JP2005346987A - Porous film, battery and separator therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film which has high thickness precision even if the thickness is extremely thin. <P>SOLUTION: The porous film is composed of a resin composite film consisting of at least high density polyethylene resin (A) and filler material (B), wherein holes are formed on the resin composite film by extending it. Average value of the thickness of the porous film is 5 to 40 μm, and maximum value and minimum value are within the limits of the average value ± 25%. Inorganic filler which consists of barium sulfate grains or calcium carbonate grains having average grain diameter from 0.1 to 25 μm is used as the filler material (B). The filler material (B) of 50 to 400 part by mass is combined with the high density polyethylene resin (A) of 100 part by mass. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a porous film, and can be used as packaging, sanitary, livestock, agricultural, architectural, medical, separation membrane, light diffusion plate, battery separator, and particularly as a separator for non-aqueous electrolytic battery. It can be used suitably.

Conventionally, small secondary batteries are widely used as power sources for portable electronic devices such as OA, FA, home appliances, and communication devices. In particular, portable devices using lithium ion secondary batteries are increasing because they have high volumetric efficiency when connected to devices, leading to smaller and lighter devices.
On the other hand, large secondary batteries are being researched and developed in many fields related to environmental issues such as load leveling, uninterruptible power supply (UPS), and electric vehicles. Large capacity, high output, high voltage and long-term storage stability The use of lithium ion secondary batteries, which are a type of non-aqueous electrolyte secondary battery, is expanding from the point of being superior to the above.

The working voltage of a lithium ion secondary battery is usually designed with an upper limit of 4.1 to 4.2V. At such a high voltage, the aqueous solution causes electrolysis and cannot be used as an electrolyte. Therefore, so-called non-aqueous electrolytes using organic solvents are used as electrolytes that can withstand high voltages.
As the solvent for the nonaqueous electrolyte, a high dielectric constant organic solvent capable of causing more lithium ions to be present is used, and organic carbonates such as polypropylene carbonate and ethylene carbonate are used as the high dielectric constant organic solvent. Yes. As a supporting electrolyte that becomes a lithium ion source in the solvent, a highly reactive electrolyte such as lithium hexafluorophosphate is dissolved in the solvent and used.

Lithium ion secondary battery separators are in direct contact with the positive electrode and negative electrode and are interposed between the two electrodes, so that insulation is required from the viewpoint of preventing internal short circuit, and air permeability and electrolyte diffusion that serve as lithium ion passages -To provide a holding function, a fine pore structure is used, and the fine pores are required to have a shutdown function that melts and shuts off the pores when abnormal heat is generated.
From this safety point of view, it has a microporous structure, and when it reaches a high temperature (140 to 160 ° C.), the micropores are blocked, thereby blocking ion conduction inside the battery and preventing subsequent temperature rise inside the battery. There is provided a separator made of a polyolefin resin having a shutdown function that can be performed.
However, if the battery temperature continues to rise for some reason after the shutdown and exceeds the heat resistance temperature of the separator, the separator melts and the isolation between the positive electrode and the negative electrode is remarkably reduced, so that a short circuit may occur in the battery.

In order to solve the above-mentioned problems, the inorganic-containing porous material having excellent heat resistance composed of a kneaded material containing 30 to 70% by weight of mineral oil as a plasticizer with respect to a mixture of polyolefin resin and inorganic powder or inorganic fiber. A membrane separator is provided in Japanese Patent Laid-Open No. 10-50287 (Patent Document 1).
However, when manufacturing porous films for separators using polyolefin resin and inorganic powder as raw materials, the polyolefin resin and inorganic powder are mixed with a plasticizer consisting of a large amount of mineral oil and the mixture is formed into a sheet. After the primary processing to perform, secondary processing to stretch and roll the sheet to provide pores, a step of extracting and removing the blended mineral oil with an organic solvent is necessary. In this extraction step, a large amount of organic There is a problem that productivity is poor, for example, the number of processes increases with the use of a solvent.

JP-A-2001-164015 (Patent Document 2) provides a porous film made of a polypropylene resin, a filler, and an amide plasticizer.
However, the polypropylene resin has a high melting point and it is difficult to close the micropores at an abnormally high temperature, and a shutdown function cannot be expected. Therefore, although it can be used as a separator for a large-capacity battery system, it is not currently used as a separator for consumer batteries.
In addition, since the porous film disclosed in Patent Document 2 uses polypropylene, it is difficult to form a film having uniform permeability, and the film is maintained at a specific thickness and a specific thickness accuracy. Hateful. Therefore, since the thickness of the film is difficult to be uniform, a thin portion is easily torn when pressure is applied, and there is a problem in insulation. And when winding a positive electrode, a separator, and a negative electrode spirally using a winding core, since a film is easy to tear, it is difficult to cut and a battery cannot be manufactured stably.

Furthermore, JP-A-2003-82139 (Patent Document 3) includes a high-density polyethylene resin, a filler made of calcium carbonate, and the like, and a plasticizer of a low molecular weight compound such as an aliphatic hydrocarbon having a molecular weight of 200 to 500 or a higher alcohol. There is provided a porous film formed by forming a sheet from the kneaded product of the resin composition and stretching the sheet.
However, as a result of further examination by the present inventor, it was practically difficult to produce a separator having a uniform pore diameter, and it was very difficult to control the thickness accuracy to the target accuracy. Therefore, when winding the positive electrode, separator and negative electrode in a spiral shape using a cylindrical, rhombus or flat core, etc., it cannot be stored in a predetermined size and cannot be accommodated in a battery can, Even if it could be accommodated, there was a case where a short circuit occurred due to local pressure.

Japanese Patent Laid-Open No. 10-50287 JP 2001-164015 A JP 2003-82139 A

  The present invention has been made in view of the above problems, and even if the thickness is very thin, the thickness accuracy can be obtained. As a result, the battery separator is spirally wound and accommodated in a battery can. At this time, it is an object of the present invention to prevent the occurrence of a short circuit because no pressure is applied locally when the battery can be accommodated in a predetermined size and is accommodated in a battery can.

  As a result of intensive studies to solve the above problems, the inventors of the present invention have developed a film composed of a resin composition composed of at least two components of a high-density polyethylene resin (A) and a filler (B) by stretching. There is provided a porous film provided with pores, having an average thickness of 5 to 40 μm, and having a maximum value and a minimum value of ± 25% or less of the average thickness.

The porous film of the present invention has an average thickness of 5 μm or more and 40 μm or less, and can exhibit excellent insulating properties by controlling the maximum value and the minimum value of the thickness to ± 25% or less of the average thickness. Specifically, since the thickness is uniform, the thin portion is not substantially torn when pressure is applied. In addition, since the thickness is uniform, the porous film of the present invention can be used as a separator, and can be stored in a predetermined size when the positive electrode, the separator, and the negative electrode are wound in a spiral shape. Therefore, it can be easily accommodated in the battery can without any pressure being applied, and the occurrence of a short circuit can be prevented.
Furthermore, since it contains a high density polyethylene resin, it can exhibit an excellent shutdown function in an abnormally high temperature state when used as a nonaqueous electrolyte battery separator.

In addition, since it is made of a polyethylene resin, which is a polyolefin resin, the film has excellent air permeability and heat resistance. The high density polyethylene preferably has a density of 0.94 g / cm ³ or more, more preferably 0.95 g / cm ³ in order to more effectively exhibit a shutdown function in an abnormally high temperature (140 to 160 ° C.) state. More preferably. The upper limit of the density is 0.97 g / cm ³ .
For the high-density polyethylene, the melt flow rate is 1 g / 10 min or less, preferably 0.6 g / 10 min or less, particularly preferably 0.1 g / 10 in order to keep the strength of the film to be molded as required. Is less than a minute. When the melt flow rate is larger than 1 g / 10 min, stretching of 3 times or more becomes difficult, and the strength of the resulting porous film is lowered. The lower limit is 0.01 g / 10 minutes.

Specifically, the high-density polyethylene is preferably a homopolymer polyethylene or a copolymer polyethylene having an α-olefin comonomer content of 2 mol% or less, and more preferably a homopolymer polyethylene. In addition, there is no restriction | limiting in particular in the kind of alpha-olefin comonomer.
The polymerization catalyst for the high density polyethylene is not particularly limited, and may be any one such as a Ziegler type catalyst, a Phillips type catalyst, or a Kaminsky type catalyst. As a method for polymerizing polyethylene, there are one-stage polymerization, two-stage polymerization, or more multistage polymerization, and any method may be used.

The filler (B) preferably has an average particle size of 0.1 to 25 μm.
In this way, since a very small filler of about 0.1 to 25 μm is used as a filler that becomes the starting point of the pores during stretching, the thickness of the very thin film of about 5 to 40 μm High accuracy can be maintained.
The reason why the average particle diameter is within the above range is to uniformly disperse the resin in the resin composition and to obtain a desired pore size. When the average particle size is less than 0.1 μm, the dispersibility is reduced due to aggregation of the fillers, causing uneven stretching, and the contact area of the interface between the thermoplastic resin and the filler is increased to cause interfacial peeling due to stretching. This is because it is difficult to make the material porous. On the other hand, if the average particle size exceeds 25 μm, it is difficult to make the film thin, and it is difficult to increase the thickness accuracy of a very thin film of 5 to 40 μm, preferably 5 to 30 μm. This is because the mechanical strength decreases. The average particle size of the filler is preferably about 0.5 to 5 μm.

As the filler, both inorganic and organic fillers can be used, and one kind can be used alone, or two or more kinds can be used in combination.
Examples of inorganic fillers include carbonates such as calcium carbonate, magnesium carbonate and barium carbonate; sulfates such as calcium sulfate, magnesium sulfate and barium sulfate; chlorides such as sodium chloride, calcium chloride and magnesium chloride; calcium oxide, In addition to oxides such as magnesium oxide, zinc oxide, titanium oxide, and silica, silicates such as talc, clay, and mica can be used. Among these, calcium carbonate or barium sulfate is more preferable, and barium sulfate is most preferable. In order to improve the dispersibility in the resin, the inorganic filler may be hydrophobized by coating the surface of the inorganic filler with a surface treatment agent. Examples of the surface treatment agent include higher fatty acids such as stearic acid or lauric acid, or metal salts thereof.

As the organic filler, resin particles having a melting point higher than that of high-density polyethylene and low-density polyethylene are preferable so that the filler does not melt at the stretching temperature, and crosslinked resin particles having a gel content of about 4 to 10% are further included. preferable.
Examples of organic fillers include ultra high molecular weight polyethylene, polystyrene, polymethyl methacrylate, polycarbonate, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, polytetrafluoroethylene, polyimide, poly Examples thereof include thermoplastic resins and thermosetting resins such as etherimide, melamine, and benzoguanamine. Among these, crosslinked polystyrene is particularly preferable.

The blending ratio of the high density polyethylene resin (A) and the filler (B) is preferably such that the filler (B) is 50 parts by mass or more and 400 parts by mass or less with respect to 100 parts by mass of the high density polyethylene resin. More preferably, it is 100-200 mass parts.
If the blending amount of the filler is 50 to 400 parts by mass and less than 50 parts by mass, when the raw material to be blended is only high-density polyethylene, the desired good air permeability is hardly expressed, This is because the appearance and texture are liable to deteriorate, and if the amount of the filler exceeds 400 parts by mass, defects in the process such as resin burning are likely to occur during film forming, and the film strength is greatly reduced.

  The resin and the filler are essential components of the film, but it is preferable to add a plasticizer for the purpose of improving the dispersibility of the filler in the resin when these raw materials are mixed and melted.

The plasticizer used in the present invention is one that is liquid or solid at 140 ° C. or higher. Usually, since the liquid is changed to a gas, a plasticizer having a boiling point of 140 ° C. or higher is substantially used. When the boiling point is less than 140 ° C., the plasticizer volatilizes when the product having the film of the present invention becomes hot for some reason, and the product may burst. A plasticizer having a boiling point of 140 ° C. or higher is a plasticizer having a boiling point of 140 ° C. or higher at atmospheric pressure, or the weight after heating at 140 ° C. for 1 hour does not decrease by 10% or more with respect to the weight before heating. It is defined as a thing.
Further, the plasticizer has a melting point of 25 ° C. or higher. If a plasticizer having a melting point of 25 ° C. or higher is used, the film is solid at room temperature, so that the rigidity of the film is improved. For example, handling in an assembly process of a product having the film of the present invention such as a battery becomes easier. Here, a plasticizer having a melting point of 25 ° C. or higher is a plasticizer having a clear endothermic peak at 25 ° C. or higher when measured by DSC (differential scanning calorimetry), or a clear endothermic peak when measured by DSC. What is not defined is defined as having a kinematic viscosity at 25 ° C. of 100,000 mm ² / sec or more.

As a plasticizer in the present invention, an ester compound, an amide compound, a fatty acid, an alcohol compound, an amine compound, an epoxy compound, an ether compound, on the premise that the melting point is 25 ° C. or higher and the solid or liquid is 140 ° C. Mineral oil, paraffin wax, liquid silicone, fluorine oil, liquid polyethers, liquid polybutenes, liquid polybutadienes, carboxylates, sulfonates, amine salts, carboxylic acid compounds, fluorine compounds, compounds with sulfone bonds, etc. Can be mentioned. One plasticizer may be used alone, or two or more plasticizers may be used in combination.
Specifically, as a plasticizer, it describes in the item of the plasticizer of P29-64 of "Plastics compounding agent" (The Taiseisha issue November 30, 1987 2nd edition issue), P49 to P50. The plasticizers (TCP, TOP, PS ESBO, etc.) listed in Table 4 and Table 6 of P52 to P54 can be used.

Examples of the ester compound include tetraglycerin tristearate, glycerin tristearate, stearyl stearate, glycerin monostearate, sorbitan monostearate, ethylene carbonate or distearyl carbonate.
Examples of the amide compound include ethylene bis stearamide and hexamethylene bis stearamide.
Examples of the alcohol compound include stearyl alcohol, oleyl alcohol, and dodecylphenol.
Examples of the amine compound include dihydrodiethyl stearylamine and laurylamine.
Examples of the epoxy compound include epoxy soybean oil.
Examples of the ether compound include triethylene glycol.
Examples of the mineral oil include kerosene and naphthenic oil.

Examples of the carboxylate include calcium stearate and sodium oleate.
Examples of the sulfonate include sodium dodecylbenzenesulfonate.
Examples of the amine salt include stearyl dimethyl betaine or lauryl trimethyl ammonium chloride.
Examples of the carboxylic acid compound include stearic acid or oleic acid, or derivatives thereof (excluding salts).
Examples of the compound having a sulfone bond include sulfolane and dipropylsulfonic acid.

  The plasticizer used in the present invention is more preferably an ester compound, an amide compound or a compound having a sulfone bond. In particular, in the present invention, the plasticizer is an ester or amide composed of a saturated fatty acid, and more preferably a plasticizer mainly composed of ethylene carbonate.

  In this invention, it is preferable that the compounding ratio of a plasticizer is 1-30 mass parts with respect to 100 mass parts of high density polyethylene resins. A more preferable blending ratio is 1 to 20 parts by mass with respect to 100 parts by mass of the resin. If the blending ratio of the plasticizer is less than 1 part by mass, the desired good stretchability is hardly exhibited, and the appearance and texture are likely to deteriorate. On the other hand, when the blending ratio of the plasticizer exceeds 30 parts by mass, problems in the process such as resin burning are likely to occur during film formation.

Furthermore, in the porous film of the present invention, additives generally blended in the resin composition, for example, antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, neutralizers, antifogging agents, antiblocking agents. Further, an antistatic agent, a slip agent, a colorant, and the like may be blended within a range that does not impair the properties of the porous film.
Specifically, as the antioxidant described in P154 to P158 of the “plastics compounding agent”, the ultraviolet absorber described in P178 to P182, and the antistatic agent described in P271 to P275. A surfactant and a lubricant described in P283 to 294 are appropriately blended as necessary.

As described above, the porous film of the present invention is formed by processing the melt-kneaded product of the resin composition as a film, and stretching the formed film to provide pores starting from the filler. When the porous film is used as a battery separator, the average thickness is 5 μm or more and 40 μm or less, preferably 30 μm or less. This is because if the thickness is less than 5 μm, the film is easily torn, while if it exceeds 40 μm, the battery area is reduced and the battery capacity is reduced when wound in a predetermined battery can as a battery separator. by.
Further, the maximum value and the minimum value of the thickness are ± 25% or less of the average thickness, that is, the thickness fluctuation (thickness accuracy) is ± 25% or less. The maximum value and the minimum value of the thickness are more preferably ± 20% or less of the average thickness, and particularly preferably ± 15% or less. When the thickness fluctuation exceeds ± 25% of the average thickness, a partial pressure is applied when the film is wound, and the insulation is lowered when used as a battery separator.

  In addition, the average thickness of the said porous film is a value obtained by measuring 30 places unspecified in a surface with a 1/1000 mm dial gauge, and calculating the average. Moreover, the maximum value of the thickness refers to the largest value among the measured values at the 30 locations, and the minimum value of the thickness refers to the smallest value among the measured values at the 30 locations. The thickness fluctuation is a value calculated by the formula: {(maximum thickness or minimum thickness−average thickness) / average thickness} × 100 (%).

In particular, in a preferred embodiment of the present invention, the filler (B) is filled in a relatively large amount of 100 to 200 parts by mass with respect to 100 parts by mass of the high-density polyethylene (A), and the filler (B) is blended. Since the strength is relatively low as compared with the case where the amount is small or no filler is blended, it is important to sufficiently control the thickness and thickness accuracy.
The thickness of the porous film can be freely adjusted by the type or blending amount of the high-density polyethylene resin (A), filler (B) and plasticizer (C), and stretching conditions (stretching ratio, stretching temperature, etc.).

The porous film of the present invention is produced, for example, by the following method.
First, each component is mixed with a powder mixer such as a Henschel mixer, a super mixer, or a tumbler mixer. At this time, it is preferable that the high-density polyethylene is in advance in the form of powder or pellets, the filler is in the form of powder, the plasticizer is in the form of powder, and the stretching aid is in advance in the form of pellets.
Next, the mixture is heated and kneaded with a uniaxial or biaxial kneader, a kneader or the like. Thereafter, it may be pelletized and transferred to a film forming step, or may be directly formed by a molding machine without being pelletized. The pellets may be temporarily stored in equipment or containers for storing raw materials such as silos and hopper flexible containers.

  In the present invention, the moisture content of the pellets is usually 1000 ppm or less, preferably 700 ppm or less, and melt-molded to form a film. This is because if the moisture content of the pellet is larger than 1000 ppm, gel and pinball are extremely generated, which is not preferable. On the other hand, when the molten mixture is directly taken to the film forming process without pelletization, vacuum degassing or open degassing is performed in the middle from the melt kneading process to the film forming process so that the moisture content of the molten mixture is 1000 ppm or less. Preferably it is done.

  Thereafter, a film (raw sheet) is obtained by melting and forming a film using an extruder or the like under a temperature condition higher than the melting point of the high-density polyethylene resin, preferably higher than the melting point + 20 ° C. and lower than the decomposition temperature. Specific examples of the film forming method include T-die molding, calendar molding, and press molding. The thickness of the film (raw fabric sheet) formed in this way can be selected as appropriate as long as the stretchability and the like are not impaired, but is preferably in the range of 0.02 to 2 mm.

The molded resin film (raw fabric sheet) is stretched in at least a uniaxial direction (uniaxial stretching) by a method such as roll stretching, tenter stretching, simultaneous biaxial stretching, and rolling, preferably in the film longitudinal direction (longitudinal direction). Is stretched (biaxial stretching) in the biaxial direction in the transverse direction. Such stretching treatment is preferably performed in the vicinity of the softening point of the resin (measured according to JIS K6760). A number of pores can be provided by peeling the interface between the resin and the filler by the above stretching. In addition, in order to stabilize an aperture diameter, you may heat-process after extending | stretching.
The stretching ratio in the stretching step may be appropriately selected in accordance with the film breakage during stretching, the air permeability of the obtained film, the hardness of the film, or the like. Specifically, for example, in the case of biaxial stretching, the stretching ratio in the longitudinal and lateral directions is 1.5 times or more in at least one direction, preferably 2 times or more.

The pores formed by stretching the porous film have a three-dimensional network shape and are communicated with the opening on both sides of the film to form a battery separator. Gas or water vapor can be transmitted and liquid droplets cannot be transmitted. . Specifically, the pore diameter is set to 0.3 μm or less, water droplets of 100 to 3000 μm are not transmitted, and water vapor of about 0.0004 μm can be transmitted.
More specifically, the porous film of the present invention preferably has an air permeability of 50 to 1000 (sec / 100 cc), more preferably 100 to 500 (sec / 100 cc), More preferably, it is 300 (sec / 100 cc). If the air permeability is 50 to 1000 (sec / 100 cc), if it is less than 50 (sec / 100 cc), the impregnation and retention of the electrolyte will be reduced and the capacity of the secondary battery will be reduced, and the cycle performance will be reduced. May decrease. On the other hand, if the air permeability exceeds 1000 (sec / 100 cc), the ion conductivity becomes low, and sufficient battery characteristics cannot be obtained when used as a separator for a nonaqueous electrolyte battery.
In addition, the said air permeability (Gurley value) has measured the air permeability (sec / 100cc) based on JISP8117.

  The film thus obtained can be used for various applications such as packaging, hygiene, livestock, agriculture, construction, medical, separation membrane, light diffusion plate, battery separator, etc., but non-aqueous electrolyte battery separator Can be suitably used, and a good nonaqueous electrolyte battery can be obtained.

The nonaqueous electrolyte battery containing the nonelectrolyte battery separator made of the porous film of the present invention closes the fine holes opened in the separator when it is in a high temperature (140 to 160 ° C.) state from the viewpoint of safety. As a result, there is a need for a shutdown function that blocks ion conduction inside the battery and prevents subsequent temperature rise inside the battery. On the other hand, if the shutdown temperature is lower than 130 ° C., when the temperature rises to 130 ° C. or higher during charging, the shutdown occurs and the device cannot be used repeatedly any more.
Therefore, in the porous film of the present invention, the shutdown temperature is set to 130 ° C. or more and 140 ° C. or less.

  And the non-aqueous electrolyte battery of the present invention has an insulation failure rate after temperature rise of 20% or less. This is because the initial failure rate as a battery increases when the insulation failure rate after temperature rise is 25% or more. In addition, the insulation defect rate after temperature rising is measured by the method as described in the Example mentioned later.

As described above, the porous film of the present invention is a thin film of about 5 to 40 μm, and its thickness accuracy is increased to ± 25% or less to keep the thickness substantially uniform. When placed in a battery can by interposing between a plate and a negative electrode plate, the separator can be prevented from being locally loaded, and as a result, the separator can be prevented from being damaged, and insulation is reliably maintained. can do. And since the thickness of a separator is made thin with 5-40 micrometers, the filling amount of the positive electrode plate and negative electrode plate in a battery can can be increased, and battery capacity can be raised.
And since the fine filler of 0.1-25 micrometers is used as a filler, the porous film of thickness accuracy +/- 25% or less can be formed by extending | stretching with said 5-40 micrometers.

  Furthermore, since it is mainly composed of a high density polyethylene resin, it can exhibit an excellent shutdown function in a high temperature state when used in a non-aqueous electrolyte battery separator. And since it contains a high density polyethylene resin, it is hard to tear and handling property is improving. In addition, by using the polyethylene resin, a film having excellent air permeability and heat resistance can be obtained.

Furthermore, a stretching method is employed to provide a fine pore structure that maintains appropriate air permeability, and the manufacturing cost can be reduced.
Moreover, the physical properties of the porous film of the present invention can be freely adjusted by the blending amount and type of resin, filler and plasticizer, and stretching conditions (stretching ratio, stretching temperature, etc.). Therefore, a porous film having desired physical properties according to the application can be obtained by variously changing the conditions.

Hereinafter, the present invention will be specifically described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view of a porous film. A porous film 1 according to an embodiment includes a three-dimensional network of pores 1a, and the pores 1a communicate with both surfaces 1b and 1c of the porous film. The air permeability of the film is in the range of 100 to 1000 (sec / 100 cc).
The thickness of the porous film 1a is 5 to 40 μm, and the thickness fluctuation is ± 25% or less of the average thickness.

In the porous film, the resin has a density of 0.95 g / cm ³ or more and 0.97 g / cm ³ or less and a high-density polyethylene having a melt flow rate of 1 g / 10 min or less, and an average particle size of 0.1 to 25 μm. Using an inorganic filler made of barium sulfate or calcium carbonate and using ethylene carbonate, sorbitan monostearate, ethylene bis-stearic acid amide or glycerine tristearate as a plasticizer, which has a melting point of 25 ° C. or higher and is liquid or solid at 140 ° C. ing. In this embodiment, the blending ratio of the filler and the plasticizer is 100 to 200 parts by mass for the filler and 5 to 10 parts by mass for the plasticizer with respect to 100 parts by mass of the high-density polyethylene.

  The above raw materials are mixed and kneaded to disperse the filler in the resin. The kneaded material is heated and melted at a required temperature, and then formed with a T-die to form a film (raw sheet). The molding temperature can be appropriately selected according to the type of resin and the like, but is 120 to 200 ° C. The obtained film (raw sheet) has a thickness of 0.02 to 2 mm.

The film is first stretched in a biaxial stretching machine at a stretching ratio of 2 to 4.5 times in the longitudinal direction (MD direction) of the film, and then stretched at a stretching ratio of 2 to 4.5 times in the direction perpendicular to the longitudinal direction (TD direction). Stretch with. By such biaxial stretching in the vertical and horizontal directions, as shown in FIG. 2, the film 10 in which the filler 12 is dispersed in the resin 11 is peeled off at the interface between the resin 11 and the filler 12, and this peeling is caused. The part which became the void | hole 1a is obtained, and the porous film 1 is obtained. At this time, the thickness of the porous film 1 is 5 to 40 μm as described above, and the thickness fluctuation is less than ± 25. Such a porous film 1 is obtained as a continuous material and wound up in a coil shape.
The stretching is not limited to biaxial stretching, and may be uniaxial stretching, in which case the film is stretched 5 times in the longitudinal direction.

  In this embodiment, the obtained porous film 1 is cut into a required length to form a separator 1 'for a nonaqueous electrolyte battery. The separator 1 ′ is spirally wound between the positive electrode plate 21 and the negative electrode plate 22, and is accommodated in the cylindrical lithium secondary battery 20 shown in FIG. 3.

The lithium secondary battery containing the porous film of the present invention as a separator will be described in detail.
As the electrolyte, for example, an electrolyte in which a lithium salt is used as a supporting electrolyte and this is dissolved in an organic solvent is used. Examples of the organic solvent include, but are not limited to, esters such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate, methyl propionate, and butyl acetate; nitriles such as acetonitrile Ethers such as 1,2-dimethoxyethane, 1,2-dimethoxymethane, dimethoxypropane, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 4-methyl-1,3-dioxolane; Can be used. These solvents may be used alone or as a mixed solvent of two or more.
In the present embodiment, the electrolyte charged in the lithium secondary battery can equipped with the separator is composed of 1.4 mol / L of L1PF6 in a mixed solvent in which 2 parts by mass of methyl ethyl carbonate is blended with 1 part by mass of ethylene carbonate. The electrolyte is dissolved at a rate.

  As the negative electrode, a material in which an alkali metal or a compound containing an alkali metal is integrated with a current collecting material such as a stainless copper net is used. At that time, examples of the alkali metal include lithium, sodium, and potassium. Examples of the compound containing an alkali metal include an alloy of an alkali metal and aluminum, lead, indium, cadmium, tin or magnesium, a compound of an alkali metal and a carbon material, a low potential alkali metal and a metal oxide, or And compounds with sulfides. When a carbon material is used for the negative electrode, the carbon material may be any material that can be doped / undoped with lithium ions. For example, graphite, pyrolytic carbons, cokes, glassy carbons, and fired bodies of organic polymer compounds Mesocarbon microbeads, carbon fibers, activated carbon, or the like can be used.

  In the present embodiment, the negative electrode plate in the lithium secondary battery can equipped with the separator is prepared by mixing a solution prepared by dissolving vinylidene fluoride in N-methylpyrrolidone and a carbon material having an average particle size of 10 μm into a slurry. The mixture slurry is passed through a 70-mesh net to remove large particles, and then uniformly applied to both sides of a negative electrode current collector made of a strip-shaped copper foil having a thickness of 18 μm, dried, and then compressed by a roll press. Molded and cut into a strip-shaped negative electrode plate.

  As the positive electrode, a mixture obtained by appropriately adding a conductive additive, a binder such as polytetrafluoroethylene to the positive electrode active material, and using a current collector material such as a stainless copper net as a core material to form a molded body is used. It is done. Examples of the positive electrode active material include metal oxides such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, manganese dioxide, vanadium pentoxide, and chromium oxide, and metal sulfides such as molybdenum disulfide. It is done.

  In this embodiment, the positive electrode plate in the lithium secondary battery can on which the separator is mounted is, in a more preferred embodiment, added phosphorus-like graphite as a conductive additive to lithium cobalt oxide (LiCoO2) at a weight ratio of 90: 5. After mixing, this mixture and a solution in which polyvinylidene fluoride was dissolved in N-methylpyrrolidone were mixed to form a slurry. After this positive electrode mixture slurry was passed through a 70-mesh net, a large one was removed. A positive electrode current collector made of 20 μm aluminum foil is uniformly coated on both sides and dried, then compression molded by a roll press machine, and then cut into a strip-shaped positive electrode plate.

Both electrodes of the positive electrode plate 21 and the negative electrode plate 22 are overlapped with each other through the separator 1 ′, wound in a spiral shape, and stopped on the outside with a winding tape to form a wound body.
A wound body in which the positive electrode plate 21, the separator 1 ′ and the negative electrode plate 22 are integrally wound is filled in a bottomed cylindrical battery case and welded to the positive and negative electrode lead bodies 24 and 25. Next, the electrolyte is poured into the battery can and sufficiently infiltrated into the separator 1 ′, etc., and then the positive electrode lid 27 is sealed through the gasket 26 around the opening periphery of the battery can, and precharging and aging are performed. A cylindrical secondary lithium battery is manufactured.

  Since the separator made of the porous film 1 has an insulating property, it prevents a short circuit between the positive electrode plate 21 and the negative electrode plate 22 that are in direct contact with both surfaces, and lithium ions can pass through the pores 1a but cannot pass through the liquid. The electrolyte can be diffused and retained.

  The porous film of the present invention and the porous film of the comparative example were prepared, and the thickness, thickness accuracy, air permeability, shutdown temperature, and insulation failure rate after the temperature increase were measured.

"Example 1"
High-density polyethylene [HY430P manufactured by Nippon Polychem Co., Ltd., density: 0.955 g / cm ³ , melt flow rate: 0.8 g / 10 min] 100 parts by mass, barium sulfate having an average particle size of 0.66 μm [manufactured by Sakai Chemical Industry B -55] 150 parts by mass were blended and compounded, and then T-die molding was performed at a temperature of 200 ° C. to obtain an original fabric sheet. The thickness of the original fabric sheet was 100 μm on average.
Next, the obtained raw fabric sheet was uniaxially stretched by 5.0 times in the MD direction of the sheet at 80 ° C. to obtain a porous film.
The obtained porous film had a thickness average of 14 μm, a thickness fluctuation of ± 25%, and an air permeability of 560 sec / 100 cc.

"Example 2"
High density polyethylene [7000FP made by Sumitomo Mitsui Polyolefin Co., Ltd., density: 0.954 g / cm ³ , melt flow rate: 0.04 g / 10 min] 100 parts by mass, barium sulfate [B-55 by Sakai Chemical Co., Ltd.] 160 parts by mass, High castor wax as plasticizer [HCOP manufactured by Toyokuni Seiyaku Co., Ltd.
Melting point 86 ° C. 140 ° C. The weight reduction rate after heating for 1 hour was 2.4%] A raw sheet having an average thickness of 100 μm was formed in the same manner as in Example 1 except that the content was 7 parts by mass. The sheet was stretched 4.5 times in the MD direction and then biaxially stretched 2.9 times in the TD direction to obtain a porous film.
The obtained porous film had a thickness average of 27 μm after stretching, a thickness accuracy of ± 14%, and an air permeability of 75 sec / 100 cc.

"Example 3"
A raw sheet was formed in the same manner as in Example 2 except that the amount of barium sulfate added was 105 parts by mass and the high caster wax (HCOP manufactured by Toyokuni Seiyaku Co., Ltd.) was 5 parts by mass. The sheet was stretched 4.5 times in the MD direction and then stretched in the TD direction 2.9 to obtain a porous film.
The obtained porous film had an average thickness of 23 μm, a thickness accuracy of ± 17%, and an air permeability of 220 sec / 100 cc.

"Example 4"
A raw sheet was formed in the same manner as in Example 2, except that the amount of barium sulfate added was 84 parts by mass and the high caster wax (HCOP manufactured by Toyokuni Seiyaku Co., Ltd.) was 3 parts by mass. The sheet was stretched 5.5 times in the MD direction and then stretched in the TD direction 3.4 to obtain a porous film.
The obtained porous film had a thickness average of 27 μm, a thickness accuracy of ± 5%, and an air permeability of 90 sec / 100 cc.

"Example 5"
Trioctanoic acid trioctanoate as a plasticizer [Diaserizer D600, J Plus Corp. Melting point: 45 ° C. 140 ° C. Weight reduction rate after heating at 140 ° C. for 1 hour] was changed in the same manner as in Example 2. A raw sheet was formed. The sheet was stretched 4.0 times in the MD direction and then 2.8 times in the TD direction to obtain a porous film.
The obtained porous film had an average thickness of 26 μm, a thickness accuracy of ± 8%, and an air permeability of 150 sec / 100 cc.

"Example 6"
The same procedure as in Example 1 was conducted except that ethylene carbonate [reagent melting point 36 ° C. boiling point 238 ° C., manufactured by Wako Pure Chemical Industries, Ltd.] was changed to 10 parts by mass as the plasticizer.
The obtained porous film had a thickness average of 21 μm, a thickness accuracy of ± 11%, and an air permeability of 250 sec / 100 cc.

"Comparative Example 1"
Linear low density polyethylene [Ultzex 2023FP manufactured by Mitsui Chemicals, density: 0.920 g / cm ³ melt flow rate: 2.1 g / 10 min] to 100 parts by mass, barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] 160 parts by mass and 7 parts by mass of high caster wax were blended and compounded, and then T-die molding was performed at a temperature of 200 ° C. to obtain an original sheet. The thickness of the original fabric sheet was 60 μm on average.
Next, the obtained raw sheet was made into a porous film by stretching 4.0 times in the longitudinal direction (MD) of the sheet at 60 ° C. and uniaxial stretching.
The obtained porous film had an average thickness of 30 μm, a thickness fluctuation of ± 20%, and an air permeability of 1800 sec / 100 cc.

"Comparative Example 2"
The plasticizer was molded in the same manner as in Example 1 except that 3 parts by mass of glycerin trioctanoate (reagent melting point: 5 ° C., 140 ° C. for 1 hour after heating for 1 hour, 2%) manufactured by Wako Pure Chemical Industries, Ltd. was added. .
The obtained porous film had a thickness average of 35 μm, a thickness fluctuation of ± 39%, and an air permeability of 250 sec / 100 cc.

“Comparative Example 3”
Except for adding 5 parts by mass to dimethyl silicone as a plasticizer [trade name TSF451-100 manufactured by GE Toshiba Silicone Co., Ltd., kinematic viscosity at 25 ° C. 100 mm 2 / S weight reduction rate after heating at 140 ° C. for 1 hour] Molded in the same manner as in Example 1.
The obtained porous film had an average thickness of 31 μm, a thickness fluctuation of ± 45%, and an air permeability of 155 sec / 100 cc.

The thickness, thickness fluctuation, and air permeability of Examples 1 to 6 and Comparative Examples 1 to 3 were measured as follows.
(Measurement of thickness)
30 thicknesses were measured indefinitely with a 1/1000 mm dial gauge, and the average value was calculated.
(Thickness fluctuation = thickness accuracy measurement)
Based on the following equation, the thickness fluctuation was calculated from the largest value (maximum thickness), the smallest value (minimum thickness), and the calculated average value among the 30 measured values measured by the measurement method. In the table, the larger one of the values calculated by the following equation is shown.
(Maximum thickness-average thickness) / average thickness x 100 (%)
(Minimum thickness-average thickness) / average thickness x 100 (%)
(Air permeability)
In accordance with JIS P 8117, the air permeability (seconds / 100 cc) was measured.

When the porous films of Examples 1 to 6 and Comparative Examples 1 to 3 were used as battery separators, the shutdown temperature and insulation were measured by the following methods.
(Measurement of shutdown temperature)
The porous film was sandwiched between aluminum plates with 100 mmφ holes, allowed to stand in an oven for 2 minutes, and then air permeability was measured. The oven temperature of 3000 seconds / 100 cc or more is defined as the shutdown temperature.
When this temperature is higher than 140 ° C., when it is incorporated as a battery, the temperature rises abnormally during charging and discharging, and the permeability of the porous film does not fall, so there is a risk of smoke and fire.

(Insulation)
The above-mentioned 100 electrode groups were heated at a rate of 10 ° C./min, placed in an oven at 160 ° C. for 1 hour, the insulation resistance between the positive and negative electrodes was measured, and the number of 1 MΩ or less was counted. When the number of 1 MΩ or less is y, the insulation failure rate after the temperature rise is y%. If this ratio is large, the initial failure rate as a battery increases. When the insulation failure rate is 20% or less, it is indicated by “◯”, and when it exceeds 20%, it is shown as “X” in Table 1.

  Materials of Examples 1 to 6 and Comparative Examples 1 to 3, blending ratio, average thickness of porous film measured by the above method, thickness fluctuation, air permeability, shutdown temperature, insulation failure rate after temperature rise Is shown in Table 1 below. In the table, the blending ratio of the filler and the plasticizer indicates a ratio (parts by mass) to 100 parts by mass of the high-density polyethylene.

In the porous films of Examples 1 to 6, both the average thickness and thickness fluctuation were within the numerical range defined in the present invention. On the other hand, since the porous film of Comparative Example 1 used low density polyethylene instead of high density polyethylene, the air permeability was 1800 [sec / 100 cc].
Moreover, the shutdown temperature was 130 degreeC or more and 140 degrees C or less in Examples 1-6. Regarding the insulation after the temperature rise, Examples 1 to 6 were ◯, but Comparative Examples 1 to 3 were x.
From the above points, it was confirmed that the porous films of Examples 1 to 6 were superior to the porous films of Comparative Examples 1 to 3 particularly when used as battery separators.

  The film thus obtained can be used for various applications such as packaging, hygiene, livestock, agriculture, construction, medical, separation membrane, light diffusion plate, battery separator, etc., but non-aqueous electrolyte battery separator Can be suitably used, and a good nonaqueous electrolyte battery can be obtained.

It is a cross-sectional schematic diagram of the said porous film. It is explanatory drawing which shows the method in which the hole by extending | stretching is formed. It is a partially broken perspective view which shows the separator in a battery.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Porous film 1a Hole 1 'Separator 10 Film 11 Resin 12 Filler 20 Battery 21 Positive electrode plate 22 Negative electrode plate

Claims (9)

  A film made of a resin composition composed of at least two components of a high-density polyethylene resin (A) and a filler (B) is provided with pores by stretching, and has an average thickness of 5 to 40 μm and a maximum thickness. A porous film characterized in that the value and the minimum value are ± 25% or less of the average thickness.   The porous film according to claim 1, wherein the filler (B) has an average particle size of 0.1 to 25 μm.   The porous film according to claim 1 or 2, wherein the filler (B) is an inorganic filler made of barium sulfate or calcium carbonate.   The porous film according to any one of claims 1 to 3, wherein 50 to 400 parts by mass of the filler (B) is blended with respect to 100 parts by mass of the high-density polyethylene resin (A).   The porous film according to any one of claims 1 to 4, wherein a plasticizer (C) is added.   The porous film according to claim 5, wherein the plasticizer (C) is a liquid or solid plasticizer having a melting point of 25 ° C or higher and 140 ° C.   The porous film according to any one of claims 1 to 6, wherein the stretching is uniaxial stretching or biaxial stretching and is stretched 1.5 times or more in at least one direction.   A battery separator comprising the porous film according to any one of claims 1 to 7.   A battery containing the battery separator according to claim 8.

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