Porous membrane having polyamide polymer as main component, and method for producing porous membrane

Abstract

An objective of the present invention is to provide a porous membrane that has excellent strength, water permeability, and chemical resistance, and has, as the main component thereof, a polyamide polymer that is a fluorine-free polymer. The porous membrane has a structure in which spherical bodies are connected, the porous film having a polyamide polymer as the main component thereof, and the degree of crystallinity of the polyamide polymer being 27.0% to 75.0% inclusive.

Description

Porous membrane mainly composed of polyamide-based polymer and method for producing porous membrane

[0001] The present invention relates to a porous membrane mainly composed of a polyamide-based polymer and a method for producing the same.

[0002] A porous membrane using polyvinylidene fluoride (hereinafter referred to as "PVDF"), which is a fluoropolymer, is known to be excellent in strength, water permeability and chemical resistance.

[0003] Patent Document 1 discloses a PVDF porous membrane having a network structure that is particularly excellent in chemical resistance by having high crystallinity and reduced specific surface area. Further, Patent Document 2 discloses a PVDF porous membrane having a spherical structure that is superior in strength and elongation to a network structure.

[0004] On the other hand, organic fluorine compounds are concerned about their burden on the human body and the environment, and restrictions on their use are being examined along with confirmation of their safety. Polyamide-based polymers are mentioned as one of non-fluorine polymers that have low environmental burden and high chemical resistance.

[0005] Polyamide-based polymers are excellent in heat resistance, strength, acid / alkali resistance, solvent resistance and hydrophilicity, and are widely used in fields such as home electric appliances / OA equipment and automobiles. In addition, since polyamide-based polymers use raw materials that do not contain halogens, they are also characterized by low environmental burden in the polymer production process. In recent years, active development of recycling technology has also been carried out, and further reduction of environmental burden is expected.

[0006] Patent Document 3 discloses a hollow fiber membrane having a network structure obtained by dissolving polyamide in a specific solvent (sulfolane) and forming a film by solution casting.

[0007] Japanese Patent Application Laid-open No. 2014-076446, Japanese Patent Application Laid-open No. 2008-105016, International Publication No. 2010 / 038414

[0008] The hollow fiber membrane disclosed in Patent Document 3 has the advantages of excellent separation ability and a hydrophilic membrane surface that is resistant to fouling, but it suffers from the problem of low strength. Therefore, the object of the present invention is to provide a porous membrane mainly composed of a polyamide polymer, which is a non-fluorinated polymer, that has excellent strength and water permeability, and further excellent chemical resistance.

[0009] To solve the above problems, the present invention encompasses the following configurations [1] to

[19] : [1] A porous membrane mainly composed of a polyamide polymer, wherein the degree of crystallinity of the polyamide polymer is 27.0% or more and 75.0% or less, and the porous membrane has a structure in which spherical bodies are linked together. [2] The porous membrane according to [1], wherein the porosity of the porous membrane is 60% or more and 90% or less. [3] The porous membrane according to [1] or [2], wherein the average diameter of the spherical bodies in a cross-sectional image of the porous membrane is 0.1 μm or more and 15.0 μm or less. [4] The value obtained by dividing the sum of the perimeters of the pores by the sum of the areas of the pores in a cross-sectional image of the porous membrane is 1.2 μm －１ 5.0 μm or more －１ The porous membrane according to any one of [1] to [3] above, which is as follows: [5] The porous membrane according to any one of [1] to [4] above, wherein the polyamide polymer is an aliphatic polyamide polymer. [6] The aliphatic polyamide polymer is a methylene group (CH ２ A porous membrane according to [5] above, comprising a polyamide polymer in which the number of ) divided by the number of amide groups (NHCO) is 10 or more. [7] A porous membrane according to [5] or [6] above, wherein the crystallinity of the aliphatic polyamide polymer is 34.0% or more and 40.0% or less. [8] The aliphatic polyamide polymer comprises methylene groups (CH ２[5] The porous membrane according to [5] above, comprising a polyamide polymer in which the number of ) divided by the number of amide groups (NHCO) is 5 or more and 7 or less. [9] The porous membrane according to any one of [1] to [8] above, wherein the porous membrane further comprises a polymer having polar groups, and the ratio of the polymer having polar groups to 100 parts by mass of the polyamide polymer is 3 parts by mass or more and 20 parts by mass or less.

[10] The porous membrane according to [9] above, wherein the polymer having polar groups is at least one of polyvinyl alcohol and polyvinyl acetate.

[11] When the surface of the porous membrane on the side where the spherical linkage structure exists is analyzed using total internal reflection infrared absorption spectroscopy, the result is 1600 to 1700 cm⁻¹. －１ The absorption peak intensity P1 of the peak appearing at 1000-1150 cm －１[9] or

[10] above, wherein the peak intensity ratio P1 / P2 of the absorption peak intensity P2 of the peak appearing therein is 2.0 or more and 25.0 or less.

[12] A porous membrane according to any one of [1] to

[11] above, which is a hollow fiber membrane.

[13] A method for separating fluids using a porous membrane according to any one of [1] to

[12] above.

[14] A fluid separation apparatus using a porous membrane according to any one of [1] to

[12] above.

[15] A method for producing a porous membrane having a structure in which spherical bodies are linked by a solid-liquid type thermally induced phase separation method, comprising: a cooling step of cooling a polymer solution containing a polyamide polymer and a good solvent for the polyamide polymer; and a stretching step in which the ambient temperature Te (°C) during stretching satisfies Tg-35 < Te < Tg+22 with respect to the glass transition point Tg (°C) of the polyamide polymer constituting the porous membrane.

[16] The method for producing a porous membrane according to

[15] , wherein the cooling step comprises immersing the polymer solution in a cooling solution containing 60% by mass or more and less than 100% by mass of a good solvent for the polyamide polymer, and the crystallization start temperature Tc (°C) of the polymer solution and the temperature Tb (°C) of the cooling solution satisfy -65 < Tb - Tc ≤ 60.

[17] The method for producing a porous membrane according to

[15] or

[16] , wherein before the cooling step, the polymer solution is maintained at a maintenance temperature Ta (°C), and the maintenance temperature Ta and the crystallization start temperature Tc (°C) of the polymer solution satisfy 0 < Ta - Tc ≤ 40.

[18] The method for producing a porous membrane according to any one of

[15] to

[17] , wherein the time for maintaining the polymer solution at the maintenance temperature Ta (°C) is 5 seconds or more and 100 seconds or less.

[19] The HSP distance between the polyamide polymer and the good solvent is 7 MPa １／２ The method for producing a porous membrane according to any of the above

[16] to

[18] , which is as follows:

[0010] According to the present invention, a porous membrane mainly composed of polyamide, a non-fluorinated polymer, can be obtained that exhibits excellent strength, water permeability, and chemical resistance such as alkali resistance and chlorine resistance.

[0011] This is a schematic perspective view of a structure in which spherical bodies are connected according to one embodiment of the present invention. (2-a) is a schematic diagram showing the pressure loss at the pore inlet when the wall end is a corner, and (2-b), (2-c), and (2-d) are schematic diagrams showing the pressure loss at the pore inlet when the wall end is circular. This is an electron microscope image of the longitudinal cross-section of the porous membrane obtained in Example 1. This is an electron microscope image of the cross-section of the porous membrane obtained in Example 7 observed at 1,000x magnification.

[0012] The following describes in detail the morphology of the porous membrane as an embodiment of the present invention, but the present invention is not limited in any way by these embodiments. A "porous membrane" means a structure having a large number of pores. Examples of porous membrane morphologies include flat membranes and hollow fiber membranes, which are used in membrane separation processes.

[0013] In this specification, "polymer" is synonymous with "resin."

[0014] <Polyamide Polymer> The porous membrane of the present invention mainly consists of a polyamide polymer, the crystallinity of the polyamide polymer is 27.0% or more and 75.0% or less, and has a structure in which spherical bodies are linked together.

[0015] "Polyamide polymer" refers to a polymer containing a polyamide (hereinafter referred to as "PA") having an amide group (NHCO) in its main chain. The PA polymer may be a copolymer with a polymer other than PA, as long as it does not hinder the effects of the present invention, and may also contain functional groups other than amide groups. Furthermore, the PA polymer used to form the porous film may be one type or a mixture of two or more PA polymers.

[0016] The number-average molecular weight of the PA-based polymer is preferably between 5,000 and 500,000. Within this range, the viscosity of the polymer solution is more sufficiently high, making it easier to produce porous films with sufficient strength while maintaining moldability.

[0017] "Main component" refers to a component that accounts for 50% or more by mass of the constituent components. For example, in a porous membrane, the component that accounts for 50% or more by mass is the main component of the porous membrane.

[0018] From the viewpoint of further improving the strength and chemical resistance of porous membranes and optimizing membrane performance, polymer blends of PA-based polymers and various polymers known to be compatible with PA-based polymers may be used as raw materials for porous membranes. When using polymer blends, it is preferable that the PA-based polymer content is 50% by mass or more of the polymer blend. Alternatively, the PA-based polymer may be configured to include fillers.

[0019] The PA-based polymer content constituting the porous membrane according to this embodiment is 50% by mass or more, preferably 60% by mass or more, and more preferably 80% by mass or more. When the PA-based polymer content is within the above range, the characteristics of the PA-based polymer, such as high strength and high chemical resistance, are maintained, making it suitable as a porous membrane.

[0020] The fact that PA-based polymers are the main component can be identified using common analytical methods, such as nuclear magnetic resonance (NEM) measurements and infrared spectroscopy.

[0021] The PA-based polymer constituting the porous membrane according to this embodiment is preferably an aliphatic polyamide polymer (hereinafter also referred to as "aliphatic PA-based polymer").

[0022] An "aliphatic polyamide polymer" refers to a polyamide polymer that contains an aliphatic backbone in its main chain. The spherical bodies described later are formed when a portion of the PA polymer chain crystallizes while forming a regular folded structure. When the polymer chain has a folded structure, the number of crystal interfaces decreases, so the polymer chain attempts to form spherical bodies during the crystallization process. At this time, if the polymer chain is easily movable, it is easier to approach the folded structure and form spherical bodies. Therefore, it is preferable for the PA polymer to be an aliphatic PA polymer because it is less susceptible to steric hindrance, making the polymer chain more mobile and thus easier to form spherical bodies.

[0023] An "aliphatic skeleton" refers to a structure in which carbon atoms are linked together in a chain. In particular, from the viewpoint of reducing steric hindrance, it is more preferable for the aliphatic skeleton to be a saturated aliphatic hydrocarbon. It is even more preferable for the saturated aliphatic hydrocarbon to have a methylene group without substituents as its main skeleton. Examples of such aliphatic polyamide polymers include polyamide 6 (hereinafter referred to as "PA6"), polyamide 66 (hereinafter referred to as "PA66"), polyamide 610 (hereinafter referred to as "PA610"), polyamide 12 (hereinafter referred to as "PA12"), polyamide 6 / 66 copolymer (hereinafter referred to as "PA6 / 66"), and polyamide 6 / 12 copolymer (hereinafter referred to as "PA6 / 12").

[0024] Aliphatic PA polymers contain methylene groups (CH) in the polymer. ２ It is preferable that the value obtained by dividing the number of methylene groups by the number of amide groups (NHCO) is between 5 and 12. In this range, excessive hydrogen bonding by amide groups is suppressed, resulting in high solubility in organic solvents and easy processing into porous membranes. Furthermore, it is easy to obtain porous membranes with excellent chemical resistance. In particular, when the value obtained by dividing the number of methylene groups by the number of amide groups is between 10 and 12, the strength of hydrogen bonding becomes appropriate, and spherical bodies are easily formed. On the other hand, when the value obtained by dividing the number of methylene groups by the number of amide groups is between 5 and 7, the porous membrane becomes hydrophilic and easily exhibits excellent water permeability. In addition, since hydrophobic dirt is less likely to adhere to such porous membranes, the lifespan of the membrane is easily extended. Furthermore, compatibility with polymers having polar groups such as hydroxyl groups, carboxyl groups, and amide groups is increased, making it easy to obtain blended porous membranes.

[0025] The number of methylene groups and amide groups can be calculated by analyzing aliphatic PA polymers using pyrolysis GC-MS. It is also possible to combine this analysis with differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NEM). If the aliphatic PA polymer is a copolymer of two or more types, or a blend of two or more types, the number of methylene groups divided by the number of amide groups is calculated for each monomer unit constituting the aliphatic PA polymer. This value is then multiplied by the mole fraction of each monomer unit calculated using the above method, and the sum of these values ​​is taken as the number of methylene groups divided by the number of amide groups.

[0026] The melting point of the PA-based polymer used in the porous membrane according to this embodiment is preferably 175°C to 225°C, and more preferably 175°C to 200°C. A melting point of 225°C or lower is preferable because it increases the variety of organic solvents that can dissolve the PA-based polymer and polymers that are compatible with the PA-based polymer. On the other hand, a melting point of 175°C or higher makes it easier to obtain a porous membrane with excellent thermal stability, strength, and chemical resistance.

[0027] <Crystallization> The crystallinity of the PA-based polymer, which is the main component of the porous membrane of the present invention, is 27.0% or more and 75.0% or less. A crystallinity of 27.0% or more of the PA-based polymer improves the strength of the porous membrane and also improves its chemical resistance. This is thought to be because chemicals have difficulty penetrating into the regularly arranged crystalline molecules. Furthermore, a crystallinity of 75.0% or less provides a porous membrane with excellent toughness, making it less prone to breakage due to elongation. The crystallinity of the PA-based polymer is preferably 27.5% or more, more preferably 28.0% or more, and even more preferably 34.0% or more. The crystallinity of the PA-based polymer is preferably 50.0% or less, and more preferably 40.0% or less. When the crystallinity is 50.0% or less, there is a large amount of amorphous portion in the porous membrane, which allows for a large degree of elongation at break during chemical degradation.

[0028] The degree of crystallinity of PA-based polymers can be controlled, for example, by blending specific polymers with PA-based polymers or by stretching porous membranes.

[0029] In the stretching process, it is common practice to increase the crystallinity of the polymer by raising the temperature. The inventors have found that the glass transition temperature (hereinafter, "Tg") is deeply involved in the suitable temperature range for increasing the crystallinity during the stretching process in the manufacturing process of porous membranes. It is preferable that the ambient temperature Te (°C) during stretching satisfies Tg-35 < Te < Tg+22 with respect to the glass transition temperature Tg (°C) of the PA-based polymer constituting the porous membrane. Further details are described in "Method for Manufacturing Porous Membrane" below.

[0030] The crystallinity of PA-based polymers is measured using a differential scanning calorimetry (DSC) analyzer. Specifically, it is calculated using the method described in "(7) Crystallinity" of the examples described later.

[0031] <Spherical Linked Structure> The porous membrane of the present invention has a structure in which spherical bodies are linked together (hereinafter also referred to as the "spherical linked structure"). The "spherical linked structure" means a structure in which spherical bodies are linked in three dimensions. The spherical bodies may be substantially spherical or substantially ellipsoidal. The linked state is not particularly limited.

[0032] A porous membrane is formed when numerous such spherical structures, which are three-dimensionally linked, come together, and voids are formed between the solid spherical structures. The voids formed by the spherical structures are supported by the linked spheres and are therefore less prone to shrinkage. As a result, spherical structures tend to form larger voids compared to mesh structures. Consequently, porous membranes with spherical structures can maintain higher water permeability than porous membranes with mesh structures. The presence of spherical structures in a porous membrane reduces the specific surface area in contact with chemicals, resulting in excellent chemical resistance.

[0033] Figure 1 shows a schematic diagram of a part of a spherical linkage structure. In the spherical linkage structure 1 of Figure 1, multiple spherical bodies 10 are linked together. The spherical bodies 10 are approximately spherical or approximately ellipsoidal in shape. As shown in Figure 1, since the spherical bodies 10 are linked together, it is not possible to observe the entire spherical or ellipsoidal surface. However, the spherical shape of each spherical body can be extrapolated from the shape that is visible in the outer shape of each individual spherical body.

[0034] As for the state in which the spherical bodies are connected, the spherical bodies may be directly connected to each other, or the spherical bodies may be connected to each other via a non-spherical portion, for example, a constriction 11 of the spherical body.

[0035] The voids 12 between the spherical body connected structures are the voids between the aforementioned spherical body connected structures, that is, the pores of the porous membrane. In FIG. 1, the periphery of the voids 12 between the spherical body connected structures is not completely closed, but the pores of the porous membrane only need to be spaces surrounded by the spherical body connected structures.

[0036] In the cross-sectional image of the porous membrane according to the present embodiment, the average diameter of the spherical bodies is preferably 0.1 μm or more and 15 μm or less, more preferably 0.5 μm or more and 10 μm or less, and still more preferably 0.8 μm or more and 5 μm or less. When the average diameter of the spherical bodies in the cross-sectional image of the porous membrane is 0.1 μm or more, high water permeability is easily obtained; the larger the average diameter of the spherical bodies, the larger the spherical body connected structure becomes, and the larger the voids 12 between the spherical body connected structures, that is, the pores of the porous membrane become. In addition, when the average diameter of the spherical bodies in the cross-sectional image of the porous membrane is 15 μm or less, high fractionation performance as a porous membrane is easily obtained. Furthermore, when the average diameter of the spherical bodies is within the above range, appropriate flexibility can be ensured, and a structure with high strength is easily obtained. The average diameter of the spherical bodies constituting the spherical body connected structure is calculated by the method described in "(3) Average diameter of spherical bodies" in the examples described later.

[0037] In the porous membrane according to the present embodiment, the density of the spherical bodies is 10 ３ pieces / mm ２ or more and 10 ８ pieces / mm ２ or less is preferable, and 10 ３ pieces / mm ２ or more and less than 10 ４ pieces / mm ２ is more preferable. When the density of the spherical bodies in the porous membrane is 10 ３ pieces / mm ２ or more, high strength and pressure resistance, as well as appropriate flexibility can be ensured, so a structure with high strength is easily obtained. When the density of the spherical bodies is 10 ８ pieces / mm ２ or less, the water permeability is improved.

[0038] The average diameter and density of the spherical bodies can be controlled, for example, by the polymer concentration, temperature, and pressure in the polymer solution before forming the porous membrane, as well as the temperature gradient during forming (solidification). The density of the spherical bodies can be determined by taking electron microscope images, similar to the diameter measurement, and extrapolating the spherical shape of each individual spherical body from the shape revealed in the outer shape of each spherical body, using the image per unit area (1 mm²). ２ The number of spherical bodies per unit area is measured and calculated.

[0039] In the cross-sectional image of the porous membrane according to this embodiment, the value obtained by dividing the sum of the perimeter lengths of the pores by the sum of the pore areas is 1.2 μm. －１ 5.0 μm or more －１ The following is preferred: 1.2 μm －１ 2.0 μm or more －１ The following is more preferable: 1.2 μm －１ The above 1.8 μm －１ The following is even more preferable. The degree of connectivity of the spherical linkage structures forming the porous membrane can be expressed as the value obtained by dividing the sum of the perimeters of the pores (voids surrounded by the spherical linkage structures) by the sum of the pore areas in an electron microscope image of a cross section perpendicular to the surface of the porous membrane. A small perimeter relative to the area of ​​a pore means that there are few irregularities at the interface between the pore and the spherical linkage structure, and that the pores and spherical linkage structures exist together. In other words, it means that the connectivity of the spherical linkage structures is high. When the surface of the spherical linkage structures is smooth and the spherical linkage structures exist together, pore contraction is less likely to occur, and high water permeability and high strength are easily achieved.

[0040] The value obtained by dividing the sum of the perimeters of the pores in the cross-sectional image of the porous membrane by the sum of the pore areas is calculated by the method described in "(5) Perimeter and Area of ​​Pores" in the Examples described later.

[0041] One method for incorporating PA-based polymers into the connecting parts and promoting their growth is to gradually advance crystal growth during the cooling of the polymer solution in the porous membrane manufacturing method described later. Specifically, it is preferable that the crystallization start temperature Tc (°C) of the polymer solution and the temperature Tb (°C) of the cooling solution satisfy -65 < Tb - Tc ≤ 60. This causes the PA-based polymer to preferentially undergo crystallization to form spherical bodies rather than solidification, making it easier to incorporate the PA-based polymer into the connecting parts of the spherical bodies.

[0042] Furthermore, the method for manufacturing a porous membrane further includes a step of maintaining the polymer solution at a maintenance temperature Ta (°C) before the cooling step (a step of immersing the polymer solution in a cooling solution) described later, and it is also preferable that the maintenance temperature Ta and the crystallization start temperature Tc of the polymer solution satisfy 0 < Ta - Tc ≤ 40. Maintaining the polymer solution at a temperature within the above range makes it easier to induce the formation of spherical bodies before cooling and solidification, and since the time for forming spherical bodies is long, it is easy to form connected spherical bodies.

[0043] <Pressure Loss Reduction Effect of Spherical Linked Structure> The porous membrane of the present invention is characterized by having a spherical linked structure mainly composed of PA-based polymer. Here, considering the case where liquid flows from the surface to the interior of the separation membrane during liquid filtration of a porous membrane, the flow path narrows rapidly at the pore portion of the separation membrane surface, which is the inlet, resulting in a pressure loss at the inlet and obstructing the flow. In other words, the spherical shape of each spherical body constituting the spherical linked structure and the shape of the pores between the spherical linked structures determine the shape of the inlet portion of the pores. The pressure loss at this inlet portion is expressed by the following equation (1) and is known to depend on the loss coefficient (ζ) of the inlet portion. ΔP = ζ × (V ２ ) ２ / 2g ...Equation (1) Here, ΔP: pressure loss at the inlet, ζ: loss coefficient at the inlet, V ２ : is the flow velocity within the pore, and g is the acceleration due to gravity.

[0044] As shown in Figure 2, the loss coefficient (ζ) is a value determined by the shape of the flow channel. Figures (2-a) to (2-d) in Figure 2 show the pore inlet 20 and the porous membrane 21. (2-a) shows the primary flow velocity V １and the flow velocity V inside the pore ２ This also shows that (2-b) through (2-d) similarly represent the primary flow velocity V. １ and the flow velocity V inside the pore ２ Although this can occur, it is omitted from the description. As shown in (2-a), when the wall surface of the inlet is at a corner (90°), ζ = 0.5, but when there is a wall surface end 22 with a small radius R as in (2-b), ζ = 0.25, when there is a wall surface end 23 with a large radius R as in (2-c), ζ = 0.1 to 0.2, and when there is a wall surface end 24 with a very large radius R as in (2-d), ζ = 0.01 to 0.05. Thus, the loss coefficient decreases as the wall surface end is circular and its radius R increases (from 2-b to 2-d).

[0045] In other words, the porous membrane according to this embodiment has a spherical-linked structure in which spherical bodies are connected, and since the ends of the wall surface are spherical, there is little pressure loss at the inlet and the structure has little obstruction to the flow of liquid. A porous membrane having such spherical bodies and spherical-linked structure allows liquid to flow easily into the interior of the porous membrane and can exhibit high water permeability.

[0046] Furthermore, the greater the curvature of each spherical body, that is, the larger the average diameter of the spherical bodies, the larger the pores (gaps) between the spherical body interconnected structures become, and the higher the water permeability of the porous membrane. On the other hand, the fractionation performance of the porous membrane decreases. Also, if the average diameter of the spherical bodies is small, the pores (gaps) between the spherical body interconnected structures become smaller, and the water permeability of the porous membrane decreases.

[0047] Therefore, from the viewpoint of obtaining high water permeability and appropriate fractionation performance, the pore diameter of the porous membrane is preferably 0.1 μm or more and 15 μm or less, more preferably 0.3 μm or more and 10 μm or less, and even more preferably 0.5 μm or more and 5 μm or less.

[0048] The pore diameter of a porous membrane can be calculated by using a scanning electron microscope to take images of the porous membrane's surface at, for example, 1,000x or 10,000x magnification. The diameters of 50 randomly selected pores are measured, and the arithmetic mean is calculated. If the number of pores in a single image is less than 50, the calculation is performed using multiple images to arrive at 50 pores.

[0049] <Polymer Blend> The inventors have found that forming a porous film in the state of a polymer blend, which is a blend of a PA-based polymer with a specific polymer, improves the crystallinity of the PA-based polymer and exhibits excellent chemical resistance. The mechanism by which the polymer blend improves the crystallinity is not yet clear, but it is presumed that the interaction between the blended polymer and the PA-based polymer makes it easier for the molecular chains of the PA-based polymer to be arranged regularly.

[0050] As the polymer to be blended with the PA-based polymer (hereinafter referred to as the blended polymer), a polymer having polar groups such as hydroxyl groups, carboxyl groups, and amide groups is preferred from the viewpoint of appropriate interaction with the PA-based polymer. Examples of polymers having polar groups include polyvinyl alcohol (hereinafter referred to as "PVA"), polyvinyl acetate (hereinafter referred to as "PVAc"), ethylene-vinyl acetate copolymer, and polyvinylpyrrolidone (hereinafter referred to as "PVP"). It is preferable to include one or more of these. In particular, it is especially preferable to blend at least one of PVA and PVAc from the viewpoint that it is within a range that is particularly suitable for appropriate interaction with the PA-based polymer and is less likely to dissolve in water during the film formation and filtration processes.

[0051] The degree of saponification of the PVA used as a blend polymer is preferably 90% or higher, more preferably 95% or higher, and even more preferably 99% or higher. When the degree of saponification of the PVA is within the above range, the PVA is less likely to dissolve in water during the film formation and filtration processes, and the strength is also improved.

[0052] The number-average molecular weight of the PVA used as the blend polymer is preferably 5,000 to 400,000, more preferably 10,000 to 300,000, and even more preferably 40,000 to 250,000. When the number-average molecular weight of the PVA is 5,000 or higher, it is easier to obtain a porous film with high strength. Furthermore, when the number-average molecular weight of the PVA is 400,000 or lower, the solubility in organic solvents increases, making it easier to process into a porous film.

[0053] When the porous membrane according to this embodiment includes a polymer having polar groups as a blended polymer, the ratio of the polymer having polar groups to 100 parts by mass of the PA-based polymer is preferably 3 parts by mass or more and 20 parts by mass or less, and more preferably 5 parts by mass or more and 18 parts by mass or less. When the ratio of the polymer having polar groups to the PA-based polymer is 3 parts by mass or more, the degree of crystallinity of the PA-based polymer is increased by the blending of the polymer having polar groups, and the strength and chemical resistance of the porous membrane tend to improve. Furthermore, when the ratio of the polymer having polar groups to the PA-based polymer is 20 parts by mass or less, it is possible to suppress the inhibition of crystal growth of the PA-based polymer by the polymer having polar groups while maintaining appropriate solubility of the polymer blend in solvents and processability into a porous membrane, thus making it easier to achieve both high strength and chemical resistance.

[0054] The proportion of polymers containing polar groups can be determined by common analytical methods, such as nuclear magnetic resonance (NEM) measurements and infrared spectroscopy.

[0055] <Porrosion> The porous membrane of the present invention preferably has a porosity of 60% or more and 90% or less. When the porosity is 60% or more, high water permeability is easily obtained, and when it is 90% or less, deformation such as rupture, folding, and crushing of the porous membrane during use is suppressed. In general, when the porosity of a porous membrane is increased, the specific surface area of ​​the porous membrane that comes into contact with chemicals increases, so the chemical resistance of the porous membrane tends to decrease. The inventors have found that even when the porosity is 60% or more, water permeability and chemical resistance can be achieved at the same time by keeping the crystallinity of the PA-based polymer within the above range. From the above viewpoint, the porosity of the porous membrane is more preferably 63% or more and 80%, and even more preferably 65% ​​or more and 75%.

[0056] The pore diameter and porosity of the porous membrane can be controlled, similar to the average diameter and density described above, by factors such as the polymer concentration, temperature, and pressure in the polymer solution before forming the porous membrane, the temperature gradient during forming (solidification), and the magnification and temperature during stretching, as described later. The porosity of the porous membrane is calculated by the method described in "(2) Porosity" of the Examples described later.

[0057] <Fractional Particle Size> The fractional particle size of a porous membrane is measured by supplying a suspension containing standard particles of known particle size or a standard polymer of known molecular weight to the porous membrane, allowing it to permeate, and then analyzing the supplied liquid and the permeate.

[0058] Polystyrene latex particles with average particle sizes of 0.10 μm, 0.15 μm, 0.20 μm, 0.50 μm, and 1.0 μm were used as standard particles with known particle sizes. The removal rate for each particle size was calculated by measuring the particle number concentration in the feed solution and permeate. Polyethylene glycol or dextran were used as standard polymers with known molecular weights, and the molecular weight distribution in the feed solution and permeate was measured by gel permeation chromatography (GPC).

[0059] In this specification, fractionation particle size is defined as the particle size at which the removal rate is 90% or more, based on evaluation using the standard particles or standard polymer, or the particle size corresponding to the molecular weight.

[0060] The fractional particle diameter of the porous membrane according to this embodiment is preferably 0.05 μm or more and 2.0 μm or less, more preferably 0.10 μm or more and 1.5 μm or less, and even more preferably 0.15 μm or more and 1.0 μm or less. When the fractional particle diameter is 0.05 μm or more, the permeability resistance of the membrane pores is reduced, making it easier to obtain a porous membrane with high water permeability suitable for practical use. On the other hand, when the fractional particle diameter is 2.0 μm or less, the possibility of components such as turbidity leaking into the treated water can be reduced.

[0061] The fractional particle size can be controlled, similar to the average diameter and density mentioned above, by the polymer concentration, temperature and pressure in the polymer solution before forming the porous membrane, the temperature gradient during forming (solidification), and the magnification and temperature during stretching, as described later.

[0062] <Film Thickness> When the porous membrane according to this embodiment is a hollow fiber membrane, the film thickness is preferably 40 μm or more and 300 μm or less, and more preferably 40 μm or more and 250 μm or less, from the viewpoint of obtaining high water permeability and suppressing deformation such as rupture, bending, and crushing of the porous membrane during use. The film thickness of the porous membrane is measured by the method of "(11) Film Thickness" in the example described later.

[0063] <Water permeability, breaking strength, and elongation at break> The porous membrane according to this embodiment has a water permeability of 4 L / m² under a pressure of 100 kPa and at 25°C. ２ / hr or more 4,000L / m ２ Preferably, it should be less than / hr, and 40 L / m ２ / hr or more 3,000L / m ２ It is more preferable that the flow rate is less than or equal to 400 L / m ２ / hr or more 2,000L / m ２ It is even more preferable that it be less than or equal to / hr.

[0064] The porous membrane according to this embodiment preferably has virtually no macrovoids. "Macrovoid" refers to a void with a major axis of 50 μm or more observed in the cross-section of the porous membrane. "Virtually no macrovoids" means that there are 10 macrovoids / mm² in the cross-section of the porous membrane. ２ This means that the number of macrovoids present in the cross-section of the porous membrane is 5 per mm. ２ The following are preferable, and it is most preferable that they are not present at all.

[0065] The rupture strength of the porous membrane according to this embodiment is 1.0 N / mm ２ 30.0N / mm or more ２ The following is preferable: 2.0 N / mm ２ 25.0N / mm or more ２ The following is more preferable: 2.5 N / mm ２ 20.0N / mm or more ２ The following are even more preferable.

[0066] The elongation at break of the porous membrane according to this embodiment is preferably 5% to 100%, more preferably 10% to 90%, and even more preferably 15% to 80%.

[0067] When the tensile strength and tensile elongation are within the above range, sufficient water permeability can be achieved under normal usage conditions, and the rupture of the porous membrane can be suppressed.

[0068] The tensile strength and elongation at break can be controlled, like the average diameter and density of spherical bodies mainly composed of PA-based polymers, by the polymer concentration, temperature, pressure, and temperature gradient of the polymer solution before forming the porous membrane, as well as by the temperature gradient during forming (solidification). Furthermore, they can also be controlled by adjusting the crystallinity of the PA-based polymer using the stretching ratio and temperature described later. When the crystallinity of the PA-based polymer is 75.0% or less, a membrane with excellent toughness and resistance to breakage due to stretching can be obtained.

[0069] The tensile strength and elongation at break of the porous membrane are calculated by the method described in "(6) Tensile Strength and Elongation at Break" of the Examples described later.

[0070] <ATR-IR> When the surface of the porous membrane according to this embodiment on the side where the spherical linkage structure exists is analyzed using total internal reflection infrared absorbance measurement (hereinafter, "ATR-IR"), the reading is 1600 to 1700 cm². －１ The absorption peak intensity P1 of the peak appearing at 1000-1150 cm －１ The peak intensity ratio P1 / P2 of the absorption peak intensity P2 of the peak appearing is preferably 2.0 or more and 25.0 or less, more preferably 4.0 or more and 25.0 or less, and even more preferably 9.0 or more and 25.0 or less. 1600-1700 cm －１ The peaks that appear are mainly due to the C=O stretching vibration of the amide group in the PA polymer. 1000–1150 cm －１ The peaks that appear mainly originate from the stretching vibrations of the hydroxyl groups C-O-H or ether groups C-O-C of the polar group polymer blended with the PA-based polymer. Therefore, P1 / P2 is determined by the type and ratio of the PA-based polymer and the polar group polymer blended with the PA-based polymer on the surface of the porous membrane. In this invention, the surface on which the spherical linkage structure of the porous membrane exists refers to the surface on which the spherical linkage structure is more clearly observed. If the spherical linkage structure is equally clear on either surface, either surface may be measured.

[0071] When the P1 / P2 ratio is 2.0 or higher, the interactions of amide groups in the porous membrane, such as intramolecular or intermolecular hydrogen bonding, become stronger, inhibiting contact of chemical solutions containing alkalis, etc., with the amide groups, thus improving performance stability during chemical immersion. Furthermore, when the P1 / P2 ratio is 25.0 or lower, the chemical resistance is further improved by blending polymers while retaining the characteristics of PA-based polymers, which have high strength and high chemical resistance. The P1 / P2 ratio can be controlled, for example, by adjusting the blend ratio of PA-based polymers and polymers having polar groups. The ATR-IR of the porous membrane surface is measured by the method described in "(10) ATR-IR" of the examples described later.

[0072] <Chemical Resistance> In the case of a hollow fiber membrane according to this embodiment, it is preferable that the retention rate of the elongation at break of the porous membrane before and after an immersion test in which it is immersed for 4 days in an aqueous sodium hypochlorite solution with an effective chlorine concentration of 500 ppm is 60% or more, and particularly preferable that it is 90% or more. In the case of a flat membrane, it is preferable that the retention rate of the elongation at break of the porous membrane before and after an immersion test in which it is immersed for 7 days in an aqueous sodium hypochlorite solution with an effective chlorine concentration of 500 ppm is 60% or more, and particularly preferable that it is 90% or more. The retention rate of elongation at break is an indicator of chemical resistance, and the higher the retention rate of elongation at break, the smaller the structural change of the porous membrane due to chemicals, and the better the chemical resistance. Furthermore, if the retention rate of elongation at break is 60% or more, the risk of damage due to deterioration of the membrane after chemical washing is reduced, so a porous membrane with excellent chemical resistance suitable for water treatment applications can be obtained.

[0073] <Method for Manufacturing Porous Membrane> The present invention describes a method for obtaining a porous membrane having a structure in which spherical bodies mainly composed of PA-based polymer are linked together. However, the present invention is not limited in any way by these manufacturing method examples.

[0074] The inventors have discovered that a porous membrane having a structure in which spherical bodies mainly composed of PA polymer are linked together (spherical linkage structure) can be obtained by a solid-liquid type thermally induced phase separation method that includes a cooling step of cooling a polymer solution containing a PA-based polymer and a good solvent for the PA-based polymer.

[0075] Methods for producing porous membranes from PA-based polymers include, for example, thermally induced phase separation, non-solvent-induced phase separation, melt extraction, and stretching pore opening. Among these, in this embodiment, it is preferable to use a solid-liquid thermally induced phase separation method.

[0076] The "thermal-induced phase separation method" is a method in which a polymer solution is prepared by dissolving a polymer in a solvent at a high temperature, and then the polymer solution is cooled to separate the phases of the polymer solution and solidify the polymer. When using the thermal-induced phase separation method to manufacture porous membranes, a good solvent that can dissolve the PA-based polymer at 60°C or higher is preferred as the solvent for the PA-based polymer. Examples of such solvents include ethylene glycol (hereinafter referred to as "EG"), polyhydric alcohols such as glycerin, and polar solvents such as N-methylpyrrolidone (hereinafter referred to as "NMP").

[0077] Furthermore, there are mainly two types of phase separation mechanisms in thermally induced phase separation. One is a liquid-liquid type phase separation method in which a polymer solution uniformly dissolved at high temperatures separates into a concentrated phase and a dilute phase of the polymer as the solution cools due to a decrease in its solubility. The other is a solid-liquid type phase separation method in which a polymer solution uniformly dissolved at high temperatures separates into a polymer solid phase and a polymer dilute solution phase as the polymer crystallizes as it cools. In the former method, a three-dimensional network structure is mainly formed, while in the latter method, spherical bodies and spherical linkage structures are formed. The likelihood of solid-liquid phase separation varies depending on the combination of polymer and good solvent, and it is preferable to select an appropriate solvent according to the polymer. For example, if the main component of the polymer is PA6, the main component of the good solvent is preferably EG, and if the main component of the polymer is PA12, the main component of the good solvent is preferably NMP.

[0078] In the method for manufacturing a porous membrane according to this embodiment, spherical bodies and spherical body linkage structures are formed by a solid-liquid phase separation method. Compared to a three-dimensional network structure formed by a liquid-liquid phase separation method, the spherical body linkage structure formed by the solid-liquid phase separation method tends to have bulkier solid parts and larger linkages. Therefore, the voids between solid parts are less likely to shrink, making it easier to maintain high water permeability, which is preferable. For this reason, in the method for manufacturing a porous membrane according to this embodiment, it is preferable to select a polymer concentration and solvent that induce solid-liquid phase separation.

[0079] It has been known that a structure in which spherical bodies are linked (a spherical linked structure) can be formed using a solid-liquid thermally induced phase separation method with PVDF-based polymers. For example, International Publication No. 2003 / 106545 shows that spherical bodies can be formed by extruding a PVDF-based polymer from a nozzle at a temperature of 95 to 155°C and cooling it in a cooling bath at a temperature of 10 to 30°C. On the other hand, PA-based polymers, which are the main component of the porous membrane of the present invention, have a higher crystallization onset temperature and a faster crystallization rate than PVDF-based polymers. Therefore, simply applying the extrusion temperature and cooling temperature conditions used in the solid-liquid thermally induced phase separation method for PVDF-based polymers was not sufficient to form spherical bodies. As described later, the inventors focused on the crystallization onset temperature Tc of the PA-based polymer and devised methods for controlling the film formation temperature and selecting the solvent used in the polymer solution. As a result, they have created for the first time a porous membrane having a spherical linked structure with PA-based polymer as the main component.

[0080] In the method for manufacturing a porous membrane according to this embodiment, a PA-based polymer solution (hereinafter also simply referred to as "polymer solution") is discharged from a nozzle and cooled. The discharge temperature of the polymer solution is preferably 100°C to 200°C, and more preferably 110°C to 190°C. By setting the temperature of the polymer solution to 100°C or higher, polymer precipitation can be suppressed, making it easier to control the connectivity of the spherical linkage structure. On the other hand, the temperature of the polymer solution should be below the boiling point of a good solvent, but setting it to 200°C or lower is preferable because it makes the viscosity of the polymer solution easier to mold and also suppresses the evaporation of the solvent in the polymer solution.

[0081] The cooling step in the method for manufacturing a porous membrane according to this embodiment is a step of immersing the polymer solution discharged from the nozzle in a cooling solution to cool it. The cooling solution preferably contains 60% by mass or more and less than 100% by mass of a good solvent for the PA-based polymer. By containing 60% by mass or more and less than 100% by mass of a good solvent in the cooling solution, the formation of a three-dimensional network structure or skin layer due to non-solvent-induced phase separation when the polymer solution comes into contact with a poor solvent or non-solvent is suppressed, making it easier to form spherical bodies.

[0082] When a PA-based polymer solution is cooled, phase separation due to crystallization of the PA-based polymer and solidification of the PA-based polymer occur in competition. PA-based polymers are known to be crystalline polymers, and polymer solutions mainly composed of PA-based polymers have a crystallization onset temperature Tc (°C). Here, the "crystallization onset temperature Tc" is defined as follows: Using a DSC measuring device, a polymer solution containing the PA-based polymer and a solvent is sealed in a sealed DSC container, and the temperature is raised to the dissolution temperature at a heating rate of 10°C / min and held for 30 minutes to dissolve uniformly. Subsequently, the temperature at which the crystallization peak rises during the cooling process at a cooling rate of 10°C / min is defined as the crystallization onset temperature Tc.

[0083] In the cooling process, it is preferable that the crystallization start temperature Tc (°C) of the polymer solution and the temperature Tb (°C) of the cooling solution satisfy -65 < Tb - Tc ≤ 60. By satisfying this condition, the formation of spherical bodies by crystallization of the PA-based polymer can be preferentially carried out over the solidification of the PA-based polymer. It is more preferable that the crystallization start temperature Tc of the polymer solution and the temperature Tb of the cooling solution satisfy -65 < Tb - Tc ≤ 50, and even more preferable that they satisfy -65 < Tb - Tc ≤ 45.

[0084] In the cooling process, it is preferable to use a sufficient volume of cooling solution for immersing the polymer solution. When manufacturing in a batch process, it is preferable to use a cooling solution that is 100 times or more the volume of polymer solution added.

[0085] Furthermore, the specific temperature of the cooling solution is preferably between 0°C and 100°C, and more preferably between 30°C and 100°C.

[0086] The method for producing a porous membrane according to this embodiment includes a step of maintaining the polymer solution at a maintenance temperature Ta (°C) before the cooling step, and it is preferable that the maintenance temperature Ta and the crystallization start temperature Tc (°C) of the polymer solution satisfy 0 < Ta - Tc ≤ 40. Maintaining the polymer solution at a maintenance temperature Ta within the above range is preferable because it induces the formation of spherical bodies before cooling and solidification, and makes it easier to form spherical bodies. Furthermore, the time for maintaining the polymer solution within the above temperature range is preferably 5 seconds to 100 seconds, and more preferably 15 seconds to 50 seconds, from the viewpoint of easily inducing the formation of spherical bodies before cooling and solidification and making it easier to form spherical bodies. When the maintenance time is 15 seconds to 50 seconds, the progress of liquid-liquid type thermally induced phase separation is suppressed and the progress of solid-liquid type thermally induced phase separation is promoted, so even if the temperature Tb of the cooling solution does not satisfy the above preferred range (-65 < Tb - Tc ≤ 60), the PA-based polymer is more likely to form spherical bodies before cooling and solidification, which is particularly preferable. During the cooling process, solid-liquid thermally induced phase separation and liquid-liquid thermally induced phase separation occur in competition. Therefore, suppressing liquid-liquid thermally induced phase separation while maintaining the polymer solution at the maintenance temperature Ta makes it easier to form highly interconnected spherical linked structures. To suppress liquid-liquid thermally induced phase separation, it is preferable to have a small HSP distance between the PA-based polymer and the good solvent, as described later. This makes it easier to control the water permeability and strength of the porous membrane. The preferred maintenance temperature Ta depends on the crystallization start temperature Tc of the polymer solution, but a specific example of a maintenance temperature Ta is preferably 100°C to 200°C, and more preferably 110°C to 190°C.

[0087] The maintenance time of the polymer solution is the value obtained by dividing the length (m) of the pipe or other device through which the polymer solution passes at a maintenance temperature Ta ± 1 (°C) by the linear velocity (m / s) of the polymer solution inside the pipe or other device.

[0088] When maintaining the polymer solution at a temperature within the above range, applying pressure to the polymer solution is preferable as it facilitates the formation of spherical bodies. The pressure applied to the polymer solution is preferably 0.3 MPa to 30 MPa, and more preferably 0.5 MPa to 25 MPa. The time for maintaining the polymer solution at the above pressure is preferably 5 seconds to 100 seconds, and more preferably 15 seconds to 50 seconds. Specifically, by providing a retention section for the polymer solution at any point in the liquid delivery line that sends the polymer solution to the nozzle or slit die, and further providing a pressurizing means for pressurizing the retained polymer solution and a temperature adjustment means (e.g., a heating means) for adjusting the temperature of the retained polymer solution, the polymer solution can be maintained at a constant pressure and a constant temperature. The pressurizing means is not particularly limited, but by installing two or more pumps in the liquid delivery line, pressurization can be achieved at any point between the two pumps. Examples of pumps include piston pumps, plunger pumps, diaphragm pumps, wing pumps, gear pumps, rotary pumps, and screw pumps. Two or more types of pumps may be used.

[0089] The above process forms many core portions of the spherical bodies, making it easier to form a highly connective spherical body structure.

[0090] The shape of the porous membrane according to this embodiment is not particularly limited, but a flat membrane or a hollow fiber membrane is preferred. Both of these membranes can be formed by methods conventionally known to those skilled in the art. For example, a flat membrane can be manufactured by casting a polymer solution onto a substrate and immersing it in a solidification bath containing a cooling solution. In the case of a hollow fiber membrane, a hollow fiber membrane can be manufactured by discharging a polymer solution into a hollow cylindrical shape from the outer slit of a double-tubular nozzle, while simultaneously discharging an internal fluid from the inner nozzle hole and immersing the resulting mixture in a solidification bath containing a cooling solution. As the internal fluid, a fluid selected from non-solvents, poor solvents, good solvents, mixtures thereof, liquids incompatible with the polymer solution, or gases such as nitrogen or air can be used.

[0091] The concentration of the PA-based polymer in the polymer solution is preferably 5% to 60% by mass, and more preferably 10% to 50% by mass, from the viewpoint of ensuring sufficient strength while appropriately controlling the water permeability and fractionation performance of the porous membrane. Furthermore, the concentration of the polymer having polar groups in the polymer solution is preferably 2% to 20% by mass, and more preferably 4% to 10% by mass, from the viewpoint of ensuring sufficient strength while appropriately controlling the water permeability and fractionation performance of the porous membrane.

[0092] The porous membrane according to this embodiment may be stretched after solidification in the cooling process. By stretching, the degree of crystallinity of the PA-based polymer can be increased and the voids can be expanded to improve water permeability. The environment in which stretching is performed is not particularly limited, but examples include stretching in the atmosphere or stretching in a bath. When stretching in a bath, the bath liquid is preferably a non-solvent or poor solvent for the PA-based polymer. By using these solvents as the bath liquid, dissolution and reformation of the membrane structure in the bath can be prevented.

[0093] The ambient temperature Te (°C) during stretching is preferably such that Tg-35 < Te < Tg+22, more preferably Tg-35 < Te < Tg+15, even more preferably Tg-35 < Te < Tg+8, and even more preferably Tg-35 < Te < Tg+7, relative to the Tg of the PA-based polymer constituting the porous membrane. Setting Te to a temperature higher than Tg-35 increases the mobility of the molecular chains, allowing the molecular chains of the PA-based polymer to align regularly and improving the degree of crystallinity. Furthermore, setting Te to a temperature higher than Tg-35 prevents excessive stress from being applied to the porous membrane during stretching, making deformation and fracture of the porous membrane less likely. Setting Te to less than Tg+22 allows for a moderate increase in molecular chain mobility while maintaining sufficient rigidity of the porous membrane, enabling stress to be smoothly transmitted to the molecular chains during stretching, making it easier for the molecular chains to align regularly and efficiently improving the degree of crystallinity of the PA-based polymer. By improving the crystallinity of PA-based polymers, porous films with excellent chemical resistance, such as alkali resistance and chlorine resistance, can be obtained. Furthermore, when a porous film is composed of multiple PA-based polymers, it is preferable that the above formula is satisfied with respect to the Tg of the PA-based polymer with the highest mass fraction in the PA-based polymer composition constituting the porous film.

[0094] The stretching ratio is preferably 1.1 times or more and 3.0 times or less, more preferably 1.2 times or more and 2.0 times or less, and even more preferably 1.3 times or more and 1.8 times or less.

[0095] In the method for producing a porous membrane according to this embodiment, the HSP distance between the PA polymer and the good solvent is 7 MPa. １／２ The following is preferable. This improves the affinity between the PA polymer and the good solvent, allowing the dissolution temperature and maintenance temperature Ta to be controlled over a wider temperature range, making it easier to control the formation of spherical bodies and optimizing film performance such as water permeability and strength. The HSP distance between the PA polymer and the good solvent is 6 MPa. １／２ The following is more preferable: 5 MPa １／２ It is even more preferable that the following conditions be met.

[0096] Here, "HSP" refers to the Hansen solubility parameter, and the HSP distance is a parameter based on the idea that two substances with similar intermolecular interactions readily dissolve in each other. HSP consists of energy derived from intermolecular dispersion forces (δd), energy derived from intermolecular dipole interactions (δp), and energy derived from intermolecular hydrogen bonds (δh). These three parameters can be considered as coordinates in three-dimensional space (Hansen space). The HSP values ​​(δd, δp, δh) of each substance can be calculated, for example, using the computer software Hansen Solubility Parameters in Practice (HSPiP). Here, the HSP value of the PA polymer used in the polymer solution is (δd １ δp １ δh １ ) and the HSP value of the good solvent is (δd ２ δp ２ δh ２ When this is the case, the distance in HSP values ​​between the PA polymer and the good solvent (hereinafter referred to as "HSP distance") can be calculated by the following formula (2). HSP distance (MPa) １／２ ) = {4 × (δd １ -δd ２ ) ２ + (δp １ -δp ２ ) ２ + (δh １ -δh ２ ) ２} １／２ ...Equation (2) The separation device equipped with the porous membrane according to this embodiment, obtained as described above, can separate various fluids. Examples of fluid separation include separation of liquids, separation of gases, separation of solids from liquids, and separation of gases from liquids. A liquid separation device may include, for example, a raw liquid tank, a booster pump, a module filled with a porous membrane, a processing liquid tank, a washing pump, etc., but is not limited to these.

[0097] A "module" refers to a device in which a porous membrane is housed in a container so that a processed liquid can be collected by separation. Examples include a device in which multiple hollow fiber-like porous membranes are filled into a cylindrical housing and fixed at one or both ends with polyurethane or epoxy polymer to allow for the collection of the processed liquid, or a device in which a flat porous membrane is attached to a support plate and its ends are sealed to allow for the collection of the processed liquid, but the term is not limited to these examples.

[0098] Methods for separating liquids include, but are not limited to, a method in which raw materials such as factory wastewater, fermentation liquid, and culture medium are separated using a separation device equipped with a porous membrane, operated at an operating pressure of 10 kPa to 1 MPa, to remove organic matter and other substances contained in the raw materials.

[0099] Furthermore, the porous membrane according to this embodiment can also be suitably used as an adsorbent with excellent durability. Examples of adsorbents include gas adsorbents, water purification adsorbents, and wastewater purification adsorbents.

[0100] The present invention will be specifically described below with reference to examples, but the present invention is not limited in any way by these examples.

[0101] (1) Permeability Performance: When the porous membrane is a flat membrane, the permeability performance was measured using the following procedure. The flat membrane was cut into a circle with a diameter of 50 mm and set in a cylindrical filter holder. Under conditions of a temperature of 25°C and a filtration differential pressure of 10 kPa, distilled water was supplied from the surface side of the porous membrane that has a spherical interconnected structure, and the amount of water permeated over a certain period of time (m³) was measured. ３ The amount of permeate obtained was measured in terms of unit time (hr) and unit effective membrane area (m²). ２ The permeability performance was calculated by converting it to a value per 100 kPa.

[0102] If the porous membrane is a hollow fiber membrane, a module with a length of 260 mm consisting of four hollow fiber membranes is prepared, and distilled water is supplied to the module under conditions of a temperature of 25°C and a filtration differential pressure of 100 kPa, and the amount of permeate (m³) over a certain period of time is measured. ３ The permeate volume was measured. The internal volume of the module was 13 mL. The obtained permeate volume was expressed as per unit time (hr) and per unit effective membrane area (m²). ２ The permeability performance was calculated by converting it to a per-unit basis.

[0103] (2) Porosity The porosity was calculated by measuring the volume using Archimedes' principle with a porous membrane test specimen. A specific gravity measurement kit (AD-165, manufactured by A&D Co., Ltd.) was used for the measurement, and the measurement was performed according to the following procedures (a) to (e). A 2 cm x 2 cm flat membrane or a 2 cm long hollow fiber membrane was used as the test specimen, water was used as the liquid to wet the specimen, and decane was used as the liquid to measure buoyancy. (a) A wet test specimen was prepared by immersing the test specimen in ethanol for 30 minutes or more, and then immersing it in water for 1 hour or more. The wet test specimen was placed on the weighing dish in air of the specific gravity measurement kit and the zero point was set. (b) Using decane of known density, the wet test specimen was placed on the weighing dish in liquid of the specific gravity measurement kit and the buoyancy A was measured. A was taken as an absolute value. (c) The volume B of the wet test specimen was obtained by dividing A by the density of decane. (d) The wet specimens were dried, and the oven-dry mass of the specimens was measured. The volume C of the specimens when oven-dry was calculated from the oven-dry mass of the specimens and the density of the polymer used in the porous membrane. (e) The porosity was calculated using the following formula: Porosity (%) = {1 - (C / B)} × 100 The above measurement was performed on 12 different specimens, and the average value was used. Two significant figures were used.

[0104] (3) Average diameter of the spherical bodies The porous membrane was cut perpendicular to the surface of the porous membrane by the freeze-fracture method, and the exposed cross-section was photographed at 3,000x magnification using a scanning electron microscope. From the captured image, 10 spherical bodies located in the region from the surface on the side where the spherical body linkage structure exists to a thickness of 20 μm were randomly selected, and the major and minor diameters of each were measured. The above photography was performed at 5 locations, and a total of 50 major diameters and 50 minor diameters were obtained. The arithmetic mean of the obtained total of 100 diameters was calculated and used as the average diameter of the spherical bodies. Two significant figures were used.

[0105] When calculating the diameter of each spherical body, as shown in Figure 1, in the case of spherical bodies connected via a constriction 11 and whose outlines could be confirmed, a straight line 13 passing through the center in the direction of the major axis and a straight line 14 passing through the center in the direction of the minor axis were drawn, and the average of the major and minor axes was taken as the diameter of the spherical body. In the case of spherical bodies directly connected to each other, a straight line 13 passing through the center in the direction of the major axis and a straight line 14 passing through the center in the direction of the minor axis were drawn so as not to include the connecting parts between the spherical bodies, and the average of the major and minor axes was taken as the diameter of the spherical body. In the case of connected spherical bodies, if the center of one spherical body overlapped with that of an adjacent spherical body (corresponding to X1 in Figure 1), that spherical body was excluded from the measurement. Also, if two spherical bodies appeared to overlap in the depth direction of the electron microscope image, the spherical body further back (corresponding to X2 in Figure 1) was excluded from the measurement. Then, the contour line of the foreground sphere was used as the boundary line between the two spheres, the background and the foreground sphere, and only the diameter of the foreground sphere was measured and calculated. Note that if a sphere was interrupted at the edge of the electron microscope image used for measurement, that portion of the sphere was excluded from the measurement.

[0106] (4) Density of the spherical bodies Similar to "(3) Average diameter of the spherical bodies" above, a cross-section perpendicular to the surface of the porous membrane was photographed at 3,000x magnification using a scanning electron microscope (SU1510, Hitachi High-Technologies Corporation). Images were randomly taken at 10 locations, including the region from the surface on the side where the spherical body linkage structure exists up to 20 μm in the thickness direction. In the obtained cross-sectional images, the spherical shape of each spherical body was extrapolated from the shape shown in the outer shape of each individual spherical body, and the density of the image per unit area (1 mm²) was calculated. ２ The number of spherical bodies per unit area was measured. The average of the densities of the 10 obtained bodies was calculated and used as the density of the spherical bodies. Two significant figures were used.

[0107] (5) Perimeter and Area of ​​Pores Similar to "(3) Average Diameter of Spherical Bodies" above, images were randomly taken at 10 locations on a cross section perpendicular to the surface of the porous membrane using a scanning electron microscope (Hitachi High-Technologies Corporation, SU1510) at 3,000x magnification, including the region from the surface to 20 μm in the thickness direction. The obtained cross-sectional images were binarized using the image analysis software ImageJ under MaxEntropy conditions to separate the polymer structure from the pores. The sum of the perimeter and sum of the area of ​​the obtained pores were calculated by image analysis. At this time, if the pores were interrupted at the edge of the cross-sectional image, they were excluded from the analysis. The perimeter and area of ​​the pores were calculated using the 10 images described above, and the average value was used.

[0108] (6) Breaking strength and elongation at break If the porous membrane was a flat membrane, the porous membrane was cut into strips of 10 cm x 1 cm to be used as a sample. If the porous membrane was a hollow fiber membrane, the porous membrane was cut to a length of 10 cm to be used as a sample. A tensile testing machine (TENSILON® / RTM-100, manufactured by Toyo Baldwin Co., Ltd.) was used to measure the strength and elongation at break at a length of 50 mm, in an atmosphere of 25°C, and at a tensile speed of 50 mm / min. Five measurements were taken with different samples, and the average value of the obtained values ​​was taken as the breaking strength and elongation at break. Two significant figures were used.

[0109] (7) Crystallinity The degree of crystallinity was measured using a DSC6220 manufactured by Seiko Instruments Inc. Approximately 5 mg of the porous membrane was cut out, sealed in a sealed DSC container, and heated under the following conditions: starting temperature of 120°C, target temperature of 270°C, heating rate of 10°C / min, and holding time at the target temperature of 1 min. The area of ​​the melting peak (heat of fusion) observed during the heating process was measured. The degree of crystallinity was defined as the value obtained by dividing the obtained heat of fusion by the total heat of fusion of the PA-based polymer constituting the porous membrane. The measurement was performed twice, and the average value was defined as the degree of crystallinity. As the total heat of fusion, if the porous membrane is composed of a single PA-based polymer, the total heat of fusion of that PA-based polymer was used. If the porous membrane is composed of multiple PA-based polymers, the total heat of fusion of the PA-based polymer with the highest mass fraction in the PA-based polymer composition constituting the porous membrane was used. Furthermore, when the PA-based polymer is a copolymer, the total heat of fusion of the homopolymer consisting of the monomer component with the highest mass fraction among the monomer components constituting the copolymer was used. The total heat of fusion of the homopolymer was based on literature values. Specifically, when the PA-based polymer is a copolymer and the main component of the copolymer is PA6, the total heat of fusion was set to 230 J / g. When the PA-based polymer is PA12, the total heat of fusion was set to 209 J / g.

[0110] (8) Immersion Test: If the porous membrane was a flat membrane, the porous membrane was cut into strips measuring 10 cm x 1 cm and immersed in a sodium hypochlorite aqueous solution with an effective chlorine concentration of 500 ppm for 7 days. The amount of aqueous solution was 50 mL per strip. The elongation at break of the porous membrane after immersion was measured using the method described in "(6) Breaking Strength and Breaking Elongation" above, and the retention rate of the breaking elongation was calculated using the following formula: Retention rate of breaking elongation (%) = 100 × (Breaking elongation after immersion test / Breaking elongation before immersion test).

[0111] If the porous membrane was a hollow fiber membrane, after measuring the water permeability performance in "(1) Water Permeability Performance" above, an aqueous sodium hypochlorite solution prepared to an effective chlorine concentration of 500 ppm was injected into the module, and the porous membrane was immersed for 4 days. After draining the aqueous solution from the module, the porous membrane was washed by filling it with pure water three times. After washing, the module was disassembled, and the elongation at break of the obtained porous membrane was measured using the method described in "(6) Breaking Strength and Breaking Elongation". The retention rate of the breaking elongation was calculated using the following formula: Retention rate of breaking elongation (%) = 100 × (Breaking elongation after immersion test / Breaking elongation before immersion test).

[0112] (9) HSP distance The HSP distance was calculated using the HSP values ​​listed in "Hansen Solubility Parameters: A User's Handbook 2nd Edition".

[0113] (10) The ATR-IR porous membrane was vacuum-dried at 23°C for 12 hours to remove moisture. Using an IRtracer 100 manufactured by Shimadzu Corporation, the surface of the porous membrane on the side where the spherical linkage structure was present was measured using a diamond prism. The measurement conditions were a resolution of 4 cm. －１ The number of scans was set to 32, and the measurement frequency was set to 600-4000 cm. －１ The settings were adjusted accordingly. The resulting spectrum was expressed in terms of absorbance, and manual multi-point baseline correction was performed. For multi-point baseline correction, 900 cm was used, as it minimizes peak interference. －１ , 1000cm －１ , 1150cm －１ , 1600cm －１ , 1700cm －１ , 2000cm －１ and 3800 cm －１ This was performed using the wavenumber domains of these seven points.

[0114] (11) The porous membrane was cut perpendicular to its surface using the film thickness freeze-fracture method, and the film thickness was determined by observing the cross-section perpendicular to the surface of the exposed porous membrane at 1,000x magnification using an optical microscope. The film thickness was measured at 10 different locations, and the average value was used.

[0115] [Example 1] 38% by mass of PA12 polymer (manufactured by Arkema, Rilsamid® AESNO MED) and 62% by mass of NMP (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were dissolved at 170°C to obtain a homogeneous polymer solution. This polymer solution was allowed to stand at 150°C, degassed, and then cooled to 135°C in a pipe and maintained at 135°C for 15 seconds. Subsequently, the polymer solution was discharged from the outer tube of a double-tubular die for hollow fiber molding at a discharge temperature of 134°C, and then 100% by mass of NMP was discharged into the hollow part from the inner tube of the double-tubular die. The polymer solution discharged from the die passed through a dry air passage 10 cm long and then solidified in a bath of 100% by mass of NMP at -3.8°C. The resulting hollow fiber molded material was washed with water at 25°C and then stretched 1.5 times in water at 25°C to obtain a porous hollow fiber membrane having a spherical linkage structure in which spherical bodies are linked together. Figure 3 shows an electron microscope image of the longitudinal cross-section of the obtained porous hollow fiber membrane.

[0116] [Example 2] A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the stretching process involved stretching the material 1.5 times in water at 40°C.

[0117] [Example 3] A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the stretching process involved stretching the material 1.5 times in water at 60°C.

[0118] [Example 4] A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the stretching process involved stretching the material 1.5 times in water at 18°C.

[0119] [Example 5] A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the stretching process involved stretching the material 1.5 times in water at 75°C.

[0120] [Example 6] A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the stretching process involved stretching the material 1.5 times in water at 95°C.

[0121] [Reference Example 1] A porous hollow fiber membrane was obtained using the same method as in Example 1, except that stretching was not performed.

[0122] Table 1 shows the evaluation results and performance of the porous hollow fiber membrane structure obtained in Examples 1 to 6 and Reference Example 1.

[0123] The porous membrane in Reference Example 1 had a PA-based polymer crystallinity of less than 34.0%, but exhibited a certain degree of chemical resistance due to its high elongation before the immersion test, low porosity, and small contact surface area. In the porous membranes of Examples 4 to 6, which were stretched, the water permeability improved with increasing porosity, but the chemical resistance decreased due to the increased contact surface area because of the low crystallinity of the PA-based polymer. On the other hand, the porous membranes of Examples 1 to 3 according to this embodiment maintain chemical resistance even with increased porosity by setting the crystallinity of the PA-based polymer to 34.0% or higher, demonstrating that high water permeability and chemical resistance are achieved simultaneously.

[0124] [Example 7] A homogeneous polymer solution was obtained by dissolving 30% by mass of PA6 / 66 polymer (Toray Industries, Inc., CM6041), 5% by mass of PVA (Hanai Chemicals, Inc., saponification degree 99-100%, polymerization degree approximately 2000), and 65% by mass of NMP (Fujifilm Wako Pure Chemical Industries, Ltd.) at 180°C. The prepared polymer solution was dispensed onto the surface of a SUS plate and quickly and uniformly coated using a bar coater (film thickness 27 mil, 686 μm) at a coating speed of 10 m / min. The SUS plate coated with the polymer solution was quickly immersed in 60°C NMP, a cooling solution, for 60 seconds to solidify and obtain a flat porous film. An electron microscope image of the cross-section of the porous film is shown in Figure 4.

[0125] [Example 8] A flat porous film was obtained in the same manner as in Example 7, except that the polymer solution contained 2.5% by mass of PVA and 67.5% by mass of NMP.

[0126] [Comparative Example 1] A flat porous film was obtained in the same manner as in Example 7, except that the polymer solution contained 35% by mass of PA6 / 66 polymer, 65% by mass of NMP, and 0% by mass of PVA.

[0127] [Comparative Example 2] A flat porous film was obtained in the same manner as in Example 7, except that the PA6 / 66 polymer was replaced with PA6 polymer (Toray Industries, Inc., CM1041).

[0128] [Comparative Example 3] A flat porous film was obtained in the same manner as in Example 7, except that the PVA in the polymer solution was 7.5% by mass and the NMP was 62.5% by mass.

[0129] [Comparative Example 4] A flat porous membrane was obtained in the same manner as in Example 7, except that PVA (manufactured by Wako Pure Chemical Industries, Ltd., degree of saponification 86-90%, degree of polymerization approximately 1000) was used as the PVA.

[0130] [Comparative Example 5] A flat porous membrane was obtained in the same manner as in Example 7, except that the composition of the polymer solution was 30% by mass of PA6 / 66 polymer (Toray Industries, Inc., CM6041), 5% by mass of PVP (Fujifilm Wako Pure Chemical Industries, Ltd., K-30), and 65% by mass of NMP.

[0131] [Comparative Example 6] An attempt was made to prepare a homogeneous polymer solution by dissolving 29% by mass of PA6 polymer (Toray Industries, Inc., CM1041), 5% by mass of PVA (Hanai Chemicals, Inc., saponification degree 99-100%, degree of polymerization approximately 2000), 33% by mass of sulfolane (Fujifilm Wako Pure Chemical Industries, Ltd.), and 33% by mass of dimethyl sulfone (Fujifilm Wako Pure Chemical Industries, Ltd.) at 180°C. However, even after stirring for more than 5 hours, undissolved PA6 polymer pellets remained, making film formation difficult.

[0132] Table 2 shows the evaluation results and performance of the porous membrane structure obtained in Examples 7-8 and Comparative Examples 1-6. A "-" in the table indicates that measurement was not possible.

[0133]

[0134]

[0135] The porous membrane according to this embodiment has excellent chemical resistance and can therefore be suitably used for water treatment applications.

[0136] 1 Spherical linkage structure 10 Spherical body 11 Constriction of the spherical body 12 Gaps between spherical linkage structures 13 Line passing through the center in the long axis direction of the spherical body 14 Line passing through the center in the short axis direction of the spherical body 20 Inlet of pores 21 Porous membrane 22 Wall end with small radius R 23 Wall end with large radius R 24 Wall end with very large radius R V1 Primary flow velocity V2 Flow velocity within pores X1 Spherical body without measuring average diameter X2 Spherical body without measuring average diameter

Claims

1. A porous membrane mainly composed of a polyamide polymer, wherein the degree of crystallinity of the polyamide polymer is 27.0% or more and 75.0% or less, and the porous membrane has a structure in which spherical bodies are linked together.

2. The porous membrane according to claim 1, wherein the porosity of the porous membrane is 60% or more and 90% or less.

3. The porous membrane according to claim 1 or 2, wherein the average diameter of the spherical bodies in the cross-sectional image of the porous membrane is 0.1 μm or more and 15.0 μm or less.

4. In the cross-sectional image of the porous membrane, the value obtained by dividing the sum of the perimeters of the pores by the sum of the areas of the pores is 1.2 μm. －１ 5.0 μm or more －１ The porous membrane according to claim 1 or 2, which is as follows:

5. The porous membrane according to claim 1 or 2, wherein the polyamide polymer is an aliphatic polyamide polymer.

6. The aliphatic polyamide polymer has a methylene group (CH ２ The porous membrane according to claim 5, comprising a polyamide polymer whose number of ) divided by the number of amide groups (NHCO) is 10 or more.

7. The porous membrane according to claim 5, wherein the degree of crystallinity of the aliphatic polyamide polymer is 34.0% or more and 40.0% or less.

8. The aliphatic polyamide polymer has a methylene group (CH ２ The porous membrane according to claim 5, comprising a polyamide polymer in which the number of ) divided by the number of amide groups (NHCO) is 5 or more and 7 or less.

9. The porous membrane according to claim 1 or 2, wherein the porous membrane further comprises a polymer having polar groups, and the ratio of the polymer having polar groups to 100 parts by mass of the polyamide polymer is 3 parts by mass or more and 20 parts by mass or less.

10. The porous membrane according to claim 9, wherein the polymer having the polar group is at least one of polyvinyl alcohol and polyvinyl acetate.

11. When the surface of the porous membrane on the side where the spherical linkage structure exists was analyzed using total internal reflection infrared absorption spectroscopy, the reading was 1600–1700 cm⁻¹. －１ The absorption peak intensity P1 of the peak appearing at 1000-1150 cm －１ The porous membrane according to claim 9, wherein the peak intensity ratio P1 / P2 of the absorption peak intensity P2 of the peak appearing therein is 2.0 or more and 25.0 or less.

12. The porous membrane according to claim 1 or 2, wherein the membrane is a hollow fiber membrane.

13. A method for separating fluids using a porous membrane according to claim 1 or 2.

14. A fluid separation device using a porous membrane according to claim 1 or 2.

15. A method for producing a porous membrane having a structure in which spherical bodies are linked by a solid-liquid type thermally induced phase separation method, comprising: a cooling step of cooling a polymer solution containing a polyamide polymer and a good solvent for the polyamide polymer; and a stretching step in which the ambient temperature Te (°C) during stretching satisfies Tg-35 < Te < Tg+22 with respect to the glass transition point Tg (°C) of the polyamide polymer constituting the porous membrane.

16. The method for producing a porous membrane according to claim 15, wherein the cooling step comprises immersing the polymer solution in a cooling solution containing 60% by mass or more and less than 100% by mass of a good solvent for the polyamide polymer, and the crystallization start temperature Tc (°C) of the polymer solution and the temperature Tb (°C) of the cooling solution satisfy -65 < Tb - Tc ≤ 60.

17. A method for producing a porous membrane according to claim 15 or 16, comprising a step of maintaining the polymer solution at a maintenance temperature Ta (°C) prior to the cooling step, wherein the maintenance temperature Ta and the crystallization start temperature Tc (°C) of the polymer solution satisfy 0 < Ta - Tc ≤ 40.

18. The method for producing a porous membrane according to claim 17, wherein the time for maintaining the polymer solution at the maintenance temperature Ta (°C) is 5 seconds or more and 100 seconds or less.

19. The HSP distance between the polyamide polymer and the good solvent is 7 MPa. １／２ The method for producing a porous membrane according to claim 15 or 16, which is as follows:

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