Polyolefin microporous membranes, battery separators, and secondary batteries

Abstract

A microporous polyolefin film in which an orientation parameter value (fMH) of the MD direction and an orientation parameter value (fTH) of the TD direction, measured at 130°C calculated by formulas (1) and (2) using a microscopic Raman spectroscope, are both 1.70 or less. fMH = Ia (MD, 130°C) / Ib (MD, 130°C) <sb / > … formula (1) fTH = Ia (TD, 130°C) / Ib (TD, 130°C) <sb / > … formula (2) Where, Ia is the maximum intensity of the Raman band in a Raman shift band range of 1100-1170 cm-1, Ib is the maximum intensity of the Raman band in a Raman shift band range of 1040-1090 cm-1, Ia (MD, 130°C) and Ib (MD, 130°C) are the maximum intensity of the MD direction measured at 130°C, and Ia (TD, 130°C) and Ib (TD, 130°C) are the maximum intensity of the TD direction measured at 130°C. 　Provided is a microporous polyolefin film having better strength and shrinkage rate than in the prior art.

Description

[Technical Field]

[0001] This invention relates to a polyolefin microporous membrane (also called a porous polyolefin film) that is widely used as a separation membrane for separating and selectively permeating substances, and as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors. In particular, this invention is a polyolefin microporous membrane that is suitably used as a separator for non-aqueous electrolyte secondary batteries such as lithium-ion batteries, and is used as a separator that has higher safety compared to conventional polyolefin microporous membranes. [Background technology]

[0002] Polyolefin microporous membranes are used as filters, fuel cell separators, and capacitor separators. They are particularly well-suited as separators for non-aqueous electrolyte secondary batteries, such as lithium-ion batteries widely used in notebook computers and mobile phones. This is because polyolefin microporous membranes possess excellent mechanical strength, shutdown characteristics, and ion permeability.

[0003] In recent years, lithium-ion secondary batteries have seen increased capacity, primarily driven by the miniaturization of electronic devices and their application in automobiles. Consequently, thinner separators are increasingly required. However, thinning the separator reduces its strength, making it more susceptible to short circuits caused by electrodes or foreign matter (resistance to foreign matter) and film rupture when the battery is subjected to impact (reduced impact resistance), thus compromising battery safety. Therefore, even greater strength is required. Furthermore, in high-energy batteries, even with the separator's shutdown function stopping the electrochemical reaction, the internal temperature continues to rise. This can lead to thermal contraction and rupture of the separator, resulting in a short circuit between the electrodes. Therefore, separators require high strength and low shrinkage at high temperatures.

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

[0005] Patent Document 2 describes a method for improving shutdown temperature and puncture strength, using a polyolefin with a weight molecular weight of 500,000 or more as the main component, and controlling the orientation ratio of the MD direction and TD direction determined by X-ray analysis, resulting in a value of 0.24~0.75 N / (g / m²). 2 A method for obtaining a microporous membrane with a puncture strength and shutdown temperature of 139°C to 146°C has been disclosed.

[0006] Patent Document 3 discloses a method for improving mechanical strength and transmittance, which involves controlling the degree of orientation determined by infrared spectral measurement to obtain a microporous membrane with a puncture strength of 300 to 500 gf when converted to 25 μm. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-95950 [Patent Document 2] Patent No. 6671255 [Patent Document 3] Japanese Patent Publication No. 2013-199545 [Overview of the project] [Problems that the invention aims to solve]

[0008] Separators require high strength and low shrinkage at high temperatures. However, increasing the strength of the separator worsens its shrinkage characteristics at high temperatures. The microporous membranes described in Patent Documents 1 to 3 were insufficient in terms of achieving both thin separators, high strength, and low shrinkage at high temperatures, which are necessary for increasing the capacity of batteries.

[0009] In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having both higher strength and lower shrinkage rate at high temperatures than conventional ones, and a separator using the same.

Means for Solving the Problems

[0010] As a result of intensive studies to achieve the above object, the present inventors have found that a microporous membrane having orientation parameter values in the MD direction and TD direction at high temperatures calculated by microscopic Raman spectroscopy within a specific range solves the above problems and can achieve both high strength and low shrinkage rate at high temperatures, and thus have completed the present invention. That is, the present invention has the following configuration.

[0011] The orientation parameter value (fMH) in the MD direction and the orientation parameter value (fTH) in the TD direction measured at 130 °C calculated by the following formulas (1) and (2) using a microscopic Raman spectrometer are both 0.00 or more and 1.70 or less. The puncture strength, converted to a weight equivalent, is 0.8 N / (g / m²). 2 ) or higher, and the porosity is 48% or less. It is characterized by the following. fMH = I a (MD, 130 °C) / I b (MD, 130 °C) ··· Formula (1) fTH = I a (TD, 130 °C) / I b (TD, 130 °C) ··· Formula (2) Note that I a is the maximum intensity of the Raman band in the range of the Raman shift band of 1100 to 1170 cm -1 , I b is the maximum intensity of the Raman band in the range of the Raman shift band of 1040 to 1090 cm -1 , I a (MD, 130 °C), I b (MD, 130 °C) is the maximum intensity in the MD direction measured at 130 °C, I a (TD, 130 °C), I b (TD, 130 °C) is the maximum intensity in the TD direction measured at 130 °C.

Effects of the Invention

[0012] According to the present invention, a highly safe polyolefin microporous membrane can be obtained that achieves both high strength and low shrinkage rate at high temperatures. [Modes for carrying out the invention]

[0013] The polyolefin microporous membrane according to the embodiment of the present invention is useful as a separator for batteries due to its excellent strength and shrinkage rate, and possesses excellent safety. The present invention can be realized by satisfying the orientation parameters at high temperatures in the MD direction and TD direction calculated by micro-Raman spectroscopy within the range described later, and it has been found that this leads to achieving both strength and shrinkage rate, which were previously in a trade-off relationship. Note that the present invention is not limited to the embodiments described below, and the direction parallel to the direction in which the polyolefin microporous membrane is formed is referred to as the film formation direction, longitudinal direction, or MD direction, and the direction perpendicular to the film formation 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 film according to an embodiment of the present invention is characterized in that the orientation parameter value in the MD direction (fMH) and the orientation parameter value in the TD direction (fTH), measured at 130°C by the method described later, are both 1.70 or less. The orientation parameter is an index that indicates the degree of orientation of crystalline molecular chains, calculated by Raman spectroscopy, and the higher this value, the more highly oriented the crystalline molecular chains are. When fMH and fTH are 0.00 or higher, it means that the film has a strong structure that maintains its orientation state even at high temperatures in both the MD and TD directions, resulting in excellent strength. From the viewpoint of strength, fMH and fTH are 0.00 or higher, preferably 0.50 or higher, more preferably 0.90 or higher, even more preferably 1.00 or higher, and particularly preferably 1.10 or higher. However, if fMH and fTH are too high, it leads to a deterioration of the shrinkage rate due to relaxation of the crystal structure at high temperatures. Therefore, fMH and fTH are 1.70 or less, preferably 1.50 or less, and more preferably 1.20 or less. From the perspective of balancing strength and shrinkage rate, it is important that both fMH and fTH are 1.70 or less. By satisfying the above range, high strength and low shrinkage rate at high temperatures can be achieved simultaneously. Note that the above range can be controlled by the raw material design and manufacturing method described later. In order to form a strong structure that maintains its orientation at high temperatures, a weight-average molecular weight of 0.8 × 10⁻⁶ with a long relaxation time is desirable. 6 It is preferable to manufacture a film using the above polyolefin resin as the main raw material, forming a highly oriented structure by wet sequential stretching, and then performing dry re-stretching at high temperature after washing and drying. From the above viewpoint, in the molecular weight distribution of the polyolefin microporous film, a molecular weight of 0.9 × 10⁻⁶ with a long relaxation time is preferable. 6 The above components are present in the polyolefin microporous membrane at a concentration of 30% by mass or more, and have a short relaxation time and a molecular weight of 0.3 × 10⁻⁶. 6 It is even more preferable that the following components are included in an amount of less than 50% by mass. This results in a highly oriented structure with minimal orientation change even at high temperatures, and a highly safe polyolefin microporous membrane that achieves both high strength and low shrinkage at high temperatures.

[0016] In the polyolefin microporous membrane according to the embodiment of the present invention, it is preferable that the orientation parameter value in the MD direction (fML) and the orientation parameter value in the TD direction (fTL), measured at 25°C by the method described later, are both 1.70 or less. From the viewpoint of strength, higher fML and fTL are preferable, but if the highly oriented structure increases when measured at 25°C, the shrinkage rate increases due to the relaxation of molecular orientation at high temperatures. From the viewpoint of suppressing shrinkage rate, fML and fTL are preferably 1.50 or less, and more preferably 1.30 or less. The above fML and fTL are orientation parameters calculated by the following equations (3) and (4), and I a This corresponds to a Raman shift bandwidth of 1100-1170 cm². -1 Maximum intensity within the range, I b This corresponds to a Raman shift bandwidth of 1040-1090 cm². -1 The maximum intensity of the Raman band in the range, I a (MD, 25℃) and I b (MD, 25℃) is the measurement of the MD direction of a polyolefin microporous membrane at 25℃. a (TD, 25℃) and I b (TD, 25°C) is the value measured at 25°C in the TD direction of the polyolefin microporous membrane. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods described later. fML = I a (MD, 25℃) / I b (MD, 25℃) ...Equation (3) fTL = I a (TD, 25℃) / I b (TD, 25℃) ...Equation (4).

[0017] The polyolefin microporous membrane according to the embodiment of the present invention is measured by the method described later. a (MD, 25℃) and D a The ratio (fMLH) and D (MD, 130℃) a (TD, 25℃) and D aThe ratio (fTLH) of (TD, 130°C) is preferably 4.00 or less, more preferably 3.00 or less, even more preferably 2.50 or less, even more preferably 2.00 or less, and particularly preferably 1.50 or less. When it is 4.00 or less, the CC stretching vibration of the polyethylene molecular chains in the crystalline phase is maintained at 130°C, that is, the molecular chain structure of the crystal is highly maintained even at 130°C, and high strength is obtained. a This corresponds to a Raman shift bandwidth of 1100-1170 cm². -1 Maximum strength within the range and 1200cm -1 The difference in intensity, D a (MD, 130°C) indicates the measurement of the MD direction of the polyolefin microporous membrane at 130°C. a (TD, 130°C) is the measurement of the TD direction of the polyolefin microporous membrane at 130°C. a (MD, 25℃) indicates the measurement of the MD direction of the polyolefin microporous membrane at 25℃. a (TD, 25°C) is the value measured at 25°C in the TD direction of the polyolefin microporous membrane. 1130 cm -1 This band is attributed to the CC stretching vibration of polyethylene molecular chains in the crystalline phase, and the direction of the Raman tensor of the vibration corresponds to the molecular chain axis. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods described later. fMLH=D a (MD, 25℃) / D a (MD, 130℃))...Equation (5) fTLH=D a (TD, 25℃) / D a (TD, 130℃))...Equation (6).

[0018] When the crystal structure relaxes and melts at high temperatures, the orientation parameter measured at 130°C decreases compared to the orientation parameter measured at 25°C (orientation parameter measured at 25°C > orientation parameter measured at 130°C). Furthermore, in samples with high melting points, recrystallization occurs at 130°C, and the orientation parameter measured at 130°C may increase compared to the orientation parameter measured at 25°C. Therefore, the smaller the difference between the orientation parameters at 25°C and 130°C, and the closer the change is to zero, the better the crystal structure retention at high temperatures and the reduction in shrinkage rate. It is particularly preferable that the change in orientation parameters between 25°C and 130°C, calculated by micro-Raman spectroscopy, is small, and it is preferable that both the difference between fML and fMH (fML-fMH...(7)) and the difference between fTL and fTH (fTL-fTH...(8)) are 0.50 or less. More preferably, they are 0.40, and even more preferably 0.20 or less. The lower limits for the difference between fML and fMH (fML-fMH) and the difference between fTL and fTL (fTL-fTH) are -0.50 or greater, preferably -0.20 or greater, more preferably -0.10 or greater, and even more preferably 0.00 or greater. When both the difference between fML and fMH (fML-fMH) and the difference between fTL and fTH (fTL-fTH) are 0.50 or less, the relaxation of the crystal structure at 130°C is suppressed, and good shrinkage characteristics are obtained. When it is -0.50 or greater, the melting of the crystal structure at high temperatures is suppressed, and excellent strength is obtained. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods described later.

[0019] When the orientation parameters fMH, fTH, fML, and fTL satisfy the above range, particularly excellent strength and shrinkage characteristics are obtained, and this range can be controlled by the raw material design and manufacturing method described later. In order to form a strong structure that maintains the orientation state at high temperatures, a weight-average molecular weight of 0.8 × 10⁻⁶ is used, which has a long relaxation time. 6 It is preferable to manufacture a film using the above polyolefin resin as the main raw material, forming a highly oriented structure by wet sequential stretching, and then performing a dry re-stretching process at high temperature after washing and drying. From the above viewpoint, in the molecular weight distribution of the polyolefin microporous film, a molecular weight of 0.9 × 10⁻⁶ with a long relaxation time is preferable. 6The above components are present in the polyolefin microporous membrane at a concentration of 30% by mass or more, and have a short relaxation time and a molecular weight of 0.3 × 10⁻⁶. 6 The following components are preferably included in an amount of less than 50% by mass. This results in a highly oriented structure with minimal orientation change even at high temperatures, and a highly safe polyolefin microporous membrane that achieves both high strength and low shrinkage at high temperatures.

[0020] The porosity of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more, from the viewpoint of permeability and electrolyte content. When the porosity is 30% or more, the balance between permeability, strength and electrolytic solution content is improved, and non-uniformity of the battery reaction is eliminated. As a result, dendrite formation is suppressed, and it can be used without impairing the performance of conventional batteries, making it suitable for use as a separator for secondary batteries. Furthermore, while good output characteristics can be obtained by increasing the porosity, the safety of the battery is reduced due to a decrease in puncture strength and an increase in shrinkage rate. For this reason, the porosity is preferably 50% or less, and more preferably 48% or less.

[0021] The puncture strength of a polyolefin microporous film is preferable as it affects safety, such as suppressing short circuits caused by foreign matter in the battery. The puncture strength of a polyolefin microporous film, converted to a film thickness of 10 μm, is preferably 2.5 N or higher, more preferably 3.0 N or higher, even more preferably 4.0 N or higher, even more preferably 4.3 N or higher, and particularly preferably 5.0 N or higher. Furthermore, the puncture strength per unit basis weight (puncture strength converted to basis weight), an index indicating the strength of the film, normalized by the amount of resin, is 0.7 N / (g / m²). 2 Preferably, it is 0.8 N / (g / m³). 2 ) or more is more preferable, and 0.9 N / (g / m 2The above is particularly preferable. When the puncture strength is within the above range, short circuits due to foreign matter are suppressed and good battery safety is obtained. To improve puncture strength, it is preferable to use ultra-high molecular weight polyolefin as the main component in the raw material formulation and increase the strength by increasing the number of tie molecules that connect the lamellar crystals. Furthermore, from the viewpoint of suppressing the decrease in porosity due to melting in the heat setting process, it is preferable to use ultra-high molecular weight polyolefin which has few low molecular weight components and a sharp molecular weight distribution. Puncture strength can be achieved by setting the above fMH, fTH, fML, and fTL to specific ranges and adopting raw materials, molecular weights, resin concentrations, and drawing methods within the ranges described later.

[0022] Note that the puncture strength when the film thickness is converted to 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 T1 (μm) is L1 (N). The basis weight equivalent puncture strength is the measured puncture strength (L1) converted to basis weight G (g / m²). 2 This is the value obtained by dividing by ), and is calculated using the formula: L1 / G.

[0023] From the viewpoint of suppressing short circuits in the battery due to abnormal heat generation, the sum of the shrinkage rates in the MD direction and TD direction at 130°C / 1h is preferably 30% or less, more preferably 29% or less, even more preferably 28% or less, even more preferably 27% or less, and particularly preferably 25% or less. Furthermore, when a high heat-resistant coating layer such as aramid or polyimide is applied, the sum of the shrinkage rates in the MD direction and TD direction at 130°C / 1h is preferably 33% or less, more preferably 31% or less, even more preferably 30% or less, even more preferably 29% or less, and particularly preferably 28% or less. When the shrinkage rate is within this range, dimensional changes are minimal when the internal temperature of the battery rises, and insulation can be maintained, thus preventing the expansion of internal short circuits and minimizing their impact, resulting in high safety. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods described later.

[0024] In batteries, tension is applied in the MD direction, so if the shrinkage rate in the MD direction is high, the film will rupture and lead to a short circuit. Therefore, the polyolefin microporous film according to the embodiment of the present invention preferably has a shrinkage rate in the MD direction of 15% or less at 130°C / 1h, more preferably 12% or less, even more preferably 11% or less, and even more preferably 10% or less. When the shrinkage rate in the MD direction is within this range, dimensional changes are minimal when the internal temperature of the battery rises, insulation can be maintained, and high safety can be obtained. The above range can be achieved by applying the raw materials, molecular weights, and manufacturing methods described later.

[0025] Furthermore, the polyolefin microporous membrane according to the embodiment of the present invention preferably has a shrinkage rate in the TD direction of 30% or less at 130°C / 1h, more preferably 25% or less, and even more preferably 20% 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 when localized abnormal heat generation occurs, thereby maintaining safety. The above thermal shrinkage rates can be achieved by setting the fMH, fTH, fML, and fTL to specific ranges and employing the raw materials, resin concentrations, and stretching methods described later. The shrinkage rates in the MD and TD directions at 130°C / 1h can be measured by the method described in the examples.

[0026] In the polyolefin microporous membrane of the present invention, the tensile breaking strength in the MD direction (tensile breaking strength in the MD direction; hereinafter also simply referred to as "MD tensile strength") is preferably 200 MPa or more, more preferably 250 MPa or more, and even more preferably 280 MPa or more, from the viewpoint of suppressing film rupture during the battery winding process and preventing short circuits due to foreign matter in the battery.

[0027] From the perspective of balancing 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 100 MPa or more, preferably 160 MPa or more, more preferably 190 MPa or more, and even more preferably 200 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, which suppresses wrinkles and sagging of the film, and also prevents short circuits caused by the film tearing due to foreign matter inside the battery, thereby improving safety. The above tensile strength can be achieved by adopting the raw materials, resin concentration, and stretching method described later.

[0028] Furthermore, the tensile (breaking) elongation in the MD and TD directions (hereinafter also simply referred to as "MD elongation" and "TD elongation") is preferably 50% or more, more preferably 60% or more, even more preferably 90% or more, even more preferably 120% or more, and particularly preferably 150% or more. An MD elongation or TD elongation of 50% or more is preferable because it suppresses short circuits caused by winding or foreign matter inside the battery, resulting in good safety. In addition, both the MD elongation and TD elongation are preferably 200% or less, and more preferably 170% or less. An MD elongation and TD elongation of 200% or less allows for a balance between strength and elongation. The tensile strength and tensile elongation in the MD and TD directions can be measured by the method described in the examples.

[0029] In the polyolefin microporous membrane according to the embodiment of the present invention, the air permeability refers to the value measured in accordance with JIS P 8117 (2009). In this specification, unless otherwise specified regarding film thickness, the term "air permeability" is used to mean "air permeability when the film thickness is 10 μm." The air permeability (Gurley value) measured in a polyolefin microporous membrane with film thickness T1 (μm) is p1 (sec / 100cm 3 When this is the case, the air permeability p2 (sec / 100cm) is calculated by the formula: p2 = (p1 × 10) / T1. 3 ) is defined as the air permeability when the film thickness is 10 μm.

[0030] Air permeability is 200 sec / 100 cm 3Preferably, the following: 130 sec / 100 cm 3 The following is more preferable: 110 sec / 100 cm 3 It is even more preferable that the following conditions are met: Air permeability of 200 sec / 100 cm 3 The following conditions allow for good ion permeability and a reduction in electrical resistance.

[0031] In the polyolefin microporous membrane according to the embodiment of the present invention, resistance increases with increasing film thickness, and the output characteristics of the battery decrease. From the viewpoint of battery output characteristics, the film thickness is preferably 12 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. As the film thickness decreases, the strength decreases and safety decreases; therefore, from the viewpoint of safety, the film thickness is preferably 1 μm or more, and more preferably 3 μm or more.

[0032] The shutdown temperature is the temperature at which, when a polyolefin microporous membrane is heated, the resin portion shrinks and melts, closing the pores and stopping discharge and charging. This temperature is measured by the method described later. Electrodes used in high-energy-density lithium-ion secondary batteries tend to have reduced thermal stability, so it is preferable for the battery to shut down (close pores) quickly after a short circuit. The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is 143°C or lower. Preferably, it is 141°C or lower, more preferably 140°C or lower, and even more preferably 139°C or lower. The microporous membrane obtained by the present invention has excellent short-circuit resistance and the above-mentioned shutdown temperature, thus providing excellent battery safety. To set the shutdown temperature within the above range, it is preferable to set the raw material composition of the microporous membrane within the range described later.

[0033] [2] Polyolefin resin The resin raw material in the polyolefin microporous membrane according to the embodiments of the present invention may be a single composition, a composition combining a main raw material and a secondary raw material, or a polyolefin resin mixture (polyolefin resin composition) consisting of two or more polyolefin resins. The raw material form in the polyolefin microporous membrane is preferably a polyolefin resin, and examples of polyolefin resins include polyethylene and polypropylene, with a single composition being more preferable.

[0034] The polyolefin resin is preferably a homopolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc., and a homopolymer of ethylene (polyethylene) is particularly preferred. Polyethylene may also be a copolymer containing an ethylene homopolymer and other α-olefins.

[0035] Other α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, or alkenes with more than 1 carbon atoms, vinyl acetate, methyl methacrylate, and styrene.

[0036] The preferred type of polyolefin resin to use is polyethylene, with a density of 0.94 g / cm³. 3 High-density polyethylene exceeding 0.93-0.94 g / cm³ 3 Medium-density polyethylene in the range of 0.93 g / cm³ 3 Examples include even lower-density polyethylene and linear low-density polyethylene.

[0037] Furthermore, from the viewpoint of film strength and shrinkage balance, it is preferable to use ultra-high molecular weight polyolefins alone or as the main component in polyolefin resins, and ultra-high molecular weight polyolefins have a weight-average molecular weight of 8.0 × 10⁶. 5 That is all, 9.0 × 10 5 The above is preferable, 10 x 10 5 The above is more preferable, 15 × 10 5 The above is particularly preferred, and from the viewpoint of moldability, 100 × 10 5The following is preferable. It is important that auxiliary materials are added in an amount that does not impair the fibril structure formed by the main material or the moldability.

[0038] Weight-average molecular weight is 8.0 × 10⁻⁶ 5 If the above conditions are met, the longer relaxation time improves the retention of the crystal structure at high temperatures, suppressing melting and shrinkage. Furthermore, it maintains its orientation even at high temperatures, resulting in a strong structure that balances strength and shrinkage, thus improving battery safety. Additionally, the weight-average molecular weight is 9.0 × 10⁻⁶. 5 By using the above-mentioned polyolefin resins, the number of tie molecules increases, making it easier to obtain high strength. In addition, the melting of fibrils during the stretching and heat-setting processes is suppressed, resulting in good power characteristics. Furthermore, by lowering the relaxation rate of the resin, the heat-setting temperature can be increased, resulting in good shrinkage characteristics, and improving the trade-off relationship between ionic resistance, strength, and shrinkage.

[0039] From the above perspective, the weight-average molecular weight (Mw) of the polyolefin microporous membrane is 8.0 × 10⁻⁶. 5 The above is preferable, 9.0 × 10 5 The above is more preferable, 10 × 10 5 The above is even more preferable, and it is particularly preferable that the molecular weight of the raw material is maintained. In order to form a strong structure that maintains the orientation state at high temperatures, a long relaxation time of 9.0 × 10 in the molecular weight distribution of the polyolefin microporous membrane is preferred. 5 Preferably, the above components are present in the polyolefin microporous membrane at a concentration of 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 most preferably 40% by mass or more. From the viewpoint of maintaining the crystalline structure at high temperatures and suppressing melting during stretching and heat-fixing processes, a molecular weight of 3.0 × 10⁻⁶ is desirable. 5 The content of the following components is preferably less than 50% by mass, more preferably 45% by mass or less, even more preferably 40% by mass or less, and even more preferably 35% by mass or less. For low shutdown temperatures, the molecular weight should be 3.0 × 10 5The content of the following components 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. In order to obtain the molecular weight of the above polyolefin microporous film, it is preferable to produce the film by adding the antioxidant described later, kneading under a nitrogen atmosphere, or a combination of adding the antioxidant and kneading under a nitrogen atmosphere to the above raw material formulation.

[0040] 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. A narrower molecular weight distribution is preferable because it leads to a more unified system and easier acquisition of uniform micropores, but a narrower distribution reduces moldability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or higher, more preferably 5.0 or higher, and even more preferably 6.0 or higher. As the molecular weight distribution increases, the amount of low molecular weight components increases, leading to a decrease in strength and making it easier for fine fibrils to melt and fuse during stretching and heat fixing. Therefore, the upper limit is preferably 80 or lower, more preferably 50 or lower, even more preferably 20 or lower, and particularly preferably 10 or lower. By setting the molecular weight within the above range, good moldability can be obtained, and uniform micropores can be obtained because the system is unified.

[0041] In the manufacturing process of a polyolefin microporous film according to an embodiment of the present invention, it is preferable to add a plasticizer for the purpose of improving moldability. The mixing ratio of polyolefin resin and plasticizer may be adjusted as appropriate within a range that does not impair moldability, but it is preferable that the proportion of polyolefin resin is 10 to 50% by mass, with the total of polyolefin resin and plasticizer being 100% by mass. When the proportion of polyolefin resin is 10% by mass or more (the proportion of plasticizer is 90% by mass or less), swells and neck-in at the exit of the die can be suppressed when forming into a sheet, improving the moldability and film-forming properties of the sheet. When the proportion of polyolefin resin is 50% by mass or less (the proportion of plasticizer is 50% by mass or more), the pressure rise in the film-forming process can be suppressed, and good moldability can be obtained. When the total of polyolefin resin and plasticizer is 100% by mass, the proportion of polyolefin resin is preferably 10% by mass or more, and more preferably 20% by mass or more.

[0042] When using a polyolefin resin with a weight-average molecular weight (Mw) of 900,000 or more as the main component or alone, from the viewpoint of pressure and tensile stress in the film-forming process, 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 most preferably less than 25% by mass, with the total of the polyolefin resin and plasticizer being 100% by mass.

[0043] The weight-average molecular weight (Mw) of the high-density polyethylene used in the embodiments of the present invention, separate from ultra-high molecular weight polyethylene, obtained by high-temperature gel permeation chromatography (GPC) measurement, is 1 × 10⁻⁶. 5 The following is preferable: When Mw is within the above range, it is less likely to inhibit the structure formed by the ultra-high molecular weight polyethylene, making it easier to form low-melting-point crystals and reduce shrinkage force during melting. This makes it possible to achieve a balance between mechanical strength, shrinkage, and shutdown characteristics.

[0044] The melting point (°C) of the high-density polyethylene used in the embodiments of the present invention, as determined by differential scanning calorimeter (DSC), is preferably 132°C or lower. More preferably, the melting point of the high-density polyethylene is 127°C or higher, even more preferably 130°C or higher, and particularly preferably 131°C or higher. When the melting point of the high-density polyethylene is within the above range, the melting point of the pre-stretched structure can be lowered within an appropriate range, which makes it less likely to inhibit the structure formed by the ultra-high molecular weight polyethylene when it is made into a polyolefin microporous membrane, and makes it easier to form low-melting-point crystals. This makes it possible to improve the shutdown characteristics of the polyolefin microporous membrane.

[0045] The low molecular weight polyethylene used in the embodiments of the present invention preferably has a heat of fusion ΔH (J / g) of 200 J / g or more, more preferably 210 J / g or more, and even more preferably 220 J / g or more, obtained from a differential scanning calorimeter (DSC). The upper limit of ΔH is not particularly limited, but due to the properties of polyethylene, it is typically 260 J / g or less. When ΔH is within the above range, it is easier to form low-melting-point crystals without excessively reducing the amount of crystals in the polyolefin microporous film. This makes it possible to achieve both shutdown characteristics and permeability.

[0046] Furthermore, the polyolefin microporous membrane according to the embodiments of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, and even blocking inhibitors and fillers, to the extent that they do not impair the effects of the present invention. In particular, it is preferable to add antioxidants in order to suppress oxidative degradation of the polyolefin resin due to its thermal history.

[0047] As an antioxidant, it is preferable to use one or more selected from, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox"® 1330: molecular weight 775.2), tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7), etc.

[0048] The properties of the polyolefin microporous membrane can be adjusted or enhanced by appropriately selecting the type and amount of antioxidants and heat stabilizers. It is preferable to use an amount of antioxidant that does not increase the MFR of the gel-like sheet, as measured by the method described in JIS K7210-1 (2014). The amount of antioxidant added is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, 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, relative to the amount of resin. From the viewpoint of film formation properties such as preventing eye discharge and streaks, the upper limit should be 3.0% by mass or less, and it is particularly preferable to suppress oxidative degradation by combining the addition of antioxidants and kneading under a nitrogen atmosphere.

[0049] Furthermore, the layer structure of the polyolefin microporous film according to the embodiment of the present invention may be single-layer or laminated, and laminated is preferred from the viewpoint of balancing physical properties. When using a laminated layer of the above-described polyolefin resin formulation, it is preferable that the above layer is contained in 50% by mass or more of the total film thickness.

[0050] [3] Method for producing polyolefin microporous membrane Next, a 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 has the following steps (a) to (e).

[0051] (a) A step of melt-kneading a polymer material containing one or more types of polyolefin resins and a solvent as necessary to prepare a polyolefin resin solution. (b) The process of extruding the obtained molten mixture, forming it into a sheet, and cooling and solidifying it. (c) A step of stretching the obtained sheet by a sequential stretching method including a roll method or a tenter method. (d) The process of extracting a plasticizer from the stretched film and drying the film. (e) A process of performing heat treatment / re-stretching by a stretching method including a roll method or a tenter method.

[0052] In particular, it is especially preferable that in step (a), an antioxidant in the amount described later is added and the mixture is kneaded under a nitrogen atmosphere in order to prevent a decrease in molecular weight, (c) wet sequential stretching is performed in the longitudinal and transverse directions, and (e) heat treatment / re-stretching is carried out at a temperature of 130°C or higher using a tenter method.

[0053] The following describes each step.

[0054] (a) Preparation of polyolefin resin solution The above polymer material is heated and dissolved in a plasticizer to prepare a polyolefin resin solution. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, but it is preferable that the solvent is liquid at room temperature in order to enable relatively high-magnification stretching.

[0055] Examples of solvents include aliphatic, cyclic aliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate.

[0056] As a liquid solvent, it is preferable to use a non-volatile liquid solvent such as liquid paraffin in order to obtain a stable gel-like sheet.

[0057] While the solvent is miscible with the polyolefin resin in a molten and kneaded state, a solid solvent 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.

[0058] The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40°C. A viscosity of 20 cSt or higher at 40°C reduces the likelihood of the sheet extruded from the die containing the polyolefin resin solution becoming non-uniform. On the other hand, a viscosity of 200 cSt or lower at 40°C facilitates the removal of the liquid solvent. The viscosity of the liquid solvent is measured at 40°C using an Ubbelohde viscometer.

[0059] (b) Formation of extruded material and formation of gel sheet The method for uniformly melting and kneading the polyolefin resin solution is not particularly limited, but when a high-concentration polyolefin resin solution is to be prepared, it is preferable to do so 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.

[0060] In the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin completely melts. The melt-mixing temperature varies depending on the polyolefin resin used, but it is preferably between (melting point of polyolefin resin + 10°C) and (melting point of polyolefin resin + 120°C). More preferably, it is between (melting point of polyolefin resin + 20°C) and (melting point of polyolefin resin + 100°C).

[0061] Here, the melting point refers to the value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987) (the same applies hereinafter). For example, when the polyolefin resin is polyethylene, the melting and kneading temperature of the polyethylene resin is preferably in the range of 140 to 250°C. More preferably it is 150 to 230°C, and particularly preferably 150 to 200°C. Specifically, since polyethylene compositions have a melting point of about 130 to 140°C, the melting and kneading temperature is preferably 140 to 250°C.

[0062] From the viewpoint of suppressing the degradation of polyolefin resin, a lower melt-mixing temperature is preferable. However, if the temperature is lower than the above-mentioned temperature, unmelted material may be generated in the extruded product from the die, which may cause film rupture or other problems in the subsequent stretching process. Conversely, if the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin resin becomes more severe, which may worsen the physical properties of the resulting polyolefin microporous film, such as strength and porosity. Furthermore, decomposition products may precipitate on chill rolls or rolls in the stretching process and adhere to the sheet, leading to deterioration of appearance. Therefore, it is preferable to mix the polyolefin resin within the above-mentioned range. In addition, the smaller the Q / Ns value, 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 more kneadable the resin becomes, resulting in a more uniform solution. However, a decrease in Q / Ns leads to greater shear heat generation, accelerating the degradation of the resin, and preventing the acquisition of molecular weight components within the above-mentioned range in the film. Low molecular weight components accumulate in the bleed-out plasticizer and adhere to the sheet, worsening the appearance. A high Q / Ns value suppresses resin degradation, but it results in insufficient kneadability, making it impossible to obtain a uniform solution. Therefore, it is particularly preferable to appropriately adjust the Q / Ns according to the molecular weight and solubility of the resin used, and to suppress oxidative degradation by combining the addition of antioxidants and kneading under a nitrogen atmosphere.

[0063] Next, a gel-like sheet is obtained by cooling the resulting extruded material. Cooling allows for the immobilization of the microphase of the polyolefin resin separated by the solvent. It is preferable to cool the gel-like sheet to 10-50°C during the cooling process. This is to ensure that the final cooling temperature is below the crystallization completion temperature, and by refining the higher-order structure, uniform stretching becomes easier during subsequent stretching. Therefore, it is preferable to cool at a rate of 30°C / min or more until the temperature is at least below the gelation temperature.

[0064] Generally, a slow cooling rate leads to the formation of relatively large crystals, resulting in a coarser higher-order structure of the gel-like sheet and a larger gel structure. Conversely, a fast cooling rate results in the formation of small, uniform crystals, leading to a denser higher-order structure of the gel-like sheet and enabling uniform stretching.

[0065] Cooling methods include direct contact with cold air, cooling water, or other cooling media; contact with rolls cooled by a refrigerant; and the use of casting drums, etc.

[0066] While the case of a single-layer polyolefin microporous membrane has been described so far, the polyolefin microporous membrane according to the embodiments of the present invention is not limited to a single layer, but may be a laminate. There is no particular limit to the number of layers; it may be a two-layer laminate or a laminate of three or more layers. In addition to the polyolefin resin described above, the laminated portion may contain other desired resins to an extent that does not impair the effects of the present invention.

[0067] Conventional methods can be used to form a laminate of polyolefin microporous membranes. For example, one method involves preparing the desired resins as needed, supplying these resins separately to an extruder and melting them at the desired temperature, then combining them in a polymer tube or die, and finally extruding them through a slit-shaped die to the desired thickness of each laminate to form the laminate.

[0068] (c) Stretching process The resulting gel-like sheet (including laminated sheets) is stretched. Methods of stretching include uniaxial stretching in the sheet transport direction (MD direction) using rolling or a roll stretcher, uniaxial stretching in the sheet width direction (TD direction) using a tenter, sequential biaxial stretching using a combination of a roll stretcher and a tenter, or two tenters, and simultaneous biaxial stretching using a simultaneous biaxial tenter. However, sequential biaxial stretching is preferred from the viewpoint of controlling orientation in the MD and TD directions.

[0069] From the perspective of fMLH and fTLH, uniaxial stretching in the sheet transport direction (MD direction) by a roll stretcher is preferably performed with a pressure of 0.1 MPa or higher between the stretching roll and the nip roll. By forming the film within this range, the crystalline molecular chains can be more oriented. If the film is formed with a pressure of less than 0.1 MPa between the stretching roll and the nip roll, slippage occurs on the rolls, making it difficult to apply stretching stress, and in some cases, the crystalline molecular chains cannot be sufficiently oriented.

[0070] 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, but it is preferable to stretch it to 5 times or more in either direction, more preferably to 6 times or more in both the MD and TD directions from the viewpoint of orientation control, and from the viewpoint of maintaining the crystal structure at high temperatures, the stretching ratio in the MD direction is preferably 7 times or more, the area ratio is preferably 40 times or more, more preferably 45 times or more, and even more preferably 50 times or more.

[0071] The stretching temperature is preferably 10°C or less above the melting point of the gel-like sheet, and more preferably in the range of (crystal dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5°C). Specifically, in the case of polyethylene compositions, the crystal dispersion temperature is about 90 to 110°C, so the stretching temperature is preferably 100 to 130°C, and more preferably 110 to 120°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 above upper limit, molecular relaxation is promoted, making it impossible to sufficiently orient the molecular chains by stretching. When the stretching temperature is within the above range, film breakage due to stretching of the polyolefin resin is suppressed, and the crystalline molecular chains can be more oriented while enabling high-magnification stretching.

[0072] The stretching described above causes cleavage of the higher-order structure of the gel sheet, refines the crystalline phase, and forms a fibril structure oriented in the stretching direction. As a result, a microporous film is obtained that maintains excellent crystalline structure retention even at high temperatures, forming a structure that maintains its orientation and achieving both excellent strength and resistance to high-temperature shrinkage. Therefore, the polyolefin microporous film according to the embodiment of the present invention is suitable for battery separators, and the polyolefin microporous film of this application enables a significant improvement in battery safety compared to conventional technology.

[0073] (d) Plasticizer extraction (washing) and drying process Next, the plasticizer (solvent) remaining in the gel-like sheet is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a polyolefin microporous film is obtained by removing the solvent.

[0074] Examples of cleaning solvents include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain-like fluorocarbons such as trifluorinated ethane.

[0075] These cleaning solvents have low surface tension (e.g., 24 mN / m or less at 25°C). By using cleaning solvents with low surface tension, the shrinkage of the microporous network structure due to surface tension at the gas-liquid interface during drying after cleaning is suppressed, resulting in a polyolefin microporous film with excellent porosity and permeability. These cleaning solvents are appropriately selected depending on the plasticizer and used individually or in mixtures.

[0076] Cleaning methods include immersing the gel sheet in a cleaning solvent for extraction, showering the gel sheet with the cleaning solvent, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but generally, it is preferable to use 300 parts by mass or more per 100 parts by mass of the gel sheet.

[0077] The washing temperature should be 15-30°C, and may be heated to 80°C or below if necessary. At this time, from the viewpoint of enhancing the cleaning effect of the washing solvent, ensuring that the physical properties of the resulting polyolefin microporous membrane (e.g., physical properties in the TD direction and / or MD direction) are not non-uniform, and improving the mechanical and electrical properties of the polyolefin microporous membrane, the longer the time the gel sheet is immersed in the washing solvent, the better.

[0078] The cleaning described above is preferably carried out until the residual solvent in the gel-like sheet, i.e., the polyolefin microporous membrane, after cleaning is less than 1% by mass.

[0079] Subsequently, the solvent in the polyolefin microporous film is dried and removed in a drying process. There are no particular limitations on the drying method, and methods such as using a metal heating roll or using hot air can be selected. The drying temperature is preferably 40 to 100°C, and more preferably 40 to 80°C. If drying is insufficient, the porosity of the polyolefin microporous film will decrease in subsequent heat treatment, and the permeability will deteriorate.

[0080] (e) Heat treatment / re-stretching process The dried polyolefin microporous membrane may be stretched (re-stretched) in at least one axial direction. Re-stretching can be performed by heating the polyolefin microporous membrane and using the Tenter method or the like, similar to the stretching described above. Re-stretching may be uniaxial or biaxial. In the case of multi-stage stretching, it can be performed by combining simultaneous biaxial or sequential stretching.

[0081] The re-drawing temperature is preferably below the melting point of the polyolefin resin composition, and 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-drawing temperature is preferably 70 to 140°C, more preferably 110 to 140°C, even more preferably 120 to 140°C, and still more preferably 130 to 140°C. 135 to 140°C is still more preferably.

[0082] In particular, the polyolefin microporous membrane according to the embodiment of the present invention has a long relaxation time and a weight-average molecular weight of 0.9 × 10 6 By using polyethylene as the main raw material, we discovered that stretching and heat fixing are possible at high temperatures above 130°C. By stretching at the magnification described later within the above temperature range, the orientation relaxation of polyolefin molecular chains is suppressed, resulting in a highly oriented structure and the formation of a thermally stable structure. The resulting microporous membrane has a high orientation parameter at 130°C, a small difference in orientation parameters between 25°C and 130°C, and exhibits both good basis weight puncture strength and thermal shrinkage characteristics.

[0083] Weight-average molecular weight is 0.9 × 10 6 If the weight-average molecular weight is less than 0.9 × 10, the relaxation time is short, and heat treatment at temperatures above 130°C will lead to a decrease in porosity. In contrast, if the weight-average molecular weight is 0.9 × 10, 6 Because the polyethylene described above has a long relaxation time, it can suppress the decrease in porosity even when stretching and heat-fixing are performed at temperatures above 130°C, and because heat-fixing can be performed at high temperatures, orientation relaxation at high temperatures is suppressed, and a highly oriented structure can be obtained. Therefore, the weight-average molecular weight is 0.9 × 10⁻⁶. 6 It is preferable to use the above polyethylene and perform heat fixation at a temperature higher than 130°C.

[0084] For uniaxial stretching, the re-stretching ratio is preferably 1.01 to 3.0 times, with a particularly preferred ratio of 1.1 to 1.2 times in the TD direction, and more preferably 1.2 to 1.7 times. When biaxial stretching is performed, it is preferable to stretch by 1.01 to 2.0 times in both the MD and TD directions. Note that the re-stretching ratios may differ between the MD and TD directions, and multi-stage stretching combining sequential stretching is preferred. The dry stretching process is effective for controlling the orientation of molecular chains measured at 25°C using Raman spectroscopy, and high puncture strength can be obtained by performing dry stretching at the above stretching ratios.

[0085] From the viewpoint of shrinkage rate and wrinkles and sagging, the relaxation rate from the maximum re-stretching magnification is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. When the relaxation rate is 20% or less, a uniform fibril structure can be obtained.

[0086] (f) Other processes Furthermore, depending on the application, the polyolefin microporous membrane can be subjected to hydrophilic treatment. Hydrophilic treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, etc. It is preferable to perform monomer grafting after crosslinking treatment.

[0087] It is preferable to crosslink polyolefin microporous membranes by irradiation with ionizing radiation such as alpha rays, beta rays, gamma rays, or electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad and an acceleration voltage of 100 to 300 kV are preferred. Crosslinking increases the meltdown temperature of the polyolefin microporous membrane.

[0088] In the case of surfactant treatment, nonionic surfactants, cationic surfactants, anionic surfactants, or amphoteric surfactants can be used, but nonionic surfactants are preferred. The polyolefin microporous membrane is immersed in a solution obtained by dissolving the surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by the doctor blade method.

[0089] The polyolefin microporous membrane according to the embodiment of the present invention may be surface coated with a porous fluororesin such as polyvinylidene fluoride or polytetrafluoroethylene, or a porous material such as polyimide or polyphenylene sulfide, or an inorganic coating such as ceramic, in order to improve the meltdown characteristics and heat resistance when used as a battery separator. In particular, the polyolefin porous membrane obtained by the present invention has high strength and low thermal shrinkage, making it easy to control tension during coating, suppressing shrinkage during the drying process, and exhibiting excellent coatability.

[0090] The polyolefin microporous membrane obtained as described above can be used in various applications such as filters, fuel cell separators, and capacitor separators, but it is particularly safe when used as a battery separator. Therefore, this separator can be preferably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as those used in electric vehicles. [Examples]

[0091] 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. Unless otherwise specified, evaluations in this application were performed under conditions of 23°C and 65% humidity. The evaluation and analysis methods used in the examples are as follows.

[0092] (1) Weight average molecular weight (Mw) The molecular weight distribution of polyolefins (weight-average molecular weight, molecular weight distribution, and content of specified components) was measured using high-temperature gel permeation chromatography (GPC). The molecular weight distribution of the film was measured using the stretched polyolefin microporous membrane, and the molecular weight distribution of the polyolefin raw material was measured using the polyolefin raw material itself, 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 pieces) (φ7.8mm x 30cm, manufactured by Showa Denko) Solvent: 1,2,4-Trichlorobenzene (TCB, manufactured by Wako Pure Chemical Industries) (0.1% BHT added) Flow rate: 1.0mL / min Column temperature: 145℃ Sample preparation: 5 mg of the sample was added to 5 mL of the measurement solvent, heated and stirred at 160-170°C for approximately 30 minutes, and the resulting solution was filtered through a metal filter (pore size 0.5 μm). Injection volume: 0.200mL Standard sample: Monodisperse polystyrene (manufactured by Tosoh Corporation) (PS) Data processing: GPC data processing system manufactured by TRC

[0093] (2) Film thickness (μm) The film thickness at five points within a 50 mm × 50 mm range of the polyolefin microporous membrane was measured using a contact thickness gauge, Mitutoyo Corporation's LiteMatic VL-50 (super-hard spherical probe with a diameter of 10.5 mm, measurement load of 0.01 N), and the average value was taken as the film thickness (μm).

[0094] (3) Air permeability (sec / 100 cm 3 ) For the polyolefin microporous membrane with a film thickness of T1 (μm), in accordance with JIS P8117:2009, the air permeability (seconds / 100 cm 3 ) was measured using a Wang-type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in an atmosphere of 25°C. Also, the air permeability (converted to 10 μm) (seconds / 100 cm 3 ) was calculated using the following formula when the film thickness was 10 μm.

[0095] Formula: Air permeability (converted to 10 μm) (seconds / 100 cm 3 ) = Air permeability (seconds / 100 cm 3 ) × 10 (μm) / Film thickness (μm) of the polyolefin microporous membrane.

[0096] (4) Porosity (%) A sample was cut from the polyolefin microporous membrane into a 50 mm × 50 mm square, and its volume (cm 3 ) and mass (g) were measured at room temperature of 25°C. From these values and the film density (g / cm 3 ), the porosity of the polyolefin microporous membrane was calculated using the following formula.

[0097] Porosity (%) = (Volume - Mass / Film density) / Volume × 100 Note that the film density was calculated assuming a constant value of 0.99 g / cm 3 .

[0098] (5) 10 μm equivalent puncture strength (N) and basis weight equivalent puncture strength (N / (g / m 2 )) Puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was set to 2 mm / second. Using a force gauge (DS2-20N, manufactured by Imada Corporation), the maximum load (N) was measured (L1) when a 1.0 mm diameter needle with a spherical tip (radius of curvature R: 0.5 mm) was punctured into a polyolefin microporous membrane in a 25°C atmosphere, and the puncture strength (L2) converted to a film thickness of 10 μm was calculated using the following formula. Formula: L2 = L1 × 10 (μm) / Thickness of polyolefin microporous membrane (μm).

[0099] The basis weight equivalent strength was calculated by measuring the maximum load (N) (L1) when puncturing a polyolefin microporous membrane in an atmosphere of 25°C, and then calculating the basis weight equivalent puncture strength (L3) using the following formula. Formula: L3 = L1 / Basis weight of polyolefin microporous membrane.

[0100] 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 its mass (g) at room temperature (25°C), and using the following formula. Formula: Basis weight (g / m 2 )=mass(g) / (50(mm)×50(mm))×10 6 .

[0101] (6) Tensile breaking strength (MPa) Tensile tests were conducted using a Shimadzu Autograph AGS-J tensile testing machine in accordance with JIS K7127:1999. The strength at which the sample broke was divided by the cross-sectional area of ​​the sample before the test to obtain the tensile breaking strength (MPa). The measurement conditions were: temperature; 23±2℃, sample shape; width 10mm × length 50mm, chuck distance; 20mm, tensile speed; 100mm / min. A paper frame with a width 40 × 60mm and a 20 × 20mm cutout in the center was used as the sample holder. A 10mm wide × 50mm long sample was placed in the sample holder and chucked at a pressure of 0.4MPa. After that, both ends of the sample holder were cut and the measurement was performed. The above measurements were taken at three different points on the same film in both the MD and TD directions, and the average value of these three points was taken as the tensile breaking strength in each direction (MD tensile breaking strength, TD tensile breaking strength).

[0102] (7) Tensile elongation at break (%) Tensile tests were performed using a Shimadzu Autograph AGS-J tensile testing machine. The tensile elongation at break was calculated from the gauge length L0 (mm) of the specimen before testing and the gauge length L (mm) at break using the following formula. The measurement conditions were: temperature; 23±2℃, sample shape; width 10mm × length 50mm, chuck distance; 20mm, tensile speed; 100mm / min. A paper frame with a 40×60mm width and a 20×20mm cutout in the center was used as the sample holder. A 10mm width × length 50mm sample was placed in the sample holder and chucked at a pressure of 0.4MPa. After that, both ends of the sample holder were cut and the measurement was performed. The above measurements were performed at three different locations on the same film in both the MD and TD directions, and the average value of these three points was taken as the tensile elongation at break in each direction (MD tensile elongation at break, TD tensile elongation at break). Tensile elongation at break (%) = ((L - L0) / L) × 100.

[0103] (8) Shrinkage rate at 130℃ / 1h (%) A 5cm x 5cm square sample was cut from the polyolefin microporous membrane, with two sides parallel to the MD direction. The sample length in the MD direction was measured at the center of the cut sample in the TD direction, and this was used as the length before MD shrinkage (L).1MD ) and the sample length in the TD direction at the center in the MD direction was measured and designated as the TD pre-shrinkage length (L 1TD ). Next, the sample was put into an oven with the temperature in the tank set at 130°C and heated, and it was taken out 1 hour after being put in. The length in the MD direction at the location where the MD pre-shrinkage length was measured before was measured and designated as the MD post-shrinkage length (L 2MD ). Also, the length in the TD direction at the location where the TD pre-shrinkage length was measured before was measured and designated as the TD post-shrinkage length (L 2TD ). Using these values, the thermal shrinkage rate after 1 hour at 130°C was calculated by the following formula. Also, this measurement was performed at three arbitrary locations within the sample plane, and the average value was calculated as the thermal shrinkage rate (%) after 1 hour at 130°C. Formula: Thermal shrinkage rate (%) in the MD direction after 1 hour at 130°C = 100×(L 1MD - L 2MD ) / L 1MD Formula: Thermal shrinkage rate (%) in the TD direction after 1 hour at 130°C = 100×(L 1TD - L 2TD ) / L 1TD .

[0104] (9) Raman spectroscopy A 2 cm × 2 cm square sample was cut out from the polyolefin microporous membrane such that two sides were parallel to the MD direction. The polarized Raman spectrum of the polyolefin microporous membrane was measured as follows using a microscopic Raman spectrometer JASCO NRS-5100, and the orientation parameter of the crystal molecular chains was calculated.

[0105] 〈Raman measurement conditions〉 · Laser: 532 nm · Grating: 2400 Line / mm · Lens: 20× · Slit: 200×1000 μm · Aperture: φ4000 μm 1. A laser polarized in the machine direction of the polyolefin microporous membrane using a polarizer was made to enter the test piece, and the scattered light was collected through an analyzer oriented in the machine direction. 2. The obtained Raman spectrum at 1130 cm- 1 and 1060cm- 1 The ratio of the Raman bands I 1130 / I 1060 This was defined as the Raman orientation parameter, and its value was calculated.

[0106] The Raman spectrum was obtained with the polarizer positioned parallel to the longitudinal direction of the film (0° / 0°) as the MD direction and perpendicular to it (90° / 90°) as the TD direction. 1130cm- 1 This band is attributed to the CC stretching vibration of polyethylene molecular chains in the crystalline phase. Since the direction of the vibrational Raman tensor coincides with the molecular chain axis, it allows us 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.

[0107] <Calculation of peak and orientation parameters> I a Raman shift bandwidth: 1100-1170 cm -1 Maximum intensity of the Raman band in the range I b: Raman shift bandwidth: 1040-1090 cm -1 Maximum intensity of the Raman band in the range I a (MD, 25℃): Value in the MD direction measured at 25℃ I a (TD, 25℃): Value in the TD direction measured at 25℃ I b (MD, 25℃): Value in the TD direction measured at 25℃ I b (TD, 25℃): Value in the TD direction measured at 25℃ I a (MD, 130°C): MD direction value after heating at 130°C for 60 min using a heating stage. I a (TD, 130°C): TD value after heating at 130°C for 60 minutes using a heating stage. I b (MD, 130°C): MD direction value after heating at 130°C for 60 min using a heating stage. I b(TD, 130°C): TD value 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=I a (MD, 25℃) / I a (MD, 130℃))...Equation (5) fTLH=I a (TD, 25℃) / I a (TD, 130℃))...Equation (6) fML-fMH ···(7) fTL-fTH...Equation (8).

[0108] Note that I in equations (5) and (6) a This corresponds to a Raman shift bandwidth of 1100-1170 cm². -1 Maximum strength within the range and 1200cm -1 This is the difference in intensity, I a (MD, 130°C) is measured at 130°C in the MD direction, I a (TD, 130°C) is measured at 130°C in the TD direction. a (MD, 25℃) was measured at 25℃ in the TD direction. a (TD, 25°C) is the value measured at 25°C in the TD direction.

[0109] Furthermore, when measuring at 130°C using a heating stage, the MD and TD four sides of the polyolefin microporous membrane were fixed with Kapton tape before measurement.

[0110] (10) Shutdown temperature While heating a polyolefin microporous membrane at a heating rate of 5°C / min, the air permeability resistance was measured using a Wang Ren type air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.), and the air permeability resistance was measured until it reached the detection limit of 99999 seconds / 100cm. 3 The temperature at which the air reached was defined as the shutdown temperature (°C).

[0111] The measurement cell was constructed from an aluminum block with a thermocouple directly beneath a polyolefin microporous membrane. A sample was cut into a 5cm x 5cm square and measured by raising the temperature while securing the edges with O-rings.

[0112] (11) Short-circuit test Short-circuit resistance was evaluated using a desktop precision universal tester, Autograph AGS-X (manufactured by Shimadzu Corporation). A laminate was fabricated consisting of a polypropylene insulator (thickness 0.2 μm) / negative electrode (for lithium-ion batteries (copper foil (thickness approx. 0.9 μm), active material: artificial graphite (particle size approx. 13 μm)) / separator / 500 μm diameter chromium sphere (material: chromium (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 with a diameter of approximately 500 μm (material: chromium (SUJ-2)) was placed between the separator and aluminum foil in the sample laminate. A UJ-2)) was placed. Then, it was pressed under conditions of 0.3 mm / min, and the resistance to foreign matter was evaluated by the amount of displacement until the battery short-circuited. In the compression load change, the point where the leakage current value began to rise was used as the starting point, and the moment the above circuit was formed via the metal ball and current was detected was used as the short-circuit point, and the displacement was measured. Samples that did not short-circuit even with a high amount of displacement had better resistance to foreign matter, and the relationship between the amount of displacement and resistance to foreign matter was defined as the following four stages. A level of B or higher is acceptable for practical purposes, but a level of A or higher is preferable as batteries are becoming more energy denser and larger in capacity. S: Displacement (mm) / Separator thickness (μm) is greater than 0.025 A: The displacement (mm) / separator thickness (μm) is greater than 0.024 and less than or equal to 0.025. B: The displacement (mm) / separator thickness (μm) is greater than 0.020 and less than or equal to 0.024. C: Displacement (mm) / separator thickness (μm) is 0.020 or less.

[0113] [Melting point of polyolefin resin raw materials] The melting point of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) based on JIS K7121:1987. A 6.0 mg sample was sealed 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 Parking Elmer PYRIS Diamond DSC. After heating from 30°C to 230°C at a rate of 10°C / min (first heating), the pan was held at 230°C for 5 minutes, cooled at a rate of 10°C / min, and then heated again from 30°C to 230°C at a rate of 10°C / min (second heating), obtaining each melting endothermic curve. The temperature at the peak top of the melting endothermic curve obtained during the second heating was defined as the melting point of the polyolefin resin raw material.

[0114] [Heat of fusion (ΔH) of polyolefin resin raw materials] The heat of fusion of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) based on JIS K7121:1987. A 6.0 mg sample was sealed 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 Parking Elmer PYRIS Diamond DSC. After heating from 30°C to 230°C at a rate of 10°C / min (first heating), the sample was held at 230°C for 5 minutes, cooled at a rate of 10°C / min, and then heated again from 30°C to 230°C at a rate of 10°C / min (second heating), obtaining each endothermic fusion curve. The heat of fusion on the endothermic fusion curve obtained in the second heating was integrated from 60°C to 160°C to obtain the ΔH (J / g) of the polyolefin resin raw material.

[0115] The present invention will be specifically described below with reference to examples, but the present invention is not limited in any way by these examples.

[0116] [Example 1] The raw material is Mw 15 x 10 5Using ultra-high molecular weight polyethylene, 20 parts by mass of ultra-high molecular weight polyethylene were mixed with 80 parts by mass of liquid paraffin. 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 per 20 parts by mass of ultra-high molecular weight polyethylene and mixed to prepare a polyethylene resin solution. The obtained polyethylene resin solution was fed into a twin-screw extruder and kneaded at 180°C to prepare a polyethylene solution. The obtained polyethylene solution was supplied to a T-die, and the extruded material was cooled with a cooling roll controlled to 35°C to form a gel-like sheet. The obtained gel-like sheet was longitudinally stretched using a roll method at a stretching temperature of 115°C to a stretching ratio of 7.0 times. At this time, the pressure between the stretching roll and the nip roll was 0.3 MPa. Subsequently, it was led to a tenter and transversely stretched at a stretching temperature of 120°C to a stretching ratio of 7.0 times. The stretched film was washed in a methylene chloride washing tank to remove liquid paraffin. The washed film was dried and then re-stretched laterally at 135°C using a tenter method to a stretch ratio of 1.4 times to obtain a polyolefin microporous film.

[0117] [Examples 2-4] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the raw material formulation and film formation conditions were changed as shown in Table 1.

[0118] [Example 5] The raw material is Mw 15 x 10 5 70 parts by mass of ultra-high molecular weight polyethylene and Mw is 1 × 10 5A polyethylene (PE) mixture consisting of 30 parts by mass of high-density polyethylene with a melting point of 131.5°C and a ΔH of 225 (J / g) was prepared by adding 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 to 70 parts by mass of ultra-high molecular weight polyethylene to obtain a polyethylene mixture. 25 parts by mass of the obtained mixture was added to 75 parts by mass of liquid paraffin and fed into a twin-screw extruder, where it was kneaded at 180°C to prepare a polyethylene solution. The obtained polyethylene solution was supplied to a T-die, and the extruded material was cooled on a cooling roll controlled to 35°C to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched 5 × 5 times using a tenter method at a stretching temperature of 105°C. The stretched film was washed in a methylene chloride washing tank to remove the liquid paraffin. The washed film was dried, then re-stretched longitudinally to 1.7 times its original size using a roll stretching method at 100°C, and then re-stretched transversely to 1.7 times its original size using a tenter method at 137°C to obtain a polyolefin microporous film.

[0119] [Examples 6-8] Polyolefin microporous membranes were prepared in the same manner as in Example 1, except that the raw material formulation and film-forming conditions were changed as shown in Table 1. The high-density polyethylene used in Examples 6 and 7 was the same high-density polyethylene used in Example 5.

[0120] [Comparative Examples 1-2] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the raw material formulation and film formation conditions were changed as shown in Table 2.

[0121] The evaluation results of the obtained polyolefin microporous membranes are shown in Tables 3 and 4. In Tables 1 and 2, "UHPE" refers to ultra-high molecular weight polyethylene, and "HDPE" refers to high-density polyethylene.

[0122] [Table 1]

[0123] [Table 2]

[0124] [Table 3]

[0125] [Table 4]

[0126] Examples 1-8 all yielded microporous films with small orientation parameters at high temperatures, as calculated by micro-Raman spectroscopy, achieving both excellent strength and low shrinkage. Examples 1-3 and 5-7, in particular, achieved excellent strength and low shrinkage, resulting in microporous films with superior short-circuit performance. In contrast, Comparative Examples 1-2, which used high-density polyethylene as the main component, had large orientation parameters at high temperatures, resulting in inferior strength and low shrinkage.

Claims

1. Using a micro-Raman spectrometer, the orientation parameter values ​​in the MD direction (fMH) and the orientation parameter values ​​in the TD direction (fTH), measured at 130°C and calculated by equations (1) and (2) below, are both between 0.00 and 1.70, respectively, and the puncture strength equivalent to the base weight is 0.8 N / (g / m²). ２ A polyolefin microporous membrane having a porosity of 48% or less, The raw material resin in the polyolefin microporous membrane is polyethylene. The polyolefin microporous membrane contains 50% by weight or less of polyethylene with a weight-average molecular weight of 3.0 × 10⁵ or less, and contains 30% or more of polyethylene with a weight-average molecular weight of 9.0 × 10⁵ or more. Polyolefin microporous membrane. 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) Note that I ａ is the maximum intensity of the Raman band in the range of the Raman shift band of 1100 to 1170 cm －１ , I ｂ is the maximum intensity of the Raman band in the range of the Raman shift band of 1040 to 1090 cm －１ , I ａ (MD, 130 °C), I ｂ (MD, 130 °C) is the maximum intensity in the MD direction measured at 130 °C, I ａ (TD, 130 °C), I ｂ (TD, 130 °C) is the maximum intensity in the TD direction measured at 130 °C.

2. The polyolefin microporous membrane according to claim 1, wherein the values ​​calculated using a micro-Raman spectrometer satisfy the following equations (5) and (6). fMLH = D ａ (MD, 25 °C) / D ａ (MD, 130 °C) ≤ 4 ··· Equation (5) fTLH = D ａ (TD, 25 °C) / D ａ (TD, 130 °C) ≤ 4... (Equation 6) D ａ This corresponds to a Raman shift bandwidth of 1100–1170 cm². －１ Maximum strength within the range and 1200 cm －１ The difference in intensity, D ａ (MD, 130°C) was measured at 130°C in the MD direction, D ａ (TD, 130°C) is measured at 130°C in the TD direction, D ａ (MD, 25°C) was measured at 25°C in the TD direction, D ａ (TD, 25°C) is the value measured at 25°C in the TD direction.

3. The polyolefin microporous membrane according to claim 1 or 2, wherein the values ​​calculated using a micro-Raman spectrometer satisfy the following equations (7) and (8). 0.00 ≤ fML - fMH ≤ 0.50 ... (7) 0.00≦fTL-fTH≦0.50...Equation (8) Note that fML and fTL are the orientation parameter values ​​in the MD direction (fML) and TD direction (fTL) measured at 25°C, calculated by equations (3) and (4) below. ａ This corresponds to a Raman shift bandwidth of 1100–1170 cm². －１ Maximum Raman band intensity within the range, I ｂ This corresponds to a Raman shift bandwidth of 1040–1090 cm². －１ Maximum Raman band intensity within the range, I ａ (MD, 25°C), I ｂ (MD, 25°C) is the maximum intensity in the MD direction measured at 25°C, I ａ (TD, 25℃), I ｂ (TD, 25°C) represents the maximum intensity 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)

4. A polyolefin microporous membrane according to any one of claims 1 to 3, wherein the tensile breaking strength in the MD direction is 200 MPa or more.

5. A polyolefin microporous membrane according to any one of claims 1 to 4, wherein the sum of the shrinkage rates in the MD direction and the TD direction at 130°C / 1h is 30% or less.

6. A polyolefin microporous membrane according to any one of claims 1 to 5, wherein the shrinkage rate in the MD direction at 130°C / 1h is 15% or less.

7. A polyolefin microporous membrane according to any one of claims 1 to 6, wherein the shutdown temperature is 143°C or lower.

8. A polyolefin microporous membrane according to any one of claims 1 to 7, wherein the weight-average molecular weight of the polyolefin microporous membrane is 800,000 or more.

9. A polyolefin microporous membrane according to any one of claims 1 to 8, obtained by stretching including at least wet sequential biaxial stretching.

10. A battery separator using a polyolefin microporous membrane according to any one of claims 1 to 9.

11. A secondary battery using the battery separator described in claim 10.

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