JP2020163793A - Precursor film for porous film - Google Patents

Abstract

To provide a precursor film capable of producing a porous film excellent in air permeability with high quality, and a porous film obtained using the precursor film.SOLUTION: A precursor film for porous film includes a polyethylene-based resin layer containing a polyethylene-based resin, and a polypropylene-based resin layer containing a polypropylene-based resin, in which the polyethylene-based resin included in the polyethylene-based resin layer has a semi-crystallization time (t1/2) in isothermal crystallization test at 124°C of 400 seconds or less, and the polypropylene-based rein included in the polypropylene-based resin layer has a content of a component having a molecular weight of 100,000 or less of 12 mass% or more, and a content of an ethylene component of 10 mass% or less.SELECTED DRAWING: None

Description

The present invention relates to a precursor film for obtaining a porous film, and a porous film obtained by using such a precursor film.

Power storage devices such as lithium secondary batteries are widely used as power sources for mobile phones, small electronic devices such as notebook computers, and electric vehicles. Such a lithium secondary battery is composed of a positive electrode, a negative electrode, a separator for preventing a short circuit interposed between the positive electrode and the negative electrode, and a non-aqueous electrolytic solution. As the separator, for example, a porous film made of polyolefin as a raw material and formed into a thin film to be porous is used.

As a method for producing a separator made of a porous film made of such a polyolefin as a raw material, a wet method and a dry method are known. In the wet method, for example, a resin composition obtained by mixing an additive such as a solvent with a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is formed into a film, and then the additive is added from the formed film. A porous film is produced by stretching a film that has been extracted and then extracted with additives.

On the other hand, as a dry method, for example, as disclosed in Patent Document 1, a method of obtaining a porous film by forming voids by utilizing cleavage during stretching is known. According to the technique disclosed in Patent Document 1, a polyethylene-based resin film and a polypropylene-based resin film are integrated by heat-sealing, and then the film obtained by the integration is stretched to be porous. Quality film is manufactured.

When the dry method is adopted as the method for producing the porous film, the wet method does not require an extraction step of additives such as a solvent, which is indispensable, and therefore, the productivity is generally superior to that of the wet method.

Japanese Patent Application Laid-Open No. 10-100434

On the other hand, when a porous film is produced by a laminating method in which a polyethylene-based resin film and a polypropylene-based resin film are integrated by heat-sealing as in Patent Document 1, polyethylene as a polyethylene-based resin layer A porous resin film is heat-treated, and a polypropylene-based resin film is laminated on both sides of the polyethylene-based resin film to be stretched and porous to form a porous film. However, in the laminating method, the polyethylene-based resin film or polypropylene-based resin film before laminating may be torn at the time of film formation.

The present invention has been made in view of such circumstances, and a precursor film capable of producing a porous film having excellent air permeability with high quality, and a porous film obtained by stretching and porous the precursor film. The purpose is to provide.

In order to achieve the above object, the present inventors focused on polyethylene-based resin and polypropylene-based resin as raw materials, and as a result of repeated studies, the semi-crystallization time in the isothermal crystallization test at 124 ° C. was 400 seconds. A polyethylene-based resin layer containing the following polyethylene-based resin and a polypropylene-based resin having a molecular weight of 100,000 or less and a component content of 12% by mass or more and an ethylene component content of 10% by mass or less. It has been found that the above object can be achieved by the precursor film provided with the polypropylene-based resin layer containing the mixture, and the present invention has been completed.

That is, the present invention provides the following [1] to [7].
[1] A precursor film for a porous film, comprising a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin.
The polyethylene-based resin contained in the polyethylene-based resin layer has a semi-crystallization time (t1 / 2) of 400 seconds or less in an isothermal crystallization test at 124 ° C.
A porous film in which the polypropylene-based resin contained in the polypropylene-based resin layer has a molecular weight of 100,000 or less and a component content of 12% by mass or more and an ethylene component content of 10% by mass or less. Precursor film for.

[2] The precursor film for a porous film according to [1], wherein the crystallization heat generation energy (ΔH) of the polyethylene resin at 124 ° C. is 110 J / g or more.

[3] The precursor film for a porous film according to [1] or [2], wherein the polypropylene-based resin has a weight average molecular weight (Mw) of 250,000 to 1.2 million.

[4] The precursor film for a porous film according to any one of [1] to [3], wherein the polypropylene-based resin has a mesopentad fraction of 90 to 98%.

[5] A porous film obtained by stretching and porous a precursor film for the porous film according to any one of [1] to [4].

[6] A separator for a power storage device having the porous film according to [5].

[7] A power storage device including the separator for a power storage device according to [6], a positive electrode, and a negative electrode.

According to the present invention, it is possible to provide a precursor film capable of producing a porous film having excellent air permeability with high quality, and a porous film obtained by using such a precursor film.

The precursor film of the present invention is a film used for producing a porous film.
A polyethylene resin layer containing a polyethylene resin and a polypropylene resin layer containing a polypropylene resin are provided.
The polyethylene-based resin contained in the polyethylene-based resin layer has a semi-crystallization time of 400 seconds or less in an isothermal crystallization test at 124 ° C.
The polypropylene-based resin contained in the polypropylene-based resin layer has a content of a component having a molecular weight of 100,000 or less of 12% by mass or more, and a content of an ethylene component of 10% by mass or less.

Here, the precursor film of the present invention is a film including a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin, and is used for producing a porous film. However, as a method for producing such a porous film by a dry method, a laminating method and a coextrusion method are known.

In the laminating method, a polyethylene-based resin film and a polypropylene-based resin film are separately prepared, heat-treated separately, and then bonded to each other, and the bonded film is stretched and made porous to produce a porous film. The method. Further, in the coextrusion method, a polyethylene-based resin and a polypropylene-based resin are collectively extruded into a film to obtain a precursor film, and the obtained precursor film is stretched and made porous. This is a method for producing a film.

That is, in the laminating method, in order to obtain a precursor film, it is necessary to first mold the polyethylene-based resin film and the polypropylene-based resin film individually, then heat-treat them, and then bond them together. On the other hand, according to the coextrusion method, by extruding the polyethylene-based resin and the polypropylene-based resin at once, the precursor film having the polyethylene-based resin layer and the polypropylene-based resin layer can be molded at once. , A precursor film can be produced with high efficiency as compared with the laminating method.

On the other hand, when the present inventors diligently studied the production of the precursor film by the coextrusion method and adopted the method of making the precursor film obtained by the coextrusion method porous by stretching, It is usually difficult to form sufficient pores, and on the other hand, polyethylene-based resins and polypropylene-based resins, which are raw materials, have specific characteristics and are combined. It has been found that a porous film having sufficient pores can be obtained with high quality when the film is made porous by stretching, and the present invention has been completed.

Hereinafter, the precursor film of the present invention will be described in detail.
As described above, the precursor film of the present invention includes a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin.

[Polyethylene resin]
In the present invention, as the polyethylene-based resin for forming the polyethylene-based resin layer, a polyethylene-based resin having a semi-crystallization time (t1 / 2) of 400 seconds or less in an isothermal crystallization test at 124 ° C. is used, and is semi-crystallized. The crystallization time (t1 / 2) is preferably 350 seconds or less, more preferably 330 seconds or less, still more preferably 300 seconds or less, and particularly preferably 250 seconds or less.

In the present invention, the "semi-crystallization time at 124 ° C. (t1 / 2)" means that a molten polyethylene resin is subjected to a differential scanning calorimeter (DSC) up to 124 ° C. at −500 ° C./min. It is the time until the integral value of the crystallization curve displayed when the temperature is maintained at the temperature after quenching reaches half of the integral value of the curve until the transition to the crystallization state. The temperature conditions may be appropriately changed according to the purpose of measurement.

When the semi-crystallization time at 124 ° C. exceeds 400 seconds, it becomes difficult to form pores when the precursor film including the polyethylene-based resin layer containing the polyethylene-based resin is stretched and porousd, and the resulting porosity is obtained. The air permeability of the quality film becomes insufficient, and the charge / discharge characteristics become insufficient when used as a separator for a power storage device.

On the other hand, when the semi-crystallization time at 124 ° C. is 50 seconds or more, the mechanism is not necessarily clear, but it is possible to facilitate the formation of pores when stretched and porous. Therefore, the lower limit of the semi-crystallization time at 124 ° C. is preferably 50 seconds or longer, more preferably 60 seconds or longer, still more preferably 70 seconds or longer, and particularly preferably 80 seconds or longer.

Further, as the polyethylene resin, the crystallization heat generation energy (ΔH) at 124 ° C. is preferably 110 J / g or more, more preferably 110 to 190 J / g, and further preferably 120 to 175 J / g. , Particularly preferably 135 to 163 J / g. When the crystallization heat generation energy (ΔH) at 124 ° C. is 110 J / g or more, the crystallinity of the polyethylene-based resin layer constituting the precursor film is increased, and the pore-opening property when stretched and porous is enhanced. Can be done. Further, the crystallization heat generation energy (ΔH) is determined from the viewpoint of ensuring sufficient peel strength between the polyethylene-based resin layer constituting the precursor film and the polypropylene-based resin layer when stretched and porous. It is preferably 190 J / g or less.

The crystallization heat generation energy (ΔH) at 124 ° C. is maintained at the temperature after quenching the molten polyethylene resin to 124 ° C. at −500 ° C./min by, for example, a differential scanning calorimeter (DSC). It can be obtained from the integrated value of the crystallization curve displayed at the time of. The temperature conditions may be appropriately changed according to the purpose of measurement.

The polyethylene-based resin preferably has a differential scanning calorimetry (DSC heat generation energy) of 200 J / g or more, more preferably 205 J / g or more, and further preferably 210 J / g or more. The upper limit of the differential scanning calorimetry (DSC heat generation energy) is not particularly limited, but is preferably 240 J / g or less.

The polyethylene-based resin preferably has a weight average molecular weight of 220,000 or more and 600,000 or less, and more preferably 250,000 or more and 500,000 or less. The weight average molecular weight of the polyethylene resin can be determined as a polystyrene-equivalent value by, for example, gel permeation chromatography (GPC) measurement. The ratio of the components having a molecular weight of 10,000 to 100,000 can be obtained from, for example, the distribution of the molecular weight obtained as a polystyrene-equivalent value from the result of gel permeation chromatography (GPC) measurement. Further, the polyethylene-based resin preferably has a melting point of 125 ° C. or higher and 140 ° C. or lower.

The polyethylene-based resin contains 80% by mass or more, preferably 90% by mass or more of polyethylene units, but the content of the propylene component in the polyethylene-based resin is 0.7% by mass or less. It is preferable that the content of the propylene component is too large, the propylene component may inhibit the crystallization of the polyethylene-based resin, and the crystallinity may be lowered. Further, the content of the butene component in the polyethylene resin is preferably 0.3% by mass or less, and if the butene content is too large, the butene component inhibits the crystallization of the polyethylene resin and crystallizes. It may reduce the degree.

In the present invention, the content ratio of the polyethylene-based resin in the polyethylene-based resin layer is preferably 99.80% by mass or more. When the content ratio of the polyethylene-based resin is 99.80% by mass or more, crystal defects in the lamella crystals in the polyethylene-based resin constituting the polyethylene-based resin layer are reduced, and the crystallinity is improved, which is preferable.

As a method for producing the polyethylene-based resin for forming the polyethylene-based resin layer, a conventionally known method can be used and is not particularly limited. For example, the polyethylene-based resin can be produced by using a catalyst such as a Ziegler-Natta catalyst or a metallocene catalyst, and a method using a Ziegler-Natta catalyst is particularly preferable. The polymerization method may be either single-stage or multi-stage, but multi-stage is preferable. The semi-crystallization time of the polyethylene resin at 124 ° C., the crystallization heat generation energy at 124 ° C., the weight average molecular weight and the like can be controlled by adjusting various production conditions such as the polymerization temperature and the catalyst amount.

In multistage polymerization, it is preferable to carry out polymerization sequentially and continuously with a plurality of polymerization reactors connected in series or in parallel. The polymerization can be carried out in an organic solvent, a liquid monomer, or a gas phase. In the multi-stage polymerization, for example, first, in the first stage, ethylene or polyethylene and α-olefin are polymerized / copolymerized to produce polyethylene as a high molecular weight component. Next, in the second stage, ethylene and hydrogen are introduced into the polymerization system to produce polyethylene as a low molecular weight component. That is, in the multistage polymerization, a polyethylene-based resin containing a high molecular weight component and a low molecular weight component can be produced, and it is preferable to adopt such an embodiment. According to such a method, the weight average molecular weight and the molecular weight A polyethylene-based resin having a content ratio of 10,000 to 100,000 components in a desired range can be obtained. Alternatively, a method of sequentially producing polyethylene having a low molecular weight component in the first stage and polyethylene having a high molecular weight component in the second stage may be adopted. Examples of the α-olefin copolymerized with ethylene include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, 1-octene, 1-nonene, 1 -Decenes and the like can be mentioned.

[Polypropylene resin]
The propylene-based resin forming the polypropylene-based resin layer may be a resin containing 80% by mass or more, preferably 90% by mass or more of propylene units, and is a propylene homopolymer, which is a combination of propylene and other olefins. Examples include polymers. As the propylene-based resin, it may be used alone or in combination of two or more. Further, the copolymer of propylene and other olefins may be either a block copolymer or a random copolymer. Examples of the olefin copolymerized with propylene include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, 1-octene, 1-nonene, 1-decene and the like. Α-olefin and the like.

In the present invention, the polypropylene-based resin forming the polypropylene-based resin layer having a molecular weight of 100,000 or less and a component content of 12% by mass or more and an ethylene component content of 10% by mass or less is used. Use.

The content of the component having a molecular weight of 100,000 or less in the polypropylene resin is 12% by mass or more, preferably 14% by mass or more, and more preferably 16% by mass or more. If the content of the component having a molecular weight of 100,000 or less in the polypropylene resin is less than 12% by mass, the porosity of the porous film obtained by stretching and porosifying the precursor film becomes insufficient. It ends up. The upper limit of the content of the component having a molecular weight of 100,000 or less in the polypropylene resin is not particularly limited, but it is said that the strength of the porous film obtained by stretching and porosifying the precursor film is sufficient. From the viewpoint, it is preferably 35% by mass or less, more preferably 30% by mass or less. The proportion of the component having a molecular weight of 100,000 or less can be obtained from the distribution of the molecular weight obtained as a polystyrene-equivalent value from the result of gel permeation chromatography (GPC) measurement.

The content of the ethylene component in the polypropylene resin is 10% by mass or less, preferably 5% by mass or less, and more preferably 3% by mass or less. When the content of the ethylene component exceeds 10% by mass, it becomes difficult to form pores when the precursor film having the polypropylene-based resin layer containing the polypropylene-based resin is stretched and made porous, and the obtained porous film becomes difficult to form. The air permeability of the film becomes insufficient, and the charge / discharge characteristics become insufficient when it is used as a separator for a power storage device. The lower limit of the content of the ethylene component in the polypropylene resin is not particularly limited, but it is preferably substantially 0% by mass.

The weight average molecular weight (Mw) of the polypropylene-based resin is not particularly limited, but is preferably 250,000 to 1.2 million, more preferably 500,000 to 1,000,000, still more preferably 500,000 to 800,000, and particularly preferably. It is 550,000 to 800,000. According to the polypropylene-based resin having a weight average molecular weight in the above range, it is possible to provide a porous film having good pores even by a coextrusion method. In particular, when a polypropylene resin having a weight average molecular weight of 600,000 to 750,000 is used, the mechanism is not always clear, but the occurrence of tearing can be suppressed more appropriately when producing a porous film. As a result, the production of the porous film can be performed more stably.

The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the polypropylene resin is not particularly limited, but is preferably 7.0 to 15.0, more preferably 7.5 to 14.5, and further. It is preferably 8.0 to 14.0. By setting the molecular weight distribution in the above range, the aperture ratio of the porous film obtained by stretching and porosifying the precursor film can be further increased, and thereby the galley value of the porous film can be further increased. .. The weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polypropylene-based resin can be determined as polystyrene-equivalent values by, for example, gel permeation chromatography (GPC) measurement.

The mesopentad fraction of the polypropylene resin is not particularly limited, but is preferably 90% or more, more preferably 92% or more, still more preferably 94% or more, and particularly preferably 96% or more. According to the polypropylene resin having a pentad fraction of 90% or more, the lamella crystals can be grown more appropriately when the precursor film is produced by the coextrusion method, whereby the precursor film is stretched and porous. When the film is formed into a porous film, the porous film can have a good porosity. The upper limit of the mesopentad fraction is not particularly limited, but is preferably 98% or less, and more preferably 97.8% or less. The mesopentad fraction means, for example, the ratio of five consecutive propylene monomer units having the same three-dimensional structure in the polypropylene-based resin quantified based on the peak assignment of the ¹³ C-nuclear magnetic resonance spectrum. It is commonly used as an indicator of regularity.

As a method for producing the polypropylene-based resin for forming the polypropylene-based resin layer, a conventionally known method can be used and is not particularly limited. For example, the polypropylene-based resin can be produced by using a catalyst such as a Ziegler-Natta catalyst or a metallocene catalyst, and a method using a Ziegler-Natta catalyst is particularly preferable. The polymerization method may be either single-stage or multi-stage. The content, weight average molecular weight, molecular weight distribution, mesopentad fraction, etc. of a polypropylene resin having a molecular weight of 100,000 or less can be controlled by adjusting various production conditions such as polymerization temperature and catalyst amount. .. Further, the content of the ethylene component in the polypropylene resin can be controlled by adjusting the amount of ethylene used for the polymerization.

[Precursor film]
The precursor film of the present invention includes a polyethylene-based resin layer containing the above-mentioned polyethylene-based resin (hereinafter, PE layer) and a polypropylene-based resin layer containing the above-mentioned polypropylene-based resin (hereinafter, PP layer). The structure of the precursor film is not particularly limited as long as it has a PP layer on the PE layer, for example, PE layer / PP layer / PE layer, PP layer / PE layer / PP layer. Examples thereof include a three-layer structure, a five-layer structure such as a PP layer / PE layer / PP layer / PE layer / PP layer, and a PE layer / PP layer / PE layer / PP layer / PE layer. Among these, a configuration in which a PP layer containing a pair of polypropylene-based resins is arranged on both sides of a PE layer containing a polyethylene-based resin, that is, a three-layer structure of a PP layer / PE layer / PP layer is preferable.

The thickness of the precursor film is not particularly limited, but is preferably 8 to 40 μm, and more preferably 12 to 35 μm. By setting the thickness of the precursor film in the above range, the porous film obtained by stretching and porousning such a precursor film shall have good pores with a film thickness of 10 to 30 μm. Can be done.

[Manufacturing method of precursor film]
The precursor film of the present invention can be produced by a method of forming a film by collectively extruding the above-mentioned polyethylene-based resin and the above-mentioned polypropylene-based resin into a film, that is, a method of forming a film by coextrusion. it can. For example, in the case of producing a precursor film in which the PE layer is used as an intermediate layer and PP layers are provided on both sides thereof, a polyethylene resin is used so that the PE layer is used as an intermediate layer and PP layers are formed on both sides thereof. And a polypropylene-based resin can be produced by forming a film by coextrusion.

When coextruding a polyethylene resin and a polypropylene resin, the apparatus used for coextrusion is not particularly limited, and conventionally known devices can be used. Examples of such a coextruding device include a coextruder equipped with a circular die, a coextruder equipped with a feed block or a multi-manifold type T-die, and the like.

The die temperature (that is, the film forming temperature) at the time of coextrusion using a coextruder is preferably 185 to 240 ° C, more preferably 190 to 235 ° C, and further preferably 195 to 230 ° C. By setting the die temperature to 185 ° C. or higher, a precursor film can be formed without breaking during film formation. Further, by setting the die temperature to 240 ° C. or lower, deterioration of the polypropylene resin due to heat can be reduced, and deterioration of the characteristics of the finally obtained porous film can be suppressed.

[Porous film]
The porous film of the present invention is a microporous film obtained by stretching and porosifying the precursor film of the present invention described above, and is preferably used as a separator for a power storage device, for example. is there.

The porous film of the present invention has a film thickness of preferably 40 μm or less, more preferably 35 μm or less, still more preferably 30 μm or less. When the film thickness is 40 μm or less, the precursor film is easily opened when the porous film is produced, which is preferable because a porous film having a low galley value (air permeability) can be more easily obtained. The thinner the film thickness of the precursor film, the easier it is to open holes. However, when producing a porous film having a film thickness of less than 10 μm, it is difficult to uniformly control the film thickness. Since it is often the case, the lower limit of the film thickness of the porous film is preferably 10 μm or more.

The porosity of the porous film of the present invention is not particularly limited, but is preferably 45 to 70%, more preferably 48 to 65%, and most preferably 48 to 60%.

When the porous film of the present invention is used as a separator for a power storage device, the galley value (air permeability) is preferably 400 seconds / 100 cc or less, and more preferably 380 seconds / 100 cc or less. When the galley value of the porous film is 400 seconds / 100 cc or less, an increase in impedance during high-temperature storage can be effectively suppressed in a power storage device using the porous film as a separator. If the galley value of the porous film is too low, a short circuit is likely to occur when the power storage device is operated in the power storage device using the porous film as the separator. Therefore, the galley value of the porous film is preferably 50 seconds / 100 cc or more, more preferably 65 seconds / 100 cc or more, and further preferably 75 seconds / 100 cc or more.

[Manufacturing method of porous film]
The porous film of the present invention is produced by stretching and porosifying the precursor film of the present invention described above.

When the precursor film is stretched and porous, it is preferable to heat-treat the precursor film in advance before stretching and porous. By subjecting the precursor film to heat treatment in advance, the polyethylene-based resin and polypropylene-based resin constituting the precursor film can be crystallized, whereby the precursor film can be made more suitable for porosity by stretching, and can be stretched. The porosity can be more preferably performed. Further, the crystallinity of the precursor film can be controlled by the heat treatment conditions, and the pore-opening characteristics of the porous film obtained by the stretched porosity can be adjusted by controlling the crystallinity by the heat treatment conditions. Can (ie, can be controlled indirectly).

Examples of the heat treatment method include a method of bringing the precursor film into contact with a preheated roll, a method of passing the precursor film under an environment heated to a predetermined temperature, and the like, and an appropriate method is used. It is good.

The heat treatment temperature is preferably 110 ° C. or higher and 145 ° C. or lower, more preferably 128 ° C. or higher and 140 ° C. or lower, and further preferably 130 ° C. or higher and 136 ° C. or lower. By setting the heat treatment temperature in the above range, the crystallinity of the precursor film can be increased more appropriately, and the galley value (air permeability) of the obtained porous film can be made sufficiently low. The heat treatment time is preferably 20 seconds to 60 minutes, more preferably 40 seconds to 40 minutes, and further preferably 70 seconds to 20 minutes. The heat treatment time may be appropriately determined according to the temperature of the heat treatment described above.

The layer thickness ratio between the PE layer and the PP layer may be appropriately changed according to the purpose. For example, in the case of a three-layer structure of PP layer / PE layer / PP layer, the PP layer on each surface layer is formed from the viewpoint of surely performing heat treatment of the PE layer to form good pores in the PP layer. The thickness of is at least the same as the thickness of the PE layer, or thinner than the thickness of the PE layer. With such a layer structure, the PE layer can be reliably heat-treated, and good pores can be formed in the PE layer during stretching. Further, with such a configuration, when the obtained porous film is incorporated into a power storage device as a separator, when the power storage device generates abnormal heat, a sufficient amount of polyethylene resin is melted, thereby forming a PP layer. The formed holes can be appropriately closed, and the battery function can be appropriately lost (shut down).

The lamellar thickness of the PE layer of the heat-treated precursor film is a value measured by the small-angle X-ray scattering method, preferably 150 to 300 Å, and the lower limit thereof is more preferably 170 Å or more, and 180 Å or more. It is even more preferable to have. When the lamella thickness is 150 Å or more, pores having a relatively large pore diameter can be formed in the PE layer by undergoing the stretching and porous treatment, and the galley value (air permeability) of the obtained porous film can be sufficiently obtained. Can be as low as possible. On the other hand, when the lamella thickness of the PE layer is less than 150 Å, the obtained porous film has good withstand voltage characteristics, but the galley value becomes too high, and the porous film is used as a separator for a power storage device. If so, there is a possibility that it will not be suitable as a separator for a power storage device. It is also not preferable from the viewpoint of withstand voltage characteristics.

Then, the precursor film heat-treated in this way is made porous by stretching (stretched porous), whereby a porous film can be obtained.

The stretching method in stretching and porous is not particularly limited, but is next to the uniaxial stretching method in the mechanical direction (MD), the uniaxial stretching method in the width direction (TD) substantially perpendicular to the mechanical direction, and the mechanical direction (MD). Examples thereof include a sequential biaxial stretching method of stretching in the width direction (TD), a simultaneous biaxial stretching method of stretching in the mechanical direction (MD) and the width direction (TD) almost simultaneously, and a tubular biaxial stretching method. Among these, it is advisable to adopt a stretching method suitable for the purpose.

As a specific method of stretching and porosity, for example, the heat-treated precursor film is stretched at a low temperature in a low-temperature stretching zone and then stretched in a high-temperature stretching zone at a temperature higher than that of the low-temperature stretching. This includes a method of obtaining a porous film. Only one of the low-temperature stretching and the high-temperature stretching may not be sufficient to make both the polypropylene-based resin and the polyethylene-based resin sufficiently porous, and a porous film may not be formed.

The temperature of the low temperature stretching is not particularly limited, but is preferably −20 ° C. or higher and + 50 ° C. or lower, and more preferably + 20 ° C. or higher and + 40 ° C. or lower. If the temperature of low-temperature stretching is too low, the precursor film is likely to break during stretching, which is not preferable. On the other hand, if the temperature of low-temperature stretching is too high, the polyethylene-based resin in the precursor film is difficult to open, which is not preferable.

The draw ratio for low-temperature stretching is not particularly limited, but is preferably in the range of 3% or more and 200% or less, and more preferably in the range of 5% or more and 100% or less. When the draw ratio of low-temperature stretching is 3% or more, it becomes easy to obtain a porous film having a sufficiently low galley value. On the other hand, when the draw ratio of low-temperature stretching exceeds 200%, the generated craze changes into cracks, which causes film rupture. Therefore, the draw ratio of low temperature stretching is preferably 200% or less.

The temperature of high-temperature stretching is preferably 70 ° C. or higher and 150 ° C. or lower, and more preferably 80 ° C. or higher and 145 ° C. or lower. By setting the temperature of high-temperature stretching within this range, the porosity in high-temperature stretching can be made sufficient, and a porous film having a sufficiently low galley value can be easily obtained.

The draw ratio for high-temperature stretching is not particularly limited, but is preferably in the range of 100% or more and 400% or less. If the draw ratio of high-temperature stretching is too low, the galley value of the porous film may not be sufficiently low. Further, if the draw ratio of high-temperature stretching is too high, the galley value of the porous film may become too low.

According to the present invention, by adopting such a production method, a porous film having a film thickness of 18 μm or less and a galley value of 400 seconds or less can be suitably produced.

As described above, according to the present invention, there is provided a precursor film capable of producing a porous film having excellent air permeability with high quality, and a porous film obtained by using such a precursor film. In particular, the precursor film of the present invention can be suitably produced by a coextrusion method in which a polyethylene-based resin and a polypropylene-based resin are collectively extruded in a film-like state. Therefore, according to the present invention, a precursor film can be produced with high efficiency, and further, when the film is stretched and made porous to obtain a porous film, the film is torn (that is, peculiar to the laminating method). It is possible to suppress defects), thereby producing a porous film with high quality.

[Power storage device]
The power storage device of the present invention includes the above-mentioned separator for a power storage device containing the porous film of the present invention, a positive electrode, and a negative electrode.

(Non-aqueous electrolyte)
The power storage device of the present invention usually contains a non-aqueous electrolytic solution in addition to the above-mentioned separator for a power storage device containing the porous film of the present invention, a positive electrode, and a negative electrode. Preferred examples of the non-aqueous solvent used in the non-aqueous electrolyte solution include cyclic carbonates and chain esters. It preferably contains a chain ester, more preferably a chain carbonate, and contains both a cyclic carbonate and a chain carbonate, as it synergistically improves the electrochemical properties over a wide temperature range, especially at high temperatures. Is most preferable. The term "chain ester" is used as a concept including a chain carbonate and a chain carboxylic acid ester.

Examples of the cyclic carbonate include one or more selected from ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC), and a combination of EC and VC and a combination of PC and VC are particularly preferable.

Further, when the non-aqueous solvent contains ethylene carbonate and / or propylene carbonate, the stability of the film formed on the electrode is increased, and the high temperature and high voltage cycle characteristics are improved, which is preferable. The content of ethylene carbonate and / or propylene carbonate is preferably 3% by volume or more, more preferably 5% by volume or more, still more preferably 7% by volume or more, based on the total volume of the non-aqueous solvent. The upper limit thereof is preferably 45% by volume or less, more preferably 35% by volume or less, and further preferably 25% by volume or less.

Examples of the chain ester include methyl ethyl carbonate (MEC) as the asymmetric chain carbonate, dimethyl carbonate (DMC) and diethyl carbonate (DEC) as the symmetric chain carbonate, and ethyl acetate (hereinafter, EA) as the chain carboxylic acid ester. Is preferably mentioned. Among the chain esters, a combination of asymmetric and ethoxy group-containing chain esters such as MEC and EA is possible.

The content of the chain ester is not particularly limited, but it is preferably used in the range of 60 to 90% by volume with respect to the total volume of the non-aqueous solvent. If the content of the chain ester is 60% by volume or more, the viscosity of the non-aqueous electrolyte solution does not become too high, and if it is 90% by volume or less, the electrical conductivity of the non-aqueous electrolyte solution decreases and a wide temperature range, particularly The above range is preferable because there is little possibility that the electrochemical properties at high temperatures will deteriorate.

The volume ratio of EA in the chain ester is preferably 1% by volume or more, more preferably 2% by volume or more in a non-aqueous solvent. The upper limit is more preferably 10% by volume or less, and further preferably 7% by volume or less. The asymmetric chain carbonate is more preferably having an ethyl group, and methyl ethyl carbonate is particularly preferable.
The ratio of the cyclic carbonate to the chain ester is preferably 10:90 to 45:55, preferably 15:85, from the viewpoint of improving the electrochemical properties in a wide temperature range, particularly at a high temperature. ~ 40:60 is more preferable, and 20:80 to 35:65 is particularly preferable.

As the electrolyte salt used in the non-aqueous electrolyte solution, a lithium salt is preferably mentioned. As the lithium salt, one or more selected from the group consisting of LiPF ₆ , LiBF ₄ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ is preferable, and LiPF ₆ , LiBF ₄ and LiN ( SO _{2 F)} 1 kind or more preferably two or more selected from _2, it is more preferred to use LiPF _6.

The non-aqueous electrolyte solution can be obtained, for example, by mixing the above-mentioned non-aqueous solvent and adding a composition obtained by mixing the above-mentioned electrolyte salt and solubilizing agent at a specific mixing ratio. At this time, it is preferable that the compound to be added to the non-aqueous solvent and the non-aqueous electrolytic solution to be used is purified in advance within a range that does not significantly reduce the productivity and has as few impurities as possible.

The separator for a power storage device containing the porous film of the present invention can be used, for example, in the following first and second power storage devices. Further, in this case, as the non-aqueous electrolyte, not only a liquid electrolyte but also a gelled electrolyte can be used. Among them, it is preferably used as a separator for a lithium ion battery (first power storage device) or a lithium ion capacitor (second power storage device) that uses a lithium salt as an electrolyte salt, and more preferably for a lithium ion battery. , It is more preferable to use it for a lithium ion secondary battery.

(First power storage device)
The lithium ion secondary battery as the first power storage device has a positive electrode, a negative electrode, and the non-aqueous electrolytic solution described above. Constituent members such as a positive electrode and a negative electrode can be used without particular limitation.

For example, as the positive electrode active material for a lithium ion secondary battery, a composite metal oxide containing one or more selected from the group consisting of cobalt, manganese, and nickel is used. These positive electrode active materials may be used alone or in combination of two or more.

Examples of such lithium composite metal oxides include LiCoO ₂ , LiCo _1-x M _x O ₂ (where M is Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, and One or more elements selected from Cu), LiMn ₂ O ₄ , LiNiO ₂ , LiCo _1-x NixO ₂ , LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _{0 .3} Co _0.2 Mn _0.3 O ₂ , LiNi _0.8 Mn _0.1 Co _0.1 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , Li ₂ MnO ₃ and LiMO ₂ (M is a transition metal such as Co, Ni, Mn, Fe) and one or more selected from LiNi _1/2 Mn _3/2 O ₄ are preferably mentioned.

For the positive electrode, the above-mentioned positive electrode active material is acetylene black, a conductive agent, and polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a styrene and butadiene copolymer (SBR), and an acrylonitrile and butadiene copolymer (S). After mixing with a binder such as NBR) and carboxymethyl cellulose (CMC), adding a solvent to the binder and kneading the mixture to make a positive electrode mixture, this positive electrode mixture is applied to an aluminum foil of a current collector, a stainless steel plate, etc. It can be produced by coating, drying, pressure molding, and then heat-treating under predetermined conditions. The conductive agent is not particularly limited as long as it is an electron conductive material that does not cause a chemical change, and is, for example, one or more selected from natural graphite (scaly graphite, etc.), graphite such as artificial graphite, acetylene black, and the like. Carbon black and the like.

Negative negative active materials for lithium ion secondary batteries include lithium metals, lithium alloys, and carbon materials capable of occluding and releasing lithium, tin (single unit), tin compounds, silicon (single unit), silicon compounds, or Li. ₄ Lithium titanate compounds such as Ti ₅ O ₁₂ can be used alone or in combination of two or more.

Among these, it is more preferable to use a highly crystalline carbon material such as artificial graphite or natural graphite from the viewpoint of high ability to occlude and release lithium ions.

In particular, artificial graphite particles having a massive structure in which a plurality of flat graphite fine particles are assembled or bonded in a non-parallel manner to each other, and mechanical actions such as compressive force, frictional force, and shearing force are repeatedly applied to form scaly natural graphite spherical. It is preferable to use the treated particles.

The negative electrode is kneaded with a conductive agent, a binder, and a high boiling point solvent similar to those for producing the positive electrode described above to form a negative electrode mixture, and then this negative electrode mixture is applied to a copper foil or the like of a current collector. It can be produced by drying, pressure molding, and then heat-treating under predetermined conditions.

The structure of the lithium ion secondary battery as the first power storage device is not particularly limited, and a coin-type battery, a cylindrical battery, a square battery, a laminated battery, or the like can be applied.

The wound lithium ion secondary battery has, for example, a configuration in which an electrode body is housed in a battery case together with a non-aqueous electrolyte solution. The electrode body is composed of a positive electrode, a negative electrode, and a separator. At least a part of the non-aqueous electrolyte solution is impregnated in the electrode body.

In the winding type lithium ion secondary battery, the positive electrode includes a long sheet-shaped positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material and provided on the positive electrode current collector. The negative electrode includes a long sheet-shaped negative electrode current collector and a negative electrode mixture layer containing a negative electrode active material and provided on the negative electrode current collector. The separator is formed in the shape of a long sheet like the positive electrode and the negative electrode. The positive electrode and the negative electrode are wound in a tubular shape with a separator interposed therebetween.

The battery case includes a bottomed cylindrical case body and a lid that closes an opening of the case body. The lid and case body are made of metal, for example, and are insulated from each other. The lid is electrically connected to the positive electrode current collector, and the case body is electrically connected to the negative electrode current collector. The lid may serve as the positive electrode terminal, and the case body may serve as the negative electrode terminal.

The lithium ion secondary battery can be charged and discharged at -40 to 100 ° C, preferably -10 to 80 ° C. Further, as a countermeasure against an increase in the internal pressure of the wound lithium ion secondary battery, a method of providing a safety valve on the battery lid or a method of making a notch in a member such as a battery case body or a gasket can be adopted. Further, as a safety measure to prevent overcharging, a current cutoff mechanism that senses the internal pressure of the battery and cuts off the current can be provided on the lid.

As an example, the manufacturing procedure of the lithium ion secondary battery will be described below.
First, a positive electrode, a negative electrode, and a separator are produced. Next, the electrode body is assembled by superimposing them and winding them in a cylindrical shape. Next, the electrode body is inserted into the case body, and the non-aqueous electrolyte solution is injected into the case body. As a result, the electrode body is impregnated with the non-aqueous electrolytic solution. After injecting the non-aqueous electrolyte solution into the case body, the case body is covered with a lid, and the lid and the case body are sealed. The shape of the electrode body after winding is not limited to the cylindrical shape. For example, the positive electrode, the separator, and the negative electrode may be wound and then pressure may be applied from the side to form a flat shape.

The above-mentioned lithium ion secondary battery can be used as a secondary battery for various purposes. For example, it is mounted on a vehicle such as an automobile and can be suitably used as a power source for a drive source such as a motor for driving the vehicle. The type of vehicle is not particularly limited, and examples thereof include a hybrid vehicle, a plug-in hybrid vehicle, an electric vehicle, and a fuel cell vehicle. Such a lithium ion secondary battery may be used alone, or may be used by connecting a plurality of batteries in series and / or in parallel.

Although the wound lithium ion secondary battery has been illustrated above, the present invention is not limited to such an embodiment and may be applied to a laminated lithium ion secondary battery.

(Second power storage device)
Further, as a second power storage device, a lithium ion capacitor can be mentioned. The lithium ion capacitor has, as a separator, a separator for a power storage device having the porous film of the present invention described above, a non-aqueous electrolyte solution, a positive electrode, and a negative electrode. The lithium ion capacitor can store energy by utilizing the intercalation of lithium ions with a carbon material such as graphite which is a negative electrode. Examples of the positive electrode include those using an electric double layer between the activated carbon electrode and the electrolytic solution, those using the doping / dedoping reaction of the π-conjugated polymer electrode, and the like. The electrolytic solution contains at least a lithium salt such as LiPF ₆ .

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples, and includes various combinations that can be easily inferred from the gist of the invention.

Using the polyethylene-based resin and polypropylene-based resin having the characteristics shown in Table 1, the precursor films of Examples 1 to 16 and Comparative Examples 1 to 4 were molded by the methods shown below, and then each precursor film was stretched. By making the film porous, the porous films of Examples 1 to 16 and Comparative Examples 1 to 4 were formed. The precursor film and the porous film of Examples 1 to 16 and Comparative Examples 1 to 4 had a three-layer structure having a PP layer / PE layer / PP layer.

Each characteristic was measured by the method shown below.

[Weight average molecular weight (Mw), molecular weight distribution (Mw / Mn) of polypropylene resin, content ratio of components with molecular weight of 100,000 or less]
The weight average molecular weight (Mw), molecular weight distribution (Mw / Mn), and content ratio of components having a molecular weight of 100,000 or less of the polypropylene resin were determined by standard polystyrene conversion using a gel permeation chromatograph manufactured by Agilent. Two Agilent PLgel Olexis were used as columns, and the measurement was carried out at 145 ° C. in ortodichlorobenzene prepared at 0.05 wt / vol%. A differential refractometer (RI) was used as the detector.

[Mesopentad fraction [mmmm] of polypropylene resin (NMR (nuclear magnetic resonance) measurement)]
Polypropylene resin was dissolved in ODCB (1,2-dichlorobenzene) / C ₆ D ₆ (4/1) solvent at a concentration of 10 wt / vol%, and ¹³ C-NMR measurement was performed at a resolution of 100 MHz, a temperature of 130 ° C., and the number of integrations. It was carried out under the condition of 8000 times, and the mesopentad fraction was calculated by the peak height method.

[Content ratio of ethylene component in polypropylene resin]
¹³ C-NMR measurement was performed in the same manner as the above mesopentad fraction measurement, and from the ¹³ C-NMR spectrum assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry), the peak height method was used. The content ratio of the ethylene component in the polypropylene resin was determined. In this measurement, the ratio of those having no branched structure was defined as the content ratio of the ethylene component.

[Weight average molecular weight (Mw) of polyethylene resin]
The weight average molecular weight (Mw) of the polyethylene-based resin was determined by standard polystyrene conversion using a gel permeation chromatograph manufactured by Agilent. Two Agilent PLgel Olexis were used as columns, and the measurement was carried out at 145 ° C. in ortodichlorobenzene prepared at 0.05 wt / vol%. A differential refractometer (RI) was used as the detector.

[Density of polyethylene resin]
The density of the polyethylene resin (kg / m ³ ) was measured under the condition of 25 ° C. in accordance with JIS K7112.

[Semi-crystallization time of polyethylene resin at 124 ° C. (t1 / 2)]
The polyethylene resin is heated from a scanning temperature range of 50 ° C. to 250 ° C. at a heating rate of 200 ° C./min using an input compensation type differential scanning calorimeter (trade name: DSC8500) manufactured by PerkinElmer. After scanning, heat treatment was performed for 5 minutes. Then, the temperature was lowered to 124 ° C. at −500 ° C./min, and the temperature was kept for 90 minutes. Then, the area of the crystallization heat generation energy ΔH at this time was integrated, and the time when the area was halved was defined as the semi-crystallization time (t1 / 2).

[Crystallization heat generation energy of polyethylene resin at 124 ° C (ΔH)]
For polyethylene resin, use a PerkinElmer input compensation type differential scanning calorimeter (trade name: DSC8500) to raise the temperature from 50 ° C to 250 ° C in the scanning temperature range at a heating rate of 200 ° C / min, and then perform heat treatment scanning. Later, heat treatment was performed for 5 minutes. Then, the temperature was lowered to 124 ° C. at −500 ° C./min, and the temperature was kept for 90 minutes. Then, the exothermic energy at this time was defined as the crystallization exothermic energy (ΔH).

[Measurement of differential scanning calorimetry (DSC heat generation energy) of polyethylene resin]
Differential scanning of polyethylene-based resin using an input-compensated differential scanning calorimeter (trade name: Diamond DSC) manufactured by PerkinElmer under a scanning temperature range of 30 ° C to 230 ° C and 10 ° C / min. The DSC calorific value was measured by performing calorie analysis.

[Film thickness of precursor film and porous film]
The film thicknesses of the precursor film and the porous film were measured using a dial gauge (manufactured by Ozaki Seisakusho Co., Ltd., trade name "PEACOCK No. 25").

[Gare value (air permeability) of porous film]
A test piece of 80 mm in MD and full width was collected from the manufactured porous film, and a B-type galley type densometer (manufactured by Toyo Seiki Co., Ltd.) was collected at the center and the left and right ends (50 mm inside from the end face). The measurement was carried out according to JIS P8117. The average value of the three points was evaluated as the galley value. In addition, the galley value per 1 μm of thickness was also calculated from the results of the film thickness of the porous film measured above and the galley value.

[Porosity of porous film]
Two 100 mm × 100 mm test pieces were collected from both ends in the width direction of the porous film using a mold, and the weight of each of the two collected test pieces was measured up to 0.1 mg. The porosity was calculated from the measured weight using the following formula.
Pore ratio (%) = {(1-Test piece weight (g) / Density (g / cm ³ )) / (Test piece area (100 cm ² ) x film thickness (cm))} x 100

<Example 1>
Using polypropylene-based resin and polyethylene-based resin having the characteristics shown in Table 1, using a co-extruder equipped with a multi-manifold type T-die, the PE layer is an intermediate layer at a die temperature (co-extrusion temperature) of 210 ° C. By co-extruding the polyethylene-based resin and the polypropylene-based resin so that PP layers are formed on both sides thereof, a precursor film having a three-layer structure of PP layer / PE layer / PP layer was obtained. .. The thickness ratio of each layer in the precursor film was PP layer / PE layer / PP layer = 1/1/1.

Then, the obtained precursor film was heat-treated in a temperature range of 130.2 ° C. to 135.7 ° C. for 30 minutes, and then the heat-treated precursor film was subjected to 18% in a cold stretching zone at 35 ° C. It was stretched at a low temperature at the stretching ratio (initial stretching ratio) of. Then, the precursor film stretched at a low temperature is stretched at a high temperature in a heat stretching zone at 130 ° C. at a stretching ratio of 190% (maximum stretching ratio), and then heat-relaxed until the stretching ratio reaches 125% (final stretching ratio). It was. Then, a porous film was prepared by heat-fixing at a temperature of 133 ° C. Then, with respect to the produced porous film, the galley value (total galley value) was measured and the galley value per 1 μm thickness was calculated. In this example, the precursor film was stretched by a uniaxial stretching method in the mechanical direction (MD). The results are shown in Table 1.

<Examples 2 to 16, Comparative Examples 1 to 4>
The same as in Example 1 except that the polypropylene-based resin and the polyethylene-based resin having the characteristics shown in Table 1 were used and the thickness ratio of the PP layer / PE layer / PP layer was as shown in Table 1. , Precursor film and porous film were prepared and evaluated in the same manner. The results are shown in Table 1.
In each of the polyethylene-based resins used in Examples 1 to 16, the content of the propylene component and the content ratio of the butene component were both substantially 0% by mass.

As shown in Table 1, according to Examples 1 to 16, a precursor film by co-extrusion can be suitably produced by using a polypropylene-based resin and a polyethylene-based resin having the characteristics shown in Table 1, respectively. Furthermore, by using the precursor film thus obtained, it was possible to obtain a porous film having a galley value of 400 seconds / 100 cc or less and a film thickness of 25 μm or less. In Examples 1 to 16, when the porous film was produced, the porous film could be reliably formed without tearing in the middle of the production. Moreover, when the porosity was measured for the porous films obtained in Examples 1 to 16, the porosity was in the range of 45 to 70%. From this result, it can be said that the porous films obtained in Examples 1 to 16 can be suitably used as, for example, a separator for a lithium ion battery.

On the other hand, in Comparative Examples 1 and 2 in which the polyethylene-based resin having a semi-crystallization time (t1 / 2) at 124 ° C. of more than 400 seconds was used, the PE layer made of the polyethylene-based resin was sufficiently opened. It did not progress, and the result was that the galley value exceeded 10000 seconds / 100 cc.
Further, in Comparative Example 3 in which the polypropylene-based resin having an ethylene component content of more than 10% by mass was used, the opening of the PP layer made of the polypropylene-based resin did not proceed sufficiently, and the galley value was 10,000. The result exceeded seconds / 100 cc.
Further, in Comparative Example 4 in which a polypropylene-based resin having a molecular weight of 100,000 or less and a content of less than 12% by mass of a component was used, a PE layer made of a polyethylene-based resin and a PP layer made of a polypropylene-based resin were used. In all of the above, although the pores were opened, the galley value exceeded 400 seconds / 100 cc, which was an insufficient result.

The precursor film of the present invention can be used for producing a porous film preferably used as a separator for a power storage device including a lithium ion battery, a lithium ion capacitor and the like.

Claims (7)

A precursor film for a porous film, comprising a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin.
The polyethylene-based resin contained in the polyethylene-based resin layer has a semi-crystallization time (t1 / 2) of 400 seconds or less in an isothermal crystallization test at 124 ° C.
A porous film in which the polypropylene-based resin contained in the polypropylene-based resin layer has a molecular weight of 100,000 or less and a component content of 12% by mass or more and an ethylene component content of 10% by mass or less. Precursor film for. The precursor film for a porous film according to claim 1, wherein the crystallization heat generation energy (ΔH) of the polyethylene resin at 124 ° C. is 110 J / g or more. The precursor film for a porous film according to claim 1 or 2, wherein the polypropylene-based resin has a weight average molecular weight (Mw) of 250,000 to 1.2 million. The precursor film for a porous film according to any one of claims 1 to 3, wherein the polypropylene-based resin has a mesopentad fraction of 90 to 98%. A porous film obtained by stretching and porous a precursor film for a porous film according to any one of claims 1 to 4. The separator for a power storage device having the porous film according to claim 5. A power storage device including the separator for a power storage device according to claim 6, a positive electrode, and a negative electrode.