JP2023050126A - Polyolefin microporous film, laminate, and non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide a polyolefin microporous film having coatability at high speed, an excellent rate characteristic, when forming a lithium ion secondary battery therefrom, and foreign matter resistance.SOLUTION: In a polyolefin microporous film, 10 μm conversion air permeability is 80 sec/100 cm3 or more and 350 sec/100 cm3 or less, and formula 1 and formula 2 are satisfied; -0.3≤F1MD≤0.5 ...(formula 1), 0.7≤F2MD≤2.0 ...(formula 2).SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a polyolefin microporous membrane, a laminate, and a non-aqueous electrolyte secondary battery.

Thermoplastic resin microporous membranes are widely used as material separation membranes, permselective membranes, isolation membranes, and the like. Specific uses of thermoplastic resin microporous membranes include separators for non-aqueous electrolyte secondary batteries (e.g., lithium-ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc.), and electric double-layer capacitors. separators, various filters (reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, etc.), moisture-permeable waterproof clothing, medical materials, fuel cell supports, and the like.

In particular, polyolefin microporous membranes are widely used as separators for lithium ion secondary batteries. It is characterized by excellent mechanical strength, which greatly contributes to battery safety and productivity. In addition, the polyolefin microporous membrane has both electrical insulation and ion permeability through the electrolyte that permeates the micropores. block the transmission of By automatically blocking the permeation of ions, the polyolefin microporous membrane has a pore blocking function (shutdown characteristic) that suppresses an excessive temperature rise.

In addition, if the high temperature state is maintained for a certain period of time after the pore blocking function (shutdown characteristic) is exhibited, the polyolefin microporous membrane after pore blocking will partially flow due to melting and the pore blocking will not be maintained. A phenomenon (meltdown) in which ions pass through is observed. The higher the temperature at which this meltdown occurs, the better. When incorporating a polyolefin microporous membrane as a separator into a lithium-ion secondary battery, it is often the case that a heat-resistant resin layer is applied to improve the meltdown resistance, that is, to raise the meltdown temperature. High-speed coatability is becoming important from the viewpoint of improving productivity.

In order to dry the coating film stably even at high speed coating, a high drying temperature is required. Properties are becoming increasingly sought after in polyolefin microporous membranes. In addition, from the viewpoint of downsizing and cost reduction of lithium ion secondary batteries, it has become important to maintain a high meltdown temperature in a structure having a thin heat-resistant resin layer.

Regarding the stress characteristics of polyolefin microporous membranes at high temperatures, Patent Document 1 discloses that by setting the temperature of the stress transition point and the stress peak value in a thermomechanical analysis measurement to specific ranges, thermal runaway of a lithium ion secondary battery can be prevented. Techniques for suppressing are proposed.

Patent Literature 2 proposes a technique for improving the safety of lithium ion secondary batteries by setting the shrinkage stress at 80° C. in thermomechanical analysis measurement to a specific range.

Further, Patent Document 3 proposes a technique for improving the output characteristics, foreign matter resistance, etc. of a lithium ion secondary battery by setting the piercing strength, the number of holes, etc. within specific ranges.

WO2019/045077 JP 2009-269941 A WO2020/203901

Although Patent Documents 1 and 2 describe stress in thermomechanical analysis measurements, and Patent Document 3 describes the relationship between puncture strength and pore structure, both are sufficient for high-speed coating properties. It was not designed in consideration of the above, and was inadequate from the viewpoint of compatibility between productivity and lithium ion secondary battery characteristics.

An object of the present invention is to provide a polyolefin microporous membrane that eliminates the above drawbacks and has high-speed coatability, good rate characteristics when used as a lithium ion secondary battery, and foreign matter resistance. .

The present invention for solving the above problems has the following configuration.
(1) A polyolefin microporous membrane having a 10 μm equivalent air permeability of 80 sec/100 cm ³ or more and 350 sec/100 cm ³ or less and satisfying Formulas 1 and 2 below.
−0.3≦F1MD≦0.5 (formula 1)
0.7≦F2MD≦2.0 (Formula 2)
In formula 1 or formula 2,
F1MD means the shrinkage stress (MPa) at 120°C in the longitudinal direction of the polyolefin microporous membrane, determined by thermomechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement,
F2MD means the maximum shrinkage stress (MPa) in the longitudinal direction of the polyolefin microporous membrane obtained by thermomechanical analysis (TMA), which is determined by temperature rising constant length measurement from 30°C to 250°C.
(2) The polyolefin microporous membrane according to (1), which satisfies Formula 3 below.
−0.3≦F1TD≦0.3 (Formula 3)
In formula 3,
F1TD means the shrinkage stress (MPa) at 120°C in the width direction of the polyolefin microporous membrane, determined by thermo-mechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement.
(3) The polyolefin microporous membrane according to (1) or (2), which satisfies the following formula 4.
0≦|F1MD−F1TD|≦0.15 (Formula 4)
In formula 4,
F1MD means the shrinkage stress (MPa) at 120°C in the longitudinal direction of the polyolefin microporous membrane, determined by thermomechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement,
F1TD means the shrinkage stress (MPa) at 120°C in the width direction of the polyolefin microporous membrane, determined by thermo-mechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement.
(4) The polyolefin microporous membrane according to any one of (1) to (3), which has a shutdown temperature of 128° C. or more and 143° C. or less in the temperature-rising air permeability method.
(5) The polyolefin microporous membrane according to any one of (1) to (4), which has a breaking elongation in the width direction of 100% or more and 180% or less.
(6) The molecular weight distribution of polyethylene measured by the GPC method contains 10% by mass or more of a polyethylene component with a molecular weight of 100,000 or less and 20% by mass or more of a polyethylene component with a molecular weight of 1,000,000 or more, (1) to (5) ) The polyolefin microporous membrane according to any one of ).
(7) A laminate comprising the polyolefin microporous membrane according to any one of (1) to (6) and a heat-resistant resin layer.
(8) A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to any one of (1) to (6) or the laminate according to (7).

The polyolefin microporous membrane according to the embodiment of the present invention enables high-speed coating when improving functions by coating, and has good rate characteristics and foreign matter resistance when made into a lithium ion secondary battery. is. Therefore, the polyolefin microporous membrane according to the embodiment of the present invention is effective in miniaturizing the battery and improving the impact resistance while suppressing the production cost.

Hereinafter, the polyolefin microporous membrane according to the embodiment of the present invention will be described in detail. In this specification, the polyolefin microporous membrane may be simply referred to as "microporous membrane".

A polyolefin microporous membrane according to an embodiment of the present invention contains a polyolefin resin as a main component. Here, "mainly composed of a polyolefin resin" means that the microporous membrane contains more than 50% by mass and 100% by mass or less of the polyolefin resin when the total mass of the microporous membrane is 100% by mass.

Here, as the polyolefin resin according to the embodiment of the present invention, various polyethylene resins, various polypropylene resins, and the like can be mentioned.

The polyethylene resin used in the embodiment of the present invention is an aspect in which the total content of ethylene-derived components exceeds 50% by mass and is 100% by mass or less when the total mass of the polymer of the polyethylene resin is 100% by mass. means a polymer of

Further, the polypropylene resin used in the embodiment of the present invention means that the total content of propylene-derived components exceeds 50% by mass and is 100% by mass or less when the total mass of the polymer of the polypropylene resin is 100% by mass. A polymer of one aspect is meant.

The polyethylene resin used in the embodiment of the present invention is a homopolymer consisting only of ethylene, or propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-butene, 3-methyl-1-pentene, 3-ethyl-1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene, 4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1 - Chain olefins (α-olefins) such as hexene, 3-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, etc. A copolymer obtained by copolymerizing is exemplified.

The polypropylene resin used in the embodiment of the present invention includes a homopolymer consisting only of propylene, an ethylene-propylene copolymer, an ethylene-propylene-butene copolymer, a propylene-butene copolymer, and the like. The addition amount of the polypropylene-based resin is more preferably less than 5% by mass, more preferably 3% by mass or less, relative to the total mass of the polyolefin microporous membrane. By setting it as the said range, it becomes the polyolefin microporous membrane excellent in foreign material resistance.

Moreover, the polyolefin resin used in the embodiment of the present invention may be either a single substance or a mixture of two or more different polyolefin resins.

Among these various polyolefin resins, polyethylene resins are particularly preferable from the viewpoint of excellent pore blocking performance. The melting point (softening point) of the polyethylene resin is preferably 70 to 150° C. from the viewpoint of pore blocking performance of the microporous membrane.

Hereinafter, polyethylene resin will be described in detail as an example of the polyolefin resin used in the embodiments of the present invention. The types of polyethylene resins used in embodiments of the present invention include high-density polyethylene having a density exceeding 0.94 g/cm ³ and medium-density polyethylene having a density in the range of 0.93 to 0.94 g/cm ³ . , low-density polyethylene with a density lower than 0.93 g/cm ³ , linear low-density polyethylene, ultra-high molecular weight polyethylene having a specific molecular weight, etc., but the pore structure inside the polyolefin microporous membrane described later is desired. From the viewpoint of controlling the content within the range of , a configuration containing ultra-high molecular weight polyethylene as a main component is preferable when the total mass of the microporous membrane is 100% by mass.

The ultra-high molecular weight polyethylene used in the embodiment of the present invention preferably has a weight average molecular weight of 1.0×10 ⁶ or more and 1.0×10 ⁷ or less. By using ultra-high molecular weight polyethylene with a weight average molecular weight of 1.0×10 ⁶ or more, the entanglement of molecular chains increases, and when a polyolefin resin sheet is stretched to produce a microporous film, stress is applied uniformly to the sheet. loaded. Therefore, it is possible to improve permeability by promoting pore opening, and to improve puncture resistance by promoting orientation of fibrils constituting the microporous membrane.

Therefore, the weight average molecular weight of the ultra-high molecular weight polyethylene is preferably 1.0×10 ⁶ or more, more preferably 1.5×10 ⁶ or more, still more preferably 2.0×10 ⁶ or more, Particularly preferably, it is 3.0×10 ⁶ or more. The upper limit of the weight-average molecular weight (Mw) is preferably 4.0×10 ⁶ or less, more preferably 1.8×10, from the viewpoint of stability when extruding the polyolefin resin raw material from an extruder. ⁶ or less.

The ultra-high molecular weight polyethylene used in the embodiment of the present invention preferably has a melting point of 135° C. or higher as determined by differential scanning calorimetry (DSC). The correlation between melting point and crystallinity is generally known, and a high melting point of ultrahigh molecular weight polyethylene means high crystallinity of ultrahigh molecular weight polyethylene. By setting the melting point of the ultra-high molecular weight polyethylene to 135°C or higher, it is possible to improve the balance between the permeability of the polyolefin microporous membrane and the shrinkage stress at 120°C. In addition, from the viewpoint of improving stretchability when a polyolefin resin sheet is stretched to produce a microporous film, the melting point of ultra-high molecular weight polyethylene is preferably 138° C. or less.

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

The polyolefin microporous membrane according to the embodiment of the present invention preferably has a configuration mainly composed of ultrahigh molecular weight polyethylene when the total mass of the microporous membrane is 100% by mass, but from the viewpoint of quality, high density polyethylene may include The high density polyethylene used in the embodiment of the present invention preferably has a weight average molecular weight (Mw) of 1.0×10 ⁴ or more and 1.0×10 ⁶ or less. By applying high-density polyethylene having a weight-average molecular weight within the above range to the polyolefin microporous membrane according to the embodiment of the present invention, the pressure fluctuation of the resin in the extruder is less likely to occur, and the quality may be improved. .

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- It is preferable to use one or more selected from di-t-butyl-4-hydroxybenzyl)benzene, tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane and the like.

It is important that the polyolefin microporous membrane according to the embodiment of the present invention has a 10 μm equivalent air permeability of 80 sec/100 cm ³ or more and 350 sec/100 cm ³ or less. Here ^, the 10 μm equivalent air permeability is 10 μm/ Refers to air permeability converted per 10 μm by multiplying T1 (μm).

Here, T1 (μm) refers to the average value of the thickness of five points measured within a 50 mm×50 mm square range of the polyolefin microporous membrane with a contact thickness meter. When the 10 μm equivalent air permeability of the polyolefin microporous membrane exceeds 350 seconds/100 cm ³ , the lithium ion permeability becomes insufficient when the polyolefin microporous membrane is used as a lithium ion secondary battery, and the rate characteristics are insufficient. may become. In addition, if the 10 μm equivalent air permeability of the polyolefin microporous membrane is less than 80 seconds/100 cm ³ , the withstand voltage characteristics may be insufficient, or it may be difficult to achieve compatibility with other characteristics such as puncture strength. .

It is important that the polyolefin microporous membrane according to the embodiment of the present invention satisfies the following (Equation 1) when the shrinkage stress at 120° C. in the longitudinal direction is F1MD (MPa).

−0.3≦F1MD≦0.5 (formula 1)

Here, the shrinkage stress at 120°C refers to the cross-sectional area of the sample at the initial stage of measuring the shrinkage force at 120°C by continuously measuring the shrinkage force by measuring the temperature at a constant length from 30°C to 250°C in thermomechanical analysis (TMA). It is a value converted to stress by dividing by .

One of the reasons why polyolefin microporous membranes are preferably used as separators for lithium-ion secondary batteries is that they have a pore-blocking function that automatically blocks ion permeation at about 120 to 150 ° C. in the event of an abnormal reaction inside or outside the battery. That is, it has a shutdown function. When coating a heat-resistant resin layer or the like on a polyolefin microporous membrane, the drying temperature is preferably as high as possible from the viewpoint of high-speed coating. However, if the shutdown function is manifested in the coating process and the pores are blocked, the permeability of lithium ions will decrease, and the rate characteristics etc. when making it into a lithium ion secondary battery will become insufficient. The upper limit is about 120°C. Therefore, the shrinkage stress at 120° C. is important in the longitudinal direction, which is the transport direction in the coating process or the like.

If the shrinkage stress F1MD at 120°C in the longitudinal direction of the polyolefin microporous membrane exceeds 0.5 MPa, the shrinkage stress in high-temperature drying during high-speed coating becomes too large, and wrinkles may appear in the laminate after coating. Cracks may occur in the film, resulting in a poor appearance or a decrease in the meltdown temperature in the laminate structure. In addition, if the shrinkage stress F1MD at 120°C in the longitudinal direction of the polyolefin microporous membrane is less than -0.3 MPa, it cannot withstand the conveying tension during high-temperature drying during high-speed coating, causing uneven deformation in the longitudinal direction. The meltdown temperature may decrease due to partial thickness unevenness of the laminated body after coating.

The shrinkage stress F1MD at 120°C in the longitudinal direction of the polyolefin microporous membrane is preferably 0.4 MPa or less, more preferably 0.3 MPa or less, from the viewpoint of improving the appearance of the laminate and improving the meltdown resistance. preferable. In addition, F1MD is preferably -0.1 MPa or more, more preferably 0 MPa or more, from the viewpoint of suppressing partial thickness unevenness of the coated laminate in high-temperature drying and improving meltdown resistance.

It is important that the polyolefin microporous membrane according to the embodiment of the present invention satisfies the following (formula 2), where F2MD (MPa) is the maximum shrinkage stress in the longitudinal direction.

0.7≦F2MD≦2.0 (Formula 2)

Here, the maximum shrinkage stress is defined as the shrinkage force measured continuously from 30°C to 250°C by thermomechanical analysis (TMA), and the maximum shrinkage force at 30°C to 250°C. It is a value converted to stress by dividing by the cross-sectional area of the sample.

When the maximum shrinkage stress F2MD in the longitudinal direction of the polyolefin microporous membrane exceeds 2.0 MPa, the shrinkage stress propagates to the coating film of the heat-resistant resin layer when maintained at a high temperature, and cracks occur in the coating film, resulting in lamination. Meltdown temperature in the body may decrease. Further, if the maximum shrinkage stress F2MD in the longitudinal direction of the polyolefin microporous membrane is less than 0.7 MPa, the pin puncture strength may decrease, resulting in insufficient foreign matter resistance.

The maximum shrinkage stress F2MD in the longitudinal direction of the polyolefin microporous membrane is preferably 1.7 MPa or less, more preferably 1.5 MPa or less, from the viewpoint of improving the meltdown resistance in the laminate structure. Moreover, F2MD is preferably 0.8 MPa or more, more preferably 0.9 MPa or more, from the viewpoint of good foreign matter resistance.

In the polyolefin microporous membrane according to the embodiment of the present invention, the 10 μm equivalent air permeability, the shrinkage stress F1MD at 120° C. in the longitudinal direction, and the maximum shrinkage stress F2MD in the longitudinal direction are individually designed within the scope of the present application. without difficulty. For example, the 10 μm equivalent air permeability is obtained by adjusting the raw material composition (concentration of plasticizer to be finally washed and removed) and manufacturing conditions (heat setting temperature) to set the porosity and pore diameter to a specific range. It is possible to design within the range of the present application by adjusting the composition (type and molecular weight of polyolefin, amount of resin added) and manufacturing conditions (heat setting temperature and relaxation rate after stretching) to obtain a specific range.

However, if the heat setting temperature is set high in order to design the F1MD and F2MD to be lower than the specific values, the porosity will be low and the 10 μm equivalent air permeability will be large and will be outside the above range. In addition, when ultra-high molecular weight polyethylene is selected as a raw material to design F2MD to be higher than a specific value and the molecular weight is designed to be high, F1MD is also increased and falls outside the above range. Thus, it is not easy to achieve all of the 10 μm equivalent air permeability, F1MD, and F2MD within the scope of the present application.

In the polyolefin microporous membrane according to the embodiment of the present invention, the following method may be mentioned as a method for adjusting all of the 10 μm equivalent air permeability, F1MD, and F2MD within the scope of the present application. That is, the amount of polypropylene resin added is less than 5% by mass with respect to the total mass of the polyolefin microporous membrane, the weight average molecular weight is 1.0 × 10 ⁶ or more and 1.8 × 10 ⁶ or less, and the melting point is 135 ° C. or more and 138 ° C. Hereinafter, in the DSC curve obtained from differential scanning calorimetry (DSC ₎ , the ratio of the heat of fusion ΔH ₁₃₀ at 130 ° C. or less (= ΔH ₁₃₀ / ΔH _all ) of the total heat of fusion ΔH all is 45% or less. A composition containing molecular weight polyethylene as a main component is used as a raw material composition. Then, with respect to the raw material composition, the final surface magnification is 60 times or more, and after the heat setting step, it is continuously brought into contact with a plurality of rolls at 80 ° C. or more and 120 ° C. or less for 2 seconds or more, and the roll peripheral speed difference is used. For example, a method of relaxing in the direction by 0.1% or more and 4% or less.

In particular, in order to achieve both an F1MD of 0.5 MPa or less and an F2MD of 0.7 MPa or more, a specific weight-average molecular weight and highly crystalline ultra-high molecular weight polyethylene are used as the main component to achieve high orientation characteristics and high temperature regions. This can be achieved by providing a raw material composition that allows designing a structure that satisfies both relaxation responsiveness and relaxing the raw material composition in the longitudinal direction under a specific condition of 80° C. or higher and 120° C. or lower.

In the polyolefin microporous membrane according to the embodiment of the present invention, from the viewpoint of suppressing edge curling during high-speed coating and improving workability, when the shrinkage stress at 120 ° C. in the width direction is F1TD (MPa) , preferably satisfies the following (formula 3).

−0.3≦F1TD≦0.3 (Formula 3)

As a method for setting F1TD within the range of (Formula 3), there is a method of adjusting the heat setting temperature and the relaxation rate in the heat setting step to relax the residual strain in the width direction. From the viewpoint of suppressing edge curling during high-speed coating, F1TD is more preferably 0.15 MPa or less, and particularly preferably 0.13 MPa or less.

The polyolefin microporous membrane according to the embodiment of the present invention preferably satisfies the following (Formula 4) from the viewpoint of suppressing curling in a low-humidity environment in the manufacturing process of a lithium ion secondary battery.

0≦|F1MD−F1TD|≦0.15 (Formula 4)

It is known that curling is more likely to occur in a low-humidity environment than in a normal humidity environment due to the combination of the effects of changes in the amount of moisture. Smaller is better. Even in the case of a microporous membrane, where shrinkage stress measurement at room temperature shows almost no difference between the longitudinal and width directions, the shrinkage stress measurement at a high temperature such as 120°C amplifies the stress difference between the longitudinal and width directions. Therefore, it is possible to estimate the curl resistance in a low-humidity environment.

In the polyolefin microporous membrane according to the embodiment of the present invention, |F1MD-F1TD| is more preferably 0.1 MPa or less, more preferably 0.05 MPa or less, from the viewpoint of improving curl resistance in a low-humidity environment. is particularly preferred.

As a method for making |F1MD-F1TD| within the range of (Equation 4) in the polyolefin microporous membrane according to the embodiment of the present invention, a method of sufficiently propagating the strain change after stretching or after relaxation treatment to the entire microporous membrane. etc.

Specifically, after the dry stretching step in the width direction and after the heat setting step (relaxation in the width direction), straight sections L1 and L2 are arranged in which the magnification does not change, and the time T1 for passing through L1 is set. The ratio of the time T2 to pass through the length of L2 is set to 1.3 times or more and 6 times or less to reduce the shrinkage stress in the width direction, and after the heat setting process, a plurality of temperatures of 80 ° C. or more and 120 ° C. or less The shrinkage stress in the longitudinal direction is reduced by 0.1% or more and 4% or less in the longitudinal direction due to the roll peripheral speed difference while contacting the roll for 2 seconds or longer.

The polyolefin microporous membrane according to the embodiment of the present invention has a shutdown temperature of 143 ° C. or less in the temperature-rising air permeability method from the viewpoint of improving battery safety when used as a separator of a lithium ion secondary battery. is preferred. From the viewpoint of better battery safety, the shutdown temperature in the temperature-rising air permeability method is more preferably 141° C. or lower, and particularly preferably 138° C. or lower. In addition, from the viewpoint of improving foreign matter resistance while keeping F1MD and F1TD, which are shrinkage stresses at 120 ° C., within the desired range, shutdown in the temperature-rising air permeability method of the polyolefin microporous membrane according to the embodiment of the present invention The temperature is preferably 128°C or higher, more preferably 135°C or higher.

Various techniques for lowering the shutdown temperature have been proposed so far. It is not easy to further lower the shutdown temperature while forming a structure in which the composition is used as the raw material composition and the relaxation treatment is sufficiently performed while heating in both the longitudinal direction and the width direction to sufficiently progress crystallization.

As a method of setting F1MD, F1TD, and F2MD within the range of (Formula 1) to (Formula 4) and further setting the shutdown temperature to 143°C or less, the weight average molecular weight is 1.0 × 10 ⁶ or more and 1.8 × 10 ⁶ Hereinafter, the ratio of the heat of fusion ΔH 130 up to 130° C. or less out of the total heat of fusion ΔH _all in the DSC curve obtained from the melting point 135° C. or higher and 138° C. or lower (=ΔH ₁₃₀ /ΔH _all ) _is A method of obtaining a composition containing ultra-high molecular weight polyethylene of 45% or less as a main component and containing specific high-density polyethylene described later, and further kneading with a twin-screw extruder having a screw with a specific structure described later. mentioned.

Specifically, the specific high density polyethylene has a melting point of 130° C. or higher and 135° C. or lower, a weight average molecular weight of 3.0×10 ³ or higher and 1.5×10 ⁵ or lower, and differential scanning calorimetry (DSC). A high-density polyethylene having a total heat of fusion ΔH _all of 200 J/g or more and 250 J/g or less and a half width of 4.0° C. or more and 6.0° C. or less in the DSC curve obtained from .

Further, as the screw having the specific structure, the distance between the tip of the screw and the vent hole is 1.0D or more and 15.0D or less, where D is the outermost diameter of the screw. A screw with a structure that uses at least one screw piece with a length of 0.2D or more and 0.9D or less and does not use two or more screw pieces with a length of 1.5D or more in the raw material conveying direction mentioned.

A resin composition containing ultra-high molecular weight polyethylene with a specific melting point and crystallinity and high-density polyethylene with a specific melting point and crystallinity is stretched after forming into a sheet with an extruder having a screw with a specific configuration with high kneading ability. As a result, the shutdown temperature in the temperature-rising air permeability method can be set to 143° C. or lower while satisfying the ranges of (Equation 1) to (Equation 4).

The polyolefin microporous membrane according to the embodiment of the present invention preferably has a breaking elongation of 100% or more in the width direction from the viewpoint of improving foreign matter resistance when used as a separator of a lithium ion secondary battery. , more preferably 110% or more, particularly preferably 120% or more. From the viewpoint of facilitating designing F1MD, F2MD, and F1TD within the range of (Formula 1) to (Formula 4), the breaking elongation in the width direction is preferably 180% or less. Examples of the method for setting the breaking elongation in the width direction within a specific range include a method of appropriately adjusting the weight average molecular weight of ultra-high molecular weight polyethylene, the wet draw ratio in the width direction, the dry draw ratio, the relaxation rate, and the like.

The polyolefin microporous membrane according to the embodiment of the present invention has the molecular weight of polyethylene measured by the GPC method from the viewpoint of facilitating designing the shutdown temperature in the desired range in F1MD, F2MD, F1TD, and the temperature-rising air permeability method. The distribution preferably contains 10% by mass or more of a polyethylene component with a molecular weight of 100,000 or less and 20% by mass or more of a polyethylene component with a molecular weight of 1,000,000 or more.

In the polyolefin microporous membrane according to the embodiment of the present invention, as a method of making the polyethylene component with a molecular weight of 100,000 or less or the polyethylene component with a molecular weight of 1,000,000 or more in the molecular weight distribution of polyethylene measured by the GPC method a specific range, weight average A composition containing ultra high molecular weight polyethylene having a molecular weight of 1.0×10 ⁶ or more and 1.8×10 ⁶ or less and high density polyethylene having a weight average molecular weight of 3.0×10 ³ or more and 1.5×10 ^{5 or} less is used as a raw material composition. and a method of appropriately adjusting the ratio of ultra-high molecular weight polyethylene and high density polyethylene, antioxidant concentration, plasticizer concentration, and the like.

The polyolefin microporous membrane according to the embodiment of the present invention may be a laminate having a heat-resistant resin layer from the viewpoint of improving meltdown resistance when mounted in a lithium ion secondary battery. As the heat-resistant resin layer, resins such as various fluorine resins, acrylic resins, and aromatic polyamide resins, which are insoluble in the battery electrolyte and are electrically stable within the range of battery usage conditions, are preferably used.

From the viewpoint of further improving the heat resistance, the heat-resistant resin layer may contain organic powder, inorganic powder, or a mixture thereof as a filler. For example, as the organic powder, a fluororesin, a melamine resin, an aromatic polyamide resin, or the like can be used. As the inorganic powder, metal oxides, metal nitrides, metal carbides, metal hydroxides, carbonates, sulfates, etc., more specifically alumina, silica, titanium dioxide, aluminum hydroxide, calcium carbonate, etc. can be used. can be done.

A non-aqueous electrolyte secondary battery according to an embodiment of the present invention includes a polyolefin microporous membrane according to an embodiment of the present invention or a polyolefin microporous membrane laminate according to an embodiment of the present invention. Non-aqueous electrolyte secondary batteries include, for example, lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries and the like.

Next, an example of a method for producing a polyolefin microporous membrane according to an embodiment of the present invention will be described below, but the present invention should not be construed as being limited to such an example.

A method for producing a polyolefin microporous membrane according to an embodiment of the present invention preferably includes the following steps (a) to (g).

(a) Extrusion step of melt-kneading one or more polyolefin resins and a plasticizer to prepare a polyolefin resin solution (b) Casting step (c) of extruding the polyolefin resin solution, molding it into a sheet, and cooling and solidifying it ) A wet stretching step of stretching the obtained sheet by a roll method or a tenter method (d) A washing/drying step of extracting a plasticizer from the stretched film and drying the film (e) Re-stretching the film after washing/drying Dry stretching step (f) First heat setting step (g) Heating while relaxing the film after dry stretching in the width direction (g) Heating while relaxing the film after the first heat setting step in the longitudinal direction Second heat setting process

Each step will be described below.
(a) Extrusion step for preparing polyolefin resin solution A polyolefin resin solution is prepared by heating and dissolving the polyolefin resin used in the embodiment of the present invention in a plasticizer. 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 high magnification.

Solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate, Phthalic acid esters that are liquid at room temperature, such as dioctyl phthalate, can be mentioned. In order to obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

As for the solvent ratio, from the viewpoint of stably dissolving ultra-high molecular weight polyethylene as the main component of the polyolefin resin in the polyolefin resin solution, the solvent concentration is 70% by mass when the total amount of the polyolefin resin solution is 100% by mass. is preferred. A solvent that is solid at room temperature and is miscible with the polyolefin resin in a melt-kneaded state may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, paraffin wax, and the like.

The viscosity of the liquid solvent is preferably 20-200 cSt at 40°C. When the viscosity at 40° C. is 20 cSt or more, the gel-like 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) Casting step of molding into a sheet and cooling and solidifying The method of melt-kneading the polyolefin resin solution is not particularly limited, but 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). For example, when the polyolefin resin is a polyethylene resin, the melt-kneading temperature is preferably in the range of 130 to 260°C because the polyethylene resin often has a melting point in the range of about 120 to 140°C. When two or more polyolefin resins are used, the melt-kneading temperature is preferably set according to the polyolefin resin having the highest melting point.

If the melt-kneading temperature is too low, an insufficiently kneaded fine composition called unmelted material is produced, which may cause deterioration of the appearance of the microporous membrane or breakage of the membrane during stretching. On the other hand, if the melt-kneading temperature is too high, the polyolefin resin may deteriorate, resulting in a decrease in weight average molecular weight and yellowing.

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

Next, a gel-like sheet is obtained by cooling the melt-kneaded resin. Cooling methods include direct contact with cold air, cooling water, and other cooling media, contact with refrigerant-cooled rolls, and the use of casting drums, etc. The final cooling temperature can be adjusted depending on the sheet transport. A temperature of 10 to 50° C. is preferable in order to solidify to a sufficient degree.

So far, the polyolefin microporous membrane has been described as a single layer, but the polyolefin microporous membrane according to the embodiment of the present invention is not limited to a single layer, and may be a laminate of polyolefin microporous layers. . 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.

As a method of making a polyolefin microporous membrane into a laminate of polyolefin microporous layers, for example, desired resins are prepared as necessary, and these resins are separately supplied to an extruder and melted at a desired temperature, Examples include a method of merging in a polymer tube or a mouthpiece.

(c) Wet stretching step The resulting gel-like sheet is stretched while containing a plasticizer. As the stretching method, uniaxial stretching in the sheet conveying direction (MD direction) by the peripheral speed difference of the roll in the roll stretching machine, while holding the sheet with clips in the tenter, the sheet conveying direction (MD direction), or the width direction ( TD direction), sequential biaxial stretching by combining a roll stretcher and a tenter, or a tenter and a tenter, while holding the sheet in the tenter with clips in the sheet conveying direction (MD direction) and width direction (TD direction) Simultaneous biaxial stretching and the like in which stretching is performed at the same time.

From the viewpoint of enhancing the uniformity of the microporous structure, the draw ratio is preferably 5 times or more and 10 times or less in any direction.

The stretching temperature is preferably the melting point of the gel sheet + 10°C or lower, more preferably in the range of (the crystal dispersion temperature of the polyolefin resin) to (the melting point of the gel sheet + 5°C). Specifically, since polyethylene compositions often have a crystal dispersion temperature of about 90 to 110°C, the stretching temperature is preferably 100 to 130°C, more preferably 115 to 125°C. The crystal dispersion temperature Tcd can be obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012).

If the stretching temperature is lower than 90° C., the pore size may become non-uniform, and the withstand voltage characteristics and the like may become insufficient. On the other hand, if the stretching temperature is higher than 130° C., it may be difficult to form pores, resulting in insufficient permeability.

By stretching as described above, the polyolefin resin component in the gel sheet is cleaved, forming a network structure in which a large number of fibrils are connected three-dimensionally, and the fibrils are selectively arranged in the plane direction. The mechanical properties such as are improved. By stretching the film before removing the plasticizer, the polyolefin resin component can be smoothly cleaved, making it possible to make the pores finer and reduce the heat shrinkage.

(d) Washing/Drying Step The remaining plasticizer (solvent) is removed from the wet stretched film obtained in (c) using a washing solvent. Since the polyolefin resin phase and the plasticizer phase are separated, a polyolefin microporous membrane can be obtained by removing the plasticizer.

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

These cleaning solvents have a low surface tension (eg, 24 mN/m or less at 25°C). By using a cleaning solvent with a low surface tension, shrinkage due to the surface tension of the air-liquid interface when the network structure that forms the microporosity is dried is suppressed, and it has moderate pores and excellent permeability. A polyolefin microporous membrane is obtained. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

Examples of the washing method include a method of immersing the wet-stretched film in a washing solvent for extraction, a method of showering the wet-stretched film with a washing solvent, a method of combining these methods, and the like. The amount of the washing solvent used varies depending on the washing method, but generally it is preferably 300 parts by weight or more per 100 parts by weight of the wet stretched film.

The washing temperature may be from 15 to 30°C, with heating to 80°C or less if necessary. At this time, the point of enhancing the cleaning effect of the cleaning solvent, the point of uniforming the physical properties of the polyolefin microporous membrane obtained (for example, the physical properties in the TD direction and / or the MD direction), the mechanical properties and electrical properties of the porous polyolefin film From the viewpoint of improving physical properties, it is preferable that the wet-stretched film is immersed in the washing solvent for a longer period of time and the number of times of washing is increased. preferable.

Thereafter, the washing solvent is removed by drying from the wet stretched film in which the plasticizer has been replaced with the washing solvent. 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. Insufficient drying may reduce the porosity of the polyolefin microporous membrane in the subsequent heat treatment, resulting in poor permeability.

(e) Dry Stretching Step The dried film obtained in (d) may be re-stretched at least uniaxially from the viewpoint of adjusting physical properties such as pore size and puncture strength. Re-stretching can be carried out with a roll stretching machine, a tenter, or a combination thereof, but from the viewpoint of adjusting shrinkage stress in the width direction, it is preferable to include at least stretching with a tenter. The re-stretching temperature is preferably set below the melting point of the film after drying, and in the case of a composition containing polyethylene resin as a main component, it is specifically preferably 70 to 135°C. The re-stretching ratio may be either uniaxial stretching or biaxial stretching, but in the case of biaxial stretching, it is preferable to stretch 1.01 to 2.0 times in each of the transport direction and the width direction. Further, after stretching in the width direction, it is preferable to install a straight section in which the magnification does not change at a temperature equivalent to the stretching temperature, from the viewpoint of relaxing strain immediately after stretching.

(f) First heat-setting step The dry-stretched film obtained in (e) is heat-set to adjust the heat shrinkage rate and shrinkage stress. When the last drawing in the dry drawing step (e) is a tenter, heat setting may be continuously performed in the same tenter. Heat setting in the tenter is preferably performed while relaxing in the width direction from the viewpoint of reducing the heat shrinkage rate and shrinkage stress, and the relaxation rate (the width of the film after heat setting with respect to the maximum film width in dry stretching) is 5% or more and 20% or less is preferable. Moreover, the heat setting temperature is preferably 125 to 140° C. in the case of a structure containing polyethylene resin as a main component. Further, it is preferable from the viewpoint of reducing the residual strain in the entire film to provide a straight section in which the magnification does not change at a temperature equivalent to the heat setting temperature after being relaxed in the width direction. In addition, when the length of the straight section after the width direction stretching in the step (e) is L1 (m) and the length of the straight section after the relaxation in this step is L2 (m), the film passes through L2. The time T2 (seconds) for passing through L1 is preferably 1.3 to 6 times the time T1 (seconds) for passing through L1 from the viewpoint of reducing shrinkage stress in the width direction at 120°C.

(g) Second heat-setting step The film after the first heat-setting step obtained in (f) is further heat-set in a direction different from the direction in which relaxation adjustment was performed in (f). may Considering that it is efficient to continuously perform (e) and (f) with a tenter, the second heat setting process is preferably performed while relaxing in the conveying direction. In addition, when the second heat setting step is performed while relaxing in the conveying direction, from the viewpoint of improving the quality after relaxing by not fixing the width direction, the peripheral speed difference of a plurality of rolls is used to relax. A method is preferred. The temperature of each roll is preferably 80 ° C. or higher and 120 ° C. or lower from the viewpoint of promoting relaxation in the transport direction and suppressing deterioration in appearance due to heat fusion of the film, and the contact time between the film and the roll (the total contact time of a plurality of rolls time) is preferably 2 seconds or more and 30 seconds or less. In addition, the relaxation rate in the transport direction due to the difference in roll peripheral speed (the rate of decrease in the transport speed of the last roll relative to the transport speed of the first roll) is 0 from the viewpoint of promoting relaxation in the transport direction and suppressing deterioration of appearance such as wrinkles. .1% or more and 4% or less is preferable. Moreover, from the viewpoint of being able to strictly control the shrinkage stress in each direction, it is preferable to use a method in which the steps (f) and (g) are not performed in the same tenter, but are performed separately.

Methods for measuring properties and evaluating effects according to embodiments of the present invention are as follows. However, embodiments of the present invention are not limited to these examples. The evaluation method was performed at a temperature of 25° C. and a humidity of 50% unless otherwise specified.

(1) weight average molecular weight (Mw)
The weight average molecular weight of the polyethylene resin was determined by gel permeation chromatography (GPC) under the following conditions.
・Measuring device: GPC-150C manufactured by WATERS CORPORATION
・Column: SHODEX UT806M manufactured by Showa Denko K.K.
・Column temperature: 135°C
・ Solvent (mobile phase): O-dichlorobenzene ・ Solvent flow rate: 1.0 mL / min ・ Sample concentration: 0.1 wt% (dissolution condition: 135 ° C. / 1 H)
・Injection volume: 500 μL
・Detector: WATERS CORPORATION differential refractometer (RI detector)
• Calibration curve: prepared using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.

(2) Melting point of polyolefin resin Determined by differential scanning calorimetry (DSC) based on JIS K7121 (1987). 6.0 mg of the sample was sealed in an aluminum pan, and using a PYRIS Diamond DSC manufactured by Parking Elmer, the temperature was increased from 30 ° C. to 230 ° C. at a rate of 10 ° C./min under a nitrogen atmosphere, and then the temperature was increased to 230. C. for 5 minutes and cooled at a rate of 10.degree. C./min. Thereafter, the temperature was raised from 30° C. to 230° C. for the second time at a rate of 10° C./min to obtain each melting endothermic curve (DSC curve). The temperature of the maximum peak of the melting endothermic curve (DSC curve) obtained in the second heating was taken as the melting point of the polyolefin resin.

(3) ΔH ₁₃₀ /ΔH _all of polyolefin resin
For the melting endothermic curve (DSC curve) at the second temperature rise obtained in the same manner as in (2), a linear baseline was set in the range of 60 ° C. to 160 ° C. using analysis software. The total heat of fusion ΔH _all (J/g) was calculated by calculating the amount of heat from the area of the portion surrounded by the melting endothermic curve and converting the amount of heat per unit mass. In addition, the heat quantity of the area surrounded by the baseline and the melting endothermic curve at 130 ° C. or lower is calculated, and the heat quantity is converted to a unit mass, so that the heat of fusion at 130 ° C. or lower ΔH ₁₃₀ (J / g) was calculated. ΔH ₁₃₀ /ΔH _all (%) was determined from each heat of fusion obtained.

(4) DSC half width (2) From the melting endothermic curve (DSC curve) at the second heating, read the maximum maximum peak height (h), Two points of temperature showing a height of h/2 were read from the melting endothermic curve. The absolute value of the difference in temperature between the two read points was obtained and used as the DSC half width (°C). When two or more points of temperature indicating the height of h/2 were read, the two points closest to the melting point were used to obtain the half-value width.

(5) Thickness The thickness of the polyolefin microporous membrane at 5 randomly selected locations within the range of 50 mm × 50 mm is measured with a contact thickness meter (Lightmatic VL-50 manufactured by Mitutoyo Co., Ltd. (10.5 mmφ super hard spherical surface measurement The thickness was measured with a load of 0.01 N), and the average value of five points was taken as the thickness (μm).

(6) Porosity A sample of 50 mm×50 mm square was cut from the polyolefin microporous membrane, and the mass (g) was measured. Also, the volume (cm ³ ) of the sample was calculated from the thickness and cut size obtained in (5). The porosity (%) was calculated by the following formula from the obtained mass and volume, and resin density (g/cm ³ ). The resin density was calculated as 0.99 g/cm ³ .
Porosity (%) = [(volume - mass / resin density) / volume] x 100

(7) 10 μm conversion air permeability Air permeability P ₁ (seconds/100 cm ³ ) was measured with an Oken type air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P8117 (2009). bottom. When the thickness obtained in (5) was defined as T1, it was converted into air permeability per 10 μm by the following formula, and the 10 μm equivalent air permeability (second/100 cm ³ ) was obtained.
10 μm conversion air permeability (second/100 cm ³ )=P ₁ (second/100 cm ³ )×10 μm/T1 (μm)

(8) F1MD
A sample for evaluation was prepared by cutting a polyolefin microporous membrane into a rectangle of 15 mm in the MD direction and 3 mm in the TD direction. For the evaluation sample, the thickness of the evaluation sample was measured in the same manner as in (5) except that the measurement positions were three locations along the center line in the MD direction (the center and the intermediate positions between the center and each end). asked for After that, using "TMA7100" manufactured by Hitachi High Technology Co., Ltd., the sample for evaluation was fixed to the chuck so that the distance between the chucks (MD direction) was 10 mm, and the temperature was changed from 30°C to 250°C at 5°C/min in the constant length mode. The temperature was raised at a rate of The temperature and shrinkage force when the temperature was raised to 200° C. were measured at intervals of 1 second, and the shrinkage force (gf) at 120° C. was obtained from the obtained chart. The cross-sectional area S was calculated from the width and thickness of the evaluation sample before fixing to the chuck, and the shrinkage force at 120°C was divided by the cross-sectional area S to obtain the shrinkage stress F1MD (MPa) at 120°C.
In addition, polyolefin microporous membranes can be identified as MD or TD by the following method. When a polyolefin microporous membrane is obtained from a battery, the winding direction (longitudinal direction) of the battery is MD, and the longitudinal direction is basically MD even in a stack type. If the film is a cut film and the direction is unknown, the amount of heat shrinkage is measured, and the direction with the highest amount of heat shrinkage is defined as the MD. In addition, the amount of heat shrinkage can be measured by the following method.
(Amount of heat shrinkage)
A polyolefin microporous membrane is cut into a circle with a diameter of 100 mm and placed in an oven at 40° C. for 24 hours. The direction in which the dimension of the microporous polyolefin membrane after being placed in the oven is the shortest is defined as the MD direction.

(9) F2MD
The temperature and shrinkage force in the MD direction were obtained in the same manner as in (8), and the maximum shrinkage stress (MPa) was obtained by dividing the maximum shrinkage force (gf) by the cross-sectional area S of the evaluation sample.

(10) F1TD
The shrinkage force (gf) at 120° C. was obtained in the same manner as in (8) except that the sample for evaluation was a rectangle of 3 mm in the MD direction and 15 mm in the TD direction, and the sample installation direction between the chucks was the TD direction. The shrinkage force at 120°C was obtained by dividing the shrinkage force at 120°C by the cross-sectional area S.

(11) 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 (manufactured by 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 was composed of an aluminum block with a thermocouple directly below the polyolefin microporous film.

(12) TD breaking elongation A tensile test is performed using a tensile tester (Shimadzu Autograph AGS-J type), the gauge length L0 (mm) of the test piece before the test, the gauge length L at break ( mm) from the following formula. The measurement conditions were temperature: 25° C., sample shape: width 10 mm (MD direction)×length 50 mm (TD direction), distance between chucks: 20 mm, tensile speed: 100 mm/min. Measurements were taken at three different points on the same film, and the average value was taken as the TD breaking elongation (%). Tensile elongation at break (%) = ((L-L0) / L) x 100

(13) Ratio of Components with a Molecular Weight of 100,000 or Less For the polyolefin microporous membrane, the molecular weight distribution obtained by GPC in the same manner as in (1) was used to calculate the following formula.
Ratio of components with a molecular weight of 100,000 or less (% by mass) = (Amount of polyethylene components with a molecular weight of 100,000 or less) / (Amount of total polyethylene components) x 100

(14) Ratio of Components with a Molecular Weight of 1,000,000 or More For the polyolefin microporous membrane, the molecular weight distribution obtained by GPC in the same manner as in (1) was used to calculate the following formula.
Ratio of components with a molecular weight of 1,000,000 or more (% by mass) = (Amount of polyethylene components with a molecular weight of 1,000,000 or more) / (Amount of total polyethylene components) x 100

(15) Rate characteristics In order to evaluate the capacity retention rate under rapid charge/discharge conditions when configuring a lithium ion secondary battery, a non-aqueous electrolyte secondary battery consisting of a positive electrode, a negative electrode, a separator and an electrolyte was used. A charge-discharge test was performed by incorporating the membrane as a separator.

NMC532 (lithium ^nickel manganese cobalt composite oxide (Li _1.05 Ni _0.50 Mn _0.29 Co _0.21 O ₂ )) was laminated on the cathode, and the anode was laminated with natural graphite having a density of 1.45 g/cm ³ on a copper foil of width 40 mm × length 35 mm × thickness 10 μm with a unit area mass of 5.5 mg/cm ² . was used. The positive and negative electrodes were dried in a vacuum oven at 120° C. before use.

A polyolefin microporous membrane having a length of 50 mm and a width of 50 mm was dried in a vacuum oven at room temperature and used as the separator. The electrolytic solution is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30/35/35, volume ratio) in which vinylene carbonate (VC) and LiPF ₆ are dissolved, VC concentration: 0.5% by mass, LiPF ₆ Concentration: A 1 mol/L solution was prepared.

A positive electrode, a separator, and a negative electrode were stacked, the resulting laminate was placed in a laminated pouch, an electrolytic solution was injected into the laminated pouch, and the laminated pouch was vacuum-sealed to produce a lithium ion secondary battery. .

The prepared lithium ion secondary battery was charged at 35° C. and 0.1 C by 10 to 15% as an initial charge, left at 35° C. overnight (12 hours or more), and degassed. Next, a temperature of 35° C., a voltage range of 2.75 to 4.2 V, CC-CV charging with a charging current value of 0.1 C (final current condition of 0.02 C), and CC discharging with a discharge current value of 0.1 C were performed. . Next, a temperature of 35° C., a voltage range of 2.75 to 4.2 V, a CC-CV charge with a charge current value of 0.2 C (final current condition of 0.05 C), and a CC discharge with a discharge current value of 0.2 C for 3 cycles. The time point at which the test was performed was defined as the initial stage of the lithium ion secondary battery.

Next, after CC-CV charging (final current condition 0.05C) at temperature: 35°C, voltage range: 2.75 to 4.2V, charging current value: 0.2C, CC at 0.2C at 15°C Discharge was performed, and the discharge capacity at that time was defined as 0.2C capacity. Subsequently, temperature: 35 ° C., voltage range: 2.75 to 4.2 V, charging current value: after CC-CV charging (final current condition 0.05 C) at 0.5 C, lithium ion secondary battery at 15 ° C. A rate test at 10C (180 mA, 14.4 mA/cm ² ) was performed. From this result, the ratio of 10C capacity to 0.2C capacity {(10C capacity/0.2C capacity) x 100} (%) was taken as the capacity retention rate (%) under rapid charge/discharge conditions. The higher the capacity retention rate, the better the rate characteristics.

A: 65% or more B: 60% or more and less than 65% C: 50% or more and less than 60% D: 40% or more and less than 50% E: less than 40%

(16) Foreign matter resistance The foreign matter resistance of the polyolefin microporous membrane was tested 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)) / polyolefin microporous film / 500 μm diameter chromium A sphere (material: chromium (SUJ-2))/aluminum foil evaluation laminate was prepared, and the aluminum foil and negative electrode of the evaluation laminate were connected to a circuit consisting of a capacitor and a clad resistor with a cable. After charging to 1.5 V, the battery was pressed under the condition of 0.3 mm/min, and foreign matter resistance was evaluated by the amount of displacement until the battery short-circuited.In the change in compressive load, the point where the leakage current value began to rise was the start point. The displacement from the starting point to the short-circuit point was measured by setting the moment when the above circuit was formed through the point and the metal ball and the current was detected as the short-circuit point. It shows that the property is good, and was evaluated in the following 5 stages, and D or higher was regarded as a pass.

A: Displacement (mm)/polyolefin microporous film thickness (μm) of 0.025 or more B: Displacement (mm)/polyolefin microporous film thickness (μm) of 0.024 or more and less than 0.025 C: Displacement (mm) / polyolefin microporous film thickness (μm) is 0.022 or more and less than 0.024 D: transition (mm) / polyolefin microporous film thickness (μm) is 0.020 or more and less than 0.022 E: transition (mm) / polyolefin Microporous film thickness (μm) is less than 0.020

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

A: The shutdown temperature in the electrolyte is less than 144°C B: The shutdown temperature in the electrolyte is 144°C or more and less than 146°C C: The shutdown temperature in the electrolyte is 146°C or more and less than 147°C D: The shutdown temperature in the electrolyte is 147°C or higher

(18) Preparation of Coating Solution for Heat Resistant Resin Layer As the copolymer (A), vinylidene fluoride-hexafluoropropylene is prepared by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. A copolymer was synthesized. The resulting copolymer (A) had a weight average molecular weight of 1,500,000 and a molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate determined by NMR measurement of 98.0/1.5/0.5. there were.

As the polymer (B), a vinylidene fluoride-hexafluoropropylene copolymer was synthesized by a suspension polymerization method using vinylidene fluoride and hexafluoropropylene as starting materials. The obtained copolymer (B) had a weight average molecular weight of 300,000 and a vinylidene fluoride/hexafluoropropylene molar ratio of 93/7 determined by NMR measurement.

26 parts by mass of copolymer (A), 4 parts by mass of copolymer (B), and 600 parts by mass of NMP (N-methylpyrrolidone) are mixed, and alumina particles (average particle size 1.1 μm) are added while stirring with a disper. 70 parts by mass were added, and further pre-stirring was carried out at 2000 rpm for 1 hour with a disper. Then, using a dyno mill (Dyno Mill Multilab manufactured by Shinmaru Enterprises (1.46 L container, filling rate 80%, φ0.5 mm alumina beads)), under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / s After three treatments, a coating solution for a heat-resistant resin layer was obtained.

(19) Appearance of coating film in high-speed coating evaluation A coating liquid prepared by the method described in (18) using a coating apparatus for reverse gravure coating was applied to a polyolefin microporous film (width 300 mm × length 500 m). After coating on one side and solidifying by immersing in a coagulation bath consisting of an aqueous solution containing 5% by mass of NMP, it was washed with water and dried at 80°C to obtain a laminate having a polyolefin microporous membrane and a heat-resistant resin layer. . A laminate was prepared so that the thickness of the heat-resistant resin layer after drying was 2 μm, and the conveying speed during coating was set to 60 m/min, and the winding tension was set to 100 N/m. Regarding the state of the coating (heat-resistant resin layer) of the obtained laminate (length 2 m), while reflecting the light of the fluorescent lamp, the size and number of coating defects with different light reflection conditions were checked, and the following criteria were used. Evaluation was performed, and D or more was set as a pass.

A: No coating defect of 1 mm or more was observed.
B: No coating defects of 10 mm or more were found, but 2 or more and 4 or less coating defects of 1 mm or more and less than 10 mm were found.
C: No coating defects of 10 mm or more were observed, but 5 or more and 9 or less of coating defects of 1 mm or more and less than 10 mm were found.
D: No coating defects of 10 mm or more were found, but 10 or more coating defects of 1 mm or more and less than 10 mm were found.
E: One or more coating defects of 10 mm or more were observed.

(20) Edge curl in high-speed coating evaluation When a laminate was produced in the same manner as in (19), the state of edge curl of the laminate was observed during the process from the drying process to the winding process, Evaluation was performed according to the following criteria.

A: Edge curl was not observed.
B: Edge curl with a maximum floating height of less than 10 mm was observed.
C: Edge curl with a maximum floating height of 10 mm or more was observed.

(21) Meltdown Resistance of Laminate As an evaluation simulating battery safety, the resistance value in the electrolytic solution during temperature rise was evaluated. Regarding the laminate produced by ^the method of (19), the resistance was measured while heating at a temperature increase rate of 5 ° C./min in the same manner as in (17). The temperature at which the resistance dropped again to 1000 Ω·cm ² or less was read as the meltdown temperature in the electrolytic solution of the laminate. The higher the meltdown temperature in the electrolyte, the higher the safety of the battery structure.

A: The meltdown temperature in the electrolyte is 180°C or higher B: The meltdown temperature in the electrolyte is 175°C or higher and lower than 180°C C: The meltdown temperature in the electrolytic solution is 170°C or higher and lower than 175°C D: In the electrolytic solution The meltdown temperature of 160 ° C. or more and less than 170 ° C. E: The meltdown temperature in the electrolyte is less than 160 ° C.

(22) Low Humidity Curl Resistance of Laminate A sample for evaluation was prepared by cutting a 100 mm×100 mm square from the laminate produced in the same manner as in (19). The obtained sample for evaluation was moved into a low humidity dry room with a dry bulb temperature of 25 ° C. and a dew point temperature of -50 ° C., and placed on a horizontal glass plate so that the heat-resistant resin layer faced up for 3 minutes. I left it. For the sample for evaluation after 3 minutes have passed, measure the heights of the four corners that have risen vertically from the glass plate surface, and determine the maximum height of the four corners as the curl height of the sample for evaluation. and evaluated according to the following criteria.

A: The curl height is less than 1 mm B: The curl height is 1 mm or more and less than 3 mm C: The curl height is 3 mm or more and less than 5 mm D: The curl height is 5 mm or more, or the curl state is large and the corners of the evaluation sample are rounded. measurement is not possible due to

(Example 1)
Ultra high molecular weight polyethylene (melting point 136° C., ΔH ₁₃₀ /ΔH _all =33%) having a weight average molecular weight Mw of 18×10 ⁵ was used as a raw material. Add 77 parts by mass of liquid paraffin to 23 parts by mass of ultra-high molecular weight polyethylene, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass based on the total mass of the polyethylene resin. Part of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane was added as an antioxidant and mixed to prepare a polyethylene resin solution.

The obtained polyethylene resin solution was put into a twin-screw extruder, kneaded at 180° C., supplied to a T-die, and the extrudate was cooled with a cooling roll controlled at 15° C. to form a gel sheet. In addition, the screw of the twin-screw extruder has a distance of 11.5D from the tip of the screw to the vent hole when the outermost diameter of the screw is D, and a screw piece having a length of 0.5D in the raw material conveying direction in between. A screw (type 1) having a configuration using one screw piece, eight screw pieces of 1.0D, and two screw pieces of 1.5D was used.

The resulting gel-like sheet was stretched 6.5 times at 115°C in the longitudinal direction (MD direction) with a roll stretching machine, cooled, and then stretched at 125°C in the width direction (TD direction) with a tenter stretching machine. and stretched 6.5 times. After successive wet stretching, the sheet width was fixed as it was in the tenter stretching machine and held at a temperature of 125° C. for 10 seconds.

Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing bath to remove the liquid paraffin and then dried at 80°C.

The film after washing and drying is stretched 1.7 times (dry stretching) at 130°C in the width direction (TD direction) in the tenter stretching machine, and the straight section L1 with the sheet width fixed is stretched 5 times in the tenter stretching machine. After the first heat setting step, heat setting was performed at 130° C. for 25 seconds in a state of 8% relaxation in the width direction in the same tenter stretching machine.

After the first heat-set film from the tenter stretching machine is exposed to a room temperature atmosphere, as a second heat-setting step, it is passed through four metal rolls set at 100 ° C. and heat-set again. to obtain a polyolefin microporous membrane.

In addition, when performing the second heat setting, the conditions relaxed by 0.1% in the conveying direction by adding a peripheral speed difference between the rolls (the conveying speed of the last roll with respect to the conveying speed of the first roll of the four rolls was reduced by 0.1%), and the total contact time with the roll was set to 2 seconds. The total draw ratio considering relaxation was 66.0 times. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Example 2)
A microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the raw material composition and production conditions were as shown in the table, and the wet stretching was simultaneous biaxial stretching in a tenter. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Examples 3 and 4)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Example 5)
The raw materials are ultra-high molecular weight polyethylene with a weight average molecular weight Mw of 15×10 ⁵ (melting point 136° C., ΔH ₁₃₀ /ΔH _all =34%) and high density polyethylene with a weight average molecular weight Mw of 11×10 ⁴ (melting point 133° C., ΔH _all = 220 J/g, DSC half width = 4.8°C), and 75 parts by mass of liquid paraffin is added to 22 parts by mass of ultra-high molecular weight polyethylene and 3 parts by mass of high-density polyethylene to prepare a polyethylene resin solution, A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Example 6)
The raw material composition and manufacturing conditions are as shown in the table. Regarding the screw of the twin screw extruder, when the outermost diameter of the screw is D, the distance from the tip of the screw to the vent hole is 10.75D, and the length of the raw material conveying direction in between is 10.75D. Example except that one screw piece with a thickness of 0.75D and ten screw pieces of 1.0D were used, and the screw (type 2) was changed to a configuration that did not use a 1.5D screw piece. A microporous polyolefin membrane was obtained in the same manner as in 5. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Examples 7 and 8)
A polyolefin microporous membrane was obtained in the same manner as in Example 5, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Example 9)
A polyolefin microporous membrane was obtained in the same manner as in Example 5, except that the raw material composition and production conditions were as shown in the table, and the wet stretching was simultaneous biaxial stretching in a tenter. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Comparative Examples 1, 4, 7, 8, 9)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table. In the table, the content (%) of polypropylene means the content ratio (% by mass) with respect to the total raw material composition including liquid paraffin. In Comparative Example 7, the content of polypropylene in the obtained polyolefin microporous membrane was 5.2% by mass.

(Comparative Examples 2, 3, 6)
A polyolefin microporous membrane was obtained in the same manner as in Example 2, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

(Comparative Example 5)
A polyolefin microporous membrane was obtained in the same manner as in Example 9, except that the raw material composition and production conditions were as shown in the table. The properties of the obtained polyolefin microporous membrane are shown in the table.

When the polyolefin microporous membrane according to the embodiment of the present invention is applied as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, the air permeability and shrinkage stress behavior at high temperatures are reduced to a specific range. Therefore, it can be suitably used as a separator having excellent high-speed coatability, good rate characteristics when made into a lithium ion secondary battery, and foreign matter resistance. It can also be used for various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture-permeable waterproof clothing, medical materials, fuel cell supports, and the like.

Claims (8)

A polyolefin microporous membrane having a 10 μm equivalent air permeability of 80 sec/100 cm ³ or more and 350 sec/100 cm ³ or less, and satisfying Formula 1 and Formula 2 below.
−0.3≦F1MD≦0.5 (formula 1)
0.7≦F2MD≦2.0 (Formula 2)
In formula 1 or formula 2,
F1MD means the shrinkage stress (MPa) at 120°C in the longitudinal direction of the polyolefin microporous membrane, determined by thermomechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement,
F2MD means the maximum shrinkage stress (MPa) in the longitudinal direction of the polyolefin microporous membrane obtained by thermomechanical analysis (TMA), which is determined by temperature rising constant length measurement from 30°C to 250°C. The polyolefin microporous membrane according to claim 1, which satisfies Formula 3 below.
−0.3≦F1TD≦0.3 (Formula 3)
In formula 3,
F1TD means the shrinkage stress (MPa) at 120°C in the width direction of the polyolefin microporous membrane, determined by thermo-mechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement. 3. The polyolefin microporous membrane according to claim 1 or 2, which satisfies the following formula 4.
0≦|F1MD−F1TD|≦0.15 (Formula 4)
In formula 4,
F1MD means the shrinkage stress (MPa) at 120°C in the longitudinal direction of the polyolefin microporous membrane, determined by thermomechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement,
F1TD means the shrinkage stress (MPa) at 120°C in the width direction of the polyolefin microporous membrane, determined by thermo-mechanical analysis (TMA) from 30°C to 250°C temperature-rising constant length measurement. The polyolefin microporous membrane according to claim 1 or 2, wherein the shutdown temperature in the temperature-rising air permeability method is 128°C or higher and 143°C or lower. The polyolefin microporous membrane according to claim 1 or 2, having a breaking elongation in the width direction of 100% or more and 180% or less. 3. The polyolefin according to claim 1 or 2, which contains 10% by mass or more of a polyethylene component having a molecular weight of 100,000 or less and 20% by mass or more of a polyethylene component having a molecular weight of 1,000,000 or more in the molecular weight distribution of polyethylene measured by the GPC method. Microporous membrane. A laminate comprising the polyolefin microporous membrane according to claim 1 or 2 and a heat-resistant resin layer. A non-aqueous electrolyte secondary battery comprising the polyolefin microporous membrane according to claim 1 or 2.