Polyolefin microporous membrane, battery separator and secondary battery

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

[Technical Field]

[0001] The present invention relates to a polyolefin microporous membrane (also referred to as a porous polyolefin film) that is widely used as a separation membrane for substance separation, selective permeation, etc., and as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. In particular, the present invention relates to a polyolefin microporous membrane that is suitable for use as a separator for nonaqueous electrolyte secondary batteries such as lithium ion batteries, and is used as a separator that has higher safety than conventional polyolefin microporous membranes. [Background technology]

[0002] Polyolefin microporous membranes are used as filters, fuel cell separators, and capacitor separators. They are particularly suitable for use as separators for nonaqueous electrolyte secondary batteries such as lithium ion batteries, which are widely used in notebook personal computers, mobile phones, etc. This is because polyolefin microporous membranes have excellent mechanical strength, shutdown properties, and ion permeability.

[0003] In recent years, lithium-ion secondary batteries have become increasingly high-capacity due to the miniaturization of electronic devices and their deployment in automotive applications. This has led to a demand for thinner separators. However, thinner separators result in reduced strength, making them more susceptible to short circuits caused by electrodes or foreign objects (foreign object resistance) and membrane rupture upon impact (reduced impact resistance), thereby reducing battery safety. Therefore, even higher strength is required. Furthermore, in high-energy batteries, even if the separator's shutdown function stops the electrochemical reaction, the temperature inside the battery continues to rise, resulting in thermal shrinkage and membrane rupture, leading to a short circuit between the electrodes. Therefore, to improve battery safety, separators are required to have high strength, low shrinkage at high temperatures, and a shutdown function at low temperatures. However, lowering the shutdown temperature to improve safety also reduces the strength of the porous film.

[0004] For example, Patent Document 1 discloses a method for improving strength, shrinkage, and shutdown temperature by dry re-stretching in the MD (machine direction) and controlling the Raman orientation parameter value in the MD to obtain a microporous membrane with a thickness of 12 μm or less, a pin puncture strength of 230 gf or more, a thermal shrinkage rate in the TD (transverse direction) of 15% at 105°C for 8 hours, and a shutdown temperature of 135°C to 149°C.

[0005] Patent Document 2 describes a method for improving shutdown temperature and pin puncture strength by using polyolefin having a weight molecular weight of 500,000 or more as the main component, controlling the orientation ratio in the MD direction and the TD direction as determined by X-ray analysis, and achieving a temperature of 0.24 to 0.75 N / (g / m 2 ) and a shutdown temperature of 139°C to 146°C.

[0006] Patent Document 3 discloses a method for improving mechanical strength and permeability by controlling the degree of orientation determined by infrared spectroscopy to obtain a microporous membrane with a pin puncture strength of 300 to 500 gf converted to 25 μm. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2020-95950 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-103201 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-199545 Summary of the Invention [Problem to be solved by the invention]

[0008] Separators are required to have high strength and low-temperature shutdown capability, but increasing separator strength has the problem of deteriorating shutdown characteristics. The microporous membranes described in Patent Documents 1 to 3 were insufficient in terms of achieving both thin separator thickness, high strength, and low-temperature shutdown capability that are required for increasing battery capacity.

[0009] In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having a better balance between strength and shutdown temperature than conventional membranes, and a separator using the same. [Means for solving the problem]

[0010] As a result of extensive research to achieve the above-mentioned object, the present inventors have found that a microporous membrane having specific ranges of orientation parameters in the MD and TD directions at high temperatures calculated by micro-Raman spectroscopy can solve the above-mentioned problems and achieve both excellent strength and shutdown properties, thereby completing the present invention. That is, the present invention has the following configuration.

[0011] The polyolefin microporous membrane has one of an MD orientation parameter value (fMH) and a TD orientation parameter value (fTH) greater than 1.5 and the other not greater than 1.5, as measured at 130°C using a micro-Raman spectrometer according to the following equations (1) and (2): fMH = I a (MD, 130℃) / I b (MD, 130℃) ...Equation (1) fTH = I a (TD, 130℃) / I b (TD, 130℃) ...Equation (2) In addition, I a I a is the Raman shift band 1100-1170cm -1 The maximum intensity of the Raman band in the range of I b is the Raman shift band 1040-1090cm -1 The maximum intensity of the Raman band in the range of Ia (MD, 130℃), I b (MD, 130℃) is the maximum strength in the MD direction measured at 130℃, I a (TD, 130℃), I b (TD, 130°C) is the maximum strength in the TD direction measured at 130°C. [Effects of the Invention]

[0012] According to the present invention, a microporous polyolefin membrane having an excellent balance between strength and shutdown temperature and high safety can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0013] The polyolefin microporous membrane according to an embodiment of the present invention is useful as a battery separator because it has excellent strength, shutdown properties, and shrinkage, and has excellent safety. The present invention can be realized by satisfying the orientation parameters at high temperatures in the MD and TD directions calculated by micro-Raman spectroscopy within the ranges described below, and it has been discovered that this leads to an improvement in the balance between strength and shutdown temperature, which have traditionally been in a trade-off relationship. Note that the present invention is not limited to the embodiments described below. The direction parallel to the membrane-forming direction of the polyolefin microporous membrane is referred to as the membrane-forming direction, longitudinal direction, or MD direction, and the direction perpendicular to the membrane-forming direction is referred to as the width direction or TD direction.

[0014] The present invention will be described in further detail below.

[0015] [1] Polyolefin microporous membrane The polyolefin microporous membrane according to an embodiment of the present invention is characterized in that either the MD orientation parameter value (fMH) or the TD orientation parameter value (fTH), measured at 130°C by the method described below, is greater than 1.5, while the other is 1.5 or less. This means that the mobility of the crystal structure in either the MD or TD direction is controlled. The Raman orientation parameter is an index that indicates the degree of orientation of crystalline molecular chains as calculated by Raman spectroscopy; a higher value indicates a higher degree of orientation of the crystalline molecular chains. When either fMH or fTH is greater than 1.5, the membrane has a strong structure that maintains its orientation even at high temperatures, resulting in excellent strength. When either is 1.5 or less, the crystal structure melts at high temperatures, resulting in a low orientation parameter and enabling a lower shutdown temperature. From the perspective of strength, the higher the degree of orientation, the better. 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, but since this leads to a deterioration in the shrinkage rate and an increase in the shutdown temperature, either fMH or fTH is preferably 1.4 or less, more preferably 1.3 or less. From the viewpoint of the balance between strength and shutdown temperature, it is preferable that either fMH or fTH is 1.3 or less, and the other is 1.7 or more, and from the viewpoint of film formability, it is more preferable that the direction in which the orientation parameter is greater than 1.5 is the MD direction (i.e., fMH is greater than fTH). By satisfying the above range, a microporous membrane with 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 below, and in order to form a strong structure that maintains the orientation state at high temperatures, a weight average molecular weight of 9.0 × 10 with a long relaxation time is preferable. 5 The above polyolefin resins are used as the main raw material, and film formation is preferably performed by combining wet stretching, including re-longitudinal stretching after washing and drying, with dry sequential stretching. A polyolefin resin with a weight-average molecular weight of 3.0 × 10, which has a short relaxation time, is used as a crystalline melting and orientation relaxation promoter at high temperatures. 5 The following polyolefin resins are more preferably combined with the auxiliary raw material. From the above viewpoint, the polyolefin microporous membrane preferably has a molecular weight of 0.9×10 with a long relaxation time in its molecular weight distribution.6 The above components are contained in a polyolefin microporous film at 30% by mass or more, and have a molecular weight of 3.0 × 10 5 It is more preferable that the following components are contained in the range of 20 to 50% by mass.

[0016] In the polyolefin microporous membrane according to an embodiment of the present invention, the MD orientation parameter value (fML) and TD orientation parameter value (fTL) measured at 25°C by the method described below are preferably both 1.5 or less. From the viewpoint of strength, the higher the fML and fTL, the better. However, as the amount of highly oriented structure measured at 25°C increases, the shutdown temperature rises and the molecular orientation relaxes at high temperatures, increasing the shrinkage rate. From the above viewpoints, fML and fTL are preferably 1.5 or less, more preferably 1.4 or less. The fML and fTL are orientation parameters calculated by the following equations (3) and (4), and are expressed as I a is the Raman shift band 1100-1170cm -1 The maximum intensity in the range of b is the Raman shift band 1040-1090cm -1 The maximum intensity of the Raman band in the range of I a (MD, 25°C) and I b (MD, 25°C) is the measurement of the MD direction of the polyolefin microporous membrane at 25°C, and I a (TD, 25°C) and I b (TD, 25° C.) is a value measured in the TD direction of the polyolefin microporous membrane at 25° C. The above range can be achieved by using raw materials, molecular weights, and production methods within the ranges described below. fML = I a (MD, 25℃) / I b (MD, 25℃) ...Equation (3) fTL = I a (TD, 25℃) / I b (TD, 25°C) ...Equation (4).

[0017] The polyolefin microporous membrane according to an embodiment of the present invention has a D measured by the method described below.a (MD, 25℃) and D a I a (MD, 130℃) ratio (fMLH) and D a (TD, 25°C) and D a It is preferable that one of the ratios (fTLH) of (TD, 130°C) is 4.0 or more, and the other is less than 4.0. When it is 4.0 or more, the CC stretching vibration of the polyethylene molecular chain in the crystalline phase at 130°C is reduced, that is, the crystalline structure is melted, and good shutdown characteristics are obtained. When it is less than 4.0, the crystalline molecular chain structure is highly maintained even at 130°C, and high strength is obtained. a is the Raman shift band 1100-1170cm -1 and maximum intensity in the range of 1200 cm -1 D a (MD, 130℃) is the D measured in the MD direction of the polyolefin microporous membrane at 130℃ a The value of D a (TD, 130°C) is the D measured in the TD direction of the polyolefin microporous membrane at 130°C. a The value of D a (MD, 25°C) is the D measured in the MD direction of the polyolefin microporous membrane at 25°C. a The value of D a (TD, 25°C) is the D measured in the TD direction of the polyolefin microporous membrane at 25°C. a The value is 1130cm -1 is a band attributed to the C—C stretching vibration of polyethylene molecular chains in the crystalline phase, and the direction of the vibration Raman tensor is the molecular chain axis. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods within the ranges described below. fMLH=D a (MD, 25℃) / D a (MD, 130℃))...Equation (5) fTLH=D a (TD, 25℃) / D a (TD, 130℃))...Equation (6) Typically, the orientation parameter measured at 130°C decreases compared to that measured at 25°C due to relaxation of the crystalline structure at high temperatures (orientation parameter measured at 25°C > orientation parameter measured at 130°C). On the other hand, when the melting point of a polyolefin microporous membrane is high 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 may increase compared to that measured at 25°C. The smaller the difference in the orientation parameters between 25°C and 130°C, and the closer the change is to zero, the better the crystalline structure is maintained at high temperatures, resulting in reduced shrinkage and high strength. On the other hand, a small difference in the orientation parameters between 25°C and 130°C means that the crystalline structure is less melted at high temperatures, leading to poor shutdown characteristics. Therefore, from the viewpoint of the balance between strength, shrinkage, and shutdown temperature, it is preferable that the change in the orientation parameter in either the MD or TD direction calculated by micro-Raman spectroscopy between 25°C and 130°C is large and the change in the other direction is small, and from the viewpoint of shutdown properties, it is particularly preferable that the difference in the orientation parameter between 25°C and 130°C is 0.0 or less (orientation parameter measured at 25°C≦orientation parameter measured at 130°C). When the difference in the orientation parameter between 25°C and 130°C is 0.0 or less, melting of the crystalline structure is promoted at high temperatures, and excellent shutdown properties are obtained. In a preferred embodiment of the present invention, the difference between fML and fMH (fML - fMH (7)) is preferably 0.0 or less, more preferably -0.1 or less, even more preferably -0.2 or less, even more preferably -0.3 or less, and particularly preferably -0.4 or less; the difference between fTL and fTH (fTL - fTH (8)) is preferably greater than 0.0, more preferably 0.1 or more, even more preferably 0.2 or more, and preferably 0.5 or less from the viewpoint of shrinkage rate. When the difference between fML and fMH is 0.0 or less, good shutdown characteristics are obtained, and when the difference between fTL and fTH is greater than 0.0, good strength is obtained. The above ranges can be achieved by applying the raw materials, molecular weights, and manufacturing methods within the ranges described below.

[0018] By ensuring that the orientation parameters fMH, fTH, fML, and fTL are within the above ranges, excellent strength and shutdown balance can be obtained, and the above ranges can be controlled by the raw material design and manufacturing method described below. In addition, in order to form a strong structure that maintains the orientation state at high temperatures, a weight-average molecular weight with a long relaxation time of 9.0 × 10 5 The above polyolefin resins are used as the main raw material, and film formation is preferably performed by combining wet stretching, including re-longitudinal stretching after washing and drying, with dry sequential stretching. A polyolefin resin with a weight-average molecular weight of 3.0 × 10, which has a short relaxation time, is used as a crystalline melting and orientation relaxation promoter at high temperatures. 5 The following polyolefin resins are more preferably combined with the auxiliary raw material. From the above viewpoint, the polyolefin microporous membrane has a molecular weight of 9.0×10 with a long relaxation time in its molecular weight distribution. 5 The above components are contained in a polyolefin microporous film at 30% by mass or more, and the relaxation time is short. The weight-average molecular weight is 3.0 × 10 5 It is more preferable that the following components are contained in the range of 20 to 50% by mass, thereby obtaining a highly safe polyolefin microporous membrane that combines high strength with low-temperature shutdown properties.

[0019] The porosity of the polyolefin microporous membrane according to an embodiment of the present invention is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more, from the viewpoints of permeability and electrolyte content. A porosity of 30% or more provides a good balance between permeability, strength, and electrolyte content, eliminating non-uniformity in the battery reaction. As a result, dendrite formation is suppressed, allowing use without impairing conventional battery performance, making the membrane suitable for use as a separator for secondary batteries. Increasing the porosity also results in good output characteristics, but also reduces battery safety due to factors 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.

[0020] The puncture strength of a polyolefin microporous membrane is preferably as high as possible because it affects safety, such as preventing short circuits caused by foreign matter in a battery. The puncture strength of a polyolefin microporous membrane converted to a membrane 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. Furthermore, the puncture strength per unit area weight (equivalent puncture strength per unit area weight), which is an index of film strength obtained by standardizing the puncture strength by the resin amount, is 0.75 N / (g / m 2 ) or more is preferable, and 0.8N / (g / m 2 ) or more is more preferable, and 0.85N / (g / m 2 ) or more is more preferable, and 0.9N / (g / m 2 ) or more is more preferable, and 1.0 N / (g / m 2 ) or more 2.0N / (g / m 2 ) or less is particularly preferred. When the pin puncture strength is within the above range, short circuits due to foreign matter and the like are suppressed, and good battery safety is achieved. To improve pin puncture strength, it is preferable to combine crystal orientation control with the use of an ultra-high molecular weight polyolefin as the main component in the raw material formulation, and increasing the number of tie molecules connecting the lamellae to increase strength. Furthermore, from the viewpoint of suppressing a decrease in porosity due to melting in a heat setting process or the like, an ultra-high molecular weight polyolefin with a molecular weight distribution containing few low molecular weight components is preferred. Pin puncture strength can be achieved by setting the fMH, fTH, fML, and fTL values ​​within specific ranges and adopting the raw materials, resin concentrations, and stretching method described below.

[0021] The puncture strength when converted to a film thickness of 10 μm refers to the puncture strength L2 (N) calculated by the formula: L2 = (L1 × 10) / T1 when the puncture strength of a polyolefin microporous film with a film thickness of T1 (μm) is L1 (N), and the puncture strength per unit basis weight is calculated by multiplying the measured puncture strength (L1) by the basis weight G (g / m 2 ) and is calculated using the formula: L1 / G.

[0022] From the viewpoint of preventing battery short circuits due to abnormal heat generation, the total shrinkage rate in the MD and TD directions at 130°C / 1h is preferably 35% or less, more preferably 30% or less, and even more preferably 29% or less. When the shrinkage rate is within this range, there is little dimensional change when the internal temperature of the battery rises, and insulation can be maintained, preventing the expansion of internal short circuits and minimizing their impact, resulting in high safety. Note that the above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods within the ranges described below.

[0023] Because a battery is subjected to tension in the MD direction, a high MD shrinkage rate can cause membrane rupture and lead to a short circuit. Therefore, the polyolefin microporous membrane according to an embodiment of the present invention has a MD shrinkage rate of 15% or less, preferably 12% or less, and more preferably 10% or less at 130°C / 1 h. When the MD shrinkage rate is within this range, dimensional change is minimal when the internal temperature of the battery rises, insulation properties can be maintained, and high safety can be achieved. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods within the ranges described below.

[0024] Furthermore, the polyolefin microporous membrane according to an embodiment of the present invention preferably has a TD shrinkage rate of 25% or less at 130°C / 1 h, more preferably 20% or less, and even more preferably 18% or less. When the shrinkage rate is within this range, deterioration of shape stability at high temperatures can be suppressed, and internal short circuits can be suppressed in the event of localized abnormal heat generation, thereby maintaining safety. The above heat shrinkage rate can be achieved by setting the fMH, fTH, fML, and fTL values ​​within specific ranges and employing the raw material and resin concentrations and stretching method described below. The MD and TD shrinkage rates at 130°C / 1 h can be measured by the method described in the Examples.

[0025] In the polyolefin microporous membrane of the present invention, the tensile strength at break in the MD (tensile strength at break in the MD; hereinafter, also simply referred to as "MD tensile strength") is preferably 250 MPa or more, more preferably 300 MPa or more, more preferably 350 MPa or more, even more preferably 400 MPa or more, and still more preferably 450 MPa or more, from the viewpoints of preventing membrane rupture during the battery winding process and short-circuiting due to foreign matter in the battery.

[0026] From the viewpoint of the 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 250 MPa or more, preferably 300 MPa or more, more preferably 320 MPa or more, and even more preferably 350 MPa or more. When the TD tensile strength is within the above range, the balance between the MD tensile strength and the TD tensile strength is good, wrinkling and sagging of the film are suppressed, and short circuits caused by tearing of the film due to foreign matter in the battery are prevented, improving safety. The above tensile strength can be achieved by adopting the raw materials, resin concentration, and stretching method described below.

[0027] In the polyolefin microporous membrane according to the embodiment of the present invention, the air permeability is a value measured in accordance with JIS P 8117 (2009). In addition, when the air permeability measured in a polyolefin microporous membrane having a thickness of T1 (μm) is p1 (sec / 100 cm 3 ), the air permeability p2 (sec / 100 cm ) calculated by the formula: p2 = (p1 × 10) / T1 3 ) is the air permeability when the membrane thickness is 10 μm (air permeability converted to 10 μm).

[0028] Air permeability is 200 seconds / 100cm 3 Preferably, it is 120 seconds / 100 cm or less. 3 More preferably, it is 115 seconds / 100 cm or less. 3 It is more preferable that the air permeability is 200 sec / 100 cm or less. 3 When the thickness is not more than this, good ion permeability can be obtained and the electrical resistance can be reduced.

[0029] In the polyolefin microporous membrane according to an embodiment of the present invention, the resistance increases with increasing membrane thickness, resulting in a decrease in battery output characteristics. From the viewpoint of battery output characteristics, the membrane thickness is preferably 12 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. Since the thinner the membrane, the lower the strength and safety, from the viewpoint of safety, the membrane thickness is preferably 1 μm or more, and more preferably 2 μm or more.

[0030] The shutdown temperature is the temperature at which, when a polyolefin microporous membrane is heated, the resin shrinks and melts, closing the pores and thereby terminating discharge and charging. This temperature is measured by the method described below. Because electrodes used in lithium-ion secondary batteries designed for high energy density tend to have reduced thermal stability, it is preferable to shut down (close the pores) quickly after a short circuit in the battery. The shutdown temperature of the polyolefin microporous membrane according to an embodiment of the present invention is preferably 143°C or lower, more preferably 140°C or lower. Furthermore, from the viewpoint of compatibility with pin 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 the above shutdown temperature, thereby achieving excellent battery safety. To achieve the shutdown temperature in the above range, it is preferable that the raw material composition of the microporous membrane be within the range described below, and that the membrane-forming conditions for the microporous membrane be within the range described below. In particular, a method for achieving a shutdown temperature of 143°C or lower while satisfying formulas (1) and (2) is to use a polyolefin having a weight-average molecular weight of 1.0 × 10 6 Over 1.8 x 10 6 The melting point is between 135°C and 138°C, and the total heat of fusion ΔH is calculated from the differential scanning calorimetry (DSC) curve. all The heat of fusion ΔH up to 130℃ 130 The ratio (=ΔH 130 / ΔH all ) is 45% or less as the main component, and a specific high-density polyethylene is also contained in the composition, and the composition is then kneaded in a twin-screw extruder having a screw with a specific structure.

[0031] The specific high density polyethylene has a melting point of 130°C or higher and 135°C or lower, and the total heat of fusion ΔH in the DSC curve obtained by differential scanning calorimetry (DSC) is all Examples of such polyethylene include high-density polyethylene having a modulus of elasticity of 200 J / g or more and 250 J / g or less and a half-value width of 4.0°C or more and 6.0°C or less.

[0032] An example of a screw with a specific structure is one in which, when the outermost diameter of the screw is D, the distance between the tip of the screw and the vent hole is 1.0D or more and 15.0D or less, and at least one screw piece having a length in the raw material conveying direction of 0.2D or more and 0.9D or less is used between the tip and the vent hole, and no more than two screw pieces having a length in the raw material conveying direction of 1.5D or more are used.

[0033] To improve permeability, the polyolefin microporous membrane according to an embodiment of the present invention preferably has an average pore size calculated using a perm porometer of 36 to 46 nm, more preferably 38 to 45 nm, and particularly preferably 40 to 44 nm. Examples of a method for adjusting the average pore size to 36 to 46 nm include a method of stretching the membrane to a specific magnification or greater using a stretching method including at least dry sequential biaxial stretching.

[0034] [2] Polyolefin resin The resin raw material for the polyolefin microporous membrane according to the embodiment of the present invention may be a single composition, a composition combining a main raw material and an auxiliary raw material, or a polyolefin resin mixture (polyolefin resin composition) consisting of two or more polyolefin resins. The raw material form for the polyolefin microporous membrane is preferably a polyolefin resin, such as polyethylene or polypropylene, and a composition combining a main raw material and an auxiliary raw material is preferred.

[0035] The polyolefin resin is preferably a homopolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc., and is particularly preferably a homopolymer of ethylene (polyethylene). The polyethylene may be a copolymer containing the homopolymer of ethylene and another α-olefin.

[0036] Other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, or alkenes having a carbon number greater than or equal to 1, vinyl acetate, methyl methacrylate, styrene, and the like.

[0037] The type of polyolefin resin used in the embodiment of the present invention is preferably polyethylene, and has a density of 0.94 g / cm 3 High density polyethylene with a density of 0.93 to 0.94 g / cm 3 Medium density polyethylene, with a density of 0.93 g / cm 3 Examples include lower low density polyethylene and linear low density polyethylene.

[0038] In addition, from the viewpoint of film strength and shrinkage rate, it is preferable to use ultra-high molecular weight polyolefin as the main component of the polyolefin resin, and the ultra-high molecular weight polyolefin has a weight average molecular weight of 1.0 × 10 6 More than 1.5 × 10 is preferable. 6 From the viewpoint of molding processability, the weight average molecular weight is preferably 1.0×10 7 The following is preferable: It is important to add the auxiliary material within a range that does not impair the fibril structure formed by the main material or the moldability.

[0039] Weight average molecular weight is 1.0 x 10 6 If the weight average molecular weight is 1.0×10 or more, the retention of the crystalline structure at high temperatures is improved due to the long relaxation time, and melting and shrinkage are suppressed, and the polymer has a strong structure that maintains its oriented state even at high temperatures, making it possible to achieve both strength and shrinkage, thereby improving the safety of the battery. 6By using the above polyolefin resins, the number of tie molecules increases, making it easier to obtain high strength. In addition, it suppresses the melting of fibrils during the stretching and heat setting processes, resulting in good output characteristics. At the same time, by slowing down the relaxation rate of the resin, it is possible to increase the heat setting temperature, resulting in good shrinkage characteristics, and improving the trade-off between ionic resistance, strength, and shrinkage.

[0040] The ultra-high molecular weight polyethylene used in the embodiment of the present invention has a total heat of fusion ΔH in a DSC curve obtained by differential scanning calorimetry (DSC). all The heat of fusion of 130℃ or less ΔH 130 The ratio (=ΔH 130 / ΔH all ) is preferably 45% or less. 130 / ΔH all By making the ΔH of the ultra-high molecular weight polyethylene 45% or less, the crystalline components that melt at 130°C or less can be reduced, and dimensional stability at high temperatures can be improved. In addition, from the viewpoint of improving the stretchability when stretching a polyolefin resin sheet to produce a microporous membrane, it is preferable to set the ΔH of the ultra-high molecular weight polyethylene 45% or less. 130 / ΔH all is preferably 10% or more.

[0041] The auxiliary materials may be added within a range that does not impair the fibril structure formed by the main materials or the molding processability. A weight-average molecular weight of 5.0 × 10 5 The following polyolefin resins are more preferable in combination with auxiliary materials: 4.0 x 10 5 Less than 3.0 x 10 is preferable. 5 Less than 2.0 x 10 is more preferable. 5 The following is even more preferable: 1.0 x 10 5 The following is particularly preferred. From the viewpoint of achieving a good balance between the puncture strength converted into basis weight and the shutdown temperature, the polyolefin used as the secondary raw material should have a melting point of 130°C or higher and 135°C or lower, and a total heat of fusion ΔH in the DSC curve obtained by differential scanning calorimetry (DSC) of 130°C or higher. all High density polyethylene having a modulus 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.

[0042] From the above viewpoint, the weight average molecular weight of the polyolefin microporous film is 8.0 × 10 5 More than 9.0×10 is preferable. 5 More than 10 x 10 is preferable. 5 The above is more preferable, and it is particularly preferable to maintain the molecular weight of the raw material. In addition, in order to form a strong structure that maintains the orientation state at high temperatures and to exhibit high strength, it is preferable to use a polyolefin microporous membrane having a molecular weight of 9.0 × 10 with a long relaxation time in the molecular weight distribution. 5 The amount of the above components contained in the polyolefin microporous membrane is preferably 30% by mass or more, more preferably 33% by mass or more, even more preferably 35% by mass or more, even more preferably 38% by mass or more, and particularly preferably 40% by mass or more. From the viewpoint of promoting melting and relaxation of orientation of crystals and realizing a low-temperature shutdown temperature, it is preferable that the molecular weight of the polyolefin microporous membrane is 3.0 × 10 5 The content of the following polyolefin is preferably 30% by mass or more, more preferably 35% by mass or more, even more preferably 40% by mass or more, and particularly preferably 45% by mass or more. From the viewpoint of maintaining the crystalline structure at high temperatures, the main raw material is an ultra-high molecular weight polyolefin, and has a weight-average molecular weight of 3.0 × 10 5 The content of the following polyolefins is less than 50% by mass. To obtain the molecular weight of the polyolefin microporous membrane, it is preferable to produce the membrane from the above-mentioned raw material formulation by adding an antioxidant or kneading under a nitrogen atmosphere, as described below, or by combining the addition of an antioxidant and kneading under a nitrogen atmosphere.

[0043] The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of the ultra-high molecular weight polyolefin is preferably in the range of 3.0 to 100. The narrower the molecular weight distribution, the more uniform the system and the easier it is to obtain uniform micropores, so a narrower molecular weight distribution is preferable; however, the narrower the distribution, the worse the molding processability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or more, more preferably 5.0 or more, and even more preferably 6.0 or more. As the molecular weight distribution increases, the amount of low molecular weight components increases, which leads to a decrease in strength and the tendency for fine fibrils to melt and fuse during stretching and heat setting. Therefore, the upper limit is preferably 80 or less, more preferably 50 or less, even more preferably 20 or less, and particularly preferably 10 or less. By setting the molecular weight in the above range, good molding processability can be obtained, and the system can be unified, resulting in uniform micropores.

[0044] In the production process of the polyolefin microporous membrane according to an embodiment of the present invention, it is preferable to add a plasticizer to improve moldability. The blending ratio of the polyolefin resin and the plasticizer may be adjusted as appropriate within a range that does not impair moldability. However, the proportion of the polyolefin resin is preferably 10 to 50% by mass, where the total of the polyolefin resin and the plasticizer is 100% by mass. When the proportion of the polyolefin resin is 10% by mass or more (the proportion of the plasticizer is 90% by mass or less), swelling and necking at the outlet of the die during sheet formation can be suppressed, improving sheet formability and film formability. When the proportion of the polyolefin resin is 50% by mass or less (the proportion of the plasticizer is 50% by mass or more), pressure increases during the film formation process can be suppressed, resulting in good moldability. When the total of the polyolefin resin and the plasticizer is 100% by mass, the proportion of the polyolefin resin is preferably 10% by mass or more, more preferably 20% by mass or more.

[0045] When a polyolefin resin having a weight-average molecular weight of 900,000 or more is used as the main component or alone, the proportion of the polyolefin resin is preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably less than 28.5% by mass, and even more preferably less than 25% by mass, based on 100% by mass of the total of the polyolefin resin and the plasticizer, in terms of the pressure and stretching stress during the film-forming process.

[0046] In addition, the polyolefin microporous membrane according to an embodiment of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, etc., within the range that does not impair the effects of the present invention. In particular, it is preferable to add antioxidants for the purpose of suppressing oxidative degradation of the polyolefin resin due to thermal history.

[0047] As the antioxidant, it is preferable to use one or more selected from the group consisting of 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 (e.g., "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2), and tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7).

[0048] Appropriate selection of the type and amount of antioxidant and heat stabilizer is important for adjusting or enhancing the properties of the polyolefin microporous membrane, and the amount is preferably such that the MFR of the gel-like sheet (described later) measured by the method described in JIS K7210-1 (2014) does not increase. The amount of antioxidant added is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, even more preferably 1.0% by mass or more, even more preferably 1.2% by mass or more, and most preferably 1.5% by mass or more, based on the amount of resin. From the viewpoint of the quality and stable membrane formability of the polyolefin microporous membrane, the upper limit is 3.0% by mass or less, and it is particularly preferable to suppress oxidative degradation by combining the addition of an antioxidant and kneading under a nitrogen atmosphere.

[0049] The polyolefin microporous membrane according to an embodiment of the present invention may have a single-layer or multi-layer structure, and a multi-layer structure is preferred from the viewpoint of balance of physical properties. When layers having the above-described polyolefin resin formulations are used as a laminate, the content of the layers is preferably 50% by mass or more of the total membrane thickness.

[0050] [3] Manufacturing method of polyolefin microporous membrane Next, the method for producing a polyolefin microporous membrane according to an embodiment of the present invention will be specifically described. The 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, if necessary, a solvent to prepare a polyolefin resin solution. (b) A step of extruding the resulting molten mixture, forming it into a sheet, and cooling and solidifying it. (c) A step of simultaneously biaxially stretching the obtained sheet using a tenter system (d) Thereafter, the plasticizer is extracted from the resulting stretched film and the film is dried. (e) A step of re-stretching / heat treating using a stretching method including a roll method or a tenter method In particular, in step (a), it is particularly preferable to add an antioxidant in an amount described below or to knead the film under a nitrogen atmosphere in order to prevent a decrease in molecular weight, and in step (e), to perform dry longitudinal re-stretching by a roll stretching method and heat treatment / re-stretching at a temperature of 130°C or higher by a tenter method.

[0051] Each step will be described below.

[0052] (a) Preparation of polyolefin resin solution The polymer material is dissolved in a plasticizer under heating to prepare a polyolefin resin solution. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, but it is preferable that the solvent is liquid at room temperature in order to enable stretching at a relatively high ratio.

[0053] Examples of the solvent include aliphatic, alicyclic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions having boiling points corresponding to these hydrocarbons, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate.

[0054] In order to obtain a stable gel-like sheet, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0055] A solvent that is miscible with the polyolefin resin in the melt-kneaded state but is solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, using only a solid solvent may result in uneven stretching.

[0056] 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 from the polyolefin resin solution is less likely to be non-uniform. On the other hand, if the viscosity at 40°C is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is measured at 40°C using an Ubbelohde viscometer.

[0057] (b) Formation of extrudate and gel-like sheet The method for uniformly melt-kneading the polyolefin resin solution is not particularly limited, but when preparing a high-concentration polyolefin resin solution, it is preferable to carry out the melt-kneading in a twin-screw extruder. If necessary, known additives such as metal soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, and antistatic agents may be added within a range that does not impair film-forming properties or impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.

[0058] In the extruder, the polyolefin resin solution is mixed uniformly at a temperature at which the polyolefin resin is completely melted. The melt-mixing temperature varies depending on the polyolefin resin used, but is preferably (melting point of polyolefin resin + 10°C) to (melting point of polyolefin resin + 120°C). More preferably, it is (melting point of polyolefin resin + 20°C) to (melting point of polyolefin resin + 100°C).

[0059] Here, the melting point refers to a value measured by DSC (Differential Scanning Calorimetry) in accordance with JIS K7121 (1987) (the same applies hereinafter). For example, when the polyolefin resin is polyethylene, the melt-kneading temperature of the polyethylene resin is preferably in the range of 140 to 250°C, more preferably 150 to 230°C, and most preferably 150 to 200°C. Specifically, since the polyethylene composition has a melting point of approximately 130 to 140°C, the melt-kneading temperature is preferably 140 to 250°C.

[0060] A lower melt-kneading temperature is preferable to prevent deterioration of the polyolefin resin. However, if the temperature is lower than the above range, unmelted material may be generated in the extrudate extruded from the die, which may cause film rupture during the subsequent stretching process. Furthermore, if the temperature is higher than the above range, thermal decomposition of the polyolefin resin becomes severe, which may deteriorate the physical properties of the resulting polyolefin microporous film, such as strength and porosity. Furthermore, decomposition products may precipitate on the chill roll or rolls used in the stretching process and adhere to the sheet, resulting in poor appearance. Therefore, it is preferable to knead the melt-kneading temperature within the above range. Furthermore, the smaller the value of Q / Ns, which is the ratio of the extrusion rate Q (kg / h) of the polyolefin solution to the screw rotation speed Ns (rpm) of the twin-screw extruder, the better the resin kneading, resulting in a more uniform solution. However, a decrease in Q / Ns increases shear heating, accelerating resin degradation and preventing the molecular weight components in the film from falling within the above range. Low-molecular-weight components accumulate in the bled-out plasticizer and adhere to the sheet, resulting in poor appearance. If the Q / Ns value is large, resin degradation is suppressed, but kneading is insufficient and a uniform solution cannot be obtained. Therefore, it is particularly preferable to adjust Q / Ns appropriately according to the molecular weight and solubility of the resin used, and to suppress oxidative degradation by adding an antioxidant or kneading under a nitrogen atmosphere.

[0061] The screws of the twin-screw extruder preferably have a structure in which, when the outermost diameter of the screw is D, the distance between the screw tip and the vent hole is 1.0D or more and 15.0D or less, and at least one screw piece having a length in the raw material conveying direction of 0.2D or more and 0.9D or less is used between them, and no more than two screw pieces having a length in the raw material conveying direction of 1.5D or more are used. By using such a structure, it becomes possible to improve the kneading ability, particularly when two or more types of polyolefin resins are used.

[0062] The resulting extrudate is then cooled to obtain a gel-like sheet, which can solidify the microphase of the polyolefin resin separated by the solvent. In the cooling step, the gel-like sheet is preferably cooled to 10 to 50°C. This is to ensure that the final cooling temperature is below the crystallization end temperature, which refines the higher-order structure and facilitates uniform stretching in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30°C / min or more until the temperature is at least below the gelation temperature.

[0063] Generally, when the cooling rate is slow, relatively large crystals are formed, resulting in a coarse higher-order structure of the gel-like sheet and a large gel structure.In contrast, when the cooling rate is fast, small, uniform crystals are formed, resulting in a dense higher-order structure of the gel-like sheet and enabling uniform stretching.

[0064] The cooling method may be a method of directly contacting the material with cold air, cooling water or other cooling medium, a method of contacting the material with a roll cooled with a cooling medium, or a method using a casting drum or the like.

[0065] Although the polyolefin microporous membrane has been described above as a single layer, the polyolefin microporous membrane according to an embodiment of the present invention is not limited to a single layer and may be a laminate. There is no particular limitation on the number of layers, and the membrane may be a two-layer laminate or a three- or more-layer laminate. As described above, the laminated portion may contain, in addition to the polyolefin resin, other desired resins to the extent that the effects of the present invention are not impaired.

[0066] Conventional methods can be used to form a laminate from a polyolefin microporous membrane, such as preparing desired resins as needed, separately feeding these resins to an extruder to melt them at a desired temperature, joining them in a polymer tube or die, and extruding them through a slit die to the desired thickness for each layer to form a laminate.

[0067] (c) Stretching process The resulting gel-like sheet (including laminated sheets) is stretched. Examples of stretching methods include uniaxial stretching in the sheet conveying direction (MD) using rolling or a roll stretching machine, uniaxial stretching in the sheet width direction (TD) using a tenter, sequential biaxial stretching using a roll stretching machine and tenter or a combination of a tenter and tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter. The stretching ratio of the gel-like sheet may be adjusted as appropriate within a range that does not impair the orientation parameters in the MD and TD directions. However, since stretching of a gel-like sheet tends to cause orientation in the stretching axis direction, it is preferable to stretch the gel-like sheet by simultaneous biaxial stretching to a magnification of 5x or more in both directions from the perspective of controlling the orientation in the MD and TD directions at high temperatures, and then sequentially stretching in the MD and TD directions in a dry re-stretching process after washing and drying (described below), thereby controlling the orientation parameters in both the MD and TD directions at high temperatures. In this specification, re-stretching in the MD direction may be referred to as "longitudinal re-stretching," and re-stretching in the TD direction may be referred to as "transverse re-stretching."

[0068] The stretching temperature is preferably set to a temperature not higher than 10°C above the melting point of the gel-like sheet, more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the gel-like sheet + 5°C). Specifically, since a polyethylene composition has a crystal dispersion temperature of approximately 90 to 110°C, the stretching temperature is preferably 100 to 130°C, more preferably 100 to 115°C, and particularly preferably 100 to 110°C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012). If the temperature exceeds the upper limit, molecular relaxation is promoted, making it difficult to achieve sufficient pore opening through stretching. A stretching temperature within the above range prevents membrane rupture due to stretching of the polyolefin resin and produces a microporous membrane that is appropriately oriented in the MD and TD directions. This allows for control of the orientation parameters at high temperatures in both the MD and TD directions in the dry re-stretching process after washing and drying, as described below.

[0069] (d) Plasticizer extraction (washing) and drying process Next, the plasticizer (solvent) remaining in the gel-like sheet is removed using a cleaning solvent. Because the polyolefin resin phase and the solvent phase are separate, removing the solvent yields a microporous polyolefin membrane.

[0070] Examples of the cleaning 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.

[0071] These cleaning solvents have low surface tension (e.g., 24 mN / m or less at 25°C). By using a cleaning solvent with low surface tension, the surface tension at the air-liquid interface prevents the network structure that forms the micropores from shrinking during drying after washing, resulting in a polyolefin microporous membrane with excellent porosity and permeability. These cleaning solvents are selected appropriately depending on the plasticizer and used alone or in combination.

[0072] The cleaning method may be a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet.

[0073] The washing temperature may be 15 to 30° C., and is heated to 80° C. or lower as necessary. In this case, the longer the time the gel-like sheet is immersed in the washing solvent, the better, from the viewpoints of increasing the washing effect of the washing solvent, preventing non-uniform physical properties (e.g., physical properties in the TD and / or MD) of the resulting polyolefin microporous membrane, and improving the mechanical and electrical properties of the polyolefin microporous membrane.

[0074] The above-mentioned washing is preferably carried out until the residual solvent in the washed gel-like sheet, ie, the polyolefin microporous membrane, is less than 1% by mass.

[0075] Thereafter, the solvent in the polyolefin microporous membrane is removed by drying in a drying step. The drying method is not particularly limited, and a method using a metal heating roll or a method using hot air can be selected. The drying temperature is preferably 40 to 100°C, more preferably 40 to 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.

[0076] (e) Heat treatment / re-stretching process The method is characterized in that the dried polyolefin microporous membrane is stretched (re-stretched) in at least one direction. The re-stretching can be carried out by roll stretching or tenter method, as in the stretching described above. The re-stretching may be uniaxial or biaxial, but from the viewpoint of orientation control in the MD and TD directions at high temperatures, sequential stretching that combines longitudinal re-stretching using roll stretching and transverse re-stretching using tenter stretching is particularly preferred. The temperature for longitudinal re-stretching is preferably set to not more than the melting point of the film after washing and drying, and more preferably within the range of (Tcd of 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. Longitudinal re-stretching is effective for controlling the orientation parameter measured at 130°C using MD Raman spectroscopy, and the stretching ratio is 1.2 times or more, preferably 1.3 times or more, more preferably 1.4 times or more, even more preferably 1.5 times or more, and even more preferably 1.7 times or more. When the stretching ratio is 1.2 times or more, the difference in orientation parameter between 25°C and 130°C is large, forming a structure that easily promotes melting of the crystalline structure at high temperatures, and obtaining good shutdown properties. Furthermore, from the viewpoint of productivity, the stretching ratio is preferably 3.0 times or less.

[0077] The temperature for the transverse re-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°C, more preferably 110 to 140°C, even more preferably 120 to 140°C, and even more preferably 130 to 140°C. By performing stretching at the ratio described below within the above temperature range, orientation relaxation of the polyolefin molecular chains is suppressed, resulting in a highly oriented structure and a thermally stable structure. The microporous film obtained thereby has a high TD orientation parameter at 130°C and a small difference in TD orientation parameter between 25°C and 130°C, and exhibits both excellent pin puncture strength and heat shrinkage properties.

[0078] Weight average molecular weight is 9.0 x 10 5 If the weight average molecular weight is less than 9.0 × 10, the relaxation time is short, and heat treatment at 130 °C or higher leads to a decrease in porosity. 5 The above polyethylene has a long relaxation time, so even if it is stretched and heat-set at temperatures above 130°C, the decrease in porosity can be suppressed. Since heat-set can be performed at high temperatures, orientation relaxation at high temperatures is suppressed, and a highly oriented structure can be obtained. Therefore, a weight-average molecular weight of 9.0 × 10 5 It is preferable to use the above polyethylene and heat fix it at a temperature higher than 130°C.

[0079] In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 3.0 times, and in particular, the ratio in the TD direction is preferably 1.1 to 1.2 times, more preferably 1.2 to 1.7 times. In the case of biaxial stretching, it is preferable to stretch in both the MD and TD directions by 1.01 to 2.0 times. The re-stretching ratios in the MD and TD directions may be different, and multi-stage stretching combining successive stretching is preferred. The dry stretching process is effective in controlling the orientation of molecular chains as measured at 25°C using Raman spectroscopy, and high pin puncture strength can be obtained by dry stretching at the above stretching ratio.

[0080] From the viewpoint of shrinkage rate 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. When the relaxation rate is 20% or less, a uniform fibril structure is obtained.

[0081] (f) Other processes Furthermore, depending on other applications, the polyolefin microporous membrane can be subjected to a hydrophilization treatment. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, etc. Monomer grafting is preferably performed after crosslinking treatment.

[0082] The polyolefin microporous membrane is preferably crosslinked by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. The crosslinking treatment increases the meltdown temperature of the polyolefin microporous membrane.

[0083] In the surfactant treatment, any of nonionic, cationic, anionic, and amphoteric surfactants can be used, but nonionic surfactants are preferred. The polyolefin microporous membrane is immersed in a solution prepared by dissolving the surfactant 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.

[0084] For the purpose of improving the meltdown properties and heat resistance when used as a battery separator, the polyolefin microporous membrane according to an embodiment of the present invention may be surface-coated with a porous fluorine-based resin such as polyvinylidene fluoride or polytetrafluoroethylene, a porous material such as polyimide or polyphenylene sulfide, or an inorganic coating such as ceramic. In particular, the polyolefin porous membrane obtained by the present invention has high strength and low thermal shrinkage, which makes it easy to control the tension during coating, suppresses shrinkage during the drying process, and has excellent coatability.

[0085] The polyolefin microporous membrane obtained as described above can be used for various purposes such as filters, fuel cell separators, and capacitor separators, but is particularly safe when used as a battery separator. Therefore, the separator is preferably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as for electric vehicles. Furthermore, secondary batteries using such separators are suitable for use in electric vehicles, taking advantage of their properties. [Example]

[0086] The present invention will be described in more detail with reference to examples, but the embodiments of the present invention are not limited to these examples. Evaluations in this application were performed in an environment of 23°C and 65% humidity unless otherwise specified. The evaluation and analysis methods used in the examples are as follows.

[0087] (1) Weight average molecular weight (Mw) The molecular weight distribution of polyolefins (weight average molecular weight, molecular weight distribution, content of specific components, etc.) was measured by high-temperature gel permeation chromatography (GPC). The molecular weight distribution of the film was measured using a stretched polyolefin microporous membrane, and the molecular weight distribution of the polyolefin raw material was measured using the polyolefin raw material under the following conditions. Equipment: High temperature GPC equipment (Equipment No. HT-GPC, manufactured by Polymer Laboratories, PL-220) Detector: Differential refractive index detector RI Guard column: Shodex G-HT Column: Shodex HT806M (2 columns) (φ7.8 mm x 30 cm, Showa Denko) Solvent: 1,2,4-trichlorobenzene (TCB, manufactured by Wako Pure Chemical Industries, Ltd.) (0.1% BHT added) Flow rate: 1.0mL / min Column temperature: 145℃ Sample preparation: 5 mL of the measurement solvent was added to 5 mg of sample, and the mixture was heated and stirred at 160 to 170°C for about 30 minutes, after which the resulting solution was filtered through a metal filter (pore size 0.5 μm). Injection volume: 0.200mL Standard sample: Monodisperse polystyrene (manufactured by Tosoh) (PS) Data processing: GPC data processing system manufactured by Toray Research Center.

[0088] (2) Film thickness (μm) The thickness of the polyolefin microporous membrane was measured at five points within a 50 mm x 50 mm area using a contact thickness gauge, Litematic VL-50 (10.5 mmφ superhard spherical probe, measuring load 0.01 N) manufactured by Mitutoyo Corporation, and the average value was taken as the thickness (μm).

[0089] (3) Air permeability (sec / 100cm 3 ), and 10μm equivalent air permeability (sec / 100cm 3 ) For a polyolefin microporous membrane with a thickness of T1 (μm), the air permeability (sec / 100cm) was measured in accordance with JIS P-8117 using an Oken air permeability meter (manufactured by Asahi Seiko, EGO-1T) at 25°C. 3 ) was measured. In addition, the air permeability (air permeability converted to 10 μm) (sec / 100 cm) when the membrane thickness was 10 μm was calculated using the following formula: 3 ) was calculated.

[0090] Formula: 10μm equivalent air permeability (sec / 100cm 3 ) = Air permeability (sec / 100cm 3 ) × 10 (μm) / Polyolefin microporous membrane thickness (μm) (4) Porosity (%) A 50mm x 50mm square sample was cut from the polyolefin microporous membrane, and its volume (cm) at room temperature (25°C) was measured. 3 The film density (g / cm) and mass (g) were measured. 3 ) and the porosity of the polyolefin microporous membrane was calculated using the following formula:

[0091] Porosity (%) = (volume - mass / membrane density) / volume × 100 The film density is 0.99 g / cm 3 The calculation was performed assuming a constant value of

[0092] (5) 10 μm equivalent puncture strength (N) and basis weight equivalent puncture strength (N / (g / m 2 )) The puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was 2 mm / s. Using a force gauge (IMADA DS2-20N), the maximum load (N) when a polyolefin microporous membrane was punctured in an atmosphere of 25°C with a 1.0 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm) was measured (L1), and the puncture strength (L2) converted to a membrane thickness of 10 μm was calculated using the following formula: Formula: L2 = L1 × 10 (μm) / polyolefin microporous membrane thickness (μm) The puncture strength converted into basis weight (L3) was calculated from the maximum load (N) measured (L1) when the polyolefin microporous membrane was punctured in an atmosphere of 25°C using the following formula. Formula: L3 = L1 / basis weight of polyolefin microporous membrane The basis weight of the polyolefin microporous membrane was calculated by cutting a 50 mm x 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25°C, and using the following formula: Formula: Weight (g / m 2 )=mass(g) / (50(mm)×50(mm))×10 6 (6) Tensile strength (MPa) Tensile tests were performed using a Shimadzu Autograph AGS-J in accordance with JIS K7127. The strength at break was divided by the cross-sectional area of ​​the sample before the test to obtain the tensile strength (MPa). Measurement conditions were: temperature: 23±2°C, sample shape: 10mm wide x 50mm long, chuck distance: 20mm, and tensile speed: 100mm / min. A 40mm wide x 60mm long paper frame with a 20mm x 20mm hole cut out from the center was used as the sample holder. A 10mm wide x 50mm long sample was clamped between the sample holder and chucked at a pressure of 0.4MPa. The sample holder was then cut at both ends (the centers of the two sides parallel to the sample) and measurements were performed. These measurements were performed at three different points on the same film in both the MD and TD directions, and the average of the three measurements was used as the tensile strength in each direction (MD tensile strength at break, TD tensile strength at break).

[0093] (7) Tensile elongation at break (%) Tensile tests were performed using a Shimadzu Autograph AGS-J. The tensile elongation at break was calculated using the following formula, based on the gauge length L (mm) of the test specimen before the test and the gauge length L (mm) at break. The measurement conditions were: temperature: 23±2°C, sample shape: 10 mm wide x 50 mm long, chuck distance: 20 mm, and tensile speed: 100 mm / min. A 40 mm wide x 60 mm long paper frame with a 20 mm x 20 mm hole cut out in the center was used as the sample holder. A 10 mm wide x 50 mm long sample was clamped between the sample holder and chucked at a pressure of 0.4 MPa. The sample holder was then cut at both ends (the centers of the two sides parallel to the sample) and measurements were performed. The above measurements were carried out at three different points on the same film in both the MD and TD directions, and the average value of the three points was taken as the tensile elongation at break in each direction (MD tensile elongation at break, TD tensile elongation at break).

[0094] Tensile elongation at break (%) = ((L-L0) / L) × 100.

[0095] (8) Shrinkage rate (%) at 130℃ / 1h A 5 cm x 5 cm square sample was cut out of the polyolefin microporous membrane so that two sides were parallel to the MD direction. The length of the sample in the MD direction was measured at the center of the cut sample in the TD direction, and this was defined as the length before MD shrinkage (L 1MD ) In addition, the sample length in the TD direction was measured at the center of the MD direction, and this was defined as the length before TD shrinkage (L 1TD ) was used. Next, the sample was placed in an oven with the temperature inside the oven set to 130°C and heated, and one hour after being placed in the oven, the sample was taken out. The MD length was measured at the location where the length before MD shrinkage was measured, and this was used as the length after MD shrinkage (L 2MD ) was used. In addition, the length in the TD direction was measured at the location where the length before TD shrinkage was measured, and this was used as the length after TD shrinkage (L 2TD Using these values, the thermal shrinkage rate after 1 hour at 130°C was calculated using the following formula. This measurement was also carried out at three arbitrary points on the sample surface, and the average value was calculated as the thermal shrinkage rate (%) after 1 hour at 130°C.

[0096] Formula: MD direction heat shrinkage rate (%) after 1 hour at 130°C = 100 x (L 1MD -L 2MD ) / L 1MD Formula: Thermal shrinkage 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 micro-Raman spectrometer as follows, and the orientation parameter of the crystalline molecular chain was calculated. <Raman measurement conditions> Laser: 532nm Grating: 2400 Line / mm Lens: 20x Slit: 200×1000μm Aperture: φ4000μm 1. A laser beam polarized in the machine direction of the polyolefin microporous membrane using a polarizer was incident on the test piece, and the scattered light was collected through an analyzer oriented in the machine direction.

[0097] 2. 1130 cm- of the obtained Raman spectrum 1 and 1060cm- 1 Ratio of Raman bands I 1130 / I 1060 was defined as the Raman orientation parameter and its value was calculated. The Raman spectrum was obtained with the polarizer parallel to the longitudinal direction of the film (0° / 0°) as the MD direction and perpendicular to the longitudinal direction (90° / 90°) as the TD direction. 1 is a band attributed to the CC stretching vibration of polyethylene molecular chains in the crystalline phase, and since the direction of the vibration Raman tensor coincides with the molecular chain axis, it is possible to determine the orientation state of the molecular chains. A larger value of the orientation parameter indicates a higher degree of orientation of the crystalline molecular chains. Calculation of Peak and Orientation Parameters I a : Raman shift band 1100~1170cm -1 Maximum intensity of the Raman band in the range I b: Raman shift band 1040~1090cm -1 Maximum intensity of the Raman band in the range I a (MD, 25°C): MD value measured at 25°C I a (TD, 25°C): Value in the TD direction measured at 25°C I b (MD, 25°C): Value in the TD direction measured at 25°C I b (TD, 25°C): Value in the TD direction measured at 25°C I a (MD, 130℃): Value in the MD direction after heating at 130℃ for 60 minutes using a heating stage I a (TD, 130°C): Value in the TD direction after heating at 130°C for 60 minutes using a heating stage I b (MD, 130℃): Value in the MD direction after heating at 130℃ for 60 minutes using a heating stage I b(TD, 130°C): Value in the TD direction after heating at 130°C for 60 minutes using a heating stage fMH=I a (MD, 130℃) / I b (MD, 130℃) ...Equation (1) fTH=I a (TD, 130℃) / I b (TD, 130℃) ...Equation (2) fML=I a (MD, 25℃) / I b (MD, 25℃)...Equation (3) fTL=I a (TD, 25℃) / I b (TD, 25℃)...Equation (4) fMLH=D a (MD, 25℃) / D a (MD, 130℃))...Equation (5) fTLH=D a (TD, 25℃) / D a (TD, 130℃))...Equation (6) fML-fMH (7) fTL-fTH ···(8) formula In addition, D in equations (5) and (6) a is the Raman shift band 1100-1170cm -1 and maximum intensity in the range of 1200 cm -1 D a (MD, 130℃) is the D measured at 130℃ in the MD direction. a The value of D a (TD, 130℃) is the D measured at 130℃ in the TD direction. a The value of D a (MD, 25℃) is the D measured at 25℃ in the TD direction a The value of D a (TD, 25°C) is the D measured at 25°C in the TD direction. a is the value of The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0098] (10) Short-circuit test The evaluation of short-circuit resistance was carried out using a desktop precision universal testing machine, Autograph AGS-X (Shimadzu Corporation). A laminate was prepared consisting of a polypropylene insulator (thickness 0.2 μm) / negative electrode (for lithium-ion batteries (copper foil (thickness approximately 0.9 μm), active material: artificial graphite (particle size approximately 13 μm)) / separator / 500 μm diameter chrome sphere (material: chrome (SUJ-2)) / aluminum foil). The aluminum foil and negative electrode of the sample laminate were connected by cable to a circuit consisting of a capacitor and a clad resistor. The capacitor was charged to approximately 1.5 V, and a metal sphere (material: chrome (S)) approximately 500 μm in diameter was inserted between the separator and aluminum foil in the sample laminate. UJ-2)) was then placed on the battery. The battery was then pressed at a rate of 0.3 mm / min, and foreign matter resistance was evaluated based on the amount of displacement until the battery shorted out. With the change in compressive load, the point at which the leakage current value began to increase was taken as the starting point, and the moment the above circuit was formed via the metal ball and current was detected was taken as the short-circuit point, and the displacement was measured. The sample that did not short out even with a large amount of displacement had better foreign matter resistance, and the relationship between displacement and foreign matter resistance was divided into three levels as shown below. A and B are preferable as they allow for batteries to have higher energy density and capacity. In this evaluation, a grade of B or higher was considered a pass. A: Displacement (mm) / separator thickness (μm) is greater than 0.025 B: Displacement (mm) / separator thickness (μm) is greater than 0.020 and less than 0.025 C: Displacement (mm) / separator thickness (μm) is 0.020 or less.

[0099] (11) Shutdown temperature (℃) The polyolefin microporous membrane was heated at a temperature increase rate of 5°C / min while measuring the air permeability resistance using an Oken air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The air permeability resistance reached the detection limit of 99999 seconds / 100cm. 3 The temperature reached by the air was taken as the shutdown temperature (°C).

[0100] The measurement cell was made up of an aluminum block and had a structure in which a thermocouple was placed directly under the polyolefin microporous film. The sample was cut into a 5 cm x 5 cm square and heated while being fixed with an O-ring around the periphery.

[0101] (12) Melting point (°C) Differential scanning calorimetry (DSC) was performed according to JIS K7121 (1987). A 6.0 mg sample (raw polyolefin resin or polyolefin microporous membrane) was placed in an aluminum pan and heated from 30°C to 230°C at a rate of 10°C / min under a nitrogen atmosphere using a Parker Elmer PYRIS Diamond DSC. The sample was then heated from 30°C to 230°C at a rate of 10°C / min (first heating), held at 230°C for 5 minutes, cooled at a rate of 10°C / min, and heated again from 30°C to 230°C at a rate of 10°C / min (second heating). A melting endothermic curve (DSC curve) was obtained for each sample. For the raw polyolefin resin, the peak-top temperature of the melting endothermic curve obtained during the second heating was used as the melting point.

[0102] (13) ΔH of polyolefin resin 130 / ΔH all For the melting endothermic curve (DSC curve) obtained in the second heating step in the same manner as in (12), a linear baseline was set in the range of 60°C to 160°C using analysis software, and the heat quantity was calculated from the area enclosed by the baseline and the melting endothermic curve. The total heat quantity of melting ΔH was calculated by converting the heat quantity per unit mass. all The heat of fusion ΔH (J / g) was calculated by calculating the heat of fusion in the area enclosed by the baseline and the melting endothermic curve at 130°C or below, and converting this heat per unit mass. 130 (J / g) was calculated. From the obtained heat of fusion, ΔH 130 / ΔH all (%) was calculated.

[0103] (14) DSC half-width of polyolefin resin (℃) The maximum peak height (h) was read from the melting endothermic curve (DSC curve) obtained in the second heating cycle in the same manner as in (12), and two temperatures indicating a height of h / 2 were read from the melting endothermic curve in the second heating cycle. The absolute value of the difference between the temperatures read at the two points was calculated and used as the DSC half-width (°C). When two or more temperatures indicating a height of h / 2 were read, the two points closest to the melting point were used to calculate the half-width.

[0104] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0105] (15) Average pore diameter (nm) Using a Perm Porometer (CFP-1500A manufactured by PMI), the maximum pore size and average pore size were measured in the order of dry-up and wet-up. For wet-up, pressure was applied to a porous polyolefin film thoroughly soaked in Galwick (trade name) manufactured by PMI, which has a surface tension of 15.6 dynes / cm. The average pore size was calculated from the pressure at the point where the curve showing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The following formula was used to convert pressure and pore size.

[0106] 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 a constant.

[0107] (16) Shutdown characteristics To simulate battery safety, the resistance of the electrolyte solution during temperature rise was evaluated. In a 30% humidity environment, a polyolefin microporous membrane was punched into a 19 mm diameter circle and placed in a coin battery (CR2032 standard). The electrolyte was then injected and vacuum-impregnated in a vacuum dryer at -50 kPa for 1 minute. The coin battery was then sealed using a crimping machine, and the resistance was measured while heating at a rate of 5°C / min. The electrolyte used was a 1 mol / L solution of LiPF6 (EC:EMC = 4:6V%) (LiPF6: lithium hexafluorophosphate, EC: ethylene carbonate, EMC: ethyl methyl carbonate). The temperature was raised from 25°C to 180°C in 30 minutes using a thermostatic chamber. The resistance was measured at each temperature using an impedance analyzer at a frequency of 200 kHz, and the resistance was measured at 1000 Ω·cm. 2 The first temperature reached was read as the shutdown temperature in the electrolyte. The lower the shutdown temperature in the electrolyte, the higher the safety of the battery configuration, and was evaluated on a 5-point scale as shown below. In this evaluation, a temperature of D or higher was considered a pass. A: The shutdown temperature in the electrolyte is less than 138°C. B: The shutdown temperature of the electrolyte is 138°C or higher and less than 140°C. C: Shutdown temperature in the electrolyte is 140℃ or higher but less than 145℃ D: Shutdown temperature in the electrolyte is 145°C or higher but less than 146°C E: The shutdown temperature in the electrolyte is 146°C or higher.

[0108] In the above measurement, if the MD and TD directions of the polyolefin microporous membrane to be measured are unknown, the tensile breaking strength is determined in the same manner as in (6) for a total of seven directions from 0° to 90°, shifted by 15° from one direction of the film plane as the reference, and the direction with the highest tensile breaking strength is regarded as the MD direction, and the direction perpendicular to the direction with the highest tensile breaking strength is regarded as the TD direction.

[0109] [Example 1] The raw material has a weight average molecular weight (Mw) of 15 x 10 5 Ultra-high molecular weight polyethylene (melting point 136°C, ΔH 130 / ΔHall A polyethylene resin solution was prepared by adding 80 parts by mass of liquid paraffin to 20 parts by mass of ultra-high molecular weight polyethylene (ULHMWPE) using a 33% ethanol (HMWPE) emulsion. Furthermore, 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 ultra-high molecular weight polyethylene, and mixing. The resulting polyethylene resin solution was fed into a twin-screw extruder and kneaded at 180 °C to prepare a polyethylene solution. The twin-screw extruder screw was a Type 1 screw with a distance of 11.5 D from the screw tip to the vent hole, where D is the outermost diameter of the screw, and a length in the feed direction of the material was determined by adding one 0.5 D screw piece, eight 1.0 D screw pieces, and two 1.5 D screw pieces. The resulting polyethylene solution was fed into a T-die, and the extrudate was cooled on a cooling roll controlled at 35 °C to form a gel-like sheet. The resulting gel-like sheet was simultaneously biaxially stretched 5x5 times (MD magnification x TD magnification) using a tenter 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 and re-stretched longitudinally to 1.7 times using a roll stretching method at 100°C, and then re-stretched transversely to 1.6 times using a tenter at 139°C to obtain a polyolefin microporous membrane. The properties of the resulting polyolefin microporous membrane are shown in the table.

[0110] [Example 2] The raw material has a weight average molecular weight (Mw) of 1.5 x 10 6 70 parts by mass of ultra-high molecular weight polyethylene and a weight average molecular weight (Mw) of 1.0 × 10 5A polyethylene mixture was obtained by adding 30 parts by mass of high-density polyethylene (UHMWPE), 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 as antioxidants, based on the mass of the ultra-high molecular weight polyethylene. 75 parts by mass of liquid paraffin was added to 25 parts by mass of the resulting mixture and mixed in a twin-screw extruder at 180 °C to prepare a polyethylene solution. The twin-screw extruder screw was a Type 1 screw with a distance of 11.5 D from the screw tip to the vent hole, where D is the outermost diameter of the screw, and a length in the feed direction of the material was one 0.5 D screw piece, eight 1.0 D screw pieces, and two 1.5 D screw pieces. The resulting polyethylene solution was fed into a T-die, and the extrudate was cooled on a cooling roll controlled at 35 °C to form a gel-like sheet. The resulting gel-like sheet was simultaneously biaxially stretched 5x5 times using a tenter 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 and re-stretched longitudinally to 1.7 times using a roll stretching method, and then re-stretched transversely to 1.7 times using a tenter at 137°C to obtain a polyolefin microporous membrane. The properties of the resulting polyolefin microporous membrane are shown in the table.

[0111] [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.

[0112] [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.

[0113] [Example 5] A polyolefin microporous membrane was obtained in the same manner as in Example 4, except that the raw material composition and production conditions were as shown in the table, and the screw of the twin-screw extruder was changed to a screw (Type 2) in which, when the outermost diameter of the screw is D, the distance from the screw tip to the vent hole was 10.75D, and between them, one screw piece with a length of 0.75D in the raw material conveying direction, ten screw pieces with a length of 1.0D, and no 1.5D screw pieces were used. The properties of the obtained polyolefin microporous membrane are shown in the table.

[0114] [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.

[0115] [Comparative Example 1] The raw material has a weight average molecular weight (Mw) of 2.0 x 10 6 30 parts by mass of ultra-high molecular weight polyethylene and a weight average molecular weight (Mw) of 5.0 × 10 5A polyethylene mixture was obtained by adding 70 parts by mass of high-density polyethylene (UHMWPE), 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 as antioxidants, based on the mass of the ultra-high molecular weight polyethylene. 71.5 parts by mass of liquid paraffin was added to 28.5 parts by mass of the resulting mixture and fed into a twin-screw extruder and kneaded at 180 °C to prepare a polyethylene solution. The twin-screw extruder screw was a Type 1 screw with a vent hole distance of 11.5 D from the screw tip, where D is the outermost diameter of the screw, and a screw configuration consisting of one 0.5 D screw piece, eight 1.0 D screw pieces, and two 1.5 D screw pieces, with lengths in the feed direction, was used. The resulting polyethylene solution was fed into a T-die, and the extrudate was cooled on a cooling roll controlled at 35 °C to form a gel-like sheet. The resulting gel-like sheet was longitudinally stretched using a roll system at a stretching temperature of 120°C to a stretch ratio of 7.5. It was then introduced into a tenter and transversely stretched at a stretching temperature of 120°C to a stretching ratio of 9.0. The stretched membrane was washed in a washing tank of methylene chloride to remove liquid paraffin. The washed membrane was dried and then re-transversely stretched using a tenter system at a temperature of 130°C to a stretching ratio of 1.6 to obtain a polyolefin microporous membrane. The properties of the resulting polyolefin microporous membrane are shown in the table.

[0116] [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.

[0117] [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.

[0118] In Tables 1 and 2, "UHPE" means ultra-high molecular weight polyethylene, and "HDPE" means high-density polyethylene.

[0119] The evaluation results of the obtained polyolefin microporous membrane are shown in Tables 3 and 4.

[0120] [Table 1]

[0121] [Table 2]

[0122] [Table 3]

[0123] [Table 4]

Claims

1. A polyolefin microporous film in which the content of polyethylene with a molecular weight of 3.0 x 10 5 or less is 50% by mass or less and the content of polyethylene with a molecular weight of 9.0 x 10 5 or more is 30% by mass or more, wherein one of the MD orientation parameter value (fMH) and the TD orientation parameter value (fTH) measured at 130°C using a micro-Raman spectrometer and calculated according to the following equations (1) and (2) is greater than 1.5, and the other is 1.5 or less. fMH = I ａ (MD, 130 °C) / I ｂ (MD, 130 °C) ... Equation (1) fTH = I ａ (TD, 130 °C) / I ｂ (TD, 130 °C) ... Equation (2) In addition, I ａ is the Raman shift band 1100-1170 cm －１ the maximum intensity of the Raman band in the range of I ｂ is the Raman shift band 1040-1090 cm －１ the maximum intensity of the Raman band in the range of I ａ (MD, 130℃), I ｂ (MD, 130°C) is the maximum strength in the MD direction measured at 130°C, I ａ (TD, 130℃), I ｂ (TD, 130°C) is the maximum strength in the TD direction measured at 130°C.

2. The polyolefin microporous membrane according to claim 1, wherein the MD orientation parameter (fMH) measured at 130°C using a micro-Raman spectrometer according to equation (1) is greater than 1.

5.

3. 3. The polyolefin microporous membrane according to claim 1, wherein one of the values ​​(fMLH, fTLH) calculated by the following formula (5) or (6) using a micro-Raman spectrometer is 4.0 or more, and the other is less than 4.0: 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 Raman shift band 1100-1170 cm －１ and the maximum intensity in the range of 1200 cm －１ The difference in intensity, D ａ (MD, 130°C) is measured in the MD direction at 130°C, D ａ (TD, 130°C) is measured at 130°C in the TD direction, ａ (MD, 25°C) is measured in the MD direction at 25°C, D ａ (TD, 25°C) is the value measured in the TD direction at 25°C.

4. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein one of the values ​​calculated by the following formula (7) or (8) using a micro-Raman spectrometer is greater than 0.0, and the other is 0.0 or less: fML-fMH...Equation (7) fTL-fTH...Equation (8) Here, fML and fTL are the orientation parameter values ​​in the MD direction (fML) and the TD direction (fTL) measured at 25°C, calculated by the following equations (3) and (4), respectively. ａ is the Raman shift band 1100-1170 cm －１ the maximum intensity of the Raman band in the range of I ｂ is the Raman shift band 1040-1090 cm －１ the maximum intensity of the Raman band in the range of I ａ (MD, 25°C), I ｂ (MD, 25°C) is the maximum strength in the MD direction measured at 25°C, I ａ (TD, 25℃), I ｂ (TD, 25°C) is the maximum strength in the TD direction measured at 25°C. fML = I ａ (MD, 25 °C) / I ｂ (MD, 25 °C) ... Equation (3) fTL = I ａ (TD, 25 °C) / I ｂ (TD, 25 °C) ... Equation (4)

5. 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 a temperature-programmed air permeability test.

6. The polyolefin microporous membrane according to any one of claims 1 to 5, having a tensile break strength in the MD direction of 200 MPa or more.

7. Penetration strength converted to basis weight is 0.75N / (g / m ２ 7. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has a molecular weight of 1.0 or more.

8. The polyolefin microporous membrane according to any one of claims 1 to 7, having an average pore diameter calculated by perm porometry of 36 nm or more and 46 nm or less.

9. The polyolefin microporous membrane according to any one of claims 1 to 8, having a weight average molecular weight of 800,000 or more.

10. The polyolefin microporous membrane according to any one of claims 1 to 9, wherein the main component of the polyolefin microporous membrane is polyethylene.

11. The polyolefin microporous membrane according to any one of claims 1 to 10, obtained by stretching including at least dry sequential biaxial stretching.

12. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 11.

13. A secondary battery using the battery separator according to claim 12.

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