JP7334719B2 - Polyolefin Microporous Membrane, Multilayer Polyolefin Microporous Membrane, Battery - Google Patents

Description

The present invention relates to a polyolefin microporous membrane, a multi-layer polyolefin microporous membrane and a battery.

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors. Among them, a microporous film made of polyolefin as a resin material is excellent in chemical resistance, insulating properties, mechanical strength, etc., and has shut-down characteristics, and thus has been widely used as a secondary battery separator in recent years.

Secondary battery separators have mechanical strength to prevent short circuits due to foreign matter contamination in the wound body and the battery, and to suppress thermal runaway of the battery. Shutdown characteristics are required. It should be noted that the lower the temperature at which the holes close (shutdown temperature), the better the shutdown characteristics.

In Patent Document 1, a polyolefin having a characteristic viscosity of 5 × 10 ⁵ Pa s or more and 5 × 10 ⁶ Pa s or less and a non-Newtonian fluidity of 0.15 or more and 0.4 or less is included, and the crystallinity is 65% or more and 85% or less, and a heat-resistant porous layer provided on at least one side of the porous substrate and containing a heat-resistant resin is disclosed. It is It is described that a non-aqueous electrolyte battery separator having excellent mechanical strength and shutdown characteristics and excellent short-circuit resistance at high temperatures is provided.

In Patent Document 2, when measured by DSC, the degree of crystallinity is 65 to 85%, the ratio of lamellar crystals in the crystal is 30 to 85%, the crystal length is 5 nm to 50 nm, and the non- A polyolefin microporous membrane is disclosed with a crystal length of 3 nm to 30 nm. It is described that even when combined with a heat-resistant porous layer, excellent mechanical strength and shutdown characteristics are obtained, and a polyolefin microporous membrane capable of preventing the electrolyte from drying up is provided.

In Patent Document 3, the fraction (a) with a weight average molecular weight of 1×10 ⁶ or more is 21 to 60% by weight, the fraction (b) with a weight average molecular weight of 1×10 ⁴ or more and less than 1×10 ⁶ and the weight average molecular weight The sum of the fraction (c) less than 1×10 ⁴ is 40 to 79% by weight, the fraction (c) is 30% by weight or less, and the weight average molecular weight/number average molecular weight is 7 to 50, Polyolefin microporous membranes characterized by bubble point values greater than 980 kPa are disclosed. It is described that a microporous polyolefin membrane having an excellent balance of air permeability, porosity, pore size, bubble point value, mechanical strength, dimensional stability, shutdown properties and meltdown properties and a method for producing the same are provided. .

JP 2012-129115 A WO2011/118660 JP-A-2002-284918

Secondary batteries, such as lithium-ion secondary batteries, have high energy density and are widely used as batteries for personal computers, mobile phones, and the like. Secondary batteries are also expected to serve as power sources for driving motors of electric vehicles and hybrid vehicles, and as stationary storage batteries.

In recent years, there has been a demand for higher energy densities in secondary batteries, and in order to increase the volume of the electrodes, it is desired to reduce the thickness of separators and apply high-capacity electrodes. However, as the thickness of the separator is reduced, the mechanical strength is lost, and a short circuit is likely to occur due to the contamination of the wound body or the inside of the battery with foreign matter. In addition, high-nickel-based materials are being studied as high-capacity electrodes, but high-nickel-based materials have safety concerns, such as the release of oxygen from the positive electrode material crystals when the temperature rises during overcharging, which can lead to ignition or explosion. big. Therefore, microporous membranes used for battery separators are required to have improved shutdown characteristics. However, the polyolefin microporous membranes of Patent Documents 1 to 3 are not sufficiently effective in shutting down characteristics in batteries with high energy densities.

In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane that is excellent in the structural balance of the entire membrane and that exhibits excellent shutdown characteristics and high battery safety when used as a battery separator. .

The present inventors have made intensive studies to solve the above problems, and found that a highly stretched microporous membrane has excellent mechanical strength, but the structure in the in-plane direction and the film thickness direction is anisotropic, It has been found that structural changes leading to shutdown are less likely to occur at lower temperatures than in highly non-stretched microporous membranes, and as a result it is difficult to lower the shutdown temperature. Then, by setting the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction to a predetermined value, the structural balance in the film in-plane direction and the film thickness direction is adjusted, and the polyolefin microstructure that can solve the above problems. The inventors have found that a porous membrane can be provided, and have completed the present invention.

In order to solve the above problems, the porous film of the present invention has the following configuration.
(1) The difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature by wide-angle X-ray diffraction measurement is 8% or more and 25% or less, and the crystallinity in the film thickness direction is the plane A polyolefin microparticle whose crystallinity is greater than the crystallinity in the in-plane direction and the difference between the crystallinity in the in-plane direction at the shutdown temperature measured by wide-angle X-ray measurement and the crystallinity in the in-plane direction at room temperature is 58% or more and 90% or less. porous membrane.
(2) The polyolefin microporous membrane according to (1), wherein the melting peak temperature of the polyolefin microporous membrane observed in the second temperature rise in differential scanning calorimetry is 115°C or higher and 132°C or lower.
(3) The polyolefin microporous membrane according to (1) or (2), wherein the resin composition forming the polyolefin microporous membrane has a molecular weight distribution of 5 or more and 30 or less.
(4) The polyolefin microporous membrane according to any one of (1) to (3), which has a puncture strength equivalent to 9 μm of 2.25 N or more.
(5) The polyolefin microporous membrane according to any one of (1) to (4), wherein the polyolefin contains polyethylene.
(6) The polyolefin microporous membrane according to any one of (1) to (5), which has a thickness of 9 μm or less.
(7) A multi-layer microporous membrane obtained by laminating one or more other porous layers on the polyolefin microporous membrane according to any one of (1) to (6).
(8) A battery separator comprising the polyolefin microporous membrane according to any one of (1) to (6) or the multilayer microporous membrane according to (7).
(9) A battery comprising the battery separator according to (8).

The polyolefin microporous membrane of the present invention has a low shutdown temperature, and can impart excellent shutdown performance to batteries when used as a battery separator.

1. Polyolefin Microporous Membrane The polyolefin microporous membrane of the present embodiment will be described below. It should be noted that the present invention is not limited to the embodiments described below.

In the polyolefin microporous film according to the present invention, the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature by wide-angle X-ray diffraction measurement is 25% or less, and the crystallinity in the film thickness direction is is greater than the crystallinity in the in-plane direction, and the difference between the crystallinity in the in-plane direction and the crystallinity in the in-plane direction at the shutdown temperature by wide-angle X-ray measurement is 58% or more.

Here, the film thickness direction represents a direction perpendicular to the surface of the polyolefin microporous film. The in-plane direction means a direction perpendicular to the film thickness plane of the polyolefin microporous membrane. In addition, the degree of crystallinity was determined by fitting the amorphous halo and the crystalline peak from the diffraction spectrum obtained by wide-angle X-ray diffraction measurement in which the measurement sample was irradiated with X-rays while the temperature was raised from room temperature. The area is obtained and can be determined from the formula below.
Formula: Crystallinity (%) = (Peak area of crystal) / (Peak area of crystal and amorphous) x 100
The degree of crystallinity in the film thickness direction and the degree of crystallinity in the in-plane direction at room temperature are the differences between the degrees of crystallinity determined from diffraction spectra at room temperature when X-rays are irradiated in the film thickness direction and in the in-plane direction, respectively. The crystallinity in the in-plane direction at the shutdown temperature is the difference between the crystallinities obtained from the diffraction spectrum at the shutdown temperature when X-rays are irradiated in the in-plane direction. Here, the shutdown temperature is the permeability measured by an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) while heating the microporous membrane from room temperature (25 ° C.) at a temperature increase rate of 5 ° C./min. The temperature at which the resistance reaches the detection limit of 1×10 ⁵ sec/100 cm ³ Air.
(Difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25° C.))
To control the shutdown characteristics, it is important to control the balance between the structure in the film thickness direction and the structure in the in-plane direction. The balance between the structure in the film thickness direction and the structure in the in-plane direction can be known from the difference in crystallinity in each direction. The difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature by wide-angle X-ray diffraction measurement is 25% or less, and the crystallinity in the film thickness direction is the crystallinity in the in-plane direction. , the structural difference between the film thickness direction and the in-plane direction becomes small, and the structure in the in-plane direction changes at a temperature lower than the conventional shutdown temperature, improving the shutdown characteristics. When the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25° C.) exceeds 25%, the thermally stable structure in the in-plane direction increases, which is important for controlling shutdown characteristics. A change in the in-plane direction is suppressed, and the shutdown characteristic deteriorates.

The upper limit of the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25° C.) is preferably 20% or less. Although the lower limit of the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction is not particularly limited, it is preferably 5% or more, more preferably 8% or more. When the difference between the degree of crystallinity in the film thickness direction and the degree of crystallinity in the in-plane direction is 5% or more, the structure in the in-plane direction is stabilized, resulting in a film with well-balanced shutdown characteristics.

In addition, by making the degree of crystallinity in the film-thickness direction larger than that in the in-plane direction, the thermally stable structure in the in-plane direction becomes smaller than that in the film-thickness direction. Shutdown characteristics are improved by accelerating the change of .

The difference between the degree of crystallinity in the film thickness direction and the degree of crystallinity in the in-plane direction at room temperature is determined by, for example, the content of the nucleating agent and the weight average molecular weight ( Mw), molecular weight distribution (MwD), stretching conditions, particularly the stretching temperature of the film after drying, which will be described later, can be adjusted to the above range.

(Difference between in-plane crystallinity at shutdown temperature and in-plane crystallinity at room temperature)
It is also important that the structural change in the in-plane direction occurs preferentially to improve the shutdown characteristics. The structural change in the in-plane direction until shutdown is obtained from the difference between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at room temperature obtained by wide-angle X-ray measurement while increasing the temperature, which will be described later. can know. The difference between the in-plane crystallinity at shutdown and the in-plane crystallinity at room temperature is 58% or more. When this difference is less than 58%, the change in the in-plane direction (crystal melting) is suppressed, and the shutdown characteristics deteriorate.

The lower limit of the in-plane crystallinity at the shutdown temperature and the in-plane crystallinity at room temperature is preferably 60% or more, more preferably 65% or more. Although the upper limit is not particularly limited, it is preferably 90% or less, more preferably 85% or less. If it is within the above range, it indicates that crystal melting in the in-plane direction proceeds preferentially, and the shutdown characteristics are improved. If this difference exceeds 90%, the thermally unstable structure in the in-plane direction becomes too large, resulting in a decrease in mechanical strength and a significant decrease in the thermal stability of the entire film. There are concerns about thermal stability when placed in

When producing a polyolefin microporous membrane, the difference between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at the room temperature is determined by, for example, the content of the nucleating agent, the resin used as the raw material The above range can be achieved by adjusting the weight average molecular weight (Mw), molecular weight distribution (MwD) and stretching conditions of the composition, particularly the stretching temperature of the film after drying, which will be described later.

(Melting peak temperature of polyolefin microporous membrane observed at second temperature rise (2nd heat) by differential scanning calorimetry (DSC))
The melting peak temperature of the polyolefin microporous membrane is measured in a nitrogen atmosphere (20 mL/min) in a temperature range of 0°C to 200°C, with a heating rate of 10°C/min and a holding time of 10 minutes after reaching 200°C. (First temperature rise), then the temperature is lowered at a rate of 30° C./min under a nitrogen atmosphere (20 mL/min). Furthermore, in a nitrogen atmosphere (20 mL/min), the measurement temperature range is 0 ° C. to 200 ° C., and after reaching 200 ° C. at a temperature increase rate of 10 ° C./min, the temperature is raised for a second time with a holding time of 5 minutes. . Using the straight line connecting 0°C and 200°C from the DSC curve obtained in the second temperature increase as the baseline, the temperature at the melting endothermic peak was read, and the temperature was observed in the second temperature increase by DSC of the polyolefin microporous membrane. It is defined as the melting peak temperature. When two or more peaks are observed, the melting point peak temperature of the microporous membrane is read from the most endothermic peak.

The melting peak temperature of the polyolefin microporous membrane observed by DSC 2nd heat is preferably 115°C or higher and 132°C or lower, more preferably 120 or higher and 130°C or lower, and still more preferably 122°C or higher and 130°C or lower. By setting the melting peak temperature observed in the 2nd heat of DSC of the polyolefin microporous membrane to 115°C or higher and 132°C or lower, the stress during wet stretching or dry stretching is suppressed, and the in-plane direction and the film thickness direction are thermally insensitive. The number of stable structures increases, and structural changes in the in-plane direction, which are important for controlling shutdown characteristics, occur, improving shutdown characteristics. The melting peak temperature of polyolefin can be obtained by the differential scanning calorimetry (DSC) method described later.

(Molecular weight distribution (MwD) of polyolefin microporous membrane)
The molecular weight distribution of the polyolefin microporous membrane is preferably 5 or more and 30 or less, more preferably 16 or more and 25 or less, and still more preferably 17 or more and 23 or less. By setting the molecular weight distribution of the polyolefin microporous membrane within the above range, it is possible to prevent the mechanical strength from being lowered due to a significant decrease in the melting point and the crystallinity. Since the formation of a stable structure is suppressed, excellent shutdown characteristics are exhibited and battery safety is improved. The molecular weight distribution of the polyolefin microporous membrane can be determined by the gel permeation chromatography (GPC) method described later.

The molecular weight distribution of the polyolefin microporous membrane is determined in the process of producing the polyolefin microporous membrane, for example, by including at least one of branched polyethylene and antioxidant, or by adjusting kneading conditions (especially temperature, screw rotation speed). The above range can be achieved by, for example,

(film thickness)
Although the upper limit of the thickness of the polyolefin microporous membrane is not particularly limited, it is preferably 20 μm or less, more preferably 12 μm or less, and still more preferably 9 μm or less. Although the lower limit of the film thickness is not particularly limited, it is preferably 1 μm or more, more preferably 3 μm or more. When the film thickness is within the above range, the battery capacity is improved when the polyolefin microporous film is used as a battery separator.

(Porosity)
Although the lower limit of the porosity of the polyolefin microporous membrane is not particularly limited, it is, for example, 20% or more, more preferably 30% or more, and still more preferably 40% or more. Although the upper limit of the porosity is not particularly limited, for example, it is preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less. When the porosity is within the above range, it is possible to increase the retention amount of the electrolytic solution and ensure high ion permeability. Furthermore, even when the porosity is high, battery safety can be ensured due to excellent shutdown characteristics. Moreover, the lower limit of the porosity is preferably 20% or more from the viewpoint of further increasing the ion permeability and rate characteristics. The porosity can be set within the above range by adjusting the compounding ratio of the constituent components of the polyolefin, the draw ratio, the heat setting conditions, and the like in the manufacturing process.

(shutdown temperature)
The shutdown temperature of the polyolefin microporous membrane was defined as the temperature at which the air resistance obtained from temperature-rising air resistance measurement reached the detection limit of 1×10 ⁵ seconds/100 cm ³ Air. The shutdown temperature is 110° C. or more and less than 135° C., preferably 110° C. or more and 134° C. or less, more preferably 110° C. or more and 133° C. or less, and even more preferably 110° C. or more and 132° C. or less. When the shutdown temperature is less than 135° C., battery thermal runaway at high temperatures can be suppressed compared to conventional microporous membranes, improving safety. Also, if the shutdown temperature is less than 110° C., for example, there is a concern about thermal stability when placed near a vehicle-mounted high-temperature portion, and a concern that air resistance will suddenly deteriorate during heat fixation.

(Puncture strength converted to 9 μm)
The lower limit of the 9 μm equivalent puncture strength of the polyolefin microporous membrane is preferably 1.35 N or more, more preferably 1.80 N or more, and still more preferably 2.25 N. Although the upper limit of the 9 μm equivalent puncture strength is not particularly limited, it is, for example, 3.6 N or less. When the 9 μm equivalent puncture strength is within the above range, the polyolefin microporous membrane has excellent membrane strength. A secondary battery using this polyolefin microporous membrane as a separator can suppress the occurrence of a short circuit between the electrodes. The 9 μm equivalent puncture strength can be obtained by including a nucleating agent, adjusting the weight average molecular weight Mw, the molecular weight distribution MwD, and the stretching conditions (especially the stretching temperature of the film after drying, which will be described later), when producing the polyolefin microporous membrane. The above range can be achieved by, for example,

(Thermal shrinkage rate)
The heat shrinkage of the polyolefin microporous membrane in the machine direction and width direction after 8 hours at 105° C. is preferably 12% or less, more preferably 6% or less, and even more preferably 4% or less. Although the lower limits of the heat shrinkage in the machine direction and the width direction are not particularly limited, they are preferably 0.5% or more. When the heat shrinkage rate in the machine direction and the width direction is 12% or less, the risk of deformation inside the battery and short circuit at the ends when used in the battery can be reduced. Moreover, when the thermal shrinkage ratios in the machine direction and the width direction are 0.5% or more, pore clogging due to thermal shrinkage is likely to occur, and deterioration of shutdown characteristics can be prevented.

2. Method for Producing Polyolefin Microporous Membrane Examples of the method for producing the polyolefin microporous membrane include a dry film-forming method and a wet film-forming method. As the method for producing the polyolefin microporous membrane of the present embodiment, from the viewpoint of controlling the structure and physical properties of the membrane, a membrane formation method combining wet and dry processes is preferable.

A method for producing a polyolefin microporous membrane will be described below. In addition, the following description is an example of the manufacturing method, and is not limited to this method.

First, a polyolefin and a film-forming solvent are melt-kneaded to prepare a resin solution. As a melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

Polyolefins include polyethylene, polypropylene and the like. As polyethylene, various polyethylenes can be used, including ultra-high molecular weight polyethylene, high density polyethylene, branched high density polyethylene, medium density polyethylene, branched low density polyethylene, linear low density polyethylene and the like. From the viewpoint of improving the mechanical strength of the microporous membrane and improving the shutdown characteristics, it is more preferable to use branched high-density polyethylene. Polyethylene may be an ethylene homopolymer or a copolymer of ethylene and other α-olefins. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Polyethylene can be included in an amount of 50% by mass or more based on 100% by mass of the polyolefin microporous membrane.

In particular, it preferably contains high-density polyethylene. When high-density polyethylene is contained, excellent melt-extrusion properties and uniform drawing processing properties are obtained. In particular, high-density polyethylene (density: 0.920 g/m ³ or more and 0.970 g/m ³ or less) is excellent in melt extrusion properties and excellent in uniform drawing processing properties. The weight average molecular weight (Mw) of high-density polyethylene is preferably 1×10 ⁴ or more and less than 1×10 ⁶ .

The content of high-density polyethylene is preferably 50% by mass or more as a lower limit and 100% by mass or less as an upper limit with respect to 100% by mass of the polyolefin microporous membrane.

The branched high-density polyethylene preferably has a weight average molecular weight of 1×10 ⁴ or more and less than 1×10 ⁶ , a molecular weight distribution of 10 or more and 30 or less, and a melting point of 115° C. or more and 132° C. or less. When polyethylene within this range is used, the crystallinity can be easily adjusted, the molecular weight distribution of the polyolefin microporous membrane and the crystallinity difference between the film thickness direction and the in-plane direction can be easily adjusted to an appropriate range, and the shutdown characteristics can be improved. When branched high-density polyethylene is included, in-plane orientation is less likely to proceed, and the shutdown temperature can be lowered, which is more preferable.

The weight average molecular weight (Mw) of the ultrahigh molecular weight polyethylene is preferably 1×10 ⁶ or more, more preferably 1×10 ⁶ or more and 8×10 ⁶ or less. When the Mw of the ultra-high molecular weight polyethylene is within the above range, moldability is improved. Ultra high molecular weight polyethylene can comprise at least one. For example, two or more types of ultra-high molecular weight polyethylenes with different Mw may be mixed and used as a raw material.

Moreover, when ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the polyolefin microporous membrane is made thin. The content of ultra-high molecular weight polyethylene can be 0% by mass or more and 70% by mass or less with respect to 100% by mass of the polyolefin microporous membrane. When the content of ultra-high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the molecular weight distribution (MwD) of the polyolefin microporous membrane is easily controlled within a specific range described later, and extrudability, kneadability, etc. productivity tends to be excellent.

The molecular weight distribution of the polyolefin is preferably 5 or more and 30 or less, more preferably 16 or more and 25 or less, and still more preferably 17 or more and 23 or less. By setting the molecular weight distribution of the polyolefin to 15 or more and 30 or less, the stress during wet or dry stretching is suppressed, and the thermally unstable structure increases in the in-plane direction and the film thickness direction, which is important for controlling shutdown characteristics. A structural change occurs in the in-plane direction, and the shutdown characteristics are improved. The molecular weight distribution of the starting material can be determined by the gel permeation chromatography (GPC) method described later.

In the process of producing a polyolefin microporous membrane, for example, by including at least one of branched polyethylene and an antioxidant, or by adjusting kneading conditions (especially temperature, screw rotation speed), the molecular weight distribution of polyolefin can be within the above range.

Polyolefin can contain resin components other than polyethylene and polypropylene, if necessary. As the other resin component, for example, a resin that imparts heat resistance can be used. In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, crystallization retarders, etc. may be included.

The resin solution may contain components other than the polyolefin and film-forming solvent described above, and may contain, for example, a crystal nucleating agent (nucleating agent), an antioxidant, and the like. The nucleating agent is not particularly limited, and known compound-based and fine particle-based crystal nucleating agents can be used. The nucleating agent may be a masterbatch in which the nucleating agent is previously mixed and dispersed in the polyolefin.

When not containing a crystal nucleating agent, the polyolefin preferably contains the branched polyethylene and high-density polyethylene described above. Also, the polyolefin microporous membrane may contain high-density polyethylene, branched polyethylene and a nucleating agent. By including these, the puncture strength can be further improved.

The molten resin is then extruded and cooled to form a gel sheet. For example, the resin solution prepared above is fed from an extruder to one die and extruded into a sheet to obtain a formed body. A gel-like sheet is formed by cooling the obtained compact.

As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably carried out at a rate of 50° C./min or more until at least the gelation temperature. Cooling is preferably performed to 25° C. or lower. When the cooling rate is within the above range, the degree of crystallinity is kept within an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but cooling by contacting with a roll cooled with a cooling medium is preferable.

Next, the gel-like sheet is stretched. The stretching (first stretching) of the gel-like sheet is also called wet stretching. Wet stretching is carried out at least uniaxially. Since the gel-like sheet contains a solvent, it can be uniformly stretched. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, the final area draw ratio (area ratio) in wet drawing is preferably 3 times or more, and more preferably 4 times or more and 30 times or less. In the case of biaxial stretching, the final areal stretching ratio is preferably 9 times or more, more preferably 16 times or more, and even more preferably 25 times or more. The upper limit of the final area draw ratio is preferably 100 times or less, more preferably 64 times or less. Moreover, the final area draw ratio is preferably 3 times or more in both the longitudinal direction and the width direction, and the draw ratios in the machine direction and the width direction may be the same or different. When the stretching ratio is 5 times or more, an improvement in puncture strength can be expected. The wet draw ratio is the draw ratio of the gel-like sheet after the wet draw when the gel-like sheet before the wet draw is used as a reference.

The stretching temperature is preferably within the range of the crystal dispersion temperature (Tcd) of the polyolefin to Tcd + 30°C, more preferably within the range of the crystal dispersion temperature (Tcd) + 10°C to the crystal dispersion temperature (Tcd) + 28°C. , Tcd+15.degree. C. to Tcd+26.degree. When the stretching temperature is within the above range, the stress during stretching of the polyolefin is adjusted, thermally unstable structures increase in the in-plane direction and the film thickness direction, and changes in the in-plane direction, which is important for controlling shutdown characteristics, are given priority. and the shutdown characteristics are improved. Here, the crystal dispersion temperature (Tcd) is a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. The above ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene compositions have a crystal dispersion temperature of about 90-100.degree. Therefore, when polyethylene is used as a raw material, the stretching temperature can be, for example, 90° C. or higher and 130° C. or lower.

Selection of raw materials and stretching as described above cause cleavage between polyethylene lamellae, miniaturize the polyethylene phase, and form a large number of fibrils. Fibrils form a three-dimensionally irregularly connected network structure. In addition, thermally unstable structures increase in the in-plane direction and the film thickness direction, resulting in a structure that can preferentially cause changes in the in-plane direction, which is important for controlling shutdown characteristics. Therefore, it is suitable for safer and higher-performance battery separators.

Next, the membrane-forming solvent is removed from the stretched gel-like sheet to obtain a microporous membrane (film). The film-forming solvent is removed by washing with a washing solvent. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

Next, the microporous membrane from which the membrane-forming solvent has been removed is dried by a heat drying method or an air drying method. Next, the dried microporous membrane is stretched. The stretching of the microporous membrane after drying is composed of the second stretching and the third stretching, and is also called dry stretching. In dry stretching, the microporous membrane film after drying is stretched at least uniaxially. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferred. In the case of sequential stretching, it is preferable to stretch in the machine direction (second stretching) and then continuously stretch in the width direction (third stretching). The method of dry stretching can be performed with a tenter type stretching machine, a roll type stretching machine, or the like while heating.

The area draw ratio (area ratio) of the dry drawing is preferably 1.1 times or more, and more preferably 1.2 times or more and 9.0 times or less. By setting the surface magnification within the above range, the puncture strength, shutdown temperature, etc. can be easily controlled within the desired range. In the case of uniaxial stretching, the stretching is, for example, 1.2 times or more, preferably 1.2 times or more and 3.0 times or less in the machine direction or width direction. In the case of biaxial stretching, the draw ratios in the machine direction and the width direction are set to 1.0 times or more and 3.0 times or less, respectively, and the draw ratios in the machine direction and the width direction may be the same or different. It is preferable that the draw ratios in the width direction are substantially the same. In dry stretching, after stretching in the machine direction by more than 1.0 times and 3.0 times or less (second stretching), it is continuously stretched in the width direction by more than 1.0 times and 3.0 times or less (third (stretching) is preferred. In particular, when roll stretching is used, the stretching in the machine direction is preferably less than 1.4 times, more preferably less than 1.3 times. Within the above range, the stress during stretching of the polyolefin is adjusted, thermally unstable structures increase in the in-plane direction and the film thickness direction, and changes in the in-plane direction, which is important for controlling shutdown characteristics, occur preferentially. structure. The dry stretching ratio is the stretching ratio of the microporous membrane after dry stretching with respect to the microporous membrane before dry stretching. The second drawing is preferably 3-step drawing or more, more preferably 4-step drawing or more, further preferably 5-step drawing or more. By doing so, the stress around one stage is dispersed, and stretching unevenness and the like can be suppressed.

The dry stretching temperature is usually 90 to 135°C, more preferably 100 to 135°C, still more preferably 105 to 135°C, from the viewpoint of stretching stress control. In particular, the second stretching temperature is preferably 100° C. or higher and 120° C. or lower, more preferably 110° C. or higher and 120° C. or lower. Within the above range, the stress during stretching of the polyolefin is adjusted, thermally unstable structures increase in the in-plane direction and the film thickness direction, and changes in the in-plane direction, which is important for controlling shutdown characteristics, occur preferentially. structure. By setting the second stretching temperature to 120° C. or lower, it is possible to prevent partial melting of the film and the occurrence of uneven appearance of the microporous film after the second stretching.

Moreover, the microporous membrane after drying may be subjected to a heat treatment. At least one of heat setting treatment and heat relaxation treatment can be used as the heat treatment method. The heat setting treatment is a heat treatment for heating while holding both ends in the width direction so that the dimension in the width direction of the film does not change. The heat setting treatment is preferably performed by a tenter method or a roll method. The thermal relaxation treatment is a heat treatment that thermally shrinks the film in the machine direction and the width direction during heating. For example, as a thermal relaxation treatment method, there is a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099. The heat treatment temperature is preferably within the range of (Tcd) to (Tm: melting point) of the second polyolefin, more preferably within the range of the stretching temperature of the microporous membrane ±5 ° C., and the second stretching temperature of the microporous membrane ±3 C. is particularly preferred.

For example, heat treatment and thermal relaxation treatment may be performed after the third stretching. In the thermal relaxation treatment, the relaxation temperature is, for example, 80° C. or higher and 135° C. or lower, preferably 90° C. or higher and 133° C. or lower. Moreover, when thermal relaxation treatment is performed, the final dry draw ratio is, for example, 1.0 to 9.0 times, preferably 1.2 to 4.0 times. The relaxation rate can be 0% or more and 70% or less.

The polyolefin microporous membrane of the present invention may be a single layer, or may be formed by laminating resin compositions having different compositions such as different polymer species, molecular weights, and their compounding ratios.

Further, another porous layer may be laminated on the polyolefin microporous membrane of the present invention to form a multi-layer porous membrane. The other porous layer is not particularly limited. For example, a porous layer such as an inorganic particle layer containing a binder and inorganic particles may be laminated. The binder component constituting the inorganic particle layer is not particularly limited, and known components can be used. Examples include acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, and the like. can be used. The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used. For example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, and the like can be used. can. Moreover, as the multilayer polyolefin porous membrane, the porous binder resin may be laminated on at least one surface of the polyolefin microporous membrane.

EXAMPLES The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

1. Measurement method and evaluation method [Crystallinity]
Five pieces of 3 cm x 3 cm were cut from the center position of the polyolefin microporous membrane in the width direction, and the stacks were used as measurement samples, which were heated from room temperature (25°C) to about 160°C at a rate of 5°C/min in a synchrotron radiation facility. A wide-angle X-ray diffraction measurement was performed. X-rays were perpendicularly incident on the surface of the measurement sample (the surface of the polyolefin microporous film) and measured to obtain a diffraction spectrum in the film thickness direction. In addition, X-rays were incident perpendicularly to the film thickness of the measurement sample (thickness of the microporous polyolefin film) and measured to obtain a diffraction spectrum in the in-plane direction. Each direction was measured once near the center of the sample at room temperature and shutdown temperature. From the obtained diffraction spectrum, the amorphous halo and the crystalline peak were fitted to determine the crystalline peak area and the amorphous peak area. The crystallinity in each direction was calculated using the following formula.
Formula: Crystallinity (%) = (Peak area of crystal) / (Peak area of crystal and amorphous) x 100
The degree of crystallinity in the film thickness direction and the degree of crystallinity in the in-plane direction at room temperature were defined as the difference between the degrees of crystallinity obtained from diffraction spectra at room temperature when X-rays were irradiated in the film thickness direction and in the in-plane direction, respectively. The crystallinity in the in-plane direction at the shutdown temperature was defined as the difference in crystallinity obtained from the diffraction spectrum at the shutdown temperature when X-rays were irradiated in the in-plane direction. Here, the shutdown temperature was the temperature measured by the method described later.

(Apparatus/measurement conditions)
It was measured under the following conditions using synchrotron radiation wide-angle X-rays.
(1) Synchrotron radiation facility SPring-8 BL08B2
(2) Wavelength 0.10 nm
(3) Beam diameter 0.35 mm in length and 0.40 mm in width
(4) Camera length 54.2mm
(5) Detector Hamamatsu Photonics flat panel C9728DK pixel size 50um square
(6) Exposure time 5 sec (when measuring in-plane direction), 2 sec (when measuring in film thickness direction)
(7) Measurement interval About 0.4°C step
(8) Measurement range 5 ≤ q ≤ 25 nm ^-1
[Melting peak temperature observed in the second heating (2nd heat) by differential scanning calorimetry (DSC)]
Measurements were made using a Perkin-Elmer DSC 8500. Select five locations so that the center of the polyolefin microporous membrane in the width direction becomes a test piece, so that it is almost equally spaced in the width direction. packed in a container. The first temperature rise was performed in a nitrogen atmosphere (20 mL/min), with a measurement temperature range of 0°C to 200°C, a temperature rise rate of 10°C/min, and a holding time of 10 minutes when the temperature reached 200°C. After that, the temperature was lowered at a rate of 30° C./min under a nitrogen atmosphere (20 mL/min). Next, the second temperature increase (reheating) is performed under a nitrogen atmosphere (20 mL/min), the measurement temperature range is 0 ° C. to 200 ° C., the temperature increase rate is 10 ° C./min, and the temperature when reaching 200 ° C. A holding time of 5 minutes was used. From the DSC curve obtained by the second heating, the straight line connecting 0 ° C. and 200 ° C. is used as the baseline, and the temperature at the melting endothermic peak is read, and the melting observed by reheating the polyolefin microporous membrane by DSC was the peak temperature. (When two or more peaks are observed, the melting point peak temperature of the microporous membrane is read from the most endothermic peak.).

[Weight average molecular weight/molecular weight distribution of polyolefin]
The weight average molecular weight (Mw) and molecular weight distribution (MwD) of polyolefin were determined by gel permeation chromatography (GPC) under the following conditions.
・ Measuring device: GPC-150C manufactured by Waters Corporation
・Column: Shodex UT806M manufactured by Showa Denko K.K.
・Column temperature: 135°C
・ Solvent (mobile phase): o-dichlorobenzene ・ Solvent flow rate: 1.0 ml / min ・ Sample concentration: 0.1 wt% (dissolution condition: 135 ° C. / 1 h)
・Injection amount: 500 μl
・ Detector: Waters Corporation differential refractometer (RI detector)
• Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a polyethylene conversion factor (0.46).

[Weight average molecular weight/molecular weight distribution of polyolefin microporous membrane]
The weight average molecular weight (Mw) and molecular weight distribution (MwD) of the polyolefin microporous membrane were determined by gel permeation chromatography (GPC) under the following conditions.
・ Measuring device: GPC-150C manufactured by Waters Corporation
・Column: Shodex UT806M manufactured by Showa Denko K.K.
・Column temperature: 135°C
・ Solvent (mobile phase): o-dichlorobenzene ・ Solvent flow rate: 1.0 ml / min ・ Sample concentration: 0.1 wt% (dissolution condition: 135 ° C. / 1 h)
・Injection amount: 500 μl
・ Detector: Waters Corporation differential refractometer (RI detector)
• Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a polyethylene conversion factor (0.46).

[Shutdown temperature]
While heating the microporous membrane from room temperature (25 ° C.) at a temperature rising rate of 5 ° C./min, the air permeability resistance was measured with an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The temperature at which the temperature reached 1×10 ⁵ seconds/100 cm ³ Air, which is the detection limit, was determined and defined as the shutdown temperature (° C.).

The measurement cell consisted of an aluminum block and had a structure with a thermocouple directly below the microporous membrane.

[Thickness]
The thickness of the microporous membrane was measured at 5 points within an area of 50 mm×50 mm using a contact thickness meter (Lightmatic manufactured by Mitutoyo Corporation), and the average value was determined as the thickness T ₀ (μm).

[Porosity (%)]
A sample obtained by cutting a polyolefin microporous membrane into X cm x Y cm was calculated by the following formula.

Porosity (%) = [1-(10000 × W / ρ) / (X × Y × T ₀ )] × 100
W: sample weight (g) ρ: resin density of polyethylene 0.95 (g/m ² )
T ₀ : sample thickness (μm)
Note that ρ uses the density of the resin used as the raw material.

[Puncture strength (N)]
Using a puncture tester NDG5 (manufactured by KatoTech), the maximum load ( N) was measured. The puncture strength is a value obtained by the following formula in which the maximum load (N) is converted to 9 μm.
Puncture strength = maximum load (N) x 9 (μm)/thickness T ₀ (μm) of polyolefin microporous membrane
[Heat shrinkage]
The heat shrinkage in the machine direction and the heat shrinkage in the width direction at 105° C. for 8 hours were measured as follows.
(1) A test piece was prepared by cutting out a 5 cm×5 cm piece with a dumbbell cutter at the center position in the width direction of the polyolefin microporous membrane at room temperature. This was repeated three times in the longitudinal direction to obtain n=3 samples. (If it cannot be cut out to 5 cm x 5 cm, cut it with a dumbbell cutter in the longitudinal direction / width direction with a length equivalent to 50% of the width direction length of the polyolefin microporous membrane.) The length of the cut sample in the machine direction ( L _0MD ) and the length in the width direction (L _0TD ) were measured respectively.
(2) The cut sample was heat-treated in an oven at 105°C for 8 hours without applying a load or gripping, and then returned to room temperature of 25°C for equilibration.
(3) The length in the machine direction (L _1MD ) and the length in the width direction (L _1TD ) of the heat-treated samples were measured respectively.
(4) Heat shrinkage in the machine direction and width direction is expressed by the following equations.

Machine direction heat shrinkage (%) = (1-L _1MD ) ÷ L _0MD ) x 100
Heat shrinkage rate in width direction (%) = (1-L _1TD )/L _0TD ) x 100
(Example 1)
A high-density polyethylene (polyethylene 1) having an Mw of 3×10 ⁵ and a molecular weight distribution of 20 is added with liquid paraffin as a film-forming solvent so as to have a resin concentration shown in Table 1, and melt-kneaded with a twin-screw extruder to obtain a polyolefin resin. A solution was prepared. A polyolefin resin solution was supplied from a twin-screw extruder to a T-die, extruded, and cooled while being taken up by a cooling roll to form a gel-like sheet. The gel-like sheet was simultaneously biaxially stretched (first stretching) at 110° C. by 5 times in both the machine and width directions using a tenter stretching machine. The stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried. The dried film was stretched 1.1 times in the width direction at 110° C. using a batch type stretching machine (second stretching). After that, heat setting was performed at 115° C. for 10 minutes to obtain a polyolefin microporous membrane. Table 1 shows the production conditions and measurement results of the polyolefin microporous membrane.

(Example 2)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the film was simultaneously biaxially stretched at 8 times in both the machine and width directions, and the second stretching was at 100° C. at 1.2 times in the machine direction.

(Example 3)
A high-density polyethylene (polyethylene 1) having an Mw of 5×10 ⁵ and a molecular weight distribution of 5 was added with liquid paraffin as a film-forming solvent so as to have a resin concentration shown in Table 1, melt-kneaded with a twin-screw extruder, Simultaneous biaxial stretching at 5 times in both width directions, second stretching at 115 ° C. in the machine direction at 1.2 times using a roll stretching machine, and then heat setting at 125 ° C. for 10 minutes. A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that

(Comparative example 1)
Table 1 shows a polyethylene resin composition comprising a high-density polyethylene (polyethylene 1) having an Mw of 5×10 ⁵ and a molecular weight distribution of 5 shown in Table 1 and an ultra-high molecular weight polyethylene (polyethylene 2) having an Mw of 1×10 ⁶ and a molecular weight distribution of 8. Add liquid paraffin as a film-forming solvent so that the resin concentration is as shown, melt-knead with a twin-screw extruder, and perform the first stretching at 90 ° C. with a tenter stretching machine at 5 times in both the machine direction and the width direction at the same time. Polyolefin in the same manner as in Example 1 except that it was biaxially stretched, the second stretch was 1.2 times in the machine direction at 90 ° C., and then heat set at 125 ° C. for 10 minutes. A microporous membrane was obtained.

(Comparative example 2)
The first stretching is simultaneously biaxially stretched to 9 times in both the machine direction and the width direction at 90 ° C. with a tenter stretching machine, the second stretching is 1.2 times in the machine direction at 90 ° C., and then A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that heat setting was performed at 125° C. for 10 minutes.

(evaluation)
In the polyolefin microporous membranes of Examples 1 to 3, the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25 ° C.) is 25% or less, and the crystallinity in the film thickness direction is is greater than the crystallinity in the in-plane direction, the difference between the crystallinity in the in-plane direction at the shutdown temperature measured by wide-angle X-ray measurement and the crystallinity in the in-plane direction at room temperature is 58% or more, and the shutdown temperature is The temperature was 135° C. or lower, indicating excellent shutdown characteristics.

From the above, the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25 ° C.) is 25% or less, and the crystallinity in the film thickness direction is higher than the crystallinity in the in-plane direction. It is clear that a microporous film in which the difference between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at room temperature by wide-angle X-ray measurement is 58% or more has excellent shutdown characteristics. became.

The polyolefin microporous membrane of the present invention exhibits excellent shutdown characteristics when incorporated into a secondary battery as a separator. Therefore, it can be suitably used for a secondary battery separator that requires a high capacity.

Claims (9)

The difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature by wide-angle X-ray diffraction measurement is 8% or more and 25% or less, and the crystallinity in the film thickness direction is the crystallinity in the in-plane direction. A polyolefin microporous membrane having a crystallinity greater than the crystallinity and having a difference of 58% or more and 90% or less between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at the room temperature as measured by wide-angle X-ray measurement. 2. The polyolefin microporous membrane according to claim 1, wherein the melting peak temperature of the polyolefin microporous membrane observed in the second temperature rise in differential scanning calorimetry is 115[deg.] C. or more and 132[deg.] C. or less. 3. The polyolefin microporous membrane according to claim 1, wherein the resin composition forming the polyolefin microporous membrane has a molecular weight distribution of 5 or more and 30 or less. The polyolefin microporous membrane according to any one of claims 1 to 3, which has a puncture strength of 2.25 N or more in terms of 9 μm. The polyolefin microporous membrane according to any one of claims 1-4, wherein the polyolefin comprises polyethylene. The polyolefin microporous membrane according to any one of claims 1 to 5, which has a thickness of 9 µm or less. A multilayer microporous membrane obtained by laminating one or more other porous layers on the polyolefin microporous membrane according to any one of claims 1 to 6. A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 6 or the multilayer microporous membrane according to claim 7. A battery comprising the battery separator according to claim 8 .