JP2014062244A - Porous film and electricity storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film which is uniformly in contact with an electrode surface when the porous film is used as a separator for a lithium ion battery and which exhibits excellent uniformity of ionic conductivity between the separator and an electrode interface.SOLUTION: The porous film has 9.5% or less of a variance of surface roughness Ra on both surfaces obtained by use of a laser microscope.

Description

  When the present invention is used as a separator for a lithium ion battery, it is in uniform contact with the electrode surface and is excellent in ion conduction uniformity at the separator / electrode interface. The present invention relates to a porous film having excellent properties. Specifically, the present invention relates to a porous film that has a small variation in surface roughness on both surfaces of the porous film and can be suitably used for a separator application or a light reflector application of an electricity storage device.

  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 There are various uses such as a receptor member, and a polyolefin-based porous film represented by polypropylene and polyethylene is mainly used. Porous polyolefin-based films are used as separators for power storage devices and as light reflectors for surface light sources, due to their characteristics such as high permeability and high porosity.

  When the porous film is used as an electrolytic device, particularly as a separator for a lithium ion battery, it is important from the viewpoint of uniform ion conduction at the separator / electrode interface that the separator and the electrode are in uniform contact. When the separator and the electrode are not in uniform contact, there is a possibility that a large ionic conduction occurs locally and the cycle characteristics are deteriorated. It is considered that it is important that the separator that can be in uniform contact with the electrode has few uneven thicknesses and that the holes are uniformly formed.

  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, a polyolefin resin is used as a matrix resin, an extractable to be extracted after forming into a sheet is added and mixed, and only the additive is extracted using a good solvent of the extractable so that voids are formed in the matrix resin. Has been proposed (see, for example, Patent Documents 1 and 2).

  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 Documents 3 and 4). In addition, as a dry method, a so-called β is formed by utilizing a crystal density difference and crystal transition of α-type crystal (α crystal) and β-type crystal (β crystal), which are polymorphs of polypropylene, to form voids in the film. Many proposals of a method called a crystallization method have been made (for example, see Patent Documents 5 to 8).

  In order to uniformly open the porous film as described above, an extrusion casting process in which a molten resin is extruded from a die and formed into a sheet on a cooling drum is important in the film forming process. The reason is that, for example, in the lamellar stretching method and β crystal method in which voids are formed using the crystal structure of the resin, the crystal structure in the cast sheet is almost determined in the extrusion casting process, and thus the resin crystal structure is hindered. This is because such a casting method is considered to affect the uniformity of the opening.

  The cast method when extruding the molten resin from the die and forming the sheet on the cooling drum includes the air knife cast method using air pressing pressure, the air chamber cast method, the roll cast method of pressing with a releasable roll, An electrostatic application casting method using electro-adhesion may be used. The electrostatic application casting method is a useful casting method that has a small influence on the crystal structure of the cast sheet because it adheres to the cooling drum by electrostatic adhesion. On the other hand, polyolefin resin generally has a high melting specific resistance and electrostatic adhesion. Since it is weak, it is difficult to use an electrostatic application casting method, and an air knife casting method or a roll casting method is often used.

  Patent Document 9 discloses a method for producing an ultrathin polyolefin film and sheet, which uses an electrostatic application casting method in an extrusion casting process of a polyolefin-based film. However, when the film thickness is thick, the adhesion to the cooling drum is poor, and a sufficient cast sheet cannot be obtained with the porous film.

Japanese Patent Laid-Open No. 55-131028 JP 2003-231772 A Japanese Patent Publication No.55-32531 JP 2005-56851 A JP-A 63-199742 JP-A-6-100720 Japanese Patent Laid-Open No. 9-255804 JP 2008-120931 A JP-A-7-52232

  An object of the present invention is to solve the above-described problems. That is, when the molten resin was formed into a sheet from the die, it could be cast by an electrostatic application casting method, and it was used as a lithium ion battery separator by reducing the variation in surface roughness on both sides of the obtained porous film. In this case, the object is to provide a porous film and an electricity storage device that are in uniform contact with the electrode surface and have excellent ion conduction uniformity at the separator / electrode interface.

  In order to solve the above-described problems and achieve the target, the porous film of the present invention is characterized in that the variation in surface roughness Ra obtained by a laser microscope is 9.5% or less on both sides.

  When the porous film of the present invention is used as a separator for a lithium ion secondary battery, it can be suitably used as a separator because it is in uniform contact with the electrode surface and is excellent in ion conduction uniformity at the separator / electrode interface. Further, when used as a light reflecting plate, it is excellent in uniformity of light reflection, and therefore can be suitably used as a light reflecting plate for a surface light source.

  Embodiments of the present invention will be described below.

  The resin constituting the porous film of the present invention may be any of polyolefin resin, polycarbonate resin, polyamide resin, polyimide resin, polyamideimide, aromatic polyamide resin, fluorine resin, etc., but heat resistance, moldability, From the viewpoint of reduction of production cost, chemical resistance, oxidation resistance, reduction resistance, etc., it is preferable that the polyolefin resin is a main component. In addition, when single resin contains more than 50 mass% among the whole resin which comprises a porous film, the resin is made into a main component. Examples of the monomer component constituting the polyolefin-based resin include ethylene, propylene, 1-butene, 1-pentene, 3-methyl-1-pentene, 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, Examples include vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. As the resin constituting the porous film according to the present invention, a homopolymer of the above monomers and a copolymer of two or more monomers selected from the above monomers are preferably used. However, it is not limited to these. 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 the above, polypropylene resin is preferable from the viewpoint of heat resistance, air permeability, porosity, and the like.

  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 through-holes in the porous film of the present invention, either a wet method or a dry method may be used, but a dry method is preferable because the process can be simplified, and when a polypropylene resin is used, from the viewpoint of productivity. The β crystal method is particularly preferable.

  When a polypropylene resin is used as the resin constituting the porous film according to the present invention and is made porous by the β crystal method, the β crystal forming ability of the polypropylene resin is preferably 40% or more. If the β-crystal forming ability is less than 40%, the amount of β-crystals 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 40% or more, it is preferable to use a polypropylene resin having a high isotactic index or to add a β crystal nucleating agent. The β crystal forming ability is more preferably 45% or more, and particularly preferably 50% or more.

  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 polypropylene resin. For this purpose, it is added to a polyolefin resin called a β crystal nucleating agent. Thus, it is preferable to use a crystallization nucleating agent that selectively forms β crystals as an additive. 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. Examples of the amide compound include N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide, N, N′-dicyclopentyl-2,6-naphthalenedicarboxamide, N, N′-dicyclooctyl-2, 6-naphthalenedicarboxamide, N, N′-dicyclododecyl-2,6-naphthalenedicarboxamide, N, N′-dicyclohexyl-2,7-naphthalenedicarboxamide, N, N′-dicyclohexyl-4,4 ′ -Biphenyldicarboxamide, N, N'-dicyclopentyl-4,4'-biphenyldicarboxamide, N, N'-dicyclooctyl-4,4'-biphenyldicarboxamide, N, N'-dicyclododecyl-4 , 4′-biphenyldicarboxamide, N, N′-dicyclohexyl-2,2′-biphenyl Carboxamide, N, N′-diphenylhexanediamide, N, N′-dicyclohexylterephthalamide, N, N′-dicyclohexanecarbonyl-p-phenylenediamine, N, N′-dibenzoyl-1,5-diaminonaphthalene, N, N′-dibenzoyl-1,4-diaminocyclohexane, N, N′-dicyclohexanecarbonyl-1,4-diaminocyclohexane, N-cyclohexyl-4- (N-cyclohexanecarbonylamino) benzamide, N-phenyl-5- ( Tetraoxaspiro compounds such as N-benzoylamino) pentanamide and 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5.5] undecane. It can be preferably used. β crystal nucleating agents may be used in combination of two or more. As content of (beta) crystal nucleating agent, it is preferable that it is 0.05-0.5 mass part with respect to 100 mass parts of polyolefin resin (the whole mixture, when using a mixture), 0.1-0. If it is 3 mass parts, it is more preferable.

  When the porous film of the present invention is composed of a polyolefin resin, the resin is preferably isotactic polypropylene having a melt flow rate (hereinafter referred to as MFR) in the range of 4 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, MFR is 4g-20g / 10min.

  Further, the polypropylene resin contained in the porous film is preferably isotactic polypropylene, and its isotactic index 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, a commercially available resin can be used.

  The porous film of the present invention has a variation in surface roughness Ra required by a laser microscope of 9.5% or less on both sides. The variation of the surface roughness Ra is more preferably 9% or less on both surfaces, still more preferably 8.5% or less, and particularly preferably 8% or less. When the variation of the surface roughness Ra exceeds 9.5%, when the porous film of the present invention is used as a separator of a lithium ion battery, large ion conduction occurs locally between the separator and the electrode, and the cycle characteristics are deteriorated. There is a case to let you. The smaller the variation of the surface roughness Ra, the better. However, the lower limit is substantially 0.1%.

  In order to make the variation in the surface roughness Ra within the above range, from the viewpoint of uniformizing the front and back structures of the porous film, in the extrusion casting process in which the molten resin is extruded from the die and formed into a sheet on the cooling drum, It is preferable to use an applied casting method. Details will be described later.

  As for the porous film of this invention, it is preferable that surface roughness Ra calculated | required with a laser microscope is 0.1-1 micrometer on both surfaces. The surface roughness Ra is more preferably 0.2 to 0.8 μm on both sides, still more preferably 0.3 to 0.8 μm, and particularly preferably 0.4 to 0.7 μm. If the surface roughness Ra obtained by a laser microscope is less than 0.1 μm on both sides, the holes are not sufficiently opened, and the battery characteristics may be deteriorated when used for a lithium ion secondary battery. On the other hand, when the thickness exceeds 1 μm, the opening of the hole becomes large and the puncture strength may be lowered.

  In order to make the surface roughness Ra required by the laser microscope both in the above range, it is preferable to make the film porous by, for example, the β crystal method from the viewpoint of forming a porous film having a pore size that is neither too large nor too small.

  The porous film of the present invention preferably has a light reflectance variation of 9.5% or less at a wavelength of 560 nm on at least one side. The variation in the light reflectance at a wavelength of 560 nm on at least one side is more preferably 9% or less, further preferably 8.5% or less, and particularly preferably 8% or less. If the variation in light reflectance exceeds 9.5%, light reflection unevenness may occur when the porous film of the present invention is used as a light reflecting plate of a surface light source. In addition, when used as a separator of a lithium ion battery, large ion conduction may occur locally between the separator and the electrode due to unevenness of thickness, film surface layer, and internal structure, and the cycle characteristics may be deteriorated. The smaller the variation in the light reflectance, the better. However, the lower limit is substantially 0.1%.

  In order to make the variation in light reflectance within the above range, it is preferable to use an electrostatic application casting method in an extrusion casting process in which a molten resin is extruded from a die and formed into a sheet on a cooling drum. Details will be described later.

  When the porous film of the present invention is used for a battery separator or the like, it preferably has high air permeability, and the air resistance is 10 to 1,000 seconds / 100 ml, which is referred to as low internal resistance of the battery. From the viewpoint, it is more preferably 50 to 600 seconds / 100 ml, and further preferably 80 to 300 seconds / 100 ml. If the air permeability resistance is less than 10 seconds / 100 ml, the strength of the film is lowered, and metal lithium deposited on the negative electrode in the lithium ion secondary battery may penetrate the porous film and short circuit. On the other hand, if it exceeds 1,000 seconds / 100 ml, since the air permeability is low, the internal resistance of the battery is high and a high output density may not be obtained.

  In order to make the air resistance within the above range, 1 to 10% by mass of an ethylene / α-olefin copolymer may be added to the polypropylene resin composition from the viewpoint of promoting opening during stretching. preferable. Here, as the ethylene / α-olefin copolymer, an ultra-low density polyethylene having a density of 0.89 or less is preferable, and an ethylene / 1-octene copolymer obtained by copolymerizing 1-octene is particularly preferably used. be able to. Examples of the copolymer polyethylene resin include commercially available resins such as “Engage” (registered trademark) (type names: 8411, 8452, 8100, etc.) manufactured by Dow Chemical Company.

  In the above polypropylene resin composition, a high melt tension polypropylene resin may be blended in the range of 0.5 to 5% by mass from the viewpoint of improving the film forming property. High melt tension polypropylene resin is a mixture of a high molecular weight component or a component having a branched structure in the polypropylene resin, or by copolymerizing a long-chain branched component with the polypropylene resin, thereby increasing the tension in the molten state. Although it is a polypropylene resin, it is preferable to use the polypropylene resin which copolymerized the long-chain branching component especially. This high melt tension polypropylene resin is commercially available. For example, polypropylene resins PF814, PF633, and PF611 manufactured by Basel, polypropylene resin WB130HMS manufactured by Borealis, and polypropylene resins D114 and D206 manufactured by Dow can be used.

  In addition, the porous film of the present invention includes an antioxidant, a heat stabilizer, an antistatic agent, a lubricant composed of inorganic or organic particles, and an antiblocking agent and a filling agent within the range not impairing the effects of the present invention. Various additives such as an agent and 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 composition for the purpose of suppressing oxidative deterioration due to the thermal history of the polypropylene resin.

  In the porous film of the present invention, for the purpose of imparting various effects, a layer having a through hole made porous by a β crystal method may be laminated on at least one surface. The laminated structure may be a two-layer laminated structure, a three-layer laminated structure, or a larger number of laminated layers, but in order to keep the variation in surface roughness Ra within a predetermined range, the same raw material is used for both surface layers. It is particularly preferred to provide a layer of composition. As a lamination method, for example, either a feed block method by coextrusion or a multi-manifold method, or a method of laminating porous films by lamination may be used. In particular, for the purpose of improving the workability of a porous film, for example, it is preferable to laminate a layer made porous by a β crystal method without containing an ethylene / α-olefin copolymer. More preferably, it is arranged.

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

  As the polypropylene resin, 99.3 to 99.75 parts by mass of a commercially available polypropylene resin having an MFR of 4 to 30 g / 10 min, and N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide 0.05 to 0 as the β crystal nucleating agent .5 parts by mass and 0.2 parts by mass of antioxidant are mixed, and using a twin screw extruder, a raw material (A) mixed in advance at a predetermined ratio is prepared. At this time, the melting temperature is preferably 270 to 300 ° C.

  Similarly, 69.8 to 90 parts by mass of the above polypropylene resin, 9.8 to 30 parts by mass of an ultra-low density polyethylene resin (ethylene / octene-1 copolymer), which is also commercially available MFR 18 g / 10 min, an antioxidant. 0.2 parts by mass are mixed, and a raw material (B) mixed in advance at a predetermined ratio is prepared using a twin screw extruder.

  Next, 89.7 parts by mass of the raw material (A), 10 parts by mass of the raw material (B), and 0.3 parts by mass of the antioxidant were mixed by dry blending and supplied to a single screw melt extruder. Perform melt extrusion at 0 ° C. Next, after removing foreign substances and modified polymer with a filter installed in the middle of the polymer tube, the molten resin is discharged from the die onto the cooling drum to obtain an unstretched sheet.

  The manufacturing method of the porous film of the present invention forms the molten resin from the die from the viewpoint that the porous film can have a uniform structure on the front and back sides, and that the membrane vibration during casting can be suppressed and the longitudinal spots can be reduced. It is preferable to cast by an electrostatic application casting method. The electrostatic application casting method is not particularly limited as long as it is a method in which an unstretched sheet is brought into close contact with the cooling drum by electrostatic attraction. For example, a metal wire such as a tungsten wire is used as an electrode, and a support is used on the cooling drum. There is a method in which an unstretched sheet is brought into close contact with a cooling drum by inducing an electrostatic force by applying a voltage to a metal wire after fixing the resin in the immediate vicinity of the unstretched sheet and extruding the resin. At this time, it is preferable that the direction in which the metal wire is pulled is parallel to the width direction of the unstretched sheet.

  As a method for casting the porous film of the present invention, from the viewpoint of adhesion between the cooling drum and the unstretched sheet, an electrode having a wire shape, a tape shape, a needle shape, or the like can be used. From the viewpoint of safety, it is preferable to use a wire electrode. When a wire electrode is used, the wire diameter is preferably 0.05 to 0.2 mm, more preferably 0.05 to 0.1 mm. If the wire diameter is less than 0.05 mm, wire breakage due to insufficient strength may occur. When the diameter exceeds 0.2 mm, the directivity of electrostatic application is lowered, and the adhesion between the cooling sheet and the unstretched sheet may be lowered. A tape-like electrode may be used to improve the adhesion from the wire-like electrode, and the average thickness of the tape-like electrode is 0.01 to 0.3 mm from the viewpoint of the adhesion between the cooling drum and the unstretched sheet. Is more preferable, and 0.02-0.1 mm is more preferable. When the average thickness of the tape-shaped electrode is less than 0.01 mm, handling becomes difficult, and the electrode may be broken or broken when high tension is applied. If it exceeds 0.3 mm, the directivity is lowered, and adhesion failure may occur during casting. When a tape-shaped electrode is used, the width of the tape-shaped electrode is preferably 1 to 20 mm and more preferably 2 to 10 mm from the viewpoint of adhesion between the cooling drum and the unstretched sheet. When the width of the tape-like electrode is less than 1 mm, handling becomes difficult, and it may not be applied uniformly in the width direction, and a partial adhesion failure may occur. If it exceeds 20 mm, the contact point between the cooling drum and the unstretched sheet may become unstable, resulting in poor contact.

  As a casting method of the porous film of the present invention, from the viewpoint of suppressing the electrode vibration at the time of electrostatic application and uniformizing the structure of the unstretched film, the deflection of the electrode when a load of 50 gf is applied is 1 to 30 mm. It is preferably 1 to 20 mm, more preferably 1 to 10 mm. The smaller the deflection of the electrode when a load of 50 gf is applied, the better. However, when the electrode as described above is used, the lower limit is substantially 1 mm. When the deflection of the electrode when a load of 50 gf is applied exceeds 30 mm, vibration of the electrode occurs, and adhesion spots between the cooling drum and the unstretched sheet, that is, structural spots in the longitudinal direction may occur.

  As the method for casting the porous film of the present invention, the distance between the die and the cooling drum and the distance between the cooling drum and the electrode can be appropriately set from the viewpoint of the adhesion between the cooling drum and the unstretched sheet. In order to cast with good adhesion between the cooling drum and the unstretched sheet, the distance between the die and the cooling drum is preferably 5 to 50 mm, more preferably 10 to 40 mm. If the distance between the die and the cooling drum is less than 5 mm, the molten resin discharged from the die may be sheared by the die lip, resulting in streak defects. When it exceeds 50 mm, the neck-in of the molten resin discharged from the die becomes large, which may cause unevenness in the width direction. The distance between the cooling drum and the electrode is preferably 3 to 30 mm, more preferably 5 to 20 mm, and even more preferably 10 to 20 mm. If the distance between the cooling drum and the electrode is less than 3 mm, poor adhesion may occur due to discharge. If it exceeds 30 mm, application may be insufficient and adhesion failure may occur.

  As a method for casting the porous film of the present invention, from the viewpoint of controlling the adhesion between the cooling drum and the unstretched sheet and the β crystal fraction of the unstretched sheet to a high level, The temperature is preferably higher than 80 ° C, more preferably higher than 120 ° C, even more preferably higher than 122 ° C, and particularly preferably higher than 124 ° C. When the temperature of the cooling drum is 120 ° C. or lower, the adhesion between the cooling drum and the unstretched sheet is lowered, and defects such as bubbles may occur. Further, the temperature of the cooling drum is preferably lower than the melt crystallization temperature of the molten resin, and in the case of a laminated structure, it is preferably lower than the melt crystallization temperature of the surface layer of the molten resin. When the temperature of the cooling drum is higher than the melt crystallization temperature of the molten resin, the molten resin cannot be solidified on the cooling drum and a sheet may not be obtained.

  As the method for casting the porous film of the present invention, it is preferable that the electrode voltage for electrostatic application is 8 kV or more from the viewpoint of adhesion between the cooling drum and the unstretched sheet. If the electrode voltage applied electrostatically is less than 8 kV, the adhesion between the cooling drum and the unstretched sheet may deteriorate, and defects such as bubbles may occur.

  As the method for casting the porous film of the present invention, from the viewpoint of further improving the adhesion between the cooling drum and the unstretched sheet, an electrostatic application casting method and one or more cooling drum adhesion methods of spot air, air knife, and air chamber are used. It is also possible to use together.

  The thickness of the unstretched sheet can be appropriately set depending on the thickness and physical properties of the target porous film, but is preferably thicker than 200 μm. When the thickness of the unstretched sheet is 200 μm or less, pinholes may be generated due to a decrease in productivity, foreign matter, roll transfer, or the like.

  The unstretched sheet obtained after the casting step 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 110 to 130 ° C, and particularly preferably 120 to 130 ° C. As a draw ratio, it is 3 to 6 times, More preferably, it is 3 to 5.8 times. As the stretching ratio is increased, the porosity is increased. However, if the stretching ratio exceeds 6 times, the film may be easily broken in the next transverse stretching step. 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) Is preferably 40 to 90%. Considering stretching in the width direction, 50 to 80% is more preferable.

  Next, the end of the film is gripped and introduced into a tenter type stretching machine. And preferably, it heats to 130-155 degreeC, More preferably, it is 145-153 degreeC, 4-12 times in the width direction, More preferably, it is 6-11 times, More preferably, 6.5-10 times is stretched. The transverse stretching speed at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min. Subsequently, heat setting is performed in the tenter as it is, and the temperature is preferably 155 to 165 ° C, more preferably 159 to 165 ° C, and further preferably 160 to 164 ° C. Furthermore, the heat setting may be performed while relaxing in the longitudinal direction and / or the width direction of the film. In particular, the relaxation rate in the width direction is 10 to 35%, more preferably 10 to 25%, still more preferably 10 to 20. % Is preferable from the viewpoint of thermal dimensional stability.

  The porous film of the present invention has a small variation in surface roughness Ra, and thus can be used for packaging products, sanitary products, agricultural products, building products, medical products, separation membranes, light diffusion plates, and in particular, power storage. It can be suitably used as a separator of a device or a light reflector of a surface light source.

  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 cycle characteristics can be improved, but also the productivity of the battery can be improved. In addition, when coating the porous film of the present invention for imparting a function, since the variation of the surface roughness Ra is small, a coating with less unevenness is possible on both sides, for example, a heat-resistant coating separator, a slippery property, etc. It can also be suitably used as a coating substrate such as a coating separator or a shutdown coating separator.

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

(1) Surface roughness Ra, variation in surface roughness Ra Cut out from the center in the width direction of the porous film to a length of 10 mm and a width of 9.8 mm, and pasted on a slide glass so that there is no looseness or deformation Was used as a sample. First, black and white ultra-deep data was measured using an ultra-deep shape measuring microscope VK-8510 manufactured by Keyence Corporation in accordance with a user's manual. The magnification at the time of measurement was 100 × for the objective lens, 1 × for the digital zoom and optical zoom, 0.1 μm for the measurement PITCH, and adjustment of the light receiving gain and offset was performed according to the manual measurement section of the user's manual. Next, the black and white ultra-depth data was transferred to a personal computer installed with the image measurement / analysis software VK-H1W dedicated to the ultra-depth shape measurement microscope, and the surface roughness Ra was measured according to the measurement / analysis section of the reference manual. The measurement range of the surface roughness Ra was such that the length × width of the film was 111.859 × 149.146 μm. The measurement position is 10 places at 10 mm intervals in the central part in the width direction of the film in the longitudinal direction (10 samples are sampled and measured at the central part of each sample). Measure 10 points at 1mm intervals so that the distance between both edges is the same (measured for one piece of the above sample), and average the total of 16 measured values excluding the maximum and minimum values in the longitudinal and width directions. The surface roughness Ra of the porous film was used. Further, the variation in the surface roughness Ra was determined by the following formula.

((Maximum value in 16 locations-minimum value in 16 locations) / average value in 16 locations) x 100
In addition, the same measurement was performed on both surfaces (cooling drum contact surface and non-contact surface) of the same sample.

(2) Variation in light reflectivity 560 nm with φ60 integrating sphere 130-0632 (manufactured by Hitachi, Ltd.) and 10 ° tilt spacer attached to spectrophotometer U-3410 (manufactured by Hitachi, Ltd.) The reflection of was measured. The measurement positions were measured at 10 points at 10 mm intervals in the central part in the width direction of the film in the longitudinal direction, and the dispersion of the measured values at a total of 8 points excluding the maximum value and the minimum value was determined by the following formula.

((Maximum value in 8 locations-minimum value in 8 locations) / average value in 8 locations) x 100
In addition, the same measurement was performed on both surfaces (cooling drum contact surface and non-contact surface) of the same sample.

(3) β crystal forming ability 5 mg of a porous film was taken as a sample in an aluminum pan and measured using a differential scanning calorimeter (Seiko Denshi Kogyo RDC220). First, under a nitrogen atmosphere, the temperature was raised from room temperature to 260 ° C. at 20 ° C./min (first run), held for 10 minutes, and then cooled to 20 ° C. at 10 ° C./min. After maintaining for 5 minutes, the melting peak observed when the temperature is increased (second run) at 20 ° C./minute again is the melting peak of the β crystal at the melting temperature of 145 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 ΔHα, and the heat of fusion of β crystal is ΔHβ, the value calculated by the following formula is 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.

(4) Melt crystallization temperature 5 mg of the resin raw material of the porous film was sampled in an aluminum pan and measured using a differential scanning calorimeter (Seiko Denshi Kogyo RDC220). First, the temperature of the crystallization peak observed when the temperature is raised from room temperature to 260 ° C. at 20 ° C./min (first run) in a nitrogen atmosphere, held for 10 minutes, and then cooled to 20 ° C. at 10 ° C./min. Was the melting crystallization temperature.

(5) Air permeability resistance A square with a side length of 100 mm was cut 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 air resistance of the porous film.

(6) Front and back structure uniformity About the produced porous film, the dispersion | variation in surface roughness Ra of both surfaces was measured as described in said (1), and the following references | standards evaluated.

○: 8% or less for both the cooling drum contact surface and non-contact surface Δ: Both the cooling drum contact surface and non-contact surface are 9.5% or less ×: Either the cooling drum contact surface or non-contact surface is greater than 9.5% (7) Production of Battery A positive electrode sheet using a lithium cobaltate (LiCoO ₂ ) manufactured by Hosen Co., Ltd. having a thickness of 40 μm as an active material was cut into 5 cm × 4 cm (coating weight: 1.5 mAhg / cm) ³ ). Among these, a side of 4 cm × 1 cm is an uncoated portion for connecting the tab, and the active material layer is 4 cm × 4 cm. An aluminum positive electrode tab having a width of 5 mm, a length of 3 cm, and a thickness of 0.1 mm was ultrasonically welded to the uncoated positive electrode portion at a length of 1 cm.

Also, a negative electrode sheet made of Hosen Co., Ltd. and using graphite having a thickness of 50 μm as an active material was cut out to 5.5 cm × 4.5 cm (coating weight: 1.6 mAhg / cm ³ ). Of these, the side 4.5 cm × 1 cm is an uncoated portion for connecting the tab, and the active material layer is 4.5 cm × 4.5 cm. A copper negative electrode tab having the same size as the positive electrode tab was ultrasonically welded to the negative electrode uncoated portion.

  Next, the porous film is cut out to 6 cm × 6 cm, the negative electrode and the positive electrode are stacked on both sides of the separator with the active material layer separating the separator, and the positive electrode application part is disposed so as to face the negative electrode application part. Got. When the solid electrolyte layer laminate surface was a single side, the solid electrolyte laminate surface was disposed so as to face the negative electrode. The positive electrode, negative electrode, and separator were sandwiched between a piece of 14 cm × 10 cm aluminum laminate film, the long side of the aluminum laminate film was folded, and the two long sides of the aluminum laminate film were heat-sealed to form a bag.

As an electrolytic solution, 2 mass of vinylene carbonate as an additive was added to a solution obtained by dissolving LiPF ₆ as a solute at a concentration of 1 mol / liter in a mixed solvent of ethylene carbonate: diethyl carbonate = 3: 7 (volume ratio). % Added electrolyte was prepared. An electrolyte solution (1.0 g) was poured into a bag-shaped aluminum laminate film, and the short sides of the aluminum laminate film were heat-sealed while impregnating under reduced pressure to obtain a laminate type battery.

  Five laminate batteries were produced for each test item, and the average value of the three batteries from which the maximum and minimum points of each test result were removed was taken as the test value.

(Cycle characteristics)
The cycle characteristics were tested according to the following procedure and evaluated based on the life capacity retention rate.

<1st to 100th cycle>
Charging and discharging were performed as one cycle, and the following conditions were repeated 100 times.

  Charging: Charging capacity was obtained by charging to 4.2 V by constant current charging at 25 ° C. and 25 mA.

  Discharge: Discharged to 3.0 V by constant current discharge at 25 ° C. and 25 mA to obtain a discharge capacity.

<Calculation of life capacity maintenance ratio>
(Discharge capacity at the 100th cycle) / (Discharge capacity at the first cycle) × 100 was defined as the discharge capacity retention rate. A discharge capacity retention rate of 85% or more was designated as A, 80% or more and less than 85% as B, and C as a value less than 80%.

Example 1
As raw material resin for porous film, 99.55 parts by mass of homopolypropylene resin FLX80E4 (MFR 7.5 g / 10 min (condition M), PP-1) manufactured by Sumitomo Chemical Co., Ltd., and N, N ′ as β crystal nucleating agent -0.25 parts by mass of dicyclohexyl-2,6-naphthalenedicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., Nu-100), 0.1 mass of IRGANOX 1010 and IRGAFOS 168 manufactured by Ciba Specialty Chemicals as antioxidants The raw material is supplied from the weighing hopper to the twin screw extruder, melted and kneaded at 300 ° C., and discharged from the die in a strand form so as to be mixed in a ratio of 0.2 parts by mass in total, and a 25 ° C. water tank And solidified by cooling, and cut into a chip shape to obtain a chip raw material (A). The melt crystallization temperature of the chip raw material (A) was 128 ° C.

  In addition, 69.8 parts by mass of PP-1, 30 parts by mass of Dow Chemical Engage 8411 (MFR 18 g / 10 min (Condition D)) which is an ethylene / octene-1 copolymer, and antioxidant (Irganox 1010 / Irgafos168 = 1/1) is mixed at a ratio of 0.2 part by mass, the raw material is supplied from the weighing hopper to the twin screw extruder, melt kneaded at 220 ° C., discharged from the die in a strand shape, and 25 ° C. And solidified by cooling in a water tank and cut into chips to obtain a chip raw material (B).

  Next, 89.7 parts by mass of the raw material (A), 10 parts by mass of the raw material (B), and 0.3 parts by mass of an antioxidant (Irganox 1010 / Irgafos168 = 1/2) are used as the raw material for the A layer of the laminated film. 99.7 parts by mass of raw material (A) and 0.3 part by mass of antioxidant (Irganox 1010 / Irgafos 168 = 1/2) are mixed as a raw material for layer B by dry blending, and are separately uniaxial melt extrusion. Each was melt-extruded at 220 ° C. After removing the foreign matter with a 30 μm cut sintered filter, three layers are laminated at a thickness ratio of 1: 8: 1 so as to have a layer configuration of B layer | A layer | B layer using a feed block method. The sheet was discharged onto a cooling drum whose surface temperature was controlled at 125 ° C., and electrostatically cast so as to be indirectly on the drum for 18 seconds to obtain an unstretched sheet having a thickness of 220 μm. As the conditions of the electrostatic application cast at that time, a tungsten wire is used as an electrode, the wire wire diameter is 0.1 mm, the deflection of the electrode when a load of 50 gf is applied is 5 mm, the distance between the die and the cooling drum is 15 mm, The distance between the cooling drum and the electrode was 18 mm, the cooling drum speed was 6 m / min, and the electrode voltage for electrostatic application was 8 kV. Further, spot air was used at the width of 10 mm at both ends of the unstretched film for the purpose of improving edge adhesion. Next, preheating was performed using a ceramic roll heated to 120 ° C., and the film was stretched 5 times in the longitudinal direction. Thereafter, the end portion was introduced into a tenter-type stretching machine by holding it with a clip, and stretched in the width direction seven times at 150 ° C. at a stretching rate of 1,700% / min. As it was, 10% relaxation was applied in the width direction at 160 ° C., followed by heat treatment at 160 ° C. for 6.6 seconds to obtain a porous film having a thickness of 20 μm. About the porous film produced as mentioned above, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Example 2)
A porous film was obtained in the same manner as in Example 1 except that an electrode having a wire diameter of tungsten wire electrode of 0.15 mm was used. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Example 3)
A porous film was obtained in the same manner as in Example 1 except that an electrode having a deflection of 25 mm when a load of 50 gf was applied was used. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

Example 4
A porous film was obtained in the same manner as in Example 1 except that an electrode having a deflection of 10 mm when a load of 50 gf was applied was used. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Example 5)
A porous film was obtained in the same manner as in Example 1 except that an electrode having a deflection of 15 mm when a load of 50 gf was applied was used. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Example 6)
A porous film was obtained in the same manner as in Example 1 except that the distance between the cooling drum and the electrode was 24 mm. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Example 7)
A porous film was obtained in the same manner as in Example 1 except that the temperature of the cooling drum was 122 ° C. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Comparative Example 1)
A porous film was obtained in the same manner as in Example 1 except that the casting method was an air knife casting method, and an air knife was installed so that air was uniformly blown in the width direction of the unstretched film. About the porous film produced as mentioned above, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Comparative Example 2)
A porous film was obtained in the same manner as in Example 1 except that an electrode having an electrode deflection of 31 mm when a load of 50 gf was applied was used. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Comparative Example 3)
A porous film was obtained in the same manner as in Example 1 except that the temperature of the cooling drum was 120 ° C. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Comparative Example 4)
A porous film was obtained in the same manner as in Example 1 except that the distance between the cooling drum and the electrode was 31 mm. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

(Comparative Example 5)
A porous film was obtained in the same manner as in Example 1 except that the applied voltage of electrostatic application was 7 kV. Thus, about the produced porous film, it measured and evaluated by the method as described in said (1)-(7). The results are shown in Table 1.

  When used as a separator for a lithium ion battery, the porous film of the present invention can be provided as a porous film that is in uniform contact with the electrode surface and excellent in ion conduction uniformity at the separator / electrode interface.

Claims (10)

A porous film having a variation in surface roughness Ra determined by a laser microscope of 9.5% or less on both sides. The porous film of Claim 1 whose surface roughness Ra calculated | required with a laser microscope is 0.1-1 micrometer on both surfaces. The porous film according to claim 1 or 2, wherein a variation in light reflectance at a wavelength of 560 nm on at least one side is 9.5% or less. The porous film in any one of Claims 1-3 which has polyolefin resin as a main component. The porous film according to claim 4, wherein the polyolefin resin is a polypropylene resin. The porous film according to claim 5, wherein the polypropylene resin in the porous film has a β-crystal forming ability of 40% or more. The porous film according to any one of claims 1 to 6, wherein the air permeability resistance is 10 to 1,000 seconds / 100 ml. The porous film in any one of Claims 1-7 used for the separator for electrical storage devices. An electricity storage device using the porous film according to claim 8 as a separator. The porous film in any one of Claims 1-7 used for a light reflection board.