WO2022202095A1

WO2022202095A1 - Microporous polyolefin film, separator for battery, and secondary battery

Info

Publication number: WO2022202095A1
Application number: PCT/JP2022/007847
Authority: WO
Inventors: 中嶋龍太; 豊田直樹; 坂本光隆; 久万琢也; 大倉正寿
Original assignee: 東レ株式会社
Priority date: 2021-03-23
Filing date: 2022-02-25
Publication date: 2022-09-29
Also published as: KR20230160224A; CN116724371A; JPWO2022202095A1

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

Polyolefin microporous membrane, battery separator and secondary battery

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

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

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

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

Patent Document 2 describes a technique for improving shutdown temperature and puncture strength, in which polyolefin having a weight molecular weight of 500,000 or more is used as a main component, and the ratio of orientation in the MD direction and the TD direction obtained by X-ray analysis is controlled. A technique for obtaining a microporous membrane with a puncture strength of 24-0.75 N/(g/m ² ) and a shutdown temperature of 139° C.-146° C. is disclosed.

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

JP 2020-95950 A Japanese Patent No. 6671255 JP 2013-199545 A

Separators are required to have high strength and low shrinkage at high temperatures. However, it was not sufficient from the standpoint of achieving both high strength and low shrinkage at high temperatures, as well as the thickness of the separator accompanying the increase in capacity of the battery.

In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane that achieves both higher strength and lower shrinkage at high temperatures than before, and a separator using the same.

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

Both 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 0.00. Above, it is characterized by being 1.70 or less.
fMH = _Ia (MD, 130°C)/ _Ib (MD, 130°C)(1) Formula fTH = _Ia (TD, 130°C)/ _Ib (TD, 130°C)(2) where I _a is the maximum Raman band intensity in the Raman shift band range of 1100 to 1170 cm ^-1 and I _b is the Raman band maximum intensity in the Raman shift band range of 1040 to 1090 cm ^-1 , I _a (MD, 130°C) and I _b (MD, 130° C.) are the maximum intensities in the MD direction measured at 130° C. I _a (TD, 130° C.) and I _b (TD, 130° C.) are is the maximum intensity in the TD direction measured at .

According to the present invention, a highly safe polyolefin microporous membrane that achieves both high strength and low shrinkage at high temperatures can be obtained.

The polyolefin microporous membrane according to the embodiment of the present invention is useful as a battery separator because of its excellent strength and shrinkage rate, and has excellent safety. The present invention can be realized by satisfying the orientation parameters in the MD direction and the TD direction at high temperatures calculated by microscopic Raman spectroscopy within the range described later, leading to compatibility between strength and shrinkage ratio, which were conventionally in a trade-off relationship. This is what I found. The present invention is not limited to the embodiments described below. The direction in which the tape is drawn is called the width direction or the TD direction.

The present invention will be described in further detail below.

[1] Polyolefin microporous membrane The polyolefin microporous membrane according to the embodiment of the present invention has an orientation parameter value (fMH) in the MD direction and an orientation parameter value (fTH) in the TD direction measured at 130 ° C. by the method described later. ) are all 1.70 or less. The orientation parameter is an index indicating the degree of orientation of crystal molecular chains as a value calculated by Raman spectroscopy, and the higher the value, the more highly oriented the crystal molecular chains. When fMH and fTH are 0.00 or more, it means that the film has a strong structure in which the orientation state is maintained even at high temperatures in both the MD and TD directions, and excellent strength can be obtained. From the viewpoint of strength, fMH and fTH are 0.00 or more, preferably 0.50 or more, more preferably 0.90 or more, still more preferably 1.00 or more, and particularly preferably 1.10 or more. However, if fMH and fTH are too high, relaxation of the crystal structure at high temperature leads to deterioration of the shrinkage ratio. Therefore, fMH and fTH are 1.70 or less, preferably 1.50 or less, and more preferably 1.20 or less. From the viewpoint of the balance between strength and shrinkage, it is important that both fMH and fTH are 1.70 or less. By satisfying the above ranges, both high strength and low shrinkage at high temperatures can be achieved. note that. The above range can be controlled by the raw material design and manufacturing method described later, and in order to form a strong structure that maintains the orientation state at high temperatures, the main raw material is a polyolefin resin with a weight average molecular weight of 0.8 × 10 ⁶ or more with a long relaxation time. It is preferable to form a film in which a highly oriented structure is formed by wet sequential stretching, and dry re-stretching after washing and drying is performed at a high temperature. From the above viewpoint, in the molecular weight distribution of the polyolefin microporous membrane, the polyolefin microporous membrane contains 30% by mass or more of a component with a molecular weight of 0.9 × 10 ⁶ or more with a long relaxation time, and a molecular weight of 0.3 × with a short relaxation time. It is more preferable that the component having a molecular weight of 10 ⁶ or less is contained in a range of less than 50% by mass. As a result, a highly oriented structure with little change in orientation even at high temperatures can be obtained, and a highly safe polyolefin microporous membrane having both high strength and low shrinkage at high temperatures can be obtained.

The polyolefin microporous film according to the embodiment of the present invention has an orientation parameter value (fML) in the MD direction and an orientation parameter value (fTL) in the TD direction measured at 25 ° C. by the method described later, both of which are 1.70. The following are preferred. From the viewpoint of strength, the higher the fML and fTL, the better. However, when the highly oriented structure increases in the measurement at 25° C., the shrinkage rate increases due to the relaxation of the molecular orientation at high temperature. From the viewpoint of shrinkage rate suppression, fML and fTL are preferably 1.50 or less, more preferably 1.30 or less. The above fML and fTL are orientation parameters calculated by the following equations (3) and (4), I _a is the maximum intensity in the Raman shift band 1100 to 1170 cm ⁻¹ , and I _b is the Raman shift. The maximum intensities of Raman bands in the range of 1040-1090 cm ⁻¹ , I _a (MD, 25° C.) and I _b (MD, 25° C.) were measured in the MD direction of the polyolefin microporous membrane at 25° C., and I _a ( TD, 25°C) and I _b (TD, 25°C) are values measured in the TD direction of the polyolefin microporous membrane at 25°C. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.
fML = _Ia (MD, 25°C)/ _Ib (MD, 25°C)(3) Formula fTL = _Ia (TD, 25°C)/ _Ib (TD, 25°C)... (4) Formula.

The polyolefin microporous membrane according to the embodiment of the present invention has _a ratio ( _fMLH ) and Da (TD, 25°C) of _Ia (MD, 25°C) and Da (MD, 130°C) measured by the method described later. ) and Da (TD, 130 ° C.) ( _fTLH ) are both preferably 4.00 or less, more preferably 3.00 or less, still more preferably 2.50 or less, and even more preferably 2.00 or less. Preferably, 1.50 or less is particularly preferable. When it is 4.00 or less, it means that the C—C stretching vibration of the polyethylene molecular chain in the crystal phase at 130° C. is maintained, that is, the structure in which the molecular chain structure of the crystal is highly retained even at 130° C., and the strength is high. is obtained. D _a is the difference between the maximum intensity in the Raman shift band of 1100 to 1170 cm ⁻¹ and the intensity at 1200 cm ⁻¹ , D _a (MD, 130° C.) is measured in the MD direction of the polyolefin microporous membrane at 130° C. _Da (TD, 130°C) is measured in the TD direction of the polyolefin microporous membrane at 130°C, _Da (MD, 25°C) is measured in the MD direction of the polyolefin microporous membrane at 25°C, _Da (TD, 25°C) °C) is the value measured at 25°C in the TD direction of the polyolefin microporous membrane. 1130 cm ⁻¹ is a band attributed to the C—C stretching vibration of the polyethylene molecular chain in the crystal phase, and the direction of the Raman tensor of vibration is the molecular chain axis. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.
fMLH=D _a (MD, 25° C.)/D _a (MD, 130° C.)) (5) Formula fTLH=D _a (TD, 25° C.)/D _a (TD, 130° C.)) (6) formula.

When the crystal structure relaxes and melts at high temperatures, the orientation parameter measured at 130°C decreases more than the orientation parameter measured at 25°C (orientation parameter measured at 25°C > orientation parameter measured at 130°C). Also, recrystallization occurs at 130°C in samples with high melting points of the film, and the orientation parameter measured at 130°C may increase from 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 variation is to 0, the better the retention of the crystal structure at high temperatures and the smaller the shrinkage rate. It is particularly preferable that the change in the orientation parameter at 25° C. and 130° C. calculated by microscopic Raman spectroscopy is small, and the difference between fML and fMH (fML−fMH (7)) and the difference between fTL and fTH (fTL−fTH (8) are preferably 0.50 or less. It is more preferably 0.40, and more preferably 0.20 or less. The lower limit of the difference between fML and fMH (fML-fMH) and the difference between fTL and fTL (fTL-fTH) is -0.50 or more, preferably -0.20 or more, more preferably -0.10 or more, and 0 0.00 or more is more preferable. If the difference between fML and fMH (fML-fMH) and the difference between fTL and fTH (fTL-fTH) are both 0.50 or less, the relaxation of the crystal structure at 130° C. is suppressed and good shrinkage properties are obtained. When it is -0.50 or more, melting of the crystal structure is suppressed at high temperatures, and excellent strength can be obtained. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.

By satisfying the above ranges for the orientation parameters fMH, fTH, fML, and fTL, particularly excellent strength and shrinkage properties can be obtained, and the above ranges can be controlled by raw material design and manufacturing methods, which will be described later. In order to form a strong structure that maintains the oriented state at high temperature, polyolefin resin with a weight average molecular weight of 0.8 x ¹⁰⁶ or more with a long relaxation time is used as the main raw material, and a highly oriented structure is formed by wet sequential stretching. However, it is preferable to form a film in which the dry re-stretching step after washing and drying is performed at a high temperature. From the above viewpoint, in the molecular weight distribution of the polyolefin microporous membrane, the polyolefin microporous membrane contains 30% by mass or more of a component with a molecular weight of 0.9 × 10 ⁶ or more with a long relaxation time, and a molecular weight of 0.3 × with a short relaxation time. It is preferable that the component of 10 ⁶ or less is contained in the range of less than 50% by mass. As a result, a highly oriented structure with little change in orientation even at high temperatures can be obtained, and a highly safe polyolefin microporous film having both high strength and low shrinkage at high temperatures can be obtained.

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

The higher the puncture strength of the polyolefin microporous membrane, the better, because it affects the safety such as prevention of short-circuiting caused by foreign matter in the battery. The pin puncture strength of the polyolefin microporous membrane converted to a thickness of 10 μm is preferably 2.5 N or more, more preferably 3.0 N or more, still more preferably 4.0 N or more, and even more preferably 4.3 N or more. 0N or more is particularly preferable. In addition, the puncture strength per unit basis weight (converted puncture strength per basis weight), which is an index showing the strength of the film obtained by standardizing the puncture strength with the amount of resin, is preferably 0.7 N/(g/m ² ) or more, and is preferably 0.7 N/(g/m 2 ) or more. 8 N/(g/m ² ) or more is more preferable, and 0.9 N/(g/m ² ) or more is particularly preferable. When the puncture strength is within the above range, short circuits due to foreign matter or the like are suppressed, and good battery safety is obtained. In order to improve the puncture strength, it is preferable to combine the use of ultra-high-molecular-weight polyolefin as the main component in the raw material formulation and the increase in the number of tie molecules that connect the lamellar crystals to increase the strength, in addition to controlling the orientation of the crystals. In addition, from the viewpoint of suppressing a decrease in porosity due to melting in a heat setting step or the like, it is preferable to use an ultrahigh molecular weight polyolefin having a small amount of low molecular weight components and a sharp molecular weight distribution. The puncture strength can be achieved by setting the aforementioned fMH, fTH, fML, and fTL in specific ranges and adopting the raw materials, molecular weight, resin concentration, and stretching method within the ranges described later.

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

From the viewpoint of suppressing the short circuit of the battery due to abnormal heat generation, the total shrinkage rate in the MD direction and the TD direction at 130 ° C./1 h is preferably 30% or less, more preferably 29% or less, further preferably 28% or less, and 27% or less. Even more preferably, 25% or less is particularly preferable. Further, the total shrinkage ratio in the MD direction and the TD direction at 130 ° C./1 h when applying a high heat resistant coating layer such as aramid or polyimide is preferably 33% or less, more preferably 31% or less, and 30% or less. More preferably, 29% or less is even more preferable, and 28% or less is particularly preferable. If the shrinkage ratio is within this range, the battery's internal temperature will not change much, and the insulation will be maintained. be done. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.

Since the battery is under tension in the MD direction, if the shrinkage rate in the MD direction is high, the membrane will break and lead to a short circuit. Therefore, the polyolefin microporous membrane according to the embodiment of the present invention preferably has a shrinkage rate in the MD direction at 130° C./1 h of 15% or less, more preferably 12% or less, further preferably 11% or less. % or less is even more preferable. When the shrinkage ratio in the MD direction is within this range, the dimensional change is small when the internal temperature of the battery rises, and the insulation can be maintained, resulting in high safety. The above range can be achieved by applying the raw materials, molecular weight, and manufacturing method within the ranges described later.

In addition, the polyolefin microporous membrane according to the embodiment of the present invention preferably has a shrinkage rate in the TD direction at 130°C/1h of 30% or less, more preferably 25% or less, and even more preferably 20% or less. When the shrinkage ratio is within this range, deterioration of shape stability at high temperatures can be suppressed, and internal short-circuiting can be suppressed when abnormal heat is generated locally, thereby maintaining safety. The above heat shrinkage rate can be achieved by setting the fMH, fTH, fML, and fTL in specific ranges and adopting the raw materials, resin concentration, and stretching method described later. The shrinkage ratios in the MD direction and the TD direction at 130° C./1 h can be measured by the method described in Examples.

In the polyolefin microporous membrane of the present invention, the tensile breaking strength in the MD direction (tensile breaking strength in the MD direction; hereinafter simply referred to as "MD tensile strength") is effective for suppressing film breakage in the battery winding process and removing foreign matter in the battery. From the viewpoint of short circuit prevention due to the like, the 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 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 100 MPa or more, preferably 160 MPa or more. , more preferably 190 MPa or more, still 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, and wrinkles and sagging of the film are suppressed. improved sexuality. The above tensile strength can be achieved by adopting the raw materials, resin concentration, and stretching method, which will be described later.

In addition, the tensile (breaking) elongation in the MD direction and the TD direction (hereinafter also simply referred to as "MD elongation" and "TD elongation") is preferably 50% or more, more preferably 60% or more, and 90% or more. More preferably, 120% or more is even more preferable, and 150% or more is particularly preferable. When the MD elongation or the TD elongation is 50% or more, short circuits due to foreign matter during winding or in the battery can be suppressed, and good safety can be obtained, which is preferable. Both MD elongation and TD elongation are preferably 200% or less, more preferably 170% or less. When the MD elongation and TD elongation are 200% or less, both strength and elongation can be achieved. The tensile strength and tensile elongation in the MD and TD directions can be measured by the method described in Examples.

In the polyolefin microporous membrane according to the embodiment of the present invention, air permeability is a value measured according to JIS P 8117 (2009). In this specification, the term "air permeability" is used in the sense of "air permeability when the film thickness is 10 µm", unless otherwise specified for the film thickness. When the air permeability (Gurley value) measured in the polyolefin microporous membrane having a thickness of T1 (μm) is p1 (sec/100 cm ³ ), the permeability calculated by the formula: p2=(p1×10)/T1. Let the air permeability p2 (sec/100 cm ³ ) be the air permeability when the film thickness is 10 μm.

The air permeability is preferably 200 sec/100 cm ³ or less, more preferably 130 sec/100 cm ³ or less, even more preferably 110 sec/100 cm ³ or less. If the air permeability is 200 sec/100 cm ³ or less, good ion permeability can be obtained and electric resistance can be lowered.

In the polyolefin microporous film according to the embodiment of the present invention, the resistance increases as the film thickness increases, and the output characteristics of the battery deteriorate. From the viewpoint of battery output characteristics, the film thickness is preferably 12 μm or less, more preferably 10 μm or less, and even more preferably 5 μm or less. The thinner the film, the lower the strength and the lower the safety. Therefore, from the viewpoint of safety, the thickness is preferably 1 μm or more, more preferably 3 μm or more.

The shutdown temperature is the temperature at which the resin part shrinks and melts and the pores close when the polyolefin microporous membrane is heated to stop discharging and charging, and is the temperature measured by the method described later. Since the electrodes used in lithium-ion secondary batteries designed for high energy density tend to have reduced thermal stability, it is preferable to shut down (hole clogging) quickly after the battery is short-circuited. The shutdown temperature of the polyolefin microporous membrane according to the embodiment of the present invention is 143° C. or less. It is preferably 141° C. or lower, more preferably 140° C. or lower, and still more preferably 139° C. or lower. The microporous membrane obtained by the present invention has excellent short-circuit resistance and has the shutdown temperature described above, so that excellent battery safety can be obtained. In order to set the shutdown temperature within the above range, it is preferable to set the raw material composition of the microporous membrane within the range described below.

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

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

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

As the type of polyolefin resin to be used, polyethylene is preferable, and high-density polyethylene having a density exceeding 0.94 g/cm ³ , medium-density polyethylene having a density in the range of 0.93 to 0.94 g/cm ³ , and density of 0.94 g/cm 3 are used. Low density polyethylene lower than 93 g/cm ³ , linear low density polyethylene and the like are included.

In addition, from the viewpoint of film strength and shrinkage balance, it is preferable to use an ultra ^- high molecular weight polyolefin alone or as a main component of the polyolefin resin. It is preferably 9.0×10 ⁵ or more, more preferably 10×10 ⁵ or more, particularly preferably 15×10 ⁵ or more, and preferably 100×10 ⁵ or less from the viewpoint of moldability. In addition, it is important to add the auxiliary material within a range that does not impair the fibril structure formed by the main material and the moldability.

If the weight-average molecular weight is 8.0×10 ⁵ or more, the relaxation time is long, so the retention of the crystal structure at high temperatures is improved, and the melting and shrinkage are suppressed, and the orientation state is maintained even at high temperatures. It has a strong structure and can achieve both strength and shrinkage, improving the safety of the battery. In addition, by using a polyolefin resin with a weight-average molecular weight of 9.0×10 ⁵ or more, the number of tie molecules increases, making it easier to obtain high strength. In addition to obtaining good output characteristics, the heat setting temperature can be raised by lowering the relaxation rate of the resin, and good shrinkage characteristics can be obtained.

From the above viewpoint, the weight average molecular weight (Mw) of the polyolefin microporous membrane is preferably 8.0 × 10 ⁵ or more, more preferably 9.0 × 10 ⁵ or more, further preferably 10 × 10 ⁵ or more, and maintains the molecular weight of the raw material. It is particularly preferred that In order to form a strong structure that maintains the orientation state at high temperatures, the polyolefin microporous membrane contains 30% by mass or more of a component with a molecular weight distribution of 9.0 × 10 ⁵ or more that has a long relaxation time. It is preferably contained in an amount of 33% by mass or more, more preferably 35% by mass or more, even more preferably 38% by mass or more, and still more preferably 40% by mass or more. From the viewpoint of retention of the crystal structure at high temperatures and suppression of melting in the stretching and heat setting steps, the content of the component having a molecular weight of 3.0×10 ⁵ or less is preferably less than 50% by mass, more preferably 45% by mass or less. It is preferably 40% by mass or less, more preferably 35% by mass or less. The content of components having a molecular weight of 3.0×10 ⁵ or less is preferably 30% by mass or more, more preferably 35% by mass or more, even more preferably 40% by mass or more, further preferably 45% by mass, for lower shutdown temperatures. The above are particularly preferred. In order to obtain the molecular weight of the polyolefin microporous membrane, the above-described raw material formulation is added to the antioxidant and kneaded under a nitrogen atmosphere, and a combination of the addition of an antioxidant and kneading under a nitrogen atmosphere. It is preferable to form the film with.

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

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

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

The weight-average molecular weight (Mw) obtained by high-temperature gel permeation chromatography (GPC) measurement or the like of the high-density polyethylene used in the embodiment of the present invention, apart from the ultra-high molecular weight polyethylene, is preferably 1×10 ⁵ or less. When Mw is within the above range, the structure formed by the ultra-high molecular weight polyethylene is less likely to be disturbed, and low-melting crystal formation and reduction in shrinkage force during melting are likely to be possible. This makes it possible to achieve both mechanical strength, shrinkage, and shutdown characteristics.

The melting point (°C) obtained from a differential scanning calorimeter (DSC) of the high-density polyethylene used in the embodiment of the present invention is preferably 132°C or less. Further, the melting point of high-density polyethylene is more preferably 127° C. or higher, 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 structure before stretching can be lowered in an appropriate range, and when it is made into a polyolefin microporous membrane, the structure formed by the ultra-high molecular weight polyethylene is hardly hindered. , the formation of low-melting-point crystals is likely to be possible. This makes it possible to improve the shutdown characteristics of the polyolefin microporous membrane.

The low molecular weight polyethylene used in the embodiment of the present invention preferably has a heat of fusion ΔH (J / g) obtained from a differential scanning calorimeter (DSC) of 200 J / g or more, more preferably 210 J / g or more, 220 J/g or more is more preferable. Although the upper limit of ΔH is not particularly limited, it is typically 260 J/g or less due to the properties of polyethylene. When ΔH is within the above range, low-melting-point crystals can easily be formed without excessively reducing the amount of crystals in the polyolefin microporous membrane. This makes it possible to achieve both shutdown characteristics and transparency.

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

Examples of antioxidants include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5- di-t-butyl-4-hydroxybenzyl)benzene (for example, BASF "Irganox" (registered trademark) 1330: molecular weight 775.2), tetrakis[methylene-3(3,5-di-t-butyl-4 -Hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF, molecular weight 1177.7) and the like are preferably used.

The characteristics of the polyolefin microporous membrane can be adjusted or enhanced by appropriately selecting the type and amount of antioxidant and heat stabilizer to be added. It is preferable that the amount added does not increase the MFR of the gel-like sheet described later, which is measured by the method described in JIS K7210-1 (2014), and the amount of antioxidant added is 0.5 mass with respect to the resin amount. % or more is preferable, 0.7 mass % or more is more preferable, 1.0 mass % or more is still more preferable, 1.2 mass % or more is still more preferable, and 1.5 mass % or more is more preferable.
The upper limit is 3.0% by mass or less from the viewpoint of film-forming properties such as drool and streaks, and it is particularly preferable to suppress oxidative deterioration by combining the addition of an antioxidant and kneading in a nitrogen atmosphere.

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

[3] Method for Producing Microporous Polyolefin Film Next, a method for producing a microporous polyolefin film according to an embodiment of the present invention will be specifically described. A method for producing a polyolefin microporous membrane according to an embodiment of the present invention preferably includes the following steps (a) to (e).

(a) a step of melt-kneading a polymer material containing one or more polyolefin resins and optionally a solvent to prepare a polyolefin resin solution; (b) extruding the obtained molten mixture into a sheet; (c) stretching the obtained sheet by a sequential stretching method including a roll method or a tenter method; (d) extracting a plasticizer from the obtained stretched film and drying the film; Step (e) A step of heat-treating/re-stretching by a stretching method including a roll method or a tenter method.

In particular, in the step (a), for the purpose of preventing a decrease in molecular weight, an antioxidant is added in an amount to be described later and kneaded in a nitrogen atmosphere, and (c) wet sequential stretching is performed in the longitudinal direction and the lateral direction, In the step (e), it is particularly preferable to carry out heat treatment/restretching at a temperature of 130° C. or higher by a tenter method.

Each step will be explained below.

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

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

As the 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.

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

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

(b) Formation of extrudate and formation of gel-like sheet The method for uniform melt-kneading of the polyolefin resin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin resin solution, it is preferably carried out in a twin-screw extruder. . If necessary, known additives such as metallic soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, antistatic agents, etc., may be added to the extent that the effects of the present invention are not impaired without impairing the film formability. may In particular, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.

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

Here, the melting point refers to a value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987) (hereinafter the same). For example, when the polyolefin-based resin is polyethylene, the melt-kneading temperature of the polyethylene-based resin is preferably in the range of 140 to 250°C. It is more preferably 150 to 230°C, particularly preferably 150 to 200°C. Specifically, since the polyethylene composition has a melting point of about 130 to 140°C, the melt-kneading temperature is preferably 140 to 250°C.

From the viewpoint of suppressing deterioration of the polyolefin resin, a lower melt-kneading temperature is preferable, but if the temperature is lower than the above-mentioned temperature, unmelted substances are generated in the extrudate extruded from the die, and film breakage etc. occur in the subsequent stretching process. may cause it. On the other hand, if the temperature is higher than the above temperature, thermal decomposition of the polyolefin resin becomes violent, and the physical properties of the obtained polyolefin microporous membrane, such as strength and porosity, may deteriorate. In addition, the decomposition products are deposited on chill rolls, rolls in the stretching process, etc., and adhere to the sheet, leading to deterioration of the appearance. Therefore, the melt-kneading temperature is preferably kneaded within the above range. In addition, the smaller the value of Q/Ns, which is the ratio of the extrusion rate Q (kg/h) of the polyolefin solution to the screw rotation speed Ns (rpm) of the twin-screw extruder, the kneadability of the resin increases, and a uniform solution can be obtained. be done. However, a decrease in Q/Ns causes a large amount of shear heat generation, which promotes deterioration of the resin and makes it impossible to obtain the molecular weight component in the film within the range described above. Low-molecular-weight components accumulate in the bleeding-out plasticizer and adhere to the sheet, thereby deteriorating the appearance. When the value of Q/Ns is large, deterioration of the resin is suppressed, but the kneadability is insufficient and a uniform solution cannot be obtained. Therefore, it is particularly preferable to appropriately change Q/Ns in accordance with the molecular weight and solubility of the resin to be used, and to suppress oxidative deterioration by a combination of adding an antioxidant and kneading in a nitrogen atmosphere.

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

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

Cooling methods include direct contact with cold air, cooling water, and other cooling media, contact with rolls cooled with a refrigerant, and the use of casting drums.

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

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

(c) Stretching Step The resulting gel-like (including laminated sheet) sheet is stretched. The stretching method used includes uniaxial stretching in the sheet conveying direction (MD direction) by rolling or a roll stretching machine, uniaxial stretching in the sheet width direction (TD direction) by a tenter, roll stretching machine and tenter, or tenter and tenter. and simultaneous biaxial stretching using a simultaneous biaxial tenter, etc., but the sequential biaxial stretching step is preferred from the viewpoint of orientation control in the MD direction and the TD direction.

From the viewpoint of fMLH and fTLH, it is preferable to apply a pressure of 0.1 MPa or more between the stretching rolls and the nip rolls in the uniaxial stretching in the sheet conveying direction (MD direction) by the roll stretching machine. By forming the film within the above range, the crystal molecular chains can be more oriented. If the pressure between the stretching rolls and the nip rolls is less than 0.1 MPa, slippage occurs on the rolls, making it difficult to apply stretching stress, and the crystal molecular chains may not be sufficiently oriented.

The stretching ratio of the gel-like sheet may be appropriately adjusted within a range that does not impair the orientation parameters in the MD direction and the TD direction. It is more preferably 6 times or more in both directions, 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, and the area ratio is preferably 40 times or more, more preferably 45 times or more, and still more preferably. is more than 50 times.

The stretching temperature is preferably the melting point of the gel sheet + 10°C or less, more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the gel sheet + 5°C). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 110°C, the stretching temperature is preferably 100 to 130°C, more preferably 110 to 120°C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012). If the above upper limit is exceeded, the relaxation of the molecules is accelerated, and the molecular chains cannot be sufficiently oriented by stretching. When the stretching temperature is within the above range, film breakage due to stretching of the polyolefin resin is suppressed, and crystal molecular chains can be more oriented while allowing stretching at a high magnification.

As described above, the higher-order structure of the gel sheet is cleaved by stretching, and the crystal phase is refined to form a fibril structure oriented in the stretching direction. It is possible to obtain a microporous membrane that maintains a structure and has both excellent strength and high-temperature shrinkage resistance. Therefore, the polyolefin microporous membranes according to the embodiments of the present invention are suitable for battery separators, and the polyolefin microporous membranes of the present application can greatly improve the safety of batteries compared to the conventional technology.

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

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

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

Examples of the cleaning method include a method of immersing the gel-like sheet in a cleaning solvent for extraction, a method of showering the gel-like sheet with a cleaning solvent, and a combination of these methods. The amount of the cleaning solvent used varies depending on the cleaning method, but generally it is preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet.

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

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

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

(e) Heat Treatment/Re-stretching Step The dried microporous polyolefin membrane may be stretched (re-stretched) at least uniaxially. The re-stretching can be performed by a tenter method or the like while heating the polyolefin microporous membrane in the same manner as the stretching described above. Re-stretching may be uniaxial stretching or biaxial stretching. In the case of multistage stretching, simultaneous biaxial stretching or sequential stretching is combined.

The re-stretching temperature is preferably below the melting point of the polyolefin resin composition, and more preferably within the range of (Tcd-20°C of the polyolefin resin composition) to the melting point of the polyolefin resin composition. Specifically, in the case of a polyethylene composition, the re-stretching temperature is preferably 70 to 140.degree. C., more preferably 110 to 140.degree. C., still more preferably 120 to 140.degree. 135 to 140°C is even more preferred.

In particular, the polyolefin microporous membrane according to the embodiment of the present invention is made mainly of polyethylene having a weight average molecular weight of 0.9 × 10 ⁶ or more with a long relaxation time, and is stretched and heat set at a high temperature of 130 ° C. or more. It is possible to obtain a highly oriented structure by suppressing the relaxation of the orientation of the polyolefin molecular chain and forming a thermally stable structure. The resulting microporous film has a high orientation parameter at 130° C., a small difference in orientation parameter between 25° C. and 130° C., and a microporous film that satisfies both per unit weight equivalent puncture strength and thermal shrinkage characteristics.

If the weight-average molecular weight is less than 0.9×10 ⁶ , the relaxation time is short, and heat treatment at 130° C. or higher leads to a decrease in porosity. On the other hand, polyethylene with a weight-average molecular weight of 0.9×10 ⁶ or more has a long relaxation time. Since it can be fixed, it is possible to suppress the relaxation of orientation at high temperature and obtain a highly oriented structure. Therefore, it is preferable to use polyethylene having a weight-average molecular weight of 0.9×10 ⁶ or more and to heat-set at a temperature higher than 130°C.

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

From the viewpoint of shrinkage rate and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. A uniform fibril structure is obtained when the relaxation rate is 20% or less.

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

It is preferable to subject the polyolefin microporous membrane to cross-linking treatment by irradiating it with ionizing radiation such as α-rays, β-rays, γ-rays and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferred, and an acceleration voltage of 100 to 300 kV is preferred. The cross-linking treatment increases the meltdown temperature of the polyolefin microporous membrane.

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

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

The polyolefin microporous membrane obtained as described above can be used in various applications such as filters, separators for fuel cells, and separators for capacitors, and is particularly safe when used as a battery separator. Therefore, the separator can be preferably used as a battery separator for secondary batteries, such as electric vehicles, which require high energy density, high capacity, and high output.

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

(1) weight average molecular weight (Mw)
Polyolefin molecular weight distribution measurement (measurement of weight average molecular weight, molecular weight distribution, content of predetermined component, etc.) was performed by high-temperature gel permeation chromatography (GPC). The molecular weight distribution of the film was measured using the polyolefin microporous film after stretching, and the molecular weight distribution of the polyolefin raw material was measured under the following conditions.
Apparatus: high temperature GPC apparatus (Equipment No. HT-GPC, manufactured by Polymer Laboratories, PL-220)
Detector: Differential Refractive Index Detector RI
Guard column: Shodex G-HT
Column: Shodex HT806M (2 columns) (φ7.8 mm × 30 cm, manufactured by Showa Denko)
Solvent: 1,2,4-trichlorobenzene (TCB, manufactured by Wako Pure Chemical Industries) (0.1% BHT added)
Flow rate: 1.0 mL/min
Column temperature: 145°C
Sample preparation: 5 mL of a measurement solvent was added to 5 mg of a sample, and the mixture was heated and stirred at 160 to 170° C. for about 30 minutes, and the resulting solution was filtered through a metal filter (pore size: 0.5 μm).
Injection volume: 0.200 mL
Standard sample: Monodisperse polystyrene (manufactured by Tosoh) (PS)
Data processing: TRC GPC data processing system.

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

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

Formula: air permeability (converted to 10 μm) (second/100 cm ³ )=air permeability (second/100 cm ³ )×10 (μm)/thickness of polyolefin microporous membrane (μm).

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

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

(5) 10 μm conversion piercing strength (N) and basis weight conversion piercing strength (N/(g/m ² ))
The puncture strength was measured according to JIS Z 1707 (2019), except that the test speed was 2 mm/sec. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), a needle with a diameter of 1.0 mm and a spherical tip (curvature radius R: 0.5 mm) was used to pierce the polyolefin microporous membrane in an atmosphere of 25 ° C. The maximum load (N) was measured (L1), and the puncture strength (L2) converted to a film thickness of 10 μm was calculated from the following formula.
Formula: L2=L1×10 (μm)/thickness of polyolefin microporous membrane (μm).

The weight-converted strength was obtained by measuring (L1) the maximum load (N) when the polyolefin microporous membrane was pierced in an atmosphere of 25° C., and calculating the weight-converted piercing strength (L3) from the following formula.
Formula: L3=L1/weight of polyolefin microporous membrane.

The basis weight of the polyolefin microporous membrane was calculated by the following formula by cutting a 50 mm×50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25° C.
Formula: basis weight (g/m ² ) = mass (g)/(50 (mm) x 50 (mm)) x 10 ⁶ .

(6) Tensile breaking strength (MPa)
JIS K7127: Perform a tensile test using a tensile tester (Shimadzu Autograph AGS-J type) in accordance with 1999, divide the strength at the time of sample breakage by the cross-sectional area of the sample before the test, Tensile breaking strength (MPa) and The measurement conditions are temperature: 23±2° C., sample shape: width 10 mm×length 50 mm, distance between chucks: 20 mm, tensile speed: 100 mm/min. In addition, a paper frame with a width of 40 × 60 mm was hollowed out at the center of the paper frame at 20 × 20 mm as a sample holder. Both ends of the sample holder were cut and measured. The above measurements were performed at three different points in the same film in the MD direction and the TD direction, and the average value of the three points was the tensile breaking strength in each direction (MD tensile breaking strength, TD tensile breaking strength strength).

(7) Tensile elongation at break (%)
A tensile test was performed using a tensile tester (Shimadzu Autograph AGS-J type). ) was calculated from the following formula. The measurement conditions are temperature: 23±2° C., sample shape: width 10 mm×length 50 mm, distance between chucks: 20 mm, tensile speed: 100 mm/min. In addition, a paper frame with a width of 40 × 60 mm was hollowed out at the center of the paper frame at 20 × 20 mm as a sample holder. Both ends of the sample holder were cut and measured. The above measurements were performed at three different points in the same film in the MD direction and the TD direction, and the average value of the three points was the tensile elongation at break in each direction (MD tensile elongation at break, TD tensile elongation at break).
Tensile elongation at break (%)=((L−L0)/L)×100.

(8) Shrinkage rate (%) at 130°C/1h
A 5 cm x 5 cm square sample having two sides parallel to the MD direction was cut out of the polyolefin microporous membrane. The sample length in the MD direction was measured at the central portion of the cut sample in the TD direction, and this was defined as the length before MD contraction (L _1MD ). Also, the length of the sample in the TD direction was measured at the center in the MD direction, and this was defined as the TD pre-contraction length (L _1TD ). Next, the sample was placed in an oven with an internal temperature of 130° C., heated, and taken out after 1 hour. The length in the MD direction was measured at the location where the length before MD contraction was measured, and this was defined as the length after MD contraction (L _2MD ). In addition, the length in the TD direction was measured at the location where the length before TD contraction was measured, and this was defined as the length after TD contraction (L _2TD ). Using these values, the thermal shrinkage rate after 1 hour at 130° C. was calculated by the following formula. Further, this measurement was performed at arbitrary three points within the sample surface, and the average value was calculated as the thermal shrinkage rate (%) after 1 hour at 130°C.
Formula 130 ° C in MD direction, thermal shrinkage rate (%) after 1 hour = 100 × (L _1MD - L _2MD ) / L _1MD
Formula Thermal contraction rate (%) after 1 hour at 130°C in the TD direction = 100 x (L _1TD - L _2TD )/L _1TD .

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

<Raman measurement conditions>
・Laser: 532nm
・Grating: 2400 Line/mm
・Lens: 20x
・Slit: 200×1000 μm
・Aperture: φ4000μm
1. A laser polarized in the machine direction of the polyolefin microporous membrane using a polarizer was incident on the specimen and the scattered light was collected through an analyzer oriented in the machine direction.
2. The ratio I ₁₁₃₀ /I ₁₀₆₀ of the Raman bands at 1130 cm ⁻¹ and 1060 cm ⁻¹ in the obtained Raman spectrum was defined as the Raman orientation parameter and the value was calculated.

The Raman spectrum was obtained with the polarizer having the direction parallel (0°/0°) to the longitudinal direction of the film as the MD direction and the direction (90°/90°) perpendicular to the film as the TD direction. 1130 cm- ¹ is a band attributed to the C—C stretching vibration of the polyethylene molecular chain in the crystal phase, and since the direction of the Raman tensor of vibration coincides with the molecular chain axis, the orientation of the molecular chain can be known. A larger value of the orientation parameter means that the crystal molecular chains are highly oriented.

<Calculation of peak and orientation parameters>
I _a : maximum Raman band intensity I _{b in the Raman shift band range of 1100 to 1170 cm −1 :} ^Raman band maximum intensity I _a in the Raman shift band range of 1040 to 1090 cm ⁻¹ (MD, 25° C.): 25 MD value _Ia (TD, 25°C) measured in °C: TD value _Ib (MD, 25°C) measured at 25°C: TD value measured at 25°C
I _b (TD, 25° C.): value in TD measured at 25° C. I _a (MD, 130° C.): value in MD after heating at 130° C. for 60 minutes using a heating stage I _a (TD, 130° C. ): value I _b in the TD direction after heating at 130° C. for 60 minutes using a heating stage (MD, 130° C.): value I _b in the MD direction after heating at 130° C. for 60 minutes using a heating stage (TD, 130° C. ): Value in TD direction fMH= _Ia (MD, 130°C)/ _Ib (MD, 130°C) after heating at 130°C for 60min using a heating stage(1) Formula fTH= _Ia (TD, 130°C)/ _Ib (TD, 130°C)(2) Equation fML= _Ia (MD, 25°C)/ _Ib (MD, 25°C) (3) Equation fTL= _Ia (TD, 25°C)/ _Ib (TD, 25°C) °C) (4) Formula fMLH= _Ia (MD, 25°C)/ _Ia (MD, 130°C)) (5) Formula fTLH= _Ia (TD, 25°C)/ _Ia ( TD, 130° C.)) Equation (6) fML-fMH Equation (7) fTL-fTH Equation (8).

In addition, I _a in equations (5) and (6) is the difference between the maximum intensity in the Raman shift band of 1100 to 1170 cm ⁻¹ and the intensity at 1200 cm ⁻¹ , and I _a (MD, 130° C.) is the MD direction. _Ia (TD, 130°C) is measured at 130°C in the TD direction, _Ia (MD, 25°C) is measured in the TD direction at 25°C, _Ia (TD, 25°C) is measured in the TD It is a value measured at 25° C. in the direction.

In addition, when measuring at 130°C using a heating stage, the polyolefin microporous membrane was fixed on four sides MD and TD with a Kapton tape.

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

The measurement cell consisted of an aluminum block and had a structure with a thermocouple directly below the polyolefin microporous membrane. A sample was cut into a 5 cm x 5 cm square, and the temperature was measured while fixing the periphery with an O-ring.

(11) Short-circuit test Short-circuit resistance was evaluated using a desktop precision universal testing machine, Autograph AGS-X (manufactured by Shimadzu Corporation). Polypropylene insulator (thickness: 0.2 μm)/negative electrode (for lithium-ion battery (copper foil (thickness: approx. 0.9 μm), active material: artificial graphite (particle diameter: approx. 13 μm))/separator/500 μm diameter chromium ball (material: : Chromium (SUJ-2))/aluminum foil laminate was prepared.The aluminum foil and negative electrode of the sample laminate were connected with a cable to a circuit consisting of a capacitor and a clad resistor.The capacitor was charged to about 1.5 V. , A metal ball (material: chromium (SUJ-2)) with a diameter of about 500 μm was placed between the separator and aluminum foil in the sample laminate.Then, the battery was short-circuited by pressing at a rate of 0.3 mm/min. The starting point is the point where the leakage current value begins to rise, and the short-circuit point is the moment when the above circuit is formed via the metal ball and the current is detected. A sample that does not short-circuit even with a large amount of displacement has better resistance to foreign matter, and the relationship between the amount of displacement and resistance to foreign matter is made into the following four stages. A or higher is preferable because higher energy density and higher capacity will be achieved.
S: Displacement (mm)/separator thickness (μm) is greater than 0.025 A: Displacement (mm)/separator thickness (μm) is greater than 0.024 and 0.025 or less B: Displacement (mm)/separator thickness (μm) is greater than 0.020 and 0.024 or less C: Displacement (mm)/separator thickness (μm) is 0.020 or less.

[Melting point of polyolefin resin raw material]
The melting point of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) in accordance with JIS K7121:1987. 6.0 mg of the sample was sealed in an aluminum pan, and the temperature was raised from 30 ° C. to 230 ° C. at a rate of 10 ° C./min under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer. After the temperature was raised at 230°C for 5 minutes (first temperature rise), the temperature was maintained at 230°C for 5 minutes, cooled at a rate of 10°C/min, and again heated from 30°C to 230°C at a rate of temperature increase of 10°C/min. (Second heating), each melting endotherm curve was obtained. The temperature of the peak top on the melting endothermic curve obtained in the second heating was taken as the melting point of the polyolefin resin raw material.

[Heat of fusion (ΔH) of polyolefin resin raw material]
The heat of fusion of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) in accordance with JIS K7121:1987. 6.0 mg of the sample was sealed in an aluminum pan, and the temperature was raised from 30 ° C. to 230 ° C. at a rate of 10 ° C./min under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer. After the temperature was raised at 230°C for 5 minutes (first temperature rise), the temperature was maintained at 230°C for 5 minutes, cooled at a rate of 10°C/min, and again heated from 30°C to 230°C at a rate of temperature increase of 10°C/min. (Second heating), each melting endotherm curve was obtained. The heat of fusion on the melting endothermic curve obtained in the second temperature rise was integrated from 60° C. to 160° C. to obtain ΔH (J/g) of the polyolefin resin raw material.

Examples will be shown and described below, but the present invention is not limited to these examples.

[Example 1]
Ultra high molecular weight polyethylene with Mw of 15×10 ⁵ is used as a raw material, 80 parts by weight of liquid paraffin is added to 20 parts by weight of ultra high molecular weight polyethylene, and 0.5 parts by weight is added to the weight of 20 parts by weight of ultra high molecular weight polyethylene. 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 for antioxidant These were added as agents and mixed to prepare a polyethylene resin solution. The obtained polyethylene resin solution was charged into a twin-screw extruder and kneaded at 180° C. to prepare a polyethylene solution. The resulting polyethylene solution was supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 35°C to form a gel sheet. The resulting gel-like sheet was longitudinally stretched by a roll system at a stretching temperature of 115° C. at a stretching ratio of 7.0 times. At this time, the pressure between the stretching rolls and the nip rolls was 0.3 Mpa. Subsequently, the film was led to a tenter and laterally stretched at a stretching temperature of 120° C. at a stretching ratio of 7.0 times. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried and laterally stretched again at a temperature of 135° C. at a stretching ratio of 1.4 times by a tenter method to obtain a polyolefin microporous membrane.

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

[Example 5]
Polyethylene consisting of 70 parts by mass of ultra-high molecular weight polyethylene with Mw of 15×10 ⁵ and 30 parts by mass of high-density polyethylene with Mw of 1×10 ⁵ , melting point of 131.5° C., and ΔH of 225 (J/g) as raw materials. (PE) of the mixture, 0.5 parts by weight of 2,6-di-t-butyl-p-cresol and 0.7 parts by weight of tetrakis[methylene-3-(3) with respect to 70 parts by weight of ultra-high molecular weight polyethylene ,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane was added as an antioxidant to obtain a polyethylene mixture. 75 parts by mass of liquid paraffin was added to 25 parts by mass of the obtained mixture, and the mixture was introduced into a twin-screw extruder and kneaded at 180° C. to prepare a polyethylene solution. The resulting polyethylene solution was supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 35°C to form a gel sheet. The resulting gel-like sheet was simultaneously biaxially stretched 5×5 times by a tenter method at a stretching temperature of 105°C. The stretched membrane was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane is dried, longitudinally stretched again to 1.7 times at a temperature of 100°C by a roll stretching method, and then transversely stretched again to a stretching ratio of 1.7 times at a temperature of 137°C by a tenter method. A membrane was obtained.

[Examples 6 to 8]
A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the raw material formulation and membrane-forming conditions were changed as shown in Table 1. The same high-density polyethylene as used in Example 5 was used as the high-density polyethylene used in Examples 6 and 7.

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

The evaluation results of the obtained polyolefin microporous membrane are as shown in Tables 3 and 4. In Tables 1 and 2, "UHPE" means ultra high molecular weight polyethylene, and "HDPE" means high density polyethylene.

In Examples 1 to 8, a microporous film with a small orientation parameter at high temperature calculated by microscopic Raman spectroscopy is obtained, and both excellent strength and low shrinkage are achieved. No. 7 achieves particularly good strength and low shrinkage, and a microporous membrane excellent in the short-circuit test is obtained. On the other hand, Comparative Examples 1 and 2 using high-density polyethylene as a main component have large orientation parameters at high temperatures and are inferior in strength and low shrinkage.

Claims

Both 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 0.00. A polyolefin microporous membrane having a ratio of 1.70 or less.
fMH = Ia (MD, 130°C)/ Ib (MD, 130°C) 　(1) Formula fTH = Ia (TD, 130°C)/ Ib (TD, 130°C) 　(2) where I a is the maximum Raman band intensity in the Raman shift band range of 1100 to 1170 cm -1 and I b is the Raman band maximum intensity in the Raman shift band range of 1040 to 1090 cm -1 , I a (MD, 130°C) and I b (MD, 130° C.) are the maximum intensities in the MD direction measured at 130° C. I a (TD, 130° C.) and I b (TD, 130° C.) are is the maximum intensity in the TD direction measured at .
2. The polyolefin microporous membrane according to claim 1, wherein the values calculated using a microscopic Raman spectrometer satisfy the following formulas (5) and (6).
fMLH = Da (MD, 25°C)/Da (MD, 130°C) ≤ 4 (5) Formula fTLH = Da (TD, 25°C) / Da (TD, 130°C) ≤ 4 (6) Formula D a is the difference between the maximum intensity in the Raman shift band 1100 to 1170 cm −1 and the intensity at 1200 cm −1 , D a (MD, 130 ° C) is measured at 130 ° C in the MD direction, D a (TD, 130°C) was measured at 130°C in the TD direction, Da (MD, 25°C) was measured at 25°C in the TD direction, Da (TD, 25°C) was measured at 25°C in the TD direction value.
3. The polyolefin microporous membrane according to claim 1 or 2, wherein a value calculated using a microscopic Raman spectrometer satisfies the following formulas (7) and (8).
0.00≤fML-fMH≤0.50 (7) 0.00≤fTL-fTH≤0.50 (8) Note that fML and fTL are the following (3) and (4) is the orientation parameter value (fML) in the MD direction and the orientation parameter value (fTL) in the TD direction measured at 25 ° C. calculated by the formula, and I a is the Raman band in the Raman shift band 1100 to 1170 cm −1 maximum intensity, I b is the maximum intensity of the Raman band in the Raman shift band 1040-1090 cm -1 , I a (MD, 25 °C), Maximum intensity, Ia (TD, 25°C), Ib (TD, 25°C), is the maximum intensity in the TD direction measured at 25°C.
fML = Ia (MD, 25°C)/ Ib (MD, 25°C) 　(3) Formula fTL = Ia (TD, 25°C)/ Ib (TD, 25°C) 　・・・Equation (4)
The polyolefin microporous membrane according to any one of claims 1 to 3, which has a tensile strength at break in the MD direction of 200 MPa or more.
5. The polyolefin microporous membrane according to any one of claims 1 to 4, which has a per unit area equivalent puncture strength of 0.7 N/(g/m 2 ) or more.
The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the total shrinkage rate at 130°C/1h in the MD and TD directions is 30% or less.
The polyolefin microporous membrane according to any one of claims 1 to 6, which has a shrinkage rate of 15% or less at 130°C/1h in the MD direction.
The polyolefin microporous membrane according to any one of claims 1 to 7, which has a shutdown temperature of 143°C or less.
The polyolefin microporous membrane according to any one of claims 1 to 8, wherein the polyolefin microporous membrane has a weight average molecular weight of 800,000 or more.
Claim 1, wherein the content of polyethylene having a molecular weight of 3.0×10 5 or less in the polyolefin microporous membrane is 50% by weight or less, and the content of polyethylene having a molecular weight of 9.0×10 5 or more is 30% or more. 9. The polyolefin microporous membrane according to any one of items 1 to 9.
The polyolefin microporous membrane according to any one of claims 1 to 10, wherein the main component of the polyolefin microporous membrane is polyethylene.
The polyolefin microporous membrane according to any one of claims 1 to 11, which is obtained by stretching including at least wet sequential biaxial stretching.
A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 12.
A secondary battery using the battery separator according to claim 13 .