JP2017203145A - Heat-resistant synthetic resin microporous film and separator for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat-resistant synthetic resin microporous film which has low heat shrinkage property and low fluidity and is excellent in melt-down resistance.SOLUTION: A heat-resistant synthetic resin microporous film contains a synthetic resin microporous film containing an olefin resin and has a gel fraction of 75 mass% or more, a storage elastic modulus Er at 40-250°C by dynamic viscoelasticity of 0.008 MPa or more, a maximum shrinkage factor of TMA of 25% or less and a radical amount measured by electron spin resonance method of 2.0×10spins/100 mg or less, and accordingly has low heat shrinkage property and low fluidity and is excellent in melt-down resistance.SELECTED DRAWING: None

Description

  The present invention relates to a heat-resistant synthetic resin microporous film and a battery separator.

  Conventionally, lithium ion secondary batteries have been used as batteries 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 into the positive electrode. Therefore, the separator is required to have ion permeability such as lithium ions.

  As the separator, a synthetic resin microporous film using a synthetic resin such as a propylene resin is used because of its excellent insulation and low cost.

  The synthetic resin microporous film is manufactured by stretching a synthetic resin film. It has been pointed out that the synthetic resin microporous film produced by the stretching method is thermally contracted at a high temperature due to high residual stress due to stretching, and as a result, the positive electrode and the negative electrode may be short-circuited. Therefore, it has been attempted to ensure the safety of the lithium ion secondary battery by reducing the thermal shrinkage rate of the synthetic resin microporous film.

  Patent Document 1 discloses a synthetic resin microporous film processed by electron beam irradiation and having a thermomechanical analysis (TMA) value at 100 ° C. of 0% to −1% as a separator for a lithium ion secondary battery. ing.

JP 2003-22793 A

  In recent years, lithium ion secondary batteries tend to use high-energy electrodes as the capacity increases. When such an electrode is short-circuited, it generates heat rapidly and may reach a high temperature of 100 ° C. or higher. As in the separator of Japanese Patent Laid-Open No. 2003-22793, when the TMA value is simply set to 0% to -1%, when the inside of the battery becomes hot, the synthetic resin microporous film is melted by heat and soaks into the electrode. Therefore, a short circuit between the positive electrode and the negative electrode cannot be prevented, and the safety of the lithium ion secondary battery may not be ensured. Therefore, there is a need for a separator that has excellent heat shrinkability and has a property (low fluidity) that hardly melts even at an abnormally high temperature.

  Then, this invention provides the heat resistant synthetic resin microporous film excellent in the meltdown resistance which has low heat shrinkability and low fluidity | liquidity.

The heat-resistant synthetic resin microporous film of the present invention contains a synthetic resin microporous film containing an olefin resin, a gel fraction of 75% by mass or more, and a storage elastic modulus Er at 40 to 250 ° C. due to dynamic viscoelasticity. Is 0.008 MPa or more, the maximum shrinkage of TMA is 25% or less, and the radical amount measured by the electron spin resonance method is 1.0 × 10 ¹⁶ spins / 100 mg or less.

  ADVANTAGE OF THE INVENTION According to this invention, the heat resistant synthetic resin microporous film excellent in the meltdown resistance which has low heat shrinkability and low fluidity | liquidity can be provided.

  The heat-resistant synthetic resin microporous film of the present invention contains a synthetic resin microporous film containing an olefin resin.

(Synthetic resin microporous film)
The synthetic resin microporous film is not particularly limited as long as it is a microporous film that contains at least an olefin resin and is used as a separator in a battery such as a conventional secondary battery (such as a lithium ion secondary battery). Can be used.

  The synthetic resin microporous film contains at least an olefin resin. The olefin resin preferably contains an ethylene resin and / or a propylene resin, and more preferably contains an ethylene resin.

  Examples of the propylene-based resin include homopolypropylene and copolymers of propylene and other olefins. 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. The content of the propylene component in the propylene-based resin is preferably 50% by mass or more, and more preferably 80% by mass or more.

  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.

  Polymers containing ethylene resins such as homopolymers such as ultra-low density polyethylene, low density polyethylene, medium density polyethylene, and high density polyethylene, linear low density polyethylene, and ethylene-propylene copolymer Is mentioned. Moreover, the ethylene resin microporous film may contain other olefin resin as long as it contains the ethylene resin. The content of the ethylene component in the ethylene-based resin is preferably more than 50% by mass, more preferably 80% by mass or more.

  The weight average molecular weight of the olefin resin is preferably 30,000 to 500,000, and more preferably 50,000 to 480,000. 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. The weight average molecular weight of the ethylene-based resin is preferably 30,000 to 400,000, and more preferably 50,000 to 300,000. 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 that is excellent in film forming stability and in which micropores are uniformly formed.

  The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the olefin resin is preferably 5.0 to 30, and more preferably 7.5 to 25. The molecular weight distribution of the propylene-based resin is preferably 7.5 to 12, and more preferably 8 to 11. The molecular weight distribution of the ethylene-based resin is preferably 5.0 to 30, and more preferably 8.0 to 27. According to the olefin resin having a molecular weight distribution within the above range, an olefin resin microporous film having a high porosity can be provided.

  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, 6 to 7 mg of an olefin resin is sampled, the collected olefin resin is supplied to a test tube, and 0.05% by mass of BHT (dibutylhydroxytoluene) is contained in the test tube. A diluted solution is prepared by adding a DCB (orthodichlorobenzene) solution and diluting the olefin-based resin concentration to 1 mg / mL.

  The dilution liquid 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 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
TSKguardcolumn-HHR (30) HT x 1
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

  130-170 degreeC is preferable and, as for melting | fusing point of olefin resin, 133-165 degreeC is more preferable. 160-170 degreeC is preferable and, as for melting | fusing point of propylene-type resin, 160-165 degreeC is more preferable. The melting point of the ethylene-based resin is preferably 130 to 140 ° C, and more preferably 133 to 138 ° C. According to the olefin resin having a melting point within the above range, it is possible to provide an olefin resin microporous film having excellent openness.

  In the present invention, the melting point of the olefin-based resin can be measured using a differential scanning calorimeter (for example, a device name “DSC220C”, manufactured by Seiko Instruments Inc.) according to the following procedure. First, 10 mg of an olefin resin is heated from 25 ° C. to 250 ° C. at a heating rate of 10 ° C./min, and held at 250 ° C. for 3 minutes. Next, the olefin-based resin is cooled from 250 ° C. to 25 ° C. at a temperature decrease rate of 10 ° C./min, and held at 25 ° C. for 3 minutes. Subsequently, the olefin resin is reheated from 25 ° C. to 250 ° C. at a rate of temperature increase of 10 ° C./min, and the temperature at the top of the endothermic peak in this reheating step is defined as the melting point of the olefin resin.

  The olefin resin preferably has a silane cross-linked structure. The silane cross-linked structure means that molecular chains of the olefin resin are cross-linked by a bond including a siloxane bond.

  Thus, when the olefin-based resin has a silane cross-linked structure, the synthetic resin microporous film is further excellent in low heat shrinkage and low fluidity, and is further excellent in meltdown resistance.

  Examples of a method for introducing a silane cross-linked structure into an olefin resin include, for example, (1) a method of containing a silane-modified olefin resin in an olefin resin and cross-linking molecular chains of the silane-modified olefin resin, and (2) an olefin. Examples thereof include a method in which a silane coupling agent is mixed with a resin and the molecular chains of the olefin resin are cross-linked by the silane coupling agent.

  The silane-modified olefin resin can be produced, for example, by subjecting an olefin resin to a graft reaction of an unsaturated silane compound in the presence of a radical polymerization initiator.

  Examples of the unsaturated silane compound include vinyldimethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, vinyltriisopropoxysilane, Examples include vinyltri-n-propoxysilane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropylmethyldimethoxysilane.

  Any silane coupling agent may be used as long as it can introduce a silane cross-linking structure into the olefin resin. For example, vinyl dimethoxysilane, vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, vinyltriethoxy. Examples include silane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, vinyltriisopropoxysilane, vinyltrin-propoxysilane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropylmethyldimethoxysilane.

  In order to introduce a silane cross-linked structure into the olefin resin by the method (1), water is supplied to the silane-modified olefin resin, silanol groups are generated in the silane-modified olefin resin, and the generated silanol groups are bonded to each other. What is necessary is just to bridge | crosslink the molecular chains of a silane modified olefin resin by the coupling | bonding which carries out a dehydration condensation (silanol dehydration condensation reaction) and contains a siloxane bond.

  In order to introduce a silane crosslinked structure into the olefin resin by the method (2), a silane coupling agent is added to the olefin resin, and then the silane coupling agent is grafted or polymerized to the olefin resin. . Water is reacted with the introduced alkoxysilyl group to form a silanol group, and the generated silanol groups are subjected to dehydration condensation (silanol dehydration condensation reaction). What is necessary is just to bridge | crosslink the molecular chains of an olefin resin with the coupling | bonding containing the obtained siloxane bond.

  The synthetic resin microporous film includes micropores. It is preferable that the micropores communicate with each other in the film thickness direction, whereby excellent air permeability can be imparted to the heat resistant synthetic resin microporous film. Such a heat-resistant synthetic resin microporous film can transmit ions such as lithium ions in the thickness direction.

  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. According to the synthetic resin microporous film having an air permeability within the above range, a heat-resistant synthetic resin microporous film having excellent ion permeability can be provided.

  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 porosity of the synthetic resin microporous film is preferably 30 to 70%, and more preferably 35 to 65%. According to the synthetic resin microporous film having a porosity within the above range, a heat-resistant synthetic resin microporous film having excellent ion permeability can be provided.

In addition, the porosity of a synthetic resin microporous film can be calculated | required by measuring a true volume using a pycnometer. Specifically, first, a synthetic resin microporous film is cut into a 150 mm × 150 mm square. Next, after measuring the thickness of the film, the mass of the film is weighed using an electronic balance. The weighed film is measured for porosity according to the following equation using a fully automatic pycnometer (Micro-Ultrapyc 1200e, manufactured by Cantachrome Instruments Japan GK). At this time, the environmental temperature is 25 ° C., the cell size is 4.5 cm ³ , and the pressure setting of the helium gas is 17 psi.
Porosity (%) = (apparent volume−true volume) / apparent volume × 100
Apparent volume: Volume obtained from the actual measured values of the area and thickness of the cut out film True volume: Volume measured with a pycnometer

  The thickness of the synthetic resin microporous film is preferably 5 to 100 μm, and more preferably 5 to 50 μm.

  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 with a dial gauge, and the arithmetic mean value is defined as the thickness of the synthetic resin microporous film.

  As the synthetic resin microporous film, an olefin-based resin microporous film produced by a stretching method is more preferable from the viewpoint of openability and mechanical properties. 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. On the other hand, according to the coating layer of the present invention, the thermal shrinkage of the olefin-based resin microporous film can be reduced, and therefore the effects of the present invention can be further exhibited. Furthermore, when the olefin-based resin microporous film has a silane cross-linked structure, heat shrinkage can be further reduced, and the effect of the present invention can be particularly exhibited.

As a method for producing an olefin resin microporous film by a stretching method, specifically,
(1) A process of obtaining an olefin resin film by extruding an olefin resin, a process of generating and growing lamella crystals in the olefin resin film, and stretching the olefin resin film to separate the lamella crystals. And a step of obtaining an olefin resin microporous film in which micropores are formed;
(2) A step of obtaining an olefinic resin film by extruding an olefinic resin composition containing an olefinic resin and a filler, and olefinic resin and filler by uniaxially or biaxially stretching the olefinic resin film. And (3) an olefin resin and an extractable material (for example, a filler, a plasticizer, etc.), and a step of obtaining an olefin resin microporous film in which micropores are formed by peeling the interface with A process for obtaining an olefin resin film by extruding an olefin resin composition containing the olefin resin composition, a process for forming a micropore by extracting an extractable material from the olefin resin film with a solvent, and a micropore Of obtaining an olefin resin microporous film by stretching an olefin resin film having a part formed And the like.

  Among these, the method (1) is preferable because a large number of micropores are formed uniformly without using a solvent.

(Coating layer)
In the heat resistant synthetic resin microporous film, a coating layer containing a polymer having a crosslinked structure is formed on at least a part of the surface of the synthetic resin microporous film.

  By forming a coating layer on at least a part of the surface of the synthetic resin microporous film, it is possible to obtain a heat-resistant synthetic resin microporous film having excellent low heat shrinkage and low fluidity at high temperatures and improved meltdown resistance. it can.

  The coating layer includes a polymer having a polymer cross-linked structure. The coating layer contains a polymer of a polymerizable compound having two or more polymerizable functional groups in one molecule. The coating layer containing such a polymer has high hardness and moderate elasticity and elongation. Therefore, a heat-resistant synthetic resin microporous film excellent in both low heat shrinkage and low fluidity can be provided by using a coating layer containing the polymer.

  Although the coating layer should just be formed in at least one part of the surface of a synthetic resin microporous film, it is preferable to be formed in the whole surface of a synthetic resin microporous film. The entire surface of the synthetic resin microporous film is the surface including the wall surface of the micropores that continue from one surface of the synthetic resin microporous film to the opposite surface.

  Further, by using a polymerizable compound, a coating layer can be formed on the surface of the synthetic resin microporous film so as not to block the micropores of the synthetic resin microporous film. This may be performed by coating a low-viscosity polymerizable compound as it is, or by coating the surface of the synthetic resin microporous film after the polymerizable compound is diluted with a solvent to lower the viscosity. Thereby, the entire surface of the synthetic resin microporous film can be thinly coated, and a heat-resistant synthetic resin microporous film having excellent air permeability and ion permeability can be provided.

  The polymerizable compound has two or more polymerizable functional groups in one molecule. The polymerizable functional group is not particularly limited, but a radical polymerizable functional group is preferable. The radical polymerizable functional group is a functional group containing a radical polymerizable unsaturated bond that can be radically polymerized by irradiation with active energy rays. Although it does not restrict | limit especially as a radically polymerizable functional group, For example, a (meth) acryloyl group, a vinyl group, etc. are mentioned, A (meth) acryloyl group is preferable.

  The radical polymerizable compound includes a polyfunctional acrylic monomer having two or more radical polymerizable functional groups in one molecule, a vinyl oligomer having two or more radical polymerizable functional groups in one molecule, and one molecule. Modified polyfunctional (meth) acrylate having two or more (meth) acryloyl groups, dendritic polymer having two or more (meth) acryloyl groups, and urethane (meth) acrylate having two or more (meth) acryloyl groups An oligomer is mentioned.

  In the present invention, (meth) acrylate means acrylate or methacrylate. (Meth) acryloyl means acryloyl or methacryloyl. Moreover, (meth) acrylic acid means acrylic acid or methacrylic acid.

  The polyfunctional acrylic monomer only needs to have two or more radical polymerizable functional groups in one molecule, but it has three or more functional groups having three or more radical polymerizable functional groups in one molecule. A polyfunctional acrylic monomer is preferable, and a trifunctional to hexafunctional polyfunctional acrylic monomer having 3 to 6 radical polymerizable functional groups in one molecule is more preferable.

As a polyfunctional acrylic monomer,
Bifunctional polyfunctional acrylic monomers such as 1,9-nonanediol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate;
Trimethylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, ethoxylated isocyanuric acid tri (meth) acrylate, ε-caprolactone modified tris- (2-acryloxyethyl) isocyanurate, and ethoxylated glycerol tri (meth) ) A trifunctional or higher polyfunctional acrylic monomer such as acrylate can be exemplified.

  It does not specifically limit as a vinyl-type oligomer, For example, a polybutadiene-type oligomer etc. can be illustrated. The polybutadiene oligomer means an oligomer having a butadiene skeleton such as a poly (1,2-butadiene) oligomer and a poly (1,3-butadiene) oligomer.

  A commercially available product can be used as the polybutadiene oligomer. Examples of the poly (1,2-butadiene) oligomer include “B-1000”, “B-2000”, and “B-3000” manufactured by Nippon Soda Co., Ltd.

  Preferred examples of the modified polyfunctional (meth) acrylate include an alkylene oxide modified product of a polyfunctional (meth) acrylate and a caprolactone modified product of a polyfunctional (meth) acrylate.

  The alkylene oxide modified product of polyfunctional (meth) acrylate is preferably obtained by esterifying an adduct of polyhydric alcohol and alkylene oxide with (meth) acrylic acid. The polyfunctional (meth) acrylate-modified caprolactone is preferably obtained by esterifying an adduct of a polyhydric alcohol and caprolactone with (meth) acrylic acid.

  The dendritic polymer having two or more (meth) acryloyl groups in one molecule means a spherical macromolecule in which branch molecules having (meth) acryloyl groups are radially assembled.

  The dendritic polymer having a (meth) acryloyl group includes a dendrimer having two or more (meth) acryloyl groups in one molecule, and a hyperbranched polymer having two or more (meth) acryloyl groups in one molecule. Can be mentioned.

  A dendrimer means a spherical polymer obtained by integrating (meth) acrylates in a spherical shape with (meth) acrylates as branch molecules.

  A hyperbranched polymer having two or more (meth) acryloyl groups in one molecule is an ABx type polyfunctional monomer (where A and B are functional groups that react with each other, and the number X of B is 2 or more). It means a spherical polymer obtained by modifying the surface and the inside of a hyperbranched structure having an irregular branch structure obtained by polymerization with a (meth) acryloyl group.

  The urethane acrylate oligomer is obtained, for example, by reacting a polyisocyanate compound, a (meth) acrylate having a hydroxyl group or an isocyanate group, and a polyol compound.

  In the present invention, among the polymerizable compounds described above, a polyfunctional acrylic monomer is preferable, and trimethylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol. Hexa (meth) acrylate and ditrimethylolpropane tetra (meth) acrylate are preferred. According to these, excellent melt-down resistance can be imparted to the heat-resistant synthetic resin microporous film.

  When a polyfunctional acrylic monomer is used as the radical polymerizable compound, the content of the polyfunctional acrylic monomer in the radical polymerizable compound is preferably 30% by mass or more, more preferably 80% by mass or more, and 100% by mass. Is particularly preferred. By using a polymerizable compound containing 30% by mass or more of a polyfunctional acrylic monomer, the resulting heat-resistant synthetic resin microporous film is imparted with excellent melt-down resistance without reducing air permeability. Can do.

  In the present invention, as the polymerizable compound, only one kind of the above-described polymerizable compounds may be used, or two or more kinds of polymerizable compounds may be used in combination.

  The coating layer contains a polymer of the polymerizable compound described above. This polymer is preferably a polymer obtained by polymerizing a polymerizable compound by irradiation with active energy rays. That is, the polymer is preferably a polymer of a polymerizable compound by irradiation with active energy rays. The coating layer containing such a polymer has a high hardness, thereby reducing the heat shrinkage and fluidity of the heat-resistant synthetic resin microporous film at high temperatures and improving the meltdown resistance. be able to.

  The active energy ray is not particularly limited, and examples thereof include an electron beam, plasma, ultraviolet rays, α rays, β rays, and γ rays. Of these, electron beams and ultraviolet rays are preferable.

  It is preferable that a part of the polymer in the coating layer and a part of the olefin resin in the synthetic resin microporous film are chemically bonded. By using a coating layer containing such a polymer, as described above, heat shrinkage at high temperatures is reduced, heat resistance is reduced, and heat resistance synthetic resin microporous film having excellent low fluidity. Can be provided. The chemical bond is not particularly limited, and examples thereof include a covalent bond, an ionic bond, and an intermolecular bond.

  The content of the coating layer in the heat-resistant synthetic resin microporous film is preferably 1 to 50 parts by mass and more preferably 3 to 45 parts by mass with respect to 100 parts by mass of the resin constituting the synthetic resin microporous film. 5 to 40 parts by mass is particularly preferable. By setting the content of the coating layer within the above range, the coating layer can be uniformly formed without blocking the micropores on the surface of the synthetic resin microporous film. Thereby, the heat resistant synthetic resin microporous film with improved heat resistance and meltdown resistance can be provided without reducing air permeability.

Even if the coating layer does not contain inorganic particles, the meltdown resistance of the heat-resistant synthetic resin microporous film can be improved. Therefore, it is preferable that the coating layer does not contain inorganic particles. However, if necessary, the coating layer may contain inorganic particles. Examples of the inorganic particles include inorganic particles generally used for heat resistant porous layers. Examples of the material constituting the inorganic particles include Al ₂ O ₃ , SiO ₂ , TiO ₂ , and MgO.

(Heat-resistant synthetic resin microporous film)
As described above, the heat-resistant synthetic resin microporous film of the present invention includes a synthetic resin microporous film and a coating layer formed on at least a part of the surface of the synthetic resin microporous film.

  For the heat-resistant synthetic resin microporous film, the gel fraction is preferably 75% by mass or more, more preferably 78 to 99% by mass, and particularly preferably 78 to 90% by mass. When the gel fraction is 75% by mass or more, the heat-resistant synthetic resin microporous film is more excellent in meltdown resistance.

About a heat resistant synthetic resin microporous film, a gel fraction is measured in the following way. First, about 0.1 g of a test piece is obtained by cutting a heat-resistant microporous film. After weighing the weight [W ₁ (g)] of the test piece, the test piece is 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. After the mesh basket and the filtrate are dried under reduced pressure at 80 ° C. for 7 hours, the total weight [W ₂ (g)] of the mesh basket and the filtrate is weighed. Then, the gel fraction is calculated according to the following formula.
Gel fraction (mass%) = 100 × (W ₂ −W ₀ ) / W ₁

  About the heat resistant synthetic resin microporous film, the storage elastic modulus Er at 40 to 250 ° C. by dynamic viscoelasticity is 0.008 MPa or more, preferably 0.01 to 10,000 MPa, and more preferably 0.1 to 1000 MPa. When the storage elastic modulus Er at 40 to 250 ° C. due to dynamic viscoelasticity is 0.008 MPa or more, the heat resistant synthetic resin microporous film is more excellent in meltdown resistance.

About the heat resistant synthetic resin microporous film, the storage elastic modulus Er at 40 to 250 ° C. by dynamic viscoelasticity is measured as follows. A sample piece is cut into a size of 30 mm (width direction) × 35 mm (extrusion direction). The sample piece was folded along the winding length direction to a size of (width direction) 5 mm × (extrusion direction) 35 mm. The measurement was carried out in air, and the measurement was performed using a dynamic viscoelasticity measuring device (DVA-200, manufactured by IT Measurement Control Co., Ltd.), and the temperature dispersion of the storage elastic modulus was measured under the following conditions.
Deformation mode: tension, grip length: 25 mm, measurement start temperature: 40 ° C., measurement end temperature: 250 ° C., lower limit elastic modulus at the end of measurement: 10 Pa, lower limit movement at the end of measurement: 0.01 cN, rate of temperature increase: 10 C / min, data capture interval: every 1 ° C., measurement frequency: 1 Hz, strain: 0.1%, static / power ratio: 1, upper limit elongation: 50%, minimum load: 1 cN.

  The maximum heat shrinkage rate of the heat resistant synthetic resin microporous film in TMA measurement is 25% or less, preferably 0 to 20% or less, and more preferably 0 to 15%. The heat-resistant synthetic resin microporous film has an excellent melt-down resistance because thermal shrinkage at high temperatures is reduced by the coating layer. Therefore, the maximum heat shrinkage rate of the heat resistant synthetic resin microporous film can be 25% or less.

In addition, the measurement of the maximum heat shrinkage rate in TMA measurement of a heat resistant synthetic resin microporous film can be performed as follows. First, a flat rectangular test piece (width 3 mm × length 30 mm) is obtained by cutting the heat-resistant synthetic resin microporous film. At this time, the length direction (extrusion direction) of the heat-resistant synthetic resin microporous film and the length direction of the test piece are made parallel. The both ends of the length direction of a test piece are gripped with a gripper, and attached to a TMA measuring apparatus (for example, trade name “TMA-SS6000” manufactured by Seiko Instruments Inc.). At this time, the distance between the gripping tools is set to 10 mm, and the gripping tools can be moved along with the thermal contraction of the test piece. The test piece was heated in air from 25 ° C. to 250 ° C. at a heating rate of 5 ° C./min with a tension of 19.6 mN (2 gf) applied to the test piece in the length direction. Measure the distance L (mm) between the gripping tools, calculate the thermal contraction rate based on the following formula, and set the maximum value as the maximum thermal contraction rate.
Thermal contraction rate (%) = 100 × (10−L) / 10

  The puncture strength of the heat-resistant synthetic resin microporous film is not particularly limited, but is preferably 0.6N or more, more preferably 0.8N or more, and further preferably 1.0 to 7.0N. The heat-resistant synthetic resin microporous film having a puncture strength of 0.6 N or more has excellent mechanical strength, and can thereby improve battery productivity and safety.

  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 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 load until the needle penetrates is determined. The piercing strength.

  Although the air permeability of a heat resistant synthetic resin microporous film is not specifically limited, 50-600 sec / 100 mL is preferable and 100-300 sec / 100 mL is more preferable.

  In addition, as a measuring method of the air permeability of a heat resistant synthetic resin microporous film, the same method as the measuring method mentioned above of the air permeability of a synthetic resin microporous film is used.

  The porosity of the heat-resistant synthetic resin microporous film is not particularly limited, but is preferably 30 to 70% and more preferably 35 to 65%. A heat-resistant synthetic resin microporous film having a porosity within the above range is excellent in both mechanical strength and ion permeability.

  As a method for measuring the porosity of the heat-resistant synthetic resin microporous film, the same method as described above for the porosity of the synthetic resin microporous film is used.

Radical amount of heat-resistant synthetic resin microporous film, 2.0 × a 10 ¹⁶ spins / 100mg, preferably ^{2.0 × 10 14 ~2.0 × 10 16} spins / 100mg, 2.0 × 10 14 ~ 2.0 × 10 ¹⁵ spins / 100 mg is more preferable. By reducing the amount of radicals, it is possible to suppress a decrease in mechanical strength of the heat-resistant synthetic resin microporous film due to residual radicals over time.

The radical amount of the heat-resistant synthetic resin microporous film is a value measured by an electron spin resonance (ESR) method. The radical amount of the heat-resistant synthetic resin microporous film can be measured as follows. First, 80 mg of heat-resistant synthetic resin microporous film was used as a sample, and this sample was put into a quartz sample tube (length 10 cm, diameter 4 mm) for X-band measurement. Then, the sample is measured at room temperature using an electron spin resonance (ESR) measuring apparatus (for example, trade name “FA-200” manufactured by JEOL Ltd.). Confirm the microwave resonance dip of the sample and observe the signal. The measurement is performed under the following conditions.
Output: 1.01mW
Center magnetic field: 336mT
Sweep width: ± 10mT
Sweep time: 4min
Modulation width (FMW): 0.6mT
Gain: × 20
Time constant: 0.03sec
Total number of times: 1 Number of data points: 8192
Mn2 + digital marker setting: 850
Standard substance: TEMPOL (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl)
<Data processing conditions>
Baseline correction: Correction by Mn2 + 3rd and 4th signal 2 Signal wave height ratio: Mn2 + (3rd signal) overlaps with the sample signal, so the reference Mn2 + (4th signal) wave height is used, and the sample signal Displayed as a ratio of wave height to wave height. After obtaining the signal wave height ratio (S) of the sample, the signal wave height ratio (corrected signal wave height ratio) when the sample weight is set to 100 mg is obtained from the following formula (1) from the weight (Ws) of the sample measured in advance. .
Correction signal wave height ratio = S × (100 / Ws) (1)
Spin number: TEMPOL is used as a standard substance, and the spin number of the third signal of Mn2 + when the Mn2 + digital marker is set to 850 is obtained. The number of spins of the sample is calculated from the relative intensity of the third signal of Mn2 + to the signal of the measurement sample. The number of spins per 100 mg of sample is also shown.
g value: The g value is calculated by a calculation program from the g value of the third and fourth signals of Mn2 +, the magnetic field interval, and the central magnetic field of the sample signal.

(Method for producing heat-resistant synthetic resin microporous film)
A heat-resistant synthetic resin microporous film is produced by forming a coating layer on at least a part of the surface of the synthetic resin microporous film.

(Coating process)
First, an application step of applying a polymerizable compound having two or more polymerizable functional groups in one molecule to at least a part of the surface of the synthetic resin microporous film is performed.

  By applying the polymerizable compound to the surface of the synthetic resin microporous film, the polymerizable compound can be attached to the surface of the synthetic resin microporous film. At this time, the polymerizable compound may be directly applied to the surface of the synthetic resin microporous film. However, it is preferable to disperse or dissolve the polymerizable compound in a solvent to obtain a coating solution, and to apply this coating solution to the surface of the synthetic resin microporous film. By using the polymerizable compound as the coating liquid in this way, the polymerizable compound can be uniformly attached to the surface of the synthetic resin microporous film while reducing the blockage of the micropores. This makes it possible to produce a heat-resistant synthetic resin microporous film with a uniform coating layer and improved meltdown resistance without reducing air permeability.

  When ultraviolet rays are used as active energy rays in the irradiation step described later, a photopolymerization initiator may be contained in the polymerizable compound. Since a part of the polymerizable compound and the polymerizable initiator is impregnated into the synthetic resin microporous film, the polymerizable compound causes a graft reaction to the polyolefin resin constituting the synthetic resin microporous film, and A chemical bond is formed between them. Further, when the silane-modified olefin resin is contained in the polyolefin resin, the polymerizable compound is graft-polymerized to the silane-modified olefin resin, and thus a chemical bond is generated between the silane-modified olefin resin and the film. Thereby, a part of polymer in a membrane | film | coat layer and a part of olefin resin in a synthetic resin microporous film are couple | bonded chemically. By using a coating layer containing such a polymer, as described above, heat shrinkage at high temperatures is reduced and heat resistance synthetic resin microporous material having excellent low heat shrinkage and excellent low fluidity. A film can be provided.

  Furthermore, the coating liquid can smoothly wet the wall surface of the microporous part in the synthetic resin microporous film, thereby not only the surface of the synthetic resin microporous film but also the opening end of the microporous part continuous to this surface. A coating layer can also be formed on the wall surface. Thereby, a membrane | film | coat layer can be firmly integrated with the surface of a synthetic resin microporous film.

  In addition, since the polymerizable compound having a radical polymerizable functional group having two or more functional groups is excellent in wettability with respect to the synthetic resin microporous film, the polymerizable compound can be used without blocking the micropores in the synthetic resin microporous film. Can be applied. Thereby, the membrane | film | coat layer which has the through-hole penetrated in the thickness direction can be formed in the location corresponding to the micropore part of a synthetic resin microporous film. Therefore, according to such a coating layer, a heat-resistant synthetic resin microporous film having improved meltdown resistance can be provided without reducing air permeability.

  The solvent used in the coating solution is not particularly limited as long as the polymerizable compound can be dissolved or dispersed. For example, alcohols such as methanol, ethanol, propanol, isopropyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, and the like. Examples include ketones, ethers such as tetrahydrofuran and dioxane, ethyl acetate, and chloroform. Of these, ethyl acetate, toluene, xylene, and acetone are preferable. These solvents can be easily removed after the coating solution is applied to the surface of the synthetic resin microporous film. In addition, since these solvents penetrate into the amorphous part of the synthetic resin microporous film, it becomes easy to impregnate part of the polymerizable compound, and part of the olefinic resin in the synthetic resin microporous film and the coating layer. It works to promote bond formation with a part of the polymer. 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.

  1-25 mass% is preferable and, as for content of the polymeric compound in a coating liquid, 3-20 mass% is more preferable. By setting the content of the polymerizable compound within the above range, the coating layer can be formed uniformly without blocking the micropores of the synthetic resin microporous film, and thus the resistance to air permeability is not reduced. A heat-resistant synthetic resin microporous film having improved meltdown properties can be produced.

  The method of applying the polymerizable compound to the synthetic resin microporous film is not particularly limited. For example, (1) a method of applying the polymerizable compound to the surface of the synthetic resin microporous film; (2) synthesis in the polymerizable compound A method of immersing a resin microporous film and applying a polymerizable compound to the surface of the synthetic resin microporous film; (3) A coating solution is prepared by dissolving or dispersing the polymerizable compound in a solvent, and this coating solution is synthesized. A method in which a synthetic resin microporous film is heated to remove the solvent after coating on the surface of the resin microporous film; and (4) a coating solution is prepared by dissolving or dispersing the polymerizable compound in the solvent. A method of immersing the synthetic resin microporous film in the liquid and applying the coating liquid in the synthetic resin microporous film, and then heating the synthetic resin microporous film to remove the solvent can be mentioned. Of these, the methods (3) and (4) are preferred. According to these methods, the polymerizable compound can be uniformly applied to 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 30 to 130 ° C, more preferably 40 to 120 ° C. By setting the heating temperature within the above range, the applied solvent can be efficiently removed while reducing thermal deformation of the synthetic resin microporous film and blockage of the micropores.

  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 above-mentioned, a polymeric compound can be made to adhere to the synthetic resin microporous film surface and the internal pore surface by apply | coating a polymeric compound or a coating liquid to the synthetic resin microporous film surface.

(Irradiation process)
Next, the irradiation process which irradiates an active energy ray to the said synthetic resin microporous film which apply | coated the polymeric compound is implemented. Thereby, the polymerizable compound is polymerized, and the coating layer containing the polymer of the polymerizable compound can be integrally formed on at least a part of the surface of the synthetic resin microporous film, preferably the entire surface.

  By irradiating with active energy rays, a part of the synthetic resin contained in the synthetic resin microporous film reacts with a part of the polymerizable compound forming the coating layer to generate a chemical bond.

  When the synthetic resin microporous film contains an ethylene-based resin, the mechanical energy and meltdown resistance of the entire synthetic resin microporous film are improved by cross-linking the ethylene-based resins by irradiation with active energy rays. The resulting heat resistant synthetic resin microporous film also has excellent mechanical strength and meltdown resistance.

  And even if the synthetic resin microporous film is given a cross-linked structure, and the inside of the battery becomes a high temperature and becomes higher than the melting point of the ethylene-based resin, the synthetic resin microporous film has a cross-linked structure. The film shape is maintained, and an electrical short circuit between the positive and negative electrodes is effectively prevented.

  Moreover, when the synthetic resin microporous film contains an ethylene resin, a part of the synthetic resin microporous film starts to melt, and the micropores are blocked at the initial stage of abnormal temperature rise in the battery. Therefore, the secondary battery excellent in safety can be constructed by exhibiting an excellent shutdown effect, preventing the passage of ions and stopping the discharge of the secondary battery to prevent the temperature rise in the battery.

  The active energy ray is not particularly limited, and examples thereof include an electron beam, plasma, ultraviolet rays, α rays, β rays, and γ rays. Of these, electron beams and ultraviolet rays are preferable.

  When an electron beam is used as the active energy ray, the acceleration voltage of the electron beam with respect to the synthetic resin microporous film is not particularly limited, but is preferably 50 to 300 kV, and more preferably 100 to 250 kV. By setting the acceleration voltage of the electron beam within the above range, the coating layer can be formed while reducing deterioration of the synthetic resin in the synthetic resin microporous film.

  When the synthetic resin microporous film contains homopolypropylene, the irradiation dose of the electron beam to the synthetic resin microporous film is preferably 20 to 100 kGy, more preferably 30 to 80 kGy, and particularly preferably 40 to 70 kGy. Moreover, when a synthetic resin microporous film contains homopolyethylene, 20-600 kGy is preferable, 30-500 kGy is more preferable, and 50-300 kGy is especially preferable. By setting the irradiation dose of the electron beam within the above range, the polymerizable compound can be polymerized while reducing the deterioration of the synthetic resin in the synthetic resin microporous film, and thereby heat resistance excellent in meltdown resistance. A synthetic resin microporous film can be provided.

When using ultraviolet rays as the active energy ray, the accumulated light quantity of ultraviolet to the synthetic resin microporous film is preferably 1000~5000mJ / cm ^2, more preferably ^{1000~4000mJ / cm 2, 1500~3700mJ / cm} 2 is particularly preferred. In addition, when using an ultraviolet-ray as an active energy ray, it is preferable that the photoinitiator is contained in the said coating liquid. Examples of the photopolymerization initiator include benzophenone, 2,2-dimethoxy-1,2-diphenylethane-1-one benzyl, methyl-o-benzoylbenzoate, and anthraquinone.

When using a plasma as the active energy rays, plasma energy density to a synthetic resin microporous film is not particularly limited, but is preferably 5~50J / cm ^2, more preferably ^{10~45J / cm 2, 20~45J / cm} 2 Is particularly preferred.

  In the irradiation step, the active energy ray is irradiated to the synthetic resin microporous film coated with the polymerizable compound, preferably in an atmosphere having an oxygen concentration of 1000 ppm or less. The oxygen concentration in the atmosphere in the irradiation step is preferably 500 ppm or less, more preferably 1 to 400 ppm, and particularly preferably 1 to 200 ppm. Irradiation under such an oxygen concentration atmosphere can reduce the oxidative degradation of the synthetic resin contained in the synthetic resin microporous film and increase the crosslink density of the coating layer in the irradiation process. Can do. Thereby, the meltdown resistance of a heat resistant synthetic resin microporous film can be improved. Furthermore, by carrying out the irradiation step in an atmosphere having the above oxygen concentration, it is possible to suppress a decrease in mechanical strength of the heat resistant synthetic resin microporous film due to residual radicals over time.

  The irradiation step is preferably performed in an inert gas atmosphere. Thereby, the oxygen concentration in the atmosphere in the irradiation process can be easily adjusted. The inert gas is not particularly limited, and examples thereof include nitrogen, helium, neon, argon, krypton, xenon, radon, and a mixed gas thereof.

(Heating process)
Next, it is preferable to carry out a heating step of heat-treating the synthetic resin microporous film irradiated with active energy rays, preferably in an atmosphere having an oxygen concentration of 1000 ppm or less. According to the heating step, the polymerization of the polymerizable compound can be promoted based on the residual radical remaining in the irradiation step described above, and the remaining radical can also be deactivated by promoting the polymerization of the polymerizable compound. . Thereby, the mechanical strength and meltdown resistance of the heat resistant synthetic resin microporous film can be improved.

  In the heating step, the synthetic resin microporous film irradiated with active energy rays is preferably (melting point −70) ° C. to (melting point −10) ° C., more preferably (melting point −50) ° C. of the olefin resin contained in the base material. It is preferable to heat-treat at (melting point−10) ° C., particularly preferably at (melting point−30) ° C. to (melting point−10) ° C. By setting the heat treatment temperature of the synthetic resin microporous film within the above range, the polymerization of the polymerizable compound based on the residual radical remaining in the irradiation step is promoted, and the synthetic resin in the synthetic resin microporous film is deteriorated. Remaining radicals can also be deactivated by promoting the polymerization of the polymerizable compound while reducing the above.

  In the heating step, the heat treatment time of the synthetic resin microporous film irradiated with active energy rays is preferably from 0.1 minute to 3 hours, and more preferably from 0.2 minute to 3 hours. By setting the heat treatment time of the synthetic resin microporous film within the above range, the polymerization of the polymerizable compound based on the residual radical remaining in the irradiation step can be promoted and the residual radical can be deactivated. As a result, deterioration of the synthetic resin in the synthetic resin microporous film can be reduced.

  In the heating step, the synthetic resin microporous film irradiated with active energy rays is subjected to heat treatment in an atmosphere having an oxygen concentration of 1000 ppm or less. The oxygen concentration in the atmosphere in the heating step is 1000 ppm or less, preferably 500 ppm or less, preferably 1 to 400 ppm, and more preferably 1 to 200 ppm. By carrying out the heating step in such an oxygen concentration atmosphere, the oxidation degradation of the synthetic resin contained in the synthetic resin microporous film can be reduced in the heating step, and the crosslinking density of the coating layer can be reduced. Can be high. Thereby, the meltdown resistance of a heat resistant synthetic resin microporous film can be improved.

  The heating step is preferably performed in an inert gas atmosphere. Thereby, the oxygen concentration in the atmosphere in the heating process can be easily adjusted. The inert gas is not particularly limited, and examples thereof include nitrogen, helium, neon, argon, krypton, xenon, radon, and a mixed gas thereof.

(Water treatment process)
In order to obtain a silane crosslinked structure by reacting a silane-modified olefin resin or a synthetic resin microporous film containing a silane coupling agent, a water treatment step may be performed. In addition, when performing a water treatment process, although a water treatment process may be performed at any timing, in order to obtain a silane crosslinked structure efficiently, it is preferable to carry out after the irradiation process to a film layer.

  When the olefin-based resin constituting the olefin-based resin microporous film contains a silane-modified olefin-based resin, a silane crosslinked structure is formed between the molecular chains of the silane-modified olefin-based resin by the water treatment step. Is done.

  On the other hand, when a silane coupling agent is contained in the olefin resin constituting the olefin resin microporous film, the silanol dehydration condensation reaction proceeds between the silane coupling agents by the water treatment step. . Furthermore, by irradiating the olefin resin film or the olefin resin microporous film with active energy rays, the olefin resin is grafted with a silane coupling agent (for example, graft polymerization). By the silanol dehydration condensation reaction and the graft reaction, a silane cross-linking structure is formed by the silane coupling agent between the molecular chains of the olefin resin. The irradiation of the active energy ray to the olefin-based resin microporous film is preferably performed by the irradiation of the active energy ray performed at the time of forming the coating layer. On the other hand, the olefin resin molecules chemically bonded to the coating layer are bonded by a silane cross-linked structure, so that a three-dimensional network structure is obtained. In addition, the irradiation conditions of the active energy ray to the olefin resin film or the olefin resin microporous film for graft polymerization of the silane coupling agent to the olefin resin are as follows. Since the irradiation conditions are the same as those in FIG.

  The method for supplying water to the olefin resin film or the olefin resin microporous film is not particularly limited as long as the silanol dehydration condensation reaction of the silane-modified olefin resin and the silane coupling agent can proceed. Examples thereof include a method of immersing the olefin resin film or the olefin resin microporous film in water, a method of spraying water on the olefin resin film or olefin resin microporous film, and the like.

  5-100 degreeC is preferable, as for the temperature of the water supplied to an olefin resin film or an olefin resin microporous film, 25-95 degreeC is more preferable, and 50-95 degreeC is especially preferable. By making the temperature of water into the said temperature range, the silanol dehydration condensation reaction of a silane modified olefin resin and a silane coupling agent can be advanced smoothly.

  The time for supplying water to the olefin resin film or the olefin resin microporous film to cause the silanol dehydration condensation reaction is preferably 1 to 1200 minutes, more preferably 10 to 600 minutes, and particularly preferably 60 to 600 minutes. By setting the silanol dehydration condensation reaction time within the above range, the silanol dehydration condensation reaction of the silane-modified olefin resin and the silane coupling agent can proceed smoothly.

  The heat resistant synthetic resin microporous film is suitably used as a separator for a non-aqueous electrolyte secondary battery. Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery. Since the heat-resistant synthetic resin microporous film is excellent in meltdown resistance, by using such a heat-resistant synthetic resin microporous film, the inside of the battery is, for example, 100 to 150 ° C., particularly 130 to 150 ° C. Even when the temperature becomes high, the electrical short circuit between the electrodes due to the shrinkage and melt flow of the heat-resistant synthetic resin microporous film can be reduced.

  The non-aqueous electrolyte secondary battery is not particularly limited as long as it includes the heat-resistant synthetic resin microporous film as a separator, and includes a positive electrode, a negative electrode, a separator including the heat-resistant synthetic resin microporous film, and non-aqueous electrolysis. Contains liquid. The heat-resistant synthetic resin microporous film is disposed between the positive electrode and the negative electrode, thereby preventing an electrical short circuit between the electrodes. In addition, the nonaqueous electrolytic solution is filled at least in the micropores of the heat-resistant synthetic resin microporous 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 .

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

[Examples 1 to 4, Comparative Examples 1 and 3]
(Extrusion process)
Isotactic polypropylene, high-density polyethylene, silane-modified polypropylene (trade name “LINKLON XPM800HM” manufactured by Mitsubishi Chemical Corporation), silane-modified high-density polyethylene (trade name “LINKLON HM600A” manufactured by Mitsubishi Chemical Corporation), and vinyltrimethoxysilane Supply to the extruder by the predetermined amount shown in Table 1, melt knead at the resin temperature shown in Table 1, extrude into a film form from a T-die attached to the tip of the extruder, and set the surface temperature to 80 ° C The cooled olefin resin film (thickness 29 μm) was obtained with a cooled roll. The film formation rate was 25 m / min and the draw ratio was 65.

(Curing process)
The obtained olefin-based resin film was allowed to stand for 1 hour in an air oven at the atmospheric temperature (curing temperature) shown in Table 1 and cured.

(First stretching step)
The cured olefin resin film was cut into a strip shape of 300 mm in the extrusion direction (length direction) and 160 mm in the width direction. Using a uniaxial stretching apparatus, the olefin resin film was uniaxially stretched only in the extrusion direction at a stretching ratio of 1.5 times at a stretching rate of 50% / min so that the surface temperature was 23 ° C.

(Second stretching step)
Subsequently, using a uniaxial stretching apparatus, the surface temperature of the olefin resin film was adjusted to the second stretching temperature shown in Table 1 at a stretching rate of 42% / min and a stretching ratio of 2.7 times in the extrusion direction. Only uniaxially stretched.

(Annealing process)
Thereafter, the olefin resin film was left to stand for 10 minutes so that the surface temperature became the annealing temperature shown in Table 1 while being attached to the uniaxial stretching apparatus, and the olefin resin film was annealed. . This obtained the olefin resin microporous film which has a micropore part. The shrinkage rate of the olefin resin film during annealing was 10%.

(Coating process)
A coating solution containing ethyl acetate as a solvent, trimethylolpropane triacrylate (TMPTA, trade name “Light Acrylate TMP-A” manufactured by Kyoei Chemical Co., Ltd.) as a polymerizable compound, and benzophenone as a photopolymerization initiator was prepared. Table 1 shows the content of each component in the coating solution. Next, after coating the coating solution on the surface of the olefin resin microporous film, the solvent was removed by heating the olefin resin microporous film at 60 ° C. for 2 minutes. Thereby, the polymerizable compound was adhered to the entire surface of the olefin resin microporous film.

(Irradiation process)
Next, the olefin resin microporous film to which the polymerizable compound was adhered was placed in an atmosphere reduced to an oxygen concentration of 200 ppm by nitrogen purge. The polymerizable compound was polymerized by irradiating the both sides of the olefin resin microporous film coated with the polymerizable compound with ultraviolet rays so that the integrated light amount was 1700 mJ / cm ² .

(Heating process)
Subsequently, the olefin-based resin microporous film coated with the polymer of the polymerizable compound was placed in a heating furnace adjusted to an oxygen concentration of 200 ppm and heated at the heating temperature shown in Table 1 for 1 hour. As a result, a coating layer containing a polymer of a polymerizable compound was formed on the surface of the olefin-based resin microporous film and on the wall surface of the open end of the micropores continuous with the surface.

(Water treatment process)
The olefin resin microporous film is immersed in water at 90 ° C. for 300 minutes, and a silanol dehydration condensation reaction is allowed to proceed in silane-modified polypropylene, silane-modified high-density polyethylene or vinyltrimethoxysilane to carry out a crosslinking treatment, and a heat-resistant olefin system A resin microporous film was obtained. In Comparative Examples 1 and 3, the water treatment process was not performed. Table 1 shows the thickness, air permeability and porosity of the obtained heat-resistant olefin-based resin microporous film.

[Example 5, Comparative Example 2]
(Extrusion process)
Isotactic polypropylene and γ-methacryloxypropyltrimethoxysilane were supplied to the extruder in predetermined amounts as shown in Table 1, melt-kneaded at the resin temperatures shown in Table 1, and attached to the tip of the extruder. The film was extruded from a die into a film shape, taken up with a cooling roll whose surface temperature was set to 80 ° C., and a cooled olefin resin film (thickness 29 μm) was obtained. The film formation rate was 25 m / min and the draw ratio was 65. In Comparative Example 2, γ-methacryloxypropyltrimethoxysilane was not supplied to the extruder.

(Curing process)
The obtained olefin-based resin film was allowed to stand for 1 hour in a hot air oven at the curing temperature shown in Table 1 and cured.

(First stretching step)
The cured olefin resin film was cut into a strip shape of 300 mm in the extrusion direction (length direction) and 160 mm in the width direction. Using a uniaxial stretching apparatus, the olefin resin film was uniaxially stretched only in the extrusion direction at a stretching ratio of 1.5 times at a stretching rate of 50% / min so that the surface temperature was 23 ° C.

(Second stretching step)
Subsequently, using a uniaxial stretching apparatus, the surface temperature of the olefin resin film was adjusted to the second stretching temperature shown in Table 1 at a stretching rate of 42% / min and a stretching ratio of 2.7 times in the extrusion direction. Only uniaxially stretched.

(Annealing process)
Thereafter, the olefin resin film was left to stand for 10 minutes so that the surface temperature became the annealing temperature shown in Table 1 while being attached to the uniaxial stretching apparatus, and the olefin resin film was annealed. . This obtained the olefin resin microporous film which has a micropore part. The shrinkage rate of the olefin resin film during annealing was 10%. Table 1 shows the thickness, air permeability, and porosity of the obtained olefin-based resin microporous film.

(Coating process)
A coating solution containing ethyl acetate as a solvent and trimethylolpropane triacrylate (TMPTA, trade name “Light Acrylate TMP-A” manufactured by Kyoei Chemical Co., Ltd.) as a polymerizable compound was prepared. Table 1 shows the content of each component contained in the coating solution. Next, after coating the coating solution on the surface of the olefin resin microporous film, the solvent was removed by heating the olefin resin microporous film at 60 ° C. for 2 minutes. Thereby, the polymerizable compound was adhered to the entire surface of the olefin resin microporous film.

(Irradiation process)
Next, one side of the olefin resin microporous film to which the polymerizable compound is attached is irradiated with an electron beam at an accelerating voltage of 110 kV so that the absorbed dose becomes the dose shown in Table 1, and trimethylol which is a polymerizable compound. Propane triacrylate was polymerized.

  In Example 5, γ-methacryloxypropyltrimethoxysilane kneaded into an olefin resin microporous film was graft polymerized to isotactic polypropylene. Since the graft polymerized γ-methacryloxypropyltrimethoxysilane and isotactic polypropylene also reacted with the trimethylolpropane triacrylate of the coating layer, a crosslinked structure was formed in the olefin resin microporous film.

(Heating process)
Subsequently, the olefin resin microporous film coated with the polymer of the polymerizable compound after being irradiated with the electron beam was placed in a heating furnace adjusted to an oxygen concentration of 30 ppm, and the heating temperature shown in Table 1 for 1 hour. Heated. As a result, a coating layer containing a polymer of a polymerizable compound was formed on the surface of the olefin-based resin microporous film and on the wall surface of the open end of the micropores continuous with the surface.

(Water treatment process)
The olefin resin microporous film is immersed in water at 90 ° C. for 300 minutes, and a silanol dehydration condensation reaction proceeds in γ-methacryloxypropyltrimethoxysilane bonded to the olefin resin of the olefin resin microporous film to form a silane crosslinked structure. Was introduced to obtain a heat-resistant olefin resin microporous film. In Comparative Example 2, no water treatment step was performed. Table 1 shows the thickness, air permeability and porosity of the obtained heat-resistant olefin-based resin microporous film.

[Evaluation]
About heat resistant olefin resin microporous film, thickness, air permeability, porosity, gel fraction, maximum thermal shrinkage of TMA, storage elastic modulus Er at 40-250 ° C. by dynamic viscoelasticity, piercing strength and The amount of radicals was measured according to the procedure described above. These results are shown in Table 1. In Table 1, “Storage elastic modulus Er at 40 to 250 ° C. due to dynamic viscoelasticity” is the minimum storage elastic modulus Er at 40 to 250 ° C. “Storage elastic modulus Er at 40 to 250 ° C. due to dynamic viscoelasticity” was simply expressed as “storage elastic modulus Er”.

  Table 1 shows the content of the coating layer with respect to 100 parts by mass of the olefin resin microporous film for the heat resistant olefin resin microporous film.

  The heat-resistant synthetic resin microporous film of the present invention is excellent in meltdown resistance and is suitably used as a separator for non-aqueous electrolyte secondary batteries.

Claims (5)

Contains a synthetic resin microporous film containing an olefin resin, has a gel fraction of 75% by mass or more, a storage elastic modulus Er at 40 to 250 ° C. by dynamic viscoelasticity of 0.008 MPa or more, and a maximum shrinkage of TMA. A heat-resistant synthetic resin microporous film having a radical amount measured by an electron spin resonance method of 25% or less and 2.0 × 10 ¹⁶ spins / 100 mg or less.   The heat-resistant synthetic resin microporous film according to claim 1, wherein the olefin resin has a silane cross-linked structure.   2. A coating layer comprising a polymer of a polymerizable compound formed on at least a part of the surface of a synthetic resin microporous film and having two or more polymerizable functional groups in one molecule. The heat-resistant synthetic resin microporous film according to claim 1 or 2.   The heat-resistant synthetic resin microporous film according to any one of claims 1 to 3, wherein the olefin resin contains an ethylene resin and / or a propylene resin.   A battery separator comprising the heat-resistant synthetic resin microporous film according to any one of claims 1 to 4.