JP7533189B2 - Polyolefin-based microporous film, laminate, and nonaqueous electrolyte secondary battery using the same - Google Patents

Description

The present invention relates to a polyolefin-based microporous membrane, a laminate, and a nonaqueous electrolyte secondary battery using the same.

Microporous thermoplastic resin membranes are widely used as separation membranes, selective permeable membranes, and isolation membranes for substances. Specific applications of microporous membranes include, for example, separators for nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries, separators for electric double-layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture-permeable waterproof clothing, medical materials, and supports for fuel cells.

In particular, polyolefin-based microporous membranes are widely used as separators for lithium-ion secondary batteries. In addition to excellent mechanical strength, which contributes greatly to the safety and productivity of batteries, they also have ion permeability through the electrolyte that permeates the micropores while ensuring electrical insulation, and have a pore-blocking function that automatically blocks ion permeation at around 120 to 150°C when an abnormal reaction occurs inside or outside the battery, thereby suppressing excessive temperature rise.

When incorporating a polyolefin-based microporous film into a lithium-ion secondary battery, it is common to laminate a heat-resistant resin layer or an electrode layer to improve heat resistance. In the lamination process, a method is generally used in which a metal roll is rotated and the polyolefin-based microporous film is conveyed while coating or laminating. In addition, when manufacturing a polyolefin-based microporous film, a method is generally used in which an intermediate product is wound into a roll, and then the intermediate product is slit and wound onto a reel to produce a finished product.

From the viewpoint of preventing scratches on the surface of the polyolefin-based microporous membrane when it is transported by a rotating metal roll and preventing wrinkles from occurring during the production of roll-shaped intermediate products and reel products of the polyolefin-based microporous membrane, it is important that the friction coefficient of the microporous membrane is low, i.e., that the surface of the microporous membrane is slippery. On the other hand, in recent years, lithium-ion secondary batteries have become smaller and thinner, and when lithium-ion secondary batteries are manufactured by winding the positive electrode, separator, and negative electrode, it is important that the polyolefin-based microporous membrane used in the separator is smooth in order to enable the battery to be made smaller and thinner with the same number of layers.

Regarding the friction coefficient of polyolefin-based microporous membranes, Patent Document 1 proposes lowering the friction coefficient of the membrane surface by broadly setting the molecular weight distribution of the polyolefin composition that constitutes the microporous membrane within a specific range, and Patent Document 2 proposes lowering the friction coefficient of the membrane surface by including a low-molecular-weight polyethylene resin at a specific concentration range, thereby causing the polyethylene resin to bleed out onto the membrane surface.

Regarding the smoothness of polyolefin-based microporous membranes, Patent Document 3 proposes increasing the surface roughness of the polyolefin-based microporous membrane within a specific range to improve adhesion to electrodes.

Japanese Patent Application Publication No. 8-311225 JP 2014-12857 A JP 2009-91461 A

The surface of a microporous membrane with a large coefficient of friction and good slipperiness is generally rough, and the smoothness of the microporous membrane required for miniaturization of batteries cannot be obtained. In other words, the smoothness and slipperiness of the surface of a microporous membrane are contradictory properties, and it has been difficult to achieve both smoothness and slipperiness at the same time. In Patent Documents 1 and 2, the design does not take smoothness into sufficient consideration, and is insufficient in terms of miniaturization and thinning of lithium-ion batteries. In Patent Document 3, the design does not take smoothness into sufficient consideration, and is insufficient in terms of miniaturization and thinning of lithium-ion batteries.

The problem that the present invention aims to solve is to provide a polyolefin-based microporous membrane that overcomes the above-mentioned shortcomings and has the contradictory properties of slipperiness and smoothness.

In order to solve the above problems, the present invention has the following configuration.
(1) A polyolefin-based microporous membrane in which the dynamic friction coefficient μk between one surface and the other surface of the microporous membrane is 0.2 or more and 0.7 or less, and the root-mean-square height Sq of the surface between the one surface and the other surface of the microporous membrane that has a larger root-mean-square height Sq is 10 nm or more and 100 nm or less.

(2) A polyolefin-based microporous film according to (1), in which the dynamic friction coefficient μmk between at least one surface of the microporous film and a metal is 0.13 or less.

(3) The polyolefin-based microporous membrane of (1) or (2), in which the number of fibrils in a cube with a side length of 2.7 μm, as determined by the following measurement method, is 870/ ^μm3 to ^2,800 /μm3, and the average diameter of the fibrils is 25 nm to 60 nm.

Step (A): The cross section of the microporous membrane is cut to sequentially obtain images, and a three-dimensional image is created. Step (B): The resin parts and pore parts of the obtained three-dimensional image are distinguished, and a three-dimensional image of the constituent resin parts is created. Step (C): The constituent resin parts are divided into fibril units, and the number of fibrils is counted.

(4) A polyolefin-based microporous film according to any one of (1) to (3), in which the pin puncture strength of the microporous film is 180 gf or more and 700 gf or less.

(5) A polyolefin-based microporous film according to any one of (1) to (4), in which the thickness of the microporous film is 3 μm or more and 14 μm or less.

(6) The polyolefin-based microporous film according to any one of (1) to (5), which has a heat shrinkage rate of 20% or less in both the longitudinal and transverse directions at 130° C. for 1 hour.
(7) A microporous membrane roll produced by winding the microporous membrane according to any one of (1) to (6) above, the polyolefin-based microporous membrane roll having a coefficient of variation of dynamic friction μk of 10% or less.

(8) A polyolefin-based microporous membrane roll as described in (7), in which the coefficient of variation of the root mean square height Sq for one surface of the microporous membrane is 10% or less.

(9) A microporous membrane/heat-resistant resin layer laminate in which a heat-resistant resin layer is further laminated on the polyolefin-based microporous membrane described in any one of (1) to (6).

(10) A non-aqueous electrolyte secondary battery comprising the polyolefin-based microporous membrane described in any one of (1) to (6) above, or the microporous membrane/heat-resistant resin layer laminate described in (9).

When the polyolefin-based microporous membrane according to the embodiment of the present invention is used as a separator for nonaqueous electrolyte secondary batteries, such as lithium ion secondary batteries, it has excellent lubricity and smoothness, is easy to transport, and has a good appearance when wound up, and has the effect of enabling the batteries to be made smaller and thinner.

The polyolefin-based microporous membrane according to an embodiment of the present invention will be described in detail below. In this specification, the polyolefin-based microporous membrane may be simply referred to as a "microporous membrane."

The polyolefin-based microporous film of the present invention has a dynamic friction coefficient μk between one surface and the other surface of the film of 0.2 or more and 0.7 or less, and the root-mean-square height Sq of the surface between the one surface and the other surface of the microporous film that has a large root-mean-square height Sq is 10 nm or more and 100 nm or less. The dynamic friction coefficient μk is one index of the ease of sliding when films are stacked on top of each other, and the root-mean-square height Sq measured with a white light interferometer is one index of smoothness.

The polyolefin-based microporous film in the embodiment of the present invention is mainly composed of a polyolefin-based resin. Here, "mainly composed of a polyolefin-based resin" means that the film contains more than 50% by mass and not more than 100% by mass of polyolefin-based resin when the total mass of the microporous film is taken as 100% by mass. Here, examples of the polyolefin-based resin in the embodiment of the present invention include various polyethylene-based resins and various polypropylene-based resins. The polyethylene-based resin in the embodiment of the present invention means a polymer in which the total amount of ethylene-derived components is more than 50% by mass and not more than 100% by mass when the total mass of the polyethylene-based resin polymer is taken as 100% by mass.

In addition, the polypropylene-based resin in the embodiment of the present invention means a polymer in which the total amount of propylene-derived components is more than 50% by mass and not more than 100% by mass when the total mass of the polypropylene-based resin polymer is 100% by mass.

The polyethylene resin in the embodiment of the present invention may be a homopolymer consisting of ethylene alone, or a copolymer in which a chain olefin (α-olefin) such as propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-butene, 3-methyl-1-pentene, 3-ethyl-1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene, 4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1-hexene, 3-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, etc. is copolymerized.

The polypropylene-based resin in the embodiment of the present invention may be a homopolymer consisting of propylene alone, or various polypropylene-based resins such as ethylene-propylene copolymer, ethylene-propylene-butene copolymer, and propylene-butene copolymer.

In addition, the polyolefin resin in the present embodiment may be a single resin or a mixture of two or more different polyolefin resins.

Among these various polyolefin resins, polyethylene resins are particularly preferred from the viewpoint of excellent pore-blocking performance. The melting point (softening point) of the polyethylene resin is preferably 70 to 150°C from the viewpoint of the pore-blocking performance of the microporous membrane.

Hereinafter, a polyethylene resin will be described in detail as an example of the polyolefin resin used in the embodiment of the present invention. The types of polyethylene resin used in the embodiment of the present invention include high-density polyethylene having a density of more than 0.94 g/ ^cm3 , medium-density polyethylene having a density in the range of 0.93 to 0.94 g/ ^cm3 , low-density polyethylene having a density lower than 0.93 g/ ^cm3 , linear low-density polyethylene, and ultra-high molecular weight polyethylene having a specific molecular weight, but from the viewpoint of controlling the internal pore structure of the polyolefin microporous membrane described below within a desired range, it is preferable that the microporous membrane contains 80% by mass or more of ultra-high molecular weight polyethylene when the total mass of the microporous membrane is 100% by mass.

The ultra-high molecular weight polyethylene used in the embodiment of the present invention preferably has a weight average molecular weight of 1.0×10 ⁶ or more and 1.0×10 ⁷ or less. If the weight average molecular weight is 1.0×10 ⁶ or more, the relaxation time is not too short, and the increase in the stretching temperature and heat treatment temperature is suppressed, and fine fibrils are melted, and the number of pores in the microporous membrane can be prevented from decreasing. By using an ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 ⁶ or more, the entanglement of molecular chains is increased, and stress is uniformly applied to the polyethylene resin layer in the stretching process, so that it is possible to control various structures in the thickness direction described below within a desired range. Therefore, the weight average molecular weight of the ultra-high molecular weight polyethylene is 1.0×10 ⁶ or more, preferably 1.5×10 ⁶ or more, more preferably 2.0×10 ⁶ or more, and even more preferably 3.0×10 ⁶ or more. The upper limit of the weight average molecular weight (Mw) is preferably 8.0×10 ⁶ or less, more preferably 6.0×10 ⁶ or less, even more preferably 5.0×10 ⁶ or less, and still more preferably 4.0×10 ⁶ or less.

The molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of the ultra-high molecular weight polyethylene is preferably within the range of 3.0 to 100. The narrower the molecular weight distribution, the more uniform the system and the easier it is to obtain uniform micropores, so the narrower the molecular weight distribution, the lower the moldability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or more, more preferably 5.0 or more, and even more preferably 6.0 or more. As the molecular weight distribution increases, the amount of low molecular weight components increases, which leads to a decrease in strength and the tendency for fine fibrils to melt and fuse during stretching and heat setting, so the upper limit is preferably 80 or less, more preferably 50 or less, even more preferably 20 or less, and even more preferably 10 or less. By setting it in the above range, good moldability can be obtained, and the system can be unified to obtain uniform micropores.

The high-density polyethylene used in the embodiment of the present invention preferably has a weight-average molecular weight (Mw) of 1.0 × 10 ⁴ or more and 1.0 × 10 ⁶ or less, more preferably 1.0 × 10 ⁵ or more and 1.0 × 10 ⁶ or less, and even more preferably 5.0 × 10 ⁵ or more and 9.0 × 10 ⁵ or less. By using a high-density polyethylene having a weight-average molecular weight within the above range for the polyolefin-based microporous membrane of the embodiment of the present invention, pressure fluctuations of the resin in the extruder are less likely to occur, and the quality may be improved.

In addition, the polyolefin-based microporous membrane according to the embodiment of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, and fillers, within the scope of the invention, without impairing the effects of the present invention. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration of the polyolefin-based resin due to its thermal history.

As the antioxidant, it is preferable to use one or more selected from, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, etc.

In the present invention, the dynamic friction coefficient μk between one surface and the other surface of a membrane can be determined by determining the dynamic friction force between the microporous membranes in accordance with ASTM-D-1894, and then dividing the result by the normal force of the microporous membrane (i.e., the gravity of a weight placed on the membrane). As a specific measurement method, the microporous membranes are stacked one on top of the other so that one surface and the other surface are in contact with each other, and a weight called a sliding piece, weighing 200 g and having a contact surface of 63 mm square, is placed on top of the upper microporous membrane. A horizontal force is then applied to the upper microporous membrane, and the contact surfaces are slid to determine the dynamic friction force.

In an embodiment of the present invention, the dynamic friction coefficient μk between one surface and the other surface of the membrane must be 0.7 or less in order to increase the ease of sliding and improve the appearance when wound up. In addition, in order to further increase the ease of sliding and improve the appearance when wound up for transporting the microporous membrane, the dynamic friction coefficient μk between one surface and the other surface of the membrane is preferably 0.65 or less, and more preferably 0.6 or less. In order to prevent slippage after winding, it is important that the dynamic friction coefficient μk between one surface and the other surface of the membrane is 0.2 or more.

In the embodiment of the present invention, it is important that the root-mean-square height Sq is 10 nm or more and 100 nm or less on the surface where the root-mean-square height Sq is large between one surface and the other surface of the microporous membrane. The root-mean-square height Sq of the present invention is obtained by measuring the surface of the microporous membrane using a white light interferometer. As a result of various studies, it has been found that the root-mean-square height Sq in non-contact measurement in an area surrounded in mm units is an important indicator for miniaturization and thinning of a battery after winding, and it is important for miniaturization and thinning of a battery to have the root-mean-square height Sq of 100 nm or less on the surface where the root-mean-square height Sq is large between one surface and the other surface of the microporous membrane. The area surrounded in mm units is, for example, a measurement area of 2.5 mm x 2.5 mm. From the viewpoint of further miniaturization and thinning of the battery, the root mean square height Sq is preferably 90 nm or less, more preferably 80 nm or less, and particularly preferably 65 nm or less. Also, from the viewpoint of maintaining the slipperiness of the microporous membrane, it is important that the root mean square height is 10 nm or more, more preferably 20 nm or more.

If the root mean square height Sq of both surfaces of the microporous membrane is small and within a specific range, this leads to thinner and smaller batteries. Therefore, if the root mean square height Sq of the surface with a large root mean square height Sq is set to a specific range, the root mean square height Sq of the opposite surface will be smaller, and neither surface of the microporous membrane will exceed the specific range.

Regarding the relationship between the smoothness and slipperiness of a film, generally, when the smoothness of the film is reduced by roughening the surface of the film, the coefficient of dynamic friction decreases and the slipperiness increases, and the smoothness and slipperiness of the film are often contradictory. Therefore, it is not easy to achieve both of these, and as a result of various studies, it was found that a method in which 80% by mass or more of the constituent resin of the polyolefin-based microporous film is ultra-high molecular weight polyethylene, the areal ratio of wet stretching is 60 times or more, and the heat setting temperature after stretching is set to a temperature 25°C or more lower than the melting point of the polyethylene-based resin for 15 minutes or more is effective. With such a configuration and manufacturing method, a large number of fine fibrils (fibrous resin components) are formed on the surface of the microporous film to reduce the contact area between the microporous films and to exhibit slipperiness, and by performing heat setting at a low temperature for a long time, the fibril shape of the microporous film surface due to heating is maintained, while the root mean square height Sq in the area enclosed in mm units of the microporous film, i.e., the smoothness, can be improved. Microfibrils are fibrous resin structures that are irregularly connected in three dimensions and have an average diameter of several nanometers to several hundred nanometers.

In the present invention, from the viewpoint of scratch resistance when transported by a rotating roll or the like, the polyolefin-based microporous membrane preferably has a dynamic friction coefficient μmk between the membrane surface and metal of 0.13 or less on at least one surface of the membrane. From the viewpoint of improving scratch resistance, the dynamic friction coefficient μmk between the membrane surface and metal is more preferably 0.08 or less, and even more preferably 0.04 or less. The dynamic friction coefficient μmk between the membrane surface and metal can be determined by determining the dynamic friction force between the microporous membrane and metal foil (e.g., SUS foil, aluminum foil) according to ASTM-D-1894, and then dividing it by the normal force of the microporous membrane (i.e., the gravity of a weight placed on the membrane). As a specific measurement method, the microporous membrane and the metal foil are stacked one on top of the other so that one surface of the microporous membrane and the surface of the metal foil are in contact with each other, and a 200 g weight called a sliding piece with a contact surface of 63 mm square is placed on top of the upper microporous membrane. A horizontal force is then applied to the upper microporous membrane, and the contact surfaces are slid to determine the kinetic friction force.

Methods for making the dynamic friction coefficient μmk between the surface of the microporous membrane and metal 0.13 or less include a method in which the stretching ratio in the width direction is higher than that in the longitudinal direction in the wet stretching described below, and the stretching ratio in the longitudinal direction is higher than that in the width direction in the dry stretching after the wet stretching. By using such manufacturing conditions, it is possible to orient the fibrils on the surface of the microporous membrane more in the longitudinal direction, thereby improving the slipperiness when the microporous membrane is shifted on the metal in the transport direction.

From the viewpoint of achieving both easy lubrication and smoothness, the polyolefin-based microporous membrane according to an embodiment of the present invention preferably has a fibril count of 870/μm3 to ²⁸⁰⁰ / ^μm3 and an average fibril diameter of 25 nm to 60 nm in a three-dimensional image of a cube with a side length of 2.7 μm, which is created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane.

FIB-SEM measurement refers to a method of measuring continuous images at regular intervals in the depth direction by repeatedly cutting off the cross section of a microporous membrane at regular intervals (FIB cutting) with a focused ion beam (FIB) and taking SEM (scanning electron microscope) images of the cut surface. A specific example of the measurement method is to impregnate a microporous membrane with a resin that has been subjected to electronic dyeing, embedding the pores, and then using a microtome to prepare a slice of the film cross section so that the cross section of the film becomes the initial observation surface. One example is a method of cutting the obtained slice of the film cross section by 10 nm in the depth direction while sequentially taking SEM images in the depth direction. In addition, as a method of specifying the position information of each SEM image, a method is used in which a marking containing a metal component is made on a part of the observation screen of the microporous membrane to be cut with FIB, and the correlation between the positions of each image is specified based on the marking position. The image area is preferably about 3 μm square to 10 μm square, and if the observation surface is inclined, the scale may be converted taking the inclination into account.

In addition, a method for creating a three-dimensional image from each cross-sectional image may involve, for example, using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd. to perform binarization processing on the embedded resin portion (bright resin portion, i.e., the portion corresponding to the pore portion of the microporous membrane) observed as a bright portion of the polyolefin-based microporous membrane and the resin portion (constituent resin portion) constituting the polyolefin-based microporous membrane, and then creating a three-dimensional stereoscopic image based on the information from the binarization processing. Regarding the three-dimensional stereoscopic image, a method may be used in which a three-dimensional image of the constituent resin portion inside the polyolefin-based microporous membrane is created by performing a thinning process on the resin portion constituting the polyolefin-based microporous membrane using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd. In terms of the size of the three-dimensional image to be created, from the perspective of analysis time and reproducibility of analysis parameters, in this invention, the image will be a cube with sides 2.7 μm long.

The number of fibrils is determined by dividing the lines of the constituent resin parts obtained from the three-dimensional image by dividing them at the intersections and branches of the lines of the constituent resin parts that have been subjected to a dividing process using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd., and then converting the number of fibrils per ¹ μm3. As a specific example of the dividing process, an X-shaped portion is divided into four, a Y-shaped portion into three, and a T-shaped portion into three.

From the viewpoint of achieving both good lubricity and smoothness, the polyolefin-based microporous membrane according to an embodiment of the present invention has a fibril count of preferably 1000/μm3 or ^more , more preferably 1250/μm3 or more, and even more preferably 1400/ ^μm3 or ^more in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by the above-mentioned FIB-SEM measurement of the microporous membrane.

In the present invention, a method for reducing the number of fibrils from 870/μm3 to 2800/μm3 or ^less in a 2.7 μm square ^three -dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane includes a method in which 80% by mass or more of the resin constituting the polyolefin microporous membrane is ultra-high molecular weight polyethylene, the areal magnification of wet stretching is 60 times or more, and the resin concentration during production (the ratio of the total amount of each resin such as polyethylene when the total amount of the polyolefin resin solution is taken as 100% by mass) is less than 30% by mass. In an embodiment of the present invention, by using a high molecular weight polyethylene resin as 80% by mass or more of the resin constituting the polyolefin microporous membrane, spherulite growth of polyethylene in the cast sheet before stretching can be suppressed and the cast sheet structure can be made uniform, and therefore, by combining with wet stretching with a wet areal magnification of 60 times or more, uniform pore opening is possible and a large number of fine fibrils can be arranged on the surface of the microporous membrane.

From the viewpoint of improving the impact resistance of the battery when used as a separator for a lithium ion secondary battery, the polyolefin-based microporous film of the present invention preferably has a puncture strength of 180 gf or more and 700 gf or less. From the viewpoint of further improving the impact resistance of the battery when used as a separator for a lithium ion secondary battery, the puncture strength is more preferably 250 gf or more, even more preferably 350 gf or more, and particularly preferably 500 gf or more. From the viewpoint of the impact resistance of the battery, the higher the strength of the polyolefin-based microporous film, the more preferable it is, but from the viewpoint of a good balance with other physical properties such as the thermal shrinkage rate, 700 gf or less is preferable. Note that the puncture strength in the present invention is the puncture strength when the thickness is converted to 10 μm.

A method for making the pin puncture strength of a polyolefin-based microporous membrane 180 gf or more and 700 gf or less includes making 80% by mass or more of the constituent resin of the polyolefin-based microporous membrane ultra-high molecular weight polyethylene, making the areal ratio of the wet stretching 60 times or more, and making the porosity of the polyolefin-based microporous membrane 35% or more and 55% or less.

The polyolefin microporous film of the present invention has a thickness of 3 μm or more and 14 μm or less, which is preferable from the viewpoint of increasing the capacity of the battery, since when used as a separator of a lithium ion battery, the distance between electrodes can be reduced and the number of stacked battery components can be increased.As a method for making the thickness 3 μm or more and 14 μm or less, a method of adopting a wet stretching method and increasing the stretching ratio or the line speed during production is used.From the viewpoint of increasing the capacity of the battery, the thickness of the polyolefin microporous film is more preferably 12 μm or less, more preferably 10 μm or less, and even more preferably 7 μm or less.
The polyolefin microporous film of the present invention is preferably 20% or less in both the longitudinal and transverse directions at 130°C for 1 hour, because when used as a separator for a lithium ion battery, the film has good dimensional stability when held at high temperatures, and insulation failure caused by deformation at the separator end is unlikely to occur. Among polyolefin microporous films, microporous films mainly composed of polyethylene resins in particular have a shutdown function in which the pores are closed when the temperature reaches approximately 120 to 150°C, and when used as a separator for a lithium ion battery, they have a function of stopping the movement of lithium ions and suppressing thermal runaway of the battery. Therefore, the lithium ion battery may be held for a certain period of time up to a temperature just before shutdown occurs, and if the separator is significantly deformed in this temperature range (e.g., 130°C), a short circuit may occur between the positive and negative electrodes in the lithium ion battery, leading to problems such as fire.
The polyolefin microporous membrane of the present invention preferably has a heat shrinkage rate of 18% or less in both the longitudinal and transverse directions at 130° C. for 1 hour from the viewpoint of improving dimensional stability at high temperatures. Also, the heat shrinkage rate of 3% or more in both the longitudinal and transverse directions at 130° C. for 1 hour from the viewpoint of maintaining shutdown properties.
An example of a method for making the heat shrinkage rate at 130°C for 1 hour 20% or less in both the longitudinal and transverse directions is a method in which the re-stretching temperature is 130°C or higher in a step of stretching (re-stretching) a dried polyolefin-based microporous membrane in at least one axial direction, as described below.

When manufacturing polyolefin-based microporous membranes, a common method is to prepare an intermediate product wound into a roll, and then slit the intermediate product into a reel shape to produce the finished product. Polyolefin-based microporous membrane rolls, which are seen as intermediate products or products after slitting, have good and uniform slipperiness and smoothness over a wide range in the longitudinal direction. One index for evaluating uniformity is the coefficient of variation. The coefficient of variation is the standard deviation of the measured values (N pieces) divided by the average value.

From the viewpoint of achieving good smoothness over a wide range, it is preferable that the coefficient of variation of the root mean square height Sq when a 10 m long microporous membrane is cut from the roll in the longitudinal direction and 10 or more points are measured at equal intervals in the longitudinal direction of the microporous membrane is 10% or less. From the viewpoint of achieving better smoothness, it is more preferable that the coefficient of variation of the root mean square height Sq of the polyolefin microporous membrane roll is 8% or less, and even more preferably 6% or less.

In addition, from the viewpoint of improving the easy slippage over a wide range, the polyolefin-based microporous membrane roll of the present invention is preferably such that when the microporous membrane having a length of 1 m in the longitudinal direction is cut from the rolled state at 20 or more locations, the microporous membranes are stacked one on top of the other in any combination so that one surface of the microporous membrane is in contact with the other surface, and the dynamic friction force μk is measured 10 or more times, the coefficient of variation of the dynamic friction coefficient μk is 10% or less. From the viewpoint of improving the easy slippage over a wide range, the coefficient of variation of the dynamic friction coefficient μk of the polyolefin-based microporous membrane roll is more preferably 8% or less, and particularly preferably 6% or less.

Methods for setting the root mean square height Sq and the coefficient of variation of the dynamic friction coefficient μk of the microporous membrane roll within the desired range include a method in which 80% by mass or more of the constituent resin of the polyolefin-based microporous membrane is ultra-high molecular weight polyethylene, the areal ratio of the wet stretching is 60 times or more, the heat setting temperature after stretching is set to a temperature 25°C or more lower than the melting point of the polyethylene-based resin, and the film is produced stably and continuously for 15 minutes or more, thereby setting the dynamic friction coefficient μk and the root mean square height Sq within the desired range.

The polyolefin-based microporous film according to the embodiment of the present invention may be laminated with a heat-resistant resin layer to improve the heat resistance when mounted on a lithium-ion battery. As the heat-resistant resin layer, resins that are insoluble in the electrolyte of the battery and are electrically stable within the range of the battery use conditions, such as various fluorine-based resins, acrylic resins, and aromatic polyamide resins, are preferably used. In addition, the heat-resistant resin layer may contain an organic powder, an inorganic powder, or a mixture thereof as a filler to further improve the heat resistance. For example, the organic powder may be a fluorine-based resin, a melamine-based resin, an aromatic polyamide-based resin, etc., and the inorganic powder may be a metal oxide, a metal nitride, a metal carbide, a metal hydroxide, a carbonate, a sulfate, etc., more specifically, alumina, silica, titanium dioxide, aluminum hydroxide, calcium carbonate, etc. can be used.

Next, an example of a method for producing a polyolefin-based microporous membrane according to an embodiment of the present invention will be described below, but the present invention should not be interpreted as being limited to this example.

The method for producing a microporous polyolefin membrane according to an embodiment of the present invention preferably includes the following steps (a) to (e).
(a) A process for melt-kneading a polymer material containing one or more polyolefin-based resins and, if necessary, a solvent to prepare a polyolefin-based resin solution; (b) A process for extruding the melt, forming it into a sheet, and cooling and solidifying it; (c) A process for stretching the obtained sheet by a roll method or a tenter method; (d) A process for extracting the plasticizer from the obtained stretched film and drying the film; (e) A process for heat treatment/re-stretching. Each process will be explained below.

(a) Preparation of polyolefin resin solution The polyolefin resin used in the embodiment of the present invention is dissolved in a plasticizer by heating to prepare a polyolefin resin solution. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, but it is preferable that the solvent is liquid at room temperature in order to enable stretching at a relatively high ratio.

Solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. In order to obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

From the viewpoint of making it easier to control the fine fibril structure within a specific range, it is preferable that the solvent concentration is in the range of more than 70% by mass when the total amount of the polyolefin resin solution is taken as 100% by mass. A solvent that is miscible with the polyolefin resin in the melt-kneaded state but is solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, if only a solid solvent is used, there is a risk of uneven stretching, etc.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40°C. If the viscosity at 40°C is 20 cSt or more, the sheet extruded from the die of the polyolefin resin solution is less likely to be non-uniform. On the other hand, if the viscosity at 40°C is 200 cSt or less, the liquid solvent is easily removed. The viscosity of the liquid solvent is measured at 40°C using an Ubbelohde viscometer.

(b) Formation of extrudate and formation of gel-like sheet The method of homogeneous melt kneading of polyolefin resin solution is not particularly limited, but when preparing a high-concentration polyolefin resin solution, it is preferable to carry out in a twin-screw extruder. If necessary, known additives such as metal soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, antistatic agents, etc. may be added within a range that does not impair film formability and does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of polyolefin resin.

In the extruder, the polyolefin resin solution is mixed uniformly at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably between (melting point of polyolefin resin + 10°C) and (melting point of polyolefin resin + 120°C). More preferably, it is between (melting point of polyolefin resin + 20°C) and (melting point of polyolefin resin + 100°C).

The melting point here refers to a value measured by DSC (Differential Scanning Calorimetry) based on JIS K7121 (1987). For example, when the polyolefin resin is a polyethylene resin, the melt-kneading temperature of the polyethylene resin is preferably in the range of 140 to 250°C. More preferably, it is 160 to 230°C, and even more preferably, it is 170 to 200°C. Specifically, since the polyethylene resin has a melting point of about 130 to 140°C, the melt-kneading temperature is preferably 140 to 250°C, and even more preferably, 180 to 230°C.

From the viewpoint of suppressing deterioration of the polyolefin resin, a lower melt-kneading temperature is preferable, but if the temperature is lower than the above-mentioned temperature, unmelted material may be generated in the extrudate extruded from the die, which may cause film breakage in the subsequent stretching process. If the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin resin becomes intense, and the physical properties of the obtained polyolefin microporous film, such as strength and porosity, may deteriorate. In addition, the decomposition products may precipitate on the cooling roll or the rolls in the stretching process, and adhere to the sheet, leading to a deterioration in the appearance. Therefore, it is preferable to knead the melt-kneading temperature within the above-mentioned range.

Next, the resulting extrudate is cooled to obtain a gel-like sheet, and the microphase of the polyolefin resin separated by the solvent can be fixed by cooling. In the cooling step, it is preferable to cool the gel-like sheet to 10 to 50°C. This is to make the final cooling temperature below the crystallization end temperature, and by making the higher-order structure finer, it becomes easier to perform uniform stretching in the subsequent stretching. Therefore, it is preferable to cool at a rate of 30°C/min or more until the temperature is at least below the gelling temperature.

In general, when the cooling rate is slow, relatively large crystals are formed, resulting in a coarse higher-order structure of the gel-like sheet and a large gel structure. In contrast, when the cooling rate is fast, small and uniform crystals are formed, resulting in a dense higher-order structure of the gel-like sheet, which leads to uniform stretching and a reduction in unopened areas.

Cooling methods include direct contact with cold air, cooling water, or other cooling media, contact with a roll cooled with a refrigerant, or using a casting drum, etc.

So far, the polyolefin-based microporous film has been described as being single-layered, but the polyolefin-based microporous film according to the embodiment of the present invention is not limited to being single-layered, and may be a laminate. There is no particular limit to the number of layers, and the film may be a two-layer laminate or a three-layer or more laminate.

Examples of methods for forming a polyolefin-based microporous film into a laminate include a method in which desired resins are prepared as needed, these resins are fed separately to an extruder and melted at a desired temperature, and the resins are joined in a polymer tube or die, and extruded through a slit die to the desired thickness for each layer to form a laminate.

(c) Wet stretching step The gel-like sheet (including laminated sheet) thus obtained is stretched. Examples of the stretching method that can be used include uniaxial stretching in the sheet conveying direction (MD) using a roll stretching machine, uniaxial stretching in the sheet width direction (TD) using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter.

The stretching ratio varies depending on the thickness of the gel-like sheet, but it is preferable to stretch to 7 times or more in either direction from the viewpoint of uniformity of the film thickness. In addition, from the viewpoint of setting the dynamic friction coefficient μk and the root mean square height Sq of the surface in the desired range, the areal ratio is preferably 60 times or more, more preferably 80 times or more, and particularly preferably 100 times or more. In addition, from the viewpoint of suppressing breakage during the production of the polyolefin-based microporous film, the areal ratio is preferably 150 times or less. In addition, with regard to the stretching ratios in the conveying direction and the width direction, from the viewpoint of improving the easy sliding property with metal by combining with the dry stretching described later, it is preferable to set the stretching ratio in the width direction larger than the stretching direction, and the stretching ratio in the width direction is preferably 1.4 times or more, more preferably 1.6 times or more, relative to the stretching ratio in the conveying direction.

From the viewpoint of improving the stretching uniformity in the stretching process, the preferred form of the stretching ratio and raw material composition is to stretch from a wet gel-like sheet at an areal ratio of 60 times or more, and more preferably to stretch from a wet gel-like sheet at a areal ratio of 10 x 10 times or more, with the ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1 million or more being 80% by mass or more when the total mass of the total polyolefin resin contained in the gel-like sheet is taken as 100% by mass. An even more preferred form is to stretch from a wet gel-like sheet at an areal ratio of 60 times or more, and more preferably to stretch from a wet gel-like sheet at a areal ratio of 10 x 10 times or more, with the ultra-high molecular weight polyethylene having a Mw of 2 million or more being 80% by mass or more when the total mass of the cast sheet is taken as 100% by mass.

The ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1,000,000 or more preferably accounts for 90% by mass or more, and more preferably 95% by mass or more, when the total mass of all polyolefin resins contained in the gel-like sheet is taken as 100% by mass.

The stretching temperature is preferably set to the melting point of the gel-like sheet + 10°C or less, and more preferably in the range of (crystal dispersion temperature of polyolefin resin) to (melting point of gel-like sheet + 5°C). Specifically, since a polyethylene composition has a crystal dispersion temperature of about 90 to 110°C, the stretching temperature is preferably 100 to 130°C, more preferably 115 to 125°C, and even more preferably 117.5 to 125°C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of the dynamic viscoelasticity measured according to ASTM D 4065 (2012).

If the stretching temperature is less than 90°C, the pores will be insufficient due to low-temperature stretching, making it difficult to achieve uniformity in film thickness and reducing the porosity. If the stretching temperature is higher than 130°C, the sheet may melt and the pores may become easily blocked.

The stretching described above causes the higher-order structure of the gel sheet to break down, the crystal phase to become finer, and numerous fibrils to form. The fibrils form a network structure in which they are irregularly connected three-dimensionally. The stretching improves the mechanical strength and forms pores, making the polyolefin-based microporous membrane of the present invention suitable for use as a battery separator.

In addition, by stretching before removing the plasticizer, the polyolefin resin is in a sufficiently plasticized and softened state, which allows the higher-order structure to be cleaved smoothly and allows the crystal phase to be uniformly refined. In addition, by stretching before removing the plasticizer, the higher-order structure is easily cleaved, so distortion during stretching is less likely to remain, and the thermal shrinkage rate can be reduced compared to when stretching after removing the plasticizer.

(d) Plasticizer Extraction (Washing) and Drying Step Next, the plasticizer (solvent) remaining in the gel-like sheet is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a polyolefin microporous membrane is obtained by removing the solvent.

Examples of cleaning solvents include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain fluorocarbons such as ethane trifluoride.

These cleaning solvents have low surface tension (for example, 24 mN/m or less at 25°C). By using a cleaning solvent with low surface tension, the network structure that forms the micropores is prevented from shrinking due to the surface tension at the air-liquid interface during drying after cleaning, resulting in a polyolefin-based microporous membrane with excellent porosity and permeability. These cleaning solvents are selected appropriately depending on the plasticizer, and are used alone or in combination.

The cleaning method may be a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the cleaning solvent onto the gel-like sheet, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but it is generally preferable to use 300 parts by weight or more per 100 parts by weight of the gel-like sheet.

The cleaning temperature may be 15 to 30°C, and may be heated to 80°C or lower if necessary. In this case, the longer the time that the gel-like sheet is immersed in the cleaning solvent, the better, from the viewpoints of increasing the cleaning effect of the cleaning solvent, preventing the physical properties (e.g., physical properties in the TD and/or MD) of the resulting polyolefin-based microporous membrane from becoming non-uniform, and improving the mechanical and electrical properties of the porous polyolefin film.

It is preferable to carry out the above-mentioned washing process until the residual solvent in the gel-like sheet after washing, i.e., the polyolefin-based microporous membrane, is less than 1% by mass.

Then, in the drying process, the solvent in the polyolefin-based microporous membrane is dried and removed. There are no particular limitations on the drying method, and methods using metal heating rolls or hot air can be selected. The drying temperature is preferably 40 to 100°C, and more preferably 40 to 80°C. If the drying is insufficient, the porosity of the polyolefin-based microporous membrane may decrease in the subsequent heat treatment, resulting in poor permeability.

(e) Heat Treatment/Re-Stretching Step The dried polyolefin-based microporous membrane may be stretched (re-stretched) at least uniaxially. The re-stretching may be performed by a tenter method or the like while heating the porous polyolefin film, similarly to the above-mentioned stretching. The re-stretching may be uniaxial or biaxial. In the case of multi-stage stretching, simultaneous biaxial or sequential stretching may be combined.

The re-stretching temperature is preferably below the melting point of the polyolefin resin, and more preferably within the range of (Tcd of the polyolefin resin composition - 20°C) to the melting point of the polyolefin resin. Specifically, in the case of a polyethylene resin, the re-stretching temperature is preferably 70 to 135°C, more preferably 110 to 135°C, even more preferably 125 to 135°C, and even more preferably 130 to 135°C.

The re-stretching ratio may be either uniaxial or biaxial, but in the case of biaxial stretching, it is preferable to stretch 1.01 to 2.0 times in each of the conveying direction and the width direction. From the viewpoint of improving the slipperiness with metal, it is preferable to set the stretching ratio in the width direction in (c) wet stretching to be larger than the stretching ratio in the conveying direction, and then set the stretching ratio in the conveying direction in re-stretching to be larger than the stretching ratio in the width direction. By setting such stretching conditions, it is possible to orient a large number of fine fibrils formed in wet stretching in the conveying direction after dry re-stretching, and it is possible to suppress film rupture during stretching by alternately setting high-magnification directions for wet stretching and re-stretching.

After re-stretching, it is preferable to carry out a heat setting treatment to improve the thermal dimensional stability and smoothness of the polyolefin-based microporous film. From the viewpoint of maintaining slipperiness while improving smoothness, it is preferable to set the heat setting temperature at a temperature 25°C or more lower than the melting point of the polyethylene-based resin and carry out the heat treatment for 15 minutes or more. By carrying out the heat setting treatment at a lower temperature and for a longer period than before, it is possible to maintain the fine fibril shape of the surface and improve slipperiness, while mitigating waviness and distortion during stretching and improving smoothness. The heat treatment can be carried out at a low speed using a separate processing line after winding once, or by putting the wound roll into an oven.

(f) Other steps Furthermore, depending on other applications, the polyolefin-based microporous membrane may be subjected to hydrophilization treatment. The hydrophilization treatment may be performed by monomer grafting, surfactant treatment, corona discharge, etc. The monomer grafting is preferably performed after the crosslinking treatment.

The methods for measuring the characteristics and evaluating the effects in the embodiments of the present invention are as follows. However, the embodiments of the present invention are not limited to these examples.

(1) Weight average molecular weight (Mw)
The molecular weight distribution of the polyolefin (weight average molecular weight, molecular weight distribution, content of a specific component, etc.) was measured by high-temperature gel permeation chromatography (GPC) under the following measurement conditions.

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 columns) (φ7.8 mm × 30 cm, Showa Denko)
Solvent: 1,2,4-trichlorobenzene (TCB, Wako Pure Chemical Industries, Ltd.) (0.1% BHT added)
Flow rate: 1.0mL/min
Column temperature: 145 ° C.
Sample preparation: 5 mL of the measurement solvent was added to 5 mg of sample, and the mixture was heated and stirred at 160 to 170° C. for about 30 minutes, and the resulting solution was filtered through a metal filter (pore size: 0.5 μm).

Injection volume: 0.200mL
Standard sample: Monodisperse polystyrene (manufactured by Tosoh) (PS)
Data processing: TRC GPC data processing system. The obtained Mw was then converted to polyethylene (PE) using the following conversion formula:

Mw (PE conversion) = Mw (PS conversion measurement value) x 0.468
(2) Film Thickness The film thickness of the polyolefin microporous film was measured at five points within an area of 50 mm × 50 mm using a contact thickness meter, Litematic VL-50 (10.5 mmφ ultrahard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation, and the average value was recorded as the film thickness (μm).

(3) Porosity A 5 cm square sample for evaluation was cut from the polyolefin microporous membrane, and its volume ( ^cm3 ) and mass (g) at room temperature (25°C) were determined, and the porosity was calculated from these values and the resin density (g/ ^cm3 ) using the following formula. The resinity was set to a constant value of 0.99 g/ ^cm3 .

Porosity = [(volume - mass / resin density) / volume] x 100.

(4) Puncture strength Measured in accordance with JIS Z 1707 (2019) except that the test speed was 2 mm/sec. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), the maximum load (L1) was measured when a polyolefin microporous membrane was pierced with a needle having a diameter of 1.0 mm and a spherical tip (radius of curvature R: 0.5 mm) in an atmosphere of 25°C, and the puncture strength (L2) converted to a film thickness of 10 μm was calculated using the following formula.
L2(gf)=L1(gf)×10(μm)/thickness of polyolefin microporous membrane (μm) (5) Measurement of three-dimensional structure of microporous membrane (FIB-SEM)
Sequential images were measured using the FIB-SEM under the following conditions.
Sample preparation: A polyolefin-based microporous membrane was embedded in an epoxy-based resin, and then subjected to electronic staining using _OsO4 and subjected to measurement.
Observation device: FEI Helios G4
Observation conditions: Acceleration voltage 1 kV
Sample tilt: 52°
Pixel size: Image horizontal direction: 5.4 nm, image vertical direction: 6.8 nm (after tilt correction)
・FIB slice interval: 10 nm
Image alignment method: Marking was done by depositing Pt on the top of the film, and the position between each image was confirmed.
Tilt correction: Since FIB-SEM observations were performed at an angle of 52°, the SEM images were observed shrinking in the vertical direction. To make the image vertically resemble that observed from the front, it was necessary to magnify it by 1.27 (=/sin52°). The image after tilt correction was used to create the three-dimensional image described below.
Measurement size: FIB processing was performed sequentially on 5 μm×5 μm of the film cross section, and the film was sliced in the depth direction until it reached 4 μm, and measurements were performed on a volume of 5 μm×5 μm×4 μm (400 captured images).

(6) Creation of a three-dimensional image For the FIB-SEM image obtained in (5), the embedded bright resin portion (i.e., the portion corresponding to the pores of the microporous membrane) and the resin portion constituting the microporous membrane were binarized using image processing software "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd., and then a three-dimensional stereoscopic image was created based on the information from the binarization process. In the binarization process, noise was removed from the FIB-SEM image by averaging 3 pixels x 3 pixels, and then dynamic binarization was performed using a threshold value of -30 gradation from the image averaged over 21 pixels x 21 pixels, thereby dividing the image into two types, bright and dark. Thereafter, a three-dimensional image of the resin portion constituting the inside of the microporous membrane was created by performing a thinning process on the resin portion constituting the microporous membrane using image processing software "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd. As a specific example of the division process, a process method was adopted in which an X-shaped portion was divided into four, a Y-shaped portion into three, and a T-shaped portion into three. From the viewpoint of analysis time and reproducibility of analysis parameters, the size of the three-dimensional image to be created was set to a cube surrounded by a side with a length of 2.7 μm at the center out of the FIB-SEM measurement size of 5 μm × 5 μm × 4 μm.

(7) Measurement of the number of fibrils The three-dimensional image obtained in (6) was processed using image processing software "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd., to determine the number of thin lines remaining after dividing the lines of the constituent resin parts that had been subjected to a thinning process at the intersections and branching parts. The value converted into the number of lines per ¹ μm3 was taken as the number of fibrils in the present invention.

(8) Measurement of Average Diameter of Fibrils The average value of all fibril diameters contained in the three-dimensional image of 2.7 μm square obtained in (6) was calculated using image processing software "ExFact (registered trademark) Analysis for Fiber" manufactured by Japan Visual Science Co., Ltd., and this was taken as the average diameter of fibrils.

(9) Dynamic friction coefficient μk between films
In accordance with ASTM-D-1894, microporous membranes were placed one on top of the other so that one surface was in contact with the other, and a 200 g weight called a sliding piece with a contact surface of 63 mm square was placed on top of the upper microporous membrane. A horizontal force was applied to the upper microporous membrane to slide the contact surface, and the kinetic frictional force was determined by dividing the kinetic frictional force by the normal force equivalent to the gravity of the weight to determine the kinetic friction coefficient μk between the membranes.

(10) Dynamic friction coefficient between film and metal μmk
The dynamic friction coefficient μmk between the film and metal was determined in the same manner as in (9), except that the lower microporous film was changed to an aluminum foil having a thickness of 0.3 μm (As One Corporation, product number 3-2154-08).

(11) Root mean square height (Sq) of the microporous membrane
The microporous membrane was cut into 30 mm squares to prepare a measurement sample, and then a Mitsubishi Chemical Systems "VertScan 3.0 R4300C" was used as a white light interferometer to measure unevenness information in a measurement area of 2.5 mm x 2.5 mm, and the root mean square height Sq defined in ISO 25178 was obtained. Note that measurements were performed five times at different locations within the same sample, and the average value was adopted as the root mean square height Sq. In addition, measurements were performed on both sides of the sample, and the value of the surface with the largest Sq was adopted as the root mean square height Sq of the sample.

(12) Variation coefficient of dynamic friction coefficient μk A film of 10m length is cut out from a roll of 10cm wide polyolefin microporous film, and samples are further cut out from the end of the film in the longitudinal direction at intervals of 50cm in the longitudinal direction.From the width direction center part of the cut out sample, two pieces are selected at random to make a total of 10 combinations, and then μk is calculated for each of them by the same method as in (9).The standard deviation of the 10 measured values is divided by the average value to calculate the variation coefficient of dynamic friction coefficient μk.

(13) Coefficient of variation of square root height Sq After cutting out the samples, Sq was determined for each sample in the same manner as in (11), and the standard deviation of the 10 measured values was divided by the average value to determine the coefficient of variation of the square root height Sq.

(14) Appearance evaluation of microporous roll A 100 mm wide, 200 m long film (3 inch paper core) was prepared and rewound onto another 3 inch paper core under a conveying tension of 30 N/m. The appearance of the microporous membrane roll after rewinding was visually inspected while changing the conveying speed, and evaluated according to the following criteria.
A: No wrinkles were generated even when rewinding at a speed of 40 m/min.
B: When rewinding at a speed of 40 m/min, wrinkles were generated, but when rewinding at a speed of 30 m/min, no wrinkles were generated.
C: Wrinkles occurred when rewinding at a speed of 30 m/min, but no wrinkles occurred at a speed of 20 m/min.
D: Wrinkles occurred when rewinding at a speed of 20 m/min, but no wrinkles occurred at a speed of 10 m/min.
E: Wrinkles occurred when rewinding at a speed of 10 m/min.

(15) Scratch resistance The microporous membrane was slid on aluminum foil in the same manner as in (9) except that the load of the weight was 400 g and the contact surface was slid five times at the same location. Then, Al was deposited on the aluminum foil contact surface of the microporous membrane. The number of linear scratches in the direction in which the microporous membrane slid (the number included in a 63-mm width of the sliding piece length) was counted using an optical microscope and evaluated according to the following criteria.
A: Less than one.
B: 2 or more and 4 or less.
C: 5 or more and 7 or less.
D: 8 or more and 10 or less.
E: 11 or more.

(16) Size reduction characteristics after winding A long roll of a microporous membrane with a width of 100 mm was prepared, and the end of the long roll was temporarily fixed to one side of a 100 mm square, 0.2 mm thick iron plate with an adhesive tape, and then 100 turns were wound around the iron plate under a tension of 10 N/m to produce a wound body. The thickness of the obtained wound body at the center in the surface direction was measured with a dial gauge, and the thickness of the film laminated part excluding the thickness of the 0.2 mm iron plate was obtained. The thickness of the obtained film laminated part was divided by the thickness of the microporous membrane per sheet multiplied by 200 to obtain a value, and the thickness increase ratio after winding was used as an index of size reduction characteristics during battery production. The smaller this value, the more suitable it is for size reduction.
A: The thickness increase ratio after winding is 1.05 times or less.
B: The thickness increase ratio after winding is more than 1.05 times and 1.07 times or less.
C: The thickness increase ratio after winding is more than 1.07 times and 1.09 times or less.
D: The thickness increase ratio after winding is more than 1.09 times and 1.1 times or less.
E: The thickness increase ratio after winding exceeded 1.1 times.
(17) Heat shrinkage at 130°C for 1 hour A 50 mm square (longitudinal direction x width direction) sample was cut out, and the sample was sandwiched between 0.09 mm thick paper on both sides and stored in an oven heated to 130°C for 1 hour for heat treatment. The dimensions before and after the heat treatment were measured to determine the shrinkage. The length of the line connecting the center positions of the opposing sides of the 50 mm square sample was used as the measurement point for the dimensions. Five samples were heat treated and measured, and the average of the shrinkage rates of the five samples was used as the heat shrinkage rate at 130°C for 1 hour.

(18) Dimensional stability at high temperature After winding the roll of microporous membrane in the same manner as in (16), the end of the wound microporous membrane was fixed with polyimide tape to prepare a wound body. Then, the roll was sandwiched between 0.09 mm thick paper and heated in an oven heated to 130°C for 1 hour for heat treatment, and the length from the end of the iron plate to the part where the iron plate was most exposed due to the shrinkage of the microporous membrane in the direction perpendicular to the winding direction of the roll was measured for the rolled body after heat treatment. A total of three measurements were performed, and the average value of the three measurements was calculated, and the evaluation was performed according to the following criteria.
A: The length of the exposed portion is 3 mm or less.
B: The length of the exposed portion is more than 3 mm and 6 mm or less.
C: The length of the exposed portion exceeded 6 mm.

(19) Melting point A microporous membrane or a raw material was sealed in a measurement pan, and using a PYRIS DIAMOND DSC manufactured by PARKING ELMER, the temperature was raised from 30° C. to 230° C. at a rate of 10° C./min (first heating), then held at 230° C. for 5 minutes, cooled at a rate of 10° C./min, and heated again at a rate of 10° C./min from 30° C. to 230° C. (second heating). The melting endothermic peaks were read from the endothermic charts for the first and second heating, and the melting point was determined as the temperature of the melting endothermic peak in the first heating in the case of a microporous membrane, and as the temperature of the melting endothermic peak in the second heating in the case of a raw material.

Example 1
As raw materials, ultra-high molecular weight polyethylene (melting point 135°C) with a weight average molecular weight Mw of 10 x 10 ⁵ and high density polyethylene (melting point 135°C) with Mw of 5 x 10 ⁵ were used. 80 parts by mass of liquid paraffin was added to 16 parts by mass of ultra-high molecular weight polyethylene and 4 parts by mass of high density polyethylene, and further 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the total mass of the polyethylene resin and mixed to prepare a polyethylene resin solution.

The resulting polyethylene resin solution was fed into a twin-screw extruder and kneaded at 180°C, then fed into a T-die, and the extrudate was cooled with a cooling roll controlled at 15°C to form a gel-like sheet.

The resulting gel-like sheet was stretched 8 times in the machine direction (MD) at 118°C using a roll stretching machine, cooled, and then stretched 8 times in the width direction (TD) using a tenter stretching machine. The sheet width was then fixed in the tenter stretching machine and held at 118°C for 10 seconds.

The stretched gel-like sheet was then immersed in a methylene chloride bath in a cleaning tank, and after removing the liquid paraffin, dried at 80°C. The sheet was then shrunk in the width direction by 3% in an oven, relaxed, and heat-set at a temperature of 110°C for 30 minutes to obtain a polyolefin-based microporous membrane. The properties of the obtained microporous membrane are shown in Table 1. The polyolefin-based microporous membrane had good slipperiness and smoothness. It was easy to transport and had good appearance when wound, and was suitable for miniaturizing the battery (reducing the size of the wound body).

Example 2
As raw materials, ultra-high molecular weight polyethylene (melting point 135°C) with Mw of 10 x 10 ⁵ and high density polyethylene (melting point 135°C) with Mw of 5 x 10 ⁵ were used. 80 parts by mass of liquid paraffin was added to 16 parts by mass of ultra-high molecular weight polyethylene and 4 parts by mass of high density polyethylene, and further 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the total mass of the polyethylene resin and mixed to prepare a polyethylene resin solution.

The resulting polyethylene resin solution was fed into a twin-screw extruder and kneaded at 180°C, then fed into a T-die, and the extrudate was cooled with a cooling roll controlled at 15°C to form a gel-like sheet.

The resulting gel-like sheet was stretched 8 times in the machine direction (MD) at 118°C using a roll stretching machine, cooled, and then stretched 8 times in the width direction (TD) using a tenter stretching machine. The sheet width was then fixed in the tenter stretching machine and held at 118°C for 10 seconds.

The stretched gel-like sheet was then immersed in a methylene chloride bath in a washing tank, and after removing the liquid paraffin, dried at 80°C. It was then stretched 1.2 times in the longitudinal direction (MD) at 120°C using a roll stretching machine, cooled at room temperature, and stretched 1.2 times in the transverse direction (TD) at 120°C using a tenter stretching machine. It was then shrunk 3% in the transverse direction in an oven to relax it, and heat-set at 115°C for 30 minutes to obtain a polyolefin-based microporous membrane.

The properties of the obtained microporous membrane are shown in Table 1. The polyolefin-based microporous membrane had good slipperiness and smoothness. It was easy to transport and had good appearance when wound up.

Example 3
A polyolefin-based microporous membrane was obtained in the same manner as in Example 1, except that the production conditions were as shown in Table 1. The properties of the obtained microporous membrane are shown in Table 1. The polyolefin-based microporous membrane had good lubricity and smoothness. The transportability and appearance when wound up were good.

(Examples 4 to 9, 12 to 14)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 2, except that the production conditions were as shown in Table 1, Table 2, or Table 3. The properties of the obtained microporous membrane are shown in Table 1, Table 2, or Table 3. The polyolefin-based microporous membrane had good lubricity and smoothness. The transportability and appearance when wound up were good.

(Examples 10 and 11)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 6, except that the raw material composition was as shown in Table 2 or Table 3. The properties of the obtained microporous membrane are shown in Table 2 or Table 3. The polyolefin-based microporous membrane had good lubricity and smoothness. The transportability and appearance when wound up were good.

(Comparative Examples 1 to 3)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 1, except that the raw material composition and production conditions were as shown in Table 3 or Table 4. The obtained polyolefin-based microporous membrane had insufficient properties in at least one of lubricity and smoothness, and was poor in transportability or appearance when wound up.

The polyolefin-based microporous membrane according to the embodiment of the present invention has excellent lubricity and smoothness when used as a separator for nonaqueous electrolyte secondary batteries, including lithium ion secondary batteries, and is therefore suitable for use as a separator that allows for easy transport during battery production, good appearance when wound up, and allows for smaller and thinner batteries.

It can also be used in various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, as well as in moisture-permeable, waterproof clothing, medical materials, and supports for fuel cells.

Claims (10)

A polyolefin-based microporous film in which the dynamic friction coefficient μk between one surface and the other surface of the microporous film is 0.2 or more and 0.7 or less, and the root mean square height Sq of the surface between the one surface and the other surface of the microporous film that has a larger root mean square height Sq is 10 nm or more and 100 nm or less. The polyolefin-based microporous film according to claim 1, wherein the dynamic friction coefficient μmk between at least one surface of the microporous film and a metal is 0.13 or less. The polyolefin-based microporous membrane according to claim 1 or 2, wherein the number of fibrils in a cube having a side length of 2.7 μm as determined by the following measurement method is 870/ ^μm3 to 2800/ ^μm3 , and the average diameter of the fibrils is 25 nm to 60 nm.
Step (A): An image is obtained while cutting a cross section of a microporous membrane to obtain a three-dimensional image.
Step (B): From the obtained three-dimensional image, resin parts and pore parts are distinguished, and a three-dimensional image of the constituent resin parts is created.
Step (C): The constituent resin portion is divided into fibrils, and the number of fibrils is counted. The polyolefin-based microporous film according to any one of claims 1 to 3, wherein the pin puncture strength of the microporous film is 180 gf or more and 700 gf or less. The polyolefin-based microporous film according to any one of claims 1 to 4, wherein the thickness of the microporous film is 3 μm or more and 14 μm or less. The polyolefin-based microporous film according to any one of claims 1 to 5, wherein the heat shrinkage rate at 130°C for 1 hour is 20% or less in both the longitudinal and transverse directions. A microporous membrane roll wound with the microporous membrane according to any one of claims 1 to 6, the polyolefin-based microporous membrane roll having a coefficient of variation of dynamic friction μk of 10% or less. The polyolefin-based microporous membrane roll according to claim 7, wherein the coefficient of variation of the root mean square height Sq for one surface of the microporous membrane is 10% or less. A microporous membrane/heat-resistant resin layer laminate, comprising a polyolefin-based microporous membrane according to any one of claims 1 to 6 and a heat-resistant resin layer laminated thereon. A non-aqueous electrolyte secondary battery comprising the polyolefin-based microporous membrane according to any one of claims 1 to 6, or the microporous membrane/heat-resistant resin layer laminate according to claim 9.