JP2016022679A - Laminated microporous film and method for producing the same, and cell separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated microporous film having excellent physical strength and high porosity and a method for producing the same and to provide a cell separator using the same.SOLUTION: There is provided a laminated microporous film which comprises a first microporous film composed of a first resin composition and a second microporous film composed of a second resin composition, wherein the first resin composition and the second resin composition have an MFR of 1.0 g/10 min or less and the molecular weight distribution (Mw/Mn) of the resin contained in the first resin composition is 10 or more.SELECTED DRAWING: None

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 suitable for use as separators for lithium ion secondary batteries. Has been. In recent years, lithium ion secondary 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.

  Here, a lithium ion secondary battery for a hybrid electric vehicle is required to have higher output characteristics for taking out a lot of energy in a short time. Moreover, since lithium ion secondary batteries for hybrid electric vehicles are generally large and require high energy capacity, it is required to ensure higher safety.

  The battery separator included in the lithium ion secondary battery is required to have a shutdown (hereinafter also referred to as “SD”) function in order to ensure safety. The “SD function” is a function that stops the battery reaction and prevents further temperature rise by rapidly increasing the electrical resistance of the battery separator when the temperature inside the battery rises excessively. is there. 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. Thus, the temperature at which the film constituting the battery separator 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 it is excellent in safety, so that the difference between the temperature at which SD is started and the membrane breaking temperature is large.

  Furthermore, in order to improve the discharge performance at a large current and the discharge performance at a low temperature of the lithium ion secondary battery, it is necessary to reduce the resistance of ions flowing in a state where the battery separator retains the electrolyte solution as much as possible. There is a demand for a battery separator having a low electric resistance in a state where an electrolytic solution is included.

  For the purpose of providing a microporous film serving as a separator that can cope with such circumstances, for example, Patent Document 1 discloses a laminated film having a high melting point resin layer and a low melting point resin layer by a coextrusion molding method. A method for producing a laminated microporous film by molding and stretching is proposed.

  Patent Document 2 discloses a method of producing a laminated microporous film having good air permeability by separately forming a polypropylene film and a polyethylene film, then laminating, annealing, and stretching. .

Japanese Patent No. 2883726 Japanese Patent No. 3003830

  However, for example, the physical properties required for battery separators are diverse, and in particular, when battery separators are used under harsh conditions where foreign matter is mixed in or external loads are applied, for example, Strength and physical strength such as tensile strength in the length direction (referred to as machine direction, hereinafter referred to as “MD”) and width direction (referred to as direction orthogonal to the machine direction, hereinafter referred to as “TD”). Improvement is desired.

  Moreover, in order to prevent the short circuit by the foreign material etc. in a battery when winding a separator, the separator needs to have the puncture strength more than fixed and the tensile strength of MD and TD. In addition, in recent lithium ion secondary batteries, it is desirable that the battery separator has a large pore diameter in order to increase the battery output and capacity.

  The separator is said to have a higher porosity and a larger battery electrical property as the pore diameter increases. However, the increase in porosity and the increase in the pore diameter are in a relation with physical strength. For this reason, a separator having a high porosity or a large pore diameter has a problem that physical strength is insufficient even if battery electrical characteristics are good.

  The present invention has been made in view of such circumstances, and provides a laminated microporous film having excellent physical strength and high porosity, a method for producing the same, and a battery separator using the same. The task is to do.

  As a result of intensive studies to solve the above problems, the inventors of the present invention are laminated microporous films having at least two layers of microporous films, and the MFR of the resin composition constituting each layer and at least one layer The molecular weight distribution of the resin contained in the resin is within a predetermined range, thereby obtaining a laminated microporous film having excellent physical strength (hereinafter also simply referred to as “strength”) and high porosity. The headline and the present invention were completed.

That is, the present invention is as follows.
[1] A first microporous film made of the first resin composition and a second microporous film made of the second resin composition are laminated, and the first resin composition And the MFR of the second resin composition is 1.0 g / 10 min or less, and the molecular weight distribution (Mw / Mn) of the resin contained in the first resin composition is 10 or more. Laminated microporous film.
[2] The second resin composition are those having a lower melting point T _mB than the melting point T _mA of the first resin composition, the laminated microporous film according to [1].
[3] The laminated microporous film according to [1] or [2], wherein the thickness of the laminated microporous film is 16 μm or less.
[4] The laminated microporous film according to any one of [1] to [3], wherein the air permeability of the laminated microporous film is 10 to 5000 seconds / 100 cc per 20 μm thickness.
[5] The laminated microporous film according to any one of [1] to [4], wherein the first resin composition includes a polypropylene resin.
[6] The laminated microporous film according to any one of [1] to [5], wherein the second resin composition includes a polyethylene resin.
[7] A battery separator including the laminated microporous film according to any one of [1] to [6].
[8] The method for producing a laminated microporous film according to any one of [1] to [6], wherein the laminated microporous film is formed of a first fine resin composition. A laminated microporous film comprising a porous film and a second microporous film made of a second resin composition, wherein the second resin composition contains a polyethylene resin. Production method.
[9] The method for producing a laminated microporous film according to any one of [1] to [6], wherein the laminated microporous film is formed of a first fine resin composition. A porous film and a second microporous film made of the second resin composition are laminated and provided, and the melting point T of the first resin composition and the first resin composition. The second resin composition having a melting point _TmB lower than _mA is extruded by a coextrusion method, and the first resin film made of the first resin composition and the second resin composition are made. A method for producing a laminated microporous film, comprising: forming a laminated film in which a second resin film is laminated; and forming the laminated microporous film by opening the laminated film by a dry method. .

  ADVANTAGE OF THE INVENTION According to this invention, the laminated microporous film provided with the outstanding physical intensity | strength and high porosity, its manufacturing method, and the battery separator using the same can be provided.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

[Laminated microporous film]
The laminated microporous film of this embodiment includes a first microporous film made of the first resin composition and a second microporous film made of the second resin composition.

The first resin composition and the second resin composition have melting points (hereinafter simply referred to as “melting points”) T _mA and T _mB measured by a method according to JIS K-7121, which are represented by the following formula (1):
T _mA > T _mB (1)
It is preferable to satisfy the condition represented by. 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 between the melting point T _mA of the first resin composition and the melting point T _{mB of} the second resin composition is more preferably 5 ° C. or more, and further preferably 10 ° C. or more. When the relationship between the melting points satisfies the condition represented by the above formula (1), when the laminated microporous film is used as a battery separator, when the internal temperature of the battery rises due to an abnormal current, the melting point becomes lower. Even if the second microporous film is melted, the first microporous film having a higher melting point is difficult to melt. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be improved.

  The laminated microporous film of the present embodiment is a laminate comprising a first microporous film and a second microporous film, but the mode of lamination is not particularly limited, and each microporous film One or two or more films may be provided. Specific examples of the embodiment include (a) a laminate composed of one first microporous film and one second microporous film, and (b) one first microporous film and its A laminate comprising a second microporous film laminated on both sides, (c) a laminate comprising one second microporous film and a first microporous film laminated on both sides thereof, And (d) first microporous film / second microporous film / first microporous film / second microporous film, and the respective microporous films are alternately laminated. The laminated body is mentioned. In these, the aspect of said (c) is preferable from a viewpoint of exhibiting the effect of this invention more effectively and reliably.

[First microporous film]
The 1st microporous film in this embodiment consists of a 1st resin composition, and a 1st resin composition contains 1 type, or 2 or more types of resin. The resin contained in the first resin composition is not particularly limited, but is preferably a thermoplastic resin, and more preferably a polymer containing an olefinic hydrocarbon as a monomer component. The first resin composition preferably contains a thermoplastic resin, more preferably a polymer containing an olefin hydrocarbon as a monomer component, and thus has a plurality of characteristics required for a battery separator. Examples of such a polymer include polyethylene resin, polypropylene (hereinafter sometimes abbreviated as “PP”) resin, polybutene-1 resin, poly-4-methylpentene resin, and ethylene-propylene copolymer. And a copolymer containing an olefin hydrocarbon such as an ethylene-tetrafluoroethylene copolymer together with other monomer components. The first resin composition preferably contains a polymer containing olefin hydrocarbon as a monomer component as a main component. Thereby, the several characteristic calculated | required by the separator for batteries can be combined more favorably. From the same viewpoint, the first resin composition more preferably contains a polypropylene resin as a main component.

The first resin composition preferably has a melting point higher than that of the second resin composition. The melting point T _mA is, for example, 150 ° C. to 280 ° C. Is more preferable because the balance tends to be further improved. In order to obtain such a first resin composition, a resin having a melting point of 150 ° C. to 280 ° C. may be included in the first resin composition.

  The melt flow rate of the resin contained in the first resin composition (hereinafter also referred to as “MFR”. Measured under a load of 230 ° C. and 2.16 kg according to ASTM D1238. The same shall apply hereinafter). 0 g / 10 min or less, preferably 0.8 g / 10 min or less. On the other hand, the lower limit of the MFR is not particularly limited, but the MFR is preferably 0.1 g / 10 min or more, and more preferably 0.3 g / 10 min or more. By setting the MFR to 0.1 g / 10 min or more, the melt viscosity of the resin at the time of molding tends to be a value suitable for productivity. On the other hand, by setting the MFR to 1.0 g / 10 min or less, The mechanical strength of the porous film becomes more sufficient.

  The molecular weight distribution of the resin contained in the first resin composition is a ratio (hereinafter referred to as “Mw”) of a weight average molecular weight (hereinafter also referred to as “Mn”) to a number average molecular weight (hereinafter also referred to as “Mn”). Mw / Mn ”) and 10.0 or more, and preferably 11.0 or more. On the other hand, the upper limit of Mw / Mn is not particularly limited, but Mw / Mn is preferably 15.0 or less, and more preferably 14.0 or less. If Mw / Mn is 10.0 or more, heat generation during molding of the first resin composition is further suppressed, and resin deterioration is less likely to occur, and if it is 15.0 or less, it is derived from a high molecular weight component. Less unmelted material. In the present specification, 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 are described in detail in the following examples. It is measured according to the method. When the microporous film is made of a resin, the microporous film may be used directly as a sample for GPC measurement. When the microporous film contains a component other than the resin, the component is removed from the microporous film. What has been removed may be used as a sample for GPC measurement.

  The polypropylene resin that can be contained in the first resin composition is a polymer containing propylene as a monomer component, and may be a homopolymer (monopolymer) or a copolymer (copolymer). . When the PP resin is a copolymer, it may be a random copolymer or a block copolymer. Moreover, when PP resin is a copolymer, it does not specifically limit as a copolymerization component, For example, other olefins, such as ethylene, butene, and hexene, are mentioned. When the polypropylene resin is a copolymer, the copolymerization ratio of propylene is preferably 50% by mass or more, 70% by mass or more, or 90% by mass or more.

  A polypropylene resin is used individually by 1 type or in mixture of 2 or more types. The polymerization catalyst used for obtaining the polypropylene resin is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst. Moreover, there is no restriction | limiting in particular also about the stereoregularity of a polypropylene resin, An isotactic or syndiotactic polypropylene resin is used.

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

  Polypropylene resins are known modified polypropylenes as described in, for example, JP-A-44-15422, JP-A-52-30545, JP-A-6-313078, and JP-A-2006-83294. Resin may be used. Furthermore, the polypropylene resin may be a mixture in any proportion of the above-described polypropylene resin that has not been modified and the modified polypropylene resin.

  The content ratio of the resin as the main component in the first resin composition is preferably 50% by mass or more, more preferably 80% by mass, and 100% by mass with respect to the total amount of the first resin composition. That is, it is particularly preferable that the first resin composition is made of a resin. When the content ratio of the resin is equal to or more than the above lower limit value, it is possible to suppress the risk of occurrence of foreign matters such as unmelted material and oxidized degradation product.

  In addition, the first resin composition in the present embodiment may contain only the resin, but in addition to the resin, as long as the effects exerted by the present invention are not impaired, as necessary. Additional components, such as antioxidants, metal deactivators, heat stabilizers, flame retardants (organic phosphate ester compounds, ammonium polyphosphate compounds, aromatic halogen flame retardants, silicone flame retardants, etc.), Plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weather resistance (light) improvers, polyolefin nucleating agents, slip agents, inorganic or Organic fillers and reinforcing materials (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various colorants Releasing agent or the like may be added.

[Second microporous film]
The 2nd microporous film in this embodiment consists of a 2nd resin composition, and a 2nd resin composition contains 1 type, or 2 or more types of resin. The resin contained in the second resin composition is not particularly limited, but is preferably a thermoplastic resin. As such a resin, for example, a polyolefin resin which is a polymer containing an olefinic hydrocarbon such as a polyethylene resin and a polypropylene resin as a monomer component, and a saturated polyester resin such as a polyethylene terephthalate resin and a polybutylene terephthalate resin are preferable.

  The melting point of the second resin composition is preferably 100 ° C. to 150 ° C., since the safety of the battery is drastically improved when the laminated microporous film is used as a battery separator. In order to obtain such a second resin composition, a resin having a melting point of 100 ° C. to 150 ° C. may be included in the resin composition. Examples of such a resin include a polyethylene resin, and more specifically, a so-called high density polyethylene resin, medium density polyethylene resin, and low density polyethylene resin. Among these, high-density polyethylene resin is preferably used from the viewpoint of further dramatically improving battery safety.

  The MFR of the resin contained in the second resin composition is 1.0 g / 10 min or less, preferably 0.5 g / 10 min or less. On the other hand, the lower limit of the MFR is not particularly limited, but the MFR is preferably 0.05 g / 10 min or more, and more preferably 0.1 g / 10 min or more. If the MFR is 0.05 g / 10 min or more, fish eyes are less likely to be generated in the second microporous film, and if it is 1.0 g / 10 min or less, drawdown is less likely to occur. The property is further improved.

The density of the second resin composition is preferably 0.945 to 0.970 g / cm ³ , more preferably 0.955 to 0.970 g / cm ³ , and still more preferably 0.960 to 0.967 g / cm ³ , particularly preferably 0.963 to 0.967 g / cm ³ . If the density is 0.945 g / cm ³ or more, a microporous film with better air permeability is obtained, and if it is 0.970 g / cm ³ or less, the second microporous film is stretched. When producing, the breakage of the film is more effectively suppressed.

  The content ratio of the resin as the main component in the second resin composition is preferably 50% by mass or more, more preferably 80% by mass, and 100% by mass with respect to the total amount of the second resin composition. That is, it is particularly preferable that the second resin composition is made of a resin.

  In addition, the second resin composition in the present embodiment may contain only the resin, but in addition to the resin, as long as the effects exhibited by the present invention are not impaired. Additional components may be added. As an additional component, the same thing as the additional component in a 1st resin composition is mentioned, for example.

[Physical properties of laminated microporous film]
The porosity of the laminated microporous film of the present embodiment is preferably 20% to 70%, more preferably 35% to 65%, and still more preferably 45% to 60%. When the porosity is 20% or more, sufficient ion permeability can be ensured when the laminated microporous film is used as a battery separator. On the other hand, when the porosity is 70% or less, the laminated microporous film can ensure more sufficient mechanical strength.

The porosity of the laminated microporous film of the present embodiment 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 is calculated by cutting out a 10 cm × 10 cm square sample from the film and using the following formula from the volume and mass of the sample.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) × 100

  The air permeability of the laminated microporous film of the present embodiment is preferably 10 seconds / 100 cc to 5000 seconds / 100 cc, more preferably 50 seconds / 100 cc to 1000 seconds / 100 cc, more preferably 100 seconds per 20 μm film thickness. / 100 cc to 500 seconds / 100 cc. Setting the air permeability to 5000 seconds / 100 cc or less is suitable from the viewpoint of securing sufficient ion permeability of the laminated microporous film. On the other hand, setting the air permeability to 10 seconds / 100 cc or more is preferable from the viewpoint of obtaining a more uniform laminated microporous film having no defects. In addition, the air permeability of the laminated microporous film of the present embodiment 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 air permeability is measured using a Gurley type air permeability meter according to JIS P-8117.

  5-20 micrometers is preferable and, as for the film thickness of the lamination | stacking microporous film of this embodiment, 5-16 micrometers is more preferable. When the film thickness is 5 μm or more, the mechanical strength tends to be further improved, and when the film thickness is 16 μm or less, the battery tends to be more effective for downsizing.

  The puncture strength of the laminated microporous film is preferably 2 to 10 N, more preferably 3 to 10 N, from the viewpoint of battery failure due to a short circuit between the electrodes. The puncture strength is measured according to the method described in the examples.

[Method for producing laminated microporous film]
The method for producing a laminated microporous film of the present embodiment is a method for producing the laminated microporous film described above. For example, preferably, a T die or a circular die is used, and (a) a plurality of resin compositions (for example, The first resin composition and the second resin composition (the same applies hereinafter) are extruded by a co-extrusion method, and a plurality of resin films (for example, a resin film made of the first resin composition and a second resin composition). It may be a manufacturing method having a step of forming a laminated film in which a resin film made of a product (the same applies hereinafter) is laminated and a step of opening the laminated film by a dry method to form a laminated microporous film. . Alternatively, using a T-die or a circular die, (b) after extruding a plurality of resin films separately, and laminating each resin film by a laminating method to form a laminated film, and then the laminated film dry method (C) A method of forming a laminated microporous film by opening a plurality of resin films separately, and further opening each by a dry method to obtain a microporous film. The method of bonding a porous film is mentioned. In addition, as a method of opening by the above-mentioned dry method, the method of extending | stretching and making it porous is mentioned, for example. 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 of (a) is preferred. 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.

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

In any of the production methods, the draw ratio after extrusion, that is, the film winding speed (unit: m / min), the resin composition extrusion speed (linear speed in the flow direction of the molten resin passing through the die lip, unit: The value divided by 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.

  Furthermore, it is preferable to heat-treat (anneal) the film (resin film, resin film laminate, microporous film, or laminated microporous film) obtained by molding, if necessary. As the annealing method, for example, a method in which a film is brought into contact with a heating roll, a method in which the film is exposed to a heated gas phase, a method in which the film is wound on a core and exposed to a heated gas phase or a heated liquid phase, and a combination thereof are performed. A method is mentioned. The conditions for these heat treatments are appropriately determined depending on the type of material constituting the film.

The heating temperature when annealing a laminated film obtained by laminating a resin film made of the first resin composition and a resin film made of the second resin composition is the viewpoint of the balance of porosity, air permeability, and heat shrinkage rate. _Therefore , it is preferably (T _mB −30) ° 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. The heating time is preferably 10 seconds to 100 hours, more preferably 1 minute to 10 hours.

Although the manufacturing method of the lamination | stacking microporous film of this embodiment is not specifically limited, It is preferable to have at least any one process of the following (A) and (B). Having these steps is preferable from the viewpoint of improving the air permeability of the obtained laminated microporous film. In particular, when the production method includes both of these steps (A) and (B), the air permeability of the laminated microporous film is further improved.
(A) A cold stretching process in which at least one resin film is cold-stretched at least 1.05 times to 2.0 times in one direction.
(B) A hot stretching process in which the cold-stretched at least one resin film is hot-stretched at least 1.05 times to 5.0 times in at least one direction. Note that the stretching temperature in the hot stretching step is higher than the stretching temperature in the cold stretching step.

  The manufacturing method of the laminated microporous film of the present embodiment includes a first resin film having a higher melting point and a second resin having a lower melting point in advance as in the methods (a) and (b). When forming the laminated film which laminated | stacked the film, it is preferable to have the cold extending process of giving a 1st extending | stretching with respect to the laminated film, and obtaining a stretched laminated film. When the manufacturing method of a lamination | stacking microporous film has this cold stretching process, the air permeability of the lamination | stacking microporous film obtained improves. Moreover, the manufacturing method of the laminated microporous film of this embodiment includes the second resin film having a lower melting point and the first resin film having a higher melting point as in the method (c). When laminating them after separately making them porous, it is preferable to have a cold stretching step of obtaining a stretched laminated film by subjecting each resin film to a first stretching. When the manufacturing method of a lamination | stacking microporous film has this cold drawing process, the air permeability of the microporous film obtained further improves.

When cold stretching is applied to a laminated film in which a resin film made of the first resin composition having a higher melting point and a resin film made of the second resin composition having a lower melting point are laminated, The stretching temperature is preferably −20 ° C. or higher from the viewpoint of more effectively and reliably preventing film breakage, and (T _mB −60) ° C. or lower from the viewpoint of further improving the porosity and air permeability. Is preferable. From the same viewpoint, the stretching temperature is more preferably 0 ° C. or higher and 50 ° C. or lower. Here, the drawing temperature of cold drawing means the surface temperature of the film in the cold drawing process. The surface temperature of the film can be measured with a contact thermometer.

  The draw ratio in the cold drawing step is preferably 1.05 to 2.0 times, more preferably 1.1 times or more and less than 2.0 times. When the draw ratio is 1.05 times or more, the air permeability of the obtained laminated microporous film is improved, and when it is 2.0 times or less, the heat shrinkability of the obtained laminated microporous film is increased. improves. Cold stretching is performed in at least one direction, but may be performed in both the MD and TD directions of the film. In cold stretching, it is preferable to perform uniaxial stretching only on the MD of the film.

It is preferable that the manufacturing method of the lamination | stacking microporous film of this embodiment has a heat | fever extending process which gives 2nd extending | stretching after the said cold extending process, and obtains an extending | stretching laminated | multilayer film. When heat stretching is performed on a laminated film in which a resin film made of the first resin composition having a higher melting point and a resin film made of the second resin composition having a lower melting point are laminated, The stretching temperature is preferably (T _mB −60) ° C. or more from the viewpoint of more effectively and reliably preventing film breakage, and from the viewpoint of further improving the porosity and air permeability, (T _mB − 2) It is preferable that it is below ℃. From the same viewpoint, this stretching temperature is more preferably (T _mB −30) ° C. or higher and (T _mB −2) ° C. or lower. Here, the stretching temperature of heat stretching means the surface temperature of the film in the heat stretching step.

  The draw ratio in the heat drawing step is preferably 1.05 to 5.0 times, more preferably 1.1 to 5.0 times, and still more preferably 2.0 to 5.0 times. It is. When the draw ratio is 1.05 times or more, the air permeability of the obtained laminated microporous film is further improved, and when it is 5.0 times or less, the heat shrinkability of the obtained laminated microporous film is reduced. Is further improved. The hot stretching may be performed in at least one direction and may be performed in both the MD and TD directions of the film, 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.

  It is preferable that the manufacturing method of the lamination | stacking microporous film of this embodiment has the heat setting process which heat-sets a stretched laminated film after the said heat stretching process. Providing the heat setting step is a viewpoint capable of suppressing shrinkage in the stretching direction of the stretched laminated film due to residual stress acting during stretching, and a viewpoint of improving the delamination strength of the resulting laminated microporous film To preferred. As a method of heat setting, a method of heat shrinking to the extent that the length of the stretched laminated film after heat setting is reduced by 3 to 50% (hereinafter, this method is referred to as “relaxation”), and a dimension in the stretching direction is changed. The method of fixing so that it may not be mentioned.

When heat setting is performed on a laminated film in which a resin film made of the first resin composition having a higher melting point and a resin film made of the second resin composition having a lower melting point are laminated, the heat setting temperature Is preferably (T _mB −30) ° C. or higher and (T _mB −2) ° C. or lower, more preferably (T _mB −15) ° C. or higher and (T _mB −2) ° C. or lower. When the heat _setting temperature is (T _mB −30) ° C. or more, the air permeability of the obtained laminated microporous film is further improved, and when it is (T _mB −2) ° C. or less, the obtained laminated microporous material is obtained. The heat shrinkability of the conductive film is further improved. Here, the heat setting temperature means the surface temperature of the film at the time of heat setting.

  In the stretching process other than the cold stretching process, the hot stretching process, and the heat setting process, and the cold stretching process and the hot stretching process, the roll, the tenter, the autograph, etc. A method of stretching or heat setting in the biaxial direction 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.

  The laminated microporous film in the present embodiment is suitably used as a battery separator, more specifically as a lithium ion secondary battery separator. In addition, it is 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 | fusing point Melting | fusing point was measured by the method based on JISK-7121.

(2) Melt flow rate (MFR)
MFR was measured under the conditions of 230 ° C. and 2.16 kg for the polypropylene resin and 190 ° C. and 2.16 kg for the polyethylene resin according to JIS K-7210. The unit of MFR is g / 10 minutes.

(3) Molecular weight and molecular weight distribution (Mw / Mn)
The molecular weight distribution of the resin in the resin composition is a value of the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). The GPC measurement was performed using a GPC apparatus (trade name “HLC-8121GPC / HT”) manufactured by Tosoh Corporation. Using the trade name “TSKgel GMHHR-H (20)” (two) manufactured by Tosoh Corporation as the column, mobile phase o-dichlorobenzene (o-DCB), column temperature 155 ° C., flow rate 1.0 mL / min, sample concentration The measurement was performed under the conditions of 0.5 mg / mL (o-DCB), an injection amount of 500 μL, a sample dissolution temperature of 160 ° C., and a sample dissolution time of 3 hours. 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.

(4) Density The density of the resin composition was measured according to JIS K-7112. The unit of density is g / cm ³ .

(5) Film thickness (μm)
The film thickness was measured using a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(6) Porosity (%)
A 10 cm × 10 cm square sample was cut out from the film, and calculated from the volume and mass of the sample using the following formula.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) × 100

(7) Air permeability (sec / 100cc)
The air permeability of the sample was measured with a Gurley type air permeability meter according to JIS P-8117. The value obtained by converting the value of the air permeability of the sample per 20 μm thickness was defined as the air permeability.

(8) Puncture strength (N)
A handy compression tester KES-G5 type (model number) 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. A puncture test was performed at / sec, and the puncture strength of the sample was measured. The value obtained by converting the puncture strength value of the sample per 20 μm film thickness was defined as the puncture strength.
Puncture strength (N) = Puncture strength of sample × 20 / film thickness

(9) Tensile strength (kg / cm ² )
In accordance with JIS K-7127, tensile strength at break for MD and TD samples (shape: width 10 mm × length 100 mm) using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A type (trademark) (Unit: kg) was measured. In addition, the sample used the thing which stuck the cellophane tape (Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) to the single side | surface of the both ends (each 25mm) of the sample between chuck | zippers 50mm. Moreover, in order to prevent sample slipping during the test, a fluoro rubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The tensile strength (unit: kg / cm ² ) of MD and TD was determined by dividing the value of tensile strength at break by the sample cross-sectional area before the test.

[Example 1]
First, a polypropylene resin having a melting point of 165 ° C., an MFR of 0.5 g / 10 min, and an Mw / Mn of 11 is prepared as the first resin composition (a-1), and the second resin composition (b -1), a polyethylene resin having a melting point of 136 ° C., an MFR of 0.25 g / 10 min, and a density of 0.96 g / cm ³ was prepared. Next, the polypropylene resin was introduced into a single screw extruder set at 220 ° C. having a diameter of 20 mm and L / D = 30 via a feeder, and the polyethylene resin was set at 200 ° C. The resin was put into an extruder through a feeder, and those resins were 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, cold air of 25 ° C. was applied to the extruded molten resin, and the cast roll cooled to 95 ° C. was wound at a draw ratio of 200 times and a winding speed of 15 m / min. A three-layer laminated film (Af-) having a structure which is a certain first resin film (A-1) and the second resin film (B-1) having an inner layer sandwiched between both outer layers and having a lower melting point. 1) was molded. This laminated film (Af-1) was annealed for 6 hours in a hot air circulating oven heated to 130 ° C.

  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). Next, the stretched laminated film was uniaxially stretched 3.0 times in the machine direction at a temperature of 120 ° C. (thermal stretching step). Thereafter, it was relaxed by a factor of 0.8 at a temperature of 130 ° C. and heat-fixed to obtain a laminated microporous film. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

[Example 2]
Instead of the second resin composition (b-1), the second resin composition (b-2) has a melting point of 136 ° C., an MFR of 0.32 g / 10 minutes, and a density of 0.96 g / cm ³ . A laminated microporous film was obtained in the same manner as in Example 1 except that a certain polyethylene resin was used. About the obtained lamination | stacking microporous film, the film thickness, the porosity, the air permeability, and the puncture strength were calculated | required. The results are shown in Table 1.

[Comparative Example 1]
In place of the first resin composition (a-1), a polypropylene resin having a melting point of 165 ° C., an MFR of 1.7 g / 10 min, and an Mw / Mn of 10 as the first resin composition (a-2) A laminated microporous film was obtained in the same manner as in Example 2 except that was used. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

[Comparative Example 2]
In place of the first resin composition (a-1), a polypropylene resin having a melting point of 165 ° C., an MFR of 2.1 g / 10 minutes, and an Mw / Mn of 10 as the first resin composition (a-3) A laminated microporous film was obtained in the same manner as in Example 2 except that was used. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

[Comparative Example 3]
Instead of the second resin composition (b-2), the second resin composition (b-3) has a melting point of 136 ° C., an MFR of 1.3 g / 10 minutes, and a density of 0.96 g / cm ³ . A laminated microporous film was obtained in the same manner as in Comparative Example 3 except that a certain polyethylene resin was used. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

[Comparative Example 4]
In place of the second resin composition (b-1), the second resin composition (b-3) has a melting point of 136 ° C., an MFR of 1.3 g / 10 minutes, and a density of 0.96 g / cm ³ . A laminated microporous film was obtained in the same manner as in Example 1 except that a certain polyethylene resin was used. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

[Comparative Example 5]
In place of the first resin composition (a-1), a polypropylene having a melting point of 165 ° C., an MFR of 0.5 g / 10 min, and an Mw / Mn of 8 as the first resin composition (a-4) A laminated microporous film was obtained in the same manner as in Example 1 except that the resin was used. About the obtained laminated microporous film, the film thickness, porosity, air permeability, puncture strength and tensile strength were determined. The results are shown in Table 1.

  The laminated microporous films of the present embodiment (Examples 1 and 2) all exhibited excellent physical strength and good porosity. On the other hand, the comparative example which manufactured the lamination | stacking microporous film in which MFR of the 1st resin composition exceeds 1.0 g / 10min and the molecular weight distribution (Mw / Mn) of the 1st resin composition is 10 or more. In 1 and 2, although good porosity was obtained, the physical strength was low. Comparative Example 3 in which the MFR of the first and second resin compositions exceeds 1.0 g / 10 min and the molecular weight distribution (Mw / Mn) of the first resin composition is 10 or more is excellent in physical properties. Although it has a certain strength, the porosity is low. Further, Comparative Example 4 in which the MFR of the first resin composition is 1.0 g / 10 min or less and the MFR of the second resin composition exceeds 1.0 g / 10 min, the molecular weight of the first resin composition The porosity of the comparative example 5 whose distribution (Mw / Mn) is less than 10 also became low.

  From the above results, since the molecular weight distribution of the resin contained in the first resin composition is wide, it has a wide range of high molecular weight components that are easily oriented immediately after extrusion and low molecular weight components that have a high crystallization speed. It is considered that both the physical strength and the porosity of the laminated microporous film were improved as a result of containing a large amount of low molecular weight component microcrystals in the resin film.

  Since the laminated microporous film of the present invention has excellent physical strength and high porosity, it has industrial applicability as a battery separator, particularly a lithium ion secondary battery separator. .

Claims (9)

  A first microporous film made of the first resin composition and a second microporous film made of the second resin composition are laminated to provide the first resin composition and the first resin composition. Laminated microporous resin having a MFR of 1.0 g / 10 min or less and a molecular weight distribution (Mw / Mn) of the resin contained in the first resin composition of 10 or more. Sex film. The second resin composition, the one having a first melting point T _mB than the melting point T _mA resin composition, the laminated microporous film according to claim 1, wherein.   The laminated microporous film according to claim 1 or 2, wherein the thickness of the laminated microporous film is 16 µm or less.   The laminated microporous film according to any one of claims 1 to 3, wherein an air permeability of the laminated microporous film is 10 to 5000 seconds / 100 cc per 20 µm of film thickness.   The laminated microporous film according to any one of claims 1 to 4, wherein the first resin composition contains a polypropylene resin.   The laminated microporous film according to any one of claims 1 to 5, wherein the second resin composition contains a polyethylene resin.   The battery separator containing the lamination | stacking microporous film of any one of Claims 1-6. It is a manufacturing method of the lamination | stacking microporous film of any one of Claims 1-6,
The laminated microporous film is provided by laminating a first microporous film made of the first resin composition and a second microporous film made of the second resin composition. ,
A method for producing a laminated microporous film, wherein the second resin composition comprises a polyethylene resin. It is a manufacturing method of the lamination | stacking microporous film of any one of Claims 1-6,
The laminated microporous film is provided by laminating a first microporous film made of the first resin composition and a second microporous film made of the second resin composition. ,
The first resin composition and the second resin composition having a melting point T _mB lower than the melting point T _{mA of} the first resin composition are extruded by a co-extrusion method. Forming a laminated film obtained by laminating a first resin film made of a product and a second resin film made of the second resin composition;
Opening the laminated film by a dry method to form the laminated microporous film;
A method for producing a laminated microporous film.