JP7639341B2 - Polyolefin microporous membrane - Google Patents

Description

The present invention relates to a polyolefin microporous membrane that has excellent shutdown properties when used as a separator for a secondary battery.

Microporous membranes are used in a variety of fields, including as 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, etc. 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, as the energy density of lithium-ion secondary batteries has increased, the thermal stability of the electrodes used has tended to decrease (the onset temperature of thermal runaway has decreased). Therefore, the polyolefin microporous membranes used in secondary battery separators are required to shut down the battery at a lower temperature when the battery abnormally heats up due to a short circuit or overcharging, and to prevent thermal runaway by preventing ion migration between the positive and negative electrodes.

Patent Document 1 describes a microporous membrane that contains a polyolefin copolymer having a melting point of 122 to 126° C., has a shutdown temperature of 130.5° C. or lower, and has a relatively fast shutdown speed due to the use of a low-molecular-weight polyethylene component.
Patent Document 2 discloses a polyethylene microporous membrane having a pore blocking temperature Tf of 134° C. or lower and a relationship between the film melting temperature Tm and Tf such that Tm-Tf>0, and claims that when used as a battery separator, the membrane can prevent short circuits during overcharge.

Japanese Patent No. 5536872 Japanese Patent Application Publication No. 2000-204188

In Patent Documents 1 and 2, the microporous membrane achieves a low shutdown temperature by containing a polyethylene component with a low molecular weight and a low melting point, but the inclusion of a polyethylene component with a low molecular weight and a low melting point tends to result in a non-uniform pore structure. When a microporous membrane having such a pore structure is exposed to an atmosphere above the shutdown temperature, shutdown does not complete quickly, and when this membrane is used as a separator for a battery designed for high energy density, such as those being developed in recent years, safety issues may arise. Such issues are particularly noticeable in separators with high porosity and excellent permeability.

The object of the present invention is to solve the above problems. That is, to provide a polyolefin microporous membrane that, when used as a battery separator, has excellent permeability and can quickly complete shutdown when exposed to high temperatures inside the battery, thereby providing high safety.

In order to solve the above problems and achieve the object, the present invention has the following configuration.
[1]
A polyolefin microporous film having a gloss value of 45% or more in both the MD direction and the TD direction at an incidence angle of 60°, and an air permeability converted to a thickness of 10 μm of 300 sec/100 cm3 or ^less .
[2]
The polyolefin microporous film according to [1], wherein the average value of the gloss value in the MD direction and the gloss value in the TD direction at an incident angle of 60° is 50% or more.
[3]
The polyolefin microporous membrane according to [1] or [2], which has a shutdown temperature of 135° C. or lower.
[4]
The polyolefin microporous membrane according to any one of [1] to [3], which has a porosity of 35% or more.
[5]
The polyolefin microporous membrane according to any one of [1] to [4], having an average pore size of 35 nm or less.
[6]
The polyolefin microporous membrane according to any one of [1] to [5], which has a pin puncture strength of 1.5 N or more per 10 μm.
[7]
The microporous polyolefin membrane according to any one of [1] to [6], having a porous layer comprising inorganic particles and a resin binder on at least one surface thereof.

According to the present invention, it is possible to provide a polyolefin microporous membrane having high permeability and excellent shutdown properties.

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 has a gloss value of 45% or more in both the MD and TD directions at an incidence angle of 60°, and an air permeability converted to a 10 μm thickness of 300 sec/100 ^cm3 or less.

The gloss value is a value measured in accordance with JIS Z 8741:1997, and is an index showing the amount of reflected light (glossiness) when a sample is irradiated with light. The gloss value can be measured by the method described below. The gloss value of the microporous membrane according to the embodiment of the present invention at an incident angle of 60° is 45% or more in both the MD direction and the TD direction, preferably 48% or more, more preferably 50% or more, even more preferably 55% or more, and particularly preferably 60% or more.

When the gloss value of a polyolefin microporous membrane is within the above range, the pores are quickly blocked when the film is exposed to high temperatures, and when the film is used as a separator for a secondary battery, battery safety can be improved. In a polyolefin microporous membrane with a gloss value within the above range, the pores are densely and uniformly formed in the membrane surface structure, and the membrane has a structure with few voids. Therefore, it is presumed that the shutdown behavior proceeds uniformly within the film surface, and that shutdown is reached with minimal structural change, resulting in rapid completion of shutdown.

To achieve the above-mentioned gloss value range, the composition of the polyolefin raw material constituting the microporous membrane can be adjusted to the range described below, and the membrane production conditions can be set to the range described below.

The upper limit of the gloss value in the MD direction and the TD direction of the polyolefin microporous membrane according to the embodiment of the present invention is not particularly limited, but is, for example, 120% or less, and preferably 100% or less. By setting the upper limit of the gloss value in the MD direction and the TD direction in the above range, it is easy to achieve compatibility with permeability. In order to set the gloss value in the MD direction and the TD direction in the above range, it is preferable that the raw material composition of the film is within the range described later, and that the stretching conditions and heat setting conditions during film production are within the range described later.

The polyolefin microporous membrane according to the embodiment of the present invention has an air permeability (Gurley value) calculated based on a thickness of 10 μm of 300 sec/100 cm ³ or less. It is preferably 250 sec/100 cm ³ or less, more preferably 200 sec/100 cm ³ or less, and even more preferably 150 sec/100 cm ³ or less. When the air permeability is in the above range, when used as a battery separator, it has excellent ion permeability, and a secondary battery incorporating this separator has a lower impedance and improves battery output and rate characteristics. In addition, the lower limit of the air permeability calculated based on a thickness of 10 μm is, for example, 10 sec/100 cm ³ or more, preferably 50 sec/100 cm ³ or more. When the lower limit of the air permeability is in the above range, it is easy to achieve compatibility with the film strength. In order to set the air permeability calculated based on a thickness of 10 μm to the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions and heat setting conditions during film formation are in the range described later.

In the polyolefin microporous membrane according to an embodiment of the present invention, the average value (average gloss value) of the gloss value in the MD direction and the gloss value in the TD direction at an incidence angle of 60° is preferably 50% or more, more preferably 55% or more, even more preferably 60% or more, and particularly preferably 63% or more.

When the average gloss value of the polyolefin microporous membrane is within the above range, the pores are quickly blocked when the film is exposed to high temperatures, and when the film is used as a separator for a secondary battery, battery safety can be improved. In a polyolefin microporous membrane having an average gloss value within the above range, the pores are densely and uniformly formed in the membrane surface structure, and the membrane has a structure with few voids. Therefore, it is presumed that the shutdown behavior proceeds uniformly within the film surface, and that shutdown is achieved with minimal structural change, resulting in rapid completion of shutdown.

The upper limit of the average gloss value of the polyolefin microporous membrane according to an embodiment of the present invention is not particularly limited, but is, for example, 120% or less, and preferably 100% or less. By setting the upper limit of the average gloss value in the above range, it becomes easier to achieve compatibility with permeability. In order to set the average gloss value in the above range, it is preferable that the raw material composition of the film is within the range described below, and that the stretching conditions and heat setting conditions during film production are within the range described below.

The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 135°C or less. More preferably, it is 132°C or less, even more preferably, it is 130°C or less, and particularly preferably, it is 129°C or less. By controlling the shutdown temperature to 135°C or less, the battery safety is excellent when it is used as a battery separator. The lower limit of the shutdown temperature is not particularly limited, but it is preferably 100°C or more because it can suppress the decrease in ion permeability due to pore blockage during film formation. In order to set the shutdown temperature in the above range, it is preferable that the raw material composition of the film is within the range described later, and that the stretching conditions and heat setting conditions during film formation are within the range described later.

The shutdown temperature is measured using the method described below. When the polyolefin microporous membrane is heated, the resin shrinks and melts, closing the porous structure, making it essentially impossible to discharge or charge when used as a battery separator.

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, and even more preferably 45% or more. There is no particular upper limit to the porosity, but it is preferably 75% or less because this can suppress a decrease in membrane strength. By having a porosity in the above range, the output characteristics are excellent when used as a separator for a secondary battery.

The porosity can be adjusted to the above range by adjusting the blend ratio of the polyolefin components, the stretching ratio, the heat setting conditions, etc. during the manufacturing process.

The average pore size (average flow diameter) of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 35 nm or less, more preferably 32 nm or less, even more preferably 30 nm or less, and particularly preferably 27 nm or less. A separator having an average pore size in the above range has an excellent balance between strength and permeability, and when the polyolefin microporous membrane is used as a battery separator, shutdown is quickly completed in the event of abnormal heat generation, resulting in excellent battery safety. There is no particular lower limit for the average pore size, but a value of 5 nm or more is preferable because good permeability can be easily obtained.

The average pore size is measured using a method in accordance with ASTM E1294-89 (half-dry method) and can be measured using the method described below.

The puncture strength per 10 μm of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 1.5 N or more, more preferably 1.8 N or more, even more preferably 2.2 N or more, and particularly preferably 2.5 N or more. There is no particular upper limit to the puncture strength per 10 μm, but it is preferably, for example, 8.0 N or less, since this makes it easier to control the shutdown temperature within an appropriate range. When the puncture strength per 10 μm is within the above range, the occurrence of short circuits of the electrodes is suppressed in a secondary battery using this as a separator.

When producing a polyolefin microporous membrane, the 10 μm equivalent puncture strength can be adjusted to the above range by adjusting the weight average molecular weight (Mw) of the raw material resin and the stretching conditions.

The thickness of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 20 μm or less, more preferably 15 μm or less, even more preferably 13 μm or less, and particularly preferably 9 μm or less. There is no particular lower limit to the thickness, but it is preferably 1 μm or more because it can suppress the occurrence of short circuits when used as a separator for a secondary battery. When the thickness is within the above range, the battery capacity is improved by using the polyolefin microporous membrane 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. The main component referred to here refers to the component that constitutes the polyolefin microporous membrane in the largest mass%. The polyolefin resin is preferably contained in an amount of 80 mass% or more, more preferably 90 mass% or more, based on the total mass of the polyolefin microporous membrane.

Examples of polyolefin-based resins include polyethylene-based resins and polypropylene-based resins, and using polyethylene-based resins as the main component is preferable from the standpoint of functional balance such as shutdown behavior, strength, and permeability.

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

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

The polyolefin microporous membrane according to an embodiment of the present invention may be a single-layer membrane, but it can also be a laminate membrane consisting of at least two layers having different properties, with layer A being made of polyolefin resin A having a polyolefin resin as the main component, and layer B being made of a polyolefin resin (referred to as polyolefin resin B) having different properties as the main component.

Examples of layer configurations include layer A only, layer A/layer B, layer A/layer B/layer A, and layer B/layer A/layer B. Of these, layer A only and layer A/layer B/layer A configurations are preferred, and layer A/layer B/layer A configuration is particularly preferred. By using the layer configurations as described above, it becomes easy to set the gloss value of the polyolefin microporous membrane according to an embodiment of the present invention within an appropriate range, and good shutdown properties and permeability can be achieved at the same time.

In the polyolefin microporous membrane according to the embodiment of the present invention, the ratio of the thickness of layer A to the total thickness of all layers is preferably 50% or less. More preferably, it is 40% or less, even more preferably 30% or less, and most preferably 20% or less. In the polyolefin microporous membrane according to the embodiment of the present invention, by setting the raw materials constituting layer A within the range described below, it is possible to effectively impart a shutdown function even if the ratio of the thickness of layer A is reduced.

When the polyolefin microporous membrane according to the embodiment of the present invention has a plurality of A layers, the total thickness of the A layers is preferably 6.0 μm or less. More preferably, it is 5.0 μm or less, even more preferably 4.0 μm or less, and most preferably 3.0 μm or less. The polyolefin microporous membrane according to the embodiment of the present invention can effectively impart a shutdown function even if the thickness of the A layer is thin by setting the raw materials constituting the A layer to the range described below.

From the viewpoint of melt extrusion characteristics and stretch processing characteristics, the polyolefin resin A preferably contains high density polyethylene (density: 0.940 g/m ³ or more and 0.970 g/m ³ or less). The polyethylene-based resin (polyethylene (A)) used in the polyolefin resin A is preferably not only a homopolymer of ethylene but also a copolymer containing other α-olefins in order to lower the melting point and crystallinity of the polyolefin resin A. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. As the copolymer containing an α-olefin, a copolymer containing hexene-1 (ethylene-1-hexene copolymer) is preferable, and it is more preferable that the polyethylene (A) is mainly composed of an ethylene-1-hexene copolymer. In addition, the α-olefin can be confirmed by measuring with C ¹³ -NMR.

It is further preferable that polyolefin resin A contains branched polyethylene, since this allows for the formation of a uniform fine pore structure and suppresses crystal orientation during stretching, resulting in a polyolefin microporous film with excellent shutdown characteristics.

The branched polyethylene is preferably contained in an amount of 10% by mass or more, more preferably 30% by mass or more, even more preferably 50% by mass or more, and particularly preferably 70% by mass or more, based on the total mass of layer A in the polyolefin microporous membrane. This makes it easy to set the gloss value of the polyolefin microporous membrane according to the embodiment of the present invention within an appropriate range, and a polyolefin microporous membrane having excellent shutdown properties can be obtained.

The lower limit of the weight average molecular weight of the branched 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 branched polyethylene is 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 polyethylene within the above range, both a low shutdown temperature and film strength during shutdown can be achieved.

The melting point of the branched polyethylene is preferably 136°C or less, more preferably 135°C or less, and even more preferably 133°C or less. It is also preferably 115°C or more, more preferably 120°C or more, and even more preferably 125°C or more. By setting the melting point 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 polyethylene can be measured by the method described below.

Furthermore, by adding low-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymer produced by a single-site catalyst, low-molecular-weight polyethylene having a weight-average molecular weight of 1000 to 100000, or the like as polyolefin resin A, a shutdown function at low temperatures is imparted, and the properties as a battery separator can be improved. However, if the proportion of the above-mentioned low-molecular-weight polyethylene added as polyolefin resin A is large, the porosity of the microporous film decreases in the film-forming process, so it is preferable to add high-density polyethylene having a density of more than 0.94 g/ ^cm3 as an ethylene-α-olefin copolymer, and it is even more preferable to add branched polyethylene containing long-chain branches, since it is possible to maintain the porosity while improving the shutdown properties as described above.

From the above viewpoints, it is preferable that the molecular weight distribution of the polymer constituting the polyolefin microporous membrane according to an embodiment of the present invention is such that the amount of components having a molecular weight of less than 40,000 is less than 20%. More preferably, the amount of components having a molecular weight of less than 20,000 is less than 20%, and even more preferably, the amount of components having a molecular weight of less than 10,000 is less than 20%. By using the above-mentioned raw materials, it is possible to lower the shutdown temperature without significantly reducing the molecular weight, and as a result, it is possible to achieve compatibility with other physical properties such as strength and porosity.

From the viewpoints of melt extrusion characteristics, stretch processing characteristics, and the permeability and strength of the resulting polyolefin microporous film, it is preferable that polyolefin resin B contains high-density polyethylene, and it is preferable that it contains 10 mass% or more of high-density polyethylene relative to the total mass of layer B in the polyolefin microporous film, more preferably 30 mass% or more, even more preferably 50 mass% or more, and particularly preferably 60 mass% or more.

The lower limit of the weight average molecular weight of the high density polyethylene is preferably 1 x 10 ⁵ or more, more preferably 2 x 10 ⁵ or more, and even more preferably 3 x 10 ⁵ or more. The upper limit of the weight average molecular weight of the high density polyethylene is 1 x 10 ⁶ or less, more preferably 8.0 x 10 ⁵ or less, and even more preferably 6.0 x 10 ⁵ or less. By setting the weight average molecular weight of the high density polyethylene contained in polyolefin resin B within the above range, good film formation stability can be obtained when laminated with layer A, and in addition, good permeability and film strength can be obtained.

The melting point of the high-density polyethylene is preferably 135°C to 140°C. By setting the melting point of the high-density polyethylene in the above range, good permeability and membrane strength can be obtained. The melting point of the high-density polyethylene can be measured by the method described below.

The use of a polypropylene-based resin as part of the polyolefin-based resin contained in polyolefin resin A and polyolefin resin B is preferable from the viewpoint of improving the meltdown temperature when the polyolefin microporous membrane according to the embodiment of the present invention is used as a separator for a secondary battery. The meltdown temperature is the temperature at which the separator melts and breaks when the battery generates abnormal heat, and a higher temperature is preferable from the viewpoint of battery safety. In addition to homopolypropylene, block copolymers and random copolymers can also be used as the type of polypropylene-based resin. The block copolymers and random copolymers can contain copolymer components with α-olefins other than propylene, and ethylene is preferred as the α-olefin.

The amount of polypropylene resin added to the polyolefin microporous membrane is preferably 30% by mass or less, more preferably 20% by mass or less, based on the total mass of the polyolefin microporous membrane. By setting the amount to 30% by mass or less, it becomes easier to control the shutdown temperature, strength, and permeability within appropriate ranges.

Polyolefin resin A and polyolefin resin B may contain resin components other than polyethylene-based resin and polypropylene-based resin as necessary. In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be contained within a range that does not impair the effects of the present invention.

The polyolefin microporous membrane according to an embodiment of the present invention may have one or more coating layers on at least one surface.

Examples of the coating layer include other porous layers other than polyolefin resins. The other porous layers are not particularly limited, but are preferably porous layers such as an inorganic particle layer containing a resin binder and inorganic particles. The resin 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, silicon, etc.

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 following describes a wet method for producing a polyolefin microporous membrane. Note that the following description is an example of a 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), and may further include the following step (6), and may further include at least one of the following steps (7) and (8).

(1) A process for preparing a polyolefin-based resin composition by melt-kneading the polyolefin-based resin and a membrane-forming solvent; (2) A process for extruding the polyolefin-based resin composition and cooling it to form a gel-like sheet; (3) A first stretching process for preheating and stretching the gel-like sheet; (4) A process for removing the membrane-forming solvent from the gel-like sheet after the stretching; (5) A process for drying the sheet after the membrane-forming solvent has been removed; (6) A second stretching process for preheating and stretching the dried sheet; (7) A process for heat-treating the dried sheet; and (8) A process for crosslinking and/or hydrophilizing the dried sheet.

Each step will be described below. In the following description, a numerical range expressed using "~" means a range that includes the numerical values before and after "~" as the lower and upper limits.

(1) Preparation of polyolefin resin composition A polyolefin resin composition is prepared by heating and dissolving the polyolefin resin in a plasticizer (membrane-forming solvent). The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, 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, alicyclic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding to these, as well as phthalic acid esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. 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 polyolefin resin to plasticizer may be appropriately selected within a range that does not impair moldability, but the content of polyolefin resin is preferably 10 to 50 mass% relative to the total mass of the polyolefin resin composition. If the polyolefin resin 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 polyolefin resin is 50 mass% or less, shrinkage in the thickness direction is suppressed, resulting in good moldability.

The method for uniformly melt-kneading the polyolefin resin and 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 for a polyolefin resin composition consisting of polyolefin resin A, 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 200°C or lower, and even more preferably 180°C or lower.

Furthermore, for the polyolefin resin composition consisting of polyolefin resin B, the temperature is preferably 170°C or higher, more preferably 180°C or higher, and even more preferably 190°C or higher, with the upper limit being preferably 250°C or lower, more preferably 230°C or lower, and even more preferably 220°C or lower.

By keeping the temperature of the polyolefin resin composition within the above range during kneading, it is possible to prevent a decrease in strength due to deterioration of the resin, and the polyolefin resin and plasticizer can be melt-kneaded 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 0.05 or more, more preferably 0.1 or more, and even more preferably 0.15 or more. This makes it possible to prevent a decrease in strength due to resin deterioration during kneading. The upper limit is preferably 3.0 or less, more preferably 2.0 or less, and particularly preferably 1.0 or less, which allows sufficient shear to be applied to the polyolefin resin composition and allows a uniform dispersion state to be obtained.

(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 a flat die method or an inflation method. In addition, multiple polyolefin resin compositions of the same or different compositions may be fed from multiple extruders to one multi-manifold type composite T-die, laminated in layers, and extruded into a sheet of 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 higher-order structure of the gel-like sheet becomes dense, leading to uniform stretching and improved strength and elongation of the film. The cooling method at this time includes direct contact with cold air, cooling water, or other cooling medium, contact with a roll cooled with a refrigerant, and the use of a casting drum.

(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 to 120°C, and even more preferably 110 to 117°C. 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 preheating time is preferably 5 seconds or more, more preferably 15 seconds or more, even more preferably 60 seconds or more, and particularly preferably 180 seconds or more. By setting the preheating time within the above range, the temperature distribution in the gel sheet becomes uniform, the crystal structure is stabilized, and the film is uniformly stretched in the subsequent stretching step, resulting in a polyolefin microporous film having a uniform fine pore structure. There is no particular upper limit to the preheating time, but from the viewpoint of productivity, it is preferably 1000 seconds or less.

The preheated gel-like sheet 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 stretching, or multi-stage stretching (e.g., a combination of simultaneous biaxial stretching and sequential stretching) may be used.

The stretching ratio (area stretching ratio) in this process is preferably 16 times or more, more preferably 25 times or more. In addition, it is preferable that the stretching ratio is 5 times or more in both the machine longitudinal direction and the machine width direction (MD direction and TD direction), and 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 in the above range, it is possible to increase the mechanical strength and permeability. In addition, the area stretching ratio is preferably 100 times or less, more preferably 80 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 within the range of TCD + 5°C to TCD + 28°C, and particularly preferably within the range of TCD + 10°C to 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 to 120°C, and even more preferably 110 to 117°C.

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

(4) Removal of membrane-forming solvent The membrane-forming solvent is removed (washed) using a washing solvent. Since the polyolefin resin phase is phase-separated from the membrane-forming solvent phase, 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. The washing solvent and the method of removing the membrane-forming solvent using the same are publicly known, so the explanation is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Publication No. 2002-256099 can be used.

(5) Drying 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 film may be preheated before the second stretching step. The preheating temperature is preferably 90 to 140°C, more preferably 95 to 135°C, and even more preferably 100 to 130°C. The polyolefin microporous membrane may be stretched by a tenter method, a roll method, an inflation method, or the like while being heated, as described above. The stretching may be uniaxial or biaxial. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used.

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

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 The polyolefin microporous membrane after drying may be heat treated. The heat treatment stabilizes the crystals and makes the lamellae uniform. Heat setting and/or heat relaxation may be used as the heat treatment method. 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 direction during heating. The heat setting is preferably performed by a tenter method or a roll method. For example, the heat relaxation method may be the method disclosed in Japanese Patent Application Laid-Open No. 2002-256099. The heat treatment temperature is preferably within the range of the TCD to the 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 Treatment, Hydrophilization Treatment Furthermore, the dried polyolefin microporous membrane may be further subjected to a crosslinking treatment and a hydrophilization treatment.

For example, a crosslinking treatment is performed on a polyolefin microporous membrane by irradiating it with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. In the case of electron beam irradiation, an electron beam dose of 0.1 to 100 MRAD and an acceleration voltage of 100 to 300 KV are preferred. The crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane.

Hydrophilization can be achieved by monomer grafting, surfactant treatment, corona discharge, etc. It is preferable to carry out monomer grafting after crosslinking.

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

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to these examples. In addition, Examples 1, 3, 5, and 7 should be read as Reference Examples 1, 3, 5, and 7, respectively.

[Measurement method]
[Film thickness]
The thickness of the polyolefin microporous membrane was measured at five points within a 50 mm x 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 × 50 mm square sample was cut from the polyolefin microporous membrane, and its volume (cm ³ ) and mass (g) were measured at room temperature of 25° C. From these values and the membrane density (g/cm ³ ), the porosity of the polyolefin microporous membrane was calculated according to the following formula.

Porosity (%) = (volume - mass / membrane density) / volume x 100

The film density was calculated assuming a constant value of 0.99 g/cm ³ .

[10 μm equivalent puncture strength]
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 (N) 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), and the puncture strength was calculated using the following formula when the membrane thickness was 10 μm.

Formula: Puncture strength (10 μm equivalent) (N) = Maximum load (N) x 10 (μm) / Polyolefin microporous membrane thickness (μm)

[Air permeability]
For a polyolefin microporous membrane having a thickness T ₁ (μm), the air permeability (sec/100 cm ³ ) 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. In addition, the air permeability (converted to 10 μm) (sec/100 cm ³ ) when the membrane thickness was set to 10 μm was calculated using the following formula.

Formula: air permeability (10 μm equivalent) (sec/100 cm ³ )=air permeability (sec/100 cm ³ )×10 (μm)/film thickness of polyolefin microporous membrane (μm)

[Gloss value]
The measurement was carried out in accordance with JIS Z-8741 (1997) using a digital variable angle glossmeter UGV-5D manufactured by Suga Test Instruments Co., Ltd. under conditions of an incident angle of 60° and a receiving angle of 60°.

Measurements were taken at three points on both surfaces of the polyolefin microporous membrane so that the incident and received light of the glossmeter were parallel to the MD direction of the film sample, and the average of the three gloss values measured at each surface was calculated. The value of the surface with the highest average value was used as the gloss value in the MD direction of the polyolefin microporous membrane.

In addition, measurements were taken at three points on both surfaces of the polyolefin microporous membrane so that the incident and received light of the glossmeter were parallel to the TD direction of the film sample, and the average value of the three gloss values measured at each surface was calculated. The value of the surface with the highest average value was used as the gloss value in the TD direction of the polyolefin microporous membrane.

In addition, the average of the MD gloss value and the TD gloss value calculated using the above-mentioned method was calculated as the average gloss value.

[Shutdown temperature]
The polyolefin microporous membrane was heated at a heating rate of 5°C/min while measuring the air permeability resistance with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.), and the temperature at which the air permeability resistance reached the detection limit of 1 x ¹⁰⁵ seconds/100 ^cm3 Air was determined and recorded as the shutdown temperature (°C).

The measurement cell was constructed from an aluminum block and had a thermocouple directly below the polyolefin microporous membrane. The sample was cut into a 5 cm x 5 cm square and the temperature was measured while it was fixed around the periphery with an O-ring.

[Average pore size]
Measurements were performed using a perm porometer (CFP-1500A, manufactured by PMI). GALWICK (surface tension: 15.9 dynes/cm) was used as the impregnating liquid for the polyolefin microporous membrane, and measurements were performed in the order of dry-up and wet-up. The average pore size (average flow size) (nm) was calculated from the pressure at the point where the curve showing the half slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement, and this was taken as the average pore size. The following formula was used to convert between pressure and pore size of the microporous membrane.

d = C γ / P
(In the above formula, "d (nm)" is the pore size of the microporous membrane, "γ (dynes/cm)" is the surface tension of the impregnating liquid, "P (KPa)" is the pressure, and "C" is a constant, which was set to 2860.)

[Weight average molecular weight of polyolefin resin]
The weight average molecular weight (Mw) and molecular weight distribution (MwD) of the polyolefin resin were determined by gel permeation chromatography (GPC) under the following conditions.
Measurement device: GPC-150C manufactured by Waters Corporation
Column: Shodex UT806M manufactured by Showa Denko K.K.
Column temperature: 135° C.
Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml/min Sample concentration: 0.1 wt% (dissolution conditions: 135°C/1h)
Injection volume: 500 μl
Detector: Waters Corporation differential refractometer (RI detector)
Calibration curve: A calibration curve was prepared using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

Differential Scanning Calorimetry (DSC)
The melting point of the raw polyolefin resin was measured by differential scanning calorimetry (DSC). 6.0 mg of the sample was sealed in an aluminum pan, and using a Parking Elmer PYRIS Diamond DSC, the sample was heated from 30°C to 230°C at 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 heating rate of 10°C/min from 30°C to 230°C (second heating). The temperature at the peak top of the melting peak obtained in the second heating was taken as the melting point of the resin raw material.

Example 1
100% by mass of branched polyethylene having a weight average molecular weight (Mw) of 2.5×10 ⁵ and a melting point of 133° C. was used as polyolefin resin A, and 39.8% by mass of the polyolefin resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant were mixed and charged into a twin-screw extruder, 60% by mass of liquid paraffin was fed from a side feeder of the twin-screw extruder, and the mixture was melt-kneaded under the conditions of a temperature of 160° C., a cylinder rotation speed of 250 rpm, and Q/Ns=0.2 to prepare a polyolefin resin composition A. The polyolefin resin composition A was fed into a T-die, extruded into a sheet, and cooled while being taken up with a cooling roll adjusted to a temperature of 30° C. at a take-up speed of 4 m/min to form a gel-like sheet.

The gel-like sheet obtained was cut into a square of 80 mm on each side, preheated at 110°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 110°C and a stretching speed of 1000 mm/min. 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 in an electric oven at 120°C for 10 minutes to obtain a polyolefin microporous membrane.

Example 2
Polyolefin resin A was 100% by mass of branched polyethylene having a weight average molecular weight (Mw) of 2.5 x ¹⁰⁵ and a melting point of 133°C. 39.8% by mass of the polyolefin resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant were mixed and charged into a twin-screw extruder. 60% by mass of liquid paraffin was fed from a side feeder of the twin-screw extruder, and the mixture was melt-kneaded under the conditions of a temperature of 160°C, a cylinder rotation speed of 250 rpm, and Q/Ns = 0.2 to prepare polyolefin resin composition A.

As polyolefin resin B, 60 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 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 were used, and 24.8 mass% of the above polyolefin resin B and 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant were mixed and charged into a twin-screw extruder. 75 mass% of liquid paraffin was supplied from the side feeder of the twin-screw extruder, and the above mixture was melt-kneaded under the conditions of a temperature of 200°C, a cylinder rotation speed of 250 rpm, and Q/Ns = 0.2 to prepare polyolefin resin composition B. Polyolefin resin composition A and polyolefin resin composition B were fed from each twin-screw extruder to a three-layer T-die and extruded into a sheet having a layer structure of A layer/B layer/A layer with a lamination ratio of 1/3/1. This was cooled while being taken up with a cooling roll adjusted to 30° C. at a take-up speed of 8 m/min to form a gel-like sheet.

The gel-like sheet obtained was cut into a square of 80 mm on each side, 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. 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 in an electric oven at 120°C for 10 minutes to obtain a polyolefin microporous membrane.

Example 3
The same procedure as in Example 2 was repeated, except that a mixture of 80 mass% branched polyethylene having a weight average molecular weight (Mw) of 2.5 x ¹⁰⁵ and a melting point of 133°C and 20 mass% homopolypropylene having a Mw of 1.0 x ¹⁰⁶ and a melting point of 163°C was used as polyolefin resin A, and the mixture was cooled while being taken up at a take-up speed of 6 m/min to form a gel-like sheet.

Example 4
100% by mass of branched polyethylene having a weight average molecular weight (Mw) of 2.5×10 ⁵ and a melting point of 133° C. was used as polyolefin resin A, and 29.8% by mass of the polyolefin resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant were mixed and charged into a twin-screw extruder, 70% by mass of liquid paraffin was fed from a side feeder of the twin-screw extruder, and the mixture was melt-kneaded under conditions of a temperature of 160° C., a cylinder rotation speed of 250 rpm, and Q/Ns=0.2 to prepare polyolefin resin composition A. Polyolefin resin composition A and polyolefin resin composition B were fed from each twin-screw extruder to a three-layer T-die and extruded into sheets with a layer structure of A layer/B layer/A layer with a lamination ratio of 1/5/1. The gel-like sheet obtained was cut into a square of 80 mm square, preheated at 110°C for 300 seconds, and simultaneously biaxially stretched at a stretching temperature of 110°C and a stretching speed of 1000 mm/min to 5 times in the MD direction and 5 times in the TD direction. The stretched membrane was washed in a washing tank of methylene chloride to remove liquid paraffin, and the washed membrane was dried in a drying oven adjusted to 20°C and heat-set in an electric oven at 120°C for 10 minutes to obtain a polyolefin microporous membrane. The same procedure as in Example 2 was carried out except for the above.

Example 5
Polyolefin resin A was prepared by mixing 60 mass% of branched polyethylene having a weight average molecular weight (Mw) of 2.5 x 10 ⁵ and a melting point of 133°C, and 40 mass% of ultra-high molecular weight polyethylene having a Mw of 2.5 x 10 ⁶ and a melting point of 127°C. 24.8 mass% of the above polyolefin resin A and 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant. The mixture was then charged into a twin-screw extruder, and 75 mass% of liquid paraffin was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded under the conditions of a temperature of 180°C, a cylinder rotation speed of 250 rpm, and Q/Ns = 0.2 to prepare polyolefin resin composition A. Polyolefin resin composition A was fed to a T-die, extruded into a sheet, and cooled while being taken up at a take-up speed of 4 m/min using a cooling roll adjusted to 30° C. to form a gel-like sheet.

The gel-like sheet obtained was cut into a square of 80 mm on each side, preheated at 110°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 110°C and a stretching speed of 1000 mm/min. 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 in an electric oven at 115°C for 10 minutes to obtain a polyolefin microporous membrane.

Example 6
The same procedure as in Example 2 was repeated, except that 80% 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 20% by mass of branched polyethylene (Sumikathene EP CU5003 manufactured by Sumitomo Chemical Co., Ltd.) having an MFR of 0.4 g/min measured at 190°C and 2.16 kgf and a melting point of 125°C were used as polyolefin resin A, and 29.8% by mass of the polyolefin resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant were mixed and fed into a twin-screw extruder, and 70% by mass of liquid paraffin was supplied from a side feeder of the twin-screw extruder.

(Example 7)
The same procedure as in Example 5 was repeated, except that polyolefin resin A was made of 60% by mass of branched polyethylene having a weight average molecular weight (Mw) of 2.5 x ¹⁰ and a melting point of 133°C, and 40% by mass of ultra-high molecular weight polyethylene having Mw of 2.0 x ¹⁰ and a melting point of 133°C.

(Comparative Example 1)
Polyolefin resin A was prepared by mixing 25% by mass of linear low-density polyethylene having a weight average molecular weight (Mw) of 5 x ^{10 and} a melting point of 126°C, 55% by mass of high-density polyethylene having a weight average molecular weight (Mw) of 6.0 x ¹⁰ and a melting point of 136°C, and 20% by mass of ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 2.0 x 10 and a melting point of 133°C. 24.8% by mass of the polyolefin resin A and 0.2% by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant, and feeding the mixture into a twin-screw extruder. 75% by mass of liquid paraffin was fed from a side feeder of the twin-screw extruder, and the mixture was melt-kneaded under the conditions of a temperature of 160°C, a cylinder rotation speed of 250 rpm, and Q/Ns = 0.2 to prepare polyolefin resin composition A. The polyolefin resin composition A and the polyolefin resin composition B were fed from each twin-screw extruder to a three-layer T-die and extruded into a sheet having a layer structure of A layer/B layer/A layer with a lamination ratio of 1/1/1. This was cooled while being taken up at a take-up speed of 8 m/min with a cooling roll adjusted to 30° C., to form a gel-like sheet. The same procedure as in Example 2 was carried out except for the above.

(Comparative Example 2)
As polyolefin resin A, 60 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 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 were used, and 24.8 mass% of the above polyolefin resin A and 0.2 mass% of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant were mixed and charged into a twin-screw extruder. 75 mass% of liquid paraffin was supplied from the side feeder of the twin-screw extruder, and the above mixture was melt-kneaded under the conditions of a temperature of 200°C, a cylinder rotation speed of 250 rpm, and Q/Ns = 0.2 to prepare polyolefin resin composition A. Polyolefin resin composition A was fed to a T-die, extruded into a sheet, and cooled while being taken up at a take-up speed of 5 m/min using a cooling roll adjusted to 30° C. to form a gel-like sheet.

The gel-like sheet obtained was cut into a square of 80 mm on each side, 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. 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 in an electric oven at 120°C for 10 minutes to obtain a polyolefin microporous membrane.

(Comparative Example 3)
The same procedure as in Comparative Example 2 was repeated, except that the film was cooled while being taken up at a take-up speed of 8 m/min using a cooling roll adjusted to 30° C. to form a gel-like sheet, and that the film was heat-set in an electric oven at 128° C. for 10 minutes.

(evaluation)
The polyolefin microporous membranes of Examples 1 to 7 have good gloss values, so that the shutdown behavior proceeds uniformly on the film surface and the structural change until shutdown is small, and therefore, when used as a separator for a secondary battery, they can provide excellent shutdown characteristics and high safety, and also have excellent permeability and therefore excellent output characteristics. On the other hand, the polyolefin microporous membranes of Comparative Examples 1 and 2 have low gloss values and high shutdown temperatures, so that when used as a separator for a secondary battery, shutdown takes a long time and the battery stability is insufficient. The polyolefin microporous membrane of Comparative Example 3 has high air permeability and therefore has insufficient output characteristics as a separator for a secondary battery.

The polyolefin microporous membrane of the present invention, when incorporated into a secondary battery as a separator, has excellent permeability while quickly completing shutdown when exposed to high temperatures inside the battery, thereby providing high safety. Therefore, it can be suitably used as a separator with excellent safety, particularly in secondary batteries that require high capacity.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

This application is based on a Japanese patent application (Patent Application No. 2019-152107) filed on August 22, 2019, the contents of which are incorporated herein by reference.

Claims (10)

A laminated film having a gloss value of 45% or more at an incidence angle of 60° in both MD and TD directions and an air permeability of 250 sec/100 ^cm3 or less converted to a thickness of 10 μm , the laminated film including a layer A made of a polyolefin resin A containing a polyolefin resin as a main component, and a polyolefin microporous film containing 10 mass% or more of branched polyethylene relative to the total mass of the layer A. The polyolefin microporous film according to claim 1, wherein the laminated film is a laminated film consisting of at least two layers having different properties. The polyolefin microporous film according to claim 1 or 2, in which the average value of the gloss value in the MD direction and the gloss value in the TD direction at an incidence angle of 60° is 50% or more. The polyolefin microporous membrane according to any one of claims 1 to 3, which has a shutdown temperature of 135°C or less. The polyolefin microporous membrane according to any one of claims 1 to 4, having a porosity of 35% or more. The polyolefin microporous membrane according to any one of claims 1 to 5, having an average pore size of 35 nm or less. The microporous polyolefin film according to any one of claims 1 to 6, which has a puncture strength of 1.5 N or more per 10 μm. The microporous polyolefin membrane according to claim 7 , wherein the ratio of the thickness of layer A made of polyolefin resin A to the total thickness of all layers is 50% or less. The polyolefin microporous membrane according to claim 8 , which has a plurality of the A layers, and the total thickness of the A layers is 6.0 μm or less. The microporous polyolefin membrane according to any one of claims 1 to 9 , having a porous layer comprising inorganic particles and a resin binder on at least one surface thereof.