Laminated porous membrane

Abstract

To provide a thin laminated porous membrane that has little shedding of a porous layer and little curling in a dry environment.SOLUTION: A laminated porous membrane includes a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, and the thickness of the polyolefin porous membrane is 10 μm or less, the amount of carboxymethyl cellulose per 100 pts.wt. of the inorganic particles is 0.7 pt.wt. or less, the amount of the porous layer falling off during polishing is 0.70 μm or less, and the amount of curl is 1.5 mm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a laminated porous membrane.

Background Art

[0002] Thermoplastic resin porous membranes are widely used as separation materials, selective permeation and isolation materials, etc. For example, battery separators used in lithium-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, etc., separators for electric double layer capacitors, reverse osmosis filtration membranes, ultrafiltration membranes, and various filters such as microfiltration membranes, moisture permeable and waterproof clothing, and medical materials.

[0003] In particular, as a separator for lithium-ion secondary batteries, a polyolefin porous membrane having ion permeability by impregnation with an electrolytic solution, excellent electrical insulation, electrolyte resistance and oxidation resistance, and having a pore blocking effect of interrupting current at a temperature of about 120 to 150 ° C when the battery abnormally overheats to suppress excessive temperature rise is preferably used.

[0004] However, when the temperature continues to rise even after pore blockage for some reason, the polyolefin porous membrane may rupture. This phenomenon is not limited to the case where polyolefin is used, and it cannot be avoided at a temperature equal to or higher than the melting point of the resin constituting the porous membrane.

[0005] On the other hand, a laminated porous membrane in which a porous layer mainly composed of inorganic particles and a binder resin is coated on a polyolefin porous membrane has been adopted. In this laminated porous membrane, the shrinkage of the polyolefin porous membrane due to temperature rise is suppressed by the porous layer. For example, in Patent Document 1, a laminated porous membrane in which a porous layer containing inorganic particles having a major axis of 4 μm or more formed in a plate shape is laminated on at least one side of a polyolefin porous membrane to suppress the occurrence of curl in a dry environment has been proposed.

Prior Art Documents

Patent Documents

[0006] Patent Document 1 Japanese Patent Application Laid-Open No. 2019-93565 Summary of the Invention Problems to be Solved by the Invention

[0007] Due to the differences in the constituent materials and structures of the polyolefin porous membrane and the porous layer, the laminated porous membrane may curl in a dry environment, and problems such as displacement may occur when the laminated porous membrane and the electrode are laminated. Furthermore, with the improvement of battery performance, efforts have been made to minimize the ingress of moisture into the battery, and battery production is carried out in a drier environment with a lower dew point. Therefore, the laminated porous membrane is in a situation where curling is more likely to occur. In addition, with the increase in the energy density of the battery, the thickness of the laminated porous membrane has been reduced, and the polyolefin porous membrane has become thinner. When the polyolefin porous membrane becomes thinner, its rigidity decreases, and the curling of the laminated porous membrane in a dry environment tends to increase. In addition, the production speed of batteries has been increasing year by year, and the conveying speed of the laminated porous membrane during battery manufacturing has also become faster. Therefore, there is a need for a laminated porous membrane in which inorganic particles contained in the porous layer are less likely to fall off when the porous layer of the laminated porous membrane is rubbed against a conveying roll or the like. In the technology described in Patent Document 1, when the polyolefin porous membrane is thinned, it is difficult to suppress the curling of the laminated porous membrane in a dry environment, and the falling off of inorganic particles contained in the porous layer has not been sufficiently suppressed.

[0008] An object of the present invention is to provide a thin laminated porous membrane with a small amount of dropout of the porous layer and suppressed curling in a dry environment. Means for Solving the Problems

[0009] In view of the prior art, the present inventor has conducted intensive studies and found that the following configuration solves this problem. (1) A laminated porous membrane having a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, wherein the thickness of the polyolefin porous membrane is 10 μm or less, the amount of carboxymethyl cellulose relative to 100 parts by weight of the inorganic particles in the porous layer is 0.7 parts by weight or less, the amount of dropout during polishing of the porous layer is 0.70 μL or less, and the curl amount is 1.5 mm or less. (2) The laminated porous membrane according to (1) above, wherein the tensile strength in the width direction orthogonal to the film-forming direction of the polyolefin porous membrane is 150 MPa or more. (3) The laminated porous membrane according to (1) or (2) above, wherein the thickness ratio of the porous layer in the laminated porous membrane is 16% or more. (4) The laminated porous membrane according to any one of (1) to (3) above, wherein the inorganic particles are precipitated barium sulfate and the particle diameter is 0.6 μm or more and 1.6 μm or less. (5) A method for producing a laminated porous membrane having a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, the method comprising the step of applying a coating liquid containing inorganic particles and carboxymethyl cellulose to at least one side of the polyolefin porous membrane and then drying to form a porous layer containing inorganic particles and carboxymethyl cellulose, wherein the viscosity of the carboxymethyl cellulose when it is an aqueous solution containing 2% by weight of carboxymethyl cellulose is 90 mPa·s or less.

Advantages of the Invention

[0010] The laminated porous membrane having a porous layer on at least one side of the polyolefin porous membrane of the present invention has a small amount of dropout of the porous layer and suppressed curl in a dry environment, so that the battery yield is improved.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0012] The present invention relates to a laminated porous membrane having a porous layer on at least one side of a polyolefin porous membrane. More specifically, it relates to a laminated porous membrane having a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, wherein the amount of shedding during polishing of the porous layer is 0.70 μL or less, and the curl amount is 1.5 mm or less.

[0013] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below.

[0014] <Polyolefin Porous Membrane> In the embodiments of the present invention The polyolefin resin constituting the polyolefin porous membrane is not particularly limited, but polyethylene and polypropylene are preferred. Also, it may be a single substance or a mixture of two or more different polyolefin resins, for example, a mixture of polyethylene and polypropylene, or a copolymer of different olefins. The polyolefin porous membrane is preferable because, in addition to basic properties such as electrical insulation and ion permeability, it has a pore blocking effect that blocks current and suppresses excessive temperature rise when the battery abnormally overheats.

[0015] Among them, polyethylene is particularly preferred because it has excellent pore blocking performance. Hereinafter, polyethylene will be detailed as an example of the polyolefin resin used in the present invention, but the embodiments of the present invention are not limited thereto.

[0016] Examples of the polyethylene include ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene, and the like. The polymerization catalyst is not particularly limited, and examples thereof include Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like. These polyethylenes may be not only homopolymers of ethylene but also copolymers containing a small amount of other α-olefins. Preferred α-olefins other than ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, (meth)acrylic acid, (meth)acrylic acid esters, styrene, and the like.

[0017] The polyethylene may be a single substance, but is preferably a mixture composed of two or more types of polyethylene. As the polyethylene mixture, a mixture of two or more types of ultra-high molecular weight polyethylene having different weight average molecular weights (Mw), a mixture of high density polyethylene, a mixture of medium density polyethylene, and a mixture of low density polyethylene may be used, or a mixture of two or more types of polyethylene selected from the group consisting of ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene may be used.

[0018] The polyolefin porous membrane preferably has a function of closing pores during abnormal charge and discharge reactions. Therefore, the melting point (softening point) of the resin constituting the membrane is preferably 70°C or higher and 150°C or lower. More preferably, it is 80°C or higher and 140°C or lower, and even more preferably, it is 100°C or higher and 130°C or lower. When the melting point of the resin constituting the membrane is 70°C or higher and 150°C or lower, the pore closing function is exhibited during normal use, and the battery can be used without becoming unusable, and safety can be ensured by the pore closing function during abnormal reactions.

[0019] The thickness of the polyolefin porous membrane is preferably 10 μm or less. More preferably, it is 3 μm or more and 9 μm or less, and even more preferably, it is 5 μm or more and 7 μm or less. By setting the thickness of the polyolefin porous membrane to 10 μm or less, it is possible to achieve both practical film strength and pore blocking function, the area per unit volume of the battery is not restricted, and it is suitable for increasing the capacity of the battery. Also, the greater the thickness of the polyolefin porous membrane, the higher the rigidity of the polyolefin porous membrane, and the curling of the laminated porous membrane in a dry environment is suppressed.

[0020] The tensile strength in the width direction (TD: Transverse Direction) perpendicular to the film formation direction of the polyolefin porous membrane is preferably 150 MPa or more. More preferably, it is 180 MPa or more, and even more preferably, it is 210 MPa or more. The upper limit is not particularly limited, but examples include 500 MPa or less and 400 MPa or less. When the tensile strength in the TD of the polyolefin porous membrane is 150 MPa or more, the bending rigidity of the laminated porous membrane is large, the curling in a dry environment is reduced, and defects such as displacement can be reduced when laminating the laminated porous membrane and the electrode, which is preferable.

[0021] <Porous layer> In the embodiment of the present invention, the porous layer is provided on at least one side of the polyolefin porous membrane and contains inorganic particles and carboxymethyl cellulose. The porous layer can further contain a binder and an additive. When the porous layer is provided only on one side of the polyolefin porous membrane, the number of steps for forming the porous layer is small, and the production cost can be suppressed. When the porous layer is provided on both sides of the polyolefin porous membrane, the shrinkage of the polyolefin porous membrane due to heat can be suppressed more effectively by suppressing the shrinkage due to heat from both sides. Also, when porous layers having the same composition are provided on both sides of the polyolefin porous membrane with similar thicknesses (thickness ratio within 1.5 times), since the porous layers are symmetric with respect to the polyolefin porous membrane in the thickness direction of the laminated porous membrane, curling in a dry environment can be suppressed, which is preferable. More preferably, there is a configuration in which porous layers having the same composition are provided on both sides of the polyolefin porous membrane with a thickness ratio within 1.2 times, and even more preferably, there is a configuration in which porous layers having the same composition are provided on both sides of the polyolefin porous membrane with a thickness ratio within 1.1 times. When porous layers having the same composition are provided on both sides of the polyolefin porous membrane with different thicknesses, the laminated porous membrane tends to curl toward the side with the thicker porous layer in a dry environment.

[0022] <Inorganic particles> The inorganic particles used in the present invention are not particularly limited as long as they are electrochemically stable. Specifically, they are selected from the group consisting of sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, lanthanum oxide, cerium oxide, strontium oxide, vanadium oxide, zirconium oxide, magnesium silicate, calcium silicate, hydrotalcite, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, layered double hydroxide, boehmite, alumina, bismuth oxide, tin oxide, titanium oxide, zirconium oxide, zirconium phosphate, titanium phosphate, zeolite, calcium sulfate, magnesium sulfate, aluminum sulfate, barium sulfate, gypsum, silicon dioxide, silicon nitride, titanium nitride, silicon carbide, kaolinite, talc, mica, and glass fiber. These may be used alone or in combination of two or more. Among these, precipitated barium sulfate is preferable because there is little particle dropout when the porous layer is rubbed. Specifically, precipitated barium sulfate produced by a method of obtaining barium sulfate by adding sulfuric acid to barium sulfide or a method of obtaining barium sulfate by adding sodium sulfate to barium chloride can be mentioned. Precipitated barium sulfate has a more uniform particle size compared to flotation barium sulfate, so the voids between the inorganic particles forming the porous layer are likely to be uniform, and it is easy to suppress dropout when the porous layer is rubbed.

[0023] The inorganic particles used in the present invention preferably have a particle diameter of 0.1 μm or more and 1.6 μm or less, more preferably 0.3 μm or more and 1.2 μm or less, and even more preferably 0.6 μm or more and 1.0 μm or less. The particle diameter is a value measured by the same method as the particle diameter of the inorganic particles contained in the porous layer described in the examples below. When the particle diameter is equal to or greater than the above-mentioned lower limit, the voids between the inorganic particles become sufficient, and the movement path of lithium ions in the battery becomes narrow and long, thereby suppressing an increase in electrical resistivity, which is preferable. Furthermore, it is preferable because it suppresses a significant decrease in the performance of the battery due to clogging of the pores of the polyolefin porous membrane with inorganic particles. When the particle diameter is equal to or less than the above-mentioned upper limit, the contact points between the inorganic particles in the porous layer decrease, the structure of the porous layer becomes brittle, and it becomes difficult to suppress the shrinkage of the polyolefin porous membrane at high temperatures. Also, it is preferable because it suppresses an increase in the defective rate of the battery due to the porous layer falling off and mixing into the battery during the battery manufacturing process.

[0024] <Carboxymethyl cellulose> The porous layer of the laminated porous membrane of the present invention contains carboxymethyl cellulose. When carboxymethyl cellulose is included, curling of the laminated porous membrane in a dry environment is suppressed. The carboxymethyl cellulose used in the present invention preferably has a viscosity of 4 mPa·s or more and 90 mPa·s or less, more preferably 6 mPa·s or more and 50 mPa·s or less, and even more preferably 8 mPa·s or more and 20 mPa·s or less when it is an aqueous solution containing 2% by weight of carboxymethyl cellulose. When the viscosity is equal to or higher than the above-mentioned lower limit, coating defects such as repelling are less likely to occur when the porous layer is coated on the polyolefin porous membrane, which is preferable. When the viscosity is equal to or lower than the above-mentioned upper limit, the dispersibility of the inorganic particles is good, and as a result, the dropout of the inorganic particles when the porous layer is rubbed is less, which is preferable. The viscosity when it is an aqueous solution containing 2% by weight can be controlled by the molecular weight of carboxymethyl cellulose. The viscosity tends to increase as the molecular weight of carboxymethyl cellulose increases, and tends to decrease as the molecular weight of carboxymethyl cellulose decreases. The blending amount of carboxymethyl cellulose is preferably 0.2 parts by weight or more and 0.7 parts by weight or less, more preferably 0.3 parts by weight or more and 0.5 parts by weight or less with respect to 100 parts by weight of the inorganic particles. When the blending amount is 0.2 parts by weight or more, the inorganic particles can be uniformly dispersed in the dispersion medium, which is preferable. When the blending amount is 0.7 parts by weight or less, the dropout when the porous layer is rubbed is less, which is preferable.

[0025] <Binder> The binder used in the present invention preferably contains at least one functional group selected from the group consisting of an amino group, an amide group, a carbonyl group, a carboxyl group, a sulfonyl group, a phosphate group, a hydroxyl group, an alkyl group, and a halogen group in the polymer skeleton. By this, the effect of binding inorganic particles contained in the porous layer to each other and the effect of binding the inorganic particles and the polyolefin porous membrane can be exhibited. Specifically, it is selected from the group of acrylic resin, polyacrylamide, polyvinyl alcohol, polyamide, and polyethylene glycol, and may be used alone or in combination of two or more. Among these, acrylic resin and polyacrylamide are preferable. In the porous layer, the volume of the binder with respect to 100 mL of inorganic particles is preferably 6 mL or more and 17 mL or less, more preferably 10 mL or more and 16 mL or less. When the volume of the binder is 6 mL or more, the binding of inorganic particles to each other becomes sufficient, suppressing the detachment of the porous layer, which is preferable. When the volume of the binder is 16 mL or less, the increase in air permeability resistance due to the porous layer can be suppressed, which is preferable.

[0026] <Additive> The porous layer may appropriately contain a thickener, a wetting agent, etc. for the purpose of improving coatability, a crosslinking agent, a thermosetting agent, etc. for the purpose of improving heat resistance.

[0027] <Thickness of the porous layer> In the embodiment of the present invention, the thickness of the porous layer is preferably 0.5 μm or more and 4.0 μm or less. More preferably, it is 1.0 μm or more and 3.0 μm or less, and still more preferably, it is 1.5 μm or more and 2.0 μm or less. When the thickness of the porous layer is equal to or greater than the aforementioned lower limit, the effect of suppressing the shrinkage of the polyolefin porous membrane due to the temperature rise by the porous layer is sufficiently exhibited, which is preferable. When the thickness of the porous layer is equal to or less than the aforementioned upper limit, since the inter-pole distance between the positive electrode and the negative electrode in the battery increases, the electrical resistance increases, and the ratio of the laminated porous membrane in the volume of the battery increases, suppressing the decrease in the volume energy density of the battery, which is preferable.

[0028] <Laminated porous membrane> In the laminated porous membrane of the present invention, the thickness ratio of the porous layer is preferably 16% or more and 37% or less, more preferably 26% or more and 29% or less. When the thickness ratio is equal to or greater than the aforementioned lower limit, the effect of suppressing the shrinkage of the polyolefin porous membrane due to temperature rise by the porous layer is sufficiently exhibited, which is preferable. When the thickness ratio is equal to or less than the aforementioned upper limit, the curl of the laminated porous membrane in a dry environment is small, and defects such as displacement can be reduced when the laminated porous membrane and the electrode are laminated, which is preferable. Further, the higher the thickness ratio of the porous layer in the laminated porous membrane, the greater the effect of suppressing the thermal shrinkage of the polyolefin porous membrane by the porous layer.

[0029] In the laminated porous membrane of the present invention, the increase amount of the air permeability resistance obtained by the measurement method described later is preferably 60 sec / 100 mL or less, more preferably 40 sec / 100 mL or less. When the increase amount of the air permeability resistance is 60 sec / 100 mL or less, it is possible to suppress the decrease in ion permeability in the battery and the increase in electrical resistivity, which is preferable.

[0030] In the laminated porous membrane of the present invention, the amount of dropout during polishing of the porous layer obtained by the measurement method described later is preferably 0.70 μL or less, more preferably 0.40 μL or less. When the amount of dropout during polishing of the porous layer is 0.70 μL or less, it is possible to suppress the dropout of the porous layer when the porous layer comes into contact with the transport roll in the battery production process, which is preferable.

[0031] In the laminated porous membrane of the present invention, the amount of curl obtained by the measurement method described later is preferably 1.5 mm or less, more preferably 1.0 mm or less. When the amount of curl is equal to or less than the aforementioned upper limit, the curl of the laminated porous membrane in a dry environment is small, and defects such as displacement can be suppressed when the laminated porous membrane and the electrode are laminated, which is preferable.

[0032] <Manufacturing method of laminated porous membrane, method for forming porous layer> The laminated porous membrane of the present invention can be manufactured by a manufacturing method including a step of forming a porous layer containing inorganic particles and carboxymethyl cellulose by applying a coating liquid containing inorganic particles and carboxymethyl cellulose to at least one side of a polyolefin porous membrane and then drying it. At this time, it is preferable that the viscosity of the carboxymethyl cellulose is 90 mPa·s or less when it is an aqueous solution containing 2% by weight of carboxymethyl cellulose.

[0033] Hereinafter, the step of forming a porous layer containing inorganic particles and carboxymethyl cellulose by applying a coating liquid containing inorganic particles and carboxymethyl cellulose and then drying it will be described in detail. The porous layer in the present invention can be obtained by the following steps. (a) Preparation of a masterbatch liquid using inorganic particles, a dispersant, and a dispersion medium. (b) Preparation of a coating liquid for a porous layer using the masterbatch liquid, a binder, an additive, and a dispersion medium. (c) A step of applying the coating liquid for a porous layer to at least one side or both sides of a polyolefin porous membrane. (d) After the application, the dispersion medium is dried with a dryer to form a porous layer.

[0034] In the step (a), it is preferable to use water as the dispersion medium. As long as the dispersion stability of the coating liquid for a porous layer is not impaired, a mixture of water and a hydrophilic solvent such as ethanol, isopropyl alcohol, or N-methylpyrrolidone can be used as the dispersion medium. As a method for dispersing inorganic particles, known methods can be used. For example, a ball mill, a bead mill, a sand mill, a colloid mill, a roll mill, an impeller, a disper, a homogenizer, a planetary mixer, and an ultrasonic disperser can be mentioned.

[0035] In the step (b), a thickener, a wetting agent, etc. are appropriately added for the purpose of improving coatability, and a crosslinking agent, a thermosetting agent, etc. are appropriately added for the purpose of improving heat resistance.

[0036] In the step (c), a known method can be used to apply the coating liquid for the porous layer to at least one side or both sides of the polyolefin porous membrane. For example, the direct gravure coating method, reverse gravure coating method, kiss reverse gravure coating method, direct bar coating method, air knife coating method, Meyer bar coating method, pipe doctor method, blade coating method, die coating method, etc. can be mentioned, and these methods can be carried out alone or in combination.

Examples

[0037] Hereinafter, examples will be shown and specifically described, but the present invention is not limited by these examples. Also, the measured values in the examples are the values obtained by the following methods.

[0038] In the present invention, the direction parallel to the film-forming direction of the polyolefin porous membrane is referred to as the film-forming direction, longitudinal direction or MD (Machine Direction), and the direction perpendicular to the film-forming direction in the plane of the polyolefin porous membrane is referred to as the width direction or TD (Transverse Direction). In the case where the film-forming direction is unclear in the polyolefin porous membrane, the direction with the highest degree of orientation in the polyolefin porous membrane is regarded as the film-forming direction.

[0039] <Viscosity of carboxymethyl cellulose (CMC) aqueous solution (mPa·s)> Carboxymethyl cellulose was dissolved in water to prepare a 2 wt% aqueous solution, and the viscosity of this carboxymethyl cellulose aqueous solution was measured using a rheometer (manufactured by Anton Paar, MCR302) under the following conditions. Cone plate: CP50-1 Temperature: 25 °C Shear rate: 100 sec -1 。

[0040] <Thickness (μm)> The thickness of the polyolefin porous membrane and the laminated porous membrane was measured using a contact-type film thickness gauge (Mitutoyo Corporation's Lite-Matic) under the conditions of a super-hard spherical probe of φ10.5 mm and a load of 0.15 N, and the average of five measurement values was obtained. Furthermore, the thickness of the porous layer was measured by the above method for the thickness of the polyolefin porous membrane obtained by washing the laminated porous membrane with the same dispersion medium as the dispersion medium contained in the coating liquid for the porous layer and removing the porous layer, and was obtained from the following calculation formula. Thickness of porous layer (μm) = Thickness of laminated porous membrane (μm) - Thickness of polyolefin porous membrane with porous layer removed (μm).

[0041] <Tensile strength (MPa)> The tensile strength was measured using a tensile testing machine (Instron, model 5543) under the following conditions in accordance with ASTM D882. The tensile strength in the TD direction of the polyolefin porous membrane was obtained by dividing the strength at sample breakage by the thickness of the polyolefin porous membrane and the MD width of the polyolefin porous membrane of 10 mm, and the average of the measurement values of 3 samples was calculated. Sample: Polyolefin porous membrane, MD 10 mm × TD 200 mm Pulling direction: TD direction of the polyolefin porous membrane Distance between chucks: 20 mm Tensile speed: 100 mm / min Grip: Instron, 2702-018 Jaw Faces for Flats (Rubber Coated, 50 mm × 38 mm) Chuck pressure: 0.50 MPa Load cell: 500 N Temperature: 23 °C.

[0042] <Particle diameter (μm) of inorganic particles contained in the porous layer> The particle diameter of the inorganic particles contained in the porous layer was measured by observing the surface of the porous layer of the laminated porous membrane using a scanning electron microscope by the following method. (1) To prevent charging by the electron beam, platinum was vapor-deposited on the surface of the porous layer using a JFC-1600 auto fine coater manufactured by JEOL Ltd. (2) The surface of the porous layer was observed under the following conditions using a field emission scanning electron microscope (JSM-6701F, manufactured by JEOL Ltd.), and particles with a major axis of 0.1 μm or more were defined as inorganic particles. The major axis of 50 arbitrary inorganic particles was measured and averaged. Observation mode: SEM Accelerating voltage: 2 kV Image selector: LEI Magnification: 5000 - 10000 times.

[0043] <Volume (mL) of binder per 100 mL of inorganic particles> The volume of the binder per 100 mL of inorganic particles was obtained from the following calculation formula. Volume (mL) of binder per 100 mL of inorganic particles = { (Amount of water-soluble binder (g) / True density of water-soluble binder (g / mL)) + (Amount of water-dispersed binder (g) / True density of water-dispersed binder (g / mL))} / (Amount of inorganic particles (g) / True density of inorganic particles (g / mL)) × 100.

[0044] <Thickness ratio (%) of the porous layer in the laminated porous membrane> The thickness ratio of the porous layer in the laminated porous membrane was obtained from the following calculation formula using the thickness measured by the above method. Thickness ratio (%) of the porous layer in the laminated porous membrane = Thickness of the porous layer (μm) / Thickness of the laminated porous membrane (μm) × 100.

[0045] <Increase amount of air permeability resistance (sec / 100 mL)> The increase amount of air permeability resistance was measured by the following method using a King's air permeability resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P 8117. (1) The air permeability resistance of three points of the laminated porous membrane was measured and averaged to obtain the air permeability resistance of the laminated porous membrane. (2) The laminated porous membrane was washed with the same dispersion medium as the dispersion medium contained in the coating liquid for the porous layer to remove the porous layer, and then the dispersion medium was dried. The air permeability resistance of three points of the polyolefin porous membrane from which the porous layer was removed was measured and averaged to obtain the air permeability resistance of the polyolefin porous membrane from which the porous layer was removed. (3) The increase in air permeability resistance is obtained by the following calculation formula. Increase in air permeability resistance (sec / 100mL) = Air permeability resistance of the laminated porous membrane (sec / 100mL) - Air permeability resistance of the polyolefin porous membrane from which the porous layer has been removed (sec / 100mL) <Drop-off amount (μL) during polishing of the porous layer> The drop-off amount during polishing of the porous layer was measured by the following method and evaluated according to the following criteria in an environment with a temperature of 20 ± 10 °C and a humidity of 30% RH or less using a friction measuring machine (manufactured by Trinity Lab Co., Ltd., static / dynamic friction measuring machine TL201Tt). (1) Cut out the laminated porous membrane to a size of TD 49 mm × MD 110 mm and measure the weight using an electronic balance (measurement accuracy 0.01 mg). (2) Fix the laminated porous membrane to a 50 mm × 50 mm flat contact element with a single-sided adhesive cushion tape (manufactured by Trasco Nakayama Co., Ltd., edge cushion tape TEC-50BK) cut to 50 mm × 50 mm attached to the contact surface so that the cushion tape is in contact with the polyolefin porous membrane surface (in the case of a laminated porous membrane having a porous layer on one side) or the non-measured porous layer surface (in the case of a laminated porous membrane having porous layers on both sides). (3) Horizontally fix a polishing film #6000 (manufactured by 3M, lapping film sheet A3-2SHT) cut to 90 mm × 220 mm to the sliding part of the static / dynamic friction measuring machine. (4) Bring the porous layer of the laminated porous membrane fixed to the polishing film and the contact element into contact, and place a 1 kg weight on the contact element. At this time, the contact area between the polishing film and the porous layer is 49 mm × 50 mm. (5) After reciprocally sliding the sliding part of the static / dynamic friction measuring machine horizontally at a speed of 10 mm / sec and a distance of 25 mm for 5 times, remove the laminated porous membrane from the contact element and measure the weight using the electronic balance. (6) Obtain the drop-off amount (mg) during polishing of the porous layer from the weights of the laminated porous membrane before and after the test obtained by the following calculation formula. Drop-off amount (mg) during polishing of the porous layer = Weight of the laminated porous membrane before the test (mg) - Weight of the laminated porous membrane after the test (mg) (7) Convert the unit of the amount of shedding during polishing of the porous layer from weight (mg) to volume (μL) using the following calculation formula. Amount of shedding (μL) during polishing of the porous layer = (Amount of shedding (mg) during polishing of the porous layer / 1000 / True density of the porous layer (g / mL)) × 1000 Note that the true density of the porous layer is calculated using the following calculation formula. True density of the porous layer (g / mL) = (Amount of inorganic particles (g) + Amount of dispersant (g) + Amount of binder (g) + Amount of additive (g)) / {(Amount of inorganic particles (g) / True density of inorganic particles (g / mL)) + (Amount of dispersant (g) / True density of dispersant (g / mL)) + (Amount of binder (g) / True density of binder (g / mL)) + (Amount of additive (g) / True density of additive (g / mL))} A: The amount of shedding during polishing of the porous layer is 0.40 μL or less. B: The amount of shedding during polishing of the porous layer is more than 0.40 μL and 0.70 μL or less. C: The amount of shedding during polishing of the porous layer is more than 0.70 μL.

[0046] <Amount of curl (mm)> The amount of curl of the laminated porous membrane was measured by the following method. (1) Cut out three pieces of the laminated porous membrane to a size of MD 25 mm × TD 150 mm. (2) As shown in Figure 1, place the laminated porous membrane on a SUS430 plate with the porous layer side facing down (SUS430 plate side) in an environment of temperature 20 ± 5°C and humidity 40 ± 20% RH, and align one side of TD with the reference line. Fix the other side of TD with a 10 mm × 25 mm magnet sheet. The part not fixed by the magnet sheet in TD is 140 mm. In the case of a laminated porous membrane having porous layers on both sides of a polyolefin porous membrane, first place the laminated porous membrane on a horizontal surface in an environment with a temperature of 20 ± 10°C and a dew point of -45 ± 10°C. When the laminated porous membrane curls and rolls up, check the porous layer surface that comes inside the arc. Then, in an environment with a temperature of 20 ± 5°C and a humidity of 40 ± 20%RH, fix the laminated porous membrane so that the porous layer surface that comes inside the arc contacts a SUS430 plate. If the porous layer surface that is outside the arc when the laminated porous membrane curls and rolls up is fixed to contact the SUS430 plate and the curl is measured in a dry environment, the laminated porous membrane rolls up upward and the curl amount cannot be measured. (3) After leaving the above sample in a horizontal place for 3 minutes in an environment with a temperature of 20 ± 10°C and a dew point of -45 ± 10°C, as shown in Figure 2, measure the distance between the reference line and the "midpoint of the TD1 side of the laminated porous membrane" with a straightedge (JIS Class 1, scale pitch of 0.5 mm). (4) Obtain the curl amount by averaging the measured values of 3 samples. A: The curl amount is 1.0 mm or less. B: The curl amount is greater than 1.0 mm and 1.5 mm or less. C: The curl amount is greater than 1.5 mm.

[0047] (Example 1) Hereinafter, the addition amounts of the materials used in the coating liquid for the porous layer are described as the blending amounts as solid components or active ingredients. While stirring the water contained in a container with a stirrer equipped with a disperser-type blade (manufactured by Shin-Tong Science Co., Ltd., Three One Motor), sodium carboxymethyl cellulose was added to obtain an aqueous solution with a concentration of 2% by weight. The viscosity of this aqueous solution was 12 mPa·s. Next, with respect to 100 parts by weight of precipitated barium sulfate, the 2% by weight aqueous sodium carboxymethyl cellulose solution was prepared so that the amount of sodium carboxymethyl cellulose was 0.5 part by weight. While stirring at 800 rpm with the stirrer, water and 100 parts by weight of the precipitated barium sulfate were added, and the mixture was stirred at 1200 rpm for 3 hours to obtain a mixed liquid with a solid content of 55% by weight.

[0048] Zirconia beads with a bead diameter of 0.5 mm (manufactured by Toray Industries, Inc., TRESERAM φ0.5 mm, density 6.06 g / cm 3 ) (133 g) were filled into a bead mill disperser (manufactured by Asada Iron Works Co., Ltd., Pico Mill PCM-LR, dispersion unit volume 0.048 L), and the mixed solution was dispersed to obtain a masterbatch solution.

[0049] While stirring the obtained masterbatch solution at 800 rpm with the stirrer, 2.5 parts by weight of an acrylic resin (BM-950B, manufactured by Nippon Zeon Co., Ltd.) as a water-dispersible binder, 0.4 parts by weight of a wetting agent (SN Wet 366, manufactured by San Nopco Ltd.) as an additive, and water were added to 100 parts by weight of the precipitated barium sulfate. Then, the mixture was stirred at 800 rpm for 30 minutes with the stirrer to obtain a coating solution for the porous layer with a solid content of 45% by weight.

[0050] The obtained coating solution for the porous layer was coated on one side of a polyolefin porous membrane with a thickness of 7 μm by kiss reverse gravure method and dried at 60 °C to obtain a laminated porous membrane.

[0051] Table 1 shows the results of evaluating the particle diameter of inorganic particles contained in the porous layer, the volume of the binder with respect to 100 mL of inorganic particles, the thickness of the porous layer, the thickness ratio of the porous layer in the laminated porous membrane, the increase in air permeability resistance, the amount of dropout during polishing of the porous layer, and the amount of curl for the obtained laminated porous membrane.

[0052] (Examples 2 to 17, Reference Examples 1 to 2, Comparative Examples 1 to 3, 6 to 7) A laminated porous membrane was obtained in the same manner as in Example 1 except that the materials and compounding amounts shown in Table 1 were changed. The addition amounts of the dispersant, water-soluble binder, and water-dispersible binder are the addition amounts (parts by weight) as the solid content or active ingredient with respect to 100 parts by weight of inorganic particles.

[0053] (Comparative Example 4) The carboxymethylcellulose sodium of Example 13 was changed to a polyacrylic acid-based dispersant (manufactured by Toagosei Co., Ltd., Aron (registered trademark)), and 1.5 parts by weight of polyacrylamide (Polystron, manufactured by Arakawa Chemical Industries, Ltd.) was added as a water-soluble binder to the masterbatch solution. A laminated porous membrane was obtained in the same manner as in Example 13 except that the polyolefin porous membrane was changed to one with a thickness of 7 μm.

[0054] (Comparative Example 5) A laminated porous membrane was obtained in the same manner as in Comparative Example 4 except that the polyolefin porous membrane of Example 4 was changed to one with a thickness of 10 μm and a tensile strength TD of 181 MPa.

[0055] [Table 1] [Industrial Applicability]

[0056] The laminated porous membrane of the present invention can be used for battery separators used in lithium-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, polymer batteries, etc., separators for electric double layer capacitors, filters, moisture-permeable and waterproof clothing, etc. In particular, it is preferably used as a separator for lithium-ion secondary batteries. [Explanation of Symbols]

[0057] 1: SUS430 plate 2: Laminated porous membrane 3: Magnet sheet 4: Reference line 5: Distance between the reference line and the midpoint of the TD1 side of the "laminated porous membrane"

Claims

1. A laminated porous membrane having a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, wherein the thickness of the polyolefin porous membrane is 10 μm or less, the amount of carboxymethyl cellulose relative to 100 parts by weight of the inorganic particles in the porous layer is 0.7 parts by weight or less, the amount of shedding during polishing of the porous layer is 0.70 μL or less, and the curl amount is 1.5 mm or less.

2. The laminated porous membrane according to Claim 1, wherein the tensile strength in the width direction orthogonal to the film-forming direction of the polyolefin porous membrane is 150 MPa or more.

3. The laminated porous membrane according to Claim 1 or 2, wherein the thickness ratio of the porous layer in the laminated porous membrane is 16% or more.

4. The laminated porous membrane according to Claim 1 or 2, wherein the inorganic particles are precipitated barium sulfate and the particle diameter is 0.6 μm or more and 1.6 μm or less.

5. A method for producing a laminated porous membrane having a porous layer containing inorganic particles and carboxymethyl cellulose on at least one side of a polyolefin porous membrane, the method comprising a step of forming a porous layer containing inorganic particles and carboxymethyl cellulose by applying a coating liquid containing inorganic particles and carboxymethyl cellulose to at least one side of the polyolefin porous membrane and then drying, wherein the viscosity of the carboxymethyl cellulose when it is an aqueous solution containing 2% by weight of carboxymethyl cellulose is 90 mPa·s or less.

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