KR20120032539A - Polyolefin microporous membrane - Google Patents

Abstract

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

Description

Polyolefin microporous membrane {POLYOLEFIN MICROPOROUS MEMBRANE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to separators for separation of substances, selective permeation, and the like, and to microporous membranes widely used as separators for electrochemical reaction devices such as alkali, lithium secondary batteries, fuel cells, capacitors, and the like, and particularly preferably as separators for lithium ion batteries. The present invention relates to a polyolefin microporous membrane used.

Polyolefin microporous membranes are widely used for separation of various materials, selective permeation membranes, and separators. Examples of applications include the use of microfiltration membranes, fuel cell separators, capacitor separators, or functional materials in holes to create new functions. The base material of the functional film for this, a battery separator, etc. are mentioned. In these applications, it is especially preferably used as a separator for lithium ion batteries widely used for notebook computers, mobile phones, digital cameras, and the like. The reason for this is that the membrane has mechanical strength and hole occlusion property.

The hole occlusion refers to the performance of ensuring the safety of the battery by melting and blocking the battery when the inside of the battery is overheated in an overcharged state, etc., and the lower the temperature at which the hole occlusion occurs, The effect is considered to be high.

In addition, in order to prevent the short circuit caused by foreign matters in the battery or the like when winding the separator, the puncture strength and the longitudinal direction (referred to as machine direction, hereinafter referred to as MD) of the separator, and the width direction (referred to the machine direction and vertical direction, hereinafter also referred to as TD). Tensile strength) must have a certain degree or more of strength. In addition, in recent lithium ion secondary batteries, in order to increase the output power and capacity of the battery, the separator needs to be excellent in thermal shrinkage under high temperature as well as large pore size.

It is considered that the higher the porosity of the separator and the larger the pore size, the better the electrical characteristics of the battery. However, the high porosity and the large porosity have a relationship opposite to the magnitude and strength of the heat shrinkage rate. For this reason, even if the separator with high porosity and large pore hardening has good battery electrical characteristics, there is a problem that the shrinkage is large or the strength is insufficient under the high temperature of the battery oven test.

As a means to solve these problems, the present applicant proposes a method of kneading a polymer, a filler, and a plasticizer by phase-separation in the following Patent Document 1, followed by stretching after extraction. As a result, a microporous membrane having high porosity and large pore size and low heat shrinkage has been proposed, but it is difficult to achieve low heat shrinkage while developing sufficient strength in all directions in stretching after extraction.

Moreover, the applicant of this patent proposes the microporous film which prescribed | regulated the water permeation amount / air permeation amount ratio, while being in the specific pore size range through the extraction and extending | stretching process specific to patent document 2 below. However, not only the thermal shrinkage tends to increase in the membranes subjected to such extraction and stretching processes, but also the electrical properties tend to be insufficient in recent high-output lithium ion secondary batteries and the like in the water permeation amount / air permeation amount described in the literature. .

In Patent Document 3 below, although a microporous membrane having a large pore size is proposed using a high molecular weight polyolefin, it has not been reached to a microporous membrane having excellent balance such as high heat resistance, high strength, and large pore diameter.

In addition, although the following patent document 4 proposes a high heat resistance and a large pore size microporous membrane, it is difficult to increase the strength of the membrane in such a production method.

In addition, although the high-strength microporous membrane is proposed by the following patent document 5 by the specific polyolefin blend, since the low density polyethylene is blended, heat fixation at high temperature becomes difficult.

Japanese Patent No.3258737 Japanese Patent Laid-Open No. 2004-323820 Japanese Patent Laid-Open No. 10-258462 Japanese Patent No. 3050021 Japanese Patent Publication No. 8-34873

An object of the present invention is to provide a polyolefin microporous membrane having a high pore size and excellent electrical characteristics and excellent strength and low heat shrinkage property without degrading the properties of a conventional polyolefin microporous membrane.

MEANS TO SOLVE THE PROBLEM As a result of earnestly researching in order to achieve the objective mentioned above, the present inventors found that the polyolefin microporous membrane which adjusted the bubble point, the tensile strength of the longitudinal direction and the width direction, and the thermal contraction rate of the width direction at 130 degreeC to the specific range was made. While finding a large pore size and excellent in strength and low heat shrinkage, the present invention was completed. That is, this invention is as follows.

(1) A polyolefin microporous membrane having a bubble point of 1 MPa or less, a tensile strength in the longitudinal direction and a tensile strength in the width direction of 50 MPa or more and a thermal shrinkage in the width direction at 130 ° C of 20% or less, respectively.

(2) The polyolefin microporous membrane as described in said (1) containing polypropylene.

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

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

(5) The polyolefin microporous membrane of any one of said (1)-(4) containing the ultrahigh molecular weight polyethylene whose viscosity average molecular weight is 500,000 or more, and the polyethylene whose viscosity average molecular weight is less than 500,000.

(6) The polyolefin microporous membrane of any one of said (1)-(5) whose porosity is 20% or more and 60% or less.

(7) The battery separator which consists of the polyolefin microporous film in any one of said (1)-(6).

(8) A nonaqueous electrolyte secondary battery provided with the battery separator as described in said (7).

(9) process of melt-kneading and extruding a resin composition containing at least a polyolefin and a plasticizer to obtain a sheet-like substance, stretching the sheet-like substance to obtain a film, and extracting a plasticizer from the sheet-like substance or the film The manufacturing method of the polyolefin microporous film whose total draw ratio is 50 times or more including the process of heat-fixing the said film.

The polyolefin microporous membrane of the present invention has excellent strength and low heat shrinkability while being largely hardened in comparison with a conventional polyolefin microporous membrane. For this reason, it is possible to improve battery characteristics and battery safety by using the polyolefin microporous membrane of this invention for a battery separator.

BRIEF DESCRIPTION OF THE DRAWINGS The sectional drawing of the cell used for the measurement test of a high speed thermal film resistance (break resistance) is shown.

Best Mode for Carrying Out the Invention Hereinafter, the best mode for carrying out the present invention (hereinafter also referred to as "the present embodiment") will be described in detail. In addition, this invention is not limited to the following embodiment, It can variously deform and implement within the range of the summary.

The polyolefin microporous membrane of the present embodiment has a bubble point of 1 MPa or less, a tensile strength in the longitudinal direction and a tensile strength in the width direction of 50 MPa or more, and a thermal shrinkage in the width direction at 130 ° C. of 20% or less, respectively.

The bubble point of the polyolefin microporous membrane needs to be 1.0 MPa or less, and preferably 0.8 MPa or less in order to prevent the hole from becoming too dense. As a minimum of a bubble point, Preferably it is 0.1 MPa or more, More preferably, it is 0.3 MPa or more. If it is less than 0.1 MPa, the hole is coarsened, which may cause a decrease in film strength.

This bubble point method is known as a simple method of showing the maximum pore size, but apart from the bubble point side, the water permeation rate and air permeation ratio (water permeation amount / air permeation amount) of the microporous membrane have a correlation between the average pore size and the average pore size. Have It is preferable that this ratio is 1.7 * 10 <^-3> or more. If it is less than 1.7 * 10 <^-3>, permeability will become inadequate easily, and there exists a tendency for the capacity retention rate of a battery to fall. Although there is no definition in an upper limit, it is preferable that it is less than 2.3 * 10 <^-3> , More preferably, it is less than 2.1 * 10 <^-3> . If it is 2.3x10 <^-3> or more, there exists a possibility that a hole may become large too much and may become short of strength, or the short by lithium dendrite may arise easily. If the bubble point is 1.0 MPa or less, and the water permeation amount / air permeation ratio is in the above range, the average pore size balance is excellent, and high strength and low heat shrinkability are easily maintained while maintaining the permeability. It is particularly preferable because it brings good performance.

In addition, the polyolefin microporous membrane of the present embodiment needs to have a tensile strength of 50 MPa or more in both the longitudinal direction (MD) and the width direction (TD), more preferably 70 MPa or more, and even more preferably 100 MPa or more. Do. If the tensile strength is weak (less than 50 MPa), the battery winding property is deteriorated, or a short circuit is likely to occur due to an external battery shock test, foreign matter in the battery, or the like.

In addition, the polyolefin microporous membrane of the present embodiment has a thermal shrinkage in the width direction (TD) at 130 ° C of 20% or less, preferably 17% or less, and more preferably 15% from the viewpoint of ensuring safety in an oven test or the like. It is as follows. Although the thermal contraction rate in the longitudinal direction MD in 130 degreeC does not have a restriction | limiting in particular, Like the width direction, Preferably it is 20% or less, More preferably, it is 17% or less, More preferably, 15 from a viewpoint of ensuring safety. It is% or less.

The polyolefin microporous membrane of the present embodiment preferably contains polypropylene. By including polypropylene in the microporous membrane, not only the heat resistance can be improved but also it tends not to break even under a high draw ratio. Furthermore, it becomes easy to adjust MD and TD tensile elongation mentioned later to a preferable range, As a result, the impact resistance of the obtained battery can be improved and the risk of a short circuit can be reduced. As content of polypropylene, Preferably it is 1-80 mass% with respect to a polymer material, More preferably, it is 2-50 mass%, More preferably, it is 3-30 mass%. When it is less than 1 mass%, there exists a tendency for an effect to become difficult to express, and when it exceeds 80 mass%, there exists a tendency which becomes difficult to ensure permeability.

Moreover, it is preferable that MD and TD tensile elongation are respectively 10 to 200%, as for the polyolefin microporous film of this embodiment, it is more preferable that it is 10 to 150%, and it is still more preferable that it is 10 to 120%. It is preferable that the sum total of MD tensile elongation and TD tensile elongation is 20 to 250%, 20 to 230% is more preferable, It is further more preferable that it is 20 to 200%. Microporous membranes in which the MD and TD tensile elongations are in the above ranges have good battery winding properties, and the wound body is less likely to be deformed in a battery impact test or the like. When the tensile elongation exceeds the above range, the elongation of the microporous membrane becomes large, which may easily deform with respect to repeated shock in a battery impact test or the like, resulting in increased risk of short circuit.

In order to obtain the microporous membrane whose MD and TD tensile elongation are in the said range, it is necessary to combine several methods, for example, it can achieve by adjusting the draw ratio mentioned later and the various conditions in the extending | stretching and relaxation operation after extraction. Can be. In addition, as described above, mixing polypropylene in the polymer is also one of the effective methods.

It is preferable that the polyolefin microporous membrane of this embodiment contains the ultrahigh molecular weight polyethylene whose viscosity average molecular weight is 500,000 or more, and the polyethylene whose viscosity average molecular weight is less than 500,000. By containing the said various polyethylenes, not only melt viscosity increases at the time of separator melting, but also there exists a tendency for a breakwater resistance to improve by premature relaxation of melt tension.

It is preferable that the porosity of the polyolefin microporous membrane of this embodiment is 20% or more from a viewpoint of permeability, and 60% or less from a viewpoint of membrane strength, breakdown voltage, and a heat shrink rate. More preferably, they are 25% or more and 60% or less, More preferably, they are 30% or more and 55% or less.

Although the air permeability of the polyolefin microporous membrane of this embodiment is so preferable that it is low, in terms of balance with thickness and porosity, Preferably it is 1 second or more, More preferably, it is 50 second or more. In terms of permeability, it is preferably 1000 seconds or less, more preferably 500 seconds or less.

It is preferable that it is 1 micrometer or more from a viewpoint of film strength, and, as for the thickness of the polyolefin microporous film of this embodiment, 5 micrometers or more are more preferable. Moreover, it is preferable that it is 50 micrometers or less from a viewpoint of permeability, and 30 micrometers or less are more preferable.

Moreover, it is preferable that it is 0.2 N / micrometer or more, and, as for the puncture strength of the polyolefin microporous film of this embodiment, 0.22 N / micrometer or more is more preferable. When the puncture strength is low (less than 0.2 N / μm), when used as a battery separator, the microporous membrane is perforated by sharp parts such as an electrode material, whereby pinholes and cracks are likely to occur, or an external battery is impacted. It tends to be easily deformed in a test or the like.

Next, although the manufacturing method of the polyolefin microporous membrane of this embodiment is demonstrated, if the obtained microporous membrane has the said characteristic, a polymer kind, a solvent kind, an extrusion method, an extending | stretching method, an extraction method, a pore method, heat fixation? There is no limitation in the heat treatment method or the like.

As a manufacturing method of the polyolefin microporous film of this embodiment, the process of melt-kneading and extrusion of the resin composition containing a polyolefin and a plasticizer at least, and obtaining a sheet-like substance, the process of extending | stretching the said sheet-like substance, and obtaining a film, the said sheet-like It is preferable to include the process of extracting a plasticizer from a substance or the said film, and the process of heat-fixing the said film.

The polyolefin microporous membrane of this embodiment is obtained by the method containing the process of the following (a)-(e), for example.

(a) The polymer material of any one of a polyolefin single piece, a polyolefin mixture, a polyolefin solvent mixture, and a polyolefin kneaded material is melt-kneaded.

(b) The melt is extruded, molded into a sheet, and cooled and solidified. The plasticizer and the inorganic agent are extracted as necessary.

(c) The obtained sheet is stretched in one or more directions.

(d) After stretching, the plasticizer and the inorganic agent are extracted as necessary.

(e) Subsequently, heat fixing and heat treatment are performed.

The polyolefin used in the present embodiment is a homopolymer of ethylene or propylene or a copolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene and norbornene. It may be a mixture of polymers. Especially, polyethylene and its copolymer are preferable at the point which improves the performance of a microporous membrane. As a polymerization catalyst of such polyolefin, a Ziegler-Natta type catalyst, a Philips type catalyst, a metallocene catalyst, etc. are mentioned. The polyolefin may be obtained by a one-stage polymerization method or may be obtained by a multistage polymerization method.

As a composition of the polymer to supply, it is preferable to blend two or more types of polyolefin. This makes it possible to adjust the fuse temperature and the short temperature. More preferably, it is a blend of two or more types of polyethylene, It is preferable to contain the ultrahigh molecular weight polyethylene whose viscosity average molecular weight (Mv) is 500,000 or more, and the polyethylene whose viscosity average molecular weight (Mv) is less than 500,000. The polyethylene to be blended is preferably a high-density homopolymer in that the pores are not blocked and can be thermally fixed at a higher temperature.

Moreover, it is preferable that the viscosity average molecular weights (Mv) of the whole polymer material are 100,000 or more and 1.2 million or less. More preferably, they are 300,000 or more and 800,000 or less. If the viscosity average molecular weight (Mv) is less than 100,000, there is a risk that the breakage resistance at the time of melting will not be sufficient, if the viscosity average molecular weight (Mv) is more than 1.2 million, the extrusion process is difficult, the shrinkage force at the time of melting is slow, and there is a fear that the heat resistance is poor. have.

Blending polypropylene, which is a polyolefin having a higher melting point than polyethylene, to these not only improves heat resistance, but also makes it possible to operate at a higher temperature than polyethylene alone in the stretching and relaxation step after extraction, and also the strength and thermal contraction rate of the microporous membrane. As a result, it is possible to reduce the tensile elongation while maintaining the pore size. Furthermore, although the reason is not clear, it is especially preferable because there exists an effect that it does not fracture well even under high draw ratio.

The improvement of the heat resistance by such a blend is preferable because it is combined with the low heat shrinkability of this application, since the breakage resistance at high temperature becomes more favorable.

Moreover, well-known additives, such as metal soaps, such as a calcium stearate and a zinc stearate, an ultraviolet absorber, an optical stabilizer, an antistatic agent, an antifog agent, and a coloring pigment, can also be mixed and used.

Moreover, inorganic agents as represented by alumina, titania, etc. can also be added. The inorganic agent may extract all or part of any one of all the processes and may remain in the product.

The plasticizer used in the present embodiment refers to an organic compound capable of forming a uniform solution with a polyolefin at a temperature below the boiling point, and specifically, decalin, xylene, dioctylphthalate, dibutyl phthalate, stearyl alcohol, and oleyl. Alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, paraffin oil and the like. Among these, paraffin oil and dioctyl phthalate are preferable.

Although the ratio of a plasticizer is not specifically limited, 20 mass% or more is preferable from a porosity viewpoint of the microporous film obtained, and 90 mass% or less is preferable from a viscosity viewpoint. More preferably, they are 50 mass% or more and 70 mass% or less.

As an extraction solvent used for extraction of a plasticizer, it is preferable that a boiling point is lower than melting | fusing point of a polyolefin, although it is a poor solvent with respect to a polyolefin and a good solvent with respect to a plasticizer. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane and fluorocarbons, alcohols such as ethanol and isopropanol, and acetone. And ketones such as 2-butanone. It selects from these and uses individually or in mixture. These extraction solvents may be regenerated by distillation or the like after extraction of the plasticizer and used again.

The total weight ratio of the plasticizer and the inorganic agent in the total mixture melt-kneaded is preferably from 20 to 95% by mass, more preferably from 30 to 80% by mass in terms of permeability and film forming properties of the membrane.

In addition, in view of preventing thermal deterioration and thus deterioration in quality during melt kneading, it is preferable to blend an antioxidant in the mixture. 0.3 mass% or more is preferable with respect to the total polyolefin weight, and, as for the density | concentration of antioxidant, 0.5 mass% or more is more preferable. Moreover, 5.0 mass% or less is preferable, and 3.0 mass% or less is more preferable.

As antioxidant, the phenolic antioxidant which is a primary antioxidant is preferable, and 2, 6- di-t- butyl- 4-methyl phenol and pentaerythryl- tetrakis- [3- (3, 5- di-t -Butyl-4-hydroxyphenyl) propionate, octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, and the like. In addition, a secondary antioxidant can also be used in combination, tris (2,4-di-t-butylphenyl) phosphite and tetrakis (2,4-di-t-butylphenyl) -4,4-biphenyl Phosphorus antioxidants such as lene-diphosphonite, sulfur-based antioxidants such as dilauryl-thio-dipropionate, and the like.

As a method of melt kneading and extrusion, first or part of the raw materials are first premixed with a Henschel mixer, a ribbon blender, a tumbler blender or the like as necessary. In small cases, it may be stirred by hand. Subsequently, all raw materials are melt-kneaded with a screw extruder, a kneader, a mixer, etc., such as a single screw extruder and a twin screw extruder, and are extruded from a T type die or a cyclic die, etc.

Melt kneading is preferably carried out in a state in which the raw material polymer is mixed with an antioxidant at a predetermined concentration, replaced with a nitrogen atmosphere, and maintained in a nitrogen atmosphere. 160 degreeC or more is preferable and, as for the temperature at the time of melt kneading, 180 degreeC or more is more preferable. Moreover, less than 300 degreeC is preferable, less than 240 degreeC is more preferable, and less than 230 degreeC is still more preferable.

The melt mentioned in this embodiment may also contain the unmelted inorganic agent which can be extracted by an inorganic substance extraction process. In addition, the melt kneaded and homogenized melt is preferably passed through the screen to improve the film quality.

Next, shaping | molding of a gel sheet is demonstrated. As a shaping | molding method of a gel sheet, it is preferable to melt-knead | mix and melt the extruded melt by compression cooling. Examples of the cooling method include a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll or press machine cooled with a refrigerant, and a method of contacting a roll or press machine cooled with a refrigerant with excellent thickness control. It is preferable at the point.

There is no restriction | limiting in particular about the order, the method, and the frequency | count of the extending | stretching and plasticizer extraction, or extending | stretching and plasticizer extraction and inorganic agent extraction which are carried out subsequently. Inorganic extraction may not be performed as needed.

Examples of the stretching method include MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching by a simultaneous biaxial tenter or inflation molding. Can be mentioned. The draw ratio is a total plane magnification, and in order to obtain desired tensile strength and tensile elongation, 8 times or more is preferable, 15 times or more is more preferable, 30 times or more is more preferable, 40 times or more is especially preferable. Especially, simultaneous or sequential biaxial stretching is preferable. Moreover, 50 times or more are preferable and 60 times or more of the total draw ratio of all processes is the same reason.

In plasticizer extraction, a plasticizer is extracted by immersing in an extraction solvent or showering. Thereafter, it is sufficiently dried.

As a method of heat setting, the relaxation operation is performed at a predetermined temperature atmosphere and a predetermined relaxation rate. It can carry out using a tenter or a roll stretching machine. Relaxation refers to the reduction of the membrane into MD and / or TD. The relaxation rate is the MD dimension of the membrane after the relaxation operation divided by the MD dimension of the membrane before the operation, or the TD dimension after the relaxation operation divided by the TD dimension of the membrane before the operation, or MD, when both MD and TD are relaxed. Is the product of the relaxation rate of TD. As a predetermined relaxation rate, 0.9 or less are preferable and, as for a heat shrinkage rate, it is more preferable that it is 0.8 or less. In addition, in view of the prevention of wrinkles, porosity and permeability, it is preferably 0.6 or more. Although the relaxation operation can be performed in both MD and TD directions, the thermal contraction rate can be reduced not only in the operation direction but also in the operation and vertical directions by the relaxation operation of only MD or TD. By extending | stretching 1.5 times or more, More preferably, 1.8 times or more before the said relaxation operation, it becomes easy to obtain a high-porosity microporous membrane while being high strength.

The stretching and relaxation operation after the plasticizer extraction is preferably performed in the TD direction. In terms of thermal contraction rate and large pore curing, the temperature in the stretching step before the relaxation operation and the relaxation operation is preferably 125 ° C or higher, and at least one of them is preferably 130 ° C or higher, and more preferably 132 ° C or higher. When the temperature in the stretching step before the relaxation operation and the relaxation operation is within the above range, when the polyolefin is polyethylene, the stretching and relaxation operation is performed near the melting point, and it is easy to obtain a large pore size and a low thermal shrinkage rate compared with the conventional microporous membrane. . Furthermore, although the reason is not clear, even if it is a low elongation film | membrane, the microporous film which is excellent in a breakability is easy to be obtained. It is preferable that polypropylene other than polyethylene is blended as a polyolefin from the viewpoint that it can be extended | stretched and relaxed by higher temperature conditions different from such a conventional thing, and also it does not fracture well under conditions, such as large total draw ratio.

Moreover, the polyolefin microporous film of this embodiment can also be surface-treated, such as electron beam irradiation, plasma irradiation, surfactant application | coating, and chemical modification.

<Example>

An Example is shown to the following and this embodiment is demonstrated in detail.

[How to measure]

Measurement methods such as physical properties in the present specification are as follows.

(1) Viscosity Average Molecular Weight (Mv)

Based on ASTM-D4020, the intrinsic viscosity [eta] at 135 degreeC in a decalin solvent is calculated | required. Mv of polyethylene was computed by following Formula.

[η] = 6.77 × 10 ^-4 Mv ^0.67

About polypropylene, Mv was computed by following Formula.

[η] = 1.10 × 10 ^-4 Mv ^0.80

(2) film thickness (μm)

It measured at room temperature 23 +/- 2 degreeC using KBM (trademark) which is a micro thickness measuring instrument by Toyo Seiki.

(3) Porosity (%)

A 10 cm × 10 cm side sample was cut out of the microporous membrane to obtain its volume (cm ³ ) and mass (g), and calculated from these and membrane density (g / cm ³ ) using the following equation.

Porosity = (volume-mass / membrane density) / volume × 100

In addition, the film density was calculated from the material density.

(4) air permeability (sec)

In accordance with JIS P-8117, a Gurley air permeability meter (Toyo Seiki Co., Ltd. make, G-B2 (trademark)) was used. The inner cylinder weight was 567 g, and the time which 100 ml of air passes through the areas of diameter 28.6 mm and 645 mm <^2> was measured.

(5) air permeation amount

Permeation rate constant Rgas of air was calculated | required from the air permeability (second) using the following formula. The measurement was performed indoors at room temperature of 23 ° C.

Rgas (m ³ / (m ² ? Sec? Pa)) = 0.0001 / Air permeability / 0.0006424 / (0.01276 × 101325)

(6) water permeation amount

The water permeation amount when the microporous membrane previously immersed in alcohol was set in a 41 mm diameter stainless steel permeation cell, the alcohol of the membrane was washed with water, and water was passed at a differential pressure of about 50000 Pa and then elapsed for 120 seconds. The water permeation amount per unit time, unit pressure, and unit area was calculated from cm ³ ), and this was set as water permeability (cm ³ / (cm ² ? Sec? Pa)). The measurement was performed indoors at room temperature of 23 ° C. The permeation rate constant Rliq of water was determined from the water permeability (cm ³ / (cm ² sec.Pa)) using the following equation.

Rliq (m ³ / (m ² ? Sec? Pa)) = water permeability / 100

(7) puncture strength (N / μm)

The microporous membrane was fixed with the sample holder of 11.3 mm in diameter of the opening part using the handy compression tester KES-G5 (trademark) manufactured by Kato Tech. Next, 1 / film thickness (micrometer) is set to the maximum puncture load N by performing a puncture test in 25 degreeC atmosphere with the curvature radius of 0.5 mm of a needle tip, and a puncturing speed of 2 mm / sec of a fixed microporous membrane. The multiplied puncture strength (N / μm) was calculated.

(8) tensile strength (MPa), tensile elongation (%)

Based on JIS K7127, it measured about MD and TD sample (shape; width 10mm x length 100mm) using the Shimadzu Corporation Corporation tensile tester and Autograph AG-A type | brand (trademark). In addition, the sample used 50 mm of chuck | zippers, and attached the cellophane tape (Nitto Denko Co., Ltd. make, brand name: N.29) to one surface of both ends (25 mm each) of a sample. Further, in order to prevent sample slipping during the test, a 1 mm thick fluororubber was adhered to the inside of the chuck of the tensile tester.

Tensile elongation (%) was calculated | required by dividing the elongation amount (mm) to break by the inter-chuck distance (50 mm), and multiplying by 100.

Tensile strength (MPa) was calculated | required by dividing the strength at break by the sample cross section before a test.

In addition, the total (%) of MD tensile elongation and TD tensile elongation was calculated | required by adding the value of MD and TD. In addition, the measurement was performed at the temperature of 23 +/- 2 degreeC, chuck | zipper pressure 0.30 MPa, and tensile velocity 200mm / min (400% / min of strain rate in the sample which cannot secure 50 mm of intervertebral distance).

(9) 130 ° C heat shrink rate (%)

100 mm in MD direction and 100 mm in TD direction were cut out, and it left still in 130 degreeC oven for 1 hour. At this time, it sandwiched between two sheets so that a warm air might not directly contact a sample. After taking out from the oven and cooling, the length (mm) was measured, and the heat shrinkage ratios of MD and TD were calculated by the following equation (as far as the sample length could not be secured, within a range of 100 mm x 100 mm. One long sample).

MD heat shrink rate (%) = (length of TD after 100-heating) / 100 × 100

TD heat shrinkage (%) = (length of TD after 100-heating) / 100 x 100

(10) bubble point (MPa)

It measured with the ethanol solvent based on ASTM F316-86. The point where continuous foam was confirmed was made into the bubble point.

(11) High speed thermal film resistance (breakproof)

Nickel foil A (length 100 mm x width 25 mm), thickness 10 micrometers, nickel foil B (length 100 mm x width 15 mm), the separator which immersed in electrolyte solution for 30 minutes or more (MD length 75 mm x TD length 25 mm), Aramid film (trademark), a slide glass (75 mm long x 25 mm wide), and a glass plate (25 mm long x 20 mm wide) provided with a window of 10 mm x 10 mm in the center are prepared.

As shown in FIG. 1, the slide glass, nickel foil A, a separator, an aramica film, nickel foil B, and a glass plate were overlapped in order, and it fixed with the clip.

A thermocouple was connected to the cell and left in the oven. Then, it heated up at the speed | rate of 5 degree-C / min, and after reaching 150 degreeC, it hold | maintained at 150 degreeC for 1 hour. The impedance change at this time was measured by the LCR meter on 10 mV alternating current and 1 kHz conditions. In this measurement, from the time when the impedance was maintained at 150 ° C., the A was able to maintain an insulation state of at least 1000? What was able to be able to hold D and 5 minutes was set to E.

In addition, the composition ratio of the prescribed electrolyte solution was as follows.

Composition ratio (volume ratio) of solvent: propylene carbonate / ethylene carbonate / (gamma) -butyrolactone = 1/1/2

Composition ratio of solute: LiBF ₄ was dissolved in the solvent so as to have a concentration of 1 mol / liter.

[Manufacture and Evaluation of Battery]

(1) Preparation of Positive Electrode

92.2 mass% of lithium cobalt composite oxide LiCoO ₂ as an active substance, 2.3 mass% of flaky graphite and acetylene black as a conductive agent, and 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder, respectively, N-methylpyrrolidone (NMP). ) To prepare a slurry. This slurry was apply | coated to the one side | surface of the aluminum foil with a thickness of 20 micrometers used as a positive electrode collector by a die coater, and it dried by 130 minute (s) for 3 minutes, and was compression-molded by the roll press. At this time, the active material application amount of the positive electrode was 250 g / m ² , and the active material bulk density was 3.00 g / cm ³ . This was cut into a width of about 40 mm to form a band.

(2) Production of negative electrode

96.9 mass% of artificial graphite as an active substance, 1.4 mass% of ammonium salt of carboxymethylcellulose as a binder, and 1.7 mass% of styrene-butadiene copolymer latex were dispersed in purified water to prepare a slurry. This slurry was apply | coated to one side of the copper foil of thickness 12micrometer used as a negative electrode electrical power collector with a die coater, and it dried at 120 degreeC for 3 minutes, and then compression-molded by the roll press. At this time, the active material application amount of the negative electrode was 106 g / m ² , and the active material bulk density was 1.35 g / cm ³ . This was cut into a width of about 40 mm to form a band.

(3) Preparation of nonaqueous electrolyte

It was prepared by dissolving LiPF ₆ as a solute to a concentration of 1.0 mol / liter in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio).

(4) battery assembly

The electrode plate laminated body was manufactured by superimposing a separator, a belt-shaped positive electrode, and a belt-shaped negative electrode using the polyolefin microporous membrane in the order of a belt-shaped negative electrode, a separator, a belt-shaped positive electrode, and a separator, and winding in vortex multiple times. The electrode plate laminate was pressed into a flat plate, and then housed in an aluminum container, the lead made of aluminum was taken out of the positive electrode current collector, the lead made of nickel was led out of the negative electrode current collector, and welded to the bottom of the container. In addition, the above-mentioned nonaqueous electrolyte was injected into this container and sealed. The lithium ion battery thus produced was designed to have a size of 6.3 mm long (thickness), 30 mm wide and 48 mm high, with a nominal discharge capacity of 620 mAh.

(5) Battery evaluation (under 25 degreeC atmosphere)

The lithium ion battery assembled as described above was charged with constant current constant voltage (CCCV) for 6 hours under conditions of a current value of 310 mA (0.5C) and a final battery voltage of 4.2V. At this time, the current value immediately before the end of charging became almost zero. Then, it was left to stand (aging) for 1 week in 25 degreeC atmosphere.

Next, a cycle of charging a constant current constant voltage (CCCV) for 3 hours under the condition of a current value of 620 mA (1.0C) and a final battery voltage of 4.2 V, and discharging the battery voltage to 3.0 V at a constant current value (CC) of 620 mA. Was performed. The discharge capacity at this time was made into the initial discharge capacity.

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

(b) Separately, in order to perform the impact test of the battery before the cycle test of (a), it dropped repeatedly 10 times on the concrete floor from the height of 1.9 m. Then, the battery was disassembled and observed. It was set as A that the deformation | transformation of the winding body was hardly seen, and what was able to confirm the deformation | transformation B easily and to C which was seen insignificantly was C.

Example 1

47 mass% of homopolymer polyethylene with Mv of 700,000, 46 mass% of homopolymer polyethylene of Mv 300,000, and 7 mass% of polypropylene of Mv 400,000 were dry blended using a tumbler blender. 1 mass% of pentaerythritol-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] is added to 99 mass% of the obtained pure polymer mixture, Dry blending was carried out again using a tumbler blender to obtain a mixture such as a polymer. The obtained polymer and the like were substituted with nitrogen, and then supplied to a twin screw extruder by a feeder under a nitrogen atmosphere. In addition, flow paraffin (kinematic viscosity 7.59 × 10 ⁻⁵ m ² / s at 37.78 ° C.) was injected into the extruder cylinder by a plunger pump.

The feeder and the pump were adjusted so that the flow paraffin content ratio in the total mixture melt-kneaded and extruded might be 65 mass%. Melt-kneading conditions were set temperature 200 degreeC, it was performed by screw rotation speed 240 rpm, and discharge amount 12 kg / h.

Then, the melt-kneaded material was extrusion-cast on the cooling roll controlled at the surface temperature of 25 degreeC via the T-die, and the gel sheet of 2000 micrometers in thickness was obtained.

Next, the biaxial stretching was performed by inducing a simultaneous biaxial tenter stretching machine. The setting stretching conditions were MD magnification 7.0 times, TD magnification 7.0 times and set temperature 125 degreeC.

Next, the mixture was led to a methyl ethyl ketone bath, sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then methyl ethyl ketone was dried and removed.

Next, thermal fixation was performed by induction to the TD tenter. The stretching temperature and the magnification at the time of heat setting were performed at 128 degreeC-2.0 times, and the temperature and relaxation rate at the time of relaxation after that were 133 degreeC and 0.80.

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

[Example 2]

It carried out similarly to Example 1 except having a biaxial stretching temperature of 120 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 3

Except having made the disk thickness after extrusion 900 degreeC, biaxial stretching temperature 122 degreeC, extending | stretching temperature and magnification at the time of heat setting at 130 degreeC-2.0 times, and setting temperature and relaxation rate at the time of subsequent relaxation to 135 degreeC and 0.80, It carried out similarly to Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 4

Using 30 mass% of homopolymer polyethylene with Mv of 2.5 million, and 70 mass% of homopolymer polyethylene with Mv of 250,000, the disk thickness after extrusion is made into 2400 micrometers and the draw temperature and the magnification at the time of heat setting are 125 degreeC-1.9 times, It carried out similarly to Example 1 except having set the temperature and relaxation rate at the time of the relaxation after that to 132 degreeC and 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 whose Mv is 500,000 for 99 mass% of pure polymer mixtures, and extending | stretching temperature at the time of heat setting to 125 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 6]

The film thickness after extrusion is 1800 µm, the biaxial stretching ratio is 5 x 5 times, the biaxial stretching temperature is 115 deg. C, and the stretching temperature and magnification at the time of heat setting are 125 deg. C to 1.7 times, and the temperature and relaxation rate at the time of subsequent relaxation. It carried out similarly to Example 4 except having set it as 131 degreeC and 0.70. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 7

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

Example 8

Example 1 except having made biaxial stretching temperature into 123 degreeC using the blend of 45 mass% of homopolymer polyethylenes with Mv of 700,000, 40 mass% of homopolymer polyethylenes with Mv 300,000, and 15 mass% of polypropylene of Mv 400,000. The same was done as. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 9

Example 1 except having made biaxial stretching temperature into 123 degreeC using the blend of 45 mass% of homopolymer polyethylenes with Mv of 700,000, 30 mass% of homopolymer polyethylenes with Mv 300,000, and 25 mass% of polypropylene of Mv 400,000. The same was done as. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 10

It carried out similarly to Example 1 except having set the thickness of the gel sheet to 1600 micrometers, the extending | stretching temperature at the time of heat setting to 125 degreeC, and the temperature at the time of the relaxation | relieving after that to 130 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Example 11

The stretching-relaxation temperature at the time of heat setting is 128 degreeC and 133 degreeC using 30 mass% of homopolymer polyethylenes with 2.5 million Mv, 60 mass% of homopolymer polyethylenes with Mv 250,000, and 10 mass% of polypropylene of Mv 400,000. It carried out similarly to Example 4 except having made into. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Comparative Example 1

It carried out similarly to Example 1 except having extended | stretched temperature and magnification at the time of heat setting at 120 degreeC-1.5 time, and setting temperature and relaxation rate at the time of relaxation after that to 125 degreeC and 0.80. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Comparative Example 2

Stretching temperature at the time of heat setting-magnification was performed at 122 degreeC-1.3 times, and it carried out similarly to Example 2 except not performing relaxation. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

Comparative Example 3

60 mass% of homopolymers of Mv 270,000 and 40 mass% of homopolymers of Mv 950,000 were dry blended using a tumbler blender. 1 mass% of pentaerythritol-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] is added to 99 mass% of the obtained pure polymer mixture, Dry blending was carried out again using a tumbler blender to obtain a mixture such as a polymer. The obtained polymer and the like were substituted with nitrogen, and then supplied to a twin screw extruder by a feeder under a nitrogen atmosphere. In addition, flow paraffin (kinematic viscosity 7.59 × 10 ⁻⁵ m ² / s at 37.78 ° C.) was injected into the extruder cylinder by a plunger pump.

The feeder and the pump were adjusted so that the flow paraffin content ratio in the total mixture melt-kneaded and extruded was 62 mass%. Melt-kneading conditions were set temperature 200 degreeC, it was performed by screw rotation speed 240 rpm, and discharge amount 12 kg / h.

Subsequently, the melt-kneaded material was extruded on the cooling roll controlled at the surface temperature of 25 degreeC through the T-die, and it cast at the rolling ratio 4 times, and the gel sheet of 200 micrometers in thickness was obtained.

Next, the obtained sheet was guided to a TD tenter stretching machine and stretched transversely before extraction at a draw temperature of 115 ° C. and a draw ratio 5 times, followed by 10% heat relaxation.

Next, the mixture was led to a methyl ethyl ketone bath, sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then methyl ethyl ketone was dried and removed.

Next, the film after the extraction was guided to a multi-stage roll type stretching machine, and the film was extracted and stretched so that the magnification in the stretching temperature of 110 ° C. and the MD direction was doubled to obtain a microporous membrane.

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

[Comparative Example 4]

30 mass% of homopolymer polyethylene with Mv of 700,000, 15 mass% of homopolyethylene with Mv of 300,000, 5 mass% of homopolymer polypropylene of Mv 400,000, 30.6 mass% of dioctylphthalate (DOP), 18.4 mass% of fine silica, Pentaerythryl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added as an antioxidant and 1 mass% was mixed. The obtained polymer and the like were substituted with nitrogen, and then supplied to a twin screw extruder by a feeder under a nitrogen atmosphere.

Melt-kneading conditions were set temperature 200 degreeC, it was performed by screw rotation speed 240 rpm, and discharge amount 12 kg / h.

Subsequently, the melt-kneaded material was extrusion-cast on the cooling roll controlled at the surface temperature of 80 degreeC via T-die, and the gel sheet of 110 micrometers in thickness was obtained.

DOP and finely divided silica were extracted and removed from this gel sheet to obtain a microporous membrane. Two sheets of the microporous membrane were overlapped and stretched in the longitudinal direction 5 times at 110 ° C., and then drawn twice in the transverse direction at 130 ° C. by induction in a TD tenter. Thereafter, the TD relaxation ratio was set to 0.80 at 130 ° C.

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

The following results are understood from the results in Table 1.

(1) The polyolefin microporous membranes of Examples 1 to 11 in which the bubble point, the tensile strength in the longitudinal direction and the width direction, and the thermal contraction rate in the width direction at 130 ° C. were adjusted to a specific range had a good balance of impact resistance and breakage resistance. And, the battery produced using this was excellent in capacity retention.

(2) In the polyolefin microporous membranes of Comparative Examples 1 and 2, the bubble point exceeded 1 MPa and did not have sufficient pore size, resulting in poor capacity retention.

(3) The polyolefin microporous membrane of Comparative Example 3 had a TD heat shrinkage of more than 20% at 130 ° C and a puncture strength of 0.20 N / µm or less. fell.

(4) Since the polyolefin microporous membrane of Comparative Example 4 had a TD tensile strength of less than 50 MPa, and the total MD tensile elongation and TD tensile elongation exceeded 250%, the microporous membrane is likely to deform with respect to repeated shocks. Impact resistance fell.

(5) Compared with Examples 5 and 7, the polyolefin microporous membrane of Example 11 had excellent membrane resistance and impact resistance because Mv was blended with polyethylene of 500,000 or more, polyethylene with less than 500,000, and polypropylene. .

(6) The polyolefin microporous membranes of Examples 8 and 9 have a higher content of polypropylene than in Example 1. For this reason, not only thermal fixation is possible at high temperature, but also tensile elongation is low, and both the breaking resistance and impact resistance were excellent.

(7) Compared with Example 6, the polyolefin microporous membrane of Example 4 had a high total draw ratio and a low tensile elongation, and thus was excellent in impact resistance.

(8) Compared with Example 10, the polyolefin microporous membrane of Example 1 had a low porosity, so that the capacity retention rate was slightly decreased. However, the polyolefin microporous membrane was excellent in both break resistance and impact resistance because of its high strength and low heat shrinkage.

From the above results, the polyolefin microporous membrane of the present embodiment had a large pore diameter and excellent strength, tensile elongation, and low heat shrinkability. Therefore, by using the polyolefin microporous membrane of this embodiment for a battery separator, the secondary battery excellent in the balance of battery characteristics and battery safety can be obtained.

[Industry availability]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to polyolefin microporous membranes used for separation of materials, selective permeation separation membranes, and separators, and the like, and particularly has industrial applicability as separators used in lithium ion batteries and the like.

1: Separator
2: nickel foil A
3: nickel foil B
4: aramika film
5: glass plate
6: slide glass
7: thermocouple
8: window of 10 × 10 mm

Claims (9)

The polyolefin microporous membrane whose bubble point is 1 MPa or less, and the tensile strength of the longitudinal direction and the tensile strength of the width direction are respectively 50 MPa or more and the thermal contraction rate of the width direction in 130 degreeC is 20% or less. The polyolefin microporous membrane of Claim 1 containing polypropylene. The polyolefin microporous membrane of Claim 1 whose sum of MD tensile elongation and TD tensile elongation is 20 to 250%. The polyolefin microporous membrane of Claim 3 whose sum of MD tensile elongation and TD tensile elongation is 20 to 200%. The polyolefin microporous membrane of Claim 1 containing the ultrahigh molecular weight polyethylene whose viscosity average molecular weight is 500,000 or more, and the polyethylene whose viscosity average molecular weight is less than 500,000. The polyolefin microporous membrane of Claim 1 whose porosity is 20% or more and 60% or less. The battery separator which consists of the polyolefin microporous film in any one of Claims 1-6. A nonaqueous electrolyte secondary battery provided with the battery separator of Claim 7. Melt-kneading and extruding a resin composition containing at least a polyolefin and a plasticizer to obtain a sheet-like substance,
Stretching the sheet-like substance to obtain a film,
Extracting a plasticizer from the sheet-like material or the film,
Heat fixing the film
The manufacturing method of the polyolefin microporous film of any one of Claims 1-6 containing these.

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