JP2022151659A - Polyolefin microporous film, separator for battery and secondary battery - Google Patents

Abstract

To provide a polyolefin microporous film having both mechanical strength and shutdown temperature by restraining a change of orientation at room and high temperatures, and to provide a separator for battery using the microporous film.SOLUTION: There is disclosed, a polyolefin microporous film characterized in that either one of orientation parameter values (fMH) and (fTH) measured at 130°C in MD and TD directions, calculated bybelow-mentioned formulae (1) and (2) using a Raman spectrum device, is larger than 1.5 and the other is 1.5 or less. Formula (1) is fMH=Ia(MD,130°C)/Ib(MD,130°C) and formula (2) is fTH=Ia(TD,130°C)/Ib(TD,130°C) (herein, Ia is maximum mechanical strength of Raman band in 1100-1170 cm-1; Ib is maximum strength of the above band in 1040-1090 cm-1; Ia(MD,130°C) and Ib(MD,130°C) are maximum strengths at 130°C in the MD direction; and Ia(TD,130°C) and Ib(TD,130°C) are maximum strengths at 130°C in the TD direction).SELECTED DRAWING: None

Description

The present invention is a separation membrane used for material separation, selective permeation, etc., and a polyolefin microporous membrane ( Also referred to as porous polyolefin film). In particular, the present invention relates to a polyolefin microporous membrane suitable for use as a separator for non-aqueous electrolyte secondary batteries such as lithium ion batteries, and is used as a separator having higher safety than conventional polyolefin microporous membranes.

Polyolefin microporous membranes are used as filters, separators for fuel cells, and separators for capacitors. In particular, it is suitably used as a separator for non-aqueous electrolyte secondary batteries such as lithium ion batteries widely used in notebook personal computers, mobile phones and the like. The reason for this is that the polyolefin microporous membrane has excellent mechanical strength, shutdown characteristics, and ion permeation performance.

In recent years, the capacity of lithium-ion secondary batteries has been increasing, mainly due to the miniaturization of electronic devices and the development of in-vehicle applications. Along with this, there is a growing demand for thinner separators. However, thinning the separator reduces its strength, making it more likely that the electrode or foreign matter will cause a short circuit (foreign object resistance), or if the battery receives an impact, the membrane will rupture (decrease in impact resistance), reducing battery safety. descend. Therefore, higher strength than ever before is required. In addition, in a battery with high energy, even if the progress of the electrochemical reaction is stopped by the shutdown function of the separator, the temperature inside the battery continues to rise. is short-circuited. Therefore, in order to improve battery safety, separators are required to have high strength, low shrinkage at high temperatures, and a shutdown function at low temperatures. However, when the shutdown temperature is lowered to improve safety, there is a problem that the strength of the porous film is lowered.

For example, in Patent Document 1, dry re-stretching is performed in the MD direction (machine direction) as a method for improving the strength, shrinkage rate, and shutdown temperature, and the film thickness is 12 μm or less by controlling the Raman orientation parameter value in the MD direction. A technique for obtaining a microporous membrane having a strength of 230 gf or more, a heat shrinkage rate of 15% in the TD (width direction) at 105° C. for 8 hours, and a shutdown temperature of 135° C. to 149° C. is disclosed.

In Patent Document 2, as a method for improving shutdown temperature and puncture strength, polyolefin having a weight molecular weight of 500,000 or more is used as a main component, and the orientation ratio in the MD and TD directions obtained by X-ray analysis is controlled to 0.24. A method for obtaining a microporous membrane with a puncture strength of ~0.75 N/(g/m ² ) and a shutdown temperature of 139°C to 146°C is disclosed.

Patent Document 3 discloses, as a method for improving mechanical strength and permeability, a method for obtaining a microporous film having a puncture strength of 300 to 500 gf converted to 25 μm by controlling the degree of orientation determined by infrared spectrum measurement. .

Japanese Patent Application Laid-Open No. 2020-95950 Japanese Patent Application Laid-Open No. 2017-103201 JP 2013-199545 A

Separators are required to have high strength and low-temperature shutdown, but there is a problem that the shutdown characteristics deteriorate when the strength of the separator is increased. However, it was not sufficient from the viewpoint of achieving both thin film, high strength, and low-temperature shutdown.

In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having a superior balance between strength and shutdown temperature, and a separator using the membrane.

The present inventors have made intensive studies to achieve the above-mentioned object, and as a result, a microporous film having a specific range of orientation parameters at high temperature in the MD and TD directions calculated by microscopic Raman spectroscopy solves the above-mentioned problems. The present inventors have solved the problem and found that both excellent strength and shutdown characteristics can be achieved, thereby completing the present invention. That is, the present invention has the following configurations.

Either one of the orientation parameter value (fMH) in the MD direction and the orientation parameter value (fTH) in the TD direction measured at 130 ° C. calculated by the following formulas (1) and (2) using a microscopic Raman spectrometer is 1. 0.5 and the other is 1.5 or less.
fMH = _Ia (MD, 130°C)/ _Ib (MD, 130°C) (1) Formula fTH = _Ia (TD, 130°C)/ _Ib (TD, 130°C) _{. _} ^_ _{_} ^_ Maximum strength, _Ia (MD, 130°C), _Ib (MD, 130°C) are maximum strengths in the MD direction measured at 130°C, _Ia (TD, 130°C), _Ib (TD, 130°C) are It is the maximum intensity in the TD direction measured at 130°C.

INDUSTRIAL APPLICABILITY According to the present invention, a polyolefin microporous membrane having excellent balance between strength and shutdown temperature and having high safety can be obtained.

The polyolefin microporous membrane according to the embodiment of the present invention is useful as a battery separator because of its excellent strength, shutdown characteristics and shrinkage rate, and has excellent safety. The present invention can be realized by satisfying the orientation parameters under high temperature in the MD and TD directions calculated by microscopic Raman spectroscopy in the range described later, and it is possible to improve the balance between the strength and the shutdown temperature, which were conventionally in a trade-off relationship. I found that they are connected. The present invention is not limited to the embodiments described below. The direction in which the tape is drawn is called the width direction or the TD direction.

The present invention will be described in further detail below.

[1] Polyolefin microporous membrane The polyolefin microporous membrane according to the embodiment of the present invention has an orientation parameter value (fMH) in the MD direction and an orientation parameter value (fTH) in the TD direction measured at 130 ° C. by the method described later. ) is greater than 1.5 and the other is less than or equal to 1.5. This means controlling the mobility of the crystal structure in either the MD or TD direction. The Raman orientation parameter is an index indicating the degree of orientation of crystal molecular chains as a value calculated by Raman spectrometry, and the higher the value, the more highly oriented the crystal molecular chains. When either one of fMH and fTH is greater than 1.5, a strong structure in which the orientation state is maintained even at high temperatures is obtained, resulting in excellent strength. If one is 1.5 or less, the crystal structure melts at high temperatures, resulting in a low orientation parameter and a low shutdown temperature. From the viewpoint of strength, the higher the degree of orientation, the better, and the value of either fMH or fTH is preferably 1.7 or more, more preferably 1.8 or more. From the viewpoint of strength, the higher the degree of orientation, the better. However, since this leads to a deterioration in shrinkage and an increase in shutdown temperature, one of fMH and fTH is preferably 1.4 or less, more preferably 1.3 or less. From the viewpoint of the balance between strength and shutdown temperature, one of fMH and fTH is preferably 1.3 or less, and the other is preferably 1.7 or more. More preferably, the direction greater than .5 is the MD direction (ie, fMH is greater than fTH). By satisfying the above range, a microporous membrane having an excellent balance between strength and shutdown temperature can be obtained. note that. The above range can be controlled by the raw material design and manufacturing method described later, and in order to form a strong structure that maintains the orientation state at high temperatures, a polyolefin resin with a weight average molecular weight of 9.0 × 10 ⁵ or more with a long relaxation time is the main raw material, and wet stretching, including re-longitudinal stretching after washing and drying, and dry sequential stretching are preferably combined to form a film. It is more preferable to combine a polyolefin resin of 3.0 × ^{10 5} or less with an auxiliary raw material. More preferably, the film contains 30% by mass or more of the component with a short relaxation time and a molecular weight of 3.0×10 ⁵ or less in the range of 20 to 50% by mass.

The polyolefin microporous membrane according to the embodiment of the present invention has an orientation parameter value (fML) in the MD direction and an orientation parameter value (fTL) in the TD direction measured at 25 ° C. by the method described later, both of which are 1.5. The following are preferred. From the viewpoint of strength, the higher the fML and fTL, the better, but when the highly oriented structure increases in the measurement at 25 ° C., the shutdown temperature rises and the shrinkage rate increases due to the relaxation of molecular orientation at high temperatures. From the above viewpoint, fML and fTL are preferably 1.5 or less, more preferably 1.4 or less. The above fML and fTL are orientation parameters calculated by the following equations (3) and (4), I _a is the maximum intensity in the Raman shift band 1100 to 1170 cm ⁻¹ , and I _b is the Raman shift. The maximum intensities of Raman bands in the range of 1040-1090 cm ⁻¹ , I _a (MD, 25° C.) and I _b (MD, 25° C.) were measured in the MD direction of the polyolefin microporous membrane at 25° C., and I _a ( TD, 25°C) and I _b (TD, 25°C) are values measured in the TD direction of the polyolefin microporous membrane at 25°C. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.
fML = _Ia (MD, 25°C)/ _Ib (MD, 25°C) (3) Formula fTL = _Ia (TD, 25°C)/ _Ib (TD, 25°C) ... (4) formula.

The polyolefin microporous membrane according to the embodiment of the present invention has _a ratio ( _fMLH ) and _{Da (} TD, 25° C.) to Da (TD, 130° C.) ( _fTLH ) is preferably 4.0 or more and the other is less than 4.0. If it is 4.0 or more, the C—C stretching vibration of the polyethylene molecular chain in the crystal phase at 130° C. is reduced, that is, the crystal structure melts, resulting in good shutdown characteristics. Even at 130°C, the molecular chain structure of the crystal is highly maintained, and high strength can be obtained. D _a refers to the difference between the maximum intensity in the Raman shift band of 1100 to 1170 cm ⁻¹ and the intensity at 1200 cm ⁻¹ , and D _a (MD, 130° C.) is the MD direction of the polyolefin microporous membrane at 130° C. The measured _Da value, _Da (TD, 130°C) is the value of _Da measured in the TD direction of the polyolefin microporous membrane at 130°C, and _Da (MD, 25°C) is the MD direction of the polyolefin microporous membrane. is the value of _Da measured at 25°C, and _Da (TD, 25°C) is the value of _Da measured in the TD direction of the polyolefin microporous membrane at 25°C. 1130 cm ⁻¹ is a band attributed to the C—C stretching vibration of the polyethylene molecular chain in the crystal phase, and the direction of the Raman tensor of vibration is the molecular chain axis. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.
fMLH=D _a (MD, 25° C.)/D _a (MD, 130° C.)) (5) fTLH=D _a (TD, 25° C.)/D _a (TD, 130° C.)) (6) Ordinarily, the crystal structure relaxes at high temperatures, so the orientation parameter measured at 130°C is smaller than the orientation parameter measured at 25°C (orientation parameter measured at 25°C>orientation parameter measured at 130°C ). On the other hand, when the polyolefin microporous membrane has a high melting point due to a combination of a specific polyolefin composition and dry sequential stretching, recrystallization occurs at 130°C, and the orientation parameter measured at 130°C is higher than the orientation parameter measured at 25°C. may also increase. The smaller the difference in the orientation parameters between 25° C. and 130° C. and the closer the variation is to 0, the better the retention of the crystal structure at high temperatures, the lower the shrinkage rate and the higher the strength. On the other hand, if the difference in orientation parameter between 25° C. and 130° C. is small, it means that the crystal structure melts less at high temperatures, leading to deterioration in shutdown characteristics. Therefore, from the viewpoint of the balance of strength, shrinkage, and shutdown temperature, the change in the orientation parameter at 25°C and 130°C in either the MD direction or the TD direction calculated by microscopic Raman spectroscopy should be large, and the change in the other direction should be small. However, from the viewpoint of shutdown characteristics, the difference in orientation parameter between 25° C. and 130° C. is preferably 0.0 or less (orientation parameter measured at 25° C.≦orientation parameter measured at 130° C.). If the difference in orientation parameter between 25° C. and 130° C. is 0.0 or less, melting of the crystal structure is promoted at high temperatures, and excellent shutdown characteristics are obtained. In a preferred embodiment of the present invention, the difference between fML and fMH (fML-fMH (7) formula) is preferably 0.0 or less, more preferably −0.1 or less, and further preferably −0.2 or less. -0.3 or less is even more preferable, -0.4 or less is particularly preferable, and the difference between fTL and fTH (fTL-fTH: formula (8)) is preferably greater than 0.0, and 0.4 or less. It is more preferably 1 or more, more preferably 0.2 or more, and preferably 0.5 or less from the viewpoint of shrinkage. Good shutdown characteristics are obtained when the difference between fML and fMH is 0.0 or less, and good strength is obtained when the difference between fTL and fTH is greater than 0.0. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.

By satisfying the above ranges for the orientation parameters fMH, fTH, fML, and fTL, excellent strength and shutdown balance can be obtained, and the above ranges can be controlled by raw material design and manufacturing methods, which will be described later. In addition, in order to form a strong structure that maintains the orientation state at high temperatures, polyolefin resin with a weight average molecular weight of 9.0 × 10 ⁵ or more with a long relaxation time is used as the main raw material, and longitudinal re-stretching after washing and drying is performed. A polyolefin resin having a short relaxation time and a weight average molecular weight of 3.0×10 ⁵ or less is used as an auxiliary raw material as an aid for accelerating the melting and orientation relaxation of crystals at high temperatures. From the above viewpoint, the polyolefin microporous membrane contains 30% by mass or more of a component with a molecular weight of 9.0 × 10 ⁵ or more, which has a long relaxation time in the molecular weight distribution of the polyolefin microporous membrane, and the relaxation time is short. More preferably, the content of components having a weight-average molecular weight of 3.0×10 ⁵ or less is in the range of 20 to 50 mass %. As a result, a highly safe polyolefin microporous membrane having both high strength and low-temperature shutdown properties can be obtained.

The porosity of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 30% or more, more preferably 35% or more, and still more preferably 40% from the viewpoint of permeability and electrolyte content. That's it. When the porosity is 30% or more, the balance between permeability, strength and electrolyte content is improved, and non-uniformity in battery reaction is eliminated. As a result, the generation of dendrites is suppressed, and the separator can be used without impairing the performance of conventional batteries, and can be suitably used as a separator for secondary batteries. In addition, although good output characteristics can be obtained by increasing the porosity, the safety of the battery deteriorates, such as a decrease in puncture strength and an increase in shrinkage. Therefore, the porosity is preferably 50% or less, more preferably 48% or less.

The higher the puncture strength of the polyolefin microporous membrane, the better, because it affects the safety such as prevention of short-circuiting caused by foreign matter in the battery. The pin puncture strength of the polyolefin microporous membrane converted to a thickness of 10 μm is preferably 5.5 N or more, more preferably 6.0 N or more, and even more preferably 6.5 N or more. In addition, the puncture strength per unit basis weight (converted puncture strength), which is an index showing the strength of the film obtained by standardizing the puncture strength with the amount of resin, is preferably 0.75 N/(g/m ² ) or more, and is preferably 0.75 N/(g/m 2 ) or more. 8 N/(g/m ² ) or more is more preferable, 0.85 N/(g/m ² ) or more is more preferable, 0.9 N/(g/m ² ) or more is still more preferable, and 1.0 N/(g/m 2 ) or more is more preferable. /m ² ) or more and 2.0 N/(g/m ² ) or less is particularly preferable. When the puncture strength is within the above range, short circuits due to foreign matter or the like are suppressed, and good battery safety is obtained. In order to improve the puncture strength, it is preferable to combine the control of the orientation of crystals and the use of ultra-high molecular weight polyolefin as the main component in the raw material formulation to increase the strength by increasing the number of tie molecules that connect lamellar crystals. In addition, from the viewpoint of suppressing a decrease in porosity due to melting in a heat setting step or the like, an ultra-high molecular weight polyolefin having a molecular weight distribution with a small amount of low molecular weight components is preferable. The puncture strength can be achieved by setting the fMH, fTH, fML, and fTL in specific ranges and adopting the raw material, resin concentration, and stretching method described later.

The puncture strength when the film thickness is converted to 10 μm is expressed by the formula: L2 = (L1 × 10)/ It refers to the puncture strength L2 (N) calculated by T1, and the puncture strength per unit basis weight is a value obtained by dividing the actually measured puncture strength (L1) by the basis weight G (g/m ² ), and the formula: L1 /G is calculated.

From the viewpoint of suppressing short-circuiting of the battery due to abnormal heat generation, the total shrinkage rate in the MD direction and the TD direction at 130° C./1 h is preferably 35% or less, more preferably 30% or less, and even more preferably 29% or less. If the shrinkage ratio is within this range, the dimensional change is small and the insulation can be maintained when the internal temperature of the battery rises. . The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.

Since the battery is under tension in the MD direction, if the shrinkage rate in the MD direction is high, the membrane will break and lead to a short circuit. Therefore, the polyolefin microporous membrane according to the embodiment of the present invention has a shrinkage rate in the MD direction at 130°C/1h of 15% or less, preferably 12% or less, and more preferably 10% or less. When the shrinkage ratio in the MD direction is within this range, the dimensional change is small when the internal temperature of the battery rises, and the insulation can be maintained, resulting in high safety. The above range can be achieved by applying the starting material, molecular weight, and manufacturing method within the ranges described later.

In addition, the polyolefin microporous membrane according to the embodiment of the present invention preferably has a shrinkage rate in the TD direction at 130° C./1h of 25% or less, more preferably 20% or less, and even more preferably 18% or less. When the shrinkage ratio is within this range, deterioration of shape stability at high temperatures can be suppressed, and internal short-circuiting can be suppressed when abnormal heat is generated locally, thereby maintaining safety. The above heat shrinkage rate can be achieved by setting the fMH, fTH, fML, and fTL in specific ranges and adopting the raw materials, resin concentration, and stretching method described later. The shrinkage ratios in the MD direction and the TD direction at 130° C./1 h can be measured by the method described in Examples.

In the polyolefin microporous membrane of the present invention, the tensile breaking strength in the MD direction (tensile breaking strength in the MD direction; hereinafter simply referred to as "MD tensile strength") is effective for suppressing film breakage in the battery winding process and removing foreign matter in the battery. From the viewpoint of short-circuiting due to the like, the MD tensile strength is preferably 250 MPa or more, preferably 300 MPa or more, more preferably 350 MPa or more, still more preferably 400 MPa or more, and still more preferably 450 MPa or more.

From the viewpoint of balance with the MD tensile strength, the tensile breaking strength in the TD direction (tensile breaking strength in the TD direction; hereinafter simply referred to as "TD tensile strength") is preferably TD tensile strength 250 MPa or more, preferably It is 300 MPa or higher, more preferably 320 MPa or higher, and still more preferably 350 MPa or higher. When the TD tensile strength is within the above range, the balance between the MD tensile strength and the TD tensile strength is good, and wrinkles and sagging of the film are suppressed. improved sexuality. The above tensile strength can be achieved by adopting the raw materials, resin concentration, and stretching method, which will be described later.

In the polyolefin microporous membrane according to the embodiment of the present invention, air permeability is a value measured according to JIS P 8117 (2009). Further, when the air permeability measured in the polyolefin microporous membrane having a thickness of T1 (μm) is p1 (seconds/100 cm ³ ), the air permeability calculated by the formula: p2=(p1×10)/T1 Let p2 (second/100 cm ³ ) be the air permeability when the film thickness is 10 μm (10 μm equivalent air permeability).

The air permeability is preferably 200 sec/100 cm ³ or less, more preferably 120 sec/100 cm ³ or less, and even more preferably 115 sec/100 cm ³ or less. If the air permeability is 200 sec/100 cm ³ or less, good ion permeability can be obtained and electric resistance can be lowered.

In the polyolefin microporous membrane according to the embodiment of the present invention, as the membrane thickness increases, the resistance increases and the output characteristics of the battery deteriorate. From the viewpoint of battery output characteristics, the film thickness is preferably 12 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. The thinner the film, the lower the strength and the lower the safety. Therefore, from the viewpoint of safety, the thickness is preferably 1 μm or more, more preferably 2 μm or more.

The shutdown temperature is the temperature at which the resin portion shrinks and melts and the pores close when the polyolefin microporous membrane is heated to stop discharging and charging, and is the temperature measured by the method described later. Since the electrodes used in lithium-ion secondary batteries designed for high energy density tend to have reduced thermal stability, it is preferable to shut down (hole clogging) quickly after the battery is short-circuited. The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 143°C or lower, more preferably 140°C or lower. Moreover, from the viewpoint of compatibility with puncture strength, the shutdown temperature of the polyolefin microporous membrane is preferably 128° C. or higher. The microporous membrane obtained by the present invention has excellent short-circuit resistance and has the shutdown temperature described above, so that excellent battery safety can be obtained. In order to set the shutdown temperature within the above range, it is preferable to set the raw material composition of the microporous membrane within the range described later and the film forming conditions of the microporous membrane within the range described later. In particular, as a method of setting the shutdown temperature to 143°C or less while satisfying the formulas ( ¹ ) and ( ² ), ° C. or lower, in the DSC curve obtained from differential scanning calorimetry (DSC), the ratio of the heat of fusion ΔH ₁₃₀ up to 130 ° C. or lower (= ΔH ₁₃₀ / ΔH _all ) of the total heat of fusion ΔH _all A method of obtaining a composition containing a specific high-density polyethylene containing molecular weight polyethylene as a main component, and further kneading the mixture with a twin-screw extruder having a screw of a specific structure.

The specific high-density polyethylene has a melting point of 130 ° C. or higher and 135 ° C. or lower, and a DSC curve obtained from differential scanning calorimetry (DSC), in which the total heat of fusion ΔH _all is 200 J / g or more and 250 J / g or less, and the half width is High-density polyethylene having a temperature of 4.0°C or higher and 6.0°C or lower can be mentioned.

As a screw with a specific structure, when the outermost diameter of the screw is D, the distance between the tip of the screw and the vent hole is 1.0 D or more and 15.0 D or less, and the length in the raw material conveying direction is between Examples include a screw having a structure in which at least one screw piece with a length of 0.2D or more and 0.9D or less is used and two or more screw pieces with a length of 1.5D or more in the raw material conveying direction are not used.

From the viewpoint of good permeability, the polyolefin microporous membrane according to the embodiment of the present invention preferably has an average pore diameter of 36 nm or more and 46 nm or less, more preferably 38 nm or more and 45 nm or less, as calculated from a perm porometer. It is preferably 40 nm or more and 44 nm or less. Examples of the method for adjusting the average pore size to 36 nm or more and 46 nm or less include a method of stretching at a specific ratio or more by a stretching method including at least dry sequential biaxial stretching.

[2] Polyolefin resin The resin raw material in the polyolefin microporous membrane according to the embodiment of the present invention may be a single composition, or may be a composition in which a main raw material and an auxiliary raw material are combined. It may be a polyolefin resin mixture (polyolefin resin composition). The raw material form of the polyolefin microporous membrane is preferably a polyolefin resin. Examples of the polyolefin resin include polyethylene, polypropylene, etc. A composition obtained by combining a main raw material and an auxiliary raw material is preferable.

Polyolefin resins are preferably homopolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc., and particularly preferably ethylene homopolymers (polyethylene). Polyethylene may be a homopolymer of ethylene and a copolymer containing other α-olefins.

Other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, alkenes having more carbon atoms, vinyl acetate, methyl methacrylate, styrene, and the like. mentioned.

As the type of ^polyolefin resin used in the embodiment of the present invention ^, polyethylene is preferable. Density polyethylene, low-density polyethylene having a density lower than 0.93 g/cm ³ , linear low-density polyethylene, and the like.

In addition, from the viewpoint of film strength and shrinkage ratio, it is preferable to use an ultra ^- high molecular weight polyolefin as the main component of the polyolefin resin. ×10 ⁶ or more is more preferable. Moreover, from the viewpoint of moldability, the weight average molecular weight is preferably 1.0×10 ⁷ or less. In addition, it is important to add the auxiliary material within a range that does not impair the fibril structure formed by the main material and the moldability.

When the weight-average molecular weight is 1.0×10 ⁶ or more, the relaxation time is long, so the retention of the crystal structure at high temperatures is improved, and the melting and shrinkage are suppressed, and the orientation state is maintained even at high temperatures. It has a strong structure and can achieve both strength and shrinkage, improving the safety of the battery. In addition, by using a polyolefin resin with a weight average molecular weight of 1.0×10 ⁶ or more, the number of tie molecules increases and high strength can be easily obtained. In addition to obtaining good output characteristics, the heat setting temperature can be raised by lowering the relaxation rate of the resin, and good shrinkage characteristics can be obtained.

The ultra-high molecular weight polyethylene used in the embodiment of the present invention has a DSC curve obtained from differential _{scanning calorimetry} ( _DSC ). ΔH _all ) is preferably 45% or less. By setting ΔH ₁₃₀ /ΔH _all to 45% or less, it is possible to reduce crystal components that melt at 130° C. or less and improve dimensional stability at high temperatures. Moreover, from the viewpoint of improving the stretchability when a polyolefin resin sheet is stretched to produce a microporous membrane, the ΔH ₁₃₀ /ΔH _all of the ultra-high molecular weight polyethylene is preferably 10% or more.

The auxiliary raw material may be added within a range that does not impair the ^fibril structure formed by the main raw material and the molding processability. More preferably, the following polyolefin resins are combined with auxiliary raw materials, more preferably 4.0×10 ⁵ or less, even more preferably 3.0×10 ⁵ or less, and even more preferably 2.0×10 ⁵ or less. 1.0×10 ⁵ or less is particularly preferred. Regarding polyolefin as an auxiliary raw material, from the viewpoint of achieving both per unit weight equivalent puncture strength and shutdown temperature within a good range, the melting point is 130 ° C or higher and 135 ° C or lower, and the DSC curve obtained from differential scanning calorimetry (DSC) A high-density polyethylene having a heat of fusion ΔH _all of 200 J/g or more and 250 J/g or less and a half width of 4.0° C. or more and 6.0° C. or less is preferably used.

From the above viewpoint, the weight average molecular weight of the polyolefin microporous membrane is preferably 8.0 × 10 ⁵ or more, preferably 9.0 × 10 ⁵ or more, more preferably 10 × 10 ⁵ or more, and maintains the molecular weight of the raw material. is particularly preferred. In order to form a strong structure that maintains the orientation state at high temperatures and to develop high strength, the amount of components with a molecular weight of 9.0 × 10 ⁵ or more with a long relaxation time in the molecular weight distribution of the polyolefin microporous membrane is required. It is preferably contained in the polyolefin microporous membrane in an amount of 30% by mass or more, more preferably 33% by mass or more, even more preferably 35% by mass or more, and even more preferably 38% by mass or more. Preferably, 40% by mass or more is particularly preferable. From the viewpoint of promoting melting and relaxation of crystal orientation and developing a low-temperature shutdown temperature, the content of polyolefin having a molecular weight of 3.0×10 ⁵ or less is preferably 30% by mass or more, more preferably 35% by mass or more, and 40% by mass or more. % by mass or more is more preferable, and 45% by mass or more is particularly preferable. From the standpoint of retention of the crystal structure at high temperatures, the main raw material is ultra-high molecular weight polyolefin, and the content of polyolefin having a weight average molecular weight of 3.0×10 ⁵ or less is less than 50% by mass. In order to obtain the molecular weight of the polyolefin microporous membrane, the above-described raw material formulation is added to the antioxidant and kneaded under a nitrogen atmosphere, and a combination of the addition of an antioxidant and kneading under a nitrogen atmosphere. It is preferable to form the film with.

The molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of the ultrahigh molecular weight polyolefin is preferably in the range of 3.0-100. The narrower the molecular weight distribution, the more uniform the system and the easier it is to obtain uniform micropores. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or higher, more preferably 5.0 or higher, and even more preferably 6.0 or higher. As the molecular weight distribution increases, the amount of low-molecular-weight components increases, resulting in a decrease in strength and the melting and fusion of fine fibrils during stretching and heat setting. is 20 or less, particularly preferably 10 or less. By setting the amount within the above range, good moldability can be obtained, and since the system is unified, uniform micropores can be obtained.

In the manufacturing process of the polyolefin microporous membrane according to the embodiment of the present invention, it is preferable to add a plasticizer for the purpose of improving moldability. The blending ratio of the polyolefin resin and the plasticizer may be appropriately adjusted within a range that does not impair the molding processability. is preferred. When the proportion of the polyolefin resin is 10% by mass or more (the proportion of the plasticizer is 90% by mass or less), it is possible to suppress swelling and neck-in at the outlet of the die when forming a sheet, thereby improving the moldability and the manufacturability of the sheet. The film properties are improved, and when the proportion of the polyolefin resin is 50% by mass or less (the proportion of the plasticizer is 50% by mass or more), the pressure rise in the film forming process can be suppressed, and good moldability can be obtained. The ratio of the polyolefin resin is preferably 10% by mass or more, more preferably 20% by mass or more, when the total of the polyolefin resin and the plasticizer is 100% by mass.

When a polyolefin resin having a weight-average molecular weight of 900,000 or more is used as a main component or alone, the ratio of the polyolefin resin from the viewpoint of the pressure and stretching stress in the film forming process is 100% by mass as the total of the polyolefin resin and the plasticizer. 35 mass % or less is preferable, 30 mass % or less is more preferable, less than 28.5 mass % is still more preferable, and less than 25 mass % is more preferable.

In addition, the polyolefin microporous membrane according to the embodiment of the present invention may contain an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent, and a filler within the range that does not impair the effects of the present invention. You may contain various additives, such as. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to heat history of the polyolefin resin.

Examples of antioxidants include 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, BASF "Irganox" (registered trademark) 1330: 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) and the like are preferably used.

Appropriately selecting the types and amounts of antioxidants and heat stabilizers added is important for adjusting or enhancing the properties of the polyolefin microporous membrane, and is measured by the method described in JIS K7210-1 (2014). The amount of antioxidant added is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, based on the amount of resin. 0% by mass or more is more preferable, 1.2% by mass or more is even more preferable, and 1.5% by mass or more is even more preferable. From the viewpoint of the quality and stable film-forming properties of the polyolefin microporous membrane, the upper limit is 3.0% by mass or less, and it is particularly possible to suppress oxidative deterioration by combining the addition of an antioxidant and kneading in a nitrogen atmosphere. preferable.

Moreover, the layer structure of the polyolefin microporous membrane according to the embodiment of the present invention may be a single layer or a laminate, and the laminate is preferable from the viewpoint of physical property balance. When the layers of the above-mentioned polyolefin resin prescription are laminated and used, it is preferable that the above layers are contained in the total film thickness in an amount of 50% by mass or more.

[3] Method for Producing Microporous Polyolefin Film Next, a method for producing a microporous polyolefin film according to an embodiment of the present invention will be specifically described. A method for producing a polyolefin microporous membrane according to an embodiment of the present invention preferably includes the following steps (a) to (e).
(a) a step of melt-kneading a polymer material containing one or more polyolefin resins and optionally a solvent to prepare a polyolefin resin solution; (b) extruding the resulting molten mixture into a sheet; (c) a step of simultaneously biaxially stretching the obtained sheet by a tenter method; (d) a step of extracting a plasticizer from the obtained stretched film and drying the film; (e) a roll method. Alternatively, a step of re-stretching/heat-treating by a stretching method including a tenter method, in particular, in the step (a), an additive amount of an antioxidant described later is added for the purpose of preventing a decrease in molecular weight, and kneading is performed in a nitrogen atmosphere. In the step (e), it is particularly preferable to carry out the heat treatment/re-stretching at a temperature of 130° C. or higher by a dry longitudinal re-stretching by a roll-stretching method and a tenter method.

Each step will be described below.

(a) Step of preparing polyolefin resin solution The polymer material is heated and dissolved in a plasticizer to prepare a polyolefin resin solution. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but the solvent is preferably liquid at room temperature in order to enable stretching at a relatively high magnification.

Solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate, Phthalic acid esters that are liquid at room temperature, such as dioctyl phthalate, can be mentioned.

In order to obtain a stable gel-like sheet, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

A solvent that is solid at room temperature and is miscible with the polyolefin resin in a melt-kneaded state may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, paraffin wax, and the like. However, if only a solid solvent is used, there is a possibility that stretching unevenness or the like may occur.

The viscosity of the liquid solvent is preferably 20-200 cSt at 40°C. If the viscosity at 40° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin resin solution from the die is less likely to be uneven. On the other hand, if the viscosity at 40° C. is 200 cSt or less, the liquid solvent can be 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 for uniform melt-kneading of the polyolefin resin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin resin solution, it is preferably carried out in a twin-screw extruder. . If necessary, known additives such as metallic soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, antistatic agents, etc., may be added to the extent that the effects of the present invention are not impaired without impairing the film formability. may In particular, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.

In the extruder, the polyolefin resin solution is uniformly mixed 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 from (the melting point of the polyolefin resin +10° C.) to (the melting point of the polyolefin resin +120° C.). More preferably, it is (melting point of polyolefin resin +20° C.) to (melting point of polyolefin resin +100° C.).

Here, the melting point refers to a value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987) (the same shall apply hereinafter). For example, when the polyolefin-based resin is polyethylene, the melt-kneading temperature of the polyethylene-based resin is preferably in the range of 140 to 250°C. More preferably 150 to 230°C, most preferably 150 to 200°C. Specifically, since the polyethylene composition has a melting point of about 130-140°C, the melt-kneading temperature is preferably 140-250°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 substances are generated in the extrudate extruded from the die, and film breakage etc. occur in the subsequent stretching process. may cause it. On the other hand, if the temperature is higher than the above temperature, thermal decomposition of the polyolefin resin becomes violent, and the physical properties of the obtained polyolefin microporous membrane, such as strength and porosity, may deteriorate. In addition, the decomposition products are deposited on chill rolls, rolls in the stretching process, etc., and adhere to the sheet, leading to deterioration of the appearance. Therefore, the melt-kneading temperature is preferably kneaded within the above range. In addition, the smaller the value of Q/Ns, which is the ratio of the output Q (kg/h) of the polyolefin solution to the screw rotation speed Ns (rpm) of the twin-screw extruder, the kneadability of the resin increases, and a uniform solution can be obtained. be done. However, a decrease in Q/Ns causes a large amount of shear heat generation, which promotes deterioration of the resin and makes it impossible to obtain the molecular weight component in the film within the range described above. Low-molecular-weight components accumulate in the bleeding-out plasticizer and adhere to the sheet, thereby deteriorating the appearance. When the value of Q/Ns is large, deterioration of the resin is suppressed, but the kneadability is insufficient and a uniform solution cannot be obtained. Therefore, it is particularly preferable to appropriately change Q/Ns in accordance with the molecular weight and solubility of the resin to be used, and to suppress oxidative deterioration by a combination of adding an antioxidant and kneading in a nitrogen atmosphere.

The screw of the twin-screw extruder has a distance of 1.0 D or more and 15.0 D or less between the tip of the screw and the vent hole, where D is the outermost diameter of the screw, and a length of 0.0 D between them in the raw material conveying direction. A structure that uses at least one screw piece with a length of 2D or more and 0.9D or less and does not use two or more screw pieces with a length of 1.5D or more in the raw material conveying direction is preferably used. By adopting such a structure, it becomes possible to improve the kneadability particularly when using two or more kinds of polyolefin resins.

The resulting extrudate is then cooled to obtain a gel-like sheet, and the cooling can fix the microphases of the polyolefin resin separated by the solvent. It is preferable to cool the gel-like sheet to 10 to 50° C. in the cooling step. This is because the final cooling temperature is set to the crystallization finish temperature or lower, and by making the higher-order structure finer, it becomes easier to perform uniform stretching in subsequent stretching. Therefore, it is preferable to cool at least at a rate of 30° C./min or more until the gelling temperature or lower.

In general, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarser, and the gel structure that forms it also becomes larger. On the other hand, when the cooling rate is high, small and uniform crystals are formed, so that the high-order structure of the gel-like sheet becomes dense and uniform stretching becomes possible.

Cooling methods include a method of direct contact with cold air, cooling water, or other cooling medium, a method of contact with rolls cooled with a refrigerant, and a method of using a casting drum or the like.

Although the case where the polyolefin microporous membrane is a single layer has been described so far, the polyolefin microporous membrane according to the embodiment of the present invention is not limited to a single layer, and may be a laminate. The number of layers to be laminated is not particularly limited, and may be a two-layer lamination or a lamination of three or more layers. The laminated portion may contain a desired resin other than the polyolefin resin as described above, to the extent that the effect of the present invention is not impaired.

A conventional method can be used as a method of forming a polyolefin microporous membrane into a laminate. For example, the desired resins may be prepared as desired, fed separately to an extruder, melted at the desired temperature, and combined in a polymer tube or die to achieve the desired thickness of each laminate. There is a method of forming a laminate by, for example, extruding from a slit-shaped die.

(c) Stretching Step The resulting gel-like (including laminated sheet) sheet is stretched. The stretching method used includes uniaxial stretching in the sheet conveying direction (MD direction) by rolling or a roll stretching machine, uniaxial stretching in the sheet width direction (TD direction) by a tenter, roll stretching machine and tenter, or tenter and tenter. and simultaneous biaxial stretching using a simultaneous biaxial tenter. The stretching ratio of the gel-like sheet may be appropriately adjusted within a range that does not impair the orientation parameters in the MD direction and the TD direction. From the viewpoint of orientation control in the and TD directions, the stretching of the gel-like sheet is performed by simultaneous biaxial stretching, stretching 5 times or more in any direction, and sequentially in the MD and TD directions in the dry re-stretching process after washing and drying, which will be described later. A method of stretching and controlling orientation parameters at high temperatures in each of the MD and TD directions is preferred. In this specification, re-stretching in the MD direction may be referred to as re-stretching in the machine direction, and re-stretching in the TD direction may be referred to as re-stretching in the TD direction.

The stretching temperature is preferably set to the melting point of the gel sheet +10° C. or lower, more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the gel sheet +5° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 110° C., the stretching temperature is preferably 100 to 130° C., more preferably 100 to 115° C., and particularly preferably 100° C. ~110°C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012). If the upper limit is exceeded, the relaxation of molecules is accelerated, making it difficult to obtain sufficient porosity by stretching. When the stretching temperature is within the above range, film rupture due to stretching of the polyolefin resin is suppressed, and a microporous membrane appropriately oriented in the MD and TD directions is obtained, and dry re-stretching after washing and drying, which will be described later. It is possible to control the orientation parameters under high temperature in the MD direction and the TD direction respectively in the process.

(d) Plasticizer Extraction (Washing)/Drying Step 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, the polyolefin microporous membrane can be obtained by removing the solvent.

Examples of washing 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.

These cleaning solvents have a low surface tension (eg, 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 suppressed from shrinking due to the surface tension of the air-liquid interface during drying after cleaning, resulting in a polyolefin microporous membrane with excellent porosity and permeability. is obtained. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

Examples of the washing method include a method of immersing the gel-like sheet in a washing solvent for extraction, a method of showering the gel-like sheet with a washing solvent, and a method of combining 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 per 100 parts by mass of the gel-like sheet.

The washing temperature may be from 15 to 30°C, with heating to 80°C or less if necessary. At this time, from the viewpoint of enhancing the cleaning effect of the cleaning solvent, from the viewpoint of preventing the physical properties of the obtained polyolefin microporous membrane (for example, the physical properties in the TD and / or MD directions) from becoming uneven, and the mechanical properties of the polyolefin microporous membrane From the viewpoint of improving physical properties and electrical properties, the longer the gel-like sheet is immersed in the cleaning solvent, the better.

The washing as described above is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the polyolefin microporous membrane is less than 1% by mass.

Thereafter, the solvent in the polyolefin microporous membrane is dried and removed in a drying step. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40-100°C, more preferably 40-80°C. If the drying is insufficient, the porosity of the polyolefin microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

(e) Heat treatment/re-stretching step The dried microporous polyolefin membrane is stretched (re-stretched) at least uniaxially. Re-stretching can be carried out by roll stretching or a tenter method in the same manner as the stretching described above. Re-stretching may be uniaxial stretching or biaxial stretching, but from the viewpoint of orientation control at high temperatures in the MD and TD directions, sequential stretching is performed by combining re-longitudinal stretching using roll stretching and re-transverse stretching using tenter stretching. is particularly preferred.
The temperature for re-stretching in the longitudinal direction is preferably below the melting point of the film after washing and drying, more preferably within the range of (Tcd of the polyolefin resin composition-20° C.) to the melting point of the polyolefin resin composition. Specifically, in the case of a polyethylene composition, the re-stretching temperature is preferably 70 to 130°C, more preferably 90 to 120°C, and even more preferably 100 to 115°C.
Re-longitudinal stretching is effective in controlling orientation parameters measured at 130° C. using Raman spectroscopy in the MD direction, and the stretching ratio is 1.2 times or more, preferably 1.3 times or more, and 1.4 times or more. More preferably, 1.5 times or more is even more preferable, and 1.7 times or more is even more preferable. When the draw ratio is 1.2 times or more, the difference in orientation parameters between 25° C. and 130° C. is large, a structure is formed in which melting of the crystal structure is facilitated at high temperatures, and good shutdown characteristics are obtained. Moreover, from the viewpoint of productivity, the draw ratio is preferably 3.0 times or less.

The temperature for re-lateral stretching is preferably below the melting point of the film, more preferably within the range of (Tcd of the polyolefin resin composition-20° C.) to the melting point of the polyolefin resin composition. Specifically, in the case of a polyethylene composition, the re-stretching temperature is preferably 70 to 140.degree. C., more preferably 110 to 140.degree. C., still more preferably 120 to 140.degree. By stretching in the above temperature range at a ratio described later, the relaxation of the orientation of the polyolefin molecular chains can be suppressed, a highly oriented structure can be obtained, and a thermally stable structure can be formed. The resulting microporous film has a high orientation parameter in the TD direction at 130°C and a small difference between the orientation parameters in the TD direction at 25°C and 130°C.

If the weight-average molecular weight is less than 9.0×10 ⁵ , the relaxation time is short, so heat treatment at 130° C. or higher leads to a decrease in porosity. On the other hand, polyethylene with a weight-average molecular weight of 9.0×10 ⁵ or more has a long relaxation time. Since it can be fixed, it is possible to suppress the relaxation of orientation at high temperature and obtain a highly oriented structure. Therefore, it is preferable to use polyethylene having a weight average molecular weight of 9.0×10 ⁵ or more and to heat-set at a temperature higher than 130°C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 3.0 times, particularly preferably 1.1 to 1.2 times, more preferably 1.2 to 1.7 times in the TD direction. . When the film is biaxially stretched, it is preferably stretched 1.01 to 2.0 times in each of the MD and TD directions. Note that the re-stretching ratio may be different in the MD direction and the TD direction, and multi-stage stretching combining successive stretching is preferred. The dry stretching process is effective in controlling the orientation of molecular chains measured at 25° C. using Raman spectroscopy, and high puncture strength can be obtained by dry stretching at the above stretching ratio.

From the viewpoint of shrinkage and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 30% or less, more preferably 250% or less, and even more preferably 20% or less. A uniform fibril structure is obtained when the relaxation rate is 20% or less.

(f) Other Steps Further, the polyolefin microporous membrane may be subjected to hydrophilization treatment according to other uses. Hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably carried out after the cross-linking treatment.

It is preferable to subject the polyolefin microporous membrane to cross-linking treatment 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 dose of 0.1 to 100 Mrad is preferred, and an acceleration voltage of 100 to 300 kV is preferred. The cross-linking treatment increases the meltdown temperature of the polyolefin microporous membrane.

In the case of surfactant treatment, nonionic surfactants, cationic surfactants, anionic surfactants or amphoteric surfactants can be used, but nonionic surfactants are preferred. A polyolefin microporous membrane is immersed in a solution of a surfactant dissolved in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by a doctor blade method.

The polyolefin microporous membrane according to the embodiment of the present invention is a fluororesin porous material such as polyvinylidene fluoride, polytetrafluoroethylene, etc., for the purpose of improving meltdown characteristics and heat resistance when used as a battery separator. Surface coating of porous materials such as polyimide, polyphenylene sulfide, etc., inorganic coating such as ceramics, etc. may be performed. In particular, since the polyolefin porous membrane obtained by the present invention has high strength and low thermal shrinkage, tension control during coating is facilitated, and shrinkage during the drying process is suppressed, resulting in excellent coatability.

The polyolefin microporous membrane obtained as described above can be used in various applications such as filters, separators for fuel cells, and separators for capacitors, and is particularly safe when used as a battery separator. Therefore, the separator can be preferably used as a battery separator for secondary batteries, such as electric vehicles, which require high energy density, high capacity, and high output. Moreover, a secondary battery using such a separator is suitably used for an electric vehicle or the like, taking advantage of its characteristics.

The present invention will be described in more detail with reference to Examples, but embodiments of the present invention are not limited to these Examples. In addition, the evaluation in this application was performed under an environment of temperature 23° C. and humidity 65% unless otherwise specified. Evaluation methods and analysis methods used in the examples are as follows.

(1) weight average molecular weight (Mw)
Polyolefin molecular weight distribution measurement (measurement of weight average molecular weight, molecular weight distribution, content of predetermined component, etc.) was performed by high-temperature gel permeation chromatography (GPC). The molecular weight distribution of the film was measured using the polyolefin microporous film after stretching, and the molecular weight distribution of the polyolefin raw material was measured under the following conditions.
Apparatus: high temperature GPC apparatus (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, manufactured by Showa Denko)
Solvent: 1,2,4-trichlorobenzene (TCB, manufactured by Wako Pure Chemical Industries) (0.1% BHT added)
Flow rate: 1.0 mL/min
Column temperature: 145°C
Sample preparation: 5 mL of a measurement solvent was added to 5 mg of a 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: 0.200 mL
Standard sample: Monodisperse polystyrene (manufactured by Tosoh) (PS)
Data processing: GPC data processing system manufactured by Toray Research Center.

(2) Film thickness (μm)
The film thickness of the polyolefin microporous film at 5 points within the range of 50 mm × 50 mm was measured with a contact thickness meter, Mitutoyo Co., Ltd. Lightmatic VL-50 (10.5 mmφ super hard spherical probe, measurement load 0.01 N). , and the average value was taken as the film thickness (μm).

(3) Air permeability (sec/100 cm ³ ) and 10 μm equivalent air permeability (sec/100 cm ³ )
In accordance with JIS P-8117, the Oken type air permeability meter (Asahi Seiko, EGO-1T) is used for the polyolefin microporous membrane with a thickness of T ₁ (μm) in an atmosphere of 25 ° C. degrees (seconds/100 cm ³ ) were measured. Also, the air permeability (10 μm equivalent air permeability) (sec/100 cm ³ ) when the film thickness was 10 μm was calculated according to the following formula.

Formula: 10 μm conversion air permeability (seconds/100 cm ³ ) = air permeability (seconds/100 cm ³ ) x 10 (μm)/thickness of polyolefin microporous membrane (μm)
(4) Porosity (%)
A 50 mm×50 mm square sample was cut from the polyolefin microporous membrane, and its volume (cm ³ ) and mass (g) at room temperature of 25° C. were measured. From these values and the film density (g/cm ³ ), the porosity of the polyolefin microporous film was calculated by the following equation.

Porosity (%) = (volume - mass / film density) / volume x 100
The film density was calculated assuming a constant value of 0.99 g/cm ³ .

(5) 10 μm conversion piercing strength (N) and basis weight conversion piercing strength (N/(g/m ² ))
The puncture strength was measured according to JIS Z 1707 (2019), except that the test speed was 2 mm/sec. Using a force gauge (Imada DS2-20N), a needle with a spherical tip (curvature radius R: 0.5 mm) and a diameter of 1.0 mm was used to pierce the polyolefin microporous membrane in an atmosphere of 25 ° C. The maximum The load (N) was measured (L1), and the puncture strength (L2) converted to a film thickness of 10 μm was calculated from the following formula.
Formula: L2 = L1 × 10 (μm) / film thickness of polyolefin microporous membrane (μm)
The maximum load (N) when the polyolefin microporous membrane is pierced in an atmosphere of 25° C. is measured (L1), and the basis weight conversion piercing strength (L3) is calculated from the following formula.
Formula: L3 = L1 / basis weight of polyolefin microporous membrane The basis weight of the polyolefin microporous membrane is obtained by cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane and measuring the mass (g) at room temperature 25 ° C. , was calculated by the following formula.
Formula: basis weight (g/m ² ) = mass (g)/(50 (mm) x 50 (mm)) x 10 ⁶
(6) Tensile breaking strength (MPa)
A tensile test was performed using a tensile tester (Shimadzu Autograph AGS-J type) in accordance with JIS K7127, and the strength at break of the sample was divided by the cross-sectional area of the sample before the test, and the tensile breaking strength (MPa) was obtained. . The measurement conditions are temperature: 23±2° C., sample shape: width 10 mm×length 50 mm, distance between chucks: 20 mm, tensile speed: 100 mm/min. In addition, a paper frame with a width of 40 × 60 mm was hollowed out at the center of the paper frame at 20 × 20 mm as a sample holder. Measurement was performed by cutting both ends of the sample holder (central portions of two sides parallel to the sample among the four sides of the sample holder). The above measurements were performed at three different points in the same film in the MD direction and the TD direction, and the average value of the three points was the tensile breaking strength in each direction (MD tensile breaking strength, TD tensile breaking strength strength).

(7) Tensile elongation at break (%)
A tensile test was performed using a tensile tester (Shimadzu Autograph AGS-J type). ) was calculated from the following formula. The measurement conditions are temperature: 23±2° C., sample shape: width 10 mm×length 50 mm, distance between chucks: 20 mm, tensile speed: 100 mm/min. In addition, a paper frame with a width of 40 × 60 mm was hollowed out at the center of the paper frame at 20 × 20 mm as a sample holder. Both ends of the sample holder (central portions of two sides parallel to the sample among the four sides of the sample holder) were cut and measured. I made a measurement. The above measurements were performed at three different points in the same film in the MD direction and the TD direction, and the average value of the three points was the tensile elongation at break in each direction (MD tensile elongation at break, TD tensile elongation at break).

Tensile elongation at break (%)=((L−L0)/L)×100.

(8) Shrinkage rate (%) at 130°C/1h
A 5 cm x 5 cm square sample having two sides parallel to the MD direction was cut out of the polyolefin microporous membrane. The sample length in the MD direction was measured at the central portion of the cut sample in the TD direction, and this was defined as the length before MD contraction (L _1MD ). Also, the length of the sample in the TD direction was measured at the center in the MD direction, and this was defined as the TD pre-contraction length (L _1TD ). Next, the sample was placed in an oven with an internal temperature of 130° C., heated, and taken out after 1 hour. The length in the MD direction was measured at the location where the length before MD contraction was measured, and this was defined as the length after MD contraction (L _2MD ). In addition, the length in the TD direction was measured at the location where the length before TD contraction was measured, and this was defined as the length after TD contraction (L _2TD ). Using these values, the thermal shrinkage rate after 1 hour at 130° C. was calculated by the following formula. Further, this measurement was performed at arbitrary three points within the sample surface, and the average value was calculated as the thermal shrinkage rate (%) after 1 hour at 130°C.

Formula: Thermal contraction rate (%) after 1 hour at 130°C in the MD direction = 100 x (L _1MD - L _2MD )/L _1MD
Formula: Thermal contraction rate (%) after 1 hour at 130°C in the TD direction = 100 x (L _1TD - L _2TD )/L _1TD
(9) Raman Spectroscopy The polarized Raman spectrum of the polyolefin microporous film was measured using a JASCO NRS-5100 microscopic Raman spectrometer as follows, and the orientation parameter of the crystal molecular chains was calculated.
<Raman measurement conditions>
・Laser: 532 nm
・Grating: 2400 Line/mm
・Lens: 20x
・Slit: 200×1000 μm
・Aperture: φ4000μm
1. A laser polarized in the machine direction of the polyolefin microporous membrane using a polarizer was incident on the specimen and the scattered light was collected through an analyzer oriented in the machine direction.

2. The ratio I ₁₁₃₀ /I ₁₀₆₀ of the Raman bands at 1130 cm ⁻¹ and 1060 cm ⁻¹ in the obtained Raman spectrum was defined as the Raman orientation parameter and the value was calculated.
The Raman spectrum was obtained with the polarizer having the direction parallel (0°/0°) to the longitudinal direction of the film as the MD direction and the direction (90°/90°) perpendicular to the film as the TD direction. 1130 cm- ¹ is a band attributed to the C—C stretching vibration of the polyethylene molecular chain in the crystal phase, and since the direction of the Raman tensor of vibration coincides with the molecular chain axis, the orientation of the molecular chain can be known. A larger value of the orientation parameter means that the crystal molecular chains are highly oriented.
<Calculation of peak and orientation parameters>
I _a : maximum Raman band intensity I _{b in the Raman shift band range of 1100 to 1170 cm −1 :} ^Raman band maximum intensity I _a in the Raman shift band range of 1040 to 1090 cm ⁻¹ (MD, 25° C.): 25 MD value _Ia (TD, 25°C) measured in °C: TD value _Ib (MD, 25°C) measured at 25°C: TD value measured at 25°C
I _b (TD, 25° C.): value in TD measured at 25° C. I _a (MD, 130° C.): value in MD after heating at 130° C. for 60 min using a heating stage I _a (TD, 130° C.) ): value I _b in the TD direction after heating at 130° C. for 60 minutes using a heating stage (MD, 130° C.): value I _b in the MD direction after heating at 130° C. for 60 minutes using a heating stage (TD, 130° C. ): Value in TD direction fMH=I _a (MD, 130° C.)/I _b (MD, 130° C.) after heating at 130° C. for 60 min using a heating stage (1) Formula fTH= _Ia (TD, 130°C)/ _Ib (TD, 130°C) (2) Equation fML= _Ia (MD, 25°C)/ _Ib (MD, 25°C) (3) Equation fTL= _Ia (TD, 25°C)/ _Ib (TD, 25°C) °C) (4) Formula fMLH=D _a (MD, 25°C)/D _a (MD, 130°C)) (5) Formula fTLH=D _a (TD, 25°C)/D _a ( TD, 130° C.)) Equation (6) fML-fMH Equation (7) fTL- _fTH Equation (8) Incidentally, Da in equations (5) and (6) is the Raman shift Refers to the difference between the maximum intensity in the range of 1100-1170 cm ⁻¹ and the intensity at 1200 cm ⁻¹ , D _a (MD, 130° C.) is the value of D _a measured at 130° C. in the MD direction, D _a (TD, 130 °C) is the value of _Da measured at 130 °C in the TD direction, _Da (MD, 25 °C) is the value of _Da measured at 25 °C in the TD direction, and _Da (TD, 25 °C) is the value of Da measured in the TD direction at 25 °C. The values of _Da measured at 25° C. will be described in detail below with reference to examples, but the present invention is not limited to these examples.

(10) Short-circuit test Short-circuit resistance was evaluated using a desk-top precision universal testing machine Autograph AGS-X (manufactured by Shimadzu Corporation). Polypropylene insulator (thickness: 0.2 μm)/negative electrode (for lithium-ion battery (copper foil (thickness: approx. 0.9 μm), active material: artificial graphite (particle diameter: approx. 13 μm))/separator/500 μm diameter chromium ball (material: : Chromium (SUJ-2))/aluminum foil laminate was prepared.The aluminum foil and the negative electrode of the sample laminate were connected by a cable to a circuit consisting of a capacitor and a clad resistor.The capacitor was charged to about 1.5 V. A metal ball (material: chromium (SUJ-2)) with a diameter of about 500 μm was placed between the separator and aluminum foil in the sample laminate, then pressed at 0.3 mm/min until the battery shorted. The starting point is the point where the leakage current value begins to rise, and the short-circuit point is the moment when the above-mentioned circuit is formed via the metal ball and the current is detected. Displacement was measured.The sample that does not short circuit even with a high displacement has better resistance to foreign matter, and the relationship between displacement and foreign matter resistance is made into the following three stages.Because batteries are becoming higher energy density and higher capacity, A , and B. In this evaluation, B or higher was regarded as acceptable.
A: Displacement (mm) / separator thickness (μm) is greater than 0.025 B: Displacement (mm) / separator thickness (μm) is greater than 0.020 and 0.025 or less C: Displacement (mm) / separator thickness (μm) is 0.020 or less.

(11) Shutdown temperature (°C)
While heating the polyolefin microporous membrane at a temperature increase rate of 5 ° C./min, the air permeability resistance was measured with an Oken type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The temperature at which the detection limit of 99999 seconds/100 cm ³ Air was reached was defined as the shutdown temperature (°C).

The measurement cell consisted of an aluminum block and had a structure with a thermocouple directly below the polyolefin microporous film.

(12) Melting point (°C)
It was measured by a differential scanning calorimetry (DSC) method based on JIS K7121 (1987). A 6.0 mg sample (raw material polyolefin resin or polyolefin microporous membrane) was sealed in an aluminum pan, and the temperature was raised from 30 ° C. to 230 ° C. at 10 ° C./min under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer. Then, after the temperature was raised from 30 ° C. to 230 ° C. at 10 ° C./min (first temperature rise), the temperature was maintained at 230 ° C. for 5 minutes, cooled at a rate of 10 ° C./min, and the temperature was raised again at 10 ° C./min. The temperature was raised from 30° C. to 230° C. at a high rate (second heating) to obtain each melting endothermic curve (DSC curve). For the polyolefin resin raw material, the peak top temperature on the melting endothermic curve obtained in the second heating was adopted as the melting point.

(13) ΔH ₁₃₀ /ΔH _all of polyolefin resin
For the melting endothermic curve (DSC curve) at the second temperature increase obtained in the same manner as in (12), a linear baseline was set in the range of 60 ° C. to 160 ° C. using analysis software. The total heat of fusion ΔH _all (J/g) was calculated by calculating the amount of heat from the area of the portion surrounded by the melting endothermic curve and converting the amount of heat per unit mass. In addition, the heat quantity of the area surrounded by the baseline and the melting endothermic curve at 130 ° C. or lower is calculated, and the heat quantity is converted to a unit mass, so that the heat of fusion at 130 ° C. or lower ΔH ₁₃₀ (J / g) was calculated. ΔH ₁₃₀ /ΔH _all (%) was determined from each heat of fusion obtained.

(14) DSC half width of polyolefin resin (°C)
From the melting endothermic curve (DSC curve) at the second temperature rise obtained in the same manner as in (12), the maximum maximum peak height (h) was read, and from the melting endothermic curve at the second temperature rise h/ Two readings of the temperature indicating the height of 2 were taken. The absolute value of the difference in temperature between the two read points was obtained and used as the DSC half width (°C). When two or more points of temperature indicating the height of h/2 were read, the two points closest to the melting point were used to obtain the half-value width.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

(15) Average pore size (nm)
Using a perm porometer (manufactured by PMI, CFP-1500A), the maximum pore size and average pore size were measured in the order of Dry-up and Wet-up. For Wet-up, pressure is applied to a porous polyolefin film sufficiently soaked in Galwick (trade name) manufactured by PMI with a surface tension of 15.6 dynes/cm, and Dry-up measurement is performed at 1/2 of the pressure and flow rate curve. The average pore diameter was converted from the pressure at the intersection of the slope curve and wet-up measurement curve. The following formula was used for conversion of pressure and pore size.

d=C・γ/P
(In the above formula, "d (nm)" 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 the constant.

(16) Shutdown characteristics As an evaluation simulating battery safety, the resistance value in the electrolytic solution during temperature rise was evaluated. In an atmosphere with a humidity of 30%, a polyolefin microporous membrane was punched out into a circle of φ19 mm, placed in a coin battery (CR2032 standard), injected with an electrolytic solution, and vacuum impregnated with a vacuum dryer at a pressure of -50 kPa for 1 minute. rice field. After that, it was sealed with a coin battery caulking machine, and the resistance was measured while heating at a rate of temperature increase of 5° C./min. A 1 mol/L solution of LiPF ₆ (EC:EMC=4:6V%) was used as the electrolyte (LiPF ₆ : lithium hexafluorophosphate, EC: ethylene carbonate, EMC: ethylmethyl carbonate). The temperature was raised from 25° C. to 180° C. in 30 minutes using a constant temperature bath. At a frequency of 200 kHz, the resistance value at each temperature was read with an impedance analyzer, and the first temperature reaching 1000 Ω·cm ² was read as the shutdown temperature in the electrolyte. The lower the shutdown temperature in the electrolytic solution, the higher the safety of the battery structure, which was evaluated in the following five stages. In this evaluation, D or higher was regarded as a pass.
A: Shutdown temperature in electrolyte is less than 138°C B: Shutdown temperature in electrolyte is 138°C or higher and less than 140°C C: Shutdown temperature in electrolyte is 140°C or more and less than 145°C D: Shutdown temperature in electrolyte is 145° C. or more and less than 146° C. E: The shutdown temperature in the electrolyte is 146° C. or more.

In the above-mentioned measurement, if the MD direction and TD direction of the polyolefin microporous film to be measured are not known, the polyolefin microporous film is shifted by 15° with respect to one direction of the film plane, and The tensile strength at break is obtained in the same manner as in (6) for a total of 7 directions up to °.

[Example 1]
Ultra high molecular weight polyethylene (melting point 136°C, ΔH ₁₃₀ /ΔH _all = 33%) with a weight average molecular weight (Mw) of 15 × 10 ⁵ was used as a raw material, and 80 parts by weight of liquid paraffin was added to 20 parts by weight of ultra high molecular weight polyethylene. and 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 was added as an antioxidant and mixed to prepare a polyethylene resin solution. The obtained polyethylene resin solution was charged into a twin-screw extruder and kneaded at 180° C. to prepare a polyethylene solution. In addition, the screw of the twin-screw extruder has a distance of 11.5D from the tip of the screw to the vent hole when the outermost diameter of the screw is D, and a screw piece having a length of 0.5D in the raw material conveying direction in between. A screw (type 1) having a configuration using one screw piece, eight screw pieces of 1.0D, and two screw pieces of 1.5D was used. The resulting polyethylene solution was supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 35°C to form a gel sheet. The resulting gel-like sheet was subjected to simultaneous biaxial stretching at a stretching temperature of 105° C. to 5×5 times (MD magnification×TD magnification) by a tenter method. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane is dried, longitudinally stretched again to 1.7 times at a temperature of 100°C by a roll stretching method, and then transversely stretched again to a stretching ratio of 1.6 times at a temperature of 139°C by a tenter method. A membrane was obtained. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Example 2]
As raw materials, 70 parts by mass of ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1.5 × 10 ⁶ and 30 parts by mass of high-density polyethylene having a weight average molecular weight (Mw) of 1.0 × 10 ⁵ , ultra-high molecular weight polyethylene 0.5 parts by weight of 2,6-di-t-butyl-p-cresol and 0.7 parts by weight of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxy A polyethylene mixture was obtained in which phenyl)-propionate]methane was added as an antioxidant. 75 parts by mass of liquid paraffin was added to 25 parts by mass of the obtained mixture, and the mixture was introduced into a twin-screw extruder and kneaded at 180° C. to prepare a polyethylene solution. In addition, the screw of the twin-screw extruder has a distance of 11.5D from the tip of the screw to the vent hole when the outermost diameter of the screw is D, and a screw piece having a length of 0.5D in the raw material conveying direction in between. A screw (type 1) having a configuration using one screw piece, eight screw pieces of 1.0D, and two screw pieces of 1.5D was used. The resulting polyethylene solution was supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 35°C to form a gel sheet. The resulting gel-like sheet was simultaneously biaxially stretched 5×5 times by a tenter method at a stretching temperature of 105°C. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried, longitudinally stretched again to 1.7 times by a roll stretching method, and then laterally stretched again to a stretching ratio of 1.7 times by a tenter method at a temperature of 137° C. to obtain a polyolefin microporous membrane. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Example 3]
A polyolefin 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 the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Example 4]
A polyolefin microporous membrane was obtained in the same manner as in Example 2, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Example 5]
The raw material composition and manufacturing conditions are as shown in the table. Regarding the screw of the twin screw extruder, when the outermost diameter of the screw is D, the distance from the tip of the screw to the vent hole is 10.75D, and the length of the raw material conveying direction in between is 10.75D. Example except that one screw piece with a thickness of 0.75D and ten screw pieces of 1.0D were used, and the screw (type 2) was changed to a configuration that did not use a 1.5D screw piece. A microporous polyolefin membrane was obtained in the same manner as in Example 4. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Examples 6 to 10]
A polyolefin microporous membrane was obtained in the same manner as in Example 5, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Comparative Example 1]
As raw materials, 30 parts by mass of ultra-high molecular weight polyethylene with a weight average molecular weight (Mw) of 2.0 × 10 ⁶ and 70 parts by mass of high-density polyethylene with a weight average molecular weight (Mw) of 5.0 × 10 ⁵ , ultra-high molecular weight polyethylene 0.5 parts by weight of 2,6-di-t-butyl-p-cresol and 0.7 parts by weight of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxy A polyethylene mixture was obtained in which phenyl)-propionate]methane was added as an antioxidant. 71.5 parts by mass of liquid paraffin was added to 28.5 parts by mass of the obtained mixture, and the mixture was introduced into a twin-screw extruder and kneaded at 180° C. to prepare a polyethylene solution. In addition, the screw of the twin-screw extruder has a distance of 11.5D from the tip of the screw to the vent hole when the outermost diameter of the screw is D, and a screw piece having a length of 0.5D in the raw material conveying direction in between. A screw (type 1) having a configuration using one screw piece, eight screw pieces of 1.0D, and two screw pieces of 1.5D was used. The resulting polyethylene solution was supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 35°C to form a gel sheet. The resulting gel-like sheet was longitudinally stretched by a roll method at a stretching temperature of 120° C. so as to have a tensile strength of 7.5. Subsequently, the film was led to a tenter and laterally stretched at a stretching temperature of 120° C. so that the stretching ratio would be 9.0 times. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried and laterally stretched again at a temperature of 130° C. at a stretching ratio of 1.6 times by a tenter method to obtain a polyolefin microporous membrane. The properties of the obtained polyolefin microporous membrane are shown in the table.

[Comparative Examples 2 to 5]
A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the raw material composition and membrane-forming conditions were changed as shown in the table.

[Comparative Examples 6 and 7]
A polyolefin 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 the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

In Tables 1 and 2, "UHPE" means ultra high molecular weight polyethylene, and "HDPE" means high density polyethylene.

Evaluation results of the obtained polyolefin microporous membrane are shown in Tables 3 and 4.

Claims (14)

Either one of the orientation parameter value (fMH) in the MD direction and the orientation parameter value (fTH) in the TD direction measured at 130 ° C. calculated by the following formulas (1) and (2) using a microscopic Raman spectrometer is 1. A polyolefin microporous membrane that is greater than .5 and the other is less than or equal to 1.5.
fMH = _Ia (MD, 130°C)/ _Ib (MD, 130°C) ... (1) formula
fTH = _Ia (TD, 130°C)/ _Ib (TD, 130°C) ... (2) formula
Here, I _a is the maximum intensity of the Raman band in the range of Raman shift band 1100 to 1170 cm ⁻¹ , I _b is the maximum intensity of the Raman band in the range of Raman shift band 1040 to 1090 cm ⁻¹ , I _a (MD, 130 °C), _Ib (MD, 130°C) is the maximum intensity in the MD direction measured at 130°C, _Ia (TD, 130°C), _Ib (TD, 130°C) is the maximum intensity in the TD direction measured at 130°C strength. 2. The polyolefin microporous membrane according to claim 1, wherein the orientation parameter value (fMH) in the MD direction measured at 130[deg.] C. using a microscopic Raman spectrometer and calculated according to formula (1) is greater than 1.5. One of the values (fMLH, fTLH) calculated by the following formulas (5) and (6) using a microscopic Raman spectrometer is 4.0 or more, and the other is less than 4.0. 3. or the polyolefin microporous membrane according to 2.
fMLH=D _a (MD, 25° C.)/D _a (MD, 130° C.) (5) Formula fTLH=D _a (TD, 25° C.)/D _a (TD, 130° C.) (6 )formula
D _a is the difference between the maximum intensity and the intensity at 1200 cm ⁻¹ in the Raman shift band of 1100 to 1170 cm ⁻¹ , D _a (MD, 130° C.) is measured at 130° C. in the MD direction, D _a (TD, 130° C. ) is measured at 130° C. in the TD direction, Da (MD, 25° C.) is measured at 25° C. in the TD direction, and _Da (TD, 25° C.) is _a value measured at 25° C. in the TD direction. Any one of claims 1 to 3, wherein one of the values calculated by the following formulas (7) and (8) using a microscopic Raman spectrometer is greater than 0.0 and the other is 0.0 or less. The polyolefin microporous membrane according to the item.
fML-fMH ・・・(7) formula fTL-fTH ・・・(8) formula
Note that fML and fTL are the orientation parameter value (fML) in the MD direction and the orientation parameter value (fTL) in the TD direction measured at 25° C. calculated by the following equations (3) and (4), and _Ia is Raman. The maximum intensity of the Raman band in the range of shift band 1100-1170 cm ⁻¹ , I _b is the maximum intensity of Raman band in the range of Raman shift band 1040-1090 cm ⁻¹ , I _a (MD, 25° C.), I _b ( MD, 25°C) is the maximum strength in the MD direction measured at 25°C, and _Ia (TD, 25°C) and _Ib (TD, 25°C) are the maximum strengths in the TD direction measured at 25°C.
fML = _Ia (MD, 25°C)/ _Ib (MD, 25°C) ・・・Equation (3)
fTL = _Ia (TD, 25°C)/ _Ib (TD, 25°C) ・・・Equation (4) The polyolefin microporous membrane according to any one of claims 1 to 4, which has a shutdown temperature of 128°C or higher and 143°C or lower in the temperature-rising air permeability method. The polyolefin microporous membrane according to any one of claims 1 to 5, which has a tensile strength at break in the MD direction of 200 MPa or more. 7. The polyolefin microporous membrane according to any one of claims 1 to 6, which has a per unit area-equivalent puncture strength of 0.75 N/(g/m ² ) or more. The polyolefin microporous membrane according to any one of claims 1 to 7, which has an average pore size of 36 nm or more and 46 nm or less as calculated from a perm porometer. The polyolefin microporous membrane according to any one of claims 1 to 8, wherein the polyolefin microporous membrane has a weight average molecular weight of 800,000 or more. The content of polyethylene having a molecular weight of 3.0×10 ⁵ or less in the polyolefin microporous membrane is 50% by mass or less, and the content of polyethylene having a molecular weight of 9.0×10 ⁵ or more is 30% by mass or more. The polyolefin microporous membrane according to any one of 1 to 9. The polyolefin microporous membrane according to any one of claims 1 to 10, wherein the main component of the polyolefin microporous membrane is polyethylene. The polyolefin microporous membrane according to any one of claims 1 to 11, which is obtained by stretching including at least dry sequential biaxial stretching. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 12. A secondary battery using the battery separator according to claim 13 .