Polyethylene microporous membrane, method for producing the same, and separator containing the microporous membrane

Abstract

To provide a polyethylene microporous membrane excellent in the mechanical strength and permeability and significantly improved in the heat resistance at high temperature, a method for manufacturing the same, and a separator including the microporous membrane.SOLUTION: According to an aspect, a polyethylene microporous membrane contains a polyethylene with a weight average molecular weight of 1×105 g / mol to 10×105 g / mol. The microporous membrane has a thickness of 3 μm to 20 μm, a puncture strength of 0.25 N / μm or more, a gas permeability of 1.5×10-5 Darcy or more, a shrinkage rate in the transverse direction of 10% or less as measured after being left at 131°C for 1 hour, a tensile strength in the machine direction of 1500 kg / cm2 or more, a tensile strength in the transverse direction of 2000 kg / cm2 or more, and a ratio between the tensile strength in the machine direction and the tensile strength in the transverse direction of 0.5 to 0.7.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a polyethylene microporous membrane, a method for producing the same, and a separator containing the microporous membrane. In one embodiment, the disclosure relates to a polyethylene microporous membrane with improved heat resistance, a method for producing the same, and a separator containing the microporous membrane. [Background technology]

[0002] Polyethylene microporous membranes are used in a variety of fields, including separation filters, separators for secondary batteries, separators for fuel cells, and separators for supercapacitors. In particular, they are widely used as separators for secondary batteries due to their excellent electrical insulation and ion permeability.

[0003] In recent years, secondary batteries have become larger and more powerful for applications in electric vehicles, ESS (Energy Storage Systems), and other applications, making battery safety an even more crucial factor. For example, when batteries are exposed to or operated in high-temperature environments, the separator may contract, causing an internal short circuit, which could lead to a fire. Therefore, there is a need to develop a heat-resistant polyethylene microporous membrane that can withstand the temperature rise of the battery. Along with heat resistance, high mechanical strength is required to improve safety during the battery manufacturing process and use, and high permeability is required to improve capacity and output.

[0004] As a method to enhance such safety, Korean Patent Publication No. 10-2016-0106177 describes a method where the air permeability resistance (calculated at 16 μm) is 100-220 sec / 100 cc, the puncture strength (calculated at 16 μm) is 550 gf or more, and the semi-crystallization time (t) during isothermal crystallization at 117°C is t. 1 / 2A polyolefin microporous membrane is disclosed with a shrinkage time of 10 to 35 minutes. However, such a microporous membrane has the disadvantage of being unsuitable for high-capacity and large-scale batteries because its lateral shrinkage rate at 105°C is only about 6%, resulting in poor battery safety in high-temperature environments such as hot-box testing.

[0005] Therefore, there is a need to develop polyethylene microporous membranes that not only have high mechanical strength and permeability, but also significantly improved heat resistance at higher temperatures, and in particular, can achieve excellent battery safety in hot box evaluations at 130°C, which is an evaluation index for high-temperature safety. [Overview of the project] [Problems that the invention aims to solve]

[0006] To solve the above-mentioned problems, this disclosure aims to provide a polyethylene microporous membrane that is excellent in mechanical strength and permeability and has significantly improved heat resistance at high temperatures, a method for manufacturing the same, and a separator containing the microporous membrane.

[0007] In particular, the objective is to provide a polyethylene microporous membrane that can achieve excellent battery safety even after battery assembly and evaluation at a hot box of 130°C.

[0008] The microporous membrane of this disclosure is widely applicable to green technology fields such as electric vehicles, battery charging stations, and solar and wind power generation that use batteries. Furthermore, the microporous membrane of this disclosure can be used in environmentally friendly electric vehicles and hybrid vehicles that reduce air pollution and greenhouse gas emissions and prevent climate change. The microporous membrane can be used as a separator in secondary batteries. [Means for solving the problem]

[0009] As one means for achieving the above problems, the present disclosure includes polyethylene having a weight average molecular weight of 1×10 5 g / mol to 10×10 5 g / mol, the thickness of the microporous membrane is 3 μm to 20 μm, the puncture strength is 0.25 N / μm or more, the gas permeability is 1.5×10 -5 Darcy or more, the lateral shrinkage rate measured after leaving at 131° C. for 1 hour is 10% or less, the tensile strength in the longitudinal direction (TS MD ) is 1500 kg / cm 2 or more, the tensile strength in the lateral direction (TS TD ) is 2000 kg / cm 2 or more, and the ratio of the tensile strength in the longitudinal direction to the tensile strength in the lateral direction (TS MD / TS TD ) is 0.5 to 0.7, and provides a polyethylene microporous membrane.

[0010] In one aspect, the polyethylene microporous membrane may have a tensile strength in the longitudinal direction of 2000 kg / cm 2 or more and a tensile strength in the lateral direction of 2800 kg / cm 2 or more.

[0011] In one aspect, the average value of the tensile strength in the longitudinal direction and the tensile strength in the lateral direction may be 2500 kg / cm 2 or more. The average value is calculated by (TS MD +TS TD ) / 2.

[0012] In one aspect, the polyethylene microporous membrane may have a puncture strength of 0.50 N / μm or more.

[0013] In one aspect, the polyethylene microporous membrane may have a porosity of 30% to 70%.

[0014] In one aspect, the polyethylene microporous membrane may be produced by a wet method including a sequential biaxial stretching process.

[0015] Furthermore, as another means to achieve the above-mentioned objectives, this disclosure provides (a) a weight-average molecular weight of 1 × 10 5 g / mol ~ 10 × 10 5 The process includes the steps of: (b) producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent in g / mol using an extruder; (c) forming the molten product into a sheet by extruding it; (d) forming a film by sequentially biaxially stretching the sheet in the longitudinal and transverse directions; (e) extracting diluent from the stretched film and drying it; and (c) heat-treating the dried film at a temperature at which 20% to 50% of the crystals of the dried film melt, wherein the longitudinal stretch ratio (SR) of step (c) is MD ) is 4.0 times or more, and the lateral elongation ratio (SR) TD ) is 5.3 times or more, and the ratio of the vertical stretch ratio to the horizontal stretch ratio (SR MD / SR TD The present invention provides a method for manufacturing a polyethylene microporous membrane in which the coefficient of porosity (C) is 0.5 to 0.75.

[0016] In one embodiment, the temperature at which 20% to 50% of the crystals of the film dried in step (e) melt may be 133°C to 145°C.

[0017] In other embodiments of this disclosure, a separator comprising the polyethylene microporous membrane is provided. The preparation of the separator from the microporous membrane may include cutting the microporous membrane to a desired size depending on the application. Furthermore, the porosity and thickness of the microporous membrane may be adjusted within a range of such porosity and thickness depending on the specific application.

[0018] In one embodiment, an electrochemical element including the separator is provided.

[0019] In one embodiment, the electrochemical element may be a secondary battery that includes the separator between the positive electrode and the negative electrode.

[0020] Furthermore, as another means of achieving the above-mentioned objectives, this disclosure provides a separator comprising the polyethylene microporous membrane described above. [Effects of the Invention]

[0021] The polyethylene microporous membrane according to this disclosure has excellent mechanical strength and permeability, and can also ensure significantly improved heat resistance at high temperatures.

[0022] Furthermore, the polyethylene microporous membrane relating to this disclosure has a perforation strength of 0.25 N / μm or more and a tensile strength in the longitudinal direction (machine direction, MD) of 1500 kg / cm². 2 In summary, the tensile strength in the transverse direction (TD) is 2000 kg / cm². 2 The above results show that the gas permeability is 1.5 × 10⁻⁶. -5 It can be Darcy or better.

[0023] Furthermore, the polyethylene microporous membrane according to this disclosure may have a lateral shrinkage rate of 10% or less, measured after being left at 131°C for 1 hour.

[0024] Furthermore, by including a polyethylene microporous membrane according to one embodiment, this disclosure can provide a battery with excellent high-temperature thermal safety, which does not smoke or ignite in high-temperature environments. In detail, this disclosure can provide a battery with excellent high-temperature thermal safety, which does not smoke or ignite in hot box evaluation at a high temperature of 130°C. [Modes for carrying out the invention]

[0025] The embodiments described herein can be modified into various other forms, and the technology relating to one aspect is not limited to the embodiments described below. Furthermore, embodiments of one aspect are provided to give a more complete explanation of this disclosure to a person with average skill in the art.

[0026] Furthermore, the singular form used in the specification and the attached claims is intended to include plural forms unless otherwise indicated in the context.

[0027] Furthermore, the numerical ranges used herein include lower and upper limits, all values ​​within those limits, increments logically derived from the form and width of the defined range, all double-limited values, and all possible combinations of upper and lower limits of numerical ranges limited in different forms. Unless otherwise defined herein, values ​​outside the numerical range that may occur due to experimental error or rounding up are also included in the defined numerical range.

[0028] Furthermore, throughout the specification, "including" a certain component means, unless otherwise stated, that it may include other components rather than excluding them.

[0029] This disclosure relates to the longitudinal tensile strength (TS MD ) and tensile strength in the lateral direction (TS TD ) ratio (TS MD / TS TD This invention was conceived with the observation that, when controlled within a specific range, it is possible to achieve excellent mechanical strength and transparency even at very thin thicknesses, while also ensuring significantly improved heat resistance at high temperatures.

[0030] According to one embodiment, the weight-average molecular weight is 1 × 10 5 g / mol ~ 10 × 10 5 It contains polyethylene in a concentration of g / mol and has a longitudinal tensile strength (TS) above a specified value. MD ) and tensile strength in the lateral direction (TS TD ) satisfies the ratio of the tensile strength in the longitudinal direction to the tensile strength in the transverse direction (TS MD / TS TD When controlling the material to a specific range, even with a thin thickness in the range of 3 μm to 20 μm, the puncture strength is 0.25 N / μm or higher, and the gas permeability is 1.5 × 10⁻⁶. -5 We can provide a polyethylene microporous membrane that is darcy or higher and has a lateral shrinkage rate of 10% or less measured after being left at 131°C for 1 hour.

[0031] A secondary battery according to one embodiment can ensure both excellent battery performance and safety by including a polyethylene microporous membrane that satisfies both of the above-mentioned physical properties. In particular, this disclosure can provide a battery with excellent high-temperature thermal safety, in which the battery does not smoke or catch fire during hot box evaluation at a high temperature of 130°C. That is, the polyethylene microporous membrane of this disclosure is suitably applicable to high-power / high-capacity batteries because it satisfies both of the above-mentioned physical properties.

[0032] In one embodiment, a polyethylene microporous membrane that satisfies both of the above-mentioned physical properties can be manufactured by a stretching process under specific conditions, thereby achieving a ratio of longitudinal tensile strength to transverse tensile strength within a specific range.

[0033] In one embodiment, the polyethylene microporous membrane can be manufactured by a stretching process under specific conditions, along with a heat setting process under specific conditions, thereby achieving a ratio of longitudinal tensile strength to transverse tensile strength within a specific range.

[0034] In one embodiment, the polyethylene microporous membrane may be manufactured by a wet process including a sequential biaxial stretching step with a specific stretch ratio or higher, thereby providing a polyethylene microporous membrane that satisfies all of the above-mentioned physical properties.

[0035] Specifically, the polyethylene microporous membrane is manufactured by extruding a mixture of polyethylene resin in which diluent is dissolved, and then sequentially biaxially stretching it to a specific stretch ratio to produce a film, and then extracting diluent from the film. It can be manufactured by a wet process that includes a normal sequential biaxial stretching step known to those skilled in the art, and is not limited as long as a microporous membrane having the above-mentioned physical properties can be produced.

[0036] One embodiment is the longitudinal elongation ratio (SR MD ) is 4.0 times or more, and the lateral elongation ratio (SR) TD ) is 5.3 times or more, and the ratio of the vertical stretch ratio to the horizontal stretch ratio (SR MD / SR TDThe ratio may be 0.5 to 0.75. As a result, the polyethylene microporous membrane can have excellent mechanical strength and permeability even at a thin thickness, as well as significantly improved heat resistance at high temperatures.

[0037] The polyethylene microporous membrane will be described in more detail below.

[0038] In one embodiment, the ratio of the tensile strength in the longitudinal direction to the tensile strength in the transverse direction (TS MD / TS TD ) may be 0.5 or greater, 0.65 or less, or 0.7 or less, for example, 0.5 to 0.7 or 0.5 to 0.65.

[0039] In one embodiment, the polyethylene microporous membrane has a longitudinal tensile strength (TS MD ) is 1500 kg / cm³ 2 More than 1800kg / cm 2 Above 2000 kg / cm³ 2 The above is sufficient, TS MD There is no particular upper limit on the value, but for example, 3500 kg / cm³ 2 Below 3000kg / cm 2 The following may be the case. In a specific example, the TS MD The value is 1500 kg / cm³. 2 ~3500 kg / cm 2 , 1800 kg / cm 2 ~3500 kg / cm 2 , or 2000 kg / cm² 2 ~3000 kg / cm 2 This may be the case, but it is not the only option.

[0040] In one embodiment, the microporous membrane disclosed herein has a transverse tensile strength (TS TD ) is 2000 kg / cm³ 2 More than 2500kg / cm 2 More than 2800kg / cm 2 Above 3000 kg / cm³ 2 The above is sufficient, TS TDThere is no particular upper limit on the value, but for example, 5000 kg / cm³ 2 Below 4500kg / cm 2 The following may be the case. In a specific example, the TS TD The value is 2000 kg / cm³. 2 ~5000 kg / cm 2 , 2500 kg / cm 2 ~5000 kg / cm 2 , 2800 kg / cm 2 ~4500 kg / cm 2 , or 3000 kg / cm² 2 ~4500 kg / cm 2 This may be the case, but it is not the only option.

[0041] In one embodiment, the average of the longitudinal tensile strength and the transverse tensile strength is 2500 kg / cm². 2 More than 2600kg / cm 2 Above 2700 kg / cm³ 2 The above is sufficient, and there is no particular upper limit on the average value, but for example, 4250 kg / cm³ 2 Below 4000kg / cm 2 The following, or 3750 kg / cm³ 2 The following may apply: In one specific embodiment, the average value is 2500 kg / cm³. 2 ~4250 kg / cm 2 , 2600 kg / cm 2 ~4000 kg / cm 2 , or 2700 kg / cm² 2 ~3750 kg / cm 2 This may be the case, but it is not the only option.

[0042] TS within the above range MD Value, TS TD These average values ​​can be achieved in the manufacturing of polyethylene microporous membranes by controlling the longitudinal and transverse stretching ratios in the stretching process to a specific ratio, or by controlling the temperature in the heat-setting process to a specific range, but are not necessarily limited to these methods.

[0043] A microporous membrane according to one embodiment, despite having a thin thickness, has the above-mentioned range of TS MD / TS TD In addition to possessing a value, it has significantly improved tensile strength, resulting in less thermal shrinkage and significantly increased heat resistance. Specifically, as a result of the following high-temperature hot-box evaluation of a battery manufactured using a microporous membrane according to one embodiment as a separator, the battery does not produce smoke or fire in a high-temperature environment, thus demonstrating excellent battery safety. The manufacturing and evaluation method of the battery for the above evaluation is as follows: A positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material are wound together with the microporous membrane of this disclosure and placed in an aluminum pouch to manufacture a battery. Next, an electrolyte in which 1M lithium hexafluorophosphate (LiPF6) is dissolved in a solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2 is injected into the inside of the battery and sealed to produce a 2Ah capacity battery. Next, the battery is subjected to aging and degassing processes, fully charged to 4.2V, placed in an oven and heated to 5°C / min, then left for 30 minutes after reaching 130°C, and checked for any smoke or fire from the battery.

[0044] In one embodiment, the thickness of the polyethylene microporous membrane is 3 μm to 20 μm, specifically 3 μm to 15 μm, more specifically 3 μm to 10 μm, and even more specifically 3 μm to 7 μm. The microporous membrane of this disclosure can achieve excellent puncture strength, gas permeability, thermal shrinkage rate, and tensile strength even in a very thin film state with a thickness within the above range, and in particular, it has been shown to have excellent thermal safety in hot box evaluation using the evaluation method of the following examples. Furthermore, it has excellent resistance to external stress generated during battery manufacturing and to temperature rise and dendrite formation that occur during battery charging and discharging, and because the internal resistance of the battery is low, the battery's charging and discharging performance can be improved.

[0045] In one aspect, even if the polyethylene microporous membrane has a thickness within the above range, it can have a puncture strength of 0.25 N / μm or more. Specifically, the puncture strength may be 0.40 N / μm or more, or 0.50 N / μm or more, and the upper limit is not particularly limited, but for example, it may be 1.0 N / μm or less. In a specific aspect, the puncture strength may be 0.25 N / μm to 1.0 N / μm, 0.40 N / μm to 1.0 N / μm, or 0.50 N / μm to 1.0 N / μm, but is not limited thereto. By satisfying the puncture strength within the above range, it is excellent in resistance to external stress generated during battery manufacturing and dendrites generated during battery charging and discharging, and can ensure battery safety. In addition, the separator for secondary batteries can be thinned and is suitably applicable to high-capacity / high-output batteries.

[0046] In one aspect, the polyethylene microporous membrane can have a gas permeability of 1.5×10 -5 Darcy or more. Specifically, the gas permeability may be 1.6×10 -5 Darcy or more, 3.0×10 -5 Darcy or less, or 5.0×10 -5 Darcy or less. In a specific aspect, the gas permeability may be 1.5×10 -5 Darcy to 5.0×10 -5 Darcy or 1.6×10 -5 Darcy to 3.0×10 -5 Darcy, but is not limited thereto. By satisfying the gas permeability within the above range, it can have excellent ionic conductivity, and the charging and discharging characteristics of the battery can be improved due to the low internal resistance of the battery.

[0047] In one aspect, the polyethylene microporous membrane may have a lateral shrinkage rate, measured after being left at 131°C for 1 hour, of 10% or less, 8% or less, or 6% or less, and the lower limit is not particularly limited, and may be, for example, 0.1%, 0.5%, or 1%. In a specific aspect, the shrinkage rate may be 0.1% to 10%, 0.5% to 8%, or 1% to 6%, but is not limited thereto.

[0048] In one aspect, from the viewpoints of mechanical strength and ionic conductivity, the polyethylene microporous membrane may have a porosity of 30% to 70%, may be 40% to 70% or 50% to 70%, but is not limited thereto. The porosity is calculated by the following mathematical formula. Specifically, a sample with a horizontal length of A cm, a vertical length of B cm, and a thickness of T cm is prepared, and its mass is measured, and the porosity is calculated from the ratio of the resin mass of the same volume to the mass of the microporous membrane.

[0049] Porosity (%) = 100 × {1 - M / (A × B × T × ρ)} In the above mathematical formula, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm 3 ) of the polyethylene resin constituting the microporous membrane.

[0050] In one aspect, the polyethylene microporous membrane may be produced by a wet method including a sequential biaxial stretching process, whereby a polyethylene microporous membrane satisfying the above physical properties can be provided.

[0051] Specifically, the polyethylene microporous membrane is produced by extruding and sequentially biaxially stretching a mixture of a polyethylene resin and a diluent into a film shape, and extracting the diluent from the film, and can be produced by a wet method including a normal sequential biaxial stretching process known to those skilled in the art, and is not limited as long as a microporous membrane having the above physical properties can be produced.

[0052] As one aspect, the longitudinal draw ratio (SR MD ) is 4.0 times or more, and the lateral draw ratio (SR TD) is 5.3 times or more, and the ratio of the vertical stretch ratio to the horizontal stretch ratio (SR MD / SR TD The ratio may be 0.5 to 0.75. As a result, the polyethylene microporous membrane can have excellent mechanical strength and permeability even at a thin thickness, as well as significantly improved heat resistance at high temperatures.

[0053] The method for manufacturing the polyethylene microporous membrane described herein will be explained below.

[0054] A method for producing a polyethylene microporous membrane according to one embodiment includes (a) a weight-average molecular weight of 1 × 10 5 g / mol ~ 10 × 10 5 The process includes the steps of: (b) producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent in g / mol using an extruder; (c) forming the molten product into a sheet by extruding it; (d) forming a film by sequentially biaxially stretching the sheet in the longitudinal and transverse directions; (e) extracting diluent from the stretched film and drying it; and (c) heat-treating the dried film at a temperature at which 20% to 50% of the crystals of the dried film melt, wherein the longitudinal stretch ratio (SR) of step (c) is MD ) is 4.0 times or more, and the lateral elongation ratio (SR) TD ) is 5.3 times or more, and the ratio of the vertical stretch ratio to the horizontal stretch ratio (SR MD / SR TD ) may be between 0.5 and 0.75.

[0055] The following describes each manufacturing step.

[0056] First, step (a) is a step of producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent using an extruder, wherein the mixture contains polyethylene resin and diluent in a weight ratio of 10-60:90-40 for pore formation, specifically in a weight ratio of 20-40:80-60, but is not particularly limited as long as the objectives of this disclosure are achieved. When the weight ratio within the above range is satisfied, the molten product has sufficient fluidity, a uniform sheet can be easily formed in the subsequent steps, and sufficient orientation is achieved during the stretching process, ensuring mechanical strength, thus preventing problems such as breakage during the stretching process.

[0057] The polyethylene resin may be high-density polyethylene or contain high-density polyethylene, from the viewpoint of strength, extrusion and kneadability, stretchability, and heat resistance of the final microporous membrane.

[0058] In one embodiment, the polyethylene resin has a weight-average molecular weight of 1 × 10 5 g / mol ~ 10 × 10 5 It may be g / mol, specifically 3 × 10 5 g / mol ~ 8 × 10 5 The expression may be in g / mol, but is not necessarily limited to this.

[0059] In one embodiment, the polyethylene resin may have a melting temperature of 130°C or higher, specifically 130°C to 140°C, but is not necessarily limited thereto. The melting temperature of the polyethylene resin can be determined by DSC.

[0060] In one embodiment, the diluent can be any organic compound that forms a single phase with the polyethylene resin at the extrusion temperature, without any limitations. For example, the diluent may be one or more combinations selected from the group consisting of aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraffin oil, and paraffin wax; phthalic acid esters such as dibutyl phthalate and dioctyl phthalate; C10-C20 fatty acids such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; and C10-C20 fatty alcohols such as cetyl alcohol, stearyl alcohol, and oleyl alcohol. One specific example of the aforementioned diluent is, but is not limited to, paraffin oil having a kinetic viscosity of 20 cSt to 200 cSt at 40°C.

[0061] Furthermore, the mixture may further contain one or more of the usual additives used to improve specific functions, such as oxidation stabilizers, UV stabilizers, and antistatic agents, to the extent that the properties of the microporous membrane do not significantly deteriorate.

[0062] (b) Step is a step of extruding the molten material into a sheet, which may be carried out in a manner known to those skilled in the art, and may be, for example, by extruding the molten material with a T-die and forming it into a sheet by casting or calendering while cooling to a temperature of 10°C to 80°C.

[0063] (c) Step, the longitudinal stretch ratio (SR MD ) is 4.0 times or more, and the lateral elongation ratio (SR) TD ) is 5.3 times or more, and the ratio of the vertical stretch ratio to the horizontal stretch ratio (SR MD / SR TD The tensile strength in the longitudinal direction (TS) may be between 0.5 and 0.75. MD ) and tensile strength in the lateral direction (TS TD ) ratio (TS MD / TS TD By controlling the properties within a specific range, it is possible to manufacture polyethylene microporous membranes that have excellent mechanical strength and permeability even at thin thicknesses, as well as significantly improved heat resistance at high temperatures.

[0064] The longitudinal stretch ratio may be specifically 4 to 10 times, more specifically 5 to 8 times, and the transverse stretch ratio may be 5.3 to 15 times, more specifically 6 to 10 times. By satisfying the above ranges for the stretch ratios in each direction, a polyethylene microporous membrane having the physical properties targeted in this disclosure can be manufactured.

[0065] The aforementioned SR MD / SR TD By satisfying the range of 0.5 to 0.75, the ratio of longitudinal to transverse tensile strengths that this disclosure aims to achieve, along with 1500 kg / cm², is achieved. 2 The above longitudinal tensile strength and 2000 kg / cm² 2 The above lateral tensile strength can be ensured.

[0066] (c) The stretching in step (c) is carried out by a sequential stretching method using a roll or tenter, and may be performed at a temperature in the range of 60°C below the melting temperature of polyethylene to the melting temperature of polyethylene. When stretching is performed at the above-mentioned temperature range, the fluidity of the polyethylene resin can be ensured for effective stretching. Specifically, the sheet is stretched uniformly throughout and no breakage occurs due to stretching, so the stretching can be performed stably. As a result, a high-quality microporous membrane can be manufactured with uniform physical properties such as gas permeability, perforation strength, and tensile strength throughout the entire membrane. As an example, the stretching may be carried out at 80°C to 140°C or 90°C to 120°C, but is not limited thereto.

[0067] Step (d) is to extract the diluent from the stretched film and dry it, which is done by extracting the diluent from the film using an organic solvent and drying the organic solvent with the film in which the diluent has been replaced by the organic solvent. The organic solvent is not particularly limited as long as it can extract the diluent. Specifically, methyl ethyl ketone, methylene chloride, hexane, etc. may be used as the organic solvent because they have high extraction efficiency and dry quickly.

[0068] (d) Step is preferably carried out at a high temperature to increase the solubility of the diluent and the organic solvent, but may be carried out at a temperature of 40°C or lower from the viewpoint of safety due to the boiling of the organic solvent.

[0069] (e) Step is to heat-treat the dried film at a temperature at which 20% to 50% of the crystals in the dried film melt, using a roll-type or tenter-type apparatus, and applying heat while forcibly holding the film so as not to cause dimensional changes in the longitudinal and transverse directions, thereby removing residual stress in the film and reducing the thermal shrinkage rate of the final polyethylene microporous membrane.

[0070] The ratio of the vertical stretch ratio to the horizontal stretch ratio within the above range (SR MD / SR TDWhen a film stretched using the above method is heat-treated at the temperatures described above, a polyethylene microporous film can be manufactured that has physical properties that cannot be achieved with microporous films manufactured by conventional methods. Microporous films with such properties are suitably applicable to high-power / high-capacity batteries. High-power / high-capacity batteries refer to secondary batteries that can transmit a considerable amount of power during discharge and can be used in applications requiring high power, such as electric vehicles, power tools, energy storage systems, and portable electronic devices.

[0071] In one embodiment, the temperature at which 20% to 50% of the crystals in the dried film melt varies depending on the molecular weight of the polyethylene used and the elongation rate of the film. In this disclosure, by using polyethylene with a high molecular weight as the polyethylene resin for manufacturing polyethylene microporous membranes, the temperature at which 20% to 50% of the film crystals melt is 133°C to 145°C, and specifically may be 135°C to 140°C. Conventionally, heat treatment was carried out at a temperature below 133°C, but microporous membranes manufactured by heat treatment at this temperature have the drawback of not satisfying sufficient physical properties. In contrast, in this disclosure, the above physical properties can be obtained by using highly crystalline polyethylene with a high melting point. For example, conventional microporous membranes either do not satisfy sufficient levels of gas permeability and puncture strength, or even if they have excellent gas permeability and puncture strength, they have the drawback of not being suitable for use as separators for secondary batteries because their thermal shrinkage rate is poor.

[0072] In one embodiment, the heat treatment in step (e) may be carried out at a temperature in the range of 133°C to 145°C, specifically, at a temperature in the range of 135°C to 140°C. By performing the heat treatment at the above-mentioned temperature range, a polyethylene microporous film satisfying all of the physical properties for which this disclosure is intended can be produced.

[0073] In one embodiment, step (e) may be thermal stretching and thermal relaxation followed by thermal fixing. That is, the tension can be adjusted during the heat treatment, and the heat treatment can be carried out in a variety of ways. The heat treatment may be repeated 1 to 3 times, but is not necessarily limited thereto.

[0074] Step (e) above may include, for example, a heat stretching step of stretching in the longitudinal or transverse direction, a heat setting step of applying heat while fixing the longitudinal and transverse length / width, and a heat relaxation step of relaxing (shrinking) in the longitudinal or transverse direction. For example, the heat relaxation step may be relaxing to 80% to 99% or 90% to 99% of the transverse width before the heat relaxation step, and the heat stretching step may be stretching to 120% to 160% or 140% to 160% of the transverse width before the heat stretching step.

[0075] This disclosure provides a separator comprising a polyethylene microporous membrane as described above, the separator may be any separator used in any known energy storage device, and is not particularly limited, but a non-limiting example is a separator used in a lithium secondary battery.

[0076] Examples and experimental cases are described below with specific illustrations. However, the examples and experimental cases described below are merely illustrative and the technology described herein is not limited thereto.

[0077] [Physical property measurement method] 1. Weight average molecular weight (g / mol) The weight-average molecular weight (Mw) was measured using high-temperature GPC (Gel Permeation Chromatography) from Agilent Technologies. PLgel Guard and PLgel Olexis were used as GPC columns, 1,2,4-trichlorobenzene (TCB) as the solvent, and polystyrene as the standard sample, and the analysis was performed at 140°C.

[0078] 2. Thickness of microporous membrane (μm) The thickness of the microporous membrane was measured using a contact-type thickness measuring instrument with a thickness accuracy of 0.1 μm. The measurement was performed using a TESA Mu-Hite Electronic Height Gauge from TESA Corporation, with a measurement pressure of 0.63 N.

[0079] 3.Punching strength (N / μm) Penetration strength was measured using an INSTRON UTM (Universal Test Machine) 3345, with a pin tip having a diameter of 1.0 mm and a radius of curvature of 0.5 mm attached, and by pressing a microporous membrane at a speed of 120 mm / min. The penetration strength was calculated by dividing the load (N) at which the microporous membrane ruptured by the thickness of the microporous membrane (μm).

[0080] 4. Gas permeability (Darcy) Gas permeability was measured using a porometer (CFP-1500-AEL from PMI). Generally, gas permeability is expressed by the Gurley number, but the Gurley number does not correct for the effect of thickness, making it difficult to understand the relative permeability due to the porosity structure. To solve this, the gas permeability in this disclosure is measured using the Darcy permeability constant calculated by the following mathematical formula 1. Nitrogen was used as the gas. 689.5~1379kPa( 100-200 psi ) The average value of the Darcy transmission constant measured in the region was calculated.

[0081] [Mathematical formula 1] Darcy transmission constant (C)=(8F·T·V) / (πD 2 (P 2 -1)) F=Flow rate (cc / min) T = Sample thickness (mm) V = viscosity of the gas (0.185 cp for N2) D = Diameter of the sample (mm) P = pressure (kPa)( (psi This is a reference value; 1 psi = 6.895 kPa ) )

[0082] 5. Tensile strength (kg / cm 2 ) Tensile strength was measured according to ASTM D882, using an INSTRON UTM (Universal Test Machine) 3345, by pulling the microporous membrane in both the lateral and mechanical directions at a speed of 100 mm / min, and measuring the strength at which the microporous membrane ruptured.

[0083] 6. Lateral contraction rate (%) A 15cm x 15cm microporous membrane, with its length and width indicated on a 10cm strip, was placed in a temperature-stabilized oven (DKN612, Yamato Scientific Co., Ltd.) at 131°C for 1 hour. The change in length was then measured, and the lateral shrinkage rate was calculated using the method shown in Mathematical Formula 2 below.

[0084] [Mathematical formula 2] Lateral shrinkage rate (%) = {(Lateral length before heating - Lateral length after heating) / Lateral length before heating} × 100

[0085] 7. Hotbox Evaluation The hot box evaluation was performed using a battery assembled with a polyethylene microporous membrane as the separator. Specifically, a positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material were wound together with the manufactured microporous membrane and placed in an aluminum pouch. An electrolyte solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2, with 1M lithium hexafluorophosphate (LiPF6) dissolved in it was then injected and sealed to assemble a 2Ah capacity battery. After aging and degassing the assembled battery, it was fully charged to 4.2V, placed in an oven and heated at 5°C / min until it reached 130°C, then left for 30 minutes to measure the changes in the battery.

[0086] If the battery emitted smoke or caught fire after being left at 130°C for 30 minutes, it was judged as a "Fail." If there was no change in the battery's voltage / current and neither smoke nor fire occurred, it was judged as a "Pass."

[0087] 8. Thermal property analysis of the film The phenomenon of the film crystals melting in response to temperature changes was analyzed using a Discovery DSC250 differential scanning calorimeter (DSC) from TA Instrument Co., Ltd., under conditions of a sample weight of 5 mg and a scan speed of 10°C / min.

[0088] In detail, the temperature at which 20% of the crystals in the film introduced into the heat fixing device melt (T m1 ) and the temperature at which 50% melts (T m2 ) represents the temperature (T) at the point where the heat capacity is 20% of the total heat capacity (Heat of fusion) of the film introduced into the heat fixing device. m1 ) and the temperature (T) at the point showing 50% heat capacity m2 It was measured using a method that calculates ).

[0089] 9.Porosity (%) The porosity was calculated using the following mathematical formula. Specifically, a sample with dimensions A cm (width), B cm (length), and T cm (thickness) was prepared, its mass was measured, and the porosity was calculated from the ratio of the resin mass to the microporous membrane mass for the same volume.

[0090] Porosity (%)=100×{1-M / (A×B×T×ρ)} In the above mathematical formula, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm³) of the polyethylene resin forming the microporous membrane. 3 )

[0091] <Example 1> Weight-average molecular weight is 5.0 × 10⁻⁶ 5 High-density polyethylene resin (0.95 g / cm³) has a melting point of 135°C and is expressed in g / mol. 3 A mixture containing paraffin oil, which has a kinematic viscosity of 80 cSt at 40°C, in a weight ratio of 30:70, was melt-kneaded using a twin-screw extruder to produce a molten product.

[0092] The molten material was continuously extruded using a T-die, and a sheet with a width of 300 mm and an average thickness of 700 μm was produced using a casting roll set to 30°C. The sheet was then stretched longitudinally using a roll method to increase its size sixfold at a stretching temperature of 100°C, and subsequently guided to a tenter for transverse stretching eightfold at a stretching temperature of 118°C.

[0093] Paraffin oil was extracted from the film, which had been stretched longitudinally and transversely at 25°C using methylene chloride, and the film from which the paraffin oil had been extracted was dried at 50°C. The dried film was then heat-stretched transversely to 150% at 136°C, the temperature at which 38% of the crystals in the dried film melt, using a tenter-type heat-fixing apparatus. After heat-fixing for 10 seconds, the film was heat-relaxed (shrinked) to 93% of its width before the heat relaxation process, thereby producing a microporous film with a thickness of 5 μm. The melting temperature was measured using a Discovery DSC250 differential scanning calorimeter (DSC) provided by TA Instruments, under conditions of a sample weight of 5 mg and a scan speed of 10°C / min.

[0094] The physical properties of the ultimately manufactured microporous membrane and the performance of the battery to which it was applied are recorded in Table 2 below.

[0095] <Examples 2-4 and Comparative Examples 1-5> A polyethylene microporous membrane was manufactured in the same manner as in Example 1, except that the stretch ratio in the longitudinal / horizontal direction and the heat-fixing temperature were changed to the conditions listed in Table 1. The physical properties of the finally manufactured microporous membrane and the performance of the battery to which it was applied are recorded in Table 2 below.

[0096] [Table 1]

[0097] [Table 2]

[0098] Referring to Tables 1 and 2 above, the microporous films of the examples satisfy all the physical properties targeted by this disclosure, and the batteries to which they are applied pass hot box evaluations, achieving excellent thermal safety.

[0099] The microporous membranes of Comparative Example 1 and Comparative Example 2 had a ratio of longitudinal stretch ratio to transverse stretch ratio (SR) greater than 0.75. MD / SR TD As a result of performing the stretching step in ), the ratio of the tensile strength in the longitudinal direction to the tensile strength in the transverse direction (TS MD / TS TD Since the coefficient of thermal expansion exceeds 0.7 and the lateral shrinkage rate at 131°C exceeds 10%, there is a drawback in that the thermal safety of batteries to which it is applied is significantly reduced.

[0100] The microporous membrane of Comparative Example 3 has a ratio of 0.75 between the longitudinal stretch ratio and the transverse stretch ratio (SR MD / SR TD The stretching step was performed at a lower temperature than in the example, but the ratio of the tensile strength in the longitudinal direction to the tensile strength in the transverse direction (TS MD / TS TD Since the coefficient of thermal expansion exceeds 0.7 and the lateral shrinkage rate at 131°C exceeds 10%, there is a drawback in that the thermal safety of batteries to which it is applied is significantly reduced.

[0101] The microporous membrane of Comparative Example 4 had a longitudinal stretch ratio of less than 4.0 times, and the ratio of the longitudinal stretch ratio to the transverse stretch ratio (SR) was less than 0.5. MD / SR TD As a result of performing the stretching step, the tensile strength in the longitudinal direction was low, causing the film to break during the process, making it impossible to manufacture the film.

[0102] The microporous membrane of Comparative Example 5 had a transverse stretch ratio of less than 5.3 times, and the ratio of the longitudinal stretch ratio to the transverse stretch ratio (SR) was greater than 0.75. MD / SR TD As a result of performing the stretching step in this disclosure, the gas permeability intended for this disclosure cannot be achieved, and the battery to which it is applied cannot achieve the performance required of a battery and suffers from the disadvantage of significantly reduced thermal safety.

[0103] As described above, this disclosure has been explained through specific matters and limited embodiments, which are provided only for a more general understanding of the disclosure, and the disclosure is not limited to the embodiments described above. A person with ordinary skill in the art to which this disclosure belongs can make various modifications and variations from such descriptions.

Claims

1. Weight-average molecular weight is 1 × 10 ５ g / mol~10×10 ５ It contains polyethylene in g / mol, The thickness of the microporous membrane is 3 μm to 20 μm, the puncture strength is 0.25 N / μm or more, the gas permeability is 1.48×10−17 m2 (1.5×10 －５ Darcy) or more, the lateral shrinkage rate measured after leaving it standing at 131 °C for 1 hour is 10% or less, and the longitudinal tensile strength (TS ＭＤ ) is 2000 kg / cm ２ or more, the lateral tensile strength (TS ＴＤ ) is 2800 kg / cm ２ or more, and the ratio of the longitudinal tensile strength to the lateral tensile strength (TS ＭＤ / TS ＴＤ ) is 0.5 to 0.

7. A polyethylene microporous membrane.

2. The average of the longitudinal and transverse tensile strengths is 2500 kg / cm². ２ The polyethylene microporous membrane according to claim 1 is as described above.

3. The polyethylene microporous membrane according to claim 1, wherein the perforation strength is 0.50 N / μm or more.

4. A polyethylene microporous membrane according to claim 1, wherein the porosity is 30% to 70%.

5. A method for producing a polyethylene microporous membrane according to claim 1, A method for producing polyethylene microporous membranes by a wet process including a sequential twin-screw centrifugation step.

6. (a) Weight-average molecular weight is 1 × 10 ５ g / mol~10×10 ５ A step of producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent in g / mol using an extruder, (b) The step of extruding the molten material to form it into a sheet, (c) A step of forming the sheet into a film by successively biaxially stretching it in the longitudinal and transverse directions, (d) A step of extracting diluent from the stretched film and drying it, (e) The step of heat-treating the dried film at 133°C to 145°C, The longitudinal stretch ratio (SR) of the (c) step ＭＤ The ratio is 4.0 times or more, and the lateral elongation ratio (SR ＴＤ ) is 5.3 times or more, and the ratio of the elongation ratio in the vertical direction to the elongation ratio in the horizontal direction (SR ＭＤ / SR ＴＤ A method for producing a polyethylene microporous membrane, wherein the coefficient of porosity is 0.5 to 0.

75.

7. A separator comprising a polyethylene microporous membrane according to any one of claims 1 to 4.

8. An electrochemical element comprising the separator described in claim 7.

9. The electrochemical element according to claim 8, which is a secondary battery comprising the separator between the positive electrode and the negative electrode.

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