JP6958586B2 - Porous film wound body and its manufacturing method - Google Patents

Description

The present invention relates to a porous film wound body and a method for producing the same, which have excellent air permeability and mechanical strength and can be suitably used for a separator for a battery, particularly a lithium ion secondary battery.

Demand for secondary batteries is increasing with the shift to OA (Office Automation) and FA (Factory Automation), and is also widely used as a power source for portable devices such as home appliances and communication devices. In particular, in portable devices, the use of lithium ion secondary batteries is increasing because of excellent volumetric efficiency, miniaturization and weight reduction of the devices to be loaded.
On the other hand, in the fields related to energy and the environment, for example, uninterruptible power supply such as road leveling, so-called UPS (Uninterruptible Power Supply), and large secondary batteries in electric vehicles are being researched and developed, and have a large capacity. From the viewpoint of high output, high voltage and excellent long-term storage, the use of lithium ion secondary battery, which is a kind of uninterruptible electrolyte secondary battery, is expanding.

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. Naturally, this battery separator is required to have insulating properties due to its role, and further, it is also required to have 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 micropore structure, and a porous film is used.

Further, in the separator for a lithium ion secondary battery, as described above, although it is porous with air permeability, even if thermal runaway occurs due to an abnormality in the battery, film rupture or shrinkage occurs between the electrodes. Is short-circuited, and it is required to be able to prevent accidents such as ignition caused by abnormal heat generation of the battery. Therefore, in order to enhance the safety of the separator, a film having higher mechanical strength, specifically, a porous film having high piercing strength is required.

Further, in a lithium ion secondary battery, the thickness of the separator is expected to be further reduced due to the increase in capacity, and the demand for a thin film and high puncture strength is increasing.
The thinning of the separator increases the difficulty of film formation, and the loss of productivity due to breakage during stretching is also a concern.

The porous film used as a separator for a lithium ion secondary battery generally contains a polyolefin resin as a main component, and the main manufacturing methods thereof are a wet method, a dry uniaxial stretching method, and a dry biaxial stretching method. It is roughly divided.

In the wet method, since an ultra-high molecular weight polymer can be used, a porous film having high piercing strength can be obtained, but a step of extracting a plasticizer with a solvent is required for porosity, so that the manufacturing cost and environment It had a problem that the load was high.

The dry uniaxial stretching method is cheaper than the wet method and has a lower environmental load because it can be manufactured without using a solvent. Since it undergoes stretching, efficient continuous production is difficult and productivity is inferior.

On the other hand, in the dry biaxial stretching method as shown in Patent Documents 1 to 4, a general-purpose stretching device is used, so that the productivity is high.

Japanese Unexamined Patent Publication No. 11-297297 JP 2015-4017 Japanese Unexamined Patent Publication No. 2012-7156 Japanese Unexamined Patent Publication No. 2017-171796

The above-mentioned Patent Document 1 describes that the laminated film produced by the dry biaxial stretching method has high puncture strength, but the preferable thickness is 15 to 50 μm, and further thinning has not been studied.
Patent Documents 2 and 3 describe that a porous film is obtained by a dry biaxial stretching method in which an unstretched film-like material made of a polypropylene-based resin having β-crystal activity is longitudinally stretched and then laterally stretched. In each case, only a puncture strength of 15 gf / μm or less was obtained.
Patent Document 4 describes that a porous film having high puncture strength can be obtained by a dry biaxial stretching method in which an unstretched film-like material made of a polypropylene resin having β crystal activity is laterally stretched and then longitudinally stretched. However, further thinning has not been investigated, and stretch fracture has not been described.

The present invention has been made to solve the above problems, and is a thin film, has high piercing strength, is excellent in air permeability, and is excellent in productivity, and is particularly suitable for a battery separator. Focusing on the process conditions of the dry biaxial stretching method, which is a body and its manufacturing method and capable of continuous production, a porous film wound body capable of suppressing stretch breakage, which impairs productivity, even though it is a thin film, and The purpose is to provide the manufacturing method.

As a result of diligent studies to achieve the above problems, the present inventor has completed the present invention focusing on solving such problems. That is, the gist of the present invention is a porous film wound body in which a porous film made of a polypropylene resin composition having β crystal activity is wound around a core, and the thickness of the porous film is 10 μm or less. It is a porous film wound body in which the length of the porous film is 200 m or more.

Further, the polypropylene-based resin composition having β-crystal activity is melt-kneaded in an extruder including a screw having a supply portion, a compression portion, and a measurement portion, and the length of the compression portion is 20% or more of the total length. Then, after performing a molding process in which the film-like material is extruded into a film-like material and cooled and solidified, and the film-like material is laterally stretched in the vertical direction (TD) perpendicular to the flow direction, the film-like material is subjected to lateral stretching in the flow direction (MD). A stretching step in which longitudinal stretching is performed and the ratio of the ratio of the transverse stretching to the longitudinal stretching ratio is 3.0 or more, and the molding step and the porous film obtained by the stretching step are wound around the core. It is a method for producing a porous film wound body having a winding step to be performed.

The porous film wound body that can be produced by the present invention is thin and has high piercing strength, is excellent in air permeability, and is also excellent in productivity. Therefore, the porous film wound body can be suitably used as a battery separator, and can contribute to the improvement of battery capacity and safety.
Further, according to the production method of the present invention, the porous film wound body can be produced easily and efficiently.

Hereinafter, the present invention will be described in detail.

[Porosity film]
(Polypropylene resin)
Examples of the polypropylene-based resin used in the present invention include homopolypropylene (propylene homopolymer), random copolymers of propylene and other α-olefins, block copolymers, and the like. These may be used alone or in combination of two or more.
Specific examples of α-olefins other than propylene, which are the components derived from the constituent units 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 structural unit derived from the α-olefin (α-olefin content) shall be 10 mol% or less from the viewpoint of facilitating the porosity of the polypropylene resin by stretching. Is preferable, and more preferably 5 mol% or less. For the same reason, 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-based 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%, and further preferably 90 to 99%.
Here, the "isotactic pentad fraction" means that all five methyl groups that are side chains with respect to the main chain formed by a carbon-carbon bond composed of five consecutive propylene units are in the same direction. It means the ratio of the three-dimensional structure located in. ¹³ Obtained from the pentad unit in the polypropylene molecular chain measured by the signal in the methyl group region of the C-NMR spectrum. The attribution of the signal in the methyl group region shall be in accordance with the method described in A. Zambelli, et al., “Macromolecules” Volume 8, Issue 5, pp.687-689 (1975).

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

The melt flow rate (MFR) of the polypropylene-based resin is not particularly limited, but is preferably 0.5 to 15 g / 10 minutes, and 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 the molding process is high, and good productivity can be ensured. Further, 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-based resin is not particularly limited, and a polymerization method using a known polymerization catalyst may be used. For example, a multisite catalyst typified by a Ziegler-Natta type catalyst or a metallocene catalyst may be used. A polymerization method using a typical single-site catalyst or the like can be used.

As polypropylene resins, for example, "Novatec PP", "Wintech", "Waymax" (manufactured by Japan Polypropylene Corporation); "Notio", "Toughmer XR" (manufactured by Mitsui Chemicals, Inc.); "Zeras", "Thermoran" (manufactured by Mitsubishi Chemicals, Inc.); "Sumitomo Noblen", "Tough Serene" (manufactured by Sumitomo Chemicals, Ltd.); "Prime Polypropylene" "Prime TPO" (manufactured by Prime Polymer Co., Ltd.); "Adflex" , "Adsil", "HMS-PP (PF-814)" (manufactured by Sun Aroma Co., Ltd.); "Inspire", "Versify" (manufactured by Dow Chemicals, Inc.) and the like can be used.

Further, from the viewpoint of promoting porosity and imparting moldability, polypropylene-based resins may be used in combination with two or more polypropylene-based resins as long as the effects of the present invention are not impaired, as well as modified polyolefin resins and alicyclic saturateds. A hydrocarbon resin or a modified product thereof, an ethylene-based copolymer, low molecular weight polypropylene, a thermoplastic elastomer, or the like may be added. Examples of the thermoplastic elastomer include styrene-butadiene-based, polyolefin-based, urethane-based, polyester-based, polyamide-based, 1,2-polybutadiene, polyvinyl chloride-based, ionomer and the like.
Further, from the viewpoints of moldability, productivity, improvement and adjustment of various physical properties of the porous film, etc., recycled resin, silica, talc, kaolin and calcium carbonate generated by trimming loss or the like are not impaired within the range that does not impair the effects of the present invention. Inorganic particles such as titanium oxide and pigments such as carbon black, flame retardants, weather resistance stabilizers, heat stabilizers, antistatic agents, melt viscosity improvers, cross-linking agents, lubricants, nucleating agents, plasticizers, anti-aging agents, etc. Various additives such as antioxidants, light stabilizers, ultraviolet absorbers, neutralizers, antifogging agents, antiblocking agents, slip agents, and colorants may be added.

The polypropylene-based resin constituting the porous film of the present invention preferably has β-crystal activity. It can be said that the fact that the film has β-crystal activity indicates that the film-like material of the polypropylene-based resin before the film was made porous produced β-crystals. If β crystals are formed in the film-like material, suitable micropores are likely to be formed in the drawing 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 the crystal melting peak temperature derived from β-crystal in differential scanning calorimetry (DSC). Specifically, it can be determined by the method described in Examples described later.

Further, the possession of β-crystal activity can also be determined by detecting a diffraction peak in the 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 exceeding the crystal melting peak temperature of polypropylene-based resin, slowly cooled, and then wide-angle X-ray measurement is performed, and the film is derived from the (300) plane of β crystals. If 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 film-like material of the polypropylene resin before the film is made porous, and many uniform micropores are formed in the drawing step of making the film porous. A porous film that is easy to use, 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 it is preferable that the β crystal activity is close to 100%.

The β-crystal activity is determined as the ratio of ΔHmβ to the total of the detected α-crystal-derived crystal melting heat amount ΔHmα and β-crystal-derived crystal melting heat amount ΔHmβ.
For example, when the polypropylene-based 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 these ΔHmα and ΔHmβ, the β crystal activity of homopolypropylene can be calculated.
Further, for example, when the polypropylene-based resin is a random copolymer containing 1 to 4 mol% of ethylene, ΔHmβ is mainly detected in the temperature range of 120 ° C. or higher and lower than 140 ° C., and ΔHmα is mainly detected. 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 in which a substance that promotes the formation of α-crystals is not added to the polypropylene-based resin, or a method in which a polypropylene-based resin that has been treated to generate radical peroxides is added ( (See 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 expressing β-crystal activity, it is preferable to obtain by a method of adding a β-crystal nucleating agent.

<β crystal nucleating agent>
As described above, the β-crystal nucleating agent is formulated to obtain a polypropylene-based resin having β-crystal activity. As long as it can promote the formation and growth of β crystals of the polypropylene-based resin, it is not particularly limited, and known ones can be used.
For example, amide compounds; tetraoxaspirones; quinacridones; nano-sized iron oxides; alkaline or alkaline earth metals of carboxylic acids such as potassium 1,2-hydroxystearate, magnesium benzoate, magnesium succinate, magnesium phthalate. Salts; aromatic sulfonic acid compounds such as sodium benzene sulfonate and sodium naphthalene sulfonate; 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 composed of an oxide, a hydroxide or a salt of a group 2 element metal; a composition composed of a cyclic phosphorus compound and a magnesium compound. In addition, 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.

As a commercially available β-crystal nucleating agent, "NJester NU-100" (manufactured by Shin Nihon Rika Co., Ltd.), and as a commercially available polypropylene-based resin containing a β-crystal nucleating agent, 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 β-crystal nucleating agent, the composition of the polypropylene-based resin, etc., but is blended at a ratio of 0.0001 to 5 parts by mass with respect to 100 parts by mass of the polypropylene-based resin. Is preferable. It is more preferably 0.001 to 3 parts by mass, still more preferably 0.01 to 2 parts by mass. When the compounding ratio of the β crystal nucleating agent is 0.0001 parts by mass or more, it is possible to promote the formation and growth of β crystals and to have sufficient β crystal activity in the porous film. Further, when it 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.

(Thickness)
The thickness of the porous film of the present invention is 10 μm or less, preferably 9 μm or less, and more preferably 8 μm or less. When the thickness is 10 μm or less, the electrical resistance of the porous film itself can be reduced when used as a battery separator, and a sufficient battery capacity is secured. On the other hand, the lower limit of the thickness is not particularly limited, but is preferably 1 μm or more, more preferably 2 μm or more, and further preferably 3 μm or more. When the thickness is 1 μm or more, when the porous film is used as a battery separator, sufficient electrical insulation is obtained, short circuits are unlikely to occur, and sufficient safety of the battery is ensured.
The thickness is specifically measured by the method described in Examples described later.

(Porosity)
The porosity of the porous film of the present invention is preferably 20 to 80%, more preferably 25 to 70%, and even more preferably 30 to 60%. When the porosity is in the above range, a porous film having a good balance between high puncture strength and air permeability can be obtained.
The porosity is specifically measured by the method described in Examples described later.

(Air permeability 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 / (dL · μm) or less.
The air permeation resistance referred to in the present invention represents the time required for 1 dL (100 mL) of air to pass in the thickness direction as measured by the Garley method in accordance with JIS P 8117: 2009. Therefore, the smaller the value, the easier it is for air to pass through.
If the air permeation resistance per 1 μm of the thickness is less than 10 seconds / (dL · μm), an accident such as an internal short circuit may occur when the porous film is used as a battery separator, which is sufficiently safe. I can't get sex. It is preferably 15 seconds / (dL · μm) or more, and more preferably 20 seconds / (dL · μm) or more. On the other hand, if the air permeation resistance per 1 μm of thickness exceeds 1000 seconds / (dL · μm), the electrical resistance becomes high and the battery performance deteriorates when the porous film is used as a battery separator, which is preferable. No. It is preferably 500 seconds / (dL · μm) or less, more preferably 200 seconds / (dL · μm) or less.
The air permeation resistance is specifically measured by the method described in Examples described later.

(Puncture strength)
The puncture strength per 1 μm of the thickness of the porous film of the present invention is preferably 20 gf / μm or more, more preferably 21 gf / μm or more. If the piercing strength per 1 μm of thickness is 20 gf / μm or more, it can be said that the porous film has sufficient piercing strength even if it is a thin film of 10 μm or less. 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 film physical characteristics.
The puncture strength is specifically measured by the method described in Examples described later.

The lower limit of the ratio of the puncture strength to the air permeability resistance (piercing strength / air permeability resistance) of the porous film of the present invention is preferably 0.75 or more, more preferably 0.80 or more, and 0.90 or more. More preferred. On the other hand, the upper limit is not particularly limited, but is preferably 3.0 or less, more preferably 2.0 or less, and even more preferably 1.5 or less.
The puncture strength / air permeability resistance in the above range suggests that the porous film has both mechanical strength and air permeability. Further, when used as a battery separator, sufficient battery performance and safety are ensured.

(Heat shrinkage rate)
The heat shrinkage rate in the flow direction (MD) of the porous film of the present invention is preferably 25% or less, more preferably 20% or less, still more preferably 15% or less. When the heat shrinkage rate of the MD is 25% or less, when the porous film is used as a battery separator, the occurrence of an internal short circuit can be suppressed due to the heat shrinkage of the porous film when the battery is overheated.

The heat shrinkage in the vertical direction (TD) perpendicular to the flow direction of the porous film of the present invention is preferably 5% or less, more preferably 3% or less, still more preferably 1% or less. Assuming that the heat shrinkage rate of TD is 5% or less, when a porous film is used as a battery separator, the positive electrode and the negative electrode are formed at the end of the wound body due to the heat shrinkage of the porous film when the battery is overheated. It is possible to prevent the occurrence of internal short circuit due to contact.

(Laminated structure)
The porous film of the present invention has excellent air permeability and high piercing strength even if it is a single layer, but other layers may be laminated and used as long as the function of the porous film is not impaired. For example, a high melting temperature resin layer, a heat resistant layer made of a coating layer made of heat resistant particles and a binder, or a shutdown layer made of a low melting temperature resin layer can be laminated.
The number of layers in the case of a laminated structure can be appropriately selected depending on the intended use and purpose of use of the film, and is preferably 2 to 7 layers, more preferably from the viewpoint of productivity, cost and the like. It has two or three layers. The thickness of each layer, the number of repeated layers, and the like can also be appropriately adjusted according to the intended use and purpose of use of the film.

[Perforated film wound body]
The porous film wound body of the present invention is obtained by winding the above-mentioned porous film around a core.
In the porous film wound body of the present invention, the length of the porous film is 200 m or more. When the length is 200 m or more, for example, when used as a battery separator, it is sufficiently possible to continuously produce a battery member in which a porous film is laminated between a positive electrode and a negative electrode. The upper limit of the length is not particularly limited, but is preferably 10,000 m or less.

(core)
The core is a cylindrical core used for winding a porous film. The core can be formed of, for example, paper, resin-impregnated paper, acrylonitrile-butadiene-styrene copolymer (ABS resin), FRP, phenol resin, inorganic resin, adhesive, or the like.

The material of the core is not particularly limited, but plastic, a thermosetting resin, or the like is preferable from the viewpoints of having a small coefficient of thermal expansion, high rigidity, low swelling property with respect to humidity, and excellent turning property.
When the material of the core is paper, the desired characteristics can be easily obtained by coating the surface thereof with a resin or the like. Further, the core is preferably a resin-impregnated paper tube from the viewpoint of surface smoothness. When the core material is a resin, it can be specifically formed of an acrylonitrile-butadiene-styrene copolymer (ABS resin), FRP, a phenol resin, an inorganic substance-containing resin, or the like.

The outer diameter of the core is preferably 5 inches or more (about 12.7 cm or more), more preferably 6 inches or more, still more preferably 8 inches, from the viewpoint of relaxing the winding tightness after winding the porous film. It is inches or more, particularly preferably 9 inches or more. The upper limit of the outer diameter of the core is not particularly limited, but from the viewpoint of handling, it is preferably 20 inches or less, and more preferably 15 inches or less.

The width of the core is preferably 10 mm or more and 2,000 mm or less, more preferably 15 mm or more and 1,900 mm or less, and further preferably 20 mm or more and 1,700 mm or less. Since the width of the core is 10 mm or more, the porous film is easily affected by the quality of the core, which is particularly useful in the present embodiment.

[Manufacturing method of porous film wound body]
In the method for producing a porous film wound body as described above, a polypropylene-based resin composition containing a β-crystal nucleating agent has a supply portion, a compression portion, and a measurement portion, and the length of the compression portion is long. A molding process in which the film is melt-kneaded with an extruder containing a screw, which is 20% or more of the total length, extruded into a polypropylene resin film and cooled and solidified, and the vertical direction in which the film is orthogonal to the flow direction ( A stretching step in which the transverse stretching in the TD) is performed, then the longitudinal stretching is performed in the flow direction (MD), and the ratio of the ratio of the transverse stretching to the magnification of the longitudinal stretching is 3.0 or more. The porous film obtained by the molding step and the stretching step has a winding step of being wound around a core.
According to such a manufacturing method, a porous film wound body having excellent air permeability and high piercing strength can be manufactured with high productivity without stretching and breaking.
Hereinafter, the above-mentioned manufacturing method will be described in the order of each step.

(Molding process)
First, in the molding step, a polypropylene-based resin composition containing a β-crystal nucleating agent is melt-kneaded and cooled and solidified to form a film-like material.

A known method can be used for the melt kneading and molding. For example, a polypropylene-based 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 thereof include a method of cutting an object into a flat surface. Of these, the extrusion molding method is more preferable. Specifically, it is preferable to carry out by the following method.

In melt-kneading, polypropylene-based resin, β-crystal nucleating agent, and additives added as needed are mixed by hand blending using a Henschel mixer, super mixer, tumbler type mixer, etc., or in a bag. After mixing, it is preferable to pellet the resin composition by performing it with a uniaxial or biaxial extruder, a kneader or the like.

The kneading temperature for pelletization depends on the type and amount of the β crystal nucleating agent, but is preferably 230 to 380 ° C. from the viewpoint of the dispersibility of the β crystal nucleating agent and the deterioration due to heating of the polypropylene resin. , More preferably 240-350 ° C. When a β-crystal nucleating agent that can be dissolved in a high-temperature polypropylene-based 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.

The pellets of the resin composition obtained after melt-kneading are put into an extruder and extruded from a T-die to form a film-like product.
The extruder is provided with a screw in a cylinder, and the screw has a supply portion, a compression portion, and a weighing portion. In extrusion molding, it is important that the length of the compressed portion of the screw is 20% or more of the total length, preferably 25% or more, and more preferably 30% or more. On the other hand, the upper limit is not particularly limited, but is preferably 80% or less, and more preferably 75% or less.
When the length of the compressed portion is 20% or more of the total length, the compressed portion is gently compressed during melt-kneading. As a result, the friction of the pellets of the resin composition against the cylinder wall surface and the aggregation of the β-crystal nucleating agent due to a sudden change in resin pressure are suppressed, and the β-crystal structure of the polypropylene-based resin becomes more dense, resulting in high puncture strength and high puncture strength. It is considered that excellent air permeability can be obtained. Further, it is considered that by stabilizing the melt extrusion and increasing the strength of the film-like material, stretching fracture is suppressed, continuous winding is possible, and a porous film wound body having a length of 200 m or more can be obtained. Be done.
When extruding from a T-die to form a film-like material, the gap of the T-die is determined by various conditions such as a desired porous film thickness, stretching conditions, draft ratio, etc., but productivity and film forming stability are determined. From the viewpoint of the above, it is generally 0.1 to 3.0 mm, preferably 0.3 to 2.5 mm.

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

The cooling and solidification 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 crystals in the film-like material and from the viewpoint of film formation stability and the like. It is 140 ° C., more preferably 100 to 135 ° C.

(Stretching process)
Next, in the stretching step, the film-like material is stretched to obtain a porous film. By stretching a film-like material made of a resin composition having β crystals obtained in the molding step, a large number of micropores are formed, and a porous film having communication in the thickness direction can be obtained.

As the stretching method, there are methods such as a roll stretching method, a rolling method, a tenter stretching method, and a simultaneous biaxial stretching method, but in the manufacturing method according to the present invention, a sequential biaxial stretching method is used.
In the sequential biaxial stretching method, the film-like material is laterally stretched to TD and then longitudinally stretched to MD. Such a stretching step is excellent in productivity because it is not necessary to rewind the sheet obtained by each stretching and it can be continuously performed.

In either or both of the transverse stretching and the longitudinal stretching, a preheating step before stretching may be provided. The stretching step may be performed by one-step stretching or multi-step stretching. Further, it may have a heat fixing step and a heat relaxing step after that.

The stretching temperature in the transverse stretching is preferably 60 to 140 ° C., more preferably 70 to 130 ° C. from the viewpoint of good stretchability and efficient porosity.
The lateral stretching ratio is preferably 5.0 to 12 times, more preferably 5.5 to 11 times, still more preferably 6.0 to 10 times, from the viewpoint of obtaining a porous film having high puncture strength. It is double.

The strain rate of the transverse stretching is preferably 10 to 12000% / min, more preferably 50 to 10000% / min, still more preferably 50 to 10000% / min, from the viewpoint of the air permeability of the porous film and the ease of adjusting the tensile strength. It is 100 to 8000% / minute.

Further, the longitudinal stretching ratio is preferably 1.1 to 4.0 times from the viewpoint of obtaining a porous film having excellent air permeability and high piercing strength while suppressing breakage during stretching. More preferably, it is 1.2 to 3.5 times.
When the longitudinal stretching is performed by the roll stretching method, it is performed by making a difference in the speed of the rolls in a facility having two or more rolls having a temperature control function, a touch roll (nip roll), and a driving function. At this time, the longitudinal stretching may be performed in one step or in two or more steps. When the longitudinal stretching is performed in two or more steps, the product of the respective stretching ratios is the longitudinal stretching ratio.

On the other hand, the stretching temperature in the longitudinal stretching is 60 to 160 from the viewpoint of performing uniform stretching without breaking the film and from the viewpoint of efficiently expanding the pores without melting the polypropylene-based resin. The temperature is preferably 70 to 155 ° C, more preferably 80 to 150 ° C.

The strain rate of longitudinal stretching is preferably 10 to 3000% / min, more preferably 50 to 2000% / min, and even more preferably 50 to 2000% / min, from the viewpoint of air permeability of the porous film and ease of adjusting the tensile strength. It is 100 to 1000% / minute.
When the longitudinal stretching is performed by the roll stretching method, the length L [%] of the stretching section is located on the downstream side next to the position where the touch roll of the low-speed roll located on the upstream side of the stretching step is applied. It is defined as the length of the pass line of the film to the position where the touch roll of the high-speed roll is applied.
When the draw drawn between the two rolls was D [%] and the peripheral speed of the low-speed roll on the upstream side was S [m / min], the strain rate was calculated by the following equation 1.

The ratio of the lateral stretching ratio to the longitudinal stretching ratio [transverse stretching ratio / longitudinal stretching ratio] is 3.0 or more. That is, the magnification of the transverse stretching performed first is made larger than the magnification of the longitudinal stretching performed later, and the ratio of [transverse stretching ratio / longitudinal stretching ratio] is set to 3.0 or more, and the transverse stretching and the longitudinal stretching are performed in this order.
If the ratio of [transverse stretching ratio / longitudinal stretching ratio] is less than 3.0, it is difficult to obtain a porous film having excellent air permeability and high piercing strength.

From the viewpoint of improving dimensional stability and the like, when heat treatment is performed after either or both of transverse stretching and longitudinal stretching, the heat treatment temperature is preferably 100 to 170 ° C, more preferably 120 to 165 ° C. be. The heat treatment time is preferably 1 to 120 seconds, more preferably 3 to 90 seconds.
Further, the porous film of the present invention can be used for surface treatment such as corona treatment, plasma treatment, printing, coating, vapor deposition, perforation processing, etc. May be applied.

(Winding process)
Subsequently, in the winding step, the porous film wound body can be obtained by winding the porous film obtained in the stretching step around the core. In the winding process, the conditions are not particularly limited.

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

[Physical property evaluation and measurement]
Various physical properties were evaluated and measured by the methods shown below for each of the porous film wound bodies produced in the following Examples and Comparative Examples.

<Thickness>
A sample piece having a diameter of 40 mm was cut out from the porous film wound body, and the thickness was measured at any five points on the film surface with a dial gauge having a scale of 1/1000 mm, and the average value was calculated.

<Porosity>
The actual mass W ₁ measured for the specimens prepared in the thickness measurement, the porosity of the mass W ₀ when the porosity of 0% calculated from the density and the thickness calculated from the raw materials of the resin composition by the following formula 2 (%) Was calculated.

<Air permeation resistance (Garley value)>
Using the sample piece prepared by the thickness measurement, the air permeation resistance (seconds / 100 mL) was measured by the Garley method in accordance with JIS P 8117: 2009, and converted per 1 μm of thickness.

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

<Heat shrinkage rate>
Cut out strip-shaped sample pieces (dimensions of 220 mm on the long side and 10 mm on the short side) from the porous film so that MD and TD are in the long side direction, and write marked lines with an interval of 200 mm in the long side direction. Then, the sample piece was heated in a hot air oven at 105 ° C. for 30 minutes. The heat shrinkage of MD and TD was calculated based on the following formula 3 with the distance between the marked lines after heating as χ (mm).

<Presence / absence of β crystal activity (DSC)>
Weigh 10 mg of the porous film into a sample, and use a differential scanning calorimeter (DSC-7, manufactured by PerkinElmer Japan Co., Ltd .; the same applies hereinafter) to heat the film from 25 ° C to 200 ° C in a nitrogen gas atmosphere. After raising the temperature at 10 ° C./min and holding for 1 minute, lower the temperature from 200 ° C. to 25 ° C. at a cooling rate of 10 ° C./min for 1 minute, and then raise again from 25 ° C. to 200 ° C. at a heating rate of 10 ° C./min. It was warm. When the peak was detected at 145 to 160 ° C., which is the crystal melting peak temperature Tmβ derived from the β crystal of the polypropylene resin at the time of reheating, it was defined as “having β crystal activity”.
It was confirmed that the porous films of the following Examples and Comparative Examples both had β crystal activity.

<β-crystal activity (DSC) of membranous material>
About 10 mg of the film-like substance before stretching was weighed and used as a sample, and the temperature was raised from 25 ° C. to 200 ° C. at a heating rate of 10 ° C./min with a differential scanning calorimeter under a nitrogen gas atmosphere. It was confirmed that the polypropylene-based resin "has β-crystal activity" by detecting a peak at 145 to 160 ° C., which is a crystal melting peak temperature Tmβ derived from β-crystals of the polypropylene-based resin when the temperature is raised.
The β-crystal activity of the film-like material was calculated by the following formula 4 from the crystal melting heat amount ΔHmα derived from the α crystal of the polypropylene resin detected by DSC and the crystal melting heat amount ΔHmβ derived from the β crystal.

<Continuous film forming property>
When a wound body was manufactured by continuously performing a molding step, a stretching step, and a winding step, the continuous film-forming property was evaluated as follows based on the length of the porous film that could be wound and collected without stretching breakage. ..
◯: Length of porous film is 200 m or more Δ: Length of porous film is 100 m or more and less than 200 m ×: Length of porous film is less than 100 m

[Manufacturing of porous film wound body]
The raw materials used for producing the porous film wound bodies of Examples and Comparative Examples are as follows.

<Polypropylene>
Homopolypropylene, MFR = 2.4 g / 10 minutes (230 ° C, 2.16 kg load), isotactic pentad fraction = 98%, Mw = 470,000, Mw / Mn = 4.0
<β crystal nucleating agent>
-3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5,5] undecane <antioxidant>
・ "IRGANOX B 225" (manufactured by BASF Japan Ltd.)

<Preparation of pellet raw material A>
0.2 parts by mass of β crystal nucleating agent and 0.1 parts by mass of antioxidant are mixed with 100 parts by mass of polypropylene, and a biaxial extruder in the same direction (diameter 40 mm, effective length L and outer diameter D of screw). L / D = 32), and melt-kneaded at a cylinder temperature of 280 ° C. The strands extruded from the strand die were rapidly cooled in a water tank to solidify and cut with a pelletizer to prepare a pellet raw material A.

(Example 1)
Pellet raw material A is used in a single-screw extruder (diameter 40 mm, effective length L of screw to outer diameter D ratio L / D = 26.5, compression section length to total screw length = 38%, compression ratio = 2.7). After the temperature is adjusted to 200 ° C. and melt-kneaded, the molten resin sheet extruded from a T-die with a gap of 0.5 mm and 200 ° C. is taken up by a cast roll at 129 ° C., cooled and solidified, and a film having a thickness of about 35 μm is obtained. Got The β-crystal activity of this film-like substance was 67%.
The obtained film-like material is passed through a film tenter facility (manufactured by Kyoto Machinery Co., Ltd.), and while being heated at 110 ° C., it is transversely stretched 7.0 times at a strain rate of 280% / min and then at 155 ° C. for 60 seconds. Heat treatment was performed and the mixture was slowly cooled to obtain a transversely stretched sheet. Subsequently, while heating this transversely stretched sheet at 140 ° C., a longitudinally stretched draw of 18% was applied at a strain rate of 269% / min, and then a longitudinally stretched draw of 18% was further applied at a strain rate of 197% / min. , The porous film obtained by longitudinally stretching at 1.4 times was wound by a winder to obtain a porous film wound body.

(Example 2)
While heating the transversely stretched sheet at 140 ° C., a 27% longitudinally stretched draw was applied at a strain rate of 397% / min, and then a 10% longitudinally stretched draw was further applied at a strain rate of 115% / min to 1.4. It was the same as in Example 1 except that it was longitudinally stretched by a factor of two.

(Example 3)
While heating the transversely stretched sheet at 140 ° C., a 10% longitudinally stretched draw was applied at a strain rate of 147% / min, and then a 27% longitudinally stretched draw was further applied at a strain rate of 270% / min, 1.4. It was the same as in Example 1 except that it was longitudinally stretched by a factor of two.

(Comparative Example 1)
Pellet raw material A is used in a single-screw extruder (diameter 40 mm, effective length L of screw to outer diameter D ratio L / D = 26.5, compression section length to total screw length = 15%, compression ratio = 2.1). After the temperature is adjusted to 200 ° C. and melt-kneaded, the molten resin sheet extruded from a T-die with a gap of 0.5 mm and 200 ° C. is taken up by a cast roll at 125 ° C., cooled and solidified, and a film having a thickness of about 35 μm is obtained. Got The β-crystal activity of this film-like substance was 67%.
The obtained film-like material is passed through a film tenter facility (manufactured by Kyoto Machinery Co., Ltd.), and while being heated at 110 ° C., it is transversely stretched 7.0 times at a strain rate of 280% / min and then at 155 ° C. for 60 seconds. Heat treatment was performed and the mixture was slowly cooled to obtain a transversely stretched sheet. Subsequently, while heating this transversely stretched sheet at 120 ° C., a longitudinally stretched draw of 22.5% was applied at a strain rate of 331% / min, and then a longitudinally stretched sheet of 22.5% was further stretched at a strain rate of 251% / min. The porous film obtained by applying a draw and longitudinally stretching at 1.5 times was wound by a winder to obtain a porous film wound body.

(Comparative Example 2)
While heating the transversely stretched sheet at 110 ° C., a 32.3% longitudinally stretched draw is applied at a strain rate of 475% / min, and then a 32.3% longitudinally stretched draw is further applied at a strain rate of 388% / min. It was the same as in Comparative Example 1 except that it was longitudinally stretched 1.8 times.

(Comparative Example 3)
The same as in Comparative Example 1 was carried out except that the transversely stretched sheet was longitudinally stretched while being heated at 130 ° C.

(Comparative Example 4)
The same as in Comparative Example 1 was carried out except that the transversely stretched sheet was longitudinally stretched while being heated at 140 ° C.

(Comparative Example 5)
While heating the transversely stretched sheet at 140 ° C., a 18.3% longitudinally stretched draw was applied at a strain rate of 269% / min, and a 18.3% longitudinally stretched draw was further applied at a strain rate of 197% / min. It was the same as in Comparative Example 1 except that it was longitudinally stretched at 1.4 times.

The production conditions for the porous film wound body are summarized in Table 1 below.

The measurement and evaluation results of various physical properties of each porous film wound body produced in Examples and Comparative Examples are summarized in Table 2 below.

As can be seen from the results shown in Table 2, the porous film wound bodies of Examples 1 to 3 are thin films, have high puncture strength and excellent air permeability, and are continuously wound without stretch breakage. I was able to take it.
On the other hand, it was found that the porous film wound bodies of Comparative Examples 1 to 5 had insufficient piercing strength, were easily stretched and broken, could not be continuously wound, and were not highly productive. rice field.
That is, in the molding process, by using the screw used in the examples, the pellet raw material A is gently compressed during melt kneading, so that the pellet raw material A is rubbed against the cylinder wall surface and the β crystal nucleating agent is aggregated due to a sudden change in resin pressure. It is considered that high puncture strength and excellent air permeability were obtained because the β crystal structure of the polypropylene-based resin became denser.
Furthermore, the melt extrusion is stabilized by gentle compression and the strength of the film-like material is increased, so that stretch fracture is suppressed, continuous winding is possible, and a porous film wound body having a length of 200 m or more is wound. Is considered to have been obtained.

As described above, the porous film wound body that can be produced by the present invention has a thin film, high puncture strength, and excellent air permeability, and is a nickel hydrogen battery, a lithium ion secondary battery, an aluminum electrolytic capacitor, and the like. It can be suitably used for various power storage devices such as electric double-layer capacitors and lithium ion capacitors, and above all, separators for batteries, particularly lithium ion secondary batteries.
Furthermore, packaging for body fluid absorption pads such as disposable paper omelets, medical materials such as surgical gowns, clothing materials such as jumpers and raincoats, building materials such as house waterproofing materials and heat insulating materials, desiccants, and disposable cairo. It is expected to be widely used in various applications that require air permeability, such as materials, various filters, and materials such as liquid treatment materials such as industrial filtration membranes.

Claims (9)

A porous film wound body in which a porous film made of a polypropylene-based resin composition having β-crystal activity is wound around a core, the thickness of the porous film is 10 μm or less, and piercing per 1 μm thickness. A porous film wound body having a strength of 20 gf / μm or more and a length of the porous film of 200 m or more. The porous film wound body according to claim 1, wherein the air permeation resistance per 1 μm of the thickness of the porous film is 10 to 1000 seconds / (dL · μm). The porous film wound body according to claim 1 or 2, wherein the heat shrinkage rate in the vertical direction (TD) perpendicular to the flow direction of the porous film is 5% or less. The porous film wound body according to any one of claims 1 to 3, wherein the heat shrinkage rate in the flow direction (MD) of the porous film is 25% or less. A polypropylene-based resin composition having β-crystal activity is melt-kneaded by an extruder including a screw having a supply portion, a compression portion, and a measurement portion, and the length of the compression portion is 20% or more of the total length. The molding process, which is extruded into a film and cooled and solidified,
After the film-like material is laterally stretched in the vertical direction (TD) perpendicular to the flow direction, it is longitudinally stretched in the flow direction (MD), and the ratio of the lateral stretching to the magnification of the longitudinal stretching is The stretching process with a ratio of 3.0 or more and
The porous film obtained by the molding step and the stretching step is, possess a winding process to be wound around the core,
A method for producing a porous film wound body , wherein the thickness of the porous film is 10 μm or less. The method for producing a porous film wound body according to claim 5 , wherein the puncture strength per 1 μm of the thickness of the porous film is 20 gf / μm or more. The method for producing a porous film wound body according to claim 5 or 6 , wherein the air permeation resistance per 1 μm of the thickness of the porous film is 10 to 1000 seconds / (dL · μm). The method for producing a porous film wound body according to any one of claims 5 to 7 , wherein the heat shrinkage rate in the vertical direction (TD) perpendicular to the flow direction of the porous film is 5% or less. The method for producing a porous film wound body according to any one of claims 5 to 8 , wherein the heat shrinkage rate in the flow direction (MD) of the porous film is 25% or less.