Polyolefin microporous membrane and battery separator

Abstract

To provide a polyolefin microporous film having superior shutdown characteristics at thermal runaway and having superior impact resistance.SOLUTION: In a polyolefin microporous film, a total degree of orientation resulting from 360° measurement in 15° steps by Raman spectroscopy is 70 or more and 90 or less, and the proportion of amorphous component (α135) at 135°C measured by the solid echo technique of pulse NMR is 35% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a microporous polyolefin membrane and a battery separator using the same. [Background technology]

[0002] Polyolefin microporous membranes are used in various fields, such as filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among these, polyolefin microporous membranes are widely used as separators for secondary batteries because of their excellent chemical resistance, insulating properties, mechanical strength, and shutdown properties.

[0003] In particular, separators for lithium-ion batteries are related to battery characteristics, productivity, and safety, and are required to have mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking properties (shutdown properties), meltdown properties, etc. In particular, in recent years, development has been progressing with the aim of increasing battery size and achieving higher energy density, capacity, and output, primarily for automotive applications, and as a result, the safety requirements for separators have become even higher.

[0004] The shutdown characteristic is the ability to ensure the safety of a battery by causing the separator to melt and close the pores, cutting off the battery reaction, when the inside of the battery becomes overheated due to overcharging. The lower the shutdown temperature, at which the pores close, the greater the safety effect.

[0005] Furthermore, as battery capacity increases, components (separators) are becoming thinner, and in order to prevent short circuits when the battery is wound or due to foreign matter inside the battery, there is a demand for separators with increased puncture strength, MD (machine direction or longitudinal direction) and TD (direction perpendicular to MD) tensile strength, and elongation. Methods for increasing strength include controlling orientation by increasing the stretch ratio and using high-molecular-weight polyolefins, while methods for low-temperature shutdown involve lowering the melting point of the raw materials by reducing the molecular weight.

[0006] For example, Patent Document 1 describes a method for determining the stretching ratio of a gel-like sheet using an orientation distribution function calculated from peak area intensities obtained by polarized Raman spectroscopy.

[0007] Patent Document 2 also describes that, when an ethylene polymer is measured by the solid echo method of pulsed NMR and the free induction decay at 130°C is approximated using three components, the composition ratio of the least mobile component (α130) and the composition ratio β / γ of the intermediate mobile component (β) and the most mobile component (γ) have predetermined values, thereby enabling the production of a microporous membrane or the like that has an excellent appearance and an excellent balance between maintaining porosity and maintaining low thermal shrinkage, even during high-speed production.

[0008] Patent Document 3 describes that a polyolefin microporous membrane with excellent safety and output characteristics can be obtained when the porosity, shutdown temperature, and the lowest melting point among the melting points of each layer of the polyolefin microporous membrane satisfy a predetermined relationship.

[0009] Patent Document 4 describes that by using polyethylene having a crystal relaxation temperature below a specific temperature as the material for a polyolefin microporous membrane, a polyolefin microporous membrane that has high strength at room temperature and can retain its shape even at high temperatures can be obtained.

[0010] Patent Document 5 describes that by defining the orientation parameter in a specific direction of a polyolefin microporous membrane, calculated by Raman spectroscopy, within a specific range, a polyolefin microporous membrane with excellent winding properties and coatability can be obtained. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2017-197634 [Patent Document 2] International Publication No. 2018 / 088209 [Patent Document 3] Japanese Patent Application Publication No. 2019-143142 [Patent Document 4] Japanese Patent Application Publication No. 2019-157060 [Patent Document 5] International Publication No. 2017 / 115799 Summary of the Invention [Problem to be solved by the invention]

[0012] However, in Patent Document 1, the orientation is measured by polarized Raman spectroscopy as a method for determining the stretching conditions, but the relationship between these orientation parameters and the safety and impact resistance of the battery has not been considered at all.

[0013] In Patent Document 5, orientation is measured by polarized Raman spectroscopy, but only orientation parameters in the direction of 45° or 135° relative to the MD, TD, or MD of the film surface are measured, and orientation parameters in other directions are not disclosed. Furthermore, Example 1 describes high strength (47 cN / μm) physical properties, but the orientation parameters at 45° and 135° are low at 2.5 and 2.8, respectively, meaning that omnidirectional orientation is low and impact resistance is an issue.

[0014] In Patent Document 2, an ethylene polymer is measured by a solid echo method using pulsed NMR, but the measurement of a microporous membrane molded product was not considered, even though the proportions of each component contained in the microporous membrane molded product vary greatly depending on the kneading and stretching conditions. Also, the focus is on the balance between appearance, porosity, and low thermal shrinkage, and no effect on shutdown property is obtained.

[0015] In Patent Document 3, Example 5 describes the physical properties of a high-strength (49 cN / μm) and low-shutdown product (129°C), but the film thickness is as thick as 20 μm and the heat shrinkage is as high as 20% in MD and 16% in TD. Therefore, when heat is generated after incorporating the microporous membrane into a battery, the microporous membrane shrinks, causing a short circuit between the electrodes, and there is an issue that safety cannot be ensured.

[0016] With regard to Patent Document 4, the high strength range of 50 cN / μm or more, which has been required in recent years, has not been achieved even when looking at the examples and comparative examples.

[0017] An object of the present invention is to provide a microporous polyolefin membrane that is excellent in shutdown property during thermal runaway and impact resistance. [Means for solving the problem]

[0018] As a result of extensive research conducted by the inventors to achieve the above-mentioned object, they discovered that by adjusting the parameters of Raman spectroscopy and pulsed NMR to specific ranges using the advanced film-forming technology described below, it is possible to provide a battery separator that is excellent in battery safety and impact resistance, and thus completed the present invention.

[0019] That is, the present invention has the following configuration. [1] The polyolefin microporous membrane is made of polyethylene as a resin, A microporous polyolefin film having a total degree of orientation of 70 to 90 as measured over 360° at 15° intervals by Raman spectroscopy, and a proportion of amorphous components (α135) at 135°C of 35% or more as measured by the solid echo method of pulsed NMR. [2] The polyolefin microporous membrane according to [1], wherein the ratio of the 0° orientation degree (MD orientation degree) to the 90° orientation degree (TD orientation degree) obtained by Raman spectroscopy is in the range of 0.8 or more and 1.3 or less. [3] The polyolefin microporous membrane according to [1] or [2], wherein the degree of orientation obtained by measuring the maximum value (Rmax) over 360° at 15° intervals by Raman spectroscopy, divided by the minimum value (Rmin), is in the range of 1.0 or more and 1.5 or less. [4] The microporous polyolefin film according to any one of [1] to [3], which has a thickness in the range of 5 to 12 μm. [5] A battery separator using the polyolefin microporous membrane according to any one of [1] to [4]. [6] The battery separator according to [5], wherein a porous layer is laminated on the polyolefin microporous membrane. [7] The battery separator according to [6], wherein the porous layer contains at least one resin selected from the group consisting of fluorine-based resins, acrylic resins, polyvinyl alcohol-based resins, and carboxymethyl cellulose-based resins, and inorganic particles. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a polyolefin microporous membrane that is excellent in shutdown property during thermal runaway and impact resistance. DETAILED DESCRIPTION OF THE INVENTION

[0021] An embodiment of the present invention will be described below. The polyolefin microporous membrane of the present invention has a total omnidirectional orientation degree of 70 or more and 90 or less when measured over 360° at 15° intervals by Raman spectroscopy. Here, the omnidirectional orientation degree is the sum of the orientation degrees at 24 locations obtained by measuring 360° (all around) at 15° intervals, with a certain axis set as 0°. In the examples of the present invention, the MD is set as the 0° direction and the direction is counterclockwise. However, since the MD orientation degree, TD orientation degree, and omnidirectional orientation degree have similar values even in the clockwise direction, clockwise orientation may also be used. When the in-plane omnidirectional orientation degree is 70 or more, a thin film is less likely to short-circuit when subjected to external impact, improving battery safety. From the perspective of improving safety, a higher omnidirectional orientation degree is preferable, but since it is difficult to increase the in-plane omnidirectional orientation degree when attempting to lower the shutdown temperature, the upper limit is set to 90. The degree of orientation in all directions in the plane is more preferably 78 to 85, which provides a particularly good balance between strength and shutdown temperature.

[0022] The degree of orientation in all directions can be measured by the method described in the Examples. In order to set the degree of orientation in all directions within the above range, the raw material composition of the film and the stretching conditions during film production can be appropriately adjusted within the ranges described below. (Microscopic Raman measurement equipment and measurement conditions) Measurement equipment: inVia micro-Raman spectroscopy system (Renishaw) Measurement conditions: ·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. (Polarization conditions) The laser was incident perpendicularly to the film surface (XY plane) and polarized using a polarizer. The polarizers for the incident and scattered light were positioned parallel to each other. The sample was also rotated in the incident polarization plane to obtain Raman spectra in each direction. (Calculation of peak intensity) The intensity of each peak is 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 at 1130 cm was calculated by peak fitting using a Gaussian-Lorentzian mixed function approximation. The degree of orientation in each direction was calculated from the peak intensity. The degree of orientation was calculated by the peak intensity at 1130 cm measured by Raman spectroscopy at each measurement angle. -1 and 1060cm -1 It is calculated from the peak intensity ratio (I1130 / I1060) of 1130cm -1 and 1060cm -1 The peaks are due to the symmetric and antisymmetric vibration modes of the CC stretching band, respectively, and both show strong anisotropy with respect to the polarization angle. Therefore, the peak intensity ratio (I1130 / I1060) is a parameter that correlates with the degree of molecular chain orientation in the incident polarization direction. Note that a degree of orientation of 1.7 indicates no orientation, and values above 1.7 indicate advanced molecular orientation.

[0023] The polyolefin microporous membrane of the present invention has an amorphous component (α135) ratio of 35% or more at 135°C as measured by the solid echo method of pulsed NMR. When the free induction decay, which is the result of analyzing values measured at 135°C using pulsed NMR with the solid echo method, is approximated to three components: a crystalline component (A), an intermediate component (B), and an amorphous component (C), the amorphous component ratio is 35% or more. The amorphous component (α135) ratio is preferably 36% or more. If the amorphous component ratio is less than 35%, the shutdown temperature will be high, which may reduce safety when used as a battery separator for secondary batteries that require high energy density, high capacity, and high power output, such as in electric vehicles.

[0024] The proportion of the amorphous component (α135) can be measured by the method described in the Examples. To set the proportion of the amorphous component (α135) within the above range, the raw material composition, stretching conditions during film formation, and heat setting conditions can be appropriately adjusted within the ranges described below. (Pulse NMR measurement equipment and measurement conditions) Device: Bruker mq20 Measurement method: Solid echo method Measured nuclear frequency: 19.95 MHz (1H square) Pulse width: 2.18 seconds (90°C pulse) Pulse repetition time: 3s Measurement temperature: 135℃ (Three-component approximation of free induction decay obtained from pulsed NMR) The free induction decay (M(t)) of the polyolefin microporous membrane of the present invention, obtained by the solid echo method in pulsed NMR measured at a specific temperature, was approximated to the three components described above by fitting Equation 1 by the least squares method, assuming that the film resin is composed of the three components described above. M(t)=A×exp(-(1 / 2)(t / Ta) 2 )sinet / et+B×exp(-(1 / Wd)(t / Tb)Wd)+C×exp(-t / Tc) Equation 1 A: Crystalline component composition (%) Ta: relaxation time of crystalline components (msec) B: Composition rate of intermediate component (%) Tb: Relaxation time of intermediate component (msec) C: Amorphous component ratio (%) Tc: Relaxation time of amorphous components (msec) t: Observation time (msec) Wd: Shape factor (1 <Wd<2) e: Shape factor (0.1 <e<0.2) 。

[0025] The polyolefin microporous membrane of the present invention preferably has a MD / TD orientation ratio measured by Raman spectroscopy within the range of 0.8 to 1.3. The MD orientation is the degree of orientation in the 0° direction when MD is defined as the 0° direction. The TD orientation is the degree of orientation in the 90° direction when MD is defined as the 0° direction. The MD / TD orientation ratio is more preferably within the range of 0.8 to 1.3, and even more preferably within the range of 0.9 to 1.2. By keeping the MD / TD orientation ratio within the range of 0.8 to 1.3, a thin film is less likely to short-circuit when subjected to external impact, improving battery safety. To achieve the MD / TD orientation ratio within the above range, it is preferable that the raw material composition of the film be within the range described below, and that the stretching conditions during film formation be within the range described below.

[0026] The polyolefin microporous membrane of the present invention preferably has an Rmax / Rmin ratio, calculated by dividing the largest value (Rmax) by the smallest value (Rmin) among 24 orientation degrees, in the range of 1.0 to 1.5. It is more preferably in the range of 1.0 to 1.3. If the Rmax / Rmin ratio exceeds 1.51, a thin film may be more susceptible to short-circuiting upon external impact, potentially reducing the safety of the battery. To achieve an Rmax / Rmin ratio within the above range, it is preferable that the raw material composition of the film be within the range described below, and that the stretching conditions during film formation be within the range described below.

[0027] Next, the polyolefin microporous membrane of the present invention will be described, but is not necessarily limited thereto. The polyolefin microporous membrane of the present invention is a film containing a polyolefin resin as a main component. Here, in the present invention, "main component" means that the proportion of the polyolefin microporous membrane is 50% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 99% by mass or more. The polyolefin microporous membrane preferably contains 50% by mass or more of polyethylene, and from the viewpoints of permeability, porosity, mechanical strength, and shutdown property, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0028] Examples of polyolefin resins include polyethylene, polypropylene, and mixtures thereof, with polyethylene being preferred. The polyethylene is preferably a homopolymer of ethylene, and more preferably a copolymer containing other α-olefins to lower the melting point of the raw material. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Of these, hexene-1 is the most preferred copolymer containing an α-olefin. The α-olefin can be confirmed by C13-NMR measurement.

[0029] The melting point of the polyethylene is preferably higher than 130° C. and lower than 140° C. If the melting point is higher than 130° C., a decrease in porosity can be suppressed, and if it is lower than 140° C., an increase in the shutdown temperature can be suppressed.

[0030] The density of polyethylene is 0.94 g / cm 3 High density polyethylene (HDPE) with a density of 0.93 to 0.94 g / cm 3 Medium density polyethylene, with a density of 0.93 g / cm 3Examples of suitable polyethylenes include low-density polyethylene (LDPE) and linear low-density polyethylene (LDPE). High-density polyethylene is preferable for increasing tensile strength. Because high-density polyethylene has a relatively low weight-average molecular weight, it tends to swell and neck significantly at the die outlet when molded into a sheet, resulting in poor sheet formability. Therefore, it is preferable to add ultra-high molecular weight polyethylene (UHMwPE) to increase the viscosity and strength of the sheet and improve process stability.

[0031] The polyolefin resin may be a mixture of two or more polyolefins, and a polyolefin composition consisting of ultra-high molecular weight polyethylene and high-density polyethylene is preferred. The ultra-high molecular weight polyethylene has a weight-average molecular weight of 1.0 × 10 6 Over 4.0 x 10 6 The content of the ultra-high molecular weight polyethylene is less than 100% by mass, and by including it, the pores of the microporous membrane can be made finer, high heat resistance can be imparted, and pin puncture strength can be improved. The content of the ultra-high molecular weight polyethylene, based on 100% by mass of the entire polyolefin resin, is preferably at least 30% by mass, more preferably at least 40% by mass, and is preferably at most 70% by mass, more preferably at most 60% by mass. When the content of the ultra-high molecular weight polyethylene is within the above preferred range, the ultra-high molecular weight polyethylene is sufficiently dispersed, making it easy to control the crystallinity in the membrane, and the balance between tensile strength and tensile elongation can be appropriately controlled by the membrane production method described below.

[0032] From the viewpoint of lowering the shutdown temperature, the high-density polyethylene may be a high-density polyethylene having a terminal vinyl group concentration of 2.0 or more and 10.0 or less per 10,000 carbon atoms and containing a branch having 3 or more carbon atoms (also called a long-chain branch), and having a weight-average molecular weight of 1.0 × 10 3 Over 1.0 x 10 5 The following high-density polyethylene (also referred to as low-molecular-weight high-density polyethylene) may also be used: The terminal vinyl group concentration and weight-average molecular weight can be measured by infrared spectroscopy and gel permeation chromatography, respectively, as described in the Examples.

[0033] From the viewpoint of lowering the shutdown temperature, the high-density polyethylene is preferably long-chain branched and has a terminal vinyl group concentration of 2.0 to 10.0 per 10,000 carbon atoms as measured by infrared spectroscopy. When the terminal vinyl group concentration of the high-density polyethylene is 2.0 to 10.0 per 10,000 carbon atoms, the proportion of amorphous components (α135) at 135°C as measured by the solid echo method of pulsed NMR increases, and the shutdown temperature decreases. The terminal vinyl group concentration can be determined, for example, by infrared spectroscopy as described in the examples below.

[0034] When high-density polyethylene containing long-chain branches is used as the high-density polyethylene, the proportion of amorphous components (α135) at 135°C measured by the solid echo method of pulsed NMR increases, and the shutdown temperature decreases.

[0035] The weight average molecular weight (Mw) of high-density polyethylene having a terminal vinyl group concentration of 2.0 or more and 10.0 or less per 10,000 carbon atoms is 1.0 x 10 5 It is preferable that the ratio is equal to or greater than 1.8×10 5 That's all. The upper limit is 1.0 × 10 6 It is preferably equal to or less than 5.0 × 10 5 More preferably, it is 3.5×10 5 When Mw is within the above preferred range, it is easy to achieve both the final tensile strength and the tensile elongation. 5 By setting the temperature at or above 1.0×10, it is possible to prevent pore clogging during heat treatment due to the lower melting point of the raw material, and to suppress deterioration of output characteristics due to a decrease in porosity. 6 By setting the temperature to the range below, it is possible to suppress an increase in the shutdown temperature due to an increase in the melting point.

[0036] A particularly preferred high density polyethylene as a raw material has a terminal vinyl group concentration of 2.0 to 10.0 per 10,000 carbon atoms and a weight average molecular weight of 1.0 x 105 ~1.0×10 6 The polyolefin resin is a high-density polyethylene, and is contained in an amount of 50% by mass or more when the entire polyolefin resin is taken as 100% by mass.

[0037] When low-molecular-weight high-density polyethylene is used as the high-density polyethylene, the content of the low-molecular-weight high-density polyethylene is preferably 50% by mass or less, and more preferably 40% by mass or less, based on 100% by mass of the entire polyolefin resin. If the content of the low-molecular-weight high-density polyethylene is 50% by mass or less, the proportion of amorphous components (α135) at 135°C measured by the solid echo method of pulsed NMR can be maintained high, while preventing a decrease in porosity due to pore clogging. Furthermore, the content of UHMwPE is preferably 50% by mass or more, from the viewpoint of the balance between shutdown temperature and pin puncture strength.

[0038] By using the above preferred polyolefin resin and appropriately adjusting the production conditions described below, a polyolefin microporous membrane can be prepared so that the total degree of orientation is 70 to 90 and the proportion of amorphous components (α135) at 135°C measured by the solid echo method of pulsed NMR is 35% or more. As a result, a polyolefin microporous membrane can be obtained that maintains pin puncture strength and tensile strength / elongation even in a thin film and has an excellent balance between impact resistance and shutdown performance.

[0039] The polyolefin resin may contain various additives, such as antioxidants, heat stabilizers, antistatic agents, UV absorbers, antiblocking agents, and fillers, as long as the effects of the present invention are not impaired. Addition of an antioxidant is particularly preferred for the purpose of suppressing oxidative degradation of the polyethylene resin due to thermal history. The antioxidant is preferably one or more selected from the group consisting of 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"® 1330: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7). Appropriate selection of the type and amount of antioxidant and heat stabilizer is important for adjusting or enhancing the properties of the microporous membrane.

[0040] The polyolefin microporous membrane of the present invention can be obtained by biaxially stretching the above-mentioned raw materials. The biaxial stretching method can be any of inflation, simultaneous biaxial stretching, and sequential biaxial stretching, but among these, simultaneous biaxial stretching and sequential biaxial stretching are preferred in terms of membrane production stability, thickness uniformity, and control of high film rigidity and dimensional stability.

[0041] [1] Manufacturing method of polyolefin microporous membrane Next, the method for producing the polyolefin microporous membrane of the present invention will be described, but is not necessarily limited thereto. The method for producing the polyolefin microporous membrane of the present invention comprises the following steps (a) to (e): (a) A polyolefin solution is prepared by kneading and dissolving polymer materials including a polyolefin simple substance, a polyolefin mixture, a polyolefin solvent mixture (plasticizer), additives, and a polyolefin mixture. (b) extruding the melt, forming it into a sheet, and cooling and solidifying it; (c) The obtained sheet is stretched by a roll method or a tenter method. (d) The plasticizer is then extracted from the resulting stretched film and the film is dried. (e) Then, heat treatment / re-stretching is carried out. (f) Perform aging treatment as necessary. Each step will be described below.

[0042] (a) Preparation of polyolefin solution The polyolefin resin is heated and dissolved in a plasticizer to prepare a polyolefin solution. The plasticizer is not particularly limited as long as it can sufficiently dissolve polyethylene. However, to enable relatively high stretching ratios, the solvent is preferably liquid at room temperature. Examples of solvents include aliphatic, cycloaliphatic, 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. To obtain a gel-like sheet with a stable liquid solvent content, a nonvolatile liquid solvent such as liquid paraffin is preferably used. A solvent that is miscible with polyethylene in the melt-kneaded state but solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, using a solid solvent alone may result in uneven stretching.

[0043] The blending ratio of polyolefin resin and plasticizer is 100% by mass, and the polyolefin resin content may be appropriately selected within a range that does not impair moldability, but is generally 10 to 50% by mass. If the polyolefin resin content is less than 10% by mass (if the plasticizer content is 90% by mass or more), swelling and necking will be significant at the die outlet when molding into a sheet, resulting in poor sheet moldability and reduced film formability. On the other hand, if the polyolefin resin content exceeds 50% by mass (if the plasticizer content is 50% by mass or less), shrinkage in the thickness direction will be significant and moldability will also be reduced. The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40°C. If the viscosity at 40°C is 20 cSt or more, the sheet extruded from the die 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. The viscosity of the liquid solvent is measured at 40°C using an Ubbelohde viscometer.

[0044] (b) Formation of extrudate and gel-like sheet The method for uniformly melt-kneading the polyolefin solution is not particularly limited, but when a high-concentration polyolefin solution is to be prepared, it is preferable to carry out the melt-kneading in a twin-screw extruder. If necessary, various additives such as antioxidants may be added within a range that does not impair the effects of the present invention. Addition of an antioxidant is particularly preferable to prevent oxidation of polyethylene.

[0045] In the extruder, the polyolefin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but 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). Here, the melting point refers to the value measured by DSC in accordance with JIS K7121 (1987) (the same applies hereinafter). For example, in the case of polyethylene, the melt-kneading temperature is preferably in the range of 140 to 250°C, more preferably 160 to 230°C, and most preferably 170 to 200°C. Specifically, since the polyethylene composition has a melting point of approximately 130 to 140°C, the melt-kneading temperature is preferably 140 to 250°C, more preferably 180 to 230°C.

[0046] 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.

[0047] The resulting extrudate is then cooled to obtain a gel-like sheet, which solidifies the polyethylene microphase separated by the solvent. The cooling step is preferably performed to 10 to 50°C. This is because the final cooling temperature is preferably below the crystallization end temperature. By reducing the high-order structure, uniform stretching becomes easier 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 of less than 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, which results 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, which results in a dense high-order structure in the gel-like sheet, which not only allows for uniform stretching but also leads to high film toughness.

[0048] The cooling method may be a method of directly contacting the material with cold air, cooling water or other cooling medium, a method of contacting the material with a roll cooled with a cooling medium, or a method using a casting drum or the like.

[0049] Furthermore, the polyolefin microporous membrane of the present invention is not limited to a single layer and may be a laminate. There is no particular limitation on the number of layers, and it may be a two-layer laminate or a three-layer or more laminate. As described above, the laminated portion may contain desired resins in addition to polyethylene to the extent that the effects of the present invention are not impaired. Conventional methods can be used to form a polyolefin microporous membrane into a laminate. For example, there is a method of forming a laminate by preparing desired resins as needed, separately feeding these resins into an extruder and melting them at the desired temperature, joining them in a polymer tube or die, and extruding them through a slit die to the desired lamination thickness.

[0050] (c) Stretching process The resulting gel-like sheet (including laminated sheets) is stretched. Stretching methods include MD uniaxial stretching using a roll stretcher, TD uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretcher and a tenter or a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter. The stretching ratio varies depending on the thickness of the gel-like sheet, but stretching of 5 times or more in either direction is preferred from the viewpoint of membrane thickness uniformity. The area ratio is preferably 25 times or more, more preferably 36 times or more, and even more preferably 49 times or more. If the area ratio is less than 25 times, stretching is insufficient, membrane uniformity is easily impaired, and a microporous membrane with excellent strength cannot be obtained. The area ratio is preferably 100 times or less. If the area ratio is high, breakage tends to occur frequently during the production of the microporous membrane, reducing productivity, and the orientation and crystallinity increase, improving the melting point and strength of the porous substrate. However, increasing the crystallinity means decreasing the amorphous portion, which increases the film melting point and shutdown temperature. By adjusting the stretching conditions appropriately within the above ranges, the proportion of amorphous components (α135) at 135°C and the Raman orientation can be adjusted within the above ranges.

[0051] The stretching temperature is preferably set to the melting point of the gel-like sheet + 10°C or less, 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 130°C, more preferably 90 to 125°C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it may be determined from NMR. If the temperature is lower than 90°C, the pores will not be sufficiently opened due to low-temperature stretching, making it difficult to achieve a uniform membrane thickness and resulting in a low porosity. If the temperature is higher than 130°C, the sheet will melt, making the pores more likely to become blocked.

[0052] The stretching described above causes cleavage of the higher-order structure formed in the gel 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 gel sheet suitable for battery separators. Furthermore, by stretching before removing the plasticizer, the polyolefin 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 is less likely to remain during stretching, resulting in a lower thermal shrinkage rate than when stretching is performed after removing the plasticizer.

[0053] (d) Washing and drying process Next, the solvent remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyethylene phase and the solvent phase are separate, removing the solvent yields a microporous membrane. 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 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 washing due to the surface tension at the air-liquid interface, resulting in a microporous membrane with high porosity and permeability. These cleaning solvents are selected appropriately depending on the plasticizer and used alone or in combination.

[0054] Washing can be performed by immersing the gel-like sheet in a washing solvent for extraction, showering the gel-like sheet with the washing solvent, or a combination of these. The amount of washing solvent used varies depending on the washing method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet. The washing temperature may be 15 to 30°C, and may be heated to 80°C or less as needed. 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 non-uniformity in the TD and / or MD properties of the resulting microporous membrane, and improving the mechanical and electrical properties of the microporous membrane. The above-described washing is preferably performed until the residual solvent in the washed gel-like sheet, i.e., the microporous membrane, is less than 1% by mass.

[0055] Thereafter, the solvent in the microporous membrane is removed by drying in a drying step. The drying method is not particularly limited, and methods using metal heating rolls or hot air can be selected. The drying temperature is preferably 40 to 100°C, more preferably 40 to 80°C. If the drying is insufficient, the porosity of the microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

[0056] (e) Heat treatment / re-stretching process The dried microporous membrane may be stretched (restretched) at least uniaxially. Restretching can be carried out by a tenter method or the like while heating the microporous membrane, similar to the stretching described above. Restretching may be uniaxial or biaxial. Multistage stretching is carried out by combining simultaneous biaxial and / or sequential stretching.

[0057] 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, the temperature is preferably 70 to 135°C, more preferably 110 to 132°C, and most preferably 120 to 130°C.

[0058] In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 1.6 times, particularly 1.1 to 1.6 times in TD, and more preferably 1.2 to 1.4 times. In the case of biaxial stretching, it is preferably 1.01 to 1.6 times in both MD and TD. The re-stretching ratios in MD and TD may be different. Stretching within the above ranges can increase porosity and permeability, but stretching at a ratio of 1.6 or more promotes orientation, raising the melting point of the film and the shutdown temperature. From the viewpoints of heat shrinkage and wrinkles and sagging, the relaxation ratio from the maximum re-stretching ratio is preferably 0.9 or less, more preferably 0.8 or less. By appropriately adjusting the stretching conditions within the above ranges, the proportion of amorphous components (α135) at 135°C and the Raman orientation can be adjusted to the above ranges.

[0059] If necessary, at least one surface of the stretched membrane or polyolefin microporous membrane can be subjected to corona discharge treatment in an atmosphere of air, nitrogen, or a mixture of carbon dioxide and nitrogen. After the above-described steps are completed, the polyolefin microporous membrane is wound around a core to obtain a wound body.

[0060] (f) Aging treatment process The polyolefin microporous membrane wound into a roll is subjected to aging treatment in a constant-temperature chamber. Rapid cooling is performed to preserve the amorphous portion, raising concerns about width shrinkage of the polyolefin microporous membrane over time during downstream processes or transportation. Therefore, to prevent width shrinkage during normal handling while preserving the amorphous portion of the polyolefin microporous membrane, aging treatment is preferably performed by storing the membrane at a constant temperature lower than the crystal dispersion temperature. The aging temperature is preferably 40°C to 80°C, more preferably 45°C to 70°C, and even more preferably 50°C to 70°C. Temperatures below 40°C may cause shrinkage due to the heat of the ambient temperature in summer or during transportation by ship, etc.; temperatures above 80°C may cause excessive shrinkage due to the temperature being close to the crystal dispersion temperature, potentially reducing the width-cutting yield in the coating process.

[0061] (g) Other processes Furthermore, depending on other applications, the microporous membrane can also be subjected to a hydrophilization treatment. Hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably performed after crosslinking treatment. The polyolefin microporous membrane is preferably crosslinked by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. Crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane.

[0062] In the surfactant treatment, any of nonionic, cationic, anionic, and amphoteric surfactants can be used, but nonionic surfactants are preferred. The multi-layer, 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 multi-layer, microporous membrane by a doctor blade method.

[0063] For the purpose of improving the meltdown properties and heat resistance when used as a battery separator, the polyolefin microporous membrane may be surface coated with a porous fluorine-based resin such as polyvinylidene fluoride or polytetrafluoroethylene, or a porous material such as polyimide or polyphenylene sulfide, or may be coated with an inorganic coating such as ceramic.

[0064] The polyolefin microporous membrane obtained as described above can be used for various purposes such as filters, fuel cell separators, and capacitor separators. 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] [Porous layer] A porous layer can be provided on at least one side of the polyolefin microporous membrane. The porous layer imparts or improves at least one of the functions, such as heat resistance, adhesion to electrode materials, and electrolyte permeability. The porous layer contains a resin that imparts or improves the above functions and may further contain inorganic particles. The resin preferably contains at least one resin selected from the group consisting of fluorine-based resins, acrylic resins, polyvinyl alcohol-based resins, and carboxymethyl cellulose-based resins.

[0066] [1] Structure and properties of microporous polyolefin membranes Other physical properties of the polyolefin microporous membrane are listed below. (1) Thickness of the polyolefin microporous membrane The upper limit of the thickness of the polyolefin microporous membrane is 12 μm. Furthermore, the upper limit of the thickness of the polyolefin microporous membrane is more preferably 9 μm, and the lower limit is preferably 4 μm, more preferably 5 μm, and even more preferably 7 μm. When the thickness of the polyolefin microporous membrane is within the above range, it can have practical puncture strength and pore-blocking function, and is also suitable for increasing the capacity of batteries.

[0067] (2) Porosity The upper limit of the porosity of the polyolefin microporous membrane is preferably 60%, more preferably 55%, and most preferably 50%. The lower limit of the porosity is preferably 35%, more preferably 38%. If the porosity is 60% or less, sufficient mechanical strength and insulating properties are easily obtained, and short circuits are less likely to occur during charge and discharge. Furthermore, if the porosity is 35% or more, good ion permeability can be obtained, resulting in good battery charge and discharge characteristics.

[0068] (3) Tensile strength The polyolefin microporous membrane preferably has a tensile strength of 200 MPa or more and 350 MPa or less, more preferably 250 MPa or more and 300 MPa or less, in both MD and TD. Within this range, mechanical strength is easily obtained and short circuits are less likely to occur during charging and discharging. If either the MD or TD tensile strength exceeds the upper limit of the above range, the viscoelasticity of the polyolefin microporous membrane may decrease, potentially reducing process stability during conveying and coating. If the tensile strength is below the lower limit of the above range, the polyolefin microporous membrane may elongate significantly during conveying, causing wrinkles in the polyolefin microporous membrane and reducing productivity after coating. The MD and TD tensile strengths can be measured using a tensile tester. Regarding the MD / TD tensile strength ratio, a higher MD strength is preferred to improve conveyability during coating, and it is preferably 1.00 or more and 1.50 or less. It is more preferably 1.10 or more and 1.45 or less, and even more preferably 1.15 or more and 1.40 or less. If the strength ratio is greater than 1.50, the film retention strength when an impact is applied in the MD and TD will differ greatly, which may lead to deflection in one direction and cause a short circuit.

[0069] (4) Air resistance The upper limit of the air resistance of a polyolefin microporous membrane, assuming a thickness of 10 μm, is 300 seconds / 100 cc air / 10 μm, more preferably 250 seconds / 100 cc air / 10 μm, and even more preferably 200 seconds / 100 cc air / 10 μm. The lower limit of the air resistance is 30 seconds / 100 cc air / 10 μm, preferably 50 seconds / 100 cc air / 10 μm, and even more preferably 80 seconds / 100 cc air / 10 μm. If the air resistance is 300 seconds / 100 cc air / 10 μm or less, ion permeability is good, allowing for high-speed charging and discharging, and sufficient permeability can be obtained even after coating. Furthermore, if the air resistance is 30 seconds / 100 cc air / 10 μm or more, self-discharge and battery degradation can be prevented.

[0070] (5)Piercing strength The polyolefin microporous membrane has a pin puncture strength of 40 cN / μm or more, preferably 45 cN / μm. When the polyolefin microporous membrane has a pin puncture strength of 40 cN / μm or more, when the polyolefin microporous membrane is incorporated into a battery as a separator, the membrane has high foreign matter resistance, does not short-circuit electrodes, and improves battery safety.

[0071] (6) Heat shrinkage rate The upper limit of the heat shrinkage rate of the polyolefin microporous membrane at 100°C / 1 hour or 120°C / 1 hour is 10%, and more preferably 8% or less. If the heat shrinkage rate is 10% or less, the microporous membrane can be maintained even when heated when used in a battery, making short circuits less likely to occur and improving battery safety.

[0072] (7) Shutdown temperature The shutdown temperature of the polyolefin microporous membrane is 140°C or lower, preferably 139°C or lower. If the shutdown temperature is 140°C or lower, thermal runaway is less likely to occur when the polyolefin microporous membrane is incorporated into a battery as a separator, improving the safety of the battery. Furthermore, if the pin puncture strength is 50 cN / μm or higher, it has been difficult to lower the shutdown temperature to 140°C or lower. However, if the raw material composition is a polyolefin microporous membrane with a weight-average molecular weight of 1.0 × 10 6 Over 4.0 x 10 6 and an ultra-high molecular weight polyolefin resin having a terminal vinyl group concentration of 2.0 or more per 10,000 carbon atoms as determined by infrared spectroscopy and a weight average molecular weight of 1.0 x 10 6 We found that these properties can be achieved by using a polyethylene resin containing long chain branches of less than 1000 or a low molecular weight high density polyethylene resin with a weight average molecular weight of 1000 to 100000 and examining the MD / TD stretching conditions.

[0073] [3]Usage The microporous polyolefin membrane is suitable as a separator (insulating material) for electrochemical reaction devices such as batteries and capacitors, and is particularly suitable for use as a separator for non-aqueous electrolyte secondary batteries, particularly lithium secondary batteries. [Example]

[0074] The methods for measuring the physical properties of the resins used in the examples and the polyolefin microporous membranes obtained in the examples are described below.

[0075] (1) Weight average molecular weight The weight average molecular weights of the ultra-high molecular weight polyethylene and high-density polyethylene were determined by gel permeation chromatography under the following measurement conditions. Measurement equipment: Waters Corporation GPC-150C Column: Shodex UT806M manufactured by Showa Denko K.K. Column temperature: 135℃ Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1% by mass (dissolution conditions: 135°C / 1h) Injection volume: 500 μl Detector: Waters Corporation differential refractometer (RI detector) Calibration curve: A calibration curve was prepared from a monodisperse polystyrene standard sample using a polyethylene conversion factor (0.46).

[0076] (2) Terminal vinyl group concentration The high-density polyethylene resin was hot-pressed at 210°C, then rapidly cooled at 25°C and molded into a sample with a thickness of approximately 1 mm. An infrared spectrum was then obtained using a Fourier transform infrared spectrophotometer (model number: FREEXACT-II, manufactured by Horiba, Ltd.). -1 The absorbance of the absorption peak at 10000 carbon atoms [A = log(I0 / I) (where A represents absorbance, I0 represents the transmitted light intensity of the blank cell, and I represents the transmitted light intensity of the sample cell)] was measured, and the number of terminal vinyl groups per 10,000 carbon atoms in the high-density polyethylene (number / 10,000C) was calculated using the following formula: Terminal vinyl group concentration (number / 10,000C) = (11.4 × absorbance A) / (density of polyethylene (g / cm 3 ) × sample thickness (mm)].

[0077] (3) Thickness (average film thickness) The thickness of the polyolefin microporous membrane was measured at five points within a 50 mm x 50 mm area at room temperature (23°C) using a contact thickness meter (Litematic manufactured by Mitutoyo Corporation, contact pressure 0.01 N, 10.5 mmφ probe), and the average value was calculated.

[0078] (4) Porosity (%) The polyolefin microporous membrane was cut into a size of 5 cm x 5 cm, and its volume (cm 3 ) and mass (g), and compare them with the film density (g / cm 3 ) was calculated using the following formula: Porosity = ((volume - mass / membrane density) / volume) x 100 The film density is set to 0.95 to 0.99 depending on the density of the polyethylene used, but here the film density was set to 0.99. The volume was calculated using the thickness measured in (1) above.

[0079] (5) Tensile strength The tensile strength (MPa) for each direction was measured using an Instron 5543 tensile testing machine in accordance with ASTM D882 under the following conditions: Sample shape: 100mm x 10mm rectangular; Measurement direction: MD (length direction), TD (transverse direction); Chuck distance: 20mm; Tensile speed: 100mm / min; Grip: Instron 2702-018 Jaw Faces for Flats (Rubber Coated, 50 x 38mm); Load cell: 500N; Chuck pressure: 0.50MPa; Temperature: 23°C. The tensile strength (MPa) was calculated by dividing the strength at sample break by the cross-sectional area of the sample before testing. The tensile strength was measured at five points in each direction and the average value was calculated.

[0080] (6) Air resistance The polyolefin microporous membrane was measured for air resistance, which is the time required for 100 cc of gas to permeate, using an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P8117.

[0081] (7)Piercing strength A Kato Tech KES-G5 needle with a spherical tip and a diameter of 1 mm was used to pierce a polyolefin microporous membrane fixed to a sample holder with a thickness of T1 (μm) at a speed of 2 mm / sec, and the maximum load was measured. Detailed conditions are shown below: Diameter of sample holder opening: 11.3 mm; Radius of curvature of needle tip: 0.5 mm; Piercing speed: 2 mm / sec; Ambient temperature: 23°C. The measured maximum load, La, was converted to the maximum load, Lb, for a membrane thickness of 1 μm using the formula: Lb = (La × 10) / T1, and this was used as the piercing strength (N / μm).

[0082] (8) Heat shrinkage rate The polyolefin microporous membrane was heated at 105°C for 8 hours, and the MD shrinkage was measured three times, and the average value was used as the MD heat shrinkage. The same measurement was also performed in the TD to determine the TD heat shrinkage.

[0083] (9) Evaluation of shutdown temperature and shutdown capability during thermal runaway The polyolefin microporous membrane was heated from room temperature (25°C) at a temperature increase rate of 5°C / min, while measuring the air resistance with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). When the air resistance reached the detection limit of 1 x 10 5 seconds / 100cm 3 The temperature reached by the air was determined and taken as the shutdown temperature (°C). The measurement cell was constructed from an aluminum block with a thermocouple located directly below the polyolefin microporous membrane. A sample was cut into a 5cm x 5cm square and heated while being fixed with an O-ring. The results were used to evaluate the shutdown performance during thermal runaway, with a rating of ◯ (good) when the measurement temperature was less than 140°C and × (unacceptable) when the measurement temperature was 140°C or higher.

[0084] (10) Impact resistance test A cylindrical battery was prepared according to the following procedure and subjected to an impact test. <Preparation of positive electrode> A slurry was prepared by dispersing 92.2% by mass of lithium-cobalt composite oxide (LiCoO2) as the active material, 2.3% by mass each of flake graphite and acetylene black as conductive agents, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil (positive electrode current collector) using a die coater with an active material coating weight of 250 g / m2 and an active material bulk density of 3.00 g / cm3. The mixture was then dried at 130 °C for 3 minutes, compression molded using a roll press, and cut into strips approximately 57 mm wide. <Preparation of negative electrode> A slurry was prepared by dispersing 96.9% by mass of artificial graphite as the active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as the binder in purified water. This slurry was applied to a substrate with an active material coating amount of 106 g / m 2 , active material bulk density 1.55g / cm 3 The paste was applied to one side of a 12 μm thick copper foil negative electrode current collector using a die coater at a high packing density of 100 μm. The foil was then dried at 120°C for 3 minutes, compression molded using a roll press, and cut into strips approximately 58 mm wide. <Preparation of non-aqueous electrolyte> The solution was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 1 / 2 (volume ratio) to a concentration of 1.0 mol / L. <separator> The separators described in the Examples and Comparative Examples were slit into strips of 60 mm. <Battery assembly> An electrode plate laminate was produced by stacking a strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator in that order, and spirally winding the stack multiple times under a winding tension of 250 gf. This electrode plate laminate was placed in a stainless steel container with an outer diameter of 18 mm and a height of 65 mm, and an aluminum tab extending from the positive electrode current collector was welded to the container lid terminal, and a nickel tab extending from the negative electrode current collector was welded to the container wall. After drying for 12 hours at 80°C under vacuum, the nonaqueous electrolyte was poured into the container in an argon box and the container was sealed. <Impact resistance test> The assembled batteries were first charged at a constant current of 500 mA. After the battery voltage reached 4.20 V, they were charged at each constant voltage until the current value fell below 10 mA, resulting in fully charged batteries. Next, the fully charged cylindrical batteries were placed with the long side facing horizontal, and a 9.1 kg rod with a diameter of 15.8 mm was dropped from a height of 61 cm onto the flat center surface of each battery to impact each battery. Batteries that generated heat of 90°C or higher due to the impact were rated as × (fail), those that generated heat of 100°C or higher were rated as △ (fail), those that did not generate heat of 100°C or higher were rated as ○ (good), and those that did not generate heat of 110°C or higher were rated as ◎ (excellent).

[0085] (11) Overall rating An overall evaluation was made based on the results of the shutdown capability in the event of thermal runaway and the shock resistance. If one was good (◯) and the other excellent (◎), the overall evaluation was excellent (◎); if both were good (◯), the overall evaluation was good (◯); and if either was poor (△, ×), the overall evaluation was poor (×).

[0086] (12) Pulsed NMR The amorphous component (α135) at 135°C was determined by pulse NMR. [Apparatus and measurement conditions] Device: Bruker mq20 Measurement method: Solid echo method Measured nuclear frequency: 19.95 MHz (1H square) Pulse width: 2.18 seconds (90°C pulse) Pulse repetition time: 3s Measurement temperature: 135℃ [Three-component approximation of free induction decay obtained from pulsed NMR] Assuming that the resin constituting the polyolefin microporous membrane is in three states: a crystalline component (A), an intermediate component (B), and an amorphous component (C), the polyolefin microporous membranes obtained in the examples were measured by pulse NMR (solid echo method) and the free induction decay (M(t)) was fitted using Equation 1 by the least squares method to approximate the three components: a crystalline component (A), an intermediate component (B), and an amorphous component (C), and the proportion of the amorphous component was calculated. M(t)=A×exp(-(1 / 2)(t / Ta) 2 )sinet / et+B×exp(-(1 / Wd)(t / Tb)Wd)+C×exp(-t / Tc) Equation 1 A: Crystalline component composition (%) Ta: relaxation time of crystalline components (msec) B: Composition rate of intermediate component (%) Tb: Relaxation time of intermediate component (msec) C: Amorphous component composition (%) Tc: Relaxation time of amorphous components (msec) t: Observation time (msec) Wd: Shape factor (1 <Wd<2) e: Shape factor (0.1 <e<0.2)。

[0087] (13) Raman spectroscopy The degree of orientation was determined by Raman spectroscopy. 〔Device〕 The measurement device used was the inVia micro-Raman spectroscopy system (manufactured by Renishaw). 180° backscattering configuration, 250mm spectral length, 3000 lines / mm diffraction grating, 532nm excitation laser 50x objective lens (NA=0.75) Spot size (spatial resolution) 5μm [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 each direction at 15° intervals, with MD being 0°. [Calculation of peak intensity] The Raman spectrum obtained was -1More than 1160cm -1 Obtain a baseline by linear approximation in the following region, 1060 cm -1 and 1130cm -1 The peak intensities were calculated by peak fitting using a Gaussian-Lorentzian mixed function approximation. [Orientation degree] 1130cm -1 and 1060cm -1 The peak intensity ratio (I1130 / I1060) was taken as the degree of orientation. The machine direction axis was set at 0°, and measurements were taken over 360° in 15° increments to determine the sum of the degrees of orientation in each direction (15° × n (1≦n≦24 (n is an integer))). The maximum value (Rmax) of the degrees of orientation in each direction was divided by the minimum value (Rmin) to determine the value of Rmax / Rmin. The MD orientation / TD orientation was calculated from the degree of orientation at 0° (MD orientation) and the degree of orientation at 90° (TD orientation).

[0088] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0089] Example 1 Weight average molecular weight (Mw) is 2.4 x 10 6 and 40% by mass of ultra-high molecular weight polyethylene (UHMwPE), with a terminal vinyl group concentration of 8.3 per 10,000 carbon atoms and Mw of 3.0 × 10 5100 parts by mass of a polyethylene (PE) composition consisting of 60% by mass of high-density polyethylene (HDPE) containing C3 or longer long-chain branches was dry-blended with 0.375 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)propionate]methane to obtain a mixture. 25.0 parts by mass of the resulting mixture was fed into a high-mixing twin-screw extruder, and 75.0 parts by mass of liquid paraffin was fed from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 210°C to prepare a polyethylene solution. The resulting polyethylene solution was fed from the twin-screw extruder to a T-die and extruded to form a sheet-like molded product. The output of the twin-screw extruder was adjusted so that the final film thickness when fed to the T-die would be the value listed in Table 1. The extruded molded product was taken up with a cooling roll to form a gel-like sheet. The resulting gel-like sheet was longitudinally stretched using a roll method at a stretching temperature of 115°C to 5.5 times its original size. The membrane was then introduced into a tenter and transversely stretched to a stretching ratio of 9.0 at a stretching temperature of 125.0°C. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried and re-stretched in the tenter at 130.0°C to a stretching ratio of 1.5, followed by heat relaxation. The wound body was placed in a thermostatic chamber at 60°C for 24 hours for aging treatment to obtain a 9 μm polyolefin microporous membrane.

[0090] Example 2 A microporous membrane was obtained in the same manner as in Example 1, except that the ratio and temperature of longitudinal stretching, transverse stretching, and transverse re-stretching were changed to the conditions shown in Table 1, and the output of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 1.

[0091] Example 3 A microporous membrane was obtained in the same manner as in Example 1, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those conditions shown in Table 1, and the discharge rate of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 1.

[0092] Example 4 Weight average molecular weight is 1.5 x 10 6and 40% by mass of ultra-high molecular weight polyethylene having a terminal vinyl group concentration of 6.5 per 10,000 carbon atoms and a Mw of 3.0 × 10 5 A microporous membrane was obtained in the same manner as in Example 1, except that a polyethylene composition consisting of 60% by mass of high-density polyethylene having long chain branches was used, the ratio and temperature of longitudinal stretching, transverse stretching, and transverse re-stretching were changed to those shown in Table 1, and the output rate of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 1. Example 5 Weight average molecular weight is 1.8 x 10 6 60% by mass of ultra-high molecular weight polyethylene and Mw of 9.0 × 10 4 and 40% by mass of a low-molecular-weight, high-density polyethylene, a microporous membrane was obtained in the same manner as in Example 1, except that the longitudinal stretching ratio and temperature, transverse stretching ratio and temperature, and transverse re-stretching ratio and temperature were changed to those shown in Table 1, and the output rate of the twin-screw extruder was adjusted so that the final membrane thickness would be as shown in Table 1. Examples 6 to 9 A microporous membrane was obtained in the same manner as in Example 5, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to the conditions shown in Table 1, and the output of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 1.

[0093] Comparative Example 1 A microporous membrane was obtained in the same manner as in Example 1, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those conditions shown in Table 2, and the discharge rate of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 2.

[0094] Comparative Example 2 A microporous membrane was obtained in the same manner as in Example 1, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those conditions shown in Table 2, and the discharge rate of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 2.

[0095] Comparative Example 3 Weight average molecular weight is 2.4 x 10 6 and 40% by mass of ultra-high molecular weight polyethylene having a terminal vinyl group concentration of 1.0 or less per 10,000 carbon atoms and a Mw of 3.0 × 105 A microporous membrane was obtained in the same manner as in Example 1, except that a polyethylene composition consisting of 60 mass% of high-density polyethylene containing no long chain branches was used, the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those shown in Table 2, and the output rate of the twin-screw extruder was adjusted so that the final membrane thickness would be as shown in Table 2.

[0096] Comparative Example 4 A microporous membrane was obtained in the same manner as in Comparative Example 3, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those conditions shown in Table 2, and the output of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 2.

[0097] Comparative Example 5 A microporous membrane was obtained in the same manner as in Comparative Example 3, except that the ratio and temperature of longitudinal stretching, the ratio and temperature of transverse stretching, and the ratio and temperature of re-transverse stretching were changed to those conditions shown in Table 2, and the output of the twin-screw extruder was adjusted so that the final film thickness would be as shown in Table 2.

[0098] The resin compositions, film-forming conditions, physical properties, pulse NMR results, and Raman orientation degrees of the polyolefin microporous membranes obtained in Examples 1 to 4 and Comparative Examples 1 to 8 are shown in Tables 1 and 2, and the results of the shutdown properties during thermal runaway, impact resistance, and overall evaluation are shown in Table 3.

[0099] [Table 1]

[0100] [Table 2]

[0101] [Table 3]

[0102] As can be seen from Tables 1 to 3, the polyolefin microporous membranes of Examples 1 to 9 have excellent shutdown properties during thermal runaway and excellent impact resistance.

Claims

1. A polyolefin microporous membrane comprising polyethylene as the resin constituting the membrane, wherein the total degree of orientation obtained by measuring over 360° in 15° increments by Raman spectroscopy is between 70 and 90, and the proportion of amorphous components (α135) at 135°C measured by the solid echo method of pulsed NMR is 35% or more.

2. The polyolefin microporous membrane according to claim 1, wherein the ratio of the degree of orientation at 0° (MD orientation) to the degree of orientation at 90° (TD orientation) obtained by Raman spectroscopy is in the range of 0.8 to 1.

3.

3. 3. The polyolefin microporous membrane according to claim 1, wherein the value obtained by dividing the largest orientation degree (Rmax) by the smallest orientation degree (Rmin) obtained by measuring 360° at 15° intervals by Raman spectroscopy is in the range of 1.0 or more and 1.5 or less.

4. The polyolefin microporous membrane according to any one of claims 1 to 3, having a membrane thickness in the range of 5 to 12 µm.

5. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 4.

6. The battery separator according to claim 5, wherein a porous layer is laminated on the polyolefin microporous film.

7. 7. The battery separator according to claim 6, wherein the porous layer contains at least one resin selected from the group consisting of a fluorine-based resin, an acrylic-based resin, a polyvinyl alcohol-based resin, and a carboxymethyl cellulose-based resin, and inorganic particles.

Images