CN101568575A - Polyolefin microporous membrane - Google Patents

Abstract

The present invention provides a polyolefin microporous film having a bubble point of 1 MPa or less, tensile strength in the longitudinal direction and tensile strength in the width direction of 50 MPa or more each, and a thermal shrinkage rate in the width direction at 130° C. of 20% the following. The polyolefin microporous membrane of the present invention has a large pore size, excellent strength and low thermal shrinkage.

Description

Polyolefin Microporous Membrane

technical field

The present invention relates to microporous membranes widely used as separation membranes for material separation and selective permeation, separators for electrochemical reaction devices such as alkaline secondary batteries, lithium secondary batteries, fuel cells, and capacitors. In particular, the present invention relates to a polyolefin microporous membrane suitable as a separator for lithium ion batteries.

Background technique

Microporous membranes made of polyolefins are widely used as separation and selective permeation separation membranes and separator materials for various substances. Base materials for functional films that fill pores with materials to express new functions, separators for batteries, etc. Among these uses, it is particularly preferably used as a separator for lithium ion batteries widely used in notebook personal computers, mobile phones, digital cameras, and the like. The reason for this is that such a membrane has mechanical strength and pore clogging property.

The pore clogging property refers to the performance of ensuring the safety of the battery by melting the microporous membrane when the battery is overheated due to an overcharged state or the like, clogging the pores and cutting off the battery reaction. The lower the temperature at which pore clogging occurs, the higher the safety effect.

In addition, when winding the separator, and in order to prevent short circuit due to foreign matter in the battery, etc., the piercing strength of the separator (piercing strength) and the longitudinal direction (referring to the machine direction, hereinafter also referred to as MD), the width direction (is It refers to the direction perpendicular to the machine direction, hereinafter also referred to as TD) The tensile strength needs to have a certain degree of strength. In addition, in recent lithium ion secondary batteries, due to the high output and high capacity of the battery, not only the pore diameter of the separator is increased, but also excellent heat shrinkability at high temperature is required.

The higher the porosity and the larger the pore diameter of the separator, the better the electrical characteristics of the battery, however, there is an inverse relationship between the increase of the porosity and the increase of the pore diameter and the size and strength of the thermal shrinkage rate. Therefore, even if a separator with a high porosity and a large pore size has good battery electrical characteristics, it suffers from large shrinkage or insufficient strength at high temperatures in a battery oven test.

As means for solving these problems, the present applicant proposed in Patent Document 1 a method of kneading a polymer, a filler, and a plasticizer, phase-separating them, extracting them, and stretching them. Thus, a microporous membrane with high porosity and large pore diameter and low thermal shrinkage is provided. However, it is difficult to achieve both sufficient strength in all directions and low thermal shrinkage in stretching after extraction.

In addition, in Patent Document 2, the present applicant proposes a microporous membrane having a specific range of pore diameters and a predetermined water permeability/air permeability ratio through specific extraction and stretching steps. However, the film produced through this extraction and stretching process not only has a tendency to increase the thermal shrinkage rate, but also under the water permeability/air permeability described in this document, the lithium ion secondary battery with high output power in recent years In batteries and the like, electrical characteristics often become insufficient.

Patent Document 3 proposes a large-pore-diameter microporous membrane prepared using a high-molecular-weight polyolefin. However, a so-called high-heat-resistant, high-strength, large-pore-diameter, and well-balanced microporous membrane has not been obtained so far.

In addition, in Patent Document 4, a microporous membrane with high heat resistance and large pore diameter is proposed. However, it is difficult to increase the strength of the membrane by this method.

In addition, in Patent Document 5, a high-strength microporous film prepared from a specific polyolefin mixture is proposed, however, since low-density polyethylene is blended, heat fixation at high temperatures becomes difficult.

Patent Document 1: Japanese Patent No. 3258737

Patent Document 2: Japanese Patent Laid-Open No. 2004-323820

Patent Document 3: Japanese Patent Application Laid-Open No. 10-258462

Patent Document 4: Japanese Patent No. 3050021

Patent Document 5: Japanese Patent Application Laid-Open No. 8-34873

Contents of the invention

The problem to be solved by the invention

An object of the present invention is to provide a polyolefin microporous membrane which has excellent electrical properties due to its large pore size without degrading the properties of conventional polyolefin microporous membranes, and which is also excellent in strength and low thermal shrinkage.

solutions to problems

In order to achieve the above objects, the present inventors conducted intensive studies, and as a result, found that the bubble point, the elongation strength in the longitudinal direction and the width direction, and the heat shrinkage rate in the width direction at 130°C were adjusted within specific ranges. Microporous film made of polyolefin It has a large pore size, and has excellent strength and low thermal shrinkage, thereby completing the present invention. That is, the present invention is as follows.

(1) A microporous film made of polyolefin, wherein the microporous film has a bubble point of 1 MPa or less, an elongation strength in the longitudinal direction and an elongation strength in the width direction of 50 MPa or more, respectively, and a heat resistance in the width direction at 130° C. The shrinkage rate is below 20%.

(2) The polyolefin microporous membrane according to (1) above, wherein the microporous membrane contains polypropylene.

(3) The polyolefin microporous film according to (1) or (2) above, wherein the sum of MD elongation and TD elongation is 20 to 250%.

(4) The polyolefin microporous film according to (1) or (2) above, wherein the sum of MD elongation and TD elongation is 20 to 200%.

(5) The polyolefin microporous membrane according to any one of (1) to (4) above, wherein the microporous membrane contains ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and a viscosity average molecular weight of 500,000 or more. Polyethylene with a molecular weight below 500,000.

(6) The polyolefin microporous film according to any one of (1) to (5) above, wherein the porosity is 20% or more and 60% or less.

(7) A battery separator comprising the polyolefin microporous membrane according to any one of (1) to (6) above.

(8) A nonaqueous electrolyte secondary battery comprising the separator described in (7) above.

(9) A method for producing a polyolefin microporous film having a total elongation ratio of 50 times or more, the method comprising melt-kneading and extruding a resin composition containing at least polyolefin and a plasticizer to obtain a sheet shape a step of stretching the sheet to obtain a film; a step of extracting a plasticizer from the sheet or the film; and a step of heat-fixing the film.

The effect of the invention

Compared with conventional polyolefin microporous membranes, the polyolefin microporous membrane of the present invention has a larger pore size and has excellent strength and low thermal shrinkage. Therefore, by using the polyolefin microporous membrane of the present invention as a battery separator, battery characteristics and battery safety can be improved.

Description of drawings

FIG. 1 shows a cross-sectional view of a battery used in a measurement test of high-speed thermal membrane rupture (membrane rupture resistance).

Explanation of reference signs

1...diaphragm; 2...nickel foil A; 3...nickel foil B; 4...Aramica film; 5...glass plate; 6...glass slide; 7...thermocouple ;8...10×10mm windows

Detailed ways

The best mode for carrying out the present invention (hereinafter also referred to as "the present embodiment") will be described in detail below. In addition, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the gist.

The polyolefin microporous membrane of this embodiment has a bubble point of 1 MPa or less, a longitudinal tensile strength and a widthwise tensile strength of 50 MPa or greater, and a heat shrinkage rate of 20% or less in the width direction at 130°C.

The bubble point of the polyolefin microporous membrane needs to be 1.0 MPa or less, preferably 0.8 MPa or less, in order not to make the pores too dense. The lower limit of the bubble point is preferably 0.1 MPa or more, more preferably 0.3 MPa or more. When the bubble point is lower than 0.1 MPa, pores may become coarse, which may result in a decrease in membrane strength.

The bubble point method is known as a simple method for expressing the maximum pore diameter. Unlike the bubble point, the ratio of water permeability to air permeability (water permeability/air permeability) of a microporous membrane has a correlation with the average pore diameter. This ratio is preferably 1.7×10 ^-3 or more. When the ratio is less than 1.7×10 ^-3 , the permeability tends to be insufficient, and the capacity retention rate of the battery tends to decrease. The upper limit of this ratio is not limited, but it is preferably less than 2.3×10 ^-3 , more preferably less than 2.1×10 ^-3 . When the ratio is 2.3×10 ^-3 or more, the pores may become too large, the strength may be insufficient, or a short circuit may easily occur due to lithium dendrites. When the bubble point is less than 1.0MPa and the water permeability/air permeability is within the above range, it has excellent average pore size balance and high strength and low heat shrinkage while maintaining permeability. It is a lithium-ion battery in recent years. Since it imparts good properties, it is particularly preferred.

In addition, the tensile strength of the polyolefin microporous membrane of the present embodiment in the longitudinal direction (MD) and the width direction (TD) needs to be 50 MPa or more, more preferably 70 MPa or more, and still more preferably 100 MPa or more. When the elongation strength is low (less than 50 MPa), the winding property of the battery is deteriorated, and a short circuit is likely to occur due to a battery impact test from the outside, foreign objects in the battery, and the like.

In addition, from the viewpoint of ensuring safety in an oven test, etc., the polyolefin microporous membrane according to the present embodiment has a heat shrinkage rate in the width direction (TD) of 20% or less at 130°C, preferably 17% or less, more preferably 17% or less. 15% or less. The thermal shrinkage rate in the longitudinal direction (MD) at 130° C. is not particularly limited, and is the same as in the width direction. From the viewpoint of ensuring safety, it is preferably 20% or less, more preferably 17% or less, and even more preferably 15% or less .

The polyolefin microporous membrane of the present embodiment preferably includes polypropylene. By including polypropylene in the microporous membrane, not only heat resistance can be improved, but also breakage tends to be less likely to occur even at high elongation ratios. In addition, the MD and TD elongation elongation ratios described later can be easily adjusted within appropriate ranges, and as a result, the impact resistance of the obtained battery can be improved and the risk of short circuit can be reduced. The content of polypropylene is preferably 1 to 80% by mass, more preferably 2 to 50% by mass, and still more preferably 3 to 30% by mass based on the polymer material. When the polypropylene content is less than 1% by mass, it may be difficult to express the effect, and when the polypropylene content exceeds 80% by mass, it may be difficult to ensure permeability.

In addition, the MD and TD elongation elongations of the polyolefin microporous membrane according to the present embodiment are each preferably from 10 to 200%, more preferably from 10 to 150%, and still more preferably from 10 to 120%. The total of the MD elongation and the TD elongation is preferably 20 to 250%, more preferably 20 to 230%, and still more preferably 20 to 200%. A microporous membrane having MD and TD elongation elongation within the above-mentioned range not only has good battery windability, but also hardly deforms the wound body in a battery impact test or the like. When the elongation exceeds the above range, the elongation of the microporous membrane increases, and deformation is likely to occur against repeated impacts in battery impact tests and the like, and as a result, the risk of short circuit may increase.

In order to obtain a microporous membrane having MT and TD elongation elongations within the above-mentioned ranges, several methods need to be combined. For example, it can be realized by adjusting various conditions in the elongation ratio, elongation after extraction, and relaxation operation described later. Also as mentioned above, mixing polypropylene in the polymer is also an effective method.

The polyolefin microporous membrane of the present embodiment preferably contains ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of less than 500,000. By containing the above-mentioned various polyethylenes, not only the melt viscosity when the separator is melted increases, but also the membrane rupture resistance tends to be improved by early relaxation of the melt tension.

From the viewpoint of permeability, the porosity of the polyolefin microporous membrane of the present embodiment is preferably 20% or more; from the viewpoint of membrane strength, withstand voltage, and thermal shrinkage rate, the porosity is preferably 60%. the following. The porosity is more preferably 25% to 60%, and still more preferably 30% to 55%.

The air permeability of the polyolefin microporous membrane according to the present embodiment is preferably as low as possible, but from the viewpoint of the balance between thickness and porosity, it is preferably 1 sec or more, and more preferably 50 sec or more. In addition, from the viewpoint of permeability, the air permeability is preferably 1000 sec or less, more preferably 500 sec or less.

From the viewpoint of membrane strength, the thickness of the polyolefin microporous membrane according to the present embodiment is preferably 1 μm or more, more preferably 5 μm or more. In addition, from the viewpoint of permeability, the thickness is preferably 50 μm or less, more preferably 30 μm or less.

In addition, the piercing strength of the polyolefin microporous membrane according to the present embodiment is preferably 0.2 N/μm or more, and more preferably 0.22 N/μm or more. When the piercing strength is low (less than 0.2N/μm), when it is used as a battery separator, the sharp part of the electrode material and the like penetrates into the microporous membrane, and pinholes or cracks are likely to occur. It tends to be easily deformed in impact tests and the like.

Next, the method for producing a polyolefin microporous membrane according to this embodiment will be described; as long as the obtained microporous membrane has the above-mentioned characteristics, the type of polymer, the type of solvent, the method of extrusion, the method of stretching, the method of extraction, the method of opening holes, the thermal Fixing and heat treatment methods, etc. are not limited in any way.

The method for producing a polyolefin microporous film according to the present embodiment preferably includes the steps of: melt-kneading and extruding a resin composition containing at least a polyolefin and a plasticizer to obtain a sheet; stretching the sheet; A process of obtaining a film from a sheet or film; a process of extracting a plasticizer from the sheet or film; and a process of heat-fixing the film.

The polyolefin microporous membrane of the present embodiment can be obtained, for example, by a method including the following steps (a) to (e).

(a) Melting and kneading any polymer material of polyolefin monomer, polyolefin mixture, polyolefin solvent mixture and polyolefin kneaded product.

(b) The melt is extruded, shaped into a sheet and allowed to cool and solidify. Extract plasticizers and inorganics as needed.

(c) Extending the obtained sheet in more than one axis direction.

(d) After stretching, plasticizers and inorganic substances are extracted as necessary.

(e) Followed by heat fixing and heat treatment.

The polyolefin used in this embodiment is a homopolymer of ethylene or propylene, or ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, norbornene A copolymer of the above polymers may also be a mixture of the above polymers. Among them, polyethylene and its copolymers are preferable from the viewpoint of improving the performance of the microporous membrane. Examples of such polyolefin polymerization catalysts include Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like. Polyolefins can be obtained by a one-step polymerization method, or by a multi-step polymerization method.

As a composition of the supplied polymer, it is preferable to blend two or more polyolefins. The melting temperature and the short-circuit temperature can thus be controlled. More preferably, two or more polyethylenes are blended, preferably comprising ultra-high molecular weight polyethylene with a viscosity average molecular weight (Mw) of 500,000 or more and polyethylene with a viscosity average molecular weight (Mv) of less than 500,000. The polyethylene to be blended is preferably a high-density homopolymer from the standpoint of non-clogging of pores and thermal fixability at higher temperatures.

In addition, the viscosity average molecular weight (Mv) of the polymer material as a whole is preferably not less than 100,000 and not more than 1.2 million, more preferably not less than 300,000 and not more than 800,000. When the viscosity-average molecular weight (Mv) is less than 100,000, the film rupture resistance during melting may become insufficient, and when the viscosity-average molecular weight exceeds 1,200,000, the extrusion process may become difficult and the melting When the contraction force relaxes slowly, the heat resistance is poor.

The blending of these polyethylenes with polypropylene, which is a polyolefin with a higher melting point than polyethylene, not only improves heat resistance, but also can be used at higher temperatures than polyethylene monomers in the stretching and relaxation processes after extraction. In addition, while maintaining the strength, thermal shrinkage rate, and pore diameter of the microporous membrane, the elongation rate can be reduced. In addition, although the reason is not clear, it is particularly preferable because it has the effect of being difficult to break even at a high stretching ratio.

It is preferable to improve the heat resistance through the above-mentioned blending and the low heat shrinkage of the present application, because the film rupture resistance at high temperature becomes better.

In addition, known additives such as metallic soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, photostabilizers, antistatic agents, antifogging agents, and coloring pigments may be mixed and used.

In addition, inorganic substances represented by alumina, titanium dioxide, and the like may also be added. The whole amount or a part of the inorganic substance may be extracted in any one of the whole steps, and the inorganic substance may remain in the product.

The plasticizer used in this embodiment is an organic compound that can form a homogeneous solution with polyolefin at a temperature below the boiling point. Specifically, decahydronaphthalene, xylene, dioctyl phthalate, phthalate Dibutyl formate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonanol, diphenyl ether, n-decane, n-dodecane, paraffin oil, etc. Among these, paraffin oil, dioctyl phthalate are preferable.

The ratio of the plasticizer is not particularly limited, but from the viewpoint of the porosity of the obtained microporous membrane, the ratio of the plasticizer is preferably 20% by mass or more; from the viewpoint of viscosity, it is preferably 90% by mass or more, more preferably Preferably it is 50 mass % - 70 mass %.

The extraction solvent used for extracting the plasticizer is a poor solvent for polyolefin and a good solvent for plasticizer, and a solvent having a boiling point lower than the melting point of polyolefin is preferable. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as dichloromethane, 1,1,1-trichloroethane, and fluorocarbons; alcohols such as ethanol and isopropanol; Classes, acetone, 2-butanone and other ketones. One of these may be selected or a combination of a plurality of them may be selected and used. These extraction solvents can be regenerated and reused by distillation or the like after extraction of the plasticizer.

From the viewpoint of film permeability and film-forming properties, the total weight ratio of the plasticizer and the inorganic substance to the entire melt-kneaded mixture is preferably 20 to 95% by mass, more preferably 30 to 80% by mass.

In addition, from the viewpoint of preventing thermal deterioration during melt-kneading and resulting deterioration in quality, an antioxidant is preferably added to the mixture. The concentration of the antioxidant relative to the weight of the entire polyolefin is preferably 0.3% by mass or more, more preferably 0.5% by mass or more. In addition, the concentration is preferably 5.0% by mass or less, more preferably 3.0% by mass or less.

As antioxidant, the phenolic antioxidant as main antioxidant is preferred, can enumerate 2,6-di-tert-butyl-4-methylphenol, pentaerythritol-tetra[3-(3,5-di-tert Butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, etc. In addition, secondary antioxidants can be used in combination, such as tris(2,4-di-tert-butylphenyl) phosphite, tetrakis(2,4-di-tert-butylphenyl)-4,4-biphenylene Phosphorus antioxidants such as dilauryl thiodipropionate, sulfur antioxidants such as dilauryl thiodipropionate, etc.

The methods of melt kneading and extrusion include firstly premixing a part or all of the raw materials with a Henschel mixer, ribbon blender, drum mixer, etc. as needed. Agitation by hand is acceptable in a few cases. Then, it is melt-kneaded with a screw extruder such as a single-screw extruder or a twin-screw extruder, a kneader, a mixer, and the like, and extruded with a T-die, a ring die, or the like.

It is preferable to carry out melt-kneading with the nitrogen atmosphere maintained after the antioxidant is mixed with the base polymer at a predetermined concentration and replaced with a nitrogen atmosphere. The temperature at the time of melt-kneading is preferably 160°C or higher, more preferably 180°C or higher. In addition, the temperature is preferably lower than 300°C, more preferably lower than 240°C, even more preferably lower than 230°C.

The molten material described in this embodiment may contain unmelted inorganic substances that can be extracted in the inorganic substance extraction step. In addition, in order to improve the film quality, it is preferable to pass the melt kneaded and homogenized through a screen.

Next, the formation of the gel sheet will be described. As a method of forming the gel sheet, it is preferable to solidify the melt kneaded and extruded by compression cooling. Examples of cooling methods include a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll or a press machine cooled with a cooling medium, and the like. From the viewpoint of excellent thickness control, a method of contacting with a roll or a press machine cooled with a cooling medium is preferable.

There is no particular limitation on the order, method and number of times of the extension and plasticizer extraction or the extension and plasticizer extraction and inorganic substance extraction to be performed next. Extraction of inorganic substances may not be performed as required.

Examples of stretching methods include MD uniaxial stretching with a roll stretcher, TD uniaxial stretching with a tenter, sequential double stretching with a combination of a roll stretcher and a tenter, or a combination of a tenter and a tenter. Axial stretching, simultaneous biaxial stretching with a simultaneous biaxial tenter or blow molding, etc. In order to obtain the desired tensile strength and elongation, the total area ratio is preferably 8 times or more, more preferably 15 times or more, still more preferably 30 times or more, particularly preferably 40 times or more. Among them, simultaneous or sequential biaxial stretching is preferable. In addition, for the same reason, the total stretching ratio of all steps is preferably 50 times or more, and more preferably 60 times or more.

In plasticizer extraction, the plasticizer is extracted by dipping or spraying in an extraction solvent. Dry well thereafter.

The thermal fixation method performs a relaxation operation at a predetermined relaxation rate in a predetermined temperature atmosphere. It can be performed using a tenter frame or a roll stretcher. A relaxation operation refers to a shrinking operation in the TD and/or TD direction of the film. The relaxation rate refers to the value obtained by dividing the MD size of the film after the relaxation operation by the MD size of the film before operation, or the value obtained by dividing the TD size after the relaxation operation by the TD size of the film before operation, Or, when relaxation is performed in both MD and TD directions, it is a value obtained by multiplying the relaxation rate in MD by the relaxation rate in TD. From the viewpoint of thermal contraction rate, the predetermined relaxation rate is preferably 0.9 or less, more preferably 0.8 or less. In addition, the predetermined relaxation rate is preferably 0.6 or more from the viewpoint of prevention of wrinkle generation, porosity, and permeability. The relaxation operation can be performed in both directions of MD and TD, even if the relaxation operation is performed in only one direction of MD or TD, not only the thermal shrinkage in the operation direction can be reduced, but also the direction perpendicular to the operation direction can be reduced on the heat shrinkage rate. By stretching 1.5 times or more, more preferably 1.8 times or more before this relaxation operation, a microporous membrane with high strength and large pore size can be easily obtained.

The stretching and relaxation operations after extraction of the plasticizer are preferably performed in the TD direction. From the viewpoint of heat shrinkage rate and pore size increase, the temperature of both the relaxation operation and the stretching step before the relaxation operation is preferably 125°C or higher, and at least one of them is preferably 130°C or higher, more preferably 132°C or higher. When the temperature of the relaxation operation and the stretching step before the relaxation operation is within the above range, when the polyolefin is polyethylene, stretching and relaxation operations are performed near the melting point, which is easier than conventional microporous membranes. A microporous membrane with large pore size and low heat shrinkage is obtained. In addition, although the reason is unknown, it is easy to obtain a microporous membrane excellent in membrane rupture property even for a membrane with a low elongation. From the viewpoint of being able to stretch and relax under such higher temperature conditions different from conventional ones, and being difficult to break even under the condition of a large total elongation ratio, as polyolefin, it is preferable to blend polyolefins in addition to polyethylene. propylene.

In addition, the polyolefin microporous membrane of this embodiment can be subjected to surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, and chemical modification.

Example

The present embodiment will be described in more detail below by way of examples.

[test methods]

The measuring methods of the physical properties etc. in this specification are as follows.

(1) Viscosity average molecular weight (Mv)

The intrinsic viscosity [η] at 135° C. in a decahydronaphthalene solvent was determined according to ASTM-D4020. Mv of polyethylene was calculated by the following formula.

[η]=6.77×10 ^-4 Mv ^0.67

Mv of polypropylene was calculated by the following formula.

[η]=1.10×10 ^-4 Mv ^0.80

(2) Film thickness (μm)

Measurement was carried out at a room temperature of 23±2° C. using a micro thickness gauge KBM (trademark) manufactured by Toyo Seiki.

(3) Porosity (%)

A 10 cm x 10 cm square sample was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were obtained, and the porosity was calculated from the volume, mass, and film density (g/cm ³ ) using the following formula.

Porosity=(volume-mass)/volume×100

Additionally, the film density is calculated from the material density.

(4) Air permeability (sec)

According to JIS P-8117, a Gurley-type air permeability meter (manufactured by Toyo Seiki Co., Ltd., G-B2 (trademark)) was used. Measure the time for 100ml of air to pass through an inner cylinder with a weight of 567g, a diameter of 28.6mm, and an area of ^645mm2 .

(5) air flow

The air permeation rate constant Rgas was obtained from the air permeability (sec) using the following formula. The measurement was performed in a room at a room temperature of 23°C.

Rgas(m ³ /(m ² sec Pa))＝0.0001/air permeability/0.0006424/(0.01276×101325)

(6) Water permeability

A microporous membrane pre-soaked in ethanol was set in a stainless steel permeable tank with a diameter of 41mm, and after the ethanol of the membrane was washed with water, water was permeated under a pressure difference of about 50000Pa. Quantity (cm ³ ) The water permeation amount per unit time, unit pressure, and unit area was calculated, and this was defined as water permeability (cm ³ /(cm ² ·sec·Pa)). The measurement was performed in a room at a room temperature of 23°C. The water permeation rate constant Rliq was obtained from the water permeability (cm ³ /(cm ² ·sec·Pa)) using the following formula.

Rliq(cm ³ /(cm ² .sec.Pa))=water permeability/100

(7) (puncture strength (N/μm)

Using a portable compression tester KES-G5 (trademark) manufactured by KATO TECH Co., Ltd., the microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. Then, under the conditions of a needle tip curvature radius of 0.5mm, a piercing speed of 2mm/sec, and an atmosphere of 25°C, a piercing test was performed on the central part of the fixed microporous membrane, and the maximum piercing load (N) was multiplied by 1/film thickness. (μm) of the piercing strength (N/μm).

(8) Tensile strength (MPa) and elongation (%)

According to JIS K7127, MD and TD samples (shape; width 10 mm×length 100 mm) were measured using an extension tester Autograph AG-A type (trademark) manufactured by Shimadzu Corporation. In addition, as a sample, a sample in which the distance between clamps was set to 50 mm, and cellophane tape (manufactured by Nitto Denko Packaging System Co., Ltd., trade name: N.29) was attached to one surface of both ends of the sample (25 mm each) was used. In addition, in order to prevent the sample from slipping during the test, fluororubber with a thickness of 1 mm was attached to the inner side of the jig of the tensile testing machine.

Elongation elongation (%) was calculated|required by dividing the elongation amount (mm) at the time of breaking by the clamp distance (50 mm), and multiplying by 100.

The tensile strength (MPa) was obtained by dividing the strength at break by the cross-sectional area of the sample before the test.

In addition, the sum (%) of MD elongation and TD elongation was calculated by adding the values of MD and TD. In addition, the measurement is carried out under the conditions of temperature 23±2°C, clamp pressure 0.30 MPa, and extension speed 200 mm/min (for samples that cannot ensure a clamp distance of 50 mm, the strain rate is 400%/min).

(9) Heat shrinkage rate at 130°C (%)

A sample of 100 mm in the MD direction and 100 mm in the TD direction was cut out and left to stand in an oven at 130° C. for 1 hour. At this time, in order not to directly blow the hot air to the sample, the sample was sandwiched between two sheets of paper. The length (mm) was measured after taking out from an oven and cooling, and calculated the thermal contraction rate of MD and TD by the following formula. (When the length of the sample cannot be ensured, use the longest possible sample within the range of 100mm×100mm.)

MD heat shrinkage rate (%)=(100-MD length after heating)/100×100

TD heat shrinkage rate (%)=(100-TD length after heating)/100×100

(10) Bubble point (MPa)

Determined in ethanol solvent according to ASTM F316-86. The point at which continuous bubbles are confirmed is set as the bubble point.

(11) High-speed thermal membrane rupture (membrane rupture resistance)

Prepare nickel foil A (length 100mm x width 25mm) and nickel foil B (length 100mm x width 15mm) with a thickness of 10μm, and a separator soaked in the electrolyte for more than 30 minutes (MD length 75mm x TD length 25mm), centered 10mm x Aramica film (trademark) with a window of 10 mm, slide glass (length 75 mm x width 25 mm), glass plate (length 25 mm x width 20 mm).

As shown in Figure 1, stack the glass slide, nickel foil A, separator, Aramica film, nickel foil B, and glass plate in order, and fix them with clips.

The above-mentioned battery was connected to a thermocouple, and left to stand in an oven. Thereafter, the temperature was raised at a rate of 5°C/min, and after reaching 150°C, it was held at 150°C for 1 hour. The resistance change at this time was measured with an LCR meter under the conditions of AC 10 mV and 1 kHz. In this measurement, from the time when the temperature is kept at 150°C, the evaluation that the impedance can be kept in an insulating state of 1000Ω or more for 60 minutes or more is rated as A, the evaluation of keeping it for 30 minutes or more is rated as B, and the evaluation that can be kept for 10 minutes or more is rated as C. The evaluation of holding for 5 minutes or more was D, and the evaluation of being unable to hold for 5 minutes was E.

In addition, the composition ratio of the predetermined electrolytic solution is as follows.

The composition ratio (volume ratio) of the solvent: dipropyl carbonate/diethyl carbonate/γ-butyrolactone=1/1/2

Compositional ratio of solute: LiBF ₄ was dissolved in the above solvent at a concentration of 1 mol/L.

[Production and Evaluation of Batteries]

(1) Production of positive electrode

In N-methylpyrrolidone (NMP), 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3% by mass of flake graphite and acetylene black as a conductive agent, and 3.2% by mass of viscose The binder polyvinylidene fluoride (PVDF) was used to prepare the slurry. This slurry was coated on one side of a 20-μm-thick aluminum foil constituting the positive electrode current collector with a die coater, dried at 130° C. for 3 minutes, and then compression-molded with a roll press. At this time, the coating amount of the active material on the positive electrode was 250 g/m ² , and the volume density of the active material was 3.00 g/cm ³ . This was cut into a strip with a width of about 40 mm.

(2) Production of negative electrode

Disperse 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethylcellulose as a binder, and 1.7% by mass of styrene-butadiene copolymer latex in purified water to prepare a slurry . This slurry was coated on one surface of a 12-μm-thick copper foil constituting the negative electrode current collector with a die coater, dried at 120° C. for 3 minutes, and then compression-molded with a roll press. At this time, the coating amount of the active material on the negative electrode was 106 g/m ² , and the volume density of the active material was 1.35 g/cm ³ . This was cut into a strip with a width of about 40 mm.

(3) Preparation of non-aqueous electrolyte

In a mixed solvent of diethyl carbonate: ethyl methyl carbonate = 1:2 (volume ratio), LiPF ₆ as a solute was dissolved to a concentration of 1.0 mol/L to prepare a non-aqueous electrolytic solution.

(4) Battery assembly

A separator made of a polyolefin microporous film, a strip-shaped positive electrode, and a strip-shaped negative electrode are laminated in the order of a strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator, and wound multiple times to form an electrode plate laminate. . Press the electrode plate laminate into a flat plate shape, put it into an aluminum container, lead out an aluminum lead from the positive electrode collector, and weld it on the battery cover, lead out a nickel lead from the negative electrode collector, and weld it to the bottom of the container . In addition, the above-mentioned non-aqueous electrolytic solution was poured into the container and sealed. The lithium-ion battery produced in this way was designed to have a size of 6.3 mm in the longitudinal direction (thickness), 30 mm in the lateral direction, and 48 mm in height, and had a nominal discharge capacity of 620 mAh.

(5) Battery evaluation (under 25°C atmosphere)

In the lithium ion battery assembled as described above, constant current constant voltage (CCCV) charging was performed for 6 hours under the conditions of a current value of 310 mA (0.5 C) and a final battery voltage of 4.2 V. At this time, the current value before the end of charging is approximately zero. Thereafter, it was left to stand in a 25° C. atmosphere for 1 week (aging).

After that, the following cycle is carried out: charge with constant current and constant voltage (CCCV) for 3 hours under the conditions of current value 620mA (1.0C) and end battery voltage 4.2V, and then discharge at a certain current value (CC) 620mA to battery voltage 3.0V . The discharge capacity at this time was set as the initial discharge capacity.

(a) The above cycle was further repeated 300 times. In this cycle, the ratio (%) of the discharge capacity at the 300th cycle to the initial discharge capacity was the capacity retention rate. A high capacity retention rate means good cycle characteristics.

(b) In addition, in order to perform the battery impact test before the cycle test of (a), it was dropped on the concrete floor from a height of 1.9 m, and it was repeated 10 times. The battery is then disassembled for observation. The evaluation was A when almost no deformation of the wound body was observed, the evaluation was B when slight deformation was observed, and the evaluation was C when deformation could be easily confirmed.

[Example 1]

47% by mass of Mv700,000 homopolymer polyethylene, 46% by mass of Mv300,000 homopolymer polyethylene, and 7% by mass of Mv400,000 polypropylene were dry blended using a tumble mixer. 1% by mass of pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant to 99% by mass of the pure polymer mixture obtained, Dry blending was performed again with a drum mixer to obtain a mixture of polymers and the like. The mixture of the obtained polymer and the like was replaced with nitrogen, and then supplied to a twin-screw extruder through a feeder under a nitrogen atmosphere. In addition, liquid paraffin (dynamic viscosity at 37.78°C: 7.59×10 ⁻⁵ m ² /s) was injected into the barrel of the extruder by a plunger pump.

The feeder and the pump were adjusted so that the ratio of the amount of liquid paraffin to the entire mixture of melt kneading and extrusion was 65% by mass. The melt-kneading conditions were a set temperature of 200° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg/h.

Then, the molten kneaded product was extruded through a T-shaped die onto a cooling roll whose surface temperature was controlled at 25° C., and cast to obtain a gel sheet with a thickness of 2000 μm.

Then, it is introduced into a simultaneous biaxial tenter stretching machine, and biaxially stretched. The stretching conditions set were 7.0 times MD magnification, 7.0 times TD magnification, and a set temperature of 125°C.

Then, it was introduced into a methyl ethyl ketone tank, fully immersed in methyl ethyl ketone, extracted to remove liquid paraffin, and then dried to remove methyl ethyl ketone.

Then, it is introduced into a TD tenter and heat-fixed. The stretching temperature at the time of heat fixing was 128° C. and the stretching ratio was 2.0 times, and the temperature at the time of relaxation thereafter was 133° C., and the relaxation rate was 0.80.

Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 2]

Except that the biaxial stretching temperature was 120° C., it was carried out in the same manner as in Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 3]

Except that the original film thickness after extrusion is 900 μm, the biaxial stretching temperature is 122°C, the stretching temperature at the time of heat setting is 130°C, and the stretching ratio is 2.0 times, and the temperature at the time of relaxation thereafter is 135°C and the relaxation rate is 0.80. Other than that, it carried out similarly to Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 4]

In addition to using 30% by mass of Mv2.5 million homopolymer polyethylene and 70% by mass of Mv250,000 homopolymer polyethylene, the thickness of the original film after extrusion is 2400 μm, and the stretching temperature at heat setting is 125°C and the stretching ratio It was performed in the same manner as in Example 1, except that the temperature at the time of subsequent relaxation was 1.9 times and the relaxation rate was 132° C. and the relaxation rate was 0.7. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 5]

It carried out similarly to Example 1 except having used the homopolymer polyethylene which Mv is 500,000 for 99 mass % of pure polymer mixtures, and the temperature at the time of heat setting was 125 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 6]

Except that the thickness of the original film after extrusion is 1800 μm, the biaxial stretching ratio is 5×5 times, the biaxial stretching temperature is 115°C, the stretching temperature at heat fixation is 125°C and the stretching ratio is 1.7 times, and the stretching ratio during subsequent relaxation The same procedure as in Example 4 was performed except that the temperature was 131° C. and the relaxation rate was 0.70. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 7]

It carried out similarly to Example 5 except having used the homopolymer polyethylene whose Mv was 1.2 million, and the biaxial stretching temperature being 128 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 8]

In addition to using a blend of 45% by mass of homopolymer polyethylene with Mv of 700,000, 40% by mass of homopolymer polyethylene with Mv of 300,000, and 15% by mass of polypropylene with Mv of 400,000, and biaxial Except that stretching temperature was 123 degreeC, it carried out similarly to Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 9]

In addition to using a blend of 45% by mass of homopolymer polyethylene with Mv of 700,000, 30% by mass of homopolymer polyethylene with Mv of 300,000, and 25% by mass of polypropylene with Mv of 400,000, and biaxial Except that stretching temperature was 123 degreeC, it carried out similarly to Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 10]

The same procedure as in Example 1 was carried out except that the thickness of the gel sheet was 1600 μm, the stretching temperature at the time of heat setting was 125° C., and the temperature at the time of relaxation thereafter was 130° C. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 11]

In addition to using 30% by mass of homopolymer polyethylene with Mv of 2.5 million, 60% by mass of homopolymer polyethylene with Mv of 250,000, and 10% by mass of polypropylene with Mv of 400,000, and the extension and Except that the relaxation temperature was 128°C and 133°C, it was carried out in the same manner as in Example 4. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative example 1]

The same procedure as in Example 1 was carried out except that the stretching temperature at the time of heat setting was 120° C. and the stretching ratio was 1.5 times, and the temperature at the time of subsequent relaxation was 125° C. and the relaxation rate was 0.80. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative example 2]

The same procedure as in Example 2 was carried out except that the stretching temperature at the time of heat setting was 122° C., the stretching ratio was 1.3 times, and the relaxation was not performed. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative example 3]

60% by mass of a homopolymer of Mv 270,000 and 40% by mass of a homopolymer of Mv 950,000 were dry blended using a tumble mixer. 1% by mass of pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant to 99% by mass of the pure polymer mixture obtained, again Dry blending is performed with a tumble mixer to obtain a mixture of polymers and the like. The mixture of the obtained polymer and the like was replaced with nitrogen, and then supplied to a twin-screw extruder through a feeder under a nitrogen atmosphere. In addition, liquid paraffin (dynamic viscosity at 37.78°C: 7.59×10 ⁻⁵ m ² /s) was injected into the barrel of the extruder by a plunger pump.

The feeder and the pump were adjusted so that the ratio of the amount of liquid paraffin to the entire mixture of melt kneading and extrusion was 62% by mass. The melt-kneading conditions were a set temperature of 200° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg/h.

Then, the molten kneaded product was extruded through a T-shaped die onto a cooling roll whose surface temperature was controlled at 25° C., and cast with a 4-fold calendering ratio to obtain a gel sheet with a thickness of 200 μm.

Then, the obtained sheet was introduced into a TD tenter stretcher, and stretched in the transverse direction before extraction at a stretching temperature of 115° C. and a stretching ratio of 5 times, and then subjected to 10% thermal relaxation.

Then, it was introduced into a methyl ethyl ketone tank, fully immersed in methyl ethyl ketone, extracted to remove liquid paraffin, and then dried to remove methyl ethyl ketone.

Then, the above-mentioned extracted membrane was introduced into a multi-stage roll-type stretching machine in the longitudinal direction, and extracted and stretched under the conditions of a stretching temperature of 110° C. and a stretching ratio of 2 times in the MD direction to obtain a microporous membrane.

Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative example 4]

Add 30% by mass of homopolymer polyethylene with Mv of 700,000, 15% by mass of homopolymer polyethylene with Mv of 300,000, 5% by mass of homopolymer polypropylene with Mv of 400,000, 30.6% by mass of Dioctyl phthalate (DOP), 18.4% by mass of finely powdered silica, and 1% by mass of pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid as an antioxidant esters] and mix. The resulting mixture of the polymer and the like was replaced with nitrogen, and then supplied to a twin-screw extruder through a feeder under a nitrogen atmosphere.

The melt-kneading conditions were a set temperature of 200° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg/h.

Then, the molten kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled at 80° C., and cast to obtain a gel sheet with a thickness of 110 μm.

DOP and fine powdered silica were extracted and removed from this gel sheet to obtain a microporous membrane. Two sheets of this microporous film were stacked, stretched 5 times in the longitudinal direction at 110°C, introduced into a TD tenter, and stretched 2 times in the lateral direction at 130°C. Thereafter, the TD relaxation rate at 130°C was 0.80.

Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

^* 1 The evaluation was A when almost no disassembly and deformation of the wound body after the test were observed; the evaluation was B when slight disassembly and deformation of the wound body were found after the test; and the evaluation was C when deformation was easily confirmed.

^* 2 The evaluation of the insulation state of 1000Ω or more maintained for more than 60 minutes is A; the insulation state of 1000Ω or more maintained for 30 minutes is evaluated as B; the insulation state of 1000Ω or more maintained for 10 minutes is evaluated as C; the insulation state of 5 minutes is maintained The above insulation state of 1000Ω or more was evaluated as D, and the insulation state of 1000Ω or more that could not be maintained for 5 minutes was evaluated as E.

The following conclusions can be drawn from the results in Table 1:

(1) The polyolefin microporous membranes of Examples 1 to 11 in which the bubble point, elongation strength in the longitudinal direction and width direction, and thermal shrinkage rate in the width direction at 130° C. are adjusted within specific ranges have good impact resistance Balanced with membrane rupture resistance, batteries manufactured using them have excellent capacity retention.

(2) The polyolefin microporous membranes of Comparative Examples 1 and 2 had a bubble point of more than 1 MPa and did not have sufficient pore diameters, so the capacity retention rate was poor.

(3) The polyolefin microporous membrane of Comparative Example 3 has a TD heat shrinkage rate of more than 20% at 130° C., and has a puncture strength of 0.20 N/μm or less and insufficient strength. The balance of membrane rupture is poor.

(4) The microporous membrane made of polyolefin of Comparative Example 4 is less than 50 MPa in TD elongation strength, and the sum of MD elongation elongation and TD elongation elongation exceeds 250%, therefore, the microporous membrane is easily deformed against repeated impacts, Poor impact resistance.

(5) Compared with Examples 5 and 7, the polyolefin microporous film of Example 11 uses polyethylene with an Mv of 500,000 or more and polyethylene with an Mv of less than 500,000, and further blends with polypropylene, so Both film rupture resistance and impact resistance were excellent.

(6) Compared with Example 1, the polypropylene content of the polyolefin microporous membranes of Examples 8 and 9 was high. As a result, not only heat fixation at high temperature becomes possible, but also the elongation elongation becomes low, and both film rupture resistance and impact resistance are excellent.

(7) Compared with Example 6, the polyolefin microporous membrane of Example 4 has excellent impact resistance because of its high total stretch ratio and low elongation elongation.

(8) Compared with Example 10, although the capacity retention rate of the polyolefin microporous membrane in Example 1 is slightly lower due to its low porosity, it has better resistance to membrane rupture and Both impact properties were excellent.

From the above results, it can be seen that the polyolefin microporous membrane according to the present embodiment has a large pore size and has excellent balance of strength, elongation, and low thermal shrinkage. Therefore, by using the polyolefin microporous membrane of the present embodiment as a battery separator, a secondary battery having excellent balance between battery characteristics and battery safety can be obtained.

Industrial availability

The present invention relates to a polyolefin microporous membrane used for material separation and selective permeation separation membrane, separator, etc., and can be industrially utilized as a separator used in lithium ion batteries and the like.

Claims (9)

1. polyolefin microporous film, the bubble point of wherein said micro-porous film is below the 1MPa, and the extension strength of length direction and the extension strength of width are respectively more than the 50MPa, and the percent thermal shrinkage of the width under 130 ℃ is below 20%.

2. polyolefin microporous film according to claim 1, wherein, described micro-porous film comprises polypropylene.

3. polyolefin microporous film according to claim 1 and 2, wherein, the summation that MD extends elongation and TD extension elongation is 20～250%.

4. polyolefin microporous film according to claim 3, wherein, the summation that MD extends elongation and TD extension elongation is 20-200%.

5. according to each described polyolefin microporous film of claim 1～4, wherein, it is that ultrahigh molecular weight polyethylene(UHMWPE) and viscosity-average molecular weight more than 500,000 is lower than 500,000 polyethylene that described micro-porous film contains viscosity-average molecular weight.

6. according to each described polyolefin microporous film of claim 1～5, wherein, void content is more than 20% below 60%.

7. battery separator, described barrier film is formed by each described polyolefin microporous film of claim 1～6.

8. nonaqueous electrolytic solution secondary battery, it has battery separator according to claim 7.

9. manufacture method that the total elongation multiplying power is the polyolefin microporous film more than 50 times, described method comprises:

With contain at least polyolefine and softening agent the resin combination melting mixing, extrude and obtain the operation of flap;

Extend described flap and obtain the operation of film;

The operation of extracting elasticizer from described flap or described film; And

Operation with described film heat setting.