CN118019787A

CN118019787A - Polyolefin microporous film, separator for battery, and secondary battery

Info

Publication number: CN118019787A
Application number: CN202280065520.3A
Authority: CN
Inventors: 西村直哉; 下川床辽; 久万琢也; 大仓正寿
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2021-09-29
Filing date: 2022-09-12
Publication date: 2024-05-10
Also published as: WO2023053930A1

Abstract

A polyolefin microporous film comprising a polyethylene resin as a main component, wherein the polyolefin microporous film has an arithmetic average roughness Sa (A) of 1.0 [ mu ] m or less as measured by a scanning white light interference microscope method after heat treatment at 160 ℃, and a puncture strength of 500 mN/(g/m ²) or more in terms of weight per unit area, a separator for a battery and a secondary battery using the same.

Description

Polyolefin microporous film, separator for battery, and secondary battery

Technical Field

The present invention relates to a separation membrane used for separation and selective permeation of substances, and a polyolefin microporous membrane widely used as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. Further, the present invention relates to a separator for a battery and a secondary battery.

Background

The polyolefin microporous membrane is mainly used as a filter, a separator for a fuel cell, and a separator for a capacitor. In particular, the present invention is suitable for use as a separator for nonaqueous electrolyte secondary batteries such as lithium ion batteries widely used in notebook-type personal computers, mobile phones, and the like. The reason for this is that polyolefin microporous membranes have excellent mechanical strength, shutdown temperature, and ion permeability.

In addition, the separator is required to have a function of ensuring safety when the battery abnormally heats. The shutdown temperature is preferably low because the polyolefin microporous membrane melts to block pores and thus block current. On the other hand, the temperature inside the battery also continues to rise for a certain time after shutdown, sometimes a phenomenon of melting occurs in which the separator is perforated at a high temperature compared to the shutdown temperature without maintaining insulation, and the higher the temperature at which the melting occurs (melting temperature) is more preferable.

In recent years, lithium ion secondary batteries have been developed to increase the capacity of the batteries, mainly in the miniaturization of electronic devices and the development of vehicle-mounted applications, and as the capacity of the batteries increases, the thermal stability of electrode materials used tends to decrease. In the separator used in such a high-capacity battery, a laminate film in which a heat-resistant porous layer composed of inorganic particles, a binder, and the like is provided on a polyolefin microporous film is widely used for the purpose of improving heat resistance. On the other hand, although the laminated film has an effect of preventing thermal shrinkage of the separator and suppressing exposure of the electrode at the end of the battery, there is a case where the laminated film is melted by leaving pinholes in a partial portion of the separator, and further improvement in the melting temperature is also required for the laminated film.

For example, patent document 1 describes a separator for a secondary battery, which has both shutdown characteristics of a polyethylene microporous membrane and heat resistance of a polypropylene-containing layer, by laminating a microporous membrane containing polyethylene and polypropylene as essential components and a polyethylene microporous membrane.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2002-321323

Disclosure of Invention

Problems to be solved by the invention

Although the microporous membrane described in patent document 1 has improved heat resistance by blending polypropylene having a higher melting point than polyethylene, the phase separation structure of polyethylene and polypropylene may deteriorate other basic characteristics such as membrane strength as a separator. Further, no mention is made of heat resistance in the case where a heat-resistant porous layer is provided on a microporous film.

The present invention addresses the above problems. That is, the polyolefin microporous membrane is provided with a heat-resistant porous layer, and when used as a battery separator, the polyolefin microporous membrane can provide high safety against abnormal heat generation of a battery, and can realize low resistance and high capacity of the battery by providing excellent membrane strength.

Means for solving the problems

In order to solve the above problems, the present invention has the following configuration. In the following description, the numerical range indicated by "to" is a range including the numerical values described before and after "to" as the lower limit value and the upper limit value.

〔1〕

A polyolefin microporous film comprising a polyethylene resin as a main component, wherein the polyethylene resin has an arithmetic average roughness Sa (A) of 1.0 [ mu ] m or less as measured by a scanning white light interference microscope method after heat treatment at 160 ℃ and a puncture strength of 500 mN/(g/m ²) or more in terms of weight per unit area.

〔2〕

The polyolefin microporous membrane according to item [ 1 ], wherein the sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction, as measured by thermal mechanical analysis, is 12.0 mN/(g/m ²) or less.

〔3〕

According to the polyolefin microporous membrane of the above [ 1] or [ 2 ], when the arithmetic average roughness before the heat treatment at 160 ℃ is set to be Sa (B), sa (a)/Sa (B) is 20 or less.

〔4〕

The polyolefin microporous membrane according to any of the above [1 ] to [ 3 ], wherein the Sa (B) is 0.05 μm or more.

〔5〕

The polyolefin microporous membrane according to any one of the above [ 1 ] to [ 4 ], which has a surface open pore ratio of 15% or more.

〔6〕

The polyolefin microporous membrane according to any one of the above [1] to [ 5 ], wherein the area ratio of the polyethylene resin component having a molecular weight of 5 ten thousand or less is 10% or more and the area ratio of the polyethylene resin component having a molecular weight of 100 ten thousand or more is 10% or more with respect to the peak area of the entire molecular weight components in the differential molecular weight distribution curve of the polyethylene resin measured by a Gel Permeation Chromatography (GPC) method.

〔7〕

A separator for a battery, which uses the polyolefin microporous membrane described in any one of [ 1] to [ 6 ].

〔8〕

A secondary battery using the separator for a battery described in [ 7 ].

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, when the heat-resistant porous layer is provided and used as a separator for a battery, it is possible to provide a polyolefin microporous membrane which can impart high safety to abnormal heat generation of the battery, is excellent in permeability and membrane strength, and can realize low resistance and high capacity of the battery.

Detailed Description

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the embodiments described below.

[ Polyolefin microporous film ]

The polyolefin microporous membrane according to the embodiment of the present invention contains a polyethylene resin as a main component, and has an arithmetic average roughness Sa (A) of 1.0 [ mu ] m or less as measured by a scanning white light interference microscope method after heat treatment at 160 ℃ and a puncture strength of 500 mN/(g/m ²) or more in terms of weight per unit area.

The polyolefin microporous membrane according to the embodiment of the present invention has an arithmetic average roughness Sa (a) of 1.0 μm or less, preferably 0.5 μm or less, more preferably 0.2 μm or less, still more preferably 0.1 μm or less, and particularly preferably 0.06 μm or less, as measured by a scanning white light interference microscopy method after heat treatment at 160 ℃. When the heat-resistant porous layer is provided by setting Sa (a) of the polyolefin microporous film after heat treatment at 160 ℃ to the above range, the film shape-retaining property, that is, the melting property is excellent when the laminated film is produced, and the safety is excellent when the laminated film is used as a separator for a battery. The lower limit of Sa (A) after heat treatment at 160℃is not particularly limited from the above standpoint, but is, for example, 0.01 μm or more from the standpoint of productivity.

Specifically, sa (a) and Sa (B) described below can be measured by the method described in examples.

In order to set Sa (A) after heat treatment at 160℃to the above range, the raw material composition and the film forming conditions of the microporous film are preferably set to the following range.

In the polyolefin microporous membrane according to the embodiment of the present invention, when the arithmetic average roughness before heat treatment at 160 ℃ measured by a scanning white light interference microscope method is Sa (B), it is preferable that Sa (a)/Sa (B) be 20 or less. The ratio Sa (A)/Sa (B) is more preferably 10 or less, still more preferably 5 or less, particularly preferably 2 or less, and most preferably 1 or less. When the ratio of Sa (a)/Sa (B) is in the above range, the film shape retention performance at high temperature is excellent, and the heat-resistant porous layer is provided on the polyolefin microporous film to form a laminated film, and when the laminated film is used as a battery separator, the adhesion to the heat-resistant porous layer is excellent, and therefore a battery excellent in safety can be produced. The lower limit of Sa (a)/Sa (B) is not particularly limited from the above viewpoint, but is, for example, 0.1 or more from the viewpoint of productivity.

In order to set Sa (A)/Sa (B) to the above range, the raw material composition of the microporous membrane and the membrane forming conditions are preferably set to the below range.

The above-mentioned Sa (B) of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 0.05 μm or more, more preferably 0.06 μm or more, still more preferably 0.08 μm or more, and particularly preferably 0.10 μm or more. When the polyolefin microporous membrane is used as a battery separator by providing a heat-resistant porous layer on the polyolefin microporous membrane by setting Sa (B) to the above range, high safety can be provided because of excellent adhesion to the heat-resistant porous layer. The upper limit of Sa (B) is not particularly limited from the above viewpoint, but is, for example, 0.5 μm or less from the viewpoint of both the strength and the film forming property of the microporous film.

In order to set Sa (B) to the above range, the raw material composition of the polyolefin microporous membrane is preferably set to the following range.

The puncture strength in terms of weight per unit area of the polyolefin microporous membrane according to the embodiment of the present invention is 500 mN/(g/m ²) or more, preferably 600 mN/(g/m ²) or more, more preferably 800 mN/(g/m ²) or more, and still more preferably 1000 mN/(g/m ²) or more. When the puncture strength in terms of weight per unit area is in the above range, the microporous membrane can be made thin to achieve a high capacity, and the separator can be made thin to improve resistance to impact from outside the battery and foreign matter inside the battery, thereby improving safety. The upper limit of the puncture strength in terms of weight per unit area is not particularly limited, but is preferably 2000 mN/(g/m ²) or less, for example, in view of easy control of Sa (a) after heat treatment at 160 ℃.

Here, the puncture strength in terms of weight per unit area means a puncture strength of L1 (mN) in the polyolefin microporous membrane having a weight per unit area T1 (g/m ²) by the formula: l2=l1/T1, and the puncture strength L2 calculated.

The puncture strength in terms of weight per unit area can be specifically measured by the method described in the examples.

In order to set the puncture strength in terms of weight per unit area within the above range, the raw material composition and the film forming conditions of the microporous film are preferably set within the ranges described below.

The surface open pore ratio of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 15% or more, more preferably 17% or more, further preferably 20% or more, and particularly preferably 22% or more. When the polyolefin microporous membrane is used as a battery separator by providing a heat-resistant porous layer on the polyolefin microporous membrane to form a laminated membrane, the surface aperture ratio of the polyolefin microporous membrane is in the above range, and besides the low resistance of the polyolefin microporous membrane can be achieved, the polyolefin microporous membrane can provide high safety due to excellent adhesion to the heat-resistant porous layer. The upper limit of the surface aperture ratio is not particularly limited from the above viewpoint, but is, for example, 50% or less from the viewpoint of the strength of the microporous membrane.

The surface open porosity can be evaluated and calculated by the method described below.

In order to set the surface open pore ratio within the above range, the raw material composition of the microporous membrane and the membrane forming conditions are preferably set within the following range.

The polyolefin microporous membrane according to the embodiment of the present invention preferably has a sum of maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction, as measured by Thermal Mechanical Analysis (TMA), of 12.0 mN/(g/m ²) or less, more preferably 10 mN/(g/m ²) or less, and still more preferably 8 mN/(g/m ²) or less. When the polyolefin microporous membrane is used as a separator for a battery, the sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction measured by TMA is in the above range, and therefore, when the separator is used as a battery, thermal shrinkage of the separator during abnormal heat generation of the battery can be suppressed, and high safety can be provided. The lower limit of the sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction is not particularly limited from the above viewpoints, but is, for example, 1 mN/(g/m ²) or more from the viewpoint of both the strength and the permeability of the microporous membrane.

The sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction can be specifically measured by the method described in the examples.

In order to set the sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction to the above range, the raw material composition and the film forming conditions of the microporous film are preferably set to the following ranges.

In the polyolefin microporous membrane according to the embodiment of the present invention, it is preferable that the area ratio of the polyethylene resin component having a molecular weight of 5 ten thousand or less is 10% or more and the area ratio of the polyethylene resin component having a molecular weight of 100 ten thousand or more is 10% or more with respect to the peak area of the entire molecular weight components in a differential molecular weight distribution curve of the polyethylene resin measured by a Gel Permeation Chromatography (GPC) method described later. The area ratio of the polyethylene resin component having a molecular weight of 5 ten thousand or less is more preferably 15% or more, and still more preferably 20% or more. The area ratio of the polyethylene resin component having a molecular weight of 5 ten thousand or less is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. The area ratio of the polyethylene resin component having a molecular weight of 100 ten thousand or more is more preferably 15% or more, and still more preferably 20% or more. The area ratio of the polyethylene resin component having a molecular weight of 100 ten thousand or more is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. When the amount of the polyethylene resin component having a molecular weight of 5 ten thousand or less and the polyethylene resin component having a molecular weight of 100 ten thousand or more in the polyolefin microporous membrane is in the above range, the polyolefin microporous membrane can be made high in strength while suppressing heat shrinkage, and when the polyolefin microporous membrane is used as a battery separator, the melting property is excellent.

In order to set the amount of the polyethylene resin component having a molecular weight of 5 ten thousand or less and the polyethylene resin component having a molecular weight of 100 ten thousand or more in the polyolefin microporous membrane to the above-mentioned ranges, it is preferable to set the raw material composition and the kneading conditions of the microporous membrane to the below-mentioned ranges.

The polyolefin microporous membrane according to the embodiment of the present invention preferably has an area ratio of a polyethylene resin component having a molecular weight of 200 ten thousand or more of 10% or less, more preferably 8% or less, and even more preferably 6% or less, in a molecular weight distribution of the polyethylene resin measured by a GPC method. The area ratio of the polyethylene resin component having a molecular weight of 200 ten thousand or more is preferably 1% or more, more preferably 3% or more. When the amount of the polyethylene resin component having a molecular weight of 200 ten thousand or more in the polyolefin microporous membrane is in the above range, the polyolefin microporous membrane can be made high in strength while suppressing heat shrinkage, and when used as a battery separator, the polyolefin microporous membrane is excellent in melting characteristics.

The polyolefin microporous membrane according to the embodiment of the present invention has an air permeability in terms of thickness of preferably 30 seconds/100 cm ³/μm or less, more preferably 20 seconds/100 cm ³/μm or less, and still more preferably 15 seconds/100 cm ³/μm or less. The lower limit of the air permeability in terms of thickness is not particularly set, but is preferably 1 second/100 cm ³/μm or more from the viewpoint of easy compatibility with film strength. When the air permeability in terms of thickness is in the above range, a microporous membrane having excellent charge and discharge characteristics can be produced when the microporous membrane is used as a battery separator.

The air permeability in terms of thickness can be adjusted to the above range by adjusting the blending ratio of the raw materials, the stretching ratio, the heat setting conditions, and the like in the manufacturing process.

The thickness of the polyolefin microporous membrane according to the embodiment of the present invention can be appropriately adjusted according to the application, but is preferably 20 μm or less, more preferably 15 μm or less, still more preferably 10 μm or less, and particularly preferably 8 μm or less. Further, it is preferably 2 μm or more. When the thickness of the polyolefin microporous membrane is in the above range, the safety and the high battery capacity can be both achieved when the membrane is used as a battery separator.

The thickness may be within the above range by appropriately adjusting film forming conditions such as extrusion conditions.

The polyolefin microporous membrane according to the embodiment of the present invention preferably has a porosity of 30% or more, more preferably 35% or more, and even more preferably 40% or more. The upper limit of the porosity is not particularly set, but is preferably 80% or less in view of suppressing a decrease in film strength. When the porosity is in the above range, the output characteristics are excellent when the microporous membrane is used as a separator for a secondary battery.

The porosity may be in the above range by adjusting the raw material prescription, stretching ratio, heat setting condition, and the like in the manufacturing process.

The polyolefin microporous film according to the embodiment of the present invention preferably has a heat shrinkage ratio in the MD and TD directions of 15% or less, more preferably 10% or less, and even more preferably 8% or less after the polyolefin microporous film is stored at 120 ℃ for 1 hour. If the thermal shrinkage rate of the polyolefin microporous film is in the above range, safety at abnormal heat generation is excellent in the case of using the polyolefin microporous film as a separator for a battery. The lower limit of the heat shrinkage in the MD and TD after 1 hour of storage at 120 ℃ is not particularly set, but is preferably 0% or more in view of suppressing the occurrence of wrinkles and sagging and making the film quality preferable.

In order to set the heat shrinkage ratio within the above range, the raw material composition, kneading conditions, and film forming conditions of the microporous film are preferably set within the following ranges.

The specific constitution of the polyolefin microporous membrane of the present embodiment will be described below, but the present invention is not necessarily limited thereto.

The polyolefin microporous film according to the embodiment of the present invention contains a polyethylene resin as a main component. The main component shown here means the component having the largest content in mass% among the components constituting the polyolefin microporous membrane. The polyethylene resin component in the polyolefin microporous membrane is preferably 80% by mass or more, more preferably 90% by mass or more, still more preferably 96% by mass or more, and particularly preferably 99% by mass or more. When the content of the polyethylene resin component in the polyolefin microporous membrane is in the above range, the microporous membrane is excellent in the membrane forming property and uniformity, and the membrane strength and permeability are excellent in the balance of performance as a separator for a battery. Here, 2 or more polyethylene resins may be contained in the polyolefin microporous membrane, and in this case, the total amount of the polyethylene resins may be the amount of the polyethylene resin component constituting the polyolefin microporous membrane. The content of the polyethylene resin in the polyolefin microporous membrane can be measured by the method described below.

The polyolefin microporous membrane according to the embodiment of the present invention may use various polyethylene resins, and examples thereof include ultrahigh molecular weight polyethylene, high density polyethylene, medium density polyethylene, branched low density polyethylene, and linear low density polyethylene. The polyethylene resin may be an ethylene homopolymer or a copolymer of ethylene and another α -olefin. Examples of the α -olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Here, the polyethylene resin contains ethylene in an amount exceeding 50 mol% with respect to the entire raw material monomer components.

The polyolefin microporous membrane according to the embodiment of the present invention preferably contains an ultrahigh molecular weight polyethylene (hereinafter, described as a resin a) in the polyethylene, and more preferably contains a resin a and a high density polyethylene (hereinafter, described as a resin B).

The weight average molecular weight (Mw) of the ultra-high molecular weight polyethylene used as the resin a is preferably 80 ten thousand or more, more preferably 90 ten thousand or more, and still more preferably 100 ten thousand or more. The weight average molecular weight (Mw) is preferably 250 ten thousand or less, more preferably 200 ten thousand or less, and further preferably 140 ten thousand or less. When the weight average molecular weight of the resin a is in the above range, the heat shrinkage of the polyolefin microporous film is suppressed, and when the resin a is used as a battery separator, the safety at abnormal heat generation is excellent.

The melting point of the resin A is preferably 133℃or higher, more preferably 135℃or higher. When the melting point of the resin a is in the above range, a microporous membrane having excellent permeability and strength is obtained. The melting point can be measured by the DSC method described later.

The content of the resin a in the polyolefin microporous membrane is preferably 30 mass% or more, more preferably 50 mass% or more, and still more preferably 60 mass% or more. Further, the content is preferably 95% by mass or less, more preferably 90% by mass or less, and still more preferably 80% by mass or less. When the content of the resin a in the polyolefin microporous membrane is in the above range, the membrane strength is excellent and heat shrinkage is suppressed when the polyolefin microporous membrane is used as a battery separator, whereby excellent battery safety can be provided.

The weight average molecular weight (Mw) of the high-density polyethylene (density: 0.940g/m ³ or more and 0.970g/m ³ or less) used as the resin B is preferably 1 ten thousand or more, more preferably 2 ten thousand or more, and still more preferably 5 ten thousand or more. The weight average molecular weight (Mw) is preferably 20 ten thousand or less, more preferably 15 ten thousand or less, and further preferably 10 ten thousand or less. When the weight average molecular weight of the resin B is in the above range, excellent battery safety can be imparted by suppressing heat shrinkage when the polyolefin microporous membrane is used as a battery separator.

The melting point of the resin B is more preferably 128℃or higher, and still more preferably 130℃or higher. Further, the temperature is preferably 135℃or lower, more preferably 134℃or lower. When the melting point of the resin B is in the above range, the polyolefin microporous membrane is used as a battery separator, and therefore, excellent shutdown properties can be achieved, and heat shrinkage can be suppressed, thereby providing excellent battery safety.

The crystallization heat of fusion (. DELTA.H) of the resin B, as measured by differential scanning calorimetric analysis (DSC), is preferably 200J/g or more, more preferably 210J/g or more, and still more preferably 220J/g or more. When the crystallization heat of fusion (Δh) of the resin B is set to the above range, in the case of using the polyolefin microporous membrane as a separator for a battery, the membrane strength can be improved while suppressing deformation of the membrane accompanying crystallization fusion, and therefore excellent battery safety can be imparted. The upper limit of the crystallization heat of fusion (Δh) of the resin B is not particularly set from the above viewpoint, but is preferably 280J/g or less from the viewpoint of film formability.

In the temperature distribution curve of the crystallization heat of fusion of the resin B measured by differential scanning calorimetric analysis (DSC), the half width of the crystallization heat of fusion peak is preferably 6 ℃ or less, more preferably 5 ℃ or less. Further, it is preferably 1℃or higher, more preferably 2℃or higher. When the content is within the above range, the deformation of the polyolefin microporous membrane due to the crystal melting is suppressed, and the safety is excellent when the polyolefin microporous membrane is used as a separator for a battery.

The content of the resin B in the polyolefin microporous film is preferably 5 mass% or more, more preferably 10 mass% or more, and further preferably 20 mass% or more. The content is preferably 70% by mass or less, more preferably 50% by mass or less, and still more preferably 40% by mass or less. When the content of the resin B in the polyolefin microporous membrane is in the above range, the membrane strength is excellent and heat shrinkage is suppressed when the polyolefin microporous membrane is used as a battery separator, whereby excellent battery safety can be provided.

The polyolefin microporous membrane according to the embodiment of the present invention may contain a resin other than the polyethylene resin, and for example, from the viewpoint of improving the heat resistance of the microporous membrane, it is preferable to add a polypropylene resin. The polypropylene resin may be a block copolymer or a random copolymer in addition to the homopolypropylene. The block copolymer and the random copolymer may contain a copolymer component with an α -olefin other than propylene, and examples of the α -olefin include ethylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, and the like. Here, the polypropylene resin contains propylene in an amount exceeding 50 mol% with respect to the total raw material monomer components.

The amount of the polypropylene resin to be added is preferably 20 mass% or less, more preferably 5 mass% or less, and even more preferably 3 mass% or less, based on the total mass of the polyolefin microporous membrane. When the content is within the above range, a polyolefin microporous membrane having excellent productivity, quality and strength can be obtained.

The polyolefin microporous membrane may contain a resin component other than the polyethylene-based resin and the polypropylene-based resin, as required. In addition, various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent, a filler, a crystallization nucleating agent, and a crystallization retarder may be contained within a range that does not impair the effects of the present invention.

The polyolefin microporous membrane of the present invention is preferably used as a laminate membrane having 1 or more heat-resistant porous layers on at least one surface. The heat-resistant porous layer is not particularly limited, and for example, it preferably contains a binder made of a resin and inorganic particles. As the binder component, for example, an acrylic resin, a poly 1, 1-difluoroethylene resin, a polyamideimide resin, a polyamide resin, an aromatic polyamide resin, a polyimide resin, a polyvinyl alcohol resin, a cellulose ether resin, or the like can be used. As the inorganic particles constituting the heat-resistant porous layer, for example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, zeolite, glass filler, kaolin, talc, mica, titanium dioxide, calcium fluoride, lithium fluoride, and the like can be used.

The average particle diameter of the inorganic particles is preferably 0.3 μm or more and 1.8 μm or less, more preferably 0.5 μm or more and 1.5 μm or less. The average particle diameter of the particles can be measured by a measuring device using a laser diffraction method or a dynamic light scattering method. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe may be measured by a particle size distribution measuring apparatus (makrolon HRA, manufactured by daily necator), and the value of the particle diameter (D50) when 50% is accumulated from the small particle side in terms of volume may be set as the average particle diameter. The shape of the particles may be a regular sphere, a substantially sphere, a plate, or a needle, but is not particularly limited.

In addition to the above raw materials, the heat-resistant porous layer may contain a component for adjusting wettability such as a surfactant in order to improve the coating property.

The proportion of the inorganic particles in the heat-resistant porous layer is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 95% by mass or more. Further, it is preferably 99 mass% or less. By setting the proportion of the inorganic particles in the heat-resistant porous layer to the above range, the heat resistance and the permeability of the laminated film can be both achieved.

The thickness of the heat-resistant porous layer is preferably 0.5 to 5 μm, more preferably 1 to 4 μm, from the viewpoint of both heat resistance when used as a separator for a battery and high capacity of the battery.

The method for forming the heat-resistant porous layer is not particularly limited, and examples thereof include a reverse roll/coating method, a gravure/coating method, a kiss/coating method, a roll brush method, a spray method, an air knife coating method, a wire bar coating method, a tube doctor blade method, a blade coating method, and a die coating method. Further, after the coating liquid is coated on the polyolefin microporous membrane by the above method, the solvent is dried under the conditions of a drying temperature of 40 to 100 ℃ and a drying time of 3 to 120 seconds, whereby a heat-resistant porous layer can be formed.

The solid content concentration of the coating liquid for forming the heat-resistant porous layer is not particularly limited as long as it can be uniformly coated, but is preferably 20 mass% or more and 90 mass% or less, more preferably 30 mass% or more and 80 mass% or less.

The solvent used for the coating liquid is not particularly limited as long as it can uniformly disperse the binder and the inorganic particles, and examples thereof include water, alcohols, and acetone.

[ Method for producing microporous polyolefin film ]

Next, a method for producing a polyolefin microporous membrane according to an embodiment of the present invention will be shown. Examples of the method for producing the polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. As the method for producing the polyolefin microporous membrane in the present embodiment, a wet membrane production method is preferable from the viewpoints of control of the structure and physical properties of the membrane.

Hereinafter, a method for producing a wet polyolefin microporous membrane will be described. The following description is an example of a manufacturing method, and is not limited to this method.

The method for producing a polyolefin microporous membrane according to the embodiment of the present invention preferably includes the following steps (1) to (5) in this order, and may further include the following step (6), or may further include the following step (7) after the step (6) or instead of the step (6).

(1) A step of melt-kneading the polyolefin resin and the film-forming solvent to prepare a polyolefin resin composition

(2) Extruding the polyolefin resin composition, and cooling to form a gel sheet

(3) A1 st stretching step of preheating the gel sheet and stretching the same

(4) A step of removing the film-forming solvent from the stretched gel-like sheet

(5) Drying the sheet from which the film-forming solvent has been removed

(6) A2 nd stretching step of preheating the dried sheet and stretching the sheet

(7) A step of heat-treating the dried sheet

(1) Preparation Process of polyolefin resin composition

A polyolefin resin composition in which a plasticizer (film-forming solvent) is dissolved by heating is prepared. The plasticizer is not particularly limited as long as it is a solvent capable of uniformly dispersing the polyolefin resin, but the solvent is preferably a liquid at room temperature in order to enable stretching at a high magnification. Examples of the solvent include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, mineral oil fractions having boiling points corresponding to these hydrocarbons, and phthalates such as dibutyl phthalate and dioctyl phthalate which are liquid at room temperature. In order to obtain a gel-like sheet having a stable content of the liquid solvent, a nonvolatile liquid solvent such as liquid paraffin is preferably used.

The blending ratio of the polyolefin resin to the plasticizer is preferably such that the content of the polyolefin resin is 10 to 50% by mass based on the total mass of the polyolefin resin composition. When the content of the polyolefin resin is in the above range, the dispersion state of the polyolefin resin and the plasticizer is good, and the obtained microporous membrane is excellent in strength, permeability, and heat resistance. Further, when the sheet is molded, the expansion and contraction amount at the outlet of the die becomes appropriate, and the moldability and film forming property of the sheet become good.

From the viewpoint of obtaining a uniform kneaded state, the melt-kneading of the polyolefin resin and the plasticizer is preferably performed in a twin-screw extruder. In addition, from the viewpoint of suppressing torque fluctuation during extrusion and uniformly dispersing a polyolefin resin and a plasticizer each composed of a plurality of types, it is preferable to adjust the distance between the screw tip and the vent hole, the distance between the vent holes, or the screw configuration of the twin-screw extruder, specifically, it is preferable to use at least 1 screw element having a distance between the screw tip and the vent hole of 1.0D to 15.0D and a length of 0.2D to 0.9D in the raw material conveying direction therebetween when the outermost diameter of the screw is D, and not to use 2 or more screw elements having a length of 1.5D or more in the raw material conveying direction.

The resin temperature during kneading is preferably 150℃or higher, more preferably 160℃or higher, further preferably 180℃or higher, and the upper limit is preferably 250℃or lower, more preferably 240℃or lower, further preferably 230℃or lower. By setting the temperature of the polyolefin resin composition in the kneading to the above range, the strength decrease due to the deterioration of the resin can be prevented, and the polyolefin resin and the plasticizer can be uniformly melt-kneaded.

In kneading in a twin-screw extruder, the ratio of the extrusion mass Q (kg/hr) to the screw rotation speed Ns (rpm) is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.1 or more. This can prevent the strength from being lowered due to the deterioration of the resin during kneading. The upper limit is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less, whereby sufficient shear can be applied to the polyolefin resin composition, and a uniformly dispersed state can be obtained.

(2) Gel sheet forming step

The melt of the polyolefin resin composition is fed from the extruder to the die, and extruded in a sheet form. The extrusion method may be any one of a flat die method and an inflation method. Further, a plurality of polyolefin resin compositions having the same or different compositions may be fed from a plurality of extruders to 1 multi-manifold type composite T-die to be laminated and extruded into a sheet shape constituted by lamination. The extrusion temperature is preferably 140 to 250℃and the extrusion speed is preferably 0.2 to 15 m/min.

The sheet-like melt-extruded resin composition is cooled and solidified to form a gel-like sheet. Preferably, the cooling is performed in the cooling step to a temperature of 10 to 50 ℃. This is because it is preferable to set the final cooling temperature to a temperature equal to or lower than the crystallization completion temperature, and the higher-order structure is made finer, whereby uniform stretching is facilitated in the subsequent stretching. The cooling rate at this time is preferably 50℃per minute or more, more preferably 100℃per minute or more, and still more preferably 150℃per minute or more. Generally, if the cooling rate is low, a relatively large crystal is formed, and therefore the higher-order structure of the gel-like sheet becomes thicker, and the gel structure thereof is also large. In contrast, if the cooling rate is high, relatively small crystals are formed, and therefore the higher-order structure of the gel-like sheet becomes dense, and the strength and elongation of the film are improved in addition to uniform stretching. The cooling method in this case includes a method of directly contacting with cold air, cooling water, or another cooling medium, a method of contacting with a roll cooled with a cooling medium, a method of using a casting drum, and the like.

The shrinkage of the sheet-like melt-extruded resin is preferably 70% or more, more preferably 80% or more, and even more preferably 85% or more. The tenter ratio is represented by the following formula, where a represents the width of the gel sheet after cooling and solidification, and B represents the width of the discharge port of the die: when the value calculated in a/b×100 is set to the above range, the molecular chain orientation of the resin at the time of melt extrusion is suppressed, and crystallization is delayed, whereby the surface of the microporous film is roughened. Since the microporous membrane having a high degree of roughness has excellent adhesion to the heat-resistant porous layer, it can be used as a separator for a battery to produce a battery having excellent safety. The upper limit of the reduction ratio is not particularly set from the above viewpoint, but is preferably 99% or less from the viewpoint of film formation stability. In order to adjust the neck-in ratio to the above range, the composition can be adjusted by adjusting the formulation of the polyolefin resin composition, adjusting the resin temperature at the time of extrusion, adjusting the interval between the casting drum and the die lip, or assisting the adhesion of the casting drum and the gel sheet with an air knife, an air chamber or the like.

(3) 1 St stretching step

Next, the obtained gel-like sheet is stretched at least in the uniaxial direction, but the gel-like sheet is preferably preheated before stretching. The preheating temperature is preferably 90 to 130 ℃, more preferably 105 ℃ or higher, further preferably 110 ℃ or higher, and further preferably 120 ℃ or lower, further preferably 117 ℃ or lower. By performing the preheating temperature under the above conditions, the polyolefin microporous membrane having a uniform and fine pore structure can be obtained by uniformly stretching in the stretching step.

The preheated gel sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multistage stretching (e.g., a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.

The stretching ratio (area stretching ratio) in this step is preferably 16 times or more, more preferably 25 times or more. The stretching ratio is preferably 4 times or more, more preferably 5 times or more, in both the machine direction (MD direction) and the machine direction (TD direction). The stretching ratios in the MD direction and the TD direction may be the same or different from each other. By setting the area stretching ratio to the above range, mechanical strength and permeability can be improved. The area stretching ratio in this step is preferably 100 times or less, more preferably 64 times or less, whereby a polyolefin microporous membrane having excellent membrane strength can be produced while preventing membrane rupture. The stretching ratio in this step means the area stretching ratio of the polyolefin microporous film immediately before the next step, based on the polyolefin microporous film immediately before the present step.

The stretching temperature in this step is preferably in the range of (TCD) to (tcd+30) c, more preferably (tcd+5) c or more, particularly preferably (tcd+10) c or more, still more preferably (tcd+28) c or less, and particularly preferably (tcd+26) c or less, of the crystallization dispersion temperature of the polyethylene resin. If the stretching temperature is within the above range, film breakage due to stretching is suppressed, and high-rate stretching can be performed.

The crystallization dispersion Temperature (TCD) was determined by measuring the dynamic viscoelasticity temperature characteristics by using ASTM D4065. When a polyethylene resin is used as the polyolefin resin, the ultra-high molecular weight polyethylene, the polyethylene other than the ultra-high molecular weight polyethylene, and the polyethylene resin composition have a crystal dispersion temperature of about 100 to 110 ℃, and therefore the stretching temperature is preferably 90 to 130 ℃, more preferably 105 ℃ or higher, still more preferably 110 ℃ or higher, still more preferably 120 ℃ or lower, still more preferably 117 ℃ or lower.

The above stretching causes cracking in the polyethylene sheet, and the polyethylene resin phase is refined to form a plurality of fibrils. The fibrils form a three-dimensionally irregularly connected network.

(4) Removing step of film-forming solvent

The film forming solvent is removed (washed) using a washing solvent. The polyolefin resin phase is phase-separated from the film-forming solvent phase. Therefore, if the film-forming solvent is removed, a porous film composed of fibrils forming a fine three-dimensional network structure and having pores (voids) irregularly communicating in three dimensions is obtained. The method for removing the cleaning solvent and the film-forming solvent using the same are well known, and therefore, the description thereof will be omitted. For example, a method disclosed in Japanese patent application laid-open No. 2132327 and Japanese patent application laid-open No. 2002-256099 can be used.

(5) Drying process

The polyolefin microporous membrane from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably not higher than the crystal dispersion Temperature (TCD) of the polyolefin resin, and particularly preferably not lower than 5 ℃ below TCD. The drying is preferably performed until the residual washing solvent becomes 5 parts by mass or less, more preferably 3 parts by mass or less, based on 100 parts by mass (dry mass) of the total mass of the polyolefin microporous membrane.

(6) 2 Nd stretching step

The dried polyolefin microporous membrane may be stretched at least in the uniaxial direction (stretching step 2). The polyolefin microporous membrane may be preheated prior to the 2 nd stretching process. The preheating temperature is preferably 90 to 140 ℃, more preferably 95 ℃ or higher, further preferably 100 ℃ or higher, and further preferably 150 ℃ or lower, further preferably 140 ℃ or lower. The stretching of the polyolefin microporous membrane can be performed by a tenter method, a roll method, an inflation method, or the like in the same manner as described above while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, any of simultaneous biaxial stretching and sequential biaxial stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.

The area stretch ratio in this step is preferably 4.0 times or less, more preferably 2.0 times or less, further preferably 1.7 times or less, and particularly preferably 1.5 times or less. In the case of biaxial stretching, stretching ratios in the MD direction and the TD direction may be the same or different from each other. The stretching ratio in this step means a stretching ratio of the polyolefin microporous film immediately before the next step, based on the polyolefin microporous film immediately before the present step.

(7) Heat treatment process

Further, the polyolefin microporous membrane after drying may be subjected to a heat treatment after the above step (6) or instead of the above step (6). The crystals were stabilized by heat treatment and the sheets were homogenized. As the heat treatment method, a heat setting treatment and/or a heat relaxation treatment may be used. The heat setting treatment is a heat treatment in which heat is applied while maintaining the film in a constant size. The thermal relaxation treatment is a thermal treatment for thermally shrinking the film in the MD direction and the TD direction during heating. The heat-setting treatment is preferably performed by a tenter method or a roll method. The relaxation rate in the relaxation treatment is a value obtained by dividing the size of the film after the relaxation treatment by the size of the film before the relaxation treatment, and the relaxation rate of the film in both MD and TD directions is preferably 1.0 or less, more preferably 0.98 or less, and still more preferably 0.96 or less. From the viewpoint of flatness of the microporous membrane, the thickness is preferably 0.80 or more, more preferably 0.90 or more. The heat treatment temperature is preferably in the range of TCD to melting point of the polyolefin resin. The melting point can be measured by a Differential Scanning Calorimeter (DSC) based on JIS K7121 (1987).

The polyolefin microporous membrane obtained in the above-described manner can be used in various applications such as filters, separators for secondary batteries, separators for fuel cells, separators for capacitors, and the like.

[ Separator for Battery and Secondary Battery ]

The present invention also relates to a separator for a battery using the above-mentioned polyolefin microporous membrane, and particularly preferably a separator for a battery using a laminate membrane having 1 or more heat-resistant porous layers on at least one surface of the polyolefin microporous membrane. When the heat-resistant porous layer is provided and used as a battery separator, it is possible to provide high safety against abnormal heat generation of the battery, and the battery has excellent permeability and film strength, thereby achieving low resistance and high capacity. The details of the heat resistant porous layer are as described above.

The present invention also relates to a secondary battery using the above battery separator.

Examples

The present application will be described in further detail by way of examples, but embodiments of the present application are not limited to these examples. The evaluation in the present application was performed in an environment of a temperature of 23℃and a humidity of 65%, unless otherwise specified. The evaluation method and analysis method used in the examples are as follows.

[ Method of measurement ]

[ Thickness ]

The thickness of a polyolefin microporous membrane at any 5 points in the range of 50mm×50mm was measured by contacting a thick film meter, a wafer made by Kabushiki Kaisha, with a plug VL-50 (10.5 mm phi super hard spherical probe, measuring load 0.01N), and the average value was set to be the thickness (. Mu.m).

[ Porosity ]

Samples were cut from the polyolefin microporous membrane in a square of 50mm×50mm square, and the volume (cm ³) and mass (g) thereof were measured. From their values and film densities (g/cm ³), the porosity of the polyolefin microporous films was calculated by the following formula. In addition, the film density was calculated assuming a constant value of 0.99g/cm ³. In this measurement, samples were cut out from any 3 positions of the polyolefin microporous membrane, and the average value of the obtained porosities was calculated by measuring the samples.

The formula: porosity (%) = [ volume-mass/membrane density)/volume ] x 100

[ Air permeability ]

For the polyolefin microporous membrane, according to JIS P-8117: in 2009, air permeability (sec/100 cm ³) was measured in an atmosphere at 25℃using a Wang Yan type air permeability meter (EGO-1T, manufactured by Asahi Kabushiki Kaisha). Further, the air permeability in terms of thickness was calculated by dividing the thickness (μm) of the polyolefin microporous membrane measured by the above method.

[ Puncture Strength in terms of weight per unit area ]

The puncture strength was measured in accordance with JIS Z1707 (2019) except that the test speed was set to 2 mm/sec. A polyolefin microporous membrane was punctured with a needle having a diameter of 1.0mm and a spherical surface (radius of curvature R:0.5 mm) at the tip thereof using a dynamometer (DS 2-20N manufactured by Demar Co., ltd.) under an atmosphere at 25℃to measure the maximum load (mN) at that time, and the puncture strength (mN/(g/m ²)) in terms of weight per unit area was determined from the following formula.

The formula: puncture strength (mN/(g/m ²))=maximum load (mN)/weight per unit area of polyolefin microporous membrane (g/m ²) in terms of weight per unit area

The weight per unit area of the polyolefin microporous membrane was calculated by cutting a sample from the polyolefin microporous membrane in a square of 50mm×50mm square, and measuring the mass (g) at room temperature of 25 ℃.

The formula: weight per unit area (g/m ²) =mass (g)/(50 (mm) ×50 (mm))×10 ⁶

[ Gel Permeation Chromatography (GPC) ]

The weight average molecular weight (Mw) of the polyolefin resin, the area ratio of the polyethylene component having a molecular weight of 5 ten thousand or less to the peak area of all the molecular weight components in the polyolefin microporous membrane, the area ratio of the polyethylene component having a molecular weight of 100 ten thousand or more to the peak area of all the molecular weight components, and the area ratio of the polyethylene component having a molecular weight of 200 ten thousand or more to the peak area of all the molecular weight components were determined by GPC under the conditions shown below. In the differential molecular weight distribution curve obtained by GPC, the area ratio of each molecular weight component was obtained from the ratio of the area of each molecular weight region to the peak area of all the molecular weight components.

Measurement device: GPC-150C manufactured by Waters Corporation

Column: shodex UT806M manufactured by Showa Denko Co., ltd

Column temperature: 135 DEG C

Solvent (mobile phase): o-dichlorobenzene

Solvent flow rate: 1.0 ml/min

Sample concentration: 0.1 mass% (dissolution conditions: 135 ℃ C./1 h)

Sample introduction amount: 500 μl

Detector: waters Corporation differential refractometer (RI detector)

Standard curve: a standard curve obtained by using a monodisperse polystyrene standard sample was prepared by using a polyethylene conversion factor (0.46).

Differential scanning calorimetric analysis (DSC)

6.0Mg of the sample was sealed in an aluminum pan, and after heating from 30℃to 230℃at 10℃per minute (1 st heating), the sample was kept at 230℃for 5 minutes, cooled at a rate of 10℃per minute, and heated again from 30℃to 230℃at a rate of 10℃per minute (2 nd heating) using PYRISDiamond DSC manufactured by PARKING ELMER. The heat of crystallization and the half width of the peak of crystallization and the melting point of the polyolefin resin as the raw material were calculated from the peak of crystallization and melting obtained by drawing a base line between 60℃and 200℃in the temperature distribution curve of the heat absorption amount measured at the 2 nd temperature rise measured by DSC. The melting point is the temperature at the time point at which the maximum value of the heat absorption is exhibited, and the heat of crystallization and melting is calculated from the area of the peak of crystallization and melting. The content of the polyethylene resin in the polyolefin microporous film was calculated from the area (Δh1) of the crystal melting peak obtained by drawing a base line between 60 ℃ and 155 ℃ and the area (Δh2) of the crystal melting peak obtained by drawing a base line between 155 ℃ and 200 ℃ in the temperature distribution curve of the heat absorption amount measured at the 2 nd temperature rise measured by the DSC described above, by the following formula. Polyethylene resin content (%) =100×Δh1/(Δh1+Δh2) of the polyolefin microporous membrane

[ Method for measuring Sa (A) (preparation of sample for evaluation) ]

The polyolefin microporous membrane was cut out to 15mm square, and adhered to the center of a polyimide tape (API-114 AFR manufactured by zhongchen corporation, 19mm in width) cut out to have a length of 20mm without introducing wrinkles. The polyolefin microporous membrane attached to the polyimide tape was placed on an aluminum plate having a thickness of 2mm and a square of 5cm with the polyimide tape facing downward, and four sides of the microporous membrane were fixed to the aluminum plate with the polyimide tape (19 mm tape width, API-114AFR manufactured by zhongji chemical Co., ltd.) so as not to cause wrinkles. In fixing the four sides, the polyolefin microporous membrane was attached so that the outer circumferential width of the membrane was about 2mm and the polyimide tape was fixed. The sample was put into an oven having a temperature of 160℃in the tank, and taken out 15 minutes after the putting. In addition to the evaluation samples produced in the 1 st step, samples were produced in which the surfaces of the polyolefin microporous films at the time of the first tape adhesion were reversed, and a total of 2 evaluation samples were produced.

[ Method for measuring Sa (A) ]

The surface roughness of the sample produced by the above method was measured by using a scanning white light interference microscope VS-1540 manufactured by hitachin, inc. The measurement was performed under the following conditions, and the three-dimensional surface roughness parameter Sa (a) was calculated according to ISO 25178. For each sample for evaluation, 4-point measurement was performed on 2 samples prepared under the same conditions, and the obtained values were averaged.

Objective lens: 5 times of

Tube lens: 0.5 times

Wavelength filter: 530 white light

Camera: high pixel

Measurement mode: wave

Cut-off: without any means for

[ Method for measuring Sa (B) ]

The polyolefin microporous membrane cut out to 5cm×5cm was stuck to a metal frame having an outer dimension of 6cm square and an inner dimension of 3cm square without introducing wrinkles, and Sa (B) was calculated in the same manner as described above using a scanning white light interference microscope VS-1540 manufactured by Hitachi-device and by Hitachi-device. The polyolefin microporous membrane was measured at 4 points on each surface and 8 points in total under the same conditions, and the measured values were averaged.

[ Surface open porosity ]

For the polyolefin microporous film on which Pt was deposited, a scanning electron microscope (JSM-6701F, manufactured by japan electronics corporation) was used to set the acceleration voltage to 2kV, the Working Distance (WD) to 8 μm, and the secondary electron image of the surface was observed in SEI mode at a magnification of 10000 times. The image used in the binarization was an image having an observation area of 11.7 μm×9.4 μm (1280 pixels×1024 pixels) and an 8bit (256 gray scale) gray scale, and the area ratio (surface aperture ratio) of the opening portion of the surface of the microporous membrane was extracted by performing the binarization using HALCON of MVTec Software corporation.

As an image processing method, the surface SEM image was subjected to noise removal by 3-pixel×3-pixel averaging, and then dynamic binarization processing was performed with-30 gray scale as a threshold from the image subjected to 21-pixel×21-pixel averaging, thereby extracting a dark portion. The individual dark portions were all counted as holes, and the total of the counted hole areas was added up to calculate the area of the opening in the SEM observation region, and the ratio of the area of the opening to the area of the observation region (11.7 μm×9.4 μm=110 μm ²) was set as the surface opening ratio. The above measurement was performed for 5 points each and 10 points in total on both surfaces of the polyolefin microporous membrane, and the measurement was averaged.

[ Sum of maximum shrinkage forces in terms of weight per unit area in the longitudinal and width directions ]

A test piece was placed using a thermal mechanical analyzer (cell コ, TMA/SS6100, product of Confucius, inc.) so that the sample size of the measurement portion became 10mm in length and 3mm in width. After an initial load of 9.8mN was applied to the sample, the change in shrinkage force of the sample with respect to the change in temperature was measured by detecting the load applied to the probe while the temperature was raised from room temperature (23 ℃) to 160℃at a temperature-raising rate of 5℃per minute, with the deformation amount of the sample being constant. The maximum shrinkage force is set to a value at which the shrinkage force becomes maximum between room temperature and 160 ℃. Regarding the maximum shrinkage force in the longitudinal direction, the above measurement was sampled so that the longitudinal direction of the test piece became the longitudinal direction of the polyolefin microporous membrane, and the average value was calculated by measuring 3 times in the same procedure. Regarding the maximum shrinkage force in the width direction, the above measurement was sampled so that the longitudinal direction of the test piece became the width direction of the polyolefin microporous membrane, and the average value was calculated by measuring 3 times in the same procedure. The sum of the maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction is calculated from the maximum shrinkage forces in the longitudinal direction and the width direction obtained by the above method by the following equation.

The formula: sum of maximum shrinkage forces (mN/(g/m ²)) in terms of weight per unit area in the longitudinal direction and the width direction= (maximum shrinkage force in the longitudinal direction (mN) +maximum shrinkage force in the width direction (mN))/weight per unit area of the polyolefin microporous membrane (g/m ²)

[ Heat shrinkage ]

Square samples of 100mm each in the MD direction and the TD direction were cut out from the polyolefin microporous membrane.

Next, the sample was put into an oven having a temperature of 120 ℃ in the tank and heated, and after 1 hour from the time of putting into the oven, the sample lengths in the MD direction and the TD direction were measured. The length in the MD direction after the oven was put into was L _MD (mm), the length in the TD direction was L _TD (mm), and the heat shrinkage after 1 hour of storage at 120℃was calculated by the following formula. The measurement was performed at any 3 points in the surface of the polyolefin microporous membrane, and the average value was calculated.

Formula 1: heat shrinkage (%) = { (100-L _MD)/100) ×100 after storage at 120 ℃ in MD direction for 1 hour

Formula 2: heat shrinkage (%) = { (100-L _TD)/100) ×100 after 1 hour storage at 120 ℃ in TD direction

[ Melting temperature of polyolefin microporous film ]

A round sample with a diameter of 19mm cut out of a polyolefin microporous film and members (upper cover, lower cover, gasket (PFA), spacer (cylindrical with a diameter of 15.5mm and a thickness of 1.0 mm), wave gasket) of a 2032 type coin cell were prepared. The members of the 2032 type coin cell were all purchased from baoquan corporation. Hereinafter, the production steps of the evaluation cell are shown, and the operations are all performed in a drying chamber having a dew point temperature of-35 ℃ or lower.

A sample for measurement and a gasket were placed in this order from the lower cover side on the inner bottom of the lower cover of the member of the 2032 type coin cell. Then, a solution in which 0.3 mass% of surfactant F-444 (manufactured by DIC corporation) was added to an electrolyte solution (manufactured by dow chemical company) obtained by dissolving LiBF ₄ in a mixed solvent (EC/pc=50/50 [ mass ratio ]) of Ethylene Carbonate (EC) and Propylene Carbonate (PC) so as to have a concentration of 1mol/L was prepared, and 0.1mL of the solution was injected into the coin cell. Next, after a spacer was placed on the sample for measurement in the hollow portion of the gasket, the sample was allowed to stand for 1 minute in an atmosphere of-50 kPa by gauge pressure, and this operation was performed 2 times to impregnate the polyolefin microporous membrane with the electrolyte. Then, a wave washer and an upper cover were placed on the separator in this order from the separator side, and the separator was sealed with a coin cell caulking machine (manufactured by baoquan corporation) to prepare a battery for evaluation.

The above-mentioned battery for evaluation was sandwiched by coaxial contact probes provided in an oven, and the resistance of the battery was measured with an LCR tester (LCR field motor system) at an amplitude of 50mV and a frequency of 1 kHz. The temperature of the coin cell was monitored by bringing a temperature measuring resistor into close contact with the cover of the cell, and the temperature of the coin cell was raised from room temperature to 50℃and allowed to stand for 10 minutes, and then the resistance was measured while raising the temperature to 180℃at a rate of 5℃per minute. The temperature at which the resistance of the evaluation battery initially exceeded 1kΩ cm ² was set as the shutdown temperature of the polyolefin microporous membrane, the temperature was continuously raised from the shutdown temperature, and the temperature at which the resistance again became 1kΩ cm ² was set as the melting temperature. The above measurement was performed by cutting out 2 arbitrary sites of the polyolefin microporous membrane, and the average value was calculated. The safety of the separator for a battery when used as a separator for a battery was determined based on the measured melting temperature and resistance value at 180 ℃, and A, B, C was determined to be acceptable.

A: the melting temperature is above 180 ℃, and the resistance value at 180 ℃ is above 10kΩ cm ²

B: a melting temperature of 180 ℃ or higher, and a resistance value at 180 ℃ of 1kΩ cm ² or higher and less than 10kΩ cm ²

C: the melting temperature is more than 170 ℃ and less than 180 DEG C

D: the melting temperature is above 160 ℃ and less than 170 DEG C

E: a melting temperature of less than 160 DEG C

[ Melting temperature of laminate film ]

An acrylic emulsion (produced by Showa Denko Co., ltd., "polyethylene glycol" (registered trademark) AT-731, a nonvolatile content of 47%), alumina particles having an average particle diameter of 0.5 μm, and ion-exchanged water were each prepared as a mixture of 2:55:43, and the resultant mixture was added to a polypropylene container together with zirconia beads, and the mixture was dispersed for 12 hours by a paint shaker (manufactured by Toyo Seisakusho Co., ltd.). Then, the mixture was filtered through a filter having a filtration limit of 5. Mu.m, to obtain a coating liquid. The coating liquid was coated on the polyolefin microporous membrane using a wire bar, and dried for 1 minute with a hot air oven set to 50 ℃ to obtain a laminated membrane provided with a heat-resistant porous layer. The wire rod was selected and coated so that the thickness of the heat-resistant porous layer after drying became 3. Mu.m. With respect to the laminated film obtained by the above steps, the melting temperature was measured as described above.

Example 1

The polyolefin material used was 70 mass% of an ultra-high molecular weight polyethylene having a Mw of 1.0X10 ⁶ and a melting point of 136℃as the resin A, and 30 mass% of a high density polyethylene having a Mw of 6X 10 ⁴, a melting point of 132℃and a DeltaH of 220J/g and a half width of a crystal melting peak of 4℃as the resin B. To 25 mass% of the polyolefin material, 75 mass% of liquid paraffin was added, and based on the mass of the ultra-high molecular weight polyethylene, 0.5 mass part of 2, 6-di-t-butyl-p-cresol and 0.7 mass part of tetrakis [ methylene-3- (3, 5-di-t-butyl-4-hydroxyphenyl) -propionate ] methane were added as antioxidants, followed by mixing, to prepare a polyolefin resin composition. The obtained polyolefin resin composition was fed into a twin-screw extruder composed of screws shown in Table 1, and kneaded at 180℃to prepare a polyethylene solution. The resulting polyethylene solution was fed to a T-die having a temperature of 200℃and the extrudate was cooled by a casting drum controlled to 35℃to form a gel-like sheet. The sheet conveyance direction when the gel sheet is formed is defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction in the film surface is defined as the width direction. In this case, the gap between the apex of the casting drum and the die lip was adjusted so that the web shrinkage became 85%. After the obtained gel-like sheet was cut out to form a square of 80mm square and placed in a batch biaxial stretching machine, preheating was performed at 115℃for 300 seconds, and simultaneous biaxial stretching was performed at a stretching temperature of 115℃and a stretching speed of 1000 mm/min so that the length direction of the square-cut sheet became 8 times and the width direction became 8 times. The stretched film was washed in a washing tank of methylene chloride, liquid paraffin was removed, the washed film was dried in a drying oven adjusted to 20 ℃, and heat-setting treatment (heat treatment) was performed in an electric oven at 125 ℃ for 10 minutes, thereby obtaining a polyolefin microporous film.

Example 2

The same procedure as in example 1 was repeated except that 90 mass% of an ultra-high-molecular-weight polyethylene having a Mw of 1.0X10 ⁶ and a melting point of 136℃was used as the polyolefin material, and 10 mass% of a high-density polyethylene having a Mw of 6X 10 ⁴, a melting point of 132℃and a DeltaH of 220J/g and a half width of a crystal melting peak of 4℃was used as the resin B.

Example 3

The same procedure as in example 1 was repeated except that 80 mass% of an ultra-high-molecular-weight polyethylene having a Mw of 1.5X10 ⁶ and a melting point of 136℃was used as the polyolefin material, and 20 mass% of a high-density polyethylene having a Mw of 6X 10 ⁴, a melting point of 132℃and a DeltaH of 220J/g and a half width of a crystal melting peak of 4℃was used as the resin B.

Example 4

The same procedure as in example 1 was carried out except that the gap between the apex of the casting drum and the die lip was adjusted at the time of forming the gel sheet, so that the shrinkage of the sheet was adjusted to 91%.

Example 5

The same procedure as in example 1 was carried out except that the gel sheet was biaxially stretched at the same time so as to be 7 times in the longitudinal direction and 7 times in the width direction, then the liquid paraffin was removed, the washed film was dried in a drying oven adjusted to 100 ℃ for 10 minutes, cut out so as to be 80mm square, and placed in a batch biaxial stretching machine, and the gel sheet was preheated and heat-set at 130 ℃ for 10 minutes, stretched at a stretching temperature of 130 ℃ at a stretching speed of 1000 mm/min so as to be 2.0 times in the width direction, and then heat-relaxed so as to be 0.90 times in the width direction.

Comparative example 1

The same procedure as in example 1 was carried out except that 40 mass% of an ultra-high molecular weight polyethylene having a Mw of 2.5X10 ⁶ and a melting point of 133℃was used as the polyolefin material, 60 mass% of a high density polyethylene having a Mw of 3.5X10 ⁵, a melting point of 135℃and a DeltaH of 190J/g and a half width of a crystal melting peak of 6℃was used as the resin B, the gel-like sheet was cut out in the form of a square 80mm square, preheating was carried out at 115℃for 300 seconds, and simultaneous biaxial stretching was carried out in the form of a sheet cut out at 115℃at a stretching speed of 1000 mm/min to give a square 7-fold length and 7-fold width-wise.

Comparative example 2

The same procedure as in example 1 was repeated except that 100 mass% of an ultra-high molecular weight polyethylene having a Mw of 1.0X10 ⁶ and a melting point of 136℃was used as the polyolefin material as the resin A.

Comparative example 3

The same procedure as in example 1 was repeated except that 25 mass% of a high-density polyethylene (resin B) having a Mw of 6.0X10 ⁴, a melting point of 132℃and a DeltaH of 220J/g and a half width of a crystal melting peak of 4℃and 5 mass% of a polypropylene (PP) having a Mw of 1.0X10 ⁶, which was different from that of resin A and resin B, were used as the polyolefin material.

Comparative examples 4 to 6

The procedure of example 1 was repeated except that the distance between the screw tip and the vent hole and the structure of the extrusion screw were changed as shown in table 3.

Comparative example 7

The same procedure as in example 1 was conducted except that 50 mass% of an ultra-high molecular weight polyethylene having a Mw of 1.0X10 ⁶ and a melting point of 136℃was used as the polyolefin material, 50 mass% of a high density polyethylene having a Mw of 6X 10 ⁴, a melting point of 132℃and a DeltaH of 220J/g and a half width of a crystal melting peak of 4℃was used as the resin B, that gel-like sheets were cut out so as to form square shapes having a square shape of 80mm square, and the gel-like sheets were left to stand in a batch biaxial stretching machine, then preheated at 110℃for 300 seconds, simultaneously biaxially stretched at 110℃to remove liquid paraffin, and the washed film was dried in a drying oven adjusted to 20℃and heat-set at 120℃for 10 minutes (heat treatment) in an electric oven.

Comparative example 8

The same procedure as in example 1 was carried out except that the gap between the apex of the casting drum and the die lip was adjusted at the time of forming the gel sheet, so that the shrinkage of the sheet was 69%.

Tables 1 to 4 show examples and comparative examples.

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In examples 1 to 5, the arithmetic average roughness Sa (A) measured by a scanning white light interference microscope method after heat treatment at 160℃was 1.0 μm or less, and the puncture strength in terms of weight per unit area was 500 mN/(g/m ²) or more. Therefore, good results were shown in the evaluation of the melting characteristics after the heat-resistant porous layer was provided. On the other hand, in comparative examples 1,2, 7 and 8, the penetration strength in terms of Sa (a) and weight per unit area was not in the above range, and the melting property was poor after the heat-resistant porous layer was provided. In comparative examples 3 to 6, samples having uniform and high quality were not obtained, and in particular, samples that were worthy of evaluation as microporous films were not obtained in comparative examples 4 and 5. Further, with comparative examples 3 and 6, the heat-resistant porous layer could not be uniformly formed on the polyolefin microporous film due to the unevenness of the polyolefin microporous film, and the melting characteristics were poor.

Industrial applicability

The polyolefin microporous membrane of the present invention, when used as a separator for a battery, can impart high safety against abnormal heat generation of the battery and is excellent in membrane strength, and therefore, can be suitably used as a separator for a secondary battery requiring a high capacity.

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

The present application is based on japanese patent application (japanese patent application publication No. 2021-160022) filed on 9/29 of 2021, the contents of which are incorporated herein by reference.

Claims

1. A polyolefin microporous film comprising a polyethylene resin as a main component, wherein the polyethylene resin has an arithmetic average roughness Sa (A) of 1.0 [ mu ] m or less as measured by a scanning white light interference microscope method after heat treatment at 160 ℃ and a puncture strength of 500 mN/(g/m ²) or more in terms of weight per unit area.

2. The microporous polyolefin membrane according to claim 1, wherein the sum of maximum shrinkage forces in terms of weight per unit area in the longitudinal direction and the width direction measured by thermomechanical analysis is 12.0 mN/(g/m ²) or less.

3. The polyolefin microporous membrane according to claim 1 or 2, wherein Sa (a)/Sa (B) is 20 or less, assuming that an arithmetic average roughness before the heat treatment at 160 ℃ is Sa (B).

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the Sa (B) is 0.05 μm or more.

5. The polyolefin microporous membrane according to claim 1 or 2, which has a surface open pore ratio of 15% or more.

6. The polyolefin microporous membrane according to claim 1 or 2, wherein the polyethylene resin component having a molecular weight of 5 ten thousand or less has an area ratio of 10% or more and the polyethylene resin component having a molecular weight of 100 ten thousand or more has an area ratio of 10% or more with respect to the peak area of the entire molecular weight components in the differential molecular weight distribution curve of the polyethylene resin measured by gel permeation chromatography, GPC.

7. A separator for a battery using the polyolefin microporous film according to claim 1 or 2.

8. A secondary battery using the separator for a battery according to claim 7.