Separation membrane, separation membrane module, gas separation method, gas separation device, and method for producing separation membrane

Abstract

To provide a separation membrane, a separation membrane module, a separation device, and a gas separation method which are excellent in productivity and workability, and can effectively separate gas having a high warming coefficient.SOLUTION: A separation membrane contains a thermoplastic resin as a main component, wherein in measurement of dynamic viscoelasticity, when the separation membrane is measured while changing the frequency to 1 to 100 Hz, an inclination A of a logarithmic approximate expression obtained from a frequency F and a tanδ peak temperature T plot is 1.4 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separation membrane, in particular a gas separation membrane capable of separating and recovering gases with a high global warming potential, a gas separation membrane module, a gas separation method, a gas separation apparatus, and a method for producing a separation membrane. [Background technology]

[0002] Gases with high global warming potentials include fluorinated gases such as perfluorocarbons (PFCs), SF6, hydrofluorocarbons (HFCs), and NF3, as well as chlorofluorocarbons (CFCs). These gases are essential for a wide range of applications, including the semiconductor industry, metal refining, and electronic and electrical equipment. Emissions are expected to increase annually, particularly in developed countries, due to increased emissions from device manufacturing processes in the growing semiconductor industry, the widespread use of electronic and electrical equipment, and the mass disposal of used electrical appliances. Due to their high greenhouse potential, fluorinated gases are released into the atmosphere through combustion decomposition. However, because fluorinated gases are chemically stable, their combustion decomposition requires high temperatures. Establishing technologies capable of separating and recovering these gases could potentially reduce combustion decomposition costs and greenhouse gas emissions.

[0003] Membrane separation is a known gas separation method for selectively separating and purifying specific gases from various mixed gases. Membrane separation is attracting attention because it is a more energy-efficient method than other gas separation methods. Examples of membrane separation methods include zeolite, carbon-based membranes, and polyimide membranes, as proposed in Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-345545 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-185212 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the findings of the present inventors, these separation membranes have been used to separate fluorine-containing gases, such as nitrogen, helium, argon, and carbon dioxide, which are contained in air and do not contain fluorine atoms, from fluorine-containing gases, but have not been used to separate fluorine-containing mixed gases. Furthermore, since they use special raw materials and undergo complicated processes, they have the problem of being expensive and having poor productivity. Furthermore, zeolite membranes, in particular, have the problem of being difficult to process into a desired shape.

[0006] In view of the above-mentioned problems of the conventional technology, the present inventors have an object to provide a separation membrane that can effectively separate fluorine-containing mixed gases and has excellent productivity and processability, as well as a separation membrane module, a gas separation method, and a gas separation apparatus that use the same. [Means for solving the problem]

[0007] As a result of intensive research to solve the above problems, the inventors discovered that a separation membrane containing a thermoplastic resin as its main component and exhibiting specific behavior in dynamic viscoelasticity measurements has high productivity and high gas separation properties, and thus completed the present invention.

[0008] That is, the separation membrane of the present invention has a thermoplastic resin as its main component, and when measured in dynamic viscoelasticity measurement by changing the frequency in the range of 1 to 100 Hz, the slope A of the logarithmic approximation equation obtained from the plot of frequency F and tan δ peak temperature T is 1.4 or more. [Effects of the Invention]

[0009] According to the present invention, a separation membrane capable of separating specific fluorine-containing gases with high productivity is provided by using a thermoplastic resin having excellent processability. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 1 is an example of an image of a cross section of a separation membrane cut in the thickness direction, taken with a scanning electron microscope (hereinafter referred to as SEM) at a magnification of 10,000 times. [Figure 2] Figure 2 shows the image in Figure 1 after binarization and noise removal. [Figure 3] FIG. 3 shows only voids with a major axis greater than 10 nm extracted from the image of FIG. 2, and shows their outlines and the thickness of the dense layer. DETAILED DESCRIPTION OF THE INVENTION

[0011] The separation membrane of the present invention is characterized in that it contains a thermoplastic resin as a main component, and in that, when measured in dynamic viscoelasticity measurement by varying the frequency in the range of 1 to 100 Hz, the slope A of a logarithmic approximation equation obtained from a plot of frequency F and tan δ peak temperature T is 1.4 or more. In this specification, proportions (percentages, parts, etc.) based on mass are the same as proportions (percentages, parts, etc.) based on weight.

[0012] (Resin composition constituting separation membrane) The resin composition constituting the separation membrane of the present invention is primarily composed of a thermoplastic resin. The term "primary component" refers to the component that is most abundant by mass among all components of the separation membrane. Specific examples of preferred thermoplastic resins that can be the primary component include polyester resins such as polyethylene terephthalate, polyethylene-2,6-naphthalate, polypropylene terephthalate, and polybutylene terephthalate; polyolefin resins such as polyethylene, polystyrene, polypropylene, polyisobutylene, polybutene, polymethylpentene, and poly(4-methyl-1-pentene); polyamide resins; polyimide resins (excluding those that do not exhibit thermoplasticity); polyether resins, polyesteramide resins, polyetherester resins, and acrylic resins such as polymethyl methacrylate; polyurethane resins; polycarbonate resins; polyvinyl chloride resins; polyphenylene sulfide resins; polyethersulfone resins; polysulfone resins; and copolymers and mixtures thereof. Among these, polyester resins, polyolefin resins, polyamide resins, acrylic resins, and mixtures thereof are particularly preferred from the viewpoints of molding processability and crosslinking reactivity upon irradiation with radiation (described later). Polyolefin resins are more preferred because they have an excellent balance between reactivity to radiation, mechanical properties, and resistance to gases such as PFC, SF, HFC, NF, and CFC.

[0013] The resin composition constituting the separation membrane of the present invention is particularly preferably composed mainly of poly(4-methyl-1-pentene) shown in (1) below. In addition to (1), the resin composition may contain the following components (2) and (3).

[0014] (1) Poly(4-methyl-1-pentene) (hereinafter referred to as "PMP") The separation membrane of the present invention preferably contains PMP as a main component, where the main component refers to the component that is contained in the largest amount by mass of all the components of the separation membrane.

[0015] PMP may be a homopolymer of 4-methyl-1-pentene or a copolymer of a monomer other than 4-methyl-1-pentene that is copolymerizable with 4-methyl-1-pentene, as long as it has a repeating unit derived from 4-methyl-1-pentene. Specific examples of the monomer copolymerizable with 4-methyl-1-pentene include olefins having 2 to 20 carbon atoms other than 4-methyl-1-pentene.

[0016] Examples of the olefin having 2 to 20 carbon atoms that can be copolymerized with 4-methyl-1-pentene include ethylene, propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.

[0017] The olefin having 2 to 20 carbon atoms to be copolymerized with 4-methyl-1-pentene may be one type or a combination of two or more types.

[0018] The density of the PMP used in the separation membrane of the present invention is 825 to 840 (kg / m 3 ), and 830 to 835 (kg / m 3 If the density is lower than the above range, the mechanical strength of the separation membrane may decrease, and problems such as the tendency for defects to occur may occur. On the other hand, if the density is higher than the above range, the gas permeability tends to decrease.

[0019] The melt flow rate (MFR) of PMP measured at 260°C under a 5 kg load is not particularly limited as long as it is in a range that allows easy mixing with a plasticizer described below and allows co-extrusion, but is preferably 1 to 200 g / 10 min, and more preferably 5 to 30 g / 10 min. If the MFR is in the above range, it is easy to extrude to a relatively uniform film thickness.

[0020] PMP may be produced directly by polymerizing olefins or by thermal decomposition of a high-molecular-weight 4-methyl-1-pentene polymer. The 4-methyl-1-pentene polymer may be purified by solvent fractionation, which separates the polymer based on the difference in solubility in a solvent, or molecular distillation, which separates the polymer based on the difference in boiling point.

[0021] In addition to the PMP produced as described above, commercially available polymers such as "TPX" manufactured by Mitsui Chemicals, Inc. may also be used.

[0022] The content of PMP in the separation membrane is preferably 70 to 100 mass%, more preferably 80 to 100 mass%, and even more preferably 90 to 100 mass%, when all components of the separation membrane are taken as 100 mass%. When the content of PMP in the separation membrane is 70 mass% or more, sufficient gas permeability is achieved.

[0023] Furthermore, the content of PMP in the raw materials for producing the separation membrane is preferably 10 to 50% by mass, when the total components constituting the raw materials are taken as 100% by mass. A content of 10% by mass or more improves the membrane strength of the separation membrane. On the other hand, a content of 50% by mass or less improves the permeability performance of the separation membrane. The content is more preferably 15 to 50% by mass, even more preferably 20 to 45% by mass, and particularly preferably 25 to 40% by mass.

[0024] (2) Plasticizer for thermoplastic resin The plasticizer for the thermoplastic resin is not particularly limited as long as it is a compound that plasticizes the thermoplastic resin. The plasticizer for the thermoplastic resin may be one type of plasticizer or two or more types of plasticizers may be used in combination.

[0025] In particular, when the thermoplastic resin is mainly composed of PMP, examples of the plasticizer include palm kernel oil, dibutyl phthalate, dioctyl phthalate, dibenzyl ether, coconut oil, and mixtures thereof. Among these, dibutyl phthalate and dibenzyl ether are preferably used in terms of compatibility and spinnability. In the process for producing the separation membrane of the present invention, a resin mixture containing a thermoplastic resin and a plasticizer is first melt-kneaded to obtain a "resin composition." The content of the plasticizer for the thermoplastic resin in the resin composition is preferably 50% by mass or more and 90% by mass or less, when the total mass of the components constituting the resin composition is taken as 100% by mass.

[0026] A content of 90% by mass or less improves the membrane strength of the separation membrane. Furthermore, a content of 50% by mass or more improves the permeability of the separation membrane. The content is more preferably 50% by mass or more and 85% by mass or less, even more preferably 55% by mass or more and 80% by mass or less, and particularly preferably 60% by mass or more and 75% by mass or less.

[0027] The resin composition is then melted, discharged from a discharge nozzle or the like, and cooled to obtain a "molded resin product." The plasticizer of the thermoplastic resin is preferably eluted from the molded resin product. From the viewpoint of increasing the permeability of the separation membrane, the content of the plasticizer in the molded resin product after elution from the thermoplastic resin, i.e., the content of the plasticizer in the separation membrane of the present invention, is preferably 1000 ppm or less by mass, more preferably 500 ppm or less, and particularly preferably 100 ppm or less.

[0028] (3) Additives The resin molded product constituting the separation membrane of the present invention may contain additives other than those described in (2) as long as the effects of the present invention are not impaired.

[0029] Examples of additives include resins other than the thermoplastic resin that is the main component, such as cellulose ether, polyacrylonitrile, polyolefin, polyvinyl compound, polycarbonate, poly(meth)acrylate, polysulfone, or polyethersulfone; organic lubricants, crystal nucleating agents, organic particles, inorganic particles, end-capping agents, chain extenders, ultraviolet absorbers, infrared absorbers, color inhibitors, matting agents, antibacterial agents, antistatic agents, deodorizers, flame retardants, weather resistance agents, antistatic agents, antioxidants, ion exchange agents, antifoaming agents, color pigments, fluorescent brighteners, or dyes.

[0030] (Separation membrane shape) The separation membrane of the present invention is preferably a hollow fiber separation membrane (hereinafter referred to as a "hollow fiber membrane"), which is preferred because it can be efficiently packed into a module and can provide a large effective membrane area per unit volume of the module.

[0031] The shape of the separation membrane in the present invention, i.e., the thickness of the separation membrane, the outer and inner diameters of the hollow fiber membrane, and the hollowness can be determined by, for example, applying stress to a hollow fiber membrane sufficiently cooled in liquid nitrogen, cutting it in the thickness direction, and observing the cross section (hereinafter referred to as the "diametric cross section") using an optical microscope or SEM. Specific methods are described in detail in the Examples.

[0032] The thickness of the separation membrane is preferably 10 to 500 μm from the viewpoint of achieving both permeability and membrane strength. The thickness is more preferably 30 μm or more, and even more preferably 50 μm or more. The thickness is more preferably 200 μm or less, even more preferably 150 μm or less, and particularly preferably 100 μm or less.

[0033] From the viewpoint of achieving both an effective membrane area when packed in a module and membrane strength, the outer diameter of the hollow fiber membrane is preferably 50 to 2500 μm. The outer diameter of the hollow fiber membrane is more preferably 100 μm or more, even more preferably 200 μm or more, and particularly preferably 300 μm or more. The outer diameter is more preferably 1000 μm or less, even more preferably 500 μm or less, and particularly preferably 450 μm or less.

[0034] In addition, considering the relationship between the pressure loss of the fluid flowing through the hollow portion and the buckling pressure, the inner diameter of the hollow fiber membrane is preferably 20 to 1000 μm. The inner diameter of the hollow fiber membrane is more preferably 50 μm or more, even more preferably 100 μm or more, and particularly preferably 150 μm or more. The inner diameter is more preferably 500 μm or less, even more preferably 300 μm or less, and particularly preferably 250 μm or less.

[0035] In addition, considering the relationship between the pressure loss of the fluid flowing through the hollow portion and the buckling pressure, the hollow fiber membrane preferably has a hollow ratio of 15 to 70%, more preferably 20% or more, and even more preferably 25% or more, and more preferably 60% or less, even more preferably 50% or less, and particularly preferably 40% or less.

[0036] Figure 1 shows an example of an image of a cross section of a separation membrane cut in the thickness direction, taken with an SEM at 10,000x magnification. Figure 2 shows the image from Figure 1 after noise has been removed and binarized. Figure 3 shows the outlines of voids with a major axis greater than 10 nm extracted from the image in Figure 2, along with the thickness of the dense layer.

[0037] The method for adjusting the outer diameter, inner diameter, and hollow ratio of each hollow fiber membrane to fall within the above ranges is not particularly limited. For example, the adjustments can be made by appropriately changing the shape of the discharge hole of the spinneret used to produce the hollow fiber membrane, or the draft ratio, which can be calculated from the take-up speed / discharge speed.

[0038] As will be described later, the hollow fiber membrane of this embodiment can be produced by forming a hollow fiber membrane from a membrane-forming solution containing a polymer, and high selectivity can be obtained by irradiating the membrane with radiation.

[0039] (Average pore size converted from NKP plot) The separation membrane of the present invention preferably has an average pore size calculated from the NKP plot of more than 0.60 nm. When the average pore size calculated from the NKP plot exceeds 0.60 nm, the permeability of CF4 is good. Furthermore, it is preferably 1.0 nm or less. When it is 1.0 nm or less, it is possible to suppress the permeation of gases such as CF8 and CF6, thereby obtaining a separation membrane with high selectivity. The average pore size calculated from the NKP plot is preferably more than 0.61 nm, more preferably 0.62 nm or more. It is preferably 0.90 nm or less, and even more preferably 0.85 nm or less, 0.80 nm or less, or even 0.70 nm or less. The method for controlling the pore size is not particularly limited, but pores exceeding 0.6 nm can be formed by distorting the molecular chains due to stress remaining inside the membrane during the stretching and drying processes in the washing process described below. Furthermore, the radiation irradiation process described below results in pores of 1.0 nm or less, which exhibit gas selectivity, and is within the preferred pore size range. The average pore size converted from the NKP plot can be estimated using Normalized Knudsen-based Permeance (NKP), as described in the literature (Membrane. 2011, 383, 152-158.). The formula defined by NKP is expressed as the following formula (1), based on the molecular dependency of the gas permeability coefficient at a given temperature, when He is used as the standard and there is no significant difference in activation energy for each gas species.

[0040]

number

[0041]

number

[0042] Here, P i is the gas permeability coefficient of component i expressed by equation (2), J is the gas flux, d is the membrane thickness, and P H and P L are the partial pressures of the gas on the supply side and the permeation side, respectively. i is the molecular weight of component i, MHe is the molecular weight of He, d p is the average pore diameter converted from the NKP plot, d i is the molecular diameter of component i, d He is the molecular diameter of He, and P i / P He The value of can be replaced with the separation factor β(i / He) between component i and He, which will be described later. The gas species used as component i is not limited as long as there is no significant difference in activation energy; for example, H2, O2, N2, CH4, etc. are preferably used. In the present invention, N2 was used because of its easy availability and ease of handling. Furthermore, the formula used in this calculation is only suitable for separation membranes that have selectivity due to the molecular sieve effect, and therefore only applies to separation membranes that do not have coarse pores on the outer surface, i.e., separation membranes in which no surface pores are observed by surface observation using an SEM or the like. (Dense layer of separation membrane) The separation membrane of this embodiment preferably has a dense layer on at least one surface layer. In the case of a hollow fiber membrane, the dense layer may be on the outer surface side, the inner surface side, or both the outer surface and the inner surface. i However, when (a) D1 ≥ D2 ≥ D3 ≥ D4 ≥ D5 or (b) D5 ≥ D4 ≥ D3 ≥ D2 ≥ D1 is satisfied, it is preferable that the dense layer be provided on the surface of D5 in (a) or D1 in (b). By providing a dense layer on the surface of D5 in (a) or D1 in (b), defects of the dense layer can be suppressed and low leakage can be achieved. Furthermore, when the separation membrane has a hollow fiber shape, it is more preferable that the dense layer be provided on the outer surface of the separation membrane. By providing a dense layer on the outer surface of the separation membrane, the effective membrane area per unit volume of the module can be increased.

[0043] The dense layer refers to the area of ​​a hollow fiber membrane observed at 10,000x magnification using an SEM, where a line is drawn perpendicular to the outer surface from any point on the membrane's outer surface toward the inner surface, or from the inner surface toward the outer surface, up to the first pores larger than 10 nm. In the case of a sheet membrane, the dense layer refers to the area where a line is drawn perpendicular to the sheet surface from one surface to the other, up to the first pores larger than 10 nm. For example, a hollow fiber membrane sufficiently cooled in liquid nitrogen is subjected to stress (using a razor, microtome, or broad ion beam, as necessary), and a diameter cross section or a cross section perpendicular to the diameter cross section, i.e., a cross section parallel to the longitudinal direction of the membrane (hereinafter referred to as the "longitudinal cross section"), is observed using an SEM. The resulting image is then binarized using the image analysis software "ImageJ," where only pores with a major axis larger than 10 nm are extracted, and a line is drawn perpendicular to the outer surface to measure the distance to the pore. In the case of a sheet-like film, the Young's modulus can be obtained by applying stress in the same manner and then observing and analyzing a cross section of the sheet parallel to the direction in which the Young's modulus is highest. Specific measurement methods will be described in detail in the Examples.

[0044] The thickness of the dense layer is preferably 0.1 to 2.0 μm. When the thickness of the dense layer is 0.1 μm or more, low leakage is favorable, and when the thickness is 2.0 μm or less, permeability is favorable.

[0045] The thickness of the dense layer is more preferably 0.1 to 1.5 μm, further preferably 0.1 to 1.0 μm, and particularly preferably 0.1 to 0.4 μm. (viscoelastic properties) When the separation membrane of the present invention is measured in dynamic viscoelasticity measurement (hereinafter referred to as DMA) while changing the frequency in the range of 1 to 100 Hz, it is preferable that the slope A of the logarithmic approximation equation obtained from the plot of frequency F and tan δ peak temperature T is 1.4 or more. Specifically, the gradient A here is a value obtained by the following steps (1) to (3). (1) Using a method conforming to JIS K-7244-4 (1999), the frequency dependence of dynamic viscoelasticity (frequency dispersion, driving frequency is 1 to 100 Hz) is measured under the following measurement conditions: tension mode, chuck distance 10 mm, strain 0.02 to 0.05%, heating rate 3°C / min, temperature range 0 to 150°C. (2) For each frequency, find the temperature at which tan δ is maximum, and plot the frequency F on the horizontal axis and the tan δ maximum temperature T on the vertical axis. If there are multiple temperatures at which tan δ is maximum, T is the temperature at which the maximum value (peak height) of tan δ is largest. (3) From the obtained plot, the logarithmic approximation formula (T = A × log e (F) + B) and find the slope A of the equation The slope A determined by the above procedure correlates with the mobility of the amorphous polymer, particularly the crystalline polymer, constituting the separation membrane; a larger slope A indicates lower amorphous mobility. Gases such as C4F8 and C4F6 easily permeate the membrane due to distortion of the polymer chains at the boundary between the amorphous and crystalline portions. However, when the slope A is 1.4 or greater, the permeability of these gases can be reduced due to the reduction in free volume caused by crosslinking, thereby improving selective separation. The slope A is more preferably 1.5 or greater, even more preferably 2.0 or greater, and particularly preferably 2.5 or greater. The upper limit of the slope A is not particularly limited, but is essentially 20. A slope A of 1.4 or greater can be achieved by increasing the heating temperature or relaxation rate in the heating step described below, or by adjusting the irradiation intensity, dose, etc. in the radiation irradiation step.

[0046] In the separation membrane of the present invention, the temperature at which tan δ is maximized when measured in the DMA at a frequency of 10 Hz is preferably equal to or higher than the glass transition temperature (Tg) of the main thermoplastic resin + 1°C. It is more preferably equal to or higher than Tg + 3°C, and even more preferably equal to or higher than Tg + 4°C. The upper limit of the temperature at which tan δ is maximized when measured at a frequency of 10 Hz is not particularly specified, but is essentially Tg + 60°C. When the main component of the thermoplastic resin is PMP, the temperature at which tan δ is maximized when measured at a frequency of 10 Hz is preferably equal to or higher than 48°C. It is more preferably equal to or higher than 50°C, and even more preferably equal to or higher than 51°C. The upper limit of the temperature at which tan δ is maximized when measured at a frequency of 10 Hz is not particularly specified, but is essentially 100°C. By setting the temperature in this range, the polymer constituting the separation membrane can be more effectively constrained, thereby reducing the permeability of gases such as C4F8 and C4F6 and improving the selective separation ability.

[0047] In the separation membrane of the present invention, the width at 3 / 4 from the tan δ peak height when measured in the DMA at a frequency of 10 Hz is preferably at least the width at 3 / 4 from the tan δ peak height of the main thermoplastic resin + 2°C. More preferably, it is at least the width at 3 / 4 from the tan δ peak height of the main thermoplastic resin + 3°C, and even more preferably, it is at least the width at 3 / 4 from the tan δ peak height of the main thermoplastic resin + 4°C. The upper limit of the width at 3 / 4 from the tan δ peak height is not particularly specified, but is essentially the width at 3 / 4 from the tan δ peak height of the main thermoplastic resin + 60°C. When the main component of the thermoplastic resin is PMP, the width at 3 / 4 from the tan δ peak height when measured in the DMA at a frequency of 10 Hz is preferably at least 19°C. More preferably, it is at least 20°C, and even more preferably at least 21°C. The upper limit of the width at 3 / 4 from the tan δ peak height is not particularly specified, but is essentially 80°C. By setting the temperature within this range, the polymers constituting the separation membrane can be more tightly constrained, resulting in reduced permeability to gases such as C4F8 and C4F6 and improved selective separation. The width at the ¾ position from the tan δ peak height can be set to at least the width at the ¾ position from the tan δ peak height of the main thermoplastic resin + 2°C, or 19°C or higher when the main component of the thermoplastic resin is PMP, by increasing the heating temperature or increasing the relaxation rate in the heating step described below, or by adjusting the irradiation intensity, irradiation amount, etc. in the radiation irradiation step.

[0048] (porosity) The separation membrane of the present invention preferably has a porosity of 40 to 70%. A porosity of 40% or more results in good permeability, while a porosity of 70% or less results in good membrane strength. 45 to 60% is preferred, and 53 to 60% is particularly preferred. To obtain a porosity within this range, structure formation using thermally induced phase separation, which will be described later, is preferably used. Note that the porosity of the present invention refers to the porosity of the entire separation membrane, and a specific method for measuring the porosity will be described in detail in the Examples.

[0049] (pore structure of separation membrane) The separation membrane of this embodiment has an average pore diameter D i Coefficient of variation α i It is preferable that the values ​​of are all 150% or less. i The average pore diameter D is the average pore diameter in each region when the region is divided into 5 equal parts at equal intervals from one surface in the thickness direction and each region is numbered 1 to 5 from the outer surface to the inner surface, and the average pore diameters of each region are denoted as D1, D2, D3, D4, and D5. i The diameter cross section of a hollow fiber membrane is exposed using a microtome after being sufficiently cooled in liquid nitrogen, and then observed using an SEM. The image obtained is then binarized using the image analysis software "ImageJ" and only pores with a diameter larger than 10 nm are extracted. The areas to be observed are five areas divided into five equal intervals in the thickness direction so that each surface of the separation membrane is the edge when the diameter cross section is observed using an SEM. The coefficient of variation α i is the standard deviation of the pore diameters of all pores observed in a specific area, expressed as the average pore diameter D i The coefficient of variation is the value obtained by dividing by the coefficient of variation and multiplying by 100. When the coefficient of variation is 150% or less, the pore size is uniform throughout the region, which suppresses localized liquid leakage due to large pores, and it is possible to achieve both permeability and low leakage. The coefficient of variation is preferably 120% or less, more preferably 100% or less, and even more preferably 80% or less. There is no particular lower limit for the coefficient of variation, but it is essentially 0%. Average pore size D i The average pore diameter D is preferably 100 to 1000 nm. i When the average pore diameter D is 100 nm or more, the permeability is good, and when it is 1000 nm or less, the strength of the separation membrane is good. i The average pore diameter D is preferably 120 to 800 nm, more preferably 150 to 600 nm, further preferably 170 to 400 nm, and particularly preferably 200 to 300 nm. i The measurement method will be described in detail in the Examples.

[0050] In addition, the separation membrane of the present invention has the above-mentioned average pore diameter D i However, it is preferable that the following requirement (a) or (b) is satisfied.

[0051] (a) D1 ≥ D2 ≥ D3 ≥ D4 ≥ D5 (b) D5 ≥ D4 ≥ D3 ≥ D2 ≥ D1 Average pore diameter D i However, by gradually increasing or decreasing the size of the pores toward one of the surfaces, i.e., the outer or inner surface, uniform separation is possible, which suppresses localized liquid leakage due to large pores and achieves both high permeability and low leakage.

[0052] (voids larger than 10 μm in the support layer) In the separation membrane of this embodiment, the support layer preferably has three or fewer voids larger than 10 μm. The support layer refers to the layer excluding the dense layer when the hollow fiber membrane has a dense layer, and has multiple voids. The voids refer to depressions with a diameter of more than 10 nm when the support layer in the radial cross section or longitudinal cross section of the hollow fiber membrane is observed at 2,000x magnification using an SEM. Furthermore, the multiple voids refer to 10 or more voids per field of view when the support layer in the radial cross section or longitudinal cross section is observed at 2,000x magnification using an SEM. Having three or fewer voids larger than 10 μm in the support layer improves the strength of the hollow fiber membrane. The voids of 10 μm or more in the support layer are determined by applying stress to a hollow fiber membrane sufficiently cooled in liquid nitrogen (using a razor, microtome, or broad ion beam, if necessary), observing the radial or longitudinal cross section using an SEM, and binarizing the resulting image using the image analysis software "ImageJ," after which only pores with an average diameter of 10 μm or more are extracted. The number of voids of 10 μm or more in the support layer is preferably two or less, even more preferably one or less, and particularly preferably zero. The method for measuring the number of voids of 10 μm or more in the support layer is described in detail in the Examples.

[0053] (Average porosity of cross section) The average cross-sectional porosity of the separation membrane of the present invention is preferably 25 to 50%. The average cross-sectional porosity can be determined by exposing a cross-section of a hollow fiber membrane sufficiently cooled in liquid nitrogen using a microtome, observing the cross-section using an SEM, binarizing the obtained image using the image analysis software "ImageJ," and calculating the proportion of pores with a diameter greater than 10 nm relative to the field of view area. An average cross-sectional porosity of 25% or more results in good permeability, while an average cross-sectional porosity of 50% or less results in good separation membrane strength. The average cross-sectional porosity is preferably 30 to 45%, more preferably 35 to 40%. Specific methods for measuring the average cross-sectional porosity will be described in detail in the Examples.

[0054] (Separation coefficient β(CF4 / C4F8)) The separation membrane of the present invention preferably has a ratio of CF4 permeability to C4F8 permeability (separation factor β(CF4 / C4F8)) of 10 or more at 100 kPa and 37°C. The separation factor β(CF4 / C4F8) is preferably 20 or more, more preferably 30 or more, even more preferably 40 or more, and particularly preferably 50 or more. The calculation method will be explained in detail in the Examples. In the separation membrane of the present invention, by setting the ratio within the above range, it becomes easier to separate gases with a high global warming potential more efficiently. A separation factor β(CF4 / C4F8)) of 10 or more can be achieved by lowering the cooling temperature in the cooling step described below or by adjusting the irradiation amount in the radiation irradiation step. The higher the separation factor β, the better, and although there is no set upper limit, the practical upper limit is 1,000.

[0055] (gas permeability) The separation membrane of the present invention preferably has a CF4 permeability of 5 GPU or more at 100 kPa and 37°C. The CF4 permeability is more preferably 10 GPU or more, even more preferably 20 GPU or more, particularly preferably 30 GPU or more, and even more particularly preferably 40 GPU or more. The calculation method will be explained in detail in the Examples. By setting the CF4 permeability within the above range in the separation membrane of the present invention, it becomes easier to separate gases with a high global warming potential more efficiently. A CF4 permeability of 10 GPU or more can be achieved by using a washing solvent having a solubility parameter distance Ra for the thermoplastic resin (described later) of 8 or more, or by adjusting the irradiation dose in the radiation crosslinking step. The higher the CF4 permeability, the better, and there is no set upper limit. However, considering compatibility with selectivity β, a practical upper limit is 500 GPU.

[0056] (Method of manufacturing separation membrane) An example of the method for producing the separation membrane of the present invention is shown below, which includes steps (1) to (4). (1) A preparation step of melt-kneading a resin mixture containing 10% by mass or more and 50% by mass or less of a thermoplastic resin and 50% by mass or more and 90% by mass or less of a plasticizer to obtain a resin composition. (2) A molding step in which the resin composition is melted, discharged from a discharge die, and cooled in a cooling bath to obtain a resin molded product. (3) A washing step in which the plasticizer contained in the resin molded product is extracted into a solvent to obtain a resin molded product from which the plasticizer has been removed. (4) A radiation exposure step of irradiating the resin molded product from which the plasticizer has been removed with radiation.

[0057] Next, the method for producing a separation membrane of the present invention will be specifically described using an example in which the separation membrane is a hollow fiber membrane.

[0058] (Preparation process) In the preparation process for obtaining a resin composition for producing the separation membrane of the present invention, a mixture containing 10 to 50 mass% of a thermoplastic resin and 50 to 90 mass% of a plasticizer is melt-kneaded. The mixture preferably contains 15 to 50 mass% of a thermoplastic resin and 50 to 85 mass% of a plasticizer, more preferably 20 to 45 mass% of a thermoplastic resin and 55 to 80 mass% of a plasticizer, and particularly preferably 25 to 40 mass% of a thermoplastic resin and 60 to 75 mass% of a plasticizer. The melt-kneading of the mixture can be performed using a kneader, roll mill, Banbury mixer, or mixer such as a single-screw or twin-screw extruder. Among these, a twin-screw extruder is preferred from the viewpoint of uniformly dispersing the plasticizer, and a twin-screw extruder with a vent hole is more preferred from the viewpoint of removing volatile substances such as moisture and low-molecular-weight substances. In addition, from the viewpoint of increasing the mixing strength and improving the uniform dispersion of the plasticizer, it is preferable to use a twin-screw extruder equipped with a screw having a kneading disk portion.

[0059] The resin composition obtained in the preparation step may be pelletized, remelted, and used in melt film formation, or may be directly introduced into a die and used in melt film formation. When pelletizing, it is preferable to dry the pellets and use a resin composition with a moisture content of 200 ppm (by mass) or less. By keeping the moisture content at 200 ppm (by mass) or less, deterioration of the resin can be suppressed.

[0060] (molding process) In the hollow fiber membrane formation process, a hollow fiber membrane is obtained from a molten mixture of a thermoplastic resin and a plasticizer, i.e., a resin composition, by utilizing phase separation. Specifically, the resin composition obtained in the preparation process is discharged into a gas atmosphere, for example, from a discharge nozzle having a double annular nozzle with a gas flow path in the center, and introduced into a cooling bath to cause phase separation of the resin composition, thereby obtaining a resin molded product.

[0061] Specifically, the resin composition in a molten state is discharged from the outer tube of a double-tube spinning nozzle while a hollow-forming gas is discharged from the inner ring of a double-tube spinneret. The discharged resin composition is then run through a gas atmosphere and then cooled and solidified in a cooling bath to obtain a resin molded product.

[0062] Here, we will explain the cooling bath that cools the resin composition discharged from the discharge die. It is important to select the solvent for the cooling bath based on its affinity with the thermoplastic resin and the plasticizer. The solvent for the cooling bath should have a solubility parameter Ra of 5.0 to 18.0 MPa for the thermoplastic resin. 1 / 2 and the solubility parameter distance Rb for the plasticizer is 1.0 to 2.9 MPa. 1 / 2 Or 6.5 to 10.0 MPa 1 / 2 It is preferable to use a solvent in the cooling bath that is in the range of Ra and Rb. By having Ra and Rb in the above ranges, the size of the micropores in the dense layer can be controlled, and both high permeability and selective permeability can be achieved. The reason for this is that Ra is 5.0 to 18.0 MPa 1 / 2 When Rb is in the range of 1.0 to 2.9 or 6.5 to 10.0 MPa, solidification occurs before crystallization or aggregation of the thermoplastic resin. 1 / 2 It is estimated that by keeping the Ra in the range of 5.0 to 7.0 MPa, the exchange of solvent and plasticizer can be carried out at an appropriate speed, suppressing the formation of large pores that cause liquid leakage, while forming fine pores that only gas can pass through. 1 / 2 and Rb is 6.5 to 10.0 MPa. 1 / 2 It is more preferable to use a solvent in the cooling bath that is in the range of

[0063] The affinity between a thermoplastic resin and a solvent can be estimated using the three-dimensional Hansen solubility parameter, as described in the literature (Ind. Eng. Chem. Res. 2011, 50, 3798-3817.). Specifically, the smaller the solubility parameter distance (Ra) in the following equation (3), the higher the affinity of the solvent for the thermoplastic resin.

[0064]

number

[0065] where δ Ad , δ Ap and δ Ah are the dispersion, polar and hydrogen bonding terms of the thermoplastic solubility parameter, and δ Cd , δ Cp and δ Ch are the dispersion, polar and hydrogen bonding terms of the solubility parameter of the solvent.

[0066] The affinity between a plasticizer and a cooling solvent can be estimated in a similar manner. Specifically, the smaller the solubility parameter distance (Rb) in the following formula (4), the higher the affinity of the solvent for the plasticizer.

[0067]

number

[0068] where δ Bd , δ Bp and δ Bh are the dispersion term, polar term, and hydrogen bonding term of the solubility parameter of the thermoplastic resin, and δ Cd , δ Cp and δ Ch are the dispersion, polar and hydrogen bonding terms of the solubility parameter of the solvent.

[0069] When the solvent is a mixed solvent, the solubility parameter (δ Mixture ) can be calculated using the following formula (5).

[0070]

number

[0071] where φ i , δ iare the volume fraction and solubility parameter of component i, and are valid for the dispersion term, polarity term, and hydrogen bonding term, respectively. Here, "volume fraction of component i" refers to the ratio of the volume of component i before mixing to the sum of the volumes of all components before mixing. The 3D Hansen solubility parameters of solvents were used when listed in the literature (Ind. Eng. Chem. Res. 2011, 50, 3798-3817.). For solvent parameters not listed, the values ​​included in the software "Hansen Solubility Parameters in Practice" developed by Charles Hansen et al. were used. The 3D Hansen solubility parameters of solvents and polymers not listed in the software can be calculated using the Hansen sphere method using the software.

[0072] In the separation membrane production method of the present application, the solvent used in the cooling bath in the formation step is not particularly limited as long as it is within the above-mentioned ranges. However, when the thermoplastic resin contains PMP as the main component and dibutyl phthalate is used as the plasticizer, triacetin, dimethyl phthalate, and γ-butyrolactone are preferred, and among these, triacetin is more preferred because its Ra and Rb are within the above-mentioned more preferred ranges.

[0073] The cooling bath temperature is preferably 15 to 45°C. A temperature of 15°C or higher suppresses sudden temperature changes, stabilizes winding, and reduces fluctuations in the yarn shape. On the other hand, a temperature of 45°C or lower suppresses excessive crystallization and improves permeability. The temperature of the cooling bath is preferably 15 to 40°C, more preferably 15 to 25°C.

[0074] Furthermore, the resin composition discharged from the nozzle die is preferably exhausted at a wind speed of 8 to 20 m / s from at least one of its surfaces, preferably the surface on which a dense layer is to be formed, before cooling in a cooling bath. By exhausting at a wind speed of 8 to 20 m / s, the evaporated plasticizer is immediately exhausted, preventing the plasticizer from liquefying while remaining on the film surface. This allows for the formation of a dense layer with appropriate micropores upon cooling in the aforementioned cooling bath. The exhaust air volume is preferably 8 to 17 m / s, more preferably 10 to 14 m / s. The gas used in the gaseous atmosphere in the exhaust section is not particularly limited, but air or nitrogen is preferably used. The gaseous atmosphere generally has a temperature lower than the nozzle die temperature. Furthermore, the resin composition discharged from the nozzle die is preferably introduced into a cooling bath after passing through a free-running section with a distance of 30 to 80 mm from the nozzle face to the solvent surface. Passing through a free-running section of 30 to 80 mm results in a dense layer with a good thickness. The free-running section refers to the space from the nozzle surface to the solvent surface in the cooling bath. In this application, the "distance from the nozzle surface to the solvent surface" is sometimes referred to as the "free-running distance."

[0075] The free running distance is preferably 30 to 70 mm, more preferably 35 to 60 mm, and particularly preferably 40 to 50 mm.

[0076] In the molding process for producing the separation membrane of the present invention, the resin composition discharged from the discharge nozzle is wound up by a winding device. In this case, the draft ratio calculated by (winding speed by the winding device) / (discharge speed from the discharge nozzle) is preferably 3.0 to 20.0. A draft ratio of 3.0 or more stabilizes winding and reduces fluctuations in the thread shape. A draft ratio of 20.0 or less can prevent excessive stretching of the resin composition discharged from the nozzle, thereby preventing leakage due to defects in the dense layer. The draft ratio is preferably 3.0 to 10.0, more preferably 4.5 to 8.0.

[0077] (Cleaning process) The resin composition thus obtained is immersed in a solvent that does not dissolve the thermoplastic resin but is miscible with the plasticizer, thereby eluting the plasticizer, thereby increasing the porosity. In this case, using a solvent or mixed solvent that has a suitable affinity for the plasticizer allows for good solvent exchange and high cleaning efficiency. While there are no particular limitations on the solvent as long as it does not dissolve the polymer used here but is miscible with the plasticizer, it is preferable to use a solvent whose Ra is in the range of 8 to 35 and whose solubility parameter distance Rb with respect to the plasticizer is in the range of 5 to 35. In this case, using a solvent or mixed solvent that has a suitable affinity for both the polymer and the plasticizer allows for good solvent exchange without causing defects on the film surface, thereby increasing cleaning efficiency.

[0078] When the solubility parameter distance Ra of the cleaning solvent to the thermoplastic resin is 8 or more, the shape stability of the resin composition is good, and when it is 35 or less, the resin composition swells appropriately, resulting in high cleaning efficiency. The range of Ra is preferably 10 to 25, and particularly preferably 12 to 22. When the solubility parameter distance Rb of the cleaning solvent to the plasticizer is 35 or less, good solvent exchange is achieved, resulting in high cleaning efficiency. The range of Rb is preferably 10 to 25, and particularly preferably 12 to 22.

[0079] The solvent used in the washing step is not particularly limited as long as it is within the above-mentioned ranges. However, when the thermoplastic resin contains PMP as the main component and dibutyl phthalate is used as the plasticizer, methanol, ethanol, isopropyl alcohol, acetone, and acetonitrile are preferred. Among these, methanol, ethanol, and isopropyl alcohol are more preferred because their Ra and Rb are within the above-mentioned particularly preferred ranges.

[0080] This washing step allows a resin molded product from which the plasticizer has been removed to be obtained, but the "plasticizer-free resin molded product" of the present invention also includes a resin molded product from which the plasticizer has not been completely removed and from which the plasticizer still remains.

[0081] Stretching the membrane in the longitudinal direction during the washing step results in a separation membrane with good gas permeability and good gas selectivity. The stretching speed is preferably 0.01% / sec to 10% / sec. The stretching speed is a value indicating the percentage of the total length of the membrane stretched per hour. If the stretching speed exceeds 10% / sec, a large tension will be generated in the membrane, which may cause irreversible destruction. If the stretching speed is less than 0.01%, it will take a long time to stretch, which is not preferable from the viewpoint of productivity. The stretching speed is more preferably 0.05% / sec to 5.0% / sec, and even more preferably 0.1% / sec to 3.0% / sec.

[0082] The stretching ratio is preferably 1.05 times or more and less than 1.2 times. The stretching ratio is a value representing the number of times the total length of the membrane is stretched. If the stretching ratio is 1.05 times or more and less than 1.2 times, a membrane with high gas selectivity can be obtained due to strain formed in the amorphous portion of the resin. If the stretching ratio is less than 1.05 times, no strain is formed, and a membrane with high gas selectivity cannot be obtained. If the stretching ratio is 1.2 times or more, the boundary between the crystalline portion and the amorphous portion may be cleaved, forming voids. If voids are formed, and the size of the voids is sufficiently large compared to the gas molecules, gas selectivity will decrease. The stretching ratio is more preferably 1.05 times to 1.15 times, and even more preferably 1.1 times to 1.15 times.

[0083] (Heating process) After the washing step and before the radiation exposure step described below, it is preferable to include a heating step in which the film is heated to a temperature equal to or higher than the glass transition temperature of the thermoplastic resin. This heating step not only efficiently removes the solvent that adhered during the washing step and dries the film, but also relieves the internal stress remaining in the film during the molding and washing steps described above, thereby improving the dimensional stability of the film, and also makes it possible to increase the crosslinking reactivity during the radiation exposure step described below. Specifically, the heating step is preferably performed at room temperature to the glass transition temperature (Tg) of the thermoplastic resin + 120°C, and when the thermoplastic resin is mainly composed of PMP, it is preferably performed at room temperature to 150°C.

[0084] After the heating, the separation membrane is cooled, and at this time, it is preferable to relax the separation membrane by about 1 to 20%, more preferably 5 to 15%. By relaxing the separation membrane, it is possible to prevent the accumulation of internal stress in the membrane during cooling and improve the dimensional stability of the membrane, as well as to further increase the crosslinking reactivity in the radiation exposure step described below. (Radiation irradiation process) By irradiating the film mainly composed of thermoplastic resin obtained in the above-mentioned process with radiation, the polymer chains of the thermoplastic resin are crosslinked, and as a result, when dynamic viscoelasticity is measured by changing the frequency in the range of 1 to 100 Hz, the slope A of the logarithmic approximation equation obtained from the plot of frequency F and tan δ peak temperature T can be made 1.4 or more.

[0085] The intensity and amount of radiation to be irradiated are appropriately selected depending on the thickness, shape, etc. of the separation membrane to be produced. The atmosphere in which radiation is irradiated is not particularly limited, but it is more preferable to irradiate in an inert gas atmosphere, since this can suppress decomposition of the polymer film due to irradiation and promote the crosslinking reaction.

[0086] Examples of radiation that can be used include electron beams, alpha rays, beta rays, gamma rays, X-rays, and neutron rays. Among these, electron beams and gamma rays are more preferred because they are easy to handle and the crosslinking reaction is easily controlled.

[0087] Furthermore, the radiation exposure step preferably includes a heating step after radiation exposure. By including a heating step, crosslinking between polymer chains is further promoted, making it possible to obtain a membrane with a higher separation factor β. Specifically, the exposure is preferably carried out at a temperature between room temperature and the glass transition temperature (Tg) of the thermoplastic resin + 120°C, and when the thermoplastic resin is mainly composed of PMP, the exposure is preferably carried out at a temperature between room temperature and 150°C.

[0088] (module) The separation membrane of the present invention obtained as described above can be packed into a case by a conventionally known method or the like to form a module containing the separation membrane. For example, a hollow fiber membrane module comprises a plurality of hollow fiber membranes and a cylindrical case. The plurality of hollow fiber membranes are bundled and inserted into a cylindrical case, and then their ends are fixed and sealed to the case with a thermosetting resin such as polyurethane or epoxy resin. The ends of the hollow fiber membranes cured with the thermosetting resin are cut to obtain an open surface of the hollow fiber membrane, and a module is fabricated. This allows for the separation of gases with a high global warming potential. (Separation Apparatus) The separation membrane module of the present invention obtained as described above can be installed in a gas separation process by a conventionally known method or the like to form a gas separation apparatus incorporating a separation membrane module. For example, the gas separation apparatus mainly comprises a pump for sending and / or sucking gas, piping through which gas flows, and a gas separation module. These components make it possible to form a gas separation apparatus capable of separating gases with a high global warming potential.

[0089] (Gas separation method) Using the device of the present invention obtained as described above, gas separation can be performed by conventionally known methods. For example, by introducing a mixed gas into a gas separation device and supplying the gas to a gas separation membrane module using a pump and / or sucking gas from the gas separation membrane module using a pump, gases with a high global warming potential can be separated through the separation membrane module. Examples of such devices include, but are not limited to, those described in International Publication No. 2002 / 058826, JP 2000-185212 A, JP 11-345545 A, JP 2005-161187 A, and JP 4-284814 A. Note that multiple separation membrane modules may be used as needed depending on the purity of the gas supplied and the purity required after separation. [Example]

[0090] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples in any way. [Measurement and evaluation methods] The respective property values ​​in the examples were determined by the following methods.

[0091] A. Outer and inner diameters of hollow fiber membrane (μm) After freezing the hollow fiber membrane with liquid nitrogen, stress was applied (using a razor or microtome as necessary), and the exposed cross-section was observed under an optical microscope. The average values ​​of the outer and inner diameters at 10 randomly selected points were taken as the outer and inner diameters of the hollow fiber membrane, respectively.

[0092] B. Hollow fiber membrane hollow ratio (%) The hollow ratio of the hollow fiber membrane was calculated from the outer diameter and inner diameter determined in (1) above using the following formula.

[0093] Hollowness ratio (%) = 100 × [inner diameter (μm 2 )] 2 / [Outer diameter (μm 2 )] 2 C. Gas permeability (GPU) A small module with an effective length of 100 mm was fabricated using three hollow fiber membranes. Specifically, three hollow fiber membranes were bundled and inserted into a plastic pipe, which served as a cylindrical case. The gaps between the membranes and the pipe at the end of the hollow fiber membrane bundle were sealed with a thermosetting resin. The end of the sealed hollow fiber membrane was cut to obtain an open surface for the hollow fiber membrane, creating a small module for evaluation. Gas permeation flow rates were measured using this small module. Nitrogen, helium, and SF6 were used individually as the measurement gases, and the pressure change per unit time on the permeation side of carbon dioxide or nitrogen was measured using an external pressure system at a measurement temperature of 37°C in accordance with the pressure sensor method of JIS K7126-1 (2006). The pressure difference between the supply side and the permeation side was set to 100 kPa.

[0094] Next, the gas permeation flow rate Q was calculated using the following formula (6), and the calculated value was taken as the gas permeability. As described above, J is the gas flux, d is the membrane thickness, and P H and P Lare the partial pressures of the gas on the supply side and the permeation side, respectively. GPU is a general unit that indicates the gas permeation flow rate Q, and 1 GPU = 3.35 × 10 -10 mol / m 2 The pressure is s·Pa. The ratio of the gas permeation flow rate Q of each component was defined as the separation factor β. The membrane area was calculated from the outer diameter and length of the region contributing to gas permeation.

[0095]

number

[0096] D. Average pore size (nm) calculated from the NKP plot The gas permeability of N2 and He was calculated using method C above, and the ratio, the separation factor β(N2 / He), was calculated. This value was substituted into the above-mentioned formula (2) to determine the average pore size. Note that the formula used for this calculation is only suitable for separation membranes that have separation function due to the molecular sieving effect, and therefore only applies to separation membranes that do not have coarse pores on the outer surface, i.e., membranes in which no surface pores are visible through surface observation using SEM or other methods. Therefore, measurements were not possible when the separation factor β(N2 / He) was 2.7 or less, which is considered to have coarse pores on the outer surface and no molecular sieving effect.

[0097] E. Separation coefficient β (CF4 / C4F8) (nm) The gas permeability of CF4 and C4F8 was calculated using the method C above, and the ratio thereof, the separation factor β (CF4 / C4F8), was determined.

[0098] F. Dense layer thickness (μm) Following the same procedure as in Method A above, the separation membrane was frozen with liquid nitrogen and then fractured (using a razor blade, microtome, or broad ion beam, as necessary) to expose the radial or longitudinal cross section. Subsequently, platinum sputtering was performed under the following conditions to pretreat the radial or longitudinal cross section. After observation at 10,000x magnification using an SEM, the thickness of the dense layer was determined by drawing a line perpendicular to the outer surface from any point on the outer surface of the separation membrane toward the inner surface. The length of the line extending from the outer surface to the first pore exceeding 10 nm was defined as the thickness of the dense layer. The pores were extracted after binarizing the analysis image using the image analysis software "ImageJ." The binarization was performed by plotting the distribution of pixel counts. The horizontal axis represents the brightness of the analysis image, and the vertical axis represents the number of pixels at the corresponding brightness. When the number of pixels at the highest brightness is defined as A, the binarization was performed by matching the lower brightness of the two points where the number of pixels is 1 / 2A. Furthermore, the resulting binarized image was subjected to a noise removal process (equivalent to Despeckle in ImageJ) in which all pixels were replaced with the median value of the 3 × 3 pixel neighborhood around that pixel, and the resulting image was used as the analysis image. A line perpendicular to the outer surface was drawn from any point on the outer surface of the obtained image toward the inner surface, and the length up to the first pore larger than 10 nm was calculated. Measurements were performed at 10 random locations, and the average value was used as the dense layer thickness. (Sputtering) Equipment: Hitachi High-Technologies Corporation (E-1010) Deposition time: 40 seconds Current value: 20mA (SEM) Equipment: Hitachi High-Technologies Corporation (SU1510) Accelerating voltage: 5 kV Probe current: 30pA G. Porosity (%) The hollow fiber membrane was dried in a vacuum at 25°C for 8 hours, and the fiber length L (mm) and mass M (g) were measured. The density ρ1 (g / mm 3 ) was calculated from the following formula using the outer diameter (mm) and inner diameter (mm) measured in (1) above.

[0099] ρ1=M / [π×{(outer diameter / 2) 2 -(inner diameter / 2) 2}×L] The porosity ε (%) was calculated from the following formula.

[0100] ε=1-ρ1 / ρ2 where ρ2 is the density of the polymer.

[0101] H.Average porosity of cross section (%) As in Method A above, the separation membrane was frozen with liquid nitrogen and then fractured (using a razor blade, microtome, or broad ion beam, as needed) to expose the radial or longitudinal cross section. Subsequently, the radial or longitudinal cross section was pretreated by platinum sputtering under the following conditions. After observation at 10,000x magnification using an SEM, the percentage of pores with a major diameter greater than 10 nm was calculated. Pores were extracted using the image analysis software "ImageJ" after binarizing the analysis image (Huang's binarization). Furthermore, the binarized image was subjected to a single noise removal process (equivalent to Despeckle in ImageJ) in which all pixels were replaced with the median value of a 3x3 pixel neighborhood. The pores were extracted using the Analyze Particles command in ImageJ, and the ratio of the total area of ​​the resulting pores to the area of ​​the field of view was calculated as the pore openness. The measurement was carried out at any five points, and the average value was taken as the average cross-sectional area ratio.

[0102] I. Average pore size D i (nm), pore size variation coefficient As in Method A above, the separation membrane was frozen with liquid nitrogen and then fractured (using a razor blade, microtome, or broad ion beam, as needed) to expose the radial or longitudinal cross section. Subsequently, platinum sputtering was performed under the following conditions to pretreat the radial or longitudinal cross section. After observation at 10,000x magnification using an SEM, pores with a major diameter greater than 10 nm were extracted from the field of view. The pores were extracted using the image analysis software "ImageJ" after binarizing the analysis image (Huang's binarization). The binarized image was then subjected to a single noise removal process (equivalent to Despeckle in ImageJ) in which every pixel was replaced with the median value of a 3x3 pixel neighborhood. The resulting image was used as the analysis image. Pores were extracted using the Analyze Particles command in ImageJ, and the average pore size was calculated from the average pore area. The measurement was carried out on five regions that were divided into five equal intervals in the thickness direction so that each surface of the separation membrane was an edge, and the average pore diameters of each region were designated D1 to D5, starting from one side face. Furthermore, the standard deviation of the pore diameters for the five regions was divided by the arithmetic mean of the pore diameters, and the result was multiplied by 100 to obtain the coefficient of variation.

[0103] J. Viscoelastic properties, slope A, tan δ peak temperature at 10 Hz, tan δ peak width at 3 / 4 position at 10 Hz The viscoelastic properties and slope A were determined by the following steps (1) to (3): (1) The dynamic viscoelasticity was measured using a Seiko Instruments DMS6100 dynamic viscoelasticity measuring device in accordance with JIS-K7244 (1999). The viscoelasticity was evaluated at frequencies of 1 Hz, 10 Hz, and 100 Hz under the following measurement conditions: tension mode, chuck distance of 10 mm, strain of 0.02 to 0.05%, and heating rate of 3°C / min. (2) From the measurement results at each frequency, the temperature T at which tan δ is maximum was determined, and the horizontal axis was plotted with frequency F, and the vertical axis was plotted with tan δ maximum temperature T. If there are multiple temperatures at which tan δ is maximum, T was taken as the temperature at which the maximum value (peak height) of tan δ was the largest. (3) From the obtained plot, the logarithmic approximation formula (T = A × log e (F)+B) and found the slope A of the equation. (4) Furthermore, the temperature at which tan δ reaches its maximum value from the measurement results at 10 Hz was defined as the tan δ peak temperature at 10 Hz. The temperatures at which the maximum value (peak height) of tan δ is 3 / 4 of the peak height were determined on the lower and higher sides of the tan δ peak temperature, and the difference between these temperatures was defined as the 3 / 4 position width of the tan δ peak at 10 Hz. [Thermoplastic resin] The following thermoplastic resins were prepared:

[0104] PMP:TPX DX845 (density: 833kg / m 3 , MFR: 9.0g / 10min) Polypropylene (PP): Commercially available (density: 900-910 kg / m 3 , MFR: 9.0g / 10min) Polyethylene (PE): Commercially available (density: 940-970 kg / m 3 , MFR: 13.0g / 10min) [Other ingredients] Plasticizer: Dibutyl phthalate Example 1 35% by mass of PMP and 65% by mass of dibutyl phthalate were fed into a twin-screw extruder and melt-kneaded at 290°C. The mixture was then introduced into a melt spinning pack set at 245°C and spun from the outer annular portion of the nozzle (double-tube type, 2.0 mm diameter) with one nozzle hole. The spun hollow fiber was introduced into a triacetin cooling bath at 30°C and wound on a winder to a draft ratio of 6.0. The idle running distance was set to 20 mm. A metal filter with a diameter of 200 μm was used as the filter inside the melt spinning pack. The wound hollow fiber was introduced into a washing device equipped with multiple rolls and washed with isopropanol. It was then vacuum-dried at room temperature using multiple rolls to remove the isopropanol. It was then introduced into a drying and cooling device equipped with multiple rolls, dried at 30°C, and cooled to room temperature.

[0105] Next, the hollow fiber membrane was irradiated with an electron beam at an irradiation intensity of 300 kV and an irradiation dose of 100 kGy in an air atmosphere to obtain a hollow fiber membrane with an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%.

[0106] The physical properties of the obtained hollow fiber membrane are shown in Table 1. It had the ability to selectively separate CF4 / C4F8.

[0107] Example 2 A hollow fiber membrane was obtained in the same manner as in Example 1, except that the spun hollow fiber membrane was introduced into a triacetin cooling bath at 20°C. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. It had a higher CF4 / C4F8 selective separation property than Example 1.

[0108] Example 3 A hollow fiber membrane was obtained in the same manner as in Example 2, except that the PMP content was 17.5% by mass and the PP content was 17.5% by mass. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had the ability to selectively separate CF4 / C4F8.

[0109] Example 4 A hollow fiber membrane was obtained in the same manner as in Example 2, except that the PMP content was 17.5% by mass and the PE content was 17.5% by mass. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had the ability to selectively separate CF4 / C4F8.

[0110] Example 5 A hollow fiber membrane was obtained in the same manner as in Example 2, except that the electron beam irradiation dose was set to 25 kGy. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. It had the ability to selectively separate CF4 / C4F8.

[0111] Example 6 A hollow fiber membrane was obtained in the same manner as in Example 2, except that the atmosphere during electron beam irradiation was a nitrogen atmosphere. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. It had a higher CF4 / C4F8 selective separation property than Examples 1 to 5.

[0112] Example 7 A hollow fiber membrane was obtained in the same manner as in Example 2, except that during washing with isopropanol, the membrane was stretched to a draw ratio of 1.16 at a draw rate of 0.2% / sec while the washing step was being carried out. The resulting hollow fiber membrane had an outer diameter of 360 μm, an inner diameter of 190 μm, a hollowness of 27%, and a porosity of 50%. The physical properties of the resulting hollow fiber membrane are shown in Table 1. The membrane had the ability to selectively separate CF4 / C4F8.

[0113] Example 8 A hollow fiber membrane was obtained in the same manner as in Example 2, except that during washing with isopropanol, the membrane was stretched to a stretch ratio of 1.05 at a stretching speed of 0.2% / sec while the washing step was being carried out. The obtained hollow fiber membrane had an outer diameter of 370 μm, an inner diameter of 193 μm, a hollowness of 27%, and a porosity of 50%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had the ability to selectively separate CF4 / C4F8.

[0114] (Comparative Example 1) A hollow fiber membrane was obtained in the same manner as in Example 1, except that the electron beam irradiation step was not performed. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had lower selective separation ability between CF4 and C4F8 compared to the examples.

[0115] (Comparative Example 2) A hollow fiber membrane was obtained in the same manner as in Example 3, except that the electron beam irradiation step was not performed. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had lower selective separation ability between CF4 and C4F8 than the examples.

[0116] (Comparative Example 3) A hollow fiber membrane was obtained in the same manner as in Example 4, except that the electron beam irradiation step was not performed. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. The membrane had lower selective separation ability between CF4 and C4F8 than the examples.

[0117] Comparative Example 4 A hollow fiber membrane was obtained in the same manner as in Example 1, except that the drying temperature was 70°C and the electron beam irradiation step was not performed. The obtained hollow fiber membrane had an outer diameter of 380 μm, an inner diameter of 200 μm, a hollowness of 28%, and a porosity of 55%. The physical properties of the obtained hollow fiber membrane are shown in Table 1. Compared with the Examples, the membrane had lower CF4 gas permeability and lower selective separation ability between CF4 and C4F8.

[0118] [Table 1]

[0119] [Table 2] [Industrial Applicability]

[0120] The separation membrane, separation membrane module, separation device, and gas separation method of the present invention can be suitably used for separating and recovering gases with high global warming potential. For example, it is possible to selectively separate and recover CF4 / C4F8 from fluorine-based mixed gases generated in semiconductor manufacturing, metal refining, electrical appliance manufacturing processes, and gas recovery processes from discarded electrical appliances.

Claims

1. A separation membrane containing a thermoplastic resin as a main component, wherein when dynamic viscoelasticity measurement is performed while changing the frequency in the range of 1 to 100 Hz, the slope A of a logarithmic approximation equation obtained from a plot of frequency F and tan δ peak temperature T is 1.4 or more.

2. 2. The separation membrane according to claim 1, wherein the width at 3 / 4 of the tan δ peak height measured at a frequency of 10 Hz in the dynamic viscoelasticity measurement is 19° C. or more.

3. 3. The separation membrane according to claim 1, wherein the main component of the thermoplastic resin is poly(4-methyl-1-pentene).

4. The separation membrane according to claim 3, having an average pore diameter calculated from an NKP plot of more than 0.60 nm.

5. The separation membrane according to claim 4, wherein the separation membrane is divided into five equal regions at equal intervals in the thickness direction from one surface thereof, and the regions are designated as regions 1 to 5, and the coefficient of variation αi of the average pore size in each of regions 1 to 5 is 150% or less.

6. The separation membrane according to claim 5, wherein the separation membrane is divided into five regions at equal intervals in the thickness direction from one surface thereof, and the regions are designated as regions 1 to 5. The average pore diameter Di (where i is an integer from 1 to 5) in each of regions 1 to 5 satisfies the following requirement (a) or (b): (a) D1≧D2≧D3≧D4≧D5 (b) D5≧D4≧D3≧D2≧D1

7. The separation membrane according to claim 4, wherein the average porosity of the cross section of the separation membrane is 25% to 50%.

8. The separation membrane according to claim 4, which has a dense layer.

9. The separation membrane according to claim 4, wherein the dense layer has a thickness in the range of 0.1 μm to 2.0 μm.

10. The separation membrane according to claim 4 , wherein the dense layer is present on at least one surface side.

11. CF of the separation membrane at 100 kPa 4 Permeability and C 4 F 8 The ratio of permeability (separation factor β (CF 4 / C 4 F 8 5. The separation membrane according to claim 4, wherein )) is 10 or more.

12. CF of the separation membrane at 100 kPa 4 The separation membrane according to claim 4, having a permeability of 5 GPU or more.

13. The separation membrane according to claim 4 , wherein the separation membrane is in the form of a hollow fiber.

14. The separation membrane according to claim 4 , wherein the surface having the dense layer is an outer surface of the hollow fiber separation membrane.

15. A separation membrane module incorporating the separation membrane according to claim 4.

16. A method for producing a separation membrane, comprising the following steps (1) to (4): (1) A preparation step of melt-kneading a resin mixture containing 10% by mass or more and 50% by mass or less of a thermoplastic resin and 50% by mass or more and 90% by mass or less of a plasticizer to obtain a resin composition. (2) A molding step in which the resin composition is melted, discharged from a discharge die, and cooled in a cooling bath to obtain a resin molded product. (3) A washing step in which the plasticizer contained in the resin molded product is extracted into the solvent using a solvent to obtain a resin molded product from which the plasticizer has been removed. (4) A radiation exposure step of irradiating the resin molded product from which the plasticizer has been removed with radiation.

17. The method for producing a separation membrane according to claim 16, wherein the resin molded product is stretched at a stretching ratio of 1.05 times or more but less than 1.2 times in the washing step.

18. the solvent has a solubility parameter distance Ra of 8 to 35 with respect to the thermoplastic resin of the resin molded article, The method for producing a separation membrane according to claim 16 or 17, wherein the solubility parameter distance Rb for the plasticizer is in the range of 5 to 35.

19. The method for producing a separation membrane according to claim 16 or 17, wherein the thermoplastic resin is mainly composed of poly(4-methyl-1-pentene).

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