JP2015214688A - Microporous resin film and production method of the same, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microporous resin film excellent in permeability for lithium ions and heat resistance, and a production method of the film.SOLUTION: The microporous resin film includes micropores formed by stretching a crystalline resin film containing a crystalline resin. The microporous resin film has an air permeability of 300 sec/100 mL or less and a maximum contraction stress of 2.2 MPa or less in the stretching direction when heated from 30°C to 250°C at an elevation rate of 5°C/min.

Description

  The present invention relates to a microporous resin film and a method for producing the same, a separator for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

  Conventionally, lithium ion batteries have been used as batteries for portable electronic devices and automobiles. This lithium-on battery generally includes a positive electrode, a negative electrode, a separator, and an electrolytic solution. The positive electrode is formed by applying lithium cobalt oxide or lithium manganate to the surface of the aluminum foil. The negative electrode is formed by applying carbon to the surface of the copper foil. And the separator is arrange | positioned so that a positive electrode and a negative electrode may be partitioned off, and the electrical short circuit between electrodes is prevented.

  When the lithium ion battery is charged, lithium ions are released from the positive electrode and move into the negative electrode. On the other hand, when the lithium ion battery is discharged, lithium ions are released from the negative electrode and move into the positive electrode. Therefore, the separator used in the lithium ion secondary battery is required to be able to transmit lithium ions satisfactorily.

  As the separator, a microporous resin film containing an olefin resin or the like is used because it is excellent in insulation and cost. As a method for producing a microporous resin film, a dry method and a wet method are known.

  The dry method includes a step of obtaining an olefin resin film by melt kneading and extruding an olefin resin, a step of generating and growing a lamellar crystal in the olefin resin film, and a lamella crystal by stretching the olefin resin film. And obtaining a microporous resin film in which micropores are formed by separating them.

  In the wet method, an olefin resin and a plasticizer were melt-kneaded and extruded to obtain an olefin resin film, and a hole was formed by extracting the plasticizer from the olefin resin film and biaxially stretching. And obtaining a microporous resin film.

  However, the microporous resin film undergoes large thermal shrinkage near the melting point of the olefin resin. For example, when the separator is damaged due to the mixing of metal foreign matter and a short circuit occurs between the electrodes, the battery temperature may rise due to the generation of Joule heat. As the battery temperature rises, the microporous resin film thermally shrinks, causing a short circuit between the electrodes. Therefore, it is desired to ensure the excellent safety of the lithium ion battery by improving the heat resistance of the microporous resin film.

  Therefore, Patent Document 1 discloses a microporous resin film having communication holes and having a shrinkage stress (TMA) in the length direction at 80 ° C. of 15 to 40 N / m.

  Patent Document 2 discloses a method for producing a microporous resin film by a wet method using a plasticizer. Specifically, in Patent Document 2, (A) a step of kneading a polyolefin and a plasticizer, extruding from a T die, sandwiching between two rolls, and forming a gel sheet after cooling, (B) a gel sheet A method for producing a microporous resin film is disclosed which includes a step of stretching the film and (C) a step of extracting a plasticizer after the stretching. In the above method, in step (A), the peripheral speed Av of the first roll and the peripheral speed Bv of the second roll satisfy the relationship of 2 ≧ Av / Bv> 1, whereby the thickness is uniform, and MD ( A microporous resin film having reduced shrinkage stress in the machine direction) can be obtained.

JP 2009-269941 A JP 2004-335255 A

  However, the microporous resin film of Patent Document 1 has insufficient heat resistance, and therefore further improvement in heat resistance is required.

  Moreover, the method of patent document 2 is a technique used only for the wet method using a plasticizer. Therefore, if the method of Patent Document 2 is applied to a dry method that does not use a plasticizer, lamellar crystals cannot be grown uniformly and the openability of the micropores is reduced. Ion permeability decreases.

  An object of this invention is to provide the microporous resin film excellent in lithium ion permeability | transmittance and heat resistance, and its manufacturing method. Furthermore, this invention aims at providing the separator for nonaqueous electrolyte secondary batteries and the nonaqueous electrolyte secondary battery containing the said microporous resin film.

[Microporous resin film]
The microporous resin film of the present invention is a microporous resin film in which micropores are formed by stretching a crystalline resin film containing a crystalline resin,
The air permeability is 300 sec / 100 mL or less, and the maximum shrinkage stress in the stretching direction when heated from 30 ° C. to 250 ° C. at a heating rate of 5 ° C./min is 2.2 MPa or less.

  In the present invention, a crystalline resin is preferably used. The crystalline resin is a resin having a property of becoming a crystal in a solid state, and a clear crystal diffraction pattern can be confirmed by X-ray diffraction. It means a resin having a transition point and a melting point.

  Examples of the crystalline resin include an olefin resin, a styrene resin, and polyethylene terephthalate. Examples of the olefin resin include ethylene resin and propylene resin. Especially, since the microporous resin film excellent in heat resistance and air permeability is obtained, an olefin resin is preferable and a propylene resin is more preferable.

  Examples of the propylene-based resin include a propylene homopolymer, a copolymer of propylene and another olefin, and the like. Propylene-type resin may be used independently, or 2 or more types may be used together. Further, the copolymer of propylene and another olefin may be a block copolymer or a random copolymer.

  Examples of the olefin copolymerized with propylene include α such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene and 1-decene. -Olefin etc. are mentioned.

  The propylene-based resin is preferably a propylene homopolymer (homopolypropylene), more preferably isotactic homopolypropylene and syndiotactic homopolypropylene, and particularly preferably isotactic homopolypropylene because of having excellent crystallinity.

  The weight average molecular weight of the propylene-based resin is preferably 250,000 to 500,000, and more preferably 280,000 to 480,000. According to the propylene-based resin having a weight average molecular weight within the above range, it is possible to provide a microporous resin film that is excellent in film formation stability and in which micropores are uniformly formed.

  The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the propylene-based resin is preferably 7.5 to 12.0, more preferably 8.0 to 11.5, and particularly preferably 8.0 to 11.0. . According to the propylene resin having a molecular weight distribution within the above range, it is possible to provide a microporous resin film having a high surface opening ratio, excellent ion permeability, and excellent mechanical strength. .

  Here, the weight average molecular weight and the number average molecular weight of the propylene-based resin are values converted by polystyrene using a GPC (gel permeation chromatography) method. Specifically, 6 to 7 mg of propylene-based resin is collected, and the collected propylene-based resin is supplied to a test tube, and then the o-DCB (0.05% by weight of BHT (dibutylhydroxytoluene) is contained in the test tube. (Orthodichlorobenzene) solution is added to dilute the propylene resin concentration to 1 mg / mL to prepare a diluted solution.

  The dilution liquid is shaken for 1 hour at 145 ° C. and a rotation speed of 25 rpm using a dissolution filter, and the propylene resin is dissolved in the o-DCB solution to obtain a measurement sample. Using this measurement sample, the weight average molecular weight and the number average molecular weight of the propylene-based resin can be measured by the GPC method.

The weight average molecular weight and number average molecular weight in the propylene-based resin can be measured, for example, with the following measuring apparatus and measurement conditions.
Product name "HLC-8121GPC / HT" manufactured by TOSOH
Measurement conditions Column: TSKgelGMHHR-H (20) HT x 3
TSKguardcolumn-HHR (30) HT x 3
Mobile phase: o-DCB 1.0 mL / min
Sample concentration: 1 mg / mL
Detector: Bryce refractometer
Reference material: Polystyrene (Molecular weight: 500-8420000, manufactured by TOSOH)
Elution conditions: 145 ° C
SEC temperature: 145 ° C

  160-170 degreeC is preferable and, as for melting | fusing point of propylene-type resin, 163-170 degreeC is more preferable. According to the propylene-based resin having a melting point within the above range, it is possible to provide a microporous resin film that is excellent in film formation stability and suppressed in mechanical strength at high temperatures.

  The heat of fusion obtained by differential scanning calorimetry (DSC) in the propylene-based resin is preferably 85 mJ / mg or more, and more preferably 90 mJ / mg or more. A propylene resin having a heat of fusion of 85 mJ / mg or more has high orientation, and can form micropores uniformly in the crystalline resin film.

  The melting point of the propylene resin and the heat of fusion obtained by DSC can be measured in the following manner. First, 10 mg of propylene-based resin is collected. Next, the propylene-based resin is heated from 0 ° C. to 250 ° C. at a heating rate of 10 ° C./min, and held at 250 ° C. for 3 minutes. Next, the propylene-based resin is cooled from 250 ° C. to 0 ° C. at a temperature decrease rate of 10 ° C./min and held at 0 ° C. for 3 minutes. Subsequently, the propylene-based resin is reheated from 0 ° C. to 250 ° C. at a heating rate of 10 ° C./min. The melting peak top temperature in this reheating step is taken as the melting point, and the heat of fusion is calculated from the total area of the melting peak. can do. The DSC of the propylene-based resin can be performed using, for example, DSC220C manufactured by Seiko Instruments Inc.

  The microporous resin film of the present invention comprises a crystalline resin stretched film formed by uniaxially stretching or biaxially stretching a crystalline resin film containing a crystalline resin. The crystalline resin film is obtained by extruding after melting and kneading the crystalline resin.

  The air permeability of the microporous resin film is 300 sec / 100 mL or less, preferably 50 to 300 sec / 100 mL, more preferably 50 to 250 sec / 100 mL, and particularly preferably 50 to 200 sec / 100 mL. A microporous resin film having an air permeability within the above range is excellent in both mechanical strength and ion permeability.

  The air permeability of the microporous resin film was measured at 10 points in the length direction of the microporous resin film in accordance with JIS P8117 in an atmosphere having a temperature of 23 ° C. and a relative humidity of 65%. The value is obtained by calculating the average value.

  The maximum shrinkage stress in the stretching direction of the microporous resin film when the microporous resin film is heated from 30 ° C. to 250 ° C. at a rate of temperature increase of 5 ° C./min is 2.2 MPa or less, but 2.0 MPa or less. Preferably, 1.7 MPa or less is more preferable, and 1.5 MPa or less is particularly preferable. A microporous resin film having a maximum shrinkage stress of 2.2 MPa or less is excellent in heat resistance. Therefore, by using such a microporous resin film, even when the temperature inside the non-aqueous electrolyte secondary battery becomes high, an electrical short circuit between the electrodes due to thermal contraction of the microporous resin film. Can be reduced to a high level. The lower limit of the maximum shrinkage stress in the stretching direction of the microporous resin film when the microporous resin film is heated from 30 ° C. to 250 ° C. at a heating rate of 5 ° C./min is not particularly limited. 0.3 MPa.

  The maximum shrinkage stress of the microporous resin film refers to the highest shrinkage stress in the plane of the microporous resin film and corresponds to the stretching direction of the crystalline resin film. The maximum shrinkage stress of the microporous resin film can be measured using a thermomechanical analyzer (for example, TMA / SS7100C manufactured by Hitachi High-Tech Science Co., Ltd.). Specifically, it is as follows. First, an arbitrary portion of the microporous resin film is cut to obtain a strip having a width of 3 mm and a length of 50 mm. At this time, the length direction of the strip is set to be the stretching direction of the microporous resin film. The both ends of the strip in the length direction are gripped by a pair of grips and attached to the thermomechanical analyzer. At this time, the distance between the grippers is 10 mm. An initial load of 0.261 MPa is applied in the length direction of the band to prevent the band from sagging between the grippers. Furthermore, the gripping tool is fixed so as not to move during the measurement. Thereafter, the strip was heated from 30 ° C. to 250 ° C. at a rate of 5 ° C./min, the shrinkage stress (MPa) at each temperature was measured, and the maximum value was the maximum shrinkage stress (MPa) of the microporous resin film. ). When the microporous resin film is formed by biaxially stretching a crystalline resin film, the maximum shrinkage stress is measured in the above-described manner for each stretching direction of the microporous resin film, and in each stretching direction, Of the maximum shrinkage stress, the larger maximum shrinkage stress is defined as the maximum shrinkage stress (MPa) of the microporous resin film.

  The surface opening ratio of the microporous resin film is preferably 25 to 55%, more preferably 30 to 50%. A microporous resin film having a surface opening ratio within the above range is excellent in both mechanical strength and ion permeability.

  In addition, the surface opening ratio of a microporous resin film can be measured in the following way. First, in an arbitrary part on the surface of the microporous resin film, a measurement part having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is determined, and this measurement part is photographed at a magnification of 10,000 times.

Next, each micropore formed in the measurement portion is surrounded by a rectangle. The rectangle is adjusted so that both the long side and the short side have the minimum dimension. Let the area of the said rectangle be an opening area of each micropore part. The total opening area S (μm ² ) of the micropores is calculated by summing the opening areas of the micropores. This is the total opening area S of the minute hole ([mu] m ²⁾ of 122.88μm ² (9.6μm × 12.8μm) surface porosity values multiplied by 100 and divided by the (%). In addition, about the micropore part which exists over the measurement part and the part which is not a measurement part, only the part which exists in the measurement part in the micropore part is made into a measuring object.

  The maximum major axis of the open end of the micropore in the microporous resin film is preferably 1 μm or less, and more preferably 100 nm to 900 nm. According to the microporous resin film having a maximum long diameter of 1 μm or less at the opening end of the microporous part, even if dendrites (dendritic crystals) grow on the electrode surface by repeating charge and discharge of the battery, Generation of minute internal short circuit (dendritic short circuit) can be reduced.

  The average major axis of the open ends of the micropores in the microporous resin film is preferably 500 nm or less, and more preferably 10 nm to 400 nm. The microporous resin film having an average major axis at the opening end of the micropore portion of 500 nm or less has uniform ion permeability, and this can reduce the generation of dendrites.

  In addition, the maximum major axis and the average major axis of the open end of the micropores in the microporous resin film are measured as follows. First, the surface of the microporous resin film is coated with carbon. Next, 10 arbitrary positions on the surface of the microporous resin film are photographed at a magnification of 10,000 using a scanning electron microscope. The photographing range is a plane rectangular range of 9.6 μm long × 12.8 μm wide on the surface of the microporous resin film.

  The major axis of the opening end of each micropore appearing in the obtained photograph is measured. The major axis of the open end of the microhole is defined as the diameter of a perfect circle having the smallest diameter that can surround the open end of the microhole. Micropores that exist across the imaging range and the non-imaging range are excluded from the measurement target. And let the largest long diameter among the long diameters of the opening end in the micropore part in a photograph be the maximum long diameter of the opening end of the micropore part in a microporous resin film. The arithmetic mean value of the major axis of the opening end in each micropore in the photograph is taken as the average major axis of the opening end of the micropore in the microporous resin film.

The pore density of the microporous resin film is preferably 15 / μm ² or more, and more preferably 17 / μm ² or more. A microporous resin film having a pore density of 15 / μm ² or more is excellent in mechanical strength and ion permeability.

In addition, the pore density of a microporous resin film is measured in the following way. First, in an arbitrary part on the surface of the microporous resin film, a measurement part having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is determined, and this measurement part is photographed at a magnification of 10,000 times. Then, the number of micropores is measured in the measurement part, and the pore density can be calculated by dividing this number by 122.88 μm ² (9.6 μm × 12.8 μm).

  The porosity of the microporous resin film is preferably 45 to 70%, more preferably 47 to 67%. A microporous resin film having a porosity in the above range is excellent in air permeability and mechanical strength.

In addition, the porosity of a microporous resin film can be measured in the following way. First, the microporous resin film is cut to obtain a test piece having a plane square shape (area 100 cm ² ) of 10 cm in length and 10 cm in width. Next, the weight W (g) and thickness T (cm) of the test piece are measured, and the apparent density ρ (g / cm ³ ) is calculated by the following formula (1). In addition, the thickness of a test piece measures 15 thickness of a test piece using a dial gauge (for example, signal ABS Digimatic indicator by Mitutoyo Corporation), and makes it the arithmetic mean value. Then, using this apparent density ρ (g / cm ³ ) and the density ρ ₀ (g / cm ³ ) of the crystalline resin itself, the porosity P (%) of the microporous resin film based on the following formula (2) Can be calculated.
Apparent density ρ (g / cm ³ ) = W / (100 × T) (1)
Porosity P [%] = 100 × [(ρ ₀ −ρ) / ρ ₀ ] (2)

  Although the thickness in particular of a microporous resin film is not restrict | limited, 1-100 micrometers is preferable and 1-50 micrometers is more preferable. In addition, the thickness of a microporous resin film measures 15 thickness of a microporous resin film using a dial gauge (for example, signal ABS Digimatic indicator by Mitutoyo Corporation), and makes it the arithmetic mean value.

[Production method of microporous resin film]
The microporous resin film of the present invention comprises the following steps:
An extrusion step of supplying a crystalline resin to an extruder, melt-kneading, and obtaining a crystalline resin film by extruding from a T-die attached to the tip of the extruder;
Curing step for curing the crystalline resin film obtained in the extrusion step at (Tm-30) to (Tm-5) ° C;
A first stretching step in which the crystalline resin film after the curing step is uniaxially stretched in the extrusion direction at a stretching ratio of 1.05 to 2 times at a surface temperature of -20 to 100 ° C;
By uniaxially stretching the crystalline resin film after the first stretching step in the extrusion direction at a surface temperature of 100 to 150 ° C. at a stretching ratio of 2.8 to 4.0 times and a stretching speed of 5 to 400% / min. A second stretching step for obtaining a crystalline resin stretched film;
While the crystalline resin stretched film after the second stretching step is heated at a surface temperature of (Tm-35) ° C. or higher and below the melting point of the crystalline resin, the shrinkage rate is 5 to 25% in the extrusion direction. An annealing process to shrink with,
It can manufacture by the method which has this. “Tm” means the melting point of the crystalline resin.

(Extrusion process)
In the method of the present invention, first, a crystalline resin is supplied to an extruder, melted and kneaded, and then extruded from a T die attached to the tip of the extruder to obtain a crystalline resin film.

  The temperature of the crystalline resin when melt-kneading the crystalline resin with an extruder is preferably (Tm + 20) to (Tm + 100) ° C, more preferably (Tm + 25) to (Tm + 80) ° C, and (Tm + 25) to (Tm + 60). ° C is particularly preferred. By setting the temperature of the crystalline resin during the melt-kneading within the above range, a crystalline resin film having a uniform thickness and improved orientation of the crystalline resin can be obtained.

  50-300 are preferable and, as for the draw ratio at the time of extruding crystalline resin to a film form from an extruder, 70-230 are more preferable. By setting the draw ratio to 50 or more, the tension applied to the crystalline resin can be improved. This makes it possible to sufficiently align the crystalline resin and promote the generation of lamellae. Moreover, the film-forming stability of a crystalline resin film can be improved by making a draw ratio into 300 or less. This makes it possible to obtain a microporous resin film having a uniform thickness and width.

  The draw ratio is a value obtained by dividing the clearance of the lip of the T die by the thickness of the crystalline resin film extruded from the T die. The clearance of the lip of the T die is measured by measuring the clearance of the lip of the T die at 10 or more locations using a clearance gauge in conformity with JIS B7524 (for example, JIS clearance gauge manufactured by Nagai Gauge Manufacturing Co., Ltd.), and the arithmetic average thereof This can be done by determining the value. Further, the thickness of the crystalline resin film extruded from the T die is 10 or more in the thickness of the crystalline resin film extruded from the T die using a dial gauge (for example, signal ABS Digimatic Indicator manufactured by Mitutoyo Corporation). It can be performed by measuring and calculating the arithmetic mean value.

  Furthermore, the deposition rate of the crystalline resin film is preferably 10 to 300 m / min, more preferably 15 to 250 m / min, and particularly preferably 15 to 200 m / min. By setting the deposition rate of the crystalline resin film to 10 m / min or more, the tension applied to the crystalline resin can be improved. This makes it possible to sufficiently align the crystalline resin molecules and promote the generation of lamellae. Moreover, the film-forming stability of a crystalline resin film can be improved by making the film-forming speed | rate of a crystalline resin film into 300 m / min or less. This makes it possible to obtain a microporous resin film having a uniform thickness and width.

  Then, by cooling the crystalline resin film extruded from the T die until the surface temperature becomes (Tm-100) ° C. or less, the crystalline resin constituting the crystalline resin film is crystallized to form a lamella. Generate. In the present invention, the crystalline resin film is cooled by extruding the melt-kneaded crystalline resin so that the crystalline resin molecules constituting the crystalline resin film are oriented in advance. The portion where the is oriented can promote the formation of lamellae.

  The surface temperature of the cooled crystalline resin film is preferably (Tm-100) ° C. or less, more preferably (Tm-140) to (Tm-110) ° C., and (Tm-135) to (Tm-120) ° C. Is particularly preferred. By cooling the surface temperature of the crystalline resin film to the above range, the crystalline resin can be crystallized to highly generate lamellae.

(Curing process)
Next, the crystalline resin film obtained by the extrusion process described above is cured. This curing process is performed in order to grow the lamella formed in the crystalline resin film in the extrusion process. As a result, it is possible to promote a laminated lamella structure in which crystallized portions (lamellar) and amorphous portions are alternately arranged in the extrusion direction of the crystalline resin film. It is possible to generate a crack between lamellas, not within the lamella, and to form a minute through hole (microhole part) starting from this crack.

  The curing process is performed by curing the crystalline resin film obtained by the extrusion process at a curing temperature of (Tm-30) to (Tm-5) ° C.

  The curing temperature of the crystalline resin film is (Tm-30) to (Tm-5) ° C, but (Tm-25) to (Tm-10) ° C is preferable. By setting the curing temperature of the crystalline resin film to (Tm-30) ° C. or higher, crystallization of the crystalline resin film can be sufficiently promoted. Moreover, collapse of the lamellar structure due to relaxation of the molecular orientation of the crystalline resin can be reduced by setting the curing temperature of the crystalline resin film to (Tm-5) ° C. or lower.

  The curing temperature of the crystalline resin film is the surface temperature of the crystalline resin film. However, when the surface temperature of the crystalline resin film cannot be measured, for example, when the crystalline resin film is cured in a roll shape, the curing temperature of the crystalline resin film is the atmospheric temperature and To do. For example, when curing is performed in a state where the crystalline resin film is wound in a roll shape inside a heating apparatus such as a hot stove, the temperature inside the heating apparatus is set as the curing temperature.

  Curing of the crystalline resin film may be performed while the crystalline resin film is running, or may be performed in a state where the crystalline resin film is wound into a roll.

  When the curing of the crystalline resin film is performed while running the crystalline resin film, the curing time of the crystalline resin film is preferably 1 minute or more, and more preferably 5 minutes to 60 minutes.

  When the crystalline resin film is cured in the state of being wound in a roll shape, the curing time is preferably 1 hour or longer, and more preferably 15 hours or longer. By curing the crystalline resin film in a state of being wound in such a curing time, the curing of the crystalline resin film as a whole can be performed at the curing temperature described above. Thereby, the lamella can be sufficiently grown in the crystalline resin film. In addition, from the viewpoint of reducing thermal deterioration of the crystalline resin film, the curing time is preferably 35 hours or less, and more preferably 30 hours or less.

  When the crystalline resin film is cured in a rolled state, the crystalline resin film is unwound from the crystalline resin film roll after the curing process, and a stretching process and an annealing process described later are performed. Good.

(First stretching process)
Next, a first stretching process is performed in which the crystalline resin film after the curing process is uniaxially stretched in the extrusion direction at a surface temperature of −20 to 100 ° C. and a stretching ratio of 1.05 to 2 times. In the first stretching step, the lamellae in the crystalline resin film are hardly melted, and by separating the lamellae by stretching, the fine cracks are efficiently generated independently in the non-crystalline part between the lamellae. A large number of micropores can be reliably formed starting from this crack.

  In the first stretching step, the surface temperature of the crystalline resin film is -20 to 100 ° C, preferably 0 to 80 ° C, particularly preferably 10 to 40 ° C. By setting the surface temperature within the above range, breakage of the crystalline resin film at the time of stretching can be reduced, and cracks can be generated in the non-crystalline part between lamellae.

  In the first stretching step, the stretching ratio of the crystalline resin film is 1.05 to 2 times, preferably 1.1 to 1.8 times, and particularly preferably 1.2 to 1.5 times. By setting the draw ratio to 1.05 times or more, micropores can be formed in the non-crystalline part between lamellae. Moreover, the microporous resin film in which the micropore part was formed uniformly can be formed by making a draw ratio into 2 times or less.

  In addition, in this invention, the draw ratio of a crystalline resin film means the value which remove | divided the length of the crystalline resin film after extending | stretching with the length of the crystalline resin film before extending | stretching.

  In the first stretching step, the stretching rate of the crystalline resin film is preferably 20% / min or more, more preferably 20 to 3000% / min, and particularly preferably 20 to 2000% / min. By setting the stretching speed to 20% / min or more, the micropores can be formed uniformly in the non-crystalline part between lamellae. By setting the stretching speed to 3000% / min or less, it is possible to suppress breakage of the olefin resin film due to stretching.

  In addition, in this invention, the extending | stretching speed of a crystalline resin film means the change rate of the dimension in the extrusion direction (stretching direction) of the crystalline resin film per unit time.

  The method for stretching the crystalline resin film in the first stretching step is not particularly limited as long as the crystalline resin film can be uniaxially stretched. Examples thereof include a method of uniaxially stretching a crystalline resin film at a predetermined temperature using a longitudinal uniaxial stretching apparatus using rolls having different peripheral speeds.

(Second stretching step)
Next, the crystalline resin film after the first stretching step is uniaxially stretched in the extrusion direction at a surface temperature of 100 to 150 ° C. at a stretching ratio of 2.8 to 4.0 times and a stretching speed of 5 to 400% / min. The 2nd extending process which obtains a crystalline resin stretched film by this is implemented. By performing such a second stretching step, a large number of micropores formed in the crystalline resin film in the first stretching step can be grown.

  In the second stretching step, the surface temperature of the crystalline resin film is 100 to 150 ° C, but preferably 110 to 140 ° C. By setting the surface temperature of the crystalline resin film to 100 ° C. or higher, the micropores can be highly grown. Moreover, the blockage | closure of a micropore part can be reduced by making the surface temperature of a crystalline resin film into 150 degrees C or less.

  In the second stretching step, the stretching ratio of the crystalline resin film is 2.8 to 4.0 times, preferably 2.9 to 3.9 times, and more preferably 3.1 to 3.9 times. By setting the draw ratio of the crystalline resin film to 2.8 times or more, the micropores can be grown. Thereby, the microporous resin film which has the outstanding air permeability can be provided. Furthermore, by setting the draw ratio of the crystalline resin film to 2.8 times or more, the number of micropores is increased by promoting the opening between crystals, thereby improving the orientation of the fibrillated amorphous part. Can be reduced. Thereby, the heat shrink of a microporous resin film can be reduced and heat resistance can be improved. Moreover, it becomes possible to suppress obstruction | occlusion of a micropore part by making the draw ratio of a crystalline resin film 4.0 times or less. Furthermore, by setting the draw ratio of the crystalline resin film to 4.0 times or less, it is possible to reduce the orientation of the fibrillated non-crystalline part more than necessary, and thereby heat shrinkage of the microporous resin film. Can be reduced to improve heat resistance.

  In the second stretching step, the stretching rate of the crystalline resin film is 5 to 400% / min, preferably 5 to 300% / min, and more preferably 5 to 200% / min. By setting the stretching speed to 5% / min or more, it is possible to reduce the excessive growth of the micropores, thereby providing a microporous resin film having excellent mechanical strength. Moreover, a micropore part can be made to grow uniformly by making extending | stretching speed into 400% / min or less.

  In the second stretching step, the method for stretching the crystalline resin film is not particularly limited as long as the crystalline resin film can be uniaxially stretched. Examples thereof include a method of uniaxially stretching a crystalline resin film at a predetermined temperature using a longitudinal uniaxial stretching apparatus using rolls having different peripheral speeds.

(Annealing process)
Next, an annealing process is performed in which the crystalline resin stretched film that has been uniaxially stretched in the second stretching process is annealed. In the annealing step, the crystalline resin stretched film is contracted in the extrusion direction while heating the crystalline resin stretched film after the second stretching step.

  This annealing step is performed in order to relieve the residual strain generated in the crystalline resin stretched film due to the stretching applied in the stretching step described above. In the method of the present invention, the annealing step is performed after the crystalline resin stretched film is stretched at a relatively high stretch ratio in the second stretching step described above. Thereby, the heat shrink under high temperature is highly reduced, and the microporous resin film excellent in heat resistance can be provided.

  In the annealing step, the crystalline resin stretched film after the second stretching step is heated at a surface temperature of (Tm-35) ° C. or higher and below the melting point of the crystalline resin in the extrusion direction from 5 to 25. It is performed by shrinking at a shrinkage rate of%.

  In the annealing step, the surface temperature of the stretched crystalline resin film is (Tm−35) ° C. or higher and below the melting point of the crystalline resin, but (Tm−25) to (Tm−1) ° C. is preferable. By setting the surface temperature of the crystalline resin film to (Tm−35) ° C. or higher, the strain remaining in the stretched crystalline resin film by stretching can be sufficiently relaxed. Thereby, a microporous resin film having high heat resistance can be provided. Moreover, the blockage | closure of the micropore part formed at the extending | stretching process can be reduced by making the surface temperature of a crystalline resin stretched film below into the temperature of melting | fusing point of crystalline resin.

  In the annealing step, the shrinkage ratio of the crystalline resin stretched film is 5 to 25%, preferably 8 to 22%. By setting the shrinkage ratio of the stretched crystalline resin film to 5% or more, the strain remaining in the stretched crystalline resin film is sufficiently relaxed by stretching, and the fibrillated amorphous part is oriented more than necessary. The heat resistance of the resulting microporous resin film can be improved. Further, by setting the shrinkage rate of the crystalline resin stretched film to 25% or less, it is possible to uniformly anneal the crystalline resin stretched film without slackening while suppressing the blocking of the micropores.

  The shrinkage rate of the crystalline resin stretched film is the shrinkage length of the crystalline resin stretched film in the extrusion direction (stretching direction) during the annealing process, and the crystallinity in the extrusion direction (stretching direction) after the second stretching process. A value obtained by dividing by the length of the stretched resin film and multiplying by 100.

  According to the method of the present invention described above, a microporous resin film containing a large number of micropores formed through the film front and back surfaces can be produced. Such a microporous resin film has excellent air permeability and can smoothly and uniformly transmit lithium ions. Furthermore, the microporous resin film obtained by the method of the present invention has a low residual strain generated by stretching of the crystalline resin film. Therefore, such a crystalline resin film has high heat shrinkage at high temperatures and is excellent in heat resistance.

  Therefore, the microporous resin film of the present invention is suitably used as a separator for nonaqueous electrolyte secondary batteries. Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery. Since the microporous resin film of the present invention is excellent in lithium ion permeability, a non-aqueous electrolyte secondary battery that can be charged and discharged at a high current density by using this microporous resin film Can be provided. Furthermore, since the microporous resin film of the present invention is also excellent in heat resistance, by using such a microporous resin film, the inside of the battery has a high temperature of, for example, 100 to 150 ° C., particularly 130 to 150 ° C. Even in such a case, it is possible to provide a non-aqueous electrolyte secondary battery in which the electrical short circuit between the electrodes due to the shrinkage of the microporous resin film is highly reduced.

  The nonaqueous electrolyte secondary battery is not particularly limited as long as it contains the microporous resin film of the present invention as a separator, and includes a positive electrode, a negative electrode, a microporous resin film, and a nonaqueous electrolyte. A microporous resin film is arrange | positioned between a positive electrode and a negative electrode, and can prevent the electrical short circuit between electrodes by this. In addition, the nonaqueous electrolytic solution is filled at least in the micropores of the microporous resin film, so that lithium ions can move between the electrodes during charging and discharging.

  The positive electrode is not particularly limited, but preferably includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer preferably includes a positive electrode active material and voids formed between the positive electrode active materials. When the positive electrode active material layer includes voids, the voids are also filled with the non-aqueous electrolyte. The positive electrode active material is a material capable of occluding and releasing lithium ions, and examples of the positive electrode active material include lithium cobaltate and lithium manganate. Examples of the current collector used for the positive electrode include aluminum foil, nickel foil, and stainless steel foil. The positive electrode active material layer may further contain a binder, a conductive auxiliary agent, and the like.

  The negative electrode is not particularly limited, but preferably includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector. The negative electrode active material layer preferably includes a negative electrode active material and voids formed between the negative electrode active materials. When the negative electrode active material layer contains voids, the voids are also filled with the non-aqueous electrolyte. The negative electrode active material is a material capable of occluding and releasing lithium ions. Examples of the negative electrode active material include graphite, carbon black, acetylene black, and ketjen black. Examples of the current collector used for the negative electrode include copper foil, nickel foil, and stainless steel foil. The negative electrode active material layer may further contain a binder, a conductive auxiliary agent, and the like.

A nonaqueous electrolytic solution is an electrolytic solution in which an electrolyte salt is dissolved in a solvent that does not contain water. Examples of the non-aqueous electrolyte used in the lithium ion secondary battery include a non-aqueous electrolyte obtained by dissolving a lithium salt in an aprotic organic solvent. Examples of the aprotic organic solvent include a mixed solvent of a cyclic carbonate such as propylene carbonate and ethylene carbonate and a chain carbonate such as diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiN (SO ₂ CF ₃ ) ₂ .

  ADVANTAGE OF THE INVENTION According to this invention, the microporous resin film excellent in lithium ion permeability | transmittance and heat resistance, and its manufacturing method can be provided.

  Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto.

[Example 1]
(Extrusion process)
Isotactic homopolypropylene (weight average molecular weight Mw 400,000, number average molecular weight Mn 37,000, molecular weight distribution (Mw / Mn) 10.8, melting point 163 ° C., heat of fusion 96 mJ / mg) is supplied to the extruder and the resin temperature After melt-kneading at 200 ° C. and extruding into a film form from a T die attached to the tip of the extruder, it is cooled until the surface temperature reaches 30 ° C., and is a long shape having a thickness of 30 μm and a width of 200 mm A homopolypropylene film was obtained. The extrusion rate was 10 kg / hour, the film formation rate was 25 m / min, and the draw ratio was 85.

(Curing process)
Next, the long homopolypropylene film 400 m was wound around a cylindrical core body having an outer diameter of 178 mm to obtain a homopolypropylene film roll. The homopolypropylene film roll was allowed to stand for 24 hours in a hot air oven in which the atmospheric temperature of the place where the homopolypropylene film roll was installed was 150 ° C. for curing. At this time, the temperature of the homopolypropylene film was entirely the same as the temperature inside the hot stove from the surface to the inside of the homopolypropylene film roll.

(First stretching process)
Next, after the homopolypropylene film was unwound from the cured homopolypropylene film roll, the surface temperature of the homopolypropylene film was 23 ° C., and the draw ratio was 1.2 times at a draw rate of 240% / min. The film was uniaxially stretched only in the extrusion direction using a uniaxial stretching apparatus.

(Second stretching step)
Next, the homopolypropylene film was uniaxially stretched only in the extrusion direction at a stretching ratio of 2.95 times at a stretching rate of 47% / min using a uniaxial stretching apparatus so that the surface temperature was 115 ° C. A polypropylene stretched film was obtained.

(Annealing process)
Thereafter, the stretched homopolypropylene film was annealed by heating for 45 seconds so that the surface temperature was 143 ° C., thereby obtaining a microporous resin film (thickness: 26 μm). The shrinkage rate in the extrusion direction of the homopolypropylene stretched film during annealing was 6%.

[Examples 2 to 4 and Comparative Examples 1 to 2]
Example 1 except that the surface temperature of the homopolypropylene film in the second stretching step, the stretching ratio and the stretching speed, and the surface temperature and shrinkage of the homopolypropylene stretched film in the annealing step were changed as shown in Table 1, respectively. Similarly, a microporous resin film was obtained.

[Evaluation]
About the microporous resin films obtained in the examples and comparative examples, the thickness, the air permeability, the porosity, the surface opening ratio, the hole density, and the maximum major axis and the average major axis of the opening end of the micropores are as described above. Measured according to Furthermore, the maximum shrinkage stress in the stretching direction (extrusion direction) when the microporous resin films obtained in the examples and comparative examples were heated from 30 ° C. to 250 ° C. at a temperature rising rate of 5 ° C./min was determined according to the above-described procedure. It was measured. These results are shown in Table 1.

Claims (7)

A microporous resin film in which micropores are formed by stretching a crystalline resin film containing a crystalline resin,
A microporous material having an air permeability of 300 sec / 100 mL or less and a maximum shrinkage stress in a stretching direction of 2.2 MPa or less when heated from 30 ° C. to 250 ° C. at a heating rate of 5 ° C./min. Resin film.   The microporous resin film according to claim 1, wherein the crystalline resin contains a propylene-based resin.   The microporous material according to claim 2, wherein the propylene-based resin has a weight average molecular weight of 25 to 500,000, a molecular weight distribution of 7.5 to 12.0, and a melting point of 160 to 170 ° C. Resin film.   A separator for a non-aqueous electrolyte secondary battery comprising the microporous resin film according to any one of claims 1 to 3.   A non-aqueous electrolyte secondary battery comprising a positive electrode, a separator for a non-aqueous electrolyte secondary battery according to claim 4, a negative electrode, and a non-aqueous electrolyte. An extrusion step of supplying a crystalline resin to an extruder, melt-kneading, and obtaining a crystalline resin film by extruding from a T-die attached to the tip of the extruder;
Curing step for curing the crystalline resin film obtained in the extrusion step at (Tm-30) to (Tm-5) ° C;
A first stretching step in which the crystalline resin film after the curing step is uniaxially stretched in the extrusion direction at a stretching ratio of 1.05 to 2 times at a surface temperature of -20 to 100 ° C;
By uniaxially stretching the crystalline resin film after the first stretching step in the extrusion direction at a surface temperature of 100 to 150 ° C. at a stretching ratio of 2.8 to 4.0 times and a stretching speed of 5 to 400% / min. A second stretching step for obtaining a crystalline resin stretched film;
While the crystalline resin stretched film after the second stretching step is heated at a surface temperature of (Tm-35) ° C. or higher and below the melting point of the crystalline resin, the shrinkage rate is 5 to 25% in the extrusion direction. An annealing process to shrink with,
(The above-mentioned Tm is defined as the melting point of the crystalline resin).   The method for producing a microporous resin film according to claim 6, wherein in the extrusion step, the crystalline resin is melt-kneaded at (Tm + 20) to (Tm + 100) ° C. with an extruder.