JP7604877B2 - Polyolefin microporous membrane - Google Patents

Description

The present invention relates to a polyolefin microporous membrane, and in particular to a polyolefin microporous membrane that has excellent charge/discharge characteristics and battery safety when used as a separator for a secondary battery.

Microporous membranes are used in a variety of fields, including filters such as filtration membranes and dialysis membranes, as well as separators for secondary batteries and electrolytic capacitors. Among these, polyolefin microporous membranes, which use polyolefin as the resin material, have excellent chemical resistance, insulating properties, mechanical strength, and shutdown properties, and therefore have been widely used in recent years as separators for secondary batteries.

Secondary batteries, such as lithium-ion secondary batteries, have a high energy density and are therefore widely used in personal computers, mobile phones, and other devices. Secondary batteries are also expected to be used as power sources for driving motors in electric and hybrid vehicles, and as stationary storage batteries.

In recent years, the energy density of lithium-ion secondary batteries has been increasing. Some new electrode materials designed for high energy density have the characteristic of easily expanding during charging. As a result, pressure is applied to the separator during charging, compressing the pores and releasing the electrolyte, which deteriorates the charge/discharge characteristics. In addition, the battery manufacturing process sometimes involves a pressing process to mold the cells and improve the adhesion between the components, but this can cause the pores in the separator to collapse, compromising the ion permeability of the separator. Due to these issues, there is a demand for improved compression resistance in separators.

On the other hand, the electrodes used in lithium-ion secondary batteries designed for high energy density tend to have reduced thermal stability. For this reason, in the event of abnormal heat generation due to short circuit or overcharging, it is necessary to prevent thermal runaway by shutting down the separator (blocking pores) at a lower temperature and preventing ion movement between the positive and negative electrodes.

The compression resistance of microporous membranes can be improved by increasing the membrane strength through the raw material formulation and stretching conditions. However, increasing the strength also increases the melting point of the resin, which deteriorates the shutdown characteristics. For this reason, it has been difficult to achieve both compression resistance and shutdown characteristics.

Patent Document 1 proposes a microporous membrane with excellent compression resistance, for example, by using a specific raw material formulation that contains ultra-high molecular weight polyethylene and/or a nucleating agent, and by adjusting the weight-average molecular weight Mw and the stretching ratio.

In Patent Document 2, the essential components are a polyolefin with a viscosity average molecular weight (Mv) of less than 300,000, a polyolefin with an Mv of 500,000 or more, and electrochemically inactive particles larger than the film thickness. The height A (μm) of the part where the particles protrude from the film surface and the film thickness B (μm) satisfy the relationship 0<A/B×100<25, and pressure is selectively applied to the protruding particles, reducing the pressure on the entire film surface and suppressing an increase in air resistance after pressing.

Patent Document 3 proposes that a polyolefin microporous membrane that exhibits an increase in air permeability of 500 seconds/100 cc or less after being heated and compressed at 90°C for 5 minutes under a pressure of 5 MPa has an excellent balance of heat compression resistance, permeability, mechanical properties, and heat shrinkability.

International Publication No. 2018/164056 JP 2007-262203 A JP 2004-161899 A

In contrast to Patent Document 1, for example, while high-magnification stretching is essential to improve compression resistance, this also increases the melting point of the resin. Therefore, improvements are desired in consideration of compatibility with the shutdown temperature. In Patent Document 2, there is concern that coarse particles will form coarse pores, which may worsen shutdown characteristics. Patent Document 3 describes the film thickness after release from heat compression, but does not discuss separator deformation during heat compression.

The object of the present invention is to solve the above problems. That is, to provide a polyolefin microporous film that, when used as a battery separator, has excellent charge/discharge characteristics due to high compression resistance, while quickly shutting down the battery in the event of abnormal heat generation, thereby providing high safety.

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 including the numerical values before and after "to" as the lower and upper limits.
[1] A polyolefin microporous membrane having a membrane thickness reduction rate/porosity ratio of 0.20 or less when pressurized at 70°C and 2.3 MPa, and a shutdown temperature of 140°C or less.
[2] The polyolefin microporous membrane according to [1] above, having a porosity of 35% or more.
[3] The polyolefin microporous membrane according to [1] or [2] above, which has a porosity of 30% or more when pressurized at 70°C and 2.3 MPa.
[4] The polyolefin microporous membrane according to any one of [1] to [3] above, comprising 50 to 99% by mass of a polyethylene resin and 1 to 50% by mass of a polymer incompatible with the polyethylene resin.
[5] The polyolefin microporous membrane according to [4] above, wherein the polymer incompatible with the polyethylene resin is an amorphous polymer.
[6] The polyolefin microporous membrane according to [4] above, wherein the polymer incompatible with the polyethylene resin is a cyclic olefin polymer.
[7] The polyolefin microporous membrane according to any one of [1] to [6] above, wherein a first phase made of a polyethylene-based resin has a continuous structure, and a second phase made of a polymer incompatible with the polyethylene-based resin has a discontinuous structure, and the second phase has a domain diameter of 10 to 300 nm.
[8] The polyolefin microporous membrane according to any one of [1] to [7] above, which has a heat shrinkage rate of 5% or less after 8 hours at 105°C.
[9] The polyolefin microporous membrane according to any one of [1] to [8] above, having an air permeability per 1 μm thickness of 30 sec/100 cm3 or ^less .
[10] The polyolefin microporous membrane according to any one of [1] to [9] above, having a pin puncture strength of 10 gf/μm or more per 1 μm thickness.
[11] The polyolefin microporous membrane according to any one of [1] to [10] above, having a thickness of 5 μm or more.

The polyolefin microporous membrane of the present invention has high compression resistance and the ability to shut down (block pores) at lower temperatures. Therefore, when used as a separator for a secondary battery, it is possible to provide a polyolefin microporous membrane with excellent charge/discharge characteristics and safety.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the embodiment described below.

The polyolefin microporous membrane according to an embodiment of the present invention (hereinafter, sometimes simply referred to as "microporous membrane") has a value expressed as the membrane thickness reduction rate/porosity when pressurized at 70°C and 2.3 MPa of 0.20 or less, and a shutdown temperature of 140°C or less.

The value represented by the film thickness reduction rate/porosity when pressurized at 70°C and 2.3 MPa (hereinafter, simply referred to as "film thickness reduction rate/porosity") can be measured and calculated by the method described below. The film thickness reduction rate/porosity is 0.20 or less, preferably 0.18 or less, more preferably 0.15 or less, even more preferably 0.12 or less, and particularly preferably 0.10 or less. When the film thickness reduction rate/porosity is in the above range, the film thickness reduction rate/porosity is excellent in compression resistance. That is, when the microporous membrane is used as a battery separator, even if the separator is in a pressurized state during electrode expansion or press processing, the pore structure and permeability are maintained, and a battery with excellent charge/discharge characteristics can be obtained. The lower limit of the film thickness reduction rate/porosity is not particularly limited, but is, for example, 0.01 or more. By setting the lower limit of the film thickness reduction rate/porosity in the above range, it is easy to achieve compatibility with the shutdown temperature. To achieve the above range of membrane thickness reduction rate/porosity, it is preferable that the raw material composition of the microporous membrane be within the range described below, and that the membrane production conditions for the microporous membrane be within the range described below.

The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is 140°C or less. It is preferably 139°C or less, more preferably 138°C or less, even more preferably 136°C or less, and particularly preferably 134°C or less. By controlling the shutdown temperature within the above range, the battery safety is excellent when used as a battery separator. The lower limit of the shutdown temperature is not particularly limited, but it is preferably 110°C or more because it is easy to achieve compatibility with permeability. In order to set the shutdown temperature within the above range, it is preferable that the raw material composition of the microporous membrane is within the range described below, and the film formation conditions of the microporous membrane are within the range described below. The shutdown temperature is a temperature measured by the method described below, and is a temperature at which, when the polyolefin microporous membrane is heated, the resin portion shrinks and melts, closing the porous structure, and when used as a battery separator, it is substantially impossible to discharge or charge.

The porosity of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 35% or more, more preferably 40% or more, even more preferably 45% or more, and particularly preferably 50% or more. There is no particular upper limit to the porosity, but it is preferably 80% or less because this can suppress a decrease in membrane strength. When the porosity is in the above range, the microporous membrane has excellent output characteristics when used as a separator for a secondary battery. The porosity can be adjusted to the above range by adjusting the blending ratio of the raw materials, the stretching ratio, the heat setting conditions, etc. during the manufacturing process.

The porosity of the polyolefin microporous membrane according to the embodiment of the present invention when pressurized at 70°C and 2.3 MPa is preferably 30% or more, more preferably 33% or more, even more preferably 40% or more, and particularly preferably 45% or more. There is no particular upper limit for the porosity when pressurized at 70°C and 2.3 MPa, but it is preferably 80% or less because it can suppress a decrease in membrane strength. When the porosity when pressurized at 70°C and 2.3 MPa is in the above range, the microporous membrane has excellent output characteristics when used as a separator for a secondary battery. The porosity when pressurized at 70°C and 2.3 MPa can be set to the above range by adjusting the blending ratio of the raw materials, the stretching ratio, the heat setting conditions, etc. during the manufacturing process. The porosity when pressurized at 70°C and 2.3 MPa can be measured and calculated by the method described below.

In the polyolefin microporous membrane according to the embodiment of the present invention, the first phase made of a polyethylene resin preferably has a continuous structure, and the second phase made of a polymer incompatible with the first phase preferably has a discontinuous structure. The domain diameter of the second phase forming the discontinuous structure is preferably 10 to 300 nm, particularly preferably 20 nm or more, more preferably 250 nm or less, and particularly preferably 150 nm or less.

When the second phase has the above-mentioned domain diameter, a uniform pore structure is formed, and the polyolefin microporous membrane is better able to improve both the compression resistance and high permeability. The phase separation structure in the microporous membrane can be observed by the method described below, and the domain diameter can be calculated. The domain diameter of the second phase that forms a discontinuous structure can be adjusted to the above range by adjusting the resin components that form the second phase, their blending ratios in the manufacturing process, kneading conditions, stretching conditions, etc.

The polyolefin microporous membrane according to the embodiment of the present invention has a heat shrinkage ratio before and after heating at 105°C for 8 hours of preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. There is no particular lower limit for the heat shrinkage ratio after 8 hours at 105°C, but it is preferably -0.1% or more because the flatness of the microporous membrane is good. By setting the heat shrinkage ratio after 8 hours at 105°C in the above range, when the microporous membrane is used as a battery separator, it is possible to suppress end short-circuiting due to shrinkage during heat generation. The heat shrinkage ratio after 8 hours at 105°C can be calculated by the method described below. The heat shrinkage ratio after 8 hours at 105°C can be set to the above range by adjusting the blending ratio of the raw materials, the stretch ratio, the heat setting conditions, etc. during the manufacturing process.

The polyolefin microporous membrane according to the embodiment of the present invention has an air permeability calculated based on a thickness of 1 μm of preferably 30 sec/100 cm ³ or less, more preferably 20 sec/100 cm ³ or less, even more preferably 15 sec/100 cm ³ or less, and particularly preferably 10 sec/100 cm ³ or less. There is no particular lower limit for the air permeability calculated based on a thickness of 1 μm, but it is preferably 1 sec/100 cm ³ or more since it is easy to achieve compatibility with the membrane strength. By setting the air permeability calculated based on a thickness of 1 μm within the above range, a microporous membrane having excellent charge/discharge characteristics can be obtained when used as a battery separator. The air permeability calculated based on a thickness of 1 μm can be set to the above range by adjusting the blending ratio of the raw materials, the stretching ratio, the heat setting conditions, etc. in the manufacturing process.

The polyolefin microporous membrane according to the embodiment of the present invention has a puncture strength of 10 gf/μm or more, more preferably 15 gf/μm or more, even more preferably 20 gf/μm or more, and particularly preferably 30 gf/μm or more, calculated based on a thickness of 1 μm. The upper limit of the puncture strength calculated based on a thickness of 1 μm is not particularly limited, but it is preferably 100 gf/μm or less, for example, because it is easy to control the shutdown temperature within an appropriate range. When the puncture strength calculated based on a thickness of 1 μm is in the above range, the microporous membrane is resistant to external impacts and has excellent safety when used as a battery separator. The puncture strength calculated based on a thickness of 1 μm can be adjusted to the above range by adjusting the blending ratio of the raw materials, the stretching ratio, the heat setting conditions, etc., during the manufacturing process.

The tensile strength (tensile breaking strength) in the MD direction of the polyolefin microporous membrane is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 200 MPa or more. The upper limit of the tensile strength in the MD direction is not particularly limited, but is, for example, 500 MPa or less, since this makes it easy to achieve compatibility with the shutdown temperature. When the tensile strength in the MD direction is within the above range, the membrane is unlikely to break even when a high tension is applied in the MD direction, and it can be used in applications requiring high durability.

The tensile strength (tensile breaking strength) in the TD direction of the polyolefin microporous membrane is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 200 MPa or more. The upper limit of the tensile strength in the TD direction is not particularly limited, but is, for example, 500 MPa or less, since this makes it easy to achieve compatibility with the shutdown temperature. When the tensile strength in the TD direction is within the above range, the membrane is unlikely to break even when a high tension is applied in the TD direction, and it can be used in applications requiring high durability.

In the polyolefin microporous membrane, the ratio of MD tensile strength to TD tensile strength (MD tensile strength/TD tensile strength) is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, and particularly preferably 0.8 or more. In addition, such a ratio is preferably 2.0 or less, more preferably 1.7 or less, even more preferably 1.5 or less, and particularly preferably 1.3 or less. By setting the ratio of MD tensile strength to TD tensile strength in the above range, the microporous membrane is resistant to external impacts and has excellent safety when used as a battery separator.

The thickness of the polyolefin microporous film according to the embodiment of the present invention can be adjusted appropriately depending on the application. It is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. It is also preferably 30 μm or less, more preferably 28 μm or less, even more preferably 25 μm or less, and particularly preferably 21 μm or less. By setting the thickness of the polyolefin microporous film within the above range, it is possible to achieve both safety and high battery capacity when used as a separator for a secondary battery.

The specific configuration of the polyolefin microporous membrane of this embodiment is described below, but is not necessarily limited thereto.

The polyolefin microporous membrane according to an embodiment of the present invention contains a polyolefin resin as a main component (hereinafter, the main polyolefin resin is referred to as "resin A"). The main component referred to here is the component that is contained in the polyolefin microporous membrane with the largest content in mass %. When two or more polyolefin resins that are resin A are contained, the total content of these polyolefin resins is taken as the content of resin A, and this total content may be the component that is contained in the polyolefin microporous membrane with the largest content in mass %.

Examples of resin A include polyethylene-based resins such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, and low-crystalline or amorphous ethylene-α-olefin copolymers; polypropylene-based resins such as homopolypropylene, propylene-ethylene copolymers (random copolymers and/or block copolymers), propylene-α-olefin copolymers, and propylene-ethylene-α-olefin copolymers; and cyclic olefin-based resins containing alicyclic structures such as cycloalkanes and cycloalkenes as repeating units. The α-olefins are not particularly limited as long as they can be copolymerized with propylene or ethylene, and examples of such resins include 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-pentene, and 1-heptene. Among the above-mentioned resins, resin A preferably contains a polyethylene-based resin, and more preferably contains high-density polyethylene and ultra-high molecular weight polyethylene, from the viewpoint of achieving both compression resistance and shutdown characteristics. Resin A may be selected from two or more of the above-mentioned resins.

Resin A preferably contains high density polyethylene (density: 0.940 g/m ³ or more and 0.970 g/m ³ or less) in terms of melt extrusion characteristics and stretch processing characteristics. The polyethylene-based resin used in resin A (hereinafter sometimes referred to as "polyethylene (A)") is preferably not only an ethylene homopolymer but also a copolymer containing other α-olefins in order to reduce the melting point and crystallinity of polyethylene (A). Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. As a copolymer containing α-olefin, a copolymer containing hexene-1 (ethylene-1-hexene copolymer) is preferable, and it is more preferable that polyethylene (A) is mainly composed of ethylene-1-hexene copolymer. In addition, α-olefin can be confirmed by measuring with ¹³ C-NMR. The main component shown here refers to the component that is the component that constitutes polyethylene (A) and has the highest content in mass %.

The lower limit of the weight average molecular weight of the high-density polyethylene is preferably 1 × 10 ⁴ or more, more preferably 1 × 10 ⁵ or more, and even more preferably 1.5 × 10 ⁵ or more. The upper limit of the weight average molecular weight of the high-density polyethylene is preferably 1 × 10 ⁶ or less, more preferably 8.0 × 10 ⁵ or less, and even more preferably 6.0 × 10 ⁵ or less. By setting the weight average molecular weight of the branched high-density polyethylene within the above range, both a low shutdown temperature and film strength during shutdown can be achieved.

The melting point of the branched high-density polyethylene is preferably 138°C or lower, more preferably 136°C or lower, and even more preferably 134°C or lower. The melting point is preferably 115°C or higher, more preferably 120°C or higher, and even more preferably 125°C or higher. By setting the melting point of the high-density polyethylene within the above range, it is possible to achieve both a low shutdown temperature and the permeability of the polyolefin microporous membrane.

Resin A preferably contains branched high-density polyethylene, because this allows the formation of a uniform fine pore structure and the suppression of crystal orientation during stretching to produce a polyolefin microporous film with excellent compression resistance and shutdown characteristics. Branched high-density polyethylene is polyethylene that is polymerized using a catalyst in which a chromium compound is supported on a carrier made of an inorganic compound such as silica or alumina.

The branched high-density polyethylene content is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, based on the total mass of resin A. This allows for a polyolefin microporous film with superior compression resistance and shutdown properties to be obtained.

The lower limit of the weight average molecular weight of the branched high-density polyethylene is preferably 1 × 10 ⁴ or more, more preferably 1 × 10 ⁵ or more, and even more preferably 1.5 × 10 ⁵ or more. The upper limit of the weight average molecular weight of the high-density polyethylene is preferably 1 × 10 ⁶ or less, more preferably 8.0 × 10 ⁵ or less, and even more preferably 6.0 × 10 ⁵ or less. By setting the weight average molecular weight of the branched high-density polyethylene within the above range, both a low shutdown temperature and film strength during shutdown can be achieved.

The melting point of the branched high-density polyethylene is preferably 138°C or lower, more preferably 136°C or lower, and even more preferably 134°C or lower. The melting point is preferably 115°C or higher, more preferably 120°C or higher, and even more preferably 125°C or higher. By setting the melting point of the branched high-density polyethylene within the above range, it is possible to achieve both a low shutdown temperature and the permeability of the polyolefin microporous membrane. The melting point of the branched high-density polyethylene can be measured by the method described below.

The content of resin A in the polyolefin microporous membrane is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and is preferably 99% by mass or less, more preferably 97% by mass or less, even more preferably 95% by mass or less. By setting the content of resin A in the polyolefin microporous membrane within the above range, it becomes easier to control the strength, permeability, and shutdown characteristics of the microporous membrane, and the microporous membrane has better compression resistance.

When preparing a polyolefin resin composition for producing a polyolefin microporous membrane, a membrane-forming solvent such as liquid paraffin is used as a plasticizer. Since the membrane-forming solvent is removed during the production process of the microporous membrane, the composition of the polyolefin resin composition differs from the composition of the polyolefin microporous membrane. In other words, the content of resin A in the polyolefin microporous membrane corresponds to the content of resin A relative to the composition of the polyolefin resin composition excluding the components to be removed, such as the membrane-forming solvent.

From the viewpoint of achieving both compression resistance and shutdown characteristics, it is preferable to mix a resin (hereinafter referred to as "resin B") that is a polymer incompatible with resin A in the polyolefin microporous membrane according to an embodiment of the present invention.

Resin B is incompatible with resin A and preferably forms a phase-separated structure in the microporous membrane. Resin B can be either a crystalline polymer or an amorphous polymer.

Examples of crystalline polymers include polypropylene-based resins such as homopolypropylene, propylene-ethylene copolymers (random copolymers and/or block copolymers), propylene-α-olefin copolymers, and propylene-ethylene-α-olefin copolymers, polymethylpentene, polyester resins, polyamides, polyacetals, fluororesins, polyphenylene sulfide, and polyether ether ketones. The above-mentioned α-olefins are not particularly limited as long as they can be copolymerized with propylene or ethylene, and examples of such α-olefins include 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-pentene, and 1-heptene.

Amorphous polymers include cyclic olefin polymers that contain alicyclic structures such as cycloalkanes and cycloalkenes as repeating units, polyvinyl chloride, acrylic resins, polycarbonates, polystyrenes, polybutadienes, styrene-based elastomers, polysulfones, ABS resins, silicone resins, etc.

Among the above-mentioned resins, it is preferable to use a resin that is an amorphous polymer for resin B, and a resin that is a cyclic olefin polymer is more preferable.

By selecting resin B from the above-mentioned polymers, a uniform and fine domain structure is formed in the microporous membrane, and when used as a battery separator, the compression resistance of the microporous membrane is improved without impairing battery performance such as ion permeability or battery safety performance such as shutdown characteristics.

When a resin that is an amorphous polymer is used as resin B, the glass transition temperature of resin B is preferably 60°C or higher, more preferably 80°C or higher, even more preferably 100°C or higher, and particularly preferably 120°C or higher. The glass transition temperature is preferably 250°C or lower, more preferably 200°C or lower, even more preferably 180°C or lower, and particularly preferably 150°C or lower. By setting the glass transition temperature of resin B within the above range, a phase separation structure consisting of resin A phase, resin B phase, and plasticizer phase is formed uniformly and finely when the molten kneaded resin is cooled, and the resulting microporous film has excellent compression resistance.

The content of resin B in the polyolefin microporous membrane is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less. It is also preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more. By setting the content of resin B within the above range, it becomes easier to control the balance between compression resistance and permeability.

The layer structure of the polyolefin microporous membrane according to the embodiment of the present invention may be a single layer membrane or a laminated membrane consisting of at least two layers having different properties. When it is a laminated membrane, it is preferable that at least one layer is composed of the above-mentioned resin A and resin B.

The polyolefin microporous membrane according to the embodiment of the present invention may be provided with a coating layer. Examples of the coating layer include other porous layers other than polyolefin resins. The other porous layer is not particularly limited, but is preferably, 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, polyamideimide resin, polyamide resin, aromatic polyamide resin, polyimide resin, etc.

The inorganic particles constituting 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.

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

The wet method for producing a polyolefin microporous membrane is described below. Note that the following description is only one example of the production method, and is not limited to this method.

The method for producing a polyolefin microporous membrane in an embodiment of the present invention preferably includes the following steps (1) to (5) in order, and may further include the following step (6). After step (6) or instead of step (6), the method may further include at least one of the following steps (7) and (8).

(1) A step of melt-kneading the polyolefin resin and the membrane-forming solvent to prepare a polyolefin resin composition (2) A 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 step of removing the membrane-forming solvent from the gel-like sheet after the stretching (5) A step of drying the sheet after the membrane-forming solvent removal (6) A second stretching step of preheating and stretching the dried sheet (7) A step of heat-treating the dried sheet (8) A step of crosslinking and/or hydrophilizing the dried sheet (1) A step of preparing a polyolefin resin composition Resin A and resin B are heated and dissolved in a plasticizer (membrane-forming solvent) to prepare a polyolefin resin composition. The plasticizer is not particularly limited as long as it is a solvent that can uniformly disperse resin A and resin B, but in order to enable relatively high-magnification stretching, it is preferable that the solvent is liquid at room temperature. Examples of the solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, etc., mineral oil fractions having boiling points corresponding thereto, and phthalic acid esters that are liquid at room temperature such as dibutyl phthalate, dioctyl phthalate, etc. In order to obtain a gel-like sheet with a stable content of the liquid solvent, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

The blending ratio of resin A, resin B, and plasticizer may be appropriately selected within a range that does not impair moldability, but the combined content of resin A and resin B is preferably 10 to 50 mass% relative to the total mass of the polyolefin resin composition. If the combined content of resin A and resin B is 10 mass% or more, swelling and necking at the outlet of the die are not large when molded into a sheet, resulting in good moldability and film-forming properties of the sheet. On the other hand, if the combined content of resin A and resin B is 50 mass% or less, shrinkage in the thickness direction is suppressed, resulting in good moldability.

The content of resin B in the polyolefin resin composition is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less. The content of resin B is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. By setting the content of resin B in the polyolefin resin composition within the above range, resin B forms a good dispersion state in the polyolefin resin composition, and a polyolefin microporous film with better compression resistance can be obtained.

The method for uniformly melt-kneading resin A, resin B, and the plasticizer is not particularly limited, but is preferably carried out in a twin-screw extruder.

The resin temperature during kneading is preferably 140°C or higher, more preferably 150°C or higher, and even more preferably 160°C or higher, with the upper limit being preferably 250°C or lower, more preferably 240°C or lower, and even more preferably 230°C or lower. By keeping the temperature of the polyolefin resin composition during kneading within the above range, it is possible to prevent a decrease in strength due to deterioration of the resin, and it is possible to melt and knead resin A, resin B, and the plasticizer uniformly.

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

(2) Gel-Like Sheet Forming Step The melt of the polyolefin-based resin composition is fed from the extruder to a die and extruded into a sheet.

The extrusion method may be either the flat die method or the 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 in layers, and extruded into a sheet having a laminated structure. The extrusion temperature is preferably 140 to 250°C, and the extrusion speed is preferably 0.2 to 15 m/min.

The resin composition melt-extruded into a sheet is cooled and solidified to become a gel-like sheet. In the cooling process, it is preferable to cool to 10 to 50°C. This is because it is preferable to set the final cooling temperature below the crystallization end temperature, and by making the high-order structure finer, it becomes easier to perform uniform stretching in the subsequent stretching. In addition, the cooling rate at this time is preferably 50°C/min or more, more preferably 100°C/min or more, and even more preferably 150°C/min or more. In general, if the cooling rate is slow, relatively large crystals are formed, so the high-order structure of the gel-like sheet becomes coarse, and the gel structure that forms it also becomes large. On the other hand, if the cooling rate is fast, relatively small crystals are formed, so the high-order structure of the gel-like sheet becomes dense, which leads to uniform stretching as well as improved strength and elongation of the film. The cooling method at this time includes a method of directly contacting the film with cold air, cooling water, or other cooling medium, a method of contacting a roll cooled with a refrigerant, and a method using a casting drum, etc.

(3) First Stretching Step Next, the obtained gel-like sheet is stretched at least uniaxially, but it is preferable to preheat the gel-like sheet before stretching. The preheating temperature is preferably 90 to 130°C, more preferably 105°C or higher, even more preferably 110°C or higher, and more preferably 120°C or lower, even more preferably 117°C or lower. By preheating at the above-mentioned temperature conditions, a polyolefin microporous membrane that is uniformly stretched in the stretching step and has a uniform and fine pore structure can be obtained.

The gel-like sheet after preheating is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination of these. 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 multi-stage stretching (e.g., a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, with sequential biaxial stretching being preferred from the viewpoint of increasing compression resistance.

The stretching ratio (area stretching ratio) in this process is preferably 16 times or more, more preferably 25 times or more, and particularly preferably 49 times or more. In addition, it is preferably 4 times or more in both the machine longitudinal direction (MD direction) and the machine width direction (TD direction), and more preferably 5 times or more. The stretching ratios in the MD direction and the TD direction may be the same or different from each other. By setting the area stretching ratio within the above range, it is possible to increase the mechanical strength and permeability. In addition, the area stretching ratio in this process is preferably 150 times or less, more preferably 100 times or less, which can prevent membrane rupture and suppress an increase in the shutdown temperature. The stretching ratio in this process refers to the area stretching ratio of the polyolefin microporous membrane immediately before being subjected to the next process, based on the polyolefin microporous membrane immediately before this process.

The stretching temperature in this process is preferably within the range of the crystal dispersion temperature (TCD) of the polyolefin resin to (TCD + 30) °C, more preferably at least (TCD + 5) °C, particularly preferably at least (TCD + 10) °C, and more preferably at most (TCD + 28) °C, particularly preferably at most (TCD + 26) °C. When the stretching temperature is within the above range, film rupture due to stretching of the polyolefin resin is suppressed, and high-magnification stretching is possible.

The crystal dispersion temperature (TCD) is determined by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. When a polyethylene resin is used as the polyolefin resin, ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene resin compositions have a crystal dispersion temperature of approximately 100 to 110°C, so the stretching temperature is preferably 90 to 130°C, more preferably 105°C or higher, even more preferably 110°C or higher, more preferably 120°C or lower, and even more preferably 117°C or lower.

The stretching process described above causes cleavage between the polyethylene lamellae, resulting in the polyethylene resin phase becoming finer and the formation of numerous fibrils. The fibrils form a network structure that is irregularly connected three-dimensionally.

(4) Membrane-forming solvent removal process The membrane-forming solvent is removed (washed) using a washing solvent. The polyolefin resin phase is phase-separated from the membrane-forming solvent phase. Therefore, when the membrane-forming solvent is removed, a porous membrane is obtained that is made of fibrils that form a fine three-dimensional network structure and has holes (voids) that are three-dimensionally irregularly connected. Washing solvents and methods for removing membrane-forming solvents using them are publicly known, so their explanation is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Laid-Open No. 2002-256099 can be used.

(5) Drying step The polyolefin microporous membrane from which the membrane-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably equal to or lower than the crystal dispersion temperature (TCD) of the polyolefin resin, and is particularly preferably at least 5°C lower than the TCD. The drying is preferably performed until the 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.

(6) Second stretching step The dried polyolefin microporous membrane may be stretched at least uniaxially (second stretching step). The polyolefin microporous membrane may be preheated before the second stretching step. The preheating temperature is preferably 90 to 140°C, more preferably 95°C or higher, even more preferably 100°C or higher, and more preferably 135°C or lower, even more preferably 130°C or lower. The polyolefin microporous membrane may be stretched by a tenter method, a roll method, an inflation method, or the like while heating as described above. The stretching may be uniaxial or biaxial. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.

The stretching temperature in this process is not particularly limited, but is usually 90 to 140°C, more preferably 95°C or higher, even more preferably 100°C or higher, and more preferably 130°C or lower.

The areal stretching ratio in this process is preferably 16.0 times or less, more preferably 4.0 times or less, and even more preferably 2.0 times or less. In the case of biaxial stretching, the stretching ratios in the MD direction and the TD direction may be the same or different. The stretching ratio in this process refers to the stretching ratio of the polyolefin microporous membrane immediately before being subjected to the next process, based on the polyolefin microporous membrane immediately before this process.

(7) Heat Treatment Step After or instead of the step (6), the polyolefin microporous membrane after drying may be heat treated. The heat treatment stabilizes the crystals and makes the lamellae uniform. As the heat treatment method, heat setting and/or heat relaxation can be used. Heat setting is a heat treatment in which the membrane is heated while maintaining its dimensions. Heat relaxation is a heat treatment in which the membrane is thermally shrunk in the MD or TD directions during heating. The heat setting is preferably performed by a tenter method or a roll method. For example, the heat relaxation method is the method disclosed in JP-A-2002-256099. The heat treatment temperature is preferably within the range of TCD to melting point of the polyolefin resin. The melting point can be measured by a differential scanning calorimeter (DSC) based on JIS K7121 (1987).

(8) Crosslinking and hydrophilization treatment steps The dried polyolefin microporous membrane may be subjected to a crosslinking and hydrophilization treatment after or instead of the step (7).

For example, the crosslinking treatment is performed by irradiating the polyolefin microporous membrane with ionizing radiation such as A-rays, B-rays, G-rays, or electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 to 100 MRAD, and the acceleration voltage is preferably 100 to 300 kV. The crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane.

Hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably performed after crosslinking treatment.

The polyolefin microporous membrane obtained in the above manner can be used for various applications, such as filters, secondary battery separators, fuel cell separators, and capacitor separators. In particular, when used as a battery separator, it has excellent permeability and can provide high safety, and can be more preferably used as a secondary battery separator for electric vehicles and other devices that require high energy density, high capacity, and high output.

The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.
[Measurement method]
[Thickness]
The thickness of the polyolefin microporous membrane was measured at any five points within a 50 mm × 50 mm area using a contact thickness meter, Litematic VL-50 (10.5 mmφ ultrahard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation, and the average value was recorded as the thickness (μm).
[Porosity]
A 50 mm x 50 mm square sample was cut from the polyolefin microporous membrane, and its volume ( ^cm3 ) and mass (g) were measured at room temperature of 25° C. From these values and the membrane density (g/ ^cm3 ), the porosity of the polyolefin microporous membrane was calculated by the following formula. The membrane density was assumed to be a constant value of 0.99 g/ ^cm3 .

Formula: Porosity (%)=(Volume-Mass/Membrane Density)/Volume×100
[Puncture strength converted to 1 μm thickness]
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 (gf) was measured when a polyolefin microporous membrane was punctured in an atmosphere of 25°C with a needle having a diameter of 1.0 mm and a spherical tip (radius of curvature R: 0.5 mm). Using the thickness value measured by the above-mentioned method, the value calculated from the following formula was used as the puncture strength (gf/μm) converted to a thickness of 1 μm.

Formula: puncture strength converted into 1 μm thickness (gf/μm)=maximum load (gf)/polyolefin microporous membrane thickness (μm)
[Air permeability converted into thickness of 1 μm]
The air permeability (sec/100 cm ³ ) of the polyolefin microporous membrane was measured in an atmosphere of 25° C. using an Oken air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117: 2009. In addition, the air permeability (sec/100 cm ³ ) converted to a thickness of 1 μm was calculated using the thickness value measured by the above-mentioned method according to the following formula.

Formula: air permeability per 1 μm thickness (sec/100 cm ³ )=air permeability (sec/100 cm ³ )/thickness of polyolefin microporous membrane (μm)
[Tensile strength]
The tensile strength in the MD direction and the tensile strength in the TD direction were measured in accordance with the method of ASTM D882 using a universal testing machine AGS-J manufactured by Shimadzu Corporation. A rectangular test piece having a width of 10 mm was used as the test piece, and measurements were taken at three arbitrary points on the sample surface in both the MD direction and the TD direction, and the average value was taken as the tensile strength in the relevant direction.
[Shutdown temperature]
The polyolefin microporous membrane was heated at a heating rate of 5°C/min while measuring the air resistance with an Oken air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.), and the temperature at which the air resistance reached the detection limit of 99,999 sec/100 ^cm3 Air was defined as the shutdown temperature (°C).

The measurement cell was composed of an aluminum block and had a structure in which a thermocouple was placed directly under the polyolefin microporous membrane. A sample was cut into a 5 cm x 5 cm square and heated while being fixed with an O-ring around the periphery.
[Heat shrinkage rate after 8 hours at 105°C]
A square sample of 5 cm x 5 cm was cut out from the polyolefin microporous membrane, with two sides parallel to the MD direction. The length of the sample in the MD direction was measured at the center of the TD direction of the sample, and this was the length before MD shrinkage (L _1MD ). The length of the sample in the TD direction was also measured at the center of the MD direction, and this was the length before TD shrinkage (L _1TD ).

Next, the sample was placed in an oven with an internal temperature of 105°C and heated, and was taken out 8 hours after placement. The MD length at the location where the above-mentioned length before MD shrinkage was measured was measured, and this was taken as the length after MD shrinkage (L _2MD ). In addition, the TD length at the location where the above-mentioned length before TD shrinkage was measured was measured, and this was taken as the length after TD shrinkage (L _2TD ). Using these values, the heat shrinkage rate after 8 hours at 105°C was calculated using the following formula. This measurement was also performed at three arbitrary locations on the sample surface, and the average value was calculated as the heat shrinkage rate (%) after 8 hours at 105°C.

Formula: Heat shrinkage rate (%) after 8 hours at 105°C = 100 x ( _L1MD - _L2MD + _L1TD - _L2TD ) / ( _L1MD + _L1TD )
[Domain diameter in polyolefin microporous membrane]
The polyolefin microporous membrane was stained with _RuO4 , and a thin section was prepared for observing the cross section in the thickness direction-MD direction by the microtome method. A transmission electron microscope (JEM1400Plus type manufactured by JEOL Ltd.) was used for observation at an acceleration voltage of 100 kV and a magnification of 50,000. Since the resin A part, the resin B part, and the pore part were stained differently, an observation image having contrast in shade was obtained. In the above observation image, the area of the domain was calculated for the region having a domain-like contrast derived from resin B, and the diameter of the domain was calculated assuming that the cross section of the domain was circular, and this value was taken as the domain diameter. The domain diameter value was measured and calculated as described above for any 10 domains, and the average value was taken as the domain diameter (nm) in the polyolefin microporous membrane.
[Film thickness reduction rate when pressurized at 70° C. and 2.3 MPa, film thickness reduction rate/porosity when pressurized at 70° C. and 2.3 MPa, and porosity when pressurized at 70° C. and 2.3 MPa]
A sample was prepared by stacking 100 polyolefin microporous membranes and punching them into a circle with a diameter of 14 mm using a punching machine. Measurements were performed in compression mode using a circular compression tool with a diameter of 4 mm using a viscoelasticity measuring device (RSA-G2 manufactured by TA Instruments). The sample was set on the sample stage, and the temperature in the tank was raised to 70°C while the sample was held down with a pressure of 70 KPa using the compression tool described above. The gap between the measurement tool and the sample stage at the time when the temperature was raised to 70°C was taken as the sample thickness _T1 . After the temperature in the tank reached 70°C, the compression pressure was changed to 2.3 MPa and maintained for 1 hour, and the value at which the gap between the measurement tool and the sample stage became the narrowest was taken as the sample thickness under pressure _T2 . Using the porosity (%) of the polyolefin microporous membrane calculated by the measurement method described above in [Porosity], the membrane thickness reduction rate/porosity when pressurized at 70°C and 2.3 MPa was calculated using the following formula 1, the membrane thickness reduction rate/porosity when pressurized at 70°C and 2.3 MPa was calculated using the following formula 2, and the porosity when pressurized at 70°C and 2.3 MPa was calculated using the following formula 3.

Formula 1: Film thickness reduction rate (%) when pressurized at 70° C. and 2.3 MPa=100×(T ₁ −T ₂ )/T ₁
Equation 2: Membrane thickness reduction rate when pressurized at 70° C. and 2.3 MPa/porosity (−)=100×(T ₁ −T ₂ )/(T ₁ ×porosity)
Equation 3: Porosity (%) at 70° C. and pressurized at 2.3 MPa=100×{T ₁ ×Porosity/100−(T ₁ −T ₂ )}/T ₂
Example 1
Resin A was a mixture consisting of 60% by mass of high-density polyethylene having a weight-average molecular weight (Mw) of 6.0 x 10 ⁵ and a melting point of 136° C., and 40% by mass of ultra-high molecular weight polyethylene having a weight-average molecular weight (Mw) of 2.0 x 10 ⁶ and a melting point of 133° C. Resin B was a cyclic olefin copolymer (COC) (COC TOPAS (registered trademark) 6017S-04 manufactured by Polyplastics) having a glass transition temperature of 178° C.

A mixture of 17% by mass of resin A, 8% by mass of resin B, 0.2% by mass of the antioxidant tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane, and 74.8% by mass of liquid paraffin was placed in a kneading evaluation test device (Labo Plastomill, manufactured by Toyo Seiki Co., Ltd.) and melt-kneaded for 15 minutes at a temperature of 200°C and a cylinder rotation speed of 50 rpm to prepare a polyolefin-based resin composition. The polyolefin-based resin composition was sandwiched between Teflon sheets and pressed at 200°C, after which a sample was removed and sandwiched between metal plates adjusted to 25°C, and cooled to form a gel-like sheet.

The gel-like sheet thus obtained was cut into a square shape of 80 mm square, preheated at 115°C for 300 seconds, and simultaneously biaxially stretched 5 times in the MD direction and 5 times in the TD direction at a stretching temperature of 115°C and a stretching speed of 1000 mm/min (first stretching). The stretched membrane was washed in a washing tank of methylene chloride to remove liquid paraffin, dried in a drying oven adjusted to 20°C, and heat-set (heat treatment) in an electric oven at 120°C for 10 minutes to obtain a polyolefin microporous membrane.
Example 2
Resin A was a mixture consisting of 60% by mass of branched high-density polyethylene having a weight-average molecular weight (Mw) of 4.0×10 ⁵ and a melting point of 136° C., and 40% by mass of ultra-high molecular weight polyethylene having a weight-average molecular weight (Mw) of 2.0×10 ⁶ and a melting point of 133° C. Resin B was not used. A mixture of 25% by mass of Resin A, 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant, and 74.8% by mass of liquid paraffin was charged into a kneading evaluation test device (Labo Plastomill manufactured by Toyo Seiki Co., Ltd.) and melt-kneaded at a temperature of 200° C. and a cylinder rotation speed of 50 rpm for 15 minutes to prepare a polyolefin-based resin composition. The polyolefin resin composition was sandwiched between Teflon sheets and pressed at 200° C., and then a sample was taken out and sandwiched between metal plates whose temperature was adjusted to 25° C., and cooled to form a gel-like sheet.

The gel-like sheet thus obtained was cut into a square shape of 80 mm square, preheated at 115°C for 300 seconds, and sequentially biaxially stretched at a stretching temperature of 115°C and a stretching speed of 1000 mm/min, first 9 times in the MD direction and then 9 times in the TD direction (first stretching). The stretched membrane was washed in a washing tank of methylene chloride to remove liquid paraffin, dried in a drying oven adjusted to 20°C, and heat-set (heat treatment) in an electric oven at 125°C for 10 minutes to obtain a polyolefin microporous membrane.
Example 3
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that a polyolefin resin composition was prepared using a mixture of 20 mass% of Resin A, 5 mass% of Resin B, 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant, and 74.8 mass% of liquid paraffin, using a cyclic olefin copolymer (COC) having a glass transition temperature of 138°C (COC TOPAS (registered trademark) 6013F-04 manufactured by Polyplastics) as Resin B.
Example 4
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that a polyolefin resin composition was prepared using a mixture of 20 mass% of Resin A, 5 mass% of Resin B, 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant, and 74.8 mass% of liquid paraffin, using a cyclic olefin polymer (COP) (COP ZEONOR (registered trademark) 1420R manufactured by Zeon Corporation) having a glass transition temperature of 136°C as Resin B.
(Comparative Example 1)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that resin A was a mixture of 80% by mass of high-density polyethylene and 20% by mass of ultra-high molecular weight polyethylene, resin B was not used, and a polyolefin resin composition was prepared using a mixture of 25% by mass of resin A, 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant, and 74.8% by mass of liquid paraffin.
(Comparative Example 2)
The same procedure as in Example 2 was repeated, except that Resin A was a mixture consisting of 60% by mass of high-density polyethylene having a weight-average molecular weight (Mw) of 6.0 x ^{105 and} a melting point of 136°C, and 40% by mass of ultra-high molecular weight polyethylene having a weight-average molecular weight (Mw) of 2.0 x 106 and a melting point of 133°C, to obtain a polyolefin microporous membrane.
(Comparative Example 3)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that a polyolefin resin composition was prepared using a mixture of 12 mass% of Resin A, 13 mass% of Resin B, 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant, and 74.8 mass% of liquid paraffin.
(Comparative Example 4)
The same procedure as in Example 3 was repeated, except that crosslinked acrylic beads (acrylic particles) having an average particle size of 2.2 μm (Art Pearl J-4P, manufactured by Negami Chemical Industrial Co., Ltd.) were used as Resin B, to obtain a polyolefin microporous membrane.
(evaluation)
The polyolefin resin composition and film-forming conditions used to produce each polyolefin microporous membrane, and the evaluation results of the resulting polyolefin microporous membrane are shown in Table 1. In Table 1, "-" in "Polyolefin resin composition" indicates that the component was not contained. Also, "-" in "Domain diameter in polyolefin microporous membrane" indicates that the presence of a domain could not be confirmed and therefore could not be measured.

The polyolefin microporous membranes of Examples 1 to 4 all have a value of 0.20 or less, which is expressed as the rate of decrease in membrane thickness when pressurized at 70°C and 2.3 MPa/porosity, and at the same time, they have good compression resistance and a low shutdown temperature of 140°C or less. Therefore, when used as a separator for a secondary battery, they can provide excellent output characteristics and high safety. On the other hand, the polyolefin microporous membranes of Comparative Examples 1 to 4 have poor compression resistance, high shutdown temperatures, or both, and therefore do not provide sufficient output characteristics or battery stability when used as a separator for a secondary battery.

When the polyolefin microporous membrane of the present invention is used as a separator for a secondary battery, it has excellent charge/discharge characteristics due to its high compression resistance, and can quickly shut down the battery when it abnormally heats up, thereby providing high safety, and therefore can be suitably used as a separator for a secondary battery that is particularly required to have a high capacity.

Claims (10)

A polyolefin microporous membrane having a value expressed as a membrane thickness reduction rate/porosity when pressurized at 70°C and 2.3 MPa of 0.20 or less, a shutdown temperature of 140°C or less, and comprising 50 to 99% by mass of a polyethylene-based resin and 1 to 50% by mass of a polymer incompatible with the polyethylene-based resin . The polyolefin microporous membrane according to claim 1, having a porosity of 35% or more. The polyolefin microporous membrane according to claim 1 or 2, which has a porosity of 30% or more when pressurized at 70°C and 2.3 MPa. The microporous polyolefin membrane according to any one of claims 1 to 3 , wherein the polymer incompatible with the polyethylene resin is an amorphous polymer. The microporous polyolefin membrane according to any one of claims 1 to 4 , wherein the polymer incompatible with the polyethylene resin is a cyclic olefin polymer. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein a first phase made of a polyethylene-based resin has a continuous structure, and a second phase made of a polymer incompatible with the polyethylene-based resin has a discontinuous structure, and the second phase has a domain diameter of 10 to 300 nm. The microporous polyolefin membrane according to any one of claims 1 to 6 , which has a heat shrinkage rate of 5% or less after 8 hours at 105°C. The microporous polyolefin membrane according to any one of claims 1 to 7 , having an air permeability per 1 µm thickness of 30 sec/100 ^cm3 or less. The microporous polyolefin membrane according to any one of claims 1 to 8 , having a pin puncture strength of 10 gf/μm or more per 1 μm thickness. The microporous polyolefin membrane according to any one of claims 1 to 9 , having a thickness of 5 µm or more.