JP2011162773A - Porous film and electricity storage device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film which has preferable working handleability and exhibits superior characteristics when being used as a separator. <P>SOLUTION: The porous film has an area proportion of fibril of 8 to 50% on at least one surface thereof and has a Gurley air permeability of 10 to 400 s/100 mL. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a porous film having excellent slipperiness which is important for process handling in addition to high air permeability. Specifically, by increasing the ratio of the fibril part on the surface of the porous film, it is suitable for the separator application of the electricity storage device by eliminating the deterioration of the slipperiness which becomes a problem with the porous film and having high air permeability. It is related with the porous film which can be used for.

  Porous films include various separators such as batteries and electrolytic capacitors, various separation membranes (filters), absorbent articles represented by diapers and sanitary products, moisture-permeable waterproof members for clothing and medical use, members for heat-sensitive paper, inks The uses of the receptor member and the like are diverse, and polyolefin-based porous films represented by polypropylene and polyethylene are mainly used. Porous polyolefin films are particularly used as power storage device separators because of their high permeability and high porosity.

  An electricity storage device is one of the extremely important electrical devices that support today's ubiquitous society because it can take out electrical energy whenever and wherever it is needed. On the other hand, with the widespread use of portable devices such as video cameras, personal computers, mobile phones, portable music players, and portable game machines, the need for high capacity, small size and light weight for power storage devices (especially secondary batteries) is increasing year by year. . Among them, lithium ion batteries have a high energy density per unit volume and mass as compared with other power storage devices, and have a high output density. Therefore, demand for lithium ion batteries is greatly increasing as power storage devices that satisfy the above needs.

Furthermore, in recent years, global warming, air pollution, oil depletion, CO ₂ emission regulations, and the like have become problems, and the environmental burden of automobiles is becoming a major problem. Therefore, electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles can be one of the solutions such as environmental measures (cleaning), energy saving measures (improvement of fuel consumption), and next-generation fuel measures (new energy development). Development and commercialization of (FCV) and the like are being actively studied. As these main power supply and auxiliary power supply, for example, lithium ion batteries, electric double layer capacitors, and the like are attracting attention, and application studies are being rapidly promoted.

  Various proposals have been made as a method for making a polyolefin film porous. Porous methods can be broadly classified into wet methods and dry methods. As a wet method, polyolefin is used as the matrix resin, the extractables to be extracted after sheeting are added and mixed, and only the additives are extracted using the good solvent of the extractables, creating voids in the matrix resin. Proposal has been made for a method of squeezing (for example, see Patent Document 1). On the other hand, as a dry method, for example, by adopting low-temperature extrusion and a high draft ratio at the time of melt extrusion, the lamella structure in the film before stretching formed into a sheet is controlled, and this is uniaxially stretched so that at the lamella interface There has been proposed a method (so-called lamellar stretching method) in which cleavage occurs to form voids (see, for example, Patent Document 2). Further, as a dry method, a large amount of incompatible resin is added as inorganic particles or polypropylene, which is a matrix resin, and a sheet is formed and stretched to cause cleavage at the interface between the particles and the polypropylene resin. A forming method has also been proposed (see, for example, Patent Document 3). Furthermore, there is a so-called β crystal method in which voids are formed in the film by utilizing the crystal density difference and crystal transition between α type crystal (α crystal) and β type crystal (β crystal), which are polymorphs of polypropylene. Many proposals of the called method are also made (for example, refer patent documents 4-6).

  When the porous film manufactured by the method as described above is used as an electrolytic device, particularly as a separator for a lithium ion battery, the required characteristics include processing handleability. The film subjected to the porous treatment is considered to have a lower stiffness and a higher coefficient of friction, which has a problem of impeding handling in the processing step.

  In a porous film, Patent Document 7 discloses a polypropylene resin, a β crystal nucleating agent, and polypropylene as a proposal for achieving both improvement in workability due to high friction coefficient and low slipperiness and high air permeability. A method has been proposed in which a resin composition made of a resin that is incompatible with the resin is formed into a sheet and made porous by biaxial orientation. However, if the mixing amount of the incompatible resin with the polypropylene resin is small, the slipping property is insufficient, and if the mixing amount is large, the air permeability is insufficient. Therefore, the good slipping property and air permeability achieved by the present invention are achieved. It was not possible to achieve both.

  Patent Document 8 proposes a method of coating particles on the film surface. However, in the method of coating particles on the film surface, when particles are dropped from the inside during handling of the film, or particles are released in the electrolyte during use as a separator and move onto the electrode. As a result, there is a possibility of insulation, which is insufficient as a countermeasure.

  In Patent Document 9, a porous film having good slipperiness obtained by forming a resin composition comprising a polyolefin resin containing a modified polyolefin resin and a polysiloxane gum into a film shape and making it porous by an extraction method is disclosed. Disclosure. However, the porous film thus obtained has poor air permeability, and when used as a separator for a lithium ion battery, the internal resistance of the battery is high and a high output density cannot be obtained.

Japanese Patent Laid-Open No. 55-131028 Japanese Patent Publication No.55-32531 JP-A-57-203520 JP-A 63-199742 JP-A-6-100720 Japanese Patent Laid-Open No. 9-255804 JP 2005-171230 A JP 2009-19118 A JP 2000-7819 A

  An object of the present invention is to solve the above-described problems. In other words, by providing the area ratio of the fibrils on the surface of the porous polyolefin film within a specific range and increasing the air permeability, it is possible to provide a separator imparted with slipperiness in addition to good battery characteristics. An object of the present invention is to provide a porous film that can be suitably used for a lithium ion secondary battery.

  The above-described problems can be achieved by a porous film having an area ratio of fibrils of at least one side of 8 to 50% and a Gurley air permeability of 10 to 400 seconds / 100 ml.

  The porous film of the present invention is excellent in air permeability and processability suitable for a lithium ion secondary battery separator, and can be suitably used as a separator.

The porous film according to the present invention is ion-coated using an IB-5 type ion coater manufactured by Eiko Engineering Co., Ltd., and the surface of the film is 1000 times using a field emission scanning electron microscope (JSM-6700F) manufactured by JEOL Ltd. It is an observed photograph substitute drawing. About the observation result of FIG. 1, Image-ProPlus Ver. FIG. 4 is a schematic diagram in which the image analysis described in the example is performed using 4.5 and only the fibril portion is displayed in black (scale bar is added after image analysis).

  The porous film of the present invention has a plurality of through holes that penetrate both surfaces of the film and have air permeability. As a method for forming the through-hole in the film, either a wet method or a dry method may be used, but a dry method is desirable because the process can be simplified.

In the porous film of the present invention, the area ratio of fibrils on at least one side is preferably 8 to 50%, more preferably 10 to 35%, and further preferably 12 to 25%. When the ratio of fibrils on the surface of the porous film is less than 8%, the stiffness of the porous film is lowered, and the slipperiness may be deteriorated. On the other hand, if it exceeds 50%, the air permeability deteriorates, and when used as a battery separator, good battery characteristics may not be obtained. Also, the frictional resistance tends to increase. The fibril refers to a portion of the resin that has been divided into fibers when the resin composition is made into a sheet and then made porous by a wet method or a dry method. In the present invention, the area measured by the method described later. Means 0.5 μm ² or more. The part where the area is less than 0.5 μm ² is not a fibril. In addition, the void portion and the uncut portion are not part of the splitting in the first place. In the present invention, it is important that a large fibrillation portion (fibril) having an area of 0.5 μm ² or more exists on the surface of the porous film. When the splitting portion on the surface of the porous film is only thin or small with a thickness of less than 0.5 μm ^{2, the} air permeability is good, but the stiffness of the film is insufficient and the slipperiness may be deteriorated. When the surface of the porous film has many uncleaved portions, the slipperiness is improved, but sufficient air permeability may not be obtained.

  FIG. 1 is an SEM image of the surface of the porous film where fibrils are present, and the fibril area ratio is the ratio of the fibril area to the measurement area. FIG. 2 is an image obtained by analyzing the image of FIG. 1 by a predetermined method described later and displaying only the fibrils in black.

  Since the porous film of the present invention is used for a battery separator or the like, it preferably has high air permeability. The Gurley air permeability is preferably in the range of 10 to 400 seconds / 100 ml from the viewpoint of reducing the internal resistance of the battery and further improving the output density, and more preferably in the range of 50 to 200 seconds / 100 ml. . If the Gurley air permeability is less than 10 seconds / 100 ml, the strength of the film is lowered, and metallic lithium deposited on the negative electrode in the lithium ion secondary battery may penetrate through the porous film and cause a short circuit. is there. On the other hand, when the Gurley air permeability exceeds 400 seconds / 100 ml, the internal resistance of the battery is high due to poor air permeability, and a high output density may not be obtained.

  As a method for adjusting the Gurley air permeability to the above-described preferable range, 1 to 10% by mass of an ethylene / α-olefin copolymer is added to the polypropylene resin from the viewpoint of generating a large amount of fibrils by assisting in the splitting (separation). It is preferable to do. Here, examples of the ethylene / α-olefin copolymer include linear low density polyethylene and ultra-low density polyethylene. Among them, ethylene / octene-1 copolymer obtained by copolymerization of octene-1 is preferable. Can be used. As this ethylene-octene-1 copolymer, a commercially available resin can be used. Furthermore, as will be described later, as the stretching temperature in the longitudinal direction, it is preferable to employ a temperature of 90 to 140 ° C, more preferably 100 to 130 ° C. As the stretching temperature in the width direction, it is preferable to employ a temperature of 140 to 155 ° C. The transverse stretching speed at this time is preferably 100 to 3,000% / minute, more preferably 100 to 2,000% / minute.

  As a method of controlling the area ratio of the fibrils on the film surface and the Gurley permeability, controlling the temperature when heat-fixing the film (heat treatment after transverse stretching), and in addition, the film after longitudinal stretching This is possible by starting the transverse stretching while maintaining the temperature without cooling. As the heat setting temperature, it is preferable to employ a temperature of 140 to 160 ° C. from the viewpoint of growing the fibrils in the film thickly. When the heat setting temperature is less than 140 ° C., the fibril growth is insufficient and the slipperiness may be deteriorated. Moreover, when it exceeds 160 degreeC, a hole may close by heat shrink of a porous film, and air permeability may deteriorate.

  As a method of starting transverse stretching in a state where the temperature is maintained without cooling the film after longitudinal stretching, a method using a rotating roll controlled to a predetermined temperature, a method using a hot air oven, or the like can be adopted. From the viewpoint of growing the fibrils inside thickly, it is preferable to employ a temperature of preferably 90 to 155 ° C, more preferably 90 to 140 ° C, and still more preferably 100 to 130 ° C. If the temperature maintained after longitudinal stretching is less than 90 ° C., the film is cooled and fibrils do not grow, and the slipperiness of the resulting porous film may deteriorate. Moreover, when it exceeds 130 degreeC, a fibril grows excessively and the air permeability of the porous film obtained may deteriorate, and when it exceeds 155 degreeC, this tendency becomes remarkable. When the heat setting temperature is increased, the porous film is increased in strength and improved in slipperiness, but the air permeability is deteriorated. Conversely, when the heat solid temperature is lowered, the air permeability is improved. The slipperiness, strength, and heat shrinkage rate will deteriorate. In order to control the area ratio and air permeability of the fibrils on the surface of the porous film to a preferable range, it is possible to grow the fibrils in the film after longitudinal stretching thickly by maintaining the film after longitudinal stretching at a high temperature. The fibrils are cleaved by further stretching the uniaxially stretched uniaxially stretched film with the thickly grown fibrils, but since the fibrils before the cleavage are thick, the fibrils after the cleavage are sufficiently thick and slippery. And air permeability can be achieved.

  The porous film of the present invention preferably contains a polyolefin resin from the viewpoint of porosity. Examples of the monomer component constituting the polyolefin-based resin include ethylene, propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methyl-1-butene, 1-hexene, 4-methyl- 1-pentene, 5-ethyl-1-hexene, 1-heptene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, vinylcyclohexene Styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene, at least two copolymers selected from these homopolymers and the above monomer components, and these homopolymers, Examples include, but are not limited to, blends of copolymers. In addition to the above monomer components, for example, vinyl alcohol and maleic anhydride may be copolymerized or graft polymerized, but are not limited thereto. Among these, polypropylene is preferable from the viewpoints of heat resistance, air permeability, porosity, and the like.

  As a dry method for forming through-holes in the film, it is preferable to use the β crystal method from the viewpoint that the film is biaxially oriented and uniform physical properties and high strength can be maintained while being a thin film.

  In order to make the polyolefin film porous by the β crystal method, the β crystal forming ability of the polyolefin resin constituting the film is preferably 30 to 100%. If the β-crystal forming ability is less than 30%, the amount of β-crystal is small at the time of film production, so the number of voids formed in the film is reduced by utilizing the transition to α-crystal, and as a result, the film with low permeability There are cases where it can only be obtained. In order to make the β crystal forming ability in the range of 30 to 100%, it is preferable to add the above β crystal nucleating agent as well as using a polypropylene resin having a high isotactic index. The β-crystal forming ability is more preferably 35 to 80%, and particularly preferably 40 to 70%.

  In order to make the β crystal forming ability within the above-mentioned preferable range, it is important to form a large amount of β crystals in the polyolefin resin. For this purpose, it is necessary to add the β crystal nucleating agent to the polyolefin resin. It is preferable to use, as an additive, a crystallization nucleating agent that selectively forms β crystals. Examples of the β crystal nucleating agent include various pigment compounds and amide compounds. In particular, amide compounds disclosed in JP-A-5-310665 can be preferably used. As content of (beta) crystal nucleating agent, when the whole polyolefin resin is 100 mass parts, it is preferable that it is 0.05-0.5 mass part, and more preferable if it is 0.1-0.3 mass part. .

  The polyolefin resin contained in the porous film is preferably an isotactic polypropylene resin having a melt flow rate (hereinafter referred to as MFR, measurement conditions are 230 ° C., 2.16 kg) in the range of 2 to 30 g / 10 min. . If the MFR is out of the above preferred range, it may be difficult to obtain a biaxially stretched film. More preferably, the MFR is 3 to 20 g / 10 minutes.

  The isotactic index of the isotactic polypropylene resin is preferably 90 to 99.9%. If the isotactic index is less than 90%, the crystallinity of the resin is low, and it may be difficult to achieve high air permeability. As the isotactic polypropylene resin, a commercially available resin can be used.

  Moreover, it is preferable that said polypropylene resin contains a high melt tension polypropylene in 0.5-5 mass% at the point of film forming property improvement. High melt tension polypropylene is a polypropylene resin that has increased tension in the molten state by mixing high molecular weight components or components having a branched structure into the polypropylene resin or by copolymerizing a long-chain branched component with the polypropylene. However, among these, it is preferable to use a polypropylene resin copolymerized with a long-chain branching component. This high melt tension polypropylene is commercially available. For example, polypropylene resins PF814, PF633, and PF611 manufactured by Basell, polypropylene resin WB130HMS manufactured by Borealis, and polypropylene resins D114 and D206 manufactured by Dow can be used.

  In the porous film of the present invention, an antioxidant, a heat stabilizer, an antistatic agent, a lubricant composed of inorganic or organic particles, and further an antiblocking agent and filler, as long as the effects of the present invention are not impaired. Various additives such as an incompatible polymer may be contained. In particular, it is preferable to contain 0.01 to 0.5 parts by mass of an antioxidant with respect to 100 parts by mass of the polypropylene resin for the purpose of suppressing oxidative deterioration due to the thermal history of the polypropylene resin.

  The porous film of the present invention has a layer containing an ethylene / α-olefin copolymer, and when this layer is used as the first layer, the amount of the first layer is less than that of the first layer on at least one side of the first layer. Laminating the second layer containing the ethylene / α-olefin copolymer or the third layer not containing the ethylene / α-olefin copolymer is a physical property balance between the fibril area ratio and the Gurley permeability. It is preferable from the viewpoint that a good film can be obtained. Having the first layer containing the ethylene / α-olefin copolymer is preferable from the viewpoint of forming voids. Further, a second layer having a content of ethylene / α-olefin copolymer less than that of the first layer on at least one surface of the first layer or a third layer not containing ethylene / α-olefin copolymer. Is preferable from the viewpoint of suppressing the formation of voids and increasing the area ratio of fibrils. That is, by appropriately providing the first to third layers and controlling the ethylene / α-olefin content and the lamination ratio, the balance of physical properties such as the Gurley air permeability can be well controlled within a desired range.

  The content of the ethylene / α-olefin copolymer in the first layer is preferably 1 to 10% by mass. The content of the ethylene / α-olefin copolymer in the second layer is preferably 0.01 to 9% by mass, and more preferably 0.01 to 6% by mass from the viewpoint of slipperiness. If the content of the ethylene / α-olefin copolymer in the second layer exceeds 9% by mass, voids may be formed excessively and slipperiness may deteriorate. As the laminated structure, as long as the second layer or the third layer is laminated on at least one surface of the first layer, it may be a two-layer laminated structure, a three-layer laminated structure, or a larger number of laminated layers. As a lamination method, either a feed block method or a multi-manifold method may be used. As the lamination thickness ratio, the sum of the thicknesses of the first layers is preferably 0.5 to 30 when the sum of the thicknesses of the second layer or the third layer is 1, from the viewpoint of air permeability. 2 to 20 is more preferable. If the sum of the thicknesses of the first layer and the second layer or the third layer is less than 0.5, the air permeability may be deteriorated. On the other hand, when it exceeds 30, there are few thick fibrils and the stiffness of the film is lowered, and the slipperiness may be deteriorated.

  In the case of a single film of only the first layer without laminating the second layer or the third layer, there is a possibility that the area ratio of fibrils that can provide the required workability even if the air permeability is good cannot be achieved. . Further, in the case of a single film of only the second layer or the third layer, even if the fibril area ratio can be achieved, the air permeability may be excessively deteriorated. Therefore, the second layer has a first layer containing an ethylene / α-olefin copolymer, and contains an ethylene / α-olefin copolymer in an amount smaller than that of the first layer on at least one side of the first layer. It is easy to control the area ratio of fibrils and the Gurley air permeability within a favorable range by laminating the above layer or the third layer containing no ethylene / α-olefin copolymer.

  The porosity of the porous film of the present invention is preferably 50 to 90% from the viewpoint of reducing the internal resistance of the battery, further improving the output density, and from the viewpoint of film strength. More preferred. When the porosity is less than 50%, the internal resistance increases when used as a separator, and excellent battery characteristics may not be obtained. On the other hand, when the porosity exceeds 90%, the strength of the film decreases, and pinholes are easily generated, causing a short circuit, or torn when wound to be stored inside the battery. The handleability may be inferior, for example.

  Below, the manufacturing method of the porous film of this invention is demonstrated concretely. In addition, the manufacturing method of the film of this invention is not limited to this.

  As a polypropylene resin, 96 parts by mass of a commercially available homopolypropylene resin having an MFR of 8 g / 10 minutes, 1 part by mass of a commercially available MFR of 2.5 g / 10 minutes, a high melt tension polypropylene resin, and an ultra low density polyethylene resin having a melt index of 18 g / 10 minutes 3 parts by mass of N, N′-dicyclohexyl-2,6-naphthalene dicarboxyamide 0.2 part by mass was mixed in advance using a twin-screw extruder at a predetermined ratio (A of the laminated film). Layer material) is prepared. At this time, the melting temperature is preferably 270 to 300 ° C. Similarly, 0.2 parts by mass of N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide is added to 98 parts by mass of the above-mentioned homopolypropylene resin, 1 part by mass of high melt tension polypropylene resin, and 1 part by mass of ultra-low density polyethylene resin. The raw material (the raw material for B layer of a laminated | multilayer film) previously mixed by the predetermined ratio using a twin-screw extruder is prepared.

  Next, these mixed raw materials are supplied to separate single-screw melt extruders, and melt extrusion is performed at 200 to 230 ° C. And after removing foreign substances and modified polymer with a filter installed in the middle of the polymer tube, laminating by feed block method or multi-manifold method, discharging onto the cast drum from T-die, and unstretched sheet obtain. At this time, the surface temperature of the cast drum is preferably 105 to 130 ° C. from the viewpoint of controlling the β crystal fraction of the cast film to be high. At this time, particularly, the forming of the end portion of the sheet affects the subsequent stretchability, and therefore it is preferable that the end portion is sprayed with spot air to be in close contact with the drum. Further, air may be blown over the entire surface using an air knife as necessary from the state in which the entire sheet is in close contact with the drum.

  Next, the obtained unstretched sheet is biaxially oriented to form pores in the film. As a biaxial orientation method, the film is stretched in the longitudinal direction of the film and then stretched in the width direction, or the sequential biaxial stretching method in which the film is stretched in the width direction and then stretched in the longitudinal direction. The simultaneous biaxial stretching method can be used, but it is preferable to adopt the sequential biaxial stretching method in that it is easy to obtain a highly air permeable film, and in particular, stretching in the longitudinal direction and then stretching in the width direction. Is preferred.

  As specific stretching conditions, first, the temperature is controlled so that the unstretched sheet is stretched in the longitudinal direction. As a temperature control method, a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like can be adopted. The stretching temperature in the longitudinal direction is preferably 90 to 140 ° C, more preferably 100 to 130 ° C. As a draw ratio, it is 3-6 times, More preferably, it is 3-5 times. When the film is stretched in the longitudinal direction, a so-called neck-down phenomenon is observed in which the film width decreases. To achieve high air permeability, the neck-down rate (film width after stretching / before stretching) The film width is preferably 30 to 70%. Considering stretching in the width direction, 40 to 65% is more preferable. Next, the film end is held and introduced into a stenter type stretching machine without cooling the film through a hot air oven controlled at 90 to 155 ° C., preferably 90 to 140 ° C. The time from the end of longitudinal stretching (stretching in the longitudinal direction) to the start of lateral stretching (stretching in the width direction) is preferably 5 seconds to 60 seconds, and more preferably 8 seconds to 20 seconds. And it heats to 140-155 degreeC preferably, and extends 5 to 12 times in the width direction, More preferably, it extends 6 to 10 times. The transverse stretching speed at this time is preferably 100 to 3,000% / minute, more preferably 100 to 2,000% / minute. Next, heat setting is performed in the stenter as it is, but the temperature is preferably 140 to 160 ° C. Further, the heat setting may be performed while relaxing in the longitudinal direction and / or the width direction of the film, and in particular, the relaxation rate in the width direction is preferably 7 to 12% from the viewpoint of thermal dimensional stability.

  The porous film of the present invention is not only excellent in air permeability but also excellent in slipperiness, so that it can be suitably used particularly as a separator for an electricity storage device.

  Here, examples of the electricity storage device include a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery, and an electric double layer capacitor such as a lithium ion capacitor. Since such an electricity storage device can be repeatedly used by charging and discharging, it can be used as a power supply device for industrial devices, household equipment, electric vehicles, hybrid vehicles, and the like. When the porous film obtained by the present invention is used as a separator, not only the output density can be improved, but also the productivity of the battery can be increased.

  Hereinafter, the present invention will be described in detail by way of examples. The characteristics were measured and evaluated by the following methods.

(1) Area ratio of fibrils A porous film is ion-coated using an IB-5 type ion coater manufactured by Eiko Engineering, and the film surface is photographed using a field emission scanning electron microscope (JSM-6700F) manufactured by JEOL Ltd. Observation was performed at a magnification of 1000 times. The obtained image data (the image of the observation part only without the display of the scale bar or the like) was transferred to Image-ProPlus Ver. Image analysis was performed using 4.5, and the area ratio of the fibril part was calculated. As an image analysis method, first, a flattening filter (dark, 10 pixels) was executed once to correct luminance spots, and then a median filter (kernel size 3 × 3) was executed once to remove noise. Next, a local equalization filter (logarithmic distribution, small window 100, step 1) was executed once to highlight the fibrils brightly. Further, the black and white of the image was inverted to display the fibril portion in black, and contrast adjustment (contrast 100) was performed. Finally, spatial calibration is performed, and in the count / size item, the lower limit value of the area detection is set to 0.5 μm ² and the count is performed, so that only fibrils of 0.5 μm ² or less and black spots (noise) are removed. Detected. The area ratio of the fibrils was calculated by calculating the area ratio with respect to the total area of the detected fibrils by measuring the area ratio of the count / size item. Measurements were made at 10 points on both sides of the same porous film, and the value of the surface with the higher average value was taken as the fibril area ratio of the sample.

(2) β-crystal forming ability A resin or 5 mg of a film was sampled in an aluminum pan and measured using a differential scanning calorimeter (Seiko Denshi Kogyo RDC220). First, the temperature is raised from room temperature to 240 ° C. at 10 ° C./min (first run) in a nitrogen atmosphere, held for 10 minutes, and then cooled to 30 ° C. at 10 ° C./min. After holding for 5 minutes, the melting peak observed when the temperature is raised again at 10 ° C./min (second run) is the melting peak of the β crystal at 145 ° C. to 157 ° C., 158 ° C. The melting at which the peak is observed is defined as the melting peak of the α crystal, and the melting heat amount of the α crystal is obtained from the baseline and the area of the region surrounded by the peak drawn from the flat portion on the high temperature side. Is the ΔHα, and the heat of fusion of the β crystal is ΔHβ, the value calculated by the following formula is the β crystal forming ability. The heat of fusion was calibrated using indium.

β crystal forming ability (%) = [ΔHβ / (ΔHα + ΔHβ)] × 100
In addition, the β crystal fraction in the state of the sample can be calculated by calculating the abundance ratio of the β crystal in the same manner from the melting peak observed in the first run.

(3) Melt flow rate (MFR)
The MFR of polypropylene and thermoplastic elastomer is measured according to the condition M (230 ° C., 2.16 kg) of JIS K 7210 (1995). The polyethylene resin is measured in accordance with the condition D (190 ° C., 2.16 kg) of JIS K 7210 (1995).

(4) Gurley air permeability A square with a side length of 100 mm was cut out from the film and used as a sample. The permeation time of 100 ml of air was measured three times at 23 ° C. and a relative humidity of 65% using a JIS P 8117 (1998) B-shaped Gurley tester. The average value of the permeation time was defined as the Gurley air permeability of the film.

(5) Porosity The film was cut into a size of 30 mm × 40 mm and used as a sample. Using an electronic hydrometer (SD-120L manufactured by Mirage Trading Co., Ltd.), the specific gravity was measured in an atmosphere at a room temperature of 23 ° C. and a relative humidity of 65%. The measurement was performed three times, and the average value was defined as the specific gravity ρ of the film.

  Next, the measured film was hot-pressed at 280 ° C. and 5 MPa, and then rapidly cooled with water at 25 ° C. to prepare a sheet from which pores were completely erased. The specific gravity of this sheet was measured in the same manner as described above, and the average value was defined as the specific gravity (d) of the resin. In the examples described later, the specific gravity d of the resin was 0.91 in any case. From the specific gravity of the film and the specific gravity of the resin, the porosity was calculated by the following formula.

Porosity (%) = [(d−ρ) / d] × 100
(6) Film thickness Film using a dial gauge thickness gauge (JIS B-7503 (1997), UPAIGHT DIAL GAUGE (0.001 × 2 mm), No. 25, measuring element 5 mmφ flat type, 125 gf load) manufactured by PEACOCK Ten points were measured at 10 mm intervals in the vertical direction and the horizontal direction, and the average value thereof was taken as the film thickness of the sample.

(7) Lamination Thickness Ratio A cross section in the width direction of the porous film was cut out and observed with a field emission scanning electron microscope (JSM-6700F) manufactured by JEOL Ltd. at a magnification of 5000 times, and the thickness of each layer was measured. The thickness of each layer was determined by measuring the distance from the interface of each layer to the interface. Measurements were made at five arbitrary positions in the film width direction, and the average value was taken as the thickness of each layer of the sample. The lamination thickness ratio was calculated from the thickness of each layer.

(8) Workability A slit film obtained by slitting the film into a tape shape having a width of 12.7 mm was run on a stainless steel guide pin (surface roughness: Ra is 100 nm) using a tape running tester (running speed of 25 m). / Min, winding angle 60 °, exit side tension 90 g, number of running times 1). At this time, whether or not wrinkles were generated on the film was visually observed for 100 m. The measurement was performed 5 times, and the determination was made according to the following criteria based on the number of wrinkles generated.

○: 0 pieces Δ: 1-2 pieces ×: 3 pieces or more (9) Evaluation as a separator Preparation of Electrolytic Solution After dissolving LiC ₄ F ₉ SO ₃ in trimethyl phosphate, propylene carbonate was added and mixed, and LiC ₄ F ₉ was added to a mixed solvent having a volume ratio of propylene carbonate and trimethyl phosphate of 1: 2. An organic electrolyte solution in which SO ₃ was dissolved at 0.6 mol / liter was prepared.

B. Production of Battery A lithium cobaltate positive electrode manufactured by Hosen Co., Ltd. having a thickness of 40 μm was cut into a width of 200 mm and a length of 4,000 mm. A spherical graphite negative electrode made by Hosen Co., Ltd. having a thickness of 50 μm was cut into a width of 200 mm and a length of 4,000 mm. Furthermore, the porous film of each Example and Comparative Example was slit to a width of 220 mm and cut to a length of 4,050 mm.

  Next, the above-mentioned belt-like positive electrode was overlapped with the above-mentioned sheet-like negative electrode through the separator film of each Example / Comparative Example, and wound into a spiral shape to obtain a spiral electrode body. The prepared electrode body was filled in a bottomed cylindrical battery case, and the lead body of the positive electrode and the negative electrode was welded. Then, the electrolyte solution was injected into the battery case. The opening of the battery case was sealed and the battery was precharged to produce a cylindrical organic electrolyte secondary battery.

C. Battery characteristics For each secondary battery produced, charging and discharging operations were performed under an atmosphere of 25 ° C., with charging being performed at 1,600 mA up to 4.2 V for 3.5 hours, and discharging at 1,600 mA up to 2.7 V. The capacity was examined. Further, a charging / discharging operation was performed in which charging was performed at 1,600 mA to 4.2 V for 3.5 hours, and discharging was performed at 16,000 mA to 2.7 V, and the discharge capacity was examined.

  The value obtained by the formula of [(discharge capacity at 16,000 mA) / (discharge capacity at 1,600 mA)] × 100 was evaluated according to the following criteria.

○: 85% or more Δ: 80% or more and less than 85% ×: less than 80% (Example 1)
As a raw material resin for a porous polyolefin film, 96 parts by mass of homopolypropylene FLX80E4 (hereinafter referred to as PP-1) manufactured by Sumitomo Chemical Co., Ltd., Polypropylene PF-814 (hereinafter referred to as PPell) manufactured by Basell, which is a high melt tension polypropylene resin. 1 part by mass of HMS-PP) and Engage 8411 (melt index: 18 g / 10 min, hereinafter simply referred to as Engage) made by Dow Chemical, which is an ethylene-octene-1 copolymer, are added to 3 parts by mass. N, N′-dicyclohexyl-2,6-naphthalene dicarboxyamide (Nippon Rika Co., Ltd., Nu-100, hereinafter simply referred to as β crystal nucleating agent), which is a β crystal nucleating agent, is 0. 2 parts by mass, IRGANOX1010, IRGA manufactured by Ciba Specialty Chemicals, which is an antioxidant Twin screw extrusion from the weighing hopper so that the OS168 is mixed at a ratio of 0.15 and 0.1 parts by mass (hereinafter simply referred to as an acid inhibitor, and a mass ratio of 3: 2 unless otherwise specified). The raw material is supplied to the machine, melt kneaded at 300 ° C., discharged from the die in a strand shape, cooled and solidified in a water bath at 25 ° C., cut into a chip shape, and the chip raw material (A layer (first layer of laminated film) Layer)).

  In addition, a ratio of 98 parts by mass of PP-1, 1 part by mass of HMS-PP, 1 part by mass of Engage, 0.2 parts by mass of β crystal nucleating agent, and 0.25 parts by mass of an antioxidant. The raw material is supplied from the weighing hopper to the twin screw extruder so as to be mixed at 300 ° C, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Thus, a chip raw material (raw material for layer B (second layer) of the laminated film) was obtained.

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into the tenter type stretching machine by holding the clip with a clip and stretched 6 times at 150 ° C. in the width direction at a stretching rate of 1,500% / min. did. The time from the end of stretching in the longitudinal direction to the start of transverse stretching was 10 seconds. As it was, heat treatment was performed at 155 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous film having a total thickness of 20 μm and a laminate thickness ratio of 1: 10: 1 of B / A / B.

(Example 2)
A porous film was obtained in the same manner as in Example 1 except that the stacking ratio of Example 1 was 1: 60: 1.

(Example 3)
A porous film was obtained in the same manner as in Example 1 except that the stacking ratio of Example 1 was 1: 1: 1.

(Example 4)
A porous film was obtained in the same manner as in Example 1 except that PP-1 was changed to 90 parts by mass and Engage was set to 9 parts by mass in the raw material for layer B of Example 1.

(Example 5)
A porous film was obtained in the same manner as in Example 1 except that the temperature was maintained at 90 ° C. so as not to cool the film after longitudinal stretching in Example 1.

(Example 6)
A porous film was obtained in the same manner as in Example 1 except that the temperature was maintained at 140 ° C. so that the film was not cooled after the longitudinal stretching in Example 1.

(Example 7)
Add 96 parts by weight of PP-1, 1 part by weight of HMS-PP, 3 parts by weight of Engage, mix 0.2 parts by weight of β-crystal nucleating agent, and mix 0.25 parts by weight of an antioxidant. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  Also, 99 parts by mass of PP-1 and 1 part by mass of HMS-PP are added, so that the β crystal nucleating agent is mixed in an amount of 0.2 parts by mass, and the antioxidant is mixed in a ratio of 0.25 parts by mass. The raw material is supplied from the weighing hopper to the twin screw extruder, melted and kneaded at 300 ° C., discharged from the die in a strand shape, cooled and solidified in a 25 ° C. water tank, cut into a chip shape, and the chip raw material (laminated) Film B layer (third layer) raw material).

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into the tenter type stretching machine by holding the clip with a clip and stretched 6 times at 150 ° C. in the width direction at a stretching rate of 1,500% / min. did. The time from the end of stretching in the longitudinal direction to the start of transverse stretching was 10 seconds. As it was, heat treatment was performed at 160 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous film having a total thickness of 20 μm and a B / A / B lamination thickness ratio of 1: 15: 1.

(Example 8)
Add PP-1 to 94 parts by mass, HMS-PP to 1 part by mass, Engage to 5 parts by mass, β-crystal nucleating agent to 0.2 parts by mass, and an antioxidant to 0.25 parts by mass. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  Further, 96 parts by mass of PP-1, 1 part by mass of HMS-PP, 3 parts by mass of Engage are added, 0.2 parts by mass of β-crystal nucleating agent, and 0.25 parts by mass of an antioxidant. The raw material is supplied from the weighing hopper to the twin screw extruder so as to be mixed at 300 ° C, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Thus, a chip raw material (raw material for layer B (second layer) of the laminated film) was obtained.

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end was introduced into a tenter type stretching machine by holding the clip with a clip, and stretched 6 times at 150 ° C. in the width direction at a stretching rate of 100% / min. The time from the end of stretching in the longitudinal direction to the start of transverse stretching was 10 seconds. As it was, heat treatment was performed at 155 ° C. for 7 seconds while applying 5% relaxation in the width direction to obtain a porous film having a total thickness of 16 μm and a laminate thickness ratio of 1: 10: 1 of B / A / B.

Example 9
A porous film was obtained in the same manner as in Example 1 except that the layer structure of Example 1 was laminated to be a two-layer structure of A layer / B layer, and the lamination thickness ratio was 5: 1.

(Example 10)
Add PP-1 to 98 parts by mass, HMS-PP to 1 part by mass, Engage to 1 part by mass, β-crystal nucleating agent to 0.2 parts by mass, and antioxidant to 0.25 parts by mass. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used. This composition was supplied to a single screw extruder and melt-extruded at 220 ° C. After removing the foreign matter with a 30 μm cut sintered filter, it was discharged from a T-die to a cast drum whose surface temperature was controlled at 120 ° C. and cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into the tenter type stretching machine by holding the clip with a clip and stretched 6 times at 150 ° C. in the width direction at a stretching rate of 1,500% / min. did. The time from the end of stretching in the longitudinal direction to the start of transverse stretching was 10 seconds. As it was, heat treatment was performed at 158 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous polypropylene film having a thickness of 20 μm.

(Comparative Example 1)
Add 96 parts by weight of PP-1, 1 part by weight of HMS-PP, 3 parts by weight of Engage, mix 0.2 parts by weight of β-crystal nucleating agent, and mix 0.25 parts by weight of an antioxidant. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  In addition, a ratio of 98 parts by mass of PP-1, 1 part by mass of HMS-PP, 1 part by mass of Engage, 0.2 parts by mass of β crystal nucleating agent, and 0.25 parts by mass of an antioxidant. The raw material is supplied from the weighing hopper to the twin screw extruder so as to be mixed at 300 ° C, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Thus, a chip raw material (raw material for layer B (second layer) of the laminated film) was obtained.

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into a tenter-type stretching machine by holding the clip with a clip, and stretched at 150 ° C. six times at a stretching speed of 1,500% / min. As it was, heat treatment was performed at 165 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous polypropylene film having a total thickness of 20 μm and a B / A / B lamination thickness ratio of 1: 10: 1.

(Comparative Example 2)
Add 96 parts by weight of PP-1, 1 part by weight of HMS-PP, 3 parts by weight of Engage, mix 0.2 parts by weight of β-crystal nucleating agent, and mix 0.25 parts by weight of an antioxidant. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw material was used.

  This composition was supplied to a single screw extruder and melt-extruded at 220 ° C. After removing the foreign matter with a 30 μm cut sintered filter, it was discharged from a T-die to a cast drum whose surface temperature was controlled at 120 ° C. and cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into the tenter type stretching machine by holding the clip with a clip and stretched 6 times at 150 ° C. in the width direction at a stretching rate of 1,500% / min. did. As it was, heat treatment was performed at 155 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous polypropylene film having a thickness of 20 μm.

(Comparative Example 3)
Add 96 parts by weight of PP-1, 1 part by weight of HMS-PP, 3 parts by weight of Engage, mix 0.2 parts by weight of β-crystal nucleating agent, and mix 0.25 parts by weight of an antioxidant. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  In addition, a ratio of 98 parts by mass of PP-1, 1 part by mass of HMS-PP, 1 part by mass of Engage, 0.2 parts by mass of β crystal nucleating agent, and 0.25 parts by mass of an antioxidant. The raw material is supplied from the weighing hopper to the twin screw extruder so as to be mixed at 300 ° C, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Thus, a chip raw material (raw material for layer B (second layer) of the laminated film) was obtained.

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Once cooled to room temperature, the end was introduced into a tenter-type stretching machine by gripping with a clip, and stretched 6 times at 150 ° C. at a stretching rate of 1,500% / min. As it was, heat treatment was performed at 155 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous polypropylene film having a total thickness of 20 μm and a laminate thickness ratio of 1: 10: 1 of B / A / B.

(Comparative Example 4)
Add PP-1 to 98 parts by mass, HMS-PP to 1 part by mass, Engage to 1 part by mass, β-crystal nucleating agent to 0.2 parts by mass, and antioxidant to 0.25 parts by mass. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  Also, 99 parts by mass of PP-1 and 1 part by mass of HMS-PP are added, so that the β crystal nucleating agent is mixed in an amount of 0.2 parts by mass, and the antioxidant is mixed in a ratio of 0.25 parts by mass. The raw material is supplied from the weighing hopper to the twin screw extruder, melted and kneaded at 300 ° C., discharged from the die in a strand shape, cooled and solidified in a 25 ° C. water tank, cut into a chip shape, and the chip raw material (laminated) Film B layer (third layer) raw material).

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into a tenter-type stretching machine by holding the clip with a clip, and stretched at 150 ° C. six times at a stretching speed of 1,500% / min. As it was, heat treatment was performed at 160 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous film having a total thickness of 20 μm and a B / A / B lamination thickness ratio of 1: 10: 1.

(Comparative Example 5)
Add 96 parts by weight of PP-1, 1 part by weight of HMS-PP, 3 parts by weight of Engage, mix 0.2 parts by weight of β-crystal nucleating agent, and mix 0.25 parts by weight of an antioxidant. The raw material is supplied from the weighing hopper to the twin-screw extruder, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Chip raw materials (raw materials for layer A (first layer) of the laminated film) were used.

  In addition, a ratio of 88 parts by mass of PP-1, 1 part by mass of HMS-PP, 11 parts by mass of Engage, 0.2 parts by mass of β crystal nucleating agent, and 0.25 parts by mass of an antioxidant The raw material is supplied from the weighing hopper to the twin screw extruder so as to be mixed at 300 ° C, melted and kneaded at 300 ° C, discharged from the die into strands, cooled and solidified in a 25 ° C water bath, and cut into chips. Thus, a chip raw material (raw material for layer B (second layer) of the laminated film) was obtained.

  These compositions were supplied to separate single-screw extruders and melt-extruded at 220 ° C., respectively. After removing the foreign matter with a 30μm cut sintered filter, three layers are laminated using the feed block method to form a layer structure of B layer / A layer / B layer, and the surface temperature is controlled from the T die to 120 ° C. Then, it was cast so as to be indirectly on the drum for 15 seconds to obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 110 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Through an oven controlled at 110 ° C. so as not to cool the film, the end portion was introduced into a tenter-type stretching machine by holding the clip with a clip, and stretched at 150 ° C. six times at a stretching speed of 1,500% / min. As it was, heat treatment was performed at 155 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous film having a total thickness of 20 μm and a laminate thickness ratio of 1: 10: 1 of B / A / B.

(Comparative Example 6)
A porous film was obtained in the same manner as in Example 1 except that the temperature was maintained at 160 ° C. so as not to cool the film after longitudinal stretching in Example 1.

  The porous film of the present invention can be provided as a porous film having excellent processability and good battery characteristics.

DESCRIPTION OF SYMBOLS 1 ... Fibril part 2 ... Cavity part 3 ... Uncleaved part

Claims (5)

A porous film having an area ratio of fibrils of at least one side of 8 to 50% and a Gurley air permeability of 10 to 400 seconds / 100 ml. A second layer having a first layer containing an ethylene / α-olefin copolymer and containing an ethylene / α-olefin copolymer in an amount smaller than that of the first layer on at least one side of the first layer. The porous film according to claim 1, wherein a third layer not containing an ethylene / α-olefin copolymer is laminated. The porous film according to claim 1 or 2, wherein the porosity is 50 to 90%. The porous film in any one of Claims 1-3 used for the separator for electrical storage devices. An electricity storage device using the porous film according to claim 4 as a separator.