JP6208187B2 - Polyolefin microporous membrane, power storage device separator, and power storage device - Google Patents

Description

  The present invention relates to a polyolefin microporous membrane, a separator for an electricity storage device, and an electricity storage device.

  BACKGROUND ART A polyolefin microporous membrane (hereinafter sometimes simply referred to as “PO microporous membrane”) is widely used as a separator for various substances, a permselective separation membrane, a separator, and the like. Applications include, for example, microfiltration membranes; separators for batteries such as lithium ion batteries and fuel cells; separators for capacitors; base materials for functional membranes for filling functional materials into holes and causing new functions to appear. Is mentioned. Among these, a PO microporous film is suitably used as a separator for lithium ion batteries widely used in notebook personal computers (PCs), mobile phones, tablet PCs, digital cameras, and the like.

In recent years, power storage devices such as lithium ion batteries have been increasing in capacity and the energy density per cell has increased, so ensuring safety has become important.
Regarding the safety of batteries, one of the important factors is that they will not ignite or explode in the event of a collision or collapse.

  Ignition and explosion in the event of a collision or crushing is due to an internal short circuit due to the film breaking of the separator. Therefore, it is required to prevent internal shortages due to impact or the like by increasing the film breaking strength of the separator.

  In addition, when a lithium ion secondary battery generates heat, a separator made of a PO microporous membrane is closed by melting and shrinking of the separator, and can be insulated from each other by shutting down, reducing the risk of ignition, explosion, etc. Can be lowered.

  On the other hand, if the shrinkage of the separator is too large, the electrode at the end of the film is exposed, a short circuit occurs, and further heat generation may occur. Therefore, it is preferable that the separator does not shrink until the melting point of the polyolefin resin where the shutdown temperature occurs.

  On the other hand, in recent years, thorium ion batteries for in-vehicle use have been actively developed, and there is a demand for maintaining performance in a high temperature environment such as use in an engine room or in a high temperature area.

  Therefore, if the film shrinks at a high temperature of about 100 ° C., the internal resistance increases and the battery capacity decreases.

  For these reasons, it is required that the separator does not shrink up to the shutdown temperature when the cell generates heat, but quickly shrinks when the shutdown occurs. Furthermore, high mechanical strength is required because an internal short circuit does not occur when an impact such as a collision is applied.

  Further, since the amount of the electrode and separator wound in a unit volume is increased by increasing the capacity of the cell, the winding tension of the electrode and separator tends to be increased. As a result, there is a problem that the winding pin for winding the separator and the electrode is difficult to come off. For this reason, there has been a demand for separators that have good pin pull-out properties during winding.

  In this respect, Patent Documents 1 and 2 show that the external maximum stress 2 mN / μm standardized by the isolation film thickness is added, and the maximum transverse shrinkage of the TMA (Thermal Mechanical Analysis) measurement performed in the low stress mode is 0. When the microporous membrane is left at 120 ° C. for 1 hour, a PO microporous membrane having a shrinkage rate of 10% or less in the vertical and horizontal directions, high perforation strength, excellent film breaking property and heat stability It is disclosed. Such a PO microporous membrane does not expose the electrodes when the battery becomes hot, prevents internal short circuit and reduces the risk of ignition and explosion.

Patent No. 5127671 Japanese Patent No. 5592745

  However, such a PO microporous membrane does not shrink even when it reaches a temperature at which shutdown begins, and there is a problem in that insulation between electrodes due to instantaneous shutdown does not occur.

  The present invention has been made in view of the above-mentioned problems, and is excellent in performance at high temperatures when used as a lithium ion secondary battery and other separator for electrochemical devices, and can impart high heat safety. Another object of the present invention is to provide a polyolefin microporous membrane, a power storage device separator comprising the polyolefin microporous membrane, and a power storage device using the power storage device separator.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by having a thermomechanical analysis primary differential curve having a predetermined characteristic in thermomechanical analysis measurement in the film width direction. It came to make this invention.

That is, the present invention is as follows.
[1]
In the thermomechanical analysis first derivative curve obtained by thermomechanical analysis measurement in the membrane width direction of the polyolefin microporous membrane,
The maximum value of the differential value in the region of less than the polyolefin microporous membrane 10 ° C. lower than the melting point Tm of (Tm-10 ° C.) is state, and are 0.1 or less,
The polyolefin microporous membrane 10 ° C. lower than the melting point Tm of (Tm-10 ° C.) or higher, the maximum value of a differential value in the region lower than the melting point Tm of the polyolefin microporous membrane, Ru der 0.15 or more,
Polyolefin microporous membrane.
[2]
In the thermomechanical analysis first derivative curve obtained by thermomechanical analysis measurement in the membrane width direction of the polyolefin microporous membrane,
The maximum value of the differential value in the region below the temperature (Tm−10 ° C.) 10 ° C. lower than the melting point Tm of the polyolefin microporous membrane is 0.1 or less,
Both the longitudinal tensile strength and the membrane width direction tensile strength are 1500 kg / cm ² or more.
Polyolefin microporous membrane.
[3]
In the thermomechanical analysis first derivative curve,
The maximum value of the differential value in the region below 10 ° C. lower than the melting point Tm of the polyolefin microporous membrane (Tm−10 ° C.) is 0.05 or less.
The polyolefin microporous membrane according to [1] or [2].
[4]
In the thermomechanical analysis first derivative curve,
The maximum value of the differential value in the region below the melting point Tm of the polyolefin microporous membrane is 0.2 or more, at a temperature lower than the melting point Tm of the polyolefin microporous membrane by 10 ° C (Tm-10 ° C) or higher.
[1] to [3] The polyolefin microporous membrane according to any one of [1] to [3].
[5]
In the thermomechanical analysis first derivative curve,
The minimum value of the thermomechanical analysis differential value in the region below the melting point Tm of the polyolefin microporous membrane and less than the temperature (Tm + 5 ° C) 5 ° C higher than the melting point Tm of the polyolefin microporous membrane is 0.2 or more.
[1] to [4] The polyolefin microporous membrane according to any one of [1] to [4].
[6]
The dynamic friction coefficient in the film width direction is 0.4 or less,
[1] to [5] The polyolefin microporous membrane according to any one of [1] to [5].
[7]
The film thickness 12 μm equivalent air permeability is 150 seconds / 100 cc or less,
[1] to [6] The polyolefin microporous membrane according to any one of [1] to [6] .
[8]
The film thickness is 5 μm or more and 20 μm or less,
[1] to [7] The polyolefin microporous membrane according to any one of [1] to [7] .
[9]
[1] A separator for an electricity storage device, comprising the polyolefin microporous membrane according to [8] .
[10]
[9] An electricity storage device comprising the electricity storage device separator according to [9] .

  According to the present invention, it is possible to provide a PO microporous membrane that is excellent in performance at high temperatures and can provide high heat safety when used as a separator for lithium ion secondary batteries and other electrochemical devices. it can.

The thermomechanical analysis primary differential curve in Example 2 is shown.

  Hereinafter, although the form for implementing this invention (henceforth abbreviated as "this embodiment") is demonstrated in detail, this invention is not limited to this and does not deviate from the summary. Various modifications are possible within the range.

[Polyolefin microporous membrane]
The PO microporous membrane of the present embodiment is a temperature (Tm−) lower than the melting point Tm of the polyolefin microporous membrane in the first-order differential curve of thermomechanical analysis obtained by TMA measurement in the membrane width direction (TD) of the PO microporous membrane. The maximum value of the differential value in the region below 10 ° C. (hereinafter also referred to as “region I”) is 0.1 or less. Here, the “thermomechanical analysis primary differential curve” is a curve obtained by linearly differentiating a thermomechanical analysis curve (hereinafter also referred to as “TMA curve”) composed of compressive stress and temperature.

(Maximum differential value in region I)
The maximum value of the differential value in the region I is 0.1 or less, preferably 0.075 or less, and more preferably 0.05 or less. Further, the lower limit of the maximum differential value in the region I is not particularly limited, but is 0 or more.

  In TMA measurement, a sample cut into 15 mm × 3 mm so that the longitudinal direction of the sample is parallel to TD is used. “TD” refers to a direction perpendicular to MD when the longitudinal direction (raw material resin discharge direction and machine direction) in the extrusion process is MD (hereinafter abbreviated as “MD”) (hereinafter referred to as “TD”). Abbreviation). This sample (PO microporous membrane) is fixed to the chuck with a width of 10 mm in the longitudinal direction, the initial load is 0.0049 N (0.5 gf), and the constant length mode is 250 at a rate of 30 ° C. to 10 ° C./min. The temperature is raised to ° C. At this time, the chucked test piece is held in a constant displacement state, and the stress at each temperature is measured at intervals of 1 second. A TMA curve showing the relationship between temperature and shrinkage stress is drawn from the obtained data, and a TMA primary differential curve is obtained by differentiating the stress value with temperature. The TMA first derivative curve shows the contraction state of the film as the temperature rises. A large first derivative value of the TMA curve indicates that the shrinkage force at that temperature tends to increase.

  The maximum value of the differential value in the region I is in the above range, which indicates that the contraction force does not increase until the shutdown temperature is reached even when the PO microporous membrane is exposed to a high temperature. That is, it indicates that an internal short circuit is not likely to occur due to the electrode being exposed by contraction before the shutdown temperature. Therefore, when the maximum value of the differential value in the region I is within the above range, the risk of ignition or explosion is low even during thermal runaway or when exposed to high temperatures, and high safety can be ensured. The effect can be confirmed in a heat safety test such as an oven test. Furthermore, since the maximum value of the differential value in the region I is in the above range, the contraction force does not increase until the shutdown temperature is reached, and the closed hole due to the contraction does not occur. Rise is suppressed. Therefore, the battery performance under high temperature, particularly the cycle characteristics can be kept good. The melting point Tm of the PO microporous membrane refers to the temperature indicated by the maximum peak value of the second temperature increase in DSC measurement (Differential Scanning Calorimetric).

  The maximum value of the differential value in region I can be controlled high by adjusting the primary stretching ratio and primary stretching temperature, adjusting the heat treatment relaxation ratio and the fixed temperature, and adjusting the maximum strain rate ratio before and after the extraction process. By doing so, it can be controlled low.

(Maximum differential value in region II)
Further, the maximum value of the differential value of the TMA primary differential curve in the region between the temperature lower than the melting point Tm-10 ° C. of the PO microporous membrane and below the melting point Tm (hereinafter also referred to as “region II”) is preferably It is 0.15 or more, More preferably, it is 0.2 or more, More preferably, it is 0.26 or more. When the maximum value of the differential value of the TMA linear differential curve in region II is in the above range, the shrinkage force of the PO microporous membrane increases instantaneously when the PO microporous membrane is exposed to a high temperature and reaches the melting point Tm. However, there is a tendency that the hole is closed more quickly and shuts down. Thereby, it exists in the tendency which can ensure high heat-resistant safety. When the shutdown temperature is reached, the shutdown occurs instantaneously, and the ion reaction path is partially formed, so that the electrode reaction becomes partly active and the starting point of thermal runaway is further suppressed. It tends to be possible. The upper limit of the maximum differential value in region II is not particularly limited, but is preferably 0.8 or less.

  The maximum differential value in region II can be measured by the same method as the maximum differential value in region I. The maximum value of the differential value in the region II can be controlled to be high by lowering the stretching temperature, and can be controlled to be low by adjusting the maximum strain rate ratio before and after the extraction process.

(Maximum differential value in region III)
Furthermore, the minimum value of the differential value of the TMA primary differential curve in the region between the melting point Tm of the PO microporous membrane and the melting point Tm + 5 ° C. (hereinafter also referred to as “region III”) is 0.2 or more. More preferably, it is 0.22 or more, More preferably, it is 0.23 or more. When the maximum value of the differential value in the region III is in the above range, the contraction stress continuously increases when the PO microporous membrane is exposed to a high temperature above the melting point and shutdown is started, and the ion conduction path Therefore, there is a tendency that the danger such as ignition and explosion at a high temperature is further reduced. The upper limit of the maximum differential value in region III is not particularly limited, but is preferably 0.8 or less.

  The maximum differential value in region III can be measured by the same method as the maximum differential value in region I. The maximum value of the differential value in the region III can be controlled to be high by increasing the maximum strain rate ratio before and after extraction.

  The dynamic friction coefficient in the TD direction of the PO microporous membrane in the present embodiment is preferably 0.4 or less, more preferably 0.3 or less, and further preferably 0.25 or less. A PO microporous film having a TD direction dynamic friction coefficient in the above-mentioned range tends to exhibit an effect in the battery manufacturing process by reducing pin dropout defects in the winding process. Here, the “pin missing defect” means that the separator is tied to the winding pin when the winding pin that is a winding core at the time of winding is pulled out from the wound body. The lower limit of the dynamic friction coefficient in the TD direction is not particularly limited and is 0 or more. The dynamic friction coefficient in the TD direction can be controlled to be high by decreasing the stretching temperature, and can be controlled to be low by increasing the heat treatment fixing temperature. The dynamic friction coefficient in the TD direction can also be measured by the method described in the examples.

The MD and TD tensile strengths (longitudinal tensile strength and tensile strength in the membrane width direction) of the PO microporous membrane in the present embodiment are preferably 1500 kg / cm ² or more, more preferably 2000 kg / cm ² or more. More preferably, it is 2300 kg / cm ² or more. A PO microporous membrane having MD and TD tensile strengths in the above range tends to be less susceptible to deformation in a battery impact test or the like. This is advantageous in that an internal short circuit due to a film breakage or the like does not occur when a battery impact is applied. The upper limits of the MD and TD tensile strengths of the PO microporous membrane are both preferably 4000 kg / cm ² or less. MD and TD tensile strength can be controlled to be high by increasing the total draw ratio, and can be controlled to be low by reducing the draw ratio. Moreover, MD and TD tensile strength can be measured by the method as described in an Example.

  The 12 μm equivalent air permeability of the PO microporous membrane in the present embodiment is preferably 150 seconds / 100 cc or less, more preferably 120 seconds / 100 cc or less, and further preferably 100 seconds / 100 cc or less. A PO microporous membrane having a film thickness of 12 μm equivalent air permeability in the above range tends to be more excellent in output characteristics and cycle characteristics because of good ion permeability. The lower limit of the 12 μm equivalent air permeability of the PO microporous membrane is not particularly limited, and is 0 second / 100 cc or more. The 12 μm equivalent air permeability can be controlled to be high by decreasing the porosity, and can be controlled to be low by increasing the air permeability. Moreover, the air permeability of a film thickness of 12 μm can be measured by the method described in the examples.

  The film thickness of the PO microporous membrane in the present embodiment is preferably 5 μm or more, more preferably 6 μm or more, and further preferably 8 μm or more. A PO microporous film having a film thickness in the above range tends to maintain strength to prevent film breakage when an impact such as collision is applied. The film thickness of the PO microporous membrane is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. Moreover, when the PO microporous film having a film thickness in the above range is used as a battery, it can increase the amount of electrode winding per volume, and tends to exhibit an effect of increasing the capacity. The film thickness can be measured by the method described in the examples.

  The porosity of the PO microporous membrane in the present embodiment is not particularly limited, but is preferably 30% or more, more preferably 40% or more, and further preferably 45% or more from the viewpoint of output characteristics. In addition, the porosity of the PO microporous membrane is preferably 70% or less, more preferably 65% or less, and further preferably 60% or less, from the viewpoint of film resistance. The porosity can be measured by the method described in the examples.

  The puncture strength of the PO microporous membrane in the present embodiment is not particularly limited, but is preferably 2N or more, more preferably 3N or more, and further preferably 4N or more from the viewpoint of mechanical strength.

[Production method of polyolefin microporous membrane]
The production method of the PO microporous membrane of the present embodiment is not particularly limited. For example, an extrusion step (a) in which a resin composition containing a polyolefin resin and a pore-forming material is melt-kneaded and extruded, and the step (a ) And a primary stretching step of stretching the sheet-shaped molded product obtained in the step (b) at least once in at least a uniaxial direction ( c), the extraction step (d) for extracting the pore-forming material from the primary stretched membrane obtained in the step (c), and the primary stretched membrane obtained in the step (d) are stretched at least in the uniaxial direction. Examples include a method having a secondary stretching step (e) and a heat setting step (f) in which the secondary stretched film obtained in the step (e) is heat-set at a predetermined temperature.

  By using the above-mentioned PO microporous membrane production method, a PO microporous membrane that has excellent performance at high temperatures and can provide high heat safety when used as a separator for lithium ion secondary batteries and other electrochemical devices Can be provided. In addition, the manufacturing method of PO microporous film of this embodiment is not limited to the said manufacturing method, A various deformation | transformation is possible in the range which does not deviate from the summary.

[Extrusion step (a)]
The extrusion step (a) is a step in which a resin composition containing a polyolefin resin and a pore-forming material is melt-kneaded and extruded. In addition, in an extrusion process (a), you may mix another component as needed. Although it does not specifically limit as another component, For example, an inorganic material etc. are mentioned.

(Polyolefin resin)
The polyolefin resin (hereinafter also referred to as “PO”) used in the melt-kneading in the extrusion step of the step (a) is not particularly limited. For example, a homopolymer of polyolefin such as ethylene and propylene; ethylene and propylene , 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and a copolymer obtained by polymerizing at least two monomers selected from the group consisting of norbornene. As said homopolymer, the homopolymer of ethylene or propylene is preferable.

  The PO is not particularly limited, and one type may be used alone, or two or more types may be used. When using two or more kinds of PO, after making a mixture in which two or more kinds of PO are mixed in advance, it may be mixed with other components such as pore forming materials, or various POs may be added separately to other components Then, the mixture may be used by mixing with each other (hereinafter, the description of “mixture” may include both cases). Use of two or more kinds of PO is preferable from the viewpoint of facilitating the control of the fuse temperature and the short-circuit temperature of the separator, and improving the high-temperature cycle characteristics and the heat resistance in the oven test. For example, a mixture of an ultrahigh molecular weight PO having a viscosity average molecular weight (hereinafter sometimes abbreviated as “Mv”) of 500,000 or more and a PO having a viscosity of less than Mv 500,000 has a strength of the separator due to its moderate molecular weight distribution. It is more preferable from the viewpoint of achieving both transparency and improved output characteristics and impact resistance in a collision test.

  The Mv of the entire PO microporous membrane is preferably 100,000 to 1,200,000, more preferably 300,000 to 800, whether PO is used alone or in combination of two or more. 000. When the Mv of the entire PO microporous membrane is 100,000 or more, the TMA primary differential value in the region II tends to increase, and the heat resistance in the impact test and the oven test tends to improve. Moreover, when the Mv of the entire PO microporous membrane is 1,200,000 or less, the TMA primary differential value in the region I tends to be small, and the high-temperature cycle characteristics tend to be good. In addition, the orientation of the PO resin in the microporous film is suppressed, and the uniformity of the entire microporous film tends to be easily developed.

  In this embodiment, it is preferable to use a mixture of polyethylene and polypropylene as the polyolefin resin from the viewpoint of improving the results of the collision test and the oven test.

  The polyethylene content is preferably 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more based on the total amount of the polyolefin resin. When the polyethylene content is within the above range, the fuse function of the separator tends to be further improved, and the heat resistance safety in the crash test and the oven test can be improved. The upper limit of the polyethylene content is not particularly limited, but is 100% by mass or less.

  Further, the content of polypropylene is preferably 1% by mass or more, more preferably 3% by mass or more, and further preferably 5% by mass or more with respect to the total amount of the polyolefin resin. An endotherm due to melting of the polypropylene occurs around 160 ° C. Therefore, when the content of polypropylene is within the above range, high temperature cycle characteristics are improved in the case of local heat generation due to a short circuit inside the battery, and heat resistance safety such as a crash test and an oven test is further improved. Tend to. The content of polypropylene is preferably 50% by mass or less, more preferably 20% by mass or less, and still more preferably 10% by mass or less with respect to the total amount of the polyolefin resin. When the content of polypropylene is 50 mass or less, the shutdown function of the separator tends to be further improved.

  The blending ratio of PO is not particularly limited, but is preferably 1 to 60% by weight, more preferably 10 to 10% by weight based on the total weight of PO, the pore-forming material (plasticizer) and the inorganic material blended as necessary. 40% by mass.

(Hole forming material)
The pore-forming material refers to a plasticizer and an inorganic material to be blended as necessary, and corresponds to a portion that becomes a pore of the microporous membrane when the pore-forming material is removed in the subsequent extraction step (d). Material.

  Although it does not specifically limit as a plasticizer, It is preferable to use the non-volatile solvent which can form a uniform solution above melting | fusing point of polyolefin. Specific examples of such a non-volatile solvent include, for example, hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol. . Among the plasticizers, liquid paraffin is highly compatible with the polyolefin resin if it is polyethylene or polypropylene, and even when the melt-kneaded product is stretched, the interface separation between the resin and the plasticizer hardly occurs and is uniform. This is preferable because it tends to facilitate easy stretching. Note that these plasticizers may be recovered and reused after extraction by an operation such as distillation.

  The ratio of the polyolefin resin composition and the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and formed into a sheet shape. For example, the mass fraction of the plasticizer in the composition comprising the polyolefin resin composition and the plasticizer is preferably 20 to 90 mass%, more preferably 30 to 80 mass%. When the mass fraction of the plasticizer is 90% by mass or less, the melt tension at the time of melt molding is hardly insufficient and the moldability tends to be improved. On the other hand, when the mass fraction of the plasticizer is 20% by mass or more, the polyolefin chain is not broken even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, and the uniform and fineness required for the separator is obtained. It is easy to form a pore structure and the strength is also likely to increase.

  The inorganic material is not particularly limited. For example, oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide; silicon nitride, titanium nitride, nitride Nitride ceramics such as boron; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite , Ceramics such as asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. These may be used individually by 1 type and may be used in combination of 2 or more type. Among these, at least one selected from the group consisting of silica, alumina, and titania is preferable from the viewpoint of electrochemical stability.

  The blending ratio of the inorganic material is not particularly limited, but for example, from the viewpoint of obtaining good separability with respect to the total mass of PO and the inorganic material, 5 mass% or more is preferable, and 10 mass% or more is more preferable. From the viewpoint of securing high strength, 99% by mass or less is preferable, and 95% by mass or less is more preferable.

  The inorganic material can be used as a pore-forming material, but from the viewpoint of improving heat resistance, it can be contained in the PO microporous membrane without being used as a pore-forming material.

(Optional additive)
In the step (a), the resin composition containing PO may contain any additive. Although it does not specifically limit as an additive, For example, Polymers other than polyolefin resin; Antioxidants, such as a phenol type compound, a phosphorus compound, and a sulfur type compound; Metal soaps, such as a calcium stearate and a zinc stearate; Agents; light stabilizers; antistatic agents; antifogging agents; colored pigments and the like. The total amount of these additives is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less with respect to 100 parts by mass of the polyolefin resin.

  The method of kneading in step (a) is not particularly limited. For example, after mixing some or all of the raw materials in advance using a Henschel mixer, a ribbon blender, a tumbler blender or the like as necessary, all the raw materials are mixed. And a screw extruder such as a single-screw extruder or a twin-screw extruder; a kneader; a method of melt-kneading with a mixer or the like.

  In this embodiment, the kneading is not particularly limited, but after the antioxidant is mixed with the raw material PO at a predetermined concentration, the periphery of the mixture is replaced with a nitrogen atmosphere, and the mixture is melted while maintaining the nitrogen atmosphere. It is preferable to perform kneading. The temperature during melt kneading is preferably 160 ° C. or higher, and more preferably 180 ° C. or higher. The temperature is preferably less than 300 ° C.

  In the step (a), the kneaded material obtained through the kneading is extruded by an extruder such as a T-die or an annular die. At this time, it may be single layer extrusion or laminated extrusion. Various conditions at the time of extrusion are not specifically limited, For example, a well-known method is employable.

[Sheet forming step (b)]
The sheet forming step (b) is a step of forming the extrudate obtained in the extrusion step (a) into a sheet shape. The sheet-like molded product obtained by the sheet molding step (b) may be a single layer or a laminate. Although it does not specifically limit as a method of sheet forming, For example, the method of solidifying an extrudate by compression cooling is mentioned. The compression cooling method is not particularly limited, and examples thereof include a method in which the extrudate is brought into direct contact with a cooling medium such as cold air and cooling water; a method in which the extrudate is brought into contact with a metal roll cooled with a refrigerant, a press machine, and the like. It is done. Among these, the method of bringing the extrudate into contact with a metal roll cooled by a refrigerant, a press machine, or the like is preferable in terms of easy film thickness control. Although the cooling temperature in this case will not be specifically limited if it is the temperature which an extrudate solidifies, For example, 20-90 degreeC is preferable.

[Primary stretching step (c)]
The primary stretching step (c) is a step of stretching the sheet-like molded product obtained in the sheet forming step (b) at least once in at least a uniaxial direction. This stretching step (stretching step performed before the next extraction step (d)) will be referred to as “primary stretching”, and the film obtained by the primary stretching will be referred to as “primary stretched film”. In the primary stretching, the sheet-like molded product can be stretched in at least one direction, and may be performed in both MD and TD directions, or only one of MD and TD.

  The stretching method for primary stretching is not particularly limited. For example, uniaxial stretching with a roll stretching machine; TD uniaxial stretching with a tenter; sequential biaxial stretching with a roll stretching machine and a tenter or a combination of a plurality of tenters; simultaneous biaxial tenter Or simultaneous biaxial stretching by inflation molding, etc. are mentioned.

  The draw ratio of MD and / or TD for primary stretching is preferably 2 times or more, more preferably 3 times or more. When the stretching ratio of MD and / or TD for primary stretching is within the above range, the strength of the obtained PO microporous film tends to be further improved. Further, the draw ratio of MD and / or TD for primary stretching is preferably 10 times or less, more preferably 5 times or less. When the draw ratio of primary stretch MD and / or TD is within the above range, stretch breakage tends to be further suppressed. When performing biaxial stretching, sequential stretching or simultaneous biaxial stretching may be used, but the stretching ratio in each axial direction is preferably 2 to 10 times, more preferably 3 to 5 times, respectively. is there.

  The primary stretching temperature is not particularly limited, and can be selected with reference to the raw material resin composition and concentration contained in the PO composition. The stretching temperature is preferably in the range of the melting point Tm of the PO resin (Tm-30 to Tm ° C.) from the viewpoint of preventing breakage due to excessive stretching stress and balancing strength and heat shrinkage. When the main composition resin of the PO microporous membrane is polyethylene, the stretching temperature is preferably 110 ° C. or higher, and preferably 130 ° C. or lower from the viewpoint of increasing the strength of the microporous membrane. Specifically, the stretching temperature is preferably 100 to 135 ° C, more preferably 110 to 130 ° C, and still more preferably 115 to 129 ° C.

[Extraction step (d)]
The extraction step (d) is a step of obtaining a porous film by extracting the pore-forming material from the primary stretched film obtained in the primary stretch process (c). Examples of the method for removing the pore-forming material include a method in which the primary stretched film is immersed in an extraction solvent to extract the pore-forming material and sufficiently dried. The method for extracting the hole forming material may be either a batch type or a continuous type. Further, the residual amount of the pore forming material, particularly the plasticizer, in the porous film is preferably less than 1% by mass with respect to the mass of the entire porous film.

  The extraction solvent used for extracting the pore-forming material should be a poor solvent for the polyolefin resin and a good solvent for the pore-forming material plasticizer and having a boiling point lower than the melting point of the polyolefin resin. Is preferred. Such an extraction solvent is not particularly limited, and examples thereof include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; hydrofluoroether and hydrofluorocarbon. Non-chlorinated halogenated solvents such as ethanol; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by an operation such as distillation. Moreover, when using an inorganic material as a pore formation material, aqueous solution, such as sodium hydroxide and potassium hydroxide, can be used as an extraction solvent. In addition, there is no restriction | limiting in particular about the order of extraction, a method, and the frequency | count.

[Secondary stretching step (e)]
The secondary stretching step (e) is a step of stretching the primary stretched film obtained in the extraction step (d) in at least a uniaxial direction. This stretching step (stretching step performed after the extraction step (d)) will be referred to as “secondary stretching”, and the film obtained by the secondary stretching will be referred to as “secondary stretching film”. In the secondary stretching, the porous film obtained through the extraction step of step (d) can be stretched in at least one direction, and may be performed in both MD and TD directions, or only one of MD or TD. May be.

  The draw ratio of MD in secondary stretching is preferably 1.1 times or more, more preferably 2.0 times or more, and further preferably 3.0 times or more. Moreover, the draw ratio of TD of secondary stretching becomes like this. Preferably it is 1.1 times or more, More preferably, it is 1.5 times or more, More preferably, it is 2.0 times or more. When stretching in the biaxial direction, 1.1 times or more is preferable in at least one of MD and TD, and 2.0 times or more is more preferable. When the draw ratio of secondary stretching is in the above range, it is possible to balance the reduction of the differential value of the TMA primary differential curve in region I and the increase of the differential value of the TMA primary differential curve in region II.

  Further, the stretching step of the present embodiment includes primary stretching and secondary stretching, and the final total stretching ratio in each axial direction after both steps is preferably 3 times or more, More preferably, it is 5 times or more, and more preferably 10 times or more. Moreover, the final total area stretch ratio after going through both steps (c) and (e) is preferably 9 times or more, more preferably 16 times or more, and further preferably 50 times or more. It is. When the total draw ratio and the total area draw ratio are within the above ranges, the strength and permeability of the resulting PO microporous membrane tend to be further improved. Further, from the viewpoint of dimensional stability and prevention of breakage during stretching, the total stretching ratio in each axial direction is preferably less than 20 times, and the total area ratio is preferably 200 times or less.

The stretching step of the present embodiment includes primary stretching and secondary stretching. The maximum strain rate ratio (primary stretching maximum strain rate / secondary stretching maximum strain rate) in each stretching step is preferably 0.4 or less. More preferably, it is 0.3 or less, More preferably, it is 0.2 or less. The reason why the maximum strain rate ratio is in the above-mentioned range is not clear, but the PO microporous membrane has a melting point less than −10 ° C., a decrease in the differential value of the TMA curve, and a melting point less than + 5 ° C. This is effective for increasing the differential value of the TMA curve. The strain rate is obtained as follows.
Strain rate (% / sec) = (stretch ratio-1) × 100 ÷ stretch time (sec)
Stretching time (seconds) = Stretching distance (m) ÷ Stretching speed (m / s)

  In each stretching process, when stretching is performed in multiple stages, the strain rate may not take a constant value. In that case, the strain rate at the stage where the strain rate is maximized is set as the maximum strain rate in the stretching step. For example, when stretching is performed using a roll stretching machine having a plurality of stages, the maximum strain rate is calculated using the inter-roll distance and the inter-roll speed at which the strain rate is maximum as the stretching distance and the stretching speed.

The secondary stretching temperature is not particularly limited, and can be selected with reference to the raw material resin composition and concentration contained in the PO composition. The stretching temperature is preferably in the range of (the melting point of the PO microporous film−30 ° C.) to the melting point of the main composition resin from the viewpoint of preventing breakage due to excessive stretching stress. When the main composition resin of the PO microporous membrane is polyethylene, the stretching temperature is preferably 110 ° C. or higher, and preferably 130 ° C. or lower from the viewpoint of increasing the strength of the microporous membrane. The stretching temperature is more preferably 115 to 129 ° C, still more preferably 118 to 127 ° C.
In addition, the secondary stretching causes pore expansion and increases the porosity. The porosity of the secondary stretched film is preferably 50% or more, and more preferably 60% or more. When the porosity of the secondary stretched film is 50% or more, the heat setting temperature can be set high in the next step (f), and the differential value of the TMA primary differential curve in region I tends to be reduced. The porosity of the secondary stretched film is preferably 90% or less from the viewpoint of strength.

[Heat setting step (f)]
The heat setting step (f) is a step of heat-setting the secondary stretched film obtained in the secondary stretch step (e) at a predetermined temperature. The heat treatment method in this case is not particularly limited, and examples thereof include a heat setting method in which stretching and relaxation operations are performed using a tenter or a roll stretching machine.

  The stretching operation in the heat setting step (f) is an operation of stretching the PO microporous film in at least one direction of MD and TD, and may be performed in both the MD and TD directions, or one of MD or TD. You may just go. The draw ratio of MD and TD in the heat setting step (f) is preferably 1.5 times or more, more preferably 1.6 times or more, respectively. In addition, although the upper limit of the draw ratio of MD and TD in a heat setting process (f) is not specifically limited, 5 times or less are preferable. When the draw ratio is within the above range, shrinkage stress in the vicinity of the melting point remains, and the differential value of the TMA primary differential curve in the regions II and III tends to increase. Moreover, it exists in the tendency for the intensity | strength and porosity of a porous film to improve more because a draw ratio exists in the said range.

  The stretching temperature in this stretching operation is not particularly limited, but is preferably at least 15 ° C. lower than the melting point Tm of the PO microporous membrane, and is in the range of 10 ° C. to 10 ° C. higher than the melting point Tm of the PO microporous membrane (Tm−10 ° C. to Tm + 10 ° C.) is more preferable, and a temperature range from 5 ° C. to 8 ° C. higher than the melting point Tm of the PO microporous membrane (Tm−5 ° C. to Tm + 8 ° C.) is more preferable. When the stretching temperature is within the above range, the thermal contraction rate of the obtained PO microporous membrane tends to be further reduced, and the porosity and strength tend to be further improved.

  The relaxation operation in the heat setting step (f) is an operation of reducing the PO microporous film in at least one direction of MD and TD, and may be performed in both the MD and TD directions. Only one of the two may be done. The relaxation rate in the heat setting step (f) is preferably 1.0 or less, more preferably 0.97 or less, and still more preferably 0.95 or less. When the relaxation rate in the heat setting step (f) is 1.0 or less, the heat resistance in the oven test tends to be further improved. The relaxation rate is preferably 0.8 or more from the viewpoint of film quality, and more preferably 0.85 or more from the viewpoint of increasing the relaxation temperature. Further, the relaxation rate being in the above range can balance the reduction of the differential value of the TMA primary differential curve in region I and the increase of the differential value of the TMA primary differential curve in region II and region III. Here, the “relaxation rate” is a value obtained by dividing the dimension of the film after the relaxation operation by the dimension of the film before the relaxation operation. When both MD and TD are relaxed, the MD relaxation rate and the TD It is a value multiplied by the relaxation rate.

  Although the relaxation temperature in this relaxation operation is not particularly limited, it is preferably not more than 10 ° C. higher than the melting point of the PO microporous membrane Tm, and is in the range of −5 ° C. to + 8 ° C. from the melting point Tm of the PO microporous membrane (Tm −5 ° C. to Tm + 8 ° C.), more preferably in the range of −3 ° C. to + 5 ° C. (Tm−3 ° C. to Tm + 5 ° C.) from the melting point Tm of the PO microporous membrane, and −1 from the melting point Tm of the PO microporous membrane. The range from C to + 3 ° C. (Tm−1 ° C. to Tm + 3 ° C.) is particularly preferable. When the temperature in the relaxation operation is in the above range, not only the residual stress due to the stretching process can be removed, but also the orientation of the molecular chain can be firmly fixed. The differential value can be reduced. Moreover, it is preferable that the temperature in the relaxation operation is in the above range from the viewpoint of roughening the surface form of the PO microporous membrane and reducing the dynamic friction coefficient.

[Other processes]
The production method of the PO microporous membrane of the present embodiment can include other steps than the above steps (a) to (f). Other steps are not particularly limited. For example, in addition to the above-described heat setting step, a plurality of single-layer PO microporous membranes are stacked as a step for obtaining a PO microporous membrane that is a laminate. A laminating process is mentioned. In addition, the PO microporous membrane manufacturing method of the present embodiment is a surface treatment that performs surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, chemical modification, etc. on the surface of the PO microporous membrane. A process may be included. Furthermore, the above-mentioned inorganic material may be applied to one or both sides of the PO microporous membrane to obtain a PO microporous membrane having an inorganic material layer.

  Further, after the post-processing step, the master roll wound with the microporous film may be subjected to an aging treatment at a predetermined temperature, and then the master roll may be rewound. This tends to make it easier to obtain a PO microporous membrane with higher thermal stability. In the above case, the temperature at the time of aging treatment of the master roll is not particularly limited, but is preferably 35 ° C or higher, more preferably 45 ° C or higher, and further preferably 60 ° C or higher. Further, from the viewpoint of maintaining the permeability of the PO microporous membrane, the temperature at the time of aging treatment of the master roll is preferably 120 ° C. or lower. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or longer because the above-described effect is easily exhibited.

[Use]
The polyolefin microporous membrane can be suitably used for an electricity storage device separator, particularly a lithium ion battery separator. The separator for an electricity storage device of this embodiment is composed of the above-mentioned polyolefin microporous film. Further, the electricity storage device of this embodiment includes the polyolefin microporous film.

  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 as long as the gist thereof is not exceeded.

(1) TMA measurement (Thermomechanical analysis: Thermomechanical Analysis)
TMA measurement of the polyolefin microporous film was performed using Shimadzu Corporation TMA50 (trademark). First, a sample (polyolefin microporous membrane) cut out to about 15 mm in the TD direction and 3 mm in the MD direction was fixed to the chuck so that the distance between chucks (TD direction) was 10 mm, and set on a dedicated probe. The initial load was 0.0049 N (0.5 gf), and the probe was heated to 250 ° C. at a rate of 10 ° C./min from 30 ° C. in the constant length mode. While reaching 250 ° C., temperature and stress were sampled at 1 second intervals. Based on the obtained data, a TMA curve showing the relationship between temperature and shrinkage stress was prepared. A tensile type was used as the dedicated probe.

(2) TMA first derivative curve:
Obtained by TMA measurement was determined differential value I _n of the stress at time n with the data of temperature and stress. I _n is represented by the following formula, T _n, F _n is the temperature at each n seconds, it shows the stress shows a T n _{+ 1,} F n _{+ 1} is respectively n + 1 second temperature and stress.
_{I n = F n + 1 -F} n / T n + 1 -T n
Horizontal axis all T _n in the measurement range, to obtain a curve showing the relationship between the differential value of the temperature and stress in TMA measurement by taking the vertical axis to I _n. Further subjected to smoothing by moving average approximation of the 5-point interval of I _n, and a TMA first derivative curve.

(3) DSC measurement (Differential Scanning Calorimetric)
DSC was measured using DSC60 manufactured by Shimadzu Corporation. First, a separator 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 sample was laid on an aluminum open sample pan having a diameter of 5 mm, a clamping cover was placed thereon, and the sample was fixed in the aluminum pan by a sample sealer. In a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a temperature increase rate of 10 ° C./min, held at 200 ° C. for 5 minutes, and then decreased from 200 ° C. to 30 ° C. at a temperature decrease rate of 10 ° C./min. Then, after hold | maintaining for 5 minutes at 30 degreeC, it heated up again from 30 degreeC to 200 degreeC with the temperature increase rate of 10 degree-C / min. In the melting endothermic curve at this time, the maximum temperature was defined as the melting point of the PO microporous membrane. When there were a plurality of maximum values, the temperature at which the maximum value of the melting endothermic curve was the maximum was adopted as the melting point of the PO microporous membrane.

(4) Tensile breaking strength (kg / cm ² )
A tensile test was performed using a tensile tester (Shimadzu Autograph AG-A type), and the strength at the time of sample break was divided by the cross-sectional area of the sample before the test to obtain tensile break strength (kg / cm ² ). The measurement conditions are: temperature: 23 ± 2 ° C., sample shape: width 10 mm × length 100 mm, distance between chucks: 50 mm, tensile speed: 200 mm / min. The tensile elongation (%) was obtained by dividing the amount of elongation (mm) up to fracture by the distance between chucks (50 mm) and multiplying by 100.

(5) Film thickness (μm)
The film thickness of the PO microporous membrane was measured at room temperature (23 ± 2 ° C.) using Toyo Seiki's microthickness meter KBM (trademark).

(6) Porosity (%)
A sample was obtained by cutting a 10 cm × 10 cm square from the PO microporous membrane, and its volume (cm ³ ) and mass (g) at room temperature 23 ± 2 ° C. were measured. From these values and the membrane density (g / cm ³ ), the porosity of the PO microporous membrane was calculated by the following equation.
Porosity (%) = (volume−mass / membrane density) / volume × 100
The film density was calculated assuming a constant value of 0.95 g / cm ³ .

(7) Air permeability (sec / 100cc)
In accordance with JIS P-8117, using a Gurley type air permeability meter (G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd.), the air resistance of the PO microporous membrane at room temperature 23 ± 2 ° C. (Hereinafter also referred to as air permeability) was measured. “Air permeability” refers to the time (seconds) required for 100 cc of air to pass through a sample area of 6.452 cm ² . Moreover, the air permeability of a film thickness of 12 μm can be obtained by the following formula.
Film thickness 12 μm equivalent air permeability (second / 100 cc) = air permeability (second / 100 cc) / film thickness (μm) * 12 μm

(8) Coefficient of dynamic friction Using a KES-SE friction tester manufactured by Kato Tech Co., Ltd., load 50 g, contact area 10 × 10 = 100 mm 2 (0.5 mmφ hard stainless steel wire SUS304 piano wires without any gaps, and Measured 3 times on TD with a sample size of width 50mm x measuring direction 200mm under the conditions of contact feed speed 1mm / sec, tension 6kPa, temperature 23 ° C, humidity 50%. The average was obtained. Note that the value of the dynamic friction coefficient was the value of the surface in contact with the negative electrode when the battery was produced.

[Battery Manufacturing Method]
The output characteristics, cycle characteristics, high temperature cycle characteristics, pin pull-out property, impact test, and oven test were performed by creating a simple assembled battery. The method for producing the assembled battery and the method for evaluating each characteristic are shown below.

a. Production of Positive Electrode A lithium cobalt composite oxide LiCoO ₂ as a positive electrode active material, graphite and acetylene black as conductive materials were dispersed in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders to prepare a slurry. This slurry was applied to a 15 μm-thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. The obtained molded body was slit to a width of 57.0 mm to obtain a positive electrode.

b. Manufacture of Negative Electrode An artificial graphite as a negative electrode active material, an ammonium salt of carboxymethyl cellulose and a styrene-butadiene copolymer latex as a binder were dispersed in purified water to prepare a slurry. This slurry was applied to a copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. The obtained molded body was slit to a width of 58.5 mm to obtain a negative electrode.

c. Preparation of non-aqueous electrolyte LiPF ₆ as a solute was dissolved in a mixed solvent of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate: = 1: 1: 2 (volume ratio) to a concentration of 1 mol / L, and non-aqueous An electrolyte solution was prepared.

d. Battery assembly After laminating the positive electrode, the PO microporous membrane described later, and the negative electrode, a wound electrode body was prepared by a conventional method. The number of windings was adjusted according to the thickness of the PO microporous film. The outermost peripheral end portion of the obtained wound electrode body was fixed by applying an insulating tape. The negative electrode lead was welded to the battery can, the positive electrode lead was welded to the safety valve, and the wound electrode body was inserted into the battery can. Thereafter, 5 g of the non-aqueous electrolyte was poured into the battery can, and the lid was caulked to the battery can via a gasket, to obtain a cylindrical secondary battery having an outer diameter of 18 mm and a height of 65 mm. This cylindrical secondary battery was charged to a battery voltage of 4.2 V at a current value of 0.2 C (current that is 0.2 times the hourly rate (1 C) of the rated electric capacity) in an atmosphere of 25 ° C. The battery was charged for a total of 3 hours by a method of starting to reduce the current value so as to hold 2 V. Subsequently, the battery was discharged to a battery voltage of 3.0 V at a current value of 0.2 C, and the battery capacity at that time was set to X mAh.

e. Cycle test Using the battery assembled as described above, (i) current amount 0.5C, upper limit voltage 4.2V
, Constant current constant voltage charging for a total of 8 hours, (ii) 10 minutes of rest, (iii) constant current discharge of current amount 0.5C, end voltage 2.5V, (iv) 10 minutes of rest For convenience, the battery was charged and discharged 50 times. All the charge / discharge treatments were performed in an atmosphere of 25 ° C. Thereafter, the ratio of the discharge capacity at the 50th cycle to the battery capacity XmAh was multiplied by 100 to obtain the capacity retention rate (%). The evaluation was made according to the following criteria.
A: Capacity maintenance rate (%) is 70% or more.
○: Capacity maintenance rate (%) is 50% or more and less than 70%.
Δ: Capacity maintenance rate (%) is 30% or more and less than 30%.
X: Capacity maintenance rate (%) is less than 30%.

f. High-temperature cycle test Using the battery assembled as described above, (i) current amount 0.5C, upper limit voltage 4.2V
, Constant current constant voltage charging for a total of 8 hours, (ii) 10 minutes of rest, (iii) constant current discharge of current amount 0.5C, end voltage 2.5V, (iv) 10 minutes of rest For convenience, the battery was charged and discharged 50 times. All the charge / discharge treatments were performed in an atmosphere of 90 ° C. Thereafter, the ratio of the discharge capacity at the 50th cycle to the battery capacity XmAh was multiplied by 100 to obtain the capacity retention rate (%). The evaluation was made according to the following criteria.
A: Capacity maintenance rate (%) is 70% or more.
○: Capacity maintenance rate (%) is 50% or more and less than 70%.
Δ: Capacity maintenance rate (%) is 30% or more and less than 30%.
X: Capacity maintenance rate (%) is less than 30%.

g. Output characteristics test The batteries assembled as described above were measured for 1 C discharge capacity and 5 C discharge capacity up to a discharge end voltage of 3 V in a constant temperature state of 25 ° C., and 5 C capacity / 1 C capacity was defined as an output characteristic value. The evaluation was made according to the following criteria.
A: Output characteristic value is 0.85 or more.
○: Output characteristic value is 0.75 or more and less than 0.85.
Δ: Output characteristic value is 0.65 or more and less than 0.75.
X: Output characteristic value is less than 0.5.

h. Oven Test Using the battery assembled as described above, the battery after charging was heated from room temperature to 150 ° C. at a rate of 5 ° C./min, left at 150 ° C. for a predetermined time, and the ignition status was confirmed. The evaluation was made according to the following criteria.
◎: Those that did not ignite even after standing time of 1 hour or more ○: Those that ignited after standing time 30 minutes to 1 hour △: Those that ignited after standing time 10 minutes to 30 minutes ×: Those that ignited after standing time 10 minutes or less

i. Collision test Using the battery assembled as described above, a SUS rod with a diameter of 15.8 mmφ was placed perpendicular to the length direction of the battery, and then a weight of 9.1 kg was applied from a position of 61 cm in height. The batteries were allowed to fall freely and collided, and the battery temperature was increased after the collision and the ignition status was confirmed in 5 cells. The evaluation was made according to the following criteria.
A: Surface temperature rise is 30 ° C. or less in all 5 cells. ○: Surface temperature rise is more than 30 ° C. and 80 ° C. or less in 1-2 cells, and surface temperature rise of other cells is 30 ° C. or less. In 5 cells, the surface temperature rise is over 30 ° C. and below 80 ° C., the surface temperature rise of other cells is under 30 ° C. x: In 1 to 5 cells, the surface temperature rise is over 80 ° C., or the cell ignites.

j. Pin pull-out test Pull the pin that is the winding shaft by hand from the wound electrode body produced as described above, and the pin pull-out characteristics (pulled by the pin when the pin is pulled out of the wound electrode body) It was confirmed that the figure was shifted by 2 mm or more). 100 wound electrode bodies were confirmed, and pin pullability was evaluated according to the following evaluation criteria, based on the number of wound electrode bodies that were pulled by the pin and the winding shape was shifted by 2 mm or more.
◎: The number of wound electrode bodies that are pulled by a pin and the winding shape is shifted by 2 mm or more is one. ○: The number of wound electrode bodies that are pulled by the pin and the winding shape is shifted by 2 mm or more is two to four. The number of wound electrode bodies that are pulled and displaced by 2 mm or more is 5 to 9 ×: The number of wound electrode bodies that are pulled by a pin and displaced by 2 mm or more is 10 or more

[Example 1]
47.5 parts by mass of homopolymer polyethylene (melting point: 135.5 ° C.) having an Mv of 700,000 and 47.5 parts by mass of homopolymer polyethylene having an Mv of 300,000 (melting point: 135.5 ° C.) And 5 parts by mass of polypropylene having Mv of 400,000 were dry blended using a tumbler blender. To 99 parts by mass of the obtained PO mixture, 1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added, and the tumbler is added again. The mixture was obtained by dry blending using a blender. The obtained mixture was supplied to the twin screw extruder by a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump. The operating conditions of the feeder and the pump were adjusted so that the ratio of liquid paraffin in the total mixture to be extruded was 65% by mass and the polymer concentration (hereinafter sometimes abbreviated as “PC”) was 35% by mass. . These were then melt kneaded in a twin screw extruder.

  Subsequently, the obtained melt-kneaded product was cooled and solidified to obtain a sheet-like molded product. The obtained sheet-like molded product was led to an MD roll stretching machine, and then led to a TD uniaxial tenter to obtain a primary stretched film (primary stretching step). The set stretching conditions were an MD magnification of 3 times, a TD magnification of 3 times, an MD stretching temperature of 120 ° C., and a TD stretching temperature of 117 ° C. Next, the obtained primary stretched membrane was introduced into a methylene chloride bath and sufficiently immersed to extract and remove liquid paraffin as a plasticizer, and then methylene chloride was removed by drying to obtain a porous membrane.

  The obtained porous film was led to an MD uniaxial roll stretching machine, and then led to a TD uniaxial tenter to obtain a secondary stretched film (secondary stretching step). The set stretching conditions for this secondary stretching were MD 4 times, TD magnification 2 times, MD stretching temperature 120 ° C., and TD stretching temperature 120 ° C. Subsequently, the secondary stretched film was guided to a TD uniaxial tenter for heat setting. The maximum strain rate ratio (primary stretching maximum strain rate / secondary stretching maximum strain rate) was 0.12.

  As a heat setting step, after a stretching operation at a stretching temperature of 127 ° C. and a stretching ratio of 1.9 times, a relaxing operation at a relaxation temperature of 135.5 ° C. and a relaxation rate of 0.89 times was performed. Various characteristics of the obtained PO microporous membrane were evaluated by the above methods. The results are shown in Table 1.

[Examples 2 to 17 and Comparative Examples 1 to 4]
A PO microporous membrane was obtained in the same manner as in Example 1 except that the primary stretching ratio, primary stretching temperature, secondary stretching ratio, secondary stretching temperature, relaxation temperature, and relaxation rate were set as shown in Table 1, respectively. In addition, when the draw ratio is not described in the table, the film is not passed through the MD uniaxial roll stretching machine and the TD uniaxial tenter. Various characteristics of the obtained PO microporous membrane were evaluated by the above methods. The results are shown in Table 2. Moreover, the thermomechanical analysis primary differential curve in Example 2 is shown in FIG.

* Tm: DSC 2nd heat Maximum peak temperature (melting point)
Region I: Range below Tm−10 ° C. in the primary differential curve of thermomechanical analysis Region II: Range above Tm−10 ° C. in the primary differential curve of thermomechanical analysis and below Tm Region III: Above Tm in the primary differential curve of thermomechanical analysis, Tm + 5 ° C or less range

  The present invention has industrial applicability as a polyolefin microporous film suitable for a separator, particularly a lithium ion battery separator.

Claims (10)

In the thermomechanical analysis first derivative curve obtained by thermomechanical analysis measurement in the membrane width direction of the polyolefin microporous membrane,
The maximum value of the differential value in the region of less than the polyolefin microporous membrane 10 ° C. lower than the melting point Tm of (Tm-10 ° C.) is state, and are 0.1 or less,
In the thermomechanical analysis first derivative curve,
The polyolefin microporous membrane 10 ° C. lower than the melting point Tm of (Tm-10 ° C.) or higher, the maximum value of a differential value in the region lower than the melting point Tm of the polyolefin microporous membrane, Ru der 0.15 or more,
Polyolefin microporous membrane.   In the thermomechanical analysis first derivative curve obtained by thermomechanical analysis measurement in the membrane width direction of the polyolefin microporous membrane,
  The maximum value of the differential value in the region below 10 ° C. lower than the melting point Tm of the polyolefin microporous membrane (Tm−10 ° C.) is 0.1 or less,
  Both longitudinal tensile strength and tensile strength in the film width are 1500 kg / cm ² That's it,
  Polyolefin microporous membrane. In the thermomechanical analysis first derivative curve,
The maximum value of the differential value in the region below 10 ° C. lower than the melting point Tm of the polyolefin microporous membrane (Tm−10 ° C.) is 0.05 or less.
The polyolefin microporous membrane according to claim 1 or 2. In the thermomechanical analysis first derivative curve,
The maximum value of the differential value in the region below the melting point Tm of the polyolefin microporous membrane is 0.2 or more, at a temperature lower than the melting point Tm of the polyolefin microporous membrane by 10 ° C (Tm-10 ° C) or higher.
The polyolefin microporous film according to any one of claims 1 to 3. In the thermomechanical analysis first derivative curve,
The minimum value of the thermomechanical analysis differential value in the region below the melting point Tm of the polyolefin microporous membrane and less than the temperature (Tm + 5 ° C) 5 ° C higher than the melting point Tm of the polyolefin microporous membrane is 0.2 or more.
The polyolefin microporous membrane according to any one of claims 1 to 4. The dynamic friction coefficient in the film width direction is 0.4 or less,
The polyolefin microporous membrane according to any one of claims 1 to 5. The film thickness 12 μm equivalent air permeability is 150 seconds / 100 cc or less,
The polyolefin microporous membrane according to any one of claims 1 to 6 . The film thickness is 5 μm or more and 20 μm or less,
The polyolefin microporous membrane according to any one of claims 1 to 7 . The separator for electrical storage devices which consists of the polyolefin microporous film of Claims 1-8 . An electricity storage device comprising the electricity storage device separator according to claim 9 .