JP6345513B2 - Method for producing microporous film - Google Patents

Description

  The present invention relates to a method for producing a microporous film.

  Microporous films, particularly polyolefin microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like, and in particular, lithium ion battery separators. In recent years, lithium-ion batteries have been used for small electronic devices such as mobile phones and laptop computers, and are also being applied to electric vehicles and hybrid vehicles.

  Here, a lithium ion battery used for an electric vehicle or a hybrid vehicle is required to have higher output characteristics for taking out a lot of energy in a short time. Further, since lithium ion batteries used for hybrid electric vehicle applications are generally large and have high energy capacity, it is required to ensure higher safety and to be low in cost. As a separator having such a role, a microporous film by a dry method as shown in Patent Documents 1 and 2 has been proposed.

  As a method for ensuring safety, a method of forming a layer mainly composed of an inorganic filler on a microporous film has been proposed (Patent Document 3). Furthermore, as a method of improving the productivity of the separator coated with the inorganic filler, there is a method in which two microporous films are bonded together by laminating, and then the inorganic filler is coated on both sides and the two films are peeled off. Illustrated. (Patent Document 4)

Special table 2003-519723 gazette Japanese Patent No. 3003830 JP 2012-109249 A Special table 2013-511818 gazette

  The method of Patent Document 4 is excellent in that two inorganic coated separators can be obtained at the same time, but it is necessary to laminate in advance, and further improvement from the viewpoint of productivity (specifically, lamination can be omitted). Method). In addition, the exemplified production method uses a single-layer microporous film made of a polyethylene resin, and is a dry-type three-layer microporous film as in Patent Document 1 (the outer layer is made of a polypropylene resin, with an intermediate layer). When an application to a layer made of a polyethylene resin is attempted, there is a problem that the polyethylene resin of the intermediate layer melts during lamination and the permeability deteriorates.

  This invention is made | formed in view of the said problem, and it aims at providing the manufacturing method of the microporous film which generation | occurrence | production of a wrinkle is suppressed and is excellent in permeability | transmittance.

  As a result of intensive studies to solve the above-described problems, the inventors of the present invention have developed a microporous structure in which two sheets are bonded together without lamination, if a raw film containing a specific polypropylene resin is overlaid and stretched. As a result, the inventors have found that two microporous films coated with an inorganic filler on one side can be obtained as in Patent Document 4, and the present invention has been completed.

That is, the present invention is as follows.
[1]
A single resin comprising at least one polypropylene resin selected from the group consisting of a polypropylene resin (A) having a pentad fraction of 90 to 97% and a polypropylene resin (B) having a propylene content of 98 to 99.5 mol%. A film forming step of obtaining a layer original film, or a laminated original film having an outermost layer including one or more kinds of polypropylene resins selected from the group consisting of the polypropylene resin (A) and the polypropylene resin (B); ,
Two or more layers of the single layer original film or the laminated original film obtained in the film formation step are stacked and held at 90 ° C. to 160 ° C., and at least 1.05 times to 5.0 times in one direction. Stretching step to obtain a porous laminate,
After coating a dispersion of an inorganic filler on both surfaces of the laminate obtained in the stretching step to form a coating layer, the layers stacked in the laminate are peeled off to form the coating layer A coating process for obtaining at least two microporous films having:
In this order, a method for producing a microporous film.
[2]
The method for producing a microporous film according to [1] above, wherein the film thickness of the microporous film obtained in the peeling step is 10 μm to 14 μm.

  According to the present invention, it is possible to provide a method for producing a microporous film that suppresses generation of wrinkles and is excellent in permeability.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

[Method for producing microporous film]
The manufacturing method of the microporous film of this embodiment is:
A single resin comprising at least one polypropylene resin selected from the group consisting of a polypropylene resin (A) having a pentad fraction of 90 to 97% and a polypropylene resin (B) having a propylene content of 98 to 99.5 mol%. A film forming step of obtaining a layer original film, or a laminated original film having an outermost layer including one or more kinds of polypropylene resins selected from the group consisting of the polypropylene resin (A) and the polypropylene resin (B); ,
In a state where two sheets of the single layer original film or the laminated original film obtained in the film forming step are stacked and held at 90 ° C. to 160 ° C., at least 1.05 times to 5.0 times in one direction Stretching to obtain a laminate by stretching,
After coating a dispersion of an inorganic filler on both sides of the laminate obtained in the stretching step, the layer of the laminate is peeled off and at least two microporous layers coated with an inorganic filler on one side A coating process for obtaining a film;
Are included in this order.

[Film formation process]
The film forming step includes at least one selected from the group consisting of a polypropylene resin (A) having a pentad fraction of 90 to 97% and a polypropylene resin (B) having a propylene content of 98 to 99.5 mol%. A single layer original film containing a polypropylene resin or a laminated original film having a layer containing one or more polypropylene resins selected from the group consisting of the polypropylene resin (A) and the polypropylene resin (B) as an outermost layer. It is a process to obtain.

  A method for producing a single layer original film or a laminated original film (hereinafter collectively referred to as “original film”) in the film forming process is not particularly limited, and examples thereof include T-die extrusion molding, inflation molding, and calendar. Examples of the sheet forming method include molding and Skyf's method. Among these, T-die extrusion molding is preferable from the viewpoints of physical properties and applications required for the microporous film of the present embodiment.

  The draw ratio after extrusion in the film forming step, that is, the film winding speed (unit: m / min) is the resin extrusion speed (linear velocity in the flow direction of the molten resin passing through the die lip. Unit: m / min). The divided value is preferably 10 to 500, more preferably 50 to 300, and further preferably 100 to 250. Moreover, the winding speed of the film when winding the raw film is preferably about 2 to 400 m / min, more preferably 10 to 200 m / min, and further preferably 15 to 30 m / min. When the draw ratio is in the above range, it is preferable from the viewpoint of improving the permeability of the obtained microporous film.

[Polypropylene resin (A)]
The polypropylene resin (A) is a polypropylene having a pentad fraction of 90 to 97%. “Pentad fraction” refers to the proportion of isotactic chains existing in pentad units in the molecular chain. The proportion of propylene monomer units at the center of a chain in which five propylene monomer units are continuously meso-bonded. Rate.

  As the Pendat fraction approaches 100%, the stereoregularity becomes higher and the polypropylene resin has higher crystallinity. As illustrated in the examples of Patent Document 1, a microporous film having good permeability can be obtained by using a polypropylene resin exceeding 98%.

  In contrast, in the present embodiment, by using a polypropylene resin having a pentad fraction of 90 to 97%, which has a slightly lower crystallinity, it is possible to achieve both the press bonding property and the permeability in the stretching step described later. The pentad fraction is 90 to 97%, preferably 92 to 96%, and more preferably 94 to 96%. When the pentad fraction is 90% or more, the opening property at the time of stretching is improved, and good permeability can be obtained. On the other hand, when the pentad fraction is 97% or less, it is possible to obtain an original film that can be pressure-bonded at an appropriate temperature in the stretching step. The pentad fraction can be measured by the method described in the examples.

  The polypropylene resin (A) is preferably homopolypropylene. When the polypropylene resin (A) is homopolypropylene, the permeability tends to be further improved.

[Polypropylene resin (B)]
The polypropylene resin (B) is propylene having a propylene content of 98 to 99.5 mol%. The polypropylene resin (B) is preferably a propylene-based copolymer obtained by copolymerizing ethylene and / or α-olefin. When the polypropylene resin (B) is a propylene-based copolymer, the α-olefin serves as a tie molecule that connects the polypropylene crystals, and the destruction that occurs between the crystals tends to be further suppressed. The α-olefin is not particularly limited, and examples thereof include butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, and octene-1.

  In a conventional method for producing a microporous film, a propylene homopolymer is used as in Patent Document 1 in order to increase crystallinity and improve permeability. On the other hand, in this embodiment, 0.5 to 2.0 mol% of ethylene and / or α-olefin is copolymerized, thereby making it possible to achieve both press bonding and permeability in the stretching step. Propylene content is 98-99.5 mol%, Preferably it is 98.5-99.5 mol%, More preferably, it is 99-99.5 mol%. The propylene content can be measured by the method described in the examples.

  In addition, ethylene and α-olefin play a role of tie molecules that connect between polypropylene crystals, and tend to suppress destruction that occurs between the crystals. From this point of view, ethylene and α-olefin are preferably present dispersed in the molecular chain, and the propylene-based copolymer is preferably a random copolymer.

  The pentad fraction of the polypropylene resin (B) is preferably 95 to 99.9%, more preferably 97 to 99.9%, and further preferably 98 to 99.9%. When the pentad fraction is 95% or more, the openability during stretching is improved and the permeability tends to be further improved.

  The melt flow rate (MFR) of the polypropylene resin (A) and the polypropylene resin (B) is preferably 0.1 to 5.0 g / 10 minutes, more preferably 0.5 to 4.0 g / 10 minutes. More preferably, it is 2.0 to 4.0 g / 10 minutes. When the MFR is 0.1 g / 10 min or more, there is a tendency that breakage in the film forming process hardly occurs. Moreover, it exists in the tendency for the puncture strength of the microporous film obtained to become favorable because MFR is 5.0 g / 10min or less. MFR can be measured by the method described in the Examples.

[Polyethylene resin]
In the present embodiment, when a laminated microporous film having a layer containing a polyethylene resin is produced, a polyethylene resin can also be used. The MFR of the polyethylene resin is preferably 0.01 to 10 g / 10 minutes, more preferably 0.1 to 1.0 g / 10 minutes, and further preferably 0.1 to 0.5 g / 10 minutes. . When MFR is 0.01 g / 10 min or more, it tends to be difficult to generate fish eyes. Moreover, it exists in the tendency for permeability to improve more because MFR is 10 g / 10min or less. MFR can be measured by the method described in the Examples.

The density of the polyethylene resin is preferably 955 to 970 kg / m ³ , more preferably 955 to 967 kg / m ³ , and further preferably 958 to 965 kg / m ³ . When the density is 955 kg / m ³ or more, a microporous film with better air permeability tends to be obtained. Moreover, when it is 970 kg / m ³ or less, it tends to be difficult for the membrane to break during stretching. The density can be measured by the method described in the examples.

  The polypropylene resin (A), the polypropylene resin (B), and the polyethylene resin used as necessary in the present embodiment may contain other additional components as necessary in addition to the above components. Although it does not specifically limit as an additional component, For example, an olefin type elastomer, antioxidant, a metal deactivator, a heat stabilizer, a flame retardant (an organic phosphate ester type compound, an ammonium polyphosphate type compound, an aromatic halogen) Flame retardants, silicone flame retardants, etc.), fluoropolymers, plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weather resistance (light) Property improver, polyolefin nucleating agent, slip agent, inorganic or organic filler or reinforcing material (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various colors Agents, release agents and the like. The total content of these additional components is preferably 20 parts by mass or less, more preferably 100 parts by mass of the polypropylene resin (A), the polypropylene resin (B), or the polyethylene resin used as necessary. Is 10 parts by mass or less, more preferably 5 parts by mass or less.

  The configuration of the laminated raw film is not particularly limited as long as it has a layer containing one or more polypropylene resins selected from the group consisting of polypropylene resin (A) and polypropylene resin (B) in the outermost layer. , Polypropylene resin layer / polypropylene resin layer, polypropylene resin layer / polyethylene resin layer, polypropylene resin layer / polyethylene resin layer / polypropylene resin layer, and the like.

[Heat treatment process]
The raw film produced as described above is preferably subjected to a heat treatment before the cold drawing step described later. The heat treatment method is not particularly limited. For example, the method of bringing the raw film into contact with a heating roll, the method of exposing the laminated body to a heated gas phase before winding the laminate, and winding the raw film onto the core The method of exposing in a heating gas phase or a heating liquid phase, and the method of combining these are mentioned.

  In the case of a single layer raw film, the temperature of the heating roll and / or the temperature in the heated gas phase (hereinafter also referred to as “heat treatment temperature”) is preferably 120 ° C. to 160 ° C., more preferably 130 ° C. to 150 ° C. More preferably, it is 130-140 degreeC. When the heat treatment temperature is within the above range, the permeability tends to be further improved.

  On the other hand, in the case of a laminated raw film having a layer containing a polyethylene resin, the treatment temperature is preferably 110 to 130 ° C, more preferably 120 to 128 ° C, and further preferably 125 to 128 ° C. When the heat treatment temperature is within the above range, the permeability tends to be further improved.

  The time for performing heat treatment (hereinafter also referred to as “heat treatment time”) is preferably 10 seconds to 100 hours, more preferably 1 to 60 minutes, and further preferably 10 to 30 minutes. When heat treatment is performed in such a temperature range and a treatment time range, the degree of crystallinity of the raw film is increased, and a microporous film having good permeability tends to be obtained.

  Moreover, it can also heat-process in the state which accumulated two or more original film. In a state where two or more original fabric films are stacked, a laminate in which the respective original fabric films are bonded can be obtained by performing heat treatment in the preferable temperature range described above. In this case as well, it is possible to obtain a microporous film in which two sheets are bonded together without performing lamination before the coating process, as in the case of bonding films in a stretching process described later.

[Stretching process]
The stretching step is 1.05 times to 5.0 times in at least one direction in a state where two single layer original films or laminated original films obtained in the film forming step are stacked and held at 90 ° C. to 160 ° C. Is a step of obtaining a laminate. By this stretching, the layer constituting the laminate is made porous.

  Although it does not specifically limit as an extending | stretching method, For example, the method of extending | stretching to a uniaxial direction and / or a biaxial direction with a roll, a tenter, an autograph, etc. is mentioned. In particular, from the viewpoint of physical properties and applications required for the microporous film of the present embodiment, two or more stages of uniaxial stretching with a roll are preferable.

  The stretching step preferably includes the following cold stretching step and thermal stretching step in this order. In the cold stretching step, the laminate obtained in the heat treatment step is preferably stretched at a temperature of −20 ° C. to 80 ° C. in at least one direction 1.03 times to 2.0 times.

  The stretching temperature for cold stretching in the cold stretching step is preferably −20 ° C. or higher and lower than 80 ° C., more preferably 0 ° C. or higher and 50 ° C. or lower, and further preferably 20 ° C. or higher and 40 ° C. or lower. When the stretching temperature is −20 ° C. or higher, the microporous film tends to be difficult to break. Moreover, it exists in the tendency for the permeability | transmittance of the microporous film obtained to become favorable because extending | stretching temperature is less than 80 degreeC. Here, the drawing temperature of cold drawing shows the surface temperature of the film in the cold drawing process. The surface temperature of the film can be measured by providing a non-contact type thermocouple in the stretching roll machine.

  The draw ratio of cold drawing in the cold drawing step is preferably 1.03 to 1.5 times, more preferably 1.05 to 1.3 times, and even more preferably 1.1 times or more. 1.3 times or less. When the draw ratio is 1.03 times or more, the permeability tends to be good. Moreover, it exists in the tendency for a film to become difficult to fracture | rupture because a draw ratio is 1.5 times or less. The cold stretch of the raw film may be performed in at least one direction and may be performed in two directions. Preferably, the original film is uniaxially stretched only in the film extrusion direction (hereinafter also referred to as “MD direction”).

  In the hot stretching step, it is preferable to perform hot stretching at least 1.05 times to 5.0 times in the state where the temperature is kept at 90 ° C. to 160 ° C. after cold drawing as described above.

  In the case of a single layer original film, the stretching temperature of the heat stretching is preferably 90 ° C to 160 ° C, more preferably 120 ° C to 150 ° C or less, and more preferably 120 ° C to 140 ° C. By heat-stretching at 90 ° C. or higher, the film is hardly broken. Moreover, the permeability | transmittance of the microporous film obtained becomes favorable by heat-stretching below 160 degreeC. Here, the stretching temperature of the heat stretching indicates the surface temperature of the film in the heat stretching process.

  On the other hand, in the case of a laminated raw film having a layer containing a polyethylene resin, the stretching temperature for hot stretching is 90 ° C to 160 ° C, preferably 90 ° C to 130 ° C, more preferably 110 ° C to 125 ° C. It is. By heat-stretching at 90 ° C. or higher, the film is hardly broken. Moreover, it exists in the tendency for the permeability | transmittance of the microporous film obtained to become favorable by heat-stretching at 130 degrees C or less.

[Heat setting process]
The manufacturing method of the microporous film of the present embodiment includes a heat setting step in which the raw film obtained through the stretching step is preferably heat-treated at a temperature higher by 0 ° C. to 30 ° C. than the heat stretching temperature. Is preferred. This heat setting method includes a method of heat shrinking to such an extent that the length of the microporous film after heat setting is reduced by 3 to 50% with respect to the length of the microporous film before heat setting (hereinafter referred to as this method). The method is referred to as “relaxation”), and a method of heat setting so that the dimension in the stretching direction does not change. By this heat setting, a microporous film having much better dimensional stability, that is, having a small heat shrinkage rate can be obtained.

  In the case of a laminate composed of a single-layer raw film, the heat setting temperature is preferably 120 ° C to 160 ° C, more preferably 140 ° C to 160 ° C or less, and further preferably 145 to 155 ° C. . When the heat setting temperature is within the above range, the permeability tends to be further improved. Here, the heat setting temperature indicates the surface temperature of the microporous film in the heat setting process.

  In the case of a laminated raw film having a layer containing a polyethylene resin, the heat setting temperature is preferably 120 ° C to 130 ° C, more preferably 120 ° C to 125 ° C, and still more preferably 123 ° C to 125 ° C. When the heat setting temperature is within the above range, the permeability tends to be further improved.

[Coating process]
In the coating step, at least two sheets of the inorganic filler coated on one side were peeled off after coating the dispersion of the inorganic filler on both sides of the laminate obtained in the stretching step. This is a step of obtaining a microporous film.

  Specifically, an inorganic filler-containing resin solution in which an inorganic filler and a resin binder are dissolved or dispersed in a solvent (hereinafter, also referred to as “dispersion liquid”) is obtained on both surfaces of a laminate obtained by a stretching process or a heat setting process. The laminated body which has an inorganic coating layer on both surfaces is obtained by apply | coating to and drying. Furthermore, the microporous films of the obtained laminate are peeled to obtain at least two microporous films coated with an inorganic filler on one side.

  When the resin constituting the microporous film is made of only a polyethylene resin as exemplified in Patent Document 4, the width of the film becomes wide (specifically, 1000 mm or more) and / or the thickness of the film becomes thin ( Specifically, wrinkles are likely to occur during drying in the coating process.

  On the other hand, in the case of a microporous film containing a polypropylene resin, wrinkles tend not to occur. However, when a dispersion is applied to only one sheet, it is difficult to completely eliminate the wrinkles. On the other hand, when two microporous films containing a polypropylene resin are applied together as in this embodiment, wrinkles are generated even under the conditions of the wide and thin films as described above. A microporous film coated with an inorganic filler can be obtained.

  As the solvent, it is preferable to use a solvent in which the inorganic filler and the resin binder can be dissolved or dispersed uniformly and stably. Such a solvent is not particularly limited, and examples thereof include N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, and hexane. Also, in order to stabilize the inorganic filler-containing resin solution or to improve the coating property to the polyolefin resin porous membrane, the dispersion includes a dispersant such as a surfactant, a thickener, a wetting agent, an antifoaming agent. Various additives such as an agent, a pH adjusting agent including acid and alkali, and the like may be added. These additives are preferably those that can be removed upon solvent removal or plasticizer extraction, but are electrochemically stable in the range of use of the lithium ion secondary battery, do not inhibit the battery reaction, and are up to about 200 ° C. If stable, it may remain in the battery (in the multilayer porous membrane).

  The method for dissolving or dispersing the inorganic filler and the resin binder in the solvent is not particularly limited. For example, 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, and a disperser. And a mechanical stirring method using a homogenizer, a high-speed impact mill, ultrasonic dispersion, a stirring blade, and the like.

  The method for applying the dispersion on both surfaces of the laminate is not particularly limited as long as it can realize the required layer thickness and application area. Examples of such coating methods include 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 Examples include a coater method, a squeeze coater method, a cast coater method, a die coater method, a screen printing method, and a spray coating method.

  The solvent is preferably a solvent that can be removed from the dispersion applied to the laminate. The method for removing the solvent is not particularly limited as long as it does not adversely affect the microporous film. For example, the method of drying at a temperature below the melting point while fixing the laminate, or drying under reduced pressure at a low temperature. Examples thereof include a method, a method of immersing in a poor solvent for a resin binder to solidify the resin binder, and simultaneously extracting the solvent.

  From the viewpoint of improving heat resistance, the thickness of the coating layer formed by coating the inorganic filler dispersion is preferably 0.5 μm or more, more preferably 2 μm or more, and even more preferably 3 μm. It is above, Especially preferably, it is 4 micrometers or more. The upper limit of the layer thickness of the coating layer is preferably 100 μm or less, more preferably 50 μm or less, further preferably 30 μm or less, and particularly preferably 20 μm or less, from the viewpoint of permeability and battery capacity increase. Most preferably, it is 10 μm or less.

[Inorganic filler]
Although it does not specifically limit as an inorganic filler, For example, the thing which has melting | fusing point of 200 degreeC or more, is high in electrical insulation, and is electrochemically stable in the use range of a lithium ion secondary battery is preferable. Such inorganic fillers are not particularly limited, but include, for example, oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide; silicon nitride, titanium nitride, boron nitride, etc. Nitride ceramics: silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos And ceramics such as zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber. These can be used alone or in combination of two or more. Among these, alumina and titania are more preferable from the viewpoint of electrochemical stability.

  The average particle size of the inorganic filler is preferably 0.1 μm or more, more preferably 0.2 μm or more, and further preferably 0.3 μm or more. The average particle size of the inorganic filler is preferably 3.0 μm or less, more preferably 1.0 μm or less. When the average particle size is 0.1 μm or more, it is preferable from the viewpoint of reducing the thermal shrinkage rate of the multilayer porous film and making it difficult to break the film, and from the viewpoint of realizing a high short-circuit temperature. On the other hand, when the average particle size is 3.0 μm or less, it is preferable from the viewpoint of reducing the thermal shrinkage of the microporous film and making it difficult to break the membrane.

  The proportion of the inorganic filler in the coating layer (mass fraction) is preferably 50% or more, more preferably 55% or more, further preferably 60% or more, from the viewpoint of heat resistance. Particularly preferably, it is 65% or more. The proportion of the inorganic filler in the coating layer is preferably less than 100%, preferably 99.99% or less, more preferably 99.9% or less, and particularly preferably 99% or less. It is.

  On the other hand, the resin binder is not particularly limited. For example, an inorganic filler can be bound, it is insoluble in the electrolyte solution of the lithium ion secondary battery, and electrochemically within the range of use of the lithium ion secondary battery. It is preferable that it is stable.

  Such a resin binder is not particularly limited. For example, polyolefin such as polyethylene and polypropylene; fluorine-containing resin such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer Fluorine-containing rubber such as ethylene and tetrafluoroethylene copolymer; styrene-butadiene copolymer and hydride thereof; acrylonitrile-butadiene copolymer and hydride thereof; acrylonitrile-butadiene-styrene copolymer and hydride thereof; Methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, polyacetic acid Rubbers such as vinyl; polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, melting point and / or glass transition temperature of such polyesters are 180 ° C. or more resins. These can be used alone or in combination of two or more.

  The viscosity average molecular weight of the polyolefin used for the resin binder is preferably 1000 or more, more preferably 2000 or more, further preferably 5000 or more, and preferably 12 million as the upper limit from the viewpoint of moldability. Less than, more preferably less than 2 million, and even more preferably less than 1 million.

  When polyvinyl alcohol is used as the resin binder, the saponification degree is preferably 85% or more and 100% or less, more preferably 90% or more and 100% or less, and further preferably 95% or more and 100% or less. Particularly preferably, it is 99% or more and 100% or less. When the degree of saponification is 85% or more, the short-circuit temperature can be significantly improved, and variations in the short-circuit temperature can be suppressed, and good safety performance tends to be realized.

  The average degree of polymerization of polyvinyl alcohol (measurement method: JIS K-6726) is preferably 200 or more, more preferably 300 or more, and even more preferably 500 or more. The average degree of polymerization of polyvinyl alcohol is preferably 5000 or less, more preferably 4000 or less, and further preferably 3500 or less. When the average degree of polymerization is 200 or more, the inorganic filler tends to be firmly bound in a small amount, and the increase in the air permeability of the microporous film can be further suppressed while maintaining the mechanical strength of the coating layer. It is in. On the other hand, when the average degree of polymerization is 5000 or less, gelation or the like tends to be further prevented when preparing a dispersion with an inorganic filler. In addition, a commercial item can be used as said polyvinyl alcohol, and the catalog value of a commercial item can be used also as the average degree of polymerization.

  The content of the resin binder in the total amount of the inorganic filler and the resin binder is a volume fraction, and is preferably 0.5% by volume or more, more preferably 0.7% by volume or more, and further preferably 1. It is 0% by volume or more, particularly preferably 2% by volume or more, and most preferably 2.5% by volume or more. The content of the resin binder is preferably 8% by volume or less. It is preferable from the viewpoint of sufficiently binding the inorganic filler and making it difficult to cause peeling, missing, etc., from the viewpoint that the content of the resin binder is 0.5% by volume or more (from the viewpoint of sufficiently ensuring good handling properties). is there. On the other hand, when the content of the resin binder is 8% by volume or less, the ion permeability of the microporous film tends to be further improved.

[Microporous film]
Although it does not specifically limit as a microporous film of this embodiment, For example, the lamination | stacking microporous film which has a layer containing the layer containing a polypropylene resin or the layer containing a polypropylene resin is mentioned, for example. Among these, a laminated laminated microporous film having a layer containing a polypropylene resin as the outermost layer is preferable. As a specific example of the embodiment, a three-layer laminated microporous film having a layer containing a polyethylene resin as an intermediate layer and layers containing a polypropylene resin on both sides thereof can be mentioned.

  The film thickness excluding the coating layer of the microporous film is 16 μm or less, preferably 10 to 14 μm, more preferably 12 to 14 μm. When the film thickness of the microporous film is 16 μm or less, the permeability tends to be further improved. Moreover, it becomes difficult to generate | occur | produce a wrinkle because the film thickness of a microporous film is 10 micrometers or more. The film thickness can be measured by the method described in the examples.

  As a manufacturing method of the lamination | stacking microporous film of this embodiment, said method is mentioned, for example. Specifically, using a T-die or a circular die, a method of forming a laminated film in which each resin film is laminated by a coextrusion method, and then stretching the laminated film to make it porous; extruding each resin film separately The laminated film which laminated | stacked each resin film by the laminating method bonded after doing is formed, Then, the method of extending | stretching and making the laminated film porous is mentioned.

  Next, although an example and a comparative example are given and explained more concretely, the present invention is not limited to the following examples.

  In addition, about the raw material used in the Example and the comparative example, and the evaluation method of various characteristics are as follows. When two microporous films were obtained in each example and comparative example, the average value thereof was shown in Table 1 as an evaluation result.

(1) MFR of polypropylene resin
Based on JIS K7210, it is a value measured under conditions of a temperature of 210 ° C. and a load of 2.16 kg, and its unit is g / 10 minutes.

(2) MFR of polyethylene resin
Based on JIS K7210, it is a value measured under conditions of a temperature of 190 ° C. and a load of 2.16 kg, and its unit is g / 10 minutes.

(3) Pentad fraction (%)
The pentad fraction of the polypropylene resin was calculated by the peak height method from the 13C-NMR spectrum assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Analytical Chemical Society). The 13C-NMR spectrum was measured using ECS-400 manufactured by JEOL Ltd., and the microporous film was dissolved in o-dichlorobenzene-d under the conditions of a measurement temperature of 130 ° C. and a cumulative number of 10,000 times. It was.

(4) Propylene content (mol%), ethylene content (mol%)
The propylene content and the ethylene content of the polypropylene resin were calculated by the peak height method from the 13C-NMR spectrum assigned based on the description in the Polymer Analysis Handbook (edited by the Analytical Society of Japan). The 13C-NMR spectrum was measured using ECS-400 manufactured by JEOL Ltd., and the microporous film was dissolved in o-dichlorobenzene-d under the conditions of a measurement temperature of 130 ° C. and a cumulative number of 10,000 times. It was.

(5) Density Based on JIS K7112, it is a value measured under the condition of 25 ° C., and its unit is kg / m ³ .

(6) Molecular weight distribution (Mw / Mn)
The molecular weight distribution of the resin in the polyethylene resin is a value of the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). The GPC measurement was performed using a GPS device (trade name “HLC-8121GPC / HT”) manufactured by Tosoh Corporation. Using the trade name “TSKgel GMHHR-H (20)” (two) manufactured by Tosoh Corporation as the column, mobile phase o-dichlorobenzene (o-DCB), column temperature 155 ° C., flow rate 1.0 mL / min, sample concentration The measurement was performed under the conditions of 0.5 mg / mL (o-DCB), an injection amount of 500 μL, a sample dissolution temperature of 160 ° C., and a sample dissolution time of 3 hours. The molecular weight was calibrated with polystyrene, and Mw and Mn were obtained from the polystyrene-equivalent molecular weight to derive the molecular weight distribution.

(7) Film thickness (μm)
The film thickness of the microporous film was measured using a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(8) Air permeability (sec / 100cc)
The air permeability of the microporous film was measured with a Gurley type air permeability meter based on JIS P-8117.

(9) Wrinkles The wrinkle occurrence frequency during drying in the coating process was evaluated according to the following criteria.
○: No wrinkles occur.
Δ: Wrinkles occur intermittently.
X: Wrinkles always occur.

In addition, the used polypropylene resin and polyethylene resin are as follows.
Polypropylene (A): propylene homopolymer, MFR = 3, pentad fraction = 96%
Polypropylene (B): propylene homopolymer, MFR = 3, pentad fraction = 90%
Polypropylene (C): propylene copolymer, MFR = 3, pentad fraction = 98%, propylene content = 99.5 mol%, ethylene content = 0.5 mol%
Polypropylene (D): propylene copolymer, MFR = 3, pentad fraction = 98%, propylene content = 98 mol%, ethylene content = 2 mol%
Polypropylene (E): propylene homopolymer, MFR = 3, pentad fraction = 98%
Polyethylene (A): MFR = 0.5, density = 963 kg / m ³ , Mw / Mn = 15

[Example 1]
Polypropylene (A) having a diameter of 40 mm, L / D = 30 (L: distance from raw material supply port to discharge port of single screw extruder (m), D: inner diameter of single screw extruder (m), and so on) Then, it was charged into a single screw extruder having a cylinder temperature of 200 ° C. via a feeder, and polyethylene (A) was charged into a single screw extruder having a diameter of 30 mm, L / D = 30 and a cylinder temperature of 200 ° C. via a feeder. A film having polypropylene (A) as a surface layer and polyethylene (A) as an intermediate layer was extruded from a two-type three-layer coextrusion T die having a width of 1500 mm and a lip thickness of 3.0 mm installed at the tip of the single screw extruder. . The ratio of the extrusion weight of the surface layer to the intermediate layer was 2: 1, and the T die temperature (also referred to as “film formation temperature”) was 190 ° C. Immediately apply cold air of 25 ° C to the molten resin after extrusion, and then wind it on a core with a cast roll cooled to 95 ° C at a draw ratio of 200 and a winding speed of 15 m / min. Obtained (film formation step).

  Prepare two laminated raw films wound up on the core, and pass them through a furnace heated to 128 ° C. for 30 minutes in a state where the two sheets are overlapped, and then cool to 25 ° C. It was led to a roll and wound up to obtain a laminate (heat treatment step).

  The obtained laminate was uniaxially stretched 1.3 times in the MD direction at a temperature of 25 ° C., and then uniaxially stretched 2.5 times in the MD direction at a temperature of 110 ° C. (stretching step). It was relaxed by a factor of 0.9 at a temperature of 130 ° C. and heat-set (heat-setting step) to obtain a laminate in which two microporous films were bonded together.

  An aqueous solution in which 95 parts by mass of alumina particles (average particle size 0.7 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by mass of water, respectively. Was prepared. The obtained aqueous solution was applied to both surfaces of the laminate by dip coating, and then dried at 60 ° C. to remove water, so that an inorganic layer having a thickness of 4 μm (a binder volume fraction of 3.4%) was formed on both surfaces. The laminated body formed was obtained. The microporous film of the obtained laminate was peeled off to obtain two microporous films having a width of 1000 mm each having an inorganic layer formed on one side (coating process). The film thickness before and after the coating process and the air permeability after the coating process were measured. The results are shown in Table 1.

[Example 2]
Polypropylene (A) was introduced into a single screw extruder having a diameter of 40 mm, L / D = 30, and a cylinder temperature of 200 ° C. via a feeder, and extruded from a single layer T die having a width of 1500 mm and a lip thickness of 3.0 mm. The T die temperature was 190 ° C. The extruded molten resin is immediately applied with cold air at 25 ° C., and then wound on a core with a cast roll cooled to 95 ° C. under a draw ratio of 200 and a winding speed of 15 m / min. (Film formation step). After the heat treatment step, the same operation as in Example 1 was performed to obtain two microporous films having a width of 1000 mm with an inorganic layer formed on one side. The evaluation results are shown in Table 1.

[Examples 3 to 5]
Two microporous films having a width of 1000 mm each having an inorganic layer formed on one surface were obtained in the same manner as in Example 1 except that the polypropylene (A) used in the film forming step was changed to the polypropylene shown in Table 1. The evaluation results are shown in Table 1.

Example 6
Two laminated microporous films having a width of 1000 mm each having an inorganic layer formed on one surface were obtained in the same manner as in Example 1 except that the lip thickness in the film forming step was changed to 4 mm. The evaluation results are shown in Table 1.

[Comparative Example 1]
After obtaining a laminated raw film in the film forming step in the same manner as in Example 1, a heat treatment step, a stretching step, and a heat setting step were carried out with only one sheet to obtain a laminated microporous film having a width of 1000 mm.

  An aqueous solution in which 95 parts by mass of alumina particles (average particle size 0.7 μm) and 5 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) are uniformly dispersed in 150 parts by mass of water, respectively. Was prepared. The obtained aqueous solution was applied to one side of the laminated microporous film using a gravure coater, dried at 60 ° C. to remove water, and a 4 μm thick inorganic layer (binder volume fraction 3.4). %) Was obtained on one side (a coating process). When drying the coating process, wrinkles were always generated, resulting in poor quality.

[Comparative Example 2]
A laminated microporous film having a width of 1000 mm in which an inorganic layer was formed on one side was obtained in the same manner as in Comparative Example 1 except that the lip thickness in the film forming step was changed to 4 mm. When drying the coating process, wrinkles were intermittently generated, resulting in poor quality.

[Comparative Example 3]
After obtaining a single-layer original film in the film forming step in the same manner as in Example 2, a heat treatment step, a stretching step, and a heat setting step were carried out with only one sheet to obtain a microporous film having a width of 1000 mm. After the heat treatment step, similarly to Comparative Example 1, a microporous film having a width of 1000 mm having an inorganic layer formed on one surface was obtained. When drying the coating process, wrinkles were always generated, resulting in poor quality.

[Comparative Example 4]
A laminated raw film was obtained in the same manner as in Example 1 except that the polypropylene (A) used in the film forming step was changed to polypropylene (E). The heat treatment step, the stretching step, and the heat setting step were carried out in the same manner as in Example 1, but the pressure-bonding property of the film was insufficient, and it was not possible to obtain a laminate in which two laminated microporous films were bonded together It was.

[Comparative Example 5]
The film formation process, the heat treatment process, the stretching process, and the heat setting process were performed in the same manner as in Comparative Example 1 to obtain a laminated microporous film having a width of 1000 mm. With reference to Patent Document 4, two laminated microporous films were bonded together using a pressure roll pressurized at 1.5 kg / cm under a temperature condition of 130 ° C. to obtain a laminate. Similar to the coating process of Example 1, a laminated microporous film having a width of 1000 mm having an inorganic layer formed on one surface was obtained. As shown in Table 1, the air permeability was inferior.

  The microporous film of the present invention has industrial applicability as a battery separator, more specifically as a lithium secondary battery separator.

Claims (2)

A single resin comprising at least one polypropylene resin selected from the group consisting of a polypropylene resin (A) having a pentad fraction of 90 to 97% and a polypropylene resin (B) having a propylene content of 98 to 99.5 mol%. A film forming step of obtaining a layer original film, or a laminated original film having an outermost layer including one or more kinds of polypropylene resins selected from the group consisting of the polypropylene resin (A) and the polypropylene resin (B); ,
Two or more layers of the single layer original film or the laminated original film obtained in the film formation step are stacked and held at 90 ° C. to 160 ° C., and at least 1.05 times to 5.0 times in one direction. Stretching step to obtain a porous laminate,
After coating a dispersion of an inorganic filler on both surfaces of the laminate obtained in the stretching step to form a coating layer, the layers stacked in the laminate are peeled off to form the coating layer A coating process for obtaining at least two microporous films having:
In this order, a method for producing a microporous film.   The manufacturing method of the microporous film of Claim 1 whose film thickness of the said microporous film obtained at the said peeling process is 10 micrometers-14 micrometers.