Polyolefin microporous membrane, battery separator, and secondary battery

Abstract

To provide a polyolefin microporous membrane that provides particularly excellent safety when used as a battery separator.SOLUTION: A polyolefin microporous membrane shows an average degree of in-plane molecular orientation of 2.9 or more when analyzed by Raman spectroscopy and exhibits a melting peak(s) at a temperature of 140°C or higher, which occupies 5% or more and 40% or less of the total area of all the melting peak(s), when measured with a differential scanning calorimeter (DSC).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyolefin microporous membrane, a battery separator, and a secondary battery. [Background technology]

[0002] Polyolefin microporous membranes are used as filters, fuel cell separators, and capacitor separators. They are particularly well-suited for use as separators for lithium-ion batteries, which are widely used in laptop computers, smartphones, electric vehicles, and other devices. This is due to the excellent mechanical strength and shutdown characteristics of polyolefin microporous membranes. In particular, in recent years, development of lithium-ion secondary batteries, primarily for automotive applications, has been progressing with the aim of increasing battery size and achieving higher energy density, capacity, and output. Consequently, the demand for separator safety has become even higher. The shutdown (SD) characteristic is the ability of the separator to close pores and shut off the battery reaction when the temperature inside the battery becomes abnormally high, such as when the battery is overheated during overcharging, thereby ensuring battery safety. Generally, the lower the shutdown temperature, the greater the safety effect. Furthermore, as battery capacity increases, the separator is becoming thinner, and separators are required to have lower resistance to further improve output characteristics, as well as higher mechanical strength to prevent short circuits caused by foreign objects inside the battery or when the separator is wound.

[0003] Patent Document 1 describes a method for producing a microporous membrane that is highly safe and has both high permeability and high mechanical strength, by sequentially stretching polyethylene having a relatively high molecular weight. The resulting microporous membrane achieves high permeability and strength.

[0004] Patent Document 2 discloses a polyethylene microporous membrane containing 20% ​​or more of high-density polyethylene having a molecular weight of less than 10,000, having a pore-clotting temperature Tf of 134°C or less, and having a relationship between the film melting temperature Tm and Tf such that Tm-Tf>0, and that when used as a battery separator, it can prevent short circuits during overcharge. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-108323 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-204188 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in the above documents, methods for increasing strength include controlling orientation by increasing the draw ratio and using high-molecular-weight polyethylene. Meanwhile, methods for lowering shutdown temperatures mainly involve increasing the pore-closing component in the microporous membrane by using low-molecular-weight raw materials. However, Patent Document 1 achieves both high mechanical strength and permeability, but the shutdown (pore-closing) temperature is high. On the other hand, Patent Document 2 has an excellent pore-closing temperature due to the high content of low-molecular-weight components, but is presumed to have insufficient mechanical strength for use as a current separator. As described above, there is a trade-off between the shutdown properties of a microporous membrane and mechanical strength and permeability, and it has been difficult to achieve both at a high level.

[0007] An object of the present invention is to provide a polyolefin microporous membrane that exhibits excellent mechanical strength and low resistance while shutting down at low temperatures, and that exhibits excellent safety and output characteristics when used as a battery separator. [Means for solving the problem]

[0008] The present inventors conducted extensive studies to solve the above-mentioned problems and found that the polyolefin microporous membrane of the present invention can achieve low-temperature shutdown, high mechanical strength, and low resistivity at high levels, all at the same time, compared to conventional polyolefin microporous membranes.

[0009] That is, the present invention is a polyolefin microporous membrane having an in-plane average degree of molecular orientation of 2.9 or more as measured by Raman spectroscopy, and an area ratio of 140°C or higher in the melting peak as measured by differential scanning calorimetry (DSC) of 5% to 40%.

[0010] The present invention also relates to a battery separator using the polyolefin microporous membrane of the present invention.

[0011] The present invention also relates to a secondary battery using the battery separator of the present invention. [Effects of the Invention]

[0012] The polyolefin microporous membrane of the present invention has excellent mechanical strength, low resistance, and shutdown characteristics, and therefore, when used as a battery separator, it has excellent safety and also excellent output characteristics, making it suitable for use as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles, while maintaining safety. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described.

[0014] In the present invention, the direction parallel to the film-forming direction of the polyolefin microporous membrane is referred to as the film-forming direction, longitudinal direction, or MD direction, and the direction perpendicular to the film-forming direction within the plane of the polyolefin microporous membrane is referred to as the width direction or TD direction. In the present invention, the melting endothermic curve obtained by differential scanning calorimetry (DSC) is also referred to as the melting curve or melting peak, the temperature at the apex of the melting peak is referred to as the peak top temperature, and the endothermic amount at the peak top temperature is referred to as the peak top intensity.

[0015] The microporous polyolefin membrane of the present invention preferably contains at least one polyethylene resin.

[0016] The polyethylene resin in the present invention may be a homopolymer of ethylene, or may be a copolymer containing other α-olefins to lower the melting point, as described below. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. The presence and type of α-olefins can be determined by the C 13 -This can be confirmed by measuring with NMR.

[0017] The polyethylene resin in the present invention has a weight average molecular weight Mw of 5.0 × 10 obtained by high-temperature gel permeation chromatography (GPC). 5 It is preferable that the value is 7.0×10 or more. 5 More preferably, 1.0×10 6 More preferably, it is 2.0×10 6 Less than 1.5 x 10 is preferable. 6 The following is more preferable: When Mw is within the above range, stretching stress is transmitted efficiently and the unstretched portion is reduced, thereby suppressing an excessive rise in shutdown temperature and enabling improved mechanical strength and low resistance.

[0018] The polyethylene resin in the present invention preferably has a melting point, as determined by differential scanning calorimetry (DSC), of 135°C or lower, more preferably 134°C or lower. It is also preferably 120°C or higher, more preferably 125°C or higher. A melting point within the above range allows the melting point of the pre-stretched structure to be lowered, and when made into a polyolefin microporous membrane, the thermal stability can be controlled within an appropriate range, resulting in low-temperature shutdown and low resistance.

[0019] The polyolefin microporous membrane of the present invention contains polyethylene having a molecular weight of 3.5×10 in the molecular weight distribution obtained by gel permeation chromatography (GPC). 5 The proportion of the following polyethylene is preferably 30% by mass or more and less than 50% by mass: Molecular weight 3.5×10 5 By ensuring that the proportion of the following polyethylene is within the above range, stress is transmitted efficiently during stretching, and the unstretched portion that causes an increase in resistance can be reduced, thereby enabling low resistance and improving mechanical strength. 5 The proportion of the following polyethylene is more preferably 35% by mass or more, and even more preferably 40% by mass or more. 5 The proportion of the following polyethylene is more preferably 48 mass % or less.

[0020] Molecular weight 3.5×10 5 To set the proportion of the following polyethylene within the above range, it is preferable to use the raw material composition and film-forming process conditions for the polyolefin microporous film described below.

[0021] The polyolefin microporous membrane of the present invention has a molecular weight of 9.0 × 10 in the molecular weight distribution of polyethylene obtained by gel permeation chromatography (GPC). 5 The proportion of the polyethylene is preferably 30% by mass or more and 50% by mass or less. 5 When the proportion of the polyethylene is within the above range, there are many molecules that can efficiently transmit stress during stretching, making it difficult for unstretched portions that cause an increase in resistance to be formed, making it possible to reduce resistance and also improve mechanical strength. 5 The proportion of the polyethylene is more preferably 32% by mass or more, and even more preferably 34% by mass or more. 5 The upper limit of the polyethylene content is more preferably 40% by mass or less.

[0022] Molecular weight of polyolefin microporous membrane: 9.0 x 10 5In order to set the proportion of polyethylene within the above range, it is preferable to use the raw material composition and film-forming process conditions for the polyolefin microporous film described below.

[0023] The polyethylene resin of the present invention preferably has a crystalline melting fraction (hereinafter referred to as "high-temperature crystalline melting fraction" H) of 0.1% to 6.0%, more preferably 0.1% to 4.5%, and even more preferably 0.1% to 3.5%, at or above the temperature corresponding to 50% of the peak top intensity of the melting peak obtained by differential scanning calorimetry (DSC) measurement, which is on the higher side. The high-temperature crystalline melting fraction H is an indicator of recrystallization ability, and polyethylene resins with high recrystallization ability may exhibit a peak or shoulder attributed to recrystallization at temperatures higher than the peak top temperature. When the high-temperature crystalline melting fraction H is within the above range, the formation of high-melting-point crystals that are likely to form by recrystallization during or after stretching is easily suppressed during crystal stabilization. This allows thermal stability to be controlled within an appropriate range, thereby enabling low-temperature shutdown.

[0024] The content of the polyethylene resin in the microporous polyolefin film is preferably 50% or more, more preferably 60% or more.

[0025] The polyolefin microporous membrane of the present invention preferably has an intrinsic viscosity of 3.0 dL / g or more. The intrinsic viscosity of the polyolefin microporous membrane is calculated by dissolving the polyolefin microporous membrane in a decalin solution to different concentrations and extrapolating the reduced viscosity measured at 135°C for each concentration to a concentration of 0. When the polyolefin microporous membrane has an intrinsic viscosity of 3.0 dL / g or more, preferably 4.0 dL / g or more, more preferably 5.0 dL / g or more, and even more preferably 5.5 dL / g or more, the molecular chains become more entangled, resulting in lower resistance and improved mechanical strength of the polyolefin microporous membrane. Furthermore, an intrinsic viscosity of 10 dL / g or less, preferably 9 dL / g or less, results in excellent membrane formability. To achieve the intrinsic viscosity of the polyolefin microporous membrane within the above range, the raw material composition of the polyolefin microporous membrane is preferably within the range described below.

[0026] The polyolefin microporous membrane of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, and fillers, as long as the effects of the present invention are not impaired.

[0027] In particular, it is preferable to add an antioxidant to suppress oxidative degradation of polyolefin resins due to their thermal history. Examples of the antioxidant include 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).

[0028] The microporous polyolefin membrane of the present invention has an in-plane average degree of molecular orientation measured by Raman spectroscopy of 2.9 or more.

[0029] The in-plane average degree of molecular orientation of a polyolefin microporous membrane measured by Raman spectroscopy is the average value of 24 measurements of I calculated from the spectrum obtained by Raman spectroscopy using the following formula, with the MD direction set to 0° and measured up to 345° in 15° increments. I=I 1130 / I 1063 Here, I: Degree of molecular orientation I 1130 :1130cm -1 Peak intensity at I 1063 :1060cm -1 Peak intensity at

[0030] When the degree of in-plane molecular orientation is 2.9 or more, preferably 3.0 or more, more preferably 3.2 or more, and even more preferably 3.3 or more, the molecular chains are efficiently oriented in the in-plane direction, improving the mechanical strength of the polyolefin microporous membrane and reducing the unstretched portions, thereby reducing resistance.

[0031] Furthermore, by setting the degree of in-plane molecular orientation to 4.0 or less, more preferably 3.7 or less, excessive molecular orientation can be suppressed, and deterioration of shutdown characteristics due to excessive constraint of molecular chains can be suppressed.

[0032] To achieve the degree of in-plane molecular orientation within the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the ranges described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0033] In the microporous polyolefin membrane of the present invention, the proportion S of the area of ​​the melting peak at 140° C. or higher as measured by a differential scanning calorimeter (DSC) is 5% or more and 40% or less.

[0034] The melting peak area of ​​a polyolefin microporous membrane is measured by differential scanning calorimetry (DSC) in accordance with JIS K 7121: 2012. The proportion S of the area of ​​the melting peak at 140°C or higher is calculated using the following formula: S=(S 140 / S all ) x 100 Here, S: Percentage of the area above 140°C in the melting peak (%) S 140 : Area of ​​the melting peak above 140°C S all : Area of ​​the entire melting peak.

[0035] By setting the S content to 5% or more, preferably 8% or more, and more preferably 10% or more, the pores are less likely to collapse and low resistance is maintained. Also, by setting the S content to 40% or less, preferably 37% or less, more preferably 35% or less, and even more preferably 30% or less, high-melting-point crystals that inhibit shutdown are suppressed, and shutdown characteristics at low temperatures can be exhibited.

[0036] To set S within the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the range described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0037] The polyolefin microporous membrane of the present invention preferably has a peak-top temperature Tm1 on the melting curve obtained during the first heating cycle by differential scanning calorimetry (DSC) of 130°C or higher and 140°C or lower. Tm1 indicates the crystalline melting point of the polyolefin microporous membrane. A Tm1 of 130°C or higher, more preferably 132°C or higher, and even more preferably 135°C or higher can suppress an increase in resistance due to collapse of the micropores. Furthermore, a Tm1 of 140°C or lower, more preferably 138°C or lower, and even more preferably 137°C or lower allows for shutdown at a lower temperature.

[0038] The melting curve measured by a differential scanning calorimeter (DSC) can be obtained by a measurement method conforming to JIS K 7121:2012.

[0039] To set Tm1 within the above range, it is preferable that the raw material composition and film-forming conditions of the polyolefin microporous film be within the ranges described below.

[0040] The polyolefin microporous membrane of the present invention preferably has a peak top temperature Tm2 on the melting curve obtained during the second heating cycle by differential scanning calorimetry (DSC) of 137.5°C or lower. Tm2 indicates the crystallization ability of the polyolefin microporous membrane. A Tm2 of 137.5°C or lower, more preferably 137.3°C or lower, and even more preferably 137.0°C or lower, allows the polyolefin microporous membrane to have a low melting point before stretching, enabling a lower shutdown temperature. Furthermore, a Tm2 of 130°C or higher, more preferably 132°C or higher, can suppress an increase in resistance due to collapse of the micropores.

[0041] To set Tm2 within the above range, the raw material composition of the polyolefin microporous membrane is preferably within the range described below.

[0042] In the polyolefin microporous membrane of the present invention, the difference ΔTm between Tm1 and Tm2 (=Tm1-Tm2) is preferably greater than -1.0°C and less than 2.0°C. ΔTm is expected to indicate changes in the crystalline structure of the polyolefin microporous membrane during membrane production. By setting ΔTm within the above range, it is expected that the crystalline structure, such as lamellar thickness and crystallite size, will be appropriately deformed in the polyolefin microporous membrane state, making it easier to achieve a balance between mechanical strength, shutdown property, and resistance of the polyolefin microporous membrane. ΔTm is more preferably greater than -0.5°C. ΔTm is more preferably 1.5°C or less, and even more preferably 1.0°C or less.

[0043] To set ΔTm within the above range, it is preferable that the raw material composition and film-forming conditions of the polyolefin microporous film be within the ranges described below.

[0044] The polyolefin microporous membrane of the present invention preferably has an average pore size of 15 nm or more as measured by a perm porometer according to JIS K 3832-1990. An average pore size of 15 nm or more, more preferably 18 nm or more, and even more preferably 20 nm or more can suppress an increase in resistance.

[0045] To achieve an average pore size within the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the range described below, and that the stretching conditions during production of the polyolefin microporous membrane be within the range described below.

[0046] The polyolefin microporous membrane of the present invention preferably has a bubble point pore size of 20 nm or more as measured by a perm porometer according to JIS K 3832-1990. A bubble point pore size of 20 nm or more, more preferably 25 nm or more, and even more preferably 30 nm or more can suppress an increase in resistance.

[0047] To achieve a bubble point pore size within the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the range described below, and that the stretching conditions during production of the polyolefin microporous membrane be within the range described below.

[0048] The polyolefin microporous membrane of the present invention has a microporous elasticity of 100 cm as measured by the Oken type testing machine method of JIS P-8117:2009. 3 It is preferable that the air resistance when air passes through the separator is 200 seconds or less, calculated as a 5-μm thickness. By setting the air resistance at 200 seconds or less, more preferably 110 seconds or less, and even more preferably 100 seconds or less, the ion permeability can be maintained, and the output characteristics when used as a battery separator can be improved. Furthermore, by setting the air resistance at 30 seconds or less, calculated as a 5-μm thickness, an excellent balance between strength and heat resistance can be achieved.

[0049] To achieve an air resistance equivalent to a 5-µm thickness within the above range, it is preferable that the raw material composition and lamination structure of the microporous membrane be within the ranges described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0050] The polyolefin microporous membrane of the present invention preferably has a porosity of 25% or more. A porosity of 25% or more, more preferably 30% or more, even more preferably 35% or more, and even more preferably 37% or more allows the membrane to maintain mechanical strength and ion permeability when used as a battery separator, thereby maintaining the output characteristics and safety of the battery. In addition, a porosity of 60% or less is preferred from the viewpoint of the mechanical strength of the microporous membrane.

[0051] To achieve a porosity within the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the ranges described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0052] The polyolefin microporous membrane of the present invention preferably has a thickness of 1 μm or more and 25 μm or less. A thickness within this range provides good handleability and productivity, maintains safety when made into a battery, and suppresses deterioration of output characteristics. The membrane thickness is more preferably 15 μm or less, even more preferably 12 μm or less, even more preferably 10 μm or less, and even more preferably 7 μm or less.

[0053] The film thickness can be adjusted by the number of revolutions of the extruder screw, the width of the unstretched sheet, the film-forming speed, the stretching ratio, etc., within a range that does not deteriorate other physical properties.

[0054] The polyolefin microporous membrane of the present invention has a peak top temperature (Tm) on the melting curve obtained during the second heating cycle measured by a differential scanning calorimeter (DSC) when containing liquid paraffin. L The upper limit of the peak top temperature is preferably 124°C or lower, more preferably 123°C or lower, and even more preferably 122.5°C or lower. The lower limit of the peak top temperature is preferably 118°C or higher, more preferably 120°C or higher, and even more preferably 122°C or higher. LThe Tm is obtained by adding liquid paraffin, a liquid plasticizer, to a polyolefin microporous membrane, melting it once at a heating rate of 10°C / min in DSC measurement, rapidly cooling it at a cooling rate of 300°C / min, and then melting it again at a heating rate of 10°C / min. L indicates the crystalline melting point when the polyolefin resin in the polyolefin microporous membrane is crystallized under the same conditions as during membrane formation. L When the Tm of the polyolefin microporous membrane is within the above range, the crystalline structure of the polyolefin microporous membrane before stretching can be easily controlled within an appropriate range, and the polyolefin microporous membrane can easily achieve a balance between mechanical strength, shutdown property, and resistance. L In order to achieve the above range, it is preferable that the raw material composition and film-forming conditions of the polyolefin microporous film be within the ranges described below.

[0055] The resistance value of the polyolefin microporous membrane of the present invention at room temperature is 0.5 Ω cm when converted to a membrane thickness of 5 μm. 2 Preferably it is equal to or less than 0.45 Ω cm, more preferably 0.45 Ω cm 2 Less than or equal to 0.4 Ω cm, more preferably 2 The resistance at room temperature is 0.5 Ω cm for a film thickness of 5 μm. 2 If the content is less than or equal to the above, the separator can be suitably used as a battery separator for secondary batteries that require high output, such as those used in electric vehicles.

[0056] To set the converted resistance within the above range, it is preferable that the raw material composition constituting the polyolefin microporous membrane be within the range described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0057] The polyolefin microporous membrane of the present invention preferably has a pin puncture strength of 1.5 N or more, calculated as a thickness of 5 μm. A pin puncture strength of 1.5 N or more, more preferably 1.7 N or more, even more preferably 2.0 N or more, even more preferably 2.2 N or more, and even more preferably 2.4 N or more can suppress short circuits caused by foreign matter inside the battery or during winding when made into a thin film, thereby improving battery safety. Furthermore, a pin puncture strength of 6.0 N or less is preferred from the viewpoint of improving shutdown characteristics.

[0058] To achieve the pin puncture strength in the above range, it is preferable that the raw material composition of the polyolefin microporous membrane be within the range described below, and that the stretching conditions during production of the polyolefin microporous membrane be within the range described below.

[0059] The polyolefin microporous membrane of the present invention preferably has a shutdown temperature of 139°C or lower, as measured by a temperature-programmed air permeability test. A shutdown temperature of 139°C or lower, more preferably 137°C or lower, even more preferably 135°C or lower, and even more preferably 134°C or lower, allows for the provision of highly safe batteries when used as battery separators for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles. Furthermore, the shutdown temperature is preferably 100°C or higher. A shutdown temperature of 100°C or higher prevents pore closure and deterioration of output characteristics under normal operating conditions and during battery production processes.

[0060] To set the shutdown temperature within the above range, it is preferable that the raw material composition constituting the polyolefin microporous membrane be within the range described below, and that the stretching conditions and heat setting conditions during production of the polyolefin microporous membrane be within the ranges described below.

[0061] The polyolefin microporous membrane of the present invention has a weight average molecular weight of 1.4 × 10 6 Over 4.0 x 10 6The raw material may contain an ultra-high molecular weight polyolefin resin (UHMwPO) of less than 1000 kJ / cm2. By including the ultra-high molecular weight polyolefin resin, the stretching stress is transmitted efficiently, making it easier to reduce the unstretched portion, thereby improving the mechanical strength and reducing the resistance.

[0062] The UHMwPO is preferably ultra-high molecular weight polyethylene (UHMwPE). UHMwPE may be a homopolymer of ethylene or a copolymer containing a small amount of other α-olefins. Examples of other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene.

[0063] The melting point of UHMwPO measured by differential scanning calorimetry (DSC) is preferably 137°C or lower, more preferably 136°C or lower, and even more preferably 134°C or lower. The melting point of UHMwPO is preferably 120°C or higher, more preferably 125°C or higher, and even more preferably 130°C or higher. When the melting point is within the above range, the thermal stability of the resulting microporous membrane can be controlled within an appropriate range, resulting in low-temperature shutdown and low resistance.

[0064] The polyolefin microporous membrane of the present invention can be used in various applications, such as filters, fuel cell separators, capacitor separators, etc. In particular, when used as a battery separator, it exhibits excellent safety and output characteristics, and is therefore preferably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles.

[0065] The polyolefin microporous membrane of the present invention is preferably produced by the following steps (a) to (f). Examples of methods for producing a polyolefin microporous membrane using the above-mentioned raw materials are described below. However, the polyolefin microporous membrane of the present invention is not limited to those obtained by the production methods described below.

[0066] (a) Preparation of polyolefin resin solution A polyolefin resin solution is prepared by dissolving a polyolefin resin and various additives in a plasticizer under heating.

[0067] The plasticizer may be any solvent capable of sufficiently dissolving the polyolefin resin, and a liquid solvent that is liquid at room temperature is preferred to suppress uneven stretching and enable relatively high stretching ratios. Examples of liquid solvents include aliphatic, alicyclic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. Among these, nonvolatile liquid solvents such as liquid paraffin are preferred to obtain a gel-like sheet with a stable liquid solvent content.

[0068] The viscosity of the liquid solvent is preferably 20 cSt or more and 200 cSt or less at 40° C. If the viscosity is 20 cSt or more, the sheet extruded from the die from the polyolefin resin solution is less likely to be non-uniform. On the other hand, if the viscosity is 200 cSt or less, the liquid solvent can be easily removed.

[0069] The viscosity of the liquid solvent can be measured at 40°C using an Ubbelohde viscometer.

[0070] A solid solvent that is miscible with the polyolefin in the melt-kneaded state but remains solid at room temperature may be mixed with the liquid solvent, such as stearyl alcohol, ceryl alcohol, paraffin wax, etc.

[0071] The blending ratio of the plasticizer is preferably more than 50% by mass and less than 90% by mass, with the total of the polyolefin resin and the plasticizer being 100% by mass. By using a plasticizer content of more than 50% by mass, shrinkage in the thickness direction can be suppressed, improving moldability. Furthermore, by using a plasticizer content of less than 90% by mass, swelling and necking at the outlet of the die during molding into a sheet can be suppressed, improving sheet moldability and film formability.

[0072] The melt-kneading of the polyolefin resin solution is preferably carried out in a twin-screw extruder in order to prepare a highly concentrated polyolefin resin solution.

[0073] In the extruder, the polyolefin resin solution is mixed uniformly at a temperature at which the polyolefin resin is completely melted.

[0074] The melt-kneading temperature is preferably (melting point of polyolefin resin + 10°C) to (melting point of polyolefin resin + 120°C). More preferably, it is (melting point of polyolefin resin + 20°C) to (melting point of polyolefin resin + 100°C). For example, when the polyolefin resin is a polyethylene resin, the melt-kneading temperature is preferably 140 to 250°C, since polyethylene resins have a melting point of approximately 130 to 140°C. It is more preferably 150 to 210°C, even more preferably 160 to 230°C, and even more preferably 170 to 200°C.

[0075] From the viewpoint of suppressing deterioration of the resin, a lower melt-kneading temperature is preferable, but if the temperature is lower than the above-mentioned temperature, unmelted material may be generated in the extrudate extruded from the die, which may cause membrane rupture or the like in the subsequent stretching process, and if the temperature is higher than the above-mentioned temperature, thermal decomposition of the polyolefin becomes severe, and the physical properties of the obtained microporous membrane, such as strength and porosity, may be deteriorated. In addition, decomposition products may precipitate on the chill roll or the rolls in the stretching process and adhere to the sheet, leading to deterioration of the appearance. Therefore, it is preferable to knead within the above-mentioned range.

[0076] The melting point is measured by DSC based on JIS K7121:2012.

[0077] After melt-kneading, it is preferable to remove foreign matter and modified polymers using a filter.

[0078] (b) Formation of a gel-like sheet The melt-kneaded resin solution is extruded through a die and cooled to obtain a gel-like sheet. Cooling solidifies the polyethylene resin microphase separated by the solvent. The cooling process is preferably performed to 10 to 50°C. This is because a final cooling temperature below the crystallization end temperature is preferable for finely nucleating the high-order structure. This finely nucleating the high-order structure facilitates uniform stretching in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30°C / min or more until the temperature is at least below the gelation temperature. A cooling rate below 30°C / min increases the crystallinity, making it difficult to obtain a gel-like sheet suitable for stretching. Generally, a slow cooling rate results in the formation of relatively large crystals, resulting in a coarse high-order structure in the gel-like sheet and a large gel structure. In contrast, a fast cooling rate results in the formation of relatively small crystals, resulting in a dense high-order structure in the gel-like sheet, which not only allows for uniform stretching but also improves the strength and elongation of the film.

[0079] The melting point of the gel-like sheet obtained by DSC is preferably 127°C, and particularly preferably 126°C or lower. Considering the effect on heat setting after stretching, the lower limit of the melting point is preferably 115°C or higher. When the melting point of the gel-like sheet is within the above range, the crystalline structure after stretching can be easily lowered in melting point, allowing for low-temperature shutdown. Furthermore, the flexible structure reduces the number of unstretched portions, enabling low resistance. The melting point of the gel-like sheet can be adjusted by the melting point of the polyolefin resin, the proportion of plasticizer, cooling conditions, etc.

[0080] The cooling method may be, for example, direct contact with cold air, cooling water or other cooling medium, contact with a roll cooled with a cooling medium, or use of a casting drum.

[0081] (c) Stretching The resulting gel-like sheet is biaxially stretched. Biaxial stretching can be achieved by any of inflation, simultaneous biaxial stretching, and sequential biaxial stretching. Among these, simultaneous biaxial stretching or sequential biaxial stretching is preferred in terms of film formation stability, thickness uniformity, and controlling the high rigidity and dimensional stability of the film. Examples of simultaneous biaxial stretching include a method using a simultaneous biaxial tenter. Examples of sequential biaxial stretching include a method using a combination of MD stretching using a roll stretcher and TD stretching using a tenter, or sequential biaxial stretching using a combination of a tenter and a tenter.

[0082] The stretching ratio is preferably 5 times or more in both the MD and TD directions.

[0083] The area stretching ratio is preferably 25 times or more. By setting the area stretching ratio to 25 times or more, more preferably 49 times or more, and even more preferably 64 times or more, it is easy to obtain uniformity in the membrane and less likely that unstretched portions will remain, resulting in a microporous membrane that is excellent in terms of strength and resistance. Furthermore, the area stretching ratio is preferably 150 times or less. Setting the area stretching ratio to 150 times or less prevents breakage during production of the microporous membrane, improving productivity, and also prevents excessive orientation and suppresses an increase in the shutdown temperature due to an increase in the melting point of the microporous membrane.

[0084] The stretching temperature is preferably set to the melting point of the gel-like sheet + 10°C or less, and more preferably in the range of (crystal dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5°C). Specifically, since a polyethylene composition has a crystal dispersion temperature of approximately 90 to 100°C, the stretching temperature is preferably 90 to 125°C, more preferably 90 to 120°C. A temperature of 90°C or higher ensures sufficient pore opening, making it easier to achieve uniform membrane thickness and a high porosity. A temperature of 125°C or lower prevents pore clogging due to melting of the sheet. The crystal dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065.

[0085] The stretching described above causes cleavage of the higher-order structure formed in the gel-like sheet, resulting in a finer crystalline phase and the formation of numerous fibrils. The fibrils form a network structure in which the crystalline phase is irregularly connected in three dimensions. Stretching improves mechanical strength and enlarges the pores, making the sheet suitable for battery separators. Furthermore, by stretching before removing the plasticizer, the polyolefin resin is in a sufficiently plasticized and softened state, which facilitates smooth cleavage of the higher-order structure and allows for uniform refinement of the crystalline phase. Furthermore, because cleavage is easy, strain during stretching is less likely to remain, resulting in a lower thermal shrinkage rate than when stretching is performed after removing the plasticizer.

[0086] (d) Plasticizer extraction (washing) The plasticizer (solvent) remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyolefin resin phase and the solvent phase are separated, removing the solvent yields a microporous membrane.

[0087] Examples of cleaning solvents include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain fluorocarbons such as trifluoroethane. These cleaning solvents preferably have low surface tension (e.g., 24 mN / m or less at 25°C). By using a cleaning solvent with low surface tension, the network structure that forms the micropores is prevented from shrinking during drying after cleaning due to the surface tension at the gas-liquid interface, resulting in a microporous membrane with porosity and permeability. These cleaning solvents are selected appropriately depending on the plasticizer and used alone or in combination.

[0088] Examples of the cleaning method include a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, or a combination of these methods.

[0089] The amount of cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet.

[0090] The washing temperature may be 15 to 30°C, and is heated to 80°C or lower as necessary. The longer the time the gel-like sheet is immersed in the washing solvent, the better, from the viewpoints of enhancing the washing effect of the solvent, preventing the resulting polyolefin microporous membrane from having nonuniform microporous membrane properties in the TD and / or MD, and improving the mechanical and electrical properties of the polyolefin microporous membrane. The above-described washing is preferably carried out until the residual solvent in the washed gel-like sheet, i.e., the polyolefin microporous membrane, is less than 1% by mass.

[0091] (e) Drying In the drying step, the solvent in the polyolefin microporous membrane is dried and removed. If the drying is insufficient, the porosity of the polyolefin microporous membrane will decrease in the subsequent heat treatment, resulting in poor permeability. Drying methods that can be selected include methods using metal heating rolls and methods using hot air.

[0092] (f) Heat treatment / re-stretching process The dried polyolefin microporous membrane may be stretched (restretched) at least uniaxially. Restretching can be performed by a tenter method or the like while heating the polyolefin microporous membrane, similar to the stretching described above. Restretching may be uniaxial or biaxial. Multistage stretching is performed by combining simultaneous biaxial and / or sequential stretching.

[0093] The re-stretching temperature is preferably equal to or lower than the melting point of the polyethylene composition, more preferably within the range of (Tcd-20°C) to the melting point, specifically preferably 70 to 140°C, more preferably 110 to 138°C, and even more preferably 120 to 135°C.

[0094] In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 3.0 times, and particularly in the TD direction, it is preferably 1.01 to 2.0 times, more preferably 1.2 to 1.8 times, and particularly preferably 1.3 to 1.6 times. In the case of biaxial stretching, it is preferably 1.01 to 1.6 times in both the MD and TD directions. The re-stretching ratios may be different in the MD and TD directions. Stretching within the above ranges can improve mechanical strength and resistance. In addition, an increase in the shutdown temperature due to an increase in the melting point of the microporous membrane caused by progress in crystal orientation can be suppressed.

[0095] It is also preferable to further perform a relaxation treatment. The relaxation ratio is preferably 0.9 or less relative to the maximum stretched state in the previous re-stretching. By setting the relaxation ratio to 0.9 or less, preferably 0.85 or less, it is possible to suppress heat shrinkage, as well as wrinkles and sagging. Furthermore, by setting the relaxation ratio to 0.7 or more, it is possible to suppress the occurrence of wrinkles and deterioration of permeability.

[0096] (g) Other processes Furthermore, depending on other applications, the polyolefin microporous membrane may be subjected to a crosslinking treatment or a hydrophilization treatment.

[0097] Crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane. Crosslinking treatment can be performed by irradiating the polyolefin microporous membrane with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. When irradiating with electron beams, the electron beam dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV.

[0098] The hydrophilization treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, etc. The monomer grafting is preferably carried out after the crosslinking treatment.

[0099] In the surfactant treatment, any of nonionic, cationic, anionic, and amphoteric surfactants can be used, but nonionic surfactants are preferred. The polyolefin microporous membrane is immersed in a solution prepared by dissolving the surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by a doctor blade method.

[0100] The polyolefin microporous membrane of the present invention may be formed into a multilayer polyolefin porous membrane by laminating a porous layer containing a resin other than the polyolefin resin by coating or vapor deposition, for the purpose of imparting functions such as meltdown properties, heat resistance, and adhesiveness.

[0101] The porous layer may be, for example, an inorganic particle layer containing a binder and inorganic particles. Examples of binder components that can be used to form the inorganic particle layer include acrylic resin, polyvinylidene fluoride resin, polyamide-imide resin, polyamide resin, aromatic polyamide resin, and polyimide resin. Examples of inorganic particles that can be used to form the inorganic particle layer include alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

[0102] The porous layer may be one in which the resins exemplified as the binder are made porous. [Example]

[0103] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0104] First, the measurement and evaluation methods will be explained. Unless otherwise specified, the measurements were carried out at a temperature of 25±2°C (room temperature) and a humidity of 50±10%.

[0105] (1) Melting point of polyolefin resin raw material The melting points of the raw polyolefin resins were measured by differential scanning calorimetry (DSC) according to JIS K7121:2012. A 6.0 mg sample was placed in an aluminum pan and heated from 30°C to 230°C at 10°C / min under a nitrogen atmosphere using a PerkinElmer PYRIS Diamond DSC. The sample was then heated from 30°C to 230°C at 10°C / min (first heating), held at 230°C for 5 minutes, cooled at 10°C / min, and then heated again at 10°C / min (second heating). The melting endothermic curves were obtained by measuring the peak top temperature of the melting endothermic curve obtained in the second heating.

[0106] (2) Molecular weight of polyolefin raw material The molecular weight of the polyolefin was determined by gel permeation chromatography (GPC) under the following conditions. Measurement equipment: Waters Corporation GPC-150C Column: Shodex UT806M manufactured by Showa Denko K.K. Column temperature: 160℃ Solvent (mobile phase): 1,2,4-trichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1 wt% (dissolution conditions: 160°C / 1h) Injection volume: 500 μl Detector: Waters Corporation differential refractometer (RI detector) Calibration curve: A calibration curve was created using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

[0107] (3) High-temperature crystalline melting ratio H of polyolefin resin The high-temperature crystalline melting fraction (H) of polyolefin resins was measured by differential scanning calorimetry (DSC). A 6.0 mg sample was sealed in an aluminum pan and heated from 30°C to 230°C at 10°C / min using a PerkinElmer PYRIS Diamond DSC under a nitrogen atmosphere. The sample was then heated from 30°C to 230°C at 10°C / min (first heating), held at 230°C for 5 minutes, cooled at 300°C / min, and heated again from 30°C to 230°C at a heating rate of 10°C / min (second heating). Each melting endothermic curve was obtained. A linear baseline was set in the range of 60°C to 160°C for the melting endothermic curve obtained during the second heating. The heat of melting was calculated from the area enclosed by the linear baseline and the melting endothermic curve. This was converted to the total heat of fusion (H) per sample mass. all In addition, the amount of heat was calculated from the area enclosed by the linear baseline and the melting endothermic curve at temperatures higher than the peak top temperature and showing half-maximum intensity of the peak top, and converted into the high-temperature crystal fusion heat H ht The total heat of fusion H all and the high-temperature crystal fusion heat H ht was substituted into the following equation to determine the high temperature crystalline melting ratio H of the polyolefin resin. H(%)=(H ht / H all )×100…(formula).

[0108] (4) Melting point of gel sheet The melting point of the gel-like sheet of the polyolefin microporous membrane before stretching was measured by differential scanning calorimetry (DSC) according to JIS K7121:2012. A 20 mg sample was placed in an aluminum pan and heated from 30°C to 230°C at a rate of 10°C / min using a PerkinElmer PYRIS Diamond DSC to obtain a melting endothermic curve. The peak-top temperature on the obtained melting endothermic curve was taken as the melting point of the gel-like sheet before stretching.

[0109] (5) Intrinsic viscosity of polyolefin microporous membrane The intrinsic viscosity of the polyolefin microporous membrane was calculated by dissolving the polyolefin microporous membrane in a decalin solution to different concentrations, and extrapolating the reduced viscosity measured at 135°C for each concentration to a concentration of 0.

[0110] (6) Molecular weight of polyolefin microporous membrane: 3.5 × 10 5 The following percentage of polyethylene (percentage of molecular weight 350,000 or less): Molecular weight of polyolefin microporous membrane: 3.5 x 10 5 The following polyethylene proportions were calculated according to the following formula using the molecular weight distribution obtained by gel permeation chromatography (GPC) measured under the following conditions. Molecular weight of polyolefin microporous membrane: 3.5 x 10 5 The percentage of polyethylene (mass%) below = (molecular weight 3.5 × 10 5 (amount of polyethylene component below) ÷ (amount of polyethylene component of total molecular weight) × 100 Measurement equipment: Waters Corporation GPC-150C Column: Shodex UT806M manufactured by Showa Denko K.K. Column temperature: 160℃ Solvent (mobile phase): 1,2,4-trichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1 wt% (dissolution conditions: 160°C / 1h) Injection volume: 500 μl Detector: Waters Corporation differential refractometer (RI detector) Calibration curve: A calibration curve was created using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

[0111] (7) Molecular weight of polyolefin microporous membrane: 9.0 × 10 5 Percentage of polyethylene above (percentage of molecular weight 900,000 or more) Molecular weight of polyolefin microporous membrane: 9.0 x 10 5The above polyethylene proportion was calculated by the following formula using the molecular weight distribution obtained by measuring under the same conditions as in the GPC method described above. Specifically, it is as follows: Molecular weight of polyolefin microporous membrane: 9.0 x 10 5 The percentage of polyethylene (mass%) above = (molecular weight 9.0 × 10 5 (amount of polyethylene components above) ÷ (amount of polyethylene components of total molecular weight) × 100.

[0112] (8) Film Thickness The thickness of the polyolefin microporous membrane was measured at five points within a 50 mm x 50 mm area using a contact thickness meter (Mitutoyo Corporation's "Litematic" VL-50, 10.5 mmφ superhard spherical probe) with a measurement load of 0.01 N, and the average value was taken as the thickness (μm).

[0113] (9) Porosity A 5cm x 5cm square was cut out from the polyolefin microporous membrane to prepare a test piece, and its volume (cm) at room temperature (25°C) was measured. 3 ) and mass (g). These values ​​and the constant film density of 0.99 g / cm 3 From this, the porosity of the polyolefin microporous membrane was calculated using the following formula. Porosity (%) = (volume - mass / membrane density) / volume × 100 ... (formula).

[0114] (10) Bubble point pore size (BP pore size) and average pore size The following measurements were carried out at three different points in the same microporous polyolefin membrane, and the average values ​​of the bubble point pore size and average pore size were calculated.

[0115] Based on JIS K 3832:1990, bubble point pore size and average pore size were measured using a perm porometer (PMI, CFP-1500A) in the order of dry-up and wet-up. For wet-up, pressure was applied to a microporous membrane thoroughly soaked in PMI's GALWICK (trade name) with a known surface tension, and the pore size calculated from the pressure at which air begins to penetrate was taken as the maximum pore size. The average pore size was calculated from the pressure at the point where the curve showing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The following formula was used to convert pressure and average pore size. D=C·Γ / P Here, D: Average pore size of the polyolefin microporous membrane (μm) Γ: Surface tension of the liquid (15.9 mN / m) P: Pressure (Pa) C: Constant (2.86×10 3 ).

[0116] (11) 5μm equivalent air resistance Measurements were made using the Oken air resistance tester method of JIS P-8117:2009. The measurement pressure was 0.05 MPa and the air resistance was 100 cm using an Oken air resistance tester (manufactured by Asahi Seiko Co., Ltd., EGO-1T). 3 The air resistance P1 (seconds) of the polyolefin microporous membrane when air was passed through it was measured, and the air resistance P2 converted to a membrane thickness of 5 μm was calculated using the following formula. P2=(P1×5) / T Here, P2: 5μm equivalent air resistance (sec / 5μm) P1: Air permeability resistance (seconds) T: Thickness of the polyolefin microporous membrane (μm).

[0117] (12) Peak top temperature Tm in the liquid plasticizer-containing state L Peak top temperature Tm of microporous polyolefin membranes containing liquid plasticizer LThe values ​​were measured by differential scanning calorimetry (DSC) based on JIS K7121:2012, except for the conditions described below. First, 6.0 mg of a polyolefin microporous membrane was placed in an aluminum pan. Then, liquid paraffin (containing 6.0 mg of naphthenic ring carbons) was poured into the aluminum pan containing the polyolefin microporous membrane. N : 30% ± 3%, aromatic ring carbon content C A 24 mg of a liquid plasticizer (1% or less, viscosity 35 cSt at 40°C) was added and sealed. After standing at 25°C for at least 1 hour, the samples were heated from 30°C to 230°C at a rate of 10°C / min (first heating) under a nitrogen atmosphere using a PerkinElmer PYRIS Diamond DSC, held at 230°C for 10 minutes, cooled at a rate of 300°C / min, and then heated again from 30°C to 230°C at a heating rate of 10°C / min (second heating), and each melting endothermic curve was obtained. The peak top temperature on the melting endothermic curve obtained in the second heating was taken as the peak top temperature Tm in the liquid plasticizer-added state. L It was decided.

[0118] (13) Peak top temperature Tm1 on the melting curve obtained during the first heating cycle by DSC The peak-top temperature (Tm1) on the melting curve of a polyolefin microporous membrane obtained during the first heating run using a differential scanning calorimeter (DSC) was measured by differential scanning calorimetry (DSC) according to JIS K7121:2012. A 6.0 mg sample was sealed in an aluminum pan and heated from 30°C to 230°C at a rate of 10°C / min using a PerkinElmer PYRIS Diamond DSC to obtain a melting endothermic curve. The peak-top temperature on the resulting melting endothermic curve was designated as Tm1 for the polyolefin microporous membrane.

[0119] (14) Peak top temperature Tm2 on the melting curve obtained during the second heating cycle by DSC The peak-top temperature (Tm2) on the melting curve obtained during the second heating run of a polyolefin microporous membrane measured by differential scanning calorimetry (DSC) was measured according to JIS K7121:2012. A 6.0 mg sample was placed in an aluminum pan and heated from 30°C to 230°C at 10°C / min under a nitrogen atmosphere using a PerkinElmer PYRIS Diamond DSC. The sample was then heated from 30°C to 230°C at 10°C / min (first heating run), held at 230°C for 20 minutes, cooled at 10°C / min, and then heated again at 10°C / min (second heating run). The peak-top temperature (Tm2) on the melting endothermic curve obtained during the second heating run was recorded.

[0120] (15)ΔTm ΔTm was calculated using the following formula. ΔTm=Tm1-Tm2…(formula).

[0121] (16) Percentage of melting peak area above 140°C by DSC (S) The crystalline melting rate of polyolefin microporous membranes at 140°C or higher was calculated based on the results of differential scanning calorimetry (DSC) analysis in accordance with JIS K7121:2012. A 6.0 mg sample was placed in an aluminum pan and heated from 30°C to 230°C at a rate of 10°C / min under a nitrogen atmosphere using a PerkinElmer PYRIS Diamond DSC to obtain a melting endothermic curve. A linear baseline was set in the range of 60°C to 160°C for the obtained melting endothermic curve. The heat of melting was calculated from the area enclosed by the linear baseline and the melting endothermic curve, and this was converted to a value per sample mass to obtain the total heat of fusion, S all In addition, the heat of fusion S above 140°C was calculated from the area enclosed by the linear baseline and the melting endothermic curve, and converted to a value per sample mass. 140 The total heat of fusion S all and 140℃ or more heat of fusion S 140 was substituted into the following equation to determine the crystalline melting fraction S of the polyolefin microporous membrane at 140°C or higher. S(%)=(S140 / S all ) x 100 Here, S: Percentage of the area above 140°C in the melting peak (%) S 140 : Area of ​​the melting peak above 140°C S all : Area of ​​the entire melting peak.

[0122] (17) In-plane average molecular orientation measured by Raman spectroscopy I ave The in-plane orientation index of the polyolefin microporous membrane was measured and calculated by Raman spectroscopy.

[0123] The laser was incident perpendicularly to the film surface (XY plane) and polarized using a polarizer. The measurement sample was rotated, and Raman spectra were obtained in each direction at 15° intervals, with MD set to 0°. The Raman spectra obtained were analyzed at 1020 cm -1 More than 1160cm -1 Obtain a baseline by linear approximation in the following region, 1060 cm -1 and 1130cm -1 The peak intensity was calculated by peak fitting using a Gaussian-Lorentzian mixed function approximation. -1 and 1060cm -1 Peak intensity ratio (I 1130 / I 1060 The average orientation of each direction (15° × n (1≦n≦24 (n is an integer))) was calculated by measuring 360° at 15° intervals, with the MD axis being 0°. Equipment: Raman microscope spectroscopy system (Renishaw "inVia") ·180° backscatter arrangement ·Spectral length 250mm Diffraction grating 3000 lines / mm Excitation laser 532nm 50x objective lens (NA=0.75) Spot size (spatial resolution) 5μm.

[0124] (18) 5μm equivalent puncture strength Measurements were performed in accordance with JIS Z 1707:2019, except that the test speed was 2 mm / sec. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), the maximum load (N) was measured when a polyolefin microporous membrane was pierced with a 1.0 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm) in an atmosphere of 25°C, and the piercing strength converted to a membrane thickness of 5 μm was calculated using the following formula. 5μm equivalent puncture strength (N / 5μm) = maximum load (N) x 5 / T Here, T: Thickness of the polyolefin microporous membrane (μm).

[0125] (19) Shutdown temperature The microporous membrane was heated at a heating rate of 5°C / min while measuring the air resistance with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.). When the air resistance reached the detection limit of 1.0 x 10 5 seconds / 100cm 3 The temperature reached by air was determined and used as the shutdown temperature (°C) by the temperature-programmed air permeability method. The measurement cell was made up of an aluminum block with a thermocouple located directly below the microporous membrane. The sample was cut into a 50 mm x 50 mm square and heated while being fixed with an O-ring around the periphery.

[0126] (20) 5μm equivalent resistance value R2 The polyolefin microporous membrane was cut to a diameter of 19 mm in an atmosphere of 30% ± 10% humidity to prepare test specimens. One test specimen was placed in a coin battery case (CR2032 standard) along with other materials and electrolyte was poured into it. The other materials and electrolyte used were LiCoO2 for the positive electrode, artificial graphite for the negative electrode, and a 1 mol / L solution of LiPF6 in a solvent with a volume ratio of EC to EMC of 4:6. Here, LiPF6: Lithium hexafluorophosphate EC: Ethylene carbonate EMC: Ethyl methyl carbonate Same as below. The coin battery material was vacuum-impregnated with the electrolyte using a vacuum dryer at a gauge pressure of -50 kPa for 1 minute, and then sealed using a crimping machine to prepare a coin battery for evaluation.

[0127] The resistance value of the produced coin battery was measured at a frequency of 200 kHz in an atmosphere of 25°C using an impedance analyzer.

[0128] Since the obtained resistance value includes resistance other than that of the polyolefin microporous membrane, the above measurement was performed by changing the number of polyolefin microporous membranes, and the resistance value per polyolefin microporous membrane (Ω·cm 2 ) was calculated. Then, the resistance converted into a film thickness of 5 μm was calculated using the following formula. R2=R1×5 / T Here, R2: Resistance value converted to 5μm (Ω·cm 2 / 5μm) R1: Resistance per microporous polyolefin membrane (Ω cm 2 ) T: Thickness of the polyolefin microporous membrane (μm).

[0129] (21) Foreign object resistance Using a tensile tester (SHIMAZU "AUTOGRAPH" AGS-X), a 1.5V capacitor, and a data logger, a simple battery consisting of a negative electrode, a polyolefin microporous membrane, a 500μm diameter chrome ball, and aluminum foil was pressed at 0.3mm / min, and the amount of displacement until the battery shorted out was measured. The sample that did not short out even with a large amount of displacement had better foreign matter resistance, and the relationship between displacement and foreign matter resistance was evaluated on a four-point scale as shown below. A: Displacement (mm) / separator thickness (μm) was 0.04 or more. B: Displacement (mm) / separator thickness (μm) was 0.03 or more and less than 0.04. C: Displacement (mm) / separator thickness (μm) was 0.02 or more and less than 0.03. D: Displacement (mm) / separator thickness (μm) was less than 0.020.

[0130] (22) Overcharge resistance characteristics A polyolefin microporous membrane was cut to a diameter of 19 mm in an atmosphere of 30% ± 10% humidity to prepare a test piece. One test piece was placed in a coin battery case (CR2032 standard) along with other materials similar to those described in (20) above, and the same electrolyte solution as described in (20) above was poured into it. The coin battery material was vacuum-impregnated with the electrolyte in a vacuum dryer at a gauge pressure of -50 kPa for 1 minute. The coin battery was then sealed using a crimping machine to prepare a coin battery for evaluation.

[0131] The prepared coin batteries were heated in a thermostatic chamber from room temperature (25°C) to 180°C at a rate of 5°C / min. The resistance values ​​at 25°C and 140°C were measured at a frequency of 200 kHz using an impedance analyzer, and the resistance increase was calculated using the following formula. The greater this increase, the easier it is to suppress thermal runaway during battery overcharge, and the overcharge resistance characteristics were evaluated using the following four-point scale. Resistance increase rate = resistance value at 140℃ / resistance value at 25℃ A: The resistance increased by more than 5000 times. B: The resistance increase was 500 times or more but less than 5000 times. C: The resistance increase was 50 times or more but less than 500 times. D: The resistance increased less than 50 times.

[0132] (23) Rate characteristics (capacity retention rate at 15C) To evaluate the rate characteristics of the polyolefin microporous membrane, the polyolefin microporous membrane was incorporated as a separator into a nonaqueous electrolyte secondary battery consisting of a positive electrode, a negative electrode, a separator, and an electrolyte, and a charge-discharge test was carried out.

[0133] In an atmosphere of 30% ± 10% humidity, a weight of 9.5 mg / cm was applied to an aluminum foil measuring 38 mm wide x 33 mm long x 20 μm thick. 2 NMC532 (lithium nickel manganese cobalt composite oxide (Li 1.05 Ni 0.50 Mn 0.29 Co 0.21O2)) and a 40 mm wide x 35 mm long x 10 μm thick copper foil with a density of 1.45 g / cm 3 of natural graphite with a unit area mass of 5.5 mg / cm 2 The positive and negative electrodes were dried in a vacuum oven at 120°C before use.

[0134] The separator used was a polyolefin microporous film 50 mm long and 50 mm wide, which had been dried in a vacuum oven at room temperature.

[0135] Vinylene carbonate (VC) and LiPF6 were dissolved in a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio 30 / 35 / 35) to prepare a solution with a VC concentration of 0.5 mass% and a LiPF6 concentration of 1 mol / L as an electrolyte.

[0136] The positive electrode, separator, and negative electrode were stacked, and the resulting laminate was placed in a laminate pouch. An electrolyte solution was poured into the laminate pouch, and the laminate pouch was vacuum-sealed to prepare a nonaqueous electrolyte secondary battery.

[0137] The fabricated nonaqueous electrolyte secondary batteries were initially charged to 10-15% capacity at 0.1 C at 35°C, and then left overnight (12 hours or more) at 35°C to allow for degassing. Next, they were subjected to constant-current-constant-voltage (CC-CV) charging at 35°C, a voltage range of 2.75-4.2 V, and a charge current of 0.1 C (cut-off current: 0.02 C), followed by constant-current (CC) discharging at 0.1 C. Next, three cycles of CC-CV charging at 35°C, a voltage range of 2.75-4.2 V, and a charge current of 0.2 C (cut-off current: 0.05 C), followed by CC discharging at 0.2 C, were performed. This was the initial state of the nonaqueous electrolyte secondary batteries.

[0138] Next, the battery was subjected to CC-CV charging at a temperature of 35°C, a voltage range of 2.75 to 4.2V, and a charge current of 0.2C (end current condition: 0.05C), followed by CC discharging at a discharge current of 0.2C, and the discharge capacity at this time was recorded as the 0.2C capacity. Next, the battery was subjected to CC-CV charging at a temperature of 35°C, a voltage range of 2.75 to 4.2V, and a charge current of 0.5C (end current condition: 0.05C), followed by CC discharging at a discharge current of 15C (306 mA, 24.48 mA / cm) at 35°C. 2 The capacity retention rate was calculated using the following formula and evaluated as rate characteristics on a 5-point scale. Capacity maintenance rate (%)=(15C capacity / 0.2C capacity)×100 A: The capacity retention rate was 73% or more. B: The capacity retention rate was 69% or more and less than 73%. C: The capacity retention rate was 65% or more and less than 69%. D: The capacity retention rate was 61% or more and less than 65%. E: The capacity retention rate was less than 61%.

[0139] [Example 1] (Preparation of the mixture) Weight average molecular weight (Mw) is 8.0 x 10 5 High density polyethylene (HDPE) (density 0.950 g / cm 3 , melting point 133.5 ° C) 60 mass % and Mw 2.0 × 10 6 A mixture was prepared by blending 100 parts by mass of a polyolefin resin consisting of 40% by mass of ultra-high molecular weight polyethylene (UHMwPE) with 0.2 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane as an antioxidant.

[0140] (Preparation of polyolefin resin solution) 25 parts by mass of the mixture was put into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 75 parts by mass of liquid paraffin (viscosity 35 cSt at 40°C) was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0141] (Formation of gel-like sheet) The polyolefin resin solution was passed through a filter to remove foreign matter, and then fed from the twin-screw extruder to a T-die. The molded product extruded from the T-die was cooled while being taken up at a take-up speed of 5 m / min using a cooling roll controlled to a temperature of 30°C, thereby forming a gel-like sheet.

[0142] (Stretching) The gel-like sheet was simultaneously biaxially stretched 9 times in both the MD and TD directions at 115° C. using a tenter stretching machine.

[0143] (Washing and drying) The stretched gel-like sheet was fixed to a 30 cm x 30 cm aluminum frame, immersed in a methylene chloride bath adjusted to 25°C, and shaken at 100 rpm for 10 minutes to remove the liquid paraffin. The sheet was then air-dried at room temperature to obtain a dry film.

[0144] (Heat treatment) The dried film was heat-set at 125°C for 10 minutes to obtain a microporous polyolefin film.

[0145] The resulting microporous polyolefin membrane had a thickness of 6 μm. Table 1 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0146] [Example 2] A 7 μm-thick polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the thickness of the extruded molded body in the gel-like sheet formation step was adjusted for this example, and the simultaneous biaxial stretching ratios in the MD and TD directions in the stretching step were each 7 times.

[0147] [Example 3] (Preparation of the mixture) Mw is 1.45 x 10 6 A mixture was prepared by blending 0.2 parts by mass of the same antioxidant as used in Example 1 with 100 parts by mass of a polyolefin resin consisting of 100% by mass of UHMwPE (melting point 125°C).

[0148] (Preparation of polyolefin resin solution) 20 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 80 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0149] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0150] (Stretching) The gel-like sheet was simultaneously biaxially stretched 9 times in both the MD and TD directions at 110° C. using a tenter stretching machine.

[0151] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0152] (Heat treatment) The dried film was heat-set at 115°C for 10 minutes to obtain a microporous polyolefin film.

[0153] The resulting microporous polyolefin membrane had a thickness of 5 μm. Table 1 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0154] [Example 4] (Preparation of the mixture) Mw is 8.0 × 10 5 HDPE (density 0.950g / cm 3 , melting point 133.5 ° C) 60 mass % and Mw 1.5 × 10 6 A mixture was prepared by blending 0.2 parts by mass of the same antioxidant as used in Example 1 with 100 parts by mass of a polyolefin resin consisting of 40% by mass of UHMwPE (melting point 136°C).

[0155] (Preparation of polyolefin resin solution) 25 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 75 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0156] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0157] (Stretching) Simultaneous biaxial stretching was carried out in the same manner as in Example 1, except that the gel-like sheet was used.

[0158] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0159] (Heat treatment) The dried film was heat-set at 125°C for 10 minutes to obtain a microporous polyolefin film.

[0160] The resulting microporous polyolefin membrane had a thickness of 7 μm. Table 1 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0161] [Example 5] A polyolefin microporous membrane with a thickness of 6 μm was obtained in the same manner as in Example 1, except that the thickness of the extruded molded body in the gel-like sheet formation step was adjusted for this example and the stretching temperature in the stretching step was set to 110°C.

[0162] [Example 6] (Preparation of the mixture) Mw is 8.0 × 10 5 HDPE (density 0.950g / cm 3A mixture was prepared by blending 100 parts by mass of a polyolefin resin consisting of 100% by mass of polyolefin (polymer having a melting point of 133.5°C) with 0.2 parts by mass of the same antioxidant as used in Example 1.

[0163] (Preparation of polyolefin resin solution) 30 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 70 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0164] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0165] (Stretching) The gel-like sheet was simultaneously biaxially stretched 7 times in both the MD and TD directions at 115° C. using a tenter stretching machine.

[0166] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0167] (Heat treatment) The dried film was heat-set at 125°C for 10 minutes to obtain a microporous polyolefin film.

[0168] The resulting microporous polyolefin membrane had a thickness of 9 μm. Table 2 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0169] [Example 7] (Preparation of the mixture) Mw is 8.0 × 10 5 HDPE (density 0.950g / cm 3 , melting point 133.5 ° C) 60 mass % and Mw 1.5 × 10 6A mixture was prepared by blending 0.2 parts by mass of the same antioxidant as used in Example 1 with 100 parts by mass of a polyolefin resin consisting of 40% by mass of UHMwPE (melting point 135°C).

[0170] (Preparation of polyolefin resin solution) 25 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 75 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0171] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0172] (Stretching) The gel-like sheet was stretched 6.0 times in the MD direction at 115°C using a roll stretching machine, and then stretched 8 times in the TD direction at 125°C using a tenter stretching machine (sequential biaxial stretching).

[0173] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0174] (Re-stretching) The dried membrane was re-stretched in the TD direction in a tenter at a stretching temperature of 132.0°C and a stretch ratio of 1.6, and then heat-relaxed to obtain a microporous polyolefin membrane.

[0175] The resulting microporous polyolefin membrane had a thickness of 9 μm. Table 2 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0176] [Comparative Example 1] (Preparation of the mixture) Mw is 3.5 × 10 5 HDPE (density 0.955g / cm3 , melting point 135 ° C) 60 mass % and Mw 2.0 × 10 6 A mixture was prepared by blending 100 parts by mass of a polyolefin resin composed of 40% by mass of UHMwPE of the above with 0.2 parts by mass of the same antioxidant as used in Example 1.

[0177] (Preparation of polyolefin resin solution) 25 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 75 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0178] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0179] (Stretching) The gel-like sheet was simultaneously biaxially stretched 5 times in both the MD and TD directions at 115° C. using a tenter stretching machine.

[0180] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0181] (Heat treatment) The dried film was heat-set at 125°C for 10 minutes to obtain a microporous polyolefin film.

[0182] The resulting microporous polyolefin membrane had a thickness of 9 μm. Table 2 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0183] Comparative Example 2 A 10 μm-thick polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the thickness of the extruded molded body in the gel-like sheet formation step was adjusted for this example, and the simultaneous biaxial stretching ratios in the MD and TD directions in the stretching step were set to 10 times.

[0184] Comparative Example 3 (Preparation of the mixture) Mw is 1.8 x 10 5 HDPE (density 0.953g / cm 3 A mixture was prepared by blending 100 parts by mass of a polyolefin resin consisting of 100% by mass of polyolefin (polyolefin copolymer, melting point 132°C) with 0.2 parts by mass of the same antioxidant as used in Example 1.

[0185] (Preparation of polyolefin resin solution) 40 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 60 parts by mass of the same liquid paraffin as used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and a rotation speed of 200 rpm to prepare a polyolefin resin solution.

[0186] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0187] (Stretching) Simultaneous biaxial stretching was carried out in the same manner as in Example 1, except that the gel-like sheet was used.

[0188] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0189] (Heat treatment) The dried film was heat-set at 120°C for 10 minutes to obtain a microporous polyolefin film.

[0190] The resulting microporous polyolefin membrane had a thickness of 5 μm. Table 3 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0191] Comparative Example 4 A 10 μm-thick polyolefin microporous membrane was obtained in the same manner as in Example 3, except that the thickness of the extruded molded body in the gel-like sheet formation step was adjusted for this example, and the simultaneous biaxial stretching ratios in the MD and TD directions in the stretching step were set to 5 times.

[0192] Comparative Example 5 (Preparation of the mixture) Mw is 1.5 × 10 6 UHMwPE (density 0.955g / cm 3 A mixture was prepared by blending 100 parts by mass of a polyolefin resin consisting of 100% by mass of polyolefin (polyolefin copolymer, melting point 136°C) with 0.2 parts by mass of the same antioxidant as used in Example 1.

[0193] (Preparation of polyolefin resin solution) 21 parts by mass of the mixture was fed into a high-mixing type twin-screw extruder (inner diameter 58 mm, L / D = 42), and 79 parts by mass of liquid paraffin similar to that used in Example 1 was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 180°C and 200 rpm to prepare a polyolefin resin solution.

[0194] (Formation of gel-like sheet) A gel-like sheet was formed using the polyolefin resin solution in the same manner as in Example 1, except that the thickness of the extruded molded product was adjusted for this example.

[0195] (Stretching) Simultaneous biaxial stretching was carried out in the same manner as in Example 1, except that the gel-like sheet was used.

[0196] (Washing and drying) Washing and drying were carried out in the same manner as in Example 1, except that the gel-like sheet stretched in the above step was used, to obtain a dry film.

[0197] (Heat treatment) The dried film was heat-set at 125°C for 10 minutes to obtain a microporous polyolefin film.

[0198] The resulting microporous polyolefin membrane had a thickness of 7 μm. Table 3 shows the blending ratio of each component, production conditions, evaluation results, etc.

[0199] [Table 1]

[0200] [Table 2]

[0201] [Table 3]

[0202] The polyolefin microporous membranes of Examples 1 to 7 were confirmed to have excellent resistance and puncture strength as battery separators while shutting down at low temperatures, demonstrating that they combine excellent mechanical strength, resistance, and shutdown properties. Furthermore, battery performance evaluation confirmed that the polyolefin microporous membranes of Examples 1 to 7 also combined foreign matter resistance, overcharge resistance, and rate performance, confirming that when used as battery separators, they offer excellent safety and output properties.

[0203] On the other hand, the polyolefin microporous membranes of Comparative Examples 1 to 5 had poorer physical properties as battery separators, indicating that they did not simultaneously achieve mechanical strength, resistance, and shutdown properties. Furthermore, the polyolefin microporous membranes of Comparative Examples 1 to 5 had poorer battery properties, confirming that they did not simultaneously achieve each property.

[0204] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. [Industrial Applicability]

[0205] The polyolefin microporous membrane of the present invention has excellent mechanical strength and resistance, as well as excellent shutdown characteristics, and when used as a battery separator, it exhibits excellent safety and output characteristics, and is therefore suitable for use as a separator for secondary batteries, which require high battery capacity and thin film thickness.

Claims

1. A polyolefin microporous membrane, wherein in the molecular weight distribution of the polyethylene component of said polyolefin microporous membrane, the proportion of polyethylene with a molecular weight of 3.5 x 10 5 or less is less than 50 mass% and the proportion of polyethylene with a molecular weight of 9.0 x 10 5 or more is 30 mass% or more, the in-plane average degree of molecular orientation measured by Raman spectroscopy is 2.9 or more, the difference ΔTm (= Tm 1 - Tm 2 ) between the peak top temperature Tm 1 on the melting curve obtained during the first heating cycle and the peak top temperature Tm 2 on the melting curve obtained during the second heating cycle measured by differential scanning calorimetry (DSC) is greater than -1.0°C and less than 2.0°C, and the proportion of the area of ​​140°C or higher in the melting peak is 5% or more and 40% or less.

2. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has an intrinsic viscosity of 3.0 dL / g or more.

3. The peak top temperature Tm on the melting curve obtained during the second heating cycle using a differential scanning calorimeter (DSC) ２ The polyolefin microporous membrane according to claim 1 or claim 2, wherein the melting point is 137.5°C or less.

4. The polyolefin microporous membrane according to any one of claims 1 to 3, which has an average pore size of 15 nm or more and a bubble point pore size of 20 nm or more, as measured with a perm porometer according to JIS K 3832-1990.

5. 100 cm measured by the Oken type testing machine method of JIS P-8117:2009 ３ The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the air resistance when air passes through the membrane is 200 seconds or less when converted into a thickness of 5 µm.

6. The Tm １ The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the temperature is 130°C or higher and 140°C or lower.

7. The peak top temperature Tm on the melting curve obtained during the second heating cycle measured by a differential scanning calorimeter (DSC) in the presence of liquid paraffin Ｌ The polyolefin microporous membrane according to any one of claims 1 to 6, wherein the melting point is 124°C or less.

8. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 7.

9. A secondary battery using the battery separator according to claim 8.

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