JPH0696753A - Polyolefine micro-cellular porous film - Google Patents

Abstract

PURPOSE:To obtain a separator for battery having the reliability on the safety by using a polyolefine micro-cellular porous film. CONSTITUTION:This micro-cellular porous film is made of the mixture of high molecular weight polyethylene and high molecular weight polypropylene, and has a mechanical direction elasticity at 3000kgf/cm<2> or more and a porosity at 40-80%, and has the impedance transition temperature, at which the impedance in a range of temperature from 95 deg.C to 150 deg.C becomes 500 times or more of the impedance at 25 deg.C, and has the impedance lowering temperature, at which the impedance at 175 deg.C becomes lower than 500 times of the impedance at 25 deg.C.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a polyolefin fine pore used for various separation membranes, separators for batteries, separators for electrolytic capacitors, etc., particularly separators for non-aqueous solvent batteries such as lithium batteries. Porous porous membrane.

[0002]

2. Description of the Related Art Microporous membranes are used in various separation membranes, battery separators, electrolytic capacitor separators and the like. Particularly in lithium batteries, since lithium metal, lithium ions, etc. are used, a protic electrolyte cannot be used. For organic solvents such as γ-butyrolactone, polypropylene carbonate, dimethoxyethane, LiBF ₄ , LiClO _4, etc. An electrolyte solution in which a lithium salt is dissolved is used. Therefore, as the separator installed between the positive electrode and the negative electrode, an olefinic material such as polyethylene or polypropylene which is insoluble in the above organic solvent is processed into a microporous membrane or a non-woven fabric and used as the separator.

Further, such a separator for a non-aqueous solvent battery such as a lithium battery has high strength, low electric resistance, high temperature characteristics and the like in view of assembly workability, safety and reliability. It is required, and further, low cost is required. The high strength relates to the assembling workability, and the higher the strength, the higher the production speed at the time of assembling the battery.

The low electrical resistance means that a so-called aprotic electrolyte solution obtained by dissolving a lithium salt in the above organic solvent generally has a high internal resistance. Therefore, in order to cover this drawback, the resistance is increased by the separator. Is required to suppress the The high temperature characteristic relates to safety and means the following performance. That is,
When the temperature inside the battery rises due to heat generated when the battery is externally short-circuited, the microporous membrane shrinks due to the temperature rise, the pore diameter of the microporous membrane decreases, and the impedance rises. As the impedance transition temperature, which is 500 times or more the impedance at 0 ° C., is lower, it is possible to prevent the permeation of ions at a low temperature, and suppress a rapid temperature increase in the battery temperature. As described above, in order to suppress the excessive temperature rise due to the abnormal current, it is necessary to increase the impedance by at least two digits from the value at 25 ° C. It is recognized that the impedance is increased by 500 times or more.

In the microporous membrane with increased impedance, the melt viscosity of the resin decreases as the temperature further rises,
Impedance decreases due to resin flow, breakage, etc. at a specific temperature (temperature such as film breakage). The temperature at this time is called the impedance lowering temperature. Therefore, the lower the impedance transition temperature and the greater the difference between the impedance transition temperature and the impedance lowering temperature, the more
It is considered that the battery separator can have good high temperature characteristics and high safety.

However, for example, Japanese Patent Publication No. 63-29
Japanese Patent No. 891 discloses a microporous porous membrane made of polyethylene having a weight average molecular weight of 200,000 to 500,000 and uniaxially stretched in the rolling and machine directions, but the membrane obtained in the publication is high. Although it has a film strength, it has a low impedance lowering temperature because it is a composition of high-density polyethylene having a low melting point, and its high-temperature characteristics are poor, causing a problem in safety. In addition, JP-A-46-40119 and JP-A-1-
The microporous porous film made of polypropylene disclosed in Japanese Patent No. 113442 has a high strength and a high impedance lowering temperature, but has a high impedance transition temperature and a high temperature characteristic because of a single composition of a high melting point resin. It is defective and there is a problem with safety.

In order to improve such high temperature characteristics, in JP-A-63-308866 and JP-A-2-77108, a single film made of polyethylene and polypropylene is laminated so that polyethylene contributes to the impedance transition temperature. , Has disclosed a method of contributing polypropylene to an impedance lowering temperature to obtain a microporous porous film having high strength and excellent high-temperature characteristics. However, the impedance is lowered in the middle of the melting points of polyethylene and polypropylene, so that it is safe. The surface reliability is poor.
Further, in the case of stacking as in Japanese Patent Laid-Open No. 63-308866, the process of winding becomes complicated, and thus, Japanese Patent Laid-Open No. 2-771.
In JP-A-08, since the method of laminated extrusion is adopted, the manufacturing process is complicated and the productivity is inferior in terms of manufacturing cost.

On the other hand, as a technique for achieving a high impedance lowering temperature, there is a method of using an ultrahigh molecular weight polyolefin. Japanese Unexamined Patent Publication No. 60-242035 and Japanese Unexamined Patent Publication No. Sho 6-242035 to which so-called ultra-high molecular weight polyethylene spinning as disclosed in Japanese Unexamined Patent Publication No. 58-5228 is applied.
0-255107 discloses a high-strength microporous membrane made of ultra-high molecular weight polyethylene, but it takes time to obtain a uniform composition due to the use of ultra-high molecular weight polyethylene, and production. It cannot be said that the resistance is poor and the impedance transition temperature is not sufficiently low, and the safety remains questionable.

A technique for satisfying both a low impedance transition temperature and a high impedance lowering temperature and obtaining a high-strength film is a method in which ultra-high molecular weight polyethylene is partially blended with high-density polyethylene. Since high molecular weight polyethylene is used, it takes time to obtain a uniform composition, resulting in poor productivity. There is also a method of blending ultra-high molecular weight polyethylene with a part of high-density polyethylene and polypropylene, but this also takes time to obtain a uniform composition due to the use of ultra-high molecular weight polyethylene, resulting in poor productivity.

On the other hand, as a technique for obtaining a high-strength film from only polyethylene and polypropylene without using ultrahigh-molecular-weight polyethylene, polyethylene and polypropylene are incompatible on the molecular order (for example, polymer blend <CM. It is known to be difficult, but, for example, as described in JP-A-50-111174, a molded product made of polyethylene and polypropylene is biaxially stretched, or polyethylene and polypropylene are used. There is a method of obtaining a film by annealing a molded product made of (1) after stretching or stretching after annealing, but the obtained film has insufficient heat resistance and the impedance decreases at 170 ° C. . In addition, JP-A-4
As described in JP-A-206257, since annealing is performed, it takes time to obtain a molded product because of annealing, resulting in poor productivity. Also, JP-A-57-49
Japanese Patent No. 629 also discloses a microporous membrane made of polyethylene and polypropylene. The membrane obtained in this publication is a blend of ultra-high molecular weight polyethylene and a part of high-density polyethylene. ,
Since the inorganic filler is used in the manufacturing process, it is disadvantageous in terms of manufacturing cost in terms of maintenance of the molding machine such as deviation of the molding machine due to the inorganic filler and treatment of the inorganic filler.

[0011]

DISCLOSURE OF THE INVENTION The present invention provides a single-layer, microporous porous membrane that does not have the above-mentioned drawbacks, that is, has a high performance, a high strength, a low electric resistance and an excellent safety and is a thin film at a low cost. Is intended. As a result of diligent research, the present inventors have succeeded in developing a single-layer, microporous membrane having a high strength, a low electric resistance, and excellent safety at a low cost, and completed the present invention.

[0012]

Means for Solving the Problems That is, the present invention comprises a mixture of a high molecular weight polyethylene and a high molecular weight polypropylene and has a micropore having a machine direction elastic modulus of 3000 kgf / cm ² or more and a porosity of 40 to 80%. It is a porous film with a temperature range of 95 ℃ to 150 ℃.
Polyolefin microporosity characterized by having an impedance transition temperature of 500 times or more of the impedance at 0 ° C and having an impedance lowering temperature at which the impedance becomes lower than 500 times of the impedance at 25 ° C at 175 ° C or more. It relates to a porous membrane.

The high molecular weight polyethylene and the high molecular weight polypropylene in the present invention are those used for ordinary extrusion, injection, inflation or blow molding, and have a viscosity average molecular weight of 1,000,000 or less, preferably 700,000 or less, more preferably Is less than 500,000. Therefore, the polyolefin microporous membrane of the present invention is
From the integral curve of the (gel permeation chromatography) measurement, it was found that the molecular weight of the whole system was 8 or less.
It becomes 0 wt% or more. A polymer having a viscosity average molecular weight of 1,000,000 or more is usually called ultra-high molecular weight polyethylene, and the high-molecular weight component increases. Therefore, a composition containing ultra-high molecular weight polyethylene takes time to obtain a uniform composition. Or
Due to the non-uniformity, poor film thickness uniformity or pinholes are generated in the film, resulting in poor productivity.

As the polyethylene used in the polyolefin microporous membrane of the present invention, low density, medium density or high density polyethylene, linear low density polyethylene and the like can be used, but high density polyethylene is particularly desirable. . Further, as the polypropylene used in the present invention, isotactic polypropylene, atactic polypropylene, ethylene propylene random copolymer, ethylene propylene block copolymer and the like can be used, but isotactic polypropylene is particularly preferable. Further, if desired, an appropriate amount of additives such as a filler, a colorant, an antiaging agent, a flame retardant can be mixed.

The polyolefin microporous membrane of the present invention comprises:
It is essential that the impedance transition temperature is 95 to 150 ° C and the impedance lowering temperature is 175 ° C or higher. When the impedance transition temperature is higher than 150 ° C., the temperature inside the battery rises, which may cause a phenomenon such as ignition, explosion or explosion. Further, in the present invention, the impedance lowering temperature of 175 ° C. or higher means that the film shape is maintained although it is melted to form a non-porous film even at a temperature higher than the melting point of the polypropylene used in the present invention. means. This is probably because of the special dispersion state of polyethylene and polypropylene formed by rolling, the special film structure and the special orientation structure of polyethylene polymer chains, polypropylene polymer chains and crystalline phases. In fact, when the sample is heated from room temperature at a heating rate of 2 ° C./min under a fixed length, the contraction stress works, and the load passes the maximum value,
Gradually lower. However, when the film was observed at 175 ° C., it was not broken although it was non-porous.

The polyolefin microporous membrane of the present invention comprises
Film thickness 10 to 100 μm, average pore diameter 1 μm or less, porosity 4
It is characterized in that the elastic modulus in the machine direction is 0 to 80%, 3000 kgf / cm ² or more, and the electric resistance is 10 Ωcm ² or less. The polyolefin microporous membrane of the present invention has a machine direction elastic modulus of 3000 kgf / cm ² or more, preferably 50.
It is not less than 00 kgf / cm ² . 3000 kgf / cm
^If it is less than ² , elongation occurs when the battery is wound and assembled,
Poor dimensional stability and poor assembly processability when used as a battery separator.

The porosity of the polyolefin microporous membrane of the present invention must be 40-80%,
Furthermore, it is desirable that it is 45 to 70%. If it is less than 40%, the impregnation property of the electrolytic solution particularly when used as a battery separator is deteriorated, and the electric resistance of the separator itself increases, which is not desirable from the viewpoint of the internal resistance of the battery. When it is more than 80%, the film strength in the machine direction is inferior, and the workability in arranging it between the positive and negative electrode plates for winding and assembling is inferior.

The film thickness of the polyolefin microporous membrane of the present invention is preferably 10 to 100 μm, more preferably 20 to 50 μm. If the thickness is 10 μm or less, the battery separator may suffer from an internal short circuit, resulting in poor reliability. When the thickness is 100 μm or more, the electrical resistance of the separator itself increases, and the area of the electrode plate decreases due to an increase in the volume occupied by the separator itself, resulting in poor battery performance, which is not desirable.

The electrical resistance of the polyolefin microporous membrane of the present invention is desirably 10 .OMEGA.cm ² or less, still more when is 4Omucm ² below, from the viewpoint of the internal resistance when used as a separator for a battery . 1
At 0 Ωcm ² or more, electric energy cannot be effectively extracted from the battery because of large internal resistance. The average pore size as referred to in the present invention is determined by the half dry method, and is preferably 1 μm or less, more preferably 0.3 μm or less. If it is 1 μm or more, the pore diameter is large, and when it is used as a battery separator, in particular, the performance of preventing permeation of fine particles is inferior and there is a fear of internal short circuit.

In the present invention, the maximum pore size is not particularly limited, but from the viewpoint of the pore size uniformity of the microporous membrane, it is 2 μm.
It is preferably m or less. If it is 2 μm or more, the performance of preventing the permeation of fine particles is deteriorated and the uniformity of the pore size is deteriorated, which causes a problem in safety. The polyolefin microporous membrane of the present invention is obtained by, for example, mixing polyethylene and polypropylene having a viscosity average molecular weight of 500,000 or less, and an organic liquid or solid, melt-kneading, extrusion-molding, extracting, drying, and stretching. Is obtained by Here, the order of the extraction and stretching steps is not limited at all. In addition, when at least a rolling step is included in the stretching step, it is desirable that the microporous porous film obtained has a higher elastic modulus in the machine direction.

Polyethylene resin, polypropylene resin,
The composition weight ratio of the organic liquid or solid is, for example, 10 to 50% by weight, 1 to 20% by weight, and 30 to 90% by weight, respectively. The term "organic liquid or solid" as used in the present invention means liquid paraffin, paraffin wax, mineral oil such as process oil, dioctyl phthalate, and di-n phthalate.
-Butyl, phthalates such as dicyclohexyl phthalate, sebacates such as di-n-butyl sebacate, and phosphates such as tri-n-butyl phosphate.

The rolling step is performed by a rolling roll at a predetermined temperature selected from the range of the surface temperature of 60 to 160 ° C. The extraction step refers to an operation of immersing in a poor solvent of resin and a good solvent of organic liquid or solid, preferably at 20 to 90 ° C. to remove the organic liquid or solid from the sheet-shaped molded product.

Examples of the extraction solvent include alcohols such as methanol, ethanol and 2-propanol, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran, and halogenated hydrocarbons such as 1,1,1-trichloroethane. Solvents may be mentioned. The stretching step refers to means for stretching the sheet-shaped molded product to a desired film thickness.
Especially when a stretching machine is used, the stretching temperature is 80 to 140 ° C.
Then, by stretching the sheet in the machine direction (uniaxial direction) or in the biaxial direction, a microporous porous film adjusted to a desired film thickness can be obtained.

[0024]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the examples. The measuring method and the evaluating method in the examples are as follows. (1) Film thickness A dial gauge (minimum scale: 1 μm) was used. (2) Porosity Calculated from the following formula.

Porosity = Pore volume ÷ Total membrane volume × 100 Pore volume = Total membrane volume-Membrane weight ÷ Resin density (3) Machine direction elastic modulus According to ASTM-D-882, Instron type tensile tester It was measured at. (4) Average Pore Size Based on ASTM-F-316-70, the half-dry method evaluated. The upper limit of the measurement pressure is 10 kgf / c
It was set to m ² . (5) Maximum pore size Based on ASTM-E-128-61, it was calculated from the bubble point in ethanol. (6) Electric resistance The electric resistance was measured with an Ando Denki AG-4311-type LCR meter.

[0026] Electrolyte: propylene carbonate 50 vol% dimethoxyethane 50 vol% lithium perchlorate 1 mol / dm ³ Conditions: platinum black electrode between the plates a distance of about 3mm plate exposed area 0.785 cm ² exchanges 1 kH _Z Assembly: FIG. 1. Electrical resistance (Ω · cm ² ) = {(resistance value with film (Ω)) − (resistance value without film (Ω))} × electrode plate exposed area (7) melt Index Unless otherwise specified, it is based on ASTM-D-1238. (8) Viscosity average molecular weight (Mv) The intrinsic viscosity [η] at a measurement temperature of 135 ° C was measured using a solvent (decalin) and calculated from the following formula.

[Η] = 6.2 × 10 ⁻⁴ Mv ^0.7 (Chiang's equation) (9) Impedance transition temperature and impedance reduction temperature FIG. 2 shows an outline of the impedance transition temperature and impedance reduction temperature measurement device defined in the present invention. Indicates. Figure 2
In (A), 1A and 1B are Ni foil having a thickness of 10 μm and are connected to the impedance measuring device 8. As shown in FIG. 2B, the Ni foil 1A is masked with the Teflon tape 6 leaving a rectangle having a length of 15 mm and a width of 10 mm. Reference numeral 3 denotes a separator impregnated with a prescribed electrolytic solution, which is disposed between 1A and 1B, and its four sides are fixed with Teflon tape. Reference numeral 5 is a thermocouple for measuring the temperature, which is a glass plate 2B made of Teflon tape.
Pasted on. A specified electrolytic solution is filled between the glass plates 2A and 2B.

The Ni foils 1A and 1B, the glass plates 2A and 2B, the separator 3 and the thermocouple 5 are shown in FIG. 2 (C).
It is used by storing it in case 4 shown in. Reference numeral 9 is a recording device for recording the temperature and the measured impedance. As the electrolytic solution, a 1M-lithium borofluoride / propylene carbonate solution is used. Measurement frequency is 1kHz
z.

Using the device shown in FIG. 2, while continuously measuring the impedance, from 25 ° C. to 180 ° C., 2 ° C.
The temperature of the battery part is raised in an oven set to a heating rate of / min. The temperature at which a value of 500 times the impedance at 25 ° C. is first reached is measured and this temperature is taken as the impedance transition temperature. The temperature is further raised, and the first temperature at which the temperature drops below the value of 500 times the impedance at 25 ° C. is defined as the impedance lowering temperature. (10) Fraction with a molecular weight of 1,000,000 or less in the whole system It can be obtained from an integral curve of GPC measurement.

GPC (gel permeation chromatography) instrument: WATERS 150-GPC temperature: 140 ° C. solvent: 1,2,4-trichlorobenzene concentration: 0.05% (injection amount: 500 μl) column: Shodex GPC AT- 807 / S 1
This Tosoh TSK-GEL GMH ₆ -HT 2 pieces Dissolution condition: 160 ° C., 2.5 hours Calibration curve: A polystyrene standard sample was measured, and a polyethylene conversion constant (0.48) was used.
Third-order calculation (11) High-temperature characteristics Seiko Denshi Kogyo Co., Ltd. heat / stress / strain measuring device
Using TMA / SS120, sample length 10mm, width 4
The sample was evaluated from the state when the sample was heated from room temperature at a temperature rising rate of 2 ° C./min under a constant length of 0.1 mm with an initial load of 0.1 g and no air flow. (12) Melting point, differential scanning differential calorimeter DSC manufactured by Seiko Instruments Inc.
Using a 210 type, about 7 mg of a sample was evaluated from the endothermic peak temperature when measured from room temperature at a temperature rising rate of 10 ° C./min under a nitrogen stream.

[0031]

Example 1 Melt index (measurement load 5 kg, 1
90 ° C.) 24% by weight of polyethylene of 0.25 g / 10 min, and melt index of 0.5 g / 10 min
Of 6% by weight of polypropylene, a viscosity of 75.8c
70 wt% of liquid paraffin of St (37.8 ° C.) was added, and the mixture was supplied to a film forming apparatus equipped with a 30 m / mφ twin-screw extruder and a T die of 650 m / m width to obtain a sheet-shaped raw material having a thickness of 290 μm. A film was obtained. The film was rolled to a thickness of 120 μm with a rolling roll at 95 ° C., immersed in 1,1,1-trichloroethane for 20 minutes, liquid paraffin was extracted and removed, and dried to obtain a rolled film. Furthermore, the membrane was stretched 3 times in the machine direction under the conditions of a temperature of 120 ° C. and a speed of 3000 mm / min using a biaxial stretching tester to obtain a microporous membrane.

The membrane obtained as described above had the properties shown in Table 1, high strength, low electric resistance and small pore size. When the melting point was measured, the peak at 138 ° C., which is the melting point of polyethylene, and the melting point of polypropylene, 16
A peak at 1.7 ° C is observed. The melt index used (measurement load 5 kg, 190 ° C) 0.25 g /
The viscosity average molecular weight of polyethylene for 10 minutes was 350,000.

Further, when the molecular weight of the obtained film was measured by GPC measurement, a chart as shown in FIG. 3 was obtained, and the fraction of the whole system having a molecular weight of 1,000,000 or less was 92 wt.
%Met. Further, when the impedance of the obtained microporous membrane was measured, the impedance transition temperature was 141 ° C. as shown in FIG. 4, and the impedance at 180 ° C. was 20 KΩ or more, which is 500 times or more the impedance at 25 ° C. It was The high temperature characteristics of this film are 11
The load reached its maximum at 5.4 ° C., and then the load decreased, but the film remained a non-porous film even at 175 ° C.

[0034]

[Example 2] A microporous membrane was obtained in the same manner as in Example 1 except that the speed was 300 mm / min. As shown in Table 1, the obtained film showed lower electric resistance by decreasing the stretching speed. The impedance at 25 ° C. was 12Ω, and the impedance transition temperature was 141 ° C. as in Example 1, and was 20 KΩ or more at 180 ° C.

[0035]

Example 3 Melt index (measurement load: 5 kg, 1
90 ° C.) 0.25 g / 10 min polyethylene 18% by weight, melt index (measurement load 2.16 kg, 1
90 ° C.) A mixture of 6% by weight of polyethylene having 5 g / 10 min and 6% by weight of polypropylene having melt index of 0.5 g / 10 min has a viscosity of 75.8 cSt (3
7.8 ° C) 70% by weight of liquid paraffin,
The m / mφ twin-screw extruder was supplied to a film forming apparatus equipped with a 650 m / m width T die to obtain a sheet-like raw film having a thickness of 290 μm. After that, a microporous membrane was obtained in the same manner as in Example 1.

When the impedance of the obtained microporous membrane was measured, the impedance transition temperature was 135 ° C. as shown in FIG. 4, and the impedance was 20 even at 180 ° C.
It remained overscaled above 0 KΩ.

[0037]

Comparative Example 1 The microporous membrane was prepared in the same manner as in Example 2 except that the raw film of Example 1 obtained by extrusion molding was stretched 10 times without rolling, extracted and dried. Got The film obtained as described above has a low machine direction elastic modulus,
The film strength was insufficient.

[0038]

Example 4 Polyethylene, polypropylene, and liquid paraffin as described in Example 1, 28.7 and 65% by weight, respectively, were applied to a film forming apparatus in which a 650 m / m width T die was attached to a 30 m / mφ twin-screw extruder. It was supplied to obtain a sheet-shaped raw film having a thickness of 370 μm. The film was rolled to a thickness of 150 μm with a rolling roll at 95 ° C., and then the extraction described in Example 1,
After the drying process, the film was stretched 4.5 times in the machine direction to obtain a microporous membrane.

The impedance at 25 ° C. was 25Ω and the impedance transition temperature was 141 ° C. and 20 KΩ or more even at 180 ° C. The film obtained as described above had the properties shown in Table 1.

[0040]

Comparative Example 2 Melt index 0.80 g / 10 mi
n of polyethylene, 18% by weight of ultra-high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, and 6% by weight of polypropylene described in Example 1, a viscosity of 75.
70 wt% of liquid paraffin of 8 cSt (37.8 ° C.) was added, and the mixture was supplied to a film forming apparatus equipped with a 30 m / mφ twin-screw extruder and a 650 m / m width T die, and extruded into a sheet. The obtained molded product had a shark's-eye shape and lacked film-forming properties.

The melt index used was 0.80.
The viscosity average molecular weight of polyethylene of g / 10 min was 200,000.

[0042]

Comparative Example 3 Celgard K, which is a polyethylene porous film
-878 and Celgard 2 which is a polypropylene porous membrane
What was simply overlapped with 500 was evaluated as a separator. When the impedance was measured, the impedance was lowered between the melting point of polyethylene and the melting point of polypropylene as shown in FIG.

[0043]

Comparative Example 4 Melt Index (Measurement load 2.16k
g, 190 ° C) 0.8g / 10min polyethylene 5
0% by weight, and melt index 14 g / 10 mi
Using 50% by weight of polypropylene of n melt-kneaded, the same method as in Example 1 of JP-A-4-206257 was performed as shown below.

A melt-kneaded product of polyethylene and polypropylene was extruded from a T-die having a die temperature of 240 ° C. to a thickness of 27.
A long film-like product having a thickness of μ was obtained. This film-like material was heated at a temperature of 120 ° C. for 60 minutes to anneal, then uniaxially stretched at a temperature of 25 ° C. in a longitudinal direction so that a stretching ratio was 35%, and then at a temperature of 120 ° C. in the same direction as described above. Direction was uniaxially stretched so that the stretching ratio was 65% to make it porous, and further heated at 120 ° C. for 1 minute for heat setting to obtain a microporous membrane. The length in the stretching direction was not changed during heat setting.

When the impedance of this film is measured, FIG.
As shown in Fig. 4, the impedance transition temperature was 132 ° C, but the impedance lowering temperature was as low as 165 ° C.

[0046]

[Table 1]

[0047]

EFFECT OF THE INVENTION According to the polyolefin microporous membrane of the present invention, in terms of performance, it has a high elastic modulus, a low electric resistance, a small pore size, excellent workability and productivity, and low internal resistance. However, since it has a low impedance transition temperature and a high impedance lowering temperature, it can be applied as a battery separator that is also reliable in terms of safety.

[Brief description of drawings]

FIG. 1 is a schematic view of the assembly of the microporous membrane of the present invention for measuring the electrical resistance.

FIG. 2 is a diagram showing an impedance measuring device defined in the present invention.

FIG. 3 is a chart of GPC measurement results of Example 1.

FIG. 4 is a chart of impedance measurement results of Example 1 and Example 3.

5 is a chart of impedance measurement results of Comparative Example 3 and Comparative Example 4. FIG.

[Explanation of symbols]

 1A, 1B Ni foil 2A, 2B Glass plate 3 Separator 4 Case 5 Thermocouple 6 Teflon tape 8 Impedance measuring device 9 Recorder

Claims (1)

[Claims] 1. A mixture of high molecular weight polyethylene and high molecular weight polypropylene comprising 3000 kgf / cm ^2.
It is a microporous membrane having the above machine direction elastic modulus and a porosity of 40 to 80%, and has an impedance transition temperature of 500 times or more of the impedance at 25 ° C in a temperature range of 95 ° C or more and 150 ° C or less. Have and 17
A polyolefin microporous membrane having an impedance lowering temperature at which the impedance is lower than 500 times the impedance at 25 ° C at a temperature of 5 ° C or higher.