JP2009242631A - Microporous polyolefin membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microporous polyolefin membrane suitable as a separator capable of both the long-term reliability and the high output of an electricity storage device. <P>SOLUTION: A microporous polyolefin membrane includes a polyolefin resin and inorganic particles and is characterized by having a puncture strength of 2.4 N/20 μm or higher, a porosity of 50-90%, a lateral shrinkage (at 140°C) of 33% or lower, and a retention of a membrane thickness in penetration creep of 16% or higher. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polyolefin microporous membrane and a method for producing the same.

In recent years, power storage devices such as lithium ion secondary batteries and electric double layer capacitors (including those called lithium ion capacitors and non-aqueous lithium power storage elements) have been actively developed. In a power storage device, a microporous membrane (separator) is usually provided between the positive and negative electrodes. Such a separator has a function of preventing contact between positive and negative electrodes and transmitting ions.
Here, the separator is required to have a certain physical strength or more from the viewpoint of ensuring good safety of the electricity storage device. That is, pressure from the electrodes may be applied to the separator as the electricity storage device is charged / discharged, and the electrodes may break through the separator and cause a short circuit between the electrodes.
The separator is also required to have a low electrical resistance from the viewpoint of achieving high output of the electricity storage device.

  Under such circumstances, for example, Patent Document 1 proposes an electric double layer capacitor using a separator made of paper made of inorganic fibers. Patent Document 2 proposes an electric double layer capacitor using a separator made of polyolefin and inorganic powder. Patent Document 3 proposes a microporous film using ultrahigh molecular weight polyethylene. Furthermore, Patent Document 4 discloses an inorganic particle-containing polyolefin microporous membrane.

JP 2002-158143 A JP 2003-297678 A Japanese Patent No. 2794179 International Publication No. 2006/0253323 Pamphlet

However, all of the separators described in Patent Documents 1 to 4 have long-term reliability of the electricity storage device (short-circuiting due to separator film breakage is prevented for a long time, and safety as an electricity storage device is ensured for a long time. From the viewpoint of achieving both high output and high output, there is still room for improvement.
An object of the present invention is to provide a polyolefin microporous membrane suitable as a separator that can achieve both long-term reliability and high output of an electricity storage device.

The present inventors have focused not only on indices normally used for microporous membranes such as tensile strength and puncture strength, but also on long-term compression resistance in a microscopic region of the membrane as an index for improving long-term reliability in an electricity storage device.
That is, a model in which the active material slides down from the electrode due to friction between the electrode and the separator when charging and discharging are repeated many times in the electric storage device is conceivable. The slipped active material can cause the separator to break through. In addition, the active material shape of the electrode may not be smooth, and such a non-smooth shape may cause the separator to break through. In recent years, energy storage devices have been required to have higher energy density and higher output, and their winding structures and current collection structures have been diversified. However, the diversified shapes of energy storage devices have a negative pressure distribution in the battery. It tends to lead to homogenization. If the pressure distribution in the battery is non-uniform, it is considered that the above-mentioned slid active material or non-smooth active material shape tends to cause a deterioration in long-term reliability of the electricity storage device.
Then, the present inventors adopted the film thickness retention rate in piercing creep as an index indicating long-term compression resistance in a minute region of the film, set the index within a specific range, The present inventors have found that a polyolefin microporous membrane formed by appropriately selecting these parameters is suitable as a separator that can achieve both long-term reliability and high output of an electricity storage device, and has completed the present invention.

That is, the present invention is as follows.
[1] A film containing a polyolefin resin and inorganic particles, having a puncture strength of 2.4 N / 20 μm or more, a porosity of 50% or more and 90% or less, and a shrinkage ratio in the width direction at 140 ° C. of 33% or less. A polyolefin microporous membrane having a thickness retention of 16% or more.
[2] The polyolefin microporous membrane according to [1], wherein the thickness reduction rate during piercing creep is 10% or less.
[3] The polyolefin microporous membrane according to [1] or [2], wherein the average pore diameter is 0.2 μm or less and the curvature is 2.0 or less.
[4] The polyolefin microporous membrane according to any one of [1] to [3], wherein the content of the inorganic particles is 30% by mass or more and 70% by mass or less.
[5] The polyolefin microporous membrane according to any one of [1] to [4], wherein the inorganic particles have an average particle size of 1 nm to 100 nm.
[6] The polyolefin microporous film according to any one of [1] to [5], wherein the inorganic particles are silicon oxide.
[7] The polyolefin microporous membrane according to any one of [1] to [6], wherein the polyolefin resin has a viscosity average molecular weight of 50,000 to 10,000,000.
[8] The polyolefin microporous membrane according to any one of [1] to [7], wherein the polyolefin resin contains polypropylene in a proportion of 1% by mass to 50% by mass.
[9] A power storage device separator using the polyolefin microporous film according to any one of [1] to [8].
[10] An electricity storage device comprising the electricity storage device separator according to [9], a positive electrode, a negative electrode, and an electrolytic solution.
[11] A method for producing a polyolefin microporous membrane according to any one of [1] to [8], wherein the following steps (1) to (5):
(1) a kneading step of kneading a polyolefin resin, inorganic particles, and a plasticizer to form a kneaded product,
(2) After the kneading step, a molding step for processing the kneaded product into a sheet-like molded body,
(3) After the forming step, the sheet-like formed body is biaxially stretched at a surface magnification of 20 times to 200 times to form a stretched product,
(4) After the stretching step, a porous body forming step of forming a porous body by extracting a plasticizer from the stretched product,
(5) A heat treatment step in which, after the porous body forming step, heat treatment is performed on the porous body under a temperature condition of the melting point of the polyolefin resin or higher and the melting point + 40 ° C. or lower.
A method for producing a polyolefin microporous membrane comprising:

  The polyolefin microporous membrane of the present invention is suitable as a separator that can achieve both long-term reliability of an electricity storage device and high output.

  Hereinafter, the best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyolefin microporous film of the present embodiment is formed of a polyolefin resin composition containing a polyolefin resin and inorganic particles.
Examples of the polyolefin resin used in the present embodiment include polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene ( Homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more.
Examples of the polyolefin resin include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. It is done.

Here, from the viewpoint of reducing the melting point of the polyolefin microporous membrane or improving the puncture strength, the polyolefin resin preferably contains high-density polyethylene.
The proportion of the high density polyethylene in the polyolefin resin is preferably 10% by mass or more, more preferably 30% by mass or more, still more preferably 50% by mass or more, and may be 100% by mass.

From the viewpoint of improving the heat resistance of the polyolefin microporous membrane, the polyolefin resin preferably contains polypropylene.
The proportion of polypropylene in the polyolefin resin is preferably 1% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, and the upper limit is preferably 50% by mass or less, more preferably. It is 40 mass% or less, Most preferably, it is 30 mass% or less. Setting the ratio to 1% by mass or more is preferable from the viewpoint of improving the heat resistance of the polyolefin microporous membrane. On the other hand, setting the ratio to 50% by mass or less is preferable from the viewpoint of realizing a microporous film having good stretchability and high piercing strength.

Viscosity average molecular weight of the polyolefin resin (measured according to the measurement method in Examples described later. When a plurality of polyolefin resins are used, it means a value measured for each polyolefin resin). Is preferably 50,000 or more, more preferably 100,000 or more, and the upper limit is preferably 10 million or less, more preferably 3 million or less. Setting the viscosity average molecular weight to 50,000 or more is from the viewpoint of maintaining high melt tension during melt molding and ensuring good moldability, or from the viewpoint of increasing the strength of the microporous film by imparting sufficient entanglement. preferable. On the other hand, setting the viscosity average molecular weight to 10 million or less is preferable from the viewpoint of achieving uniform melt-kneading and improving sheet formability, particularly thickness stability. A viscosity average molecular weight of 3 million or less is preferable from the viewpoint of improving moldability.
From the viewpoint of improving moldability, it is preferable to use a mixture of several types of polyolefin resins having different viscosity average molecular weights.

  If necessary, the polyolefin resin composition may include an antioxidant such as phenol, phosphorus, or sulfur; a metal soap such as calcium stearate or zinc stearate; an ultraviolet absorber, a light stabilizer, an antistatic agent, Various additives such as an antifogging agent and a coloring pigment can be mixed and used.

  Examples of the inorganic particles include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide, and nitride such as silicon nitride, titanium nitride, and boron nitride. Ceramics, silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, zeolite And ceramics such as calcium silicate, magnesium silicate, diatomaceous earth, and silica sand, and glass fiber. These can be used alone or in combination of two or more. Among these, silica, alumina, and titanium are more preferable from the viewpoint of electrochemical stability. Silica is particularly preferable.

The average particle size of the inorganic particles is preferably 1 nm or more, more preferably 6 nm or more, still more preferably 10 nm or more, and the upper limit is preferably 100 nm or less, preferably 80 nm or less, more preferably 60 nm or less.
When the average particle size is 100 nm or less, even when stretching or the like is performed, peeling between the polyolefin resin and the inorganic particles tends not to occur, which is preferable from the viewpoint of suppressing the generation of macrovoids. Here, it is preferable that peeling between the polyolefin resin and the inorganic particles hardly occurs from the viewpoint of increasing the hardness of the fibril itself constituting the microporous membrane, and tends to be excellent in compression resistance in a local region of the polyolefin microporous membrane. Or a tendency to be excellent in heat resistance is observed, which is preferable. In addition, the close contact between the polyolefin resin and the inorganic particles improves the affinity of the electricity storage device separator with the non-aqueous electrolyte, and realizes a separator excellent in output retention performance, cycle retention performance, etc. To preferred.
On the other hand, setting the average particle size to 1 nm or more is preferable from the viewpoint of securing the dispersibility of the inorganic particles and improving the compression resistance in the local region.

Furthermore, blending inorganic particles having a particle size of 1 nm or more and 100 nm or less with respect to a composition containing polyethylene and polypropylene improves compatibility between polyethylene and polypropylene, and suppresses phase separation between the two. This is preferable from the viewpoint of securing stretchability.
The average particle size of the inorganic particles can be measured with a scanning electron microscope or a transmission electron microscope. That is, a 10 μm × 10 μm field of view magnified with a scanning electron microscope (SEM) is directly or after being printed on a photo from a negative, is read into an image analyzer, and the circle-converted diameter (area of each particle calculated from this is calculated. The number average value of the diameters of the same circles) can be the average particle diameter of the inorganic filler. However, if the staining boundary is unclear when inputting from the photograph to the image analysis apparatus, the photograph can be traced and input to the image analysis apparatus using this figure.

  Further, the oil absorption amount of the plasticizer (described later) of the inorganic particles is preferably 150 ml / 100 g or more, and the upper limit is preferably 1000 ml / 100 g or less, more preferably 500 ml / 100 g or less. Setting the oil absorption to 150 ml / 100 g or more is preferable from the viewpoint of suppressing formation of aggregates in a kneaded product containing a polyolefin resin, inorganic particles, and a plasticizer, and ensuring good moldability. Further, when a polyolefin microporous membrane is used as a separator for an electricity storage device, it is excellent from the viewpoint of ensuring the impregnation property and liquid retention property of the nonaqueous electrolytic solution, and maintaining the electricity storage device productivity and performance maintenance in long-term use. On the other hand, setting the oil absorption to 1000 ml / 100 g or less is preferable from the viewpoint of handleability of inorganic particles when producing a polyolefin microporous membrane.

  The proportion of the inorganic particles in the polyolefin microporous membrane is preferably 30% by mass or more, more preferably 35% by mass or more, and the upper limit is usually 70% by mass or less, preferably 67% by mass or less. . Setting the ratio to 30% by mass or more improves the heat shrinkage rate in the horizontal direction (width direction, TD direction) of the polyolefin microporous membrane at 140 ° C. from the viewpoint of forming the polyolefin microporous membrane with high porosity. It is preferable from the viewpoint of adjusting the film thickness retention ratio in the piercing creep and adjusting the film thickness reduction ratio small. On the other hand, setting the ratio to 70% by mass or less is preferable from the viewpoint of improving the film formability at a high draw ratio and improving the puncture strength of the polyolefin microporous film. Moreover, it is preferable that the said ratio shall be 30 mass% or more from a viewpoint of improving heat resistance.

As a manufacturing method of the polyolefin microporous film of this Embodiment, the manufacturing method including each process of following (1)-(5) can be used, for example.
(1) a kneading step of kneading a polyolefin resin, inorganic particles, and a plasticizer to form a kneaded product,
(2) After the kneading step, a molding step for processing the kneaded product into a sheet-like molded body,
(3) After the forming step, the sheet-like formed body is biaxially stretched at a surface magnification of 20 times to 200 times to form a stretched product,
(4) After the stretching step, a porous body forming step of forming a porous body by extracting a plasticizer from the stretched product,
(5) A heat treatment step in which, after the porous body forming step, heat treatment (thermal fixation and thermal relaxation) is performed on the porous body under a temperature condition of the melting point of the polyolefin resin or higher and the melting point + 40 ° C. or lower.

The plasticizer used in the step (1) is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin. Moreover, it is preferable that it is a liquid at normal temperature.
Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as diethylhexyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol; and the like.
In particular, when polyethylene is contained in the polyolefin resin, the use of liquid paraffin as a plasticizer suppresses interfacial peeling between the polyolefin resin and the plasticizer, and from the viewpoint of implementing uniform stretching, or from the viewpoint of realizing high piercing strength. preferable. In addition, it is preferable to use diethylhexyl phthalate from the viewpoint of increasing the load when melt-extruding the kneaded product and improving the dispersibility of the inorganic particles (realizing a high-quality film).

  The proportion of the plasticizer in the kneaded product is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 80% by mass or less, preferably 70% by mass or less. Setting the ratio to 80% by mass or less is preferable from the viewpoint of maintaining high melt tension during melt molding and ensuring moldability. On the other hand, setting the ratio to 30% by mass or more is preferable from the viewpoint of securing moldability and efficiently extending the lamellar crystals in the crystal region of the polyolefin resin. Here, the fact that the lamellar crystal is efficiently stretched means that the polyolefin chain is efficiently stretched without causing the polyolefin chain to be broken, and the formation of a uniform and fine pore structure, the strength of the polyolefin microporous film, It can contribute to improvement of crystallinity.

Examples of the method for kneading the polyolefin resin, the inorganic particles, and the plasticizer include the following methods (a) and (b).
(A) A method in which a polyolefin resin and inorganic particles are put into a resin kneading apparatus such as an extruder or a kneader, and a plasticizer is further introduced and kneaded while the resin is heated and melt-kneaded.
(B) A step of pre-kneading a polyolefin resin, inorganic particles and a plasticizer in advance at a predetermined ratio using a Henschel mixer or the like, and then introducing the kneaded product into an extruder and introducing a plasticizer while heating and melting. Kneading method.

In the preliminary kneading in the method (b), from the viewpoint of improving the dispersibility of the inorganic particles and carrying out stretching at a high magnification without breaking the film, the following formula (1) is applied to the polyolefin resin and the inorganic particles. It is preferable to pre-knead by blending an amount of plasticizer set in the range.
0.6 ≦ plasticizer weight / (plasticizer oil absorption amount × inorganic particle weight × plasticizer density) × 100 ≦ 1.2 (1)

  The step (2) is, for example, a step of extruding the kneaded material into a sheet shape via a T-die or the like and bringing it into contact with a heat conductor to cool and solidify. As the heat conductor, metal, water, air, plasticizer itself, or the like can be used. Moreover, it is preferable to cool and solidify by sandwiching between rolls from the viewpoint of increasing the film strength of the sheet-like molded body and improving the surface smoothness of the sheet-like molded body.

Examples of the stretching method in the step (3) include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Among these, it is preferable to employ the simultaneous biaxial stretching method from the viewpoint of increasing the puncture strength of the polyolefin microporous film and making the film thickness uniform.
Further, the surface magnification in the step (3) is preferably 20 times or more, preferably 25 times or more, and the upper limit is preferably 200 times or less, more preferably 100 times or less, and further preferably 70 times or less. is there. Setting the surface magnification to 20 times or more is preferable from the viewpoint of bringing the interface between the polyolefin resin and the inorganic particles into close contact and improving the compression resistance in a local and minute region of the polyolefin microporous membrane.

  The stretching temperature in the step (3) is preferably a melting point temperature of −50 ° C. or higher, more preferably a melting point temperature of −30 ° C. or higher, more preferably a melting point temperature of −20 ° C. or higher, with the melting point temperature of the polyolefin resin as a reference temperature. The upper limit is preferably a melting point temperature of −2 ° C. or lower, more preferably a melting point temperature of −3 ° C. or lower. Setting the stretching temperature to -50 ° C. or higher allows the interface between the polyolefin resin and the inorganic particles or the interface between the polyolefin resin and the plasticizer to adhere well, in a local and micro region of the polyolefin microporous membrane. It is preferable from the viewpoint of improving compression resistance. For example, when high density polyethylene is used as the polyolefin resin, the stretching temperature is preferably 115 ° C. or higher and 132 ° C. or lower. When a plurality of polyolefin resins are mixed and used, the melting point of the polyolefin resin having the larger heat of fusion can be used as a reference.

  The step (4) is preferably performed after the step (3) from the viewpoint of improving the puncture strength of the polyolefin microporous membrane. Examples of the extraction method include a method of immersing the stretched product in the plasticizer solvent. The residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by mass.

The step (5) is preferably a step of performing heat fixation and / or heat relaxation.
Here, the draw ratio in the step (5) is preferably less than 4 times, more preferably less than 3 times as the surface magnification. Setting the surface magnification to less than 4 times is preferable from the viewpoint of suppressing the generation of macrovoids and a decrease in puncture strength.
The heat treatment temperature is preferably the melting point temperature + 40 ° C. or lower, more preferably the melting point temperature + 30 ° C. or lower, based on the melting point temperature of the polyolefin resin, and the lower limit is preferably the melting point temperature or higher. Setting the heat treatment temperature to be equal to or higher than the melting point temperature is preferable from the viewpoint of suppressing the occurrence of film breakage and the like and reducing the heat shrinkage rate of the polyolefin microporous film under the condition of 140 ° C. On the other hand, setting the melting point temperature to 40 ° C. or lower is preferable from the viewpoint of suppressing the shrinkage of the polyolefin resin and reducing the thermal shrinkage rate of the polyolefin microporous membrane.

  In addition, you may post-process with respect to the obtained polyolefin microporous film after the process of said (5). Examples of such post-treatment include hydrophilization treatment with a surfactant and the like, and crosslinking treatment with ionizing radiation and the like.

The polyolefin microporous membrane of the present embodiment has a puncture strength (measured according to the measurement method in Examples described later) of 2.4 N / 20 μm or more, preferably 4 N / 20 μm or more. Preferably it is 20 N / 20 micrometers or less, More preferably, it is 10 N / 20 micrometers or less. Setting the puncture strength to 2.4 N / 20 μm or more is preferable from the viewpoint of suppressing membrane breakage due to the dropped active material or the like during battery winding. Moreover, the concern that a short circuit may occur due to the expansion and contraction of the electrode accompanying charging and discharging can be suppressed. On the other hand, setting it to 20 N / 20 micrometers or less is preferable from a viewpoint which can reduce the width | variety shrinkage by the orientation relaxation at the time of a heating.
The piercing strength can be adjusted by a method of adjusting the polyethylene molecular weight, the ratio of the polyolefin resin, the stretching temperature and the stretching ratio in the step (3), and the like.

The porosity of the microporous membrane (measured according to the measurement method in Examples described later) is 50% or more, preferably 55% or more, and the upper limit is preferably 90% or less, preferably 80% or less. It is. Setting the porosity to 50% or more is preferable from the viewpoint of securing output. On the other hand, it is preferable to set it as 90% or less from a viewpoint of ensuring piercing strength.
The porosity can be adjusted by adjusting the stretching temperature and stretching ratio in the step (3) and / or adjusting the temperature and magnification in the heat setting and thermal relaxation step in (5). is there.

The heat shrinkage in the width direction at 140 ° C. of the microporous membrane (measured according to the measurement method in Examples described later) is preferably 33% or less, more preferably 20% or less. Setting the heat shrinkage rate in the width direction at 140 ° C. to 33% or less reduces the possibility that shrinkage occurs and the electrodes come into contact with each other and a short circuit occurs when there is a heating process when creating an electricity storage device. obtain. Also, small shrinkage is preferable from the viewpoint of ensuring long-term reliability.
In addition, the said heat shrinkage rate can be adjusted with the method of adjusting the temperature of the heat fixation and heat relaxation process of said (5), and a magnification.

The film thickness retention rate (measured in accordance with the measurement method in Examples described later) of the microporous membrane is preferably 16% or more, more preferably 20% or more, and further preferably 25. The upper limit is preferably 95% or less, more preferably 90% or less, still more preferably 80% or less, and particularly preferably 60% or less. Setting the retention rate to 95% or less is preferable from the viewpoint of imparting sufficient flexibility to the microporous membrane for winding. On the other hand, the content of 16% or more is preferable from the viewpoint of realizing a polyolefin microporous film suitable as a separator capable of achieving both long-term reliability of an electricity storage device and high output.
The film thickness retention rate can be adjusted by a method of adjusting the polyethylene molecular weight, the inorganic ratio, the porosity, and the like.

The thickness reduction rate (measured according to the measurement method in the examples described later) of the microporous membrane in piercing creep is preferably 10% or less, more preferably 9% or less, and still more preferably 8 %, And the lower limit is preferably 0.1% or more. Setting the reduction rate to 10% or less is preferable from the viewpoint of realizing a polyolefin microporous film suitable as a separator capable of achieving both long-term reliability of an electricity storage device and high output. On the other hand, the content of 0.1% or more is preferable from the viewpoint of imparting sufficient flexibility to the microporous membrane to wind.
The film thickness reduction rate can be adjusted by a method for adjusting the polyethylene molecular weight, the inorganic ratio, the porosity, and the like.

The average pore diameter of the microporous membrane (measured according to the measurement method in the examples described later) is preferably 0.2 μm or less, more preferably 0.18 μm or less, and the lower limit is preferably 0.03 μm or more. It is. Setting the average pore size to 0.2 μm or less is preferable from the viewpoint of suppressing self-discharge of the electricity storage device and suppressing capacity reduction.
In addition, the said average hole diameter can be adjusted with the method etc. which adjust the temperature of the heat setting of said (5) and a thermal relaxation process, and a magnification.

The curvature of the microporous membrane (measured according to the measurement method in Examples described later) is preferably 2.0 or less, more preferably 1.8 or less, and preferably 0.5 as the lower limit. That's it. Setting the curvature to 2.0 or less is suitable from the viewpoint of obtaining a high-output power storage device with low electrical resistance.
In addition, the said curvature can be adjusted with the method etc. which adjust the temperature of the heat fixation of the said (5) and a thermal relaxation process, and a magnification.

  The final film thickness of the microporous membrane (measured according to the measurement method in Examples described later) is preferably 2 μm or more, more preferably 5 μm or more, and the upper limit is preferably 100 μm or less. More preferably, it is 60 micrometers or less, More preferably, it is 50 micrometers or less. Setting the film thickness to 2 μm or more is preferable from the viewpoint of improving the mechanical strength. On the other hand, a thickness of 100 μm or less is preferable because the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery.

The air permeability of the microporous membrane (measured according to the measurement method in Examples described later) is preferably 10 seconds or more, more preferably 50 seconds or more, and the upper limit is preferably 1000 seconds or less, preferably Is 500 seconds or shorter, more preferably 300 seconds or shorter. Setting the air permeability to 10 seconds or more is preferable from the viewpoint of suppressing self-discharge of the electricity storage device. On the other hand, setting it to 1000 seconds or less is preferable from the viewpoint of obtaining good charge / discharge characteristics.
In addition, the said air permeability can be adjusted with the method of adjusting the temperature of the heat fixation of the said (5) and a thermal relaxation process, a magnification, etc.

The microporous membrane is particularly useful as a power storage device separator using a non-aqueous electrolyte. In addition, the electricity storage device of the present embodiment uses the polyolefin microporous film described above as a separator, and includes a positive electrode, a negative electrode, and an electrolytic solution.
In the electricity storage device, for example, the microporous membrane is prepared as a vertically long separator having a width of 10 to 500 mm (preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000 to 4000 m). -Separator-negative electrode-separator or negative electrode-separator-positive electrode-separator are stacked in this order and wound into a circular or flat spiral shape to obtain a wound body, which is then housed in a battery can and further electrolyzed. It can be manufactured by injecting a liquid.
The electric storage device is manufactured through a process of laminating a flat plate in the order of positive electrode-separator-negative electrode-separator or negative electrode-separator-positive electrode-separator, laminating with a bag-like film, and injecting an electrolyte solution. You can also.

  The power storage device of this embodiment is particularly useful as an electric vehicle or a hybrid vehicle because it has high output and excellent long-term reliability.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Film thickness It measured at room temperature 23 degreeC using the micro thickness measuring device (type KBM by Toyo Seiki).

(2) Porosity A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula: did.
Porosity (%) = (volume-mass / density of mixed composition) / volume × 100
In addition, the value calculated | required by calculation from the density and mixing ratio of each used polyolefin resin and an inorganic particle was used for the density of a mixed composition.

(3) Air permeability It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117.

(4) Puncture strength A puncture test was performed using a Kato Tech, trademark, KES-G5 handy compression tester under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. Strength (N) was used.

(5) Piercing creep (film thickness retention rate, film thickness reduction rate)
The measurement was performed with a constant pressure thickness measuring instrument “PEACOCK FFA-1 manufactured by Ozaki Mfg. Co., Ltd.” using a needle-shaped measuring probe (XT-4 manufactured by Ozaki Mfg. Co., Ltd.) having a tip of φ0.1 mm as shown in FIG.
With the measurement force of 1.25 N being continuously applied, the film thickness after 1 hour is d _[1h] , the film thickness after 24 hours is d _[24h], and the film thickness before measurement is D. The film thickness retention rate and the film thickness reduction rate were calculated using the following equations.
Measurement was carried out by arbitrarily selecting five locations, and the average value was taken as the characteristic value. In piercing creep evaluation, when the time axis is a common logarithm (log [time]), the film thickness retention rate decreases approximately linearly, so the slope (reduction rate) is defined as the film thickness reduction rate. did.
Film thickness retention rate (%) = (d _[24h] / D) × 100
Film thickness reduction rate (%) = [{(d _[1h] −d _[24h] ) / D} × 100] / log ₁₀ 24

(6) Viscosity average molecular weight (Mv)
In order to prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol is dissolved in decahydronaphthalene to a concentration of 0.1 w%, and this (hereinafter abbreviated as DHN) is used as the sample solvent. .
A sample solution is prepared by dissolving the sample in DHN at 150 ° C. to a concentration of 0.1 w%. 10 ml of the prepared sample solution is sampled, and the number of seconds passing through the marked line (t) at 135 ° C. is measured with a Canon Fenceke viscometer (SO100). When inorganic particles contained in the microporous membrane, a solution in which the microporous membrane was dissolved in DHN was filtered, and after removing the inorganic particles, a sample was collected. When inorganic particles can be dissolved and removed, a microporous film from which inorganic particles have been previously removed may be used. Further, after heating the DHN to 0.99 ° C., and 10ml collected to measure the number of seconds passing between marked lines of the viscometer in the same manner _{(t B).} The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and tB.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834
Mv was calculated from the obtained [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67

(7) Average pore diameter (μm), bending rate (curvature)
It is known that the fluid inside the capillary follows the Knudsen flow when the mean free path of the fluid is larger than the pore size of the capillary, and the Poiseuille flow when it is small. Therefore, it is assumed that the air flow in the measurement of the permeability of the microporous membrane follows the Knudsen flow, and the water flow in the measurement of the permeability of the microporous membrane follows the Poiseuille flow.
In this case, the average pore diameter d (μm) and the bending rate τ (dimensionless) are the air transmission rate constant R _gas (m ³ / (m ² · sec · Pa)) and the water transmission rate constant R _liq (m ³ / (M ² · sec · Pa)), air molecular velocity ν (m / sec), water viscosity η (Pa · sec), standard pressure P _s (= 101325 Pa), porosity ε (%), film thickness From L (μm), it can be obtained using the following equation.
d = 2ν × (R _liq / R _gas ) × (16η / 3P _s ) × 10 ⁶
τ = (d × (ε / 100) × ν / (3L × P _s × R _gas )) ^1/2
Here, R _gas is obtained from the air permeability (sec) using the following equation.
R _gas = 0.0001 / (air permeability × (6.424 × 10 ⁻⁴ ) × (0.01276 × 101325))
R _liq is obtained from the water permeability (cm ³ / (cm ² · sec · Pa)) using the following equation.
R _liq = water permeability / 100
In addition, water permeability is calculated | required as follows. A microporous membrane previously immersed in alcohol is set in a stainless steel permeation cell having a diameter of 41 mm, and after the alcohol in the membrane is washed with water, water is allowed to permeate at a differential pressure of about 50000 Pa, and 120 seconds have elapsed. The water permeability per unit time, unit pressure, and unit area was calculated from the water permeability (cm ³ ) at the time, and this was taken as the water permeability.
Ν is a gas constant R (= 8.314), an absolute temperature T (K), a circumference ratio π, and an average molecular weight M of air (= 2.896 × 10 ⁻² kg / mol), using the following formula. Desired.
ν = ((8R × T) / (π × M)) ^1/2

(8) Measurement of shrinkage in the width direction at 140 ° C (140 ° C TD shrinkage)
Marks at intervals of 100 mm are placed in the lateral direction (TD direction) of the sample cut into 120 mm × 120 mm. Measure the space between marks with a gold scale and set it to T0.
Place between copy paper and leave in an oven heated to 140 ° C. After 1 hour, it was taken out from the oven and cooled in an atmosphere at 23 ° C. for 1 hour, and then the distance between the marks was measured using a metal ruler and defined as T1. The heat shrinkage rate is calculated by the following formula.
Shrinkage rate (%) in the width direction at 140 ° C. = (T0−T1) / T0 × 100

(9) Internal resistance (measurement of internal resistance of electric double layer capacitor)
On the aluminum foil, a conductive paste containing fine particle graphite powder and ethylene-acrylic acid resin as a binder was applied and dried to provide a 5 μm conductive layer. Next, a commercially available activated carbon, carbon black as a conductor, and PTFE (polytetrafluoroethylene) 8: 1: 1 mixed as a binder were applied to one side of the aluminum foil to prepare an electrode, and an electrolyte solution A single-layer laminate cell using a 1.5M triethylmethylammonium tetrafluoroborate propylene carbonate solution was prepared, and the internal resistance of the separator was measured. The measurement value index was expressed with the measurement value of Example 1 as 100. A low internal resistance of the electricity storage device means that the electricity storage device has a high output.

(10) Melting point It measured using DSC60 by Shimadzu Corporation. A perforated sheet was punched into a circle with a diameter of 5 mm, and several sheets were overlapped to give 3 mg as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endothermic curve. The peak top temperature of the obtained melting endotherm curve was defined as the melting point (° C.).
(11) Long-term reliability The single-layer laminate cell prepared by measuring the internal resistance in (9) was used. The initial discharge capacity (I) was measured, and a float test (a constant temperature bath at a temperature of 60 ° C., a constant voltage of 2.8 V) was performed. The discharge capacity (A) after 500 hours was measured. The capacity retention ratio (A / I × 100%) at this time is calculated, and the judgment of acceptance is that 95% or more is sufficiently acceptable (◎), 90% or more but less than 95% is acceptable (◯), and capacity maintenance ratio is 90%. Less than was rejected (x).

[Example 1]
18 parts by mass of high-density polyethylene “SH800” (manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 270,000, and 12 parts by mass of ultrahigh molecular weight polyethylene “UH850” (manufactured by Asahi Kasei Chemicals Corporation) having an Mv of 2 million. 20 parts by mass of silica “DM10C” (manufactured by Tokuyama Corporation) having an average primary particle size of 15 nm, and 30 parts by mass of liquid paraffin “Smoyl P-350P” (manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer A mixture of 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. In addition, liquid paraffin is side-fed to the twin-screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) to be melt kneaded and extruded is 60 parts by mass. After that, the sheet was extruded between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin resin composition having a thickness of 2000 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 126 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Further, in order to perform heat treatment (HS), it was led to a horizontal tenter stretching machine and stretched 1.7 times in the lateral direction (maximum magnification), and then the final outlet was relaxed to 1.5 times (exit magnification) and wound. . The set temperature of the transversely stretched portion is 143 ° C. and the set temperature of the relaxed portion is 148 ° C. The film forming conditions and film characteristics are shown in Table 1.

[Example 2]
The silica “DM10C” (manufactured by Tokuyama Corporation) having an average primary particle size of 15 nm is added to 15 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1, 10 parts by mass of ultra high molecular weight polyethylene of Mv 2 million. 40 parts by mass of liquid paraffin was added to parts by mass and premixed with a super mixer. The heat treatment conditions were relaxed so that the final outlet became 1.4 times after stretching by 1.7 times in the transverse direction. The set temperature of the transversely stretched portion is 143 ° C. and the set temperature of the relaxed portion is 148 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 3]
The condition of transverse stretching in Example 2 was relaxed so that the final outlet became 1.05 times after stretching 1.45 times in the transverse direction, and winding was performed. The set temperature of the transversely stretched portion is 145 ° C and the set temperature of the relaxed portion is 150 ° C. A microporous membrane was obtained in the same manner as Example 2 except for these. The film forming conditions and film characteristics are shown in Table 1.

[Example 4]
The silica of “DM10C” (trademark, (1) having an average primary particle size of 15 nm with the polyethylene of Example 1 being 10.8 parts by mass of high density polyethylene of Mv 270,000 and 7.2 parts by mass of ultra high molecular weight polyethylene of Mv 2 million. Tokuyama Co., Ltd.) was 22 parts by mass, and the transverse stretching conditions were stretched 1.7 times in the transverse direction, and then the final outlet was relaxed to 1.4 times and wound up. The set temperature of the transverse stretch portion is 140 ° C., and the set temperature of the relaxation portion is 150 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 5]
The polyethylene of Example 1 was changed to 30 parts by mass of Mv 270,000 high-density polyethylene, and the conditions of transverse stretching were relaxed so that the final exit became 1.4 times after stretching 1.5 times in the transverse direction, and winding was performed. It was. The set temperature of the transversely stretched portion is 145 ° C and the set temperature of the relaxed portion is 150 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 6]
The polyethylene of Example 1 was changed to 20 parts by mass of Mv 900,000 high-density polyethylene, 30 parts by mass of silica “DM10C” (trademark, manufactured by Tokuyama Corporation) having an average primary particle size of 15 nm, and liquid paraffin “Smoyl” as a plasticizer. 45 parts by mass of “P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.), and the condition of transverse stretching is relaxed so that the final exit becomes 1.4 times after stretching 1.5 times in the transverse direction. Winding was performed. The set temperature of the transversely stretched portion is 145 ° C and the set temperature of the relaxed portion is 150 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.
[Example 7]
18 parts by mass of high-density polyethylene of Mv 270,000 of Example 1, 12 parts by mass of ultra high molecular weight polyethylene of Mv 2 million, and 36 parts of alumina “AluC” (trademark, manufactured by Degussa) having an average primary particle size of 13 nm. 40 parts by mass of liquid paraffin in the part and premixed with a super mixer, and after conducting a heat treatment to the transverse tenter and stretching 1.3 times in the transverse direction, the final outlet is relaxed to 1.1 times Winding was performed. The set temperature of the transverse stretch portion is 145 ° C., and the set temperature of the relaxation portion is 148 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.
[Example 8]
To heat-treat the polyethylene of Example 1 with 22 parts by mass of Mv 2 million ultra high molecular weight polyethylene and 41 parts by mass of silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having an average primary particle size of 15 nm. After being led to a transverse tenter and stretched 1.4 times in the transverse direction, the final exit was relaxed to 1.3 times and wound up. The set temperature of the transverse stretch portion is 145 ° C., and the set temperature of the relaxation portion is 148 ° C. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 9]
12 parts by mass of high density polyethylene “SH850” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH650” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with an Mv of 800,000 20 parts by mass of silica “NIPSIL-LP” (trademark, manufactured by Tosoh Silica Co., Ltd.) having an average primary particle size of 15 nm, 45 parts by mass of ethylhexyl phthalate as a plasticizer, and pentad as an antioxidant A material to which 0.3 parts by mass of erythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added was adjusted with a supermixer, led to an extruder, a gear pump, Extruded between cooling rolls controlled to a surface temperature of 25 ° C. through a conduit and a T-die, a sheet-like polyolefin having a thickness of 1300 μm To obtain a resin composition. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 4 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 126 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. In order to perform further heat treatment, the film was led to a transverse tenter and stretched 1.7 times in the transverse direction, and then the final outlet was relaxed to 1.4 times and wound up. The set temperature of the transversely stretched portion is 143 ° C. and the set temperature of the relaxed portion is 148 ° C. The film forming conditions and film characteristics are shown in Table 1.

[Example 10]
The silica of Example 1 was changed to silica “QS-10” (trademark, manufactured by Tokuyama Co., Ltd.) having an average primary particle size of 15 nm, and the transverse stretching conditions were finalized by 1.7 times in the transverse direction. The outlet was relaxed and wound up to 1.4 times. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 11]
14 parts by mass of a high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 270,000 of Example 1 and an ultrahigh molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals (trade name, manufactured by Asahi Kasei Chemicals Corporation) Co., Ltd.) was changed to 10 parts by mass, and the homopolypropylene “H-100M” (prime polymer) with Mv 400,000 was changed to 6 parts by mass. Was relaxed to 1.4 times and wound up. Except for these, a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 12]
The conditions for transverse stretching in Example 1 were set so that the final outlet became 1.4 times after stretching by a factor of 1.7 in the transverse direction. The procedure was the same as in Example 1 except that the temperature was 130 ° C. The film forming conditions and film characteristics are shown in Table 1.

[Comparative Example 1]
12.8 parts by mass of the high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 270,000 in Example 1 and ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei) having an Mv of 2 million 19.2 parts by mass of Chemicals Co., Ltd. and 8 parts by mass of silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having an average primary particle size of 15 nm are introduced into a horizontal tenter for heat treatment and transverse direction Then, after the film was stretched 1.7 times, the final outlet was relaxed so as to be 1.4 times and wound up. The set temperature of the transversely stretched portion is 143 ° C. and the set temperature of the relaxed portion is 148 ° C. Except for these, a film was formed in the same manner as in Example 1, but the film after the heat treatment became transparent and a microporous film could not be obtained. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 2]
The conditions for performing the heat treatment in Comparative Example 1 were the same as those in Comparative Example 1 except that the heat treatment was performed at a set temperature of 130 ° C. for the transversely stretched portion and a set temperature of 135 ° C. for the relaxed portion. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 3]
24 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with an Mv of 2 million In a super mixer, 16 parts by mass and 0.3 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant were added. Premixed. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. In addition, liquid paraffin is side-fed to the twin-screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) to be melt kneaded and extruded is 60 parts by mass. After that, the sheet was extruded between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin resin composition having a thickness of 2000 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 126 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. In order to perform further heat treatment (HS), it was led to a horizontal tenter stretching machine and stretched 1.7 times in the lateral direction (maximum magnification), and then the final outlet was relaxed to 1.4 times (exit magnification) and wound. . The set temperature of the transverse stretch portion is 128 ° C., and the set temperature of the relaxation portion is 128 ° C. Table 2 shows the film forming conditions and film characteristics.
[Comparative Example 4]
High-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000 is 45 parts by mass. After that, the final exit was relaxed so as to be 1.5 times and wound up. The set temperature of the transverse stretch portion is 125 ° C., and the set temperature of the relaxation portion is 130 ° C. Except these, it carried out similarly to the comparative example 3. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 5]
8 parts by mass of high-density polyethylene “GUR2122” (trademark, manufactured by Ticona) having a viscosity average molecular weight (Mv) of 2,680,000 and silica “Aerosil200” (trademark, manufactured by Nippon Aerosil Co., Ltd.) having an average primary particle size of 12 nm are 10 82 parts by mass of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer and pentaerythrityl-tetrakis- [3- (3,5-di -T-Butyl-4-hydroxyphenyl) propionate] was added in advance by a Henschel mixer. The obtained mixture is fed to the feed port of the twin screw co-axial screw extruder by a feeder, extruded through a gear pump, a conduit, and a T die, and rapidly cooled by a water bath to obtain a sheet-like molded product having a thickness of about 5 mm. It was. The obtained sheet-like molded product was preheated to 150 ° C. and then heated and rolled at 120 ° C. to obtain a sheet-like molded product having a thickness of 0.2 mm. After removing liquid paraffin from the sheet-like molded product using methylene chloride, biaxial stretching was performed at 120 ° C. twice in the longitudinal direction and twice in the transverse direction using a biaxial stretching machine manufactured by Iwamoto Seisakusho. Next, the remaining liquid paraffin was removed in heptane with the four sides fixed with a stainless steel frame, and then dried at room temperature to obtain a microporous membrane. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 6]
Silica “Aerosil200” (trademark, Nippon Aerosil Co., Ltd.) having 22 parts by mass of an ultrahigh molecular weight polyethylene “UH850” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 2 million. ) Manufactured by Hydrophobic treatment not carried out) 25 parts by mass, liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as plasticizer, 53 parts by mass, and pentaerythrityl-tetrakis- as antioxidant What added [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] 0.3 parts by mass was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. The melt kneading conditions in the extruder were performed at a preset temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded material was extruded between cooling rolls controlled to a surface temperature of 25 ° C. through a gear pump, a conduit, and a T-die each set to 200 ° C. to obtain a sheet-like polyolefin resin composition having a thickness of 200 μm. . Next, it was continuously guided to a roll stretching machine, and uniaxial stretching was performed 6 times in the longitudinal direction. At this time, the set temperature of the roll stretching machine is 120 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 7]
36 parts by mass of high density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 270,000, 9 parts by mass of homopolypropylene “H-100M” (manufactured by Prime Polymer) having an Mv of 400,000 What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was premixed with a Henschel mixer. The resulting mixture is fed to the feed port of the twin screw co-axial screw extruder by a feeder, led to a transverse tenter for heat treatment and stretched 1.7 times in the transverse direction, and the final outlet is 1.4 times. Relaxed and wound up. The set temperature of the transversely stretched portion is 133 ° C., and the set temperature of the relaxed portion is 133 ° C. Except these, it carried out similarly to the comparative example 2, and obtained the microporous film. Table 2 shows the film forming conditions and film characteristics.

  As is clear from the results in Tables 1 and 2, the electricity storage device formed using the microporous membrane of the present embodiment achieves both long-term reliability and high output of the electricity storage device.

It is the schematic of the needle-shaped measuring element used for a front-end | tip part in a piercing creep test.

Claims (11)

Including polyolefin resin and inorganic particles,
The puncture strength is 2.4 N / 20 μm or more, the porosity is 50% or more and 90% or less, the shrinkage ratio in the width direction at 140 ° C. is 33% or less, the film thickness retention in piercing creep is 16% or more,
A polyolefin microporous membrane characterized by the above.   The polyolefin microporous film according to claim 1, wherein the film thickness reduction rate in piercing creep is 10% or less.   The polyolefin microporous membrane according to claim 1 or 2, wherein the average pore diameter is 0.2 µm or less and the curvature is 2.0 or less.   The polyolefin microporous film according to any one of claims 1 to 3, wherein the content of the inorganic particles is 30% by mass or more and 70% by mass or less.   5. The polyolefin microporous membrane according to claim 1, wherein the inorganic particles have an average particle size of 1 nm to 100 nm.   The polyolefin microporous film according to claim 1, wherein the inorganic particles are silicon oxide.   The polyolefin microporous membrane according to any one of claims 1 to 6, wherein the polyolefin resin has a viscosity average molecular weight of 50,000 to 10,000,000.   The polyolefin microporous membrane according to any one of claims 1 to 7, wherein the polyolefin resin contains polypropylene in a proportion of 1% by mass to 50% by mass.   The separator for electrical storage devices formed using the polyolefin microporous film in any one of Claims 1-8.   The electrical storage device containing the separator for electrical storage devices of Claim 9, a positive electrode, a negative electrode, and electrolyte solution. It is a manufacturing method of the polyolefin microporous film in any one of Claims 1-8, Comprising: Each process of the following (1)-(5),
(1) a kneading step of kneading a polyolefin resin, inorganic particles, and a plasticizer to form a kneaded product,
(2) After the kneading step, a molding step for processing the kneaded product into a sheet-like molded body,
(3) After the forming step, the sheet-like formed body is biaxially stretched at a surface magnification of 20 times to 200 times to form a stretched product,
(4) After the stretching step, a porous body forming step of forming a porous body by extracting a plasticizer from the stretched product,
(5) A heat treatment step in which, after the porous body forming step, heat treatment is performed on the porous body under a temperature condition of the melting point of the polyolefin resin or higher and the melting point + 40 ° C. or lower.
A method for producing a polyolefin microporous membrane comprising:

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