JP6356000B2 - Laminated microporous film, method for producing the same, and battery separator - Google Patents

Description

  The present invention relates to a laminated microporous film, a method for producing the same, and a battery separator.

  Microporous films, especially polyolefin microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, etc., and are particularly preferably used as lithium ion battery separators. Yes. In recent years, lithium ion batteries have been used for small electronic devices such as mobile phones and notebook personal computers, while being applied to hybrid electric vehicles and the like.

  The battery separator included in the lithium ion battery is required to have a shutdown (SD) function in order to ensure safety. The SD function is a function that, when the temperature inside the battery rises excessively, stops the battery reaction by abruptly increasing the electric resistance of the battery separator and prevents further temperature rise. As a mechanism for expressing the SD function, for example, in the case of a battery separator made of a microporous film, when the internal temperature of the battery rises to a predetermined temperature, the porous structure is lost to make it non-porous and block ion permeation. Can be mentioned. However, even if the pores are made non-porous in this way and the ion permeation is blocked, if the temperature further rises and the entire film melts and breaks, the electrical insulation cannot be maintained. The temperature at which the film cannot maintain its shape and cannot block ion permeation is called the membrane breaking temperature. It can be said that the higher the film breaking temperature, the better the battery separator. Moreover, it can be said that the greater the difference between the temperature at which SD starts and the membrane breaking temperature, the better the safety.

  For example, Patent Document 1 has a laminated structure in which a polypropylene microporous film is laminated on a conventional polyethylene microporous film for the purpose of providing a microporous film serving as a separator that can cope with such circumstances. A composite microporous film (battery separator) has been proposed.

  Patent Document 2 discloses a laminated microporous film formed from polypropylene having a specific weight average molecular weight.

Japanese Patent No. 3003830 Japanese Patent No. 3939778

  Since the polypropylene disclosed in Patent Document 1 has a relatively low molecular weight, the heat shrinkability is good even when the strain rate by stretching is large, but it is considered insufficient when considering the balance with air permeability. It is done. The same can be considered for the microporous film disclosed in Patent Document 2, and there is room for further improvement.

  The present invention has been made in view of the above problems, and a laminated microporous film having a good balance between air permeability and heat shrinkability even when the strain rate due to stretching is large, a method for producing the same, and a method for using the same. An object of the present invention is to provide a battery separator.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by a laminated microporous film having a predetermined configuration, and have completed the present invention. .

That is, the present invention is as follows.
[1]
A first microporous film comprising a first resin composition having a melting point T _mA ;
And a second microporous film comprising a second resin composition having a low melting point T _mB than the melting point T _mA,
The second resin composition is an ethylene-based resin composition;
The extensional viscosity is 18000-40000 Pa · s,
A laminated microporous film having a shear viscosity of 5000 to 10,000 Pa · s.
[2]
The laminated microporous film according to [1], wherein the film thickness is 16 μm or less.
[3]
The laminated microporous film according to [1] or [2] above, wherein the air permeability is 10 to 5000 seconds / 100 cc.
[4]
The laminated microporous film according to any one of [1] to [3], wherein the first resin composition contains two or more types of polypropylene .
[5]
A battery separator comprising the laminated microporous film according to any one of [1] to [4].
[6]
The method for producing a laminated microporous film according to any one of [1] to [4] above,
Laminate having a first resin film comprising a first resin composition having a melting point T _mA, and a second resin film comprising a second resin composition having a low melting point T _mB than the melting point T _mA A coextrusion step of forming a film by a coextrusion method;
The laminated film obtained in the co-extrusion step is stretched by a dry method so that the strain rate is 0.10 to 4.0 / second to form a laminated microporous film. Yes in order
And
The second resin composition is Ru ethylene resin composition der,
A method for producing a laminated microporous film.

  According to the present invention, a laminated microporous film having a sufficiently good balance between air permeability and heat shrinkability even when the strain rate in stretching is large, a method for producing the same, and a battery separator using the same are provided. Can do.

  Hereinafter, the embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. However, the present invention is not limited to this, and various modifications are possible without departing from the scope of the present invention. It is.

[Laminated microporous film]
The laminated microporous film of this embodiment includes a first microporous film containing a first resin composition having a melting point T _mA and a second resin composition having a melting point T _mB lower than the melting point T _mA. A second microporous film containing a product, an extensional viscosity of 18000 to 40000 Pa · s, and a shear viscosity of 5000 to 10,000 Pa · s.

  The inventors of the present invention have a good balance between the air permeability and the heat shrinkage rate of the laminated microporous film obtained even when the strain rate by stretching is large by setting the extensional viscosity and shear viscosity within the above ranges. I found it. As this factor, by increasing the elongational viscosity to some extent with respect to the shear viscosity, the molten resin in the first resin composition is easily oriented in the extrusion direction during the formation of the laminated microporous film. As a result, the crystal orientation of the resin is improved, and in each stretching process described later, it is considered that the opening between the crystals occurs uniformly and the gas permeability is improved. Another factor is that by lowering the shear viscosity to some extent relative to the extension viscosity, the porosity and shrinkage relaxation behavior during stretching is better balanced, resulting in a higher strain rate during stretching. Even so, it is considered that the air permeability and heat shrinkability of the obtained microporous film are improved. However, the factors are not limited to the above.

The first resin composition and the second resin composition have melting points (hereinafter also simply referred to as “melting points”) T _mA and T _mB measured by a method according to JIS K-7121, where T _mA > T _mB As long as the above is satisfied, the composition may be the same or different. Here, T _mA represents the melting point of the first resin composition, and T _mB represents the melting point of the second resin composition.

The difference (T _mA −T _mB ) between the melting point T _mA of the first resin composition and the melting point T _{mB of} the second resin composition is preferably 5.0 ° C. or more, more preferably 10 ° C. or more. Yes, more preferably 15 ° C or higher. When the difference in melting point (T _mA −T _mB ) is 5.0 ° C. or higher, when the laminated microporous film is used as a battery separator, the low melting point when the internal temperature of the battery rises due to abnormal current. Even if the resin layer melts, the high melting point resin layer tends to be held without melting. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be further improved. On the other hand, the difference (T _mA −T _mB ) between the melting point T _mA of the first resin composition and the melting point T _{mB of} the second resin composition is preferably 150 ° C. or less, more preferably 100 ° C. or less. And more preferably 50 ° C. or lower. When the difference in melting point (T _mA -T _mB ) is 150 ° C. or less, when a laminated microporous film is used as a battery separator, a resin having a low melting point when the internal temperature of the battery rises due to an abnormal current. Even if the layer melts, the high melting point resin layer tends to be held without melting. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be further improved.

  The laminated microporous film is a laminate of the first microporous film and the second microporous film, but the lamination mode is not particularly limited. As a specific example of the embodiment, a laminated microporous film (a) composed of one first microporous film and one second microporous film, one first microporous film and its A laminated microporous film (b) comprising a second microporous film laminated on both sides, comprising one second microporous film and a first microporous film laminated on both sides thereof The laminated microporous film (c), the first microporous film-the second microporous film-the first microporous film-the second microporous film, and so on. Is a laminated microporous film (d). Among these, the aspect of the laminated microporous film (c) is preferable from the viewpoint of more effectively and reliably exhibiting the effects of the present invention.

[First and second microporous films]
The first and second microporous films are preferably obtained by stretching and porousizing the first and second resin films described later.

(First resin composition)
Although it does not specifically limit as a 1st resin composition, The thing containing resin and the arbitrary additive which can be added as needed is mentioned. In the present specification, the “resin composition” is a concept that includes only one kind of resin (polymer material), and may be a mixture of two or more kinds of resins, and any arbitrary An additive may be contained.

  The resin contained in the first resin composition is not particularly limited, and examples thereof include polyethylene (PE), polypropylene (PP), polybutene-1, poly-4-methylpentene, and ethylene-propylene copolymer. Polyolefins; Polymers containing olefinic hydrocarbons as monomer components such as ethylene-tetrafluoroethylene copolymers. Among these, polyolefin is preferable and polypropylene is more preferable. By using such a resin, it is possible to better combine a plurality of characteristics required for battery separators. The resin contained in the first resin composition may be used alone or in combination of two or more.

  The polypropylene is not particularly limited as long as it is a polymer containing polypropylene as a monomer component, and may be a homopolymer or a copolymer. When it is a copolymer, it may be a random copolymer or a block copolymer. Moreover, when it is a copolymer, there is no limitation in a copolymerization component, For example, ethylene, butene, and a hexene are mentioned. A copolymerization component may be used individually by 1 type, or may use 2 or more types together. When polypropylene is a copolymer, the copolymerization ratio of polypropylene is preferably 50% by mass or more, more preferably 70% by mass or more, and further preferably 90% by mass or more.

  Polypropylene may be used alone or in combination of two or more. The polymerization catalyst used for obtaining polypropylene is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst.

  Also, the stereoregularity of polypropylene is not particularly limited, and isotactic or syndiotactic polypropylene is used.

  Polypropylene may have any crystallinity or melting point. Further, two or more kinds of polypropylene having different crystallinity and melting point may be blended at a specific blending ratio depending on the properties and applications of the obtained microporous film.

  The polypropylene is a known modified polypropylene as described in JP-A-44-15422, JP-A-52-30545, JP-A-6-313078, and JP-A-2006-83294. Also good. Furthermore, the polypropylene may be a mixture of any proportion of the above-described polypropylene and modified polypropylene.

  The melt flow rate (MFR) of the first resin composition (according to ASTM D1238, measured at 230 ° C. under a load of 2.16 kg, the same applies hereinafter) is preferably 0.10 to 20 g / 10 min. More preferably, it is 0.10-10 g / 10min, More preferably, it is 0.1-5.0 g / 10min. When the MFR is within the above range, the moldability of the laminated microporous film tends to be further improved.

  The molecular weight distribution of the resin contained in the first resin composition is preferably 5.0 to 15.0, more preferably 6.0 to 12.0, and further preferably 10.0 to 12.0. It is. When the molecular weight distribution is 5.0 or more, heat generation during molding of the first resin composition is suppressed, and resin degradation tends to hardly occur. Moreover, when the molecular weight distribution is 15.0 or less, unmelted materials derived from high molecular weight components tend to decrease. The molecular weight distribution is the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (hereinafter referred to as “Mw / Mn”). Further, Mw and Mn are obtained from gel permeation chromatography (hereinafter referred to as “GPC”) of a microporous film using polystyrene as a standard sample, and in detail according to the method described in the following examples. Measured.

  The content of the resin contained in the first resin composition is preferably 50% by mass or more, more preferably 80% by mass or more, and still more preferably 100% with respect to the total amount of the first resin composition. % By mass.

The first resin composition has a higher melting point than the second resin composition. Melting point T _mA of the first resin composition is preferably 150 to 280 ° C., more preferably from 150 to 250 ° C., more preferably 150 to 230 ° C.. When the melting point T _mA of the first resin composition is within the above range, the resulting laminated microporous film tends to have a better balance between film breaking temperature and film formability. Melting _| fusing point _TmA of a 1st resin composition can be adjusted with the additive used as needed and the kind of resin to be used.

The density of the second resin composition is preferably 900 to 930 kg / m ³ , more preferably 910 to 930 kg / m ³ , still more preferably 910 to 920 kg / m ³ , most preferably 910 to 915 kg / m ³ . When the density of the second resin composition is 900 kg / m ³ or more, a laminated microporous film having better air permeability tends to be obtained. Moreover, when the density of the second resin composition is 930 kg / m ³ or less, the film tends to be difficult to break when stretched.

(Second resin composition)
Although it does not specifically limit as long as it has melting | fusing point TmB lower than melting | fusing point TmA as a 2nd resin composition, For example, what contains resin and the arbitrary additive which can be added as needed is mentioned.

  The resin contained in the second resin composition is not particularly limited, but is, for example, a polyolefin that is a polymer containing an olefin hydrocarbon such as polyethylene or polypropylene as a monomer component; saturated with polyethylene terephthalate, polybutylene terephthalate, or the like. Examples include polyester. Among these, the second resin composition is preferably an ethylene-based resin composition containing polyethylene, more preferably a so-called high-density polyethylene, medium-density polyethylene, and a resin composition containing low-density polyethylene, More preferably, the resin composition contains high-density polyethylene. By using such a resin composition, it is possible to more appropriately combine a plurality of characteristics required for battery separators. The resin contained in the second resin composition may be used alone or in combination of two or more.

  “Polyethylene” refers to a polymer whose main monomer component is ethylene. Here, the “main component” means that ethylene accounts for 50% by mass or more with respect to the total amount of the monomer of the polyethylene resin, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably. It means 95% or more, still more preferably 98% or more, particularly preferably 100% by mass, ie occupying the total amount.

The second resin composition has a lower melting point than the first resin composition. The melting point T _mB of the second resin composition is preferably 100 ° C. to 150 ° C., more preferably 100 to 145 ° C., and further preferably 100 to 140 ° C. When the melting point T _mB of the second resin composition is within the above range, when the obtained laminated microporous film is used as a battery separator, the safety of the battery tends to be dramatically improved. The melting point _TmB of the second resin composition can be adjusted by the type of resin to be used and the additive to be used as necessary.

  The content of the resin contained in the second resin composition is preferably 50% by mass or more, more preferably 80% by mass or more, further preferably 100%, based on the total amount of the first resin composition. % By mass.

  The MFR of the second resin composition is preferably 0.010 to 10 g / 10 minutes, more preferably 0.10 to 3.0 g / 10 minutes, and further preferably 0.10 to 2.0 g / minute. 10 minutes, most preferably 0.10 to 1.0 g / 10 minutes. When the MFR is 0.010 g / 10 min or more, fish eyes tend not to be generated in the second microporous film. Further, when the MFR is 10 g / 10 min or less, drawdown hardly occurs and the film formability tends to be good.

The density of the second resin composition is preferably 945 to 970 kg / m ³ , more preferably 955 to 970 kg / m ³ , still more preferably 960 to 967 kg / m ³ , most preferably 963 to 967 kg / m ³ . When the density of the second resin composition is 945 kg / m ³ or more, a laminated microporous film having better air permeability tends to be obtained. Moreover, when the density of the second resin composition is 970 kg / m ³ or less, the film tends to be difficult to break when stretched.

  The first and second resin compositions may contain any additive. Although it does not specifically limit as arbitrary additives, For example, an olefin elastomer, antioxidant, a metal deactivator, a heat stabilizer, a flame retardant (an organic phosphate ester compound, an ammonium polyphosphate compound, an aromatic Halogen flame retardants, silicone flame retardants, etc.), fluoropolymers, plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weather resistance (light ) Property improver, polyolefin nucleating agent, slip agent, inorganic or organic filler and reinforcing material (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various colors Agents, release agents and the like.

  The porosity of the laminated microporous film is preferably 20% to 70%, more preferably 35% to 65%, and further preferably 45% to 60%. When the porosity is 20% or more, when the laminated microporous film is used as a battery separator, the ion permeability tends to be further improved. On the other hand, when the porosity is 70% or less, the mechanical strength of the laminated microporous film tends to be further improved. The porosity of the laminated microporous film can be adjusted to the above range by appropriately setting the composition of the resin composition constituting each layer, the stretching temperature, the stretching ratio, and the like. The porosity of the laminated microporous film can be measured by the method described in the examples.

  The air permeability of the laminated microporous film is preferably 10 to 5000 seconds / 100 cc, more preferably 50 to 1000 seconds / 100 cc, and further preferably 100 to 500 seconds / 100 cc. When the air permeability is 5000 seconds / 100 cc or less, the ion permeability of the laminated microporous film tends to be further improved. On the other hand, when the air permeability is 10 seconds / 100 cc or more, there are few defects in the laminated microporous film, and the quality tends to be more uniform. In addition, the air permeability of the laminated microporous film of this embodiment can be adjusted to the above-mentioned range by appropriately setting the composition of the resin composition constituting each layer, the stretching temperature, the stretching ratio, and the like. The air permeability can be measured by the method described in the examples.

  The film thickness of the laminated microporous film is preferably 5.0 μm or more, more preferably 10 μm or more, and further preferably 15 μm or more. When the film thickness is 5.0 μm or more, the mechanical strength tends to be further improved. The film thickness of the laminated microporous film is preferably 16 μm or less, more preferably 15 μm or less, and further preferably 12 μm or less. When the film thickness is 16 μm or less, it tends to be more effective in reducing the size of the battery. The film thickness of the laminated microporous film can be adjusted to the above range by appropriately setting the thickness of each resin film, the draw ratio, and the like. Moreover, the film thickness of a lamination | stacking microporous film can be measured by the method as described in an Example.

  The extensional viscosity is 10,000 to 100,000 Pa · s, preferably 20,000 to 80,000 Pa · s, and more preferably 18,000 to 40,000 Pa · s. When the extensional viscosity is 10,000 Pa · s or more, the ion permeability is further improved when the laminated microporous film is used as a battery separator. Moreover, the film-formability of a lamination | stacking microporous film improves more because elongation viscosity is 100000 Pa * s or less. The elongational viscosity can be adjusted by mixing two or more kinds of resins.

  The shear viscosity is 5000 to 10000 Pa · s, preferably 7000 to 8000 Pa · s, and more preferably 7500 to 8000 Pa · s. When the shear viscosity is 5000 Pa · s or more, the porosity and shrinkage relaxation behavior at the time of stretching is further improved. Moreover, when the shear viscosity is 10,000 Pa · s or less, the film forming property of the laminated microporous film is further improved. Furthermore, when the shear viscosity is in the above preferred range, the thermal shrinkage of the laminated microporous film tends to be further reduced. The shear viscosity can be adjusted by mixing two or more kinds of resins.

  The elongational viscosity and shear viscosity can be measured by the method described in the examples.

  The puncture strength of the laminated microporous film is preferably 2.0 to 10N, more preferably 3.0 to 10N, and still more preferably 4.0 to 10N. When the puncture strength is within the above range, battery failure due to a short circuit between the electrodes tends to be further suppressed. The puncture strength can be measured by the method described in the examples.

[Method for producing laminated microporous film]
As a manufacturing method of the lamination | stacking microporous film of this embodiment, after shape | molding the lamination | stacking film which laminated | stacked each resin film by the co-extrusion method using T die and a circular die, for example, the lamination film is extended | stretched and porous. (A): a method of extruding each resin film separately, then laminating each resin film by a laminating method to form a laminated film, and then stretching the laminated film to make it porous ( b); a method (c) of extruding each resin film separately and further stretching to obtain a microporous film that has been made porous, and then bonding the microporous film. Among these, from the viewpoint of physical properties required for the obtained laminated microporous film and initial / running cost, a laminated film in which each resin film is laminated by a coextrusion method is formed, and then the laminated film is stretched to be porous. The method (a) is preferably used. On the other hand, although the air permeability is slightly inferior to the method (a), the method (c) is also preferable from the viewpoint that the heat shrinkage rate of the laminated microporous film can be reduced.

Hereinafter, the method (a) will be described in detail. Method for manufacturing a laminated microporous film according to the present embodiment, the second resin having a first resin film comprising a first resin composition having a melting point T _mA, a low melting point T _mB than the melting point T _mA A co-extrusion step of forming a laminated film having a second resin film containing the composition by a co-extrusion method, and a strain rate of 0.10 by the dry method of the laminated film obtained by the co-extrusion step. A stretching step of stretching in a order of ˜4.0 / sec to form a laminated microporous film.

  In the present specification, the “resin film” refers to a resin composition formed into a film, and a “microporous film” can be obtained by stretching the resin composition to make it porous.

[Co-extrusion process]
The co-extrusion step includes a first resin film containing a first resin composition having a melting point T _mA and a second resin film containing a second resin composition having a melting point T _mB lower than the melting point T _mA. Is a step of forming a laminated film having a coextrusion method.

  In any of the production methods (a) to (d), the draw ratio after extrusion, that is, the film winding speed (unit: m / min) is set to the extrusion speed of the resin composition (molten resin passing through the die lip). The value divided by the linear velocity in the flow direction (unit: m / min) is preferably in the range of 10 to 500, more preferably 100 to 400, and still more preferably 150 to 350. When this value is 10 to 500, the air permeability of the obtained laminated microporous film is further improved. The film is wound so that the film winding speed is preferably 2 to 400 m / min, more preferably 10 to 200 m / min. When the winding speed is 2 to 400 m / min, the air permeability of the obtained laminated laminated microporous film is further improved.

[Laminated film]
Laminated film, a first resin film comprising a first resin composition having a melting point T _mA, and the second resin film comprising a second resin composition having a low melting point T _mB than the melting point T _mA Have. The first resin composition and the second resin composition have melting points (hereinafter also simply referred to as “melting points”) T _mA and T _mB measured by a method according to JIS K-7121, where T _mA > T _mB As long as the above is satisfied, the composition may be the same or different. Here, T _mA represents the melting point of the first resin composition, and T _mB represents the melting point of the second resin composition.

The difference (T _mA −T _mB ) between the melting point T _mA of the first resin composition and the melting point T _{mB of} the second resin composition is preferably 5.0 ° C. or more, more preferably 10 ° C. or more. Yes, more preferably 15 ° C or higher. When the difference in melting point (T _mA −T _mB ) is 5.0 ° C. or higher, when the laminated microporous film is used as a battery separator, the low melting point when the internal temperature of the battery rises due to abnormal current. Even if the resin layer melts, the high melting point resin layer tends to be held without melting. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be further improved. On the other hand, the difference (T _mA −T _mB ) between the melting point T _mA of the first resin composition and the melting point T _{mB of} the second resin composition is preferably 50 ° C. or less, more preferably 100 ° C. or less. More preferably, it is 150 ° C. or lower. When the difference in melting point (T _mA -T _mB ) is 150 ° C. or less, when a laminated microporous film is used as a battery separator, a resin having a low melting point when the internal temperature of the battery rises due to an abnormal current. Even if the layer melts, the high melting point resin layer tends to be held without melting. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be further improved.

  The laminated film is a laminated body of the first resin film and the second resin film, but the aspect of the lamination is not particularly limited. As a specific example of the embodiment, a laminated film (a) comprising one first microporous film and one second microporous film (a), one first microporous film and laminated on both sides thereof A laminated film (b) composed of the second microporous film formed, a laminated film (c) composed of one second microporous film and the first microporous film laminated on both sides thereof, Laminated film (d) in which the respective resin films are arranged alternately such as first microporous film-second microporous film-first microporous film-second microporous film Is mentioned. Among these, the aspect (c) is preferable from the viewpoint of more effectively and reliably exhibiting the effects of the present invention.

(First resin composition)
It does not specifically limit as a 1st resin composition, The thing similar to the above is mentioned.

(Second resin composition)
It does not specifically limit as a 2nd resin composition, The thing similar to the above is mentioned. Among these, an ethylene resin composition is preferable. By using an ethylene-based resin composition, a plurality of characteristics required for battery separators can be combined better.

[Heat treatment process]
The manufacturing method of the laminated microporous film of the present embodiment preferably further includes a heat treatment step for performing a heat treatment (annealing) on the obtained laminated film after the coextrusion step and before the stretching step. The annealing method is not particularly limited. For example, a method in which a laminated film is brought into contact with a heating roll; a method in which the laminated film is exposed to a heated gas phase; a laminated film is wound on a core, and a heated gas phase or a heating liquid is used. And a method of exposing in the phase; and a method of combining them.

In the method (a), the heating temperature when annealing the laminated film is preferably (T _mB −30) ° C. or more and (T _mB −2.0) ° C. or less, more preferably (T _mB −15) ° C. The temperature is (T _mB −2.0) ° C. or lower, more preferably (T _mB −10) ° C. or higher and (T _mB −2.0) ° C. or lower. When the heating temperature is within the above range, the balance of porosity, air permeability, and heat shrinkage of the obtained laminated microporous film tends to be further improved.

  In the method (a), the heating time when the laminated film is annealed is preferably 10 seconds to 100 hours, more preferably 1 minute to 10 hours, and further preferably 1 hour to 6 hours. When the heating time is within the above range, the balance of porosity, air permeability, and heat shrinkage of the obtained laminated microporous film tends to be further improved.

In the method (b) or (c), the heating temperature when annealing the first resin film composed of the first resin composition (hereinafter also referred to as “high melting point resin film”) alone is preferably Is (T _mA- 50) ° C. or more and (T _mA −2.0) ° C. or less, more preferably (T _mA −40) ° C. or more and (T _mA −10) ° C. or less, and more preferably (T _mA It is −40) ° C. or higher and (T _mB −5.0) ° C. or lower. When the heating temperature is within the above range, the balance of porosity, air permeability, and heat shrinkage of the obtained laminated microporous film tends to be further improved. The heating time is appropriately determined and is not particularly limited.

On the other hand, in the method (b) or (c), the heating temperature when the second resin film composed of the second resin composition (hereinafter also referred to as “low melting point resin film”) is annealed alone is It is preferably (T _mB -30) ° C. or more and (T _mB −2.0) ° C. or less, more preferably (T _mB −15) ° C. or more and (T _mB −2.0) ° C. or less, and still more preferably. Is (T _mB −10) ° C. or higher and (T _mB −2.0) ° C. or lower. When the heating temperature is within the above range, the balance of porosity, air permeability, and heat shrinkage of the obtained laminated microporous film tends to be further improved. The heating time is appropriately determined and is not particularly limited.

  In addition, the conditions of the said heat processing can be suitably determined with the composition etc. of the resin composition which comprises a laminated | multilayer film.

  As described above, annealing may be performed before each resin film is laminated, or may be performed after lamination. When annealing is performed before lamination, annealing can be performed under conditions suitable for each of the high melting point resin film and the low melting point resin film. As a result, a laminated microporous film having a better balance among porosity, air permeability, and heat shrinkage tends to be obtained. On the other hand, when annealing is performed after laminating, that is, when annealing is performed on the laminated film, the resin films composed of the resin before annealing having a high ratio of the amorphous part are laminated. A laminated microporous film having a high adhesive strength tends to be obtained. The resin films may be laminated by, for example, thermocompression bonding.

[Stretching process]
The stretching step is a step of stretching the laminated film obtained in the coextrusion step by a dry method so that the strain rate is 0.50 to 3.0 / second to form a laminated microporous film. . Here, the “dry method” refers to a stretch opening method that does not use a solvent.

  Although it does not specifically limit as a extending | stretching method, For example, cold stretching and hot stretching are mentioned, More specifically, the following hot stretching is mentioned. In the case where the stretching step includes a cold stretching step and a hot stretching step, at least one step may satisfy the following strain rate, and among these, the hot stretching step preferably satisfies the following strain rate. When the thermal stretching process satisfies the following strain rate, there is a tendency to obtain a laminated microporous film obtained by further improving the balance between the strength and the porosity, which are in a trade-off relationship from the conventional one.

  The strain rate is 0.10 to 4.0 / second, preferably 0.30 to 4.0 / second, and more preferably 0.50 to 3.0 / second. When the strain rate is 0.10 / second or more, the strength is further improved. Moreover, a porosity improves more because a strain rate is 4.0 / sec or less. On the other hand, when the strain rate is higher than 4.0 / second, the balance between the strength and the porosity of the obtained laminated microporous film becomes insufficient.

Here, the “strain rate” is defined by the following equation. In the case of a roll type stretching machine, V1 and V2 are derived from the roll peripheral speed, and L corresponds to the distance between the rolls.
Strain rate (/ sec) = (V2-V1) / L
V1: Stretching speed at the start of stretching (m / sec)
V2: Stretching speed at the end of stretching (m / sec)
L: Stretching length (m)

  Although it does not specifically limit as a extending process, For example, it is preferable that a cold drawing process and / or a hot drawing process are included, it is more preferable that at least a hot drawing process is included, and it is further including a cold drawing process and a hot drawing process. preferable. Hereinafter, each step will be described in more detail.

  In addition, when forming the laminated film which laminated | stacked 1st resin film and 2nd resin film previously like said method (a) and method (b), it is 1st with respect to the laminated film. It is preferable to include a cold stretching step of stretching to obtain a stretched laminated film. In addition, when the first resin film and the second resin film are separately made porous after lamination as in the above method (c), the first stretching is applied to each resin film. It is preferable to include a cold stretching step for obtaining a stretched laminated film.

(Cold drawing process)
In the method (a) and the method (b), when cold stretching is performed on the laminated film, the stretching temperature in the cold stretching step is preferably −20 ° C. or more and (T _mB −60) ° C. or less, more preferably. It is 0 degreeC or more and 50 degrees C or less, More preferably, it is 0 degreeC or more and 45 degrees C or less. When the stretching temperature in the cold stretching step is −20 ° C. or higher, the breakage tends to be further suppressed. Moreover, when the stretching temperature in the cold stretching step is (T _mB −60) ° C. or lower, the porosity and air permeability of the resulting laminated microporous film tend to be further improved. Here, the “stretching temperature in the cold stretching process” means the surface temperature of the film in the cold stretching process. The surface temperature of the film can be measured with a contact thermometer (the same applies hereinafter).

  In the method (c), the stretching temperature when cold-stretching the high melting point resin film alone is preferably −20 ° C. or higher and 90 ° C. or lower, more preferably 0 ° C. or higher and 50 ° C. or lower, and further preferably 0. It is not lower than 45 ° C. When the stretching temperature in the cold stretching step is −20 ° C. or higher, the breakage tends to be further suppressed. Moreover, when the stretching temperature in the cold stretching step is 90 ° C. or less, the porosity and air permeability of the obtained laminated microporous film tend to be further improved.

On the other hand, in the method (c), the stretching temperature when the low melting point resin film is cold-stretched alone is preferably (T _mB −30) ° C. or more and (T _mB −2) ° C. or less, more preferably (T _mB −25) ° C. or more and (T _mB −2) ° C. or less, more preferably (T _mB −15) ° C. or more and (T _mB −2) ° C. or less. When the stretching temperature in the cold stretching step is within the above range, the balance of the porosity, air permeability, and heat shrinkage of the obtained laminated microporous film tends to be further improved.

  In the methods (a) to (c), the draw ratio in the cold drawing step is preferably 1.05 to 2.0 times, more preferably 1.1 to 2.0 times, and still more preferably. 1.1 times to 1.18 times. When the draw ratio in the cold drawing step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved. Cold stretching may be performed in at least one direction, and may be performed in both the film extrusion direction (MD) and the film width direction (TD). Among these, from the viewpoint of air permeability of the obtained laminated microporous film, it is preferable to perform uniaxial stretching only in the film extrusion direction.

(Heat drawing process)
The manufacturing method of the laminated microporous film of the present embodiment includes a hot stretching step of obtaining a laminated microporous film by performing a second stretching at a temperature higher than the stretching temperature of the cold stretching after the cold stretching step. Is preferred.

In the method (a) and the method (b), when heat stretching is performed on the laminated film, the stretching temperature in the heat stretching step is preferably (T _mB −60) ° C. or more and (T _mB −2.0) ° C. or less. More preferably, it is (T _mB −45) ° C. or more and (T _mB −2.0) ° C. or less, and more preferably (T _mB −30) ° C. or more and (T _mB −2.0) ° C. or less. . It exists in the tendency which can suppress a fracture _| rupture more because the extending _| stretching temperature in a hot extending process is ( _TmB- 60) _degreeC or more. Moreover, when the stretching temperature in the heat stretching step is (T _mB −2.0) ° C. or lower, the porosity and air permeability of the obtained laminated microporous film tend to be further improved. Here, the “stretching temperature in the heat stretching process” means the surface temperature of the film in the heat stretching process.

  In the method (a) and the method (b), the draw ratio in the heat drawing step is preferably 1.05 times to 5.0 times, more preferably 1.1 times to 5.0 times, and further preferably. Is 2.0 times to 5.0 times. When the draw ratio in the heat drawing step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved. The thermal stretching is performed in at least one direction and may be performed in both MD and TD directions. Among these, from the viewpoint of air permeability of the obtained laminated microporous film, the hot stretching is preferably performed in the same direction as the cold stretching direction, and uniaxial stretching is performed only in the same direction as the cold stretching direction. It is more preferable.

  In the method (c), when the high melting point resin film is hot stretched alone, the stretching temperature in the hot stretching step is preferably 90 ° C. or higher and 150 ° C. or lower, more preferably 100 ° C. or higher and 150 ° C. or lower, Preferably they are 110 degreeC or more and 150 degrees C or less. When the stretching temperature in the heat stretching step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved.

  In the method (c), when the high melting point resin film is hot stretched alone, the draw ratio in the hot stretching step is preferably 1.05 times or more and 5.0 times or less, more preferably 1.1 times. It is 5.0 times or less and more preferably 2.0 times or more and 5.0 times or less. When the draw ratio in the hot drawing step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved. The hot stretching may be performed in at least one direction and may be performed in both the MD and TD directions, but is preferably performed in the same direction as the cold stretching direction, and more preferably in the same direction as the cold stretching direction. The only uniaxial stretching is performed.

On the other hand, in the method (c), when the low melting point resin film is hot stretched alone, the stretching temperature in the hot stretching step is preferably (T _mB −60) ° C. or higher and (T _mB −2.0) ° C. or lower. More preferably, it is (T _mB −45) ° C. or more and (T _mB −2.0) ° C. or less, and further preferably (T _mB −30) ° C. or more and (T _mB −2.0) ° C. or less. When the stretching temperature in the heat stretching step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved.

  Further, in the method (c), when the low melting point resin film is hot stretched alone, the stretch ratio in the hot stretching step is preferably 1.05 times or more and 5.0 times or less, more preferably 1.1 times. It is 5.0 times or less and more preferably 2.0 times or more and 5.0 times or less. When the draw ratio in the hot drawing step is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved. The hot stretching may be performed in at least one direction and may be performed in both the MD and TD directions, but is preferably performed in the same direction as the cold stretching direction, and more preferably in the same direction as the cold stretching direction. The only uniaxial stretching is performed.

  The order in which the cold drawing process and the hot drawing process are performed is not particularly limited, and examples thereof include a method of performing the hot drawing process after the cold drawing process and a method of performing the cold drawing process after the hot drawing process. Among these, from the viewpoint of air permeability of the obtained laminated microporous film, a method of performing a hot stretching step after the cold stretching step is preferable.

[Heat setting process]
It is preferable that the manufacturing method of the lamination | stacking microporous film of this embodiment includes the heat setting process which heat-sets a lamination | stacking microporous film after an extending process. By having a heat setting step, it is possible to suppress the shrinkage in the stretching direction of the laminated microporous film due to residual stress acting during stretching, and to improve the delamination strength of the resulting laminated microporous film. is there. As a method of this heat setting, a method of heat shrinking to such an extent that the length of the laminated microporous film after heat setting is reduced by 3 to 50% from the length before heat setting (hereinafter referred to as “relaxation”). And a method of fixing so that the dimension in the stretching direction does not change before and after heat setting.

In the method (a) and the method (b), when heat setting is performed on the laminated film, the heat setting temperature is preferably (T _mB −30) ° C. or more and (T _mB −2.0) ° C. or less. More preferably, it is (T _mB −25) ° C. or more and (T _mB −2.0) ° C. or less, and more preferably T _mB −15) ° C. or more and (T _mB −2.0) ° C. or less. When the heat setting temperature is within the above range, the air permeability of the obtained laminated microporous film tends to be further improved. Here, the “heat setting temperature” means the surface temperature of the film in the heat setting process.

  In the method (c), when the high melting point resin film is heat-set alone, the heat-setting temperature is preferably 100 ° C. or higher and 160 ° C. or lower, more preferably 120 ° C. or higher and 160 ° C. or lower, and further preferably 130. It is 155 degreeC or more. When the heat setting temperature is within the above range, the thermal contraction rate of the obtained laminated microporous film tends to be further reduced.

On the other hand, in the method (c), when the low melting point resin film is heat-set alone, the heat setting temperature is preferably (T _mB -30) ° C. or higher and (T _mB −2) ° C. or lower, more preferably ( T _mB −25) ° C. or more and (T _mB −2) ° C. or less, more preferably (T _mB −15) ° C. or more and (T _mB −2) ° C. or less. When the heat setting temperature is within the above range, the thermal contraction rate of the obtained laminated microporous film tends to be further reduced.

  In the cold drawing process, the hot drawing process, the other drawing process and the heat setting process, a roll, a tenter, an autograph, etc. are used in one step or two steps or more, in one axis direction and / or biaxial direction, heat A fixing method may be employed. Among these, from the viewpoint of physical properties and applications required for the laminated microporous film obtained in the present embodiment, it is preferable to perform two or more stages of uniaxial stretching and heat setting with a roll.

  In the method (b), from the viewpoint of the peel strength between the resin films in the laminated film, it is preferable to pass the high melting point resin film and the low melting point resin film between heated rolls and perform thermocompression bonding. In this case, each resin film may be laminated by being unwound from an original roll stand, niped between heated rolls, and pressure-bonded. At the time of lamination, it is preferable to perform thermocompression bonding so that the elastic recovery rate after heat treatment of each resin film does not substantially decrease. For such thermocompression bonding, for example, the thermocompression bonding temperature may be adjusted. The same applies to the case where each resin film is extruded separately and further stretched to obtain a microporous film which has been made porous, and then the microporous film is bonded.

  In the method (b), the temperature of the heated roll, in other words, the thermocompression bonding temperature is preferably 120 ° C. or higher and 140 ° C. or lower, more preferably 125 ° C. or higher and 135 ° C. or lower, and further preferably 127 ° C. or higher and 132 ° C. or lower. It is below ℃. When the thermocompression bonding temperature is 120 ° C. or higher, the peel strength between the resin films in the laminated film becomes high, and peeling tends to hardly occur in each subsequent stretching step. In addition, when the thermocompression bonding temperature is 140 ° C. or lower, the laminated microporous film that can solve the intended problem can be further suppressed by dissolving the low melting point resin film and reducing its elastic recovery rate, Further, it tends to be easily obtained.

In the method (b), the nip pressure in thermocompression bonding is preferably 1.0 to 3.0 kg / cm ² . The unwinding speed is preferably 0.50 to 8.0 m / min. Furthermore, the peel strength of the laminated film is preferably 3.0 to 60 g / 15 mm, and can be controlled by adjusting the thermocompression bonding temperature as is apparent from the above. Note that the peel strength of the laminated film is such that, in a sample in which one end of the laminated film is partly peeled, one end of both peeled films is set at a predetermined position of a tensile tester and stretched in the length direction at a speed of 300 mm / min. It can measure by measuring the tension at the time of peeling.

[Separator for battery]
The battery separator according to this embodiment includes the laminated microporous film. The laminated microporous film in the present embodiment is suitably used as a battery separator, more specifically as a lithium secondary battery separator. In addition, it is also used as various separation membranes.

  In addition, each physical property in this specification can be measured according to the method described in the following Examples, unless otherwise specified.

Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist.
The evaluation methods for various characteristics in Examples and Comparative Examples are as follows.

(1) Melting point The melting point of the resin composition was measured by a method based on JIS K-7121. This measurement was performed at least three times, and the average value was taken as the melting point value.

(2) Melt flow rate (MFR)
According to JIS K7210, MFR (unit: g / 10 minutes) was measured under conditions of 230 ° C. and 2.16 kg for polypropylene and 190 ° C. and 2.16 kg for polyethylene. This measurement was performed at least three times, and the average value was taken as the MFR value.

(3) Molecular weight, molecular weight distribution (Mw / Mn)
The molecular weight distribution of the resin in the resin composition was calculated as the value of the ratio Mw / Mn between the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). Specifically, the molecular weight was calibrated with polystyrene, and Mw and Mn were obtained from the polystyrene-equivalent molecular weight to derive the molecular weight distribution. The GPC measurement conditions are shown below.
GPC device: Product name “HLC-8121GPC / HT” manufactured by Tosoh Corporation
Column: Tosoh Corporation, trade name “TSKgel GMHHR-H (20)” (two) used in series Mobile phase: o-dichlorobenzene (o-DCB)
Column temperature: 155 ° C
Flow rate: 1.0 mL / min Sample concentration: 0.5 mg / mL (o-DCB)
Injection volume: 500 μL
Sample melting temperature: 160 ° C
Sample dissolution time: 3 hours

(4) Density The density (unit: kg / m ³ ) of the resin composition was measured according to JIS K7112. This measurement was performed at least three times, and the average value was taken as the density value.

(5) Elongation Viscosity, Shear Viscosity Elongation viscosity / shear viscosity of a laminated microporous film is measured by using an inflow pressure loss method according to Cogswell's theory [Polymer Engineering Science, 12, 64 (1972)]. Obtained. As a measuring apparatus, a twin capillary rheometer manufactured by Rosand was used, and for the orifice, the following long die and short die were used, and measurement was performed under the conditions of a temperature of 200 ° C. and an elongation strain rate of 10 s ⁻¹ .
Long die: length 16mm, diameter 1mm, inflow angle 180 °
Short die: Length 0.25mm, diameter 1mm, inflow angle 180 °

(6) Film thickness (μm)
The film thickness of the laminated microporous film was measured with a dial gauge (trade name “PEACOCK No. 25” manufactured by Ozaki Seisakusho Co., Ltd.). This measurement was performed at least 5 times, and the average value was taken as the density value.

(7) Porosity (%)
A 10 cm × 10 cm square sample was cut out from the laminated microporous film, and calculated from the volume and mass of the sample using the following formula. This measurement was performed at least 5 times, and the average value was taken as the porosity value.
Porosity (%) = (volume of laminated microporous film (cm ³ ) −mass of laminated microporous film (g) / density of resin composition constituting laminated microporous film (g / cm ³ )) / Volume of laminated microporous film (cm ³ ) × 100

(8) Air permeability (sec / 100cc)
The air permeability of the laminated microporous film was measured with a Gurley type air permeability meter based on JIS P-8117. In addition, it converted into the air permeability per 20 micrometers of film thickness based on the measured value of a film thickness, the said measurement was implemented at least 5 times, and the average value was made into the value of air permeability.

(9) Heat shrinkage rate A sample of 12 cm × 12 cm square was cut out from the laminated microporous film, two marks were made at 10 cm intervals in the MD direction of the sample, and the sample was sandwiched between papers in an oven at 100 ° C. For 60 minutes. After removing the sample from the oven and cooling, the length (cm) between the marks was measured, and the thermal shrinkage in the MD direction was calculated by the following formula.
Thermal contraction rate (%) = (10−length between marks after heating (cm)) / 10 × 100

(10) Puncture strength (N)
A handy compression tester “KES-G5 type” manufactured by Kato Tech Co., Ltd. is equipped with a needle having a diameter of 1 mm and a curvature radius of 0.5 mm at the tip, a temperature of 23 ± 2 ° C., and a needle moving speed of 0.2 cm / The piercing test of the laminated microporous film was conducted in sec. In addition, based on the measured value of a film thickness, what was converted per 20 micrometers of film thickness was made into puncture strength. That is, the puncture strength was obtained based on the following formula. This measurement was performed at least 5 times, and the average value was used as the value of puncture strength.
Puncture strength (N) = Measured puncture strength × 20 / film thickness

In addition, the used resin is as follows.
Polypropylene (a-1): Propylene homopolymer, prime polymer E111G, melting point 165 ° C., MFR 0.5 g / 10 min. Polypropylene (a-2): Propylene homopolymer, prime polymer J105G, melting point 165 ° C. MFR: 9.0 g / 10 min. Polypropylene (a-3): Propylene homopolymer, MA4AHB manufactured by Nippon Polypro Co., Ltd., melting point: 165 ° C., MFR: 5.9 g / 10 min. Polyethylene (b-1): manufactured by Tosoh Corp., melting point: 136 ° C, MFR 0.25 g / 10 min Polyethylene (b-2): Prime polymer, melting point 136 ° C, MFR 0.32 g / 10 min Polyethylene (b-3): Asahi Kasei Chemicals, melting point 136 ° C, MFR 1.3g / 10min

[Example 1]
Polypropylene (A-1) in which 50 parts by mass of polypropylene (a-1) and 50 parts by mass of polypropylene (a-3) are mixed has a diameter of 20 mm, L / D (L: from the raw material supply port to the discharge port of the extruder) Distance (m), D: Inner diameter of the extruder (m). The same applies hereinafter.) = 30, put into a single screw extruder set at 220 ° C. through a feeder, and polyethylene (b-1) having a diameter of 20 mm. , L / D = 30, was introduced into a single-screw extruder set at 200 ° C. through a feeder, and extruded from a co-pressed T die having a lip thickness of 3.0 mm installed at the tip of the extruder. Immediately after that, 25 ° C. cold air was applied to the molten resin, and the film was wound with a cast roll cooled to 95 ° C. under a draw ratio of 200 times and a winding speed of 15 m / min. The outer layer was a high melting point resin film and the inner layer was a low melting point. A three-layer laminated film (Af-1) having a resin film structure was formed (coextrusion step). The laminated film (Af-1) was annealed for 6 hours in a hot air circulating oven heated to 130 ° C. (annealing step). In addition, melting | fusing point of polypropylene (A-1) was 165 degreeC, and MFR was 1.9 g / 10min.

  Next, the annealed laminated film was uniaxially stretched 1.3 times in the machine direction at a temperature of 25 ° C. to obtain a stretched laminated film (cold stretching process). Subsequently, the stretched laminated film was uniaxially stretched 3.0 times in the longitudinal direction at a temperature of 120 ° C. (strain rate: 2.0 / second) to obtain a laminated microporous film (thermal stretching step). Thereafter, it was relaxed by a factor of 0.8 at a temperature of 130 ° C. and heat-set (heat setting step) to obtain a laminated microporous film. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, the thermal contraction rate, and the puncture strength were measured. The results are shown in Table 1.

[Example 2]
Instead of polypropylene (A-1), polypropylene (A-2) in which 70 parts by mass of polypropylene (a-1) and 30 parts by mass of polypropylene (a-2) are mixed is used instead of polyethylene (b-1). A three-layer laminate in which a coextrusion step and an annealing step are performed in the same manner as in Example 1 except that polyethylene (b-2) is used, and the outer layer has a high melting point resin film and the inner layer has a low melting point resin film structure. A film (Af-2) was obtained. The melting point of polypropylene (A-2) was 165 ° C., and the MFR was 1.2 g / 10 minutes.

  Next, a cold stretch process, a heat stretch process, and a heat setting process were performed on the three-layer laminated film (Af-2) by the same method as in Example 1 to obtain a laminated microporous film. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, the thermal contraction rate, and the puncture strength were measured. The results are shown in Table 1.

[Comparative Example 1]
In the same manner as in Example 1 except that polypropylene (A-2) was used instead of polypropylene (A-1) and polyethylene (b-2) was used instead of polyethylene (b-1). An extrusion process and an annealing process were performed to obtain a three-layer laminated film (Af-3) having a structure in which the outer layer has a high melting point resin film and the inner layer has a low melting point resin film.

  Next, with the same method as in Example 1 except that the magnification in the heat setting step was 0.95 times, a cold drawing step, a heat drawing step, and a heat setting for the three-layer laminated film (Af-3). The process was performed to obtain a laminated microporous film. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, the thermal contraction rate, and the puncture strength were measured. The results are shown in Table 1.

[Comparative Example 2]
Instead of polypropylene (A-1), polypropylene (A-3) in which 50 parts by mass of polypropylene (a-1) and 50 parts by mass of polypropylene (a-2) are mixed is used instead of polyethylene (b-1). A three-layer laminate in which a coextrusion step and an annealing step are performed in the same manner as in Example 1 except that polyethylene (b-2) is used, and the outer layer has a high melting point resin film and the inner layer has a low melting point resin film structure. A film (Af-4) was obtained. The melting point of polypropylene (A-3) was 165 ° C., and the MFR was 2.1 g / 10 minutes.

  Next, a cold stretch process, a heat stretch process, and a heat setting process were performed on the three-layer laminated film (Af-4) by the same method as in Example 1 to obtain a laminated microporous film. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, the thermal contraction rate, and the puncture strength were measured. The results are shown in Table 1.

[Comparative Example 3]
In the same manner as in Example 1 except that polypropylene (A-3) was used instead of polypropylene (A-1) and polyethylene (b-3) was used instead of polyethylene (b-1). An extrusion process and an annealing process were performed to obtain a three-layer laminated film (Af-5) having a structure in which the outer layer has a high melting point resin film and the inner layer has a low melting point resin film.

  Next, a cold stretch process, a heat stretch process, and a heat setting process were performed on the three-layer laminated film (Af-4) by the same method as in Example 1 to obtain a laminated microporous film. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, the thermal contraction rate, and the puncture strength were measured. The results are shown in Table 1.

  As shown in Table 1, the laminated microporous films of Examples 1 and 2 having specific elongation viscosity and shear viscosity have a good balance of gas permeability and heat shrinkability, and further have a specific elongation viscosity and shear viscosity. It was found that the microporous film of Example 1 having a good balance between air permeability and heat shrinkability even when the strain rate by hot stretching was large.

  The laminated microporous film of the present invention has industrial applicability as a battery separator, particularly a lithium ion secondary battery separator.

Claims (6)

A first microporous film comprising a first resin composition having a melting point T _mA ;
And a second microporous film comprising a second resin composition having a low melting point T _mB than the melting point T _mA,
The second resin composition is an ethylene-based resin composition;
The extensional viscosity is 18000-40000 Pa · s,
A laminated microporous film having a shear viscosity of 5000 to 10,000 Pa · s.   The laminated microporous film according to claim 1, wherein the film thickness is 16 μm or less.   The laminated microporous film according to claim 1 or 2, wherein the air permeability is 10 to 5000 seconds / 100 cc. The laminated microporous film according to any one of claims 1 to 3, wherein the first resin composition contains two or more kinds of polypropylene .   The battery separator containing the lamination | stacking microporous film of any one of Claims 1-4. A method for producing a laminated microporous film according to any one of claims 1 to 4,
Laminate having a first resin film comprising a first resin composition having a melting point T _mA, and a second resin film comprising a second resin composition having a low melting point T _mB than the melting point T _mA A coextrusion step of forming a film by a coextrusion method;
The laminated film obtained in the co-extrusion step is stretched by a dry method so that the strain rate is 0.10 to 4.0 / second to form a laminated microporous film. possess in order,
The second resin composition is Ru ethylene resin composition der,
A method for producing a laminated microporous film.