JP2015131874A - Method for producing heat-resistant synthetic resin microporous film, heat-resistant synthetic resin microporous film, separator for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat-resistant synthetic resin microporous film which have excellent permeability to ions such as lithium ions and excellent heat resistance and does not cause the contamination of a production line, and to provide a method for producing the same.SOLUTION: There is provided a method for producing a heat-resistant synthetic resin microporous film in which a radical-polymerizable monomer containing a tri-functional or more acrylic monomer is coated on the surface of a synthetic resin microporous film and then the synthetic resin microporous film is subjected to plasma processing. According to the heat-resistant synthetic resin microporous film obtained by the method of the present invention, there can be provided a non-aqueous electrolyte secondary battery having high output and excellent safety.

Description

  The present invention relates to a method for producing a heat-resistant synthetic resin microporous film used for a separator of a secondary battery such as a lithium ion secondary battery, a heat-resistant synthetic resin microporous film, a nonaqueous electrolyte secondary battery separator, The present invention relates to a water electrolyte secondary battery.

  Conventionally, lithium ion secondary batteries have been used as power sources for portable electronic devices. This lithium ion secondary battery is generally configured by disposing a positive electrode, a negative electrode, and a separator in 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 secondary 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 secondary battery is discharged, lithium ions are released from the negative electrode and move to the positive electrode. Therefore, the separator is required to have excellent ion permeability such as lithium ions.

  As the separator, a polyolefin-based resin microporous film is used because it is excellent in insulation and cost. However, the polyolefin resin microporous film undergoes large thermal shrinkage near the melting point of the polyolefin resin. For example, when the separator is damaged due to the mixing of metal foreign matter or the like and a short circuit occurs between the electrodes, the battery temperature rises due to the generation of Joule heat, which causes the polyolefin resin microporous film to thermally shrink. Due to the shrinkage of the polyolefin resin microporous film, a short circuit between the electrodes further occurs and the battery temperature rises.

  In recent years, lithium ion secondary batteries have been desired to be able to ensure high output and excellent safety. Therefore, the separator is also required to improve heat resistance.

  Patent Document 1 discloses a lithium secondary battery separator that is processed by electron beam irradiation and has a thermomechanical analysis (TMA) value at 100 ° C. of 0% to −1%. However, the heat resistance of the lithium secondary battery separator is insufficient only by treatment with electron beam irradiation.

  Patent Document 2 discloses a porous film in which divinylbenzene or a crosslinked polymer composed of divinylbenzene and ethylvinylbenzene is held on at least a part of the surface of the porous film composed of polyethylene or polypropylene. Yes. This porous membrane is used as a separation membrane in the water purification field, the blood treatment field, the air purification field, the food industry field and the like.

  However, when the technique disclosed in Patent Document 2 is simply applied to a separator, the separator cannot be sufficiently crosslinked, and sufficient heat resistance cannot be imparted to the separator.

  Therefore, when the amount of divinylbenzene held in the separator is increased in order to give sufficient heat resistance to the separator, the holes in the separator are extremely smaller than the holes in the separation membrane. As a result, the lithium ion permeability decreases. Such a separator cannot be used for a lithium ion secondary battery.

  Further, Patent Document 3 includes a porous base material having a large number of pores, and a porous coating layer coated on at least one surface of the porous base material and containing inorganic particles and a binder. A separator in which is cross-linked is disclosed.

  However, when the battery is manufactured, the inorganic particles contained in the porous coating layer fall off from the surface of the porous substrate and contaminate the production line.

JP 2003-22793 A Japanese Patent Laid-Open No. 3-193125 Special table 2011-505663 gazette

  The present invention provides a heat-resistant synthetic resin microporous film that is excellent in both permeability and heat resistance of ions such as lithium ions and that does not cause contamination of the production line, and a method for producing the same. Furthermore, this invention provides the separator for nonaqueous electrolyte secondary batteries and the nonaqueous electrolyte secondary battery using the said heat resistant synthetic resin microporous film.

  The method for producing a heat-resistant synthetic resin microporous film according to the present invention comprises: applying a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer to the surface of a synthetic resin microporous film; A plasma treatment is performed on the substrate.

  The synthetic resin microporous film used in the present invention contains a synthetic resin. Furthermore, the synthetic resin microporous film includes micropores penetrating the film front and back surfaces. The fine pores can impart excellent ion permeability to the heat-resistant synthetic resin microporous film. Thereby, the heat-resistant synthetic resin microporous film can transmit ions such as lithium ions in the thickness direction.

  5-100 micrometers is preferable and, as for the thickness of a synthetic resin microporous film, 10-50 micrometers is more preferable.

  In the present invention, the thickness of the synthetic resin microporous film can be measured according to the following procedure. That is, arbitrary 10 places of a synthetic resin microporous film are measured using a dial gauge, and the arithmetic mean value is defined as the thickness of the synthetic resin microporous film.

  The air permeability of the synthetic resin microporous film is preferably 50 to 600 sec / 100 mL, and more preferably 100 to 300 sec / 100 mL. A synthetic resin microporous film having an air permeability within the above range is excellent in both mechanical strength and ion permeability.

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

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

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

Next, each micropore formed in the measurement portion is surrounded by a rectangle whose one of the long side and the short side is parallel to the extending direction. 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 across the measurement part and the part which is not a measurement part, only the part which exists in a measurement part among micropores is set as a measuring object.

  The maximum major axis of the open end of the micropore in the synthetic resin microporous film is preferably 1 μm or less, and more preferably 100 nm to 800 nm. A synthetic resin microporous film having a maximum long diameter of 1 μm or less at the opening end of the micropore portion is excellent in mechanical strength and has uniform ion permeability. Such a synthetic resin microporous film can reduce the occurrence of minute internal short circuits (dendritic short circuits) due to the growth of dendrites (dendritic crystals).

  The average major axis of the open ends of the micropores in the synthetic resin microporous film is preferably 500 nm or less, and more preferably 200 nm to 500 nm. A synthetic resin microporous film having an average major axis at the opening end of the micropores of 500 nm or less has a uniform ion permeability, which can reduce the occurrence of minute internal short circuits (dendritic shorts).

  In addition, the maximum major axis and the average major axis of the open ends of the micropores in the synthetic resin microporous film are measured as follows. First, the surface of the synthetic resin microporous film is coated with carbon. Next, 10 arbitrary positions on the surface of the synthetic resin microporous 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 synthetic resin microporous film.

  The major axis of the opening end of each micropore appearing in the obtained photograph is measured. Among the long diameters of the opening ends in the microhole portion, the maximum long diameter is set as the maximum long diameter of the opening end of the microhole portion. The arithmetic mean value of the major axis of the open end in each micropore is defined as the average major axis of the open end of the micropore. 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.

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

The pore density of the synthetic resin microporous film is measured in the following manner. First, in an arbitrary portion of the surface of the synthetic resin microporous film, a measurement portion having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is determined, and this measurement portion 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 synthetic resin microporous film can be used without particular limitation as long as it is a porous film used as a separator in a conventional secondary battery such as a lithium ion secondary battery. As the synthetic resin microporous film, an olefin resin microporous film is preferable. The olefin-based resin microporous film is likely to be deformed or contracted due to melting of the olefin-based resin at a high temperature. On the other hand, according to the coating layer of the present invention, excellent heat resistance can be imparted to the olefin-based resin microporous film as described later. Therefore, the effect of this invention can be exhibited more by forming a membrane | film | coat layer integrally in an olefin resin microporous film.

  The olefin resin microporous film contains an olefin resin. As the olefin resin, ethylene resin and propylene resin are preferable, and propylene resin is more preferable.

  Examples of the propylene-based resin include homopolypropylene and copolymers of propylene and other olefins. When the synthetic resin microporous film is produced by the stretching method described later, homopolypropylene is preferable. Propylene-type resin may be used independently, or 2 or more types may be used together. 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. -An olefin etc. are mentioned, Ethylene is preferable.

As an index indicating the crystallinity of homopolypropylene, there is an isotactic pendart fraction (mmmm fraction) measured by ¹³ C-NMR method. The isotactic pendart fraction measured by ¹³ C-NMR method of homopolypropylene is a side chain with respect to the main chain formed by the carbon-carbon bond composed of any five consecutive propylene units. The ratio of the three-dimensional structure in which all of the methyl groups are located in the same direction occupies the whole molecular chain of homopolypropylene.

90% or more is preferable and, as for the isotactic pendart fraction measured by the ¹³ C-NMR method of homopolypropylene, 95% or more is more preferable. By setting the isotactic pendant fraction to 90% or more, it is possible to provide a synthetic resin microporous film in which micropores are uniformly formed.

  The weight average molecular weight of the olefin resin is preferably 250,000 to 500,000, and more preferably 280,000 to 480,000. According to the olefin resin having a weight average molecular weight within the above range, it is possible to provide an olefin resin microporous film having excellent film forming stability and having uniform micropores.

  The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the olefin resin is preferably 7.5 to 12, and more preferably 8 to 11. According to the olefin resin having a molecular weight distribution within the above range, an olefin resin microporous film having a high surface opening ratio, excellent ion permeability and excellent mechanical strength is provided. Can do.

  Here, the weight average molecular weight and number average molecular weight of the olefin resin are polystyrene-converted values measured by a GPC (gel permeation chromatography) method. Specifically, after collecting 6 to 7 mg of olefin resin, supplying the collected olefin resin to a test tube, o-DCB containing 0.05% by weight of BHT (dibutylhydroxytoluene) 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 solution is shaken for 1 hour at 145 ° C. and a rotation speed of 25 rpm using a dissolution filter, and the olefin resin is dissolved in the BHT o-DCB solution to obtain a measurement sample. Using this measurement sample, the weight average molecular weight and number average molecular weight of the olefin resin can be measured by the GPC method.

The weight average molecular weight and the number average molecular weight in the olefin 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
TSK guard column-HHR (30) HT x 1
Mobile phase: o-DCB 1.0 mL / min
Sample concentration: 1 mg / mL
Detector: Bryce refractometer
Standard substance: 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 olefin resin, 160-165 degreeC is more preferable. According to the olefin resin having a melting point within the above range, it is possible to provide an olefin resin microporous film that has excellent film forming stability and suppresses a decrease in mechanical strength at high temperatures.

  As a synthetic resin microporous film, the olefin resin microporous film manufactured by the extending | stretching method is more preferable. The olefin-based resin microporous film produced by the stretching method is particularly susceptible to thermal shrinkage at high temperatures due to residual strain generated by stretching. Therefore, the effect of the production method of the present invention can be particularly exhibited by using such an olefin-based resin microporous film in the production method of the present invention.

  As a method for producing an olefin resin microporous film by a stretching method, specifically, (1) a step of obtaining an olefin resin film by extruding the olefin resin, and generating a lamellar crystal in the olefin resin film And a step of growing, and a step of obtaining an olefin-based resin microporous film in which micropores are formed by stretching the olefin-based resin film and separating lamella crystals; and (2) an olefin-based method A process of obtaining an olefin resin film by extruding an olefin resin composition containing a resin and a filler, and an interface between the olefin resin and the filler by uniaxially or biaxially stretching the olefin resin film Olefin-based resin microporous film in which micropores are formed by peeling And a method and a that step. The method (1) is preferable because an olefin-based resin microporous film in which a large number of micropores are uniformly formed is obtained.

As the method for producing an olefin-based resin microporous film, particularly preferably, the following steps:
The olefin resin was melt-kneaded at a temperature not lower than 20 ° C. higher than the melting point of the olefin resin and not higher than 100 ° C. higher than the melting point of the olefin resin in an extruder, and attached to the tip of the extruder. An extrusion step of obtaining an olefin-based resin film by extruding from a die;
Curing step for curing the olefin resin film after the extrusion step at a temperature 30 ° C. lower than the melting point of the olefin resin and 1 ° C. lower than the melting point of the olefin resin;
A first stretching step in which the olefin resin film after the curing step is uniaxially stretched at a stretching ratio of 1.2 to 1.6 times at a surface temperature of −20 ° C. or more and less than 100 ° C .;
A second stretching step in which the olefin-based resin film stretched in the first stretching step is uniaxially stretched at a surface magnification of 1.2 to 2.2 times at a surface temperature of 100 to 150 ° C;
And an annealing step of annealing the olefin-based resin film that has been stretched in the second stretching step.

  According to the above method, it is possible to obtain an olefin resin microporous film in which a large number of micropores penetrating the film front and back surfaces are formed. According to such an olefin-based resin microporous film, there is provided a heat-resistant synthetic resin microporous film that has excellent air permeability and can smoothly and uniformly transmit ions such as lithium ions. be able to. Therefore, according to such a heat resistant synthetic resin microporous film, the internal resistance of the secondary battery can be reduced. Such a secondary battery can be charged and discharged at a high current density even in a high output application such as a vehicle such as an electric vehicle. In addition, even when the inside of the secondary battery becomes hot due to occurrence of an abnormal situation such as overcharge, the electrical safety between the electrodes can be suppressed to a high level. Is secured.

(Extrusion process)
The olefin-based resin film containing the olefin-based resin can be manufactured by supplying the olefin-based resin to an extruder, melt-kneading, and then extruding from a T-die attached to the tip of the extruder.

  The temperature of the olefin resin when the olefin resin is melt-kneaded by an extruder is preferably 20 ° C. higher than the melting point of the olefin resin and 100 ° C. higher than the melting point of the olefin resin. More preferably, the temperature is 25 ° C. higher than the melting point of the olefin-based resin and 80 ° C. higher than the melting point of the olefin-based resin. It is particularly preferable that the temperature be 50 ° C. or higher than the melting point. By setting the temperature of the olefin resin at the time of melt kneading to a temperature that is 20 ° C. higher than the melting point of the olefin resin, an olefin resin microporous film having a uniform thickness can be obtained. Moreover, the orientation of an olefin resin can be improved and the production | generation of a lamella can be accelerated | stimulated by making the temperature of the olefin resin at the time of melt-kneading into the temperature below 100 degreeC higher than melting | fusing point of an olefin resin.

  50-300 are preferable, as for the draw ratio at the time of extruding an olefin resin from an extruder to a film form, 65-250 are more preferable, and 70-250 are especially preferable. By setting the draw ratio to 50 or more, the tension applied to the olefin resin can be improved. As a result, the olefin resin can be sufficiently oriented to promote the production of lamellae. Moreover, the film-forming stability of an olefin resin film can be improved by making a draw ratio into 300 or less. This makes it possible to obtain an olefin-based resin microporous film having a uniform thickness and width.

  The draw ratio refers to a value obtained by dividing the clearance of the lip of the T die by the thickness of the olefin 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. The thickness of the olefin resin film extruded from the T die is 10 or more in the thickness of the olefin 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.

  The film forming speed of the olefin resin film is preferably 10 to 300 m / min, more preferably 15 to 250 m / min, and particularly preferably 15 to 30 m / min. By setting the film forming speed of the olefin resin film to 10 m / min or more, the tension applied to the olefin resin can be improved. As a result, the olefin resin molecules can be sufficiently oriented to promote the formation of lamellae. Moreover, the film-forming stability of an olefin resin film can be improved by making the film-forming speed | rate of an olefin resin film into 300 m / min or less. This makes it possible to obtain an olefin-based resin microporous film having a uniform thickness and width.

  And the olefin resin which comprises the olefin resin film is cooled by cooling the olefin resin film extruded from T-die until the surface temperature becomes below 100 degreeC lower than melting | fusing point of the said olefin resin. Crystallizes and lamella is highly produced. In the present invention, the olefin resin film constituting the olefin resin film is oriented in advance by extruding the melt-kneaded olefin resin, and then the olefin resin film is cooled. Thereby, the part in which the olefin resin is oriented can promote the generation of lamellae.

  The surface temperature of the cooled olefin resin film is preferably 100 ° C. or lower than the melting point of the olefin resin, more preferably 140 to 110 ° C. lower than the melting point of the olefin resin, and more than the melting point of the olefin resin. Also, a temperature lower by 135 to 120 ° C is particularly preferable. By cooling the surface temperature of the olefin resin film to the above range, the olefin resin can be crystallized and a lamella can be highly produced.

(Curing process)
Subsequently, the olefin resin film obtained by the extrusion process described above is cured. The curing process of the olefin resin is performed to grow the lamella formed in the olefin resin film in the extrusion process. As a result, it is possible to form a laminated lamella structure in which crystallized portions (lamellar) and amorphous portions are alternately arranged in the extrusion direction of the olefin-based 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 olefin resin film obtained by the extrusion process at a temperature not lower than 30 ° C. lower than the melting point of the olefin resin and not higher than 1 ° C. lower than the melting point of the olefin resin.

  The curing temperature of the olefin resin film is preferably 30 ° C. lower than the melting point of the olefin resin and 1 ° C. lower than the melting point of the olefin resin, preferably 25 ° C. lower than the melting point of the olefin resin. And a temperature lower by 10 ° C. than the melting point of the olefin resin is more preferable. By setting the curing temperature of the olefin resin film to 30 ° C. lower than the melting point of the olefin resin, crystallization of the olefin resin film can be sufficiently promoted. Moreover, collapse of the lamella structure by relaxation of the molecular orientation of an olefin resin can be reduced by making the curing temperature of an olefin resin film below 1 degreeC lower than melting | fusing point of an olefin resin.

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

  The curing of the olefin-based resin film may be performed while the olefin-based resin film is running, or may be performed in a state where the olefin-based resin film is wound into a roll.

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

  When the olefin-based resin film is cured in a rolled state, the curing time is preferably 1 hour or longer, and more preferably 15 hours or longer. By curing the olefin-based resin film in a state wound in such a curing time, the curing of the olefin-based resin film as a whole can be performed sufficiently with the curing temperature described above. Thereby, a lamella can be fully grown in an olefin resin film. Moreover, from a viewpoint of reducing the thermal deterioration of an olefin resin film, the curing time is preferably 35 hours or less, and more preferably 30 hours or less.

  If the olefin resin film is cured in a roll shape, the olefin resin film is unwound from the olefin resin film roll after the curing process, and the stretching process and the annealing process described later are performed. Good.

(First stretching process)
Next, a first stretching step is performed on the olefin-based resin film after the curing step, in which the surface temperature is -20 ° C. or higher and lower than 100 ° C., and uniaxial stretching is performed at a stretching ratio of 1.2 to 1.6 times. In the first stretching step, the olefin resin film is preferably uniaxially stretched only in the extrusion direction. In the first stretching step, the lamellae in the olefin-based resin film are hardly melted, and by separating the lamellae by stretching, a fine crack is efficiently generated independently in the non-crystalline part between the lamellae. A large number of micropores are reliably formed starting from this crack.

  In the first stretching step, the surface temperature of the olefin resin film is preferably −20 ° C. or higher and lower than 100 ° C., more preferably 0 to 80 ° C., and particularly preferably 10 to 40 ° C. By setting the surface temperature of the olefin-based resin film to −20 ° C. or higher, breakage of the olefin-based resin film during stretching can be reduced. Moreover, a crack can be generated in the amorphous part between lamellae by setting the surface temperature of the olefin resin film to less than 100 ° C.

  In the first stretching step, the stretching ratio of the olefin resin film is preferably 1.2 to 1.6 times, and more preferably 1.25 to 1.5 times. By setting the draw ratio to 1.2 times or more, micropores can be formed in the non-crystalline part between lamellae. Moreover, a micropore part can be uniformly formed in an olefin resin microporous film by making a draw ratio into 1.6 times or less.

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

  The stretching speed in the first stretching step of the olefin resin film is preferably 20% / min or more, more preferably 20 to 500% / min, and particularly preferably 20 to 70% / 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 500% / min or less, it is possible to suppress breakage of the olefin resin film in the first stretching step.

  In addition, in this invention, the extending | stretching speed of an olefin resin film means the change rate of the dimension in the extending | stretching direction of the olefin resin film per unit time.

  The method for stretching the olefin resin film in the first stretching step is not particularly limited as long as the olefin resin film can be uniaxially stretched. For example, a plurality of rolls having different peripheral speeds are used for the olefin resin film. Examples thereof include a method of uniaxially stretching at a predetermined temperature using a stretching apparatus.

(Second stretching step)
Next, a second stretching step is performed in which the olefin resin film after the first stretching step is subjected to a uniaxial stretching process at a surface temperature of 100 to 150 ° C. and a stretching ratio of 1.2 to 2.2 times. Also in the second stretching step, the olefin resin film is preferably uniaxially stretched only in the extrusion direction. By performing the stretching treatment in the second stretching step, a large number of micropores formed in the olefin resin film in the first stretching step can be grown.

  In the second stretching step, the surface temperature of the olefin resin film is preferably 100 to 150 ° C, and more preferably 110 to 140 ° C. By setting the surface temperature of the olefin resin film to 100 ° C. or higher, the micropores formed in the olefin resin film in the first stretching step can be highly grown. Moreover, the obstruction | occlusion of the micropore part formed in the olefin resin film in a 1st extending | stretching process can be highly reduced by making the surface temperature of an olefin resin film into 150 degrees C or less.

  In the second stretching step, the stretching ratio of the olefin-based resin film is preferably 1.2 to 2.2 times, and more preferably 1.5 to 2 times. By setting the draw ratio of the olefin resin film to 1.2 times or more, the micropores formed in the olefin resin film during the first drawing step can be grown. Thereby, the olefin resin microporous film having excellent air permeability can be provided. Moreover, it becomes possible by suppressing the draw ratio of an olefin resin film to 2.2 times or less to block | close the micropore part formed in the olefin resin film in a 1st extending | stretching process.

  In the second stretching step, the stretching speed of the olefin resin film is preferably 500% / min or less, more preferably 400% / min or less, and particularly preferably 15 to 60% / min. By setting the stretching speed of the olefin resin film within the above range, micropores can be uniformly formed in the olefin resin film.

  The method for stretching the olefin resin film in the second stretching step is not particularly limited as long as the olefin resin film can be uniaxially stretched. For example, a plurality of rolls having different peripheral speeds are used for the olefin resin film. Examples thereof include a method of uniaxially stretching at a predetermined temperature using a stretching apparatus.

(Annealing process)
Next, the annealing process which anneal-treats to the olefin resin film in which the uniaxial stretching was given in the 2nd extending process is performed. This annealing step is performed to alleviate the residual strain generated in the olefin resin film due to the stretching applied in the stretching step described above, and to suppress the heat shrinkage caused by heating in the resulting olefin resin microporous film. Is called.

  The surface temperature of the olefin resin film in the annealing step is preferably not less than the surface temperature of the olefin resin film in the second stretching step and 10 ° C. or lower than the melting point of the olefin resin. By setting the surface temperature of the olefin resin film to be equal to or higher than the surface temperature of the olefin resin film at the time of the second stretching step, the strain remaining in the olefin resin film can be sufficiently relaxed. Thereby, it becomes possible to improve the dimensional stability at the time of the heating of an olefin resin microporous film. Moreover, blockage | closure of the micropore part formed at the extending process can be suppressed by making the surface temperature of an olefin resin film below 10 degrees C lower than melting | fusing point of an olefin resin.

  The shrinkage ratio of the olefin resin film in the annealing step is preferably 25% or less. By setting the shrinkage rate of the olefin resin film to 25% or less, the occurrence of sagging of the olefin resin film can be reduced, and the olefin resin film can be annealed uniformly.

  The shrinkage of the olefin-based resin film is 100 by dividing the shrinkage length of the olefin-based resin film in the stretching direction during the annealing step by the length of the olefin-based resin film in the stretching direction after the second stretching step. The value multiplied by.

(Coating process)
In the method of the present invention, a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer is applied to the surface of the above-described synthetic resin microporous film.

  The radical polymerizable monomer means a monomer having a functional group having a radical polymerizable unsaturated bond capable of radical polymerization by plasma treatment. The radically polymerizable monomer includes at least a trifunctional or higher polyfunctional acrylic monomer.

  The trifunctional or higher polyfunctional acrylic monomer contains 3 or more functional groups having a radical polymerizable unsaturated bond that can be radically polymerized by plasma treatment, and contains an acryloyl group or a methacryloyl group. Just do it. The functional group having a radical polymerizable unsaturated bond capable of radical polymerization is preferably an acryloyl group or a methacryloyl group.

  As the polyfunctional acrylic monomer, a trifunctional or higher polyfunctional acrylic monomer is used, and a trifunctional to hexafunctional polyfunctional acrylic monomer is preferably used. In addition, trifunctional or more polyfunctional acrylic monomers may be used alone or in combination of two or more.

  The trifunctional polyfunctional acrylic monomer is not particularly limited. For example, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, ethoxylated isocyanuric acid triacrylate, ε- Examples include caprolactone-modified tris- (2-acryloxyethyl) isocyanurate and ethoxylated glycerin triacrylate.

  The tetrafunctional polyfunctional acrylic monomer is not particularly limited. For example, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, ditrimethylolpropane tetraacrylate, ditrimethylolpropane tetramethacrylate, ethoxylated pentaerythritol tetraacrylate, and ethoxy And pentaerythritol tetramethacrylate.

  The hexafunctional polyfunctional acrylic monomer is not particularly limited, and examples thereof include dipentaerythritol hexaacrylate and dipentaerythritol hexamethacrylate.

  Of these, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate are preferable as the trifunctional or higher polyfunctional acrylic monomer.

  The radical polymerizable monomer may further contain a bifunctional polyfunctional acrylic monomer. As the bifunctional polyfunctional acrylic monomer, two functional groups having a radical polymerizable unsaturated bond capable of radical polymerization by plasma treatment are contained in one molecule, and an acryloyl group or a methacryloyl group is contained. Good. The functional group having a radical polymerizable unsaturated bond capable of radical polymerization is preferably an acryloyl group or a methacryloyl group.

  Examples of the bifunctional polyfunctional acrylic monomer include 1,9-nonanediol dimethacrylate, 1,9-nonanediol diacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, tripropylene glycol diacrylate, 2-hydroxy-3-acryloyloxypropyl methacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, 1,10-decanediol methacrylate, Neopentyl glycol dimethacrylate, glycerin dimethacrylate and the like can be mentioned. The bifunctional polyfunctional acrylic monomer may be used alone or in combination of two or more.

  The content of the trifunctional or higher polyfunctional acrylic monomer in the radical polymerizable monomer is preferably 10% by weight or more, more preferably 30% by weight or more, particularly preferably 80% by weight or more, and most preferably 100% by weight. . By using a radically polymerizable monomer containing 10% by weight or more of a trifunctional or higher polyfunctional acrylic monomer, the resulting heat resistant synthetic resin microporous film has excellent heat resistance without reducing air permeability. Can be granted.

  A radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer is applied to the surface of the synthetic resin microporous film. At this time, the radical polymerizable monomer may be directly applied to the surface of the synthetic resin microporous film. However, it is preferable to disperse or dissolve the radical polymerizable monomer in a solvent to obtain a coating solution, and to apply this coating solution to the surface of the synthetic resin microporous film. Thus, by using a radically polymerizable monomer as a coating liquid, the radically polymerizable monomer can be uniformly attached to the surface of the synthetic resin microporous film. This makes it possible to produce a heat-resistant synthetic resin microporous film having a uniform coating layer and high heat resistance. Furthermore, by using a radically polymerizable monomer as the coating liquid, it is possible to highly reduce the blockage of the micropores in the synthetic resin microporous film by the radically polymerizable monomer. Therefore, it is possible to improve the heat resistance of the heat-resistant synthetic resin microporous film without reducing the air permeability.

  The solvent used in the coating liquid is not particularly limited as long as it can dissolve or disperse the radical polymerizable monomer. For example, alcohols such as methanol, ethanol, propanol, isopropyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone Ketones such as, ethers such as tetrahydrofuran and dioxane, ethyl acetate, chloroform and the like. Of these, ethyl acetate, ethanol, methanol, and acetone are preferable. These solvents can be removed smoothly after the coating solution is applied to the surface of the synthetic resin microporous film. Furthermore, the solvent has low reactivity with an electrolyte solution constituting a secondary battery such as a lithium ion secondary battery, and is excellent in safety.

  The content of the radical polymerizable monomer in the coating liquid is preferably 3 to 20% by weight, and more preferably 5 to 15% by weight. By setting the content of the radical polymerizable monomer within the above range, a coating layer can be uniformly formed on the surface of the synthetic resin microporous film without clogging the micropores, thus reducing the air permeability. A heat-resistant synthetic resin microporous film with improved heat resistance can be produced.

  The method for applying the radical polymerizable monomer to the surface of the synthetic resin microporous film is not particularly limited. For example, (1) a method of applying a radical polymerizable monomer to the surface of the synthetic resin microporous film; (2) radical polymerization A method in which a synthetic resin microporous film is immersed in a polymerizable monomer, and a radical polymerizable monomer is applied to the surface of the synthetic resin microporous film; (3) a coating solution obtained by dissolving or dispersing the radical polymerizable monomer in a solvent And after applying this coating liquid on the surface of the synthetic resin microporous film, the synthetic resin microporous film is heated to remove the solvent; and (4) the radical polymerizable monomer is dissolved in the solvent or Disperse to prepare a coating liquid, immerse the synthetic resin microporous film in this coating liquid, coat the coating liquid in the synthetic resin microporous film, and then synthesize the synthetic resin microporous film. Heating the beam and a method of removing the solvent. Of these, the methods (3) and (4) are preferred. According to these methods, the radical polymerizable monomer can be uniformly coated on the surface of the synthetic resin microporous film.

  In the methods (3) and (4) above, the heating temperature of the synthetic resin microporous film for removing the solvent can be set according to the type and boiling point of the solvent used. The heating temperature of the synthetic resin microporous film for removing the solvent is preferably 50 to 140 ° C, more preferably 70 to 130 ° C. By setting the heating temperature within the above range, the coated solvent can be efficiently removed while highly reducing heat shrinkage and blockage of the micropores of the synthetic resin microporous film.

  In the methods (3) and (4) above, the heating time of the synthetic resin microporous film for removing the solvent is not particularly limited, and can be set according to the type and boiling point of the solvent used. The heating time of the synthetic resin microporous film for removing the solvent is preferably 0.02 to 60 minutes, more preferably 0.1 to 30 minutes.

  As described above, the radical polymerizable monomer can be attached to the surface of the synthetic resin microporous film by coating the surface of the synthetic resin microporous film with the radical polymerizable monomer or the coating liquid.

  The amount of the radical polymerizable monomer attached to the synthetic resin microporous film is preferably 5 to 80 parts by weight, more preferably 7 to 50 parts by weight, and more preferably 10 to 40 parts by weight with respect to 100 parts by weight of the synthetic resin microporous film. Is particularly preferred. By making the adhesion amount of the radical polymerizable monomer within the above range, the coating layer can be uniformly formed on the surface of the synthetic resin microporous film without blocking the micropores. This makes it possible to produce a heat-resistant synthetic resin microporous film with improved heat resistance without reducing air permeability.

(Plasma treatment process)
Next, in the method of the present invention, the synthetic resin microporous film coated with the radical polymerizable monomer is subjected to a plasma treatment. As a result, the radically polymerizable monomer can be polymerized to form a film layer containing a polymer of the radically polymerizable monomer integrally on at least a part of the surface of the synthetic resin microporous film, preferably the entire surface.

  As described above, the coating layer contains a polymer of a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer. By using the coating layer containing such a polymer, It is possible to provide a heat-resistant synthetic resin microporous film having a high heat shrinkage and having excellent heat resistance. In addition, since the trifunctional or higher polyfunctional acrylic monomer has excellent compatibility with the synthetic resin microporous film, a coating layer can be formed without blocking the micropores of the synthetic resin microporous film. . Therefore, according to the coating layer, excellent heat resistance can be imparted to the synthetic resin microporous film without reducing air permeability.

  In the present invention, plasma treatment is performed after applying a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer on the surface of the synthetic resin microporous film. At this time, since the plasma has high energy, the plasma also reaches the synthetic resin microporous film, and the C—C bonds in the synthetic resin contained in the surface portion of the synthetic resin microporous film Synthetic resins can also be activated to generate radicals by molecular chain cleavage such as C—H bonds. Thereby, a part of the synthetic resin and a part of the polymer of the radical polymerizable monomer can be chemically bonded. Although details are not clear, the polymer of the radical polymerizable monomer contained in the coating layer is graft-polymerized to the synthetic resin contained on the surface of the synthetic resin microporous film by chemical bonding. it is conceivable that. By forming such chemical bonds, it is possible to form a coating layer that is firmly integrated with the surface of the synthetic resin microporous film. Thereby, the heat resistance of the synthetic resin microporous film can be further improved.

  Furthermore, by subjecting the synthetic resin microporous film to plasma treatment, a part of the synthetic resin is decomposed to lower the molecular weight, and as a result, the stress relaxation of the synthetic resin microporous film can proceed. It is considered that such stress relaxation effectively prevents thermal shrinkage of the heat-resistant synthetic resin microporous film at high temperatures. Moreover, since plasma has a moderately high energy, it can suppress the fall of the mechanical strength of the synthetic resin microporous film by excessive decomposition | disassembly of a synthetic resin. Examples of the synthetic resin microporous film that can provide a stress relaxation effect include polypropylene resin microporous films.

  The plasma treatment can be performed, for example, by exposing a synthetic resin microporous film coated with a radical polymerizable monomer in plasma generated by discharge in a plasma generating gas. By such plasma treatment, the radically polymerizable monomer can be activated and polymerized.

  The plasma processing can be performed using a known plasma processing apparatus. FIG. 1 shows a schematic diagram of a plasma processing apparatus suitably used in the method of the present invention.

  A plasma processing apparatus A shown in FIG. 1 includes a plasma generation apparatus 10 and a plasma generation gas introduction apparatus 20.

  The plasma generator 10 includes a pair of electrodes 11a and 11b and a power source 12 which are arranged to face each other with a predetermined distance therebetween, and one electrode 11a has a flat plate shape. The other electrode 11b has a roll shape. The shapes of the electrodes 11a and 11b are not particularly limited. Both the electrodes 11a and 11b may have a flat plate shape or a roll shape. Alternatively, the other electrode 11b may have a roll shape, and the one electrode 11a may have an arc shape so as to follow the outer peripheral surface of the other electrode 11b. At least one of the opposing surfaces of the electrodes 11a and 11b is covered with a solid dielectric.

  One electrode 11a is disposed on the outer peripheral surface of the other electrode 11b at a predetermined interval, and a space 13 is formed between the pair of electrodes 11a and 11b. One electrode 11a is connected to the power source 12, and the other electrode 11b is electrically grounded.

  The plasma generation gas introduction device 20 is provided with a gas supply source 21 filled with a plasma generation gas and a blowout port (not shown) for blowing the plasma generation gas into the space 13 at the lower end. The gas supply source 21 and the nozzle 22 are connected by a pipe 23.

  The synthetic resin microporous film B coated with the radical polymerizable monomer is passed over the guide roll 14 disposed on the film feeding side and guided to the other electrode 11b formed in a roll shape, After passing over the upper outer peripheral surface of the other electrode 11b about a half turn so as to pass between the pair of electrodes 11a, 11b, it is passed over a guide roll 15 disposed on the film delivery side. The other electrode 11b is rotatable by a rotation mechanism (not shown). In addition, the drive roll 16 is disposed in contact with the guide roll 15 disposed on the film delivery side, and the guide roll 15 can be driven to rotate by the drive roll 16. The synthetic resin microporous film B can be continuously conveyed by rotating the electrode 11b and the guide roll 15.

  A temperature control path 17 is disposed inside the electrode 11b, and the surface temperature of the electrode 11b can be adjusted by circulating a temperature control medium such as temperature-controlled water in the temperature control path 17. . Thereby, the surface temperature of the synthetic resin microporous film B stretched over the outer peripheral surface of the electrode 11b can be adjusted.

  Next, a method for plasma processing the synthetic resin microporous film B coated with the radical polymerizable monomer using the plasma processing apparatus described above will be described. First, the synthetic resin microporous film B is spanned over the guide roll 14, the other electrode 11b, and the guide roll 15, respectively, and then the electrode 11b and the guide roll 15 are rotated, so that the synthetic resin microporous film B Convey continuously while passing 13 through. By applying a pulsed voltage from the power source 12 to the electrode 11a, the space 13 is made a discharge space. On the other hand, after the plasma generating gas is introduced from the gas supply source 21 into the nozzle 22 via the pipe 23, the plasma generating gas is blown out from the outlet (not shown) of the nozzle 22 into the space 13. As a result, the plasma generating gas is turned into plasma in the discharge space 13, and the plasma processing can be performed by exposing the synthetic resin microporous film B to the plasma.

  In the plasma treatment step, the surface temperature of the synthetic resin microporous film B coated with the radical polymerizable monomer is preferably 15 to 100 ° C. By setting the surface temperature within the above range, generation of wrinkles due to thermal expansion of the synthetic resin microporous film B can be reduced.

  As the plasma generating gas, an inert gas is preferable. Examples of the inert gas include nitrogen gas, argon gas, and helium gas. By using the inert gas, the oxygen concentration in the discharge space 13 can be reduced, and the inhibition of the polymerization reaction of the radical polymerizable monomer by oxygen can be reduced.

  The oxygen concentration (volume concentration) in the discharge space 13 is preferably 3000 ppm or less, and more preferably 0 to 2000 ppm. Thus, by making oxygen concentration low, inhibition of the polymerization reaction of the radically polymerizable monomer by oxygen can be reduced.

The plasma treatment step can be performed in an atmospheric pressure atmosphere or a vacuum (reduced pressure) atmosphere, but is preferably performed in an atmospheric pressure atmosphere. The plasma treatment process under an atmospheric pressure atmosphere does not require equipment or time required for decompression, and can improve productivity. When the plasma treatment step is performed under an atmospheric pressure atmosphere, the pressure in the discharge space 13 is preferably 1.013 × 10 ^{4 to} 50.663 × 10 ⁴ Pa, and 1.333 × 10 ^{4 to} 10.664 × 10 ^4. Pa is more preferable, and 9.331 × 10 ^{4 to} 10.9797 × 10 ⁴ Pa is particularly preferable.

  The voltage may be supplied to either one of the pair of electrodes 11a and 11b, or may be supplied to both. The voltage supplied to the electrodes is not limited to a pulse wave shape, and may be a continuous wave shape such as a sine wave, but a pulse wave shape is preferable.

  The rise time of the pulse wave voltage is preferably 100 μs or less. Thereby, the plasma generating gas can be efficiently converted into plasma. On the other hand, the falling time of the pulse wave voltage is preferably 100 μs or less. Thereby, the plasma generating gas can be efficiently converted into plasma. In the present invention, the rise time of the pulse wave voltage means a time during which the voltage change is continuously positive. In the present invention, the falling time of the pulse wave voltage means a time during which the voltage change is continuously negative.

  From the viewpoint of efficiently converting the plasma generating gas into plasma, the electric field strength of the pulse wave voltage is preferably 10 to 1000 kV / cm. From the same viewpoint, the frequency of the pulse wave voltage is preferably 0.5 to 100 kHz.

In the plasma treatment step, the synthetic resin microporous film coated with the radical polymerizable monomer is preferably treated with plasma having an energy density of 5 to 50 J / cm ² . 5-50 J / cm < ² > is preferable and, as for the energy density of the plasma with respect to the synthetic resin microporous film with which the radically polymerizable monomer was coated, 10-45 J / cm < ² > is more preferable. If the energy density of the plasma is too low, the amount of plasma that reaches the synthetic resin microporous film is reduced, so that the chemical bond between the synthetic resin in the synthetic resin microporous film and the polymer of the radical polymerizable monomer is sufficient. May not be formed. Moreover, when the energy density of plasma is too high, the synthetic resin in the synthetic resin microporous film may deteriorate, and the mechanical strength of the resulting heat-resistant synthetic resin microporous film may be reduced.

According to the method of the present invention described above, a heat-resistant synthetic resin microporous film having excellent heat resistance can be provided. Such heat-resistant synthetic resin microporous film is
A synthetic resin microporous film containing a synthetic resin;
And a coating layer that is formed integrally with at least a part of the surface of the synthetic resin microporous film and includes a polymer of a radical polymerizable monomer including a trifunctional or higher polyfunctional acrylic monomer. It is characterized by that.

  The coating layer contains a polymer of a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer. And it is preferable that a part of polymer in a membrane | film | coat layer and a part of synthetic resin in a synthetic resin microporous film have couple | bonded chemically. By using a coating layer containing such a polymer, it is possible to provide a heat-resistant synthetic resin microporous film that has a high heat shrinkage at high temperatures and has excellent heat resistance.

The coating layer can improve the heat resistance of the heat-resistant synthetic resin microporous film without using inorganic particles. Therefore, it is preferable that the coating layer does not contain inorganic particles. Examples of the inorganic particles include inorganic particles generally used for porous coating layers. Examples of the material constituting the inorganic particles include Al ₂ O ₃ , SiO ₂ , TiO ₂ , and MgO.

  In addition, by using a polyfunctional acrylic monomer having three or more functional groups, a coating layer can be formed without blocking the micropores of the synthetic resin microporous film. It is highly reduced that the air permeability of the synthetic resin microporous film is lowered.

  The gel fraction of the heat resistant synthetic resin microporous film is preferably 5% or more, and more preferably 10% or more. By setting the gel fraction to 5% or more, a strong coating layer is formed, and thereby the heat shrinkage of the heat-resistant synthetic resin microporous film can be highly reduced. Further, the gel fraction of the heat resistant synthetic resin microporous film is preferably 99% or less. By making the gel fraction 99% or less, it is possible to reduce the brittleness of the heat-resistant synthetic resin microporous film.

In the present invention, the gel fraction of the heat-resistant synthetic resin microporous film can be measured according to the following procedure. First, by cutting the heat-resistant synthetic resin microporous film, 0.1 g of a test piece is obtained, and the weight [W ₁ (g)] of this test piece is weighed and filled into a test tube. Next, 20 ml of xylene is poured into the test tube, and the entire test piece is immersed in xylene. The test tube is covered with an aluminum lid and immersed in an oil bath heated to 130 ° C. for 24 hours. The contents in the test tube taken out from the oil bath are immediately opened in a stainless steel mesh basket (# 200) before the temperature drops, and insoluble matter is filtered. The weight [W ₀ (g)] of the mesh cage is weighed in advance. The mesh basket and the filtrate are dried under reduced pressure at 80 ° C. for 7 hours, and then the weight [W ₂ (g)] of the mesh basket and the filtrate is measured. Then, the gel fraction is calculated according to the following formula.
Gel fraction [%] = 100 × (W ₂ −W ₀ ) / W ₁

  The air permeability of the heat-resistant synthetic resin microporous film is not particularly limited, but is preferably 50 to 600 sec / 100 mL, and more preferably 100 to 300 sec / 100 mL. As described above, the heat-resistant synthetic resin microporous film of the present invention has a high reduction in air permeability due to the formation of the coating layer. Therefore, the air permeability of the heat resistant synthetic resin microporous film of the present invention can be within the above range. The air permeability of the heat-resistant synthetic resin microporous film can be measured by the same method as the method for measuring the air permeability of the synthetic resin microporous film described above.

  When the heat-resistant synthetic resin microporous film is heated from 25 ° C. to 180 ° C. at a heating rate of 5 ° C./min, the maximum heat shrinkage rate of the heat-resistant synthetic resin microporous film is preferably 25% or less, 20% The following is more preferable, and 5 to 17% is particularly preferable. The heat resistant synthetic resin microporous film is provided with excellent heat resistance by the coating layer. In addition, the maximum heat shrinkage rate of a heat resistant synthetic resin microporous film can be measured by the method as described in the Example mentioned later.

  The puncture strength of the heat-resistant synthetic resin microporous film is preferably 0.5 N / 25 μm or more, and more preferably 0.7 N / 25 μm or more. According to the heat-resistant synthetic resin microporous film having a puncture strength within the above range, when the heat-resistant synthetic resin microporous film is cut in the battery manufacturing process, tearing of the heat-resistant synthetic resin microporous film can be reduced. it can.

  In the present invention, the puncture strength of the heat-resistant synthetic resin microporous film can be measured according to JIS Z1707 (1998). Specifically, a needle having a diameter of 1.0 mm and a tip having a semicircular shape with a radius of 0.5 mm is pierced into a heat-resistant synthetic resin microporous film at a speed of 50 mm / min, and the maximum stress until the needle penetrates is determined. The piercing strength.

  The heat-resistant synthetic resin microporous film of the present invention is suitably used as a separator for non-aqueous electrolyte secondary batteries. Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery. Since the heat-resistant synthetic resin microporous film of the present invention is excellent in heat resistance, even when the inside of the battery becomes high temperature by using such a heat-resistant synthetic resin microporous film as a separator, It is possible to provide a non-aqueous electrolyte secondary battery in which an electrical short circuit between electrodes is highly suppressed.

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 .

  The heat-resistant synthetic resin microporous film of the present invention has a coating layer containing a polymer of a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer. According to this coating layer, the wettability of the heat-resistant synthetic resin microporous film with respect to the non-aqueous electrolyte can also be improved. For this reason, the heat-resistant synthetic resin microporous film allows the nonaqueous electrolytic solution to easily enter into the micropores, and can uniformly hold a large amount of the nonaqueous electrolytic solution. Therefore, by using a heat-resistant synthetic resin microporous film as a separator, it is possible to provide a non-aqueous electrolyte secondary battery that is excellent in productivity and has a high reduction in lifetime due to deterioration of the electrolyte. it can.

  According to the method for producing a synthetic resin microporous film of the present invention, as described above, it is possible to produce a heat-resistant synthetic resin microporous film having improved heat resistance without reducing ion permeability such as lithium ions. it can. Therefore, according to such a heat-resistant synthetic resin microporous film, it is possible to provide a non-aqueous electrolyte secondary battery that can charge and discharge at a high current density while reducing internal resistance. In addition, such a non-aqueous electrolyte secondary battery can reduce the electrical short circuit between the electrodes to a high level even when the internal temperature becomes high due to abnormal heat generation due to overcharging or the like.

  The heat-resistant synthetic resin microporous film produced by the method for producing a synthetic resin microporous film of the present invention does not require the use of a porous coating layer containing inorganic particles, and thus has excellent lightweight properties and a production process. There is no contamination of the production line due to the dropping of inorganic particles.

The schematic diagram of the plasma processing apparatus used suitably for the method of this invention is shown.

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

[Example 1]
1. Production of homopolypropylene microporous film (extrusion process)
Homopolypropylene (weight average molecular weight: 400,000, number average molecular weight: 37000, melt flow rate: 3.7 g / 10 min, isotactic pendart fraction measured by ¹³ C-NMR method: 97%, melting point: 165 ° C. ) Was fed to a single screw extruder and melt kneaded at a resin temperature of 200 ° C. Next, the melt-kneaded homopolypropylene was extruded from a T die attached to the tip of a single-screw extruder onto a cast roll at 95 ° C., and cooled to a surface temperature of 30 ° C. by applying cold air. This obtained the homopolypropylene film (width 200mm) which has the thickness shown in Table 1 of the elongate shape. The extrusion rate was 10 kg / hour, the film formation rate was 22 m / min, and the draw ratio was 83.

(Curing process)
The obtained long homopolypropylene film 50 m was wound around a cylindrical core having an outer diameter of 3 inches in a roll shape to obtain a homopolypropylene film roll. The homopolypropylene film roll was allowed to cure for 24 hours in a hot air oven where the atmospheric temperature of the place where the roll was installed was 150 ° C. At this time, the temperature of the homopolypropylene film was entirely the same as the temperature inside the hot stove from the surface of the roll to the inside.

(First stretching process)
Next, the homopolypropylene film is continuously unwound from the roll, and after the surface temperature of the homopolypropylene film is set to 20 ° C., the homopolypropylene film is sequentially passed over the first stretching roll and the second stretching roll, and the peripheral speed of the second stretching roll Was rotated so as to be larger than the peripheral speed of the first stretching roll, so that the homopolypropylene film was uniaxially stretched only in the transport direction (extrusion direction) at a stretching ratio of 1.4 times at a stretching speed of 50% / min. .

(Second stretching step)
Next, the homopolypropylene film fed from the second stretching roll is supplied into the heating furnace, the surface temperature of the homopolypropylene film is set to 120 ° C., and there is a predetermined interval in the vertical direction in the heating furnace and The homopolypropylene film is passed up and down in a zigzag manner in each of the seven stretching rolls arranged zigzag in the transport direction of the homopolypropylene film, and the circumferential speed of each of the stretching rolls is homopolypropylene. The homopolypropylene film was uniaxially stretched only in the transport direction at a stretch ratio of 2.0 times at a stretch rate of 42% / min by rotating the film so as to increase sequentially toward the transport direction of the film.

(Annealing process)
Next, the homopolypropylene film is sequentially supplied to the first roll and the second roll that are arranged above and below in the hot air furnace so that the surface temperature of the homopolypropylene film becomes 155 ° C. and tension is applied to the homopolypropylene film. As a result, the homopolypropylene film was annealed by being conveyed in a hot air oven for 4 minutes to obtain a homopolypropylene microporous film (thickness: 25 μm, basis weight: 11 g / m ² ). The shrinkage ratio of the homopolypropylene film in the annealing process was 5%.

The resulting homopolypropylene microporous film has an air permeability of 190 sec / 100 mL, a surface opening ratio of 30%, a maximum major axis of the open end of the microporous part is 530 nm, an average major axis of the open end of the microporous part is 320 nm, The pore density was 20 / μm ² .

2. Formation of coating layer (Coating process)
A coating solution was prepared by dissolving 95 parts by weight of ethyl acetate as a solvent and 5 parts by weight of trimethylolpropane triacrylate (TMPTA) as a radical polymerizable monomer. This coating solution was applied to the surface of a homopolypropylene microporous film.

  Thereafter, the homopolypropylene microporous film was heated at 80 ° C. for 2 minutes to evaporate and remove ethyl acetate. In the homopolypropylene microporous film, only 16 parts by weight of the radical polymerizable monomer was attached to 100 parts by weight of the homopolypropylene microporous film.

(Plasma treatment process)
Next, the homopolypropylene microporous film B was subjected to plasma treatment once as follows using the plasma treatment apparatus shown in FIG. The homopolypropylene microporous film B is passed over the guide roll 14, the other electrode 11b, and the guide roll 15, respectively, and then the electrode 11b and the guide roll 15 are rotated so that the homopolypropylene microporous film B is paired with a pair of electrodes. It was continuously transported at a transport speed of 1 m / min while passing between 11a and 11b. Water whose temperature was adjusted to 15 ° C. was circulated in the temperature adjustment path 17 disposed inside the electrode 11b.

By applying a pulsed voltage from the power source 12 to the electrode 11a under the following conditions, the space 13 was made a discharge space. At this time, the pressure in the discharge space 13 was 10.1 × 10 ⁴ Pa (atmospheric pressure). On the other hand, nitrogen gas was introduced from the gas supply source 21 into the nozzle 22 through the pipe 23 as a plasma generating gas, and then nitrogen gas was blown into the space 13 from the outlet (not shown) of the nozzle 22. As a result, the nitrogen gas was turned into plasma in the discharge space 13, and the homopolypropylene microporous film B was exposed to the plasma to perform plasma treatment. The oxygen concentration in the space 13 between the pair of electrodes 11a and 11b was 480 ppm. The energy density of the plasma with respect to the homopolypropylene microporous film was 5.8 J / cm ² .
<Voltage application conditions>
Glow discharge Pulse width: 9μsec
Rise time: 5 μs
Fall time: 5μs
Discharge frequency: 15 kHz
Dead time: 2.0 sec
DC voltage: 620V
Current value: 1.0A
Input power: 0.62kW

  A film layer containing a polymer of radical polymerizable monomer on the entire surface of the homopolypropylene microporous film and the wall surface of the opening end of the micropore continuous to the surface by polymerizing the radical polymerizable monomer by the plasma treatment described above. Were integrally formed. The heat-resistant homopolypropylene microporous film had the thickness shown in Table 1. Table 1 shows the content of the coating layer with respect to 100 parts by weight of the homopolypropylene microporous film in the heat-resistant homopolypropylene microporous film.

[Example 2]
A plasma treatment was performed three times on the homopolypropylene microporous film in the same procedure as in Example 1, and the plasma energy density for the homopolypropylene microporous film was changed to 17.4 J / cm ² . A heat-resistant homopolypropylene microporous film was prepared according to the procedure.

[Example 3]
The homopolypropylene microporous film was subjected to plasma treatment six times in the same procedure as in Example 1, and the plasma energy density for the homopolypropylene microporous film was changed to 34.8 J / cm ² , and the same as in Example 1. A heat-resistant homopolypropylene microporous film was prepared according to the procedure.

[Comparative Example 1]
Only a homopolypropylene microporous film was produced according to the same procedure as in Example 1 without forming a film layer.

[Comparative Example 2]
A heat-resistant homopolypropylene microporous film was prepared according to the same procedure as in Example 1 except that the plasma treatment was performed only once without performing the coating step of coating the homopolypropylene microporous film with the radical polymerizable monomer. Produced.

[Comparative Example 3]
The heat-resistant homopolypropylene microporous film was subjected to the same procedure as in Example 1 except that the homopolypropylene microporous film after the coating process was irradiated with an electron beam at an acceleration voltage of 200 kV and an irradiation dose of 70 kGy without performing plasma treatment. A film was prepared.

[Evaluation]
The gel fraction of the heat resistant homopolypropylene microporous film was measured as described above. The air permeability of the heat-resistant homopolypropylene microporous film was measured in the same manner as described above for the air permeability of the synthetic resin microporous film. The puncture strength of the heat resistant homopolypropylene microporous film was measured as described above. These results are shown in Table 1.

  The maximum heat shrinkage rate, tensile strength, and elongation at break of the heat-resistant homopolypropylene microporous film were measured as follows. Further, a nail penetration test was performed on the heat resistant homopolypropylene microporous film in the following manner. These results are shown in Table 1.

(Maximum heat shrinkage)
By cutting the heat-resistant homopolypropylene microporous film, a flat rectangular test piece having a width of 3 mm and a length of 30 mm was obtained. At this time, the length direction (extrusion direction) of the heat-resistant homopolypropylene microporous film was made parallel to the length direction of the test piece. Both ends in the length direction of the test piece were held by a gripper and attached to a TMA measuring apparatus (trade name “TMA-SS6000” manufactured by Seiko Instruments Inc.). At this time, the distance between the grips was set to 10 mm, and a tension of 19.6 mN (2 gf) was applied to the test piece in the length direction. Then, the test piece is heated from 25 ° C. to 180 ° C. at a rate of 5 ° C./min, the distance L (mm) between the gripping tools is measured at each temperature, and the heat shrinkage rate is calculated based on the following formula. did. Table 1 shows the maximum thermal shrinkage in the measurement range from 25 ° C to 180 ° C.
Thermal contraction rate (%) = 100 × (10−L) / 10

(Nail penetration test)
A composition for forming a positive electrode containing nickel-cobalt-lithium manganate (1: 1: 1) as a positive electrode active material was prepared. This positive electrode forming composition was applied to one surface of an aluminum foil as a positive electrode current collector and dried to prepare a positive electrode active material layer. Thereafter, a positive electrode current collector having a positive electrode active material layer formed on one surface was punched out to obtain a positive electrode having a plane rectangular shape of 48 mm long × 117 mm wide.

  Next, a negative electrode forming composition containing natural graphite as a negative electrode active material was prepared. This negative electrode forming composition was applied to one surface of an aluminum foil as a negative electrode current collector and dried to prepare a negative electrode active material layer. Thereafter, a negative electrode current collector in which the negative electrode active material layer was formed on one surface was punched out to obtain a flat rectangular negative electrode having a length of 50 mm and a width of 121 mm.

  Then, a heat-resistant homopolypropylene microporous film was punched out to obtain a planar rectangular shape of 52 mm long × 124 mm wide.

Next, a laminate was obtained by alternately laminating 10 positive electrode layers and 11 negative electrode layers one by one through a heat-resistant homopolypropylene microporous film. Thereafter, a tab lead was joined to each electrode by ultrasonic welding. After storing the laminate in an exterior material made of aluminum laminate foil, the exterior material was heat sealed to obtain a laminate element. A surface pressure of 1 kgf / cm ² was applied to the obtained laminate element, and it was confirmed by resistance measurement that there was no short circuit.

Next, after drying a laminated body element at 80 degreeC under pressure reduction for 24 hours, electrolyte solution was inject | poured under normal temperature normal pressure in the dry box (50 degreeC or less of dew points). As the electrolytic solution, a LiPF ₆ solution (1 mol / L) containing ethylene carbonate and dimethyl carbonate in a volume ratio of 3: 7 was used as a solvent. After the electrolyte solution was injected into the laminated body element, aging, vacuum impregnation, and temporary vacuum sealing were performed.

  Next, after storing the laminated element after the temporary vacuum seal at 20 ° C. for 24 hours, initial charge was performed under the conditions of charge 0.2 CA, constant current CC−constant voltage CV 4.2V 12 hours cut-off. .

Next, the laminated body element was degassed under reduced pressure and sealed, and then aged for one week in a charged state (SOC 100%). Subsequently, the multilayer element was subjected to initial discharge at 0.2 CA, 2nd charge / discharge at 0.2 CA, and a 5-cycle capacity confirmation test at 1 CA. Subsequently, AC resistance (ACR) and DC resistance (DCR) were measured under the following conditions for each cell.
ACR (SOC 50% 1kHz), DCR (SOC 50% 1CA, 2CA, 3CA x 10 seconds discharge)

And a laminated body element is 0.2C, C.I. C. -C. V. The battery was charged until the battery was fully charged (SOC 100%) under the condition of 4.2 V, 10 hours cut-off. Thereafter, a nail penetration test was performed in which a nail having a thickness of 3φ mm and a tip angle of 60 ° was pierced at a piercing speed of 10 mm / sec. In Table 1, “◯” and “x” are as follows.
○: Smoke and ignition did not occur in the laminate element after the test.
X: At least one of smoke and ignition occurred in the laminate element after the test.

(Tensile strength)
A test piece having a width of 10 mm and a length of 150 mm was prepared by cutting the heat-resistant homopolypropylene microporous film. At this time, the length direction (extrusion direction) of the heat-resistant homopolypropylene microporous film was made parallel to the length direction of the test piece. And the tensile strength (MPa) in the length direction of a test piece was measured based on JIS K7127 / 2/300, and the obtained value was shown in the column of “length direction” in Table 1.

  Further, the tensile strength (MPa) in the length direction of the test piece was the same as the above procedure except that the test piece was prepared by making the length direction of the heat-resistant homopolypropylene microporous film parallel to the width direction of the test piece. Was measured, and the obtained value is shown in the column of “width direction” in Table 1.

(Elongation at break)
A test piece having a width of 10 mm and a length of 150 mm was prepared by cutting the heat-resistant homopolypropylene microporous film. At this time, the length direction (extrusion direction) of the heat-resistant homopolypropylene microporous film was made parallel to the length direction of the test piece. And the breaking elongation (%) in the length direction of a test piece was measured based on JIS K7127 / 2/300, and the obtained value was shown in the column of “length direction” in Table 1.

  Further, the elongation at break (%) in the length direction of the test piece was the same as the above procedure except that the test piece was prepared by making the length direction of the heat-resistant homopolypropylene microporous film parallel to the width direction of the test piece. Was measured, and the obtained value is shown in the column of “width direction” in Table 1.

11 Plasma generator
11a, 11b electrode
12 Power supply
13 space (discharge space)
14 Guide roll
15 Guide roll
16 Drive roll
17 Temperature control
20 Gas generator for plasma generation
21 Gas supply source
22 nozzles
23 Piping A Plasma processing equipment B Synthetic resin microporous film

Claims (12)

  The synthetic resin microporous film is coated with a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer and then subjected to plasma treatment on the synthetic resin microporous film. A method for producing a porous film.   The method for producing a heat-resistant synthetic resin microporous film according to claim 1 or 2, wherein the content of the trifunctional or higher polyfunctional acrylic monomer in the radical polymerizable monomer is 10% by weight or more. .   The heat resistant synthesis according to any one of claims 1 to 3, wherein a coating liquid in which a radical polymerizable monomer is dispersed or dissolved in a solvent is applied to the surface of the synthetic resin microporous film. Manufacturing method of resin microporous film.   The heat-resistant synthetic resin fine film according to claim 3, wherein the solvent is removed by heating the synthetic resin microporous film coated with the coating liquid before the plasma treatment is performed on the synthetic resin microporous film. A method for producing a porous film.   By applying a radical polymerizable monomer containing a trifunctional or higher polyfunctional acrylic monomer on the surface of the synthetic resin microporous film, the radical polymerizable monomer is added to 100 parts by weight of the synthetic resin microporous film. The method for producing a heat-resistant synthetic resin microporous film according to any one of claims 1 to 4, wherein 80 parts by weight is adhered. Method for producing a heat resistant synthetic resin microporous film according to any one of claims 1 to 5, the energy density of the plasma in the synthetic resin microporous film, and irradiating with 5~50J / cm ^2. A synthetic resin microporous film containing a synthetic resin;
And a coating layer that is formed integrally with at least a part of the surface of the synthetic resin microporous film and includes a polymer of a radical polymerizable monomer including a trifunctional or higher polyfunctional acrylic monomer. A heat-resistant synthetic resin microporous film.   The heat resistant synthetic resin microporous film according to claim 7, wherein the gel fraction is 5% or more.   9. The heat-resistant synthetic resin microporous film according to claim 7, wherein the air permeability is 50 to 600 sec / 100 mL.   When the heat-resistant synthetic resin microporous film is heated from 25 ° C. to 180 ° C. at a heating rate of 5 ° C./min, the maximum heat shrinkage of the heat-resistant synthetic resin microporous film is 25% or less. The heat-resistant synthetic resin microporous film according to any one of claims 7 to 9.   A separator for a non-aqueous electrolyte secondary battery comprising the heat-resistant synthetic resin microporous film according to any one of claims 7 to 10.   A nonaqueous electrolyte secondary battery using the separator for a nonaqueous electrolyte secondary battery according to claim 11.

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