JP2008218085A - Polyolefin fine porous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fine porous membrane which is superior in permeability and thermal resistance and especially, suitable for a separator for storage battery or the like. <P>SOLUTION: The polyolefin fine porous membrane includes an island structure containing a polyolefin resin and fine particles and fibrils connecting the island structures, and the fibrils contain the polyolefin resin and are arranged in substance in one direction. Its manufacturing method and a separator for power storage device using it are provided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a polyolefin microporous membrane, a method for producing the same, and a separator for an electricity storage device using the same.

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, separation membranes used for water treatment and concentration utilizing the sieving effect due to the difference in pore diameter, adsorption sheets used for water absorption, oil absorption, deodorizing materials utilizing large surface area and porous space due to microporosity, difference in molecular size Moisture permeable waterproof sheet using the feature that allows air and water vapor to pass through but not water, multi-functionalized by filling various materials in the porous space, polymer electrolyte membrane and humidifying membrane useful for fuel cells, etc. Furthermore, they are used as liquid crystal materials and battery materials.
In the separation membrane field, ensuring selective permeability and maintaining the initial permeation amount are always required issues. For this reason, optimization of the pore shape and affinity control between the membrane substrate and the filtrate have been studied in the past, but with the diversification of the filtrate and filtration method, further improvements have been required for the membrane substrate. Yes.
In recent years, research and development of energy storage devices such as lithium ion secondary batteries (LIB), lithium ion capacitors (LIC), electric double layer capacitors (EDLC), and application development have been studied from the viewpoint of energy and resource saving. It is being actively conducted. In these electricity storage devices, 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. Various performances are required depending on the intended use of the electricity storage device.

For example, high output characteristics and high safety for automotive applications, etc., and higher capacity and higher energy density for applications such as personal computers and mobile phones, uninterruptible power supply (UPS) and As development for power storage system applications and the like, high capacity and high reliability are required for power storage devices. A separator, which is one of the storage device constituent members, is also strongly required to achieve both improved electrical characteristics and safety / reliability. However, this is a trade-off relationship and is not always satisfactory.
In Patent Document 1, an open cell film is proposed as a film suitable for applications such as filter media and medical clothing, and a large number of stretched non-porous surface regions and many bonded to each other. A microporous film comprising a plurality of parallel fibrils, which are substantially perpendicular to each other, is disclosed.

Patent Document 2 discloses a microporous polypropylene film composed of a three-dimensional network structure and fibrils arranged in one direction as a film suitable for aseptic packaging sheets, capacitor separators and battery separators.
Patent Document 3 discloses a microporous membrane in which a polyolefin resin, inorganic particles, and a plasticizer are melt-kneaded and formed into a sheet shape, biaxially stretched at a high magnification, and the plasticizer is extracted. The microporous membrane disclosed in the present technology is said to be suitable as a separator for a storage battery because it has a high puncture strength and a short circuit resistance at high temperatures and is excellent in electrolyte impregnation.
Patent Document 4 discloses a method for producing a separator for a non-aqueous battery comprising a porous film composed of a polyolefin resin and an inorganic powder. In this technique, low shrinkage is achieved by heat treatment at a relaxation rate of 5 to 50% in the stretching direction at a temperature equal to or higher than the melting point of polyolefin.
Japanese Patent Publication No.55-32531 Japanese Patent No. 2503007 WO2006-25323 JP 2001-266831 A

However, the membrane obtained by the above prior art is not sufficient in terms of permeability and heat resistance. It is difficult to say that the microporous film disclosed in Patent Document 1 has sufficient permeability. In addition, there remains a problem in heat resistance at high temperatures (for example, the melting point of the base material or higher), and there is concern about safety when used as a separator for an electricity storage device. The microporous film disclosed in Patent Document 2 has a problem in the uniformity of the pore shape, and the local permeation performance has a wide distribution. As a result, for example, it is used as a separator for an electricity storage device. In this case, since the movement of ions concentrates on the highly permeable part, the output characteristics and long-term characteristics cannot be maintained sufficiently. Further, the base material is stretched below the melting point of polypropylene or heat-set, and the heat resistance above the melting point is inferior. Therefore, when used as a power storage device separator, there is a concern about safety. In the Example of the above-mentioned Patent Document 3, the stretching temperature is lower than the melting point of the polyethylene as the base material, and there remains anxiety in shrinkability. Therefore, further improvement was necessary in terms of heat resistance. In addition, the hole shape in this example is presumed to be a three-dimensional network structure in which fibrils are randomly arranged by the manufacturing method, and there remains a problem in the uniformity of the hole shape, and the local permeation performance has a wide distribution. As a result, when used as, for example, a separator for an electricity storage device, the movement of ions concentrates on the highly permeable part, so that the output characteristics and long-term characteristics cannot be maintained sufficiently. In the heat treatment disclosed in Patent Document 4, that is, the heat treatment at a melting point or higher and a relaxation rate of 5 to 50%, the permeability is greatly reduced. Anxiety remains in the characteristics.
An object of the present invention is to provide a microporous membrane excellent in high permeability and heat resistance. Furthermore, microporous membranes suitable for filtration membranes and humidification membranes that require permeability and durability, and separators for storage batteries that are required to have excellent electrical characteristics such as output characteristics and safety, etc. An object is to provide a porous membrane.

The inventor includes an island-like structure containing a polyolefin resin and fine particles, and fibrils connecting the island-like structures, and the fibrils are arranged substantially in one direction. The inventors have found that the membrane has both high permeability and heat resistance, and have made the present invention.
That is, the present invention is as follows.
(1) A polyolefin comprising island-like structures containing a polyolefin resin and fine particles and fibrils connecting the island-like structures, wherein the fibrils contain a polyolefin resin and are arranged substantially in one direction. Microporous membrane.
(2) The polyolefin according to (1), wherein the fine particles are inorganic particles, organic particles having a higher melting point than the polyolefin resin, or organic particles having no melting point and a glass transition point higher than that of the polyolefin resin. Microporous membrane.
(3) The polyolefin microporous membrane according to (1) or (2) above, wherein the primary particle size of the fine particles is 1 nm or more and less than 1 μm.
(4) The polyolefin microporous membrane according to any one of the above (1) to (3), wherein the fibril density in the sea part constituted by the fibrils and the fibril gap is 100 pieces / 10 μm or less.
(5) The polyolefin microporous film according to any one of (1) to (4), wherein the fibril length connecting the island-shaped structures is 0.5 μm or more.
(6) The polyolefin microporous film according to any one of (1) to (5), wherein the fibrils are substantially arranged in the length direction.
(7) A method for producing a polyolefin microporous membrane in which a porous sheet containing a polyolefin resin and fine particles is stretched at least once in a uniaxial direction at a stretching temperature satisfying the following formulas (a) and (b).
(A) Stretching temperature ≧ Tme−5 ° C.
(B) Stretching temperature> Tm
(However, Tme in the above formula (a) is the end-set temperature of the polyolefin resin in the porous sheet measured with a differential scanning calorimeter, and Tm in (b) is with a differential scanning calorimeter. Each of the melting points of the polyolefin resin in the porous sheet to be measured is shown.)
(8) The method for producing a polyolefin microporous membrane according to (7), wherein the porosity of the porous sheet is 25% or more.
(9) The polyolefin microporous membrane according to any one of (1) to (6), which is obtained by the production method of (7) or (8).
(10) A power storage device separator comprising the polyolefin microporous membrane according to any one of (1) to (6) and (9) above.

  The microporous membrane of the present invention has both high permeability and heat resistance. Therefore, according to the present invention, it is possible to provide a microporous membrane that is particularly suitable as a separator for a storage battery that is required to have excellent output characteristics and safety.

First, the microporous membrane of the present invention will be described focusing on its preferred form.
The microporous membrane in the present invention has island structures and fibrils connecting the island structures. The island structure contains a polyolefin resin and fine particles, and the fibril contains a polyolefin resin.
The total content of polyolefin resin and fine particles in 100% by mass of the microporous membrane is preferably 50% by mass or more from the viewpoint of durability, and each content of polyolefin resin and fine particles is 10 from the point of heat resistance. It is preferable that it is mass% or more. In particular, the content of fine particles in the microporous membrane of the present invention is preferably 20% by mass to 80% by mass, more preferably 20% by mass to 60% by mass, and still more preferably 40% by mass to 60% by mass. It is as follows. A content of fine particles of 20% by mass or more is preferable because the thickening effect when the polyolefin resin is melted is further large and the heat resistance is excellent. Further, it is preferable to stretch at a temperature higher than the melting point of the polyolefin resin because a microporous membrane having good permeability can be easily obtained. When the content of the fine particles is 80% by mass or less, stretching at a higher magnification is possible, and the flexibility of the hole shape becomes higher, which is preferable.

Island structures are connected by fibrils. Here, the island-like structures do not have to be connected by fibrils in a one-to-one relationship. For example, the island-like structure A is connected to the other island-like structures B by fibrils, and at the same time, the other island-like structures C and D may be connected by another fibril.
One feature of the present invention is that the fibrils are arranged substantially in one direction. Here, “substantially in one direction” means that 90% or more of fibrils are included in an angle range of ± 20 degrees in a desired one direction. By arranging the fibrils substantially in one direction, the pore size distribution formed by the gaps between the fibrils becomes uniform, and a microporous membrane having uniform permeability and selective permeability can be easily obtained.

The microporous membrane of the present invention has a structure in which island-like structures containing polyolefin resin and fine particles are connected by fibrils, and the fibrils are arranged substantially in one direction. It can be said that it shows. Although it is inference, the reason why the microporous membrane of the present invention exhibits the above heat-resistant behavior is considered as follows. Under a high temperature equal to or higher than the melting point of the polyolefin resin constituting the island structure, the island structure has poor fluidity due to the thickening effect of the fine particles even though the polyolefin resin is in a molten state. On the other hand, since the fibrils connecting the island-like structures are presumed to be remarkably oriented, contraction stress is generated by heating and melting. However, since the fibril has a structure connected to an island structure having poor fluidity, the shrinkage of the fibril is reduced, and as a result, good heat resistance can be obtained. In addition, since the fibrils are arranged substantially in one direction, the shrinkage stress works uniformly in one direction. Unlike the case where the fibrils are arranged in a random direction, the film breakage due to stress concentration is suppressed. Think.
The fibrils are preferably arranged in the length direction. A length direction means the advancing direction (MD: Machine Direction) at the time of manufacturing a microporous film. The direction perpendicular to the surface is the TD (Transverse Direction) or the width direction. When the fibrils are arranged in the length direction, the shrinkage stress to the TD is extremely small, and when used as a separator for a storage battery generally having a wound structure, it is preferable because higher safety is obtained.

  The ratio of the island-shaped structures on the surface of the microporous membrane is preferably 5% to 70%, more preferably 10% to 60%, and still more preferably 20% to 50%. If it is 5% or more, better heat resistance is obtained, and if it is 70% or less, better permeability is obtained, which is preferable. The surface of the microporous membrane is composed of three regions of a pore region formed by an island structure region, a fibril region, and a fibril gap (in this specification, the pore region formed by the fibril region and the fibril gap is combined, Sometimes referred to as “Kaibe”). The ratio of the island-like structures on the surface of the microporous membrane can be calculated by separating the island-like structures and the sea from a surface image obtained using, for example, a scanning microscope.

The size per island structure is not particularly limited, but the length in the direction in which the fibrils are arranged is 0.1 to 50 μm from the viewpoint of achieving both high permeability and heat resistance. Is preferable, and it is more preferable that it is 0.5-10 micrometers.
The length of the fibril connecting the island-like structures is preferably 0.5 μm or more and 50 μm or less, more preferably 1 μm or more and 20 μm or less, and further preferably 1.5 μm or more and 10 μm or less. If it is 0.5 μm or more, better permeability can be obtained. For example, when it is used with a filtration membrane, the reduction in permeability due to permeability clogging is also less preferable. If it is 50 micrometers or less, since more favorable heat resistance is obtained, it is preferable.

  The fibril density in the sea is preferably 100 pieces / 10 μm or less. 100/10 μm is the number of fibrils present per length of 10 μm in a direction perpendicular to the fibrils. More preferably, it is 5/10 μm or more and 80/10 μm or less, and further preferably 10/10 μm or more and 50/10 μm or less. If it is 100/10 μm or less, it is preferable for excellent permeability, and if it is 5/10 μm or more, it is preferable for excellent strength. The fibril density can be measured by a method described later using a scanning electron microscope (SEM). If the fibril density is large or the fibrils are very small and difficult to measure, the surface photograph observed at a high magnification may be used to calculate the fibril density per 10 μm using the number of fibrils measured per 1 μm length. I do not care.

The microporous membrane of the present invention is not particularly limited as long as it has a structure containing island-like structures containing polyolefin and fine particles, and fibrils arranged in substantially one direction connecting the island-like structures. Including the case where the original network structure is partially present, from the viewpoint of permeability and heat resistance, the structure is substantially composed of the island structure and the fibril, and further, a three-dimensional network. A structure that does not include a structure is preferable.
The polyolefin resin used in the present invention includes polyolefin resins that can be used for ordinary extrusion, injection, inflation, blow molding, and the like. For example, ethylene, propylene, 1-butene, 4-methyl-1-pentene, Homopolymers and copolymers such as 1-hexene and 1-octene, 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, and ethylene propylene rubber. . When the microporous membrane of the present invention is used as a battery separator, it is preferably a low-melting point resin. In addition, high-density polyethylene is particularly used as a main component (for example, 10 parts by mass in 100 parts by mass of polyolefin resin) because of the required performance of high strength. Part or more) is preferably used.

The viscosity average molecular weight of the matrix resin of the polyolefin resin or microporous membrane used in the present invention is preferably 50,000 or more and less than 10 million, more preferably 200,000 or more and less than 3 million, more preferably 400,000 or more and less than 1 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 10 million or less, it tends to be easy to obtain uniform melt kneading, and it is preferable because it tends to be excellent in sheet formability, particularly thickness stability.
Polyolefin resins include phenolic, phosphorus and sulfur-based antioxidants, metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light as necessary, as long as the advantages of the present invention are not impaired. Additives such as stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.

  The fine particles are preferably inorganic particles, organic particles having a melting point higher than that of the polyolefin resin, or organic particles having no melting point and a glass transition point higher than that of the polyolefin resin. When the organic particles have both a melting point and a glass transition point, the melting point is preferably higher than the melting point of the polyolefin resin, and the glass transition point may be lower than the melting point of the polyolefin resin. Specifically, the inorganic particles are preferably oxides and nitrides such as silicon, aluminum, titanium and magnesium, and carbonates and sulfates such as calcium and barium. Moreover, the inorganic particle which performed the surface treatment suitably can be used. For example, when producing a microporous membrane for filtration applications using an aqueous solvent or a separator for an aqueous electrolyte storage device, inorganic particles subjected to hydrophilic treatment are suitable, and a separator for a nonaqueous electrolyte storage device is manufactured. In this case, inorganic particles that have been subjected to hydrophobic treatment are suitable. 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.

  The fine particles preferably have a primary particle size of 1 nm or more and less than 1 μm. More preferably, it is 1 nm or more and less than 100 nm. The particle size can be measured with a scanning electron microscope or a transmission electron microscope. In view of achieving good dispersion in the polyolefin resin, the primary particle size is preferably 1 nm or more. It is considered that a sufficient thickening effect can be imparted by showing good dispersion, and the heat resistance is more excellent. Similarly, the primary particle size is preferably less than 1 μm from the viewpoint that a sufficient thickening effect can be obtained. Further, when stretched, the primary particle size is preferably less than 1 μm from the viewpoint that the interfacial separation between the polyolefin resin and the fine particles hardly occurs and a uniform pore size distribution is likely to occur.

It is preferable that the fine particles have substantially no internal surface area inside the primary particles, that is, the primary particles themselves have substantially no fine pores. When such fine 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.
The final film thickness of the microporous membrane of the present invention 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. If the film thickness is 2 μm or more, the mechanical strength is sufficient, and if the film thickness is 100 μm or less, the occupied volume of the separator is reduced, which tends to be more 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.
The air permeability is preferably in the range of 1 second to 500 seconds, more preferably 5 seconds to 200 seconds, and even more preferably 7 seconds to 100 seconds. When the air permeability is 1 second or more, self-discharge is small when used as a battery separator, and when it is 500 seconds or less, good charge / discharge characteristics are obtained, which is preferable.

The bubble point of the microporous membrane is preferably from 0.1 MPa to 1 MPa, more preferably from 0.2 MPa to 0.7 MPa. A bubble point of 1 MPa or less is preferable because the permeability is good and the influence of clogging is small. If it is 0.1 MPa or more, it is preferable because it has a low possibility of self-discharge when used as a battery separator and is reliable.
The heat shrinkage at high temperature is preferably 30% or less. Both the MD direction and the TD direction are preferably small, but when used as a battery separator, it is preferably low shrinkage in the TD direction. The shrinkage in the TD direction at a high temperature (for example, 150 ° C.) is preferably 30% or less because the safety of the battery can be secured. More preferably, it is 20% or less, More preferably, it is 10% or less. Although a minimum in particular is not restrict | limited, 1% or more is preferable from an adhesive viewpoint with an electrode.

Next, although a suitable example is described regarding the manufacturing method for obtaining the microporous membrane of this invention, the manufacturing method of the microporous membrane of this invention is not necessarily limited to this example. For example, the microporous membrane of the present invention can be obtained by a method including the following steps (1) to (4).
(1) Step of melt-kneading polyolefin resin, fine particles and plasticizer (2) Step of extruding the melt, forming into a sheet, cooling and solidifying (3) Step of extracting the plasticizer (4) At least uniaxially Step of stretching There are no particular restrictions on the order and number of times of these steps.

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 when mixed with the polyolefin resin. 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 preferable because it is highly compatible with polyethylene and is difficult to cause interface peeling between the resin and the plasticizer during stretching.
The ratio of the polyolefin resin, fine particles, and plasticizer 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. It ’s fine. Specifically, the mass fraction of the plasticizer in the composition comprising the polyolefin resin, fine particles, and the plasticizer is preferably 30 to 80 mass%, more preferably 40 to 70 mass%. 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. In addition, since the plasticizing effect is sufficient, the crystalline folded lamellar crystal can be efficiently stretched, and when stretched at a high magnification, the polyolefin chain is not broken and a uniform and fine pore structure is obtained and the strength is easily increased. The method of melt-kneading polyolefin resin, fine particles and plasticizer is to introduce polyolefin resin and fine particles into a resin kneading apparatus such as an extruder or kneader, and introduce a plasticizer at an arbitrary ratio while heating and melting the resin, Furthermore, a method of obtaining a uniform solution by kneading a composition comprising a resin, fine particles and a plasticizer is preferred. As a more preferable method, a polyolefin resin, fine particles, and a plasticizer are previously kneaded at a predetermined ratio using a Henschel mixer or the like, and the kneaded product is put into an extruder and plasticized at an arbitrary ratio while being heated and melted. Introducing an agent and further kneading.

  The step (2) of extruding the melt and cooling and solidifying it to produce a sheet-like microporous membrane precursor is a sheet-like solution of a uniform solution of polyolefin resin, fine particles and plasticizer via a T-die or the like. It is preferable to carry out by extruding, contacting with a heat conductor, and cooling to a temperature sufficiently lower than the crystallization temperature of the resin. As the heat conductor used for cooling and solidification, metal, water, air, plasticizer itself, or the like can be used. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable. Further, it is more preferable that the metal roll is sandwiched between the rolls because the heat conduction efficiency is further increased, the sheet is oriented and the film strength is increased, and the surface smoothness of the sheet is also improved. The die lip interval when extruding into a sheet form from a T die is preferably 400 μm or more and 3000 μm or less, and more preferably 500 μm or more and 2500 μm. A die lip spacing of 400 μm or more is preferable because it reduces the meander and the like, has less influence on the film quality such as streaks and defects, and prevents film breakage in the subsequent stretching process. A thickness of 3000 μm or less is preferable because the cooling rate is high and uneven cooling can be prevented and the thickness stability can be maintained.

The method of extracting the plasticizer of (3) may be either a batch type or a continuous type. However, the plasticizer is extracted by immersing the microporous membrane in an extraction solvent and sufficiently dried. It is preferable to remove substantially from. 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.
It is desirable that the extraction solvent is a poor solvent for the polyolefin resin and fine particles and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin microporous membrane. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and non-chlorine-based 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 stretching step (4), at least uniaxial stretching is performed. 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 method in which stretching in the MD direction and stretching in the TD direction are performed simultaneously, and the deformation rate in each direction 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 in an unconstrained state or a constant state. It is in a state of being fixed to the length. The stretching step may be performed in a step before the step of extracting the plasticizer in (3), in a subsequent step, or may be performed a plurality of times before and after. A step of performing stretching a plurality of times in the pre-step and the post-step of the extraction step (3) is preferable.

  The stretching temperature (T [° C.]) may be any number as long as the stretching is possible. As preferred stretching conditions, (I) a temperature (Tme-5) higher than the melting point (Tm [° C]) of the polyolefin in the porous sheet and 5 ° C lower than the end-set temperature (Tme [° C]) of the melting endothermic peak. (° C.) or higher (T [° C.]> Tm [° C.] and T [° C.] ≧ Tme [° C.] − 5 ° C.). Furthermore, it is more preferable to use (II) stretching at a temperature not higher than the melting point of the polyolefin in the porous sheet (T [° C.] ≦ Tm [° C.]), and (3) from the step of extracting the plasticizer. It is most preferable that the stretching step of (II) is included in the previous step and the stretching step of (I) is included in the subsequent step rather than the step of extracting the plasticizer of (3). The melting point (Tm [° C.]) here refers to the peak top temperature of the melting endothermic curve measured with a differential scanning calorimeter (DSC). When a single polyolefin resin or two or more types of polyolefin resins are used and there are two or more melting endothermic peaks, the peak top temperature having the largest melting endotherm is regarded as the melting point in the present invention. The end set temperature (Tme [° C.]) of the melting endothermic peak refers to the end temperature of the melting endothermic peak. When there are two or more melting endothermic peaks, the end temperature of the melting endothermic peak at the highest temperature is regarded as the end-set temperature in the present invention. The end set temperature is obtained from the intersection of the tangent line of the melting endothermic peak and the tangent line of the baseline.

It is preferable to include the stretching step (I) because it is easy to form fibrils that connect the island structures. The stretching step (II) is preferable because high strength is easily achieved. The porosity of the porous sheet used in the stretching step (I) is preferably 25% or more and 90% or less, more preferably 30% or more and 70% or less, and further preferably 35% or more and 60% or less. If it is 25% or more, interfacial peeling from the polyolefin resin hardly occurs during stretching, and the hole shape can be easily controlled. If it is 90% or less, there is little concern that the porosity will unnecessarily increase due to stretching, and the obtained microporous film tends to have high strength.
The stretching ratio is preferably in the range of 20 times to less than 200 times in terms of total surface magnification, more preferably in the range of 20 times to 100 times, and further preferably in the range of 25 times to 50 times. When the total surface magnification is 20 times or more, sufficient 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 present invention provides the above-mentioned preferred production method, that is, a polyolefin in which a porous sheet containing a polyolefin resin and fine particles is stretched at least once in a uniaxial direction at a stretching temperature satisfying the following formulas (a) and (b): A method for producing a microporous membrane is also included.
(A) Stretching temperature ≧ Tme−5 ° C.
(B) Stretching temperature> Tm
(However, Tme in the above formula (a) is the end-set temperature of the polyolefin resin in the porous sheet measured with a differential scanning calorimeter, and Tm in (b) is with a differential scanning calorimeter. Each of the melting points of the polyolefin resin in the porous sheet to be measured is shown.)
In the production method of the present invention, a heat treatment step such as heat fixation and heat relaxation may be provided subsequent to or after each stretching process, and such an embodiment is preferable because it further suppresses shrinkage of the microporous membrane.

The microporous membranes of the present invention may be used in layers with other base materials.
The manufacturing method of the present invention may include a post-processing step. Examples of the post-treatment include hydrophilization treatment with a surfactant and the like, cross-linking treatment with ionizing radiation, and the like, coating a thermoplastic resin, inorganic particles, and the like on one side or both sides.
This invention includes the separator for electrical storage devices using the said microporous film. Examples of the electricity storage device include a lithium ion secondary battery (LIB), a lithium ion capacitor (LIC), and an electric double layer capacitor (EDLC).

EXAMPLES Next, although an Example demonstrates this invention further in detail, these do not restrict | limit the scope of the present invention. The test methods in the examples are as follows.
<Evaluation of microporous membrane>
(1) Viscosity average molecular weight 2,6-di-tert-butyl-4-methylphenol was dissolved in decahydronaphthalene to prevent deterioration of the sample to a concentration of 0.1 w%, and this (hereinafter abbreviated as DHN) ) As a sample solvent. The microporous membrane is dissolved in DHN at 150 ° C. to a concentration of 0.1 w%. The solution is filtered, fine particles are removed and used as a sample solution. Alternatively, a microporous film in which the fine particles are extracted and removed first by immersing the microporous film in a solution in which the fine particles dissolve but the polyolefin resin does not dissolve or react may be used. 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

(2) Primary particle diameter It measured using the scanning electron microscope (SEM) "model S-4800, the product made by HITACHI." The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV.
(3) Film thickness It measured at room temperature 23 degreeC using the micro thickness measuring device (type KBM by Toyo Seiki).
(4) Porosity A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were obtained, and calculated from these and the film density (g / cm ³ ) using the following formula: did.
Volume (cm ³ ) = 10 × 10 × film thickness (μm) / 10000
Porosity (%) = (volume-mass / density of mixed composition) / volume × 100
As the density of the mixed composition, a value calculated from the density and mixing ratio of each of the used polyolefin resin and fine particles was used.
(5) Air permeability It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117.

(6) Shrinkage A sample of MD120 mm × TD120 mm square was cut out from the microporous membrane and marked with an oil-based pen at three locations at intervals of TD100 mm. The microporous film was sandwiched between copy paper (manufactured by KOKYUYO) having an A4 size, a basis weight of 64 g / m ² , and a paper thickness of 0.092 mm, and the sides of the copy paper were stapled. It was placed horizontally in an oven at 150 ° C. and left for 1 hour. Then, it air-cooled and measured TD length (mm) between marks. The shrinkage rate was calculated from the average value at three locations.
Shrinkage rate (%) = (1-TD length (mm) / 100) × 100
(7) Fibril density The fibril density of the microporous membrane was measured using a scanning electron microscope (SEM) “Model S-4800, manufactured by HITACHI”. The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV. The surface image was printed by observing the fibril array in the vertical direction. A line was drawn in the horizontal direction in the sea (hole region formed by the fibril region and the fibril gap), and the number of fibrils crossing the line was measured. The length of the line is a length corresponding to 0.5 μm to 50 μm in the SEM observation image. The fibril density was calculated from the average value of a total of three places in three different visual fields. In Example 1, it was observed and measured at a magnification of 20000 times.
Fibril density (line / 10 μm) = number of crossed fibrils (line) / line length corresponding to SEM observation image (μm) × 10

(8) Fibril length The fibril length of the microporous membrane was measured using a scanning electron microscope (SEM) “Model S-4800, manufactured by HITACHI”. The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV. The surface image was printed by observing the fibril array in the vertical direction. Twenty fibrils were arbitrarily selected, the fibril length was measured, and the average value was calculated. The same operation was performed for a total of three fields, and the average value of the three fields was the fibril length. In Example 1, it was observed and measured at a magnification of 3000 times.
(9) Island Ratio The island ratio of the microporous membrane was measured using a scanning electron microscope (SEM) “Model S-4800, manufactured by HITACHI”. The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV. Observation and printing were performed at a magnification such that about 3 to 100 island-shaped structures existed on a surface photograph 10 cm × 10 cm, and island structures were cut out from the surface photograph. The ratio of the island structure was calculated from the ratio of the weight of the surface photograph 10 cm × 10 cm measured in advance and the weight of the island structure cut out from the surface photograph 10 cm × 10 cm to obtain the island ratio. In Example 1, it was observed and measured at a magnification of 3000 times.
Island ratio (%) = weight of cut-out island structure (g) / surface photograph weight (g) × 100

(10) Melting point and end set temperature Measurement was performed using a differential scanning calorimeter “DSC60” (trademark, manufactured by Shimadzu Corporation). A perforated sheet was punched into a circle with a diameter of 5 mm, and several sheets were stacked to give 3 mg, which was used as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endothermic curve. The peak top temperature of the obtained melting endothermic curve was defined as the melting point (Tm [° C.]), and the peak end temperature was defined as the end set temperature (Tme [° C.]). The melting point and end set temperature were read from the melting endothermic curve using a thermal analysis workstation (Shimadzu, TA-60WS).

(11) Rate characteristic evaluation a. Preparation of 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) as a binder A slurry is prepared by dispersing 2% by 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 is prepared by dispersing 96.6% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. . 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 non-aqueous electrolyte solution It is adjusted 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. Rate characteristic evaluation The discharge capacity at the time of 6.0 mA (1C) discharge in each cell was taken as 100%, and the 12.0 mA (2C) discharge capacity and the 18 mA (3C) discharge capacity were compared. Charging / discharging was performed in the following order using a charging / discharging device (model HJ-201BS, manufactured by Hokuto Denko).
(CH1) 6.0 mA charge, (DC1) 6.0 mA discharge, (CH2) 6.0 mA charge, (DC2) 6.0 mA discharge, (CH3) 6.0 mA charge, (DC3) 12.0 mA discharge, (CH4 ) 6.0 mA charge, (DC4) 6.0 mA discharge, (CH5) 6.0 mA charge, (DC5) 18.0 mA discharge.
Charging was performed for a total of 3 hours by charging to a battery voltage of 4.2 V at a specified current value and starting to narrow the current value from the specified value so as to hold 4.2 V after reaching the specified voltage value. The battery was discharged at a specified current value to a battery voltage of 3.0V. The discharge capacities during 2C discharge and 3C discharge in each cell were compared.
2C discharge capacity (%) = (DC3) discharge capacity / (DC2) discharge capacity × 100
3C discharge capacity (%) = (DC5) discharge capacity / (DC2) discharge capacity × 100

(12) Evaluation of heat resistance a. Cell preparation A positive electrode 75 mm × 25 mm, a negative electrode 75 mm × 25 mm, a microporous membrane 50 mm × 50 mm, and a PET film 75 mm × 75 mm are cut out. As the positive electrode and the negative electrode, the same positive electrode and negative electrode prepared by rate characteristic evaluation were used. Using a SUS plate 70 x 70 mm and a double clip (manufactured by KOKYUYO, small specification, mouth width 19 mm), the SUS plate, PET film, negative electrode, microporous membrane, positive electrode, PET film, SUS plate are stacked in this order, and then with a double clip Hold the four corners and fix. The positive electrode and the negative electrode are placed so that one end of the positive electrode and the negative electrode protrudes from the PET film so that the active material surfaces of the positive electrode and the negative electrode face each other. A microporous film is disposed in a portion where the positive electrode and the negative electrode overlap each other, and is superposed so that the positive electrode and the negative electrode do not directly contact each other. Using a tester (manufactured by HIOKI, HIOKI3560 AC Milliome High Tester), connect the terminal to the positive and negative electrodes protruding from the PET film, measure the resistance value between the positive and negative electrodes, and confirm that it is 10 ⁶ Ω or more. The appearance of the cell is shown in FIGS.
b. Evaluation The cell was placed in an oven set at a predetermined temperature, taken out after a predetermined time, and after the cell was sufficiently cooled, the resistance between the positive and negative electrodes was measured. When the resistance value was 1000Ω or more, it was judged as acceptable (◯), and when it was less than 1000Ω, it was judged as unacceptable (x). The oven temperature and time were implemented under two conditions: (1) 180 ° C./30 minutes, and (2) 200 ° C./10 minutes.

[Example 1]
22.5 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 15 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having a primary particle diameter of 15 nm, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Co., Ltd.) as a plasticizer. 37.5 parts by mass of Petroleum Research Institute Co., Ltd. and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant This was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1480 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter was 121 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a porous sheet. The melting endothermic curve of the obtained porous sheet is shown in FIG. As shown in FIG. 3, the melting point was 137 ° C., and the end set temperature was 151 ° C. Further, the porous sheet was MD-stretched at a roll temperature of 150 ° C. using a roll stretching machine and then wound to obtain a microporous film. The winding speed / feeding speed ratio in MD stretching was set to 2.0 times. Surface SEM images of the obtained microporous membrane are shown in FIGS. The island ratio and fibril length of the microporous membrane were measured from FIGS. 4 and 5, respectively. The fibril density was determined from FIG. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Example 2]
A microporous membrane was obtained in the same manner as in Example 1 except that the winding speed / feeding speed ratio during MD stretching of the porous sheet was changed to 3.0 times. Surface SEM images of the obtained microporous membrane are shown in FIGS. The island ratio and fibril length of the microporous membrane were measured from FIGS. 6 and 7, respectively. The fibril density was determined from FIG. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Example 3]
17.5 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 12.5 parts by mass, alumina “AluC” (trademark, manufactured by Degussa) having a primary particle size of 13 nm is 37 parts by mass, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Oil Co., Ltd.) as a plasticizer. Made by adding 33 parts by mass of a laboratory) and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant Premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt kneading was performed at a preset temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 2200 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 124 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried, and further, led to a horizontal tenter with the exit magnification set to 0.96 times, heat relaxed in the TD direction, and wound to obtain a porous sheet. At this time, the set temperature of the TD relaxation part was 150 ° C. The melting endothermic curve of the obtained porous sheet showed two peak tops (130 ° C. and 150 ° C.). The melting point determined from the peak with the highest melting endotherm was 130 ° C., and the end-set temperature determined from the highest peak was 154 ° C. Next, the porous sheet was subjected to MD stretching at a roll temperature of 150 ° C. using a roll stretching machine to obtain a microporous film. The winding speed / feeding speed ratio in MD stretching was set to 2.5 times. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Example 4]
Example 3 except that titania “TTO-55 (S)” (Ishihara Sangyo Co., Ltd.) having a primary particle size of 40 nm was used instead of alumina “AluC” (trademark, manufactured by Degussa) of Example 3. In the same manner, a microporous membrane was obtained. The film forming conditions and film characteristics are shown in Table 1. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Example 5]
12.8 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 “UH650” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 1 million 19.2 parts by mass, silica "QS10" (trademark, manufactured by Tokuyama Corporation) having a primary particle size of 15 nm, 20 parts by mass, dioctyl phthalate (DOP) as a plasticizer, 48 parts by mass, antioxidant What added 0.3 mass part of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an agent was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between rolls controlled at a surface temperature of 140 ° C. to obtain a sheet-like polyolefin composition having a thickness of 100 μm. Next, it was sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a porous sheet. The resulting porous sheet had a melting point of 133 ° C. and an end set temperature of 137 ° C. The obtained sheet was stretched 2.5 times in the longitudinal direction at 140 ° C. using a biaxial stretching machine manufactured by Iwamoto Seisakusho to obtain a microporous film. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Comparative Example 1]
A microporous membrane was obtained in the same manner as in Example 1 except that MD stretching of the porous sheet was performed at a roll temperature of 145 ° C. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Comparative Example 2]
Instead of MD stretching the porous sheet with a roll stretching machine, the same procedure as in Example 1 was performed, except that the transverse tenter was TD stretched (the exit magnification was set to 1.6 times and the set temperature of the TD stretched portion was set to 130 ° C.). Thus, a porous film was obtained. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Comparative Example 3]
Instead of MD stretching the porous sheet with a roll stretching machine, the same procedure as in Example 1 was performed except that the TD heat relaxation was performed with a horizontal tenter (the exit magnification was set to 0.92 times and the temperature of the TD relaxation part was set to 150 ° C.). Thus, a microporous membrane was obtained. The characteristics of the obtained microporous membrane are shown in Table 1 together with the film forming conditions. The evaluation results of the obtained microporous membrane are shown in Table 2.

[Comparative Example 4]
Pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-) as an antioxidant in high-density polyethylene “B161” (trademark, manufactured by Asahi Kasei Chemicals Corporation) having a viscosity average molecular weight (Mv) of 130,000 What added 0.3 part by mass of 4-hydroxyphenyl) propionate] was mixed with a Henschel mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. The melt-kneading conditions were a set temperature of 180 ° C., a screw rotation speed of 100 rpm, and a discharge rate of 12 kg / h. Subsequently, the polyolefin film was wound up through a roll whose roll surface temperature was controlled at 25 ° C. while melt-kneading the melt-kneaded material through a T die so as to have a draft ratio of 100. Next, heat treatment was performed at 115 ° C. for 30 minutes. The obtained sheet was stretched 1.5 times in the machine direction at 25 ° C. using a roll stretching machine, and finally stretched to 120 times in the machine direction at 120 ° C., followed by heat treatment at 125 ° C. To obtain a microporous membrane. Table 1 shows the film forming conditions and the characteristics of the microporous film. The evaluation results of the obtained microporous membrane are shown in Table 2.

As is apparent from Table 1, the microporous membrane of the present invention has a low air permeability and better permeability than the comparative example. Further, the shrinkage is small and an island / fibril structure is formed.
As shown in Table 2, the results of the examples of the present invention are clearly superior in heat resistance and rate characteristics than the comparative examples. In particular, it can be said to be a microporous membrane excellent in output characteristics and safety as a separator for a storage battery such as a non-aqueous electrolyte battery.

The microporous membrane of the present invention can be suitably used as a separator for a storage battery such as a non-aqueous electrolyte battery excellent in output characteristics and safety, as a component of a fuel cell, a humidifying membrane, a filtration membrane, or the like. In particular, it is useful in the field of batteries for electric vehicles and hybrid vehicles that require output characteristics and safety.
1 Microporous membrane

It is the schematic (top view) of the cell used in heat resistance evaluation. It is the schematic (sectional drawing) of the cell used in heat resistance evaluation. 3 is a melting endothermic curve by DSC of the porous sheet of Example 1. 2 is a surface SEM image (3000 times) of the microporous membrane of Example 1. FIG. 2 is a surface SEM image (20,000 times) of the microporous membrane of Example 1. FIG. 4 is a surface SEM image (3000 times) of the microporous membrane of Example 2. FIG. 4 is a surface SEM image (20,000 times) of the microporous membrane of Example 2.

Explanation of symbols

2 Positive electrode 3 Negative electrode 4 PET film 5 SUS plate 6 Double clip

Claims (10)

A polyolefin microporous membrane comprising an island-like structure containing a polyolefin resin and fine particles and fibrils connecting the island-like structures, wherein the fibril contains a polyolefin resin and is arranged substantially in one direction . The polyolefin microporous according to claim 1, wherein the fine particles are inorganic particles, organic particles having a melting point higher than that of the polyolefin resin, or organic particles having no melting point and a glass transition point higher than that of the polyolefin resin. film. The polyolefin microporous membrane according to claim 1 or 2, wherein the primary particle diameter of the fine particles is 1 nm or more and less than 1 µm. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein a fibril density in a sea part constituted by fibrils and fibril gaps is 100 pieces / 10 µm or less. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein a fibril length connecting the island-shaped structures is 0.5 µm or more. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the fibrils are substantially arranged in the length direction. A method for producing a polyolefin microporous membrane, in which a porous sheet containing a polyolefin resin and fine particles is stretched at least once in a uniaxial direction at a stretching temperature satisfying the following formulas (a) and (b).
(A) Stretching temperature ≧ Tme−5 ° C.
(B) Stretching temperature> Tm
(However, Tme in the above formula (a) is the end-set temperature of the polyolefin resin in the porous sheet measured with a differential scanning calorimeter, and Tm in (b) is with a differential scanning calorimeter. Each of the melting points of the polyolefin resin in the porous sheet to be measured is shown.) The method for producing a polyolefin microporous membrane according to claim 7, wherein the porosity of the porous sheet is 25% or more. The polyolefin microporous membrane according to any one of claims 1 to 6, obtained by the production method according to claim 7 or 8. The electrical storage device separator which consists of a polyolefin microporous film as described in any one of Claims 1-6.

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