JP7540336B2 - Polyolefin microporous membrane, battery - Google Patents

Description

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

Microporous membranes are used in a variety of fields, including filters such as filtration membranes and dialysis membranes, as well as separators for batteries and electrolytic capacitors. Among these, microporous membranes made of polyolefin as the resin material have excellent chemical resistance, insulation properties, mechanical strength, and shutdown properties, and therefore have been widely used in recent years as separators for secondary batteries.

Separators for secondary batteries are required to have mechanical strength to prevent short circuits caused by foreign matter entering the wound body or the battery, and to have shutdown characteristics that block the current flowing between the electrodes by closing the pores in the separator at high temperatures to suppress thermal runaway in the battery. Generally, it is considered that the lower the temperature at which the pores close (shutdown temperature), the better the shutdown characteristics.

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

Patent Document 2 discloses a polyolefin microporous membrane that, when measured by DSC (differential scanning calorimetry), has a degree of crystallinity of 65-85%, a ratio of lamellar crystals to the crystals of 30-85%, a crystal length of 5 nm-50 nm, and an amorphous length of 3 nm-30 nm. It also discloses that, even when combined with a heat-resistant porous layer, a polyolefin microporous membrane is provided that has excellent mechanical strength and shutdown properties and is capable of preventing the electrolyte from drying up.

Patent Document 3 discloses a polyolefin microporous membrane that is formed using high-density polyethylene containing 20% or more of molecules with a molecular weight of less than 10,000 and having a weight-average molecular weight of 100,000 to 4,000,000, and that is characterized by having a pore-clotting temperature Tf of 134°C or less and a melting temperature Tm and Tf that can be expressed by the relationship Tm-Tf>0. It also discloses that the membrane provides polyethylene microporous membranes that can be used as separators for batteries, particularly high-charge capacity batteries, to prevent short circuits during overcharging of the batteries.

JP 2012-129115 A Patent No. 4920122 Patent No. 4230584

Because of their high energy density, secondary batteries, such as lithium-ion secondary batteries, are widely used in personal computers, mobile phones, and more recently drones. Secondary batteries are also expected to be used as power sources for driving motors in electric and hybrid vehicles, and as stationary storage batteries.

In recent years, there has been a demand for high-density energy density in secondary batteries to reduce size and improve performance, and it is desirable to apply high-capacity electrodes and thin the separator to increase the electrode volume. However, high-nickel materials have been considered as high-capacity electrodes, but the high-nickel materials are more likely to cause fires and explosions due to temperature rises in the battery during overcharging, etc., compared to current electrode materials. Therefore, it is desirable for the microporous membrane used in the battery separator to have a further reduction in shutdown temperature in order to block the pores before the battery reaches a runaway temperature. In addition, as the separator becomes thinner, the distance between the electrodes becomes shorter, so that the separator deformation at high temperatures, for example, dimensional changes in the thickness direction, increases the possibility of short circuits occurring. Therefore, in addition to the reduction in the shutdown temperature, a shutdown characteristic is required that minimizes dimensional changes in the thickness direction in the temperature range during shutdown and maintains the distance between the electrodes. The polyolefin microporous membranes of Patent Documents 1 to 3 do not have sufficient shutdown characteristics in batteries with high energy density.

In view of the above circumstances, the present invention aims to provide a polyolefin microporous membrane that has high permeability and excellent shutdown characteristics when used as a battery separator, and has high output characteristics and safety when used in a battery.

Therefore, the present inventors have conducted intensive studies to solve the above problems, and have found that the shutdown function of the separator is largely related to not only the structural change caused by the pore blockage in the membrane plane, but also the structural change in the membrane thickness direction. Here, the membrane thickness direction refers to the normal direction of the surface of the polyolefin microporous membrane. The in-plane direction refers to the normal direction of the plane perpendicular to the surface of the polyolefin microporous membrane. Furthermore, although a microporous membrane designed to be highly stretched like a general microporous membrane has excellent mechanical strength, the thermal stability in the membrane in-plane direction is excessively high compared to the membrane thickness direction because it is stretched to form the membrane. Therefore, the structural change in the membrane plane required to achieve shutdown is unlikely to occur unless the temperature is higher than that of a microporous membrane that is not highly stretched, and as a result, the change in the membrane thickness direction during shutdown is excessively large. Furthermore, it was found that a microporous membrane designed to be stretched to a low degree for the purpose of lowering the shutdown temperature or the like deteriorates in performance as a microporous membrane, such as decreased mechanical strength and decreased permeability, and also makes it difficult to form a thermally stable structure required in the thickness direction, and tends to experience large dimensional change in the thickness direction upon shutdown.

In response to the above, we have discovered that by adjusting the raw materials and composition and performing stretching under appropriate conditions, the change in melting point of the microporous membrane from the basic structure formed by the raw material composition can be kept within an appropriate range, thereby controlling the balance of thermally stable structures in the in-plane direction and thickness direction of the membrane, and providing a polyolefin microporous membrane that can solve the above problems while maintaining the basic properties of the microporous membrane, such as permeability, and thus have completed the present invention.

The polyolefin microporous membrane of the present invention is characterized in that it contains 90% by mass or more of polyethylene relative to 100% by mass of the polyolefin microporous membrane, has a 10 μm converted air resistance of 50 sec/100 cc or more and 400 sec/100 cc or less, a membrane thickness change rate at shutdown of 30 % or less, and a shutdown temperature measured by an air permeability method of 133° C. or less .

The battery separator of the present invention is made using the above-mentioned polyolefin microporous membrane.

The battery of the present invention is equipped with the above-mentioned battery separator.

The polyolefin microporous membrane of the present invention has excellent shutdown characteristics while retaining the permeability of a microporous membrane, and when used as a battery separator, it provides excellent battery safety.

The present embodiment of the present invention is described below.

The polyolefin microporous membrane of the present invention contains 90% by mass or more of polyethylene relative to 100% by mass of the polyolefin microporous membrane, has a 10 μm converted air resistance of 50 sec/100 cc or more and 400 sec/100 cc or less, a membrane thickness change rate at shutdown of 30 % or less, and a shutdown temperature measured by the air permeability method of 133° C. or less .

The polyolefin microporous membrane of the present invention contains polyolefin as a main component. The polyolefin is preferably contained in an amount of 90% by mass or more relative to 100% by mass of the polyolefin microporous membrane. Examples of polyolefin include polyethylene and polypropylene. As the 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. The polyethylene may be an ethylene homopolymer or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. Hexene-1 is the most preferable copolymer containing an α-olefin, and it is preferable that the polyethylene is mainly composed of an ethylene-1-hexene copolymer. The α-olefin can be confirmed by measuring with C ¹³ -NMR. Since the ethylene/1-hexene copolymer contains transition metals (Mn, Cr, etc.) and their supports (silicon dioxide, aluminum oxide, etc.) as catalyst residues in the raw materials, the polyolefin microporous membrane of the present invention containing the polyethylene may also contain the above catalyst residues. In addition, the polyethylene may be contained in an amount of 50% by mass or more relative to 100% by mass of the polyolefin microporous membrane.

The 10 μm-equivalent air permeability resistance of the polyolefin microporous membrane of the present invention is 50 sec/100 cc or more and 400 sec/100 cc or less.

The upper limit is preferably 400 seconds/100cc, more preferably 350 seconds/100cc, even more preferably 300 seconds/100cc, and particularly preferably 250 seconds/100cc. If the upper limit exceeds 400 seconds/100cc, the ion permeability is deteriorated and the electrical resistance increases when used as a battery separator, which is not preferable. The lower limit is not particularly limited, but is preferably 50 seconds/100cc, more preferably 75 seconds/100cc, and even more preferably 100 seconds/100cc or more. If the lower limit of the 10 μm converted puncture strength of the polyolefin microporous membrane is less than 50 seconds/100cc, more structural changes are required to block the pores, so the shutdown temperature increases and sufficient insulation cannot be obtained when used as a battery separator, which is not preferable. The 10 μm equivalent pin puncture strength of the polyolefin microporous membrane can be adjusted to the above range by adjusting the raw material composition and stretching conditions during production of the polyolefin microporous membrane.

The thickness change rate of the polyolefin microporous membrane of the present invention at shutdown is 33% or less. The thickness change rate of the polyolefin microporous membrane at shutdown is an index of the dimensional change in the thickness direction at shutdown, and is more preferably 30% or less, more preferably 27% or less, and particularly preferably 25% or less. The lower limit of the thickness change rate at shutdown is not particularly limited, but is preferably 0.5% or more, for example. The time of shutdown here refers to the time when the air permeability resistance obtained from the temperature increase air permeability resistance measurement described later reaches the detection limit of 1 x ¹⁰⁵ seconds/100 ccAir (shutdown temperature). If the thickness change rate at shutdown exceeds 33%, the dimensional change in the thickness direction is large, and there is a risk of deformation of the microporous membrane inside the battery or short circuit at the end. In addition, if the thickness change rate at shutdown is less than 0.5%, there is a concern that pore blockage does not occur and the shutdown function does not appear.

The rate of change in membrane thickness at shutdown can be adjusted to the above range when producing a polyolefin microporous membrane, for example, by adjusting the stretching conditions (particularly the stretching temperature and stretching ratio) and the raw material composition, etc., to bring the change in the melting point of the microporous membrane from the basic structure into an appropriate range. Details will be described later.

As described above, there is usually a trade-off between the permeability and shutdown properties of a microporous membrane; however, by adjusting the raw materials and composition and performing stretching under appropriate conditions as described in this application, it is possible to provide a polyolefin microporous membrane that can achieve the above relationship by keeping the change in melting point of the microporous membrane from the basic structure formed by the raw material composition within an appropriate range.

Next, we will explain the polyolefin microporous membrane of this embodiment.

(Shutdown temperature)
The shutdown temperature of the polyolefin microporous membrane was evaluated as the temperature at which the air resistance obtained from the temperature-elevating air resistance measurement described below reached the detection limit of 1×10 ⁵ sec/100 cc Air. The shutdown temperature is preferably 133° C. or less, more preferably 131° C. or less, even more preferably 130° C. or less, and particularly preferably 128° C. or less. When the shutdown temperature is 133° C. or less, the battery thermal runaway at high temperatures can be suppressed and safety is improved compared to conventional microporous membranes. In addition, the lower limit of the shutdown temperature is not particularly limited, but when it is less than 110° C., there is a concern about thermal stability when placed near a high-temperature part of a vehicle, or a concern that the air resistance will rapidly deteriorate during heat fixation. When producing a polyolefin microporous membrane, for example, the stretching conditions (particularly the stretching temperature and stretching ratio) and the raw material composition can be adjusted to set the melting point change of the microporous membrane from the basic structure within an appropriate range, and this will be described in detail later.

(Melting points Tm ₁ and Tm ₂ of microporous polyolefin membrane)
The melting points Tm ₁ and Tm ₂ of the polyolefin microporous membrane are determined by DSC. In the present application, the melting peak obtained from the first heating measurement, which indicates the melting point of the crystal structure of the polyolefin microporous membrane, is defined as Tm ₁ , and the melting peak obtained from the second heating measurement, which is derived from the basic structure of the raw material composition in the polyolefin microporous membrane, is defined as Tm _2. The lower limit of Tm ₁ is preferably 125°C or higher, more preferably 127°C or higher, and even more preferably 129°C or higher. The upper limit of _{Tm 1} is preferably 133°C or lower, more preferably 132°C or lower, even more preferably 131°C or lower, and particularly preferably 130°C or lower. If the lower limit of Tm ₁ is less than 125°C, the shutdown property is improved, but the permeability is significantly deteriorated, and the property balance as a microporous membrane is significantly deteriorated. If the upper limit of _Tm1 exceeds 133° C., the proportion of thermally stable structures will be higher than necessary, which is undesirable since it will result in a deterioration in the shutdown temperature and a large dimensional change in the film thickness direction during shutdown.

The lower limit of Tm ₂ is preferably 125°C or more, more preferably 127°C or more, and even more preferably 129°C or more. The upper limit of Tm ₂ is preferably 133°C or less, more preferably 132°C or less, even more preferably 131°C or less, and particularly preferably 130°C or less. If the lower limit of Tm ₂ is less than 125°C, the shutdown property is improved, but the permeability is significantly deteriorated, and the balance of properties as a microporous membrane is significantly deteriorated. If the upper limit of Tm ₂ exceeds 134°C or more, the proportion of thermally stable structure becomes higher than necessary, and the shutdown property is deteriorated, which is not preferable. Tm ₁ and Tm ₂ of the polyolefin microporous membrane can be controlled, for example, by adjusting the raw material composition and stretching conditions (especially the gel-like sheet stretching temperature and stretching ratio) when producing a polyolefin microporous membrane. Details will be described later.

(Melting point difference ΔTm of polyolefin microporous membrane)
The melting point difference ΔTm of the polyolefin microporous membrane is an index showing the change in the crystal structure due to the membrane formation, and is defined as the difference between the melting peak Tm ₂ obtained from the second heating measurement of DSC and the melting peak Tm ₁ obtained from the first heating measurement (ΔTm = Tm ₂ - Tm ₁ ). The lower limit of ΔTm is preferably greater than -1.4°C, more preferably greater than -1.0°C, even more preferably greater than -0.5°C, and particularly preferably greater than 0°C. If the lower limit of ΔTm is -1.4°C or less, the change in the proportion of the thermally stable structure from the basic structure is large, and in particular the proportion of the thermally stable structure in the in-plane direction of the membrane becomes higher than necessary, which is undesirable because it contributes to the deterioration of the shutdown temperature and the increase in the dimensional change in the membrane direction at the time of shutdown. The upper limit of ΔTm is preferably less than 1.0°C, more preferably less than 0.5°C. If the upper limit of ΔTm is 1.0°C or more, the change of the thermally stable structure from the basic structure is small, and it is difficult to form the thermally stable structure required for the entire microporous membrane, and the mechanical strength may decrease or the dimensional change in the thickness direction at shutdown may become large, which is undesirable.When producing a polyolefin microporous membrane, the ΔTm of the polyolefin microporous membrane can be controlled by adjusting, for example, the raw material composition and the stretching conditions (particularly the gel-like sheet stretching temperature and stretching ratio). Details will be described later.

(10 μm equivalent puncture strength)
The lower limit of the 10 μm equivalent pin puncture strength of the polyolefin microporous membrane is preferably 1.0 N or more, more preferably 1.2 N or more, and even more preferably 1.5 N or more. The upper limit of the 10 μm equivalent pin puncture strength is not particularly limited. When the 10 μm converted pin puncture strength is within the above range, the polyolefin microporous membrane has excellent membrane strength. The occurrence of short circuits of electrodes due to foreign matter, etc. is suppressed. The 10 μm converted pin puncture strength of the polyolefin microporous membrane can be determined by adding a nucleating agent, adjusting the weight average molecular weight Mw, the raw material composition, and the stretching conditions during the production of the polyolefin microporous membrane. It is possible to adjust the above range by adjusting the above.

(TD/MD tensile breaking strength ratio)
The lower limit of the TD/MD tensile breaking strength ratio of the polyolefin microporous membrane is preferably 0.5 or more, more preferably 0.7 or more, and even more preferably 0.9 or more.The TD/MD tensile breaking strength ratio is preferably 1.5 or less, more preferably 1.3 or less, and even more preferably 1.1 or less.When the TD/MD tensile breaking strength ratio exceeds the above range, since it is highly oriented in one direction, it is not preferable that the orientation is not eliminated unless the temperature is higher, and the blocking of pores is suppressed, and the shutdown property is deteriorated.

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

(Porosity)
The lower limit of the porosity of the polyolefin microporous membrane is not particularly limited, but is preferably 35% or more, more preferably 38% or more, even more preferably 40% or more, particularly preferably 42% or more, and extremely preferably 43% or more. The upper limit of the porosity is not particularly limited, but is preferably 70% or less, more preferably 60% or less, and even more preferably 50%. By having the porosity in the above range, the retention amount of the electrolyte can be increased and high ion permeability can be ensured. In addition, when the porosity is in the above range, the rate characteristics are improved. In addition, from the viewpoint of further improving the ion permeability and rate characteristics, the porosity is preferably 35% or more. The porosity can be set to the above range by adjusting the blending ratio of the polyolefin components, the stretching conditions, the heat fixing conditions, etc. in the manufacturing process.

Next, we will explain the manufacturing method of polyolefin microporous membrane.

Note that the following description is only one example of a manufacturing method and is not limited to this method.

The polyolefin microporous membrane can be produced by a known method for producing a polyolefin microporous membrane. Examples of the method for producing a polyolefin microporous membrane include a dry membrane production method and a wet membrane production method. From the viewpoint of controlling the structure and physical properties of the membrane, the polyolefin microporous membrane of the present invention is preferably produced by a wet method alone or a combination of a wet method and a dry method.

First, a resin solution is prepared by melt-kneading polyolefin and a membrane-forming solvent. As a melt-kneading method, for example, a method using a twin-screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. The melt-kneading method is well known, so a description is omitted.

The lower limit of the melting point of the polyolefin raw material is preferably 126°C or higher, more preferably 128°C or higher, and even more preferably 130°C or higher. The upper limit of the melting point of the polyolefin raw material is preferably 135°C or lower, more preferably 134°C or lower. If it is less than 126°C, the shutdown characteristics are improved, but the permeability is significantly deteriorated, and the balance of characteristics as a microporous membrane is significantly deteriorated. If the melting point of the polyolefin exceeds 135°C, the thermally stable structure becomes excessively large, and the shutdown characteristics including the shutdown temperature are deteriorated. The melting point of the polyolefin raw material can be determined by the DSC method described later.

For the production of the polyolefin microporous membrane of the present invention, it is preferable to use high density polyethylene (density: 0.940 g/cm ³ or more and 0.970 g/cm ³ or less) because it has excellent melt extrusion properties and excellent uniform stretch processing properties. In particular, branched high density polyethylene is more preferable because it is difficult for in-plane orientation to proceed, changes in the crystal structure can be suppressed, and the shutdown temperature can be reduced. The weight average molecular weight (Mw) of the high density polyethylene is preferably 1.0×10 ⁴ or more and less than 1.0×10 ⁶ , more preferably 1.0×10 ⁵ or more and 3.5×10 ⁵ or less, even more preferably 1.5×10 ⁵ or more and 2.0×10 ⁵ or less, and particularly preferably 1.5×10 ⁵ or more and 1.8×10 ⁵ or less. When Mw is within the above range, excessive in-plane orientation is difficult to proceed during membrane production, and changes in the crystal structure of the microporous membrane are easily controlled within an appropriate range, improving the shutdown properties and suppressing deterioration of permeability.

The content of high-density polyethylene is preferably at least 50% by mass and at most 100% by mass, relative to 100% by mass of the polyolefin microporous membrane.

Furthermore, the addition of low-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymer produced by a single-site catalyst, or low-molecular-weight polyethylene having a weight-average molecular weight of 1,000 to 100,000 can provide a shutdown function at low temperatures and improve the properties of the battery separator. However, if the proportion of the above-mentioned low-molecular-weight polyethylene is high, the porosity of the microporous membrane decreases during the membrane production process, so high-density polyethylene that is an ethylene-α-olefin copolymer and has a density of more than 0.94 g/ ^cm3 is preferred, and branched high-density polyethylene is even more preferred.

From the above viewpoints, when the molecular weight distribution of the polyolefin microporous membrane of the present invention is measured, it is preferable that the amount of components having a molecular weight of less than 40,000 is less than 20%. More preferably, the amount of components having a molecular weight of less than 20,000 is less than 20%, and even more preferably, the amount of components having a molecular weight of less than 10,000 is less than 20%. In the present invention, by using the above-mentioned raw materials, it is possible to lower the shutdown temperature without significantly reducing the molecular weight, and as a result, it is possible to achieve compatibility with other physical properties such as strength and porosity.

The polyolefin microporous membrane of the present invention can contain ultra-high molecular weight polyethylene. The Mw of the ultra-high 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 in the above range, the moldability is good. The ultra-high molecular weight polyethylene can contain one or more kinds. For example, two or more kinds of ultra-high molecular weight polyethylenes having different Mw may be mixed and used as a raw material. In addition, when the ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the polyolefin microporous membrane is thinned.

The content of ultra-high molecular weight polyethylene can be 0% by mass or more and 70% by mass or less, based on 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 productivity, such as extrudability and kneadability, tends to be excellent.

The polyolefin microporous membrane of the present invention may contain two or more types of polyolefins as necessary, and may contain other resin components other than polyethylene and polypropylene. Examples of other resin components that can be used include resins that impart heat resistance. In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be contained within a range that does not impair the effects of the present invention.

The proportion of polyolefin in the resin solution when forming the polyolefin microporous membrane of the present invention is calculated from the following formula. Although it depends on the raw material composition, a range of 20% to 50% is preferable, a range of 20% to 40% is more preferable, and a range of 25% to 35% is even more preferable. If the ratio of polyolefin to membrane-forming solvent is less than 20%, it is not preferable because it increases the possibility of poor molding of the gel sheet and membrane rupture during stretching. If the ratio of polyolefin to membrane-forming solvent is 50% or more, it is not preferable because the stress during stretching increases, and the thermal stabilization of the structure by stretching proceeds more than necessary.

Percentage of polyolefin in resin solution (%)
= Weight of polyolefin ÷ (Weight of polyolefin + Weight of membrane-forming solvent) × 100
The resin solution may contain components other than the polyolefin and the membrane-forming solvent, such as a crystal nucleating agent (nucleating agent), an antioxidant, etc. 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 master batch in which the nucleating agent is mixed and dispersed in advance in the polyolefin.

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

As a method for forming a gel-like sheet, for example, the methods disclosed in Japanese Patent Publication No. 2132327 and Japanese Patent Publication No. 3347835 can be used. Cooling is preferably performed at a rate of 50°C/min or more at least up to the gelation temperature. Cooling is preferably performed to 25°C or less. By cooling, the microphase of the polyolefin separated by the membrane-forming solvent can be fixed. If the cooling rate is within the above range, the crystallinity is maintained within an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, etc. can be used, but it is preferable to cool by contacting with a roll cooled with a refrigerant.

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

In wet stretching, the final area stretch ratio (area ratio) is preferably 3 times or more in the case of uniaxial stretching, and more preferably 4 times or more and 30 times or less. In the case of biaxial stretching, the final area stretch ratio is preferably 16 times or more, and more preferably 25 times or more. The upper limit of the final area stretch ratio is preferably 81 times or less, and more preferably 49 times or less. In addition, the final stretch ratio is preferably 3 times or more in both the longitudinal direction (machine direction: MD direction) and the transverse direction (width direction: TD direction), and the stretch ratios in the MD direction and the TD direction may be the same or different. If the stretch ratio is 5 times or more, an improvement in the puncture strength can be expected. The wet stretch ratio refers to the stretch ratio of the gel-like sheet after wet stretching, based on the gel-like sheet before wet stretching.

The lower limit of the area ratio after the first stretching of the polyolefin microporous membrane of the present invention is preferably 10 times or more, more preferably 16 times or more, and even more preferably 25 times or more. On the other hand, the upper limit of the final area ratio is preferably 81 times or less, more preferably 49 times or less, and even more preferably 36 times or less. If the final area ratio is less than 10 times, it is difficult to form a thermally stable structure required for a microporous membrane, which may cause a decrease in mechanical strength, and is therefore not preferred. In addition, if the final area ratio is 81 times or more, it is not preferred because the strain in the film thickness direction increases and the change in the film thickness direction at high temperatures becomes large.

The stretching temperature of the first stretching 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, and particularly preferably within the range of Tcd + 15 ° C to Tcd + 26 ° C. When the stretching temperature is within the above range, the stress during polyolefin stretching is adjusted, the balance of the thermally stable structure in the film is appropriately adjusted, and the change in the in-plane direction, which is important for controlling the shutdown temperature, occurs preferentially, and the dimensional change in the film thickness direction during shutdown is also suppressed, improving the shutdown characteristics. Here, the crystal dispersion temperature (Tcd) refers to a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. The above polyethylene composition has a crystal dispersion temperature of about 90 to 100 ° C. Therefore, when polyethylene is used as the raw material, the preferred stretching temperature for the first stretching can be, for example, 90°C or more and 130°C or less, more preferably 100°C or more and 120°C or less, and particularly preferably 105°C or more and 115°C or less.

The above-mentioned raw material selection and stretching causes cleavage between the polyethylene lamellae, resulting in finer polyethylene phases and the formation of numerous fibrils. The fibrils form a mesh structure in which they are irregularly connected in three dimensions. Furthermore, it becomes possible to appropriately control the balance of thermally stable structures within the film, resulting in a polyolefin microporous film structure that exhibits shutdown properties. This makes it suitable for safer, higher performance battery separators.

Next, the membrane-forming solvent is removed from the stretched gel-like sheet to form a microporous membrane (film). The membrane-forming solvent is removed by washing with a washing solvent. Since the polyolefin phase is phase-separated from the membrane-forming solvent phase, when the membrane-forming solvent is removed, a porous membrane is obtained that is made of fibrils that form a fine three-dimensional mesh structure and has holes (voids) that are irregularly connected in three dimensions. Washing solvents and methods for removing the membrane-forming solvent using them are publicly known, so a description is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Publication 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. The drying temperature is preferably equal to or lower than the crystal dispersion temperature (Tcd) of the polyolefin, and is particularly preferably 5°C or more lower than Tcd. The drying is preferably performed until the remaining washing solvent is 5% by mass or less, and more preferably 3% by mass or less, based on 100% by mass (dry weight) of the microporous membrane film. If the remaining washing solvent is within the above range, the porosity of the polyolefin microporous membrane is maintained when the subsequent microporous membrane film stretching and heat treatment steps are performed, and deterioration of permeability is suppressed.

Next, the dried microporous membrane is stretched. The stretching of the dried microporous membrane consists of second stretching and third stretching, and is also called dry stretching. In dry stretching, the dried microporous membrane film is stretched at least in one axial direction. 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 MD direction (second stretching) and then continuously stretch in the TD direction (third stretching). The dry stretching method can be performed using a tenter-type stretching machine or a roll-type stretching machine while heating.

The area stretching ratio (area ratio) of the dry stretching is preferably 1.1 times or more, and more preferably 1.2 times or more and 9.0 times or less. By setting the area ratio in the above range, it is possible to control the puncture strength, etc. to the desired range. In the case of uniaxial stretching, for example, the stretching ratio in the MD direction or TD direction is 1.2 times or more, preferably 1.2 times or more and 3.0 times or less. In the case of biaxial stretching, the stretching ratios in the MD direction and the TD direction are each 1.0 times or more and 3.0 times or less, and the stretching ratios in the MD direction and the TD direction may be the same or different, but it is preferable that the stretching ratios in the MD direction and the TD direction are approximately the same. In the dry stretching, it is preferable to stretch in the MD direction at more than 1.0 times and 3.0 times or less (second stretching), and then continuously stretch in the TD direction at more than 1.0 times and 3.0 times or less (third stretching). The dry stretching ratio refers to the stretching ratio of the microporous membrane after dry stretching based on the microporous membrane before dry stretching. The second stretching is preferably three-stage stretching or more, more preferably four-stage stretching or more, and even more preferably five-stage stretching or more. This distributes the stress per stage, and can suppress uneven stretching.

From the viewpoint of stretching stress control, the dry stretching temperature is usually 90 to 135°C, more preferably 100 to 135°C, and even more preferably 105 to 135°C. In particular, the second stretching temperature is preferably 100°C or higher and 120°C or lower, and more preferably 110°C or higher and 120°C or lower. By setting the temperature within the above range, the stress during stretching of the polyolefin microporous membrane is adjusted, the balance of the thermally stable structure in the membrane is appropriately adjusted, and the change in the in-plane direction, which is important for controlling the shutdown temperature, occurs preferentially. The dimensional change in the thickness direction during shutdown is also suppressed, and the shutdown characteristics are improved. At temperatures of 120°C or higher, the membrane may partially melt, causing the microporous membrane to have an uneven appearance after the second stretching.

The dried microporous membrane may be heat-treated. The heat treatment stabilizes the crystals and makes the lamellae uniform. At least one of heat setting and heat relaxation can be used as the heat treatment method. Heat setting is a heat treatment in which the membrane is heated while holding both ends in the TD direction so that the dimensions in the TD direction of the membrane do not change. The heat setting is preferably performed by a tenter method or a roll method. Heat relaxation is a heat treatment in which the membrane is thermally shrunk in the MD direction or the TD direction while being heated. For example, the heat relaxation method is the method disclosed in JP-A-2002-256099. The heat treatment temperature is preferably within the range of (Tcd) to (Tm: melting point) of the polyolefin used, more preferably within the range of the stretching temperature of the microporous membrane ±5°C, and particularly preferably within the range of the second stretching temperature of the microporous membrane ±3°C.

For example, after the third stretching, heat treatment and heat relaxation treatment may be performed. In the heat relaxation treatment, the relaxation temperature is, for example, preferably 80°C to 135°C, more preferably 90°C to 133°C, even more preferably 105°C to 125°C, and particularly preferably 110°C to 120°C. In addition, when the heat relaxation treatment is performed, the final dry stretch ratio is, for example, 1.0 to 9.0 times, preferably 1.2 to 4.0 times. The relaxation ratio can be 0% to 70%.

The lower limit of the final area ratio after heat setting of the polyolefin microporous membrane is preferably 10 times or more, more preferably 16 times or more, and even more preferably 25 times or more. On the other hand, the upper limit of the final area ratio is preferably 81 times or less, more preferably 49 times or less, and even more preferably 36 times or less. If the final area ratio is less than 10 times, it is difficult to form the thermally stable structure required for a microporous membrane, and the mechanical strength decreases or the thermally stable structure required in the film thickness direction is not formed, which is not preferable. In addition, if the final area ratio is 81 times or more, it is not preferable because the distortion in the film thickness direction increases and the change in the film thickness direction at high temperatures becomes large.

The polyolefin microporous film may be a single layer, or may be configured as a multilayer polyolefin microporous film by laminating one or more other microporous layers made of a polyolefin microporous film on the polyolefin microporous film described above. The multilayer polyolefin microporous film may have two or more layers made of a polyolefin microporous film. In the case of a multilayer polyolefin microporous film, the composition of the polyolefin constituting each layer may be the same composition or different compositions.

In addition, the polyolefin microporous film or the multilayer polyolefin microporous film described above may be laminated with a porous layer other than polyolefin to form a multilayer polyolefin porous film. The other porous layer is not particularly limited, but 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, for example, acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, polyimide resin, etc. 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, etc. can be used. In addition, the multilayer polyolefin porous film may be one in which the porous binder resin is laminated on at least one surface of the polyolefin microporous film.

The present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to these examples. Note that, in the following, Examples 7 to 8 should be read as Reference Examples 7 to 8, respectively.

(Measurement and evaluation methods)
[Film thickness]
The thickness of the microporous membrane was measured at five points within an area of 50 mm x 50 mm using a contact thickness meter (Litematic, manufactured by Mitutoyo Corporation), and the average value was calculated.

[Porosity]
The weight of the microporous membrane, w ₁ , was compared with the weight of an equivalent non-porous polymer, w ₂ (a polymer of the same width, length and composition), and was determined according to the following formula:

Porosity (%) = (w ₂ - w ₁ )/w ₂ ×100
[Puncture strength]
The 10 μm converted puncture strength is the maximum load (N) when a polyolefin microporous membrane is punctured at a speed of 2 mm/sec with a 1 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm) and converted to 10 μm. This is the value calculated using the formula:

Formula: puncture strength (converted to 10 μm)=maximum load (N)×10 (μm)/film thickness T ₁ (μm) of polyolefin microporous membrane
[Air permeability resistance]
For a microporous membrane having a thickness _T1 (μm), the air permeability resistance _P1 (sec/100 cm3) was measured using an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P 8117: 2009. In addition, the air permeability resistance _P2 (converted to 10 μm) (sec/100 ^cm3 / ¹⁰ μm) when the membrane thickness was set to 10 μm was calculated using the formula: _P2 = ( _P1 × 10)/ _T1 .

[Polyolefin weight average molecular weight/molecular weight distribution]
The weight average molecular weight (Mw) and molecular weight distribution (MwD) of the polyolefin were determined by gel permeation chromatography (GPC) under the following conditions.

Measurement 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 conditions: 135°C/1h)
Injection volume: 500 μl
Detector: Waters Corporation differential refractometer (RI detector)
Calibration curve: A calibration curve was prepared using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

[Polyolefin raw material melting point]
Measurement was performed at three different points in the same microporous membrane using the following method, and the melting point of the polyolefin raw material was determined. The polyolefin raw material was sealed in a measurement pan, and the temperature was raised from 30°C to 230°C at a heating rate of 10°C/min using a PYRIS DIAMOND DSC manufactured by PARKING ELMER (1st heating), then held at 230°C for 5 minutes, cooled at a rate of 10°C/min, and heated again from 30°C to 230°C at a heating rate of 10°C/min (2nd heating). The peak top of the melting chart of the obtained 2nd heating was determined as the melting point of the polyolefin raw material.

[Polyolefin microporous membrane melting points Tm ₁ , Tm ₂ , and melting point difference ΔTm]
Measurements were performed at three different points in the same microporous membrane by the following method, and the melting point obtained in the first heating was designated Tm ₁ , and the melting point obtained in the second heating was designated Tm _2. The polyolefin microporous membrane was sealed in a measurement pan, and using a PYRIS DIAMOND DSC manufactured by PARKING ELMER, the temperature was raised from 30° C. to 230° C. at a heating rate of 10° C./min (first heating), then held at 230° C. for 5 minutes, cooled at a rate of 10° C./min, and again raised from 30° C. to 230° C. at a heating rate of 10° C./min (second heating). From the melting charts obtained for each heating, the temperature at which the peak top intensity was the highest and the melting enthalpy of that peak was 50% of the entire peak was determined as the melting point of the polyolefin microporous membrane (°C), the melting point obtained from the first heating was _Tm1 , and the melting point obtained from the second heating was _Tm2 . The melting point difference ΔTm (°C) was calculated from _Tm2 - _Tm1 .

[TD/MD tensile breaking strength ratio]
A tensile test was performed using a tensile tester (Shimadzu Autograph AGS-J type), and the strength at the time of sample breakage was divided by the cross-sectional area of the sample before the test to obtain the tensile break strength (MPa). The measurement conditions were: temperature; 23±2°C, sample shape; width 10 mm × length 50 mm, chuck distance; 20 mm, and tensile speed; 100 mm/min. The above measurements were performed in MD and TD at different locations in the same microporous membrane, three points each, and the average value of the TD tensile break strength and the MD tensile break strength was calculated as the TD/MD tensile break strength ratio.

[Shutdown temperature]
The shutdown temperature was evaluated by measuring the temperature-rising air resistance as described below. The air resistance was measured with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) while heating the microporous membrane at a temperature-rising rate of 5°C/min, and the temperature at which the air resistance reached the detection limit of 1.0 x ¹⁰⁵ seconds/100 cc Air was determined and taken as the shutdown temperature (°C).

The measurement cell was constructed from an aluminum block and had a thermocouple directly below the microporous membrane. The sample was cut into a 50 mm x 50 mm square and the temperature was measured while it was fixed around the periphery with an O-ring.

[Film thickness change rate at shutdown]
The film thickness change rate at shutdown was measured as follows.
(1) The shutdown temperature of a polyolefin microporous membrane is measured by the method described above. Using another test piece in the same membrane, the thickness T ₁ (μm) of a test piece of the polyolefin microporous membrane before heat treatment at room temperature (25° C.) is measured by the method described above.
(2) The polyolefin microporous membrane is heat-treated for 30 minutes at the shutdown temperature described above while being fixed in all directions in a 100mm x 100mm frame as a test piece of 50mm x 50mm. The frame used must be able to withstand the shrinkage of the microporous membrane when heated and maintain its dimensions after the test. The air resistance is measured by the method described above and confirmed to be 1.0 x ¹⁰⁵ seconds/100cc or more. If it is less than 1.0 x ¹⁰⁵ seconds/100cc, the polyolefin microporous membrane is heat-treated again for 30 minutes at a temperature 1°C higher than the shutdown temperature, and the air resistance is measured by the method described above and confirmed to be 1.0 x ¹⁰⁵ seconds/100cc or more.
(3) The polyolefin microporous membrane that was heat-treated and confirmed to have an air resistance of 1.0 × ¹⁰⁵ sec/100 cc or more by the above-mentioned method was measured for thickness at five points within an area of 45 mm × 45 mm with a contact thickness meter (Litematic, manufactured by Mitutoyo Corporation) to calculate the average value, which was designated as the thickness after heat treatment _T2 .
(4) The membrane thickness _T2 of the microporous membrane after heat treatment was divided by the membrane thickness _T1 of the microporous membrane before heat treatment, the obtained value was subtracted from 1, and the value was expressed as a percentage (%) to calculate the membrane thickness change rate at shutdown. The specific formula is as follows:

Film thickness change rate=(1−T ₂ ÷T ₁ )×100.

[Shutdown characteristics]
The shutdown characteristics of each sample were evaluated on a 5-point scale using the following indices. When the evaluations differed between the shutdown temperature and the rate of change in film thickness at shutdown, the lower evaluation was used.
A: Shutdown temperature is 128°C or lower and the film thickness change rate at shutdown is 25% or less. B: Shutdown temperature is 130°C or lower and the film thickness change rate at shutdown is 30% or less. C: Shutdown temperature is 133°C or lower and the film thickness change rate at shutdown is 33% or less. D: Shutdown temperature is 135°C or higher and the film thickness change rate at shutdown is 35% or more. E: Shutdown temperature exceeds 135°C and the film thickness change rate at shutdown exceeds 35%.

Example 1
A branched high-density polyethylene (ethylene-1-hexene copolymer, density 0.953 g/cm ³ ) having a weight average molecular weight (Mw) of 1.8×10 ⁵ as the first polyethylene was melt-kneaded with liquid paraffin at a polyolefin ratio in the resin solution shown in Table 1 in a strong kneading type twin-screw extruder while maintaining the screw rotation speed Ns at 200 rpm at a temperature of 180° C. to prepare a polyolefin solution. The polyolefin solution was fed from the twin-screw extruder to a T-die, and the extruded molded product was cooled while being taken up by a cooling roll set at 20° C. or less to form a gel-like sheet. The gel-like sheet was simultaneously biaxially stretched at 110° C. by a tenter stretching machine at 5 times the MD and TD directions. The stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin, then dried and heat-set at 115° C. for 10 minutes. The evaluation results of the obtained polyolefin microporous membrane are shown in Table 1.

Example 2
A microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the gel-like sheet was stretched at 115°C.

Example 3
A branched high-density polyethylene (ethylene-1-hexene copolymer, density 0.953 g/cm ³ ) having a weight average molecular weight (Mw) of 2.0×10 ⁵ as the first polyethylene was melt-kneaded with liquid paraffin in a twin-screw extruder at the polyolefin ratio in the resin solution shown in Table 1 to prepare a polyolefin solution. The polyolefin solution was supplied to a T-die from the twin-screw extruder, and the extruded molded product was cooled while being taken up by a cooling roll to form a gel-like sheet. A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the gel-like sheet was simultaneously biaxially stretched at 105° C. by a tenter stretching machine at 5 times in both the MD and TD directions.

Example 4
A polyolefin microporous membrane was obtained in the same manner as in Example 3, except that the ratio of polyolefin in the resin solution was changed to 35% and the gel-like sheet was simultaneously biaxially stretched at 115°C by 5 times in both the MD and TD directions using a tenter stretching machine.

Example 5
A microporous polyolefin membrane was obtained in the same manner as in Example 1, except that a crystal nucleating agent CN-002 (manufactured by Riken Vitamin Co., Ltd.) was added in the composition shown in Table 1.

Example 6
The stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin, then dried, stretched 1.5 times in the width direction at 120°C, and relaxed 10% in the width direction to obtain a polyolefin microporous membrane in the same manner as in Example 1.

(Example 7)
A polyolefin solution was prepared by melt-kneading a branched high-density polyethylene (ethylene-1-hexene copolymer, density 0.953 g/cm ³ ) having a weight average molecular weight (Mw) of 1.8×10 ⁵ as the first polyethylene and an ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 2.4×10 ⁶ as the second polyethylene with liquid paraffin in a twin-screw extruder at the polyolefin ratio in the resin solution shown in Table 1. A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the polyolefin solution was supplied to a T-die from the twin-screw extruder and the extruded product was cooled while being taken up by a cooling roll to form a gel-like sheet.

(Example 8)
A polyolefin solution was prepared by melt-kneading in a twin-screw extruder a branched high-density polyethylene (ethylene-1-hexene copolymer, density 0.953 g/cm ³ ) having a weight average molecular weight (Mw) of 1.8×10 ⁵ as polyethylene and an ultra-high molecular weight polypropylene having a weight average molecular weight (Mw) of 10×10 ⁵ with liquid paraffin in the polyolefin ratio in the resin solution shown in Table 1. A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the polyolefin solution was supplied to a T-die from the twin-screw extruder, and the extruded product was cooled while being taken up by a cooling roll to form a gel-like sheet.

The evaluation results of the polyolefin microporous membranes obtained in Examples 2 to 8 are shown in Table 1.

(Comparative Example 1)
A high-density polyethylene having a weight average molecular weight (Mw) of 3.5×10 ⁵ , which is the first polyethylene, was melt-kneaded with liquid paraffin in a twin-screw extruder at the polyolefin ratio in the resin solution shown in Table 2 to prepare a polyolefin solution. The polyolefin solution was fed from the twin-screw extruder to a T-die, and the extruded extrusion was cooled while being taken up by a cooling roll to form a gel-like sheet. The gel-like sheet was simultaneously biaxially stretched at 115° C. in both the MD and TD directions at 5 times the original length by a tenter stretching machine. The stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin, then dried and heat-set at 125° C. for 10 minutes. The evaluation results of the obtained polyolefin microporous membrane are shown in Table 2.

(Comparative Example 2)
A polyolefin solution was prepared by melt-kneading a high-density polyethylene having a weight-average molecular weight (Mw) of 3.5× ¹⁰⁵ as the first polyethylene and an ultra-high molecular weight polyethylene having a weight-average molecular weight (Mw) of 2.4× ¹⁰⁶ as the second polyethylene with liquid paraffin in the polyolefin ratio in the resin solution shown in Table 2 in a twin-screw extruder. A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the polyolefin solution was supplied to a T-die from the twin-screw extruder and the extruded product was cooled while being taken up by a cooling roll to form a gel-like sheet.

(Comparative Example 3)
A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the gel-like sheet was simultaneously biaxially stretched at 9 times in both the MD and TD directions using a tenter stretching machine.

(Comparative Example 4)
A polyolefin microporous membrane was obtained in the same manner as in Comparative Example 1, except that the ratio of polyolefin to the membrane-forming solvent was changed to 25%.

(Comparative Example 5)
A polyolefin microporous membrane was obtained in the same manner as in Example 2, except that the gel-like sheet was simultaneously biaxially stretched at 9 times in both the MD and TD directions using a tenter stretching machine.

(Comparative Example 6)
A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the gel-like sheet was simultaneously biaxially stretched at 3 times in both the MD and TD directions using a tenter stretching machine.

(Comparative Example 7)
A high-density polyethylene having a weight-average molecular weight (Mw) of 3.5×10 ⁵ as the first polyethylene, an ultra-high-molecular-weight polyethylene having a weight-average molecular weight (Mw) of 2.4×10 ⁶ as the second polyethylene, and a linear low-density polyolefin having a weight-average molecular weight (Mw) of 0.7×10 ⁵ as the third polyethylene (density 0.938 cm ³ ) shown in Table 2 were melt-kneaded with liquid paraffin in a twin-screw extruder at the polyolefin ratio in the resin solution shown in Table 2 to prepare a polyolefin solution. The polyolefin solution was fed from the twin-screw extruder to a T-die, and the extruded extrusion molded body was cooled while being taken up by a cooling roll to form a gel-like sheet. The gel-like sheet was simultaneously biaxially stretched at 115° C. by a tenter stretching machine at 5 times in both the MD and TD directions. The stretched gel-like sheet was immersed in a methylene chloride bath to remove the liquid paraffin, then dried and heat-set at 120° C. for 10 minutes.

The evaluation results of the polyolefin microporous membranes obtained in Comparative Examples 1 to 7 are shown in Table 2.

(evaluation)
The polyolefin microporous membranes of Examples 1 to 7 had excellent air resistance as battery separators, while having a shutdown temperature of 133° C. or less and a membrane thickness change rate during shutdown of 33% or less, indicating that they had excellent shutdown properties.

The microporous polyolefin membrane of the present invention has excellent shutdown characteristics when incorporated as a separator in a secondary battery, and can therefore be suitably used as a separator for secondary batteries that require high battery capacity and thin film thickness.

Claims (9)

A polyolefin microporous membrane comprising 90% by mass or more of polyethylene based on 100% by mass of the polyolefin microporous membrane, the polyethylene consisting solely of branched high-density polyethylene having a weight-average molecular weight of 1 x 104 ^or more and less than 1 x 106 ^, a 10 μm converted air resistance of 50 sec/100 cc or more and 400 sec/100 cc or less, a rate of change in membrane thickness at shutdown of 30% or less, and a shutdown temperature measured by an air permeability method of 133° C. or less. The microporous polyolefin membrane according to claim 1, wherein the melting point difference ΔTm (°C) obtained by DSC is -1.4<ΔTm<1.0. The microporous polyolefin membrane according to claim 1 or 2, having a melting point Tm ₁ obtained by DSC of 133° C. or lower. The microporous polyolefin film according to any one of claims 1 to 3, wherein the raw polyethylene melting point is 126°C or higher and 135°C or lower. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the ratio of tensile breaking strength in the TD direction to the MD direction is 0.5 or more and 1.5 or less. The polyolefin microporous film according to any one of claims 1 to 5, having a 10 μm equivalent puncture strength of 1.0 N or more. The microporous polyolefin membrane according to any one of claims 1 to 6 , wherein the polyethylene contained is an ethylene/1-hexene copolymer. A battery separator comprising the microporous polyolefin membrane according to any one of claims 1 to 7 . A battery comprising the battery separator according to claim 8 .