Polyolefin microporous membrane, battery separator, and lithium ion secondary battery

Abstract

The puncture strength converted to basis weight is 1.18N / (g / m 2 ) or more 1.96N / (g / m 2 ) or less, and in a differential scanning calorimetry (DSC) curve detected in the first heating run in DSC measurement, the ratio H(T1) / H(T2), where H(T1) is the first peak height and H(T2) is the second peak height, determined under the following conditions, is 0.8 or more and less than 2.0. The polyolefin microporous membrane has high compression resistance and suppresses an increase in shrinkage stress in the rolling direction, resulting in excellent heat resistance.

Description

[Technical Field]

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

[0002] Polyolefin microporous membranes are used in a variety of applications, including battery separators, various filters, breathable and waterproof clothing, reverse osmosis filtration membranes, and microfiltration membranes. When used as a separator for lithium-ion batteries, their performance is deeply related to battery characteristics, productivity, and safety. In recent years, separators have been required to have high porosity in order to improve the charging speed and power output characteristics of lithium-ion batteries. However, as the porosity increases, the separator's compression resistance decreases, making it more susceptible to compression due to electrode expansion during battery charging and discharging. As a result, the separator's pore structure changes over time during repeated battery charging and discharging, resulting in a decline in power output characteristics. Therefore, improved compression resistance is essential for separators. Furthermore, improving separator compression resistance is important during the battery manufacturing process, as separators are compressed by winding, which reduces the battery's power output characteristics.

[0003] Pouch-type lithium-ion batteries are widely used in electric vehicles, where high-density and lightweight batteries are required. The battery exterior of pouch-type cells is more flexible than that of prismatic or cylindrical batteries, and the constraints in the winding direction (MD) are looser. Therefore, when the battery heats up abnormally, the separator's MD deformation causes the battery to swell and distort, resulting in insufficient insulation within the battery and potentially causing thermal runaway and fire in the lithium-ion battery. Therefore, dimensional stability at high temperatures, especially shrinkage stress in the MD, is considered an important performance factor for separators.

[0004] Patent Document 1 proposes a microporous polyolefin membrane having a maximum shrinkage stress in the longitudinal direction of 0.7 MPa or more and 2 MPa or less and a shutdown temperature of 128°C or more and 143°C or less.

[0005] In Patent Document 2, the puncture strength converted into basis weight is 80 gf / (g / m 2 ) or higher and a melting point (Tm) of 120°C or higher and 135°C or lower has been proposed.

[0006] Patent Document 3 proposes a polyolefin microporous membrane having a lower limit of pin puncture strength of 0.45 N / μm or more, a first peak melting point of more than 137°C and not more than 140°C, a second peak melting point of 145°C or more, and a second peak melting point of 0.5 or more and less than 1.0, as detected in the first heating run in differential scanning calorimetry, and when the melting endotherm of the first peak is set to 1.0, the melting endotherm of the second peak is 0.5 or more and less than 1.0.

[0007] Patent Document 4 proposes a microporous polyolefin membrane having a pin puncture strength of 1000 to 2000 gf. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2023-50126 [Patent Document 2] Japanese Patent Publication No. 2022-51238 [Patent Document 3] International Publication No. 2021 / 065283 [Patent Document 4] International Publication No. 2022 / 127224 Summary of the Invention [Problem to be solved by the invention]

[0009] Improving the mechanical strength is an effective way to improve the compression resistance of polyolefin microporous membranes, and efforts to improve mechanical strength have been made to date, such as in the aforementioned patent documents, by improving raw material composition, stretching conditions, etc. For example, adjusting membrane-forming conditions, such as low-temperature stretching or high dry stretching ratios, can improve the mechanical strength of polyolefin microporous membranes. However, these methods tend to result in high residual stress after stretching, which increases shrinkage stress. This creates a trade-off between improved compression resistance and dimensional stability at high temperatures (especially low shrinkage stress in the MD direction). Therefore, in order to achieve the required compression resistance in future lithium-ion secondary battery separators, the high shrinkage stress at high battery temperatures may prevent sufficient safety from being ensured.

[0010] In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane that has high compression resistance and excellent heat resistance, particularly in which an increase in MD shrinkage stress is suppressed. [Means for solving the problem]

[0011] In order to solve the above problems, the present invention has the following features. [I] The puncture strength converted to basis weight is 1.18N / (g / m 2 ) or more 1.96N / (g / m 2 ) or less, and in a differential scanning calorimetry (DSC) curve detected in the first heating run in a DSC measurement, the ratio H(T1) / H(T2) of the first peak height H(T1) to the second peak height H(T2), determined under the following conditions, is 0.8 or more and less than 2.0. [Measurement conditions for first peak height H(T1) and second peak height H(T2)] In a DSC curve obtained by heating from 30°C to 230°C at a rate of 10°C / min using a differential scanning calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER), the first peak height H(T1) and the second peak height H(T2) are determined by the following procedure.

[0012] (i) For the DSC curve during the first temperature rise, heat flow (mW) is plotted against temperature (°C), and the line connecting 70°C and 170°C is used as the baseline, and the baseline is subtracted from the original data. The data from which the baseline has been subtracted is approximated using a smoothing spline in the temperature range of 70°C to 170°C to obtain H(T), which represents the DSC curve as a function of temperature T. The UnivariateSpline module of Python is used for the approximation calculation, and the smoothing spline function is a quartic function with a smoothing parameter of 1. Note that the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, but since H(T) is differentiated twice to evaluate the curvature as described below, it is preferable to use a quartic or higher function for the above function. Furthermore, the error between the data points and the approximated value at each temperature is evaluated using the following formula, and H(T) is determined so that the value obtained from the following formula is in the range of 0 to 1.5. Formula: ((Y(T i )-H(T i )) 2 ) 1 / 2 ) x 100 where Y(T i ) and H(T i ) are the temperatures T i The subscript i represents each temperature in the raw data. (ii) Calculate the curvature C(T) at each temperature using the following formula: C(T)=(d 2 H / dT 2 ) / (1+(dH / dT) 2 ) 3 / 2 (iii) Within the range of 130 to 155°C, among the temperatures at which the minimum value of curvature is negative, the temperature at which the heat flow value is highest is defined as T1, and the highest temperature higher than T1 is defined as T2. The values ​​of the DSC curve at each temperature are defined as the first peak height H(T1) and the second peak height H(T2). [II] The polyolefin microporous film according to [I], which has a porosity of 37% or more after heating and pressurizing at 7.8 MPa / 70°C / 10 sec. [III] The maximum shrinkage stress in the machine direction (MD direction) of the film converted into basis weight is 0 or more and 10 mN / (g / m 2The polyolefin microporous membrane according to [I] or [II], wherein: [IV] The polyolefin microporous membrane according to any one of [I] to [III], wherein the total value of Raman orientation parameters measured under the following conditions is 75 or more, and the MD / TD ratio of the Raman orientation parameters is 0.75 or more and less than 0.92. [Raman orientation parameter measurement conditions] 〔Device〕 Measurement equipment: inVia micro-Raman spectroscopy system (Renishaw) ·Focusing conditions: 180° backscattering arrangement ·Spectral length: 250mm Diffraction grating: 3000 lines / mm Excitation laser: 532nm Lens: 50x objective lens (NA=0.75) Spot size (spatial resolution): 5 μm [Polarization conditions] The laser was incident perpendicularly to the film surface (XY plane) and polarized using a polarizer. The measurement sample was rotated to obtain Raman spectra in 24 directions up to 345° in 15° increments, with the MD being set to 0°. [Calculation of peak intensity] The Raman spectrum obtained was -1 More than 1160cm -1 Obtain a baseline by linear approximation in the following region, 1060 cm -1 and 1130cm -1 The maximum values ​​of the Raman bands are calculated as the peak intensities I1130 and I1160, respectively. [Orientation parameters] 1130cm -1 and 1060cm -1The peak intensity ratio (I1130 / I1060) is used as the orientation parameter, and the MD orientation parameter / TD orientation parameter is calculated from the 0° orientation parameter (MD orientation parameter) and the 90° orientation parameter (TD orientation parameter). In addition, the MD is set to 0° and measurements are taken in 24 directions up to 345° in 15° increments, and the sum of the orientation parameters for each direction (15°×n (1≦n≦24 (n is an integer))) is calculated as the total value of the Raman orientation parameter. [V] The polyolefin microporous membrane according to any one of [I] to [III], which has a shutdown temperature of less than 146°C as measured by a temperature-programmed air permeation resistance method. [VI] The microporous polyolefin film according to any one of [I] to [V], which has a bubble point diameter of 20 nm or more. [VII] In the differential molecular weight distribution curve obtained by gel permeation chromatography (GPC), the number of peaks determined by the following method is only one, and the weight average molecular weight Mw is 5.0 × 10 5 Over 2.0 x 10 6 and the molecular weight is 2.0 × 10 6 The polyolefin microporous film according to any one of [I] to [VI], wherein the above proportion is 5% to 20% of the total. [GPC measurement conditions and peak number detection method] Measurement equipment: Agilent high-temperature GPC PL-GPC220 Column: Agilent PL1110-6200 (20 μm 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 weight (dissolution conditions: 160°C / 3.5 hours) Injection volume: 500 μL Detector: Agilent refractive index detector (RI detector) ·Viscometer: Agilent viscosity detector Calibration curve: Create a universal calibration curve using monodisperse polystyrene standard samples (EASIVIAL PS-H, PL2014-9001, PL2013-6001). The number of peaks is determined using the differential molecular weight distribution obtained by the GPC method described above, using the following procedure. (i) Plot dw / dlogM, the concentration fraction differentiated by the logarithm of molecular weight, on the vertical axis and logM, the logarithm of molecular weight, on the horizontal axis. A smoothing spline approximation is performed on the plotted points to obtain G(logM), which represents dw / dlogM as a function of logM. The approximation calculation uses the Python UnivariateSpline module, with the smoothing spline function being a quartic function and a smoothing parameter of 1. While the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, it is preferable to use a quartic or higher function because G(logM) is differentiated twice to evaluate the curvature, as described below. Furthermore, the error between the data points and the approximated value at each temperature is evaluated using the following equation, and G(logM) is determined so that the value obtained from the following equation is in the range of 0 to 5.0. Formula: ((Y(Mi)-G(logMi)) 2 ) 1 / 2 ) x 100 where Y(Mi) and G(logMi) represent the raw data and the approximated value at molecular weight Mi, respectively, and the subscript i represents each molecular weight of the raw data. Note that w is the concentration fraction and M is the molecular weight. (ii) Calculate the curvature D(logM) for each logM using the following formula: D(T)=(d 2 G / dlogM 2 ) / (1+(dG / dlogM) 2 ) 3 / 2 (iii) In the range of logM between 3 and 7, among the logM values ​​where the minimum value of curvature D(T) is a negative value, the number of logM values ​​where curvature D(T) is -0.2 or less is counted and this is taken as the number of peaks. The molecular weight is 2.0 × 10 6 The proportions of the above components are determined by the following procedure. (i) A differential molecular weight distribution curve is obtained by plotting log(M) and dw / dlog(M) for the data obtained from GPC measurement. (ii) The molecular weight of 2.0 × 10 when the area enclosed by the obtained differential molecular weight distribution curve and the baseline (usually the horizontal axis) is taken as 100%. 6 The area ratio of the above components is calculated. The area of ​​each region is determined as the actual area from the differential molecular weight distribution curve graph. [VIII] The polyolefin microporous membrane according to any one of [I] to [VII], which has a porous layer on at least one surface of the polyolefin microporous membrane. [IX] A battery separator comprising the polyolefin microporous membrane according to any one of [I] to [VIII]. [X] A lithium ion secondary battery comprising the polyolefin microporous membrane according to any one of [I] to [VIII]. [Effects of the Invention]

[0013] The present invention provides a polyolefin microporous membrane having excellent mechanical strength and shrinkage stress properties. In particular, the polyolefin microporous membrane of the present invention is suitable for use as a battery separator. Furthermore, the polyolefin microporous membrane of the present invention is suitable for use in lithium ion secondary batteries. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, this embodiment of the present invention will be described. However, the present invention is not limited to the embodiment described below. In the present invention, the direction parallel to the film-forming direction of a polyolefin microporous membrane is referred to as the film-forming direction, longitudinal direction, or MD (Machine Direction), and the direction perpendicular to the film-forming direction in the plane of the polyolefin microporous membrane is referred to as the width direction or TD (Transverse Direction).

[0015] 1. Polyolefin microporous membrane (hereinafter simply referred to as "microporous membrane") [Puncture strength converted into basis weight] The polyolefin microporous membrane of the present invention has a puncture strength converted into basis weight of 1.18 N / (g / m 2 ) or more, and more preferably 1.27 N / (g / m 2 ) or more, and more preferably 1.37 N / (g / m 2 ) or more. The puncture strength converted into basis weight is 1.96N / (g / m 2 ) or less, and more preferably 1.86 N / (g / m 2 ) or less, and more preferably 1.76 N (g / m 2 ) is as follows.

[0016] The puncture strength converted to basis weight is 1.18N / (g / m 2 ) or less than 1.96N / (g / m 2 ), the balance between the permeability and compression resistance of the microporous membrane is compromised. Generally, the higher the porosity of a microporous membrane, the better the permeability tends to be, but the amount of resin supporting the membrane in the membrane thickness direction decreases, resulting in a decrease in compression resistance. That is, there is a trade-off between permeability and compression resistance. However, when the permeability-equivalent pin puncture strength is within the above-mentioned preferred range, the microporous membrane is mechanically stable and has good strength in the membrane thickness direction, making it less likely to be compressed even with a low resin amount, and high permeability can be maintained. A secondary battery using this polyolefin microporous membrane as a separator can maintain its pore structure even when the microporous membrane is compressed due to electrode expansion during charge and discharge, thereby achieving both charge and discharge characteristics and cycle performance. The permeability-equivalent pin puncture strength can be adjusted to a predetermined range by adjusting the membrane-forming conditions, such as the molecular weight and compounding ratio of the polyolefin, the stretching temperature, and the stretching ratio, in the production process.

[0017] [Melting endothermic peak height ratio] In the polyolefin microporous membrane of the present invention, in a differential scanning calorimetry (DSC) curve detected during the first heating run under the conditions described below, the ratio H(T1) / H(T2) of the first peak height H(T1) to the second peak height H(T2) determined under the following conditions is 0.8 or more, more preferably 0.9 or more, and even more preferably 1.0 or more. Furthermore, in the polyolefin microporous membrane of the present invention, the ratio H(T1) / H(T2) of the first peak height H(T1) to the second peak height H(T2) is less than 2.0, more preferably 1.6 or less, even more preferably 1.5 or less, and particularly preferably 1.18 or less.

[0018] The first peak height H(T1) is an index representing the most meltable component in the polyolefin microporous membrane, and the second peak height H(T2) is an index representing the high-melting-point structure formed by stretching. When the ratio of the first peak height H(T1) to the second peak height H(T2) is within the above range, it means that the polyolefin molecules are sufficiently stretched and oriented, and maintaining a highly crystalline state contributes to fibril strength, resulting in a polyolefin microporous membrane with excellent compression resistance. When the melting endothermic peak height ratio is below 0.8, stretching is insufficient, resulting in reduced compression resistance. When the melting endothermic peak height ratio is above 2.0, residual stress due to stretching is large, resulting in increased shrinkage stress and shrinkage rate when the microporous membrane is heated to high temperatures. The melting endothermic peak height ratio of the microporous membrane can be set within the above range by, for example, setting production conditions such as the molecular weight and content of ultra-high polyethylene, stretching temperature, and stretching ratio, which will be described later.

[0019] [Porosity after heating and pressurization, change in film thickness after heating and pressurization] The polyolefin microporous membrane of the present invention preferably has a porosity of 37% or more after heating and pressurization at 7.8 MPa / 70°C / 10 sec, more preferably 38% or more, and even more preferably 40% or more. The upper limit is preferably 60% or less, more preferably 50% or less.

[0020] Furthermore, the polyolefin microporous membrane of the present invention preferably has a thickness change rate after heating under pressure at 7.8 MPa / 70°C / 10 sec of 17% or less, more preferably 15% or less, and even more preferably 10% or less. When the porosity and thickness change rate after heating under pressure are within the above ranges, the pore structure can be maintained even when compressed by an external force, and when the polyolefin microporous membrane is used as a battery separator, the rate performance and cycle performance are improved. There is no particular upper limit for the porosity after heating under pressure, but it is preferably 60% or less. The porosity after heating and pressure and the rate of change in membrane thickness after heating and pressure can be set within the above ranges by appropriately selecting the raw materials and compositions of the microporous membrane and the membrane-forming conditions.

[0021] [Maximum shrinkage stress in MD direction converted into basis weight] The polyolefin microporous membrane of the present invention has a maximum shrinkage stress in the MD direction converted into basis weight of 0 mN / (g / m 2 ) or more 10mN / (g / m 2 ) or less, and more preferably 0 mN / (g / m 2 ) or more 9mN / (g / m 2 ) or less, and more preferably 0 mN / (g / m 2 ) or more 8mN / (g / m 2 ) is as follows.

[0022] When the maximum MD shrinkage stress converted into basis weight of the microporous membrane is within the above range, the shrinkage force at high temperatures is suppressed, and a secondary battery using this polyolefin microporous membrane as a separator has a reduced possibility of thermal runaway due to abnormal heat generation in the battery, improving battery safety. The maximum MD shrinkage stress converted into basis weight can be kept within a predetermined range by adjusting the film-forming conditions, such as the molecular weight and blending ratio of the polyolefin, the stretching temperature, and the stretching ratio, in the production process.

[0023] [Raman orientation parameters] The polyolefin microporous membrane of the present invention preferably has a total value of Raman orientation parameters measured by the method described below of 75 or more, more preferably 78 or more, and even more preferably 80 or more. While there are no particular upper limits, from the viewpoint of film formability, it is preferably 120 or less, more preferably 100 or less. Furthermore, the ratio of the Raman orientation parameters in the longitudinal direction (MD) to the transverse direction (TD) is preferably 0.75 or more, more preferably 0.80 or more, and even more preferably 0.83 or more. The upper limit of the ratio of the Raman orientation parameters in MD to TD is preferably 0.92 or less, and even more preferably 0.87 or less. The Raman orientation parameter can be adjusted within a predetermined range by adjusting the film-forming conditions, such as the molecular weight and compounding ratio of the polyolefin, the stretching temperature, and the stretching ratio, in the production process.

[0024] [Shutdown temperature] The polyolefin microporous membrane of the present invention preferably has a shutdown temperature of less than 146°C as measured by the temperature-programmed air resistance method. When the shutdown temperature is within the above preferred range, when the polyolefin microporous membrane is used as a battery separator, it quickly shuts off current when the battery is placed at high temperatures, resulting in excellent heat resistance. The shutdown temperature can be adjusted to a predetermined range by adjusting the membrane-forming conditions, such as the molecular weight and blending ratio of the polyolefin, the stretching temperature, and the stretching ratio, in the production process. While the lower limit is not particularly limited, from the viewpoint of membrane formability, it is preferably 100°C or higher, and more preferably 140°C or higher.

[0025] [Bubble point diameter] The polyolefin microporous membrane of the present invention preferably has a bubble point diameter of 20 nm or more, more preferably 25 nm or more, as measured by the method described below. While there are no particular upper limits, from the viewpoint of membrane formability, it is preferably 70 nm or less, more preferably 50 nm or less. When the bubble point diameter is within the above range, the membrane has excellent ion permeability, and sufficient output characteristics can be obtained when used as a battery separator.

[0026] The Raman orientation parameter can be adjusted within a predetermined range by adjusting the film-forming conditions, such as the molecular weight and compounding ratio of the polyolefin, the stretching temperature, and the stretching ratio, in the production process.

[0027] [Weight average molecular weight, molecular weight 2.0×10 6 or more percentage] The polyolefin constituting the polyolefin microporous membrane of the present invention has a weight average molecular weight (Mw) determined by gel permeation chromatography (GPC) of 5.0 × 10, from the viewpoint of facilitating control of the basis weight-equivalent pin puncture strength, H(T1) / H(T2) value, porosity after pressurization and heating, membrane thickness change rate after pressurization and heating, basis weight-equivalent MD maximum shrinkage stress, Raman orientation parameter, shutdown temperature, and bubble point diameter. 5 ~2.0×10 6 The range is preferably 5.6×10 5 ~1.0×10 6 In addition, it is more preferable that the differential molecular weight distribution curve obtained by the GPC method has only one peak and the molecular weight is within the range of 2.0 × 10 6 The above proportion is preferably 5% to 20% of the total, and more preferably 7% to 15%.

[0028] By setting the weight-average molecular weight of the polyolefin constituting the polyolefin microporous membrane within the above range, it becomes possible to increase the pin puncture strength per unit area and easily set the value of H(T1) / H(T2) within a predetermined range. The weight-average molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by GPC under the conditions described below.

[0029] The polyolefin microporous membrane of the present invention is preferably produced at a combined stretching ratio of 125 times or more, more preferably 150 times or more, and even more preferably 175 times or more, comprising the first and second stretching steps in the production process described below. Generally, increasing the stretching ratio increases pin puncture strength, but the H(T1) / H(T2) ratio also increases. However, in the present invention, it has been discovered that wet stretching at a ratio exceeding a normal stretching ratio results in a high degree of orientation of the polyolefin molecules, thereby improving pin puncture strength while keeping the H(T1) / H(T2) ratio within a predetermined range. When the stretching ratio is below the above range, the oriented structure does not develop, making it impossible to obtain sufficient pin puncture strength. Furthermore, the H(T1) / H(T2) ratio increases along with the pin puncture strength per unit area. However, by setting the stretching ratio within the above range, the oriented structure develops, sufficient pin puncture strength is obtained, and an increase in H(T1) / H(T2) can be prevented. There is no particular upper limit to the stretching ratio, but it is preferably 400 times or less in order to stably form a film without rupture.

[0030] The polyolefin microporous membrane of the present invention preferably has a membrane thickness of 1 μm or more and 30 μm or less, more preferably 3 μm or more and 20 μm or less.

[0031] The polyolefin microporous membrane of the present invention has a basis weight of 1 g / m 2 More than 20g / m 2 Preferably, it is 3 g / m or less. 2 More than 15g / m 2 More preferably, it is:

[0032] The polyolefin microporous membrane of the present invention preferably has a porosity of 20% or more and 70% or less, more preferably 30% or more and 60% or less.

[0033] The polyolefin microporous membrane of the present invention has an air resistance of 400 sec / 100 cm 2 It is preferable that the speed is less than 300 sec / 100 cm. 2The lower limit is not particularly limited, but from the viewpoint of preventing overcurrent, it is preferably 50 sec / 100 cm 2 When the air resistance is in the above range, good ion permeability can be obtained.

[0034] (Method for producing a polyolefin microporous membrane) The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or a multi-layer microporous membrane consisting of multiple layers. The polyolefin resin composition in the single layer form will be described below.

[0035] (1) Polyolefin resin composition The polyolefin resin composition may contain polyethylene. (polyethylene) The polyethylene has a weight-average molecular weight (Mw) of 5.0 × 10 to facilitate control of the strength and stretchability of the microporous membrane. 5 Over 2.0 x 10 6 Preferably, the melting point of the polyethylene is less than 128°C and less than 137°C. The polyethylene may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene. Preferred α-olefin copolymers other than ethylene include propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of the α-olefin other than ethylene is preferably 5 mol% or less, assuming the α-olefin copolymer to be 100 mol%.

[0036] The polyolefin resin composition may contain other resin components in addition to the polyethylene as needed. Examples of such other resin components include resins that further impart heat resistance. Furthermore, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be added within the range that does not impair the effects of the present invention.

[0037] In this embodiment, a microporous membrane may be formed by laminating a porous layer on at least one side of the polyolefin microporous membrane. The porous layer is not particularly limited, but for example, a porous layer made of a 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, and magnesium hydroxide. Examples include sodium, magnesium carbonate, and silicon.

[0038] (2) Method for producing a polyolefin microporous membrane The method for producing the polyolefin microporous membrane of the present invention is not particularly limited, but preferably includes the following steps, each of which will be described in detail below. (a) Preparation of the solution (b) Forming of gel-like sheet (c) First extension (d) Second extension (e) Removal of plasticizer and drying (f) Third Extension (g) Heat treatment (a) Preparation of the solution A plasticizer is added to a polyolefin resin composition in a twin-screw extruder, and the mixture is melt-kneaded to prepare a solution. The polyolefin resin composition preferably contains 10% by mass or more and 30% by weight or less of 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 outlet can be prevented when the polyolefin solution is extruded, resulting in good moldability and appearance of the extrusion molded product. The solution is fed from the extruder to a die and extruded into a sheet to obtain an extrusion molded product. The extrusion method may be either a flat die method or an inflation method. The flat die gap is 0.1 mm or more and 5 m The extrusion temperature is preferably 140° C. or higher and lower than 240° C., and the extrusion speed is preferably 0.2 to 15 m / min. (b) Forming of gel-like sheet The resulting extrusion is cooled to form a gel-like sheet. Cooling methods include contacting the extrusion with a refrigerant such as cold air or cooling water, or contacting a cooling roll, but contacting a roll cooled with a refrigerant is preferred. Cooling is preferably carried out at a rate of 50°C / min or more to at least the gelation temperature. Cooling is preferably carried out to 25°C or below. A cooling rate within the above range maintains the crystallinity within an appropriate range, resulting in a gel-like sheet suitable for stretching.

[0039] (c) First extension Next, the gel-like sheet is stretched. After preheating, the gel-like sheet is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial or biaxial. The stretching ratio (area stretching ratio) is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretching ratios in MD and TD may be the same or different, and the stretching ratios in both MD and TD are preferably 3 times or more. The first stretching temperature is preferably 110°C or higher and 130°C or lower.

[0040] (d) Second extension The second stretching is preferably performed by preheating followed by stretching at a predetermined ratio using a tenter method, a roll method, an inflation method, or a combination of these. The stretching may be uniaxial or biaxial. The preheating and stretching temperatures are preferably 110°C or higher and 130°C or lower. The stretching ratio is preferably 5 times or higher, more preferably 6 times or higher, and even more preferably 7 times or higher. The stretching ratios in MD and TD may be the same or different.

[0041] The polyolefin microporous membrane of the present invention is preferably prepared by a stretching ratio of 125 times or more, more preferably 150 times or more, and even more preferably 175 times or more, which is the total stretching ratio of the first stretching and the second stretching in the production process described below. Generally, increasing the stretching ratio increases the pin puncture strength, but the H(T1) / H(T2) ratio also increases. However, in the present invention, it has been discovered that by wet-stretching at a ratio exceeding the conventional stretching ratio to highly orient the polyolefin molecules, it is possible to improve the pin puncture strength while keeping the H(T1) / H(T2) ratio within a predetermined range. If the stretching ratio is below the above range, the oriented structure does not develop, making it impossible to obtain sufficient pin puncture strength. Furthermore, the H(T1) / H(T2) ratio tends to increase along with the pin puncture strength converted to basis weight. By setting the stretching ratio within the above range, it is possible to prevent an increase in H(T1) / H(T2) while developing an oriented structure and obtaining sufficient pin puncture strength. There is no particular upper limit to the stretching ratio, but it is preferably 400 times or less in order to stably form a film without rupture.

[0042] (e) Removal of plasticizers Next, the gel-like sheet is dried and the plasticizer contained therein is removed using a washing solvent. Washing solvents and methods for removing plasticizers using them are well known, so a detailed description is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and JP-A-2002-256099 can be used. After the plasticizer is removed, the sheet is dried by heat drying or air drying. Any method capable of removing the washing solvent may be used, including conventional methods such as heat drying and air drying (moving air). (f) Third Extension The film from which the plasticizer has been removed may then be subjected to a third stretching. The third stretching may be uniaxial or biaxial, and the biaxial stretching may be either simultaneous or sequential biaxial stretching. The stretching ratio is preferably 1 to 10 times, more preferably 1.2 to 7 times, and even more preferably 1.2 to 5 times. The stretching ratios in MD and TD may be the same or different. The preheating and stretching temperatures for the second stretching are preferably 100°C to 145°C, more preferably 105°C to 140°C. In the case of sequential stretching, the stretching temperatures in MD and TD may be different.

[0043] (g) Heat treatment After the plasticizer is removed, the film is heat-treated while being held with clips to fix its width. The heat treatment is preferably performed at a temperature of 115.0°C to 140°C. By setting the heat treatment temperature within this range, the heat shrinkage of the polyolefin microporous membrane can be reduced.

[0044] (h) Formation of porous layer The laminated polyolefin microporous membrane of the present invention preferably further comprises one or more porous layers on at least one surface of the polyolefin microporous membrane of the present invention.

[0045] The porous layer is not particularly limited, but may be made porous by laminating, for example, a coating layer containing a binder component and inorganic particles. As described below, the binder resin shrinks and becomes porous by drying and extracting the solvent of the binder component. The binder component is not particularly limited, and known resins can be used, such as acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, polyimide resin, and polyvinyl alcohol. These resins can be dissolved or diluted with a solvent such as an organic solvent or water. The inorganic particles are not particularly limited. There is no particular restriction on the material, and known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

[0046] The method for forming the porous layer is not particularly limited, and for example, the porous layer can be obtained by the following steps. (i) A step of preparing a coating dispersion for a porous layer using a binder resin, inorganic particles, and a solvent. (ii) A step of coating at least one surface or both surfaces of a polyolefin microporous membrane with the porous layer coating dispersion. (iii) A step of drying the solvent to form a porous layer.

[0047] The step (iii) may be replaced by a step (iv) of forming a porous layer using an organic solvent in which the solvent dissolves but the binder resin does not, and then removing the organic solvent. The battery separator of the present invention preferably includes the polyolefin microporous membrane of the present invention. The polyolefin microporous membrane of the present invention is preferably used as a separator for secondary batteries such as lithium ion secondary batteries, lithium polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, and silver-zinc secondary batteries, and is particularly preferably used as a separator for lithium ion secondary batteries. Known electrodes and electrolytes can be used in lithium ion secondary batteries using a separator made of the polyolefin microporous membrane of the present invention. Furthermore, known structures can be used in lithium ion secondary batteries using a separator made of the polyolefin microporous membrane of the present invention. [Example]

[0048] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0049] [Measurement method] (1) Film Thickness The thickness of the polyolefin microporous membrane was measured at five points within a 95 mm × 95 mm area: the upper left, upper right, center, lower left, and lower right using a contact thickness meter (a Litematic manufactured by Mitutoyo Corporation, contact pressure 0.01 N, 10.5 mmφ probe used), and the average value was taken as the thickness (μm). If the sample size cannot be made into a 95 mm × 95 mm size, it is also possible to cut out a sample of any size and measure the five points: the upper left, upper right, center, lower left, and lower right.

[0050] (2) Metsuke The polyolefin microporous membrane was cut into 5 cm squares, and the weight of each piece was measured using a precision balance (5 significant digits (0.0000 g)). 2 If the sample size cannot be made 5cm x 5cm, it may be cut to any size and the measured weight may be divided by the area.

[0051] (3) Porosity The polyolefin microporous membrane was cut into a size of 95 mm x 95 mm, and its volume (cm 3 ) and weight (g), and compare them with the film density (g / cm 3 ) and the porosity (%) was calculated using the following formula: If the sample size cannot be made 95 mm x 95 mm, it is also possible to cut out a piece of any size and determine its volume and weight. Formula: Porosity = ((volume (cm 3 )-Weight (g) / Membrane density (g / cm 3 )) / volume(cm 3 ))×100 Here, the film density is 0.99 g / cm 3 The film thickness measured in (1) above was used to calculate the volume.

[0052] (4) Air resistance The air resistance (sec / 100cm) of the polyolefin microporous membrane was measured using an air resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117:2009. 3 ) was measured.

[0053] The 9 μm equivalent air resistance was calculated using the following formula. Formula: 9μm equivalent air resistance = air resistance (sec / 100cm 3 )×9 / film thickness (μm). (5) Puncture strength converted into basis weight Using a needle with a diameter of 1 mm (tip 0.5 mmR), the weight W (g / m 2 The maximum load S (N) when the polyolefin microporous membrane of 100 mm was punctured was measured, and the puncture strength converted into basis weight was calculated using the following formula. Formula: Piercing strength converted to basis weight = S(N) / W(g / m 2 ).

[0054] (6) Endothermic peak temperature and height in DSC measurements The endothermic peak temperature and height in DSC measurements of polyolefin resin or polyolefin microporous membrane were determined using a differential scanning calorimeter (PARKING ELMER PYRIS DIAMOND DSC). A reference pan and a pan containing polyolefin resin or polyolefin microporous membrane were placed in left and right holders, respectively, and the temperature was raised from 30°C to 230°C at a rate of 10°C / min, then held at 230°C for 3 minutes, and cooled to 30°C at a rate of 10°C / min. After the temperature increase and decrease, the polyolefin resin was again heated from 30°C to 230°C at a rate of 10°C / min. The DSC curve of the sample was determined from the difference in the calorimetric curves of the pan containing the sample and the reference pan. The melting point of the polyolefin resin and the endothermic peak temperature and height of the polyolefin microporous membrane were then determined using the following procedure.

[0055] [Melting point of polyolefin resin] The melting point of the polyolefin resin was determined as the temperature at which the endothermic DSC curve reached its maximum value during the second temperature rise.

[0056] [First peak height H(T1) and second peak height H(T2) of the polyolefin microporous membrane] The first peak height H(T1) and the second peak height H(T2) were determined by the following procedure.

[0057] (i) For the DSC curve during the first temperature rise, heat flow (mW) is plotted against temperature (°C), and the line connecting 70°C and 170°C is used as the baseline, and the baseline is subtracted from the original data. The data from which the baseline has been subtracted is approximated using a smoothing spline in the temperature range of 70°C to 170°C to obtain H(T), which represents the DSC curve as a function of temperature T. The UnivariateSpline module of Python is used for the approximation calculation, and the smoothing spline function is a quintic function with a smoothing parameter of 1. Note that the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, but since H(T) is differentiated twice to evaluate the curvature as described below, it is preferable to use a quartic or higher function. Furthermore, the error between the data points and the approximated value at each temperature is evaluated using the following formula, and H(T) is determined so that the value obtained from the following formula is in the range of 0 to 1.5. Formula: ((Y(T i )-H(T i )) 2 ) 1 / 2 ) x 100 where Y(T i ) and H(T i ) are the temperatures T i The subscript i represents each temperature in the raw data.

[0058] (ii) Calculate the curvature C(T) at each temperature using the following formula: Formula:C(T)=(d 2 H / dT 2 ) / (1+(dH / dT) 2 ) 3 / 2 (iii) Among the temperatures in the range of 130 to 155°C where the curvature minimum is a negative value, the temperature at which the heat flow value is highest is designated as T1, and the highest temperature higher than T1 is designated as T2, and the values ​​of the DSC curve at these temperatures are designated as the first peak height H(T1) and the second peak height H(T2). (If there is only one temperature in the range of 130 to 155°C where the curvature minimum is a negative value, there are no values ​​for T2 and H(T2), and these are indicated as "-" in the table.) (7) Compression resistance evaluation (film thickness change rate after heating and pressurization (%), porosity rate after heating and pressurization (%)) A 50mm x 50mm (MD x TD) sample was cut from the microporous membrane. The cut sample was sandwiched between 50µm thick PET films and placed on two sheets of A4 paper to form a test specimen. A 40mm x 40mm x 10mm thick bakelite plate was preheated at 70°C for at least 30 minutes using a compression device (CYPT-20 special, manufactured by Shinto Kogyo Co., Ltd.). The test specimen was then inserted into the compression device, and the preheated bakelite plate was placed on top of the microporous membrane. The test specimen was then heated and compressed at 70°C under a pressure of 7.8MPa for 10 seconds. The heat compression was released, the sample was removed, and the polyolefin multilayer, microporous membrane was left at room temperature for 3 hours. The membrane thickness was measured at a total of 9 points (3 vertical points × 3 horizontal points) on the four sides of the part where the bakelite had been placed, 4 points near the corners, and 1 point in the center, and the average membrane thickness after release from pressure and heat was calculated. The membrane thickness change rate after heat compression was calculated using the following formula 1, and the porosity after heat compression was calculated using the following formula 2.

[0059] Formula 1: Film thickness change rate after pressure and heating = [(average film thickness before pressure and heating (μm) - average film thickness after pressure and heating (μm)) / (average film thickness before pressure and heating (μm))] × 100 Here, the average thickness before compression was the thickness measured in (1) above. Equation 2: Porosity after heating and pressurization (%) = (Volume after heating and compression (cm 3 ) - Weight after heat compression (g) / Film density (g / cm 3 )) / Volume after heating and compression (cm 3 ))×100 Here, the film density is 0.99 g / cm 3 The volume was calculated using the average film thickness measured above after the pressure and heating were released. From the obtained porosity after heating and pressure, the compression resistance was evaluated as follows. Porosity after heating and pressurization is 40% or more: A After heating and pressurization, the porosity is not included in A, but is 38% or more and less than 40%: B After heating and pressurization, the porosity is not included in A or B, and is 37% or more but less than 38%: C After heating under pressure, the porosity is not included in A, B, or C: D.

[0060] (8) Shrinkage stress evaluation (maximum shrinkage stress in MD direction converted into basis weight) The specimen was cut into 15mm x 3mm rectangles in the MD and TD measurement directions to prepare evaluation samples. Using a Hitachi High-Technologies Corporation "TMA7100," the evaluation sample was fixed to the chuck so that the chuck distance was 10mm, and the temperature was raised from 30°C to 200°C at a rate of 5°C / min in fixed length mode. The temperature and shrinkage stress were measured at 1-second intervals as the temperature rose to 200°C, and the maximum value of the shrinkage stress was taken as the maximum thermal shrinkage stress O (mN). The maximum shrinkage stress in the MD direction, calculated as a basis weight, was calculated using the following formula: Formula: Maximum shrinkage stress in MD direction converted into basis weight = O (mN) / W (g / m 2 ). From the obtained maximum shrinkage stress value in the MD direction converted into basis weight, shrinkage stress resistance was evaluated as follows. The maximum shrinkage stress in the MD direction converted to basis weight is 8 (mN) / (g / m 2 ) or less:A The maximum shrinkage stress in the MD direction converted to basis weight is 8 (mN) / W (g / m 2 ) over 9 (mN) / (g / m 2 ) or less: B The maximum shrinkage stress in the MD direction converted to basis weight is 9 (mN) / W (g / m 2 ) over 10(mN) / (g / m 2 ) and below:C. The maximum shrinkage stress in the MD direction converted to basis weight is 10(mN) / (g / m 2 ) :D.

[0061] (9) Raman orientation parameters 〔Device〕 Measurement equipment: inVia micro-Raman spectroscopy system (Renishaw) ·Focusing conditions: 180° backscattering arrangement ·Spectral length: 250mm Diffraction grating: 3000 lines / mm Excitation laser: 532nm Lens: 50x objective lens (NA=0.75) Spot size (spatial resolution): 5 μm (polarization conditions) The laser was incident perpendicularly to the film surface (XY plane) and polarized using a polarizer. The sample was rotated to obtain Raman spectra in 24 directions, from MD (0°) to 345° in 15° increments. [Calculation of peak intensity] The Raman spectrum obtained was -1 More than 1160cm -1 Obtain a baseline by linear approximation in the following region, 1060 cm -1 and 1130cm -1 The maximum values ​​of the Raman bands were determined as peak intensities I1130 and I1160, respectively. [Orientation parameters] 1130cm -1 and 1060cm -1 The peak intensity ratio (I1130 / I1060) was used as the orientation parameter, and the MD orientation parameter / TD orientation parameter was calculated from the 0° orientation parameter (MD orientation parameter) and the 90° orientation parameter (TD orientation parameter). In addition, the MD was set to 0° and measurements were taken in 24 directions up to 345° in 15° increments, and the sum of the orientation parameters for each direction (15° × n (1≦n≦24 (n is an integer))) was calculated as the total value of the Raman orientation parameter.

[0062] (10) Shutdown temperature (temperature-rising air permeability resistance method) A polyolefin microporous membrane punched into a 45 mm diameter circle was exposed to an atmosphere of 20°C, and the air resistance was measured while the temperature was raised at a rate of 5°C / min. When the air resistance reached 100,000 sec / 100cm 3 The temperature at which the temperature reached was defined as the shutdown temperature, and the average value of two measurements was used. The air resistance was measured using an air resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P8117:2009.

[0063] (11) Meltdown temperature (temperature-rising air resistance method) A polyolefin microporous membrane punched into a 45 mm diameter circle was exposed to a 20°C atmosphere and the air resistance was measured while the temperature was raised at a rate of 5°C / min. The air resistance was measured to be 100,000 sec / 100cm.3 Continue to increase the temperature even after reaching the air resistance of 100,000 sec / 100cm 3 The temperature at which the temperature becomes lower than this value was defined as the meltdown temperature. The air resistance was measured in accordance with JIS P8117:2009 using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T).

[0064] (12) Mean flow diameter, bubble point diameter The mean flow diameter and bubble point diameter of the polyolefin microporous membrane were determined using a mean flow diameter perm porometer (PMI CFP-1500A). Galwick (surface tension: 15.9 dynes / cm) was used as the impregnation liquid for the polyolefin microporous membrane, and measurements were performed in the dry-up and wet-up stages. The mean flow diameter (nm) was measured according to ASTM E1294-89 (1999) (half-dry method), and the pore size was calculated from the pressure (kPa) at the point where the curve representing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The bubble point diameter (nm) was calculated from the bubble point pressure (kPa) measured according to the bubble point method (JIS K3832 (1990)). The following equation was used to convert pressure to pore size for both the mean flow diameter and bubble point diameter: Formula: d=C·γ / P (In the above formula, "d (nm)" is the mean flow diameter or bubble point diameter of the microporous membrane, "γ (dynes / cm)" is the surface tension of the impregnation liquid, "P (kPa)" is the pressure determined by the half-dry method or the bubble point method, and "C" is a constant set to 2860.) (13) Weight average molecular weight (Mw) in differential molecular weight distribution, number of peaks, molecular weight 2.0 × 10 6 The proportion of the above ingredients The weight average molecular weight (Mw) of the polyolefin resin or the polyolefin microporous membrane was determined by gel permeation chromatography (GPC) under the following conditions. ·Measuring device: AMR high temperature GPC device PL-GPC220 Column: PL1110-6200, PLgel 20um MIXED-A 7.5 x 7300mm Column temperature: 160°C (heating rate: 0.5°C / min) Solvent (mobile phase): 1,2,4-Trichlorobenzene (TCB) Sigma-Aldrich 256412-2L for HPLC Additive (antioxidant): 4,4'-Thiobis (6-tert-butyl-methyl-cresol), add 250 mg of additive to TCB (2 L). Solvent flow rate: 1 ml / min Sample concentration: 0.067mg / ml (dissolution conditions: 160℃ / 30min) Injection volume: 500 μl Detector: Agilent 1260 Infinity 2 Multi-Detector GPC / SEC System (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. [Number of peaks] The number of peaks is determined by the following procedure for the differential molecular weight distribution obtained by the above GPC method. (i) Plot dw / dlogM, the concentration fraction differentiated by the logarithm of molecular weight, on the vertical axis and logM, the logarithm of molecular weight, on the horizontal axis. A smoothing spline approximation is performed on the plotted points to obtain G(logM), which represents dw / dlogM as a function of logM. The approximation calculation uses the Python UnivariateSpline module, with the smoothing spline function being a quartic function and a smoothing parameter of 1. While the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, it is preferable to use a quartic or higher function because G(logM) is differentiated twice to evaluate the curvature, as described below. Furthermore, the error between the data points and the approximated value at each temperature is evaluated using the following equation, and G(logM) is determined so that the value obtained from the following equation is in the range of 0 to 5.0. Formula: ((Y(Mi)-G(logMi))2)1 / 2)×100 where Y(Mi) and G(logMi) represent the raw data and the approximated value at molecular weight Mi, respectively, and the subscript i represents each molecular weight of the raw data. Note that w is the concentration fraction and M is the molecular weight. (ii) Calculate the curvature D(logM) for each logM using the following formula: Formula: D(T)=(d 2 G / dlogM 2 ) / (1+(dG / dlogM) 2 ) 3 / 2 (iii) In the range of logM between 3 and 7, among the logM values ​​where the minimum value of curvature D(T) is a negative value, the number of logM values ​​where curvature D(T) is -0.2 or less is counted and this is taken as the number of peaks.

[0065] The molecular weight is 2.0 × 10 6 The proportions of the above components are determined by the following procedure. (i) A differential molecular weight distribution curve is obtained by plotting log(M) and dw / dlog(M) for the data obtained from GPC measurement. (ii) The molecular weight of 2.0 × 10 when the area enclosed by the obtained differential molecular weight distribution curve and the baseline (usually the horizontal axis) is taken as 100%. 6 The area ratio of the above components is calculated. The area of ​​each region is determined as the actual area from the differential molecular weight distribution curve graph.

[0066] [Example 1] (1) Preparation of polyolefin solution Weight average molecular weight Mw is 1.0 x 10 6 Polyethylene having a melting point of 135.0°C was melt-kneaded with liquid paraffin in a twin-screw extruder to give a resin concentration of 20% by mass, to prepare a polyolefin solution. (2) Forming gel-like sheets The polyolefin solution was fed from a twin-screw extruder to a T-die and extruded. The extruded product was cooled while being taken up by a cooling roll controlled at 15°C, to form a gel-like sheet. (3) First Extension The gel-like sheet was simultaneously stretched at 115°C by 5.0 times in the MD and 5.0 times in the TD (area stretching ratio: 25 times) using a clip stretching machine. (4) Second stretching, removal of membrane-forming solvent, and drying The film after the first stretching was simultaneously stretched 2.45 times in the MD and 2.45 times in the TD (area stretching ratio: 6 times) at 115°C using a clip stretching machine (total areal stretching ratio including the first stretching: 150 times). After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried to obtain a dried microporous membrane. (5) Heat treatment The dried microporous membrane was heat-set at 127° C. to obtain a microporous polyolefin membrane. The conditions (1) to (5) above are shown in Table 1.

[0067] [Examples 2 to 6, Comparative Examples 1 to 6] Stretching was carried out in the same manner as in Example 1, except that the raw materials, stretching, and heat setting conditions were as shown in the table, to obtain a microporous polyolefin membrane.

[0068] [Comparative Examples 7 to 8] In the first stretching, the gel-like sheet was simultaneously stretched 10 times in MD and 10 times in TD (area stretching ratio 100 times) at 115°C using a clip stretching machine in the same manner as in Example 1, except that the raw material, stretching, and heat setting conditions were as shown in the table, to obtain a polyolefin microporous membrane.

[0069] Comparative Example 9 (1) Preparation of polyolefin solution Weight average molecular weight Mw is 2.4 x 10 6 Polyethylene having a melting point of 133.0°C was melt-kneaded with liquid paraffin in a twin-screw extruder to give a resin concentration of 10% by mass, to prepare a polyolefin solution. (2) Forming gel-like sheets The polyolefin solution was fed from a twin-screw extruder to a T-die and extruded. The extruded product was cooled while being taken up by a cooling roll controlled at 15°C, to form a gel-like sheet. (3) First Extension The gel-like sheet was simultaneously stretched at 115°C by 5.0 times in the MD and 5.0 times in the TD (area stretching ratio: 25 times) using a clip stretching machine. (4) Second stretching, removal of membrane-forming solvent, and drying After the first stretching, the film was immersed in a methylene chloride bath without being subjected to a second stretching to remove the liquid paraffin, and then dried to obtain a dried microporous membrane. (5) Third Extension The dried microporous membrane was sequentially stretched at 130°C by 2.0 times in the MD and 3.0 times in the TD (area stretching ratio 6.0 times) using a clip stretching machine. (6) Heat treatment The microporous membrane after the third stretching was heat set at 130°C to obtain a microporous polyolefin membrane.

[0070] [Comparative Example 10] The film was stretched in the same manner as in Comparative Example 9, except that the third stretching was performed by sequentially stretching the film to 1.75 times in MD and 1.75 times in TD (area stretching ratio: 3.06 times), to obtain a microporous polyolefin film.

[0071] [result] Measurement results of the physical properties of the obtained polyolefin microporous membranes are shown in Table 2. The polyolefin microporous membranes obtained in the Examples had higher pin puncture strengths converted into basis weight compared to the Comparative Examples, and the ratio H(T1) / H(T2) of the first peak height H(T1) to the second peak height H(T2) obtained by DSC measurement was also kept within a predetermined range.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] [Table 4]

[0076] In the table, PE stands for polyethylene. [Industrial Applicability]

[0077] When used as a battery separator, the polyolefin microporous membrane of the present invention can provide a polyolefin microporous membrane that can maintain a safe state even when the battery is subjected to an external impact or when the internal temperature of the battery rises due to heat generation.Taking advantage of its properties, the battery separator using the polyolefin microporous membrane of the present invention can be suitably used in lithium-ion batteries for electric vehicles and consumer applications.

Claims

1. The puncture strength converted to basis weight is 1.18 N / (g / m ２ ) or more 1.96N / (g / m ２ ) or less, and in a differential scanning calorimetry (DSC) curve detected in the first heating run, the ratio H(T1) / H(T2) of a first peak height H(T1) to a second peak height H(T2), determined under the following conditions, is 0.8 or more and less than 2.

0. [Measurement conditions for first peak height H(T1) and second peak height H(T2)] In a DSC curve obtained by heating from 30°C to 230°C at a rate of 10°C / min using a differential scanning calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER), the first peak height H(T1) and the second peak height H(T2) are determined by the following procedure. (i) Heat flow (mW) is plotted against temperature (°C) for the DSC curve during the first temperature rise, and the straight line connecting 70°C and 170°C is used as the baseline, and the baseline is subtracted from the original data. The data from which the baseline has been subtracted is approximated using a smoothing spline in the temperature range of 70°C to 170°C, to obtain H(T), which represents the DSC curve as a function of temperature T. The approximation calculation uses Python's UnivariateSpline module, and the smoothing spline function is a quartic function with a smoothing parameter of 1. Note that the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, but since H(T) is differentiated twice to evaluate the curvature as described below, it is preferable to use a quartic or higher function. Furthermore, the error between the data points and the approximate value at each temperature is evaluated using the following equation, and H(T) is determined so that the value obtained from the following equation is in the range of 0 to 1.

5. Formula: ((Y(T ｉ )) - H(T ｉ )) ２ )) １／２ )) × 100 Here, Y(T ｉ ) and H(T ｉ ) are the temperatures T ｉ The subscript i represents each temperature of the raw data. (ii) The curvature C(T) at each temperature is calculated using the following formula: C(T)=(d ２ H / dT ２ ) / (1+(dH / dT) ２ ) ３／２ (iii) Within the range of 130 to 155°C, among the temperatures at which the minimum value of curvature is negative, the temperature at which the heat flow value is highest is defined as T1, and the highest temperature higher than T1 is defined as T2, and the values ​​of the DSC curve at these temperatures are defined as the first peak height H(T1) and the second peak height H(T2).

2. The polyolefin microporous membrane according to claim 1, which has a porosity of 37% or more after heating and pressurizing at 7.8 MPa at 70°C for 10 seconds.

3. The maximum shrinkage stress in the machine direction (MD direction) of the film converted into basis weight is 0 or more and 10 mN / (g / m ２ 3. The polyolefin microporous membrane according to claim 1 or 2, wherein the polyolefin microporous membrane has a viscosity of 1000 MPa or less.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the total value of Raman orientation parameters measured under the following conditions is 75 or more, and the MD / TD ratio of the Raman orientation parameters is 0.75 or more and less than 0.

92. [Raman Orientation Parameter Measurement Conditions] 〔Device〕 Measurement device: inVia microscopic Raman spectroscopy system (manufactured by Renishaw) ・Light condensing condition: 180° backscattering arrangement ・Spectral length: 250mm Diffraction grating: 3000 lines / mm Excitation laser: 532 nm Lens: 50x objective lens (N.A. = 0.75) Spot size (spatial resolution): 5 μm [Polarization conditions] The laser was incident perpendicularly to the film surface (XY plane) and polarized using a polarizer. The measurement sample was rotated to obtain Raman spectra in 24 directions up to 345° in 15° increments, with the MD at 0°. [Calculation of peak intensity] The Raman spectrum obtained was －１ 1160cm or more －１ The baseline was obtained by linear approximation in the following region: 1060 cm －１ and 1130 cm －１ The maximum values ​​of the Raman bands are determined as peak intensities I1130 and I1160, respectively. [Orientation parameters] 1130cm －１ and 1060 cm －１ The peak intensity ratio (I1130 / I1060) is used as the orientation parameter, and the MD orientation parameter / TD orientation parameter is calculated from the 0° orientation parameter (MD orientation parameter) and the 90° orientation parameter (TD orientation parameter). Also, the sum of the orientation parameters in each direction (15° × n (1≦n≦24 (n is an integer))) measured in 24 directions up to 345° in 15° increments, with MD at 0°, is calculated as the total value of the Raman orientation parameter.

5. The polyolefin microporous membrane according to claim 1 or 2, which has a shutdown temperature of less than 146°C as measured by a temperature-programmed air resistance method.

6. The microporous polyolefin membrane according to claim 1 or 2, having a bubble point diameter of 20 nm or more.

7. In the differential molecular weight distribution curve obtained by gel permeation chromatography (GPC), the number of peaks determined by the following method is only one, and the weight average molecular weight Mw is 5.0 × 10 ５ Above 2.0 x 10 ６ and the molecular weight is 2.0 × 10 ６ The polyolefin microporous membrane according to claim 1 or 2, wherein the above proportion is 5% or more and 20% or less of the total. [GPC measurement conditions and peak number detection method] Measurement equipment: Agilent high-temperature GPC equipment PL-GPC220 Column: Agilent PL1110-6200 (20 μm MIXED-A) x 2 Column temperature: 160°C Solvent (mobile phase): 1,2,4-trichlorobenzene Solvent flow rate: 1.0 mL / min Sample concentration: 0.1% by weight (dissolution conditions: 160°C / 3.5H) Injection volume: 500 μL Detector: Agilent differential refractive index detector (RI detector) ・Viscometer: Agilent viscosity detector Calibration curve: Prepared by the universal calibration curve method using monodisperse polystyrene standard samples (EASIVIAL PS-H, PL2014-9001, PL2013-6001). The number of peaks is determined for the differential molecular weight distribution obtained by the GPC method using the following procedure. (i) Plot dw / dlogM, the value obtained by differentiating the concentration fraction with the logarithm of the molecular weight, on the vertical axis and logM, the logarithm of the molecular weight, on the horizontal axis. Approximation is performed using a smoothing spline for the plotted points to obtain G(logM), which represents dw / dlogM as a function of logM. The approximation calculation uses Python's UnivariateSpline module, with the smoothing spline function being a quartic function and the smoothing parameter being 1. Note that the calculation method, smoothing spline function, and smoothing parameter are not limited to those described above, but since G(logM) is differentiated twice to evaluate the curvature as described below, it is preferable to use a quartic or higher function. Furthermore, the error between the data points and the approximate value at each temperature is evaluated using the following formula, and G(logM) is determined so that the value obtained from the following formula is in the range of 0 to 5.

0. Formula: ((Y(Mi) - G(logMi)) ２ ) １／２ ) × 100 where Y(Mi) and G(logMi) represent the raw data and the approximated value at molecular weight Mi, respectively, and the subscript i represents each molecular weight of the raw data. Note that w is the concentration fraction and M is the molecular weight. (ii) The curvature D(logM) at each logM is calculated using the following formula: D(T)=(d ２ G / dlogM ２ ) / (1+(dG / dlogM) ２ ) ３／２ (iii) In the range of log M of 3 or more and 7 or less, among the log M values ​​where the minimum value of the curvature D(T) is a negative value, the number of log M values ​​where the curvature D(T) is −0.2 or less is counted and this is taken as the number of peaks. In addition, the molecular weight is 2.0 × 10 ６ The proportions of the above components are determined by the following procedure. (i) A differential molecular weight distribution curve is obtained by plotting log(M) and dw / dlog(M) for the data obtained by GPC measurement. (ii) The molecular weight of 2.0 × 10 when the area surrounded by the obtained differential molecular weight distribution curve and the baseline (usually the horizontal axis) is taken as 100%. ６ The area ratio of the above components is calculated. The area of ​​each region is determined as the actual area from the differential molecular weight distribution curve graph.

8. The polyolefin microporous membrane according to claim 1 or 2, which has a porous layer on at least one surface thereof.

9. A battery separator comprising the polyolefin microporous membrane according to claim 1 or 2.

10. A lithium ion secondary battery comprising the polyolefin microporous membrane according to claim 1 or 2.

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