 32) Column: TSKgel GMPW ＸＬ 2, G3000PW ＸＬ 1 tube (φ7.8 mm×30 cm, manufactured by Tosoh) Solvent: water Flow rate: 0.7 mL / min Column temperature: 23° C. Injection volume: 0.1 mL Standard sample: monodisperse polyethylene oxide (PEO) and polyethylene glycol (PEG) manufactured by Tosoh and Agilent.

[0089] The molecular weight distribution was normalized by setting the maximum value of the peak derived from polyethylene glycol with Mw 400 to 1, and then the maximum value (P) of the peak detected in the molecular weight range of 10,000 to 1,000,000 was determined. In addition, the molecular weight distribution obtained by measuring the aqueous polyethylene glycol solution before filtration by GPC was normalized in the same manner as above by setting the maximum value of the peak derived from polyethylene glycol with Mw 400 to 1, and then the maximum value of the peak in the molecular weight range of 10,000 to 1,000,000 was determined as P １ Let P / P１ The filtration accuracy and permeability of the polyolefin microporous membrane were evaluated according to the following criteria, and a membrane with a rating of A or B was deemed to pass. １ A: Less than 0.50 B: 0.50 or more and less than 0.80 C: 0.80 or more Permeability (g / min / cm ２ ) A: 0.15 or more B: 0.10 or more, less than 0.15 C: Less than 0.10.

[0090] [Example 1] (Preparation process) A polyolefin raw material having a weight average molecular weight (Mw) of 1.0 × 10 ６ A polyolefin resin composition for layer A was prepared by using 100% by mass of polyethylene having a melting point of 135°C, adding 80% by mass of liquid paraffin to 20% by mass of the above polyolefin raw material, and further adding 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 based on the mass of the polyolefin raw material. The mixture was then charged into a twin-screw extruder and kneaded at 180°C to prepare a polyolefin resin composition for layer A. 100% by mass of polypropylene having a melting point of 163°C was used as the polyolefin raw material for layer B, and 70% by mass of liquid paraffin was added to 30% by mass of the above polyolefin raw material. Furthermore, 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the mass of the polyolefin raw material, and the mixture was charged into a twin-screw extruder and kneaded at 180°C to prepare a polyolefin resin composition for layer B.

[0091] (Gel-like sheet forming process) The polyolefin resin compositions for Layer A and Layer B were passed through filters from each twin-screw extruder to remove foreign matter, and then fed to a three-layer T-die and extruded to form a layer structure of Layer A / Layer B / Layer A, with the extrusion rate ratio of each layer adjusted to 42.5% / 15% / 42.5% (Layer A / Layer B / Layer A). The extrudate was taken up by a cooling roll controlled to 20°C, and a gel-like sheet with a thickness of approximately 0.5 mm was formed.

[0092] (First Stretching Step) The first stretching was carried out using a simultaneous biaxial tenter stretching machine at 121° C. with a stretch ratio of 5 in both the MD and TD directions and a stretching speed of 1000% / min in both the MD and TD.

[0093] (Removing Step and Drying Step) The wet-stretched gel-like sheet was immersed in a methylene chloride bath to thoroughly remove the liquid paraffin, and then air-dried to obtain a sheet.

[0094] (Heat Treatment Step) The above sheet was subjected to heat treatment at 100° C. for 1 minute using only the heating mechanism of the tenter-type stretching machine without performing the second stretching, to produce a polyolefin microporous membrane.

[0095] Other Examples and Comparative Examples Other examples and comparative examples were carried out in the same manner as in Example 1, except that the raw materials and film-forming conditions were changed as shown in the table. In Comparative Example 8, 0.2 parts by mass of a nucleating agent, NA-11 (manufactured by ADEKA Corporation), was added to Layer B per 100 parts by mass of the polyolefin raw material.

[0096] Examples and Comparative Examples are shown in Tables 1 and 2. Note that the sample of Comparative Example 7 failed to produce a polyolefin microporous membrane with stable quality.

[0097] [Evaluation] The polyolefin microporous membranes of Examples 1 to 5 satisfied all of the above-mentioned formulas 1 to 3, 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 6 and 8 did not satisfy at least one of formulas 1 to 3.

[0098]

[0099]

[0100] The polyolefin microporous membrane of the present invention, when used as a battery separator, has excellent dendrite resistance and output characteristics, and therefore can be suitably used 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 therefore can be suitably used as a high-precision liquid filter that requires the removal of minute foreign matter, such as in semiconductor processes.

Claims

1. The bubble point pore size determined by the porometer method is A (nm), and the water permeability under a pressure of 90 kPa is B (mL / min / cm ２ ) a polyolefin microporous membrane that satisfies the following formulas 1, 2, and 3. Formula 1: A - 20 × B ≦ 14 Formula 2: A ≦ 30 Formula 3: 0.05 ≦ B ≦ 1.0 2. The puncture strength converted into unit basis weight is 600mN / (g / m ２ 2. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has a viscosity of 1000 MPa or less.

3. The pore volume determined by nitrogen gas adsorption is 0.6 cm ３ The polyolefin microporous membrane according to claim 1 or 2, wherein the polyolefin microporous membrane has a viscosity of 1 / g or less.

4. The microporous polyolefin membrane according to claim 1 or 2, which has a porosity of 40% or more.

5. Air resistance is 300 seconds / 100 cm ３ The polyolefin microporous membrane according to claim 1 or 2, wherein:

6. The microporous polyolefin membrane according to claim 1 or 2, having a thickness of 15 μm or less.

7. Polypropylene resin is used at a weight of 0.1 g / m ２ 2.5g / m or more ２ The polyolefin microporous membrane according to claim 1 or 2, comprising:

8. The microporous polyolefin membrane according to claim 1 or 2, which has a laminated structure of two or more layers each composed mainly of a different resin.

9. The microporous polyolefin membrane according to claim 1 or 2, which has at least one layer containing 90% by mass or more of a polypropylene-based resin.

10. A battery separator comprising the microporous polyolefin membrane according to claim 1 or 2.

11. A battery comprising the battery separator of claim 10.

12. A liquid filter comprising the polyolefin microporous membrane according to claim 1 or 2.

13. A filtration unit comprising a liquid filter according to claim 12.

Images