KR20240026878A - Polyolefin microporous membrane and battery separator - Google Patents

Abstract

The purpose is to provide a polyolefin microporous film that is thin when used as a separator, has a low shutdown temperature, and has both mechanical strength and insulation properties after melting. The film thickness is 6 ㎛ or less, the puncture strength equivalent to 5 ㎛ is 1.7 N or more, the shutdown temperature measured by the elevated temperature permeability method is 80 ℃ or more and 138 ℃ or less, and the crystallinity of polypropylene when reaching 169 ℃ is 3 ppm or more and 200 ppm or less. It is a polyolefin microporous membrane characterized by:

Description

Polyolefin microporous membrane and battery separator

The present invention relates to polyolefin microporous membranes and battery separators.

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, and separators for batteries and electrolytic capacitors. Among these, microporous films made of polyolefin as a resin material are excellent in chemical resistance, insulation, mechanical strength, etc., and have shutdown characteristics, so they are widely used as separators for secondary batteries. In addition, batteries need to satisfy safety standards such as self-discharge characteristics, static test, hot box test, and impact resistance test to prevent long life and capacity reduction, and polyolefin microporous films must have insulation, mechanical strength, and shutdown characteristics. Improvements are required.

From the viewpoint of thermal safety of the battery, in Patent Document 1, the polyolefin laminated microporous membrane consists of at least three layers, the film thickness is in the range of 3 to 25 μm, the meltdown temperature is in the range of 159 to 200 ° C., and the The degree is in the range of 50 to 300 seconds, the puncture strength is in the range of 100 to 550 gf, only the inner layer of the three layers contains polypropylene, and at least one layer forming the surface layer has a melt flow rate of 50 to 150 g/10 minutes and a melting point. A separator film characterized by containing a resin with a temperature of 120 to 130° C. has been proposed. In addition, in Patent Document 2, it is a laminated microporous membrane made of polyethylene and polypropylene with a film thickness of 5 to 20 μm, the microporous film contains polypropylene in a ratio of 3 to 50%, and the difference between the shutdown temperature and the film rupture temperature is 33 ° C. or more. , a separator film characterized by a shutdown temperature of 140°C or lower and a film rupture temperature of 150°C or higher has been proposed. In Patent Document 3, in order to ensure battery safety at high temperatures, it is microporous, characterized by containing polymethylpentene with a Tm of 200.0°C or higher and an MFR of 80.0dg/min or lower, and has a meltdown temperature of 180.0°C or higher. , a separator film has been proposed that has a shutdown temperature of 131.0°C or less and a 170°C TD heat shrinkage of 30.0% or less.

Japanese Patent Publication No. 2015-208894 Japanese Patent Publication No. 2011-63025 Japanese Patent Publication No. 2012-530802

Recently, for the purpose of improving the driving performance of electric vehicles and transitioning from 4G to 5G in Internet communication, lithium ion secondary batteries are required to have higher capacity and higher safety. Therefore, separators are required to be thinner, maintain insulation at high temperatures inside the battery, and improve mechanical strength and shutdown characteristics.

However, as the separator becomes thinner, it becomes difficult to maintain mechanical strength and insulation properties at high temperatures. Additionally, in order to improve the mechanical strength of the separator, for example, film forming conditions can be adjusted, but this method tends to increase the shutdown temperature. As a result, the temperature inside the battery rises excessively when abnormal heat is generated, and the insulation at high temperatures cannot be sufficiently maintained. In the case of the separators of Patent Documents 1 to 3, when thin films are made, a sufficient balance between maintenance of insulation at high temperatures and mechanical strength cannot be achieved.

In view of the above circumstances, the purpose of the present invention is to provide a polyolefin microporous film that is thin, has a low shutdown temperature, and has both high mechanical strength and insulating properties after melting.

The polyolefin microporous film of the first form of the present invention has a film thickness of 6 ㎛ or less, a 5 ㎛ equivalent puncture strength of 1.7 N or more, and a shutdown temperature measured by the elevated temperature permeability method of 80 ℃ or more and 138 ℃ or less, reaching 169 ℃. It is characterized in that the crystallinity of the polypropylene is 3ppm or more and 200ppm or less.

Additionally, the polyolefin microporous membrane may be a multilayer microporous membrane composed of a plurality of layers.

In addition, the polyolefin microporous film may have peaks in the GPC chart with a molecular weight in the range of 5.0×10 ⁴ to 1.0×10 ⁵ and in the range of 3.0×10 ⁵ to 7.0×10 ⁵ .

Additionally, polyethylene having a weight average molecular weight of 4.0×10 ⁵ or more and 1.0×10 ⁶ or less may be contained.

Additionally, the polyolefin microporous film may have a polypropylene concentration of 3.5% by mass or more and 10.0% by mass or less.

Additionally, the polyolefin microporous membrane may have a porous layer laminated on at least one side of the polyolefin microporous membrane.

The battery separator of the second aspect of the present invention includes the polyolefin microporous film.

According to the present invention, it is possible to provide a polyolefin microporous film that is thin, has a low shutdown temperature, and has both puncture strength and insulating properties after melting. In particular, polyolefin microporous films are suitably used as battery separators.

Hereinafter, this embodiment of the present invention will be described. Additionally, the present invention is not limited to the embodiments described below.

The polyolefin microporous film of the present invention has a film thickness of 6 ㎛ or less, a 5 ㎛ equivalent puncture strength of 1.7 N or more, a shutdown temperature of 80 ℃ or more and 138 ℃ or less as measured by the elevated temperature permeability method, and polypropylene when it reaches 169 ℃. The crystallinity is 3ppm or more and 200ppm or less.

The upper limit of the film thickness of the polyolefin microporous film of the present invention is 6 μm or less. If the film thickness exceeds 6 μm, it cannot support the high capacity of the battery. The upper limit of the film thickness is preferably 4.7 μm or less, and more preferably 4.5 μm or less. The lower limit of the film thickness is preferably 1 μm or more, more preferably 3.0 μm or more, from the viewpoint of puncture strength and insulation properties at high temperatures. If the film thickness is in the above preferred range, when a polyolefin microporous film is used as a battery separator, the amount of active material in the electrode can be increased by decreasing the film thickness, resulting in an improvement in battery capacity. The film thickness can be kept within a predetermined range by adjusting the extrusion discharge amount and heat setting temperature.

The polyolefin microporous film of the present invention has a puncture strength equivalent to a thickness of 5 µm (puncture strength equivalent to 5 µm) of 1.7 N or more, and a shutdown temperature of 80°C or more and 138°C or less. Due to these characteristics, even when high tension is applied, the film is difficult to break, so it has high durability and has excellent self-discharge characteristics when introduced into a battery. Additionally, when the battery generates abnormal heat, it can be shut down more quickly and temperature rise can be prevented. In addition, it is preferable that the shutdown temperature is 80°C or higher because unnecessary shutdown does not occur in hot areas or periods and there is a low possibility of damaging the battery function. The balance between 5-μm equivalent puncture strength and shutdown temperature can be kept within a predetermined range by adjusting the combination of film forming conditions such as polyolefin molecular weight, mixing ratio, and stretching temperature during the manufacturing process. The lower limit of the 5㎛ equivalent puncture strength is preferably 1.7N or more, more preferably 1.9N or more, from the viewpoint of suppressing the defect rate in the battery process, maintaining the self-discharge characteristics of the battery, and compression resistance, and the upper limit is not particularly limited, but is 3.0. N or less is preferred. From the viewpoint of more quickly suppressing abnormal heat generation in the battery, the upper limit of the shutdown temperature is preferably 137°C or lower, and more preferably 136°C or lower.

The polyolefin microporous film of the present invention has a polypropylene crystallinity of 3 ppm or more and 200 ppm or less when the temperature is raised and reaches 169°C (hereinafter sometimes referred to as the polypropylene crystallinity when 169°C is reached). The fact that the crystallinity of polypropylene is 3 ppm or more indicates that the regular structure of polypropylene sufficiently remains when 169°C is reached, and since it is difficult to relax even when the temperature rises, the shape retention characteristics after melting are excellent and the insulation condition is maintained well. You can. In addition, when the crystallinity of polypropylene is 200 ppm or less, the amount of ordered structure is not excessive, which suppresses phase separation between polyolefin and polypropylene at high temperatures, makes it difficult to create holes, and can further suppress short circuits. The crystallinity of polypropylene is preferably 10 ppm or more and 170 ppm or less, more preferably 20 ppm or more and 150 ppm or less, from the viewpoint of suppressing short circuit due to film rupture at high temperatures.

Here, the crystallinity of polypropylene upon reaching 169°C can be determined by differential scanning calorimetry (DSC), which will be described later. The crystallinity can be kept within a predetermined range, for example, by adding polypropylene with a predetermined molecular weight and melting point. In this polyolefin microporous membrane, it is necessary to adjust the crystallinity of polypropylene when reaching 169°C, and the molecular weight and melting point of other polyolefins in consideration of compatibility with polyolefins other than those being mixed.

In order to satisfy the above puncture strength, shutdown temperature, and crystallinity of polypropylene upon reaching 169°C, the polyolefin microporous membrane may be a multilayer microporous membrane composed of a plurality of layers.

The polyolefin constituting the polyolefin microporous membrane of the present invention has a molecular weight in the range of 5.0×10 ⁴ to 1.0×10 ⁵ and 3.0×10 ⁵ to 7.0×10 in the GPC chart from the viewpoint of easy control of strength and shutdown characteristics. It is desirable to have each peak in the range of ⁵ .

The polyolefin microporous film of the present invention preferably contains polyethylene with a weight average molecular weight of 4.0×10 ⁵ or more and 1.0×10 ^{6 or} less. It is more preferable that the weight average molecular weight of polyethylene is 4.0×10 ⁵ or more and 1.0×10 ⁶ or less. By making the polyolefin constituting the polyolefin microporous membrane a polyolefin resin composition within the above range, strength is maintained even when formed into a thin film, and the shutdown characteristic is excellent because it is easy to melt. The weight average molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by GPC method.

The polyolefin microporous film of the present invention contains polyethylene and isotactic polypropylene, and the isotactic polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene is preferably 3.5% by mass or more and 10.0% by mass or less. More preferably, it is 4.0 mass% or more and 6.0 mass% or less. If the lower limit of the polypropylene concentration is within the above preferred range, the polypropylene component remains even after the polyolefin is melted, has a sufficient network, and heat resistance can be maintained. If the upper limit of the polypropylene concentration is within the above preferable range, a decrease in the puncture strength of the entire film is suppressed, puncture becomes difficult, and short circuit can be further suppressed. The polypropylene concentration in the polyolefin microporous film can be determined by infrared spectroscopy (IR measurement) described later. Additionally, the polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane can be controlled by the polypropylene concentration contained in the polyolefin resin raw material forming the polyolefin microporous membrane. The polypropylene concentration contained in the total mass of polyethylene and isotactic polypropylene in the polyolefin resin raw material is preferably 1.0 mass% or more and 10.0 mass% or less, more preferably 2.0 mass% or more and 6.0 mass% or less, and further. Preferably it is 3.0 mass% or more and 5.5 mass% or less. The lower limit of the porosity of the polyolefin microporous membrane of the present invention is not particularly limited, but is, for example, 20% or more, and more preferably 30% or more. The lower limit of the porosity is not particularly limited, but is, for example, 70% or less, preferably 60% or less. When the porosity is within the above range, the retention amount of the electrolyte can be increased and high ion permeability can be secured. Additionally, when the porosity is within the above range, the rate characteristics are improved. Additionally, from the viewpoint of further improving ion permeability and rate characteristics, it is preferable that the porosity is 20% or more. The porosity can be within the above range by adjusting the mixing ratio of polyolefin components, draw ratio, heat setting conditions, etc. during the manufacturing process.

The heat shrinkage rate of the polyolefin microporous membrane of the present invention in the machine direction is, for example, 10% or less, preferably 9% or less, and more preferably 8% or less. The heat shrinkage rate of the polyolefin microporous membrane in the width direction at 120°C for 1 hour is, for example, 10% or less, preferably 9% or less, and more preferably 7% or less. The lower limit of the heat shrinkage rate in the machine direction and the lower limit of the heat shrinkage rate in the width direction are not particularly limited, but are preferably -2.0% or more. When the upper limits of the heat shrinkage rate in the machine direction and the heat shrinkage rate in the width direction are within the above range, the risk of deformation inside the battery or short circuit at the ends can be reduced, and battery safety is improved. The heat shrinkage of the polyolefin microporous membrane can be within the above range by adjusting the mixing ratio of polyolefin components, stretching ratio, heat setting conditions, etc. during the manufacturing process.

(Method for producing polyolefin microporous membrane)

The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or may be a multilayer microporous membrane composed of a plurality of layers. The layer composition is preferably two or more layers, more preferably three layers, and it is particularly preferred that the A layer and B layer having different resin compositions are A layer/B layer/A layer or B layer/A layer/B layer. . Polyolefin resin composition A and polyolefin resin composition B constituting the A layer and B layer are explained below.

(1) Polyolefin resin composition A

Polyolefin resin composition A may contain polyethylene a1 and polyethylene a2.

(polyethylene a1)

Polyethylene a1 is polyethylene with a weight average molecular weight (Mw) of 7.0×10 ⁵ or more. Polyethylene a1 may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene, but it is preferable to use a monopolymer of ethylene. Preferred α-olefin copolymers other than ethylene include propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of α-olefins other than ethylene is preferably 5 mol% or less based on 100 mol% of the α-olefin copolymer. From the viewpoint of uniformity of the pore structure of the polyolefin microporous film, it is preferable that it is a monopolymer of ethylene.

From the viewpoint of making it easier to control the strength, elongation, and melting of the microporous membrane, polyethylene a1 preferably has a weight average molecular weight (Mw) of 7.0×10 ⁵ to 2.0×10 ⁶ , and 1.0×10 ⁶ to 1.8×10 ⁶ It is more preferable that it is below. Additionally, the melting point of polyethylene a1 is preferably 134°C or higher and 137°C or lower, and more preferably 134°C or higher and 136°C or lower. The content of polyethylene a1 is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on 100% by mass of polyolefin resin composition A. The upper limit is 95% by mass.

(polyethylene a2)

From the viewpoint of making it easier to control the melting of the microporous membrane, polyethylene a2 has a weight average molecular weight (Mw) of 5.0×10 ⁴ to 7.0×10 ⁵ , preferably 3.0×10 ⁵ or less, and more preferably 2.0×10 ⁵ or less. desirable. In addition, polyethylene a2 is preferably a low melting point component, and the melting point is preferably 130°C or higher and 134°C or lower, more preferably 130°C or higher and 133°C or lower, and even more preferably 130°C or higher and 132°C or lower. Polyethylene a2 is preferably at least one selected from the group consisting of high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene, and may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene. Preferred α-olefin copolymers other than ethylene include propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of α-olefins other than ethylene is preferably 10 mol% or less, based on 100 mol% of the α-olefin copolymer. The content of polyethylene a2 is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or more, based on 100% by mass of polyolefin resin composition A.

(2) Polyolefin resin composition B

Polyolefin resin composition B may contain polyethylene b1 and polypropylene.

(polyethylene b1)

Polyethylene b1 can be the same as polyethylene a1 mentioned above. However, the same meaning means that it is polyethylene with a molecular weight and melting point in the same range as polyethylene a1.

(polypropylene)

The type of polypropylene is not particularly limited as long as it is polypropylene that satisfies the following molecular weight and melting point. From the viewpoint of phase separation and shape maintenance of the microporous membrane at high temperatures, the weight average molecular weight (Mw) of polypropylene is preferably 1×10 ⁶ or more, more preferably 1.2×10 ⁶ or more, and 1.2×10 6 to ⁴ ×10 ⁶ This is more preferable. Moreover, the melting point of polypropylene is preferably 155 to 175°C, and more preferably 160 to 170°C.

Polypropylene may be a monopolymer of propylene, a copolymer of propylene and other α-olefins and/or diolefins (propylene copolymer), or a mixture of two or more types selected from these, but the monopolymer of propylene may be used alone. It is more preferable to use As the propylene copolymer, either a random copolymer or a block copolymer can be used. The α-olefin in the propylene copolymer is preferably an α-olefin having 8 or less carbon atoms. Examples of α-olefins having 8 or less carbon atoms include ethylene, butene-1, pentene-1,4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, styrene, and combinations thereof. The diolefin in the propylene copolymer is preferably a diolefin having 4 to 14 carbon atoms. Examples of diolefins having 4 to 14 carbon atoms include butadiene, 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene. It is desirable to adjust the content of other α-olefins and diolefins in the propylene copolymer so that the polypropylene falls within the above-mentioned preferred melting point range. The content of polypropylene is preferably 10 mass% or more and 30 mass% or less, and more preferably 10 mass% or more and 20 mass% or less, based on 100 mass% of polyolefin resin composition B.

The polyolefin resin compositions A and B may, if necessary, contain other resin components other than polyethylene a1, a2, b1, and polypropylene. As other resin components, for example, resins that provide heat resistance can be further contained. Additionally, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, anti-blocking agents, fillers, crystal nucleating agents, and crystallization retarders may be contained within the range that does not impair the effect of the present invention.

When the layer composition of the polyolefin microporous membrane is A layer/B layer, the thickness ratio of A layer/B layer is preferably 5/95 to 90/10, more preferably 30/70 to 80/20, and more preferably 30/70 to 80/20. Preferably it is 35/65 to 75/25. As a result, even thin films can have high heat resistance while maintaining puncture strength.

In this embodiment, a porous layer may be laminated on at least one side of a polyolefin microporous membrane to form a microporous membrane. The porous layer is not particularly limited, but for example, a porous layer made of resin may be laminated. The resin used here is not particularly limited, and known resins can be used, and examples include acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, and polyimide resin. The porous layer may further contain inorganic particles, and the inorganic particles are not particularly limited, and known materials can be used, examples of which include alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

(3) Method for producing polyolefin microporous membrane

The method for producing a polyolefin microporous membrane of the present invention includes the following steps. Each process is explained in detail.

(a) Preparation of solutions for layer A and layer B

(b) Forming of gel sheet

(c) first elongation

(d) Removal of plasticizer, drying

(e) Second elongation

(f) heat treatment

(a) Preparation of solutions for layer A and layer B

The A layer and the B layer are composed of the above-described polyolefin resin composition A and polyolefin resin composition B, respectively. A plasticizer is added to the polyolefin resin composition in a twin-screw extruder, melt-kneaded, and solutions for the A layer and B layer are respectively prepared. It is preferable that the polyolefin resin composition contains 10% by mass or more and 30% by mass or less relative to the entire resin solution. By keeping the concentration of the polyolefin resin composition within the above range, when extruding the polyolefin solution, melt fracture or neck-in at the die exit can be prevented, and the moldability and appearance of the extruded article become good.

The solutions of layer A and layer B are each supplied from an extruder to one die, and both solutions are extruded there into a layered sheet to obtain an extruded body. The extrusion method may be either a flat die method or an inflation method. In either method, the solution is supplied to each manifold and stacked in layers at the lip inlet of the multilayer die (multiple manifold method), or the solution is supplied to the die as a layered flow in advance (block method). can be used. The multiple manifold method and block method can be applied as usual methods. The gap of the multi-layer flat die can be set to 0.1 mm or more and 5 mm or less. The extrusion temperature is preferably 140°C or higher and 250°C or lower, and the extrusion speed is preferably 0.2 to 15 m/min. By controlling the extrusion amount of the solution of each layer, the film thickness ratio of the layers can be adjusted.

The thickness of each sheet is preferably 5/95 to 90/10, more preferably 30/70 to 80/20, and even more preferably 5/95 to 90/10. Preferably it is 35/65 to 75/25. A polyolefin microporous film with an excellent balance of strength and melting is obtained while maintaining the polypropylene network within the preferred ranges of the polyolefin composition, the thickness ratio of each sheet, and the stretching conditions described later. In the polyolefin microporous membrane, when the polypropylene concentration of polyolefin resin composition B is 10% by mass or more and 30% by mass or less, from the viewpoint of maintaining the polypropylene network, rather than 0/100 (single layer structure of B layer), e.g. For example, it is preferable to intensively arrange the polyolefin resin composition B in multiple layers, such as 50/50. As a result, even thin films can have higher heat resistance while maintaining puncture strength.

(b) Forming of gel sheet

A gel-like sheet is formed by cooling the obtained extruded body. As a cooling method, a method of contacting a coolant such as cold air or cooling water, a method of contacting a cooling roll, etc. can be used, but it is preferable to cool it by contacting it with a roll cooled with a coolant. Cooling is preferably performed at a rate of 50°C/min or higher, at least up to the gelation temperature. Cooling is preferably performed to 25°C or lower. If the cooling rate is within the above range, the crystallinity is maintained in an appropriate range, resulting in a gel-like sheet suitable for stretching.

(c) first elongation

Next, the gel sheet is stretched. After preheating, the gel sheet is preferably stretched to a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial stretching or biaxial stretching. The draw ratio (cotton draw ratio) is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The draw ratio in the longitudinal direction (hereinafter sometimes referred to as MD) and the transverse direction (hereinafter sometimes referred to as TD) may be the same or different, and the draw ratio in either MD or TD is preferably 3 times or more. .

The lower limit of the first stretching temperature is preferably 100°C or higher and 130°C or lower, and more preferably 110°C or higher and 120°C or lower. By using the above-described polyolefin compositions A, B and layer configuration, and keeping the stretching temperature within the above preferred range, film rupture due to stretching of the polyolefin resin containing the low melting point component is suppressed, and stretching at a high magnification is possible. As a result, the puncture strength of the polyolefin microporous membrane is improved, and the increase in shutdown temperature is easily suppressed. Additionally, phase separation of polyethylene and polypropylene under high temperatures is suppressed, and shape retention is improved. Therefore, when used as a battery separator, even if it is a thin film, it is excellent in mechanical strength and battery safety.

(d) Removal of plasticizers

Next, a cleaning solvent is used to remove the plasticizer contained in the gel-like sheet and dry it. Since the cleaning solvent and the method for removing the plasticizer using it are well known, description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Application Laid-Open No. 2002-256099 can be used. After removing the plasticizer, it is dried using a heat drying method or an air drying method. Any method capable of removing the cleaning solvent may be used, including conventional methods such as heat drying and air drying (moving air).

(e) Second elongation

A polyolefin microporous membrane can be obtained by preheating the dried sheet and then stretching it in at least one direction (dry stretching). The second stretching can be performed by a tenter method or the like while heating. The final draw ratio of the second stretching is preferably 1.1 times or more, and more preferably 1.4 times or more. By setting the final stretch ratio within the above range, the puncture strength can be easily controlled to a desired range. However, if stretched at a high magnification, the shutdown temperature and heat shrinkage increase, so it is preferable that the ratio is 9 times or less. When the above-described polyolefin compositions A and B and the layer structure are used, film rupture due to stretching of the polyolefin resin of the low melting point component is suppressed and can be easily controlled.

(f) heat treatment

After the second stretching, heat treatment is performed while the width is fixed while being held by a clip. Heat treatment is preferably performed at 115.0°C or higher and 135.0°C or lower. By setting the heat treatment temperature within the above range, the heat shrinkage rate of the polyolefin microporous film can be suppressed. During heat treatment, heat relaxation treatment may be performed. When performing thermal relaxation treatment, the relaxation rate can be 5% or more and 30% or less, assuming 100% of the previous length. By setting the relaxation rate within the above range, the heat shrinkage rate can be reduced and variation in the process of the microporous membrane after relaxation can be suppressed.

Example

Hereinafter, the present invention will be explained in more detail by examples. Additionally, the present invention is not limited to these examples.

[measurement method]

(1) Film thickness

The film thickness of the polyolefin microporous membrane at 5 points (top left, top right, center, bottom left, bottom right) within a range of 95 mm ), and the average value was referred to as film thickness (μm). Additionally, if the sample size cannot be created at 95 mm x 95 mm, it may be cut to an arbitrary size and measured at five points: upper left, upper right, center, lower left, and lower right.

(2) Unit weight

Polyolefin microporous films cut to 5 cm Additionally, if the sample size cannot be 5 cm x 5 cm, it may be cut to an arbitrary size and calculated by dividing the measured mass by the area.

(3) public rate

The polyolefin microporous membrane was cut to a size of 95 mm x 95 mm, its volume (cm3) and mass (g) were determined, and the porosity (%) was calculated from these and the membrane density (g/cm3) using the following equation.

Formula: Porosity=((Volume-Mass/Film Density)/Volume)×100

Here, the membrane density was set to 0.99 g/cm3. In addition, the film thickness measured in (1) above was used to calculate the volume.

(4) Speculation resistance

For the polyolefin microporous film, the air permeability resistance (sec/100 cm3) was measured using an air permeability resistance meter (manufactured by ASAHI SEIKO CO., LTD., EGO-1T) in accordance with JIS P-8117:2009.

(5) Tolja Robber

A needle with a diameter of 1 mm (the tip is 0.5 mmR) was used and the maximum load value S (N) was measured when a polyolefin microporous membrane with a film thickness (T (μm)) was traversed at a speed of 2 mm/sec. The converted puncture intensity with a film thickness of 5 μm was calculated using the following formula.

Formula: Converted puncture intensity=S(N)×5(㎛)/T(㎛)

(6) Shutdown temperature (also known as SD temperature)

A polyolefin microporous film punched into a circular shape with a diameter of 45 mm is exposed to an atmosphere at 20°C, and the air permeability resistance is measured while raising the temperature at a rate of 5°C/min. When the air permeation resistance reaches 100,000 seconds/100 cm3, The temperature was defined as the shutdown temperature, and the average value of two measurements was used. The air permeability resistance was measured using an air permeability resistance meter (manufactured by ASAHI SEIKO CO., LTD., EGO-1T) in accordance with JIS P8117:2009.

(7) Meltdown temperature (also called MD temperature)

A polyolefin microporous film punched into a circular shape with a diameter of 45 mm is exposed to an atmosphere at 20°C, and the air permeability resistance is measured while raising the temperature at a rate of 5°C/min. The temperature is increased even after the air permeation resistance reaches 100,000 seconds/100 cm3. The process was further continued, and the temperature at which the permeation resistance was less than 100,000 seconds/100 cm3 was defined as the meltdown temperature. The air permeability resistance was measured using an air permeability resistance meter (manufactured by ASAHI SEIKO CO., LTD., EGO-1T) in accordance with JIS P8117:2009.

(8) Heat shrinkage rate

The polyolefin microporous membrane was cut to a size of 95 mm After exposing the test piece to a temperature of 105°C for 8 hours without applying a load, return the test piece to room temperature, measure the length (mm) after shrinkage in the machine direction and width direction, and calculate the machine direction and width using the following equations from the obtained test piece length. The thermal contraction rate (%) in each direction was determined.

Formula: MD thermal shrinkage rate (%)=(1-length after shrinkage in the machine direction/length before shrinkage in the machine direction)×100

TD heat shrinkage rate (%)=(1-length after shrinkage in the width direction/length before shrinkage in the width direction)×100

(9) Weight average molecular weight

The weight average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of the polyolefin resin and polyolefin microporous membrane were determined by gel permeation chromatography (GPC) using the following measurement conditions.

Measuring conditions

·Measurement device: Agilent high-temperature GPC device PL-GPC220

Column: Agilent PL1110-6200 (20㎛ MIXED-A) × 2

·Column temperature: 160℃

Solvent (mobile phase): 1,2,4-trichlorobenzene

·Solvent flow rate: 1.0mL/min

·Sample concentration: 0.1% by mass (dissolution conditions: 160℃/3.5H)

·Injection amount: 500㎕

・Detector: Differential refractive index detector (RI detector) manufactured by Agilent

·Viscometer: Viscosity detector made by Agilent

· Calibration curve: Prepared using the universal calibration curve method using monodisperse polystyrene standard samples.

Additionally, the peak positions on the low molecular weight side and the high molecular weight side were estimated as follows.

· Peak position on the low molecular weight side: Peak position of the Gaussian function on the low molecular weight side when the molecular weight distribution is fitted with two Gaussian functions.

· Peak position on the high molecular weight side: Molecular weight at the maximum value of the molecular weight distribution.

(10) Melting point and melting peak

The melting point of the polyolefin resin and the melting peak of the polyolefin microporous film were determined by a scanning differential calorimeter (PYRIS DIAMOND DSC, manufactured by PARKING ELMER). The polyolefin resin and the polyolefin microporous membrane were each left standing in a sample holder, heated from 30°C to 230°C to completely melt, then held at 230°C for 3 minutes, and then lowered to 30°C at a rate of 10°C/min. This was used as the first temperature increase, and the same measurement was repeated again, and the melting point (Tm) of the polyolefin resin and the heat of fusion of the polyolefin microporous film were respectively determined from the endothermic peak during the second temperature increase. Additionally, the baseline when calculating the heat of fusion was a straight line connecting 30°C and 230°C. For polyolefin resin, a peak with a heat of fusion of 70 J/g or more was regarded as an endothermic peak, and for a polyolefin microporous film, a peak with a heat of fusion of 0.1 J/g or more was regarded as an endothermic peak.

(11) Floor ratio

The layer ratio of the polyolefin microporous film was observed using a transmission electron microscope (TEM) under the following measurement conditions.

Measuring conditions

· Sample preparation: A polyolefin microporous membrane is stained with ruthenium tetroxide and cross-sectioned with an ultramicrotome.

Measuring device: Transmission electron microscope (JEM1400 Plus type manufactured by JEOL, Ltd.)

·Observation conditions: acceleration voltage 100kV

·Observation direction: TD/ND

(12) Crystallinity of polypropylene when reaching 169℃

The crystallinity of polypropylene in the polyolefin microporous film upon reaching 169°C was determined by the following measurement.

Measuring conditions

Measuring device: Scanning differential calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER)

·Sample mass: 6mg

·Atmospheric gas: nitrogen

·Start temperature: 30℃

·Temperature increase rate: 5℃/min

·Temperature reached: 169℃

·Holding time at reached temperature: 5 minutes

·Temperature reduction rate: 30℃/min

·End temperature: 30℃

In the temperature lowering process of the above measurement, when the regular structure remains, an exothermic peak due to crystallization of polypropylene is detected in a temperature range of 120°C or higher. In the present invention, the crystallinity χ for the entire polypropylene resin was obtained from the exothermic peak detected when the temperature was raised to 169°C and then cooled using the following equation.

Equation: χ=ΔH _PP /ΔH _PP ^f ×(isotactic polypropylene concentration)

Here, ΔH _PP represents the enthalpy of crystallization (J/g) during the temperature lowering process of the polypropylene structure. ΔH _PP refers to the value divided by the mass of the measured sample divided by the area surrounded by the line connecting the start and end temperatures of the exothermic peak due to crystallization of polypropylene on the DSC curve during the temperature lowering process and the DSC curve. In addition, the isotactic polypropylene concentration is a value obtained from IR measurement described later. For example, if the polypropylene is isotactic polypropylene, ΔH _PP ^f represents the enthalpy of complete fusion of polypropylene (J/g). ΔH _PP ^f was calculated by adopting 170 J/g.

(13) Measurement of polypropylene concentration relative to the total mass of polyethylene and isotactic polypropylene in the polyolefin microporous membrane

The isotactic polypropylene concentration relative to the sum of the masses of polyethylene and isotactic polypropylene in the polyolefin microporous membrane is the peak intensity of 1462 cm ^-1 derived from polyethylene and 1376 cm ^-1 derived from isotactic polypropylene obtained by IR measurement. Saved by the rain. The measurement conditions are as follows.

Measuring device: FT-IR device (FT/IR-6600 manufactured by JASCO Corporation)

·Measurement temperature: 25℃

·Aperture: X=300㎛, Y=300㎛

・Number of integration: 16 times

·Resolution: 4cm ^-1

Formula: Isotactic polypropylene concentration (%) = (1462 cm ^-1 peak height × conversion factor 1)/(1376 cm ^-1 peak height × conversion factor 2) × 100. Here, conversion factor 1 is 20 and conversion factor 2 is 10.

(14) Hot box characteristics

Battery safety was evaluated by the hot box characteristics shown below.

battery production

In a lithium ion secondary battery, a positive electrode and a negative electrode are stacked with a separator interposed between them, and the separator contains an electrolyte solution (electrolyte). As the positive electrode active material, lithium cobalt composite oxide LiCoO ₂ was used, graphite was used as the negative electrode active material, and 1 mol/L LiPF ₆ prepared in a mixed solvent of DC/dimethyl carbonate (DMC) was used as the electrolyte solution. To assemble the battery, a positive electrode, a separator made of a microporous membrane, and a negative electrode are laminated, then a wound electrode body is manufactured by a conventional method, inserted into a battery can, impregnated with an electrolyte solution, and then a battery that also serves as a positive electrode terminal equipped with a safety valve. The lid was closed through a gasket.

hot box testing

The assembled battery was charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant constant voltage of 4.2V, and then discharged at a current of 0.2C to a final voltage of 3.0V. Next, constant current charging was performed at a current value of 0.2C up to 4.2V, and then constant voltage charging at 4.2V was performed as pretreatment. The pretreated battery was placed in an oven, raised from room temperature at 5°C/min, and then left at 150°C for 30 minutes. A decrease in charging voltage of more than 50% within 15 minutes after reaching 150°C was considered unacceptable, a decrease in charging voltage of 20 to 50% was considered possible, and a decrease of 20% or less was considered excellent.

(15) Self-discharge characteristics

Self-discharge characteristics (K value) were evaluated by the following method. After constant current charging to a battery voltage of 3.85 V with a current value of 0.5 C for the secondary battery for testing assembled in the following (method for manufacturing an evaluation battery), constant voltage charging was performed until the battery voltage reached 0.05 C at 3.85 V. The open circuit voltage of this battery was measured after being left for 24 hours, and this value was designated as V1. For this battery, the open circuit voltage was measured after being left for an additional 24 hours, that is, after being left for a total of 48 hours after charging, and this value was designated as V2. The K value was calculated from the obtained values of V1 and V2 using the following formula.

Formula: K value=(V1-V2)/24

[Example 1]

(1) Preparation of solution for layer A

90% ^by mass of polyethylene with a weight average molecular weight of ^1.5 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

70% by mass of polyethylene with a weight average molecular weight ^{of 1.5} , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of A layer/solution of B layer/solution of A layer was extruded so that the layer thickness ratio was 25/50/25. The extruded molded body was cooled while being pulled out with a cooling roll controlled at 25°C to form a gel-like sheet.

(4) First stretching, removal of film forming solvent, drying

The gel sheet was simultaneously stretched 5 times in both MD and TD at 110.0°C using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath, the liquid paraffin was removed, and then dried to obtain a dried microporous membrane.

(5) Second stretching, heat treatment

Afterwards, it was preheated at 128.0°C, stretched 1.6 times in TD using a tenter stretching machine, then relaxed by 15.0% in TD, and heat-set at 128.0°C while held in the tenter to obtain a polyolefin microporous film. Table 1 shows the properties of the obtained polyolefin microporous membrane.

[Example 2]

Polyolefin solutions A and B were stretched in the same manner as in Example 1 except that the solution of the B layer/solution of the A layer/solution of the B layer was pushed out so that the layer thickness ratio was 25/50/25, and a polyolefin microporous membrane was obtained.

[Example 3]

(1) Preparation of solution for layer A

70% ^by mass of polyethylene with a weight average molecular weight of ^1.5 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

85% ^by mass of polyethylene with a weight average molecular weight of ^1.5 , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of A layer/solution of B layer/solution of A layer was extruded so that the layer thickness ratio was 20/60/20. The extruded molded body was cooled while being pulled out with a cooling roll heated at 25°C to form a gel-like sheet.

(4) First stretching, removal of film forming solvent, drying

Except that the gel-like sheet was stretched at a stretching temperature of 112.5°C, the gel sheet was stretched, liquid paraffin was removed, and dried to obtain a dried microporous membrane in the same manner as in Example 1.

(5) Second stretching, heat treatment

After preheating at 127.0°C, stretching 1.5 times in TD using a tenter stretching machine, 15.0% relaxation was performed in TD, and heat setting at 127.0°C while holding in the tenter to obtain a polyolefin microporous film.

[Example 4]

Polyolefin solutions A and B are supplied from a twin-screw extruder to the three-layer T die, and the solution of layer B/solution of layer A/solution of layer B is extruded so that the layer thickness ratio is 30/40/30, and the first stretching temperature is set to A polyolefin microporous film was obtained in the same manner as in Example 3 except that the second stretching temperature and heat setting temperature were 113.5°C and 126.0°C.

[Example 5]

A polyolefin microporous film was obtained in the same manner as in Example 4 except that the first stretching temperature was 114.5°C, the second stretching temperature and heat setting temperature were 126.0°C, and the relaxation rate was 10.0%.

[Example 6]

A polyolefin microporous film was obtained in the same manner as in Example 5 except that the first stretching temperature was 115.0°C.

[Example 7]

The solution of layer B/solution of layer A/solution of layer B was pushed out so that the layer thickness ratio was 25/50/25, the first stretching temperature was set to 115.5°C, and the relaxation rate was set to 15.0%, in the same manner as in Example 5. A polyolefin microporous membrane was obtained.

[Example 8]

(1) Preparation of solution for layer A

85% ^by mass of polyethylene with a weight average molecular weight ^of 7.5 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

90% by mass of polyethylene with a weight average molecular weight ^{of 7.5} , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of B layer/solution of A layer/solution of B layer was extruded so that the layer thickness ratio was 15/70/15. The extruded molded body was cooled while being pulled out with a cooling roll heated at 25°C to form a gel-like sheet.

(4) First stretching, removal of film forming solvent, drying

The gel sheet was stretched in the same manner as in Example 1 except that the stretching temperature was set to 112.0°C, the liquid paraffin was removed, and the gel sheet was dried to obtain a dried microporous membrane.

(5) Second stretching, heat treatment

After preheating at 130.0°C, stretching 1.8 times in TD using a tenter stretching machine, relaxation of 15.0% was performed in TD, and heat setting at 130.0°C while holding in the tenter to obtain a polyolefin microporous film.

[Example 9]

The ratios of polyethylene and polypropylene in polyolefin solution B are set to 90% and 10%, respectively, and the solution of layer A/solution of layer B/solution of layer A is extruded so that the layer thickness ratio is 30/40/30, and the second stretching ratio is A polyolefin microporous film was obtained in the same manner as in Example 5, except that the was increased to 1.8 times and the relaxation rate was set to 15.0.

[Example 10]

A polyolefin microporous membrane was obtained in the same manner as in Example 8, except that the first stretching temperature was 113.0°C and the second stretching temperature was 125.0°C.

[Example 11]

Example except that the solution of layer A/solution of layer B/solution of layer A was pushed out so that the layer thickness ratio was 25/50/25, the first stretching temperature was set to 111.0°C, and the relaxation rate of the second stretching was set to 12.0%. In the same manner as in 1, a polyolefin microporous film was obtained.

[Example 12]

Example except that the solution of layer A/solution of layer B/solution of layer A was pushed out so that the layer thickness ratio was 35/30/35, the first stretching temperature was set to 112.0°C, and the relaxation rate of the second stretching was set to 10.0%. In the same manner as in 8, a polyolefin microporous film was obtained.

[Comparative Example 1]

(1) Preparation of A-layer solution

40% ^by mass of polyethylene with a weight average molecular weight ^of 2.0 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

80% ^by mass of polyethylene with a weight average molecular weight of ^3.0 , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of B layer/solution of A layer/solution of B layer was extruded so that the layer thickness ratio was 10/80/10. The extruded molded body was cooled while being pulled out with a cooling roll heated at 25°C to form a gel-like sheet. At this time, the discharge was adjusted so that the thickness after stretching was around 4.0 μm by stretching described later.

(4) First stretching, removal of film forming solvent, drying

The gel sheet was simultaneously stretched 5 times in both MD and TD at 115.0°C using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath, the liquid paraffin was removed, and then dried to obtain a dried microporous membrane.

(5) Second stretching, heat treatment

Afterwards, it was preheated at 125.5°C, stretched 1.5 times in TD using a tenter stretching machine, then relaxed by 15.0% in TD, and heat set at 125.5°C while held in a tenter to obtain a polyolefin microporous film.

[Comparative Example 2]

(1) Preparation of solution for layer A

40% ^by mass of polyethylene with a weight average molecular weight ^of 1.6 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

70% ^by mass of polyethylene with a weight average molecular weight of ^3.0 , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of A layer/solution of B layer/solution of A layer was extruded so that the layer thickness ratio was 40/20/40. The extruded extruded body was cooled while being pulled out with a cooling roll maintained at 25°C to form a gel-like sheet.

(4) First stretching, removal of film forming solvent, drying

The gel sheet was simultaneously stretched 5 times in both MD and TD at 110.0°C using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath, the liquid paraffin was removed, and then dried to obtain a dried microporous membrane.

(5) Second stretching, heat treatment

Afterwards, it was preheated at 128.0°C, stretched 1.6 times in TD using a tenter stretching machine, then relaxed by 15.0% in TD, and heat-set at 128.0°C while held in the tenter to obtain a polyolefin microporous film.

[Comparative Example 3]

(1) Preparation of A-layer solution

30% by mass of polyethylene with a weight average ^molecular weight ^of 2.0 Melt-kneading was performed to prepare polyolefin solution A.

(2) Preparation of solution for layer B

50% ^by mass of polyethylene with a weight average molecular weight of ^3.0 , polyolefin solution B was prepared.

(3) Forming of gel sheet

Polyolefin solutions A and B were supplied from a twin-screw extruder to a three-layer T die, and the solution of A layer/solution of B layer/solution of A layer was extruded so that the layer thickness ratio was 35/30/35. The extruded molded body was cooled while being pulled out with a cooling roll heated at 25°C to form a gel-like sheet.

(4) First stretching, removal of film forming solvent, drying

The gel sheet was simultaneously stretched 5 times in both MD and TD at 114.0°C using a tenter stretching machine. After stretching, the sheet was immersed in a methylene chloride bath, the liquid paraffin was removed, and then dried to obtain a dried microporous membrane.

(5) Second stretching, heat treatment

Afterwards, it was preheated at 125.0°C, stretched 1.5 times in TD using a tenter stretching machine, then relaxed by 10.0% in TD, and heat set at 125°C while held in the tenter to obtain a polyolefin microporous film.

[Comparative Example 4]

A gel-like sheet was formed using only the polyolefin solution A of Comparative Example 1, and the polyolefin mida was prepared in the same manner as in Comparative Example 1, except that the first stretching temperature was 114.0°C, the second stretching and heat setting temperature was 130.0°C, and the relaxation rate was 20%. I got the sclera.

[Reference Example 1]

A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the extrusion discharge was adjusted so that the film thickness of the polyolefin microporous membrane was about 9.0 μm.

[result]

The physical property measurement results of the obtained polyolefin microporous membrane are shown in Table 2. Compared to the comparative example, the polyolefin microporous film obtained in the example is a thin film and has a low shutdown temperature, so it has both puncture strength and insulation after melting, and the battery used as a battery separator is excellent in battery safety and self-discharge characteristics, as represented by a hot box.

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

When the polyolefin microporous membrane of the present invention is used as a battery separator, it is possible to provide a polyolefin microporous membrane that is safe even when the battery is in a high temperature state, even if it is a thin film.

Claims (7)

The film thickness is 6 ㎛ or less, the puncture strength equivalent to 5 ㎛ is 1.7 N or more, the shutdown temperature measured by the elevated temperature permeability method is 80 ℃ or more and 138 ℃ or less, and the crystallinity of polypropylene when reaching 169 ℃ is 3 ppm or more and 200 ppm or less. Polyolefin microporous membrane. According to claim 1,
A polyolefin microporous membrane, which is a multilayer microporous membrane composed of multiple layers. The method of claim 1 or 2,
A polyolefin microporous membrane having peaks in the GPC chart with molecular weights in the range of 5.0×10 ⁴ to 1.0×10 ⁵ and 3.0×10 ⁵ to 7.0×10 ⁵ . The method according to any one of claims 1 to 3,
A polyolefin microporous film containing polyethylene with a weight average molecular weight of 4.0×10 ⁵ or more and 1.0×10 ⁶ or less. The method according to any one of claims 1 to 4,
A polyolefin microporous film containing polyethylene and isotactic polypropylene, and having an isotactic polypropylene concentration of 3.5 mass% or more and 10.0 mass% or less relative to the total mass of polyethylene and isotactic polypropylene. The method according to any one of claims 1 to 5,
A polyolefin microporous membrane having a porous layer on at least one side. A battery separator comprising the polyolefin microporous film according to any one of claims 1 to 6.