JP5213768B2 - Polyolefin microporous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous membrane suitable for a separator capable of improving cycle characteristics of an electric storage device. <P>SOLUTION: The polyolefin microporous membrane includes a first microporous layer containing polypropylene and polyethylene, and a second microporous layer laminated on the first microporous layer. The first microporous layer forms a surface layer. Melting heat of the polypropylene is not lower than 90 J/g. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

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

In recent years, power storage devices such as lithium ion secondary batteries and electric double layer capacitors (including those called lithium ion capacitors and non-aqueous lithium power storage elements) have been actively developed. In a power storage device, a microporous membrane (separator) is usually provided between the positive and negative electrodes. Such a separator has a function of preventing contact between positive and negative electrodes and transmitting ions.
Here, from the viewpoint of further improving the safety of the electricity storage device, the battery reaction is quickly stopped in the case of abnormal heating in addition to a certain mechanical property that does not break during repeated charging and discharging. Characteristics (shutdown characteristics), performance that maintains the shape even at high temperatures and prevents a dangerous situation in which the positive electrode material and the negative electrode material react directly are required (heat-resistant film-breaking property).

  Under such circumstances, for example, Patent Document 1 discloses a first type layer that is deformed into a substantially non-porous film-like sheet while maintaining dimensions substantially at a temperature of about 80 ° C. to 150 ° C. Further, there is disclosed a laminated sheet comprising a microporous structure and a second type layer that substantially maintains the dimensions at room temperature to a temperature at least about 10 ° C. higher than the transformation temperature of the first type layer.

JP-A-62-1010857

  However, the laminated sheet described in Patent Document 1 still has room for improvement in terms of characteristics (cycle characteristics) for maintaining good battery capacity after repeated charging and discharging. An object of the present invention is to provide a polyolefin microporous membrane suitable as a separator capable of improving the cycle characteristics of an electricity storage device.

  As a result of intensive studies in view of the above circumstances, the present inventors include a first microporous layer containing polypropylene and polyethylene, and a second microporous layer laminated on the first microporous layer, A polyolefin microporous membrane characterized in that one microporous layer forms a surface layer and the heat of fusion of the polypropylene is 90 J / g or more is suitable as a separator that can improve the cycle characteristics of an electricity storage device. As a result, the present invention has been completed.

That is, the present invention is as follows.
[1]
A first microporous layer comprising polypropylene and polyethylene, and a second microporous layer that is laminated to the first microporous layer and is different from the first microporous layer,
The first microporous layer forms a surface layer;
Polyolefin microporous membrane [2], wherein the heat of fusion of polypropylene in the microporous layer is 90 J / g or more
2. The polyolefin microporous membrane according to claim 1, wherein the proportion of the polypropylene in the total amount of the polyolefin resin used in the first microporous layer is 40 to 95% by mass.
[3]
The first microporous layer further contains inorganic particles, and the proportion of the inorganic particles in the total amount of the polyolefin resin and inorganic particles used in the first microporous layer is 5 to 60% by mass. Item 2. The polyolefin microporous membrane according to Item 1.
[4]
4. The polyolefin microporous membrane according to claim 1, wherein the first microporous layer has a thickness of 0.2 μm or more and 5 μm or less.
[5]
The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the second microporous layer contains polyethylene as a main component.
[6]
The separator for electrical storage devices formed using the polyolefin microporous film in any one of Claims 1-5.

  The polyolefin microporous membrane of the present invention is suitable as a separator that can improve the cycle characteristics of an electricity storage device.

The best mode for carrying out the present invention (hereinafter abbreviated as “embodiment”) will be described in detail below. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.
The polyolefin microporous membrane of the present embodiment includes a first microporous layer containing polypropylene and polyethylene, and a second microporous layer laminated on the first microporous layer, which is different from the first microporous layer. And a layer. Both the first microporous layer and the second microporous layer are preferably formed of a polyolefin resin composition containing a polyolefin resin. The “different” may be a difference in raw materials or a difference in structure such as porosity.

Examples of the polyolefin resin used in the present embodiment include polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene ( Homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more.
Examples of the polyolefin resin include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. It is done. In addition, it is preferable that the said polyolefin resin composition contains the said polyolefin resin as a main component. In the present embodiment, the “main component” is preferably a proportion in which the specific component is also in the matrix component containing the specific component, preferably 5% by mass or more, more preferably 70% by mass or more, and still more preferably 90% by mass. That is the above, meaning that it may be 100% by mass.
The second microporous layer preferably contains polyethylene as a main component.

From the viewpoint of reducing the melting point of the polyolefin microporous membrane or improving the piercing strength, the polyolefin resin preferably contains high-density polyethylene.
The proportion of the high-density polyethylene in the polyolefin resin is preferably 10% by mass or more, and more preferably 30% by mass or more.
Here, from the viewpoint of improving the heat resistance of the polyolefin microporous membrane, the polyolefin resin forming the first microporous layer preferably includes polypropylene.
The proportion of polypropylene in the polyolefin resin is preferably 40% by mass or more, more preferably 60% by mass or more, and the upper limit is preferably 95% by mass or less, more preferably 80% by mass or less. Setting the ratio to 40% by mass or more is preferable from the viewpoint of improving the heat resistance (or dimensional stability) of the polyolefin microporous membrane. On the other hand, setting the ratio to 95% by mass or less is preferable from the viewpoint of realizing a microporous film having good stretchability and high piercing strength.

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

In particular, the molecular weight of the polypropylene resin is preferably 100,000 or more and 600,000 or less, and more preferably 100,000 or more and 500,000 or less from the viewpoints of ensuring good moldability, sheet formability, and permeability.
In addition, the heat of fusion ΔH of the polypropylene resin is preferably 70 J / g or more, more preferably 80 J / g or more, and 90 J / g from the viewpoint of increasing the crystallinity of the microporous film and making the pore diameter uniform. The above is more preferable.

If necessary, the polyolefin resin composition used for the first microporous layer or the second microporous layer may be phenol-based, phosphorus-based, sulfur-based antioxidants; calcium stearate, zinc stearate, etc. Metal soaps; UV absorbers, light stabilizers, crystal nucleating agents, antistatic agents, antifogging agents, various additives such as color pigments, and inorganic fillers can be mixed and used.
Examples of the inorganic filler include alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide, and other oxide ceramics, silicon nitride, titanium nitride, boron nitride, and the like. Nitride ceramics, silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, asbestos, Examples include zeolite, calcium silicate, magnesium silicate, diatomaceous earth, ceramics such as silica sand, and glass fiber. These can be used alone or in combination of two or more. Among these, silica, alumina, and titanium are more preferable from the viewpoint of electrochemical stability. Silica is particularly preferable.

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

On the other hand, setting the average particle size to 1 nm or more is preferable from the viewpoint of securing the dispersibility of the inorganic particles and improving the compression resistance in the local region.
In addition, the average particle diameter in this Embodiment is a value measured by the Coulter method.
Further, the oil absorption amount of the plasticizer (described later) of the inorganic particles is preferably 150 ml / 100 g or more, and the upper limit is preferably 1000 ml / 100 g or less, more preferably 500 ml / 100 g or less. Setting the oil absorption to 150 ml / 100 g or more is preferable from the viewpoint of suppressing formation of aggregates in a kneaded product containing a polyolefin resin, inorganic particles, and a plasticizer, and ensuring good moldability. Further, when a polyolefin microporous membrane is used as a separator for an electricity storage device, it is excellent from the viewpoint of ensuring the impregnation property and liquid retention property of the nonaqueous electrolytic solution, and maintaining the electricity storage device productivity and performance maintenance in long-term use. On the other hand, setting the oil absorption to 1000 ml / 100 g or less is preferable from the viewpoint of handleability of inorganic particles when producing a polyolefin microporous membrane.

The oil absorption amount in the present embodiment is a value measured by a FRONTEX S410 plasticizer oil absorption amount measuring device. 5 g of inorganic particles were added, and a plasticizer was added dropwise while kneading. The amount of plasticizer added (ml) when the torque at the time of kneading increased and decreased to 70% of the maximum torque was determined, and was calculated from the weight of the inorganic particles (g) using the following formula.
Plasticizer oil absorption (ml / 100 g) = Plasticizer addition / Inorganic particle weight × 100

  The proportion of the inorganic particles in the total amount of the polyolefin resin and the inorganic particles is preferably 5% by mass or more, more preferably 20% by mass or more, and the upper limit is usually 60% by mass or less, preferably 40%. It is below mass%. Setting the ratio to 5% by mass or more is preferable from the viewpoint of forming a polyolefin microporous film with a high porosity and the impregnation property of a nonaqueous electrolytic solution. On the other hand, it is preferable that the ratio is 60% by mass or less from the viewpoint of improving the film formability at a high draw ratio and improving the puncture strength of the polyolefin microporous film.

As a manufacturing method of the polyolefin microporous film of this Embodiment, the manufacturing method including each process of following (1)-(5) can be used, for example.
(1) A kneading step in which a kneaded product is formed by kneading a polyolefin resin, inorganic particles, and a plasticizer as raw materials for each layer according to a desired configuration.
(2) A molding step in which two or more types of the kneaded material are laminated, cooled and solidified, and processed into a sheet-like laminated molded body after the kneading step.
(3) A stretching process in which, after the molding process, the sheet-shaped molded body is biaxially stretched at a surface magnification of 20 to 200 times to form a stretched product.
(4) A porous body forming step of forming a porous body by extracting a plasticizer from the stretched product after the stretching step.
(5) A heat treatment step in which, after the porous body forming step, the porous body is heat-treated at a melting point or less of the polyolefin resin and stretched in the width direction.

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

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

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

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

  The step (2) is, for example, a step of obtaining a gel sheet by extruding the kneaded material into a sheet shape through a T-die or the like and bringing it into contact with a heat conductor to cool and solidify. The gel sheet can be produced by either a method in which the gel sheets constituting each layer are integrated from each extruder and co-extruding with a single die, or a method in which the gel sheets constituting each layer are stacked and heat-sealed. Co-extrusion is more preferred because it is easy to obtain high interlayer adhesive strength, and it is easy to form communication holes between layers, so that high permeability can be easily maintained and productivity is excellent. As the heat conductor, metal, water, air, plasticizer itself, or the like can be used. Moreover, it is preferable to cool and solidify by sandwiching between rolls from the viewpoint of increasing the film strength of the sheet-like molded body and improving the surface smoothness of the sheet-like molded body.

Examples of the stretching method in the step (3) include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Among these, it is preferable to employ the simultaneous biaxial stretching method from the viewpoint of increasing the puncture strength of the polyolefin microporous film and making the film thickness uniform.
Further, the surface magnification in the step (3) is preferably 20 times or more, preferably 25 times or more, and the upper limit is preferably 200 times or less, more preferably 100 times or less. Setting the surface magnification to 20 times or more is preferable from the viewpoint of securing sufficient strength as a separator.

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

The step (4) is preferably performed after the step (3) from the viewpoint of improving the puncture strength of the polyolefin microporous membrane. Examples of the extraction method include a method of immersing the stretched product in the plasticizer solvent. The residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by mass.
The step (5) is preferably a step of performing heat fixation and / or heat relaxation.

Here, the draw ratio in the step (5) is preferably less than 4 times, more preferably less than 3 times as the surface magnification. Setting the surface magnification to less than 4 times is preferable from the viewpoint of suppressing the generation of macrovoids and a decrease in puncture strength.
The heat treatment temperature is preferably 100 ° C. or higher based on the melting point temperature of the polyolefin resin, and the upper limit is preferably lower than the melting point temperature of polyethylene. Setting the heat treatment temperature to 100 ° C. or higher suppresses the occurrence of film breakage and the like. On the other hand, setting the melting point of polyethylene at or below ° C is suitable from the viewpoint of suppressing the shrinkage of the polyolefin resin and reducing the thermal shrinkage rate of the polyolefin microporous membrane.
In addition, you may post-process with respect to the obtained polyolefin microporous film after the process of said (5). Examples of such post-treatment include hydrophilization treatment with a surfactant and the like, and crosslinking treatment with ionizing radiation and the like.

The polyolefin microporous membrane of the present embodiment has a puncture strength (measured according to the measurement method in Examples described later) of 240 g / 20 μm or more, preferably 400 g / 20 μm or more, preferably as an upper limit. It is 2000 g / 20 μm or less, more preferably 1000 g / 20 μm or less. Setting the puncture strength to 240 g / 20 μm or more is preferable from the viewpoint of suppressing film breakage due to the dropped active material or the like during battery winding. Moreover, the concern that a short circuit may occur due to the expansion and contraction of the electrode accompanying charging and discharging can be suppressed. On the other hand, setting it to 2000 g / 20 μm or less is preferable from the viewpoint of reducing width shrinkage due to orientation relaxation during heating.
The puncture strength can be adjusted by adjusting the draw ratio and the draw temperature.

The porosity of the microporous membrane (measured according to the measurement method in Examples described later) is 20% or more, preferably 35% or more, and the upper limit is preferably 90% or less, preferably 80% or less. It is. Setting the porosity to 20% or more is preferable from the viewpoint of ensuring the permeability of the separator. On the other hand, it is preferable to set it as 90% or less from a viewpoint of ensuring piercing strength.
The porosity can be adjusted by changing the draw ratio.

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

The thickness of the first microporous layer containing polypropylene and polyethylene of the microporous membrane is preferably 0.2 μm or more, more preferably 0.5 μm or more, still more preferably 2.0 μm or more, preferably as an upper limit. Is 5 μm or less, more preferably 4 μm or less. A film thickness of 0.5 μm or more is preferable from the viewpoint of heat resistance. On the other hand, it is preferable to set it as 5 micrometers or less from a viewpoint of ensuring piercing strength.
The film thickness of the first microporous layer can be adjusted by changing the extrusion amount or the like when the microporous film is produced using the coextrusion method. In the case of a two-type three-layer configuration in which the second microporous layer is held between the first microporous layers, the film thickness of the first microporous layer is the film thickness of the microporous layer located on one side. Means.

The air permeability of the microporous membrane (measured according to the measurement method in Examples described later) is preferably 10 seconds or more, more preferably 50 seconds or more, and the upper limit is preferably 1000 seconds or less. Preferably it is 500 seconds or less. Setting the air permeability to 10 seconds or more is preferable from the viewpoint of suppressing self-discharge of the electricity storage device. On the other hand, setting it to 1000 seconds or less is preferable from the viewpoint of obtaining good charge / discharge characteristics.
The air permeability can be adjusted by changing the stretching temperature and the stretching ratio.

The heat of fusion of the polypropylene in the microporous membrane (measured according to the measurement method in Examples described later) is preferably 90 J / g or more, more preferably 95 J / g or more, and even more preferably 100 J / g. The upper limit is preferably 165 J / g or less, and more preferably 150 J / g or less. In this embodiment, the heat of fusion of polypropylene in the microporous membrane is specified, but by adjusting the cooling conditions after extrusion, the heat of fusion of propylene in the microporous membrane is specified above a specific value. It is considered that the lamellar layer becomes moderately thick. When a large number of lamellar layers with an appropriate thickness are generated, a microporous film is formed to maintain the pore structure skeleton in the stretching process, extraction process, and heat treatment process, and to prevent uneven crushing in the film thickness direction. In this case, it is considered that a large number of holes having a uniform size are generated. Therefore, it is considered that a large number of pores having a uniform size are generated when the microporous film is formed. When such a microporous membrane having a large number of uniformly sized pores is arranged inside the battery, the ion permeation flow path will be present in a uniform and uniform manner, and clogging is unlikely to occur during repeated charge and discharge. It is presumed that the cycle characteristics as a battery are improved. Setting the heat of fusion of the polypropylene in the microporous film to 90 J / g or more is preferable from the viewpoint of improving cycle characteristics and heat resistance. On the other hand, it is preferable to set it as 165 J / g or less from a viewpoint of ensuring piercing strength.
The heat of fusion of polypropylene in the microporous membrane can be adjusted by a method of adjusting kneading conditions, a method of adjusting cooling conditions after kneading, or the like.

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

  Since the electricity storage device using the polyolefin microporous membrane of the present embodiment as a separator contains polypropylene having a specific heat of fusion in the surface layer of the separator, cycle characteristics can be improved.

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

(1) Viscosity average molecular weight (Mv)
Based on ASRM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent was determined, and Mv of polyethylene was calculated by the following equation.
[Η] = 0.00068 × Mv ^0.67
Mv of polypropylene was calculated from the following formula.
[Η] = 1.10 × Mv ^0.80
The Mv of the layer was calculated using the polyethylene equation.

(2) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a microthickness measuring instrument (type KBM manufactured by Toyo Seiki Co., Ltd.).

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

(4) Air permeability (sec / 100cc)
It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117.

(5) Puncture strength (g)
A piercing test was conducted 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 trademark, KES-G5 handy compression tester manufactured by Kato Tech, and the maximum piercing load was defined as the piercing strength (N). .

(6) Heat of melting of polypropylene (J / g)
Measurement was performed using DSC60 manufactured by Shimadzu Corporation. The surface layer was peeled off from the microporous polyolefin membrane, punched into a circle with a diameter of 5 mm, and several sheets were stacked to give 3 mg as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endothermic curve. About the obtained melting endothermic curve, a linear baseline is set in the range of 85 ° C. to 175 ° C., and the amount of heat is calculated from the area of the portion surrounded by the linear baseline and the endothermic melting curve. In conversion, the heat of fusion of polypropylene in the microporous membrane was further calculated from the content of polypropylene in the microporous membrane. Similarly, the melting point of polypropylene was measured using the melting heat ΔH of the polypropylene resin and the minimum temperature in the endothermic melting curve as the melting point.
When a plurality of polyolefin resins are present in the microporous membrane and the peaks partially overlap, the maximum value and the maximum of the endothermic melting curve in the range of 85 ° C. to 175 ° C. are analyzed using the analysis software TA-60 manufactured by Shimadzu Corporation. Connect the value or maximum value, the endothermic melting curve and the point where the baseline is in contact with a straight line, calculate the amount of heat from the area surrounded by the straight line and the endothermic melting curve, and similarly calculate the heat of fusion of polypropylene in the microporous membrane. Calculated. At this time, in the endothermic melting curve of the microporous membrane, the temperature at the minimum value has a value closer to the melting point of the polypropylene resin, and the heat of fusion calculated from the area obtained from the above analysis method is the polypropylene in the microporous membrane. Of heat of fusion.

(7) Surface layer thickness (μm)
Calculation was carried out by multiplying the film thickness of all layers by the ratio of the extrusion amount to the total surface layer.

(8) Cycle characteristics (% / 100 times)
A lithium ion battery using an electrolyte prepared by adding LiPF ₆ 1M to a solvent composed of ethylene carbonate (EC): methylene carbonate (MEC) = 1: 2 (weight ratio), using a carbon electrode as a negative electrode and LiCoO ₂ as a positive electrode. Was made. A cycle test in which this battery was charged to 4.2 V and then discharged was repeated 100 times at 25 ° C., and the change in battery capacity after the cycle test was examined.

(9) Thermal film breaking temperature (° C.), heat-resistant film breaking property Two nickel foils (A, B) having a thickness of 10 μm are prepared, and one nickel foil A is a square of 10 mm length and 10 mm width on a slide glass. Mask and fix with Teflon (registered trademark) tape leaving the part.
Another nickel foil B was placed on a ceramic plate connected with a thermocouple, a microporous film of a measurement sample immersed in a specified electrolyte solution for 3 hours was placed thereon, and a nickel foil was attached thereon. Place a slide glass and then put silicon rubber.
After this was set on a hot plate, the temperature was raised at a rate of 2 ° C./min with a pressure of 1.5 MPa applied by a hydraulic press. The change in impedance at this time was measured under conditions of AC 1 V and 1 kHz. In this measurement, the temperature when the impedance reached 1000Ω was defined as the hole closing temperature, and the temperature at which the impedance again decreased below 1000Ω after reaching the hole closed state was defined as the thermal film breaking temperature.

The composition ratio of the prescribed electrolyte is as follows.
Composition ratio (volume ratio) of solvent: propylene carbonate / ethylene carbonate / δ-butyllactone = 1/1/2
Solute composition ratio: LiBF ₄ is dissolved in the above solvent to a concentration of 1 mol / liter.
A film having a thermal film breaking temperature of 200 ° C. or higher was rated as heat-resistant film breaking property, a film having a thermal film breaking temperature of 170 ° C. or higher and lower than 200 ° C.

(10) Film-forming property The film 250 m was visually observed during film formation, and there were no inorganic particle aggregates or gels, and no breakage occurred during stretching. A sample that was observed but not broken during stretching was shown as Δ, and an aggregate of inorganic particles or a gel-like material was seen, and a sample that fractured during stretching was shown as x.

[Example 1]
As raw materials for the surface layer, 25.6 parts by mass of polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 25) 6.4) parts by mass, silica (oil absorption 200 mL / 100 g, average particle size 15 nm) 8 parts by mass, bis (p-ethylbenzylidene) sorbitol as the nucleating agent 0.3 parts by mass, tetrakis- [ Methylene- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate] 0.2 parts by mass of methane, liquid paraffin (kinematic viscosity at 37.78 ° C., 75.90 cSt, density) 0.868) 9.6 parts by mass were stirred with a mixer to prepare a raw material. As a raw material for the intermediate layer, 42 parts by mass of high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000), tetrakis- [methylene- (3 ′, 5′-di-t-butyl-) as an antioxidant 4′-Hydroxyphenyl) propionate] methane was mixed in an amount of 0.2 parts by mass to prepare a raw material. Each compound was charged through two twin screw extruder feeders having a diameter of 25 mm and L / D = 48.
Further, the surface layer has 45 parts by mass of liquid paraffin with respect to 25.6 parts by mass of the polypropylene used for the surface layer, and the intermediate layer has liquid paraffin with respect to 42 parts by mass of the high-density polyethylene used for the intermediate layer. 58 parts by mass were injected into each extruder by side feed, and the extrusion amounts of both surface layers and intermediate layers were adjusted to 4 kg and 16 kg per hour, respectively, and kneaded at 200 ° C. and 200 rpm. Then, it extruded on the conditions of 240 degreeC from T die which can be co-extruded (2 types and 3 layers) attached to the front-end | tip of an extruder. Immediately, the sheet was cooled and solidified with a cast roll adjusted to 70 ° C. to form a sheet having a thickness of 1.4 mm. This sheet was stretched 7 × 7 times at 126 ° C. with a simultaneous biaxial stretching machine, then dipped in methylene chloride, extracted after removing liquid paraffin, and dried in a transverse direction at 125 ° C. with a tenter stretching machine. The film was stretched 1.7 times. Thereafter, this stretched sheet is heat-treated at 127 ° C. in the 23% width direction to obtain a microporous membrane having a two-layer three-layer structure in which the two layers of the surface layer have the same composition and the intermediate layer has a different composition. It was. The obtained microporous film had a film thickness of 23.5 μm, a porosity of 47.4%, an air permeability of 256 sec / 100 cc, and a puncture strength of 450 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Example 2]
19.2 parts by mass of raw material polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 25) A microporous membrane was prepared in the same manner as in Example 1 except that 12.8 parts by mass was selected. The obtained microporous film had a thickness of 17.6 μm, a porosity of 44.0%, an air permeability of 257 sec / 100 cc, and a puncture strength of 432 g. Table 1 shows the heat of fusion of polypropylene in the microporous membrane, cycle characteristics, heat-resistant film-breaking properties, and film-forming properties.

[Example 3]
9.6 parts by mass of raw material polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 25) A microporous membrane was prepared in the same manner as in Example 1 except that 22.4 parts by mass was selected. The resulting microporous film had a thickness of 16.8 μm, a porosity of 43.1%, an air permeability of 240 sec / 100 cc, and a puncture strength of 487 g. Table 1 shows the heat of fusion of polypropylene in the microporous membrane, cycle characteristics, heat-resistant film-breaking properties, and film-forming properties.

[Example 4]
11.2 parts by mass of raw material polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 25) A microporous membrane was prepared in the same manner as in Example 1, except that 2.8 parts by mass and silica (oil absorption 200 mL / 100 g, average particle size 15 nm) were 26 parts by mass. The resulting microporous film had a thickness of 16.5 μm, a porosity of 48.0%, an air permeability of 205 sec / 100 cc, and a puncture strength of 462 g. Table 1 shows the heat of fusion of polypropylene in the microporous membrane, cycle characteristics, heat-resistant film-breaking properties, and film-forming properties.

[Example 5]
25.6 parts by mass of polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000) 6.4 mass Parts, silica (oil absorption 200 mL / 100 g, average particle size 15 nm) 8 parts by mass, bis (p-ethylbenzylidene) sorbitol as a nucleating agent 0.3 parts by mass, tetrakis- [methylene- (3 ′, 5'-di-t-butyl-4'-hydroxyphenyl) propionate] 0.2 parts by weight of methane, liquid paraffin (kinematic viscosity at 37.78 ° C., 75.90 cSt, density 0.868) as plasticizer 54. Six parts by mass were stirred with a mixer to adjust the raw materials. The blend was kneaded for 10 minutes at 200 ° C. and 50 rpm using a batch melt kneader (Toyo Seiki Co., Ltd .: Labo Plast Mill). The obtained kneaded product was molded with a 200 ° C., 5 MPa superheated press and heat-treated for 3 minutes as it was, and then cooled and pressed at 5 Mpa with a water-cooled press to prepare a sheet (A) having a thickness of 150 μm.
Similarly, 42 parts by mass of high-density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000), tetrakis- [methylene- (3 ′, 5′-di-t-butyl-4′-) as an antioxidant Hydroxyphenyl) propionate] 0.2 parts by mass of methane and 58 parts by mass of liquid paraffin were blended to prepare raw materials. The blend was kneaded for 10 minutes at 200 ° C. and 50 rpm using a batch melt kneader (Toyo Seiki Co., Ltd .: Labo Plast Mill). The obtained kneaded material was molded with a 200 ° C., 5 MPa superheated press and heat treated for 3 minutes, and then cooled and pressed at 5 Mpa with a water-cooled press to prepare a sheet (B) having a thickness of 1100 μm.
Sheets A and B were laminated so as to be A / B / A, and pressure bonded under conditions of a heat seal temperature of 130 ° C. and a heat seal pressure of 5 MPa. The obtained laminated film was stretched 7 × 7 times at 126 ° C. using a simultaneous biaxial stretching machine (manufactured by Toyo Seiki Co., Ltd.), and then paraffin oil was extracted and removed with methylene chloride, dried, and then simultaneously biaxial. The film was stretched 1.7 times in the transverse direction under the condition of 125 ° C. by a stretching machine. Thereafter, this stretched sheet is heat-treated at 127 ° C. in the 23% width direction to obtain a microporous membrane having a two-layer three-layer structure in which the two layers of the surface layer have the same composition and the intermediate layer has a different composition. It was. The obtained microporous film had a film thickness of 23.1 μm, a porosity of 46.8%, an air permeability of 266 sec / 100 cc, and a puncture strength of 470 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Example 6]
The surface layer material is 32.0 parts by mass of polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g), high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000). ) Is 8.0 parts by mass, bis (p-ethylbenzylidene) sorbitol is 0.3 parts by mass as a nucleating agent, and tetrakis- [methylene- (3 ′, 5′-di-t-butyl-4 ′] is an antioxidant. -Hydroxyphenyl) propionate] 0.2 parts by mass of methane, plasticizer, liquid paraffin (dynamic viscosity at 37.78 ° C., 75.90 cSt, density 0.868) 9.6 parts by mass was stirred with a mixer to prepare the raw materials A microporous membrane was prepared in the same manner as in Example 1 except that. The obtained microporous film had a film thickness of 21.1 μm, a porosity of 45.4%, an air permeability of 302 sec / 100 cc, and a puncture strength of 460 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Example 7]
Surface material is polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 300,000, ΔH: 80 J / g) 25.6 parts by mass, high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000) 6 4 parts by mass, (oil absorption 200 mL / 100 g, average particle size 15 nm) 8 parts by mass, intermediate layer raw material is high density polyethylene (density: 0.95 g / cm ³ , viscosity average molecular weight 250,000) 19.0 parts by mass 1. High-density polyethylene (density: 0.95 g / cm ³ , viscosity average molecular weight 700,000) 19.0 parts by mass, polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g) A microporous membrane was prepared in the same manner as in Example 1 except that the amount was adjusted to 0 part by mass and the amount of liquid paraffin injected from the side feed during extrusion was 65 parts by mass in the intermediate layer. The resulting microporous film had a thickness of 23.6 μm, a porosity of 48.4%, an air permeability of 255 sec / 100 cc, and a puncture strength of 490 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Example 8]
High-density polyethylene (density: 0.95 g / cm ³ , viscosity average molecular weight 250,000) 19 parts by mass, high-density polyethylene (density: 0.95 g / cm ³ , viscosity average molecular weight 700,000) 19 parts by mass, polypropylene (Density: 0.95 g / cm ³ , viscosity average molecular weight 400,000) 2 parts by mass, intermediate layer raw material is polypropylene (density 0.90 g / cm ³ , viscosity average molecular weight 400,000, ΔH: 92 J / g) 9.6 mass Parts, high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 250,000) 2.6 parts by mass, high density polyethylene (density 0.95 g / cm ³ , viscosity average molecular weight 2 million) 3.8 parts by mass, Silica (oil absorption 200 mL / 100 g, average particle size 15 nm) 24 parts by mass, as a plasticizer, liquid paraffin (kinematic viscosity at 37.78 ° C. 75.90 cSt, density 0 868) 9.6 parts by mass with a mixer to adjust the raw materials, and the liquid paraffin injection amount from the side feed at the time of extrusion is 75 parts by mass for the surface layer and 40 parts by mass for the intermediate layer. A microporous membrane was prepared in the same manner as in Example 1 except that the amount of extrusion of the intermediate layer was adjusted to 6 kg and 8 kg per hour, respectively. The resulting microporous film had a thickness of 18.0 μm, a porosity of 68.0%, an air permeability of 230 sec / 100 cc, and a puncture strength of 320 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Example 9]
A microporous film was prepared in the same manner as in Example 1 except that the sheet was prepared by cooling and solidifying with a cast roll adjusted to 80 ° C. The obtained microporous film had a film thickness of 23.0 μm, a porosity of 48.5%, an air permeability of 236 sec / 100 cc, and a puncture strength of 420 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

[Comparative Example 1]
A microporous membrane was prepared in the same manner as in Example 1 except that the conditions for extruding from the T-die were 220 ° C and the temperature of the cast roll was 30 ° C. The obtained microporous film had a thickness of 17.6 μm, a porosity of 44.4%, an air permeability of 321 sec / 100 cc, and a puncture strength of 477 g. Table 1 shows the heat of fusion of polypropylene in the microporous membrane, cycle characteristics, heat-resistant film-breaking properties, and film-forming properties.

[Comparative Example 2]
A microporous membrane was prepared in the same manner as in Example 2 except that the conditions for extruding from the T-die were 220 ° C and the temperature of the cast roll was 30 ° C. The obtained microporous film had a thickness of 17.0 μm, a porosity of 44.4%, an air permeability of 279 sec / 100 cc, and a puncture strength of 479 g. Table 1 shows the heat of fusion of polypropylene in the microporous membrane, cycle characteristics, heat-resistant film-breaking properties, and film-forming properties.

[Comparative Example 3]
A microporous film was prepared in the same manner as in Example 6 except that the conditions for extruding from the T die were 220 ° C. and the temperature of the cast roll was 30 ° C. The obtained microporous film had a thickness of 20.3 μm, a porosity of 42.4%, an air permeability of 402 sec / 100 cc, and a puncture strength of 430 g. Table 1 shows the heat of fusion, cycle characteristics, heat-resistant film breaking property, and film forming property of polypropylene in the microporous film.

  From the results shown in Table 1, the polyolefin microporous membrane of the present embodiment has a specific heat range for the melting heat of polypropylene contained in the surface layer, and therefore the cycle characteristics of the battery using the microporous membrane are good and practical. It was rich.

Claims (6)

A first microporous layer comprising polypropylene and polyethylene, and a second microporous layer that is laminated to the first microporous layer and is different from the first microporous layer,
The first microporous layer forms a surface layer;
The polyolefin microporous film, wherein the heat of fusion of the polypropylene in the microporous layer is 90 J / g or more.   2. The polyolefin microporous membrane according to claim 1, wherein the proportion of the polypropylene in the total amount of the polyolefin resin used in the first microporous layer is 40 to 95% by mass.   The first microporous layer further contains inorganic particles, and the proportion of the inorganic particles in the total amount of the polyolefin resin and inorganic particles used in the first microporous layer is 5 to 60% by mass. Item 2. The polyolefin microporous membrane according to Item 1.   4. The polyolefin microporous membrane according to claim 1, wherein the first microporous layer has a thickness of 0.2 μm or more and 5 μm or less.   The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the second microporous layer contains polyethylene as a main component.   The separator for electrical storage devices formed using the polyolefin microporous film in any one of Claims 1-5.