JP2020164824A - Polyolefin microporous membrane, and battery - Google Patents

Abstract

To provide a polyolefin microporous membrane that is excellent in safety property and output characteristics when used as a battery separator.SOLUTION: A polyolefin microporous membrane, which is a laminated film comprising at least 2 layers of an A layer having a polyolefin-based resin as a main component and a B layer having a polyethylene-based resin as a main component, and in which, defining the melting point obtained in the first temperature increasing process in the measurement by DSC method for B layer as Tm1(°C) and the melting point obtained in the second temperature increasing process as Tm2(°C), the following equation is satisfied. Tm1-Tm2≤2.SELECTED DRAWING: None

Description

The present invention relates to a polyolefin microporous membrane that is excellent in safety and output characteristics when used as a battery separator.

The microporous polyolefin membrane is used as a filter, a separator for a fuel cell, a separator for a capacitor, and the like. In particular, it is suitably used as a separator for lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, and the like. The reason is that the polyolefin microporous membrane has excellent mechanical strength and shutdown characteristics of the membrane. In particular, in recent years, lithium-ion secondary batteries have been developed with the aim of increasing the size of batteries and increasing energy density, capacity, and output, mainly for in-vehicle applications, and along with this, the safety of separators has been improved. The required characteristics are also becoming higher.

The shutdown characteristic is the performance of ensuring the safety of the battery by melting and closing the holes when the inside of the battery is overheated in an overcharged state and blocking the battery reaction, and is the temperature at which the holes are closed. It is said that the lower the shutdown temperature, the higher the safety effect.

In addition, as the battery capacity increases, the thickness of the member (separator) is becoming thinner, and in order to prevent short circuits due to foreign matter in the battery or when winding, the puncture strength of the separator, MD (machine direction), and TD (machine) Increases in tensile strength and elongation in the vertical direction) are required.

As a method for increasing the strength, orientation control by increasing the draw ratio and a method using high molecular weight polyolefin are adopted, and as a method for low temperature shutdown, the melting point of the raw material is lowered by lowering the molecular weight.

Patent Document 1 describes a method for producing polyethylene having a relatively large molecular weight by sequential stretching as a method for providing a microporous membrane having high safety, high permeation performance and high mechanical strength. The resulting microporous membrane achieves high permeability and strength, and also has a high breakthrough temperature when the separator is exposed to high temperatures and has good heat shrinkage properties.

Patent Document 2 describes a method of achieving shutdown characteristics and high strength by using polyethylene having an average viscosity of 100,000 to 300,000 and polyethylene having an average viscosity of 700,000 or more.

Further, for example, Patent Document 3 and Patent Document 4 describe a microporous film in which a film is laminated and aimed at high strength and low temperature shutdown.

Japanese Unexamined Patent Publication No. 2009-108323 Japanese Unexamined Patent Publication No. 2008-266457 JP-A-2015-208893 Japanese Patent No. 4098401

In Patent Document 1, however, since polyethylene having a relatively large molecular weight is used, the melting point is high, and in addition, since the polymer is manufactured by sequential stretching, the polymer is highly oriented, resulting in a high melting point and a shutdown temperature. Is high.

In Patent Document 2, since a component having a relatively large molecular weight is used as a main raw material in order to maintain strength, the shutdown temperature is as high as 137 ° C., and sufficient shutdown performance is not obtained. Usually, when polyethylene having a low molecular weight is used, the melting point is lowered, so that the pores are closed during the heat treatment and the porosity is lowered. In Patent Document 2, high clogging is suppressed and a high porosity is maintained by adding inorganic particles, but it is said that the film structure tends to be non-uniform because the pores are formed by using the inorganic particles. There are disadvantages.

As described above, the shutdown characteristics of the microporous membrane and other physical properties such as permeability and strength are usually in a trade-off relationship, and it has been difficult to achieve both at a high level.

In Patent Document 3 and Patent Document 4, good safety performance is obtained with a shutdown temperature of about 130 ° C., but since polyethylene having a low molecular weight and a low melting point is used, the porosity is low and sufficient strength is obtained. Has not been obtained. Further, in order to obtain the required shutdown characteristics, it is necessary to increase the thickness of the layer using polyethylene having a low molecular weight and a low melting point, and the strength and permeability tend to decrease.

As mentioned above, there is room for improvement in the development of highly safe separators that do not impair battery performance in response to the diversifying customer needs associated with higher energy densities, higher capacities, and higher outputs.

An object of the present invention is to solve the above-mentioned problems. That is, it is an object of the present invention to provide a polyolefin microporous membrane excellent in shutdown characteristics and other physical properties such as strength, heat resistance, and permeability, and excellent safety and output characteristics when used as a battery separator.

In order to solve the above-mentioned problems and achieve the object, the polyolefin microporous film of the present invention is composed of at least two layers, an A layer containing a polyolefin resin as a main component and a B layer containing a polyethylene resin as a main component. The melting point obtained in the first heating process of the B layer measured by the DSC (Differential Scanning Calorimetry) method is Tm1 (° C), and the melting point obtained in the second heating process is Tm2 (° C). ), It is characterized by satisfying the following equation.
Tm1-Tm2 ≤ 2

Since the polyolefin microporous membrane of the present invention is excellent in safety and output characteristics when used as a battery separator, a secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like. It can be suitably used as a battery separator for electric vehicles.

Hereinafter, embodiments of the present invention will be described.

The polyolefin microporous film of the present invention is a laminated film composed of at least two layers, an A layer containing a polyolefin resin as a main component and a B layer containing a polyethylene resin as a main component, and the B layer was measured by the DSC method. When the melting point obtained in the first temperature raising process is Tm1 (° C.) and the melting point obtained in the second temperature rising process is Tm2 (° C.), the following equation is satisfied.
Tm1-Tm2 ≤ 2
Since the polyolefin microporous membrane of the present invention simultaneously satisfies various physical properties such as shutdown characteristics and other strength, heat resistance, and permeability, it is preferable to have a laminated structure of at least two layers.

Here, Tm1 and Tm2, which are the melting points of the B layer of the polyolefin microporous membrane, are obtained by DSC measurement, and in the present application, the melting peaks obtained from the first temperature rise process are melted from the Tm1 and second temperature rise measurements. Let the peak be Tm2. It is considered that Tm1 is related to the melting point of the crystal structure of the polyolefin microporous film, and Tm2 is derived from the basic structure composed of the raw material composition in the polyolefin microporous film.

In a normal polyolefin microporous film, the melting point is raised due to the crystal orientation of the polyolefin in the stretching step, the temperature of Tm1 becomes high, the value of (Tm1-Tm2) becomes large, the shutdown temperature rises, and the necessary shutdown characteristics are imparted. Therefore, the thickness of the shutdown layer becomes thicker, and the strength, heat yield, and porosity may deteriorate.

In the present invention, the raw material composition is set to the range described later by using a specific polyolefin described later as a raw material, and the stretching condition and the heat fixing condition at the time of film formation are set to the range described later (Tm1-Tm2). By setting the value of 2 or less, the rise in shutdown temperature is suppressed even when stretched, and the shutdown characteristics are compatible with other physical properties such as strength, heat resistance, and permeability, and when used as a battery separator, It has made it possible to obtain a polyolefin microporous film with excellent safety and output characteristics.

The value of (Tm1-Tm2) is more preferably 1.5 or less, and further preferably 1.0 or less. The lower limit is not particularly limited, but about -5.0 is substantially the lower limit. In order to set the value of (Tm1-Tm2) in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions and heat fixing conditions at the time of film formation are in the range described later.

In the polyolefin microporous membrane of the present invention, the Tm1 of the B layer is preferably 125 ° C. or higher, more preferably 127 ° C. or higher, and even more preferably 130 ° C. or higher. The upper limit of Tm1 is preferably 135 ° C. or lower, more preferably 133 ° C. or lower, further preferably 132 ° C. or lower, and particularly preferably 131 ° C. or lower. When the lower limit of Tm1 is less than 125 ° C., the shutdown characteristic is improved, but the permeability is remarkably deteriorated, and the characteristic balance as a microporous membrane is remarkably deteriorated. If the upper limit of Tm1 exceeds 135 ° C., the proportion of the thermally stable structure becomes larger than necessary, and the shutdown characteristics may deteriorate. In order to set the value of Tm1 in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions and heat fixing conditions at the time of film formation are in the range described later.

In the polyolefin microporous membrane of the present invention, the Tm2 of the B layer is preferably 125 ° C. or higher, more preferably 127 ° C. or higher, and even more preferably 130 ° C. or higher. The upper limit of Tm2 is preferably 135 ° C. or lower, more preferably 133 ° C. or lower, further preferably 132 ° C. or lower, and particularly preferably 131 ° C. or lower. When the lower limit of Tm2 is less than 125 ° C., the shutdown characteristic is improved, but the permeability is significantly deteriorated, and the characteristic balance as a microporous membrane is significantly deteriorated. If the upper limit of Tm2 exceeds 135 ° C. or higher, the proportion of the thermally stable structure becomes larger than necessary, and the shutdown characteristics may deteriorate. In order to set the value of Tm2 in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions and heat fixing conditions at the time of film formation are in the range described later.

The microporous polyolefin membrane of the present invention preferably has a shutdown temperature of 135 ° C. or lower. It is more preferably 133 ° C. or lower, further preferably 130 ° C. or lower, and most preferably 128 ° C. or lower. When the shutdown temperature is 135 ° C or less, it provides a highly safe battery when used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as electric vehicles. can do. When the shutdown temperature is 100 ° C. or lower, the holes are closed even under a normal operating environment and the battery characteristics are deteriorated. Therefore, the lower limit of the shutdown temperature is about 100 ° C. In order to set the shutdown temperature within the above range, it is preferable that the raw material composition and the laminated structure for forming the B layer are within the range described later, and the stretching conditions and heat fixing conditions at the time of film formation are within the range described later.

The polyolefin microporous membrane of the present invention preferably has an air permeation resistance equivalent to a thickness of 10 μm of 300 seconds or less. It is more preferably 250 seconds or less, still more preferably 200 seconds or less. If the air permeation resistance exceeds 300 seconds, the ion permeability deteriorates and the output characteristics when used as a battery separator deteriorates, which is not preferable. From the viewpoint of output characteristics, the lower the air permeation resistance is, the more preferable it is, but from the viewpoint of the balance between strength and heat resistance, the lower limit is about 50 seconds. In order to set the air permeation resistance within the above range, it is preferable that the raw material composition and the laminated structure of the film are within the range described later, and the stretching conditions and the heat fixing conditions at the time of film formation are within the range described later.

The polyolefin microporous membrane of the present invention preferably has a puncture strength of 2 N or more in terms of a thickness of 10 μm. It is more preferably 2.2 N or more, further preferably 2.5 N or more, and most preferably 2.8 N or more. If the puncture strength is less than 2.0 N, a short circuit is likely to occur during winding or due to foreign matter in the battery when the thin film is formed, which may reduce the safety of the battery. From the viewpoint of improving safety, the higher the puncture strength is, the more preferable it is, but lowering the shutdown temperature and improving the piercing strength are often trade-offs, and the upper limit is about 8.0 N. In order to set the puncture strength in the above range, it is preferable that the raw material composition and the laminated structure of the film are in the range described later, and the stretching conditions at the time of film formation are in the range described later.

The thickness of the microporous polyolefin membrane of the present invention is appropriately adjusted depending on the intended use and is not particularly limited, but is preferably 1 μm or more and 25 μm or less. If the thickness is less than 1 μm, handling may become difficult or the safety of the battery may decrease. If the thickness exceeds 25 μm, the amount of resin increases and productivity decreases, or the output characteristics deteriorate. May decrease. The thickness is more preferably 2 μm or more and 15 μm or less, further preferably 2 μm or more and 10 μm or less, and most preferably 2 μm or more and 8 μm or less. The thickness can be adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched sheet, the film forming speed, the stretching ratio, etc. within a range that does not deteriorate other physical properties.

The microporous polyolefin membrane of the present invention preferably has a porosity of 35% or more. The porosity is more preferably 40% or more, further preferably 42% or more, and most preferably 45% or more. If the porosity is less than 35%, the ion permeability may be insufficient when used as a battery separator, and the output characteristics of the battery may be deteriorated. The higher the porosity is, the more preferable it is from the viewpoint of output characteristics, but if it is too high, the strength may decrease, so the upper limit is about 70%. In order to set the porosity within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions and heat fixing conditions at the time of film formation are within the range described later.

Microporous polyolefin membrane of the present invention, when the tensile strength in a longitudinal direction of the film _{M MD,} the tensile strength in the width direction was set to _{M TD,} it is preferable _{M MD} and _{M TD} is both 80MPa or more. The tensile strength is more preferably 90 MPa or more, further preferably 100 MPa or more, and most preferably 120 MPa or more. If the tensile strength is less than 80 MPa, a short circuit is likely to occur during winding or due to foreign matter in the battery when the thin film is formed, which may reduce the safety of the battery. From the viewpoint of improving safety, the higher the tensile strength is, the more preferable it is, but lowering the shutdown temperature and improving the tensile strength are often trade-offs, and the upper limit is about 200 MPa. In order to set the tensile strength in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions at the time of film formation are in the range described later.

In the present invention, the direction parallel to the film forming direction of the film is referred to as the film forming direction, the longitudinal direction, or the MD direction, and the direction orthogonal to the film forming direction in the film surface is referred to as the width direction or the TD direction. ..

Next, the raw material used for the polyolefin microporous membrane of the present invention will be described, but the present invention is not necessarily limited thereto.

The microporous polyolefin film of the present invention is a laminated film composed of at least two layers, an A layer composed of a polyolefin composition A containing a polyolefin resin as a main component and a B layer containing a polyethylene resin as a main component. Here, the polyolefin composition A contains a polyolefin-based resin, and may contain other resins and additives.

Here, in the present invention, the "main component" means that the ratio of the specific component to all the components is 50% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more. , Most preferably 95% by mass or more.

The polyolefin-based raw material A used for the layer A preferably contains polyolefin as a main component, and may be a polyolefin composition. Examples of the polyolefin include polyethylene and polypropylene, and two or more kinds of these may be blended and used.

Polyolefin which is the main component of the polyolefin raw material A used in the present invention preferably has a weight average molecular weight (hereinafter referred to as Mw) is 1.0 × 10 ⁵ or more, more preferably 1.5 × 10 ⁵ or more, 1 .8 × 10 ⁵ or more is more preferable. Preferably 1.0 × 10 ⁶ or less as the upper limit, more preferably 5.0 × 10 ⁵ or less, more preferably 3.5 × 10 ⁵ or less.

Mw is low molecular weight is less than 1.0 × 10 ^5, and a high melting point due to the orientation during stretching, occur pore closing such during heat treatment due to lower the melting point of the material, the porosity is lowered, lowering the output characteristic May be done. When Mw is more than 1.0 × 10 ^6, there is a case where the thermal shrinkage rate increases.

The melting point of the polyolefin, which is the main component of the polyolefin-based raw material A used in the present invention, is preferably higher than 130 ° C. When the melting point is higher than 130 ° C., it is possible to suppress a decrease in the porosity and a heat shrinkage.

The polyolefin-based raw material A used in the present invention preferably contains polyethylene as a main component. In order to improve the permeability, the porosity, and the mechanical strength, the proportion of polyethylene is preferably 70% by mass or more, more preferably 80% by mass or more, with the entire polyolefin-based raw material A as 100% by mass. Preferably, polyethylene is used alone, more preferably.

Here, as the types of polyethylene, high-density polyethylene having a density exceeding 0.94 g / cm ³ , medium-density polyethylene having a density in the range of 0.93 to 0.94 g / cm ^3, and a density of 0.93 g / cm ³ are used. Examples thereof include low-density polyethylene lower than cm ^{3 and} linear low-density polyethylene. In order to increase the film strength, it is preferable to use high-density polyethylene and medium-density polyethylene, and even if they are used alone, they may be used alone. It may be used as a mixture.

Further, when polypropylene is added, the meltdown temperature can be improved when the microporous polyolefin membrane of the present invention is used as a battery separator. As the type of polypropylene, in addition to homopolymers, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer can contain a copolymer component with α-ethylene other than propylene, and ethylene is preferable as the other α-ethylene. However, when polypropylene is added, the mechanical strength tends to decrease as compared with the use of polyethylene alone. Therefore, the amount of polypropylene added is preferably 20% by mass or less in the polyolefin-based raw material A.

When blending two or more kinds of polyolefin to a polyolefin-based material A used in the present invention, the weight average molecular weight it is preferred to use 1.0 × 10 ⁶ or more 4.0 × 10 under ⁶ ultrahigh molecular weight polyolefin resin. By containing the ultra-high molecular weight polyolefin resin, the pores can be made finer and the heat resistance can be improved, and the strength and elongation can be further improved.

As the ultra-high molecular weight polyolefin resin (UHMwPO), it is preferable to use ultra-high molecular weight polyethylene (UHMwPE). The ultrahigh molecular weight polyethylene may be a copolymer containing a small amount of other α-olefins as well as a homopolymer of ethylene. Other α-olefins other than ethylene may be the same as above.

The blending ratio of the polyolefin-based raw material A and the plasticizer may be appropriately selected within a range in which the total of the polyolefin-based raw material A and the plasticizer is 100% by mass and the content of the polyolefin-based raw material A does not impair the molding processability. , 10 to 50% by mass. When the polyolefin-based raw material A is less than 10% by mass (when the plasticizer is 90% by mass or more), when molding into a sheet, swells and neck-ins are large at the outlet of the base, and the formability of the sheet deteriorates. Membrane property is reduced. On the other hand, when the polyolefin-based raw material A exceeds 50% by mass (when the plasticizer is 50% by mass or less), the shrinkage in the thickness direction becomes large and the molding processability also deteriorates.

The polyethylene-based raw material B used for the B layer of the polyolefin microporous membrane of the present invention has excellent melt extrusion characteristics and uniform stretching processing characteristics. Therefore, high-density polyethylene (density: 0.920 g / cm ³ or more and 0.970 g / It is preferable to use cm ³ or less). Such polyethylene is preferably not only a homopolymer of ethylene, but also a copolymer containing another α-olefin in order to lower the melting point of the raw material. Examples of the α-olefin include propylene, butene-1, hexene-1, penten-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like. Among them, hexene-1 is most preferable as the copolymer containing α-olefin, and the B layer of the polyolefin microporous film of the present invention most preferably contains ethylene / 1-hexene copolymer as a main component. .. The α-olefin can be confirmed by measuring with C ^13- NMR. By using 50% or more of the branched high-density polyethylene in the raw material of the B layer and setting the film forming conditions described later, in-plane orientation is difficult to proceed, changes in the crystal structure can be suppressed, and the shutdown temperature can be lowered, which is more preferable. ..

The weight average molecular weight of high density polyethylene (Mw) of preferably from 1.0 × 10 ⁴ or more 1.0 × 10 than ^6, more not less 1.0 × 10 ⁵ or more 3.5 × 10 ⁵ or less preferably, still more preferably 1.5 × 10 ⁵ or more 2.5 × 10 ⁵ or less, particularly preferably 1.5 × 10 ⁵ or more 2.0 × 10 ⁵ or less. When the weight average molecular weight is within the above range, excessive orientation in the plane does not easily proceed during film formation, and changes in the crystal structure of the microporous film can be easily controlled within an appropriate range. It can be improved and the deterioration of permeability can be suppressed.

The melting point of the polyethylene-based raw material B is preferably 130 ° C. or higher, and preferably 135 ° C. or lower. When the melting point is 130 ° C. or higher, the decrease in the porosity can be suppressed, and when the melting point is 135 ° C. or lower, the increase in the shutdown temperature can be suppressed.

That is, particularly preferred form of the polyethylene raw material B used in the present invention had an Mw is 1.5 × ^{10 5} ~2.5 × ¹⁰ 5 and a melting point polyethylene of 130 to 135 ° C..

The content of the polyethylene-based raw material B in the B layer is preferably 50% by mass or more with respect to 100% by mass of the polyolefin resin in the B layer. It is more preferably 80% by mass or more, further preferably 90% by mass or more, and most preferably 99% by mass or more. By increasing the content of the polyethylene-based raw material B, it is possible to impart excellent shutdown characteristics even if the B layer is thinned, and as a result, it is compatible with other physical properties such as strength, heat resistance, and permeability. Is preferable because it enables.

In addition, the polyolefin-based raw material A and the polyethylene-based raw material B used as raw materials for the A layer and the B layer of the polyolefin microporous film of the present invention may contain antioxidants, heat stabilizers and antistatic agents as long as the effects of the present invention are not impaired. It may contain an inhibitor, an ultraviolet absorber, and various additives such as an antistatic agent and a filler.

In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyethylene resin. Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris (3,5-di). -T-Butyl-4-hydroxybenzyl) Benzene (for example, BASF "Irganox" (registered trademark) 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4-hydroxy) It is preferable to use one or more selected from [phenyl) propionate] methane (for example, "Irganox" (registered trademark) 1010: molecular weight 1177.7 manufactured by BASF). Appropriate selection of the type and amount of antioxidant and heat stabilizer added is important for adjusting or enhancing the characteristics of the microporous membrane.

Further, when the distribution of the weight average molecular weight (Mw) of the microporous polyolefin 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% by mass. More preferably, the amount of components having a molecular weight of less than 20,000 is less than 20% by mass, further preferably the amount of components having a molecular weight of less than 10,000 is less than 20% by mass, and most preferably the amount of components having a molecular weight of less than 10,000 is less than 10% by mass. In the present invention, by using the above-mentioned raw materials, the shutdown temperature can be lowered without significantly lowering the molecular weight, and as a result, compatibility with other physical properties such as strength and porosity can be achieved. The component amount of the weight average molecular weight (Mw) can be evaluated by gel permeation chromatography (GPC) described later.

Next, the configuration of the polyolefin microporous membrane using the above raw materials will be described.

In the polyolefin microporous membrane of the present invention, at least one layer (hereinafter referred to as layer A) in the laminated structure may contain the polyolefin composition A as a main component from the viewpoints of strength, heat resistance, permeability and the like. preferable. When the content of the polyolefin composition A in the layer A is less than 50% by mass, the orientation of the film is lowered and the strength is lowered, and when there are many additive components having low heat resistance, the heat shrinkage is large. Or the transparency may deteriorate.

Further, in the polyolefin microporous membrane of the present invention, it is preferable that at least one layer (hereinafter referred to as layer B) of the laminated structure contains polyethylene-based raw material B as a main component from the viewpoint of shutdown characteristics. If the content of the polyethylene-based raw material B in the layer B is less than 50% by mass, the shutdown characteristics may deteriorate.

In the polyolefin microporous membrane of the present invention, the ratio of the total thickness of the B layer to the total thickness is preferably 50% or less. It is more preferably 45% or less, further preferably 40% or less, and most preferably 30% or less. The polyolefin microporous membrane of the present invention can effectively impart a shutdown function even if the ratio of the total thickness of the B layer is reduced by setting the raw material constituting the B layer in the above range. If the ratio of the total thickness of the layer B exceeds 50%, the ratio of the thickness of the layer A may decrease, and the strength, heat resistance, and permeability may decrease. Further, the ratio of the total thickness of the B layer to the total thickness is preferably 10% or more. It is more preferably 15% or more, further preferably 18% or more, and most preferably 20% or more. If the ratio of the total thickness of the B layer is less than 10%, sufficient shutdown characteristics may not be obtained. In the polyolefin microporous membrane of the present invention, the total thickness of the B layer is preferably 4.0 μm or less. It is more preferably 3.8 μm or less, further preferably 3.5 μm or less, and most preferably 3.0 μm or less. In the polyolefin microporous membrane of the present invention, the shutdown function can be effectively imparted even if the total thickness of the B layer is reduced by setting the raw materials constituting the B layer in the above range. If the total thickness of the B layer exceeds 4.0 μm, the strength, heat resistance, and permeability may decrease. Further, the total thickness of the B layer is preferably 1.0 μm or more. It is more preferably 1.5 μm or more, further preferably 1.8 μm or more, and most preferably 2.0 μm or more. If the total thickness of the B layer is less than 1.0 μm, sufficient shutdown characteristics may not be obtained.

In the polyolefin microporous film of the present invention, it is preferable that the B layer is arranged on at least one side of the A layer, and the B layer forms the surface layer of at least one side of the polyolefin microporous film. By forming the surface layer of at least one side of the B layer, when heat is applied to the film, the holes can be quickly closed, the shutdown characteristics can be improved, and the quality such as uneven stacking thickness can be improved. Further, it is more preferable to have a three-layer laminated structure of B layer / A layer / B layer. By adopting a three-layer laminated structure, it is possible to suppress curling of the polyolefin microporous film and improve the shutdown characteristics.

The polyolefin microporous membrane of the present invention can be obtained by biaxial stretching using the above-mentioned raw materials. The biaxial stretching method can be obtained by any of the inflation method, the simultaneous biaxial stretching method, and the sequential biaxial stretching method. Among them, film forming stability, thickness uniformity, high rigidity and dimensional stability of the film are obtained. It is preferable to adopt the simultaneous biaxial stretching method or the sequential biaxial stretching method in terms of controlling the above, and it is preferable that the steps (a) to (e) are as follows. An example of a method for forming a microporous polyolefin membrane using the above raw materials will be described below, but the method is not necessarily limited to this.

(A) A polyolefin solution is prepared by kneading and dissolving a polymer material containing a single polyolefin, a polyolefin mixture, a polyolefin solvent mixture (plasticizer), an additive, and a polyolefin kneaded product.
(B) The melt is extruded, molded into a sheet, cooled and solidified, and then cooled and solidified.
(C) The obtained sheet is stretched by a roll method or a tenter method.
(D) Then, the plasticizer is extracted from the obtained stretched film and the film is dried.
(E) Subsequently, heat treatment / re-stretching is performed.

Hereinafter, each step will be described.

(A) Preparation of polyolefin solution Prepare a polyolefin solution in which a polyolefin resin is heated and dissolved in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving polyethylene, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification. Solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, mineral oil distillates having corresponding boiling points, and dibutylphthalate. Examples thereof include phthalates that are liquid at room temperature, such as dioctyl phthalates. In order to obtain a gel-like sheet having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with polyethylene, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The blending ratio of the polyolefin resin and the plasticizer may be 100% by mass based on the total of the polyolefin resin and the plasticizer, and the content of the polyolefin resin may be appropriately selected within a range that does not impair the moldability, but is 10 to 50% by mass. Is. When the polyolefin resin is less than 10% by mass (when the plasticizer is 90% by mass or more), when molding into a sheet, swells and neck-ins are large at the outlet of the base, and the formability of the sheet deteriorates and the film forming property is deteriorated. Tends to decrease. On the other hand, when the amount of the polyolefin resin exceeds 50% by mass (when the amount of the plasticizer is 50% by mass or less), the shrinkage in the thickness direction becomes large, and the molding processability tends to decrease.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 ° C. When the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin solution from the die is unlikely to become non-uniform. On the other hand, if it is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

(B) Formation of Extruded Product and Formation of Gel-like Sheet The uniform melt-kneading of the polyolefin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin solution, it is preferably performed in a twin-screw extruder. If necessary, various additives such as antioxidants may be added as long as the effects of the present invention are not impaired. In particular, it is preferable to add an antioxidant to prevent the oxidation of polyethylene.

In the extruder, the polyolefin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of polyolefin resin + 10 ° C.) to (melting point of polyolefin resin + 120 ° C.). More preferably, it is (melting point of polyolefin resin + 20 ° C.) to (melting point of polyolefin resin + 100 ° C.). Here, the melting point means a value measured by DSC based on JIS K7121 (1987) (hereinafter, the same applies). For example, in the case of polyethylene, the melt-kneading temperature is preferably in the range of 140 to 250 ° C. More preferably, it is 160 to 230 ° C, and most preferably 170 to 200 ° C. Specifically, since the polyethylene composition has a melting point of about 130 to 140 ° C., the melt-kneading temperature is preferably 140 to 250 ° C., more preferably 180 to 230 ° C.

From the viewpoint of suppressing deterioration of the resin, it is preferable that the melt-kneading temperature is low, but if it is lower than the above-mentioned temperature, unmelted material is generated in the extruded product extruded from the die, causing film rupture or the like in the subsequent stretching step. If the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin becomes intense, and the physical properties of the obtained microporous film, for example, strength and porosity may be inferior. In addition, the decomposed product precipitates on a chill roll or a roll in the stretching process and adheres to the sheet, which leads to deterioration of the appearance. Therefore, it is preferable to knead within the above range.

The microporous polyolefin film of the present invention is a laminated film composed of at least two layers, an A layer containing a polyolefin resin as a main component and a B layer containing a polyethylene resin as a main component. The laminated structure is not particularly limited, and the preferred form is as described above. First, the polyolefin-based raw material A for the A layer is supplied to a twin-screw extruder, kneaded and extruded under the above-mentioned conditions. Preferably, the filter is then used to remove foreign matter and modified polymers. On the other hand, the polyethylene-based raw material B for the B layer is supplied to another twin-screw extruder, kneaded and extruded under the above-mentioned conditions. Then, preferably, a filter is used to remove foreign substances and modified polymers. The raw materials extruded from each melt extruder are supplied to a multi-manifold type composite T-die. Then, they are laminated with a die so as to have a predetermined lamination thickness ratio and discharged onto a cast drum to obtain a sheet having a two-layer structure.

Next, a gel-like sheet is obtained by cooling the obtained extruded product, and the microphase of polyethylene separated by a solvent can be immobilized by cooling. It is preferable to cool to 10 to 50 ° C. in the cooling step. This is because the final cooling temperature is preferably set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower. If the cooling rate is less than 30 ° C./min, the crystallinity increases and it is difficult to obtain a gel-like sheet suitable for stretching. Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure forming the gel structure also becomes large. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to improvement in film strength and elongation in addition to uniform stretching.

Examples of the cooling method include a method of directly contacting with cold air, cooling water, and other cooling media, a method of contacting with a roll cooled with a refrigerant, a method of using a casting drum, and the like.

(C) Stretching Step The obtained gel-like sheet is stretched. As the stretching method used, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching by a simultaneous biaxial tenter, etc. Can be mentioned. The draw ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, but it is preferable to stretch 5 times or more in any direction. The area magnification is preferably 25 times or more, more preferably 36 times or more, and even more preferably 49 times or more. If the area magnification is less than 25 times, the stretching is insufficient and the uniformity of the film is likely to be impaired, and an excellent microporous film cannot be obtained from the viewpoint of strength. The area magnification is preferably 100 times or less. When the area magnification is increased, tearing is likely to occur frequently during the production of the microporous film, the productivity is lowered, and when the orientation is advanced and the crystallinity is high, the melting point and strength of the porous substrate are improved. However, the higher crystallinity means that the amorphous part is reduced, and the melting point and shutdown temperature of the film are raised.

The stretching temperature is preferably in the range of (melting point of the gel-like sheet + 10 ° C. or lower) to (crystal dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5 ° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 100 ° C., the stretching temperature is preferably 90 to 125 ° C., more preferably 90 to 120 ° C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it may be obtained from NMR. If the temperature is lower than 90 ° C., the pores are insufficiently opened due to low-temperature stretching, it is difficult to obtain uniformity in film thickness, and the pore ratio is also low. If the temperature is higher than 125 ° C., the sheet melts and the pores are likely to be closed.

Due to the above stretching, the higher-order structure formed on the gel sheet is cleaved, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Although the mechanical strength is improved by stretching, the melting point is generally raised by the orientation of fibrils, and the value of Tm1-Tm2 becomes large. Further, by stretching before removing the plasticizer, the polyolefin is in a state of being sufficiently plasticized and softened, so that the higher-order structure can be cleaved smoothly and the crystal phase can be uniformly refined. .. Further, since the cleavage is easy, strain at the time of stretching is unlikely to remain, and the heat shrinkage rate can be lowered as compared with the case of stretching after removing the plasticizer.

(D) Cleaning / Drying Step Next, the solvent remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyethylene phase and the solvent phase are separated, a microporous membrane can be obtained by removing the solvent. Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like can be mentioned. These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent with a low surface tension, shrinkage is suppressed by the surface tension of the gas-liquid interface when the network structure forming microporous is washed and dried, and a microporous membrane with porosity and permeability can be obtained. .. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

The cleaning method can be carried out by a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, a method using a combination thereof, or the like. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the gel sheet. The washing temperature may be 15 to 30 ° C., and if necessary, heat to 80 ° C. or lower. At this time, from the viewpoint of enhancing the cleaning effect of the solvent, from the viewpoint of preventing the physical properties of the obtained microporous membrane from becoming non-uniform in the TD direction and / or the MD direction, the mechanical properties and electricity of the microporous membrane. From the viewpoint of improving the physical properties, the longer the gel-like sheet is immersed in the cleaning solvent, the better. The above-mentioned washing is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the microporous membrane becomes less than 1% by mass.

Then, in the drying step, the solvent in the microporous membrane is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. If the drying is insufficient, the porosity of the microporous membrane will decrease in the subsequent heat treatment, and the permeability will deteriorate.

(E) Heat treatment / re-stretching step The dried microporous membrane may be stretched (re-stretched) at least in the uniaxial direction. The re-stretching can be performed by the tenter method or the like in the same manner as the above-mentioned stretching while heating the microporous membrane. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial and / and sequential stretching are combined.

The re-stretching temperature is preferably equal to or lower than the melting point of the polyethylene composition, and more preferably in the range of (Tcd-20 ° C.) to the melting point. Specifically, 70 to 135 ° C. is preferable, and 110 to 132 ° C. is more preferable. Most preferably, it is 120 to 130 ° C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.0 to 1.6 times, particularly preferably 1.1 to 1.6 times in the TD direction, and more preferably 1.2 to 1.4 times. In the case of biaxial stretching, it is preferable to make it 1.01 to 1.6 times in the MD direction and the TD direction, respectively. The re-stretching magnification may be different in the MD direction and the TD direction. By stretching within the above range, the porosity and permeability can be increased, but when stretching at a magnification of 1.6 or more, the orientation progresses, the melting point of the film rises, and Tm1-Tm2 The value increases and the shutdown temperature rises.
From the viewpoint of heat shrinkage and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 0.9 or less, and more preferably 0.8 or less.

(F) Other Steps Further, depending on other uses, the microporous membrane can be hydrophilized. The hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge, or the like. The monomer graft is preferably carried out after the cross-linking treatment. It is preferable that the porous polyolefin film is crosslinked by irradiation with ionizing radiation such as α-ray, β-ray, γ-ray, and electron beam. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an accelerating voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the porous polyolefin film.

In the case of surfactant treatment, any of nonionic surfactant, cationic surfactant, anionic surfactant or amphoteric surfactant can be used, but nonionic surfactant is preferable. The multilayer microporous membrane is immersed in water or a solution prepared by dissolving a surfactant in a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the multilayer microporous membrane by the doctor blade method.

The polyolefin microporous membrane of the present invention is formed as a multilayer polyolefin porous membrane by laminating porous layers other than polyolefin by coating or vapor deposition for the purpose of imparting functions such as meltdown characteristics, heat resistance, and adhesiveness. May be good. The other porous layer is not particularly limited, and 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 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. it can. Further, the multilayer polyolefin porous membrane may be one in which the porous binder resin is laminated on at least one surface of the polyolefin microporous membrane.

The polyolefin microporous film obtained as described above can be used for various purposes such as a filter, a separator for a fuel cell, and a separator for a capacitor, and is particularly excellent in safety and output characteristics when used as a battery separator. Therefore, it can be preferably used as a battery separator for a secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.

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

First, the measurement method and the evaluation method will be described.

[Film thickness]
The film thickness of 5 points within the range of 95 mm × 95 mm of the polyolefin microporous membrane was measured with a contact thickness meter (using a contact thickness 0.01N, 10.5 mmφ probe manufactured by Mitutoyo Co., Ltd.), and the average value was measured. The film thickness was T ₁ (μm).

[Thickness ratio of each layer]
The thickness of each layer was determined by SEM observation of the cross section of the polyolefin microporous membrane. The cross-section test piece was prepared by using the cryo-CP method, and in order to prevent charge-up due to an electron beam, a small amount of metal fine particles were vapor-deposited and SEM observation was performed, the thickness of each layer was measured, and the thickness ratio was determined. For the measurement, 3 points were evaluated for each sample and the average value was used.
・ Measuring device Field emission scanning electron microscope (FE-SEM) S-4800 (manufactured by Hitachi High-Technologies Co., Ltd.)
Cross Section Polisher (CP) SM-9010 (manufactured by JEOL Ltd.)
-Measurement conditions Acceleration voltage: 1.0 kV.

[Vacancy rate]
The microporous polyolefin membrane was cut into a size of 5 cm × 5 cm and the weight was measured. In addition, the volume (cm ³ ) was obtained from the thickness and area of the porous membrane, and the volume (cm ³ ) was calculated from them and the membrane density (g / cm ³ ) using the following equation.

Formula: Pore ratio = ((volume-weight / film density) / volume) x 100
Here, the film density was set to 0.99. In addition, the thickness measured in (1) above was used for calculating the volume.

[Puncture strength]
The 10 μm equivalent piercing strength is the following, which is the maximum load (N) converted to 10 μm when a polyolefin microporous film is pierced at a speed of 2 mm / sec with a needle having a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm. It is a value obtained by the formula of.

Formula: Puncture strength (10 μm conversion) = maximum load (N) × 10 (μm) / film thickness of polyolefin microporous membrane T ₁ (μm).

[Air permeation resistance]
For a microporous membrane with a film thickness of T ₁ (μm), an air permeability resistance P ₁ (sec) is used with an air permeability meter (made by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117 (2009). / 100 cm ³ ) was measured. Further, by the _formula of _{P 2 = (P 1 × 10} ) / T 1, to calculate the air resistance _P 2 when a 10 [mu] m film thickness (10 [mu] m in terms ^{of) (sec / 100cm 3 / 10μm} ).

[Weight average molecular weight / molecular weight distribution of polyolefin]
The weight average molecular weight (Mw) of the polyolefin resin and the polyolefin microporous membrane was determined by the gel permeation chromatography (GPC) method under the following conditions. In addition, the content of low molecular weight components was determined from the integrated molecular weight distribution curve of the obtained polyolefin microporous membrane.

-Measuring device: GPC-150C manufactured by Waters Corporation
-Column: Showa Denko Corporation Shodex UT806M
・ Column temperature: 135 ℃
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow velocity: 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: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using polyethylene or a conversion coefficient (0.46).

[Polyolefin microporous membrane melting points Tm1, Tm2, ΔTm]
Approximately 5 mg of the components of each layer was scraped from the microporous membrane to prepare a sample for evaluation. For the sample of each layer, the melting point obtained in the first temperature raising process was Tm1, and the melting point obtained in the second temperature raising process was Tm2. A microporous polyolefin film was sealed in a measuring 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 (the first temperature increase), and then 230. It was held at ° C. for 5 minutes, cooled at a rate of 10 ° C./min, and again heated from 30 ° C. to 230 ° C. at a heating rate of 10 ° C./min (second temperature rise). From the obtained melting chart of each temperature rise, the temperature at which the melting enthalpy is 50% of the whole is defined as the melting point of the microporous membrane, and the melting point obtained from the first temperature rise is obtained from Tm1 and the second temperature rise. The melting point was Tm2, and ΔTm was calculated by the following formula. For ΔTm in the table, the smallest value among the ΔTm measured in each layer was described.
ΔTm = Tm1-Tm2
[Shutdown temperature]
While heating the microporous membrane at a heating rate of 5 ° C./min, the air permeability resistance was measured with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.), and the air permeability resistance is the detection limit. The temperature at which 0 × 10 ⁵ seconds / 100 ccAir was reached was determined and used as the shutdown temperature (° C.).

The measurement cell was composed of an aluminum block and had a structure having a thermocouple directly under the microporous membrane. The sample was cut into a 50 mm × 50 mm square, and the temperature was measured while fixing the periphery with an О ring.

(Example 1)
As a first polyolefin solution for the layer A, the weight-average molecular weight (Mw) of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene (UHMwPE) 40% by weight and Mw of 3.0 × 10 ⁵ high density Porichiren (HDPE : Density 0.955 g / cm ³ , melting point 135 ° C.) In 100 parts by mass of the first polyolefin resin consisting of 60% by mass, the antioxidant tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl) ) -Propionate] 0.2 parts by mass of methane was blended to prepare a mixture. 25 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] 75 mass from the side feeder of the twin-screw extruder. Parts were supplied and melt-kneaded at 210 ° C. and 250 rpm to prepare a first polyolefin solution.

As a second polyolefin solution for layer B, the weight average molecular weight (Mw) of the second polyolefin resin 100 mass parts consisting of branched high density polyethylene 1.8 × 10 ^5, antioxidant tetrakis [methylene-3- (3,5-Diter-Shari-Butyl-4-hydroxyphenyl) -Propionate] 0.2 parts by mass of methane was added to prepare a mixture. 30 parts by mass of the obtained mixture was put into another twin-screw extruder of the same type as above, and 70 parts by mass of liquid paraffin [35 cSt (40 ° C.)] was supplied from the side feeder of the twin-screw extruder. A second polyolefin solution was prepared by melt-kneading under the same conditions.

The first and second polyolefin solutions are filtered from each twin-screw extruder to remove foreign substances, and then supplied to the three-layer T-die to supply the second polyolefin solution / first polyolefin solution / second polyolefin solution ( B layer / A layer / B layer) was extruded. The extruded product was cooled while being taken up by a cooling roll whose temperature was adjusted to 30 ° C. at a taking-up speed of 2 m / min to form a gel-like three-layer sheet.

The gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 110 ° C. by a tenter stretching machine. The stretched gel-like three-layer sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to a temperature of 25 ° C., and shaken at 100 rpm for 3 minutes to remove liquid paraffin and at room temperature. It was air dried.

The obtained dry film was heat-fixed at 120 ° C. for 10 minutes. The thickness of the obtained porous polyolefin membrane was 12 μm, and the thickness ratio of each layer was 1/7/1 (B layer / A layer / B layer). Table 1 shows the blending ratios of the constituent components, manufacturing conditions, evaluation results, and the like. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 2)
A polyolefin microporous film was obtained in the same manner as in Example 1 except that the temperature of the heat fixing treatment was set to 125 ° C. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 3)
A polyolefin microporous film was obtained in the same manner as in Example 1 except that the stretching temperature of the gel-like three-layer sheet in the tenter stretching machine was 115 ° C. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 4)
The stretching temperature of the gel-like three-layer sheet in the tenter stretching machine is 115 ° C., the stretching ratio in both the MD direction and the TD direction is 7 times, the temperature of the heat fixing treatment is 125 ° C., and the discharge of the twin-screw extruder is performed. A microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the amount was adjusted so that the thickness of the porous polyolefin membrane was 10 μm and the thickness ratio of each layer was 1/7/1. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 5)
A microporous polyolefin membrane was obtained in the same manner as in Example 1 except that the discharge amount of the twin-screw extruder was adjusted so that the thickness ratio of each layer of the porous polyolefin membrane was 1/4/1. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 6)
As a second polyolefin solution for layer B, the weight average molecular weight (Mw) to the first polyolefin resin 100 mass parts consisting of branched high density polyethylene 1.8 × 10 ^5, antioxidant tetrakis [methylene-3- (3,5-Ditershally butyl-4-hydroxyphenyl) -propionate] 0.2 parts by mass of methane was added to prepare a mixture. 30 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] 70 mass from the side feeder of the twin-screw extruder. Parts were supplied and melt-kneaded under the conditions of 210 ° C. and 250 rpm to prepare a second polyolefin solution.

As a first polyolefin solution for the layer A, the weight-average molecular weight (Mw) of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene (UHMwPE) 40% by weight and Mw of 3.0 × 10 ⁵ high density Porichiren (HDPE: In 100 parts by mass of the second polyolefin resin having a density of 0.955 g / cm ³ , melting point 135 ° C.) and 60% by mass, the antioxidant tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl)) was added. -Propionate] 0.2 parts by mass of methane was blended to prepare a mixture. 15 parts by mass of the obtained mixture was put into another twin-screw extruder of the same type as above, and 85 parts by mass of liquid paraffin [35 cSt (40 ° C.)] was supplied from the side feeder of the twin-screw extruder. A first polyolefin solution was prepared by melt-kneading under the same conditions.

After removing foreign substances from each twin-screw extruder through a filter, the first and second polyolefin solutions are supplied to the three-layer T-die to supply the first polyolefin solution / second polyolefin solution / first polyolefin solution ( It was extruded so as to form A layer / B layer / A layer). The extruded product was cooled while being taken up by a cooling roll whose temperature was adjusted to 30 ° C. at a taking-up speed of 2 m / min to form a gel-like three-layer sheet.

The gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 110 ° C. by a tenter stretching machine. The stretched gel-like three-layer sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to a temperature of 25 ° C., and shaken at 100 rpm for 3 minutes to remove liquid paraffin and at room temperature. It was air dried.

The obtained dry film was heat-fixed at 120 ° C. for 10 minutes. The thickness of the obtained porous polyolefin membrane was 15 μm, and the thickness ratio of each layer was 3/1/3 (A layer / B layer / A layer). Table 1 shows the blending ratios of the constituent components, manufacturing conditions, evaluation results, and the like. In addition, ΔTm in Table 1 is the value of the B layer.

(Example 7)
A microporous polyolefin membrane was obtained in the same manner as in Example 6 except that the discharge amount of the twin-screw extruder was adjusted so that the thickness of the porous polyolefin membrane was 16 μm and the thickness ratio of each layer was 1/1/1. .. In addition, ΔTm in Table 1 is the value of the B layer.

(Comparative Example 1)
As a first polyolefin solution for the layer A, the weight-average molecular weight (Mw) of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene (UHMwPE) 40% by weight and Mw of 3.0 × 10 ⁵ high density Porichiren (HDPE : Density 0.955 g / cm ³ , melting point 135 ° C.) In 100 parts by mass of the first polyolefin resin consisting of 60% by mass, the antioxidant tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl) ) -Propionate] 0.2 parts by mass of methane was blended to prepare a mixture. 30 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] 70 mass from the side feeder of the twin-screw extruder. Parts were supplied and melt-kneaded at 210 ° C. and 250 rpm to prepare a first polyolefin solution.

The first polyolefin solution was supplied to a T-die and extruded after removing foreign matter from a twin-screw extruder through a filter. The extruded product was cooled while being taken up by a cooling roll whose temperature was adjusted to 30 ° C. at a taking-up speed of 2 m / min to form a gel-like sheet.

The gel-like sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 115 ° C. by a tenter stretching machine. The stretched gel-like sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to a temperature of 25 ° C., shaken at 100 rpm for 3 minutes to remove liquid paraffin, and air-dried at room temperature. ..

The obtained dry film was heat-fixed at 125 ° C. for 10 minutes. The thickness of the obtained porous polyolefin membrane was 15 μm. Table 1 shows the blending ratios of the constituent components, manufacturing conditions, evaluation results, and the like. In addition, ΔTm in Table 1 is the value of the A layer (single layer configuration).

(Comparative Example 2)
As a second polyolefin solution for layer B, the weight average molecular weight (Mw) to the first polyolefin resin 100 mass parts consisting of branched high density polyethylene 1.8 × 10 ^5, antioxidant tetrakis [methylene-3- (3,5-Ditershally butyl-4-hydroxyphenyl) -propionate] 0.2 parts by mass of methane was added to prepare a mixture. 30 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] 70 mass from the side feeder of the twin-screw extruder. Parts were supplied and melt-kneaded under the conditions of 210 ° C. and 250 rpm to prepare a second polyolefin solution.

The second polyolefin solution was supplied to the T-die and extruded after removing foreign matter from the twin-screw extruder through a filter. The extruded product was cooled while being taken up by a cooling roll whose temperature was adjusted to 30 ° C. at a taking-up speed of 2 m / min to form a gel-like sheet.

The gel-like sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 110 ° C. by a tenter stretching machine. The stretched gel-like sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to a temperature of 25 ° C., shaken at 100 rpm for 3 minutes to remove liquid paraffin, and air-dried at room temperature. ..

The obtained dry film was heat-fixed at 120 ° C. for 10 minutes. The thickness of the obtained porous polyolefin membrane was 15 μm. Table 1 shows the blending ratios of the constituent components, manufacturing conditions, evaluation results, and the like. In addition, ΔTm in Table 1 is the value of the B layer (single layer structure).
(Comparative Example 3)
As a first polyolefin-based resin solution for the layer A, the weight-average molecular weight (Mw) of 3.0 × 10 ⁵ high density Porichiren (HDPE: density 0.955 g / cm ^3, melting point 135 ° C.) 70% by weight and Mw to but 2.0 × 10 ultra-high molecular weight polyethylene (UHMwPE) first 100 parts by weight of polyolefin resin comprising 30 wt% of ^6, antioxidant tetrakis [methylene-3- (3,5-ditertiary butyl-4 -Hydroxyphenyl) -propionate] 0.2 parts by mass of methane was added to prepare a mixture. 23 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] 77 mass from the side feeder of the twin-screw extruder. Parts were supplied and melt-kneaded under the conditions of 180 ° C. and 200 rpm to prepare a first polyolefin-based resin solution.

As a second polyolefin resin solution for layer B, the weight average molecular weight (Mw) of 3.0 × 10 ⁵ high density Porichiren (HDPE: density 0.955 g / cm ^3, melting point 135 ° C.) 54.5 wt% and Mw of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene (UHMwPE) 18 wt%, Mw4 × 10 ⁴ branched high-density polyethylene as the polyethylene raw material B (C-HDPE: density 0.948 g / cm ^3, melting point 125 ℃) In 100 parts by mass of the second polyolefin resin consisting of 27.5% by mass, the antioxidant tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl) -propionate] methane 0.2. The parts by mass were mixed to prepare a mixture. 27.5 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and liquid paraffin [35 cst (40 ° C.)] was introduced from the side feeder of the twin-screw extruder. A second polyolefin resin solution was prepared by supplying 72.5 parts by mass and melt-kneading under the conditions of 180 ° C. and 200 rpm.

After removing foreign substances from each twin-screw extruder through a filter, the first and second polyolefin resin solutions are supplied to the three-layer T-die to supply the second polyolefin resin solution / first polyolefin resin solution /. It was extruded so as to be a second polyolefin resin solution (B layer / A layer / B layer). The extruded product was cooled while being taken up by a cooling roll whose temperature was adjusted to 30 ° C. at a taking-up speed of 5 m / min to form a gel-like three-layer sheet.

The gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 112 ° C. by a tenter stretching machine. The stretched gel-like three-layer sheet was fixed to a 20 cm × 20 cm aluminum frame plate, immersed in a methylene chloride bath adjusted to a temperature of 25 ° C., and shaken at 100 rpm for 3 minutes to remove liquid paraffin and at room temperature. It was air dried.

The obtained dry film was heat-fixed at 120 ° C. for 10 minutes. The thickness of the obtained porous polyolefin membrane was 10 μm, and the thickness ratio of each layer was 1/2/1 (B layer / A layer / B layer). Table 1 shows the blending ratios of the constituent components, manufacturing conditions, evaluation results, and the like.

(Evaluation)
It is shown that the microporous polyolefin membranes of Examples 1 to 7 have excellent air permeation resistance and puncture strength as a battery separator, and can lower the shutdown (SD) temperature, and have excellent shutdown characteristics. Was done. However, the films of Examples 6 and 7 had large lamination unevenness during lamination, and the flatness of the polyolefin microporous film was slightly deteriorated. On the other hand, the polyolefin microporous film of Comparative Example 1 had a high shutdown temperature, and the polyolefin microporous film of Comparative Example 2 was a film having low puncture strength.

The microporous polyolefin membrane of the present invention is excellent in shutdown characteristics and other physical properties such as strength, heat resistance, and permeability, and is excellent in safety and output characteristics when used as a battery separator. Therefore, it can be suitably used for a separator for a secondary battery, which requires a high battery capacity and a thin film.

Claims (10)

It is a laminated film consisting of at least two layers, an A layer containing a polyolefin resin as a main component and a B layer containing a polyethylene resin as a main component, and is obtained in the first heating process of the B layer measured by the DSC method. A polyolefin microporous film satisfying the following formula when the melting point is Tm1 (° C.) and the melting point obtained in the second temperature raising process is Tm2 (° C.).
Tm1-Tm2 ≤ 2 The polyolefin microporous film according to claim 1, wherein the ratio of the total thickness of the B layer to the total thickness of the polyolefin microporous film is 50% or less. The polyolefin microporous membrane according to claim 1 or 2, wherein the total thickness of the B layer is 4 μm or less. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the polyethylene-based resin constituting the B layer has a weight average molecular weight Mw of 150,000 or more and 250,000 or less. The polyolefin microporous film according to any one of claims 1 to 4, wherein the B layer is arranged on at least one side of the A layer, and the B layer forms a surface layer on at least one side. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the shutdown temperature is 135 ° C. or lower. The polyolefin microporous membrane according to any one of claims 1 to 6, wherein the air permeation resistance in terms of thickness of 10 μm is 300 seconds or less. The polyolefin microporous membrane according to any one of claims 1 to 7, wherein the puncture strength in terms of thickness of 10 μm is 2N or more. A battery separator using the polyolefin microporous membrane according to any one of claims 1 to 8. A secondary battery using the battery separator according to claim 9.