JP2012117047A - Porous polypropylene film, and separator for electricity storing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous polypropylene film which has a high strength in the traverse direction, is excellent in an electrolyte solution absorbing property, and exhibits excellent battery characteristics and safety when used as a separator.SOLUTION: The porous polypropylene film has a break strength in the traverse direction PTD of ≥80 MPa and ≤200 MPa, a ratio (PTD/PMD) of the break strength in the traverse direction PTD and a break strength in the machine direction PMD of 1 to 2, and a propylene carbonate soaking rate in the thickness direction of 0.01 to 10 sec/20 μm.

Description

  The present invention relates to a porous polypropylene film and an electricity storage device separator. More specifically, it can be suitably used for a separator used in a non-aqueous solvent battery or a capacitor, has high strength in the width direction, excellent liquid absorption of an electrolytic solution, and excellent battery characteristics and safety when used as a separator. And a separator for an electricity storage device using the same as a separator.

  A non-aqueous solvent battery such as a lithium battery or a lithium ion battery has a problem that the electrolytic solution used is an organic solvent and is inferior in safety against heat generation of the battery as compared with an aqueous solvent of an aqueous battery. Therefore, conventionally, in order to improve the safety of non-aqueous solvent batteries, particularly lithium ion batteries having a large energy density, a separator using a microporous porous film of an olefin-based material mainly composed of polyethylene has been used.

  In recent years, large batteries such as hybrid vehicle (HEV) batteries, tool batteries, etc. have been increased in output and rapidly rise to temperatures higher than 130 ° C, so they are shut down at an appropriate temperature (around 130 ° C). The function to perform is not necessarily required, and high safety is required. Furthermore, it is important for HEV batteries to be able to guarantee a long service life of 10 years or more and more stringent safety.

  A separator using a polypropylene porous membrane has been proposed as a separator for a non-aqueous solvent battery such as a lithium ion battery (for example, Patent Documents 1 and 2). However, polypropylene has low surface free energy and poor electrolyte absorption in the thickness direction. Therefore, a polypropylene porous membrane made by a conventional manufacturing method causes uneven absorption of liquid, particularly in a large battery, so that the resistance increases locally and the initial capacity and long-term stability may be inferior.

  A separator using polyethylene (for example, Patent Documents 3, 4, and 5) has good electrolyte solution absorbability in the thickness direction, but is susceptible to shrinkage at a temperature of 120 ° C. or less, and heat is generated due to short circuit between electrodes. Inferior to the problem.

  In addition, a proposal has been made to use a polypropylene non-woven fabric that is excellent in liquid absorption and heat resistance and suitable for high-power applications such as a large battery as a separator (for example, Patent Document 6). However, in this case, since the base material is a nonwoven fabric made of fibers, it has a large average pore diameter of about several μm, so there is a possibility that it has a defect due to the through hole. It is very high, and it is suggested that a fine short circuit is likely to occur, an initial failure at the time of winding the battery is likely to occur, and it is not possible to sufficiently compensate for a long life as in the HEV battery and a further severe safety. Furthermore, as long as the nonwoven fabric is used, the film thickness becomes large and the volume increase is inevitable, and there is also a problem that goes against the trend of the era of battery miniaturization and weight reduction.

  In addition, a proposal has been made to laminate a polyolefin-based porous film and a porous film using a resin having a high softening point (for example, Patent Document 7). However, the liquid absorbency is low, the electrolyte injection time becomes long, the battery assembly workability may be lowered, and the two types of films are laminated, so that the thickness is increased or the processing cost is increased. There is a case.

  In addition, proposals have been made to apply organic particles or inorganic particle layers having a high softening point to the surface of a porous substrate (for example, Patent Document 8). In this case, the liquid absorption is improved by the organic particle or inorganic particle layer, but the porosity and air permeability are low, and it is not suitable for a high output battery such as a HEV battery.

Japanese Patent Laid-Open No. 01-103634 JP 2008-248231 A JP-A-11-130899 JP-A-11-130900 JP 2001-19791 A JP-A-60-52 JP 2006-269359 A JP 2007-273443 A

  An object of the present invention is to solve the above-described problems. In other words, while using polypropylene with low surface free energy, it is excellent in electrolyte absorption in the thickness direction, has high strength in the width direction, and has a good balance with the strength in the longitudinal direction, and is excellent when used as a separator. Another object of the present invention is to provide a porous polypropylene film exhibiting battery characteristics and safety.

  The present invention for achieving the above object has a width direction breaking strength PTD of 80 MPa or more and 200 MPa or less, and a ratio (PTD / PMD) of the width direction breaking strength PTD to the longitudinal direction breaking strength PMD is 1 to 1. 2 and a porous polypropylene film having a propylene carbonate permeation rate in the thickness direction of 0.01 to 10 seconds / 20 μm.

  The porous polypropylene film of the present invention has a high strength in the width direction, a good balance with the strength in the longitudinal direction, excellent electrolyte absorption in the thickness direction, and excellent battery characteristics when used as a separator. A porous polypropylene film exhibiting safety can be provided.

  The porous polypropylene film in this invention means the polypropylene film which has the through-hole which penetrates both surfaces of a film and has air permeability.

  As a method for forming a through-hole in a film, either a wet method or a dry method may be used, but a dry method is desirable because the process can be simplified, and in particular, while the film is biaxially oriented to make physical properties uniform and thin. In view of maintaining high strength, it is preferable to use the β crystal method.

  In order to form through-holes in the film using the β crystal method, it is important to form a large amount of β crystals in the polypropylene resin. For this purpose, it is added to the polypropylene resin, which is called a β crystal nucleating agent. Thus, a crystallization nucleating agent that selectively forms β crystals is preferably used as an additive. Examples of the β crystal nucleating agent include various pigment compounds and amide compounds. In particular, amide compounds disclosed in JP-A-5-310665 can be preferably used. As content of (beta) crystal nucleating agent, when the whole polypropylene resin is 100 mass parts, it is preferable that it is 0.05-0.5 mass part, and more preferable if it is 0.1-0.3 mass part. .

  The polypropylene resin contained in the porous polypropylene film of the present invention is an isotactic polypropylene having a melt flow rate (hereinafter referred to as MFR, measurement conditions are 230 ° C., 2.16 kg) in the range of 2 to 30 g / 10 min. A resin is preferred. If the MFR is out of the above preferred range, it may be difficult to obtain a biaxially stretched film. More preferably, the MFR is 3 to 20 g / 10 minutes.

  The isotactic index of the isotactic polypropylene resin is preferably 90 to 99.9%. When the isotactic index is less than 90%, the crystallinity of the resin is low, and it may be difficult to achieve high strength and workability. A commercially available resin can be used as the isotactic polypropylene resin.

  Of course, a homopolypropylene resin can be used for the porous polypropylene film of the present invention, and from the viewpoint of stability in the film-forming process, film-forming properties, and uniformity of physical properties, an ethylene component and butene are added to polypropylene. You may use resin which copolymerized alpha-olefin components, such as hexene and octene, in 5 mass% or less. The form of the comonomer introduced into the polypropylene may be either random copolymerization or block copolymerization.

  The polypropylene resin contained in the porous polypropylene film of the present invention improves the air gap formation efficiency at the time of stretching, and the air permeability is improved by expanding the pore diameter. Therefore, 80 to 99% by mass of polypropylene and ethylene / α-olefin It is preferable to make it a mixture with 20-1 mass% of copolymers. Here, examples of the ethylene / α-olefin copolymer include linear low density polyethylene and ultra-low density polyethylene. Among them, ethylene / octene-1 copolymer obtained by copolymerization of octene-1 is preferably used. be able to. Examples of the ethylene-octene-1 copolymer include commercially available resins such as “Engage (registered trademark)” (type names: 8411, 8452, 8100, etc.) manufactured by Dow Chemical.

  Since the porous polypropylene film of the present invention is preferably made porous by the β crystal method, the β resin forming ability of the polypropylene resin contained in the film is preferably 50% or more. If the β-crystal forming ability is less than 50%, the amount of β-crystal is small at the time of film production, so the number of voids formed in the film is reduced by utilizing the transition to α-crystal, and as a result, only a film with low permeability is obtained. It may not be possible. Preferably, the β crystal forming ability is 50% to 90%. When the β crystal forming ability exceeds 90%, coarse pores may be formed, and the function as a separator for an electricity storage device may not be provided. In order to make the β crystal forming ability in the range of 50 to 90%, it is preferable to add the above β crystal nucleating agent as well as using a polypropylene resin having a high isotactic index. The β crystal forming ability is more preferably 60 to 90%.

  In the porous polypropylene film of the present invention, it is important that the ratio (PTD / PMD) of the breaking strength PTD in the width direction and the breaking strength PMD in the longitudinal direction is 1-2. When the value of PTD / PMD is less than 1, the strength balance in the longitudinal direction and the width direction is deteriorated, for example, a wound battery (the separator is fixed in the longitudinal direction but not fixed in the width direction). When an impact is applied in a straight line perpendicular to the winding axis, the separator may be severely split in the width direction and short-circuited. The value of PTD / PMD is preferably 1 to 1.7 from the viewpoint of safety. When the value of PTD / PMD exceeds 2, it may be excessively shrunk in the longitudinal direction, may be easily torn, or may have poor transmission performance.

  In addition, if the strength itself is low, the above-described safety may be inferior, so it is important that the breaking strength in the width direction of the porous polypropylene film of the present invention is 80 MPa or more. More preferably, it is 100 MPa or more from the viewpoint of safety. On the other hand, if the breaking strength in the width direction is too high, it may be excessively shrunk in the longitudinal direction, may be easily torn, or may be inferior in permeation performance. The safety described above can be determined by, for example, a so-called collision test in which a large force is locally applied to the wound product of the electrode and the separator to check the fracture mode.

  The breaking strength can be controlled by the crystallinity of polypropylene, the air permeation resistance of the resulting porous polypropylene film, the crystal orientation in the film plane, and the like. There is a tendency that the air permeability resistance becomes stronger by decreasing within 10 to 1,000 seconds / 100 ml, and becomes weaker by increasing it. Here, even with the same air permeation resistance, the strength can be increased as the plane orientation is increased. Therefore, the control of the orientation state is important. For example, when the film is produced by stretching in at least one direction in the film forming process, the plane orientation of the porous film can be increased as the stretching condition is higher. In particular, when manufacturing using the longitudinal-lateral sequential biaxial stretching method, in order to set the PTD / PMD value to 1-2, it is effective to keep the stretching ratio in the longitudinal direction low and increase the lateral stretching ratio. It is. Specifically, it is preferable to set the value of the stretching ratio in the width direction / the stretching ratio in the longitudinal direction to 2 or more (hereinafter, the width direction may be referred to as horizontal and the longitudinal direction may be referred to as vertical). More preferably, the value of lateral stretch ratio / longitudinal stretch ratio is 3 or more. In order to increase the breaking strength in the width direction, it is effective to increase the transverse draw ratio. Specifically, it is preferable to set the transverse draw ratio to a value exceeding 8 times. More preferably, it is 10 times or more. When the transverse draw ratio is 8 times or less, the air permeability resistance becomes high, battery characteristics may be deteriorated, and productivity may be lowered. If the transverse stretching ratio exceeds 20, the film may be easily broken.

  It is important that the porous polypropylene film of the present invention has a propylene carbonate penetration rate in the thickness direction of 0.01 to 10 seconds / 20 μm. When the propylene carbonate penetration rate in the thickness direction exceeds 10 seconds / 20 μm, the battery processability deteriorates and the productivity decreases, and when used as a separator, the electrolyte is locally absorbed, and the battery is partially absorbed. The internal resistance may increase and the initial discharge capacity may decrease. Preferably, it is 0.01 to 5 seconds / 20 μm.

  In the present invention, the propylene carbonate penetration rate in the thickness direction is preferably as high as possible, but it is difficult to measure a value substantially less than 0.01 sec / 20 μm. The propylene carbonate permeation rate in the thickness direction is the time from when the propylene carbonate droplet contacts the porous polypropylene film surface until it penetrates the back surface, converted to the time per 20 μm of the porous polypropylene film. is there.

  The propylene carbonate penetration rate in the thickness direction can be controlled in the film forming process. In particular, when producing using the longitudinal-lateral sequential biaxial stretching method, it is effective to keep the longitudinal stretching ratio low, increase the lateral stretching ratio, and further set the longitudinal stretching or transverse stretching temperature within a specified range. . Specifically, it is preferable to set the value of the transverse draw ratio / longitudinal draw ratio to 2 or more. More preferably, it is 2.5 or more, more preferably 3 or more. Moreover, it is preferable to make a horizontal stretch ratio into the value exceeding 8 times. More preferably, it is 9 times or more, and further preferably 10 times or more. Furthermore, it is effective that the longitudinal stretching temperature is 110 to 135 ° C, more preferably 120 to 135 ° C, and the lateral stretching temperature is 140 to 155 ° C, more preferably 145 to 153 ° C.

  In general, polypropylene has low surface free energy. Therefore, unlike liquid paraffin with a hydrocarbon structure, etc., the affinity with carbonates typified by propylene carbonate is poor, and porous polypropylene films made by conventional manufacturing methods have a propylene carbonate penetration rate in the thickness direction. It was slow. In general, it is necessary to perform contradictory control in order to achieve both liquid absorbency and breaking strength. However, in the present invention, in the above-described manufacturing method, the breaking strength ratio in the width direction and the longitudinal direction and the breaking strength in the width direction are kept high. However, the propylene carbonate penetration rate in the thickness direction could be increased. This mechanism is not yet clear, but by increasing the draw ratio in the width direction with respect to the draw ratio in the longitudinal direction, even polypropylene with a low surface free energy has a structure in which propylene carbonate can easily penetrate. it is conceivable that.

  In addition, the porous polypropylene film produced by the above-described production method has a flat continuous hole because the fibrils are entangled with each other and has a three-dimensional network structure, and a cavity in which the electrolytic solution is accumulated. Can be secured. Therefore, not only the penetration rate of propylene carbonate in the thickness direction is high, but also an organic solvent can be suitably retained. Therefore, long-term characteristics can be maintained even after repeated charging and discharging.

  The thickness of the porous polypropylene film of the present invention is preferably 10 to 40 μm. When the thickness is less than 10 μm, dendrite growth from the negative electrode cannot be suppressed, long-term stability and safety are insufficient, and the positive electrode and the negative electrode may be short-circuited during assembly and use as a battery. When the thickness is larger than 40 μm, the separator resistance becomes high, the output characteristics may become insufficient, and the positive electrode and the negative electrode active material cannot be sufficiently filled in the battery, and the energy density may decrease. .

  Since the porous polypropylene film of the present invention is suitably used for a separator of an electricity storage device, the air permeability resistance is within the range of 10 to 1,000 seconds / 100 ml, which reduces the internal resistance of the battery and further improves the output density. From the viewpoint of If the air permeability resistance is less than 10 seconds / 100 ml, the porosity may become high or the pore diameter may become too large, and the strength may not be sufficiently maintained, or the long-term stability and safety of the battery when used as a separator. May be shorter. On the other hand, if it exceeds 1,000 seconds / 100 ml, the output characteristics when used as a separator will be insufficient. More preferably, it is 10-300 seconds / 100 ml, More preferably, it is 50-200 seconds / 100 ml from a viewpoint of a separator characteristic.

  Here, the air permeation resistance is an index of the air permeability of the sheet and is shown in JIS P 8117 (1998). Air permeation resistance refers to stretching conditions (stretching direction (longitudinal or transverse), stretching method (longitudinal or transverse uniaxial stretching, longitudinal-horizontal or transverse-longitudinal biaxial stretching, simultaneous biaxial stretching, biaxial stretching) Redrawing, etc.), draw ratio, drawing speed, drawing temperature, etc.). Compared to the case of uniaxial stretching, the gas permeation resistance is lower in the case of performing biaxial stretching, and the gas permeation resistance is decreased when the draw ratio is increased, and the gas permeation resistance is increased when the stretching ratio is decreased.

  Below, the manufacturing method of the porous film of this invention is demonstrated concretely. In addition, the manufacturing method of the film of this invention is not limited to this.

  First, as the polypropylene resin constituting the porous film, 90 to 99 parts by mass of a commercially available homopolypropylene resin with an MFR of 8 g / 10 minutes, 1 to 10 parts by mass of a commercially available MFR of 8 g / 10 minutes with a very low density polyethylene resin, N, N′— A raw material is prepared by mixing 0.1 to 0.3 parts by mass of dicyclohexyl-2,6-naphthalenedicarboxyamide and mixing in advance at a predetermined ratio using a twin screw extruder. At this time, the melting temperature is preferably 270 to 300 ° C. It is preferable to remove foreign matters by providing a high-accuracy filter that can remove foreign matters of, for example, 20 μm or more on the downstream side of the twin screw extruder.

  Next, the above-mentioned mixed raw material is supplied to a single screw melt 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. At this time, for example, it is preferable to remove the foreign matter by providing a high-accuracy filter that can remove foreign matters of 20 μm or more.

  Moreover, it is preferable from the viewpoint of controlling the β crystal fraction in the unstretched sheet that the cast drum for obtaining the unstretched sheet has a surface temperature of 105 to 130 ° C. At this time, in particular, since the forming of the end portion of the sheet affects the subsequent stretchability, 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 based on the state of close contact of the entire sheet on the drum.

  Next, the obtained unstretched sheet 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, it is possible to stretch in the width direction after stretching in the longitudinal direction. preferable.

  As specific stretching conditions, first, the 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. As the stretching temperature in the longitudinal direction, it is preferable to employ a temperature of 110 to 135 ° C., more preferably 120 to 135 ° C., from the viewpoint of film characteristics and uniformity. The draw ratio is preferably 2 to 4.5 times, more preferably 2.5 to 4 times. If the draw ratio is less than 2, the air permeability resistance may be lowered, the battery characteristics may be deteriorated, and the productivity may be lowered. The higher the draw ratio is, the lower the air resistance becomes. However, if the stretch exceeds 4.5 times, the fibrils are excessively oriented in the longitudinal direction, so that the fibril orientation degree is sufficiently lowered when stretched in the width direction. Cannot be obtained, and the film has a slow propylene carbonate penetration rate in the thickness direction.

  Next, the uniaxially stretched polypropylene film is introduced into a tenter stretcher while gripping the film end, and stretched in the width direction to obtain a biaxially stretched film. The stretching temperature is preferably 140 to 155 ° C, and more preferably 145 to 153 ° C because a film having excellent propylene carbonate permeability in the thickness direction and low air resistance can be obtained without being excessively oriented in the transverse direction. The value of the transverse direction stretching ratio / longitudinal stretching ratio is preferably 2 or more, more preferably 2.5 or more, and further preferably 3 or more. When the value of transverse stretch ratio / longitudinal stretch ratio is less than 2, air resistance increases, battery characteristics may be deteriorated, and productivity may be deteriorated. In addition, as the value of the transverse draw ratio / longitudinal draw ratio is increased, the air permeation resistance decreases. However, if the transverse draw ratio / longitudinal draw ratio exceeds 5, the film may be easily broken. is there. The transverse stretching speed at this time may be constant, or the speed may be changed in the first, middle, and latter stages of stretching.

  Next, heat setting is performed in the stenter as it is, and the temperature is preferably from the transverse stretching temperature to 160 ° C., and the heat setting time is preferably 5 to 30 seconds. Furthermore, during heat setting, the film may be relaxed in the longitudinal direction and / or the width direction of the film. Particularly, the relaxation rate in the width direction is set to 5 to 12%, so that propylene carbonate permeability in the thickness direction and thermal dimensional stability are achieved. From the viewpoint of sex.

  Since the porous polypropylene film of the present invention has a high air permeability, the resistance as a separator is low, and among the above electricity storage devices, it can be preferably used for lithium ion batteries and lithium ion capacitors.

  Examples of the electricity storage device of the present invention include a non-aqueous electrolyte secondary battery using an organic solvent and an electric double layer capacitor. In particular, a lithium ion battery is suitable from the balance of battery capacity and output density. Since it can be used repeatedly by charging and discharging, it can be used for power supplies of IT equipment, daily life equipment, hybrid cars, electric cars and the like. In particular, a lithium ion battery is suitable for the above-mentioned use from the balance of battery capacity and output density. Since the electrical storage device using the porous film of the present invention has high air permeability, it can be suitably used as a power source for hybrid vehicles, 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.

(1) Thickness measurement At 25 ° C. and 65% RH, reading was performed up to 0.1 μm using an upright dial gauge R1-A (measuring element 10 mmφ, measurement load 50 g) manufactured by Ozaki Mfg. Co., Ltd. About the site | part periphery used for evaluation, it changed the place and measured 3 times and made the average value film thickness.

(2) Propylene carbonate penetration rate in the thickness direction The test was carried out at 25 ° C. and 65% RH. A sample having a known thickness (T: μm) is cut into a longitudinal direction of 50 mm and a width direction of 50 mm, and placed on a glass plate placed 15 mm above a 250 W metal halide light source. A detector (CCD line sensor camera E7450D manufactured by Electrosensory Devices Co., Ltd.) equipped with a camera lens AiMicro-Nikkor 55 mmF2.8S manufactured by Nikon Corporation is installed on the glass plate 250 mm. In this state, the amount of light transmitted through the film portion of the light irradiated from the metal halide light source was detected by a detector. 0.5 ml of propylene carbonate (manufactured by Nacalai Tesque Co., Ltd., Extra Pure Regent) was collected using a micropipette and dropped from 2 cm above the sample. Time measurement is started at the same time as the dripping, and the time (S: seconds) when the transmitted light amount is three times that before the dripping is converted into a value per 20 μm thickness according to the following formula. The speed (second / 20 μm) was used.

Propylene carbonate penetration rate in the thickness direction (sec / 20 μm) = S / T × 20
(3) Breaking strength According to JIS K 7127 (1999, test piece type 2), the film strength / elongation measuring device (AMF / RTA-100) manufactured by Orientec Co., Ltd. was used, and the temperature was 25 ° C. and 65% RH. The breaking strength was measured.

  For the breaking strength PTD in the width direction, a sample having a known thickness is cut into a size of 1 cm in the longitudinal direction and 15 cm in the width direction, and stretched at an original length of 50 mm and a pulling speed of 300 mm / min to measure the breaking strength (unit: MPa). did. The same measurement was performed 5 times, and the average value of the obtained breaking strengths was defined as the strength in the width direction of the sample.

  For the longitudinal direction breaking strength PMD, a sample having a known thickness is cut into a size of 1 cm in the width direction and 15 cm in the longitudinal direction, and the breaking strength PMD in the longitudinal direction is determined by the same measurement method and analysis method as the breaking strength PTD in the width direction. Obtained.

(4) β crystal forming ability 5 mg of a porous film was taken as a sample in an aluminum pan and measured using a differential scanning calorimeter (RDC220 manufactured by Seiko Denshi Kogyo). First, the temperature was raised from room temperature to 260 ° C. at 10 ° C./min (first run) in a nitrogen atmosphere, held for 10 minutes, and then cooled to 20 ° C. at 10 ° C./min. About the melting peak observed when the temperature is increased (second run) again at 10 ° C./min after holding for 5 minutes, the melting having a peak in the temperature range of 145 to 157 ° C. is the melting peak of β crystal, 158 ° C. or more 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
In addition, the β crystal fraction in the state of the sample can be calculated by calculating the abundance ratio of the β crystal in the same manner from the melting peak observed in the first run.

(5) Collision test Using a lithium ion battery automatic winder manufactured by Minato Seisakusho Co., Ltd., the porous polypropylene film of the present invention, an aluminum foil with a thickness of 100 μm, and a copper foil with a thickness of 100 μm were combined into a film / copper foil / film / It was piled up so that it might become aluminum foil, and it wound up by 1 m and the winding tension of 25 N / m in the longitudinal direction of a porous polypropylene film, and created the cylindrical layered product. A round bar was placed across the center of the cylindrical laminate. The round bar was placed so as to be parallel to the longitudinal direction of the separator. A heavy object of 20 kg, 30 kg, or 40 kg was dropped on the round bar from a height of 60 cm. The cylindrical laminate was disassembled and the state of the porous polypropylene film was evaluated according to the following criteria.

◎: No tearing at 20 kg, 30 kg, or 40 kg ○: Torn at 40 kg, but not torn at 20 kg and 30 kg ) Initial discharge capacity evaluation The positive electrode was manufactured by using a positive electrode having a lithium cobalt oxide (LiCoO ₂ ) thickness of 40 μm manufactured by Hosen Co., Ltd., and punching it into a square having a side of 100 mm.

  The negative electrode was manufactured by using a graphite negative electrode having a thickness of 50 μm manufactured by Hosen Co., Ltd., and punching it into a square having a side of 105 mm.

In the electrolyte solution, propylene carbonate (PC) was used as an organic solvent, and LiPF ₆ 1.0 mol / L was used as an indicator salt.

  The separator was produced by punching out a square with a side of 110 mm so that the area was larger than both the positive electrode and the negative electrode. Here, the porous polypropylene film of each example and comparative example was used as the separator.

  The positive electrode and the negative electrode were arranged so that the active material layers of the positive electrode and the negative electrode face each other, a separator was sandwiched between them, and a laminate film was further arranged on the exterior part, and three-sided sealing was performed. Make sure that the tip of each tab is pulled out to the outside of the laminate cell, and then fill the electrolyte solution in the laminate cell, then vacuum deaerate to seal the laminate cell, Assembled. About each produced secondary battery, it aged for 0.5 hour in 25 degreeC atmosphere. Thereafter, a charge / discharge operation was performed in which charging was performed at 300 mA to 4.2 V for 0.5 hour, and discharging was performed at 300 mA up to 2.7 V, and the discharge capacity was examined.

  The value obtained by the formula of [(discharge capacity) / (charge capacity)] × 100 was evaluated according to the following criteria. In addition, five test pieces were measured, and the average value was evaluated.

○: 90% or more Δ: 80% or more and less than 90% ×: One or more batteries that are less than 80% or less than 20% The present invention will be described more specifically based on the following examples. Needless to say, the present invention is not limited to these.

Example 1
97 parts by mass of homopolypropylene FSX80E4 (hereinafter referred to as PP-1; MFR = 8) manufactured by Sumitomo Chemical Co., Ltd. as a raw material resin for porous polypropylene film, manufactured by Dow Chemical Co., which is an ethylene-octene-1 copolymer N, N'-dicyclohexyl-2,6-naphthalenedicarboxyamide (Nippon Nippon Chemical Co., Ltd.), a β-crystal nucleating agent, was added to 3 parts by mass of Engage 8411 (melt index: 18 g / 10 min, hereinafter simply referred to as PE). Co., Ltd., Nu-100, hereinafter simply expressed as β crystal nucleating agent) is 0.3 part by mass, and further, IRGANOX1010 and IRGAFOS168 manufactured by Ciba Specialty Chemicals, which are antioxidants, are 0.1 and 0.1, respectively. The raw material is fed from the weighing hopper to the twin screw extruder so that the mass parts are mixed at this ratio, and melt kneading is performed at 300 ° C. There, ejected from the die into strands, cooled and solidified at 25 ° C. water bath, and a chip raw material and cut into chips.

  This chip is supplied to a single screw extruder and melt extruded at 220 ° C., passed through a 50 μm cut screen filter, discharged from a T-die to a cast drum whose surface temperature is controlled at 120 ° C., and indirectly in the drum for 15 seconds. To obtain an unstretched sheet. Next, preheating was performed using a ceramic roll heated to 125 ° C., and the film was stretched 3 times in the longitudinal direction of the film. Next, the end portion was introduced into a tenter type stretching machine by holding it with a clip, and after extending 11.2 times at 150 ° C., heat treatment was performed at 155 ° C. for 7 seconds while relaxing 10% in the width direction. . As a result, a 20 μm thick porous polypropylene film was obtained.

  The obtained porous polypropylene film had a high propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was good. Moreover, it had a high strength and a well-balanced strength ratio, and the result of the collision test was good.

(Example 2)
A porous polypropylene film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that the stretching ratio in the longitudinal direction was 4 times and the stretching temperature in the width direction was 154 ° C.

  The obtained porous polypropylene film had a high propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was good. Moreover, it had a high strength and a well-balanced strength ratio, and the result of the collision test was good.

(Example 3)
A porous polypropylene film having a thickness of 20 μm was obtained by performing the same operation as in Example 1 except that the draw ratio in the longitudinal direction was 4 times and the draw ratio in the width direction was 89.2 times.

  The obtained porous polypropylene film had a high propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was good. Moreover, it had a high strength and a well-balanced strength ratio, and the result of the collision test was good.

(Comparative Example 1)
A porous polypropylene film having a thickness of 20 μm is obtained by performing the same operation as in Example 1 except that the stretching ratio in the longitudinal direction is 4 times, the stretching ratio in the width direction is 6 times, and the temperature of the heat treatment is 160 ° C. It was.

  The obtained porous polypropylene film had a slow propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was somewhat insufficient. Further, the strength in the width direction was low, and the result of the collision test was insufficient.

(Comparative Example 2)
A porous polypropylene film having a thickness of 20 μm was carried out in the same manner as in Example 1 except that the draw ratio in the longitudinal direction was 4.5 times, the draw ratio in the width direction was 5 times, and the temperature of the heat treatment was 160 ° C. Got.

  The obtained porous polypropylene film had a slow propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was somewhat insufficient. Further, the strength in the width direction was low, and the result of the collision test was insufficient.

(Comparative Example 3)
A porous polypropylene film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that the draw ratio in the longitudinal direction was 5 times and the draw ratio in the width direction was 4.5 times.

  The obtained porous polypropylene film had a slow propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was insufficient. Further, the strength in the width direction was low, and the result of the collision test was insufficient.

(Comparative Example 4)
Example 1 except that the draw ratio in the longitudinal direction is 4 times, the draw ratio in the width direction is 8.5 times, the stretch temperature in the width direction is 135 ° C., and the relaxation rate in the width direction is 5%. Thus, a porous polypropylene film having a thickness of 20 μm was obtained.

  The obtained porous polypropylene film had a slightly slow penetration rate of propylene carbonate in the thickness direction, and the results of initial discharge capacity evaluation were somewhat insufficient. Moreover, although the intensity | strength of the width direction was high, the balance with the intensity | strength of a longitudinal direction was bad, and the result of the collision test was inadequate.

(Comparative Example 5)
The same operation as in Example 1 except that the stretching ratio in the longitudinal direction was 4 times, the stretching temperature in the longitudinal direction was 105 ° C., the stretching ratio in the width direction was 8 times, and the relaxation rate in the width direction was 5%. A porous polypropylene film having a thickness of 20 μm was obtained.

  The obtained porous polypropylene film had a slow propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was somewhat insufficient. Further, the strength in the width direction was low, and the result of the collision test was insufficient.

(Comparative Example 6)
A porous polypropylene film having a thickness of 20 μm was obtained by performing the same operation as in Comparative Example 6 except that the stretching temperature in the longitudinal direction was 140 ° C.

  The obtained porous polypropylene film had high strength, and the result of the collision test was good. However, the air permeability was poor and the propylene carbonate penetration rate in the thickness direction was slow, and the results of initial discharge capacity evaluation were insufficient.

(Comparative Example 7)
Commercially available Celgard “Celguard” 2400 was used as Comparative Example 37. “Celguard” 2400 is a microporous polypropylene film using a lamellar stretching method.

  The film had a slow propylene carbonate penetration rate in the thickness direction, and the initial discharge capacity evaluation was insufficient. Further, the strength in the width direction was low, and the result of the collision test was insufficient.

  The porous polypropylene film according to the present invention has a high strength in the width direction, a good balance with the strength in the longitudinal direction, excellent electrolyte absorption in the thickness direction, and excellent battery characteristics when used as a separator. A porous polypropylene film exhibiting safety can be provided.

Claims (5)

The breaking strength PTD in the width direction is 80 MPa or more and 200 MPa or less, the ratio of the breaking strength PTD in the width direction to the breaking strength PMD in the longitudinal direction (PTD / PMD) is 1 to 2, and the propylene carbonate penetration rate in the thickness direction Porous polypropylene film having a thickness of 0.01 to 10 seconds / 20 μm The porous polypropylene film according to claim 1, wherein the air permeability resistance is 10 to 1,000 seconds / 100 ml. The porous polypropylene film according to claim 1 or 2, which is used as a separator for an electricity storage device. A method for producing a porous polypropylene film by biaxial stretching, characterized in that when stretching an unstretched film made of a polypropylene-based resin having β crystal forming ability, the stretching ratio in the width direction exceeds 8 times. The manufacturing method of the porous polypropylene film in any one of Claims 1-3. The method for producing a porous polypropylene film according to claim 4, wherein the film is stretched in the width direction at a stretch ratio of 2 times or more than the stretch ratio in the longitudinal direction.