KR101843806B1 - Polyolefin microporous membrane, separator for battery, and battery - Google Patents

Abstract

(S ₂ / S ₂ ) of the pore volume (S ₂ ) in the range of 100 to 1,000 nm with respect to the pore volume (S ₁ ) in the pore radius of 10 to 1,000 nm in the pore diameter distribution curve obtained by the mercury porosimetry, S ₁ × 100) of 25% or more and a ratio (S ₃ / S ₁ × 100) of the pore volume (S ₃ ) in the range of the pore radius of 500 to 1,000 nm is 5% or less.
The polyolefin microporous membrane of the present invention is intended to provide a thin polyolefin microporous membrane having a high porosity and excellent permeability and mechanical strength, a battery separator using such a microporous membrane, and a battery using such a separator.

Description

TECHNICAL FIELD [0001] The present invention relates to a polyolefin microporous membrane, a separator for a battery, and a battery. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a thin polyolefin microporous membrane having a high porosity and excellent permeability and mechanical strength, a battery separator using such a microporous membrane, and a battery using such a separator.

The lithium ion secondary battery has a higher voltage than other secondary batteries such as a nickel-hydrogen secondary battery and a nickel-cadmium secondary battery, so that a high energy density is obtained. However, if an internal short circuit occurs, the secondary battery may generate heat rapidly. Therefore, the separator for a lithium ion secondary battery needs to have a function (shutdown function) for stopping the battery reaction when an internal short circuit occurs. Conventionally, a polyolefin microporous membrane has been used as a battery separator. The polyolefin microporous membrane closes the pores due to heat generation of the battery, thereby blocking ion conduction of the electrolyte, thereby shutting down the cell reaction.

However, LiCoO ₂ The crystals of the positive electrode active material such as lithium are destroyed and generate heat violently. As a method for suppressing the heat generation at the time of overcharging, there is a method of deliberately forming a minute short circuit between the anode and the cathode to prevent the overcharge from proceeding, in addition to a method of preventing overheating by a battery separator (WO 2005/117167). An electrode having a porous film having a convex portion with a thickness of several micrometers on the surface facing the other electrode is used as the short-circuited portion, and lithium or a transition metal or the like is concentrated on the convex portion of the porous film at the time of overcharging And precipitating the dendrites so as to penetrate the separator. When this method is used, it is preferable to use a polyolefin microporous membrane having a high porosity in order to facilitate the formation of minute shorts. The microporous membrane having a high porosity can be produced by, for example, a method of forming a void by a hole forming agent made of an inorganic material, but such a microporous membrane has a problem that the piercing strength is lowered.

WO 2006/106783 (Patent Document 1), the pore diameter is large, the permeability and the excellent mechanical strength of the polyolefin microporous membrane has a weight average molecular weight of 5 × 10 ⁵ or more ultra-high molecular weight polyethylene with a weight average molecular weight of 1 × 10 ⁴ more than 5 × 10 A polyethylene composition of polyethylene having a weight average molecular weight of less than ⁵ and a solvent for film formation is melt kneaded and then extruded from a die and cooled to form a gel sheet. The gel sheet is subjected to first stretching in at least one axial direction, And the second stretching is performed in at least one axial direction with respect to the stretched product after removing the solvent, and the second stretching is performed in at least one axial direction.

WO 2006/106783

However, since the polyolefin microporous membrane of Patent Document 1 contains a large amount of polyethylene having a weight average molecular weight of less than 5 10 ⁵ , it is difficult to obtain a good balance of porosity, permeability and piercing strength when a relatively thin microporous membrane is formed there was.

In recent years, a thin separator, particularly a separator having a thickness of 19 탆 or less, is required for a small lithium ion secondary battery used in a cellular phone or the like. Therefore, a thin polyolefin microporous membrane having a high porosity, excellent permeability and piercing strength, and suitable for preventing heat generation during overcharging is desired.

Accordingly, an object of the present invention is to provide a thin polyolefin microporous membrane having a high porosity and excellent permeability and mechanical strength, a battery separator using such a microporous membrane, and a battery using such a separator.

As a result of intensive studies in view of the above object, the present inventors have found that a mixture of a first polyethylene having a weight average molecular weight of 5 × 10 ⁵ to 9 × 10 ⁵ and a second polyethylene having a weight average molecular weight of 1 × 10 ⁶ or more, A polyolefin microporous membrane having a large pore volume in a suitable diameter range and having excellent permeability and mechanical strength even when it is thin can be obtained by subjecting the polyolefin microporous membrane to stretching, heat setting, washing, .

That is, the polyolefin microporous membrane of the present invention has a pore diameter distribution curve in which the ratio of the pore volume in the pore radius range of 100 to 1,000 nm is 25% or more , And the ratio of the pore volume in the pore radius range of 500 to 1,000 nm is 5% or less.

The polyolefin constituting the polyolefin microporous membrane preferably comprises a first polyethylene having a weight average molecular weight of 5 10 ⁵ to 9 10 ⁵ and a second polyethylene having a weight average molecular weight of 1 10 ⁶ or more. It is preferable that the total content of the first polyethylene and the second polyethylene is 100 mass% and the content of the second polyethylene is 10 mass% to 25 mass%. The terminal vinyl group concentration of the first polyethylene is preferably less than 0.2 per 10,000 carbon atoms. The polyolefin microporous membrane may contain 10% by mass or less of inorganic filler based on 100% by mass of the microporous membrane.

The polyolefin microporous membrane according to the preferred embodiment of the present invention has an average thickness of 19 μm or less and a porosity of 45% or more, and the peak of the pore diameter distribution curve is in a range of 50 nm or more of the pore radius. The polyolefin microporous membrane according to another preferred embodiment of the present invention has a heat shrinkage ratio (TD) of less than 7.5% (under the condition of 105 ° C and 8 hours) and a maximum shrinkage ratio of less than 10% in the transverse direction (TD) Just before melting). The polyolefin microporous membrane according to still another preferred embodiment of the present invention has a porosity of 50% or more, a piercing strength of 100 mN / 占 퐉 or more, and an average thickness of 18 占 퐉 or less.

The battery separator of the present invention is characterized by being formed by the polyolefin microporous membrane.

The battery of the present invention is characterized by including the battery separator. The separator for a battery of the present invention is suitable for a small lithium ion secondary battery.

(Effects of the Invention)

The polyolefin microporous membrane of the present invention is thin, has high porosity, excellent permeability and mechanical strength, and has particularly excellent piercing strength. The small lithium ion secondary battery using the separator made of the polyolefin microporous membrane of the present invention has excellent safety. Therefore, the polyolefin microporous membrane of the present invention is particularly suitable for a separator of a small-sized lithium ion secondary battery for a cellular phone.

1 is a graph showing an example of a typical pore diameter distribution curve.
2 is a graph schematically showing a temperature-dimensional change rate curve for obtaining a maximum shrinkage rate immediately before melting.
3 is a graph showing the hole diameter distribution curves of Example 1 and Comparative Examples 1, 2 and 6. Fig.

[1] Polyolefin microporous membrane

(A) Composition

The polyolefin constituting the polyolefin microporous membrane is preferably a composition of a first polyethylene having a weight average molecular weight (Mw) of 5 10 ⁵ to 9 10 ⁵ and a second polyethylene having a Mw of 1 10 ⁶ or more. The content of the second polyethylene is preferably 10 to 25% by mass, more preferably 15 to 25% by mass, based on 100% by mass of the entire polyethylene composition. If the content is 10 to 25 mass%, the polyolefin microporous film obtained has a good balance of porosity and mechanical strength.

(1) a first polyethylene

The first polyethylene is preferably high density polyethylene, medium density polyethylene, branched low density polyethylene and chain low density polyethylene, more preferably high density polyethylene. The first polyethylene may be not only a homopolymer of ethylene but also an ethylene /? - olefin copolymer containing a small amount of? -Olefin other than ethylene. As the? -Olefins other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1 and octene-1 are preferable. In addition to these, vinyl acetate, methyl methacrylate and styrene may be contained. The melting point of the first polyethylene can be made to be 132 占 폚 or higher by copolymerizing ethylene and an? -Olefin other than ethylene. The melting point is determined by differential scanning calorimetry (DSC) according to JIS K 7121. The content of the? -Olefin other than ethylene is preferably 5 mol% or less.

The Mw of the first polyethylene is preferably 5 × 10 ⁵ to 8 × 10 ⁵ , more preferably 5.5 × 10 ⁵ to 7 × 10 ⁵ . The molecular weight distribution (weight average molecular weight / number average molecular weight (Mw / Mn)) of the first polyethylene is preferably 50 or less, more preferably 2 to 50, further preferably 3 to 15, and most preferably 4 to 10 .

It is preferable that the terminal vinyl group concentration of the first polyethylene is less than 0.2 per 10,000 carbon atoms in order to obtain a polyolefin microporous membrane having high permeability and mechanical strength. As a commercially available product of such polyethylene, for example, grades "SH-800" and "SH-810" of Sun Fine (registered trademark, product of Asahi Chemical Industry Co., Ltd.) are listed. The terminal vinyl group concentration of these commercial products is 0.05 to 0.14 per 10,000 carbon atoms. Such a polyethylene can be produced, for example, by a Chigler-Natta catalyst or a single site polymerization catalyst. The terminal vinyl group concentration can also be measured by the method described in WO 1997/23554.

As long as the terminal vinyl group concentration is less than 0.2, polyethylene having a terminal vinyl group concentration of less than 0.2 may be mixed with polyethylene having a terminal vinyl group concentration of 0.2 or more. The shutdown characteristic of the polyolefin microporous membrane is improved by the polyethylene having the terminal vinyl group concentration of 0.2 or more. As a commercially available product of polyethylene having a terminal vinyl group concentration of 0.2 or more, for example, Lupolen (registered trademark, manufactured by Basell) and the like can be mentioned. The terminal vinyl group concentration of these commercial products is 0.6 to 10.0 per 10,000 carbon atoms. Such polyethylene can be produced by a chromium-containing catalyst.

Polyethylene having a melting point of 130 占 폚 or lower may be added to the first polyethylene for the purpose of lowering the shutdown temperature to, for example, 130 占 폚 or less.

(2) a second polyethylene

The second polyethylene is preferably ultra-high molecular weight polyethylene. The ultrahigh molecular weight polyethylene may be not only a homopolymer of ethylene but also an ethylene /? - olefin copolymer containing a small amount of? -Olefin other than ethylene. Examples of? -Olefins other than ethylene include propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1 and octene-1. In addition, vinyl acetate, methyl methacrylate, and styrene may also be used. The content of the? -Olefin other than ethylene is preferably 5 mol% or less. Mw of the second polyethylene is ^{1 × 10 6 ~ 5 × 10} 6 is preferable, and more preferably is ^{1 × 10 6 ~ 3 × 10} 6. The Mw / Mn of the second polyethylene is preferably 1.2 to 50, more preferably 3 to 20, further preferably 4 to 15, most preferably 4 to 10.

The second polyethylene may be produced by, for example but not limited to, a Chigler-Natta catalyst or a single site polymerization catalyst. The melting point of the second polyethylene is preferably 134 캜 or higher. As a commercially available product of ultrahigh molecular weight polyethylene, for example, a grade "240M" of Highjax Million (registered trademark, manufactured by Mitsui Kagaku K.K.) can be mentioned.

(3) Other components

The polyethylene composition may contain other components such as an inorganic filler and a heat-resistant polymer. The inorganic filler preferably contains silicon and / or aluminum atoms. As the heat resistant polymer, those described in WO 2007/132942 and WO 2008/016174 are preferable. The content of each of the inorganic filler and the heat resistant polymer is preferably 10% by mass or less based on 100% by mass of the microporous membrane.

(B) Manufacturing method

The method for producing a polyolefin microporous membrane comprises the steps of (1) melt-kneading the polyolefin and a solvent for film formation to prepare a polyolefin solution, (2) extruding the polyolefin solution from the die, (3) (6) removing the solvent for film formation from the gel sheet, (7) drying the obtained microporous membrane, (8) drying the microporous membrane, and Stretching the sclera (re-stretching), and (9) heat-treating the sclera. After the step (9), (10) a crosslinking treatment step by ionizing radiation, (11) a hydrophilization treatment step may be carried out, if necessary.

(1) Preparation of polyolefin solution

Various additives such as antioxidants and microparticulate silicic acid (pore-forming agent) may be added to the polyolefin solution prepared by melt-kneading the polyolefin and the solvent for film formation, as needed, within the range not to impair the effects of the present invention.

In order to enable the stretching at a relatively high magnification, the solvent for film formation is preferably liquid at room temperature. Examples of the liquid solvent include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, para-xylene, undecane, dodecane and liquid paraffin, mineral oil fractions corresponding to these boiling points, and dibutyl phthalate, And phthalic acid esters in liquid state at room temperature such as dioctyl phthalate. In order to obtain a gel sheet in which the content of the liquid solvent is stable, it is preferable to use a nonvolatile liquid solvent such as liquid paraffin. Further, a solvent which is miscible with the polyolefin in the melt-kneading state but is solid at room temperature may be mixed with a liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax, and the like. However, if only a solid solvent is used, there is a possibility that elongation deviation or the like may occur.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 占 폚. When the viscosity at 40 占 폚 is 20 cSt or more, the sheet extruded from the die by the polyolefin solution does not become uneven. On the other hand, when it is 200 cSt or less, the liquid solvent can be easily removed.

The homogeneous melt kneading of the polyolefin solution is not particularly limited, but it is preferable to carry out the melt kneading in a twin-screw extruder in order to prepare a high-concentration polyolefin solution. The solvent for forming the film may be added before or during the kneading, while the latter may be preferably added from the middle of the twin-screw extruder during kneading.

The melting and kneading temperature is preferably (melting point (Tm) + 10 ° C) to (Tm + 120 ° C) of the polyethylene composition. Specifically, since the polyethylene composition has a melting point of about 130 to 140 ° C, the melt-kneading temperature is preferably 140 to 250 ° C, more preferably 180 to 230 ° C, and most preferably 210 to 230 ° C.

The ratio (L / D) of the length (L) to the diameter (D) of the screw of the twin-screw extruder is preferably in the range of 20 to 100, more preferably in the range of 35 to 70. When L / D is 20 or more, melt-kneading becomes sufficient. When L / D is 100 or less, the residence time of the polyolefin solution does not excessively increase. The inner diameter of the cylinder of the twin-screw extruder is preferably 40 to 100 mm.

The content of the polyolefin is preferably 20 to 30% by mass, more preferably 20 to 28% by mass, based on 100% by mass of the polyolefin solution. When the content of the polyolefin is 20 to 30% by mass, the gel sheet is excellent in moldability.

(2) Extrusion

The melt-kneaded polyolefin solution in the extruder is extruded from the sheet die. The gap of the sheet die is preferably 0.1 to 5 mm, and it is preferable to heat the sheet die to 140 to 250 캜 at the time of extrusion. The extrusion speed of the heating solution is preferably 0.2 to 15 m / min.

(Q / Ns (kg / h)) of the extruded amount (Q) (kg / h) of the polyolefin solution to the screw rotation speed (Ns) (rpm) of the biaxial extruder in order to uniformly disperse the polyolefin in the extrusion- ) Is preferably 0.4 kg / h / rpm or less, and more preferably 0.35 kg / h / rpm or less. The lower limit of Q / Ns is not particularly limited, but 0.01 kg / h / rpm is preferable. Q / Ns also depends on the shape of the screw (for example, the diameter, the depth of the screw groove, etc.). The screw rotation speed (Ns) is preferably 50 rpm or more. The upper limit of the screw rotation speed (Ns) is not particularly limited, but is preferably 500 rpm.

(3) Formation of a gel sheet

The gel-shaped sheet is obtained by cooling the formed body extruded from the die. The cooling is preferably carried out at least to a gelation temperature at a rate of 30 DEG C / min or more, preferably 50 DEG C / min or more. It is also preferable to cool to 10 to 45 占 폚. The microphase of the polyolefin separated by the solvent for film formation by cooling can be immobilized. Generally, if the cooling rate is low, a relatively high polyolefin crystal is formed, so that the gel sheet has a high-order structure. However, if the cooling rate is high, relatively small polyolefin crystals are formed, so that the gel sheet has a high-order structure. When the cooling rate is 30 DEG C / min or more, the rise of crystallinity is prevented, and a gel sheet suitable for stretching is obtained. Examples of the cooling method include a method of making direct contact with a cooling medium such as cold air or cooling water, a method of contacting with a roll cooled with a refrigerant, and the like. The thickness of the gel sheet is preferably 0.5 to 5 mm, more preferably 0.7 to 3 mm.

(4) Stretching of the gel sheet

The gel sheet is stretched in at least one axial direction. Since the gel sheet contains a solvent for film formation, it can be simply and uniformly stretched. After heating, the gel sheet is stretched at a predetermined magnification by the tenter method or the like. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is preferred.

In the case of uniaxial stretching, the stretching magnification is preferably at least 2 times, more preferably from 3 to 30 times, and more preferably at least three times in any direction in biaxial stretching. The area ratio is preferably 9 times or more, more preferably 16 times or more, and most preferably 20 times or more. By making the area magnification 9 times or more, the piercing strength is further improved. However, since it is difficult to make the area magnification of 400 times or more in view of the drawing apparatus and drawing operation, the upper limit of the area magnification is practically 400 times. The stretching magnifications in the longitudinal direction (MD) and the transverse direction (TD) in the biaxial stretching need not be the same.

The stretching temperature is preferably not higher than the melting point (Tm) of the polyethylene composition, more preferably not lower than the crystalline dispersion temperature (Tcd) and less than the melting point (Tm) of the polyethylene composition. When the stretching temperature is not higher than the melting point (Tm), the polyethylene composition is not melted and orientation of molecular chains by stretching can be achieved. When the stretching temperature is not lower than the crystal dispersion temperature (Tcd), softening of the polyethylene composition is sufficient and stretching at a high magnification can be carried out without stretching at the time of stretching. The crystal dispersion temperature (Tcd) is determined from the temperature characteristic of dynamic viscoelasticity measured according to ASTM D 4065. Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 100 캜, the stretching temperature is 90 to 130 캜, preferably 100 to 120 캜, more preferably 110 to 120 캜, And most preferably 115 to 120 ° C.

By the stretching as described above, cleavage occurs between the polyethylene lamellae and the polyethylene phase is refined, and a large number of fibrils are formed. The fibrils form three-dimensionally irregularly connected network structures. Stretching improves the mechanical strength and enlarges the pores, which makes it suitable for battery separators.

The polyolefin microporous membrane having a higher mechanical strength can be obtained by forming a temperature distribution in the film thickness direction according to desired physical properties. Details of the method are described in Japanese Patent No. 3347854.

(5) Heat fixation treatment process

The stretched gel sheet is subjected to heat fixing treatment (heat treatment in a state where it is fixed to the tenter). The crystal of the gel sheet is stabilized by the heat fixing treatment, and the lamellar layer is homogenized. Therefore, a mesopore structure composed of fibril formed by stretching is stabilized, and a microporous membrane having a large pore diameter, excellent mechanical strength and low heat shrinkage ratio can be produced by the solvent removal treatment for the rear stage. The heat fixing process is performed by a tenter method, a roll method, or a rolling method. The heat setting treatment temperature is within the temperature range of (Tcd-20 ° C) to Tm.

(6) Removal of solvent for film formation

A cleaning solvent is used to remove the solvent for forming the film. Since the polyolefin phase and the solvent for film formation are separated, a porous film can be obtained by removing the solvent for film formation. As the washing solvent, for example pentane, hexane, ketones such as chlorinated hydrocarbons, diethyl ether, dioxane and the like ethers, methyl ethyl ketone, saturated hydrocarbons, methylene chloride, carbon tetrachloride, and the like, such as heptane, trifluoride ethane, C ₆ Chain fluorocarbons such as F ₁₄ and C ₇ F ₁₆ , C ₅ H ₃ F ₇ Cyclic hydrofluorocarbons such as C ₄ F ₉ OCH ₃ , C ₄ F ₉ OC ₂ H ₅ , And perfluoro ethers such as C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ . These cleaning solvents have low surface tension (e.g., 24 mN / m or less at 25 캜). By using a cleaning solvent having a low surface tension, the mesh structure that forms the micropores is inhibited from shrinking due to the surface tension of the gas-liquid interface at the time of drying after cleaning, and a microporous membrane having higher porosity and permeability is obtained.

The removal of the solvent for film formation can be performed by a method of immersing the stretched film in a cleaning solvent, a method of showering a cleaning solvent on the stretched film, or a combination thereof. The amount of the washing solvent to be used varies depending on the washing method, but it is generally preferably 300 to 30,000 parts by mass based on 100 parts by mass of the drawn film. The cleaning temperature may be 15 to 30 占 폚, and if necessary, it is heated to 80 占 폚 or less. It is preferable to remove the solvent for film formation until the residual amount becomes less than the original amount of 1 mass%.

(7) Drying

The polyolefin microporous membrane obtained by removing the solvent for film formation is dried by a heat drying method, a blow drying method, or the like. The drying temperature is preferably not higher than the crystal dispersion temperature (Tcd) of the polyethylene composition, and particularly preferably not higher than Tcd-5 占 폚. The drying is preferably carried out until the dry weight of the microporous membrane is 100 mass% and the residual washing solvent is 5 mass% or less, more preferably 3 mass% or less. If the drying is sufficient, there is no fear that the porosity of the microporous membrane is lowered in the subsequent heat treatment, and the permeability is kept good.

(8) stretching of the microporous membrane

The dried microporous membrane is stretched (re-stretched) in at least one axial direction. The reheating can be performed by a tenter method or the like as described above while heating the microporous membrane. As the tenter device, for example, the device described in WO 2009/084722 can be used. The re-stretching may be uniaxial stretching or biaxial stretching, but in the case of uniaxial stretching, it is preferable to perform the stretching in the transverse direction (TD). In the case of biaxial stretching, simultaneous biaxial stretching and sequential stretching may be used, but simultaneous biaxial stretching is preferred. Since the re-stretching is usually performed on the long-sheet-like microporous film obtained from the stretched gel sheet, the longitudinal direction (MD) and the transverse direction (TD) in the re-stretching are the MD direction and the TD direction .

The temperature of the recrystallization is preferably not higher than the melting point (Tm) of the polyethylene composition, more preferably within the range of (Tcd-20 ° C) to Tm. Specifically, the temperature is preferably 70 to 135 占 폚, more preferably 110 to 132 占 폚, and most preferably 120 to 130 占 폚.

In the case of uniaxial stretching, the magnification of the re-stretching is preferably 1.01 to 1.6 times, particularly 1.1 to 1.6 times, more preferably 1.2 to 1.5 times in the TD direction. And in the case of biaxial stretching, it is preferable to be 1.01 to 1.6 times in the MD direction and in the TD direction, respectively. Further, although it may be different in the MD direction and the TD direction, it is preferable that the TD direction is smaller than the MD direction.

The speed of re-winding is preferably not less than 3% / second in both the MD and TD directions, more preferably not less than 5% / second. The upper limit is preferably 50% / second, more preferably 25% / second. The re-rolling speed may be set independently of each other in the MD direction and the TD direction.

(9) Heat treatment

Heat the microporous membrane after drying. Crystallization is stabilized by the heat treatment, and the lamellar layer is homogenized. The heat treatment time is preferably 1,000 seconds or less, more preferably 1 to 800 seconds. The heat treatment is heat fixing treatment and / or heat relaxation treatment. The heat fixing process is a heat treatment performed so that there is no dimensional change in both the MD direction and the TD direction, and the heat relaxation process is heat shrink treatment. The heat fixing treatment can be performed by heating in a state of being fixed to the tenter. The heat setting treatment temperature is preferably in the range of Tcd to Tm, more preferably in the range of the stretching (re-stretching) temperature of the microporous film ± 5 ° C, and particularly preferably in the range of the re- The heat relaxation treatment can be performed by moving the inside of the heating furnace, for example, by a belt conveyor or an air floating method, or narrowing it in the TD direction while being heated while being held by the tenter. The thermal relaxation treatment is carried out at a temperature below the melting point (Tm), preferably within a temperature range of 60 占 폚 to (Melting point (Tm) -5 占 폚). Shrinkage in the TD direction due to thermal relaxation treatment is long in the TD direction prior reenactment Shin (L ₁₎ to the relative length of the TD direction after the heat relaxation treatment (L ₂₎ that it is preferable to so abnormal, 91%, 95% or more Is more preferable. By the thermal relaxation treatment as described above, a high strength microporous membrane having good permeability can be obtained.

(10) Cross-linking treatment of microporous membrane

The microporous membrane may be subjected to crosslinking treatment by irradiation with ionizing radiation such as? Rays,? Rays,? Rays, electron beams, and the like. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane.

(11) Hydrophilization treatment

The microporous membrane may be subjected to hydrophilization treatment depending on the application. The hydrophilizing treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge, or the like. The monomer graft is preferably carried out after the crosslinking treatment.

In the case of the surfactant treatment, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant, and a bionic surfactant can be used, but a nonionic surfactant is preferred. The 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 microporous membrane by the doctor blade method.

(C) Properties

(1) an average film thickness of 19 mu m or less

If the average film thickness exceeds 19 탆, it is not suitable for a separator of a small-sized lithium ion secondary battery for a cellular phone. The average film thickness is preferably 12 to 18 mu m, more preferably 14 to 17 mu m. The average film thickness of the microporous membrane can be controlled mainly by appropriately adjusting the thickness and stretching ratio of the gel sheet.

(2) Distribution of pore volume

As shown in Fig. 1, the pore radius distribution curve (the curve obtained by plotting the pore radius r and the log differential pore volume [dVp / dLog (r)) of the polyolefin microporous membrane obtained by the mercury porosimetry has a pore radius of 10 to 1,000 (Represented by the area (S ₁ ) of the hatching area in the range of 10 to 1,000 nm of the pore radius) in the range of 100 to 1,000 nm (the hatching radius in the range of 100 to 1,000 nm of the pore radius in the range of 100 to 1,000 nm) (S ₂ / S ₁ × 100) of the pore radius (represented by the area (S ₂ ) of the pore radius (S ₂ / S ₁ × 100) of the pore radius of 500 to 1,000 nm (S ₃ / S ₁ x 100) of the area (represented by the area (S ₃ ) of the portion) is 5% or less. (S ₂ / S ₁ × 100) is preferably 30% or more, more preferably 35% or more. (S ₃ / S ₁ × 100) is preferably 4.5% or less.

It is preferable that the peak of the hole diameter distribution curve (the highest peak when there are a plurality of peaks) is in the range of the pore radius of 50 nm or more, more preferably the pore radius is in the range of 70 to 500 nm, Nm is most preferable.

Since the microporous membrane of the present invention has a pore volume distribution as described above, it has a high porosity suitable for the above-mentioned mechanism for preventing heat generation during overcharging (a mechanism for deliberately forming minute short-circuit portions between the gaps to avoid overcharging) It is also excellent in permeability and mechanical strength.

(3) porosity of 25 to 80%

If the porosity is 25% or more, the polyolefin microporous membrane has good air permeability. On the other hand, when the polyolefin microporous membrane is used as a separator for a battery, the mechanical strength is sufficient and the electrode is not short-circuited. The porosity is preferably 45% or more, more preferably 50 to 55%.

(4) Specification of 10 sec / 100 cm 3 / 탆 or less

(Air permeability) measured in accordance with JIS P 8117 is 10 sec / 100 cm 3 / 탆 or less, the above-mentioned mechanism for preventing the occurrence of overheating (a minute short circuit is deliberately formed between the gaps, Mechanism). ≪ / RTI > The air permeability is preferably 1 sec / 100 cm 3 / 탆 to 10 sec / 100 cm 3 / 탆, more preferably 2 sec / 100 cm 3 / 탆 to 9 sec / 100 cm 3 / 탆. Here, the degree of air permeability is defined as the air permeability (P ₁ ) measured in accordance with JIS P 8117 with respect to the microporous membrane of the average film thickness (T _AV ) by a formula P ₂ = P ₁ / T _AV (P ₂ ), which is the value converted to the permeability.

(5) Piercing strength of 1.0 x 10 < ² > mN /

The piercing strength of the microporous membrane is represented by the maximum load when a needle having a diameter of 1 mm, whose tip has a spherical surface (radius of curvature R: 0.5 mm), is pierced into the microporous membrane at a speed of 2 mm / sec. When the piercing strength is 1.0 × 10 ² mN / μm or more, there is no possibility of short-circuiting when the polyolefin microporous membrane is mounted on a battery as a battery separator. The piercing strength is preferably 1.3 × 10 ² mN / μm or more, more preferably 1.5 × 10 ² mN / μm or more. Here, the piercing strength is the piercing strength (S) (mN) measured on the microporous membrane of the average film thickness (T _AV ) by the formula of S '= S / T _AV and the piercing strength (S ') (mN / m).

(6) Tensile breaking strength of 5 x 10 ⁴ ㎪ or more

When the tensile fracture strength measured by ASTM D882 is 5 x 10 < ⁴ > or more in both the MD and TD directions, there is no fear that the film will be broken when used as a battery separator. In particular, the tensile breaking strength in the MD direction is ^{6 × 10 4 ~ 2.5 × 10} 5 ㎪ is more preferred, and the tensile strength at break in the TD direction is more preferably ^{5 × 10 4 ~ 1.5 × 10} 5 ㎪, 5 × 10 4 ~ 1.0 x 10 < ^-5 > is most preferable.

(7) Tensile elongation at break of 100% or more

When the tensile elongation at break measured by ASTM D882 is 100% or more in both the MD direction and the TD direction, there is no worry that the film will be broken when used as a battery separator. The tensile elongation at break is preferably 110 to 300%. In particular, tensile elongation at break in the MD direction is more preferably 125 to 250%, and tensile elongation at break in the TD direction is more preferably 140 to 300%.

(8) Heat shrinkage rate of 10% or less

The heat shrinkage rate when held at 105 占 폚 for 8 hours is 10% or less in both the MD and TD directions. The heat shrinkage ratio in the TD direction is preferably 8% or less, more preferably 7.5% or less, and most preferably 6% or less.

(9) Maximum shrinkage immediately before melting of not more than 25%

As clearly shown in Fig. 2, when the microporous membrane is heated under a load, the microporous membrane continues to contract, and the rate of dimensional change (shrinkage ratio) becomes maximum at temperature (T) (占 폚). When the temperature (T) (占 폚) is exceeded, the microporous membrane expands rapidly. This is presumably due to the melting of the microporous membrane. The maximum shrinkage percentage (P) at the temperature (T) (占 폚) (immediately before melting) is an index of the content melting shrinkage. The maximum shrinkage ratio in the MD direction is preferably 10% or less. The maximum shrinkage ratio in the TD direction is preferably 15% or less, more preferably 12% or less.

[2] Battery separator

The polyolefin microporous membrane of the present invention is thin and excellent in permeability, mechanical strength and heat shrinkage resistance, and is therefore suitable for a battery separator, particularly a separator of a small-sized lithium ion secondary battery for a cellular phone.

[3] batteries

The polyolefin microporous membrane of the present invention is suitable for a separator for a secondary battery such as a lithium ion secondary battery, a lithium polymer secondary battery, a nickel-hydrogen secondary battery, a nickel-cadmium secondary battery, a nickel-zinc secondary battery or a silver- It is preferable for a separator for a lithium ion secondary battery. Hereinafter, a lithium ion secondary battery will be described.

In a lithium ion secondary battery, a positive electrode and a negative electrode are laminated via a separator containing an electrolyte (electrolyte). The structure of the electrode is not particularly limited. For example, the electrode structure (coin type) in which the disk-shaped positive electrode and the negative electrode are opposed to each other, the electrode structure (laminate type) in which the plate-shaped positive electrode and the negative electrode are alternately stacked, An electrode structure (wound type) in which a positive electrode and a negative electrode are overlapped and wound, or the like.

The positive electrode usually has (a) a current collector and (b) a layer formed on the surface thereof and including a positive electrode active material capable of intercalating and deintercalating lithium ions. Examples of the positive electrode active material include inorganic oxides such as transition metal oxides, complex oxides of lithium and transition metals (lithium complex oxides) and transition metal sulfides, and transition metals such as V, Mn, Fe, Co and Ni . Preferable examples of the lithium composite oxide include lithium nickel oxide, lithium cobaltate, lithium manganese oxide, and layered lithium composite oxide having a? -NaFeO ₂ type structure as a matrix. The negative electrode has (a) a current collector, and (b) a layer formed on the surface thereof and including a negative electrode active material. Examples of the negative electrode active material include carbonaceous materials such as natural graphite, artificial graphite, cokes, and carbon black.

The electrolytic solution is obtained by dissolving a lithium salt in an organic solvent. Examples of the lithium salt include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , LiN C ₂ F ₅ SO ₂ ) ₂ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , lower aliphatic carboxylic acid lithium salt and LiAlCl ₄ . These may be used alone or in combination of two or more. Examples of the organic solvent include organic solvents having a high boiling point and high dielectric constant such as ethylene carbonate, propylene carbonate, ethylmethyl carbonate and? -Butyrolactone; organic solvents such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl And organic solvents having a low boiling point and a low viscosity such as carbonate, diethyl carbonate and the like. These may be used alone or in combination of two or more. Since an organic solvent having a high dielectric constant has a high viscosity and an organic solvent having a low viscosity has a low dielectric constant, it is preferable to use a mixture of both.

An electrolyte solution for imparting ion permeability to a separator attached to a battery is impregnated by a dipping method or the like. When the cylindrical battery is assembled, for example, a positive electrode sheet, a microporous membrane separator, a negative electrode sheet and a microporous membrane separator are laminated in this order, wound into a battery can, impregnated with an electrolytic solution, Cover the battery cover with a gasket.

The present invention will be described in further detail with reference to the following examples, but the present invention is not limited to these examples.

Example

Example 1

82% by mass of high density polyethylene (HDPE) having a weight average molecular weight (Mw) of 5.6 x 10 ⁵ , a molecular weight distribution (Mw / Mn) of 4.1, a terminal vinyl group concentration of 10,000 carbon atoms per 0.1, ^6, and 18 mass% of ultrahigh molecular weight polyethylene (UHMWPE) having Mw / Mn of 5 was prepared. The melting point (Tm) of the polyethylene composition was 135 占 폚, and the crystal dispersion temperature (Tcd) was 100 占 폚.

Mw and Mw / Mn of UHMWPE and HDPE were determined by gel permeation chromatography (GPC) method according to the method described in " Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (Hereinafter the same).

Measurement apparatus: Polymer Laboratories PL-GPC220

Column: Polymer Laboratories' Three PLgel Mixed-B Columns

Column temperature: 145 ° C

Solvent (mobile phase): 1,2,4-trichlorobenzene (containing approximately 1,000 ppm butylated hydroxytoluene, manufactured by Aldrich)

Solvent flow rate: 0.5 ml / min

Sample concentration: 0.25 to 0.75 mg / mL (dissolution condition: 160 ° C / 2h)

· Injection volume: 300 μL

· Detector: Differential refractometer

Calibration curve: The calibration curve was prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

25 parts by mass of the polyethylene composition was fed into a twin-screw extruder, and 75 parts by mass of liquid paraffin [50 cSt (40 占 폚)] was fed from the side feeder of the twin-screw extruder and melted and kneaded at 210 占 폚 and 350 rpm to prepare a polyethylene solution. This polyethylene solution was extruded from a T-die provided in a twin-screw extruder and cooled while being taken up by a cooling roll adjusted to 40 캜 to form a gel sheet having a thickness of 1.2 mm. The obtained gel sheet was simultaneously biaxially stretched at a ratio of 5 times in both the MD and TD directions at 118.0 占 폚 by a tenter stretching machine, and heat-settled at 95 占 폚.

The stretched gel sheet was then immersed in a methylene chloride bath to remove liquid paraffin. The washed film was air-dried and re-stretched at a temperature of 126.8 DEG C in the TD direction by a tenter stretching device at a magnification of 1.4 times, and then heat-fixed at a temperature of 126.8 DEG C so as not to change dimensions in both the MD direction and the TD direction while being fixed to the tenter stretching device (Total time of re-smearing and subsequent heat fixation treatment: 26 seconds) to obtain a microporous membrane.

Comparative Example 1

Wherein the polyethylene composition has a composition of 70% by mass of HDPE and 30% by mass of UHMWPE, the concentration of the polyethylene composition of the polyethylene solution is 23% by mass, the stretching temperature is 115.0 占 폚, the re- L ₂ / L ₁ where L ₁ represents the length of the microporous membrane before re-stretch in the TD direction, L ₂ represents the length of the microporous film after the thermal relaxation treatment The length of the microporous membrane in the TD direction) was 1.0, and the total time of the re-stretching, the subsequent thermal relaxation treatment and the heat setting treatment was 26 seconds, A polyethylene microporous membrane having a thickness of 16 mu m was produced.

Comparative Example 2

The stretching temperature was set to 118.0 deg. C, the re-stretching magnification was set to 1.4, the temperature of the re-stretching and the subsequent heat fixing treatment was set to 126.9 deg. C, and the total relaxation time Was changed to 26 seconds, a polyethylene microporous membrane having an average film thickness of 16 占 퐉 was produced.

Comparative Example 3

The composition of the polyethylene composition was a polyethylene composition having a weight average molecular weight of 7.5 x 10 ⁵ , a molecular weight distribution of 11.8, a terminal vinyl group concentration of 0.7 carbon atoms per 10,000 carbon atoms and 70 wt% of HDPE and 30 wt% of UHMWPE, Of 23% by mass, a stretching temperature of 116.5 占 폚, a re-stretching magnification of 1.1 times, a temperature of 124.2 占 폚 for the re-stretch and subsequent heat fixation treatment, and L ₂ / L ₁ was set to 0.95, and the total amount of the polyethylene microporous film having an average film thickness of 16 占 퐉 was obtained in the same manner as in Example 1 except that the total time of the re-stretching, the thermal relaxation treatment and the heat- .

Comparative Example 4

The composition of the polyethylene composition was 60% by mass of HDPE and 40% by mass of UHMWPE, the drawing temperature was 115.0 占 폚, the re-drawing magnification was 1.08 times, the temperature of the re-drawing and the subsequent heat setting was 124.5 占 폚, The thermal relaxation treatment was performed at 124.5 캜 so that L ₂ / L ₁ was 0.96 between the subsequent heat fixation treatments, and the total time of the re-stretch and the subsequent thermal relaxation treatment and heat setting treatment was changed to 26 seconds, A polyethylene microporous membrane having an average film thickness of 16 mu m was produced.

Comparative Example 5

The concentration of the polyethylene solution the polyethylene composition to 25% by weight and reproduce not performed put L ₂ / L ₁ is to be processed relaxation heat from 126.0 ℃ to be 0.95 that shrinkage in the TD direction and then heat-treated at 126.0 ℃ and thermal relaxation A polyethylene microporous membrane having an average film thickness of 20 占 퐉 was produced in the same manner as in Comparative Example 1, except that the total time of the treatment and subsequent heat fixation treatment was 26 seconds.

Comparative Example 6

The composition of the polyethylene composition was changed to 98 mass% of HDPE and 2 mass% of UHMWPE, the concentration of the polyethylene composition of the polyethylene solution was 39 mass%, the stretching temperature was 118.7 占 폚, the re-stretching magnification was 1.4 times, A polyethylene microporous membrane having an average film thickness of 19 占 퐉 was produced in the same manner as in Example 1 except that the temperature of the subsequent heat fixing treatment was 130.2 占 폚.

The physical properties of the polyethylene microporous membrane obtained in Example 1 and Comparative Examples 1 to 6 were measured by the following methods. The results are shown in Table 1 and Fig.

(1) Average film thickness

The average film thickness of the microporous membrane was measured by using a contact thickness meter (RC-1 made by Meisan K.K.) at intervals of 1 cm over a width of 10 cm in a plurality of places on the test piece, Were obtained.

(2) Air permeability (sec / 100 cm 3 / 탆)

The air permeability is a value obtained when the permeability (P ₁ ) measured in accordance with JIS P8117 with respect to the microporous membrane of the average film thickness (T _AV ) is ₁ μm by the formula of P ₂ = P ₁ / T _AV (P ₂ ). ≪ tb >< TABLE >

(3) porosity (%)

The porosity is calculated from the mass (w ₁ ) of the microporous membrane and the mass (w ₂ ) of the membrane of the same size composed of the polyethylene composition such as the microporous membrane to the porosity (%) = (w ₂ -w ₁ ) / w ₂ × 100 . ≪ / RTI >

(4) Piercing strength (mN / 탆)

A needle having a diameter of 1 mm and having a spherical surface (radius of curvature R: 0.5 mm) was pierced on the microporous membrane having an average film thickness (T _AV ) at a rate of 2 mm / sec to obtain a maximum load S Load (mN)] was measured, and the load S 'at the time when the film thickness was 1 m was determined by the formula of S' = S / T _{AV to obtain} the piercing strength (mN / m).

(5) Tensile breaking strength and tensile elongation at break

The test piece was measured by ASTM D882 using a rectangular test piece having a width of 10 mm.

(6) Heat shrinkage (%)

The shrinkage percentage in the MD direction and the TD direction when the microporous membrane was held at 105 占 폚 for 8 hours were measured three times, respectively, and the average value was obtained.

(7) Maximum shrinkage rate immediately before melting

A 50 mm x 3 mm rectangular test piece cut from the microporous membrane was set at a chuck distance of 10 mm on a thermomechanical analyzer (Seiko Instruments Inc., TMA / SS6000) and a load of 19.6 mN was applied to the lower end of the test piece The temperature was raised at a rate of 5 DEG C / min while being applied, and the dimensional change was measured. The rate of change of the dimension with respect to the dimension of the test piece at 23 캜 was calculated to prepare a temperature-dimensional change rate curve shown in Fig. The maximum value (P) of the dimensional change rate (shrinkage ratio) in the temperature range of 135 to 140 占 폚 was defined as the maximum shrinkage rate immediately before melting.

(8) Distribution of pore volume

The pore volume distribution was determined by mercury porosimetry according to the method described in paragraphs 82 to 83 of WO 2009/044227 under the following conditions.

Measuring device: Micromeritics Co., Ltd. Pore Sizer 9320

Mercury: contact angle 141.3 DEG, surface tension 484 dyne / cm

Pressure range: 3.6 ~ 207MPa

Cell volume: 15 cm

The ratio of the pore volume (S ₂ ) in the pore radius (S ₂ ) in the range of 100 to 1,000 nm and the pore volume (S ₃ ) in the range of the pore radius in the range of 500 to 1,000 nm to the pore volume It was determined from the S ₂ / S ₁ and S ₃ / S ₁ shown in Fig.

Note: (1) Mw represents the weight average molecular weight.

     (2) Weight average molecular weight / number average molecular weight (Mw / Mn).

     (3) 10,000 vinyl groups per terminal vinyl group concentration as determined by infrared spectroscopy.

     (4) MD represents the longitudinal direction.

     (5) TD represents the horizontal direction.

(6) L ₁ represents the length of the microporous membrane before re-stretching in the TD direction, and L ₂ represents the length of the microporous membrane after the thermal relaxation treatment in the TD direction.

(7) was obtained from S ₂ / S ₁ shown in FIG.

(8) was obtained from S ₃ / S ₁ shown in FIG.

     (9) Rate of dimensional change at 135 占 폚. It had already elongated at the point when it reached 135 DEG C and exceeded the reference dimension at 23 DEG C. [

As clearly shown in Table 1, the microporous membrane of Example 1 had a pore diameter distribution curve in which the ratio of the pore volume in the range of 100 to 1,000 nm in pore radius with respect to the pore volume in the range of 10 to 1,000 nm 25% or more, and the ratio of the pore volume in the range of the pore radius in the range of 500 to 1,000 nm was 5% or less. Therefore, the microporous membrane of Example 1 had an average thickness of 19 탆 or less and a high porosity of 50% or more and a piercing strength of 100 mN / 탆 or more, and was excellent in tensile fracture strength and heat shrinkage resistance.

On the other hand, in the microporous membranes of Comparative Examples 1 to 5, since the content of UHMWPE in the polyethylene composition exceeds 25% by mass, the ratio of the pore volume in the range of 500-1,000 nm to the pore volume in the pore radius of 10-1,000 nm 5%. Therefore, the microporous membranes of Comparative Examples 1 to 5 were inferior to the microporous membrane of Example 1 in at least one of the permeability, the porosity, the heat shrinkability, and the maximum shrinkage rate immediately before melting. The microporous membrane of Comparative Example 6 had less than 10% by mass of UHMWPE in the polyethylene composition, so that the ratio of the pore volume in the range of 100 to 1,000 nm of the pore radius to the pore volume in the pore radius of 10 to 1,000 nm was less than 25%. Therefore, the microporous membrane of Comparative Example 6 was inferior to the microporous membrane of Example 1 in porosity, tensile elongation at break, and maximum shrinkage rate immediately before melting.

≪ Industrial Availability >

The polyolefin microporous membrane of the present invention is thin, has a high porosity, excellent permeability, mechanical strength, particularly excellent piercing strength, and the small lithium ion secondary battery using the separator made of the polyolefin microporous membrane of the present invention has excellent safety. Therefore, the polyolefin microporous membrane of the present invention can be suitably used particularly for a separator of a small-sized lithium ion secondary battery for a cellular phone.

S ₁ : area of the hatched area in the range of pore radius 10 to 1,000 nm
S ₂ : area of the hatched area in the range of 100-1,000 nm of the pore radius
S ₃ : the area of the hatched area in the pore radius 500 to 1,000 nm
P: maximum value (%) of rate of change (shrinkage ratio) in a temperature range of 135 to 140 캜
T: Temperature (캜)

Claims (10)

The ratio of the pore volume in the pore radius range of 100 to 1,000 nm is 25% or more and the pore radius is in the range of 500 to 1,000 nm in the pore diameter range of the pore radius in the range of 10 to 1,000 nm in the pore diameter distribution curve obtained by the mercury intrusion method. Wherein the ratio of the pore volume in the range of 1 to 1,000 nm is 5% or less. The method according to claim 1,
A first polyethylene having a weight average molecular weight of 5 10 ⁵ to 9 10 ⁵ and a second polyethylene having a weight average molecular weight of 1 10 ⁶ or more. 3. The method of claim 2,
Wherein the total content of the first polyethylene and the second polyethylene is 100 mass% and the content of the second polyethylene is 10-25 mass%. The method according to claim 2 or 3,
Wherein the first polyethylene has an end vinyl group concentration of less than 0.2 per 10,000 carbon atoms and an inorganic filler having a mass of the microporous membrane of 100 mass% or less and 10 mass% or less of the inorganic filler. 4. The method according to any one of claims 1 to 3,
The average thickness is 19 占 퐉 or less, the porosity is 45% or more, and the peak of the pore diameter distribution curve has a pore radius of 50 nm or more. 4. The method according to any one of claims 1 to 3,
Wherein a heat shrinkage ratio in the transverse direction (TD) when held at 105 占 폚 for 8 hours is not more than 7.5%, and a maximum shrinkage rate in the transverse direction (TD) immediately before the melting is not more than 10%. 4. The method according to any one of claims 1 to 3,
A porosity of 50% or more, a piercing strength of 100 mN / 占 퐉 or more, and an average thickness of 18 占 퐉 or less. A separator for a battery, characterized by comprising the polyolefin microporous membrane according to any one of claims 1 to 3. A battery comprising the battery separator according to claim 8. 10. The method of claim 9,
A battery characterized by being a small lithium ion secondary battery.

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