JP6682942B2 - Polypropylene resin porous film and method for producing the same - Google Patents

Description

  The present invention relates to a polypropylene resin porous film having excellent air permeability and mechanical strength, which can be suitably used for a separator for batteries, particularly for lithium ion secondary batteries, and a method for producing the same.

Demand for secondary batteries has increased along with the adoption of OA (Office Automation) and FA (Factor Automation), and is also widely used as a power source for portable appliances such as household electric appliances and communication equipment. In particular, in portable devices, the use of lithium ion secondary batteries is increasing because they are excellent in volumetric efficiency and the size and weight of the devices to be loaded can be reduced.
On the other hand, in the fields related to energy and environment, for example, research and development of load leveling, an uninterruptible power supply device such as so-called UPS (Uninterruptible Power Supply), and a large secondary battery in an electric vehicle are being advanced, and a large capacity From the viewpoints of high output, high voltage, and excellent long-term storage stability, lithium ion secondary batteries, which are a type of non-aqueous electrolyte secondary battery, are widely used.

  A battery separator is provided between the positive electrode and the negative electrode of the lithium ion secondary battery from the viewpoint of preventing an internal short circuit. This battery separator is naturally required to have an insulating property due to its role, and is further required to have an air permeability to serve as a passage for lithium ions and a function of diffusing and holding an electrolytic solution. In order to meet these requirements, the battery separator needs to have a fine pore structure, and a porous film is used.

  Further, in the lithium ion secondary battery separator, as described above, even when the battery is porous and has a thermal runaway due to an abnormality in the battery, a membrane rupture or contraction occurs, causing a gap between the electrodes. It is required to prevent accidents such as ignition due to short circuit of the battery and abnormal heat generation of the battery. Therefore, in order to improve the safety of the separator, a porous film having higher mechanical strength, specifically, high puncture strength is required.

Such a porous film generally contains a polyolefin-based resin as a main component, and the main production methods thereof are roughly classified into three types: a wet method, a dry uniaxial stretching method, and a dry biaxial stretching method.
Since the wet method can use a polymer having an ultrahigh molecular weight, a porous film having high puncture strength can be obtained, but the manufacturing cost is high because a step of extracting the plasticizer with a solvent is required for porosification. Had a problem.

  On the other hand, for example, Patent Documents 1 to 5 disclose a dry uniaxial stretching method or a dry biaxial stretching method, and these dry methods can be manufactured using a general-purpose stretching device. Therefore, the cost is lower than the wet method and the productivity is excellent.

Japanese Patent Publication No. 2003-517723 JP, 2014-70092, A JP, 2013-23673, A JP, 11-297297, A JP, 2005-4017, A

  Although Patent Documents 1 to 3 describe obtaining a polypropylene resin porous film by a dry uniaxial stretching method, the method described in Patent Document 1 cannot provide sufficient puncture strength. Further, in Patent Documents 2 and 3, although a higher piercing strength is obtained, an unstretched raw film is once wound up, annealed, and then cold-stretched and hot-stretched. Therefore, efficient continuous production is possible. Difficult and less productive.

On the other hand, Patent Document 4 describes that the laminated film produced by the dry biaxial stretching method has high puncture strength, but the puncture strength per 1 μm of thickness is not clear. Further, in the method described in the same document, since low-temperature stretching, high-temperature longitudinal stretching and high-temperature transverse stretching are carried out, the film is frequently rewound, efficient continuous production is difficult, and the stretching ratio is also low. Therefore, productivity is poor.
Further, although Patent Document 5 describes that a polypropylene-based resin porous film having β-crystal activity is obtained by a dry biaxial stretching method in which transverse stretching is performed after longitudinal stretching, a dry uniaxial stretching method is described. No puncture strength equal to or higher than that of

  Therefore, a battery separator is required to have a porous film having excellent air permeability and high puncture strength, and a method capable of efficiently producing such a film is also required. .

  The present invention has been made in order to solve the above problems, excellent in air permeability, and has a high puncture strength, also excellent in productivity, particularly preferably used for a battery separator It is an object of the present invention to provide a polypropylene-based resin porous film that can be manufactured and a method for producing the same.

  The porous film of the present invention was examined by focusing on the process conditions of the dry biaxial stretching method capable of continuous production, and based on the finding of a method capable of improving the puncture strength as compared with the conventional method, an excellent transparent film was obtained. It has both temper and high puncture strength.

That is, the present invention provides the following [1] to [7].
[1] The air resistance per 1 μm in thickness is 10 to 1000 seconds / 100 mL, and according to JIS K 7127: 1999, a test piece having a length of 80 mm, a width of 15 mm, and a thickness of 15 to 28 μm is used. , A distance between chucks of 40 mm and a tensile speed S _{MD of} the film in the machine direction (MD: Machine Direction) measured at a test speed of 200 mm / min, and a tensile strength in a direction (TD: Transverse Direction) perpendicular to the machine direction. A polypropylene resin porous film in which S _TD satisfies the relationships of the following formulas (1) and (2).
0.13 ≦ S _MD / S _TD ≦ 0.80 (1)
170 MPa ≤ S _MD + S _TD ≤ 400 MPa (2)
[2] The polypropylene resin porous film according to the above [1], which has a puncture strength of 21.5 gf or more per 1 μm in thickness.
[3] The polypropylene resin porous film according to the above [1] or [2], wherein the polypropylene resin has β-crystal activity.

[4] A battery separator having the polypropylene resin porous film according to any one of [1] to [3].
[5] A battery having the battery separator according to the above [4].

[6] A molding step of melt-kneading a polypropylene resin composition containing a β-crystal nucleating agent to form a polypropylene resin film, and a stretching step of stretching the film to obtain a porous film. In the stretching step, the film material is transversely stretched in a direction (TD) perpendicular to a flow direction (MD) of the film, and then longitudinally stretched in a flow direction (MD) of the film. The method for producing a polypropylene-based resin porous film, wherein the ratio of the transverse stretching ratio to the longitudinal stretching ratio is 3.0 or more.
[7] The method for producing a polypropylene-based resin porous film according to the above [6], wherein the transverse stretching ratio is 5.5 to 12.0 times.

The polypropylene resin porous film of the present invention is excellent in air permeability, has high puncture strength, and is also excellent in productivity. Therefore, this polypropylene-based resin porous film can be suitably used as a battery separator and can contribute to the improvement of battery safety.
Further, according to the production method of the present invention, the polypropylene-based resin porous film can be produced simply and efficiently.

Hereinafter, the present invention will be described in detail.
[Polypropylene resin porous film]
The polypropylene resin porous film of the present invention has an air resistance of 10 to 1000 seconds / 100 mL per 1 μm in thickness, and has a length of 80 mm in the measurement direction, a width of 15 mm and a thickness of 15 according to JIS K 7127: 1999. The tensile strength S _{MD in} the machine direction (MD) of the film and the direction perpendicular to the machine direction (TD) measured at a chuck distance of 40 mm and a test speed of 200 mm / min using a test piece of ˜28 μm. The tensile strength S _TD satisfies a predetermined relational expression.
The porous film of the present invention achieves both excellent air permeability and high puncture strength by satisfying predetermined tensile strengths in the MD and TD directions.

(Polypropylene resin)
Examples of the polypropylene-based resin used in the present invention include homopolypropylene (propylene homopolymer), and a random copolymer or block copolymer of propylene and another α-olefin. These may be used alone or in combination of two or more.
Specific examples of the α-olefin other than propylene, which is the component derived from the structural unit of each copolymer, include ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, and 1-nonene. , 1-decene and the like. These α-olefins may be used alone or in combination of two or more.
In each of the copolymers, the content of the constituent unit derived from the α-olefin (α-olefin content) is 10 mol% or less from the viewpoint of easily making the polypropylene resin porous by stretching. Is preferable, and more preferably 5 mol% or less. From the same viewpoint, the content of the structural unit derived from propylene in each of the copolymers is preferably 90 mol% or more, more preferably 95 mol% or more.

The polypropylene resin is preferably homopolypropylene from the viewpoint of maintaining mechanical strength and heat resistance. In this case, from the viewpoint of mechanical strength, the isotactic pentad fraction showing stereoregularity is preferably 80% or more, more preferably 85 to 99%, further preferably 90 to 99%.
Here, "isotactic pentad fraction" means that all of the five methyl groups that are side chains with respect to the main chain formed by carbon-carbon bonds composed of arbitrary five continuous propylene units have the same direction. Means the proportion of the three-dimensional structure located at. It is determined from the pentad unit in the polypropylene molecular chain measured by the signal in the methyl group region of the ¹³ C-NMR spectrum. The attribution of the signal in the methyl group region is based on the method described in A. Zambelli, et al., “Macromolecules” Volume 8, Issue 5, pp.687-689 (1975).

The molecular weight of the polypropylene resin has a weight average molecular weight (Mw) of 100,000 to 100,000 measured by high temperature gel permeation chromatography (high temperature GPC method) from the viewpoints of film forming stability, productivity, mechanical strength of the film, and the like. It is preferably 3,000,000, and more preferably 100,000 to 1,000,000.
From the same viewpoint, the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn), which is a parameter indicating the molecular weight distribution, is preferably 1.5 to 10.0, more preferably 2. It is 0 to 8.0, and more preferably 2.0 to 6.0.
The weight average molecular weight and the number average molecular weight are determined by the GPC method, and the closer the Mw / Mn is to 1, the narrower the molecular weight distribution.

The melt flow rate (MFR) of the polypropylene resin is not particularly limited, but is preferably 0.5 to 15 g / 10 minutes, more preferably 1.0 to 10 g / 10 minutes. . If it is 0.5 g / 10 minutes or more, the melt viscosity of the resin during molding is high, and good productivity can be secured. If it is 15 g / 10 minutes or less, the mechanical strength of the film can be sufficiently maintained.
The MFR is a value measured under the conditions of a temperature of 230 ° C. and a load of 2.16 kg in accordance with JIS K 7210-1: 2014.

  The method for producing the polypropylene resin is not particularly limited, and a known polymerization method using a polymerization catalyst may be used.For example, a multi-site catalyst typified by a Ziegler-Natta type catalyst or a metallocene catalyst. A polymerization method using a typical single-site catalyst can be used.

  Commercially available polypropylene-based resins include, for example, "Novatech PP", "Wintech" (manufactured by Japan Polypro Co., Ltd.); "Notio", "Tufmer XR" (manufactured by Mitsui Chemicals, Inc.); "Zelas", " Thermoran "(manufactured by Mitsubishi Chemical Co., Ltd.);" Sumitomo Noblen "," Tufselen "(manufactured by Sumitomo Chemical Co., Ltd.);" Prime Polypro "," Prime TPO "(manufactured by Prime Polymer Co., Ltd.);" Adflex "," Adsil " , "HMS-PP (PF-814)" (manufactured by Sun Allomer Co., Ltd.); "Inspire", "Versify" (manufactured by Dow Chemical Co.) and the like can be used.

Further, from the viewpoint of accelerating porosity and imparting molding processability, the polypropylene-based resin is a combination of two or more polypropylene-based resins, a modified polyolefin resin, an alicyclic saturated resin, and the like, as long as the effects of the present invention are not impaired. Hydrocarbon resin or its modified product, ethylene copolymer, low molecular weight polypropylene, thermoplastic elastomer and the like may be added. Examples of the thermoplastic elastomers include styrene / butadiene type, polyolefin type, urethane type, polyester type, polyamide type, 1,2-polybutadiene, polyvinyl chloride type, ionomer and the like.
Further, from the viewpoints of molding processability and productivity, improvement and adjustment of various physical properties of the porous film, etc., recycled resin, silica, talc, kaolin and calcium carbonate, which are caused by trimming loss, etc., within a range not impairing the effects of the present invention. Inorganic particles such as, pigments such as titanium oxide and carbon black, flame retardants, weather resistance stabilizers, heat resistance stabilizers, antistatic agents, melt viscosity improvers, crosslinking agents, lubricants, nucleating agents, plasticizers, antiaging agents, Various additives such as an antioxidant, a light stabilizer, an ultraviolet absorber, a neutralizing agent, an antifogging agent, an antiblocking agent, a slip agent and a coloring agent may be added.

  The polypropylene resin forming the porous film of the present invention preferably has β crystal activity. It can be said that having the β-crystal activity indicates that the polypropylene-based resin film material before the film was made porous forms β-crystals. If β crystals are generated in the film-like material, suitable fine pores are easily formed in the stretching step of making the film porous, and a porous film having excellent air permeability can be obtained.

  The fact that the polypropylene-based resin has β-crystal activity can be determined by detecting a crystal melting peak temperature derived from β-crystal in a differential scanning calorimetry (DSC). Specifically, it can be determined by the method described in Examples described later.

  The β-crystal activity can also be determined by detecting a diffraction peak in a diffraction profile obtained by wide-angle X-ray diffraction measurement. Specifically, the film is heat-treated at 170 to 190 ° C., which is a temperature higher than the crystal melting peak temperature of the polypropylene-based resin, and slowly cooled, and then wide-angle X-ray measurement is performed, which results from the (300) plane of β crystal. When the diffraction peak is detected in the range of 2θ = 16.0 ° to 16.5 °, it is judged to have β-crystal activity (“Macromolecular Chemistry and Physics” Volume 187, Issue 3, pp. 643-652 (1986), “Progress in Polymer Science” Volume 16, Issues 2-3, pp.361-404 (1991), “Macromolecular Symposia” Volume 89, Issue 1, pp.499-511 (1995), “ Macromolecular Chemistry and Physics ”Volume 75, Issue 1, pp.134-158, (1964)).

  The β crystal activity, which is the degree of β crystal activity in the porous film, is preferably 20% or more, more preferably 40% or more, still more preferably 60% or more. It can be said that the higher the β-crystal activity, the more β-crystals are generated in the polypropylene resin film material before the film is made porous, and in the stretching step of making the film porous, many uniform fine pores are formed. A porous film that is easy, has excellent air permeability, and has high mechanical strength can be obtained. The upper limit of the β-crystal activity is not particularly limited, but the closer to 100%, the better.

The β-crystal activity is determined as the ratio of ΔHmβ in the total of the detected amount of crystal melting heat ΔHmα derived from α crystal and the amount of crystal melting heat ΔHmβ derived from β crystal.
For example, when the polypropylene resin is homopolypropylene, ΔHmβ is mainly detected in the temperature range of 145 ° C. or higher and lower than 160 ° C., and ΔHmα is mainly detected in the temperature range of 160 to 170 ° C. From this ΔHmα and ΔHmβ, the β-crystal activity of homopolypropylene can be calculated.
Further, for example, when the polypropylene resin is a random copolymer containing 1 to 4 mol% of ethylene, ΔHmβ is mainly detected in a temperature range of 120 ° C. or higher and lower than 140 ° C., and ΔHmα is mainly It is detected in the temperature range of 140 to 165 ° C. From the ΔHmα and ΔHmβ, the β crystal activity of the random copolymer can be calculated.

  As a method for obtaining a polypropylene-based resin having β-crystal activity, a method of not adding a substance that promotes α-crystal formation to the polypropylene-based resin or a method of adding a polypropylene-based resin that has been treated to generate a peroxide radical ( Japanese Patent No. 3739481), a method of adding a β crystal nucleating agent to the resin composition, and the like. Of these, from the viewpoint of efficiently exhibiting β-crystal activity, it is preferable to obtain it by a method of adding a β-crystal nucleating agent.

(Air resistance)
The porous film of the present invention has sufficient air permeability, and the air permeability resistance per 1 μm of thickness is 10 to 1000 seconds / 100 mL.
The air permeation resistance referred to in the present invention represents the time required for 100 mL of air to pass in the thickness direction, which is measured by the Gurley method according to JIS P 8117: 2009. Therefore, it is indicated that the smaller the value, the easier the air is to pass.
If the air resistance is less than 10 seconds / 100 mL, an accident such as an internal short circuit may occur when the porous film is used as a battery separator, and sufficient safety cannot be obtained. It is preferably 15 seconds / 100 mL or more, more preferably 20 seconds / 100 mL or more. On the other hand, when the air resistance is more than 1000 seconds / 100 mL, when the porous film is used as a battery separator, the electrical resistance becomes high and the battery performance is deteriorated, which is not preferable. It is preferably 800 seconds / 100 mL or less, and more preferably 600 seconds / 100 ml or less.
The air permeability resistance is specifically measured by the method described in Examples described later.

(Tensile strength)
According to JIS K 7127: 1999, the porous film of the present invention uses a test piece having a length of 80 mm in the measuring direction, a width of 15 mm and a thickness of 15 to 28 μm at a chuck distance of 40 mm and a test speed of 200 mm / min. The tensile strength S _{MD of MD} and the tensile strength S _{TD of} TD of the film, which are respectively measured, satisfy the relationships of the following formulas (1) and (2).
0.13 ≦ S _MD / S _TD ≦ 0.80 (1)
170 MPa ≤ S _MD + S _TD ≤ 400 MPa (2)
When the tensile strength S _MD and the tensile strength S _TD satisfy the relationships shown in the above formulas (1) and (2), the polypropylene resin porous film has excellent air permeability and high puncture strength. Can be configured.
The tensile strength is specifically measured by the method described in Examples below.

The ratio S _MD / S _TD of the tensile strength S _MD and the tensile strength S _TD is 0.13 to 0.80, preferably 0.2 to 0.75. When S _MD / S _TD is less than 0.13, the porosity of the film is low and sufficient air permeability cannot be obtained. On the other hand, when it exceeds 0.80, a film having sufficient puncture strength cannot be obtained.
Further, the sum S _MD + S _TD of the tensile strength S _MD and the tensile strength S _TD is 170 to 400 MPa, preferably 200 to 350 MPa. If S _MD + S _TD is less than 170 MPa, a film having sufficient puncture strength cannot be obtained. On the other hand, when it exceeds 400 MPa, the porosity of the film is low and there is a possibility that sufficient air permeability cannot be obtained.

(Puncture strength)
The puncture strength per 1 μm in thickness of the porous film of the present invention is preferably 21.5 gf / μm or more, more preferably 22.0 gf / μm or more. If the puncture strength per thickness of 1 μm is 21.5 gf / μm or more, it can be said that the polypropylene-based resin porous film has sufficient puncture strength even when it is made thin. On the other hand, the upper limit of the puncture strength is not particularly limited, but is preferably 35 gf / μm or less in consideration of the balance with other physical properties of the film.
The puncture strength is specifically measured by the method described in Examples below.

(Thickness)
The thickness of the porous film of the present invention is preferably 1 to 500 μm, more preferably 2 to 100 μm, still more preferably 3 to 50 μm, still more preferably 5 to 30 μm. When the thickness is 1 μm or more, when the porous film is used as a battery separator, sufficient electric insulation is obtained, a short circuit hardly occurs, and sufficient battery safety is secured. Also, it is not easily torn when wound and stored in a battery. When the thickness is 500 μm or less, the electric resistance of the porous film itself becomes small when used as a battery separator, and sufficient battery performance is ensured.
The thickness is specifically measured by the method described in Examples below.

(Layered structure)
The polypropylene resin porous film of the present invention has excellent air permeability and high puncture strength in a single layer, but within a range that does not hinder the function of the porous film, it can be used by laminating other layers. Good. For example, a high melting temperature resin layer, a heat resistant layer such as a coating layer made of heat resistant particles and a binder, or a shutdown layer such as a low melting temperature resin layer may be laminated.
The number of layers in the case of a laminated structure can be appropriately selected depending on the application and purpose of use of the film, and is preferably 2 to 7 layers, and more preferably from the viewpoint of productivity and cost. There are two or three layers. The thickness of each layer, the number of repeated layers, and the like can be appropriately adjusted according to the application and purpose of use of the film.

[Method for producing polypropylene-based resin porous film]
The polypropylene resin porous film as described above is not particularly limited in its production method, but a polypropylene resin composition containing a β crystal nucleating agent is melt-kneaded to form a polypropylene resin film. And a stretching step of stretching the film to obtain a porous film. In the stretching step, the film is transversely stretched in TD and then longitudinally in MD. It can be suitably produced by the production method of the present invention in which stretching is performed and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is 3.0 or more.
According to such a manufacturing method, a polypropylene-based resin porous film having excellent air permeability and high puncture strength can be manufactured with high productivity.
The manufacturing method will be described below in order of steps.

(Molding process)
First, in the molding step, a polypropylene resin composition containing a β crystal nucleating agent is melt-kneaded to mold a polypropylene resin film.

<Β crystal nucleating agent>
The β crystal nucleating agent is blended in order to obtain a polypropylene resin having β crystal activity, as described above. The compound is not particularly limited as long as it can promote the formation and growth of β crystal of the polypropylene resin, and known compounds can be used.
For example, amide compounds; tetraoxaspiro compounds; quinacridones; nano-sized iron oxides; alkali or alkaline earth metals of carboxylic acids such as potassium 1,2-hydroxystearate, magnesium benzoate, magnesium succinate, and magnesium phthalate. Salts; aromatic sulfonic acid compounds such as sodium benzenesulfonate and sodium naphthalenesulfonate; di- or triesters of dibasic or tribasic carboxylic acids; phthalocyanine pigments such as phthalocyanine blue; and component a which is an organic dibasic acid. Examples thereof include a two-component compound consisting of a component b which is an oxide, hydroxide or salt of a Group 2 element metal; a composition consisting of a cyclic phosphorus compound and a magnesium compound. In addition, the substances specifically described in JP-A-2003-306585, JP-A-8-144122 or JP-A-9-194650 can also be used. These may be used alone or in combination of two or more.

  A commercially available β crystal nucleating agent is “Ngester NU-100” (manufactured by Shin Nippon Rika Co., Ltd.), and a polypropylene resin containing a β crystal nucleating agent is commercially available as polypropylene “Bepol B-”. 022SP "(manufactured by Aristech), polypropylene" Beta (β) -PP BE60-7032 "(manufactured by Borealis), polypropylene" BNX BETAPP-LN "(manufactured by Mayzo) and the like.

  The blending amount of the β crystal nucleating agent is appropriately adjusted depending on the type of the β crystal nucleating agent, the composition of the polypropylene resin, and the like, but is blended in a ratio of 0.0001 to 5 parts by mass with respect to 100 parts by mass of the polypropylene resin. Preferably. It is more preferably 0.001 to 3 parts by mass, and even more preferably 0.01 to 2 parts by mass. When the blending ratio of the β-crystal nucleating agent is 0.0001 parts by mass or more, the formation and growth of β-crystals can be promoted and the porous film can have sufficient β-crystal activity. Further, when the content is 5 parts by mass or less, bleeding of the β-crystal nucleating agent on the surface of the porous film is suppressed, and it is preferable from the viewpoint of cost.

<Melting and molding>
The melt kneading and molding can be carried out by known methods. For example, a polypropylene resin composition containing a β crystal nucleating agent is melt kneaded using an extruder, extruded from a T die, and cooled and solidified by a cast roll. Examples of the method include a method by extrusion molding, and a method in which a film-like material produced by the tubular method is cut open to form a flat surface. Of these, the method by extrusion molding is more preferable. Specifically, the following method is preferable.

  Melt-kneading is carried out by hand blending polypropylene resin, β crystal nucleating agent, and additives added as necessary using a Henschel mixer, a super mixer, a tumbler type mixer, or the like, or by accommodating them in a bag. After mixing, it is preferable to pelletize the resin composition by carrying out with a single-screw or twin-screw extruder, a kneader or the like.

  The kneading temperature depends on the type and blending amount of the β crystal nucleating agent, but is preferably 230 to 380 ° C., more preferably from the viewpoint of dispersibility of the β crystal nucleating agent and deterioration of the polypropylene resin due to heating. It is 240-350 degreeC. When a β crystal nucleating agent that can be dissolved in a high temperature polypropylene resin is used, it is preferable to melt-knead at a temperature at which the β crystal nucleating agent is completely dissolved from the viewpoint of dispersibility of the β crystal nucleating agent.

  It is preferable that the pellets of the resin composition obtained after the melt-kneading are put into an extruder and extruded from a T die to form a film. The T-die gap is determined by various conditions such as the desired thickness of the porous film, the stretching conditions, and the draft ratio, but is generally 0.1 to 0.1 from the viewpoint of productivity, film-forming stability, and the like. It is 3.0 mm, preferably 0.3 to 1.5 mm.

  The extrusion temperature is appropriately adjusted according to the fluidity and moldability of the resin composition, but is 160 to 380 ° C. from the viewpoint of productivity, film-forming stability, deterioration of the polypropylene resin due to heating, and the like. The temperature is preferably 165 to 350 ° C, more preferably 170 to 300 ° C.

  The cooling and solidifying temperature by the cast roll is preferably 80 to 150 ° C., more preferably 90 to 150 ° C., in order to sufficiently increase the ratio of β crystal in the film material, and from the viewpoint of film forming stability and the like. The temperature is 140 ° C, more preferably 100 to 135 ° C.

(Stretching process)
Next, in the stretching step, the film material is stretched to obtain a porous film. By stretching the film-like material made of the resin composition having β crystal obtained in the molding step, a porous film having a large number of fine pores and having continuity in the thickness direction can be obtained.
Examples of the stretching method include a roll stretching method, a rolling method, a tenter stretching method, and a simultaneous biaxial stretching method. In the manufacturing method according to the present invention, the sequential biaxial stretching method is used.
In the sequential biaxial stretching method, the film material is transversely stretched in TD and then longitudinally stretched in MD. Such a stretching step is excellent in productivity because it does not need to rewind the sheet obtained by each stretching and can be continuously performed.

The ratio of the transverse stretching ratio to the longitudinal stretching ratio is 3.0 or more. That is, the ratio of the transverse stretching performed first is made larger than the ratio of the longitudinal stretching performed later, and the ratio of [transverse stretching ratio / longitudinal stretching ratio] is 3.0 or more, and the transverse stretching and the longitudinal stretching are performed in this order.
As described above, after the film-like material is transversely stretched at a high draw ratio within a range that does not stretch and break, the resulting transversely drawn sheet is longitudinally drawn at a draw ratio lower than that of the transverse draw, so that the tensile strength S _TD is more tensile. higher than the intensity _{S MD,} S MD _{/ S TD} satisfies the condition shown in the equation to be 0.13 or more 0.80 or less (1), _{and, S} sum of _MD and _{S TD S} MD _{+ S TD} Can suitably obtain a polypropylene resin porous film that satisfies the condition shown in the above formula (2), which is 170 MPa or more and 400 MPa or less. Such a porous film has excellent air permeability, and has high puncture strength that cannot be achieved by the conventional dry uniaxial stretching method or dry biaxial stretching method.
When the ratio of [transverse draw ratio / longitudinal draw ratio] is less than 3.0, it is difficult to obtain a porous film having excellent air permeability and high puncture strength.

The stretching temperature in the transverse stretching is preferably 60 to 140 ° C, and more preferably 70 to 130 ° C from the viewpoint of good stretchability and efficient porosification.
The horizontal stretching ratio is improved S _TD, excellent permeability, and, from the viewpoint of obtaining a porous film having high puncture strength is preferably from 5.5 to 12.0 times, more preferably It is 6.0 to 11.0 times, and more preferably 6.5 to 10.0 times.

On the other hand, the stretching temperature in the longitudinal stretching is 60 to 160 ° C. from the viewpoint of performing uniform stretching without breaking the film, and from the viewpoint of efficiently expanding the pores without melting the polypropylene resin. Is preferable, more preferably 70 to 155 ° C, and further preferably 80 to 150 ° C.
The longitudinal stretching ratio is preferably 1.2 to 4.0 times from the viewpoint of obtaining a porous film having excellent air permeability and high puncture strength while suppressing breakage during stretching. It is more preferably 1.3 to 3.5 times.

The strain rate of transverse stretching is preferably 100 to 12000% / min, more preferably 800 to 10000% / min, and even more preferably, from the viewpoint of the air permeability of the porous film and the ease of adjusting the tensile strength. 1000-8000% / min.
On the other hand, the strain rate of the longitudinal stretching is preferably 10 to 3000% / min, more preferably 20 to 2000% / min, from the viewpoint of air permeability of the porous film and ease of adjusting tensile strength. It is preferably 50 to 1000% / min.

From the viewpoint of improving dimensional stability, heat treatment may be performed after either or both of the transverse stretching and the longitudinal stretching. The heat treatment temperature is preferably 100 to 170 ° C, more preferably 120 to 170 ° C. The heat treatment time is preferably 1 to 120 seconds, more preferably 3 to 60 seconds.
Furthermore, the porous film of the present invention is a surface treatment such as corona treatment, plasma treatment, printing, coating, vapor deposition, and perforation treatment according to the application and the purpose of use, etc. within a range that does not impair the effects of the present invention. May be given.

  Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

[Production of polypropylene-based resin porous film]
The raw materials used for producing the polypropylene resin porous films of Examples and Comparative Examples are as follows.

<Polypropylene>
-"Novatech PP FY6HA" (manufactured by Japan Polypro Co., Ltd.); MFR: 2.4 g / 10 min (230 ° C, 2.16 kg load), isotactic pentad fraction 98%, Mw: 470,000, Mw / Mn: 4.0
<Β crystal nucleating agent>
B-1: 2,6-naphthalenedicarboxylic acid dicyclohexylamide; "Ngester NU-100" (manufactured by Shin Nippon Rika Co., Ltd.)
B-2: 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5,5] undecane <antioxidant>
・ "IRGANOX B 225" (manufactured by BASF Japan Ltd.)

(Example 1)
0.2 parts by mass of the β crystal nucleating agent (B-1) and 0.1 parts by mass of an antioxidant are mixed with 100 parts by mass of polypropylene, and the same-direction twin-screw extruder (manufactured by Toshiba Machine Co., 40 mm, the ratio L / D = 32) of the effective length L of the screw and the outer diameter D, and the mixture was melt-kneaded at a set temperature of 280 ° C. The obtained strand was cooled and solidified in a water tank and cut with a pelletizer to prepare pellets.
The produced pellets were melt-kneaded at 200 ° C. using a single-screw extruder (manufactured by Mitsubishi Heavy Industries, Ltd.), and a molten resin sheet extruded from a T die having a gap of 0.8 mm and 200 ° C. was cast with a 127 ° C. cast roll. It was taken out, cooled and solidified to obtain a film-like material having a thickness of about 120 μm. The β-crystal activity of this film was 62%.
The obtained film-like material is passed through a film tenter facility (manufactured by Kyoto Kikai Co., Ltd.) to preheat to 80 ° C. and transversely stretched, then heat treated at 155 ° C. for 10 seconds and gradually cooled to obtain a transversely stretched sheet. It was Subsequently, this transversely stretched sheet was longitudinally stretched at 100 ° C. to obtain a polypropylene resin porous film. This porous film had β-crystal activity.
The stretching conditions for transverse stretching (stretching 1) and longitudinal stretching (stretching 2) are as shown in Table 1.

(Examples 2 to 5, Comparative Examples 2 and 3)
A polypropylene-based resin porous film was produced in the same manner as in Example 1 except that the stretching conditions in Example 1 were changed as shown in Table 1.
In addition, in Example 5, B-2 was used instead of B-1 as a β crystal nucleating agent. Further, the thickness of the film composition was about 80 μm.

(Comparative Example 1)
In Example 1, a polypropylene-based resin porous film was produced without performing longitudinal stretching (stretching 2).

(Comparative example 4)
A polypropylene-based resin porous film was produced in the same manner as in Example 1 except that the longitudinal stretching was performed as the stretching 1 and the transverse stretching was performed as the stretching 2 in Example 1.

[Physical property evaluation and measurement]
With respect to each polypropylene resin porous film produced in the above Examples and Comparative Examples, various physical properties were evaluated and measured by the following methods.

<Tensile strength>
It was measured by a method according to JIS K 7127: 1999. As the measuring device, a tensile compression tester (200X type, manufactured by Intesco Co., Ltd.) was used. As the test piece, a polypropylene resin porous film cut into a rectangle having a length of 80 mm and a width of 15 mm in the measuring direction was used. Both ends of the test piece in the length direction were chucked at a chuck distance of 40 mm, pulled at a crosshead speed of 200 mm / min, and the stress at the breaking point was measured 5 times as tensile strength, and the average value was obtained.

<Thickness>
A sample piece having a diameter of 40 mm was cut out from the polypropylene-based resin porous film, the thickness was measured at any 5 points on the film surface with a dial gauge having a unit weight of 1/1000 mm, and the average value was obtained.

<Porosity>
It was calculated by the following formula from the substantial amount W ₁ measured for the sample piece prepared by the thickness measurement and the mass W ₀ when the porosity was 0% calculated from the density calculated from the raw material of the resin composition and the thickness.
Porosity [%] = {(W ₀ −W ₁ ) / W ₀ } × 100

<Air resistance (Gurley value)>
Using the sample piece prepared by the thickness measurement, the air permeation resistance (second / 100 mL) was measured by the Gurley method according to JIS P 8117: 2009, and converted into 1 μm thickness.

<Puncture strength>
The sample piece prepared by the thickness measurement is fixed to a holder (measurement part: a circle having a diameter of 10 mm), and a metal (SUS440C) needle having a diameter of 1 mm and a tip curvature radius of 0.5 mm is pierced in the thickness direction at a speed of 300 mm / min. The maximum load at which holes were opened was measured and converted per 1 μm of thickness.

<Presence of β-crystal activity (DSC)>
A polypropylene-based resin porous film (10 mg) was weighed to give a sample, which was measured with a differential scanning calorimeter (DSC-7, manufactured by Perkin Elmer Japan Co., Ltd .; hereinafter the same) under a nitrogen gas atmosphere at 25 ° C to 200 ° C. Heating rate up to 10 ° C / min for 1 minute and then cooling rate from 200 ° C to 25 ° C at 10 ° C / minute cooling rate and holding for 1 minute, then heating rate from 25 ° C to 200 ° C at 10 ° C / minute The temperature was raised again. A case where a peak was detected at a crystal melting peak temperature Tmβ of 145 to 160 ° C. derived from β crystal of the polypropylene resin at the time of reheating, was defined as “having β crystal activity”.
It was confirmed that the film-like polypropylene-based resin porous films of the following Examples and Comparative Examples each had β-crystal activity.

<Β crystal activity (DSC) of the film material>
About the film-form material before stretching, 10 mg was weighed as a sample, and the temperature was raised from 25 ° C. to 200 ° C. at a heating rate of 10 ° C./min in a nitrogen gas atmosphere with a differential scanning calorimeter. It was confirmed that “having β-crystal activity” by detecting a peak at 145 to 160 ° C., which is a crystal melting peak temperature Tmβ derived from β-crystal of polypropylene resin at the time of temperature rise.
The β-crystal activity of the film material was calculated from the α-crystal-derived crystal fusion heat amount ΔHmα and the β-crystal-derived crystal fusion heat amount ΔHmβ of the polypropylene resin detected by DSC according to the following formula.
β crystal activity [%] = {ΔHmβ / (ΔHmα + ΔHmβ)} × 100
The β-crystal activities of the film materials of the following Examples and Comparative Examples were both 62%.

  Table 1 below shows the results of measurement and evaluation of various physical properties of the polypropylene-based resin porous films produced in Examples and Comparative Examples.

As can be seen from the results shown in Table 1, a polypropylene-based resin porous film having tensile strengths S _MD and S _TD obtained by carrying out longitudinal stretching after transverse stretching (Examples 1 to 5). ) Is excellent in air permeability and has high puncture strength.
On the other hand, when the tensile strength did not meet the predetermined conditions (Comparative Examples 1 to 4), neither sufficient air permeability nor puncture strength was obtained.

As described above, the polypropylene resin porous film of the present invention is excellent in air permeability and has high puncture strength, and has a nickel hydrogen battery, a lithium ion secondary battery, an aluminum electrolytic capacitor, an electric double layer capacitor. It can be suitably used for various electric storage devices such as lithium-ion capacitors, and especially for batteries, especially separators for lithium-ion secondary batteries.
In addition, pads for absorbing body fluids such as disposable paper diapers, medical materials such as surgical gowns, clothing materials such as jumpers and rainwear, building waterproofing materials such as house waterproofing materials, heat insulating materials, desiccants, packaging of disposable warmers, etc. It is expected to be widely used in various applications where air permeability is required, such as materials, various filters, and materials such as liquid treatment materials such as industrial filtration membranes.

Claims (4)

The air permeation resistance per thickness 1 μm is 10 to 1000 seconds / 100 mL, the puncture strength per thickness 1 μm is 21.5 gf or more, and the length in the measuring direction is 80 mm and the width is 15 mm according to JIS K 7127: 1999. And a tensile strength S _MD in the film flow direction (MD) measured at a chuck distance of 40 mm and a test speed of 200 mm / min using a test piece having a thickness of 15 to 28 μm, and a direction perpendicular to the flow direction ( tensile strength _{S TD} of TD) is, meets the following relationship formula (1) and (2),
A polypropylene-based resin porous film, wherein the polypropylene-based resin constituting the film has β-crystal activity and the porosity is 50.5% or less .
0.13 ≦ S _MD / S _TD ≦ 0.80 (1)
170 MPa ≤ S _MD + S _TD ≤ 400 MPa (2) A battery separator comprising the polypropylene-based resin porous film according to claim 1 . A battery comprising the battery separator according to claim 2 . A polypropylene resin composition containing a β crystal nucleating agent is melt-kneaded, and includes a molding step of molding a polypropylene resin film, and a stretching step of stretching the film to obtain a porous film,
In the stretching step, the film material is transversely stretched in a direction (TD) perpendicular to the flow direction (MD) of the film, and then longitudinally stretched in the flow direction (MD) of the film,
The method for producing a polypropylene resin porous film, wherein the transverse stretching ratio is 5.5 to 12.0 times, and the ratio of the transverse stretching ratio to the longitudinal stretching ratio is 3.0 or more.