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

Abstract

A porous film having both thinness and air permeability is provided, the porous film being formed from [I] a resin composition containing (A) a polyolefin resin and (B) a thermoplastic styrenic elastomer. The porous film contains 20-50% of the (B) thermoplastic styrenic elastomer by mass, relative to the combined mass of the (A) polyolefin resin and the (B) thermoplastic styrenic elastomer. In the porous film, the air permeability (S) is less than 10 sec / 100 cc and the bubble point pressure (PBA) is 5-80 kPa.

Description

Porous film, ion exchange membrane support and electricity storage device

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

[0002] Porous polymer films having a large number of interconnecting fine 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 mainly composed of polyolefin resins are used as separators in electricity storage devices such as various capacitors, various batteries, various electrolytic capacitors, etc. Such porous films mainly composed of polyolefin resins are produced, for example, by a method of controlling the crystallization of the polyolefin resin in the cooling and solidifying process from 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 to generate 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 solvent-extracting them.

[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, followed by biaxial stretching. These techniques allow for the adjustment of the porous structure depending on the type of styrene-based thermoplastic elastomer. Patent Document 5 also proposes that a polyolefin microporous film having a predetermined thickness and a specific tortuosity exhibits excellent isotropic mass transfer. Patent Document 6 describes a laminated porous film having an inorganic particle-containing layer on at least one side of a porous polyolefin film, and proposes that by adjusting the storage modulus at a predetermined temperature, the film can exhibit good air permeability, strength, and heat resistance.

[0005] JP 2016-141786 A JP 2017-222823 A JP 2015-230743 A JP 2019-199529 A JP 2020-066716 A JP 2014-205344 A

[0006] In recent years, there has been a demand for thinner porous films and improved air permeability. However, film thickness and air permeability are in a trade-off relationship, and the thinner the film, the worse the air permeability tends to be. The techniques described in Patent Documents 1 to 4 above 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 yet has good air permeability. Therefore, a first object of the present invention is to provide a porous film that is both thin and has good air permeability.

[0007] Furthermore, as the film thickness becomes thinner, the film loses stiffness and tends to wrinkle easily during the film production process. For example, when the film is wound into a roll, the roll tends to wrinkle easily. The techniques described in Patent Documents 1 to 4 above do not have sufficient air permeability, and the technique described in Patent Document 5 does not have sufficient thickness. Therefore, a porous film that is thin and stiff yet has good air permeability is desired. Therefore, a second object of the present invention is to provide a porous film that has good air permeability, while also being thin and having film stiffness.

[0008] Furthermore, the techniques described in the above Patent Documents 1 to 4 and 6 do not have sufficient air permeability, and the technique described in Patent Document 5 has insufficient heat resistance, so there is a demand for a porous film that is highly heat resistant and also has good air permeability. Therefore, a third object of the present invention is to provide a porous film that has both heat resistance and air permeability.

[0009] To achieve the above object, the present invention proposes a porous film having the following aspects, and also proposes an ion exchange membrane support and an electricity storage device using the porous film.

[0010] [1] A porous film according to a 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), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more but less than 50% by mass, the air permeability (S) is less than 10 seconds / 100 cc, and the bubble point pressure (P ＢＡ [2] The porous film of the second aspect of the present invention is the porous film of the first aspect, wherein the air permeability (S) (sec / 100 cc) and the bubble point pressure (P ＢＡ ) (kPa) ratio (S / P ＢＡ ) is 0.5 seconds / (100 cc·kPa) or less.

[0011] [3] A third aspect of the porous film 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), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin 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 at 23°C in the machine direction (MD) of 30 MPa or more, a porosity of 80% or less, and an air permeability (S) of less than 10 seconds / 100 cc.

[0012] [4] A fourth aspect of the present invention provides a porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more and less than 50% by mass, and the bubble point pressure (P ＢＡ[5] The porous film of the fifth aspect of the present invention is a porous film in which the air permeability (S) (sec / 100cc) and bubble point pressure (P) of the porous film of the fourth aspect are different from each other in that the air permeability (S) (sec / 100cc) and bubble point pressure (P ＢＡ ) (kPa) ratio (S / P ＢＡ ) is 0.5 seconds / (100 cc·kPa) or less.

[0013] [6] A sixth aspect of the porous film 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), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin 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 storage modulus (E') of 1 MPa or more at 150°C and 10 Hz in the machine direction (MD), and an air permeability (S) of less than 10 seconds / 100 cc.

[0014] [7] A seventh aspect of the present invention is a porous film according to any one of the first to sixth aspects, 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. [8] A eighth aspect of the present invention is a porous film according to any one of the first to seventh aspects, 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. [9] A ninth aspect of the present invention is a porous film according to any one of the first to eighth aspects, wherein the polyolefin-based resin (A) is a polypropylene-based resin.

[10] The porous film of the tenth aspect of the present invention is a porous film in any one of the first to ninth 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.

[11] The porous film of the eleventh aspect of the present invention is a porous film in any one of the first to tenth aspects, wherein the thickness (T) is less than 50 μm.

[12] The porous film of the twelfth aspect of the present invention is a porous film in any one of the first to eleventh aspects, wherein the resin composition [I] contains a crystal nucleating agent (C).

[0015]

[13] An ion exchange membrane support according to a thirteenth aspect of the present invention is an ion exchange membrane support made of the porous film according to any one of the first to twelfth aspects.

[14] An electricity storage device according to a fourteenth aspect of the present invention is an electricity storage device including the ion exchange membrane support according to the thirteenth aspect.

[0016] According to the first and second aspects of the present invention, a porous film having a large pore size and excellent air permeability can be obtained despite being a thin film. According to the third aspect of the present invention, a porous film having an excellent tensile modulus and film strength can be obtained despite being a thin film. Therefore, a porous film having both thin film properties and film stiffness while maintaining good air permeability can be provided. According to the fourth and fifth aspects of the present invention, a porous film having a large pore size and excellent air permeability while being a thin film, and further having an excellent tensile modulus and film strength can be obtained. Therefore, a porous film having both thin film properties and film stiffness while maintaining good air permeability can be provided. According to the sixth aspect of the present invention, a porous film having both heat resistance and air permeability can be obtained. As described above, the porous films of all aspects of the present invention have excellent heat resistance and air permeability, and therefore can be suitably used as ion exchange membrane supports, particularly ion exchange membrane supports for electricity storage devices.

[0017] 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.

[0018] In the present invention, the term "film" encompasses sheets. Furthermore, in the present invention, when it is stated that "x to y" (x and y are any numbers), it means "x or more and y or less" unless otherwise specified, and also includes the meaning of "preferably greater than x" or "preferably smaller than y." Furthermore, "x or more" (x is any number) includes the meaning of "preferably greater than x" unless otherwise specified, and "y or less" (y is any number) includes the meaning of "preferably smaller than y" unless otherwise specified.

[0019] <<Porous Film of the Present Invention>> A porous film according to one embodiment of the present invention (hereinafter also referred to as "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 above-mentioned "main component" means the component having the highest mass proportion among the components constituting the resin composition [I]. The content of the main component can be assumed to 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].

[0020] <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.

[0021] The components constituting the resin composition [I] will be described in detail below.

[0022] [Polyolefin Resin (A)] Examples of the polyolefin resin (A) include homopolypropylene (propylene homopolymer), and polyolefin resins such as random copolymers 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 alone or in combination of two or more. Among these, homopolypropylene is preferred from the viewpoint of mechanical strength.

[0023] 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 by the upper limit currently available industrially, but this may not apply in the future if resins with higher regularity are developed at the industrial level. The isotactic pentad fraction refers to a stereostructure or the proportion thereof in which all five methyl groups, which are side chains, are positioned in the same direction relative to the main chain formed by carbon-carbon bonds composed of any five consecutive propylene units. The signal assignments in the methyl group region are as described in A. Zambelli et al., Macromol. , 8,687 (1975).

[0024] 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. Note that Mw / Mn in the present invention is a value measured by GPC (gel permeation chromatography) method.

[0025] The melt flow rate (MFR(A)) of the polyolefin resin (A) at a temperature of 230°C and a load of 2.16 kg is preferably 1 g / 10 min or more. When the styrene-based thermoplastic elastomer (B) described below is dispersed in the polyolefin resin (A), the in-plane orientation of the styrene-based thermoplastic elastomer (B) can be suppressed, and the dispersion diameter of the styrene-based thermoplastic elastomer (B) can be increased, thereby increasing the interconnectivity in the thickness direction and, as a result, improving the air permeability of the porous film of the present invention. 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 ensured. When two or more polyolefin resins (A) are used in combination, it is sufficient that at least one polyolefin resin (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) at a temperature of 230 ° C and a load of 2.16 kg.

[0026] The tensile modulus of the polyolefin resin (A) at 23°C is preferably 1000 MPa to 3000 MPa. Since the tensile modulus of the polyolefin resin (A) affects the air permeability and bubble point pressure of the porous film of the present invention, in order to adjust the air permeability and bubble point pressure of the porous film of the present invention within a predetermined range, the tensile modulus of the polyolefin resin (A) is preferably within the above 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. On the other hand, it is preferably 3000 MPa or less, more preferably 2700 MPa or less, and even more preferably 2400 MPa or less.

[0027] Examples of the polyolefin resin (A) that can be used include commercially available products under the trade names "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).

[0028] 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 resin is stretched.

[0029] [Styrene-based thermoplastic elastomer (B)] The porous film of the present invention preferably contains a styrene-based thermoplastic elastomer (B). By kneading and melting the styrene-based thermoplastic elastomer (B) with the polyolefin-based resin (A) to form 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.

[0030] 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 is one in which the carbon double bonds of the copolymer have been hydrogenated to convert them into single bonds.

[0031] 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.

[0032] 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 a styrene-butadiene-styrene copolymer (SBS), a styrene-butadiene-butylene-styrene copolymer (SBBS), a styrene-ethylene-butadiene-styrene copolymer (SEBS), a styrene-isoprene-styrene copolymer (SIS), a styrene-ethylene-propylene-styrene copolymer (SEPS), and a styrene-ethylene-ethylene-propylene-styrene copolymer (SEEPS). Examples of the styrene-olefin copolymer (B2) include a styrene-butadiene copolymer (SBR), a hydrogenated styrene-butadiene copolymer (SEB), a styrene-isoprene copolymer (SIR), and a styrene-ethylene-propylene copolymer (SEP). The above-listed styrene-based thermoplastic elastomers (B) may be used alone or in combination of two or more.

[0033] Among the above, from the viewpoint of the formability of the 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), as the main component. That is, from the viewpoint of the formability of the 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 the above viewpoints, it is more preferable to use SEPS or SEEPS as the main component, and among these, it is particularly preferable to use SEEPS as the main component. The "main component" in the styrene-based thermoplastic elastomer (B) refers to a resin that is contained in the largest amount as 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.

[0034] From the viewpoint of achieving both air permeability and mechanical properties at a higher level, 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.

[0035] In the porous film of the present invention, pores are formed within the domains of the styrene-based thermoplastic elastomer (B) from cleavage that occurs during stretching. The structure of the domains is closely related to the pore structure; the larger the domains, the larger the pores and the improved air permeability. Among styrene-based thermoplastic elastomers (B), styrene-olefin-styrene copolymer (B1) is preferred because it is easy to obtain large domains. Furthermore, uneven distribution of the styrene-olefin copolymer (B2) on the surface of the domains improves the dispersibility of the domains and increases the uniformity of the pore structure. Furthermore, within the domains, pores formed by the styrene-olefin copolymer (B2) are formed between the large pores formed by the styrene-olefin-styrene copolymer (B1), thereby improving the interconnectivity of the pores and further improving the air permeability.

[0036] The styrene content of the styrene-based thermoplastic elastomer (B) is preferably 20% by mass or more but 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). If the styrene content is equal to or more than the lower limit, domains can be effectively formed in the polyolefin-based resin (A). If the styrene content is equal to or less than the upper limit, excessively large domain formation can be suppressed. When two or more types of styrene-based thermoplastic elastomers (B) are contained in the resin composition [I], the styrene content refers to the total amount of all the styrene-based thermoplastic elastomers (B).

[0037] 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. 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.

[0038] 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 explained below. Generally, when a resin composition having a sea-island structure of matrix / domains is melt-extruded and cooled and solidified, the resin composition extruded from a shaping device such as a die or nozzle is cooled and solidified using a cooling and solidification device such as a cast roll (cooling roll) or air or water cooling. In this case, the resin composition melts and elongates in the gap (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), which forms the domains, is small, a resin composition in which the domains are elongated in the flow direction (extrusion direction) is obtained. If 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, if the weight-average molecular weight of the styrene-based thermoplastic elastomer (B) that constitutes the domains is large, the domains are less susceptible to melt elongation, and the domains in the resulting resin composition before stretching tend to maintain their spherical shape. Spherical domains are more likely to cleave within the domains due to stress in the flow direction (extrusion direction), which is thought to result in sufficient formation of a porous structure. For these reasons, it is preferable that the weight-average molecular weight (Mw) of the main resin in the styrene-based thermoplastic elastomer (B) be 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 main resin in the styrene-based thermoplastic elastomer (B) be within the above range, since this tends to result in uniform dispersion diameters of the formed domains.

[0039] The melt flow rate (MFR(B)) of the styrene-based thermoplastic elastomer (B) contained in the resin composition [I], measured at 200°C under a load of 10 kg, is preferably 2 g / 10 min or less. The shape of the styrene-based thermoplastic elastomer (B) dispersed in the resin composition changes depending on the viscosity difference with the polyolefin-based resin (A). However, if the styrene-based thermoplastic elastomer (B) has an MFR below the upper limit, it is easy to increase the viscosity difference with the polyolefin-based resin (A), and the shape is likely to become spherical. Spherically dispersed domains tend to have a more uniform porous structure in the subsequent stretching process than domains with a large aspect ratio, allowing for the production of a thin porous film with excellent physical property stability. From this perspective, the melt flow rate (MFR(B)) is preferably 2 g / 10 min or less, more preferably 1 g / 10 min or less, even 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 typically 0 g / 10 min, i.e., a state where there is no flow. When two or more types of styrene-based thermoplastic elastomers (B) are used, it is sufficient that at least one of the styrene-based thermoplastic elastomers (B) has the above MFR value. In particular, if a styrene-based thermoplastic elastomer having an MFR within the above range is the main component of the components forming the domains, it is preferable because stress tends to concentrate inside the domains during the stretching process, making it easier for pore openings to occur and resulting in porosity. The "main component" refers to the component with the highest mass ratio among the components forming the domains. 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 forming 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) at a temperature of 200°C and a load of 10 kg.

[0040] 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 there has been a problem in that thinner films and improved air permeability are mutually exclusive, and the thinner the film, the worse the air permeability. More specifically, when obtaining a porous film having a thickness of 50 μm or less, particularly 30 μm or less, it has been difficult to achieve an air permeability of 10 sec / 100 mL or less. In consideration of the above-mentioned problems, the present inventors conducted extensive research and found that, by adjusting the combination of polyolefin resin and styrene-based thermoplastic elastomer and film-forming conditions, excellent air permeability can be maintained, especially when the melt flow rates (MFR) of the polyolefin resin and the styrene-based thermoplastic elastomer are significantly different, even when the film is thin. 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. In this case, if the domains are stretched in the MD, stress is less likely to concentrate in the domains, making it difficult for holes to form. It is believed that a large difference in the melt flow rates (MFR) of the polyolefin resin and the styrene-based thermoplastic elastomer increases the dispersion diameter of the styrene-based thermoplastic elastomer domains, which serve as the origin of openings, and suppresses in-plane deformation of the domains. As a result, it is believed that the interconnectivity in the thickness direction is improved and the air permeability can be reduced. 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.From the viewpoint of film formability, the difference (MFR(A)-MFR(B)) is preferably 20 g / 10 min or less, more preferably 15 g / 10 min or less. When the resin composition [I] contains two or more polyolefin resins (A) and / or two or more styrene thermoplastic elastomers (B), the difference 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.

[0041] In the resin composition [I], the content of the styrene thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene thermoplastic elastomer (B) is preferably 20% by mass or more but 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 thermoplastic elastomer (B) 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 thermoplastic elastomer (B) serves as the domain, and a porous structure can be easily formed when the resin composition is stretched.

[0042] [Crystal nucleating agent (C)] The resin composition [I] preferably contains a crystal nucleating agent (C) because it is believed that increasing the rigidity of the polyolefin resin (A) is effective in forming voids when the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) are stretched to form pores.

[0043] The content of the crystal 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).

[0044] 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.

[0045] 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").

[0046] 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.

[0047] 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 polyolefin resin (A) and the styrene-based thermoplastic elastomer (B). If the content of the α-crystal nucleating agent is equal to or more than the lower limit, α-crystal formation can be sufficient. On the other hand, if 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 during the film-forming process or secondary processing process, and this is preferable in terms of reducing costs.

[0048] Examples of β-crystal nucleating agents include amide compounds, tetraoxaspiro compounds, quinacridones, nanoscale iron oxides, alkali or alkaline earth metal salts of carboxylic acids such as potassium 1,2-hydroxystearate, magnesium benzoate, magnesium succinate, and magnesium phthalate, aromatic sulfonic acid compounds such as sodium benzenesulfonate and sodium naphthalenesulfonate, or di- or triesters of tribasic carboxylic acids, phthalocyanine pigments such as phthalocyanine blue, two-component compounds comprising Component A, an organic dibasic acid, and Component B, 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. Specific types of nucleating agents are described in JP-A-2003-306585, JP-A-06-289566, and JP-A-09-194650.

[0049] 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 a β-crystal nucleating agent include the polypropylene "Bepol B-022SP" manufactured by Aristech, the polypropylene "Beta(β)-PPBE60-7032" manufactured by Borealis, and the polypropylene "BNXBETAPP-LN" manufactured by Mayzo.

[0050] The β-crystal nucleating agent is preferably incorporated into 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 perspective, 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 resin components, i.e., the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B). Of these, 0.001 parts by mass or more or 3.0 parts by mass or less is even more preferred, and 0.01 parts by mass or more or 1.0 part by mass or less is even more preferred. If the content of the β-crystal nucleating agent is equal to or greater than the above-mentioned lower limit, β-crystals of the polyolefin resin (A) can be sufficiently generated and grown during production, ensuring sufficient β-crystal activity, and the desired air permeability performance can be obtained when the ion exchange membrane support is used. In addition, if it is less than the upper limit, it is economically advantageous, and there is no bleeding of the β-crystal nucleating agent onto the surface of the porous film, which is preferable.In addition, if a layer containing polyolefin is laminated in addition to the resin layer made of 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.

[0051] [Other Components] The resin composition [I] may contain various additives, such as heat stabilizers, antioxidants, UV absorbers, light stabilizers, colorants, antistatic agents, hydrolysis inhibitors, lubricants, and flame retardants, to the extent that the properties of the resin composition are not impaired. It 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, and vapor deposition, as well as perforation, as necessary, within the scope of the present invention. It is also possible to use several porous films of the present invention stacked together depending on the application.

[0052] <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.

[0053] (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) can contribute to thinner and more highly integrated cell stacks. 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, so that the film can maintain sufficient strength. 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.

[0054] (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. The lower limit is usually 0.25 seconds / 100cc. The air permeability (S) in the present invention is measured at 25°C in accordance with JIS P8117:2009. In order to reduce the air permeability (S) of the porous film of the present invention, the porosity of the continuous pores may be increased to increase the diameter of the continuous pores. To achieve this, for example, the viscosity difference (e.g., MFR difference) between the polyolefin resin (A) and the styrene thermoplastic elastomer (B) may be increased to increase the pore diameter, or a polyolefin resin (A) with a relatively low molecular weight may be used to increase the pore diameter, or the thickness of the film may be adjusted. In addition, the thickness can be adjusted by other conditions such as the casting temperature and the stretching ratio during film formation, but is not limited to these.

[0055] (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 the pressure at which the first bubble appears when the film is immersed in a liquid with low surface tension, such as alcohol, and air is injected from the top or bottom of the film, gradually increasing the pressure. 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. While a larger maximum pore size of the porous film of the present invention can improve the air permeability characteristics, if it is too large, it may cause breakage or the like when the film is thin. From this perspective, in order to adjust the maximum pore size 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, and even more preferably 14 kPa or more. On the other hand, from the viewpoint of breathability, it is preferably 80 kPa or less, 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, for example, by 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 stretch ratio during film formation. However, it is not limited to these.

[0056] (Maximum pore diameter) The maximum pore diameter in the porous film of the present invention is preferably 0.5 μm or more and 9.0 μm or less. ＢＡ) is the maximum pore diameter calculated from the above. As mentioned above, if the maximum pore diameter of the porous film of the present invention is large, the air permeability can be improved, but if it is too large, it may cause breakage or the like when the film thickness is thin. Therefore, the maximum pore diameter of the porous film of the present invention is preferably 0.5 μm or more, more preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. On the other hand, it is preferably 9.0 μm or less, more preferably 7.0 μm or less, more preferably 5.0 μm or less, and even more preferably 3.0 μm or less.

[0057] (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 / 100 cc) to the air permeability (S) (kPa) of the porous film of the present invention (S / P ＢＡ ) is correlated with the film thickness and porosity, and the ratio (S / P ＢＡ If the ratio (S / P) of the porous film of the present invention is small, the film thickness tends to be thin and the porosity tends to be high, which is preferable. 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, 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 The thickness of the film can be adjusted by, for example, the chemical structure, molecular weight, MFR, content, casting temperature, stretching ratio, etc. of the polyolefin resin (A) and the styrene thermoplastic elastomer (B), but is not limited to these methods.

[0058] (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 making the ratio (S / T) of the air permeability (S) to the thickness (T) 0.5 seconds / (100cc μm) or less, a porous film can be obtained that is thin (for example, 50 μm or less) but has excellent air permeability properties. On the other hand, the lower limit is 0.01 seconds / (100cc μm) from the viewpoint of pore diameter and porosity. From this viewpoint, the value of (S / T) is preferably 0.5 seconds / (100cc μm) or less, more preferably 0.45 seconds / (100cc μm) or less, more preferably 0.40 seconds / (100cc μm) or less, 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, it is preferably 0.01 seconds / (100cc μm) or more, more preferably 0.02 seconds / (100cc μm) or more, more preferably 0.03 seconds / (100cc μm) or more, 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.

[0059] (Porosity) From the viewpoint of improving the air permeability, the porous film of the present invention preferably has a porosity of 50% or more, more preferably 55% or more, and even more preferably 60% or more. On the other hand, the upper limit of the porosity is not particularly limited, but it is practical to set it to 90% or less. In particular, from the viewpoint of ensuring the stiffness of the film, the porosity of the film is preferably 80% or less, more preferably 78% or less, and even more preferably 76% or less. In order to adjust the porosity of the porous film of the present invention to the above range, 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 can be adjusted. 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.

[0060] (MD tensile modulus) The porous film of the present invention preferably has a tensile modulus of 25 MPa or more in the machine direction (MD) at 23 ° C. When the tensile modulus of the porous film of the present invention in the machine direction (MD) at 23 ° C. is 25 MPa or more, it is possible to make the roll less likely to wrinkle when the film is wound into a roll. Furthermore, when it is 30 MPa or more, it is possible to suppress the occurrence of wrinkles during the film production process, and it is possible to make the roll less likely to wrinkle when the film is wound into a roll. From this viewpoint, the tensile modulus of the porous film of the present invention in the machine direction (MD) at 23 ° C. is preferably 25 MPa or more, more 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. The upper limit is not particularly limited, but 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).

[0061] (MD storage modulus) The porous film of the present invention preferably has a storage modulus (E') of 1 MPa or more at 150 ° C. and 10 Hz in the machine direction (MD). If the storage modulus (E') of the porous film of the present invention is 1 MPa or more at 150 ° C. and 10 Hz in the machine direction (MD), it can be said to have heat resistance. From this viewpoint, the storage modulus (E') of the porous film of the present invention is preferably 1 MPa or more at 150 ° C. and 10 Hz in the machine direction (MD), more preferably 3 MPa or more, more preferably 5 MPa or more, and even more preferably 7 MPa or more. The upper limit is not particularly limited, but can 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).

[0062] (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. When the average tensile strength of the porous film of the present invention is below the upper limit, it is preferable 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.

[0063] (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 in the above range, excellent mechanical properties can be obtained. The average tensile elongation at break in the present invention is the average value of the tensile elongation at break 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 elongation at break in an arbitrary length direction and the width direction perpendicular thereto may be used.

[0064] The average tensile strength and average tensile elongation at break can be adjusted by the chemical structure, molecular weight, MFR, and content of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B), the casting temperature during film formation, the stretching ratio, etc. 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 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 resin (A) and the styrene-based thermoplastic elastomer (B) to fall within the above-mentioned preferred range, adjusting the temperature of the cast roll during film formation in the production method described below to fall within a preferred range, or, in the case of biaxial stretching, adjusting the longitudinal and transverse stretching ratios and stretching ratios to fall within preferred ranges, etc. 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 a pore structure and the film thickness, and is therefore thought to lead to the formation of an appropriate porous structure and the improvement of the air permeability characteristics.

[0065] <Method for producing 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] containing a polyolefin resin (A) and a styrene thermoplastic elastomer (B) to obtain a pre-stretched sheet, and a step (II) of stretching the pre-stretched sheet to obtain a porous film.

[0066] [Step (I)] First, in step (I), the resin composition [I], for example, the polyolefin resin (A), the styrene-based thermoplastic elastomer (B), and the crystal nucleating agent (C) are 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 to obtain a non-porous sheet before stretching. In this case, a method for molding the non-porous sheet can be, for example, T-die molding.

[0067] 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 cast roll is above the lower limit, crystallization of the polyolefin resin (A) proceeds sufficiently, so that a porous structure is sufficiently formed. On the other hand, when the temperature of the cast roll is below the upper limit, problems such as the sheet before stretching fusing with the roll and film rupture are less likely to occur.

[0068] 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 casting roll to the above lower limit or more, productivity is improved. On the other hand, by setting the speed of the casting roll to the above upper limit or less, it becomes easier to maintain the domains in the sheet before stretching in a spherical shape, and it becomes easier to form a sufficient porous structure after stretching.

[0069] [Step (I-a)] 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 (I-a) 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.

[0070] 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.

[0071] [Step (II)] Next, the obtained unstretched sheet is uniaxially or biaxially stretched. The uniaxial stretching may be longitudinal uniaxial stretching or transverse uniaxial stretching. The biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching.

[0072] To produce the porous film of the present invention having gas permeability, it is sufficient to select the stretching conditions in each stretching step, and it is more preferable to adopt sequential biaxial stretching, which makes it easier to control the porous structure. Stretching in the machine direction (MD) of the sheet is called "longitudinal stretching," and stretching in the direction perpendicular to the machine direction (TD) is called "transverse stretching."

[0073] 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.

[0074] It is desirable to perform 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 lower than 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 higher than the lower limit, breakage during stretching can be suppressed, which is preferable.

[0075] The longitudinal (MD) stretching ratio at low temperatures is preferably 1.1 to 5.0, more preferably 1.2 or more and 4.5 or less, and even more preferably 1.3 or more and 4.0 or less. By setting the stretching ratio at low temperatures to the above lower limit or more, pore formation progresses, suggesting that sufficient porosity is generated by stretching. Furthermore, by setting the stretching ratio at low temperatures to the above upper limit or less, breakage during stretching can be suppressed.

[0076] Furthermore, the longitudinal stretching (MD) may be a two-stage stretching process in which stretching at a low temperature is followed by stretching at a high temperature. Specifically, the stretching temperature at a high 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 high-temperature longitudinal stretching temperature to the above-mentioned lower limit or higher, film rupture during stretching can be suppressed. On the other hand, by setting the high-temperature longitudinal stretching temperature to the above-mentioned upper limit or lower, closure of pores formed by low-temperature stretching can be suppressed.

[0077] The longitudinal stretching (MD) ratio at high temperatures is preferably 1.1 to 5.0 times, more preferably 1.2 times or more and 4.5 times or less, even more preferably 1.3 times or more and 4.0 times or less, and even more preferably 1.5 times or more and 3.0 times or less. By setting the stretching ratio at high temperatures to the above lower limit or more, it is possible to enlarge the pores formed by longitudinal stretching at low temperatures. Furthermore, by setting the stretching ratio at high temperatures to the above upper limit or less, it is possible to adjust the film properties (thickness, air permeability, and mechanical properties) within suitable ranges and to prevent film breakage during stretching.

[0078] The temperature of 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 longitudinal stretching are enlarged, thereby increasing the porosity of the porous layer and providing sufficient air permeability and mechanical properties.

[0079] The transverse stretching (TD) ratio can be selected arbitrarily, but is preferably 1.1 to 10, more preferably 1.5 to 8.0, and even more preferably 2.0 to 6.0. By stretching at the above transverse stretching ratio, it is possible to obtain a sufficient porosity without deforming the pores generated during longitudinal stretching.

[0080] Furthermore, when the porous film of the present invention is produced by biaxial stretching, the stretching ratio of longitudinal stretching to transverse stretching is preferably longitudinal:transverse = 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 suitable ranges. When the longitudinal stretching is performed in two stages at low temperature and high temperature, it is sufficient that the (longitudinal stretching ratio at low temperature × longitudinal stretching ratio at high temperature): transverse stretching ratio is within the above range.

[0081] <Uses> The porous film of the present invention can be used in various fields, such as separation membranes used in the production of ultrapure water, purification of chemical solutions, water treatment, etc., waterproof and moisture-permeable films used in clothing and sanitary materials, separators used in electronic components such as capacitors, batteries, electrolytic capacitors, etc. In particular, the porous film of the present invention can be suitably used as a support for ion exchange membranes.

[0082] (Ion exchange membrane support) The porous film of the present invention is useful as an ion exchange membrane support. That is, the ion exchange membrane support 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.

[0083] (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 electricity 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.

[0084] 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.

[0085] <Measurement method> (1) Bubble point pressure (P ＢＡ ) and mean flow diameter pressure (P Ａｐ Using a Perm Porometer (manufactured by 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" (manufactured by Porous Materials, surface tension: 15.6 dynes / cm) as a reagent. ＢＡ The 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.

[0086] (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: bubble point pressure P ＢＡ

[0087] (3) Air permeability (S) The air permeability resistance of the porous film (sample) was measured in accordance with JIS P8117: 2009 under an air atmosphere at 25°C. As a measuring instrument, a digital Oken type air permeability dedicated machine (manufactured by Asahi Seiko Co., Ltd.) was used.

[0088] (4) Melt Flow Rate (MFR) The MFR(A) of the polyolefin resin (A) was measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K7210-1 (2014). 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. For samples that did not flow in the MFR measurement, the MFR value was set to 0 in the calculation.

[0089] (5) Thickness (Film Thickness) (T) A 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.

[0090] (6) Porosity A porous film (sample) was cut into a 10 cm square, and the density ρ1 (apparent density) was measured. The density ρ0 (true density) when the porosity was 0% was calculated. The porosity was calculated from these values ​​according to the following formula: Porosity (%) = (1 - ρ1 / ρ0) x 100

[0091] (7) Tensile strength and tensile elongation at break A tensile tester (Shimadzu Corporation tensile tester AG-1kNXplus) was used as the measuring device. Test pieces were cut out from porous films (samples) into rectangles with a length of 100 mm in the measurement direction and a width of 15 mm. Both ends of the test piece in the longitudinal direction 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 elongation at break at five points in the width direction, and the average values ​​were calculated. The above tensile test was performed on both the MD and TD tensile tests of the film. In addition, the average tensile strength and average tensile elongation at break were calculated by averaging the MD and TD values ​​of tensile strength and tensile elongation at break, respectively. The measurement was performed at room temperature (25 ° C.).

[0092] (8) Tensile Modulus A tensile tester (Shimadzu Corporation, tensile tester AGS-X) was used as the measuring device. Test pieces were cut out from porous films (samples) into rectangles with a length of 150 mm in the measurement direction and a width of 10 mm. 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 tensile test was carried out in both the MD and TD 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).

[0093] (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 carried out under conditions of a measurement temperature of 30 to 200°C, a strain of 0.1%, a frequency of 10 Hz, and a temperature rise 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, the MD measurement was carried out using a sample having an MD gauge length of 2 cm and a TD width of 0.4 cm, and the TD measurement was carried out using a sample having a TD gauge length of 2 cm and an MD width of 0.4 cm.

[0094] <Materials> (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) - (A-3) Ethylene / α-olefin copolymer (weight average molecular weight (Mw): 298,000, molecular weight distribution (Mw / Mn): 4.72, MFR (230°C, 2.16 kg): 0.98 g / 10 min, flexural modulus at 23°C: 1410 MPa, melting point: 133°C) The tensile modulus is a value measured using a tensile tester in accordance with JIS K7161 and JIS K7162, and the flexural modulus is a value measured using a flexural tester in accordance with JIS K7171.

[0095] (Styrene-based thermoplastic elastomers (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) - (B-4) Styrene-ethylene-butene-styrene block copolymer (SEBS) (weight average molecular weight (Mw): 125,000, molecular weight distribution (Mw / Mn): 1.06, MFR (230°C, 2.16 kg): less than 0.1, MFR(B) (200°C, 10 kg): less than 0.1, styrene content: 31% by mass)

[0096] (Crystal nucleating agent (C)) (C-1) α 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) (C-3) Crystal nucleating agent for polyethylene (manufactured by Riken Vitamin Co., Ltd., trade name: Rikemaster CN-002)

[0097] (Additives (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) Phenol-based 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)

[0098] (Additives (E)) (E-1) Metal soap-based external lubricant (1,2-hydroxystearic acid soap, metal component: Zn (zinc), metal content: 9.0 to 10.0%)

[0099] <Preparation of porous film> <Example 1-1> Polyolefin resin (A-1) 60% by mass, styrene-based thermoplastic elastomer (B-1) 20% by mass, styrene-based thermoplastic elastomer (B-2) 20% by mass, crystalline nucleating agent (C-2) was blended at a ratio of 0.05 parts by mass relative to 100 parts by mass of the resin component, and (D-1) 0.1 parts by mass relative to 100 parts by mass of the resin component, (D-2) 0.1 parts by mass relative to 100 parts by mass of the resin component was added to a φ44 mm twin-screw extruder, melt-kneaded at a set temperature of 205 ° C. to produce full compound pellets. Next, the produced pellets were added to a φ65 mm single-screw extruder, melt-kneaded at a set temperature of 220 ° C., and then molded into a sheet using a T-die. The sheet was then placed on a cast roll set under the conditions shown in Table 1 and cooled and solidified to obtain a pre-stretched sheet having 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 Table 1 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, and then stretched in the transverse direction at a stretching temperature of 145°C at the ratio shown in Table 1, 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.

[0100] Example 1-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 1-1, 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 1-1, 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 1-3 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 1-1, except that the contents of the polyolefin resin (A-2), styrene-based thermoplastic elastomers (B-1), (B-2), 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 shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0102] Example 1-4 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 1-1, except that the contents of the polyolefin resin (A-2), 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 1-1, 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] Example 1-5 A pre-stretched sheet having a thickness of 95 μm was obtained in the same manner as in Example 1-1, 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 solidification 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 1-1, 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] Example 1-6 A pre-stretched sheet having a thickness of 120 μm was obtained in the same manner as in Example 1-1, 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 solidification 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 1-1, 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] Example 1-7 A pre-stretched sheet having a thickness of 115 μm was obtained in the same manner as in Example 1-1, 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 solidification 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 1-1, 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.

[0106] Comparative Example 1-1 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 1-1, 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 1-1, 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.

[0107] Comparative Example 1-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 1-1, 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 shown in Table 1. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0108] <Comparative Example 1-3> A material containing 70% by mass of polyolefin resin (A-2), 30% by mass of styrene-based thermoplastic elastomer (B-2), and 0.1 parts by mass of crystal nucleating agent (C-1) per 100 parts by mass of the resin component was fed into a φ40 mm twin-screw extruder, melt-kneaded at a set temperature of 205 ° C, and then molded into a sheet using a T-die. The sheet was then placed on a cast roll set at 127 ° C and cooled and solidified to obtain a pre-stretched sheet having a thickness of 420 μm. Thereafter, a porous film (sample) was obtained in the same manner as in Example 1-1, except that the obtained pre-stretched sheet was stretched at the stretch ratios shown in Table 1. The evaluation results of the obtained film are summarized in Table 1.

[0109] Comparative Example 1-4 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 1-1, 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 solidification were carried out under the cast roll conditions as shown in Table 1. Thereafter, when the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 1, the sheet broke during longitudinal stretching, and no porous film (sample) was obtained.

[0110]

[0111] As shown in Table 1, in Examples 1-1 to 1-7, porous films were obtained that were both thin and air permeable. These films did not suffer from uneven adhesion or breakage during film formation and had good mechanical properties. On the other hand, the films with an air permeability (S) of less than 10 seconds / 100 cc and a bubble point pressure (P ＢＡ The porous films of Comparative Examples 1-1 to 1-4, which did not satisfy the condition that the compressive strength is 5 to 80 kPa, were unable to achieve both thinning and air permeability.

[0112] In Comparative Example 1-1, the viscosity of the polyolefin resin (A) was high and the proportion of the styrene-based thermoplastic elastomer domain component was low, which is presumably why pores were less likely to form, the bubble point pressure was low, and the air permeability was poor. In Comparative Example 1-2, the viscosity of the polyolefin resin (A) was high and the casting speed was relatively fast, which presumably caused the domain shape to be stretched in the MD, which resulted in pores being less likely to form, the bubble point pressure was low, and the air permeability was poor. In Comparative Example 1-3, the viscosity of the polyolefin resin (A) was high and the proportion of the styrene-based thermoplastic elastomer domain component was low, and the film was thick, which is presumably why the air permeability was poor. In Comparative Example 1-4, the proportion of the styrene-based thermoplastic elastomer domain component was low, which is presumably why stress was more likely to concentrate in the polyolefin resin matrix component, leading to breakage during low-temperature longitudinal stretching.

[0113] From the results of the above Examples 1-1 to 1-7 and the tests conducted by the present inventors, it has been found that the porous film formed from the resin composition [I] containing the polyolefin resin (A) as the main component has an air permeability (S) of less than 10 seconds / 100 cc and a bubble point pressure (P ＢＡ It has been found that by setting the compressive strength to 5 to 80 kPa, the air permeability can be improved even if the film thickness is thin, specifically, even if the film thickness is 50 μm or less.

[0114] <Preparation of porous film> <Example 2-1> Polyolefin resin (A-1) 60% by mass, styrene-based thermoplastic elastomer (B-1) 20% by mass, styrene-based thermoplastic elastomer (B-2) 20% by mass, crystalline nucleating agent (C-2) was blended at a ratio of 0.05 parts by mass relative to 100 parts by mass of the resin component, and (D-1) 0.1 parts by mass relative to 100 parts by mass of the resin component, (D-2) 0.1 parts by mass relative to 100 parts by mass of the resin component was added to a φ44 mm twin-screw extruder, melt-kneaded at a set temperature of 205 ° C. to produce full compound pellets. Next, the produced pellets were fed into a φ65 mm single-screw extruder, melt-kneaded at a set temperature of 220 ° C., and then molded into a sheet using a T-die. The sheet was then placed on a cast roll set under the conditions shown in Table 2 and cooled and solidified to obtain a pre-stretched sheet having a thickness of 80 μm. Thereafter, the obtained pre-stretched sheet was subjected to low-temperature stretching at the ratio shown in Table 2 between a roll (X) set at 25 ° C. and a roll (Y) set at 25 ° C. 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, and then stretched in the transverse direction at a stretching temperature of 145 ° C. at the ratio shown in Table 2, and then heat-treated at 155 ° C. to obtain a porous film (sample). The evaluation results of the obtained film are summarized in Table 2.

[0115] Example 2-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidifying were carried out under the cast roll conditions shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0116] Example 2-3 A pre-stretched sheet having a thickness of 120 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidifying were carried out under the cast roll conditions shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0117] Example 2-4 A pre-stretched sheet having a thickness of 115 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidifying were carried out under the cast roll conditions shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0118] Comparative Example 2-1 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidifying were carried out under the cast roll conditions shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0119] Comparative Example 2-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidifying were carried out under the cast roll conditions shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0120] Comparative Example 2-3 A pre-stretched sheet having a thickness of 95 μm was obtained in the same manner as in Example 2-1, 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 2, and cooling and solidification were carried out under the cast roll conditions as shown in Table 2. Thereafter, a porous film (sample) was obtained in the same manner as in Example 2-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 2. The evaluation results of the obtained film are summarized in Table 2.

[0121] Comparative Example 2-4 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 2-1, except that the contents of the polyolefin resin (A-1), the styrene-based thermoplastic elastomer (B-3), the crystal nucleating agent (C-2), the additives (D-1), (D-2), and the additive (E-1) were as shown in Table 2, and cooling and solidifying were carried out under the cast roll conditions as shown in Table 2. Thereafter, when the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 2, the sheet broke during longitudinal stretching, and no porous film (sample) was obtained.

[0122]

[0123] As shown in Table 2, in Examples 2-1 to 2-4, porous films having good air permeability characteristics while being thin and stiff were obtained. 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 2-1 to 2-4, which did not satisfy the conditions of a porous film having an MD tensile modulus (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, were unable to achieve both thin film properties and film stiffness while having good air permeability characteristics.

[0124] In Comparative Example 2-1, the viscosity of the PP was high and the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, which is presumably why pores were less likely to open and the air permeability was poor. In Comparative Example 2-2, the viscosity of the PP was high and the casting speed was relatively fast, which is presumably why the domain shape was stretched in the MD, which was less likely to open pores and why the air permeability was poor. In Comparative Example 2-3, the mean flow meter pressure and bubble point pressure were low, which resulted in large pore diameters, especially large maximum pore diameters, and the MD stretch ratio was also 3x, which is presumably why the MD tensile modulus (23°C) was low. In Comparative Example 2-4, the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, which is presumably why stress was more likely to concentrate in the polyolefin-based resin, which is the matrix component, leading to breakage during low-temperature longitudinal stretching.

[0125] From the results of the above Examples 2-1 to 2-4 and the tests that the present inventors have conducted so far, it has been found that in the porous film formed from the resin composition [I] mainly composed of polyolefin resin (A), by making the tensile modulus at 23°C in the machine direction (MD) 30 MPa or more, making the porosity 80% or less, and making the air permeability (S) less than 10 seconds / 100cc, it is possible to obtain a porous film that is thin and has stiffness, yet has better air permeability properties.

[0126] <Preparation of porous film> <Example 3-1> Polyolefin resin (A-1) 60% by mass, styrene-based thermoplastic elastomer (B-1) 20% by mass, styrene-based thermoplastic elastomer (B-2) 20% by mass, crystalline nucleating agent (C-2) was blended at a ratio of 0.05 parts by mass relative to 100 parts by mass of the resin component, and (D-1) 0.1 parts by mass relative to 100 parts by mass of the resin component, (D-2) 0.1 parts by mass relative to 100 parts by mass of the resin component was added to a φ44 mm twin-screw extruder, melt-kneaded at a set temperature of 205 ° C. to produce full compound pellets. Next, the produced pellets were added to a φ65 mm single-screw extruder, melt-kneaded at a set temperature of 220 ° C., and then molded into a sheet using a T-die. The sheet was then placed on a cast roll set under the conditions shown in Table 3 and cooled and solidified to obtain a pre-stretched sheet having a thickness of 80 μm. Thereafter, the obtained pre-stretched sheet was subjected to low-temperature stretching at the ratio shown in Table 3 between a roll (X) set at 25 ° C. and a roll (Y) set at 25 ° C. 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, and then stretched in the transverse direction at a stretching temperature of 145 ° C. at the ratio shown in Table 3, and then heat-treated at 155 ° C. to obtain a porous film (sample). The evaluation results of the obtained film are summarized in Table 3.

[0127] Example 3-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 3-1, 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 3, and cooling and solidifying were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0128] Example 3-3 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 3-1, except that the contents of the polyolefin resin (A-2), styrene-based thermoplastic elastomers (B-1), (B-2), crystal nucleating agent (C-2), and additives (D-1) and (D-2) were as shown in Table 3, and cooling and solidifying were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0129] Example 3-4 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 3-1, except that the contents of the polyolefin resin (A-2), styrene-based thermoplastic elastomer (B-3), crystal nucleating agent (C-2), and additives (D-1) and (D-2) were as shown in Table 3, and cooling and solidifying were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0130] Example 3-5 A pre-stretched sheet having a thickness of 120 μm was obtained in the same manner as in Example 3-1, 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 3, and cooling and solidification were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0131] Example 3-6 A pre-stretched sheet having a thickness of 115 μm was obtained in the same manner as in Example 3-1, 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 3, and cooling and solidification were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0132] Comparative Example 3-1 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 3-1, 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 3 and cooling and solidifying were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0133] Comparative Example 3-2 A pre-stretched sheet having a thickness of 90 μm was obtained in the same manner as in Example 3-1, 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 3, and cooling and solidifying were carried out under the cast roll conditions shown in Table 3. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0134] <Comparative Example 3-3> A material containing 70% by mass of polyolefin resin (A-3), 30% by mass of styrene-based thermoplastic elastomer (B-4), and 1.75 parts by mass of crystal nucleating agent (C-3) per 100 parts by mass of the resin component was introduced into a φ25 mm twin-screw extruder, melt-kneaded at a set temperature of 220 ° C, and then molded into a sheet using a T-die. The sheet was then placed on a cast roll set at 110 ° C and cooled and solidified to obtain a pre-stretched sheet having a thickness of 200 μm. Thereafter, a porous film (sample) was obtained in the same manner as in Example 3-1, except that the obtained pre-stretched sheet was stretched at the stretch ratio shown in Table 3. The evaluation results of the obtained film are summarized in Table 3.

[0135] Comparative Example 3-4 A pre-stretched sheet having a thickness of 100 μm was obtained in the same manner as in Example 3-1, except that the contents of the polyolefin resin (A-1), the styrene-based thermoplastic elastomer (B-3), the crystal nucleating agent (C-2), the additives (D-1), (D-2), and the additive (E-1) were as shown in Table 3, and cooling and solidification were carried out under the cast roll conditions as shown in Table 3. Thereafter, when the obtained pre-stretched sheet was stretched at the stretch ratio as shown in Table 3, the sheet broke during longitudinal stretching, and no porous film (sample) was obtained.

[0136]

[0137] As shown in Table 3, in Examples 3-1 to 3-6, porous films having both heat resistance and air permeability were obtained. These films did not suffer from uneven adhesion or film breakage during film formation. On the other hand, the porous films of Comparative Examples 3-1 to 3-4, which did not satisfy the conditions of a storage modulus (E') of 1 MPa or more at 150 ° C. and 10 Hz in the machine direction (MD) and an air permeability (S) of less than 10 seconds / 100 cc, were unable to achieve both heat resistance and air permeability.

[0138] In Comparative Example 3-1, the viscosity of the PP was high and the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, which is presumably why pores were less likely to form and the air permeability was poor. In Comparative Example 3-2, the viscosity of the PP was high and the casting speed was relatively fast, which is presumably why the domain shape was stretched in the MD, which was less likely to form pores and why the air permeability was poor. In Comparative Example 3-3, the viscosity of the ethylene-α-olefin copolymer was high and the melting point was low, which is presumably why the air permeability and heat resistance were poor. In Comparative Example 3-4, the proportion of the styrene-based thermoplastic elastomer, which is the domain component, was low, which is presumably why stress was more likely to concentrate in the polyolefin-based resin, which is the matrix component, which led to breakage during low-temperature longitudinal stretching.

[0139] From the results of the test carried out by the inventors in above-mentioned Example 3-1 to 3-6 and the porous film formed by the resin composition [I] that is main component of polyolefin resin (A), by making the storage modulus (E') of machine direction (MD) at 150 ° C, 10 Hz be 1 MPa or more, and by making the air permeability (S) be less than 10 seconds / 100cc, it can be found that both heat resistance and air permeability characteristics can be improved.

Claims

1. A porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more but less than 50% by mass, the air permeability (S) is less than 10 seconds / 100 cc, and the bubble point pressure (P ＢＡ ) is 5 to 80 kPa.

2. Air permeability (S) (sec / 100cc) and bubble point pressure (P ＢＡ ) (kPa) ratio (S / P ＢＡ 2. The porous film according to claim 1, wherein the compressive strength (kPa) is 0.5 seconds / (100 cc·kPa) or less.

3. A porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more but 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.

4. A porous film formed from a resin composition [I] containing a polyolefin resin (A) and a styrene-based thermoplastic elastomer (B), wherein the content of the styrene-based thermoplastic elastomer (B) relative to the total amount of the polyolefin resin (A) and the styrene-based thermoplastic elastomer (B) is 20% by mass or more but less than 50% by mass, the storage modulus (E') in the machine direction (MD) at 150°C and 10 Hz is 1 MPa or more, and the air permeability (S) is less than 10 seconds / 100 cc.

5. The porous film according to claim 1, 3 or 4, 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.

6. The porous film according to claim 1, 3 or 4, wherein the melt flow rate (MFR(B)) of the styrene-based thermoplastic elastomer (B) at a temperature of 200°C under a load of 10 kg is 2 g / 10 min or less.

7. The porous film according to claim 1, 3 or 4, wherein the polyolefin resin (A) is a polypropylene resin.

8. The porous film according to claim 1, 3 or 4, 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.

9. The porous film according to claim 1, 3 or 4, having a thickness (T) of less than 50 μm.

10. A porous film according to claim 1, 3 or 4, wherein the resin composition [I] contains a crystal nucleating agent (C).

11. An ion exchange membrane support comprising the porous film according to any one of claims 1 to 10.

12. An electricity storage device comprising the ion exchange membrane support according to claim 11.

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