Porous membrane

Abstract

Provided is a porous membrane that has high filtration performance and is less likely to deteriorate in water permeability due to scratches on the membrane surface in a method for removing turbidity from natural water, domestic sewage, and suspended water that is treated water thereof by a membrane filtration method. A porous membrane characterized in that the ratio of the porosity in the thickness of 0.12% from the surface of the filtered liquid side surface to the opening ratio of the filtered liquid side surface is 1.05 or more.

Description

Technical Field

[0001] This invention relates to porous membranes. Background Technology

[0002] Water purification is the process of obtaining drinking water or industrial water from natural water sources such as rivers, lakes, and groundwater, which are considered suspended water. Wastewater treatment is the process of treating domestic sewage and other wastewater to obtain reclaimed water for other uses or clean water that can be discharged. Both treatments require solid-liquid separation (turbidity removal) to remove suspended solids. In water purification, turbidity (clay, colloids, bacteria, etc.) from natural water sources is removed. In wastewater treatment, suspended solids in wastewater, as well as suspended solids (sludge, etc.) in treated water after biological treatment (secondary treatment) using activated sludge, are removed.

[0003] Previously, these turbidity removal operations were mainly carried out through sedimentation, sand filtration, or coagulation sedimentation sand filtration. However, in recent years, membrane filtration has gradually become more widespread. The advantages of membrane filtration include, for example, the following.

[0004] (1) The turbidity removal level of the obtained water is high and stable (the obtained water is highly safe).

[0005] (2) The filter device requires little space.

[0006] (3) It is easy to operate automatically.

[0007] For example, in water treatment, membrane filtration has been used as an alternative to coagulation-sedimentation sand filtration, or as a method installed downstream of coagulation-sedimentation sand filtration to further improve the quality of treated water after coagulation-sedimentation sand filtration. Regarding wastewater treatment, the use of membrane filtration for separating sludge from secondary wastewater treatment has also been explored.

[0008] In these membrane filtration-based turbidity removal operations, hollow fiber ultrafiltration membranes or microfiltration membranes (pore sizes ranging from several nm to hundreds of nm) are mainly used. There are two main filtration methods using hollow fiber membranes: internal pressure filtration, which filters from the inner surface of the membrane to the outer surface, and external pressure filtration, which filters from the outer surface to the inner surface. Of these two methods, external pressure filtration is advantageous because it allows for a larger membrane surface area on the side in contact with the suspended raw water, thus reducing the turbidity load per unit membrane surface area. Hollow fibers and their manufacturing methods are disclosed in Patent Documents 1-3.

[0009] Turbidity removal based on membrane filtration has become increasingly popular in water purification and wastewater treatment due to its numerous advantages over conventional sedimentation and sand filtration methods, as mentioned above. However, the lack of a technology for long-term stable membrane filtration operation hinders its widespread adoption (see Non-Patent Literature 1). The main reason for the instability of membrane filtration operation is the deterioration of membrane permeability. The primary cause of this deterioration is membrane clogging (fouling) caused by turbid substances (see Non-Patent Literature 1). Additionally, the membrane surface may be rubbed by turbid substances, further reducing permeability.

[0010] On the other hand, a known method for manufacturing porous membranes is thermally induced phase separation. In this method, a thermoplastic resin and an organic liquid are used. The organic liquid is a solvent that is insoluble in the thermoplastic resin at room temperature but soluble at high temperatures, i.e., a latent solvent. The thermally induced phase separation method involves mixing the thermoplastic resin and the organic liquid at high temperature, dissolving the thermoplastic resin in the organic liquid, and then cooling to room temperature, thereby inducing phase separation and removing the organic liquid to produce a porous membrane. This method has the following advantages.

[0011] (a) Even with polymers such as polyethylene, which do not have a suitable solvent that can dissolve them at room temperature, film formation can be achieved.

[0012] (b) Since the film is formed by melting at high temperature and then cooling and solidifying, crystallization is promoted during film formation, especially when the thermoplastic resin is a crystalline resin, making it easy to obtain a high-strength film.

[0013] Based on the above advantages, it is widely used as a method for manufacturing porous membranes (see, for example, non-patent documents 2-5).

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 60-139815

[0017] Patent Document 2: Japanese Patent Application Publication No. 3-215535

[0018] Patent Document 3: Japanese Patent Application Publication No. 4-065505

[0019] Non-patent literature

[0020] Non-patent literature 1: Y. Watanabe, R. Bian, Membrane, 24(6), 1999, pp. 310-318.

[0021] Non-Patent Document 2: Editorial Committee of the Encyclopedia of Plastics and Functional Polymer Materials, Encyclopedia of Plastics and Functional Polymer Materials, Industry Survey Association, February 2004, pp. 672-679

[0022] Non-Patent Literature 3: Hideto Matsuyama, "Preparation of Polymer Porous Membranes by Thermally Induced Phase Separation (TIPS) Method", Chemical Engineering Journal, Chemical Industry Press, June 1998, pp. 45-56

[0023] Non-Patent Literature 4: Akira Takizawa, "Membranes", IPC Corporation, January 2017, pp. 404-406

[0024] Non-patent literature 5: DRLloyd et al., Journal of Membrane Science, 64, 1991, pp. 1-11 Summary of the Invention

[0025] The problem to be solved by the present invention

[0026] The object of this invention is to provide a porous membrane with high filtration performance and minimal degradation of permeability due to rubbing of the membrane surface. This porous membrane can be appropriately used, for example, in methods for removing turbidity from natural water, domestic sewage, and suspended water as treated water thereof via membrane filtration.

[0027] means for solving problems

[0028] The inventors conducted in-depth research to solve the aforementioned problems and discovered that by making the ratio of porosity to open porosity of the surface from the outermost surface of the filtered liquid side to a thickness of 0.12% of the membrane thickness 1.05 or higher, high filtration performance and high abrasion resistance are achieved. Furthermore, it was found that increasing the polymer backbone size at this location yields even better results.

[0029] Previously, it was known to suppress the degradation of water permeability caused by clogging by using membranes with high porosity in filtration (International Publication No. 2001 / 053213). In addition, it is possible to improve water permeability by increasing the overall porosity of the membrane, but it is difficult to maintain strength while diluting the concentration of the polymer constituting the membrane.

[0030] The reduction in permeability due to membrane surface rubbing is believed to occur not during filtration operation, but primarily during air washing or other methods to remove impurities deposited on the membrane surface due to external pressure filtration. However, this phenomenon is not fully understood, and few technologies have been developed to address the degradation of permeability caused by membrane surface rubbing. Japanese Patent Application Publication No. 11-138164 only discloses the use of a membrane with high tensile strength as a method to suppress changes in membrane performance caused by air bubbling washing.

[0031] In addition, International Publication No. 2015 / 104871 describes a method for suppressing rubbing by adjusting the aperture ratio, but using only the aperture ratio as a method for suppressing rubbing is not enough.

[0032] The inventors discovered that by making the ratio of porosity to open porosity of the surface from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness 1.05 or more, high filtration performance can be achieved without reducing strength, thereby enabling the manufacture of a membrane with high abrasion resistance, thus completing the present invention.

[0033] The present invention provides the following invention. [1]

[0035] A porous membrane, characterized in that the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness to the open porosity of the liquid being filtered side surface is 1.05 or more. [2]

[0037] According to the porous membrane described in [1], the porosity of the filtered liquid side surface is 25% or more. [3]

[0039] According to the porous membrane described in [1] or [2], the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness is 35% or more. [4]

[0041] The porous membrane according to any one of [1] to [3], wherein the polymer skeleton size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 100 nm or more. [5]

[0043] The porous membrane according to any one of [1] to [4], wherein the porosity of the filtered liquid side surface is 35% or more. [6]

[0045] The porous membrane according to any one of [1] to [5], wherein the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness to the open porosity of the liquid being filtered side surface is 0.7 or more. [7]

[0047] The porous membrane according to any one of [1] to [6], wherein the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness is 20% or more. [8]

[0049] The porous membrane according to any one of [1] to [7], wherein the polymer skeleton size from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the membrane thickness is 100 nm or more. [9]

[0051] The porous membrane according to any one of [1] to [8], wherein the porosity of the liquid-to-filter side surface is 35% or more, and the porosity from the outermost surface of the liquid-to-filter side surface to a thickness of 0.12% of the membrane thickness is 40% or more.

[10]

[0053] The porous membrane according to any one of [1] to [9], wherein the cross-sectional pore size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 300 nm or less.

[11]

[0055] The porous membrane according to any one of [1] to

[10] , wherein the membrane thickness is 100 μm or more and 500 μm or less.

[12]

[0057] The porous membrane according to any one of [1] to

[11] , wherein the porous membrane is a hollow fiber membrane composed of thermoplastic resin.

[13]

[0059] According to the porous membrane described in

[12] , the thermoplastic resin contains a fluoropolymer as the main component.

[14]

[0061] According to the porous membrane of

[13] , the fluororesin contains at least one selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof.

[15]

[0063] A porous membrane is characterized in that the product of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness and the open porosity of the liquid being filtered side surface is 860% or more.

[16]

[0065] According to the porous membrane described in

[15] , the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness is 35% or more.

[17]

[0067] According to

[15] or

[16] , the porous membrane has an opening ratio of 25% or more on the surface of the filtered liquid.

[18]

[0069] The porous membrane according to any one of

[15] to

[17] , wherein the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness to the open porosity of the liquid being filtered side surface is 1.05 or more.

[19]

[0071] The porous membrane according to any one of

[15] to

[18] , wherein the polymer skeleton size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 100 nm or more.

[20]

[0073] The porous membrane according to any one of

[15] to

[19] , wherein the product of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness and the open porosity of the liquid being filtered side surface is 1140% or more. [twenty one]

[0075] The porous membrane according to any one of

[15] to

[20] , wherein the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness to the open porosity of the liquid being filtered side surface is 0.7 or more. [twenty two]

[0077] The porous membrane according to any one of

[15] to

[21] , wherein the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness is 20% or more. [twenty three]

[0079] The porous membrane according to any one of

[15] to

[22] , wherein the polymer skeleton size from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the membrane thickness is 100 nm or more. [twenty four]

[0081] The porous membrane according to any one of

[15] to

[23] , wherein the porosity of the liquid-to-filter side surface is 35% or more, and the porosity from the outermost surface of the liquid-to-filter side surface to a thickness of 0.12% of the membrane thickness is 40% or more.

[25]

[0083] The porous membrane according to any one of

[15] to

[24] , wherein the cross-sectional pore size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 300 nm or less.

[26]

[0085] The porous membrane according to any one of

[15] to

[25] , wherein the membrane thickness is 100 μm or more and 500 μm or less.

[27]

[0087] The porous membrane according to any one of

[15] to

[26] , wherein the porous membrane is a hollow fiber membrane composed of a thermoplastic resin.

[28]

[0089] According to the porous membrane described in

[27] , the thermoplastic resin contains a fluoropolymer as the main component.

[29]

[0091] According to the porous membrane of

[28] , the fluororesin contains at least one selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof.

[0092] Invention Effects

[0093] According to the present invention, a porous membrane with high filtration performance and abrasion resistance is provided. Attached Figure Description

[0094]

Figure 1

[0095]

【 Figure 2 [Image 1] is a diagram showing the structure of an apparatus for manufacturing porous hollow fiber membranes.

[0096]

【 Figure 3A The diagram illustrates the method for determining the boundary of a layer, and it also illustrates the method for determining the location of the line used to determine the length of the hole used in the boundary determination.

[0097]

【 Figure 3B The diagram above illustrates the method for determining the boundaries of layers. Figure 3A A diagram illustrating the method for determining the length of the hole in the line.

[0098]

【 Figure 4 The process of obtaining a binary image containing only the pores on the outermost surface of the membrane is called 'the process of obtaining a binary image containing only the pores on the outermost surface of the membrane'.

[0099]

【 Figure 5 [Image 1] is an electron microscope image of a cross-section near the filtered liquid side of the porous hollow fiber membrane obtained in Example 1.

[0100]

【 Figure 6 [Image 1] is an electron microscope image of a cross-section near the filtered liquid side of the porous hollow fiber membrane obtained in Example 8.

[0101]

【 Figure 7 The image shows the filter assembly used in the water permeability test. Detailed Implementation

[0102] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0103] The porous membrane of this embodiment is a porous membrane in which the ratio of porosity from the outermost surface of the liquid being filtered to the open porosity of the liquid being filtered side surface is 1.05 or higher, or a porous membrane in which the product of the porosity from the outermost surface of the liquid being filtered to the open porosity of the liquid being filtered side surface is 860% or higher. Any porous membrane possesses high filtration performance and abrasion resistance.

[0104] In this embodiment, the porous membrane preferably has a porosity ratio of 1.05 or higher relative to the open porosity of the filtered liquid side surface from the outermost surface to a thickness of 0.12% of the membrane thickness, and the product of the porosity and the open porosity is 860% or higher.

[0105] The porous membrane of this embodiment will be described below.

[0106] The porous membrane of this embodiment preferably contains a fluoropolymer, such as a vinylidene fluoride-based or trifluorochloroethylene-based resin, as the main component, and serves as the polymer component (e.g., a thermoplastic resin) constituting the membrane. Here, "containing...as the main component" means containing 50% or more by mass, calculated based on the solid content of the polymer component. The aforementioned polymer component may be a single type or a combination of multiple types.

[0107] Furthermore, the weight-average molecular weight (Mw) of the vinylidene fluoride resin is not particularly limited, but is preferably 100,000 or more and 1,000,000 or less, more preferably 150,000 or more and 1,500,000 or less. Moreover, it is not limited to a single molecular weight vinylidene fluoride resin; multiple vinylidene fluoride resins with different molecular weights can be mixed. In this embodiment, the weight-average molecular weight (Mw) can be determined by gel permeation chromatography (GPC) based on a standard resin with a known molecular weight.

[0108] On the other hand, porous membranes may also contain other polymeric components. There are no particular limitations on these other polymeric components, but substances compatible with vinylidene fluoride resins are preferred; for example, fluorinated resins that exhibit high drug resistance similar to vinylidene fluoride resins can be appropriately used.

[0109] As a form of the aforementioned porous membrane, it can be, for example, a membrane structure having hollow fiber membranes. Here, hollow fiber membrane refers to a membrane with a hollow ring-like shape. Because porous membranes have a hollow fiber membrane structure, the membrane area per unit volume of the module can be increased compared to planar membranes.

[0110] However, the porous membrane in this embodiment is not limited to porous membranes with a hollow fiber membrane structure (hollow fiber porous membrane), but may also have other membrane structures such as flat membranes and tubular membranes.

[0111] The porous membrane of this embodiment is preferably a hollow fiber membrane containing a thermoplastic resin, but it may also be a hollow fiber membrane composed solely of a thermoplastic resin. The thermoplastic resin preferably contains a fluoropolymer as a main component, but it may also be composed solely of a fluoropolymer. As the fluoropolymer, it is preferably composed of at least one selected from the group consisting of vinylidene fluoride resin (PVDF), trifluorochloroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof, or it may be composed solely of at least one selected from the group consisting of vinylidene fluoride resin (PVDF), trifluorochloroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof.

[0112] The porous membrane of this embodiment is preferably a porous membrane in which the ratio of the porosity to the open porosity of the filtered liquid side surface from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness (from the outermost surface to the position where the outermost surface is 0.12% of the membrane thickness in the membrane thickness direction relative to 100% of the membrane thickness) is 1.05 or more.

[0113] When the ratio is 1.05 or higher, the communication between the pores on the surface and the pores near the surface inside the membrane is good, and the pores near the surface, which are most conducive to filtration, are less likely to become clogged. Furthermore, due to the good communication between the pores near the surface, dirt can be easily removed by backwashing or cross-flow effects, thus exhibiting high filtration performance. The ratio is preferably 1.10 or higher, more preferably 1.10 or higher and 2.50 or lower. When the ratio is 2.50 or lower, the polymer forming the pores on the surface is less prone to deformation, maintaining its filtering performance.

[0114] Ideally, the ratio of 1.05 or higher should be maintained within the thickness from the outermost surface of the liquid being filtered to 0.12% of the membrane thickness, as described above. This is important for ensuring high filtration performance due to the high porosity near the surface in contact with the liquid being filtered. This is because the membrane surface in contact with the liquid being filtered has the highest concentration of fouling, leading to pore blockage and affecting the overall performance of the membrane.

[0115] In accordance with the same principle, the ratio of the porosity of the porous membrane from the outermost surface of the liquid being filtered to a thickness of 0.10% of the membrane thickness to the open porosity of the liquid being filtered side surface is preferably 1.05 or more, more preferably 1.10 or more, and even more preferably 1.10 or more and 2.50 or less.

[0116] Furthermore, in this embodiment, the ratio of the porosity of the porous membrane from the outermost surface of the liquid being filtered to a thickness of 0.2% of the membrane thickness to the open porosity of the liquid being filtered side surface is preferably 1.05 or more, more preferably 1.10 or more and 2.50 or less, and even more preferably 1.10 or more and 1.50 or less.

[0117] In this embodiment, the porous membrane preferably has an opening ratio of 25% or more on the surface of the filtered liquid side.

[0118] When the porosity is 25% or higher, high filtration performance can be achieved. A high porosity results in a low membrane fouling load per pore and fewer pores becoming completely clogged, thus presumably leading to high filtration performance. The aforementioned porosity is preferably 30% or higher, more preferably 35% or higher, and even more preferably 37% or higher. Alternatively, the aforementioned porosity can be 60% or lower.

[0119] The porous membrane of this embodiment preferably has a porosity of 35% or more, more preferably 40% or more, from the surface of the filtrate to a thickness of 0.12% of the membrane thickness. When the porosity is 40% or more, it can exhibit high filtration performance for a wider range of filtrate properties.

[0120] When the porosity is 35% or higher, similar to the open porosity, the membrane fouling load per pore is small, and fewer pores are completely clogged, thus it is presumed that high filtration performance can be exhibited. Preferably, it is 35% or higher and 85% or lower, more preferably 38% or higher and 80% or lower, further preferably 40% or higher and 78% or lower, and particularly preferably 44% or higher and 75% or lower. When the porosity is 85% or lower, it can practically possess sufficient strength.

[0121] The porous membrane of this embodiment preferably has an opening ratio of 35% or more on the surface of the liquid being filtered and a porosity of 40% or more from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness.

[0122] In this embodiment, the porosity of the porous membrane from the surface of the filtered liquid to a thickness of 0.10% of the membrane thickness is preferably 35% or more, and more preferably 40% or more.

[0123] Furthermore, in this embodiment, the porosity of the porous membrane from the outermost surface of the filtrate side to a thickness of 0.2% of the membrane thickness is preferably 35% or more, more preferably 35% or more and 85% or less, further preferably 38% or more and 80% or less, even more preferably 40% or more and 78% or less, and particularly preferably 44% or more and 75% or less. When the porosity is 35% or more, the membrane fouling load per pore is small, and fewer pores are completely blocked, thus exhibiting high filtration performance. When it is 40% or more, it can exhibit high filtration performance for a wider range of filtrate properties, and when it is 85% or less, it has sufficient strength for practical use.

[0124] In this embodiment, the porous membrane preferably has a porosity ratio of 0.7 or higher relative to the open porosity of the filtered liquid side surface from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the membrane thickness.

[0125] When the ratio is 0.7 or higher, the communication between the pores on the surface and the pores near the surface inside the membrane is good, maximizing the use of the surface pores for filtration and exhibiting high filtration performance. The ratio is preferably 0.7 or higher and 1.1 or lower. When the ratio is 1.1 or lower, the polymer forming the surface pores is less prone to deformation, maintaining its filtering properties.

[0126] In this embodiment, the ratio of the porosity of the porous membrane from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness to the open porosity of the liquid being filtered side surface (sometimes referred to as "0.12% ratio" in this specification) is preferably greater than the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness to the open porosity of the liquid being filtered side surface (sometimes referred to as "0.04% ratio" in this specification). The larger the 0.12% ratio, the more consistent or improved the connectivity becomes as it extends from the surface towards the membrane thickness, resulting in higher filtration performance.

[0127] The difference between the proportion of 0.12% and the proportion of 0.04% (“proportion of 0.12%” - “proportion of 0.04%”) is preferably 0.1 or more and 0.8 or less, more preferably 0.2 or more and 0.7 or less, and even more preferably 0.25 or more and 0.6 or less.

[0128] In this embodiment, the porous membrane, with the same aim as the ratio of 0.04%, preferably has a porosity ratio of 0.7 or more from the outermost surface of the filtered liquid side surface to a thickness of 0.02% of the membrane thickness relative to the open porosity of the filtered liquid side surface.

[0129] Furthermore, in this embodiment, the porous membrane preferably has a porosity ratio of 0.7 or more relative to the open porosity of the surface of the filtered liquid side surface, from the outermost surface of the filtered liquid side surface to a thickness of 0.067% of the membrane thickness, which is 0.7 or more and 1.1 or less, and even more preferably 0.8 or more and 1.0 or less.

[0130] In this embodiment, the porous membrane preferably has a porosity of 20% or more from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the membrane thickness.

[0131] When the porosity is 20% or higher, similar to the open porosity, the membrane fouling load per pore is small, and fewer pores are completely clogged, thus it is presumed that high filtration performance can be exhibited. The porosity is preferably 20% or higher and 80% or lower, more preferably 25% or higher and 75% or lower, and even more preferably 30% or higher and 70% or lower. When the porosity is 80% or lower, the membrane structure can be maintained under pressure, and it can practically possess sufficient strength.

[0132] In this embodiment, the porous membrane preferably has a porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness greater than the porosity from the outermost surface of the liquid being filtered to a thickness of 0.04% of the membrane thickness. The difference between the porosity to a thickness of 0.12% and the porosity to a thickness of 0.04% (“porosity to a thickness of 0.12% (%)” - “porosity to a thickness of 0.04% (%)”) is preferably 5% or more and 30% or less, more preferably 10% or more and 25% or less.

[0133] The porous membrane of this embodiment aims to achieve the same porosity as the membrane thickness up to 0.04%, preferably with a porosity of 20% or more from the outermost surface of the filtered liquid side to a thickness of 0.02%.

[0134] Furthermore, in this embodiment, the porous membrane preferably has a porosity of 20% or more from the outermost surface of the filtrate side to a thickness of 0.067% of the membrane thickness, more preferably 20% or more and 80% or less, further preferably 25% or more and 75% or less, and even more preferably 30% or more and 70% or less. When the porosity is 20% or more, similar to the open porosity, the membrane fouling load per pore is small, and fewer pores are completely blocked, thus exhibiting high filtration performance; when it is 80% or less, the membrane structure can be maintained under pressure, and it can have sufficient strength in practical applications.

[0135] In this embodiment, the porous membrane preferably has a polymer backbone size of 100 nm or more from the outermost surface of the filtered liquid to a thickness of 0.12% of the membrane thickness.

[0136] When the polymer backbone size is 100 nm or more, it can suppress the decrease in water permeability caused by rubbing during circumferential shaking of the membrane due to air washing, etc., and is therefore preferred. This is because when the polymer backbone size is 100 nm or more, the polymer constituting the porous membrane has sufficient strength to maintain the structure and prevent pore deformation due to rubbing, thus suppressing the decrease in water permeability. The polymer backbone size is preferably 100 nm or more and 300 nm or less, more preferably 105 nm or more and 260 nm or less.

[0137] The porous membrane of this embodiment, with the same aim as the polymer backbone size in the thickness up to 0.12% of the membrane thickness, preferably has a polymer backbone size of 100 nm or more from the outermost surface of the filtered liquid to 0.10% of the membrane thickness. Furthermore, the porous membrane of this embodiment preferably has a polymer backbone size of 100 nm or more from the outermost surface of the filtered liquid to 0.2% of the membrane thickness, more preferably 100 nm or more and 300 nm or less, and even more preferably 105 nm or more and 260 nm or less.

[0138] In this embodiment, the porous membrane preferably has a polymer backbone size of 100 nm or more from the outermost surface of the filtered liquid to a thickness of 0.04% of the membrane thickness.

[0139] When the polymer backbone size is 100 nm or more, it can suppress the decrease in water permeability caused by rubbing during membrane circumferential movement due to air washing, etc., and is therefore preferred. The polymer backbone size is preferably 100 nm or more and 300 nm or less, and more preferably 110 nm or more and 200 nm or less.

[0140] In this embodiment, the difference between the polymer backbone size from the outermost surface of the filtrate side to a thickness of 0.12% and the polymer backbone size from the outermost surface of the filtrate side to a thickness of 0.04% (“polymer backbone size to 0.12% thickness” - “polymer backbone size to 0.04% thickness”) of the porous membrane is preferably within ±15 nm, more preferably within ±10 nm. Furthermore, the polymer backbone size to a thickness of 0.12% may be greater than or equal to the polymer backbone size to a thickness of 0.04%.

[0141] In this embodiment, the porous membrane, with the same aim as the polymer backbone size in the thickness up to 0.04% of the membrane thickness, preferably has a polymer backbone size of 100 nm or more from the outermost surface of the filtered liquid side to the thickness up to 0.02% of the membrane thickness.

[0142] Furthermore, in this embodiment, the porous membrane preferably has a polymer skeleton size of 100 nm or more from the outermost surface of the filtered liquid to a thickness of 0.067% of the membrane thickness, more preferably 100 nm or more and 300 nm or less, and even more preferably 110 nm or more and 200 nm or less.

[0143] The porous membrane of this embodiment preferably has a cross-sectional pore size of 300 nm or less from the outermost surface of the filtered liquid to a thickness of 0.12% of the membrane thickness.

[0144] The aforementioned cross-sectional aperture is preferably 100 nm or more and 300 nm or less, more preferably 120 nm or more and 280 nm or less, and even more preferably 150 nm or more and 250 nm or less. When the cross-sectional aperture is 300 nm or less, it can practically provide sufficient blocking performance.

[0145] The porous membrane of this embodiment aims to achieve the same cross-sectional pore size as the membrane thickness up to 0.12%, preferably with a cross-sectional pore size of 300 nm or less from the outermost surface of the filtered liquid to a thickness of 0.10%.

[0146] Furthermore, the porous membrane of this embodiment preferably has a cross-sectional pore size of 300 nm or less from the outermost surface of the filtered liquid to a thickness of 0.2% of the membrane thickness, more preferably 100 nm or more and 300 nm or less, even more preferably 120 nm or more and 280 nm or less, and even more preferably 150 nm or more and 250 nm or less.

[0147] In this embodiment, the porous membrane preferably has a porosity product of 860%·% from the surface of the filtered liquid to a thickness of 0.12% of the membrane thickness and the open porosity of the surface of the filtered liquid. Preferably, it is 1000%·% or more, more preferably 1140%·% or more. When the product is 860%·% or more, since the membrane fouling load on each pore is very small in both the surface and thickness directions, very few pores are completely blocked, thus high filtration performance is expected. Alternatively, it can be 5000%·% or less.

[0148] When the porous membrane in this embodiment is a hollow fiber membrane, the inner diameter is preferably 0.3 mm or more and 5 mm or less. When the inner diameter is 0.3 mm or more, the pressure loss of the liquid flowing inside the hollow fiber membrane will not become too large, and when the inner diameter is 5 mm or less, sufficient compressive strength and burst strength can be easily exhibited with a relatively thin membrane thickness. More preferably, the inner diameter is 0.4 mm or more and 3 mm or less, and even more preferably, 0.5 mm or more and 2 mm or less.

[0149] Furthermore, the membrane thickness is preferably 0.1 mm or more and 1 mm or less. When the membrane thickness is 0.1 mm or more, it is easy to exhibit sufficient compressive strength and burst strength, and when the membrane thickness is 1 mm or less, it is easy to exhibit sufficient water permeability. More preferably, the membrane thickness is 0.15 mm or more and 0.8 mm or less, further preferably 0.16 mm or more and 0.6 mm or less, or 0.17 mm or more and 0.5 mm or less. In addition, 0.1 mm or more and 0.5 mm or less is preferred.

[0150] The outer diameter is preferably 0.5 mm or more and 5 mm or less. When the outer diameter is 0.5 mm or more, sufficient tensile strength is achieved. When the outer diameter is 5 mm or less, the number of filler strands can be increased in the container holding the porous membrane (preferably a porous hollow fiber membrane). More preferably, the outer diameter is 0.6 mm or more and 4 mm or less, and even more preferably, 0.7 mm or more and 3 mm or less.

[0151] The preferred pure water permeability of the porous membrane in this embodiment is 1000 L / m³. 2 / hr and above and 20000L / m 2 / hr and below. Within this range, the pure water permeability can balance filtration and retention performance. The preferred pure water permeability is 1200 L / m³. 2 / hr and above and 18000L / m 2 / hr and below, more preferably 3000L / m 2 / hr and above and 12000L / m 2 / hr and below.

[0152] The porous membrane of this embodiment preferably has an overall porosity of 50% or more from the viewpoint of water permeability, and preferably 90% or less from the viewpoint of strength. More preferably, it has a porosity of 55% or more and 85% or less, and even more preferably, it has a porosity of 65% or more and 80% or less.

[0153] The inventors have discovered that factors including the porosity of the electrode surface (differences that cannot be detected by measuring the overall porosity of the membrane) affect filtration performance. Specifically, they found that filtration performance is improved by controlling the ratio of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness to the open porosity of that liquid being filtered side surface, or the product of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the membrane thickness and the open porosity of that liquid being filtered side surface.

[0154] The aforementioned porous membrane (preferably a porous hollow fiber membrane) is expected to have a three-dimensional network structure. The three-dimensional network structure referred to in this application means, in a pattern such as... Figure 1The structure shown is as follows. For example, thermoplastic resin a bonds to form a mesh, and voids b are formed. In the three-dimensional mesh structure, the so-called spherulitic resin masses are hardly observed. The voids b of the three-dimensional mesh structure are preferably surrounded by thermoplastic resin a, and the various parts of the voids b are interconnected. Since most of the thermoplastic resin used forms a three-dimensional mesh structure that contributes to the strength of the porous membrane (preferably a hollow fiber membrane), a high-strength support layer can be formed. In addition, chemical resistance is also improved. Although the reason for the improved chemical resistance is not yet clear, it is believed that it may be due to the large amount of thermoplastic resin forming the mesh that contributes to the strength, so that even if a part of the mesh is eroded by a chemical, it will not have a significant impact on the overall strength of the layer.

[0155] The aforementioned porous membrane (preferably a porous hollow fiber membrane) can be a single-layer structure or a multi-layer structure with two or more layers. The layer having the surface of the filtrate is designated as layer (A), and the layer having the surface of the filtrate is designated as layer (B).

[0156] For example, layers (A) and (B) share the function as follows: layer (A) is designated as a barrier layer, utilizing its small surface pore size to prevent foreign matter contained in the treated liquid (raw water) from permeating through the membrane; layer (B) is designated as a support layer, ensuring high mechanical strength while minimizing reduction in permeability. The division of functions between layers (A) and (B) is not limited to the above. In this embodiment, the porous membrane may have only one surface facing the filtered liquid.

[0157] The following describes a two-layer structure where layer (A) is a barrier layer and layer (B) is a support layer. The thickness of layer (A) is preferably 1 / 100 or more of the film thickness and less than 40 / 100. By making layer (A) thicker, it can be used even if the raw water contains insoluble substances such as sand or condensate. This is because even with some abrasion, the surface pore size does not change. Within this thickness range, a balance between the desired barrier performance and high permeability can be achieved. The thickness of layer (A) is more preferably 2 / 100 or more of the film thickness and less than 30 / 100. The thickness of layer (A) is preferably 1 μm or more and less than 100 μm, more preferably 2 μm or more and less than 80 μm.

[0158] The specific manufacturing method for hollow fiber membranes is explained.

[0159] The method for manufacturing the porous membrane (preferably a porous hollow fiber membrane) according to this embodiment preferably includes: a step of forming a hollow fiber-shaped molten compound by extruding a molten compound containing a thermoplastic resin, an organic liquid, and inorganic micropowder from a spinneret having an annular nozzle; and a step of extracting and removing the organic liquid and inorganic micropowder after the hollow fiber-shaped molten compound has solidified to produce a porous membrane (preferably a porous hollow fiber membrane). The molten compound may consist of two components, a thermoplastic resin and a solvent, or it may consist of three components, a thermoplastic resin, inorganic micropowder, and a solvent.

[0160] The thermoplastic resin used in the manufacturing method of the porous membrane (preferably a porous hollow fiber membrane) of this embodiment is a resin that is elastic and does not exhibit plasticity at room temperature, but exhibits plasticity upon appropriate heating, thereby enabling molding. Furthermore, the thermoplastic resin is a resin that, upon cooling and temperature reduction, reverts to its original elastomer state without undergoing chemical changes such as molecular structure during this period (see, for example, the Chemical Encyclopedia 6 Abridged Edition, edited by the Chemical Encyclopedia Editorial Committee, Kyoritsu Publishing, pp. 860 and 867, 1963).

[0161] Examples of thermoplastic resins include the resins listed under the thermoplastic plastics entry (pp. 829-882) of "Chemical Products of 12695" (Kakogyo Nihon Sho, 1995) and the resins listed on pages 809-810 of "Chemical Handbook Application Edition, Revised 3rd Edition" (edited by the Japanese Chemical Society, Maruzen, 1980). Specific examples of thermoplastic resins include polyolefins such as polyethylene and polypropylene, fluorinated resins such as polyvinylidene fluoride, ethylene-vinyl alcohol copolymers, polyamides, polyetherimides, polystyrene, polysulfones, polyvinyl alcohol, polyphenylene ether, polyphenylene sulfide, cellulose acetate, and polyacrylonitrile. From the perspective of strength performance, crystalline thermoplastic resins such as polyolefins, fluorinated resins such as polyvinylidene fluoride, ethylene-vinyl alcohol copolymers, and polyvinyl alcohol are preferred. More preferably, fluorinated resins such as polyolefins and polyvinylidene fluoride are used because they are hydrophobic and therefore have high water resistance, and durability in the filtration of ordinary aqueous liquids is expected. Specifically, the aforementioned fluororesin preferably contains one or more of the following as main components: vinylidene fluoride resin (PVDF), trifluorochloroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures of these resins; more preferably, it consists solely of resins composed of one or more of these resins combined individually. Particularly preferably, the aforementioned fluororesin may use polyvinylidene fluoride (PVDF), which exhibits excellent chemical durability, such as chemical resistance. Examples of PVDF include PVDF homopolymers and PVDF copolymers with a PVDF content of 50 mol% or more. Examples of PVDF copolymers include copolymers of PVDF with one or more monomers selected from tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, or ethylene. PVDF homopolymers are particularly preferred as PVDF.

[0162] The desired concentration of thermoplastic resin in the melt blend is 30% to 48% by mass, preferably 32% to 45% by mass. Mechanical strength is easily guaranteed at a concentration of 30% by mass or higher, while no reduction in water permeability occurs at a concentration of 48% by mass or lower.

[0163] Furthermore, in the case where the porous membrane is a two-layer structure, the concentration of thermoplastic resin in the melt compound of layer (B) is preferably 34% to 48% by mass, and more preferably 35% to 45% by mass.

[0164] The concentration of thermoplastic resin in the melt blend of layer (A) is preferably 10% by mass or more and 35% by mass or less, more preferably 12% by mass or more and 35% by mass or less. If the concentration is 10% by mass or more, both surface pore size and mechanical strength can be taken into account, and if the concentration is 35% by mass or less, the water permeability will not decrease.

[0165] For the organic liquid, an organic liquid that is a potential solvent relative to the thermoplastic resin used in this embodiment is used. In this embodiment, a potential solvent is a solvent that hardly dissolves the thermoplastic resin at room temperature (25°C), but can dissolve the thermoplastic resin at temperatures above room temperature. The organic liquid only needs to be liquid at the melt mixing temperature with the thermoplastic resin, and does not need to be liquid at room temperature.

[0166] When the thermoplastic resin is polyethylene, examples of organic liquids include phthalate esters such as dibutyl phthalate, diheptyl phthalate, dioctyl phthalate, di(2-ethylhexyl) phthalate, diisodecyl phthalate, and tridecyl phthalate; sebacate esters such as dibutyl sebacate; adipate esters such as dioctyl adipate; trimellitic anhydride esters such as trioctyl trimellitic anhydride; phosphate esters such as tributyl phosphate and trioctyl phosphate; glycerides such as propylene glycol didecanoate and propylene glycol dioleate; paraffins such as liquid paraffin; and mixtures thereof.

[0167] When the thermoplastic resin is polyvinylidene fluoride (PVDF), examples of organic liquids include phthalates such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dicyclohexyl phthalate, diheptyl phthalate, dioctyl phthalate, and di(2-ethylhexyl) phthalate; sebacate such as dibutyl sebacate; adipate such as dioctyl adipate; benzoate such as methyl benzoate and ethyl benzoate; phosphates such as triphenyl phosphate, tributyl phosphate, and tricresyl phosphate; ketones such as γ-butyrolactone, ethylene carbonate, propylene carbonate, cyclohexanone, acetophenone, and isophorone; and mixtures thereof.

[0168] Examples of inorganic micropowders include silica, alumina, titanium dioxide, zirconium dioxide, and calcium carbonate. Silica micropowders with an average primary particle size of 3 nm or more and 500 nm or less are particularly preferred. More preferably, they are 5 nm or more and 100 nm or less. Even more preferred are hydrophobic silica micropowders that are not easily agglomerated and have good dispersibility. Hydrophobic silica with a MW (methanol wettability) value of 30% or more is even more preferred. The MW value here refers to the capacity percentage of methanol required to completely wet the powder. Specifically, the MW value is determined by adding silica to pure water and then adding methanol below the surface while stirring, calculating the capacity percentage of methanol in the aqueous solution when 50% by mass of silica settles. The "average primary particle size of the inorganic micropowder" mentioned above refers to a value obtained from the analysis of electron micrographs. That is, firstly, a group of inorganic micropowders is pretreated using the method of ASTM D3849. Subsequently, the diameters of 3,000 to 5,000 particles captured in the transmission electron microscope images were measured, and their values ​​were taken as an arithmetic mean to calculate the average primary particle size of the inorganic micropowder.

[0169] Regarding the amount of inorganic powder added, it is preferable that the mass ratio of inorganic powder in the melt mixture is 5% by mass or more and 50% by mass or less. More preferably, it is 10% by mass or more and 40% by mass or less. If the proportion of inorganic powder is 5% by mass or more, the effect of inorganic powder mixing can be fully expressed, and if it is 40% by mass or less, stable spinning can be achieved.

[0170] Melt blending can be carried out using conventional melt blending methods, such as extrusion. The following describes the use of extrusion, but melt blending methods are not limited to extrusion. Figure 2 An example of a manufacturing apparatus for implementing the manufacturing method of this embodiment is shown.

[0171] Figure 2 The apparatus for manufacturing porous hollow fiber membranes shown includes: an extruder 10, a nozzle 20 for forming hollow fibers, a coagulation bath 30 for storing a solution for solidifying the membrane-forming solution, and multiple rollers 50 for conveying and winding the porous hollow fiber membrane 40. 60 is a suction machine, and 70 is a high-temperature container. Figure 2 The space S shown is the air gap through which the film-forming solution sprayed from the nozzle 20 of the hollow fiber forming machine passes before reaching the solution in the coagulation bath 30.

[0172] A hollow fiber forming nozzle 20, having one or more annular nozzles configured in a concentric circle, is installed at the front end of an extruder 10. The molten compound is extruded from the hollow fiber forming nozzle 20 by the extruder 10. In the case of manufacturing multilayer films, there are methods such as: installing a hollow fiber forming nozzle 20 having two or more annular nozzles at the front end of an extruder 10, and extruded by supplying molten compound to each annular nozzle using a different extruder 10; or coating the remaining layers after manufacturing one layer of the multilayer. For example, in the former method of manufacturing using different extruders, by merging and overlapping the separately supplied molten compound at the nozzles, a hollow fiber extrudate with a multilayer structure can be obtained. In this case, by extruding molten compounds of different compositions from adjacent annular nozzles, a multilayer film with different pore sizes of adjacent layers can be obtained. Different compositions refer to different constituent substances in the molten compound, or even if the constituent substances are the same, different composition ratios. Even with the same thermoplastic resin, if the molecular weight and molecular weight distribution are significantly different, they are considered to be different constituent substances. The confluence position of melt-mixed compounds with different compositions may be at the lower end face of the hollow fiber forming nozzle 20, or it may not be at the lower end face of the hollow fiber forming nozzle 20.

[0173] When extruding the molten compound from an annular nozzle, it is preferable to extrude it with a nozzle ejection parameter R (1 / s) of 10 or more and 1000 or less, which results in high productivity, spinning stability, and ultimately a high-strength film. Here, the nozzle ejection parameter R is the value obtained by dividing the ejection linear velocity V (m / s) by the nozzle slit width d (m). The ejection linear velocity V (m / s) is the ejection capacity of the molten compound per unit time (m³ / s). 3 / second) divided by the cross-sectional area of ​​the nozzle (m²) 2 The value obtained is R. If R is 10 or higher, the filament diameter of the hollow extruder does not have problems such as pulsating fluctuations, and spinning can be carried out with good productivity and stability. In addition, if R is 1000 or lower, the elongation at break, which is one of the important strengths of the resulting porous hollow fiber membrane, can be kept sufficiently high. Elongation at break is the elongation relative to the original length when stretched along the length direction of the membrane.

[0174] In the case of a multi-layered porous hollow fiber membrane, the extrusion linear velocity V of the melt-mixed compound formed by the confluence of resins is divided by the slit width d of the nozzle as the extrusion parameter R. R is more preferably 50 or more and 1000 or less.

[0175] The hollow fiber-shaped molten compound ejected from the nozzle solidifies using a cooling medium such as air or water. However, depending on the type of porous hollow fiber membrane being processed, after passing through the air gap S formed by the air layer, it passes through a coagulation bath 30 containing water or the like. That is, the air gap S extends from the nozzle 20 for hollow fiber forming to the water surface in the coagulation bath 30. If necessary, a container such as a cylinder can be used in the air gap S from the nozzle. After passing through the coagulation bath 30, it is wound onto a reel or similar device as needed.

[0176] Especially after the molten compound passes through the nozzle, it is best to immediately pass it through a high-temperature container such as a cylindrical container for 0.015 seconds or more. If it is 0.015 seconds or more, the solvent vapor accumulated in the high-temperature container is suppressed, closing the surface pores. This reduces the resin concentration on the surface of the molten compound by absorbing the solvent vapor, thereby increasing the open area ratio and the porosity near the surface. To control the pore size of the surface, 0.18 seconds or less is preferred. More preferably, 0.018 seconds or more and 0.14 seconds or less is preferred, and even more preferably, 0.021 seconds or more and 0.12 seconds or less is preferred.

[0177] In non-solvent-induced phase separation methods, a cylinder is sometimes used throughout the air gap to utilize the moisture in the air gap for phase separation. In this invention, a new insight has been discovered regarding the control of porosity relative to the surface opening ratio by solvent vapor.

[0178] The set temperature of the aforementioned high-temperature container relative to the ejection temperature T of the molten compound is preferably (T-60)℃ to (T+60)℃. More preferably, it is (T-50)℃ to (T+50)℃. If it is (T-60)℃ or higher, the effect of the solvent vapor can be fully realized. While there is no particular reason, it is preferable to set the temperature to (T+60)℃ or lower to prevent excessive increase in the set temperature and thus deterioration of the resin compound.

[0179] The time it takes for the molten compound to pass through the air gap S is called the air gap time, which is preferably 0.20 seconds or more. When the air gap time is 0.20 seconds or more, the polymer molecules orient themselves within the air gap, thereby further improving the compressive strength. The air gap time is further preferably 0.20 seconds or more and 2.0 seconds or less. If it is 2.0 seconds or less, stable manufacturing can be achieved. A time of 0.30 seconds or more and 1.5 seconds or less is desirable, and more preferably 0.40 seconds or more and 1.1 seconds or less is preferred.

[0180] In the air gap, it is desirable to use a suction device or similar means to blow cooling air at a speed of 0.80 m / sec or less in a direction perpendicular to the ejection direction. Although the reason is not yet certain, it is presumed that if the speed is 0.80 m / sec or less, the solvent vapor will be moderately retained on the surface of the molten compound, thus suppressing the closure of the surface, i.e., the pores, by the solvent vapor, thereby reducing the resin concentration on the surface of the molten compound by absorbing the solvent vapor, thereby increasing the open porosity and the porosity near the surface.

[0181] In the solidified hollow fiber, the polymer-rich phase and the organic liquid-rich phase are finely separated. Furthermore, for example, when inorganic powder is added, and if the inorganic powder is silica powder, the silica powder is unevenly distributed within the organic liquid-rich phase. By extracting and removing the organic liquid and inorganic powder from the hollow fiber, the organic liquid-rich phase becomes porous. Therefore, a porous hollow fiber membrane can be obtained.

[0182] For the extraction and removal of organic liquids and inorganic powders, if the same solvent can be used for extraction and removal, they can be carried out simultaneously. However, extraction and removal are usually performed separately.

[0183] For the extraction and removal of organic liquids, a liquid is used that does not dissolve or modify the thermoplastic resin used and is suitable for mixing and extraction with the organic liquid. Specifically, this can be achieved by contacting the liquid with it through methods such as impregnation. For easy removal from the hollow fiber membrane after extraction, the liquid is preferably volatile. Examples of such liquids include alcohols and dichloromethane. If the organic liquid is water-soluble, water can also be used as the extraction liquid.

[0184] Extraction and removal of inorganic micropowders typically utilizes aqueous liquids. For example, when the inorganic micropowder is silica, it can be removed by first contacting it with an alkaline solution to convert the silica into silicates, and then contacting it with water for extraction.

[0185] The extraction and removal of organic liquids and inorganic powders can be performed either first. However, when the organic liquid and water are immiscible, it is preferable to perform the extraction and removal of the organic liquid first, followed by the extraction and removal of the inorganic powders. This is advantageous because the organic liquid and inorganic powders typically coexist and mix within the thicker organic liquid phase, allowing for the smooth extraction and removal of the inorganic powders.

[0186] In this way, a porous hollow fiber membrane can be obtained by extracting and removing organic liquids and inorganic powders from a solidified porous hollow fiber membrane.

[0187] Furthermore, for the solidified hollow fiber membrane, stretching along the length direction of the porous hollow fiber membrane can be performed at any of the following stages, within a stretching ratio of 3 times or less: (i) before extraction to remove organic liquid and inorganic powder; (ii) after extraction to remove organic liquid and before extraction to remove inorganic powder; (iii) after extraction to remove inorganic powder and before extraction to remove organic liquid; and (iv) after extraction to remove organic liquid and inorganic powder. Generally, stretching a hollow fiber membrane along its length improves its water permeability, but reduces its compressive strength (bursting strength and compressive strength), thus often preventing it from achieving practical strength. However, the porous hollow fiber membrane obtained by the manufacturing method of this embodiment has high mechanical strength. Therefore, stretching with a stretching ratio of 1.1 times or more and up to 3.0 times is possible. Stretching improves the water permeability of the porous hollow fiber membrane. The stretching ratio referred to here is the value obtained by dividing the length of the hollow fiber after stretching by the length of the hollow fiber before stretching. For example, when a porous hollow fiber membrane with a hollow fiber length of 10 cm is stretched to a hollow fiber length of 20 cm, the stretching ratio is 2 times according to the following formula.

[0188] 20cm ÷ 10cm = 2

[0189] The stretching process is preferably performed at a room temperature of 0°C or higher and 160°C or lower. Temperatures above 160°C result in larger stretch marks, reduced elongation at break, and decreased water permeability, making this less desirable. Temperatures at 0°C or lower also increase the likelihood of tensile fracture, making this impractical. A room temperature of 10°C or higher and 140°C or lower is more preferred, and even more preferably 20°C or higher and 100°C or lower is more desirable.

[0190] In this invention, it is preferable to stretch a hollow fiber membrane containing an organic liquid. Compared to a hollow fiber membrane without an organic liquid, a hollow fiber membrane containing an organic liquid experiences less breakage during stretching. Furthermore, for hollow fiber membranes containing an organic liquid, the shrinkage of the stretched hollow fiber membrane can be increased, thus increasing the degree of freedom in setting the shrinkage rate after stretching.

[0191] Furthermore, it is preferable to stretch hollow fiber membranes containing inorganic microparticles. Regarding hollow fiber membranes containing inorganic microparticles, the presence of these microparticles gives the hollow fiber membrane rigidity, making it difficult to flatten during stretching. Additionally, it prevents the final hollow fiber membrane from having excessively small pore sizes or filament diameters.

[0192] In this invention, it is more desirable to stretch hollow fiber membranes containing both organic liquids and inorganic micropowders.

[0193] Based on the above reasons, compared with stretching a hollow fiber membrane after extraction, it is more preferable to stretch a hollow fiber membrane containing either an organic liquid or an inorganic micropowder. Furthermore, compared with stretching a hollow fiber membrane containing either an organic liquid or an inorganic micropowder, it is even more preferable to stretch a hollow fiber membrane containing both organic liquid and inorganic micropowder.

[0194] Furthermore, the method of extraction using a stretched hollow fiber membrane has the advantage of allowing the extraction solvent to easily penetrate into the interior of the membrane because stretching increases the surface and internal porosity of the hollow fiber membrane. Additionally, the method of extraction after a stretching and subsequent shrinking process, as described later, results in a hollow fiber membrane with a low tensile modulus and high flexibility. Therefore, during extraction in a liquid flow, the hollow fiber membrane is easily agitated by the liquid flow, increasing the stirring effect and allowing for efficient extraction in a short time.

[0195] In this invention, because the process includes stretching the hollow fiber membrane and then shrinking it, a hollow fiber membrane with a low tensile modulus of elasticity can be obtained. Here, "low tensile modulus of elasticity" means that the fibers easily elongate under a small force and return to their original shape when the force is removed. If the tensile modulus of elasticity is low, the hollow fiber membrane will not flatten, is easy to bend, and is easily shaken by the water flow during filtration. Because the fibers shake in a non-fixed manner with the bending of the fibers as the water flows, the layer of pollutants deposited on the membrane surface cannot grow and is easily peeled off, thus maintaining a high level of filtered water flow. Furthermore, when the fibers are forcibly shaken by rinsing or air washing, the shaking becomes greater, and the washing recovery effect becomes higher.

[0196] Regarding the degree of fiber length shrinkage during stretching and then shrinking, it is desirable that the fiber length shrinkage rate relative to the fiber length increment caused by stretching is in the range of 0.3 or higher and 0.9 or lower. For example, when a 10cm fiber is stretched to 20cm and then shrinks back to 14cm, according to the following formula...

[0197] Fiber length shrinkage rate = {(maximum fiber length during stretching) - (fiber length after shrinkage)} / [(maximum fiber length during stretching) - (initial fiber length)] = (20-14) / (20-10) = 0.6. The fiber length shrinkage rate is 0.6.

[0198] When the fiber length shrinkage rate is 0.9 or higher, the water permeability tends to decrease; when it is less than 0.3, the tensile modulus of elasticity tends to increase, and therefore it is not preferred. In this invention, a fiber length shrinkage rate in the range of 0.50 or higher and 0.85 or lower is more preferred.

[0199] In addition, by employing a process of stretching the hollow fiber membrane to its maximum fiber length and then shrinking it, the resulting hollow fiber membrane will not break when stretched to its maximum fiber length during use.

[0200] Here, when the stretch ratio is set as X and the fiber length shrinkage rate relative to the fiber length increment caused by stretching is set as Y, the rate Z representing the guarantee of the breaking elongation can be defined by the following formula.

[0201] Z = (Maximum fiber length during stretching - Fiber length after shrinkage) / Fiber length after shrinkage = (XY - Y) / (X + Y - XY)

[0202] Preferably, Z is 0.2 or higher and 1.5 or lower, and more preferably Z is 0.3 or higher and 1.0 or lower. If Z is too small, the guarantee of elongation at break decreases; if Z is too large, the permeability decreases compared to the high probability of breakage during tension.

[0203] Furthermore, in the manufacturing method of the present invention, since the process of stretching and then shrinking is included, there are very few fractures at low elongation for tensile breaking elongation, which can narrow the distribution of tensile breaking elongation.

[0204] From the perspective of shrinkage time and physical properties, it is desirable that the temperature in the space during the stretching and subsequent shrinking process be within the range of 0°C or higher and 160°C or lower. If the temperature is below 0°C, shrinkage takes longer and is therefore impractical; if the temperature exceeds 160°C, the elongation at break and permeability decrease, making it undesirable.

[0205] In this invention, it is also preferable to curl the hollow fiber membrane during the shrinkage process. This allows for the production of a hollow fiber membrane with high curl without crushing or damaging it.

[0206] Typically, hollow fiber membranes are straight tubular without bending. Therefore, when bundled into filter modules, the gaps between the hollow fibers are eliminated, resulting in a high probability of forming fiber bundles with low porosity. Conversely, if hollow fiber membranes with high crimp are used, the bending of each fiber causes the gaps between the hollow fiber membranes to widen evenly, forming fiber bundles with high porosity. Furthermore, filter modules made of hollow fiber membranes with low crimp, especially when used under external pressure, have fewer gaps in the fiber bundles, increasing flow resistance and preventing the filtration pressure from being effectively transmitted to the center of the fiber bundles. Furthermore, the washing effect inside the fiber bundles deteriorates when backwashing or flushing to remove filter deposits from the hollow fiber membrane. Fiber bundles made of hollow fiber membranes with high crimp have high porosity, maintaining the gaps between the hollow fiber membranes even under external pressure filtration, and are less prone to flow deviation.

[0207] In this invention, a curl degree of 1.5 or higher and 2.5 or lower is preferred. A curl degree of 1.5 or higher is preferred for the reasons stated above; furthermore, if it is less than 2.5, the reduction in filter area per unit volume can be suppressed.

[0208] Examples of methods for rolling hollow fiber membranes include placing the hollow fiber membrane between, for example, a pair of gear rollers with periodic irregularities or a pair of sponge-like strips with irregularities during the stretching and then shrinking process, and removing them from there during the shrinking process.

[0209] Furthermore, in this invention, it is preferable to use a removal device consisting of a pair of opposing infinite track belts for stretching. In this case, removal devices are used on both the upstream and downstream sides of the stretching process. In each removal device, the hollow fiber membrane is placed between opposing belts, and the fibers are transported by moving the two belts in the same direction at the same speed. Additionally, in this case, it is preferable to stretch the fiber at a higher speed on the downstream side than on the upstream side. If stretching is performed in this manner, no tensile tension is generated during stretching, slippage is prevented, and fiber flattening is avoided.

[0210] Here, the inner side of the infinite track type belt that contacts the drive roller is preferably made of a highly elastic belt such as a fiber-reinforced belt, while the outer surface that contacts the hollow fiber membrane is made of an elastomer. Furthermore, the compressive modulus of elasticity of the elastomer along its thickness direction is 0.1 MPa or more and 2 MPa or less; more preferably, the thickness of the elastomer is 2 mm or more, up to 20 mm. From the viewpoint of chemical resistance and heat resistance, it is particularly preferable that the elastomer on the outer surface is made of silicone rubber.

[0211] Additionally, the stretched membrane can be heat-treated as needed to improve compressive strength. Heat treatment is ideally performed at 80°C or higher and 160°C or lower. Temperatures of 160°C or lower can suppress the decrease in elongation at break and water permeability; temperatures of 100°C or higher can improve compressive strength. Furthermore, from the viewpoint of minimizing changes in fiber diameter, porosity, pore size, and water permeability, heat treatment of the hollow fiber membrane after extraction is desirable.

[0212] When using PVDF (polyvinylidene fluoride) as a thermoplastic resin, the solvent for PVDF needs to be appropriately selected to balance high open porosity and high compressive strength. Firstly, methods to increase open porosity include reducing the concentration of PVDF and, as previously mentioned, increasing the temperature of the fluid used to form the hollow portion. When using a method that reduces the PVDF concentration for film formation, the pore size also increases; therefore, a solvent capable of achieving both high open porosity and small pore size needs to be selected. The following parameter P is a formula relating the three-dimensional solubility parameters of PVDF and the solvent, used to evaluate the solubility of PVDF in the solvent. The right-hand side of the formula shows the three-dimensional solubility range of the Hansen solubility parameters and quantitatively represents the distance from the three-dimensional solubility parameters of PVDF (σdp, σpp, σhp) to the three-dimensional solubility parameters of the solvent (σdm, σpm, σhm).

[0213] P=((σdm-σdp) 2 +(σpm-σpp) 2 +(σhm-σhp) 2 ) 0.5

[0214] [In the formula, σdm and σdp represent the dispersion force terms of the solvent and polyvinylidene fluoride, respectively; σpm and σpp represent the dipole coupling force terms of the solvent and polyvinylidene fluoride, respectively; σhm and σhp represent the hydrogen bonding terms of the solvent and polyvinylidene fluoride, respectively.]

[0215] Furthermore, the above concept is not limited to PVDF.

[0216] In the case of a porous membrane with a two-layer structure, the parameter P between the solvent and PVDF used to prepare the melt-mixed compound B for forming the layer (B) is preferably greater than 7.88, more preferably 7.88 to 10.0. If this value is 7.88 or higher, the reduction in water permeability can be suppressed.

[0217] In the preparation of the melt-mixed compound A forming layer (A), the parameter P between the solvent and PVDF is preferably 7.88, more preferably 0 to 7.88, and even more preferably 1.00 to 7.88. If this value is 7.88 or less, high porosity and small pore size can be achieved.

[0218] Example

[0219] The present embodiment will be further described in detail below through examples and comparative examples, but the present embodiment is not limited to these examples.

[0220] In addition, the measurement method used in this embodiment is described below.

[0221] Unless otherwise specified, all the following measurements were performed at 25°C. Following the explanation of the evaluation methods, the manufacturing methods and evaluation results of the examples and comparative examples will be described.

[0222] In addition, the membrane's composition, manufacturing conditions, and various properties are shown in Tables 1 and 2.

[0223] (1) Measurement of outer diameter, inner diameter, and film thickness (mm)

[0224] The hollow fiber membrane was thinly sliced ​​at 15cm intervals in a direction perpendicular to the membrane length using a razor or similar tool. The major and minor axes of the inner and outer diameters of the cross-sections were measured using a microscope. The inner and outer diameters were calculated using formulas (2) and (3) respectively. The inner diameter was subtracted from the calculated outer diameter, and the result divided by 2 was used as the membrane thickness. Measurements were taken at 20 points, and the average value was used as the inner diameter, outer diameter, and membrane thickness under these conditions.

[0225]

Number 1

[0226]

[0227]

Number 2

[0228]

[0229] (2) Pure water permeability (L / m 2 / hr)

[0230] Hollow fiber membranes were immersed in a 50% (w / w) ethanol aqueous solution for 30 minutes, followed by immersion in water for 30 minutes to wet the membranes. One end of a 10cm length of the wetted hollow fiber membrane was sealed, and an injection needle was inserted into the hollow section at the other end. Pure water at 25°C was injected into the hollow section through the injection needle at a pressure of 0.1 MPa. The permeate flow rate of pure water to the outer surface was measured, and the pure water permeate flux was determined using the following formula. Here, the effective membrane length refers to the net membrane length excluding the portion into which the injection needle is inserted. Ten measurement points were used, and the average value was taken as the pure water permeability under each condition.

[0231]

Number 3

[0232]

[0233] (3) Breaking strength (MPa), elongation at break (%)

[0234] The load and displacement at tension and fracture were measured under the following conditions.

[0235] Sample: Moistened hollow fiber membrane prepared using method (2)

[0236] Measuring instrument: Instron tensile testing machine (Shimadzu Corporation: AGS-X) Chuck distance: 5cm

[0237] Stretching speed: 20cm / min

[0238] The breaking strength and elongation at break are determined by the following formula.

[0239]

Number 4

[0240]

[0241]

[0242] The cross-sectional area of ​​the membrane is calculated using the following formula.

[0243]

Number 5

[0244]

[0245] (4) Overall membrane porosity

[0246] The overall porosity of the membrane can be determined by the following formula.

[0247] Porosity (total membrane) % = 100 × (Wet membrane weight [g] - Dry membrane weight [g]) / Specific gravity of water [g / cm³] 3 ] / (membrane volume [cm 3 Here, a wet membrane refers to a membrane whose pores are filled with pure water but whose hollow portion does not contain pure water. Specifically, a wet membrane can be obtained by immersing a sample membrane with a length of 10-20 cm in ethanol to fill the pores with ethanol, and then repeating the immersion in pure water 4-5 times to completely replace the interior of the pores with pure water. Then, hold one end of the hollow fiber and shake it thoroughly about 5 times, and then hold the other end and shake it thoroughly about 5 times to remove the water in the hollow portion. Alternatively, a dry membrane can be obtained by measuring the weight of the aforementioned wet membrane and then drying it in an oven at, for example, 60°C until a constant weight is achieved.

[0248] Membrane volume can be expressed by the following formula: membrane volume [cm] 3 The solution is calculated as π × {(outer diameter [cm] / 2)^2 - (inner diameter [cm] / 2)^2} × membrane length [cm].

[0249] When the weight of a single membrane is too small and the error in weight measurement becomes large, multiple membranes can be used.

[0250] (4) Methods for determining the boundaries between layers in multi-layer structures

[0251] The cross-section of the membrane was observed using a Hitachi SU8000 series electron microscope at an accelerating voltage of 3 kV. In this embodiment and comparative example, images were taken near the boundaries between layers at 1000x magnification. Based on the images, if the boundary lines between layers can be distinguished, these boundary lines are taken as the boundaries between layers. In the porous hollow fiber membranes of this embodiment and comparative example, since the boundaries can also be distinguished, these boundary lines are also taken as the boundaries between layers.

[0252] When the boundaries between layers cannot be identified using the methods described above, the boundaries can be determined using the following methods. For example, in the case of a two-layer porous hollow fiber membrane, the method for determining the boundary between layer (A) and layer (B) will be explained. The following is the method when layer (A) is designated as a barrier layer and layer (B) as a support layer.

[0253] Cross-sections of the hollow fiber membrane were photographed using an electron microscope, and images were taken that confirmed the shape of 20 or more pores. Multiple images were taken to allow for a complete observation of the cross-section. In this example and comparative example, measurements were performed at 5000x magnification. The electron microscope samples of the cross-sections were obtained by slicing membrane samples frozen in ethanol into circular pieces.

[0254] Using the commercially available image analysis software Winroof 6.1.3, the image was processed as follows: Figure 3A As shown, 100 lines L, each equidistant from the surface FA, are drawn at intervals of 101, dividing the entire film thickness into 101 equal parts. (That is, lines connecting points of equal film thickness.) Figure 3B As shown, the length Lh of the portion corresponding to the pore h in the cross-sectional image of line L is measured. The average length Lh of this cross-section is calculated by arithmetic mean, and the cross-sectional pore diameter of each film thickness is determined. When the magnification of the scanning electron microscope image is sufficiently high, lines equidistant from the surface FA can be approximated as straight lines. Using the maximum value of the determined cross-sectional pore diameter, the cross-sectional pore diameter of each film thickness is normalized, and the point that first reaches the normalized value closest to 0.7 from the surface FA is taken as the boundary layer of the layer.

[0255] (5) Three-dimensional solubility parameters

[0256] The three-dimensional solubility parameters are cited from the following reference: Hansen, Charles (2007). Hansen Solubility Parameters: A user's handbook, Second Edition. Boca Raton, Flavouring Center: CRC Press. (ISBN 978-0-8493-7248-3)

[0257] (6) Hole diameter and porosity of inner and outer surfaces

[0258] Using the same electron microscope as (4), the surface of the filtered liquid was photographed. The photographs were taken at a magnification that allowed the shape of 20 or more pores to be confirmed; in this embodiment and comparative example, the photographs were taken at 10,000x magnification.

[0259] Using the captured image, as described in International Publication No. 2001 / 53213, a transparent film was overlaid on a copy of the image, and the perforated areas were blacked out with a black pen. The transparent film was then copied onto white paper, clearly distinguishing the perforated areas as black and the non-perforated areas as white. Subsequently, the commercially available image analysis software Winroof 6.1.3 was used to binarize the image using discriminant analysis. By calculating the area occupied by the resulting binarized image, the porosity of surfaces FA and FB was determined.

[0260] The aperture is determined as follows: For each hole on the surface, calculate the equivalent circle diameter. Starting with the hole with the largest aperture, sum the areas of each hole in sequence. The aperture diameter is determined when the sum reaches 50% of the total area of ​​all holes.

[0261] (7) Resistance to abrasion of the film surface

[0262] This is an indicator used to determine the degree of degradation in water permeability caused by membrane surface abrasion. A wetted hollow fiber membrane (sample length: 100 mm), which had been repeatedly impregnated with pure water after being soaked in ethanol, was placed on a metal plate. A suspension of fine sand (130 μm particle size: Fuji Brown FRR#120) at 20% by weight suspended in water was sprayed onto the outer surface of the membrane from a nozzle positioned 70 cm above the membrane at a pressure of 0.1 MPa. After 15 minutes of spraying, the membrane was flipped over and sprayed for another 15 minutes. The pure water flux was measured before and after spraying, and the membrane surface abrasion resistance rate was calculated using the following formula.

[0263] Membrane surface abrasion resistance [%%] = 100 × (pure water flux after spraying) / (pure water flux before spraying)

[0264] (8) Porosity, polymer skeleton size, cross-sectional pore diameter

[0265] Porous hollow fiber membranes are cut into rings with a cross-section orthogonal to the fiber length direction and then embedded in epoxy resin. After trimming, the sample cross-section is processed using BIB (Biologically Injectable Bonding) to create a smooth cross-section, and then treated with electrical conductivity to prepare microscopic examination samples. Microscopic examination samples are prepared at one cut position on each sample.

[0266] Electron microscopy (SEM) images of the membrane cross-section of the prepared microscopic examination specimens were obtained using a Hitachi SU7000 electron microscope. The image acquisition conditions were as follows: for each microscopic examination specimen, five fields of view were captured, including the outer surface area.

[0267] Image acquisition conditions

[0268] Accelerating voltage: 1kV

[0269] Detector: Reflected electron detector

[0270] Camera magnification: 50,000x (device's display magnification)

[0271] Image resolution: 2560×1920 pixels

[0272] Image analysis was performed using ImageJ. First, a Plugins-Bilateral Filter Fiji was applied (10 times with spatial radius = 3 and range radius = 50) to perform filtering. Then, Threshold processing (Image-Adjust-Treshold: Maximum Entropy method (MaxEntropy selected) was applied to the filtered SEM image to binarize the pores (the parts of the pores embedded in the resin) and the polymer skeleton.

[0273] Using the top of the binarized image as a reference, the pixel of the film portion closest to the top of the image is designated as the location with a film thickness of 0 nm. Regions of a predetermined thickness (e.g., 100 nm, sometimes 50 nm) are continuously cut along the film thickness direction, and the porosity, polymer skeleton size, and cross-sectional pore size are calculated from each image using the method shown below. Here, for example, the porosity, polymer skeleton size, and cross-sectional pore size for the 0–300 nm region are set as the sum of the average values ​​of the porosity, polymer skeleton size, and cross-sectional pore size for continuously cut regions of 0–100 nm, 100–200 nm, and 200–300 nm, respectively, after the above calculations are performed. In addition, for example, for the porosity, polymer skeleton size, and cross-sectional pore size in the 0–1250 nm region, the average values ​​of the porosity, polymer skeleton size, and cross-sectional pore size in the 0–50 nm, 50 nm–100 nm…1150 nm–1200 nm, and 1200 nm–1250 nm regions, which are continuously cut and calculated as above, are set respectively.

[0274] Furthermore, since the 0–100 nm region includes the openings on the outermost surface of the membrane, it is necessary to define the outermost surface of the membrane and calculate the values ​​based on a binary image containing only the pores when calculating the porosity and cross-sectional pore size. To obtain a binary image containing only the pores, the pencil tool in Adobe Photoshop Elements 9 was used. The boundaries of the openings / embedded resin portions on the outermost surface of the membrane were manually defined, and then the embedded resin portions were painted over to obtain a binary image containing only the pores. Specific operations are as follows: Figure 4 As shown. For an image where the polymer backbone is at a brightness of 255 (white) and the pores and embedding resin are at a brightness of 0 (black), a line is drawn using the pencil tool to connect the two ends of the openings on the outermost surface of the film. Then, using the smear tool, the embedding resin portion is smeared to a brightness of 0 (black), thus obtaining a binary image of only pores in the 0-100 nm region. The positions of the two ends of the openings on the outermost surface of the film are determined arbitrarily by the operator.

[0275] Porosity (%): For binary images with holes (holes are 0% black), apply Analyze-Analyze Particles to calculate the porosity by summing the values ​​of the five fields of view in the %area of ​​the Summary. The AnalyzeParticles settings are as follows.

[0276] Size (Pixel 2): ​​0-infinity

[0277] Circularity: 0-1.00

[0278] Summarize: The checkbox is checked.

[0279] Exclude on edges: The checkbox is unchecked.

[0280] Include Holes: The checkbox is unchecked.

[0281] Polymer skeleton size: For the binary image of the film (equivalent to an image where the polymer portion is 0% black), apply ImageJ's Plugins-BoneJ-Thickness to obtain the LocalThickness image. For the LocalThickness image, apply Analyze-Histogram to obtain detailed numerical information about the LocalThickness. Based on the obtained LocalThickness values ​​from the five fields of view, calculate the summation average, which is defined as the polymer skeleton size.

[0282] Cross-sectional aperture: For the binary image of the aperture (equivalent to an image where the aperture portion is 0% black), apply ImageJ's Plugins-BoneJ-Thickness to obtain the LocalThickness image. For the LocalThickness image, apply Analyze-Histogram to obtain detailed numerical information about the LocalThickness. Based on the obtained LocalThickness values ​​from the five fields of view, calculate the summation average, which is defined as the cross-sectional aperture.

[0283] (9) Permeability test

[0284] Hollow fiber membrane 12 is used to make such a product. Figure 7 The filter assembly 11 is shown. The filter assembly 11 consists of an effective membrane length of 1 m and 300 hollow fibers, with the hollow fibers at both ends sealed with an epoxy-based sealing material 13. At the upper end of the assembly, the hollow portion of the hollow fiber membrane is open; conversely, at the lower end, the hollow portion of the hollow fiber membrane is sealed. River water with a turbidity of 2-4 degrees is filtered through the raw water and air inlet 14 from the outer surface of the hollow fibers, and filtered water is obtained from the inner surface of the upper end. The set flux (set flux (m³ / day) is equal to the filtration flow rate (m³ / day)) is gradually increased. 3 / day) divided by membrane outer surface area (m²) 2 The value obtained is used as the critical Flux (m / day) before the transmembrane pressure difference begins to rise sharply. The sharp rise in transmembrane pressure difference is judged by an increase rate of about 50 kPa / 5 days.

[0285] (Example 1)

[0286] The thermoplastic resin used was vinylidene fluoride homopolymer (KF-W#1000 manufactured by Kureha Corporation); the organic liquid used was a mixture of di(2-ethylhexyl) phthalate (DEHP) (manufactured by CG ESTER Corporation) and dibutyl phthalate (DBP) (manufactured by CG ESTER Corporation); and the inorganic powder used was silica powder (manufactured by AEROSIL Corporation of Japan, trade name: AEROSIL-R972, primary particle size approximately 16 nm). The hollow fiber membrane was melt-extruded using an extruder through a hollow fiber forming nozzle. The melt blend used was a homopolymer of vinylidene fluoride, di(2-ethylhexyl) phthalate, dibutyl phthalate, and silica powder in a mass ratio of 40.0:30.8:6.20:23.0. Air was used as the fluid for forming the hollow part. The mixture was extruded together from a hollow fiber forming nozzle with an outer diameter of 2.0 mm and an inner diameter of 0.9 mm at an extrusion temperature of 240°C.

[0287] The hollow fibrous melt-mixed compound, extruded at an ejection temperature of 240°C, passes through a high-temperature container at a set temperature of 240°C for 0.053 seconds. After a total of 0.60 seconds of air movement within the high-temperature container, it is introduced into a coagulation bath containing water at 30°C. It is then removed at a speed of 30 m / min, stretched on a conveyor belt at 60 m / min, and then contracted at 45 m / min while being blown with hot air at 140°C, before being wound onto a reel. The air velocity in the air gap is set to 0.80 m / s.

[0288] The obtained hollow fiber material was impregnated in isopropanol to extract and remove di(2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Next, it was impregnated in a 50% (w / w) aqueous ethanol solution for 30 minutes, then in water for 30 minutes, and finally in a 20% (w / w) aqueous sodium hydroxide solution at 70°C for 1 hour. This process was repeated with water washing to extract and remove silica micropowder, yielding a porous hollow fiber membrane.

[0289] The resulting porous hollow fiber membrane is a porous membrane with its outer surface (outer diameter side surface) serving as the side surface of the filtered liquid.

[0290] Table 1 shows the detailed composition and conditions.

[0291] Figure 5 This is an electron microscope image of a cross-section near the filtered liquid side of the obtained porous hollow fiber membrane.

[0292] (Example 2)

[0293] Except that the composition of the melt-blended compound was set as follows: vinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica micropowder = 34.0: 32.5: 8.10: 25.4 (mass ratio), a porous hollow fiber membrane was obtained by the same method as in Example 1.

[0294] Table 1 shows the detailed composition and conditions.

[0295] (Example 3)

[0296] Except for setting the passage time of the high-temperature container through the air gap to 0.018 seconds, a porous hollow fiber membrane was obtained by the same method as in Example 1.

[0297] Table 1 shows the detailed composition and conditions.

[0298] (Example 4)

[0299] Except for setting the air velocity in the air gap to 1.8 m / s, a porous hollow fiber membrane was obtained using the same method as in Example 1.

[0300] Table 1 shows the detailed composition and conditions.

[0301] (Example 5)

[0302] Except that the organic liquid used was a mixture of di(2-ethylhexyl)octyl adipate (DOA) (manufactured by Tokyo Chemical Industry Co., Ltd.) and dibutyl sebacate (DBS) (manufactured by Fujifilm and Wako Pure Chemical Co., Ltd.), and the composition of the melt compound was set as vinylidene fluoride homopolymer: di(2-ethylhexyl)octyl adipate: dibutyl sebacate: silica micropowder = 40.0: 25.0: 12.0: 23.0 (mass ratio), a porous hollow fiber membrane was obtained by the same method as in Example 1.

[0303] Table 1 shows the detailed composition and conditions.

[0304] (Example 6)

[0305] Except for setting the outer diameter to 0.9 mm and the inner diameter to 0.6 mm, a porous hollow fiber membrane was obtained by the same method as in Example 1.

[0306] Table 1 shows the detailed composition and conditions.

[0307] (Example 7)

[0308] Except for the absence of stretching and shrinking processes, porous hollow fiber membranes were obtained using the same method as in Example 1. Table 1 shows the detailed composition and conditions.

[0309] (Example 8)

[0310] A porous hollow fiber membrane with a two-layer structure is manufactured by setting layer (A) as the outer surface side of the hollow fiber membrane and layer (B) as the inner surface side of the hollow fiber membrane. Polyvinylidene fluoride homopolymer was used as the thermoplastic resin, a mixture of di(2-ethylhexyl) phthalate and dibutyl phthalate was used as the organic liquid, and silica powder was used as the inorganic powder. The composition of the melt-blended compound for layer (A) was set as follows: vinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica powder = 34.0:32.5:8.1:25.4 (mass ratio). The composition of the melt-blended compound for layer (B) was set as follows: vinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica powder = 40.0:31.7:5.3:23.0 (mass ratio). Hollow fiber membranes were melt-extruded using two extruders. Air was used as the fluid for forming the hollow section, and the melt-blended compound was extruded from a nozzle for forming triple-ring hollow fibers at an extrusion temperature of 250°C. The outermost diameter of the nozzle for forming the triple-ring hollow fiber was set to 2.0 mm, the innermost diameter to 0.9 mm, and the diameter of the portion of layers (A) and (B) corresponding to the boundary of the molten compound outlet was set to 1.8 mm. The process following the ejection of the molten compound was carried out using the same method as in Example 1 to obtain a porous hollow fiber membrane.

[0311] The resulting porous hollow fiber membrane is a porous membrane with its outer surface (outer diameter side surface) serving as the side surface of the filtered liquid.

[0312] Table 2 shows the detailed composition and conditions.

[0313] Figure 6 This is an electron microscope image of a cross-section near the filtered liquid side of the obtained porous hollow fiber membrane.

[0314] (Example 9)

[0315] Except that the composition of the melt blend of layer (A) is set as polyvinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica micropowder = 25.0:35.9:10.3:28.8 (mass ratio), a porous hollow fiber membrane is obtained by the same method as in Example 8.

[0316] Table 2 shows the detailed composition and conditions.

[0317] (Example 10)

[0318] Except that the composition of the melt-blended compound of layer (A) was set as follows: polyvinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica micropowder = 20.0:38.3:10.9:30.8 (mass ratio) and the air gap time was set to 0.42 seconds, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0319] Table 2 shows the detailed composition and conditions.

[0320] (Example 11)

[0321] Except for setting the passage time of the high-temperature container through the air gap to 0.018 seconds, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0322] Table 2 shows the detailed composition and conditions.

[0323] (Example 12)

[0324] Except for setting the air velocity in the air gap to 1.8 m / s, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0325] Table 2 shows the detailed composition and conditions.

[0326] (Example 13)

[0327] Except for setting the outer diameter to 0.9 mm and the inner diameter to 0.6 mm, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0328] Table 2 shows the detailed composition and conditions.

[0329] (Comparative Example 1)

[0330] Except for setting the passage time of the high-temperature container through the air gap to 0.012 seconds, a porous hollow fiber membrane was obtained by the same method as in Example 1.

[0331] Table 1 shows the detailed composition and conditions.

[0332] (Comparative Example 2)

[0333] Except for setting the air velocity in the air gap to 2.1 m / s, a porous hollow fiber membrane was obtained using the same method as in Example 1.

[0334] Table 1 shows the detailed composition and conditions.

[0335] (Comparative Example 3)

[0336] Except for setting the passage time of the high-temperature container through the air gap to 0.012 seconds, the porous hollow fiber membrane was obtained by the same method as in Example 7.

[0337] Table 1 shows the detailed composition and conditions.

[0338] (Comparative Example 4)

[0339] Except that the composition of the melt-mixed compound was set as follows: polyvinylidene fluoride homopolymer: di(2-ethylhexyl) phthalate: dibutyl phthalate: silica micropowder = 34.0:32.5:8.10:25.4 (mass ratio), the high-temperature container passage time in the air gap was set to 0.012 seconds, and the air velocity in the air gap was set to 2.1 m / s, the porous hollow fiber membrane was obtained by the same method as in Example 1.

[0340] Table 1 shows the detailed composition and conditions.

[0341] (Comparative Example 5)

[0342] Except for setting the passage time of the high-temperature container through the air gap to 0.012 seconds, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0343] Table 2 shows the detailed composition and conditions.

[0344] (Comparative Example 6)

[0345] Except for setting the air velocity in the air gap to 2.1 m / s, a porous hollow fiber membrane was obtained by the same method as in Example 8.

[0346] Table 2 shows the detailed composition and conditions.

[0347] (Comparative Example 7)

[0348] Except for setting the passage time of the high-temperature container through the air gap to 0.012 seconds, the porous hollow fiber membrane was obtained by the same method as in Example 10.

[0349] Table 2 shows the detailed composition and conditions.

[0350] Table 1

[0351]

[0352] Table 2

[0353]

[0354] Industrial availability

[0355] According to the present invention, a porous membrane with high filtration performance and abrasion resistance is provided.

Claims

1. A porous membrane, which is a hollow fiber membrane composed of thermoplastic resin, characterized in that, The ratio of porosity to open porosity of the filtration liquid side surface from the outermost surface to a thickness of 0.12% of the film thickness is 1.05 or higher. The porosity from the outermost surface of the filtered liquid side to a thickness of 0.12% of the membrane thickness is 35% or more. The thermoplastic resin contains a fluoropolymer as the main component, and the fluoropolymer contains polyvinylidene fluoride resin (PVDF).

2. The porous membrane according to claim 1, wherein, The porosity of the side surface of the filtered liquid is 25% or more.

3. The porous membrane according to claim 1, wherein, The polymer backbone size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness is 100 nm or more.

4. The porous membrane according to claim 1, wherein, The porosity of the side surface of the filtered liquid is 35% or more.

5. The porous membrane according to claim 1, wherein, The ratio of the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the film thickness to the open porosity of the filtered liquid side surface is 0.7 or more.

6. The porous membrane according to claim 1, wherein, The porosity from the outermost surface of the filtered liquid side to a thickness of 0.04% of the membrane thickness is 20% or more.

7. The porous membrane according to claim 1, wherein, The polymer backbone size from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the film thickness is 100 nm or more.

8. The porous membrane according to claim 1, wherein, The porosity of the filtered liquid side surface is 35% or more, and the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness is 40% or more.

9. The porous membrane according to claim 1, wherein, The cross-sectional pore size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 300 nm or less.

10. The porous membrane according to claim 1, wherein the membrane thickness is 0.1 mm or more and 1 mm or less.

11. The porous membrane according to claim 1, wherein, The film thickness is 100 μm or more and 500 μm or less.

12. The porous membrane according to claim 11, wherein, The inner diameter is 0.3mm or more and 5mm or less.

13. The porous membrane according to claim 11, wherein, Outer diameter ≥ 0.5mm and ≤ 5mm.

14. The porous membrane according to claim 11, wherein, The fluororesin further comprises at least one selected from the group consisting of trifluorochloroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof.

15. A porous membrane, which is a hollow fiber membrane composed of thermoplastic resin, characterized in that, The product of the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the film thickness and the open porosity of the liquid being filtered side surface is 860% or more, the porosity from the outermost surface of the liquid being filtered to a thickness of 0.12% of the film thickness is 35% or more, the thermoplastic resin contains a fluoropolymer as the main component, and the fluoropolymer contains polyvinylidene fluoride resin (PVDF).

16. The porous membrane according to claim 15, wherein, The porosity of the side surface of the filtered liquid is 25% or more.

17. The porous membrane according to claim 15, wherein, The ratio of the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness to the open porosity of the filtered liquid side surface is 1.05 or higher.

18. The porous membrane according to claim 15, wherein, The polymer backbone size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness is 100 nm or more.

19. The porous membrane according to claim 15, wherein, The product of the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness and the open porosity of the filtered liquid side surface is 1140% or more.

20. The porous membrane according to claim 15, wherein, The ratio of the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the film thickness to the open porosity of the filtered liquid side surface is 0.7 or more.

21. The porous membrane according to claim 15, wherein, The porosity from the outermost surface of the filtered liquid side to a thickness of 0.04% of the membrane thickness is 20% or more.

22. The porous membrane according to claim 15, wherein, The polymer backbone size from the outermost surface of the filtered liquid side surface to a thickness of 0.04% of the film thickness is 100 nm or more.

23. The porous membrane according to claim 15, wherein, The porosity of the filtered liquid side surface is 35% or more, and the porosity from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the film thickness is 40% or more.

24. The porous membrane according to claim 15, wherein, The cross-sectional pore size from the outermost surface of the filtered liquid side surface to a thickness of 0.12% of the membrane thickness is 300 nm or less.

25. The porous membrane according to claim 15, wherein the membrane thickness is 0.1 mm or more and 1 mm or less.

26. The porous membrane according to claim 15, wherein, The film thickness is 100 μm or more and 500 μm or less.

27. The porous membrane according to claim 15, wherein, The inner diameter is 0.3mm or more and 5mm or less.

28. The porous membrane according to claim 15, wherein, Outer diameter ≥ 0.5mm and ≤ 5mm.

29. The porous membrane according to claim 15, wherein, The fluororesin further comprises at least one selected from the group consisting of trifluorochloroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures thereof.

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