JP2015101713A - Separator for power storage device and power storage device made using separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for power storage device which makes it possible to obtain a power storage device having excellent lithium ion diffusibility and high output.SOLUTION: This invention provides a porous polypropylene film in which a diffusion coefficient D[m/s] in thickness direction measured by magnetic field gradient NMR method satisfies formula (1): 5.0×10≤D≤10.0×10and air permeability T[sec/100 cc μm] satisfies formula (2): 5≤T≤25.

Description

  The present invention relates to a porous polypropylene film. Furthermore, the present invention relates to a power storage device separator having an appropriate diffusion coefficient in the thickness direction and having excellent battery performance.

  In recent years, microporous resin films have been actively developed as separators for power storage devices such as lithium ion secondary batteries. Here, the control of the pore opening property of the membrane as a function of adjusting the passage of ions is most important in properly exhibiting the separator function.

  In order to achieve a high output of the electricity storage device, it is required that the hole has high porosity and low electrical resistance, while in order to ensure the safety of the electricity storage device, high mechanical strength is required. It is necessary to adjust the openness to some extent.

  Here, various methods for making a polypropylene film microporous and applying it to a separator have been proposed. Microporous methods can be roughly classified into wet methods and dry methods.

  In the wet method, polypropylene is used as the matrix resin, the extractables are added and mixed, and then sheeted, and then only the extractables are extracted using the good solvent of the extractables. For example, Patent Document 1 has been proposed. On the other hand, for example, in Patent Document 2, as a dry method, by adopting low temperature extrusion and high draft ratio at the time of melt extrusion, the lamella structure in the film before stretching formed into a sheet is controlled, and this is uniaxially stretched in the longitudinal direction. Thus, there has been proposed a method (so-called lamellar stretching method) in which cleavage at the lamella interface is generated to form voids. Also, for example, in Patent Document 3, as a dry method, a large amount of incompatible resin is added as inorganic particles or polypropylene as a matrix resin as particles, and a sheet is formed and stretched to form an interface between the particles and the polypropylene resin. Also proposed is a method of cleaving to form voids. Further, for example, in Patent Documents 4 to 6, in the film, the difference in crystal density between the α-type crystal (α crystal) and the β-type crystal (β crystal), which are polymorphs of polypropylene, is utilized. A so-called β crystal method for forming voids has also been proposed.

  In addition, for example, Patent Document 7 proposes a microporous film having high film rigidity by defining stretching conditions when microporousizing a polyolefin film by a β crystal method, and Patent Document 8 describes stretching, heat treatment, By adjusting the relaxation conditions in detail, the friction coefficient of the fill is controlled, and a microporous film with a high coating process is proposed. However, in any case, the cast drum contact method uses an air knife, and there is a possibility that a uniform hole opening property is not sufficiently obtained.

  Patent Document 9 discloses a separator for an electricity storage device having a high temperature storage characteristic by defining a diffusion coefficient and an air permeability in a film thickness direction within a certain range by a magnetic field gradient NMR method. Patent Document 10 discloses a separator for an electricity storage device that defines diffusion coefficients not only in the film thickness direction but also in the MD and TD directions by a magnetic field gradient NMR method, and has oven resistance characteristics. However, since all of these proposed separators have a small lithium ion diffusion coefficient, the lithium ion permeability is insufficient even when used as a separator, and it may be difficult to obtain a high output.

Japanese Patent Laid-Open No. 55-131028 Japanese Patent Publication No.55-32531 JP-A-57-203520 JP-A 63-199742 JP-A-6-100720 Japanese Patent Laid-Open No. 9-255804 JP 2013-213193 A JP 2013-1117013 A JP 2011-81994 A JP 2011-81995 A

  An object of the present invention is to solve the above-described problems. That is, an object of the present invention is to provide an electricity storage device separator that is excellent in lithium ion diffusibility and capable of obtaining a high output electricity storage device.

  In order to solve the above-described problems, the present invention has the following features.

(1) The diffusion coefficient D [m ² / s] in the thickness direction measured by the magnetic field gradient NMR method satisfies the equation (1), and the air permeability T [sec / 100 cc · μm] satisfies the equation (2). Filling porous polypropylene film.

5.0 × 10 ⁻¹¹ ≦ D ≦ 10.0 × 10 ⁻¹¹ (1)
5 ≦ T ≦ 25 (2)
(2) A separator for an electricity storage device using the porous polypropylene film described in (1) above.

  (3) An electricity storage device comprising the porous polypropylene film according to (1) or the electricity storage device separator according to (2).

  By using the porous polypropylene film of the present invention as a separator for an electricity storage device, an electricity storage device having excellent output characteristics, particularly high output, can be obtained.

In the porous polypropylene film of the present invention, the diffusion coefficient D [m ² / s] in the thickness direction measured by the magnetic field gradient NMR method satisfies 5.0 × 10 ⁻¹¹ ≦ D ≦ 10.0 × 10 ⁻¹¹ . The air permeability T [sec / 100 cc · μm] satisfies 5 ≦ T ≦ 25. The present inventors have found that excellent battery characteristics can be obtained by simultaneously satisfying the diffusion coefficient range and the air permeability range.

  Hereinafter, detailed means for adjusting the values of the diffusion coefficient D and the air permeability T to the specific ranges will be described. The feature is in the casting condition. More specifically, (1) the cast film contact method is a nip cast method in which the molten resin is pressed against the cast drum with a nip roll, and (2) the nip roll temperature is controlled to be equal to or lower than the nip roll temperature and the film. (3) The nip roll is processed with a satin finish to firmly peel off the nip roll and the film. If these conditions are not met, the cast film may be held on the nip roll without being peeled off, and may not be formed. In the present invention, by paying attention to the above-mentioned viewpoints, the diffusion coefficient D and the air permeability T can be controlled within the above ranges, and a separator excellent in battery characteristics has been successfully obtained.

  The polypropylene resin contained in the porous polypropylene film of the present invention is preferably an isotactic polypropylene resin having a melt flow rate (hereinafter referred to as MFR) in the range of 2 to 30 g / 10 minutes. An MFR of 5 to 20 g / 10 min is more preferable in terms of achieving both high porosity and film forming stability. Here, MFR is an index indicating the melt viscosity of a resin defined in JIS K7210 (1995), and is widely used as a physical property value indicating the characteristics of a polyolefin resin. In the case of polypropylene resin, measurement is performed under condition M of JIS K7210 (1995), that is, at a temperature of 230 ° C. and a load of 2.16 kg.

  Moreover, it is preferable that the isotactic index of the isotactic polypropylene resin is 90 to 99.9% because voids can be efficiently formed in the film because of high crystallinity. When the isotactic index is within this range, a highly permeable porous film can be easily obtained.

  The porous polypropylene film of the present invention may be composed of 100% by mass of the above-described isotactic polypropylene resin. However, from the viewpoint of realizing high air permeability and porosity, 90 to 99.9 isotactic polypropylene resin is used. You may be comprised from the polyolefin resin containing the mass%. From the viewpoint of heat resistance, it is more preferable that 92 to 99% by mass is a polypropylene resin. Here, the polypropylene resin may be a polypropylene copolymer obtained by copolymerizing a comonomer as well as a homopolypropylene which is a homopolymer of propylene. The comonomer is preferably an unsaturated hydrocarbon, and examples thereof include 1-butene, 1-pentene, 4-methylpentene-1, and 1-octene which are ethylene and α-olefin. The copolymerization rate of these comonomers in the polypropylene copolymer is preferably 5% by mass or less, more preferably 3% by mass or less.

  The porous polypropylene film of the present invention is preferably composed of a polypropylene resin containing 0.1 to 10% by mass of an ethylene / α-olefin copolymer from the viewpoint of realizing high air permeability and high porosity. . More preferably, it is 2-10 mass%, and it is further more preferable if it is 3-8 mass%. Any preferred lower limit can be combined with any preferred upper limit. Here, examples of the ethylene / α-olefin copolymer include linear low-density polyethylene and ultra-low-density polyethylene. Among them, an ethylene / 1-octene copolymer obtained by copolymerizing 1-octene is used. Ultra-low density polyethylene can be preferably used. As these ethylene / α-olefin copolymers, commercially available “Engage (registered trademark)” (type names: 8411, 8452, 8100, etc.) manufactured by Dow Chemical can be used.

  The porous polypropylene film of the present invention has pores that penetrate both surfaces of the film and have air permeability (hereinafter referred to as through-holes). This through hole is preferably formed in the film by, for example, biaxial stretching.

  Specific examples of the method include a β crystal method. In order to form a through-hole in a film using the β crystal method, it is preferable that the β crystal forming ability of the polypropylene resin composition as a raw material for the porous polypropylene film is 50 to 90%. By making the β-crystal forming ability of the polypropylene resin 50% or more, a sufficient amount of β-crystal is present at the time of film production, and a sufficient number of voids are formed in the film using the transition to α-crystal. A film having excellent permeability can be obtained. On the other hand, by making the β-crystal forming ability of polypropylene resin 90% or less, it is not necessary to add a large amount of β-crystal nucleating agent, and it is not necessary to make the stereoregularity of the polypropylene resin to be used extremely high. The film formation stability is good. Industrially, the β-crystal forming ability is more preferably 60 to 90%, and further preferably 70 to 90%.

  As a method for controlling the β-crystal forming ability of the polypropylene resin to 50 to 90%, use of a polypropylene resin having a high isotactic index, or a crystallization nucleus that selectively forms a β-crystal when added to the polypropylene resin. An agent (β crystal nucleating agent) is used as an additive. As the β crystal nucleating agent, various pigment compounds and amide compounds can be preferably used. For example, amide compounds; tetraoxaspiro compounds; quinacridones; iron oxides having a nanoscale size; carboxylic acids represented by potassium 1,2-hydroxystearate, magnesium benzoate or magnesium succinate, magnesium phthalate, etc. Alkali or alkaline earth metal salts; aromatic sulfonic acid compounds represented by sodium benzenesulfonate or sodium naphthalenesulfonate; ditriesters or triesters of dibasic or tribasic carboxylic acids; represented by phthalocyanine blue Phthalocyanine pigments; binary compounds composed of organic dibasic acids and Group IIA metal oxides, hydroxides or salts of periodic table; compositions composed of cyclic phosphorus compounds and magnesium compounds. In particular, amide compounds disclosed in JP-A-5-310665 can be preferably used. As content when making a polypropylene resin composition contain (beta) crystal nucleating agent, it is preferable that it is 0.05-0.50 mass%, when based on the whole polypropylene resin composition, 0.40% by mass is more preferable. Any preferred lower limit can be combined with any preferred upper limit.

  The porous polypropylene film of the present invention preferably has a total film thickness of 10 to 50 μm. By making the total thickness of the film 10 μm or more, the film is difficult to break during use. On the other hand, by setting it to 50 μm or less, the volume ratio of the porous film occupying in the electricity storage device is moderate, and a high energy density can be obtained. it can. The total film thickness is more preferably 12 to 30 μm, and even more preferably 14 to 25 μm. Any preferred lower limit can be combined with any preferred upper limit.

  The porous polypropylene film of the present invention may be a laminated film formed by laminating a plurality of layers having different compositions or the same composition. A laminated film is preferable because the film surface characteristics and the overall film characteristics may be individually controlled within a preferable range. In that case, it is preferable to use a three-layer stack of A layer / B layer / A layer type, but there is a problem even if a two-layer stack of A layer / B layer type is selected or a multilayer stack of four or more layers. Absent. In addition, a lamination | stacking thickness ratio is not restrict | limited in the range which does not impair the effect of this invention.

The diffusion coefficient D [m ² / s] of the porous polypropylene film of the present invention is preferably in the range of 5.0 × 10 ⁻¹¹ to 10 × 10 ⁻¹¹ . More preferable to be ^{5.5 × 10 -11 ~8.5 × 10 -11} , further preferably at ^{6.0 × 10 -11 ~7.0 × 10 -11} . The air permeability T [sec / 100 cc · μm] is preferably 5 to 20, and more preferably 7 to 18. By setting the cast film contact method to the nip cast method and setting the nip roll temperature to a constant temperature as will be described later, the values of the diffusion coefficient D and the air permeability T can be controlled to the above preferable values in a balanced manner.

  The porous polypropylene film of the present invention may contain various additives such as an antioxidant, a heat stabilizer and an antistatic agent, and further an antiblocking agent and a filler as long as the effects of the present invention are not impaired. Good. In particular, for the purpose of suppressing oxidative deterioration due to the thermal history of the polypropylene resin, it is preferable to add 0.01 to 0.50 parts by mass of an antioxidant with respect to 100 parts by mass of the polypropylene resin.

  The example of the manufacturing method of the porous polypropylene film of this invention is demonstrated concretely below.

  First, as a polypropylene resin, 80 to 100 parts by mass of a commercially available homopolypropylene resin having an MFR of 4 to 10 g / 10 min, and further, 1 to 10 parts by mass of an ultra-low density polyethylene resin having a melt index of 10 to 30 g / 10 min, Prepare a raw material by mixing 0.1 to 0.5 parts by mass of β-crystal nucleating agent such as N′-dicyclohexyl-2,6-naphthalenedicarboxamide and mixing in advance at a predetermined ratio using a twin screw extruder To do. At this time, the melting temperature is preferably 290 to 310 ° C. By setting the melting temperature to 290 ° C. or higher, the β crystal nucleating agent is easily dissolved in the polypropylene resin, so that it becomes difficult to cause filter clogging and film defects. On the other hand, when the melting temperature is 310 ° C. or lower, the polypropylene resin is less likely to be thermally deteriorated.

  Next, the above-mentioned mixed raw material is supplied to a single-screw melt extruder, and melt extrusion is performed at 200 to 240 ° C. At this time, it is preferable to add an antioxidant from the viewpoint of preventing gelation of ultra-low density polyethylene. 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. At this time, the surface temperature of the cast drum is preferably 105 to 130 ° C. from the viewpoint of controlling the β crystal fraction of the cast film to be high. The β crystal fraction is preferably 40 to 80% from the viewpoint of making the formation of through holes uniform and improving the uniformity of air permeability and porosity. The β crystal fraction is more preferably 45 to 80%, and further preferably 50 to 75%. Any preferred lower limit can be combined with any preferred upper limit. Further, a nip cast method is preferable for bringing the sheet into close contact with the drum. The characteristic is that the unstretched sheet solidified while being heated by the cast drum is then brought into contact with a roll heated to 60 to 95 ° C., preferably 70 to 90 ° C., for 0.05 to 5.0 seconds and then gradually cooled. This is preferable from the viewpoint of improving uniformity.

  If the nip roll temperature of the cast is too high, the film will not be cooled appropriately and will not be peeled off from the cast drum, and if it is too low, β crystals may not be properly generated. For this reason, it is preferable that it is 80-120 degreeC, and 90-115 degreeC is more preferable.

  From the viewpoint of the peelability between the cast film and the nip roll, the surface of the nip roll is preferably subjected to a satin finish. This improves the peelability. For example, when a mirror roll having a surface roughness of 0.8 s or less and plated with hard chrome is used as a nip roll, it is difficult to form a film without peeling off the cast film.

  The unstretched sheet thus obtained is biaxially stretched to form through holes in the film. As a biaxial stretching 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 of stretching in the longitudinal direction after stretching in the width direction, or the longitudinal direction and the width direction of the film are stretched almost simultaneously. 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 highly air permeable film, and in particular, the longitudinal stretching in the longitudinal direction and then the transverse stretching in the width direction. It is preferable.

  As specific stretching conditions, first, an unstretched sheet is controlled to a temperature at which it can be stretched in the longitudinal direction. 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 longitudinal stretching temperature in the longitudinal direction is preferably 120 to 140 ° C., more preferably 130 to 138 ° C., from the viewpoint of improving film properties and improving uniformity. The longitudinal draw ratio is preferably 4.5 to 6 times, more preferably 4.8 to 6 times, and further preferably 5 to 6 times. In the longitudinal stretching step in the longitudinal direction, all the β crystals of polypropylene formed in the unstretched sheet are transformed into α crystals.

  Next, the uniaxially stretched polypropylene film is introduced into a stenter type stretching machine by gripping the film end. And preferably it heats to 140-155 degreeC, Preferably it is 4-10 times in the width direction, More preferably, 5-9 times transverse stretching is performed. The transverse stretching speed at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min. Any preferred lower limit can be combined with any preferred upper limit. In particular, it is preferably 3,000% / min or less until the transverse draw ratio reaches 4 times. In the region where the transverse stretching ratio exceeds 4 times and reaches the final magnification, even if high-speed stretching is performed from the viewpoint of productivity, there is little influence on achieving uniformity of physical properties.

  Next, heat setting is performed in a stenter as it is, and the heat setting temperature is preferably a transverse stretching temperature + 5 ° C. to a transverse stretching temperature + 15 ° C. and a heat setting temperature of 163 ° C. or less. The heat setting time is preferably 5 to 30 seconds. Furthermore, the heat setting may be performed while relaxing in the longitudinal direction and / or the width direction of the film, and in particular, the relaxation rate in the width direction is preferably 7 to 15% from the viewpoint of thermal dimensional stability.

  The porous polypropylene film of the present invention is excellent in air permeability and electrical characteristics, and can be suitably used as a separator for an electricity storage device. Specifically, since it has excellent output characteristics, it can be suitably used for non-aqueous electrolyte secondary batteries for electric vehicles and the like.

  Hereinafter, the present invention will be described in detail by way of examples. The characteristics were measured and evaluated by the following methods.

[Diffusion coefficient D of lithium ion by magnetic field gradient NMR method]
1. Measurement conditions-Equipment used: ECA-500 (manufactured by JEOL Ltd.)
・ Measurement frequency: 194.4 MHz
・ Locking solvent: None ・ Number of integrations: 16 times ・ Measurement temperature: 30 ° C.
・ Pulse sequence: BPPLED (Bipolar Pulse Paris Longitudinal Eddy current Delay)
Magnetic field gradient strength is 0.1 to 6.0 T / m, gradient magnetic field application time is 0.5 ms to 5 ms, diffusion time is 20 ms to 200 ms, and magnetic field gradient strength with respect to peak strength when magnetic field gradient strength is the smallest. The parameters were set so that the peak intensity attenuation rate was about 50 to 90% when the maximum value was.

2. Sample Preparation 2.1 Introduction of Film into NMR Tube Several films were stacked and hollowed in the thickness direction, laminated and introduced into the NMR tube. When laminating, the film orientation was aligned. In this analysis, a microcell for measuring the diffusion coefficient was used as the NMR tube.

2.2 Preparation of sample for measurement As shown in section 2.1, the NMR tube into which the film was introduced was put into a glove box, and 1M LiTFSI / EC-MEC * (prepared in the glove box) (1: 2 VOL%) The solution was added.

*) LiTFSI: Lithium bis (Trifluoromethanesulfonimide)
EC: Ethylene carbonate
MEC: Methylyl carbonate
3. Calculation of diffusion coefficient D (regression calculation analysis)
The peak intensity and the diffusion coefficient D are expressed by the following equation.

I = I ₀ exp [−γ ² g ² δ ² D (Δ−δ / 3)] (1)
I: Peak intensity when PFG is applied I ₀ : Peak intensity when g = 0 (PFG is the smallest) γ: Gyromagnetic ratio (value intrinsic to the nucleus)
g: PFG intensity δ: PFG application time Δ: Diffusion time Other than I, I ₀ , g, and D, constants are obtained. When the equation (1) is modified and the constant term is summarized as a, Ln (I / I ₀ ) = -Ag ² D (2)
It can be expressed as At this time, the PFG intensity was changed by several points, the peak intensity at each PFG intensity was observed, Ln (I / I ₀ ) against ag ² was plotted, and the diffusion coefficient D was calculated from the slope.

[Air permeability T]
A square having a size of 100 mm × 100 mm was cut from the film and used as a sample. Using the Oken type tester method of JISP 8117 (2009), the permeation time of 100 ml of air was measured at 23 ° C. and relative humidity 65%. The measurement was performed three times with the sample changed, and the average value of the permeation time was defined as the air permeability of the film. In addition, it can confirm that the through-hole is formed in the film that this air permeability value is a finite value.

[Battery characteristics]
A positive electrode with a lithium cobalt oxide (LiCoO ₂ ) thickness of 40 μm made by Hosen Co., Ltd. is used, punched into a circle with a diameter of 15.9 mm, and a graphite negative electrode made by Hosen Co., Ltd. with a thickness of 50 μm is used. Then, the film was punched into a circle having a diameter of 16.2 mm, and then the porous films of the examples and comparative examples were punched to a diameter of 24.0 mm. From the bottom, the negative electrode, the separator, and the positive electrode were stacked in this order and stored in a small stainless steel container with a lid. The container and the lid are insulated, the container is in contact with the negative electrode copper foil, and the lid is in contact with the positive electrode aluminum foil. In this vessel, an organic solvent in which ethylene carbonate and dimethyl carbonate are mixed at a mass ratio of 3: 7 is used as an organic solvent, and an electrolyte solution in which LiPF ₆ 1.0 mol / L is dissolved as an indicator salt is injected into the container. The battery was sealed and aged for 1 hour after injecting the electrolyte, and batteries were prepared for each of the examples and comparative examples.

  About each produced secondary battery, charging / discharging operation which charges to 4.2V at 1 mA for 3 hours and discharge to 2.7V at 3 mA was performed in 25 degreeC atmosphere, and discharge capacity A was investigated. Further, charging / discharging operation was performed by charging at 1 mA up to 4.2 V for 3 hours and discharging at 30 mA up to 2.7 V, and the discharge capacity B was examined.

  A value obtained by a calculation formula of [(discharge capacity B) / (discharge capacity A)] × 100 (%) was evaluated according to the following criteria. In addition, 20 test pieces were measured, and the average value was evaluated.

○: 85% or more Δ: 80% or more and less than 85% ×: less than 80% or one or more batteries that are less than 20% in each battery (Example 1)
N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide (Nippon Nippon Kayaku Co., Ltd., NU-100), a β crystal nucleating agent, in 99.5 parts by mass of homopolypropylene FLX80E4 manufactured by Sumitomo Chemical Co., Ltd. ) 0.3 parts by mass, and further 0.1 parts by mass of “IRGANOX” (registered trademark) 1010 and “IRGAFOS” (registered trademark) 168 manufactured by BASF Japan, which are antioxidants, are mixed at this ratio. The raw material is fed from a measuring hopper to a twin screw extruder, melt kneaded at 300 ° C., discharged from a die in a strand shape, cooled and solidified in a water bath at 25 ° C., cut into a chip shape, and a polypropylene composition ( MFR: 8 g / 10 minutes (hereinafter simply referred to as βPP) was prepared as a raw material resin for the film. In addition, 30 parts by mass of “engage (registered trademark)” 8411 (melt index: 18 g / 10 min) manufactured by Dow Chemical, which is an ultra-low density polyethylene resin, and homopolypropylene FLX80E4 (MFR: 7 g) manufactured by Sumitomo Chemical Co., Ltd. / 10 min) To 63 parts by mass, 5 parts by mass of “IRGANOX” (registered trademark) 1010 manufactured by BASF Japan as an antioxidant and 2 parts by mass of “IRGAFOS” (registered trademark) 168 were added and mixed. The product is fed to a twin screw extruder, melt kneaded at 220 ° C, extruded into a strand, solidified with cooling water at 25 ° C, and cut into chips by cutting with a chip cutter. , Simply expressed as PE master).

  Mixing 90 parts by weight of βPP and 10 parts by weight of PE master, supplying them to a single screw extruder, performing melt extrusion at 210 ° C, and controlling the surface temperature from a T die to 120 ° C through a 40 µm cut sintered filter. The molten polypropylene resin was closely adhered to the cast drum in the form of a sheet using a nip roll whose surface temperature was controlled to 120 ° C. and the surface was textured. Furthermore, it was gradually cooled on a metal roll whose surface temperature was controlled at 80 ° C. for 3 seconds to obtain an unstretched sheet.

  Next, heating was performed using a ceramic roll heated to 125 ° C., and the film was stretched 5 times in the longitudinal direction of the film using the difference in peripheral speed between the roll heated to 125 ° C. and the roll cooled to 30 ° C. It was. At this time, the film temperature in the stretching section was measured with a radiation thermometer (emissivity (ε) = 0.95) and found to be 132 ° C.

  Next, the end portion was introduced into a tenter-type stretching machine by holding the clip with a clip, and stretched 6.5 times in the width direction at 150 ° C. At that time, the transverse stretching was performed at a constant 1,800% / min from the beginning to the end of stretching. Then, heat treatment was performed at 158 ° C. for 6 seconds while applying 10% relaxation in the width direction. The evaluation results of the obtained porous polypropylene film are shown in Table 1.

(Example 2)
In Example 1, 96 parts by mass of βPP and 4 parts by mass of PE master were mixed, and cast in the same manner except that the nip roll temperature of casting was 110 ° C. to obtain an unstretched sheet.

  Next, heating is performed using a ceramic roll heated to 120 ° C. and a radiation heater (output 3 kW), and the difference in peripheral speed between the roll heated to 128 ° C. and the roll cooled to 30 ° C. is used. 4.8 times longitudinal stretching was performed in the longitudinal direction. At this time, the film temperature in the stretching section was 127 ° C.

  Next, the end portion was introduced into a tenter-type stretching machine by gripping with a clip, and stretched 7 times in the width direction at 153 ° C. At that time, the stretching was performed at a constant stretching rate of 900% / min from the start to the end of the stretching. Then, heat treatment was performed at 158 ° C. for 6 seconds while applying 10% relaxation in the width direction to obtain a porous polypropylene film. The evaluation results are shown in Table 1.

(Example 3)
An unstretched sheet was obtained by casting in the same manner as in Example 1 except that the cast nip roll temperature was 100 ° C. and the cast drum temperature was 125 ° C.

  Next, heating is performed using a ceramic roll heated to 123 ° C. and a radiation heater (output 3 kW), and the difference in peripheral speed between the roll heated to 123 ° C. and the roll cooled to 30 ° C. is used. The film was stretched 5 times in the longitudinal direction. At this time, the film temperature in the stretching section was 122 ° C.

  Next, the end portion was introduced into a tenter-type stretching machine by holding the clip with a clip, and stretched 6.5 times in the width direction at 150 ° C. At that time, the stretching was performed at a constant stretching rate of 2000% / min from the start to the end of the stretching. Then, heat treatment was performed at 160 ° C. for 6 seconds while applying 10% relaxation in the width direction to obtain a porous polypropylene film. The evaluation results are shown in Table 1.

Example 4
An unstretched sheet was obtained by casting in the same manner as in Example 1 except that the nip roll temperature of casting was 120 ° C. and the cast drum temperature was 125 ° C.

  Next, heating is performed using a ceramic roll heated to 120 ° C., and the film is stretched 4.8 times in the longitudinal direction in the longitudinal direction of the film using the difference in peripheral speed between the roll heated to 120 ° C. and the roll cooled to 30 ° C. Went. At this time, the film temperature in the stretching section was 119 ° C.

  Next, the end portion was introduced into a tenter type stretching machine by holding it with a clip, and stretched 6.8 times in the width direction at 152 ° C. At that time, the transverse stretching was performed at a constant 2300% / min from the start to the end of stretching. Then, heat treatment was performed at 160 ° C. for 6 seconds while applying 10% relaxation in the width direction to obtain a porous polypropylene film. The evaluation results are shown in Table 1.

(Comparative Example 1)
In Example 1, 100 parts by mass of βPP was used, and the same was performed except that the nip roll temperature of the cast whose surface was mirror-finished was 70 ° C., the stretching roll temperature before longitudinal stretching was 130 ° C., and the lateral stretching speed was 3,300%. Film formation was carried out. The evaluation results of the obtained porous film are shown in Table 1.

(Comparative Example 2)
A non-stretched sheet was obtained in the same manner as in Example 1 except that 96 parts by mass of βPP and 4 parts by mass of PE master were mixed and an air knife was used for the method of closely contacting the cast drum.

  Next, heating is performed using a ceramic roll heated to 135 ° C. and a radiation heater (output 3 kW), and the difference in peripheral speed between the roll heated to 135 ° C. and the roll cooled to 30 ° C. is used. 4.8 times longitudinal stretching was performed in the longitudinal direction. At this time, the film temperature in the stretching section was 127 ° C. Further, the neck down in the longitudinal stretching in the longitudinal direction was 24%.

  Next, the end portion was introduced into a tenter-type stretching machine by holding it with a clip, and stretched by 7 times in the width direction at 155 ° C. At that time, the transverse stretching was performed at a constant 1500% / min from the beginning to the end of stretching. Then, heat treatment was performed at 158 ° C. for 6 seconds while applying 10% relaxation in the width direction. The evaluation results of the obtained porous film are shown in Table 1.

(Comparative Example 3)
The same operation as in Example 1 was performed except that the nip roll temperature of casting was 140 ° C. A cast film was wound around the nip roll, and the film could not be formed.

  Since the porous film of the present invention is excellent not only in air permeability but also in output characteristics, it can be suitably used when used for a separator for an electricity storage device. In particular, it can be suitably used as a separator of a lithium ion battery which is a nonaqueous electrolyte secondary battery.

Claims (3)

Porosity in which the diffusion coefficient D [m ² / s] in the thickness direction measured by the magnetic field gradient NMR method satisfies the formula (1) and the air permeability T [sec / 100 cc · μm] satisfies the formula (2) Polypropylene film.
5.0 × 10 ⁻¹¹ ≦ D ≦ 10.0 × 10 ⁻¹¹ (1)
5 ≦ T ≦ 25 (2) The separator for electrical storage devices using the porous polypropylene film of Claim 1. An electricity storage device comprising the porous polypropylene film according to claim 1 or the electricity storage device separator according to claim 2.