JP7532843B2 - Polyolefin microporous membrane, battery separator, and secondary battery - Google Patents

Description

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

Microporous membranes are widely used as separation membranes, selective permeable membranes, and isolation membranes for substances. Specific applications of microporous membranes include, for example, battery separators used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries, separators for electric double-layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture-permeable waterproof clothing, medical materials, and supports for fuel cells.

In particular, polyethylene microporous membranes are widely used as separators for lithium-ion secondary batteries. In addition to excellent mechanical strength, which contributes greatly to the safety and productivity of batteries, polyethylene microporous membranes also have the advantage of being electrically insulating while allowing ions to pass through the electrolyte that permeates the micropores. Furthermore, polyethylene microporous membranes have a pore-blocking function that automatically blocks ion passage at around 120 to 150°C in the event of an abnormal reaction inside or outside the battery, thereby preventing excessive temperature rise.

The fields of use for lithium-ion batteries are expanding from conventional small consumer applications such as mobile phones and PC batteries to fields requiring large capacity and large capacity, such as power tools, automobile and bicycle storage batteries, and large-scale power storage equipment. To meet these demands, the adoption of electrodes that provide high capacity in battery structures is progressing. In light of the increase in battery size, battery safety is becoming more important than ever before.

To meet these demands, efforts have been made to develop separators (Patent Documents 1 to 3).

Microporous membranes having high strength and high permeability have been made using ultra-high molecular weight polyolefins (hereinafter referred to as "UHMWPO"). For example, Patent Documents 1 and 2 disclose UHMWPO membranes made by a process including a step of forming a gel-like sheet made by extruding a solution containing UHMWPO having an average molecular weight of 5 x ¹⁰⁵ or more and a solvent, a step of removing the solvent from the gel-like sheet, and then a step of stretching the gel-like sheet.

Patent Document 3 discloses a polyolefin microporous membrane that has large pores, excellent strength, and low heat shrinkage.

International Publication No. 2000/049073 International Publication No. 2000/049074 International Publication No. 2008/069216

The technology disclosed in Patent Document 1 discloses a microporous membrane with reduced heat shrinkage in the transverse direction (TD) and high tensile elongation, but low strength. The technology disclosed in Patent Document 2 achieves both high strength and high porosity, but shows a deterioration in heat shrinkage at 105°C. An example is given of an improved heat shrinkage, but the problem is a decrease in pin puncture strength. Patent Document 3 uses a relatively low molecular weight polyethylene as a raw material and increases the stretch ratio to obtain a microporous membrane with a large pore size, but the tensile elongation is low.

The strength required for a microporous membrane is important not only for preventing short circuits caused by protrusions on the surface of the electrodes when used in a battery, such as puncture strength, but also for maintaining insulation properties without rupturing the membrane when an external force is applied to the battery. Microporous membranes are expected to be resistant to impact when they have high elongation, but the frequency of short circuits during battery production increases due to low puncture strength, resulting in a drawback in that battery productivity decreases. Tensile strength is also expected to play an important role in impact resistance, and an improvement method can be adopted in which an increase in the stretch ratio is used to increase the tensile strength, but this does not lead to improvement of impact resistance because it leads to a decrease in tensile elongation. In addition, an increase in the stretch ratio leads to an increase in the thermal shrinkage rate, and when the battery is in an abnormal state, i.e., when heated from the outside or when internal heat is generated due to overcharging, etc., the microporous membrane used as an insulating film is more likely to shrink, which increases the possibility of causing a short circuit and causing a battery accident.

There is a demand for a film with higher impact resistance by improving both tensile strength and tensile elongation while preventing an increase in thermal shrinkage.

The inventors conducted extensive research to solve the above problems, and discovered that the following configuration could solve the problems, leading to the present invention. That is, the present invention is as follows.

(1) A polyolefin microporous membrane having an average tensile strength of 150 MPa or more, an average tensile elongation of 185% or more, an average solid heat shrinkage calculated from the solid heat shrinkage measured after heating at 105°C for 8 hours of 10.0% or less, and a TD heat shrinkage of 14% or less at the shutdown temperature determined by thermomechanical analysis.
(2) The polyolefin microporous membrane according to (1), wherein the average tensile elongation is 190% or more.
(3) The polyolefin microporous membrane according to (1) or (2), wherein the average tensile strength is 160 MPa or more and the average tensile elongation is 195% or more.
(4) The polyolefin microporous membrane according to any one of (1) to (3), wherein the polyolefin resin forming the polyolefin microporous membrane contains a polyethylene resin as a main component.
(5) The polyolefin microporous membrane according to any one of (1) to (4), having a pin puncture strength of 6000 mN/20 μm or more.
(6) A battery separator using the polyolefin microporous membrane according to any one of (1) to (5).
(7) A secondary battery using the battery separator according to (6).

According to the present invention, it is possible to provide a polyolefin microporous membrane having high impact resistance by achieving both high tensile strength and high tensile elongation, which was previously difficult to achieve, while at the same time achieving a low thermal shrinkage rate. The polyolefin microporous membrane of the present invention has high tensile strength and high tensile elongation in addition to high puncture strength, which is important in battery production, and therefore is safer when used in batteries and is also suitable as a separator for lithium ion secondary batteries.

The present invention was arrived at through extensive research by the inventors to obtain a polyolefin microporous membrane with excellent battery productivity and safety, and the discovery that by controlling the stretching conditions at certain conditions, it is possible to control the amount of amorphous components and crystallinity derived from ultra-high molecular weight polyethylene, thereby obtaining a polyolefin microporous membrane with high puncture strength and high impact resistance.

The present invention will be described in detail below.

The polyolefin microporous membrane according to an embodiment of the present invention has an average tensile strength of 150 MPa or more, an average tensile elongation of 185% or more, an average solid heat shrinkage calculated from the solid heat shrinkage measured after heating at 105°C for 8 hours of 10.0% or less, and a TD heat shrinkage of 14% or less at the shutdown temperature as determined by thermomechanical analysis.

(Raw materials)
(Resin type)
The polyolefin microporous membrane according to the embodiment of the present invention contains a polyolefin resin as a main component. The polyolefin resin is preferably contained in an amount of 80% by mass or more, more preferably 90% by mass or more, based on the total mass of the polyolefin microporous membrane. It is particularly preferable that the polyolefin microporous membrane contains polyethylene as a main component.

The polyolefin resin may be a single substance or a mixture of two or more different polyolefin resins. An example of a mixture of two or more different polyolefin resins is a mixture of polyolefin resins selected from polyethylene, polypropylene, polybutene, and poly-4-methyl-1-pentene. A preferred mixture of two or more different polyolefin resins is a mixture of polyethylene and another polyolefin resin. The polyolefin resin is not limited to a homopolymer, and may be a copolymer of different olefins. For example, a polyethylene-based resin or a polypropylene-based resin is preferred as the polyolefin resin.

Among these polyolefin resins, polyethylene-based resins are particularly preferred from the viewpoint of excellent pore-blocking performance. Therefore, the polyolefin resin forming the polyolefin microporous membrane preferably contains a polyethylene-based resin as a main component, and such polyolefin resin is more preferably a polyethylene-based resin. The melting point (softening point) of the polyethylene-based resin is preferably 70 to 150°C from the viewpoint of pore-blocking performance.

The polyolefin resin used in the present invention will be described in detail below using a polyethylene resin as an example.

As the polyethylene-based resin, various polyethylenes can be used, including ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. In addition, there is no particular restriction on the polymerization catalyst for the polyethylene-based resin, and Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like can be used. These polyethylene-based resins may be not only homopolymers of ethylene, but also copolymers containing small amounts of other α-olefins. Suitable α-olefins other than ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, (meth)acrylic acid, esters of (meth)acrylic acid, styrene, and the like. The polyethylene-based resin may be a single substance, but is preferably a polyethylene mixture consisting of two or more types of polyethylene.

The polyethylene mixture may be a mixture of two or more ultra-high molecular weight polyethylenes having different weight average molecular weights (Mw), a mixture of high density polyethylenes, a mixture of medium density polyethylenes, or a mixture of low density polyethylenes, or a mixture of two or more polyethylenes selected from the group consisting of ultra-high molecular weight polyethylenes, high density polyethylenes, medium density polyethylenes, and low density polyethylenes. The polyethylene mixture is preferably a mixture of ultra-high molecular weight polyethylenes having an Mw of 1×10 ⁶ or more and polyethylenes having an Mw of 1×10 ⁴ or more and less than 7×10 ⁵ .

The ultra-high molecular weight polyethylene used has a weight average molecular weight of preferably 1.0×10 ⁶ or more and 1.0×10 ⁷ or less. The lower limit of the weight average molecular weight is more preferably 1.2×10 ⁶ or more, even more preferably 1.5×10 ⁶ or more, even more preferably 2.0×10 ⁶ or more, and most preferably 3.0×10 ⁶ or more. The upper limit of the weight average molecular weight is more preferably 8.0×10 ⁶ or less, even more preferably 6.0×10 ⁶ or less, even more preferably 5.0×10 ⁶ or less, and most preferably 4.0×10 ⁶ or less. By having a weight average molecular weight of 1.0×10 ⁶ or more, high puncture strength can be achieved. Furthermore, by having a weight average molecular weight of 3.0×10 ⁶ or more, the entanglement density of the amorphous region increases, which is preferable for achieving both tensile strength and tensile elongation.

From the viewpoint of mechanical strength, the molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of the ultra-high molecular weight polyethylene is preferably in the range of 3.0 to 100. The lower limit of the molecular weight distribution is more preferably 4.0 or more, even more preferably 5.0 or more, even more preferably 6.0 or more, and most preferably 8.0 or more. The upper limit of the molecular weight distribution is more preferably 80 or less, even more preferably 50 or less, even more preferably 20 or less, and most preferably 17 or less. Ultra-high molecular weight polyethylene may be used as a mixture with polyethylene, but when used alone, a molecular weight distribution of 3.0 or more provides excellent processability, and a molecular weight distribution of 100 or less reduces the possibility of defects occurring during processing due to an increase in low molecular weight components. It is believed that the molecular weight distribution of ultra-high molecular weight polyethylene is at least a certain level, making it easier for the high melting characteristics due to the high molecular weight components to be expressed when melted.

The melting point (Tm) of the ultra-high molecular weight polyethylene is preferably 122°C or higher and 140°C or lower. By setting the melting point of the ultra-high molecular weight polyethylene to 122°C or higher, it is easy to obtain a polyolefin microporous film with good permeability. Furthermore, by setting the melting point of the ultra-high molecular weight polyethylene to 140°C or lower, it is possible to obtain a polyolefin microporous film with excellent shutdown characteristics in which the pores of the polyolefin microporous film are blocked when an abnormal state occurs during battery use. The lower limit of the melting point of the ultra-high molecular weight polyethylene is more preferably 124°C or higher, and even more preferably 126°C or higher. Furthermore, the upper limit of the melting point of the ultra-high molecular weight polyethylene is more preferably 138°C or lower, even more preferably 136°C or lower, particularly preferably 134°C or lower, and most preferably 132°C or lower. In general, the melting point of a polyethylene resin is controlled by copolymerization with other α-olefins. In the case of ultra-high molecular weight polyethylene, as in the case of low molecular weight polyethylene, the number of branches per unit introduced by copolymerization is related to the melting point. Furthermore, because ultra-high molecular weight polyethylene has an extremely high molecular weight, the number of branches per molecule of ultra-high molecular weight polyethylene makes a big difference not only in the melting point but also in the physical properties of the product (the properties as a microporous polyolefin membrane) and the behavior when melted. Specifically, this is manifested in the elongation during tensile deformation and the melting properties when melted, which are the focus of this application.

The melting point is measured according to JIS K7122:2012. That is, the measurement sample (a 0.5 mm thick molded product melt-pressed at 210°C) is placed in the sample holder of a differential scanning calorimeter (PerkinElmer Pyris Diamond DSC) at ambient temperature, heat-treated at 230°C for 3 minutes in a nitrogen atmosphere, cooled to 30°C at a rate of 10°C/min, held at 30°C for 3 minutes, and heated to 230°C at a rate of 10°C/min.

In addition to the ultra-high molecular weight polyethylene, a polyethylene having a lower molecular weight may be used. The melting point (Tm) of the polyethylene having a lower molecular weight is, for example, preferably 131.0° C. or more, more preferably 131.0° C. to 135° C. The weight average molecular weight is, for example, preferably less than 1.0×10 ⁶ , more preferably 1.0×10 ⁵ or more and less than 1.0×10 ⁶ , and even more preferably 2×10 ⁵ to 9.5×10 ^5. Tm is measured in the same manner as the ultra-high molecular weight polyethylene. The polyethylene having a lower molecular weight optionally has a molecular weight distribution (MWD) of 1.0×10 ² or less, for example 50.0 or less, for example in the range of 3.0 to 20.0. For example, the polyethylene having a lower molecular weight may be one or more of high density polyethylene (HPDE), medium density polyethylene, branched low density polyethylene, or linear low density polyethylene. If desired, high density polyethylene is used as the polyethylene having a lower molecular weight.

Optionally, the polyethylene may be a polyethylene having terminal unsaturation. For example, the polyethylene may have a terminal unsaturation level of 0.20 or more per 10,000 carbon atoms, such as 5.0 or more per 10,000 carbon atoms, e.g., 10.0 or more per 10,000 carbon atoms. The terminal unsaturation level can be measured, for example, according to the procedures described in WO 1997/023554. In another embodiment, the polyethylene has a terminal unsaturation level of less than 0.20 per 10,000 carbon atoms.

From the viewpoint of tensile strength, the content of ultra-high molecular weight polyethylene in the polyethylene mixture is preferably 1 to 95% by mass. The content of ultra-high molecular weight polyethylene in the polyethylene mixture is more preferably 5 to 90% by mass, even more preferably 20 to 85% by mass. It is even more preferably 30 to 80% by mass, and most preferably 40 to 75% by mass. When the ultra-high molecular weight polyethylene is present in the polyethylene mixture at 1% by mass or more, high puncture strength can be obtained. When the content of ultra-high molecular weight polyethylene in the polyethylene mixture exceeds 95% by mass, resin extrusion productivity is likely to decrease. In addition, when the ultra-high molecular weight polyethylene is present in the polyethylene mixture at 20% by mass or more, both high tensile strength and tensile elongation can be achieved.

If desired, the amount of polyethylene other than ultra-high molecular weight polyethylene in the polyethylene mixture is, for example, preferably 99.0% by mass or less, more preferably 5.0% by mass to 99.0% by mass, even more preferably 30.0% by mass to 95.0% by mass, and even more preferably 40.0% by mass to 85.0% by mass, based on the mass of the layer in which it is present.

From the viewpoint of mechanical strength, the molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of the polyethylene mixture is preferably within the range of 3.0 to 200. The lower limit of the molecular weight distribution is more preferably 4.0 or more, even more preferably 5.0 or more, even more preferably 6.0 or more, and most preferably 8.0 or more. The upper limit of the molecular weight distribution is more preferably 180 or less, even more preferably 150 or less, even more preferably 120 or less, and most preferably 100 or less.

The polyethylene mixture preferably contains at least 2% by mass of ultra-high molecular weight polyethylene components with a weight-average molecular weight of 1,000,000 or more.

In addition, in the polyolefin microporous film obtained after film formation, a polyethylene mixture containing 10% by mass or more of ultra-high molecular weight components (f(≧2330k)) having a molecular weight of 2.33 million or more is preferable. A polyethylene mixture containing 11% by mass or more of ultra-high molecular weight components (f(≧2330k)) having a molecular weight of 2.33 million or more is more preferable. By containing 10% by mass or more of polyethylene components having a molecular weight of 2.33 million or more in the polyethylene mixture in the polyolefin microporous film obtained after film formation, it is possible to achieve high tensile elongation and low thermal shrinkage while having high pin puncture strength and tensile strength, and it is possible to achieve both high battery productivity and high safety.

Methods for controlling ultra-high molecular weight components with a molecular weight of 2.33 million or more (f(≧2330k)) in terms of the raw materials include adjusting 1) the weight average molecular weight of ultra-high molecular weight polyethylene, 2) the molecular weight distribution of ultra-high molecular weight polyethylene, 3) the ratio of ultra-high molecular weight polyethylene in the raw materials, and 4) the weight average molecular weight, molecular weight distribution, and ratio in the raw materials of polyethylene components other than ultra-high molecular weight polyethylene. In addition, as a film-forming condition, ultra-high molecular weight components with a molecular weight of 2.33 million or more (f(≧2330k)) can also be controlled by adjusting the amount of antioxidant added or by applying a kneading technique that introduces an inert gas such as nitrogen, and the raw materials and film-forming conditions can be adjusted from among these.

(Solvent type)
The diluent is not particularly limited as long as it is a substance that can be mixed with polyolefin resin or dissolve polyolefin resin. A liquid diluent may be used, or a solvent that is miscible with polyolefin when melt-kneaded with polyolefin resin but is in a solid state at room temperature may be mixed with the diluent. Examples of such solid diluents include stearyl alcohol, ceryl alcohol, paraffin wax, etc. Examples of liquid diluents include aliphatic, cycloaliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions whose boiling points correspond (are the same or similar to) those of these aliphatic, cycloaliphatic, or aromatic hydrocarbons, as well as phthalic acid esters that are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate, vegetable oils such as soybean oil, castor oil, sunflower oil, and cotton oil, and other fatty acid esters. In order to obtain a gel-like sheet (gel-like molded product) with a stable content of liquid diluent, it is more preferable to use a non-volatile diluent such as liquid paraffin. For example, the viscosity of the liquid diluent at 40° C. is preferably 20 to 500 cSt, more preferably 30 to 400 cSt, and even more preferably 50 to 350 cSt. If the viscosity of the liquid diluent is less than 20 cSt, it will be non-uniformly discharged from the die and kneading will be difficult. If the viscosity of the liquid diluent is more than 500 cSt, it will be difficult to remove the solvent (diluent).

The blending ratio of the polyolefin resin and the diluent is preferably 1 to 60% by mass, with the total of the polyolefin resin and the diluent being 100% by mass, from the viewpoint of improving the moldability of the extrudate. The ratio of the polyolefin resin to the mixture of the polyolefin resin and the diluent is more preferably 10 to 55% by mass, and even more preferably 15 to 50% by mass. By making the ratio of the polyolefin resin to the mixture of the polyolefin resin and the diluent 1% by mass or more, swelling and necking at the nozzle outlet during extrusion can be suppressed, and the film-forming properties of the gel-like sheet are easily improved. On the other hand, by making the ratio of the polyolefin resin to the mixture of the polyolefin resin and the diluent 60% by mass or less, the differential pressure at the nozzle can be kept small, and the gel-like sheet can be stably produced. The homogeneous melt-kneading process of the polyolefin resin solution is not particularly limited, but examples include a process using a calendar, various mixers, and an extruder with a screw.

(Production method)
The method for producing a polyolefin microporous membrane of the present invention includes, for example, the following steps (1) to (5) and may further include steps (6) and/or (7).
(1) adding a membrane-forming solvent (diluent) to the polyolefin resin, and then melt-kneading the mixture to prepare a polyolefin resin solution;
(2) A step of extruding the polyolefin resin solution through a die lip and then cooling it to form a gel-like molding;
(3) A step of stretching the gel-like molding in at least one axial direction (first stretching step);
(4) removing the membrane-forming solvent;
(5) drying the resulting film;
(6) stretching the dried membrane again in at least one direction (second stretching step); and (7) heat-treating the membrane.

If necessary, a heat setting process, a hot roll process, or a hot solvent process may be performed before the membrane-forming solvent removal process (4).

In addition to steps (1) to (7), steps such as a drying step, a heat treatment step, a cross-linking step using ionizing radiation, a hydrophilization step, and a surface coating step may also be included.

(1) Preparation of polyolefin resin solution After adding a suitable membrane-forming solvent to the polyolefin resin, melt-kneading is performed to prepare a polyolefin resin solution. The melt-kneading method is known, so the description will be omitted. As the melt-kneading method, for example, the method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. However, the polyolefin resin concentration of the polyolefin resin solution is 10 to 60 mass%, preferably 15 to 50 mass%, of the polyolefin resin, with the total of the polyolefin resin and the membrane-forming solvent being 100 mass%. If the proportion of the polyolefin resin is 10 mass% or more, it is preferable because the productivity is excellent. In addition, if the proportion of the polyolefin resin is 60 mass% or less, the moldability of the gel-like molding is good.

The ratio (L/D) of the screw length (L) to the diameter (D) of the twin-screw extruder is preferably in the range of 20 to 100, more preferably in the range of 35 to 70. If L/D is less than 20, melt kneading tends to be insufficient. If L/D is more than 100, the residence time of the polyolefin resin solution increases too much. The shape of the screw is not particularly limited, and may be a known one. The cylinder inner diameter of the twin-screw extruder is preferably 40 to 200 mm. When the polyolefin resin is fed into the twin-screw extruder, it is preferable to set the ratio Q/Ns of the input amount Q (kg/h) of the polyolefin resin solution to the screw rotation speed Ns (rpm) to 0.03 to 2.0 kg/h/rpm. If Q/Ns is 0.03 kg/h/rpm or more, excessive shear fracture of the polyolefin resin can be prevented, and a decrease in strength and meltdown temperature can be prevented. Furthermore, if Q/Ns is 2.0 kg/h/rpm or less, the mixture can be kneaded uniformly. It is more preferable that the ratio Q/Ns is 0.05 to 1.8 kg/h/rpm. It is preferable that the screw rotation speed Ns is 50 rpm or more. There is no particular upper limit for the screw rotation speed Ns, but it is preferably 500 rpm or less.

The preferred temperature range of the polyolefin resin solution in the extruder varies depending on the resin; for example, it is 140 to 250°C for a polyethylene composition, and 160 to 270°C when polypropylene is included. The temperature of the polyolefin resin solution in the extruder is indirectly monitored by installing a thermometer inside the extruder or in the cylinder, and the heater temperature, rotation speed, and discharge rate in the cylinder are appropriately adjusted to reach the target temperature. The solvent may be added before the start of kneading, or it can be added midway through kneading. It is preferable to add an antioxidant to prevent oxidation of the polyolefin resin during melt kneading. As the antioxidant, it is preferable to use one or more selected from, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7).

(2) Formation process of gel-like molding The polyolefin resin solution melted and kneaded in the extruder is cooled to form a resin composition containing a solvent. At this time, it is preferable to extrude the resin composition from a die having a slit-shaped opening to form a sheet-like resin composition, but it is also possible to use the so-called inflation method in which the resin composition is solidified by extruding the resin composition from a blown film die having a circular opening.

The extrusion temperature is preferably 140 to 250°C, more preferably 160 to 240°C, and even more preferably 180 to 230°C. By setting the extrusion temperature at 140°C or higher, it is possible to prevent the pressure at the nozzle from increasing too much, while by setting the temperature at 250°C or lower, it is possible to prevent deterioration of the material. The extrusion speed is preferably 0.2 to 20 m/min. A gel-like sheet (gel-like molding) is formed by cooling the polyolefin resin solution extruded into a sheet shape.

The cooling method can be a method of contacting with a refrigerant such as cold air or cooling water, or a method of contacting with a cooling roll, and it is preferable to cool by contacting with a roll cooled with a refrigerant. For example, an unstretched gel-like sheet can be formed by contacting the polyethylene resin solution extruded in a sheet form with a rotating cooling roll set to a surface temperature of 20°C to 40°C with a refrigerant. The extruded polyethylene resin solution is preferably cooled to 25°C or less. The cooling rate at this time is preferably 50°C/min or more. By cooling in this manner, the polyolefin phase can be microphase separated from the solvent. This makes it easier for the unstretched gel-like sheet to have a dense structure, and also prevents the crystallinity from increasing excessively, resulting in a structure suitable for stretching.

As a cooling method, in order to improve the cooling efficiency and flatness of the sheet, two or more types of rolls may be placed close to each other, and the resin solution discharged onto one roll may be pressed by one or more rolls to cool the polyolefin resin solution. In addition, in order to form a gel-like sheet at high speed, a chamber may be used in which the sheet is brought into close contact with the rolls. The film thickness can be adjusted by adjusting the extrusion amount of each polyolefin resin solution. As an extrusion method, for example, the methods disclosed in JP-B-06-104736 and JP-A-3347835 may be used.

(3) First stretching step The obtained sheet-like gel-like molding is stretched at least in one axial direction. The first stretching causes cleavage between polyolefin (polyethylene) crystal lamellae, refines the polyolefin (polyethylene) phase, and forms a large number of fibrils. The resulting fibrils form a three-dimensional mesh structure (a network structure in which the fibers are irregularly connected in three dimensions). The gel-like molding contains a membrane-forming solvent, so it is easy to stretch uniformly.

As a stretching method, two or more stages of stretching in a state containing a solvent are preferable. There are no particular limitations on the stretching method at each stage. Uniaxial stretching/simultaneous biaxial stretching and simultaneous biaxial stretching/uniaxial stretching are also preferable. Considering productivity and investment costs, uniaxial stretching/uniaxial stretching is also preferable. The stretching direction can be the sheet conveying direction (MD) or the sheet width direction (TD), but the order can be either MD/TD or TD/MD. After heating, the gel-like sheet can be stretched by a tenter method, a roll method, a rolling method, or a combination of these.

The stretching ratio varies depending on the thickness of the gel-like molding, but in uniaxial stretching, it is preferably 2 times or more, and more preferably 3 to 30 times. In biaxial stretching, it is preferable to stretch at least 3 times in both directions, that is, by setting the area ratio to 9 times or more, since this improves the pin puncture strength. If the area ratio is 9 times or more, the stretching is sufficient, and a polyolefin microporous membrane with high elasticity and high strength can be obtained. The area ratio is preferably 12 times or more, more preferably 16 times or more, even more preferably 18 times or more, and most preferably 20 times or more. On the other hand, if the area ratio is 400 times or less, there are few restrictions in terms of the stretching device, stretching operation, etc. It is preferably 45 times or less, more preferably 40 times or less, even more preferably 36 times or less, and most preferably 32 times or less.

The temperature of the first stretching is preferably in the range of the crystal dispersion temperature of the polyolefin resin or higher to the crystal dispersion temperature + 30°C, more preferably in the range of the crystal dispersion temperature + 10°C to the crystal dispersion temperature + 25°C, and particularly preferably in the range of the crystal dispersion temperature + 15°C to the crystal dispersion temperature + 20°C. If the stretching temperature is the crystal dispersion temperature + 30°C or lower, the orientation of the molecular chains after stretching is good. On the other hand, if the stretching temperature is higher than the crystal dispersion temperature, the resin is sufficiently softened, preventing film breakage due to stretching and allowing high-magnification stretching. Here, the crystal dispersion temperature refers to the value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. When a polyethylene-based resin is used as the main component of the polyolefin resin, the crystal dispersion temperature is generally 90 to 100°C. Therefore, when the polyolefin resin is composed of 90% by mass or more of polyethylene-based resin, the stretching temperature is usually set to the range of 90 to 130°C, preferably 100 to 125°C, and more preferably 105 to 120°C.

When preheating the sheet before stretching, the temperature may be set higher than the stretching temperature in the subsequent stage. This allows the effective temperature of the sheet to be raised in a short period of time, which contributes to improved productivity.

During the first stretching, multiple stages of stretching at different temperatures may be performed. In this case, it is preferable to stretch at two different temperatures, with the temperature in the latter stage being higher than the temperature in the former stage. As a result, a polyolefin microporous membrane with a high-order structure having a large pore size and high permeability is obtained without a decrease in strength or physical properties in the width direction. Although not limited, it is preferable that the difference in stretching temperature between the former stage and the latter stage is 5°C or more. When increasing the temperature of the membrane from the former stage to the latter stage, (a) the temperature may be increased while continuing stretching, or (b) stretching may be stopped during the temperature increase and a predetermined temperature is reached before starting the latter stage of stretching, but the former (a) is preferable. In either case, it is preferable to heat the membrane rapidly during the temperature increase. Specifically, it is preferable to heat the membrane at a temperature increase rate of 0.1°C/sec or more, and it is more preferable to heat the membrane at a temperature increase rate of 1 to 5°C/sec. Needless to say, it is preferable that the stretching temperatures in the former and latter stages and the total stretching ratio are each within the above range.

Depending on the desired physical properties, the film may be stretched with a temperature distribution in the film thickness direction, resulting in a polyolefin microporous film with even greater mechanical strength. For example, the method disclosed in Japanese Patent No. 3347854 can be used.

(4) Membrane-forming solvent removal process A cleaning solvent is used to remove (wash) the membrane-forming solvent. Since the polyolefin phase is phase-separated from the membrane-forming solvent, a porous membrane is obtained by removing the membrane-forming solvent. Cleaning solvents and methods for removing the membrane-forming solvent using the same are publicly known, so a description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Publication No. 2002-256099 can be used.

(5) Membrane Drying Step The polyolefin microporous membrane obtained by removing the membrane-forming solvent is dried by a heat drying method, an air drying method or the like.

(6) Second stretching process The dried membrane may be stretched again in at least one axial direction. The second stretching may be performed by a tenter method or roll stretching while heating the membrane, similarly to the first stretching. The second stretching may be uniaxial or biaxial. The stretching direction may be the sheet conveying direction (MD) or the sheet width direction (TD), and may be in either the MD/TD or TD/MD order.

The temperature of the second stretching is preferably in the range of the crystal dispersion temperature of the polyolefin resin constituting the polyolefin microporous membrane to the crystal dispersion temperature + 40°C, and more preferably in the range of the crystal dispersion temperature + 10°C to the crystal dispersion temperature + 40°C. If the temperature of the second stretching is more than the crystal dispersion temperature + 40°C, the permeability decreases and the physical properties in the width direction of the sheet when stretched in the lateral direction (width direction: TD direction) become more variable. In particular, the variation in the air permeability in the width direction of the stretched sheet becomes larger. On the other hand, if the temperature of the second stretching is less than the crystal dispersion temperature, the polyolefin resin is not softened sufficiently, the membrane is easily broken during stretching, and it is difficult to stretch uniformly. When the polyolefin resin is a polyethylene resin, the stretching temperature may be, for example, in the range of 90 to 140°C, and preferably in the range of 100 to 140°C.

The uniaxial stretching ratio of the second stretching is preferably 1.1 to 3.0 times. For example, in the case of uniaxial stretching, the stretching ratio is 1.1 to 3.0 times in the longitudinal direction (machine direction: MD direction) or TD direction. In the case of biaxial stretching, the stretching ratio is 1.1 to 3.0 times in each of the MD and TD directions. In the case of biaxial stretching, the stretching ratios in the MD and TD directions may be different in each direction as long as they are 1.1 to 3.0 times. If this ratio is less than 1.0, the productivity per hour of the polyolefin microporous membrane is poor. On the other hand, if this ratio is more than 3.0, the polyolefin microporous membrane is more likely to break during production, and the pore size becomes larger, which may cause problems with voltage resistance when used in a battery. It is more preferable that the second stretching ratio is 1.2 to 2.0 times.

The speed of the second stretching is preferably 3%/sec or more in the stretching axis direction. For example, in the case of uniaxial stretching, it is preferably 3%/sec or more in the MD or TD direction. In the case of biaxial stretching, it is preferably 3%/sec or more in the MD and TD directions. The stretching speed (%/sec) in the stretching axis direction represents the ratio of the length stretched per second in the region where the membrane (sheet) is re-stretched, with the length in the stretching axis direction before re-stretching being 100%. If the stretching speed is less than 3%/sec, the permeability decreases, or when stretched in the TD direction, the variation in physical properties in the sheet width direction increases. In particular, the variation in air permeability in the stretched sheet width direction is likely to increase. The speed of the second stretching is preferably 5%/sec or more, and more preferably 10%/sec or more. In the case of biaxial stretching, the stretching speeds in the MD and TD directions may be different from each other as long as they are 3%/sec or more, but are preferably the same. There is no particular upper limit to the speed of the second stretching, but it is preferable that it be 300%/sec or less to prevent breakage.

(7) Heat Treatment Step The membrane after the second stretching is heat treated. Heat setting and/or heat relaxation may be used as the heat treatment method. In particular, heat setting stabilizes the crystals of the membrane. Therefore, the network structure of fibrils formed by the second stretching is maintained, and a polyolefin microporous membrane with large pore size and excellent strength can be produced.

The heat setting is carried out, for example, within a temperature range from the crystal dispersion temperature to the melting point of the polyolefin resin constituting the polyolefin microporous membrane. The heat setting temperature is preferably within the range of the second stretching temperature ±5°C, which makes it easier to stabilize the physical properties. This temperature is more preferably within the range of the second stretching temperature ±3°C. Even if the second stretching is not carried out, the heat setting is preferably carried out within a temperature range from the crystal dispersion temperature to the melting point of the polyolefin resin constituting the polyolefin microporous membrane, more preferably 90°C to 135°C. The heat setting is carried out by a tenter method, a roll method, or a rolling method.

The heat relaxation treatment method can be, for example, the method disclosed in JP-A-2002-256099.

The polyethylene microporous membrane (polyolefin microporous membrane) may be a single-layer membrane, or may have a layer structure consisting of two or more layers with different molecular weights or average pore diameters. In the case of a layer structure consisting of two or more layers, it is preferable that the molecular weight and molecular weight distribution of the polyethylene resin (polyolefin resin) of at least one of the outermost layers satisfy the above-mentioned ranges.

A multilayer polyethylene microporous membrane (multilayer polyolefin microporous membrane) consisting of two or more layers can be produced by, for example, a method of heating and melting and kneading each of the polyethylenes (polyolefin resins) constituting layers A and B with a molding solvent, supplying the resulting resin solutions from each extruder to one die and integrating them to be co-extruded, a method of overlapping and heat fusing the gel-like sheets constituting each layer, a method of heat fusing after stretching each, or a method of heat fusing after removing the solvent. The co-extrusion method is preferred because it is easier to obtain adhesive strength between layers, it is easier to form communicating pores between layers, it is easier to maintain high permeability, and it is also superior in productivity.

By using the above manufacturing method, it is possible to obtain a polyolefin microporous membrane that achieves high puncture strength and tensile strength while also achieving high tensile elongation and low thermal shrinkage.

Although not limited thereto, it is preferable to adopt an in-line method in which the first stretching, removal of the membrane-forming solvent, drying treatment, second stretching, and heat treatment are performed continuously on a line. However, if necessary, an offline method may be adopted in which the membrane after drying treatment is temporarily wound into a film, and the second stretching and heat treatment are performed while unwinding it.

(8) Other steps (a) Heat setting step, hot roll treatment step, and hot solvent treatment step before washing, after washing, and during the second stretching step Any of the heat setting step, hot roll treatment step, and hot solvent treatment step may be provided before removing the membrane-forming solvent from the gel-like molding that has been subjected to the first stretching. Also, a step of heat setting the membrane after washing or during the second stretching step may be provided.

(i) Heat Setting Treatment The method for heat setting the stretched gel-like molding before and/or after washing, and the membrane during the second stretching step may be the same as that described above.

(ii) Hot Roll Treatment Process At least one surface of the stretched gel-like molded product before washing may be brought into contact with a hot roll (hot roll treatment). For the hot roll treatment, for example, the method described in JP-A-2007-106992 can be used. When the method described in JP-A-2007-106992 is used, the stretched gel-like molded product is brought into contact with a heating roll whose temperature is adjusted to a temperature between the crystal dispersion temperature of the polyolefin resin +10° C. and less than the melting point of the polyolefin resin. The contact time between the heating roll and the stretched gel-like molded product is preferably 0.5 seconds to 1 minute. The contact may be made with heating oil held on the roll surface. The heating roll may be either a smooth roll or an uneven roll that may have a suction function.

(iii) Hot Solvent Treatment Step The stretched gel-like molded product before washing may be subjected to a treatment of contacting with a hot solvent. As a hot solvent treatment method, for example, the method disclosed in WO 2000/020493 can be used.

(b) Membrane Crosslinking Treatment Step The heat-treated polyolefin microporous membrane may be crosslinked by ionizing radiation using α-rays, β-rays, γ-rays, electron beams, etc., thereby improving the meltdown temperature. This treatment can be performed with an electron beam dose of 0.1 to 100 Mrad and an accelerating voltage of 100 to 300 kV.

(c) Hydrophilization Treatment Step The heat-treated polyolefin microporous membrane may be hydrophilized by monomer grafting treatment, surfactant treatment, corona discharge treatment, plasma treatment, or the like.

(d) Surface Coating Treatment Step The surface of the polyolefin microporous membrane after heat treatment is coated with a fluororesin porous body such as polyvinylidene fluoride or polytetrafluoroethylene, or a porous body such as PA (polyamide), PAI (polyamideimide), PI (polyimide), or PPS (polyphenylene sulfide), thereby improving the meltdown property when used as a battery separator. A coating layer containing PP (polypropylene) may be formed on at least one surface of the polyolefin microporous membrane after the second stretching. Examples of PP for coating include those disclosed in International Publication No. WO 2005/054350.

In order to produce a polyolefin microporous film having high impact resistance, pin puncture strength and low heat shrinkage, it is preferable to simultaneously satisfy the following: ultra-high molecular weight polyethylene having a melting point of less than 133°C, a weight average molecular weight of 1.0 x ¹⁰⁶ or more and a molecular weight distribution of less than 18.0 is contained at 30% by mass or more; the component having a molecular weight of 2.33 million or more is 10% by mass or more in the entire polyolefin microporous film; and the total area stretching ratio before solvent extraction is 20 times or more and 45 times or less. This improves the pin puncture strength and tensile strength, and can simultaneously achieve high tensile elongation and low heat shrinkage. For example, it is preferable because it is possible to obtain a polyolefin microporous film having a pin puncture strength of 6000 mN (converted into a film thickness of 20 μm) or more, an average tensile strength of 150 MPa or more, an average tensile elongation of 185%, an average solid heat shrinkage of 10% or less, and a heat shrinkage in the TD direction of 14% or less at the shutdown temperature determined by thermomechanical analysis (TMA).

The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties:

(1) Film Thickness (μm)
The thickness of the polyolefin microporous membrane is preferably 3 to 25 μm, more preferably 3 to 22 μm, and even more preferably 5 to 20 μm, because batteries have become increasingly dense and have higher capacity in recent years. By making the membrane thickness 3 μm or more, a separator with guaranteed insulation can be obtained. By making the membrane thickness 25 μm or less, it is suitable for safety, high output, and high capacity.

(2) Air permeability (sec/100cm ³ ) and normalized air permeability (sec/100cm ³ /μm)
The normalized air permeability (Gurley value) is preferably 100 sec/100 cm ³ /μm or less. If the normalized air permeability is 100 sec/100 cm ³ /μm or less, good ion conductivity is obtained when the material is used in a battery. In addition, the air permeability is preferably 20 sec/100 cm3 ^or more. When a polyolefin microporous membrane is used in a battery, the air permeability is not too low, i.e., the permeability is not high, regardless of the membrane thickness. If the air permeability is too high, a short circuit is likely to occur during battery production, and even when used as a battery, discharge is likely to progress during storage, so the air permeability is preferably 20 seconds/100 cm3 or ^more . The air permeability can be adjusted by the resin composition (melting point and molecular weight distribution of polyethylene mixtures such as ultra-high molecular weight polyethylene), the stretching temperature and stretching ratio before solvent extraction, the dry stretching temperature and dry stretching ratio after washing, and the resin composition. It is possible.

(3) Porosity (%)
The porosity is preferably 25 to 80%. When the porosity is 25% or more, good air permeability and normalized air permeability can be obtained. When the porosity is 80% or less, the strength is sufficient when the polyolefin microporous membrane is used as a battery separator, and short circuits can be suppressed. The porosity is more preferably 25 to 60%, and even more preferably 25 to 50%. When the porosity is within this range, it is suitable for achieving both tensile strength and tensile elongation.

(4) 20 μm conversion puncture strength (mN)
The pin puncture strength is preferably 6000 mN (612 gf) or more when converted to a 20 μm membrane. If the pin puncture strength is 6000 mN or more when converted to a 20 μm membrane, when the polyolefin microporous membrane is incorporated into a battery as a battery separator, This is preferable in terms of preventing a decrease in yield during battery production and ensuring storage stability.

(5) Tensile strength (MPa), average tensile strength (MPa)
The tensile strength is preferably 80 MPa or more in both the MD and TD directions. If the tensile strength is in this range, the risk of membrane rupture is reduced. The tensile strength in the MD and TD directions is preferably 110 MPa or more, more preferably 140 MPa or more, and even more preferably 160 MPa or more. If the tensile strength is in the above preferred range, the membrane tends not to rupture when an impact is applied to the battery. The tensile strength in each direction of the polyolefin microporous membrane can be determined by the measurement method described in the Examples.

The average tensile strength is calculated from the tensile strength in the MD direction (TMD) and the tensile strength in the TD direction (TTD) according to the following calculation formula (1).
Average tensile strength (MPa) = (TMD x TTD) ^0.5 ... Formula (1)

The average tensile strength must be 150 MPa or more, preferably 155 MPa or more, more preferably 160 MPa or more, and even more preferably 165 MPa or more. By making the average tensile strength 150 MPa or more, the effect of making it difficult to deform due to external forces can be obtained. There is no particular limit to the upper limit of the average tensile strength, but 500 MPa or less is preferable, and 400 MPa or less is more preferable.

The average tensile strength can be adjusted by appropriately combining and adjusting several of the following factors as the raw material for the polyolefin microporous membrane: 1) ultra-high molecular weight polyethylene ratio, 2) weight average molecular weight of ultra-high molecular weight polyethylene, 3) molecular weight distribution of ultra-high molecular weight polyethylene, and 4) melting point of ultra-high molecular weight polyethylene. It can also be adjusted by controlling the stretching ratio. A high ultra-high molecular weight polyethylene ratio and weight average molecular weight, a wide molecular weight distribution, and a low melting point tend to increase the average tensile strength. The higher the stretching ratio, the higher the average tensile strength tends to be.

(6) Tensile elongation (%), average tensile elongation (%)
The tensile elongation is preferably 100% or more in both the MD and TD directions. This reduces the possibility of the separator breaking during battery production and when an external force is applied to the battery. The tensile elongation in the MD and TD directions is preferably 130% or more, more preferably 140% or more, and even more preferably 160% or more. When the tensile elongation is in the above preferred range, the battery tends to easily absorb energy when an impact is applied. The tensile elongation in each direction of the polyolefin microporous membrane can be determined by the measurement method described in the Examples.

The average tensile elongation can be calculated from the tensile elongation in the MD direction (EMD) and the tensile elongation in the TD direction (ETD) according to the following formula.
Average tensile elongation (%) = (EMD x ETD) ^0.5 ... formula (2)

The average tensile elongation must be 185% or more, preferably 190% or more, more preferably 195% or more, and even more preferably 200% or more. By making the average tensile elongation 185% or more, the effect of absorbing shock can be obtained. There is no particular upper limit on the average tensile elongation, but 500% or less is preferable, and 400% or less is more preferable.

The average tensile elongation can be adjusted by appropriately combining and adjusting several of the following raw materials for the polyolefin microporous membrane: 1) ultra-high molecular weight polyethylene ratio, 2) weight average molecular weight of ultra-high molecular weight polyethylene, 3) molecular weight distribution of ultra-high molecular weight polyethylene, and 4) melting point of ultra-high molecular weight polyethylene. It can also be adjusted by controlling the stretching ratio. A high ultra-high molecular weight polyethylene ratio and weight average molecular weight, a wide molecular weight distribution, and a low melting point tend to increase the average tensile elongation. On the other hand, a lower stretching ratio tends to increase the average tensile elongation.

(7) Heat shrinkage rate after heating at 105°C for 8 hours (solid heat shrinkage rate) (%)
The heat shrinkage in the MD direction (HSMD) and the heat shrinkage in the TD direction (HSTD) after heating at 105° C. for 8 hours are preferably less than 10%. If the heat shrinkage is less than 10%, the occurrence of short circuits between electrodes can be suppressed even when the polyolefin microporous membrane is used as a separator for a large lithium battery by applying an inorganic coating treatment or the like. In order to suppress short circuits between electrodes even when the battery is heated, the heat shrinkage is more preferably 8% or less, even more preferably 6% or less, and even more preferably less than 5% in both the MD direction and the TD direction.

The heat shrinkage rate is preferably less than 5% in the TD direction. The reason for this is that in a wound battery (a type of battery manufactured by stacking and winding up electrodes and a polyolefin microporous membrane as a separator), shrinkage in the MD direction can be suppressed, but little stress acts in the TD direction, which is the width direction of the polyolefin microporous membrane, and the membrane is prone to shrinkage when the temperature inside the battery rises. The heat shrinkage rate of the polyolefin microporous membrane can be determined by the measurement method described in the Examples.

The MD heat shrinkage (HSMD) and the TD heat shrinkage (HSTD) were averaged by the following formula (3) (referred to as the average solid heat shrinkage).
Average solid thermal shrinkage (%) = (HSMD x HSTD) ^0.5 ... formula (3)

The average solid thermal shrinkage is within 10.0%, more preferably within 6.0%, and even more preferably within 5.0%. When used in a laminated type lithium ion battery, the averaged thermal shrinkage may affect safety. The average solid thermal shrinkage can be adjusted by adjusting the stretching temperature and heat treatment temperature.

(8) Shutdown temperature (SDT) (°C) and meltdown temperature (MDT) (°C)
The shutdown (SD) temperature is preferably 140° C. or lower, more preferably 138° C. or lower, and further preferably 136° C. or lower. When the shutdown temperature is in this range, the pores are closed at a lower temperature, blocking the movement of lithium ions and allowing the battery function to be safely stopped.

The meltdown (MD) temperature is preferably 145°C or higher, more preferably 147°C or higher, and even more preferably 149°C or higher. The greater the difference between the meltdown temperature and the shutdown temperature, the more preferable it is. The greater the difference between the meltdown temperature and the shutdown temperature, the more the battery function can be stopped without restarting lithium ion conduction even if the temperature inside the battery overheats (overshoots) when the flow of lithium ions stops due to abnormal heat generation inside the battery. The difference between the meltdown temperature and the shutdown temperature is preferably 10°C or higher, more preferably 12°C or higher, and even more preferably 13°C or higher.

The shutdown temperature is strongly affected by the melting point of the raw material. It can also be adjusted by adjusting the stretching ratio and stretching temperature. Meanwhile, the meltdown temperature can also be adjusted by adjusting the melting point of the raw material and the molecular weight of the ultra-high molecular weight polyethylene. When using ultra-high molecular weight polyethylene with a weight-average molecular weight of more than 1 million, it is possible to achieve a meltdown temperature of 140°C or more, which exceeds the melting point of polyethylene. This is thought to be due to the high melt viscosity that is a characteristic of ultra-high molecular weight polyethylene.

(9) Average toughness (MPa%)
The average toughness is calculated using the average tensile strength and the average tensile elongation according to the following formula (4).
Average toughness (MPa%) = average tensile strength (MPa) × average tensile elongation (%) Formula (4)

In verifying the absorbed energy when impacted, since the energy is absorbed by the surface, the strength and elongation in the MD and TD directions are averaged to obtain an index. The average toughness is preferably 2×10 ⁴ MPa% or more, more preferably 2.4×10 ⁴ MPa% or more, even more preferably 2.8×10 ⁴ MPa% or more, still more preferably 3.0×10 ⁴ MPa% or more, and even more preferably 3.2×10 ⁴ MPa% or more.

(10) Corrected toughness (MPa%)
The average toughness calculated by formula (4) is used to calculate the corrected toughness by formula (5) below.
Corrected toughness (MPa%) = average toughness (MPa%) × (25 - MD solid heat shrinkage rate) / 25 Formula (5)

(11) Impact resistance strength (kN/ ^m2 )
The impact on the polyolefin microporous membrane in the battery is simulated and calculated by the following experiment. A polyolefin microporous membrane cut to 60 (mm) x 65 (mm) is fixed to a paper frame (outer frame: 75 (mm) x 80 (cm), inner frame: 35 (mm) x 50 (cm)) so that the same direction is longer, and the measurement is performed. When fixing the four sides of the polyolefin microporous membrane to the paper frame, fix it so that sagging does not occur. As a means for applying the impact, an apparatus used in the "weight drop resistance test (DuPont impact test)" for evaluating the mechanical properties of a coating film specified in JIS K 5600-5-3 1999 can be used. The above-mentioned apparatus is not limited, and it is sufficient to fix the polyolefin microporous membrane to the paper frame, drop a specified weight from above, and determine the upper limit height at which the membrane does not break.

In a drop weight resistance tester, the radius of the plunger and the mass of the weight are adjusted according to the impact resistance strength of the test piece. Such machines often have heights set in 5 cm increments, and the conditions are set so that the membrane does not break at the minimum height of 5 cm. If the membrane does not break, the weight height is increased by one step (by 5 cm) and the test is performed again, and if the membrane breaks, the height is decreased by one step and the test is repeated. The maximum height (H (m)) at which the membrane does not break is calculated. If the membrane breaks even at the minimum height with the lightest weight, the radius of the plunger used is increased to adjust the conditions so that the membrane does not break even at the minimum height.

In a normal weight drop resistance test, the 50% breaking height is the average of the maximum height at which the membrane does not break and the minimum height at which the membrane breaks, but in evaluating the breaking behavior of a polyolefin microporous membrane, the height at which the membrane does not break 100%, i.e., the maximum height at which the membrane does not break, is measured. This is because not breaking is important for ensuring the safety of the battery. The impact resistance can be calculated using the weight mass (W (kg)) and the contact area (S) between the pusher and the support (Equation (6)).
Impact resistance strength (kN/m ² )=W×H/S (6)

The contact area can be calculated from the plunger radius and the depth of the support, or the contact area can be determined using a test piece that did not rupture after the test.

As an example, the analytical method is shown below. If the radius of the pusher is d, the depth of the support is L, the tip of the pusher is a sphere with a radius of r, and the angle of the contact part between the tip of the pusher and the support is θ,
L=r×(1-cosθ)
2d=2×r×sinθ
If C = d/L, then
sinθ=2/C×(C ² /(C ² +1))
From this, r and θ can be calculated, and the contact area S of the support is S = 2 × π × r ² × (1-cos θ)
The contact area of the support can be calculated from the depth of the support and the radius of the pusher, and the impact resistance can be calculated from equation (6).

Impact resistance is easily affected by tensile strength and tensile elongation, and is generally considered to be able to be organized by the average toughness calculated by formula (4). However, while working on improving impact resistance, it was surprisingly found that impact resistance cannot be explained by toughness alone, and that there is a strong correlation between impact resistance and the corrected toughness corrected using MD solid heat shrinkage (solid heat shrinkage in the MD direction). The mechanism by which such physical properties are expressed is still under investigation, but it is believed that when impact is applied, a shrinking force acts within the polyolefin microporous membrane surface, and the smaller the in-plane shrinkage force, the better the impact resistance. Since the impact resistance test was performed in a temperature range where the molecular mobility of polyethylene crystals is low, it is believed that there is a relationship with the solid heat shrinkage rate. It was surprising that the solid heat shrinkage rate affects impact resistance, and it became clear that the behavior of a polyolefin microporous membrane when it is impacted is complex and cannot be controlled by tensile strength and tensile elongation alone.

(12) Thermal shrinkage rate (%) in the TD direction at the shutdown (SD) temperature determined by thermomechanical analysis (TMA method)
The thermal shrinkage rate (%) in the TD direction at the shutdown (SD) temperature determined by thermomechanical analysis (TMA method) is 14% or less. It is preferably 13% or less, more preferably 12% or less, even more preferably 11% or less, and most preferably 10% or less. The SD function is a function that safely stops the battery function when the battery is placed in an abnormal state, and plays an important role in lithium ion secondary batteries. When this function works, the polyolefin microporous film stops the movement of lithium ions between the positive and negative electrodes. However, if the polyolefin microporous film used between the electrodes shrinks at this time, a short circuit occurs between the electrodes, and the abnormal reaction of the battery cannot be stopped. In particular, when electrodes and polyolefin microporous membranes are wound and used in a prismatic battery, the polyolefin microporous membrane is restrained in the longitudinal direction (also called the machine direction, or MD direction) because it is wound, but the force restraining the polyolefin microporous membrane in the width direction (TD direction) is weak, and if the temperature near the polyolefin microporous membrane rises, it may shrink in the width direction, increasing the possibility of a short circuit between the electrodes.

When shutting down, some of the resin in the polyolefin microporous membrane melts, making it more likely to shrink. For this reason, a low shrinkage rate during shutdown is preferable. Polyolefin microporous membranes that have other functions, such as low air permeability and high pin puncture strength, tend to have a high shrinkage rate, making it difficult to achieve both. The TD shrinkage rate (%) at the SD temperature determined by the TMA method for the polyolefin microporous membrane can be determined by the measurement method described in the Examples.

The heat shrinkage in the TD direction at the SD temperature determined by the TMA method can be adjusted by appropriately adjusting the following factors as the raw material for the polyolefin microporous membrane: 1) the ultra-high molecular weight polyethylene ratio, 2) the weight average molecular weight of the ultra-high molecular weight polyethylene, 3) the molecular weight distribution of the ultra-high molecular weight polyethylene, and 4) the melting point of the ultra-high molecular weight polyethylene. The heat shrinkage in the TD direction at the SD temperature determined by the TMA method can also be adjusted by controlling the heat treatment temperature and the SDT of the polyolefin microporous membrane.

The entanglements formed by ultra-high molecular weight polyethylene components with a molecular weight of 2.33 million or more are uniformly formed within the polyolefin microporous film, and the amount of amorphous components derived from ultra-high molecular weight polyethylene can be controlled by controlling the weight average molecular weight, molecular weight distribution, and melting point. By combining with appropriate stretching conditions, appropriate deformation of the amorphous region becomes easy, and it is possible to achieve both tensile strength and tensile elongation. The puncture strength and heat shrinkage rate, which are affected by the crystallinity of polyethylene-based resins including ultra-high molecular weight polyethylene, i.e., the solid heat shrinkage rate and the heat shrinkage rate in the TD direction at the shutdown temperature, can also be achieved by appropriately controlling the stretching conditions and the structure of ultra-high molecular weight polyethylene (melting point, molecular weight distribution).

[Battery separator and secondary battery]
The present invention also relates to a battery separator using the above-mentioned polyolefin microporous membrane, and a secondary battery using the battery separator.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

The physical properties of the polyolefin microporous membrane were measured using the following methods.

(1) Film Thickness (μm)
The thickness of the polyolefin microporous membrane was measured at five points within a 95 mm x 95 mm area using a contact thickness meter (Litematic manufactured by Mitutoyo Corporation, contact pressure 0.01 N, 10.5 mmφ probe), and the average value was taken as the thickness _T1 (μm).

(2) 20 μm equivalent air permeability (sec/100 cm ³ )
For a polyolefin microporous membrane having a thickness T ₁ (μm), the air permeability (sec/s) was measured using an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117:2009. The 20 μm-equivalent air permeability (sec/100 cm ³ ) was calculated using the following formula, assuming ^the membrane thickness to be 20 μm.
Formula: 20μm conversion air permeability (sec/100cm ³ ) = air permeability (sec/100cm ³ ) x 20 (μm)/film thickness T ₁ (μm)

(3) Porosity (%)
The polyolefin microporous membrane was cut into a piece measuring 5 cm x 5 cm, and its volume (cm ³ ) and weight (g) were determined. From these and the membrane density (g/cm ³ ), calculation was performed using the following formula.
Formula: Porosity = ((volume - weight / membrane density) / volume) x 100
Here, the film density was set to 0.99. The volume was calculated using the thickness (film thickness) measured in (1) above.

(4) Puncture strength (mN) and 20 μm equivalent puncture strength (mN/20 μm)
The puncture strength was defined as the maximum load (P ₁ ) when a needle with a diameter of 1 mm (tip diameter of 0.5 mmR) was used to puncture a polyolefin microporous membrane with a thickness of T ₁ (μm) at a speed of 2 mm/sec. The 20 μm equivalent puncture strength was calculated using the following formula, assuming a membrane thickness of 20 μm.
Formula: P ₂ = (P ₁ ×20)/T ₁

(5) Tensile strength (MPa) and average tensile strength (MPa)
The tensile strength was measured using a 10 mm wide strip specimen according to ASTM D882. The average tensile strength (MPa) obtained by averaging the tensile strength in the MD direction (TMD) and the tensile strength in the TD direction (TTD) was calculated according to formula (1).
Average tensile strength (MPa) = (TMD x TTD) ^0.5 ... Formula (1)

(6) Tensile elongation (%) and average tensile elongation (%)
The tensile elongation was determined by taking three 10 mm-wide rectangular test pieces from the center of the width direction of the polyolefin microporous membrane, measuring each piece according to ASTM D 882, and calculating the average of the measurement results. The average tensile elongation (%) was determined by averaging the tensile elongation in the MD direction (EMD) and the tensile elongation in the TD direction (ETD) using formula (2).
Average tensile elongation (%) = (EMD x ETD) ^0.5 ... formula (2)

(7) 105°C heat shrinkage (solid heat shrinkage measured after heating at 105°C for 8 hours) (%) and average 105°C heat shrinkage (average solid heat shrinkage) (%)
A polyolefin microporous membrane was cut into a size of 50 mm x 50 mm, and the initial length was measured, and then the midpoint of each side of the sample was marked, and the sample was sandwiched between paper and left in an oven at 105 ° C for 8 hours. After removing the sample from the oven and cooling, the length (mm) between the marks in the MD and TD directions was measured. The MD 105 ° C heat shrinkage (HSMD) and TD 105 ° C heat shrinkage (HSTD) were measured for three samples, and the average value of the values calculated using the following formula was used.
Formula: 105°C heat shrinkage rate (%) = [(initial length between marks (mm) - length between marks after heating (mm)) / initial length between marks (mm)] x 100

The average 105°C heat shrinkage rate (%) obtained by averaging the MD 105°C heat shrinkage rate and the TD 105°C heat shrinkage rate was calculated according to formula (3).
Average 105°C heat shrinkage (average solid heat shrinkage) (%) = (HSMD x HSTD) ^0.5 ... formula (3)

(8) Shutdown temperature (SDT) and meltdown temperature (MDT) (°C)
The shutdown temperature and meltdown temperature were measured by the method disclosed in International Publication No. 2007/052663. According to this method, the polyolefin microporous membrane was exposed to an atmosphere of 30°C and heated at a rate of 5°C/min, during which the air permeability of the membrane was measured. The temperature at which the air permeability of the polyolefin microporous membrane first exceeded 100,000 sec/100 ^cm3 was defined as the shutdown temperature of the polyolefin microporous membrane. The meltdown temperature was defined as the temperature at which the air permeability measured while continuing to heat the membrane after reaching the shutdown temperature again reached 100,000 sec/100 ^cm3 . The air permeability of the polyolefin microporous membrane was measured according to JIS P8117:2009 using an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.).

(9) Impact resistance strength (kN/ ^m2 )
The impact strength was calculated by the following experiment, which simulated the impact on the polyolefin microporous membrane in the battery. The impact strength was measured by fixing a sample cut to 60 (mm) x 65 (mm) to a paper frame (outer frame: 75 (mm) x 80 (cm), inner frame: 35 (mm) x 50 (cm)) so that the same direction was longer. The impact tester used was a DuPont impact tester H-50 manufactured by Toyo Seiki Seisakusho Co., Ltd. The pusher was selected from radii of 12.7 mm, 6.3 mm, and 4.7 mm, and measurements were performed. The maximum height at which the membrane did not break and the weight mass were recorded.

The polyolefin microporous membrane is fixed to a paper frame on all four sides. At this time, it is fixed so that sagging does not occur. As a means of applying the impact, an apparatus used in the "weight drop resistance test (Dupont impact test)" for evaluating the mechanical properties of coating films specified in JIS K 5600-5-3 1999 can be used. The apparatus is not limited to the above, and it is sufficient to drop a specified weight from above after fixing the polyolefin microporous membrane to the paper frame and determine the upper limit height at which the membrane does not break.

In a drop weight resistance tester, the radius of the plunger and the mass of the weight are adjusted according to the impact resistance strength of the test piece. Such machines often have heights set in 5 cm increments, and the conditions are set so that the membrane does not break at the minimum height of 5 cm. If the membrane does not break, the weight height is increased by one step (by 5 cm) and the test is performed again, and if the membrane breaks, the height is decreased by one step and the test is repeated. The maximum height (H (m)) at which the membrane does not break is calculated. If the membrane breaks even at the minimum height with the lightest weight, the radius of the plunger used is increased to adjust the conditions so that the membrane does not break even at the minimum height.

In a normal weight drop resistance test, the 50% breaking height is the average of the maximum height at which the membrane does not break and the minimum height at which the membrane breaks, but in evaluating the breaking behavior of a polyolefin microporous membrane, the height at which the membrane does not break 100%, i.e., the maximum height at which the membrane does not break, is measured. This is because not breaking is important for ensuring the safety of the battery. The impact resistance can be calculated using the weight mass (W (kg)) and the contact area (S) between the pusher and the support (Equation (6)).
Impact resistance (kN/m2) = W × H/S Formula (6)

(10) Melting point (°C)
The raw resin was sealed in a measurement pan, and using a PYRIS DIAMOND DSC manufactured by PARKING ELMER, the temperature was raised to 230°C and completely melted, then the temperature was held at 230°C for 3 minutes, and the temperature was lowered to 30°C at a rate of 10°C/min. For example, in the case of polyethylene, the temperature was raised from 30°C to 230°C at a rate of 10°C/min, held at 230°C for 3 minutes, and the temperature was lowered to 30°C at a rate of 10°C/min. This was the first temperature rise, and the same measurement was repeated two more times, and the melting point (Tm) was obtained from the endothermic peak during the second temperature rise.

(11) Polyethylene Molecular Weight and Molecular Weight Distribution The weight average molecular weight (Mw) and molecular weight distribution (MWD) of ultra-high molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) were determined by gel permeation chromatography (GPC) under the following conditions.
・Measuring device: Agilent high temperature GPC device PL-GPC220
Column: Agilent PL1110-6200 (20 μm MIXED-A) × 2 Column temperature: 160 ° C.
Solvent (mobile phase): 1,2,4-trichlorobenzene Solvent flow rate: 1.0 mL/min Sample concentration: 0.1 wt% (dissolution conditions: 160°C/3.5H)
Injection volume: 500 μL
Detector: Agilent refractive index detector (RI detector)
Viscometer: Agilent viscosity detector Calibration curve: Prepared by the universal calibration curve method using a monodisperse polystyrene standard sample.

(12) Thermal shrinkage rate (%) in the TD direction at the SD temperature determined by thermomechanical analysis (TMA method)
Using a thermomechanical analyzer (TMA/SS 6100 manufactured by Hitachi High-Tech Science Corporation), the temperature was raised and scanned to measure the shrinkage load (mN). The measurement conditions were as follows: sample shape; width 3 mm x length 10 mm, load: 19.6 mN, temperature scanning range 30 to 210 ° C., and heating rate; 10 ° C./min. The measurement was performed by sampling so that the TD direction was the measurement direction. In the range of 110 ° C. or higher and the maximum shrinkage temperature or lower, the inflection point of the displacement was obtained and taken as the shutdown temperature, and the shrinkage rate at the time of shutdown was calculated using the following formula from the displacement at that time. The above measurements were performed at three points each at different points in the same polyolefin microporous membrane for TD.
Formula: TD shrinkage rate at SD temperature (%) = [(initial sample length (mm) - TMA displacement at SD temperature (mm)) / initial sample length (mm)] x 100

Example 1
(Production of polyolefin microporous membrane)
A polyethylene composition was prepared, which consisted of 40% by mass of ultra-high molecular weight polyethylene (PE1) having a weight average molecular weight (Mw) of 3.32×10 ⁶ , a molecular weight of 2.33 million or more (f(≧2330k)) of 31.6% of the total, a molecular weight distribution (MWD) of 16.8, and a melting point (Tm) of 127°C, and 60% by mass of high density polyethylene (PE2) having a weight average molecular weight of 4.97×10 ⁵ , a molecular weight distribution of 5.7, and a melting point of 135°C. In the polyethylene composition, the ultra-high molecular weight component (f(≧2330k)) having a molecular weight of 2.33 million or more accounted for 18.2% of the total composition. 0.375 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant was dry-blended with 100 parts by mass of the polyethylene composition to prepare a polyethylene composition.

25 parts by mass of the obtained polyethylene composition was fed into a twin-screw extruder. In addition, 75 parts by mass of liquid paraffin was fed from a side feeder of the twin-screw extruder, melted and kneaded to prepare a polyethylene resin solution in the extruder. Next, the polyethylene resin solution was extruded at 210°C from a die installed at the tip of the extruder, and an unstretched gel-like sheet was formed while being taken up by a cooling roll whose internal cooling water temperature was kept at 25°C. The cooled extrudate was then subjected to simultaneous biaxial stretching. The simultaneous biaxial stretching was performed by preheating, stretching, and then heat setting in that order. The preheating temperature, stretching temperature, and heat setting temperature were controlled to 115°C, and the simultaneous biaxial stretching was performed by a tenter at a stretch ratio of 5 times in both TD and MD. The stretched gel-like sheet was fixed to a 20 cm x 20 cm aluminum frame and immersed in methylene chloride at 25 degrees, after which the liquid paraffin was removed by vibrating at 100 rpm for 3 minutes, and then dried with air at room temperature. The size of the membrane remained constant during this process, and the membrane was then heat-treated at 125°C for 10 minutes while still fixed, forming the final polyolefin microporous membrane.

[Examples 2 to 8, Comparative Examples 1 to 6]
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the resin composition and other conditions were changed to those shown in Tables 1 and 2.

For the above examples and comparative examples, the physical properties of the raw polyolefin resin and process conditions are shown in Table 1, and the physical properties of the obtained polyolefin microporous membrane are shown in Table 2.

[Discussion]
As can be seen from Tables 1 and 2 showing the above results, the results shown in Examples 1 to 8 achieved higher pin puncture strength, higher tensile strength, and higher tensile elongation than the results shown in the Comparative Examples, while reducing the solid heat shrinkage at 105° C. and the TD heat shrinkage at a shutdown temperature of about 130° C. By having these properties, a polyolefin microporous membrane capable of preventing a short circuit between electrodes when an impact is applied inside a battery could be obtained.

In Examples 1 to 5, 7, and 8, by containing 30% by mass or more of ultra-high molecular weight polyethylene having a melting point of less than 133° C. and a weight average molecular weight of 1.0×10 ⁶ or more, and using a raw material containing 10% by mass or more of ultra-high molecular weight components (f(≧2330k)) having a molecular weight of 2.33 million or more in the polyolefin microporous membrane, and by setting the total area stretching ratio before solvent extraction to 20 times or more and 45 times or less, it is possible to achieve both high tensile strength and high tensile elongation, and also high pin puncture strength and low heat shrinkage at the same time. Since the ultra-high molecular weight polyethylene having a melting point of less than 133° C. is a copolymer with a monomer other than ethylene, it is believed that the entanglement in the amorphous region increases, and both the tensile strength and the tensile elongation can be improved.

It was also found that even when using ultra-high molecular weight polyethylene with a melting point of 135°C, the entanglement can be increased by setting the weight average molecular weight to 3.0 x ¹⁰⁶ or more and the molecular weight distribution to 14.0 or more, and similar effects can be obtained (Example 6). Such a polyolefin microporous membrane not only had high tensile strength and high tensile elongation, and achieved high toughness, but also had a low thermal shrinkage rate, and thus exhibited high impact strength that could not be achieved by toughness alone.

At the same time, the thermal shrinkage rate (%) in the TD direction at the SD temperature determined by the TMA method, which is the thermal shrinkage rate during shutdown, also achieved 12% or less. During shutdown, some of the resin melts and proceeds, but in Examples 1 to 8, which have a large amount of entangled components, it is believed that the entanglement of the ultra-high molecular weight polyethylene components acts as a resistance to the shrinkage stress resulting from melting during shutdown, suppressing the shrinkage behavior during shutdown.

In Comparative Examples 1, 4, and 5, the components with a molecular weight of 2.33 million or more in the entire polyolefin microporous film were less than 10% by mass, so high tensile strength and high tensile elongation could not be achieved at the same time. In Comparative Example 2, the polyolefin microporous film contains 10% by mass or more of ultra-high molecular weight components with a molecular weight of 2.33 million or more, but the weight average molecular weight is not 3.0 x 10 ⁶ or more as in Example 6, and the melting point is 133°C, which is more than 132°C, so it is thought that the entanglement between polyethylenes is insufficient, and high tensile strength and high tensile elongation could not be achieved. As explained above, even in ultra-high molecular weight polyethylene, the melting point is affected by the number of branches per number of polymerized carbons, but since the molecular weight is significantly large, the number of branches in one molecule of ultra-high molecular weight polyethylene becomes large, which brings about a large difference in viscoelastic behavior and has a large effect on the properties of the polyolefin microporous film. By increasing the stretch ratio, the tensile strength improves, but the elongation decreases (Comparative Examples 3 and 6), so it is difficult to improve the tensile strength and tensile elongation at the same time.

The thermal shrinkage rate (%) in the TD direction at the SD temperature determined by the TMA method tended to be lower in Examples 1 to 8 than in Comparative Examples 1 to 6. In addition to the components constituting the polyolefin microporous membrane, it is also affected by the film-forming conditions and the SD temperature of the obtained polyolefin microporous membrane (which itself varies depending on the components constituting the polyolefin microporous membrane and the film-forming conditions). Table 2 shows the temperature difference (SDT-Tm(PE1)) between the shutdown temperature (SDT) and the melting point (Tm) of the ultra-high molecular weight polyethylene (represented as PE1 in the table), and the temperature difference (heat treatment temperature-Tm(PE1)) between the melting point (Tm) of the ultra-high molecular weight polyethylene (represented as PE1 in the table) and the heat treatment temperature.

The larger the SDT-Tm(PE1), the greater the amount of ultra-high molecular weight polyethylene that melts at the shutdown temperature, and the greater the influence of the melt viscosity of the ultra-high molecular weight polyethylene. On the other hand, the smaller the heat treatment temperature-Tm(PE1), the more the ultra-high molecular weight polyethylene component is sufficiently relaxed during film formation, which is thought to lead to a decrease in the heat shrinkage rate. In Examples 1 to 3, SDT-Tm(PE1) is large, and the melting point of the ultra-high molecular weight polyethylene used is low (presumably strengthening the entanglement characteristics) and the molecular weight distribution is also wide (increasing the high molecular weight component), so it is thought that the ultra-high molecular weight polyethylene partially melts at the SD temperature, and the entanglement effect makes it easier to suppress shrinkage. In addition, since the heat treatment temperature-Tm(PE1) is small, it is thought that the relaxation of the structure derived from the ultra-high molecular weight polyethylene progresses, and the heat shrinkage rate (%) in the TD direction at the SD temperature is reduced. In Examples 4 and 6 to 8, it is presumed that the reason why the heat shrinkage rate in the TD direction at the SD temperature is slightly higher is because one of these effects is small.

A similar tendency was observed in the comparative examples, and in comparative examples 5 and 6, the TD shrinkage rate (%) at the SD temperature was reduced due to these effects. On the other hand, in comparative examples 1 to 4, the heat shrinkage rate (%) in the TD direction at the SD temperature was poor due to the narrow molecular weight distribution of the ultra-high molecular weight polyethylene, the high melting point, and the inappropriate film-forming conditions. The reason why the heat shrinkage rate (%) in the TD direction at the SD temperature was relatively small in comparative example 5 is thought to be that the molecular weight of the ultra-high molecular weight polyethylene used was reduced, which promoted relaxation of the polyolefin microporous film and reduced the shrinkage rate. However, in comparative example 5, the molecular weight of the ultra-high molecular weight polyethylene was reduced, so the amount of components with a molecular weight of 2.33 million or more in the polyolefin microporous film was reduced, the tensile elongation was reduced, and the impact strength was not sufficiently improved.

From these results, it was found that by controlling the molecular weight, molecular weight distribution, and melting point of the ultra-high molecular weight polyethylene, and by controlling the amount of polyethylene with a molecular weight of 2.33 million or more in the polyolefin microporous membrane, while selecting an appropriate stretching ratio, stretching temperature, and heat treatment temperature, it was possible to improve the balance between tensile strength and tensile elongation, simultaneously achieving a low thermal shrinkage rate, and thereby achieving high impact resistance that had not been achieved before.

Claims (6)

A polyolefin microporous membrane having an average tensile strength of 150 MPa or more, an average tensile elongation of 185% or more, an average solid heat shrinkage of 10.0% or less calculated from the solid heat shrinkage measured after heating at 105°C for 8 hours, and a TD heat shrinkage of 14% or less at a shutdown temperature determined by thermomechanical analysis , wherein the polyolefin resin forming the polyolefin microporous membrane contains a polyethylene resin as a main component. The polyolefin microporous membrane according to claim 1, wherein the average tensile elongation is 190% or more. The polyolefin microporous membrane according to claim 1 or 2, wherein the average tensile strength is 160 MPa or more and the average tensile elongation is 195% or more. The microporous polyolefin membrane according to any one of claims 1 to 3 , which has a pin puncture strength of 6000 mN/20 µm or more. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 4 . A secondary battery using the battery separator according to claim 5 .