Porous film, ion exchange membrane support, and electricity storage device

Abstract

To provide a porous film having both thin film properties and film stiffness while having good air permeability characteristics.SOLUTION: The porous film is formed from a resin composition [I] containing a polyolefin-based resin (A) and a styrene-based thermoplastic elastomer (B), wherein the content ratio of the styrene-based thermoplastic elastomer (B) to the total amount of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B) is 20 mass% or more and less than 50 mass%, the tensile modulus at 23 °C in the machine direction (MD) is 30 MPa or more, the porosity is 80% or less, and the air permeability (S) is less than 10 seconds / 100 cc.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a porous film, an ion exchange membrane support, and an electricity storage device. [Background technology]

[0002] Porous polymer films with numerous fine interconnecting pores are used in a variety of fields, including as separation membranes used in the production of ultrapure water, the purification of chemical solutions, and water treatment, as waterproof and breathable films used in clothing and sanitary materials, and as separators used in electronic components such as capacitors, batteries, and electrolytic capacitors.

[0003] Among these, porous films containing polyolefin resins as a main component are used as separators in electricity storage devices such as various capacitors, various batteries, and various electrolytic capacitors. Such porous films containing polyolefin resins as the main component are produced, for example, by a method of controlling the crystallization of the polyolefin resin in the cooling and solidifying process following melt extrusion, and stretching a film of the crystallized polyolefin resin to make it porous; a method of applying strain such as stretching to a polyolefin molded product in which different solids are microdispersed, thereby generating voids between the different solids to make it porous; or a method of microdispersing fine powders of different polymers, paraffins, waxes, etc. in a polyolefin resin and then extracting them with a solvent.

[0004] Patent Documents 1 to 4 propose a technique for obtaining a stretched porous film by blending a polypropylene resin as a matrix with a styrene-based thermoplastic elastomer that is partially compatible with the matrix as a domain, melt-extruding the matrix through a T-die, and melt-solidifying the resulting sheet using a cast roll. These techniques allow the porous structure to be adjusted by changing the type of styrene-based thermoplastic elastomer. Furthermore, Patent Document 5 proposes that a microporous polyolefin membrane having a predetermined thickness and a specific tortuosity exhibits excellent isotropy in mass transfer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-141786 [Patent Document 2] Japanese Patent Application Publication No. 2017-222823 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-230743 [Patent Document 4] Japanese Patent Application Publication No. 2019-199529 [Patent Document 5] Japanese Patent Publication No. 2020-066716 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, there has been a demand for thinner porous films with improved air permeability. However, as the film thickness decreases, the film loses stiffness and tends to wrinkle easily during the film-making process. For example, when the film is wound into a roll, the roll tends to wrinkle easily. The techniques described in the above Patent Documents 1 to 4 do not have sufficient air permeability, and the technique described in Patent Document 5 does not have sufficient thickness, so there is a demand for a porous film that is thin and stiff yet has good air permeability.

[0007] Therefore, an object of the present invention is to provide a porous film that has good air permeability, thinness, and stiffness. [Means for solving the problem]

[0008] The present invention proposes the following porous film as a porous film having good air permeability while also being thin and stiff. It also proposes an ion exchange membrane support and an electricity storage device using the porous film.

[0009] [1] The porous film of the first aspect of the present invention is a porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), a content ratio of the styrene-based thermoplastic elastomer (B) to the total amount of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more and less than 50% by mass, The porous film has a tensile modulus of elasticity in the machine direction (MD) at 23°C of 30 MPa or more, a porosity of 80% or less, and an air permeability (S) of less than 10 seconds / 100 cc.

[0010] [2] The porous film of the second aspect of the present invention is the porous film of the first aspect, wherein the ratio (S / T) of the air permeability (S) (sec / 100cc) to the thickness (T) (μm) of the porous film is 0.5 sec / (100cc μm) or less.

[0011] [3] A third aspect of the porous film of the present invention is the porous film of the first or second aspect, wherein the polyolefin resin (A) is a polypropylene resin. [4] A fourth aspect of the porous film of the present invention is the porous film of any one of the first to third aspects, wherein the polyolefin resin (A) has a melt flow rate (MFR(A)) of 1 g / 10 min or more at a temperature of 230°C and a load of 2.16 kg.

[0012] [5] A fifth aspect of the porous film of the present invention is the porous film of any one of the first to fourth aspects, wherein the melt flow rate (MFR(B)) of the styrene thermoplastic elastomer (B) at a temperature of 200°C and a load of 10 kg is 2 g / 10 min or less.

[0013] [6] A sixth aspect of the porous film of the present invention is the porous film of any one of the first to fifth aspects, wherein the thickness (T) is less than 50 μm. [7] The porous film of the seventh aspect of the present invention is the porous film of any one of the first to sixth aspects, wherein the resin composition [I] contains a crystal nucleating agent (C).

[0014] [8] An ion exchange membrane support according to an eighth aspect of the present invention is an ion exchange membrane support comprising the porous film according to any one of the first to seventh aspects. [9] An electricity storage device according to a ninth aspect of the present invention is an electricity storage device including the ion exchange membrane support according to the eighth aspect. [Effects of the Invention]

[0015] According to the present invention, a porous film can be obtained which is thin but has excellent tensile modulus and film strength, and thus has good air permeability while being thin and having film stiffness. Furthermore, the porous film of the present invention has excellent air permeability despite being a thin film, and therefore can be suitably used as an ion exchange membrane support, particularly as an ion exchange membrane support for an electricity storage device. DETAILED DESCRIPTION OF THE INVENTION

[0016] A porous film and a method for producing the same will be described in detail below as an example of an embodiment of the present invention, although the scope of the present invention is not limited to the embodiment described below.

[0017] The term "film" in the present invention includes sheets. Furthermore, in the present invention, when it is written "x to y" (x and y are any numbers), unless otherwise specified, it means "greater than x and less than y", as well as "preferably greater than x" or "preferably smaller than y". Furthermore, unless otherwise specified, "x or more" (x is any number) also includes the meaning of "preferably greater than x," and "y or less" (y is any number) also includes the meaning of "preferably smaller than y."

[0018] <Porous film of the present invention> A porous film according to one embodiment of the present invention (hereinafter also referred to as "the porous film of the present invention") is a porous film formed from a resin composition [I] containing a polyolefin resin (A) as a main component and further containing a styrene-based thermoplastic elastomer (B). Here, the "main component" refers to the component that accounts for the largest proportion by mass among the components constituting the resin composition [I]. The content of the main component can be 50 mass % or more, 60 mass % or more, 70 mass % or more, 80 mass % or more, or 90 mass % or more of the components constituting the resin composition [I].

[0019] <Resin composition [I]> The resin composition [I] preferably contains a polyolefin resin (A) as a main component, and further contains a styrene-based thermoplastic elastomer (B) as a domain component that forms a sea-island structure with the polyolefin resin (A) as a matrix, and may further contain additives such as a crystal nucleating agent (C) and an antioxidant.

[0020] The components constituting the resin composition [I] will be described in detail below.

[0021] [Polyolefin resin (A)] Examples of the polyolefin resin (A) include homopolypropylene (propylene homopolymer), and random or block copolymers of propylene with an α-olefin such as ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, or 1-decene. The polyolefin resins (A) exemplified above may be used singly or in combination of two or more. Among these, homopolypropylene is preferred from the viewpoint of mechanical strength.

[0022] When the polyolefin resin (A) is a polypropylene resin, its isotactic pentad fraction is preferably 80 mol% or more and 99 mol% or less, more preferably 83 mol% or more or 98 mol% or less, and even more preferably 85 mol% or more or 97 mol% or less. The isotactic pentad fraction indicates stereoregularity, and if the isotactic pentad fraction is too low, sufficient porosity may not be achieved by stretching. On the other hand, the upper limit of the isotactic pentad fraction is specified as the upper limit that can be obtained industrially at present, but this may not apply in the future if a resin with higher regularity is developed at the industrial level. The isotactic pentad fraction refers to the conformation or proportion of the conformation in which the five methyl groups in the side chains are all aligned in the same direction relative to the main chain formed by carbon-carbon bonds of any five consecutive propylene units. The signal assignment in the methyl group region follows A. Zambelli et al., Macromol., 8, 687 (1975).

[0023] The Mw / Mn of the polyolefin resin (A) is preferably 1.5 or more and 10.0 or less, more preferably 2.0 or more or 8.0 or less, and even more preferably 2.5 or more or 6.0 or less. Mw / Mn is a parameter indicating molecular weight distribution, and a smaller Mw / Mn means a narrower molecular weight distribution. By setting Mw / Mn to the above lower limit or more, sufficient extrusion moldability can be obtained, making industrial mass production possible. On the other hand, by setting Mw / Mn to the above upper limit or less, sufficient mechanical strength can be ensured. In the present invention, Mw / Mn is a value measured by GPC (gel permeation chromatography).

[0024] The polyolefin resin (A) preferably has a melt flow rate (MFR(A)) of 1 g / 10 min or more at a temperature of 230° C. and a load of 2.16 kg. When the melt flow rate (MFR(A)) is 1 g / 10 min or more, when the styrene thermoplastic elastomer (B) described later is dispersed in the polyolefin resin (A), the in-plane orientation of the styrene thermoplastic elastomer (B) can be suppressed, and the dispersed diameter of the styrene thermoplastic elastomer (B) increases, thereby increasing the interconnectivity in the thickness direction, and as a result, the air permeability of the porous film of the present invention can be improved. From this viewpoint, the melt flow rate (MFR(A)) of the polyolefin resin (A) is preferably 1 g / 10 min or more, more preferably 1.5 g / 10 min or more, even more preferably 3 g / 10 min or more, even more preferably 5 g / 10 min or more, and even more preferably 7 g / 10 min or more. On the other hand, the upper limit is preferably 20 g / 10 min or less, more preferably 18 g / 10 min or less, and even more preferably 16 g / 10 min or less. By setting the MFR to the upper limit or less, the strength of the porous film of the present invention can be sufficiently maintained. When two or more polyolefin resins (A) are used in combination, it is sufficient that at least one of the polyolefin resins (A) has the above MFR value. The MFR of the polyolefin resin (A) in the present invention is a value measured in accordance with JIS K7210-1 (2014) under conditions of a temperature of 230°C and a load of 2.16 kg.

[0025] The tensile modulus of elasticity of the polyolefin resin (A) at 23°C is preferably 1000 MPa to 3000 MPa. The tensile modulus of the polyolefin resin (A) affects the air permeability and bubble point pressure of the porous film of the present invention, so that the tensile modulus of the polyolefin resin (A) is preferably within the above range in order to adjust the air permeability and bubble point pressure of the porous film of the present invention within a predetermined range. From this viewpoint, the tensile modulus of the polyolefin resin (A) at 23°C is preferably 1000 MPa or more, more preferably 1300 MPa or more, and even more preferably 1600 MPa or more, and is preferably 3000 MPa or less, more preferably 2700 MPa or less, and even more preferably 2400 MPa or less.

[0026] Examples of the polyolefin resin (A) that can be used include commercially available products such as "Novatec PP," "WINTEC," and "Waymax" (manufactured by Japan Polypropylene Corporation); "Notio" and "Tafmer XR" (manufactured by Mitsui Chemicals, Inc.); "Zelas" and "Thermorun" (manufactured by Mitsubishi Chemical Corporation); "Sumitomo Noblen" and "Tafthren" (manufactured by Sumitomo Chemical Co., Ltd.); "Prime PP" and "Prime TPO" (manufactured by Prime Polymer Co., Ltd.); "Adflex," "Adsyl," and "HMS-PP (PF814)" (manufactured by Sunallomer Co., Ltd.); and "Versify" and "Inspire" (manufactured by The Dow Chemical Company).

[0027] The content of the polyolefin resin (A) in the resin composition [I] is preferably 50% by mass or more and 95% by mass or less, more preferably 55% by mass or more or 90% by mass or less, even more preferably 60% by mass or more or 85% by mass or less, and particularly preferably 70% by mass or less. When the content of the polyolefin resin (A) is within the above range, a sea-island structure can be formed in which the polyolefin resin (A) serves as the matrix and the styrene-based thermoplastic elastomer (B) serves as the domain, and a porous structure can be easily formed when the film is stretched.

[0028] [Styrene-based thermoplastic elastomer (B)] The porous film of the present invention preferably further contains a styrene-based thermoplastic elastomer (B). By kneading and melting the styrene-based thermoplastic elastomer (B) into the polyolefin-based resin (A) and forming a sea-island structure in which the styrene-based thermoplastic elastomer (B) acts as a domain in the matrix of the polyolefin-based resin (A), a porous film having a highly uniform porous structure can be obtained, and the shape and diameter of the pores can be easily controlled.

[0029] The styrene-based thermoplastic elastomer (B) is a type of thermoplastic elastomer resin based on a styrene component, and is a copolymer consisting of a continuum of a soft component (e.g., a butadiene component) and a hard component (e.g., a styrene component). Specifically, there are copolymers in which the carbon double bonds of the copolymer have been hydrogenated to convert them into single bonds.

[0030] The copolymers may be random copolymers, block copolymers, or graft copolymers, and the block copolymers may have a linear block structure or a radially branched block structure.

[0031] Examples of the styrene-based thermoplastic elastomer (B) include a styrene-olefin-styrene copolymer (B1) and a styrene-olefin copolymer (B2). Examples of the styrene-olefin-styrene copolymer (B1) include styrene-butadiene-styrene copolymer (SBS), styrene-butadiene-butylene-styrene copolymer (SBBS), styrene-ethylene-butadiene-styrene copolymer (SEBS), styrene-isoprene-styrene copolymer (SIS), styrene-ethylene-propylene-styrene copolymer (SEPS), and styrene-ethylene-ethylene-propylene-styrene copolymer (SEEPS). Examples of the styrene-olefin copolymer (B2) include styrene-butadiene copolymer (SBR), hydrogenated styrene-butadiene copolymer (SEB), styrene-isoprene copolymer (SIR), and styrene-ethylene-propylene copolymer (SEP). The styrene-based thermoplastic elastomers (B) exemplified above may be used alone or in combination of two or more.

[0032] Among the above, from the viewpoint of the ability to form a porous structure, it is preferable to use a resin having a structure in which both ends are styrene polymers as the main component, since poor compatibility with the polyolefin resin (A) tends to facilitate the formation of a porous structure. On the other hand, in order to efficiently disperse the styrene-based thermoplastic elastomer (B) in the resin composition [I], it is preferable to use a resin containing an ethylene component (hydrogenated butadiene component), an ethylene-propylene component (hydrogenated isoprene component), or a butylene component, which have high compatibility with the polyolefin resin (A). That is, from the viewpoint of the ability to form a porous structure, it is preferable to use a styrene-olefin-styrene copolymer (B1) as the main component, and from the viewpoint of the dispersibility of the styrene-based thermoplastic elastomer (B), it is preferable to use SEP, SEPS, SEBS, or SEEPS as the main component. From both of these viewpoints, it is more preferable to use SEPS or SEEPS as the main component, and among these, SEEPS is particularly preferable. The "major component" in the styrene-based thermoplastic elastomer (B) refers to the resin that is contained in the largest amount of the styrene-based thermoplastic elastomer (B) contained in the resin composition [I], and more specifically refers to a resin that accounts for preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more, with 100% by mass being the upper limit, when the total amount of the styrene-based thermoplastic elastomer (B) is taken as 100% by mass.

[0033] In order to achieve both high levels of air permeability and mechanical properties, the styrene-based thermoplastic elastomer (B) contained in the resin composition [I] preferably contains a styrene-olefin-styrene copolymer (B1) and a styrene-olefin copolymer (B2). By combining two or more types of styrene-based thermoplastic elastomers (B), the uniformity of the porous structure obtained by the stretching step can be improved.

[0034] In the porous film of the present invention, pores are formed inside the domains of the styrene-based thermoplastic elastomer (B) due to cleavage that occurs during stretching. The structure of the domains is closely related to the porous structure, and the larger the domains, the larger the pores, resulting in improved air permeability. Among the styrene-based thermoplastic elastomers (B), a styrene-olefin-styrene copolymer (B1) is preferred because it is easy to obtain large domains. Furthermore, the styrene-olefin copolymer (B2) is unevenly distributed on the surface of the domains, which improves the dispersibility of the domains and increases the uniformity of the porous structure. Furthermore, inside the domain, holes formed by the styrene-olefin copolymer (B2) are formed between large holes formed by the styrene-olefin-styrene copolymer (B1), thereby improving the interconnectivity of the holes and further improving the air permeability.

[0035] The styrene content of the styrene-based thermoplastic elastomer (B) is preferably 20% by mass or more and less than 50% by mass, more preferably 25% by mass or more or 45% by mass or less, even more preferably 30% by mass or more or 45% by mass or less, and particularly preferably 35% by mass or more or 40% by mass or less, based on the total amount of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B). When the styrene content is equal to or greater than the lower limit, domains can be effectively formed in the polyolefin resin (A), and when the styrene content is equal to or less than the upper limit, the formation of excessively large domains can be suppressed. When the resin composition [I] contains two or more types of styrene-based thermoplastic elastomers (B), the styrene content refers to the total amount of all the styrene-based thermoplastic elastomers (B).

[0036] The weight average molecular weight (Mw) of the resin that is the main component of the styrene-based thermoplastic elastomer (B) contained in the resin composition [I] is preferably 100,000 or more, more preferably 150,000 or more or 1,000,000 or less, more preferably 800,000 or less, even more preferably 600,000 or less, even more preferably 500,000 or less, even more preferably 400,000 or less, even more preferably 350,000 or less, and particularly preferably 300,000 or less. Furthermore, the Mw / Mn of the resin that is the main component of the styrene-based thermoplastic elastomer (B) contained in the resin composition [I] is preferably 1.00 or more and 1.50 or less, more preferably 1.40 or less, and even more preferably 1.05 or more or 1.20 or less.

[0037] When a film made of the resin composition [I] is stretched at least uniaxially to form a porous film, the microstructure (morphology) of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) in the resin composition before stretching is one of the factors that affect the formation of the porous structure. Specifically, it is preferable that the styrene-based thermoplastic elastomer (B), which exists as domains in a matrix mainly composed of the polyolefin resin (A), exists in a spherical shape, from the viewpoint of ease of drilling holes, as will be explained below. Generally, when a resin composition having a matrix / domain sea-island structure is melt-extruded and cooled to solidify, the resin composition is extruded from a shaping device such as a die or nozzle and then cooled and solidified using a cooling and solidification device such as a cast roll (cooling roll) or air or water cooling. During this process, the resin composition melts and elongates in the gap between the shaping device and the cooling and solidification device. Therefore, if the weight-average molecular weight of the styrene-based thermoplastic elastomer (B) forming the domains is small, a resin composition is obtained in which the domains are elongated in the flow direction (extrusion direction). When the domains are elongated in the flow direction (extrusion direction), the area of ​​the domain particles in the flow direction increases, making it difficult for cleavage to occur within the domains due to stress in the flow direction (extrusion direction), and this can result in insufficient formation of a porous structure. On the other hand, when the weight-average molecular weight of the domains of the styrene-based thermoplastic elastomer (B) is large, the domains are less susceptible to the effects of melt elongation, and the domains in the resulting resin composition before stretching tend to maintain a spherical shape. When the domains are spherical, cleavage within the domains is more likely to occur due to stress in the flow direction (extrusion direction), which is thought to result in the formation of a sufficient porous structure. For these reasons, it is preferable that the weight-average molecular weight (Mw) of the resin that is the main component of the styrene-based thermoplastic elastomer (B) is within the above range. Furthermore, it is more preferable that the ratio (molecular weight distribution) Mw / Mn of the number-average molecular weight Mn to the weight-average molecular weight (Mw) of the resin that is the main component of the styrene-based thermoplastic elastomer (B) is within the above range, because the dispersion diameter of the formed domains tends to be uniform.

[0038] The styrene-based thermoplastic elastomer (B) contained in the resin composition [I] preferably has a melt flow rate (MFR(B)) measured at a temperature of 200° C. under a load of 10 kg of 2 g / 10 min or less. The styrene-based thermoplastic elastomer (B) dispersed in the resin composition changes its shape depending on the viscosity difference with the polyolefin-based resin (A), but if the styrene-based thermoplastic elastomer has an MFR not greater than the above upper limit, the viscosity difference with the polyolefin-based resin (A) can be easily increased, and the shape is likely to become spherical. The spherically dispersed domains tend to have a more uniform porous structure in the subsequent stretching step than domains with a large aspect ratio, and a porous film can be obtained that is thin but has excellent stability in physical properties. From this viewpoint, the melt flow rate (MFR(B)) is preferably 2 g / 10 min or less, more preferably 1 g / 10 min or less, more preferably 0.5 g / 10 min or less, and even more preferably 0.1 g / 10 min or less. The lower limit is not particularly limited, but is usually 0 g / 10 min, ie, a state where the material does not flow. When two or more types of styrene-based thermoplastic elastomers (B) are used, it is sufficient that at least one type of styrene-based thermoplastic elastomer (B) has the above MFR value. Among these, if a styrene-based thermoplastic elastomer having an MFR within the above range is the main component of the components forming the domains, stress tends to concentrate inside the domains during the stretching step, which makes it easier for pore origins to occur and makes the film porous. The "main component" refers to the component that accounts for the largest proportion by mass of the components that form the domains. The content of the main component can be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more of the components that form the domains. The MFR of the styrene-based thermoplastic elastomer (B) in the present invention is a value measured in accordance with JIS K7210-1 (2014) under conditions of a temperature of 200°C and a load of 10 kg.

[0039] In the resin composition [I], the difference (MFR(A) - MFR(B)) between the melt flow rate (MFR(A)) of the polyolefin resin (A) at a temperature of 230°C and a load of 2.16 kg and the melt flow rate (MFR(B)) of the styrene thermoplastic elastomer (B) at a temperature of 200°C and a load of 10 kg is preferably 1 g / 10 min or more. In recent years, there has been a demand for thinner porous films and improved air permeability, but these two properties are mutually exclusive, and the thinner the film, the worse the air permeability becomes. More specifically, when obtaining a porous film with a thickness of 50 μm or less, especially 30 μm or less, it has been difficult to achieve an air permeability of 10 sec / 100 mL or less. As a result of extensive research in light of the above-mentioned problems, the present inventors have found that, by adjusting the combination of polyolefin resin and styrene-based thermoplastic elastomer and film-forming conditions, etc., extremely good air permeability properties can be maintained even when the thickness is reduced, particularly when the melt flow rates (MFR) of the polyolefin resin and the styrene-based thermoplastic elastomer are significantly different from each other. When producing the porous film of the present invention, the domains are easily stretched in the MD when the film is drawn down into a cast, and holes are formed during MD stretching, originating from the styrene-based thermoplastic elastomer. If the domains are stretched in the MD, stress is less likely to concentrate on the domains, making it difficult for holes to form. It is believed that a large difference in melt flow rate (MFR) between the polyolefin resin and the styrene-based thermoplastic elastomer increases the dispersion diameter of the styrene-based thermoplastic elastomer domains, which are the origin of openings, and suppresses in-plane deformation of the domains, resulting in improved interconnectivity in the thickness direction and reduced air permeability. From the above viewpoints, the difference (MFR(A)-MFR(B)) between the melt flow rate (MFR(A)) of the polyolefin resin (A) and the melt flow rate (MFR(B)) of the styrene-based thermoplastic elastomer (B) is preferably 1 g / 10 min or more, more preferably 3 g / 10 min or more, even more preferably 5 g / 10 min or more, and even more preferably 7 g / 10 min or more. Furthermore, from the viewpoint of film formability, the difference (MFR(A)-MFR(B)) is preferably 20 g / 10 min or less, and more preferably 15 g / 10 min or less. In addition, when the resin composition [I] contains two or more polyolefin resins (A) and / or two or more styrene thermoplastic elastomers (B), the MFR(A) value is the difference between the polyolefin resin (A) having the highest MFR(A) value and the styrene thermoplastic elastomer (B) having the lowest MFR(B) value.

[0040] In the resin composition [I], the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B) is preferably 20% by mass or more and less than 50% by mass, more preferably 25% by mass or more or 45% by mass or less, even more preferably 30% by mass or more or 43% by mass or less, and particularly preferably 32% by mass or more or 40% by mass or less. When the content of the styrene-based thermoplastic elastomer (B) is within the above range, a sea-island structure can be formed in which the polyolefin-based resin (A) serves as the matrix and the styrene-based thermoplastic elastomer (B) serves as the domain, and a porous structure can be easily formed when the film is stretched.

[0041] [Nucleating agent (C)] The resin composition [I] preferably contains a crystal nucleating agent (C) because increasing the rigidity of the polyolefin resin (A) is considered to be effective in forming voids when the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) are stretched to form pores.

[0042] The content of the nucleating agent (C) is preferably 0.001 parts by mass or more and 10 parts by mass or less, more preferably 0.005 parts by mass or more or 5 parts by mass or less, even more preferably 0.01 parts by mass or more or 2 parts by mass or less, and even more preferably 0.05 parts by mass or more or 1.5 parts by mass or less, relative to 100 parts by mass of the resin components in the resin composition [I], i.e., the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B).

[0043] The crystal nucleating agent (C) is preferably an α-crystal nucleating agent or a β-crystal nucleating agent, with the α-crystal nucleating agent being preferred from the viewpoint of excellent heat resistance, while the β-crystal nucleating agent is preferred from the viewpoint of heat fusion properties.

[0044] Examples of α-crystal nucleating agents that can be used include dibenzylidene sorbitol (manufactured by New Japan Chemical Co., Ltd., trade name "Gelall MD"), "ADK STAB NA-11", "ADK STAB NA-27", "ADK STAB NA-902", "ADK STAB NA-21", and "ADK STAB NA-71" (all five of which are manufactured by ADEKA Corporation), a masterbatch containing a mixture of magnesium stearate and silica (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., trade name "Hi-Cycle Master"), and a masterbatch containing the sodium salt of 2-hydroxy-2-oxo-4,6,10,12-tetra-tert-butyl-1,3,2-dibenzo[d,g]perhydrodioxaphosphalocin (manufactured by ADEKA Corporation, trade name "ADK STAB M-701").

[0045] Other examples of α-crystal nucleating agents include inorganic nucleating agents such as silica, talc, and calcium carbonate. Organic (metal carboxylate-type) nucleating agents include calcium stearate, sodium benzoate, aluminum benzoate, aluminum dibenzoate, potassium benzoate, lithium benzoate, sodium β-naphthalate sodium cyclohexyl carboxylate, metal pimelic acid salt, and metal rosinate. Other examples include benzylidene sorbitol and its derivatives, and metal phosphate esters. Furthermore, polymer-type nucleating agents include poly-3-methylbutene-1, polyvinylcycloalkane, polyvinyltrialkylsilane, EPR, Kevlar® fiber, and sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate.

[0046] The content of the α-crystal nucleating agent in the resin composition [I] is preferably 0.001 parts by mass or more and 10 parts by mass or less, more preferably 0.005 parts by mass or more or 5 parts by mass or less, even more preferably 0.01 parts by mass or more or 2 parts by mass or less, and even more preferably 0.05 parts by mass or more or 1.5 parts by mass or less, relative to 100 parts by mass of the resin components in the resin composition [I], i.e., the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B). When the content of the α-crystal nucleating agent is equal to or greater than the lower limit, the formation of α-crystals can be sufficient. On the other hand, when the content of the α-crystal nucleating agent is equal to or less than the upper limit, there is no risk of bleeding, plate-out, powder falling, etc. of the α-crystal nucleating agent occurring during the film-forming process, secondary processing, etc., and costs are reduced, which is preferable.

[0047] Examples of β-crystal nucleating agents include amide compounds; tetraoxaspiro compounds; quinacridones; iron oxides having nanoscale sizes; alkali or alkaline earth metal salts of carboxylic acids, such as potassium 1,2-hydroxystearate, magnesium benzoate or magnesium succinate, and magnesium phthalate; aromatic sulfonic acid compounds, such as sodium benzenesulfonate or sodium naphthalenesulfonate; or di- or triesters of tribasic carboxylic acids; phthalocyanine pigments, such as phthalocyanine blue; two-component compounds comprising component A, which is an organic dibasic acid, and component B, which is an oxide, hydroxide, or salt of a metal from Group IIA of the periodic table; and compositions comprising a cyclic phosphorus compound and a magnesium compound. In addition, specific types of nucleating agents are described in JP-A Nos. 2003-306585, 06-289566, and 09-194650.

[0048] Examples of commercially available β-crystal nucleating agents include the β-crystal nucleating agent "Njestar NU-100" manufactured by New Japan Chemical Co., Ltd., and specific examples of polypropylenes containing β-crystal nucleating agents include the polypropylene "BepolB-022SP" manufactured by Aristech, the polypropylene "Beta(β)-PPBE60-7032" manufactured by Borealis, and the polypropylene "BNXBETAPP-LN" manufactured by Mayzo.

[0049] The β-crystal nucleating agent is preferably blended with the polyolefin resin (A). In this case, the ratio of the β-crystal nucleating agent to the polyolefin resin (A) is preferably adjusted appropriately depending on the type of β-crystal nucleating agent or the composition of the polyolefin resin (A). From this viewpoint, the content of the β-crystal nucleating agent is preferably 0.0001 to 5.0 parts by mass relative to 100 parts by mass of the total amount of the resin components, i.e., the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B), more preferably 0.001 part by mass or more or 3.0 parts by mass or less, and even more preferably 0.01 part by mass or more or 1.0 part by mass or less. If the content of the β-crystal nucleating agent is equal to or greater than the lower limit, β-crystals of the polyolefin resin (A) can be sufficiently generated and grown during production, sufficient β-crystal activity can be ensured, and the desired air permeability can be obtained when the film is used as an ion exchange membrane support. Furthermore, if the content is equal to or less than the upper limit, this is economically advantageous and also preferred because there is no bleeding of the β-crystal nucleating agent onto the surface of the porous film. Furthermore, if a layer containing a polyolefin is laminated in addition to the resin layer made of the polyolefin resin (A), the amount of the β-crystal nucleating agent added to each layer may be the same or different. By changing the amount of the β-crystal nucleating agent added, the porous structure of each layer can be appropriately adjusted.

[0050] [Other ingredients] The resin composition [I] may contain various additives, such as heat stabilizers, antioxidants, ultraviolet absorbers, light stabilizers, colorants, antistatic agents, hydrolysis inhibitors, lubricants, and flame retardants, as appropriate, to the extent that the properties of the resin composition are not impaired. The resin composition [I] may also contain other resins to the extent that the properties of the resin composition are not impaired. Furthermore, the resin composition [I] may be subjected to surface treatments such as corona treatment, plasma treatment, printing, coating, vapor deposition, and even perforation, as needed, within the scope of the present invention, and several sheets of the porous film of the present invention may be stacked and used depending on the application.

[0051] <Porous film of the present invention> The porous film of the present invention may be formed solely from a porous layer made of the resin composition, or may have other layers laminated thereon as long as the features of the present invention are not impaired.

[0052] (Thickness) The thickness (T) of the porous film of the present invention is preferably less than 50 μm. When the porous film of the present invention is used as an ion exchange membrane support in an electricity storage device, the thickness (T) of the porous film of the present invention can contribute to a thinner cell stack and higher integration. On the other hand, the lower limit of the thickness (T) of the porous film of the present invention is preferably 1 μm or more in order to maintain sufficient strength as a film. From this viewpoint, the thickness (T) of the porous film of the present invention is preferably less than 50 μm, more preferably 1 μm or more or 30 μm or less, even more preferably 2 μm or more or 25 μm or less, and even more preferably 5 μm or more or 20 μm or less.

[0053] (Air permeability (S)) The air permeability (S) of the porous film of the present invention is preferably less than 10 seconds / 100cc, more preferably 9 seconds / 100cc or less, even more preferably 8 seconds / 100cc or less, and even more preferably 7 seconds / 100cc or less, with the lower limit usually being 0.25 seconds / 100cc. The air permeability (S) in the present invention is measured under the condition of 25°C in accordance with JIS P8117:2009. In order to reduce the air permeability (S) of the porous film of the present invention, it is sufficient to increase the porosity of the continuous pores and increase the diameter of the continuous pores. For example, to achieve this, the pore diameter can be increased by increasing the difference in viscosity (e.g., the difference in MFR) between the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B), or by using a polyolefin resin (A) with a relatively low molecular weight to increase the pore diameter, or by adjusting the thickness of the film. In addition, the air permeability (S) can also be adjusted by changing conditions such as the casting temperature and the stretch ratio during film formation. However, the air permeability (S) is not limited to these.

[0054] (porosity) In order to ensure the stiffness of the porous film of the present invention, the porosity of the film is preferably 80% or less, more preferably 78% or less, and even more preferably 76% or less. On the other hand, in order to improve the air permeability, the porosity of the film is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. The porosity of the porous film of the present invention can be adjusted within the above range by, for example, changing the chemical structure, molecular weight, MFR, tensile modulus and content of the polyolefin resin (A), the chemical structure, molecular weight, MFR and content of the styrene thermoplastic elastomer (B), as well as the casting temperature and stretching ratio during film formation. When the porous film of the present invention has a layer other than the porous layer, it is sufficient that the porosity of only the porous layer is within the above range. The porosity is the total porosity of interconnected pores and non-interconnected pores, and can be calculated from the density of the resin composition by the method described in the examples.

[0055] (MD tensile modulus) The porous film of the present invention preferably has a tensile modulus at 23° C. in the machine direction (MD) of 30 MPa or more. When the tensile modulus of the porous film of the present invention in the machine direction (MD) at 23°C is 30 MPa or more, the occurrence of wrinkles during the film production process can be suppressed, and the roll of the film can be made less likely to wrinkle when wound into a roll. From this viewpoint, the tensile modulus of elasticity in the machine direction (MD) of the porous film of the present invention at 23° C. is preferably 30 MPa or more, more preferably 35 MPa or more, even more preferably 40 MPa or more, and even more preferably 45 MPa or more. There is no particular upper limit, but it can be, for example, 500 MPa or less. The tensile modulus at 23° C. of the porous film of the present invention can be adjusted by the molecular weight, melting point, MFR, content, stretching conditions, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B).

[0056] (MD storage modulus) The porous film of the present invention preferably has a storage modulus (E') in the machine direction (MD) at 150°C and 10 Hz of 1 MPa or more. When the storage modulus (E') of the porous film of the present invention in the machine direction (MD) at 150°C and 10 Hz is 1 MPa or more, it can be said to have heat resistance. From this viewpoint, the storage modulus (E') of the porous film of the present invention in the machine direction (MD) at 150°C and 10 Hz is preferably 1 MPa or more, more preferably 3 MPa or more, even more preferably 5 MPa or more, and even more preferably 7 MPa or more. There is no particular upper limit, but it can be assumed to be, for example, 100 MPa or less. The MD storage modulus of the porous film of the present invention at 150° C. and 10 Hz can be adjusted by the molecular weight, melting point, MFR, content, stretching conditions, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B).

[0057] (S / T) The ratio (S / T) of the air permeability (S) (seconds / 100cc) to the thickness (T) (μm) of the porous film of the present invention, i.e., the ratio (S / T) of the air permeability (S) (seconds / 100cc) to the thickness (T) (μm), is preferably 0.5 seconds / (100cc·μm) or less. By keeping the ratio (S / T) of air permeability (S) to thickness (T) at 0.5 s / (100 cc μm) or less, a porous film with excellent air permeability can be obtained despite its thin thickness (e.g., 50 μm or less). On the other hand, the lower limit is 0.01 s / (100 cc μm) from the viewpoint of pore size and porosity. From this viewpoint, the (S / T) value is preferably 0.5 seconds / (100cc·μm) or less, more preferably 0.45 seconds / (100cc·μm) or less, even more preferably 0.40 seconds / (100cc·μm) or less, even more preferably 0.35 seconds / (100cc·μm) or less, and even more preferably 0.30 seconds / (100cc·μm) or less. On the other hand, from the viewpoint of pore size and porosity, the (S / T) value is preferably 0.01 seconds / (100cc·μm) or more, even more preferably 0.02 seconds / (100cc·μm) or more, even more preferably 0.03 seconds / (100cc·μm) or more, even more preferably 0.04 seconds / (100cc·μm) or more, and even more preferably 0.05 seconds / (100cc·μm) or more. The value of (S / T) can be adjusted by, for example, the chemical structure, molecular weight, MFR, content, casting temperature during film formation, stretching ratio, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B), but is not limited to these methods.

[0058] (Bubble Point Pressure) The bubble point pressure (P BA ) is preferably 5 kPa or more and 80 kPa or less. The bubble point pressure of a film is determined by immersing the film in a liquid with low surface tension, such as alcohol, injecting air into the top or bottom of the film, and gradually increasing the pressure, and the pressure is measured at which the first bubble appears. The bubble point pressure of a film is proportional to the surface tension of the liquid and inversely proportional to the pore size, and therefore correlates with the largest pore in the film, i.e., the maximum pore size. For details on the method for measuring the bubble point pressure of the porous film of the present invention, please refer to the measurement method in the Examples. The larger the maximum pore diameter of the porous film of the present invention, the higher the air permeability can be, but if it is too large, it may cause breakage or the like when the film is thin. From this viewpoint, in order to adjust the maximum pore diameter of the porous film of the present invention within a preferred range, the bubble point pressure correlated therewith is preferably 5 kPa or more, more preferably 8 kPa or more, even more preferably 11 kPa or more, even more preferably 14 kPa or more. On the other hand, from the viewpoint of breathability, it is preferably 80 kPa or less, even more preferably 75 kPa or less, even more preferably 70 kPa or less, even more preferably 65 kPa or less. The bubble point pressure of the porous film of the present invention can be adjusted by, for example, the chemical structure, molecular weight, MFR, tensile modulus and content of the polyolefin resin (A), the chemical structure, molecular weight, MFR and content of the styrene thermoplastic elastomer (B), as well as the casting temperature and stretching ratio during film formation, but is not limited thereto.

[0059] (Maximum pore diameter) The maximum pore size in the porous film of the present invention is preferably 0.5 μm or more and 9.0 μm or less. The maximum pore size is determined by the bubble point pressure (P BA ) is the maximum pore diameter calculated from As mentioned above, the larger the maximum pore diameter of the porous film of the present invention, the higher the air permeability can be, but if it is too large, it may cause breakage or the like when the film is thin. Therefore, the maximum pore size of the porous film of the present invention is preferably 0.5 μm or more, more preferably 1.0 μm or more, even more preferably 1.5 μm or more, even more preferably 2.0 μm or more, and is preferably 9.0 μm or less, even more preferably 7.0 μm or less, even more preferably 5.0 μm or less, even more preferably 3.0 μm or less.

[0060] (S / P BA ) The air permeability (S) (sec / 100cc) and bubble point pressure (P BA ) (kPa) ratio (S / P BA ), i.e., bubble point pressure (P BA ) (kPa) of the porous film of the present invention (S) (seconds / 100cc) to the air permeability (S / PBA ) is correlated with the film thickness and porosity, and the ratio (S / P BA ) is preferable because the film tends to be thin and have a high porosity. From this viewpoint, the ratio (S / P) of the porous film of the present invention BA ) is preferably 0.5 seconds / (100cc kPa) or less, more preferably 0.4 seconds / (100cc kPa) or less, even more preferably 0.3 seconds / (100cc kPa) or less, and even more preferably 0.2 seconds / (100cc kPa) or less. The lower limit is usually 0.003 seconds / (100cc kPa) from the viewpoint of film thickness and porosity. In the porous film of the present invention, the ratio (S / P BA ) can be adjusted by, for example, the chemical structure, molecular weight, MFR, content, casting temperature during film formation, stretching ratio, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B). However, it is not limited to these methods.

[0061] (average tensile strength) The average tensile strength of the porous film of the present invention is preferably 2 MPa or more. When the average tensile strength of the porous film of the present invention is 2 MPa or more, excellent mechanical properties can be obtained. From this viewpoint, the average tensile strength of the porous film of the present invention is more preferably 4 MPa or more, more preferably 6 MPa or more, more preferably 8 MPa or more, and even more preferably 10 MPa or more. The upper limit is not particularly limited, but may be, for example, 400 MPa or less, 300 MPa or less, 200 MPa or less, or 100 MPa or less. The porous film of the present invention having an average tensile strength of not more than the upper limit is preferred in terms of excellent handleability. The average tensile strength in the present invention is the average value of the tensile strength in MD and TD measured by the method described in the Examples. When the MD and TD are unknown, the average value of the tensile strength in an arbitrary length direction and the width direction perpendicular thereto may be used.

[0062] (Average tensile elongation at break) The average tensile elongation at break of the porous film of the present invention is preferably 1% or more, more preferably 10% or more, even more preferably 20% or more, and even more preferably 30% or more. The upper limit is not particularly limited, but may be, for example, 200% or less. When the average tensile elongation at break of the porous film of the present invention is within the above range, excellent mechanical properties can be obtained. The average tensile breaking elongation in the present invention is the average value of the tensile breaking elongation in MD and TD measured by the method described in the Examples. When MD and TD are unknown, the average value of the tensile breaking elongation in an arbitrary length direction and the width direction perpendicular thereto may be used.

[0063] The average tensile strength and average tensile elongation at break can be adjusted by the chemical structure, molecular weight, MFR, content, casting temperature during film formation, stretching ratio, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B). For example, the average tensile strength and average tensile elongation at break can be adjusted by using a styrene-based thermoplastic elastomer (B) that has low compatibility with the polyolefin-based resin (A) as the main component; using a resin with a high weight-average molecular weight (Mw) as the main component of the styrene-based thermoplastic elastomer (B); using a resin with a low MFR (i.e., high viscosity) as the styrene-based thermoplastic elastomer (B); adjusting the content ratio of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B) to be within the above-mentioned preferred range; adjusting the temperature of the cast roll during film formation to be within a preferred range in the production method described below; and, in the case of biaxial stretching, adjusting the longitudinal and transverse stretch ratios and stretch ratios to be within preferred ranges. The application of the above-mentioned means not only controls the morphology of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B), but also leads to the control of the formation of the pore structure and the film thickness, which is thought to result in the formation of an appropriate porous structure and improve the air permeability properties.

[0064] <Method of manufacturing the porous film of the present invention> The method for producing the porous film of the present invention (hereinafter also referred to as "the method") preferably comprises a step (I) of melt-extruding a resin composition [I] mainly containing a polyolefin resin (A), preferably a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B) to obtain a pre-stretched sheet, and a step (II) of stretching the pre-stretched sheet to obtain a porous film.

[0065] [Process (I)] First, in step (I), a resin composition [I], such as a polyolefin resin (A), a styrene-based thermoplastic elastomer (B), and a crystal nucleating agent (C), is melted and kneaded using an extruder or the like under a temperature condition of not less than the melting point and not more than the decomposition temperature of the polyolefin resin (A), and then molded into a non-porous sheet before stretching. In this case, the non-porous sheet can be formed, for example, by T-die forming.

[0066] When the kneaded material is cooled and molded into a sheet, the temperature of the cast roll is preferably 60°C or higher and 150°C or lower, more preferably 70°C or higher or 140°C or lower, even more preferably 80°C or higher or 130°C or lower, and even more preferably 90°C or higher or 120°C or lower. When the temperature of the casting roll is equal to or higher than the lower limit, the crystallization of the polyolefin resin (A) proceeds sufficiently, resulting in the formation of a sufficient porous structure. On the other hand, when the temperature of the casting roll is equal to or lower than the upper limit, problems such as the sheet before stretching fusing with the roll and rupturing are less likely to occur.

[0067] The speed of the casting roll is preferably 1.0 m / min or more and 5.0 m / min or less, more preferably 1.2 m / min or more and 4.0 m / min or less, and even more preferably 1.5 m / min or more and 3.0 m / min or less. By setting the speed of the cast roll to at least the above lower limit, productivity is improved, whereas by setting the speed of the cast roll to at most the above upper limit, the domains in the sheet before stretching tend to remain spherical, and a sufficient porous structure tends to be formed after stretching.

[0068] [Process (Ia)] In the present production method, when the resin composition is melt-extruded into a sheet and brought into close contact with a casting roll in the step (I), a step (Ia) of bringing the sheet into close contact with a casting roll can be included. Specific examples of the method include a touch roll, pinning, and touch rollers at both ends in the sheet width direction.

[0069] By closely adhering the sheet to the cast roll, uneven adhesion is less likely to occur, which in turn suppresses uneven pore formation in the film after stretching, resulting in a porous film with uniform properties.

[0070] [Process (II)] Next, the obtained unstretched sheet is stretched uniaxially or biaxially. The uniaxial stretching may be longitudinal uniaxial stretching or transverse uniaxial stretching, and the biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching.

[0071] To produce the porous film of the present invention having gas permeability, the stretching conditions may be selected appropriately in each stretching step, and it is more preferable to employ sequential biaxial stretching, which makes it easier to control the porous structure. Stretching of the sheet in the machine direction (MD) is called "longitudinal stretching", and stretching in the direction perpendicular to the machine direction (TD) is called "transverse stretching".

[0072] When sequential biaxial stretching is used, it is relatively easy to control the porous structure and to balance it with other physical properties such as mechanical strength, shrinkage rate, etc. The stretching temperature may be appropriately selected depending on the composition, crystalline melting peak temperature, crystallinity, etc. of the resin composition used.

[0073] It is desirable to carry out the longitudinal stretching at a low temperature, and the specific stretching temperature is preferably 0 to 50°C, more preferably 5°C or higher or 40°C or lower. If the longitudinal stretching temperature is below the upper limit, stress tends to concentrate inside the domain during stretching, making voids more likely to form. On the other hand, if the longitudinal stretching temperature is above the lower limit, breakage during stretching can be suppressed, which is preferable.

[0074] The longitudinal stretching (MD) ratio at low temperatures is preferably 1.1 to 5.0 times, more preferably 1.2 times or more or 4.5 times or less, and even more preferably 1.3 times or more or 4.0 times or less. It is suggested that by setting the stretching ratio at low temperature to the above lower limit or more, pore formation progresses and sufficient porosity is generated by stretching. Furthermore, by setting the stretching ratio at low temperature to the above upper limit or less, breakage during stretching can be suppressed.

[0075] Furthermore, the longitudinal stretching (MD) may be a two-stage stretching process in which the above-mentioned low-temperature stretching is followed by high-temperature stretching. The specific high-temperature stretching temperature is preferably 60 to 155°C, more preferably 70°C or higher or 140°C or lower, and even more preferably 80°C or higher or 130°C or lower. By setting the temperature for longitudinal stretching at a high temperature to be equal to or higher than the above lower limit, it is possible to prevent film breakage during stretching, whereas by setting the temperature for longitudinal stretching at a high temperature to be equal to or lower than the above upper limit, it is possible to prevent pores formed during stretching at a low temperature from being blocked.

[0076] The longitudinal stretching (MD) ratio at high temperature is preferably 1.1 to 5.0 times, more preferably 1.2 times or more or 4.5 times or less, even more preferably 1.3 times or more or 4.0 times or less, and even more preferably 1.5 times or more or 3.0 times or less. By setting the stretching ratio at high temperature to the above lower limit or more, the pores formed by longitudinal stretching at low temperature can be enlarged. Also, by setting the stretching ratio at high temperature to the above upper limit or less, the film properties (thickness, air permeability, and mechanical properties) can be adjusted to preferred ranges, and film breakage during stretching can be suppressed.

[0077] The temperature for transverse stretching (TD) is preferably 100 to 155°C, more preferably 110°C or higher or 150°C or lower. When the transverse stretching temperature is within the above range, the pores generated during the longitudinal stretching are enlarged, thereby increasing the porosity of the porous layer and providing sufficient air permeability and mechanical properties.

[0078] The transverse stretching (TD) ratio can be selected arbitrarily, but is preferably 1.1 to 10 times, more preferably 1.5 times or more or 8.0 times or less, and even more preferably 2.0 times or more or 6.0 times or less. By stretching at the above-mentioned transverse stretching ratio, the pores generated during longitudinal stretching are not deformed, and a sufficient porosity can be obtained.

[0079] When the porous film of the present invention is produced by biaxial stretching, the stretching ratio between the longitudinal and transverse directions is preferably 0.5 to 1.5:1, more preferably 0.7 to 1.2:1. When the stretching ratio is within the above range, the film properties (thickness, air permeability and mechanical properties) can be adjusted to preferred ranges. When the longitudinal stretching is carried out in two stages at low temperature and high temperature, it is sufficient that the transverse stretching ratio (longitudinal stretching ratio at low temperature x longitudinal stretching ratio at high temperature) is within the above range.

[0080] <Application> The porous film of the present invention can be used in various fields, for example, as a separation membrane used in the production of ultrapure water, the purification of chemical solutions, water treatment, etc., a waterproof and moisture-permeable film used in clothing and sanitary materials, etc., and a separator used in electronic components such as capacitors, batteries, and electrolytic capacitors. In particular, the porous film of the present invention can be suitably used as a support for an ion exchange membrane.

[0081] (ion exchange membrane support) The porous film of the present invention is useful as a support for an ion exchange membrane. That is, the support for an ion exchange membrane of the present invention comprises the porous film of the present invention. The preferred embodiments and preferred properties of the ion exchange membrane support of the present invention are the same as those described for the porous film.

[0082] (Electricity storage device) The electricity storage device of the present invention preferably includes the ion exchange membrane support of the present invention. Examples of the power storage device include lithium secondary batteries such as redox flow batteries, nickel-hydrogen batteries, lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries, as well as capacitor-based devices such as aluminum electrolytic capacitors, electric double layer capacitors, and lithium ion capacitors. Among these, redox flow batteries and lithium secondary batteries are preferred. [Example]

[0083] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded.

[0084] <Measurement method> (1) Bubble point pressure (P BA ) and mean flow diameter pressure (P Ap ) Using a perm porometer (Porous Materials), the bubble point pressure (P) of the porous film (sample) was measured in accordance with ASTM F316-86 using a polyhexafluoropropene liquid "GALWICK" (Porous Materials, surface tension: 15.6 dynes / cm) as a reagent. BA ) was measured. Bubble point pressure (P BA ) and plot the wet flow curve, dry flow curve, and 1 / 2 dry flow curve on a graph, and calculate the mean flow diameter pressure (P Ap ) was sought.

[0085] (2) Maximum pore diameter The bubble point pressure (P BA ) was used to calculate the maximum pore diameter d of the porous film (sample) from the following formula. d=cγ / P c: constant 2860 γ: Surface tension of the liquid "GALWICK": 15.6 dynes / cm P: the bubble point pressure P BA

[0086] (3) Air permeability (S) The air resistance of the porous film (sample) was measured in accordance with JIS P8117: 2009 in an air atmosphere at 25° C. The measuring device used was a digital Oken-type dedicated air permeability machine (manufactured by Asahi Seiko Co., Ltd.).

[0087] (4) Melt flow rate (MFR) The MFR(A) of the polyolefin resin (A) was measured in accordance with JIS K7210-1 (2014) at a temperature of 230°C and a load of 2.16 kg. In addition, the MFR(B) of the styrene-based thermoplastic elastomer (B) was measured at a temperature of 200°C and a load of 10 kg in accordance with JIS K7210-1 (2014). The difference (MFR(A)-MFR(B)) was calculated from the obtained MFR(A) and MFR(B) values. In addition, the MFR value of those that did not flow in the above MFR measurement was set to 0 in the calculation.

[0088] (5) Thickness (film thickness) (T) The porous film (sample) was cut into a 10 cm square, and measurements were taken at nine points using a dial gauge with a graduation of 1 / 1000 mm, and the average value was taken as the thickness.

[0089] (6) Porosity The porous film (sample) was cut into a 10 cm square, the density ρ1 (apparent density) was measured, and the density ρ0 (true density) when the porosity was 0% was calculated. The porosity was calculated from these values ​​using the following formula. Porosity (%)=(1-ρ1 / ρ0)×100

[0090] (7) Tensile strength, tensile elongation at break The measurement device used was a tensile tester (Shimadzu Corporation AG-1kNXplus tensile tester). Test specimens were rectangular specimens cut from porous film (samples) with a length of 100 mm in the measurement direction and a width of 15 mm. Both longitudinal ends of the specimen were chucked with a chuck distance of 50 mm and pulled at a crosshead speed of 50 mm / min. The maximum strength from the start of the test to break was measured as the tensile strength, and the elongation at break was measured as the tensile breaking elongation. Five measurements were taken at five points in the width direction, and the average values ​​were calculated. The tensile tests were conducted in both the MD and TD directions of the film. The average tensile strength and average tensile breaking elongation were calculated by averaging the MD and TD values ​​for tensile strength and tensile breaking elongation, respectively. Measurements were performed at room temperature (25°C).

[0091] (8) Tensile modulus The measurement device used was a tensile tester (Shimadzu Corporation, AGS-X tensile tester). Test pieces were cut out from porous film (sample) into rectangles measuring 150 mm in the measurement direction and 10 mm in width. Both ends of the test piece in the longitudinal direction were chucked with a chuck distance of 100 mm and pulled at a crosshead speed of 5 mm / min to determine the tensile modulus. Measurements were carried out at five points in the width direction, and the average value was calculated. The above tensile tests were carried out in both the MD and TD directions of the film. The average tensile modulus was calculated by averaging the MD and TD values ​​for each tensile modulus. Measurements were carried out at room temperature (25°C).

[0092] (9) Storage modulus (E') Using a dynamic viscoelasticity measuring device ("DVA-200" manufactured by IT Measurement Control Co., Ltd.) and a tensile jig, measurements were taken at a measurement temperature of 30 to 200°C, a strain of 0.1%, a frequency of 10 Hz, and a heating rate of 3°C / min, and the storage modulus (E') of the porous film (sample) at 150°C is shown in the table. In this case, MD measurements were taken using a sample with an MD gauge length of 2 cm and a TD width of 0.4 cm, and TD measurements were taken using a sample with a TD gauge length of 2 cm and an MD width of 0.4 cm.

[0093] <Material> (Polyolefin resin (A)) (A-1) Homopolypropylene (weight average molecular weight (Mw): 254,300, molecular weight distribution (Mw / Mn): 5.03, MFR(A) (230°C, 2.16 kg): 10 g / 10 min, tensile modulus at 23°C: 2050 MPa, flexural modulus: 2050 MPa, melting point: 169°C) (A-2) Homopolypropylene (weight average molecular weight (Mw): 538,000, molecular weight distribution (Mw / Mn): 3.22, MFR(A) (230°C, 2.16 kg): 1.9 g / 10 min, tensile modulus at 23°C: 1700 MPa, flexural modulus: 1800 MPa, melting point: 167°C) The tensile modulus is a value measured using a tensile tester in accordance with JIS K7161 and JIS K7162.

[0094] (Styrene-based thermoplastic elastomer (B)) (B-1) Styrene-ethylene-propylene block copolymer (SEP) (weight average molecular weight (Mw): 148,000, molecular weight distribution (Mw / Mn): 1.04, MFR (230°C, 2.16 kg): not flowing, MFR(B) (200°C, 10 kg): 1.8 g / 10 min, styrene content: 36% by mass) (B-2) Styrene-ethylene-propylene-styrene block copolymer (SEPS) (weight average molecular weight (Mw): 271,000, molecular weight distribution (Mw / Mn): 1.09, MFR (230°C, 2.16 kg): not flowing, MFR(B) (200°C, 10 kg): not flowing, styrene content: 20% by mass) (B-3) Styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS) (weight average molecular weight (Mw): 196,000, molecular weight distribution (Mw / Mn): 1.07, MFR (230°C, 2.16 kg): not flowing, MFR(B) (200°C, 10 kg): not flowing, styrene content: 32% by mass)

[0095] (Nucleating Agent (C)) (C-1) Alpha crystal nucleating agent (1,3:2,4-bis-O-(4-methylbenzylidene)-D-sorbitol) (C-2) β-crystal nucleating agent (3,6-bis[4-(N-cyclohexylcarbamoyl)phenyl]-2,4,8,10-tetraoxaspiro[5.5]undecane)

[0096] (Additive (D)) (D-1) Phosphorus-based antioxidant (3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane) (D-2) Phenolic antioxidant (3,9-bis[2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane)

[0097] (Additive (E)) (E-1) Metal soap-based external lubricant (1,2-hydroxystearic acid soap, metal component: Zn (zinc), metal content: 9.0 to 10.0%)

[0098] <Preparation of porous film> Example 1 A material containing 60% by mass of polyolefin resin (A-1), 20% by mass of styrene-based thermoplastic elastomer (B-1), and 20% by mass of styrene-based thermoplastic elastomer (B-2), 0.05 parts by mass of nucleating agent (C-2) per 100 parts by mass of the resin components, 0.1 parts by mass of (D-1) per 100 parts by mass of the resin components, and 0.1 parts by mass of (D-2) per 100 parts by mass of the resin components, was fed into a φ44 mm twin-screw extruder and melt-kneaded at a set temperature of 205°C to produce full compound pellets. Next, the prepared pellets were fed into a φ65 mm single-screw extruder, melt-kneaded at a set temperature of 220°C, and then formed into a sheet using a T-die. The sheet was then placed on a cast roll set under the conditions shown in the table and cooled to solidify, yielding a pre-stretched sheet with a thickness of 80 μm. The obtained pre-stretched sheet was then low-temperature stretched between a roll (X) set at 25°C and a roll (Y) set at 25°C at the ratio shown in the table to obtain an MD-stretched porous film. Next, the obtained MD-stretched porous film was preheated at a preheating temperature of 145°C in a film tenter facility, and then stretched in the transverse direction at a stretching temperature of 145°C at the ratio shown in the table, and then heat-treated at 155°C to obtain a porous film (sample). The evaluation results of the obtained film are summarized in Table 1.

[0099] <Example 2> A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-1), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), and additives (D-1) and (D-2) were as shown in Table 1, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0100] Example 3 A pre-stretched sheet having a thickness of 120 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-1), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), additives (D-1), (D-2), and additive (E-1) were as shown in Table 1, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0101] Example 4 A pre-stretched sheet having a thickness of 115 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-1), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), additives (D-1), (D-2) and additive (E-1) were as shown in Table 1 and cooling solidification was carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0102] <Comparative Example 1> A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-2), the styrene thermoplastic elastomers (B-1), (B-2), and the crystal nucleating agent (C-1) were as shown in Table 1, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0103] <Comparative Example 2> A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-2), the styrene thermoplastic elastomers (B-1), (B-2), and the crystal nucleating agent (C-1) were as shown in Table 1, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0104] <Comparative Example 3> A pre-stretched sheet having a thickness of 95 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-1), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), additives (D-1), (D-2), and additive (E-1) were as shown in Table 1, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0105] <Comparative Example 4> A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 1, except that the contents of the polyolefin resin (A-1), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), additives (D-1), (D-2), and additive (E-1) were set as shown in Table 1, and cooling and solidification was carried out under the cast roll conditions shown in Table 1. Thereafter, when the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 1, the sheet broke during longitudinal stretching, and no porous film (sample) was obtained.

[0106] [Table 1]

[0107] As shown in Table 1, in Examples 1 to 4, porous films were obtained that were thin and stiff, yet had good air permeability. These films did not suffer from uneven adhesion or film breakage during film formation, and film wrinkles during the film formation process were also reduced. On the other hand, the porous films of Comparative Examples 1 to 4, which did not satisfy the conditions that the MD tensile modulus (23°C) of the porous film was 30 MPa or more, the porosity was 80% or less, and the air permeability (S) was less than 10 seconds / 100 cc, had good air permeability properties but were unable to achieve both thinness and stiffness of the film.

[0108] In Comparative Example 1, the viscosity of the PP was high and the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, so it is presumed that holes were difficult to form and the air permeability was poor. In Comparative Example 2, the viscosity of the PP was high and the casting speed was relatively fast, so it is presumed that the domain shape was stretched in the MD, making it difficult to open pores and resulting in poor air permeability. In Comparative Example 3, the mean flow meter pressure and bubble point pressure were low, so the pore diameter was large, especially the maximum pore diameter, and the MD stretch ratio was also 3 times, which is presumably why the MD tensile modulus (23°C) was low. In Comparative Example 4, the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, so stress was likely to be concentrated in the polyolefin-based resin, which is the matrix component, and it is presumed that this led to breakage during longitudinal stretching, which is low-temperature stretching.

[0109] From the above examples and comparative examples and the results of the tests conducted by the present inventors, it has been found that in a porous film formed from a resin composition [I] containing a polyolefin resin (A) as a main component, by making the tensile modulus at 23°C in the machine direction (MD) 30 MPa or more, the porosity 80% or less, and the air permeability (S) less than 10 sec / 100 cc, it is possible to obtain a porous film that is thin and stiff, yet has better air permeability properties.

Claims

1. A porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), a content ratio of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin-based resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more but less than 50% by mass, The tensile modulus at 23°C in the machine direction (MD) is 30 MPa or more, The porosity is 80% or less, and A porous film having an air permeability (S) of less than 10 seconds / 100 cc.

2. The porous film according to claim 1, wherein the ratio (S / T) of the air permeability (S) (seconds / 100 cc) to the thickness (T) (μm) of the porous film is 0.5 seconds / (100 cc μm) or less.

3. 2. The porous film according to claim 1, wherein the polyolefin resin (A) is a polypropylene resin.

4. 2. The porous film according to claim 1, wherein the polyolefin resin (A) has a melt flow rate (MFR(A)) of 1 g / 10 min or more at a temperature of 230° C. and a load of 2.16 kg.

5. 2. The porous film according to claim 1, wherein the styrene-based thermoplastic elastomer (B) has a melt flow rate (MFR(B)) of 2 g / 10 min or less at a temperature of 200° C. and a load of 10 kg.

6. 2. The porous film according to claim 1, having a thickness (T) of less than 50 μm.

7. The porous film according to claim 1, wherein the resin composition [I] contains a crystal nucleating agent (C).

8. An ion exchange membrane support comprising the porous film according to any one of claims 1 to 7.

9. An electricity storage device comprising the ion exchange membrane support according to claim 8 .

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