Hollow fiber membrane, method for manufacturing a hollow fiber membrane, and method for removing endotoxin

Abstract

The objective is to provide a hollow fiber membrane that, even after being dried following manufacturing, possesses the same water permeability as when wet, and has excellent handling properties. [Solution] The hollow fiber membrane 11 comprises a hollow fiber porous substrate 21 and a composition layer 22 covering the porous substrate 21, wherein the composition layer 22 contains a wetting and moisturizing agent, a surfactant, and water, and the ratio of the wetting and moisturizing agent to the surfactant is 2 to 200 times by mass.

Description

[Technical Field]

[0001] The present invention relates to a hollow fiber membrane, a method for manufacturing a hollow fiber membrane, and a method for removing endotoxins. [Background technology]

[0002] Separation technology using hollow fiber membranes is used in various fields because the hollow fiber membrane modules, which are modularized versions of hollow fiber membranes, are small, making it easy to miniaturize the equipment equipped with them, and because it can produce high-quality treated water. Specifically, separation technology using hollow fiber membranes is widely used in environmental technology fields such as water purification and wastewater treatment, as well as in the medical field, pharmaceutical manufacturing field, and food industry field. Among these, for example, in the field of water production where biocompatibility is required, such as water for dialysis and water for injection, hollow fiber membranes capable of removing endotoxins are widely used as endotoxin retentive filters (ETRFs). Endotoxins are lipopolysaccharides (LPS) that make up the outer membrane of Gram-negative bacteria, and if they enter the human body, they can cause a strong inflammatory response. Therefore, it is necessary to remove endotoxins in the production of water used as liquids that enter the human body, such as water for dialysis and water for injection. For this reason, there is a need for the development of hollow fiber membranes that can achieve endotoxin removal.

[0003] Examples of hollow fiber membranes used in endotoxin capture filters and the like include the hollow fiber membranes described in Patent Documents 1 and 2.

[0004] Patent Document 1 describes a hollow fiber membrane module having a case housing and a hollow fiber membrane bundle housed within the case housing, wherein the inner wall surface of the case housing and at least one end of the hollow fiber membrane bundle are bonded together with an adhesive, and the amount of wetting agent at the end of the hollow fiber membrane bundle is 0.6 g or less per gram of membrane. According to Patent Document 1, since a hollow fiber membrane bundle with a reduced glycerin content is used in advance, even if the hollow fiber membrane module is heat-sterilized, no cracks (gaps) will occur in the bonded parts between the hollow fiber membranes.

[0005] Patent Document 2 describes a hollow fiber porous membrane containing polyethersulfone or polysulfone, wherein the hollow fiber porous membrane has an inner diameter of 300 to 600 μm and a thickness of 70 to 200 μm, a molecular weight cutoff of 10,000 or less, and multiple pores with a pore diameter of 0.1 to 0.5 μm are present across the entire outer surface of the hollow fiber porous membrane, and the swelling rate is less than 5%. According to Patent Document 2, it is disclosed that the membrane has a low risk of leakage and is highly reliable, making it suitable for use as an endotoxin capture filter. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2006-247444 [Patent Document 2] International Publication No. 2020 / 059752 [Overview of the project] [Problems that the invention aims to solve]

[0007] In order to effectively remove endotoxins, hollow fiber membranes used in endotoxin capture filters not only have small pore sizes, but also utilize highly hydrophobic materials as the constituent material of the hollow fiber membrane. By using highly hydrophobic materials, an interaction occurs with the hydrophobicity of endotoxins, promoting hydrophobic-hydrophobic interaction, thereby effectively removing endotoxins.

[0008] When hollow fiber membranes are modularized to manufacture hollow fiber membrane modules, their ends are bonded together after drying. The manufactured hollow fiber membrane modules may also be transported. Therefore, the hollow fiber membranes require high handling properties, such as resistance to adhesion between dried hollow fiber membranes during end bonding and transport. Furthermore, while the water permeability of hollow fiber membranes may differ between dry and wet states, to enable the use of hollow fiber membrane modules with dried hollow fiber membranes as quickly as possible, the membranes must be able to pass water appropriately even when dry. Specifically, it is known that highly hydrophobic hollow fiber membranes, once dried, become difficult to allow water to pass through due to their surface tension, resulting in a significant decrease in water permeability. On the other hand, it is also known that even with such highly hydrophobic hollow fiber membranes, once water is passed through, the surface of the hollow fiber membrane becomes covered with water, and the water permeability recovers. Therefore, hollow fiber membranes are required to exhibit the same water permeability as when wet, even after being dried following manufacturing.

[0009] This invention was made in view of the above circumstances, and aims to provide a hollow fiber membrane that has the same water permeability as when wet, even when dried after manufacturing, and has excellent handling properties, as well as a method for manufacturing the same. Furthermore, this invention aims to provide a method for removing endotoxins using the above-mentioned hollow fiber membrane. [Means for solving the problem]

[0010] As a result of various studies, the inventors have found that the above objective can be achieved by the present invention described below.

[0011] A hollow fiber membrane according to a first aspect of the present invention comprises a hollow fiber porous substrate and a composition layer covering the porous substrate, wherein the composition layer contains a humectant, a surfactant, and water, and the ratio of the humectant to the surfactant is 2 to 200 times by mass.

[0012] With this configuration, a composition layer containing a humectant, a surfactant, and water is provided on the hollow fiber porous substrate, with the humectant and surfactant in the aforementioned proportions. By providing such a composition layer, even if the porous substrate is hydrophobic, sufficient permeability can be achieved after drying, even when using water as the permeable liquid (rather than a liquid with lower surface tension than water). In other words, the presence of the composition layer effectively suppresses the decrease in water permeability due to drying. Furthermore, even with the composition layer on the porous substrate, it is possible to suppress the tendency for the hollow fiber membranes to stick together after drying during end bonding or transportation. Therefore, a hollow fiber membrane with excellent handling properties can be obtained. From these findings, it is possible to provide a hollow fiber membrane that, even after drying, possesses the same water permeability as when wet and exhibits excellent handling properties.

[0013] A hollow fiber membrane according to a second aspect of the present invention is a hollow fiber membrane according to a first aspect of the present invention, wherein the water content is 50% by mass or less with respect to the composition layer.

[0014] With this configuration, the composition layer provided on the porous substrate is not only determined by the content ratio of the wetting humectant and the surfactant, but also by the composition layer containing the wetting humectant, the surfactant and water such that the water content is as described above. As a result, the difference between the water permeability of the hollow fiber membrane dried after manufacturing and the water permeability of the hollow fiber membrane when wet is reduced, and a hollow fiber membrane with better handling properties is obtained.

[0015] A hollow fiber membrane according to a third aspect of the present invention is a hollow fiber membrane according to a first or second aspect of the present invention, wherein the mass ratio of the hollow fiber membrane to the porous substrate is 1.03 to 2 times.

[0016] With this configuration, the composition layer is provided such that the mass ratio of the hollow fiber membrane to the porous substrate is the above ratio. This allows the effects of the composition layer, such as reducing the difference between the water permeability of the hollow fiber membrane when wet and the water permeability of the hollow fiber membrane when dried after manufacturing, and improving handling properties, to be more favorably achieved. Therefore, a hollow fiber membrane is obtained in which the difference between the water permeability of the hollow fiber membrane when wet and the water permeability of the hollow fiber membrane when dried after manufacturing is smaller, resulting in better handling properties.

[0017] A hollow fiber membrane according to a fourth aspect of the present invention is a hollow fiber membrane according to any one of the first to third aspects of the present invention, wherein the composition layer is coated not only on the surface of the porous substrate but also on the surface of the pores inside the porous substrate.

[0018] With this configuration, since the composition layer is also coated on the surface of the pores inside the porous substrate through which water passes, the decrease in water permeability due to drying can be more effectively suppressed. Therefore, a hollow fiber membrane can be obtained in which the difference between the water permeability of the hollow fiber membrane dried after manufacturing and the water permeability of the hollow fiber membrane when wet is smaller.

[0019] A hollow fiber membrane according to a fifth aspect of the present invention is a hollow fiber membrane according to any one of the first to fourth aspects of the present invention, wherein the porous substrate contains a hydrophobic resin, has a contact angle with water of 80° or more, and has an endotoxin removal performance of 3 or more in terms of logarithmic removal rate.

[0020] Since the porous base material contains a hydrophobic resin such that the contact angle with water is 80° or more, it becomes less permeable to water when dried. In addition, since the porous base material has an endotoxin removal performance of 3 or more in terms of logarithmic removal rate, the pores inside the porous base material are relatively small, and it cannot be said that the water permeability is extremely high. For this reason as well, the water permeability tends to be insufficient when dried. Even in the case of a hollow fiber membrane comprising such a porous base material, by comprising the composition layer, the hollow fiber membrane dried after production has water permeability similar to that in a wet state, and becomes a hollow fiber membrane with excellent handleability. Further, as described above, the porous base material contains a hydrophobic resin such that the contact angle with water is 80° or more, and the endotoxin removal performance is 3 or more in terms of logarithmic removal rate, so it is excellent in endotoxin removal performance. The hollow fiber membrane can also exhibit the excellent endotoxin removal performance of this porous base material. Therefore, even the hollow fiber membrane dried after production has water permeability similar to that in a wet state, is excellent in handleability, and furthermore, a hollow fiber membrane excellent in endotoxin removal performance can be obtained.

[0021] The hollow fiber membrane according to the sixth aspect of the present invention is the hollow fiber membrane according to the fifth aspect of the present invention, wherein the hydrophobic resin is at least one of polysulfone and polyethersulfone.

[0022] According to such a configuration, since the porous base material is a base material containing polysulfone, polyethersulfone, or polysulfone and polyethersulfone as the hydrophobic resin, the base material is more excellent in endotoxin removal performance. In addition, even in the case of a hollow fiber membrane comprising such a porous base material, by comprising the composition layer, even the hollow fiber membrane dried after production has water permeability similar to that in a wet state, and can be a hollow fiber membrane with excellent handleability. Therefore, even the hollow fiber membrane dried after production has water permeability similar to that in a wet state, is excellent in handleability, and furthermore, a hollow fiber membrane more excellent in endotoxin removal performance can be obtained.

[0023] A hollow fiber membrane according to the seventh aspect of the present invention is a hollow fiber membrane according to any one of the first to sixth aspects of the present invention, wherein the wetting and moisturizing agent is a non-volatile or low-volatility hydrophilic liquid, and the surfactant is a non-volatile or low-volatility surfactant.

[0024] With this configuration, even after drying the hollow fiber membrane after manufacturing, the wetting and moistening agent and the surfactant remain in the composition layer, reducing the difference between the water permeability of the hollow fiber membrane when wet and the water permeability of the hollow fiber membrane when dried after manufacturing, and allowing the effects of the composition layer, such as improved handling properties, to be more favorably achieved. Therefore, a hollow fiber membrane can be obtained in which the difference between the water permeability of the hollow fiber membrane when wet and the water permeability of the hollow fiber membrane when dried after manufacturing is smaller, resulting in a hollow fiber membrane with better handling properties.

[0025] The hollow fiber membrane according to the eighth aspect of the present invention is a hollow fiber membrane according to any one of the first to seven aspects of the present invention, which is used to remove endotoxins from a liquid to be treated that contains endotoxins.

[0026] The hollow fiber membrane has the same water permeability as when wet and is easy to handle. Therefore, even if water is used as the liquid to be permeated after the hollow fiber membrane has been manufactured and dried, rather than a liquid with a lower surface tension than water, sufficient permeability can be achieved. For this reason, there is a low risk of liquids with a lower surface tension than water being mixed into the liquid obtained after removing endotoxins, making it suitable as a hollow fiber membrane used to remove endotoxins from a liquid to be treated that contains endotoxins.

[0027] A method for manufacturing a hollow fiber membrane according to the ninth aspect of the present invention is a method for manufacturing a hollow fiber membrane according to any one of the first to eight aspects of the present invention, wherein the porous substrate is immersed in an aqueous solution in which the wetting and moistening agent and the surfactant are dissolved, and the porous substrate immersed in the aqueous solution is dried to coat the porous substrate with the composition layer.

[0028] With this configuration, the hollow fiber membrane can be manufactured suitably.

[0029] A method for producing a hollow fiber membrane according to a tenth aspect of the present invention is a method for producing a hollow fiber membrane according to a ninth aspect of the present invention, wherein the content of the wetting and moisturizing agent is 5 to 70% by mass relative to the aqueous solution, and the content of the surfactant is 0.05 to 2% by mass relative to the aqueous solution.

[0030] With this configuration, the hollow fiber membrane can be manufactured more favorably.

[0031] An endotoxin removal method according to the 11th aspect of the present invention is an endotoxin removal method that removes the endotoxin from a liquid to be treated by filtering the liquid containing the endotoxin with a hollow fiber membrane according to any one of the 1st to 8th aspects of the present invention.

[0032] The hollow fiber membrane has the same water permeability as when wet and is easy to handle. Therefore, even if water is used as the liquid to be permeated after the hollow fiber membrane is manufactured and dried, rather than a liquid with a lower surface tension than water, sufficient permeability is possible. As a result, there is little risk of liquids with a lower surface tension than water being mixed into the liquid obtained from which endotoxins have been removed. Therefore, filtration of a liquid to be treated containing endotoxins using the hollow fiber membrane is suitable as a filtration method for removing endotoxins from a liquid to be treated containing endotoxins.

[0033] A method for removing endotoxins according to a twelfth aspect of the present invention is a method for removing endotoxins according to an eleventh aspect of the present invention, wherein the composition layer is removed from the hollow fiber membrane by passing water through the hollow fiber membrane before filtering the liquid to be treated.

[0034] With this configuration, water permeability can be ensured even when the hollow fiber membrane is dry, and even if the composition layer is removed from the hollow fiber membrane by passing water through it before filtering the liquid to be treated, water can still pass through. Therefore, it is more suitable as a filtration method for removing endotoxins from a liquid to be treated that contains endotoxins. [Effects of the Invention]

[0035] According to the present invention, it is possible to provide a hollow fiber membrane that has the same water permeability as when wet, even after being dried after manufacturing, and has excellent handling properties, as well as a method for manufacturing the same. Furthermore, according to the present invention, it is possible to provide a method for removing endotoxins using the hollow fiber membrane. [Brief explanation of the drawing]

[0036] [Figure 1] Figure 1 is a partial perspective view showing an example of a hollow fiber membrane according to an embodiment of the present invention. [Figure 2] Figure 2 is a schematic diagram showing an example of the cross-sectional structure of the hollow fiber membrane shown in Figure 1. [Figure 3] Figure 3 is a schematic diagram showing an example of a membrane filtration apparatus equipped with a hollow fiber membrane according to an embodiment of the present invention. [Modes for carrying out the invention]

[0037] As mentioned above, hollow fiber membranes are sometimes required to be able to pass water favorably even when dry. To meet this requirement, hollow fiber membranes have sometimes been used that have been immersed in a humectant such as glycerin or polyethylene glycol and then dried. By doing so, even if the hollow fiber membrane has high hydrophobicity, the surface is coated with the humectant, which can keep the contact angle with water low and improve the water permeability of the dried hollow fiber membrane.

[0038] However, the inventors have found that, in the case of highly hydrophobic hollow fiber membranes used in endotoxin capture filters, simply coating them with the aforementioned wetting and moistening agent does not ensure sufficient water permeability when dry. Therefore, when using hollow fiber membranes coated with the aforementioned wetting and moistening agent, it is necessary to allow a liquid with a surface tension lower than water, such as an aqueous surfactant solution containing about 1% by mass of a surfactant like sodium lauryl sulfate or an aqueous ethanol solution, to permeate before allowing water to pass through. Specifically, Patent Document 1 describes as an example a hollow fiber membrane made of polyethersulfone that was immersed in a 30% glycerin aqueous solution at 40°C for 4 seconds and then dried with hot air. Even with this hollow fiber membrane, sufficient water permeability could not be ensured when dry, and it was found that in order to allow water to pass through, it was necessary to allow a liquid with a surface tension lower than water to permeate before water could pass through. Furthermore, Patent Document 2 describes an example in which a hollow fiber porous membrane containing polyethersulfone is immersed in a 45% by mass aqueous glycerin solution as a humectant, and twice the amount of glycerin relative to the amount of resin constituting the hollow fiber porous membrane is attached, and then dried in a dryer at 55°C. Even with this hollow fiber membrane, sufficient water permeability could not be ensured when dried, and it was found that in order to allow water to pass through, it was necessary to allow a liquid with a surface tension lower than water to pass through before water could permeate. This can be seen from the fact that the amount of air leakage of the obtained hollow fiber membrane was measured, and there was a change in the pressure retention rate during the measurement.

[0039] Furthermore, even highly hydrophobic hollow fiber membranes that are impermeable to water when dry can be made wet by allowing a liquid with a lower surface tension than water to permeate them. This covers the surface of the hollow fiber membrane with water, resulting in a very small contact angle with water, thus ensuring good water permeability. The fact that a very small contact angle with water ensures good water permeability can also be seen from equation (1) below. P = 4γcosθ / d (1)

[0040] P represents the pressure (Pa) at which the liquid (water) permeates through the hollow fiber membrane, γ represents the surface tension (N / m) of the permeating liquid (water), θ represents the contact angle (°) of the hollow fiber membrane with respect to the liquid, and d represents the pore diameter (m) of the hollow fiber membrane. From this equation, it can be seen that not only does reducing the surface tension of the permeating liquid lower the pressure at which the liquid permeates through the hollow fiber membrane, but also that reducing the contact angle of the hollow fiber membrane with respect to the liquid lowers the pressure at which the liquid permeates through the hollow fiber membrane.

[0041] In conventional hollow fiber membranes as described above, it was necessary to allow a liquid with a lower surface tension than water to pass through before allowing water to pass through. In such cases, there is a risk that the liquid (with a lower surface tension than water) may be mixed into the water obtained by filtering through the hollow fiber membrane (dialysis water, water for injection, etc.). Also, glycerin and other substances attached to the hollow fiber membrane are washed away by the permeation of a liquid with a lower surface tension than water, but this glycerin and other substances may also be mixed into the water obtained by filtering through the hollow fiber membrane. It is necessary to prevent the mixing of glycerin and other substances into the water obtained by filtering through the hollow fiber membrane. For this reason, if the cleaning performance is low, cleaning will take a long time. Therefore, high cleaning performance is required for hollow fiber membranes to which glycerin and other substances are attached. Furthermore, cleaning with a liquid with a lower surface tension than water tends to take longer than cleaning with water. For this reason, in order to improve the cleaning performance, it is also required that water can be permeated from the beginning, rather than having to pass a liquid with a lower surface tension than water through before allowing water to pass through. In other words, hollow fiber membranes are required to have the same water permeability as when wet, even after being dried following manufacturing.

[0042] Based on the above, hollow fiber membranes are required to have the same water permeability as when wet, even after being dried after manufacturing, and to have excellent handling properties. Furthermore, these hollow fiber membranes are also required to have high washability.

[0043] As a result of various studies, the inventors have found that the above objective—to provide a hollow fiber membrane that has the same water permeability as when wet, even after being dried after manufacturing, and that is easy to handle—can be achieved by the present invention described below.

[0044] The embodiments of the present invention will be described below, but the present invention is not limited thereto.

[0045] [Hollow fiber membrane] The hollow fiber membrane according to an embodiment of the present invention comprises a hollow fiber porous substrate and a composition layer covering the porous substrate, wherein the composition layer contains a wetting and moistening agent, a surfactant, and water, and the ratio of the wetting and moistening agent to the surfactant is 2 to 200 times by mass. With such a hollow fiber membrane, even if the porous substrate is hydrophobic, and even if water is used as the liquid to be permeated after drying, rather than a liquid with a lower surface tension than water, sufficient permeability can be achieved. In other words, the composition layer can sufficiently suppress the decrease in water permeability due to drying. This water permeability during drying can be ensured to be, for example, 95% or more of the water permeability during wet conditions. Furthermore, even if the composition layer is provided on the porous substrate, the composition layer can suppress the tendency for the hollow fiber membranes to stick together after drying during end bonding or transportation. Therefore, a hollow fiber membrane with high handling properties can be obtained. Based on these factors, the hollow fiber membrane maintains the same water permeability as when wet, even after drying following manufacturing, and exhibits excellent handling properties. Furthermore, the composition layer is easily removed by passing water through the hollow fiber membrane. Additionally, since the hollow fiber membrane can permeate water from the outset, rather than liquids with a surface tension lower than water, the composition layer is also easily removed. Therefore, the hollow fiber membrane also exhibits excellent washability.

[0046] Hollow fiber membranes are generally wet upon completion of manufacturing. In the case of hollow fiber membranes used in ETRF (for example, those made of hydrophobic resin with a logarithmic removal rate of 3 or higher), the pores are very small, and because the membrane is made of hydrophobic resin, such hollow fiber membranes cannot pass water if they are dried. Even liquids with a lower surface tension than water (such as surfactants like sodium lauryl sulfate: SLS or water containing ethanol) cannot permeate them. Furthermore, when hollow fiber membranes are modularized and transported, it is common practice to dry the hollow fiber membranes. For this reason, conventional technology does not dry the hollow fiber membranes directly after manufacturing, but rather immerses them in a glycerin aqueous solution before drying. This coats the surface with glycerin, preventing the hydrophobic resin from being exposed. These dried hollow fiber membranes, with improved handling properties, are then used for modularization. The resulting hollow fiber membranes of the modules are thoroughly cleaned by passing an SLS aqueous solution, which has a lower surface tension than water, through them, followed by water, before being shipped and used (for medical purposes, etc.). However, using an SLS aqueous solution in this way presents the problem that cleaning the SLS takes a very long time.

[0047] In contrast, the hollow fiber membrane according to this embodiment contains not only a humectant such as glycerin but also a surfactant such as SLS in the composition layer coated on the porous substrate. Therefore, water can pass through it even after it has been dried after manufacturing. This is because the surfactant dissolves from the composition layer into the water passing through the hollow fiber membrane, resulting in a liquid with a lower surface tension than water. Furthermore, the liquid is automatically prepared to have a lower surface tension than water within the micropores of the membrane where the lowest possible surface tension is needed. As a result, there is no need to pass an SLS aqueous solution through the membrane before water can pass through it. In conventional methods of flowing an SLS aqueous solution, the SLS aqueous solution is also flowed into piping and pumps, increasing the amount of SLS aqueous solution used. Therefore, rinsing (cleaning the SLS) takes a very long time. When using the hollow fiber membrane according to this embodiment, at least the surfactant aqueous solution does not flow into the piping and pumps upstream of the hollow fiber membrane, thus reducing rinsing time. Furthermore, when using the hollow fiber membrane according to this embodiment, it is not necessary to pass the SLS aqueous solution through it before water is passed through it, and although surfactants are eluted from the composition layer provided in the hollow fiber membrane according to this embodiment, the amount eluted is small, thus reducing the risk of contamination in medical settings.

[0048] As shown in Figure 1, the hollow fiber membrane 11 according to an embodiment of the present invention is a hollow fiber membrane. The hollow fiber membrane 11 also comprises a hollow fiber porous substrate 21 and a composition layer 22, as shown in Figure 2. Figure 1 is a partial perspective view showing an example of the hollow fiber membrane 11 according to an embodiment of the present invention. Figure 2 is an enlarged view of a part A of the hollow fiber membrane 11 shown in Figure 1, showing the cross-sectional structure of the hollow fiber membrane 11. Figure 2 is a schematic diagram that shows the positional relationship of the layers and does not specifically show the relationship of the layer thicknesses.

[0049] (Porous base material) The porous substrate 21 is not particularly limited as long as it is a porous substrate and has a hollow fiber structure, for example, a hollow fiber membrane containing a hydrophobic resin. Even if a hollow fiber membrane containing a hydrophilic resin is used as the porous substrate 21, the effects achieved by providing the composition layer can be achieved, but there is a tendency that the composition layer does not need to be provided. From this point of view, using a hollow fiber membrane containing a hydrophobic resin can more clearly demonstrate the effects achieved by providing the composition layer. Furthermore, when the hollow fiber membrane according to this embodiment is used to remove endotoxins from a liquid to be treated that contains endotoxins, that is, when it is used as an endotoxin capture filter (ETRF), a hollow fiber membrane containing a hydrophobic resin is used as the porous substrate 21.

[0050] The hydrophobic resin is not particularly limited, but examples include polysulfone, polyethersulfone, polyacrylonitrile, cellulose acetate such as cellulose triacetate and partial cellulose acetate, polymethyl methacrylate, polyvinylidene fluoride, polyacrylonitrile, polyethylene, polytetrafluoroethylene, polyketone, and copolymers thereof. When the hollow fiber membrane is used as an endotoxin capture filter, polysulfone and polyethersulfone are preferred as the hydrophobic resin in terms of their excellent endotoxin removal performance and treated water quality. That is, when the hollow fiber membrane is used as an endotoxin capture filter, the porous substrate 21 is preferably a hollow fiber membrane containing at least one of polysulfone and polyethersulfone. Furthermore, the hydrophobic resin may be used individually or in combination of two or more of the exemplified hydrophobic resins.

[0051] The porous substrate 21 preferably has a contact angle with water of 80° or more, more preferably 85° or more, even more preferably 89° or more, and particularly preferably 90° or more. Even if the contact angle is low, the effects achieved by providing the composition layer can be exhibited, but there is a tendency for the composition layer not to be necessary. From this point of view, the higher the contact angle, the more significantly the effects achieved by providing the composition layer can be exhibited. Furthermore, the porous substrate 21 is preferable to have a high contact angle because a higher contact angle results in higher endotoxin removal performance. Therefore, it is preferable for the porous substrate 21 to have a high contact angle, but in reality, the limit is often around 150°, and the upper limit of the contact angle is preferably 150°. Therefore, the contact angle is preferably 80 to 150°, more preferably 85 to 150°, even more preferably 89 to 150°, and particularly preferably 90 to 150°. The contact angle is obtained by dropping a water droplet onto a porous substrate (or, if the porous substrate has both dense and sparse surfaces, onto the dense surface), taking an image at that moment, and measuring the angle between the surface of the water droplet and the surface of the porous substrate. A general contact angle measuring device can be used for this measurement; for example, the Drop Master 700 manufactured by Kyowa Interface Science Co., Ltd. can be used.

[0052] As described above, the porous substrate 21 preferably contains the hydrophobic resin, but may also contain components other than the hydrophobic resin (such as a hydrophilic resin). When the porous substrate 21 contains components other than the hydrophobic resin, it is preferable that the porous substrate 21 contains the hydrophobic resin in such a way that it is within the range of the contact angle, and it is preferable that the hydrophobic resin is present in an amount of, for example, 80 mass or more relative to the porous substrate. Furthermore, it is more preferable that the porous substrate 21 is made of the hydrophobic resin, and even more preferable that it is made of at least one of polysulfone and polyethersulfone.

[0053] The porous substrate 21 preferably has an endotoxin removal performance with a log reduction value (LRV) of 3 or higher, and more preferably 4 or higher. A higher LRV indicates higher endotoxin removal performance, which is preferable. That is, the higher the LRV, the more sufficient the endotoxin removal performance will be even when used as an ETRF, and sufficient treated water quality can be ensured. Furthermore, a higher LRV tends to reduce water permeability, making it more difficult for water to pass through when dried, but the presence of the composition layer allows water to pass through. Therefore, a higher LRV is preferable, but since a higher LRV tends to reduce water permeability, taking this into consideration, it is preferable to have an LRV of 7 or less, and more preferably 5 or less.

[0054] The logarithmic removal rate here is an indicator of the degree to which endotoxins are removed from porous substrates, and is expressed by the following formula (2). LRV=log 10 (C0 / C) (2)

[0055] Here, LRV represents the logarithmic removal rate, C0 represents the endotoxin concentration (EU / mL: Endotoxin Units per milliliter) contained in the liquid before permeation of the porous substrate, and C0 represents the endotoxin concentration (EU / mL) contained in the liquid after permeation of the porous substrate.

[0056] For example, an LRV of 3 indicates that the endotoxin concentration has been reduced to 1 / 1000 by permeation through the hollow fiber membrane (i.e., the endotoxin removal rate is 99.9%). An LRV of 4 indicates that the endotoxin concentration has been reduced to 1 / 10000 by permeation through the hollow fiber membrane (i.e., the endotoxin removal rate is 99.99%). Therefore, it is preferable that the porous substrate 21 has an endotoxin removal rate of 99.9% or higher, and more preferably 99.99% or higher. Specifically, the LRV can be measured using a test solution with an endotoxin concentration of 100 EU / mL, in accordance with the "Test Method for Endotoxin Inhibition Performance of Ultrafiltration Membranes" in JIS K 3824.

[0057] The porous substrate 21 has a pure water permeability of 100 to 2000 L / m³ in terms of water permeability (water flow rate) at a membrane differential pressure of 0.1 MPa. 2 The permeability is preferably liters per square meter per hour (LMH), more preferably 200 to 1800 LMH, even more preferably 200 to 1500 LMH, and particularly preferably 300 to 1500 LMH. If the permeability is too low, the permeability performance tends to be poor. If the permeability is too high, the fractionation characteristics such as endotoxin blocking performance tend to decrease. Therefore, if the permeability is within the above range, a hollow fiber membrane with superior permeability performance and fractionation characteristics (endotoxin blocking performance, etc.) can be obtained.

[0058] The permeability at a membrane differential pressure of 0.1 MPa can be determined, for example, as follows: First, a module having 20 porous substrates with an effective length of 20 cm, each sealed at one end of the porous substrate, is used. Pure water is used as the raw water, and the water is filtered under conditions of a filtration pressure of 0.1 MPa and a temperature of 25°C to measure the permeability per unit time. Specifically, the housing of the module is filled with pure water, and external pressure is applied so that the membrane differential pressure becomes 0.1 MPa. At this time, the amount of water flowing out from the opening at the other end is measured for 1 minute. From this measured permeability, the permeability per unit membrane area and per unit time is converted to the permeability (L / m³) at a membrane differential pressure of 0.1 MPa. 2 / Time: LMH) is obtained.

[0059] The porous substrate 21 preferably has a gradient structure in which the pores within the porous substrate 21 gradually increase in size from one of the inner circumferential surface and the outer circumferential surface toward the other. This gradient structure may be one in which the pores within the porous substrate 21 gradually increase in size from the inner circumferential surface toward the outer circumferential surface, or one in which the pores gradually increase in size from the outer circumferential surface toward the inner circumferential surface. Alternatively, the porous substrate 21 may have a gradient structure in which the pores of the porous substrate 21 gradually increase in size from the inner circumferential surface toward the central part, and gradually decrease in size from the central part toward the outer circumferential surface. The gradient structure is preferably one in which the pores gradually increase in size from the inner circumferential surface toward the outer circumferential surface.

[0060] As the porous substrate 21, commercially available products can be used, for example, UF film 6401 and UF film 6021 manufactured by Kuraray Co., Ltd.

[0061] (composition layer) The composition layer 22 covers the porous substrate 21, and may cover the surface 23a of the porous substrate 21, or it may cover the surface 23b of the pores 24 inside the porous substrate 21. Preferably, as shown in Figure 2, the composition layer 22 covers not only the surface 23a of the porous substrate 21, but also the surface 23b of the pores 24 inside the porous substrate 21. This allows the surface tension to be reduced even within the pores (micropores) 24 in the film where it is most necessary to reduce the surface tension, and allows for higher water permeability from the dry state. The composition layer 22 contains a wetting humectant, a surfactant, and water.

[0062] (Moisturizing agent) The aforementioned wetting and moisturizing agent is not particularly limited as long as it can moisturize the surface of the porous substrate 21 when applied to its surface. By including the wetting and moisturizing agent in the composition layer 22 that covers the porous substrate 21, the surface of the resulting hollow fiber membrane 11 can be moisturized. Furthermore, the wetting and moisturizing agent is preferably a non-volatile or low-volatility hydrophilic liquid. Non-volatile means that it hardly volatilizes at room temperature and atmospheric pressure, and specifically means that even when left for 24 hours at room temperature and atmospheric pressure, it retains 99% or more of its mass. Low-volatility means that it does not volatilize easily at room temperature and atmospheric pressure, and for example, its vapor pressure is lower than that of water, and more preferably, its vapor pressure at 25°C is 3200 Pa or less. Both non-volatile and low-volatility mean that it volatilizes less easily than water. The wetting and moisturizing agent is preferably a hydrophilic liquid, and more preferably a liquid that is easily washed off the hollow fiber membrane 11 with water. Examples of the humectants include polyhydric alcohols, urea, amino acids, hyaluronic acid, and polysaccharides. Examples of polyhydric alcohols include polyethylene glycol, polypropylene glycol, sorbitol, glycerin, ethylene glycol, triethylene glycol, polyglycerin, polyvinyl alcohol, propylene glycol, xylitol, diglycerin, dipropylene glycol, and 1,3-butylene glycol. Among these, polyhydric alcohols are preferred as humectants, and glycerin is more preferred from the viewpoint of safety and cost. Furthermore, the humectants may be used individually or in combination of two or more.

[0063] (Surfactants) The surfactant is not particularly limited as long as it can reduce the contact angle of the porous substrate 21 with respect to water by being attached to the surface of the porous substrate 21. That is, by including the surfactant in the composition layer 22 that covers the porous substrate 21, the contact angle of the resulting hollow fiber membrane 11 with respect to water can be reduced to be lower than the contact angle of the porous substrate 21 with respect to water. Furthermore, the surfactant is preferably a non-volatile or low-volatility surfactant. More preferably, the surfactant is a surfactant that is easily washed away from the hollow fiber membrane 11 with water. The surfactant may be a cationic surfactant, anionic surfactant, nonionic surfactant, or amphoteric surfactant, but anionic surfactants are preferred from the viewpoint of cost and general use. Specifically, cationic surfactants include didecyldimethylammonium methyl sulfate, benzalkonium chloride, and cetyltrimethylammonium chloride. Specifically, anionic surfactants include alkylbenzene sulfonates, alkyl ether sulfates, sodium lauryl sulfate (SLS), sodium laureth sulfate, and sodium alkylbenzene sulfonate. Examples of nonionic surfactants include sorbitan fatty acid esters, sucrose fatty acid esters, glycerin fatty acid esters, fatty alcohol ethoxylates, polyoxyethylene alkylphenyl ethers, polysorbate 20, and sorbitan monolaurate. Examples of biionic surfactants include cocamidopropyl betaine, lauryldimethyl betaine, alkyldiaminoethylglycine hydrochloride, sodium laurylaminopropionate, and coconut oil fatty acid amidopropyl betaine. Among these surfactants, anionic surfactants are preferred, and sodium lauryl sulfate is more preferred from the viewpoint of safety and cost. Furthermore, these surfactants may be used individually or in combination of two or more.

[0064] The composition layer is a liquid layer, for example, in which the wetting and moisturizing agent is a liquid and the surfactant is either a liquid or dissolved in the wetting and moisturizing agent. Therefore, the composition layer is not limited to this and may be a liquid layer. That is, the composition layer 22 may be a liquid layer containing the wetting and moisturizing agent, the surfactant and water, or it may be a liquid layer consisting of the wetting and moisturizing agent, the surfactant and water.

[0065] (Content) The ratio of the humectant to the surfactant (humectant / surfactant) is 2 to 200 times by mass, preferably 3 to 200 times, more preferably 3 to 150 times, and even more preferably 4 to 150 times. If this ratio is too low or too high, it tends to be difficult to fully achieve the effect of allowing water to pass through even in a dry state by providing the composition layer. In other words, by keeping the ratio within this range, a hollow fiber membrane with the same water permeability as when wet can be obtained even after drying following manufacturing. It also exhibits excellent handling and washability. This ratio can be measured, for example, using gas chromatography, liquid chromatography, ultraviolet-visible light spectrophotometer, and total organic carbon meter. Furthermore, if non-volatile or low-volatility materials are used as the surfactant and humectant, the ratio of their charge amounts will be the ratio mentioned above, as they are less volatile.

[0066] The water content is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, relative to the composition layer. If the water content is too high, the hollow fiber membranes tend to stick together, reducing handling properties. Therefore, if the water content is within the above range, handling properties are excellent. For this reason, the lower limit of the water content is not particularly limited, but if the wetting and moistening agent is a hydrophilic liquid, it absorbs moisture (moisture from the air, etc.), so the lower limit of the water content is about 3% by mass, and 5% by mass in a space with a relative humidity of 10%. Reducing the water content improves handling properties, but reducing it to a low level requires a huge amount of energy, and in a normal space, it becomes 3% by mass or more, or 5% by mass or more, resulting in a large energy loss. Therefore, the water content is preferably 3% by mass or more, and more preferably 5% by mass or more.

[0067] The water content can be measured by the following method. After manufacturing the hollow fiber membrane, place it in a storage environment and measure the weight of the hollow fiber membrane when the moisture reaches equilibrium. This weight is denoted as W1. Then, measure the weight of the hollow fiber membrane after the water has evaporated using a dryer or the like. This weight is denoted as W2. Also, measure the weight of the porous substrate. This weight is denoted as W3. Alternatively, the weight of the porous substrate may be measured after passing 90°C water through the hollow fiber membrane for 30 minutes or more to remove all the composition layers present in the hollow fiber membrane, and then drying the porous substrate. Using W1, W2, and W3, the water content Cw (%) is measured from the following formula (3). Cw(%)=(W1-W2) / (W2-W3)×100 (3)

[0068] (Mass ratio of hollow fiber membrane to porous substrate: Amount of composition layer) The mass ratio of the hollow fiber membrane to the porous substrate (hollow fiber membrane / porous substrate) is preferably 1.03 to 2 times, more preferably 1.05 to 2 times, and even more preferably 1.05 to 1.8 times. If the mass ratio of the hollow fiber membrane to the porous substrate is small, the amount of the composition layer is relatively small, and if the mass ratio is large, the amount of the composition layer is relatively large. This mass ratio represents the amount of the composition layer. If the mass ratio is too small, the amount of the composition layer is small, and the effect achieved by having the composition layer, namely the effect of exhibiting the same water permeability even when dry as when wet, tends not to be fully achieved. If the mass ratio is too large, the amount of the composition layer becomes too large, and the hollow fiber membranes tend to stick together, reducing handling properties. Therefore, if the mass ratio of the hollow fiber membrane to the porous substrate is within the above range, a hollow fiber membrane can be obtained that has the same water permeability as when wet, even after being dried after manufacturing, and has excellent handling and washability.

[0069] The mass ratio of the hollow fiber membrane to the porous substrate is determined by measuring the mass ratio Cm (times) of the hollow fiber membrane to the porous substrate using the following formula (4). Cm (times) = W2 / W3 (4)

[0070] The method of using the hollow fiber membrane is not particularly limited, but the hollow fiber membrane contains a hydrophobic resin, and even if the hollow fiber membrane has small pores within the membrane, it retains the same water permeability as when wet even after drying after manufacturing, making it suitable for use in ETRF. In other words, the hollow fiber membrane is preferably used to remove endotoxins from a liquid to be treated that contains endotoxins.

[0071] The hollow fiber membrane, even when dried after manufacturing, has the same water permeability as when wet. It is possible to ensure that the water permeability when dry is, for example, 95% or more of the water permeability when wet. Specifically, it is preferable that the water permeability of the hollow fiber membrane when dry (amount of water permeable at a differential pressure of 0.1 MPa between membranes) (water permeability when dry / water permeability when wet) is 95% or more, and more preferably 90% or more. The water permeability of the hollow fiber membrane when dry (amount of water permeable at a differential pressure of 0.1 MPa between membranes) can be measured by the same method as the water permeability of the porous substrate at a differential pressure of 0.1 MPa described above (specifically, in this method, the hollow fiber membrane dried after manufacturing can be used instead of the porous substrate). The water permeability of the hollow fiber membrane in the dry state at a membrane differential pressure of 0.1 MPa is preferably 100 to 2000 LMH, and more preferably 300 to 1500 LMH. Furthermore, the composition layer is removed from the hollow fiber membrane by passing water through it. In addition, the water permeability of the hollow fiber membrane in the wet state at a membrane differential pressure of 0.1 MPa is equivalent to the water permeability of the porous substrate in the wet state, and specifically, it is preferably 100 to 2000 LMH, more preferably 200 to 1800 LMH, even more preferably 200 to 1500 LMH, and particularly preferably 300 to 1500 LMH.

[0072] [Method for manufacturing hollow fiber membranes] The method for manufacturing the hollow fiber membrane is not particularly limited as long as it is a manufacturing method that can produce the hollow fiber membrane. Specifically, examples include a method in which the porous substrate is immersed in an aqueous solution in which the wetting humectant and the surfactant are dissolved, and the porous substrate immersed in the aqueous solution is dried to coat the porous substrate with the composition layer. Here, drying does not mean removing all the water contained in the composition layer, but rather drying the water so that the composition layer can be formed. If the wetting humectant and the surfactant are non-volatile or low-volatility, the drying can be easily achieved by drying the water. On the other hand, if the wetting humectant and the surfactant are not non-volatile or low-volatility, it is necessary to dry them while taking into account that the wetting humectant and the surfactant will volatilize when the water dries. That is, an example is a manufacturing method in which the porous substrate is immersed in an aqueous solution in which the wetting humectant and the surfactant are dissolved, and the porous substrate immersed in the aqueous solution is dried so that the composition layer can be formed on the porous substrate.

[0073] The aqueous solution is not particularly limited as long as it is an aqueous solution capable of forming the composition layer, but examples include aqueous solutions containing the humectant and surfactant. Since the aqueous solution contains the surfactant, etc., the aqueous solution can suitably cover the surface of the porous substrate by immersing the porous substrate in the aqueous solution. By drying the porous substrate in this state (the porous substrate immersed in the aqueous solution), the composition layer is coated at least on the surface of the porous substrate. Furthermore, since the aqueous solution contains the surfactant, etc., it is preferable that the aqueous solution penetrates into the pores within the porous substrate when the porous substrate is immersed in the aqueous solution. By drying the porous substrate in this state, the composition layer is coated not only on the surface of the porous substrate but also on the surface of the pores inside the porous substrate. Note that if the humectant and surfactant are non-volatile or low-volatility, they are not removed by this drying, and only the water content decreases. As a result, it is possible to achieve a state in which the humectant and surfactant are coated at a high concentration, which is preferable.

[0074] The content of the wetting and moistening agent is preferably 5 to 70% by mass, more preferably 5 to 65% by mass, and even more preferably 5 to 60% by mass, relative to the aqueous solution. The content of the surfactant is preferably 0.05 to 2% by mass, more preferably 0.1 to 2% by mass, and even more preferably 0.1 to 1% by mass, relative to the aqueous solution. When the content of the wetting and moistening agent and the surfactant are within the above ranges, a hollow fiber membrane that can suitably provide the effects of having the composition layer can be suitably manufactured. Specifically, if the content of the wetting and moistening agent is too low, it tends to be difficult to ensure water permeability during drying that is not inferior to that during wetting. Also, if the content of the wetting and moistening agent is too high, it tends to reduce handling properties. If the content of the surfactant is too low, it tends to be difficult to ensure water permeability during drying that is not inferior to that during wetting. Also, if the content of the surfactant is too high, it tends to increase the time required to rinse the hollow fiber membrane.

[0075] The drying process is not particularly limited as long as it can dry the porous substrate immersed in the aqueous solution so that the composition layer is formed on the porous substrate, and can be carried out using, for example, a general-purpose dryer. The dryer is not particularly limited, but examples include a forced-air dryer, a constant-temperature dryer, a hot-air dryer, an electric dryer, a gas dryer, and an incubator, among which a hot-air dryer is preferred. The drying conditions are also not particularly limited, but for example, if the drying temperature is too low, the drying time tends to be prolonged. On the other hand, if the drying temperature is too high, there is a risk of decomposing the wetting humectant and the surfactant. Therefore, the drying temperature is preferably 40 to 150°C, more preferably 45 to 130°C, and even more preferably 50 to 120°C. As for the drying time, it is preferable to select a time appropriate for the formation of the composition layer according to the drying temperature and other factors.

[0076] [How to use hollow fiber membranes] The hollow fiber membrane according to this embodiment can be used for membrane filtration. Specifically, for example, the hollow fiber membrane can be modularized as follows, and this modularized structure can be used for membrane filtration. More specifically, the hollow fiber membrane according to this embodiment is bundled in a predetermined number of strands, cut to a predetermined length, filled into a casing of a predetermined shape, and the ends of the hollow fiber bundles are fixed to the casing with a thermosetting resin such as polyurethane resin or epoxy resin to form a module. Various module structures are known, such as a type in which both ends of the hollow fiber membrane are open and fixed, and a type in which one end of the hollow fiber membrane is open and fixed, and the other end is sealed but not fixed, and the hollow fiber membrane according to this embodiment can be used in any of these module structures.

[0077] The hollow fiber membrane according to this embodiment is modularized as described above and can be incorporated into a membrane filtration device, for example, as shown in Figure 3. Figure 3 is a schematic diagram showing an example of a membrane filtration device equipped with the hollow fiber membrane according to this embodiment. The membrane filtration device 31 includes a membrane module 32 in which the hollow fiber membrane is modularized as described above. For example, the upper end 33 of the hollow fiber membrane has an open hollow section, and the lower end 34 has the hollow section sealed with an epoxy resin. Another example of the membrane module 32 is one which uses 20 hollow fiber membranes with an effective membrane length of 20 cm. In this membrane filtration device 31, the liquid to be processed is introduced from the inlet 35, and the filtered liquid (filtered water) etc., filtered by the membrane module 32, is discharged from the outlet 36. In this way, filtration using the hollow fiber membrane is performed. The air introduced into the membrane filtration device 31 is discharged from the air vent 37.

[0078] The hollow fiber membrane according to this embodiment is modularized in this way and used in various applications such as water purification, drinking water production, industrial water production, and wastewater treatment. More specifically, as described above, the hollow fiber membrane is preferably used to remove endotoxins from a liquid to be treated that contains endotoxins.

[0079] (Methods for removing endotoxins) The method for removing endotoxins is not particularly limited as long as it uses the hollow fiber membrane. For example, one method of removing endotoxins is to filter the liquid to be treated containing endotoxins through the hollow fiber membrane to remove the endotoxins from the liquid to be treated. For example, one method of removal is to use the membrane filtration device 31. In this removal method, it is preferable to remove the composition layer from the hollow fiber membrane by passing water through it before filtering the liquid to be treated. Even if the composition layer is removed from the hollow fiber membrane by passing water through it before filtering the liquid to be treated, the liquid to be treated can still be passed through. Therefore, this method is more suitable for filtering out endotoxins from a liquid to be treated containing endotoxins. Furthermore, the composition layer can be removed almost completely by passing water through it. Therefore, there is no risk of foreign matter originating from the composition layer being mixed into the liquid obtained by filtering the liquid to be treated containing endotoxins. The removal of the composition layer by water flow can be seen, for example, from the fact that the water permeability of the hollow fiber membrane after removal becomes the same as that of the porous substrate. It can also be seen from the fact that the LRV of the hollow fiber membrane after removal becomes the same as that of the porous substrate.

[0080] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited thereto. [Examples]

[0081] [Example 1] First, a polysulfone hollow fiber membrane (UF membrane 6401 manufactured by Kuraray Co., Ltd.) was permeated with an SLS aqueous solution, and then water was passed through it to remove glycerin from the polysulfone hollow fiber membrane. This polysulfone hollow fiber membrane, from which the glycerin had been removed, was used as a porous substrate.

[0082] Furthermore, the pure water permeability performance of this porous substrate (the polysulfone hollow fiber membrane from which glycerin has been removed) was measured by the following method. Specifically, the porous substrate was modularized to form a membrane filtration device as shown in Figure 3 (membrane area: 0.01 m²). 2A porous substrate was prepared and adjusted to a membrane differential pressure of 0.1 MPa, and the amount of water (permeability) permeating through the porous substrate was measured. By dividing the permeability per unit time obtained in this way by the membrane area, the permeability (permeation rate) of pure water at a membrane differential pressure of 0.1 MPa was calculated. As a result, the pure water permeability performance of the porous substrate was 790 LMH in terms of permeability at a membrane differential pressure of 0.1 MPa.

[0083] The LRV of the porous substrate was measured by the following method. Specifically, using a test solution with an endotoxin concentration of 100 EU / mL, the LRV of the porous substrate was measured in accordance with the "Test Method for Endotoxin Inhibition Performance of Ultrafiltration Membranes" in JIS K 3824. As a result, the LRV of the porous substrate was 4.39.

[0084] The contact angle of the porous substrate with respect to water was measured using a Drop Master 700 manufactured by Kyowa Interface Science Co., Ltd. The result showed that the contact angle of the porous substrate with respect to water was 95°. The porous substrate was used in a dry state during the measurement. The amount of water dropped was 1.0 μl, and the contact angle was measured 2 seconds after the start of water droplet application to the porous substrate. The measurement was performed in an atmosphere of room temperature (25°C) and relative humidity (50%).

[0085] Next, the porous substrate (the porous substrate after removing glycerin from the polysulfone hollow fiber membrane) was immersed in an aqueous solution containing a humectant and a surfactant without drying it, while it was still wet. The aqueous solution contained 10% by mass of glycerin (Gly) as the humectant, 0.125% by mass of sodium lauryl sulfate (SLS) as the surfactant, and the remainder was water.

[0086] The porous substrate, immersed in the aqueous solution, was dried for 24 hours using a hot air dryer (DKN602 manufactured by Yamato Scientific Co., Ltd.) under the drying temperature (55°C) shown in Table 1, and then stored for 48 hours in an environment of 25°C and 50% relative humidity. As a result, a hollow fiber membrane was obtained in which the composition layer consisting of the wetting and moistening agent, the surfactant, and water covered not only the surface of the porous substrate but also the surface of the pores inside the porous substrate.

[0087] (Mass ratio: moisturizing agent / surfactant) The ratio of the humectant to the surfactant in the composition layer provided on the resulting hollow fiber membrane was calculated from the concentrations of the humectant and surfactant in the aqueous solution used, given that the humectant and surfactant are non-volatile. The result was the value shown in Table 1 (80 times) in mass ratio.

[0088] (Water content) The water content in the composition layer provided in the obtained hollow fiber membrane was measured. Specifically, first, the mass of the hollow fiber membrane was measured. This mass was designated as W1. Next, the hollow fiber membrane was dried for 24 hours at 65°C using the hot air dryer (DKN602 manufactured by Yamato Scientific Co., Ltd.). The mass at this time was designated as W2. Using W1 and W2, the water content Cw (mass%) was calculated from formula (3). As a result, the value shown in Table 1 (21 mass%) was obtained.

[0089] (Mass ratio: hollow fiber membrane / porous base material) The mass ratio of the obtained hollow fiber membrane to the porous substrate was measured. Specifically, first, the weight of the porous substrate was measured. This weight was denoted as Wa. Next, the weight of the hollow fiber membrane was measured. This weight was denoted as Wb. Using Wa and Wb, the mass ratio Cm (times) of the hollow fiber membrane to the porous substrate was calculated from equation (4). As a result, the value shown in Table 1 (1.15 times) was obtained.

[0090] [evaluation] The hollow fiber membrane obtained as described above was evaluated as follows.

[0091] (Water permeability when dry) In a method for measuring the pure water permeability of a porous substrate, the obtained hollow fiber membrane was modularized using the hollow fiber membrane instead of the porous substrate, resulting in a membrane filtration device as shown in Figure 3 (membrane area: 0.01 m²). 2 A sample was prepared and measured. The hollow fiber membrane used was stored under the conditions described above and was therefore in a dry state. As a result, the water permeability rate in the dry state (water permeability at a differential pressure between membranes of 0.1 MPa) was the value shown in Table 1 (774 LMH).

[0092] (Water permeability when wet) After filtering water for 30 minutes using a membrane filtration system that measured the water permeability rate when dry, the amount of water permeating through the porous substrate (water permeability) was measured. By dividing the obtained water permeability per unit time by the membrane area, the water permeability of pure water at a membrane differential pressure of 0.1 MPa was calculated. The value obtained here is the water permeability rate when wet (water permeability at a membrane differential pressure of 0.1 MPa), which is the value shown in Table 1 (790 LMH).

[0093] Furthermore, LRV (Light Range Value) was also measured using the hollow fiber membrane in this state. The result was 4.39, the same as the LRV of the porous substrate.

[0094] (Water permeability in dry conditions / Water permeability in wet conditions) The water permeability rate in the dry state obtained as described above was divided by the water permeability rate in the wet state. The obtained values ​​(98%) are shown in Table 1.

[0095] (Handling) Membrane area 1m 2In order to modularize the hollow fiber membranes so as to achieve the above, a membrane bundle formed by bundling 2000 of the hollow fiber membranes was prepared. The membrane bundle was visually checked in a state where no particular operation of separating the hollow fiber membranes from each other in the membrane bundle was performed (a state where the hollow fiber membranes were left to their own weight). As a result, when no adhesion between the hollow fiber membranes in the membrane bundle was confirmed, it was judged that the adhesion between the hollow fiber membranes was sufficiently suppressed, and the hollow fiber membranes could be suitably modularized, and this was evaluated as "Good". In addition, when adhesion between the hollow fiber membranes in the membrane bundle was confirmed, it could not be said that the adhesion between the hollow fiber membranes was sufficiently suppressed, and when modularizing the hollow fiber membranes, there was a risk that defects such as poor adhesion (for example, leakage caused by the resin used for sealing during modularization not being placed at an appropriate position if the hollow fiber membranes are adhered to each other) would occur, and this was evaluated as "Poor". When the hollow fiber membrane according to Example 1 was used, it was evaluated as "Good". The obtained evaluation results are shown in Table 1.

[0096] (Detergency) Membrane area 1 m 2 The hollow fiber membranes were modularized so as to achieve the above, to produce a membrane filtration device as shown in Figure 3. Hot water at 90°C was passed through the obtained membrane filtration device at a filtration flux of 10 m 3 / m 2 / day for 90 minutes. Thereafter, ultrapure water having an electrical resistivity of 18 MΩ·cm was passed through at a filtration flux of 10 m 3 / m 2 / day, and the time until the obtained permeate reached 15 MΩ·cm or more was measured. The shorter this time, the higher the detergency. Specifically, when this time was 6 hours or less, it was judged as "Good", and when it exceeded 6 hours, it was judged as "Poor". When the hollow fiber membrane according to Example 1 was used, it was evaluated as "Good". The obtained evaluation results are shown in Table 1.

[0097] [Examples 2 to 7, and Comparative Examples 1 to 6] The same procedure as in Example 1 was carried out, except that the concentration of the wet humectant and the concentration of the surfactant in the aqueous solution were changed to the concentrations shown in Table 1, respectively. These evaluation results are also shown in Table 1.

[0098] Furthermore, when the hollow fiber membrane according to Comparative Example 1 was used, it was not possible to pass water through it when measuring the water permeability rate during dry conditions. Therefore, in the case of the membrane filtration apparatus using the hollow fiber membrane according to Comparative Example 1, a 70°C SLS aqueous solution (1% by mass concentration) was added to the membrane filtration apparatus, and the hollow fiber membrane provided in the membrane filtration apparatus was immersed in the SLS aqueous solution (1% by mass concentration) for 1 hour. After that, the water was filtered through the membrane filtration apparatus for 30 minutes, and then the water permeability rate during wet conditions was measured.

[0099] Furthermore, when the hollow fiber membrane according to Comparative Example 1 was used, water could not be passed through it when measuring the cleaning performance, similar to when measuring the water permeability during drying. Therefore, it was not possible to evaluate the cleaning performance. However, even when using the hollow fiber membrane according to Comparative Example 1, it was possible to pass water through it after immersion in a 70°C SLS aqueous solution (concentration 1% by mass). However, in this case, it is also necessary to wash this SLS aqueous solution, and the time required for washing is thought to exceed 6 hours. For these reasons, the hollow fiber membrane according to Comparative Example 1 was evaluated as not having high cleaning performance, and since it was not possible to evaluate the cleaning performance, the evaluation result was "-".

[0100] [Table 1]

[0101] As can be seen from Table 1, hollow fiber membranes (hollow fiber membranes according to Examples 1 to 7) in which a composition layer containing the wetting and moistening agent, the surfactant, and water coats a hollow fiber porous substrate, and the ratio of the wetting and moistening agent to the surfactant is 2 to 200 times by mass ratio, exhibited excellent handling and washability. Furthermore, the hollow fiber membranes according to Examples 1 to 7 had a high ratio of water permeability when dry to water permeability when wet. From this, it was found that the hollow fiber membranes according to Examples 1 to 7, even when dried after manufacturing, have the same water permeability as when wet. In addition, it was found that even when the surfactant is not included, if the amount of the wetting and moistening agent is large (Comparative Example 3), or even when both the wetting and moistening agent and the surfactant are included, but the ratio of the content is less than 2 times (Comparative Example 6), the ratio of water permeability when dry to water permeability when wet is high, but the handling and washability are poor. Furthermore, it was found that when only one of the humectant and the surfactant was included, and the amount of the humectant was not as large as in Comparative Example 3 (Comparative Examples 2, 4, and 5), and when both were included but the content ratio exceeded 200 times (Comparative Example 1), the water permeability could not be exhibited in the same way as when wet, as with the hollow fiber membranes of Examples 1 to 7.

[0102] From the above, it was found that the hollow fiber membranes according to Examples 1 to 7, compared to cases using hollow fiber membranes other than those using hollow fiber membranes in which a composition layer containing the wetting and moistening agent, the surfactant, and water covers a hollow fiber porous substrate, and the content ratio of the wetting and moistening agent to the surfactant is 2 to 200 times by mass ratio (Comparative Examples 1 to 6), not only have the same water permeability as when wet, even when dried after manufacturing, but also have excellent handling properties and excellent washability. [Explanation of symbols]

[0103] 11 Hollow fiber membrane 21 Porous substrate 22 Composition layer 23a, 23b surface 24 holes 31 Membrane filtration system 32 membrane modules 33 Upper end 34 Lower end 35 Inlet 36 Outlet 37 Air vent

Claims

1. The invention comprises a hollow fiber-like porous substrate and a composition layer that covers the porous substrate. The aforementioned composition layer It contains a humectant, surfactant, and water. A hollow fiber membrane in which the content ratio of the wetting and moisturizing agent to the surfactant is 2 to 200 times by mass.

2. The hollow fiber membrane according to claim 1, wherein the water content is 50% by mass or less relative to the composition layer.

3. The hollow fiber membrane according to claim 1, wherein the mass ratio of the hollow fiber membrane to the porous substrate is 1.03 to 2 times.

4. The hollow fiber membrane according to claim 1, wherein the composition layer is coated not only on the surface of the porous substrate but also on the surface of the pores inside the porous substrate.

5. The porous substrate is Contains hydrophobic resin, The contact angle with water is 80° or more. The hollow fiber membrane according to claim 1, wherein the endotoxin removal performance is 3 or more in terms of logarithmic removal rate.

6. The hollow fiber membrane according to claim 5, wherein the hydrophobic resin is at least one of polysulfone and polyethersulfone.

7. The aforementioned wetting and moisturizing agent is a non-volatile or poorly volatile hydrophilic liquid. The hollow fiber membrane according to claim 1, wherein the surfactant is a non-volatile or low-volatility surfactant.

8. A hollow fiber membrane according to claim 1, used for removing endotoxins from a liquid to be treated that contains endotoxins.

9. A method for producing a hollow fiber membrane according to any one of claims 1 to 8, The porous substrate is immersed in an aqueous solution in which the wetting and moistening agent and the surfactant are dissolved. A method for producing a hollow fiber membrane, comprising drying the porous substrate that has been immersed in the aqueous solution, thereby coating the porous substrate with the composition layer.

10. The content of the aforementioned humectant is 5 to 70% by mass relative to the aqueous solution. The method for producing a hollow fiber membrane according to claim 9, wherein the content of the surfactant is 0.05 to 2% by mass relative to the aqueous solution.

11. A method for removing endotoxins, comprising filtering a liquid to be treated containing endotoxins with a hollow fiber membrane according to any one of claims 1 to 8 to remove the endotoxins from the liquid to be treated.

12. The method for removing endotoxins according to claim 11, wherein the composition layer is removed from the hollow fiber membrane by passing water through the hollow fiber membrane before filtering the liquid to be treated.

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