Polyolefin microporous membranes and battery separators

Abstract

The objective of the present invention is to provide a polyolefin microporous membrane which is a thin membrane when used as a separator, has a low shutdown temperature, and has both mechanical strength and insulating properties after melting. The polyolefin microporous membrane is characterized in that: the membrane has a thickness of at most 6 μm; the puncture strength for an equivalent of 5 μm is at least 1.7 N; the shutdown temperature as measured through temperature-rising air permeability is 80-138 °C; and the crystallinity of polypropylene when reaching 169 °C is 3-200 ppm.

Description

[Technical Field]

[0001] This invention relates to polyolefin microporous membranes and battery separators. [Background technology]

[0002] Microporous membranes are used in various fields, including filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among these, microporous membranes made from polyolefin resin are widely used as separators for secondary batteries because they excel in chemical resistance, insulation, and mechanical strength, and possess shutdown characteristics. Furthermore, batteries need to meet safety requirements such as self-discharge characteristics to extend lifespan and prevent capacity degradation, as well as nail puncture tests, hot box tests, and impact resistance tests. For polyolefin microporous membranes, improvements in insulation, mechanical strength, and shutdown characteristics are required.

[0003] From the perspective of the thermal safety of the battery, in Patent Document 1, the polyolefin-based laminated microporous membrane consists of at least three layers, the film thickness is in the range of 3 to 25 μm, the melt-down temperature is in the range of 159 to 200 °C, the air permeability is in the range of 50 to 300 seconds, the puncture strength is in the range of 100 to 550 gf, only the inner layer of the three layers contains polypropylene, and at least one layer forming the surface layer contains a resin with a melt flow rate of 50 to 150 g / 10 min and a melting point of 120 to 130 °C. A separator film has been proposed. Also, in Patent Document 2, a laminated microporous membrane with a film thickness of 5 to 20 μm made of polyethylene and polypropylene, containing 3 to 50% of polypropylene in the microporous membrane, with the difference between the shutdown temperature and the membrane rupture temperature being 33 °C or more, the shutdown temperature being 140 °C or less, and the membrane rupture temperature being 150 °C or more. A separator film has been proposed. In Patent Document 3, in order to ensure the battery safety at high temperatures, it is characterized by containing polypentene having a Tm of 200.0 °C or higher and an MFR of 80.0 dg / min or less, being microporous, having a melt-down temperature of 180.0 °C or higher, having a shutdown temperature of 131.0 °C or less, and having a 170 °C TD thermal shrinkage of 30.0% or less. A separator film has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, lithium-ion secondary batteries have been required to have even higher capacity and greater safety in line with the improvement of electric vehicle performance and the transition from 4G to 5G internet communication. Therefore, separators need to be made thinner, maintain insulation at high temperatures inside the battery, and have improved mechanical strength and shutdown characteristics.

[0006] However, thinning the separator makes it difficult to maintain both mechanical strength and insulation at high temperatures. Furthermore, while it is possible to improve the mechanical strength of the separator by adjusting the film deposition conditions, such methods tend to increase the shutdown temperature. As a result, the internal temperature of the battery rises too high during abnormal heat generation, making it impossible to adequately maintain insulation at high temperatures. In the separators described in Patent Documents 1 to 3, when thinned, it is not possible to adequately balance the maintenance of insulation at high temperatures with mechanical strength.

[0007] In view of the above circumstances, the present invention aims to provide a polyolefin microporous film that is a thin film, has a low shutdown temperature, and possesses both high mechanical strength and insulating properties after melting. [Means for solving the problem]

[0008] The polyolefin microporous film according to the first aspect of the present invention is characterized by having a film thickness of 6 μm or less, a puncture strength equivalent to 5 μm of 1.7 N or more, a shutdown temperature measured by the temperature rise air permeability method being 80°C or higher and 138°C or lower, and a crystallinity of polypropylene of 3 ppm or higher and 200 ppm or lower when 169°C is reached.

[0009] Furthermore, the polyolefin microporous membrane may be a multilayer microporous membrane consisting of multiple layers.

[0010] Furthermore, the above polyolefin microporous membrane has a molecular weight of 5.0 × 10⁶ in the GPC chart. 4 ~1.0×10 5 The range and 3.0 × 10 5 ~7.0×10 5 Each of these ranges may have its own peak.

[0011] Furthermore, the weight-average molecular weight is 4.0 × 10⁻⁶. 5 The above 1.0 × 10 6 It may contain polyethylene as follows:

[0012] Furthermore, the polyolefin microporous membrane may have a polypropylene concentration of 3.5% by mass or more and 10.0% by mass or less.

[0013] Furthermore, the above-mentioned polyolefin microporous membrane may have a porous layer laminated on at least one side thereof.

[0014] A battery separator according to a second aspect of the present invention comprises the above-mentioned polyolefin microporous membrane. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a polyolefin microporous film that is a thin film, has a low shutdown temperature, and possesses both puncture strength and insulating properties after melting. In particular, the polyolefin microporous film is suitably used as a separator for batteries. [Modes for carrying out the invention]

[0016] The following describes embodiments of the present invention. However, the present invention is not limited to the embodiments described below.

[0017] The polyolefin microporous membrane of the present invention has a film thickness of 6 μm or less, a puncture strength equivalent to 5 μm of 1.7 N or more, a shutdown temperature measured by the temperature rise air permeability method of 80°C to 138°C, and a crystallinity of polypropylene of 3 ppm to 200 ppm when 169°C is reached.

[0018] The polyolefin microporous membrane of the present invention has an upper limit of 6 μm or less in thickness. If the thickness exceeds 6 μm, it cannot accommodate high-capacity batteries. The upper limit of the thickness is preferably 4.7 μm or less, and more preferably 4.5 μm or less. The lower limit of the thickness is preferably 1 μm or more, and more preferably 3.0 μm or more, from the viewpoint of puncture strength and insulation at high temperatures. When the thickness is within the above preferred range, when the polyolefin microporous membrane is used as a battery separator, the amount of active material in the electrodes can be increased by the amount the thickness is reduced, which in turn leads to an improvement in battery capacity. The thickness can be kept within a predetermined range by adjusting the extrusion discharge amount and the heat-fixing temperature.

[0019] The polyolefin microporous film of the present invention has a puncture strength (5μm equivalent puncture strength) of 1.7N or more per 5μm thickness, and a shutdown temperature of 80°C to 138°C. Due to these properties, the film is less likely to break even when subjected to high tension, exhibiting high durability, and also has excellent self-discharge characteristics when incorporated into a battery. Furthermore, it can shut down more quickly in the event of abnormal battery overheating, preventing temperature rise. A shutdown temperature of 80°C or higher is preferable because it prevents unnecessary shutdowns in extremely hot regions or seasons, thus reducing the possibility of impairing the battery's function. The balance between the 5μm equivalent puncture strength and the shutdown temperature can be adjusted within a predetermined range by combining film formation conditions such as the molecular weight of the polyolefin, the blending ratio, and the stretching temperature during the manufacturing process. From the viewpoint of suppressing the defect rate in the battery process, maintaining the battery's self-discharge characteristics, and compressibility, the lower limit of the 5μm equivalent puncture strength is preferably 1.7N or more, more preferably 1.9N or more, and the upper limit is not particularly limited, but preferably 3.0N or less. From the viewpoint of suppressing abnormal heat generation of the battery more quickly, the shutdown temperature is preferably 137°C or lower, and more preferably 136°C or lower.

[0020] The polyolefin microporous membrane of the present invention has a crystallinity of polypropylene (hereinafter sometimes referred to as the crystallinity of polypropylene at the time of reaching 169°C) of 3 ppm or more and 200 ppm or less when heated to reach 169°C. That the crystallinity of polypropylene is 3 ppm or more indicates that the regular structure of polypropylene remains sufficiently at the time of reaching 169°C, and since it is difficult to relax even when the temperature rises, it has excellent shape retention characteristics after melting and can maintain a good insulating state. Also, that the crystallinity of polypropylene is 200 ppm or less means that the amount of the regular structure does not become excessive, phase separation between the polyolefin and polypropylene at high temperatures is suppressed, holes are less likely to form, and short circuits can be further suppressed. From the viewpoint of suppressing short circuits due to film breakage at high temperatures, the crystallinity of polypropylene is preferably 10 ppm or more and 170 ppm or less, more preferably 20 ppm or more and 150 ppm or less.

[0021] Here, the crystallinity of polypropylene at the time of reaching 169°C can be determined by differential scanning calorimetry (DSC) described later. The crystallinity can be made within a predetermined range, for example, by adding polypropylene having a predetermined molecular weight and melting point. In this polyolefin microporous membrane, it is necessary to adjust the molecular weight and melting point of other polyolefins in consideration of the compatibility with other polyolefins to be mixed when determining the crystallinity of polypropylene at the time of reaching 169°C.

[0022] In order to satisfy the above puncture strength, shutdown temperature, and crystallinity of polypropylene at the time of reaching 169°C, the polyolefin microporous membrane can also be a multilayer microporous membrane composed of a plurality of layers.

[0023] The polyolefin constituting the polyolefin microporous membrane of the present invention preferably has peaks in the range of molecular weight of 5.0×10 4 ~1.0×10 5 and in the range of 3.0×10 5 ~7.0×10 5 in the GPC chart from the viewpoint of easily controlling the strength and shutdown characteristics.

[0024] The polyolefin microporous membrane of the present invention has a weight-average molecular weight of 4.0 × 10⁶ 5 The above 1.0 × 10 6 It is preferable to contain polyethylene as follows: The weight-average molecular weight of polyethylene is 4.0 × 10⁶. 5 The above 1.0 × 10 6 The following is more preferable: By using a polyolefin resin composition within the above range as the polyolefin constituting the polyolefin microporous membrane, strength is maintained even when it is made into a thin film, and furthermore, it is easily melted, resulting in excellent shutdown characteristics. The weight-average molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by the GPC method.

[0025] The polyolefin microporous membrane of the present invention contains polyethylene and isotactic polypropylene, and preferably the concentration of isotactic polypropylene relative to the total mass of polyethylene and isotactic polypropylene is 3.5% by mass or more and 10.0% by mass or less. More preferably, it is 4.0% by mass or more and 6.0% by mass or less. When the lower limit of the polypropylene concentration is within the above preferred range, the polypropylene component remains even after the polyolefin melts, maintaining a sufficient network and heat resistance. When the upper limit of the polypropylene concentration is within the above preferred range, the decrease in the puncture strength of the entire membrane is suppressed, making it less likely for holes to form and further suppressing short circuits. The polypropylene concentration in the polyolefin microporous membrane can be determined by infrared spectroscopy (IR measurement) as described later. The polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane can be controlled by the polypropylene concentration contained in the polyolefin resin raw material that forms the polyolefin microporous membrane. The polypropylene concentration in the polyolefin resin raw material, relative to the total mass of polyethylene and isotactic polypropylene, is preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 2.0% by mass or more and 6.0% by mass or less, and even more preferably 3.0% by mass or more and 5.5% by mass or less. The lower limit of the porosity of the polyolefin microporous membrane of the present invention is not particularly limited, but for example, it is 20% or more, and more preferably 30% or more. The lower limit of the porosity is not particularly limited, but for example, it is 70% or less, and preferably 60% or less. By having the porosity within the above range, the amount of electrolyte held can be increased and high ion permeability can be ensured. In addition, when the porosity is within the above range, the rate characteristics are improved. Furthermore, from the viewpoint of further improving ion permeability and rate characteristics, it is preferable that the porosity be 20% or more. The porosity can be set to the above range by adjusting the blending ratio of the polyolefin components, the stretching ratio, the heat setting conditions, etc. during the manufacturing process.

[0026] The thermal shrinkage rate of the polyolefin microporous membrane of the present invention in the mechanical direction is, for example, 10% or less, preferably 9% or less, and more preferably 8% or less. The thermal shrinkage rate of the polyolefin microporous membrane in the width direction at 120°C for 1 hour is, for example, 10% or less, preferably 9% or less, and more preferably 7% or less. The lower limit of the thermal shrinkage rate in the mechanical direction and the lower limit of the thermal shrinkage rate in the width direction are not particularly limited, but are preferably -2.0% or more. When the upper limit of the thermal shrinkage rate in the mechanical direction and the thermal shrinkage rate in the width direction are within the above range, the risk of deformation inside the battery and short circuits at the ends can be reduced, and battery safety can be improved. The thermal shrinkage of the polyolefin microporous membrane can be set to the above range by adjusting the blending ratio of the polyolefin components, the stretching ratio, the heat setting conditions, etc. during the manufacturing process.

[0027] (Method for manufacturing polyolefin microporous membranes) The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or a multilayer microporous membrane consisting of multiple layers. The layer configuration is preferably two or more layers, more preferably three layers, and particularly preferably A / B / A or B / A / B layers, with A and B layers having different resin compositions. The polyolefin resin compositions A and B constituting A and B layers are described below.

[0028] (1) Polyolefin resin composition A Polyolefin resin composition A may contain polyethylene a1 and polyethylene a2.

[0029] (Polyethylene A12) Polyethylene a1 has a weight-average molecular weight (Mw) of 7.0 × 10⁻⁶. 5 The above is polyethylene. Polyethylene a1 is an α-olefin other than ethylene. of A copolymer containing a small amount of ethylene is acceptable, but it is preferable to use an ethylene monopolymer. Other α-olefins besides ethylene are also acceptable. andPreferably, the α-olefins are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of α-olefins other than ethylene is preferably 5 mol% or less, with the α-olefin copolymer being 100 mol%. From the viewpoint of uniformity of the pore structure of the polyolefin microporous membrane, it is preferable that it be an ethylene monopolymer.

[0030] Polyethylene a1 is chosen because it allows for easier control of the strength, stretchability, and melting of microporous membranes, and has a weight-average molecular weight (Mw) of 7.0 × 10⁻⁶. 5 The above 2.0 × 10 6 Preferably less than 1.0 × 10 6 The above 1.8 × 10 6 The following is more preferable. The melting point of polyethylene a1 is preferably 134°C to 137°C, and more preferably 134°C to 136°C. The content of polyethylene a1 is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on 100% by mass of the polyolefin resin composition A. The upper limit is 95% by mass.

[0031] (Polyethylene a2) Polyethylene a2 is chosen because it facilitates control over the melting of microporous membranes, and has a weight-average molecular weight (Mw) of 5.0 × 10⁻⁶. 4 The above 7.0 x 10 5 It is less than 3.0 × 10 5 Preferably, it is 2.0 × 10 5 The following is more preferable. Furthermore, polyethylene a2 is preferably a low melting point component, preferably with a melting point of 130°C or higher and less than 134°C, more preferably 130°C or higher and 133°C or lower, and even more preferably 130°C or higher and 132°C or lower. Polyethylene a2 is preferably at least one selected from the group consisting of high-density polyethylene, medium-density polyethylene, branched low-density polyethylene and linear low-density polyethylene, and other α-olefins other than ethylene. ofIt may also contain a small amount of copolymer. α-olefins other than ethylene. and The following are preferred: propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of α-olefins other than ethylene is preferably 10 mol% or less, with α-olefin copolymer being 100 mol%. The content of polyethylene a2 is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass, based on 100% by mass of polyolefin resin composition A. below That is the case.

[0032] (2) Polyolefin resin composition B Polyolefin resin composition B may contain polyethylene b1 and polypropylene.

[0033] (Polyethylene b1) Polyethylene b1 may be the same as polyethylene a1 in the above section. However, "same" means polyethylene having the same molecular weight and melting point as polyethylene a1.

[0034] (polypropylene) The type of polypropylene is not particularly limited as long as it satisfies the following molecular weight and melting point requirements. From the viewpoint of phase separation and shape retention of microporous membranes at high temperatures, the weight-average molecular weight (Mw) of polypropylene is 1 × 10⁻⁶. 6 The above is preferable, 1.2 × 10 6 The above is more preferable, 1.2 × 10 6 ~4×10 6 This is even more preferable. The melting point of polypropylene is preferably 155 to 175°C, and more preferably 160 to 170°C.

[0035] Polypropylene may be a propylene monopolymer, a copolymer of propylene with other α-olefins and / or diolefins (propylene copolymer), or a mixture of two or more selected from these, but it is more preferable to use a propylene monopolymer alone. Either a random copolymer or a block copolymer can be used as the propylene copolymer. As the α-olefin in the propylene copolymer, an α-olefin having 8 or fewer carbon atoms is preferred. Examples of α-olefins having 8 or fewer carbon atoms include ethylene, butene-1, pentene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, and combinations thereof. As the diolefin in the propylene copolymer, a diolefin having 4 to 14 carbon atoms is preferred. Examples of diolefins having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene. The content of other α-olefins and diolefins in the propylene copolymer is preferably adjusted so that the polypropylene falls within the preferred melting point range described above. The polypropylene content is preferably 10% to 30% by mass, and more preferably 10% to 20% by mass, relative to 100% by mass of the polyolefin resin composition B.

[0036] The polyolefin resin compositions A and B described above may optionally contain other resin components besides polyethylene a1, a2, b1, and polypropylene. These other resin components may include, for example, resins that further impart heat resistance. Furthermore, within limits that do not impair the effects of the present invention, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, blocking inhibitors, fillers, crystal nucleating agents, and crystallization retarders may be included.

[0037] When the polyolefin microporous film has an A / B layer structure, the thickness ratio of the A / B layers is preferably 5 / 95 to 90 / 10, more preferably 30 / 70 to 80 / 20, and even more preferably 35 / 65 to 75 / 25. This allows the thin film to maintain puncture strength while also having high heat resistance.

[0038] In this embodiment, a porous layer may be laminated on at least one side of a polyolefin microporous membrane to form a microporous membrane. The porous layer is not particularly limited, but for example, a porous layer made of resin may be laminated. The resin used here is not particularly limited, and known resins can be used, such as acrylic resin, polyvinylidene fluoride resin, polyamide-imide resin, polyamide resin, aromatic polyamide resin, and polyimide resin. The porous layer may further contain inorganic particles, and the inorganic particles are not particularly limited, and known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

[0039] (3) Method for producing polyolefin microporous membranes The method for producing the polyolefin microporous membrane of the present invention includes the following steps. Each step will be described in detail. (a) Preparation of solutions for layers A and B (b) Forming of gel-like sheet (c) First extension (d) Removal of plasticizers and drying (e) Second extension (f) Heat treatment.

[0040] (a) Preparation of solutions for layers A and B Layer A and Layer B consist of the aforementioned polyolefin resin composition A and polyolefin resin composition B, respectively. A plasticizer is added to the polyolefin resin composition in a twin-screw extruder, and the mixture is melt-kneaded to prepare the solutions for Layer A and Layer B. Preferably, the polyolefin resin composition is contained in an amount of 10% to 30% by mass relative to the total resin solution. By keeping the concentration of the polyolefin resin composition within the above range, melt fracture and neck-in at the die exit can be prevented when extruding the polyolefin solution, resulting in good moldability and appearance of the extruded product.

[0041] The A and B layer solutions are each supplied from the extruder to a single die, where both solutions are extruded into a layered sheet to obtain an extruded molded body. Either the flat die method or the inflation method can be used for extrusion. In either method, a method in which the solutions are supplied to separate manifolds and stacked in layers at the lip inlet of the multilayer die (multiple manifold method), or a method in which the solutions are supplied to the die in a pre-formed layered flow (block method) can be used. Conventional methods can be applied to both the multiple manifold method and the block method. The gap of the multilayer flat die can be set to 0.1 mm or more and 5 mm or less. The extrusion temperature is preferably 140°C or more and 250°C or less, and the extrusion speed is preferably 0.2 to 15 m / min. The layer thickness ratio can be adjusted by adjusting the amount of each layer's solution extruded.

[0042] The thickness of each sheet is preferably such that the sheet thickness of the solution constituting layer A / the sheet thickness of the solution constituting layer B is 5 / 95 to 90 / 10, more preferably 30 / 70 to 80 / 20, and even more preferably 35 / 65 to 75 / 25. Within the preferred range of the polyolefin composition, the thickness ratio of each sheet, and the stretching conditions described later, a polyolefin microporous film with an excellent balance of strength and melting is obtained while maintaining the polypropylene network. In the polyolefin microporous film, when the polypropylene concentration of polyolefin resin composition B is 10% by mass or more and 30% by mass or less, from the viewpoint of maintaining the polypropylene network, it is preferable to concentrate the polyolefin resin composition B in multiple layers, for example, 50 / 50, rather than 0 / 100 (single-layer structure of layer B). This makes it possible to have higher heat resistance while maintaining puncture strength even in a thin film.

[0043] (b) Forming of gel-like sheet A gel-like sheet is formed by cooling the obtained extruded product. Cooling methods include contact with a refrigerant such as cold air or cooling water, or contact with a cooling roll; however, cooling by contact with a roll cooled with a refrigerant is preferred. Cooling is preferably carried out at a rate of 50°C / min or more, at least up to the gelation temperature. Cooling is preferably carried out to 25°C or below. When the cooling rate is within the above range, the crystallinity is maintained within an appropriate range, resulting in a gel-like sheet suitable for stretching.

[0044] (c) First extension Next, the gel sheet is stretched. After preheating, the gel sheet is preferably stretched to a predetermined magnification by the tenter method, roll method, inflation method, or a combination thereof. Stretching may be uniaxial or biaxial. The stretching magnification (surface stretching magnification) is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretching magnification in the longitudinal direction (hereinafter sometimes referred to as MD) and the width direction (hereinafter sometimes referred to as TD) may be the same or different, and it is preferable that the stretching magnification in both the MD and TD directions is 3 times or more.

[0045] The first stretching temperature is preferably between 100°C and 130°C, and more preferably between 110°C and 120°C. By using the polyolefin compositions A and B and the layer configuration described above, and setting the stretching temperature within the above preferred range, film rupture due to stretching of the low-melting-point polyolefin resin is suppressed, and high-magnification stretching is possible. As a result, the puncture strength of the polyolefin microporous film is improved, and the rise in the shutdown temperature is more easily suppressed. Furthermore, phase separation of polyethylene and polypropylene at high temperatures is suppressed, and shape retention is improved. Therefore, when used as a battery separator, even a thin film exhibits excellent mechanical strength and battery safety.

[0046] (d) Removal of plasticizers Next, a washing solvent is used to remove the plasticizer contained in the gel-like sheet and to dry it. The washing solvent and the method of removing the plasticizer using it are well known, so a detailed explanation is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Publication No. 2002-256099 can be used. After removing the plasticizer, the sheet is dried by a heat drying method or an air drying method. Any method capable of removing the washing solvent may be used, including conventional methods such as heat drying and air drying (moving air).

[0047] (e) Second extension A polyolefin microporous film can be obtained by preheating the dried sheet and then stretching it in at least one direction (dry stretching). The second stretching can be performed by the Tenter method or the like while heating. The final stretching ratio of the second stretching is preferably 1.1 times or more, and more preferably 1.4 times or more. By setting the final stretching ratio within the above range, the puncture strength can be easily controlled to the desired range. However, stretching to a high ratio increases the shutdown temperature and thermal shrinkage, so it is preferable to keep it at 9 times or less. When using the polyolefin compositions A and B and layer configuration described above, film breakage due to stretching of the low-melting-point polyolefin resin is suppressed and easier to control.

[0048] (f) Heat treatment After the second stretching, the material is held with clips and heat-treated while maintaining its width. The heat treatment is preferably performed at a temperature of 115.0°C to 135.0°C. By setting the heat treatment temperature within the above range, the thermal shrinkage rate of the polyolefin microporous film can be suppressed. A heat relaxation treatment may be performed during the heat treatment. If a heat relaxation treatment is performed, the relaxation rate can be set to 5% to 30%, with the length immediately before the treatment being 100%. By setting the relaxation rate within the above range, the thermal shrinkage rate can be reduced, and fluttering of the microporous film during the subsequent processing can be suppressed. [Examples]

[0049] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

[0050] [Measurement method] (1) Film thickness The film thickness at five points (upper left, upper right, center, lower left, and lower right) within a 95mm x 95mm area of ​​the polyolefin microporous membrane was measured using a contact thickness gauge (Mitutoyo Lightmatic, contact pressure 0.01N, 10.5mmφ probe), and the average value was taken as the film thickness (μm). If the sample size cannot be 95mm x 95mm, the sample may be cut to any size, and the five points (upper left, upper right, center, lower left, and lower right) may be measured.

[0051] (2) Inspection Prepare a 5cm square of polyolefin microporous membrane, measure the mass of each using a precision balance (5 significant figures (0.0000g)), and measure the mass of each 25cm square. 2 The calculation was performed by dividing by [the specified area]. If the sample size cannot be 5cm x 5cm, the sample can be cut to any size, and the measured mass can be divided by the area to calculate the result.

[0052] (3) Porosity A polyolefin microporous membrane was cut to a size of 95 mm x 95 mm, and its volume (cm³) 3 Calculate the weight and mass (g), and then compare them with the membrane density (g / cm³). 3 The porosity (%) was calculated using the following formula. Formula: Porosity = ((Volume - Mass / Membrane Density) / Volume) × 100 Here, the membrane density is 0.99 g / cm³. 3 This was done. Furthermore, the film thickness measured in (1) above was used to calculate the volume.

[0053] (4) Air permeability resistance For polyolefin microporous membranes, the air permeability resistance (sec / 100cm) was measured using an air permeability resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117:2009. 3 ) was measured.

[0054] (5)Piercing strength The maximum load value S(N) was measured when a polyolefin microporous membrane with a thickness T (μm) was punctured using a needle with a diameter of 1 mm (tip radius of 0.5 mmR) at a speed of 2 mm / second. The equivalent puncture strength for a 5 μm film thickness was calculated using the following formula. Formula: Converted puncture strength = S(N) x 5(μm) / T(μm).

[0055] (6) Shutdown temperature (also known as SD temperature) A 45mm diameter circular polyolefin microporous membrane was exposed to a 20°C atmosphere, and its air permeability resistance was measured while the temperature was increased at a rate of 5°C / min. If the air permeability resistance was 100,000 seconds / 100cm, 3 The temperature at which the airflow resistance was reached was defined as the shutdown temperature, and the average of two measurements was used. The airflow resistance was measured in accordance with JIS P8117:2009 using an airflow resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.).

[0056] (7) Meltdown temperature (also called MD temperature) A 45mm diameter circular polyolefin microporous membrane was exposed to a 20°C atmosphere, and its air permeability resistance was measured while the temperature was increased at a rate of 5°C / min. If the air permeability resistance was 100,000 seconds / 100cm, 3 Even after reaching this point, the temperature continues to rise, and the air permeability resistance reaches 100,000 seconds / 100cm. 3 The temperature below which the value falls was defined as the meltdown temperature. Air permeability resistance was measured using an air permeability resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P8117:2009.

[0057] (8) Thermal shrinkage A polyolefin microporous membrane was cut to a size of 95 mm x 95 mm. The length (mm) of the test specimen before shrinkage at room temperature (25°C) was measured in both the mechanical and width directions. The polyolefin microporous membrane test specimen was exposed to a temperature of 105°C for 8 hours without any load applied. After returning the test specimen to room temperature, the length (mm) after shrinkage in both the mechanical and width directions was measured. The thermal shrinkage rate (%) in the mechanical and width directions was calculated from the obtained test specimen lengths using the following formula. Formula: MD heat shrinkage rate (%) = (1 - length after shrinkage in the machine direction / length before shrinkage in the machine direction) × 100 TD thermal shrinkage rate (%) = (1 - length after shrinkage in the width direction / length before shrinkage in the width direction) × 100 (9) Weight average molecular weight The weight-average molecular weight (Mw) and molecular weight distribution (Mw / Mn) of polyolefin resins and polyolefin microporous membranes were determined by gel permeation chromatography (GPC) under the following measurement conditions. Measurement conditions • Measurement device: Agilent PL-GPC220 high-temperature GPC device • Columns: Agilent PL1110-6200 (20μm MIXED-A) x 2 Column temperature: 160℃ • Solvent (mobile phase): 1,2,4-trichlorobenzene • Solvent flow rate: 1.0 mL / min ·Sample concentration: 0.1% by mass (dissolution conditions: 160℃ / 3.5H) Injection volume: 500 μL • Detector: Agilent differential refractive index detector (RI detector) ·Viscometer: Agilent viscosity detector • Calibration curve: A universal calibration curve was created using monodisperse polystyrene standard samples. The peak positions on the low molecular weight and high molecular weight sides were estimated as follows. • Low molecular weight side peak position: The position of the peak of the Gaussian function on the low molecular weight side when the molecular weight distribution is fitted with two Gaussian functions. • High molecular weight peak position: Molecular weight at the maximum value of the molecular weight distribution.

[0058] (10) Melting point and melting peak The melting points of polyolefin resins and the melting peaks of polyolefin microporous membranes were determined using a scanning differential calorimeter (PARKING ELMER PYRIS DIAMOND DSC). The polyolefin resin and polyolefin microporous membranes were placed in separate sample holders and heated from 30°C to 230°C until completely melted. They were then held at 230°C for 3 minutes and cooled to 30°C at a rate of 10°C / min. This was considered the first heating cycle, and the same measurement was repeated. The melting point (Tm) of the polyolefin resin and the heat of fusion of the polyolefin microporous membrane were determined from the endothermic peaks during the second heating cycle. The baseline for calculating the heat of fusion was a straight line connecting 30°C and 230°C. For polyolefin resins, peaks with a heat of fusion of 70 J / g or more were considered endothermic peaks, and for polyolefin microporous membranes, peaks with a heat of fusion of 0.1 J / g or more were considered endothermic peaks.

[0059] (11) Layer ratio The layer ratio of the polyolefin microporous membrane was observed using a transmission electron microscope (TEM) under the following measurement conditions. Measurement conditions • Sample preparation: A polyolefin microporous membrane is stained with ruthenium tetroxide and cross-sectionally cut using an ultramicrotome. • Measuring device: Transmission electron microscope (JEOL JEM1400Plus model) Observation conditions: Acceleration voltage 100kV Observation direction: TD / ND.

[0060] (12) Crystallinity of polypropylene at 169°C The crystallinity of polypropylene in a polyolefin microporous film at 169°C was determined by the following measurement. Measurement conditions • Measuring device: Scanning differential calorimeter (PARKING ELMER PYRIS DIAMOND DSC) • Sample mass: 6 mg • Atmosphere gas: Nitrogen ·Starting temperature: 30℃ • Heating rate: 5°C / min ·Achieved temperature: 169℃ • Holding time at the target temperature: 5 minutes ·Cooling rate: 30℃ / min ·End temperature: 30℃.

[0061] During the cooling process of the above measurement, when the ordered structure remains, an exothermic peak due to the crystallization of polypropylene is detected in the temperature range of 120°C or higher. In this invention, the crystallinity χ for the entire polypropylene resin was determined from the exothermic peak detected when the temperature was raised to 169°C and then cooled, using the following formula. Equation: χ = ΔH PP / ΔH PP f × (Iso-tactic polypropylene concentration) Here ΔH PP ΔH represents the crystallization enthalpy (J / g) during the cooling process of the polypropylene structure. PP This refers to the area enclosed by the line connecting the start and end temperatures of the exothermic peak due to polypropylene crystallization on the DSC curve during the cooling process, and the DSC curve itself, divided by the mass of the measured sample. The isotactic polypropylene concentration is a value obtained from the IR measurement described later. For example, if the polypropylene is isotactic polypropylene, ΔH PP f ΔH represents the complete melting enthalpy (J / g) of polypropylene. PP f The value was calculated using 170 J / g.

[0062] (13) Measurement of polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in a polyolefin microporous membrane The isotactic polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane was 1462 cm³ derived from polyethylene, as obtained by IR measurement. -1 and 1376 cm² derived from isotactic polypropylene -1 The peak intensity ratio was used to determine the value. The measurement conditions were as follows: • Measurement device: FT-IR spectrometer (JASCO FT / IR-6600) ·Measurement temperature: 25℃ • Aperture: X=300μm, Y=300μm • Total number of times: 16 ·Resolution: 4cm -1 Formula: Isotactic polypropylene concentration (%) = (1462 cm³) -1 Peak height × conversion factor 1) / (1376cm) -1 (Peak height × Conversion factor 2) × 100. Here, conversion factor 1 is 20 and conversion factor 2 is 10.

[0063] (14) Hotbox characteristics Battery safety was evaluated based on the hot box characteristics shown below.

[0064] Battery manufacturing In a lithium-ion secondary battery, the positive and negative electrodes are stacked with a separator in between, and the separator contains an electrolyte. Lithium cobalt composite oxide (LiCoO2) was used as the positive electrode active material, graphite as the negative electrode active material, and 1 mol / L LiPF6 prepared in a mixed solvent of DC / dimethyl carbonate (DMC) was used as the electrolyte. The battery was assembled by stacking the positive electrode, a separator consisting of a microporous membrane, and the negative electrode, then fabricating a wound electrode body by a conventional method, inserting it into a battery case, impregnating it with the electrolyte, and then crimping the battery cover, which also serves as the positive electrode terminal equipped with a safety valve, via a gasket.

[0065] Hot box testing The assembled batteries were charged with a constant current of 1C to a voltage of 4.2V, then charged with a constant voltage of 4.2V, and subsequently discharged with a current of 0.2C to a cutoff voltage of 3.0V. Next, they were charged with a constant current of 0.2C to 4.2V, followed by constant voltage charging at 4.2V as pretreatment. The pretreated batteries were placed in an oven, heated from room temperature at 5°C / min, and then left at 150°C for 30 minutes. Batteries whose charging voltage dropped by more than 50% within 15 minutes of reaching 150°C were deemed unacceptable, those with a charging voltage drop of 20-50% were deemed acceptable, and those with a drop of 20% or less were deemed excellent.

[0066] (15) Self-discharge characteristics The self-discharge characteristics (K value) were evaluated using the following method. A test secondary battery, 0.5C, assembled according to the (Method for Manufacturing an Evaluation Battery) described below, was charged with a constant current up to a battery voltage of 3.85V. Then, it was charged with a constant voltage until the battery voltage reached 0.05C at 3.85V. The open-circuit voltage of this battery was measured after leaving it for 24 hours, and this value was defined as V1. The open-circuit voltage of this battery was measured again after leaving it for another 24 hours, i.e., a total of 48 hours after charging, and this value was defined as V2. The K value was calculated from the obtained values ​​of V1 and V2 using the following formula. Formula: K value = (V1 - V2) / 24.

[0067] [Example 1] (1) Preparation of the solution in layer A Weight-average molecular weight is 1.5 × 10⁻⁶ 6 , 90% by mass of polyethylene with a melting point of 136.0℃ and a weight-average molecular weight of 1.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 10% by mass of polyethylene with a melting point of 132.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 17% by mass.

[0068] (2) Preparation of the solution in layer B Weight-average molecular weight is 1.5 × 10⁻⁶ 6 , 70% by mass of polyethylene with a melting point of 136.0℃ and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 30% by mass of polypropylene with liquid paraffin in a twin-screw extruder to a resin concentration of 20% by mass.

[0069] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution A / solution B / solution A was 25 / 50 / 25. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet.

[0070] (4) First stretching, removal of film-forming solvent, drying The gel-like sheets were simultaneously stretched five times in both the MD and TD directions using a tenter stretcher at 110.0°C. After stretching, the sheets were immersed in a methylene chloride bath to remove the liquid paraffin, and then dried to obtain a microporous film.

[0071] (5) Second stretching and heat treatment Subsequently, the material was preheated to 128.0°C and then stretched to 1.6 times its original length (TD) using a tenter stretcher. After 15.0% relaxation of the TD, the material was heat-set at 128.0°C while held in the tenter to obtain a polyolefin microporous film. The properties of the obtained polyolefin microporous film are shown in Table 1.

[0072] [Example 2] Polyolefin solutions A and B were stretched in the same manner as in Example 1, except that the solution for layer B / solution for layer A / solution for layer B were extruded so that the layer thickness ratio was 25 / 50 / 25, to obtain a polyolefin microporous film.

[0073] [Example 3] (1) Preparation of the solution in layer A Weight-average molecular weight is 1.5 × 10⁻⁶ 6 , 70% by mass of polyethylene with a melting point of 135.0℃ and a weight-average molecular weight of 1.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 30% by mass of polyethylene with a melting point of 132.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 20% by mass.

[0074] (2) Preparation of the solution in layer B Weight-average molecular weight is 1.5 × 10⁻⁶ 6 , polyethylene with a melting point of 135.0℃, 85% by mass, and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 15% by mass of polypropylene with liquid paraffin in a twin-screw extruder to achieve a resin concentration of 20% by mass.

[0075] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution A / solution B / solution A was 20 / 60 / 20. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet.

[0076] (4) First stretching, removal of film-forming solvent, drying The gel sheet was stretched in the same manner as in Example 1, except that the stretching temperature was set to 112.5°C. The liquid paraffin was removed and the sheet was dried to obtain a microporous film after drying.

[0077] (5) Second stretching and heat treatment After preheating to 127.0°C, the material was stretched to 1.5 times its original length (TD) using a tenter stretcher. Then, 15.0% relaxation was applied to the TD, and the material was heat-set at 127.0°C while held in the tenter to obtain a polyolefin microporous film.

[0078] [Example 4] Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of the B layer solution / A layer solution / B layer solution was 30 / 40 / 30. A polyolefin microporous film was obtained in the same manner as in Example 3, except that the first stretching temperature was 113.5°C and the second stretching temperature and heat setting temperature were 126.0°C.

[0079] [Example 5] A polyolefin microporous film was obtained in the same manner as in Example 4, except that the first stretching temperature was 114.5°C, the second stretching temperature and heat-fixing temperature were 126.0°C, and the relaxation rate was 10.0%.

[0080] [Example 6] A polyolefin microporous film was obtained in the same manner as in Example 5, except that the first stretching temperature was set to 115.0°C.

[0081] [Example 7] A polyolefin microporous film was obtained in the same manner as in Example 5, except that the B layer solution / A layer solution / B layer solution were extruded to a thickness ratio of 25 / 50 / 25, the first stretching temperature was 115.5°C, and the relaxation rate was 15.0%.

[0082] [Example 8] (1) Preparation of the solution in layer A Weight-average molecular weight is 7.5 × 10⁻⁶ 5 , polyethylene with a melting point of 136.0℃, 85% by mass, and a weight-average molecular weight of 1.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 15% by mass of polyethylene with a melting point of 132.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 25% by mass.

[0083] (2) Preparation of the solution in layer B Weight-average molecular weight is 7.5 × 10⁻⁶ 5 , 90% by mass of polyethylene with a melting point of 136.0℃ and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 10% by mass of polypropylene with liquid paraffin in a twin-screw extruder to achieve a resin concentration of 15% by mass.

[0084] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution B / solution A / solution B was 15 / 70 / 15. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet.

[0085] (4) First stretching, removal of film-forming solvent, drying The gel sheet was stretched in the same manner as in Example 1, except that the stretching temperature was set to 112.0°C. The liquid paraffin was removed and the sheet was dried to obtain a microporous film after drying.

[0086] (5) Second stretching and heat treatment After preheating to 130.0°C, the material was stretched to 1.8 times its original length (TD) using a tenter stretcher. Then, 15.0% relaxation was applied to the TD, and the material was heat-set at 130.0°C while held in the tenter to obtain a polyolefin microporous film.

[0087] [Example 9] A polyolefin microporous film was obtained in the same manner as in Example 5, except that the ratio of polyethylene to polypropylene in polyolefin solution B was set to 90% and 10%, respectively, and the layer thickness ratio of the A layer solution / B layer solution / A layer solution was 30 / 40 / 30, and the second stretching ratio was set to 1.8 times and the relaxation ratio to 15.0.

[0088] [Example 10] A polyolefin microporous film was obtained in the same manner as in Example 8, except that the first stretching temperature was 113.0°C and the second stretching temperature was 125.0°C.

[0089] [Example 11] A polyolefin microporous film was obtained in the same manner as in Example 1, except that the solution of layer A / solution of layer B / solution of layer A was extruded so that the layer thickness ratio was 25 / 50 / 25, the first stretching temperature was 111.0°C, and the relaxation rate of the second stretching was 12.0%.

[0090] [Example 12] A polyolefin microporous film was obtained in the same manner as in Example 8, except that the solution of layer A / solution of layer B / solution of layer A was extruded so that the layer thickness ratio was 35 / 30 / 35, the first stretching temperature was 112.0°C, and the relaxation rate of the second stretching was 10.0%.

[0091] [Comparative Example 1] (1) Preparation of A layer solution Weight-average molecular weight is 2.0 × 10⁻⁶ 6 , 40% by mass of polyethylene with a melting point of 133.0℃ and a weight-average molecular weight of 3.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 60% by mass of polyethylene with a melting point of 136.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 25% by mass.

[0092] (2) Preparation of the solution in layer B Weight-average molecular weight is 3.0 × 10⁻⁶ 5 , polyethylene with a melting point of 135.0℃, 80% by mass, and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 20% ​​by mass of polypropylene with liquid paraffin in a twin-screw extruder to a resin concentration of 25% by mass.

[0093] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution B / solution A / solution B was 10 / 80 / 10. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet. At this time, the extrusion was adjusted so that the thickness after stretching, as described later, would be around 4.0 μm.

[0094] (4) First stretching, removal of film-forming solvent, drying The gel-like sheets were simultaneously stretched five times in both the MD and TD directions using a tenter stretcher at 115.0°C. After stretching, the sheets were immersed in a methylene chloride bath to remove the liquid paraffin, and then dried to obtain a microporous film.

[0095] (5) Second stretching and heat treatment Subsequently, the material was preheated to 125.5°C, then stretched to 1.5 times its original length in a tenter stretcher. After that, 15.0% relaxation was applied to the TD, and the material was heat-set at 125.5°C while held in the tenter to obtain a polyolefin microporous film.

[0096] [Comparative Example 2] (1) Preparation of the solution in layer A Weight-average molecular weight is 1.6 × 10⁻⁶ 6 , 40% by mass of polyethylene with a melting point of 134.0℃ and a weight-average molecular weight of 3.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 60% by mass of polyethylene with a melting point of 135.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 28.5% by mass.

[0097] (2) Preparation of the solution in layer B Weight-average molecular weight is 3.0 × 10⁻⁶ 5 , polyethylene with a melting point of 135.0℃, 70% by mass, and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 30% by mass of polypropylene with liquid paraffin in a twin-screw extruder to a resin concentration of 25% by mass.

[0098] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution A / solution B / solution A was 40 / 20 / 40. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet.

[0099] (4) First stretching, removal of film-forming solvent, drying The gel-like sheets were simultaneously stretched five times in both the MD and TD directions using a tenter stretcher at 110.0°C. After stretching, the sheets were immersed in a methylene chloride bath to remove the liquid paraffin, and then dried to obtain a microporous film.

[0100] (5) Second stretching and heat treatment Subsequently, the material was preheated to 128.0°C, stretched to 1.6 times its original length in a tenter stretcher, then relaxed by 15.0% in the tenter, and heat-set at 128.0°C while held in the tenter to obtain a polyolefin microporous film.

[0101] [Comparative Example 3] (1) Preparation of A layer solution Weight-average molecular weight is 2.0 × 10⁻⁶ 6 , 30% by mass of polyethylene with a melting point of 133.0℃ and a weight-average molecular weight of 3.0 × 10 5 Polyolefin solution A was prepared by melt-kneading 70% by mass of polyethylene with a melting point of 135.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 28.5% by mass.

[0102] (2) Preparation of the solution in layer B Weight-average molecular weight is 3.0 × 10⁻⁶ 5, 50% by mass of polyethylene with a melting point of 135.0℃ and a weight-average molecular weight of 2.0 × 10 6 Polyolefin solution B was prepared by melt-kneading 50% by mass of polypropylene with liquid paraffin in a twin-screw extruder to achieve a resin concentration of 30% by mass.

[0103] (3) Molding of gel-like sheet Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T-die, and extruded so that the layer thickness ratio of solution A / solution B / solution A was 35 / 30 / 35. The extruded molded body was cooled while being taken up by a cooling roll temperature-controlled at 25°C to form a gel-like sheet.

[0104] (4) First stretching, removal of film-forming solvent, drying The gel-like sheets were simultaneously stretched five times in both the MD and TD directions using a tenter stretcher at 114.0°C. After stretching, the sheets were immersed in a methylene chloride bath to remove the liquid paraffin, and then dried to obtain a microporous film.

[0105] (5) Second stretching and heat treatment Subsequently, the material was preheated to 125.0°C, stretched to 1.5 times its original length in a tenter stretcher, then relaxed by 10.0% in the tenter, and heat-set at 125°C while held in the tenter to obtain a polyolefin microporous film.

[0106] [Comparative Example 4] A gel-like sheet was formed using only the polyolefin solution A of Comparative Example 1. A polyolefin microporous film was obtained in the same manner as in Comparative Example 1, except that the first stretching temperature was 114.0°C, the second stretching and heat-fixing temperature was 130.0°C, and the relaxation rate was 20%.

[0107] [Reference example 1] A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the extrusion discharge was adjusted so that the film thickness of the polyolefin microporous membrane was approximately 9.0 μm.

[0108] [result] Table 2 shows the results of the physical property measurements of the obtained polyolefin microporous film. Compared to the comparative example, the polyolefin microporous film obtained in the example is a thin film with a lower shutdown temperature, possesses both puncture strength and insulation after melting, and the battery in which it was used as a battery separator exhibits excellent battery safety, such as that of a hot box, and superior self-discharge characteristics.

[0109] [Table 1-1]

[0110] [Table 1-2]

[0111] [Table 2-1]

[0112] [Table 2-2] [Industrial applicability]

[0113] The polyolefin microporous membrane of the present invention, when used as a separator for batteries, can provide a safe polyolefin microporous membrane even when it is a thin film and even when the battery is under high-temperature conditions.

Claims

1. The film thickness is 6 μm or less, the puncture strength equivalent to 5 μm is 1.7 N or more, the shutdown temperature measured by the temperature rise air permeability method is 80°C or higher and 138°C or lower, and the crystallinity of the polypropylene at 169°C is 3 ppm or higher and 200 ppm or lower. A polyolefin microporous membrane composed of two or more layers with different polyolefin compositions, comprising a layer containing only polyethylene as the polyolefin.

2. In the GPC chart, the molecular weight is 5.0 × 10 ４ ~1.0 x 10 ５ The range and 3.0 × 10 ５ ~7.0 x 10 ５ A polyolefin microporous membrane according to claim 1, having peaks in each of the following ranges.

3. Weight-average molecular weight is 4.0 × 10⁻⁶ ５ The above 1.0 x 10 ６ A polyolefin microporous membrane according to claim 1, containing polyethylene as follows.

4. A polyolefin microporous membrane according to claim 1, comprising polyethylene and isotactic polypropylene, wherein the concentration of isotactic polypropylene relative to the total mass of polyethylene and isotactic polypropylene is 3.5% by mass or more and 10.0% by mass or less.

5. A battery separator comprising a polyolefin microporous membrane as described in claim 1.

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