JP6295641B2 - Microporous membrane and separator using the same - Google Patents

Description

  The present invention relates to a microporous membrane and a separator using the same, and more particularly to a microporous membrane having a fine and uniform pore diameter and excellent tensile strength and permeability and a separator using the same.

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

  Since the separator for lithium ion batteries is wound while applying high tension, it is desirable that the separator has excellent mechanical strength in the MD direction. On the other hand, if the pore diameter is not uniform, local dielectric breakdown is likely to occur and the withstand voltage performance is reduced, and the air permeability (air resistance) and mechanical strength are locally biased. Therefore, partial short-circuiting due to dendrite generation / growth tends to occur. However, the mechanical strength of the microporous membrane and the uniformity of the pore size distribution are in a contradictory relationship, and it has been difficult to efficiently produce a thin film separator excellent in these balances.

  For example, in Patent Document 1, after melt-kneading ultrahigh molecular weight polyethylene and high-density polyethylene and simultaneous biaxial stretching, heat setting is performed at a predetermined temperature, and then the solvent is removed to produce a microporous film. It is described. However, although this microporous membrane has high permeability and low thermal shrinkage, the tensile strength is not always sufficient.

  Patent Document 2 discloses that a polyethylene resin is melt-kneaded and simultaneously biaxially stretched, and further stretched in the TD direction, thereby increasing the tensile strength in the TD direction, further compressing resistance, electrolyte injection property, mechanical Manufacturing a microporous membrane excellent in mechanical properties, permeability and heat shrinkage resistance is described. However, this microporous membrane has a structure having relatively fine pores and relatively coarse pores, and the variation in pore diameter is large. As a result, there have been various problems such as the above-mentioned dielectric breakdown easily occurring.

International Publication No. 2000/020492 JP 2008-081513 A

  Accordingly, an object of the present invention is to provide a microporous membrane that is a thin film and has a uniform fine pore diameter, tensile strength, and high permeability and excellent physical property balance.

In order to solve the above problems, a microporous membrane and a separator according to the present invention are characterized by the following.
(1) When the pore diameter showing the maximum peak of the pore diameter distribution in the pore diameter distribution curve obtained from the pressure change of the air flow rate in the dry state and the wet state is Dp (nm), the pore diameter is 0.9 Dp to 1.1 Dp. The total value of the pore diameter distribution of a certain hole is 75% or more of the whole hole, the maximum pore diameter is 45 nm or less, the MD direction tensile strength S _MD is 1000 to 3500 kgf / cm ² , and the MD direction tensile strength S _A microporous membrane having a ratio S _MD / S _TD of a tensile strength S _{TD in} the _MD and TD directions of 1.1 to 2.5.
(2) In the pore size distribution curve, the region where the pore size distribution is 10% or more is in the range of the pore size of 20 to 30 nm, and the air resistance when the film thickness is 20 μm is 1000 sec / 100 mL or less. The microporous membrane of (1) above.
(3) The microporous film according to (1) or (2), wherein the average film thickness is 20 μm or less.
(4) The microporous film according to any one of (1) to (3), wherein the dielectric breakdown voltage is 3.2 to 4.0 kV when the film thickness is 20 μm.
(5) The microporous film according to any one of (1) to (4), wherein the puncture strength when the film thickness is 20 μm is 200 gf / 20 μm or more.
(6) The microporous film according to any one of (1) to (5), wherein the porosity is 20 to 80%.
(7) A separator using the microporous film of any one of (1) to (6) above.

  Since the microporous membrane of the present invention has a smaller pore size and a uniform pore size distribution as compared with the prior art, there is no unevenness in withstand voltage performance, there is little deviation in air permeability and mechanical strength, and resistance to dendrites is high. Further, by using such a homogeneous microporous membrane as a battery separator, it is possible to prevent battery malfunction due to non-uniformity of the membrane and improve the yield rate.

  Since the microporous membrane of the present invention has excellent MD tensile strength, the membrane is not easily broken even when high tension is applied, and can be suitably used for applications requiring high durability. Moreover, by using such a microporous membrane having excellent strength as a battery separator, it is possible to prevent a short circuit during battery production or use.

  Furthermore, by making the microporous membrane of the present invention a thin film having a thickness of 20 μm or less, it can be suitably used for applications that require both miniaturization and high durability. Further, when such a thin and excellent microporous membrane is used as a battery separator, the thin film separator can be wound while being pressed with a high tension, thereby further increasing the capacity of the battery.

It is a related figure which shows the pore diameter distribution curve of the microporous film obtained by the Example and comparative example of this invention. It is a schematic diagram which shows the example of the aeration curve of a dry sample and a wet sample. It is a schematic diagram showing an example of a pore diameter distribution curve, (A) is an example when the total value of the pore diameter distribution of the pores having a diameter in the range of 0.9 Dp to 1.1 Dp is 75% or more, (B) represents an example when the region where the pore size distribution is 10% or more is in the range of 20 to 30 nm in diameter.

  The microporous membrane of the present invention has a pore diameter (pore diameter) showing the maximum peak of the pore diameter distribution in a pore diameter distribution curve obtained from a pressure change of the air flow rate in a dry state and a wet state when Dp (nm) is set. The total pore diameter distribution of pores having a pore diameter of 0.9 Dp to 1.1 Dp is 75% or more of the whole pores. Since such a microporous membrane has a uniform pore size distribution as compared with the prior art, the air permeability and mechanical strength are less biased and the resistance to dendrites is high. Further, by using such a homogeneous microporous membrane as a battery separator, it is possible to prevent battery malfunction due to non-uniformity of the membrane and improve the yield rate. In this specification, the “pore diameter showing the maximum peak” is the pore diameter of the most distributed pore among the ratio of the total pores distributed in the microporous membrane, and is determined from the pore diameter distribution curve. it can. The point with the highest pore size distribution value (%) is taken as the maximum peak.

  The pore diameter distribution curve can be measured by the following method using a porometer. First, a porometer is used for each of a dry sample (hereinafter also simply referred to as “dry sample”) and a wet sample (hereinafter also simply referred to as “wet sample”) filled with the measurement liquid in the pores. The relationship between the air pressure and the air flow rate is measured, and the aeration curve (Dry Curve) of the dry sample and the aeration curve (Wet Curve) of the wet sample are obtained as shown in FIG.

The wet sample in which the measurement liquid is filled in the pores shows the same characteristics as the capillary filled with the liquid. When the wet sample is set on the porometer and the air pressure is gradually increased, the air pressure overcomes the surface tension of the measurement liquid in the pores in order from the large diameter pores, and the measurement liquid is pushed out of the pores. Along with this, the air flow rate gradually increases, and the sample finally becomes dry. Therefore, the pore diameter can be calculated by measuring the pressure when the liquid is pushed out of the pore. Assuming that the shape of the pores is substantially cylindrical, the conditions for air of pressure P to enter the pores of diameter D are that the surface tension of the measurement liquid is γ and the contact angle of the measurement liquid is θ. It is expressed by the Washburn formula shown in the following formula 1.
PD = 4γ cos θ (Formula 1)
In particular, a measurement point (measurement point indicating the maximum pore diameter) at which bubble generation is first detected is referred to as a bubble point. As a standard measurement method of a bubble point, the method as described in ASTM F316-86 is mentioned, for example.

  The average pore size of the microporous membrane is determined based on the half dry method defined in ASTM E1294-89 using the above-mentioned dry sample aeration curve (Dry Curve) and wet sample aeration curve (Wet Curve). be able to. As shown in FIG. 2, the average flow rate is the pressure at the point where the half-curve curve (Half-Dry Curve) of the dry sample aeration curve (Dry Curve) and the wet sample aeration curve (Wet Curve) intersect. An average pore diameter (Mean Flow Pore Diameter) of the microporous membrane is calculated by obtaining the mean flow diameter pressure and substituting this average flow diameter pressure into the above (Equation 1).

On the other hand, when the air flow rate of the wet sample at pressure P _j is F _{w, j} and the air flow rate of the dry sample is F _{d, j} , the cumulative filter flow rate (CFF: Cumulative Filter Flow, unit:%) and the pore size distribution ( PSF (Pore Size Frequency, unit:%) is calculated by the following formula, respectively.
CFF = [(F _{w, j} / F _{d, j} ) × 100] (Expression 2)
PSF = (CFF) _{j + 1} − (CFF) _j (Formula 3)

  By combining the above (Formula 1) to (Formula 3), a pore size distribution curve showing the relationship between the pore diameter D and the pore size distribution PSF is obtained based on the pressure change of the air flow rate in the dry state and the wet state. be able to. An example of such a pore size distribution curve is shown in FIG. In the microporous membrane of the present invention, when the pore diameter showing the maximum peak in the pore diameter distribution curve is Dp (nm), the total pore diameter distribution of pores having a pore diameter of 0.9 Dp to 1.1 Dp is It is characterized by being 75% or more of the whole. In addition, it is preferable that the total value of the pore diameter distribution of the hole whose hole diameter is 0.9 Dp-1.1 Dp is 80% or more of the whole hole, and it is more preferable that it is 85% or more of the whole hole. The higher the total value, the more uniform pore diameter is present, and the less the deviation of air permeability and mechanical strength. The higher this value, the better.

  In the microporous membrane of the present invention, the maximum pore diameter measured based on ASTM F316-86 (bubble point method) is usually 45 nm or less, preferably 40 nm or less, and particularly preferably 38 nm or less. By setting the maximum hole diameter to 45 nm or less, partial short-circuiting can be prevented and excellent voltage resistance can be ensured. In the present specification, the “maximum pore size” indicates the maximum pore size among all pores distributed in the microporous membrane, and can be measured by a bubble point method.

  In the microporous membrane of the present invention, the average pore diameter measured based on ASTM E1294-89 is preferably 10 to 40 nm, more preferably 12 to 35 nm, and 14.5 to 24.5 nm. It is particularly preferred. By setting the average pore diameter to 40 nm or less, excellent voltage resistance can be ensured. Moreover, the microporous film excellent in the permeability can be obtained by setting the average pore diameter to 10 nm or more.

In the microporous membrane of the present invention, MD direction tensile strength (hereinafter may be simply referred to as "MD tensile _{strength") S MD} is usually 1000~3500kgf / cm ^2, preferably at 1200~2900kgf / cm ² Yes, more preferably 1500 to 2800 kgf / cm ² . Further, MD direction tensile strength _{S MD} and TD directions of the tensile strength (hereinafter may be simply referred to as "TD tensile _strength") ratio _S MD _{/ S TD} of _{S TD} is usually 1.1 to 2.5 Yes, preferably 1.3 to 2.5, more preferably 1.4 to 2.5. Since such a microporous membrane has excellent MD tensile strength, the membrane is not easily broken even when high tension is applied, and can be suitably used for applications requiring high durability. In addition, when such a microporous membrane with excellent strength is used as a battery separator, it is possible to prevent short-circuiting during battery production and use, and it is possible to wind the separator by applying high tension. High capacity can also be achieved. In addition, as a measuring method of tensile strength (tensile breaking strength), the method as described in ASTM D882 can be employ | adopted, for example.

  In the pore size distribution curve, the region where the pore size distribution is 10% or more is preferably in the range of a pore size of 20 to 30 nm as illustrated in FIG. Since such a microporous membrane has a small variation in pore size distribution, it has higher resistance to dendrites, and therefore, it is possible to further improve the yield rate of battery separators.

The microporous membrane of the present invention preferably has an air resistance of 1000 sec / 100 mL or less when the film thickness is 20 μm. Here, the air resistance when formed into a 20μm thickness, the microporous film having a thickness _{T 1 (μm),} the air resistance was measured in accordance with JIS _P 8117 (2009) P ₁ Is the air resistance P ₂ calculated by the formula: P ₂ = (P ₁ × 20) / T ₁ . In the following description, “air permeability resistance” is used to mean “air resistance when the film thickness is 20 μm” unless otherwise specified.

  When the microporous membrane is used as a battery separator, the air permeability resistance is preferably a low value. When the air resistance exceeds 1000 sec / 100 mL, the output of the battery may be small. In addition, as an upper limit of air permeability resistance, 600 sec / 100 mL or less is more preferable, and 400 sec / 100 mL or less is especially preferable. The lower limit of the air resistance is not particularly limited, but from the viewpoint of ensuring the shutdown performance, the air resistance is preferably 100 sec / 100 mL or more.

  The average film thickness of the microporous membrane is preferably 20 μm or less, more preferably less than 18 μm, and particularly preferably less than 15 μm. A method for measuring the average film thickness will be described later. By using a microporous film that is thin and has high MD strength, it can be suitably used for applications that require both space saving and high durability. Further, when such a microporous membrane is used as a battery separator, the thin film separator can be wound while being pressed with a high tension, so that it is possible to further increase the capacity of the battery.

The microporous film preferably has a withstand voltage (dielectric breakdown voltage) of 3.2 to 4.0 kV, more preferably 3.3 to 3.9 kV when the film thickness is 20 μm. It is particularly preferable that the voltage is 4 to 3.8 kV. Here, the withstand voltage when the film thickness is 20 μm means that the dielectric breakdown voltage in the microporous film having the film thickness T ₁ (μm) is V ₁ (kV): V ₂ = (V ₁ × refers to the breakdown voltage V ₂ calculated by 20) / T _1, when the measurement was performed a plurality of times shall refer to their mean values. When the measurement is performed a plurality of times, the minimum withstand voltage when the film thickness is 20 μm is preferably 2.8 kV or more, and the maximum withstand voltage when the film thickness is 20 μm is It is preferable that it is 3.6 kV or more.

From the viewpoint of improving durability, the microporous membrane preferably has a puncture strength of 200 gf / 20 μm or more, more preferably 500 gf / 20 μm or more, and 600 gf / 20 μm or more when the film thickness is 20 μm. More preferably, it is more preferably 650 gf / 20 μm or more. Here, the piercing strength when formed into a 20μm film thickness, when the puncture strength of the microporous film having a thickness _{T 1} ([mu] m) (maximum load) and _{L 1, L 2 = (L} 1 × 20) / The puncture strength L ₂ calculated by the formula of T ₁ is indicated.

  The porosity of the microporous membrane is preferably 20 to 80%. When the porosity is less than 20%, the microporous membrane does not have good air permeability. On the other hand, if it exceeds 80%, when the microporous membrane is used as a battery separator, the mechanical strength becomes insufficient, and the risk of a short circuit of the electrode increases. The porosity is more preferably 30 to 65%, particularly preferably 40 to 45%.

  As a material constituting the microporous membrane, polyolefin is preferably used, and polyethylene is more preferably used. The polyethylene content is preferably 90% by weight or more, more preferably 95% by weight or more, and particularly preferably 99% by weight or more, based on 100% by weight of the entire polyolefin. By setting the polyethylene content to 90% by weight or more, physical property unevenness due to a decrease in homogeneity can be prevented, and a uniform fine pore diameter can be realized.

When polyethylene is used as the material for the microporous membrane, the polyethylene is polyethylene having a weight average molecular weight of 5.0 × 10 ^{5 to} 9.0 × 10 ⁵ (high-density polyethylene; hereinafter, also simply referred to as “HDPE”). A composition with polyethylene having a weight average molecular weight of 1.5 × 10 ^{6 to} 3.0 × 10 ⁶ (ultra high molecular weight polyethylene; hereinafter also simply referred to as “UHMWPE”) is preferable. By using HDPE and UHMWPE having a relatively large molecular weight as main components (preferably, by using substantially only HDPE and UHMWPE as polyolefin), a microporous film having excellent strength can be provided. The content of HDPE is preferably 90% by weight or less, more preferably 80% by weight or less, and particularly preferably 75% by weight or less, based on 100% by weight of the entire polyethylene composition. Further, the content of UHMWPE is preferably 10% by weight or more, more preferably 20% by weight or more, and particularly preferably 25% by weight or more, based on 100% by weight of the entire polyethylene composition.

  When using a composition of HDPE and UHMWPE as described above, the molecular weight distribution of HDPE (ratio of weight average molecular weight Mw to number average molecular weight Mn, ie Mw / Mn) is preferably in the range of 3-20, The molecular weight distribution (Mw / Mn) of UHMWPE is preferably in the range of 3-20. If the molecular weight distribution is too narrow, the amount of high molecular weight components increases, and the fluidity during extrusion molding decreases. Moreover, there exists a possibility of leading to a membrane breakage due to a decrease in stretchability during film formation. On the other hand, if the molecular weight distribution is too wide, the proportion of low molecular weight components increases and it may be difficult to ensure strength.

Although the manufacturing method of the said microporous film is not specifically limited, From the viewpoint of ensuring excellent MD tensile strength, after performing simultaneous biaxial stretching in the MD direction and TD direction, further stretching in the MD direction. It is preferable to carry out. That is, the method for producing a microporous membrane preferably includes the following steps (1) and (2) from the viewpoint of improving the strength.
(1) The step of simultaneously biaxially stretching the sheet extruded from the die in the MD direction and the TD direction at a stretching temperature of 100 to 120 ° C. (2) The simultaneously biaxially stretched sheet is stretched at a temperature of 100 to 120 ° C. Step of additional stretching in the MD direction at a stretch ratio of 1.3 to 2.0 times By carrying out these steps during production, the strength is higher than that of a conventional polyolefin microporous membrane having the same stretch ratio. Can be obtained. The additional stretching ratio in the MD direction is more preferably 1.4 to 2.0 times, and particularly preferably 1.6 to 2.0 times.

  Specific examples of the present invention will be described below with reference to examples, but the present invention is not limited thereto.

(Measuring method)
1. Film thickness The thickness of the microporous film was measured at a randomly selected MD position using a contact-type thickness meter. Measurements were taken at 5 mm intervals over a distance of 30 cm at points along the TD (width) of the membrane. And the measurement along said TD was performed 5 times and the arithmetic mean was made into the thickness of a sample.

2. Air permeability resistance The air resistance resistance P ₁ was measured with a gas permeability meter (Asahi Seiko Co., Ltd., EGO-1T) with respect to a microporous film having a film thickness T ₁ . Further, by the _formula of _{P 2 = (P 1 × 20} ) / T 1, to calculate the air resistance of _{P 2} when a 20μm thickness.

3. Porosity The porosity is calculated from the mass w1 of the microporous membrane and the mass w2 of the same size non-porous membrane made of the same polyethylene composition as the microporous membrane, and the porosity (%) = (w2-w1). ) / W2 × 100.

4). Pore size distribution The pore size distribution curve was calculated by the following method using a palm porometer (model number: CFP-1500A) manufactured by PMI as a measuring device and Galwick (15.9 dyn / cm) as a measuring solution. First, for each of the dry sample and the wet sample, the relationship between the air pressure and the air flow rate was measured using a porometer, and as shown in FIG. 2, the dry sample aeration curve (Dry Curve) and the wet sample aeration curve (Wet Curve). ) Cumulative filter flow rate (CFF, unit:%) and pore diameter based on Equation 1 and Equation 2 below, where F _{w, j} is the air flow rate of the wet sample at pressure P _j and F _{d, j} is the air flow rate of the dry sample. Distribution (PSF, unit:%) was calculated.
CFF = [(F _{w, j} / F _{d, j} ) × 100] (Equation 1)
PSF = (CFF) _{j + 1} − (CFF) _j (Formula 2)
The surface tension of the measurement liquid is γ, the contact angle of the measurement liquid is θ, and the pore diameter D corresponding to the pressure P is calculated by the following equation 3, and the relationship between the pore diameter D and the pore diameter distribution PSF is calculated. The pore size distribution curve shown was obtained.
PD = 4γ cos θ (Formula 3)
And the total value of the pore diameter distribution of the hole diameter Dp (nm) which shows the maximum peak in the obtained pore diameter distribution curve, and the hole diameter in the range of 0.9Dp-1.1Dp was calculated | required. Further, it is determined whether or not the region having a pore size distribution of 10% or more is in the range of the pore size of 20 to 30 nm. If the pore size is in the range of 20 to 30 nm, ○ (good); (Out of range).

5. Maximum pore diameter The maximum pore diameter of the microporous membrane was measured by a method (bubble point method) based on ASTM F316-86. In addition, a palm porometer (model number: CFP-1500A) manufactured by PMI was used as a measuring instrument, and Galwick was used as a measuring solution.

6). Average pore diameter The average pore diameter of the microporous membrane was measured by a method (half dry method) based on ASTM E1294-89. In addition, a palm porometer (model number: CFP-1500A) manufactured by PMI was used as a measuring instrument, and Galwick was used as a measuring solution.

7). Puncture strength A needle with a diameter of 1 mm having a spherical surface (curvature radius R: 0.5 mm) at the tip is pierced into a microporous film with an average film thickness T ₁ (μm) at a speed of 2 mm / sec, and the maximum load L ₁ (penetration) The load immediately before the measurement, unit: gf) was measured, and the puncture strength L ₂ (gf / 20 μm) when the film thickness was 20 μm was calculated by the formula L ₂ = (L ₁ × 20) / T ₁ .

8). Tensile strength MD tensile strength S _MD and TD tensile strength S _TD were measured by a method based on ASTM D882 using a strip-shaped test piece having a width of 10 mm.

9. The withstand voltage performance 150mm square on aluminum plates, placed a microporous film having a thickness _{T 1} which is cut out in a diameter of 60 mm, at a cylinder electrode brass 50mm diameter thereon, Kikusui Electronics Ltd. TOS5051A resistance A voltage tester was connected. 0.2 kV / sec went energized at a rate of rise of the read value _{V 1} of the when dielectric breakdown, conversion _equation: V 2 = based on _{(V 1 × 20) / T} 1, per thickness 20μm It was calculated withstand voltage V ₂ of. Measurement of withstand voltage V ₂ is performed 15 times, the maximum value, to obtain a mean value and a minimum value.

Example 1
(A) 30% by weight of ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight Mw1 of 2.89 × 10 ⁶ and a molecular weight distribution Mw1 / Mn1 of 5.28; and (b) a weight average molecular weight Mw2 of 5. 30 parts by weight of a polyethylene composition consisting of 70% by weight of high-density polyethylene (HDPE) having a molecular weight distribution Mw2 / Mn2 of 4.81 and 72 × 10 ⁵ is charged into the twin-screw extruder. 70 parts by weight of liquid paraffin were supplied from the side feeder, melt-kneaded at 210 ° C. and 200 rpm, and a polyethylene resin solution was prepared in an extruder. Subsequently, the polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and a gel-like molded product was formed while taking the obtained sheet-shaped extrudate with a 25 ° C. cooling roll. The obtained gel-like molded product was subjected to simultaneous biaxial stretching at 115 ° C. so as to be 5 × 5 times, and then stretched in the MD direction at a temperature of 115 ° C. and a magnification of 1.3 times. The stretched gel-like sheet was immersed in methylene chloride at 25 ° C. to remove liquid paraffin. Thereafter, the film was air-dried at room temperature, and heat-treated at 125 ° C. for 10 minutes to produce a polyethylene microporous film.
The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. This microporous membrane had a small pore size, a sharp pore size distribution, and a small variation in pore size. Moreover, the obtained microporous film had high MD tensile strength and had excellent permeability.

(Example 2)
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the draw ratio was changed to 1.4 times in the MD direction after 5 × 5 times stretching. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. This microporous membrane had a small pore size, a sharp pore size distribution, and a small variation in pore size. Moreover, although the obtained microporous film was a thin film, it had high MD tensile strength and had excellent permeability and high withstand voltage performance.

(Example 3)
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio in the MD direction after 5 × 5 stretching was 1.6 times. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. This microporous membrane had a small pore size, a sharp pore size distribution, and a small variation in pore size. Moreover, although the obtained microporous film was a thin film, it had high MD tensile strength and had excellent permeability.

Example 4
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio in the MD direction after 5 × 5 stretching was 2.0 times. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. This microporous membrane had a small pore size, a sharp pore size distribution, and a small variation in pore size. Moreover, although the obtained microporous film was a thin film, it had high MD tensile strength and had excellent permeability and high withstand voltage performance.

(Comparative Example 1)
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching in the MD direction was not performed after 5 × 5 stretching. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. Although this microporous membrane had a small pore diameter, MD tensile strength was not sufficient.

(Comparative Example 2)
Liquid paraffin was removed and dried at room temperature after 5 × 5 stretching, and then simultaneous biaxial stretching was performed under the operating conditions of a temperature of 132 ° C., a magnification of 1.8 times in the MD direction, and a magnification of 1.4 times in the TD direction. Otherwise, a polyethylene microporous membrane was obtained in the same manner as in Example 1. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. Although this microporous membrane had high MD tensile strength, the pore diameter was large.

(Comparative Example 3)
A polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the draw ratio in the MD direction after 5 × 5 drawing was 1.1 times. The membrane characteristics of the obtained microporous membrane are shown in Table 1 and FIG. Although this microporous membrane had a small pore size, the pore size variation was slightly large.

  In the present application, the notation “A to B” indicating a numerical range is used in a range including the lower limit value A and the upper limit value B, that is, “A or more and B or less”.

  The microporous membrane according to the present invention can be particularly suitably used as a lithium ion battery separator.

Claims (7)

A microporous membrane made of polyolefin, and when the pore diameter showing the maximum peak of the pore diameter distribution in a pore diameter distribution curve obtained from the pressure change of the air flow rate in a dry state and a wet state is Dp (nm), the pore diameter is and the sum of the pore size distribution of a 0.9Dp~1.1Dp holes more than 75% of the total pore, maximum pore size is not more than 45 nm, MD direction tensile strength _{S MD} is at 1000~3500kgf / cm ² A microporous membrane having a ratio S _MD / S _TD of a tensile strength S _{MD in the MD} direction and a tensile strength S _{TD in} the TD direction of 1.1 to 2.5.   2. The microporous membrane according to claim 1, wherein a region in which the pore size distribution is 10% or more in the pore size distribution curve is in a range of 20 to 30 nm in pore size, and the air resistance is 1000 sec / 100 mL or less.   The microporous membrane according to claim 1 or 2, wherein the average film thickness is 20 µm or less.   The microporous film according to any one of claims 1 to 3, wherein a dielectric breakdown voltage when the film thickness is 20 µm is 3.2 to 4.0 kV.   The microporous membrane according to any one of claims 1 to 4, wherein a puncture strength when the film thickness is 20 µm is 200 g / 20 µm or more.   The microporous membrane according to any one of claims 1 to 5, wherein the porosity is 20 to 80%.   The separator which uses the microporous film in any one of Claims 1-6.

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