JP2005343936A - Porous film, separator for non-aqueous electrolyte cell and non-aqueous electrolyte cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film suitable as a non-aqueous electrolyte cell separator having improved tear resistance and handling property and excellent electrical insulation to effectively prevent the generation of short circuit, and exhibiting shut-down function at high temperature (140-160°C) to prevent the temperature increase in the cell. <P>SOLUTION: A film composed of a resin composition containing (A) a resin containing a high-density polyethylene resin and a low-density polyethylene resin and (B) a filler having an average particle diameter of 0.1-25μm is drawn to form pores. The film has an average thickness of ≥5μm and ≤40μm and the maximum value and the minimum value of the thickness are fallen within the range of ±25% of the average thickness. <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 a high volumetric efficiency and are reduced in size and weight when installed in 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 non-aqueous 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. ing. 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.

The separator of the lithium ion secondary battery is in direct contact with the positive electrode and the negative electrode and is interposed between the two electrodes, so that insulation is required from the viewpoint of preventing an internal short circuit, and the permeability of the lithium ion passage and the electrolyte solution In order to provide a diffusion / holding function, a fine pore structure is used, and the fine pores are required to have a shutdown function for melting and blocking 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. A separator made of a polyolefin resin having a shutdown function capable of being provided is provided.
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, a plasticizer consisting of a large amount of mineral oil is mixed into the polyolefin resin and inorganic powder, and this 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.
Moreover, since the porous film disclosed in Patent Document 2 uses a polypropylene resin, it is difficult to produce 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. In addition, when the positive electrode, the separator, and the negative electrode are wound in a spiral shape by using a winding core, the battery is difficult to cut and cannot be stably manufactured because it is easy to tear.

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 inventors, 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, has improved handling properties that are difficult to tear, and exhibits excellent insulation when used as a nonaqueous electrolyte battery separator, effectively preventing the occurrence of short circuits. An object of the present invention is to provide a porous film that can perform a shutdown function when it is in a high temperature (140 to 160 ° C.) state and can prevent a temperature rise inside the battery.

  The present invention is a film comprising a resin composition containing a resin (A) containing a high-density polyethylene resin and a low-density polyethylene resin, and a filler (B), and the film starts from the filler (B) by stretching. Provided with a porous film, wherein the average thickness is 5 μm or more and 40 μm or less, and the maximum value and the minimum value of the thickness are less than ± 25% of the average thickness. ing.

Since the porous film of the present invention contains a high-density polyethylene resin, it can exhibit an excellent shutdown function in an abnormally high temperature state when used as a non-aqueous electrolyte battery separator. Furthermore, since the porous film of the present invention contains a low density polyethylene resin, it is difficult to tear, the handling property is improved, the tensile property is improved, and the film thickness system is improved. In addition, since it is made of a polyethylene resin, which is a polyolefin resin, the film has excellent air permeability and heat resistance.
Furthermore, by setting the average thickness of the film to 5 μm or more and 40 μm or less and controlling the maximum value and the minimum value of the film to less than ± 25% of the average thickness, excellent insulating properties can be exhibited. Specifically, since the thickness is uniform, the thin portion is not substantially torn when pressure is applied. In addition, since it has a predetermined thickness, 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 being subjected to a certain pressure, and the occurrence of a short circuit can be prevented.

The high density polyethylene used in the present invention preferably has a density of 0.94 g / cm ³ or more, and more preferably 0.95 g in order to more effectively exhibit a shutdown function in an abnormally high temperature (140 to 160 ° C.) state. / Cm ³ or more is more preferable. The upper limit of the density is 0.97 g / cm ³ .
For the high-density polyethylene, the melt flow rate is 1.5 g / 10 min or less, preferably 1.1 g / 10 min or less, more preferably 0.6 g in order to maintain the strength of the film to be molded. / 10 min or less, particularly preferably 0.1 g / 10 min or less. When the melt flow rate is larger than 1.5 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.

On the other hand, with low-density polyethylene used in the present invention, it is preferable density to hardly torn the film is less than 0.86 g / cm ³ or more 0.94 g / cm ^3.
Examples of the low density polyethylene include linear low density polyethylene and low density polyethylene radical-polymerized by a high pressure method.
The linear low density polyethylene is a copolymer of ethylene and an α-olefin, and examples of the α-olefin include propylene, butene, and hexene.
The low density polyethylene radically polymerized by the high pressure method may be a homopolymer or a copolymer with an α-olefin. Examples of the α-olefin comonomer include propylene, butene, and hexene.

  The present invention is characterized by using a combination of a high-density polyethylene resin and a low-density polyethylene. However, other thermoplastic resins may be used in combination. Examples of other thermoplastic resins include amorphous thermoplastic resins (preferably resins having a glass transition temperature Tg of 10 ° C. to 200 ° C.), polypropylene, polybutene, propylene ethylene block copolymer, propylene ethylene random copolymer, and the like. Is mentioned. However, the resin (A) used in the present invention is preferably composed of a high density polyethylene resin and a low density polyethylene.

  The ratio of the high-density polyethylene resin and the low-density polyethylene in the resin component can be appropriately selected depending on the use or physical properties of the obtained porous film. When the resin (A) is 100 parts by mass, the high-density polyethylene resin is 99 to 99 parts by mass. It is preferable that 70 mass parts and low density polyethylene shall be 1-30 mass parts. When the high density polyethylene resin is less than 70 parts by mass and the low density polyethylene is more than 30 parts by mass, the mechanical strength is lowered.

  The filler used in the present invention is in the form of a filler having an average particle size of 0.1 to 25 μm. 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 becomes difficult to make the film thin, and the mechanical strength of the film is lowered. 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 resin and the filler is preferably 50 parts by mass or more and 400 parts by mass or less with respect to 100 parts by mass of the resin. When the blending amount of the filler is less than 50 parts by mass, the desired good air permeability is hardly expressed, and the appearance and texture are liable to deteriorate. On the other hand, when the blending amount of the filler exceeds 400 parts by mass, problems in the process such as resin burning are likely to occur during film forming, and the film strength is greatly reduced.
More preferably, the filler is blended at a ratio of 60 parts by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the resin.

  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 preferably has a boiling point of 140 ° C. or higher. When a plasticizer having a boiling point of less than 140 ° C. is used, 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 preferably has a boiling point of 140 ° C. or higher and a melting point of 25 ° C. or higher. If a plasticizer having a melting point of 25 ° C. or higher is used, the rigidity of the film is improved, and handling in the assembling 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.

Usable as plasticizers in the present invention are ester compounds, amide compounds, fatty acids, alcohol compounds, amine compounds, epoxy compounds, ether compounds, mineral oil, paraffin wax, liquid silicone, fluorine oil, liquid polyethers, liquid polybutene , Liquid polybutadienes, carboxylates, sulfonates, amine salts, carboxylic acid compounds, fluorine-based compounds, compounds having a sulfone bond, and the like. 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.
However, those having a melting point of 25 ° C. or higher and a boiling point of 140 ° C. or higher specified by the present invention are 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 this invention, it is preferable that ethylene carbonate or sulfolane is included as a plasticizer, and it is preferable that these ratios are 80-100 mass parts in 100 mass parts of components of a plasticizer.

  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 resin. A more preferable blending ratio is 1 to 20 parts by mass with respect to 100 parts by mass of the resin. When the blending ratio of the plasticizer is less than 1 part by mass, the desired good stretchability is hardly expressed, and the appearance and texture are liable 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, more preferably 5 to 30 μm. When the thickness is less than 5 μm, the film is easily broken, whereas when the thickness exceeds 40 μm. When being wound and stored in a predetermined battery can as a battery separator, the battery area is reduced, and consequently the battery capacity is reduced.
In addition, the maximum value and the minimum value of the thickness are less than ± 25% of the average thickness, that is, the thickness fluctuation is less than ± 25%. The average thickness of the maximum value and the minimum value of the thickness is more preferably ± 20% or less, particularly ± 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 (%).

Particularly, in a preferred embodiment of the present invention, the filler (B) is filled in a relatively large amount of 50 to 400 parts by mass with respect to 100 parts by mass of the thermoplastic resin (A), and the blending of the filler (B) is small. Since the strength is relatively low as compared with the case where 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 types or blending amounts of the thermoplastic 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 to preliminarily prepare the thermoplastic resin in powder or pellet form, the filler in powder form, the plasticizer in powder form, and the stretching aid in pellet form.
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, the film (raw sheet) is melted and formed into a film using an extruder or the like under a temperature condition that is higher than the melting point of the high-density polyethylene resin and the low-density polyethylene resin, preferably higher than the melting point + 20 ° C. and lower than the decomposition temperature Have gained. 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 vertical and horizontal directions is 1.5 times or more in at least one direction, preferably 2.0 or more in either the vertical or horizontal direction.

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 500 (sec / 100 cc), more preferably 100 to 500 (sec / 100 cc), More preferably, it is 300 (sec / 100 cc).
The air permeability is set to 50 to 500 (sec / 100 cc). When the air permeability is less than 50 (sec / 100 cc), the impregnating / holding ability of the electrolytic solution is lowered and the capacity of the secondary battery is reduced. There is a risk that the sex will decline. On the other hand, if the air permeability exceeds 500 (sec / 100 cc), the ion conductivity becomes low, and it becomes difficult to obtain sufficient battery characteristics 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.

  When the porous form of the present invention is used as a battery separator, the porous form of the present invention is usually interposed between a positive electrode and a negative electrode, wound in a spiral shape, and stored in a battery can. In the present invention, the winding failure rate at this time is preferably 15% or less, and more preferably 10% or less. When the winding defect rate is 15% or less, it is more suitable for industrial production, such as good handling in the battery assembling process and low occurrence rate of defective products. The winding failure rate is measured by the method described in the examples described later.

  The non-aqueous electrolyte battery containing the non-electrolyte battery separator made of the porous film of the present invention closes the fine holes opened in the separator when 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 can block ion conduction inside the battery and prevent a subsequent temperature rise inside the battery. As an index of this function, the non-aqueous electrolyte battery of the present invention has an insulation failure rate after temperature rise of 20% or less, more preferably 10% or less. This is because the initial failure rate as a battery increases when the insulation failure rate after temperature rise is 20% 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, since the porous film of the present invention contains a high-density polyethylene resin, it can exhibit an excellent shutdown function in a high temperature state when used in a nonaqueous electrolyte battery separator. Furthermore, since the porous film of the present invention contains a low-density polyethylene resin, it is difficult to tear and the handling property is improved. In addition, by using these polyethylene resins, a film excellent in air permeability and heat resistance can be obtained.

  In addition, since the porous film of the present invention has a high thickness accuracy and the thickness is substantially uniform, it is interposed between a positive electrode plate and a negative electrode plate as a battery separator and wound in a spiral shape in a battery can. When accommodated, it is possible to suppress a local load on the separator, and as a result, the separator can be prevented from being damaged and the insulation can be reliably maintained. 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.

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. The porous film 1 includes three-dimensional network-like pores 1a, and the pores 1a communicate with both surfaces 1b and 1c of the porous film. The temperament is in the range of 100 to 500 (sec / 100 cc).
The thickness of the porous film 1a is 5 to 40 μm, and the thickness fluctuation is less than ± 25% of the average thickness.

In the porous film, the resin has a density of 0.95 g / cm ^{3 to} 0.97 g / cm ³ and a melt flow rate of 1.5 g / 10 min or less, and a density of 0.86 g / cm ^3. The low density polyethylene which is 0.94 g / cm ³ or more is used.
Barium sulfate or calcium carbonate having an average particle size of 0.1 to 25 μm is used as a filler, and ethylene carbonate, sorbitan monostearate, ethylene bis-stearic acid amide having a melting point of 25 ° C. or higher and a boiling point of 140 ° C. or higher as a plasticizer Or glycerin tristearate is used. The compounding ratio of the filler and the plasticizer is such that the filler is 100 to 150 parts by mass and the plasticizer is 5 to 10 parts by mass with respect to 100 parts by mass of the resin.

  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 the resin, but is 150 to 250 ° C, preferably 180 to 220 ° C. The obtained film (raw sheet) has a thickness of 0.02 to 2 mm.

  The film is first stretched in the longitudinal direction (longitudinal direction) of the film at a stretching ratio of 4 to 4.5 times with a biaxial stretching machine, and then stretched in the direction perpendicular to the longitudinal direction (transverse direction) to be 4 to 4.5 times. 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 that 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.

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

The lithium secondary battery containing the porous film of the present invention as a separator will be described in detail.
As the electrolytic solution, for example, an electrolytic solution 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 an example to be described later, an electrolytic solution filled in a lithium secondary battery can equipped with a separator is 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.

  A negative electrode plate in a lithium secondary battery can equipped with a separator in an example described later is prepared by mixing a solution of vinylidene fluoride dissolved in N-methylpyrrolidone and a carbon material having an average particle diameter of 10 μm into a slurry. The agent 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 and dried, and then compression molded by a roll press. Then, it is cut into a strip-like 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 a more preferred embodiment, a positive electrode plate in a lithium secondary battery can in which a separator is mounted in an example described later is obtained by adding phosphorous graphite as a conductive additive to lithium cobalt oxide (LiCoO ₂ ) 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 outside with a winding tape to form a wound body.
A wound body integrally wound with the positive electrode plate 21, the separator 1 'and the negative electrode plate 22 is filled in a bottomed cylindrical battery case and welded to the positive and negative electrode lead bodies 24 and 25. Next, the electrolyte solution is poured into the battery can, and the electrolyte solution is sufficiently infiltrated into the separator 1 ′, etc., and then the positive electrode lid 27 is sealed around the opening periphery of the battery can via the gasket 26, 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 pass through the pores 1a but cannot pass through the liquid. The electrolyte can be diffused and retained.

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

"Example 1"
80 parts by mass of high-density polyethylene [HI-ZEX3300F manufactured by Mitsui Chemicals, density: 0.950 g / cm ³ , melt flow rate: 1.1 g / 10 min] pulverized to a size of 20 mesh-thru or less Density polyethylene [Nippon Polychem HF230, density: 0.923 g / cm ³ ] mixed resin with 20 parts by mass, plasticizer, sulfolane [Wako Pure Chemical Industries, Ltd. reagent melting point 29 ° C., boiling point 287 ° C.] 5 parts by mass Then, 100 parts by mass of barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] as a filler was blended and compounded. The barium sulfate had an average particle size of 0.66 μm.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm.
Next, the obtained raw sheet is stretched successively at 80 ° C. in the longitudinal direction (MD) of the sheet at 4.5 times, and then at a stretching ratio of 4 times in the transverse direction (TD) orthogonal at 125 ° C. The porous film which concerns on was produced.

"Example 2"
70 parts by mass of high-density polyethylene [HI-ZEX7000FP manufactured by Mitsui Chemicals, density: 0.956 g / cm ³ , melt flow rate: 0.04 g / 10 min], and 30 parts by mass of low-density polyethylene used in Example 1 As a plasticizer, ethylenebis stearamide [Alfro H-50L, melting point 144 ° C., 140 ° C. after heating at 140 ° C. for 1 hour is 0.3%] 10 parts by mass as a plasticizer 120 parts by mass of calcium carbonate [NC # 2310 manufactured by Nitto Flour Chemical Co., Ltd.] was blended and compounded. The calcium carbonate had an average particle size of 0.97 μm.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm. Using the obtained raw sheet, a porous film according to the present invention was produced in the same manner as in Example 1.

"Example 3"
A mixed resin of 90 parts by mass of the high-density polyethylene used in Example 2 and 10 parts by mass of the low-density polyethylene used in Example 1 was mixed with ethylene carbonate [a reagent manufactured by Wako Pure Chemical Industries, Ltd., melting point 39 ° C., boiling point] 238 ° C.] 5 parts by mass and sorbitan monostearate [Reodol SP-S10 made by Kao Corporation, melting point 51 ° C., 140 ° C./0.1% weight loss after heating for 1 hour] 5 parts by mass, barium sulfate as filler [B-55 manufactured by Sakai Chemical Co., Ltd.] 120 parts by mass were blended and compounded.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm. Using the obtained raw sheet, a porous film according to the present invention was produced in exactly the same manner as in Example 1.

"Example 4"
To a mixed resin of 80 parts by mass of the high-density polyethylene used in Example 2 and 20 parts by mass of the low-density polyethylene used in Example 1, glycerin tristearate [reagent melting point: 72 ° C., manufactured by Wako Pure Chemical Industries, Ltd.] The compound was compounded by blending 5 parts by mass with a weight loss rate of 1.1% after heating at 140 ° C. for 1 hour and 120 parts by mass of barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] as a filler.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm. Using the obtained raw sheet, a porous film according to the present invention was produced in the same manner as in Example 1.

"Example 5"
To a mixed resin of 50 parts by mass of the high-density polyethylene used in Example 2 and 50 parts by mass of the low-density polyethylene used in Example 1, ethylene carbonate [reagent made by Wako Pure Chemical Industries, Ltd., melting point 39 ° C., boiling point Compound was performed by blending 10 parts by mass of 238 ° C. and 122 parts by mass of barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] as a filler.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm. Using the obtained raw sheet, a porous film according to the present invention was produced in the same manner as in Example 1.

"Comparative Example 1"
A mixed resin of 80 parts by mass of the high-density polyethylene used in Example 2 and 20 parts by mass of the low-density polyethylene used in Example 1, ethylene carbonate as a plasticizer [reagent made by Wako Pure Chemical Industries, Ltd., melting point 39 ° C., boiling point Compounding was performed by blending 10 parts by mass of 238 ° C. and 110 parts by mass of barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] as a filler.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm. The obtained raw sheet is stretched at 80 ° C. in the longitudinal direction (MD) of the sheet at 4.5 times, and then at 125 ° C. in the transverse direction (TD) at a stretching ratio of 4 times to obtain a porous film. Produced.

"Comparative Example 2"
A raw sheet was prepared in exactly the same manner as in Comparative Example 1. The obtained raw sheet was stretched at 80 ° C. in the longitudinal direction (MD) of the sheet at 4 times, and then at a stretching ratio of 3.3 times in the transverse direction (TD) orthogonal at 125 ° C. to obtain a porous film. Produced.

“Comparative Example 3”
High density polyethylene [HI-ZEX7000FP, manufactured by Mitsui Chemicals, density: 0.956 g / cm ³ , melt flow rate: 0.04 g / 10 min], 100 parts by mass of ethylene carbonate [made by Wako Pure Chemical Industries, Ltd., reagent Compounding was carried out by blending 6 parts by mass of melting point 39 ° C. and boiling point 238 ° C. and 110 parts by mass of barium sulfate [B-55 manufactured by Sakai Chemical Co., Ltd.] as a filler.
Next, T-die molding was performed at a temperature of 200 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was an average of 250 μm.
Next, the obtained raw sheet is stretched successively at 80 ° C. in the longitudinal direction (MD) of the sheet at 4.5 times, and then at a stretching ratio of 4 times in the transverse direction (TD) orthogonal at 125 ° C. The porous film which concerns on was produced.

About the porous film produced in Examples 1-5 and Comparative Examples 1-3, thickness, thickness fluctuation | variation, and air permeability 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.

  The porous films of Examples 1 to 5 and Comparative Examples 1 to 3 were used as battery separators, and the above-described electrolyte, positive electrode plate, and negative electrode plate were used as the electrolyte solution, positive electrode plate, and negative electrode plate of the secondary battery.

The positive electrode plate and the negative electrode plate were wound in a spiral manner so as to overlap each other through any of the porous films of Examples 1 to 5 and Comparative Examples 1 to 3, and the outside was stopped with a winding tape. It was a round body. The obtained wound body was filled in a bottomed cylindrical battery case having an outer diameter of 18 mm, and the positive and negative lead bodies were welded.
An electrolytic solution in which L1PF6 was dissolved at a ratio of 1.4 mol / liter in a mixed solvent containing 2 parts by mass of methylethyl carbonate with respect to 1 part by mass of ethylene carbonate was prepared. After injecting this electrolyte into the battery can and sufficiently infiltrating the separator into the separator, etc., the positive electrode lid is sealed around the opening periphery of the battery can via a gasket, precharging and aging are performed. A secondary lithium battery was produced.

The obtained secondary lithium battery was evaluated as follows.
(Measurement of winding defect rate)
Using a positive electrode plate and a negative electrode plate having a length of 50 cm and a width of 59 mm, using two separators made of the porous films of Examples 1 to 5 and Comparative Examples 1 to 3, the two separators, the positive electrode plate, and the negative electrode plate Are alternately stacked, applying a force of 3.92 N / cm to the positive electrode plate, 3.92 N / cm to the negative electrode plate, 0.29 N / cm to the separator, and inserting a slit that sandwiches two separators with a diameter of 4 mm. A certain metal core was used to wind around the core. After completion, the metal core was pulled out.
100 of the above-mentioned entrained electrode groups were made, and the number of the separators that were torn was confirmed. If the number of the separators torn is x, the winding defect rate is x%.

(Insulation failure rate after temperature rise)
The above-mentioned 100 electrode groups were heated at a rate of 10 ° C./min, placed in an oven at 130 ° C. for 1 hour, and 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.

  Materials of Examples 1-5 and Comparative Examples 1-3, blending ratio, average thickness of porous film measured by the above method, thickness fluctuation, air permeability, winding failure rate, insulation after temperature increase The defective rate is shown in Table 1 below. In the table, the resin ratio means high density polyethylene (parts by mass) / low density polyethylene (parts by mass). Moreover, the compounding ratio of the filler and the plasticizer indicates a ratio (parts by mass) to 100 parts by mass of the thermoplastic resin.

  In the porous films of Examples 1 to 5, both the average thickness and thickness fluctuation were within the numerical range defined in the present invention. On the other hand, the porous film of Comparative Example 1 was very thin with an average thickness of 4 μm. The porous film of Comparative Example 2 had a thickness average of 30 μm and was within the specified range of the present invention, but the thickness fluctuation was ± 83% and the uniformity was very poor.

  Regarding the winding failure rate, the porous film of Examples 1 to 5 was 2 to 12%, whereas the porous film of Comparative Example 1 was 85%. Therefore, it was confirmed that the handling of the porous films of Examples 1 to 4 was significantly improved as compared with the porous film of Comparative Example 1. Further, in Comparative Example 3 in which only the high-density polyethylene resin is used as the resin component, the winding failure rate is 25%, which is sufficiently suitable for practical use. However, the addition of the low-density polyethylene resin further improves the winding failure rate. I was able to confirm.

  Regarding the insulation failure rate after the temperature rise, the porous films of Examples 1 to 5 are 3 to 19%, whereas the porous film of Comparative Example 1 is 89% and the porous film of Comparative Example 2 is 77%. %Met. Therefore, it was confirmed that when the porous films of Examples 1 to 5 were used as battery separators as compared with the porous films of Comparative Examples 1 and 2, the initial failure rate of the batteries was significantly reduced. Moreover, it has confirmed that the porous film of this invention was maintaining the insulation property comparable as the comparative example 3 which used only the high density polyethylene resin as a resin component.

  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 (10)

  A film comprising a resin composition comprising a resin (A) containing a high-density polyethylene resin and a low-density polyethylene resin, and a filler (B), wherein the film is provided with pores starting from the filler by stretching. The porous film is characterized in that the average thickness is 5 μm or more and 40 μm or less, and the maximum value and the minimum value of the thickness are less than ± 25% of the average thickness. The high-density polyethylene resin has a density of 0.94 g / cm ³ or more and 0.97 g / cm ³ or less, and the low-density polyethylene resin has a density of 0.86 g / cm ³ or more and less than 0.94 g / cm ^3. Item 2. The porous film according to Item 1.   The porous film according to claim 1 or 2, wherein the resin (A) is 100 parts by mass, the high-density polyethylene resin is 99 to 70 parts by mass, and the low-density polyethylene resin is 1 to 30 parts by mass.   The porous material according to any one of claims 1 to 3, wherein the filler (B) is in the form of a filler having an average particle size of 0.1 µm or more and 25 µm or less, and is made of an inorganic filler containing barium sulfate or calcium carbonate. Sex film.   The porous film according to any one of claims 1 to 4, wherein the resin composition contains a plasticizer (C) having a melting point of 25 ° C or higher and a boiling point of 140 ° C or higher.   The porous film according to claim 5, wherein the filler (B) is 50 to 400 parts by mass and the plasticizer (C) is 1 to 30 parts by mass with respect to 100 parts by mass of the thermoplastic resin (A).   The porous film according to any one of claims 1 to 6, wherein the stretching is biaxially stretched at a stretch ratio of 1.5 times or more in any one of the vertical and horizontal directions.   A separator for a non-aqueous electrolyte battery comprising the porous film according to claim 1.   The separator for a nonaqueous electrolyte battery according to claim 8, wherein the air permeability of the porous film is 50 (sec / 100 cc) to 500 (sec / 100 cc).   A nonaqueous electrolyte battery containing the separator for a nonelectrolyte battery according to claim 8 or 9.

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