Polyolefin microporous membrane, laminate, and battery

Abstract

The present invention pertains to a polyolefin micro porous film that is a monolayer containing a polyethylene (A) and a polyolefin (B) other than polyethylene and that has a film thickness of 8 µm or less, a shutdown temperature of 135°C or less, and a meltdown temperature of 160°C or more.

Description

Technical Field

[0001] The present invention relates to a polyolefin microporous membrane, a laminate, and a battery using the same, which are excellent in safety and output characteristics when used as a separator for a battery.

Background Art

[0002] Polyolefin microporous membranes are used as filters, separators for fuel cells, separators for capacitors, etc. In particular, they are preferably used as separators for lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, etc. The reason is that polyolefin microporous membranes have excellent mechanical strength and shut-down characteristics of the membrane. In particular, in recent years, in lithium-ion secondary batteries, development has been promoted aiming at high energy density, high capacity, and high output, mainly for in-vehicle applications. Along with this, the required characteristics for the safety of separators have become even higher.

[0003] When the inside of the battery overheats in an overcharged state, the separator needs to have a function (shut-down function) of melting and clogging the pores to cut off the current in order to prevent accidents such as ignition. The temperature at which this shut-down function appears (shut-down temperature) is preferably lower. Also, immediately after shut-down, the temperature inside the battery continues to rise. Therefore, at temperatures above the shut-down temperature, the shape of the separator itself must be maintained to prevent short-circuiting of the electrodes, and the melt-down temperature of the separator is preferably higher. Therefore, it is necessary to achieve both low shut-down and high melt-down, and it can be said that the greater the temperature difference between the shut-down temperature and the melt-down temperature, the higher the safety. As a method for lowering the shut-down temperature, there is a method of lowering the melting point of the raw material by reducing the molecular weight of the material constituting the separator. As a method for raising the melt-down temperature, there is a method of adding a high-melting polyolefin such as polypropylene. Also, regarding the shut-down function, it is necessary for safety to quickly cut off the current, and the shut-down speed is also an important characteristic.

[0004] On the other hand, with the increase in the battery capacity, the thickness of the separator tends to be thinned. In order to prevent short circuits caused by winding or foreign objects in the battery, higher strength of the separator is required. Generally, in order to increase the strength of the separator, methods such as controlling the crystal orientation of polyolefin by high-stretch ratio or increasing the molecular weight of the raw material can be mentioned. However, when the crystals are highly oriented, the melting point becomes higher and the shutdown temperature also becomes higher. Therefore, increasing the strength and lowering the shutdown temperature are in a trade-off relationship.

[0005] As a technology for achieving both low shutdown and high melt-down, Patent Document 1 and Patent Document 2 describe a technology for achieving both by laminating a layer for lowering the shutdown temperature and a layer for raising the melt-down temperature.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, since it is a laminated film formation, it is difficult to thin the film. Also, even if the film is thinned, the thickness of each layer may become too thin, resulting in significant non-uniformity in physical properties such as shutdown characteristics and melt-down characteristics. Although Patent Document 3 exemplifies a single-layer thin film microporous membrane, it has been difficult to achieve both low shutdown and high melt-down.

[0008] There is room for improvement in the development of a highly safe separator without sacrificing battery performance in response to the diverse needs of customers associated with high energy density, high capacity, and high output as described above.

[0009] An object of the present invention is to solve the above-described problems. That is, to provide a polyolefin microporous membrane excellent in safety and output characteristics when used as a separator for a battery.

Means for Solving the Problems

[0010] In order to solve the above-described problems and achieve the object, the present invention has the following configuration. 〔1〕 A single-layer polyolefin microporous membrane containing polyethylene (A) and a polyolefin (B) other than polyethylene, having a film thickness of 8 μm or less, a shutdown temperature of 135 °C or less, and a melt-down temperature of 160 °C or more. 〔2〕 When the tensile strength in the TD direction is M TD and M TD is 50 MPa or more, the polyolefin microporous membrane according to 〔1〕. 〔3〕 When the tensile strength in the MD direction is M MD and M MD / M TD is 0.5 to 2.0, the polyolefin microporous membrane according to 〔1〕 or 〔2〕. 〔4〕 The polyolefin microporous membrane according to any one of 〔1〕 to 〔3〕, having a puncture strength of 1.0 N or more in terms of a thickness of 5 μm. 〔5〕 The polyolefin microporous membrane according to any one of 〔1〕 to 〔4〕, having an air permeability resistance of 50 seconds / 100 cm 3 or more and 1000 seconds / 100 cm 3 or less in terms of a thickness of 5 μm. 〔6〕 The polyolefin microporous membrane according to any one of 〔1〕 to 〔5〕, having an average pore diameter of 50 nm or less. 〔7〕 A polyolefin microporous membrane according to any one of [1] to [6], wherein the ratio of the average pore diameter to the maximum pore diameter (average pore diameter / maximum pore diameter) is 0.56 to 1.0. 〔8〕 A polyolefin microporous membrane according to any one of [1] to [7], wherein a polyethylene-based resin is the main component. 〔9〕 A polyolefin microporous membrane according to any one of [1] to [8], wherein the content of the polyolefin (B) is 5 to 40% by mass. 〔10〕 A polyolefin microporous membrane according to any one of [1] to [9], wherein the polyolefin (B) is a polypropylene-based resin. 〔11〕 A polyolefin microporous membrane according to any one of [1] to

[10] , which has peaks at less than 150 °C and at 150 °C or higher in DSC, and the half-width of the peak at less than 150 °C is 10 °C or less. 〔12〕 A laminate in which a coating layer is provided on at least one side of a polyolefin microporous membrane according to any one of [1] to

[11] . 〔13〕 A battery using a polyolefin microporous membrane according to any one of [1] to

[11] or the laminate according to

[12] .

Advantages of the Invention

[0011] The polyolefin microporous membrane of the present invention is excellent in strength while being a thin film, and has low shutdown characteristics and high melt-down characteristics. Therefore, when used as a separator for a battery, it is excellent in safety and output characteristics. Therefore, it can be suitably used as a separator and a laminate for a battery that requires high energy density, high capacity, and high output, such as an electric vehicle, and a secondary battery.

Embodiments for Carrying Out the Invention

[0012] The polyolefin microporous membrane according to an embodiment of the present invention is a single-layer polyolefin microporous membrane containing polyethylene (A) and a polyolefin (B) other than polyethylene. The membrane thickness is 8 μm or less, the shutdown temperature is 135°C or less, and the melt-down temperature is 160°C or more.

[0013] The polyolefin microporous membrane according to an embodiment of the present invention (hereinafter, may be simply referred to as "microporous membrane") has a membrane thickness of 8 μm or less. More preferably, it is 7 μm or less, still more preferably 6 μm or less, and most preferably 5 μm or less. When the membrane thickness exceeds 8 μm, sufficient output characteristics and energy density may not be obtained when used as a separator for future high-capacity batteries. From the above viewpoints, a thinner membrane thickness is preferable. However, since the safety may decrease or the handling may become difficult, the lower limit of the membrane thickness is preferably 2 μm or more. The membrane thickness can be adjusted by the discharge amount of the extruder, the film-forming speed, the stretching ratio, the stretching temperature, etc. within a range that does not deteriorate other physical properties.

[0014] The polyolefin microporous membrane in the embodiment of the present invention has a single-layer structure containing polyethylene (A) and a polyolefin (B) other than polyethylene. The single layer mentioned here means a structure in which layers having different compositions, raw materials used, and physical properties are not arranged in the membrane thickness direction of the polyolefin microporous membrane. If the polyolefin microporous membrane is single-layer, not only the manufacturing process becomes simpler but also the film can be made thinner compared to a laminated structure in which two or more layers having different compositions, raw materials used, and physical properties are arranged in the membrane thickness direction of the polyolefin microporous membrane. Therefore, it is preferably single-layer.

[0015] The shutdown temperature of the polyolefin microporous membrane of the present invention is 135°C or lower. More preferably, it is 134°C or lower, still more preferably 133°C or lower, and most preferably 132°C or lower. When the shutdown temperature is 135°C or lower, the safety is improved when it is used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as electric vehicles. From the viewpoint of safety, the lower the shutdown temperature, the more preferable. However, when the shutdown temperature is 80°C or lower, the pores are closed even under normal use conditions, and the battery characteristics deteriorate. Therefore, the shutdown temperature is preferably about 80°C as the lower limit. To set the shutdown temperature within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions and heat fixation conditions during film formation of the film are within the range described later.

[0016] The melt-down temperature of the polyolefin microporous membrane of the present invention is 160°C or higher. More preferably, it is 162°C or higher, still more preferably 165°C or higher, and most preferably 168°C or higher. When the melt-down temperature is 160°C or higher, the safety is improved when it is used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as electric vehicles. From the viewpoint of safety, the higher the melt-down temperature, the more preferable. However, from the viewpoint of balance with other characteristics, about 250°C is the upper limit. To set the melt-down temperature within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions and heat fixation conditions during film formation of the film are within the range described later.

[0017] As described above, the polyolefin microporous membrane of the present application is a thin film with a thickness of 8 μm or less, and has excellent shutdown characteristics and melt-down characteristics. Usually, in order to achieve both shutdown characteristics and melt-down characteristics, a method of laminating a layer for lowering the shutdown temperature and a layer for raising the melt-down temperature has been generally used. However, in the case of a microporous membrane with a thin film thickness required in the future, if it is a laminate, the film thickness of each layer becomes too thin, so it is difficult to exhibit the characteristics of each layer, or the thickness unevenness and lamination unevenness increase, and the physical properties may vary greatly. On the other hand, in order to achieve both shutdown characteristics and melt-down characteristics in a single-layer microporous membrane, it is necessary to uniformly knead raw materials with different characteristics. However, in the prior art, uniform kneading is difficult, and in the case of a thin film, the non-uniformity of kneading becomes more prominent. Therefore, it has been difficult to obtain a single-layer microporous membrane that is thin and has excellent shutdown characteristics and melt-down characteristics.

[0018] When the tensile strength in the longitudinal direction of the film of the polyolefin microporous membrane of the present invention is M MD and the tensile strength in the width direction is M TD , it is preferably that M TD is 50 MPa or more. The tensile strength M TD is more preferably 80 MPa or more, still more preferably 100 MPa or more, and most preferably 120 MPa or more. When the tensile strength M TD is less than 50 MPa, when made into a thin film, wrinkles are likely to occur in the film during post-processing such as coating, and the handleability deteriorates, or a short circuit is likely to occur due to winding or foreign matter in the battery, etc., and the safety of the battery may decrease. From the viewpoint of improving safety, the higher the tensile strength, the better. However, lowering the shutdown temperature and improving the tensile strength often result in a trade-off, and about 200 MPa is the upper limit. In order to make the tensile strength within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation of the film are within the range described later.

[0019] In the present invention, the direction parallel to the film forming direction of the film is referred to as the film forming direction, the longitudinal direction, or the MD direction, and the direction perpendicular to the film forming direction in the film plane is referred to as the width direction or the TD direction.

[0020] The polyolefin microporous membrane of the present invention has a tensile strength M MD preferably of 80 MPa or more. The tensile strength M MD is more preferably 100 MPa or more, still more preferably 120 MPa or more, and most preferably 160 MPa or more. When the tensile strength is less than 80 MPa, short circuits are likely to occur due to winding during thinning or foreign matter in the battery, etc., and the safety of the battery may decrease. From the viewpoint of improving safety, the higher the tensile strength, the better, but in many cases, lowering the shutdown temperature and improving the tensile strength are in a trade-off relationship, and about 200 MPa is the upper limit. To set the tensile strength within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation of the film are within the range described later.

[0021] The polyolefin microporous membrane of the present invention preferably has a value of M MD / M TD of 0.5 to 2.0. The value of M MD / M TD is more preferably 0.7 to 1.8, still more preferably 0.8 to 1.6, and most preferably 1.0 to 1.6. When the value of M MD / M TD is less than 0.5 or exceeds 2.0, the anisotropy of the film becomes too large, and when the film is thinned, the film is likely to be avoided and the handleability may decrease. To set the ratio of the tensile strength within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation of the film are within the range described later.

[0022] The tensile elongation (elongation at break) in the MD direction of the polyolefin microporous membrane is not particularly limited, but for example, it is preferably 40% or more and 300% or less, more preferably 60% or more and 200% or less, and still more preferably 70% or more and 150% or less. When the elongation at break in the MD direction is within the above range, it is difficult to deform even when a high tension is applied during coating, and wrinkles are also less likely to occur. Therefore, the occurrence of coating defects is suppressed, and the flatness of the coating surface is good, which is preferable.

[0023] The tensile elongation (elongation at break) in the TD direction of the polyolefin microporous membrane is preferably 60% or more, and more preferably 70% or more. When the elongation at break in the TD direction is within the above range, it has excellent impact resistance that can be evaluated by an impact test or the like. Also, when the polyolefin microporous membrane is used as a separator, the separator can follow the unevenness of the electrode, the deformation of the battery, and the generation of internal stress due to battery heat generation, etc., which is preferable.

[0024] The MD tensile elongation and TD tensile elongation are values measured by a method conforming to ASTM D882.

[0025] The polyolefin microporous membrane of the present invention preferably has a puncture strength of 1.0 N or more for a film converted to a thickness of 5 μm. More preferably 1.2 N or more, still more preferably 1.4 N or more, and most preferably 1.6 N or more. When the puncture strength is 1.0 N or more, even when used as a thin film separator, short circuits due to winding or foreign objects in the battery are less likely to occur, and the safety of the battery can be improved. However, in many cases, increasing the puncture strength and lowering the shutdown temperature are in a trade-off relationship, and 5 N is the upper limit. To make the puncture strength within the above range, it is preferable to set the raw material composition of the film within the range described later, and also to set the stretching conditions during film formation within the range described later. Generally, it is possible to increase the strength by increasing the stretching ratio.

[0026] The polyolefin microporous membrane of the present invention has an air permeability resistance of 50 seconds / 100 cm for a film converted to a thickness of 5 μm 3 or more and 1000 seconds / 100 cm 3It is preferably as follows. More preferably, it is 50 seconds / 100 cm 3 or more and 300 seconds / 100 cm 3 or less. Even more preferably, it is 50 seconds / 100 cm 3 or more and 200 seconds / 100 cm 3 or less. Most preferably, it is 70 seconds / 100 cm 3 or more and 200 seconds / 100 cm 3 or less. When the air permeability resistance is 50 seconds / 100 cm 3 or more, when used as a separator of a thin film, the film has excellent strength and good handling properties. When used as a separator for a high-output battery, dendrite-induced micro-short circuits are less likely to occur. When the air permeability resistance is 1000 seconds / 100 cm 3 or less, when used as a battery separator, the ion permeability is sufficient and the output characteristics of the battery are excellent. To make the air permeability resistance within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation of the film are within the range described later.

[0027] The polyolefin microporous membrane of the present invention preferably has an average pore diameter of 50 nm or less. More preferably, it is 40 nm or less, even more preferably 30 nm or less, and most preferably 25 nm or less. If the average pore diameter is 50 nm or less, dendrite-induced micro-short circuits are less likely to occur when the thin film is used as a separator for a high-output battery. From the above viewpoints, the smaller the average pore diameter, the more preferable. However, if it is too small, the ion permeability becomes insufficient and the output characteristics of the battery may deteriorate. Therefore, about 10 nm is the lower limit. To make the average pore diameter within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation of the film are within the range described later.

[0028] The polyolefin microporous membrane of the present invention preferably has a ratio of average pore diameter to maximum pore diameter (average pore diameter / maximum pore diameter) of 0.56 to 1.0. More preferably, it is 0.60 to 1.0, still more preferably 0.65 to 1.0, and most preferably 0.68 to 1.0. When (average pore diameter / maximum pore diameter) is 0.56 or more, the pore size uniformity is high, so dendrite-induced short circuits can be suppressed even when used as a separator for thin-film high-output batteries. The upper limit is 1.0 due to the measurement principle. To make (average pore diameter / maximum pore diameter) within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions during film formation are within the range described later.

[0029] When the polyolefin microporous membrane of the present invention is heated with temperature increase based on JIS K7121 using a differential scanning calorimeter (DSC), it preferably has peaks at less than 150°C and at 150°C or more, respectively. Having a peak as referred to here means having a maximum value when the horizontal axis is temperature and the vertical axis is heat flow for the results obtained by DSC. For the polyolefin microporous membrane in the present invention, it is preferable that the temperatures at which the maximum values occur are less than 150°C and at 150°C or more. Also, the temperature at the maximum of the peak at less than 150°C is preferably 135°C or lower. The lower limit value is 120°C, preferably 123°C or higher. When it is higher than the above range, shutdown may occur at a higher temperature when used as a battery separator. Also, when the temperature at the maximum of the peak at less than 150°C is lower than the above range, the shrinkage rate at high temperatures increases, and the electrodes in the battery may come into contact and short-circuit.

[0030] Furthermore, the polyolefin microporous membrane of the present invention preferably has a half-width of the peak below 150°C of 10.0°C or less, more preferably 9.5°C or less, still more preferably 9.3°C or less, even more preferably 9.1°C or less, and most preferably 9.0°C or less. The smaller the half-width, the easier it is for the resin to melt all at once when the polyolefin microporous membrane is used as a separator of a battery at a certain temperature, so the shutdown speed increases, leading to an improvement in the safety of the battery, which is preferable. The half-width of the peak referred to here means the temperature at which the heat generation amount becomes 0.5 times the maximum heat generation amount Q in the region below 150°C, respectively T 1 / 2 , T 1 , T 2 (T 1 < T 2 ). When there are two or more maximum values in the region below 150°C and three or more temperatures at which Q 2 -T 1 means the value of. In the case where there are three or more temperatures at which the maximum value is two or more in the region below 150°C and Q 1 / 2 is obtained, the minimum temperature of the corresponding temperature is T 1 , and the maximum temperature is T 2 , and the half-width is calculated. To make the half-width within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions and heat fixation conditions during film formation of the film are within the range described later.

[0031] In addition, the porosity of the polyolefin microporous membrane of the present invention is preferably 30% or more, more preferably 35% or more, and still more preferably 40% or more. The upper limit is preferably 70% or less, more preferably 65% or less, and still more preferably 60% or less. When the porosity is lower than the above range, when used as a separator of a battery, the ion permeability becomes insufficient and the output characteristics of the battery deteriorate, so it is preferably 30% or more. Also, when it is higher than the above range, the strength decreases and short circuits are likely to occur due to winding or foreign matters in the battery, so it is preferably 70% or less. To make the porosity within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions and heat fixation conditions during film formation of the film are within the range described later.

[0032] In the present invention, by using a specific polyethylene-based resin described later as a raw material and setting the raw material composition within the range described later, and also setting the stretching conditions and heat fixation conditions during film formation within the ranges described later, both the reduction of the shutdown temperature and the increase of the melt-down temperature are achieved even in the thin film.

[0033] Next, the raw materials of the polyolefin microporous membrane according to the embodiment of the present invention will be described, but it is not necessarily limited thereto. The polyolefin microporous membrane according to the embodiment of the present invention is a film mainly composed of a polyolefin resin. Here, in the embodiment of the present invention, the "main component" means that the proportion of a specific component in all components is 50% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, and most preferably 99% by mass or more.

[0034] The polyolefin resin used in the embodiment of the present invention is preferably a polyolefin-based resin, and may also be a polyolefin composition. Examples of the polyolefin-based resin include polyethylene-based resins, polypropylene-based resins, etc., and two or more of these may be blended and used.

[0035] The polyolefin microporous membrane according to the embodiment of the present invention preferably has a polyethylene-based resin as the main component.

[0036] The polyolefin microporous membrane according to the embodiment of the present invention contains polyethylene (A) and a polyolefin (B) other than polyethylene. First, polyethylene (A) will be described.

[0037] Polyethylene (A) includes a polyethylene-based resin. As the polyethylene-based resin, various polyethylenes can be used, such as ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene, etc.

[0038] The polyethylene (A) used in the polyolefin microporous membrane according to the embodiment of the present invention has excellent melt extrusion characteristics and excellent uniform stretching process characteristics. Therefore, it is preferable to use high-density polyethylene (density: 0.920 g / cm 3 or more and 0.970 g / cm 3 or less).

[0039] As such a polyethylene resin, it is preferably not only a homopolymer of ethylene but also a copolymer containing other α-olefins in order to lower the melting point and crystallinity of the raw material. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. As the copolymer containing α-olefin (ethylene·α-olefin copolymer), a copolymer containing hexene-1 is preferable, and it is more preferable that the polyethylene (A) is mainly composed of an ethylene·1-hexene copolymer. Further, the α-olefin can be confirmed by measurement with C 13 -NMR.

[0040] Among high-density polyethylenes, it is preferable to include branched high-density polyethylene (branched HDPE). Branched high-density polyethylene is more preferable because the in-plane crystal orientation is difficult to progress, the change in crystal structure can be suppressed, and the shutdown temperature can be lowered. Furthermore, even when the draw ratio is increased, the crystal orientation is difficult to progress, the generation of high-melting-point components can be suppressed, and an increase in the half-value width of the peak in DSC can also be suppressed. As a result, it is possible to achieve high strength and thin film formation by high magnification stretching while maintaining the shutdown speed.

[0041] The weight average molecular weight (Mw) of the high-density polyethylene is preferably 1.0×10 4 or more and 1.0×10 6 or less, more preferably 5.0×10 4 or more and 3.5×10 5 or less, still more preferably 8.0×10 4 or more and 2.5×10 5 or less, and further preferably 1.5×105 The above 2.0×10 5 It is particularly preferable that the weight average molecular weight is within the above range. When the weight average molecular weight is within the above range, excessive crystal orientation in the plane is less likely to progress during film formation, and it is possible to easily control the change in the crystal structure of the polyolefin microporous membrane within an appropriate range, so that the shutdown characteristics can be improved and the deterioration of permeability can also be suppressed.

[0042] In addition, the melting point of the high-density polyethylene is preferably 130°C or higher and preferably 135°C or lower. When the melting point is 130°C or higher, a decrease in the porosity can be suppressed, and when it is 135°C or lower, an increase in the shutdown temperature can be suppressed.

[0043] That is, a particularly preferred form of the main raw material or the raw material used for the purpose of lowering the shutdown temperature in the embodiment of the present invention is polyethylene having a Mw of 1.0×10 5 ~1.0×10 6 and a melting point of 130 to 135°C, and it is preferable that this polyethylene is contained in an amount of 50% by mass or more based on 100% by mass of the entire polyolefin resin.

[0044] In addition, when a low molecular weight polyethylene such as low density polyethylene, linear low density polyethylene, ethylene·α-olefin copolymer produced by a single site catalyst, or low molecular weight polyethylene having a weight average molecular weight of 1000 to 100000 is added to the polyethylene (A), a shutdown function at low temperature is imparted, and the characteristics as a battery separator can be improved. However, in the polyethylene (A), if the content ratio of the above-mentioned low molecular weight polyethylene is large, a decrease in the porosity of the microporous membrane occurs in the film formation process. Therefore, the content ratio of the low molecular weight polyethylene is adjusted so that the density becomes a high density polyethylene exceeding 0.94 g / cm 3 It is preferable to adjust the density by adding a branched high density polyethylene having a long chain branch component.

[0045] Also, from the above viewpoints, the molecular weight distribution of the polymer constituting the polyolefin microporous membrane according to the embodiment of the present invention preferably has a component amount of less than 20% with a molecular weight of less than 40,000. More preferably, the component amount with a molecular weight of less than 20,000 is less than 20%, and even more preferably, the component amount with a molecular weight of less than 10,000 is less than 20%. By using the above-described raw materials, it is possible to lower the shutdown temperature without significantly reducing the molecular weight. As a result, it becomes possible to achieve compatibility with other physical properties such as strength and porosity.

[0046] When blending two or more types of polyethylene in the polyethylene (A) used in the embodiment of the present invention, it is preferable to use ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0×10 6 or more and less than 4.0×10 6 By containing ultra-high molecular weight polyethylene, it is possible to make the pores finer and improve the heat resistance, and furthermore, the strength and elongation can be improved. The ultra-high molecular weight polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount of other α-olefins. The other α-olefins other than ethylene may be the same as those described above.

[0047] Furthermore, since the main raw material described above or the raw material used for the purpose of lowering the shutdown temperature has a relatively small molecular weight, when forming into a sheet, the swell and neck are large at the outlet of the die, and the formability of the sheet tends to deteriorate. By adding ultra-high molecular weight polyethylene as an auxiliary material, the viscosity and strength of the sheet increase, and the process stability increases. Therefore, it is preferable to add ultra-high molecular weight polyethylene. However, if the proportion of ultra-high molecular weight polyethylene is 50% by mass or more, the extrusion load increases and the extrusion moldability decreases. Therefore, the addition amount of ultra-high molecular weight polyethylene is preferably less than 50% by mass based on the total amount of polyethylene (A).

[0048] In addition, the polyolefin microporous membrane according to the embodiment of the present invention contains a polyolefin (B) other than polyethylene for the purpose of improving the melt-down characteristics. The polyolefin (B) is not particularly limited, and polypropylene-based resins, polymethylpentene-based resins, polybutene-based resins, polyacetal-based resins, styrene-based resins, polyphenylene ether-based resins, etc. can be used. Among them, from the viewpoints of kneadability and electrical stability when used as a separator, polypropylene-based resins are preferred. As for the types of polypropylene-based resins, in addition to homopolymers of propylene, block copolymers and random copolymers can also be used. The block copolymers and random copolymers can contain copolymer components with other α-olefins other than propylene, and ethylene is preferred as the other α-olefin. Note that the polyolefin (B) and the polyethylene (A) are different resins.

[0049] The upper limit of the content of the polyolefin (B) in the polyolefin microporous membrane is preferably 40% by mass or less, more preferably 35% by mass or less, based on the total mass of the polyolefin microporous membrane. The lower limit of the addition amount of the polyolefin (B) is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more, and most preferably 20% by mass or more. If the polyolefin (B) is 40% by mass or less, the pore diameter of the microporous membrane becomes large, sufficient permeability can be obtained, it has excellent strength, and the increase in the shutdown temperature can be suppressed. If it is 5% by mass or more, it has a co-continuous structure with the polyolefin resin as the main component, and the effect of improving the melt-down temperature due to the addition of the polyolefin (B) is likely to be manifested.

[0050] Also, the melting point of the added polyolefin (B) is preferably 150°C or higher, more preferably 155°C or higher, and still more preferably 160°C or higher.

[0051] Furthermore, the molecular weight of the polyolefin (B) is preferably 5.0×10 5 or more, more preferably 10×105 or more, more preferably 15×10 5 or more. The upper limit of the weight average molecular weight is preferably 10×10 6 or less, more preferably 8.0×10 6 or less, still more preferably 5.0×10 6 or less, and most preferably 3.0×10 6 or less. When the molecular weight is within the above range, it is preferable because the strength of the obtained polyolefin microporous membrane becomes sufficient. When it is 10×10 6 or less, during the melt kneading in the manufacturing process of the polyolefin microporous membrane described later, the viscosity does not become too high and uniform kneading can be achieved, which is preferable.

[0052] The blending ratio of the polyolefin resin and the plasticizer may be appropriately selected within a range that does not impair the molding processability, with the total of the polyolefin resin and the plasticizer being 100% by mass, and the content of the polyolefin resin being 10 to 50% by mass. If the polyolefin resin is less than 10% by mass (when the plasticizer is 90% by mass or more), when forming into a sheet, the swell and neck-in at the die outlet are large, the formability of the sheet deteriorates, and the film-forming property decreases. On the other hand, if the polyolefin resin exceeds 50% by mass (when the plasticizer is 50% by mass or less), the shrinkage in the film thickness direction increases, and the molding processability also decreases.

[0053] In addition, the polyolefin microporous membrane according to the embodiment of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, and further anti-blocking agents and fillers, etc., as long as the effects of the present invention are not impaired. In particular, for the purpose of suppressing the oxidative degradation due to the thermal history of the polyethylene resin, it is preferable to add an antioxidant. As the antioxidant, 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 (for example, "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2), tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7), etc., it is preferable to use one or more selected therefrom. Appropriately selecting the types and addition amounts of the antioxidant and the heat stabilizer is important for adjusting or enhancing the characteristics of the microporous membrane.

[0054] The polyolefin microporous membrane according to the embodiment of the present invention is obtained by biaxially stretching using the above-mentioned raw materials. As the method of biaxial stretching, it can be obtained by any of the inflation method, the simultaneous biaxial stretching method, and the sequential biaxial stretching method. Among them, it is preferable to adopt the simultaneous biaxial stretching method or the sequential biaxial stretching method in terms of controlling the film-forming stability, thickness uniformity, high rigidity of the film, and dimensional stability.

[0055] Next, the manufacturing method of the polyolefin microporous membrane according to the embodiment of the present invention will be described, but it is not necessarily limited thereto. The manufacturing method of the polyolefin microporous membrane according to the embodiment of the present invention comprises the following steps (a) to (e).

[0056] (a) Knead and dissolve a polymer material containing a polyolefin alone, a polyolefin mixture, a polyolefin solvent (plasticizer) mixture, an additive, and a polyolefin kneaded product to prepare a polyolefin solution (b) Extrude the dissolved material, mold it into a sheet shape, and cool and solidify it (c) Stretch the obtained sheet by a roll method or a tenter method. (d) Then, extract the plasticizer from the obtained stretched film and dry the film. (e) Subsequently, perform heat treatment / re-stretching / heat setting.

[0057] Hereinafter, each step will be described.

[0058] (a) Preparation of polyolefin resin solution A polyolefin resin solution in which the polyolefin resin used in the embodiment of the present invention is heated and dissolved in a plasticizer is prepared. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but in order to enable relatively high magnification stretching, it is preferable that the solvent is liquid at room temperature. Examples of the solvent include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding thereto, and phthalic acid esters such as dibutyl phthalate and dioctyl phthalate that are liquid at room temperature. In order to obtain a stable gel-like sheet, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, a solvent that is miscible with the polyolefin resin but solid at room temperature may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, and paraffin wax. However, if only a solid solvent is used, uneven stretching or the like may occur.

[0059] 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 with the polyolefin resin solution is less likely to be non-uniform. On the other hand, if it is 200 cSt or less, the removal of the liquid solvent is easy. The viscosity of the liquid solvent is the viscosity measured at 40 °C using an Ubbelohde viscometer.

[0060] (b) Formation of extrudate and formation of gel-like sheet Although the uniform melt-kneading of the polyolefin resin solution is not particularly limited, when it is desired to prepare a high-concentration polyolefin resin solution, it is preferably carried out in a twin-screw extruder. If necessary, various additives such as antioxidants may be added within a range that does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent the oxidation of the polyolefin resin.

[0061] Since the polyolefin microporous membrane according to the embodiment of the present invention is a microporous membrane of a single film containing polyethylene (A) and a polyolefin (B) other than polyethylene, it is necessary to uniformly knead and extrude raw materials having different melting points. If the kneading state is not uniform, the strength and melt-down temperature of the microporous membrane may decrease, or the variation in pore diameter may increase. In order to knead uniformly, when the melting point of the raw material with the lowest melting point among the polyethylene (A) and polyolefin (B) used is set as Tm1 in the first half of the extruder, it is preferably set to Tm1 + 30°C or lower, and uniformly mixed in a state before the raw materials melt. Next, in the second half of the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which polyethylene (A) and polyolefin (B) are completely melted. When the melting point of the raw material with the highest melting point among the polyethylene (A) and polyolefin (B) used is set as Tm2, the melt-kneading temperature is preferably (Tm2 - 10°C) to (Tm2 + 120°C). More preferably, it is (Tm2 + 20°C) to (Tm2 + 100°C). Here, the melting point refers to the value measured by DSC based on JIS K7121 (1987) (the same shall apply hereinafter). For example, when using a polyethylene-based resin and a polypropylene-based resin, the melt-kneading temperature is preferably 160°C or lower in the first half of the extruder and in the range of 150 to 280°C in the second half.

[0062] From the perspective of suppressing resin deterioration, a lower melt-kneading temperature is preferable. However, if the temperature is lower than the above-mentioned temperature, unmelted materials may occur in the extrudate extruded from the die, which may cause film breakage or other problems in the subsequent stretching process. If the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin-based resin will become intense, and the physical properties of the obtained microporous membrane, such as strength and porosity, may be inferior. In addition, the decomposition products may deposit on the chill roll or the roll in the stretching process and adhere to the sheet, leading to deterioration of the appearance. Therefore, it is preferable to knead within the above range.

[0063] Next, a gel-like sheet is obtained by cooling the obtained extrudate. By cooling, the microphase of the polyolefin-based resin separated by the solvent can be fixed. In the cooling process, it is preferable to cool to 10 to 50 °C. This is because it is preferable that the final cooling temperature is below the crystallization end temperature. By making the higher-order structure finer, uniform stretching becomes easier in the subsequent stretching. Therefore, it is preferable that the cooling is performed at a rate of 30 °C / min or more at least until it reaches below the gelation temperature. If the cooling rate is less than 30 °C / min, the crystallinity increases, and it is difficult to obtain a gel-like sheet suitable for stretching. Generally, when the cooling rate is slow, relatively large crystals are formed, so the higher-order structure of the gel-like sheet becomes coarser, and the gel structure forming it also becomes larger. On the contrary, when the cooling rate is fast, relatively small crystals are formed, so the higher-order structure of the gel-like sheet becomes dense, leading to higher strength and uniform pore size.

[0064] As the cooling method, there are methods such as direct contact with cold air, cooling water, or other cooling media, contact with a roll cooled by a refrigerant, and use of a casting drum or the like.

[0065] (c) Stretching process The obtained gel-like sheet (including the laminated sheet) is stretched. Examples of the stretching method used include uniaxial stretching in the sheet conveyance direction (MD direction) by a roll stretching machine, uniaxial stretching in the sheet width direction (TD direction) by a tenter, sequential biaxial stretching by a combination of a roll stretching machine and a tenter, or a combination of two tenters, and simultaneous biaxial stretching by a simultaneous biaxial tenter. The stretching ratio varies depending on the thickness of the gel-like sheet from the viewpoint of film thickness uniformity, but it is preferably stretched 5 times or more in any direction. In terms of the area ratio, it is preferably 25 times or more, more preferably 36 times or more, even more preferably 49 times, and most preferably 64 times or more. If the area ratio is less than 25 times, the stretching is insufficient and the film uniformity is easily impaired, and an excellent microporous membrane cannot be obtained from the viewpoint of strength. The area ratio is preferably 100 times or less. When the area ratio is increased, breaks are likely to occur frequently during the production of the microporous membrane, productivity decreases, and as the orientation progresses and the crystallinity increases, the melting point and strength of the microporous membrane improve. However, an increase in crystallinity means a decrease in the amorphous part, and the melting point and shutdown temperature of the film increase.

[0066] The stretching temperature is preferably set to be equal to or lower than the melting point of the gel-like sheet + 10 °C, and more preferably in the range of (crystalline dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5 °C). Specifically, in the case of a polyethylene composition, since it has a crystalline dispersion temperature of about 90 to 100 °C, the stretching temperature is preferably 90 to 125 °C, and more preferably 90 to 120 °C. The crystalline dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it may be determined from NMR. If it is less than 90 °C, the pore formation is insufficient due to low-temperature stretching, it is difficult to obtain film thickness uniformity, and the porosity also decreases. If it is higher than 125 °C, the sheet melts and pore blockage is likely to occur.

[0067] Due to the stretching as described above, the high-order structure formed in the gel sheet undergoes cleavage, the crystal phase is refined, and a large number of fibrils are formed. The fibrils form a three-dimensionally irregularly connected network structure. As the stretching improves the mechanical strength and enlarges the pores, it becomes suitable for use as a battery separator. Also, by stretching before removing the plasticizer, the polyolefin resin is in a sufficiently plasticized and softened state, so that the cleavage of the high-order structure becomes smooth and the refinement of the crystal phase can be performed uniformly. Moreover, since the cleavage is easy, strain during stretching is less likely to remain, and the thermal shrinkage rate can be made lower than in the case of stretching after removing the plasticizer.

[0068] (d) Plasticizer extraction (washing) and drying process Next, the plasticizer (solvent) remaining in the gel sheet is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a microporous membrane can be obtained by removing the solvent. Examples of the washing solvent 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 trifluoroethane. These washing solvents have a low surface tension (for example, 24 mN / m or less at 25°C). By using a washing solvent with a low surface tension, in the network structure forming the micropores, shrinkage due to the surface tension at the gas-liquid interface during drying after washing is suppressed, and a microporous membrane having good porosity and permeability can be obtained. These washing solvents are appropriately selected according to the plasticizer and used alone or in combination.

[0069] The cleaning method can be carried out by a method of immersing the gel sheet in a cleaning solvent for extraction, a method of showering the cleaning solvent onto the gel sheet, or a combination of these methods. The amount of the cleaning solvent used varies depending on the cleaning method, but generally it is preferably 300 parts by mass or more with respect to 100 parts by mass of the gel sheet. The cleaning temperature may be 15 to 30 °C, and it is heated to 80 °C or lower as necessary. At this time, from the viewpoint of enhancing the cleaning effect of the solvent, the viewpoint of preventing the physical properties in the TD direction and / or MD direction of the obtained polyolefin microporous membrane from becoming non-uniform, and the viewpoint of improving the mechanical and electrical physical properties of the polyolefin microporous membrane, the longer the time the gel sheet is immersed in the cleaning solvent, the better. The cleaning as described above is preferably carried out until the residual solvent in the gel sheet after cleaning, that is, in the polyolefin microporous membrane, becomes less than 1% by mass.

[0070] Thereafter, in the drying step, the solvent in the polyolefin microporous membrane is dried and removed. The drying method is not particularly limited, and methods such as using a metal heating roll or using 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 microporous membrane decreases and the permeability deteriorates during the subsequent heat treatment.

[0071] (e) Heat treatment / Re-stretching / Heat fixing step The dried polyolefin microporous membrane may be stretched (re-stretched) in at least one axial direction. The re-stretching can be carried out by a tenter method or the like in the same manner as the above-mentioned stretching while heating the microporous membrane. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, it is carried out by combining simultaneous biaxial and / or sequential stretching.

[0072] The temperature of the re-stretching is preferably below the melting point of the polyolefin-based composition, and more preferably within the range of (Tcd - 20 °C) to the melting point. Specifically, 70 to 135 °C is preferable, and 110 to 132 °C is more preferable. Most preferably, it is 120 to 130 °C.

[0073] In the case of uniaxial stretching, the magnification of re-stretching is preferably 1.01 to 1.6 times, particularly preferably 1.1 to 1.6 times in the TD direction, and more preferably 1.2 to 1.4 times. In the case of biaxial stretching, it is preferably 1.01 to 1.6 times in both the MD direction and the TD direction. Note that the magnification of re-stretching may be different between the MD direction and the TD direction. By stretching within the above-mentioned range, the porosity and permeability can be increased. However, when stretching at a magnification of 1.6 or more, the orientation progresses, the melting point of the film rises, and the shutdown temperature rises. From the viewpoints of heat shrinkage rate and wrinkle and sag, the relaxation rate from the maximum re-stretching magnification is preferably 0.9 or less, and more preferably 0.8 or less.

[0074] It is preferable to perform heat setting on the film after re-stretching while fixing the width of the film. The heat setting temperature is preferably 70 to 135°C, more preferably 110 to 132°C. Most preferably, it is 115 to 130°C. The heat setting time is not particularly limited, but is about 1 second to 10 minutes.

[0075] (f) Other processes Furthermore, depending on other applications, the microporous membrane can also be subjected to a hydrophilic treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably performed after cross-linking treatment. It is preferable to perform a cross-linking treatment on the polyolefin microporous membrane by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of electron beam irradiation, an electron beam dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment increases the melt-down temperature of the polyolefin microporous membrane.

[0076] In the case of surfactant treatment, any of nonionic surfactants, cationic surfactants, anionic surfactants, or amphoteric surfactants can be used, but nonionic surfactants are preferable. The microporous membrane is immersed in a solution obtained by dissolving the surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the microporous membrane by the doctor blade method.

[0077] The polyolefin microporous membrane may be surface-coated with a porous fluororesin such as polyvinylidene fluoride or polytetrafluoroethylene, or a porous material such as polyimide or polyphenylene sulfide, or an inorganic coating such as ceramic, for the purpose of improving the melt-down characteristics and heat resistance when used as a separator for a battery.

[0078] It is also preferable that the polyolefin microporous membrane according to the embodiment of the present invention is a laminate provided with a coat layer on at least one side.

[0079] The polyolefin microporous membrane obtained as described above can be used in various applications such as a filter, a separator for a fuel cell, and a separator for a capacitor. In particular, when used as a separator for a battery, it not only has low shut-down characteristics and high melt-down characteristics, but also has high strength despite being a thin film. Since it combines high safety functions and output characteristics, it can be preferably used as a separator for a secondary battery that requires high energy density, high capacity, and high output, such as an electric vehicle.

[0080] The present invention also relates to a battery using the polyolefin microporous membrane or laminate according to the embodiment of the present invention.

Examples

[0081] Hereinafter, the present invention will be described in detail with reference to examples. The characteristics were measured and evaluated by the following methods.

[0082] 1. Measurement of molecular weight distribution of polyolefin The molecular weight distribution of polyolefin (measurement of weight average molecular weight, molecular weight distribution, content of a predetermined component, etc.) was measured by high-temperature GPC (gel permeation chromatography). The measurement conditions were as follows.

[0083] Apparatus: High-temperature GPC apparatus (instrument No. HT-GPC, manufactured by Polymer Laboratories, PL-220) Detector: Differential Refractometer RI Guard Column: Shodex G-HT Column: Shodex HT806M (2 pieces) (φ7.8mm×30cm, manufactured by Showa Denko) Solvent: 1,2,4-Trichlorobenzene (TCB, manufactured by Wako Pure Chemical Industries, Ltd.) (added with 0.1% BHT) 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 after heating and stirring at 160 - 170°C for about 30 minutes, the resulting solution was filtered through a metal filter (pore size 0.5 μm). Injection Volume: 0.200 mL Standard Sample: Monodisperse Polystyrene (manufactured by Tosoh Corporation) (PS) Data Processing: GPC Data Processing System manufactured by TRC

[0084] After that, the obtained Mw and Mn were converted to polyethylene (PE). The conversion formula is as follows. Mw (converted to PE) = Mw (measured value converted to PS) × 0.468 Mn (converted to PE) = Mn (measured value converted to PS) × 0.468

[0085] 2. Film Thickness The thickness of the microporous membrane was measured at randomly selected MD positions using a contact thickness gauge, Mitutoyo Corporation's Lite-Matic VL-50 (10.5 mmφ carbide spherical probe, measurement load 0.01 N). The measurement was carried out at points along the TD (width) of the membrane at 5 mm intervals over a distance of 30 cm. Then, the above measurement along the TD was performed 5 times, and the arithmetic mean was taken as the thickness of the sample.

[0086] 3. Air Permeability Resistance For the porous film with a film thickness of T1, in accordance with JIS P-8117, the air permeability resistance G1, which is the time required to permeate 100 cm 3 of gas, was measured using a King-type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The air permeability resistance G2 when the film thickness is 5 μm was calculated using the formula: G2 = (G1 × 5) / T1.

[0087] 4. Puncture strength Using a force gauge (DS2-20N manufactured by IMADA Co., Ltd.), a needle with a diameter of 1 mm having a spherical surface (radius of curvature R: 0.5 mm) at the tip was punctured into a microporous membrane with an average film thickness T1 (μm) at a speed of 2 mm / second. Otherwise, in accordance with JIS Z 1707 (2019), the maximum load L1 (load immediately before penetration, unit: N) was measured, and the puncture strength L2 (N) when the film thickness was 5 μm was calculated using the formula L2 = (L1 × 5) / T1.

[0088] 5. Porosity (%) A 5 cm square sample was cut out from the microporous membrane, and its volume (cm 3 ) and mass (g) were determined. From these and the polymer density (g / cm 3 ), the following formula was used for calculation. The above measurements were performed at three different locations in the same microporous membrane, and the average value of the porosity was determined. Porosity = [(Volume - Mass / Polymer density) / Volume] × 100 Note that the polymer density was calculated assuming a constant value of 0.99 g / cm 3 .

[0089] 6. Tensile strength, elongation at break Tensile strength M MD and tensile strength M TD , and the elongation at break in the MD direction and the elongation at break in the TD direction were measured using a strip-shaped test piece with a width of 10 mm in accordance with ASTM D882 at a speed of 100 mm / min.

[0090] 7. Shutdown temperature The above-mentioned air permeability resistance was measured while raising the temperature from room temperature at 5°C / min. The temperature when the air permeability resistance reached 100,000 seconds / 100 cm 3 was defined as the shutdown temperature (SD temperature) (°C). The measurement cell was composed of an aluminum block, with a thermocouple directly below the polyolefin microporous membrane. The sample was cut into a 5 cm × 5 cm square, and the temperature was measured while heating with the periphery fixed with an O-ring.

[0091] 8. Meltdown temperature A microporous membrane with a 50-mm angle is sandwiched using a pair of metal block frames having holes with a diameter of 12 mm, and a tungsten carbide sphere with a diameter of 10 mm is placed on the microporous membrane. The microporous membrane is installed so as to have a plane in the horizontal direction. Starting from 30 °C, the temperature is increased at a rate of 5 °C / min. The temperature at which the microporous membrane is broken by the sphere was measured and taken as the melt-down temperature (MD temperature).

[0092] 9. DSC Measurement The melting point and the half-width are determined by a differential scanning calorimeter (DSC). This DSC was performed using MDSC2920 or Q1000Tzero-DSC of TA Instruments. Based on JIS K7121, the temperature was increased from 30 °C to 230 °C at a rate of 10 °C / min, and the temperature (peak temperature) at the maximum value of the obtained melting peak was evaluated. The peak temperature in the region below 150 °C was designated as P1, and the peak temperature at 150 °C or higher was designated as P2.

[0093] The half-width is the temperature at which the heat generation amount becomes 0.5 times the maximum heat generation amount Q in the region below 150 °C 1 / 2 for each of T 1 , T 2 (T 1 < T 2 ). The value of T 2 - T 1 was calculated. When there are two or more maximum values in the region below 150 °C and three or more temperatures at which Q 1 / 2 is obtained, the minimum temperature of the corresponding temperatures was taken as T 1 , and the maximum temperature was taken as T 2 to calculate the half-width.

[0094] 10. Maximum Pore Diameter and Average Pore Diameter Using a palm porometer (manufactured by PMI, CFP-1500A), the maximum pore diameter and the average pore diameter were measured in the order of Dry-up and Wet-up. For Wet-up, pressure was applied to a porous polyolefin film sufficiently immersed in Galwick (trade name) manufactured by PMI with a surface tension of 1.59 × 10 -2 N / m, and the pore diameter converted from the pressure at which air begins to penetrate was taken as the maximum pore diameter.

[0095] Regarding the average diameter, the pore diameter was converted from the pressure at the point where the curve showing the slope of 1 / 2 of the pressure and flow rate curves in the Dry-up measurement intersects with the curve in the Wet-up measurement. The conversion between pressure and pore diameter was performed using the following mathematical formula. d = C·γ / P

[0096] In the above formula, "d (μm)" is the pore diameter of the porous polyolefin film, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant determined by the wetting tension of the immersion liquid, contact angle, etc.

[0097] Hereinafter, examples will be shown for specific explanation, but the present invention is not limited by these examples at all.

[0098] [Example 1] 54.6 parts by mass of branched high-density polyethylene (branched HDPE) with a weight average molecular weight (Mw) of 1.8×10 5 , a melting point of 133°C, 23.4 parts by mass of ultra-high molecular weight polyethylene (UHPE) (Mw 2.0×10 6 , a melting point of 133°C), and 22.0 parts by mass of polypropylene (PP) (Mw 1.1×10 6 , a melting point of 165°C) were respectively mixed to obtain a polyolefin composition. 71.5 parts by mass of liquid paraffin was added to 28.5 parts by mass of the polyolefin composition. Further, 0.5 part by mass of 2,6-di-t-butyl-p-cresol and 0.7 part by mass of tetrakis [methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate] methane were added as antioxidants based on the mass of the polyolefin in the mixture and mixed to prepare a polyethylene resin solution.

[0099] The obtained polyethylene resin solution was introduced into a twin-screw extruder, kneaded with the first half of the extruder at 150°C and the second half at 180°C, supplied to a T-die, and finally extruded into a sheet shape so that the thickness of the microporous membrane became 5 μm. Then, the extrudate was cooled with a cooling roll controlled at 15°C to form a gel-like sheet.

[0100] The obtained gel sheet was held at the four sides by clips with a film stretcher, stretched 7 times in the longitudinal direction at 115 °C, and then stretched 7 times in the width direction (sequential stretching (area magnification: 49 times)). Subsequently, the sheet width was fixed as it was in the film stretcher and held at a temperature of 115 °C for 10 seconds and then taken out.

[0101] Next, the stretched gel sheet was fixed to a metal frame, immersed in a methylene chloride bath in a washing tank, and after removing the liquid paraffin, it was dried to obtain a polyolefin microporous membrane.

[0102] Finally, the polyolefin microporous membrane fixed to the metal frame was introduced into a hot air oven and heat-fixed at 120 °C for 10 minutes.

[0103] The raw material properties, film-forming conditions, and evaluation results of the polyolefin microporous membrane are shown in Table 1.

[0104] [Examples 2 to 3, Comparative Examples 1 to 4] Except that the raw material composition and film-forming conditions were changed as shown in Tables 1 and 2, polyolefin laminated microporous membranes were produced in the same manner as in Example 1. The raw material properties, film-forming conditions, and evaluation results of the obtained polyolefin microporous membranes are as shown in Tables 1 and 2.

[0105] Note that "linear HDPE" described in Tables 1 and 2 indicates linear high-density polyethylene.

[0106] [Example 4] 20 parts by mass of branched high-density polyethylene (branched HDPE) with a weight average molecular weight (Mw) of 9.0×10 4 and a melting point of 131 °C, 70 parts by mass of ultra-high molecular weight polyethylene (UHPE) (melting point 136 °C, Mw 1.0×10 6 ), and polypropylene (PP) (Mw 1.1×10 610.0 parts by mass of ) and were each mixed to obtain a polyolefin composition. 77 parts by mass of liquid paraffin was added to 23 parts by mass of the polyolefin composition, 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 mass of the polyolefin in the mixture and mixed to prepare a polyethylene resin solution.

[0107] The obtained polyethylene resin solution was charged into a twin-screw extruder, kneaded with the first half of the extruder at 150 °C and the second half at 180 °C, supplied to a T-die, and finally extruded into a sheet so that the thickness of the microporous membrane became 6 μm. Then, the extrudate was cooled with a cooling roll controlled at 15 °C to form a gel-like sheet.

[0108] The obtained gel-like sheet was gripped at the four sides with clips by a film stretcher and stretched 5 times in the longitudinal and width directions at 115 °C (simultaneous stretching (areal magnification 25 times)), and the sheet width was fixed in the film stretcher as it was and held at a temperature of 115 °C for 10 seconds and then taken out.

[0109] Next, the stretched gel-like sheet was fixed to a metal frame, immersed in a methylene chloride bath in a washing tank, dried after removing the liquid paraffin to obtain a polyolefin microporous membrane.

[0110] Finally, the polyolefin microporous membrane fixed to the metal frame was introduced into a hot air oven and heat-fixed at 130 °C for 10 minutes.

[0111] The raw material characteristics, film-forming conditions of the polyolefin microporous membrane, and the evaluation results of the microporous membrane are described in Table 1.

[0112] [Example 5] The stretching ratio by the film stretcher was 7 times in the longitudinal and width directions (simultaneous stretching (areal magnification 49 times)), and a porous membrane with a thickness of 6 μm was obtained in the same manner as in Example 4 except for this.

[0113] Table 1 shows the raw material properties, film-forming conditions, and evaluation results of the polyolefin microporous membrane.

[0114] [Table 1]

[0115] [Table 2]

Industrial Applicability

[0116] Although the polyolefin microporous membrane of the present invention is a thin film, it has excellent strength, low shutdown characteristics, and high melt-down characteristics. Therefore, when used as a separator for batteries, it has excellent safety and output characteristics. Therefore, it can be suitably used as a battery separator and a laminate for batteries that require high energy density, high capacity, and high output, such as electric vehicles, and secondary batteries.

[0117] Although the present invention has been described in detail with reference to specific embodiments, it is obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

[0118] This application is based on Japanese Patent Application No. 2019-152106 filed on August 22, 2019, the content of which is incorporated herein by reference.

Claims

1. A single-layer polyolefin microporous membrane containing polyethylene (A) and a polyolefin (B) other than polyethylene, wherein the polyethylene (A) contains branched high-density polyethylene, the membrane thickness is 8 μm or less, the puncture strength in terms of a thickness of 5 μm is 1.4 N or more, the shutdown temperature is 134 °C or less, the melt-down temperature is 160 °C or more, and the ratio of the average pore diameter to the maximum pore diameter (average pore diameter / maximum pore diameter) is 0.56 to 1.

0. A polyolefin microporous membrane.

2. The tensile strength in the TD direction is M ＴＤ When, M ＴＤ The polyolefin microporous membrane according to claim 1, wherein is 50 MPa or more.

3. When the tensile strength in the MD direction is M ＭＤ , the value of M ＭＤ / M ＴＤ is 0.5 to 2.0, and the polyolefin microporous membrane according to claim 1 or 2.

4. The air permeability resistance in terms of a thickness of 5 μm is 50 seconds / 100 cm ３ or more and 1000 seconds / 100 cm ３ The polyolefin microporous membrane according to any one of claims 1 to 3, which is below

5. The polyolefin microporous membrane according to any one of Claims 1 to 4, wherein the average pore diameter is 50 nm or less.

6. The polyolefin microporous membrane according to any one of Claims 1 to 5, wherein the polyethylene-based resin is the main component.

7. The polyolefin microporous membrane according to any one of Claims 1 to 6, wherein the content of the polyolefin (B) is 5 to 40% by mass.

8. The polyolefin microporous membrane according to any one of Claims 1 to 7, wherein the polyolefin (B) is a polypropylene-based resin.

9. The polyolefin microporous membrane according to any one of Claims 1 to 8, which has peaks at less than 150 °C and 150 °C or more in DSC, and the half-value width of the peak at less than 150 °C is 10 °C or less.

10. A laminate having a coating layer provided on at least one side of the polyolefin microporous membrane according to any one of Claims 1 to 9.

11. A battery using the polyolefin microporous membrane according to any one of Claims 1 to 9 or the laminate according to Claim 10.

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