阅读说明：本技术 聚烯烃多层微多孔膜及其制造方法 (Polyolefin multilayer microporous film and method for producing same ) 是由 中嶋木乃美 松下胜彦 李丹 金子慧 于 2020-01-15 设计创作，主要内容包括：课题为提供聚烯烃多层微多孔膜,其在伴随着高能量密度化的薄膜隔膜中不仅具有迄今为止的外部短路性、高温形状保持特性,且还能改善高温时的耐异物性。解决手段为聚烯烃多层微多孔膜,其中,包含超高分子量聚丙烯和高密度聚乙烯的第1层形成于包含超高分子量聚乙烯和高密度聚乙烯的第2层的两面,在第1层中,聚丙烯含量小于20％的区域为30％以上且60％以下,聚丙烯含量为20％以上的区域的最大直径的平均值为0.1μm以上且10μm以下,上述聚丙烯含量是由利用AFM-IR测定的1465cm~(-1)的激光照射时和1376cm~(-1)的激光照射时的AFM悬臂的位移求出的,所述聚烯烃多层微多孔膜在90℃时的穿刺伸长率为0.40mm/μm以上。(The object is to provide a polyolefin multilayer microporous film which has not only external short-circuiting properties and high-temperature shape retention properties as in the past but also improved resistance to foreign matter at high temperatures in a film separator accompanied by an increase in energy density. The solution is a polyolefin multilayer microporous film, wherein 1 st layers comprising ultra-high molecular weight polypropylene and high density polyethylene are formed on both sides of 2 nd layers comprising ultra-high molecular weight polyethylene and high density polyethylene, the 1 st layers have a polypropylene content of less than 20% of the regions of 30% to 60%, and the average value of the maximum diameter of the regions having a polypropylene content of 20% or more is 0.1 [ mu ] m to 10 [ mu ] m, the polypropylene content being measured by AFM-IR1465cm ‑1 At the time of laser irradiation of 1376cm ‑1 The polyolefin multilayer microporous membrane has a puncture elongation at 90 ℃ of 0.40mm/μm or more, as determined from the displacement of the AFM cantilever during laser irradiation.)

1. A polyolefin multilayer microporous membrane wherein a 1 st layer comprising ultra-high molecular weight polypropylene and high density polyethylene is formed on both sides of a 2 nd layer comprising ultra-high molecular weight polyethylene and high density polyethylene,

in the 1 st layer, the region having a polypropylene content of less than 20% is 30% to 60%, and the average value of the maximum diameters of the regions having a polypropylene content of 20% or more is 0.1 μm to 10 μm, the polypropylene content being 1465cm measured by AFM-IR^-1At the time of laser irradiation of 1376cm^-1The displacement of the AFM cantilever during laser irradiation of (1),

the polyolefin multilayer microporous membrane has a puncture elongation at 90 ℃ of 0.40mm/μm or more.

2. The polyolefin multilayer microporous film according to claim 1, wherein the high density polyethylene of the 2 nd layer has a molecular weight distribution (Mw/Mn) of 11 or more.

3. The polyolefin multilayer microporous membrane according to claim 1 or 2, wherein a porous layer is laminated on at least one surface of the polyolefin multilayer microporous membrane.

4. The polyolefin multilayer microporous film according to any one of claims 1 to 3, which is used as a separator for a battery.

5. The method for producing a polyolefin multilayer microporous membrane according to any one of claims 1 to 4, comprising the steps (a) to (f),

a step (a) of adding a plasticizer to a polyolefin resin comprising a high-density polyethylene resin and an ultrahigh-molecular-weight polypropylene resin constituting the 1 st layer, and then melt-kneading the mixture in a range where the twin-screw extruder has an inner diameter of 58mm and an L/D of 42 and the Q/Ns (discharge amount/rotation speed) is 0.15 or more and less than 0.30 and the screw rotation speed (Ns) of the twin-screw extruder is 50rpm or more and less than 150rpm, thereby preparing a solution of the 1 st layer;

a step (b) of adding a plasticizer to the high-density polyethylene resin and the ultrahigh-molecular-weight polyethylene resin constituting the layer 2, and then melt-kneading the mixture to prepare a solution for the layer 2;

a step (c) of extruding the solution of the layer 1 and the solution of the layer 2 obtained in the steps (a) and (b) from a die, cooling at least one surface thereof at a rate at which the microphase is immobilized, and molding the resulting material into a gel-like multilayer sheet;

a step (d) of stretching the gel-like multilayer sheet in the machine direction and the width direction to obtain a multilayer stretch molded product;

a step (e) of extracting and removing the plasticizer from the multilayer stretched product, and drying the product to obtain a multilayer porous product;

and (f) heat-treating the multilayer porous molded product to obtain a polyolefin multilayer microporous membrane.

Technical Field

The present invention relates to a polyolefin multilayer microporous film and a method for producing the same.

Background

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among these, polyolefin microporous membranes using polyolefin as a resin material have been widely used in recent years as battery separators because they are excellent in chemical resistance, insulation properties, mechanical strength, and the like, and have shutdown characteristics.

Secondary batteries (for example, lithium ion secondary batteries) are widely used as batteries for personal computers, mobile phones, and the like because of their high energy density. In recent years, lithium ion secondary batteries, which are mounted as driving batteries for automobiles that cope with environmental problems, such as electric automobiles and hybrid automobiles, and are intended to increase the travel distance (longer than gasoline automobiles), have been increasingly required to have higher performance, such as higher energy density, and higher safety, and have been increasingly required to be improved at higher levels.

In particular, in the case of a large-sized high-capacity lithium ion battery, characteristics are important as a battery, and at the same time, higher reliability is also important. Specifically, for example, the thermal runaway temperature is lowered due to an increase in energy density, and thus, it is required to secure safety at a higher level. In view of safety, separators used in batteries are required to have high internal short-circuit resistance in addition to characteristics such as external short-circuit resistance and high-temperature heat resistance.

At this time, as a method for securing safety in the separator, there are known: by designing a method of preventing short-circuiting by avoiding film rupture with high strength, temperature rise inside the battery can be suppressed by controlling the behavior of the separator when exposed to high heat.

The nail penetration test is one of representative tests among main methods for evaluating safety against an internal short circuit. It is a test for observing the behavior of a battery when a nail is inserted into the battery to forcibly cause an internal short circuit, and it is known that the behavior of the battery at this time is controlled by the thermal shrinkage and melting characteristics of the separator.

In addition, the safety test that is generally performed differs depending on the kind of the secondary battery (for example, lithium ion secondary battery). In a safety test of a cylindrical battery in which a positive electrode, a negative electrode, and a separator are wound and housed in a can, a test called an impact test is performed in which a weight is dropped from the outside of the battery, and the presence or absence of short circuit, explosion, or ignition due to direct contact of the electrodes due to breakage of the separator is confirmed. On the other hand, in the case of a laminate type (also referred to as a pouch type, which is a structure in which positive and negative electrodes are laminated and sealed with a separator interposed therebetween without being housed in a can), the separator is reliably broken to short the inside thereof by the nail penetration test described above, and whether or not short-circuiting and explosion, ignition, and/or the degree thereof are caused by direct contact of the electrodes is confirmed.

Further, the polyolefin multilayer microporous membrane used for the separator is also required to have a shutdown (shut down) function for preventing a temperature rise of the lithium ion secondary battery. The shutdown function is a function of stopping the battery reaction by melting polyolefin constituting the separator at a high temperature to block pores, and shutdown characteristics at a lower temperature have been required in recent high energy density designs.

Further, the polyolefin multilayer microporous film used for the separator is required to have meltdown (melt down) characteristics in addition to the shutdown function. The meltdown property means a molten shape retaining property capable of preventing short circuit between electrodes by melting of the separator even when the temperature in the battery is further increased after shutdown.

The battery separator has insulation properties for preventing short circuit between both electrodes in a battery, ensures safety, has ion permeability by retaining an electrolyte in pores thereof, and plays an important role in battery safety, battery characteristics (for example, capacity, output characteristics, and cycle characteristics), and the like. In particular, the demand for the separator has become very strict in recent years, and further improvement of the separator is urgently required.

Patent document 1 discloses a microporous film which is formed from a thermoplastic resin composition containing 5 to 90 parts by mass of a polyphenylene ether resin per 100 parts by mass of a polypropylene resin, has a sea-island structure formed from a sea portion containing the polypropylene resin as a main component and an island portion containing the polyphenylene ether resin as a main component, has a high film rupture temperature at an interface between the sea portion and the island portion and a microporous film having pores formed in the sea portion, and has a good balance among permeability, puncture strength, electrical resistance and heat shrinkage when used as a battery separator.

Patent document 2 discloses a polyolefin multilayer microporous membrane including at least a 1 st microporous layer and a 2 nd microporous layer, the 1 st microporous layer being formed of a 1 st polyolefin resin containing polypropylene, the 2 nd microporous layer being formed of a 2 nd polyolefin resin containing ultrahigh molecular weight polyethylene, the polyolefin multilayer microporous membrane having a membrane thickness of 25 μm or less, a membrane thickness (μm) and a void ratio (%) being void ratio (%)/membrane thickness (μm) ≥ 3.0, and an air permeability in terms of 16 μm of 100 seconds/100 cc or more and 300 seconds/100 cc or less.

Patent document 3 discloses a multilayer microporous membrane comprising 1 st, 2 nd and 3 rd layers, wherein each of the 1 st and 3 rd layers comprises 40 to 97 wt% of a microporous membrane having a thickness of 1.0 × 10 based on the weight of the 1 st or 3 rd layer⁶Ethylene/alpha-olefin copolymer having the following Mw, and 0 to 25% by weight of an ethylene/alpha-olefin copolymer having a molecular weight of more than 1.0X 10⁶The Mw of the layer 2 comprising from 15 to 40 weight percent polypropylene, from 0 to 10 weight percent of a polyethylene having a molecular weight greater than 1.0 x 10, based on the weight of the layer 2⁶And 50 to 85% by weight of a polyethylene having an Mw ofHas a thickness of 1.0 × 10⁶The polyethylene has an Mw of 130.5 ℃ or lower, and the film has a shutdown temperature of 170.0 ℃ or higher.

Patent document 4 discloses that the composition comprises at least 2 layers, has a shutdown temperature of 129.5 to 135.0 ℃, an air permeability of 50 to 300 seconds/100 cc, a film thickness of 3 to 16 μm, a puncture strength of 100 to 400gf, and a shutdown speed of 1.55 × 10⁴～3.00×10⁴A polyolefin multilayer microporous film containing polyethylene as a main component in the range of seconds. It is described that the separator has excellent puncture strength and gas resistance, and can provide excellent safety in a nail penetration test, a hot box test, and the like when used in a lithium ion battery.

Patent document 5 discloses a microporous film comprising polypropylene, which has a weight average molecular weight Mw of 82 to 100 ten thousand, a pentad fraction (pentad fraction) of 90 to 95%, and a film thickness of 10 to 15 μm. Further, patent document 5 describes that a polyolefin microporous film is provided which has improved ion conductivity associated with high output characteristics of a battery, i.e., has good air permeability and puncture strength in a well-balanced manner, and particularly has an excellent balance between air permeability and strength in the case of a thin film, and has high safety and practicability as a separator for a lithium ion secondary battery.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 05528361 gazette

Patent document 2: international publication No. 2015/194667

Patent document 3: japanese Kohyo publication No. 2012-522354

Patent document 4: japanese patent laid-open publication No. 2015-208893

Patent document 5: japanese laid-open patent publication No. 2013-23673

Disclosure of Invention

Problems to be solved by the invention

Although the above documents and the like have achieved various improvements in performance, they have not provided a polyolefin multilayer microporous film and a battery separator and the like having excellent puncture elongation, which have high safety in terms of excellent shutdown characteristics and meltdown characteristics at the time of abnormal heat generation, and which have improved short-circuit resistance due to foreign matter or the like at high temperatures in the normal use range.

The present invention aims to provide a polyolefin multilayer microporous film, a battery separator, and the like, which have excellent shutdown properties and meltdown properties and have good puncture elongation at high temperatures.

Means for solving the problems

The polyolefin multilayer microporous membrane of the present invention has the following characteristics (1) to (5).

(1) A polyolefin multilayer microporous membrane wherein a 1 st layer comprising ultra-high molecular weight polypropylene and high density polyethylene is formed on both sides of a 2 nd layer comprising ultra-high molecular weight polyethylene and high density polyethylene,

in the 1 st layer, the region having a polypropylene content of less than 20% is 30% to 60%, and the average value of the maximum diameters of the regions having a polypropylene content of 20% or more is 0.1 μm to 10 μm, the polypropylene content being 1465cm measured by AFM-IR^-1At the time of laser irradiation of 1376cm^-1The displacement of the AFM cantilever during laser irradiation of (1),

the polyolefin multilayer microporous membrane has a puncture elongation at 90 ℃ of 0.40mm/μm or more.

(2) The polyolefin multilayer microporous membrane according to the above (1), wherein the high-density polyethylene of the 2 nd layer has a molecular weight distribution (Mw/Mn) of 11 or more.

(3) The polyolefin multilayer microporous membrane according to the above (1) or (2), wherein a porous layer is laminated on at least one surface of the polyolefin multilayer microporous membrane.

(4) The polyolefin multilayer microporous membrane according to any one of the above (1) to (3), which is used as a battery separator.

(5) The method for producing a polyolefin multilayer microporous membrane according to any one of the above (1) to (4), which comprises the following steps (a) to (f),

a step (a) of adding a plasticizer to a polyolefin resin comprising a high-density polyethylene resin and an ultrahigh-molecular-weight polypropylene resin constituting the 1 st layer, and then melt-kneading the mixture in a range where the twin-screw extruder has an inner diameter of 58mm and an L/D of 42 and the Q/Ns (discharge amount/rotation speed) is 0.15 or more and less than 0.30 and the screw rotation speed (Ns) of the twin-screw extruder is 50rpm or more and less than 150rpm, thereby preparing a solution of the 1 st layer;

a step (b) of adding a plasticizer to the high-density polyethylene resin and the ultrahigh-molecular-weight polyethylene resin constituting the layer 2, and then melt-kneading the mixture to prepare a solution for the layer 2;

a step (c) of extruding the solution of the layer 1 and the solution of the layer 2 obtained in the steps (a) and (b) from a die, cooling at least one surface thereof at a rate at which the microphase is immobilized, and molding the resulting material into a gel-like multilayer sheet;

a step (d) of stretching the gel-like multilayer sheet in the machine direction and the width direction to obtain a multilayer stretch molded product;

a step (e) of extracting and removing the plasticizer from the multilayer stretched product, and drying the product to obtain a multilayer porous product;

and (f) heat-treating the multilayer porous molded product to obtain a polyolefin multilayer microporous membrane.

Effects of the invention

According to the present invention, a polyolefin multilayer microporous film having both shutdown properties and meltdown properties and having good puncture elongation at high temperatures can be provided. When used as a separator, the battery can have improved safety. The puncture elongation at high temperature is not related to the puncture strength which is a conventional physical property. That is, even if the puncture strength at normal temperature is high, the puncture elongation is not necessarily high. In addition, even if the puncture strength and puncture elongation at normal temperature are high, the puncture elongation at high temperature is not necessarily high. In the present invention, the possibility of internal short-circuiting of the battery with increased pressure can be greatly reduced by increasing the puncture elongation at 90 ℃, which is a high-temperature use range of a normal battery.

Drawings

FIG. 1 is a map of the polypropylene content obtained by AFM-IR measurement.

Detailed Description

The followingThe present invention will be described in detail. In the polyolefin multilayer microporous membrane of the present invention, the 1 st layer comprising ultrahigh molecular weight polypropylene and high density polyethylene is formed on both sides of the 2 nd layer comprising ultrahigh molecular weight polyethylene and high density polyethylene, the 1 st layer has a polypropylene content of less than 20% of a region of 30% to 60% inclusive, and the average value of the maximum diameter of the region having a polypropylene content of 20% or more is 0.1 μm to 10 μm inclusive, the polypropylene content being 1465cm measured by AFM-IR^-1At the time of laser irradiation of 1376cm^-1The polyolefin multilayer microporous membrane has a puncture elongation at 90 ℃ of 0.40mm/μm or more, as determined from the displacement of the AFM cantilever during laser irradiation.

[ layer 1]

In layer 1, the polypropylene content (which is 1465cm as measured by AFM-IR) was measured by^-1At the time of laser irradiation of 1376cm^-1Obtained from the displacement of the AFM cantilever during laser irradiation) of less than 20% is 30% to 60% and the average value of the maximum diameter of the region having a polypropylene content of 20% or more is 0.1 μm to 10 μm, and excellent puncture elongation, melting temperature, and gas resistance can be obtained, and the safety of the battery can be improved.

As for the polypropylene content, 1465cm can be measured in the AFM-IR measurement of the sample^-1At the time of laser irradiation of 1376cm^-1The displacement of the AFM cantilever during laser irradiation of (1) is determined from the intensity ratio thereof. Through 1465cm^-1The CH deflection angle of polyethylene was measured by laser irradiation of (2)^-1Laser irradiation of polypropylene CH₃The content ratio of polyethylene to polypropylene can be determined by measuring the angle.

The average value of the maximum diameter of the region having the polypropylene content of 20% or more can be calculated by extracting the region having the polypropylene content of 20% or more by binarizing the image obtained by AFM-IR measurement using HALCON13 of MVTec Software, inc.

The ratio of the region having a polypropylene content of less than 20% and the average value of the maximum diameters of the regions having a polypropylene content of 20% or more can be adjusted as follows: the regulation is carried out by forming a certain kneaded state in such a manner that the uneven structure is maintained and forming a sea-island structure of polyethylene and polypropylene when the molten resin is solidified during the casting cooling thereof, so that the high molecular weight polypropylene exists in each of the micrometer sizes.

(1) Ultra-high molecular weight polypropylene

The weight average molecular weight (Mw) of the ultrahigh molecular weight polypropylene contained in the 1 st layer was 1X 10⁶The isotactic polypropylene is used as the main component. Other polypropylene components may also be included. The kind of the polypropylene is not particularly limited, and may be any of a homopolymer of propylene, a copolymer of propylene with other α -olefin and/or diene (propylene copolymer), or a mixture of 2 or more kinds selected from them. From the viewpoint of mechanical strength, reduction in the size of the through-hole diameter, and the like, it is preferable to use at least a homopolymer of isotactic propylene as the main component (70% by mass or more, preferably 80% by mass, more preferably 90% by mass or more in the polypropylene component), and it is more preferable to use a homopolymer of propylene alone.

As the propylene copolymer, any of a random copolymer and a block copolymer can be used. The α -olefin in the propylene copolymer is preferably an α -olefin having 8 or less carbon atoms. Examples of the α -olefin having 8 or less carbon atoms include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-octene, vinyl acetate, methyl methacrylate, styrene, and a combination thereof. The diolefin in the propylene copolymer is preferably a diolefin having 4 to 14 carbon atoms. Examples of the diolefin 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% when the propylene copolymer is 100 mol%.

The isotactic polypropylene contained in the 1 st layer preferably has a weight average molecular weight of 1X 10⁶Above, more preferably 1.2 × 10⁶Above, 1.2X 10 is particularly preferable⁶～4×10⁶. When Mw is within the above range, the polyolefinThe hydrocarbon multilayer microporous membrane is excellent in strength, gas barrier property and heat resistance.

Further, Mw was 5X 10 relative to 100 mass% of polypropylene contained in the 1 st layer⁴The content of the polypropylene below is preferably 1% by mass or more and 5% by mass or less. Mw of 5X 10⁴When the content of the polypropylene falls within the above range, the shutdown initiation temperature is lowered due to the presence of a small amount of low-molecular-weight components, and the safety is improved.

The polypropylene preferably has a molecular weight distribution (Mw/Mn) of 1.01 to 100, more preferably 1.1 to 50, and still more preferably 2.0 to 20. When the Mw/Mn is within the above range, the polyolefin multilayer microporous film of the present embodiment is excellent in strength and meltdown characteristics.

As the polypropylene, isotactic polypropylene having a pentad isotactic fraction (mmmm fraction) of 92% to 98%, more preferably 93% to 97%, still more preferably 94% to 96% is preferably used. When the mmmm fraction is 92.0% or more, the balance between the puncture elongation at 90 ℃ and the strength is good, and the foreign matter resistance is good. When the isotactic pentad fraction is in the above range, the puncture elongation at 90 ℃ is improved in addition to the high-temperature melting property, the very good gas barrier property and the appearance.

As other types of polypropylene that are also conventionally used, syndiotactic polypropylene and atactic polypropylene are mentioned, but when used as a main component, it is not suitable for forming appropriate crystallinity and a layer separation structure, and improvement of puncture elongation cannot be expected.

The Mw, Mw/Mn, and mmmm fraction are values measured by the methods described later.

The polypropylene content in the 1 st layer is preferably 4 mass% or more and less than 10 mass% when the entire polyolefin multilayer microporous membrane resin is 100 mass%. When the content of polypropylene is within the above range, the polyolefin multilayer microporous film has good strength and gas resistance.

(2) High density polyethylene

The high density polyethylene contained in layer 1 was a polyethylene having a density of 0.94g/cm³The above polyethylene. Here, the weight average molecular weight (Mw) of the high density polyethylene is preferably 1 × 10⁵Above and less than 1 × 10⁶More preferably 1.5X 10⁵Above and 9X 10⁵Hereinafter, more preferably 2 × 10⁵Above and 8 × 10⁵The following. When Mw is within the above range, strength and appearance are good.

When the total amount of the resin contained in the 1 st layer is 100 mass%, the content of the high-density polyethylene contained in the 1 st layer is preferably 50 mass% or more, more preferably 60 mass% or more, and still more preferably 60 mass% or more and 80 mass% or less. When the content of the high-density polyethylene is 50% by mass or more, the strength when the film is formed is excellent and the appearance is good.

(3) Island structure

In the prior art, when 2 different raw materials are mixed, it is generally considered that the raw materials are mixed as well as possible. The invention finds the following techniques: the high molecular weight polypropylene is present in the micron size, and good puncture elongation, melt resistance and gas barrier properties are obtained, without mixing the high molecular weight polypropylene and the high density polyethylene uniformly as much as possible, but forming a certain kneaded state so as to maintain an uneven structure, and forming a sea-island structure of polyethylene and polypropylene when the molten resin is solidified during the casting and cooling process, thereby maintaining the sea-island structure even when the microporous membrane is formed.

The sea-island structure referred to herein is a structure in which a region having a polypropylene content of less than 20% and a polyethylene content of 80% or more is a sea portion, and a region having a polypropylene content of 20% or more and a polyethylene content of less than 80% is an island portion. In the layer 1, the sea portion is preferably 30% or more and less than 60%. The average value of the maximum diameter of the island portion is preferably 0.1 μm to 10 μm in size. By setting the average value of the maximum diameters of the sea region and the island region within the above range, favorable puncture elongation, melting temperature, and gas resistance can be obtained, and the safety of the battery can be improved.

[ layer 2]

(1) Ultra-high molecular weight polyethylene

The 2 nd layer contains a polymer having a weight average molecular weight (Mw) of 1X 10⁶More than ten thousand ultra high molecular weight polyethylenes. In the present invention, when the ultra-high molecular weight polyethylene is added to the layer 1, the compatibility is significantly reduced due to the viscosity difference with the polypropylene contained in the layer 1, and uniform mixing is difficult. As a result, the uniformity of the formed film is deteriorated, the manufacturing process becomes unstable, and quality variation is likely to occur. From the above viewpoint, the ultra-high molecular weight polyethylene having low compatibility with polypropylene is contained in the 2 nd layer, not the 1 st layer. The ultra-high-molecular-weight polyethylene is not particularly limited within the range satisfying the Mw described above, and a commonly used material may be used, and an ethylene- α -olefin copolymer may be used in addition to the homopolymer of ethylene.

The content of the ultrahigh-molecular-weight polyethylene is preferably 20 mass% or more and less than 50 mass% with respect to 100 mass% of the entire polyolefin multilayer microporous membrane resin. When the content of the ultra-high molecular weight polyethylene is within the above range, the polyolefin multilayer microporous film is excellent in strength and appearance.

(2) High density polyethylene

Layer 2 also comprises high density polyethylene. The high density polyethylene preferably has a density of 0.94g/cm³And a molecular weight distribution (Mw/Mn) of 10 or more. When Mw/Mn is within the above range, the shutdown temperature and puncture elongation become good, and the safety of the battery is improved. The content of the high-density polyethylene in the 2 nd layer is preferably 50 mass% or more, more preferably 60 mass% or more, and further preferably 60 mass% or more and 80 mass% or less, assuming that the total amount of the resin contained in the 2 nd layer is 100 mass%. When the content of the high-density polyethylene is 50% by mass or more, the strength when the film is formed and the puncture elongation at 90 ℃ are excellent, and the appearance is good.

[ method for producing polyolefin multilayer microporous film ]

The method for producing a polyolefin multilayer microporous membrane of the present invention comprises the following steps.

(A) Preparation of layer 1 and layer 2 solutions

(B) Forming of gel-like multilayer sheet

(C) Stretching No. 1

(D) Removal of plasticizers

(E) Drying

(F) Stretch No. 2 (optional)

(G) Thermal treatment

(H) Formation of other porous layers.

(A) Preparation of layer 1 and layer 2 solutions

A plasticizer was added to the polyolefin resin in a twin-screw extruder, and melt-kneading was performed to prepare a layer 1 solution and a layer 2 solution, respectively. The plasticizer is added in at least 2 stages, i.e., the first half and the second half of kneading. In the addition in the 1 st stage, the plasticizer is incorporated into the resin to sufficiently swell the resin, and the resin is mixed. The delivery of the molten resin in the extruder is enhanced by the addition of the next stage 2. The ratio of the plasticizer added in the 1 st stage and the 2 nd stage is preferably 70% or more and 90% or less in the 1 st stage, and 10% or more and 30% or less in the 2 nd stage. When the addition ratio in the 1 st stage exceeds 90%, the amount of the plasticizer incorporated into the resin increases, and thus the viscosity of the molten resin increases. In addition, since the addition ratio of the plasticizer in the 2 nd stage is decreased, the transportation of the molten resin in a high viscosity state becomes difficult, and the possibility of the occurrence of a feed bottleneck (feed sock) is increased. If the ratio of the 1 st stage is less than 70%, the plasticizer necessary for swelling the resin is insufficient, and the resin is not melted due to insufficient kneading, resulting in poor appearance. Even if the addition ratio of the plasticizer in the 2 nd stage is increased, the swelling of the resin is not affected in the constitution of the extruder.

From the viewpoint of phase separation, when the addition ratio of the plasticizer in the 1 st stage exceeds 90%, the resin concentration of polyethylene and polypropylene in the plasticizer is low. Since the molecules are present at a sufficiently spaced distance from each other, the dispersed phase size becomes fine and the effect of the 90 ℃ puncture elongation is reduced. When the addition ratio of the plasticizer in the 1 st stage is less than 70%, the resin concentration of polyethylene and polypropylene in the plasticizer becomes high, and the intermolecular distance becomes short. Different kinds of polyolefins cause entropy repulsion, and polyolefin resins of the same kind aggregate, and thus the dispersed phase size increases. As a result, problems such as unevenness of the film due to tensile stress concentration occur due to this aspect.

When the addition ratio of the plasticizer in the 1 st stage and the plasticizer in the 2 nd stage are within the above range, a good viscosity for the transportation of the molten resin can be obtained, and the polyethylene and the polypropylene form an appropriate phase separation structure, so that the puncture elongation at 90 ℃. The plasticizer is added in stages at an appropriate addition ratio, whereby the kneading property of the resin, the transportability of the molten resin, and the phase separation structure can be controlled.

The blending ratio of the polyolefin resin and the plasticizer in the layer 1 is preferably 20 to 25% by weight, based on 100% by weight of the total of the polyolefin resin and the plasticizer. When the polyolefin resin concentration of the layer 1 is within the above range, the porosity after film formation is reduced, and the permeability is improved, thereby improving the battery performance.

In addition, the compounding ratio of the polyolefin resin and the plasticizer in the 2 nd layer is preferably 20 to 30% by mass when the total amount of the polyolefin resin and the plasticizer is 100% by weight. When the polyolefin resin concentration of the 2 nd layer is in the above range, swelling (swell) and necking at the die outlet can be prevented when the polyolefin solution is extruded, and the moldability and self-supporting property of the extruded article can be improved. From the viewpoint of phase separation, by setting the content of the polyolefin resin of the 1 st layer within the above range, the polyethylene and the polypropylene can maintain an appropriate intermolecular distance, forming a phase separation structure effective for a puncture elongation at 90 ℃.

The 1 st and 2 nd layer solutions were supplied from the extruders to 1 die, and the two solutions were combined in a layer form so that the 1 st layer was formed on both surfaces of the 2 nd layer at the die, and extruded into a sheet form to obtain an extruded molded body. The extrusion method may be either a flat die method or an inflation method. In any of these methods, a method of supplying the solutions to respective manifolds and laminating the solutions at an inlet of a lip of a Multi-layer die (Multi-manifold method) or a method of flowing the solutions in layers in advance and supplying the solutions to the die (block method) may be employed. The multi-manifold method and the feedblock method can be applied to the general methods. The gap between the flat dies for multi-layer use can be set to 0.1 to 5 mm. The extrusion temperature is preferably 140-250 ℃, and the extrusion speed is preferably 0.2-15 m/min.

By adjusting the extrusion amounts of the solutions for the 1 st layer and the 2 nd layer, the film thickness ratio of the microporous layers a and B can be adjusted.

In the present invention, the high molecular weight polypropylene and the high density polyethylene are not uniformly mixed as much as possible, but are formed in a certain kneaded state in such a manner that an uneven structure is maintained, and a sea-island structure of polyethylene and polypropylene is formed when a molten resin is solidified during the casting cooling thereof.

The method for forming the sea-island structure is not particularly limited, and specific methods are shown below. First, when the raw material of the layer 1 is kneaded in the extruder, when the twin-screw extruder has an inner diameter of 58mm and an L/D of 42, the Q/Ns (discharge amount/rotation speed) is preferably 0.15 or more and less than 0.30, and the screw rotation speed (Ns) of the twin-screw extruder is preferably 50rpm or more and less than 150 rpm. Further, by setting the set temperature of the extruder to 140 ℃ to 210 ℃ and controlling the resin temperature during kneading to 210 ℃ or lower, it is possible to simultaneously suppress the decrease in molecular weight and form an uneven structure, and it is possible to obtain good puncture elongation, melt resistance, and air resistance.

When the Q/Ns is less than 0.15 and the resin temperature is higher than 210 ℃, molecular deterioration is promoted by shear and heat during kneading, and the deterioration of the strength, the lowering of the melting temperature, and the falling of low-molecular-weight components are caused, resulting in deterioration of the workability. When the Q/Ns is 0.30 or more and the resin temperature is less than 140 ℃, although the puncture elongation is expected to be improved, the resin is not sufficiently melted, the separation of polyethylene and polypropylene is large, the physical properties in the product fluctuate greatly, and the appearance is adversely affected.

Further, the Q/Ns (discharge amount/rotation speed) can be increased as much as possible by further increasing the inner diameter of the extruder, changing the screw length, and the like, but it is important that the high molecular weight polypropylene is present in the micrometer size by keeping the dispersion degree at a certain level or less in addition to good puncture elongation.

By kneading within the above-specified range, excessive deterioration of molecules can be suppressed, the gas barrier can be maintained at a low level, the resistance due to the battery output characteristics can also be suppressed at a low level, and a good puncture elongation at 90 ℃ and a low shutdown temperature can also be obtained.

(B) Forming of gel-like multilayer sheet

The obtained extrusion molded article was cooled to mold a gel-like multilayer sheet. Upon cooling, the microphase (micro phase) of the solutions of the layer 1 and the layer 2 separated by the plasticizer can be fixed. In general, when the cooling rate is lowered, the pseudo cell units are increased, and the higher-order structure of the obtained gel-like multi-layer sheet is coarsened, but when the cooling rate is raised, the cell units become compact. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, or a method of contacting with a cooling roller can be used.

The cooling temperature may be set arbitrarily, but is preferably 15 to 40 ℃. The cooling rate is preferably in the range of 0.1 to 100 ℃/sec to 50 ℃ or less, more preferably 0.5 to 50 ℃/sec, and particularly preferably 1.0 to 30 ℃/sec. When the cooling rate is in the above range, the strength of the polyolefin multilayer microporous film becomes good. When the cooling speed is less than 0.1 ℃/s, not only can a uniform gel sheet not be formed, but also the gas resistance is improved due to excessive phase separation development of polypropylene; when the cooling rate exceeds 100 ℃/sec, phase separation of polypropylene does not occur, and a structure effective for 90 ℃ puncture elongation cannot be formed.

(C) Stretching No. 1

The obtained gel-like multilayer sheet was stretched at least in a uniaxial direction. The gel-like multilayer sheet can be uniformly stretched by containing a plasticizer. The gel-like multilayer sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof after heating. The stretching may be uniaxial stretching or biaxial stretching, and biaxial stretching is preferable. In the case of biaxial stretching, simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

The stretch ratio (area stretch ratio) is preferably 2 times or more, and more preferably 3 to 30 times in the case of uniaxial stretching. In the case of biaxial stretching, the stretching ratio is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretch ratios in both the machine direction and the width direction are preferably 3 or more, and the stretch ratios in the machine direction and the width direction may be the same or different from each other. The stretching ratio in this step is an area stretching ratio of the microporous membrane immediately before the microporous membrane is subjected to this step and immediately before the microporous membrane is subjected to the next step.

The lower limit of the stretching temperature is preferably 90 ℃ or more, more preferably 110 ℃ or more, more preferably 112 ℃ or more, and further preferably 113 ℃ or more. The upper limit of the stretching temperature is preferably 135 ℃ or lower, more preferably 132 ℃ or lower, and still more preferably 130 ℃ or lower. When the stretching temperature is within the above range, film breakage is suppressed by stretching the polyolefin resin having a low melting point component, and high-ratio stretching can be achieved. Further, the polyolefin phase is refined to form a large number of fibrils three-dimensionally. By stretching in an appropriate temperature range, the through-hole diameter can be controlled, and a high porosity can be achieved even with a thin film thickness. This makes it possible to form a film suitable for a safer and higher-performance battery separator.

(D) Removal of plasticizers

The removal (washing) of the plasticizer is performed using a washing solvent. Since a washing solvent and a method for removing a plasticizer using the washing solvent are well known, the description thereof is omitted. For example, the method disclosed in the specification of Japanese patent No. 2132327 and Japanese patent application laid-open No. 2002-256099 can be used.

(E) Drying

And drying the multilayer microporous membrane after the plasticizer is removed by a heating drying method or an air drying method. Any method capable of removing the washing solvent, including conventional methods such as heat drying and air drying (air blowing), can be used. The treatment conditions for removing volatile substances such as a washing solvent can be the same as those disclosed in, for example, PCT patent publication nos. WO2008/016174 and WO 2007/132942.

(F) Stretch No. 2 (optional)

The dried multilayer microporous film is preferably stretched again at least in a uniaxial direction. The multilayer microporous film can be stretched by a tenter method or the like in the same manner as the above-described first stretching while heating. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferable. In the case of biaxial stretching, simultaneous biaxial stretching and sequential stretching may be used, but simultaneous biaxial stretching is preferable. The stretching temperature is not particularly limited, but is usually preferably 90 to 135 ℃ and more preferably 95 to 130 ℃. When the film is stretched again within the above range, the film is stretched while keeping the temperature sufficiently, and therefore, the film is not easily broken during stretching, and the phase separation structure of the polypropylene can be maintained.

(G) Thermal treatment

The 2 nd stretched multilayer microporous film is preferably subjected to heat treatment. The multilayer microporous membrane is subjected to heat treatment in a state where the width is fixed while being held by a clip (widthwise heat-fixing treatment step). The heat treatment is preferably 115 ℃ to 135 ℃. By performing the heat treatment at 115 to 135 ℃, the multi-layer microporous membrane becomes stable in crystallization at that temperature, the flaky crystals become uniform, and the shrinkage in the width direction can be reduced.

(H) Formation of other porous layers

The obtained multilayer microporous membrane may be provided with a layer other than the 1 st and 2 nd layers on at least one surface thereof. Examples of the other layer include a porous layer (coating layer) formed using a filler-containing resin solution containing a filler and a resin binder, or using a heat-resistant resin solution. The coating treatment can be performed as needed as described in PCT patent publication No. WO2008/016174, for example.

[ lithium ion Secondary Battery ]

An example of a lithium ion secondary battery to which the polyolefin multilayer microporous film of the present embodiment is applied includes a battery element in which a negative electrode and a positive electrode are disposed to face each other with a separator interposed therebetween, and an electrolyte solution. The electrode structure is not particularly limited, and a conventionally known structure may be used, and examples thereof include an electrode structure in which disk-shaped positive and negative electrodes are arranged so as to face each other (button type), an electrode structure in which flat plate-shaped positive and negative electrodes are alternately laminated (laminated type), and an electrode structure in which a strip-shaped positive and negative electrodes are laminated and wound (wound type). The current collector, the positive electrode active material, the negative electrode active material, and the electrolyte solution used in the lithium ion 2-time battery are not particularly limited, and conventionally known materials may be used in appropriate combinations.

Examples

The present invention will be described in more detail with reference to examples, but the embodiments of the present invention are not limited to the following examples. The evaluation methods, analysis methods and materials used in the examples are as follows.

(1) Weight average molecular weight (Mw), molecular weight distribution (Mw/Mn)

The weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw/Mn) of polypropylene, ultrahigh molecular weight polyethylene, and high density polyethylene were determined by Gel Permeation Chromatography (GPC) under the following conditions.

The measurement device: GPC-150C manufactured by Waters Corporation

Column chromatography: shodex UT806M manufactured by Showa Denko K.K

Column temperature: 135 deg.C

Solvent (mobile phase): ortho-dichlorobenzene

Solvent flow rate: 1.0ml/min

Sample concentration: 0.1 wt% (dissolution conditions: 135 ℃/1h)

Sample size: 500. mu.l

The detector: differential refractometer (RI detector) manufactured by Waters Corporation

Standard curve: the calibration curve obtained using the monodisperse polystyrene standard sample was prepared using a predetermined conversion constant.

(2) Isotactic pentad fraction (mmmm fraction)

Isotactic pentad fraction (mmmm fraction) represents the proportion of isotactic chains in pentad units in a molecular chain, and is the fraction of propylene monomer units in the center of a chain in which 5 propylene monomer units are continuously meso-bonded. Pentad fraction of propylene homopolymer¹³C-NMR was measured under the following conditions, and the isotactic pentad fraction was (peak area at 21.7 ppm)/(peak area at 19 to 23 ppm).

The measurement device: JNM-Lambada400 (manufactured by Japan electronic Co., Ltd.)

Resolution: 400MHz

Measurement temperature: 125 deg.C

Solvent: 1, 2, 4-trichlorobenzene/deuterated benzene ═ 7/4

Pulse width: 7.8 μ sec (microsecond)

Pulse interval: 5 seconds

Cumulative number of times: 2000 times

Displacement reference: TMS 0ppm

Mode: single pulse broadband decoupling.

(3) Film thickness (mum)

A sample piece of 95 mm. times.95 mm square was cut out, and the film thickness at 5 positions in the range was measured by a contact thickness meter (Litimatic, manufactured by Sanfeng corporation), and the average value was defined as the film thickness.

(4) Air resistance (second/100 cc)

The microporous membrane was subjected to air resistance (seconds/100 cm) measurement by air permeability meter (EGO-1T, manufactured by Asahi Seiki Kasei K.K.) according to JIS P-8117 Wangshan tester method³) And (4) carrying out measurement.

(5) Pricking Strength (gf/μm) at 90 ℃

The maximum load at the time of piercing the microporous membrane with a needle having a diameter of 1mm and a spherical tip (radius of curvature R: 0.5mm) at a speed of 2 mm/sec was measured at 90 ℃. The measurement was performed 3 times, and the average of the maximum load per unit film thickness was defined as the puncture strength at 90 ℃.

(6)90 ℃ spur strength (mm)

The distance of movement of the needle tip from the contact with the microporous membrane to the puncture and rupture of the needle having a diameter of 1mm and a spherical tip (radius of curvature R: 0.5mm) was measured at a speed of 2 mm/sec in an atmosphere of 90 ℃. The measurement was performed 3 times, and the average value of the moving distance of the needle tip per unit film thickness was defined as the puncture elongation at 90 ℃.

(7) Shutdown temperature, melting temperature

The microporous membrane was heated at a temperature rise rate of 5 ℃/min, and the gas resistance was measured by a Wang gas resistance meter (EGO-1T, manufactured by Asahi Seiki Seisakusho K.K.) to determine that the gas resistance reached the detection limit (i.e., 1X 10⁵Seconds/100 cc) as the off temperature (. degree. C.). After the shutdown, the heating was continued, and the air resistance was determined to be less than 1X 10 again⁵The temperature in seconds/100 cc was taken as the melting temperature (. degree. C.).

(8) AFM-IR measurement

The microporous polyolefin membrane obtained in example was cut out, and cut by a microtome to obtain a cross section in the machine direction, and the cross section having a thickness of 500nm was sliced to obtain a sample. The sample was fixed to a ZnSe prism for AFM-IR, and the cross section of the 1 st layer was irradiated with an infrared laser beam from the prism side under ATR conditions, and the thermal expansion of the sample due to light absorption was detected by the displacement of the AFM cantilever.

The sample was irradiated with an infrared laser under the following conditions to perform measurement.

The measurement device: NanoIR Spectroscopy System (manufactured by Anasys Instruments Co., Ltd.)

Light source: tunable Pulsed Laser (1kHz)

AFM mode: contact mode

Determination of wavenumber range: 1575-1200 cm^-1

Wavenumber resolution: 2cm^-1

Coaverages (co-mean): 32

Cumulative number of times: 2 times or more

Angle of polarization: 45 degree

Number of measurement points: 2

In order to visualize the distribution of polypropylene in the cross section of the 1 st layer, AFM-IR measurement was performed on a region corresponding to the 1 st layer of the sample (a region having a mechanical direction of 10 μm and including all the 1 st layer from the surface layer portion of the thin film in the thickness direction). For AFM-IR measurement, 1465cm was measured on the sample^-1Is irradiated with laser light of 1376cm^-1The AFM cantilever displacement during laser irradiation of (1) was measured, and the polypropylene content was determined from the intensity ratio and mapped (FIG. 1). By 1465cm^-1When irradiated with laser, the angle of change of CH of polyethylene was 1376cm^-1CH of polypropylene upon laser irradiation₃The contents of polyethylene and polypropylene can be determined by angle change measurement. Further, a region having a polypropylene content of 20% or more (reference sign a in fig. 1) and a region having a polypropylene content of less than 20% (reference sign b in fig. 1) were separated, and the ratio of the region having a polypropylene content of less than 20% in the region of the layer 1 was determined. Further, the image obtained by AFM-IR measurement was binarized by using HALCON13 of MVTec Software to extract a region having a polypropylene content of 20% or more, and the average value of the maximum diameter was calculated. The area of layer 1 was determined from an optical microscope image of the specimen.

In the polyolefin microporous membranes obtained in the respective examples, a membrane having a polypropylene content of less than 20% (sea) of 30% to 60% was evaluated as "o", and the other membranes were evaluated as "x". Further, a film having an average value of the maximum diameters of the regions (islands) having a polypropylene content of 20% or more in the layer 1 of 0.1 μm or more and 10 μm or less was evaluated as "o", and other films were evaluated as "x".

(9) Output characteristic

When used as a battery separator, the ionic resistance is reduced to improve the output characteristics of the battery. The microporous membrane was evaluated as good (O) when the gas barrier was less than 200 seconds/100 cc and as bad (X) when the gas barrier was not less than 200 seconds/100 cc.

(10) Resistance to foreign matter

In the case where foreign matter is present in the high-temperature battery, the membrane preferably has a high elongation in order to prevent the rupture of the separator due to the foreign matter, and the puncture elongation at 90 ℃ which is a high-temperature use range of a conventional battery is preferably high. The puncture elongation at 90 ℃ of the microporous membrane was evaluated as good (. smallcircle.) at 0.35 mm/. mu.m or more and poor (. smallcircle.) at less than 0.35 mm/. mu.m.

(11) High temperature shape retention characteristics

In order to maintain an insulating state and withstand inertial heat generation even after the battery abnormally generates heat and exhibits a shutdown function, the film preferably has high heat resistance, and specifically, the microporous film preferably has a high melting temperature. Therefore, the melting temperature, which is a high-temperature shape-retaining property of the microporous film, was evaluated as good (. largecircle.) at 170 ℃ or higher, which could not be achieved by the low-melting PE alone, and as bad (. largecircle.) at a temperature lower than 170 ℃.

(example 1)

(1) Preparation of layer 1 polyolefin resin solution

Mw of 2.0X 10 in 20 mass%⁶And (3) an ultrahigh molecular weight polypropylene (isotactic, isotactic pentad fraction 95.5%) and an Mw of 80 mass% of 4.0X 10⁵The high-density polyethylene (2) was blended with 0.2 mass% of tetrakis [ methylene-3- (3, 5-di-tert-butyl-4-hydroxyphenyl) -propionate ] as an antioxidant in 100 mass% of polyolefin]Methane, to produce a polyolefin mixture. The obtained polyolefin mixture was charged into a twin-screw extruder (inner diameter: 58mm, L/D: 42), and liquid paraffin was added from a 2-side feeder of the twin-screw extruder so that the concentration of the polyolefin resin became 23 mass%. The liquid paraffin was added in a ratio of 75% from the upstream side feeder and 25% from the downstream side feeder. The polyolefin resin solution of the layer 1 was prepared while maintaining the discharge amount (Q) of the polyolefin mixture at 33.9kg/h, the kneading temperature at 200 ℃ and the screw rotation speed (Ns) at 138rpm (maintaining the discharge amount/rotation speed (Q/Ns) at 0.25 kg/h/rpm).

(2) Preparation of layer 2 polyolefin resin solution

Mw of 60 mass% of the total⁵And 40 mass% of a high density polyethylene (Mw/Mn of 15) having an Mw of 2.0X 10⁶The same antioxidant as in layer 1 was added in an amount of 0.2 mass% to 100 mass% of the polyolefin of the ultrahigh molecular weight polyethylene to prepare a polyolefin mixture. The obtained polyolefin mixture was charged into a twin-screw extruder (inner diameter: 58mm, L/D: 42), and liquid paraffin was added from a 2-side feeder of the twin-screw extruder so that the concentration of the polyolefin resin became 25 mass%. The liquid paraffin was added in a ratio of 75% from the upstream side feeder and 25% from the downstream side feeder. The polyolefin resin solution of layer 2 was prepared while maintaining the discharge amount (Q) of the polyolefin mixture at 72.1kg/h, the kneading temperature at 200 ℃ and the screw rotation speed (Ns) at 292rpm (maintaining the Q/Ns at 0.25 kg/h/rpm).

(3) Extrusion

Each resin solution was fed from a twin-screw extruder to a T-die for three layers so as to be a resin solution for layer 1/a resin solution for layer 2/a resin solution for layer 1, and extruded so that the layer thickness ratio became 1/8/1. The extruded molded article was cooled while being pulled at a pulling speed of 4m/min by a cooling roll adjusted to 25 ℃ to form a gel-like three-layer sheet.

(4) Stretching, removal of film-forming solvent, and drying

The gel-like three-layer sheet was simultaneously biaxially stretched 5-fold in both the machine direction and the width direction (No. 1 stretch) by a tenter stretcher at 119 ℃, and in this state, the sheet width was directly fixed in the tenter stretcher, and heat-fixing was performed at a temperature of 110 ℃. Then, the stretched gel-like three-layer sheet was immersed in a dichloromethane bath in a washing tank, and the liquid paraffin was removed and air-dried at room temperature.

(5) 2 nd stretching and Heat treatment

Then, after preheating at 125 ℃, 1.5-fold stretching (2 nd stretching) was performed in the width direction by a tenter stretcher, and after performing relaxation of 4% in the width direction, heat-setting was performed at 126 ℃ while keeping on the tenter, to obtain a polyolefin multilayer microporous film. The film characteristics and battery characteristics of the resulting polyolefin multilayer microporous film are shown in table 1.

(example 2)

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the resin composition was changed to 25 mass% of ultrahigh-molecular-weight polypropylene and 75 mass% of high-density polyethylene in the preparation of the polyolefin resin solution of the layer 1.

(example 3)

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was replaced with Mw of 2.0X 10⁶The polyolefin multilayer microporous membrane was obtained in the same manner as in example 1, except that the resin composition of the ultrahigh-molecular-weight polypropylene (isotactic/pentad fraction: 94.8%) in the preparation of the polyolefin resin solution of the layer 2 was changed to 70 mass% of high-density polyethylene and 30 mass% of ultrahigh-molecular-weight polyethylene.

(example 4)

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was replaced with Mw of 2.0X 10⁶A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the resin composition was changed to 75 mass% of high-density polyethylene and 25 mass% of ultrahigh-molecular-weight polyethylene in the preparation of the polyolefin resin solution of the layer 2 (isotactic/pentad fraction 94.8%).

(example 5)

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was replaced with Mw of 2.0X 10⁶In the preparation of the polyolefin resin solution of the 2 nd layer, the high-density polyethylene of example 1 was replaced with the high-density polyethylene having Mw of 4.0 × 10 (isotactic, pentad fraction: 95.6%)⁵A polyolefin multilayer microporous membrane was obtained in the same manner as in example 1 except for using the high density polyethylene (Mw/Mn: 10).

(example 6)

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the screw rotation speed (Ns) was changed to 145rpm and the Q/Ns was adjusted to 0.24kg/h/rpm in the preparation of the polyolefin resin solution of layer 1.

(example 7)

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the screw rotation speed (Ns) was changed to 130rpm and the Q/Ns was adjusted to 0.27kg/h/rpm in the preparation of the polyolefin resin solution of layer 1.

Comparative example 1

In the preparation of the polyolefin resin solution of layer 1, the resin composition was changed to not contain the ultrahigh molecular weight polypropylene but to have a Mw of 70 mass% of 4.0X 10⁵30 mass% of Mw of 2.0X 10⁶A polyolefin single-layer microporous membrane was obtained in the same manner as in example 1, except that the resin concentration of the polyolefin resin solution of the 1 st layer was set to 25% and the 2 nd layer was not formed.

Comparative example 2

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the resin composition was changed to 15 mass% of ultrahigh-molecular-weight polypropylene and 85 mass% of high-density polyethylene in the preparation of the polyolefin resin solution of the layer 1.

Comparative example 3

A polyolefin multilayer microporous film was obtained in the same manner as in example 1 except that the resin composition was changed to 50 mass% of ultrahigh-molecular-weight polypropylene and 50 mass% of high-density polyethylene so that the resin concentration in the polyolefin resin solution of layer 1 was 30 mass%, and the resin composition was changed to 70 mass% of high-density polyethylene and 30 mass% of ultrahigh-molecular-weight polyethylene so that the resin concentration in the polyolefin resin solution of layer 2 was 28.5% in the preparation of the polyolefin resin solution of layer 2, and extrusion was performed so that the layer thickness ratio of layer 2/layer 1/layer 2 was 38/24/38.

Comparative example 4

A polyolefin multilayer microporous film was obtained in the same manner as in example 1 except that the resin composition was changed to 50 mass% of ultrahigh-molecular-weight polypropylene and 50 mass% of high-density polyethylene so that the resin concentration in the polyolefin resin solution of layer 1 was 30 mass%, and the resin composition was changed to 82 mass% of high-density polyethylene and 18 mass% of ultrahigh-molecular-weight polyethylene so that the layer thickness ratio of the polyolefin resin solution of layer 2/the polyolefin resin solution of layer 1/the polyolefin resin solution of layer 2 was 38/24/38 in the preparation of the polyolefin resin solution of layer 2.

Comparative example 5

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was replaced with Mw of 2.0X 10⁶In the preparation of the polyolefin resin solution of the 2 nd layer, the high-density polyethylene of example 1 was replaced with the high-density polyethylene having Mw of 4.0 × 10 (isotactic, pentad fraction of 94.8%)⁵A polyolefin multilayer microporous membrane was obtained in the same manner as in example 1 except for using the high density polyethylene (Mw/Mn: 5).

Comparative example 6

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the amount of liquid paraffin added to the twin-screw extruder was changed to 100% in the preparation of the polyolefin resin solution of the layer 1.

Comparative example 7

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was changed to Mw of 2.0X 10⁶A polyolefin multilayer microporous membrane was obtained in the same manner as in example 1 except for the above-mentioned ultrahigh-molecular-weight polypropylene (isotactic/pentad fraction: 86.0%).

Comparative example 8

A polyolefin multilayer microporous film was obtained in the same manner as in example 1, except that the screw rotation speed was changed to 240rpm and the Q/Ns was adjusted to 0.18kg/h/rpm in the preparation of the polyolefin resin solution of layer 1.

Comparative example 9

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was changed to Mw of 1.0X 10⁶A polyolefin multilayer microporous membrane was obtained in the same manner as in example 1, except for using the above ultra-high-molecular-weight polypropylene (syndiotactic).

Comparative example 10

In the preparation of the polyolefin resin solution of layer 1, the ultra-high molecular weight polypropylene of example 1 was changed to Mw of 1.0X 10⁶A polyolefin multilayer microporous membrane was obtained in the same manner as in example 1, except for the above-mentioned ultrahigh-molecular-weight polypropylene (atactic structure).

[ Table 1]

[ Table 2]

Description of the reference numerals

a region having a polypropylene content of 20% or more

b zone with polypropylene content less than 20%