JP2017105925A - Microporous film, separator for battery and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microporous film, a separator for battery and a battery good in cycle property of the battery, winding property of a battery wound body and balance of smoothness of surface coating.SOLUTION: The microporous film is a microporous film for battery separator containing a polyolefin resin and average of power spectrum density Pobtained by Fourier transformation of a surface shape profile of the microporous film satisfies 4400(mm nm)≤P≤84000(mm nm) in range of space frequency 0.37 (/mm) to 3.3 (/mm).SELECTED DRAWING: None

Description

  The present invention relates to a microporous membrane, a battery separator, and a battery.

  Polyolefin microporous membranes made of polyolefin are widely used as separators in permeation separation membranes, fuel cells, capacitors and the like because they exhibit excellent electrical insulation and ion permeability. In particular, in recent years, polyolefin microporous membranes are suitably used as separators for lithium ion batteries widely used in notebook personal computers, mobile phones, digital cameras, vehicle-mounted devices, and the like.

  In recent years, the performance of these electronic devices has been remarkably improved, and there is a need to increase the size and capacity of batteries. Along with this, there is a rapid increase in demand for ensuring the life of large-sized batteries. In particular, various technical developments have been made in the characteristics (cycle characteristics) for maintaining good battery capacity after repeated charge and discharge of the battery. For example, Patent Documents 1 and 2 disclose separators having good cycle characteristics. Conventionally, as described in Patent Document 3, an average center roughness Ra or the like is used as an index for evaluating the separator surface.

JP 2009-269941 A JP 2010-202828 A JP 2014-62244 A

  However, Patent Document 3 does not discuss the winding property of the battery winding body and the smoothness of the surface coating at all. Moreover, in patent document 1, 2, examination regarding the said PSD is not performed at all about the separator for storage batteries.

  The present invention has been made in view of the above circumstances, and provides a microporous membrane, a battery separator, and a battery that have a good balance of battery cycle characteristics, battery winding properties, and surface coating smoothness. The purpose is to do.

  As a result of intensive studies, the present inventors have found that the above-described object is achieved when the power spectral density obtained by Fourier transforming the surface shape profile data of the microporous membrane is within a predetermined range, and the present invention is completed. It came to.

That is, the present invention is as follows.
[1]
A microporous membrane for a battery separator containing a polyolefin resin,
In the range where the average value P _ave of power spectral density obtained by Fourier transforming the surface shape profile data of the microporous membrane is a spatial frequency of 0.37 (/ mm) or more and 3.3 (/ mm) or less, 4400 ( mm · nm ² ) ≦ P _ave ≦ 84000 (mm · nm ² ).
[2]
The microporous membrane according to [1], wherein the P _ave satisfies 18000 (mm · nm ² ) ≦ P _ave ≦ 73000 (mm · nm ² ).
[3]
The microporous membrane according to [1] or [2], wherein the P _ave satisfies 31500 (mm · nm ² ) ≦ P _ave ≦ 73000 (mm · nm ² ).
[4]
In the roll wound with the microporous film, the difference between the maximum value and the minimum value of the outer diameter of the roll is 0.1 to 1.0 (mm / 2000 m), any of [1] to [3] A microporous membrane according to any one of the above.
[5]
A battery separator comprising the microporous film according to any one of [1] to [4].
[6]
A battery comprising the battery separator according to [5].

  ADVANTAGE OF THE INVENTION According to this invention, the microporous film excellent in the balance of the cycling characteristics of a battery, the winding property of a battery winding body, and the smoothness of a surface coating can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter sometimes abbreviated as “this embodiment”) will be described in detail. The present invention is not limited to the present embodiment described below, and can be implemented with various modifications within the scope of the gist.

The microporous membrane of this embodiment is a microporous membrane for battery separators containing a polyolefin resin, and is an average value of power spectral density (PSD) obtained by Fourier transforming the surface shape profile data of the microporous membrane. P _ave is the spatial frequency 0.37 (/ mm) or more 3.3 (/ mm) or less in the range satisfying the ^{4400 (mm · nm 2) ≦} P ave ≦ 84000 (mm · nm 2). Hereinafter, the range of _Pave is also simply referred to as “PSD range”.
Microporous membrane of this embodiment, since it is constructed as described above, especially, since the P _ave of the microporous membrane is in the above range, the cycle characteristics of the battery, winding of the battery wound body, In addition, the balance of the smoothness of the surface coating is excellent. Furthermore, the microporous membrane of this embodiment can realize a lithium ion secondary battery that is excellent in battery characteristics and productivity.

  Moreover, in the roll which wound up the microporous film of this embodiment, it is preferable that the difference between the maximum value and the minimum value of the outer diameter of the roll is 0.1 to 1.0 (mm / 2000 m). When the difference in outer diameter is within the above range, the microporous membrane of the present embodiment tends to have better surface coating smoothness, and can realize a lithium ion secondary battery with better productivity.

<Polyolefin microporous membrane>
The microporous membrane of the present embodiment is preferably a microporous membrane obtained from copolymerized high-density polyethylene and a polyolefin mixture containing high-density polyethylene (hereinafter also simply referred to as “mixture”).

(Copolymerized high density polyethylene)
The copolymerized high-density polyethylene is a polyethylene obtained by copolymerization of ethylene and another monomer, and has a high density.

  The viscosity-average molecular weight (Mv) of the copolymerized high-density polyethylene is not particularly limited, but is preferably 10,000 to 300,000. Mv can be measured by the method described in Examples described later.

  The comonomer of the copolymerized high-density polyethylene is preferably an α-olefin having 3 or more carbon atoms (hereinafter also simply referred to as “comonomer”), and is not limited to the following, but, for example, propylene, butene, pentene, hexene, heptene , Octene and the like. Among these, propylene having 3 carbon atoms is more preferable from the viewpoint of affinity with other polyethylene. The content of the α-olefin unit having 3 or more carbon atoms is preferably 0.1 mol% or more, more preferably 0.1 to 1 mol%, relative to the ethylene unit of the copolymerized high-density polyethylene. More preferably, it is 0.2-0.8 mol%. When the amount is 1 mol% or less, sufficient crystallinity can be secured, and sufficient permeability of the microporous membrane tends to be secured. In addition, the said content can be calculated | required by the method as described in the Example mentioned later.

The density of the copolymerized high-density polyethylene is related to the content of α-olefin units having 3 or more carbon atoms, but is set to a high density from the viewpoint of melting point and permeability. The "high density" (unit: g / cm < ³ >) here is 0.93-0.97, Preferably it is 0.94-0.96. In addition, in this embodiment, the density of polyethylene means the value measured in accordance with D) density gradient tube method described in JIS K7112 (1999).

  The copolymerized high-density polyethylene used in the present embodiment can be produced by various known methods, and is not limited to the following. For example, a chromium compound-supported catalyst as disclosed in Japanese Patent Publication No. 1-17777 Or a magnesium compound-containing Ziegler catalyst or a metallocene catalyst.

(High density polyethylene)
In this embodiment, the high-density polyethylene is polyethylene having a comonomer unit content of less than 0.1%, and homopolyethylene containing no comonomer is preferable. Here, “high density” has the same definition as “high density” for the copolymerized high density polyethylene.

  The Mv of the high density polyethylene is preferably 100,000 or more and less than 800,000, more preferably the polyethylene having an Mv of 200,000 or more and less than 700,000, and several types of polyethylene may be blended. When the Mv of the high-density polyethylene is 200,000 or more, sufficient mechanical strength tends to be ensured, and when it is less than 800,000, it is possible to prevent the molecular weight distribution from being widened and increase the defect rate (defects). Tend to be able to prevent.

(Composition of the mixture)
The proportion of the copolymerized high-density polyethylene in the mixture is preferably 10 to 90% by mass, more preferably 20 to 75% by mass, and still more preferably 25 to 70% by mass. When the ratio is 10 to 90% by mass, the mechanism is not clear, but the desired PSD range of the present embodiment tends to be obtained.

  The proportion of the high-density polyethylene in the mixture is preferably 10 to 90% by mass, more preferably 20 to 80% by mass, and still more preferably 30 to 70% by mass. When it is 10% by mass or more, sufficient heat resistance tends to be obtained, and when it is 90% by mass or less, the fuse response time tends to be sufficiently short.

  Moreover, it is preferable that the microporous film of this embodiment further contains polypropylene from the viewpoint of improving heat resistance. That is, it is preferable that the mixture further contains polypropylene. In this case, the ratio of polypropylene to the total amount of the mixture and polypropylene is preferably 1 to 35% by mass, more preferably 3 to 20% by mass, from the viewpoint of achieving both heat resistance and a good shutdown function. More preferably, it is 4-10 mass%.

  Arbitrary additives can be contained in the mixture in this embodiment. Examples of additives include polymers other than polyolefin resins; inorganic fillers; phenol-based, phosphorus-based and sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers An antistatic agent, an antifogging agent, a coloring pigment, and the like. The total addition amount of these additives is preferably 20 parts by mass or less with respect to a total of 100 parts by mass of the copolymerized high-density polyethylene and high-density polyethylene in the mixture from the viewpoint of improving the shutdown performance and the like. Preferably it is 10 mass parts or less, More preferably, it is 5 mass parts or less.

The viscosity average molecular weight of the microporous membrane of the present embodiment is preferably 15 to 550,000, more preferably 150,000 to 450,000. When the viscosity average molecular weight is from 1 to 550,000, the mechanism is not clear, but the desired PSD range of the present embodiment tends to be obtained. The viscosity average molecular weight can be adjusted to the above range, for example, by changing the composition ratio of raw material polymers having different molecular weights. In other words, the viscosity average molecular weight can be calculated from the viscosity average molecular weight of the raw polymer and the blending ratio thereof. On the other hand, a viscosity average molecular weight can also be measured based on ASTM-D4020. That is, a polymer (polyolefin) raw material or a microporous membrane is dissolved in a decalin solution at 135 ° C., the intrinsic viscosity [η] is measured, and the viscosity average molecular weight (Mv) can be calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For polypropylene, Mv can be calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

  Since the microporous membrane of the present embodiment has a porous structure in which a large number of very small holes are gathered to form dense communication holes, the ion conductivity is very excellent and the withstand voltage characteristics are also good. Moreover, it has high strength. The microporous film may be a single layer film made of the above-described material or a laminated film.

  The film thickness of the microporous membrane of this embodiment is preferably from 0.1 μm to 100 μm, more preferably from 1 μm to 50 μm, and even more preferably from 3 μm to 25 μm. From the viewpoint of mechanical strength, 0.1 μm or more is preferable, and from the viewpoint of increasing the capacity of the battery, 100 μm or less is preferable. The film thickness of the microporous film can be adjusted by controlling the die lip interval, the draw ratio in the drawing step, and the like. The film thickness can be measured with a dial gauge (Ozaki Seisakusho: “PEACOCK No. 25” (trademark)).

The porosity of the microporous membrane of this embodiment is preferably 25% or more and 95% or less, more preferably 30% or more and 65% or less, and further preferably 35% or more and 55% or less. 25% or more is preferable from the viewpoint of improving ion conductivity, and 95% or less is preferable from the viewpoint of withstand voltage characteristics. The porosity of the microporous membrane can be controlled by controlling the mixing ratio of the polyolefin resin composition and the plasticizer, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat fixing, the relaxation rate during heat fixing, It can be adjusted by combining. The porosity can be measured by taking a sample obtained by cutting a microporous membrane into a 10 cm square and calculating from the volume and mass using the following formula.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of polymer composition) / volume (c
m ³ ) × 100

  The air permeability of the microporous membrane of this embodiment is preferably 100 to 600 seconds, more preferably 120 to 550 seconds, and still more preferably 150 to 500 seconds. When the air permeability is 600 seconds or less, sufficient permeability tends to be secured, and when the air permeability is 100 seconds or more, the pore diameter tends to be prevented from becoming excessively large. The air permeability of the microporous membrane can be controlled by controlling the mixing ratio of the polyolefin resin composition and the plasticizer, stretching temperature, stretching ratio, heat setting temperature, stretching ratio during heat fixing, relaxation rate during heat fixing, Can be adjusted by combining. The air permeability can be measured by a Gurley type air permeability meter according to JIS P-8117.

  The puncture strength of the microporous membrane of this embodiment is preferably 1 to 20 N / 20 μm, more preferably 2 to 18 N / 20 μm, from the viewpoint of rupture resistance during battery winding and battery failure due to short circuit between electrodes. is there. The puncture strength of the microporous membrane controls the mixing ratio of the polyolefin resin composition and the plasticizer, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat fixing, the relaxation rate during heat fixing, It can be adjusted by combining. The piercing strength can be measured by performing a piercing test under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec using a “KES-G5 Handy Compression Tester” (trademark) manufactured by Kato Tech . That is, the maximum piercing load (N) obtained as described above is defined as the piercing strength.

<Power spectral density (PSD)>
The power spectral density is obtained by obtaining the amplitude intensity for each spatial frequency component by Fourier transform of the measured surface data of the microporous membrane. That is, the surface shape profile of the microporous membrane of the present embodiment is measured, and the surface shape profile is obtained by Fourier transform in a spatial frequency range of 0.37 (/ mm) to 3.3 (/ mm). The average value P _ave of the power spectral density is 4400 (mm · nm ² ) or more and 84000 (mm · nm ² ) or less, preferably 18000 (mm · nm ² ) or more and 73000 (mm · nm ² ) or less. , more preferably 31500 (mm · nm ²⁾ or 73000 (mm · nm ²⁾ or less.
When P _ave is 4400 (mm · nm ² ) or more, the cycle characteristics of the battery using the microporous membrane of this embodiment is excellent. Moreover, when it is 84000 (mm * nm < ² >) or less, it is excellent in a winding property with an electrode in the winding process of the battery using the microporous film of this embodiment. Here, when P _ave is in this range, the mechanism that provides excellent cycle characteristics is not clear, but is presumed to be as follows. As the battery is repeatedly charged and discharged, the battery performance deteriorates due to the deterioration of the electrolytic solution. This is thought to be because the electrolytic solution is decomposed by the electrochemical reaction, and decomposition gas is generated. When gas accumulates, the movement of lithium ions is hindered and the cycle characteristics tend to deteriorate. On the other hand, the fine if membrane P _ave of is in the above range, moderate undulation of the microporous membrane, a gap of minimum between the contact surfaces of the microporous membrane and the electrode, easily out cracked gas from which It is estimated that the cycle characteristics are improved. In addition, it is difficult for outside air to enter through the extremely small gap, and when manufacturing the electrode winding body, it is difficult to generate wrinkles or distortion due to air on the contact surface between the microporous membrane and the electrode. Presumed. The mechanism by which the value of P _ave increases or decreases is not clear, but among various factors that affect the value of P _ave , it is estimated that there is a high correlation with the viscosity average molecular weight of the microporous membrane and the wind speed during heat treatment during production. There is a tendency that the value of P _ave increases as the viscosity average molecular weight decreases and as the wind speed increases. The above _Pave can be measured by the method described in Examples described later.

<Outside diameter difference of microporous membrane roll>
The difference between the maximum value and the minimum value of the outer diameter of the roll around which the microporous membrane of this embodiment is wound is preferably 0.1 to 1.0 (mm / 2000 m), more preferably 0.1 to 0.1 mm. It is 0.6 (mm / 2000 m), more preferably 0.1 to 0.4 (mm / 2000 m). Here, when the difference in the outer diameter is within the range, the mechanism of excellent smoothness of the surface coating is not clear, but is estimated as follows. When the outer diameter difference of the roll is large, the distortion on the inner side in the width direction of the wound roll is large, and wrinkles due to the distortion are likely to occur. Therefore, in the coating process, it is estimated that the gravure coater cannot contact the wrinkled surface and a coating defect occurs. There are various factors that affect the above outer diameter difference, but it can be adjusted to the above range by adjusting each factor such as die lip adjustment, oscillation width adjustment, heat setting temperature, magnification, and winding tension. It can be measured by the method described in Examples described later.

(Production method)
It does not specifically limit as a method of manufacturing a microporous film, A well-known manufacturing method is employable. For example, the following method is mentioned.
(1) After melt-kneading a polyolefin resin composition (composition containing copolymerized high-density polyethylene and high-density polyethylene, the same shall apply hereinafter) and a pore-forming material into a sheet, and stretching as necessary , A method of making a pore by extracting a pore-forming material,
(2) A method in which a polyolefin resin composition is dissolved and then immersed in a poor solvent for polyolefin to solidify the polyolefin, and at the same time, the solvent is removed to make it porous.

  Hereinafter, as an example of a method for producing a microporous film, a method for extracting a pore-forming material after melt-kneading a polyolefin resin composition and a pore-forming material to form a sheet will be described.

  First, the polyolefin resin composition and the hole forming material are melt-kneaded. The melt-kneading method is not limited to the following, for example, by introducing a polyolefin resin and, if necessary, other additives into a resin kneading apparatus such as an extruder, kneader, lab plast mill, kneading roll, Banbury mixer, etc. A method of introducing and kneading the pore forming material at an arbitrary ratio while the components are heated and melted may be mentioned.

(Hole forming material)
Examples of the hole forming material include, but are not limited to, a plasticizer, an inorganic material, or a combination thereof.

  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, but are not limited to, 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 Etc. Note that these plasticizers may be recovered and reused after extraction by an operation such as distillation.

  Among the plasticizers, liquid paraffin is highly compatible with polyolefin resins such as polyethylene and polypropylene, and even when the melt-kneaded product is stretched, the interface between the resin and the plasticizer does not easily peel, and uniform stretching is possible. Is preferable because it tends to be easily implemented.

  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 tends to be sufficient for improving moldability. On the other hand, when the mass fraction of the plasticizer is 20% by mass or more, even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, the polyolefin molecular chain is not broken, and the pore structure is uniform and fine. Are easily formed, and the strength is also easily increased.

(Film forming process)
Next, the melt-kneaded product is formed into a sheet shape. The method for producing the sheet-shaped molded body is not limited to the following, but for example, the melt-kneaded material is extruded into a sheet shape via a T die or the like, and is brought into contact with the heat conductor to be sufficiently higher than the crystallization temperature of the resin component. And a method of solidifying by cooling to a low temperature. The heat conductor used for cooling and solidification is not limited to the following, and examples thereof include metals, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high heat conduction efficiency. In addition, when the extruded kneaded product is brought into contact with a metal roll, sandwiching between the rolls further increases the efficiency of heat conduction, and the sheet is oriented to increase the film strength, and the surface smoothness of the sheet is also improved. This is more preferable. The die lip interval when the melt-kneaded product is extruded from the T die into a sheet is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, the meander and the like are reduced, and there is little influence on the film quality such as streaks and defects, and the risk of film breakage or the like tends to be reduced in the subsequent stretching process. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high and uneven cooling can be prevented and the thickness stability of the sheet tends to be maintained.

(Plasticizer removal process)
Next, the pore-forming material is removed from the sheet-like molded body to form a microporous film. The method for removing the hole forming material is not limited to the following, and for example, a method of extracting the hole forming material by immersing the sheet-like molded body in an extraction solvent and sufficiently drying it may be mentioned. The method for extracting the hole forming material may be either a batch type or a continuous type. In order to suppress the shrinkage of the microporous membrane, it is preferable to constrain the end of the sheet-like molded body during a series of steps of immersion and drying. Moreover, it is preferable that the amount of pore-forming materials remaining in the microporous membrane is less than 1% by mass with respect to the mass of the entire microporous membrane.

  As the extraction solvent used when extracting the pore-forming material, it is preferable to use a solvent that is a poor solvent for the polyolefin resin and a good solvent for the pore-forming material and has a boiling point lower than the melting point of the polyolefin resin. . Examples of such extraction solvents include, but are not limited to, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; hydrofluoroether, hydro Non-chlorinated halogenated solvents such as fluorocarbons; 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.

(Stretching process)
Moreover, it is preferable to extend | stretch the said sheet-like molded object or a microporous film. The stretching may be performed before extracting the hole forming material from the sheet-like molded body. Moreover, you may carry out with respect to the microporous film which extracted the pore formation material from the said sheet-like molded object. Furthermore, it may be performed before and after extracting the hole forming material from the sheet-like molded body.

  As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferred from the viewpoint of improving the strength and the like of the obtained microporous film. When the sheet-like molded body is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, and the finally obtained microporous film is hardly torn and has high puncture strength. Examples of the stretching method include, but are not limited to, methods such as simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoints of improvement of puncture strength, uniformity of stretching, and shutdown property.

  Here, simultaneous biaxial stretching is a stretching method in which stretching in the MD (machine direction of continuous microporous membrane) stretching and TD (in the direction crossing the MD of the microporous membrane at an angle of 90 °) are performed simultaneously. The stretching ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD are stretched independently. When MD or TD is stretched, the other direction is fixed in an unconstrained state or a constant length. State.

  The draw ratio is preferably in the range of 20 times to 100 times in terms of surface magnification, and more preferably in the range of 25 times to 70 times. The draw ratio in each axial direction is preferably in the range of 4 to 10 times in MD, 4 to 10 times in TD, 5 to 8 times in MD, and 5 to 8 times in TD. More preferably, it is the range. When the total area magnification is 20 times or more, there is a tendency that sufficient strength can be imparted to the resulting microporous membrane. On the other hand, when the total area magnification is 100 times or less, membrane breakage in the stretching process is prevented and high production is achieved. There is a tendency to gain.

(Heat fixing)
The microporous membrane is preferably subjected to heat treatment for the purpose of heat setting from the viewpoint of suppressing shrinkage. As a heat treatment method, a stretching operation performed at a predetermined temperature, a wind speed and a predetermined stretching rate for the purpose of adjusting physical properties, and / or a relaxation performed at a predetermined temperature and a predetermined relaxation rate for the purpose of reducing stretching stress. Operation is mentioned. These heat treatments can be performed using a tenter or a roll stretching machine. In addition, it is preferable that the said wind speed is 13-18 m / s, More preferably, it is 14-17 m / s, More preferably, it is 15-17 m / s.

  For the stretching operation, it is preferable to stretch the MD and / or TD of the membrane by 1.0 times or more, more preferably 1.1 times or more from the viewpoint of obtaining a microporous membrane having further high strength and high porosity. .

  The relaxation operation is an operation for reducing the film to MD and / or TD. 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, it is a value obtained by multiplying the MD relaxation rate and the TD relaxation rate. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both the MD and TD directions, but only one of the MD and TD may be performed.

  The stretching and relaxation operations after this plasticizer extraction are preferably performed at TD. The temperature in the stretching and relaxation operation is preferably lower than the melting point of the polyolefin resin. When the temperature in the stretching and relaxation operation is in the above range, it is preferable from the viewpoint of the balance between the reduction in thermal shrinkage and the porosity.

(Battery separator and battery)
The microporous membrane of the present embodiment is particularly preferably used for applications as a battery separator. That is, the battery separator of the present embodiment includes the microporous film of the present embodiment. The battery of this embodiment includes the microporous film of this embodiment.

  Hereinafter, the present embodiment will be described in more detail by way of examples. However, the present embodiment is not limited to only these examples. In the following examples and comparative examples, all parts are parts by mass.

  The test method of the characteristic shown in an Example and a comparative example is as follows.

(Comonomer unit content (content of α-olefin unit having 3 or more carbon atoms))
In the 13C-NMR spectrum, the molar conversion value (A) of the integrated value of the signal intensity derived from the comonomer is divided by the sum of (A) and the molar converted value (B) of the integrated value of the signal intensity derived from the ethylene unit. By multiplying the obtained quotient by 100, the comonomer unit content (mol%) was determined. However, when polypropylene is contained in the polyolefin microporous membrane, the value obtained by removing the comonomer unit content (mol%) corresponding to the polypropylene content (wt%) determined above is used as the comonomer unit content of the microporous membrane ( Mol%).
For example, when propylene is used as a comonomer, in the following structural model, I1, I1 ′, I2, I3, Iα, Iβ, Iγ, Im, and IM are assumed to be signal intensities of 13C-NMR spectra derived from the corresponding carbons, respectively. The comonomer unit content is represented by the following formula.
Comonomer unit content (mol%) = (A) / [(A) + (B)] × 100
(Here, (A) = (I1 + Im + Iα / 2) / 3, (B) = (I1 + I2 + I3 + IM + Iα / 2 + Iβ + Iγ) / 2). )
Here, since the influence of the terminal is small, it can be ignored, and when I1, I2, and I3 are Im, Iα, Iβ, and Iγ are 2Im and the above formula is arranged, the comonomer unit content is expressed by the following formula.
Comonomer unit content (mol%) = Im / [Im + (IM + 5Im) / 2] × 100

(Power spectral density (PSD) measurement obtained by Fourier transform of surface profile data)
The surface shape profile was 2,000 × 2000 μm using microporous membranes obtained in Examples and Comparative Examples using a stylus type surface shape measuring instrument (product name: DEKTAK XT-A (manufactured by ULVAC, Inc.)). ^Two ranges were measured. The measurement conditions were a stylus with a needle pressure of 1 mg and a needle diameter radius of 12.5 μm, and the sampling interval for each data point was 2000 (μm) / 200 (pt) = 10 in the winding direction of the microporous membrane. (Μm / pt) and film width direction 2000 (μm) / 3000 (pt) = 0.67 (μm / pt). In the calculation of PSD, the program calculated the power spectral density of the average roughness curve of 200 lines each in the film width direction. The output result represents the average profile power at each spatial frequency.
This power spectral density function is the square of the result of Fourier transformation of the original surface profile, and the average PSD (f) when a length L profile having N points is digitized is given by the following equation.

In the above formula, i represents a number that is squared to −1, d ₀ represents a data sampling interval, Z _j represents an amplitude function, and j is a profile of length (measurement range) L existing at N points. , And the spatial frequency f is equal to K / L, where K is an integer and takes a value between 1 and N / 2. In Examples and Comparative Examples described later, the d ₀ was 0.67 μm, and the L was 2000 μm.

(Roll outer diameter change (outer diameter difference))
The outer diameter of the roll obtained by winding the microporous membrane obtained in Examples and Comparative Examples at a speed of 20 m / min was measured using a laser dimension measuring device (measurement unit: manufactured by Keyence Corporation, sensor head LS-3060, controller unit). : Controller LS-3000 manufactured by Keyence Corporation).
Specifically, by moving a dimension measuring device composed of two sets of a projector and a receiver in parallel to the direction of the rotation axis of the winding body, the outer diameter on the line at all positions in the direction of the rotation axis of the winding body is reduced. It was measured. The difference between the maximum value and the minimum value of the inner and outer diameter values in the width direction was determined for the obtained char of the outer diameter of the roll. The roll had a width of 1200 mm to 1300 mm and a length of 2000 m.

(Cycle characteristics)
Using the microporous membranes obtained in Examples and Comparative Examples, the strip-shaped positive electrode and the strip-shaped negative electrode are stacked in the order of the strip-shaped negative electrode, the separator, the strip-shaped positive electrode, and the separator, and wound in a spiral shape to obtain an electrode plate laminate. Produced. This electrode plate laminate is pressed into a flat plate shape, and then stored in an aluminum container. The aluminum lead is led out from the positive electrode current collector to the battery lid, and the nickel lead is led out from the negative electrode current collector to the bottom of the container. Welded. Furthermore, a non-aqueous electrolyte (prepared by dissolving LiPF ₆ as a solute to a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio)) in this container. Infused and sealed. The lithium ion battery thus produced was designed to have a length (thickness) of 6.3 mm, a width of 30 mm, a height of 48 mm and a nominal discharge capacity of 620 mAh. The lithium ion battery assembled as described above was charged with a constant current and constant voltage (CCCV) for 6 hours under the conditions of a current value of 310 mA (0.5 C) and a final battery voltage of 4.2 V. At this time, the current value immediately before the end of charging was almost zero. Then, it was left (aged) for 1 week in an atmosphere at 25 ° C.
Next, the battery was charged at a constant current and constant voltage (CCCV) for 3 hours under the conditions of a current value of 620 mA (1.0 C) and a termination battery voltage of 4.2 V, and discharged to a battery voltage of 3.0 V at a constant current value (CC) of 620 mA. The cycle of doing. The discharge capacity at this time was defined as the initial discharge capacity. Thereafter, the above cycle was further repeated 300 times. In this cycle, the ratio (%) of the capacity at the 300th cycle to the initial discharge capacity was defined as the capacity maintenance rate. When the capacity retention rate was high, the cycle characteristics were good, and the evaluation was made according to the following criteria.
◎: 90% or more ○: 85% to 90%
Δ: 80% to 85%
X: Less than 85%

(Windability of the battery winding body)
The electrode winding body created using the microporous membranes obtained in Examples and Comparative Examples was wound using a winding machine (manual winding machine manufactured by Minato Seisakusho Co., Ltd.). Details are as follows. Using the microporous membranes obtained in the examples and comparative examples, the battery cathodes were obtained by winding the belt-like positive electrode and the belt-like negative electrode in the order of the belt-like negative electrode, the separator, the belt-like positive electrode and the separator in a spiral manner. Produced. The widths of the strip-like positive electrode, strip-like negative electrode, and separator were 50 mm, 52 mm, and 56 mm, respectively, and the length was 3 m. In the winding process for producing the electrode body, when the widthwise center of the negative electrode coincides with the widthwise center of the microporous film, the width (W) of the portion where both ends of the microporous film protrude from the negative electrode is expressed by the following formula: As can be seen, each of them is 2 mm, which is a winding standard.
W (mm) = (width of microporous membrane−width of negative electrode) / 2 = 2 mm

When the center in the width direction of the negative electrode does not coincide with the center in the width direction of the microporous film due to wrinkles or distortion of the microporous film, the widths of both ends of the microporous film protrude from the negative electrode to W ₁ and W ₂ , respectively. It was. That is, as shown in the following formula, W ₁ was defined to be larger than ₂ mm and W ₂ was smaller than ₂ mm.
W ₁ + W ₂ (mm) = width of microporous film−width of negative electrode = 4 mm
W ₁ > 2 mm, W ₂ <2 mm

In the winding process for producing the electrode body, the width (d) of winding deviation due to wrinkles or distortion was calculated from the following equation.
d (mm) = W ₁ −W = W−W ₂
In addition, when the width (d) of the winding deviation was small, it was evaluated according to the following criteria on the assumption that the winding property was good.
○: Less than 1.0 mm Δ: 1.0-1.5 mm
×: 1.5 mm or more

(Smoothness of surface coating)
After performing the corona discharge treatment (discharge amount 50 W) on the surface using the microporous membrane obtained in Examples and Comparative Examples, alumina particles (average particle size 0.7 μm)) 98.2 on the treated surface side. An aqueous solution in which 1.8 parts by mass of polyvinyl alcohol (average polymerization degree 1700, saponification degree 99% or more) 1.8 parts by mass was uniformly dispersed in 150 parts by mass of water was applied using a gravure coater. Then, it dried at 60 degreeC and water was removed, and the multilayer porous membrane in which the porous layer with a thickness of 2 micrometers was formed on the microporous membrane was obtained.
Twenty samples having a size of 100 cm ² were cut out from the produced multilayer porous membrane, and using the maximum value (W _MAX ) and the minimum value (W _MIN ) of mass (W) per unit area of the 20 porous layers, The mass change value (R) was calculated from the following formula.
R value = W _MAX -W _MIN

W measured the mass of the multilayer porous membrane, then measured the mass of the microporous membrane from which the porous layer had been removed, and calculated from the following formula.
W (g / m ² ) = (mass of multilayer porous membrane−mass of polyolefin resin porous membrane) × 1000
When the R value was low, the smoothness of the surface coating was good, and the following criteria were evaluated.
○: 0.01 g / m ^{2 to} 0.30 g / m ²
Δ: 0.30 g / m ^{2 to} 0.50 g / m ²
×: 0.50 g / m ² or more

[Example 1]
19 parts high density polyethylene (density 0.95) with Mv 250,000, 19 parts high density polyethylene (density 0.95) with Mv 700,000, homopolymer polypropylene (density 0.91) with Mv 400,000 2 parts, 0.3 parts of tetrakis- [methylene-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate] methane are mixed with the composition as an antioxidant. Then, it was put into a twin screw extruder through a feeder. Further, 60 parts of liquid paraffin (P-350 (trademark) manufactured by Matsumura Oil Co., Ltd.) was injected into the extruder by side feed, kneaded at 200 ° C., extruded from a T-die installed at the tip of the extruder, and immediately The mixture was cooled and solidified with a cast roll cooled to 30 ° C. to form a gel sheet having a thickness of 1500 μm. This gel-like sheet was stretched 7 × 7 times at 120 ° C. with a simultaneous biaxial stretching machine, and then this stretched film was introduced into a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Methylene chloride was removed by drying to obtain a microporous membrane. The obtained microporous membrane was guided to a TD tenter to perform heat setting (sometimes abbreviated as “HS”), heat setting temperature 131 ° C., calorie wind speed 13.2 m / s, and draw ratio 1.5 times. After HS, a relaxation operation of 0.85 times (that is, HS relaxation rate is 0.85 times) was performed to obtain a film. Table 1 shows the results of evaluating various characteristics of the obtained film.

[Example 2]
A film was formed in the same manner as in Example 1 except that HS was performed at a heat setting temperature of 131.5 ° C. and a calorie wind speed of 13.7 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 3
12 parts of copolymerized high density polyethylene (comonomer: propylene, propylene unit content 0.6 mol%, density 0.95) with Mv of 150,000, 14 parts of high density (density 0.95) polyethylene with Mv of 250,000 12 parts of high density polyethylene (density 0.95) with an Mv of 700,000 and 2 parts of polyolefin raw material with a homopolymer polypropylene (density 0.91) with an Mv of 400,000. Further, a film was formed in the same manner as in Example 1 except that HS was performed at a heat fixing temperature of 128 ° C. and a calorie wind speed of 14.5 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 4
A film was formed in the same manner as in Example 3 except that HS was performed at a calorie wind speed of 15.7 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 5
A laminated porous film having a three-layer structure of surface layer / intermediate layer / surface layer was produced as follows. On the surface layer, 14 parts of high density polyethylene (density 0.95) with an Mv of 250,000, 14 parts of high density polyethylene (density 0.95) with an Mv of 700,000, and a homopolymer polypropylene (density 0. 91) and raw material of 7.4 parts of inorganic filler were used. As the inorganic filler, talc (SK-C2 manufactured by Katsuyama Co., Ltd.) was used. The intermediate layer is made of 15.7 parts of high density polyethylene (density 0.95) with Mv of 250,000, 15.7 parts of high density polyethylene (density 0.95) with Mv of 700,000, and homopolymer of Mv 400,000 A polypropylene raw material of 1.6 parts of polypropylene was used.
Next, the raw material for the surface layer and the raw material for the intermediate layer were respectively charged into separate twin screw extruders. Liquid paraffin was injected into the middle part of the cylinders of both extruders so that the surface layer was 63% by mass and the surface layer was 67% by mass. The die used was a T-die capable of multi-manifold coextrusion. In the die, the surface layer was divided into approximately equal parts and stacked on both sides of the intermediate layer. The melted film raw material coming out of the die was cooled and solidified with a cast roll. Furthermore, a film was formed in the same manner as in Example 1 except that HS was performed at a heat setting temperature of 132.5 ° C. and a calorie wind speed of 15.9 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 6
A film was formed in the same manner as in Example 1 except that HS was performed at a heat setting temperature of 132 ° C. and a calorie wind speed of 16.1 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 7
A film was formed in the same manner as in Example 3 except that HS was performed at a heat setting temperature of 128.5 ° C. and a calorie wind speed of 16.6 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 8
20 parts of copolymerized high density polyethylene (comonomer: propylene, propylene unit content 0.6 mol%, density 0.95) with Mv of 150,000, 18 parts of high density polyethylene (density 0.95) with Mv of 250,000 , Mv 400,000 high density polypropylene (density 0.91) using 2 parts of polyolefin raw material. Further, a film was formed in the same manner as in Example 1 except that HS was performed at a heat fixing temperature of 123 ° C. and a calorie wind speed of 17.1 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Example 9
A film was formed in the same manner as in Example 8 except that HS was performed at a heat setting temperature of 123.5 ° C. and a calorie wind speed of 17.5 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 1]
A film was formed in the same manner as in Example 1 except that HS was performed at a heat setting temperature of 130 ° C. and a calorie wind speed of 12.4 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 2]
A film was formed in the same manner as in Example 5 except that HS was performed at a heat setting temperature of 134 ° C. and a calorie wind speed of 18.2 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

[Comparative Example 3]
A film was formed in the same manner as in Example 8 except that HS was performed at a heat setting temperature of 124 ° C. and a calorie wind speed of 18.6 m / s. Table 1 shows the physical properties of the obtained microporous membrane.

Claims (6)

A microporous membrane for a battery separator containing a polyolefin resin,
In the range where the average value P _ave of power spectral density obtained by Fourier transforming the surface shape profile data of the microporous membrane is a spatial frequency of 0.37 (/ mm) or more and 3.3 (/ mm) or less, 4400 ( mm · nm ² ) ≦ P _ave ≦ 84000 (mm · nm ² ). The microporous membrane according to claim 1, wherein the P _ave satisfies 18000 (mm · nm ² ) ≦ P _ave ≦ 73000 (mm · nm ² ). The microporous membrane according to claim 1 or 2, wherein the P _ave satisfies 31500 (mm · nm ² ) ≤ P _ave ≤ 73000 (mm · nm ² ).   The roll which wound up the said microporous film WHEREIN: The difference of the maximum value and the minimum value of the outer diameter of the said roll is 0.1-1.0 (mm / 2000m), The any one of Claim 1 thru | or 3 The microporous membrane according to item.   The battery separator containing the microporous film of any one of Claims 1 thru | or 4.   A battery comprising the battery separator according to claim 5.