JP2009242779A - Polyolefin fine porous membrane and separator for storage battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fine porous membrane especially suitable for a separator or the like for a nonaqueous electrolyte battery, required to be excellent in heat resistance and long-term reliability. <P>SOLUTION: The fine porous membrane comprising a polyolefin resin, a modified polyolefin and a filler is regulated so that the content of the filler is >10 mass% and ≤80 mass% based on the whole mass of the fine porous membrane. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

  Microporous membranes have various pore diameters, pore shapes, and numbers of pores, and are used in a wide range of fields because of their characteristics that can be manifested by their unique structures. For example, microporous membranes are water treatment membranes that use the sieving effect due to the difference in pore size, separation membranes that are used for concentration, etc., and water absorption, oil absorption, and deodorizing materials that use a large surface area and porous space due to microporosity. Sheets, moisture permeable waterproof sheets that use air and water vapor but not water due to the difference in molecular size, polymers that are multifunctional by filling various materials into porous spaces, and are useful for fuel cells, etc. They are used as electrolyte membranes, humidified membranes, liquid crystal materials, and battery materials.

  In recent years, the introduction and examination of electric vehicles (PEVs) and hybrid vehicles (HEVs) have been actively promoted, especially in the automobile industry, from the viewpoint of energy saving and resource saving against the backdrop of increasing global environmental protection activities. Fuel cells and large-sized lithium ion secondary batteries are being actively developed as motor drive power supplies and auxiliary power supplies. An electric double layer capacitor capable of instantaneously charging and discharging a large current is expected as an auxiliary power source for HEV and is being developed. In storage batteries such as lithium ion secondary batteries and electric double layer capacitors, a porous film holding an electrolytic solution called a separator having a function of preventing contact between positive and negative electrodes and transmitting ions is provided between the positive and negative electrodes. . With the further increase in energy density of storage batteries, there is a strong demand for long-term reliability with the development of safety and automotive applications. Further, with the increase in energy density and output of storage batteries, the winding structure and current collection structure are diversified every day, and the separator in the battery is required to have heat resistance in various states.

Under such circumstances, for example, Patent Document 1 discloses that a microporous film having a specific composition ratio of polyethylene, layered mineral, and acid-modified polyolefin is excellent in heat resistance.
Patent Document 2 discloses a microporous membrane made of a polyolefin resin and inorganic particles. The microporous membrane has a puncture strength of 3 N / 20 μm or more, and a film thickness retention ratio of puncture creep of 16% or more. It discloses that the porous membrane is excellent in safety and long-term reliability and is suitable as a separator for a nonaqueous electrolyte battery.

JP 2003-183440 A International Publication No. 2006/25323 Pamphlet

However, none of the microporous membranes disclosed in the above cited references 1 and 2 are sufficiently satisfactory from the viewpoints of heat resistance and reliability.
That is, in Patent Documents 1 and 2, the heat resistance evaluation method disclosed in the examples is an evaluation in a state in which the microporous film is pressure-bonded and fixed in the surface direction with Ni foil or positive and negative electrodes, for example, in the surface direction. There was still room for improvement in heat resistance in the evaluation under severe conditions that were not fixed by crimping. Moreover, there is no description regarding the reliability under long-term use as a battery separator. In particular, in a storage battery (storage device) such as a non-aqueous electrolyte battery, it is required to achieve both heat resistance and long-term reliability at a higher level.
An object of the present invention is to provide a polyolefin microporous membrane particularly suitable as a separator for a nonaqueous electrolyte battery that is required to have excellent heat resistance and long-term reliability.

  As a result of repeated investigations focusing on heat-resistant membranes under severe conditions where the microporous membrane is not crimped and fixed in the plane direction, the present inventors have obtained a polyolefin microporous membrane containing a polyolefin resin, a modified polyolefin, and a filler. A polyolefin microporous membrane characterized in that the content of the filler is greater than 10% by mass and 80% by mass or less based on the total mass of the microporous membrane, and has excellent heat resistance, The present inventors have found that it can be suitably used as a separator for a non-aqueous electrolyte battery that is required to have excellent long-term reliability, and has reached the present invention.

That is, the present invention is as follows.
[1]
A microporous membrane containing a polyolefin resin, a modified polyolefin, and a filler, wherein the content of the filler is greater than 10% by mass and 80% by mass or less based on the total mass of the polyolefin resin, the modified polyolefin, and the filler. Polyolefin microporous membrane.
[2]
The polyolefin microporous membrane according to [1], wherein the content of the filler is greater than 30% by mass and equal to or less than 60% by mass.
[3]
The polyolefin microporous membrane according to [1] or [2], wherein the filler is an inorganic filler.
[4]
The polyolefin microporous membrane according to any one of [1] to [3], wherein the infrared absorption spectrum of the modified polyolefin has the following specific relationship.
0.010 <(SA + SB) / SC <0.1
SA: 1792cm peak area of peak A from the carbonyl group is observed in the vicinity of ^-1 SB: peak area of a peak derived from a carbonyl group is observed near 1712cm ^-1 B SC: 1472cm ^-1 methylene groups observed near Peak area of peak C derived from [5]
The polyolefin microporous film according to any one of [1] to [4], wherein the polyolefin resin has a viscosity average molecular weight of 200,000 or more.
[6]
The polyolefin microporous membrane according to any one of [1] to [5], wherein the content of the modified polyolefin is 5% by mass or more and 50% by mass or less with respect to the total polymer mass of the microporous membrane.
[7]
A separator for a storage battery comprising the polyolefin microporous film according to any one of [1] to [6].

  According to the present invention, there is provided a polyolefin microporous membrane that is particularly suitable as a separator for a nonaqueous electrolyte battery that is required to have excellent heat resistance and reliability.

2 is an infrared absorption spectrum of the modified polyolefin used in Example 1.

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 microporous membrane in the present embodiment is a microporous membrane containing a polyolefin resin, a modified polyolefin, and a filler, and the content of the filler is greater than 10% by mass with respect to the total mass of the microporous membrane. It is a polyolefin microporous film having a mass% or less.

The polyolefin resin used in the present embodiment is a polymer containing an olefin hydrocarbon as a monomer component, and includes a carboxyl group and / or a carboxylic acid derivative group (an acid anhydride group) in the molecular chain and in the side chain. Etc.) is not actively introduced. Here, “not being actively introduced” means that the value of [(SA + SB) / SC] described later is 0.010 or less.
Specific examples include polyolefin resins used for normal extrusion, injection, inflation, blow molding, and the like, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Homopolymers and copolymers such as, multistage polymers and the like can be used. In addition, polyolefins selected from the group of these homopolymers, copolymers, and multistage polymers can be used alone or in combination. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, and the like. These can be used alone or in combination. When the microporous membrane of the present embodiment is used as a battery separator, it is preferable to use a resin mainly composed of high-density polyethylene because it is a low melting point resin and requires high piercing strength. In the present embodiment, the “main component” means that the proportion of the specific component in the matrix is preferably 10% by mass or more, preferably 30% by mass or more, and more preferably 50% by mass or more. And it means that it may be 100% by mass.

The viscosity average molecular weight of the polyolefin resin or microporous membrane used in the present embodiment (may be abbreviated as “Mv”. In addition, when measured for a mixture, it means Mv of the entire mixture). It is preferably 50,000 or more and less than 10 million. A viscosity average molecular weight of 50,000 or more is preferable because the melt tension at the time of melt molding is increased and the moldability is easily improved, and sufficient entanglement is easily imparted and the strength is easily increased. If the viscosity average molecular weight is less than 10 million, it tends to be easy to obtain uniform melt-kneading and is preferable because it tends to be excellent in sheet formability, particularly thickness stability. A more preferable viscosity region is 100,000 or more and less than 2 million, and further 500,000 or more and less than 1 million. If it is less than 2 million, melt kneading at a high polymer concentration tends to be easy and productivity improvement can be expected, and heat shrinkage tends to be reduced, which is preferable. For example, instead of using a polyolefin having a viscosity average molecular weight of less than 2 million alone, a mixture of polyethylene having a viscosity average molecular weight of 2 million and 270,000 may be used, and the viscosity average molecular weight of the mixture may be less than 2 million or 500,000 or more.
In addition, Mv in this Embodiment is measured according to the measuring method in the Example mentioned later.

As the polyolefin resin, a plurality of different types of monomers, molecular weight, density and the like may be mixed and used.
From the viewpoint of heat resistance, it is preferable to use a mixture of polypropylene and another polyolefin resin as the polyolefin resin. In particular, it is preferable to use a mixture of high-density polyethylene and polypropylene because it can have both high puncture strength and heat resistance. The content of polypropylene is preferably 1% by mass or more and less than 50% by mass, more preferably 5% by mass or more and less than 40% by mass, and still more preferably 20% by mass or more and less than 30% by mass with respect to the total amount of the polyolefin resin. If it is 1% by mass or more, it is preferable because good heat resistance can be obtained, and if it is less than 50% by mass, stretchability tends to be good, and a porous film having high puncture strength is easily obtained.

  The proportion of the polyolefin resin in the total amount of the polyolefin resin in the microporous membrane and the modified polyolefin described later is preferably 50% by mass or more, and more preferably 80% by mass or more. The upper limit is not particularly limited and may be less than 99% by mass. When the polyolefin resin is 50% by mass or more, high puncture strength is easily obtained and long-term reliability tends to be excellent.

  In addition, the polyolefin resin used in the present embodiment includes a phenol-based, phosphorus-based, or sulfur-based antioxidant, as necessary, within a range that does not impair the advantages of the present embodiment; calcium stearate or stearic acid Metal soaps such as zinc; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; color pigments; additives such as polymers other than polyolefin resins and modified polyolefins can be mixed and used.

The modified polyolefin referred to in the present embodiment is a compound in which a carboxyl group and / or a carboxylic acid derivative group (including an acid anhydride group or the like) is introduced into the molecular chain or side chain of the polyolefin resin. (Ie, “polyolefin resin” is a concept that does not include “modified polyolefin”).
This modified polyolefin can be obtained by a known method. For example, a method of grafting an α, β-unsaturated carboxylic acid or an acid derivative thereof with a polyolefin such as polyethylene in an organic solvent or in the presence of a radical generator in the melt, or an α, β-unsaturated carboxylic acid or an acid derivative thereof And a method of radical copolymerizing ethylene and an olefin such as ethylene. Examples of the α, β-unsaturated carboxylic acid or acid derivative thereof include acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, citraconic acid, mesaconic acid, and esters thereof, acid anhydrides. Etc. Examples of acid anhydrides include maleic anhydride, citraconic anhydride, itaconic anhydride, dichloromaleic anhydride, dimethylmaleic anhydride, and the like.

In the present embodiment, the modified polyolefin has the following effects. In order to achieve both good heat resistance and long-term reliability that should be noted, it is difficult to achieve with a microporous membrane comprising a polyolefin resin (unmodified) and a filler. When used as a storage battery separator, there is still room for improvement in terms of high-temperature storage characteristics.
On the other hand, by using a polyolefin resin, a modified polyolefin, and a filler in combination, it is possible to achieve both good heat resistance and long-term reliability that should be noted. In a microporous film composed only of a polyolefin resin and a modified polyolefin, it is difficult to develop the heat resistance described in the present embodiment. Further, even a microporous film made only of a modified polyolefin and a filler is difficult to exhibit the heat resistance described in the present embodiment. Furthermore, since the interaction within the modified polyolefin molecule is large, it tends to be difficult to obtain a microporous membrane having high puncture strength and low permeability, and it is difficult to use as a separator for a storage battery.
The modified polyolefin is highly compatible with the polyolefin resin and has an appropriate polar group so that it has an affinity with the filler. By increasing the affinity between the polyolefin resin and the filler, the heat resistance of the microporous membrane is improved. It is inferred that it is producing improvements. Moreover, by acting with a filler, it can be excellent in reliability as a stable microporous membrane even under severe conditions such as storage at high temperatures. Specific preferred characteristics of the modified polyolefin include the following.

  The number of functional groups that are carboxyl groups or carboxylic acid derivative groups in the modified polyolefin is desirably about 0.05 to 50 on average in one molecule of the modified polyolefin. If it is 0.05 or more, the heat resistance tends to be excellent in a microporous membrane containing polyolefin, modified polyolefin, and filler. Moreover, if it is 0.05 or more, when the microporous membrane of the present embodiment is used as a separator for non-aqueous batteries, there is a tendency that battery performance deterioration such as capacity reduction is difficult to occur. If it is 50 or less, even if the polyolefin is, for example, polyethylene having a viscosity average molecular weight of 200,000 or more or high-density polyethylene in a microporous membrane containing polyolefin, modified polyolefin, and inorganic particles, the compatibility is high and the stretchability is excellent. It can easily become high strength.

Also, the modified polyolefin, the spectrum obtained by infrared spectroscopy, (around 1792Cm ^-1) Peak A from the carbonyl group, preferably a modified polyolefin having a peak B (1712 cm around ^-1). Moreover, it is preferable that the area of the peak A, the peak B, and the methylene group-derived peak C (near 1472 cm ⁻¹ ) has the following specific range.

0.010 <(SA + SB) / SC <0.1
SA: 1765cm ^-1 and 1805cm area surrounded by the baseline and the peak A connecting ^-1 SB: 1680 cm ^-1 and the area surrounded by the baseline and the peak B connecting 1740cm ^-1 SC: 1400cm ^-1 and 1520 cm ^- Area surrounded by baseline and peak C connecting ¹

  The relationship [(SA + SB) / SC] indicates a specific relationship between the modified polyolefin and the filler, and the modified polyolefin and the polyolefin resin. When the numerical value represented by the above relational expression is small, the compatibility between the modified polyolefin and the polyolefin resin tends to be good, and when the numerical value is large, the modified polyolefin and the filler tend to exhibit good affinity. It is in. Specifically, when it is larger than 0.010, it is excellent in reliability such as heat resistance and high temperature storage. Preferably it is 0.020 or more, More preferably, it is 0.030 or more. When it is smaller than 0.1, when the polyolefin resin is high-density polyethylene or the viscosity average molecular weight is 200,000 or more, it tends to have heat resistance and high strength. Preferably it is 0.08 or less, More preferably, it is 0.07 or less.

  Specifically, “Modic” (manufactured by Mitsubishi Chemical Corporation), “Admer” (manufactured by Mitsui Chemicals) and the like are commercially available as modified polyolefins.

  In the microporous membrane of the present embodiment, the content of the modified polyolefin is preferably 5% by mass or more and 50% by mass or less, and more preferably 8% by mass or more and 30% by mass or less with respect to the total polymer mass of the microporous membrane. . If it is 5% by mass or more, good heat resistance can be easily obtained, and if it is 50% by mass or less, storage characteristics are good, and it is easy to obtain high strength because of excellent stretchability.

  The proportion of the filler in the total amount of the polyolefin resin, modified polyolefin, and filler of the present embodiment is greater than 10% by mass and 80% by mass or less. More preferably, it is 20 mass% or more and 60 mass% or less, More preferably, it is 40 mass% or more and less than 60 mass%. When it is larger than 10% by mass, it is easy to obtain a microporous film excellent in heat resistance in a microporous film containing polyolefin and modified polyolefin. If it is 80 mass% or less, a high draw ratio is possible and high strength is easily obtained. Further, when used as a battery separator, the capacity is hardly reduced during high-temperature storage, and the reliability tends to be excellent.

  The filler used in the present embodiment is preferably inert to the polyolefin resin, and may be organic particles or inorganic particles. In the case of organic particles, organic particles having a melting point higher than the melting point of the polyolefin resin or organic particles having no melting point and a glass transition point higher than the melting point of the polyolefin resin are preferable. In the case of organic particles having a melting point higher than the melting point of the polyolefin resin, or organic particles having no glass melting point and a glass transition point higher than the melting point of the polyolefin resin, good heat resistance tends to be obtained. . As the organic particles, particles such as homopolymers such as methyl methacrylate, ethyl methacrylate, methyl acrylate, acrylonitrile, styrene and the like, copolymers selected from two or more types of monomers, and crosslinked products thereof are preferable.

When the filler is inorganic particles, it is preferably inorganic particles having an electrical resistivity at room temperature of 10 ⁻² Ω · cm or more. Preferably, it is 10 ¹⁴ Ω · cm or more. If the electrical resistivity is 10 ⁻² Ω · cm or more, it is possible to maintain electronic insulation between electrodes when a microporous film containing polyolefin resin, modified polyolefin, and inorganic particles is used as a separator for an electricity storage device. Therefore, it can be a microporous film excellent as a separator. When the electrical resistivity is high, the electronic insulation is improved and there is no particular upper limit.

The shape of the inorganic particles may be any shape such as a spherical shape, a rod shape, a needle shape, or a plate shape. From the viewpoint of heat resistance, a spherical shape, a rod shape, and a needle shape are preferable. The structure of the inorganic particles may be, for example, a porous structure, a layered structure, or a nonporous structure. A non-porous structure is preferred. The non-porous structure is a structure that does not substantially have an internal surface area inside the primary particles, that is, has substantially no fine pores in the primary particles themselves.
The inorganic particles having substantially no fine pores in the primary particles themselves are inorganic particles having a porosity (P / S) in the range of 0.1 to 3.0. P / S is the ratio of the specific surface area P to the surface area S per unit weight calculated from the primary particle diameter D (unit: μm) and the density d (unit: g / cm ³ ) of the substance constituting the particles. is there. When the particles are spherical, the surface area per particle is πD ² × 10 ⁻¹² (unit: m ² ), and the weight of one particle is (πD ³ d / 6) × 10 ⁻¹² (unit). : G), the surface area S per unit weight is S = 6 / (Dd) (unit: m ² / g). The specific surface area P is obtained from a nitrogen adsorption isotherm at −196 ° C. based on the BET equation. When such inorganic particles are used, for example, when used as a separator for a non-aqueous electrolyte battery, there is a tendency that performance deterioration such as capacity reduction is difficult to occur. The reason is not clear, but if it does not substantially have fine pores inside the primary particles, it is easy to remove adsorbed water etc. in the normal drying process, so it is difficult to cause capacity reduction due to moisture mixing. Guessed. Further, when inorganic particles having a nonporous structure are used for a microporous membrane containing both a polyolefin resin and a modified polyolefin, a microporous membrane excellent in heat resistance tends to be obtained even with a small amount of the modified polyolefin.

  Specific inorganic particles are preferably oxides and nitrides such as silicon, aluminum, titanium and magnesium, and carbonates and sulfates such as calcium and barium. Silicon oxide is light in weight and low in cost, and aluminum oxide is excellent in chemical resistance, particularly low reactivity with hydrofluoric acid that may occur when used as a separator for non-aqueous batteries. Titanium oxide is more preferable because a highly permeable microporous film can be easily obtained. Moreover, the inorganic particle which performed the surface treatment suitably can be used. For example, when producing microporous membranes for filtration applications using aqueous solvents and separators for aqueous electrolyte storage devices, hydrophilic particles are suitable, and separators for nonaqueous electrolyte storage devices are manufactured. In this case, inorganic particles that have been subjected to hydrophobic treatment are suitable.

  The inorganic particles used in the present embodiment preferably have a particle size of 1 nm or more and less than 100 nm, more preferably 6 nm or more and less than 80 nm, and still more preferably 10 nm or more and less than 60 nm. The particle size is measured according to the measurement method in the examples described later. When the particle size is less than 100 nm, even when stretching or the like is performed, peeling between the polyolefin and the inorganic particles is difficult to occur, so that generation of macrovoids tends to be suppressed and high strength tends to be obtained. Further, since the inorganic particles are dispersed and fused in the polyolefin, the heat resistance tends to be excellent.

The microporous membrane of the present embodiment can be manufactured, for example, as follows.
(1) a step of melt-kneading a polyolefin resin, a modified polyolefin, a filler and a plasticizer;
(2) A step of transferring the melt and forming it into a sheet shape, followed by cooling and solidification,
(3) a step of stretching at least in a uniaxial direction at a surface magnification of 20 times or more and less than 200 times;
(4) A step of extracting a plasticizer before or after the stretching step of (3),
Is obtained by a method comprising

  The plasticizer added 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 and the modified polyolefin when mixed with the polyolefin resin and the modified polyolefin. Preferably it is liquid at normal temperature. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol. In particular, when the polyolefin resin is polyethylene, liquid paraffin is highly compatible with polyethylene because it does not have a polar group, and the interface between the resin and the plasticizer does not easily peel during stretching, so a microporous membrane with excellent heat resistance can be obtained. It is easy to be done. Moreover, it is preferable because uniform stretching can be easily performed and high piercing strength can be easily obtained.

  The ratio of the plasticizer to be used is a ratio that enables uniform melt-kneading, is a ratio that is sufficient to form a sheet-like microporous membrane precursor, and does not impair productivity. Is preferred. Specifically, the mass fraction of the plasticizer in 100% by mass of the composition comprising the polyolefin resin, the modified polyolefin, the filler and the plasticizer is preferably 30 to 80% by mass, more preferably 40 to 70% by mass. It is. When the plasticizer has a mass fraction of 80% by mass or less, the melt tension at the time of melt molding is hardly insufficient and the moldability tends to be improved, which is preferable. On the other hand, when the mass fraction is 30% by mass or more, it is preferable because the film becomes thinner in the thickness direction as the draw ratio increases and a thin film can be obtained. Further, since the plasticizing effect is sufficient, the crystalline folded lamellar crystals can be efficiently stretched. Even when stretched at a high magnification, the polyolefin chain is not broken and the pore structure is uniform and fine, and the strength is likely to increase.

  The method of melt-kneading polyolefin resin, modified polyolefin, filler and plasticizer is to introduce the plasticizer at an arbitrary ratio while charging the mixture into a resin kneading apparatus such as an extruder or kneader, Furthermore, a method of obtaining a uniform solution by kneading a composition comprising a resin, a filler and a plasticizer is preferred.

  The step (2) is a step of manufacturing the microporous membrane precursor by transferring the melt, forming it into a sheet, and cooling and solidifying it. In this step, a uniform solution of polyolefin resin, modified polyolefin, filler, and plasticizer is extruded into a sheet form via a T-die, etc., and is brought into contact with a heat conductor to cool to a temperature sufficiently lower than the crystallization temperature of the resin. It is preferable to do so.

  Next, in the stretching step (3), the stretching direction is at least uniaxial stretching. When the film is stretched at a high magnification in the biaxial direction, it has a stable structure that is difficult to tear because of molecular orientation in the plane direction, and a high puncture strength is obtained. The stretching method may be simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, multiple stretching, etc., either alone or in combination, but if the stretching method is simultaneous biaxial stretching, the piercing strength is increased. And from the viewpoint of uniform film thickness. Here, the simultaneous biaxial stretching is a technique in which stretching in the MD direction (resin flow direction, film length direction) and stretching in the TD direction (film width direction) are performed simultaneously. The deformation rate may be different. Sequential biaxial stretching is a technique in which stretching in the MD direction or TD direction is performed independently. When stretching is performed in the MD direction or TD direction, the other direction is unconstrained or fixed. It is in a state of being fixed to the length. The draw ratio is in the range of 20 times to less than 200 times in terms of surface magnification, preferably 20 times to 100 times, and more preferably 25 times to 50 times. When the total area magnification is 20 times or more, sufficient puncture strength can be imparted to the film, and when it is less than 200 times, film breakage is prevented and high productivity is obtained, which is preferable.

  The stretching ratio in each axial direction is preferably 4 to 10 times in the MD direction, preferably 4 to 10 times in the TD direction, 5 to 8 times in the MD direction, and 5 to 8 times in the TD direction. The range of is more preferable. When the film is stretched 4 times or more in the MD direction and 4 times or more in the TD direction, it is preferable because a product with small film thickness unevenness is easily obtained in both the MD direction and the TD direction. When the draw ratio in the MD direction or TD direction is 10 times or less, the winding property is excellent.

  The stretching temperature is preferably a melting point temperature of the polyolefin of −50 ° C. or higher and lower than the melting point temperature, more preferably a melting point temperature of the polyolefin of −30 ° C. or higher and a melting point temperature of −2 ° C. or lower. More preferably, it is not higher than ° C. When the melting point temperature of the polyolefin is −50 ° C. or higher, interfacial peeling between the polyolefin and the filler or between the polyolefin and the plasticizer hardly occurs, and the heat resistance tends to be excellent. Less than the melting point temperature of polyolefin is preferable because high puncture strength can be easily obtained and uneven stretching can be reduced. For example, when high-density polyethylene is used, it is preferably 110 ° C. or higher, more preferably 120 ° C. or higher, and the upper limit is preferably 132 ° C. or lower. When a plurality of polyolefins are mixed and used, the melting point of the polyolefin having the larger heat of fusion may be used as a reference.

The step of extracting the plasticizer of (4) is preferably performed after the stretching step of (3) from the viewpoint of heat resistance and high puncture strength.
It is preferable to extract the plasticizer by immersing the microporous membrane in an extraction solvent and sufficiently dry it to substantially remove the plasticizer from the microporous membrane. In order to suppress the shrinkage of the microporous membrane, it is preferable to constrain the end of the microporous membrane during a series of steps of immersion and drying. Further, the residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by mass.

  The extraction solvent is a poor solvent for the polyolefin resin, the modified polyolefin, and the filler, and a good solvent for the plasticizer, and the boiling point is desirably lower than the melting point of the polyolefin microporous membrane. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and non-chlorine solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone.

In the microporous membrane of the present embodiment, adding a heat treatment step such as heat fixation and thermal relaxation after each stretching process within a range not impairing the advantages of the present embodiment will reduce the shrinkage of the microporous membrane. Further, there is an effect of suppressing, which is preferable.
Moreover, you may perform a post-process in the range which does not impair the advantage of this Embodiment. Examples of the post-treatment include a hydrophilic treatment with a surfactant and the like, and a crosslinking treatment with ionizing radiation.

The polyolefin microporous membrane of the present embodiment is useful as a separator for batteries, particularly for nonaqueous electrolyte batteries.
The final film thickness of the microporous membrane is preferably in the range of 2 μm to 100 μm, more preferably in the range of 5 μm to 40 μm, and still more preferably in the range of 5 μm to 35 μm. When the film thickness is 2 μm or more, the mechanical strength is sufficient, and when the film thickness is 100 μm or less, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery.

The porosity is preferably in the range of 25% to 90%, more preferably 40% to 80%, and still more preferably 50% to 80%. When the porosity is 25% or more, the permeability is hardly lowered, while when the porosity is 90% or less, there is little possibility of self-discharge when used as a battery separator, which is preferable.
Note that the parameter in the present embodiment is a value measured according to a measurement method in an example described later.

The air permeability is preferably in the range of 10 seconds to 1000 seconds, more preferably 20 seconds to 500 seconds, and still more preferably 40 seconds to 200 seconds. When the air permeability is 10 seconds or more, self-discharge is small when used as a battery separator, and when it is 1000 seconds or less, good charge / discharge characteristics are obtained, which is preferable.
Note that the parameter in the present embodiment is a value measured according to a measurement method in an example described later.

The puncture strength is preferably in the range of 3.0 N / 20 μm to 20 N / 20 μm, more preferably 4.0 N / 20 μm to 15 N / 20 μm, and still more preferably 5.0 N / 20 μm to 10 N / 20 μm. When the puncture strength is 3.0 N / 20 μm or more, when used as a battery separator, when wound together with the electrode, defects such as film breakage hardly occur and excellent winding property, and at 20 N / 20 μm or less, good heat resistance Is preferable.
Note that the parameter in the present embodiment is a value measured according to a measurement method in an example described later.

The bubble point of the microporous membrane is preferably 0.3 MPa or more and 2 MPa or less, more preferably 0.5 MPa or more and 1.8 MPa or less, and further preferably 0.7 MPa or more and 1.5 MPa or less. If the bubble point is 2 MPa or less, the permeability is good and the influence of clogging and the like is small. If it is 0.3 MPa or more, it is preferable because it has a low possibility of self-discharge and is reliable when used as a battery separator.
Note that the parameter in the present embodiment is a value measured according to a measurement method in an example described later.

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

(1) Film thickness The film thickness was measured at a room temperature of 23 ± 2 ° C. using a micro thickness gauge manufactured by Toyo Seiki, KBM (trademark).

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

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

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

(5) Bubble point (MPa)
Based on ASTM E-128-61, the bubble point in ethanol was calculated.

(6) (SA + SB) / SC
For the measurement, a two-dimensional infrared spectrophotometer (FTS60A) manufactured by Nippon Bio-Rad Laboratories Co., Ltd. was used. The modified polyolefin was compression molded into a thin film and sandwiched between crystal plates for measurement.
The peak near 1792 cm ⁻¹ is defined as peak A, and the area is defined as SA. The area surrounded by the base line connecting 1765 cm ⁻¹ and 1805 cm ⁻¹ and the peak A was calculated as SA. The peak near 1712 cm ⁻¹ is defined as peak B, and the area is defined as SB. The area surrounded by the base line connecting 1680 cm ⁻¹ and 1740 cm ⁻¹ and the peak B was calculated as SB. The peak near 1472 cm ⁻¹ is defined as peak C, and the area is defined as SC. The area surrounded by the base line connecting 1400 cm ⁻¹ and 1520 cm ⁻¹ and the peak C was calculated as SC.

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

(8) Particle size The particle size of the filler is determined by measuring the area of any 100 particles using a transmission microscope “Hitachi H700-H” (manufactured by Hitachi, Ltd.) and approximating the area of each particle to a circle. By the method, the diameter was defined as the particle size.
(9) P / S
The specific surface area (P) was calculated by measuring the adsorption isotherm of the filler using a specific surface area measuring device “ASAP-2400” (manufactured by Shimadzu Corporation) and calculating the BET method.
The primary particle diameter (D) of the filler was measured by the same method as in (8).
S (m ² / g) = 6 / (D · d) was calculated (d is the true density of the filler), and P / S was calculated.
(10) Evaluation of heat resistance A microporous membrane is cut into 60 mm × 60 mm squares. The microporous membrane is fixed by a SUS frame (width 6 mm, thickness 1 mm) having an outer width of 50 mm × 50 mm square and an inner width of 44 mm × 44 m square. It arrange | positions so that a microporous film may protrude evenly from a frame, the part which protruded is turned up, a microporous film is fixed with 12 double clips (the product made from KOKYO), and a sample is created. After leaving the sample in an oven heated to a predetermined temperature, the sample is taken out after 15 minutes. After cooling, the state of the microporous membrane is visually observed and judged. If a ruptured film is observed, if BR> Negative, pass (○), and if a ruptured film is observed, it is regarded as rejected (×).

(11) High temperature storage (capacity maintenance rate (%))
a. Positive electrode 92.2% by mass of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) 3.2 as a binder, respectively. A slurry is prepared by dispersing mass% in N-methylpyrrolidone (NMP). This slurry is applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode is 250 g / m ² , and the active material bulk density is 3.00 g / cm ³ .
b. Preparation of negative electrode A slurry was prepared by dispersing 96.6% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. To do. This slurry is applied to one side of a 12 μm-thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the negative electrode is set to 106 g / m ² , and the active material bulk density is set to 1.35 g / cm ³ .
c. Preparation of Nonaqueous Electrolytic Solution Prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / L.
d. Cell assembly A separator is cut into a circle of 30 mmφ, and a positive electrode and a negative electrode are cut into a circle of 16 mmφ, and the negative electrode, the separator, and the positive electrode are stacked in this order so that the active material surfaces of the positive electrode and the negative electrode face each other. The container and the lid are insulated, the container is in contact with the negative copper foil, and the lid is in contact with the positive aluminum foil. The non-aqueous electrolyte described above is injected into this container and sealed. After standing at room temperature for a day, the battery was charged to a battery voltage of 4.2 V at a current value of 2.0 mA (0.33 C) in a 25 ° C. atmosphere, and the current value was set to 2 to maintain 4.2 V after reaching the battery voltage. The first charge after making the battery for a total of 8 hours is performed by starting the squeezing from 0.0 mA. Subsequently, the battery is discharged to a battery voltage of 3.0 V at a current value of 2.0 mA (0.33 C).
e. Evaluation of Capacity Maintenance Rate Charging / discharging was performed in the following order using a charging / discharging device (model HJ-201BS, manufactured by Hokuto Denko). Charging to a battery voltage of 4.2V with a current value of 6mA (1.0C) in an atmosphere of 25 ° C, and charging the battery for 3 hours in total by starting to reduce the current value from 6mA while maintaining 4.2V. I do. Next, the battery is discharged to a battery voltage of 3.0 V at a current value of 6 mA (1.0 C). The discharge capacity at that time is A (mAh).
Next, in a 25 ° C. atmosphere, the battery voltage is charged to 4.2 V at a current value of 6 mA (1.0 C), and after reaching the voltage, the current value starts to be reduced from 6 mA so as to maintain 4.2 V. Charge for hours. Subsequently, the cell is removed from the charge / discharge device and stored in an oven set at 70 ° C. for 10 days. Then, the cell is taken out from the oven and left until the cell temperature is stabilized at room temperature.
Next, the battery is discharged to a battery voltage of 3.0 V at a current value of 6 mA (1.0 C) in a 25 ° C. atmosphere. Subsequently, charging is performed to a battery voltage of 4.2 V at a current value of 6 mA (1.0 C), and charging is performed for a total of 3 hours by a method in which the current value starts to be reduced from 6 mA so as to maintain 4.2 V after reaching. Next, the battery is discharged to a battery voltage of 3.0 V at a current value of 6 mA (1.0 C). Let the discharge capacity at that time be B (mAh).
From the discharge capacity A (mAh) and B (mAh), the capacity retention rate was defined by the following formula.
Capacity maintenance rate (%) = B / A × 100
In addition, it means that it is excellent in long-term reliability, so that a capacity | capacitance maintenance factor is large.

[Example 1]
13.0 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 8.6 parts by mass, modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation), 2.4 parts by mass, and silica “DM10C” (trademark, average primary particle size of 15 nm) 16.0 parts by mass of Tokuyama Corporation), 60.0 parts by mass of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer, and pentaerythrityl as an antioxidant -Toyo Seiki manufactured by adding 0.1 parts by mass of tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] It was heated mixed using a company manufactured plastomill. The infrared absorption spectrum of the modified polyolefin used is shown in FIG. The calculated (SA + SB) / SC was 0.035. The heating and mixing were performed for 10 minutes with the temperature of the plastmill set to 200 ° C. and the rotation speed set to 50 rpm. The molten mixture is taken out from the plastmill and cooled, and the obtained solidified product is sandwiched between metal plates through a polyimide film, compressed at 10 MPa for 5 minutes using a hot press set at 200 ° C., and then cooled to obtain a thick film. A sheet having a thickness of 1000 μm was prepared. The obtained sheet was simultaneously biaxially stretched 7 times in the longitudinal direction and 7 times in the transverse direction at 110 ° C. using a biaxial stretching machine manufactured by Iwamoto Seisakusho. Next, the four sides were fixed with a stainless frame, the plasticizer was removed in methylene chloride, and then dried at room temperature to obtain a microporous membrane. The obtained microporous film had a film thickness of 24 μm, a porosity of 64%, an air permeability of 270 seconds, and a puncture strength of 4.8 N. The heat resistance evaluation was a result of no film breakage at 157 ° C. In addition, the capacity retention rate in the high temperature storage evaluation was 91%, indicating a good result. The film forming conditions and characteristics are shown in Table 1.

[Example 2]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) is used in 11.5 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 7.7 parts by mass of the ultrahigh molecular weight polyethylene of Mv 2 million. ) Was 4.8 parts by mass, and a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 3]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) is used in 13.7 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 9.1 parts by mass of the ultra high molecular weight polyethylene of Mv 2 million. ) Was changed to 1.2 parts by mass in the same manner as in Example 1 to obtain a microporous membrane. The film forming conditions and film characteristics are shown in Table 1.

[Example 4]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) was used in 13.1 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 8.7 parts by mass of ultra high molecular weight polyethylene of Mv 2 million. ) And 5.4 parts by mass of silica, “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having an average primary particle size of 15 nm was changed to 12.8 parts by mass in the same manner as in Example 1, and the microporous membrane. Got. The film forming conditions and film characteristics are shown in Table 1.

[Example 5]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) is used in 16.9 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 11.3 parts by mass of ultra high molecular weight polyethylene of Mv 2 million. ) 7.0 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Corporation) having an average primary particle size of 15 nm was 4.8 parts by mass, and the stretching temperature for simultaneous biaxial stretching was 115 ° C. A microporous membrane was obtained in the same manner as Example 1 except for the above. The film forming conditions and film characteristics are shown in Table 1.

[Example 6]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) was used in 20.0 parts by mass of the high-density polyethylene of Mv 270,000 of Example 5 and 13.4 parts by mass of the ultrahigh molecular weight polyethylene of Mv 2 million. ) Was changed to 1.8 parts by mass, and a microporous membrane was obtained in the same manner as in Example 5. The film forming conditions and film characteristics are shown in Table 1.

[Example 7]
A microporous membrane was obtained in the same manner as in Example 2, except that the modified polyolefin of Example 2 was changed to “Modic-AP M504” (trademark, manufactured by Mitsubishi Chemical Corporation). The film forming conditions and film characteristics are shown in Table 1.

[Example 8]
A microporous membrane was obtained in the same manner as in Example 2 except that the modified polyolefin of Example 2 was changed to “Modic-AP L503” (trademark, manufactured by Mitsubishi Chemical Corporation). The film forming conditions and film characteristics are shown in Table 1.

[Example 9]
9.6 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 6.4 parts by mass of modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation), alumina “AluC” (trademark, average particle size of 13 nm) 30.0 parts by mass of Degussa), 50.0 parts by mass of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer, and pentaerythrityl-tetrakis- as an antioxidant Toyo Seiki Seisakusho Co., Ltd. was prepared by adding 0.1 part by mass of [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate]. It was heated mixed using a manufacturing plastomill. Subsequent steps were carried out in the same manner as in Example 1 to obtain a microporous membrane. The film forming conditions and film characteristics are shown in Table 1.

[Example 10]
Example 2 was the same as Example 2 except that 4.8 parts by mass of modified polyolefin modified with maleic anhydride was used instead of modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation). Thus, a microporous membrane was obtained. The modified polyolefin has a melt flow rate of 1.0 g / 10 min (measured according to JIS K7210, 190 ° C., weight 21.1 N), 100 parts by mass of low density polyethylene, 2.0 parts by mass of maleic anhydride, and tertiary butyl chloride. A product obtained by melt kneading 0.05 parts by mass of mill peroxide at 230 ° C. in a nitrogen atmosphere was used. The film forming conditions and film characteristics are shown in Table 1.

[Example 11]
11. 7.7 parts by mass of the Mv 270,000 high-density polyethylene of Example 2 and Mv 120,000 high-density polyethylene “S360” (trademark, manufactured by Asahi Kasei Chemicals Corporation) instead of the Mv 2 million ultrahigh molecular weight polyethylene A microporous membrane was obtained in the same manner as in Example 2 except that the amount was changed to 5 parts by mass. The film forming conditions and film characteristics are shown in Table 1.

[Example 12]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) is used in 6.5 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 4.3 parts by mass of ultra high molecular weight polyethylene of Mv 2 million. ) Was changed to 13.2 parts by mass, and a microporous membrane was obtained in the same manner as in Example 1. The film forming conditions and film characteristics are shown in Table 1.

[Example 13]
10.6 parts by mass of high-density polyethylene "SH800" (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene "UH850" (trademark, Asahi Kasei Chemicals Corporation) with an Mv of 2 million Made of silica, “DM10C” (trademark, 17.6 parts by mass of modified polyolefin “Modic-AP L503” (trademark, manufactured by Mitsubishi Chemical Corporation) and having an average primary particle size of 15 nm. 4.8 parts by mass) (manufactured by Tokuyama Corporation), 60.0 parts by mass of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer, and pentaerythrityl as an antioxidant -Toyo Seiki manufactured by adding 0.1 parts by mass of tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] It was heated mixed using a company manufactured plastomill. (SA + SB) / SC calculated from the infrared absorption spectrum of the modified polyolefin used was 0.022. The heating and mixing were performed for 10 minutes with the temperature of the plastmill set to 200 ° C. and the rotation speed set to 50 rpm. The molten mixture is taken out from the plastmill and cooled, and the obtained solidified product is sandwiched between metal plates through a polyimide film, compressed at 10 MPa for 5 minutes using a hot press set at 200 ° C., and then cooled to obtain a thick film. A sheet having a thickness of 1000 μm was prepared. The obtained sheet was simultaneously biaxially stretched 7 times in the longitudinal direction and 7 times in the transverse direction at 110 ° C. using a biaxial stretching machine manufactured by Iwamoto Seisakusho. Next, the four sides were fixed with a stainless frame, the plasticizer was removed in methylene chloride, and then dried at room temperature to obtain a microporous membrane. Table 4 shows the film forming conditions and characteristics.
[Examples 14 to 19]
A microporous membrane was obtained in the same manner as in Example 13 with the composition ratio shown in Table 3. Table 4 shows the film forming conditions and characteristics.
Moreover, about the microporous film | membrane of Examples 15-19, the heat resistance with respect to the ultra-high heat of 160 degreeC or more was evaluated according to the method of (10). The results are shown in Table 5.

[Comparative Example 1]
In Example 1, except that the high density polyethylene of Mv 270,000 was 14.4 parts by mass, the ultra high molecular weight polyethylene of Mv 2 million was 9.6 parts by mass, and the modified polyolefin was not added, Similarly, a microporous membrane was obtained. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 2]
The modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corporation) is used in 19.7 parts by mass of the high-density polyethylene of Mv 270,000 of Example 1 and 13.1 parts by mass of the ultra high molecular weight polyethylene of Mv 2 million. ) Is 3.6 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having an average primary particle size of 15 nm is 3.6 parts by mass, and the stretching temperature for simultaneous biaxial stretching is 115 ° C. A microporous membrane was obtained in the same manner as in Example 1 except that. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 3]
18.7 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with an Mv of 2 million 12.4 parts by mass, modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Corp.) 3.9 parts by mass, liquid paraffin “Smoyl P-350P” (trademark, ) 65.0 parts by mass of Matsumura Oil Research Co., Ltd., 0.1 mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant The added component was heated and mixed using a plast mill manufactured by Toyo Seiki Seisakusho. The heating and mixing were performed for 10 minutes with the temperature of the plastmill set to 200 ° C. and the rotation speed set to 50 rpm. The molten mixture is taken out from the plastmill and cooled, and the obtained solidified product is sandwiched between metal plates through a polyimide film, compressed at 10 MPa for 5 minutes using a hot press set at 200 ° C., and then cooled to obtain a thick film. A sheet having a thickness of 1000 μm was prepared. The obtained sheet was simultaneously biaxially stretched 7 times in the longitudinal direction and 7 times in the transverse direction at 115 ° C. using a biaxial stretching machine manufactured by Iwamoto Seisakusho. Next, the four sides were fixed with a stainless frame, the plasticizer was removed in methylene chloride, and then dried at room temperature to obtain a microporous membrane. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 4]
4.9 parts by mass of the high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Co., Ltd.) having a viscosity average molecular weight (Mv) of 270,000 in Example 1 and ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei) having an Mv of 2 million Chemicals Co., Ltd.) 3.2 parts by mass, modified polyolefin “Modic-AP H511” (trademark, manufactured by Mitsubishi Chemical Co., Ltd.) 0.9 parts by mass, average primary particle size of 15 nm silica “DM10C (Trademark, manufactured by Tokuyama Co., Ltd.) 41.0 parts by mass, liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer, 50.0 parts by mass, antioxidant As an addition of 0.1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] It was heated and mixed by using the machine Seisakusho Co., Ltd. plastomill. Subsequent steps were carried out in the same manner as in Example 1 to obtain a microporous membrane. Table 2 shows the film forming conditions and film characteristics.

[Comparative Example 5]
83.0 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 270,000, and 2.0 parts by mass of stearic acid “Lunac” (manufactured by Kao Corporation) Then, 15.0 parts by mass of silica “Aerosil 300” (trademark, manufactured by Nippon Aerosil Co., Ltd.) having an average primary particle size of 7 nm was added and mixed by heating using a plast mill manufactured by Toyo Seiki Seisakusho. In the heat mixing, the temperature of the plastmill is 200 ° C. and the rotation speed is 5
The setting was performed at 0 rpm for 10 minutes. The molten mixture is taken out from the plastmill and cooled, and the obtained solidified product is sandwiched between metal plates through a polyimide film, compressed at 10 MPa for 5 minutes using a hot press set at 200 ° C., and then cooled to obtain a thick film. A sheet having a thickness of 800 μm was prepared. The obtained sheet was stretched 1.3 times in the longitudinal direction at 70 ° C. using a biaxial stretching machine manufactured by Iwamoto Seisakusho Co., Ltd., and then biaxially stretched twice in the lateral direction at 105 ° C. to obtain a microporous film. . Table 2 shows the film forming conditions and film characteristics.

  As suggested from the examples and comparative examples, the microporous membrane of the present embodiment is excellent in heat resistance and also excellent in high-temperature storage characteristics when used as a separator for storage batteries such as non-aqueous electrolyte batteries. It can be said that the polyolefin microporous membrane is particularly suitable as a separator for a nonaqueous electrolyte battery that is required to have excellent heat resistance and long-term reliability.

  The polyolefin microporous membrane of the present invention can be suitably used as a storage battery separator, as a component of a fuel cell, a humidifying membrane, a filtration membrane, or the like. In particular, it is useful as a separator for a non-aqueous electrolyte battery that is required to have excellent heat resistance and long-term reliability.

Claims (7)

  A microporous membrane containing a polyolefin resin, a modified polyolefin, and a filler, wherein the content of the filler is greater than 10% by mass and 80% by mass or less based on the total mass of the polyolefin resin, the modified polyolefin, and the filler. Polyolefin microporous membrane.   The polyolefin microporous membrane according to claim 1, wherein the content of the filler is greater than 30% by mass and equal to or less than 60% by mass.   The polyolefin microporous membrane according to claim 1 or 2, wherein the filler is an inorganic filler. The polyolefin microporous film according to any one of claims 1 to 3, wherein the infrared absorption spectrum of the modified polyolefin has the following specific relationship.
0.010 <(SA + SB) / SC <0.1
SA: 1792cm peak area of peak A from the carbonyl group is observed in the vicinity of ^-1 SB: peak area of a peak derived from a carbonyl group is observed near 1712cm ^-1 B SC: 1472cm ^-1 methylene groups observed near Peak area of peak C derived from   The polyolefin microporous film according to any one of claims 1 to 4, wherein the viscosity average molecular weight of the polyolefin resin is 200,000 or more.   The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the content of the modified polyolefin is 5% by mass or more and 50% by mass or less with respect to the total polymer mass of the microporous membrane.   The separator for storage batteries which consists of a polyolefin microporous film of any one of Claims 1-6.