CN117203843A

CN117203843A - Polyolefin microporous membrane and separator for battery

Info

Publication number: CN117203843A
Application number: CN202280030525.2A
Authority: CN
Inventors: 竹田健人; 窪田隆; 李丹
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2021-06-30
Filing date: 2022-05-16
Publication date: 2023-12-08

Abstract

The object is to provide a polyolefin microporous membrane which is a film when used as a separator, has a low shutdown temperature, and has both mechanical strength and insulation after melting. A polyolefin microporous membrane having a film thickness of 6 [ mu ] m or less, a puncture strength of 1.7N or more in terms of 5 [ mu ] m, a shutdown temperature of 80 ℃ or more and 138 ℃ or less as measured by a temperature-rise ventilation method, and a crystallinity of polypropylene of 3ppm or more and 200ppm or less when the temperature reaches 169 ℃.

Description

Polyolefin microporous membrane and separator for battery

Technical Field

The present invention relates to a polyolefin microporous membrane and a separator for a battery.

Background

Microporous membranes are used in various fields such as filters including filtration membranes and dialysis membranes, separators for batteries, and separators for electrolytic capacitors. Among them, microporous films made of polyolefin as a resin material are widely used as separators for secondary batteries because they are excellent in chemical resistance, insulation, mechanical strength, and the like and have shutdown characteristics. Further, in batteries, there is a need for improvement in insulation, mechanical strength, shutdown characteristics, and the like of polyolefin microporous films, which satisfy the requirements of long life, self-discharge characteristics for preventing capacity reduction, and safety typified by a nail penetration test, a hot box test, and an impact resistance test.

From the viewpoint of thermal safety of a battery, patent document 1 proposes a separator film characterized in that a laminated microporous film made of polyolefin is composed of at least three layers, the film thickness is in the range of 3 to 25 μm, the melting temperature is in the range of 159 to 200 ℃, the air permeability is in the range of 50 to 300 seconds, the puncture strength is in the range of 100 to 550gf, polypropylene is contained only in the inner layer among the three layers, and at least 1 layer forming the surface layer contains a resin having a melt flow rate of 50 to 150g/10 minutes and a melting point of 120 to 130 ℃. Patent document 2 proposes a separator film comprising a laminated microporous film of polyethylene and polypropylene having a film thickness of 5 to 20 μm, wherein the microporous film contains polypropylene in a proportion of 3 to 50%, the difference between the shutdown temperature and the rupture temperature is 33 ℃ or more, the shutdown temperature is 140 ℃ or less, and the rupture temperature is 150 ℃ or more. Patent document 3 proposes a microporous film comprising polymethylpentene having Tm of 200.0 ℃ or higher and MFR of 80.0 dg/min or lower, and a separator film characterized by having a melting temperature of 180.0 ℃ or higher, a shutdown temperature of 131.0 ℃ or lower, and TD heat shrinkage of 30.0% or lower at 170 ℃ in order to ensure battery safety at high temperatures.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2015-208894

Patent document 2: japanese patent laid-open publication No. 2011-63025

Patent document 3: japanese patent application laid-open No. 2012-53082

Disclosure of Invention

Problems to be solved by the invention

In recent years, in order to improve the running performance of electric vehicles and to shift from 4G to 5G in internet communication, lithium ion secondary batteries are further required to have a high capacity and high safety. Therefore, the separator is required to be thin and to have improved insulation properties, mechanical strength, and shutdown characteristics at high temperatures inside the battery.

However, the separator is thin, and therefore it is difficult to maintain mechanical strength and insulation at high temperatures. In addition, for example, the film forming conditions may be adjusted in order to improve the mechanical strength of the separator, but in such a method, the shutdown temperature tends to be increased. As a result, the battery internal temperature excessively increases during abnormal heat generation, and therefore, the insulation at high temperatures cannot be sufficiently maintained. In the separators of patent documents 1 to 3, when the films are made thin, the balance between the insulation properties at high temperatures and the mechanical strength cannot be sufficiently obtained.

In view of the above, an object of the present invention is to provide a polyolefin microporous membrane which is a film, has a low shutdown temperature, and has both high mechanical strength and insulation after melting.

Means for solving the problems

The polyolefin microporous membrane according to claim 1 of the present invention is characterized in that the membrane thickness is 6 μm or less, the puncture strength in terms of 5 μm is 1.7N or more, the shutdown temperature measured by the temperature-elevating air permeability method is 80℃or more and 138℃or less, and the crystallinity of polypropylene at 169℃is 3ppm or more and 200ppm or less.

The polyolefin microporous membrane may be a multilayer microporous membrane composed of a plurality of layers.

In addition, the polyolefin microporous membrane may have a molecular weight of 5.0X10 in GPC chart ⁴ ～1.0×10 ⁵ And 3.0X10 s ⁵ ～7.0×10 ⁵ Respectively having peaks in the ranges of (2)。

In addition, may contain a weight average molecular weight of 4.0X10 ⁵ Above and 1.0X10 ⁶ The following polyethylenes.

The polypropylene concentration of the polyolefin microporous membrane may be 3.5 mass% or more and 10.0 mass% or less.

The polyolefin microporous membrane may have a porous layer laminated on at least one surface of the polyolefin microporous membrane.

The separator for a battery according to claim 2 of the present invention comprises the polyolefin microporous membrane described above.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polyolefin microporous membrane having a low shutdown temperature, a puncture strength and insulation after melting can be provided. In particular, polyolefin microporous membranes are suitable for use as battery separators.

Detailed Description

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

The polyolefin microporous membrane of the present invention has a film thickness of 6 [ mu ] m or less, a puncture strength of 1.7N or more in terms of 5 [ mu ] m, a shutdown temperature of 80 ℃ or more and 138 ℃ or less as measured by a temperature-elevating air permeability method, and a crystallinity of polypropylene of 3ppm or more and 200ppm or less when reaching 169 ℃.

The upper limit of the film thickness of the polyolefin microporous membrane of the present invention is 6 μm or less. If the film thickness exceeds 6. Mu.m, the high capacity of the battery cannot be handled. The upper limit of the film thickness is preferably 4.7 μm or less, 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 viewpoints of puncture strength and insulation at high temperature. If the film thickness is within the above-mentioned preferred range, the amount of active material in the electrode can be increased in accordance with the degree of thinning the film thickness when the polyolefin microporous film is used as a separator for a battery, resulting in an improvement in the battery capacity. The film thickness can be adjusted to a predetermined range by adjusting the extrusion discharge amount and the heat fixing temperature.

The polyolefin microporous membrane of the present invention has a puncture strength (puncture strength in terms of 5 μm) of 1.7N or more in terms of 5 μm thickness, and a shutdown temperature of 80 ℃ or more and 138 ℃ or less. By these characteristics, the film is less likely to break even when high tension is applied, has high durability, and is excellent in self-discharge characteristics when incorporated into a battery. Further, the battery is turned off more quickly when abnormal heat is generated, and the temperature can be prevented from rising. Further, if the shutdown temperature is 80 ℃ or higher, unnecessary shutdown does not occur in a hot region or a time, and there is a low possibility of impairing the function as a battery, which is preferable. The balance between the puncture strength in terms of 5 μm and the shutdown temperature can be within a predetermined range by adjusting the molecular weight, the blending ratio, the stretching temperature, and other film forming conditions of the polyolefin in combination in the production process. The lower limit of the puncture strength in terms of 5 μm is preferably 1.7N or more, more preferably 1.9N or more, and the upper limit is not particularly limited, but is preferably 3.0N or less, from the viewpoints of suppression of the defective rate in the battery step, maintenance of the self-discharge characteristics of the battery, and compression resistance. The upper limit of the shutdown temperature is preferably 137 ℃ or less, more preferably 136 ℃ or less, from the viewpoint of suppressing abnormal heat generation of the battery more rapidly.

The polyolefin microporous membrane of the present invention has a crystallinity of polypropylene at 169 ℃ or higher (hereinafter, also referred to as a crystallinity of polypropylene at 169 ℃) of 3ppm or higher and 200ppm or lower when the temperature is raised. The crystallinity of 3ppm or more means that the regular structure of polypropylene remains sufficiently at 169 ℃ and is not liable to relax even at an elevated temperature, so that the shape retention property after melting is excellent and the insulating state can be maintained well. In addition, when the crystallinity of polypropylene is 200ppm or less, the amount of the structured structure does not become excessive, phase separation of polyolefin and polypropylene at high temperature can be suppressed, and the pores are not easily opened, and further short circuit can be suppressed. From the viewpoint of suppressing short-circuiting caused by film breakage at high temperature, the crystallinity of polypropylene is preferably 10ppm or more and 170ppm or less, more preferably 20ppm or more and 150ppm or less.

The crystallinity of polypropylene at 169℃can be determined by Differential Scanning Calorimetry (DSC) described later. The crystallinity can be within a predetermined range by adding polypropylene having a predetermined molecular weight and melting point, for example. In this microporous polyolefin membrane, the crystallinity of polypropylene at 169℃is required to be adjusted in consideration of compatibility with other polyolefin to be mixed, for example, the molecular weight and melting point of other polyolefin.

In order to satisfy the puncture strength, the shutdown temperature, and the crystallinity of polypropylene at 169 ℃, the polyolefin microporous membrane may be a multilayer microporous membrane composed of a plurality of layers.

From the viewpoint of easy control of strength and shutdown characteristics, the polyolefin constituting the polyolefin microporous membrane of the present invention preferably has a molecular weight of 5.0X10 in GPC chart ⁴ ～1.0×10 ⁵ And 3.0X10 s ⁵ ～7.0×10 ⁵ The ranges of (2) each have a peak.

The polyolefin microporous membrane of the present invention preferably contains a polyolefin having a weight average molecular weight of 4.0X10 ⁵ Above and 1.0X10 ⁶ The following polyethylenes. The weight average molecular weight of the polyethylene is more preferably 4.0X10 ⁵ Above and 1.0X10 ⁶ The following is given. When the polyolefin constituting the polyolefin microporous film is a polyolefin resin composition in the above range, the polyolefin microporous film can be easily melted while maintaining the strength even when it is formed into a film, and therefore has excellent shutdown characteristics. The weight average molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by GPC method.

The polyolefin microporous membrane of the present invention preferably contains polyethylene and isotactic polypropylene, and has a concentration of 3.5 mass% or more and 10.0 mass% or less relative to the total mass of the polyethylene and the isotactic polypropylene. More preferably 4.0 mass% or more and 6.0 mass% or less. When the lower limit of the polypropylene concentration is within the above preferred range, the polypropylene component remains even after the polyolefin is melted, and the polypropylene composition has a sufficient network, and can maintain heat resistance. If the upper limit of the polypropylene concentration is within the above preferred range, the decrease in puncture strength of the whole film can be suppressed, the pores are less likely to be opened, and the short circuit can be further suppressed. The polypropylene concentration in the polyolefin microporous membrane can be determined by infrared spectrometry (IR measurement) described later. In addition, the polypropylene concentration in the polyolefin microporous membrane relative to the total mass of polyethylene and isotactic polypropylene can be controlled by the polypropylene concentration contained in the polyolefin resin raw material forming the polyolefin microporous membrane. The polypropylene concentration to be contained 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 still more preferably 3.0 mass% or more and 5.5 mass% or less, with respect to the total mass of polyethylene and isotactic polypropylene in the polyolefin resin raw material. The lower limit of the void ratio of the polyolefin microporous membrane of the present invention is not particularly limited, and is, for example, 20% or more, and more preferably 30% or more. The lower limit of the void ratio is not particularly limited, and is, for example, 70% or less, preferably 60% or less. When the porosity is in the above range, the holding amount of the electrolyte can be increased, and high ion permeability can be ensured. Further, if the void ratio is in the above range, the rate characteristics are improved. In addition, from the viewpoint of further improving ion permeability and rate characteristics, the void ratio is preferably 20% or more. The void ratio may be within the above range by adjusting the blending ratio, stretching ratio, heat setting condition, etc. of the constituent components of the polyolefin in the production process.

The mechanical heat shrinkage of the polyolefin microporous membrane of the present invention is, for example, 10% or less, preferably 9% or less, and more preferably 8% or less. The polyolefin microporous membrane has a heat shrinkage in the width direction of, for example, 10% or less, preferably 9% or less, and more preferably 7% or less at 120 ℃ for 1 hour. The lower limit of the heat shrinkage in the machine direction and the lower limit of the heat shrinkage in the width direction are not particularly limited, and are preferably-2.0% or more. When the upper limit of the mechanical heat shrinkage and the width heat shrinkage is within the above ranges, the deformation of the battery and the risk of short-circuiting of the ends can be reduced, and the battery safety can be improved. The thermal shrinkage of the polyolefin microporous membrane can be within the above range by adjusting the blending ratio, stretching ratio, thermal setting conditions, etc. of the constituent components of the polyolefin in the production process.

(method for producing polyolefin microporous film)

The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or a multi-layer microporous membrane composed of a plurality of layers. The layer structure is preferably two or more layers, more preferably three layers, and particularly preferably a layer and a layer B having different resin compositions are a layer a/B layer a or B layer a/B layer B. The polyolefin resin composition a and the polyolefin resin composition B constituting the a layer and the B layer are described below.

(1) Polyolefin resin composition A

The polyolefin resin composition a may contain polyethylene a1 and polyethylene a2.

(polyethylene a 1)

Polyethylene a1 has a weight average molecular weight (Mw) of 7.0X10 ⁵ The polyethylene above. The polyethylene a1 may be a copolymer containing a small amount of other alpha-olefin copolymer other than ethylene, but a homopolymer of ethylene is preferably used. As the alpha-olefin copolymer other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene are preferable. Regarding the content of the α -olefin other than ethylene, the α -olefin copolymer is set to 100mol% and preferably 5mol% or less. From the viewpoint of the uniformity of the pore structure of the polyolefin microporous membrane, a homopolymer of ethylene is preferable.

The weight average molecular weight (Mw) of the polyethylene a1 is preferably 7.0X10 from the viewpoint of easy control of the strength, stretchability, and melting of the microporous membrane ⁵ Above and below 2.0X10 ⁶ More preferably 1.0X10 ⁶ Above and 1.8X10 ⁶ The following is given. The melting point of the polyethylene a1 is preferably 134 to 137 ℃, more preferably 134 to 136 ℃. The content of the polyethylene A1 is preferably 50 mass% or more, more preferably 60 mass% or more, and even more preferably 70 mass% or more, based on 100 mass% of the polyolefin resin composition a. The upper limit is 95 mass%.

(polyethylene a 2)

From the viewpoint of easy control of melting of the microporous film, the weight average molecular weight (Mw) of polyethylene a2 was 5.0X10 ⁴ Above and below 7.0X10 ⁵ Preferably 3.0X10 ⁵ Hereinafter, it is more preferably 2.0X10 ⁵ The following is given. Furthermore, polyethylene a2 is preferablyThe low melting point component preferably has a melting point of 130 ℃ or higher and less than 134 ℃, more preferably 130 ℃ or higher and 133 ℃ or lower, and still more preferably 130 ℃ or higher and 132 ℃ or lower. The 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 alpha-olefin copolymer other than ethylene. As the alpha-olefin copolymer other than ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate and styrene are preferable. Regarding the content of the α -olefin other than ethylene, the α -olefin copolymer is set to 100mol% and preferably 10mol% or less. The content of polyethylene a2 is preferably 50 mass% or less, more preferably 40 mass% or less, and even more preferably 30 mass% or more, based on 100 mass% of the polyolefin resin composition a.

(2) Polyolefin resin composition B

The polyolefin resin composition B may comprise polyethylene B1 and polypropylene.

(polyethylene b 1)

The polyethylene b1 may be the same as the polyethylene a1 of the above item. However, the same meaning means polyethylene having the same range of molecular weight and melting point as those of polyethylene a 1.

(Polypropylene)

The type of polypropylene is not particularly limited as long as it satisfies the following molecular weight and melting point. From the viewpoint of phase separation and shape retention at high temperature of the microporous membrane, the weight average molecular weight (Mw) of polypropylene is preferably 1X 10 ⁶ The above is more preferably 1.2X10 ⁶ The above is more preferably 1.2X10 ⁶ ～4×10 ⁶ . The melting point of polypropylene is preferably 155 to 175℃and more preferably 160 to 170 ℃.

The polypropylene may be any of a homopolymer of propylene, a copolymer of propylene with other α -olefins and/or dienes (propylene copolymer), and a mixture of 2 or more selected from them, but a homopolymer of propylene alone is more preferable. As the propylene copolymer, both random copolymers and block copolymers can be used. The α -olefin in the propylene copolymer is preferably an α -olefin having 8 or less carbon atoms. Examples of the α -olefin 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 diene in the propylene copolymer is preferably a diene having 4 to 14 carbon atoms. Examples of the diolefin having 4 to 14 carbon atoms include butadiene, 1, 5-hexadiene, 1, 7-octadiene, and 1, 9-decadiene. The content of other α -olefin and diene in the propylene copolymer is preferably adjusted so that the polypropylene falls within the above-mentioned preferable melting point range. The content of polypropylene is preferably 10% by mass or more and 30% by mass or less, more preferably 10% by mass or more and 20% by mass or less, relative to 100% by mass of the polyolefin resin composition B.

The polyolefin resin compositions a and B may contain other resin components than polyethylene a1, a2, B1, and polypropylene, as required. As the other resin component, for example, a resin imparting heat resistance may be further contained. Further, various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent, a filler, a crystallization nucleating agent, and a crystallization retarder may be contained within a range not impairing the effect of the present invention.

When the layer of the polyolefin microporous membrane is formed as a layer A/B, the thickness ratio of the layer A/B is preferably 5/95 to 90/10, more preferably 30/70 to 80/20, and even more preferably 35/65 to 75/25. Thus, the puncture strength of the film can be maintained and the film has high heat resistance.

In the present embodiment, a microporous membrane may be produced by laminating a porous layer on at least one surface of a polyolefin microporous membrane. The porous layer is not particularly limited, and for example, a porous layer formed of a resin may be laminated. The resin used here is not particularly limited, and known resins may be used, and examples thereof include acrylic resins, poly-1, 1-difluoroethylene resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, and the like. The porous layer may further contain inorganic particles, and the inorganic particles are not particularly limited, and known materials may be used, and examples thereof include alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

(3) Process for producing polyolefin microporous membrane

The method for producing a polyolefin microporous membrane of the present invention comprises the following steps. Details of each step will be described.

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

(b) Shaping of gel-like sheets

(c) First stretching

(d) Removal and drying of plasticizer

(e) Second stretching

(f) And (5) heat treatment.

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

The layer a and the layer B are formed of the polyolefin resin composition a and the polyolefin resin composition B, respectively. A plasticizer was added to the polyolefin resin composition in a twin-screw extruder, and the mixture was melt kneaded to prepare solutions of the layer A and the layer B, respectively. The polyolefin resin composition preferably contains 10 mass% or more and 30 mass% or less with respect to the entire resin solution. When the concentration of the polyolefin resin composition is in the above range, melt fracture and shrinkage (negin) at the die outlet can be prevented when the polyolefin solution is extruded, and the moldability and appearance of the extrusion molded article can be improved.

The solutions of the a layer and the B layer were supplied from the extruder to 1 die, respectively, where the two solutions were extruded in a layered sheet form, to obtain an extrusion molded body. The extrusion method may be any one of a flat die method and an inflation method. Either method may be a method of supplying a solution to each manifold and laminating the solution in layers at the die lip inlet of a multilayer die (multi-manifold method), or a method of supplying a solution to a die by preliminarily forming a laminar flow (block method). The multi-manifold method and the block method can be applied to a general method. The interval between the multilayer flat dies may be set to 0.1mm or more and 5mm or less. The extrusion temperature is preferably 140℃to 250℃inclusive, and the extrusion rate is preferably 0.2 to 15 m/min. By adjusting 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 still more preferably 35/65 to 75/25, of the thickness of the sheet of the solution constituting the layer A/the thickness of the sheet of the solution constituting the layer B. A polyolefin microporous film excellent in balance between strength and melting while maintaining a network of polypropylene in a preferable range of a polyolefin composition, a thickness ratio of each sheet, and a stretching condition described later can be obtained. In the polyolefin microporous film, when the polypropylene concentration of the polyolefin resin composition B is 10 mass% or more and 30 mass% or less, the polyolefin resin composition B is preferably arranged in a concentrated manner in a plurality of layers, for example, as 50/50, as compared with 0/100 (a single-layer structure of the B layer) from the viewpoint of network maintenance of polypropylene. Thus, even a film can maintain the puncture strength and has higher heat resistance.

(b) Shaping of gel-like sheets

The obtained extrusion molded body was cooled to mold a gel-like sheet. As the cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roller, or the like can be used, but it is preferable to contact with a roller cooled with a cooling medium and cool it. The cooling is preferably carried out at a rate of 50 ℃ per minute or more up to at least the gelation temperature. The cooling is preferably carried out up to below 25 ℃. If the cooling rate is within the above range, the crystallinity is maintained within a proper range, and a gel sheet suitable for stretching is obtained.

(c) First stretching

Next, the gel-like sheet was stretched. The gel sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof after preheating. The stretching may be uniaxial stretching or biaxial stretching. The stretching ratio (surface stretching ratio) is preferably 9 times or more, more preferably 16 times or more, particularly preferably 25 times or more. The stretching ratios in the machine direction (hereinafter, may be referred to as MD.) and the width direction (hereinafter, may be referred to as TD.) may be the same or different, and the stretching ratios in both MD and TD are preferably 3 times or more.

The lower limit of the first stretching temperature is preferably 100 ℃ or more and 130 ℃ or less, more preferably 110 ℃ or more and 120 ℃ or less. By using the polyolefin composition A, B and the layer composition, the stretching temperature is in the above-described preferable range, and thus the rupture of the polyolefin resin having a low melting point component due to stretching is suppressed, and high-rate stretching can be performed. As a result, the puncture strength of the polyolefin microporous membrane is improved, and the rise in the shutdown temperature is easily suppressed. In addition, phase separation of polyethylene and polypropylene at high temperature is suppressed, and shape retention is improved. Therefore, when used as a battery separator, the film is excellent in mechanical strength and battery safety.

(d) Removal of plasticizers

Next, the plasticizer contained in the gel-like sheet is removed and dried using a washing solvent. The washing solvent and the method for removing the plasticizer using the same are well known, and therefore, the description thereof is omitted. For example, a method disclosed in Japanese patent application No. 2132327 and Japanese patent application laid-open No. 2002-256099 can be used. After the plasticizer is removed, drying is performed by a heat drying method or an air drying method. Any method that can remove the washing solvent can be used, including conventional methods such as heat drying and air drying (air-drying).

(e) Second stretching

The polyolefin microporous membrane can be obtained by stretching (dry stretching) in at least one direction after preheating the dried sheet. The second stretching may be performed by a tenter method or the like while heating. The final stretch ratio of the second stretching is preferably 1.1 times or more, more preferably 1.4 times or more. By setting the final stretch ratio to the above range, the puncture strength can be easily controlled to a desired range. However, if the stretching is performed at a high rate, the shutdown temperature and heat shrinkage increase, and therefore, it is preferably 9 times or less. When the polyolefin composition A, B and the layer composition are used, film breakage of the polyolefin resin having a low melting point component due to stretching is suppressed, and control is easy.

(f) Heat treatment of

After the second stretching, heat treatment is performed in a state where the width is fixed in a state where the heat treatment is held by a jig. The heat treatment is preferably carried out at 115.0 ℃ or higher and 135.0 ℃ or lower. By setting the heat treatment temperature to the above range, the thermal shrinkage rate of the polyolefin microporous membrane can be suppressed. In the heat treatment, a heat relaxation treatment may be performed. In the case of performing the thermal relaxation treatment, the relaxation rate may be 5% or more and 30% or less, assuming that the length immediately before the treatment is 100%. By setting the relaxation rate to the above range, the thermal shrinkage rate can be reduced, and the jitter in the step of the microporous film after relaxation can be suppressed.

Examples

The present invention will be described in further detail with reference to examples. The present invention is not limited to these examples.

[ measurement method ]

(1) Film thickness

The thickness of 5 points on the upper left, upper right, center, lower left, and lower right of the polyolefin microporous membrane in the range of 95mm×95mm was measured by a contact thickness meter (a probe having a contact pressure of 0.01N and 10.5mm phi), and the average value was set to be the thickness (μm). In addition, when the sample size was not 95mm×95mm, the sample was cut out in any size, and 5 points of the sample were measured at the upper left, upper right, center, lower left and lower right.

(2) Pay-per-view (weight per unit area)

A polyolefin microporous film cut to 5cm square was prepared, and each mass was measured by a precision balance (effective number 5 (0.0000 g)) and divided by 25cm ² Thus, the result was calculated. When the sample size cannot be 5cm×5cm, the sample may be cut out in any size, and the measured mass may be divided by the area.

(3) Void fraction

The polyolefin microporous membrane was cut into a size of 95mm by 95mm, and the volume (cm) was determined ³ ) And mass (g), by means of them and film density (g/cm) ³ ) The void fraction (%) was calculated by the following formula.

The formula: void fraction = ((volume-mass/membrane density)/volume) x 100

Here, the film density was set to 0.99g/cm ³ . The film thickness measured by the above (1) was used for the calculation of the volume.

(4) Resistance to ventilation

Regarding the polyolefin microporous membrane, according to JIS P-8117:2009, air permeation resistance (sec/100 cm) was measured using an air permeation resistance meter (EGO-1T, manufactured by Asahi Kabushiki Kaisha Co., ltd.) ³ )。

(5) Puncture strength

A polyolefin microporous membrane having a film thickness T (μm) was pierced at a speed of 2 mm/sec using a needle having a diameter of 1mm (0.5 mmR at the tip), and the maximum load value S (N) at this time was measured. The puncture strength was calculated by the following equation with a film thickness of 5. Mu.m.

The formula: converted puncture strength=s (N) ×5 (μm)/T (μm).

(6) Shutdown temperature (also referred to as SD temperature.)

A polyolefin microporous membrane punched into a round shape with a diameter of 45mm was exposed to an atmosphere at 20℃and the air permeation resistance was measured while heating up at a rate of 5℃per minute, whereby the air permeation resistance was 100,000 seconds/100 cm ³ The temperature at which the measurement was carried out was defined as the off temperature, and an average of 2 measurements was used. Air permeation resistance according to JIS P8117:2009, measurement was performed using a permeation resistance meter (EGO-1T, manufactured by asahi corporation).

(7) Melting temperature (also referred to as MD temperature.)

A polyolefin microporous membrane punched into a round shape with a diameter of 45mm was exposed to an atmosphere at 20℃and the air permeation resistance was measured while heating up at a rate of 5℃per minute, and the air permeation resistance was 100,000 seconds/100 cm ³ Further heating to make the air permeability resistance smaller than 100,000 seconds/100 cm ³ Is defined as the melting temperature. Air permeation resistance according to JIS P8117:2009, measurement was performed using a permeation resistance meter (EGO-1T, manufactured by asahi corporation).

(8) Heat shrinkage rate

The polyolefin microporous membrane was cut into a size of 95mm×95mm, the length (mm) before shrinkage of the test piece at room temperature (25 ℃) was measured for both the machine direction and the width direction, the test piece of the polyolefin microporous membrane was exposed to a temperature of 105 ℃ for 8 hours without applying a load, and after the test piece was returned to room temperature, the length (mm) after shrinkage in the machine direction and the width direction was measured, and the thermal shrinkage (%) in the machine direction and the width direction was determined from the obtained test piece length using the following formula.

The formula: MD heat shrinkage (%) = (1-machine direction post-shrinkage length/machine direction pre-shrinkage length) ×100

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

(9) Weight average molecular weight

The weight average molecular weight (Mw) and the molecular weight distribution (Mw/Mn) of the polyolefin resin and the polyolefin microporous membrane were determined by a Gel Permeation Chromatography (GPC) method using the following measurement conditions.

Measurement conditions

Measurement device: agilent high temperature GPC apparatus PL-GPC220

Column: PL1110-6200 (20 μm MIXED-A). Times.2 roots manufactured by Agilent Co., ltd

Column temperature: 160 DEG C

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

Solvent flow rate: 1.0 mL/min

Sample concentration: 0.1 mass% (dissolution conditions: 160 ℃ C./3.5H)

Sample introduction amount: 500 mu L

Detector: agilent differential refractive index detector (RI detector)

Viscometer: agilent viscosity detector

Standard curve: was prepared by a universal standard curve method using a monodisperse polystyrene standard sample.

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

Low molecular weight side peak position: peak positions of gaussian functions on the low molecular weight side when the molecular weight distribution was fitted with 2 gaussian functions.

High molecular weight side peak position: molecular weight at maximum of molecular weight distribution.

(10) Melting point and melting peak

The melting point of the polyolefin resin and the melting peak of the polyolefin microporous membrane were obtained by a scanning differential calorimeter (PYRISDIMOND DSC, manufactured by PARKING ELMER). The polyolefin resin and the polyolefin microporous membrane were respectively left in the sample holder, and after being completely melted by heating from 30℃to 230℃they were kept at 230℃for 3 minutes, and then cooled at a rate of 10℃per minute to 30 ℃. The same measurement was repeated again with the temperature rise at time 1, and the melting point (Tm) of the polyolefin resin and the heat of fusion of the polyolefin microporous membrane were obtained from the endothermic peak at temperature rise at time 2. The baseline at which the heat of fusion was calculated was a straight line obtained by connecting 30℃and 230 ℃. Regarding the polyolefin resin, a peak having a heat of fusion of 70J/g or more is regarded as an endothermic peak, and regarding the polyolefin microporous membrane, a peak having a heat of fusion of 0.1J/g or more is regarded as an endothermic peak.

(11) Layer ratio

The layer ratio of the polyolefin microporous membrane was observed under the following measurement conditions using a Transmission Electron Microscope (TEM).

Measurement conditions

Sample preparation: the polyolefin microporous membrane was dyed with ruthenium tetroxide, and the cross-section was cut with an ultra-thin microtome.

Measurement device: transmission electron microscope (JEM 1400Plus type of Japanese electronic system)

Observation conditions: accelerating voltage of 100kV

Viewing direction: TD/ND.

(12) Crystallinity of polypropylene at 169℃

The crystallinity of polypropylene in the polyolefin microporous membrane at 169℃was determined by the following measurement.

Measurement conditions

Measurement device: scanning differential calorimeter (PARKING ELMER PYRIS DIAMOND DSC)

Sample mass: 6mg of

Atmosphere gas: nitrogen gas

Start temperature: 30 DEG C

Temperature increase rate: 5 ℃/min

Reach temperature: 169 DEG C

Hold time at reached temperature: for 5 minutes

Cooling rate: 30 ℃/min

End temperature: 30 ℃.

In the above-described temperature reduction process, when the structure remains, an exothermic peak due to crystallization of polypropylene was detected in a temperature range of 120℃or higher. In the present invention, the crystallinity χ of polypropylene with respect to the whole resin was determined from the exothermic peak detected when the temperature was raised to 169 ℃ and then cooled.

The formula: χ=Δh _PP /ΔH _PP ^f X (isotactic polypropylene concentration)

Here ΔH _PP Represents the crystallization enthalpy (J/g) during the cooling of the polypropylene structure. So-called delta H _PP The term "the value obtained by dividing the area surrounded by the line connecting the start temperature and the end temperature of the exothermic peak caused by crystallization of polypropylene on the DSC curve during the cooling process and the DSC curve by the mass of the measurement sample. The isotactic polypropylene concentration is a value obtained by IR measurement described later. For example, in the case where the polypropylene is isotactic polypropylene, ΔH _PP ^f Indicating the complete melting enthalpy (J/g) of the polypropylene. ΔH _PP ^f Calculated using 170J/g.

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

The concentration of isotactic polypropylene in the polyolefin microporous film relative to the total mass of polyethylene and isotactic polypropylene was determined from 1462cm from polyethylene obtained by IR measurement ^-1 And 1376cm derived from isotactic polypropylene ^-1 Is obtained by the peak intensity ratio of (2). The measurement conditions are as follows.

Measurement device: FT-IR device (FT/IR 6600 made by Japan spectroscopic system)

Measurement temperature: 25 DEG C

Aperture (aperture): x=300 μm, y=300 μm

Cumulative number of times: 16 times

Resolution: 4cm ^-1

The formula: isotactic polypropylene concentration (%) = (1462 cm ^-1 Peak height x conversion factor 1)/(1376 cm ^-1 Peak height x conversion coefficient 2) x 100. Here, the conversion coefficient 1 is 20, and the conversion coefficient 2 is 10.

(14) Characteristics of hot box

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

Battery fabrication

The positive electrode and the negative electrode of the lithium ion secondary battery are laminated via a separator, and the separator contains an electrolyte solution (electrolyte). As the positive electrode active material, lithium cobalt composite oxide LiCoO was used ₂ Graphite was used as the negative electrode active material, and LiPF was used as the electrolyte at 1mol/L in a mixed solvent of DC/dimethyl carbonate (DMC) ₆ . In the battery assembly, a positive electrode, a separator made of a microporous film, and a negative electrode are stacked, a wound electrode body is produced by a conventional method, the wound electrode body is inserted into a battery can, an electrolyte is impregnated into the battery can, and a battery cover serving as a positive electrode terminal including a safety valve is crimped via a gasket.

Hot box test

The assembled battery was charged at a constant current of 1C until a voltage of 4.2V, then charged at a constant voltage of 4.2V, and then discharged at a current of 0.2C until a termination voltage of 3.0V. Then, constant current charging was performed at a current value of 0.2C until constant voltage charging of 4.2V was performed after 4.2V was reached, as a pretreatment. The pretreated battery was put into an oven, heated from room temperature at 5 ℃ per minute, and then left at 150 ℃ for 30 minutes. The case where the charging voltage is reduced by 50% or more within 15 minutes after reaching 150 ℃ is regarded as failure, the case where the charging voltage is reduced by 20 to 50% is regarded as failure, and the case where the charging voltage is reduced by 20% or less is regarded as preferable.

(15) Self-discharge characteristics

The self-discharge characteristics (K value) were evaluated by the following methods. The test secondary battery assembled by the following (method of manufacturing the evaluation battery) was subjected to constant current charging at a current value of 0.5C until the battery voltage was 3.85V, and then subjected to constant voltage charging at a battery voltage of 3.85V until the battery voltage was 0.05C. The open circuit voltage after the battery was left for 24 hours was measured, and this value was set to V1. The battery was further left for 24 hours, that is, the open circuit voltage after 48 hours in total after charging was measured, and this value was set to V2. From the obtained values of V1 and V2, the K value was calculated by the following formula.

The formula: k value= (V1-V2)/24.

Example 1

(1) Preparation of solution for layer A

The weight average molecular weight was 1.5X10 by means of a twin-screw extruder ⁶ 90% by mass of polyethylene having a melting point of 136.0deg.C and a weight average molecular weight of 1.0X10 ⁵ 10 mass% of polyethylene having a melting point of 132.0℃was melt-kneaded with liquid paraffin so that the resin concentration became 17 mass%, to prepare a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 1.5X10 by means of a twin-screw extruder ⁶ 70% by mass of polyethylene having a melting point of 136.0deg.C and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 30 mass% of (a) was melt-kneaded with liquid paraffin so that the resin concentration became 20 mass%, to prepare a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the A layer to the solution of the B layer to the solution of the A layer became 25/50/25. The extruded molded body was cooled while being pulled by a cooling roll temperature-controlled at 25℃to form a gel-like sheet.

(4) Removing and drying the first stretching and film forming solvent

The gel-like sheet was simultaneously stretched to 5 times at 110.0 ℃ both MD and TD using a tenter stretcher. After stretching, the sheet was immersed in a methylene chloride bath, after removing liquid paraffin, it was dried, and a dried microporous membrane was obtained.

(5) Second stretching and heat treatment

Then, after preheating at 128.0 ℃, stretching to 1.6 times in TD by a tenter stretcher, 15.0% relaxation was performed in TD, and the resulting film was thermally fixed at 128.0 ℃ while being held in the tenter, to obtain a polyolefin microporous film. The properties of the resulting polyolefin microporous membrane are shown in table 1.

Example 2

A polyolefin microporous membrane was obtained by stretching the same procedure as in example 1, except that the polyolefin solutions a and B were extruded so that the layer thickness ratio of the solution of the B layer to the solution of the a layer to the solution of the B layer was 25/50/25.

Example 3

(1) Preparation of solution for layer A

The weight average molecular weight was 1.5X10 by means of a twin-screw extruder ⁶ 70% by mass of polyethylene having a melting point of 135.0deg.C and a weight average molecular weight of 1.0X10 ⁵ 30 mass% of polyethylene having a melting point of 132.0℃was melt-kneaded with liquid paraffin so as to have a resin concentration of 20 mass%, to prepare a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 1.5X10 by means of a twin-screw extruder ⁶ 85% by mass of polyethylene having a melting point of 135.0℃and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 15 mass% of (a) was melt-kneaded with liquid paraffin so as to have a resin concentration of 20 mass%, thereby preparing a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the A layer to the solution of the B layer to the solution of the A layer became 20/60/20. The extruded molded body was cooled while being pulled by a cooling roll temperature-controlled at 25℃to form a gel-like sheet.

(4) Removing and drying the first stretching and film forming solvent

A microporous membrane was obtained by stretching, removing liquid paraffin, and drying in the same manner as in example 1, except that the gel sheet was set to a stretching temperature of 112.5 ℃.

(5) Second stretching and heat treatment

After preheating at 127.0 ℃, stretching to 1.5 times in TD by a tenter, 15.0% relaxation was performed in TD, and the resulting film was thermally fixed at 127.0 ℃ while being held in the tenter, to obtain a polyolefin microporous film.

Example 4

A microporous polyolefin membrane was obtained in the same manner as in example 3, except that the polyolefin solutions a and B were fed from the twin screw extruder to a T-die for 3 layers, and extruded so that the layer thickness ratio of the solution for B layer/the solution for a layer/the solution for B layer was 30/40/30, the first stretching temperature was 113.5 ℃, and the second stretching temperature and the heat setting temperature were 126.0 ℃.

Example 5

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

Example 6

A polyolefin microporous membrane was obtained in the same manner as in example 5, except that the first stretching temperature was 115.0 ℃.

Example 7

A microporous polyolefin membrane was obtained in the same manner as in example 5, except that the solution of the B layer/the solution of the a layer/the solution of the B layer was extruded so that the layer thickness ratio was 25/50/25, the first stretching temperature was 115.5 ℃, and the relaxation rate was 15.0%.

Example 8

(1) Preparation of solution for layer A

The weight average molecular weight was 7.5X10 by means of a twin-screw extruder ⁵ 85% by mass of polyethylene having a melting point of 136.0deg.C and a weight average molecular weight of 1.0X10 ⁵ 15 mass% of polyethylene having a melting point of 132.0℃was melt-kneaded with liquid paraffin so as to have a resin concentration of 25 mass%, to prepare a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 7.5X10 by means of a twin-screw extruder ⁵ 90% by mass of polyethylene having a melting point of 136.0deg.C and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 10 mass% of (a) was melt-kneaded with liquid paraffin so as to have a resin concentration of 15 mass%, to prepare a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the B layer to the solution of the A layer to the solution of the B layer became 15/70/15. The extruded molded body was cooled while being pulled by a cooling roll temperature-controlled at 25℃to form a gel-like sheet.

(4) Removing and drying the first stretching and film forming solvent

A microporous membrane was obtained by stretching, removing liquid paraffin, and drying in the same manner as in example 1, except that the gel sheet was set to a stretching temperature of 112.0 ℃.

(5) Second stretching and heat treatment

After preheating at 130.0 ℃, stretching to 1.8 times in TD by a tenter, 15.0% relaxation was performed in TD, and the resulting film was thermally fixed at 130.0 ℃ while being held in the tenter, to obtain a polyolefin microporous film.

Example 9

A microporous polyolefin membrane was obtained in the same manner as in example 5, except that the polyethylene and polypropylene in the polyolefin solution B were extruded so that the ratio of the solution of the layer a to the solution of the layer B to the solution of the layer a became a layer thickness ratio of 30/40/30, respectively, and the second stretching ratio was 1.8 times and the relaxation ratio was 15.0.

Example 10

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

Example 11

A polyolefin microporous membrane was obtained in the same manner as in example 1, except that the solution of the layer a/the solution of the layer B/the solution of the layer a was extruded so that the layer thickness ratio was 25/50/25, the first stretching temperature was 111.0 ℃ and the relaxation rate of the second stretching was 12.0%.

Example 12

A microporous polyolefin membrane was obtained in the same manner as in example 8, except that the solution of the layer a/the solution of the layer B/the solution of the layer a was extruded so that the layer thickness ratio was 35/30/35, the first stretching temperature was 112.0 ℃ and the relaxation rate of the second stretching was 10.0%.

Comparative example 1

(1) Preparation of layer A solution

The weight average molecular weight was 2.0X10 by using a twin-screw extruder ⁶ 40% by mass of polyethylene having a melting point of 133.0deg.C and a weight average molecular weight of 3.0X10 ⁵ 60 mass% of polyethylene having a melting point of 136.0℃was melt-kneaded with liquid paraffin so as to have a resin concentration of 25 mass%, to prepare a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 3.0X10 by using a twin-screw extruder ⁵ 80 mass% of polyethylene having a melting point of 135.0deg.C and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 20 mass% of (a) was melt-kneaded with liquid paraffin so as to have a resin concentration of 25 mass%, to prepare a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the B layer to the solution of the A layer to the solution of the B layer became 10/80/10. The extruded molded body was cooled while being pulled by a cooling roll temperature-controlled at 25℃to form a gel-like sheet. In this case, the discharge was adjusted by stretching to be described later so that the thickness after stretching became about 4.0. Mu.m.

(4) Removing and drying the first stretching and film forming solvent

The gel-like sheet was simultaneously stretched to 5 times at 115.0 ℃ both MD and TD using a tenter stretcher. After stretching, the sheet was immersed in a methylene chloride bath, after removing liquid paraffin, it was dried, and a dried microporous membrane was obtained.

(5) Second stretching and heat treatment

Then, after preheating at 125.5 ℃, stretching to 1.5 times in TD by a tenter stretcher, 15.0% relaxation was performed in TD, and the film was thermally fixed at 125.5 ℃ while being held in the tenter, to obtain a polyolefin microporous film.

Comparative example 2

(1) Preparation of solution for layer A

The weight average molecular weight was 1.6X10 by means of a twin-screw extruder ⁶ 40% by mass of polyethylene having a melting point of 134.0deg.C and a weight average molecular weight of 3.0X10 ⁵ 60 mass% of polyethylene having a melting point of 135.0℃was melt-kneaded with liquid paraffin so as to have a resin concentration of 28.5 mass%, to prepare a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 3.0X10 by using a twin-screw extruder ⁵ 70 mass% of polyethylene having a melting point of 135.0deg.C and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 30 mass% of (a) was melt-kneaded with liquid paraffin so as to have a resin concentration of 25 mass%, to prepare a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the A layer to the solution of the B layer to the solution of the A layer became 40/20/40. The extruded molded body was cooled while being pulled by a cooling roll whose temperature was adjusted at 25℃to form a gel-like sheet.

(4) Removing and drying the first stretching and film forming solvent

(5) Second stretching and heat treatment

Then, after preheating at 128.0 ℃, stretching to 1.6 times in TD by a tenter stretcher, 15.0% relaxation was performed in TD, and the resulting film was thermally fixed at 128.0 ℃ while being held in the tenter, to obtain a polyolefin microporous film.

Comparative example 3

(1) Preparation of layer A solution

The weight average molecular weight was 2.0X10 by using a twin-screw extruder ⁶ 30% by mass of polyethylene having a melting point of 133.0deg.C and a weight average molecular weight of 3.0X10 ⁵ 70 mass% of polyethylene having a melting point of 135.0℃was melt-kneaded with liquid paraffin so as to have a resin concentration of 28.5 mass%, thereby preparing a polyolefin solution A.

(2) Preparation of solution for layer B

The weight average molecular weight was 3.0X10 by using a twin-screw extruder ⁵ 50% by mass of polyethylene having a melting point of 135.0deg.C and a weight average molecular weight of 2.0X10 ⁶ The polypropylene 50 mass% of (a) was melt-kneaded with liquid paraffin so as to have a resin concentration of 30 mass%, to prepare a polyolefin solution B.

(3) Shaping of gel-like sheets

The polyolefin solutions A and B were fed from the twin-screw extruder to a 3-layer T die, and extruded so that the layer thickness ratio of the solution of the A layer/the solution of the B layer/the solution of the A layer became 35/30/35. The extruded molded body was cooled while being pulled by a cooling roll temperature-controlled at 25℃to form a gel-like sheet.

(4) Removing and drying the first stretching and film forming solvent

The gel-like sheet was simultaneously stretched to 5 times at 114.0 ℃ both MD and TD using a tenter stretcher. After stretching, the sheet was immersed in a methylene chloride bath, after removing liquid paraffin, it was dried, and a dried microporous membrane was obtained.

(5) Second stretching and heat treatment

Then, after preheating at 125.0 ℃, stretching to 1.5 times in TD by a tenter stretcher, 10.0% relaxation was performed in TD, and the microporous polyolefin film was obtained while being thermally fixed at 125 ℃ in the tenter.

Comparative example 4

A microporous polyolefin membrane was obtained in the same manner as in comparative example 1, except that a gel sheet was formed only in the polyolefin solution a of comparative example 1, the first stretching temperature was 114.0 ℃, the second stretching and heat setting temperatures were 130.0 ℃, and the relaxation rate was 20%.

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 membrane thickness of the polyolefin microporous membrane became about 9.0 μm.

Results (results)

The physical properties of the obtained polyolefin microporous membrane are shown in table 2. The polyolefin microporous membrane obtained in examples was a film as compared with comparative examples, and was low in shutdown temperature, and had both puncture strength and insulation after melting, and the battery used as a battery separator was excellent in battery safety and self-discharge characteristics, represented by a hot box.

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Industrial applicability

The polyolefin microporous membrane of the present invention can provide a polyolefin microporous membrane which is safe in a high-temperature state even if it is a film when used as a separator for a battery.

Claims

1. A polyolefin microporous membrane having a film thickness of 6 [ mu ] m or less, a puncture strength of 1.7N or more in terms of 5 [ mu ] m, a shutdown temperature of 80 ℃ or more and 138 ℃ or less as measured by a temperature-rise ventilation method, and a crystallinity of polypropylene of 3ppm or more and 200ppm or less when the temperature reaches 169 ℃.

2. The polyolefin microporous membrane of claim 1, which is a multi-layer microporous membrane composed of a plurality of layers.

3. The polyolefin microporous membrane according to claim 1 or 2, having a molecular weight of 5.0 x 10 in GPC diagram ⁴ ～1.0×10 ⁵ And 3.0X10 of ⁵ ～7.0×10 ⁵ Respectively, have peaks within the range of (c).

4. The polyolefin microporous membrane according to any of claims 1 to 3, which contains a polyolefin having a weight average molecular weight of 4.0X10 ⁵ Above and 1.0X10 ⁶ The following polyethylenes.

5. The polyolefin microporous membrane according to any one of claims 1 to 4, comprising polyethylene and isotactic polypropylene, wherein the concentration of isotactic polypropylene relative to the total mass of polyethylene and isotactic polypropylene is 3.5 mass% or more and 10.0 mass% or less.

6. The polyolefin microporous membrane according to any one of claims 1 to 5, which has a porous layer on at least one surface.

7. A separator for a battery comprising the polyolefin microporous film according to any one of claims 1 to 6.