JP6729366B2 - Polyolefin microporous membrane and coating substrate using polyolefin microporous membrane - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polyolefin microporous film and a coating substrate using the same.

Conventionally, microporous membranes have been generally used in applications such as battery separators, capacitor films, and filters. Among them, various properties such as heat resistance, strength, and electrolyte holding power are required for battery separator applications. As a method of realizing such required characteristics, a method of using a film obtained by coating a microporous film is becoming mainstream.

Lithium-ion batteries are expected not only to be used in small mobile devices such as mobile phones and laptop computers, but also to be used in automobiles in the future, and there is a demand for improved productivity. From the viewpoint of productivity, it is desired that the strength in the machine direction (or the longitudinal direction, hereinafter MD direction) is strong in order to carry out high-speed conveyance, but if the strength in the MD direction alone is increased too much, the anisotropy of the microporous membrane will be increased. Is so strong that it tends to tear during transport, making it difficult to control the tension during coating. Therefore, it is necessary to appropriately adjust the strength balance in the MD direction and the width direction (hereinafter, TD direction).

In particular, in-vehicle lithium-ion batteries are required to have high energy density, high capacity, and high output, and as a result, separators are required to have higher transparency and safety. .. Therefore, it is essential to form (coat) a functional layer having various functions on the microporous membrane. However, if the coating material closes the pores of the microporous membrane during the coating, the battery output will be reduced due to the reduced permeability, and the battery characteristics will be degraded. Therefore, it is necessary to perform coating without deteriorating the performance of the microporous membrane. Furthermore, if the coating is not applied uniformly, the coating unevenness leads to the unevenness of the physical properties, so it is essential to apply the coating material uniformly to the base material.

For example, in Patent Document 1, a microporous film is manufactured by wet simultaneous different-ratio stretching, and the strength balance in MD direction and TD direction (ratio of elastic modulus in length direction to elastic modulus in width direction) is adjusted. It has been shown that a good slit property can be obtained. Further, it is described that good withstand voltage characteristics can be obtained by adjusting the maximum pore size.

Further, in Patent Document 2, a microporous membrane is manufactured by mixing a polyolefin, a plasticizer, and inorganic particles and sequentially stretching. The microporous membrane obtained by the above method has a uniform pore size, and has achieved both good withstand voltage characteristics and excellent permeability and strength.

Further, in Patent Document 3, a heat-resistant porous layer is formed by coating one or both sides of a base material (microporous film) and adjusting the density and viscosity of the coating material and the pore size of the base material. The microporous membrane described above is described, and the technology capable of preventing a decrease in ionic conductivity is described. However, there is no description of a method for uniformly coating.

In all of these references, the stretching method and coating material are adjusted to achieve good transportability, permeability, and withstand voltage characteristics, and the deterioration of physical properties after coating is suppressed, but coating unevenness during coating is suppressed. The method to do is not described. In addition, various types of coating materials have been conventionally adjusted and coating methods have been used to achieve uniform coating, but all of these are only from the perspective of the coating material side. The performance required on the material side has not been clarified. In order to carry out high speed conveyance and uniform coating, there is a limit only by adjusting the coating material and the coating method. Therefore, in the future, it is considered that a microporous film that allows uniform coating from the viewpoint of the substrate side will be required.

JP, 2010-007053, A International Publication No. 2005-061599 JP, 2011-204587, A

As described above, in order to carry out high-speed transportation and uniform coating, it is necessary not only to adjust the coating material but also to provide a microporous film that can be uniformly coated from the viewpoint of the base material. However, in the prior art, the performance required for the base material is only the strength in the MD direction required for high-speed transportation, and the characteristics required for the base material (microporous membrane) for uniform coating. Yes it wasn't clear yet.

In view of the above reasons, it is an object of the present invention to provide a microporous film that can be uniformly coated.

As a result of intensive studies to solve the above problems, the present inventors have found that a polyolefin microporous film having a narrow pore size distribution (uniform pore size) and a small pore size can be applied uniformly. And has reached the present invention.

That is, the present invention is as follows.
(1) A polyolefin microporous membrane having a content of polyolefin having a molecular weight of 1.0×10 ⁶ or more of 20% by mass or more,
A microporous polyolefin membrane characterized in that, when a plurality of measurement regions for measuring the distribution of pore diameters are set, the measurement results obtained in each of these measurement regions satisfy the following relational expression (A).
σ(Dexp)÷Dp×100<24 (A)
However, each of the measurement regions has a size of a circle having a diameter of 3 cm, and is separated from each other along the TD direction,
When the width of the sample in the TD direction is 18 cm or more, the samples are arranged at four places at intervals of 5 cm (distance between circle centers),
When the width of the sample in the TD direction is less than 18 cm, the samples are arranged at three positions at 3 cm intervals (distance between circle centers). Further, σ(Dexp) is a standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is the mode value (pore diameter) of the pore size distribution obtained in each measurement region. Is the average value of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj))
(2) The polyolefin microporous membrane according to (1), wherein the σ(Dexp) is less than 0.00600.
(3) The polyolefin microporous membrane according to (1), wherein the σ(Dexp) is less than 0.00100.
(4) The polyolefin microporous membrane according to any one of (1) to (3), wherein the Dp is 0.010 to 0.024 μm.
(5) A polyolefin microporous membrane having a content of polyolefin having a molecular weight of 1.0×10 ⁶ or more of 20% by mass or more,
The ratio of the tensile strength in the MD direction and the tensile strength in the TD direction is 1.4 to 2.5,
A microporous polyolefin membrane characterized in that, when a plurality of measurement regions for measuring the distribution of pore diameters are set, the measurement results obtained in each of these measurement regions satisfy the following relational expression (B).
σ(Dexp)÷Dp×100<400 (B)
However, each of the measurement regions has a size of a circle having a diameter of 3 cm, and the measurement regions are arranged at three positions spaced apart from each other in the MD direction at intervals of 5 cm (distance between centers of circles). Further, σ(Dexp) is a standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is the mode value (pore diameter) of the pore size distribution obtained in each measurement region. Is the average value of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj))
(6) The polyolefin microporous membrane according to (5), wherein the Dp is 0.010 to 0.024 μm.
(7) A polyolefin microporous membrane having a content of polyolefin having a molecular weight of 1.0×10 ⁶ or more of 20% by mass or more,
The ratio of the tensile strength in the MD direction and the tensile strength in the TD direction is 1.4 to 2.5,
Separated from each other along the TD direction,
When the width of the sample in the TD direction is 18 cm or more, the samples are arranged at four places at intervals of 5 cm (distance between circle centers),
When the width of the sample in the TD direction is less than 18 cm, RTD=σ(Dexp)/calculated from the measurement results obtained in a plurality of measurement areas arranged at three positions at 3 cm intervals (distance between circle centers). Dp x 100,
RMD=σ(Dexp)÷Dp×100 calculated from the measurement results obtained in a plurality of measurement regions arranged at three locations at 5 cm intervals (distance between circle centers) along the MD direction. A microporous polyolefin membrane characterized by satisfying the following relational expression (C).
250<RTD+RMD <420...(C)
However, each of the measurement regions is the size of a circle having a diameter of 3 cm, σ(Dexp) is the standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is each It is a value obtained by averaging the mode values (pore diameters) of the pore diameter distribution obtained in the measurement region of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj))
(8) The polyolefin microporous membrane according to any one of (1) to (7), which has a tensile strength in the MD direction of 1600 to 4500 kgf/cm ² .
(9) A coating substrate comprising the polyolefin microporous membrane according to any one of (1) to (8).

The polyolefin microporous film of the present invention has a narrow pore size distribution and a dense pore size, so that coating unevenness of the coating material and coating wrinkles can be significantly suppressed, and therefore it can be suitably used as a coating base material. it can. Further, since the strength balance between the MD direction and the TD direction is adjusted, high speed conveyance is possible. Further, since it has good permeability and withstand voltage characteristics as a base material, it also has excellent characteristics as a battery separator.

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 which shows the example of a pore diameter distribution curve. It is a schematic diagram which shows a molecular weight distribution curve.

Preferred embodiments of the present invention will be described below with reference to the drawings.

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

The wet sample in which the measurement liquid is filled in the pores exhibits the same characteristics as a liquid-filled capillary tube. When the wet sample is set in the porometer and the air pressure is gradually increased, the pores having a large diameter are sequentially pressed, and the air pressure overcomes the surface tension of the measurement liquid in the pores and the measurement liquid is extruded from the pores. Along with this, the air flow rate gradually increases, and finally the sample becomes dry. Here, assuming that the shape of the pores is substantially cylindrical, the conditions under which the air having the pressure P enters the pores having the diameter D are that the surface tension of the measurement liquid is γ and the contact angle of the measurement liquid is θ. Is expressed by the Washburn equation shown in the following Equation 1.
P=(4γ cos θ)/D (Equation 1)
Therefore, the pore diameter can be calculated by measuring the pressure when the liquid is pushed out from the pores.

On the other hand, when the air flow rate of the wet sample at the 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 formulas, respectively.
CFF=[(F _w,j /F _d,j )×100] (Equation 2)
PSF=(CFF) _j+1− (CFF) _j (Equation 3)

By combining Equations 1 to 3 above, a pore diameter distribution curve showing the relationship between the diameter D of the pores and the pore diameter distribution PSF can be obtained based on the pressure change of the air flow rate in the dry state and the wet state. .. An example of such a pore size distribution curve is shown in FIG.

Various physical property values regarding pores can be obtained from the pore size distribution curve shown in FIG. For example, the pore diameter showing the mode value in the pore diameter distribution curve (pore diameter represented as Dp in FIG. 2, hereinafter also simply referred to as “peak pore diameter”) and the value of the pore diameter distribution corresponding to the pore diameter D _j (pore diameter D _When the frequency of _j ) is (PSF) _j , the pore diameter distribution D _exp calculated by the following equation 4 and the like can be mentioned.
D _exp =Σ{D _j ×(PSF) _j } (Equation 4)

However, the physical property values obtained from the pore size distribution curve are basically the values obtained by measuring one small sample. That is, such measurement of the pore size distribution is performed on a relatively small area. Therefore, in order to study the uniformity of the pore size distribution in the entire microporous membrane in more detail, samples (measurement areas) were sampled from multiple points in the microporous membrane, and the statistical values of the physical property values obtained for each sample were collected. It is desirable to verify the variation. The inventors of the present invention have found that in the coating substrate, the uniformity of the pore size affects the amount of coating unevenness. That is, the physical property values of a plurality of samples taken at predetermined intervals are measured, and when the average value or standard deviation thereof meets the required level, the coating unevenness is small, and the polyolefin is excellent as a coating base material. They have found that a microporous membrane can be obtained, and have reached the present invention. The present invention is particularly applicable to the MD direction by scraping the coating material or adjusting the coating amount by using blades such as a blade (doctor blade) and a knife such as a gravure coating method and a blade coating method. It is useful for the method of simultaneously applying the coating material in the TD direction.

The polyolefin microporous membrane of the present invention is characterized by having a uniform pore size.
In order to perform uniform coating, the average pore diameter (Dp) obtained when a plurality of pore diameter distributions (Dexp) and peak pore diameters of the microporous polyolefin membrane are separated from each other in the MD direction or the TD direction and measured. The relative value (%) (standard deviation/peak pore size) (variation coefficient) of the standard deviation (σ(Dexp)) of the peak pore size is important. The relative value is calculated by the following formula.
Relative value (%)=σ(Dexp)÷Dp×100 (Equation 5)

Specifically, measurement regions of arbitrary dimensions are provided at four equal intervals per 20 cm in the TD direction, and the pore size distribution is measured for each measurement region. Then, the peak pore size and the distribution (Dexp) are calculated from the pore size distributions obtained in these measurement areas. Then, the standard deviation (σ(Dexp)) of the distribution (Dexp) obtained in these measurement regions is calculated, and the average value (Dp) of the peak pore diameters is calculated. The relative value (variation coefficient B) obtained by substituting the standard deviation (σ(Dexp)) and the average value (Dp) into the above-mentioned (Formula 5) is less than 24% (the following formula (A) Meeting) is important.
σ(Dexp)÷Dp×100<24 Equation (A)
Further, the coefficient of variation B is more preferably less than 19%, further preferably less than 18%. When the relative value is less than 24%, the variation in the pore diameter in the TD direction becomes small, and the coating material evenly spreads on the substrate surface in the TD direction during coating. Further, when the relative value is less than 24%, the pore diameter is uniform, so that the content of the coating material does not differ between the portion having a large pore diameter and the portion having a small pore diameter, and unevenness is less likely to occur during coating. Further, when the relative value is less than 24%, the coating material is applied uniformly, so that heat is uniformly transferred in the TD direction to the base material in the drying step of the coating material, resulting in dry wrinkles and peeling of the coating material. Can be suppressed. In particular, dry wrinkles have a great effect because the coating material unevenness in the TD direction has a great influence. Therefore, it is important that the relative value is smaller than 24%. The smaller this relative value is, the more uniform the pore diameter in the TD direction is, which is preferable, but the lower limit is substantially about 5%.

In the polyolefin microporous membrane of the present invention, the standard deviation of the pore size distribution measured at four equal intervals per 20 cm in the TD direction is preferably less than 0.00600, more preferably less than 0.00450, and 0 More preferably, it is less than 0.00454. When the standard deviation of the expected value of the hole diameter in the TD direction is less than 0.00600, the hole diameter is uniform, so that the content of the coating material does not differ between the large hole portion and the small hole portion, resulting in uneven coating. It's hard to do. Furthermore, since the coating material is applied uniformly, heat is evenly transferred to the base material in the step of drying the coating material, and it is possible to suppress dry wrinkles and peeling of the coating material. The lower limit of the standard deviation in the TD direction in the expected value of the pore diameter is smaller, the smaller the variation of the pore diameter in the TD direction is, and the smaller the lower limit is. However, the lower limit is substantially 0.00100, but is less than 0.00100. You can have it.

The relative value of the standard deviation of the peak pore diameter in the TD direction to the average value of the peak pore diameters measured at four equal intervals per 20 cm in the TD direction (standard deviation of the peak pore diameter/average value of the peak pore diameter) (variation coefficient A) is 5 %, preferably 3% or less, more preferably 1% or less. When the relative value is less than 5%, the variation in the mode value of the pore size distribution in the TD direction becomes small and the pore size becomes uniform. Therefore, the content of the coating material does not differ between the portion having a large pore diameter and the portion having a small pore diameter, and unevenness is less likely to occur during coating. Furthermore, if the relative value is less than 5%, the coating material is applied uniformly, so that heat is evenly transferred in the TD direction to the base material in the drying step of the coating material, and dry wrinkles and peeling of the coating material are removed. Can be suppressed. In particular, dry wrinkles have a great effect because the coating material unevenness in the TD direction has a great influence. Therefore, it is important that the relative value is less than 5%. The smaller this relative value is, the more uniform the pore size in the TD direction is, which is preferable, but the lower limit is substantially about 0.01%.

Further, since a wide range of products from narrow width to wide width can be obtained, it is necessary to make the coating material uniform not only in a wide range but also in a narrow range. Therefore, it is necessary to make the pore diameter uniform even in a narrow width. That is, when applying the coating material to the base material, in addition to using a wide base material, the base material may be pre-slit to a narrow width according to the size of the separator to be actually incorporated in the battery. .. Therefore, such a narrow product is incorporated into the battery as it is after the coating material is applied, and it is difficult to use it in a location where the coating state is poor (for example, both ends in the width direction). A uniform coating film is required in the width direction.

The uniformity of physical properties (pore diameter) required for such narrow products and the method for measuring the physical properties will be described below. Specifically, measurement regions having arbitrary dimensions are provided at 1 cm intervals at three equal intervals in the TD direction, and the pore size distribution is measured for each measurement region. Then, the peak pore size and the distribution (Dexp) are calculated from the pore size distributions obtained in these measurement areas. Then, the standard deviation σ (Dexp) of the distribution (Dexp) obtained in these measurement regions is calculated, and the average value (Dp) of the peak pore diameters is calculated. The calculation result (variation coefficient D) obtained by substituting the standard deviation (σ(Dexp)) and the average value (Dp) into (Expression 5) is less than 24% (Expression (A Satisfying) is important. The coefficient of variation D is more preferably less than 19%, and even more preferably less than 18%. When the relative value (variation coefficient D) is less than 24%, the variation in the pore diameter in the TD direction becomes small, and the coating material evenly spreads on the substrate surface in the TD direction during coating. Further, when the relative value (variation coefficient D) is less than 24%, the pore diameter is uniform, so that the content of the coating material does not differ between the portion having a large pore diameter and the portion having a small pore diameter, resulting in unevenness during coating. Hateful. Furthermore, when the relative value (variation coefficient D) is less than 24%, the coating material is applied uniformly, so that heat is evenly transferred in the TD direction to the base material in the drying step of the coating material, resulting in dry wrinkles, The peeling of the coating material can be suppressed. In particular, dry wrinkles have a great effect because the unevenness of the coating material in the TD direction has a great influence. Therefore, it is important that the relative value is less than 24%. The smaller this relative value is, the more uniform the pore size in the TD direction is, which is preferable. However, the lower limit is practically about 5%.

The standard deviation of the pore size distribution measured at 3 points at 1 cm intervals in the TD direction is preferably less than 0.00600, more preferably less than 0.00450, and even more preferably less than 0.00445. When the standard deviation of the expected value of the hole diameter in the TD direction is less than 0.00600, the hole diameter is uniform, so that the content of the coating material does not differ between the large hole portion and the small hole portion, resulting in uneven coating. It's hard to do. Furthermore, if the standard deviation of the expected value of the pore diameter in the TD direction is less than 0.00600, the coating material is applied uniformly, so that heat is evenly transferred to the base material during the drying step of the coating material, and the dry wrinkles are generated. The peeling of the coating material can be suppressed. The smaller the lower limit of the standard deviation in the TD direction in the expected value of the pore diameter, the smaller the variation in the pore diameter in the TD direction. Therefore, the lower limit is substantially 0.00100.

The relative value of the standard deviation in the TD direction of the peak pore diameter (the standard deviation of the peak pore diameter/the average value of the peak pore diameters) (the coefficient of variation C) is less than 5% with respect to the average value of the peak pore diameters measured at 3 points at 1 cm intervals in the TD direction. Is preferable and 3% or less is more preferable. When the relative value is less than 5%, the variation in the mode value of the pore size distribution in the TD direction becomes small and the pore size becomes uniform. Therefore, the content of the coating material does not differ between the portion having a large pore diameter and the portion having a small pore diameter, and unevenness is less likely to occur during coating. Furthermore, if the relative value is less than 5%, the coating material is applied uniformly, so that heat is evenly transferred in the TD direction to the base material in the drying step of the coating material, and dry wrinkles and peeling of the coating material are removed. Can be suppressed. In particular, dry wrinkles have a great effect because the coating material unevenness in the TD direction has a great influence. Therefore, it is important that the relative value is less than 5%. The smaller this relative value is, the more uniform the pore size in the TD direction is, which is preferable, but the lower limit is substantially about 0.01%.

The standard deviation in the TD direction of the peak pore diameter measured at 3 points at 1 cm intervals in the TD direction is preferably 0.00100 or less, more preferably 0.00070 or less, and 0.00050 or less. More preferable. When the standard deviation of the peak pore diameter in the TD direction is 0.00100 or less, the pore diameter is uniform, and shrinkage in the width direction occurs uniformly, so that wrinkles can be suppressed. The smaller the lower limit of the standard deviation of the peak pore diameter in the TD direction, the more preferable it is, but the lower limit is substantially about 0.00005. In this way, by setting the variation of the pore diameter within the above range, uniform coating is possible, and the quality of the coated product is uniform, so that an excellent coating substrate can be obtained.

In the polyolefin microporous membrane (wide product or narrow product), the average value of the peak pore diameter in the TD direction is preferably 0.024 μm or less, more preferably 0.022 μm or less. Conventionally, it has been known that good withstand voltage characteristics can be obtained by reducing the pore size of a microporous polyolefin membrane. By setting the average value of the peak pore diameter in the TD direction to 0.024 μm or less, good withstand voltage characteristics can be obtained, and short circuit at low voltage can be prevented. Further, by keeping the pore size small, it is possible to suppress the growth of dendrites and prevent a short circuit. The average value of the peak pore size in the TD direction is preferably 0.001 or more. By setting the average value of the peak pore diameters in the TD direction to be 0.001 or more, the permeability of the microporous polyolefin membrane can be ensured.

When the hole diameter in the MD direction is uniform, the content of the coating material does not differ between the large hole portion and the small hole portion, unevenness on the surface can be reduced, unevenness in drying can be reduced, and high-speed conveyance becomes possible. That is, when the pore diameter is non-uniform in the MD direction, when the coating material is applied to the surface of the base material, the coating material is absorbed in a portion having a large pore diameter more than other portions (a portion having a small pore diameter). As a result, there is a possibility that the coating material may be insufficient in the portion having the large pore diameter. Therefore, the uniform pore size in the MD direction is also important.

That is, measurement regions of arbitrary dimensions are provided at three locations at 5 cm intervals in the MD direction, and the pore size distribution is measured for each measurement region. Then, the peak pore size and the distribution (Dexp) are calculated from the pore size distributions obtained in these measurement areas. Then, the standard deviation (σ(Dexp)) of the distribution (Dexp) obtained in these measurement regions is calculated, and the average value (Dp) of the peak pore diameters is calculated. The value (variation coefficient F) obtained by substituting the standard deviation (σ(Dexp)) and the average value (Dp) into the above-described (Formula 5) is less than 400% (Formula (B) is satisfied. )is important.
σ(Dexp)÷Dp×100<400 Equation (B)
The coefficient of variation F is more preferably less than 330%, even more preferably less than 320%. When the coefficient of variation F is less than 400%, the variation in the hole diameter in the MD direction becomes small, and uneven coating in the MD direction can be suppressed. Therefore, it is possible to suppress drying unevenness during high-speed transportation.

The standard deviation of the distribution measured at three points in the MD direction at 5 cm intervals is preferably less than 0.10000, and more preferably less than 0.07000.

The relative value of the standard deviation in the MD direction of the peak pore diameter to the average value of the peak pore diameters measured at 3 points at 5 cm intervals in the MD direction (standard deviation of the peak pore diameter/average value of the peak pore diameter) (variation coefficient E) is less than 5%. Is preferred.

The standard deviation in the MD direction of the peak pore diameter measured at 3 points at 5 cm intervals in the MD direction is preferably 0.00100 or less, and more preferably 0.00060 or less.

The average value of the peak pore diameters in the MD direction of the polyolefin microporous membrane is preferably less than 0.024 μm, and more preferably 0.022 μm or less.

The sum of the variation coefficient B and the variation coefficient F is preferably 450% or less. When the sum of the coefficient of variation B and the coefficient of variation F is 450% or less, the thickness unevenness, the drying unevenness, and the wrinkles of the coating material can be reduced in both the MD direction and the TD direction, and the quality of the entire film after coating is uniform. Therefore, an excellent coating substrate can be obtained. The lower the sum of the coefficient of variation B and the coefficient of variation F is, the more preferable in order to obtain uniform quality after coating, but the lower limit is substantially about 50%.

In microporous polyolefin membrane of the present invention, the tensile strength in the MD direction (tensile strength at break in the MD direction. Hereinafter, simply referred to as "MD tensile strength".) Is preferably 4500kgf / cm ² or less, is 3000 kgf / cm ² or less More preferably, 2800 kgf/cm ² or less is even more preferable. When the MD tensile strength is 4500 kgf/cm ² or less, extreme orientation in the MD direction can be suppressed, and the microporous membrane can be prevented from tearing in the carrying step or the winding step. Further, considering the transport tension during coating, the MD tensile strength is preferably 1600 kgf/cm ² or more. When the MD tensile strength is 1600 kgf/cm ² or more, high-speed transportation during coating is possible and film breakage can be prevented in the winding step.

Intensity ratio in the MD direction of the tensile strength _{S MD} and TD directions of the tensile strength _{S TD S} MD _{/ S} TD (MD tensile strength / TD tensile strength) is preferably 1.4 or more, is 1.5 or more It is more preferable that it be 1.6 or more. When S _MD /S _TD is 1.4 or more, deformation is less likely to occur during MD winding. Further, when S _MD /S _TD is 1.4 or more, the tension control becomes easy, so that the coating can be performed while holding the base material at a suitable tension, so that a uniform coating is possible and the coating substrate Good properties as a material can be obtained. The S _MD /S _TD is preferably 2.5 or less, more preferably 2.2 or less, still more preferably 1.95 or less. When the strength ratio is 2.5 or less, it is possible to prevent only the orientation in the MD direction from becoming strong, so that the winding deviation at the time of slitting is suppressed and the slitting property becomes good.

In the polyolefin microporous film of the present invention, it is preferable that the heat shrinkage in the MD direction when kept at 105° C. for 8 hours is 5% or less. When the thermal shrinkage in the MD direction when kept at 105° C. for 8 hours is 5% or less, it is possible to prevent the microporous membrane from shrinking and short-circuiting when abnormal heat is generated, and secure sufficient safety. I can do things. Further, when the thermal shrinkage in the MD direction when kept at 105° C. for 8 hours is 5% or less, the shrinkage of the base material in the drying step at the time of coating can be suppressed, which may be caused by dry wrinkles or peeling of the coating film. Coating unevenness can be suppressed. Therefore, the smaller the thermal shrinkage at 105° C. in the MD direction, the more preferable, but the lower limit is substantially about 0.00001%.

The air permeation resistance of the polyolefin microporous membrane of the present invention is a value measured in accordance with JIS P 8117 (2009). In this specification, the term “air permeability resistance” is used to mean “air permeability resistance when the film thickness is 20 μm” unless otherwise specified. Specifically, when the air permeation resistance measured for a microporous membrane having a film thickness T ₁ (μm) is P ₁ , the permeability calculated by the formula: P ₂ =(P ₁ ×20)/T ₁ The air resistance P ₂ is the air resistance when the film thickness is 20 μm. The air resistance is preferably 1000 sec/100 cc or less, more preferably 800 sec/100 cc or less, and further preferably 500 sec/100 cc. When the air permeation resistance is 1000 sec/100 cc or less, good ion permeability is obtained, and the electric resistance can be reduced. The air resistance is preferably 100 sec/100 cc or more. When the air resistance is 100 sec/100 cc or more, good strength can be obtained.

Conventionally, it has been known that the permeability is deteriorated by making the pore size dense (making the pore size small and the distance between adjacent pore sizes small), but the microporous membrane obtained by the present invention Is excellent in that it has good permeability even though the pore size is uniform and small (the pore size is dense). As a result, good output characteristics can be obtained when used as a separator.

In the polyolefin microporous film of the present invention, the withstand voltage (dielectric breakdown voltage) when the film thickness is 20 μm is preferably 2.4 kV or more, and more preferably 2.6 kV or more from the viewpoint of the insulating property of the separator. More preferable. Here, the withstand voltage when the film thickness is 20 μm means the formula: V ₂ =(V ₁ ×when the dielectric breakdown voltage in the microporous film having the film thickness T ₁ (μm) is V ₁ (kV). 20)/T ₁ refers to the dielectric breakdown voltage V ₂ , and when the measurement is performed a plurality of times, it refers to the average value thereof. The withstand voltage and the pore diameter have a close relationship, and generally, as the pore diameter increases, the withstand voltage of the microporous membrane decreases and sufficient insulation cannot be obtained. On the other hand, when the pore size is small, the withstand voltage can be increased, but good permeability cannot be obtained. The microporous membrane obtained by the present invention is superior to conventional microporous membranes in that it has fine and uniform pore size, but also has good permeability and withstand voltage.

In the present specification, the term "puncture strength" is used to mean "puncture strength when the film thickness is 20 μm" unless otherwise specified. The puncture strength when the film thickness is 20 μm is expressed by the formula: L ₂ =(L ₁ ×20)/T ₁ when the puncture strength is L ₁ in a microporous film having a film thickness T ₁ (μm). It refers to the calculated puncture strength L ₂ . The puncture strength is preferably 450 gf or more. When the puncture strength is 450 gf or more, pinholes and cracks caused by the sharp parts such as the electrode material piercing the microporous film can be suppressed, and the defective rate at the time of battery assembly can be reduced.

Further, if the puncture strength is low, high tension cannot be applied at the time of conveyance, so that high-speed conveyance becomes difficult and stability during coating deteriorates. Along with this, the coating layer may be peeled off during high-speed transportation, leading to contamination in the coating layer. By adjusting the puncture strength within the above range, it is possible to suppress the occurrence of such a defect, and it is possible to speed up the coating.

From the viewpoint of film strength, the upper limit of the porosity of the polyolefin microporous film of the present invention is preferably 70%, more preferably 60%. In addition, the lower limit of the porosity is preferably 20%, and more preferably 40%, from the viewpoints of permeability and electrolyte content. By setting the porosity within the above range, the balance of the permeability, the strength, and the content of the electrolytic solution is improved, the nonuniformity of the cell reaction is eliminated, and the dendrite generation is suppressed. As a result, good safety, strength and permeability are obtained.

Regarding the polyolefin constituting the microporous polyolefin membrane of the present invention, from the viewpoint of membrane strength, it is preferable that the content of the polyolefin component having a large molecular weight is large. Specifically, when the total amount of polyolefin is 100% by weight, the content of the polyolefin component having a molecular weight of 1,000,000 or more is preferably 20% by weight or more, more preferably 25% by weight or more. By setting the content of the polyolefin component having a molecular weight of 1,000,000 or more to 20% by weight or more, excellent film strength can be obtained, and at the same time excellent storage characteristics at high temperature prevent short circuit, resulting in high strength and safety. It can be compatible.

Further, with regard to the polyolefin constituting the microporous polyolefin membrane of the present invention, it is preferable to use polyethylene. The content of polyethylene 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. The polyethylene preferably has a polyethylene component having a molecular weight of 500,000 or less and a polyethylene component having a molecular weight of 1,000,000 or more. Polyolefin other than polyethylene may include polypropylene, copolymerized polyolefin, and the like.

The molecular weight distribution, content of a predetermined component, weight average molecular weight (Mw) and number average molecular weight (Mn) of polyolefin can be measured by the method of GPC (gel permeation chromatography) analysis described later. FIG. 3 shows a relationship diagram of a molecular weight distribution curve of polyethylene obtained by GPC, in which the horizontal axis is the logarithmic value of the molecular weight and the vertical axis is the value obtained by differentiating the concentration fraction of polyethylene by the logarithmic value of the molecular weight. In FIG. 3, the region (a) corresponds to the “polyethylene component having a molecular weight of 500,000 or less”, and the region (b) corresponds to the “polyethylene component having a molecular weight of 1 million or more”.

The content of the polyethylene component having a molecular weight of 500,000 or less is preferably 70% by weight or less, more preferably 65% by weight or less, and 60% by weight or less, based on 100% by weight of the whole polyethylene. Is particularly preferable. When the content of the polyethylene component having a molecular weight of 500,000 or less is 70% by weight or less, the decrease in strength of the microporous membrane can be suppressed.

The content of the polyethylene component having a molecular weight of 1,000,000 or more is preferably 20% by weight or more, and more preferably 25% by weight or more, based on 100% by weight of polyethylene as a whole. By setting the content of the polyethylene component having a molecular weight of 1,000,000 or more to 20% by weight or more, excellent film strength can be obtained, and short-circuiting can be prevented easily due to excellent storage characteristics at high temperature, and strength and safety can be kept at a high level. It can be compatible.

In particular, in the case of producing a polyolefin microporous membrane, when the stretching step has two stages of a uniaxial stretching step and a simultaneous biaxial stretching step as described below, the molecular weight of polyethylene is important in order to obtain a uniform and dense pore size. Becomes When the content of low-molecular-weight polyethylene is high or when the content of high-molecular-weight polyethylene is low, the entanglement of polyethylene molecules is weak, and when uniaxially stretched, strong and weak parts of polyethylene molecules are generated. .. As a result, the pore diameter becomes non-uniform (the variation in pore diameter increases) in the stage of the simultaneous biaxial stretching step in which the pores are opened, and it becomes difficult to control the pore diameter. By setting the content of the component having a molecular weight of 1,000,000 or more to 20% or more, the entanglement of polyethylene molecules can be sufficiently ensured, so that the draw ratio of uniaxial stretching can be increased, and the MD is uniform and has a small pore size. The strength of the direction can be increased. From the above, it is preferable that the polyethylene component having a molecular weight of 500,000 or less is 70% or less and the polyethylene component having a molecular weight of 1,000,000 or more is 20% or more.

More specifically, polyethylene as a material for the microporous membrane 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”). , And a 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”).

When the composition of HDPE and UHMWPE is used as described above, the molecular weight distribution (Mw/Mn) of both HDPE and UHMWPE is preferably 3 or more. When the molecular weight distribution is 3 or more, a high molecular weight component and a low molecular weight component are present in a suitable amount, so that the moldability is improved and the reduction in stretchability during film formation can be suppressed. The molecular weight distribution of HDPE and UHMWPE is preferably 20 or less. When the molecular weight distribution is 20 or less, a decrease in strength due to an increase in low molecular weight components can be suppressed.

The method for producing a polyolefin microporous membrane of the present invention uses a so-called wet method, but includes a step of stretching in a uniaxial direction in the state where a solvent and a solute (polyolefin) are mixed and a step of performing simultaneous biaxial stretching in this order. It is important to be out. By stretching in a uniaxial direction with the solvent and solute mixed together, it becomes possible to control the anisotropy in the stretching direction and to densify the pore structure. This gives excellent properties as a coating substrate. The microporous membrane produced by the above steps has a uniform and small pore size, and has good strength balance in the MD and TD directions, permeability, and withstand voltage characteristics. Therefore, the following (1), (2) It is important that it is manufactured by the process of 1).
(1) Step of stretching the sheet extruded from the die in the MD direction at a stretching temperature of 90 to 115° C. and a stretching ratio of 1.4 to 2.0 times. (2) Stretching temperature of the sheet stretched in the MD direction Step of simultaneously stretching in MD direction and TD direction at 100 to 120° C. By performing the above steps at the time of production, compared with a polyolefin microporous membrane produced by a conventional production method and having a similar draw ratio, A finer and more uniform pore size can be obtained, and more favorable strength balance in the MD and TD directions, permeability and withstand voltage characteristics can be obtained. This makes it possible to obtain the polyolefin microporous membrane of the present invention having excellent properties as a coating substrate.

Further, by controlling the molecular weight and content of the polyethylene component as described above prior to carrying out the above steps (1) and (2), micropores having very excellent properties as a coating substrate. A membrane can be obtained. By selecting the optimum content ratio and carrying out the step (1), the molecular chains are appropriately entangled. By carrying out the step (2) in this state, a microporous membrane having no unevenness in the pore size distribution in the TD direction can be obtained, and the pore size can be densified. However, if the stretching ratio in the above step (1) is too high, the balance of the entanglement between the molecular chains is disturbed, and the variation in the pore size in the TD direction tends to increase when the stretching in the MD direction is performed. In the step 1), it is necessary to select the optimum MD draw ratio.

The present inventors have found that in order to obtain a polyolefin microporous film, a draw ratio of 1.4 times or more in the step (1) is important. If the stretching ratio in the MD direction is 1.4 times or more, a microporous film having a uniform and small pore size can be obtained, and the strength balance in the MD direction and the TD direction can be improved. The draw ratio in the MD direction is preferably 2.0 times or less. When the stretching ratio in the MD direction is 2.0 times or less, the degree of orientation in the MD direction can be adjusted, and the microporous membrane can be prevented from tearing in the carrying step or the winding step. Further, when the draw ratio in the MD direction is 2.0 times or less, the unevenness of the pore size distribution in the TD direction can be suppressed, and the uniform pore size without the unevenness of the pore size distribution can be obtained.

The stretching ratio in the step (2) is preferably 16 times or more, more preferably 25 times or more in terms of area ratio. When the area magnification in the step (2) is 16 times or more, the film can be uniformly stretched without unevenness in stretching, and unevenness in physical properties and unevenness in pore size distribution can be suppressed. The area magnification in the step (2) is preferably 49 times or less. When the area ratio in the step (2) is 49 times or less, it is possible to suppress the membrane rupture due to the increase in the total area ratio and improve the productivity. The stretching in the step (2) is preferably simultaneous stretching at the same ratio in the TD and MD directions.

The polyolefin microporous membrane of the present invention can be suitably used as a coating substrate. Therefore, the coating layer preferably applied to the present invention will be described below. The coating material used for coating is not particularly limited, but the coating layer applied to the present invention preferably contains a water-soluble resin or a water-dispersible resin and fine particles. By using a water-soluble resin or a water-dispersible resin and fine particles, not only excellent heat resistance can be obtained, but also cost reduction can be achieved, and it is preferable from the viewpoint of environmental load in the manufacturing process.

The solvent used for the coating material is not only a liquid that dissolves the water-soluble resin or the water-dispersible resin, but also broadly includes a dispersion medium used to disperse the water-soluble resin or the water-dispersible resin into particles. It is preferable that the main component is water. The water used is preferably ion-exchanged water or distilled water. The solvent may be only water, but a water-soluble organic solvent such as alcohol can be used if necessary. By using these water-soluble organic solvents, the drying speed and coatability can be improved.

In the coating layer, a surfactant, an antistatic agent and the like can be included as a composition other than the water-soluble resin or the water-dispersible resin and the fine particles within a range not impairing the object of the present invention. The fine particles may be inorganic particles or organic particles.

The shape of the particles may be a spherical shape, a substantially spherical shape, a plate shape, or a needle shape, but is not particularly limited.

Examples of the method for applying the coating material include a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brushing method, a spray coating method, an air knife coating method, a Meyer bar coating method, a pipe doctor method, and a blade coating. Method, die coating method and the like, and these methods can be performed alone or in combination. As described above, the method of simultaneously applying the coating material in the TD direction using a blade or knife such as a gravure coating method or a blade coating method is preferably applied to the polyolefin microporous film of the present invention. The solvent is generally removed by drying.

The present invention provides a coating substrate comprising the polyolefin microporous film described above. Since the pore diameter of such a coating substrate is controlled to be uniform and small, it is possible to uniformly coat the coating material while suppressing problems such as uneven coating, peeling of the coating material, and wrinkles. , The quality of the coated product is improved. Further, since the tensile strength in the MD direction is high and the balance between the MD tensile strength and the TD tensile strength is excellent, high tension can be applied during coating. Therefore, it is also suitable for high-speed coating. Further, it is excellent in that it has both a small pore diameter and a uniform pore diameter, and good permeability and withstand voltage characteristics, and is also excellent as a battery separator.

Hereinafter, specific examples of the present invention will be described using examples, but the present invention is not limited thereto.

(Measuring method)
1. Pore size distribution, its standard deviation in MD or TD direction Pore size distribution (Dexp) was calculated as follows. Along the TD direction of the microporous membrane, four samples having a diameter of 3 cm are arranged at intervals of 5 cm (distance between centers of circles), and samples having a diameter of 3 cm are arranged at intervals of 3 cm (distance between centers of circles) along the TD direction. Three samples having a diameter of 3 cm were collected at an interval of 5 cm (distance between centers of circles) in the MD direction. Since these samples were collected in different areas, a total of 10 samples were obtained.
The pore size distribution of the obtained sample was measured with a palm porometer (model number: CFP-1500A, measurement solution: Galwick) manufactured by PMI Co., Ltd. at a measurement pressure of 0 to 3500 MPa, and then the pore size distribution of each sample was measured. (Dexp) was calculated based on Equation 4 above. Then, for four measurement samples taken at 5 cm intervals (center distance between circles) along the TD direction, further, three measurement samples taken at 3 cm intervals (center distance between circles) along the TD direction. The standard deviation of the distribution was calculated for each of the measurement samples or for three measurement samples taken at 5 cm intervals (distance between centers of circles) in the MD direction. These standard deviations were defined as “standard deviation (σ(Dexp)) in the MD (or TD) direction of the pore size distribution””.

2. Peak Pore Diameter, Mean Value/Standard Deviation in TD Direction The peak pore diameter was calculated as follows. Along the TD direction of the microporous membrane, four samples having a diameter of 3 cm are arranged at intervals of 5 cm (distance between centers of circles), and samples having a diameter of 3 cm are arranged at intervals of 3 cm (distance between centers of circles) along the TD direction. Three samples having a diameter of 3 cm were collected at an interval of 5 cm (distance between centers of circles) in the MD direction. Since these samples were collected in different areas, a total of 10 samples were obtained.
For each of the obtained samples, a PMI palm porometer (model number: CFP-1500A, measurement solution: Galwick) was used to measure the pressure in the range of 0 to 3500 MPa, and the peak pore diameter (pore diameter at the most frequent value) was measured. I asked. Then, for four measurement samples taken at 5 cm intervals (center distance between circles) along the TD direction, further, three measurement samples taken at 3 cm intervals (center distance between circles) along the TD direction. The average value and standard deviation of the peak pore diameters were calculated for each of the measurement specimens or for three measurement specimens sampled at 5 cm intervals (distance between centers of circles) in the MD direction. ) Direction and "standard deviation of peak pore size in MD (or TD) direction".

3. Measurement of Molecular Weight Distribution of Polyolefin The molecular weight distribution of polyolefin was measured by high temperature GPC (measurement of weight average molecular weight, molecular weight distribution, content of predetermined component, etc.). The measurement conditions were as follows.
Equipment: High temperature GPC equipment (equipment No. HT-GPC, manufactured by Polymer Laboratories, PL-220)
Detector: Differential refractive index detector RI
Guard column: Shodex G-HT
Column: Shodex HT806M (2 pieces) (φ7.8 mm x 30 cm, Showa Denko)
Solvent: 1,2,4-trichlorobenzene (TCB, Wako Pure Chemical Industries, Ltd.) (0.1% BHT added)
Flow rate: 1.0 mL/min
Column temperature: 145°C
Sample preparation: 5 mL of the measurement solvent was added to 5 mg of the sample, and the mixture was heated and stirred at 160 to 170° C. for about 30 minutes, and the obtained solution was filtered with a metal filter (pore size 0.5 μm).
Injection volume: 0.200 mL
Standard sample: Monodisperse polystyrene (manufactured by Tosoh)
Data processing: TRC GPC data processing system Calibration curve: Calculated using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.
The areas of components having a molecular weight of 500,000 or less and components having a molecular weight of 1,000,000 or more were calculated based on the curves obtained by the above test. Then, the ratio of the component having a molecular weight of 500,000 or less and the component having a molecular weight of 1 million or more to the total area was determined.

4. Thickness The thickness of the microporous membrane was measured at a randomly selected MD position using a contact thickness gauge. The measurements were taken along the TD direction of the membrane at 5 mm intervals over a distance of 30 cm. Then, the measurement along the TD direction was performed 5 times at different MD positions, and the arithmetic average thereof was used as the thickness of the sample.

5. Air Permeation Resistance The air permeation resistance P ₁ was measured by a method described in JIS-P8117 using a gas permeation meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) with respect to the microporous membrane having the film thickness T ₁ , and the formula was obtained. : P ₂ =(P ₁ ×20)/T ₁
Thus, the air permeation resistance P ₂ when the film thickness was 20 μm was calculated.

6. Puncture strength A needle with a diameter of 1 mm having a spherical surface (radius of curvature R: 0.5 mm) at the tip is pierced into a microporous membrane having 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 of L ₂ =(L ₁ ×20)/T ₁ .

7. Porosity The porosity is calculated from the mass w1 of the microporous membrane and the mass w2 of the same size of the same polyethylene composition as the microporous membrane without pores, and the porosity (%)=(w2-w1 )/W2×100.

8. Thermal shrinkage in MD direction The microporous film was cut into 5 cm×5 cm, and the shrinkage in MD direction when treated (non-fixed) at 105° C. for 8 hours was measured 3 times (measured for 3 samples), The average value thereof was defined as the heat shrinkage rate in the MD direction.

9. Tensile Strength MD tensile strength and TD tensile strength were measured by using a strip-shaped test piece having a width of 10 mm, according to the method according to ASTM D882.

10. Withstanding voltage performance A 150 mm square aluminum plate was placed with a microporous membrane with a thickness T ₁ cut out to a diameter of 60 mm, and a brass cylindrical electrode with a diameter of 50 mm was placed thereon, and TOS5051A resistant to Kikusui Electronics was used. A voltage tester was connected. The voltage V ₁ is applied at a boosting rate of 0.2 kV/sec, the value V ₁ at the time of dielectric breakdown is read, and based on the conversion formula: V ₂ =(V ₁ ×20)/T ₁ , the film thickness per 20 μm The withstand voltage V _{2 of each} was calculated. The withstand voltage V ₂ was measured 3 times and the average value was obtained.

11. Coating unevenness Carboxymethyl cellulose (CMC) (manufactured by Daicel FineChem Ltd., product number 2200) 0.8 wt% was added with a solvent (water) 60.8 wt% and stirred for 2 hours. Subsequently, 38.4 parts by mass of substantially spherical alumina fine particles having an average particle diameter of 0.5 μm were added, and the mixture was stirred for 2 hours to sufficiently disperse the alumina fine particles, and then the filtration particle size (initial filtration efficiency: 95%) was 10 μm. Was micro-filtered with a felt type polypropylene filter to prepare a coating solution. At this time, the volume ratio of the resin component to the fine particles was 5:95 (calculated with a specific gravity of CMC of 1.6 g/cm ³ and a specific gravity of alumina of 4.0 g/cm ³ ).
The obtained coating liquid is coated on the microporous membrane by hand coating, dried at 70° C. for 1 minute, and then a sample of 15 cm×10 cm is randomly sampled and visually checked for uneven coating. went. Specifically, a transmissive light was used, and light was applied to the collected sample, and the area where the tint was different from the other areas was treated as coating unevenness. After calculating the work unevenness area ratio, the judgment was made based on the following judgment criteria.
Coating uneven area ratio (%) = (area of uneven area/total area (15 x 10 cm)) x 100 (Equation 6)
<Criteria for coating unevenness>
Coating uneven area ratio 0 to 1%: ◎ (best)
Coating unevenness area ratio 1-2%: ○ (excellent)
Coating uneven area ratio 2 to 6%: △ (good)
Non-uniform coating area ratio 6% or more: × (impossible)
There is no practical problem if the coating uneven area ratio is 2% or less.

12. Coating wrinkles The obtained coating solution is coated on the microporous membrane by hand coating and dried at 70°C for 1 minute, then a sample of 15 cm × 10 cm is randomly sampled and the number of wrinkles is visually confirmed. did. The criteria for judgment are as follows.
Number of wrinkles 0 to 1: ◎ (best)
Number of wrinkles 2-3: ○ (excellent)
Number of wrinkles 4 to 7: △ (good)
Number of wrinkles 8 or more: × (impossible)
If the number of wrinkles is 3 or less, there is no practical problem.

(Example 1)
30% by weight of ultra high molecular weight polyethylene (UHMWPE(PE1)) having a weight average molecular weight (Mw) of 2.89×10 ⁶ and a molecular weight distribution Mw/Mn of 5.28, and a weight average molecular weight Mw of 5.72. × a 10 ^5, high density polyethylene (HDPE (PE2)) molecular weight distribution Mw / Mn is 4.81 was prepared polyethylene composition consisting of 70 wt%. The content of the polyethylene component having a molecular weight of 500,000 or less was 59% by weight, and the content of the polyethylene component having a molecular weight of 1,000,000 or more was 26% by weight, based on 100% by weight of the whole polyethylene composition. 71.5% by weight of liquid paraffin was added to 28.5% by weight of this polyethylene composition, and 0.5% by weight of 2,6-di-t-butyl-p-cresol was added based on the weight of polyethylene in the mixture. And 0.7% by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxylphenyl)-propionate]methane were added as an antioxidant and mixed to prepare a polyethylene resin solution. This polyethylene resin solution was supplied to a T-die from a twin-screw extruder and extruded into a sheet having a thickness of about 1.0 mm, and then the extrudate was cooled by a cooling roll controlled at 25° C. to form a gel sheet. Formed. While subjecting the obtained gel-like sheet to a temperature of 115° C., an MD stretching step of stretching in a MD direction at a stretching ratio of 1.4 times using a batch type stretching machine was performed, and then the sheet was exposed to a temperature of 115° C. Meanwhile, a simultaneous biaxial stretching step was performed in which simultaneous biaxial stretching was performed at a magnification of 5×5 in both the MD direction and the TD direction. The stretched sheet was fixed on a 20 cm×20 cm aluminum frame plate, immersed in a methylene chloride washing bath, washed by shaking for 10 minutes to remove liquid paraffin, and then the washed membrane was air-dried at room temperature. Then, the membrane was held at 125° C. for 10 minutes to prepare a polyolefin microporous membrane. The membrane characteristics of the obtained polyolefin microporous membrane are shown in Table 1.

(Example 2)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that MD stretching and simultaneous biaxial stretching were performed at a stretching temperature of 110°C. The membrane characteristics of the obtained polyolefin microporous membrane are shown in Table 1.

(Example 3)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the stretching ratio in the MD stretching step was 1.8 times. The membrane characteristics of the obtained polyolefin microporous membrane are shown in Table 1.

(Example 4)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the amount ratio of PE1 and PE2 was 20/80. The content of the polyethylene component having a molecular weight of 500,000 or less was 64% by weight, and the content of the polyethylene component having a molecular weight of 1,000,000 or more was 20% by weight, based on 100% by weight of the total polyethylene composition. The membrane characteristics of the obtained polyolefin microporous membrane are shown in Table 1.

(Example 5)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the stretching ratio in the MD stretching step was 2.0. The membrane characteristics of the obtained polyolefin microporous membrane are shown in Table 1.

(Comparative Example 1)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the amount ratio of PE1 and PE2 was 10/90. The content of the polyethylene component having a molecular weight of 500,000 or less was 70% by weight, and the content of the polyethylene component having a molecular weight of 1,000,000 or more was 14% by weight, based on 100% by weight of the whole polyethylene composition. Table 2 shows the film characteristics of the obtained polyolefin microporous film.

(Comparative example 2)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the stretching ratio in the MD stretching step was 1.3 times. Table 2 shows the film characteristics of the obtained polyolefin microporous film.

(Comparative example 3)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the MD stretching step was not performed and only the simultaneous biaxial stretching step was performed as the stretching step. Table 2 shows the film characteristics of the obtained polyolefin microporous film.

(Comparative Example 4)
A polyolefin microporous membrane was produced in the same manner as in Comparative Example 3 except that the stretching ratio in the simultaneous biaxial stretching step was 7×5 times. Table 2 shows the film characteristics of the obtained polyolefin microporous film.

(Comparative example 5)
A polyolefin microporous membrane was produced in the same manner as in Comparative Example 3 except that the stretching ratio in the simultaneous biaxial stretching step was 6×6 times. Table 2 shows the film characteristics of the obtained polyolefin microporous film.

The microporous membrane according to the present invention can be suitably used as a base material for coating, which is in great demand in the fields of battery separators, condenser films, filters and the like.

Claims (9)

A polyolefin microporous membrane having a content of polyolefin having a molecular weight of 1.0×10 ⁶ or more of 20% by mass or more,
A microporous polyolefin membrane characterized in that, when a plurality of measurement regions for measuring the distribution of pore diameters are set, the measurement results obtained in each of these measurement regions satisfy the following relational expression (A).
σ(Dexp)÷Dp×100<24 (A)
However, each of the measurement regions has a size of a circle having a diameter of 3 cm, and is separated from each other along the TD direction,
When the width of the sample in the TD direction is 18 cm or more, the samples are arranged at four places at intervals of 5 cm (distance between circle centers),
When the width of the sample in the TD direction is less than 18 cm, the samples are arranged at three positions at 3 cm intervals (distance between circle centers).
Further, σ(Dexp) is a standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is the mode value (pore diameter) of the pore size distribution obtained in each measurement region. Is the average value of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj)) The polyolefin microporous film according to claim 1, wherein the σ(Dexp) is less than 0.00600. The polyolefin microporous membrane according to claim 1, wherein the σ(Dexp) is less than 0.00100. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the Dp is 0.010 to 0.024 µm. A polyolefin microporous membrane having a content of polyolefin having a molecular weight of 1.0×10 ⁶ or more of 20% by mass or more,
The ratio of the tensile strength in the MD direction and the tensile strength in the TD direction is 1.4 to 2.5,
A microporous polyolefin membrane characterized in that, when a plurality of measurement regions for measuring the distribution of pore diameters are set, the measurement results obtained in each of these measurement regions satisfy the following relational expression (B).
σ(Dexp)÷Dp×100<400 (B)
However, each of the measurement regions has a size of a circle having a diameter of 3 cm, and the measurement regions are arranged at three positions spaced apart from each other in the MD direction at intervals of 5 cm (distance between centers of circles). Further, σ(Dexp) is a standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is the mode value (pore diameter) of the pore size distribution obtained in each measurement region. Is the average value of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj)) The polyolefin microporous film according to claim 5, wherein the Dp is 0.010 to 0.024 μm. Molecular weight 1.0×10 ⁶ A polyolefin microporous membrane having the above polyolefin content of 20% by mass or more,
The ratio of the tensile strength in the MD direction and the tensile strength in the TD direction is 1.4 to 2.5,
Separated from each other along the TD direction,
When the width of the sample in the TD direction is 18 cm or more, the samples are arranged at four places at intervals of 5 cm (distance between circle centers),
When the width of the sample in the TD direction is less than 18 cm, RTD=σ(Dexp)/calculated from the measurement results obtained in a plurality of measurement areas arranged at three positions at 3 cm intervals (distance between circle centers). Dp x 100,
RMD=σ(Dexp)÷Dp×100 calculated from the measurement results obtained in a plurality of measurement regions arranged at three locations at 5 cm intervals (distance between circle centers) along the MD direction. A microporous polyolefin membrane characterized by satisfying the following relational expression (C).
250<RTD+RMD<420...(C)
However, each of the measurement regions is the size of a circle having a diameter of 3 cm, σ(Dexp) is a standard deviation calculated using Dexp calculated by the following formula for each measurement region, and Dp is each It is a value obtained by averaging the mode values (pore diameters) of the pore diameter distribution obtained in the measurement region of.
Dexp=Σ{Dj×(PSF)j}
(Dj: Pore diameter, (PSF)j: Pore diameter distribution value (frequency of pore diameter Dj)) MD direction tensile strength is 1600~4500kgf / cm ^2, the polyolefin microporous membrane according to any one of claims 1 to 7. A base material for coating, which comprises the polyolefin microporous film according to claim 1.

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