JP5596768B2 - Polyethylene microporous membrane and battery separator - Google Patents

Description

  The present invention relates to a polyethylene microporous membrane having a small change in film thickness and air permeability during pressurization and a high absorption rate of an electrolytic solution, a method for producing the same, and a battery separator.

  Polyolefin microporous membranes are widely used in applications such as battery separators for lithium batteries, electrolytic capacitor diaphragms, moisture-permeable and waterproof clothing, and various filtration membranes. When a polyolefin microporous membrane is used as a battery separator, its performance is closely related to battery characteristics, productivity, and safety. Therefore, excellent permeability, mechanical characteristics, heat shrinkage characteristics, shutdown characteristics, meltdown characteristics, and the like are required. For example, when the mechanical strength is low, the voltage of the battery decreases when used as a battery separator.

  As a method for improving the physical properties of the polyolefin microporous membrane, it has been proposed to optimize the raw material composition, stretching conditions, heat treatment conditions, and the like. For example, Japanese Patent Application Laid-Open No. 2-94356 (Patent Document 1) discloses a polyethylene microporous film for a lithium battery separator having good assembly processability and low electrical resistance, having a mass average molecular weight (Mw) of 400,000 to 2 million and a molecular weight. A high density polyethylene resin having a distribution [mass average molecular weight / number average molecular weight (Mw / Mn)] of 25 or less is melt-kneaded together with inorganic fine powder and organic liquid, and the resulting melt-kneaded product is extruded from a die and cooled. Thus, a polyethylene microporous membrane produced by forming a gel-like sheet, removing inorganic fine powder and organic liquid, and then stretching at a rate of 1.5 times or more is proposed. However, this polyethylene microporous film has insufficient strength because the surface pore diameter is too large.

  Japanese Patent Application Laid-Open No. 5-9332 (Patent Document 2) discloses an ultra-high molecular weight polyethylene having a high average strength and an appropriate pore size, and having a viscosity average molecular weight of 2 million or more as an inorganic fine powder. Plasticizer [a mixture of plasticizers having SP (Solubility Parameter) values of 7.5 to 8.4 and 8.5 to 9.5. The amount of plasticizer added with an SP value of 7.5 to 8.4 is 10 to 150% by mass of polyethylene. The resulting melt-kneaded product is extruded from a die and cooled to form a gel-like sheet, the inorganic fine powder and the plasticizer are removed, dried, and then stretched only in the uniaxial direction. Proposed microporous membranes to be manufactured. However, this microporous membrane also has insufficient strength because the surface pore diameter is too large.

Therefore, the applicant of the present invention has a polyolefin fine composition comprising a polyolefin composition containing 1% by mass or more of Mw of 7 × 10 ⁵ or more, Mw / Mn of 10 to 300, and the degree of orientation changes in the film thickness direction. A porous membrane was proposed [Patent No. 3347854 (Patent Document 3)]. This polyolefin microporous membrane is obtained by melt-kneading the polyolefin composition and a film-forming solvent, extruding the obtained melt-kneaded product from a die, and cooling to form a gel-like sheet. The sheet is obtained by removing the film-forming solvent after being stretched while being heated so that a temperature distribution is generated in the film thickness direction, and is excellent in mechanical strength.

The present applicant also has a crystal composed of a fine fibril made of a polyolefin having a Mw of 5 × 10 ⁵ or more or a polyolefin composition containing the polyolefin and having an average pore diameter of 0.05 to 5 μm and an angle θ with respect to the film surface of 80 to 100 degrees. A polyolefin microporous membrane having a lamella ratio of 40% or more in each cross section in the machine direction and the width direction has been proposed [WO 2000/20492 (Patent Document 4)]. This microporous membrane is a gel-like molded product formed by extruding from a die a solution comprising 10-50% by mass of the polyolefin or polyolefin composition and 50-90% by mass of a film-forming solvent and cooling. The obtained gel-like molded product is stretched as necessary, and then heat-set in a temperature range of from the crystal dispersion temperature of the polyolefin or the polyolefin composition to the melting point + 30 ° C. to remove the film-forming solvent. And has excellent permeability.

The present applicant also has a polyolefin microporous structure comprising a polyolefin having an Mw of 5 × 10 ⁵ or more or a polyolefin composition containing the polyolefin, and having an average pore diameter that gradually decreases from at least one surface toward the center in the thickness direction. A film was proposed [WO 2000/20493 (Patent Document 5)]. This microporous membrane is prepared by extruding a solution comprising 10-50% by mass of the polyolefin or polyolefin composition and 50-90% by mass of a film-forming solvent from a die, and cooling the obtained extruded product to form a gel. It is obtained by forming a sheet, contacting the gel-like sheet with a heating solvent and then removing the film-forming solvent, or removing the film-forming solvent from the gel-like sheet and then bringing it into contact with the heating solvent. Are better.

  Recently, however, not only the permeability and mechanical strength, but also characteristics related to battery life such as cycle characteristics and characteristics related to battery productivity such as electrolyte injection properties have been emphasized as separator characteristics. . In particular, an electrode of a lithium ion battery expands due to insertion of lithium during charging and contracts due to desorption of lithium during discharging. However, with the recent increase in capacity of batteries, the expansion rate during charging tends to increase. Since the separator is compressed when the electrode is expanded, the separator is required to have a small change in permeability due to the compression and a small change in film thickness due to the compression. However, the microporous membrane described in each of the above documents cannot be said to have sufficient compression resistance. If the compression characteristics of the microporous membrane are poor, there is a high risk of battery capacity shortage (deterioration of cycle characteristics) when used as a battery separator.

JP-A-2-94356 Japanese Patent Laid-Open No. 5-9332 Patent No. 3347854 International Publication No. 00/20492 Pamphlet International Publication No. 00/20493 Pamphlet

  Accordingly, an object of the present invention is to provide a polyethylene microporous membrane having a small change in film thickness and air permeability during pressurization and a high absorption rate of an electrolyte, a method for producing the same, and a battery separator comprising such a polyethylene microporous membrane. Is to provide.

  As a result of diligent research in view of the above object, the present inventors have extruded a melt-kneaded product of a polyethylene-based resin having a ratio of ultrahigh molecular weight polyethylene of 15% by mass or less and a film-forming solvent from a die, and obtained extrusion After forming the gel sheet by cooling the molded body so as to generate a temperature distribution in the film thickness direction, it is stretched at a temperature of crystal dispersion temperature + 10 ° C. to crystal dispersion temperature + 30 ° C. After removing the solvent and stretching again at a rate of 1.05-1.45 times, a dense structure layer (dense structure region) with an average pore diameter of 0.01-0.05 μm and a coarseness of 1.2-5.0 times the average pore diameter of the dense structure layer It has been found that a polyethylene microporous membrane having a structural layer (coarse structure region), a change in film thickness and a change in air permeability at the time of pressurization, and a high absorption rate of the electrolyte can be obtained, and the present invention has been conceived. . Since the dense structure layer and the coarse structure layer are not layers having a clear boundary surface, it is more accurate to refer to the dense structure region and the coarse structure region, respectively. However, in the present application, the dense structure layer and the coarse structure layer are more convenient. Sometimes called.

That is, the polyethylene microporous membrane of the present invention is a microporous membrane made of a polyethylene resin having a mass average molecular weight of 1 × 10 ⁶ or more and a proportion of ultrahigh molecular weight polyethylene of 15% by mass or less, and is adjacent in the thickness direction. A single membrane having a dense structure region having an average pore diameter of 0.01 to 0.05 μm and a coarse structure region having an average pore diameter of 1.2 to 5.0 times that of the dense structure region, and the coarse structure region is formed on at least one surface. It is characterized by being.

  The polyethylene resin is preferably composed of the ultra high molecular weight polyethylene and high density polyethylene. The ratio represented by (thickness of the coarse structure region) / (thickness of the dense structure region) is preferably 5/1 to 1/10.

The polyethylene microporous membrane is obtained by extruding a melt-kneaded product of a polyethylene-based resin having a mass average molecular weight of 1 × 10 ⁶ or more of ultrahigh molecular weight polyethylene of 15% by mass or less and a film forming solvent from a die. The extruded molded body is cooled to have a temperature distribution in the film thickness direction to form a gel sheet, and stretched at least uniaxially at a temperature of the polyethylene resin crystal dispersion temperature + 10 ° C to the crystal dispersion temperature + 30 ° C, The film-forming solvent is preferably removed and stretched again at a rate of 1.05 to 1.45 times at least in a uniaxial direction.

  In a preferred example of such a production method, one side of the extruded product is rapidly cooled and the other side is gradually cooled. Thereby, a coarse structure layer (coarse structure region) can be formed on one surface of the microporous film. In order to rapidly cool one side of the extruded molded body, the extruded molded body is placed in a cooling roll whose temperature is adjusted to a crystallization temperature of the polyethylene resin of −115 ° C. or higher and a crystallization temperature of −25 ° C. or lower for 1 to 30 seconds. It is preferable to make contact for a period of time. It is preferable that the other surface of the extrusion-molded product is gradually cooled by exposure to air at room temperature.

  In another preferred example of the production method of the present invention, the film-forming solvent is removed after heat-treating the stretched gel-like sheet. When the stretched gel-like sheet containing the film-forming solvent is heat-set, a coarse structure layer (coarse structure region) can be formed on both surfaces of the microporous film.

  In still another preferred example of the production method of the present invention, the stretched gel sheet and / or the microporous film from which the film-forming solvent has been removed are brought into contact with a heated solvent. This also makes it possible to form a coarse structure layer (coarse structure region) on both surfaces of the microporous film.

  The battery separator of the present invention is formed of the polyethylene microporous film.

  The polyethylene microporous membrane of the present invention has small changes in film thickness and air permeability during pressurization, has a high absorption rate of the electrolytic solution, and is excellent in mechanical properties, permeability, and heat shrinkage properties. By using such a polyethylene microporous membrane as a battery separator, a battery excellent in safety such as compression resistance and productivity can be obtained.

[1] Polyethylene-based resin The polyethylene microporous membrane of the present invention (hereinafter sometimes simply referred to as “microporous membrane”) has a mass average molecular weight (Mw) of 1 × 10 ⁶ or more and a ratio of ultrahigh molecular weight polyethylene of 15 It consists of a polyethylene resin of less than mass%. The polyethylene resin is (a) a polyethylene composition comprising an ultra high molecular weight polyethylene having an Mw of 1 × 10 ⁶ or more and other polyethylene, (b) a polyethylene other than the ultra high molecular weight polyethylene, or (c) a polyethylene composition. Or it is preferable that it is a mixture (polyolefin composition) of polyethylene other than ultra high molecular weight polyethylene and polyolefins other than polyethylene.

(a) When composed of a polyethylene composition Ultra high molecular weight polyethylene is not limited to ethylene homopolymer, but may be a copolymer containing a small amount of other α-olefin. Examples of α-olefins other than ethylene include propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The Mw of the ultra high molecular weight polyethylene is preferably in the range of 1 × 10 ^{6 to} 3 × 10 ⁶ . By making the Mw of ultrahigh molecular weight polyethylene 3 × 10 ⁶ or less, melt extrusion can be facilitated.

The polyethylene other than the ultra high molecular weight polyethylene has an Mw of less than 1 × 10 ⁶ and is preferably at least one selected from the group consisting of high density polyethylene, medium density polyethylene, branched low density polyethylene, and chain low density polyethylene, High density polyethylene is more preferred. The polyethylene other than the ultrahigh molecular weight polyethylene may be a copolymer containing not only a homopolymer of ethylene but also a small amount of other α-olefin. Other α-olefins other than ethylene may be the same as described above. Such a copolymer is preferably produced by a single site catalyst. The Mw of polyethylene other than ultra high molecular weight polyethylene is preferably in the range of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ . Among them Mw of high-density polyethylene is more preferably less than 7 × 10 ⁴ or more to 5 × 10 ^5, less than 2 × 10 ⁵ or more to 5 × 10 ⁵ is particularly preferred. Two or more types of polyethylenes other than ultra-high molecular weight polyethylenes having different Mw or density may be used.

Mw is preferably 1 × 10 ⁶ or less of the polyethylene composition, more preferably in the range of ^{1 × 10 5 ~1 × 10 6} , in the range of 2 × 10 ⁵ ~1 × 10 ⁶ being particularly preferred. When the Mw of the polyethylene composition exceeds 1 × 10 ⁶ , a coarse structure layer having an average pore diameter of 1.2 to 5.0 times that of the dense structure layer is not formed. When the Mw of the polyethylene composition is less than 1 × 10 ^5, it is difficult to obtain a suitable polyethylene microporous membrane because breakage tends to occur during stretching.

  The ratio of the ultrahigh molecular weight polyethylene in the polyethylene composition is 15% by mass or less, where the total of the ultrahigh molecular weight polyethylene and the other polyethylene is 100% by mass. When this ratio exceeds 15% by mass, a coarse structure layer is not formed. This proportion is preferably 12% by mass or less, particularly preferably 10% by mass or less. Although not limited, in order to obtain excellent mechanical strength, the lower limit of this ratio is preferably 1% by mass.

(b) When made of polyethylene other than ultra high molecular weight polyethylene The polyethylene resin may be made of polyethylene other than ultra high molecular weight polyethylene. The polyethylene other than the ultra high molecular weight polyethylene may be the same as described above.

(c) When composed of a polyolefin composition The polyolefin composition is a polyethylene composition or a mixture of polyethylene other than ultra-high molecular weight polyethylene and polyolefin other than polyethylene. The polyethylene other than the polyethylene composition and the ultrahigh molecular weight polyethylene may be the same as described above.

Polyolefins other than polyethylene, each having a Mw of 1 × 10 ⁴ to 4 × 10 ⁶ , polypropylene, polybutene-1, polypentene-1, polyhexene-1, poly-4-methylpentene-1, polyoctene-1, polyvinyl acetate, At least one selected from the group consisting of polymethyl methacrylate, polystyrene, ethylene / α-olefin copolymer, and polyethylene wax having Mw of 1 × 10 ³ to 1 × 10 ⁴ can be used. Polypropylene, polybutene-1, polypentene-1, polyhexene-1, poly-4-methylpentene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not only homopolymers, but also other α-olefins. It may be a copolymer. The proportion of polyolefin other than polyethylene is preferably 20% by mass or less, more preferably 10% by mass or less, based on 100% by mass of the entire polyolefin composition.

  When the polyethylene microporous membrane contains polypropylene, when used as a battery separator, meltdown characteristics and high-temperature storage characteristics of the battery are improved. As the polypropylene, a homopolymer is preferable. When a copolymer of propylene and another α-olefin, or a mixture of a homopolymer and a copolymer is used, either a block copolymer or a random copolymer can be used as the copolymer. Other α-olefins are preferably ethylene.

  When the polyolefin composition is a mixture of a polyethylene composition and a polyolefin other than polyethylene, the ratio of ultrahigh molecular weight polyethylene is 15% by mass or less, where the total of the polyethylene composition and polyolefin other than polyethylene is 100% by mass. This proportion is preferably 12% by mass or less, particularly preferably 10% by mass or less. The lower limit of this ratio is preferably 1% by mass.

(D) Molecular weight distribution Mw / Mn
Mw / Mn is a measure of molecular weight distribution. The larger this value, the wider the molecular weight distribution. The Mw / Mn of polyethylene other than the polyethylene composition and ultrahigh molecular weight polyethylene is not limited, but is preferably 5 to 30, and more preferably 10 to 25. When Mw / Mn is less than 5, the high molecular weight component is too much and melt extrusion is difficult, and when Mw / Mn is more than 30, the low molecular weight component is too much and the strength of the microporous membrane is reduced. Mw / Mn of polyethylene (homopolymer and ethylene / α-olefin copolymer) can be appropriately adjusted by multistage polymerization. As the multi-stage polymerization method, a two-stage polymerization in which a high molecular weight polymer component is generated in the first stage and a low molecular weight polymer component is generated in the second stage is preferable. In the case of a polyethylene composition, the greater the Mw / Mn, the greater the difference in mass average molecular weight between ultrahigh molecular weight polyethylene and other polyethylenes, and vice versa. Mw / Mn of the polyethylene composition can be appropriately adjusted depending on the molecular weight and mixing ratio of each component.

[2] Method for producing polyethylene microporous membrane
(a) First production method The first method for producing the polyethylene microporous membrane of the present invention includes (1) a step of preparing a polyethylene solution by melt-kneading a polyethylene resin and a film-forming solvent, (2) A step of extruding a polyethylene solution from a die, (3) a step of cooling the obtained extruded product to form a temperature distribution in the film thickness direction to form a gel sheet, (4) a first stretching step, (5 ) Having a film forming solvent removing step, (6) a drying step, and (7) a second stretching step. After the step (7), (8) a heat treatment step, (9) a crosslinking step by ionizing radiation, (10) a hydrophilization treatment step, (11) a surface coating treatment step and the like may be performed as necessary.

(1) Preparation process of polyethylene solution A polyethylene resin and a film-forming solvent are melt-kneaded to prepare a polyethylene solution. Various additives such as an antioxidant, an ultraviolet absorber, an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added to the polyethylene solution as necessary, as long as the effects of the present invention are not impaired. For example, finely divided silicic acid can be added as a pore forming agent.

  Either a liquid solvent or a solid solvent can be used as the film-forming solvent. Examples of the liquid solvent include nonane, decane, decalin, paraxylene, undecane, dodecane, aliphatic hydrocarbons such as liquid paraffin, and mineral oil fractions having boiling points corresponding to these. In order to obtain a gel-like sheet having a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. The solid solvent preferably has a melting point of 80 ° C. or lower, and examples of such a solid solvent include paraffin wax, ceryl alcohol, stearyl alcohol, dicyclohexyl phthalate, and the like. A liquid solvent and a solid solvent may be used in combination.

  The viscosity of the liquid solvent is preferably in the range of 30 to 500 cSt at a temperature of 25 ° C., more preferably in the range of 50 to 200 cSt. When the viscosity is less than 30 cSt, the polyethylene solution is not uniformly discharged from the die lip, and kneading is difficult. On the other hand, if it exceeds 500 cSt, it is difficult to remove the liquid solvent.

  The uniform melt kneading of the polyethylene solution is not particularly limited, but it is preferably performed in a twin screw extruder. Melt kneading in a twin screw extruder is suitable for preparing a highly concentrated polyethylene solution. The melt kneading temperature is not limited to the case where the polyethylene resin is any of the above [1] (a) to (c), but the polyethylene resin contains (a) a polyethylene composition or (b) other than ultrahigh molecular weight polyethylene. The melting point of polyethylene + 10 ° C to the melting point + 100 ° C is preferred. Therefore, when the polyethylene resin is the above (a) or (b), the melting and kneading temperature is preferably the melting point of the polyethylene resin + 10 ° C. to the melting point + 100 ° C. Specifically, the melt kneading temperature is preferably 140 to 250 ° C, more preferably 170 to 240 ° C. The “melting point” is determined by differential scanning calorimetry (DSC) based on JIS K7121.

  However, if the polyethylene resin is (c) a polyolefin composition and the melting point of the other polyolefin is higher than that of the polyethylene composition or polyethylene other than ultrahigh molecular weight polyethylene, the lower limit of the melt kneading temperature is the melting point of the other polyolefin. Is more preferable. For example, when polypropylene is used as the other polyolefin, the melt kneading temperature is preferably from the melting point of polypropylene to the melting point of polyethylene other than the polyethylene composition or ultrahigh molecular weight polyethylene + 100 ° C.

  The film-forming solvent may be added before the start of kneading or may be added from the middle of the extruder during kneading, but the latter is preferred. In melt kneading, it is preferable to add an antioxidant in order to prevent oxidation of the polyethylene resin.

  In the polyethylene solution, the blending ratio of the polyethylene-based resin and the film-forming solvent is 100% by mass, preferably 15-50% by mass of the polyethylene-based resin, more preferably 20-45% by mass. It is particularly preferably 25 to 40% by mass. When the ratio of the polyethylene resin is less than 15% by mass, productivity is lowered, which is not preferable. Moreover, when extruding the polyethylene solution, swell and neck-in are increased at the die outlet, and the moldability and self-supporting property of the gel-like molded product are lowered. On the other hand, when the proportion of the polyethylene resin exceeds 50% by mass, the moldability of the gel-like molded product is lowered.

(2) Extrusion Step The melt-kneaded polyethylene solution is extruded from the die directly from the extruder or via another extruder, or once cooled and pelletized, it is extruded again from the die via the extruder. As the die lip, a sheet die lip having a rectangular base shape is usually used, but a double cylindrical hollow die lip, an inflation die lip, or the like can also be used. In the case of a sheet die lip, the gap of the die lip is usually in the range of 0.1 to 5 mm, and is heated to a temperature of 140 to 250 ° C. during extrusion. The extrusion rate of the heated solution is preferably in the range of 0.2 to 15 m / min.

(3) Gel Sheet Forming Step A molded body extruded from a die lip is cooled to form a temperature distribution in the film thickness direction to form a gel sheet. As a method of cooling so that a temperature distribution is generated in the film thickness direction of the extruded molded body, a method of rapidly cooling one side of the extruded molded body and gradually cooling the other side is preferable.

  As a method for rapidly cooling one side of the extrusion-molded body, a method in which it is brought into contact with a cooling roll, a method in which it is brought into contact with a refrigerant such as cold air or cooling water, and the like can be used. The temperature of the cooling roll may be any other than the polyethylene resin (a) polyethylene composition or (b) ultrahigh molecular weight polyethylene, regardless of whether the polyethylene resin is any of the above [1] (a) to (c). The polyethylene has a crystallization temperature of −115 ° C. or higher and a crystallization temperature of −25 ° C. or lower. Therefore, when the polyethylene resin is the above (a) or (b), the temperature of the cooling roll is preferably a crystallization temperature of the polyethylene resin of −115 ° C. or more and a crystallization temperature of −25 ° C. or less. When the temperature of the cooling roll is lower than the crystallization temperature of −115 ° C., the molded body is rapidly cooled as a whole, and the temperature distribution hardly occurs in the film thickness direction. On the other hand, if the temperature of the cooling roll is higher than the crystallization temperature −25 ° C., sufficient rapid cooling cannot be achieved. The temperature of the cooling roll is more preferably a crystallization temperature of −115 ° C. or more and a crystallization temperature of −55 ° C. or less, and particularly preferably a crystallization temperature of −105 ° C. or more and a crystallization temperature of −80 ° C. or less. The “crystallization temperature” is determined according to JIS K7121.

  The crystallization temperature of polyethylene other than (a) polyethylene composition and (b) ultrahigh molecular weight polyethylene is generally 102-108 ° C. Therefore, the temperature of the cooling roll is preferably within the range of −10 to 80 ° C., more preferably within the range of −10 to 50 ° C., and particularly preferably within the range of 0 to 25 ° C. The contact time between the cooling roll and the extruded molded body is preferably 1 to 30 seconds, more preferably 2 to 15 seconds. When this contact time is less than 1 second, temperature distribution in the film thickness direction is unlikely to occur. On the other hand, if it exceeds 30 seconds, the extruded product is rapidly cooled as a whole, and the temperature distribution in the film thickness direction disappears.

  Although the conveyance speed by the cooling roll of an extrusion molded object should just be a grade which does not raise | generate a neck-in in an extrusion molded object, 0.5-20 m / min is preferable and 1-10 m / min is more preferable. The diameter of the cooling roll is preferably 10 to 150 cm, more preferably 15 to 100 cm. When the diameter is less than 10 cm, the contact time between the extruded product and the roll is short, and sufficient rapid cooling cannot be achieved. On the other hand, if it exceeds 150 cm, the equipment becomes too large. Although the number of cooling rolls may be normally one, as long as the contact time of a cooling roll and a molded object is in the said range, multiple may be sufficient as needed.

  As a method of gradually cooling the other surface of the extruded molded body, a method of exposing to the air at room temperature is preferable. Here, room temperature assumes a temperature of 10 to 40 ° C., but is not limited to this range. However, a method of blowing air by air cooling means may be used as the slow cooling method. Examples of the air cooling means include a blower and a nozzle. The temperature of the air flow of the air cooling means is not particularly limited as long as it can be gradually cooled, but is, for example, 10 to 100 ° C, and preferably 15 to 80 ° C.

  The extrusion molded body as described above is finally cooled to at least the gelation temperature or lower. Specifically, it is preferably performed up to 35 to 50 ° C, more preferably up to 25 ° C or less.

  In the present invention, since the ratio of ultrahigh molecular weight polyethylene in the polyethylene resin is 15% by mass or less, the gel obtained by rapidly cooling one side of the extruded molded body and gradually cooling the other side as described above In the sheet, a quenching layer and a slow cooling layer having different crystal structure densities are formed. Specifically, a rapid cooling layer consisting of a dense high-order crystal layer (a layer with a high density of network structure in which there are many connections in the three-dimensional network structure constituting the polyethylene resin network) and a coarse high layer A gel-like sheet having a crystallized layer having a next structure and a slow cooling layer composed of a crystalline layer (a layer in which the network structure of the polyethylene resin is low in the three-dimensional network structure and has a low network structure density) is formed. In each of the rapid cooling layer and the slow cooling layer, a structure in which a polyethylene resin phase is microphase-separated by a film forming solvent (a gel structure including a polyethylene resin phase and a film forming solvent phase) is fixed.

  The gel-like sheet formed as described above is subjected to a first stretching step, a film forming solvent removal step, and a second stretching step described below, thereby providing a fine structure layer and a coarse structure layer. A porous membrane can be produced.

(4) First stretching step The obtained gel-like sheet is stretched in at least a uniaxial direction. Stretching causes cleavage between polyethylene crystal lamella layers, and the polyethylene phase is refined to form a large number of fibrils. Since the gel-like sheet contains a film-forming solvent, it can be stretched uniformly. After heating, the gel-like sheet is stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, a rolling method, or a combination of these methods. The first stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching or multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used, but simultaneous biaxial stretching is particularly preferable.

  In the case of uniaxial stretching, the draw ratio is preferably 2 times or more, more preferably 3 to 30 times. In biaxial stretching, it is preferably at least 3 times in any direction, preferably 9 times or more in area magnification, and more preferably 25 times or more in area magnification. If the area magnification is less than 9 times, stretching is insufficient, and a highly elastic and high-strength microporous film cannot be obtained. On the other hand, when the area magnification exceeds 400 times, there are restrictions in terms of stretching apparatus, stretching operation, and the like.

  The temperature of the first stretching is not limited to the case where the polyethylene resin is any of the above [1] (a) to (c). The crystal dispersion temperature of polyethylene other than polyethylene is in the range of + 10 ° C to crystal dispersion temperature + 30 ° C. Therefore, when the polyethylene resin is the above (a) or (b), the first stretching temperature is in the range of the crystal dispersion temperature of the polyethylene resin + 10 ° C. to the crystal dispersion temperature + 30 ° C. When the stretching temperature is higher than the crystal dispersion temperature + 30 ° C., the orientation of molecular chains after stretching deteriorates. On the other hand, when the crystal dispersion temperature is lower than + 10 ° C., vein-like fibrils are not formed in the slow cooling layer, and the pore diameter is reduced or the compression resistance is lowered. The stretching temperature is preferably in the range of crystal dispersion temperature + 15 ° C to crystal dispersion temperature + 25 ° C. “Crystal dispersion temperature” is determined by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D 4065. The crystal dispersion temperature of polyethylene other than (a) polyethylene composition and (b) ultrahigh molecular weight polyethylene is 90-100 ° C. Accordingly, the stretching temperature is preferably in the range of 105 to 130 ° C, more preferably in the range of 110 to 125 ° C, and particularly preferably in the range of 115 to 125 ° C.

  The first stretching may be performed at two or more different temperatures. In this case, it is preferable to perform two-stage stretching in which the temperature at the subsequent stage is higher than that at the previous stage, and thereby the lamella layer is made uniform. As a result, a microporous film having high permeability can be obtained without lowering strength or reducing physical properties in the width direction. The difference in the stretching temperature between the former stage and the latter stage is preferably 5 ° C or more. When raising the temperature of the gel-like sheet from the former stage to the latter stage, (i) the temperature may be raised while continuing the stretching, or (ii) while the temperature rises, the stretching is stopped and the temperature reaches the predetermined temperature and then the latter stage Although stretching may be started, the former (i) is preferred. In any case, it is preferable to rapidly heat at the time of temperature rise. Specifically, it is preferable to heat at a temperature rising rate of 0.1 ° C./second or more, and more preferable to heat at a temperature rising rate of 1 to 5 ° C./second. Needless to say, the stretching temperature and the total stretching ratio of the former stage and the latter stage are within the above ranges, respectively.

  By the first stretching as described above, in the slow cooling layer of the gel-like sheet, the fibrils to be formed have a vein shape and the trunk fibers are relatively thick. Therefore, a microporous film having excellent strength can be obtained by the subsequent solvent removal treatment for film formation. Here, “leaf vein-like fibril” refers to a state in which the fibril is composed of a thick trunk fiber and a thin fiber connected to the outside thereof, and the thin fiber forms a complex network structure. On the other hand, in the rapid cooling layer of the gel-like sheet, the formed fibrils have a uniform and dense structure.

  Depending on the desired physical properties, the film may be stretched by providing a temperature distribution in the film thickness direction within a range that does not impair the effects of the present invention, thereby obtaining a microporous film having further excellent mechanical strength. The method is specifically described in Japanese Patent No. 3347854.

(5) Film forming solvent removal step A cleaning solvent is used to remove (clean) the film forming solvent. Since the polyethylene-based resin phase is separated from the film-forming solvent phase, if the film-forming solvent is removed, it is composed of fibrils that form a fine three-dimensional network structure, and pores (voids) that are irregularly communicated three-dimensionally. Is obtained. Examples of the washing solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, ethane trifluoride, Chain fluorocarbons such as C ₆ F ₁₄ and C ₇ F ₁₆ , cyclic hydrofluorocarbons such as C ₅ H ₃ F _7, hydrofluoroethers such as C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , C ₄ Examples include readily volatile solvents such as perfluoroethers such as F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ . These cleaning solvents have a low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a low surface tension cleaning solvent, the network structure forming the micropores is prevented from shrinking due to the surface tension of the gas-liquid interface during drying after cleaning, and thus has a high porosity and permeability. A porous membrane is obtained.

  The gel-like sheet after stretching can be washed by a method of immersing in a washing solvent, a method of showering the washing solvent, or a combination thereof. The washing solvent is preferably used in an amount of 300 to 30,000 parts by mass with respect to 100 parts by mass of the stretched film. The washing temperature may usually be 15 to 30 ° C. and may be heated and washed as necessary. The temperature for the heat washing is preferably 80 ° C. or lower. The washing with the washing solvent is preferably performed until the residual amount of the liquid solvent becomes less than 1% by mass of the initial addition amount.

(6) Membrane drying step The polyethylene microporous membrane obtained by stretching and removing the solvent for film formation is dried by a heat drying method or an air drying method. The drying temperature is preferably equal to or lower than the crystal dispersion temperature of polyethylene other than (a) the polyethylene composition or (b) ultrahigh molecular weight polyethylene contained in the polyethylene resin, and is preferably 5 ° C. or more lower than the crystal dispersion temperature. Drying is preferably performed until the residual cleaning solvent is 5% by mass or less, more preferably 3% by mass or less, with the microporous membrane being 100% by mass (dry weight). Insufficient drying is not preferable because the porosity of the microporous membrane is lowered in the subsequent second stretching step and heat treatment step, and the permeability is deteriorated.

(7) Second stretching step The dried film is stretched again in at least a uniaxial direction. The second stretching can be performed by a tenter method or the like, similar to the first stretching, while heating the film. The second stretching may be uniaxial stretching or biaxial stretching.

  The magnification of the second stretching is 1.05 to 1.45 times in the stretching axis direction. For example, in the case of uniaxial stretching, it is 1.05 to 1.45 times in the longitudinal direction (MD) and the transverse direction (TD). In the case of biaxial stretching, it is 1.05 to 1.45 times in the MD direction and TD direction, respectively. In the case of biaxial stretching, as long as the draw ratio in the MD direction and the TD direction is 1.05 to 1.45 times, they may be different from each other in the MD direction and the TD direction, but are preferably the same. When this magnification is less than 1.05, the compression resistance is insufficient. On the other hand, when the magnification is more than 1.45 times, the average pore diameter of the coarse structure layer becomes small and the fibrils also become thin. The second stretching ratio is more preferably 1.1 to 1.4 times.

  The temperature of the second stretching is not limited to the case where the polyethylene resin is any of the above [1] (a) to (c). It is preferably within the range of the crystal dispersion temperature of polyethylene other than polyethylene to the crystal dispersion temperature + 40 ° C. or less. Therefore, when the polyethylene resin is the above (a) or (b), the temperature of the second stretching is preferably in the range from the crystal dispersion temperature of the polyethylene resin to the crystal dispersion temperature + 40 ° C. or less. When the temperature of the second stretching exceeds the crystal dispersion temperature + 40 ° C, the permeability and compression resistance are reduced, and when the film is stretched in the TD direction, variations in physical properties (especially air permeability) increase in the sheet width direction. Or On the other hand, when the temperature of the second stretching is lower than the crystal dispersion temperature, the polyethylene resin is not sufficiently softened, the film is easily broken during stretching, and the stretching cannot be performed uniformly. The temperature of the second stretching is more preferably in the range of crystal dispersion temperature + 10 ° C or higher to crystal dispersion temperature + 40 ° C or lower. Specifically, the temperature of the second stretching is preferably in the range of 90 to 140 ° C, and more preferably in the range of 100 to 135 ° C.

  By the second stretching performed after removing the solvent as described above, excellent electrolyte solution absorbability can be obtained. Although not limited, it is preferable to employ an in-line method in which the first stretching, the solvent removal for film formation, the drying treatment, and the second stretching are continuously performed on a series of lines. However, if necessary, an off-line method may be employed in which the film after the drying treatment is temporarily used as a wound sheet and the second stretching is performed while the film is rewound.

(8) Heat treatment step It is preferable to heat-treat the film after the second stretching. The crystal is stabilized by the heat treatment, and the lamellar layer is made uniform. As the heat treatment method, heat setting treatment and / or heat relaxation treatment may be used. In particular, the heat-fixing treatment stabilizes the crystal of the film, retains a network composed of fibrils refined by the second stretching, and can produce a microporous film excellent in electrolyte solution absorbability and strength. The heat setting treatment is carried out within the temperature range of (a) polyethylene composition or (b) polyethylene other than ultrahigh molecular weight polyethylene contained in the polyethylene-based resin + 30 ° C. or lower, preferably from the crystal dispersion temperature to the melting point. The heat setting time is not particularly limited, but is preferably 0.1 second to 100 hours. If it is less than 0.1 seconds, the crystal stabilization effect is small, and if it exceeds 100 hours, the productivity is low. The heat setting treatment is performed by a tenter method, a roll method or a rolling method.

  You may perform a heat relaxation process using a belt conveyor or an air floating heating furnace other than the said system. The thermal relaxation treatment is carried out within the temperature range of (a) polyethylene composition or (b) polyethylene other than ultrahigh molecular weight polyethylene contained in the polyethylene resin, preferably 60 ° C. or higher and melting point −5 ° C. or lower. The shrinkage due to the heat relaxation treatment is preferably such that the length in the direction of the second stretching is 91% or more before the second stretching, and more preferably 95% or more. If the shrinkage is less than 91%, the physical property balance in the width direction of the sheet after the second stretching, particularly the balance of permeability, is deteriorated. By the heat relaxation treatment as described above, a high-strength microporous film having good permeability can be obtained. Moreover, you may carry out combining many heat setting processes and heat relaxation processes.

(9) Film Crosslinking Process Step The microporous film subjected to the second stretching may be subjected to a crosslinking process by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. 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 meltdown temperature of the polyethylene microporous membrane is increased by the crosslinking treatment.

(10) Hydrophilization treatment step The microporous membrane subjected to the second stretching may be subjected to a hydrophilic treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge or the like. Monomer grafting is preferably performed after the crosslinking treatment.

  In the case of the surfactant treatment, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant or a zwitterionic surfactant can be used, but a nonionic surfactant is preferred. The microporous membrane is immersed in a solution obtained by dissolving a surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, or the solution is applied to the microporous membrane by a doctor blade method.

(11) Surface coating treatment step The second stretched microporous membrane is a polypropylene porous body; a fluororesin porous body such as polyvinylidene fluoride or polytetrafluoroethylene; a porous body such as polyimide or polyphenylene sulfide. By covering the surface with a body or the like, the meltdown characteristics when used as a battery separator are improved. The polypropylene for the coating layer preferably has an Mw in the range of 5,000 to 500,000, and preferably has a solubility in 100 g of toluene at a temperature of 25 ° C. of 0.5 g or more. This polypropylene preferably has a fraction of racemic dyad (a structural unit in which two consecutive monomer units are in an enantiomeric relationship) of 0.12 to 0.88.

(b) Second manufacturing method The second manufacturing method is only the point of removing the film-forming solvent after heat-fixing the gel sheet subjected to the first stretching with respect to the first manufacturing method. However, the other processes are the same. The heat setting method may be the same as described above. However, it is preferable to heat the stretched gel sheet from both sides or only from the slow cooling layer side. By heat-setting a stretched gel-like sheet containing a film-forming solvent, the pore diameter on both sides of the microporous membrane is increased, especially the average pore diameter on the surface of the quenched layer and its vicinity is 1.2 to 5.0 times that before the treatment. It becomes. However, if the temperature and time conditions of the heat setting treatment are within the above ranges, the dense structure layer does not disappear inside the film. Therefore, a coarse structure layer can be formed on both surfaces of the microporous membrane by subjecting the stretched gel-like sheet to heat setting.

(c) Third production method The third production method is a gel-like sheet subjected to the first stretching and / or a microporous membrane from which the film-forming solvent has been removed, compared to the first production method. Only the point of contact with the hot solvent is different, and the other steps are the same. Therefore, only the hot solvent treatment step will be described below.

  The hot solvent treatment is preferably performed on the stretched gel-like sheet before washing. As the solvent used in the heat treatment, the above liquid film-forming solvent is preferable, and liquid paraffin is more preferable. However, the solvent for heat treatment may be the same as that used in preparing the polyethylene solution, or may be different.

  The thermal solvent treatment method is not particularly limited as long as the stretched gel sheet or microporous membrane can be brought into contact with the thermal solvent. For example, the stretched gel sheet or microporous membrane is directly brought into contact with the thermal solvent. A method (hereinafter, simply referred to as “direct method” unless otherwise specified), a method in which a gelled sheet or a microporous membrane after stretching is brought into contact with a cold solvent and then heated (hereinafter, unless otherwise specified). Simply called “indirect method”). The direct method includes a method of immersing the stretched gel sheet or microporous membrane in a hot solvent, a method of spraying the hot solvent on the stretched gel sheet or microporous membrane, and a gel shape after stretching the hot solvent. Although there is a method of applying to a sheet or a microporous film, a dipping method is preferable, and thereby a more uniform treatment is possible. As an indirect method, the gel-like sheet or microporous membrane after stretching is immersed in a cold solvent, or the cold solvent is sprayed on the gel-like sheet or microporous membrane after stretching, or the gel-like sheet after stretching the cold solvent. Or after apply | coating to a microporous film, the method of making it contact with a hot roll, heating in oven, or immersing in a hot solvent is mentioned.

  The temperature of the hot solvent is not limited to (a) polyethylene composition or (b) ultrahigh molecular weight polyethylene contained in the polyethylene resin, regardless of whether the polyethylene resin is any of the above [1] (a) to (c). The temperature is preferably in the range of from the crystal dispersion temperature of polyethylene to the melting point + 10 ° C. or less. Specifically, the hot solvent temperature is preferably 110 to 140 ° C, more preferably 115 to 135 ° C. The contact time is preferably 0.1 second to 10 minutes, more preferably 1 second to 1 minute. By such a hot solvent treatment, the pore diameters on both sides of the microporous membrane are increased, and in particular, the average pore diameter on the surface of the quenched layer and in the vicinity thereof is 1.2 to 5.0 times that before the treatment. However, if the temperature and time conditions of the hot solvent treatment are within the above ranges, the dense layer does not disappear inside the film. Therefore, a coarse structure layer can be formed on both surfaces of the microporous membrane by subjecting the stretched gel-like sheet to a hot solvent treatment.

  Moreover, the vein-like fibrils formed by the first stretching can be further strengthened by the hot solvent treatment. Therefore, the vein-like fibrils are not re-refined by the second stretching, and the average pore diameter of the slow cooling layer is not reduced. If the hot solvent temperature is less than the crystal dispersion temperature or the contact time is less than 0.1 seconds, the effect of the hot solvent treatment is hardly obtained. On the other hand, if the temperature of the hot solvent is higher than the melting point + 10 ° C. or if the contact time is longer than 10 minutes, the strength of the microporous film is lowered or the microporous film is broken.

  The stretched gel-like sheet and / or microporous membrane is treated with a hot solvent and then washed to remove the remaining heat treatment solvent. Since the cleaning method may be the same as the film forming solvent removing method, description thereof is omitted. Needless to say, when the hot solvent treatment is performed on the stretched gel-like sheet before washing, the solvent for heat treatment can be removed by performing the above-mentioned solvent removal treatment for film formation.

  In addition, the heat setting process before washing | cleaning is not limited only to a 2nd manufacturing method, You may exist in a 3rd manufacturing method. That is, in the third production method, the gel-like sheet before and / or after the hot solvent treatment may be heat set.

[3] Structure and physical properties of polyethylene microporous membrane The polyethylene microporous membrane of the present invention has a dense structure layer having an average pore diameter of 0.01 to 0.05 μm and at least one surface, and the average pore diameter is 1.2 to that of the dense structure layer. With a 5.0 times coarser layer. The average pore diameter of the coarse structure layer is preferably 1.5 to 3.0 times the average pore diameter of the dense structure layer. The coarse structure layer may be formed only on one side or may be formed on both sides. The average pore diameter of the dense structure layer and the coarse structure layer was determined from a transmission electron microscope (TEM) photograph of the cross section of the microporous membrane.

  The shape of the through hole is not particularly limited. Usually, both the dense structure layer and the coarse structure layer have pores composed of voids communicating irregularly three-dimensionally. The microporous membrane obtained by the second and third production methods has an average pore diameter that is greater than that of the membrane obtained by the first production method for both the dense structure layer and the coarse structure layer.

  Since the polyethylene microporous membrane of the present invention has a dense structure layer and a coarse structure layer, it is excellent in compression resistance and electrolyte absorption. Specifically, the film thickness change and the air permeability change during pressurization are small, and the absorption rate of the electrolytic solution is fast. The change in thickness of the polyethylene microporous membrane of the present invention during pressurization is small because the fibers constituting the fibrils of the dense structure layer are relatively thick, the strength of the dense structure layer is high, and the resistance to compression is high. It is thought that. The ratio of the coarse structure layer to the dense structure layer is preferably 5/1 to 1/10 in the following ratio: (thickness of coarse structure layer) / (thickness of dense structure layer). More preferably, it is 1/5. When this ratio is more than 5/1, the film thickness changes greatly when the pressure is applied, and the mechanical strength is low. On the other hand, if it is less than 1/10, the electrolyte absorbability is low. This ratio can be determined from a transmission electron microscope (TEM) photograph of the cross section of the microporous membrane. This ratio can be adjusted, for example, by adjusting the temperature of the cooling roll or the contact time between the cooling roll and the molded body.

  The polyethylene microporous membrane according to a preferred embodiment of the present invention has the following physical properties.

(a) Porosity of 25 to 80% When the porosity is less than 25%, the polyethylene microporous membrane does not have good air permeability. On the other hand, if it exceeds 80%, the strength when the microporous membrane is used as a battery separator is insufficient, and there is a great risk that the electrode is short-circuited.

(b) Air permeability of 20 to 400 seconds / 100 cm ³ (film thickness converted to 20 μm)
When the air permeability is 20 to 400 seconds / 100 cm ³ , when the polyethylene microporous membrane is used as a battery separator, the battery capacity is increased and the cycle characteristics of the battery are also improved. If the air permeability is less than 20 seconds / 100 cm ³ , shutdown may not be performed sufficiently when the temperature inside the battery rises.

(c) Puncture strength of 3,000 mN / 20 μm or more When the puncture strength is less than 3,000 mN / 20 μm, a short circuit may occur when a polyethylene microporous membrane is incorporated in a battery as a battery separator. The puncture strength is preferably 3,500 mN / 20 μm or more.

(d) Tensile rupture strength of 80,000 kPa or more When the tensile rupture strength is 80,000 kPa or more in both the longitudinal direction (MD) and the transverse direction (TD), there is no concern about film breakage when used as a battery separator. . The tensile strength at break is preferably 100,000 kPa or more in both the MD direction and the TD direction.

(e) Tensile rupture elongation of 100% or more If the tensile rupture elongation is 100% or more in both the longitudinal direction (MD) and the transverse direction (TD), there is a risk of film breakage when used as a battery separator. There is no.

(f) Thermal shrinkage of 10% or less
When the thermal shrinkage after exposure to 105 ° C for 8 hours exceeds 10% in both the longitudinal direction (MD) and the transverse direction (TD), when the polyethylene microporous membrane is used as a battery separator, Shrinkage and the possibility of a short circuit occurring at the end increases. The thermal shrinkage rate is preferably 8% or less in both the MD direction and the TD direction.

(g) Film thickness change rate after heat compression of 30% or less
The rate of change in film thickness after heating and compression at 90 ° C. for 5 minutes under a pressure of 2.2 MPa (22 kgf / cm 2) is 30% or less with the film thickness before compression as 100%. When the rate of change in film thickness is 30% or less, when the microporous membrane is used as a battery separator, the battery capacity is large and the cycle characteristics of the battery are also good. The film thickness change rate is preferably 20% or less.

(h) Ultimate air permeability of 700 sec / 100 cm ³ or less The ultimate air permeability (Gurley value) after heat compression under the above conditions is 700 sec / 100 cm ³ or less. When the ultimate air permeability is 700 sec / 100 cm ³ or less, when used as a battery separator, the battery capacity is large and the cycle characteristics of the battery are good. The ultimate air permeability is preferably 650 sec / 100 cm ³ or less.

  Thus, the microporous membrane of the present invention has small changes in film thickness and air permeability even when pressurized, and is excellent in permeability, mechanical properties, and heat shrinkage properties. Further, the absorption rate of the electrolytic solution is at least 1.5 times or more that of a film having no coarse structure layer. Therefore, it is particularly suitable as a battery separator.

[4] Battery separator The battery separator comprising the polyethylene microporous membrane can be appropriately selected depending on the type of battery, but the thickness is preferably 5 to 50 μm, more preferably 10 to 35 μm. preferable.

[5] Battery The polyethylene microporous membrane of the present invention is a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. However, it is particularly preferable to use as a separator for a lithium secondary battery. Hereinafter, a lithium secondary battery will be described as an example.

  In a lithium secondary battery, a positive electrode and a negative electrode are laminated via a separator, and the separator contains an electrolytic solution (electrolyte). The structure of the electrode is not particularly limited, and may be a known structure. For example, an electrode structure (coin type) in which disc-shaped positive electrodes and negative electrodes are opposed to each other, an electrode structure in which flat plate-like positive electrodes and negative electrodes are alternately stacked (stacked type), and belt-shaped positive electrodes and negative electrodes are stacked. It is possible to obtain a wound electrode structure (winding type) or the like.

The positive electrode usually has (i) a current collector and (ii) a layer containing a positive electrode active material formed on the surface thereof and capable of occluding and releasing lithium ions. Examples of the positive electrode active material include transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), and inorganic compounds such as transition metal sulfides. Transition metals include V, Mn, and Fe. , Co, Ni and the like. Preferable examples of the lithium composite oxide include lithium nickelate, lithium cobaltate, lithium manganate, and a layered lithium composite oxide based on an α-NaFeO ₂ type structure. The negative electrode has (i) a current collector and (ii) a layer formed on the surface thereof and containing 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 can be obtained by dissolving a lithium salt in an organic solvent. Lithium salts 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 , Examples thereof include LiN (C ₂ F ₅ SO ₂ ) ₂ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , a lower aliphatic carboxylic acid lithium salt, LiAlCl _{4 and the} like. These may be used alone or as a mixture of two or more. Examples of the organic solvent include organic solvents having a high boiling point and a high dielectric constant such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and γ-butyrolactone, and tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, and diethyl carbonate. Examples include organic solvents having a low boiling point and a low viscosity. These may be used alone or as a mixture of two or more. In particular, a high dielectric constant organic solvent has a high viscosity, and a low viscosity organic solvent has a low dielectric constant. Therefore, it is preferable to use a mixture of the two.

  When assembling the battery, the separator is impregnated with the electrolytic solution. Thereby, ion permeability can be imparted to the separator (microporous membrane). Usually, the impregnation treatment is performed by immersing the microporous membrane in an electrolytic solution at room temperature. When assembling a cylindrical battery, for example, a positive electrode sheet, a separator made of a microporous film, and a negative electrode sheet are laminated in this order, and the obtained laminate is wound from one end to form a wound electrode element. The obtained electrode element is inserted into a battery can, impregnated with the above electrolyte, and a battery lid that also serves as a positive electrode terminal provided with a safety valve is caulked through a gasket to produce a battery.

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

Example 1
Polyethylene comprising 5% by mass of ultra high molecular weight polyethylene (UHMWPE, Mw / Mn = 8) with Mw of 1.5 × 10 ⁶ and 95% by mass of high density polyethylene (HDPE, Mw / Mn = 8.6) with Mw of 3.0 × 10 ⁵ To 100 parts by mass of the composition, 0.375 parts by mass of tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant was dry blended. The measured Mw for the PE composition consisting of UHMWPE and HDPE is 3.8 × 10 ⁵ , Mw / Mn is 10.2, the melting point is 134 ° C., the crystal dispersion temperature is 100 ° C., and the crystallization temperature is 105 ° C. Met.

Mw and Mw / Mn of UHMWPE, HDPE and PE composition were determined by gel permeation chromatography (GPC) method under the following conditions.
・ Measurement device: GPC-150C made by Waters Corporation
・ Column: Shodex UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
・ Solvent (mobile phase): o-dichlorobenzene ・ Solvent flow rate: 1.0 ml / min ・ Sample concentration: 0.1 wt% (dissolution condition: 135 ° C./1 h)
・ Injection volume: 500μl
Detector: Waters Corporation differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample, using a predetermined conversion constant.

  30 parts by mass of the resulting mixture was put into a twin screw extruder (inner diameter 58 mm, L / D = 52.5), 70 parts by mass of liquid paraffin was supplied from the side feeder of the twin screw extruder, 210 ° C. and 200 rpm The polyethylene solution was prepared by melt-kneading under the following conditions. Subsequently, this polyethylene solution was extruded through a T-die installed at the tip of the extruder, and the extruded molded body was taken up by a cooling roll adjusted to 15 ° C. (contact time: 10 seconds) and opposite to the cooling roll. The side was exposed to room temperature air and slowly cooled to form a gel sheet.

  Using a tenter stretching machine, the gel-like sheet was simultaneously biaxially stretched at 116 ° C. so that the longitudinal direction (MD) and the transverse direction (TD) are 5 times (first stretching). The obtained stretched film was fixed to a 20 cm × 20 cm aluminum frame, immersed in methylene chloride adjusted to 25 ° C., and washed with rocking at 100 rpm for 3 minutes. The resulting membrane was air dried at room temperature. The dried film was stretched again at a temperature of 128 ° C. in the transverse direction (TD) at a magnification of 1.1 (second stretching) using a batch stretching machine. While the obtained re-stretched film was fixed to a batch-type stretching machine, a heat setting treatment was performed at 128 ° C. for 10 minutes to produce a polyethylene microporous film.

Example 2
A polyethylene composition (Mw: 4.0 × 10 ⁵ , Mw / Mn: 11.0, melting point: 134.5 ° C.) consisting of 5% by mass of ultra high molecular weight polyethylene (Mw / Mn = 8) having an Mw of 2.0 × 10 ⁶ and 95% by mass of HDPE , Crystal dispersion temperature: 100 ° C., crystallization temperature: 105 ° C.), the first stretching temperature is 117.5 ° C., and the second stretching is performed at a temperature of 130.5 ° C. to 1.35 times in the TD direction, A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the heat setting treatment temperature was 130.5 ° C.

Example 3
A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the same polyethylene composition as in Example 2 was used, and the temperature of the second stretching and the heat setting treatment temperature were 129 ° C.

Example 4
A polyethylene composition (Mw: 5.5 × 10 ⁵ , Mw / Mn: 11.9, melting point: 135 ° C.) consisting of 10% by mass of ultra high molecular weight polyethylene (Mw / Mn = 8) with Mw of 2.0 × 10 ⁶ and 90% by mass of HDPE Crystal dispersion temperature: 100 ° C., crystallization temperature: 105 ° C.), the melt-kneaded product has a polyethylene concentration of 35 mass%, the first stretching temperature is 118.5 ° C., and the second stretching is 129.5 ° C. A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the heat fixing treatment temperature was set to 129.5 ° C. in the lower TD direction.

Example 5
Using the same polyethylene composition as in Example 4, when the extruded polyethylene molded body is taken up by a cooling roll, air (temperature: 20 ° C.) is blown onto the opposite side of the cooling roll (air amount: 100 ml / m ² ) in a gel state. A sheet was formed, and a polyethylene microporous membrane was produced in the same manner as in Example 1 except that the temperature of the first stretching was 117 ° C., and the temperature of the second stretching and the heat setting treatment temperature were 129.2 ° C.

Example 6
A polyethylene microporous membrane was prepared in the same manner as in Example 4 except that the temperature of the cooling roll was 45 ° C., the temperature of the first stretching was 117 ° C., and the temperature of the second stretching and the heat setting treatment temperature were 129.2 ° C. Produced.

Example 7
The same polyethylene composition as in Example 4 was used, the temperature of the first stretching was 117 ° C., the heat setting treatment was performed at 125 ° C. for 15 seconds after the first stretching, washing to remove liquid paraffin, and the second stretching A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the temperature and the heat setting treatment temperature were 129.5 ° C.

Example 8
Using the same polyethylene composition as in Example 4, the temperature was increased from 117 ° C. to 125 ° C. at a rate of 1 ° C./second during the first stretching (when the film was stretched 2.5 times × 2.5 times). The temperature was raised, heat fixed at 125 ° C. for 15 seconds after the first stretching, then washed to remove liquid paraffin, and the temperature of the second stretching and the heat setting temperature were set to 129 ° C. Thus, a polyethylene microporous membrane was produced.

Example 9
Using the same polyethylene composition as in Example 4, the gel-like sheet subjected to the first stretching was fixed to a frame plate [size: 20 cm × 20 cm, made of aluminum] and placed in a liquid paraffin bath adjusted to 130 ° C. After immersion for 3 seconds, washing was performed to remove liquid paraffin, and a polyethylene microporous membrane was produced in the same manner as in Example 1 except that the temperature of the second stretching and the heat setting temperature were 129 ° C.

Example 10
Polyethylene composition consisting of 10% by mass of ultra high molecular weight polyethylene (Mw / Mn = 8) with Mw of 2.0 × 10 ⁶ and 90% by mass of high density polyethylene with Mw of 3.5 × 10 ⁵ (HDPE, Mw / Mn = 13.5) (Mw: 5.5 × 10 ⁵ , Mw / Mn: 18.5, melting point: 135 ° C., crystal dispersion temperature: 100 ° C., crystallization temperature: 105 ° C.), the first stretching temperature is 118 ° C., and the second Was stretched 1.1 times in the MD direction at a temperature of 127 ° C., and a polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the heat setting temperature was 127 ° C.

Comparative Example 1
A polyethylene composition comprising 20% by mass of ultra high molecular weight polyethylene (Mw / Mn = 8) having an Mw of 2.0 × 10 ⁶ and 80% by mass of high density polyethylene (Mw / Mn = 13.5) having an Mw of 3.5 × 10 ⁵ ( Mw: 6.9 × 10 ⁵ , Mw / Mn: 21.5, melting point: 135 ° C., crystal dispersion temperature: 100 ° C., crystallization temperature: 105 ° C.), the temperature of the cooling roll is 18 ° C., and the temperature of the first stretching Was made 115 ° C., the second stretching was not performed, and the heat setting treatment condition was 124 ° C. for 10 seconds to produce a polyethylene microporous membrane in the same manner as in Example 1.

Comparative Example 2
A polyethylene composition comprising 30% by mass of ultra high molecular weight polyethylene (Mw / Mn = 8) having an Mw of 2.0 × 10 ⁶ and 70% by mass of high density polyethylene (Mw / Mn = 8.6) having an Mw of 3.5 × 10 ⁵ ( Mw: 8.5 × 10 ⁵ , Mw / Mn: 23.8, melting point: 135 ° C., crystal dispersion temperature: 100 ° C., crystallization temperature: 105 ° C.), the polyethylene concentration of the melt-kneaded product is 28.5% by mass, A polyethylene microporous membrane was prepared in the same manner as in Example 1 except that the temperature was 18 ° C, the first stretching temperature was 115 ° C, the second stretching was not performed, and the heat setting temperature was 126 ° C. .

Comparative Example 3
Using the same polyethylene composition as in Comparative Example 2, the polyethylene composition concentration of the melt-kneaded product is 28.5 mass%, the temperature of the cooling roll is 18 ° C., the temperature of the first stretching is 115 ° C., and the second stretching is performed. A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the heat fixing treatment temperature was set to 126 ° C. at a temperature of 126 ° C. so as to be 1.8 times in the TD direction.

  The physical properties of the polyethylene microporous membranes obtained in Examples 1 to 10 and Comparative Examples 1 to 3 were measured by the following methods. The results are shown in Table 1.

(1) Average film thickness (μm)
The film thickness was measured with a contact thickness meter at an interval of 5 mm over a length of 30 cm in the transverse direction (TD) at any longitudinal position of the polyethylene microporous film, and the measured values of the film thickness were averaged.

(2) Air permeability (sec / 100 cm ³ / 20μm)
When the air permeability P ₁ measured according to JIS P8117 for a polyethylene microporous film with a film thickness T ₁ is 20 μm according to the formula: P ₂ = (P ₁ × 20) / T ₁ It was converted to air permeability P _2.

(3) Porosity (%)
Measured by mass method.

(4) Puncture strength (mN / 20μm)
A spherical end surface (radius of curvature R: 0.5 mm) in diameter 1mm needle The maximum load was measured when pricked polyethylene microporous film having a thickness T ₁ at a speed of 2 mm / sec. The measured value L ₁ of the maximum load was converted into the maximum load L ₂ when the film thickness was 20 μm by the formula: L ₂ = (L ₁ × 20) / T ₁ and used as the puncture strength.

(5) Tensile breaking strength and tensile breaking elongation Measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.

(6) Thermal shrinkage (%)
The shrinkage in the longitudinal direction (MD) and the transverse direction (TD) when the polyethylene microporous membrane was exposed to 105 ° C. for 8 hours was measured three times, and the average value was calculated.

(7) Higher order structure The area (A) of total thickness x 20 μm in the plane direction in the transmission electron microscope (TEM) photograph (10,000 times) of the cross section in the thickness direction of the microporous membrane is divided every 2 μm in the thickness direction. For the five pores in each rectangular area (total of 8 to 12 depending on the film thickness), the longest interval (maximum circumscribed circle diameter) and shortest interval (maximum inscribed circle Diameter) was measured and arithmetically averaged to obtain the average pore diameter of each rectangular region. The obtained rectangular area (B) having an average pore diameter of 0.01 to 0.05 μm was picked up as a dense structure area, the average pore diameter of all rectangular areas (B) was arithmetically averaged, and the average pore diameter of the dense structure layer It was. In the region (A), the rectangular region (C) other than the rectangular region (B) is a coarse structure region, the average pore diameter of all rectangular regions (C) is arithmetically averaged, and the average pore diameter of the coarse structure layer It was. The ratio of average pore diameter was determined by the formula: (average pore diameter of coarse structure layer) / (average pore diameter of dense structure layer). The total thickness of all rectangular regions (B) is the thickness of the dense structure layer, the total thickness of all rectangular regions (C) is the thickness of the coarse structure layer, and the ratio of thickness: (thickness of the coarse structure layer ) / (Thickness of dense structure layer).

(8) Rate of change in film thickness due to heat compression A microporous membrane sample was sandwiched between a pair of press plates having a high smooth surface, and this was measured with a press machine at 90 ° C under a pressure of 2.2 MPa (22 kgf / cm ² ). The film thickness change rate was calculated by heating and compressing for minutes and setting the film thickness before compression to 100%.

(9) Air permeability reached (sec / 100 cm ³ )
The air permeability measured in accordance with JIS P8117 for the polyethylene microporous film after being heated and compressed under the above conditions was defined as the ultimate air permeability.

(10) Electrolyte absorption rate Electrolyte kept at 18 ° C (electrolyte: LiPF ₆ , electrolyte concentration: 1 mol / L) using a dynamic surface tension measuring device (DCAT21 manufactured by EKO Co., Ltd., with precision electronic balance) Solvent: Ethylene carbonate / dimethyl carbonate = 3/7 (volume ratio)) is immersed in the microporous membrane for a certain period of time, and the increase in mass is examined. Absorption amount per sample mass [increase amount of membrane mass (g) / before absorption Film mass (g)] was calculated and used as an index of absorption rate. The absorption rate [absorption amount (g / g)] of the film of Comparative Example 1 was set to 1 and expressed as a relative ratio.

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Table 1 (continued)

Table 1 (continued)

Table 1 (continued)

Table 1 (continued)

Note: (1) Mw represents mass average molecular weight.
(2) Mw / Mn represents molecular weight distribution.
(3) MD represents the longitudinal direction, and TD represents the transverse direction.
(4) (Average pore diameter of coarse structure layer) / (Average pore diameter of dense structure layer).
(5) (Thickness of coarse structure layer) / (Thickness of dense structure layer)
(6) Exposure to air at room temperature.
(7) Air at 20 ° C was blown.
(8) LP represents liquid paraffin.

  From Table 1, the polyethylene microporous membranes of Examples 1 to 10 are a dense structure layer having an average pore diameter of 0.01 to 0.05 μm and a coarse structure layer having an average pore diameter of 1.2 to 5.0 times the average pore diameter of the dense structure layer. Therefore, the film thickness change and air permeability change after pressurization are small, and the absorption rate of the electrolytic solution is not only fast, but also the permeability, mechanical properties, and heat shrinkage properties are excellent.

  In contrast, the films of Comparative Examples 1 and 2 do not have a coarse structure layer because the proportion of ultrahigh molecular weight polyethylene in the polyethylene composition exceeds 15% by mass. Moreover, in Comparative Examples 1 and 2, redrawing is not performed. Therefore, compared with Examples 1-10, the film thickness change and air permeability change after pressurization were large, the absorption rate of electrolyte solution was slow, and permeability and heat resistance were also inferior. The film of Comparative Example 3 does not have a coarse structure layer because the ratio of ultrahigh molecular weight polyethylene in the polyethylene composition is more than 15% by mass. Moreover, in Comparative Example 3, the redrawing ratio is more than 1.45 times. Therefore, compared with Examples 1-10, the film thickness change after pressurization was large, the absorption rate of electrolyte solution was slow, and the tensile elongation at break and the heat shrink property were inferior.

Claims (5)

A microporous membrane made of a polyethylene-based resin having a mass average molecular weight of 1 × 10 ⁶ or more of ultrahigh molecular weight polyethylene of 1 to 15% by mass and having an average pore diameter of 0.01 to 0.05 μm adjacent in the thickness direction And a coarse structure region having an average pore diameter of 1.2 to 5.0 times that of the dense structure region, wherein the coarse structure region is formed on at least one surface. Porous membrane.   2. The polyethylene microporous membrane according to claim 1, wherein the polyethylene resin is composed of the ultrahigh molecular weight polyethylene and high-density polyethylene.   The polyethylene microporous membrane according to claim 1 or 2, wherein a ratio represented by (thickness of the coarse structure region) / (thickness of the dense structure region) is 5/1 to 1/10. Polyethylene microporous film characterized. A melt kneaded product of a polyethylene-based resin having a mass average molecular weight of 1 × 10 ⁶ or more of ultrahigh molecular weight polyethylene having a ratio of 1 to 15% by mass and a film-forming solvent is extruded from a die, and the obtained extruded product is formed into a film. The gel-like sheet is cooled so as to generate a temperature distribution in the thickness direction, and stretched at least uniaxially at a crystal dispersion temperature of the polyethylene-based resin + 10 ° C. to the crystal dispersion temperature + 30 ° C. The polyethylene microporous membrane according to any one of claims 1 to 3, wherein the polyethylene microporous membrane is produced by removing and stretching again at a rate of 1.05-1.45 times at least in a uniaxial direction.   A battery separator comprising the polyethylene microporous membrane according to claim 1.