JP2009129668A - Multilayer porous membrane - Google Patents

Abstract

Provided are a multi-layer porous membrane having a high heat resistance capable of withstanding a high temperature of 200 ° C. or higher and a method for producing the same.
SOLUTION: A multilayer porous membrane having a porous layer composed of particles mainly composed of a clay mineral and a resin binder on at least one surface of the porous membrane mainly composed of a polyolefin resin.
[Selection figure] None

Description

  The present invention relates to a multilayer porous membrane suitably used for separation and purification of various substances and a membrane disposed between a positive electrode and a negative electrode in a battery, and a method for producing the same. Furthermore, it is related with the separator for non-aqueous electrolyte batteries using the same, and a non-aqueous electrolyte battery.

Polyolefin porous membranes are widely used as separators in batteries, capacitors and the like because they exhibit excellent electrical insulation and ion permeability. In particular, in recent years, with the increase in functionality and weight of portable devices, lithium ion secondary batteries with high output density and high capacity density have been used as power sources. Polyolefin porous membranes are mainly used in such battery separators. Is used.
Lithium ion secondary batteries have high output density and capacity density, but because the electrolyte uses an organic solvent, the electrolyte decomposes due to heat generated by abnormal situations such as short circuit and overcharge. May cause fire. In order to prevent such a situation, some safety functions are incorporated in the lithium ion secondary battery, and one of them is a separator shutdown function. The shutdown function is a function that stops the progress of the electrochemical reaction by suppressing the ionic conduction in the electrolytic solution by blocking the micropores of the separator due to heat melting or the like when the battery generates abnormal heat. Generally, the lower the shutdown temperature, the higher the safety. One of the reasons why polyethylene is used as a component of the separator is that it has an appropriate shutdown temperature. However, in batteries with high energy, the temperature inside the battery continues to rise even if the electrochemical reaction is stopped by shutting down. As a result, the separator contracts due to heat shrinkage, and both electrodes are short-circuited. There is a problem of doing.

In order to solve such a problem, a method of forming a layer containing an inorganic filler between a separator and an electrode has been proposed (Patent Document 1). According to this method, even if the temperature continues to rise above the shutdown temperature and the separator breaks, since the inorganic filler-containing layer exists as an insulating layer, a short circuit between both electrodes can be prevented.
Japanese Patent No. 3756815

In recent years, however, the capacity of batteries has been increasing. In such high-capacity batteries, the amount of heat generated during abnormal heat generation is large, and once abnormal heat generation occurs, the structure in which the inorganic filler-containing layer is laminated on the separator is such that the separator cannot hold the inorganic filler-containing layer. There is a problem that the film breakage or the separator is thermally contracted with the inorganic filler-containing layer, and the short circuit between the two electrodes during abnormal heat generation cannot be prevented.
This short circuit problem during abnormal heat generation itself can be solved by using a polyolefin having a high melting point as the material of the porous film, since the heat resistance of the porous film is improved. However, when a porous membrane is used as a battery separator, it is required that the membrane is thermally melted at the shutdown temperature to close the pores (shutdown function). If a polyolefin having a high melting point is used as the material of the porous membrane, this time This causes another problem that the shutdown temperature becomes too high or the shutdown does not occur.

The short-circuit problem can also be solved by increasing the thickness of the inorganic filler-containing layer laminated on the polyolefin porous membrane using a polyolefin porous membrane having a shutdown function. However, when the thickness of the inorganic filler-containing layer is increased, the volume occupied by the separator in the battery increases, which is disadvantageous from the viewpoint of increasing the capacity of the battery. Further, when the thickness of the inorganic filler-containing layer is increased, the air permeability tends to increase.
An object of the present invention is to provide a multilayer porous membrane having a very thin inorganic filler-containing layer excellent in heat resistance and permeability. It is another object of the present invention to provide a production method capable of providing such a multilayer porous membrane, a non-aqueous electrolyte battery separator and a non-aqueous electrolyte battery that are highly safe and suitable for increasing the battery capacity.

As a result of intensive studies to solve the above problems, the inventors of the present invention have found that in a polyolefin porous film in which an inorganic filler-containing layer is laminated, by configuring the inorganic filler layer with particles mainly composed of clay mineral, It has been found that even with a thin inorganic filler layer thickness, the heat resistance is remarkably improved, the heat shrinkage rate at high temperature is remarkably suppressed, and it does not short-circuit at a high temperature of 200 ° C. or higher while having a good shutdown function. Reached.
That is, the present invention is as follows.
[1] A multilayer porous membrane having a porous layer composed of particles mainly composed of a clay mineral and a resin binder on at least one surface of the porous membrane mainly composed of a polyolefin resin.
[2] The multilayer porous membrane according to [1], wherein the clay mineral is a layered aluminosilicate mineral.
[3] The multilayer porous membrane according to [2], wherein the layered aluminosilicate mineral is a kaolin mineral.
[4] The multilayer porous film according to any one of [1] to [3], wherein the porous film is a porous film made of a resin composition containing polypropylene and a polyolefin other than polypropylene.
[5] The multilayer porous membrane according to [4], wherein the ratio of polypropylene to the total polyolefin in the resin composition is 1 to 35% by mass.
[6] The multilayer porous membrane according to [4], wherein the ratio of polypropylene to the total polyolefin in the resin composition is 3 to 20% by mass.
[7] The multilayer porous membrane according to any one of [4] to [6], wherein the polyolefin other than polypropylene is polyethylene.
[8] The multilayer porous membrane [9] [1] to [8] according to any one of [1] to [7], wherein the porous layer has a thickness of 3 μm to 6 μm. A battery separator comprising the multilayer porous membrane described.
[10] A non-aqueous electrolyte battery comprising the non-aqueous electrolyte battery separator according to [9].
[11] A step of forming a porous layer on a porous film mainly composed of a polyolefin resin by applying a coating liquid containing particles mainly composed of a clay mineral and a resin binder on at least one surface of the porous film. A method for producing a multilayer porous membrane.

  According to the present invention, the heat shrinkage rate at a high temperature is remarkably suppressed, and the multilayer porous membrane having a high heat resistance capable of withstanding a high temperature of 200 ° C. or higher, a method for producing the same, a separator for a nonaqueous electrolyte battery using the same, and A non-aqueous electrolyte battery can be provided.

The present invention is described in detail below.
The multilayer porous membrane of the present invention has a porous membrane composed mainly of a polyolefin resin, and a porous layer composed of particles mainly composed of a clay mineral and a resin binder laminated on at least one surface of the porous membrane. Yes.
In the present invention, by adopting particles mainly composed of clay mineral as the inorganic filler constituting the porous layer, thermal shrinkage at a high temperature of the porous film is suppressed even with a thinner porous layer thickness, and excellent heat resistance Is expressed. The porous layer may be laminated only on one side of the porous film or on both sides.

The clay mineral can be used without any particular limitation, but a clay mineral mainly composed of a layered silicate mineral is preferable because the porous layer can be easily thinned.
Layered silicate minerals include kaolinite, dickite, nacrite, halloysite, pyrophyllite, audinite, ferripyrophyllite, talc, willemsite, kerolite, pimelite, montmorillonite, beidellite, nontronite, bolcon score, Saponite, Hectorite, Sauconite, Steven Sight, Swinholderite, Vermiculite, Lizardite, Burcherin, Amesite, Kronsteadite, Nepoite, Keriaite, Freponite, Blindrite, Kuroko, Mica, Iron Mica, East Knight, Siderophyrite Tetraferri Iron Mica, Scale Mica, Polyriccionite, Mica, Celadon Stone, Iron Celadon Stone, Iron Alumino Ceradon Stone, Alumino Ceradon Stone, Tobe Mica, Soda Mica, C Tontnite, kinoshita, bite mica, ananda stone, pearl mica, clinochlore, chamosite, penantite, nimite, bailicroa, donbasite, kukuite, sudokuite, corrensite, hydrobiotite, arietite, crukeite, rectolite, tosudite, dodrite, Examples include lunijan light and salioite. Layered silicate minerals having no ion exchange ability such as kaolinite, dickite, nacrite, halloysite, pyrophyllite, talc and the like are preferred because they are less likely to inhibit lithium ion migration. In particular, aluminosilicate minerals such as kaolinite, dickite, nacrite, halloysite, and pyrophyllite are preferable from the viewpoint of electrochemical stability. Kaolin mainly composed of kaolin minerals such as kaolinite can be used particularly preferably because it is inexpensive and easily available. Kaolin includes calcined kaolin and wet kaolin, both of which can be preferably used.

The average particle size of the particles mainly composed of clay minerals is not particularly limited, but an average particle size of 3 μm or less is preferable because an increase in air permeability associated with the formation of the porous layer can be easily suppressed. Further, the average particle size of particles mainly composed of clay minerals is not particularly limited, but when the content ratio of particles having a particle size larger than 2 μm is 60% or less, it is easy to increase the air permeability associated with the formation of the porous layer. It is preferable because it can be suppressed. The particle size distribution of particles mainly composed of clay minerals is a value measured using a laser particle size distribution measuring apparatus with water as a dispersion medium, and the average particle size is a particle having a cumulative frequency of 50%. The value of the diameter.
The porous layer of the present invention needs to contain a resin binder for binding particles mainly composed of clay mineral on the porous film in addition to particles mainly composed of clay mineral. The type of the resin binder is not limited, but when the multilayer porous membrane of the present invention is used as a separator for a lithium ion secondary battery, it is insoluble in the electrolyte solution of the lithium ion secondary battery, and lithium ion It is preferable to use one that is electrochemically stable within the usage range of the secondary battery.

  Specific examples of such a resin binder include, for example, polyolefins such as polyethylene and polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer; Fluorine-containing rubber such as ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, methacryl Rubber such as acid ester-acrylic ester copolymer, styrene-acrylic ester copolymer, acrylonitrile-acrylic ester copolymer, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate Cellulose derivatives such as ethylcellulose, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose; melting point and / or glass transition temperature of polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester, etc. of 180 ° C. or higher And the like.

  When polyvinyl alcohol is used as the resin binder, the saponification degree is preferably 85% or more and 100% or less. When the saponification degree is 85% or more, when the multilayer porous membrane is used as a battery separator, the short-circuiting temperature (short-circuit temperature) is improved, and good safety performance is obtained, which is preferable. The saponification degree is more preferably 90% or more and 100% or less, further preferably 95% or more and 100% or less, and particularly preferably 99% or more and 100% or less. The degree of polymerization of polyvinyl alcohol is preferably 200 or more and 5000 or less, more preferably 300 or more and 4000 or less, and particularly preferably 500 or more and 3500 or less. When the degree of polymerization is 200 or more, particles of clay mineral as a main component can be firmly bound with a small amount of polyvinyl alcohol, and the air permeability of the multilayer porous membrane by forming the porous layer while maintaining the mechanical strength of the porous layer This is preferable because an increase can be suppressed. Moreover, it is preferable for the polymerization degree to be 5000 or less because gelation or the like during preparation of the coating solution can be prevented.

The mass fraction of particles mainly composed of clay mineral in the porous layer can be appropriately determined from the viewpoint of the binding properties of particles mainly composed of clay mineral, the permeability and heat resistance of the multilayer porous membrane, and the like. Is preferably 50% or more and less than 100%, more preferably 70% or more and 99.99% or less, still more preferably 90% or more and 99.9% or less, and particularly preferably 95% or more and 99% or less.
The layer thickness of the porous layer is preferably 1 μm or more from the viewpoint of heat resistance of the multilayer porous film, and preferably 50 μm or less from the viewpoint of increasing the capacity and permeability of the battery. More preferably, they are 1.5 micrometers or more and 10 micrometers or less, More preferably, they are 2 micrometers or more and 6 micrometers or less, Most preferably, they are 3 micrometers or more and 6 micrometers or less.

There is no limitation in the formation method of a porous layer, It can form by a well-known method. For example, the porous layer can be formed on at least one surface of the porous film by applying a coating liquid in which particles containing a clay mineral as a main component and a resin binder are dispersed or dissolved in a solvent to the porous film.
As a solvent for the coating solution, those capable of uniformly and stably dispersing or dissolving particles mainly composed of clay mineral and resin binder are preferable. For example, N-methylpyrrolidone, N, N-dimethylformamide, N, N- Examples include dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, hexane and the like.

Various additives such as surfactants, thickeners, wetting agents, antifoaming agents, PH preparations containing acids and alkalis are added to the coating solution in order to stabilize dispersion and improve coating properties. May be added. These additives are preferably those that can be removed when the solvent is removed, but are porous if they are electrochemically stable in the range of use of the lithium ion secondary battery, do not inhibit the battery reaction, and are stable up to about 200 ° C. It may remain in the layer.
The method of dissolving or dispersing the particles mainly composed of clay mineral and the resin binder in the solvent of the coating solution is not particularly limited as long as it is a method that can realize the dissolution or dispersion characteristics of the coating solution necessary for the coating process. Examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade.

The method for applying the coating solution to the porous film is not particularly limited as long as it can achieve the required layer thickness and coating area. For example, gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast Examples include a coater method, a die coater method, a screen printing method, and a spray coating method.
Furthermore, it is preferable to perform a surface treatment on the surface of the porous film prior to coating, because the coating liquid can be easily applied and the adhesion between the inorganic filler-containing porous layer after coating and the surface of the porous film is improved. The surface treatment method is not particularly limited as long as it does not significantly impair the porous structure of the porous film. For example, corona discharge treatment method, mechanical surface roughening method, solvent treatment method, acid treatment method, ultraviolet oxidation method Etc.

The method for removing the solvent from the coating film after coating is not particularly limited as long as it does not adversely affect the porous film. For example, a method of drying at a temperature below the melting point while fixing the porous membrane, a method of drying under reduced pressure at a low temperature, a method of immersing in a poor solvent for a resin binder to solidify the resin binder and simultaneously extracting the solvent, etc. Is mentioned.
Next, the porous film which has polyolefin resin as a main component is demonstrated.
The porous film containing a polyolefin resin as a main component means that the polyolefin resin occupies 50% or more and 100% or less of the mass fraction of the resin component constituting the porous film from the viewpoint of shutdown performance when used as a battery separator. A porous membrane is preferred. The proportion of the polyolefin resin is more preferably 60% or more and 100% or less, and most preferably 70% or more and 100% or less.

  The polyolefin resin refers to a polyolefin resin used for normal extrusion, injection, inflation, blow molding and the like, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Homopolymers and copolymers, multistage polymers, and the like can be used. In addition, polyolefins selected from the group of these homopolymers, copolymers, and multistage polymers can be used alone or in combination. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, ethylene-propylene random copolymer, polybutene. And ethylene propylene rubber.

When the multilayer porous membrane is used as a battery separator, it is preferable to use a resin mainly composed of high-density polyethylene because of its low melting point and high strength required performance.
From the viewpoint of improving the heat resistance of the porous film and the heat resistance of the multilayer porous film, it is more preferable to use a porous film made of polypropylene and a resin composition containing a polyolefin resin other than polypropylene.
Here, the three-dimensional structure of polypropylene is not limited, and any of isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene may be used.

The ratio of polypropylene to the total polyolefin in the polyolefin resin composition is preferably 1 to 35% by mass, more preferably 3 to 20% by mass, and still more preferably from the viewpoint of achieving both heat resistance and a good shutdown function. It is 4-10 mass%.
In this case, the polyolefin resin other than polypropylene is not limited, and examples thereof include homopolymers or copolymers of olefin hydrocarbons such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Can be mentioned. Specific examples include polyethylene, polybutene, and ethylene-propylene random copolymer.
When the pores are required to shut down due to thermal melting, such as when used as a battery separator, as polyolefin resins other than polypropylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high It is preferable to use polyethylene such as density polyethylene and ultrahigh molecular weight polyethylene. Among these, from the viewpoint of strength, it is more preferable to use polyethylene having a density measured according to JIS K 7112 of 0.93 g / cm ³ or more.

  The viscosity average molecular weight of the polyolefin resin is preferably from 30,000 to 12 million, more preferably from 50,000 to less than 2 million, and still more preferably from 100,000 to less than 1 million. A viscosity average molecular weight of 30,000 or more is preferable because the melt tension at the time of melt molding becomes large and the moldability becomes good and the strength becomes high due to the entanglement of the polymers. On the other hand, a viscosity average molecular weight of 12 million or less is preferable because uniform melt kneading is facilitated, and sheet formability, particularly thickness stability, is excellent. Furthermore, when the multilayer porous membrane of the present invention is used as a battery separator, it is preferable that the viscosity average molecular weight is less than 1,000,000 because a good shutdown function can be obtained because the pores are easily closed when the temperature rises. For example, instead of using a polyolefin having a viscosity average molecular weight of less than 1 million alone, a mixture of a polyolefin having a viscosity average molecular weight of 2 million and a polyolefin having a viscosity average molecular weight of 270,000 may be used, and the viscosity average molecular weight of the mixture may be less than 1 million. Good.

In addition to polyolefin, the porous film containing polyolefin resin as a main component can contain any additive depending on various purposes within a range not impairing the purpose of the present invention. Examples of such additives include polymers other than polyolefins; inorganic fillers; phenol-based, phosphorus-based and sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; Stabilizers; antistatic agents; antifogging agents; colored pigments and the like.
The total amount of these additives is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less with respect to 100 parts by mass of the polyolefin resin composition.

There is no limitation on the average pore diameter of the porous membrane, and it can be determined appropriately according to the application. However, when the multilayer porous membrane is used as a battery separator, the pore diameter is preferably 0.001 to 10 μm, more preferably 0.01 to 1 μm. It is.
In the present invention, the method for producing the porous film from the polyolefin resin composition is not limited, and a known production method can be adopted. For example, a method in which a polyolefin resin composition and a plasticizer are melt-kneaded and formed into a sheet shape, optionally stretched and then made porous by extracting the plasticizer, and a polyolefin resin composition is melt-kneaded to obtain a high draw rate. After extruding at a ratio, the polyolefin crystal interface is peeled off by heat treatment and stretching, the polyolefin resin composition and the inorganic filler are melt-kneaded and molded on a sheet, and then the polyolefin and inorganic filler are stretched And a method of making the structure porous by removing the solvent at the same time as solidifying the polyolefin by dissolving it in a poor solvent for the polyolefin after dissolving the polyolefin resin composition.

Hereinafter, as an example of a method for producing a porous film, a method for extracting a plasticizer after melting and kneading a polyolefin resin composition and a plasticizer to form a sheet will be described.
First, a polyolefin resin composition and a plasticizer are melt-kneaded. As the melt-kneading method, for example, polyolefin resin and other additives as required may be added to a resin kneading apparatus such as an extruder, kneader, lab plast mill, kneading roll, Banbury mixer, etc. A method of introducing a plasticizer at a ratio of kneading and kneading. At this time, it is preferable that the polyolefin resin, other additives, and the plasticizer are pre-kneaded in advance at a predetermined ratio using a Henschel mixer or the like before being added to the resin kneading apparatus. More preferably, only a part of the plasticizer is added in the pre-kneading, and the remaining plasticizer is kneaded while side-feeding the resin kneading apparatus. By doing so, the dispersibility of the plasticizer is increased, and when stretching a sheet-like molded product of the melt-kneaded mixture of the resin composition and the plasticizer in the subsequent step, the film is stretched at a high magnification without breaking. can do.

As the plasticizer, a non-volatile solvent capable of forming a uniform solution at a melting point or higher of the polyolefin can be used. Specific examples of such a non-volatile solvent include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol.
Among these, liquid paraffin is preferable because it has high compatibility with polyethylene and polypropylene, and even when the melt-kneaded product is stretched, the interface peeling between the resin and the plasticizer hardly occurs.

  The ratio of the polyolefin resin composition and the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and formed into a sheet shape. For example, the mass fraction of the plasticizer in the composition comprising the polyolefin resin composition and the plasticizer is preferably 30 to 80 mass%, more preferably 40 to 70 mass%. When the mass fraction of the plasticizer is 80% by mass or less, the melt tension at the time of melt molding is hardly insufficient and the moldability tends to be improved. On the other hand, when the mass fraction is 30% by mass or more, even if the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, the polyolefin chain is not broken, and a uniform and fine pore structure is formed and the strength is also improved. Easy to increase.

  Next, the melt-kneaded product is formed into a sheet shape. As a method for producing a sheet-like molded product, for example, a melt-kneaded product is extruded into a sheet shape via a T-die or the like, and brought into contact with a heat conductor to cool to a temperature sufficiently lower than the crystallization temperature of the resin component. And then solidify. As the heat conductor used for cooling and solidification, metal, water, air, plasticizer itself, or the like can be used. However, a metal roll is preferable because of high heat conduction efficiency. At this time, it is more preferable that the metal roll is sandwiched between the rolls, since the efficiency of heat conduction is further increased, the sheet is oriented and the film strength is increased, and the surface smoothness of the sheet is also improved. The die lip interval when extruding into a sheet form from a T die is preferably 400 μm or more and 3000 μm or less, and more preferably 500 μm or more and 2500 μm or less. When the die lip interval is 400 μm or more, the mess and the like are reduced, and there is little influence on the film quality such as streaks and defects, and film breakage and the like can be prevented in the subsequent stretching process. On the other hand, when the die lip interval is 3000 μm or less, the cooling rate is high and uneven cooling can be prevented and the thickness stability of the sheet can be maintained.

It is preferable to stretch the sheet-like molded body thus obtained. As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferred from the viewpoint of the strength of the porous film to be obtained. When the sheet-like molded body is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, and the finally obtained porous film is not easily torn and has a high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, multiple stretching, and the like, and simultaneous biaxial from the viewpoint of improvement of puncture strength, uniformity of stretching, and shutdown property Stretching is preferred.
Here, simultaneous biaxial stretching means stretching in which stretching in the MD direction (machine direction of the microporous membrane) and stretching in the TD direction (direction crossing the MD of the microporous membrane at an angle of 90 °) are performed simultaneously. It refers to a method, and the draw ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in the MD direction or TD direction is independently performed. When stretching is performed in the MD direction or TD direction, the other direction is unconstrained or constant length. It is assumed that it is fixed to

The draw ratio is preferably in the range of 20 times or more and 100 times or less, more preferably in the range of 25 times or more and 50 times or less in terms of surface magnification. The stretching ratio in each axial direction is preferably in the range of 4 to 10 times in the MD direction, preferably in the range of 4 to 10 times in the TD direction, 5 to 8 times in the MD direction, and 5 times in the TD direction. More preferably, it is in the range of 8 times or less. When the total area ratio is 20 times or more, sufficient strength can be imparted to the resulting porous film, while when the total area ratio is 100 times or less, film breakage in the stretching step is prevented and high productivity is obtained.
Moreover, you may roll a sheet-like molded object. Rolling can be performed, for example, by a pressing method using a double belt press or the like. Rolling can particularly increase the orientation of the surface layer portion. The rolling surface magnification is preferably greater than 1 and 3 or less, more preferably greater than 1 and 2 or less. When the rolling ratio is larger than 1, the plane orientation increases and the film strength of the finally obtained porous film increases. On the other hand, a rolling ratio of 3 times or less is preferable because the orientation difference between the surface layer portion and the center is small, and a uniform porous structure can be formed in the thickness direction of the film.

Next, the plasticizer is removed from the sheet-like molded body to form a porous film. As a method for removing the plasticizer, for example, a method of extracting the plasticizer by immersing the sheet-like molded body in an extraction solvent and sufficiently drying it can be mentioned. The method for extracting the plasticizer may be either a batch type or a continuous type. In order to suppress the shrinkage of the porous film, it is preferable to constrain the end of the sheet-like molded body during a series of steps of immersion and drying. Further, the residual amount of plasticizer in the porous film is preferably less than 1% by mass.
As the extraction solvent, it is preferable to use a solvent that is a poor solvent for the polyolefin resin and a good solvent for the plasticizer and has a boiling point lower than the melting point of the polyolefin resin. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine such as hydrofluoroether and hydrofluorocarbon. Halogenated solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by an operation such as distillation.

In order to suppress the shrinkage of the porous film, a heat treatment such as heat fixation or heat relaxation can be performed after the stretching process or after the formation of the porous film. Further, the porous film may be subjected to a post-treatment such as a hydrophilic treatment with a surfactant or the like, or a crosslinking treatment with ionizing radiation or the like.
Next, the case where the multilayer porous membrane of the present invention is used as a battery separator will be described.
Since the multilayer porous membrane of the present invention has excellent heat resistance and a shutdown function, it is suitable for a battery separator that separates a positive electrode and a negative electrode in a battery.
In particular, since the multilayer porous membrane of the present invention does not short-circuit even at a high temperature of 200 ° C. or higher, it can be safely used as a separator for a high electromotive force battery.

As such a high electromotive force battery, for example, a non-aqueous electrolyte battery can be cited, and the multilayer porous membrane of the present invention is disposed between the positive electrode and the negative electrode to hold the non-aqueous electrolyte, thereby allowing non-aqueous electrolysis. A liquid battery can be manufactured.
There is no limitation in a positive electrode, a negative electrode, and a non-aqueous electrolyte, A well-known thing can be used.
Examples of the positive electrode material include lithium-containing composite oxides such as LiCoO ₂ , LiNiO ₂ , spinel type LiMnO ₄ , and olivine type LiFePO ₄ , and examples of the negative electrode material include graphite, non-graphitizable carbonaceous, and easy graphite. Carbonaceous materials such as carbonaceous carbonaceous and composite carbon bodies; silicon, tin, metallic lithium, various alloy materials, and the like.

As the non-aqueous electrolyte, an electrolyte in which an electrolyte is dissolved in an organic solvent can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples thereof include lithium salts such as LiClO ₄ , LiBF ₄ , and LiPF ₆ .
When the multilayer porous membrane is used as a battery separator, the air permeability of the multilayer porous membrane is preferably 10 seconds / 100 cc or more and 650 seconds / 100 cc or less, more preferably 20 seconds / 100 cc or more and 500 seconds / 100 cc or less. More preferably, it is 30 seconds / 100 cc or more and 450 seconds / 100 cc or less, and particularly preferably 50 seconds / 100 cc or more and 400 seconds / 100 cc or less. When the air permeability is 10 seconds / 100 cc or more, there is little self-discharge when used as a battery separator, and when it is 650 seconds / 100 cc or less, good charge / discharge characteristics are obtained.

The rate of increase in the air permeability of the multilayer porous membrane due to the formation of the porous layer is preferably 0% or more and 200% or less, more preferably 0% or more and 100% or less, and 0% or more and 70%. It is particularly preferred that However, when the air permeability of the porous film is less than 100 seconds / 100 cc, the increase in the air permeability of the multilayer porous film after forming the porous layer can be preferably used if it is 0% or more and 500% or less.
The final film thickness of the multilayer porous membrane is preferably 2 μm to 200 μm, more preferably 5 μm to 100 μm, and even more preferably 7 μm to 30 μm.

If the film thickness is 2 μm or more, the mechanical strength is sufficient, and if it is 200 μm or less, the occupied volume of the separator is reduced, which is advantageous in terms of increasing the battery capacity.
The thermal shrinkage rate of the multilayer porous film at 150 ° C. is preferably 0% or more and 15% or less in both the MD direction and the TD direction, more preferably 0% or more and 10% or less, and further preferably 0% or more and 5% or less. It is. When the thermal shrinkage rate is 15% or less in both the MD direction and the TD direction, it is preferable because the multilayer porous film is prevented from being broken during abnormal heat generation of the battery and short-circuiting hardly occurs.
The shutdown temperature of the multilayer porous membrane is preferably 120 ° C. or higher and 160 ° C. or lower, more preferably 120 ° C. or higher and 150 ° C. or lower. It is preferable that the shutdown temperature is 160 ° C. or lower because even when the battery generates heat, current interruption is promptly promoted, and better safety performance tends to be obtained. On the other hand, it is preferable that the shutdown temperature is 120 ° C. or higher because the battery can be used at around 100 ° C.

The short-circuit temperature of the multilayer porous membrane is preferably 180 ° C. or higher and 1000 ° C. or lower, more preferably 200 ° C. or higher and 1000 ° C. or lower. When the short-circuit temperature is 180 ° C. or higher, even if abnormal heat generation occurs in the battery, a short circuit does not occur immediately, so heat can be dissipated during that time, and better safety performance can be obtained.
The short temperature can be controlled to a desired value by adjusting the content of polypropylene, the type of polyolefin other than polypropylene, the type of inorganic filler, the thickness of the inorganic filler-containing layer, and the like.

EXAMPLES Next, although an Example demonstrates this invention further in detail, these do not restrict | limit the scope of the present invention. The measurement / test methods in the examples are as follows.
(1) Viscosity average molecular weight of polyolefin (Mv)
Based on ASTM-D4020, the intrinsic viscosity [η] (dl / g) at 135 ° C. in a decalin solvent was determined.
About polyethylene, it computed by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80
(2) Thickness of porous film, layer thickness of porous layer A sample of MD 10 mm x TD 10 mm is cut out from the porous film and multilayer porous film, and nine points (3 points x 3 points) are selected in a lattice shape, and the film thickness is dial gauge (PEACOCK No. 25 (registered trademark) manufactured by Ozaki Mfg. Co., Ltd.) was used, and the average value of the measurement values at nine locations was defined as the film thickness (μm) of the porous film and multilayer porous film. Further, the difference in film thickness between the multilayer porous film and the porous film thus measured was defined as the layer thickness (μm) of the porous layer of the porous layer.

(3) Particle size distribution of inorganic filler, average particle size (μm)
After adding an inorganic filler to distilled water and adding a small amount of an aqueous solution of sodium hexametaphosphate and dispersing with an ultrasonic homogenizer for 1 minute, the particle size is measured using a laser particle size distribution analyzer (Microtrac MT3300EX manufactured by Nikkiso Co., Ltd.). Distribution was measured. The particle size at which the cumulative frequency was 50% was taken as the average particle size.
(4) Air permeability of porous membrane (second / 100cc), rate of increase in air permeability due to formation of porous layer (%)
Using a Gurley type air permeability meter (G-B2 (trademark) manufactured by Toyo Seiki, inner cylinder mass: 567 g) compliant with JIS P-8117, a porous film having an area of 645 mm ² (a circle with a diameter of 28.6 mm), multilayer porosity The time (seconds) required for 100 cc of air to pass through the membrane was measured and used as the air permeability of the porous membrane and the multilayer porous membrane.
The rate of increase in air permeability due to the formation of the porous layer was calculated by the following formula.
Permeability increase rate (%) = {(Air permeability of multilayer porous membrane−Air permeability of porous membrane) / Air permeability of porous membrane} × 100
(5) 150 ° C. thermal shrinkage (%)
The separator is cut to 100 mm in the MD direction and 100 mm in the TD direction, and left in an oven at 150 ° C. for 1 hour. At this time, the sample is sandwiched between two sheets of paper so that the hot air does not directly hit the sample. After the sample is taken out from the oven and cooled, the length (mm) is measured, and the thermal contraction rate of MD and TD is calculated by the following formula.
MD thermal shrinkage (%) = (100−MD length after heating) / 100 × 100
TD heat shrinkage rate (%) = (100−length of TD after heating) / 100 × 100

(6) Shutdown temperature and short-circuit temperature of the multilayer porous film a. Production of positive electrode 92.2 parts by mass of lithium cobalt composite oxide (LiCoO ₂ ) as the positive electrode active material, 2.3 parts by mass of flake graphite and acetylene black as the conductive material, and polyvinylidene fluoride (PVDF) as the resin binder 3.2 parts by mass were prepared, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector using a die coater so that the positive electrode active material application amount was 250 g / m ² . After drying at 130 ° C. for 3 minutes, using a roll press machine, compression molding was performed so that the bulk density of the positive electrode active material was 3.00 g / cm ³ to obtain a positive electrode.
b. Preparation of Negative Electrode 96.6 parts by mass of artificial graphite as a negative electrode active material, 1.4 parts by mass of ammonium salt of carboxymethyl cellulose and 1.7 parts by mass of styrene-butadiene copolymer latex as a resin binder were prepared and purified. A slurry was prepared by dispersing in water. This slurry was applied to one side of a 12 μm-thick copper foil serving as a negative electrode current collector using a die coater so that the negative electrode active material application amount was 106 g / m ² . After drying at 120 ° C. for 3 minutes, using a roll press machine, compression molding was performed so that the bulk density of the negative electrode active material was 1.35 g / cm ³ , thereby obtaining a negative electrode.
c. Preparation of Nonaqueous Electrolyte Solution In a mixed solvent of propylene carbonate: ethylene carbonate: γ-butyllactone = 1: 1: 2 (volume ratio), LiBF ₄ as a solute was dissolved to a concentration of 1.0 mol / L, A water electrolyte was prepared.
d. Measurement of shutdown temperature and short-circuit temperature A negative electrode cut into 65 mm × 20 mm and immersed in a non-aqueous electrolyte for 1 minute or longer, 9 μm (thickness) × 50 mm × 50 mm aramid film with a 16 mm diameter hole in the center, 65 mm × A multilayer porous membrane or porous membrane cut into 20 mm and immersed in a non-aqueous electrolyte for 1 hour or more, a positive electrode cut into 65 mm × 20 mm and immersed in a non-aqueous electrolyte for 1 minute or more, a Kapton film, and a silicon rubber with a thickness of about 4 mm It was prepared and laminated in this order on a ceramic plate to which a thermocouple was connected. This laminate is set on a hot plate, heated at a rate of 15 ° C./min with a pressure of 4.1 MPa applied by a hydraulic press, and the impedance change between the positive and negative electrodes is 1 V AC and 1 kHz. The measurement was performed up to 200 ° C.
The temperature at the time when the impedance reached 1000Ω was taken as the shutdown temperature, and the temperature at which the impedance again fell below 1000Ω after the shutdown was taken as the short-circuit temperature.

(7) Evaluation of battery separator suitability a. Production of Positive Electrode A positive electrode produced in the same manner as (6) a was punched into a circle having an area of 2.00 cm ² .
b. Production of Negative Electrode A negative electrode produced in the same manner as b in (6) was punched into a circle having an area of 2.05 cm ² .
c. Nonaqueous electrolyte solution Prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 ml / L.
d. Battery assembly The negative electrode, the multilayer porous film, and the positive electrode were stacked in this order from the bottom so that the active material surfaces of the positive electrode and the negative electrode faced each other. The laminate was stored in a stainless steel container with a lid in which the container main body and the lid were insulated so that the copper foil of the negative electrode and the aluminum foil of the positive electrode were in contact with the container main body and the lid, respectively. A non-aqueous electrolyte was poured into the container and sealed.

e. Evaluation (rate characteristics)
d. The simple battery assembled in step 1 is charged to a battery voltage of 4.2 V at a current value of 3 mA (about 0.5 C) at 25 ° C., and the current value starts to be reduced from 3 mA so as to hold 4.2 V. The first charge after battery preparation was performed for a total of about 6 hours, and then discharged to a battery voltage of 3.0 V at a current value of 3 mA.
Next, at 25 ° C., the battery is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C), and the current value starts to be reduced from 6 mA so as to hold 4.2 V. The battery was charged for a period of time and then discharged at a current value of 6 mA to a battery voltage of 3.0 V, and the discharge capacity at that time was set to 1 C discharge capacity (mAh).
Next, at 25 ° C., the battery is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C), and the current value starts to be reduced from 6 mA so as to hold 4.2 V. The battery was charged for a period of time and then discharged at a current value of 12 mA (about 2.0 C) to a battery voltage of 3.0 V, and the discharge capacity at that time was set to 2 C discharge capacity (mAh).
The ratio of the 2C discharge capacity to the 1C discharge capacity was calculated, and this value was used as the rate characteristic.
Rate characteristic (%) = (2C discharge capacity / 1C discharge capacity) × 100
(Cycle characteristics)
A simple battery that has been evaluated for rate characteristics is charged to a battery voltage of 4.2 V at a current value of 6 mA (about 1.0 C) at 60 ° C., and then the current value starts to be reduced from 6 mA so as to maintain 4.2 V. In this method, a cycle of charging for about 3 hours in total and discharging to a battery voltage of 3.0 V at a current value of 6 mA was repeated 100 times, and the discharge capacities (mAh) of the first cycle and the 100th cycle were measured.
The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first time was calculated, and this value was used as the cycle characteristics.
Cycle characteristics (%) = (100th discharge capacity / first discharge capacity) × 100

[Example 1]
(Manufacture of porous membrane)
47.5 parts by mass of Mv 700,000 homopolymer polyethylene, 47.5 parts by mass of Mv 250,000 homopolymer polyethylene, and 5 parts by mass of Mv 400,000 homopolymer polypropylene were dry blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained pure polymer mixture, and again A mixture such as a polymer was obtained by dry blending using a tumbler blender. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected as a plasticizer into the extruder cylinder by a plunger pump. The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65% by mass. The melt-kneading conditions were set at a preset temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die to obtain a sheet-like polyolefin composition having a thickness of 1300 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 6.4 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 118 ° C. Next, the solution was introduced into a methyl ethyl ketone tank, and liquid paraffin was extracted and removed, and then methyl ethyl ketone was removed by drying.
Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting temperature was 122 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having a film thickness of 16 μm, a porosity of 49%, and an air permeability of 155 seconds / 100 cc was obtained.
(Formation of porous layer)
A coating solution was prepared by uniformly dispersing 98.2 parts by mass of kaolin 1 in Table 1 and 1.8 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) in 150 parts by mass of water. The polyolefin resin porous membrane was coated on the surface using a gravure coater. Water was removed by drying at 60 ° C. to obtain a multilayer porous membrane in which a porous layer having a thickness of 3 μm was formed on the polyolefin resin porous membrane.

[Example 2]
A multilayer porous membrane was obtained in the same manner as in Example 1 except that the thickness of the porous layer was 5 μm.

[Example 3]
A multilayer porous membrane was obtained in the same manner as in Example 1 except that the kaolin 2 in Table 1 was used.

[Example 4]
A multilayer porous membrane was obtained in the same manner as in Example 1 except that the kaolin 3 in Table 1 was used.

[Example 5]
A multilayer porous membrane was obtained in the same manner as in Example 1 except that the kaolin 4 shown in Table 1 was used and the thickness of the porous layer was changed to 4 μm.

[Example 6]
A multilayer porous membrane was obtained in the same manner as in Example 1 except that the kaolin 5 in Table 1 was used.

[Example 7]
98.2 parts by weight of kaolin 2 in Table 1, 1.8 parts by weight of acrylic latex (solid content concentration 50%, minimum film forming temperature 0 ° C. or less), 1 weight of ammonium polycarboxylate aqueous solution (SN Dispersant 5468 manufactured by San Nopco) 1 part by weight of a polyoxyalkylene surfactant (San Nopco SN wet 980) is uniformly dispersed in 150 parts by weight of water to prepare a coating solution, and a porous layer having a thickness of 5 μm is formed on the polyolefin resin porous film. A multilayer porous membrane was obtained in the same manner as in Example 1 except that it was formed.

[Example 8]
(Manufacture of porous membrane)
47 parts by mass of polyethylene having an Mv of 700,000, 46 parts by mass of polyethylene having an Mv of 250,000, and 7 parts by mass of polypropylene having an Mv of 400,000 were dry blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained pure polymer mixture, and again A mixture such as a polymer was obtained by dry blending using a tumbler blender. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected as a plasticizer into the extruder cylinder by a plunger pump. The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65% by mass. The melt-kneading conditions were set at a preset temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die to obtain a sheet-like polyolefin composition having a thickness of 2000 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 7 times in the TD direction. At this time, the setting temperature of the simultaneous biaxial tenter was 125 ° C. Next, the solution was introduced into a methyl ethyl ketone tank, and liquid paraffin was extracted and removed, and then methyl ethyl ketone was removed by drying.
Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting temperature was 133 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having a film thickness of 16 μm, a porosity of 40%, and an air permeability of 165 seconds / 100 cc was obtained.
(Formation of porous layer)
A coating solution was prepared by uniformly dispersing 98.2 parts by mass of kaolin 2 in Table 1 and 1.8 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) in 150 parts by mass of water. The polyolefin resin porous membrane was coated on the surface using a gravure coater. Water was removed by drying at 60 ° C. to obtain a multilayer porous membrane in which a porous layer having a thickness of 3 μm was formed on the polyolefin resin porous membrane.

[Example 9]
(Manufacture of porous membrane)
47.5 parts by mass of polyethylene having a viscosity average molecular weight (Mv) of 200,000, 2.5 parts by mass of polypropylene having an Mv of 400,000, 30 parts by mass of liquid paraffin (LP) as a plasticizer, and pentaerythrityl-tetrakis- as an antioxidant What added 0.5 mass part of [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was premixed with a Henschel mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the liquid paraffin amount ratio in the total mixture (100 parts by mass) melt-kneaded and extruded was 50 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 200 rpm, and a discharge rate of 15 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1050 μm. Next, it was continuously led to a simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 6.4 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 118 ° C. Next, the plasticizer was removed by being led to a methyl ethyl ketone bath, and then methyl ethyl ketone was removed by drying. Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting conditions were a maximum draw ratio of 1.5 times, a final draw ratio of 1.3 times, a maximum drawing temperature of 123 ° C., and a final drawing temperature of 128 ° C. As a result, a polyolefin resin porous film having a film thickness of 16 μm, a porosity of 45%, and an air permeability of 235 seconds / 100 cc was obtained.
(Formation of porous layer)
A coating solution was prepared by uniformly dispersing 98.2 parts by mass of kaolin 2 in Table 1 and 1.8 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) in 150 parts by mass of water. The polyolefin resin porous membrane was coated on the surface using a gravure coater. Water was removed by drying at 60 ° C. to obtain a multilayer porous membrane in which a porous layer having a thickness of 3 μm was formed on the polyolefin resin porous membrane.

[Example 10]
(Manufacture of porous membrane)
16.6 parts by mass of polyethylene having a viscosity average molecular weight (Mv) of 700,000, 16.6 parts by mass of polyethylene having an Mv of 250,000, 1.8 parts by mass of polypropylene having an Mv of 400,000, 40 parts by mass of liquid paraffin (LP) as a plasticizer, What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was premixed with a Henschel mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) melt-kneaded and extruded was 65 parts by mass. The melt-kneading conditions were set at a preset temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1000 μm. Next, it was continuously led to a simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 6.4 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 118 ° C. Next, the plasticizer was removed by being led to a methyl ethyl ketone bath, and then methyl ethyl ketone was removed by drying. Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting temperature was 130 ° C. and the TD relaxation rate was 0.80. As a result, a polyolefin resin porous film having a film thickness of 12 μm, a porosity of 36%, and an air permeability of 235 seconds / 100 cc was obtained.
(Formation of porous layer)
A coating solution was prepared by uniformly dispersing 98.2 parts by mass of kaolin 2 in Table 1 and 1.8 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) in 150 parts by mass of water. The polyolefin resin porous membrane was coated on the surface using a gravure coater. Water was removed by drying at 60 ° C. to obtain a multilayer porous membrane in which a porous layer having a thickness of 4 μm was formed on the polyolefin resin porous membrane.

[Comparative Example 1]
98.2 parts by weight of barium sulfate (average particle size 0.30 μm) and 1.8 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) were uniformly dispersed in 150 parts by weight of water. A coating solution was prepared, and a multilayer porous membrane was obtained in the same manner as in Example 1 except that a porous layer having a thickness of 5 μm was formed on the polyolefin resin porous membrane.

[Comparative Example 2]
Uniformly disperse 98.2 parts by weight of heavy calcium carbonate (average particle size 0.87 μm) and 1.8 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) in 150 parts by weight of water. Thus, a coating solution was prepared, and a multilayer porous membrane was obtained in the same manner as in Example 1 except that a porous layer having a thickness of 5 μm was formed on the polyolefin resin porous membrane.

[Comparative Example 3]
Alumina (average particle size 0.40 μm) 98.2 parts by weight and polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) 1.8 parts by weight are uniformly dispersed in 150 parts by weight of water and applied. A multilayer porous membrane was obtained in the same manner as in Example 1 except that a liquid was prepared and a porous layer having a thickness of 6 μm was formed on the polyolefin resin porous membrane.

[Comparative Example 4]
96.4 parts by weight of strip boehmite (average particle size 2.9 μm) and 3.6 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by weight of water. A coating solution was prepared in the same manner as in Example 1 except that a porous layer having a thickness of 6 μm was formed on the polyolefin resin porous membrane to obtain a multilayer porous membrane.

[Comparative Example 5]
96.4 parts by weight of acicular boehmite (average particle size 4.2 μm) and 3.6 parts by weight of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by weight of water. A coating solution was prepared in the same manner as in Example 1 except that a porous layer having a thickness of 5 μm was formed on the polyolefin resin porous membrane to obtain a multilayer porous membrane.

[Comparative Example 6]
A porous membrane was obtained in the same manner as in Example 1 except that the porous layer was not formed.

[Comparative Example 7]
A porous membrane was obtained in the same manner as in Example 8 except that the porous layer was not formed.

[Comparative Example 8]
A porous membrane was obtained in the same manner as in Example 9 except that the porous layer was not formed.

[Comparative Example 9]
A porous membrane was obtained in the same manner as in Example 10 except that the porous layer was not formed.

Table 1 shows the particle size distribution and average particle size of particles mainly composed of clay mineral used in the examples.
Table 2 shows the air permeability, 150 ° C. heat shrinkage rate, shutdown temperature, and short circuit temperature of the multilayer porous membranes produced in Examples 1 to 10 and Comparative Examples 1 to 9.
The porous membranes of Comparative Examples 6 to 9 in which no porous layer was formed showed a very large value with a thermal shrinkage rate at 150 ° C. exceeding 50%.
On the other hand, the multilayer porous membranes of Comparative Examples 1 to 5 in which the porous layers using various inorganic fillers shown in Table 2 were formed were 150 ° C. compared with the porous membranes of Comparative Examples 6 to 9 in which the porous layers were not formed. Although the thermal contraction rate of the film was suppressed, even when the thickness of the porous layer was 5 μm or more, the value was 50% or more in the MD direction and 15% or more in the TD direction.
On the other hand, the multilayer porous membranes of Examples 1 to 10 in which the porous layer using particles mainly composed of clay minerals was formed at 150 ° C. despite the thin porous layer having a thickness of 3 to 5 μm. The thermal shrinkage rate of the MD direction and the TD direction was as small as 5% or less, indicating good heat shrinkage characteristics.

The air permeability of each of the multilayer porous membranes of Examples 1 to 10 was as small as 300 seconds / 100 CC or less. In particular, even when compared with the porous films of Comparative Examples 6 to 9 in which no porous layer was formed, the rate of increase in air permeability could be suppressed to 100% or less.
All of the simple batteries using the multilayer porous membranes or porous membranes of Examples 1 to 10 and Comparative Examples 6 to 9 as separators exhibited rate characteristics and cycle characteristics of 90% or more. From this, it has confirmed that the multilayer porous film or porous film manufactured in Examples 1-10 and Comparative Examples 6-9 can be used as a battery separator. Further, the shutdown temperature of these multilayer porous membranes or porous membranes was 145 to 148 ° C. and had a good shutdown function.
The porous films of Comparative Examples 6 to 9 that did not form the porous layer were short-circuited only by heating to the shutdown temperature + several degrees C. However, Examples 1 to 1 formed the porous layer composed of particles mainly composed of clay mineral. All the 10 multilayer porous membranes were not short-circuited even when heated to 200 ° C. or higher, and were extremely excellent in heat resistance.

Since the multilayer porous membrane of the present invention is excellent in heat resistance, it can be suitably used for separation and purification of various substances at high temperatures.
Moreover, since the multilayer porous membrane of this invention has a shutdown function, it can be used especially suitably for a battery separator. In particular, it is suitable for a battery separator for a lithium ion secondary battery.

Claims (11)

A multilayer porous membrane having a porous layer comprising particles mainly composed of a clay mineral and a resin binder on at least one surface of a porous membrane mainly composed of a polyolefin resin. The multilayer porous membrane according to claim 1, wherein the clay mineral is a layered aluminosilicate mineral. The multilayer porous membrane according to claim 2, wherein the layered aluminosilicate mineral is a kaolin mineral. The multilayer porous membrane according to any one of claims 1 to 3, wherein the porous membrane is a porous membrane made of a resin composition containing polypropylene and a polyolefin other than polypropylene. The multilayer porous membrane according to claim 4, wherein a ratio of polypropylene to total polyolefin in the resin composition is 1 to 35 mass%. The multilayer porous membrane according to claim 4, wherein a ratio of polypropylene to total polyolefin in the resin composition is 3 to 20% by mass. The multilayer porous membrane according to any one of claims 4 to 6, wherein the polyolefin other than polypropylene is polyethylene. The multilayer porous membrane according to any one of claims 1 to 7, wherein a layer thickness of the porous layer is 3 µm or more and 6 µm or less. The separator for nonaqueous electrolyte batteries which consists of a multilayer porous membrane of any one of Claims 1-8. A non-aqueous electrolyte battery comprising the non-aqueous electrolyte battery separator according to claim 9. A multilayer porous membrane having a step of forming a porous layer on a porous membrane mainly comprising a polyolefin resin by applying a coating liquid containing particles mainly composed of clay mineral and a resin binder on at least one surface of the porous membrane. Manufacturing method.