JP6886839B2 - Polyolefin microporous membrane - Google Patents

Description

The present invention relates to a microporous polyolefin membrane, a separator for a secondary battery, and the like.

Polyolefin microporous membranes are used in battery separators, condenser separators, fuel cell materials, microfiltration membranes, and the like, and are particularly used as lithium ion secondary battery separators. The separator prevents direct contact between the positive and negative electrodes, and also allows ions to permeate through the electrolytic solution held in the microporous structure.

In recent years, lithium-ion secondary batteries have been applied not only to small electronic devices such as mobile phones and notebook computers, but also to electric vehicles such as electric vehicles and small electric motorcycles. Since the capacity of an in-vehicle lithium-ion secondary battery tends to increase per cell, the amount of heat generated when the battery generates abnormal heat also increases. Therefore, the separator is also required to have a characteristic (fuse characteristic) of maintaining the safety of the battery by fusing at a relatively low temperature to promptly stop the battery reaction.

Raw materials or materials for polyolefin microporous membranes have been studied in relation to the required properties of the separator (Patent Documents 1 to 3).

Patent Document 1 states that in a multilayer (laminated) polyolefin porous membrane having a specific shutdown temperature and an increase rate of air permeation resistance and having a thickness of 20 μm or less, the peel strength between the base material and the modified porous layer is increased and the speed is high. In order to improve workability, it is described that one layer thereof contains a resin having a melt flow rate MFR (also referred to as melt index MI) of 25 to 150 g / 10 minutes and a melting point of 120 ° C. or higher and lower than 130 ° C. .. Specific examples of this resin include an ethylene-hexene copolymer (melting point 124 ° C., MFR 135 g / 10 min), an ethylene-butene copolymer (melting point 121 ° C., MFR 55 g / 10 min), and an ethylene-octene copolymer (melting point 125). ° C., MFR 50 to 136 g / 10 min), etc., and the peel strength and abrasion resistance of the polyolefin porous film laminate have been studied.

In Patent Document 2, from the viewpoint of the safety of the secondary battery, (A) a resin composed of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 500,000 or more and polyethylene (PE) having a weight average molecular weight (Mw) of 10,000 to 500,000. A microporous polyolefin membrane comprising the composition, (B) a linear ethylene-α-olefin copolymer having a melting point of 95-125 ° C. produced using a metallocene catalyst, and (C) polypropylene (PP) is described. There is. Specific examples of the component (B) are an ethylene-octene copolymer (melting point 100 ° C. or 108 ° C., octene content 7.5 mol% or 12.0 mol%), and the heat shrinkage rate of the polyolefin microporous film. , Shutdown temperature and meltdown temperature are being investigated. The metallocene catalyst is a single-site catalyst.

In Patent Document 3, from the viewpoint of ion permeability, mechanical strength and safety, it is produced by using (1) 20 to 98% by weight of PE having Mw of 500,000 or more or a composition containing the same, and (2) a metallocene catalyst. A polyethylene microporous film comprising a PE composition containing 2 to 80% by weight of a linear ethylene-α-olefin copolymer having a melting point of 95 to 125 ° C. is described. Specific examples of the component (2) are an ethylene-octene copolymer (melting point 121 ° C. and density 0.915, melting point 108 ° C. and density 0.915, or melting point 100 ° C. and density 0.902) and polyethylene fine. The shutdown temperature and meltdown temperature of the porous membrane are being investigated.

International Publication No. 2015/146579 Japanese Unexamined Patent Publication No. 2001-72788 Japanese Unexamined Patent Publication No. 11-269289

Regarding the fuse characteristics of the separator used in in-vehicle lithium-ion secondary batteries, simply using a resin with a relatively low melting point as the separator makes it easier for the separator to shrink at relatively high temperatures, and the safety of the secondary battery. May be damaged.

Therefore, the conventional polyolefin microporous membranes as described in Patent Documents 1 to 3 still have room for study on the relationship between the low temperature fuse property and the heat shrinkage property of the separator, and further improve the safety of the secondary battery. Is also desired.

In view of the above circumstances, the problems to be solved by the invention are a polyolefin microporous film having a low temperature fuse function, sufficient mechanical strength, and good heat resistance, and a secondary battery having excellent safety of the secondary battery. To provide a separator.

The present inventors have found that the above-mentioned problems can be solved by a polyolefin microporous membrane in which a specific polyethylene and a specific polyolefin are used in combination, and have completed the present invention.

That is, the present invention is as follows.
[1]
Ultra-high molecular weight polyethylene (HMWPE) having a viscosity average molecular weight (Mv) of 500,000 to 2,500,000, a melt index (MI) of 2 g / 10 minutes or more and less than 50 g / 10 minutes, and a melting point of 120 ° C. A polyolefin microporous film containing a polyolefin having a temperature of 137 ° C. or lower and different from the high molecular weight polyethylene.
[2]
The microporous polyolefin membrane according to [1], wherein the polyolefin has a melting point of 123 ° C. or higher and 137 ° C. or lower.
[3]
The microporous polyolefin membrane according to [1] or [2], wherein the polyolefin has a melting point of 126 ° C. or higher and 137 ° C. or lower.
[4]
The polyolefin microporous membrane according to any one of [1] to [3], wherein the MI of the polyolefin is 2 g / 10 minutes or more and less than 25 g / 10 minutes.
[5]
The polyolefin microporous membrane according to any one of [1] to [4], wherein the high molecular weight polyethylene has a viscosity average molecular weight (Mv) of 500,000 to 1,500,000.
[6]
A separator for a secondary battery containing the polyolefin microporous membrane according to any one of [1] to [5].

According to the present invention, the low temperature fuse characteristics, heat resistance and mechanical strength of the microporous polyolefin membrane can be ensured.
Further, according to the present invention, the fuse temperature of the separator can be lowered as compared with the conventional UHMWPE (melt index MI: 0.01 g / less than 10 minutes), and the conventional ultra-high MI PE (MI: Compared with 50 g / 10 minutes or more), the amount of heat shrinkage of the separator can be suppressed to increase the strength, and thus the safety of the secondary battery can be improved.

FIG. 1 (A) is a schematic view of a fuse temperature measuring device, and FIG. 1 (B) is a schematic view for explaining a microporous film fixed on a nickel foil in measuring the fuse temperature. Moreover, FIG. 1C is a schematic view for explaining the masking of the nickel foil in the measurement of the fuse temperature.

Hereinafter, a mode for carrying out the present invention (hereinafter, abbreviated as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist thereof.

<Microporous membrane>
The polyolefin microporous membrane according to this embodiment is
The first ultra-high molecular weight polyethylene (HMWPE) having a viscosity average molecular weight (Mv) of 500,000 to 2,500,000,
A second polyolefin (PO) having a melt index (MI) of 2 g / 10 minutes or more and less than 50 g / 10 minutes, a melting point of 120 ° C. or higher and 137 ° C. or lower, and different from the first HMWPE.
including.

Although not bound by theory, the Mv of the polyolefin microporous membrane is contradictory to the MI and fluidity of the polyolefin microporous membrane, and the MI and fluidity of the polyolefin microporous membrane correlate. It is believed that there is. Therefore, by balancing the Mv of the first HMWPE with the MI and fluidity of the second PO as the content of the polyolefin microporous film, both the fuse characteristics and the heat shrinkage characteristics can be achieved, and the secondary battery can be used. It is considered that the fuse temperature and heat shrinkage of the separator for use can be lowered, high strength can be maintained, and the safety of the secondary battery can be improved.

Further, the polyolefin microporous membrane preferably has low electron conductivity, ionic conductivity, high resistance to an organic solvent, and a fine pore size.

The polyolefin microporous film contains a polyolefin raw material such as a polyolefin resin, a polyolefin-based fiber woven fabric (woven fabric), and a polyolefin-based fiber non-woven fabric. If desired, the polyolefin microporous membrane can be made of polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, polytetra, in addition to the first HMWPE and second PO. It may further contain a resin such as fluoroethylene, paper, an aggregate of insulating substance particles, and the like.

The polyolefin microporous membrane can be formed from a polyolefin resin composition containing a first HMWPE, a second PO, other raw materials and the like.

[First Ultra High Molecular Weight Polyethylene (HMWPE)]
The ultra-high molecular weight polyethylene (HMWPE) described in the present specification is a PE having a viscosity average molecular weight (Mv) of 100,000 or more. Mv can be calculated by the following formula by measuring the ultimate viscosity [η] (dl / g) at 135 ° C. in a decalin solvent based on ASTM-D4020.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
In general, since the Mv of ultra high molecular weight polyethylene (UHMWPE) is 1 million or more, the HMWPE according to the present specification includes UHMWPE by definition.

As the first HMWPE according to the present embodiment, a known HMWPE having a viscosity average molecular weight (Mv) of 500,000 to 2,500,000 may be used. The Mv of HMWPE is 500,000 to 2,500,000, preferably 500,000 to 2,000,000, more preferably 500,000 to 2,000,000, from the viewpoint of fuse characteristics, heat shrinkage characteristics and strength of the separator for a secondary battery. It is 500,000 to 1,500,000.

The first HMWPE may be obtained by one-step polymerization or multi-step polymerization as long as it has a viscosity average molecular weight (Mv) of 500,000 to 2,500,000, or is a resin composition containing two or more kinds of polyethylene. Good.

[Second Polyolefin (PO)]
The second polyolefin is a polyolefin (PO) other than the first HMWPE. As the second PO according to the present embodiment, a known PO having a melt index (MI) of 2 g / 10 minutes or more and less than 50 g / 10 minutes and a melting point of 120 ° C. or higher and 137 ° C. or lower may be used.

In the present specification, the melt index (MI) is also referred to as a melt flow rate (MFR) and is an index of a molten resin, and the unit thereof is expressed as mass flow in g / 10 minutes.

From the viewpoint of fuse characteristics, heat shrinkage characteristics and strength of the separator for a secondary battery, the lower limit of MI of the second PO is 2 g / 10 minutes or more, preferably 2.5 g / 10 minutes or more, 3 g / 10 Minutes or more, 4g / 10 minutes or more, 4.9g / 10 minutes or more, 5g / 10 minutes or more, 7g / 10 minutes or more, 9g / 10 minutes or more, 10g / 10 minutes or more, or 12g / 10 minutes or more. The upper limit is less than 50 g / 10 minutes, preferably 49 g / 10 minutes or less, 49 g / 10 minutes or less, 48 g / 10 minutes or less, 47 g / 10 minutes or less, 45 g / 10 minutes or less, 40 g / 10 minutes or less. , 35 g / 10 minutes or less, 30 g / 10 minutes or less, 28 g / 10 minutes or less, 28 g / 10 minutes or less, or 25 g / 10 minutes or less, more preferably less than 25 g / 10 minutes.

In general, the melting point of a polymer refers to the temperature (° C.) corresponding to the heat absorption peak associated with the melting of a crystalline polymer in the DSC curve obtained by differential scanning calorimetry (DSC) of the polymer.

From the viewpoint of fuse characteristics, heat shrinkage characteristics and strength of the separator for a secondary battery, the lower limit of the melting point of the second PO is 120 ° C. or higher, preferably 120 ° C. or higher, 121 ° C. or higher, 121 ° C. or higher, 122 ° C. ° C. or higher, or more than 122 ° C., more preferably 123 ° C. or higher, and even more preferably 126 ° C. or higher. The upper limit is 137 ° C. or lower, preferably less than 137 ° C., 136 ° C. or lower, less than 136 ° C., 135 ° C. or lower, less than 135 ° C., 134 ° C. or lower, less than 134 ° C., 133 ° C. or lower, less than 133 ° C., 132. 5 ° C or lower, or less than 132.5 ° C.

From the viewpoint of individually improving the fuse characteristics, heat shrinkage characteristics and strength of the separator for a secondary battery, or balancing them, the density of the second PO is preferably 0.93 g / cm ^{3 to} 0. 95 g / cm ^3, more preferably ^{0.935g / cm 3 ~0.949g / cm 3} , more preferably from ^{0.936g / cm 3 ~0.948g / cm 3} .

As the second PO, for example, a homopolymer obtained by using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene or the like as a monomer, and a copolymer of those monomers. Examples thereof include polymers and multi-stage polymers of their monomers. Further, as the second PO, those polymers may be used alone or in combination of two or more.

From the viewpoint of the fuse characteristics of the separator for a secondary battery, the second PO preferably contains polyethylene, polypropylene, a copolymer thereof, or a mixture thereof as the polyolefin resin.

Specific examples of polyethylene include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene, high-density polyethylene, ultra-high-density polyethylene, and the like.
Specific examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, and the like.
Specific examples of the copolymer include ethylene-propylene random copolymer, ethylene propylene rubber and the like.

The second PO may be LLDPE obtained by using a single-site catalyst such as a metallocene catalyst, or LDPE obtained by using a multi-site catalyst such as a Ziegler-based catalyst or a Philips-based catalyst.

[Polyolefin resin composition]
The polyolefin microporous film contains 50% by mass or more and 100% by mass or less of the resin component constituting the microporous film as the first HMWPE and / or the first from the viewpoint of improving the fuse performance when used as a separator for a secondary battery. It is preferably formed by a polyolefin resin composition occupied by the second PO. In the polyolefin resin composition, the proportion of the polyolefin resin such as the first HMWPE and the second PO is more preferably 60% by mass or more and 100% by mass or less, and more preferably 70% by mass or more and 100% by mass or less. Is more preferable.

Further, from the viewpoint of improving the heat resistance of the microporous membrane, it is preferable to use a mixture of polyethylene and polypropylene as the polyolefin resin. In this case, the ratio of polypropylene to the total polyolefin resin in the polyolefin resin composition is preferably 1 to 35% by mass, more preferably 3 to 20% by mass, from the viewpoint of achieving both heat resistance and a good fuse function. %, More preferably 4 to 10% by mass.

Any additive can be contained in the polyolefin resin composition. Additives include, for example, polymers other than polyolefin resins; inorganic fillers; antioxidants such as phenol-based, phosphorus-based, and sulfur-based; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers. ; Antistatic agent; Antifogging agent; Colored pigment and the like. The total amount of these additives added is preferably 20 parts by mass or less with respect to 100 parts by mass of the polyolefin resin from the viewpoint of improving fuse performance and the like, more preferably 10 parts by mass or less, still more preferably 5 parts by mass. It is less than a part.

[Details of Polyolefin Microporous Membrane]
Since the polyolefin microporous membrane has a porous structure in which a large number of very small pores are gathered to form dense communication pores, it is extremely excellent in ionic conductivity and at the same time has good withstand voltage characteristics and high strength. Is.
The microporous membrane may be a monolayer membrane made of the above-mentioned materials or a laminated membrane.

The film thickness of the microporous membrane is preferably 0.1 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less, and further preferably 3 μm or more and 25 μm or less. From the viewpoint of mechanical strength, 0.1 μm or more is preferable, and from the viewpoint of increasing the capacity of the secondary battery, 100 μm or less is preferable. The film thickness of the microporous film can be adjusted by controlling the die lip interval, the stretching ratio in the stretching step, and the like.

The average pore size of the polyolefin microporous membrane is preferably 0.03 μm or more and 0.70 μm or less, more preferably 0.04 μm or more and 0.20 μm or less, still more preferably 0.05 μm or more and 0.10 μm or less, and particularly preferably 0.06 μm. It is 0.09 μm or less. From the viewpoint of high ionic conductivity and withstand voltage, 0.03 μm or more and 0.70 μm or less is preferable.
The average pore size can be adjusted by controlling the composition ratio, the cooling rate of the extruded sheet, the stretching temperature, the stretching ratio, the heat fixing temperature, the stretching ratio during heat fixing, and the relaxation rate during heat fixing, or by combining these. Can be done.

The porosity of the polyolefin microporous membrane is preferably 25% or more and 95% or less, more preferably 30% or more and 65% or less, and further preferably 35% or more and 55% or less. From the viewpoint of improving ionic conductivity, 25% or more is preferable, and from the viewpoint of withstand voltage characteristics, 95% or less is preferable.
The porosity of the microporous film controls the mixing ratio of the polyolefin resin composition and the plasticizer, the stretching temperature, the stretching ratio, the heat fixing temperature, the stretching ratio during heat fixing, and the relaxation rate during heat fixing. It can be adjusted by combining them.

The viscosity average molecular weight of the polyolefin microporous membrane is preferably 30,000 or more and 120,000 or less, more preferably 50,000 or more and less than 2,000,000, and further preferably 100,000 or more and 1,000. It is less than 000. When the viscosity average molecular weight is 30,000 or more, the melt tension during melt molding is increased, the moldability is improved, and the polymer tends to be entangled with each other to increase the strength, which is preferable. When the viscosity average molecular weight is 1,500,000 or less, it becomes easy to uniformly melt-knead, and the formability of the sheet, particularly the thickness stability tends to be excellent, which is preferable. Further, when the separator for a secondary battery is used, when the viscosity average molecular weight is less than 1,000,000, the pores are easily closed when the temperature rises, and a good fuse function tends to be obtained, which is preferable.

<Manufacturing method of microporous polyolefin membrane>
The method for producing the microporous polyolefin membrane is not particularly limited, and a known production method can be adopted. For example
(1) A method in which a polyolefin resin composition and a pore-forming material are melt-kneaded, formed into a sheet, stretched as necessary, and then made porous by extracting the pore-forming material.
(2) A method of melt-kneading a polyolefin resin composition, extruding it at a high draw ratio, and then exfoliating the polyolefin crystal interface by heat treatment and stretching to make it porous.
(3) A method in which a polyolefin resin composition and an inorganic filler are melt-kneaded and molded on a sheet, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching to make the polyolefin porous.
(4) A method in which a polyolefin resin composition is dissolved and then immersed in a poor solvent for the polyolefin to coagulate the polyolefin and at the same time remove the solvent to make the polyolefin porous.
And so on.

Hereinafter, as an example of a method for producing a porous film, a method in which a polyolefin resin composition and a pore-forming material are melt-kneaded to form a sheet and then the pore-forming material is extracted will be described.

First, the polyolefin resin composition and the pore-forming material described above are melt-kneaded. As a melt-kneading method, for example, a polyolefin resin and other additives, if necessary, are put into a resin kneading device such as an extruder, a kneader, a lab plast mill, a kneading roll, or a Banbury mixer to heat and melt the resin components. Examples thereof include a method of introducing a pore-forming material at an arbitrary ratio and kneading.

Examples of the pore-forming material include plasticizers, inorganic materials, and combinations thereof.

The plasticizer is not particularly limited, but it is preferable to use a non-volatile solvent capable of forming a uniform solution above the melting point of polyolefin. Specific examples of such a non-volatile solvent include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol. .. After extraction, these plasticizers may be recovered and reused by an operation such as distillation. Further, preferably, the polyolefin resin, other additives and the plasticizer are pre-kneaded at a predetermined ratio using a Henschel mixer or the like before being put into the resin kneading apparatus. More preferably, in the pre-kneading, only a part of the plasticizer is added, and the remaining plasticizer is kneaded while being appropriately heated in a resin kneading device and side-fed. By using such a kneading method, the dispersibility of the plasticizer is enhanced, and when the sheet-shaped molded product of the melt-kneaded product of the resin composition and the plasticizer is stretched in a later step, the film is not broken and the magnification is high. It tends to be able to be stretched with.

Among the plasticizers, liquid paraffin has high compatibility with polyethylene or polypropylene when the polyolefin resin is polyethylene or polypropylene, and even if the melt-kneaded product is stretched, the interface between the resin and the plasticizer is unlikely to peel off, and uniform stretching is performed. Is preferable because it tends to be easier to carry out.

The ratio of the polyolefin resin composition and the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and formed into a sheet. For example, the mass fraction of the plasticizer in the composition composed of the polyolefin resin composition and the plasticizer is preferably 20 to 90% by mass, more preferably 30 to 80% by mass. When the mass fraction of the plasticizer is 90% by mass or less, the melt tension during melt molding tends to be sufficient for improving the moldability. On the other hand, when the mass fraction of the plasticizer is 20% by mass or more, the polyolefin molecular chain is not broken even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, and a uniform and fine pore structure is formed. Is easy to form and the strength is also easy to increase.

The non-equipment is not particularly limited, and for example, oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, itria, zinc oxide, and iron oxide; silicon nitride, titanium nitride, nitride. Nitride ceramics such as boron; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite. , Asbestos, zeolite, calcium silicate, magnesium silicate, kaolin, silica sand and other ceramics; glass fiber. These may be used alone or in combination of two or more. Among these, silica, alumina, and titania are preferable from the viewpoint of electrochemical stability, and silica is particularly preferable from the viewpoint of easy extraction.

The ratio of the polyolefin resin composition to the inorganic material is preferably 5% by mass or more, more preferably 10% by mass or more, and higher than the total mass of these, from the viewpoint of obtaining good isolation. From the viewpoint of ensuring strength, it is preferably 99% by mass or less, and more preferably 95% by mass or less.

Next, the melt-kneaded product is formed into a sheet. As a method for producing a sheet-shaped molded product, for example, the melt-kneaded product is extruded into a sheet shape via a T-die or the like, brought into contact with a heat conductor, and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component. There is a method of solidifying. Examples of the heat conductor used for cooling solidification include metal, water, air, and a plasticizer. Among these, it is preferable to use a metal roll because of its high heat conduction efficiency. Further, when the extruded kneaded product is brought into contact with a metal roll, sandwiching it between the rolls further enhances the efficiency of heat conduction, and the sheet is oriented to increase the film strength and the surface smoothness of the sheet. It is more preferable because it tends to be. The die lip interval when extruding the melt-kneaded product from the T die into a sheet is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, the shavings and the like are reduced, the influence on the film quality such as streaks and defects is small, and the risk of film breakage in the subsequent stretching step can be reduced. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high, cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained.

Further, the sheet-shaped molded product may be rolled. Rolling can be carried out by, for example, a pressing method using a double belt press machine or the like. By rolling, the orientation of the surface layer portion can be increased in particular. The rolled surface magnification is preferably more than 1 time and 3 times or less, and more preferably more than 1 time and 2 times or less. When the rolling ratio exceeds 1 times, the plane orientation tends to increase and the film strength of the finally obtained porous film tends to increase. On the other hand, when the rolling ratio is 3 times or less, the orientation difference between the surface layer portion and the inside of the center is small, and a uniform porous structure tends to be formed in the thickness direction of the film.

Next, the pore-forming material is removed from the sheet-shaped molded product to form a porous film. Examples of the method for removing the pore-forming material include a method in which a sheet-shaped molded product is immersed in an extraction solvent to extract the pore-forming material and sufficiently dried. The method for extracting the pore-forming material may be either a batch method or a continuous method. In order to suppress the shrinkage of the porous film, it is preferable to restrain the end portion of the sheet-shaped molded product during a series of steps of dipping and drying. Further, the residual amount of the pore-forming material in the porous membrane is preferably less than 1% by mass with respect to the total mass of the porous membrane.

As the extraction solvent used when extracting the pore-forming material, it is preferable to use a solvent that is poor with respect to the polyolefin resin, is a good solvent with respect to the pore-forming material, and has a boiling point lower than that of the polyolefin resin. .. 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; non-chlorine type such as hydrofluoroether and hydrofluorocarbon. Halogenation solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by an operation such as distillation. When an inorganic material is used as the pore-forming material, an aqueous solution of sodium hydroxide, potassium hydroxide or the like can be used as the extraction solvent.

Further, it is preferable to stretch the sheet-shaped molded product or the porous film. Stretching may be performed before extracting the pore-forming material from the sheet-shaped molded product. Further, it may be applied to a porous film obtained by extracting a pore-forming material from the sheet-shaped molded product. Further, it may be performed before and after extracting the pore-forming material from the sheet-shaped molded product.
As the stretching treatment, either uniaxial stretching or biaxial stretching can be preferably used, but biaxial stretching is preferable from the viewpoint of improving the strength of the obtained porous film and the like. When the sheet-shaped molded product is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, the finally obtained porous film is less likely to tear, and the sheet-shaped molded product has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoint of improving puncture strength, stretching uniformity, and fuse characteristics. Further, from the viewpoint of ease of controlling the plane orientation, successive biaxial stretching is preferable.

Here, the simultaneous biaxial stretching is a stretching method in which MD (mechanical direction of continuous molding of microporous membrane) and TD (direction of crossing MD of microporous membrane at an angle of 90 °) are simultaneously stretched. No, the draw ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD are stretched independently, and when MD or TD is stretched, the other direction is fixed in an unconstrained state or a fixed length. Make it a state.

The draw ratio is preferably in the range of 20 times or more and 100 times or less in terms of surface magnification, and more preferably in the range of 25 times or more and 70 times or less. The stretching ratio in each axial direction is preferably in the range of 4 times or more and 10 times or less for MD and 4 times or more and 10 times or less for TD, 5 times or more and 8 times or less for MD, and 5 times or more and 8 times or less for TD. It is more preferable that the range is. When the total area magnification is 20 times or more, sufficient strength tends to be imparted to the obtained porous film, while when the total area magnification is 100 times or less, film breakage in the stretching step is prevented and high productivity is achieved. Tends to be obtained.

In order to suppress the shrinkage of the porous film, heat treatment may be performed for the purpose of heat fixation after the stretching step or after the formation of the porous film. Further, the porous membrane may be subjected to post-treatment such as hydrophilization treatment with a surfactant or the like, cross-linking treatment with ionizing radiation or the like.

The porous membrane is preferably heat-treated for the purpose of heat fixation from the viewpoint of suppressing shrinkage. The heat treatment method includes a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and / or relaxation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. The operation can be mentioned. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

In the stretching operation, it is preferable to stretch the MD and / or TD of the membrane 1.1 times or more, more preferably 1.2 times or more, from the viewpoint of obtaining a porous membrane having higher strength and high porosity.
The relaxation operation is a reduction operation of the membrane to MD and / or TD. The relaxation rate is a value obtained by dividing the size of the film after the relaxation operation by the size of the film before the relaxation operation. When both MD and TD are relaxed, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both MD and TD directions, but only one of MD or TD may be performed.
The stretching and relaxation operations after the plasticizer extraction is preferably performed in TD. The temperature in the stretching and relaxation operations is preferably lower than the melting point of the polyolefin resin (hereinafter, also referred to as “Tm”), and more preferably in the range of 1 ° C. to 25 ° C. lower than Tm. When the temperature in the stretching and relaxing operations is in the above range, it is preferable from the viewpoint of the balance between the reduction of heat shrinkage and the porosity.

<Separator for secondary battery>
The polyolefin microporous membrane according to this embodiment can be used as a separator for a secondary battery. Since the separator containing the microporous polyolefin membrane according to the present embodiment can have both fuse characteristics and heat shrinkage characteristics, the safety of the secondary battery can also be improved.

Unless otherwise specified, the above-mentioned measured values of various physical properties are values measured according to the measuring method in the examples described later.

Next, the present embodiment will be described in more detail with reference to Examples and Comparative Examples, but the present embodiment is not limited to the following Examples as long as the gist of the present embodiment is not exceeded. The physical properties in the examples were measured by the following methods.

(1) Viscosity Average Molecular Weight Based on ASTM-D4020, the ultimate viscosity [η] (dl / g) in a decalin solvent at 135 ° C. was determined.
For polyethylene, it was calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

(2) Melt index MI (g / 10 minutes)
MI was measured according to JIS K7210: 1999 (Plastic-Thermoplastic Melt Mass Flow Rate (MFR) and Melt Volume Flow Rate (MVR)). Specifically, in the case of polyethylene, a load of 2.16 kgf was applied at 190 ° C., the amount of resin (g) that flowed out in 10 minutes was measured, and the value rounded to the first decimal place was taken as MI. In the case of polypropylene, a load of 2.16 kgf was applied at 230 ° C., the amount of resin (g) that flowed out in 10 minutes was measured, and the value rounded to the first decimal place was taken as MI.

(3) Melting point (° C)
The melting point was measured using a differential scanning calorimetry (DSC) measuring device "DSC-60" (manufactured by Shimadzu Corporation). The value obtained by rounding off the first decimal place of the obtained value was defined as the melting point.

(4) Film thickness (μm)
The measurement was performed at room temperature of 23 ° C. using a micro thickener (Type KBM manufactured by Toyo Seiki Co., Ltd.).

(5) Porosity (%)
A 10 cm × 10 cm square sample was cut from a microporous membrane, its volume (cm ³ ) and mass (g) were determined, and ^{the porosity was calculated from them and the membrane density (g / cm 3} ) using the following equation.
Porosity (%) = (volume-mass / membrane density) / volume x 100

(6) Air permeability (sec / 100cc)
The air permeability was measured with a Garley type air permeability meter (manufactured by Toyo Seiki Co., Ltd.) conforming to JIS P-8117.

(7) Puncture strength (gf)
Using the handy compression tester "KES-G5" (manufactured by Kato Tech Co., Ltd.), the puncture strength of the sample is measured by performing the puncture test under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. did.

(8) Fuse temperature (° C)
FIG. 1A shows a schematic view of a fuse temperature measuring device. Reference numeral 1 denotes a microporous film, 2A and 2B are nickel foils having a thickness of 10 μm, and 3A and 3B are glass plates. Reference numeral 4 denotes an electric resistance measuring device "AG-4311" (manufactured by Ando Electric Co. Ltd.), which is connected to nickel foils 2A and 2B. Reference numeral 5 denotes a thermocouple, which is connected to the thermometer 6. Reference numeral 7 denotes a data collector, which is connected to the electric resistance measuring device 4 and the thermometer 6. Reference numeral 8 is an oven for heating the microporous membrane.
More specifically, as shown in FIG. 1 (B), the microporous film 1 is laminated on the nickel foil 2A, and the nickel foil 2A is formed with "Teflon (registered trademark)" tape (hatched portion in the figure) in the vertical direction. Fix. The microporous membrane 1 is impregnated with a 1 mol / L lithium borofluoride solution (solvent: propylene carbonate / ethylene carbonate / γ-butyl lactone = mixed solvent having a volume ratio of 1/1/2) as an electrolytic solution. As shown in FIG. 1 (C), "Teflon (registered trademark)" tape (shaded part in the figure) is attached on the nickel foil 2B, and masking is performed leaving a window portion of 15 mm × 10 mm in the central portion of the foil 2B. did.
The nickel foil 2A and the nickel foil 2B were superposed so as to sandwich the microporous film 1, and the two nickel foils were sandwiched between the glass plates 3A and 3B from both sides thereof. At this time, the window portion of the foil 2B and the microporous film 1 were placed at opposite positions. Next, the two glass plates 3A and 3B were fixed by sandwiching them with clips. The thermocouple 5 was fixed to a glass plate with "Teflon®" tape.
The temperature and electrical resistance of the microporous membrane 1 were continuously measured with such an apparatus. The temperature was raised from 25 ° C. to 200 ° C. at a rate of 15 ° C./min, and the electric resistance value was measured with an alternating current of 1 V and 1 kHz. Fuse temperature, electric resistance is a value obtained by rounding off the first decimal place of a temperature value when it reaches the 10 ³ Omega.

(9) Heat shrinkage rate (%)
The sample was cut into squares of 100 mm each for MD / TD, and the sample was placed in a constant temperature bath preheated to 120 ° C., and the dimensional shrinkage after 1 hour was determined. The sample was placed on a copy paper or the like so as not to adhere to the inner wall or the like of the constant temperature bath and to prevent the samples from fusing to each other.

(10) Rapid temperature rise test of lithium ion secondary battery <Preparation of positive electrode sheet>
_{LiNi 1/3} Mn _1/3 Co _1/3 O ₂ (manufactured by Nippon Kagaku Kogyo Co., Ltd.) as a positive electrode active material, acetylene black powder (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and polyvinylidene fluoride as a binder. The solution (manufactured by Kureha) was mixed at a solid content mass ratio of 90: 6: 4, N-methyl-2-pyrrolidone was added as a dispersion solvent so as to have a solid content of 40% by mass, and the mixture was further mixed. , A slurry solution was prepared. This slurry-like solution was applied to both sides of an aluminum foil as a current collecting material having a thickness of 20 μm, the solvent was dried and removed, and then the applied aluminum foil was rolled by a roll press to obtain a positive electrode sheet. ..

<Manufacturing of negative electrode sheet>
Graphite powder (manufactured by Osaka Gas Chemical Co., Ltd.) and graphite powder (manufactured by TIMCAL) as the negative electrode active material, and styrene-butadiene rubber and an aqueous solution of carboxymethyl cellulose as a binder were used as a solid at 90:10: 1.5: 1.8. The mixture was mixed at a mass ratio, water was added as a dispersion solvent so as to have a solid content of 45% by mass, and the mixture was further mixed to prepare a slurry-like solution. This slurry-like solution was applied to both sides of a copper foil as a current collecting material having a thickness of 18 μm, the solvent was dried and removed, and then the applied copper foil was rolled by a roll press to obtain a negative electrode sheet. ..

<Preparation of non-aqueous electrolyte solution>
A solution was prepared in which 1 mol / L _{of LiPF 6} as a lithium salt was contained in a mixed solvent in which ethylene carbonate as a non-aqueous solvent and ethyl methyl carbonate were mixed at a volume ratio of 1: 2.

<Manufacturing of sheet-shaped lithium-ion secondary battery>
The positive electrode sheet and the negative electrode sheet produced as described above were superposed on both sides of the microporous membranes obtained in Examples and Comparative Examples described later to obtain a laminate. The obtained laminate was inserted into a bag (exterior body) made of a laminate film in which both sides of an aluminum foil (thickness 40 μm) were coated with a resin layer while projecting positive and negative electrode terminals. Then, the non-aqueous electrolytic solution prepared as described above was injected into the bag, and the bag was vacuum-sealed to prepare a sheet-shaped lithium ion secondary battery.

<Rapid temperature rise test of lithium ion secondary battery>
The sheet-shaped lithium ion secondary battery was housed in a constant temperature bath "PLM-73S" (manufactured by Futaba Kagaku Co., Ltd.) set at 25 ° C., and connected to a charging / discharging device "ACD-01" (manufactured by Asuka Electronics Co., Ltd.). The secondary battery was then charged at a constant current of 0.1 C until it reached a voltage of 4.2 V, charged at a constant voltage of 4.2 V for 1 hour, and then at a constant current of 0.1 C. The charge / discharge cycle of discharging to 0 V was repeated three times. Then, the lithium ion secondary battery was fully charged by charging with a constant current of 0.1 C until the voltage reached 4.2 V. Note that 1C represents a current value when the entire capacity of the battery is discharged in 1 hour, and 0.1C represents a current value of 1/10 of that.

An aluminum block heater was installed in an explosion-proof constant temperature bath, and the inside of the aluminum block was filled with nonflammable fluorinated paraffin oil. A fully charged lithium ion secondary battery was taken out of a constant temperature bath and immersed in fluorinated paraffin oil in an aluminum block heater. Then, the aluminum block heater was heated to 150 ° C. at a speed of 15 ° C./min, and the behavior of the lithium ion secondary battery was observed. When 1 minute passed at 150 ° C., the evaluation result was evaluated as ◯ (good) when there was no abnormality in the lithium ion secondary battery, and × (bad) when smoked or burst.

[Example 1]
35 parts by mass of high molecular weight polyethylene (melting point 135 ° C) with an average molecular weight of 700,000, 35 parts by mass of high molecular weight polyethylene (melting point 137 ° C) with an average molecular weight of 250,000, and polypropylene (melting point 163 ° C) with an average molecular weight of 400,000. 25 parts by mass of polyethylene (melting point 126 ° C., MI: 3) with an average molecular weight of 90,000, tetrakis- [methylene- (3', 5'-di-t-butyl-4'-hydroxyphenyl) as an antioxidant Propionate] 0.2 parts by mass of methane was dry-blended with a tumbler type blender to obtain a mixed raw material.
The mixture raw material was fed to a twin-screw extruder in a nitrogen atmosphere by a feeder. Further, liquid paraffin (kinematic viscosity at 37.78 ° C., 7.59 × ^10-5 m ² / s) was injected into the extruder cylinder by a plunger pump. The operating conditions of the feeder and pump were adjusted so that the proportion of liquid paraffin in the total mixture extruded was 65 parts by mass and the proportion of polymer was 35 parts by mass. Next, they are melt-kneaded while being heated to 200 ° C. in a twin-screw extruder, and the obtained melt-kneaded product is extruded through a T-die onto a cooling roll controlled to a surface temperature of 70 ° C., and the extruded product is obtained. Was brought into contact with a cooling roll, molded and cooled and solidified to obtain a sheet-shaped molded product.
This sheet was stretched at a temperature of 120 ° C. at a magnification of 7 × 6.4 times by a simultaneous biaxial stretching machine, the stretched product was immersed in methylene chloride, liquid paraffin was extracted and removed, and then dried. Then, after stretching twice in the width direction at a temperature of 130 ° C. with a uniaxial stretching machine, the stretched sheet was relaxed by about 10% in the width direction to obtain a polyolefin microporous film. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 2]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 70,000 (melting point 128 ° C., MI: 5) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 3]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 80,000 (melting point 129 ° C., MI: 6) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 4]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 60,000 (melting point 131 ° C., MI: 12) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 5]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 50,000 (melting point 131 ° C., MI: 21) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 6]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 40,000 (melting point 131 ° C., MI: 30) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 7]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 80,000 (melting point 123 ° C., MI: 3) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 8]
Similar to Example 1 except that a polypropylene copolymer having a viscosity average molecular weight of 180,000 (melting point 126 ° C., MI: 6) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). , Polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 9]
Polyolefin microporous membrane in the same manner as in Example 5 except that high molecular weight polyethylene having a viscosity average molecular weight of 1 million (melting point 135 ° C.) was used instead of high molecular weight polyethylene having a viscosity average molecular weight of 700,000 (melting point 135 ° C.). Was produced. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 10]
Polyolefin microporous membrane in the same manner as in Example 5 except that high molecular weight polyethylene having a viscosity average molecular weight of 2 million (melting point 135 ° C.) was used instead of high molecular weight polyethylene having a viscosity average molecular weight of 700,000 (melting point 135 ° C.). Was produced. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 11]
35 parts by mass of high molecular weight polyethylene (melting point 135 ° C.) with a viscosity average molecular weight of 700,000, 35 parts by mass of high molecular weight polyethylene (melting point 137 ° C.) with a viscosity average molecular weight of 250,000, and 5 mass of polypropylene (melting point 163 ° C.) with a viscosity average molecular weight of 400,000. 40 parts by mass of high molecular weight polyethylene (melting point 135 ° C.) with a viscosity average molecular weight of 700,000 and a viscosity average molecular weight of 250,000 instead of 25 parts by mass of polyethylene (melting point 126 ° C., MI: 3) with a viscosity average molecular weight of 90,000. 40 parts by mass of high molecular weight polyethylene (melting point 137 ° C.), 5 parts by mass of polypropylene having a viscosity average molecular weight of 400,000 (melting point 163 ° C.), and 15 parts by mass of polyethylene having a viscosity average molecular weight of 50,000 (melting point 131 ° C., MI: 21) were used. Except for the above, a polyolefin microporous film was prepared in the same manner as in Example 1. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Example 12]
35 parts by mass of high molecular weight polyethylene (melting point 135 ° C.) with a viscosity average molecular weight of 700,000, 35 parts by mass of high molecular weight polyethylene (melting point 137 ° C.) with a viscosity average molecular weight of 250,000, and 5 mass of polypropylene (melting point 163 ° C.) with a viscosity average molecular weight of 400,000. 25 parts by mass of high molecular weight polyethylene (melting point 135 ° C.) with a viscosity average molecular weight of 700,000 and a viscosity average molecular weight of 250,000 instead of 25 parts by mass of polyethylene (melting point 126 ° C., MI: 3) with a viscosity average molecular weight of 90,000. 25 parts by mass of high molecular weight polyethylene (melting point 137 ° C.), 5 parts by mass of polypropylene having a viscosity average molecular weight of 400,000 (melting point 163 ° C.), and 45 parts by mass of polyethylene having a viscosity average molecular weight of 50,000 (melting point 131 ° C., MI: 21) were used. Except for the above, a polyolefin microporous film was prepared in the same manner as in Example 1. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Comparative Example 1]
In the same manner as in Example 1, polyethylene having a viscosity average molecular weight of 150,000 (melting point 131 ° C., MI: 1) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Comparative Example 2]
Similar to Example 1, except that polyethylene having a viscosity average molecular weight of 30,000 (melting point 127 ° C., MI: 50) was used instead of polyethylene having a viscosity average molecular weight of 90,000 (melting point 126 ° C., MI: 3). A polyolefin microporous film was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Comparative Example 3]
Except for the fact that 35 parts by mass of high molecular weight polyethylene (melting point 137 ° C) with a viscosity average molecular weight of 250,000 was used instead of 35 parts by mass of high molecular weight polyethylene (melting point 135 ° C.) with a viscosity average molecular weight of 700,000. A polyolefin microporous film was prepared in the same manner as in Example 5. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

[Comparative Example 4]
Polyolefin microporous in the same manner as in Example 5, except that ultra-high molecular weight polyethylene having a viscosity average molecular weight of 3 million (melting point 136 ° C.) was used instead of high molecular weight polyethylene having a viscosity average molecular weight of 700,000 (melting point 135 ° C.). A membrane was prepared. Table 1 shows the physical characteristics of the obtained polyolefin microporous membrane and the results of the rapid temperature rise test.

From Table 1, Examples 1 to 12 of the present invention have a lower fuse temperature of the microporous membrane as compared with Comparative Example 1 in which polyolefin having a high MI is not used, and the safety of the secondary battery at the time of rapid temperature rise Can be retained. It is considered that the low fuse temperature quickly insulated the electrodes and maintained the safety of the secondary battery.

Further, in Examples 1 to 12, the puncture strength of the microporous membrane is compared with Comparative Example 2 using polyethylene having an excessively high MI and Comparative Example 3 not containing high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more. The safety of the secondary battery is high at the time of rapid temperature rise. When the temperature of the secondary battery rises, the internal pressure of the secondary battery rises due to the volatilization of the organic solvent in the electrolytic solution, and if the puncture strength of the separator is low, film breakage may occur. Since the microporous membrane according to the embodiment of the present invention uses a polyolefin having an appropriate MI and has sufficient mechanical strength, it is presumed that it has withstood the temperature rise of the secondary battery. ..

Finally, Examples 1 to 12 have smaller heat shrinkage and are excellent in safety of the secondary battery at the time of rapid temperature rise as compared with Comparative Example 4 using ultra-high molecular weight polyethylene having an Mv of 3 million. Since the heat shrinkage is small, it is considered that the safety of the secondary battery is maintained as a result of maintaining the insulation without short-circuiting even when the temperature of the secondary battery becomes high.

From the above, by using a high molecular weight polyethylene having an appropriate viscosity average molecular weight and a polyolefin having an appropriate MI in combination, a low fuse temperature, sufficient puncture strength, and low heat shrinkage can be achieved, and a secondary battery can be used. It was clarified that a microporous membrane with excellent safety can be obtained.

1 Microporous film 2A, 2B Nickel foil 3A, 3B Glass plate 4 Electrical resistance measuring device 5 Thermocouple 6 Thermometer 7 Data collector 8 Oven

Claims (7)

With the first ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 500,000 to 2,500,000 ;
One or more of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and a copolymer of polyethylene and polypropylene, with a melt index (MI) of 2 g / 10 minutes or more and 50 g / 10 It is less than min, melting point of 137 ° C. or less 12 6 ° C. or higher, and a different second polyolefin with the first high molecular weight polyethylene;
Polyolefin microporous membrane containing. The melting point of the second polyolefin, 126 ° C. or higher 13 is 4 ° C. or less, the microporous polyolefin membrane according to claim 1. The microporous polyolefin membrane according to claim 1 or 2 , wherein the MI of the second polyolefin is 2 g / 10 minutes or more and less than 25 g / 10 minutes. The polyolefin according to any one of claims 1 to 3, wherein the polyolefin microporous membrane contains polypropylene, and the ratio of the polypropylene to the total polyolefin resin in the polyolefin microporous membrane is 1 to 10% by mass. Microporous membrane. The polyolefin microporous film is formed of a polyolefin resin composition in which 50% by mass or more and 100% by mass or less of the resin component constituting the polyolefin microporous film is occupied by the first high molecular weight polyethylene and / or the second polyolefin. The polyolefin microporous membrane according to any one of claims 1 to 4. In the polyolefin microporous film, the first high molecular weight polyethylene occupies 25% by mass or more and 40% by mass or less of the resin component constituting the polyolefin microporous film, and 15% by mass or more and 45% by mass or less is the first. The polyolefin microporous film according to any one of claims 1 to 4, which is formed of a polyolefin resin composition occupied by the second polyolefin. A separator for a secondary battery containing the polyolefin microporous membrane according to any one of claims 1 to 6.

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