JP2022098795A - Porous composite film, separator foe electrochemical element, and method for producing porous composite film - Google Patents

Abstract

To provide a porous composite film which has excellent battery characteristics while having a function of improving adhesion to a porous layer composed of a mixture of inorganic particles and a thermoplastic polymer; a separator having a porous layer including the inorganic particles in the porous composite film; and a method for producing the porous composite film.SOLUTION: A porous composite film is obtained by laminating a layer containing an adhesive resin as a main component on at least one surface of a porous base material, in which in a scanning electron microscope photograph (SEM) on the surface where a layer containing the adhesive resin of the porous composite film as a main component is laminated, when a ratio of a part determined to be the adhesive resin in 10 μm×10 μm area are determined in 10 parts by SEM photographic places, the value is 40% to 85%.SELECTED DRAWING: Figure 1

Description

The present invention relates to a porous composite film, a separator for an electrochemical device, and a method for producing a porous composite film.

Secondary batteries such as lithium-ion batteries are used in portable digital devices such as smartphones, tablets, mobile phones, laptop computers, digital cameras, digital video cameras, and portable game machines, power tools, electric bikes, and electric assist assisted bicycles. It is widely used in portable equipment applications and automobile applications such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles.

In a lithium ion battery, generally, a separator for a secondary battery and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. Has a configuration.

As the separator for a secondary battery, a polyolefin-based porous membrane base material is used. The characteristics required for a separator for a secondary battery include the characteristic that an electrolytic solution is contained in the porous structure to enable ion transfer, and the porous structure is closed by melting with heat when the lithium ion battery generates abnormal heat. The shutdown characteristic is that the energization is stopped by stopping the ion movement. However, the polyolefin-based porous film base material has a problem that the thermal shrinkage becomes extremely large in a temperature range of about 100 ° C. or higher due to the characteristics of the material and the manufacturing process, and the positive electrode and the negative electrode are likely to be short-circuited. In order to solve this problem, research and development aimed at improving heat resistance and reducing heat shrinkage by providing a layer containing inorganic particles and a thermoplastic polymer on at least one side of a polyolefin-based porous film substrate has progressed. The layer containing the above-mentioned inorganic particles and the thermoplastic polymer has an extremely large amount of inorganic particle components in order to have the functions of improving heat resistance and reducing the heat shrinkage rate, and contains a polyolefin-based porous film substrate, the inorganic particles and the thermoplastic polymer. Adhesion to the layer may be low. Therefore, when the porous composite film is conveyed in the electrochemical element manufacturing process, there is a problem that the layer containing the inorganic particles and the thermoplastic polymer is detached, which causes process contamination and damage to the porous composite film itself. Therefore, it is required to improve the adhesion between the polyolefin-based porous substrate and the layer containing the machine particles and the thermoplastic polymer.
On the other hand, lithium-ion batteries are also required to have excellent battery characteristics such as high output and long life, and are required to exhibit good battery characteristics without deteriorating battery productivity. There is.

In response to these demands, Patent Document 1 discloses a separator in which a porous substrate is subjected to a corona discharge treatment or a plasma discharge treatment, and then a porous layer containing an inorganic filler is provided. According to such a separator, it is said that the coating liquid can be easily applied and the adhesiveness between the inorganic filler-containing porous layer and the porous base material after coating is improved.

Further, Patent Document 2 discloses a technique of using an adhesive between the heat-resistant porous film and the thermoplastic resin porous film when they are laminated. By using such a technique, it is expected that the adhesiveness with each layer can be improved.

On the other hand, Patent Document 3 discloses a technique of providing a binding polymer in a dot shape for the purpose of imparting adhesiveness to an electrode to a separator.

Patent No. 6444122 Japanese Unexamined Patent Publication No. 2016-207649 Patent No. 6242756

However, the porous film is easily damaged by fibrils due to corona discharge treatment or plasma discharge treatment, and the separator described in Patent Document 1 has a low mechanical strength or has a defect in the plane, resulting in a short circuit or a short circuit. There is a concern that the long-term characteristics of the battery may deteriorate due to the non-uniform current distribution in the separator surface.

Further, when the adhesive layer is provided as in Patent Document 2, in the general coating method, the adhesive covers the entire surface of the porous substrate, so that there is a concern that the permeability is lowered and the output characteristics of the battery are deteriorated. be.

The technique of Patent Document 3 is applied to Patent Document 2, that is, by making the adhesive into a dot shape by using a spray method or an inkjet method, an output characteristic is provided by providing a portion where the adhesive does not exist (a portion having transparency). Seems to improve. However, the dots are separated by several hundred μm or more. Therefore, the current distribution in the separator surface becomes non-uniform, and there is a concern that the long-term characteristics of the battery may deteriorate.

In view of the above problems, an object of the present invention is to provide a porous composite film having excellent battery characteristics while having a function of improving adhesion to a porous layer made of a mixture of inorganic particles and a thermoplastic polymer. That is.

Another object of the present invention is to provide a separator having a porous layer containing inorganic particles in the porous composite film, and a method for producing the porous composite film.

The present invention satisfies the following configurations in order to solve the above problems.
<1> A porous composite film in which a layer containing an adhesive resin as a main component is laminated on at least one surface of a porous substrate, and a layer containing an adhesive resin as a main component of the porous composite film is laminated. In the scanning electron micrograph (SEM) of the surface, the ratio of the part determined to be the adhesive resin in the 10 μm × 10 μm region was obtained at 10 places by changing the SEM shooting place, and the value was 40% to 85. Perforated composite film characterized by%.
<2> The porous composite film according to <1>, wherein the adhesive resin contains at least one of a resin having a melting point at 100 ° C. or higher or an amorphous resin as a main component.
<3> On the side where the layer containing the adhesive resin as the main component of the porous composite film according to <1> or <2> is laminated, a porous layer composed of a mixture of inorganic particles and a thermoplastic polymer is further formed. A separator for an electrochemical element, which is formed and has a proportion of inorganic particles in a porous layer of 90% by weight or more.
<4> (a) Polyolefin containing polyethylene as a main component and a solvent for film formation are melt-kneaded and kneaded.
Step of preparing a polyolefin solution (b) Step of extruding the polyolefin solution and cooling to form a gel-like sheet (c) First stretching step of stretching the gel-like sheet (d) From the stretched gel-like sheet Step of removing the film-forming solvent (e) Step of drying the sheet after removing the film-forming solvent (f) Step of applying a coating liquid in which an adhesive resin is dissolved or dispersed in a solvent to the sheet (g). Step of drying the solvent (h) Method for producing a porous composite film, which comprises stretching 1.1 to 2 times in at least one axial direction <5> The steps of (g) and (h) are the same. The method for producing a porous composite film according to <4>, which is performed in a process.
<6> The method for producing a porous composite film according to <4> or <5>, wherein the stretching direction in the step (h) is the width direction.
<7> The method for producing a porous composite film according to any one of <4> to <6>, wherein the solvent contains 90% by weight or more of water.

When the porous composite film of the present invention is provided with a porous layer made of a mixture of inorganic particles and a thermoplastic polymer, electricity having good battery characteristics while having high adhesion between the porous composite film and the porous layer. A separator for a chemical element can be provided.

A surface SEM of a porous composite film according to a preferred embodiment of the present invention. In FIG. 1, the adhesive resin portion is painted with black ink.

Hereinafter, the present invention will be described in detail. The terms and words used herein and in the scope of the claim should not be construed in a general or lexicographical sense, but should be construed in accordance with the meanings and concepts consistent with the technical ideas of the present invention. It should be.

(Porous medium)
The porous composite film of the present invention has a porous substrate. The porous base material contains polyethylene as a main component. The term "main component" as used herein means that the proportion of the component is 90% by weight or more. As the component other than polyethylene, polyolefins such as polypropylene, polybutylene, and polypentene are preferable.

As the form of the porous base material, a film form or a non-woven fabric form can be used. The thickness of the porous substrate is not particularly limited, but is preferably 4 to 50 μm, the size of the pores present in the porous substrate is preferably 0.01 to 50 μm, and the porosity of the porous substrate is 10 to 10. It is preferably 95%. By having such a thickness, a pore size, and a porosity, sufficient mechanical strength and insulating properties can be obtained, and sufficient ionic conductivity can be obtained.

Here, the pore size is a through hole diameter measured according to the bubble point method (half-dry method) described in JIS K3832 and ASTM F316-86.

Further, the porosity here means that the constituent materials are composed of a, b ..., N, and the masses of the constituent materials are Wa, Wb ..., Wn (g · cm ² ), and the true densities of each are Is da, db ..., Dn (g / cm ³ ), and when the film thickness of interest is t (cm), it is a value (ε (%)) obtained by the following equation (1).
ε = {1- (Wa / da + Wb / db + ... + Wn / dn) / t} × 100 ... (1).

(Layer whose main component is adhesive resin)
A layer containing an adhesive resin as a main component is laminated on at least one surface of the porous base material. The adhesive resin is preferably a chemical species that is electrochemically stable and does not dissolve in the electrolytic solution, and is preferably a fluororesin, an acrylic resin, an olefin resin such as polyethylene or polypropylene, a styrene-butadiene resin, or a crosslinked polystyrene resin. , Methyl methacrylate-styrene copolymer, polyimide resin, melamine resin, phenol resin, polyacrylonitrile resin, silicon resin, urethane resin, polycarbonate resin, carboxymethyl cellulose resin and the like, and only one of these may be used. , A plurality of combinations may be used. Of these, from the viewpoint of electrochemical stability and solubility in an electrolytic solution (poor solubility), it is preferable to use at least one selected from a fluororesin, an acrylic resin and an olefin resin, and the fluororesin or acrylic is preferable. It is more preferable to use a resin. As the form, it may be dissolved in a solvent or dispersed (emulsion) in the solvent.

Examples of the fluororesin include homopolymers such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride and polychlorotrifluoroethylene, and copolymers such as ethylene-tetrafluoroethylene polymer and ethylene-chlorotrifluoroethylene polymer. Moreover, a copolymer of a homopolymer and tetrafluoroethylene, hexafluoropropylene, trifluoroethylene and the like can also be mentioned. Among these fluororesins, polyvinylidene fluoride, in particular, a resin composed of a copolymer of vinylidene fluoride and hexafluoropropylene is considered from the viewpoint of electrochemical stability and solubility in an electrolytic solution (poor solubility). It is preferably used.

The weight average molecular weight of the fluororesin is preferably 50,000 or more and 2 million or less, more preferably 100,000 or more and 1.5 million or less, and further preferably 200,000 or more and 1 million or less. If the weight average molecular weight of the fluororesin is less than 50,000, sufficient adhesiveness may not be obtained. Further, when the weight average molecular weight of the fluororesin is larger than 2 million, the handleability and coatability may be lowered due to the increase in viscosity.

Examples of the acrylic resin include poly (meth) acrylic acid and poly (meth) acrylic acid ester. Here, "(meth) acrylic" means acrylic or methacrylic. Examples of the poly (meth) acrylic acid monomer include acrylic acid and methacrylic acid, and examples of the poly (meth) acrylic acid ester monomer include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, and butyl. Examples include methacrylate. These may be used alone or in combination of two or more.

The adhesive resin has a melting point of 100 ° C. or higher in order to enter into the particle gaps in the porous layer composed of the unevenness of the porous substrate and the mixture of the inorganic particles and the thermoplastic polymer and exhibit a good anchoring effect. Alternatively, it is preferable to use at least one of the amorphous resins as the main component. If an adhesive resin having a melting point of less than 100 ° C. is used, it may melt and flow excessively in the manufacturing process described later and permeate into the inside of the porous substrate, so that sufficient adhesiveness may not be exhibited. On the other hand, a resin having a melting point too high generally does not exhibit adhesiveness, so the melting point is preferably 200 ° C. or lower, more preferably 150 ° C. or lower. Here, the melting point means the temperature of the endothermic peak at the time of the second temperature rise after the temperature is raised and cooled by the differential scanning calorimetry apparatus (DSC) measured according to JIS K7121 (2012).

The amorphous resin is a resin having no melting point at −20 to 300 ° C., that is, having no heat absorption peak and having a glass transition temperature of less than 150 ° C. as measured by a differential scanning calorimeter. Say. Here, the glass transition temperature is, for example, a low temperature at the time of the second temperature rise after raising and cooling in the differential scanning calorimetry (DSC) according to the provisions of "JIS K7121: 2012 Plastic transition temperature measuring method". It means the intersection of the straight line extending the baseline on the side to the high temperature side and the tangent line drawn at the point where the gradient of the curve of the stepwise change part of the glass transition is maximized.

(The state of the layer whose main component is adhesive resin)
The state of formation of the adhesive resin will be described with respect to the layer containing the adhesive resin as a main component. In the layer, it is not preferable that the adhesive resin uniformly covers the entire surface of the porous substrate. Therefore, it is necessary that the existing portion and the non-existing portion of the adhesive resin are formed. In this case, the portion where the adhesive resin is present contributes to the improvement of the adhesion to the porous layer made of the mixture of the inorganic particles and the thermoplastic polymer, and the perforated portion contributes to the improvement of the permeability (rate characteristics of the battery). , Both functions can be separated. In the present invention, in the scanning electron micrograph (SEM) of the surface, the ratio of the portion discriminated as the adhesive resin in the 10 μm × 10 μm region is determined at 10 locations by changing the SEM imaging location. It should be between 40% and 85. This means that the adhesive resin present portion and the open portion (the portion where the porous base material is exposed) are present in a constant ratio in a minute range of 10 μm × 10 μm in any place of the porous composite film. That is, it means that the presence portion and the non-existence portion of the adhesive resin are formed at a very fine pitch in the plane. It has been found that such a form can improve the long-term characteristics (cycle characteristics) of the battery while maintaining the adhesion to the porous layer and the rate characteristics, and arrived at the present invention. The mechanism is not always clear, but I think it is as follows. Inside a secondary battery such as a lithium-ion battery, ions move in the electrolyte during charging and discharging, but migration is dominant in driving the ion, and diffusion is a secondary phenomenon caused as a result of migration. Is believed to be. That is, it is considered that the ions basically move linearly. Therefore, if the opening state is non-uniform in the surface of the separator, there will be some areas where ions easily flow and some areas where ions do not easily flow. Then, the state of charge (or the state of discharge) becomes non-uniform in the plane of the electrode, the deterioration of the electrode is promoted, and the long-term characteristics (cycle characteristics) of the battery as a whole may deteriorate. In the present invention, the adhesive resin present portion (where the ions do not easily flow) and the non-existent portion (where the ions easily flow) are created in a minute range as shown in FIG. Sometimes it is thought that the distribution of ion flow is uniform. Therefore, if the proportion of the portion discriminated as the adhesive resin in the 10 μm × 10 μm region in the scanning electron micrograph (SEM) of the surface is less than 40%, it means that the amount of the adhesive resin is insufficient. , Adhesion with the porous layer may decrease. It is preferably 50% or more, more preferably 60% or more. On the other hand, if it exceeds 85%, it means that the amount of the adhesive resin is excessive, and the permeability may decrease. It is preferably 80% or less. On the other hand, even if the presence portion and the non-existence portion of the adhesive resin are separately formed in the macro region, the ratio of the portion discriminated as the adhesive resin in the 10 μm × 10 μm region is in a range wider than 60% to 85%. (That is, it means that the area of the adhesive resin portion is large and the intervals are large), the long-term characteristics (cycle characteristics) may be deteriorated. The method of forming a layer containing an adhesive resin as a main component on a porous substrate in the above-mentioned form is a known coating method, for example, a dip coating method, a reverse roll coating method, a gravure coating method, or a kiss. -It is difficult to realize by using the coating method, roll brushing method, spray coating method, inkjet method, air knife coating method, Meyer bar coating method, pipe doctor method, blade coating method, die coating method, etc. alone or in combination. It is preferably formed by the method described later.

As for the method of determining the ratio of the adhesive resin on the surface of the porous composite film, as described later in the section of Examples, the portion determined to be the adhesive resin portion is first extracted from the surface SEM photographed at 10000 times. .. This work is performed by placing an OHP film on the image on which the SEM image is printed and painting the adhesive resin portion with black ink, as will be described later in the section of Examples. At this time, the portion where the fibril structure peculiar to the porous substrate can be visually identified can be identified as not the adhesive resin portion, and the portion not present can be identified as the adhesive resin portion. When only the adhesive resin portion is actually extracted with respect to FIG. 1, the result is as shown in FIG. Then, image analysis software is used for the image obtained by extracting only the adhesive resin portion, and the ratio of the adhesive resin portion to the total area is obtained.

(Physical characteristics of porous composite film)
The air permeability of the porous composite film of the present invention is preferably 100 seconds / 100 cm ³ or more and 1,000 seconds / 100 cm ³ or less. More preferably, it is 200 seconds / 100 cm ³ or more and 1000 seconds / 100 cm ³ or less. More preferably, it is 400 seconds / 100 cm ³ or more and 1000 seconds / 100 cm ³ or less. When the air permeability is 1,000 seconds / 100 cm ³ or less, sufficient ion mobility can be obtained and the battery characteristics can be improved. When the air permeability is 100 seconds / 100 cm ³ or more, sufficient mechanical properties can be obtained.
The upper limit of the film thickness is preferably 20 μm, more preferably 16 μm. The lower limit is preferably 4 μm or more, more preferably 7 μm or more. If it is thinner than 4 μm, it may be difficult to secure sufficient mechanical strength and insulation, and if it is thicker than 20 μm, the electrode area that can be filled in the container decreases, resulting in a decrease in capacity. It can be difficult to avoid.

(Porous layer)
When a porous layer made of a mixture of inorganic particles and a thermoplastic polymer is further formed on a layer containing an adhesive resin as a main component formed on a porous composite film and used as a separator for an electrochemical element. There is.
(Inorganic particles contained in the porous layer)
In the present invention, inorganic particles are contained in the porous layer in order to ensure the strength of the porous layer. The inorganic particles are not particularly limited as long as they are electrochemically stable.

That is, the inorganic particles that can be used in the present invention are particularly limited as long as they do not undergo oxidation and / or reduction reactions within the operating voltage range of the electrochemical element used (for example, 0 to 5 V based on Li / Li ⁺ ). Not, but calcium phosphate, amorphous silica, crystalline glass filler, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, mica , Boehmite, etc. In particular, it is preferable to include at least one selected from the group consisting of titanium dioxide, alumina, and boehmite because of the crystal growth potential, cost, and availability of the fluorine-containing resin.

The shape of the inorganic particles may be a true sphere, a substantially sphere, or a plate, but is not particularly limited.

The lower limit of the content of the inorganic particles is preferably 90% by weight or more, more preferably 95% by weight or more, based on the entire porous layer. When the content of the inorganic particles is in this range, the strength of the porous layer is appropriately maintained.

The average particle size of the inorganic particles is preferably 1.5 times or more and 50 times or less the size of the pores of the porous substrate. More preferably, it is 2.0 times or more and 20 times or less. When the average particle size of the inorganic particles is within the above-mentioned preferable range, the air permeability resistance is maintained without blocking the pores of the separator in a state where the thermoplastic polymer and the inorganic particles are mixed, and the particles are further formed in the battery assembly step. Prevents it from falling out and causing serious battery defects.

(Thermoplastic polymer contained in the porous layer)
In the present invention, a thermoplastic polymer is used to bind the inorganic particles contained in the porous layer to each other and to bond the inorganic particles to the porous composite film. The thermoplastic polymer is preferably a water-soluble resin or a water-dispersible resin. By using a water-soluble resin or a water-dispersible resin, not only excellent heat resistance can be obtained, but also low cost is possible, and it is also preferable from the viewpoint of environmental load in the manufacturing process.

Specific examples of the water-soluble resin or the water-dispersible resin include carboxymethyl cellulose (CMC), polyvinyl alcohol, and acrylic resins such as polyacrylic acid, polyacrylamide, and polymethacrylic acid, and CMC and acrylic resins. Is the most preferable.

(Other components contained in the porous layer)
The porous layer may contain various additives such as a surfactant and an antistatic agent as a composition other than the inorganic particles and the thermoplastic polymer as long as the object of the present invention is not impaired.

(Physical characteristics of the porous layer)
The film thickness of the porous layer is preferably 2 to 10 μm per side, more preferably 3 to 8 μm, and even more preferably 4 to 6 μm. If the film thickness of the porous layer is less than 2 μm, the heat resistance may be insufficient. Further, if the film thickness of the porous layer exceeds 10 μm, not only the productivity is deteriorated, but also the capacity of the battery is expected to increase in the future, which is not preferable.
The porosity of the porous layer is preferably 30 to 90%, more preferably 40 to 70%. When the porosity of the porous layer is within the above-mentioned preferable range, the battery separator obtained by laminating the porous layer has a low electrical resistance of the film, a large current easily flows, and the film strength is maintained.

(Formation of porous layer)
For the formation of the porous layer on the porous composite film, a known coating method, for example, a dip coating method, in which inorganic particles, a thermoplastic polymer, and other components are dispersed or dissolved in a solvent containing water as a main component is used. , Reverse roll coat method, gravure coat method, kiss coat method, roll brush method, spray coat method, inkjet method, air knife coat method, Meyer bar coat method, pipe doctor method, blade coat method and die coat method, etc. Alternatively, it can be applied by coating with a combination and dried at 100 ° C. or lower, preferably 90 ° C. or lower, more preferably 80 ° C. or lower.

(Manufacturing method of porous composite film)
The method for producing the porous composite film of the present invention will be described. The method for producing a porous composite film of the present invention needs to include the following steps (a) to (h).

(A) A step of preparing a polyolefin solution by melt-kneading a polyolefin containing polyethylene as a main component and a solvent for film formation.
(B) A step of extruding the polyolefin solution and cooling it to form a gel-like sheet.
(C) The first stretching step of stretching the gel-like sheet,
(D) A step of removing the film-forming solvent from the stretched gel-like sheet (e) A step of drying the sheet after removing the film-forming solvent.

(F) Step of applying a coating liquid in which an adhesive resin is dissolved or dispersed in a solvent to the sheet (g) Step of drying the solvent (h) Step of stretching 1.1 to 2 times in at least one axial direction Next Each of the above steps will be described later.

(A) Step of preparing a polyolefin solution A suitable film-forming solvent is added to each of a polyolefin containing polyethylene as a main component, and then melt-kneaded to prepare a polyolefin solution. As the melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is known, the description thereof will be omitted.

The mixing ratio of the polyolefin and the film-forming solvent in the polyolefin solution is preferably 70 to 80 parts by mass with respect to 20 to 30 parts by mass of the polyolefin. When the blending ratio of the polyolefin and the film-forming solvent is within the above range, swells and neck-ins can be prevented at the die outlet when the polyolefin solution is extruded, and the formability and self-support of the extruded molded product (gel-shaped molded product) can be prevented. The sex becomes good.
(B) Step of forming a gel-like sheet A polyolefin solution is fed from an extruder to a die and extruded into a sheet. Multiple polyolefin solutions of the same or different composition can be fed from an extruder to a single die, where they can be layered and extruded into sheets.

As the extrusion method, either a flat die method or an inflation method can be adopted. The extrusion temperature is preferably 140 to 250 ° C., and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness can be adjusted by adjusting each extrusion amount of the polyolefin solution. As the extrusion method, for example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gel-like sheet is formed by cooling the obtained extruded body. As a method for forming the gel-like sheet, for example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or higher, at least up to the gelation temperature. Cooling is preferably performed to a temperature of 25 ° C. or lower. By cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallinity is maintained in an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but it is preferable to contact with a roll cooled with the refrigerant for cooling.

(C) First stretching step Next, the obtained gel-like sheet is stretched at least in the uniaxial direction. Since the gel-like sheet contains a film-forming solvent, it can be uniformly stretched. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferably used. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) can be used.

The stretching ratio (area stretching ratio) in this stretching step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Further, the draw ratios in the mechanical direction (MD) and the width direction (TD) may be the same or different from each other. The stretching ratio in this stretching step refers to the area stretching ratio of the microporous substrate immediately before being subjected to the next step, based on the gel-like sheet immediately before the stretching step.

The stretching temperature in this stretching step is preferably in the range of the polyolefin crystal dispersion temperature (Tcd) to Tcd + 30 ° C, more preferably in the range of Tcd + 5 ° C to Tcd + 28 ° C, and more preferably in the range of Tcd + 10 ° C to Tcd + 26 ° C. It is a particularly preferable embodiment to be inside. For example, in the case of polyethylene, the stretching temperature is preferably 90 to 140 ° C, more preferably 100 to 130 ° C. The crystal dispersion temperature (Tcd) is determined by measuring the temperature characteristics of dynamic viscoelasticity with ASTM D4065.

Due to the above stretching, for example, when polyethylene is used as the polyolefin, cleavage occurs between the polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibril forms a three-dimensionally irregularly connected network structure, and the gel-like sheet becomes a microporous base material. The mechanical strength is improved and the pores are expanded by stretching, but by stretching under appropriate conditions, it is possible to control the through-hole diameter and to have a high porosity even with a thinner film thickness.

Depending on the desired physical properties, the temperature distribution can be provided in the film thickness direction and the film can be stretched, whereby a microporous substrate having excellent mechanical strength can be obtained. Details of the method are described in Japanese Patent No. 3347854.

(D) Step of removing the film-forming solvent The film-forming solvent is removed (cleaned) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, it is composed of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the above is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(E) Drying step The microporous substrate from which the film-forming solvent has been removed is dried by a heat-drying method or an air-drying method. The drying temperature is preferably not less than the crystal dispersion temperature (Tcd) of the polyolefin, and particularly preferably 5 ° C. or more lower than Tcd.

Drying is preferably carried out with the microporous substrate as 100% by mass (dry weight) until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the microporous substrate is maintained when the second stretching step and the heat treatment step of the microporous substrate are further performed, and the deterioration of the permeability is suppressed. Will be done.

(F) Coating Step Next, a coating liquid in which an adhesive resin is dissolved or dispersed in a solvent is applied to one or both sides of the sheet that has been dried. Examples of the coating method include known coating methods such as dip coating method, reverse roll coating method, gravure coating method, kiss coating method, roll brushing method, spray coating method, inkjet method, and air knife coating method. The Meyer bar coat method, the pipe doctor method, the blade coat method, the die coat method and the like can be used alone or in combination.

The solvent preferably contains 90% by weight of water. The solvent may be only water, but if necessary, a water-soluble organic solvent such as alcohols can be mixed and used. By using these water-soluble organic solvents, the drying speed and coatability can be improved. As the water-soluble organic solvent, a mixed solution in which alcohols such as ethanol, isopropyl alcohol, and benzyl alcohol are mixed in a range of 0.1 to 10% by weight in the total coating solution is preferable. Further, if it is less than 1% by mass, an organic solvent other than alcohols may be mixed within a soluble range. However, the total amount of alcohols and other organic solvents in the coating liquid shall be less than 10% by weight. Further, components other than the adhesive resin may be further contained within a range that does not impair the object of the present invention. Examples of such a component include a dispersant, a pH adjuster and the like.

(G) Drying of solvent Next, after drying the solvent, a second stretching step is further performed. First, the solvent is dried from the sheet coated with the coating liquid, but the method for drying is not particularly limited, but it is performed immediately before the second stretching step described later, that is, it is dried using the heating mechanism in the second stretching step. It is preferable from the viewpoint of productivity.

(H) Second stretching step The second stretching includes MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching. Simultaneous biaxial stretching with a tenter and the like can be mentioned. By stretching after coating in this way, the layer containing the adhesive resin as the main component, which was spread over one surface in the step (f), is finely cleaved as shown in FIG. Furthermore, it is possible to form a structure that achieves both long-term characteristics (cycle characteristics) of the battery. The temperature of the second stretching is preferably not higher than the melting point of the stretched film, and more preferably within the range of (crystal decomposition temperature Tcd-20 ° C. of the stretched film) to the melting point of the stretched film. Specifically, when polyethylene is the main component, the re-stretching temperature is preferably 70 to 135 ° C, more preferably 110 to 132 ° C. Most preferably, it is 120 to 130 ° C.

The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012).
In the case of uniaxial stretching, the second stretching ratio is preferably 1.1 to 2 times, particularly preferably 1.1 to 1.8 times, and more preferably 1.3 to 1.5 times in the TD direction. When biaxial stretching is performed, it is preferable to stretch 1.1 to 1.8 times, preferably 1.2 to 1.4 times, respectively, in the MD direction and the TD direction. The second draw ratio may be different in the MD direction and the TD direction. By re-stretching within the above range, the surface of the layer containing the adhesive resin as a main component is finely cleaved, and the surface structure specified in the present invention is adopted.
From the viewpoint of heat shrinkage rate and wrinkles and sagging, it may be relaxed after the second stretching. The relaxation rate is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less.

(Electrochemical element)
The electrochemical element referred to in the present invention includes an electrode assembly and a battery case for accommodating the electrode assembly.

The electrode assembly includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

The porous composite film of the present invention can be suitably used for the above separator. Further, a porous composite film of the present invention provided with a porous layer can also be suitably used for the separator.

Examples of such an electrochemical element include a primary battery, a secondary battery, an electric double layer capacitor, an aluminum electrolytic capacitor and the like.

Examples of the primary battery include a manganese dry battery, an alkaline manganese dry battery, a graphite fluoride / lithium battery, a manganese dioxide / lithium battery, a solid electrolyte battery, a water injection battery, a thermal battery and the like.

Examples of the secondary battery include lithium secondary battery, lead storage battery, nickel / cadmium battery, nickel / hydrogen battery, nickel / iron storage battery, silver oxide / zinc storage battery, manganese dioxide / lithium secondary battery, lithium cobaltate / carbon dioxide. Examples include system secondary batteries and vanadium / lithium secondary batteries.

Among these, a secondary battery is preferable because it can be used for a long period of time, and a lithium secondary battery that realizes a high energy density by using an organic solvent is more preferable.

As the battery case, for example, a case made of aluminum, a case made of iron whose inner surface is nickel-plated, a case made of an aluminum laminated film, or the like can be used.

Examples of the shape of the battery case include a pouch type, a cylindrical type, a square type, and a coin type. Among these, the pouch type is preferable because it can realize high energy density and can freely design the shape at low cost.

The positive electrode is a positive electrode material composed of an active material, a binder resin, and a conductive auxiliary agent laminated on a current collector.

Examples of the active material include layered lithium-containing transition metal oxides such as LiCoO ₂ , LiNiO ₂ , and Li (NiCoMn) O ₂ , spinel-type manganese oxides such as LiMn ₂ O ₄ , and iron-based compounds such as LiFePO ₄ . Can be mentioned.

As the binder resin, a resin having high oxidation resistance may be used. Specific examples thereof include a fluorine-containing resin, an acrylic resin, and a styrene-butadiene resin.

Examples of the conductive auxiliary agent include carbon black, carbon materials such as graphite, and the like.

As the current collector, a metal foil is suitable, and aluminum is often used in particular.

The negative electrode is a negative electrode material composed of an active material and a binder resin laminated on a current collector.

Active materials include carbon materials such as artificial graphite, natural graphite, hard carbon and soft carbon, lithium alloy materials such as tin and silicon, metallic materials such as lithium, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). Can be mentioned.

Examples of the binder resin include a fluorine-containing resin, an acrylic resin, and a styrene-butadiene resin.

As the current collector, a metal foil is suitable, and in particular, a copper foil is often used.

The electrochemical element of the present invention preferably contains an electrolytic solution. The electrolytic solution is a place for moving ions between the positive electrode and the negative electrode in an electrochemical element such as a secondary battery, and has a structure in which the electrolyte is dissolved in an organic solvent.

Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , and the like, and LiPF ₆ is preferably used from the viewpoint of solubility in an organic solvent and ionic conductivity.

Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate and the like, and two or more kinds of these organic solvents may be mixed and used.

Hereinafter, a method for manufacturing a lithium secondary battery, which is preferably used among electrochemical devices, will be described.

As a method for producing a lithium secondary battery, first, an active material and a conductive auxiliary agent are dispersed in a binder resin solution to prepare a coating liquid for electrodes, and this coating liquid is applied onto a current collector to prepare a solvent. By drying, a positive electrode and a negative electrode can be obtained, respectively. The film thickness of the coating film after drying is preferably 50 μm or more and 500 μm or less.

A separator for a lithium secondary battery is placed between the obtained positive electrode and the negative electrode so as to be in contact with the active material layer of each electrode, sealed in an exterior material such as an aluminum laminate film, and after injecting an electrolytic solution, a negative electrode lead. And install a safety valve to seal the exterior material.

The lithium secondary battery thus obtained has high adhesiveness to electrodes, has excellent battery characteristics, and can be manufactured at low cost.

(Other uses)
Since the porous composite film of the present invention is porous, it can be used as a substance separation, selective permeation, a separating material, or the like. Specifically, in addition to the above-mentioned electrochemical elements, examples thereof include various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture permeable and waterproof clothing, and medical materials.

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

[Measurement method]
(Thickness of porous composite film and porous layer)
The measurement was performed using a contact film thickness meter (“Lightmatic” (registered trademark) series 318 manufactured by Mitutoyo Co., Ltd.). For the measurement, 20 points were measured under the condition of a weight of 0.01 N using a super-hard spherical surface stylus φ9.5 mm, and the average value of the obtained measured values was taken as the thickness. The thickness t of the porous layer was calculated using the following equation.
t = Sample thickness after forming the porous layer (t1) -Thickness of the sample before forming the porous layer (t2).

(Air permeability)
Select one randomly selected sample from three samples of 100 mm x 100 mm size, and use the Oken type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.) to JIS P 8117 (. The measurement was performed in accordance with 2009), and the average value was taken as the air permeability (seconds / 100 cm ³ ).

(Adhesion of porous layer)
A duralumin plate (5 cm x 15 cm) was prepared, and a 1.8 cm x 5 cm 3M Scotch transparent double-sided tape (acrylic adhesive, product number 665-1-18) was attached to the center. Next, the sample (3 cm × 6 cm) was attached by a hand roller so that the porous layer faces the double-sided tape side and covers the double-sided tape, and then the sample (3 cm × 6 cm) was left at room temperature for 5 minutes. After that, the duralumin plate was fixed using the force gauges ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd., and the peeling speed was 300 mm by the peeling method in which the sample end was gripped and the peeling angle was 180 degrees. A peeling test was performed at / min to check the peeling strength. At this time, the average value of the peel strength in the peel test for a length of 3 cm of the tape portion was taken as the peel strength. The peel strength was evaluated in the following four stages.
Evaluation criteria S (best): 100 gf / 25 mm or more A (good): 80 gf / 25 mm or more and less than 100 gf / 25 mm B (possible): 60 gf / 25 mm or more and less than 80 gf / 25 mm or more C (impossible): less than 60 gf / 25 mm.

(Ratio of adhesive resin on the surface of porous composite film)
First, the porous composite film is subjected to Pt vapor deposition (ion sputtering: sputtering current 20 mA, time 20 seconds) in advance, and an acceleration voltage 2 is used using an electro-emission scanning electron microscope (JSM-6701F manufactured by Nippon Denshi Co., Ltd.). SEM images were acquired at .00 kV and a magnification of 10000 times. The obtained SEM image was enlarged to A4 size and printed. An OHP film was placed on the printed image, and the adhesive resin portion was painted with ultrafine oil-based black ink in a region of 10 μm × 10 μm to create a projection image reproducing the adhesive resin portion on the OHP film. At this time, it was determined that the portion where the fibril structure peculiar to the porous substrate was visible was not the adhesive resin portion, and the portion where it was not was determined to be the adhesive resin portion. The projected image was read by a scanner and converted into image data again. The obtained image data was opened using ImageJ, a free software for image analysis published by the National Institutes of Health, and binarization operations were performed on the adhesive resin portion and the portion without the adhesive resin. Specifically, Binary and Make Binary were selected in this order in the Process tab. Furthermore, in the Image tab, Select Adjust and Threshold in that order to open the Threshold window. A threshold value of 0-50 was input, and the adhesive resin portion was inverted in red. Then, Measurement was selected from the Analyze tab, and the output area value (Area) was recorded (this is referred to as A1). For the area of the entire image, select Set Measurements from the Analysis tab, uncheck Limit to The Hold, select Measurement from the Analysis tab, and use this as the output area value (Area) (this is A2). ). Then, the value of A1 ÷ A2 × 100 was defined as the “ratio of the adhesive resin on the surface of the porous composite film” in the sample. Ten SEM images were taken at different locations, all of the above treatments were performed, and the range of the obtained adhesive resin ratio values was recorded.

(Battery production)
<Preparation of electrolytic solution>
A solvent was prepared by mixing ethylene carbonate (EC): methyl ethyl carbonate (MEC): diethyl carbonate (DEC) = 3: 5: 2 (volume ratio). LiPF ₆ (lithium hexafluorophosphate) and VC (vinylene carbonate) were added to the solvent to prepare an electrolytic solution having a LiPF ₆ concentration of 1.15 mol / L and a VC concentration of 0.5% by mass.

<Manufacturing of positive electrode>
Acetylene black graphite and polyvinylidene fluoride were added to lithium cobalt oxide (LiCoO ₂ ) and dispersed in N-methyl-2-pyrrolidone to form a slurry. This slurry was uniformly applied to both surfaces of an aluminum foil for a positive electrode current collector having a thickness of 20 μm and dried to form a positive electrode layer. Then, it was compression-molded by a roll press to prepare a strip-shaped positive electrode having a density of the positive electrode layer of 3.6 g / cm ³ excluding the current collector.

<Manufacturing of negative electrode>
An aqueous solution containing 1.0 part by mass of carboxymethyl cellulose is added to 96.5 parts by mass of artificial graphite and mixed, and 1.0 part by mass of styrene-butadiene latex as a solid content is added and mixed to form a slurry containing a negative electrode mixture. did. This negative electrode mixture-containing slurry was uniformly applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 8 μm and dried to form a negative electrode layer. Then, it was compression-molded by a roll press to prepare a strip-shaped negative electrode having a density of the negative electrode layer of 1.5 g / cm ³ excluding the current collector.

<Making batteries>
After laminating the above positive electrode, the sample obtained in each Example or Comparative Example, and the above negative electrode, a flat wound electrode body (height 2.2 mm × width 32 mm × depth 32 mm) was produced. At this time, the surface on which the porous layer was laminated was opposed to the positive electrode side. A tab with a sealant was welded to each electrode of this flat wound electrode body to form a positive electrode lead and a negative electrode lead.

Next, the flat wound electrode body portion was sandwiched between aluminum laminated films, sealed with a partial opening left, and dried in a vacuum oven at 60 ° C. for 12 hours. After drying, 0.75 mL of the electrolytic solution was immediately injected and sealed with a vacuum sealer.

Subsequently, the obtained battery was charged and discharged. The charging / discharging condition was a current value of 300 mA, and after constant current charging to a battery voltage of 4.35 V, constant voltage charging was performed at a battery voltage of 4.35 V until the battery voltage reached 15 mA. After a 10-minute pause, a constant current discharge was performed at a current value of 300 mA to a battery voltage of 3.0 V, and the 10-minute pause was performed. The above charging and discharging were carried out for 3 cycles to prepare a test secondary battery (flat winding type battery cell) having a battery capacity of 300 mAh.

(Rate characteristics)
The rate characteristics were tested by the following procedure and evaluated by the discharge capacity retention rate.

Using the flat winding type battery produced above, the discharge capacity when discharged at 0.5 C and the discharge capacity when discharged at 5 C were measured in an atmosphere of 25 ° C., and {(Discharge at 5 C). Capacity) / (Discharge capacity at 0.5C)} × 100 was used to calculate the discharge capacity retention rate.

Here, the charging condition was 0.5C, 4.35V constant current charging, and the discharging condition was 3.0V constant current discharging. The discharge capacity retention rate was defined as the average of the three measurement results obtained by producing the above-mentioned five flat-wound batteries and removing the results from which the maximum and minimum discharge capacity retention rates were removed.

The rate characteristics were evaluated based on the following criteria.
◯: The discharge capacity retention rate was 65% or more.
Δ: The discharge capacity retention rate was 55% or more and less than 65%.
X: The discharge capacity retention rate was less than 55%.

(Cycle characteristics)
The flat winding type battery produced above was tested by the following procedure in an atmosphere of 25 ° C. using a charge / discharge measuring device, and the discharge capacity retention rate was calculated.

<1st to 300th cycles>
Charging and discharging were set to one cycle, charging conditions were set to 1C and 4.35V constant current charging, and discharging conditions were set to 1C and 3.0V constant current discharging, and charging and discharging were performed 300 times.

<Calculation of discharge capacity maintenance rate>
The discharge capacity retention rate was calculated by (discharge capacity in the 300th cycle) / (discharge capacity in the first cycle) × 100. Five tests were carried out on the produced flat winding type battery cell, and the average of the three measurement results from which the results with the maximum and minimum discharge capacity retention rates were removed was taken as the discharge capacity retention rate.

The cycle characteristics were evaluated based on the following criteria.
◯: The discharge capacity retention rate was 70% or more.
Δ: The discharge capacity retention rate was 60% or more and less than 70%.
X: The discharge capacity retention rate was less than 60%.

[Example 1]
(Preparation of porous composite film)
Tetrakiss [methylene-3- (3) was added to 100 parts by mass of a composition consisting of 40% by mass of ultra-high molecular weight polyethylene having a mass average molecular weight of 2.7 × 106 and 60% by mass of high-density polyethylene having a mass average molecular weight of 2.6 × 105. , 5-Ditersary butyl-4-hydroxyphenyl) -propionate] 0.375 parts by mass of methane was dry-blended to prepare a polyethylene composition. 23 parts by weight of the obtained polyethylene composition was put into a twin-screw extruder. Further, 77 parts by weight of liquid paraffin was supplied from the side feeder of the twin-screw extruder and melt-kneaded to prepare a polyethylene resin solution in the extruder. Subsequently, a polyethylene resin solution was extruded from a die installed at the tip of this extruder at 190 ° C., and an unstretched gel-like sheet was formed while being taken up by a cooling roll kept at an internal cooling water temperature of 25 ° C.
The obtained unstretched gel-like sheet was introduced into a simultaneous biaxial stretching machine, and stretched 5 × 5 times in the sheet transport direction (MD) and the sheet width direction (TD). At this time, the temperature of preheating / stretching / heat fixing was adjusted to 115/115/100 ° C. The obtained biaxially stretched gel-like sheet was cooled to 30 ° C., liquid paraffin was removed in a washing tank of methylene chloride adjusted to 25 ° C., and dried in a drying oven adjusted to 60 ° C.
A water-based emulsion coating solution in which the main component is methacrylic acid / acrylic acid ester and the adhesive resin having a melting point of 160 ° C. is dispersed by emulsion polymerization is applied to the dried sheet using a bar coat, and then both ends in the width direction. The portion was gripped with a clip and led to the re-stretching device. After heating to 125 ° C. and drying the solvent (water) in the coating liquid, it is re-stretched so that the width direction magnification is 1.8 times the width of the inlet of the re-stretching device, and heat treatment is performed at that magnification. gone. The obtained film was heat-treated for 20 seconds, taken out from the re-stretching apparatus, and wound to obtain a porous composite film having a thickness of 20 μm (a film in which an adhesive resin was laminated on a porous substrate). The coating amount of the adhesive resin was 0.36 g / m ² (converted before re-stretching). The ratio of the adhesive resin on the surface of the obtained porous composite film was measured by the above method. The results are shown in Table 1.

(Laminating a porous layer on a porous composite film)
<Preparation of coating liquid>
Alumina (LS235, manufactured by Nippon Light Metal Co., Ltd., particle size 510 nm), acrylic aqueous dispersion emulsion (CSB130, manufactured by Toyo Ink Co., Ltd., solid content 40%, particle size 177 nm), carboxylmethylcellulose (SG-L02, Jielchem Co., Ltd.) A uniform dispersed slurry (A) was obtained by adding (manufactured by the same company, a turning radius of 25 nm) to water at a weight ratio of 98: 1: 1 and stirring.

<Lamination of porous layers>
The slurry (A) is applied by the gravure coating method to the surface on which the adhesive resin of the porous composite film conveyed in the horizontal direction is laminated, and dried in a hot air drying oven (temperature 60 ° C.) for 20 seconds to obtain a porous layer. A battery separator having a thickness of 4 μm was obtained. The air permeability, adhesion of the porous layer, rate characteristics, and cycle characteristics of the separator for this battery were evaluated by the above methods. The results are shown in Table 1.
[Examples 2 to 5 and Comparative Examples 1 and 2]
A porous composite film and a battery separator were manufactured, measured and evaluated in the same manner as in Example 1 except that the coating amount of the adhesive resin and the stretching ratio in the width direction at the time of re-stretching were changed as shown in Table 1. .. The results are shown in Table 1.

[Example 6]
(Preparation of porous composite film)
Tetrakiss [methylene-3- (3) was added to 100 parts by mass of a composition consisting of 40% by mass of ultra-high molecular weight polyethylene having a mass average molecular weight of 2.7 × 106 and 60% by mass of high-density polyethylene having a mass average molecular weight of 2.6 × 105. , 5-Ditersary butyl-4-hydroxyphenyl) -propionate] 0.375 parts by mass of methane was dry-blended to prepare a polyethylene composition. 23 parts by weight of the obtained polyethylene composition was put into a twin-screw extruder. Further, 77 parts by weight of liquid paraffin was supplied from the side feeder of the twin-screw extruder and melt-kneaded to prepare a polyethylene resin solution in the extruder. Subsequently, a polyethylene resin solution was extruded from a die installed at the tip of this extruder at 190 ° C., and an unstretched gel-like sheet was formed while being taken up by a cooling roll kept at an internal cooling water temperature of 25 ° C.
The obtained unstretched gel-like sheet was introduced into a simultaneous biaxial stretching machine, and stretched 5 × 5 times in the sheet transport direction (MD) and the sheet width direction (TD). At this time, the temperature of preheating / stretching / heat fixing was adjusted to 115/115/100 ° C. The obtained biaxially stretched gel-like sheet was cooled to 30 ° C., liquid paraffin was removed in a washing tank of methylene chloride adjusted to 25 ° C., and dried in a drying oven adjusted to 60 ° C.
A water-based emulsion coating liquid in which the main component is methacrylic acid / acrylic acid ester and the adhesive resin having a melting point of 160 ° C. is dispersed by emulsion polymerization is applied to the dried sheet using a bar coat, and then a roll stretching machine is used. The solvent (water) in the coating liquid was dried by heating to 125 ° C., and then stretched 1.2 times in the sheet transport direction (MD). Then, it was introduced into a tenter type stretching device, heated to 125 ° C., stretched so as to have a width direction magnification of 1.2 times with respect to the device inlet width, and heat treatment was performed at that magnification. The obtained film was heat-treated for 20 seconds, taken out from the re-stretching apparatus, and wound to obtain a porous composite film having a thickness of 20 μm (a film in which an adhesive resin was laminated on a porous substrate). The coating amount of the adhesive resin was 0.36 g / m2 (converted before re-stretching). The ratio of the adhesive resin on the surface of the obtained porous composite film was measured by the above method. The results are shown in Table 1.

(Laminating a porous layer on a porous composite film)
The coating liquid was adjusted and the porous layers were laminated in the same manner as in Example 1 to obtain a battery separator. The air permeability, adhesion of the porous layer, rate characteristics, and cycle characteristics of the separator for this battery were evaluated by the above methods. The results are shown in Table 1.

[Comparative Example 3]
A porous substrate and a battery separator were produced, measured and evaluated in the same manner as in Example 1 except that the porous composite film was not formed without applying the adhesive resin. The results are shown in Table 1.

[Comparative Example 4]
(Preparation of porous composite film)
Tetrakiss [methylene-3- (3) was added to 100 parts by mass of a composition consisting of 40% by mass of ultra-high molecular weight polyethylene having a mass average molecular weight of 2.7 × 106 and 60% by mass of high-density polyethylene having a mass average molecular weight of 2.6 × 105. , 5-Ditersary butyl-4-hydroxyphenyl) -propionate] 0.375 parts by mass of methane was dry-blended to prepare a polyethylene composition. 23 parts by weight of the obtained polyethylene composition was put into a twin-screw extruder. Further, 77 parts by weight of liquid paraffin was supplied from the side feeder of the twin-screw extruder and melt-kneaded to prepare a polyethylene resin solution in the extruder. Subsequently, a polyethylene resin solution was extruded from a die installed at the tip of this extruder at 190 ° C., and an unstretched gel-like sheet was formed while being taken up by a cooling roll kept at an internal cooling water temperature of 25 ° C.
The obtained unstretched gel-like sheet was introduced into a simultaneous biaxial stretching machine, and stretched 5 × 5 times in the sheet transport direction (MD) and the sheet width direction (TD). At this time, the temperature of preheating / stretching / heat fixing was adjusted to 115/115/100 ° C. The obtained biaxially stretched gel-like sheet was cooled to 30 ° C., liquid paraffin was removed in a washing tank of methylene chloride adjusted to 25 ° C., and dried in a drying oven adjusted to 60 ° C.
Both ends of the dried sheet in the width direction were gripped with clips and guided to the re-stretching device. After heating to 125 ° C. and drying the solvent (water) in the coating liquid, it is re-stretched so that the width direction magnification is 1.5 times the width of the inlet of the re-stretching device, and heat treatment is performed at that magnification. gone. The obtained film was heat-treated for 20 seconds, taken out from the re-stretching device, and wound up. This is unwound again, and a water-based emulsion coating liquid in which the main component is methacrylic acid / acrylic acid ester and the adhesive resin having a melting point of 160 ° C. is dispersed by emulsion polymerization is point-injected and dried using an inkjet injection method. A dot pattern layer was formed and wound to obtain a porous composite film having a thickness of 20 μm. The amount of the adhesive resin applied was 0.36 g / m2. The ratio of the adhesive resin on the surface of the obtained porous composite film was measured by the above method.
(Laminating a porous layer on a porous composite film)
The coating liquid was adjusted and the porous layers were laminated in the same manner as in Example 1 to obtain a battery separator. The air permeability, adhesion of the porous layer, rate characteristics, and cycle characteristics of the separator for this battery were evaluated by the above methods. The results are shown in Table 1.

From Table 1, from the comparison between Examples and Comparative Examples, those in which the ratio of the adhesive resin on the surface of the porous composite film is within the range specified in the present invention have high adhesion to the porous layer and at the same time, the rate. It was found that the battery characteristics such as characteristics and cycle characteristics are excellent.

1 Adhesive resin part 2 Part where adhesive resin does not exist

Claims (7)

A porous composite film in which a layer containing an adhesive resin as a main component is laminated on at least one surface of a porous substrate, and a surface on which a layer containing an adhesive resin as a main component of the porous composite film is laminated. In the scanning electron micrograph (SEM), the ratio of the portion determined to be the adhesive resin in the 10 μm × 10 μm region is 40% to 85% when 10 locations are determined by changing the SEM imaging location. A porous composite film characterized by this. The porous composite film according to claim 1, wherein the adhesive resin contains at least one of a resin having a melting point at 100 ° C. or higher or an amorphous resin as a main component. A porous layer made of a mixture of inorganic particles and a thermoplastic polymer is further formed on the side where the layer containing the adhesive resin as the main component of the porous composite film according to claim 1 or 2 is laminated. A separator for an electrochemical element, characterized in that the proportion of inorganic particles in the porous layer is 90% by weight or more. (A) Polyolefin containing polyethylene as a main component and a solvent for film formation are melt-kneaded and kneaded.
Step of preparing a polyolefin solution (b) Step of extruding the polyolefin solution and cooling to form a gel-like sheet (c) First stretching step of stretching the gel-like sheet (d) From the stretched gel-like sheet Step of removing the film-forming solvent (e) Step of drying the sheet after removing the film-forming solvent (f) Step of applying a coating liquid in which an adhesive resin is dissolved or dispersed in a solvent (g). Step of drying the solvent (h) 1.1 to 2 in at least uniaxial direction
A method for producing a porous composite film, which comprises stretching the film twice. The method for producing a porous composite film according to claim 4, wherein the steps (g) and (h) are performed in the same step. The method for producing a porous composite film according to claim 4 or 5, wherein the stretching direction in the step (h) is the width direction. The method for producing a porous composite film according to any one of claims 4 to 6, wherein the solvent contains 90% by weight or more of water.

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