WO2014007260A1

WO2014007260A1 - Porous polypropylene film, separator for electricity storage devices, and electricity storage device

Info

Publication number: WO2014007260A1
Application number: PCT/JP2013/068169
Authority: WO
Inventors: 大西村; 今西　康之; 大倉　正寿
Original assignee: 東レ株式会社
Priority date: 2012-07-04
Filing date: 2013-07-02
Publication date: 2014-01-09
Also published as: CN104395382A; JP5626486B2; CN104395382B; KR20150035548A; JPWO2014007260A1

Abstract

To provide a porous polypropylene film which is suppressed in increase of the separator resistance in cases where the porous polypropylene film is provided with a functional layer, which imparts or improves properties such as heat resistance and adhesion to an electrode. The present invention is a porous polypropylene film which is mainly composed of a polypropylene resin, and which is characterized in that the separator resistance (R1 (Ω)) of the porous polypropylene film and the separator resistance (R2 (Ω)) thereof after applying a simulated coating liquid that is composed of a functional polymer and an organic solvent onto the porous polypropylene film and drying the simulated coating liquid thereon satisfy formula (1). R2/R1 ≤ 1.2 (1)

Description

Porous polypropylene film, separator for electricity storage device, and electricity storage device

The present invention provides a porous polypropylene film capable of suppressing an increase in separator resistance and exhibiting excellent battery characteristics when a functional layer that imparts or improves performance such as heat resistance and adhesion to an electrode is provided, and The present invention relates to a power storage device separator using a porous polypropylene film, and a power storage device using the power storage device separator.

Porous polypropylene films are being considered for use in a wide range of applications, including separators for batteries and electrolytic capacitors, various separation membranes, clothing, moisture-permeable waterproof membranes for medical applications, reflectors for flat panel displays, and thermal transfer recording sheets. Yes. Among these, a porous film is suitable as a separator for lithium ion batteries widely used in mobile devices such as notebook personal computers, mobile phones, and digital cameras. In particular, in recent years, lithium-ion batteries have been used in electric vehicles and hybrid vehicles, and as the output of batteries increases and the capacity increases, studies on coating porous films with inorganic particle layers and heat-resistant resin layers are actively conducted. (For example, refer to Patent Documents 1 and 2). Further, since the size of the battery is increased and the area to be used is increased, cost reduction is also strongly desired.

Various proposals have been made as a method for making a polypropylene film porous. Among them, a β-crystal method can be cited as a dry method and a method capable of forming a film with high productivity by biaxial stretching. The β crystal method is a method in which voids are formed in a film by utilizing the crystal density difference and crystal transition between α type crystal (α crystal) and β type crystal (β crystal), which are polymorphs of polypropylene. Many proposals have been made (see, for example, Patent Documents 3 to 5). Furthermore, many proposals have been made on a method of coating a functional layer such as a heat-resistant layer on the surface of a porous polypropylene film by the β crystal method (see, for example, Patent Documents 6 to 15).

However, if a β-crystal porous polypropylene film is coated with a coating material containing an inorganic or organic material that forms a functional layer, the pore structure may change depending on the solvent used, or a component having a binder function may be a base material. In some cases, the air resistance changes due to, for example, remaining in the film. As described in Patent Documents 6 to 15, in the β crystal method, the use of a high molecular weight polypropylene resin is restricted from the viewpoint of pressure at the time of melt extrusion, etc., so that it is more organic than polyethylene films such as Patent Documents 1 and 2. There was a problem that the resistance to the solvent was low, and the solvent of the coating material for the functional layer had to be limited to an aqueous system.

Therefore, in the case of a porous polypropylene film by the β crystal method, the drying rate can be improved, and it is difficult to use an organic solvent having high material solubility. On the other hand, in order to freely design coating materials such as the use of organic solvents and maintain the characteristics of the porous polypropylene film after coating, it is highly porous or highly porous. Although it is necessary to use a polypropylene film as a base material, such a base material has a problem in practical use because of poor productivity or low strength.

JP 2007-273443 A JP 2006-164873 A Japanese Unexamined Patent Publication No. 63-199742 Japanese Patent Application Laid-Open No. 6-100720 Japanese Patent Laid-Open No. 9-255804 JP 2009-19118 A JP 2009-114434 A JP 2009-226746 A JP 2009-227819 A JP 2010-65088 A JP 2010-219037 A JP 2011-110704 A JP 2011-126275 A International Publication No. 2010/8003 JP 2011-171290 A

The object of the present invention is to solve the above-mentioned problems. That is, to provide a porous polypropylene film capable of suppressing an increase in separator resistance and exhibiting excellent battery characteristics when a functional layer that imparts or improves performance such as heat resistance and adhesion to an electrode is provided. is there.

In order to solve the above-described problems and achieve the object, the present invention provides a porous polypropylene film mainly composed of a polypropylene resin, the separator resistance R1 (Ω) of the porous polypropylene film, and the porous The separator resistance R2 (Ω) after applying and drying a coating simulation liquid composed of a functional polymer and an organic solvent on a polypropylene film satisfies the following formula (1).
R2 / R1 ≦ 1.2 (1)

When the porous polypropylene film of the present invention is provided with a functional layer that imparts or improves performance such as heat resistance and adhesion with an electrode, it suppresses an increase in separator resistance and can exhibit excellent battery characteristics. It can be suitably used as a separator for an electricity storage device.

FIG. 1 is an equivalent circuit diagram used when measuring the separator resistance.

The porous polypropylene film of the present invention comprises a polypropylene resin composition containing a polypropylene resin as a main component. By using a polypropylene resin as a main component, it is possible to satisfy the heat resistance necessary for preventing a short circuit of the battery when used as a separator for an electricity storage device. Here, having a polypropylene resin as a main component means that the proportion of the polypropylene resin in all the components constituting the porous polypropylene film is 50% by mass or more, preferably 80% by mass or more, more preferably It is 90 mass% or more, More preferably, it is 95 mass% or more.

The porous polypropylene film of the present invention has pores that penetrate from one surface of the film toward the other surface and have air permeability (hereinafter referred to as through-holes). Examples of the method for forming the through hole include an extraction method, a lamellar stretching method, a β crystal method, and the like. From the viewpoint of productivity and uniformity of physical properties in the longitudinal direction and the width direction, the β crystal method is preferable.

Here, the β crystal method uses a cast sheet of a polypropylene resin composition having a β crystal as a crystal structure, and longitudinally stretches the cast sheet to transfer the crystal structure of the β crystal to an α crystal and This is a technique for obtaining a film having through-holes by forming α-crystal fibrils oriented in the film direction and cleaving the fibrils in a transverse stretching process to form a network structure. Here, the cast sheet means an unstretched sheet obtained by molding a molten polypropylene resin composition into a sheet shape on a cast drum. In the β crystal method, in order to improve the physical properties of the porous polypropylene film, it is preferable to add a β crystal nucleating agent to the polypropylene resin composition to enhance the β crystal forming ability. Since the β-crystal forming ability is high, the portion of the crystal structure that causes crystal transition to the α-crystal increases, and the number of voids formed in the film can be increased. In addition, by controlling the raw material containing the β crystal nucleating agent, the orientation and denseness of the polypropylene crystals are improved, and the pores are uniformly and densely opened, whereby the porous polypropylene film is used as a separator for an electricity storage device. Reduction of separator resistance can be achieved. In addition, by improving the uniformity of the open state, coarse pores can be reduced, mechanical properties such as elastic modulus and tensile elongation can be improved, and performance such as heat resistance and adhesion to electrodes can be achieved. When a functional layer for imparting or improving the resistance is provided, an increase in separator resistance can be suppressed. In these β crystal methods, reduction of separator resistance and improvement of mechanical properties, suppression of increase in separator resistance when a functional layer that imparts or improves performance such as heat resistance and adhesion to electrodes is provided by using the raw materials described later. It can be achieved by performing film formation under specific film formation conditions.

Next, the raw materials used for the porous polypropylene film of the present invention will be described.
In the present invention, the β-crystal forming ability of the porous polypropylene film is preferably 60% or more from the viewpoint of the through-hole formability. More preferred is 65 to 90%, and particularly preferred is 65 to 85%. When the β crystal forming ability is less than 60%, the amount of β crystals is small, so that the number of voids formed in the film using the transition to α crystal is reduced, and the separator resistance of the film may be inferior. As a method of controlling the β crystal formation ability to 60% or more, a method using a polypropylene resin having a high isotactic index, a β crystal is selectively formed by adding it to a polypropylene resin called a β crystal nucleating agent. In the present invention, there is a method of using a β crystal nucleating agent described later, or a β crystal nucleating agent described later in a polypropylene resin having a high isotactic index. It is preferable to use the method used as

Examples of the β crystal nucleating agent used in the present invention include alkali or alkaline earth metal salts of carboxylic acids such as calcium 1,2-hydroxystearate and magnesium succinate, and N, N′-dicyclohexyl-2,6-naphthalene. Amide compounds represented by carboxamide, tetraoxaspiro compounds such as 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5.5] undecane And aromatic sulfonic acid compounds such as sodium benzene sulfonate and sodium naphthalene sulfonate, imide carboxylic acid derivatives, and quinacridone pigments, particularly amide compounds disclosed in JP-A-5-310665. It is preferable to use it. Further, the content of the β crystal nucleating agent varies depending on the β crystal nucleating agent to be used, but when the amide compound is used, it is 0.05 to 0.5 based on the whole polypropylene composition. The content is preferably 0.1% by mass, more preferably 0.1 to 0.3% by mass, and particularly preferably 0.22 to 0.3% by mass in order to have the effects described later. If it is less than 0.05% by mass, the formation of β crystals becomes insufficient, and the separator resistance of the porous polypropylene film may increase. Moreover, when it exceeds 0.5% by mass, coarse voids are formed in the film due to aggregation of the β-crystal nucleating agent, etc., and the coarse voids are formed, thereby imparting or improving performance such as heat resistance and adhesion to the electrode. In some cases, the degree of increase in separator resistance when the layer is provided increases.

In the present invention, an isotactic polypropylene resin having a melt flow rate (hereinafter referred to as MFR) of 2 to 30 g / 10 min is used as the polypropylene resin from the viewpoint of extrusion moldability and uniform pore formation. preferable. Here, MFR is an index indicating the melt viscosity of a resin defined in JIS K 7210 (1995), and is a physical property value indicating the characteristics of a polyolefin resin. In the present invention, it refers to a value measured at 230 ° C. and 2.16 kg. In the present invention, from the viewpoint of crystallinity, the isotactic index of the polypropylene resin is preferably in the range of 90 to 99.9%, more preferably 95 to 99%. When the isotactic index is less than 90%, the crystallinity of the resin is lowered, the film forming property may be lowered, and the strength of the film may be inferior.

The polypropylene resin used in the present invention is made of the above-mentioned isotopic material from the viewpoint of suppressing the increase in separator resistance when a functional layer is provided that makes the pore structure uniform and imparts or improves performance such as heat resistance and adhesion to electrodes. In addition to tactic polypropylene, it is preferable to add low molecular weight isotactic polypropylene having an MFR of 70 g / 10 min or more, preferably 100 g / 10 min or more, more preferably 500 g / 10 min or more. The upper limit is that the MFR is 5000 g / 10 min. If the MFR exceeds 5000 g / 10 min, it may be difficult to homogenize with the above-described isotactic polypropylene. Usually, low molecular weight isotactic polypropylene having an MFR of 70 g / 10 min or more has not been used in the field of films because it has poor film-forming properties and causes a decrease in strength. However, when the total amount of the polypropylene resin is 100% by mass, the low molecular weight isotactic polypropylene is 0.1 to 50% by mass, preferably 1 to 20% by mass, more preferably 2 to 10% by mass, and most preferably 2%. It has been found that the content of ˜5% by mass can suppress an increase in separator resistance when a functional layer that imparts or improves performance such as heat resistance and adhesion to an electrode is provided. The cause is still unknown, but low molecular weight polypropylene added in small amounts increases the molecular chain end concentration at the crystal interface and promotes pore formation at the crystal interface in longitudinal stretching, that is, it functions as a hole opening aid. When a functional layer that provides or improves performance such as heat resistance and adhesion to electrodes is provided, the coating composition is difficult to clog, or even if clogged, a large number of through-holes are formed. It is presumed that the increase in separator resistance can be suppressed in order to survive.

Here, the melting point of the low molecular weight isotactic polypropylene resin having an MFR of 70 g / 10 min or more is preferably 130 ° C. or more, more preferably 140 ° C. or more, and further preferably 150 ° C. or more. When the melting point is lower than 130 ° C., the openability of the porous polypropylene film may be lowered.

Examples of the low molecular weight isotactic polypropylene having the above-described properties include commercially available polypropylene resins S10AL, S10CL, J13B manufactured by Prime Polymer, and polypropylene resin 6936G1 manufactured by ExxonMobil.

The polypropylene resin composition forming the porous polypropylene film of the present invention includes an antioxidant, a heat stabilizer, an antistatic agent, a lubricant composed of inorganic or organic particles, and a blocking agent within the range that does not impair the effects of the present invention. You may contain various additives, such as an inhibitor, a filler, and an incompatible polymer. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polypropylene resin. The addition amount of the antioxidant is preferably 2 parts by mass or less, more preferably 1 part by mass or less, still more preferably 0.5 parts by mass or less with respect to 100 parts by mass of the polypropylene resin composition.

The polypropylene resin composition for forming the porous polypropylene film of the present invention can contain a pore-forming aid composed of inorganic particles or organic particles within a range not impairing the effects of the present invention.

The porous polypropylene film of the present invention has a separator resistance R1 (Ω) of the porous polypropylene film, and after applying a solution containing a functional polymer that is a coating simulation liquid and an organic solvent to the porous polypropylene film, The separator resistance R2 (Ω) of the film after drying the organic solvent (hereinafter referred to as a coating simulation film) satisfies the following formula (1).
R2 / R1 ≦ 1.2 (1)
R2 / R1 (unit is dimensionless), which is the ratio of the separator resistance before and after application of the coating simulation liquid, is preferably 1.1 or less, and more preferably 1.05 or less.

Here, the separator resistance means an electric resistance obtained by preparing an evaluation cell by a method described later and calculating a Cole-Cole plot measured by an AC impedance method from the equivalent circuit shown in FIG. In general, when a functional layer that imparts or improves performance such as heat resistance and adhesion to an electrode is provided on a conventionally produced porous polypropylene film, a functional polymer having a binder function as an inorganic particle or organic particle. (For example, polyvinylidene fluoride (PVdF), acrylic, cellulose and / or cellulose salt, acrylic resin, ethylene-vinyl alcohol, ethylene-acrylic acid copolymer (EEA) and other ethylene-acrylic acid copolymers, fluorine-based rubber Styrene butadiene rubber (SBR), cross-linked acrylic resin, polyurethane, polyvinyl butyral, polyethylene, polyvinyl alcohol, polytetrafluoroethylene, polyvinyl pyrrolidone, polyimide, polyamide, polysulfide, polyvinyl methyl ether, polyethylene Along with ethylene oxide, polypropylene oxide, melamine resin, polyvinyl pyridine, etc.), organic solvents (eg, acetone, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), cyclohexanone, γ-butyrolactone (GBL), dimethylacetamide, methyl ethyl ketone (MEK), diethyl ether, ethyl acetate, tetrahydrofuran (THF), etc.) are applied to a porous film, the film swells with an organic solvent, which causes the organic solvent to swell. Changes in the pore structure occur during drying, or functional polymers dissolved in organic solvents enter the pores (inorganic particles and organic particles may be filtered on the porous film surface and not enter the porous film. Yes) and organic In some cases, the functional polymer may block the pores or reduce the pore diameter by drying the solvent.

Therefore, in this application, in order to evaluate the degree of performance deterioration such as pore clogging due to the organic solvent and functional polymer as described above, PVdF resin, which is one kind of functional polymer, is changed to acetone, which is one kind of solvent. The dissolved coating simulation solution is applied and dried by the method described later to prepare a simulated coating film for evaluation. Small R2 / R1 is considered to indicate that the functional polymer clogs the pores, and that the resistance increase due to the structure change at the time of solvent swelling and solvent drying hardly occurs. R2 / R1 can be used as an index of the degree of increase in resistance relative to the base material when a functional layer for imparting or improving the performance is provided. When R2 / R1 satisfies the formula (1), it can be said that the film can suppress an increase in separator resistance when a functional layer that imparts or improves performance such as heat resistance and adhesion to an electrode is provided. When the value of R2 / R1 exceeds 1.2, it means that resistance is likely to increase when a functional layer that imparts or improves performance such as heat resistance and adhesion to electrodes is provided. There are industrial disadvantages such as restrictions. In the porous polypropylene film satisfying the formula (1), the addition amount of the β crystal nucleating agent in the raw material is within the above-mentioned range, the above-mentioned raw material is used, the temperature of the cast drum, the draw ratio and the temperature in the longitudinal direction. The transverse stretching speed, the temperature and time in the heat treatment step, and the relaxation rate in the relaxation zone can be obtained within the ranges described below.

In the porous polypropylene film of the present invention, the ratio PMD / PTD of the breaking strength PMD in the longitudinal direction and the breaking strength PTD in the width direction preferably satisfies the following formula (2). In the present application, the direction parallel to the film forming direction is referred to as the film forming direction, the longitudinal direction, the MD direction, or simply MD, and the direction perpendicular to the film forming direction in the film plane is the width direction and the TD direction. Alternatively, it may be simply referred to as TD.
0.7 ≦ PMD / PTD ≦ 2.0 (2)
The value of PMD / PTD is preferably 0.7 ≦ PMD / PTD ≦ 1.6, and more preferably 0.8 ≦ PMD / PTD ≦ 1.4. When the value of PMD / PTD is less than 0.7 or more than 2.0, when the functional layer such as a heat protection layer is provided, it shrinks excessively in the longitudinal or width direction, or becomes easy to tear. In some cases, the separator resistance is likely to increase, and the process suitability in the coating process and the battery assembly process may be insufficient. As a method for obtaining a porous polypropylene film satisfying the formula (2), the addition amount of the β crystal nucleating agent in the raw material should be within the above-mentioned range, the above-mentioned raw material is used, the temperature of the cast drum, and the longitudinal direction. The stretching ratio and temperature, the transverse stretching speed, the temperature and time in the heat treatment step, and the relaxation rate in the relaxation zone can be controlled within the ranges described below.

Further, if the breaking strength itself of the porous polypropylene film is low, the safety may be inferior or the process suitability in the coating process and the battery assembly process may be insufficient. The breaking strength is preferably 60 MPa or more in both the longitudinal direction and the width direction. More preferably, both are 80 MPa or more, and more preferably both are 100 MPa or more. As a method for obtaining a porous polypropylene film having a breaking strength of 60 MPa or more in both the longitudinal direction and the width direction, the amount of β crystal nucleating agent in the raw material is set in the above-described range, the above-described raw material is used, and the cast drum The temperature, the stretching ratio and temperature in the longitudinal direction, the transverse stretching speed, the temperature and time in the heat treatment step, and the relaxation rate in the relaxation zone can be controlled within the ranges described below.

The porous polypropylene film of the present invention preferably has an air permeability resistance of 10 to 1,000 seconds / 100 ml, more preferably 50 to 500 seconds / 100 ml, and 80 to 300 seconds / 100 ml. Particularly preferred. When the air permeation resistance is less than 10 seconds / 100 ml, mechanical strength such as breaking strength, which is an indicator of process suitability, may be lowered. When the air permeability resistance exceeds 1,000 seconds / 100 ml, the output characteristics may be deteriorated particularly when used as a separator for a high-power storage device. The air permeation resistance is determined by setting the addition amount of the β crystal nucleating agent in the raw material within the above-described range, using the above-described raw material, the temperature of the cast drum, the stretching ratio and temperature in the longitudinal direction, the transverse stretching speed, and the heat treatment step It is possible to control by controlling the temperature and time of each of them and the relaxation rate in the relaxation zone within the range described later.

The porous polypropylene film of the present invention preferably has a film thickness of 5 to 30 μm. If the thickness is less than 5 μm, the film may break during use. If the thickness exceeds 30 μm, the separator resistance increases and the output characteristics may deteriorate when used as a separator, and the porous film occupies the power storage device. In some cases, the volume ratio of becomes high, and a high energy density cannot be obtained. The film thickness is more preferably 10 to 25 μm, still more preferably 12 to 20 μm.

The porous polypropylene film of the present invention preferably has a porosity of 40 to 85% from the viewpoint of achieving both battery characteristics and strength. More preferably, it is 50 to 80%, and particularly preferably 55 to 75%. When the porosity is less than 40%, the separator resistance may increase particularly when used as a separator for a high-power electricity storage device. On the other hand, if the porosity exceeds 85%, mechanical strength such as elastic modulus and tensile strength may be lowered. The porosity is determined by setting the amount of β-crystal nucleating agent in the raw material within the above-mentioned range, using the above-described raw material, the temperature of the cast drum, the stretching ratio and temperature in the longitudinal direction, the transverse stretching speed, and the heat treatment step. It is possible to control by controlling the temperature and time of each of them and the relaxation rate in the relaxation zone within the range described later.

The porous polypropylene film of the present invention preferably has a heat shrinkage in the width direction of 10% or less when heat-treated at 135 ° C. for 60 minutes. More preferably, it is 5% or less, More preferably, it is 3% or less. When the heat shrinkage rate when heat-treated at 135 ° C. for 60 minutes exceeds 10%, it may be inferior in safety when used as a separator for an electricity storage device. For example, polyethylene is applied to the surface of the porous polyolefin film of the present invention. If the shutdown layer is used by laminating by coating or coextrusion lamination, etc., when the polyethylene melts and closes the hole at around 135 ° C, the porous polyolefin film as the base material may shrink and the battery may short circuit is there. On the other hand, the lower limit is 0.1%. The heat shrinkage ratio is within the above-mentioned range for the amount of β-crystal nucleating agent in the raw material, the raw material is used, the temperature of the cast drum, the stretching ratio and temperature in the longitudinal direction, the transverse stretching speed, and the heat treatment step. It is possible to control by controlling the temperature and time of each of them and the relaxation rate in the relaxation zone within the range described later.

The porous polypropylene film of the present invention may have a laminated structure for the purpose of imparting various effects. The number of laminations may be two-layer lamination, three-layer lamination, or a larger number of laminations. Both the laminated form having the porous polypropylene film of the present invention as at least one surface layer and the porous polypropylene film of the present invention. Any of laminated forms in which the same or different surface layers are formed on the surface may be adopted. As a lamination method, there are a feed block method by co-extrusion, a multi-manifold method, a method of laminating porous films by lamination, and the lamination method may be selected according to the physical properties of the resin to be laminated. As a laminated structure, for example, a layer containing polyethylene may be laminated for the purpose of imparting a shutdown property at a low temperature, or a layer containing particles may be laminated for the purpose of imparting strength or heat resistance.

Hereinafter, the method for producing the porous polypropylene film of the present invention will be described based on a specific example. In addition, the manufacturing method of the film of this invention is not limited to this.

As polypropylene resin, 94.5 parts by mass of commercially available homopolypropylene resin with MFR 8 g / 10 min, 5 parts by mass of commercially available low molecular weight polypropylene resin with MFR 1,000 g / 10 min, and N, N′-dicyclohexyl-2 as β crystal nucleating agent , 6-Naphthalenedicarboxamide 0.3 parts by mass, “IRGANOX (registered trademark)” 1010 as an antioxidant, 0.1 parts by mass of “IRGAFOS (registered trademark)” 168, and 0.05 mg of calcium behenate as a lubricant Feed the raw material from the weighing hopper to the twin screw extruder so that the mass part is mixed at this ratio, melt and knead, discharge the strand from the die, cool and solidify in a 25 ° C water bath, cut into chips Thus, a polypropylene resin composition (a) is prepared. At this time, the melting temperature is preferably 280 to 310 ° C., and the cross-sectional shape of the chip may be any of a circle, an ellipse, and a rectangle.

Next, the polypropylene resin composition (a) is supplied to a single screw extruder, and melt extrusion is performed at 200 to 230 ° C. And after removing a foreign material, a modified polymer, etc. with the filter installed in the middle of the polymer pipe | tube, it discharges on a cast drum from T-die, and an unstretched sheet is obtained. Here, when the film is made into a laminated structure by co-extrusion, a plurality of extruders are used to form a laminated structure by a feed block method or a multi-manifold method, and then discharged from a T-die onto a cast drum. It can be a sheet. The cast drum preferably has a surface temperature of 105 to 130 ° C. from the viewpoint of R2 / R1 value control, and more preferably 120 to 130 ° C. At this time, particularly, the forming of the end portion of the sheet affects the subsequent stretchability, and therefore it is preferable that the end portion is sprayed with spot air to be in close contact with the drum. Further, air may be blown over the entire surface using an air knife as necessary from the state in which the entire sheet is in close contact with the drum.

Next, the obtained cast sheet is biaxially oriented to form pores in the film. As a biaxial orientation method, the film is stretched in the longitudinal direction of the film and then stretched in the width direction, or the sequential biaxial stretching method in which the film is stretched in the width direction and then stretched in the longitudinal direction. The simultaneous biaxial stretching method can be used, but it is preferable to adopt the sequential biaxial stretching method in that it is easy to obtain a film having a balance between the separator resistance and the mechanical strength, particularly after stretching in the longitudinal direction. The film is preferably stretched in the width direction.

As specific stretching conditions, first, stretching is performed in the longitudinal direction while controlling the temperature of the cast sheet. As a temperature control method, a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like can be adopted. The stretching temperature in the longitudinal direction is preferably 90 to 140 ° C., more preferably 100 to 130 ° C., and particularly preferably 115 to 125 ° C., from the viewpoint of achieving both the R2 / R1 value and the mechanical strength. If it is less than 90 degreeC, a film may fracture | rupture. On the other hand, if the temperature exceeds 140 ° C., the air resistance may increase. From the viewpoint of achieving both R2 / R1 value and mechanical strength, the draw ratio is preferably 3 to 10 times. More preferably, it is 4.5 to 6 times. As the draw ratio is increased, the separator resistance decreases. However, if the film is stretched more than 10 times, film breakage is likely to occur in the next transverse stretching process, and the air permeability resistance becomes too low and the mechanical strength may decrease. is there.

Next, the film end is held by a tenter type stretching machine and introduced. The transverse stretching temperature is preferably 130 to 155 ° C., more preferably 145 to 155 ° C., from the viewpoint of achieving both the R2 / R1 value and the mechanical strength. If it is less than 130 degreeC, a film may fracture | rupture, and if it exceeds 155 degreeC, air permeability resistance may increase. The draw ratio in the width direction is preferably 2 to 12 times from the viewpoint of improving the tensile strength. More preferably, it is 4 to 11 times, and further preferably 7 to 10 times. If it is less than twice, the air resistance may increase or the tensile strength in the width direction may decrease. If it exceeds 12 times, the film may break. The transverse stretching speed at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min.

From the viewpoint of improving mechanical strength while reducing air resistance, the area ratio (longitudinal stretch ratio × transverse stretch ratio) is preferably high, specifically 20 times or more, more preferably 30 times or more. 45 times or more is particularly preferable. When the area magnification is low, specifically, when it is less than 20 times, it is difficult to reduce air resistance and improve mechanical strength. The upper limit of the area magnification is not particularly provided, but if it exceeds 60 times, the film forming property is deteriorated and may be easily broken.

続い Following transverse stretching, a heat treatment step is performed in the tenter. Here, the heat treatment step includes a heat setting zone (hereinafter referred to as HS1 zone) in which heat treatment is performed with the width after transverse stretching, and a relaxation zone (hereinafter referred to as Rx zone) in which heat treatment is performed while relaxing the film by narrowing the width of the tenter. It is preferable to divide into three zones, a heat fixing zone (hereinafter referred to as HS2 zone) in which heat treatment is performed with the width after relaxation, from the viewpoint of compatibility between separator resistance and mechanical strength, and low heat yield.

The temperature of the HS1 zone is preferably 140 to 165 ° C., more preferably 150 to 160 ° C. from the viewpoint of achieving both R2 / R1 value and mechanical strength. If it is lower than 140 ° C., the thermal shrinkage in the width direction may increase. If the temperature exceeds 165 ° C, the relaxation of the orientation of the film is too large, so that the relaxation rate cannot be increased in the subsequent Rx zone, and it may be difficult to achieve both R2 / R1 and mechanical strength. In some cases, the air resistance is increased due to melting of the polymer.

The heat treatment time in the HS1 zone is preferably 0.1 seconds or more and 10 seconds or less from the viewpoint of achieving both the thermal shrinkage in the width direction and the productivity.

The relaxation rate in the Rx zone in the present invention is preferably from 5 to 35%, more preferably from 10 to 25%, from the viewpoint of reducing the heat shrinkage rate in addition to the improvement of the value of R2 / R1 and mechanical strength. preferable. If the relaxation rate is less than 5%, the thermal shrinkage rate may increase. If it exceeds 35%, air resistance may increase, and thickness unevenness and flatness in the width direction may decrease.

The temperature of the Rx zone is preferably 155 to 170 ° C., more preferably 160 to 165 ° C., from the viewpoint of the value of R2 / R1 and the reduction of the heat shrinkage rate. When the temperature of the Rx zone is less than 155 ° C., the shrinkage stress for relaxation is lowered, and the above-described high relaxation rate may not be achieved, and the thermal shrinkage rate in the width direction may be increased. When the temperature exceeds 170 ° C., the polymer around the pores may melt due to high temperature, and the separator resistance may increase.

The relaxation rate in the Rx zone is preferably 100 to 1,000% / min, and more preferably 150 to 500% / min. When the relaxation rate is less than 100% / min, it is necessary to slow down the film forming rate or increase the tenter length, which may be inferior in productivity. If it exceeds 1,000% / min, the speed at which the film shrinks becomes slower than the speed at which the rail width of the tenter shrinks, the film flutters in the tenter and breaks, or the physical properties in the width direction are uneven and the flatness is lowered. There is a case.

The temperature of the HS2 zone is preferably 155 to 165 ° C, more preferably 160 to 165 ° C, from the viewpoint of achieving both the R2 / R1 value and the mechanical strength. When the temperature is lower than 155 ° C., the tension of the film after thermal relaxation becomes insufficient, which may cause uneven physical properties in the width direction and a decrease in flatness, or increase the heat shrinkage rate in the width direction. Further, the higher the temperature of the HS zone 2, the higher the mechanical strength tends to be. If it exceeds 165 ° C., the polymer around the pores may melt due to the high temperature and the separator resistance may increase.

In the present invention, the heat treatment time in the HS2 zone is preferably 0.1 seconds or more and 10 seconds or less from the viewpoint of physical property unevenness in the width direction and flatness and productivity. The film after the heat setting step is removed by slitting the ears gripped by the tenter clip, and wound around the core with a winder to obtain a product.
Thereafter, a coating layer may be provided on at least one side to form a porous film having a functional layer.

Since the porous polypropylene film of the present invention has a low R2 / R1 value of the separator resistance ratio before and after application of the application simulation liquid, a functional layer is formed using a coating liquid containing an organic solvent. However, good battery characteristics can be maintained. As the coating method, various methods can be used. For example, at least one organic solvent selected from acetone, ethanol, tetrahydrofuran, N-methyl-2-pyrrolidone and the like is used as a solvent, The porous polypropylene of the present invention is prepared using a die coating method or a gravure coating method by adding a functional polymer for binding them and an additive such as a thickener as necessary to prepare a coating solution. A porous film having a functional layer can be obtained by coating on at least one side of the film and drying the organic solvent using a drying oven.

The porous polypropylene film of the present invention not only has excellent heat resistance, mechanical strength, and productivity, but also has excellent extrusion stability, so that it can be used for packaging products, sanitary products, agricultural products, building products, medical products, separation membranes, light. Although it can be used for diffuser plates and reflective sheet applications, it is difficult to increase resistance when a functional layer that imparts or improves performance such as heat resistance and adhesion to electrodes is used. Can be preferably used. Here, examples of the electricity storage device include a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery, and an electric double layer capacitor such as a lithium ion capacitor. Since such an electricity storage device can be repeatedly used by charging and discharging, it can be used as a power supply device for industrial devices, household equipment, electric vehicles, hybrid electric vehicles, and the like. The separator for an electricity storage device formed by laminating a functional layer on the porous polypropylene film of the present invention is not only excellent in separator resistance and productivity, but also excellent in heat resistance and short circuit resistance. It can be preferably used as a power storage device separator for power supply devices such as electric vehicles and hybrid electric vehicles. The separator using the porous polypropylene film of the present invention, the positive electrode, the negative electrode, and the electrolytic solution can be suitably used for power supplies of industrial equipment and automobiles because of the excellent characteristics of the separator.

Hereinafter, the present invention will be described in detail by way of examples.

(1) Preparation of coating simulation film 5 parts by mass of PVdF-HFP copolymer (manufactured by Arkema: KYNAR POWERFLEX LBG) is added to 95 parts by mass of acetone (manufactured by Kanto Chemical Co., Ltd .: special grade) and stirred for 12 hours. Dissolved to create a simulated coating solution. The porous polypropylene film obtained in each Example or Comparative Example was cut into a size of 150 mm in the longitudinal direction and 100 mm in the width direction, and placed on cardboard (HSK ivory, A4 size), and the four corners were fixed with tape. After applying 1.5 mL of the coating liquid to the end of the porous film with a dropper across the width direction, the bar coating machine # 10 manufactured by Matsuo Sangyo Co., Ltd. was moved in the longitudinal direction (bar coating method) to apply the coating liquid. It was dried with hot air at 1 ° C. for 1 minute. Thereafter, the four corners were cut off to obtain a coating simulation film. In addition, in order to judge the effect of the coating liquid on the separator resistance, the coating simulation liquid was prepared without mixing additives such as heat-resistant resin, inorganic particles, and thickeners that form the functional layer. Was used.
The characteristics were measured and evaluated by the following methods.

(2) Thickness The thickness of the porous polypropylene film was measured using a contact-type film thickness meter, Mitsutyo Lightmatic VL-50A (10.5 mmφ carbide spherical surface probe, measuring load 0.06 N). The measurement was performed 10 times at different locations, and the average value was taken as the thickness of the porous polypropylene film.

(3) Air permeability resistance A square having a size of 100 mm × 100 mm was cut from a porous polypropylene film or a coating simulation film, and used as a sample. Using a JIS P 8117 (1998) B-type Gurley tester, the permeation time of 100 ml of air was measured at 23 ° C. and a relative humidity of 65%. The measurement was performed three times by changing the sample, and the average value of the permeation time was taken as the air resistance of the film.

(4) Separator resistance: R1 or R2
A porous polypropylene film or a coating simulation film was punched into a circle having a diameter of 24 mm. From the bottom, SUS plate with a diameter of 16 mm, porous polypropylene film or coating simulation film, SUS plate with a diameter of 16 mm are stacked in this order, and a stainless steel small container with a lid (manufactured by Hosen Co., Ltd., HS cell, spring pressure 1 kgf). Stowed. The container and the lid are insulated, and the container and the lid are in contact with the SUS plate. In this container, an electrolytic solution in which LiPF ₆ was dissolved as a solute in a mixed solvent of ethylene carbonate: dimethyl carbonate = 3: 7 (volume ratio) to a concentration of 1 mol / liter was injected and sealed, and an evaluation cell was obtained. Was made.
For each of the fabricated cells for evaluation, the AC impedance was measured under the conditions of a voltage amplitude of 10 mV and a frequency of 10 Hz to 100 kHz in a 25 ° C. atmosphere, and the separator resistance was obtained using the Cole-Cole plot using the equivalent circuit of FIG. It was. The measurement was performed five times with different samples. The separator resistance R1 (Ω) was the average value of the separator resistance obtained with the porous polypropylene film, and the separator resistance R2 (Ω was the average value of the separator resistance obtained with the coating simulation film. ).

(5) β crystal forming ability 5 mg of a porous polypropylene film was taken as a sample in an aluminum pan and measured using a differential scanning calorimeter (Seiko Denshi Kogyo RDC220). First, the temperature was raised from room temperature to 220 ° C. at 40 ° C./min in a nitrogen atmosphere (first run), held for 5 minutes, and then cooled to 20 ° C. at 10 ° C./min (first run). The melting peak observed when the temperature is raised again (second run) at 10 ° C / min after holding for 5 minutes is the melting peak of 145 ° C to 157 ° C. The melting of the α crystal is the melting peak of the α crystal, the melting peak of the α crystal is taken as the melting peak of the base, and the area of the region surrounded by the peak drawn from the flat portion on the high temperature side. When the heat of fusion of ΔHα and β crystals was ΔHβ, the value calculated by the following formula was defined as β crystal forming ability. The heat of fusion was calibrated using indium.
β crystal forming ability (%) = [ΔHβ / (ΔHα + ΔHβ)] × 100
However, in the above method, a melting peak having an apex at 140 to 160 ° C. exists, but if it is unclear whether it is caused by the melting of β crystal, the apex of the melting peak exists at 140 to 160 ° C. A sample prepared under the following conditions is determined to have β-crystal forming ability when the K value calculated from each diffraction peak intensity of the diffraction profile obtained by the 2θ / θ scan is 0.3 or more.

The sample preparation conditions and the measurement conditions of the wide angle X-ray diffraction method are shown below.
·sample:
The direction of the film is aligned, and the samples are stacked so that the sample thickness after hot press preparation is about 1 mm. This sample is sandwiched between two aluminum plates having a thickness of 0.5 mm, and is hot-pressed at 280 ° C. for 3 minutes to be melted and compressed to make the polymer chain substantially non-oriented. The obtained sheet is crystallized by being immersed in boiling water at 100 ° C. for 5 minutes immediately after being taken out together with the aluminum plate. Then, a sample obtained by cutting a sheet obtained by cooling in an atmosphere at 25 ° C. is used for measurement.
・ Wide-angle X-ray diffraction method measurement conditions:
In accordance with the above conditions, an X-ray diffraction profile is obtained by 2θ / θ scanning.
Here, the K value is observed in the vicinity of 2θ = 16 °, and is observed in the diffraction peak intensity of the (300) plane (referred to as Hβ1) due to the β crystal and in the vicinity of 2θ = 14, 17, 19 °, and α From the diffraction peak intensities (referred to as Hα1, Hα2, and Hα3, respectively) of the (110), (040), and (130) planes due to the crystal, it can be calculated by the following mathematical formula. The K value is an empirical value indicating the ratio of β crystals. For details of the K value, such as the calculation method of each diffraction peak intensity, see A. Turner Jones et al., “Macromoleculare Chemie” (Macromoleculare Chemie). ), 75, pages 134-158 (1964).
K = Hβ1 / {Hβ1 + (Hα1 + Hα2 + Hα3)}
The structure of the polypropylene crystal type (α crystal, β crystal), the obtained wide-angle X-ray diffraction profile, etc. are described in, for example, Edward P. Moore Jr. Written by "Polypropylene Handbook", Industrial Research Committee (1998), p. 135-163; Hiroyuki Tadokoro, “Structure of Polymer”, Kagaku Dojin (1976), p. 393; A. Turner Jones et al., “Macromoleculare Chemie”, 75, 134-158 (1964), and many reports including references cited therein. You can refer to it.

(6) Melt flow rate (MFR)
The MFR of the polypropylene resin was measured according to the condition M (230 ° C., 2.16 kg) of JIS K 7210 (1995).

(7) Breaking strength A porous polypropylene film was cut into a rectangle having a length of 150 mm and a width of 10 mm and used as a sample. In addition, the length direction of 150 mm was matched with the width direction of the film. Using a tensile tester (Orientec Tensilon UCT-100), the initial chuck distance was 50 mm, the tensile speed was 300 mm / min, and a tensile test was performed in the width direction of the film. A load applied to the film when the sample broke was read, and a value obtained by dividing the load by the cross-sectional area of the sample before the test (film thickness × width (10 mm)) was used as an index of the breaking strength. The measurement was performed 5 times for each sample, and the average value was evaluated.

(8) Heat shrinkage rate A porous polypropylene film was cut into a rectangular shape having a length of 150 mm and a width of 10 mm as a sample. In addition, the length direction of 150 mm was matched with the width direction of the film. Mark lines were drawn at 100 mm intervals in the center of the sample, and the distance L ₀ between the mark lines before heating was measured. The upper end of the sample was gripped, a weight of 3 g was applied to the lower end, the sample was suspended in a hot air oven heated to 135 ° C. and left to stand for 60 minutes for heat treatment. After the heat treatment, allowed to cool, after removing the weight, the gauge length L ₁ after heating was measured and the value calculated by the following formula was heat shrinkage. The measurement was performed 5 times for each sample, and the average value is shown in Table 1.
Thermal contraction rate (%) = (L ₀ −L ₁ ) / L ₀ × 100

(9) Melting point (Tm) of polypropylene resin
5 mg of polypropylene resin used for the porous polypropylene film was sampled in an aluminum pan and measured using a differential scanning calorimeter (Seiko Denshi Kogyo RDC220). First, the temperature was raised from room temperature to 220 ° C. at 40 ° C./min in a nitrogen atmosphere (first run), held for 5 minutes, and then cooled to 20 ° C. at 10 ° C./min (first run). The melting peak observed when the temperature was raised again (second run) at 10 ° C./min after holding for 5 minutes was taken as the melting point of the polypropylene resin.

(Example 1)
As the polypropylene resin, 94.7 parts by mass of Homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min, manufactured by Prime Polymer Co., Ltd. with MFR = 1,000 g / 10 min. 5 parts by mass of low molecular weight polypropylene S10CL (melting point = 163 ° C.) and 0, N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100) as β crystal nucleating agent .3 parts by mass, 0.05 parts by mass of calcium behenate, and 0.1 parts by mass of “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants. The raw materials are supplied from the weighing hopper to the twin screw extruder with L / D = 41 so that each is mixed at this ratio, and melt kneaded at 300 ° C. Performed by ejecting from the die was cooled and solidified at 25 ° C. water bath, to obtain a chip polypropylene composition obtained by cutting into chips (I).
The obtained polypropylene composition (I) is supplied to a uniaxial melt extruder, melt extruded at 210 ° C., foreign matter is removed with a 60 μm cut sintered filter, and the surface temperature is adjusted to 122 ° C. with a T-die. A cast sheet was obtained by discharging to a controlled cast drum. Next, preheating was performed using a ceramic roll heated to 123 ° C., and the film was stretched 5.0 times in the longitudinal direction of the film. Next, the edge part was hold | gripped with the clip and it extended | stretched 7.7 times in the width direction at 150 degreeC with the horizontal extending | stretching speed | rate of 1,800% / min.
In the subsequent heat treatment process, heat treatment was performed at 150 ° C. for 2 seconds while maintaining the distance between the clips after stretching (HS1 zone), and further relaxation was performed to achieve a relaxation rate of 17% at 163 ° C. for 5 seconds (Rx zone). Heat treatment was performed at 163 ° C. for 5 seconds while maintaining the distance between the clips (HS2 zone).
Then, the ear | edge part of the film hold | gripped with the clip was cut and removed, and the porous polypropylene film of thickness 21 micrometers and air permeation resistance 140 second / 100 ml was obtained. The air resistance of the coating simulation film prepared using this porous polypropylene film was 290 seconds / 100 ml.

(Example 2)
In Example 1, a porous polypropylene film having a thickness of 22 μm was obtained by the same raw material and film forming method as in Example 1 except that the draw ratio in the longitudinal direction was 4.5 times.

(Example 3)
As a polypropylene resin, 94.7 parts by mass of homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min, low molecular weight manufactured by Prime Polymer Co., Ltd. with MFR = 100 g / 10 min. 5 parts by mass of polypropylene S10AL (melting point = 161 ° C.), 0.3 N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100) as β crystal nucleating agent 1 part by mass of 0.1 part by mass of “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants, respectively. The raw materials are fed from the weighing hopper to the L / D = 41 twin screw extruder so that they are mixed at this ratio, and melt kneading is performed at 300 ° C. , Ejected from the die, it cooled and solidified at 25 ° C. water bath and then cut into chips to obtain a chip polypropylene composition (II).
A porous polypropylene film having a thickness of 22 μm was obtained by the same film forming method as in Example 1 using a chip of the polypropylene composition (II).

Example 4
As the polypropylene resin, 97.7 parts by mass of Homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min, manufactured by Prime Polymer Co., Ltd. with MFR = 1,000 g / 10 min. 2 parts by mass of low molecular weight polypropylene S10CL (melting point = 161 ° C.) and 0, N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100) as a β crystal nucleating agent .3 parts by mass, 0.05 parts by mass of calcium behenate, and 0.1 parts by mass of “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants. The raw materials are supplied from the weighing hopper to the twin screw extruder with L / D = 41 so that each is mixed at this ratio, and melt kneaded at 300 ° C. Performed by ejecting from the die was cooled and solidified at 25 ° C. water bath and then cut into chips to obtain chips of the polypropylene resin composition (III).
Using a chip of the polypropylene resin composition (III), a porous polypropylene film having a thickness of 23 μm was obtained by the same film forming method as in Example 1.

(Example 5)
As a polypropylene resin, 92.7 parts by mass of Homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min, manufactured by Prime Polymer Co., Ltd. with MFR = 1,000 g / 10 min. 8 parts by mass of low molecular weight polypropylene S10CL (melting point = 163 ° C.), 0 N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100) as β crystal nucleating agent .3 parts by mass, 0.05 parts by mass of calcium behenate, and 0.1 parts by mass of “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants. The raw materials are supplied from the weighing hopper to the twin screw extruder with L / D = 41 so that each is mixed at this ratio, and melt kneaded at 300 ° C. Performed by ejecting from the die was cooled and solidified at 25 ° C. water bath, was obtained by cutting into chips the polypropylene resin composition chips (IV).
Using a polypropylene resin composition (IV) chip, a porous polypropylene film having a thickness of 21 μm was obtained by the same film forming method as in Example 1.

(Example 6)
As the polypropylene resin, 94.7 parts by mass of Homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min, low molecular weight manufactured by Prime Polymer Co., Ltd. with MFR = 210 g / 10 min. 5 parts by mass of polypropylene J13B (melting point = 165 ° C.), 0.3 N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100) as β crystal nucleating agent 1 part by mass of 0.1 part by mass of “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants, respectively. The raw materials are supplied from the weighing hopper to the twin screw extruder of L / D = 41 so that they are mixed at this ratio, and melt kneading is performed at 300 ° C. Ejected from the die, it cooled and solidified at 25 ° C. water bath, was obtained by cutting into chips the polypropylene resin composition chips (V).
Using a polypropylene resin composition (V) chip, a porous polypropylene film having a thickness of 21 μm was obtained by the same film forming method as in Example 1.

(Example 7)
In Example 1, heat treatment (HS1 zone) was performed at 163 ° C. for 2 seconds while maintaining the distance between the clips after stretching in the width direction at 9.5 times at a transverse stretching speed of 1,800% / min in the width direction. Except for the above, a porous polypropylene film having a thickness of 21 μm was obtained by the same raw material and film forming method as in Example 1.

(Comparative Example 1)
As the polypropylene resin, 99.7 parts by mass of Homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. with MFR = 7.5 g / 10 min and N, N′-dicyclohexyl-2,6 as the β crystal nucleating agent -0.3 parts by weight of naphthalene dicarboxyamide (manufactured by Shin Nippon Rika Co., Ltd., NU-100), 0.05 parts by weight of calcium behenate, and IRGANOX (registered by Ciba Specialty Chemicals, an antioxidant) Trademark) “1010” and “IRGAFOS (registered trademark)” 168 are fed to a twin screw extruder of L / D = 41 from a weighing hopper so that 0.1 parts by mass of each are mixed at this ratio, and 300 ° C. The mixture is melt-kneaded, discharged from a die, cooled and solidified in a 25 ° C. water bath, cut into chips, and a polypropylene resin composition (VI) chip is formed. Obtained.
Using a chip of the polypropylene resin composition (VI), a porous polypropylene film having a thickness of 23 μm was obtained by the same film forming method as in Example 1.

(Comparative Example 2)
70 parts by mass of homopolypropylene FLX80E4 (melting point = 165 ° C.) manufactured by Sumitomo Chemical Co., Ltd. having a melting point of 165 ° C. and MFR = 7.5 g / 10 min, and an ethylene-octene-1 copolymer (Dow Chemical) as a copolymer PE resin Engage 8411, MFR: 18 g / 10 min), 30 parts by mass, and further, “IRGANOX (registered trademark)” 1010 and “IRGAFOS (registered trademark)” 168 manufactured by Ciba Specialty Chemicals, which are antioxidants, each 0.1 mass The raw material is supplied from the weighing hopper to the twin screw extruder so that the parts are mixed at this ratio, melt kneaded at 240 ° C., discharged from the die in a strand shape, cooled and solidified in a 25 ° C. water tank, and chips A chip of polypropylene resin composition (VII) was obtained.
90 parts by mass of the polypropylene resin composition (VI) obtained in Comparative Example 1 and 10 parts by mass of the polypropylene resin composition (VII) were dry blended and supplied to a uniaxial melt extruder, and melt extrusion was performed at 210 ° C. Foreign matters were removed with a 60 μm cut sintered filter, and then discharged onto a cast drum whose surface temperature was controlled at 121 ° C. with a T-die to obtain a cast sheet. Next, preheating was performed using a ceramic roll heated to 123 ° C., and the film was stretched 5.0 times in the longitudinal direction of the film. Next, the edge part was hold | gripped with the clip and it extended | stretched 7.7 times in the width direction at 150 degreeC with the horizontal extending | stretching speed | rate of 1,800% / min.
In the subsequent heat treatment process, heat treatment was performed at 150 ° C. for 2 seconds (HS1 zone) while maintaining the distance between the clips after stretching (HS1 zone), and further relaxed at a relaxation rate of 17% at 163 ° C. for 5 seconds (Rx zone). Heat treatment was performed at 163 ° C. for 5 seconds while keeping the distance (HS2 zone).
Then, the ear | edge part of the film hold | gripped with the clip was cut and removed, and the 23-micrometer-thick porous polypropylene film was obtained.

(Comparative Example 3)
In Example 1, heat treatment was performed at 150 ° C. for 2 seconds while maintaining the distance between the clips after stretching in the width direction (HS1 zone), and further relaxation was performed so that the relaxation rate became 5% at 153 ° C. for 5 seconds (Rx zone). A porous polypropylene film having a thickness of 21 μm was obtained by the same raw material and film forming method as in Example 1 except that heat treatment was performed at 153 ° C. for 5 seconds (HS2 zone) while maintaining the distance between the clips after relaxation. .

In the examples satisfying the requirements of the present invention, not only the air resistance is low and the mechanical strength is excellent, but also the value of R2 / R1 is low, and thus the function of imparting or improving the performance such as heat resistance and adhesion to the electrode. It can be suitably used as a base material for a separator for an electricity storage device having a layer. On the other hand, in the comparative example, the value of R2 / R1 is high, and it is difficult to use it as a base material for an electricity storage device separator having a functional layer that imparts or improves performance such as heat resistance and adhesion to electrodes.

Claims

A porous polypropylene film mainly composed of polypropylene resin,
Separator resistance R1 (Ω) of the porous polypropylene film and separator resistance R2 (Ω) after applying and drying a coating simulation liquid composed of a functional polymer and an organic solvent on the porous polypropylene film, The porous polypropylene film characterized by satisfying the following formula (1).
R2 / R1 ≦ 1.2 (1)
2. The porous polypropylene film according to claim 1, wherein the organic solvent is acetone.
The porous polypropylene film according to claim 1, wherein the following formula (2) is satisfied.
0.7 ≦ PMD / PTD ≦ 2.0 (2)
(PMD: breaking strength in the longitudinal direction of the porous polypropylene film, PTD: breaking strength in the width direction of the porous polypropylene film)
4. The porous polypropylene film according to claim 1, wherein the content of the polypropylene resin in the porous polypropylene film is 80% by mass or more.
The porous polypropylene film according to any one of claims 1 to 4, wherein the porous polypropylene film has a β-crystal forming ability of 60% or more.
The porous polypropylene film according to any one of claims 1 to 5, wherein the heat shrinkage in the width direction when heat-treated at 135 ° C for 60 minutes is 10% or less.
A power storage device separator using the porous polypropylene film according to any one of claims 1 to 6.
An electricity storage device separator obtained by laminating a functional layer on the porous polypropylene film according to any one of claims 1 to 6.
An electricity storage device comprising the separator for an electricity storage device according to claim 7 or 8, a positive electrode, a negative electrode, and an electrolytic solution.