JP2023117573A - Polyolefin multilayer microporous membrane - Google Patents

Abstract

To provide a polyolefin multilayer microporous membrane which has excellent melt-down characteristics, and has good impact resistance and cycle characteristics, in a battery design requiring high energy density.SOLUTION: A polyolefin multilayer microporous membrane has at least two layers of a first microporous layer containing polypropylene, and a second microporous layer containing polyethylene, wherein a porosity difference between the first microporous layer and the second microporous layer in the cross section of the polyolefin multilayer microporous membrane is 0% or more and 5% or less, and piercing strength in terms of a thickness of 9 μm and porosity of 40% is 330 cN or more.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a polyolefin multilayer microporous membrane suitable for use as a battery separator.

Polyolefin microporous membranes are widely used as battery separators. By interposing between the positive electrode and the negative electrode, they prevent short circuits due to contact between the active materials of both electrodes, and form ion conduction paths through the electrolyte retained in the pores. can do. Further, separators are required to have meltdown characteristics, permeability, mechanical characteristics, impedance characteristics and the like from the viewpoint of battery safety and battery performance. Furthermore, in recent years, it is feared that the increase in the energy density of the battery will increase the volume of the electrodes and the generation of gas, which will compress the separator and crush the fine pores, which will affect electrical resistance and cycle characteristics.

Patent Literature 1 discloses that the electrolyte solution injectability and mechanical strength are improved by setting the average pore size difference of each layer of the polyolefin multilayer microporous membrane to 10 nm or more.

In Patent Document 2, in a polyolefin multilayer microporous membrane, the pore size of the inner layer is designed to be 15 nm or more larger than the surface layer, and the layer thickness ratio is also designed so that the inner layer is thinner than the surface layer. It is disclosed that injectability and the like are improved.

Patent Document 3 describes that the meltdown property, oxidation resistance, and the like are improved by uniformly dispersing polypropylene.

Patent Document 4 discloses that a monolayer polyolefin microporous membrane containing no polypropylene having an average pore size of 50 nm or less has excellent compression resistance and improves the cycle characteristics of the battery.

Patent Document 5 discloses that in a polyolefin multilayer microporous membrane, the difference in melting point between the inner layer and the surface layer is less than 10°C and the difference in viscosity-average molecular weight is 500,000 or less, thereby improving withstand voltage and permeability. there is

WO2007/037289 JP 2008-255307 A WO2014/192862 WO2015/194504 JP 2010-36355 A

In recent years, in order to increase the energy density of batteries, the proportion of nickel in the positive electrode has been increased and silicon has been introduced in the negative electrode. In particular, silicon for the negative electrode has a theoretical capacity about 10 times that of conventional graphite. On the other hand, the volume expansion of silicon during charging is three times or more that of graphite, which compresses the separator and clogs the fine pores. As a result, there is a problem that battery characteristics, particularly cycle and characteristics, deteriorate. Therefore, the separator is required to be resistant to crushing even when compressed, that is, to have compression resistance. It has been found that the higher the weight per unit volume of the separator, ie, the lower the porosity, the better the compression resistance. However, when the porosity is lowered, the electrical resistance increases, so it is difficult to achieve both compression resistance and electrical resistance. In the prior art as described in Patent Documents 1 to 5, it is impossible to achieve both the levels of compression resistance and electrical resistance required in recent years while maintaining high-temperature meltdown characteristics related to battery safety. difficult.

An object of the present invention is to provide a multi-layer, microporous polyolefin membrane that achieves both compression resistance and electrical resistance as described above. Further, as a more preferred embodiment of the present invention, in addition to the above problems, the object is to provide a multi-layer, microporous polyolefin membrane that also has high-temperature meltdown properties.

As a result of intensive studies in view of the above problems, the present inventors have found that a polyolefin multilayer microporous membrane comprising two or more layers, a first microporous layer containing polyolefin and a second microporous layer containing polyethylene, has voids in each layer. We have found that by keeping the difference in porosity within a certain range, it is possible to form a multi-layer, microporous polyolefin membrane that achieves high levels of compression resistance and electrical resistance even when silicon, which has a high expansion coefficient, is used for the negative electrode. I have completed my invention.

The present invention mainly consists of the following configurations. i.e.
The multi-layer, microporous polyolefin membrane of the present invention has the following properties (1) to (8).
(1) A polyolefin multilayer microporous membrane having at least two layers, a first microporous layer containing polyolefin and a second microporous layer containing polyethylene, wherein the first microporous layer in the cross section of the polyolefin multilayer microporous membrane A polyolefin multilayer microfiber having a porosity difference of 0% or more and 5% or less between the porosity of the porous layer and the porosity of the second microporous layer, and having a thickness of 9 μm and a puncture strength of 330 cN or more when converted to a porosity of 40%. porous membrane.
(2) The polyolefin multilayer microporous membrane according to (1), which has a meltdown temperature of 160°C or higher.
(3) The polyolefin according to (1) or (2) above, wherein the polypropylene content in the first microporous layer is 10% by mass or more and less than 50% with respect to the entire first microporous layer. Multilayer microporous membrane.
(4) The film thickness change rate before and after compression for 10 seconds at a temperature of 70° C. and a pressure of 7.8 MPa is 13% or less with respect to the film thickness before compression according to any one of (1) to (3). Polyolefin multilayer microporous membrane.
(5) Any of (1) to (4), wherein the rate of change in electrical resistivity before and after compression for 10 seconds at a temperature of 70° C. and a pressure of 7.8 MPa is less than 60% of the electrical resistivity before heat compression. The polyolefin multilayer microporous membrane according to 1.
(6) The difference between the porosity of the first microporous layer and the porosity of the second microporous layer in the cross section of the polyolefin multilayer microporous membrane is 0% or more and 3% or less (1) to (5) The polyolefin multilayer microporous membrane according to any one of 1.
(7) The polyolefin multilayer, microporous membrane according to any one of (1) to (6), which has a porous layer on at least one surface of the polyolefin multilayer, microporous membrane.
(8) The polyolefin multilayer microporous membrane according to any one of (1) to (7), which is used as a battery separator.

INDUSTRIAL APPLICABILITY The present invention can provide a multi-layer, microporous polyolefin membrane having both excellent compression resistance and electrical resistance. Moreover, in a preferred embodiment of the present invention, it is possible to provide a polyolefin multi-layer, microporous membrane further having high-temperature meltdown properties. In addition, when the polyolefin multilayer microporous membrane of the present invention is used as a battery separator, it is expected to improve battery cycle characteristics and rate characteristics even under battery design for high energy density, so it is suitable as a battery separator. can be used.

The present embodiment of the present invention will be described below. It should be noted that the present invention should not be construed as being limited to the embodiments described below.

1. Polyolefin multilayer microporous membrane (hereinafter simply referred to as "microporous membrane")
[Porosity of each layer]
The polyolefin multi-layer, microporous membrane of the present invention comprises a polyolefin multi-layer, microporous membrane comprising at least two layers of a first microporous layer containing polypropylene and a second microporous layer containing polyethylene, and the first microporous layer and the porosity of the second microporous layer is 5% or less, more preferably 3% or less.

Setting the porosity difference within the above range means that the structural difference between the first microporous layer and the second microporous layer is small, that is, the structure is uniform, and it can be said that mechanical deformation is unlikely to occur. . Therefore, the resistance to impact and pressure from a direction perpendicular to the microporous membrane surface is enhanced. Furthermore, by improving the pressure resistance, the fine pores are less likely to be crushed, and the electrical resistance can be maintained at a high level even after compression. In addition, even when it is compressed, it is uniformly compressed in the plane, so structural variations are less likely to occur, and when it is used as a battery separator, ions are uniformly permeated, leading to improved cycle characteristics.

When the porosity difference between the first microporous layer and the second microporous layer exceeds 5%, the structure is uneven, so mechanical deformation is likely to occur, and the micropores are clogged. As a result, compression resistance and electrical resistance decrease.

The porosity is the weight average molecular weight (Mw) of polyethylene or polypropylene used for the first microporous layer or the second microporous layer, the content, the first stretching step, the second stretching step, the heat treatment step The above range can be obtained by appropriately adjusting the above. In addition, the porosity of each layer can be obtained by binarizing the cross-sectional image of the multilayer microporous membrane obtained with a scanning electron microscope (SEM) with the pores and the resin. A value obtained by the measurement method described in Examples.

[Meltdown temperature]
The polyolefin multilayer microporous membrane of the present invention preferably has a meltdown temperature of 160° C. or higher. Within the above range, safety is improved when used as a battery separator. The meltdown temperature is more preferably 170°C or higher, still more preferably 175°C or higher. Although the upper limit is not particularly limited, it is preferably 190° C. or less. The meltdown temperature can be obtained by measuring air resistance during temperature rise, and specifically refers to a value obtained by the measuring method described in Examples.

The meltdown temperature can be set within the above range by appropriately adjusting the melting point and content of the polypropylene used, the first stretching step, the second stretching step, the heat treatment step, and the like.

[Puncture Strength]
The multi-layer, microporous polyolefin membrane of the present invention has a thickness of 9 μm and a puncture strength of 330 cN or more when converted to a porosity of 40%. The pin puncture strength converted to a thickness of 9 μm and a porosity of 40% is preferably 350 cN or more, more preferably 400 cN or more and 600 cN or less. By setting it within the preferable range, the resistance to impact and pressure from the direction perpendicular to the film surface is increased, so better compression resistance is obtained, and when used as a battery separator, impact resistance and cycle characteristics are improved. Excellent for The puncture strength can be set within the above range by appropriately adjusting the Mw and content of polyethylene, the first stretching step, the second stretching step, and the like, which will be described later. Incidentally, the converted puncture strength can be specifically determined by the measuring method described in the Examples.

[Polypropylene content]
In the polyolefin multilayer microporous membrane of the present invention, the content of polypropylene contained in the first microporous layer is preferably 10% by mass or more and less than 50% by mass with respect to the entire first microporous layer, and more It is preferably 10% by mass or more and 30% by mass or less. When the content of polypropylene is within the above range, a network can be formed between polypropylenes, so a high meltdown temperature can be stably maintained, and it can be used in batteries that require heat resistance. . Moreover, by setting the polypropylene content to less than 50% by mass, the strength of the polyolefin multi-layer, microporous membrane can be maintained.

[Thickness change rate]
The polyolefin multilayer microporous membrane of the present invention has a film thickness change rate of 13% or less after heat compression for 10 seconds at a temperature of 70° C. and a pressure of 7.8 MPa, when the film thickness before heat compression is 100%. is preferred, more preferably 10% or less, and still more preferably 9% or less. By setting it within the above preferable range, when used as a battery separator, it can withstand the expansion of the electrode, and can exhibit the original function of the separator even when used repeatedly, so that the impact resistance and cycle characteristics are improved. improves.

The film thickness change rate can be set within the above range by controlling the porosity difference of each layer and adjusting the manufacturing conditions such as the stretching temperature and magnification so as to make the structure more uniform.

The film thickness change rate can be obtained by calculating the change rate of the film thickness after heat compression with respect to the film thickness 100% before heat compression, and specifically refers to the value obtained by the measurement method described in the Examples. .

[Electrical resistance change rate]
In the polyolefin multilayer microporous membrane of the present invention, the electrical resistance change rate before heat compression for 10 seconds at a temperature of 70 ° C. and a pressure of 7.8 MPa is preferably less than 60% with respect to the electrical resistance of 100% after heat compression. , more preferably 50% or less, and still more preferably 40% or less. Within the above preferable range, when used as a battery separator, even when expansion and contraction are repeated in the battery, an increase in resistance is suppressed and cycle characteristics are improved.

The electrical resistance change rate can be set within the above range by controlling the porosity difference of each layer and adjusting the manufacturing conditions such as stretching temperature and magnification so as to make the structure more uniform.

The rate of change in electrical resistance can be obtained by calculating the rate of change in electrical resistance before heating and compression with respect to 100% electrical resistance after heating and compression. .

2. Method for Producing Polyolefin Multilayer Microporous Membrane A method for producing a microporous membrane will be described below. In addition, the following description is an example of the manufacturing method, and is not limited to this method.

[Layer structure]
The polyolefin multilayer microporous membrane of the present invention has two or more polyolefin microporous layers, a first microporous layer containing polypropylene and a second microporous layer containing polyethylene. Each layer may be composed of polyolefin resins of different compositions, for example, three or more layers of two types of polyolefin resin layers (for convenience, these layers are respectively referred to as "A layer" and "B layer"). may be used. The stacking order is not particularly limited, and may be A layer/B layer/A layer or B layer/A layer/B layer. Hereinafter, the A layer means the first microporous layer, and the B layer means the second microporous layer.

[First Polyolefin Resin Containing Polypropylene]
The aspect of the first polyolefin resin that constitutes the first microporous layer containing polypropylene is as follows.

Polyethylene The first polyethylene resin preferably comprises ultra-high molecular weight polyethylene. Here, "ultra-high molecular weight polyethylene" means polyethylene having Mw of 1.0×10 ⁶ or more. The type of ultra-high molecular weight polyethylene may be a copolymer in which a small amount of α-olefin other than ethylene is copolymerized or a mixture in which a small amount of other α-olefin polymer is mixed. From the viewpoint of forming a uniform structure of the microporous membrane, it is preferably an ethylene homopolymer.

Preferred α-olefins other than ethylene are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene. The content of α-olefins other than ethylene is preferably 5 mol % or less based on 100 mol % of the polyolefin resin.

The weight average molecular weight (Mw) of the ultrahigh molecular weight polyethylene is preferably 1.0×10 ⁶ or more and less than 2.0×10 ⁶ , more preferably 1.8×10 ⁶ or less. In addition, Mw is a value measured by the GPC method mentioned later.

The content of ultra-high molecular weight polyethylene is preferably 65% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more with respect to 100% by mass of the first polyolefin resin.

The first polyolefin resin preferably does not substantially contain polyethylene other than ultra-high molecular weight polyethylene, but may contain 0% by mass or more and 15% by mass or less with respect to 100% by mass of the first polyolefin resin. . Mw of polyethylene other than ultra-high molecular weight polyethylene is preferably less than 3.0×10 ⁵ , more preferably less than 2.0×10 ⁵ . Furthermore, from the viewpoint of film strength, the lower limit of Mw is preferably 5.0×10 ⁴ or more. It is preferably at least one selected from the group consisting of high density polyethylene, medium density polyethylene, branched low density polyethylene and linear low density polyethylene.

Polypropylene The first polyolefin resin comprises polypropylene. The type of polypropylene is not particularly limited, and may be a propylene homopolymer, a copolymer of propylene and other α-olefins and/or diolefins (propylene copolymer), or a mixture of two or more selected from these. Any of them may be used, but it is more preferable to use a propylene homopolymer alone.

Either a random copolymer or a block copolymer can be used as the propylene copolymer. As the α-olefin in the propylene copolymer, α-olefins having 8 or less carbon atoms are preferred. Examples of α-olefins having 8 or less carbon atoms include ethylene, butene-1, pentene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene and combinations thereof. As the diolefin in the propylene copolymer, a diolefin having 4 to 14 carbon atoms is preferable. Examples of diolefins having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene and 1,9-decadiene. The content of other α-olefins and diolefins in the propylene copolymer is preferably less than 10 mol % with respect to 100 mol % of the propylene copolymer.

The weight average molecular weight (Mw) of polypropylene is preferably 1×10 ⁶ or more, more preferably 1.2×10 ⁶ or more, and particularly preferably 1.2×10 ⁶ to 4×10 ⁶ . The melting point of polypropylene is preferably 155 to 170°C, more preferably 160 to 165°C. The melting point is a value measured by a differential scanning calorimeter (DSC), which will be described later.

The content of polypropylene is preferably 10% by mass or more and less than 50% by mass, more preferably 10% by mass or more and 30% by mass or less with respect to 100% by mass of the first polyolefin resin. By setting the polypropylene content within the above range, the polyethylene can exist as an island structure in the polyethylene structure, and the network formed by the polypropylene can raise the meltdown temperature to 160° C. or higher.

[Second Polyolefin Resin Containing Polyethylene]
The aspect of the second polyolefin resin constituting the second microporous layer is as follows. Polyethylene The second polyethylene resin is ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1.0×10 ⁶ or more. preferably included. Since the type, Mw, and content of the ultra-high molecular weight polyethylene of the second polyolefin resin are the same as those of the ultra-high molecular weight polyethylene described in the section of the first polyolefin resin, the description is omitted.

The second polyolefin resin preferably contains polyethylene other than ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1.0×10 ⁵ or more. The content of polyethylene other than the ultra-high molecular weight polyethylene is preferably 10% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 20% by mass or less with respect to 100% by mass of the second polyolefin resin. be. The Mw and type of the polyethylene other than the ultra-high molecular weight polyethylene of the second polyolefin resin are the same as those of the polyethylene other than the ultra-high molecular weight polyethylene explained in the section of the first polyolefin resin, so the explanation is omitted.

Polypropylene The second polyolefin resin preferably does not substantially contain polypropylene, but may contain 0% by mass or more and 15% by mass or less relative to 100% by mass of the second polyolefin resin. The type, Mw, and melting point of the polypropylene are the same as the description of the polypropylene described in the section of the first polyolefin resin, so the description is omitted.

By appropriately adjusting the ultra-high molecular weight polyethylene of the first polyolefin resin and the second polyolefin resin, polyethylene other than ultra-high molecular weight polyethylene, and polypropylene within the above preferable range, the first microporous layer and the second microporous layer are formed. It leads to having a uniform structure with little porosity difference in the porous layer, that is, structural difference, and a multi-layer, microporous polyolefin membrane with high resistance to impact and pressure from the vertical direction can be obtained. In addition, since it has a uniform structure, the pores are uniformly crushed by compression, and when used as a battery, ions are less likely to concentrate in a part of the separator, so the separator is less likely to deteriorate and the cycle characteristics of the battery are improved. become a trend. Furthermore, by controlling the temperature within the above range, the meltdown temperature of the polyolefin multi-layer, microporous membrane can be maintained at 160° C. or higher, so that the heat resistance of the battery is increased and the safety is improved.

The thickness of each layer of the multi-layer, microporous polyolefin membrane is preferably 20/80 to 40/60, more preferably 20/80 to 30/70 as the ratio of A layer/B layer. In addition, when a plurality of A layers or B layers are provided, it is obtained as the sum of the layer thicknesses of the plurality of A layers or B layers.

In this example, a microporous membrane composed of two types of layers, the A layer and the B layer, is described, but it goes without saying that the microporous membrane can be composed of three or more types of layers.

(3) Method for producing multi-layer, microporous polyolefin membrane A method for producing the polyolefin multilayer microporous membrane of the invention will be described. For example, the method for producing this multi-layer, microporous polyolefin membrane includes the following steps.
(a) Preparation of layer A and layer B solutions (b) Molding of gel-like multilayer sheet (c) First stretching (d) Plasticizer removal (e) Drying (f) Second stretching (g) Heat treatment (a) Preparation of solutions for forming layers A and B A plasticizer is added to a polyolefin resin composition in a twin-screw extruder and melt-kneaded to prepare a solution for forming layers A and B. Prepare each. The blending ratio of the polyolefin resin composition and the plasticizer in the A layer and the B layer is 20% by mass or more and 30% by mass, with the total of the polyolefin resin composition and the plasticizer being 100% by mass. It is preferable to: By adjusting the concentration of the polyolefin resin composition within the above range, it is possible to prevent swelling and neck-in at the die exit during extrusion of the polyolefin solution, and to improve the moldability and self-supporting properties of the extrudate.

The plasticizer that can be used is not particularly limited as long as it is a non-volatile solvent that can form a uniform solution above the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples include hydrocarbons such as liquid paraffin and paraffin wax, Esters such as dioctyl phthalate and dibutyl phthalate can be mentioned.

The solutions for forming the A layer and the B layer are each fed from an extruder to one die, where both solutions are extruded in the form of a layered sheet to obtain an extrudate. The extrusion method may be either a flat die method or an inflation method. In either method, the solutions are fed into separate manifolds and layered at the lip inlet of the multi-layer die (multi-manifold method), or the solutions are pre-layered and fed to the die (block method). can be used. A common method can be applied to the multiple manifold method and the block method. The gap of the multilayer flat die can be set to 0.1 mm or more and 5 mm or less. The extrusion temperature is preferably 140° C. or higher and 250° C. or lower, and the extrusion speed is preferably 0.2 to 15 m/min. The film thickness ratio of the layers can be adjusted by adjusting the extrusion rate of the solution for each layer.

(b) Molding of Gel-Like Multi-Layer Sheet The resulting extrudate is cooled to form a gel-Large multi-layer sheet. Cooling can immobilize the polyolefin microphases separated by the membrane-forming solvent. When the cooling rate is within the above range, the degree of crystallinity is kept within an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but cooling by contacting with a roll cooled with a cooling medium is preferable. Cooling is preferably carried out at a rate of 50° C./min or more until at least the gelation temperature. Cooling is preferably performed to 25° C. or lower. Cooling can immobilize the microphases of the first and second polyolefins separated by the film-forming solvent. When the cooling rate is within the above range, the degree of crystallinity is maintained within an appropriate range, and a gel-like multilayer sheet suitable for stretching is obtained.

(c) First stretching Next, the gel-like sheet is stretched. The stretching of the gel-like sheet is also called wet stretching. Since the gel-like sheet contains a solvent, it can be uniformly stretched. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, the draw ratio (area draw ratio) is preferably 2 times or more, and more preferably 3 times or more and 30 times or less. In the case of biaxial stretching, it is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Moreover, the draw ratio in both MD and TD is preferably 3 times or more, and the draw ratio in MD and TD may be the same or different. The draw ratio in this step refers to the area draw ratio of the microporous membrane just before being subjected to the next step, with the microporous membrane just before this step as a reference.

The lower limit of the stretching temperature is preferably 90°C or higher, more preferably 110°C or higher. Moreover, the upper limit of the stretching temperature is 120° C. or lower, preferably 115° C. or lower. When the stretching temperature is within the above range, film breakage due to stretching of the low-melting-point polyolefin resin is suppressed, and high magnification stretching is possible.

(d) Removal of plasticizer The plasticizer is removed using a washing solvent. Since the washing solvent and the method for removing the plasticizer using the washing solvent are known, the explanation thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(e) Drying The polyolefin multi-layer, microporous membrane from which the plasticizer has been removed is dried by heat drying or air drying. Any method capable of removing the wash solvent may be used, including conventional methods such as heat drying, air drying (moving air), and the like. Processing conditions for removing volatile species such as washing solvents may be the same as disclosed, for example, in International Publication Nos. WO2008/016174 and WO2007/132942.

(f) Second Stretching Next, the dried multi-layer, microporous polyolefin membrane is stretched. The stretching of the microporous membrane after drying is referred to as the second stretching. The dried microporous membrane film is stretched at least uniaxially. The second stretching of the multi-layer, microporous polyolefin membrane can be performed by a tenter method or the like while heating in the same manner as described above. Since the present application contains two types of polyolefin resins, polyethylene and polypropylene, in the same layer, the second stretching is preferably uniaxial stretching from the viewpoint of uniformity of the lamellar structure.

The draw ratio is preferably 1.5 times or more in MD or TD, and more preferably 2.0 times or more. However, since the shutdown temperature and heat shrinkage increase, the upper limit is preferably 3.5 times in consideration of the balance. Here, the stretch ratio in the second stretching is based on the MD or TD length of the polyolefin multilayer, microporous membrane before the second stretching, and the MD or TD of the polyolefin multilayer, microporous membrane after the second stretching. Refers to length magnification. Here, the polypropylene of layer A exists as an island structure in the sea structure of polyethylene (sea-island structure). By setting the draw ratio within the above range, the island structure of the polypropylene can be maintained, and the high-temperature meltdown due to the polypropylene network can be ensured. Further, by setting the draw ratio within the above range, each layer is highly oriented, so that a uniform structure can be formed.

(g) Heat treatment A heat treatment is applied to the polyolefin multi-layer, microporous membrane after the second stretching. For example, the polyolefin multi-layer, microporous membrane is gripped with a clip and subjected to heat treatment while the width is fixed (TD heat setting treatment step). The heat treatment is preferably performed at 120° C. or higher and 135° C. or lower, more preferably 130° C. or lower. By setting the heat treatment temperature within the above range, the polyethylene other than the ultrahigh molecular weight polyethylene contained in the second microporous layer does not melt and can exist while being taken into the fibrils. The difference in porosity from the microporous layer can be within 5%.

By appropriately adjusting the first and second polyolefin resins, the first stretching step, the second stretching step, and the heat treatment step within the above preferable range, the porosity difference between the first and second microporous layers can be 5% or less, and a uniform structure can be formed. As a result, a multi-layer, microporous polyolefin membrane having a good film thickness change rate and electrical resistance during heating and compression can be obtained. In addition, high-temperature meltdown characteristics due to polypropylene are also exhibited.

Moreover, the polyolefin multi-layer microporous membrane of the present invention may be a laminated polyolefin porous membrane by laminating another porous layer on one or both surfaces of the polyolefin multi-layer microporous membrane. The other porous layer is not particularly limited, but for example, an inorganic particle layer containing a binder and inorganic particles can be mentioned. The binder component used in the inorganic particle layer is not particularly limited, and known materials can be used, such as acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, and the like. can be used. The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used. For example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, and the like can be used. can.

EXAMPLES The present invention will be described in more detail below with reference to examples. The present invention should not be construed as being limited to these examples.
[Measuring method]
(1) Film thickness The film thickness of the polyolefin multi-layer, microporous film was measured at 5 points within a range of 95 mm x 95 mm using a contact thickness gauge (Mitutoyo Corp. Lightmatic, contact pressure 0.01 N, 10.5 mmφ probe). and the average value was defined as the film thickness (μm).

(2) Porosity A polyolefin multilayer microporous membrane is cut into a size of 95 mm × 95 mm, its volume (cm ³ ) and weight (g) are obtained, and from these and the membrane density (g/cm ³ ), the following formula is obtained. calculated using
Formula: Porosity (%) = ((volume - weight / membrane density) / volume) x 100
Here, the film density was set to 0.99 (g/cm ³ ). The film thickness measured in (1) above was used to calculate the volume.

(3) Air resistance For the polyolefin multilayer microporous membrane, in accordance with JIS P-8117: 2009, air resistance (sec /100 cm ³ ) was measured.

(4) Piercing strength Using a needle with a diameter of 1 mm (the tip is 0.5 mmR), the maximum A load value S (cN) was measured. Furthermore, the converted puncture strength with a film thickness of 9 μm and a porosity of 40% was calculated according to the following formula.
Formula: Converted puncture strength (cN) = {S × 9 × 60} / {T × (100-P)}
(5) Shutdown Temperature and Meltdown Temperature The multi-layer, microporous polyolefin membrane is exposed to an atmosphere of 30° C., and the air resistance is measured while increasing the temperature at a rate of 5° C./min. The temperature at which the air resistance of the multi-layer, microporous polyolefin membrane reached 100,000 seconds/100 cm ³ was defined as the shutdown temperature. The meltdown temperature was defined as the temperature at which the temperature continued to rise after reaching the shutdown temperature and the air resistance became less than 100,000 seconds/100 cm ³ . The air resistance was measured using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P8117:2009.

(6) Thickness Change Rate after Heat Compression A polyolefin multi-layer, microporous membrane was cut out at 25 locations of 40 mm×40 mm at equal intervals along the TD, and the thickness was measured at 5 points per piece, and the average was calculated. Then, the five cut samples were stacked, and the set of samples was placed between horizontal plates and pressed under a pressure of 7.8 MPa at 70 ° C. for 10 seconds using a compression device (manufactured by Sintokogyo Co., Ltd., It was heated and compressed by CYPT-20 special). For the polyolefin multi-layer, microporous membrane 3 hours after releasing the heat compression, the thickness was measured at 5 points per sheet, the average (average thickness) was calculated, and it was calculated by the following formula.
Formula: Film thickness change rate (%) = [(average thickness before heat compression - average thickness after release of heat compression) / (average thickness before heat compression)] × 100
Here, the thickness was measured according to the method (1) described above.

(7) Electric resistance change rate after heat compression The electric resistance of the sample before and after heat compression used in (6) above was measured and calculated by the following formula.
Formula: Electrical resistance change rate (%) = [(Electrical resistance after heating and compression release - Electrical resistance before heating and compression) / (Electric resistance after heating and compression release)] x 100
Here, the electrical resistance was measured according to the method (8) described later.

(8) Electric resistance The electric resistance of the polyolefin multi-layer, microporous membrane was measured using a coin cell prepared under the following measurement conditions.

Measurement conditions Cell size: 2032
・Electrolyte: LiPF6 (1.0M) EC/EMC (4/6)
・Separator size: Φ19
・Equipment: IM3590 (manufactured by Solatron)
- Frequency: 200,000 Hz.

(9) Porosity of Each Layer A polyolefin multilayer microporous membrane was cut using an ion milling device (IM4000, manufactured by Hitachi High-Technologies Corporation) so as to obtain an MD cross section. After obtaining a cross-sectional image of the obtained sample using a scanning electron microscope (SEM), using HALCON 13 (manufactured by MVTec Software), binarize the resin and the hole part. , the porosities of the A layer and the B layer were calculated. Moreover, the porosity difference of each layer was calculated as an absolute value by the following formula.
Formula: Porosity difference = | A layer porosity obtained by binarization - B layer porosity obtained by binarization |
The interface between the A layer and the B layer was identified based on the layer ratio measurement result described later.
Ion milling and SEM observation at this time were performed under the following conditions.
Ion milling conditions Temperature: -160°C
・Acceleration voltage: 4 kV
SEM observation conditions Accelerating voltage: 2 kV
・Magnification: 7000 times.

(10) Melting point and melting peak The melting point of the polyolefin resin and the melting peak of the multi-layer, microporous polyolefin membrane were determined by a differential scanning calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER). The polyolefin resin and the multi-layer, microporous polyolefin membrane were each placed in a sample holder, heated to 230°C at a temperature elevation rate of 10°C/min under a nitrogen atmosphere (20 mL/min), and melted completely. , held at 230° C. for 3 minutes, and then cooled to 30° C. at a rate of 10° C./min (first run). After this temperature drop, the temperature was raised at a rate of 10°C/min in a nitrogen atmosphere (20 mL/min), and the temperature was again raised to 230°C to melt completely, then held at 230°C for 3 minutes and heated at 10°C/min. The temperature was lowered to 30° C. at a rate of (second run). From the obtained DSC curve, a straight line connecting 0 ° C. and 157 ° C. is used as a baseline, and the melting point (Tm) of the polyolefin resin is obtained from the temperature-melting endothermic curve in the second run, and the melting of the polyolefin multilayer microporous membrane A peak temperature was obtained.

(11) Polypropylene content A multi-layer, microporous polyolefin membrane was cut out and cut with a microtome so as to obtain an MD cross section, and the cross section was used as a sample. The sample is fixed to a ZnSe prism for AFM-IR, and infrared laser light is irradiated from the prism side to the cross section of the first microporous layer under ATR conditions, and the thermal expansion of the sample accompanying light absorption is detected as displacement of the AFM cantilever. did.
Measurement was performed by irradiating the sample with an infrared laser under the following conditions.

・ Measuring device: NanoIR Spectroscopy System (manufactured by Anasys Instruments)
・Light source: Tunable Pulsed Laser (1 kHz)
・AFM mode: contact mode ・Measurement wavenumber range: 1575 to 1200 cm ^-1
・ Wave number resolution: 2 cm ^-1
・Coverages: 32
・Number of accumulations: 2 or more ・Polarization angle: 45 degrees AFM-IR measurement was performed on the area in which all the polypropylene-containing layers were included. At this time, the displacement of the AFM cantilever when the sample was irradiated with lasers of 1465 cm ⁻¹ and 1376 cm ⁻¹ was measured, and the polypropylene content was obtained from the intensity ratio and mapped. That is, the content of polyethylene and polypropylene can be obtained by measuring the CH deformation angle of polyethylene with laser irradiation at 1465 cm ⁻¹ and the _CH3 deformation angle of polypropylene with laser irradiation at 1376 cm ⁻¹ .

(12) Analysis of Polyolefin Multilayer, Microporous Membrane The polyolefin multilayer, microporous membrane can be determined by focusing on the components contained in the layers and analyzing their distribution in the thickness direction.

For example, a catalyst is used in the polymerization of polyolefin, and analysis is possible by focusing on the catalyst remaining in the porous membrane. In this case, the thickness direction of the microporous membrane (MD/ZD or TD/ZD ) surface is analyzed by secondary ion mass spectrometry (SIMS) to measure the concentration distribution of trace metal elements originating from the catalyst. If the metal elements are uniformly distributed on the TD/ZD plane, it can be determined as a single layer, and if the metal species are different or a concentration distribution can be seen, it can be determined as a multilayer.
Measurement conditions ・Sample preparation: A polyolefin multi-layer, microporous membrane is cross-sectioned on the TD/ZD plane with an ultramicrotome.

・Measurement device: NanoSIMS50L (manufactured by CAMEKA)
・Degree of vacuum: 1.33×10 ^-8 Pa
・Primary ion: O ⁻
・Primary ion acceleration voltage: 25 kV
・Secondary ion polarity: positive ・Secondary ion detection area: 300×300 μm ²
As another method, a multi-layer, microporous polyolefin membrane is etched to separate it into a surface layer component and an inner layer component. The molecular weight distribution, melting point, and absorption spectrum are measured at each location, and if the analysis results of the surface layer and the inner layer are the same, it can be determined to be a single layer, and if there is a difference, it can be determined to be a multilayer. The molecular weight distribution and melting point are obtained by the GPC and DSC analyses, and the absorption spectrum is obtained by the following method.

Absorption Spectrum The absorption spectrum of the multi-layer, microporous polyolefin membrane was determined using infrared absorption spectrometry (IR) analysis.
Measurement conditions ・Detector: MCT
・Scan speed: 5 kHz
・Number of times of integration: 64 times ・Resolution: 4 cm ^-1
・Measurement wavelength: 4000 to 700 cm ^-1 .

(13) Layer ratio The layer ratio of each layer of the multi-layer, microporous polyolefin membrane was observed using a transmission electron microscope (TEM) under the following measurement conditions.

Measurement conditions ・Sample preparation: A multi-layer, microporous polyolefin membrane is dyed with ruthenium tetroxide and cross-sectioned with an ultramicrotome.

・Measuring device: transmission electron microscope (JEOL JEM1400Plus type)
・Observation conditions: acceleration voltage of 100 kV
Observation direction: TD/ZD.

[Example 1]
(1) Preparation of Layer A Solution 100% by mass of polyolefin composition consisting of 20% by mass of isotactic polypropylene (melting point: 162°C) with Mw of 2.0 x ¹⁰⁶ and 90% by mass of ultra-high molecular weight polyethylene with Mw of 1.35 x ¹⁰⁶ 0.2% by mass of tetrakis(methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate)methane as an antioxidant was charged into the twin-screw extruder. Furthermore, liquid paraffin was supplied from the side feeder of the twin-screw extruder, and melt-kneaded in the twin-screw extruder so that the polyolefin composition was 17% by mass and the liquid paraffin was 83% by mass, to prepare a polyolefin solution for layer A. .
(2) Preparation ^of layer B solution Tetrakis (methylene-3-(3,5-ditertiary 0.2% by weight of butyl-4-hydroxyphenyl)-propionate)methane was charged into a twin-screw extruder. Further, liquid paraffin was supplied from the side feeder of the twin-screw extruder, and melt-kneaded in the twin-screw extruder so that the polyolefin composition was 17% by mass and the liquid paraffin was 83% by mass, to prepare a polyolefin solution for layer B. .
(3) Molding of gel-like multilayer sheet Each solution is supplied from a twin-screw extruder to a three-layer T-die, and the layer thickness ratio of B layer solution/A layer solution/B layer solution is 35/30/35. extruded to become The extruded product was cooled while being taken up at a take-up speed of 4 m/min with a cooling roll controlled to 25° C. to form a gel-like three-layer sheet.
(4) First Stretching, Removal of Film Forming Agent, Drying The gel-like three-layer sheet is simultaneously biaxially stretched 5 times in both MD and TD at 115° C. with a tenter stretching machine, and the sheet width is stretched in the tenter stretching machine as it is. was fixed and heat-set at a temperature of 110°C. The stretched gel-like three-layer sheet was then immersed in a methylene chloride bath in a wash bath to remove liquid paraffin and air-dried at room temperature.
(5) Second stretching and heat treatment After that, after preheating at 127°C, the TD was stretched 1.7 times with a tenter stretching machine, and then the TD was relaxed by 14% and held at 128°C while being held in the tenter. It was heat-set to obtain a B-layer/A-layer/B-layer polyolefin multilayer microporous membrane. Table 1 shows the characteristics of the obtained polyolefin multilayer microporous membrane.

[Example 2]
A multi-layer, microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the solution adjustment of layer B, the layer thickness ratio of the gel-like multilayer sheet, and the second stretching conditions were changed as shown in Table 1.

[Example 3]
A multi-layer, microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the solution preparation of the A layer and the B layer and the second stretching conditions were changed as shown in Table 1.

[Example 4]
A multi-layer, microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the solution preparation of the A layer and the B layer, the layer structure of the gel-like multilayer sheet, and the first and second stretching conditions were changed as shown in Table 1. .

[Example 5]
A multi-layer, microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the solution adjustment of the layers A and B, the layer thickness ratio of the gel-like multilayer sheet, and the first and second stretching conditions were changed as shown in Table 1. Ta.

[Comparative Examples 1 to 3]
A multi-layer, microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the solution preparation of the A layer and the B layer, the layer structure of the gel-like multilayer sheet, and the first and second stretching conditions were changed as shown in Table 1. .

In the table, "UHMWPE" represents "ultra-high molecular weight polyethylene," "PP" represents "polypropylene," and "other PE" represents "polyethylene other than ultra-high molecular weight polyethylene."

Claims (8)

A polyolefin multilayer microporous membrane having at least two layers, a first microporous layer containing polypropylene and a second microporous layer containing polyethylene, wherein the first microporous layer in the cross section of the polyolefin multilayer microporous membrane A multi-layer, microporous polyolefin membrane having a difference between the porosity of the second microporous layer and the porosity of 0% or more and 5% or less, a thickness of 9 μm, and a puncture strength of 330 cN or more in terms of a porosity of 40%. The multi-layer, microporous polyolefin membrane according to claim 1, which has a meltdown temperature of 160°C or higher. 3. The polyolefin multi-layer microporous membrane according to claim 1 or 2, wherein the content of polypropylene contained in the first microporous layer is 10% by weight or more and less than 50% by weight based on the entire first microporous layer. The multi-layer, microporous polyolefin membrane according to any one of claims 1 to 3, wherein the film thickness change rate before and after compression for 10 seconds at a temperature of 70°C and a pressure of 7.8 MPa is 13% or less with respect to the film thickness before compression. . The polyolefin according to any one of claims 1 to 4, wherein the rate of change in electrical resistivity before and after compression for 10 seconds at a temperature of 70°C and a pressure of 7.8 MPa is less than 60% of the electrical resistivity before heat compression. Multilayer microporous membrane. The difference between the porosity of the first microporous layer and the porosity of the second microporous layer in the cross section of the polyolefin multi-layer microporous membrane is 0% or more and 3% or less according to any one of claims 1 to 5. polyolefin multi-layer microporous membrane. The multi-layer, microporous polyolefin membrane according to any one of claims 1 to 6, having a porous layer on at least one surface of the multi-layer, microporous polyolefin membrane. The polyolefin multilayer microporous membrane according to any one of claims 1 to 7, which is used as a battery separator.