WO2023145319A1

WO2023145319A1 - Polyolefin microporous membrane and method for producing same

Info

Publication number: WO2023145319A1
Application number: PCT/JP2022/047102
Authority: WO
Inventors: 李丹; 山崎高志
Original assignee: 東レ株式会社
Priority date: 2022-01-28
Filing date: 2022-12-21
Publication date: 2023-08-03
Also published as: JPWO2023145319A1; KR20240144092A

Abstract

A polyolefin microporous membrane having a meltdown temperature of 160-190°C inclusive, a puncture strength in terms of thickness of 35-75 gf/μm inclusive, and a coefficient of variation of peak absorbance at a wavenumber of 1376 cm-1 derived from -CH3 of 0-5.0% inclusive. Provided are a polyolefin microporous membrane that has a high meltdown temperature and a low shutdown temperature even when formed to be thin and strong and that offers exceptional battery safety, and a separator and a secondary battery in which the polyolefin microporous membrane is used.

Description

Polyolefin microporous membrane and method for producing the same

TECHNICAL FIELD The present invention relates to a polyolefin microporous membrane and a method for producing the same.

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, and separators for batteries and electrolytic capacitors. Among these, polyolefin microporous films containing polyolefin as a main component are excellent in chemical resistance, insulating properties, mechanical strength, etc., and have shut-down properties, and have been widely used as separators for secondary batteries in recent years. On the other hand, secondary batteries, such as lithium-ion secondary batteries, have high energy density and are widely used as batteries for personal computers, mobile phones, and the like. Lithium-ion secondary batteries are also expected to serve as power sources for driving motors of electric vehicles and hybrid vehicles.

In recent years, as the energy density of lithium-ion secondary batteries has increased, there has been a demand for thinner polyolefin microporous membranes used as separators. In particular, as the capacity of batteries for smartphones has increased, the safety of batteries has become the most important concern. There are various types of safety for smartphone batteries, but hot box safety is emphasized above all. Under these circumstances, in order to achieve hot box safety, various methods have been developed to produce polyolefin microporous membranes that have sufficient insulating properties, mechanical strength, shutdown properties, and meltdown properties while satisfying low thermal shrinkage. Consideration is underway.

Patent Document 1 discloses that the cycle characteristics of a battery are ensured by specifying the TMA behavior, film thickness, resin composition, and air permeability of a microporous membrane made of polyethylene and polypropylene.

Patent Document 2 discloses the concentration and distribution of polypropylene effective for imparting oxidation resistance to a monolayer film made of polypropylene and polyethylene.

Patent document 3 has at least a first microporous membrane layer containing a first polyethylene, a first polypropylene and a second polypropylene and a second microporous membrane layer containing the first polyethylene and a second polyethylene, and has a puncture strength , describe multi-layer microporous membranes defined for air permeability after hot compression.

In Patent Document 4, the average pore diameter measured by a porometer is smaller than 15 nm, the shutdown temperature is 140° C. or lower, the air resistance is 1000 sec/100 cm or lower, and a layer containing 80% by mass or more of a polypropylene resin is A. A porous polyolefin film is described which is characterized by having at least one layer.

Japanese Patent Application Laid-Open No. 2021-116316 International Publication No. 2013/099607 JP 2013-224033 JP 2020-069796

However, the polyolefin microporous membranes disclosed in Patent Documents 1 to 4 have a puncture strength of less than 35 gf/μm, and cannot be said to have sufficient strength at a film thickness of 9 μm or less. In addition, in order to ensure further high strength of the thin film, it is necessary to increase the high molecular weight component or increase the draw ratio, and the problem that the shutdown temperature rises and the battery safety is impaired. there were.

In view of the above circumstances, the present invention provides a polyolefin microporous membrane that has a high meltdown temperature and a low shutdown temperature even when thin and high strength, and has excellent battery safety, a separator and a secondary The purpose is to provide a battery.

As a result of intensive studies to solve the above problems, the present inventors have found that polyolefins that have excellent meltdown characteristics, high puncture strength, uniform dispersion of polypropylene in the film, and can ensure better battery safety. We have found that this problem can be solved by a microporous membrane.

That is, the present invention has the following constituent requirements.
(1) The meltdown temperature is 160° C. or higher and 190° C. or lower, the thickness-converted puncture strength is 35 gf/μm or higher and 75 gf/μm or lower, and the wavenumber derived from —CH ₃ is 1376 cm ⁻¹ , which is obtained by the following measurement method. A polyolefin microporous membrane having a coefficient of variation of the peak absorbance of 0 to 5.0%:
[Method for measuring coefficient of variation of absorbance]
A 50 mm × 50 mm sample piece cut out from the polyolefin microporous film is subjected to transmission measurement under the following conditions using a microscopic infrared spectrophotometer (FT/IR-6600, manufactured by JASCO Corporation);
・Measurement area 300 μm × 300 μm
・Number of measurement points: 40 x 40 points ・Measurement interval: 300 μm
・Total measurement area 12 x 12mm
・Measurement wavenumber range 600 cm ^-1 to 4000 cm ^-1
・Resolution 4 cm ^-1
・The number of times of integration 16 times Select the peak of wavenumber 1376 cm ^-1 in the graph plotting the wavenumber and absorbance on the horizontal axis and the vertical axis, respectively, and obtain the absorbance from the top of the peak to the baseline; the baseline is wavenumber 1330 cm Use a straight line drawn between the measured value (absorbance) at ^-1 and the absorbance at a wave number of 1400 cm ^-1 ; Determine the coefficient of variation (standard deviation/mean value) of the absorbance of polypropylene.
A further preferred embodiment is
(2) When the polyolefin microporous membrane is measured by cross fractionation chromatography (CFC method) under the following measurement conditions to obtain a molecular weight distribution curve, the molecular weight distribution curve of polypropylene and the molecular weight distribution curve of polyethylene overlap. The polyolefin microporous membrane according to claim 1, which has an area of 15% or more relative to the area of the entire molecular weight distribution curve of polypropylene:
[Measurement condition]
Using an IR4-type infrared spectrophotometer as a detector, obtain an elution curve for the eluted polymer in the following elution sections in the region of 140°C to 0°C, convert the elution time to molecular weight, and obtain a molecular weight distribution curve. ;
- Polypropylene: The peak detected in the 140-110°C region was taken as the polypropylene peak;
• Polyethylene: The strongest detection peak in the 110 to 75°C region was taken as the polyethylene peak.
(3) The shutdown temperature is 134° C. or higher and 140° C. or lower.
(4) A polyolefin microporous membrane having three layers, at least a first layer comprising polyethylene and a second layer comprising polypropylene and polyethylene.
(5) The first layer is a layer made of polyethylene having a weight average molecular weight (Mw) of 1 million or more and 2 million or less, and the second layer has a weight average molecular weight (Mw) of 1 million or more and 2 million It is a layer obtained using the following polyethylene and polypropylene having a weight average molecular weight (Mw) of 1,000,000 or more as raw materials.
(6) In the molecular weight distribution curve obtained by measuring the molecular weight distribution of the raw material polyethylene and the raw material polypropylene in the second layer by GPC method, the area where the molecular weight distribution curve of the raw material polyethylene and the molecular weight distribution curve of the raw material polypropylene overlap is the raw material. It is 20% or more with respect to the area of the entire molecular weight distribution curve of polypropylene.
(7) The layer ratio of the first layer to the total thickness of the polyolefin microporous membrane is 40 to 90% by mass, and the layer ratio of the second layer is 10 to 60% by mass.
(8) A three-layer structure having the first layer (skin layer) on both sides of the second layer (core layer).
(9) The film thickness is 9 μm or less.
(10) At least one surface is further provided with one or more coating layers.
(11) A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane.

According to the present invention, it is possible to provide a polyolefin microporous membrane that has a high meltdown temperature and a low shutdown temperature even when thin and high strength, and has excellent battery safety, as well as a separator and a secondary battery using the same.

4 shows the molecular weight distribution of raw materials polyethylene and polypropylene used in Example 4 of the present invention. 2 shows the molecular weight distribution of raw materials polyethylene and polypropylene used in Comparative Example 2 of the present invention. FIG. 4 is a two-dimensional distribution diagram showing the absorbance intensity of polypropylene of the polyolefin microporous membrane prepared in Example 4 of the present invention. FIG. FIG. 4 is a two-dimensional distribution diagram showing the absorbance intensity of polypropylene of the polyolefin microporous film prepared in Comparative Example 2 of the present invention. FIG. 2 shows the molecular weight distribution in CFC evaluation of polyethylene and polypropylene of the polyolefin microporous membrane produced in Example 6 of the present invention. 1 shows the molecular weight distribution in CFC evaluation of polyethylene and polypropylene of the polyolefin microporous membrane produced in Comparative Example 1 of the present invention.

The present invention is a method for producing a polyolefin microporous membrane containing polyethylene and polypropylene.

The polyolefin microporous membrane of this embodiment includes a polyolefin monolayer microporous membrane containing polyethylene and polypropylene, and a first microporous layer containing polyethylene and a second microporous layer containing polypropylene and polyethylene. Any polyolefin multilayer microporous membrane may be used.

The meltdown temperature of the polyolefin microporous membrane of the present invention is preferably 160°C or higher, more preferably 165°C or higher, and even more preferably 170°C or higher. The upper limit of the meltdown temperature is not particularly limited, but 190° C. or less is a substantial upper limit depending on the method of adding PP (polypropylene). When the meltdown temperature is within the above range, the membrane shape after shutdown can be maintained, and contact between the electrodes due to membrane rupture can be prevented. Therefore, the generation of current after shutdown can be suppressed, and battery safety can be ensured.

The lower limit of the thickness-converted puncture strength of the polyolefin microporous membrane, that is, the puncture strength per film thickness is 35 gf/μm or more, preferably 37 gf/μm or more, and more preferably 40 gf/μm or more. When the puncture strength is 35 gf/μm or more, when used as a battery separator, the separator is less likely to break during winding or due to foreign matter, resulting in less short-circuiting, thereby enhancing the safety of the battery. If the puncture strength is less than 35 gf/μm, when used as a battery separator, the separator may be broken during winding or by foreign matter, and short circuits may easily occur. Moreover, the upper limit of the puncture strength is preferably 75 gf/μm or less. If it is 75 gf/μm or more, the shutdown temperature becomes high and there is a risk of safety. When the puncture strength is within the above range, heat shrinkage at high temperatures can be suppressed, and deterioration of meltdown properties can be suppressed.

In order to keep the puncture strength within the above range, it is effective to adjust the molecular weight and content of the polymer component that constitutes the polyolefin microporous membrane. It is also effective to adjust the draw ratio when producing the polyolefin microporous membrane. In order to keep the puncture strength within the above range, the weight average molecular weight of polyethylene is preferably 1,000,000 or more, and more preferably 1,000,000 or more and 2,000,000 or less from the viewpoint of resin kneading. In addition, it is preferable that the high-molecular-weight polyethylene is contained in 50% by mass or more, more preferably 60% by mass or more, in 100% by mass of the polyolefin microporous membrane.

The polyolefin resin of the above polyolefin microporous membrane is mainly composed of polyethylene and polypropylene. Here, the "main component" means that the total content of polyethylene and polypropylene is 99% by mass or more in 100% by mass of the polyolefin microporous membrane.

The variation coefficient of the absorbance of the peak at wave number 1376 cm ⁻¹ derived from —CH ₃ obtained by measuring the polyolefin microporous membrane by the method described below using a micro-infrared spectrometer is preferably 5.0% or less. , is more preferably 3.0% or less, and even more preferably 2.0% or less. The peak at wave number 1376 cm ⁻¹ derived from —CH ₃ is a characteristic peak of polypropylene. A low coefficient of variation in absorbance for this peak suggests a high degree of homogeneity in the dispersion of polypropylene in the polyolefin microporous membrane. It is well known that the heat resistance of a polyolefin microporous membrane can be improved by adding polypropylene. On the other hand, when polypropylene is added and the dispersibility of polypropylene in the polyolefin microporous film is poor, a sea-island structure of polypropylene is formed. The heat resistance of the sea-island structure is good, but the heat resistance of other parts is poor, so the polyolefin microporous membrane may locally rupture at high temperatures, causing contact between electrodes and lowering safety. There is Although the lower limit of the coefficient of variation is not particularly limited, the theoretical lower limit is 0%, preferably 0.6% or more, more preferably 0.3% or more. A polyolefin microporous membrane excellent in meltdown property and strength can be obtained by setting the coefficient of variation of the absorbance at the wavenumber 1376 cm ⁻¹ peak within the above range.

The shutdown temperature of the polyolefin microporous membrane is preferably 134°C or higher and 140°C or lower, more preferably 134°C or higher and lower than 138°C, and even more preferably 134°C or higher and lower than 136°C. When the shutdown temperature is within the above range, it is possible to shut down the battery more quickly when abnormal heat is generated, thereby preventing abnormal heat generation. On the other hand, however, when the shutdown temperature is pursued, the strength of the polyolefin microporous membrane may be weakened and foreign matter resistance may be lowered. The shutdown temperature within the above range can be achieved by setting the puncture strength of the polyolefin microporous membrane to the aforementioned range.

The polyolefin microporous membrane described above is a polyolefin microporous membrane having three layers, and preferably has a first layer containing at least polyethylene and a second layer containing polypropylene and polyethylene.

A single-layer microporous membrane made of a composition of polypropylene and polyethylene may inhibit the crystallization of polyethylene due to the addition of polypropylene, which may reduce strength. By forming a multi-layer, microporous membrane having three layers having a first layer containing at least polyethylene and a second layer containing polypropylene and polyethylene, functions are assigned to each layer, and meltdown properties, strength, and shutdown properties are balanced. be suitable.

The second layer is preferably a layer obtained using polyethylene with a weight average molecular weight (Mw) of 1,000,000 to 2,000,000 and polypropylene with a weight average molecular weight (Mw) of 1,000,000 or more as raw materials. In the differential molecular weight distribution curve obtained by the gel permeation chromatography (GPC) method of the raw material polyethylene and the raw material polypropylene, the area where the molecular weight distribution curve of the raw material polyethylene and the molecular weight distribution curve of the raw material polypropylene overlap is the entire differential molecular weight distribution curve of the raw material polypropylene. It is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more of the area of .

The polypropylene contained in the second layer preferably has an Mw of 1 million or more and less than 4 million, and the polyethylene preferably has an Mw of 1 million or more and 2 million or less. The content of polypropylene in the second layer is preferably 8% by mass or more. When the polypropylene content is 8% by mass or more, more preferably 10% by mass or more, the heat resistance of the microporous membrane can be effectively improved. Moreover, the polypropylene content in the second layer is preferably 50% by mass or less. When the content of polypropylene is 50% by mass or less, more preferably 35% by mass or less, it is possible to suppress the maximum pore size of the microporous membrane from becoming small and suppress the increase in air resistance.

By setting the Mw of both polypropylene and polyethylene to 1,000,000 or more, it is possible to suppress deterioration in film strength and foreign matter resistance. Further, by setting the weight average molecular weight (Mw) of both polyethylene and polypropylene to 1,000,000 or more, it is possible to suppress strength reduction or deterioration of polypropylene dispersibility. The combination of ultra-high molecular weight polypropylene and polyethylene makes it possible to obtain a functional layer that has a high meltdown temperature while also having high strength.

Further, in the differential molecular weight distribution curve obtained by the gel permeation chromatography (GPC) method of the raw material polyethylene and the raw material polypropylene, the overlapping area of the differential molecular weight distribution curve of the raw material polyethylene and the differential molecular weight distribution curve of the raw material polypropylene is the differential of the raw material polypropylene. By making it 20% or more of the entire area of the molecular weight distribution curve, it is possible to suppress the occurrence of phase separation due to difficulty in incorporating polypropylene into the molecular chains of polyethylene during resin kneading. Therefore, it is possible to suppress the formation of a sea-island structure of polypropylene and the occurrence of local premature membrane rupture at high temperatures. By setting the content of polyethylene and polypropylene and the weight average molecular weight (Mw) of both within the above range, it becomes easy to disperse the polypropylene in the polyethylene in the melt-kneading process, and the polyolefin microporous membrane The coefficient of variation of the absorbance of the peak at wavenumber 1376 cm ⁻¹ derived from —CH ₃ can be adjusted within the range described above, and good meltdown characteristics, strength and uniformity of polypropylene distribution can be obtained.

Further, in the differential molecular weight distribution curve obtained by the gel permeation chromatography (GPC) method of the raw material polyethylene and the raw material polypropylene, the overlapping area of the differential molecular weight distribution curve of the raw material polyethylene and the differential molecular weight distribution curve of the raw material polypropylene is the differential of the raw material polypropylene. If the total area of the molecular weight distribution curve is less than 70%, the resin becomes hard due to the presence of a large amount of ultra-high molecular weight components, resulting in poor sheet appearance during sheet molding and film breakage during stretching. can be suppressed.

The polyolefin microporous membrane is analyzed using a cross fractionation chromatography (CFC) method under the measurement conditions described later, the temperature is lowered to 140 ° C. to 0 ° C., and the strongest detection peak is detected in the temperature range of 140 ° C. to 110 ° C. The peak of polypropylene, the strongest detection peak in the temperature range of 110 ° C. to 75 ° C. is the peak of polyethylene, and when obtaining a molecular weight distribution curve, the overlapping area of the molecular weight distribution curve of polyethylene and the molecular weight distribution curve of polypropylene is the molecular weight of polypropylene. It is preferably 15% or more with respect to the area of the entire distribution curve.

In the differential molecular weight distribution curve of the polymer obtained by analysis of the polyolefin microporous membrane using the CFC method, the area where the differential molecular weight distribution curve of polyethylene and the differential molecular weight distribution curve of polypropylene overlap is the area of the entire differential molecular weight distribution curve of polypropylene. is preferably 15% or more, more preferably 20% or more. When the overlapping area is within the above range, the polyethylene is oriented by the ultrahigh molecular weight component, and high strength of the polyolefin microporous membrane can be achieved. Moreover, it is preferable that the overlapping area is less than 50%, because heat shrinkage, increase in air resistance, and rise in shutdown temperature can be suppressed, and deterioration in battery performance and safety can be suppressed. By setting the overlapping area between the molecular weight distribution curve of polyethylene and the molecular weight distribution curve of polypropylene within the above range, the meltdown property, strength and air resistance are well balanced, and the strength and shutdown temperature are adjusted within the ranges described above. It is preferable because it can

In the above polyolefin microporous membrane, the layer ratio of the first layer to the total mass is preferably 40 to 90% by mass, and the layer ratio of the second layer is preferably 10 to 60% by mass. The layer ratio of the second layer to the total mass is more preferably 20 to 50% by mass, more preferably 20 to 40% by mass. When the layer ratio of the second layer is 10% by mass or more, the effect of maintaining the shape after shutdown due to the contained polypropylene is favorable, and a meltdown temperature of 160° C. or more can be achieved. Further, by setting the layer ratio of the second layer to 60% by mass or less, it is possible to suppress deterioration in shutdown characteristics due to insufficient thickness of the first layer containing polyethylene.

The polyolefin microporous membrane described above preferably has a three-layer structure in which the second layer is a core layer and the first layers are skin layers on both sides of the second layer. It is well known that polypropylene is susceptible to oxidative degradation. Especially during kneading, low-molecular-weight polypropylene is generated due to oxidative deterioration, and there is a risk that it will peel off from the conveying roll during the casting and winding processes, contaminating the process. A second layer comprising polypropylene can be placed on the core layer to reduce process contamination.

The film thickness of the polyolefin microporous membrane is preferably 9 μm or less. The film thickness is preferably 2 μm or more and 9 μm or less, more preferably 2 μm or more and 7 μm or less, and still more preferably 2 μm or more and 5 μm or less. When the film thickness is within the above range, when the polyolefin microporous film is used as a battery separator, the battery capacity can be improved and the insulation can be improved.

Preferred aspects of the resin composition that constitutes each layer and the manufacturing method are described below.

(1) Second layer polyolefin resin composition A containing polypropylene and polyethylene
The second layer is preferably a porous layer formed of polyolefin resin composition A containing ultra-high molecular weight polyethylene and polypropylene.

(a) Polyethylene Polyethylene is preferably ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1,000,000 or more. Here, "ultra-high molecular weight polyethylene" refers to polyethylene having Mw of 1,000,000 or more. The ultra-high molecular weight polyethylene may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene, but it is preferable to use an ethylene homopolymer.

As α-olefin copolymers other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene are preferred. The content of α-olefins other than ethylene is preferably 5 mol % or less based on 100 mol % of all copolymer components. From the viewpoint of uniformity of the pore structure of the polyolefin microporous membrane, it is preferably an ethylene homopolymer.

　The weight average molecular weight (Mw) of the ultra-high molecular weight polyethylene is preferably 1 million or more and 2 million or less, more preferably 1 million or more and 1.8 million or less. Mw is a value measured by the GPC method described later.

The content of ultra-high molecular weight polyethylene in polyolefin resin composition A is preferably 50% by mass or more and 92% by mass or less, more preferably 65% by mass or more, relative to 100% by mass of polyolefin resin composition A. ~90% by mass or less. More preferably, it is 80% by mass or more.

(b) Polypropylene The type of polypropylene is not particularly limited, and is a homopolymer of propylene, a copolymer of propylene and other α-olefins and/or diolefins (propylene copolymer), or two selected from these. Although any of the above mixtures may be used, it is more preferable to use a propylene homopolymer alone.

　As the propylene copolymer, either a random copolymer or a block copolymer can be used. 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 total copolymer components.

The Mw of polypropylene is preferably 1,000,000 or more, more preferably 1,200,000 or more, and particularly preferably 1,500,000 or more. Mw of polypropylene is preferably less than 4 million. 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 in the polyolefin resin composition A is preferably 8% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 35% by mass or less with respect to 100% by mass of the polyolefin resin composition A.

(2) Polyolefin resin composition B for the first layer
(a) Polyethylene The first layer is a porous layer formed of a polyolefin resin composition (hereinafter referred to as "polyolefin resin composition B") containing ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1,000,000 or more. Preferably. Since the preferred type and preferred weight average molecular weight (Mw) range of the ultra-high molecular weight polyethylene of the polyolefin resin composition B are the same as those of the polyolefin resin composition A, description thereof is omitted.

The polyolefin resin composition B preferably contains polyethylene other than ultra-high molecular weight polyethylene. The content of polyethylene other than ultra-high molecular weight polyethylene is preferably in the range of 0% by mass or more and 50% by mass or less with respect to 100% by mass of the entire polyolefin resin composition B. Mw of polyethylene other than ultra-high molecular weight polyethylene is preferably less than 300,000, more preferably less than 200,000. Furthermore, from the viewpoint of film strength, the lower limit of Mw is preferably 50,000 or more. The polyethylene other than ultra-high molecular weight polyethylene 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.

(3) Method for producing a polyolefin microporous membrane The polyolefin microporous membrane of the present embodiment is a polyolefin single-layer microporous membrane containing polyethylene and polypropylene, or a first microporous layer containing polyethylene and a second microporous layer containing polypropylene and polyethylene. Any polyolefin multilayer microporous membrane containing a microporous layer of

The manufacturing method (wet film manufacturing method) of the polyolefin microporous membrane will be explained. In addition, the following description is an example of the manufacturing method, and is not limited to this method.

The method for producing the polyolefin microporous membrane of this embodiment includes the following steps. (a) Preparation of first layer and second layer solutions (b) Formation of gel sheet (c) First stretching (d) Plasticizer removal (e) Drying (f) Second stretching (g) Heat treatment.

(a) Preparation of first layer and second layer solutions A plasticizer is added to a polyolefin resin composition in a twin-screw extruder, melt-kneaded, and a solution for the first layer and a solution for the second layer are prepared. Prepare each. The mixing ratio of the polyolefin resin composition and the plasticizer in the first layer and the second layer is 15% by mass or more and 30% by weight, with the total of the polyolefin resin composition and the plasticizer being 100% by weight. % by weight or less. 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 solution for the first layer and the solution for the second layer are each fed from an extruder to one die, where both solutions are extruded as a layered sheet to obtain a molded body. The extrusion method may be either a flat die method or an inflation method. In either method, both solutions are supplied to separate manifolds and layered at the lip inlet of a multi-layer die (multi-manifold method), or both solutions are pre-layered and fed to the die (block flow method). method) can be used. The multiple manifold method and the block method can be applied as usual. 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. By adjusting the throughput of the solution for each layer, the thickness ratio of the layers can be adjusted.

(b) Formation of gel-like sheet A gel-like sheet is obtained by cooling the obtained extrudate. As a cooling method, a method of contact with a cooling medium such as cold air or cooling water, a method of contact with a cooling roll, or the like can be used. It is preferable to cool the film by bringing it into contact with a roll cooled with a refrigerant. 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 plasticizer. 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.

(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 plasticizer, 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, 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 preferably 120° C. or less. 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 microporous membrane from which the plasticizer has been removed is dried by a heat drying method or an air drying method. 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 components such as washing solvents may be the same as disclosed, for example, in PCT Patent Publication Nos. WO2008/016174 and WO2007/132942.

(f) Second Stretching Next, the dried polyolefin microporous 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 polyolefin microporous 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 at least 1.5 times the TD, more preferably at least 2.0 times. When the second stretching is performed at 1.5 times or more, crystal molecular chains are highly oriented in the TD, so the MD/TD Raman orientation ratio can be adjusted to less than 0.8. The Raman orientation ratio can be adjusted to be smaller as the film is drawn at a higher magnification. However, since the shutdown temperature and the thermal shrinkage rate increase, the upper limit of the draw ratio is preferably 3.5 times or less in consideration of the balance. Here, the stretch ratio of TD in the second stretching is the ratio of the TD length of the polyolefin microporous membrane after the second stretching based on the TD length of the polyolefin microporous membrane before the second stretching. Say.

(g) Heat Treatment The polyolefin microporous membrane after the second stretching is preferably heat treated. As the heat treatment, a TD heat setting treatment step is preferable in which the polyolefin microporous membrane is gripped with a clip and subjected to heat treatment while the width is fixed. The heat treatment temperature is preferably 115° C. or higher and 135° C. or lower.

By appropriately adjusting the polyolefin resin composition, the first stretching step, the second stretching step, and the heat treatment step within the above preferred ranges, excellent high-temperature meltdown characteristics, high strength, and uniform distribution of polypropylene in the film can be obtained. and a polyolefin microporous membrane having low temperature shutdown properties.

(coating film)
A porous layer other than polyolefin resin may be laminated on at least one surface of the polyolefin microporous membrane to form a laminated polyolefin porous membrane (coating film). The other porous layer is not particularly limited, but for example, a coating layer containing a binder and inorganic particles may be laminated by coating.

The thickness of the other porous layer is preferably in the range of 1-5 μm, more preferably 1-4 μm, even more preferably 1-3 μm. When the thickness of the other porous layer has such a thickness, a sufficient effect of forming the porous layer (effect of improving insulation and strength, etc.) can be obtained, and product variation can be suppressed and productivity can be improved. Also, the adhesiveness to the electrode can be secured. If the thickness of the other porous layer is 5 μm or less, the bulk due to winding and lamination can be suppressed, which is suitable for increasing the capacity of the battery. Furthermore, it is possible to prevent the curl from becoming large and contribute to the improvement of productivity in the battery assembly process.

The binder component that constitutes the coating layer is not particularly limited, and known components can be used. For example, acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, and the like can be used.

The inorganic particles that make up the coating layer are not particularly limited, and known materials can be used. For example, titania, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon and the like can be used.

(Non-aqueous electrolyte secondary battery)
The polyolefin microporous membrane and coating film according to this embodiment can be suitably used as a separator for a non-aqueous electrolyte secondary battery. By using the polyolefin microporous film and the coating film according to the present embodiment for the separator, it is possible to provide a non-aqueous electrolyte secondary battery having excellent battery characteristics.

As an example of a non-aqueous electrolyte secondary battery to which the polyolefin microporous film and coating film according to the present embodiment are applied, an electrolytic solution containing an electrolyte is placed in a battery element in which a negative electrode and a positive electrode are arranged to face each other with a separator interposed therebetween. Those having a structure in which they are impregnated and enclosed in an exterior material are mentioned.

An example of a negative electrode is one in which a negative electrode mixture consisting of a negative electrode active material, a conductive aid, and a binder is formed on a current collector. As the negative electrode active material, a material capable of doping/dedoping lithium ions is used. Specific examples include carbon materials such as graphite and carbon, silicon oxides, silicon alloys, tin alloys, lithium metal, lithium alloys, and the like. Carbon materials such as acetylene black and ketjen black are used as conductive aids. Styrene-butadiene rubber, polyvinylidene fluoride, polyimide and the like are used as the binder. Copper foil, stainless steel foil, nickel foil, or the like is used as the current collector.

An example of the positive electrode is one in which a positive electrode mixture consisting of a positive electrode active material, a binder, and optionally a conductive aid is molded on a current collector. Examples of positive electrode active materials include lithium composite oxides containing at least one transition metal such as Mn, Fe, Co, and Ni. Specific examples include lithium nickelate, lithium cobaltate, and lithium manganate. Carbon materials such as acetylene black and ketjen black are used as conductive aids. Polyvinylidene fluoride or the like is used as the binder. Aluminum foil, stainless steel foil, or the like is used as the current collector.

As the electrolytic solution, for example, a solution obtained by dissolving a lithium salt in a non-aqueous solvent can be used. Lithium salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN(SO ₂ CF ₃ ) ₂ and the like. Non-aqueous solvents include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, γ-butyrolactone and the like. Generally, a mixture of two or more of these solvents is used together with various additives such as vinylene carbonate. Ionic liquids (normal temperature molten salts) such as imidazolium cations can also be used.

Exterior materials include metal cans and aluminum laminate packs. The shape of the battery includes coin type, cylindrical type, square type, laminate type, and the like.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[Measuring method]
(1) Film thickness The film thickness of the polyolefin microporous film was measured at 5 points within a range of 95 mm × 95 mm using a contact thickness meter (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 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 used. calculated by
Formula: Porosity (%) = ((volume - weight / membrane density) / volume) x 100
Here, the film density was set to 0.99. The film thickness measured in (1) above was used to calculate the volume.

(3) Air resistance For the polyolefin microporous membrane, air resistance (sec / 100 cm ³ ) was measured.

(4) Puncture strength Maximum load when a needle with a diameter of 1 mm (tip is 0.5 mmR) is used to pierce a polyolefin microporous membrane with a thickness of T (μm) and a porosity of P (%) at a speed of 2 mm/sec. The value S(gf) was measured. Further, the converted puncture strength was calculated by the following formula.
Formula: Converted puncture strength=S (gf)/film thickness (μm).

(5) Shutdown Temperature and Meltdown Temperature The polyolefin microporous membrane is exposed to an atmosphere of 30° C., and the air resistance is measured while raising the temperature at a rate of 5° C./min. The temperature at which the air resistance of the polyolefin microporous membrane reached 100,000 sec/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 was less than 100,000 sec/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. Moreover, the measurement is performed 5 times, and the average value is used as the value of the shutdown temperature and the meltdown temperature.

(6) Melting point and melting peak The melting point of the polyolefin resin and the melting peak of the polyolefin microporous membrane were determined by a scanning differential calorimeter (PYRIS DIAMOND DSC manufactured by Perkin Elmer). The polyolefin resin and the polyolefin microporous membrane were each placed in a sample holder, heated to 230°C at a rate of 10°C/min until completely melted, 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 . Using this as the first temperature rise, the same measurement was repeated, and the melting point (Tm) of the polyolefin resin and the melting peak of the polyolefin microporous membrane were determined from the endothermic peak at the second temperature rise. For the polyolefin resin, the peak with the heat of fusion of 70 J/g or more was regarded as the endothermic peak, and with the polyolefin microporous film, the peak with the heat of fusion of 3.0 J/g or more was regarded as the endothermic peak.

(7) Weight Average Molecular Weight (Mw) of Resin Raw Material, Molecular Weight Distribution, and Peak Area in Differential Molecular Weight Distribution Curve Obtained by permeation chromatography (GPC) method. Moreover, the differential molecular weight distribution curve was calculated by the following procedure. (A) A detection intensity (elution curve) versus elution time was calculated from a GPC differential refractive index detector (RI detector), and the elution time was converted to molecular weight. Here, the baseline of the elution curve was defined as the starting point of the retention time of the peak rise and the end point of the retention time of the peak end, and the peak detection interval was 0.017 minutes. (B) The intensity area was determined when the entire area of the elution curve was taken as 100%, and the concentration fraction of each molecular weight was determined. An integrated molecular weight curve was obtained by sequentially accumulating the concentration fractions and plotting the logarithmic value (log(M)) of the molecular weight on the horizontal axis and the integrated value of the concentration fraction (w) on the vertical axis. (C) The differential value of the curve at the logarithmic value of each molecular weight is obtained, the horizontal axis is the logarithmic value of the molecular weight (log (M)), and the vertical axis is the value obtained by differentiating the concentration fraction with the logarithmic value of the molecular weight (dw / dlog ( M)) was plotted to obtain a differential molecular weight distribution curve. The area where the differential molecular weight distribution curve of polypropylene and the differential molecular weight distribution curve of polyethylene overlap was calculated when the entire area of the obtained differential molecular weight distribution curve of polypropylene was taken as 100%.

[Measurement condition]
・Measurement device: Agilent high temperature GPC device PL-GPC220
・Column: Agilent PL1110-6200 (20 μm MIXED-A) x 2 ・Column temperature: 160°C
・ Solvent (mobile phase): 1,2,4-trichlorobenzene ・ Solvent flow rate: 1.0 mL / min ・ Sample concentration: 0.1 wt% (dissolution conditions: 160 ° C. / 3.5 H)
・Injection amount: 500 μL
・ Detector: Agilent differential refractive index detector (RI detector)
- Viscometer: Viscosity detector manufactured by Agilent - Calibration curve: Created by universal calibration curve method using monodisperse polystyrene standard sample.

(8) Weight average molecular weight (Mw) of polyolefin microporous membrane, molecular weight distribution and peak area in differential molecular weight distribution curve The weight average molecular weight (Mw) of the resin contained in the polyolefin microporous membrane is cross-fractionated using the following measurement conditions It was obtained by a chromatographic (CFC) method. Further, the differential molecular weight distribution curve of the polyolefin multi-layer, microporous membrane was calculated by the following procedure. (A) Measurement was performed by the CFC method under the following measurement conditions, an elution curve for the eluted polymer was obtained in the following elution sections in the range of 140°C to 0°C, and the elution time was converted to molecular weight. Here, the strongest peak in the 140-110°C region was taken as the peak of the polypropylene elution curve, and the strongest peak in the 110-75°C region was taken as the peak of the polyethylene elution curve. The baseline of the elution curve was set with the retention time at the rise of each peak as the start point and the retention time at the end of the peak as the end point, and the peak detection interval was 0.014 minutes. (B) The intensity area was determined when the overall area ratio of the elution curve was 100%, and the concentration fraction of each molecular weight was determined. An integrated molecular weight curve was obtained by sequentially accumulating the concentration fractions and plotting the logarithmic value (log(M)) of the molecular weight on the horizontal axis and the integrated value of the concentration fraction (w) on the vertical axis. (C) The differential value of the curve at the logarithmic value of each molecular weight is obtained, the horizontal axis is the logarithmic value of the molecular weight (log (M)), and the vertical axis is the value obtained by differentiating the concentration fraction with the logarithmic value of the molecular weight (dw / dlog ( M)) was plotted to obtain a differential molecular weight distribution curve. The area where the differential molecular weight distribution curve of polypropylene and the differential molecular weight distribution curve of polyethylene overlap was calculated when the entire area of the obtained differential molecular weight distribution curve of polypropylene was taken as 100%.

[Measurement condition]
Apparatus: CFC2 type cross fractionation chromatograph (Polymer Char)
Detector (built-in): IR4 infrared spectrophotometer (Polymer Char)
Detection wavelength: 3.42 μm
GPC column: Shodex HT-806M x 3 (Showa Denko)
Column temperature: 140°C
Column calibration: monodisperse polystyrene (Tosoh)
Molecular weight calibration method: standard calibration method (polystyrene conversion)
Eluent: o-dichlorobenzene (ODCB), dibutylhydroxytoluene (BHT) Addition flow rate: 1.0 mL/min
Sample concentration: 1.5 mg/mL
Injection volume: 0.5 mL
After injecting the sample into the column, the temperature was lowered from 140° C. to 0° C. over 140 minutes, then held at 0° C. for 30 minutes, and then the temperature was raised stepwise as described below to determine the eluate in each elution section. It was measured.
Elution division (°C): 0, 10, 20, 30, 40, 50, 60, 65, 70, 75, 80, 83, 86, 88, 90, 92, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 108, 110, 112, 114, 117, 120, 125, 130, 135, 140 (39 fractions above).

(9) Measurement of coefficient of variation of absorbance of polypropylene [Method for measuring coefficient of variation of absorbance]
A sample piece of 50 mm×50 mm cut out from the polyolefin microporous film was subjected to transmission measurement under the following conditions using a microscopic infrared spectrophotometer (FT/IR-6600, manufactured by JASCO Corporation).
・Measurement area 300 μm × 300 μm
・Number of measurement points: 40 x 40 points ・Measurement interval: 300 μm
・Total measurement area 12 x 12 mm
・Measurement wavenumber range 600 cm ^-1 to 4000 cm ^-1
・Resolution 4 cm ^-1
・ Number of times of accumulation 16 times In the graph plotting the wavenumber and absorbance on the horizontal axis and the vertical axis, respectively, the peak of wavenumber 1376 cm ^-1 derived from -CH ₃ was selected as a characteristic peak of polypropylene, and the height from the top of the peak to the baseline was obtained as the absorbance. A line obtained by drawing a straight line between the measured value (absorbance) at a wavenumber of 1330 cm ⁻¹ and the absorbance at a wavenumber of 1400 cm ⁻¹ was used as the baseline. Under the above conditions, the absorbance of a total of 1,600 polypropylene peaks of 40 × 40 points in the vertical and horizontal directions of the sample piece was obtained, and from the average value and standard deviation, the coefficient of variation of the absorbance of the polypropylene of the polyolefin microporous membrane (standard deviation / average value ).

(Example 1)
(1) Polyolefin solution A first polyolefin resin composition comprising 10 parts by mass of polypropylene (PP, melting point 162°C) with Mw of 2.0 x ¹⁰⁶ and 90 parts by mass of ultra-high molecular weight polyethylene with Mw of 1.5 x ¹⁰⁶ . 0.2 parts by mass of an antioxidant tetrakis[methylene-3-(3,5-ditert-butyl-4-hydroxyphenyl)-propionate]methane was added to 100 parts by mass of the product to prepare a resin mixture. 20 parts by mass of the resulting resin mixture was charged into the twin-screw extruder (1), and 80 parts by mass of liquid paraffin (35 cSt (40° C.)) was supplied as a plasticizer from the side feeder of the twin-screw extruder (1), A polyolefin solution was prepared by melt-kneading.

(2) Extrusion A polyolefin solution was supplied from a twin-screw extruder to a die and extruded to obtain an extrudate. The resulting extruded body was cooled while being taken up by a casting roll controlled to 35° C. at a take-up speed of 2 m/min to form a gel-like sheet.

(3) First Stretching, Removal of Plasticizer, and Drying The gel-like sheet was simultaneously biaxially stretched 8 times in both the MD and TD directions (first stretching) at 115° C. using a tenter stretching machine. The stretched gel-like sheet was fixed to an aluminum frame plate of 20 cm × 20 cm, immersed in a methylene chloride bath controlled at 25°C, shaken at 100 rpm for 3 minutes to remove liquid paraffin, and air-dried at room temperature to dry the film. got

(4) Heat setting The dried film was heat set at 130°C to obtain a polyolefin microporous film. Table 1 shows the blending ratio of each component, manufacturing conditions, evaluation results, etc. of the produced polyolefin microporous membrane.

(Examples 2 and 3)
In Examples 2 and 3, polyolefin microporous membranes were produced in the same manner as in Example 1 except for the conditions described in Table 1.

(Example 4)
(Preparation of polyolefin solution for first layer)
100 parts by mass of a first polyolefin resin consisting of 70 parts by mass of ultra high molecular weight polyethylene (UHMwPE) with Mw of 1.5×10 ⁶ and 30 parts by mass of high density polyethylene (HDPE) with Mw of 9.0×10 ⁴ , 0.2 parts by mass of an antioxidant tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane was added to prepare a resin mixture. 20 parts by mass of the resulting resin mixture was charged into the twin-screw extruder (1), and 80 parts by mass of liquid paraffin (35 cSt (40° C.)) was supplied as a plasticizer from the side feeder of the twin-screw extruder (1), Melt-kneading was performed to prepare a polyolefin solution for the first layer.

(Preparation of polyolefin solution for second layer)
Antioxidant tetrakis is added to the polyolefin resin of the second layer consisting of 10 parts by mass of ultra high molecular weight polypropylene with Mw of 2.0 × 10 ⁶ and 90 parts by weight of ultra high molecular weight polyethylene (UHMwPE) with Mw of 1.5 × 10 ⁶ 0.2 parts by mass of [methylene-3-(3,5-ditert-butyl-4-hydroxyphenyl)-propionate]methane was added to prepare a resin mixture. 17 parts by mass of the resulting resin mixture was charged into the twin-screw extruder (1) and another twin-screw extruder (2), and liquid paraffin (35 cSt (40° C.)) was supplied and melt-kneaded to prepare a polyolefin solution for the second layer.

In addition, in the differential molecular weight distribution curves obtained by the GPC method of the raw material polyethylene and the raw material polypropylene of the second layer of Example 4, a graph showing the overlapping area of both is shown in FIG.

(Extrusion)
The polyolefin solution for the first layer and the polyolefin solution for the second layer are supplied from each twin-screw extruder to the T-die for three layers, and the first polyolefin solution/second polyolefin solution/first polyolefin solution It was extruded in a layer ratio of 30/40/30. The extruded body was cooled while being taken up by a cooling roll to form a gel-like three-layer sheet.

(First stretching)
The gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both MD and TD (first stretching) at 110° C. with a tenter stretching machine.

(removal of plasticizer, drying)
The first stretched gel-like three-layer sheet was immersed in a methylene chloride bath in a washing tank to remove liquid paraffin, and air-dried at room temperature to obtain a dry film.

(Second stretching)
The dried film was stretched 1.35 times in the TD at 128.4°C (second stretch).

(Heat treatment)
The three-layer sheet subjected to the second stretching is subjected to a relaxation treatment of 0.85 times the TD at 128.4 ° C., and a polyolefin microporous membrane consisting of three layers of the first layer / second layer / first layer is obtained. Obtained.

Table 1 shows the mixing ratio of each component of the produced polyolefin microporous membrane, manufacturing conditions, evaluation results, etc. FIG. 3 shows a two-dimensional distribution diagram showing the absorbance intensity of polypropylene in Example 4. As shown in FIG.

[Examples 5 to 10]
A polyolefin microporous membrane was produced in the same manner as in Example 4 except for the conditions described in Tables 1 and 2.

[Comparative Example 1]
A microporous polyolefin membrane was produced in the same manner as in Example 1 except for the conditions listed in Table 3.

[Comparative Examples 2 to 7]
A microporous polyolefin membrane was produced in the same manner as in Example 4 except for the conditions listed in Table 3. FIG. 2 is a graph showing the overlapping area of the differential molecular weight distribution curves of the raw material polyethylene and the raw material polypropylene of Comparative Example 2 obtained by the GPC method. FIG. 4 shows a two-dimensional distribution diagram showing the absorbance intensity of polypropylene in Comparative Example 2. As shown in FIG.

PP Molecular weight distribution curve of raw polypropylene PE Molecular weight distribution curve of raw polyethylene 1 Overlapping area of raw polyethylene and raw polypropylene in Example 4 2 Overlapping area of raw polyethylene and raw polypropylene in Comparative Example 2 A Molecular weight distribution curve of polypropylene B Molecular weight of polyethylene Distribution curve 3 Overlapping area of polyethylene and polypropylene of the film of Example 6 in CFC evaluation 4 Overlapping area of polyethylene and polypropylene of the film of Comparative Example 1 in CFC evaluation

Claims

The meltdown temperature is 160° C. or higher and 190° C. or lower, the thickness-converted puncture strength is 35 gf/μm or higher and 75 gf/μm or lower, and the wave number 1376 cm −1 peak derived from —CH 3 is obtained by the following measurement method. Polyolefin microporous membrane having a coefficient of variation of absorbance of 0 to 5.0%:
[Method for measuring coefficient of variation of absorbance]
A 50 mm × 50 mm sample piece cut out from the polyolefin microporous film is subjected to transmission measurement under the following conditions using a microscopic infrared spectrophotometer (FT/IR-6600, manufactured by JASCO Corporation);
・Measurement area 300 μm × 300 μm
・Number of measurement points: 40 x 40 points ・Measurement interval: 300 μm
・Total measurement area 12 x 12 mm
・Measurement wavenumber range 600 cm -1 to 4000 cm -1
・Resolution 4 cm -1
・The number of times of integration 16 times Select the peak of wavenumber 1376 cm -1 in the graph plotting the wavenumber and absorbance on the horizontal axis and the vertical axis, respectively, and obtain the absorbance from the top of the peak to the baseline; the baseline is wavenumber 1330 cm Use a straight line drawn between the measured value (absorbance) at -1 and the absorbance at a wave number of 1400 cm -1 ; Determine the coefficient of variation (standard deviation/mean value) of the absorbance of polypropylene.
When the polyolefin microporous membrane was measured by cross fractionation chromatography (CFC method) under the following measurement conditions to obtain a molecular weight distribution curve, the overlapping area of the polypropylene molecular weight distribution curve and the polyethylene molecular weight distribution curve was The polyolefin microporous membrane according to claim 1, wherein the area of the entire molecular weight distribution curve of polypropylene is 15% or more:
[Measurement condition]
Using an IR4-type infrared spectrophotometer as a detector, obtain an elution curve for the eluted polymer in the following elution sections in the region of 140°C to 0°C, convert the elution time to molecular weight, and obtain a molecular weight distribution curve. ;
- Polypropylene: The peak detected in the 140-110°C region was taken as the polypropylene peak;
• Polyethylene: The strongest detection peak in the 110 to 75°C region was taken as the polyethylene peak.
The polyolefin microporous membrane according to claim 2, wherein the shutdown temperature is 134°C or higher and 140°C or lower.
The polyolefin microporous membrane according to claim 1 or 2, which has three layers and has at least a first layer containing polyethylene and a second layer containing polypropylene and polyethylene.
The first layer is a layer made of polyethylene having a weight average molecular weight (Mw) of 1 million or more and 2 million or less, and the second layer is a polyethylene having a weight average molecular weight (Mw) of 1 million or more and 2 million or less. 5. The polyolefin microporous membrane according to claim 4, which is a layer obtained using polypropylene having a weight average molecular weight (Mw) of 1,000,000 or more as a raw material.
In the molecular weight distribution curve obtained by measuring the molecular weight distribution of the raw material polyethylene and the raw material polypropylene in the second layer by GPC method, the area where the molecular weight distribution curve of the raw material polyethylene and the molecular weight distribution curve of the raw material polypropylene overlap is the molecular weight of the raw material polypropylene. 6. The polyolefin microporous membrane according to claim 5, wherein the area of the entire distribution curve is 20% or more.
The polyolefin microporous membrane according to claim 4, wherein the first layer accounts for 40 to 90% by mass and the second layer accounts for 10 to 60% by mass of the total mass of the polyolefin microporous membrane.
The polyolefin microporous membrane according to claim 4, which has a three-layer structure having the first layer (skin layer) on both sides of the second layer (core layer).
The polyolefin microporous membrane according to claim 1 or 2, which has a thickness of 9 μm or less.
A laminated polyolefin multilayer microporous membrane comprising one or more coating layers on at least one surface of the polyolefin microporous membrane according to claim 1 or 2.
A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to claim 1 or 2.