Polyolefin microporous membrane

Abstract

To provide a polyolefin microporous membrane suitable as a substrate, featuring both the ability to capture substances in fluids such as ionic substances and the ability to permit the passage of substances that are not the target of capture.SOLUTION: A polyolefin microporous membrane comprises a polyolefin and exhibits a Gurley value of 0.01 seconds / 100 mL or more and 20 seconds / 100 mL or less, a thickness of 120 μm or more, a porosity of 77% or more and 98% or less, and an average pore diameter of 0.60 μm or more and 3.0 μm or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to microporous polyolefin membranes. [Background technology]

[0002] Nonwoven fabrics are widely used as a means for collecting foreign matter in gas or liquid. For example, Patent Document 1 describes a liquid filter made of nonwoven fabric and used in an immersion type filtration cartridge. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-198833 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the case of ionic substances to be collected, a nonwoven fabric filter may be used as a substrate and surface-treated to collect the ionic substances. However, when a nonwoven fabric filter is used, the nonwoven fabric has a short curve, so it captures ionic substances relatively infrequently, making it difficult to achieve both ionic substance collection performance and fluid permeability that contains ionic substances. Therefore, a substrate that can achieve both ionic substance collection performance and fluid permeability when surface-treated is required. There are limitations to how much such required properties can be achieved with nonwoven fabrics, but a polyolefin microporous membrane is made by forming fibril polyolefin into a three-dimensional mesh shape, and therefore can ensure longer tortuous paths compared to nonwoven fabrics, making it possible for the membrane to simultaneously achieve the above required properties. In view of the above circumstances, an object of an embodiment of the present disclosure is to provide a polyolefin microporous membrane suitable as a substrate for achieving both the ability to capture substances in a fluid, such as ionic substances, and the ability to allow permeation of substances other than the targets of capture. [Means for solving the problem]

[0005] Specific means for solving the above problems include the following aspects. <1> A polyolefin microporous membrane comprising a polyolefin, having a Gurley value of 0.01 sec / 100 mL or more and 20 sec / 100 mL or less, a thickness of 120 μm or more, a porosity of 77% or more and 98% or less, and an average pore size of 0.60 μm or more and 3.0 μm or less. <2> The polyolefin comprises polyethylene. <1> The polyolefin microporous membrane according to claim 1. <3> The polyolefin contains an ultra-high molecular weight polyolefin, and the proportion of the ultra-high molecular weight polyolefin in the total polyolefin is 20 mass% or less. <1> or <2> The polyolefin microporous membrane according to claim 1. <4> The bubble point pressure is 0.005 MPa or more and 0.040 MPa or less. <1> ~ <3> 13. The microporous polyolefin membrane according to claim 12. <5> The ratio of the tensile breaking strength in the MD direction to the tensile breaking strength in the TD direction (MD direction / TD direction) is 0.50 or more and 2.0 or less. <1> ~ <4> 13. The microporous polyolefin membrane according to claim 12. <6> On the surface and / or inner surface of the polyolefin microporous membrane, (1) One or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond, or a molecule or molecular chain having such a functional group is bonded to the compound, (2) A resin having one or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond is attached to the surface of the polymer. <1> ~ <5> 13. The microporous polyolefin membrane according to claim 12. Effect of the Invention

[0006] According to an embodiment of the present disclosure, there is provided a polyolefin microporous membrane suitable as a substrate for achieving both the ability to capture substances in a fluid, such as ionic substances, and the ability to allow the permeation of substances other than the targets of capture. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Hereinafter, the embodiments of the present invention will be described. These descriptions and examples are merely illustrative of the embodiments, and do not limit the scope of the invention. The action mechanism described in this disclosure includes assumptions, and the correctness or incorrectness of such assumptions does not limit the scope of the invention.

[0008] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the lower limit and upper limit, respectively.

[0009] In the numerical ranges described in the present disclosure in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with a value shown in the examples.

[0010] In the present disclosure, the term "step" includes not only an independent step, but also a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved.

[0011] In the present disclosure, each component may contain multiple types of corresponding substances. When referring to the amount of each component in a composition in the present disclosure, if multiple substances corresponding to each component are present in the composition, the total amount of the multiple substances present in the composition is meant unless otherwise specified.

[0012] In this disclosure, "MD direction (machine direction)" of a polyolefin microporous membrane means the longitudinal direction of a polyolefin microporous membrane produced in a long shape, and "TD direction (width direction)" means the direction perpendicular to the "machine direction".

[0013] <Polyolefin microporous membrane> The polyolefin microporous membrane of the present disclosure contains a polyolefin and has a Gurley value of 0.01 sec / 100 mL or more and 20 sec / 100 mL or less, a thickness of 120 μm or more, a porosity of 77% or more and 98% or less, and an average pore size of 0.60 μm or more and 3.0 μm or less.

[0014] The polyolefin microporous membrane of the present disclosure has excellent performance in capturing substances in a fluid, such as ionic substances, and excellent performance in permeating substances other than the target to be captured. Specifically, the target to be captured is firstly assumed to be an ionic substance, and secondly assumed to be particles having a size of 0.1 μm or more. Such ionic substances or particles having the above-mentioned specific particle size that are assumed to be the target to be captured may be present in a fluid, and the fluid may be liquid or gas.

[0015] In order to improve the performance of collecting substances in a fluid, such as ionic substances, the polyolefin microporous membrane preferably has a thicker membrane thickness, specifically, 120 μm or more. However, when the membrane thickness of a polyolefin microporous membrane is thick, the Gurley value generally tends to increase. However, in the present disclosure, the average pore size of the polyolefin microporous membrane is set to a range of 0.60 μm to 3.0 μm, and the porosity is set to a range of 77% to 98%, thereby realizing a physical property of a Gurley value of 0.01 sec / 100 mL to 20 sec / 100 mL. As a result, it has been found that a substrate can be provided that realizes a good balance between the performance of collecting substances in a fluid, such as ionic substances, and the permeability of the fluid. In addition, the polyolefin microporous membrane of the present disclosure can also be suitably used as a filter for removing particles having a size of 0.1 μm or more.

[0016] In the present disclosure, the term "polyolefin microporous membrane" refers to a microporous membrane containing polyolefin. The term "microporous membrane" refers to a membrane that includes a three-dimensional network structure in which fibril-like polyolefin is connected, has a large number of micropores inside, has a structure in which these micropores are connected, and is in a state in which gas or liquid can pass from one side to the other side.

[0017] Conventional polyolefin microporous membranes have room for improvement in at least one of their performance in capturing substances in a fluid, such as ionic substances (hereinafter also referred to as capturing performance) and their performance in permeating a fluid containing substances other than the substance to be captured (hereinafter also referred to as permeation performance). The microporous polyolefin membrane of the present disclosure that satisfies the specific conditions of Gurley value, thickness, porosity, and average pore size satisfies these performances in a well-balanced manner.

[0018] In the present disclosure, the form of the substance to be collected is not particularly limited. For example, the substance to be collected may be in particulate form, or may not be in particulate form like substances contained in a fluid, such as ionic substances. When the substance to be collected is particulate, the average particle size of the substance to be collected may be in the range of 0.1 μm or more. In the present disclosure, the average particle size of the substance to be collected is determined by image analysis. Specifically, 100 particles are observed using an electron microscope or the like, and the arithmetic mean value of the particle sizes of the observed particles is taken as the average particle size of the substance to be collected. The substance to be collected may be in a liquid or gas state.

[0019] [Polyolefins that make up the microporous polyolefin membrane] The polyolefin microporous membrane of the present disclosure may be made of only polyolefin, or may be made of polyolefin and a material other than polyolefin. When the polyolefin microporous membrane is made of polyolefin and a material other than polyolefin, the proportion of polyolefin in the entire polyolefin microporous membrane is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. The polyolefin microporous film may contain only one type of polyolefin, or may contain two or more types of polyolefins differing in molecular structure, molecular weight, and the like.

[0020] The polyolefin microporous film may contain polyethylene as the polyolefin. In this case, the polyolefin microporous film may contain only polyethylene as the polyolefin, or may contain polyethylene and a polyolefin other than polyethylene (e.g., polypropylene). When the polyolefin microporous film contains polyethylene and a polyolefin other than polyethylene as the polyolefin, the proportion of polyethylene in the total polyolefin is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0021] The polyolefin microporous membrane may contain ultra-high molecular weight polyethylene (UHMWPE) as the polyethylene contained in the polyolefin. When the polyolefin microporous membrane contains ultra-high molecular weight polyethylene, the pore size of the polyolefin microporous membrane is not too large and the collection performance tends to be excellent.

[0022] When the polyolefin microporous membrane contains ultra-high molecular weight polyethylene, the proportion of ultra-high molecular weight polyethylene in the entire polyolefin is preferably 20% by mass or less. When the proportion of ultra-high molecular weight polyethylene in the entire polyolefin is 20% by mass or less, the pore size of the polyolefin microporous membrane is not too small and tends to have excellent permeability. The proportion of ultra-high molecular weight polyethylene in the entire polyolefin is preferably 18% by mass or less, more preferably 16% by mass or less. In addition, when the polyolefin microporous film contains ultra-high molecular weight polyethylene, the proportion of ultra-high molecular weight polyethylene in the entire polyolefin is preferably 1% by mass or more. When the proportion of ultra-high molecular weight polyethylene in the entire polyolefin is 1% by mass or more, the mechanical strength of the polyolefin microporous film is easily increased. The proportion of ultra-high molecular weight polyethylene in the entire polyolefin is preferably 3% by mass or more, more preferably 4% by mass or more.

[0023] When the polyolefin microporous film contains ultra-high molecular weight polyethylene and polyolefins other than ultra-high molecular weight polyethylene (hereinafter also referred to as other polyolefins) as polyolefins, the type of the other polyolefins is not particularly limited. As the other polyolefins, high density polyethylene (HDPE) is preferable. In the present disclosure, high density polyethylene refers to a polyethylene having a density of 942 kg / m 3 This refers to the above polyethylene.

[0024] In the present disclosure, ultra-high molecular weight polyethylene means polyethylene having a weight average molecular weight of 1 million or more and 6 million or less. The weight average molecular weight of the ultra-high molecular weight polyethylene is preferably 3 million or more, more preferably 4 million or more, and is preferably 5 million or less, more preferably 4.8 million or less.

[0025] In this disclosure, the weight average molecular weight of the polyolefin is measured by gel permeation chromatography. Specifically, the polyolefin to be measured is dissolved by heating in o-dichlorobenzene, and the measurement is performed by gel permeation chromatography (system: Alliance GPC 2000 manufactured by Waters, columns: GMH6-HT and GMH6-HTL) at a column temperature of 135°C and a flow rate of 1.0 mL / min. Molecular weight calibration is performed using monodisperse polystyrene (manufactured by Tosoh Corporation).

[0026] [Gurley value] The Gurley value of the polyolefin microporous membrane of the present disclosure is 0.01 sec / 100 mL or more and 20 sec / 100 mL or less. The polyolefin microporous membrane of the present disclosure has a Gurley value of 0.01 sec / 100 mL or more to ensure sufficient collection performance, and a Gurley value of 20 sec / 100 mL or less to ensure sufficient permeability. From the viewpoint of collection performance, the Gurley value of the polyolefin microporous membrane is preferably 0.1 sec / 100 mL or more, and more preferably 1 sec / 100 mL or more. From the viewpoint of permeability, the Gurley value of the polyolefin microporous membrane is preferably 15 seconds / 100 mL or less, and more preferably 13 seconds / 100 mL or less.

[0027] In the present disclosure, the Gurley value of a polyolefin microporous membrane is measured using a Gurley densometer manufactured by Toyo Seiki Co., Ltd. in accordance with JIS P8117: 2009. The measurement is performed by measuring the time it takes for 200 ml of air to pass through a 28.6 mmφ sample, and then halving that time to convert it to a value per 100 ml.

[0028] Thickness The microporous polyolefin membrane of the present disclosure has a thickness of 120 μm or more. The microporous polyolefin membrane of the present disclosure has a thickness of 120 μm or more, which can lengthen the flow paths within the microporous polyolefin membrane and expand the area for adsorbing substances in a fluid, such as ionic substances. From the viewpoint of collection performance, the thickness of the polyolefin microporous membrane is preferably 130 μm or more, and more preferably 135 μm or more. The upper limit of the thickness of the polyolefin microporous membrane is not particularly limited. From the viewpoint of permeability, the thickness of the polyolefin microporous membrane is preferably 500 μm or less, more preferably 400 μm or less, and even more preferably 300 μm or less.

[0029] In the present disclosure, the thickness of the polyolefin microporous membrane is measured using a contact thickness meter (Mitutoyo Corporation, ABS Digimatic Indicator, Model ID: ID-S112X). Specifically, the thickness of the polyolefin microporous membrane is measured at 20 points, and the arithmetic average value is taken as the thickness of the polyolefin microporous membrane. A cylindrical terminal with a diameter of 6.5 mm at the bottom is used as the contact terminal. Another method for measuring the thickness of the polyolefin microporous membrane includes using a contact thickness meter (Mitutoyo Corporation, LITEMATIC) and using a cylindrical measuring terminal with a diameter of 5 mm, adjusting the terminal so that a load of 7 g is applied during measurement, measuring at 20 points, and calculating the arithmetic average of the measured values.

[0030] [Porosity] The porosity of the microporous polyolefin membrane of the present disclosure is 77% or more and 98% or less. The microporous polyolefin membrane of the present disclosure has a porosity of 77% or more to ensure sufficient permeability, and a porosity of 98% or less to ensure sufficient membrane strength. From the viewpoint of permeability, the porosity of the polyolefin microporous membrane is preferably 78% or more, and more preferably 80% or more. From the viewpoint of membrane strength, the porosity of the polyolefin microporous membrane is preferably 95% or less, and more preferably 90% or less.

[0031] In the present disclosure, the porosity of a polyolefin microporous membrane is calculated according to the following calculation method: That is, the constituent materials of the polyolefin microporous membrane are a, b, c, ..., n, and the masses of the respective constituent materials are Wa, Wb, Wc, ..., Wn (g / cm 2 ), and the true densities of the constituent materials are da, db, dc, …, dn (g / cm 3 ) and when the thickness of the polyolefin microporous membrane is t (cm), the porosity ε (%) is calculated by the following formula: ε={1-(Wa / da+Wb / db+Wc / dc+…+Wn / dn) / t}×100

[0032] [Average pore diameter] The average pore size of the microporous polyolefin membrane of the present disclosure is 0.60 μm or more and 3.0 μm or less. The polyolefin microporous membrane of the present disclosure has an average pore size of 0.60 μm or more to ensure sufficient permeability even when the thickness of the polyolefin microporous membrane is 120 μm or more, and has an average pore size of 3.0 μm or less to ensure sufficient collection performance. From the viewpoint of permeability, the average pore size of the polyolefin microporous membrane is preferably 0.61 μm or more, and more preferably 0.62 μm or more. From the viewpoint of collection performance, the average pore size of the polyolefin microporous membrane is preferably 2.0 μm or less, and more preferably 1.5 μm or less.

[0033] In the present disclosure, the average pore size of the polyolefin microporous membrane is measured by the half-dry method specified in ASTM E1294-89 using a PMI Perm Porometer (model: CFP-1200-AEXL) and a PMI Galwick (surface tension 15.9 dyn / cm) as an immersion liquid. The measurement temperature is 25°C, and the measurement pressure is changed in the range of 0 kPa to 600 kPa.

[0034] [Bubble point pressure] The microporous polyolefin membrane of the present disclosure preferably has a bubble point pressure of 0.005 MPa or more and 0.040 MPa or less. When the bubble point pressure of the polyolefin microporous membrane is 0.005 MPa or more, the membrane tends to have excellent collection performance, and when the bubble point pressure of the polyolefin microporous membrane is 0.040 MPa or less, the membrane tends to have excellent permeation performance. From the viewpoint of collection performance, the bubble point pressure of the microporous polyolefin membrane is preferably 0.007 MPa or more, and more preferably 0.008 MPa or more. From the viewpoint of permeability, the bubble point pressure of the microporous polyolefin membrane is preferably 0.035 MPa or less, and more preferably 0.030 MPa or less.

[0035] In the present disclosure, the bubble point pressure of a polyolefin microporous membrane is measured by immersing the polyolefin microporous membrane in ethanol according to the bubble point test method of JIS K3832: 1990. The measurement is performed by changing the liquid temperature during the test to 24±2°C and increasing the applied pressure at a pressure increase rate of 2 kPa / sec.

[0036] [M / T ratio] The microporous polyolefin membrane of the present disclosure preferably has a ratio of tensile break strength in MD to tensile break strength in TD (MD / TD, also called M / T ratio) of 0.50 or more and 2.0 or less. When the M / T ratio of the polyolefin microporous membrane is within the above range, the physical strength of the polyolefin microporous membrane is sufficiently ensured, which also contributes to improving the ion-collecting performance. The M / T ratio of the microporous polyolefin membrane is more preferably 0.51 or more, and even more preferably 0.52 or more.The M / T ratio of the microporous polyolefin membrane is more preferably 1.5 or less, and even more preferably 1.2 or less.

[0037] In the present disclosure, the tensile breaking strength of the polyolefin microporous membrane is measured in accordance with JIS K7176:2014 using a Tensilon tester RTG-1225 manufactured by Orientec Co., Ltd. The sample width is 15 mm, the chuck distance (gauge distance) of the tester is 500 mm, and the sample is pulled at a speed of 200 mm / min, and the load at the time when the sample breaks is divided by the width of the sample to obtain a value. The same measurement operation is performed three times using three samples, and the obtained values ​​are averaged to obtain the tensile breaking strength value.

[0038] [Method of producing polyolefin microporous membrane] The microporous polyolefin membrane can be produced, for example, by a production process including the following steps (I) to (IV).

[0039] Step (I): preparing a solution containing a polyolefin and a volatile solvent having a boiling point of less than 210° C. at atmospheric pressure. Step (II): A step of melt-kneading the solution, extruding the resulting melt-kneaded mixture through a die, and cooling and solidifying it to obtain a first gel-like molded product. Step (III): A step of stretching the first gel-like molding in at least one direction (primary stretching) and drying the solvent to obtain a second gel-like molding. Step (IV): A step of stretching the second gel-like molding in at least one direction (secondary stretching).

[0040] Step (I) is a step of preparing a solution containing a polyolefin and a volatile solvent having a boiling point of less than 210°C at atmospheric pressure. The solution is preferably a thermoreversible sol-gel solution, and the polyolefin is dissolved by heating in a solvent to form a sol, thereby preparing a thermoreversible sol-gel solution. The volatile solvent having a boiling point of less than 210°C at atmospheric pressure is not particularly limited as long as it can sufficiently dissolve the polyolefin. Specific examples of the volatile solvent include tetralin (206°C to 208°C), ethylene glycol (197.3°C), decalin (decahydronaphthalene, 187°C to 196°C), toluene (110.6°C), xylene (138°C to 144°C), diethyltriamine (107°C), ethylenediamine (116°C), dimethylsulfoxide (189°C), hexane (69°C), and the like, and decalin or xylene is preferred. In the above examples, the temperature in parentheses is the boiling point at atmospheric pressure. The volatile solvents may be used alone or in combination of two or more kinds.

[0041] The polyolefin used in step (I) may be one kind or two or more kinds, and can be selected depending on the desired physical properties of the polyolefin microporous membrane.

[0042] From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the solution prepared in step (I) preferably has a polyolefin concentration of 10% by mass to 40% by mass, more preferably 15% by mass to 35% by mass, and even more preferably 20% by mass to 32% by mass. When the polyolefin concentration in the solution is 10% by mass or more, the occurrence of breakage during the film formation process of the polyolefin microporous membrane can be suppressed, and the mechanical strength of the polyolefin microporous membrane is increased, improving the handleability. When the polyolefin concentration in the solution is 40% by mass or less, the polyolefin microporous membrane of the present disclosure is easily obtained.

[0043] Step (II) is a step of melt-kneading the solution prepared in step (I), extruding the resulting melt-kneaded product through a die, and cooling and solidifying it to obtain a first gel-like molded product. In step (II), for example, extrusion is performed through a die in a temperature range from the melting point of the polyolefin to the melting point + 65°C to obtain an extrudate, and then the extrudate is cooled to obtain a first gel-like molded product. The first gel-like molded product is preferably shaped into a sheet. The cooling method is not particularly limited. For example, cooling may be performed by immersion in water or an organic solvent, contact with a cooled metal roll, or the like.

[0044] Step (III) is a step of stretching the first gel-like molded product in at least one direction (primary stretching) and drying the solvent to obtain a second gel-like molded product. The stretching step in step (III) may be either uniaxial stretching or biaxial stretching. The biaxial stretching may be sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed separately, or simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously. From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the stretching ratio in the primary stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 1.1 to 3 times, more preferably 1.1 to 2 times. The temperature during the primary stretching is preferably 75°C or less. The drying of the solvent in the step (III) (drying step) is preferably carried out at a temperature at which the second gel-like molding does not deform, and more preferably at 60° C. or lower.

[0045] The stretching step and the drying step in step (III) may be performed simultaneously or stepwise. For example, the first stretching may be performed while pre-drying and then main drying, or the first stretching may be performed between pre-drying and main drying. The first stretching may be performed by controlling the drying so that the solvent remains in a suitable state.

[0046] Step (IV) is a step of stretching the second gel-like molded product in at least one direction (secondary stretching). The stretching step in step (IV) is preferably biaxial stretching. The biaxial stretching may be any of the following: sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed separately; simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously; a step of stretching in the longitudinal direction multiple times and then stretching in the transverse direction; a step of stretching in the longitudinal direction and then stretching in the transverse direction multiple times; and a step of sequential biaxial stretching and then further stretching in the longitudinal direction and / or transverse direction once or multiple times.

[0047] From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the stretching ratio in the second stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 5 to 90, and more preferably 10 to 60. From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the stretching temperature in the second stretching is preferably 70° C. to 135° C., and more preferably 80° C. to 130° C.

[0048] If necessary, a heat setting treatment may be carried out after the step (IV) The temperature for the heat setting treatment is preferably 110°C to 150°C, more preferably 120°C to 140°C, from the viewpoint of controlling the porous structure of the polyolefin microporous membrane.

[0049] If necessary, the heat setting may be followed by extraction of the solvent remaining in the polyolefin microporous membrane and annealing. The extraction of the remaining solvent is carried out, for example, by immersing the heat setting sheet in a methylene chloride bath to dissolve the remaining solvent in the methylene chloride. It is preferable that the polyolefin microporous membrane immersed in the methylene chloride bath is removed from the methylene chloride bath by drying. The annealing can be carried out by conveying the polyolefin microporous membrane on a roll heated to, for example, 70°C to 140°C after the extraction of the remaining solvent, or by conveying the polyolefin microporous membrane in a heated atmosphere of 70°C to 140°C while maintaining the width dimension constant.

[0050] [Surface treatment of microporous polyolefin membrane] If necessary, the polyolefin microporous membrane may be subjected to a surface treatment, which can impart desired functions to the polyolefin microporous membrane. The method for carrying out the surface treatment is not particularly limited, and examples thereof include graft treatment, plasma treatment, electron beam treatment, corona treatment, and coating treatment with a functional polymer.

[0051] The surface-treated polyolefin microporous membrane has the following properties on the surface and / or inner surface: (1) One or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond, or a molecule or molecular chain having such a functional group is bonded to the compound, (2) It may be in a state where a resin having one or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond is attached.

[0052] The molecule having the functional group mentioned above may be a molecule having a functional group and a hydrocarbon group. A functional group or a molecule having a functional group may be further bonded to the functional group or the molecule having a functional group bonded to the polyolefin. In addition, the above-mentioned surface treatment is performed on the position of a certain hydrogen atom of the polyolefin constituting the polyolefin microporous membrane of the present disclosure, and as a result, two or more molecules having a functional group are bonded to the polyolefin, and a molecular chain is formed in a side chain form from the main chain of the polyolefin.

[0053] Examples of resins having the above-mentioned functional groups include polyvinyl alcohol, olefin-vinyl alcohol resins, acrylic-vinyl alcohol resins, methacrylic-vinyl alcohol resins, vinylpyrrolidone-vinyl alcohol resins, polyacrylic acid, polymethacrylic acid, perfluorosulfonic acid resins, polystyrene sulfonic acid, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxyalkanes, ethylene-tetrafluoroethylene copolymers, and polyvinylidene fluoride.

[0054] In an embodiment, the polyolefin microporous membrane may be surface-treated to increase its affinity for the substance to be collected or the medium containing the substance. For example, the surface treatment may increase the hydrophilicity of the polyolefin microporous membrane, thereby improving the collection performance for the hydrophilic substance or the medium containing the hydrophilic substance. In an embodiment, the polyolefin microporous membrane may be surface-treated to introduce an ionic functional group. For example, an anionic (or cationic) functional group may be introduced into the polyolefin microporous membrane by surface treatment to improve the collection performance for cationic (or anionic) substances.

[0055] [Graft loading amount] The microporous polyolefin membrane of the present disclosure has a graft loading of 2.0 g / m 2 More preferably, it is equal to or greater than this. The graft loading of the polyolefin microporous membrane is 2.0 g / m 2 When the polyolefin microporous membrane is subjected to a surface treatment, the effect of the surface treatment is sufficiently exhibited when the graft loading amount of the polyolefin microporous membrane is 3.0 g / m or more. 2 More preferably, it is 5.0 g / m or more. 2 More preferably, it is equal to or greater than this. The upper limit of the graft loading of the polyolefin microporous membrane is not particularly limited, but from the viewpoint of ensuring sufficient permeability, it is preferably 15.0 g / m 2 Preferably, the thickness is 14.0 g / m or less. 2It is more preferable that:

[0056] The graft loading of the polyolefin microporous membrane is an index showing the degree of effect of the surface treatment that can be imparted to the polyolefin microporous membrane, and does not require that the polyolefin microporous membrane is actually surface-treated.

[0057] In the present disclosure, the graft loading of a microporous polyolefin membrane is measured by the following method. The polyolefin microporous membrane is cut to prepare a sample of 10 cm x 10 cm. The sample is subjected to plasma treatment for 1 minute under argon atmosphere at a power of 20 W and a vacuum of 10 Pa. The sample is then immersed in a solution of acrylic acid (concentration: 10% by mass, solvent: pure water) and reacted at 80°C for 30 minutes. The sample is then washed with a mixture of water and ethanol (mass ratio 1:1) and dried in a vacuum dryer. The graft loading amount of the sample is calculated using the following formula. The basis weight of the sample is the value obtained by multiplying the measured mass of the sample by 100. Graft loading amount (g / m 2 ) = Grafted sample weight (g / m 2 )-Weight of sample before grafting (g / m 2 )

[0058] [Uses of microporous polyolefin membranes] The uses of the polyolefin microporous membrane are not particularly limited, but specific uses include air filters, liquid filters, moisture-permeable waterproof membranes, bags, dust-collecting sheet substrates, and the like. A suitable application of the microporous polyolefin membrane of the present disclosure is the capture of a substance to be captured by chemically reacting the substance with the surface of the microporous polyolefin membrane (eg, capture of ions through ionic bonds). When the polyolefin microporous membrane is used to capture particulate matter, the average particle size of the substance to be captured may be in the range of 0.1 μm or more and 10 μm or less, in the range of 0.2 μm or more and 7 μm or less, or in the range of 0.5 μm or more and 5 μm or less. The material to be collected may be contained in either a liquid or a gas. EXAMPLES

[0059] The microporous polyolefin membrane of the present disclosure will be described in more detail below with reference to examples. The materials, amounts, ratios, processing procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present disclosure. Therefore, the scope of the polyolefin microporous membrane of the present disclosure should not be construed as being limited by the specific examples shown below.

[0060] [Example 1] 15 parts by mass of ultra-high molecular weight polyethylene (UHMWPE) with a weight average molecular weight (Mw) of 4.6 million and 15 parts by mass of polyethylene terephthalate with a weight average molecular weight (Mw) of 560,000 and a density of 950 kg / m 3 A polyethylene composition was prepared by mixing 85 parts by mass of high density polyethylene (HDPE) having a polyethylene composition content of 30% by mass with decalin as a solvent.

[0061] The above polyethylene solution was extruded into a sheet form through a die at a temperature of 160° C., and the extrudate was then cooled in a water bath at a water temperature of 20° C. to obtain a first gel-like sheet.

[0062] The first gel-like sheet was pre-dried at 60° C. for 15 minutes, then stretched in the MD direction at 1.3 times its original size, and then dried at 55° C. for 10 minutes to obtain a second gel-like sheet. The amount of residual solvent in the second gel-like sheet was less than 1% by mass. Next, as the second stretching, the second gel-like sheet was stretched in the MD direction at a stretching ratio of 2.0 times at a temperature of 80° C., and then stretched in the TD direction at a stretching ratio of 4.7 times at a temperature of 125° C. Immediately after the second stretching, a heat treatment (heat setting) at 130° C. was performed.

[0063] The heat-set sheet was immersed in a methylene chloride bath consisting of one tank for 1 minute to extract the solvent from the sheet. After removing the sheet from the methylene chloride bath, the sheet was dried on a heated roll at 40°C to remove the methylene chloride. The sheet was then annealed in a heated atmosphere at 80°C. Through the above steps, a polyolefin microporous membrane of Example 1 was obtained. When the polyolefin microporous membrane of Example 1 was observed with a scanning electron microscope, it was observed that the membrane had a three-dimensional network structure in which fibril-like polyolefin was connected, had a large number of micropores therein, and had a structure in which these micropores were connected.

[0064] [Examples 2 to 4 and Comparative Examples 1 to 6] The polyethylene microporous membranes of Examples 2 to 4 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1, except that the composition of the polyethylene solution or the conditions for the polyolefin microporous membrane production process were changed as shown in Table 1. When the polyolefin microporous membranes of Examples 2 to 4 and Comparative Examples 1 to 6 were observed with a scanning electron microscope, it was observed that they had a three-dimensional network structure in which fibril-like polyolefin was connected, had a large number of micropores inside, and had a structure in which these micropores were connected.

[0065] [Table 1]

[0066] [Gurley value measurement] The Gurley value of the polyolefin microporous membrane was measured using a Gurley densometer manufactured by Toyo Seiki Co., Ltd. in accordance with JIS P8117:2009. The measurement was performed by measuring the time it took for 200 ml of air to pass through a 28.6 mmφ sample, and then halving that time to convert it to a value per 100 ml. The results are shown in Table 2.

[0067] [Thickness measurement] The thickness of the polyolefin microporous membrane was measured using a contact-type thickness gauge (Mitutoyo Corporation, ABS Digimatic Indicator, Model ID: ID-S112X). Specifically, the thickness of the polyolefin microporous membrane was measured at 20 points, and the arithmetic average value was taken as the thickness. A cylindrical terminal with a bottom diameter of 6.5 mm was used as the contact terminal. The results are shown in Table 2.

[0068] [Porosity measurement] The porosity of the polyolefin microporous membrane was calculated according to the following calculation method: the constituent materials of the polyolefin microporous membrane are a, b, c, ..., n, and the masses of the respective constituent materials are Wa, Wb, Wc, ..., Wn (g / cm 2 ), and the true densities of the constituent materials are da, db, dc, …, dn (g / cm 3 ) and the thickness of the polyolefin microporous membrane was t (cm), the porosity ε (%) was calculated by the following formula. The results are shown in Table 2. ε={1-(Wa / da+Wb / db+Wc / dc+…+Wn / dn) / t}×100

[0069] [Measurement of average pore size] The average pore size of the polyolefin microporous membrane was measured by the half-dry method specified in ASTM E1294-89 using a PMI Palm Porometer (model: CFP-1200-AEXL) and PMI Galwick (surface tension 15.9 dyn / cm) as the immersion liquid. The measurement temperature was 25°C, and the measurement pressure was changed in the range from 0 kPa to 600 kPa.

[0070] [Bubble point pressure measurement] The bubble point pressure of the polyolefin microporous membrane was measured by immersing the polyolefin microporous membrane in ethanol according to the bubble point test method of JIS K3832: 1990. The measurement was performed by changing the liquid temperature during the test to 24±2°C and increasing the applied pressure at a rate of 2 kPa / sec.

[0071] [Measurement of tensile breaking strength] The tensile breaking strength of the polyolefin microporous membrane was measured in accordance with JIS K7176:2014 using a Tensilon tester RTG-1225 manufactured by Orientec Co., Ltd. The sample width was 15 mm, the chuck distance (gauge distance) of the tester was 500 mm, and the sample was pulled at a speed of 200 mm / min, and the load at the time when the sample broke was divided by the width of the sample to obtain a value. The same measurement operation was performed three times using three samples, and the obtained values ​​were averaged to obtain the tensile breaking strength value. The M / T ratio was calculated from the tensile breaking strength values ​​measured in the MD and TD directions of the polyolefin microporous membrane. The results are shown in Table 2.

[0072] [Measurement of graft loading amount] The polyolefin microporous membrane was cut to prepare a sample of 10 cm x 10 cm. The sample was subjected to plasma treatment for 1 minute under an argon atmosphere at a power of 20 W and a vacuum of 10 Pa. The sample was then immersed in a solution of acrylic acid (concentration: 10% by mass, solvent: pure water) and reacted at 80°C for 30 minutes. The sample was then washed with a mixture of water and ethanol (mass ratio 1:1) and dried in a vacuum dryer. The graft loading amount of the sample was calculated by the following formula. The basis weight of the sample was determined by multiplying the measured mass of the sample by 100. The results are shown in Table 2. Graft loading amount (g / m 2 ) = Grafted sample weight (g / m 2 )-Weight of sample before grafting (g / m 2 )

[0073] [Liquid flow rate measurement] The polyolefin microporous membrane that had been subjected to the surface treatment described in the above-mentioned method for measuring the graft loading amount was cut into a circle having a diameter of 30 mm and set in a stainless steel liquid permeation cell having a diameter of 16 mm. A pre-measured amount of pure water (10 mL) was permeated at a differential pressure of 94 kPa, and the time required for the entire amount of pure water to permeate was measured. When the time required for 10 mL of pure water to pass through was 20 seconds or less, it was rated as "A", when it was more than 20 seconds but less than 50 seconds, it was rated as "B", when it was more than 50 seconds but less than 100 seconds, it was rated as "C", and when it was more than 100 seconds, it was rated as "D". The results are shown in Table 2.

[0074] [Measurement of ion collection efficiency] Copper ions were dissolved in pure water to prepare an aqueous copper ion solution (concentration: 1 ppm). The aqueous copper ion solution and the polyolefin microporous membrane that had been subjected to the surface treatment described in the above-mentioned method for measuring the graft loading amount were set in a measuring device, and the aqueous copper ion solution (10 mL) was filtered at a reduced pressure of 94 kPa. The copper ion concentration (ppm) of the filtered aqueous copper ion solution was measured using inductively coupled plasma atomic emission spectrometry. The ion collection rate of the polyolefin microporous membrane was calculated using the following formula. The ion collection performance of the polyolefin microporous membrane was evaluated as follows: when the ion collection rate was 99% or more, it was rated as "A", when it was 95% or more but less than 99%, it was rated as "B", when it was 90% or more but less than 95%, it was rated as "C", and when it was less than 90%, it was rated as "D". The results are shown in Table 2. Ion collection rate (%) = {(concentration before filtration - concentration after filtration) / concentration before filtration} x 100

[0075] [Measurement of weight average molecular weight of polyolefin] The weight average molecular weight of the polyolefin used in preparing the microporous polyolefin membrane was measured by gel permeation chromatography. Specifically, the polyolefin to be measured was dissolved in o-dichlorobenzene by heating, and the measurement was performed by gel permeation chromatography (system: Alliance GPC 2000 manufactured by Waters, columns: GMH6-HT and GMH6-HTL) under the conditions of a column temperature of 135°C and a flow rate of 1.0 mL / min. Molecular weight monodisperse polystyrene (manufactured by Tosoh Corporation) was used for molecular weight calibration.

[0076] [judgement] A filter was judged as "passed" when both the particle collection rate and the liquid flow rate were rated as A or B, and a filter was judged as "failed" when either one was rated as C or D. The results are shown in Table 2.

[0077] [Table 2]

[0078] As shown in Table 2, the polyolefin microporous membranes of the examples, each of which satisfied the conditions of the present disclosure for Gurley value, thickness, porosity, and average pore size, had superior overall evaluations of collection performance and permeation performance compared to the polyolefin microporous membranes of the comparative examples, each of which did not satisfy the conditions of the present disclosure for at least any of the Gurley value, thickness, porosity, and average pore size.

Claims

1. A polyolefin microporous membrane comprising a polyolefin, having a Gurley value of 0.01 sec / 100 mL or more and 20 sec / 100 mL or less, a thickness of 120 μm or more, a porosity of 77% or more and 98% or less, and an average pore size of 0.60 μm or more and 3.0 μm or less.

2. The microporous polyolefin membrane of claim 1 , wherein the polyolefin comprises polyethylene.

3. The polyolefin microporous membrane according to claim 2, wherein the polyethylene comprises ultra-high molecular weight polyethylene, and the proportion of the ultra-high molecular weight polyethylene in the total polyolefin is 20 mass% or less.

4. The microporous polyolefin membrane according to claim 1, having a bubble point pressure of 0.005 MPa or more and 0.040 MPa or less.

5. The polyolefin microporous membrane according to claim 1, wherein the ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction (MD direction / TD direction) is 0.50 or more and 2.0 or less.

6. On the surface and / or inner surface of the polyolefin microporous membrane, (1) One or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond, or a molecule or molecular chain having such a functional group, are bonded to the polymer; or (2) A resin having one or more functional groups selected from the group consisting of a hydroxy group, a carbonyl group, a carboxy group, a formyl group, a sulfo group, a sulfonyl group, a thiol group, an amino group, a nitrile group, a nitro group, a pyrrolidone ring group, a fluoro group, an ether bond, and an amide bond is attached to the polymer. The microporous polyolefin membrane according to claim 1.